keyword_docs.rs.html -- source
Description: Source to the Rust file `library/std/src/keyword_docs.rs`.
Keywords: rust, rustlang, rust-lang

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    #[doc(keyword = "as")]
    //
    /// Cast between types, or rename an import.
    ///
    /// `as` is most commonly used to turn primitive types into other primitive types, but it has other
    /// uses that include turning pointers into addresses, addresses into pointers, and pointers into
    /// other pointers.
    ///
    /// ```rust
    /// let thing1: u8 = 89.0 as u8;
    /// assert_eq!('B' as u32, 66);
    /// assert_eq!(thing1 as char, 'Y');
    /// let thing2: f32 = thing1 as f32 + 10.5;
    /// assert_eq!(true as u8 + thing2 as u8, 100);
    /// ```
    ///
    /// In general, any cast that can be performed via ascribing the type can also be done using `as`,
    /// so instead of writing `let x: u32 = 123`, you can write `let x = 123 as u32` (Note: `let x: u32
    /// = 123` would be best in that situation). The same is not true in the other direction, however,
    /// explicitly using `as` allows a few more coercions that aren't allowed implicitly, such as
    /// changing the type of a raw pointer or turning closures into raw pointers.
    ///
    /// Other places `as` is used include as extra syntax for [`crate`] and `use`, to change the name
    /// something is imported as.
    ///
    /// For more information on what `as` is capable of, see the [Reference]
    ///
    /// [Reference]: ../reference/expressions/operator-expr.html#type-cast-expressions
    /// [`crate`]: keyword.crate.html
    mod as_keyword {}

    #[doc(keyword = "break")]
    //
    /// Exit early from a loop.
    ///
    /// When `break` is encountered, execution of the associated loop body is
    /// immediately terminated.
    ///
    /// ```rust
    /// let mut last = 0;
    ///
    /// for x in 1..100 {
    ///     if x > 12 {
    ///         break;
    ///     }
    ///     last = x;
    /// }
    ///
    /// assert_eq!(last, 12);
    /// println!("{}", last);
    /// ```
    ///
    /// A break expression is normally associated with the innermost loop enclosing the
    /// `break` but a label can be used to specify which enclosing loop is affected.
    ///
    ///```rust
    /// 'outer: for i in 1..=5 {
    ///     println!("outer iteration (i): {}", i);
    ///
    ///     '_inner: for j in 1..=200 {
    ///         println!("    inner iteration (j): {}", j);
    ///         if j >= 3 {
    ///             // breaks from inner loop, let's outer loop continue.
    ///             break;
    ///         }
    ///         if i >= 2 {
    ///             // breaks from outer loop, and directly to "Bye".
    ///             break 'outer;
    ///         }
    ///     }
    /// }
    /// println!("Bye.");
    ///```
    ///
    /// When associated with `loop`, a break expression may be used to return a value from that loop.
    /// This is only valid with `loop` and not with any other type of loop.
    /// If no value is specified, `break;` returns `()`.
    /// Every `break` within a loop must return the same type.
    ///
    /// ```rust
    /// 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);
    /// println!("{}", result);
    /// ```
    ///
    /// For more details consult the [Reference on "break expression"] and the [Reference on "break and
    /// loop values"].
    ///
    /// [Reference on "break expression"]: ../reference/expressions/loop-expr.html#break-expressions
    /// [Reference on "break and loop values"]:
    /// ../reference/expressions/loop-expr.html#break-and-loop-values
    mod break_keyword {}

    #[doc(keyword = "const")]
    //
    /// Compile-time constants and deterministic functions.
    ///
    /// Sometimes a certain value is used many times throughout a program, and it can become
    /// inconvenient to copy it over and over. What's more, it's not always possible or desirable to
    /// make it a variable that gets carried around to each function that needs it. In these cases, the
    /// `const` keyword provides a convenient alternative to code duplication.
    ///
    /// ```rust
    /// const THING: u32 = 0xABAD1DEA;
    ///
    /// let foo = 123 + THING;
    /// ```
    ///
    /// Constants must be explicitly typed, unlike with `let` you can't ignore its type and let the
    /// compiler figure it out. Any constant value can be defined in a const, which in practice happens
    /// to be most things that would be reasonable to have a constant (barring `const fn`s). For
    /// example, you can't have a File as a `const`.
    ///
    /// The only lifetime allowed in a constant is `'static`, which is the lifetime that encompasses
    /// all others in a Rust program. For example, if you wanted to define a constant string, it would
    /// look like this:
    ///
    /// ```rust
    /// const WORDS: &'static str = "hello rust!";
    /// ```
    ///
    /// Thanks to static lifetime elision, you usually don't have to explicitly use 'static:
    ///
    /// ```rust
    /// const WORDS: &str = "hello convenience!";
    /// ```
    ///
    /// `const` items looks remarkably similar to `static` items, which introduces some confusion as
    /// to which one should be used at which times. To put it simply, constants are inlined wherever
    /// they're used, making using them identical to simply replacing the name of the const with its
    /// value. Static variables on the other hand point to a single location in memory, which all
    /// accesses share. This means that, unlike with constants, they can't have destructors, and act as
    /// a single value across the entire codebase.
    ///
    /// Constants, as with statics, should always be in SCREAMING_SNAKE_CASE.
    ///
    /// The `const` keyword is also used in raw pointers in combination with `mut`, as seen in `*const
    /// T` and `*mut T`. More about that can be read at the [pointer] primitive part of the Rust docs.
    ///
    /// For more detail on `const`, see the [Rust Book] or the [Reference]
    ///
    /// [pointer]: primitive.pointer.html
    /// [Rust Book]:
    /// ../book/ch03-01-variables-and-mutability.html#differences-between-variables-and-constants
    /// [Reference]: ../reference/items/constant-items.html
    mod const_keyword {}

    #[doc(keyword = "continue")]
    //
    /// Skip to the next iteration of a loop.
    ///
    /// When `continue` is encountered, the current iteration is terminated, returning control to the
    /// loop head, typically continuing with the next iteration.
    ///
    ///```rust
    /// // Printing odd numbers by skipping even ones
    /// for number in 1..=10 {
    ///     if number % 2 == 0 {
    ///         continue;
    ///     }
    ///     println!("{}", number);
    /// }
    ///```
    ///
    /// Like `break`, `continue` is normally associated with the innermost enclosing loop, but labels
    /// may be used to specify the affected loop.
    ///
    ///```rust
    /// // Print Odd numbers under 30 with unit <= 5
    /// 'tens: for ten in 0..3 {
    ///     '_units: for unit in 0..=9 {
    ///         if unit % 2 == 0 {
    ///             continue;
    ///         }
    ///         if unit > 5 {
    ///             continue 'tens;
    ///         }
    ///         println!("{}", ten * 10 + unit);
    ///     }
    /// }
    ///```
    ///
    /// See [continue expressions] from the reference for more details.
    ///
    /// [continue expressions]: ../reference/expressions/loop-expr.html#continue-expressions
    mod continue_keyword {}

    #[doc(keyword = "crate")]
    //
    /// A Rust binary or library.
    ///
    /// The primary use of the `crate` keyword is as a part of `extern crate` declarations, which are
    /// used to specify a dependency on a crate external to the one it's declared in. Crates are the
    /// fundamental compilation unit of Rust code, and can be seen as libraries or projects. More can
    /// be read about crates in the [Reference].
    ///
    /// ```rust ignore
    /// extern crate rand;
    /// extern crate my_crate as thing;
    /// extern crate std; // implicitly added to the root of every Rust project
    /// ```
    ///
    /// The `as` keyword can be used to change what the crate is referred to as in your project. If a
    /// crate name includes a dash, it is implicitly imported with the dashes replaced by underscores.
    ///
    /// `crate` can also be used as in conjunction with `pub` to signify that the item it's attached to
    /// is public only to other members of the same crate it's in.
    ///
    /// ```rust
    /// # #[allow(unused_imports)]
    /// pub(crate) use std::io::Error as IoError;
    /// pub(crate) enum CoolMarkerType { }
    /// pub struct PublicThing {
    ///     pub(crate) semi_secret_thing: bool,
    /// }
    /// ```
    ///
    /// `crate` is also used to represent the absolute path of a module, where `crate` refers to the
    /// root of the current crate. For instance, `crate::foo::bar` refers to the name `bar` inside the
    /// module `foo`, from anywhere else in the same crate.
    ///
    /// [Reference]: ../reference/items/extern-crates.html
    mod crate_keyword {}

    #[doc(keyword = "else")]
    //
    /// What expression to evaluate when an [`if`] condition evaluates to [`false`].
    ///
    /// `else` expressions are optional. When no else expressions are supplied it is assumed to evaluate
    /// to the unit type `()`.
    ///
    /// The type that the `else` blocks evaluate to must be compatible with the type that the `if` block
    /// evaluates to.
    ///
    /// As can be seen below, `else` must be followed by either: `if`, `if let`, or a block `{}` and it
    /// will return the value of that expression.
    ///
    /// ```rust
    /// let result = if true == false {
    ///     "oh no"
    /// } else if "something" == "other thing" {
    ///     "oh dear"
    /// } else if let Some(200) = "blarg".parse::<i32>().ok() {
    ///     "uh oh"
    /// } else {
    ///     println!("Sneaky side effect.");
    ///     "phew, nothing's broken"
    /// };
    /// ```
    ///
    /// Here's another example but here we do not try and return an expression:
    ///
    /// ```rust
    /// if true == false {
    ///     println!("oh no");
    /// } else if "something" == "other thing" {
    ///     println!("oh dear");
    /// } else if let Some(200) = "blarg".parse::<i32>().ok() {
    ///     println!("uh oh");
    /// } else {
    ///     println!("phew, nothing's broken");
    /// }
    /// ```
    ///
    /// The above is _still_ an expression but it will always evaluate to `()`.
    ///
    /// There is possibly no limit to the number of `else` blocks that could follow an `if` expression
    /// however if you have several then a [`match`] expression might be preferable.
    ///
    /// Read more about control flow in the [Rust Book].
    ///
    /// [Rust Book]: ../book/ch03-05-control-flow.html#handling-multiple-conditions-with-else-if
    /// [`match`]: keyword.match.html
    /// [`false`]: keyword.false.html
    /// [`if`]: keyword.if.html
    mod else_keyword {}

    #[doc(keyword = "enum")]
    //
    /// A type that can be any one of several variants.
    ///
    /// Enums in Rust are similar to those of other compiled languages like C, but have important
    /// differences that make them considerably more powerful. What Rust calls enums are more commonly
    /// known as [Algebraic Data Types][ADT] if you're coming from a functional programming background.
    /// The important detail is that each enum variant can have data to go along with it.
    ///
    /// ```rust
    /// # struct Coord;
    /// enum SimpleEnum {
    ///     FirstVariant,
    ///     SecondVariant,
    ///     ThirdVariant,
    /// }
    ///
    /// enum Location {
    ///     Unknown,
    ///     Anonymous,
    ///     Known(Coord),
    /// }
    ///
    /// enum ComplexEnum {
    ///     Nothing,
    ///     Something(u32),
    ///     LotsOfThings {
    ///         usual_struct_stuff: bool,
    ///         blah: String,
    ///     }
    /// }
    ///
    /// enum EmptyEnum { }
    /// ```
    ///
    /// The first enum shown is the usual kind of enum you'd find in a C-style language. The second
    /// shows off a hypothetical example of something storing location data, with `Coord` being any
    /// other type that's needed, for example a struct. The third example demonstrates the kind of
    /// data a variant can store, ranging from nothing, to a tuple, to an anonymous struct.
    ///
    /// Instantiating enum variants involves explicitly using the enum's name as its namespace,
    /// followed by one of its variants. `SimpleEnum::SecondVariant` would be an example from above.
    /// When data follows along with a variant, such as with rust's built-in [`Option`] type, the data
    /// is added as the type describes, for example `Option::Some(123)`. The same follows with
    /// struct-like variants, with things looking like `ComplexEnum::LotsOfThings { usual_struct_stuff:
    /// true, blah: "hello!".to_string(), }`. Empty Enums are similar to () in that they cannot be
    /// instantiated at all, and are used mainly to mess with the type system in interesting ways.
    ///
    /// For more information, take a look at the [Rust Book] or the [Reference]
    ///
    /// [ADT]: https://en.wikipedia.org/wiki/Algebraic_data_type
    /// [Rust Book]: ../book/ch06-01-defining-an-enum.html
    /// [Reference]: ../reference/items/enumerations.html
    mod enum_keyword {}

    #[doc(keyword = "extern")]
    //
    /// Link to or import external code.
    ///
    /// The `extern` keyword is used in two places in Rust. One is in conjunction with the [`crate`]
    /// keyword to make your Rust code aware of other Rust crates in your project, i.e., `extern crate
    /// lazy_static;`. The other use is in foreign function interfaces (FFI).
    ///
    /// `extern` is used in two different contexts within FFI. The first is in the form of external
    /// blocks, for declaring function interfaces that Rust code can call foreign code by.
    ///
    /// ```rust ignore
    /// #[link(name = "my_c_library")]
    /// extern "C" {
    ///     fn my_c_function(x: i32) -> bool;
    /// }
    /// ```
    ///
    /// This code would attempt to link with `libmy_c_library.so` on unix-like systems and
    /// `my_c_library.dll` on Windows at runtime, and panic if it can't find something to link to. Rust
    /// code could then use `my_c_function` as if it were any other unsafe Rust function. Working with
    /// non-Rust languages and FFI is inherently unsafe, so wrappers are usually built around C APIs.
    ///
    /// The mirror use case of FFI is also done via the `extern` keyword:
    ///
    /// ```rust
    /// #[no_mangle]
    /// pub extern fn callable_from_c(x: i32) -> bool {
    ///     x % 3 == 0
    /// }
    /// ```
    ///
    /// If compiled as a dylib, the resulting .so could then be linked to from a C library, and the
    /// function could be used as if it was from any other library.
    ///
    /// For more information on FFI, check the [Rust book] or the [Reference].
    ///
    /// [Rust book]:
    /// ../book/ch19-01-unsafe-rust.html#using-extern-functions-to-call-external-code
    /// [Reference]: ../reference/items/external-blocks.html
    mod extern_keyword {}

    #[doc(keyword = "false")]
    //
    /// A value of type [`bool`] representing logical **false**.
    ///
    /// `false` is the logical opposite of [`true`].
    ///
    /// See the documentation for [`true`] for more information.
    ///
    /// [`true`]: keyword.true.html
    mod false_keyword {}

    #[doc(keyword = "fn")]
    //
    /// A function or function pointer.
    ///
    /// Functions are the primary way code is executed within Rust. Function blocks, usually just
    /// called functions, can be defined in a variety of different places and be assigned many
    /// different attributes and modifiers.
    ///
    /// Standalone functions that just sit within a module not attached to anything else are common,
    /// but most functions will end up being inside [`impl`] blocks, either on another type itself, or
    /// as a trait impl for that type.
    ///
    /// ```rust
    /// fn standalone_function() {
    ///     // code
    /// }
    ///
    /// pub fn public_thing(argument: bool) -> String {
    ///     // code
    ///     # "".to_string()
    /// }
    ///
    /// struct Thing {
    ///     foo: i32,
    /// }
    ///
    /// impl Thing {
    ///     pub fn new() -> Self {
    ///         Self {
    ///             foo: 42,
    ///         }
    ///     }
    /// }
    /// ```
    ///
    /// In addition to presenting fixed types in the form of `fn name(arg: type, ..) -> return_type`,
    /// functions can also declare a list of type parameters along with trait bounds that they fall
    /// into.
    ///
    /// ```rust
    /// fn generic_function<T: Clone>(x: T) -> (T, T, T) {
    ///     (x.clone(), x.clone(), x.clone())
    /// }
    ///
    /// fn generic_where<T>(x: T) -> T
    ///     where T: std::ops::Add<Output = T> + Copy
    /// {
    ///     x + x + x
    /// }
    /// ```
    ///
    /// Declaring trait bounds in the angle brackets is functionally identical to using a `where`
    /// clause. It's up to the programmer to decide which works better in each situation, but `where`
    /// tends to be better when things get longer than one line.
    ///
    /// Along with being made public via `pub`, `fn` can also have an [`extern`] added for use in
    /// FFI.
    ///
    /// For more information on the various types of functions and how they're used, consult the [Rust
    /// book] or the [Reference].
    ///
    /// [`impl`]: keyword.impl.html
    /// [`extern`]: keyword.extern.html
    /// [Rust book]: ../book/ch03-03-how-functions-work.html
    /// [Reference]: ../reference/items/functions.html
    mod fn_keyword {}

    #[doc(keyword = "for")]
    //
    /// Iteration with [`in`], trait implementation with [`impl`], or [higher-ranked trait bounds]
    /// (`for<'a>`).
    ///
    /// The `for` keyword is used in many syntactic locations:
    ///
    /// * `for` is used in for-in-loops (see below).
    /// * `for` is used when implementing traits as in `impl Trait for Type` (see [`impl`] for more info
    ///   on that).
    /// * `for` is also used for [higher-ranked trait bounds] as in `for<'a> &'a T: PartialEq<i32>`.
    ///
    /// for-in-loops, or to be more precise, iterator loops, are a simple syntactic sugar over a common
    /// practice within Rust, which is to loop over anything that implements [`IntoIterator`] until the
    /// iterator returned by `.into_iter()` returns `None` (or the loop body uses `break`).
    ///
    /// ```rust
    /// for i in 0..5 {
    ///     println!("{}", i * 2);
    /// }
    ///
    /// for i in std::iter::repeat(5) {
    ///     println!("turns out {} never stops being 5", i);
    ///     break; // would loop forever otherwise
    /// }
    ///
    /// 'outer: for x in 5..50 {
    ///     for y in 0..10 {
    ///         if x == y {
    ///             break 'outer;
    ///         }
    ///     }
    /// }
    /// ```
    ///
    /// As shown in the example above, `for` loops (along with all other loops) can be tagged, using
    /// similar syntax to lifetimes (only visually similar, entirely distinct in practice). Giving the
    /// same tag to `break` breaks the tagged loop, which is useful for inner loops. It is definitely
    /// not a goto.
    ///
    /// A `for` loop expands as shown:
    ///
    /// ```rust
    /// # fn code() { }
    /// # let iterator = 0..2;
    /// for loop_variable in iterator {
    ///     code()
    /// }
    /// ```
    ///
    /// ```rust
    /// # fn code() { }
    /// # let iterator = 0..2;
    /// {
    ///     let mut _iter = std::iter::IntoIterator::into_iter(iterator);
    ///     loop {
    ///         match _iter.next() {
    ///             Some(loop_variable) => {
    ///                 code()
    ///             },
    ///             None => break,
    ///         }
    ///     }
    /// }
    /// ```
    ///
    /// More details on the functionality shown can be seen at the [`IntoIterator`] docs.
    ///
    /// For more information on for-loops, see the [Rust book] or the [Reference].
    ///
    /// [`in`]: keyword.in.html
    /// [`impl`]: keyword.impl.html
    /// [higher-ranked trait bounds]: ../reference/trait-bounds.html#higher-ranked-trait-bounds
    /// [Rust book]:
    /// ../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
    /// [Reference]: ../reference/expressions/loop-expr.html#iterator-loops
    mod for_keyword {}

    #[doc(keyword = "if")]
    //
    /// Evaluate a block if a condition holds.
    ///
    /// `if` is a familiar construct to most programmers, and is the main way you'll often do logic in
    /// your code. However, unlike in most languages, `if` blocks can also act as expressions.
    ///
    /// ```rust
    /// # let rude = true;
    /// if 1 == 2 {
    ///     println!("whoops, mathematics broke");
    /// } else {
    ///     println!("everything's fine!");
    /// }
    ///
    /// let greeting = if rude {
    ///     "sup nerd."
    /// } else {
    ///     "hello, friend!"
    /// };
    ///
    /// if let Ok(x) = "123".parse::<i32>() {
    ///     println!("{} double that and you get {}!", greeting, x * 2);
    /// }
    /// ```
    ///
    /// Shown above are the three typical forms an `if` block comes in. First is the usual kind of
    /// thing you'd see in many languages, with an optional `else` block. Second uses `if` as an
    /// expression, which is only possible if all branches return the same type. An `if` expression can
    /// be used everywhere you'd expect. The third kind of `if` block is an `if let` block, which
    /// behaves similarly to using a `match` expression:
    ///
    /// ```rust
    /// if let Some(x) = Some(123) {
    ///     // code
    ///     # let _ = x;
    /// } else {
    ///     // something else
    /// }
    ///
    /// match Some(123) {
    ///     Some(x) => {
    ///         // code
    ///         # let _ = x;
    ///     },
    ///     _ => {
    ///         // something else
    ///     },
    /// }
    /// ```
    ///
    /// Each kind of `if` expression can be mixed and matched as needed.
    ///
    /// ```rust
    /// if true == false {
    ///     println!("oh no");
    /// } else if "something" == "other thing" {
    ///     println!("oh dear");
    /// } else if let Some(200) = "blarg".parse::<i32>().ok() {
    ///     println!("uh oh");
    /// } else {
    ///     println!("phew, nothing's broken");
    /// }
    /// ```
    ///
    /// The `if` keyword is used in one other place in Rust, namely as a part of pattern matching
    /// itself, allowing patterns such as `Some(x) if x > 200` to be used.
    ///
    /// For more information on `if` expressions, see the [Rust book] or the [Reference].
    ///
    /// [Rust book]: ../book/ch03-05-control-flow.html#if-expressions
    /// [Reference]: ../reference/expressions/if-expr.html
    mod if_keyword {}

    #[doc(keyword = "impl")]
    //
    /// Implement some functionality for a type.
    ///
    /// The `impl` keyword is primarily used to define implementations on types. Inherent
    /// implementations are standalone, while trait implementations are used to implement traits for
    /// types, or other traits.
    ///
    /// Functions and consts can both be defined in an implementation. A function defined in an
    /// `impl` block can be standalone, meaning it would be called like `Foo::bar()`. If the function
    /// takes `self`, `&self`, or `&mut self` as its first argument, it can also be called using
    /// method-call syntax, a familiar feature to any object oriented programmer, like `foo.bar()`.
    ///
    /// ```rust
    /// struct Example {
    ///     number: i32,
    /// }
    ///
    /// impl Example {
    ///     fn boo() {
    ///         println!("boo! Example::boo() was called!");
    ///     }
    ///
    ///     fn answer(&mut self) {
    ///         self.number += 42;
    ///     }
    ///
    ///     fn get_number(&self) -> i32 {
    ///         self.number
    ///     }
    /// }
    ///
    /// trait Thingy {
    ///     fn do_thingy(&self);
    /// }
    ///
    /// impl Thingy for Example {
    ///     fn do_thingy(&self) {
    ///         println!("doing a thing! also, number is {}!", self.number);
    ///     }
    /// }
    /// ```
    ///
    /// For more information on implementations, see the [Rust book][book1] or the [Reference].
    ///
    /// The other use of the `impl` keyword is in `impl Trait` syntax, which can be seen as a shorthand
    /// for "a concrete type that implements this trait". Its primary use is working with closures,
    /// which have type definitions generated at compile time that can't be simply typed out.
    ///
    /// ```rust
    /// fn thing_returning_closure() -> impl Fn(i32) -> bool {
    ///     println!("here's a closure for you!");
    ///     |x: i32| x % 3 == 0
    /// }
    /// ```
    ///
    /// For more information on `impl Trait` syntax, see the [Rust book][book2].
    ///
    /// [book1]: ../book/ch05-03-method-syntax.html
    /// [Reference]: ../reference/items/implementations.html
    /// [book2]: ../book/ch10-02-traits.html#returning-types-that-implement-traits
    mod impl_keyword {}

    #[doc(keyword = "in")]
    //
    /// Iterate over a series of values with [`for`].
    ///
    /// The expression immediately following `in` must implement the [`IntoIterator`] trait.
    ///
    /// ## Literal Examples:
    ///
    ///    * `for _ **in** 1..3 {}` - Iterate over an exclusive range up to but excluding 3.
    ///    * `for _ **in** 1..=3 {}` - Iterate over an inclusive range up to and including 3.
    ///
    /// (Read more about [range patterns])
    ///
    /// [`IntoIterator`]: ../book/ch13-04-performance.html
    /// [range patterns]: ../reference/patterns.html?highlight=range#range-patterns
    /// [`for`]: keyword.for.html
    mod in_keyword {}

    #[doc(keyword = "let")]
    //
    /// Bind a value to a variable.
    ///
    /// The primary use for the `let` keyword is in `let` statements, which are used to introduce a new
    /// set of variables into the current scope, as given by a pattern.
    ///
    /// ```rust
    /// # #![allow(unused_assignments)]
    /// let thing1: i32 = 100;
    /// let thing2 = 200 + thing1;
    ///
    /// let mut changing_thing = true;
    /// changing_thing = false;
    ///
    /// let (part1, part2) = ("first", "second");
    ///
    /// struct Example {
    ///     a: bool,
    ///     b: u64,
    /// }
    ///
    /// let Example { a, b: _ } = Example {
    ///     a: true,
    ///     b: 10004,
    /// };
    /// assert!(a);
    /// ```
    ///
    /// The pattern is most commonly a single variable, which means no pattern matching is done and
    /// the expression given is bound to the variable. Apart from that, patterns used in `let` bindings
    /// can be as complicated as needed, given that the pattern is exhaustive. See the [Rust
    /// book][book1] for more information on pattern matching. The type of the pattern is optionally
    /// given afterwards, but if left blank is automatically inferred by the compiler if possible.
    ///
    /// Variables in Rust are immutable by default, and require the `mut` keyword to be made mutable.
    ///
    /// Multiple variables can be defined with the same name, known as shadowing. This doesn't affect
    /// the original variable in any way beyond being unable to directly access it beyond the point of
    /// shadowing. It continues to remain in scope, getting dropped only when it falls out of scope.
    /// Shadowed variables don't need to have the same type as the variables shadowing them.
    ///
    /// ```rust
    /// let shadowing_example = true;
    /// let shadowing_example = 123.4;
    /// let shadowing_example = shadowing_example as u32;
    /// let mut shadowing_example = format!("cool! {}", shadowing_example);
    /// shadowing_example += " something else!"; // not shadowing
    /// ```
    ///
    /// Other places the `let` keyword is used include along with [`if`], in the form of `if let`
    /// expressions. They're useful if the pattern being matched isn't exhaustive, such as with
    /// enumerations. `while let` also exists, which runs a loop with a pattern matched value until
    /// that pattern can't be matched.
    ///
    /// For more information on the `let` keyword, see the [Rust book][book2] or the [Reference]
    ///
    /// [book1]: ../book/ch06-02-match.html
    /// [`if`]: keyword.if.html
    /// [book2]: ../book/ch18-01-all-the-places-for-patterns.html#let-statements
    /// [Reference]: ../reference/statements.html#let-statements
    mod let_keyword {}

    #[doc(keyword = "while")]
    //
    /// Loop while a condition is upheld.
    ///
    /// A `while` expression is used for predicate loops. The `while` expression runs the conditional
    /// expression before running the loop body, then runs the loop body if the conditional
    /// expression evaluates to `true`, or exits the loop otherwise.
    ///
    /// ```rust
    /// let mut counter = 0;
    ///
    /// while counter < 10 {
    ///     println!("{}", counter);
    ///     counter += 1;
    /// }
    /// ```
    ///
    /// Like the [`for`] expression, we can use `break` and `continue`. A `while` expression
    /// cannot break with a value and always evaluates to `()` unlike [`loop`].
    ///
    /// ```rust
    /// let mut i = 1;
    ///
    /// while i < 100 {
    ///     i *= 2;
    ///     if i == 64 {
    ///         break; // Exit when `i` is 64.
    ///     }
    /// }
    /// ```
    ///
    /// As `if` expressions have their pattern matching variant in `if let`, so too do `while`
    /// expressions with `while let`. The `while let` expression matches the pattern against the
    /// expression, then runs the loop body if pattern matching succeeds, or exits the loop otherwise.
    /// We can use `break` and `continue` in `while let` expressions just like in `while`.
    ///
    /// ```rust
    /// let mut counter = Some(0);
    ///
    /// while let Some(i) = counter {
    ///     if i == 10 {
    ///         counter = None;
    ///     } else {
    ///         println!("{}", i);
    ///         counter = Some (i + 1);
    ///     }
    /// }
    /// ```
    ///
    /// For more information on `while` and loops in general, see the [reference].
    ///
    /// [`for`]: keyword.for.html
    /// [`loop`]: keyword.loop.html
    /// [reference]: ../reference/expressions/loop-expr.html#predicate-loops
    mod while_keyword {}

    #[doc(keyword = "loop")]
    //
    /// Loop indefinitely.
    ///
    /// `loop` is used to define the simplest kind of loop supported in Rust. It runs the code inside
    /// it until the code uses `break` or the program exits.
    ///
    /// ```rust
    /// loop {
    ///     println!("hello world forever!");
    ///     # break;
    /// }
    ///
    /// let mut i = 1;
    /// loop {
    ///     println!("i is {}", i);
    ///     if i > 100 {
    ///         break;
    ///     }
    ///     i *= 2;
    /// }
    /// assert_eq!(i, 128);
    /// ```
    ///
    /// Unlike the other kinds of loops in Rust (`while`, `while let`, and `for`), loops can be used as
    /// expressions that return values via `break`.
    ///
    /// ```rust
    /// let mut i = 1;
    /// let something = loop {
    ///     i *= 2;
    ///     if i > 100 {
    ///         break i;
    ///     }
    /// };
    /// assert_eq!(something, 128);
    /// ```
    ///
    /// Every `break` in a loop has to have the same type. When it's not explicitly giving something,
    /// `break;` returns `()`.
    ///
    /// For more information on `loop` and loops in general, see the [Reference].
    ///
    /// [Reference]: ../reference/expressions/loop-expr.html
    mod loop_keyword {}

    #[doc(keyword = "match")]
    //
    /// Control flow based on pattern matching.
    ///
    /// `match` can be used to run code conditionally. Every pattern must
    /// be handled exhaustively either explicitly or by using wildcards like
    /// `_` in the `match`. Since `match` is an expression, values can also be
    /// returned.
    ///
    /// ```rust
    /// let opt = Option::None::<usize>;
    /// let x = match opt {
    ///     Some(int) => int,
    ///     None => 10,
    /// };
    /// assert_eq!(x, 10);
    ///
    /// let a_number = Option::Some(10);
    /// match a_number {
    ///     Some(x) if x <= 5 => println!("0 to 5 num = {}", x),
    ///     Some(x @ 6..=10) => println!("6 to 10 num = {}", x),
    ///     None => panic!(),
    ///     // all other numbers
    ///     _ => panic!(),
    /// }
    /// ```
    ///
    /// `match` can be used to gain access to the inner members of an enum
    /// and use them directly.
    ///
    /// ```rust
    /// enum Outer {
    ///     Double(Option<u8>, Option<String>),
    ///     Single(Option<u8>),
    ///     Empty
    /// }
    ///
    /// let get_inner = Outer::Double(None, Some(String::new()));
    /// match get_inner {
    ///     Outer::Double(None, Some(st)) => println!("{}", st),
    ///     Outer::Single(opt) => println!("{:?}", opt),
    ///     _ => panic!(),
    /// }
    /// ```
    ///
    /// For more information on `match` and matching in general, see the [Reference].
    ///
    /// [Reference]: ../reference/expressions/match-expr.html
    mod match_keyword {}

    #[doc(keyword = "mod")]
    //
    /// Organize code into [modules].
    ///
    /// Use `mod` to create new [modules] to encapsulate code, including other
    /// modules:
    ///
    /// ```
    /// mod foo {
    ///     mod bar {
    ///         type MyType = (u8, u8);
    ///         fn baz() {}
    ///     }
    /// }
    /// ```
    ///
    /// Like [`struct`]s and [`enum`]s, a module and its content are private by
    /// default, unaccessible to code outside of the module.
    ///
    /// To learn more about allowing access, see the documentation for the [`pub`]
    /// keyword.
    ///
    /// [`enum`]: keyword.enum.html
    /// [`pub`]: keyword.pub.html
    /// [`struct`]: keyword.struct.html
    /// [modules]: ../reference/items/modules.html
    mod mod_keyword {}

    #[doc(keyword = "move")]
    //
    /// Capture a [closure]'s environment by value.
    ///
    /// `move` converts any variables captured by reference or mutable reference
    /// to owned by value variables.
    ///
    /// ```rust
    /// let capture = "hello";
    /// let closure = move || {
    ///     println!("rust says {}", capture);
    /// };
    /// ```
    ///
    /// 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:
    ///
    /// ```rust
    /// fn create_fn() -> impl Fn() {
    ///     let text = "Fn".to_owned();
    ///
    ///     move || println!("This is a: {}", text)
    /// }
    ///
    ///     let fn_plain = create_fn();
    ///
    ///     fn_plain();
    /// ```
    ///
    /// `move` is often used when [threads] are involved.
    ///
    /// ```rust
    /// let x = 5;
    ///
    /// std::thread::spawn(move || {
    ///     println!("captured {} by value", x)
    /// }).join().unwrap();
    ///
    /// // x is no longer available
    /// ```
    ///
    /// `move` is also valid before an async block.
    ///
    /// ```rust
    /// let capture = "hello";
    /// let block = async move {
    ///     println!("rust says {} from async block", capture);
    /// };
    /// ```
    ///
    /// For more information on the `move` keyword, see the [closure]'s section
    /// of the Rust book or the [threads] section
    ///
    /// [closure]: ../book/ch13-01-closures.html
    /// [threads]: ../book/ch16-01-threads.html#using-move-closures-with-threads
    mod move_keyword {}

    #[doc(keyword = "mut")]
    //
    /// A mutable variable, reference, or pointer.
    ///
    /// `mut` can be used in several situations. The first is mutable variables,
    /// which can be used anywhere you can bind a value to a variable name. Some
    /// examples:
    ///
    /// ```rust
    /// // A mutable variable in the parameter list of a function.
    /// fn foo(mut x: u8, y: u8) -> u8 {
    ///     x += y;
    ///     x
    /// }
    ///
    /// // Modifying a mutable variable.
    /// # #[allow(unused_assignments)]
    /// let mut a = 5;
    /// a = 6;
    ///
    /// assert_eq!(foo(3, 4), 7);
    /// assert_eq!(a, 6);
    /// ```
    ///
    /// The second is mutable references. They can be created from `mut` variables
    /// and must be unique: no other variables can have a mutable reference, nor a
    /// shared reference.
    ///
    /// ```rust
    /// // Taking a mutable reference.
    /// fn push_two(v: &mut Vec<u8>) {
    ///     v.push(2);
    /// }
    ///
    /// // A mutable reference cannot be taken to a non-mutable variable.
    /// let mut v = vec![0, 1];
    /// // Passing a mutable reference.
    /// push_two(&mut v);
    ///
    /// assert_eq!(v, vec![0, 1, 2]);
    /// ```
    ///
    /// ```rust,compile_fail,E0502
    /// let mut v = vec![0, 1];
    /// let mut_ref_v = &mut v;
    /// ##[allow(unused)]
    /// let ref_v = &v;
    /// mut_ref_v.push(2);
    /// ```
    ///
    /// Mutable raw pointers work much like mutable references, with the added
    /// possibility of not pointing to a valid object. The syntax is `*mut Type`.
    ///
    /// More information on mutable references and pointers can be found in```
    /// [Reference].
    ///
    /// [Reference]: ../reference/types/pointer.html#mutable-references-mut
    mod mut_keyword {}

    #[doc(keyword = "pub")]
    //
    /// Make an item visible to others.
    ///
    /// The keyword `pub` makes any module, function, or data structure accessible from inside
    /// of external modules. The `pub` keyword may also be used in a `use` declaration to re-export
    /// an identifier from a namespace.
    ///
    /// For more information on the `pub` keyword, please see the visibility section
    /// of the [reference] and for some examples, see [Rust by Example].
    ///
    /// [reference]:../reference/visibility-and-privacy.html?highlight=pub#visibility-and-privacy
    /// [Rust by Example]:../rust-by-example/mod/visibility.html
    mod pub_keyword {}

    #[doc(keyword = "ref")]
    //
    /// Bind by reference during pattern matching.
    ///
    /// `ref` annotates pattern bindings to make them borrow rather than move.
    /// It is **not** a part of the pattern as far as matching is concerned: it does
    /// not affect *whether* a value is matched, only *how* it is matched.
    ///
    /// By default, [`match`] statements consume all they can, which can sometimes
    /// be a problem, when you don't really need the value to be moved and owned:
    ///
    /// ```compile_fail,E0382
    /// let maybe_name = Some(String::from("Alice"));
    /// // The variable 'maybe_name' is consumed here ...
    /// match maybe_name {
    ///     Some(n) => println!("Hello, {}", n),
    ///     _ => println!("Hello, world"),
    /// }
    /// // ... and is now unavailable.
    /// println!("Hello again, {}", maybe_name.unwrap_or("world".into()));
    /// ```
    ///
    /// Using the `ref` keyword, the value is only borrowed, not moved, making it
    /// available for use after the [`match`] statement:
    ///
    /// ```
    /// let maybe_name = Some(String::from("Alice"));
    /// // Using `ref`, the value is borrowed, not moved ...
    /// match maybe_name {
    ///     Some(ref n) => println!("Hello, {}", n),
    ///     _ => println!("Hello, world"),
    /// }
    /// // ... so it's available here!
    /// println!("Hello again, {}", maybe_name.unwrap_or("world".into()));
    /// ```
    ///
    /// # `&` vs `ref`
    ///
    /// - `&` denotes that your pattern expects a reference to an object. Hence `&`
    /// is a part of said pattern: `&Foo` matches different objects than `Foo` does.
    ///
    /// - `ref` indicates that you want a reference to an unpacked value. It is not
    /// matched against: `Foo(ref foo)` matches the same objects as `Foo(foo)`.
    ///
    /// See also the [Reference] for more information.
    ///
    /// [`match`]: keyword.match.html
    /// [Reference]: ../reference/patterns.html#identifier-patterns
    mod ref_keyword {}

    #[doc(keyword = "return")]
    //
    /// Return a value from a function.
    ///
    /// A `return` marks the end of an execution path in a function:
    ///
    /// ```
    /// fn foo() -> i32 {
    ///     return 3;
    /// }
    /// assert_eq!(foo(), 3);
    /// ```
    ///
    /// `return` is not needed when the returned value is the last expression in the
    /// function. In this case the `;` is omitted:
    ///
    /// ```
    /// fn foo() -> i32 {
    ///     3
    /// }
    /// assert_eq!(foo(), 3);
    /// ```
    ///
    /// `return` returns from the function immediately (an "early return"):
    ///
    /// ```no_run
    /// use std::fs::File;
    /// use std::io::{Error, ErrorKind, Read, Result};
    ///
    /// fn main() -> Result<()> {
    ///     let mut file = match File::open("foo.txt") {
    ///         Ok(f) => f,
    ///         Err(e) => return Err(e),
    ///     };
    ///
    ///     let mut contents = String::new();
    ///     let size = match file.read_to_string(&mut contents) {
    ///         Ok(s) => s,
    ///         Err(e) => return Err(e),
    ///     };
    ///
    ///     if contents.contains("impossible!") {
    ///         return Err(Error::new(ErrorKind::Other, "oh no!"));
    ///     }
    ///
    ///     if size > 9000 {
    ///         return Err(Error::new(ErrorKind::Other, "over 9000!"));
    ///     }
    ///
    ///     assert_eq!(contents, "Hello, world!");
    ///     Ok(())
    /// }
    /// ```
    mod return_keyword {}

    #[doc(keyword = "self")]
    //
    /// The receiver of a method, or the current module.
    ///
    /// `self` is used in two situations: referencing the current module and marking
    /// the receiver of a method.
    ///
    /// In paths, `self` can be used to refer to the current module, either in a
    /// [`use`] statement or in a path to access an element:
    ///
    /// ```
    /// # #![allow(unused_imports)]
    /// use std::io::{self, Read};
    /// ```
    ///
    /// Is functionally the same as:
    ///
    /// ```
    /// # #![allow(unused_imports)]
    /// use std::io;
    /// use std::io::Read;
    /// ```
    ///
    /// Using `self` to access an element in the current module:
    ///
    /// ```
    /// # #![allow(dead_code)]
    /// # fn main() {}
    /// fn foo() {}
    /// fn bar() {
    ///     self::foo()
    /// }
    /// ```
    ///
    /// `self` as the current receiver for a method allows to omit the parameter
    /// type most of the time. With the exception of this particularity, `self` is
    /// used much like any other parameter:
    ///
    /// ```
    /// struct Foo(i32);
    ///
    /// impl Foo {
    ///     // No `self`.
    ///     fn new() -> Self {
    ///         Self(0)
    ///     }
    ///
    ///     // Consuming `self`.
    ///     fn consume(self) -> Self {
    ///         Self(self.0 + 1)
    ///     }
    ///
    ///     // Borrowing `self`.
    ///     fn borrow(&self) -> &i32 {
    ///         &self.0
    ///     }
    ///
    ///     // Borrowing `self` mutably.
    ///     fn borrow_mut(&mut self) -> &mut i32 {
    ///         &mut self.0
    ///     }
    /// }
    ///
    /// // This method must be called with a `Type::` prefix.
    /// let foo = Foo::new();
    /// assert_eq!(foo.0, 0);
    ///
    /// // Those two calls produces the same result.
    /// let foo = Foo::consume(foo);
    /// assert_eq!(foo.0, 1);
    /// let foo = foo.consume();
    /// assert_eq!(foo.0, 2);
    ///
    /// // Borrowing is handled automatically with the second syntax.
    /// let borrow_1 = Foo::borrow(&foo);
    /// let borrow_2 = foo.borrow();
    /// assert_eq!(borrow_1, borrow_2);
    ///
    /// // Borrowing mutably is handled automatically too with the second syntax.
    /// let mut foo = Foo::new();
    /// *Foo::borrow_mut(&mut foo) += 1;
    /// assert_eq!(foo.0, 1);
    /// *foo.borrow_mut() += 1;
    /// assert_eq!(foo.0, 2);
    /// ```
    ///
    /// Note that this automatic conversion when calling `foo.method()` is not
    /// limited to the examples above. See the [Reference] for more information.
    ///
    /// [`use`]: keyword.use.html
    /// [Reference]: ../reference/items/associated-items.html#methods
    mod self_keyword {}

    #[doc(keyword = "Self")]
    //
    /// The implementing type within a [`trait`] or [`impl`] block, or the current type within a type
    /// definition.
    ///
    /// Within a type definition:
    ///
    /// ```
    /// # #![allow(dead_code)]
    /// struct Node {
    ///     elem: i32,
    ///     // `Self` is a `Node` here.
    ///     next: Option<Box<Self>>,
    /// }
    /// ```
    ///
    /// In an [`impl`] block:
    ///
    /// ```
    /// struct Foo(i32);
    ///
    /// impl Foo {
    ///     fn new() -> Self {
    ///         Self(0)
    ///     }
    /// }
    ///
    /// assert_eq!(Foo::new().0, Foo(0).0);
    /// ```
    ///
    /// Generic parameters are implicit with `Self`:
    ///
    /// ```
    /// # #![allow(dead_code)]
    /// struct Wrap<T> {
    ///     elem: T,
    /// }
    ///
    /// impl<T> Wrap<T> {
    ///     fn new(elem: T) -> Self {
    ///         Self { elem }
    ///     }
    /// }
    /// ```
    ///
    /// In a [`trait`] definition and related [`impl`] block:
    ///
    /// ```
    /// trait Example {
    ///     fn example() -> Self;
    /// }
    ///
    /// struct Foo(i32);
    ///
    /// impl Example for Foo {
    ///     fn example() -> Self {
    ///         Self(42)
    ///     }
    /// }
    ///
    /// assert_eq!(Foo::example().0, Foo(42).0);
    /// ```
    ///
    /// [`impl`]: keyword.impl.html
    /// [`trait`]: keyword.trait.html
    mod self_upper_keyword {}

    #[doc(keyword = "static")]
    //
    /// A static item is a value which is valid for the entire duration of your
    /// program (a `'static` lifetime).
    ///
    /// On the surface, `static` items seem very similar to [`const`]s: both contain
    /// a value, both require type annotations and both can only be initialized with
    /// constant functions and values. However, `static`s are notably different in
    /// that they represent a location in memory. That means that you can have
    /// references to `static` items and potentially even modify them, making them
    /// essentially global variables.
    ///
    /// Static items do not call [`drop`] at the end of the program.
    ///
    /// There are two types of `static` items: those declared in association with
    /// the [`mut`] keyword and those without.
    ///
    /// Static items cannot be moved:
    ///
    /// ```rust,compile_fail,E0507
    /// static VEC: Vec<u32> = vec![];
    ///
    /// fn move_vec(v: Vec<u32>) -> Vec<u32> {
    ///     v
    /// }
    ///
    /// // This line causes an error
    /// move_vec(VEC);
    /// ```
    ///
    /// # Simple `static`s
    ///
    /// Accessing non-[`mut`] `static` items is considered safe, but some
    /// restrictions apply. Most notably, the type of a `static` value needs to
    /// implement the [`Sync`] trait, ruling out interior mutability containers
    /// like [`RefCell`]. See the [Reference] for more information.
    ///
    /// ```rust
    /// static FOO: [i32; 5] = [1, 2, 3, 4, 5];
    ///
    /// let r1 = &FOO as *const _;
    /// let r2 = &FOO as *const _;
    /// // With a strictly read-only static, references will have the same address
    /// assert_eq!(r1, r2);
    /// // A static item can be used just like a variable in many cases
    /// println!("{:?}", FOO);
    /// ```
    ///
    /// # Mutable `static`s
    ///
    /// If a `static` item is declared with the [`mut`] keyword, then it is allowed
    /// to be modified by the program. However, accessing mutable `static`s can
    /// cause undefined behavior in a number of ways, for example due to data races
    /// in a multithreaded context. As such, all accesses to mutable `static`s
    /// require an [`unsafe`] block.
    ///
    /// Despite their unsafety, mutable `static`s are necessary in many contexts:
    /// they can be used to represent global state shared by the whole program or in
    /// [`extern`] blocks to bind to variables from C libraries.
    ///
    /// In an [`extern`] block:
    ///
    /// ```rust,no_run
    /// # #![allow(dead_code)]
    /// extern "C" {
    ///     static mut ERROR_MESSAGE: *mut std::os::raw::c_char;
    /// }
    /// ```
    ///
    /// Mutable `static`s, just like simple `static`s, have some restrictions that
    /// apply to them. See the [Reference] for more information.
    ///
    /// [`const`]: keyword.const.html
    /// [`extern`]: keyword.extern.html
    /// [`mut`]: keyword.mut.html
    /// [`unsafe`]: keyword.unsafe.html
    /// [`RefCell`]: cell::RefCell
    /// [Reference]: ../reference/items/static-items.html
    mod static_keyword {}

    #[doc(keyword = "struct")]
    //
    /// A type that is composed of other types.
    ///
    /// Structs in Rust come in three flavors: Structs with named fields, tuple structs, and unit
    /// structs.
    ///
    /// ```rust
    /// struct Regular {
    ///     field1: f32,
    ///     field2: String,
    ///     pub field3: bool
    /// }
    ///
    /// struct Tuple(u32, String);
    ///
    /// struct Unit;
    /// ```
    ///
    /// Regular structs are the most commonly used. Each field defined within them has a name and a
    /// type, and once defined can be accessed using `example_struct.field` syntax. The fields of a
    /// struct share its mutability, so `foo.bar = 2;` would only be valid if `foo` was mutable. Adding
    /// `pub` to a field makes it visible to code in other modules, as well as allowing it to be
    /// directly accessed and modified.
    ///
    /// Tuple structs are similar to regular structs, but its fields have no names. They are used like
    /// tuples, with deconstruction possible via `let TupleStruct(x, y) = foo;` syntax. For accessing
    /// individual variables, the same syntax is used as with regular tuples, namely `foo.0`, `foo.1`,
    /// etc, starting at zero.
    ///
    /// Unit structs are most commonly used as marker. They have a size of zero bytes, but unlike empty
    /// enums they can be instantiated, making them isomorphic to the unit type `()`. Unit structs are
    /// useful when you need to implement a trait on something, but don't need to store any data inside
    /// it.
    ///
    /// # Instantiation
    ///
    /// Structs can be instantiated in different ways, all of which can be mixed and
    /// matched as needed. The most common way to make a new struct is via a constructor method such as
    /// `new()`, but when that isn't available (or you're writing the constructor itself), struct
    /// literal syntax is used:
    ///
    /// ```rust
    /// # struct Foo { field1: f32, field2: String, etc: bool }
    /// let example = Foo {
    ///     field1: 42.0,
    ///     field2: "blah".to_string(),
    ///     etc: true,
    /// };
    /// ```
    ///
    /// It's only possible to directly instantiate a struct using struct literal syntax when all of its
    /// fields are visible to you.
    ///
    /// There are a handful of shortcuts provided to make writing constructors more convenient, most
    /// common of which is the Field Init shorthand. When there is a variable and a field of the same
    /// name, the assignment can be simplified from `field: field` into simply `field`. The following
    /// example of a hypothetical constructor demonstrates this:
    ///
    /// ```rust
    /// struct User {
    ///     name: String,
    ///     admin: bool,
    /// }
    ///
    /// impl User {
    ///     pub fn new(name: String) -> Self {
    ///         Self {
    ///             name,
    ///             admin: false,
    ///         }
    ///     }
    /// }
    /// ```
    ///
    /// Another shortcut for struct instantiation is available, used when you need to make a new
    /// struct that has the same values as most of a previous struct of the same type, called struct
    /// update syntax:
    ///
    /// ```rust
    /// # struct Foo { field1: String, field2: () }
    /// # let thing = Foo { field1: "".to_string(), field2: () };
    /// let updated_thing = Foo {
    ///     field1: "a new value".to_string(),
    ///     ..thing
    /// };
    /// ```
    ///
    /// Tuple structs are instantiated in the same way as tuples themselves, except with the struct's
    /// name as a prefix: `Foo(123, false, 0.1)`.
    ///
    /// Empty structs are instantiated with just their name, and don't need anything else. `let thing =
    /// EmptyStruct;`
    ///
    /// # Style conventions
    ///
    /// Structs are always written in CamelCase, with few exceptions. While the trailing comma on a
    /// struct's list of fields can be omitted, it's usually kept for convenience in adding and
    /// removing fields down the line.
    ///
    /// For more information on structs, take a look at the [Rust Book][book] or the
    /// [Reference][reference].
    ///
    /// [`PhantomData`]: marker::PhantomData
    /// [book]: ../book/ch05-01-defining-structs.html
    /// [reference]: ../reference/items/structs.html
    mod struct_keyword {}

    #[doc(keyword = "super")]
    //
    /// The parent of the current [module].
    ///
    /// ```rust
    /// # #![allow(dead_code)]
    /// # fn main() {}
    /// mod a {
    ///     pub fn foo() {}
    /// }
    /// mod b {
    ///     pub fn foo() {
    ///         super::a::foo(); // call a's foo function
    ///     }
    /// }
    /// ```
    ///
    /// It is also possible to use `super` multiple times: `super::super::foo`,
    /// going up the ancestor chain.
    ///
    /// See the [Reference] for more information.
    ///
    /// [module]: ../reference/items/modules.html
    /// [Reference]: ../reference/paths.html#super
    mod super_keyword {}

    #[doc(keyword = "trait")]
    //
    /// A common interface for a group of types.
    ///
    /// A `trait` is like an interface that data types can implement. When a type
    /// implements a trait it can be treated abstractly as that trait using generics
    /// or trait objects.
    ///
    /// Traits can be made up of three varieties of associated items:
    ///
    /// - functions and methods
    /// - types
    /// - constants
    ///
    /// Traits may also contain additional type parameters. Those type parameters
    /// or the trait itself can be constrained by other traits.
    ///
    /// Traits can serve as markers or carry other logical semantics that
    /// aren't expressed through their items. When a type implements that
    /// trait it is promising to uphold its contract. [`Send`] and [`Sync`] are two
    /// such marker traits present in the standard library.
    ///
    /// See the [Reference][Ref-Traits] for a lot more information on traits.
    ///
    /// # Examples
    ///
    /// Traits are declared using the `trait` keyword. Types can implement them
    /// using [`impl`] `Trait` [`for`] `Type`:
    ///
    /// ```rust
    /// trait Zero {
    ///     const ZERO: Self;
    ///     fn is_zero(&self) -> bool;
    /// }
    ///
    /// impl Zero for i32 {
    ///     const ZERO: Self = 0;
    ///
    ///     fn is_zero(&self) -> bool {
    ///         *self == Self::ZERO
    ///     }
    /// }
    ///
    /// assert_eq!(i32::ZERO, 0);
    /// assert!(i32::ZERO.is_zero());
    /// assert!(!4.is_zero());
    /// ```
    ///
    /// With an associated type:
    ///
    /// ```rust
    /// trait Builder {
    ///     type Built;
    ///
    ///     fn build(&self) -> Self::Built;
    /// }
    /// ```
    ///
    /// Traits can be generic, with constraints or without:
    ///
    /// ```rust
    /// trait MaybeFrom<T> {
    ///     fn maybe_from(value: T) -> Option<Self>
    ///     where
    ///         Self: Sized;
    /// }
    /// ```
    ///
    /// Traits can build upon the requirements of other traits. In the example
    /// below `Iterator` is a **supertrait** and `ThreeIterator` is a **subtrait**:
    ///
    /// ```rust
    /// trait ThreeIterator: std::iter::Iterator {
    ///     fn next_three(&mut self) -> Option<[Self::Item; 3]>;
    /// }
    /// ```
    ///
    /// Traits can be used in functions, as parameters:
    ///
    /// ```rust
    /// # #![allow(dead_code)]
    /// fn debug_iter<I: Iterator>(it: I) where I::Item: std::fmt::Debug {
    ///     for elem in it {
    ///         println!("{:#?}", elem);
    ///     }
    /// }
    ///
    /// // u8_len_1, u8_len_2 and u8_len_3 are equivalent
    ///
    /// fn u8_len_1(val: impl Into<Vec<u8>>) -> usize {
    ///     val.into().len()
    /// }
    ///
    /// fn u8_len_2<T: Into<Vec<u8>>>(val: T) -> usize {
    ///     val.into().len()
    /// }
    ///
    /// fn u8_len_3<T>(val: T) -> usize
    /// where
    ///     T: Into<Vec<u8>>,
    /// {
    ///     val.into().len()
    /// }
    /// ```
    ///
    /// Or as return types:
    ///
    /// ```rust
    /// # #![allow(dead_code)]
    /// fn from_zero_to(v: u8) -> impl Iterator<Item = u8> {
    ///     (0..v).into_iter()
    /// }
    /// ```
    ///
    /// The use of the [`impl`] keyword in this position allows the function writer
    /// to hide the concrete type as an implementation detail which can change
    /// without breaking user's code.
    ///
    /// # Trait objects
    ///
    /// A *trait object* is an opaque value of another type that implements a set of
    /// traits. A trait object implements all specified traits as well as their
    /// supertraits (if any).
    ///
    /// The syntax is the following: `dyn BaseTrait + AutoTrait1 + ... AutoTraitN`.
    /// Only one `BaseTrait` can be used so this will not compile:
    ///
    /// ```rust,compile_fail,E0225
    /// trait A {}
    /// trait B {}
    ///
    /// let _: Box<dyn A + B>;
    /// ```
    ///
    /// Neither will this, which is a syntax error:
    ///
    /// ```rust,compile_fail
    /// trait A {}
    /// trait B {}
    ///
    /// let _: Box<dyn A + dyn B>;
    /// ```
    ///
    /// On the other hand, this is correct:
    ///
    /// ```rust
    /// trait A {}
    ///
    /// let _: Box<dyn A + Send + Sync>;
    /// ```
    ///
    /// The [Reference][Ref-Trait-Objects] has more information about trait objects,
    /// their limitations and the differences between editions.
    ///
    /// # Unsafe traits
    ///
    /// Some traits may be unsafe to implement. Using the [`unsafe`] keyword in
    /// front of the trait's declaration is used to mark this:
    ///
    /// ```rust
    /// unsafe trait UnsafeTrait {}
    ///
    /// unsafe impl UnsafeTrait for i32 {}
    /// ```
    ///
    /// # Differences between the 2015 and 2018 editions
    ///
    /// In the 2015 edition parameters pattern where not needed for traits:
    ///
    /// ```rust,edition2015
    /// trait Tr {
    ///     fn f(i32);
    /// }
    /// ```
    ///
    /// This behavior is no longer valid in edition 2018.
    ///
    /// [`for`]: keyword.for.html
    /// [`impl`]: keyword.impl.html
    /// [`unsafe`]: keyword.unsafe.html
    /// [Ref-Traits]: ../reference/items/traits.html
    /// [Ref-Trait-Objects]: ../reference/types/trait-object.html
    mod trait_keyword {}

    #[doc(keyword = "true")]
    //
    /// A value of type [`bool`] representing logical **true**.
    ///
    /// Logically `true` is not equal to [`false`].
    ///
    /// ## Control structures that check for **true**
    ///
    /// Several of Rust's control structures will check for a `bool` condition evaluating to **true**.
    ///
    ///   * The condition in an [`if`] expression must be of type `bool`.
    ///     Whenever that condition evaluates to **true**, the `if` expression takes
    ///     on the value of the first block. If however, the condition evaluates
    ///     to `false`, the expression takes on value of the `else` block if there is one.
    ///
    ///   * [`while`] is another control flow construct expecting a `bool`-typed condition.
    ///     As long as the condition evaluates to **true**, the `while` loop will continually
    ///     evaluate its associated block.
    ///
    ///   * [`match`] arms can have guard clauses on them.
    ///
    /// [`if`]: keyword.if.html
    /// [`while`]: keyword.while.html
    /// [`match`]: ../reference/expressions/match-expr.html#match-guards
    /// [`false`]: keyword.false.html
    mod true_keyword {}

    #[doc(keyword = "type")]
    //
    /// Define an alias for an existing type.
    ///
    /// The syntax is `type Name = ExistingType;`.
    ///
    /// # Examples
    ///
    /// `type` does **not** create a new type:
    ///
    /// ```rust
    /// type Meters = u32;
    /// type Kilograms = u32;
    ///
    /// let m: Meters = 3;
    /// let k: Kilograms = 3;
    ///
    /// assert_eq!(m, k);
    /// ```
    ///
    /// In traits, `type` is used to declare an [associated type]:
    ///
    /// ```rust
    /// trait Iterator {
    ///     // associated type declaration
    ///     type Item;
    ///     fn next(&mut self) -> Option<Self::Item>;
    /// }
    ///
    /// struct Once<T>(Option<T>);
    ///
    /// impl<T> Iterator for Once<T> {
    ///     // associated type definition
    ///     type Item = T;
    ///     fn next(&mut self) -> Option<Self::Item> {
    ///         self.0.take()
    ///     }
    /// }
    /// ```
    ///
    /// [`trait`]: keyword.trait.html
    /// [associated type]: ../reference/items/associated-items.html#associated-types
    mod type_keyword {}

    #[doc(keyword = "unsafe")]
    //
    /// Code or interfaces whose [memory safety] cannot be verified by the type
    /// system.
    ///
    /// The `unsafe` keyword has two uses: to declare the existence of contracts the
    /// compiler can't check (`unsafe fn` and `unsafe trait`), and to declare that a
    /// programmer has checked that these contracts have been upheld (`unsafe {}`
    /// and `unsafe impl`, but also `unsafe fn` -- see below). They are not mutually
    /// exclusive, as can be seen in `unsafe fn`.
    ///
    /// # Unsafe abilities
    ///
    /// **No matter what, Safe Rust can't cause Undefined Behavior**. This is
    /// referred to as [soundness]: a well-typed program actually has the desired
    /// properties. The [Nomicon][nomicon-soundness] has a more detailed explanation
    /// on the subject.
    ///
    /// To ensure soundness, Safe Rust is restricted enough that it can be
    /// automatically checked. Sometimes, however, it is necessary to write code
    /// that is correct for reasons which are too clever for the compiler to
    /// understand. In those cases, you need to use Unsafe Rust.
    ///
    /// Here are the abilities Unsafe Rust has in addition to Safe Rust:
    ///
    /// - Dereference [raw pointers]
    /// - Implement `unsafe` [`trait`]s
    /// - Call `unsafe` functions
    /// - Mutate [`static`]s (including [`extern`]al ones)
    /// - Access fields of [`union`]s
    ///
    /// However, this extra power comes with extra responsibilities: it is now up to
    /// you to ensure soundness. The `unsafe` keyword helps by clearly marking the
    /// pieces of code that need to worry about this.
    ///
    /// ## The different meanings of `unsafe`
    ///
    /// Not all uses of `unsafe` are equivalent: some are here to mark the existence
    /// of a contract the programmer must check, others are to say "I have checked
    /// the contract, go ahead and do this". The following
    /// [discussion on Rust Internals] has more in-depth explanations about this but
    /// here is a summary of the main points:
    ///
    /// - `unsafe fn`: calling this function means abiding by a contract the
    /// compiler cannot enforce.
    /// - `unsafe trait`: implementing the [`trait`] means abiding by a
    /// contract the compiler cannot enforce.
    /// - `unsafe {}`: the contract necessary to call the operations inside the
    /// block has been checked by the programmer and is guaranteed to be respected.
    /// - `unsafe impl`: the contract necessary to implement the trait has been
    /// checked by the programmer and is guaranteed to be respected.
    ///
    /// `unsafe fn` also acts like an `unsafe {}` block
    /// around the code inside the function. This means it is not just a signal to
    /// the caller, but also promises that the preconditions for the operations
    /// inside the function are upheld. Mixing these two meanings can be confusing
    /// and [proposal]s exist to use `unsafe {}` blocks inside such functions when
    /// making `unsafe` operations.
    ///
    /// See the [Rustnomicon] and the [Reference] for more informations.
    ///
    /// # Examples
    ///
    /// ## Marking elements as `unsafe`
    ///
    /// `unsafe` can be used on functions. Note that functions and statics declared
    /// in [`extern`] blocks are implicitly marked as `unsafe` (but not functions
    /// declared as `extern "something" fn ...`). Mutable statics are always unsafe,
    /// wherever they are declared. Methods can also be declared as `unsafe`:
    ///
    /// ```rust
    /// # #![allow(dead_code)]
    /// static mut FOO: &str = "hello";
    ///
    /// unsafe fn unsafe_fn() {}
    ///
    /// extern "C" {
    ///     fn unsafe_extern_fn();
    ///     static BAR: *mut u32;
    /// }
    ///
    /// trait SafeTraitWithUnsafeMethod {
    ///     unsafe fn unsafe_method(&self);
    /// }
    ///
    /// struct S;
    ///
    /// impl S {
    ///     unsafe fn unsafe_method_on_struct() {}
    /// }
    /// ```
    ///
    /// Traits can also be declared as `unsafe`:
    ///
    /// ```rust
    /// unsafe trait UnsafeTrait {}
    /// ```
    ///
    /// Since `unsafe fn` and `unsafe trait` indicate that there is a safety
    /// contract that the compiler cannot enforce, documenting it is important. The
    /// standard library has many examples of this, like the following which is an
    /// extract from [`Vec::set_len`]. The `# Safety` section explains the contract
    /// that must be fulfilled to safely call the function.
    ///
    /// ```rust,ignore (stub-to-show-doc-example)
    /// /// Forces the length of the vector to `new_len`.
    /// ///
    /// /// This is a low-level operation that maintains none of the normal
    /// /// invariants of the type. Normally changing the length of a vector
    /// /// is done using one of the safe operations instead, such as
    /// /// `truncate`, `resize`, `extend`, or `clear`.
    /// ///
    /// /// # Safety
    /// ///
    /// /// - `new_len` must be less than or equal to `capacity()`.
    /// /// - The elements at `old_len..new_len` must be initialized.
    /// pub unsafe fn set_len(&mut self, new_len: usize)
    /// ```
    ///
    /// ## Using `unsafe {}` blocks and `impl`s
    ///
    /// Performing `unsafe` operations requires an `unsafe {}` block:
    ///
    /// ```rust
    /// # #![allow(dead_code)]
    /// /// Dereference the given pointer.
    /// ///
    /// /// # Safety
    /// ///
    /// /// `ptr` must be aligned and must not be dangling.
    /// unsafe fn deref_unchecked(ptr: *const i32) -> i32 {
    ///     *ptr
    /// }
    ///
    /// let a = 3;
    /// let b = &a as *const _;
    /// // SAFETY: `a` has not been dropped and references are always aligned,
    /// // so `b` is a valid address.
    /// unsafe { assert_eq!(*b, deref_unchecked(b)); };
    /// ```
    ///
    /// Traits marked as `unsafe` must be [`impl`]emented using `unsafe impl`. This
    /// makes a guarantee to other `unsafe` code that the implementation satisfies
    /// the trait's safety contract. The [Send] and [Sync] traits are examples of
    /// this behaviour in the standard library.
    ///
    /// ```rust
    /// /// Implementors of this trait must guarantee an element is always
    /// /// accessible with index 3.
    /// unsafe trait ThreeIndexable<T> {
    ///     /// Returns a reference to the element with index 3 in `&self`.
    ///     fn three(&self) -> &T;
    /// }
    ///
    /// // The implementation of `ThreeIndexable` for `[T; 4]` is `unsafe`
    /// // because the implementor must abide by a contract the compiler cannot
    /// // check but as a programmer we know there will always be a valid element
    /// // at index 3 to access.
    /// unsafe impl<T> ThreeIndexable<T> for [T; 4] {
    ///     fn three(&self) -> &T {
    ///         // SAFETY: implementing the trait means there always is an element
    ///         // with index 3 accessible.
    ///         unsafe { self.get_unchecked(3) }
    ///     }
    /// }
    ///
    /// let a = [1, 2, 4, 8];
    /// assert_eq!(a.three(), &8);
    /// ```
    ///
    /// [`extern`]: keyword.extern.html
    /// [`trait`]: keyword.trait.html
    /// [`static`]: keyword.static.html
    /// [`union`]: keyword.union.html
    /// [`impl`]: keyword.impl.html
    /// [raw pointers]: ../reference/types/pointer.html
    /// [memory safety]: ../book/ch19-01-unsafe-rust.html
    /// [Rustnomicon]: ../nomicon/index.html
    /// [nomicon-soundness]: ../nomicon/safe-unsafe-meaning.html
    /// [soundness]: https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#soundness-of-code--of-a-library
    /// [Reference]: ../reference/unsafety.html
    /// [proposal]: https://github.com/rust-lang/rfcs/pull/2585
    /// [discussion on Rust Internals]: https://internals.rust-lang.org/t/what-does-unsafe-mean/6696
    mod unsafe_keyword {}

    #[doc(keyword = "use")]
    //
    /// Import or rename items from other crates or modules.
    ///
    /// Usually a `use` keyword is used to shorten the path required to refer to a module item.
    /// The keyword may appear in modules, blocks and even functions, usually at the top.
    ///
    /// The most basic usage of the keyword is `use path::to::item;`,
    /// though a number of convenient shortcuts are supported:
    ///
    ///   * 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}};`
    ///   * Reexporting with visibility modifiers such as `pub use a::b;`
    ///   * Importing with `_` to only import the methods of a trait without binding it to a name
    ///     (to avoid conflict for example): `use ::std::io::Read as _;`.
    ///
    /// Using path qualifiers like [`crate`], [`super`] or [`self`] is supported: `use crate::a::b;`.
    ///
    /// Note that when the wildcard `*` is used on a type, it does not import its methods (though
    /// for `enum`s it imports the variants, as shown in the example below).
    ///
    /// ```compile_fail,edition2018
    /// enum ExampleEnum {
    ///     VariantA,
    ///     VariantB,
    /// }
    ///
    /// impl ExampleEnum {
    ///     fn new() -> Self {
    ///         Self::VariantA
    ///     }
    /// }
    ///
    /// use ExampleEnum::*;
    ///
    /// // Compiles.
    /// let _ = VariantA;
    ///
    /// // Does not compile !
    /// let n = new();
    /// ```
    ///
    /// For more information on `use` and paths in general, see the [Reference].
    ///
    /// The differences about paths and the `use` keyword between the 2015 and 2018 editions
    /// can also be found in the [Reference].
    ///
    /// [`crate`]: keyword.crate.html
    /// [`self`]: keyword.self.html
    /// [`super`]: keyword.super.html
    /// [Reference]: ../reference/items/use-declarations.html
    mod use_keyword {}

    #[doc(keyword = "where")]
    //
    /// Add constraints that must be upheld to use an item.
    ///
    /// `where` allows specifying constraints on lifetime and generic parameters.
    /// The [RFC] introducing `where` contains detailed informations about the
    /// keyword.
    ///
    /// # Examples
    ///
    /// `where` can be used for constraints with traits:
    ///
    /// ```rust
    /// fn new<T: Default>() -> T {
    ///     T::default()
    /// }
    ///
    /// fn new_where<T>() -> T
    /// where
    ///     T: Default,
    /// {
    ///     T::default()
    /// }
    ///
    /// assert_eq!(0.0, new());
    /// assert_eq!(0.0, new_where());
    ///
    /// assert_eq!(0, new());
    /// assert_eq!(0, new_where());
    /// ```
    ///
    /// `where` can also be used for lifetimes.
    ///
    /// This compiles because `longer` outlives `shorter`, thus the constraint is
    /// respected:
    ///
    /// ```rust
    /// fn select<'short, 'long>(s1: &'short str, s2: &'long str, second: bool) -> &'short str
    /// where
    ///     'long: 'short,
    /// {
    ///     if second { s2 } else { s1 }
    /// }
    ///
    /// let outer = String::from("Long living ref");
    /// let longer = &outer;
    /// {
    ///     let inner = String::from("Short living ref");
    ///     let shorter = &inner;
    ///
    ///     assert_eq!(select(shorter, longer, false), shorter);
    ///     assert_eq!(select(shorter, longer, true), longer);
    /// }
    /// ```
    ///
    /// On the other hand, this will not compile because the `where 'b: 'a` clause
    /// is missing: the `'b` lifetime is not known to live at least as long as `'a`
    /// which means this function cannot ensure it always returns a valid reference:
    ///
    /// ```rust,compile_fail,E0623
    /// fn select<'a, 'b>(s1: &'a str, s2: &'b str, second: bool) -> &'a str
    /// {
    ///     if second { s2 } else { s1 }
    /// }
    /// ```
    ///
    /// `where` can also be used to express more complicated constraints that cannot
    /// be written with the `<T: Trait>` syntax:
    ///
    /// ```rust
    /// fn first_or_default<I>(mut i: I) -> I::Item
    /// where
    ///     I: Iterator,
    ///     I::Item: Default,
    /// {
    ///     i.next().unwrap_or_else(I::Item::default)
    /// }
    ///
    /// assert_eq!(first_or_default(vec![1, 2, 3].into_iter()), 1);
    /// assert_eq!(first_or_default(Vec::<i32>::new().into_iter()), 0);
    /// ```
    ///
    /// `where` is available anywhere generic and lifetime parameters are available,
    /// as can be seen with the [`Cow`](crate::borrow::Cow) type from the standard
    /// library:
    ///
    /// ```rust
    /// # #![allow(dead_code)]
    /// pub enum Cow<'a, B>
    /// where
    ///     B: 'a + ToOwned + ?Sized,
    ///  {
    ///     Borrowed(&'a B),
    ///     Owned(<B as ToOwned>::Owned),
    /// }
    /// ```
    ///
    /// [RFC]: https://github.com/rust-lang/rfcs/blob/master/text/0135-where.md
    mod where_keyword {}

    // 2018 Edition keywords

    #[doc(keyword = "async")]
    //
    /// Return a [`Future`] instead of blocking the current thread.
    ///
    /// Use `async` in front of `fn`, `closure`, or a `block` to turn the marked code into a `Future`.
    /// As such the code will not be run immediately, but will only be evaluated when the returned
    /// future is `.await`ed.
    ///
    /// We have written an [async book] detailing async/await and trade-offs compared to using threads.
    ///
    /// ## Editions
    ///
    /// `async` is a keyword from the 2018 edition onwards.
    ///
    /// It is available for use in stable rust from version 1.39 onwards.
    ///
    /// [`Future`]: future::Future
    /// [async book]: https://rust-lang.github.io/async-book/
    mod async_keyword {}

    #[doc(keyword = "await")]
    //
    /// Suspend execution until the result of a [`Future`] is ready.
    ///
    /// `.await`ing a future will suspend the current function's execution until the `executor`
    /// has run the future to completion.
    ///
    /// Read the [async book] for details on how async/await and executors work.
    ///
    /// ## Editions
    ///
    /// `await` is a keyword from the 2018 edition onwards.
    ///
    /// It is available for use in stable rust from version 1.39 onwards.
    ///
    /// [`Future`]: future::Future
    /// [async book]: https://rust-lang.github.io/async-book/
    mod await_keyword {}

    #[doc(keyword = "dyn")]
    //
    /// `dyn` is a prefix of a [trait object]'s type.
    ///
    /// The `dyn` keyword is used to highlight that calls to methods on the associated `Trait`
    /// are dynamically dispatched. To use the trait this way, it must be 'object safe'.
    ///
    /// Unlike generic parameters or `impl Trait`, the compiler does not know the concrete type that
    /// is being passed. That is, the type has been [erased].
    /// As such, a `dyn Trait` reference contains _two_ pointers.
    /// One pointer goes to the data (e.g., an instance of a struct).
    /// Another pointer goes to a map of method call names to function pointers
    /// (known as a virtual method table or vtable).
    ///
    /// At run-time, when a method needs to be called on the `dyn Trait`, the vtable is consulted to get
    /// the function pointer and then that function pointer is called.
    ///
    /// ## Trade-offs
    ///
    /// The above indirection is the additional runtime cost of calling a function on a `dyn Trait`.
    /// Methods called by dynamic dispatch generally cannot be inlined by the compiler.
    ///
    /// However, `dyn Trait` is likely to produce smaller code than `impl Trait` / generic parameters as
    /// the method won't be duplicated for each concrete type.
    ///
    /// Read more about `object safety` and [trait object]s.
    ///
    /// [trait object]: ../book/ch17-02-trait-objects.html
    /// [erased]: https://en.wikipedia.org/wiki/Type_erasure
    mod dyn_keyword {}

    #[doc(keyword = "union")]
    //
    /// The [Rust equivalent of a C-style union][union].
    ///
    /// A `union` looks like a [`struct`] in terms of declaration, but all of its
    /// fields exist in the same memory, superimposed over one another. For instance,
    /// if we wanted some bits in memory that we sometimes interpret as a `u32` and
    /// sometimes as an `f32`, we could write:
    ///
    /// ```rust
    /// union IntOrFloat {
    ///     i: u32,
    ///     f: f32,
    /// }
    ///
    /// let mut u = IntOrFloat { f: 1.0 };
    /// // Reading the fields of an union is always unsafe
    /// assert_eq!(unsafe { u.i }, 1065353216);
    /// // Updating through any of the field will modify all of them
    /// u.i = 1073741824;
    /// assert_eq!(unsafe { u.f }, 2.0);
    /// ```
    ///
    /// # Matching on unions
    ///
    /// It is possible to use pattern matching on `union`s. A single field name must
    /// be used and it must match the name of one of the `union`'s field.
    /// Like reading from a `union`, pattern matching on a `union` requires `unsafe`.
    ///
    /// ```rust
    /// union IntOrFloat {
    ///     i: u32,
    ///     f: f32,
    /// }
    ///
    /// let u = IntOrFloat { f: 1.0 };
    ///
    /// unsafe {
    ///     match u {
    ///         IntOrFloat { i: 10 } => println!("Found exactly ten!"),
    ///         // Matching the field `f` provides an `f32`.
    ///         IntOrFloat { f } => println!("Found f = {} !", f),
    ///     }
    /// }
    /// ```
    ///
    /// # References to union fields
    ///
    /// All fields in a `union` are all at the same place in memory which means
    /// borrowing one borrows the entire `union`, for the same lifetime:
    ///
    /// ```rust,compile_fail,E0502
    /// union IntOrFloat {
    ///     i: u32,
    ///     f: f32,
    /// }
    ///
    /// let mut u = IntOrFloat { f: 1.0 };
    ///
    /// let f = unsafe { &u.f };
    /// // This will not compile because the field has already been borrowed, even
    /// // if only immutably
    /// let i = unsafe { &mut u.i };
    ///
    /// *i = 10;
    /// println!("f = {} and i = {}", f, i);
    /// ```
    ///
    /// See the [Reference][union] for more informations on `union`s.
    ///
    /// [`struct`]: keyword.struct.html
    /// [union]: ../reference/items/unions.html
    mod union_keyword {}