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)
• Introduce x.use and use || syntax and UseCloned trait (Ergonomic ref counting rust#134797).
• Modify codegen so that we guarantee that x.use will do a copy if X: Copy is true after monomorphization. Right now the desugaring to clone occurs before monomorphization and hence it will call the clone method even for those instances where X is a Copy type.
• Convert x.use to a move rather than a clone if this is a last-use.
• Make x equivalent to x.use but with an (allow-by-default) lint to signal that something special is happened.

[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.