Tracking Issue for `guard` macro fragment specifier

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This is a tracking issue for the feature "guard macro fragment specifier" accepted in #152832 (comment).
The feature gate for the issue is #![feature(macro_guard_matcher)].

Introduce a new macro fragment specifier guard to match MatchArmGuard, to make it easier for macro authors to match the recently stabilized if_let_guard feature (#51114).

This resolves #152313, because we either introduce a new :guard matcher or must refactor matches! and assert_matches! into built-in macros. Considering that more users will benefit from :guard, we choose to do so.

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.
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Steps

• Implement the :guard fragment specifier for macro_rules and decl_macro
• Use :guard in mathces! and assert_matches! to support if-let guards
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

None for now.

Implementation history

• Support if-let-guard in matches! and assert_matches! #152832
  The initial attempt used :tt to match if-let guards, but :tt greedily captured the trailing comma and the assertion message. So we decide to introduce a new :guard matcher.