# Patterns

> **Syntax**\
> _Pattern_ :\
>       `|`? _PatternNoTopAlt_  ( `|` _PatternNoTopAlt_ )\*
>
> _PatternNoTopAlt_ :\
>       _PatternWithoutRange_\
>    | [_RangePattern_]
>
> _PatternWithoutRange_ :\
>       [_LiteralPattern_]\
>    | [_IdentifierPattern_]\
>    | [_WildcardPattern_]\
>    | [_RestPattern_]\
>    | [_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.

```rust
# 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](statements.md#let-statements)
* [Function](items/functions.md) and [closure](expressions/closure-expr.md)
  parameters
* [`match` expressions](expressions/match-expr.md)
* [`if let` expressions](expressions/if-expr.md)
* [`while let` expressions](expressions/loop-expr.md#predicate-pattern-loops)
* [`for` expressions](expressions/loop-expr.md#iterator-loops)

## 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`.

```rust
# 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:

```rust
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]

[BOOLEAN_LITERAL]: tokens.md#boolean-literals
[CHAR_LITERAL]: tokens.md#character-literals
[BYTE_LITERAL]: tokens.md#byte-literals
[STRING_LITERAL]: tokens.md#string-literals
[RAW_STRING_LITERAL]: tokens.md#raw-string-literals
[BYTE_STRING_LITERAL]: tokens.md#byte-string-literals
[RAW_BYTE_STRING_LITERAL]: tokens.md#raw-byte-string-literals
[INTEGER_LITERAL]: tokens.md#integer-literals
[FLOAT_LITERAL]: tokens.md#floating-point-literals

_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](https://github.com/rust-lang/rust/issues/41620)).

Literal patterns are always refutable.

Examples:

```rust
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.

```rust
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).

```rust
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:

```rust
# 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:

```rust,compile_fail
# 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:

```rust
# 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](#path-patterns) take precedence over identifier patterns. It is an error
if `ref` or `ref mut` is specified and the identifier shadows a constant.

Identifier patterns are irrefutable if the `@` subpattern is irrefutable or
the subpattern is not specified.

### 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:

```rust
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](#wildcard-pattern) (`_`), [`const` patterns](#path-patterns) of reference types,
and [reference patterns](#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.

Move bindings and reference bindings can be mixed together in the same pattern, doing so will
result in partial move of the object bound to and the object cannot be used afterwards.
This applies only if the type cannot be copied.

In the example below, `name` is moved out of `person`, trying to use `person` as a whole or
`person.name` would result in an error because of *partial move*.

Example:

```rust
# struct Person {
#    name: String,
#    age: u8,
# }
# let person = Person{ name: String::from("John"), age: 23 };
// `name` is moved from person and `age` referenced
let Person { name, ref age } = person;
```

## Wildcard pattern

> **Syntax**\
> _WildcardPattern_ :\
>    `_`

The _wildcard pattern_ (an underscore symbol) 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:

```rust
# 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.

## Rest patterns

> **Syntax**\
> _RestPattern_ :\
>    `..`

The _rest pattern_ (the `..` token) acts as a variable-length pattern which
matches zero or more elements that haven't been matched already before and
after. It may only be used in [tuple](#tuple-patterns), [tuple
struct](#tuple-struct-patterns), and [slice](#slice-patterns) patterns, and
may only appear once as one of the elements in those patterns. It is also
allowed in an [identifier pattern](#identifier-patterns) for [slice
patterns](#slice-patterns) only.

The rest pattern is always irrefutable.

Examples:

```rust
# let words = vec!["a", "b", "c"];
# let slice = &words[..];
match slice {
    [] => println!("slice is empty"),
    [one] => println!("single element {}", one),
    [head, tail @ ..] => println!("head={} tail={:?}", head, tail),
}

match slice {
    // Ignore everything but the last element, which must be "!".
    [.., "!"] => println!("!!!"),

    // `start` is a slice of everything except the last element, which must be "z".
    [start @ .., "z"] => println!("starts with: {:?}", start),

    // `end` is a slice of everything but the first element, which must be "a".
    ["a", end @ ..] => println!("ends with: {:?}", end),

    // 'whole' is the entire slice and `last` is the final element
    whole @ [.., last] => println!("the last element of {:?} is {}", whole, last),

    rest => println!("{:?}", rest),
}

if let [.., penultimate, _] = slice {
    println!("next to last is {}", penultimate);
}

# let tuple = (1, 2, 3, 4, 5);
// Rest patterns may also be used in tuple and tuple struct patterns.
match tuple {
    (1, .., y, z) => println!("y={} z={}", y, z),
    (.., 5) => println!("tail must be 5"),
    (..) => println!("matches everything else"),
}
```

## Range patterns

> **Syntax**\
> _RangePattern_ :\
>       _InclusiveRangePattern_\
>    | _HalfOpenRangePattern_\
>    | _ObsoleteRangePattern_
>
> _InclusiveRangePattern_ :\
>       _RangePatternBound_ `..=` _RangePatternBound_
>
> _HalfOpenRangePattern_ :\
>    | _RangePatternBound_ `..`
>
> _ObsoleteRangePattern_ :\
>    _RangePatternBound_ `...` _RangePatternBound_
>
> _RangePatternBound_ :\
>       [CHAR_LITERAL]\
>    | [BYTE_LITERAL]\
>    | `-`? [INTEGER_LITERAL]\
>    | `-`? [FLOAT_LITERAL]\
>    | [_PathInExpression_]\
>    | [_QualifiedPathInExpression_]

Range patterns match values within the range defined by their bounds. A range pattern may be
closed or half-open. A range pattern is closed if it has both a lower and an upper bound, and
it matches all the values between and including both of its bounds. A range pattern that is
half-open is written with a lower bound but not an upper bound, and matches any value equal to
or greater than the specified lower bound.

For example, a pattern `'m'..='p'` will match only the values `'m'`, `'n'`, `'o'`, and `'p'`. For an integer the
pattern `1..` will match 9, or 9001, or 9007199254740991 (if it is of an appropriate size), but
not 0, and not negative numbers for signed integers. The bounds can be literals or paths that point
to constant values.

A half-open range pattern in the style `a..` cannot be used to match within the context of a slice.

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

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](https://github.com/rust-lang/rust/issues/41620)).

Examples:

```rust
# 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!(),
});

# let uint: u32 = 5;
match uint {
    0 => "zero!",
    1.. => "positive number!",
};

// 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}'`.

> **Edition Differences**: Before the 2021 edition, closed range patterns may also be written using `...` as an alternative to `..=`, with the same meaning.

## Reference patterns

> **Syntax**\
> _ReferencePattern_ :\
>    (`&`|`&&`) `mut`? [_PatternWithoutRange_]

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

For example, these two matches on `x: &i32` are equivalent:

```rust
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_] \*\
>    `..`

[_OuterAttribute_]: attributes.md
[TUPLE_INDEX]: tokens.md#tuple-index

Struct patterns match struct values that match all criteria defined by its subpatterns.
They are also used to [destructure](#destructuring) a struct.

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

```rust
# 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:

```rust
# 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.

```rust
# 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_] )\* `,`?

Tuple struct patterns match tuple struct and enum values that match all criteria defined
by its subpatterns. They are also used to [destructure](#destructuring) 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_] `,`\
>    | [_RestPattern_]\
>    | [_Pattern_] (`,` [_Pattern_])+ `,`?

Tuple patterns match tuple values that match all criteria defined by its subpatterns.
They are also used to [destructure](#destructuring) a tuple.

The form `(..)` with a single [_RestPattern_] is a special form that does not
require a comma, and matches a tuple of any size.

The tuple pattern is refutable when one of its subpatterns is refutable.

An example of using tuple patterns:

```rust
let pair = (10, "ten");
let (a, b) = pair;

assert_eq!(a, 10);
assert_eq!(b, "ten");
```

## 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.

```rust
let int_reference = &3;
match int_reference {
    &(0..=5) => (),
    _ => (),
}
```

## Slice patterns

> **Syntax**\
> _SlicePattern_ :\
>    `[` _SlicePatternItems_? `]`
>
> _SlicePatternItems_ :\
>    [_Pattern_] \(`,` [_Pattern_])\* `,`?

Slice patterns can match both arrays of fixed size and slices of dynamic size.
```rust
// Fixed size
let arr = [1, 2, 3];
match arr {
    [1, _, _] => "starts with one",
    [a, b, c] => "starts with something else",
};
```
```rust
// 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 */ }
};
```

Slice patterns are irrefutable when matching an array as long as each element
is irrefutable. When matching a slice, it is irrefutable only in the form with
a single `..` [rest pattern](#rest-patterns) or [identifier
pattern](#identifier-patterns) with the `..` rest pattern as a subpattern.

Within a slice, a half-open range pattern like `a..` must be enclosed in parentheses,
as in `(a..)`, to clarify it is intended to match a single value.
A future version of Rust may give the non-parenthesized version an alternate meaning.

## 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.

## Or-patterns

_Or-patterns_ are patterns that match on one of two or more sub-patterns (e.g.
`A | B | C`). They can nest arbitrarily. Syntactically, or-patterns are allowed
in any of the places where other patterns are allowed (represented by the
_Pattern_ production), with the exceptions of `let`-bindings and function and
closure arguments (represented by the _PatternNoTopAlt_ production).

### Static semantics

1. Given a pattern `p | q` at some depth for some arbitrary patterns `p` and `q`,
   the pattern is considered ill-formed if:

   + the type inferred for `p` does not unify with the type inferred for `q`, or
   + the same set of bindings are not introduced in `p` and `q`, or
   + the type of any two bindings with the same name in `p` and `q` do not unify
     with respect to types or binding modes.

   Unification of types is in all instances aforementioned exact and
   implicit [type coercions] do not apply.

2. When type checking an expression `match e_s { a_1 => e_1, ... a_n => e_n }`,
   for each match arm `a_i` which contains a pattern of form `p_i | q_i`,
   the pattern `p_i | q_i` is considered ill formed if,
   at the depth `d` where it exists the fragment of `e_s` at depth `d`,
   the type of the expression fragment does not unify with `p_i | q_i`.

3. With respect to exhaustiveness checking, a pattern `p | q` is
   considered to cover `p` as well as `q`. For some constructor `c(x, ..)`
   the distributive law applies such that `c(p | q, ..rest)` covers the same
   set of value as `c(p, ..rest) | c(q, ..rest)` does. This can be applied
   recursively until there are no more nested patterns of form `p | q` other
   than those that exist at the top level.

   Note that by *"constructor"* we do not refer to tuple struct patterns,
   but rather we refer to a pattern for any product type.
   This includes enum variants, tuple structs, structs with named fields,
   arrays, tuples, and slices.

### Dynamic semantics

1. The dynamic semantics of pattern matching a scrutinee expression `e_s`
   against a pattern `c(p | q, ..rest)` at depth `d` where `c` is some constructor,
   `p` and `q` are arbitrary patterns, and `rest` is optionally any remaining
   potential factors in `c`, is defined as being the same as that of
   `c(p, ..rest) | c(q, ..rest)`.

### Precedence with other undelimited patterns

As shown elsewhere in this chapter, there are several types of patterns that
are syntactically undelimited, including identifier patterns, reference
patterns, and or-patterns. Or-patterns always have the lowest-precedence.  This
allows us to reserve syntactic space for a possible future type ascription
feature and also to reduce ambiguity. For example, `x @ A(..) | B(..)` will
result in an error that `x` is not bound in all patterns, `&A(x) | B(x)` will
result in a type mismatch between `x` in the different subpatterns.

[_GroupedPattern_]: #grouped-patterns
[_IdentifierPattern_]: #identifier-patterns
[_LiteralPattern_]: #literal-patterns
[_MacroInvocation_]: macros.md#macro-invocation
[_ObsoleteRangePattern_]: #range-patterns
[_PathInExpression_]: paths.md#paths-in-expressions
[_PathPattern_]: #path-patterns
[_Pattern_]: #patterns
[_PatternWithoutRange_]: #patterns
[_QualifiedPathInExpression_]: paths.md#qualified-paths
[_RangePattern_]: #range-patterns
[_ReferencePattern_]: #reference-patterns
[_RestPattern_]: #rest-patterns
[_SlicePattern_]: #slice-patterns
[_StructPattern_]: #struct-patterns
[_TuplePattern_]: #tuple-patterns
[_TupleStructPattern_]: #tuple-struct-patterns
[_WildcardPattern_]: #wildcard-pattern

[`Copy`]: special-types-and-traits.md#copy
[IDENTIFIER]: identifiers.md
[enums]: items/enumerations.md
[literals]: expressions/literal-expr.md
[structs]: items/structs.md
[tuples]: types/tuple.md
[scrutinee]: glossary.md#scrutinee
[type coercions]: type-coercions.md
