Tracking Issue for `async_fn_in_dyn_trait`

[compiler-errors]

This is a tracking issue for dyn-compatible traits with async functions in them, called "async functions in dyn trait". The feature gate for the issue is #![feature(async_fn_in_dyn_trait)].

About tracking issues

Tracking issues are used to record the overall progress of implementation. They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions. A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature. Instead, open a dedicated issue for the specific matter and add the relevant feature gate label. Discussion comments will get marked as off-topic or deleted. Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• [ ] Experiment with the design: https://github.com/rust-lang/rust/pull/133122
• [ ] Author an RFC
• [ ] Implement the RFC: (see above)
• [ ] Adjust documentation (see instructions on rustc-dev-guide)
• [ ] Formatting for new syntax has been added to the Style Guide (nightly-style-procedure)
• [ ] Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

XXX --- list all the "unresolved questions" found in the RFC to ensure they are not forgotten

Implementation history

[compiler-errors]

Nominating this since I think it may technically need a new lang team experiment, since https://rust-lang.github.io/rfcs/3185-static-async-fn-in-trait.html only RFC'd static AFIT behavior. I would expect @nikomatsakis will shepherd this from the lang side.