Type coercions - The Rust Reference

# Type coercions

Coercions are defined in RFC 401. RFC 1558 then expanded on that. A coercion is implicit and has no syntax.

## Coercion sites

A coercion can only occur at certain coercion sites in a program; these are typically places where the desired type is explicit or can be derived by propagation from explicit types (without type inference). Possible coercion sites are:

- `let` statements where an explicit type is given.

  For example, `42` is coerced to have type `i8` in the following:

      # #![allow(unused_variables)]
      #fn main() {
      let _: i8 = 42;
      #}

- `static` and `const` statements (similar to `let` statements).

- Arguments for function calls

  The value being coerced is the actual parameter, and it is coerced to the type of the formal parameter.

  For example, `42` is coerced to have type `i8` in the following:

      fn bar(_: i8) { }

      fn main() {
          bar(42);
      }

  For method calls, the receiver (`self` parameter) can only take advantage of unsized coercions.

- Instantiations of struct or variant fields

  For example, `42` is coerced to have type `i8` in the following:

      struct Foo { x: i8 }

      fn main() {
          Foo { x: 42 };
      }

- Function results, either the final line of a block if it is not semicolon-terminated or any expression in a `return` statement

  For example, `42` is coerced to have type `i8` in the following:

      # #![allow(unused_variables)]
      #fn main() {
      fn foo() -> i8 {
          42
      }
      #}

If the expression in one of these coercion sites is a coercion-propagating expression, then the relevant sub-expressions in that expression are also coercion sites. Propagation recurses from these new coercion sites. Propagating expressions and their relevant sub-expressions are:

- Array literals, where the array has type `[U; n]`. Each sub-expression in the array literal is a coercion site for coercion to type `U`.

- Array literals with repeating syntax, where the array has type `[U; n]`. The repeated sub-expression is a coercion site for coercion to type `U`.

- Tuples, where a tuple is a coercion site to type `(U_0, U_1, ..., U_n)`. Each sub-expression is a coercion site to the respective type, e.g. the zeroth sub-expression is a coercion site to type `U_0`.

- Parenthesized sub-expressions (`(e)`): if the expression has type `U`, then the sub-expression is a coercion site to `U`.

- Blocks: if a block has type `U`, then the last expression in the block (if it is not semicolon-terminated) is a coercion site to `U`. This includes blocks which are part of control flow statements, such as `if`/`else`, if the block has a known type.

## Coercion types

Coercion is allowed between the following types:

- `T` to `U` if `T` is a subtype of `U` (reflexive case)

- `T_1` to `T_3` where `T_1` coerces to `T_2` and `T_2` coerces to `T_3` (transitive case)

  Note that this is not fully supported yet

- `&mut T` to `&T`

- `*mut T` to `*const T`

- `&T` to `*const T`

- `&mut T` to `*mut T`

- `&T` or `&mut T` to `&U` if `T` implements `Deref<Target = U>`. For example:

      use std::ops::Deref;

      struct CharContainer {
          value: char,
      }

      impl Deref for CharContainer {
          type Target = char;

          fn deref<'a>(&'a self) -> &'a char {
              &self.value
          }
      }

      fn foo(arg: &char) {}

      fn main() {
          let x = &mut CharContainer { value: 'y' };
          foo(x); //&mut CharContainer is coerced to &char.
      }

- `&mut T` to `&mut U` if `T` implements `DerefMut<Target = U>`.

- TyCtor(`T`) to TyCtor(`U`), where TyCtor(`T`) is one of

  - `&T`
  - `&mut T`
  - `*const T`
  - `*mut T`
  - `Box<T>`

  and where `T` can obtained from `U` by unsized coercion.

- Non capturing closures to `fn` pointers

- `!` to any `T`

### Unsized Coercions

The following coercions are called `unsized coercions`, since they relate to converting sized types to unsized types, and are permitted in a few cases where other coercions are not, as described above. They can still happen anywhere else a coercion can occur.

Two traits, [`Unsize`] and [`CoerceUnsized`], are used to assist in this process and expose it for library use. The compiler following coercions are built-in and, if `T` can be coerced to `U` with one of the, then the compiler will provide an implementation of `Unsize<U>` for `T`:

- `[T; n]` to `[T]`.

- `T` to `U`, when `U` is a trait object type and either `T` implements `U` or `T` is a trait object for a subtrait of `U`.

- `Foo<..., T, ...>` to `Foo<..., U, ...>`, when:

  - `Foo` is a struct.
  - `T` implements `Unsize<U>`.
  - The last field of `Foo` has a type involving `T`.
  - If that field has type `Bar<T>`, then `Bar<T>` implements `Unsized<Bar<U>>`.
  - T is not part of the type of any other fields.

Additionally, a type `Foo<T>` can implement `CoerceUnsized<Foo<U>>` when `T` implements `Unsize<U>` or `CoerceUnsized<Foo<U>>`. This allows it to provide a unsized coercion to `Foo<U>`.

> Note: While the definition of the unsized coercions and their implementation has been stabilized, the traits themselves are not yet stable and therefore can't be used directly in stable Rust.