Wrong structured suggestion for mutability change in for loop pattern

[estebank]

Given:

use std::collections::HashMap;

struct Test {
    v: u32,
}

fn main() {
    let mut map = HashMap::new();
    map.insert("a", Test { v: 0 });

    for (k, v) in map.iter() {
        v.v += 1;
    }
}

we emit:

error[E0594]: cannot assign to `v.v` which is behind a `&` reference
  --> src/main.rs:12:9
   |
11 |     for (k, v) in map.iter() {
   |             - help: consider changing this to be a mutable reference: `&mut Test`
12 |         v.v += 1;
   |         ^^^^^^^^ `v` is a `&` reference, so the data it refers to cannot be written

when what it should be suggesting is

error[E0594]: cannot assign to `v.v` which is behind a `&` reference
  --> src/main.rs:12:9
   |
11 |     for (k, v) in map.iter() {
   |             - help: consider changing making this binding mutable: `mut v`
12 |         v.v += 1;
   |         ^^^^^^^^ `v` is a `&` reference, so the data it refers to cannot be written

CC #82081 #49839.

[estebank]

We now properly suggest calling .iter_mut() instead

error[E0594]: cannot assign to `v.v`, which is behind a `&` reference
  --> src/main.rs:12:9
   |
11 |     for (k, v) in map.iter() {
   |                   ----------
   |                   |   |
   |                   |   help: use mutable method: `iter_mut()`
   |                   this iterator yields `&` references
12 |         v.v += 1;
   |         ^^^^^^^^ `v` is a `&` reference, so the data it refers to cannot be written