Tracking Issue for `super let`

[m-ou-se]

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.
  • Experimental feature gate for super let #139080
• Implement super let
  • Clean up / simplify rustc_hir_analysis::check::region to make super let implementation easier
    • Clean up a few things in rustc_hir_analysis::check::region #138989
    • Remove ScopeDepth #139047
    • Remove terminating_scopes hash set. #139067
  • Implement super let #139112
• Use super let in pin!()
  • Collect pin!() tests for fun edge cases
    • Put pin!() tests in the right file. #139052
    • Add more tests for pin!(). #139097
  • Implement pin!() using super let #139114
  • Remove (now unused) #[rustc_macro_edition_2021] attribute #140067
• Use super let in format_args!()
  • Simplify expansion for format_args!(). #139131
  • Figure out what to do with Inlining/flattening of format_args!() accidentally exposed as stable through const #139136 and panic!() with formatting arguments sometimes accepted in const #139621
    • Crater run
    • Don't allow flattened format_args in const. #139624
  • Allow storing format_args!() in variable or const #139135
• ...
• 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?