error[E0271]: expected `foo` to be a fn item that returns `i32`, but it returns `()`
  --> $DIR/issue-106991.rs:5:13
   |
LL | fn bar() -> impl Iterator<Item = i32> {
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^ expected `i32`, found `()`
...
LL |     x.iter_mut().map(foo)
   |     --------------------- return type was inferred to be `Map<std::slice::IterMut<'_, Vec<u8>>, for<'a> fn(&'a mut Vec<u8>) {foo}>` here
   |
   = note: required for `Map<std::slice::IterMut<'_, Vec<u8>>, for<'a> fn(&'a mut Vec<u8>) {foo}>` to implement `Iterator`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0271`.