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[rustc.git] / src / doc / reference / src / items / generics.md

# Generic parameters

> **<sup>Syntax</sup>**\
> _GenericParams_ :\
> &nbsp;&nbsp; &nbsp;&nbsp; `<` `>`\
> &nbsp;&nbsp;  | `<` (_GenericParam_ `,`)<sup>\*</sup> _GenericParam_ `,`<sup>?</sup> `>`
>
> _GenericParam_ :\
> &nbsp;&nbsp; [_OuterAttribute_]<sup>\*</sup> ( _LifetimeParam_ | _TypeParam_ | _ConstParam_ )
>
> _LifetimeParam_ :\
> &nbsp;&nbsp; [LIFETIME_OR_LABEL]&nbsp;( `:` [_LifetimeBounds_] )<sup>?</sup>
>
> _TypeParam_ :\
> &nbsp;&nbsp; [IDENTIFIER]( `:` [_TypeParamBounds_]<sup>?</sup> )<sup>?</sup> ( `=` [_Type_] )<sup>?</sup>
>
> _ConstParam_:\
> &nbsp;&nbsp; `const` [IDENTIFIER] `:` [_Type_]

[Functions], [type aliases], [structs], [enumerations], [unions], [traits], and
[implementations] may be *parameterized* by types, constants, and lifetimes. These
parameters are listed in angle <span class="parenthetical">brackets (`<...>`)</span>,
usually immediately after the name of the item and before its definition. For
implementations, which don't have a name, they come directly after `impl`.
The order of generic parameters is restricted to lifetime parameters, then type parameters, and then const parameters.

Some examples of items with type, const, and lifetime parameters:

```rust
fn foo<'a, T>() {}
trait A<U> {}
struct Ref<'a, T> where T: 'a { r: &'a T }
struct InnerArray<T, const N: usize>([T; N]);
```

Generic parameters are in scope within the item definition where they are
declared. They are not in scope for items declared within the body of a
function as described in [item declarations].

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

### Const generics

*Const generic parameters* allow items to be generic over constant values. The
const identifier introduces a name for the constant parameter, and all
instances of the item must be instantiated with a value of the given type.

<!-- TODO: update above to say "introduces a name in the [value namespace]"
    once namespaces are added. -->

The only allowed types of const parameters are `u8`, `u16`, `u32`, `u64`, `u128`, `usize`
`i8`, `i16`, `i32`, `i64`, `i128`, `isize`, `char` and `bool`.

Const parameters can be used anywhere a [const item] can be used, with the
exception that when used in a [type] or [array repeat expression], it must be
standalone (as described below). That is, they are allowed in the following
places:

1. As an applied const to any type which forms a part of the signature of the
   item in question.
2. As part of a const expression used to define an [associated const], or as a
   parameter to an [associated type].
3. As a value in any runtime expression in the body of any functions in the
   item.
4. As a parameter to any type used in the body of any functions in the item.
5. As a part of the type of any fields in the item.

```rust
// Examples where const generic parameters can be used.

// Used in the signature of the item itself.
fn foo<const N: usize>(arr: [i32; N]) {
    // Used as a type within a function body.
    let x: [i32; N];
    // Used as an expression.
    println!("{}", N * 2);
}

// Used as a field of a struct.
struct Foo<const N: usize>([i32; N]);

impl<const N: usize> Foo<N> {
    // Used as an associated constant.
    const CONST: usize = N * 4;
}

trait Trait {
    type Output;
}

impl<const N: usize> Trait for Foo<N> {
    // Used as an associated type.
    type Output = [i32; N];
}
```

```rust,compile_fail
// Examples where const generic parameters cannot be used.
fn foo<const N: usize>() {
    // Cannot use in item definitions within a function body.
    const BAD_CONST: [usize; N] = [1; N];
    static BAD_STATIC: [usize; N] = [1; N];
    fn inner(bad_arg: [usize; N]) {
        let bad_value = N * 2;
    }
    type BadAlias = [usize; N];
    struct BadStruct([usize; N]);
}
```

As a further restriction, const parameters may only appear as a standalone
argument inside of a [type] or [array repeat expression]. In those contexts,
they may only be used as a single segment [path expression], possibly inside a
[block] (such as `N` or `{N}`). That is, they cannot be combined with other
expressions.

```rust,compile_fail
// Examples where const parameters may not be used.

// Not allowed to combine in other expressions in types, such as the
// arithmetic expression in the return type here.
fn bad_function<const N: usize>() -> [u8; {N + 1}] {
    // Similarly not allowed for array repeat expressions.
    [1; {N + 1}]
}
```

A const argument in a [path] specifies the const value to use for that item.
The argument must be a [const expression] of the type ascribed to the const
parameter. The const expression must be a [block expression][block]
(surrounded with braces) unless it is a single path segment (an [IDENTIFIER])
or a [literal] (with a possibly leading `-` token).

> **Note**: This syntactic restriction is necessary to avoid requiring
> infinite lookahead when parsing an expression inside of a type.

```rust
fn double<const N: i32>() {
    println!("doubled: {}", N * 2);
}

const SOME_CONST: i32 = 12;

fn example() {
    // Example usage of a const argument.
    double::<9>();
    double::<-123>();
    double::<{7 + 8}>();
    double::<SOME_CONST>();
    double::<{ SOME_CONST + 5 }>();
}
```

When there is ambiguity if a generic argument could be resolved as either a
type or const argument, it is always resolved as a type. Placing the argument
in a block expression can force it to be interpreted as a const argument.

<!-- TODO: Rewrite the paragraph above to be in terms of namespaces, once
    namespaces are introduced, and it is clear which namespace each parameter
    lives in. -->

```rust,compile_fail
type N = u32;
struct Foo<const N: usize>;
// The following is an error, because `N` is interpreted as the type alias `N`.
fn foo<const N: usize>() -> Foo<N> { todo!() } // ERROR
// Can be fixed by wrapping in braces to force it to be interpreted as the `N`
// const parameter:
fn bar<const N: usize>() -> Foo<{ N }> { todo!() } // ok
```

Unlike type and lifetime parameters, const parameters can be declared without
being used inside of a parameterized item, with the exception of
implementations as described in [generic implementations]:

```rust,compile_fail
// ok
struct Foo<const N: usize>;
enum Bar<const M: usize> { A, B }

// ERROR: unused parameter
struct Baz<T>;
struct Biz<'a>;
struct Unconstrained;
impl<const N: usize> Unconstrained {}
```

When resolving a trait bound obligation, the exhaustiveness of all
implementations of const parameters is not considered when determining if the
bound is satisfied. For example, in the following, even though all possible
const values for the `bool` type are implemented, it is still an error that
the trait bound is not satisfied:

```rust,compile_fail
struct Foo<const B: bool>;
trait Bar {}
impl Bar for Foo<true> {}
impl Bar for Foo<false> {}

fn needs_bar(_: impl Bar) {}
fn generic<const B: bool>() {
    let v = Foo::<B>;
    needs_bar(v); // ERROR: trait bound `Foo<B>: Bar` is not satisfied
}
```


## Where clauses

> **<sup>Syntax</sup>**\
> _WhereClause_ :\
> &nbsp;&nbsp; `where` ( _WhereClauseItem_ `,` )<sup>\*</sup> _WhereClauseItem_ <sup>?</sup>
>
> _WhereClauseItem_ :\
> &nbsp;&nbsp; &nbsp;&nbsp; _LifetimeWhereClauseItem_\
> &nbsp;&nbsp; | _TypeBoundWhereClauseItem_
>
> _LifetimeWhereClauseItem_ :\
> &nbsp;&nbsp; [_Lifetime_] `:` [_LifetimeBounds_]
>
> _TypeBoundWhereClauseItem_ :\
> &nbsp;&nbsp; [_ForLifetimes_]<sup>?</sup> [_Type_] `:` [_TypeParamBounds_]<sup>?</sup>

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

The `for` keyword can be used to introduce [higher-ranked lifetimes]. It only
allows [_LifetimeParam_] parameters.

```rust
struct A<T>
where
    T: Iterator,            // Could use A<T: Iterator> instead
    T::Item: Copy,          // Bound on an associated type
    String: PartialEq<T>,   // Bound on `String`, using the type parameter
    i32: Default,           // Allowed, but not useful
{
    f: T,
}
```

## Attributes

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

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

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

[IDENTIFIER]: ../identifiers.md
[LIFETIME_OR_LABEL]: ../tokens.md#lifetimes-and-loop-labels

[_ForLifetimes_]: ../trait-bounds.md#higher-ranked-trait-bounds
[_LifetimeParam_]: #generic-parameters
[_LifetimeBounds_]: ../trait-bounds.md
[_Lifetime_]: ../trait-bounds.md
[_OuterAttribute_]: ../attributes.md
[_Type_]: ../types.md#type-expressions
[_TypeParamBounds_]: ../trait-bounds.md

[array repeat expression]: ../expressions/array-expr.md
[arrays]: ../types/array.md
[associated const]: associated-items.md#associated-constants
[associated type]: associated-items.md#associated-types
[block]: ../expressions/block-expr.md
[const contexts]: ../const_eval.md#const-context
[const expression]: ../const_eval.md#constant-expressions
[const item]: constant-items.md
[enumerations]: enumerations.md
[functions]: functions.md
[function pointers]: ../types/function-pointer.md
[generic implementations]: implementations.md#generic-implementations
[higher-ranked lifetimes]: ../trait-bounds.md#higher-ranked-trait-bounds
[implementations]: implementations.md
[item declarations]: ../statements.md#item-declarations
[item]: ../items.md
[literal]: ../expressions/literal-expr.md
[path]: ../paths.md
[path expression]: ../expressions/path-expr.md
[raw pointers]: ../types/pointer.md#raw-pointers-const-and-mut
[references]: ../types/pointer.md#shared-references-
[structs]: structs.md
[tuples]: ../types/tuple.md
[trait object]: ../types/trait-object.md
[traits]: traits.md
[type aliases]: type-aliases.md
[type]: ../types.md
[unions]: unions.md
[attributes]: ../attributes.md