Tracking Issue for postfix match

This is a tracking issue for the RFC "Postfix Match" (rust-lang/rfcs#3295).
The feature gate for the issue is #![feature(postfix_match)].

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

• Implement the RFC
• Get consensus on design, and if we even want this feature
• 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

• Lifetime extension
  Method call chains will not lifetime extend their arguments. Match statements, however, are notorious for having lifetime extension. It is currently unclear if promoting these use-cases of match would cause more subtle bugs, or if it's negligible

• Postfix macros
  One thing that was brought up was that it may be possible to implement this with the rust-lang/rfcs#2442 feature. But this feature has not been implemented yet, so that is yet to be seen.

Implementation history

• #121619

Start with RFC rust-lang/rfcs#3295 and the postfix_match feature gate, then review the tracking checklist and its linked stabilization, documentation, and style-guide instructions. Done requires resolving the listed design questions, documenting the feature, adding syntax formatting guidance, and completing stabilization; no source file or test is named.