Opaque types (type alias impl Trait)

Opaque types are syntax to declare an opaque type alias that only exposes a specific set of traits as their interface; the concrete type in the background is inferred from a certain set of use sites of the opaque type.

This is expressed by using impl Trait within type aliases, for example:

type Foo = impl Bar;

This declares an opaque type named Foo, of which the only information is that it implements Bar. Therefore, any of Bar's interface can be used on a Foo, but nothing else (regardless of whether it implements any other traits).

Since there needs to be a concrete background type, you can (as of January 2021) express that type by using the opaque type in a "defining use site".

struct Struct;
impl Bar for Struct { /* stuff */ }
fn foo() -> Foo {
    Struct
}

Any other "defining use site" needs to produce the exact same type.

Defining use site(s)

Currently only the return value of a function can be a defining use site of an opaque type (and only if the return type of that function contains the opaque type).

The defining use of an opaque type can be any code within the parent of the opaque type definition. This includes any siblings of the opaque type and all children of the siblings.

The initiative for "not causing fatal brain damage to developers due to accidentally running infinite loops in their brain while trying to comprehend what the type system is doing" has decided to disallow children of opaque types to be defining use sites.

Associated opaque types

Associated opaque types can be defined by any other associated item on the same trait impl or a child of these associated items. For instance:

trait Baz {
    type Foo;
    fn foo() -> Self::Foo;
}

struct Quux;

impl Baz for Quux {
    type Foo = impl Bar;
    fn foo() -> Self::Foo { ... }
}