Not sure if I will ever have any use for this.

poller.rs

enum PollerEnum {
  Item(()),
  Timer,
}

struct Poller {
  timer: Interval,
  inner_iter: Option<Box<futures::stream::IterOk<std::vec::IntoIter<()>, std::io::Error>>>,
}

impl Future for Poller {
  type Item = PollerEnum;
  type Error = std::io::Error;

  fn poll(&mut self)
    -> Poll<PollerEnum, std::io::Error>
  {
    if self.inner_iter.is_some() {
      match self.inner_iter.as_mut().unwrap().poll()? {
        Async::Ready( item ) => {
          return Ok(Async::Ready(PollerEnum::Item(item.unwrap())));
        },
        Async::NotReady => {
          self.inner_iter = None;
        },
      };
    }

    match self.timer.poll()? {
      Async::Ready(_) => {
        self.inner_iter = Some(Box::new(stream::iter_ok::<_, std::io::Error>(vec![(), ()])));
        Ok(Async::Ready(PollerEnum::Timer))
      },
      Async::NotReady => {
        Ok(Async::NotReady)
      },
    }
  }
}