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rustc_borrowck: fix async closure error note to report FnOnce rather than Fn #148713

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rustc_borrowck: fix async closure error note to report FnOnce rather than Fn #148713

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10 changes: 8 additions & 2 deletions compiler/rustc_borrowck/src/diagnostics/region_name.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -331,7 +331,12 @@ impl<'tcx> MirBorrowckCtxt<'_, '_, 'tcx> {

let closure_kind = match def_ty {
DefiningTy::Closure(_, args) => args.as_closure().kind(),
DefiningTy::CoroutineClosure(_, args) => args.as_coroutine_closure().kind(),
DefiningTy::CoroutineClosure(_, args) => {
match args.as_coroutine_closure().kind() {
ty::ClosureKind::Fn => ty::ClosureKind::FnOnce,
kind => kind,
}
}
_ => {
// Can't have BrEnv in functions, constants or coroutines.
bug!("BrEnv outside of closure.");
Expand All @@ -353,7 +358,8 @@ impl<'tcx> MirBorrowckCtxt<'_, '_, 'tcx> {
can't escape the closure"
}
ty::ClosureKind::FnOnce => {
bug!("BrEnv in a `FnOnce` closure");
"closure implements `FnOnce`, so references to captured variables \
can't escape the closure"
}
};
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@cjgillot cjgillot Nov 8, 2025

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Any way this can look like

let note = format!("closure implements `{}`, so references to captured variables can't escape the closure", closure_kind.as_str());

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@InvalidPathException InvalidPathException Nov 9, 2025 •
edited
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I did think about this. Correct me if I'm wrong, RegionNameSource stores these as &'static str but format! gives String, so we cannot really get around it (at least in this way). If we do refactor then it might be a lot of diff for something we do not touch for years.


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4 changes: 2 additions & 2 deletions tests/ui/async-await/async-closures/not-lending.stderr
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Expand Up @@ -7,7 +7,7 @@ LL | let x = async move || -> &String { &s };
| | return type of async closure `{async closure body@$DIR/not-lending.rs:12:42: 12:48}` contains a lifetime `'2`
| lifetime `'1` represents this closure's body
|
= note: closure implements `Fn`, so references to captured variables can't escape the closure
= note: closure implements `FnOnce`, so references to captured variables can't escape the closure

error: lifetime may not live long enough
--> $DIR/not-lending.rs:16:31
Expand All @@ -18,7 +18,7 @@ LL | let x = async move || { &s };
| | return type of async closure `{async closure body@$DIR/not-lending.rs:16:31: 16:37}` contains a lifetime `'2`
| lifetime `'1` represents this closure's body
|
= note: closure implements `Fn`, so references to captured variables can't escape the closure
= note: closure implements `FnOnce`, so references to captured variables can't escape the closure

error: aborting due to 2 previous errors

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