Any efforts or tracking on stabilizing said associated types? Without them, you can't basically have both AsyncFn* lifetime link and ability to add constraints like Send onto returned future.
While AsyncFn* traits landed recently, there seems to be no tracking issue for async_fn_traits in general.
The associated types themselves likely won't ever be stabilized, but some form of syntax (like Return Type Notation) to constrain the future will. That's not part of the initial planned RTN though.
As of RTNs, I couldn't find mentions on how they would handle Fn*::call*, AsyncFn*::async_call* cases, since methods in Fn-like traits are inherently unstable with no intention to stabilize them, and thus no way to mention them in constraints. Meanwhile, Fn*::Output associated type is stable and can be referenced.