error: called `flat_map(|x| x)` on an `Iterator`. This can be simplified by calling `flatten().`
  --> $DIR/unnecessary_flat_map.rs:5:5
   |
LL |     iterator.flat_map(|x| x);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `-D clippy::flat-map` implied by `-D warnings`

error: aborting due to previous error