message: feat(interface-types) Simplify `FlattenInterfaceValueIterator` with `last_mut`.
diff: @@ -156,13 +156,7 @@ impl<'a> Iterator for FlattenInterfaceValueIterator<'a> {
  type Item = &'a InterfaceValue;
  fn next(&mut self) -> Option<Self::Item> {
  - if self.iterators.is_empty() {
  - return None;
  - }
  -
  - let index = self.iterators.len() - 1;
  -
  - match self.iterators[index].next() {
  + match self.iterators.last_mut()?.next() {
  // End of the current iterator, go back to the previous
  // one.
  None => {
  