Experiment with ergonomic ref-counting

[nikomatsakis]

Metadata
Point of contact	@spastorino
Team(s)	lang
Goal document	2025h1/ergonomic-rc

Summary

• Deliver a nightly implementation of the experimental use syntax for ergonomic ref-counting.
• RFC decision on the above

Tasks and status

• Implementation (@spastorino)
• Reviews (@nikomatsakis)
• Lang-team champion (lang )
• Author RFC (@joshtriplett)

[nikomatsakis]

This issue is intended for status updates only.

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

[nikomatsakis]

This issue has not seen many updates, but there has been definite progress:

• @joshtriplett opened Simplify lightweight clones, including into closures and async blocks rfcs#3680. The @rust-lang/lang team has not yet truly discussed or reached a decision on that RFC.
• @spastorino began implementation work on a prototype.

[nikomatsakis]

Year-end summary:

In 2024H2 @spastorino began work on an experimental implementation (not yet landed) and @joshtriplett authored a corresponding RFC (rust-lang/rfcs#3680), which has received substantial feedback.

We opened a proposed 2025H1 project goal to continue by (a) landing the experimental branch and (b) addressing feedback on the RFC, reading it with the lang-team, and reaching a decision.

[nikomatsakis]

This is a continuing project goal, and the updates below this comment will be for the new period 2025h1

[spastorino]

There's a PR up rust-lang/rust#134797 which implements the proposed RFC without the optimizations.
The PR is not yet merged and we need to continue now working on addressing comments to the PR until is merged and then start implementing optimizations.