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);
      }

- 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` 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(coerce_inner(`T`)), where TyCtor(`T`) is one of

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

  and where

  - coerce_inner(`[T, ..n]`) = `[T]`
  - coerce_inner(`T`) = `U` where `T` is a concrete type which implements the trait `U`.

- Non capturing closures to `fn` pointers