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

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

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41 changes: 23 additions & 18 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 @@ -111,9 +111,12 @@ impl RegionName {
| RegionNameSource::NamedEarlyParamRegion(span) => {
diag.span_label(*span, format!("lifetime `{self}` defined here"));
}
RegionNameSource::SynthesizedFreeEnvRegion(span, note) => {
RegionNameSource::SynthesizedFreeEnvRegion(span, closure_trait) => {
diag.span_label(*span, format!("lifetime `{self}` represents this closure's body"));
diag.note(*note);
diag.note(format!(
"closure implements `{closure_trait}`, so references to captured variables \
can't escape the closure"
));
}
RegionNameSource::AnonRegionFromArgument(RegionNameHighlight::CannotMatchHirTy(
span,
Expand Down Expand Up @@ -326,9 +329,15 @@ impl<'tcx> MirBorrowckCtxt<'_, '_, 'tcx> {
ty::LateParamRegionKind::ClosureEnv => {
let def_ty = self.regioncx.universal_regions().defining_ty;

let closure_kind = match def_ty {
DefiningTy::Closure(_, args) => args.as_closure().kind(),
DefiningTy::CoroutineClosure(_, args) => args.as_coroutine_closure().kind(),
let (is_lending_coroutine_closure, closure_kind) = match def_ty {
DefiningTy::Closure(_, args) => (false, args.as_closure().kind()),
DefiningTy::CoroutineClosure(_, args) => {
let args = args.as_coroutine_closure();
(
!args.tupled_upvars_ty().is_ty_var() && args.has_self_borrows(),
args.kind(),
)
}
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@lcnr lcnr Nov 18, 2025

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so the fix here would be to check whether args.has_self_borrows and if so, talk about AsyncFn(Mut) instead

_ => {
// Can't have BrEnv in functions, constants or coroutines.
bug!("BrEnv outside of closure.");
Expand All @@ -340,23 +349,19 @@ impl<'tcx> MirBorrowckCtxt<'_, '_, 'tcx> {
bug!("Closure is not defined by a closure expr");
};
let region_name = self.synthesize_region_name();
let note = match closure_kind {
ty::ClosureKind::Fn => {
"closure implements `Fn`, so references to captured variables \
can't escape the closure"
}
ty::ClosureKind::FnMut => {
"closure implements `FnMut`, so references to captured variables \
can't escape the closure"
}
ty::ClosureKind::FnOnce => {
bug!("BrEnv in a `FnOnce` closure");
}
let closure_trait = match (is_lending_coroutine_closure, closure_kind) {
(false, kind) => kind.as_str(),
(true, ty::ClosureKind::Fn) => "AsyncFn",
(true, ty::ClosureKind::FnMut) => "AsyncFnMut",
(true, ty::ClosureKind::FnOnce) => "AsyncFnOnce",
};
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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.


Some(RegionName {
name: region_name,
source: RegionNameSource::SynthesizedFreeEnvRegion(fn_decl_span, note),
source: RegionNameSource::SynthesizedFreeEnvRegion(
fn_decl_span,
closure_trait,
),
})
}

Expand Down
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 `AsyncFn`, 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 `AsyncFn`, so references to captured variables can't escape the closure

error: aborting due to 2 previous errors

4 changes: 2 additions & 2 deletions tests/ui/async-await/issue-74072-lifetime-name-annotations.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -39,7 +39,7 @@ LL | | y
LL | | })()
| |_____^ returning this value requires that `'1` must outlive `'2`
|
= note: closure implements `FnMut`, so references to captured variables can't escape the closure
= note: closure implements `AsyncFnMut`, so references to captured variables can't escape the closure

error[E0716]: temporary value dropped while borrowed
--> $DIR/issue-74072-lifetime-name-annotations.rs:13:5
Expand Down Expand Up @@ -88,7 +88,7 @@ LL | | y
LL | | })()
| |_____^ returning this value requires that `'1` must outlive `'2`
|
= note: closure implements `FnMut`, so references to captured variables can't escape the closure
= note: closure implements `AsyncFnMut`, so references to captured variables can't escape the closure

error[E0716]: temporary value dropped while borrowed
--> $DIR/issue-74072-lifetime-name-annotations.rs:23:5
Expand Down
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