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:
T_1 to T_3 where T_1 coerces to T_2 and T_2 coerces to
T_3&mut T to &T*mut T to *const T&T to *const T&mut T to *mut TT to U if T implements CoerceUnsized<U>&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 => Trait where T: TraitFoo<..., T, ...> => Foo<..., U, ...> where:
T: Unsize<U>Foo is a structFoo has type involving TT is not part of the type of any other fieldsBar<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 x: U = etakes_a_U(e)fn foo() -> U { e }Foo { some_u: e }let x: [U; 10] = [e, ..]let x: (U, ..) = (e, ..)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); }Run
<anon>:10:5: 10:8 error: the trait bound `&mut i32 : Trait` is not satisfied [E0277]
<anon>:10 foo(t);
^~~