Tracking Issue for `super let`

This is a tracking issue for the super let experiment.

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

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.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps / History

• Accept lang experiment.
  • https://github.com/rust-lang/rust/pull/139080
• Implement super let
  • Clean up / simplify rustc_hir_analysis::check::region to make super let implementation easier
    • https://github.com/rust-lang/rust/pull/138989
    • https://github.com/rust-lang/rust/pull/139047
    • https://github.com/rust-lang/rust/pull/139067
  • https://github.com/rust-lang/rust/pull/139112
• Use super let in pin!()
  • Collect pin!() tests for fun edge cases
    • https://github.com/rust-lang/rust/pull/139052
    • https://github.com/rust-lang/rust/pull/139097
  • https://github.com/rust-lang/rust/pull/139114
  • https://github.com/rust-lang/rust/pull/140067
• Use super let in format_args!()
  • https://github.com/rust-lang/rust/pull/139131
  • Figure out what to do with https://github.com/rust-lang/rust/issues/139136 and https://github.com/rust-lang/rust/issues/139621
    • Crater run
    • https://github.com/rust-lang/rust/pull/139624
  • https://github.com/rust-lang/rust/pull/140748
• ...
• Consider other syntaxes/solutions
• ...
• Write an RFC?
• Get the RFC accepted
• ...
• Adjust documentation (see instructions on rustc-dev-guide)
• Formatting for new syntax has been added to the Style Guide (nightly-style-procedure)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• What are the exact rules for super let?
• super let x; without initializer?
• super let _ = _ else { _ };?
• super let at function body scope?
• Best syntax for this feature?

Start with the super let feature gate and the tracking checklist, then review the unresolved syntax and rule questions. Read the linked rustc-dev-guide documentation and stabilization guidance, along with the Style Guide procedure. Done requires settled semantics, an accepted RFC, updated documentation and formatting, and a stabilization PR.