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Confusing error message suggestion when invoking mutable trait method implemented by a mutable reference #83241
Description
Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=e7ec4e3526917cc9fe6d42a43e2a9a6a
trait Foo {
fn bar(&mut self);
}
impl Foo for &mut String {
fn bar(&mut self) {
}
}
pub fn baz(f: &mut String) {
f.bar();
}The current output is:
error[E0596]: cannot borrow `f` as mutable, as it is not declared as mutable
--> src/lib.rs:11:5
|
11 | f.bar();
| ^
| |
| cannot borrow as mutable
| try removing `&mut` here
error: aborting due to previous error
The suggestion to remove &mut is quite confusing and does not appear actionable unless it means removing &mut from the declaration of fn bar(&mut self);. I believe the correct fix is to add a mut before the argument in baz:
pub fn baz(mut f: &mut String) {
f.bar();
}Ideally the output should be similar to the case where the trait is implemented by something that is not a mutable reference:
error[E0596]: cannot borrow `f` as mutable, as it is not declared as mutable
--> src/lib.rs:11:5
|
10 | pub fn baz(f: String) {
| - help: consider changing this to be mutable: `mut f`
11 | f.bar();
| ^ cannot borrow as mutable
error: aborting due to previous error
Reacted by Esteban Kuber
Activity
Suggestion for call on immutable binding of mutable type
Rollup merge of rust-lang#83251 - estebank:issue-83241, r=oli-obk