error: you are implementing `Iterator` on a `Copy` type
  --> $DIR/copy_iterator.rs:6:1
   |
6  | / impl Iterator for Countdown {
7  | |     type Item = u8;
8  | |
9  | |     fn next(&mut self) -> Option<u8> {
...  |
14 | |     }
15 | | }
   | |_^
   |
   = note: `-D copy-iterator` implied by `-D warnings`
   = note: consider implementing `IntoIterator` instead

error: aborting due to previous error