"Return position impl trait in traits" initiative

[nikomatsakis]

Metadata
Name	"Return position impl trait in traits" initiative
Owner	@tmandry
Liaison	@nikomatsakis
Implementation	@spastorino

Purpose

Defining and stabilizing impl Trait in return position in a trait or trait impl:

trait Foo {
    fn bar(&self) -> impl Clone;
}

See the explainer for more details.

Active questions

• How can users name these types?
• If impl Trait is used in a trait definition, must it be used in an impl? What does it mean if it is not?

[nikomatsakis]

Update as of 2022-02: We opened one draft RFC on this topic but withdrew it to ponder various extensions.

In our most recent conversation, @tmandry and I concluded that

• we are probably going to push off the work on giving names for a bit, I'll update the exploration doc with some notes
• we are going to re-open the RFC but edit it with some content to allow for impls to give concrete types