Use structured suggestion when missing `mut` on local binding

[estebank]

The following code:

use std::collections::HashMap;

fn main() {
    let x = HashMap::new();
    x.insert("a", 3);
    println!("{:?}", x.get("a"));
}

Produces

error[E0596]: cannot borrow immutable local variable `x` as mutable
 --> src/main.rs:5:5
  |
4 |     let x = HashMap::new();
  |         - consider changing this to `mut x`
5 |     x.insert("a", 3);
  |     ^ cannot borrow mutably

The suggestion for line 4 is a label, not a suggestion.