Existential Types

Existential types are essentially strong type aliases which only expose a specific set of traits as their interface and the concrete type in the background is inferred from a certain set of use sites of the existential type.

In the language they are expressed via

existential type Foo: Bar;

This is in existential type named Foo which can be interacted with via the Bar trait's interface.

Since there needs to be a concrete background type, you can currently express that type by using the existential type in a "defining use site".

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

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

Defining use site(s)

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

The defining use of an existential type can be any code within the parent of the existential type definition. This includes any siblings of the existential 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 existential types to be defining use sites.

Associated existential types

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