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:

  • Transitivity: T_1 to T_3 where T_1 coerces to T_2 and T_2 coerces to T_3
  • Pointer Weakening:
    • &mut T to &T
    • *mut T to *const T
    • &T to *const T
    • &mut T to *mut T
  • Unsizing: T to U if T implements CoerceUnsized<U>
  • Deref coercion: Expression &x of type &T to &*x of type &U if T derefs to U (i.e. T: Deref<Target=U>)

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]
  • T => dyn Trait where T: Trait
  • Foo<..., T, ...> => Foo<..., U, ...> where:
    • T: Unsize<U>
    • Foo is a struct
    • Only the last field of Foo has type involving T
    • T is not part of the type of any other fields
    • Bar<T>: Unsize<Bar<U>>, if the last field of Foo has type Bar<T>

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 to type 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 and T coerces to U, that does not constitute an implementation for T. For example, the following will not type check, even though it is OK to coerce t to &T and 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);
}

error[E0277]: the trait bound `&mut i32: Trait` is not satisfied
 --> src/main.rs:9:5
  |
9 |     foo(t);
  |     ^^^ the trait `Trait` is not implemented for `&mut i32`
  |
  = help: the following implementations were found:
            <&'a i32 as Trait>
note: required by `foo`
 --> src/main.rs:3:1
  |
3 | fn foo<X: Trait>(t: X) {}
  | ^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error