Patterns - The Rust Reference

# Patterns

> Syntax
> Pattern :
> LiteralPattern
> | IdentifierPattern
> | WildcardPattern
> | RangePattern
> | ReferencePattern
> | StructPattern
> | TupleStructPattern
> | TuplePattern
> | GroupedPattern
> | SlicePattern
> | PathPattern
> | MacroInvocation

Patterns are used to match values against structures and to, optionally, bind variables to values inside these structures. They are also used in variable declarations and parameters for functions and closures.

The pattern in the following example does four things:

- Tests if `person` has the `car` field filled with something.
- Tests if the person's `age` field is between 13 and 19, and binds its value to the `person_age` variable.
- Binds a reference to the `name` field to the variable `person_name`.
- Ignores the rest of the fields of `person`. The remaining fields can have any value and are not bound to any variables.

    #![allow(unused_variables)]
    fn main() {
    struct Car;
    struct Computer;
    struct Person {
        name: String,
        car: Option<Car>,
        computer: Option<Computer>,
        age: u8,
    }
    let person = Person {
        name: String::from("John"),
        car: Some(Car),
        computer: None,
        age: 15,
    };
    if let
        Person {
            car: Some(_),
            age: person_age @ 13..=19,
            name: ref person_name,
            ..
        } = person
    {
        println!("{} has a car and is {} years old.", person_name, person_age);
    }
    }

Patterns are used in:

- `let` declarations
- Function and closure parameters
- `match` expressions
- `if let` expressions
- `while let` expressions
- `for` expressions

## Destructuring

Patterns can be used to destructure structs, enums, and tuples. Destructuring breaks up a value into its component pieces. The syntax used is almost the same as when creating such values. In a pattern whose scrutinee expression has a `struct`, `enum` or `tuple` type, a placeholder (`_`) stands in for a single data field, whereas a wildcard `..` stands in for all the remaining fields of a particular variant. When destructuring a data structure with named (but not numbered) fields, it is allowed to write `fieldname` as a shorthand for `fieldname: fieldname`.

    #![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);
        }
    };
    }

## Refutability

A pattern is said to be refutable when it has the possibility of not being matched by the value it is being matched against. Irrefutable patterns, on the other hand, always match the value they are being matched against. Examples:

    #![allow(unused_variables)]
    fn main() {
    let (x, y) = (1, 2);               // "(x, y)" is an irrefutable pattern

    if let (a, 3) = (1, 2) {           // "(a, 3)" is refutable, and will not match
        panic!("Shouldn't reach here");
    } else if let (a, 4) = (3, 4) {    // "(a, 4)" is refutable, and will match
        println!("Matched ({}, 4)", a);
    }
    }

## Literal patterns

> Syntax
> LiteralPattern :
> BOOLEAN_LITERAL
> | CHAR_LITERAL
> | BYTE_LITERAL
> | STRING_LITERAL
> | RAW_STRING_LITERAL
> | BYTE_STRING_LITERAL
> | RAW_BYTE_STRING_LITERAL
> | `-`? INTEGER_LITERAL
> | `-`? FLOAT_LITERAL

Literal patterns match exactly the same value as what is created by the literal. Since negative numbers are not literals, literal patterns also accept an optional minus sign before the literal, which acts like the negation operator.

Floating-point literals are currently accepted, but due to the complexity of comparing them, they are going to be forbidden on literal patterns in a future version of Rust (see issue #41620).

Literal patterns are always refutable.

Examples:

    #![allow(unused_variables)]
    fn main() {
    for i in -2..5 {
        match i {
            -1 => println!("It's minus one"),
            1 => println!("It's a one"),
            2|4 => println!("It's either a two or a four"),
            _ => println!("Matched none of the arms"),
        }
    }
    }

## Identifier patterns

> Syntax
> IdentifierPattern :
> `ref`? `mut`? IDENTIFIER (`@` Pattern ) ?

Identifier patterns bind the value they match to a variable. The identifier must be unique within the pattern. The variable will shadow any variables of the same name in scope. The scope of the new binding depends on the context of where the pattern is used (such as a `let` binding or a `match` arm).

Patterns that consist of only an identifier, possibly with a `mut`, match any value and bind it to that identifier. This is the most commonly used pattern in variable declarations and parameters for functions and closures.

    #![allow(unused_variables)]
    fn main() {
    let mut variable = 10;
    fn sum(x: i32, y: i32) -> i32 {
       x + y
    }
    }

To bind the matched value of a pattern to a variable, use the syntax `variable @ subpattern`. For example, the following binds the value 2 to `e` (not the entire range: the range here is a range subpattern).

    #![allow(unused_variables)]
    fn main() {
    let x = 2;

    match x {
        e @ 1 ..= 5 => println!("got a range element {}", e),
        _ => println!("anything"),
    }
    }

By default, identifier patterns bind a variable to a copy of or move from the matched value depending on whether the matched value implements `Copy`. This can be changed to bind to a reference by using the `ref` keyword, or to a mutable reference using `ref mut`. For example:

    #![allow(unused_variables)]
    fn main() {
    let a = Some(10);
    match a {
        None => (),
        Some(value) => (),
    }

    match a {
        None => (),
        Some(ref value) => (),
    }
    }

In the first match expression, the value is copied (or moved). In the second match, a reference to the same memory location is bound to the variable value. This syntax is needed because in destructuring subpatterns the `&` operator can't be applied to the value's fields. For example, the following is not valid:

    #![allow(unused_variables)]
    fn main() {
    struct Person {
       name: String,
       age: u8,
    }
    let value = Person{ name: String::from("John"), age: 23 };
    if let Person{name: &person_name, age: 18..=150} = value { }
    }

To make it valid, write the following:

    #![allow(unused_variables)]
    fn main() {
    struct Person {
       name: String,
       age: u8,
    }
    let value = Person{ name: String::from("John"), age: 23 };
    if let Person{name: ref person_name, age: 18..=150} = value { }
    }

Thus, `ref` is not something that is being matched against. Its objective is exclusively to make the matched binding a reference, instead of potentially copying or moving what was matched.

Path patterns take precedence over identifier patterns. It is an error if `ref` or `ref mut` is specified and the identifier shadows a constant.

### Binding modes

To service better ergonomics, patterns operate in different binding modes in order to make it easier to bind references to values. When a reference value is matched by a non-reference pattern, it will be automatically treated as a `ref` or `ref mut` binding. Example:

    #![allow(unused_variables)]
    fn main() {
    let x: &Option<i32> = &Some(3);
    if let Some(y) = x {
        // y was converted to `ref y` and its type is &i32
    }
    }

Non-reference patterns include all patterns except bindings, wildcard patterns (`_`), `const` patterns of reference types, and reference patterns.

If a binding pattern does not explicitly have `ref`, `ref mut`, or `mut`, then it uses the default binding mode to determine how the variable is bound. The default binding mode starts in "move" mode which uses move semantics. When matching a pattern, the compiler starts from the outside of the pattern and works inwards. Each time a reference is matched using a non-reference pattern, it will automatically dereference the value and update the default binding mode. References will set the default binding mode to `ref`. Mutable references will set the mode to `ref mut` unless the mode is already `ref` in which case it remains `ref`. If the automatically dereferenced value is still a reference, it is dereferenced and this process repeats.

## Wildcard pattern

> Syntax
> WildcardPattern :
> `_`

The wildcard pattern matches any value. It is used to ignore values when they don't matter. Inside other patterns it matches a single data field (as opposed to the `..` which matches the remaining fields). Unlike identifier patterns, it does not copy, move or borrow the value it matches.

Examples:

    #![allow(unused_variables)]
    fn main() {
    let x = 20;
    let (a, _) = (10, x);   // the x is always matched by _
    assert_eq!(a, 10);

    // ignore a function/closure param
    let real_part = |a: f64, _: f64| { a };

    // ignore a field from a struct
    struct RGBA {
       r: f32,
       g: f32,
       b: f32,
       a: f32,
    }
    let color = RGBA{r: 0.4, g: 0.1, b: 0.9, a: 0.5};
    let RGBA{r: red, g: green, b: blue, a: _} = color;
    assert_eq!(color.r, red);
    assert_eq!(color.g, green);
    assert_eq!(color.b, blue);

    // accept any Some, with any value
    let x = Some(10);
    if let Some(_) = x {}
    }

The wildcard pattern is always irrefutable.

## Range patterns

> Syntax
> RangePattern :
> RangePatternBound `..=` RangePatternBound
> | RangePatternBound `...` RangePatternBound

> RangePatternBound :
> CHAR_LITERAL
> | BYTE_LITERAL
> | `-`? INTEGER_LITERAL
> | `-`? FLOAT_LITERAL
> | PathInExpression
> | QualifiedPathInExpression

Range patterns match values that are within the closed range defined by its lower and upper bounds. For example, a pattern `'m'..='p'` will match only the values `'m'`, `'n'`, `'o'`, and `'p'`. The bounds can be literals or paths that point to constant values.

A pattern a `..=` b must always have a ≤ b. It is an error to have a range pattern `10..=0`, for example.

The `...` syntax is kept for backwards compatibility.

Range patterns only work on scalar types. The accepted types are:

- Integer types (u8, i8, u16, i16, usize, isize, etc.).
- Character types (char).
- Floating point types (f32 and f64). This is being deprecated and will not be available in a future version of Rust (see issue #41620).

Examples:

    #![allow(unused_variables)]
    fn main() {
    let c = 'f';
    let valid_variable = match c {
        'a'..='z' => true,
        'A'..='Z' => true,
        'α'..='ω' => true,
        _ => false,
    };

    let ph = 10;
    println!("{}", match ph {
        0..=6 => "acid",
        7 => "neutral",
        8..=14 => "base",
        _ => unreachable!(),
    });

    // using paths to constants:
    const TROPOSPHERE_MIN : u8 = 6;
    const TROPOSPHERE_MAX : u8 = 20;

    const STRATOSPHERE_MIN : u8 = TROPOSPHERE_MAX + 1;
    const STRATOSPHERE_MAX : u8 = 50;

    const MESOSPHERE_MIN : u8 = STRATOSPHERE_MAX + 1;
    const MESOSPHERE_MAX : u8 = 85;

    let altitude = 70;

    println!("{}", match altitude {
        TROPOSPHERE_MIN..=TROPOSPHERE_MAX => "troposphere",
        STRATOSPHERE_MIN..=STRATOSPHERE_MAX => "stratosphere",
        MESOSPHERE_MIN..=MESOSPHERE_MAX => "mesosphere",
        _ => "outer space, maybe",
    });

    pub mod binary {
        pub const MEGA : u64 = 1024*1024;
        pub const GIGA : u64 = 1024*1024*1024;
    }
    let n_items = 20_832_425;
    let bytes_per_item = 12;
    if let size @ binary::MEGA..=binary::GIGA = n_items * bytes_per_item {
        println!("It fits and occupies {} bytes", size);
    }

    trait MaxValue {
        const MAX: u64;
    }
    impl MaxValue for u8 {
        const MAX: u64 = (1 << 8) - 1;
    }
    impl MaxValue for u16 {
        const MAX: u64 = (1 << 16) - 1;
    }
    impl MaxValue for u32 {
        const MAX: u64 = (1 << 32) - 1;
    }
    // using qualified paths:
    println!("{}", match 0xfacade {
        0 ..= <u8 as MaxValue>::MAX => "fits in a u8",
        0 ..= <u16 as MaxValue>::MAX => "fits in a u16",
        0 ..= <u32 as MaxValue>::MAX => "fits in a u32",
        _ => "too big",
    });
    }

Range patterns for (non-`usize` and -`isize`) integer and `char` types are irrefutable when they span the entire set of possible values of a type. For example, `0u8..=255u8` is irrefutable. The range of values for an integer type is the closed range from its minimum to maximum value. The range of values for a `char` type are precisely those ranges containing all Unicode Scalar Values: `'\u{0000}'..='\u{D7FF}'` and `'\u{E000}'..='\u{10FFFF}'`.

## Reference patterns

> Syntax
> ReferencePattern :
> (`&`|`&&`) `mut`? Pattern

Reference patterns dereference the pointers that are being matched and, thus, borrow them.

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);
    }

The grammar production for reference patterns has to match the token `&&` to match a reference to a reference because it is a token by itself, not two `&` tokens.

Adding the `mut` keyword dereferences a mutable reference. The mutability must match the mutability of the reference.

Reference patterns are always irrefutable.

## Struct patterns

> Syntax
> StructPattern :
> PathInExpression `{`
> StructPatternElements ?
> `}`

> StructPatternElements :
> StructPatternFields (`,` | `,` StructPatternEtCetera)?
> | StructPatternEtCetera

> StructPatternFields :
> StructPatternField (`,` StructPatternField) *

> StructPatternField :
> OuterAttribute *
> (
> TUPLE_INDEX `:` Pattern
> | IDENTIFIER `:` Pattern
> | `ref`? `mut`? IDENTIFIER
> )

> StructPatternEtCetera :
> OuterAttribute *
> `..`

Struct patterns match struct values that match all criteria defined by its subpatterns. They are also used to destructure a struct.

On a struct pattern, the fields are referenced by name, index (in the case of tuple structs) or ignored by use of `..`:

    #![allow(unused_variables)]
    fn main() {
    struct Point {
        x: u32,
        y: u32,
    }
    let s = Point {x: 1, y: 1};

    match s {
        Point {x: 10, y: 20} => (),
        Point {y: 10, x: 20} => (),    // order doesn't matter
        Point {x: 10, ..} => (),
        Point {..} => (),
    }

    struct PointTuple (
        u32,
        u32,
    );
    let t = PointTuple(1, 2);

    match t {
        PointTuple {0: 10, 1: 20} => (),
        PointTuple {1: 10, 0: 20} => (),   // order doesn't matter
        PointTuple {0: 10, ..} => (),
        PointTuple {..} => (),
    }
    }

If `..` is not used, it is required to match all fields:

    #![allow(unused_variables)]
    fn main() {
    struct Struct {
       a: i32,
       b: char,
       c: bool,
    }
    let mut struct_value = Struct{a: 10, b: 'X', c: false};

    match struct_value {
        Struct{a: 10, b: 'X', c: false} => (),
        Struct{a: 10, b: 'X', ref c} => (),
        Struct{a: 10, b: 'X', ref mut c} => (),
        Struct{a: 10, b: 'X', c: _} => (),
        Struct{a: _, b: _, c: _} => (),
    }
    }

The `ref` and/or `mut` IDENTIFIER syntax matches any value and binds it to a variable with the same name as the given field.

    #![allow(unused_variables)]
    fn main() {
    struct Struct {
       a: i32,
       b: char,
       c: bool,
    }
    let struct_value = Struct{a: 10, b: 'X', c: false};

    let Struct{a: x, b: y, c: z} = struct_value;          // destructure all fields
    }

A struct pattern is refutable when one of its subpatterns is refutable.

## Tuple struct patterns

> Syntax
> TupleStructPattern :
> PathInExpression `(` TupleStructItems? `)`

> TupleStructItems :
> Pattern ( `,` Pattern )* `,`?
> | (Pattern `,`)* `..` (`,` Pattern)* `,`?

Tuple struct patterns match tuple struct and enum values that match all criteria defined by its subpatterns. They are also used to destructure a tuple struct or enum value.

A tuple struct pattern is refutable when one of its subpatterns is refutable.

## Tuple patterns

> Syntax
> TuplePattern :
> `(` TuplePatternItems? `)`

> TuplePatternItems :
> Pattern `,`
> | Pattern (`,` Pattern)+ `,`?
> | (Pattern `,`)* `..` (`,` Pattern)* `,`?

Tuple patterns match tuple values that match all criteria defined by its subpatterns. They are also used to destructure a tuple.

This pattern is refutable when one of its subpatterns is refutable.

## Grouped patterns

> Syntax
> GroupedPattern :
> `(` Pattern `)`

Enclosing a pattern in parentheses can be used to explicitly control the precedence of compound patterns. For example, a reference pattern next to a range pattern such as `&0..=5` is ambiguous and is not allowed, but can be expressed with parentheses.

    #![allow(unused_variables)]
    fn main() {
    let int_reference = &3;
    match int_reference {
        &(0..=5) => (),
        _ => (),
    }
    }

## Slice patterns

> Syntax
> SlicePattern :
> `[` Pattern (`,` Pattern)* `,`? `]`

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 the length is not known statically */ }
    };
    }

## Path patterns

> Syntax
> PathPattern :
> PathInExpression
> | QualifiedPathInExpression

Path patterns are patterns that refer either to constant values or to structs or enum variants that have no fields.

Unqualified path patterns can refer to:

- enum variants
- structs
- constants
- associated constants

Qualified path patterns can only refer to associated constants.

Constants cannot be a union type. Struct and enum constants must have `#[derive(PartialEq, Eq)]` (not merely implemented).

Path patterns are irrefutable when they refer to structs or an enum variant when the enum has only one variant or a constant whose type is irrefutable. They are refutable when they refer to refutable constants or enum variants for enums with multiple variants.