Provide hint for assignment to &mut variable that should use *

[joshtriplett]

Consider the following code:

fn main() {
    let mut x = 5;
    {
        let r = &mut x;
        r = 10;
    }
    println!("{}", x);
}

Compiling this produces the following error:

mutref.rs:5:13: 5:15 error: mismatched types [E0308]
mutref.rs:5         r = 10;
                        ^~
mutref.rs:5:13: 5:15 help: run `rustc --explain E0308` to see a detailed explanation
mutref.rs:5:13: 5:15 note: expected type `&mut _`
mutref.rs:5:13: 5:15 note:    found type `_`
error: aborting due to previous error

Ideally, this error should include a suggestion that the user probably meant to assign to *r.

[Mark-Simulacrum]

We now sort of do the opposite -- we suggest &mut 10 which is almost certainly not what the user wanted, I believe.

error[E0308]: mismatched types
 --> test.rs:5:13
  |
5 |         r = 10;
  |             ^^ expected mutable reference, found integral variable
  |
  = note: expected type `&mut {integer}`
             found type `{integer}`
  = help: try with `&mut 10`

[Mark-Simulacrum]

Closing in favor of #33570.