Type coercions - The Rust Reference

# Type coercions

Type coercions are implicit operations that change the type of a value. They happen automatically at specific locations and are highly restricted in what types actually coerce.

Any conversions allowed by coercion can also be explicitly performed by the type cast operator, `as`.

Coercions are originally defined in RFC 401 and expanded upon in RFC 1558.

## 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, `&mut 42` is coerced to have type `&i8` in the following:

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

- `static` and `const` item declarations (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, `&mut 42` is coerced to have type `&i8` in the following:

      fn bar(_: &i8) { }

      fn main() {
          bar(&mut 42);
      }

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

- Instantiations of struct, union, or enum variant fields

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

      struct Foo<'a> { x: &'a i8 }

      fn main() {
          Foo { x: &mut 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, `x` is coerced to have type `&dyn Display` in the following:

      #![allow(unused)]
      fn main() {
      use std::fmt::Display;
      fn foo(x: &u32) -> &dyn Display {
          x
      }
      }

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 `U` can be obtained from `T` 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 following coercions are built-ins and, if `T` can be coerced to `U` with one of them, then an implementation of `Unsize<U>` for `T` will be provided:

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

- `T` to `dyn U`, when `T` implements `U + Sized`, and `U` is object safe.

- `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.

## Least upper bound coercions

In some contexts, the compiler must coerce together multiple types to try and find the most general type. This is called a "Least Upper Bound" coercion. LUB coercion is used and only used in the following situations:

- To find the common type for a series of if branches.
- To find the common type for a series of match arms.
- To find the common type for array elements.
- To find the type for the return type of a closure with multiple return statements.
- To check the type for the return type of a function with multiple return statements.

In each such case, there are a set of types `T0..Tn` to be mutually coerced to some target type `T_t`, which is unknown to start. Computing the LUB coercion is done iteratively. The target type `T_t` begins as the type `T0`. For each new type `Ti`, we consider whether

- If `Ti` can be coerced to the current target type `T_t`, then no change is made.
- Otherwise, check whether `T_t` can be coerced to `Ti`; if so, the `T_t` is changed to `Ti`. (This check is also conditioned on whether all of the source expressions considered thus far have implicit coercions.)
- If not, try to compute a mutual supertype of `T_t` and `Ti`, which will become the new target type.

### Examples:

    #![allow(unused)]
    fn main() {
    let (a, b, c) = (0, 1, 2);
    // For if branches
    let bar = if true {
        a
    } else if false {
        b
    } else {
        c
    };

    // For match arms
    let baw = match 42 {
        0 => a,
        1 => b,
        _ => c,
    };

    // For array elements
    let bax = [a, b, c];

    // For closure with multiple return statements
    let clo = || {
        if true {
            a
        } else if false {
            b
        } else {
            c
        }
    };
    let baz = clo();

    // For type checking of function with multiple return statements
    fn foo() -> i32 {
        let (a, b, c) = (0, 1, 2);
        match 42 {
            0 => a,
            1 => b,
            _ => c,
        }
    }
    }

In these examples, types of the `ba*` are found by LUB coercion. And the compiler checks whether LUB coercion result of `a`, `b`, `c` is `i32` in the processing of the function `foo`.

### Caveat

This description is obviously informal. Making it more precise is expected to proceed as part of a general effort to specify the Rust type checker more precisely.