Tracking Issue for RFC 3606: Drop temporaries in tail expressions before local variables

[traviscross]

This is a tracking issue for the RFC 3606: Drop temporaries in tail expressions before local variables

• RFC: Drop temporaries in tail expressions before local variables rfcs#3606

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

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

• Accept RFC: RFC: Drop temporaries in tail expressions before local variables rfcs#3606
• Implement under feature flag with Rust 2024 behavior.
  • Place tail expression behind terminating scope #125293
  • Ensure that tail expr receive lifetime extension #129627
• Crater run(s) to collect data for migration lints.
  • [DO NOT MERGE] crater run on the tail expression drop order lint #129604
  • [DO NOT MERGE] Tail expr drop order crater run #129607
• Implement migration lints.
  • Lint against possible change in drop order from tail expr rust-clippy#13175
  • Lint on tail expr drop order change in Edition 2024 #128662
• Decide to stabilize.
  • Stabilize shorter_tail_lifetime #131445
• Stabilize in nightly Rust 2024.
  • Stabilize shorter-tail-lifetimes #131983
• N/A: Add documentation to the dev guide.
  • See the instructions.
• Add documentation to the reference.
  • Add a section dedicated to Edition 2024 changes to temporary scopes reference#1592
  • See the instructions.
• Add documentation to the edition guide.
  • Edition 2024 guide for temporary lifetime changes edition-guide#318
• Ensure ready for Rust 2024 stabilization.

Unresolved Questions

• How uncommon are the situations where this change could affect existing code?
• How advanced should the edition lint and migration be?
• Can we make sure a lint catches the cases with unsafe code that could result in undefined behaviour?
• Do we want to warn-by-default about these cases in older editions, or only warn when upgrading to Rust 2024?
  • Lint on tail expr drop order change in Edition 2024 #128662 (comment)

Related

• Lint against possible change in drop order from tail expr rust-clippy#13175
• Lint on tail expr drop order change in Edition 2024 #128662

cc @nikomatsakis @m-ou-se

[dingxiangfei2009]

@rustbot claim

[pnkfelix]

(assigning self as lang team liason to ensure things progress here for 2024 edition)