This is a tracking issue for updating the expr fragment specifier for Rust 2024 according to the policy described in RFC 3531:

• Macro fragment specifiers edition policy rfcs#3531

The feature gate for the issue is #![feature(expr_fragment_specifier_2024)].

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 in nightly Rust 2024.
  • Update expr matcher for Edition 2024 and add expr_2021 nonterminal #123865
• Add test cases around nested macros spanning crates across different editions. For ideas, see this comment.
  • test: cross-edition metavar fragment specifiers #129755
• Update expr to match _.
  • [RFC] mbe: consider the _ in 2024 an expression #126697
• Implement migration lints.
  • migration lint for expr2024 for the edition 2024 #125627
• Remove edition gating on expr_2021 fragment specifier.
  • Make edition dependent :expr macro fragment act like the edition-dependent :pat fragment does #126700
• Stabilize new fragment specifier (expr_2021) in all editions.
  • Stabilize expr_2021 fragment specifier in all editions #129972
• N/A: Add documentation to the dev guide.
  • See the instructions.
• Add documentation to the reference.
  • Add expr_2021 macro fragment specifier reference#1580
  • 2024: Update expr macro fragment specifier reference#1639
• Add documentation to the edition guide.
  • 2024: Add macro-fragment-specifiers. edition-guide#312
• Ensure ready for Rust 2024 stabilization.

Unresolved Questions

• Is there a reasonable semantically-appropriate name for expr_2021, or should we just call it that per the RFC?
  • Resolved, keeping expr_2021, see Tracking Issue for updating expr macro fragment specifier for Rust 2024 #123742 (comment).

Related

• Add expr202x macro pattern #84364
• Non-terminals, e.g. expr, have diverged between parser and macro matcher #86730
• Update expr matcher for Edition 2024 and add expr_2021 nonterminal #123865
• Avoid checking the edition as much as possible #125828
• Make edition dependent :expr macro fragment act like the edition-dependent :pat fragment does #126700
• [RFC] mbe: consider the _ in 2024 an expression #126697
• Make edition dependent :expr macro fragment act like the edition-dependent :pat fragment does #126700

cc @rust-lang/wg-macros @eholk @vincenzopalazzo