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Fix invalid mut T suggestion for &mut T in missing lifetime error #151530

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rust-bors merged 1 commit into from
Feb 18, 2026
+231 −13
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Fix invalid mut T suggestion for &mut T in missing lifetime error #151530

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78 changes: 65 additions & 13 deletions compiler/rustc_resolve/src/late/diagnostics.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3837,25 +3837,32 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
"instead, you are more likely to want"
};
let mut owned_sugg = lt.kind == MissingLifetimeKind::Ampersand;
let mut sugg_is_str_to_string = false;
let mut sugg = vec![(lt.span, String::new())];
if let Some((kind, _span)) = self.diag_metadata.current_function
&& let FnKind::Fn(_, _, ast::Fn { sig, .. }) = kind
&& let ast::FnRetTy::Ty(ty) = &sig.decl.output
{
let mut lt_finder =
LifetimeFinder { lifetime: lt.span, found: None, seen: vec![] };
lt_finder.visit_ty(&ty);

if let [Ty { span, kind: TyKind::Ref(_, mut_ty), .. }] =
&lt_finder.seen[..]
{
// We might have a situation like
// fn g(mut x: impl Iterator<Item = &'_ ()>) -> Option<&'_ ()>
// but `lt.span` only points at `'_`, so to suggest `-> Option<()>`
// we need to find a more accurate span to end up with
// fn g<'a>(mut x: impl Iterator<Item = &'_ ()>) -> Option<()>
sugg = vec![(span.with_hi(mut_ty.ty.span.lo()), String::new())];
owned_sugg = true;
for param in &sig.decl.inputs {
lt_finder.visit_ty(&param.ty);
}
if let ast::FnRetTy::Ty(ret_ty) = &sig.decl.output {
lt_finder.visit_ty(ret_ty);
let mut ret_lt_finder =
LifetimeFinder { lifetime: lt.span, found: None, seen: vec![] };
ret_lt_finder.visit_ty(ret_ty);
if let [Ty { span, kind: TyKind::Ref(_, mut_ty), .. }] =
&ret_lt_finder.seen[..]
{
// We might have a situation like
// fn g(mut x: impl Iterator<Item = &'_ ()>) -> Option<&'_ ()>
// but `lt.span` only points at `'_`, so to suggest `-> Option<()>`
// we need to find a more accurate span to end up with
// fn g<'a>(mut x: impl Iterator<Item = &'_ ()>) -> Option<()>
sugg = vec![(span.with_hi(mut_ty.ty.span.lo()), String::new())];
owned_sugg = true;
}
}
if let Some(ty) = lt_finder.found {
if let TyKind::Path(None, path) = &ty.kind {
Expand All @@ -3875,6 +3882,7 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
lt.span.with_hi(ty.span.hi()),
"String".to_string(),
)];
sugg_is_str_to_string = true;
}
Some(Res::PrimTy(..)) => {}
Some(Res::Def(
Expand All @@ -3901,6 +3909,7 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
lt.span.with_hi(ty.span.hi()),
"String".to_string(),
)];
sugg_is_str_to_string = true;
}
Res::PrimTy(..) => {}
Res::Def(
Expand Down Expand Up @@ -3935,6 +3944,12 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
}
}
if owned_sugg {
if let Some(span) =
self.find_ref_prefix_span_for_owned_suggestion(lt.span)
&& !sugg_is_str_to_string
{
sugg = vec![(span, String::new())];
}
err.multipart_suggestion_verbose(
format!("{pre} to return an owned value"),
sugg,
Expand All @@ -3961,6 +3976,23 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
}
}
}

fn find_ref_prefix_span_for_owned_suggestion(&self, lifetime: Span) -> Option<Span> {
let mut finder = RefPrefixSpanFinder { lifetime, span: None };
if let Some(item) = self.diag_metadata.current_item {
finder.visit_item(item);
} else if let Some((kind, _span)) = self.diag_metadata.current_function
&& let FnKind::Fn(_, _, ast::Fn { sig, .. }) = kind
{
for param in &sig.decl.inputs {
finder.visit_ty(&param.ty);
}
if let ast::FnRetTy::Ty(ret_ty) = &sig.decl.output {
finder.visit_ty(ret_ty);
}
}
finder.span
}
}

fn mk_where_bound_predicate(
Expand Down Expand Up @@ -4058,6 +4090,26 @@ impl<'ast> Visitor<'ast> for LifetimeFinder<'ast> {
}
}

struct RefPrefixSpanFinder {
lifetime: Span,
span: Option<Span>,
}

impl<'ast> Visitor<'ast> for RefPrefixSpanFinder {
fn visit_ty(&mut self, t: &'ast Ty) {
if self.span.is_some() {
return;
}
if let TyKind::Ref(_, mut_ty) | TyKind::PinnedRef(_, mut_ty) = &t.kind
&& t.span.lo() == self.lifetime.lo()
{
self.span = Some(t.span.with_hi(mut_ty.ty.span.lo()));
return;
}
walk_ty(self, t);
}
}

/// Shadowing involving a label is only a warning for historical reasons.
//FIXME: make this a proper lint.
pub(super) fn signal_label_shadowing(sess: &Session, orig: Span, shadower: Ident) {
Expand Down
29 changes: 29 additions & 0 deletions tests/ui/lifetimes/mut-ref-owned-suggestion.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,29 @@
//! Regression test for <https://github.com/rust-lang/rust/issues/150077>
//! Tests that `&mut T` suggests `T`, not `mut T`, `&mut str` suggests `String`, not `str`,
//! when recommending an owned value.
fn with_fn(_f: impl Fn() -> &mut ()) {}
//~^ ERROR: missing lifetime specifier

fn with_ref_mut_str(_f: impl Fn() -> &mut str) {}
//~^ ERROR: missing lifetime specifier

fn with_fn_has_return(_f: impl Fn() -> &mut ()) -> i32 {
//~^ ERROR: missing lifetime specifier
2
}

fn with_dyn(_f: Box<dyn Fn() -> &mut i32>) {}
//~^ ERROR: missing lifetime specifier

fn trait_bound<F: Fn() -> &mut i32>(_f: F) {}
//~^ ERROR: missing lifetime specifier

fn nested_result(_f: impl Fn() -> Result<&mut i32, ()>) {}
//~^ ERROR: missing lifetime specifier

struct Holder<F: Fn() -> &mut i32> {
//~^ ERROR: missing lifetime specifier
f: F,
}

fn main() {}
137 changes: 137 additions & 0 deletions tests/ui/lifetimes/mut-ref-owned-suggestion.stderr
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Original file line number Diff line number Diff line change
@@ -0,0 +1,137 @@
error[E0106]: missing lifetime specifier
--> $DIR/mut-ref-owned-suggestion.rs:4:29
|
LL | fn with_fn(_f: impl Fn() -> &mut ()) {}
| ^ expected named lifetime parameter
|
= help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
help: consider using the `'static` lifetime, but this is uncommon unless you're returning a borrowed value from a `const` or a `static`
|
LL | fn with_fn(_f: impl Fn() -> &'static mut ()) {}
| +++++++
help: instead, you are more likely to want to return an owned value
|
LL - fn with_fn(_f: impl Fn() -> &mut ()) {}
LL + fn with_fn(_f: impl Fn() -> ()) {}
|

error[E0106]: missing lifetime specifier
--> $DIR/mut-ref-owned-suggestion.rs:7:38
|
LL | fn with_ref_mut_str(_f: impl Fn() -> &mut str) {}
| ^ expected named lifetime parameter
|
= help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
help: consider using the `'static` lifetime, but this is uncommon unless you're returning a borrowed value from a `const` or a `static`
|
LL | fn with_ref_mut_str(_f: impl Fn() -> &'static mut str) {}
| +++++++
help: instead, you are more likely to want to return an owned value
|
LL - fn with_ref_mut_str(_f: impl Fn() -> &mut str) {}
LL + fn with_ref_mut_str(_f: impl Fn() -> String) {}
|

error[E0106]: missing lifetime specifier
--> $DIR/mut-ref-owned-suggestion.rs:10:40
|
LL | fn with_fn_has_return(_f: impl Fn() -> &mut ()) -> i32 {
| ^ expected named lifetime parameter
|
= help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
= note: for more information on higher-ranked polymorphism, visit https://doc.rust-lang.org/nomicon/hrtb.html
help: consider using the `'static` lifetime, but this is uncommon unless you're returning a borrowed value from a `const` or a `static`
|
LL | fn with_fn_has_return(_f: impl Fn() -> &'static mut ()) -> i32 {
| +++++++
help: consider making the bound lifetime-generic with a new `'a` lifetime
|
LL - fn with_fn_has_return(_f: impl Fn() -> &mut ()) -> i32 {
LL + fn with_fn_has_return(_f: impl for<'a> Fn() -> &'a ()) -> i32 {
|
help: consider introducing a named lifetime parameter
|
LL - fn with_fn_has_return(_f: impl Fn() -> &mut ()) -> i32 {
LL + fn with_fn_has_return<'a>(_f: impl Fn() -> &'a ()) -> i32 {
|
help: alternatively, you might want to return an owned value
|
LL - fn with_fn_has_return(_f: impl Fn() -> &mut ()) -> i32 {
LL + fn with_fn_has_return(_f: impl Fn() -> ()) -> i32 {
|

error[E0106]: missing lifetime specifier
--> $DIR/mut-ref-owned-suggestion.rs:15:33
|
LL | fn with_dyn(_f: Box<dyn Fn() -> &mut i32>) {}
| ^ expected named lifetime parameter
|
= help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
help: consider using the `'static` lifetime, but this is uncommon unless you're returning a borrowed value from a `const` or a `static`
|
LL | fn with_dyn(_f: Box<dyn Fn() -> &'static mut i32>) {}
| +++++++
help: instead, you are more likely to want to return an owned value
|
LL - fn with_dyn(_f: Box<dyn Fn() -> &mut i32>) {}
LL + fn with_dyn(_f: Box<dyn Fn() -> i32>) {}
|

error[E0106]: missing lifetime specifier
--> $DIR/mut-ref-owned-suggestion.rs:18:27
|
LL | fn trait_bound<F: Fn() -> &mut i32>(_f: F) {}
| ^ expected named lifetime parameter
|
= help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
help: consider using the `'static` lifetime, but this is uncommon unless you're returning a borrowed value from a `const` or a `static`
|
LL | fn trait_bound<F: Fn() -> &'static mut i32>(_f: F) {}
| +++++++
help: instead, you are more likely to want to change the argument to be borrowed...
|
LL | fn trait_bound<F: Fn() -> &mut i32>(_f: &F) {}
| +
help: ...or alternatively, you might want to return an owned value
|
LL - fn trait_bound<F: Fn() -> &mut i32>(_f: F) {}
LL + fn trait_bound<F: Fn() -> i32>(_f: F) {}
|

error[E0106]: missing lifetime specifier
--> $DIR/mut-ref-owned-suggestion.rs:21:42
|
LL | fn nested_result(_f: impl Fn() -> Result<&mut i32, ()>) {}
| ^ expected named lifetime parameter
|
= help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
help: consider using the `'static` lifetime, but this is uncommon unless you're returning a borrowed value from a `const` or a `static`
|
LL | fn nested_result(_f: impl Fn() -> Result<&'static mut i32, ()>) {}
| +++++++
help: instead, you are more likely to want to return an owned value
|
LL - fn nested_result(_f: impl Fn() -> Result<&mut i32, ()>) {}
LL + fn nested_result(_f: impl Fn() -> Result<i32, ()>) {}
|

error[E0106]: missing lifetime specifier
--> $DIR/mut-ref-owned-suggestion.rs:24:26
|
LL | struct Holder<F: Fn() -> &mut i32> {
| ^ expected named lifetime parameter
|
= help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
help: consider using the `'static` lifetime, but this is uncommon unless you're returning a borrowed value from a `const` or a `static`
|
LL | struct Holder<F: Fn() -> &'static mut i32> {
| +++++++
help: instead, you are more likely to want to return an owned value
|
LL - struct Holder<F: Fn() -> &mut i32> {
LL + struct Holder<F: Fn() -> i32> {
|

error: aborting due to 7 previous errors

For more information about this error, try `rustc --explain E0106`.
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