error message for mismatched &Trait vs. &mut Trait" could be improved

[bachm]

The error message doesn't say that the problem is mismatching mutability.

trait Foo {
    fn bar(&mut self, other: &mut Foo);
}

struct Baz;

impl Foo for Baz {
    fn bar(&mut self, other: &Foo) {}
}

test.rs:8:5: 8:38 error: method `bar` has an incompatible type: expected trait Foo but found trait Foo
test.rs:8     fn bar(&mut self, other: &Foo) {}

[jfager]

Looks like this can be closed, this is now:

$ rustc r13033.rs 
r13033.rs:8:5: 8:38 error: method `bar` has an incompatible type for trait: values differ in mutability [E0053]
r13033.rs:8     fn bar(&mut self, other: &Foo) {}
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error

Edit: though now the complaint might be that it doesn't single out 'other'

[alexcrichton]

Flagging as needstest (just in case a test doesn't already exist)