Return type notation and the Send bound problem

[tmandry]

Summary

Please describe your meeting topic here. It doesn't have to be very long, but
it's always good to try and identify a concrete question to be answered or
narrow topic for discussion. Nobody likes a rambly meeting.

Discuss proposals for dealing with Send bounds on async fn and -> impl Trait in traits.

Background reading

Include any links to material that folks ought to try to read before-hand.

https://blog.rust-lang.org/inside-rust/2023/05/03/stabilizing-async-fn-in-trait.html

About this issue

This issue corresponds to a lang-team design meeting proposal. It corresponds to a possible topic of discussion that may be scheduled for deeper discussion during one of our design meetings.

[nikomatsakis]

Minutes https://github.com/rust-lang/lang-team/blob/master/design-meeting-minutes/2023-05-24-return-type-notation.md