Improve E0389 "cannot borrow data mutably in a `&` reference" error message

[mikhail-m1]

Error in next case could be more helpful

struct Foo<'a> {
    s: &'a mut String
}

impl<'a> Foo<'a> {
    fn f(&self) {
        self.s.push('x');
    }
}

fn main() {}

error:

error[E0389]: cannot borrow data mutably in a `&` reference
 --> 2.rs:7:9
  |
7 |         self.s.push('x');
  |         ^^^^^^ assignment into an immutable reference

cc @jonathandturner

[GuillaumeGomez]

How would you improve it? It seems pretty clear for me. :-/

[mikhail-m1]

I think not will be helpful

 fn f(&self) {
      ^ use '&mut self' for make mutable

and for sample from tests

    let fancy_ref = &(&mut fancy);
                    ^use '&mut (&mut fancy)' for make mutable
    fancy_ref.num = 6; //~ ERROR E0389

I'm not sure it's easy to do, but I'll try.

[GuillaumeGomez]

Oh, I actually proposed such a change here. Still some little things to debate.

[mikhail-m1]

@GuillaumeGomez I don't understand how its related, are you already work on it?

[GuillaumeGomez]

It's the first step. It suggests references and stuff. However, if the variable isn't mutable, it won't say "make your variable mutable and then use &mut". This step will need to be added.