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"block label not supported here" when the label is in the wrong place #138585

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kpreid opened this issue Mar 17, 2025 · 1 comment · Fixed by #138589
Closed

"block label not supported here" when the label is in the wrong place #138585

kpreid opened this issue Mar 17, 2025 · 1 comment · Fixed by #138589
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@zachs18
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A-diagnostics Area: Messages for errors, warnings, and lints T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@kpreid
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kpreid commented Mar 17, 2025 •
edited by rustbot
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Code

fn main() {
    loop 'a: { return; }
}

Current output

error: block label not supported here
 --> src/main.rs:2:10
  |
2 |     loop 'a: { return; }
  |          ^^^ not supported here

Desired output

error: misplaced loop label
 --> src/main.rs:2:10
  |
2 |     loop 'a: { return; }
  |          ^^^ not supported here
  help: change this to `'a: loop {`

Rust Version

1.85.0
@kpreid kpreid added A-diagnostics Area: Messages for errors, warnings, and lints T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Mar 17, 2025
@zachs18
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zachs18 commented Mar 17, 2025

@rustbot claim

@zachs18 zachs18 mentioned this issue Mar 17, 2025
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@bors bors closed this as completed in c3f74bc Mar 20, 2025
rust-timer added a commit to rust-lang-ci/rust that referenced this issue Mar 20, 2025
@rust-timer
Unrolled build for rust-lang#138589
55c9732 
Rollup merge of rust-lang#138589 - zachs18:block-label-not-supported-here-loop-body-help, r=petrochenkov

If a label is placed on the block of a loop instead of the header, suggest moving it to the header.

Fixes rust-lang#138585

If a label is placed on the block of a loop instead of the header, suggest to the user moving it to the loop header instead of ~~suggesting to remove it~~ emitting a tool-only suggestion to remove it.

```rs
fn main() {
    loop 'a: { return; }
}
```

```diff
 error: block label not supported here
  --> src/main.rs:2:10
   |
 2 |     loop 'a: { return; }
   |          ^^^ not supported here
+  |
+help: if you meant to label the loop, move this label before the loop
+  |
+2 -     loop 'a: { return; }
+2 +     'a: loop { return; }
+  |
```

Questions for reviewer:

* The "desired output" in the linked issue had the main diagnostic be "misplaced loop label". Should the main diagnostic message the changed instead of leaving it as "block label not supported here"?
* Should this be `Applicability::MachineApplicable`?
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@zachs18 zachs18

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@kpreid @zachs18