Tracking issue for way to express intraprocedural finite state machines

This is a tracking issue for some form of:

• https://github.com/rust-lang/rfcs/pull/3720

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

From #138777:

This feature is based on https://github.com/rust-lang/rfcs/pull/3720, though it does not exactly implement it. Rather, this is a language experiment to figure out how to impove that RFC. So, the RFC will eventually match the implementation we end up with, rather than the other way around.

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.
  • https://github.com/rust-lang/rust/pull/138780#issuecomment-2791302462
• Accept an RFC.
  • https://github.com/rust-lang/rfcs/pull/3720
• Implement in nightly.
  • https://github.com/rust-lang/rust/pull/138780
• 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

TODO.

Related

TODO.

cc @bjorn3 @folkertdev @rust-lang/lang

Start with RFC 3720 and implementation PR #138780, using the loop_match feature gate as the entry point. Review the unchecked dev guide, reference, style guide, and stabilization steps; done is an accepted RFC, completed documentation and style work, and a stabilization decision.