Tracking issue for ergonomic reference counting

This is a tracking issue for ergonomic reference counting, including:

• https://github.com/rust-lang/rfcs/pull/3680
• https://github.com/rust-lang/rust-project-goals/issues/107

...and other work.

The feature gate for this issue is #![feature(ergonomic_clones)].

About tracking issues

Tracking issues are used to record the overall progress of implementation. They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions. A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature. Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Approve as lang experiment.
• Modify the MIR building to create a customer terminator for "use" that selects between copy/clone/move post-monorphization
• Add tests and support for const use closures
• Resolve https://github.com/rust-lang/rust/issues/138101
• Accept an RFC.
• Implement in nightly.
• Finalize opsem (https://github.com/rust-lang/unsafe-code-guidelines/issues/564)
• Add documentation to the dev guide.
  • See the instructions.
• Add documentation to the reference.
  • See the instructions.
• Add formatting for new syntax to the style guide.
  • See the nightly style procedure.
• Stabilize.
  • See the instructions.

Unresolved Questions

• Name of UseCloned trait which signals x.use to clone
• Precise set of UseCloned impls -- in particular any blanket impls we ought to be concerned about?
• should x.use work on !Clone types? Currently the impl just generates a move

Related

• https://github.com/rust-lang/rust-project-goals/issues/107

cc @spastorino @jkelleyrtp @rust-lang/lang

This is a tracking issue rather than a self-contained task. Read RFC 3680, project goal 107, and the ergonomic_clones feature-gate context first; the checklist spans MIR building, const-use closures, tests, documentation, formatting, and stabilization. Choose a dedicated unchecked item or related issue, then define completion against that item.