Status of async fn in traits

A brief overview.

What we call "async fn in traits" is really multiple parts:

Core support: ability to write async fn in traits + impls
Bounding: ability to say T: AsyncIter but also "the future returned by next() is Send"
Dynamic dispatch: ability to use dyn AsyncIter and invoke async fn methods
RPITIT: sometimes you want an async fn that doesn't capture everything. This would most naturally be expressed with -> impl Future in the trait, though GATs + TAITs work too.

And some auxiliary/optonal parts:

Refinements: ability to write async fn in trait but -> impl Future in impl, or other more specific future types, in impl

Current status:

Part	Status
Core support	feature complete, but fixing bugs and writing tests
Bounding	have a proposal to address this that needs to be better circulated, "return type notation"
Dynamic dispatch	design space is enumerated, need to settle on a concrete design
RPITIT	feature complete, but needs cleanup and also authoring of tests; GATs + TAITs will allow this to be written in "desugared form"

Dynamic dispatch is a complex topic, but there are two main designs, and they come with their own dependencies:

Design	Dependencies
Decide alloc strategy at call site (part 9)	returning of unsized values
Decide when creating dyn what adapter to use (part 8)	`dyn*` (started, needs elaboration)