Tracking Issue for Rust 2024: Rescope temporary lifetimes with respect to `else` #124085

traviscross · 2024-04-17T16:15:09Z

This is a tracking issue for shortening the scope of temporary lifetimes in if-let blocks to end before else. E.g.:

if let Some(x) = ref_cell.write().some_method_that_returns_option() {
} else {
    // question is whether the `ref_cell` is still write-locked here;
    // currently, it is, but it wouldn't be under this change
}

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 with a feature gate.
- Rescope temp lifetime in if-let into IfElse with migration lint #107251
Do a crater run to analyze impact.
- [Crater run experiment] let-chain rescoping #125151
- [CRATER RUN DO NOT MERGE] crater run to evaluate effectiveness of lint for if_let_rescope #129466
  - [DO NOT MERGE] Force edition 2024 lint into action on crater cargo#14447
Decide to stabilize in Rust 2024.
- Proposal: stabilize if_let_rescope for Edition 2024 #131154
Implement in nightly Rust 2024.
Implement migration lints.
N/A: Add documentation to the dev guide.
- See the instructions.
Add documentation to the reference.
- See the instructions.
Add documentation to the edition guide.
- Edition 2024 guide for temporary lifetime changes edition-guide#318
Add formatting for new syntax to the style guide.
- See the nightly style procedure.
Ensure ready for Rust 2024 stabilization.

Unresolved Questions

TODO.

Implementation history

The text was updated successfully, but these errors were encountered:

dingxiangfei2009 · 2024-04-18T09:04:11Z

@rustbot claim

dingxiangfei2009 · 2024-05-04T19:53:49Z

Update: #107251 has been repurposed for this edition change.

…er-run, r=<try> [CRATER RUN DO NOT MERGE] Let chain lint crater run Tracked by rust-lang#124085 Related to rust-lang#107251 cc `@jieyouxu` for review context cc `@traviscross` for edition tracking There is one unresolved issue that `cargo fix --edition` does not emit `if-let-rescope` lint. Details in rust-lang/cargo#14447. Note that this patch is assuming that the feature gate `if_let_rescope` is always on just for this crater run.

…r=jieyouxu Rescope temp lifetime in if-let into IfElse with migration lint Tracking issue rust-lang#124085 This PR shortens the temporary lifetime to cover only the pattern matching and consequent branch of a `if let`. At the expression location, means that the lifetime is shortened from previously the deepest enclosing block or statement in Edition 2021. This warrants an Edition change. Coming with the Edition change, this patch also implements an edition lint to warn about the change and a safe rewrite suggestion to preserve the 2021 semantics in most cases. Related to rust-lang#103108. Related crater runs: rust-lang#129466.

…er-run, r=<try> [CRATER RUN DO NOT MERGE] Let chain lint crater run Tracked by rust-lang#124085 Related to rust-lang#107251 cc `@jieyouxu` for review context cc `@traviscross` for edition tracking There is one unresolved issue that `cargo fix --edition` does not emit `if-let-rescope` lint. Details in rust-lang/cargo#14447. Note that this patch is assuming that the feature gate `if_let_rescope` is always on just for this crater run.

…ope-lint, r=jieyouxu Preserve brackets around if-lets and skip while-lets r? `@jieyouxu` Tracked by rust-lang#124085 Fresh out of rust-lang#129466, we have discovered 9 crates that the lint did not successfully migrate because the span of `if let` includes the surrounding brackets `(..)` like the following, which surprised me a bit. ```rust if (if let .. { .. } else { .. }) { // ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ // the span somehow includes the surrounding brackets } ``` There is one crate that failed the migration because some suggestion spans cross the macro expansion boundaries. Surely there is no way to patch them with `match` rewrite. To handle this case, we will instead require all spans to be tested for admissibility as suggestion spans. Besides, there are 4 false negative cases discovered with desugared-`while let`. We don't need to lint them, because the `else` branch surely contains exactly one statement because the drop order is not changed whatsoever in this case. ```rust while let Some(value) = droppy().get() { .. } // is desugared into loop { if let Some(value) = droppy().get() { .. } else { break; // here can be nothing observable in this block } } ``` I believe this is the one and only false positive that I have found. I think we have finally nailed all the corner cases this time.

Rollup merge of rust-lang#131035 - dingxiangfei2009:tweak-if-let-rescope-lint, r=jieyouxu Preserve brackets around if-lets and skip while-lets r? `@jieyouxu` Tracked by rust-lang#124085 Fresh out of rust-lang#129466, we have discovered 9 crates that the lint did not successfully migrate because the span of `if let` includes the surrounding brackets `(..)` like the following, which surprised me a bit. ```rust if (if let .. { .. } else { .. }) { // ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ // the span somehow includes the surrounding brackets } ``` There is one crate that failed the migration because some suggestion spans cross the macro expansion boundaries. Surely there is no way to patch them with `match` rewrite. To handle this case, we will instead require all spans to be tested for admissibility as suggestion spans. Besides, there are 4 false negative cases discovered with desugared-`while let`. We don't need to lint them, because the `else` branch surely contains exactly one statement because the drop order is not changed whatsoever in this case. ```rust while let Some(value) = droppy().get() { .. } // is desugared into loop { if let Some(value) = droppy().get() { .. } else { break; // here can be nothing observable in this block } } ``` I believe this is the one and only false positive that I have found. I think we have finally nailed all the corner cases this time.

traviscross added T-lang Relevant to the language team, which will review and decide on the PR/issue. C-tracking-issue Category: A tracking issue for an RFC or an unstable feature. A-edition-2024 Area: The 2024 edition labels Apr 17, 2024

rustbot assigned dingxiangfei2009 Apr 18, 2024

traviscross mentioned this issue May 21, 2024

[Crater run experiment] let-chain rescoping #125151

Closed

traviscross added S-tracking-impl-incomplete Status: The implementation is incomplete. S-tracking-needs-migration-lint Status: This item needs a migration lint. S-tracking-needs-documentation Status: Needs documentation. labels May 21, 2024

traviscross added the S-tracking-design-concerns Status: There are blocking ❌ design concerns. label Jun 11, 2024

dingxiangfei2009 mentioned this issue Aug 15, 2024

Edition 2024 guide for temporary lifetime changes rust-lang/edition-guide#318

Open

traviscross mentioned this issue Aug 19, 2024

Rescope temp lifetime in if-let into IfElse with migration lint #107251

Merged

dingxiangfei2009 mentioned this issue Aug 23, 2024

[CRATER RUN DO NOT MERGE] crater run to evaluate effectiveness of lint for if_let_rescope #129466

Closed

dingxiangfei2009 mentioned this issue Aug 28, 2024

Add a section dedicated to Edition 2024 changes to temporary scopes rust-lang/reference#1592

Open

dingxiangfei2009 mentioned this issue Sep 29, 2024

Preserve brackets around if-lets and skip while-lets #131035

Merged

dingxiangfei2009 mentioned this issue Oct 2, 2024

Proposal: stabilize if_let_rescope for Edition 2024 #131154

Open

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Tracking Issue for Rust 2024: Rescope temporary lifetimes with respect to `else` #124085

Tracking Issue for Rust 2024: Rescope temporary lifetimes with respect to `else` #124085

traviscross commented Apr 17, 2024 •

edited

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dingxiangfei2009 commented Apr 18, 2024

dingxiangfei2009 commented May 4, 2024

Tracking Issue for Rust 2024: Rescope temporary lifetimes with respect to else #124085

Tracking Issue for Rust 2024: Rescope temporary lifetimes with respect to else #124085

Comments

traviscross commented Apr 17, 2024 • edited Loading

About tracking issues

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Unresolved Questions

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Implementation history

dingxiangfei2009 commented Apr 18, 2024

dingxiangfei2009 commented May 4, 2024

Tracking Issue for Rust 2024: Rescope temporary lifetimes with respect to `else` #124085

Tracking Issue for Rust 2024: Rescope temporary lifetimes with respect to `else` #124085

traviscross commented Apr 17, 2024 •

edited

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