Coercions

Types can implicitly be coerced to change in certain contexts. These changes are generally just weakening of types, largely focused around pointers and lifetimes. They mostly exist to make Rust "just work" in more cases, and are largely harmless.

Here's all the kinds of coercion:

Coercion is allowed between the following types:

CoerceUnsized<Pointer<U>> for Pointer<T> where T: Unsize<U> is implemented for all pointer types (including smart pointers like Box and Rc). Unsize is only implemented automatically, and enables the following transformations:

  • [T, ..n]`[T, ..n]=>` => [T]`[T]`
  • T`T=>` => Trait`Traitwhere` where T: Trait`T: Trait`
  • Foo<..., T, ...> => Foo<..., U, ...> where:
    • T: Unsize<U>
    • Foo`Foo` is a struct
    • Only the last field of Foo`Foohas type` has type T`T`
    • T`T` is not part of the type of any other fields

Coercions occur at a coercion site. Any location that is explicitly typed will cause a coercion to its type. If inference is necessary, the coercion will not be performed. Exhaustively, the coercion sites for an expression e`eto type` to type U`U` are:

  • let statements, statics, and consts: let x: U = e
  • Arguments to functions: takes_a_U(e)
  • Any expression that will be returned: fn foo() -> U { e }
  • Struct literals: Foo { some_u: e }
  • Array literals: let x: [U; 10] = [e, ..]
  • Tuple literals: let x: (U, ..) = (e, ..)
  • The last expression in a block: let x: U = { ..; e }

Note that we do not perform coercions when matching traits (except for receivers, see below). If there is an impl for some type U`Uand` and T`Tcoerces toU, that does not constitute an implementation forT. For example, the following will not type check, even though it is OK to coercetto` to &T`&Tand there is an impl for&T`:

trait Trait {} fn foo<X: Trait>(t: X) {} impl<'a> Trait for &'a i32 {} fn main() { let t: &mut i32 = &mut 0; foo(t); } 
trait Trait {}

fn foo<X: Trait>(t: X) {}

impl<'a> Trait for &'a i32 {}


fn main() {
    let t: &mut i32 = &mut 0;
    foo(t);
}
<anon>:10:5: 10:8 error: the trait `Trait` is not implemented for the type `&mut i32` [E0277]
<anon>:10     foo(t);
              ^~~