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.