Code shared from the Rust Playground

playground.rs

struct Flatten<I, S>
  where
    I: Iterator,
    S: Iterator,
  {
    base: I,
    inner: S,
  }

impl<I> Flatten<I, <I::Item as IntoIterator>::IntoIter>
  where
    I: Iterator,
    <I as Iterator>::Item: IntoIterator,
  {
    fn new (mut it: I) -> Flatten<I, <I::Item as IntoIterator>::IntoIter> {
      let inner = it.next().unwrap().into_iter();
      Flatten {
        base: it,
        inner,
      }
    }
}

impl<I> Iterator for Flatten<I, <I::Item as IntoIterator>::IntoIter>
  where
    I: Iterator,
    <I as Iterator>::Item: IntoIterator,
{
  type Item = <<I as Iterator>::Item as IntoIterator>::Item;
  fn next(&mut self) -> Option<<I::Item as IntoIterator>::Item> {
    loop {
      match self.inner.next() {
        Some(a) => return Some(a),
        None => {},
      }
      match self.base.next() {
        Some(a) => self.inner = a.into_iter(),
        None => return None,
      }
    }
  }
}