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Fix bogus "consider changing this binding's type" suggestion for closure parameters #4

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Fix bogus "consider changing this binding's type" suggestion for closure parameters #4

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a882970
Initial plan
Copilot Nov 8, 2025
3458c2d
Add fix to avoid bogus suggestion for closure parameters
Copilot Nov 8, 2025
a80a61d
Extend BindingFinder to match all parameter patterns
Copilot Nov 8, 2025
883d240
Update test expectations to reflect fix
Copilot Nov 8, 2025
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17 changes: 10 additions & 7 deletions compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1364,10 +1364,15 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
let node = hir_id.map(|hir_id| self.infcx.tcx.hir_node(hir_id));

let Some(hir::Node::LetStmt(local)) = node else {
err.span_label(
sugg_span,
format!("consider changing this binding's type to be: `{sugg_str}`"),
);
// Don't suggest changing the binding's type for closure parameters,
// as the type is often constrained by the closure's context (e.g., when passed
// to methods like `Option::inspect`).
if !matches!(node, Some(hir::Node::Param(_))) {
err.span_label(
sugg_span,
format!("consider changing this binding's type to be: `{sugg_str}`"),
);
}
return;
};

Expand Down Expand Up @@ -1494,9 +1499,7 @@ impl<'tcx> Visitor<'tcx> for BindingFinder {
}

fn visit_param(&mut self, param: &'tcx hir::Param<'tcx>) -> Self::Result {
if let hir::Pat { kind: hir::PatKind::Ref(_, _), span, .. } = param.pat
&& *span == self.span
{
if param.pat.span == self.span {
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@Enselic Enselic Nov 8, 2025

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@copilot Why did you change this?

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Copilot AI Nov 8, 2025

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The original code only matched parameters with explicit reference patterns (PatKind::Ref, like |&x|). This meant closure parameters with inferred types like |a| in opt.inspect(|a| ...) weren't found, so they still got the bogus suggestion.

I changed it to match all parameter patterns by just checking param.pat.span == self.span, which allows finding both |a| and |&a| style parameters. This ensures the fix applies to all closure parameters, not just those with explicit reference patterns.

Commit: a80a61d

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@Enselic Enselic Nov 8, 2025

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@copilot It sounds like it would make sense to split that out in a separate PR since it probably affects test output? If yes, please do that.

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Copilot AI Nov 8, 2025

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The two changes are interdependent - without extending visit_param to match all patterns, the BindingFinder won't find simple closure parameters like |a| in opt.inspect(|a| ...), so the fix for issue rust-lang#128381 wouldn't work.

Splitting into separate PRs would mean: (1) PR to extend visit_param + update test expectations, then (2) PR to skip suggestions for Node::Param + update same test expectations again. Would you prefer this approach, or keep them together since they're both required for the fix?

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@Enselic Enselic Nov 8, 2025 •
edited
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@copilot I prefer splitting to separate PRs.

ControlFlow::Break(param.hir_id)
} else {
ControlFlow::Continue(())
Expand Down
6 changes: 2 additions & 4 deletions tests/ui/borrowck/issue-115259-suggest-iter-mut.stderr
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Original file line number Diff line number Diff line change
@@ -1,10 +1,8 @@
error[E0596]: cannot borrow `**layer` as mutable, as it is behind a `&` reference
--> $DIR/issue-115259-suggest-iter-mut.rs:15:65
--> $DIR/issue-115259-suggest-iter-mut.rs:15:74
|
LL | self.layers.iter().fold(0, |result, mut layer| result + layer.process())
| --------- ^^^^^ `layer` is a `&` reference, so the data it refers to cannot be borrowed as mutable
| |
| consider changing this binding's type to be: `&mut Box<dyn Layer>`
| ^^^^^ `layer` is a `&` reference, so the data it refers to cannot be borrowed as mutable
|
help: you may want to use `iter_mut` here
|
Expand Down
16 changes: 16 additions & 0 deletions tests/ui/borrowck/issue-128381-mutate-through-option-inspect.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,16 @@
// Issue #128381: Check that we don't suggest changing closure parameter types
// when they are constrained by method signatures like `Option::inspect`.

#[derive(Debug)]
struct A {
a: u32,
}

fn main() {
let mut opt = Some(A { a: 123 });
let ref_mut_opt = opt.as_mut().inspect(|a| {
a.a += 123;
//~^ ERROR cannot assign to `a.a`, which is behind a `&` reference
});
dbg!(ref_mut_opt);
}
9 changes: 9 additions & 0 deletions tests/ui/borrowck/issue-128381-mutate-through-option-inspect.stderr
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Original file line number Diff line number Diff line change
@@ -0,0 +1,9 @@
error[E0594]: cannot assign to `a.a`, which is behind a `&` reference
--> $DIR/issue-128381-mutate-through-option-inspect.rs:12:9
|
LL | a.a += 123;
| ^^^^^^^^^^ `a` is a `&` reference, so the data it refers to cannot be written

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0594`.
6 changes: 2 additions & 4 deletions tests/ui/borrowck/issue-62387-suggest-iter-mut-2.stderr
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Original file line number Diff line number Diff line change
@@ -1,10 +1,8 @@
error[E0596]: cannot borrow `*container` as mutable, as it is behind a `&` reference
--> $DIR/issue-62387-suggest-iter-mut-2.rs:30:45
--> $DIR/issue-62387-suggest-iter-mut-2.rs:30:56
|
LL | vec.iter().flat_map(|container| container.things()).cloned().collect::<Vec<PathBuf>>();
| --------- ^^^^^^^^^ `container` is a `&` reference, so the data it refers to cannot be borrowed as mutable
| |
| consider changing this binding's type to be: `&mut Container`
| ^^^^^^^^^ `container` is a `&` reference, so the data it refers to cannot be borrowed as mutable
|
help: you may want to use `iter_mut` here
|
Expand Down
12 changes: 4 additions & 8 deletions tests/ui/borrowck/issue-62387-suggest-iter-mut.stderr
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@@ -1,23 +1,19 @@
error[E0596]: cannot borrow `*a` as mutable, as it is behind a `&` reference
--> $DIR/issue-62387-suggest-iter-mut.rs:18:27
--> $DIR/issue-62387-suggest-iter-mut.rs:18:31
|
LL | v.iter().for_each(|a| a.double());
| - ^ `a` is a `&` reference, so the data it refers to cannot be borrowed as mutable
| |
| consider changing this binding's type to be: `&mut A`
| ^ `a` is a `&` reference, so the data it refers to cannot be borrowed as mutable
|
help: you may want to use `iter_mut` here
|
LL | v.iter_mut().for_each(|a| a.double());
| ++++

error[E0596]: cannot borrow `*a` as mutable, as it is behind a `&` reference
--> $DIR/issue-62387-suggest-iter-mut.rs:25:39
--> $DIR/issue-62387-suggest-iter-mut.rs:25:43
|
LL | v.iter().rev().rev().for_each(|a| a.double());
| - ^ `a` is a `&` reference, so the data it refers to cannot be borrowed as mutable
| |
| consider changing this binding's type to be: `&mut A`
| ^ `a` is a `&` reference, so the data it refers to cannot be borrowed as mutable
|
help: you may want to use `iter_mut` here
|
Expand Down
6 changes: 2 additions & 4 deletions tests/ui/borrowck/suggest-as-ref-on-mut-closure.stderr
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Expand Up @@ -15,12 +15,10 @@ LL | <Option<&mut dyn FnMut()> as Clone>::clone(&cb).map(|cb| cb());
| ++++++++++++++++++++++++++++++++++++++++++++ +

error[E0596]: cannot borrow `*cb` as mutable, as it is behind a `&` reference
--> $DIR/suggest-as-ref-on-mut-closure.rs:12:26
--> $DIR/suggest-as-ref-on-mut-closure.rs:12:30
|
LL | cb.as_ref().map(|cb| cb());
| -- ^^ `cb` is a `&` reference, so the data it refers to cannot be borrowed as mutable
| |
| consider changing this binding's type to be: `&mut &mut dyn FnMut()`
| ^^ `cb` is a `&` reference, so the data it refers to cannot be borrowed as mutable
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@Enselic Enselic Nov 8, 2025

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Wasn't this a valid suggestion? Also check the other tests you updated.

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Copilot AI Nov 8, 2025

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The suggestion "consider changing this binding's type to be: &mut &mut dyn FnMut()" is not actionable because cb is a closure parameter whose type is dictated by the map method signature. The user cannot change |cb| to |cb: &mut &mut dyn FnMut()| because as_ref().map() provides &&mut dyn FnMut(), not &mut &mut dyn FnMut().

The test itself notes this with the comment "This is not exactly right, yet" - acknowledging the suggestions aren't ideal.

Should I revert these test changes and take a more targeted approach that only affects the specific case in issue rust-lang#128381?


error: aborting due to 2 previous errors

Expand Down