Async closures

[compiler-errors]

This RFC adds an async bound modifier to the Fn family of trait bounds. The combination desugars to a set of perma-unstable AsyncFn{,Mut,Once} traits that parallel the current Fn{,Mut,Once} traits.

These traits give users the ability to express bounds for async callable types that are higher-ranked, and allow async closures to return futures which borrow from the closure's captures.

This RFC also connects these traits to the async || {} closure syntax, as originally laid out in RFC 2394, and confirms the necessity of a first-class async closure syntax.

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For additional background, see this blog post (which is referenced in the RFC):

• Async closures 2024-07-01

Tracking:

• Tracking issue for #![feature(async_closure)] (RFC 2394 / RFC 3668) rust#62290