Auto merge of #115147 - estebank:issue-114311, r=davidtwco

Suggest mutable borrow on read only for-loop that should be mutable

```
error[E0596]: cannot borrow `*test` as mutable, as it is behind a `&` reference
  --> $DIR/suggest-mut-iterator.rs:22:9
   |
LL |     for test in &tests {
   |                 ------ this iterator yields `&` references
LL |         test.add(2);
   |         ^^^^ `test` is a `&` reference, so the data it refers to cannot be borrowed as mutable
   |
help: use a mutable iterator instead
   |
LL |     for test in &mut tests {
   |                  +++
```

Fix #114311.

Author: bors