Tracking issue for unsafe binder types (`unsafe_binders`)

This is a tracking issue for unsafe binder types. See https://hackmd.io/@compiler-errors/HkXwoBPaR for an initial design proposal.

The feature gate for the issue is #![feature(unsafe_binders)].

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.
  • We accepted this in the lang triage meeting on 2024-09-18.
• Accept an RFC.
• Implement in nightly.
  • https://github.com/rust-lang/rust/pull/130514
• 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

• Are unsafe binders ABI-compatible with the type they wrap? Miri currently accepts them as such but that should be either stably documented or removed from Miri.
• Terminology. This seems to be just an existential lifetime. Why come up with a new term?

Related

• https://github.com/rust-lang/rust/pull/136660
• https://github.com/rust-lang/rust/pull/134857
• https://github.com/rust-lang/rust/pull/134625
• https://github.com/rust-lang/rust/pull/134140
• https://github.com/rust-lang/rust/pull/130514

cc @compiler-errors @rust-lang/lang