borrowck: Don't mention unused vars in closure outlive errors by Enselic · Pull Request #154674 · rust-lang/rust

Enselic · 2026-04-01T10:16:08Z

When there is a closure arg lifetime error, the code does its best to find variables that correspond to the lifetimes involved. This commit stops mentioning arguments that are unused in closures, since they are not relevant. This removes the "red herring" of #113121.

The user_arg_index nomenclature can be applied more broadly, but then the diff becomes annoyingly big, so I chose not to do that.

Review each commit individually for a nicer experience.

The `user_arg_index` nomenclature can be applied more broadly, but then the diff becomes annoyingly big, so I chose not to do that.

So that the impact of the commit that fixes the test can be seen clearly.

When there is a lifetime error, the code does its best to find variables that correspond to the lifetimes involved. This commit stops mentioning arguments that are unused in closures, since they are not relevant. See the diff of the blessed test to get a good clue of what that means.

rustbot · 2026-04-01T12:13:34Z

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jdonszelmann · 2026-04-02T11:03:30Z

+    let mut used = false;
+    VisitPlacesWith(|place: Place<'_>, context: PlaceContext| {
+        if !matches!(context, PlaceContext::NonUse(_)) && place.local == target {
+            used = true;


is there a way to break out here? We don't need to continue the visit

I tried to find a way, but I don't think there is a way (without doing stuff like manually implementing a visitor specifically for this purpose, but IMHO that would be overkill). Luckily this code is only run in the face of compiler errors, so normal compilation speed is not affected.

jdonszelmann

looks good to me except the early return

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Enselic · 2026-04-02T15:34:19Z

@rustbot ready

jdonszelmann · 2026-04-05T07:45:37Z

@bors r+ rollup

rust-bors · 2026-04-05T07:45:39Z

📌 Commit 94ee70e has been approved by jdonszelmann

It is now in the queue for this repository.

…uwer Rollup of 2 pull requests Successful merges: - #154555 (Make sure we run the full suite, even when more specific filters are given) - #154674 (borrowck: Don't mention unused vars in closure outlive errors)

Rollup merge of #154674 - Enselic:unused-outlive, r=jdonszelmann borrowck: Don't mention unused vars in closure outlive errors When there is a closure arg lifetime error, the code does its best to find variables that correspond to the lifetimes involved. This commit stops mentioning arguments that are unused in closures, since they are not relevant. This removes the "red herring" of #113121. The `user_arg_index` nomenclature can be applied more broadly, but then the diff becomes annoyingly big, so I chose not to do that. Review each commit individually for a nicer experience.

…mann borrowck: Apply `user_arg_index` nomenclature more broadly No change in behavior. Only renames and a couple of added comments. As requested here: rust-lang#154674 (comment) r? jdonszelmann

Rollup merge of #154895 - Enselic:user-arg-index, r=jdonszelmann borrowck: Apply `user_arg_index` nomenclature more broadly No change in behavior. Only renames and a couple of added comments. As requested here: #154674 (comment) r? jdonszelmann

borrowck: Apply `user_arg_index` nomenclature more broadly No change in behavior. Only renames and a couple of added comments. As requested here: rust-lang/rust#154674 (comment) r? jdonszelmann

borrowck: Add user_arg_index_to_local() helper to avoid + 1 magic

a40ea0a

The `user_arg_index` nomenclature can be applied more broadly, but then the diff becomes annoyingly big, so I chose not to do that.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Apr 1, 2026

Enselic added 2 commits April 1, 2026 12:17

tests: Add borrowck/var-matching-lifetime-but-unused-not-mentioned.rs

7a092c4

So that the impact of the commit that fixes the test can be seen clearly.

Enselic force-pushed the unused-outlive branch from fcbc851 to 94ee70e Compare April 1, 2026 10:20

Enselic mentioned this pull request Apr 1, 2026

Misleading lifetime error with for loop and async capture #113121

Open

Enselic marked this pull request as ready for review April 1, 2026 12:13

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Apr 1, 2026

rustbot assigned jdonszelmann Apr 1, 2026

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Apr 1, 2026

jdonszelmann reviewed Apr 2, 2026

View reviewed changes

Comment thread compiler/rustc_borrowck/src/diagnostics/var_name.rs

jdonszelmann requested changes Apr 2, 2026

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 2, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 2, 2026

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 5, 2026

JonathanBrouwer mentioned this pull request Apr 5, 2026

Rollup of 2 pull requests #154842

Merged

rust-bors Bot merged commit 9b2b299 into rust-lang:main Apr 5, 2026
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rustbot added this to the 1.96.0 milestone Apr 5, 2026

Enselic mentioned this pull request Apr 6, 2026

borrowck: Apply user_arg_index nomenclature more broadly #154895

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borrowck: Don't mention unused vars in closure outlive errors#154674

borrowck: Don't mention unused vars in closure outlive errors#154674
rust-bors[bot] merged 3 commits intorust-lang:mainfrom
Enselic:unused-outlive

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