error[E0594]: cannot assign to `t.v`, which is behind a `&` reference
  --> $DIR/suggest-mut-method-for-loop.rs:14:9
   |
LL |     for mut t in buzz.values() {
   |                  -------------
   |                  |    |
   |                  |    help: use mutable method: `values_mut()`
   |                  this iterator yields `&` references
...
LL |         t.v += 1;
   |         ^^^^^^^^ `t` is a `&` reference, so the data it refers to cannot be written

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0594`.