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Suggest removing &mut when encountering mutability error #68723

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Suggest removing &mut when encountering mutability error #68723

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22 changes: 16 additions & 6 deletions src/librustc_mir/borrow_check/diagnostics/mutability_errors.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -257,12 +257,22 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
assert_eq!(local_decl.mutability, Mutability::Not);

err.span_label(span, format!("cannot {ACT}", ACT = act));
err.span_suggestion(
local_decl.source_info.span,
"consider changing this to be mutable",
format!("mut {}", self.local_names[local].unwrap()),
Applicability::MachineApplicable,
);

if let ty::Ref(_, _, Mutability::Mut) = local_decl.ty.kind {
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This condition seems insufficient. You'd need to check that the expected type requires &mut T and not &mut &mut T, otherwise it seems like it would trigger on this (which is a good test to add, to ensure that this behaves correctly):

struct S;

fn bar(x: &mut &mut S) {}

fn foo() {
    let x = &mut S;
    bar(&mut x);
}

...but this makes me wonder whether this is doable in borrowck if you don't know what type is expected (seems you only know that you want to take a mutable borrow and cannot due to lack of mut on the binding. On the other hand, if you do check it in typeck, then we need to check that:

struct S;

fn bar(x: &mut S) {}

fn foo() {
    let mut x = &mut S;
    bar(&mut x);
}

still compiles... which is probably not a good idea in typeck.

err.span_suggestion(
span,
"remove the unnecessary `&mut` here",
self.local_names[local].unwrap().to_string(),
Applicability::MaybeIncorrect,
);
} else {
err.span_suggestion(
local_decl.source_info.span,
"consider changing this to be mutable",
format!("mut {}", self.local_names[local].unwrap()),
Applicability::MachineApplicable,
);
}
}

// Also suggest adding mut for upvars
Expand Down
15 changes: 15 additions & 0 deletions src/test/ui/borrowck/issue-68697-remove-mut-ref.rs
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@@ -0,0 +1,15 @@
// Regression test for #68697: Suggest removing `&mut x`
// when `x: &mut T` and `&mut T` is expected type

struct A;

fn bar(x: &mut A) {}

fn foo(x: &mut A) {
bar(&mut x);
//~^ ERROR: cannot borrow `x` as mutable
//~| HELP: remove the unnecessary `&mut` here
//~| SUGGESTION: x
}

fn main() {}
12 changes: 12 additions & 0 deletions src/test/ui/borrowck/issue-68697-remove-mut-ref.stderr
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@@ -0,0 +1,12 @@
error[E0596]: cannot borrow `x` as mutable, as it is not declared as mutable
--> $DIR/issue-68697-remove-mut-ref.rs:6:9
|
LL | bar(&mut x);
| ^^^^^^
| |
| cannot borrow as mutable
| help: remove the unnecessary `&mut` here: `x`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0596`.