The Rust Reference

# Introduction

This book is the primary reference for the Rust programming language. It provides three kinds of material:

- Chapters that informally describe each language construct and their use.
- Chapters that informally describe the memory model, concurrency model, runtime services, linkage model and debugging facilities.
- Appendix chapters providing rationale and references to languages that influenced the design.

> Note: You may also be interested in the grammar.

Warning: This book is incomplete. Documenting everything takes a while. See the undocumented page for what is not documented in this book.

## What The Reference is Not

This book does not serve as an introduction to the language. Background familiarity with the language is assumed. A separate book is available to help acquire such background familiarity.

This book also does not serve as a reference to the standard library included in the language distribution. Those libraries are documented separately by extracting documentation attributes from their source code. Many of the features that one might expect to be language features are library features in Rust, so what you're looking for may be there, not here.

Similarly, this book does not usually document the specifics of `rustc` as a tool or of Cargo. `rustc` has its own book. Cargo has a book that contains a reference. There are a few pages such as linkage that still describe how `rustc` works.

This book also only serves as a reference to what is available in stable Rust. For unstable features being worked on, see the Unstable Book.

Finally, this book is not normative. It may include details that are specific to `rustc` itself, and should not be taken as a specification for the Rust language. We intend to produce such a book someday, and until then, the reference is the closest thing we have to one.

## How to Use This Book

This book does not assume you are reading this book sequentially. Each chapter generally can be read standalone, but will cross-link to other chapters for facets of the language they refer to, but do not discuss.

There are two main ways to read this document.

The first is to answer a specific question. If you know which chapter answers that question, you can jump to that chapter in the table of contents. Otherwise, you can press `s` or the click the magnifying glass on the top bar to search for keywords related to your question. For example, say you wanted to know when a temporary value created in a let statement is dropped. If you didn't already know that the lifetime of temporaries is defined in the expressions chapter, you could search "temporary let" and the first search result will take you to that section.

The second is to generally improve your knowledge of a facet of the language. In that case, just browse the table of contents until you see something you want to know more about, and just start reading. If a link looks interesting, click it, and read about that section.

That said, there is no wrong way to read this book. Read it however you feel helps you best.

### Conventions

Like all technical books, this book has certain conventions in how it displays information. These conventions are documented here.

- Statements that define a term contain that term in italics. Whenever that term is used outside of that chapter, it is usually a link to the section that has this definition.

  An example term is an example of a term beind defined.

- Notes that contain useful information about the state of the book or point out useful, but mostly out of scope, information are in blockquotes that start with the word "Note:" in bold.

  > Note: This is an example note.

- Warnings that show unsound behavior in the language or possibly confusing interactions of language features are in a special warning box.

  Warning: This is an example warning.

- Code snippets inline in the text are inside `<code>` tags.

  Longer code examples are in a syntax highlighted box that has controls for copying, executing, and showing hidden lines in the top right corner.

      # // This is a hidden line.
      fn main() {
          println!("This is a code example");
      }

- The grammar and lexical structure is in blockquotes with either "Lexer" or "Syntax" in bold superscript as the first line.

  > Syntax
  > ExampleGrammar:
  > `~` Expression
  > | `box` Expression

## Contributing

We welcome contributions of all kinds.

You can contribute to this book by opening an issue or sending a pull request to the Rust Reference repository. If this book does not answer your question, and you think its answer is in scope of it, please do not hesitate to file an issue or ask about it in the Rust docs channels on IRC or discord. Knowing what people use this book for the most helps direct our attention to making those sections the best that they can be.

# Notation

## Unicode productions

A few productions in Rust's grammar permit Unicode code points outside the ASCII range. We define these productions in terms of character properties specified in the Unicode standard, rather than in terms of ASCII-range code points. The grammar has a Special Unicode Productions section that lists these productions.

## String table productions

Some rules in the grammar — notably unary operators, binary operators, and keywords — are given in a simplified form: as a listing of printable strings. These cases form a subset of the rules regarding the token rule, and are assumed to be the result of a lexical-analysis phase feeding the parser, driven by a DFA, operating over the disjunction of all such string table entries.

When such a string in `monospace` font occurs inside the grammar, it is an implicit reference to a single member of such a string table production. See tokens for more information.

# Lexical structure

# Input format

Rust input is interpreted as a sequence of Unicode code points encoded in UTF-8. Most Rust grammar rules are defined in terms of printable ASCII-range code points, but a small number are defined in terms of Unicode properties or explicit code point lists. 1

1

Substitute definitions for the special Unicode productions are provided to the grammar verifier, restricted to ASCII range, when verifying the grammar in this document.

# Keywords

Rust divides keywords into three categories:

- strict
- reserved
- weak

## Strict keywords

These keywords can only be used in their correct contexts. They cannot be used as the names of:

- Items
- Variables and function parameters
- Fields and variants
- Type parameters
- Lifetime parameters or loop labels
- Macros or attributes
- Macro placeholders
- Crates

> Lexer:
> KW_AS : `as`
> KW_BREAK : `break`
> KW_CONST : `const`
> KW_CONTINUE : `continue`
> KW_CRATE : `crate`
> KW_ELSE : `else`
> KW_ENUM : `enum`
> KW_EXTERN : `extern`
> KW_FALSE : `false`
> KW_FN : `fn`
> KW_FOR : `for`
> KW_IF : `if`
> KW_IMPL : `impl`
> KW_IN : `in`
> KW_LET : `let`
> KW_LOOP : `loop`
> KW_MATCH : `match`
> KW_MOD : `mod`
> KW_MOVE : `move`
> KW_MUT : `mut`
> KW_PUB : `pub`
> KW_REF : `ref`
> KW_RETURN : `return`
> KW_SELFVALUE : `self`
> KW_SELFTYPE : `Self`
> KW_STATIC : `static`
> KW_STRUCT : `struct`
> KW_SUPER : `super`
> KW_TRAIT : `trait`
> KW_TRUE : `true`
> KW_TYPE : `type`
> KW_UNSAFE : `unsafe`
> KW_USE : `use`
> KW_WHERE : `where`
> KW_WHILE : `while`

## Reserved keywords

These keywords aren't used yet, but they are reserved for future use. They have the same restrictions as strict keywords. The reasoning behind this is to make current programs forward compatible with future versions of Rust by forbidding them to use these keywords.

> Lexer
> KW_ABSTRACT : `abstract`
> KW_BECOME : `become`
> KW_BOX : `box`
> KW_DO : `do`
> KW_FINAL : `final`
> KW_MACRO : `macro`
> KW_OVERRIDE : `override`
> KW_PRIV : `priv`
> KW_TYPEOF : `typeof`
> KW_UNSIZED : `unsized`
> KW_VIRTUAL : `virtual`
> KW_YIELD : `yield`

## Weak keywords

These keywords have special meaning only in certain contexts. For example, it is possible to declare a variable or method with the name `union`.

- `union` is used to declare a union and is only a keyword when used in a union declaration.

- `'static` is used for the static lifetime and cannot be used as a generic lifetime parameter

      // error[E0262]: invalid lifetime parameter name: `'static`
      fn invalid_lifetime_parameter<'static>(s: &'static str) -> &'static str { s }

- `dyn` denotes a trait object and is a keyword when used in a type position followed by a path that does not start with `::`.

> Lexer
> KW_UNION : `union`
> KW_STATICLIFETIME : `'static`
> KW_DYN : `dyn`

# Identifiers

> Lexer:
> IDENTIFIER_OR_KEYWORD :
> [`a`-`z` `A`-`Z`] [`a`-`z` `A`-`Z` `0`-`9` `_`]*
> | `_` [`a`-`z` `A`-`Z` `0`-`9` `_`]+

> RAW_IDENTIFIER : `r#` IDENTIFIER_OR_KEYWORD Except `crate`, `extern`, `self`, `super`, `Self`

> NON_KEYWORD_IDENTIFIER : IDENTIFIER_OR_KEYWORD Except a strict or reserved keyword

> IDENTIFIER :
> NON_KEYWORD_IDENTIFIER | RAW_IDENTIFIER

An identifier is any nonempty ASCII string of the following form:

Either

- The first character is a letter.
- The remaining characters are alphanumeric or `_`.

Or

- The first character is `_`.
- The identifier is more than one character. `_` alone is not an identifier.
- The remaining characters are alphanumeric or `_`.

A raw identifier is like a normal identifier, but prefixed by `r#`. (Note that the `r#` prefix is not included as part of the actual identifier.) Unlike a normal identifier, a raw identifier may be any strict or reserved keyword except the ones listed above for `RAW_IDENTIFIER`.

# Comments

> Lexer
> LINE_COMMENT :
> `//` (~[`/` `!`] | `//`) ~`\n`*
> | `//`

> BLOCK_COMMENT :
> `/*` (~[`*` `!`] | `**` | BlockCommentOrDoc) (BlockCommentOrDoc | ~`*/`)* `*/`
> | `/**/`
> | `/***/`

> INNER_LINE_DOC :
> `//!` ~[`\n` IsolatedCR]*

> INNER_BLOCK_DOC :
> `/*!` ( BlockCommentOrDoc | ~[`*/` IsolatedCR] )* `*/`

> OUTER_LINE_DOC :
> `///` (~`/` ~[`\n` IsolatedCR]*)?

> OUTER_BLOCK_DOC :
> `/**` (~`*` | BlockCommentOrDoc ) (BlockCommentOrDoc | ~[`*/` IsolatedCR])* `*/`

> BlockCommentOrDoc :
> BLOCK_COMMENT
> | OUTER_BLOCK_DOC
> | INNER_BLOCK_DOC

> IsolatedCR :
> A `\r` not followed by a `\n`

## Non-doc comments

Comments in Rust code follow the general C++ style of line (`//`) and block (`/* ... */`) comment forms. Nested block comments are supported.

Non-doc comments are interpreted as a form of whitespace.

## Doc comments

Line doc comments beginning with exactly three slashes (`///`), and block doc comments (`/** ... */`), both inner doc comments, are interpreted as a special syntax for `doc` attributes. That is, they are equivalent to writing `#[doc="..."]` around the body of the comment, i.e., `/// Foo` turns into `#[doc="Foo"]` and `/** Bar */` turns into `#[doc="Bar"]`.

Line comments beginning with `//!` and block comments `/*! ... */` are doc comments that apply to the parent of the comment, rather than the item that follows. That is, they are equivalent to writing `#![doc="..."]` around the body of the comment. `//!` comments are usually used to document modules that occupy a source file.

Isolated CRs (`\r`), i.e. not followed by LF (`\n`), are not allowed in doc comments.

## Examples

    # #![allow(unused_variables)]
    #fn main() {
    //! A doc comment that applies to the implicit anonymous module of this crate

    pub mod outer_module {

        //!  - Inner line doc
        //!! - Still an inner line doc (but with a bang at the beginning)

        /*!  - Inner block doc */
        /*!! - Still an inner block doc (but with a bang at the beginning) */

        //   - Only a comment
        ///  - Outer line doc (exactly 3 slashes)
        //// - Only a comment

        /*   - Only a comment */
        /**  - Outer block doc (exactly) 2 asterisks */
        /*** - Only a comment */

        pub mod inner_module {}

        pub mod nested_comments {
            /* In Rust /* we can /* nest comments */ */ */

            // All three types of block comments can contain or be nested inside
            // any other type:

            /*   /* */  /** */  /*! */  */
            /*!  /* */  /** */  /*! */  */
            /**  /* */  /** */  /*! */  */
            pub mod dummy_item {}
        }

        pub mod degenerate_cases {
            // empty inner line doc
            //!

            // empty inner block doc
            /*!*/

            // empty line comment
            //

            // empty outer line doc
            ///

            // empty block comment
            /**/

            pub mod dummy_item {}

            // empty 2-asterisk block isn't a doc block, it is a block comment
            /***/

        }

        /* The next one isn't allowed because outer doc comments
           require an item that will receive the doc */

        /// Where is my item?
    #   mod boo {}
    }
    #}

# Whitespace

Whitespace is any non-empty string containing only characters that have the `Pattern_White_Space` Unicode property, namely:

- `U+0009` (horizontal tab, `'\t'`)
- `U+000A` (line feed, `'\n'`)
- `U+000B` (vertical tab)
- `U+000C` (form feed)
- `U+000D` (carriage return, `'\r'`)
- `U+0020` (space, `' '`)
- `U+0085` (next line)
- `U+200E` (left-to-right mark)
- `U+200F` (right-to-left mark)
- `U+2028` (line separator)
- `U+2029` (paragraph separator)

Rust is a "free-form" language, meaning that all forms of whitespace serve only to separate tokens in the grammar, and have no semantic significance.

A Rust program has identical meaning if each whitespace element is replaced with any other legal whitespace element, such as a single space character.

# Tokens

Tokens are primitive productions in the grammar defined by regular (non-recursive) languages. Rust source input can be broken down into the following kinds of tokens:

- Keywords
- Identifiers
- Literals
- Lifetimes
- Punctuation
- Delimiters

Within this documentation's grammar, "simple" tokens are given in string table production form, and appear in `monospace` font.

## Literals

A literal is an expression consisting of a single token, rather than a sequence of tokens, that immediately and directly denotes the value it evaluates to, rather than referring to it by name or some other evaluation rule. A literal is a form of constant expression, so is evaluated (primarily) at compile time.

### Examples

#### Characters and strings

| Example | `#` sets | Characters | Escapes
--- | --- | --- | --- | ---
Character | `'H'` | 0 | All Unicode | Quote & ASCII & Unicode
String | `"hello"` | 0 | All Unicode | Quote & ASCII & Unicode
Raw | `r#"hello"#` | 0 or more* | All Unicode | `N/A`
Byte | `b'H'` | 0 | All ASCII | Quote & Byte
Byte string | `b"hello"` | 0 | All ASCII | Quote & Byte
Raw byte string | `br#"hello"#` | 0 or more* | All ASCII | `N/A`

* The number of `#`s on each side of the same literal must be equivalent

#### ASCII escapes

| Name
--- | ---
`\x41` | 7-bit character code (exactly 2 digits, up to 0x7F)
`\n` | Newline
`\r` | Carriage return
`\t` | Tab
`\\` | Backslash
`\0` | Null

#### Byte escapes

| Name
--- | ---
`\x7F` | 8-bit character code (exactly 2 digits)
`\n` | Newline
`\r` | Carriage return
`\t` | Tab
`\\` | Backslash
`\0` | Null

#### Unicode escapes

| Name
--- | ---
`\u{7FFF}` | 24-bit Unicode character code (up to 6 digits)

#### Quote escapes

| Name
--- | ---
`\'` | Single quote
`\"` | Double quote

#### Numbers

Number literals`*` | Example | Exponentiation | Suffixes
--- | --- | --- | ---
Decimal integer | `98_222` | `N/A` | Integer suffixes
Hex integer | `0xff` | `N/A` | Integer suffixes
Octal integer | `0o77` | `N/A` | Integer suffixes
Binary integer | `0b1111_0000` | `N/A` | Integer suffixes
Floating-point | `123.0E+77` | `Optional` | Floating-point suffixes

`*` All number literals allow `_` as a visual separator: `1_234.0E+18f64`

#### Suffixes

Integer | Floating-point
--- | ---
`u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64`, `u128`, `i128`, `usize`, `isize` | `f32`, `f64`

### Character and string literals

#### Character literals

> Lexer
> CHAR_LITERAL :
> `'` ( ~[`'` `\` \n \r \t] | QUOTE_ESCAPE | ASCII_ESCAPE | UNICODE_ESCAPE ) `'`

> QUOTE_ESCAPE :
> `\'` | `\"`

> ASCII_ESCAPE :
> `\x` OCT_DIGIT HEX_DIGIT
> | `\n` | `\r` | `\t` | `\\` | `\0`

> UNICODE_ESCAPE :
> `\u{` ( HEX_DIGIT `_`* )1..6 `}`

A character literal is a single Unicode character enclosed within two `U+0027` (single-quote) characters, with the exception of `U+0027` itself, which must be escaped by a preceding `U+005C` character (`\`).

#### String literals

> Lexer
> STRING_LITERAL :
> `"` (
> ~[`"` `\` IsolatedCR]
> | QUOTE_ESCAPE
> | ASCII_ESCAPE
> | UNICODE_ESCAPE
> | STRING_CONTINUE
> )* `"`

> STRING_CONTINUE :
> `\` followed by \n

A string literal is a sequence of any Unicode characters enclosed within two `U+0022` (double-quote) characters, with the exception of `U+0022` itself, which must be escaped by a preceding `U+005C` character (`\`).

Line-break characters are allowed in string literals. Normally they represent themselves (i.e. no translation), but as a special exception, when an unescaped `U+005C` character (`\`) occurs immediately before the newline (`U+000A`), the `U+005C` character, the newline, and all whitespace at the beginning of the next line are ignored. Thus `a` and `b` are equal:

    # #![allow(unused_variables)]
    #fn main() {
    let a = "foobar";
    let b = "foo\
             bar";

    assert_eq!(a,b);
    #}

#### Character escapes

Some additional escapes are available in either character or non-raw string literals. An escape starts with a `U+005C` (`\`) and continues with one of the following forms:

- A 7-bit code point escape starts with `U+0078` (`x`) and is followed by exactly two hex digits with value up to `0x7F`. It denotes the ASCII character with value equal to the provided hex value. Higher values are not permitted because it is ambiguous whether they mean Unicode code points or byte values.
- A 24-bit code point escape starts with `U+0075` (`u`) and is followed by up to six hex digits surrounded by braces `U+007B` (`{`) and `U+007D` (`}`). It denotes the Unicode code point equal to the provided hex value.
- A whitespace escape is one of the characters `U+006E` (`n`), `U+0072` (`r`), or `U+0074` (`t`), denoting the Unicode values `U+000A` (LF), `U+000D` (CR) or `U+0009` (HT) respectively.
- The null escape is the character `U+0030` (`0`) and denotes the Unicode value `U+0000` (NUL).
- The backslash escape is the character `U+005C` (`\`) which must be escaped in order to denote itself.

#### Raw string literals

> Lexer
> RAW_STRING_LITERAL :
> `r` RAW_STRING_CONTENT

> RAW_STRING_CONTENT :
> `"` ( ~ IsolatedCR )* (non-greedy) `"`
> | `#` RAW_STRING_CONTENT `#`

Raw string literals do not process any escapes. They start with the character `U+0072` (`r`), followed by zero or more of the character `U+0023` (`#`) and a `U+0022` (double-quote) character. The raw string body can contain any sequence of Unicode characters and is terminated only by another `U+0022` (double-quote) character, followed by the same number of `U+0023` (`#`) characters that preceded the opening `U+0022` (double-quote) character.

All Unicode characters contained in the raw string body represent themselves, the characters `U+0022` (double-quote) (except when followed by at least as many `U+0023` (`#`) characters as were used to start the raw string literal) or `U+005C` (`\`) do not have any special meaning.

Examples for string literals:

    # #![allow(unused_variables)]
    #fn main() {
    "foo"; r"foo";                     // foo
    "\"foo\""; r#""foo""#;             // "foo"

    "foo #\"# bar";
    r##"foo #"# bar"##;                // foo #"# bar

    "\x52"; "R"; r"R";                 // R
    "\\x52"; r"\x52";                  // \x52
    #}

### Byte and byte string literals

#### Byte literals

> Lexer
> BYTE_LITERAL :
> `b'` ( ASCII_FOR_CHAR | BYTE_ESCAPE ) `'`

> ASCII_FOR_CHAR :
> any ASCII (i.e. 0x00 to 0x7F), except `'`, `\`, \n, \r or \t

> BYTE_ESCAPE :
> `\x` HEX_DIGIT HEX_DIGIT
> | `\n` | `\r` | `\t` | `\\` | `\0`

A byte literal is a single ASCII character (in the `U+0000` to `U+007F` range) or a single escape preceded by the characters `U+0062` (`b`) and `U+0027` (single-quote), and followed by the character `U+0027`. If the character `U+0027` is present within the literal, it must be escaped by a preceding `U+005C` (`\`) character. It is equivalent to a `u8` unsigned 8-bit integer number literal.

#### Byte string literals

> Lexer
> BYTE_STRING_LITERAL :
> `b"` ( ASCII_FOR_STRING | BYTE_ESCAPE | STRING_CONTINUE )* `"`

> ASCII_FOR_STRING :
> any ASCII (i.e 0x00 to 0x7F), except `"`, `\` and IsolatedCR

A non-raw byte string literal is a sequence of ASCII characters and escapes, preceded by the characters `U+0062` (`b`) and `U+0022` (double-quote), and followed by the character `U+0022`. If the character `U+0022` is present within the literal, it must be escaped by a preceding `U+005C` (`\`) character. Alternatively, a byte string literal can be a raw byte string literal, defined below. The type of a byte string literal of length `n` is `&'static [u8; n]`.

Some additional escapes are available in either byte or non-raw byte string literals. An escape starts with a `U+005C` (`\`) and continues with one of the following forms:

- A byte escape escape starts with `U+0078` (`x`) and is followed by exactly two hex digits. It denotes the byte equal to the provided hex value.
- A whitespace escape is one of the characters `U+006E` (`n`), `U+0072` (`r`), or `U+0074` (`t`), denoting the bytes values `0x0A` (ASCII LF), `0x0D` (ASCII CR) or `0x09` (ASCII HT) respectively.
- The null escape is the character `U+0030` (`0`) and denotes the byte value `0x00` (ASCII NUL).
- The backslash escape is the character `U+005C` (`\`) which must be escaped in order to denote its ASCII encoding `0x5C`.

#### Raw byte string literals

> Lexer
> RAW_BYTE_STRING_LITERAL :
> `br` RAW_BYTE_STRING_CONTENT

> RAW_BYTE_STRING_CONTENT :
> `"` ASCII* (non-greedy) `"`
> | `#` RAW_STRING_CONTENT `#`

> ASCII :
> any ASCII (i.e. 0x00 to 0x7F)

Raw byte string literals do not process any escapes. They start with the character `U+0062` (`b`), followed by `U+0072` (`r`), followed by zero or more of the character `U+0023` (`#`), and a `U+0022` (double-quote) character. The raw string body can contain any sequence of ASCII characters and is terminated only by another `U+0022` (double-quote) character, followed by the same number of `U+0023` (`#`) characters that preceded the opening `U+0022` (double-quote) character. A raw byte string literal can not contain any non-ASCII byte.

All characters contained in the raw string body represent their ASCII encoding, the characters `U+0022` (double-quote) (except when followed by at least as many `U+0023` (`#`) characters as were used to start the raw string literal) or `U+005C` (`\`) do not have any special meaning.

Examples for byte string literals:

    # #![allow(unused_variables)]
    #fn main() {
    b"foo"; br"foo";                     // foo
    b"\"foo\""; br#""foo""#;             // "foo"

    b"foo #\"# bar";
    br##"foo #"# bar"##;                 // foo #"# bar

    b"\x52"; b"R"; br"R";                // R
    b"\\x52"; br"\x52";                  // \x52
    #}

### Number literals

A number literal is either an integer literal or a floating-point literal. The grammar for recognizing the two kinds of literals is mixed.

#### Integer literals

> Lexer
> INTEGER_LITERAL :
> ( DEC_LITERAL | BIN_LITERAL | OCT_LITERAL | HEX_LITERAL ) INTEGER_SUFFIX?

> DEC_LITERAL :
> DEC_DIGIT (DEC_DIGIT|`_`)*

> TUPLE_INDEX :
> `0` | NON_ZERO_DEC_DIGIT DEC_DIGIT*

> BIN_LITERAL :
> `0b` (BIN_DIGIT|`_`)* BIN_DIGIT (BIN_DIGIT|`_`)*

> OCT_LITERAL :
> `0o` (OCT_DIGIT|`_`)* OCT_DIGIT (OCT_DIGIT|`_`)*

> HEX_LITERAL :
> `0x` (HEX_DIGIT|`_`)* HEX_DIGIT (HEX_DIGIT|`_`)*

> BIN_DIGIT : [`0`-`1`]

> OCT_DIGIT : [`0`-`7`]

> DEC_DIGIT : [`0`-`9`]

> NON_ZERO_DEC_DIGIT : [`1`-`9`]

> HEX_DIGIT : [`0`-`9` `a`-`f` `A`-`F`]

> INTEGER_SUFFIX :
> `u8` | `u16` | `u32` | `u64` | `u128` | `usize`
> | `i8` | `i16` | `i32` | `i64` | `i128` | `isize`

An integer literal has one of four forms:

- A decimal literal starts with a decimal digit and continues with any mixture of decimal digits and underscores.
- A tuple index is either `0`, or starts with a non-zero decimal digit and continues with zero or more decimal digits. Tuple indexes are used to refer to the fields of tuples, tuple structs and tuple variants.
- A hex literal starts with the character sequence `U+0030` `U+0078` (`0x`) and continues as any mixture (with at least one digit) of hex digits and underscores.
- An octal literal starts with the character sequence `U+0030` `U+006F` (`0o`) and continues as any mixture (with at least one digit) of octal digits and underscores.
- A binary literal starts with the character sequence `U+0030` `U+0062` (`0b`) and continues as any mixture (with at least one digit) of binary digits and underscores.

Like any literal, an integer literal may be followed (immediately, without any spaces) by an integer suffix, which forcibly sets the type of the literal. The integer suffix must be the name of one of the integral types: `u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64`, `u128`, `i128`, `usize`, or `isize`.

The type of an unsuffixed integer literal is determined by type inference:

- If an integer type can be uniquely determined from the surrounding program context, the unsuffixed integer literal has that type.

- If the program context under-constrains the type, it defaults to the signed 32-bit integer `i32`.

- If the program context over-constrains the type, it is considered a static type error.

Examples of integer literals of various forms:

    # #![allow(unused_variables)]
    #fn main() {
    123;                               // type i32
    123i32;                            // type i32
    123u32;                            // type u32
    123_u32;                           // type u32
    let a: u64 = 123;                  // type u64

    0xff;                              // type i32
    0xff_u8;                           // type u8

    0o70;                              // type i32
    0o70_i16;                          // type i16

    0b1111_1111_1001_0000;             // type i32
    0b1111_1111_1001_0000i64;          // type i64
    0b________1;                       // type i32

    0usize;                            // type usize
    #}

Examples of invalid integer literals:

    // invalid suffixes

    0invalidSuffix;

    // uses numbers of the wrong base

    123AFB43;
    0b0102;
    0o0581;

    // integers too big for their type (they overflow)

    128_i8;
    256_u8;

    // bin, hex and octal literals must have at least one digit

    0b_;
    0b____;

Note that the Rust syntax considers `-1i8` as an application of the unary minus operator to an integer literal `1i8`, rather than a single integer literal.

#### Floating-point literals

> Lexer
> FLOAT_LITERAL :
> DEC_LITERAL `.` (not immediately followed by `.`, `_` or an identifier)
> | DEC_LITERAL FLOAT_EXPONENT
> | DEC_LITERAL `.` DEC_LITERAL FLOAT_EXPONENT?
> | DEC_LITERAL (`.` DEC_LITERAL)? FLOAT_EXPONENT? FLOAT_SUFFIX

> FLOAT_EXPONENT :
> (`e`|`E`) (`+`|`-`)? (DEC_DIGIT|`_`)* DEC_DIGIT (DEC_DIGIT|`_`)*

> FLOAT_SUFFIX :
> `f32` | `f64`

A floating-point literal has one of two forms:

- A decimal literal followed by a period character `U+002E` (`.`). This is optionally followed by another decimal literal, with an optional exponent.
- A single decimal literal followed by an exponent.

Like integer literals, a floating-point literal may be followed by a suffix, so long as the pre-suffix part does not end with `U+002E` (`.`). The suffix forcibly sets the type of the literal. There are two valid floating-point suffixes, `f32` and `f64` (the 32-bit and 64-bit floating point types), which explicitly determine the type of the literal.

The type of an unsuffixed floating-point literal is determined by type inference:

- If a floating-point type can be uniquely determined from the surrounding program context, the unsuffixed floating-point literal has that type.

- If the program context under-constrains the type, it defaults to `f64`.

- If the program context over-constrains the type, it is considered a static type error.

Examples of floating-point literals of various forms:

    # #![allow(unused_variables)]
    #fn main() {
    123.0f64;        // type f64
    0.1f64;          // type f64
    0.1f32;          // type f32
    12E+99_f64;      // type f64
    let x: f64 = 2.; // type f64
    #}

This last example is different because it is not possible to use the suffix syntax with a floating point literal ending in a period. `2.f64` would attempt to call a method named `f64` on `2`.

The representation semantics of floating-point numbers are described in "Machine Types".

### Boolean literals

> Lexer
> BOOLEAN_LITERAL :
> `true`
> | `false`

The two values of the boolean type are written `true` and `false`.

## Lifetimes and loop labels

> Lexer
> LIFETIME_TOKEN :
> `'` IDENTIFIER_OR_KEYWORD
> | `'_`

> LIFETIME_OR_LABEL :
> `'` NON_KEYWORD_IDENTIFIER

Lifetime parameters and loop labels use LIFETIME_OR_LABEL tokens. Any LIFETIME_TOKEN will be accepted by the lexer, and for example, can be used in macros.

## Punctuation

Punctuation symbol tokens are listed here for completeness. Their individual usages and meanings are defined in the linked pages.

Symbol | Name | Usage
--- | --- | ---
`+` | Plus | Addition, Trait Bounds, Macro Kleene Matcher
`-` | Minus | Subtraction, Negation
`*` | Star | Multiplication, Dereference, Raw Pointers, Macro Kleene Matcher
`/` | Slash | Division
`%` | Percent | Remainder
`^` | Caret | Bitwise and Logical XOR
`!` | Not | Bitwise and Logical NOT, Macro Calls, Inner Attributes, Never Type
`&` | And | Bitwise and Logcal AND, Borrow, References
`|` | Or | Bitwise and Logical OR, Closures, Match
`&&` | AndAnd | Lazy AND, Borrow, References
`||` | OrOr | Lazy OR, Closures
`<<` | Shl | Shift Left, Nested Generics
`>>` | Shr | Shift Right, Nested Generics
`+=` | PlusEq | Addition assignment
`-=` | MinusEq | Subtraction assignment
`*=` | StarEq | Multiplication assignment
`/=` | SlashEq | Division assignment
`%=` | PercentEq | Remainder assignment
`^=` | CaretEq | Bitwise XOR assignment
`&=` | AndEq | Bitwise And assignment
`|=` | OrEq | Bitwise Or assignment
`<<=` | ShlEq | Shift Left assignment
`>>=` | ShrEq | Shift Right assignment, Nested Generics
`=` | Eq | Assignment, Attributes, Various type definitions
`==` | EqEq | Equal
`!=` | Ne | Not Equal
`>` | Gt | Greater than, Generics, Paths
`<` | Lt | Less than, Generics, Paths
`>=` | Ge | Greater than or equal to, Generics
`<=` | Le | Less than or equal to
`@` | At | Subpattern binding
`_` | Underscore | Placeholder patterns, Inferred types
`.` | Dot | Field access, Tuple index
`..` | DotDot | Range, Struct expressions, Wildcard patterns
`...` | DotDotDot | Variadic functions
`..=` | DotDotEq | Inclusive Range
`,` | Comma | Various separators
`;` | Semi | Terminator for various items and statements, Array types
`:` | Colon | Various separators
`::` | PathSep | Path separator
`->` | RArrow | Function return type, Closure return type
`=>` | FatArrow | Match arms, Macros
`#` | Pound | Attributes
`$` | Dollar | Macros
`?` | Question | Question mark operator, Questionably sized

## Delimiters

Bracket punctuation is used in various parts of the grammar. An open bracket must always be paired with a close bracket. Brackets and the tokens within them are referred to as "token trees" in macros. The three types of brackets are:

Bracket | Type
--- | ---
`{` `}` | Curly braces
`[` `]` | Square brackets
`(` `)` | Parentheses

# Paths

A path is a sequence of one or more path components logically separated by a namespace qualifier (`::`). If a path consists of only one component, it refers to either an item or a variable in a local control scope. If a path has multiple components, it always refers to an item.

Two examples of simple paths consisting of only identifier components:

    x;
    x::y::z;

Path components are usually identifiers, but they may also include angle-bracket-enclosed lists of type arguments. In expression context, the type argument list is given after a `::` namespace qualifier in order to disambiguate it from a relational expression involving the less-than symbol (`<`). In type expression context, the final namespace qualifier is omitted.

Two examples of paths with type arguments:

    # #![allow(unused_variables)]
    #fn main() {
    # struct HashMap<K, V>(K,V);
    # fn f() {
    # fn id<T>(t: T) -> T { t }
    type T = HashMap<i32,String>; // Type arguments used in a type expression
    let x  = id::<i32>(10);       // Type arguments used in a call expression
    # }
    #}

Paths can be denoted with various leading qualifiers to change the meaning of how it is resolved:

- Paths starting with `::` are considered to be global paths where the components of the path start being resolved from the crate root. Each identifier in the path must resolve to an item.

    mod a {
        pub fn foo() {}
    }
    mod b {
        pub fn foo() {
            ::a::foo(); // call a's foo function
        }
    }
    # fn main() {}

- Paths starting with the keyword `super` begin resolution relative to the parent module. Each further identifier must resolve to an item.

    mod a {
        pub fn foo() {}
    }
    mod b {
        pub fn foo() {
            super::a::foo(); // call a's foo function
        }
    }
    # fn main() {}

- Paths starting with the keyword `self` begin resolution relative to the current module. Each further identifier must resolve to an item.

    fn foo() {}
    fn bar() {
        self::foo();
    }
    # fn main() {}

Additionally keyword `super` may be repeated several times after the first `super` or `self` to refer to ancestor modules.

    mod a {
        fn foo() {}

        mod b {
            mod c {
                fn foo() {
                    super::super::foo(); // call a's foo function
                    self::super::super::foo(); // call a's foo function
                }
            }
        }
    }
    # fn main() {}

## Canonical paths

Items defined in a module or implementation have a canonical path that corresponds to where within its crate it is defined. All other paths to these items are aliases. The canonical path is defined as a path prefix appended by the path component the item itself defines.

Implementations and use declarations do not have canonical paths, although the items that implementations define do have them. Items defined in block expressions do not have canonical paths. Items defined in a module that does not have a canonical path do not have a canonical path. Associated items defined in an implementation that refers to an item without a canonical path, e.g. as the implementing type, the trait being implemented, a type parameter or bound on a type parameter, do not have canonical paths.

The path prefix for modules is the canonical path to that module. For bare implementations, it is the canonical path of the item being implemented surrounded by angle (`<>`) brackets. For trait implementations, it is the canonical path of the item being implemented followed by `as` followed by the canonical path to the trait all surrounded in angle (`<>`) brackets.

The canonical path is only meaningful within a given crate. There is no global namespace across crates; an item's canonical path merely identifies it within the crate.

    // Comments show the canonical path of the item.

    mod a { // ::a
        pub struct Struct; // ::a::Struct

        pub trait Trait { // ::a::Trait
            fn f(&self); // a::Trait::f
        }

        impl Trait for Struct {
            fn f(&self) {} // <::a::Struct as ::a::Trait>::f
        }

        impl Struct {
            fn g(&self) {} // <::a::Struct>::g
        }
    }

    mod without { // ::without
        fn canonicals() { // ::without::canonicals
            struct OtherStruct; // None

            trait OtherTrait { // None
                fn g(&self); // None
            }

            impl OtherTrait for OtherStruct {
                fn g(&self) {} // None
            }

            impl OtherTrait for ::a::Struct {
                fn g(&self) {} // None
            }

            impl ::a::Trait for OtherStruct {
                fn f(&self) {} // None
            }
        }
    }

    # fn main() {}

# Macros

A number of minor features of Rust are not central enough to have their own syntax, and yet are not implementable as functions. Instead, they are given names, and invoked through a consistent syntax: `some_extension!(...)`.

There are two ways to define new macros:

- Macros by Example define new syntax in a higher-level, declarative way.
- Procedural Macros can be used to implement custom derive.

# Macros By Example

`macro_rules` allows users to define syntax extension in a declarative way. We call such extensions "macros by example" or simply "macros".

Currently, macros can expand to expressions, statements, items, or patterns.

(A `sep_token` is any token other than `*` and `+`. A `non_special_token` is any token other than a delimiter or `$`.)

The macro expander looks up macro invocations by name, and tries each macro rule in turn. It transcribes the first successful match. Matching and transcription are closely related to each other, and we will describe them together.

The macro expander matches and transcribes every token that does not begin with a `$` literally, including delimiters. For parsing reasons, delimiters must be balanced, but they are otherwise not special.

In the matcher, `$` name `:` designator matches the nonterminal in the Rust syntax named by designator. Valid designators are:

- `item`: an item
- `block`: a block
- `stmt`: a statement
- `pat`: a pattern
- `expr`: an expression
- `ty`: a type
- `ident`: an identifier or keyword
- `path`: a path
- `tt`: a token tree (a single token by matching `()`, `[]`, or `{}`)
- `meta`: the contents of an attribute
- `lifetime`: a lifetime. Examples: `'static`, `'a`.
- `vis`: a (visibility qualifier)visibility-and-privacy

In the transcriber, the designator is already known, and so only the name of a matched nonterminal comes after the dollar sign.

In both the matcher and transcriber, the Kleene star-like operator indicates repetition. The Kleene star operator consists of `$` and parentheses, optionally followed by a separator token, followed by `*` or `+`. `*` means zero or more repetitions, `+` means at least one repetition. The parentheses are not matched or transcribed. On the matcher side, a name is bound to all of the names it matches, in a structure that mimics the structure of the repetition encountered on a successful match. The job of the transcriber is to sort that structure out.

The rules for transcription of these repetitions are called "Macro By Example". Essentially, one "layer" of repetition is discharged at a time, and all of them must be discharged by the time a name is transcribed. Therefore, `( $( $i:ident ),* ) => ( $i )` is an invalid macro, but `( $( $i:ident ),* ) => ( $( $i:ident ),* )` is acceptable (if trivial).

When Macro By Example encounters a repetition, it examines all of the `$` name s that occur in its body. At the "current layer", they all must repeat the same number of times, so `( $( $i:ident ),* ; $( $j:ident ),* ) => ( $( ($i,$j) ),* )` is valid if given the argument `(a,b,c ; d,e,f)`, but not `(a,b,c ; d,e)`. The repetition walks through the choices at that layer in lockstep, so the former input transcribes to `(a,d), (b,e), (c,f)`.

Nested repetitions are allowed.

### Parsing limitations

The parser used by the macro system is reasonably powerful, but the parsing of Rust syntax is restricted in two ways:

1. Macro definitions are required to include suitable separators after parsing expressions and other bits of the Rust grammar. This implies that a macro definition like `$i:expr [ , ]` is not legal, because `[` could be part of an expression. A macro definition like `$i:expr,` or `$i:expr;` would be legal, however, because `,` and `;` are legal separators. See RFC 550 for more information.
2. The parser must have eliminated all ambiguity by the time it reaches a `$` name `:` designator. This requirement most often affects name-designator pairs when they occur at the beginning of, or immediately after, a `$(...)*`; requiring a distinctive token in front can solve the problem.

## Procedural Macros

Procedural macros allow creating syntax extensions as execution of a function. Procedural macros come in one of three flavors:

- Function-like macros - `custom!(...)`
- Derive mode macros - `#[derive(CustomMode)]`
- Attribute macros - `#[CustomAttribute]`

Procedural macros allow you to run code at compile time that operates over Rust syntax, both consuming and producing Rust syntax. You can sort of think of procedural macros as functions from an AST to another AST.

Procedural macros must be defined in a crate with the crate type of `proc-macro`.

> Note: When using Cargo, Procedural macro crates are defined with the `proc-macro` key in your manfiest:

>     [lib]
>     proc-macro = true

As functions, they must either return syntax, panic, or loop endlessly. Returned syntax either replaces or adds the syntax depending on the kind of procedural macro. Panics are caught by the compiler and are turned into a compiler error. Endless loops are not caught by the compiler which hangs the compiler.

Procedural macros run during compilation, and thus have the same resources that the compiler has. For example, standard input, error, and output are the same that the compiler has access to. Similarly, file access is the same. Because of this, procedural macros have the same security concerns that Cargo's build scripts have.

Procedural macros have two ways of reporting errors. The first is to panic. The second is to emit a `compile_error` macro invocation.

### The `proc_macro` crate

Procedural macro crates almost always will link to the compiler-provided `proc_macro` crate. The `proc_macro` crate provides types required for writing procedural macros and facilities to make it easier.

This crate primarily contains a `TokenStream` type. Procedural macros operate over token streams instead of AST nodes, which is a far more stable interface over time for both the compiler and for procedural macros to target. A token stream is roughly equivalent to `Vec<TokenTree>` where a `TokenTree` can roughly be thought of as lexical token. For example `foo` is an `Ident` token, `.` is a `Punct` token, and `1.2` is a `Literal` token. The `TokenStream` type, unlike `Vec<TokenTree>`, is cheap to clone.

All tokens have an associated `Span`. A `Span` is an opaque value that cannot be modified but can be manufactured. `Span`s represent an extent of source code within a program and are primarily used for error reporting. You can modify the `Span` of any token.

### Procedural macro hygiene

Procedural macros are unhygienic. This means they behave as if the output token stream was simply written inline to the code it's next to. This means that it's affected by external items and also affects external imports.

Macro authors need to be careful to ensure their macros work in as many contexts as possible given this limitation. This often includes using absolute paths to items in libraries (for example, `::std::option::Option` instead of `Option`) or by ensuring that generated functions have names that are unlikely to clash with other functions (like `__internal_foo` instead of `foo`).

### Function-like procedural macros

Function-like procedural macros are procedural macros that are invoked using the macro invocation operator (`!`).

These macros are defined by a public function with the `proc_macro` attribute and a signature of `(TokenStream) -> TokenStream`. The input `TokenStream` is what is inside the delimiters of the macro invocation and the output `TokenStream` replaces the entire macro invocation. It may contain an arbitrary number of items. These macros cannot expand to syntax that defines new `macro_rule` style macros.

For example, the following macro definition ignores its input and outputs a function `answer` into its scope.

    extern crate proc_macro;
    use proc_macro::TokenStream;

    #[proc_macro]
    pub fn make_answer(_item: TokenStream) -> TokenStream {
        "fn answer() -> u32 { 42 }".parse().unwrap()
    }

And then we use it a binary crate to print "42" to standard output.

    extern crate proc_macro_examples;
    use proc_macro_examples::make_answer;

    make_answer!();

    fn main() {
        println!("{}", answer());
    }

These macros are only invokable in modules. They cannot even be invoked to create item declaration statements. Furthermore, they must either be invoked with curly braces and no semicolon or a different delimiter followed by a semicolon. For example, `make_answer` from the previous example can be invoked as `make_answer!{}`, `make_answer!();` or `make_answer![];`.

### Derive mode macros

Derive mode macros define new modes for the `derive` attribute. These macros define new items given the token stream of a struct, enum, or [union]. They also define derive mode helper attributes.

Custom deriver modes are defined by a public function with the `proc_macro_derive` attribute and a signature of `(TokenStream) -> TokenStream`.

The input `TokenStream` is the token stream of the item that has the `derive` attribute on it. The output `TokenStream` must be a set of items that are then appended to the module or [block] that the item from the input `TokenStream` is in.

The following is an example of a derive mode macro. Instead of doing anything useful with its input, it just appends a function `answer`.

    extern crate proc_macro;
    use proc_macro::TokenStream;

    #[proc_macro_derive(AnswerFn)]
    pub fn derive_answer_fn(_item: TokenStream) -> TokenStream {
        "fn answer() -> u32 { 42 }".parse().unwrap()
    }

And then using said derive mode:

    extern crate proc_macro_examples;
    use proc_macro_examples::AnswerFn;

    #[derive(AnswerFn)]
    struct Struct;

    fn main() {
        assert_eq!(42, answer());
    }

#### Derive mode helper attributes

Derive mode macros can add additional attributes into the scope of the item they are on. Said attributes are called derive mode helper attributes. These attributes are inert, and their only purpose is to be fed into the derive mode macro that defined them. That said, they can be seen by all macros.

The way to define helper attributes is to put an `attributes` key in the `proc_macro_derive` macro with a comma separated list of identifiers that are the names of the helper attributes.

For example, the following derive mode macro defines a helper attribute `helper`, but ultimately doesn't do anything with it.

    # #[crate_type="proc-macro"]
    # extern crate proc_macro;
    # use proc_macro::TokenStream;

    #[proc_macro_derive(HelperAttr, attributes(helper))]
    pub fn derive_helper_attr(_item: TokenStream) -> TokenStream {
        TokenStream::new();
    }

And then usage on the derive mode on a struct:

    # #![crate_type="proc-macro"]
    # extern crate proc_macro_examples;
    # use proc_macro_examples::HelperAttr;

    #[derive(HelperAttr)]
    struct Struct {
        #[helper] field: ()
    }

### Attribute macros

Attribute macros define new attributes which can be attached to items.

Attribute macros are defined by a public function with the `proc_macro_attribute` attribute that a signature of `(TokenStream, TokenStream) -> TokenStream`. The first `TokenStream` is the attribute's metaitems, not including the delimiters. If the attribute is written without a metaitem, the attribute `TokenStream` is empty. The second `TokenStream` is of the rest of the item including other attributes on the item. The returned `TokenStream` replaces the item with an arbitrary number of items. These macros cannot expand to syntax that defines new `macro_rule` style macros.

For example, this attribute macro takes the input stream and returns it as is, effectively being the no-op of attributes.

    # #![crate_type = "proc-macro"]
    # extern crate proc_macro;
    # use proc_macro::TokenStream;

    #[proc_macro_attribute]
    pub fn return_as_is(_attr: TokenStream, item: TokenStream) -> TokenStream {
        item
    }

This following example shows the stringified `TokenStream`s that the attribute macros see. The output will show in the output of the compiler. The output is shown in the comments after the function prefixed with "out:".

    // my-macro/src/lib.rs
    # extern crate proc_macro;
    # use proc_macro::TokenStream;

    #[proc_macro_attribute]
    pub fn show_streams(attr: TokenStream, input: TokenStream) -> TokenStream {
        println!("attr: \"{}\"", attr.to_string());
        println!("item: \"{}\"', input.to_string());
        item
    }

    // src/lib.rs
    extern crate my_macro;

    use my_macro::show_streams;

    // Example: Basic function
    #[show_streams]
    fn invoke1() {}
    // out: attr: ""
    // out: item: "fn invoke1() { }"

    // Example: Attribute has a metaitem
    #[show_streams(bar)]
    fn invoke2() {}
    // out: attr: "bar"
    // out: item: "fn invoke2() {}"

    // Example: Multiple words in metaitem
    #[show_streams(multiple words)]
    fn invoke3() {}
    // out: attr: "multiple words"
    // out: item: "fn invoke3() {}"

    // Example:
    #[show_streams { delimiters }]
    fn invoke4() {}
    // out: "delimiters"
    // out: "fn invoke4() {}"

# Crates and source files

> Syntax
> Crate :
> UTF8BOM?
> SHEBANG?
> InnerAttribute*
> Item*

> Lexer
> UTF8BOM : `\uFEFF`
> SHEBANG : `#!` ~[`[` `\n`] ~`\n`*

> Note: Although Rust, like any other language, can be implemented by an interpreter as well as a compiler, the only existing implementation is a compiler,and the language has always been designed to be compiled. For these reasons, this section assumes a compiler.

Rust's semantics obey a phase distinction between compile-time and run-time.1 Semantic rules that have a static interpretation govern the success or failure of compilation, while semantic rules that have a dynamic interpretation govern the behavior of the program at run-time.

The compilation model centers on artifacts called crates. Each compilation processes a single crate in source form, and if successful, produces a single crate in binary form: either an executable or some sort of library.2

A crate is a unit of compilation and linking, as well as versioning, distribution and runtime loading. A crate contains a tree of nested module scopes. The top level of this tree is a module that is anonymous (from the point of view of paths within the module) and any item within a crate has a canonical module path denoting its location within the crate's module tree.

The Rust compiler is always invoked with a single source file as input, and always produces a single output crate. The processing of that source file may result in other source files being loaded as modules. Source files have the extension `.rs`.

A Rust source file describes a module, the name and location of which — in the module tree of the current crate — are defined from outside the source file: either by an explicit `mod_item` in a referencing source file, or by the name of the crate itself. Every source file is a module, but not every module needs its own source file: module definitions can be nested within one file.

Each source file contains a sequence of zero or more `item` definitions, and may optionally begin with any number of attributes that apply to the containing module, most of which influence the behavior of the compiler. The anonymous crate module can have additional attributes that apply to the crate as a whole.

    # #![allow(unused_variables)]
    #fn main() {
    // Specify the crate name.
    #![crate_name = "projx"]

    // Specify the type of output artifact.
    #![crate_type = "lib"]

    // Turn on a warning.
    // This can be done in any module, not just the anonymous crate module.
    #![warn(non_camel_case_types)]
    #}

A crate that contains a `main` function can be compiled to an executable. If a `main` function is present, it must take no arguments, must not declare any trait or lifetime bounds, must not have any where clauses, and its return type must be one of the following:

- `()`
- `Result<(), E> where E: Error`

> Note: The implementation of which return types are allowed is determined by the unstable `Termination` trait.

The optional UTF8 byte order mark (UTF8BOM production) indicates that the file is encoded in UTF8. It can only occur at the beginning of the file and is ignored by the compiler.

A source file can have a shebang (SHEBANG production), which indicates to the operating system what program to use to execute this file. It serves essentially to treat the source file as an executable script. The shebang can only occur at the beginning of the file (but after the optional UTF8BOM). It is ignored by the compiler. For example:

    #!/usr/bin/env rustx

    fn main() {
        println!("Hello!");
    }

1

This distinction would also exist in an interpreter. Static checks like syntactic analysis, type checking, and lints should happen before the program is executed regardless of when it is executed.

2

A crate is somewhat analogous to an assembly in the ECMA-335 CLI model, a library in the SML/NJ Compilation Manager, a unit in the Owens and Flatt module system, or a configuration in Mesa.

## Conditional compilation

Sometimes one wants to have different compiler outputs from the same code, depending on build target, such as targeted operating system, or to enable release builds.

Configuration options are boolean (on or off) and are named either with a single identifier (e.g. `foo`) or an identifier and a string (e.g. `foo = "bar"`; the quotes are required and spaces around the `=` are unimportant). Note that similarly-named options, such as `foo`, `foo="bar"` and `foo="baz"` may each be set or unset independently.

Configuration options are either provided by the compiler or passed in on the command line using `--cfg` (e.g. `rustc main.rs --cfg foo --cfg 'bar="baz"'`). Rust code then checks for their presence using the `#[cfg(...)]` attribute:

    # #![allow(unused_variables)]
    #fn main() {
    // The function is only included in the build when compiling for macOS
    #[cfg(target_os = "macos")]
    fn macos_only() {
      // ...
    }

    // This function is only included when either foo or bar is defined
    #[cfg(any(foo, bar))]
    fn needs_foo_or_bar() {
      // ...
    }

    // This function is only included when compiling for a unixish OS with a 32-bit
    // architecture
    #[cfg(all(unix, target_pointer_width = "32"))]
    fn on_32bit_unix() {
      // ...
    }

    // This function is only included when foo is not defined
    #[cfg(not(foo))]
    fn needs_not_foo() {
      // ...
    }
    #}

This illustrates some conditional compilation can be achieved using the `#[cfg(...)]` attribute. `any`, `all` and `not` can be used to assemble arbitrarily complex configurations through nesting.

The following configurations must be defined by the implementation:

- `target_arch = "..."` - Target CPU architecture, such as `"x86"`, `"x86_64"` `"mips"`, `"powerpc"`, `"powerpc64"`, `"arm"`, or `"aarch64"`. This value is closely related to the first element of the platform target triple, though it is not identical.
- `target_os = "..."` - Operating system of the target, examples include `"windows"`, `"macos"`, `"ios"`, `"linux"`, `"android"`, `"freebsd"`, `"dragonfly"`, `"bitrig"` , `"openbsd"` or `"netbsd"`. This value is closely related to the second and third element of the platform target triple, though it is not identical.
- `target_family = "..."` - Operating system family of the target, e. g. `"unix"` or `"windows"`. The value of this configuration option is defined as a configuration itself, like `unix` or `windows`.
- `unix` - See `target_family`.
- `windows` - See `target_family`.
- `target_env = ".."` - Further disambiguates the target platform with information about the ABI/libc. Presently this value is either `"gnu"`, `"msvc"`, `"musl"`, or the empty string. For historical reasons this value has only been defined as non-empty when needed for disambiguation. Thus on many GNU platforms this value will be empty. This value is closely related to the fourth element of the platform target triple, though it is not identical. For example, embedded ABIs such as `gnueabihf` will simply define `target_env` as `"gnu"`.
- `target_endian = "..."` - Endianness of the target CPU, either `"little"` or `"big"`.
- `target_pointer_width = "..."` - Target pointer width in bits. This is set to `"32"` for targets with 32-bit pointers, and likewise set to `"64"` for 64-bit pointers.
- `target_has_atomic = "..."` - Set of integer sizes on which the target can perform atomic operations. Values are `"8"`, `"16"`, `"32"`, `"64"` and `"ptr"`.
- `target_vendor = "..."` - Vendor of the target, for example `apple`, `pc`, or simply `"unknown"`.
- `test` - Enabled when compiling the test harness (using the `--test` flag).
- `debug_assertions` - Enabled by default when compiling without optimizations. This can be used to enable extra debugging code in development but not in production. For example, it controls the behavior of the standard library's `debug_assert!` macro.
- `proc_macro` - Set when the crate being compiled is being compiled with the `proc_macro` crate type.

You can also set another attribute based on a `cfg` variable with `cfg_attr`:

    #[cfg_attr(a, b)]

This is the same as `#[b]` if `a` is set by `cfg`, and nothing otherwise.

Lastly, configuration options can be used in expressions by invoking the `cfg!` macro: `cfg!(a)` evaluates to `true` if `a` is set, and `false` otherwise.

# Items and attributes

Crates contain items, each of which may have some number of attributes attached to it.

# Items

An item is a component of a crate. Items are organized within a crate by a nested set of modules. Every crate has a single "outermost" anonymous module; all further items within the crate have paths within the module tree of the crate.

Items are entirely determined at compile-time, generally remain fixed during execution, and may reside in read-only memory.

There are several kinds of items:

- modules
- `extern crate` declarations
- `use` declarations
- function definitions
- type definitions
- struct definitions
- enumeration definitions
- union definitions
- constant items
- static items
- trait definitions
- implementations
- `extern` blocks

Some items form an implicit scope for the declaration of sub-items. In other words, within a function or module, declarations of items can (in many cases) be mixed with the statements, control blocks, and similar artifacts that otherwise compose the item body. The meaning of these scoped items is the same as if the item was declared outside the scope — it is still a static item — except that the item's path name within the module namespace is qualified by the name of the enclosing item, or is private to the enclosing item (in the case of functions). The grammar specifies the exact locations in which sub-item declarations may appear.

# Modules

> Syntax:
> Module :
> `mod` IDENTIFIER `;`
> | `mod` IDENTIFIER `{`
> InnerAttribute*
> Item*
> `}`

A module is a container for zero or more items.

A module item is a module, surrounded in braces, named, and prefixed with the keyword `mod`. A module item introduces a new, named module into the tree of modules making up a crate. Modules can nest arbitrarily.

An example of a module:

    # #![allow(unused_variables)]
    #fn main() {
    mod math {
        type Complex = (f64, f64);
        fn sin(f: f64) -> f64 {
            /* ... */
    # panic!();
        }
        fn cos(f: f64) -> f64 {
            /* ... */
    # panic!();
        }
        fn tan(f: f64) -> f64 {
            /* ... */
    # panic!();
        }
    }
    #}

Modules and types share the same namespace. Declaring a named type with the same name as a module in scope is forbidden: that is, a type definition, trait, struct, enumeration, union, type parameter or crate can't shadow the name of a module in scope, or vice versa. Items brought into scope with `use` also have this restriction.

A module without a body is loaded from an external file, by default with the same name as the module, plus the `.rs` extension. When a nested submodule is loaded from an external file, it is loaded from a subdirectory path that mirrors the module hierarchy.

    // Load the `vec` module from `vec.rs`
    mod vec;

    mod thread {
        // Load the `local_data` module from `thread/local_data.rs`
        // or `thread/local_data/mod.rs`.
        mod local_data;
    }

The directories and files used for loading external file modules can be influenced with the `path` attribute.

    #[path = "thread_files"]
    mod thread {
        // Load the `local_data` module from `thread_files/tls.rs`
        #[path = "tls.rs"]
        mod local_data;
    }

# Extern crate declarations

> Syntax:
> ExternCrate :
> `extern` `crate` IDENTIFIER (`as` IDENTIFIER)? `;`

An `extern crate` declaration specifies a dependency on an external crate. The external crate is then bound into the declaring scope as the `ident` provided in the `extern_crate_decl`.

The external crate is resolved to a specific `soname` at compile time, and a runtime linkage requirement to that `soname` is passed to the linker for loading at runtime. The `soname` is resolved at compile time by scanning the compiler's library path and matching the optional `crateid` provided against the `crateid` attributes that were declared on the external crate when it was compiled. If no `crateid` is provided, a default `name` attribute is assumed, equal to the `ident` given in the `extern_crate_decl`.

Three examples of `extern crate` declarations:

    extern crate pcre;

    extern crate std; // equivalent to: extern crate std as std;

    extern crate std as ruststd; // linking to 'std' under another name

When naming Rust crates, hyphens are disallowed. However, Cargo packages may make use of them. In such case, when `Cargo.toml` doesn't specify a crate name, Cargo will transparently replace `-` with `_` (Refer to RFC 940 for more details).

Here is an example:

    // Importing the Cargo package hello-world
    extern crate hello_world; // hyphen replaced with an underscore

# Use declarations

> Syntax:
> UseDeclaration :
> (Visibility)? `use` UseTree `;`

> UseTree :
> (SimplePath? `::`)? `*`
> | (SimplePath? `::`)? `{` (UseTree ( `,` UseTree )* `,`?)? `}`
> | SimplePath `as` IDENTIFIER

A use declaration creates one or more local name bindings synonymous with some other path. Usually a `use` declaration is used to shorten the path required to refer to a module item. These declarations may appear in modules and blocks, usually at the top.

> Note: Unlike in many languages, `use` declarations in Rust do not declare linkage dependency with external crates. Rather, `extern crate` declarations declare linkage dependencies.

Use declarations support a number of convenient shortcuts:

- Simultaneously binding a list of paths with a common prefix, using the glob-like brace syntax `use a::b::{c, d, e::f, g::h::i};`
- Simultaneously binding a list of paths with a common prefix and their common parent module, using the `self` keyword, such as `use a::b::{self, c, d::e};`
- Rebinding the target name as a new local name, using the syntax `use p::q::r as x;`. This can also be used with the last two features: `use a::b::{self as ab, c as abc}`.
- Binding all paths matching a given prefix, using the asterisk wildcard syntax `use a::b::*;`.
- Nesting groups of the previous features multiple times, such as `use a::b::{self as ab, c, d::{*, e::f}};`

An example of `use` declarations:

    use std::option::Option::{Some, None};
    use std::collections::hash_map::{self, HashMap};

    fn foo<T>(_: T){}
    fn bar(map1: HashMap<String, usize>, map2: hash_map::HashMap<String, usize>){}

    fn main() {
        // Equivalent to 'foo(vec![std::option::Option::Some(1.0f64),
        // std::option::Option::None]);'
        foo(vec![Some(1.0f64), None]);

        // Both `hash_map` and `HashMap` are in scope.
        let map1 = HashMap::new();
        let map2 = hash_map::HashMap::new();
        bar(map1, map2);
    }

Like items, `use` declarations are private to the containing module, by default. Also like items, a `use` declaration can be public, if qualified by the `pub` keyword. Such a `use` declaration serves to re-export a name. A public `use` declaration can therefore redirect some public name to a different target definition: even a definition with a private canonical path, inside a different module. If a sequence of such redirections form a cycle or cannot be resolved unambiguously, they represent a compile-time error.

An example of re-exporting:

    # fn main() { }
    mod quux {
        pub use quux::foo::{bar, baz};

        pub mod foo {
            pub fn bar() { }
            pub fn baz() { }
        }
    }

In this example, the module `quux` re-exports two public names defined in `foo`.

Also note that the paths contained in `use` items are relative to the crate root. So, in the previous example, the `use` refers to `quux::foo::{bar, baz}`, and not simply to `foo::{bar, baz}`. This also means that top-level module declarations should be at the crate root if direct usage of the declared modules within `use` items is desired. It is also possible to use `self` and `super` at the beginning of a `use` item to refer to the current and direct parent modules respectively. All rules regarding accessing declared modules in `use` declarations apply to both module declarations and `extern crate` declarations.

An example of what will and will not work for `use` items:

    # #![allow(unused_imports)]
    use foo::baz::foobaz;    // good: foo is at the root of the crate

    mod foo {

        mod example {
            pub mod iter {}
        }

        use foo::example::iter; // good: foo is at crate root
    //  use example::iter;      // bad:  example is not at the crate root
        use self::baz::foobaz;  // good: self refers to module 'foo'
        use foo::bar::foobar;   // good: foo is at crate root

        pub mod bar {
            pub fn foobar() { }
        }

        pub mod baz {
            use super::bar::foobar; // good: super refers to module 'foo'
            pub fn foobaz() { }
        }
    }

    fn main() {}

# Functions

A function consists of a block, along with a name and a set of parameters. Other than a name, all these are optional. Functions are declared with the keyword `fn`. Functions may declare a set of input variables as parameters, through which the caller passes arguments into the function, and the output type of the value the function will return to its caller on completion.

When referred to, a function yields a first-class value of the corresponding zero-sized function item type, which when called evaluates to a direct call to the function.

For example, this is a simple function:

    # #![allow(unused_variables)]
    #fn main() {
    fn answer_to_life_the_universe_and_everything() -> i32 {
        return 42;
    }
    #}

As with `let` bindings, function arguments are irrefutable patterns, so any pattern that is valid in a let binding is also valid as an argument:

    # #![allow(unused_variables)]
    #fn main() {
    fn first((value, _): (i32, i32)) -> i32 { value }
    #}

The block of a function is conceptually wrapped in a block that binds the argument patterns and then `return`s the value of the function's block. This means that the tail expression of the block, if evaluated, ends up being returned to the caller. As usual, an explicit return expression within the body of the function will short-cut that implicit return, if reached.

For example, the function above behaves as if it was written as:

    // argument_0 is the actual first argument passed from the caller
    let (value, _) = argument_0;
    return {
        value
    };

## Generic functions

A generic function allows one or more parameterized types to appear in its signature. Each type parameter must be explicitly declared in an angle-bracket-enclosed and comma-separated list, following the function name.

    # #![allow(unused_variables)]
    #fn main() {
    // foo is generic over A and B

    fn foo<A, B>(x: A, y: B) {
    # }
    #}

Inside the function signature and body, the name of the type parameter can be used as a type name. Trait bounds can be specified for type parameters to allow methods with that trait to be called on values of that type. This is specified using the `where` syntax:

    # #![allow(unused_variables)]
    #fn main() {
    # use std::fmt::Debug;
    fn foo<T>(x: T) where T: Debug {
    # }
    #}

When a generic function is referenced, its type is instantiated based on the context of the reference. For example, calling the `foo` function here:

    # #![allow(unused_variables)]
    #fn main() {
    use std::fmt::Debug;

    fn foo<T>(x: &[T]) where T: Debug {
        // details elided
    }

    foo(&[1, 2]);
    #}

will instantiate type parameter `T` with `i32`.

The type parameters can also be explicitly supplied in a trailing path component after the function name. This might be necessary if there is not sufficient context to determine the type parameters. For example, `mem::size_of::<u32>() == 4`.

## Extern functions

Extern functions are part of Rust's foreign function interface, providing the opposite functionality to external blocks. Whereas external blocks allow Rust code to call foreign code, extern functions with bodies defined in Rust code can be called by foreign code. They are defined in the same way as any other Rust function, except that they have the `extern` modifier.

    # #![allow(unused_variables)]
    #fn main() {
    // Declares an extern fn, the ABI defaults to "C"
    extern fn new_i32() -> i32 { 0 }

    // Declares an extern fn with "stdcall" ABI
    # #[cfg(target_arch = "x86_64")]
    extern "stdcall" fn new_i32_stdcall() -> i32 { 0 }
    #}

Unlike normal functions, extern fns have type `extern "ABI" fn()`. This is the same type as the functions declared in an extern block.

    # #![allow(unused_variables)]
    #fn main() {
    # extern fn new_i32() -> i32 { 0 }
    let fptr: extern "C" fn() -> i32 = new_i32;
    #}

As non-Rust calling conventions do not support unwinding, unwinding past the end of an extern function will cause the process to abort. In LLVM, this is implemented by executing an illegal instruction.

## Attributes on functions

Outer attributes are allowed on functions. Inner attributes are allowed directly after the `{` inside its block.

This example shows an inner attribute on a function. The function will only be available while running tests.

    fn test_only() {
        #![test]
    }

> Note: Except for lints, it is idiomatic to only use outer attributes on function items.

The attributes that have meaning on a function are `cfg`, `deprecated`, `doc`, `export_name`, `link_section`, `no_mangle`, the lint check attributes, `must_use`, the procedural macro attributes, the testing attributes, and the optimization hint attributes.

# Type aliases

> Syntax
> TypeAlias :
> `type` IDENTIFIER Generics? WhereClause? `=` Type `;`

A type alias defines a new name for an existing type. Type aliases are declared with the keyword `type`. Every value has a single, specific type, but may implement several different traits, or be compatible with several different type constraints.

For example, the following defines the type `Point` as a synonym for the type `(u8, u8)`, the type of pairs of unsigned 8 bit integers:

    # #![allow(unused_variables)]
    #fn main() {
    type Point = (u8, u8);
    let p: Point = (41, 68);
    #}

A type alias to an enum type cannot be used to qualify the constructors:

    # #![allow(unused_variables)]
    #fn main() {
    enum E { A }
    type F = E;
    let _: F = E::A;  // OK
    // let _: F = F::A;  // Doesn't work
    #}

# Structs

> Syntax
> Struct :
> StructStruct
> | TupleStruct

> StructStruct :
> `struct` IDENTIFIER Generics? WhereClause? ( `{` StructFields? `}` | `;` )

> TupleStruct :
> `struct` IDENTIFIER Generics? `(` TupleFields? `)` WhereClause? `;`

> StructFields :
> StructField (`,` StructField)* `,`?

> StructField :
> OuterAttribute*
> Visibility IDENTIFIER `:` Type

> TupleFields :
> TupleField (`,` TupleField)* `,`?

> TupleField :
> OuterAttribute*
> Visibility Type

A struct is a nominal struct type defined with the keyword `struct`.

An example of a `struct` item and its use:

    # #![allow(unused_variables)]
    #fn main() {
    struct Point {x: i32, y: i32}
    let p = Point {x: 10, y: 11};
    let px: i32 = p.x;
    #}

A tuple struct is a nominal tuple type, also defined with the keyword `struct`. For example:

    # #![allow(unused_variables)]
    #fn main() {
    struct Point(i32, i32);
    let p = Point(10, 11);
    let px: i32 = match p { Point(x, _) => x };
    #}

A unit-like struct is a struct without any fields, defined by leaving off the list of fields entirely. Such a struct implicitly defines a constant of its type with the same name. For example:

    # #![allow(unused_variables)]
    #fn main() {
    struct Cookie;
    let c = [Cookie, Cookie {}, Cookie, Cookie {}];
    #}

is equivalent to

    # #![allow(unused_variables)]
    #fn main() {
    struct Cookie {}
    const Cookie: Cookie = Cookie {};
    let c = [Cookie, Cookie {}, Cookie, Cookie {}];
    #}

The precise memory layout of a struct is not specified. One can specify a particular layout using the `repr` attribute.

# Enumerations

> Syntax
> Enumeration :
> `enum` IDENTIFIER Generics? WhereClause? `{` EnumItems? `}`

> EnumItems :
> EnumItem ( `,` EnumItem )* `,`?

> EnumItem :
> OuterAttribute*
> IDENTIFIER ( EnumItemTuple | EnumItemStruct | EnumItemDiscriminant )?

> EnumItemTuple :
> `(` TupleFields? `)`

> EnumItemStruct :
> `{` StructFields? `}`

> EnumItemDiscriminant :
> `=` Expression

An enumeration, also referred to as enum is a simultaneous definition of a nominal enumerated type as well as a set of constructors, that can be used to create or pattern-match values of the corresponding enumerated type.

Enumerations are declared with the keyword `enum`.

An example of an `enum` item and its use:

    # #![allow(unused_variables)]
    #fn main() {
    enum Animal {
        Dog,
        Cat,
    }

    let mut a: Animal = Animal::Dog;
    a = Animal::Cat;
    #}

Enum constructors can have either named or unnamed fields:

    # #![allow(unused_variables)]
    #fn main() {
    enum Animal {
        Dog(String, f64),
        Cat { name: String, weight: f64 },
    }

    let mut a: Animal = Animal::Dog("Cocoa".to_string(), 37.2);
    a = Animal::Cat { name: "Spotty".to_string(), weight: 2.7 };
    #}

In this example, `Cat` is a struct-like enum variant, whereas `Dog` is simply called an enum variant. Each enum instance has a discriminant which is an integer associated to it that is used to determine which variant it holds. An opaque reference to this discriminant can be obtained with the `mem::discriminant` function.

## Custom Discriminant Values for Field-Less Enumerations

If there is no data attached to any of the variants of an enumeration, then the discriminant can be directly chosen and accessed.

These enumerations can be cast to integer types with the `as` operator by a numeric cast. The enumeration can optionally specify which integer each discriminant gets by following the variant name with `=` and then an integer literal. If the first variant in the declaration is unspecified, then it is set to zero. For every unspecified discriminant, it is set to one higher than the previous variant in the declaration.

    # #![allow(unused_variables)]
    #fn main() {
    enum Foo {
        Bar,            // 0
        Baz = 123,      // 123
        Quux,           // 124
    }

    let baz_discriminant = Foo::Baz as u32;
    assert_eq!(baz_discriminant, 123);
    #}

Under the default representation, the specified discriminant is interpreted as an `isize` value although the compiler is allowed to use a smaller type in the actual memory layout. The size and thus acceptable values can be changed by using a primitive representation or the `C` representation.

It is an error when two variants share the same discriminant.

    enum SharedDiscriminantError {
        SharedA = 1,
        SharedB = 1
    }

    enum SharedDiscriminantError2 {
        Zero,       // 0
        One,        // 1
        OneToo = 1  // 1 (collision with previous!)
    }

It is also an error to have an unspecified discriminant where the previous discriminant is the maximum value for the size of the discriminant.

    #[repr(u8)]
    enum OverflowingDiscriminantError {
        Max = 255,
        MaxPlusOne // Would be 256, but that overflows the enum.
    }

    #[repr(u8)]
    enum OverflowingDiscriminantError2 {
        MaxMinusOne = 254, // 254
        Max,               // 255
        MaxPlusOne         // Would be 256, but that overflows the enum.
    }

## Zero-variant Enums

Enums with zero variants are known as zero-variant enums. As they have no valid values, they cannot be instantiated.

    # #![allow(unused_variables)]
    #fn main() {
    enum ZeroVariants {}
    #}

# Unions

> Syntax
> Union :
> `union` IDENTIFIER Generics? WhereClause? `{`StructFields `}`

A union declaration uses the same syntax as a struct declaration, except with `union` in place of `struct`.

    # #![allow(unused_variables)]
    #fn main() {
    #[repr(C)]
    union MyUnion {
        f1: u32,
        f2: f32,
    }
    #}

The key property of unions is that all fields of a union share common storage. As a result writes to one field of a union can overwrite its other fields, and size of a union is determined by the size of its largest field.

A value of a union type can be created using the same syntax that is used for struct types, except that it must specify exactly one field:

    # #![allow(unused_variables)]
    #fn main() {
    # union MyUnion { f1: u32, f2: f32 }
    #
    let u = MyUnion { f1: 1 };
    #}

The expression above creates a value of type `MyUnion` with active field `f1`. Active field of a union can be accessed using the same syntax as struct fields:

    let f = u.f1;

Inactive fields can be accessed as well (using the same syntax) if they are sufficiently layout compatible with the current value kept by the union. Reading incompatible fields results in undefined behavior. However, the active field is not generally known statically, so all reads of union fields have to be placed in `unsafe` blocks.

    # #![allow(unused_variables)]
    #fn main() {
    # union MyUnion { f1: u32, f2: f32 }
    # let u = MyUnion { f1: 1 };
    #
    unsafe {
        let f = u.f1;
    }
    #}

Writes to `Copy` union fields do not require reads for running destructors, so these writes don't have to be placed in `unsafe` blocks

    # #![allow(unused_variables)]
    #fn main() {
    # union MyUnion { f1: u32, f2: f32 }
    # let mut u = MyUnion { f1: 1 };
    #
    u.f1 = 2;
    #}

Commonly, code using unions will provide safe wrappers around unsafe union field accesses.

Another way to access union fields is to use pattern matching. Pattern matching on union fields uses the same syntax as struct patterns, except that the pattern must specify exactly one field. Since pattern matching accesses potentially inactive fields it has to be placed in `unsafe` blocks as well.

    # #![allow(unused_variables)]
    #fn main() {
    # union MyUnion { f1: u32, f2: f32 }
    #
    fn f(u: MyUnion) {
        unsafe {
            match u {
                MyUnion { f1: 10 } => { println!("ten"); }
                MyUnion { f2 } => { println!("{}", f2); }
            }
        }
    }
    #}

Pattern matching may match a union as a field of a larger structure. In particular, when using a Rust union to implement a C tagged union via FFI, this allows matching on the tag and the corresponding field simultaneously:

    # #![allow(unused_variables)]
    #fn main() {
    #[repr(u32)]
    enum Tag { I, F }

    #[repr(C)]
    union U {
        i: i32,
        f: f32,
    }

    #[repr(C)]
    struct Value {
        tag: Tag,
        u: U,
    }

    fn is_zero(v: Value) -> bool {
        unsafe {
            match v {
                Value { tag: I, u: U { i: 0 } } => true,
                Value { tag: F, u: U { f: 0.0 } } => true,
                _ => false,
            }
        }
    }
    #}

Since union fields share common storage, gaining write access to one field of a union can give write access to all its remaining fields. Borrow checking rules have to be adjusted to account for this fact. As a result, if one field of a union is borrowed, all its remaining fields are borrowed as well for the same lifetime.

    // ERROR: cannot borrow `u` (via `u.f2`) as mutable more than once at a time
    fn test() {
        let mut u = MyUnion { f1: 1 };
        unsafe {
            let b1 = &mut u.f1;
                          ---- first mutable borrow occurs here (via `u.f1`)
            let b2 = &mut u.f2;
                          ^^^^ second mutable borrow occurs here (via `u.f2`)
            *b1 = 5;
        }
        - first borrow ends here
        assert_eq!(unsafe { u.f1 }, 5);
    }

As you could see, in many aspects (except for layouts, safety and ownership) unions behave exactly like structs, largely as a consequence of inheriting their syntactic shape from structs. This is also true for many unmentioned aspects of Rust language (such as privacy, name resolution, type inference, generics, trait implementations, inherent implementations, coherence, pattern checking, etc etc etc).

More detailed specification for unions, including unstable bits, can be found in RFC 1897 "Unions v1.2".

# Constant items

> Syntax ConstantItem : `const` IDENTIFIER `:` Type `=` Expression `;`

A constant item is a named constant value which is not associated with a specific memory location in the program. Constants are essentially inlined wherever they are used, meaning that they are copied directly into the relevant context when used. References to the same constant are not necessarily guaranteed to refer to the same memory address.

Constants must be explicitly typed. The type must have a `'static` lifetime: any references it contains must have `'static` lifetimes.

Constants may refer to the address of other constants, in which case the address will have elided lifetimes where applicable, otherwise – in most cases – defaulting to the `static` lifetime. (See static lifetime elision.) The compiler is, however, still at liberty to translate the constant many times, so the address referred to may not be stable.

    # #![allow(unused_variables)]
    #fn main() {
    const BIT1: u32 = 1 << 0;
    const BIT2: u32 = 1 << 1;

    const BITS: [u32; 2] = [BIT1, BIT2];
    const STRING: &'static str = "bitstring";

    struct BitsNStrings<'a> {
        mybits: [u32; 2],
        mystring: &'a str,
    }

    const BITS_N_STRINGS: BitsNStrings<'static> = BitsNStrings {
        mybits: BITS,
        mystring: STRING,
    };
    #}

## Constants with Destructors

Constants can contain destructors. Destructors are run when the value goes out of scope.

    # #![allow(unused_variables)]
    #fn main() {
    struct TypeWithDestructor(i32);

    impl Drop for TypeWithDestructor {
        fn drop(&mut self) {
            println!("Dropped. Held {}.", self.0);
        }
    }

    const ZERO_WITH_DESTRUCTOR: TypeWithDestructor = TypeWithDestructor(0);

    fn create_and_drop_zero_with_destructor() {
        let x = ZERO_WITH_DESTRUCTOR;
        // x gets dropped at end of function, calling drop.
        // prints "Dropped. Held 0.".
    }
    #}

# Static items

> Syntax
> StaticItem :
> `static` `mut`? IDENTIFIER `:` Type `=` Expression `;`

A static item is similar to a constant, except that it represents a precise memory location in the program. All references to the static refer to the same memory location. Static items have the `static` lifetime, which outlives all other lifetimes in a Rust program. Non-`mut` static items that contain a type that is not interior mutable may be placed in read-only memory. Static items do not call `drop` at the end of the program.

All access to a static is safe, but there are a number of restrictions on statics:

- The type must have the `Sync` trait bound to allow thread-safe access.
- Statics allow using paths to statics in the constant expression used to initialize them, but statics may not refer to other statics by value, only through a reference.
- Constants cannot refer to statics.

## Mutable statics

If a static item is declared with the `mut` keyword, then it is allowed to be modified by the program. One of Rust's goals is to make concurrency bugs hard to run into, and this is obviously a very large source of race conditions or other bugs. For this reason, an `unsafe` block is required when either reading or writing a mutable static variable. Care should be taken to ensure that modifications to a mutable static are safe with respect to other threads running in the same process.

Mutable statics are still very useful, however. They can be used with C libraries and can also be bound from C libraries in an `extern` block.

    # #![allow(unused_variables)]
    #fn main() {
    # fn atomic_add(_: &mut u32, _: u32) -> u32 { 2 }

    static mut LEVELS: u32 = 0;

    // This violates the idea of no shared state, and this doesn't internally
    // protect against races, so this function is `unsafe`
    unsafe fn bump_levels_unsafe1() -> u32 {
        let ret = LEVELS;
        LEVELS += 1;
        return ret;
    }

    // Assuming that we have an atomic_add function which returns the old value,
    // this function is "safe" but the meaning of the return value may not be what
    // callers expect, so it's still marked as `unsafe`
    unsafe fn bump_levels_unsafe2() -> u32 {
        return atomic_add(&mut LEVELS, 1);
    }
    #}

Mutable statics have the same restrictions as normal statics, except that the type does not have to implement the `Sync` trait.

## Using Statics or Consts

It can be confusing whether or not you should use a constant item or a static item. Constants should, in general, be preferred over statics unless one of the following are true:

- Large amounts of data are being stored
- The single-address property of statics is required.
- Interior mutability is required.

# Traits

A trait describes an abstract interface that types can implement. This interface consists of associated items, which come in three varieties:

- functions
- types
- constants

All traits define an implicit type parameter `Self` that refers to "the type that is implementing this interface". Traits may also contain additional type parameters. These type parameters, including `Self`, may be constrained by other traits and so forth as usual.

Traits are implemented for specific types through separate implementations.

Items associated with a trait do not need to be defined in the trait, but they may be. If the trait provides a definition, then this definition acts as a default for any implementation which does not override it. If it does not, then any implementation must provide a definition.

## Trait bounds

Generic items may use traits as bounds on their type parameters.

## Generic Traits

Type parameters can be specified for a trait to make it generic. These appear after the trait name, using the same syntax used in generic functions.

    # #![allow(unused_variables)]
    #fn main() {
    trait Seq<T> {
        fn len(&self) -> u32;
        fn elt_at(&self, n: u32) -> T;
        fn iter<F>(&self, F) where F: Fn(T);
    }
    #}

## Object Safety

Object safe traits can be the base trait of a trait object. A trait is object safe if it has the following qualities (defined in RFC 255):

- It must not require `Self: Sized`
- All associated functions must either have a `where Self: Sized` bound, or
  - Not have any type parameters (although lifetime parameters are allowed), and
  - Be a method that does not use `Self` except in the type of the receiver.
- It must not have any associated constants.
- All supertraits must also be object safe.

## Supertraits

Supertraits are traits that are required to be implemented for a type to implement a specific trait. Furthermore, anywhere a generic or trait object is bounded by a trait, it has access to the associated items of its supertraits.

Supertraits are declared by trait bounds on the `Self` type of a trait and transitively the supertraits of the traits declared in those trait bounds. It is an error for a trait to be its own supertrait.

The trait with a supertrait is called a subtrait of its supertrait.

The following is an example of declaring `Shape` to be a supertrait of `Circle`.

    # #![allow(unused_variables)]
    #fn main() {
    trait Shape { fn area(&self) -> f64; }
    trait Circle : Shape { fn radius(&self) -> f64; }
    #}

And the following is the same example, except using where clauses.

    # #![allow(unused_variables)]
    #fn main() {
    trait Shape { fn area(&self) -> f64; }
    trait Circle where Self: Shape { fn radius(&self) -> f64; }
    #}

This next example gives `radius` a default implementation using the `area` function from `Shape`.

    # #![allow(unused_variables)]
    #fn main() {
    # trait Shape { fn area(&self) -> f64; }
    trait Circle where Self: Shape {
        fn radius(&self) -> f64 {
            // A = pi * r^2
            // so algebraically,
            // r = sqrt(A / pi)
            (self.area() /std::f64::consts::PI).sqrt()
        }
    }
    #}

This next example calls a supertrait method on a generic parameter.

    # #![allow(unused_variables)]
    #fn main() {
    # trait Shape { fn area(&self) -> f64; }
    # trait Circle : Shape { fn radius(&self) -> f64; }
    fn print_area_and_radius<C: Circle>(c: C) {
        // Here we call the area method from the supertrait `Shape` of `Circle`.
        println!("Area: {}", c.area());
        println!("Radius: {}", c.radius());
    }
    #}

Similarly, here is an example of calling supertrait methods on trait objects.

    # #![allow(unused_variables)]
    #fn main() {
    # trait Shape { fn area(&self) -> f64; }
    # trait Circle : Shape { fn radius(&self) -> f64; }
    # struct UnitCircle;
    # impl Shape for UnitCircle { fn area(&self) -> f64 { std::f64::consts::PI } }
    # impl Circle for UnitCircle { fn radius(&self) -> f64 { 1.0 } }
    # let circle = UnitCircle;
    let circle = Box::new(circle) as Box<dyn Circle>;
    let nonsense = circle.radius() * circle.area();
    #}

# Implementations

An implementation is an item that associates items with an implementing type. Implementations are defined with the keyword `impl` and contain functions that belong to an instance of the type that is being implemented or to the type statically.

There are two types of implementations:

- inherent implementations
- trait implementations

## Inherent Implementations

An inherent implementation is defined as the sequence of the `impl` keyword, generic type declarations, a path to a nominal type, a where clause, and a bracketed set of associable items.

The nominal type is called the implementing type and the associable items are the associated items to the implementing type.

Inherent implementations associate the contained items to the implementing type. The associated item has a path of a path to the implementing type followed by the associate item's path component. Inherent implementations cannot contain associated type aliases.

A type can also have multiple inherent implementations. An implementing type must be defined within the same crate as the original type definition.

    # #![allow(unused_variables)]
    #fn main() {
    struct Point {x: i32, y: i32}

    impl Point {
        fn log(&self) {
            println!("Point is at ({}, {})", self.x, self.y);
        }
    }

    let my_point = Point {x: 10, y:11};
    my_point.log();
    #}

## Trait Implementations

A trait implementation is defined like an inherent implementation except that the optional generic type declarations is followed by a trait followed by the keyword `for`. Followed by a path to a nominal type.

The trait is known as the implemented trait. The implementing type implements the implemented trait.

A trait implementation must define all non-default associated items declared by the implemented trait, may redefine default associated items defined by the implemented trait, and cannot define any other items.

The path to the associated items is `<` followed by a path to the implementing type followed by `as` followed by a path to the trait followed by `>` as a path component followed by the associated item's path component.

    # #![allow(unused_variables)]
    #fn main() {
    # #[derive(Copy, Clone)]
    # struct Point {x: f64, y: f64};
    # type Surface = i32;
    # struct BoundingBox {x: f64, y: f64, width: f64, height: f64};
    # trait Shape { fn draw(&self, Surface); fn bounding_box(&self) -> BoundingBox; }
    # fn do_draw_circle(s: Surface, c: Circle) { }
    struct Circle {
        radius: f64,
        center: Point,
    }

    impl Copy for Circle {}

    impl Clone for Circle {
        fn clone(&self) -> Circle { *self }
    }

    impl Shape for Circle {
        fn draw(&self, s: Surface) { do_draw_circle(s, *self); }
        fn bounding_box(&self) -> BoundingBox {
            let r = self.radius;
            BoundingBox {
                x: self.center.x - r,
                y: self.center.y - r,
                width: 2.0 * r,
                height: 2.0 * r,
            }
        }
    }
    #}

### Trait Implementation Coherence

A trait implementation is considered incoherent if either the orphan check fails or there are overlapping implementation instances.

Two trait implementations overlap when there is a non-empty intersection of the traits the implementation is for, the implementations can be instantiated with the same type.

The `Orphan Check` states that every trait implementation must meet either of the following conditions:

1. The trait being implemented is defined in the same crate.

2. At least one of either `Self` or a generic type parameter of the trait must meet the following grammar, where `C` is a nominal type defined within the containing crate:

        T = C
          | &C
          | &mut C
          | Box<C>

## Generic Implementations

An implementation can take type and lifetime parameters, which can be used in the rest of the implementation. Type parameters declared for an implementation must be used at least once in either the trait or the implementing type of an implementation. Implementation parameters are written directly after the `impl` keyword.

    # #![allow(unused_variables)]
    #fn main() {
    # trait Seq<T> { fn dummy(&self, _: T) { } }
    impl<T> Seq<T> for Vec<T> {
        /* ... */
    }
    impl Seq<bool> for u32 {
        /* Treat the integer as a sequence of bits */
    }
    #}

## Attributes on Implementations

Implementations may contain outer attributes before the `impl` keyword and inner attributes inside the brackets that contain the associated items. Inner attributes must come before any associated items. That attributes that have meaning here are `cfg`, `deprecated`, `doc`, and the lint check attributes.

# External blocks

External blocks form the basis for Rust's foreign function interface. Declarations in an external block describe symbols in external, non-Rust libraries.

Functions within external blocks are declared in the same way as other Rust functions, with the exception that they may not have a body and are instead terminated by a semicolon.

Functions within external blocks may be called by Rust code, just like functions defined in Rust. The Rust compiler automatically translates between the Rust ABI and the foreign ABI.

Functions within external blocks may be variadic by specifying `...` after one or more named arguments in the argument list:

    extern {
        fn foo(x: i32, ...);
    }

A number of attributes control the behavior of external blocks.

By default external blocks assume that the library they are calling uses the standard C ABI on the specific platform. Other ABIs may be specified using an `abi` string, as shown here:

    // Interface to the Windows API
    extern "stdcall" { }

There are three ABI strings which are cross-platform, and which all compilers are guaranteed to support:

- `extern "Rust"` -- The default ABI when you write a normal `fn foo()` in any Rust code.
- `extern "C"` -- This is the same as `extern fn foo()`; whatever the default your C compiler supports.
- `extern "system"` -- Usually the same as `extern "C"`, except on Win32, in which case it's `"stdcall"`, or what you should use to link to the Windows API itself

There are also some platform-specific ABI strings:

- `extern "cdecl"` -- The default for x86_32 C code.
- `extern "stdcall"` -- The default for the Win32 API on x86_32.
- `extern "win64"` -- The default for C code on x86_64 Windows.
- `extern "sysv64"` -- The default for C code on non-Windows x86_64.
- `extern "aapcs"` -- The default for ARM.
- `extern "fastcall"` -- The `fastcall` ABI -- corresponds to MSVC's `__fastcall` and GCC and clang's `__attribute__((fastcall))`
- `extern "vectorcall"` -- The `vectorcall` ABI -- corresponds to MSVC's `__vectorcall` and clang's `__attribute__((vectorcall))`

Finally, there are some rustc-specific ABI strings:

- `extern "rust-intrinsic"` -- The ABI of rustc intrinsics.
- `extern "rust-call"` -- The ABI of the Fn::call trait functions.
- `extern "platform-intrinsic"` -- Specific platform intrinsics -- like, for example, `sqrt` -- have this ABI. You should never have to deal with it.

The `link` attribute allows the name of the library to be specified. When specified the compiler will attempt to link against the native library of the specified name.

    #[link(name = "crypto")]
    extern { }

The type of a function declared in an extern block is `extern "abi" fn(A1, ..., An) -> R`, where `A1...An` are the declared types of its arguments and `R` is the declared return type.

It is valid to add the `link` attribute on an empty extern block. You can use this to satisfy the linking requirements of extern blocks elsewhere in your code (including upstream crates) instead of adding the attribute to each extern block.

# Type and Lifetime Parameters

> Syntax
> Generics :
> `<` GenericParams `>`

> GenericParams :
> LifetimeParams
> | ( LifetimeParam `,` )* TypeParams

> LifetimeParams :
> ( LifetimeParam `,` )* LifetimeParam?

> LifetimeParam :
> OuterAttribute? LIFETIME_OR_LABEL `:` LifetimeBounds?

> TypeParams:
> ( TypeParam `,` )* TypeParam ?

> TypeParam :
> OuterAttribute? IDENTIFIER ( `:` TypeParamBounds )? ( `=` Type )?

Functions, type aliases, structs, enumerations, unions, traits and implementations may be parameterized by types and lifetimes. These parameters are listed in angle brackets (`<...>`), usually immediately after and before its definition the name of the item. For implementations, which don't have a name, they come directly after `impl`. Lifetime parameters must be declared before type parameters. Some examples of items with type and lifetime parameters:

    # #![allow(unused_variables)]
    #fn main() {
    fn foo<'a, T>() {}
    trait A<U> {}
    struct Ref<'a, T> where T: 'a { r: &'a T }
    #}

References, raw pointers, arrays, slices, tuples and function pointers have lifetime or type parameters as well, but are not referred to with path syntax.

## Where clauses

> Syntax
> WhereClause :
> `where` ( WhereClauseItem `,` )* WhereClauseItem ?

> WhereClauseItem :
> LifetimeWhereClauseItem
> | TypeBoundWhereClauseItem

> LifetimeWhereClauseItem :
> Lifetime `:` LifetimeBounds

> TypeBoundWhereClauseItem :
> ForLifetimes? Type `:` TypeParamBounds?

> ForLifetimes :
> `for` `<` LifetimeParams `>`

Where clauses provide an another way to specify bounds on type and lifetime parameters as well as a way to specify bounds on types that aren't type parameters.

Bounds that don't use the item's parameters or higher-ranked lifetimes are checked when the item is defined. It is an error for such a bound to be false.

`Copy`, `Clone` and `Sized` bounds are also checked for certain generic types when defining the item. It is an error to have `Copy` or `Clone`as a bound on a mutable reference, trait object or slice or `Sized` as a bound on a trait object or slice.

    struct A<T>
    where
        T: Iterator,            // Could use A<T: Iterator> instead
        T::Item: Copy,
        String: PartialEq<T>,
        i32: Default,           // Allowed, but not useful
        i32: Iterator,          // Error: the trait bound is not satisfied
        [T]: Copy,              // Error: the trait bound is not satisfied
    {
        f: T,
    }

## Attributes

Generic lifetime and type parameters allow attributes on them. There are no built-in attributes that do anything in this position, although custom derive attributes may give meaning to it.

This example shows using a custom derive attribute to modify the meaning of a generic parameter.

    // Assume that the derive for MyFlexibleClone declared `my_flexible_clone` as
    // an attribute it understands.
    #[derive(MyFlexibleClone)] struct Foo<#[my_flexible_clone(unbounded)] H> {
        a: *const H
    }

# Associated Items

Associated Items are the items declared in traits or defined in implementations. They are called this because they are defined on an associate type — the type in the implementation. They are a subset of the kinds of items you can declare in a module. Specifically, there are associated functions (including methods), associated types, and associated constants.

Associated items are useful when the associated item logically is related to the associating item. For example, the `is_some` method on `Option` is intrinsically related to Options, so should be associated.

Every associated item kind comes in two varieties: definitions that contain the actual implementation and declarations that declare signatures for definitions.

It is the declarations that make up the contract of traits and what it available on generic types.

## Associated functions and methods

Associated functions are functions associated with a type.

An associated function declaration declares a signature for an associated function definition. It is written as a function item, except the function body is replaced with a `;`.

The identifier if the name of the function. The generics, parameter list, return type, and where clause of the associated function must be the same as the associated function declarations's.

An associated function definition defines a function associated with another type. It is written the same as a function item.

An example of a common associated function is a `new` function that returns a value of the type the associated function is associated with.

    struct Struct {
        field: i32
    }

    impl Struct {
        fn new() -> Struct {
            Struct {
                field: 0i32
            }
        }
    }

    fn main () {
        let _struct = Struct::new();
    }

When the associated function is declared on a trait, the function can also be called with a path that is a path to the trait appended by the name of the trait. When this happens, it is substituted for `<_ as Trait>::function_name`.

    # #![allow(unused_variables)]
    #fn main() {
    trait Num {
        fn from_i32(n: i32) -> Self;
    }

    impl Num for f64 {
        fn from_i32(n: i32) -> f64 { n as f64 }
    }

    // These 4 are all equivalent in this case.
    let _: f64 = Num::from_i32(42);
    let _: f64 = <_ as Num>::from_i32(42);
    let _: f64 = <f64 as Num>::from_i32(42);
    let _: f64 = f64::from_i32(42);
    #}

### Methods

Associated functions whose first parameter is named `self` are called methods and may be invoked using the method call operator, for example, `x.foo()`, as well as the usual function call notation.

The `self` parameter must have one of the following types. As a result, the following shorthands may be used to declare `self`:

- `self` -> `self: Self`
- `&'lifetime self` -> `self: &'lifetime Self`
- `&'lifetime mut self` -> `self: &'lifetime mut Self`
- `self : Box<Self>` (no shorthand)

> Note: Lifetimes can be and usually are elided with this shorthand.

Consider the following trait:

    # #![allow(unused_variables)]
    #fn main() {
    # type Surface = i32;
    # type BoundingBox = i32;
    trait Shape {
        fn draw(&self, Surface);
        fn bounding_box(&self) -> BoundingBox;
    }
    #}

This defines a trait with two methods. All values that have implementations of this trait while the trait is in scope can have their `draw` and `bounding_box` methods called.

    # #![allow(unused_variables)]
    #fn main() {
    # type Surface = i32;
    # type BoundingBox = i32;
    # trait Shape {
    #     fn draw(&self, Surface);
    #     fn bounding_box(&self) -> BoundingBox;
    # }
    #
    struct Circle {
        // ...
    }

    impl Shape for Circle {
        // ...
    #   fn draw(&self, _: Surface) {}
    #   fn bounding_box(&self) -> BoundingBox { 0i32 }
    }

    # impl Circle {
    #     fn new() -> Circle { Circle{} }
    # }
    #
    let circle_shape = Circle::new();
    let bounding_box = circle_shape.bounding_box();
    #}

## Associated Types

Associated types are type aliases associated with another type. Associated types cannot be defined in inherent implementations nor can they be given a default implementation in traits.

An associated type declaration declares a signature for associated type definitions. It is written as `type`, then an identifier, and finally an optional list of trait bounds.

The identifier is the name of the declared type alias. The optional trait bounds must be fulfilled by the implementations of the type alias.

An associated type definition defines a type alias on another type. It is written as `type`, then an identifier, then an `=`, and finally a type.

If a type `Item` has an associated type `Assoc` from a trait `Trait`, then `<Item as Trait>::Assoc` is a type that is an alias of the type specified in the associated type definition. Furthermore, if `Item` is a type parameter, then `Item::Assoc` can be used in type parameters.

    trait AssociatedType {
        // Associated type declaration
        type Assoc;
    }

    struct Struct;

    struct OtherStruct;

    impl AssociatedType for Struct {
        // Associated type definition
        type Assoc = OtherStruct;
    }

    impl OtherStruct {
        fn new() -> OtherStruct {
            OtherStruct
        }
    }

    fn main() {
        // Usage of the associated type to refer to OtherStruct as <Struct as AssociatedType>::Assoc
        let _other_struct: OtherStruct = <Struct as AssociatedType>::Assoc::new();
    }

### Associated Types Container Example

Consider the following example of a `Container` trait. Notice that the type is available for use in the method signatures:

    # #![allow(unused_variables)]
    #fn main() {
    trait Container {
        type E;
        fn empty() -> Self;
        fn insert(&mut self, Self::E);
    }
    #}

In order for a type to implement this trait, it must not only provide implementations for every method, but it must specify the type `E`. Here's an implementation of `Container` for the standard library type `Vec`:

    # #![allow(unused_variables)]
    #fn main() {
    # trait Container {
    #     type E;
    #     fn empty() -> Self;
    #     fn insert(&mut self, Self::E);
    # }
    impl<T> Container for Vec<T> {
        type E = T;
        fn empty() -> Vec<T> { Vec::new() }
        fn insert(&mut self, x: T) { self.push(x); }
    }
    #}

## Associated Constants

Associated constants are constants associated with a type.

An associated constant declaration declares a signature for associated constant definitions. It is written as `const`, then an identifier, then `:`, then a type, finished by a `;`.

The identifier is the name of the constant used in the path. The type is the type that the definition has to implement.

An associated constant definition defines a constant associated with a type. It is written the same as a constant item.

### Associated Constants Examples

A basic example:

    trait ConstantId {
        const ID: i32;
    }

    struct Struct;

    impl ConstantId for Struct {
        const ID: i32 = 1;
    }

    fn main() {
        assert_eq!(1, Struct::ID);
    }

Using default values:

    trait ConstantIdDefault {
        const ID: i32 = 1;
    }

    struct Struct;
    struct OtherStruct;

    impl ConstantIdDefault for Struct {}

    impl ConstantIdDefault for OtherStruct {
        const ID: i32 = 5;
    }

    fn main() {
        assert_eq!(1, Struct::ID);
        assert_eq!(5, OtherStruct::ID);
    }

# Visibility and Privacy

> Syntax
> Visibility :
> EMPTY
> | `pub`
> | `pub` `(` `crate` `)`
> | `pub` `(` `in` ModulePath `)`
> | `pub` `(` `in`? `self` `)`
> | `pub` `(` `in`? `super` `)`

These two terms are often used interchangeably, and what they are attempting to convey is the answer to the question "Can this item be used at this location?"

Rust's name resolution operates on a global hierarchy of namespaces. Each level in the hierarchy can be thought of as some item. The items are one of those mentioned above, but also include external crates. Declaring or defining a new module can be thought of as inserting a new tree into the hierarchy at the location of the definition.

To control whether interfaces can be used across modules, Rust checks each use of an item to see whether it should be allowed or not. This is where privacy warnings are generated, or otherwise "you used a private item of another module and weren't allowed to."

By default, everything in Rust is private, with two exceptions: Associated items in a `pub` Trait are public by default; Enum variants in a `pub` enum are also public by default. When an item is declared as `pub`, it can be thought of as being accessible to the outside world. For example:

    # fn main() {}
    // Declare a private struct
    struct Foo;

    // Declare a public struct with a private field
    pub struct Bar {
        field: i32,
    }

    // Declare a public enum with two public variants
    pub enum State {
        PubliclyAccessibleState,
        PubliclyAccessibleState2,
    }

With the notion of an item being either public or private, Rust allows item accesses in two cases:

1. If an item is public, then it can be accessed externally from some module `m` if you can access all the item's parent modules from `m`. You can also potentially be able to name the item through re-exports. See below.
2. If an item is private, it may be accessed by the current module and its descendants.

These two cases are surprisingly powerful for creating module hierarchies exposing public APIs while hiding internal implementation details. To help explain, here's a few use cases and what they would entail:

- A library developer needs to expose functionality to crates which link against their library. As a consequence of the first case, this means that anything which is usable externally must be `pub` from the root down to the destination item. Any private item in the chain will disallow external accesses.

- A crate needs a global available "helper module" to itself, but it doesn't want to expose the helper module as a public API. To accomplish this, the root of the crate's hierarchy would have a private module which then internally has a "public API". Because the entire crate is a descendant of the root, then the entire local crate can access this private module through the second case.

- When writing unit tests for a module, it's often a common idiom to have an immediate child of the module to-be-tested named `mod test`. This module could access any items of the parent module through the second case, meaning that internal implementation details could also be seamlessly tested from the child module.

In the second case, it mentions that a private item "can be accessed" by the current module and its descendants, but the exact meaning of accessing an item depends on what the item is. Accessing a module, for example, would mean looking inside of it (to import more items). On the other hand, accessing a function would mean that it is invoked. Additionally, path expressions and import statements are considered to access an item in the sense that the import/expression is only valid if the destination is in the current visibility scope.

Here's an example of a program which exemplifies the three cases outlined above:

    // This module is private, meaning that no external crate can access this
    // module. Because it is private at the root of this current crate, however, any
    // module in the crate may access any publicly visible item in this module.
    mod crate_helper_module {

        // This function can be used by anything in the current crate
        pub fn crate_helper() {}

        // This function *cannot* be used by anything else in the crate. It is not
        // publicly visible outside of the `crate_helper_module`, so only this
        // current module and its descendants may access it.
        fn implementation_detail() {}
    }

    // This function is "public to the root" meaning that it's available to external
    // crates linking against this one.
    pub fn public_api() {}

    // Similarly to 'public_api', this module is public so external crates may look
    // inside of it.
    pub mod submodule {
        use crate_helper_module;

        pub fn my_method() {
            // Any item in the local crate may invoke the helper module's public
            // interface through a combination of the two rules above.
            crate_helper_module::crate_helper();
        }

        // This function is hidden to any module which is not a descendant of
        // `submodule`
        fn my_implementation() {}

        #[cfg(test)]
        mod test {

            #[test]
            fn test_my_implementation() {
                // Because this module is a descendant of `submodule`, it's allowed
                // to access private items inside of `submodule` without a privacy
                // violation.
                super::my_implementation();
            }
        }
    }

    # fn main() {}

For a Rust program to pass the privacy checking pass, all paths must be valid accesses given the two rules above. This includes all use statements, expressions, types, etc.

## `pub(in path)`, `pub(crate)`, `pub(super)`, and `pub(self)`

In addition to public and private, Rust allows users to declare an item as visible within a given scope. The rules for `pub` restrictions are as follows:

- `pub(in path)` makes an item visible within the provided `path`. `path` must be a parent module of the item whose visibility is being declared.
- `pub(crate)` makes an item visible within the current crate.
- `pub(super)` makes an item visible to the parent module. This is equivalent to `pub(in super)`.
- `pub(self)` makes an item visible to the current module. This is equivalent to `pub(in self)`.

Here's an example:

    pub mod outer_mod {
        pub mod inner_mod {
            // This function is visible within `outer_mod`
            pub(in outer_mod) fn outer_mod_visible_fn() {}

            // This function is visible to the entire crate
            pub(crate) fn crate_visible_fn() {}

            // This function is visible within `outer_mod`
            pub(super) fn super_mod_visible_fn() {
                // This function is visible since we're in the same `mod`
                inner_mod_visible_fn();
            }

            // This function is visible
            pub(self) fn inner_mod_visible_fn() {}
        }
        pub fn foo() {
            inner_mod::outer_mod_visible_fn();
            inner_mod::crate_visible_fn();
            inner_mod::super_mod_visible_fn();

            // This function is no longer visible since we're outside of `inner_mod`
            // Error! `inner_mod_visible_fn` is private
            //inner_mod::inner_mod_visible_fn();
        }
    }

    fn bar() {
        // This function is still visible since we're in the same crate
        outer_mod::inner_mod::crate_visible_fn();

        // This function is no longer visible since we're outside of `outer_mod`
        // Error! `super_mod_visible_fn` is private
        //outer_mod::inner_mod::super_mod_visible_fn();

        // This function is no longer visible since we're outside of `outer_mod`
        // Error! `outer_mod_visible_fn` is private
        //outer_mod::inner_mod::outer_mod_visible_fn();

        outer_mod::foo();
    }

    fn main() { bar() }

## Re-exporting and Visibility

Rust allows publicly re-exporting items through a `pub use` directive. Because this is a public directive, this allows the item to be used in the current module through the rules above. It essentially allows public access into the re-exported item. For example, this program is valid:

    pub use self::implementation::api;

    mod implementation {
        pub mod api {
            pub fn f() {}
        }
    }

    # fn main() {}

This means that any external crate referencing `implementation::api::f` would receive a privacy violation, while the path `api::f` would be allowed.

When re-exporting a private item, it can be thought of as allowing the "privacy chain" being short-circuited through the reexport instead of passing through the namespace hierarchy as it normally would.

# Attributes

> Syntax
> Attribute :
> InnerAttribute | OuterAttribute

> InnerAttribute :
> `#![` MetaItem `]`

> OuterAttribute :
> `#[` MetaItem `]`

> MetaItem :
> IDENTIFIER
> | IDENTIFIER `=` LITERAL
> | IDENTIFIER `(` MetaSeq `)`

> MetaSeq :
> EMPTY
> | MetaItem
> | LITERAL
> | MetaItem `,` MetaSeq

An attribute is a general, free-form metadatum that is interpreted according to name, convention, and language and compiler version. Attributes are modeled on Attributes in ECMA-335, with the syntax coming from ECMA-334 (C#).

Attributes may appear as any of:

- A single identifier, the attribute name
- An identifier followed by the equals sign '=' and a literal, providing a key/value pair
- An identifier followed by a parenthesized list of sub-attribute arguments which include literals

Inner attributes, written with a bang ("!") after the hash ("#"), apply to the item that the attribute is declared within. Outer attributes, written without the bang after the hash, apply to the thing that follows the attribute.

Attributes may be applied to many things in the language:

- All item declarations accept outer attributes while external blocks, functions, implementations, and modules accept inner attributes.
- Statements accept outer attributes.
- Block expressions accept outer and inner attributes, but only when they are the outer expression of an expression statement or the final expression of another block expression.
- Enum variants and struct and union fields accept outer attributes.
- Match expression arms accept outer attributes.
- Generic lifetime or type parameter accept outer attributes.

Some examples of attributes:

    # #![allow(unused_variables)]
    #fn main() {
    // General metadata applied to the enclosing module or crate.
    #![crate_type = "lib"]

    // A function marked as a unit test
    #[test]
    fn test_foo() {
        /* ... */
    }

    // A conditionally-compiled module
    #[cfg(target_os = "linux")]
    mod bar {
        /* ... */
    }

    // A lint attribute used to suppress a warning/error
    #[allow(non_camel_case_types)]
    type int8_t = i8;

    // Outer attribute applies to the entire function.
    fn some_unused_variables() {
      #![allow(unused_variables)]

      let x = ();
      let y = ();
      let z = ();
    }
    #}

There are three kinds of attributes:

- Built-in attributes
- Macro attributes
- Derive mode helper attributes

## Active and inert attributes

An attribute is either active or inert. During attribute processing, active attributes remove themselves from the thing they are on while inert attriutes stay on.

The `cfg` and `cfg_attr` attributes are active. The `test` attribute is inert when compiling for tests and active otherwise. Attribute macros are active. All other attributes are inert.

The rest of this page describes or links to descriptions of which attribute names have meaning.

## Crate-only attributes

- `crate_name` - specify the crate's crate name.
- `crate_type` - see linkage.
- `no_builtins` - disable optimizing certain code patterns to invocations of library functions that are assumed to exist
- `no_main` - disable emitting the `main` symbol. Useful when some other object being linked to defines `main`.
- `no_start` - disable linking to the `native` crate, which specifies the "start" language item.
- `no_std` - disable linking to the `std` crate.
- `recursion_limit` - Sets the maximum depth for potentially infinitely-recursive compile-time operations like auto-dereference or macro expansion. The default is `#![recursion_limit="64"]`.
- `windows_subsystem` - Indicates that when this crate is linked for a Windows target it will configure the resulting binary's subsystem via the linker. Valid values for this attribute are `console` and `windows`, corresponding to those two respective subsystems. More subsystems may be allowed in the future, and this attribute is ignored on non-Windows targets.

## Module-only attributes

- `no_implicit_prelude` - disable injecting `use std::prelude::*` in this module.
- `path` - specifies the file to load the module from. `#[path="foo.rs"] mod bar;` is equivalent to `mod bar { /* contents of foo.rs */ }`. The path is taken relative to the directory that the current module is in.

## FFI attributes

On an `extern` block, the following attributes are interpreted:

- `link_args` - specify arguments to the linker, rather than just the library name and type. This is feature gated and the exact behavior is implementation-defined (due to variety of linker invocation syntax).
- `link` - indicate that a native library should be linked to for the declarations in this block to be linked correctly. `link` supports an optional `kind` key with three possible values: `dylib`, `static`, and `framework`. See external blocks for more about external blocks. Two examples: `#[link(name = "readline")]` and `#[link(name = "CoreFoundation", kind = "framework")]`.
- `linked_from` - indicates what native library this block of FFI items is coming from. This attribute is of the form `#[linked_from = "foo"]` where `foo` is the name of a library in either `#[link]` or a `-l` flag. This attribute is currently required to export symbols from a Rust dynamic library on Windows, and it is feature gated behind the `linked_from` feature.

On declarations inside an `extern` block, the following attributes are interpreted:

- `link_name` - the name of the symbol that this function or static should be imported as.
- `linkage` - on a static, this specifies the linkage type.

See type layout for documentation on the `repr` attribute which can be used to control type layout.

## Macro-related attributes

- `macro_use` on a `mod` — macros defined in this module will be visible in the module's parent, after this module has been included.

- `macro_use` on an `extern crate` — load macros from this crate. An optional list of names `#[macro_use(foo, bar)]` restricts the import to just those macros named. The `extern crate` must appear at the crate root, not inside `mod`, which ensures proper function of the `$crate` macro variable.

- `macro_reexport` on an `extern crate` — re-export the named macros.

- `macro_export` - export a `macro_rules` macro for cross-crate usage.

- `no_link` on an `extern crate` — even if we load this crate for macros, don't link it into the output.

See the macros section of the first edition of the book for more information on `macro_rules` macro scope.

- `proc_macro` - Defines a function-like macro.

- `proc_macro_derive` - Defines a derive mode macro.

- `proc_macro_attribute` - Defines an attribute macro.

## Miscellaneous attributes

- `export_name` - on statics and functions, this determines the name of the exported symbol.
- `global_allocator` - when applied to a static item implementing the `GlobalAlloc` trait, sets the global allocator.
- `link_section` - on statics and functions, this specifies the section of the object file that this item's contents will be placed into.
- `no_mangle` - on any item, do not apply the standard name mangling. Set the symbol for this item to its identifier.

### Deprecation

The `deprecated` attribute marks an item as deprecated. It has two optional fields, `since` and `note`.

- `since` expects a version number, as in `#[deprecated(since = "1.4.1")]`
  - `rustc` doesn't know anything about versions, but external tools like `clippy` may check the validity of this field.
- `note` is a free text field, allowing you to provide an explanation about the deprecation and preferred alternatives.

Only public items can be given the `#[deprecated]` attribute. `#[deprecated]` on a module is inherited by all child items of that module.

`rustc` will issue warnings on usage of `#[deprecated]` items. `rustdoc` will show item deprecation, including the `since` version and `note`, if available.

Here's an example.

    # #![allow(unused_variables)]
    #fn main() {
    #[deprecated(since = "5.2", note = "foo was rarely used. Users should instead use bar")]
    pub fn foo() {}

    pub fn bar() {}
    #}

The RFC contains motivations and more details.

### Documentation

The `doc` attribute is used to document items and fields. Doc comments are transformed into `doc` attributes.

See The Rustdoc Book for reference material on this attribute.

### Testing

The compiler comes with a default test framework. It works by attributing functions with the `test` attribute. These functions are only compiled when compiling with the test harness. Like [main], functions annotated with this attribute must take no arguments, must not declare any [trait or lifetime bounds], must not have any [where clauses], and its return type must be one of the following:

- `()`
- `Result<(), E> where E: Error`

> Note: The implementation of which return types are allowed is determined by the unstable [`Termination`] trait.

> Note: The test harness is ran by passing the `--test` argument to `rustc` or using `cargo test`.

Tests that return `()` pass as long as they terminate and do not panic. Tests that return a `Result` pass as long as they return `Ok(())`. Tests that do not terminate neither pass nor fail.

A function annotated with the `test` attribute can also be annotated with the `ignore` attribute. The `ignore` attribute tells the test harness to not execute that function as a test. It will still only be compiled when compiling with the test harness.

A function annotated with the `test` attribute that returns `()` can also be annotated with the `should_panic` attribute. The `should_panic` attribute makes the test only pass if it actually panics.

### Conditional compilation

The `cfg` and `cfg_attr` attributes control conditional compilation of items and attributes. See the conditional compilation section for reference material on these attributes.

### Lint check attributes

A lint check names a potentially undesirable coding pattern, such as unreachable code or omitted documentation, for the static entity to which the attribute applies.

For any lint check `C`:

- `allow(C)` overrides the check for `C` so that violations will go unreported,
- `deny(C)` signals an error after encountering a violation of `C`,
- `forbid(C)` is the same as `deny(C)`, but also forbids changing the lint level afterwards,
- `warn(C)` warns about violations of `C` but continues compilation.

The lint checks supported by the compiler can be found via `rustc -W help`, along with their default settings. Compiler plugins can provide additional lint checks.

    # #![allow(unused_variables)]
    #fn main() {
    pub mod m1 {
        // Missing documentation is ignored here
        #[allow(missing_docs)]
        pub fn undocumented_one() -> i32 { 1 }

        // Missing documentation signals a warning here
        #[warn(missing_docs)]
        pub fn undocumented_too() -> i32 { 2 }

        // Missing documentation signals an error here
        #[deny(missing_docs)]
        pub fn undocumented_end() -> i32 { 3 }
    }
    #}

This example shows how one can use `allow` and `warn` to toggle a particular check on and off:

    # #![allow(unused_variables)]
    #fn main() {
    #[warn(missing_docs)]
    pub mod m2{
        #[allow(missing_docs)]
        pub mod nested {
            // Missing documentation is ignored here
            pub fn undocumented_one() -> i32 { 1 }

            // Missing documentation signals a warning here,
            // despite the allow above.
            #[warn(missing_docs)]
            pub fn undocumented_two() -> i32 { 2 }
        }

        // Missing documentation signals a warning here
        pub fn undocumented_too() -> i32 { 3 }
    }
    #}

This example shows how one can use `forbid` to disallow uses of `allow` for that lint check:

    # #![allow(unused_variables)]
    #fn main() {
    #[forbid(missing_docs)]
    pub mod m3 {
        // Attempting to toggle warning signals an error here
        #[allow(missing_docs)]
        /// Returns 2.
        pub fn undocumented_too() -> i32 { 2 }
    }
    #}

#### `must_use`

The `must_use` attribute can be used on user-defined composite types (`struct`s, `enum`s, and `union`s) and functions.

When used on user-defined composite types, if the expression of an expression statement has that type, then the `unused_must_use` lint is violated.

    #[must_use]
    struct MustUse {
      // some fields
    }

    # impl MustUse {
    #   fn new() -> MustUse { MustUse {} }
    # }
    #
    fn main() {
      // Violates the `unused_must_use` lint.
      MustUse::new();
    }

When used on a function, if the expression of an expression statement is a call expression to that function, then the `unused_must_use` lint is violated. The exceptions to this is if the return type of the function is `()`, `!`, or a zero-variant enum, in which case the attribute does nothing.

    #[must_use]
    fn five() -> i32 { 5i32 }

    fn main() {
      // Violates the unused_must_use lint.
      five();
    }

When used on a function in a trait declaration, then the behavior also applies when the call expression is a function from an implementation of the trait.

    trait Trait {
      #[must_use]
      fn use_me(&self) -> i32;
    }

    impl Trait for i32 {
      fn use_me(&self) -> i32 { 0i32 }
    }

    fn main() {
      // Violates the `unused_must_use` lint.
      5i32.use_me();
    }

When used on a function in an implementation, the attribute does nothing.

> Note: Trivial no-op expressions containing the value will not violate the lint. Examples include wrapping the value in a type that does not implement `Drop` and then not using that type and being the final expression of a block expression that is not used.

>     #[must_use]
>     fn five() -> i32 { 5i32 }

>     fn main() {
>       // None of these violate the unused_must_use lint.
>       (five(),);
>       Some(five());
>       { five() };
>       if true { five() } else { 0i32 };
>       match true {
>         _ => five()
>       };
>     }

> Note: It is idiomatic to use a let statement with a pattern of `_` when a must-used value is purposely discarded.

>     #[must_use]
>     fn five() -> i32 { 5i32 }

>     fn main() {
>       // Does not violate the unused_must_use lint.
>       let _ = five();
>     }

The `must_use` attribute may also include a message by using `#[must_use = "message"]`. The message will be given alongside the warning.

### Optimization Hints

The `cold` and `inline` attributes give suggestions to the compiler to compile your code in a way that may be faster than what it would do without the hint. The attributes are only suggestions, and the compiler may choose to ignore it.

#### `inline` Attribute

The `inline` attribute suggests to the compiler that it should place a copy of the attributed function in the caller, rather than generating code to call the function where it is defined.

This attribute can be used on functions and function prototypes, although it does not do anything on function prototypes. When this attribute is applied to a function in a [trait], it applies only to that function when used as a default function for a trait implementation and not to all trait implementations.

> Note: The compiler automatically inlines functions based on internal heuristics. Incorrectly inlining functions can actually make the program slower, so this attibute should be used with care.

There are three ways of using the inline attribute:

- `#[inline]` hints the compiler to perform an inline expansion.
- `#[inline(always)]` asks the compiler to always perform an inline expansion.
- `#[inline(never)]` asks the compiler to never perform an inline expansion.

#### `cold` Attribute

The `cold` attribute suggests to the compiler that the attributed function is unlikely to be called.

This attribute can be used on functions and function prototypes, although it does not do anything on function prototypes. When this attribute is applied to a function in a [trait], it applies only to that function when used as a default function for a trait implementation and not to all trait implementations.

### `derive`

The `derive` attribute allows certain traits to be automatically implemented for data structures. For example, the following will create an `impl` for the `PartialEq` and `Clone` traits for `Foo`, the type parameter `T` will be given the `PartialEq` or `Clone` constraints for the appropriate `impl`:

    # #![allow(unused_variables)]
    #fn main() {
    #[derive(PartialEq, Clone)]
    struct Foo<T> {
        a: i32,
        b: T,
    }
    #}

The generated `impl` for `PartialEq` is equivalent to

    # #![allow(unused_variables)]
    #fn main() {
    # struct Foo<T> { a: i32, b: T }
    impl<T: PartialEq> PartialEq for Foo<T> {
        fn eq(&self, other: &Foo<T>) -> bool {
            self.a == other.a && self.b == other.b
        }

        fn ne(&self, other: &Foo<T>) -> bool {
            self.a != other.a || self.b != other.b
        }
    }
    #}

You can implement `derive` for your own traits through procedural macros.

[trait]: items/traits.html[main]: crates-and-source-files.html [`Termination`]: ../std/process/trait.Termination.html [where clause]: items/where-clauses.html [trait or lifetime bounds]: trait-bounds.html

# Statements and expressions

Rust is primarily an expression language. This means that most forms of value-producing or effect-causing evaluation are directed by the uniform syntax category of expressions. Each kind of expression can typically nest within each other kind of expression, and rules for evaluation of expressions involve specifying both the value produced by the expression and the order in which its sub-expressions are themselves evaluated.

In contrast, statements in Rust serve mostly to contain and explicitly sequence expression evaluation.

# Statements

A statement is a component of a block, which is in turn a component of an outer expression or function.

Rust has two kinds of statement: declaration statements and expression statements.

## Declaration statements

A declaration statement is one that introduces one or more names into the enclosing statement block. The declared names may denote new variables or new items.

The two kinds of declaration statements are item declarations and `let` statements.

### Item declarations

An item declaration statement has a syntactic form identical to an item declaration within a module. Declaring an item within a statement block restricts its scope to the block containing the statement. The item is not given a canonical path nor are any sub-items it may declare. The exception to this is that associated items defined by implementations are still accessible in outer scopes as long as the item and, if applicable, trait are accessible. It is otherwise identical in meaning to declaring the item inside a module.

There is no implicit capture of the containing function's generic parameters, parameters, and local variables. For example, `inner` may not access `outer_var`.

    # #![allow(unused_variables)]
    #fn main() {
    fn outer() {
      let outer_var = true;

      fn inner() { /* outer_var is not in scope here */ }

      inner();
    }
    #}

### `let` statements

A `let` statement introduces a new set of variables, given by a pattern. The pattern is followed optionally by a type annotation and then optionally by an initializer expression. When no type annotation is given, the compiler will infer the type, or signal an error if insufficient type information is available for definite inference. Any variables introduced by a variable declaration are visible from the point of declaration until the end of the enclosing block scope.

## Expression statements

An expression statement is one that evaluates an expression and ignores its result. As a rule, an expression statement's purpose is to trigger the effects of evaluating its expression.

An expression that consists of only a block expression or control flow expression, if used in a context where a statement is permitted, can omit the trailing semicolon. This can cause an ambiguity between it being parsed as a standalone statement and as a part of another expression; in this case, it is parsed as a statement.

    # #![allow(unused_variables)]
    #fn main() {
    # let mut v = vec![1, 2, 3];
    v.pop();          // Ignore the element returned from pop
    if v.is_empty() {
        v.push(5);
    } else {
        v.remove(0);
    }                 // Semicolon can be omitted.
    [1];              // Separate expression statement, not an indexing expression.
    #}

When the trailing semicolon is omitted, the result must be type `()`.

    # #![allow(unused_variables)]
    #fn main() {
    // bad: the block's type is i32, not ()
    // Error: expected `()` because of default return type
    // if true {
    //   1
    // }

    // good: the block's type is i32
    if true {
      1
    } else {
      2
    };
    #}

## Attributes on Statements

Statements accept outer attributes. The attributes that have meaning on a statement are `cfg`, and the lint check attributes.

# Expressions

> Syntax
> Expression :
> LiteralExpression
> | PathExpression
> | BlockExpression
> | OperatorExpression
> | GroupedExpression
> | ArrayExpression
> | IndexExpression
> | TupleExpression
> | TupleIndexingExpression
> | StructExpression
> | EnumerationVariantExpression
> | CallExpression
> | MethodCallExpression
> | FieldExpression
> | ClosureExpression
> | LoopExpression
> | ContinueExpression
> | BreakExpression
> | RangeExpression
> | IfExpression
> | IfLetExpression
> | MatchExpression
> | ReturnExpression

An expression may have two roles: it always produces a value, and it may have effects (otherwise known as "side effects"). An expression evaluates to a value, and has effects during evaluation. Many expressions contain sub-expressions (operands). The meaning of each kind of expression dictates several things:

- Whether or not to evaluate the sub-expressions when evaluating the expression
- The order in which to evaluate the sub-expressions
- How to combine the sub-expressions' values to obtain the value of the expression

In this way, the structure of expressions dictates the structure of execution. Blocks are just another kind of expression, so blocks, statements, expressions, and blocks again can recursively nest inside each other to an arbitrary depth.

## Expression precedence

The precedence of Rust operators and expressions is ordered as follows, going from strong to weak. Binary Operators at the same precedence level are grouped in the order given by their associativity.

Operator/Expression | Associativity
--- | ---
Paths |
Method calls |
Field expressions | left to right
Function calls, array indexing |
`?` |
Unary `-` `*` `!` `&` `&mut` |
`as` | left to right
`*` `/` `%` | left to right
`+` `-` | left to right
`<<` `>>` | left to right
`&` | left to right
`^` | left to right
`|` | left to right
`==` `!=` `<` `>` `<=` `>=` | Require parentheses
`&&` | left to right
`||` | left to right
`..` `..=` | Require parentheses
`=` `+=` `-=` `*=` `/=` `%=` `&=` `|=` `^=` `<<=` `>>=` | right to left
`return` `break` closures |

## Place Expressions and Value Expressions

Expressions are divided into two main categories: place expressions and value expressions. Likewise within each expression, sub-expressions may occur in either place context or value context. The evaluation of an expression depends both on its own category and the context it occurs within.

A place expression is an expression that represents a memory location. These expressions are paths which refer to local variables, static variables, dereferences (`*expr`), array indexing expressions (`expr[expr]`), field references (`expr.f`) and parenthesized place expressions. All other expressions are value expressions.

A value expression is an expression that represents an actual value.

The left operand of an assignment or compound assignment expression is a place expression context, as is the single operand of a unary borrow, and the operand of any implicit borrow. The discriminant or subject of a match expression and right side of a let statement is also a place expression context. All other expression contexts are value expression contexts.

> Note: Historically, place expressions were called lvalues and value expressions were called rvalues.

### Moved and copied types

When a place expression is evaluated in a value expression context, or is bound by value in a pattern, it denotes the value held in that memory location. If the type of that value implements `Copy`, then the value will be copied. In the remaining situations if that type is `Sized`, then it may be possible to move the value. Only the following place expressions may be moved out of:

- Variables which are not currently borrowed.
- Temporary values.
- Fields of a place expression which can be moved out of and doesn't implement `Drop`.
- The result of dereferencing an expression with type `Box<T>` and that can also be moved out of.

Moving out of a place expression that evaluates to a local variable, the location is deinitialized and cannot be read from again until it is reinitialized. In all other cases, trying to use a place expression in a value expression context is an error.

### Mutability

For a place expression to be assigned to, mutably borrowed, implicitly mutably borrowed, or bound to a pattern containing `ref mut` it must be mutable. We call these mutable place expressions. In contrast, other place expressions are called immutable place expressions.

The following expressions can be mutable place expression contexts:

- Mutable variables, which are not currently borrowed.
- Mutable `static` items.
- Temporary values.
- Fields, this evaluates the subexpression in a mutable place expression context.
- Dereferences of a `*mut T` pointer.
- Dereference of a variable, or field of a variable, with type `&mut T`. Note: This is an exception to the requirement of the next rule.
- Dereferences of a type that implements `DerefMut`, this then requires that the value being dereferenced is evaluated is a mutable place expression context.
- Array indexing of a type that implements `DerefMut`, this then evaluates the value being indexed, but not the index, in mutable place expression context.

### Temporary lifetimes

When using a value expression in most place expression contexts, a temporary unnamed memory location is created initialized to that value and the expression evaluates to that location instead, except if promoted to `'static`. Promotion of a value expression to a `'static` slot occurs when the expression could be written in a constant, borrowed, and dereferencing that borrow where the expression was the originally written, without changing the runtime behavior. That is, the promoted expression can be evaluated at compile-time and the resulting value does not contain interior mutability or destructors (these properties are determined based on the value where possible, e.g. `&None` always has the type `&'static Option<_>`, as it contains nothing disallowed). Otherwise, the lifetime of temporary values is typically

- the innermost enclosing statement; the tail expression of a block is considered part of the statement that encloses the block, or
- the condition expression or the loop conditional expression if the temporary is created in the condition expression of an `if` or in the loop conditional expression of a `while` expression.

When a temporary value expression is being created that is assigned into a `let` declaration, however, the temporary is created with the lifetime of the enclosing block instead, as using the enclosing `let` declaration would be a guaranteed error (since a pointer to the temporary would be stored into a variable, but the temporary would be freed before the variable could be used). The compiler uses simple syntactic rules to decide which values are being assigned into a `let` binding, and therefore deserve a longer temporary lifetime.

Here are some examples:

- `let x = foo(&temp())`. The expression `temp()` is a value expression. As it is being borrowed, a temporary is created which will be freed after the innermost enclosing statement; in this case, the `let` declaration.
- `let x = temp().foo()`. This is the same as the previous example, except that the value of `temp()` is being borrowed via autoref on a method-call. Here we are assuming that `foo()` is an `&self` method defined in some trait, say `Foo`. In other words, the expression `temp().foo()` is equivalent to `Foo::foo(&temp())`.
- `let x = if foo(&temp()) {bar()} else {baz()};`. The expression `temp()` is a value expression. As the temporary is created in the condition expression of an `if`, it will be freed at the end of the condition expression; in this example before the call to `bar` or `baz` is made.
- `let x = if temp().must_run_bar {bar()} else {baz()};`. Here we assume the type of `temp()` is a struct with a boolean field `must_run_bar`. As the previous example, the temporary corresponding to `temp()` will be freed at the end of the condition expression.
- `while foo(&temp()) {bar();}`. The temporary containing the return value from the call to `temp()` is created in the loop conditional expression. Hence it will be freed at the end of the loop conditional expression; in this example before the call to `bar` if the loop body is executed.
- `let x = &temp()`. Here, the same temporary is being assigned into `x`, rather than being passed as a parameter, and hence the temporary's lifetime is considered to be the enclosing block.
- `let x = SomeStruct { foo: &temp() }`. As in the previous case, the temporary is assigned into a struct which is then assigned into a binding, and hence it is given the lifetime of the enclosing block.
- `let x = [ &temp() ]`. As in the previous case, the temporary is assigned into an array which is then assigned into a binding, and hence it is given the lifetime of the enclosing block.
- `let ref x = temp()`. In this case, the temporary is created using a ref binding, but the result is the same: the lifetime is extended to the enclosing block.

### Implicit Borrows

Certain expressions will treat an expression as a place expression by implicitly borrowing it. For example, it is possible to compare two unsized slices for equality directly, because the `==` operator implicitly borrows it's operands:

    # #![allow(unused_variables)]
    #fn main() {
    # let c = [1, 2, 3];
    # let d = vec![1, 2, 3];
    let a: &[i32];
    let b: &[i32];
    # a = &c;
    # b = &d;
    // ...
    *a == *b;
    // Equivalent form:
    ::std::cmp::PartialEq::eq(&*a, &*b);
    #}

Implicit borrows may be taken in the following expressions:

- Left operand in method-call expressions.
- Left operand in field expressions.
- Left operand in call expressions.
- Left operand in array indexing expressions.
- Operand of the dereference operator (`*`).
- Operands of comparison.
- Left operands of the compound assignment.

## Constant expressions

Certain types of expressions can be evaluated at compile time. These are called constant expressions. Certain places, such as in constants and statics, require a constant expression, and are always evaluated at compile time. In other places, such as in `let` statements, constant expressions may be evaluated at compile time. If errors, such as out of bounds array indexing or overflow occurs, then it is a compiler error if the value must be evaluated at compile time, otherwise it is just a warning, but the code will most likely panic when run.

The following expressions are constant expressions, so long as any operands are also constant expressions and do not cause any `Drop::drop` calls to be ran.

- Literals.
- Paths to functions and constants. Recursively defining constants is not allowed.
- Tuple expressions.
- Array expressions.
- Struct expressions.
- Enum variant expressions.
- Block expressions, including `unsafe` blocks, which only contain items and possibly a constant tail expression.
- Field expressions.
- Index expressions, array indexing or slice with a `usize`.
- Range expressions.
- Closure expressions which don't capture variables from the environment.
- Built in negation, arithmetic, logical, comparison or lazy boolean operators used on integer and floating point types, `bool` and `char`.
- Shared borrows, except if applied to a type with interior mutability.
- The dereference operator.
- Grouped expressions.
- Cast expressions, except pointer to address and function pointer to address casts.

## Overloading Traits

Many of the following operators and expressions can also be overloaded for other types using traits in `std::ops` or `std::cmp`. These traits also exist in `core::ops` and `core::cmp` with the same names.

# Literal expressions

> Syntax
> LiteralExpression :
> CHAR_LITERAL
> | STRING_LITERAL
> | RAW_STRING_LITERAL
> | BYTE_LITERAL
> | BYTE_STRING_LITERAL
> | RAW_BYTE_STRING_LITERAL
> | INTEGER_LITERAL
> | FLOAT_LITERAL
> | BOOLEAN_LITERAL

A literal expression consists of one of the literal forms described earlier. It directly describes a number, character, string, or boolean value.

    # #![allow(unused_variables)]
    #fn main() {
    "hello";   // string type
    '5';       // character type
    5;         // integer type
    #}

# Path expressions

A path used as an expression context denotes either a local variable or an item. Path expressions that resolve to local or static variables are place expressions, other paths are value expressions. Using a `static mut` variable requires an `unsafe` block.

    # #![allow(unused_variables)]
    #fn main() {
    # mod globals {
    #     pub static STATIC_VAR: i32 = 5;
    #     pub static mut STATIC_MUT_VAR: i32 = 7;
    # }
    # let local_var = 3;
    local_var;
    globals::STATIC_VAR;
    unsafe { globals::STATIC_MUT_VAR };
    let some_constructor = Some::<i32>;
    let push_integer = Vec::<i32>::push;
    let slice_reverse = <[i32]>::reverse;
    #}

# Block expressions

> Syntax
> BlockExpression :
> `{`
> InnerAttribute*
> Statements*
> Expression?
> `}`

> Statements :
> ( `;`
> | ItemDeclaration
> | LetStatement ;
> | NonControlFlowExpressionStatement ;
> | FlowControlExpressionStatement ;?
> )*

A block expression, or block, is a control flow expression and anonymouse namespace scope for items and variable declarations. As a control flow expression, a block sequentially executes its component non-item declaration statements and then its final optional expression. As an anonymous namespace scope, item declarations are only in scope inside the block itself and variables declared by `let` statements are in scope from the next statement until the end of the block.

Blocks are written as `{`, then any inner attributes, then statements, then an optional expression, and finally a `}`. Statements are usually required to be followed a semicolon, with two exceptions. Item declaration statements do not need to be followed by a semicolon. Expression statements usually require a following semicolon except if its outer expression is a flow control expression. Furthermore, extra semicolons between statements are allowed, but these semicolons do not affect semantics.

> Note: The semicolon following a statement is not a part of the statement itself. They are invalid when using the `stmt` macro matcher.

When evaluating a block expression, each statement, except for item declaration statements, is executed sequentially. Then the final expression is executed, if given.

The type of a block is the type of the final expression, or `()` if the final expression is omitted.

    # #![allow(unused_variables)]
    #fn main() {
    # fn fn_call() {}
    let _: () = {
        fn_call();
    };

    let five: i32 = {
        fn_call();
        5
    };

    assert_eq!(5, five);
    #}

> Note: As a control flow expression, if a block expression is the outer expression of an expression statement, the expected type is `()` unless it is followed immediately by a semicolon.

Blocks are always value expressions and evaluate the last expression in value expression context. This can be used to force moving a value if really needed. For example, the following example fails on the call to `consume_self` because the struct was moved out of `s` in the block expression.

    # #![allow(unused_variables)]
    #fn main() {
    struct Struct;

    impl Struct {
        fn consume_self(self) {}
        fn borrow_self(&self) {}
    }

    fn move_by_block_expression() {
        let s = Struct;

        // Move the value out of `s` in the block expreesion.
        (&{ s }).borrow_self();

        // Fails to execute because `s` is moved out of.
        s.consume_self();
    }
    #}

## `unsafe` blocks

> Syntax
> UnsafeBlockExpression :
> `unsafe` BlockExpression

See `unsafe` block for more information on when to use `unsafe`

A block of code can be prefixed with the `unsafe` keyword to permit unsafe operations. Examples:

    # #![allow(unused_variables)]
    #fn main() {
    unsafe {
        let b = [13u8, 17u8];
        let a = &b[0] as *const u8;
        assert_eq!(*a, 13);
        assert_eq!(*a.offset(1), 17);
    }

    # unsafe fn an_unsafe_fn() -> i32 { 10 }
    let a = unsafe { an_unsafe_fn() };
    #}

## Attributes on block expressions

Block expressions allow outer attributes and inner attributes directly after the opening brace when the block expression is the outer expression of an expression statement or the final expression of another block expression. The attributes that have meaning on a block expression are `cfg` and the lint check attributes.

For example, this function returns `true` on unix platforms and `false` on other platforms.

    # #![allow(unused_variables)]
    #fn main() {
    fn is_unix_platform() -> bool {
        #[cfg(unix)] { true }
        #[cfg(not(unix))] { false }
    }
    #}

# Operator expressions

> Syntax
> OperatorExpression :
> BorrowExpression
> | DereferenceExpression
> | ErrorPropagationExpression
> | NegationExpression
> | ArithmeticOrLogicalExpression
> | ComparisonExpression
> | LazyBooleanExpression
> | TypeCastExpression
> | AssignmentExpression
> | CompoundAssignmentExpression

Operators are defined for built in types by the Rust language. Many of the following operators can also be overloaded using traits in `std::ops` or `std::cmp`.

## Overflow

Integer operators will panic when they overflow when compiled in debug mode. The `-C debug-assertions` and `-C overflow-checks` compiler flags can be used to control this more directly. The following things are considered to be overflow:

- When `+`, `*` or `-` create a value greater than the maximum value, or less than the minimum value that can be stored. This includes unary `-` on the smallest value of any signed integer type.
- Using `/` or `%`, where the left-hand argument is the smallest integer of a signed integer type and the right-hand argument is `-1`.
- Using `<<` or `>>` where the right-hand argument is greater than or equal to the number of bits in the type of the left-hand argument, or is negative.

## Borrow operators

> Syntax
> BorrowExpression :
> (`&`|`&&`) Expression
> | (`&`|`&&`) `mut` Expression

The `&` (shared borrow) and `&mut` (mutable borrow) operators are unary prefix operators. When applied to a place expression, this expressions produces a reference (pointer) to the location that the value refers to. The memory location is also placed into a borrowed state for the duration of the reference. For a shared borrow (`&`), this implies that the place may not be mutated, but it may be read or shared again. For a mutable borrow (`&mut`), the place may not be accessed in any way until the borrow expires. `&mut` evaluates its operand in a mutable place expression context. If the `&` or `&mut` operators are applied to a value expression, then a temporary value is created.

These operators cannot be overloaded.

    # #![allow(unused_variables)]
    #fn main() {
    {
        // a temporary with value 7 is created that lasts for this scope.
        let shared_reference = &7;
    }
    let mut array = [-2, 3, 9];
    {
        // Mutably borrows `array` for this scope.
        // `array` may only be used through `mutable_reference`.
        let mutable_reference = &mut array;
    }
    #}

Even though `&&` is a single token (the lazy 'and' operator), when used in the context of borrow expressions it works as two borrows:

    # #![allow(unused_variables)]
    #fn main() {
    // same meanings:
    let a = &&  10;
    let a = & & 10;

    // same meanings:
    let a = &&&&  mut 10;
    let a = && && mut 10;
    let a = & & & & mut 10;
    #}

## The dereference operator

> Syntax
> DereferenceExpression :
> `*` Expression

The `*` (dereference) operator is also a unary prefix operator. When applied to a pointer it denotes the pointed-to location. If the expression is of type `&mut T` and `*mut T`, and is either a local variable, a (nested) field of a local variable or is a mutable place expression, then the resulting memory location can be assigned to. Dereferencing a raw pointer requires `unsafe`.

On non-pointer types `*x` is equivalent to `*std::ops::Deref::deref(&x)` in an immutable place expression context and `*std::ops::Deref::deref_mut(&mut x)` in a mutable place expression context.

    # #![allow(unused_variables)]
    #fn main() {
    let x = &7;
    assert_eq!(*x, 7);
    let y = &mut 9;
    *y = 11;
    assert_eq!(*y, 11);
    #}

## The question mark operator

> Syntax
> ErrorPropagationExpression :
> Expression `?`

The question mark operator (`?`) unwraps valid values or returns erroneous values, propagating them to the calling function. It is a unary postfix operator that can only be applied to the types `Result<T, E>` and `Option<T>`.

When applied to values of the `Result<T, E>` type, it propagates errors. If the value is `Err(e)`, then it will return `Err(From::from(e))` from the enclosing function or closure. If applied to `Ok(x)`, then it will unwrap the value to evaluate to `x`.

    # #![allow(unused_variables)]
    #fn main() {
    # use std::num::ParseIntError;
    fn try_to_parse() -> Result<i32, ParseIntError> {
        let x: i32 = "123".parse()?; // x = 123
        let y: i32 = "24a".parse()?; // returns an Err() immediately
        Ok(x + y)                    // Doesn't run.
    }

    let res = try_to_parse();
    println!("{:?}", res);
    # assert!(res.is_err())
    #}

When applied to values of the `Option<T>` type, it propagates `Nones`. If the value is `None`, then it will return `None`. If applied to `Some(x)`, then it will unwrap the value to evaluate to `x`.

    # #![allow(unused_variables)]
    #fn main() {
    fn try_option_some() -> Option<u8> {
        let val = Some(1)?;
        Some(val)
    }
    assert_eq!(try_option_some(), Some(1));

    fn try_option_none() -> Option<u8> {
        let val = None?;
        Some(val)
    }
    assert_eq!(try_option_none(), None);
    #}

`?` cannot be overloaded.

## Negation operators

> Syntax
> NegationExpression :
> `-` Expression
> | `!` Expression

These are the last two unary operators. This table summarizes the behavior of them on primitive types and which traits are used to overload these operators for other types. Remember that signed integers are always represented using two's complement. The operands of all of these operators are evaluated in value expression context so are moved or copied.

Symbol | Integer | `bool` | Floating Point | Overloading Trait
--- | --- | --- | --- | ---
`-` | Negation* |  | Negation | `std::ops::Neg`
`!` | Bitwise NOT | Logical NOT |  | `std::ops::Not`

* Only for signed integer types.

Here are some example of these operators

    # #![allow(unused_variables)]
    #fn main() {
    let x = 6;
    assert_eq!(-x, -6);
    assert_eq!(!x, -7);
    assert_eq!(true, !false);
    #}

## Arithmetic and Logical Binary Operators

> Syntax
> ArithmeticOrLogicalExpression :
> Expression `+` Expression
> | Expression `-` Expression
> | Expression `*` Expression
> | Expression `/` Expression
> | Expression `%` Expression
> | Expression `&` Expression
> | Expression `|` Expression
> | Expression `^` Expression
> | Expression `<<` Expression
> | Expression `>>` Expression

Binary operators expressions are all written with infix notation. This table summarizes the behavior of arithmetic and logical binary operators on primitive types and which traits are used to overload these operators for other types. Remember that signed integers are always represented using two's complement. The operands of all of these operators are evaluated in value expression context so are moved or copied.

Symbol | Integer | `bool` | Floating Point | Overloading Trait
--- | --- | --- | --- | ---
`+` | Addition |  | Addition | `std::ops::Add`
`-` | Subtraction |  | Subtraction | `std::ops::Sub`
`*` | Multiplication |  | Multiplication | `std::ops::Mul`
`/` | Division* |  | Division | `std::ops::Div`
`%` | Remainder |  | Remainder | `std::ops::Rem`
`&` | Bitwise AND | Logical AND |  | `std::ops::BitAnd`
`|` | Bitwise OR | Logical OR |  | `std::ops::BitOr`
`^` | Bitwise XOR | Logical XOR |  | `std::ops::BitXor`
`<<` | Left Shift |  |  | `std::ops::Shl`
`>>` | Right Shift** |  |  | `std::ops::Shr`

* Integer division rounds towards zero.

** Arithmetic right shift on signed integer types, logical right shift on unsigned integer types.

Here are examples of these operators being used.

    # #![allow(unused_variables)]
    #fn main() {
    assert_eq!(3 + 6, 9);
    assert_eq!(5.5 - 1.25, 4.25);
    assert_eq!(-5 * 14, -70);
    assert_eq!(14 / 3, 4);
    assert_eq!(100 % 7, 2);
    assert_eq!(0b1010 & 0b1100, 0b1000);
    assert_eq!(0b1010 | 0b1100, 0b1110);
    assert_eq!(0b1010 ^ 0b1100, 0b110);
    assert_eq!(13 << 3, 104);
    assert_eq!(-10 >> 2, -3);
    #}

## Comparison Operators

> Syntax
> ComparisonExpression :
> Expression `==` Expression
> | Expression `!=` Expression
> | Expression `>` Expression
> | Expression `<` Expression
> | Expression `>=` Expression
> | Expression `<=` Expression

Comparison operators are also defined both for primitive types and many type in the standard library. Parentheses are required when chaining comparison operators. For example, the expression `a == b == c` is invalid and may be written as `(a == b) == c`.

Unlike arithmetic and logical operators, the traits for overloading the operators the traits for these operators are used more generally to show how a type may be compared and will likely be assumed to define actual comparisons by functions that use these traits as bounds. Many functions and macros in the standard library can then use that assumption (although not to ensure safety). Unlike the arithmetic and logical operators above, these operators implicitly take shared borrows of their operands, evaluating them in place expression context:

    a == b;
    // is equivalent to
    ::std::cmp::PartialEq::eq(&a, &b);

This means that the operands don't have to be moved out of.

Symbol | Meaning | Overloading method
--- | --- | ---
`==` | Equal | `std::cmp::PartialEq::eq`
`!=` | Not equal | `std::cmp::PartialEq::ne`
`>` | Greater than | `std::cmp::PartialOrd::gt`
`<` | Less than | `std::cmp::PartialOrd::lt`
`>=` | Greater than or equal to | `std::cmp::PartialOrd::ge`
`<=` | Less than or equal to | `std::cmp::PartialOrd::le`

Here are examples of the comparison operators being used.

    # #![allow(unused_variables)]
    #fn main() {
    assert!(123 == 123);
    assert!(23 != -12);
    assert!(12.5 > 12.2);
    assert!([1, 2, 3] < [1, 3, 4]);
    assert!('A' <= 'B');
    assert!("World" >= "Hello");
    #}

## Lazy boolean operators

> Syntax
> LazyBooleanExpression :
> Expression `||` Expression
> | Expression `&&` Expression

The operators `||` and `&&` may be applied to operands of boolean type. The `||` operator denotes logical 'or', and the `&&` operator denotes logical 'and'. They differ from `|` and `&` in that the right-hand operand is only evaluated when the left-hand operand does not already determine the result of the expression. That is, `||` only evaluates its right-hand operand when the left-hand operand evaluates to `false`, and `&&` only when it evaluates to `true`.

    # #![allow(unused_variables)]
    #fn main() {
    let x = false || true; // true
    let y = false && panic!(); // false, doesn't evaluate `panic!()`
    #}

## Type cast expressions

> Syntax
> TypeCastExpression :
> Expression `as` PathInExpression

A type cast expression is denoted with the binary operator `as`.

Executing an `as` expression casts the value on the left-hand side to the type on the right-hand side.

An example of an `as` expression:

    # #![allow(unused_variables)]
    #fn main() {
    # fn sum(values: &[f64]) -> f64 { 0.0 }
    # fn len(values: &[f64]) -> i32 { 0 }
    fn average(values: &[f64]) -> f64 {
        let sum: f64 = sum(values);
        let size: f64 = len(values) as f64;
        sum / size
    }
    #}

`as` can be used to explicitly perform coercions, as well as the following additional casts. Here `*T` means either `*const T` or `*mut T`.

Type of `e` | `U` | Cast performed by `e as U`
--- | --- | ---
Integer or Float type | Integer or Float type | Numeric cast
C-like enum | Integer type | Enum cast
`bool` or `char` | Integer type | Primitive to integer cast
`u8` | `char` | `u8` to `char` cast
`*T` | `*V` where `V: Sized` * | Pointer to pointer cast
`*T` where `T: Sized` | Numeric type | Pointer to address cast
Integer type | `*V` where `V: Sized` | Address to pointer cast
`&[T; n]` | `*const T` | Array to pointer cast
Function pointer | `*V` where `V: Sized` | Function pointer to pointer cast
Function pointer | Integer | Function pointer to address cast
Closure ** | Function pointer | Closure to function pointer cast

* or `T` and `V` are compatible unsized types, e.g., both slices, both the same trait object.

** only for closures that do capture (close over) any local variables

### Semantics

- Numeric cast
  - Casting between two integers of the same size (e.g. i32 -> u32) is a no-op
  - Casting from a larger integer to a smaller integer (e.g. u32 -> u8) will truncate
  - Casting from a smaller integer to a larger integer (e.g. u8 -> u32) will
    - zero-extend if the source is unsigned
    - sign-extend if the source is signed
  - Casting from a float to an integer will round the float towards zero
    - NOTE: currently this will cause Undefined Behavior if the rounded value cannot be represented by the target integer type. This includes Inf and NaN. This is a bug and will be fixed.
  - Casting from an integer to float will produce the floating point representation of the integer, rounded if necessary (rounding strategy unspecified)
  - Casting from an f32 to an f64 is perfect and lossless
  - Casting from an f64 to an f32 will produce the closest possible value (rounding strategy unspecified)
    - NOTE: currently this will cause Undefined Behavior if the value is finite but larger or smaller than the largest or smallest finite value representable by f32. This is a bug and will be fixed.
- Enum cast
  - Casts an enum to its discriminant, then uses a numeric cast if needed.
- Primitive to integer cast
  - `false` casts to `0`, `true` casts to `1`
  - `char` casts to the value of the code point, then uses a numeric cast if needed.
- `u8` to `char` cast
  - Casts to the `char` with the corresponding code point.

## Assignment expressions

> Syntax
> AssignmentExpression :
> | Expression `=` Expression

An assignment expression consists of a place expression followed by an equals sign (`=`) and a value expression. Such an expression always has the `unit` type.

Evaluating an assignment expression drops the left-hand operand, unless it's an uninitialized local variable or field of a local variable, and either copies or moves its right-hand operand to its left-hand operand. The left-hand operand must be a place expression: using a value expression results in a compiler error, rather than promoting it to a temporary.

    # #![allow(unused_variables)]
    #fn main() {
    # let mut x = 0;
    # let y = 0;
    x = y;
    #}

## Compound assignment expressions

> Syntax
> CompoundAssignmentExpression :
> Expression `+=` Expression
> | Expression `-=` Expression
> | Expression `*=` Expression
> | Expression `/=` Expression
> | Expression `%=` Expression
> | Expression `&=` Expression
> | Expression `|=` Expression
> | Expression `^=` Expression
> | Expression `<<=` Expression
> | Expression `>>=` Expression

The `+`, `-`, `*`, `/`, `%`, `&`, `|`, `^`, `<<`, and `>>` operators may be composed with the `=` operator. The expression `place_exp OP= value` is equivalent to `place_expr = place_expr OP val`. For example, `x = x + 1` may be written as `x += 1`. Any such expression always has the `unit` type. These operators can all be overloaded using the trait with the same name as for the normal operation followed by 'Assign', for example, `std::ops::AddAssign` is used to overload `+=`. As with `=`, `place_expr` must be a place expression.

    # #![allow(unused_variables)]
    #fn main() {
    let mut x = 10;
    x += 4;
    assert_eq!(x, 14);
    #}

# Grouped expressions

> Syntax
> GroupedExpression :
> `(` Expression `)`

An expression enclosed in parentheses evaluates to the result of the enclosed expression. Parentheses can be used to explicitly specify evaluation order within an expression.

An example of a parenthesized expression:

    # #![allow(unused_variables)]
    #fn main() {
    let x: i32 = 2 + 3 * 4;
    let y: i32 = (2 + 3) * 4;
    assert_eq!(x, 14);
    assert_eq!(y, 20);
    #}

An example of a necessary use of parentheses is when calling a function pointer that is a member of a struct:

    # #![allow(unused_variables)]
    #fn main() {
    # struct A {
    #    f: fn() -> &'static str
    # }
    # impl A {
    #    fn f(&self) -> &'static str {
    #        "The method f"
    #    }
    # }
    # let a = A{f: || "The field f"};
    #
    assert_eq!( a.f (), "The method f");
    assert_eq!((a.f)(), "The field f");
    #}

# Array and array index expressions

## Array expressions

> Syntax
> ArrayExpression :
> `[` `]`
> | `[` Expression ( `,` Expression )* `,`? `]`
> | `[` Expression `;` Expression `]`

An array expression can be written by enclosing zero or more comma-separated expressions of uniform type in square brackets. This produces and array containing each of these values in the order they are written.

Alternatively there can be exactly two expressions inside the brackets, separated by a semi-colon. The expression after the `;` must be a have type `usize` and be a constant expression, such as a literal or a constant item. `[a; b]` creates an array containing `b` copies of the value of `a`. If the expression after the semi-colon has a value greater than 1 then this requires that the type of `a` is `Copy`.

    # #![allow(unused_variables)]
    #fn main() {
    [1, 2, 3, 4];
    ["a", "b", "c", "d"];
    [0; 128];              // array with 128 zeros
    [0u8, 0u8, 0u8, 0u8,];
    [[1, 0, 0], [0, 1, 0], [0, 0, 1]]; // 2D array
    #}

## Array and slice indexing expressions

> Syntax
> IndexExpression :
> Expression `[` Expression `]`

Array and slice-typed expressions can be indexed by writing a square-bracket-enclosed expression of type `usize` (the index) after them. When the array is mutable, the resulting memory location can be assigned to.

For other types an index expression `a[b]` is equivalent to `*std::ops::Index::index(&a, b)`, or `*std::ops::IndexMut::index_mut(&mut a, b)` in a mutable place expression context. Just as with methods, Rust will also insert dereference operations on `a` repeatedly to find an implementation.

Indices are zero-based for arrays and slices. Array access is a constant expression, so bounds can be checked at compile-time with a constant index value. Otherwise a check will be performed at run-time that will put the thread in a panicked state if it fails.

    # #![allow(unused_variables)]
    #fn main() {
    // lint is deny by default.
    #![warn(const_err)]

    ([1, 2, 3, 4])[2];        // Evaluates to 3

    let b = [[1, 0, 0], [0, 1, 0], [0, 0, 1]];
    b[1][2];                  // multidimensional array indexing

    let x = (["a", "b"])[10]; // warning: index out of bounds

    let n = 10;
    let y = (["a", "b"])[n];  // panics

    let arr = ["a", "b"];
    arr[10];                  // warning: index out of bounds
    #}

The array index expression can be implemented for types other than arrays and slices by implementing the Index and IndexMut traits.

# Tuple and tuple indexing expressions

## Tuple expressions

> Syntax
> TupleExpression :
> `(` `)`
> | `(` ( Expression `,` )+ Expression? `)`

Tuples are written by enclosing zero or more comma-separated expressions in parentheses. They are used to create tuple-typed values.

    # #![allow(unused_variables)]
    #fn main() {
    (0.0, 4.5);
    ("a", 4usize, true);
    ();
    #}

You can disambiguate a single-element tuple from a value in parentheses with a comma:

    # #![allow(unused_variables)]
    #fn main() {
    (0,); // single-element tuple
    (0); // zero in parentheses
    #}

## Tuple indexing expressions

> Syntax
> TupleIndexingExpression :
> Expression `.` TUPLE_INDEX

Tuples and struct tuples can be indexed using the number corresponding to the position of the field. The index must be written as a decimal literal with no underscores or suffix. Tuple indexing expressions also differ from field expressions in that they can unambiguously be called as a function. In all other aspects they have the same behavior.

    # #![allow(unused_variables)]
    #fn main() {
    # struct Point(f32, f32);
    let pair = (1, 2);
    assert_eq!(pair.1, 2);
    let unit_x = Point(1.0, 0.0);
    assert_eq!(unit_x.0, 1.0);
    #}

# Struct expressions

There are several forms of struct expressions. A struct expression consists of the path of a struct item, followed by a brace-enclosed list of zero or more comma-separated name-value pairs, providing the field values of a new instance of the struct. A field name can be any identifier, and is separated from its value expression by a colon. In the case of a tuple struct the field names are numbers corresponding to the position of the field. The numbers must be written in decimal, containing no underscores and with no leading zeros or integer suffix. A value of a union type can also be created using this syntax, except that it must specify exactly one field.

Struct expressions can't be used directly in the head of a loop or an if, if let or match expression. But struct expressions can still be in used inside parentheses, for example.

A tuple struct expression consists of the path of a struct item, followed by a parenthesized list of one or more comma-separated expressions (in other words, the path of a struct item followed by a tuple expression). The struct item must be a tuple struct item.

A unit-like struct expression consists only of the path of a struct item.

The following are examples of struct expressions:

    # #![allow(unused_variables)]
    #fn main() {
    # struct Point { x: f64, y: f64 }
    # struct NothingInMe { }
    # struct TuplePoint(f64, f64);
    # mod game { pub struct User<'a> { pub name: &'a str, pub age: u32, pub score: usize } }
    # struct Cookie; fn some_fn<T>(t: T) {}
    Point {x: 10.0, y: 20.0};
    NothingInMe {};
    TuplePoint(10.0, 20.0);
    TuplePoint { 0: 10.0, 1: 20.0 }; // Results in the same value as the above line
    let u = game::User {name: "Joe", age: 35, score: 100_000};
    some_fn::<Cookie>(Cookie);
    #}

A struct expression forms a new value of the named struct type. Note that for a given unit-like struct type, this will always be the same value.

A struct expression can terminate with the syntax `..` followed by an expression to denote a functional update. The expression following `..` (the base) must have the same struct type as the new struct type being formed. The entire expression denotes the result of constructing a new struct (with the same type as the base expression) with the given values for the fields that were explicitly specified and the values in the base expression for all other fields. Just as with all struct expressions, all of the fields of the struct must be visible, even those not explicitly named.

    # #![allow(unused_variables)]
    #fn main() {
    # struct Point3d { x: i32, y: i32, z: i32 }
    let base = Point3d {x: 1, y: 2, z: 3};
    Point3d {y: 0, z: 10, .. base};
    #}

## Struct field init shorthand

When initializing a data structure (struct, enum, union) with named (but not numbered) fields, it is allowed to write `fieldname` as a shorthand for `fieldname: fieldname`. This allows a compact syntax with less duplication.

Example:

    # #![allow(unused_variables)]
    #fn main() {
    # struct Point3d { x: i32, y: i32, z: i32 }
    # let x = 0;
    # let y_value = 0;
    # let z = 0;
    Point3d { x: x, y: y_value, z: z };
    Point3d { x, y: y_value, z };
    #}

# Enumeration Variant expressions

Enumeration variants can be constructed similarly to structs, using a path to an enum variant instead of to a struct:

    # #![allow(unused_variables)]
    #fn main() {
    # enum Message {
    #     Quit,
    #     WriteString(String),
    #     Move { x: i32, y: i32 },
    # }
    let q = Message::Quit;
    let w = Message::WriteString("Some string".to_string());
    let m = Message::Move { x: 50, y: 200 };
    #}

# Call expressions

> Syntax
> CallExpression :
> Expression `(` CallParams? `)`

> CallParams :
> Expression ( `,` Expression )* `,`?

A call expression consists of an expression followed by a parenthesized expression-list. It invokes a function, providing zero or more input variables. If the function eventually returns, then the expression completes. For non-function types, the expression f(...) uses the method on one of the `std::ops::Fn`, `std::ops::FnMut` or `std::ops::FnOnce` traits, which differ in whether they take the type by reference, mutable reference, or take ownership respectively. An automatic borrow will be taken if needed. Rust will also automatically dereference `f` as required. Some examples of call expressions:

    # #![allow(unused_variables)]
    #fn main() {
    # fn add(x: i32, y: i32) -> i32 { 0 }
    let three: i32 = add(1i32, 2i32);
    let name: &'static str = (|| "Rust")();
    #}

## Disambiguating Function Calls

Rust treats all function calls as sugar for a more explicit, fully-qualified syntax. Upon compilation, Rust will desugar all function calls into the explicit form. Rust may sometimes require you to qualify function calls with trait, depending on the ambiguity of a call in light of in-scope items.

> Note: In the past, the Rust community used the terms "Unambiguous Function Call Syntax", "Universal Function Call Syntax", or "UFCS", in documentation, issues, RFCs, and other community writings. However, the term lacks descriptive power and potentially confuses the issue at hand. We mention it here for searchability's sake.

Several situations often occur which result in ambiguities about the receiver or referent of method or associated function calls. These situations may include:

- Multiple in-scope traits define methods with the same name for the same types
- Auto-`deref` is undesirable; for example, distinguishing between methods on a smart pointer itself and the pointer's referent
- Methods which take no arguments, like `default()`, and return properties of a type, like `size_of()`

To resolve the ambiguity, the programmer may refer to their desired method or function using more specific paths, types, or traits.

For example,

    trait Pretty {
        fn print(&self);
    }

    trait Ugly {
      fn print(&self);
    }

    struct Foo;
    impl Pretty for Foo {
        fn print(&self) {}
    }

    struct Bar;
    impl Pretty for Bar {
        fn print(&self) {}
    }
    impl Ugly for Bar{
        fn print(&self) {}
    }

    fn main() {
        let f = Foo;
        let b = Bar;

        // we can do this because we only have one item called `print` for `Foo`s
        f.print();
        // more explicit, and, in the case of `Foo`, not necessary
        Foo::print(&f);
        // if you're not into the whole brevity thing
        <Foo as Pretty>::print(&f);

        // b.print(); // Error: multiple 'print' found
        // Bar::print(&b); // Still an error: multiple `print` found

        // necessary because of in-scope items defining `print`
        <Bar as Pretty>::print(&b);
    }

Refer to RFC 132 for further details and motivations.

# Method-call expressions

A method call consists of an expression (the receiver) followed by a single dot, an identifier, and a parenthesized expression-list. Method calls are resolved to associated methods on specific traits, either statically dispatching to a method if the exact `self`-type of the left-hand-side is known, or dynamically dispatching if the left-hand-side expression is an indirect trait object.

    # #![allow(unused_variables)]
    #fn main() {
    let pi: Result<f32, _> = "3.14".parse();
    let log_pi = pi.unwrap_or(1.0).log(2.72);
    # assert!(1.14 < log_pi && log_pi < 1.15)
    #}

When looking up a method call, the receiver may be automatically dereferenced or borrowed in order to call a method. This requires a more complex lookup process than for other functions, since there may be a number of possible methods to call. The following procedure is used:

The first step is to build a list of candidate receiver types. Obtain these by repeatedly dereferencing the receiver expression's type, adding each type encountered to the list, then finally attempting an unsized coercion at the end, and adding the result type if that is successful. Then, for each candidate `T`, add `&T` and `&mut T` to the list immediately after `T`.

For instance, if the receiver has type `Box<[i32;2]>`, then the candidate types will be `Box<[i32;2]>`, `&Box<[i32;2]>`, `&mut Box<[i32;2]>`, `[i32; 2]` (by dereferencing), `&[i32; 2]`, `&mut [i32; 2]`, `[i32]` (by unsized coercion), `&[i32]`, and finally `&mut [i32]`.

Then, for each candidate type `T`, search for a visible method with a receiver of that type in the following places:

1. `T`'s inherent methods (methods implemented directly on `T`).
2. Any of the methods provided by a visible trait implemented by `T`. If `T` is a type parameter, methods provided by trait bounds on `T` are looked up first. Then all remaining methods in scope are looked up.

> Note: the lookup is done for each type in order, which can occasionally lead to surprising results. The below code will print "In trait impl!", because `&self` methods are looked up first, the trait method is found before the struct's `&mut self` method is found.

>     struct Foo {}

>     trait Bar {
>       fn bar(&self);
>     }

>     impl Foo {
>       fn bar(&mut self) {
>         println!("In struct impl!")
>       }
>     }

>     impl Bar for Foo {
>       fn bar(&self) {
>         println!("In trait impl!")
>       }
>     }

>     fn main() {
>       let mut f = Foo{};
>       f.bar();
>     }

If this results in multiple possible candidates, then it is an error, and the receiver must be converted to an appropriate receiver type to make the method call.

This process does not take into account the mutability or lifetime of the receiver, or whether a method is `unsafe`. Once a method is looked up, if it can't be called for one (or more) of those reasons, the result is a compiler error.

If a step is reached where there is more than one possible method, such as where generic methods or traits are considered the same, then it is a compiler error. These cases require a disambiguating function call syntax for method and function invocation.

Warning: For trait objects, if there is an inherent method of the same name as a trait method, it will give a compiler error when trying to call the method in a method call expression. Instead, you can call the method using disambiguating function call syntax, in which case it calls the trait method, not the inherent method. There is no way to call the inherent method. Just don't define inherent methods on trait objects with the same name a trait method and you'll be fine.

# Field access expressions

> Syntax
> FieldExpression :
> Expression `.` IDENTIFIER

A field expression consists of an expression followed by a single dot and an identifier, when not immediately followed by a parenthesized expression-list (the latter is always a method call expression). A field expression denotes a field of a struct or union. To call a function stored in a struct, parentheses are needed around the field expression.

    mystruct.myfield;
    foo().x;
    (Struct {a: 10, b: 20}).a;
    mystruct.method();          // Method expression
    (mystruct.function_field)() // Call expression containing a field expression

A field access is a place expression referring to the location of that field. When the subexpression is mutable, the field expression is also mutable.

Also, if the type of the expression to the left of the dot is a pointer, it is automatically dereferenced as many times as necessary to make the field access possible. In cases of ambiguity, we prefer fewer autoderefs to more.

Finally, the fields of a struct or a reference to a struct are treated as separate entities when borrowing. If the struct does not implement `Drop` and is stored in a local variable, this also applies to moving out of each of its fields. This also does not apply if automatic dereferencing is done though user defined types.

    # #![allow(unused_variables)]
    #fn main() {
    struct A { f1: String, f2: String, f3: String }
    let mut x: A;
    # x = A {
    #     f1: "f1".to_string(),
    #     f2: "f2".to_string(),
    #     f3: "f3".to_string()
    # };
    let a: &mut String = &mut x.f1; // x.f1 borrowed mutably
    let b: &String = &x.f2;         // x.f2 borrowed immutably
    let c: &String = &x.f2;         // Can borrow again
    let d: String = x.f3;           // Move out of x.f3
    #}

# Closure expressions

> Syntax
> ClosureExpression :
> `move`?
> ( `||` | `|` FunctionParameters? `|` )
> (Expression | `->` TypeNoBounds BlockExpression)

A closure expression defines a closure and denotes it as a value, in a single expression. A closure expression is a pipe-symbol-delimited (`|`) list of patterns followed by an expression. Type annotations may optionally be added for the type of the parameters or for the return type. If there is a return type, the expression used for the body of the closure must be a normal block. A closure expression also may begin with the `move` keyword before the initial `|`.

A closure expression denotes a function that maps a list of parameters (`ident_list`) onto the expression that follows the `ident_list`. The patterns in the `ident_list` are the parameters to the closure. If a parameter's types is not specified, then the compiler infers it from context. Each closure expression has a unique anonymous type.

Closure expressions are most useful when passing functions as arguments to other functions, as an abbreviation for defining and capturing a separate function.

Significantly, closure expressions capture their environment, which regular function definitions do not. Without the `move` keyword, the closure expression infers how it captures each variable from its environment, preferring to capture by shared reference, effectively borrowing all outer variables mentioned inside the closure's body. If needed the compiler will infer that instead mutable references should be taken, or that the values should be moved or copied (depending on their type) from the environment. A closure can be forced to capture its environment by copying or moving values by prefixing it with the `move` keyword. This is often used to ensure that the closure's type is `'static`.

The compiler will determine which of the closure traits the closure's type will implement by how it acts on its captured variables. The closure will also implement `Send` and/or `Sync` if all of its captured types do. These traits allow functions to accept closures using generics, even though the exact types can't be named.

In this example, we define a function `ten_times` that takes a higher-order function argument, and we then call it with a closure expression as an argument, followed by a closure expression that moves values from its environment.

    # #![allow(unused_variables)]
    #fn main() {
    fn ten_times<F>(f: F) where F: Fn(i32) {
        for index in 0..10 {
            f(index);
        }
    }

    ten_times(|j| println!("hello, {}", j));
    // With type annotations
    ten_times(|j: i32| -> () { println!("hello, {}", j) });

    let word = "konnichiwa".to_owned();
    ten_times(move |j| println!("{}, {}", word, j));
    #}

# Loops

> Syntax
> LoopExpression :
> LoopLabel? (
> InfiniteLoopExpression
> | PredicateLoopExpression
> | PredicatePatternLoopExpression
> | IteratorLoopExpression
> )

Rust supports four loop expressions:

- A `loop` expression denotes an infinite loop.
- A `while` expression loops until a predicate is false.
- A `while let` expression tests a refutable pattern.
- A `for` expression extracts values from an iterator, looping until the iterator is empty.

All four types of loop support `break` expressions, `continue` expressions, and labels. Only `loop` supports evaluation to non-trivial values.

## Infinite loops

> Syntax
> InfiniteLoopExpression :
> `loop` BlockExpression

A `loop` expression repeats execution of its body continuously: `loop { println!("I live."); }`.

A `loop` expression without an associated `break` expression is diverging and has type `!`. A `loop` expression containing associated `break` expression(s) may terminate, and must have type compatible with the value of the `break` expression(s).

## Predicate loops

> Syntax
> PredicateLoopExpression :
> `while` Expressionexcept struct expression BlockExpression

A `while` loop begins by evaluating the boolean loop conditional expression. If the loop conditional expression evaluates to `true`, the loop body block executes, then control returns to the loop conditional expression. If the loop conditional expression evaluates to `false`, the `while` expression completes.

An example:

    # #![allow(unused_variables)]
    #fn main() {
    let mut i = 0;

    while i < 10 {
        println!("hello");
        i = i + 1;
    }
    #}

## Predicate pattern loops

> Syntax
> PredicatePatternLoopExpression :
> `while` `let` Pattern `=` Expressionexcept struct expression BlockExpression

A `while let` loop is semantically similar to a `while` loop but in place of a condition expression it expects the keyword `let` followed by a refutable pattern, an `=`, an expression and a block expression. If the value of the expression on the right hand side of the `=` matches the pattern, the loop body block executes then control returns to the pattern matching statement. Otherwise, the while expression completes.

    # #![allow(unused_variables)]
    #fn main() {
    let mut x = vec![1, 2, 3];

    while let Some(y) = x.pop() {
        println!("y = {}", y);
    }
    #}

## Iterator loops

> Syntax
> IteratorLoopExpression :
> `for` Pattern `in` Expressionexcept struct expression BlockExpression

A `for` expression is a syntactic construct for looping over elements provided by an implementation of `std::iter::IntoIterator`. If the iterator yields a value, that value is given the specified name and the body of the loop is executed, then control returns to the head of the `for` loop. If the iterator is empty, the `for` expression completes.

An example of a `for` loop over the contents of an array:

    # #![allow(unused_variables)]
    #fn main() {
    let v = &["apples", "cake", "coffee"];

    for text in v {
        println!("I like {}.", text);
    }
    #}

An example of a for loop over a series of integers:

    # #![allow(unused_variables)]
    #fn main() {
    let mut sum = 0;
    for n in 1..11 {
        sum += n;
    }
    assert_eq!(sum, 55);
    #}

## Loop labels

> Syntax
> LoopLabel :
> LIFETIME_OR_LABEL `:`

A loop expression may optionally have a label. The label is written as a lifetime preceding the loop expression, as in `'foo: loop { break 'foo; }`, `'bar: while false {}`, `'humbug: for _ in 0..0 {}`. If a label is present, then labeled `break` and `continue` expressions nested within this loop may exit out of this loop or return control to its head. See break expressions and continue expressions.

## `break` expressions

> Syntax
> BreakExpression :
> `break` LIFETIME_OR_LABEL? Expression?

When `break` is encountered, execution of the associated loop body is immediately terminated, for example:

    # #![allow(unused_variables)]
    #fn main() {
    let mut last = 0;
    for x in 1..100 {
        if x > 12 {
            break;
        }
        last = x;
    }
    assert_eq!(last, 12);
    #}

A `break` expression is normally associated with the innermost `loop`, `for` or `while` loop enclosing the `break` expression, but a label can be used to specify which enclosing loop is affected. Example:

    # #![allow(unused_variables)]
    #fn main() {
    'outer: loop {
        while true {
            break 'outer;
        }
    }
    #}

A `break` expression is only permitted in the body of a loop, and has one of the forms `break`, `break 'label` or (see below) `break EXPR` or `break 'label EXPR`.

## `continue` expressions

> Syntax
> ContinueExpression :
> `continue` LIFETIME_OR_LABEL?

When `continue` is encountered, the current iteration of the associated loop body is immediately terminated, returning control to the loop head. In the case of a `while` loop, the head is the conditional expression controlling the loop. In the case of a `for` loop, the head is the call-expression controlling the loop.

Like `break`, `continue` is normally associated with the innermost enclosing loop, but `continue 'label` may be used to specify the loop affected. A `continue` expression is only permitted in the body of a loop.

## `break` and loop values

When associated with a `loop`, a break expression may be used to return a value from that loop, via one of the forms `break EXPR` or `break 'label EXPR`, where `EXPR` is an expression whose result is returned from the `loop`. For example:

    # #![allow(unused_variables)]
    #fn main() {
    let (mut a, mut b) = (1, 1);
    let result = loop {
        if b > 10 {
            break b;
        }
        let c = a + b;
        a = b;
        b = c;
    };
    // first number in Fibonacci sequence over 10:
    assert_eq!(result, 13);
    #}

In the case a `loop` has an associated `break`, it is not considered diverging, and the `loop` must have a type compatible with each `break` expression. `break` without an expression is considered identical to `break` with expression `()`.

# Range expressions

> Syntax
> RangeExpression :
> RangeExpr
> | RangeFromExpr
> | RangeToExpr
> | RangeFullExpr
> | RangeInclusiveExpr
> | RangeToInclusiveExpr

> RangeExpr :
> Expression `..` Expression

> RangeFromExpr :
> Expression `..`

> RangeToExpr :
> `..` Expression

> RangeFullExpr :
> `..`

> RangeExpr :
> Expression `..=` Expression

> RangeToExpr :
> `..=` Expression

The `..` and `..=` operators will construct an object of one of the `std::ops::Range` (or `core::ops::Range`) variants, according to the following table:

Production | Syntax | Type | Range
--- | --- | --- | ---
RangeExpr | start`..`end | std::ops::Range | start ≤ x < end
RangeFromExpr | start`..` | std::ops::RangeFrom | start ≤ x
RangeToExpr | `..`end | std::ops::RangeTo | x < end
RangeFullExpr | `..` | std::ops::RangeFull | -
RangeInclusiveExpr | start`..=`end | std::ops::RangeInclusive | start ≤ x ≤ end
RangeToInclusiveExpr | `..=`end | std::ops::RangeToInclusive | x ≤ end

Examples:

    # #![allow(unused_variables)]
    #fn main() {
    1..2;   // std::ops::Range
    3..;    // std::ops::RangeFrom
    ..4;    // std::ops::RangeTo
    ..;     // std::ops::RangeFull
    5..=6;  // std::ops::RangeInclusive
    ..=7;   // std::ops::RangeToInclusive
    #}

The following expressions are equivalent.

    # #![allow(unused_variables)]
    #fn main() {
    let x = std::ops::Range {start: 0, end: 10};
    let y = 0..10;

    assert_eq!(x, y);
    #}

Ranges can be used in `for` loops:

    # #![allow(unused_variables)]
    #fn main() {
    for i in 1..11 {
        println!("{}", i);
    }
    #}

# `if` and `if let` expressions

## `if` expressions

> Syntax
> IfExpression :
> `if` Expressionexcept struct expression BlockExpression
> (`else` ( BlockExpression | IfExpression | IfLetExpression ) )?

An `if` expression is a conditional branch in program control. The form of an `if` expression is a condition expression, followed by a consequent block, any number of `else if` conditions and blocks, and an optional trailing `else` block. The condition expressions must have type `bool`. If a condition expression evaluates to `true`, the consequent block is executed and any subsequent `else if` or `else` block is skipped. If a condition expression evaluates to `false`, the consequent block is skipped and any subsequent `else if` condition is evaluated. If all `if` and `else if` conditions evaluate to `false` then any `else` block is executed. An if expression evaluates to the same value as the executed block, or `()` if no block is evaluated. An `if` expression must have the same type in all situations.

    # #![allow(unused_variables)]
    #fn main() {
    # let x = 3;
    if x == 4 {
        println!("x is four");
    } else if x == 3 {
        println!("x is three");
    } else {
        println!("x is something else");
    }

    let y = if 12 * 15 > 150 {
        "Bigger"
    } else {
        "Smaller"
    };
    assert_eq!(y, "Bigger");
    #}

## `if let` expressions

> Syntax
> IfLetExpression :
> `if` `let` Pattern `=` Expressionexcept struct expression BlockExpression
> (`else` ( BlockExpression | IfExpression | IfLetExpression ) )?

An `if let` expression is semantically similar to an `if` expression but in place of a condition expression it expects the keyword `let` followed by a refutable pattern, an `=` and an expression. If the value of the expression on the right hand side of the `=` matches the pattern, the corresponding block will execute, otherwise flow proceeds to the following `else` block if it exists. Like `if` expressions, `if let` expressions have a value determined by the block that is evaluated.

    # #![allow(unused_variables)]
    #fn main() {
    let dish = ("Ham", "Eggs");

    // this body will be skipped because the pattern is refuted
    if let ("Bacon", b) = dish {
        println!("Bacon is served with {}", b);
    } else {
        // This block is evaluated instead.
        println!("No bacon will be served");
    }

    // this body will execute
    if let ("Ham", b) = dish {
        println!("Ham is served with {}", b);
    }
    #}

`if` and `if let` expressions can be intermixed:

    # #![allow(unused_variables)]
    #fn main() {
    let x = Some(3);
    let a = if let Some(1) = x {
        1
    } else if x == Some(2) {
        2
    } else if let Some(y) = x {
        y
    } else {
        -1
    };
    assert_eq!(a, 3);
    #}

# `match` expressions

> Syntax
> MatchExpression :
> `match` Expressionexcept struct expression `{`
> InnerAttribute*
> MatchArms?
> `}`

> MatchArms :
> ( MatchArm `=>` ( BlockExpression `,`? | Expression `,` ) )*
> MatchArm `=>` ( BlockExpression | Expression ) `,`?

> MatchArm :
> OuterAttribute* MatchArmPatterns MatchArmGuard?

> MatchArmPatterns :
> `|`? Pattern ( `|` Pattern )*

> MatchArmGuard :
> `if` Expression

A `match` expression branches on a pattern. The exact form of matching that occurs depends on the pattern. Patterns consist of some combination of literals, destructured arrays or enum constructors, structs and tuples, variable binding specifications, wildcards (`..`), and placeholders (`_`). A `match` expression has a head expression, which is the value to compare to the patterns. The type of the patterns must equal the type of the head expression.

A `match` behaves differently depending on whether or not the head expression is a place expression or value expression. If the head expression is a value expression, it is first evaluated into a temporary location, and the resulting value is sequentially compared to the patterns in the arms until a match is found. The first arm with a matching pattern is chosen as the branch target of the `match`, any variables bound by the pattern are assigned to local variables in the arm's block, and control enters the block.

When the head expression is a place expression, the match does not allocate a temporary location; however, a by-value binding may copy or move from the memory location. When possible, it is preferable to match on place expressions, as the lifetime of these matches inherits the lifetime of the place expression rather than being restricted to the inside of the match.

An example of a `match` expression:

    # #![allow(unused_variables)]
    #fn main() {
    let x = 1;

    match x {
        1 => println!("one"),
        2 => println!("two"),
        3 => println!("three"),
        4 => println!("four"),
        5 => println!("five"),
        _ => println!("something else"),
    }
    #}

Patterns that bind variables default to binding to a copy or move of the matched value (depending on the matched value's type). This can be changed to bind to a reference by using the `ref` keyword, or to a mutable reference using `ref mut`.

Patterns can be used to destructure structs, enums, and tuples. Destructuring breaks a value up into its component pieces. The syntax used is the same as when creating such values. When destructing a data structure with named (but not numbered) fields, it is allowed to write `fieldname` as a shorthand for `fieldname: fieldname`. In a pattern whose head expression has a `struct`, `enum` or `tupl` type, a placeholder (`_`) stands for a single data field, whereas a wildcard `..` stands for all the fields of a particular variant.

    # #![allow(unused_variables)]
    #fn main() {
    # enum Message {
    #     Quit,
    #     WriteString(String),
    #     Move { x: i32, y: i32 },
    #     ChangeColor(u8, u8, u8),
    # }
    # let message = Message::Quit;
    match message {
        Message::Quit => println!("Quit"),
        Message::WriteString(write) => println!("{}", &write),
        Message::Move{ x, y: 0 } => println!("move {} horizontally", x),
        Message::Move{ .. } => println!("other move"),
        Message::ChangeColor { 0: red, 1: green, 2: _ } => {
            println!("color change, red: {}, green: {}", red, green);
        }
    };
    #}

Patterns can also dereference pointers by using the `&`, `&mut` and `box` symbols, as appropriate. For example, these two matches on `x: &i32` are equivalent:

    # #![allow(unused_variables)]
    #fn main() {
    let int_reference = &3;

    let a = match *int_reference { 0 => "zero", _ => "some" };
    let b = match int_reference { &0 => "zero", _ => "some" };

    assert_eq!(a, b);
    #}

Subpatterns can also be bound to variables by the use of the syntax `variable @ subpattern`. For example:

    # #![allow(unused_variables)]
    #fn main() {
    let x = 1;

    match x {
        e @ 1 ... 5 => println!("got a range element {}", e),
        _ => println!("anything"),
    }
    #}

Multiple match patterns may be joined with the `|` operator. An inclusive range of values may be specified with `..=`. For example:

    # #![allow(unused_variables)]
    #fn main() {
    # let x = 9;
    let message = match x {
        0 | 1  => "not many",
        2 ..= 9 => "a few",
        _      => "lots"
    };

    assert_eq!(message, "a few");
    #}

Other forms of range (e.g `..` for an exclusive range, or any range with one or both endpoints left unspecified) are not supported in matches. The syntax `...` is also accepted for inclusive ranges in patterns only, for backwards compatibility.

Range patterns only work with `char` and numeric types. A range pattern may not be a sub-range of another range pattern inside the same `match`.

Slice patterns can match both arrays of fixed size and slices of dynamic size.

    # #![allow(unused_variables)]
    #fn main() {
    // Fixed size
    let arr = [1, 2, 3];
    match arr {
        [1, _, _] => "starts with one",
        [a, b, c] => "starts with something else",
    };
    #}

    # #![allow(unused_variables)]
    #fn main() {
    // Dynamic size
    let v = vec![1, 2, 3];
    match v[..] {
        [a, b] => { /* this arm will not apply because the length doesn't match */ }
        [a, b, c] => { /* this arm will apply */ }
        _ => { /* this wildcard is required, since we don't know length statically */ }
    }
    #}

Finally, match arms can accept pattern guards to further refine the criteria for matching a case. Pattern guards appear after the pattern and consist of a bool-typed expression following the `if` keyword. A pattern guard may refer to the variables bound within the pattern they follow.

When the pattern matches successfully, the pattern guard expression is executed. If the expression is truthy, the pattern is successfully matched against. Otherwise, the next pattern, including other matches with the `|` operator in the same arm, is tested.

    # #![allow(unused_variables)]
    #fn main() {
    # let maybe_digit = Some(0);
    # fn process_digit(i: i32) { }
    # fn process_other(i: i32) { }
    let message = match maybe_digit {
        Some(x) if x < 10 => process_digit(x),
        Some(x) => process_other(x),
        None => panic!(),
    };
    #}

> Note: Multiple matches using the `|` operator can cause the pattern guard and and side effects it has to execute multiple times. For example:

>     use std::cell::Cell;
>     fn main() {
>         let i : Cell<i32> = Cell::new(0);
>         match 1 {
>             1 | _ if { i.set(i.get() + 1); false } => {}
>             _ => {}
>         }
>         assert_eq!(i.get(), 2);
>     }

## Attributes on match arms

Outer attributes are allowed on match arms. The only attributes that have meaning on match arms are `cfg`, `cold`, and the lint check attributes.

# `return` expressions

> Syntax
> ReturnExpression :
> `return` Expression?

Return expressions are denoted with the keyword `return`. Evaluating a `return` expression moves its argument into the designated output location for the current function call, destroys the current function activation frame, and transfers control to the caller frame.

An example of a `return` expression:

    # #![allow(unused_variables)]
    #fn main() {
    fn max(a: i32, b: i32) -> i32 {
        if a > b {
            return a;
        }
        return b;
    }
    #}

# Type system

# Types

Every variable, item and value in a Rust program has a type. The type of a value defines the interpretation of the memory holding it.

Built-in types are tightly integrated into the language, in nontrivial ways that are not possible to emulate in user-defined types. User-defined types have limited capabilities.

## Primitive types

Some types are defined by the language, rather than as part of the standard library, these are called primitive types. Some of these are individual types:

- The boolean type `bool` with values `true` and `false`.
- The machine types (integer and floating-point).
- The machine-dependent integer types.
- The textual types `char` and `str`.
- The never type `!`

There are also some primitive constructs for generic types built in to the language:

- Tuples
- Arrays
- Slices
- Function pointers
- References
- Pointers

### Boolean type

The `bool` type is a datatype which can be either `true` or `false`. The boolean type uses one byte of memory. It is used in comparisons and bitwise operations like `&`, `|`, and `!`.

    fn main() {
        let x = true;
        let y: bool = false; // with the boolean type annotation

        // Use of booleans in conditional expressions
        if x {
            println!("x is true");
        }
    }

## Numeric types

### Machine types

The machine types are the following:

- The unsigned word types `u8`, `u16`, `u32`, `u64`, and `u128` with values drawn from the integer intervals [0, 2^8 - 1], [0, 2^16 - 1], [0, 2^32 - 1], [0, 2^64 - 1], and [0, 2^128 - 1] respectively.

- The signed two's complement word types `i8`, `i16`, `i32`, `i64`, and `i128`, with values drawn from the integer intervals [-(2^7), 2^7 - 1], [-(2^15), 2^15 - 1], [-(2^31), 2^31 - 1], [-(2^63), 2^63 - 1], and [-(2^127), 2^127 - 1] respectively.

- The IEEE 754-2008 "binary32" and "binary64" floating-point types: `f32` and `f64`, respectively.

### Machine-dependent integer types

The `usize` type is an unsigned integer type with the same number of bits as the platform's pointer type. It can represent every memory address in the process.

The `isize` type is a signed integer type with the same number of bits as the platform's pointer type. The theoretical upper bound on object and array size is the maximum `isize` value. This ensures that `isize` can be used to calculate differences between pointers into an object or array and can address every byte within an object along with one byte past the end.

## Textual types

The types `char` and `str` hold textual data.

A value of type `char` is a Unicode scalar value (i.e. a code point that is not a surrogate), represented as a 32-bit unsigned word in the 0x0000 to 0xD7FF or 0xE000 to 0x10FFFF range. A `[char]` is effectively a UCS-4 / UTF-32 string.

A value of type `str` is a Unicode string, represented as an array of 8-bit unsigned bytes holding a sequence of UTF-8 code points. Since `str` is a dynamically sized type, it is not a first-class type, but can only be instantiated through a pointer type, such as `&str`.

## Never type

The never type `!` is a type with no values, representing the result of computations that never complete. Expressions of type `!` can be coerced into any other type.

## Tuple types

A tuple type is a heterogeneous product of other types, called the elements of the tuple. It has no nominal name and is instead structurally typed.

Tuple types and values are denoted by listing the types or values of their elements, respectively, in a parenthesized, comma-separated list.

Because tuple elements don't have a name, they can only be accessed by pattern-matching or by using `N` directly as a field to access the `N`th element.

An example of a tuple type and its use:

    # #![allow(unused_variables)]
    #fn main() {
    type Pair<'a> = (i32, &'a str);
    let p: Pair<'static> = (10, "ten");
    let (a, b) = p;

    assert_eq!(a, 10);
    assert_eq!(b, "ten");
    assert_eq!(p.0, 10);
    assert_eq!(p.1, "ten");
    #}

For historical reasons and convenience, the tuple type with no elements (`()`) is often called ‘unit’ or ‘the unit type’.

## Array, and Slice types

Rust has two different types for a list of items:

- `[T; N]`, an 'array'
- `[T]`, a 'slice'

An array has a fixed size, and can be allocated on either the stack or the heap.

A slice is a dynamically sized type representing a 'view' into an array. To use a slice type it generally has to be used behind a pointer for example as

- `&[T]`, a 'shared slice', often just called a 'slice', it doesn't own the data it points to, it borrows it.
- `&mut [T]`, a 'mutable slice', mutably borrows the data it points to.
- `Box<[T]>`, a 'boxed slice'

Examples:

    # #![allow(unused_variables)]
    #fn main() {
    // A stack-allocated array
    let array: [i32; 3] = [1, 2, 3];

    // A heap-allocated array, coerced to a slice
    let boxed_array: Box<[i32]> = Box::new([1, 2, 3]);

    // A (shared) slice into an array
    let slice: &[i32] = &boxed_array[..];
    #}

All elements of arrays and slices are always initialized, and access to an array or slice is always bounds-checked in safe methods and operators.

> Note: The `Vec<T>` standard library type provides a heap-allocated resizable array type.

## Struct types

A `struct` type is a heterogeneous product of other types, called the fields of the type.1

New instances of a `struct` can be constructed with a struct expression.

The memory layout of a `struct` is undefined by default to allow for compiler optimizations like field reordering, but it can be fixed with the `#[repr(...)]` attribute. In either case, fields may be given in any order in a corresponding struct expression; the resulting `struct` value will always have the same memory layout.

The fields of a `struct` may be qualified by visibility modifiers, to allow access to data in a struct outside a module.

A tuple struct type is just like a struct type, except that the fields are anonymous.

A unit-like struct type is like a struct type, except that it has no fields. The one value constructed by the associated struct expression is the only value that inhabits such a type.

1

`struct` types are analogous to `struct` types in C, the record types of the ML family, or the struct types of the Lisp family.

## Enumerated types

An enumerated type is a nominal, heterogeneous disjoint union type, denoted by the name of an `enum` item. 2

An `enum` item declares both the type and a number of variants, each of which is independently named and has the syntax of a struct, tuple struct or unit-like struct.

New instances of an `enum` can be constructed in an enumeration variant expression.

Any `enum` value consumes as much memory as the largest variant for its corresponding `enum` type, as well as the size needed to store a discriminant.

Enum types cannot be denoted structurally as types, but must be denoted by named reference to an `enum` item.

2

The `enum` type is analogous to a `data` constructor declaration in ML, or a pick ADT in Limbo.

## Union types

A union type is a nominal, heterogeneous C-like union, denoted by the name of a `union` item.

A union contains the value of any one of its fields. Since the accessing the wrong field can cause unexpected or undefined behaviour, `unsafe` is required to read from a union field or to write to a field that doesn't implement `Copy`.

The memory layout of a `union` is undefined by default, but the `#[repr(...)]` attribute can be used to fix a layout.

## Recursive types

Nominal types — structs, enumerations and unions — may be recursive. That is, each `enum` variant or `struct` or `union` field may refer, directly or indirectly, to the enclosing `enum` or `struct` type itself. Such recursion has restrictions:

- Recursive types must include a nominal type in the recursion (not mere type definitions, or other structural types such as arrays or tuples). So `type Rec = &'static [Rec]` is not allowed.
- The size of a recursive type must be finite; in other words the recursive fields of the type must be pointer types.
- Recursive type definitions can cross module boundaries, but not module visibility boundaries, or crate boundaries (in order to simplify the module system and type checker).

An example of a recursive type and its use:

    # #![allow(unused_variables)]
    #fn main() {
    enum List<T> {
        Nil,
        Cons(T, Box<List<T>>)
    }

    let a: List<i32> = List::Cons(7, Box::new(List::Cons(13, Box::new(List::Nil))));
    #}

## Pointer types

All pointers in Rust are explicit first-class values. They can be moved or copied, stored into data structs, and returned from functions.

### Shared references (`&`)

These point to memory owned by some other value. When a shared reference to a value is created it prevents direct mutation of the value. Interior mutability provides an exception for this in certain circumstances. As the name suggests, any number of shared references to a value may exit. A shared reference type is written `&type`, or `&'a type` when you need to specify an explicit lifetime. Copying a reference is a "shallow" operation: it involves only copying the pointer itself, that is, pointers are `Copy`. Releasing a reference has no effect on the value it points to, but referencing of a temporary value will keep it alive during the scope of the reference itself.

### Mutable references (`&mut`)

These also point to memory owned by some other value. A mutable reference type is written `&mut type` or `&'a mut type`. A mutable reference (that hasn't been borrowed) is the only way to access the value it points to, so is not `Copy`.

### Raw pointers (`*const` and `*mut`)

Raw pointers are pointers without safety or liveness guarantees. Raw pointers are written as `*const T` or `*mut T`, for example `*const i32` means a raw pointer to a 32-bit integer. Copying or dropping a raw pointer has no effect on the lifecycle of any other value. Dereferencing a raw pointer is an `unsafe` operation, this can also be used to convert a raw pointer to a reference by reborrowing it (`&*` or `&mut *`). Raw pointers are generally discouraged in Rust code; they exist to support interoperability with foreign code, and writing performance-critical or low-level functions.

When comparing pointers they are compared by their address, rather than by what they point to. When comparing pointers to dynamically sized types they also have their addition data compared.

### Smart Pointers

The standard library contains additional 'smart pointer' types beyond references and raw pointers.

## Function item types

When referred to, a function item, or the constructor of a tuple-like struct or enum variant, yields a zero-sized value of its function item type. That type explicitly identifies the function - its name, its type arguments, and its early-bound lifetime arguments (but not its late-bound lifetime arguments, which are only assigned when the function is called) - so the value does not need to contain an actual function pointer, and no indirection is needed when the function is called.

There is no syntax that directly refers to a function item type, but the compiler will display the type as something like `fn(u32) -> i32 {fn_name}` in error messages.

Because the function item type explicitly identifies the function, the item types of different functions - different items, or the same item with different generics - are distinct, and mixing them will create a type error:

    # #![allow(unused_variables)]
    #fn main() {
    fn foo<T>() { }
    let x = &mut foo::<i32>;
    *x = foo::<u32>; //~ ERROR mismatched types
    #}

However, there is a coercion from function items to function pointers with the same signature, which is triggered not only when a function item is used when a function pointer is directly expected, but also when different function item types with the same signature meet in different arms of the same `if` or `match`:

    # #![allow(unused_variables)]
    #fn main() {
    # let want_i32 = false;
    # fn foo<T>() { }

    // `foo_ptr_1` has function pointer type `fn()` here
    let foo_ptr_1: fn() = foo::<i32>;

    // ... and so does `foo_ptr_2` - this type-checks.
    let foo_ptr_2 = if want_i32 {
        foo::<i32>
    } else {
        foo::<u32>
    };
    #}

All function items implement `Fn`, `FnMut`, `FnOnce`, `Copy`, `Clone`, `Send`, and `Sync`.

## Function pointer types

Function pointer types, written using the `fn` keyword, refer to a function whose identity is not necessarily known at compile-time. They can be created via a coercion from both function items and non-capturing closures.

A function pointer type consists of a possibly-empty set of function-type modifiers (such as `unsafe` or `extern`), a sequence of input types and an output type.

An example where `Binop` is defined as a function pointer type:

    # #![allow(unused_variables)]
    #fn main() {
    fn add(x: i32, y: i32) -> i32 {
        x + y
    }

    let mut x = add(5,7);

    type Binop = fn(i32, i32) -> i32;
    let bo: Binop = add;
    x = bo(5,7);
    #}

## Closure types

A closure expression produces a closure value with a unique, anonymous type that cannot be written out. A closure type is approximately equivalent to a struct which contains the captured variables. For instance, the following closure:

    # #![allow(unused_variables)]
    #fn main() {
    fn f<F : FnOnce() -> String> (g: F) {
        println!("{}", g());
    }

    let mut s = String::from("foo");
    let t = String::from("bar");

    f(|| {
        s += &*t;
        s
    });
    // Prints "foobar".
    #}

generates a closure type roughly like the following:

    struct Closure<'a> {
        s : String,
        t : &'a String,
    }

    impl<'a> (FnOnce() -> String) for Closure<'a> {
        fn call_once(self) -> String {
            self.s += &*self.t;
            self.s
        }
    }

so that the call to `f` works as if it were:

    f(Closure{s: s, t: &t});

### Capture modes

The compiler prefers to capture a closed-over variable by immutable borrow, followed by unique immutable borrow (see below), by mutable borrow, and finally by move. It will pick the first choice of these that allows the closure to compile. The choice is made only with regards to the contents of the closure expression; the compiler does not take into account surrounding code, such as the lifetimes of involved variables.

If the `move` keyword is used, then all captures are by move or, for `Copy` types, by copy, regardless of whether a borrow would work. The `move` keyword is usually used to allow the closure to outlive the captured values, such as if the closure is being returned or used to spawn a new thread.

Composite types such as structs, tuples, and enums are always captured entirely, not by individual fields. It may be necessary to borrow into a local variable in order to capture a single field:

    # #![allow(unused_variables)]
    #fn main() {
    # use std::collections::HashSet;
    #
    struct SetVec {
        set: HashSet<u32>,
        vec: Vec<u32>
    }

    impl SetVec {
        fn populate(&mut self) {
            let vec = &mut self.vec;
            self.set.iter().for_each(|&n| {
                vec.push(n);
            })
        }
    }
    #}

If, instead, the closure were to use `self.vec` directly, then it would attempt to capture `self` by mutable reference. But since `self.set` is already borrowed to iterate over, the code would not compile.

### Unique immutable borrows in captures

Captures can occur by a special kind of borrow called a unique immutable borrow, which cannot be used anywhere else in the language and cannot be written out explicitly. It occurs when modifying the referent of a mutable reference, as in the following example:

    # #![allow(unused_variables)]
    #fn main() {
    let mut b = false;
    let x = &mut b;
    {
        let mut c = || { *x = true; };
        // The following line is an error:
        // let y = &x;
        c();
    }
    let z = &x;
    #}

In this case, borrowing `x` mutably is not possible, because `x` is not `mut`. But at the same time, borrowing `x` immutably would make the assignment illegal, because a `& &mut` reference may not be unique, so it cannot safely be used to modify a value. So a unique immutable borrow is used: it borrows `x` immutably, but like a mutable borrow, it must be unique. In the above example, uncommenting the declaration of `y` will produce an error because it would violate the uniqueness of the closure's borrow of `x`; the declaration of z is valid because the closure's lifetime has expired at the end of the block, releasing the borrow.

### Call traits and coercions

Closure types all implement `FnOnce`, indicating that they can be called once by consuming ownership of the closure. Additionally, some closures implement more specific call traits:

- A closure which does not move out of any captured variables implements `FnMut`, indicating that it can be called by mutable reference.

- A closure which does not mutate or move out of any captured variables implements `Fn`, indicating that it can be called by shared reference.

> Note: `move` closures may still implement `Fn` or `FnMut`, even though they capture variables by move. This is because the traits implemented by a closure type are determined by what the closure does with captured values, not how it captures them.

Non-capturing closures are closures that don't capture anything from their environment. They can be coerced to function pointers (`fn`) with the matching signature.

    # #![allow(unused_variables)]
    #fn main() {
    let add = |x, y| x + y;

    let mut x = add(5,7);

    type Binop = fn(i32, i32) -> i32;
    let bo: Binop = add;
    x = bo(5,7);
    #}

### Other traits

All closure types implement `Sized`. Additionally, closure types implement the following traits if allowed to do so by the types of the captures it stores:

- `Clone`
- `Copy`
- `Sync`
- `Send`

The rules for `Send` and `Sync` match those for normal struct types, while `Clone` and `Copy` behave as if derived. For `Clone`, the order of cloning of the captured variables is left unspecified.

Because captures are often by reference, the following general rules arise:

- A closure is `Sync` if all captured variables are `Sync`.
- A closure is `Send` if all variables captured by non-unique immutable reference are `Sync`, and all values captured by unique immutable or mutable reference, copy, or move are `Send`.
- A closure is `Clone` or `Copy` if it does not capture any values by unique immutable or mutable reference, and if all values it captures by copy or move are `Clone` or `Copy`, respectively.

## Trait objects

> Syntax TraitObjectType : `dyn`? TypeParamBounds

A trait object is an opaque value of another type that implements a set of traits. The set of traits is made up of an object safe base trait plus any number of auto traits.

Trait objects implement the base trait, its auto traits, and any supertraits of the base trait.

Trait objects are written as the optional keyword `dyn` followed by a set of trait bounds, but with the following restrictions on the trait bounds. All traits except the first trait must be auto traits, there may not be more than one lifetime, and opt-out bounds (e.g. `?sized`) are not allowed. Furthermore, paths to traits may be parenthesized.

For example, given a trait `Trait`, the following are all trait objects:

- `Trait`
- `dyn Trait`
- `dyn Trait + Send`
- `dyn Trait + Send + Sync`
- `dyn Trait + 'static`
- `dyn Trait + Send + 'static`
- `dyn Trait +`
- `dyn 'static + Trait`.
- `dyn (Trait)`

If the first bound of the trait object is a path that starts with `::`, then the `dyn` will be treated as a part of the path. The first path can be put in parenthesis to get around this. As such, if you want a trait object with the trait `::your_module::Trait`, you should write it as `dyn (::your_module::Trait)`.

> Note: For clarity, it is recommended to always use the `dyn` keyword on your trait objects unless your codebase supports compiling with Rust 1.26 or lower.

Two trait object types alias each other if the base traits alias each other and if the sets of auto traits are the same and the lifetime bounds are the same. For example, `dyn Trait + Send + UnwindSafe` is the same as `dyn Trait + Unwindsafe + Send`.

Warning: With two trait object types, even when the complete set of traits is the same, if the base traits differ, the type is different. For example, `dyn Send + Sync` is a different type from `dyn Sync + Send`. See issue 33140.

Due to the opaqueness of which concrete type the value is of, trait objects are dynamically sized types. Like all DSTs, trait objects are used behind some type of pointer; for example `&dyn SomeTrait` or `Box<dyn SomeTrait>`. Each instance of a pointer to a trait object includes:

- a pointer to an instance of a type `T` that implements `SomeTrait`
- a virtual method table, often just called a vtable, which contains, for each method of `SomeTrait` and its supertraits that `T` implements, a pointer to `T`'s implementation (i.e. a function pointer).

The purpose of trait objects is to permit "late binding" of methods. Calling a method on a trait object results in virtual dispatch at runtime: that is, a function pointer is loaded from the trait object vtable and invoked indirectly. The actual implementation for each vtable entry can vary on an object-by-object basis.

An example of a trait object:

    trait Printable {
        fn stringify(&self) -> String;
    }

    impl Printable for i32 {
        fn stringify(&self) -> String { self.to_string() }
    }

    fn print(a: Box<dyn Printable>) {
        println!("{}", a.stringify());
    }

    fn main() {
        print(Box::new(10) as Box<dyn Printable>);
    }

In this example, the trait `Printable` occurs as a trait object in both the type signature of `print`, and the cast expression in `main`.

### Trait Object Lifetime Bounds

Since a trait object can contain references, the lifetimes of those references need to be expressed as part of the trait object. This lifetime is written as `Trait + 'a`. There are defaults that allow this lifetime to usually be inferred with a sensible choice.

## Type parameters

Within the body of an item that has type parameter declarations, the names of its type parameters are types:

    # #![allow(unused_variables)]
    #fn main() {
    fn to_vec<A: Clone>(xs: &[A]) -> Vec<A> {
        if xs.is_empty() {
            return vec![];
        }
        let first: A = xs[0].clone();
        let mut rest: Vec<A> = to_vec(&xs[1..]);
        rest.insert(0, first);
        rest
    }
    #}

Here, `first` has type `A`, referring to `to_vec`'s `A` type parameter; and `rest` has type `Vec<A>`, a vector with element type `A`.

## Anonymous type parameters

> Note: This section is a placeholder for more comprehensive reference material.

> Note: This is often called "impl Trait in argument position".

Functions can declare an argument to be an anonymous type parameter where the callee must provide a type that has the bounds declared by the anonymous type parameter and the function can only use the methods available by the trait bounds of the anonymous type parameter.

They are written as `impl` followed by a set of trait bounds.

## Abstract return types

> Note: This section is a placeholder for more comprehensive reference material.

> Note: This is often called "impl Trait in return position".

Functions, except for associated trait functions, can return an abstract return type. These types stand in for another concrete type where the use-site may only use the trait methods declared by the trait bounds of the type.

They are written as `impl` followed by a set of trait bounds.

## Self types

The special type `Self` has a meaning within traits and implementations: it refers to the implementing type. For example, in:

    # #![allow(unused_variables)]
    #fn main() {
    pub trait From<T> {
        fn from(T) -> Self;
    }

    impl From<i32> for String {
        fn from(x: i32) -> Self {
            x.to_string()
        }
    }
    #}

The notation `Self` in the impl refers to the implementing type: `String`. In another example:

    # #![allow(unused_variables)]
    #fn main() {
    trait Printable {
        fn make_string(&self) -> String;
    }

    impl Printable for String {
        fn make_string(&self) -> String {
            (*self).clone()
        }
    }
    #}

> Note: The notation `&self` is a shorthand for `self: &Self`.

# Dynamically Sized Types

Most types have a fixed size that is known at compile time and implement the trait `Sized`. A type with a size that is known only at run-time is called a dynamically sized type (DST) or, informally, an unsized type. Slices and trait objects are two examples of DSTs. Such types can only be used in certain cases:

- Pointer types to DSTs are sized but have twice the size of pointers to sized types
  - Pointers to slices also store the number of elements of the slice.
  - Pointers to trait objects also store a pointer to a vtable.
- DSTs can be provided as type arguments when a bound of `?Sized`. By default any type parameter has a `Sized` bound.
- Traits may be implemented for DSTs. Unlike type parameters `Self: ?Sized` by default in trait definitions.
- Structs may contain a DST as the last field, this makes the struct itself a DST.

Notably: variables, function parameters, const and static items must be `Sized`.

# Type Layout

The layout of a type is its size, alignment, and the relative offsets of its fields. For enums, how the discriminant is laid out and interpreted is also part of type layout.

Type layout can be changed with each compilation. Instead of trying to document exactly what is done, we only document what is guaranteed today.

## Size and Alignment

All values have an alignment and size.

The alignment of a value specifies what addresses are valid to store the value at. A value of alignment `n` must only be stored at an address that is a multiple of n. For example, a value with an alignment of 2 must be stored at an even address, while a value with an alignment of 1 can be stored at any address. Alignment is measured in bytes, and must be at least 1, and always a power of 2. The alignment of a value can be checked with the `align_of_val` function.

The size of a value is the offset in bytes between successive elements in an array with that item type including alignment padding. The size of a value is always a multiple of its alignment. The size of a value can be checked with the `size_of_val` function.

Types where all values have the same size and alignment known at compile time implement the `Sized` trait and can be checked with the `size_of` and `align_of` functions. Types that are not `Sized` are known as dynamically sized types. Since all values of a `Sized` type share the same size and alignment, we refer to those shared values as the size of the type and the alignment of the type respectively.

## Primitive Data Layout

The size of most primitives is given in this table.

Type | `size_of::<Type>()`
--- | ---
bool | 1
u8 | 1
u16 | 2
u32 | 4
u64 | 8
u128 | 16
i8 | 1
i16 | 2
i32 | 4
i64 | 8
i128 | 16
f32 | 4
f64 | 8
char | 4

`usize` and `isize` have a size big enough to contain every address on the target platform. For example, on a 32 bit target, this is 4 bytes and on a 64 bit target, this is 8 bytes.

Most primitives are generally aligned to their size, although this is platform-specific behavior. In particular, on x86 u64 and f64 are only aligned to 32 bits.

## Pointers and References Layout

Pointers and references have the same layout. Mutability of the pointer or reference does not change the layout.

Pointers to sized types have the same size and alignment as `usize`.

Pointers to unsized types are sized. The size and alignment is guaranteed to be at least equal to the size and alignment of a pointer.

> Note: Though you should not rely on this, all pointers to DSTs are currently twice the size of the size of `usize` and have the same alignment.

## Array Layout

Arrays are laid out so that the `nth` element of the array is offset from the start of the array by `n * the size of the type` bytes. An array of `[T; n]` has a size of `size_of::<T>() * n` and the same alignment of `T`.

## Slice Layout

Slices have the same layout as the section of the array they slice.

> Note: This is about the raw `[T]` type, not pointers (`&[T]`, `Box<[T]>`, etc.) to slices.

## Tuple Layout

Tuples do not have any guarantees about their layout.

The exception to this is the unit tuple (`()`) which is guaranteed as a zero-sized type to have a size of 0 and an alignment of 1.

## Trait Object Layout

Trait objects have the same layout as the value the trait object is of.

> Note: This is about the raw trait object types, not pointers (`&Trait`, `Box<Trait>`, etc.) to trait objects.

## Closure Layout

Closures have no layout guarantees.

## Representations

All user-defined composite types (`struct`s, `enum`s, and `union`s) have a representation that specifies what the layout is for the type.

The possible representations for a type are the default representation, `C`, the primitive representations, and `packed`. Multiple representations can be applied to a single type.

The representation of a type can be changed by applying the [`repr` attribute] to it. The following example shows a struct with a `C` representation.

    #[repr(C)]
    struct ThreeInts {
        first: i16,
        second: i8,
        third: i32
    }

> Note: As a consequence of the representation being an attribute on the item, the representation does not depend on generic parameters. Any two types with the same name have the same representation. For example, `Foo<Bar>` and `Foo<Baz>` both have the same representation.

The representation of a type does not change the layout of its fields. For example, a struct with a `C` representation that contains a struct `Inner` with the default representation will not change the layout of Inner.

### The Default Representation

Nominal types without a `repr` attribute have the default representation. Informally, this representation is also called the `rust` representation.

There are no guarantees of data layout made by this representation.

### The `C` Representation

The `C` representation is designed for dual purposes. One purpose is for creating types that are interoptable with the C Language. The second purpose is to create types that you can soundly performing operations that rely on data layout such as reinterpreting values as a different type.

Because of this dual purpose, it is possible to create types that are not useful for interfacing with the C programming language.

This representation can be applied to structs, unions, and enums.

#### #[repr(C)] Structs

The alignment of the struct is the alignment of the most-aligned field in it.

The size and offset of fields is determined by the following algorithm.

Start with a current offset of 0 bytes.

For each field in declaration order in the struct, first determine the size and alignment of the field. If the current offset is not a multiple of the field's alignment, then add padding bytes to the current offset until it is a multiple of the field's alignment. The offset for the field is what the current offset is now. Then increase the current offset by the size of the field.

Finally, the size of the struct is the current offset rounded up to the nearest multiple of the struct's alignment.

Here is this algorithm described in pseudocode.

    struct.alignment = struct.fields().map(|field| field.alignment).max();

    let current_offset = 0;

    for field in struct.fields_in_declaration_order() {
        // Increase the current offset so that it's a multiple of the alignment
        // of this field. For the first field, this will always be zero.
        // The skipped bytes are called padding bytes.
        current_offset += field.alignment % current_offset;

        struct[field].offset = current_offset;

        current_offset += field.size;
    }

    struct.size = current_offset + current_offset % struct.alignment;

> Note: This algorithm can produce zero-sized structs. This differs from C where structs without data still have a size of one byte.

#### #[repr(C)] Unions

A union declared with `#[repr(C)]` will have the same size and alignment as an equivalent C union declaration in the C language for the target platform. The union will have a size of the maximum size of all of its fields rounded to its alignment, and an alignment of the maximum alignment of all of its fields. These maximums may come from different fields.

    #[repr(C)]
    union Union {
        f1: u16,
        f2: [u8; 4],
    }

    assert_eq!(std::mem::size_of::<Union>(), 4);  // From f2
    assert_eq!(std::mem::align_of::<Union>(), 2); // From f1

    #[repr(C)]
    union SizeRoundedUp {
       a: u32,
       b: [u16; 3],
    }

    assert_eq!(std::mem::size_of::<SizeRoundedUp>(), 8);  // Size of 6 from b,
                                                          // rounded up to 8 from
                                                          // alignment of a.
    assert_eq!(std::mem::align_of::<SizeRoundedUp>(), 4); // From a

#### #[repr(C)] Enums

For C-like enumerations, the `C` representation has the size and alignment of the default `enum` size and alignment for the target platform's C ABI.

> Note: The enum representation in C is implementation defined, so this is really a "best guess". In particular, this may be incorrect when the C code of interest is compiled with certain flags.

Warning: There are crucial differences between an `enum` in the C language and Rust's C-like enumerations with this representation. An `enum` in C is mostly a `typedef` plus some named constants; in other words, an object of an `enum` type can hold any integer value. For example, this is often used for bitflags in `C`. In contrast, Rust’s C-like enumerations can only legally hold the discriminant values, everything else is undefined behaviour. Therefore, using a C-like enumeration in FFI to model a C `enum` is often wrong.

It is an error for zero-variant enumerations to have the `C` representation.

For all other enumerations, the layout is unspecified.

Likewise, combining the `C` representation with a primitive representation, the layout is unspecified.

### Primitive representations

The primitive representations are the representations with the same names as the primitive integer types. That is: `u8`, `u16`, `u32`, `u64`, `usize`, `i8`, `i16`, `i32`, `i64`, and `isize`.

Primitive representations can only be applied to enumerations.

For C-like enumerations, they set the size and alignment to be the same as the primitive type of the same name. For example, a C-like enumeration with a `u8` representation can only have discriminants between 0 and 255 inclusive.

It is an error for zero-variant enumerations to have a primitive representation.

For all other enumerations, the layout is unspecified.

Likewise, combining two primitive representations together is unspecified.

### The `align` Representation

The `align` representation can be used on `struct`s and `union`s to raise the alignment of the type to a given value.

Alignment is specified as a parameter in the form of `#[repr(align(x))]`. The alignment value must be a power of two of type `u32`. The `align` representation can raise the alignment of a type to be greater than it's primitive alignment, it cannot lower the alignment of a type.

The `align` and `packed` representations cannot be applied on the same type and a `packed` type cannot transitively contain another `align`ed type.

### The `packed` Representation

The `packed` representation can only be used on `struct`s and `union`s.

It modifies the representation (either the default or `C`) by removing any padding bytes and forcing the alignment of the type to `1`.

The `align` and `packed` representations cannot be applied on the same type and a `packed` type cannot transitively contain another `align`ed type.

Warning: Dereferencing an unaligned pointer is undefined behavior and it is possible to safely create unaligned pointers to `packed` fields. Like all ways to create undefined behavior in safe Rust, this is a bug.

### The `transparent` Representation

The `transparent` representation can only be used on `struct`s that have a single non-zero sized field and any number of zero-sized fields, including `PhantomData<T>`.

Structs with this representation have the same layout and ABI as the single non-zero sized field.

This is different than the `C` representation because a struct with the `C` representation will always have the ABI of a `C` `struct` while, for example, a struct with the `transparent` representation with a primitive field will have the ABI of the primitive field.

Because this representation delegates type layout to another type, it cannot be used with any other representation.

# Interior Mutability

Sometimes a type needs be mutated while having multiple aliases. In Rust this is achieved using a pattern called interior mutability. A type has interior mutability if its internal state can be changed through a shared reference to it. This goes against the usual requirement that the value pointed to by a shared reference is not mutated.

`std::cell::UnsafeCell<T>` type is the only allowed way in Rust to disable this requirement. When `UnsafeCell<T>` is immutably aliased, it is still safe to mutate, or obtain a mutable reference to, the `T` it contains. As with all other types, it is undefined behavior to have multiple `&mut UnsafeCell<T>` aliases.

Other types with interior mutability can be created by using `UnsafeCell<T>` as a field. The standard library provides a variety of types that provide safe interior mutability APIs. For example, `std::cell::RefCell<T>` uses run-time borrow checks to ensure the usual rules around multiple references. The `std::sync::atomic` module contains types that wrap a value that is only accessed with atomic operations, allowing the value to be shared and mutated across threads.

# Subtyping and Variance

Subtyping is implicit and can occur at any stage in type checking or inference. Subtyping in Rust is very restricted and occurs only due to variance with respect to lifetimes and between types with higher ranked lifetimes. If we were to erase lifetimes from types, then the only subtyping would be due to type equality.

Consider the following example: string literals always have `'static` lifetime. Nevertheless, we can assign `s` to `t`:

    # #![allow(unused_variables)]
    #fn main() {
    fn bar<'a>() {
        let s: &'static str = "hi";
        let t: &'a str = s;
    }
    #}

Since `'static` outlives the lifetime parameter `'a`, `&'static str` is a subtype of `&'a str`.

Higher-ranked function pointers and trait objects have another subtype relation. They are subtypes of types that are given by substitutions of the higher-ranked lifetimes. Some examples:

    # #![allow(unused_variables)]
    #fn main() {
    // Here 'a is substituted for 'static
    let subtype: &(for<'a> fn(&'a i32) -> &'a i32) = &((|x| x) as fn(&_) -> &_);
    let supertype: &(fn(&'static i32) -> &'static i32) = subtype;

    // This works similarly for trait objects
    let subtype: &(for<'a> Fn(&'a i32) -> &'a i32) = &|x| x;
    let supertype: &(Fn(&'static i32) -> &'static i32) = subtype;

    // We can also substitute one higher-ranked lifetime for another
    let subtype: &(for<'a, 'b> fn(&'a i32, &'b i32))= &((|x, y| {}) as fn(&_, &_));
    let supertype: &for<'c> fn(&'c i32, &'c i32) = subtype;
    #}

## Variance

Variance is a property that generic types have with respect to their arguments. A generic type's variance in a parameter is how the subtyping of the parameter affects the subtyping of the type.

- `F<T>` is covariant over `T` if `T` being a subtype of `U` implies that `F<T>` is a subtype of `F<U>` (subtyping "passes through")
- `F<T>` is contravariant over `T` if `T` being a subtype of `U` implies that `F<U>` is a subtype of `F<T>`
- `F<T>` is invariant over `T` otherwise (no subtyping relation can be derived)

Variance of types is automatically determined as follows

Type | Variance in `'a` | Variance in `T`
--- | --- | ---
`&'a T` | covariant | covariant
`&'a mut T` | covariant | invariant
`*const T` |  | covariant
`*mut T` |  | invariant
`[T]` and `[T; n]` |  | covariant
`fn() -> T` |  | covariant
`fn(T) -> ()` |  | contravariant
`std::cell::UnsafeCell<T>` |  | invariant
`std::marker::PhantomData<T>` |  | covariant
`Trait<T> + 'a` | covariant | invariant

The variance of other `struct`, `enum`, `union` and tuple types is decided by looking at the variance of the types of their fields. If the parameter is used in positions with different variances then the parameter is invariant. For example the following struct is covariant in `'a` and `T` and invariant in `'b` and `U`.

    # #![allow(unused_variables)]
    #fn main() {
    use std::cell::UnsafeCell;
    struct Variance<'a, 'b, T, U: 'a> {
        x: &'a U,               // This makes `Variance` covariant in 'a, and would
                                // make it covariant in U, but U is used later
        y: *const T,            // Covariant in T
        z: UnsafeCell<&'b f64>, // Invariant in 'b
        w: *mut U,              // Invariant in U, makes the whole struct invariant
    }
    #}

# Trait and lifetime bounds

> Syntax
> TypeParamBounds :
> TypeParamBound ( `+` TypeParamBound )* `+`?

> TypeParamBound :
> Lifetime | TraitBound

> TraitBound :
> `?`? ForLifetimes? TraitPath
> | `(` `?`? ForLifetimes? TraitPath `)`

> LifetimeBounds :
> ( Lifetime `+` )* Lifetime?

> Lifetime :
> LIFETIME_OR_LABEL
> | `'static`
> | `'_`

Trait and lifetime bounds provide a way for generic items to restrict which types and lifetimes are used as their parameters. Bounds can be provided on any type in a where clause. There are also shorter forms for certain common cases:

- Bounds written after declaring a generic parameter: `fn f<A: Copy>() {}` is the same as `fn f<A> where A: Copy () {}`.
- In trait declarations as supertraits: `trait Circle : Shape {}` is equivalent to `trait Circle where Self : Shape {}`.
- In trait declarations as bounds on associated types: `trait A { type B: Copy; }` is equivalent to `trait A where Self::B: Copy { type B; }`.

Bounds on an item must be satisfied when using the item. When type checking and borrow checking a generic item, the bounds can be used to determine that a trait is implemented for a type. For example, given `Ty: Trait`

- In the body of a generic function, methods from `Trait` can be called on `Ty` values. Likewise associated constants on the `Trait` can be used.
- Associated types from `Trait` can be used.
- Generic functions and types with a `T: Trait` bounds can be used with `Ty` being used for `T`.

    # #![allow(unused_variables)]
    #fn main() {
    # type Surface = i32;
    trait Shape {
        fn draw(&self, Surface);
        fn name() -> &'static str;
    }

    fn draw_twice<T: Shape>(surface: Surface, sh: T) {
        sh.draw(surface);           // Can call method because T: Shape
        sh.draw(surface);
    }

    fn copy_and_draw_twice<T: Copy>(surface: Surface, sh: T) where T: Shape {
        let shape_copy = sh;        // doesn't move sh because T: Copy
        draw_twice(surface, sh);    // Can use generic function because T: Shape
    }

    struct Figure<S: Shape>(S, S);

    fn name_figure<U: Shape>(
        figure: Figure<U>,          // Type Figure<U> is well-formed because U: Shape
    ) {
        println!(
            "Figure of two {}",
            U::name(),              // Can use associated function
        );
    }
    #}

Trait and lifetime bounds are also used to name trait objects.

## `?Sized`

`?` is only used to declare that the `Sized` trait may not be implemented for a type parameter or associated type. `?Sized` may not be used as a bound for other types.

## Lifetime bounds

Lifetime bounds can be applied to types or other lifetimes. The bound `'a: 'b` is usually read as `'a` outlives `'b`. `'a: 'b` means that `'a` lasts longer than `'b`, so a reference `&'a ()` is valid whenever `&'b ()` is valid.

    # #![allow(unused_variables)]
    #fn main() {
    fn f<'a, 'b>(x: &'a i32, mut y: &'b i32) where 'a: 'b {
        y = x;                      // &'a i32 is a subtype of &'b i32 because 'a: 'b
        let r: &'b &'a i32 = &&0;   // &'b &'a i32 is well formed because 'a: 'b
    }
    #}

`T: 'a` means that all lifetime parameters of `T` outlive `'a`. For example if `'a` is an unconstrained lifetime parameter then `i32: 'static` and `&'static str: 'a` are satisfied but `Vec<&'a ()>: 'static` is not.

## Higher-ranked trait bounds

Type bounds may be higher ranked over lifetimes. These bounds specify a bound is true for all lifetimes. For example, a bound such as `for<'a> &'a T: PartialEq<i32>` would require an implementation like

    impl<'a> PartialEq<i32> for &'a T {
        // ...
    }

and could then be used to compare a `&'a T` with any lifetime to an `i32`.

Only a higher-ranked bound can be used here as the lifetime of the reference is shorter than a lifetime parameter on the function:

    # #![allow(unused_variables)]
    #fn main() {
    fn call_on_ref_zero<F>(f: F) where for<'a> F: Fn(&'a i32) {
        let zero = 0;
        f(&zero);
    }
    #}

Higher-ranked lifetimes may also be specified just before the trait, the only end of the following trait instead of the whole bound. This function is difference is the scope of the lifetime parameter, which extends only to the equivalent to the last one.

    # #![allow(unused_variables)]
    #fn main() {
    fn call_on_ref_zero<F>(f: F) where F: for<'a> Fn(&'a i32) {
        let zero = 0;
        f(&zero);
    }
    #}

# Type coercions

Coercions are defined in RFC 401. RFC 1558 then expanded on that. A coercion is implicit and has no syntax.

## Coercion sites

A coercion can only occur at certain coercion sites in a program; these are typically places where the desired type is explicit or can be derived by propagation from explicit types (without type inference). Possible coercion sites are:

- `let` statements where an explicit type is given.

  For example, `42` is coerced to have type `i8` in the following:

      # #![allow(unused_variables)]
      #fn main() {
      let _: i8 = 42;
      #}

- `static` and `const` statements (similar to `let` statements).

- Arguments for function calls

  The value being coerced is the actual parameter, and it is coerced to the type of the formal parameter.

  For example, `42` is coerced to have type `i8` in the following:

      fn bar(_: i8) { }

      fn main() {
          bar(42);
      }

  For method calls, the receiver (`self` parameter) can only take advantage of unsized coercions.

- Instantiations of struct or variant fields

  For example, `42` is coerced to have type `i8` in the following:

      struct Foo { x: i8 }

      fn main() {
          Foo { x: 42 };
      }

- Function results, either the final line of a block if it is not semicolon-terminated or any expression in a `return` statement

  For example, `42` is coerced to have type `i8` in the following:

      # #![allow(unused_variables)]
      #fn main() {
      fn foo() -> i8 {
          42
      }
      #}

If the expression in one of these coercion sites is a coercion-propagating expression, then the relevant sub-expressions in that expression are also coercion sites. Propagation recurses from these new coercion sites. Propagating expressions and their relevant sub-expressions are:

- Array literals, where the array has type `[U; n]`. Each sub-expression in the array literal is a coercion site for coercion to type `U`.

- Array literals with repeating syntax, where the array has type `[U; n]`. The repeated sub-expression is a coercion site for coercion to type `U`.

- Tuples, where a tuple is a coercion site to type `(U_0, U_1, ..., U_n)`. Each sub-expression is a coercion site to the respective type, e.g. the zeroth sub-expression is a coercion site to type `U_0`.

- Parenthesized sub-expressions (`(e)`): if the expression has type `U`, then the sub-expression is a coercion site to `U`.

- Blocks: if a block has type `U`, then the last expression in the block (if it is not semicolon-terminated) is a coercion site to `U`. This includes blocks which are part of control flow statements, such as `if`/`else`, if the block has a known type.

## Coercion types

Coercion is allowed between the following types:

- `T` to `U` if `T` is a subtype of `U` (reflexive case)

- `T_1` to `T_3` where `T_1` coerces to `T_2` and `T_2` coerces to `T_3` (transitive case)

  Note that this is not fully supported yet

- `&mut T` to `&T`

- `*mut T` to `*const T`

- `&T` to `*const T`

- `&mut T` to `*mut T`

- `&T` or `&mut T` to `&U` if `T` implements `Deref<Target = U>`. For example:

      use std::ops::Deref;

      struct CharContainer {
          value: char,
      }

      impl Deref for CharContainer {
          type Target = char;

          fn deref<'a>(&'a self) -> &'a char {
              &self.value
          }
      }

      fn foo(arg: &char) {}

      fn main() {
          let x = &mut CharContainer { value: 'y' };
          foo(x); //&mut CharContainer is coerced to &char.
      }

- `&mut T` to `&mut U` if `T` implements `DerefMut<Target = U>`.

- TyCtor(`T`) to TyCtor(`U`), where TyCtor(`T`) is one of

  - `&T`
  - `&mut T`
  - `*const T`
  - `*mut T`
  - `Box<T>`

  and where `T` can obtained from `U` by unsized coercion.

- Non capturing closures to `fn` pointers

- `!` to any `T`

### Unsized Coercions

The following coercions are called `unsized coercions`, since they relate to converting sized types to unsized types, and are permitted in a few cases where other coercions are not, as described above. They can still happen anywhere else a coercion can occur.

Two traits, [`Unsize`] and [`CoerceUnsized`], are used to assist in this process and expose it for library use. The compiler following coercions are built-in and, if `T` can be coerced to `U` with one of the, then the compiler will provide an implementation of `Unsize<U>` for `T`:

- `[T; n]` to `[T]`.

- `T` to `U`, when `U` is a trait object type and either `T` implements `U` or `T` is a trait object for a subtrait of `U`.

- `Foo<..., T, ...>` to `Foo<..., U, ...>`, when:

  - `Foo` is a struct.
  - `T` implements `Unsize<U>`.
  - The last field of `Foo` has a type involving `T`.
  - If that field has type `Bar<T>`, then `Bar<T>` implements `Unsized<Bar<U>>`.
  - T is not part of the type of any other fields.

Additionally, a type `Foo<T>` can implement `CoerceUnsized<Foo<U>>` when `T` implements `Unsize<U>` or `CoerceUnsized<Foo<U>>`. This allows it to provide a unsized coercion to `Foo<U>`.

> Note: While the definition of the unsized coercions and their implementation has been stabilized, the traits themselves are not yet stable and therefore can't be used directly in stable Rust.

# Destructors

When an initialized variable in Rust goes out of scope or a temporary is no longer needed its destructor is run. Assignment also runs the destructor of its left-hand operand, unless it's an uninitialized variable. If a struct variable has been partially initialized, only its initialized fields are dropped.

The destructor of a type consists of

1. Calling its `std::ops::Drop::drop` method, if it has one.
2. Recursively running the destructor of all of its fields.
   - The fields of a struct, tuple or enum variant are dropped in declaration order. *
   - The elements of an array or owned slice are dropped from the first element to the last. *
   - The captured values of a closure are dropped in an unspecified order.
   - Trait objects run the destructor of the underlying type.
   - Other types don't result in any further drops.

* This order was stabilized in RFC 1857.

Variables are dropped in reverse order of declaration. Variables declared in the same pattern drop in an unspecified ordered.

If a destructor must be run manually, such as when implementing your own smart pointer, `std::ptr::drop_in_place` can be used.

Some examples:

    # #![allow(unused_variables)]
    #fn main() {
    struct ShowOnDrop(&'static str);

    impl Drop for ShowOnDrop {
        fn drop(&mut self) {
            println!("{}", self.0);
        }
    }

    {
        let mut overwritten = ShowOnDrop("Drops when overwritten");
        overwritten = ShowOnDrop("drops when scope ends");
    }
    # println!("");
    {
        let declared_first = ShowOnDrop("Dropped last");
        let declared_last = ShowOnDrop("Dropped first");
    }
    # println!("");
    {
        // Tuple elements drop in forwards order
        let tuple = (ShowOnDrop("Tuple first"), ShowOnDrop("Tuple second"));
    }
    # println!("");
    loop {
        // Tuple expression doesn't finish evaluating so temporaries drop in reverse order:
        let partial_tuple = (ShowOnDrop("Temp first"), ShowOnDrop("Temp second"), break);
    }
    # println!("");
    {
        let moved;
        // No destructor run on assignment.
        moved = ShowOnDrop("Drops when moved");
        // drops now, but is then uninitialized
        moved;
        let uninitialized: ShowOnDrop;
        // Only first element drops
        let mut partially_initialized: (ShowOnDrop, ShowOnDrop);
        partially_initialized.0 = ShowOnDrop("Partial tuple first");
    }
    #}

## Not running destructors

Not running destructors in Rust is safe even if it has a type that isn't `'static`. `std::mem::ManuallyDrop` provides a wrapper to prevent a variable or field from being dropped automatically.

# Lifetime elision

Rust has rules that allow lifetimes to be elided in various places where the compiler can infer a sensible default choice.

## Lifetime elision in functions

In order to make common patterns more ergonomic, lifetime arguments can be elided in function item, function pointer and closure trait signatures. The following rules are used to infer lifetime parameters for elided lifetimes. It is an error to elide lifetime parameters that cannot be inferred. The placeholder lifetime, `'_`, can also be used to have a lifetime inferred in the same way. For lifetimes in paths, using `'_` is preferred. Trait object lifetimes follow different rules discussed below.

- Each elided lifetime in the parameters becomes a distinct lifetime parameter.
- If there is exactly one lifetime used in the parameters (elided or not), that lifetime is assigned to all elided output lifetimes.

In method signatures there is another rule

- If the receiver has type `&Self` or `&mut Self`, then the lifetime of that reference to `Self` is assigned to all elided output lifetime parameters.

Examples:

    fn print(s: &str);                                      // elided
    fn print(s: &'_ str);                                   // also elided
    fn print<'a>(s: &'a str);                               // expanded

    fn debug(lvl: usize, s: &str);                          // elided
    fn debug<'a>(lvl: usize, s: &'a str);                   // expanded

    fn substr(s: &str, until: usize) -> &str;               // elided
    fn substr<'a>(s: &'a str, until: usize) -> &'a str;     // expanded

    fn get_str() -> &str;                                   // ILLEGAL

    fn frob(s: &str, t: &str) -> &str;                      // ILLEGAL

    fn get_mut(&mut self) -> &mut T;                        // elided
    fn get_mut<'a>(&'a mut self) -> &'a mut T;              // expanded

    fn args<T: ToCStr>(&mut self, args: &[T]) -> &mut Command;                  // elided
    fn args<'a, 'b, T: ToCStr>(&'a mut self, args: &'b [T]) -> &'a mut Command; // expanded

    fn new(buf: &mut [u8]) -> BufWriter<'_>;                // elided - preferred
    fn new(buf: &mut [u8]) -> BufWriter;                    // elided
    fn new<'a>(buf: &'a mut [u8]) -> BufWriter<'a>;         // expanded

    type FunPtr = fn(&str) -> &str;                         // elided
    type FunPtr = for<'a> fn(&'a str) -> &'a str;           // expanded

    type FunTrait = dyn Fn(&str) -> &str;                   // elided
    type FunTrait = dyn for<'a> Fn(&'a str) -> &'a str;     // expanded

## Default trait object lifetimes

The assumed lifetime of references held by a trait object is called its default object lifetime bound. These were defined in RFC 599 and amended in RFC 1156.

These default object lifetime bounds are used instead of the lifetime parameter elision rules defined above when the lifetime bound is omitted entirely. If `'_` is used as the lifetime bound then the bound follows the usual elision rules.

If the trait object is used as a type argument of a generic type then the containing type is first used to try to infer a bound.

- If there is a unique bound from the containing type then that is the default
- If there is more than one bound from the containing type then an explicit bound must be specified

If neither of those rules apply, then the bounds on the trait are used:

- If the trait is defined with a single lifetime bound then that bound is used.
- If `'static` is used for any lifetime bound then `'static` is used.
- If the trait has no lifetime bounds, then the lifetime is inferred in expressions and is `'static` outside of expressions.

    // For the following trait...
    trait Foo { }

    // These two are the same as Box<T> has no lifetime bound on T
    Box<dyn Foo>
    Box<dyn Foo + 'static>

    // ...and so are these:
    impl dyn Foo {}
    impl dyn Foo + 'static {}

    // ...so are these, because &'a T requires T: 'a
    &'a dyn Foo
    &'a (dyn Foo + 'a)

    // std::cell::Ref<'a, T> also requires T: 'a, so these are the same
    std::cell::Ref<'a, dyn Foo>
    std::cell::Ref<'a, dyn Foo + 'a>

    // This is an error:
    struct TwoBounds<'a, 'b, T: ?Sized + 'a + 'b>
    TwoBounds<'a, 'b, dyn Foo> // Error: the lifetime bound for this object type
                               // cannot be deduced from context

Note that the innermost object sets the bound, so `&'a Box<dyn Foo>` is still `&'a Box<dyn Foo + 'static>`.

    // For the following trait...
    trait Bar<'a>: 'a { }

    // ...these two are the same:
    Box<dyn Bar<'a>>
    Box<dyn Bar<'a> + 'a>

    // ...and so are these:
    impl<'a> dyn Foo<'a> {}
    impl<'a> dyn Foo<'a> + 'a {}

    // This is still an error:
    struct TwoBounds<'a, 'b, T: ?Sized + 'a + 'b>
    TwoBounds<'a, 'b, dyn Foo<'c>>

## `'static` lifetime elision

Both constant and static declarations of reference types have implicit `'static` lifetimes unless an explicit lifetime is specified. As such, the constant declarations involving `'static` above may be written without the lifetimes.

    # #![allow(unused_variables)]
    #fn main() {
    // STRING: &'static str
    const STRING: &str = "bitstring";

    struct BitsNStrings<'a> {
        mybits: [u32; 2],
        mystring: &'a str,
    }

    // BITS_N_STRINGS: BitsNStrings<'static>
    const BITS_N_STRINGS: BitsNStrings<'_> = BitsNStrings {
        mybits: [1, 2],
        mystring: STRING,
    };
    #}

Note that if the `static` or `const` items include function or closure references, which themselves include references, the compiler will first try the standard elision rules. If it is unable to resolve the lifetimes by its usual rules, then it will error. By way of example:

    // Resolved as `fn<'a>(&'a str) -> &'a str`.
    const RESOLVED_SINGLE: fn(&str) -> &str = ..

    // Resolved as `Fn<'a, 'b, 'c>(&'a Foo, &'b Bar, &'c Baz) -> usize`.
    const RESOLVED_MULTIPLE: &dyn Fn(&Foo, &Bar, &Baz) -> usize = ..

    // There is insufficient information to bound the return reference lifetime
    // relative to the argument lifetimes, so this is an error.
    const RESOLVED_STATIC: &dyn Fn(&Foo, &Bar) -> &Baz = ..

# Special types and traits

Certain types and traits that exist in the standard library are known to the Rust compiler. This chapter documents the special features of these types and traits.

## `Box<T>`

`Box<T>` has a few special features that Rust doesn't currently allow for user defined types.

- The dereference operator for `Box<T>` produces a place which can be moved from. This means that the `*` operator and the destructor of `Box<T>` are built-in to the language.
- Methods can take `Box<Self>` as a receiver.
- A trait may be implemented for `Box<T>` in the same crate as `T`, which the orphan rules prevent for other generic types.

## `UnsafeCell<T>`

`std::cell::UnsafeCell<T>` is used for interior mutability. It ensures that the compiler doesn't perform optimisations that are incorrect for such types. It also ensures that `static` items which have a type with interior mutability aren't placed in memory marked as read only.

## `PhantomData<T>`

`std::marker::PhantomData<T>` is a zero-sized, minimum alignment, type that is considered to own a `T` for the purposes of variance, drop check and auto traits.

## Operator Traits

The traits in `std::ops` and `std::cmp` are used to overload operators, indexing expressions and call expressions.

## `Deref` and `DerefMut`

As well as overloading the unary `*` operator, `Deref` and `DerefMut` are also used in method resolution and deref coercions.

## `Drop`

The `Drop` trait provides a destructor, to be run whenever a value of this type is to be destroyed.

## `Copy`

The `Copy` trait changes the semantics of a type implementing it. Values whose type implements `Copy` are copied rather than moved upon assignment. `Copy` cannot be implemented for types which implement `Drop`, or which have fields that are not `Copy`. `Copy` is implemented by the compiler for

- Numeric types
- `char`, `bool` and `!`
- Tuples of `Copy` types
- Arrays of `Copy` types
- Shared references
- Raw pointers
- Function pointers and function item types

## `Clone`

The `Clone` trait is a supertrait of `Copy`, so it also needs compiler generated implementations. It is implemented by the compiler for the following types:

- Types with a built-in `Copy` implementation (see above)
- Tuples of `Clone` types
- Arrays of `Clone` types

## `Send`

The `Send` trait indicates that a value of this type is safe to send from one thread to another.

## `Sync`

The `Sync` trait indicates that a value of this type is safe to share between multiple threads. This trait must be implemented for all types used in immutable `static` items.

## Auto traits

The `Send`, `Sync`, `UnwindSafe` and `RefUnwindSafe` traits are auto traits. Auto traits have special properties.

If no explicit implementation or negative implementation is written out for an auto trait for a given type, then the compiler implements it automatically according to the following rules:

- `&T`, `&mut T`, `*const T`, `*mut T`, `[T; n]` and `[T]` implement the trait if `T` does.
- Function item types and function pointers automatically implement the trait.
- Structs, enums, unions and tuples implement the trait if all of their fields do.
- Closures implement the trait if the types of all of their captures do. A closure that captures a `T` by shared reference and a `U` by value implements any auto traits that both `&T` and `U` do.

For generic types (counting the built-in types above as generic over `T`), if an generic implementation is available, then the compiler does not automatically implement it for types that could use the implementation except that they do not meet the requisite trait bounds. For instance, the standard library implements `Send` for all `&T` where `T` is `Sync`; this means that the compiler will not implement `Send` for `&T` if `T` is `Send` but not `Sync`.

Auto traits can also have negative implementations, shown as `impl !AutoTrait for T` in the standard library documentation, that override the automatic implementations. For example `*mut T` has a negative implementation of `Send`, and so `*mut T` is not `Send`, even if `T` is. There is currently no stable way to specify additional negative implementations; they exist only in the standard library.

Auto traits may be added as an additional bound to any trait object, even though normally only one trait is allowed. For instance, `Box<dyn Debug + Send + UnwindSafe>` is a valid type.

## `Sized`

The `Sized` trait indicates that the size of this type is known at compile-time; that is, it's not a dynamically sized type. Type parameters are `Sized` by default. `Sized` is always implemented automatically by the compiler, not by implementation items.

# Memory model

Rust does not yet have a defined memory model. Various academics and industry are working on various proposals, but for now, this is an under-defined place in the language.

# Memory allocation and lifetime

The items of a program are those functions, modules and types that have their value calculated at compile-time and stored uniquely in the memory image of the rust process. Items are neither dynamically allocated nor freed.

The heap is a general term that describes boxes. The lifetime of an allocation in the heap depends on the lifetime of the box values pointing to it. Since box values may themselves be passed in and out of frames, or stored in the heap, heap allocations may outlive the frame they are allocated within. An allocation in the heap is guaranteed to reside at a single location in the heap for the whole lifetime of the allocation - it will never be relocated as a result of moving a box value.

## Memory ownership

When a stack frame is exited, its local allocations are all released, and its references to boxes are dropped.

# Variables

A variable is a component of a stack frame, either a named function parameter, an anonymous temporary, or a named local variable.

A local variable (or stack-local allocation) holds a value directly, allocated within the stack's memory. The value is a part of the stack frame.

Local variables are immutable unless declared otherwise. For example: `let mut x = ...`.

Function parameters are immutable unless declared with `mut`. The `mut` keyword applies only to the following parameter. For example: `|mut x, y|` and `fn f(mut x: Box<i32>, y: Box<i32>)` declare one mutable variable `x` and one immutable variable `y`.

Methods that take either `self` or `Box<Self>` can optionally place them in a mutable variable by prefixing them with `mut` (similar to regular arguments). For example:

    # #![allow(unused_variables)]
    #fn main() {
    trait Changer: Sized {
        fn change(mut self) {}
        fn modify(mut self: Box<Self>) {}
    }
    #}

Local variables are not initialized when allocated. Instead, the entire frame worth of local variables are allocated, on frame-entry, in an uninitialized state. Subsequent statements within a function may or may not initialize the local variables. Local variables can be used only after they have been initialized through all reachable control flow paths.

In this next example, `init_after_if` is initialized after the [`if` expression] while `uninit_after_if` is not because it is not initialized in the `else` case.

    # #![allow(unused_variables)]
    #fn main() {
    # fn random_bool() -> bool { true }
    fn initialization_example() {
        let init_after_if: ();
        let uninit_after_if: ();

        if random_bool() {
            init_after_if = ();
            uninit_after_if = ();
        } else {
            init_after_if = ();
        }

        init_after_if; // ok
        // uninit_after_if; // err: use of possibly uninitialized `uninit_after_if`
    }
    #}

# Linkage

> Note: This section is described more in terms of the compiler than of the language.

The compiler supports various methods to link crates together both statically and dynamically. This section will explore the various methods to link crates together, and more information about native libraries can be found in the FFI section of the book.

In one session of compilation, the compiler can generate multiple artifacts through the usage of either command line flags or the `crate_type` attribute. If one or more command line flags are specified, all `crate_type` attributes will be ignored in favor of only building the artifacts specified by command line.

- `--crate-type=bin`, `#[crate_type = "bin"]` - A runnable executable will be produced. This requires that there is a `main` function in the crate which will be run when the program begins executing. This will link in all Rust and native dependencies, producing a distributable binary.

- `--crate-type=lib`, `#[crate_type = "lib"]` - A Rust library will be produced. This is an ambiguous concept as to what exactly is produced because a library can manifest itself in several forms. The purpose of this generic `lib` option is to generate the "compiler recommended" style of library. The output library will always be usable by rustc, but the actual type of library may change from time-to-time. The remaining output types are all different flavors of libraries, and the `lib` type can be seen as an alias for one of them (but the actual one is compiler-defined).

- `--crate-type=dylib`, `#[crate_type = "dylib"]` - A dynamic Rust library will be produced. This is different from the `lib` output type in that this forces dynamic library generation. The resulting dynamic library can be used as a dependency for other libraries and/or executables. This output type will create `*.so` files on linux, `*.dylib` files on osx, and `*.dll` files on windows.

- `--crate-type=staticlib`, `#[crate_type = "staticlib"]` - A static system library will be produced. This is different from other library outputs in that the compiler will never attempt to link to `staticlib` outputs. The purpose of this output type is to create a static library containing all of the local crate's code along with all upstream dependencies. The static library is actually a `*.a` archive on linux and osx and a `*.lib` file on windows. This format is recommended for use in situations such as linking Rust code into an existing non-Rust application because it will not have dynamic dependencies on other Rust code.

- `--crate-type=cdylib`, `#[crate_type = "cdylib"]` - A dynamic system library will be produced. This is used when compiling a dynamic library to be loaded from another language. This output type will create `*.so` files on Linux, `*.dylib` files on macOS, and `*.dll` files on Windows.

- `--crate-type=rlib`, `#[crate_type = "rlib"]` - A "Rust library" file will be produced. This is used as an intermediate artifact and can be thought of as a "static Rust library". These `rlib` files, unlike `staticlib` files, are interpreted by the compiler in future linkage. This essentially means that `rustc` will look for metadata in `rlib` files like it looks for metadata in dynamic libraries. This form of output is used to produce statically linked executables as well as `staticlib` outputs.

- `--crate-type=proc-macro`, `#[crate_type = "proc-macro"]` - The output produced is not specified, but if a `-L` path is provided to it then the compiler will recognize the output artifacts as a macro and it can be loaded for a program. Crates compiled with this crate type must only export procedural macros. The compiler will automatically set the `proc_macro` configuration option. The crates are always compiled with the same target that the compiler itself was built with. For example, if you are executing the compiler from Linux with an `x86_64` CPU, the target will be `x86_64-unknown-linux-gnu` even if the crate is a dependency of another crate being built for a different target.

Note that these outputs are stackable in the sense that if multiple are specified, then the compiler will produce each form of output at once without having to recompile. However, this only applies for outputs specified by the same method. If only `crate_type` attributes are specified, then they will all be built, but if one or more `--crate-type` command line flags are specified, then only those outputs will be built.

With all these different kinds of outputs, if crate A depends on crate B, then the compiler could find B in various different forms throughout the system. The only forms looked for by the compiler, however, are the `rlib` format and the dynamic library format. With these two options for a dependent library, the compiler must at some point make a choice between these two formats. With this in mind, the compiler follows these rules when determining what format of dependencies will be used:

1. If a static library is being produced, all upstream dependencies are required to be available in `rlib` formats. This requirement stems from the reason that a dynamic library cannot be converted into a static format.

   Note that it is impossible to link in native dynamic dependencies to a static library, and in this case warnings will be printed about all unlinked native dynamic dependencies.

2. If an `rlib` file is being produced, then there are no restrictions on what format the upstream dependencies are available in. It is simply required that all upstream dependencies be available for reading metadata from.

   The reason for this is that `rlib` files do not contain any of their upstream dependencies. It wouldn't be very efficient for all `rlib` files to contain a copy of `libstd.rlib`!

3. If an executable is being produced and the `-C prefer-dynamic` flag is not specified, then dependencies are first attempted to be found in the `rlib` format. If some dependencies are not available in an rlib format, then dynamic linking is attempted (see below).

4. If a dynamic library or an executable that is being dynamically linked is being produced, then the compiler will attempt to reconcile the available dependencies in either the rlib or dylib format to create a final product.

   A major goal of the compiler is to ensure that a library never appears more than once in any artifact. For example, if dynamic libraries B and C were each statically linked to library A, then a crate could not link to B and C together because there would be two copies of A. The compiler allows mixing the rlib and dylib formats, but this restriction must be satisfied.

   The compiler currently implements no method of hinting what format a library should be linked with. When dynamically linking, the compiler will attempt to maximize dynamic dependencies while still allowing some dependencies to be linked in via an rlib.

   For most situations, having all libraries available as a dylib is recommended if dynamically linking. For other situations, the compiler will emit a warning if it is unable to determine which formats to link each library with.

In general, `--crate-type=bin` or `--crate-type=lib` should be sufficient for all compilation needs, and the other options are just available if more fine-grained control is desired over the output format of a crate.

## Static and dynamic C runtimes

The standard library in general strives to support both statically linked and dynamically linked C runtimes for targets as appropriate. For example the `x86_64-pc-windows-msvc` and `x86_64-unknown-linux-musl` targets typically come with both runtimes and the user selects which one they'd like. All targets in the compiler have a default mode of linking to the C runtime. Typically targets are linked dynamically by default, but there are exceptions which are static by default such as:

- `arm-unknown-linux-musleabi`
- `arm-unknown-linux-musleabihf`
- `armv7-unknown-linux-musleabihf`
- `i686-unknown-linux-musl`
- `x86_64-unknown-linux-musl`

The linkage of the C runtime is configured to respect the `crt-static` target feature. These target features are typically configured from the command line via flags to the compiler itself. For example to enable a static runtime you would execute:

    rustc -C target-feature=+crt-static foo.rs

whereas to link dynamically to the C runtime you would execute:

    rustc -C target-feature=-crt-static foo.rs

Targets which do not support switching between linkage of the C runtime will ignore this flag. It's recommended to inspect the resulting binary to ensure that it's linked as you would expect after the compiler succeeds.

Crates may also learn about how the C runtime is being linked. Code on MSVC, for example, needs to be compiled differently (e.g. with `/MT` or `/MD`) depending on the runtime being linked. This is exported currently through the `target_feature` attribute (note this is a nightly feature):

    #[cfg(target_feature = "crt-static")]
    fn foo() {
        println!("the C runtime should be statically linked");
    }

    #[cfg(not(target_feature = "crt-static"))]
    fn foo() {
        println!("the C runtime should be dynamically linked");
    }

Also note that Cargo build scripts can learn about this feature through environment variables. In a build script you can detect the linkage via:

    use std::env;

    fn main() {
        let linkage = env::var("CARGO_CFG_TARGET_FEATURE").unwrap_or(String::new());

        if linkage.contains("crt-static") {
            println!("the C runtime will be statically linked");
        } else {
            println!("the C runtime will be dynamically linked");
        }
    }

To use this feature locally, you typically will use the `RUSTFLAGS` environment variable to specify flags to the compiler through Cargo. For example to compile a statically linked binary on MSVC you would execute:

    RUSTFLAGS='-C target-feature=+crt-static' cargo build --target x86_64-pc-windows-msvc

# Unsafety

Unsafe operations are those that can potentially violate the memory-safety guarantees of Rust's static semantics.

The following language level features cannot be used in the safe subset of Rust:

- Dereferencing a raw pointer.
- Reading or writing a mutable static variable.
- Reading a field of a `union`, or writing to a field of a union that isn't `Copy`.
- Calling an unsafe function (including an intrinsic or foreign function).
- Implementing an unsafe trait.

# Unsafe functions

Unsafe functions are functions that are not safe in all contexts and/or for all possible inputs. Such a function must be prefixed with the keyword `unsafe` and can only be called from an `unsafe` block or another `unsafe` function.

# Unsafe blocks

A block of code can be prefixed with the `unsafe` keyword, to permit calling `unsafe` functions or dereferencing raw pointers within a safe function.

When a programmer has sufficient conviction that a sequence of potentially unsafe operations is actually safe, they can encapsulate that sequence (taken as a whole) within an `unsafe` block. The compiler will consider uses of such code safe, in the surrounding context.

Unsafe blocks are used to wrap foreign libraries, make direct use of hardware or implement features not directly present in the language. For example, Rust provides the language features necessary to implement memory-safe concurrency in the language but the implementation of threads and message passing is in the standard library.

Rust's type system is a conservative approximation of the dynamic safety requirements, so in some cases there is a performance cost to using safe code. For example, a doubly-linked list is not a tree structure and can only be represented with reference-counted pointers in safe code. By using `unsafe` blocks to represent the reverse links as raw pointers, it can be implemented with only boxes.

## Behavior considered undefined

Rust code, including within `unsafe` blocks and `unsafe` functions is incorrect if it exhibits any of the behaviors in the following list. It is the programmer's responsibility when writing `unsafe` code that it is not possible to let `safe` code exhibit these behaviors.

Warning: The following list is not exhaustive. There is no formal model of Rust's semantics for what is and is not allowed in unsafe code, so there may be more behavior considered unsafe. The following list is just what we know for sure is undefined behavior. Please read the Rustonomicon before writing unsafe code.

- Data races.
- Dereferencing a null or dangling raw pointer.
- Unaligned pointer reading and writing outside of `read_unaligned` and `write_unaligned`.
- Reads of undef (uninitialized) memory.
- Breaking the pointer aliasing rules on accesses through raw pointers; a subset of the rules used by C.
- `&mut T` and `&T` follow LLVM’s scoped noalias model, except if the `&T` contains an `UnsafeCell<U>`.
- Mutating non-mutable data — that is, data reached through a shared reference or data owned by a `let` binding), unless that data is contained within an `UnsafeCell<U>`.
- Invoking undefined behavior via compiler intrinsics:
  - Indexing outside of the bounds of an object with `offset` with the exception of one byte past the end of the object.
  - Using `std::ptr::copy_nonoverlapping_memory`, a.k.a. the `memcpy32`and `memcpy64` intrinsics, on overlapping buffers.
- Invalid values in primitive types, even in private fields and locals:
  - Dangling or null references and boxes.
  - A value other than `false` (`0`) or `true` (`1`) in a `bool`.
  - A discriminant in an `enum` not included in the type definition.
  - A value in a `char` which is a surrogate or above `char::MAX`.
  - Non-UTF-8 byte sequences in a `str`.

## Behavior not considered `unsafe`

The Rust compiler does not consider the following behaviors unsafe, though a programmer may (should) find them undesirable, unexpected, or erroneous.

##### Deadlocks

##### Leaks of memory and other resources

##### Exiting without calling destructors

##### Exposing randomized base addresses through pointer leaks

##### Integer overflow

If a program contains arithmetic overflow, the programmer has made an error. In the following discussion, we maintain a distinction between arithmetic overflow and wrapping arithmetic. The first is erroneous, while the second is intentional.

When the programmer has enabled `debug_assert!` assertions (for example, by enabling a non-optimized build), implementations must insert dynamic checks that `panic` on overflow. Other kinds of builds may result in `panics` or silently wrapped values on overflow, at the implementation's discretion.

In the case of implicitly-wrapped overflow, implementations must provide well-defined (even if still considered erroneous) results by using two's complement overflow conventions.

The integral types provide inherent methods to allow programmers explicitly to perform wrapping arithmetic. For example, `i32::wrapping_add` provides two's complement, wrapping addition.

The standard library also provides a `Wrapping<T>` newtype which ensures all standard arithmetic operations for `T` have wrapping semantics.

See RFC 560 for error conditions, rationale, and more details about integer overflow.

# Influences

Rust is not a particularly original language, with design elements coming from a wide range of sources. Some of these are listed below (including elements that have since been removed):

- SML, OCaml: algebraic data types, pattern matching, type inference, semicolon statement separation
- C++: references, RAII, smart pointers, move semantics, monomorphization, memory model
- ML Kit, Cyclone: region based memory management
- Haskell (GHC): typeclasses, type families
- Newsqueak, Alef, Limbo: channels, concurrency
- Erlang: message passing, thread failure, linked thread failure, lightweight concurrency
- Swift: optional bindings
- Scheme: hygienic macros
- C#: attributes
- Ruby: block syntax
- NIL, Hermes: typestate
- Unicode Annex #31: identifier and pattern syntax

# As-yet-undocumented Features

Several accepted, stabilized, and implemented RFCs lack documentation in this reference, The Book, Rust by Example, or some combination of those three. Until we have written reference documentation for these features, we provide links to other sources of information about them. Therefore, expect this list to shrink!

- `libstd` facade
- Trait reform – some partial documentation exists (the use of `Self`), but not for everything: e.g. coherence and orphan rules.
- Flexible target specification - Some---but not all---flags are documented in Conditional compilation
- `dllimport` - one element mentioned but not explained at FFI attributes
- define `crt_link`
- `impl Trait`

# Glossary

### Abstract Syntax Tree

An ‘abstract syntax tree’, or ‘AST’, is an intermediate representation of the structure of the program when the compiler is compiling it.

### Alignment

The alignment of a value specifies what addresses values are preferred to start at. Always a power of two. References to a value must be aligned. More.

### Arity

Arity refers to the number of arguments a function or operator takes. For some examples, `f(2, 3)` and `g(4, 6)` have arity 2, while `h(8, 2, 6)` has arity 3. The `!` operator has arity 1.

### Array

An array, sometimes also called a fixed-size array or an inline array, is a value describing a collection of elements, each selected by an index that can be computed at run time by the program. It occupies a contiguous region of memory.

### Associated Item

An associated item is an item that is associated with another item. Associated items are defined in implementations and declared in traits. Only functions, constants, and type aliases can be associated.

### Bound

Bounds are constraints on a type or trait. For example, if a bound is placed on the argument a function takes, types passed to that function must abide by that constraint.

### Combinator

Combinators are higher-order functions that apply only functions and earlier defined combinators to provide a result from its arguments. They can be used to manage control flow in a modular fashion.

### Dispatch

Dispatch is the mechanism to determine which specific version of code is actually run when it involves polymorphism. Two major forms of dispatch are static dispatch and dynamic dispatch. While Rust favors static dispatch, it also supports dynamic dispatch through a mechanism called ‘trait objects’.

### Dynamically Sized Type

A dynamically sized type (DST) is a type without a statically known size or alignment.

### Expression

An expression is a combination of values, constants, variables, operators and functions that evaluate to a single value, with or without side-effects.

For example, `2 + (3 * 4)` is an expression that returns the value 14.

### Initialized

A variable is initialized if it has been assigned a value and hasn't since been moved from. All other memory locations are assumed to be initialized. Only unsafe Rust can create such a memory without initializing it.

### Nominal Types

Types that can be referred to by a path directly. Specifically enums, structs, unions, and trait objects.

### Object Safe Traits

Traits that can be used as trait objects. Only traits that follow specific rules are object safe.

### Prelude

Prelude, or The Rust Prelude, is a small collection of items - mostly traits - that are imported into every module of every crate. The traits in the prelude are pervasive.

### Size

The size of a value has two definitions.

The first is that it is how much memory must be allocated to store that value.

The second is that it is the offset in bytes between successive elements in an array with that item type.

It is a multiple of the alignment, including zero. The size can change depending on compiler version (as new optimizations are made) and target platform (similar to how `usize` varies per-platform).

More.

### Slice

A slice is dynamically-sized view into a contiguous sequence, written as `[T]`.

It is often seen in its borrowed forms, either mutable or shared. The shared slice type is `&[T]`, while the mutable slice type is `&mut [T]`, where `T` represents the element type.

### Statement

A statement is the smallest standalone element of a programming language that commands a computer to perform an action.

### String literal

A string literal is a string stored directly in the final binary, and so will be valid for the `'static` duration.

Its type is `'static` duration borrowed string slice, `&'static str`.

### String slice

A string slice is the most primitive string type in Rust, written as `str`. It is often seen in its borrowed forms, either mutable or shared. The shared string slice type is `&str`, while the mutable string slice type is `&mut str`.

Strings slices are always valid UTF-8.

### Trait

A trait is a language item that is used for describing the functionalities a type must provide. It allows a type to make certain promises about its behavior.

Generic functions and generic structs can use traits to constrain, or bound, the types they accept.