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Merged
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Nov 4, 2025
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Suggest making binding mut on &mut reborrow #147141

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12 changes: 10 additions & 2 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 @@ -335,10 +335,18 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
LocalInfo::User(BindingForm::Var(mir::VarBindingForm {
binding_mode: BindingMode(ByRef::No, Mutability::Not),
opt_ty_info: Some(sp),
pat_span,
..
})) => {
if suggest {
err.span_note(sp, "the binding is already a mutable borrow");
err.span_suggestion_verbose(
pat_span.shrink_to_lo(),
"consider making the binding mutable if you need to reborrow \
multiple times",
"mut ".to_string(),
Applicability::MaybeIncorrect,
);
}
}
_ => {
Expand All @@ -356,9 +364,9 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
// give a best effort structured suggestion.
err.span_suggestion_verbose(
source_info.span.with_hi(source_info.span.lo() + BytePos(5)),
"try removing `&mut` here",
"if there is only one mutable reborrow, remove the `&mut`",
"",
Applicability::MachineApplicable,
Applicability::MaybeIncorrect,
);
} else {
// This can occur with things like `(&mut self).foo()`.
Expand Down
24 changes: 13 additions & 11 deletions tests/ui/borrowck/mut-borrow-of-mut-ref.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,22 +2,24 @@
#![crate_type = "rlib"]

pub fn f(b: &mut i32) {
//~^ ERROR cannot borrow
//~| NOTE not mutable
//~| NOTE the binding is already a mutable borrow
//~^ ERROR: cannot borrow
//~| NOTE: not mutable
//~| NOTE: the binding is already a mutable borrow
//~| HELP: consider making the binding mutable if you need to reborrow multiple times
h(&mut b);
//~^ NOTE cannot borrow as mutable
//~| HELP try removing `&mut` here
//~^ NOTE: cannot borrow as mutable
//~| HELP: if there is only one mutable reborrow, remove the `&mut`
g(&mut &mut b);
//~^ NOTE cannot borrow as mutable
//~| HELP try removing `&mut` here
//~^ NOTE: cannot borrow as mutable
//~| HELP: if there is only one mutable reborrow, remove the `&mut`
}

pub fn g(b: &mut i32) { //~ NOTE the binding is already a mutable borrow
pub fn g(b: &mut i32) { //~ NOTE: the binding is already a mutable borrow
//~^ HELP: consider making the binding mutable if you need to reborrow multiple times
h(&mut &mut b);
//~^ ERROR cannot borrow
//~| NOTE cannot borrow as mutable
//~| HELP try removing `&mut` here
//~^ ERROR: cannot borrow
//~| NOTE: cannot borrow as mutable
//~| HELP: if there is only one mutable reborrow, remove the `&mut`
}

pub fn h(_: &mut i32) {}
Expand Down
20 changes: 14 additions & 6 deletions tests/ui/borrowck/mut-borrow-of-mut-ref.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -15,36 +15,44 @@ note: the binding is already a mutable borrow
|
LL | pub fn f(b: &mut i32) {
| ^^^^^^^^
help: try removing `&mut` here
help: consider making the binding mutable if you need to reborrow multiple times
|
LL | pub fn f(mut b: &mut i32) {
| +++
help: if there is only one mutable reborrow, remove the `&mut`
|
LL - h(&mut b);
LL + h(b);
|
help: try removing `&mut` here
help: if there is only one mutable reborrow, remove the `&mut`
|
LL - g(&mut &mut b);
LL + g(&mut b);
|

error[E0596]: cannot borrow `b` as mutable, as it is not declared as mutable
--> $DIR/mut-borrow-of-mut-ref.rs:17:12
--> $DIR/mut-borrow-of-mut-ref.rs:19:12
|
LL | h(&mut &mut b);
| ^^^^^^ cannot borrow as mutable
|
note: the binding is already a mutable borrow
--> $DIR/mut-borrow-of-mut-ref.rs:16:13
--> $DIR/mut-borrow-of-mut-ref.rs:17:13
|
LL | pub fn g(b: &mut i32) {
| ^^^^^^^^
help: try removing `&mut` here
help: consider making the binding mutable if you need to reborrow multiple times
|
LL | pub fn g(mut b: &mut i32) {
| +++
help: if there is only one mutable reborrow, remove the `&mut`
|
LL - h(&mut &mut b);
LL + h(&mut b);
|

error[E0596]: cannot borrow `f` as mutable, as it is not declared as mutable
--> $DIR/mut-borrow-of-mut-ref.rs:34:5
--> $DIR/mut-borrow-of-mut-ref.rs:36:5
|
LL | f.bar();
| ^ cannot borrow as mutable
Expand Down
11 changes: 6 additions & 5 deletions tests/ui/did_you_mean/issue-31424.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -4,17 +4,18 @@ struct Struct;

impl Struct {
fn foo(&mut self) {
(&mut self).bar(); //~ ERROR cannot borrow
//~^ HELP try removing `&mut` here
(&mut self).bar(); //~ ERROR: cannot borrow
//~^ HELP: try removing `&mut` here
}

// In this case we could keep the suggestion, but to distinguish the
// two cases is pretty hard. It's an obscure case anyway.
fn bar(self: &mut Self) {
//~^ WARN function cannot return without recursing
//~^^ HELP a `loop` may express intention better if this is on purpose
//~^ WARN: function cannot return without recursing
//~| HELP: a `loop` may express intention better if this is on purpose
//~| HELP: consider making the binding mutable if you need to reborrow multiple times
(&mut self).bar(); //~ ERROR cannot borrow
//~^ HELP try removing `&mut` here
//~^ HELP: try removing `&mut` here
}
}

Expand Down
8 changes: 6 additions & 2 deletions tests/ui/did_you_mean/issue-31424.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -28,7 +28,7 @@ LL | (&mut self).bar();
= note: `#[warn(unconditional_recursion)]` on by default

error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
--> $DIR/issue-31424.rs:16:9
--> $DIR/issue-31424.rs:17:9
|
LL | (&mut self).bar();
| ^^^^^^^^^^^ cannot borrow as mutable
Expand All @@ -39,10 +39,14 @@ note: the binding is already a mutable borrow
LL | fn bar(self: &mut Self) {
| ^^^^^^^^^
help: try removing `&mut` here
--> $DIR/issue-31424.rs:16:9
--> $DIR/issue-31424.rs:17:9
|
LL | (&mut self).bar();
| ^^^^^^^^^^^
help: consider making the binding mutable if you need to reborrow multiple times
|
LL | fn bar(mut self: &mut Self) {
| +++

error: aborting due to 2 previous errors; 1 warning emitted

Expand Down
6 changes: 3 additions & 3 deletions tests/ui/did_you_mean/issue-34126.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,9 +3,9 @@ struct Z { }
impl Z {
fn run(&self, z: &mut Z) { }
fn start(&mut self) {
self.run(&mut self); //~ ERROR cannot borrow
//~| ERROR cannot borrow
//~| HELP try removing `&mut` here
self.run(&mut self); //~ ERROR: cannot borrow
//~| ERROR: cannot borrow
//~| HELP: if there is only one mutable reborrow, remove the `&mut`
}
}

Expand Down
2 changes: 1 addition & 1 deletion tests/ui/did_you_mean/issue-34126.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -9,7 +9,7 @@ note: the binding is already a mutable borrow
|
LL | fn start(&mut self) {
| ^^^^^^^^^
help: try removing `&mut` here
help: if there is only one mutable reborrow, remove the `&mut`
|
LL - self.run(&mut self);
LL + self.run(self);
Expand Down
19 changes: 10 additions & 9 deletions tests/ui/nll/issue-51191.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,16 +2,17 @@ struct Struct;

impl Struct {
fn bar(self: &mut Self) {
//~^ WARN function cannot return without recursing
//~^^ HELP a `loop` may express intention better if this is on purpose
//~^ WARN: function cannot return without recursing
//~| HELP: a `loop` may express intention better if this is on purpose
//~| HELP: consider making the binding mutable if you need to reborrow multiple times
(&mut self).bar();
//~^ ERROR cannot borrow `self` as mutable, as it is not declared as mutable [E0596]
//~^^ HELP try removing `&mut` here
//~^ ERROR: cannot borrow `self` as mutable, as it is not declared as mutable [E0596]
//~| HELP: try removing `&mut` here
}

fn imm(self) { //~ HELP consider changing this to be mutable
(&mut self).bar();
//~^ ERROR cannot borrow `self` as mutable, as it is not declared as mutable [E0596]
//~^ ERROR: cannot borrow `self` as mutable, as it is not declared as mutable [E0596]
}

fn mtbl(mut self) {
Expand All @@ -20,14 +21,14 @@ impl Struct {

fn immref(&self) {
(&mut self).bar();
//~^ ERROR cannot borrow `self` as mutable, as it is not declared as mutable [E0596]
//~^^ ERROR cannot borrow data in a `&` reference as mutable [E0596]
//~^ ERROR: cannot borrow `self` as mutable, as it is not declared as mutable [E0596]
//~| ERROR: cannot borrow data in a `&` reference as mutable [E0596]
}

fn mtblref(&mut self) {
(&mut self).bar();
//~^ ERROR cannot borrow `self` as mutable, as it is not declared as mutable [E0596]
//~^^ HELP try removing `&mut` here
//~^ ERROR: cannot borrow `self` as mutable, as it is not declared as mutable [E0596]
//~| HELP: try removing `&mut` here
}
}

Expand Down
20 changes: 12 additions & 8 deletions tests/ui/nll/issue-51191.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,7 @@ LL | (&mut self).bar();
= note: `#[warn(unconditional_recursion)]` on by default

error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
--> $DIR/issue-51191.rs:7:9
--> $DIR/issue-51191.rs:8:9
|
LL | (&mut self).bar();
| ^^^^^^^^^^^ cannot borrow as mutable
Expand All @@ -22,13 +22,17 @@ note: the binding is already a mutable borrow
LL | fn bar(self: &mut Self) {
| ^^^^^^^^^
help: try removing `&mut` here
--> $DIR/issue-51191.rs:7:9
--> $DIR/issue-51191.rs:8:9
|
LL | (&mut self).bar();
| ^^^^^^^^^^^
help: consider making the binding mutable if you need to reborrow multiple times
|
LL | fn bar(mut self: &mut Self) {
| +++

error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
--> $DIR/issue-51191.rs:13:9
--> $DIR/issue-51191.rs:14:9
|
LL | (&mut self).bar();
| ^^^^^^^^^^^ cannot borrow as mutable
Expand All @@ -39,30 +43,30 @@ LL | fn imm(mut self) {
| +++

error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
--> $DIR/issue-51191.rs:22:9
--> $DIR/issue-51191.rs:23:9
|
LL | (&mut self).bar();
| ^^^^^^^^^^^ cannot borrow as mutable

error[E0596]: cannot borrow data in a `&` reference as mutable
--> $DIR/issue-51191.rs:22:9
--> $DIR/issue-51191.rs:23:9
|
LL | (&mut self).bar();
| ^^^^^^^^^^^ cannot borrow as mutable

error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
--> $DIR/issue-51191.rs:28:9
--> $DIR/issue-51191.rs:29:9
|
LL | (&mut self).bar();
| ^^^^^^^^^^^ cannot borrow as mutable
|
note: the binding is already a mutable borrow
--> $DIR/issue-51191.rs:27:16
--> $DIR/issue-51191.rs:28:16
|
LL | fn mtblref(&mut self) {
| ^^^^^^^^^
help: try removing `&mut` here
--> $DIR/issue-51191.rs:28:9
--> $DIR/issue-51191.rs:29:9
|
LL | (&mut self).bar();
| ^^^^^^^^^^^
Expand Down
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