trait-bounds.md

Trait and lifetime bounds

^Syntax
TypeParamBounds :
   TypeParamBound ( + TypeParamBound )^* +^?

TypeParamBound :
      Lifetime | TraitBound

TraitBound :
      ?^? ForLifetimes^? TypePath
   | ( ?^? ForLifetimes^? TypePath )

LifetimeBounds :
   ( Lifetime + )^* Lifetime^?

Lifetime :
      LIFETIME_OR_LABEL
   | 'static
   | '_

Trait and lifetime bounds provide a way for generic items to restrict which types and lifetimes are used as their parameters. Bounds can be provided on any type in a where clause. There are also shorter forms for certain common cases:

• Bounds written after declaring a generic parameter: fn f<A: Copy>() {} is the same as fn f<A>() where A: Copy {}.
• In trait declarations as supertraits: trait Circle : Shape {} is equivalent to trait Circle where Self : Shape {}.
• In trait declarations as bounds on associated types: trait A { type B: Copy; } is equivalent to trait A where Self::B: Copy { type B; }.

Bounds on an item must be satisfied when using the item. When type checking and borrow checking a generic item, the bounds can be used to determine that a trait is implemented for a type. For example, given Ty: Trait

• In the body of a generic function, methods from Trait can be called on Ty values. Likewise associated constants on the Trait can be used.
• Associated types from Trait can be used.
• Generic functions and types with a T: Trait bounds can be used with Ty being used for T.

# type Surface = i32;
trait Shape {
    fn draw(&self, surface: Surface);
    fn name() -> &'static str;
}

fn draw_twice<T: Shape>(surface: Surface, sh: T) {
    sh.draw(surface);           // Can call method because T: Shape
    sh.draw(surface);
}

fn copy_and_draw_twice<T: Copy>(surface: Surface, sh: T) where T: Shape {
    let shape_copy = sh;        // doesn't move sh because T: Copy
    draw_twice(surface, sh);    // Can use generic function because T: Shape
}

struct Figure<S: Shape>(S, S);

fn name_figure<U: Shape>(
    figure: Figure<U>,          // Type Figure<U> is well-formed because U: Shape
) {
    println!(
        "Figure of two {}",
        U::name(),              // Can use associated function
    );
}

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

Copy, Clone, and Sized bounds are also checked for certain generic types when using the item, even if the use does not provide a concrete type. It is an error to have Copy or Clone as a bound on a mutable reference, trait object, or slice. It is an error to have Sized as a bound on a trait object or slice.

struct A<'a, T>
where
    i32: Default,           // Allowed, but not useful
    i32: Iterator,          // Error: `i32` is not an iterator
    &'a mut T: Copy,        // (at use) Error: the trait bound is not satisfied
    [T]: Sized,             // (at use) Error: size cannot be known at compilation
{
    f: &'a T,
}
struct UsesA<'a, T>(A<'a, T>);

Trait and lifetime bounds are also used to name trait objects.

?Sized

? is only used to relax the implicit Sized trait bound for type parameters or associated types. ?Sized may not be used as a bound for other types.

Lifetime bounds

Lifetime bounds can be applied to types or to other lifetimes. The bound 'a: 'b is usually read as 'a outlives 'b. 'a: 'b means that 'a lasts at least as long as 'b, so a reference &'a () is valid whenever &'b () is valid.

fn f<'a, 'b>(x: &'a i32, mut y: &'b i32) where 'a: 'b {
    y = x;                      // &'a i32 is a subtype of &'b i32 because 'a: 'b
    let r: &'b &'a i32 = &&0;   // &'b &'a i32 is well formed because 'a: 'b
}

T: 'a means that all lifetime parameters of T outlive 'a. For example, if 'a is an unconstrained lifetime parameter, then i32: 'static and &'static str: 'a are satisfied, but Vec<&'a ()>: 'static is not.

Higher-ranked trait bounds

ForLifetimes :
   for GenericParams

Trait bounds may be higher ranked over lifetimes. These bounds specify a bound that is true for all lifetimes. For example, a bound such as for<'a> &'a T: PartialEq<i32> would require an implementation like

# struct T;
impl<'a> PartialEq<i32> for &'a T {
    // ...
#    fn eq(&self, other: &i32) -> bool {true}
}

and could then be used to compare a &'a T with any lifetime to an i32.

Only a higher-ranked bound can be used here, because the lifetime of the reference is shorter than any possible lifetime parameter on the function:

fn call_on_ref_zero<F>(f: F) where for<'a> F: Fn(&'a i32) {
    let zero = 0;
    f(&zero);
}

Higher-ranked lifetimes may also be specified just before the trait: the only difference is the scope of the lifetime parameter, which extends only to the end of the following trait instead of the whole bound. This function is equivalent to the last one.

fn call_on_ref_zero<F>(f: F) where F: for<'a> Fn(&'a i32) {
    let zero = 0;
    f(&zero);
}

Implied bounds

Lifetime bounds required for types to be well-formed are sometimes inferred.

fn requires_t_outlives_a<'a, T>(x: &'a T) {}

The type parameter T is required to outlive 'a for the type &'a T to be well-formed. This is inferred because the function signature contains the type &'a T which is only valid if T: 'a holds.

Implied bounds are added for all parameters and outputs of functions. Inside of requires_t_outlives_a you can assume T: 'a to hold even if you don't explicitly specify this:

fn requires_t_outlives_a_not_implied<'a, T: 'a>() {}

fn requires_t_outlives_a<'a, T>(x: &'a T) {
    // This compiles, because `T: 'a` is implied by
    // the reference type `&'a T`.
    requires_t_outlives_a_not_implied::<'a, T>();
}

# fn requires_t_outlives_a_not_implied<'a, T: 'a>() {}
fn not_implied<'a, T>() {
    // This errors, because `T: 'a` is not implied by
    // the function signature.
    requires_t_outlives_a_not_implied::<'a, T>();
}

Only lifetime bounds are implied, trait bounds still have to be explicitly added. The following example therefore causes an error:

use std::fmt::Debug;
struct IsDebug<T: Debug>(T);
// error[E0277]: `T` doesn't implement `Debug`
fn doesnt_specify_t_debug<T>(x: IsDebug<T>) {}

Lifetime bounds are also inferred for type definitions and impl blocks for any type:

struct Struct<'a, T> {
    // This requires `T: 'a` to be well-formed
    // which is inferred by the compiler.
    field: &'a T,
}

enum Enum<'a, T> {
    // This requires `T: 'a` to be well-formed,
    // which is inferred by the compiler.
    //
    // Note that `T: 'a` is required even when only
    // using `Enum::OtherVariant`.
    SomeVariant(&'a T),
    OtherVariant,
}

trait Trait<'a, T: 'a> {}

// This would error because `T: 'a` is not implied by any type
// in the impl header.
//     impl<'a, T> Trait<'a, T> for () {}

// This compiles as `T: 'a` is implied by the self type `&'a T`.
impl<'a, T> Trait<'a, T> for &'a T {}