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

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 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

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 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.