consider changing this to be mutable suggested for a binding in a ref pattern

[Alexendoo]

Code

fn x(a: &char) {
    let &b = a;
    b.make_ascii_uppercase();
}

Current output

error[E0596]: cannot borrow `b` as mutable, as it is not declared as mutable
 --> src/lib.rs:3:5
  |
3 |     b.make_ascii_uppercase();
  |     ^ cannot borrow as mutable
  |
help: consider changing this to be mutable
  |
2 |     let &mut b = a;
  |          +++

Desired output

No response

Rationale and extra context

Applying the suggestion gives

error[[E0308]](https://doc.rust-lang.org/nightly/error_codes/E0308.html): mismatched types
 --> src/lib.rs:2:9
  |
2 |     let &mut b = a;
  |         ^^^^^^   - this expression has type `&char`
  |         |
  |         types differ in mutability
  |         help: to declare a mutable variable use: `mut b`
  |
  = note:      expected reference `&char`
          found mutable reference `&mut _`

Other cases

No response

Anything else?

No response

[fmease]

It should suggest let &(mut b) = a; instead.
Looks odd but it's correct.

@rustbot label -needs-triage A-suggestion-diagnostics D-invalid-suggestion

[surechen]

Hi, I'm interested in this issue and want to try to fix it.
@rustbot claim