Bring the Async Rust experience closer to parity with sync Rust

[nikomatsakis]

Metadata
Owner(s)	@tmandry, @nikomatsakis
Team(s)	compiler, lang, libs, libs-api
Goal document	2024h2/async

Summary

Over the next six months, we will deliver several critical async Rust building block features

• resolve the "Send bound" problem, which blocks the widespread usage of async functions in traits;
• reorganize the async WG, so that we can be better aligned and move more swiftly from here out;
• stabilize async closures, allowing for a much wider variety of async related APIs (async closures are implemented on nightly).

Tasks and status

• Overall program management (@tmandry, @nikomatsakis)
• "Send bound" problem (@nikomatsakis)
  • Implementation (@compiler-errors)
  • Author RFC (@nikomatsakis)
  • RFC decision: RTN (RFC: Return Type Notation rfcs#3654) (lang )
  • RFC decision: Implementable trait aliases (RFC: Implementable trait aliases rfcs#3437 or similar)
  • Stabilization decision (lang )
• Async WG reorganization (@nikomatsakis)
  • Author proposal
  • Org decision (libs, lang )
• Async closures (@compiler-errors)
  • implementation
  • Author RFC
  • RFC decision (lang )
  • Design meeting (lang )
  • Author call for usage (@compiler-errors)
  • Stabilization decision (lang )
• Trait for async iteration (@eholk)
  • Author RFC
  • RFC decision (libs-api, lang )
  • Design meeting (lang )
  • Implementation
• Dyn dispatch for AFIT (@spastorino)
  • Implementation (@spastorino)
  • Standard reviews (@tmandry)
• Async drop experiments (@petrochenkov)
  • author MCP
  • MCP decision ()
  • Implementation work
  • Design meeting (lang )
  • Standard reviews (compiler )

[nikomatsakis]

This issue is intended for status updates only.

For general questions or comments, please contact the owner(s) directly.

[tmandry]

Status update: July 30, 2024

Key developments

• Our project goal was merged :)
• Return type notation RFC was merged (RFC: Return Type Notation rfcs#3654)
• Async closures RFC is in final comment period (Async closures rfcs#3668)

Blockers

We are able to make some progress in each area, but the biggest upcoming blocker we can see is:

• Lang team deciding on final syntax for async closure bounds

Help wanted

There will be a call for testing async closures on the Inside Rust blog soon. Please check it out and give them a try in your own async code!

We'd like to see an updated RFC for implementable trait aliases that follows the guidelines laid out in this comment.

[nikomatsakis]

Executive summary.

We are generally on track with our marquee features: (1) Support for async closures is available on Nightly and the lang team arrived at a tentative consensus to keep the existing syntax (written rationale and formal decision are in progress). We issued a call for testing as well which has so far uncovered no issues. (2) Partial support for Return Type Notation is available on Nightly with the remainder under review. In addition, dynamic dispatch for async functions and experimental async drop work both made implementation progress. Async WG reorganization has made no progress.

Detailed updates

For this year's goal we are focused on three items:

• Stabilizing async closures:
  • The Async Closures RFC has been accepted and the feature is available for use on Nightly under the async_closure feature gate. We put out a call for testing; so far, no problems have been reported. Stabilization stalled because of a need to resolve the syntax. Lang team has reached a tentative internal consensus in favor of the syntax as implemented (impl async Fn). Next steps are (1) lang team to write up rationale and finalizing syntax decision; (2) author stabilization report.
• Resolving "Send Bounds" via Return Type Notation:
  • The Return Type Notation RFC has been accepted and partially implemented under the feature gate return_type_notation. Bounds like Trait<method(): Send> are available on Nightly; bounds like Trait::method(): Send will be available once PR #129629 is merged. Next steps are to (1) merge PR #129629; (2) call for testing; and (3) author stabilization report.
• Dyn dispatch for async functions in traits:
  • Async functions in traits were stabilized last year without support for dynamic dispatch. For this year we plan to release a procedural macro that can help users workaround this limitation; native support is planned for future years. The all important task of selecting a crate name was completed, with dynosaur being chosen. Basic support is now implemented (see this example). Next steps are polish and putting out a call for testing.
• Async drop experiments:
  • PRs with experimental implementation work for async drop are ready and awaiting review (PR list).

We have not made progress on Async WG reorganization.