blob: ab446cb5625d3c75922b221f05b346c005bf4b77 [file] [log] [blame]
use core::fmt;
use core::pin::Pin;
use futures_core::future::Future;
use futures_core::stream::{FusedStream, Stream};
use futures_core::task::{LocalWaker, Poll};
/// Future for the `flatten_stream` combinator, flattening a
/// future-of-a-stream to get just the result of the final stream as a stream.
///
/// This is created by the `Future::flatten_stream` method.
#[must_use = "streams do nothing unless polled"]
pub struct FlattenStream<Fut: Future> {
state: State<Fut>
}
impl<Fut: Future> FlattenStream<Fut> {
pub(super) fn new(future: Fut) -> FlattenStream<Fut> {
FlattenStream {
state: State::Future(future)
}
}
}
impl<Fut> fmt::Debug for FlattenStream<Fut>
where Fut: Future + fmt::Debug,
Fut::Output: fmt::Debug,
{
fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
fmt.debug_struct("FlattenStream")
.field("state", &self.state)
.finish()
}
}
#[derive(Debug)]
enum State<Fut: Future> {
// future is not yet called or called and not ready
Future(Fut),
// future resolved to Stream
Stream(Fut::Output),
}
impl<Fut> FusedStream for FlattenStream<Fut>
where Fut: Future,
Fut::Output: Stream + FusedStream,
{
fn is_terminated(&self) -> bool {
match &self.state {
State::Future(_) => false,
State::Stream(stream) => stream.is_terminated(),
}
}
}
impl<Fut> Stream for FlattenStream<Fut>
where Fut: Future,
Fut::Output: Stream,
{
type Item = <Fut::Output as Stream>::Item;
fn poll_next(mut self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<Option<Self::Item>> {
loop {
// safety: data is never moved via the resulting &mut reference
let stream = match &mut unsafe { Pin::get_unchecked_mut(self.as_mut()) }.state {
State::Future(f) => {
// safety: the future we're re-pinning here will never be moved;
// it will just be polled, then dropped in place
match unsafe { Pin::new_unchecked(f) }.poll(lw) {
Poll::Pending => {
// State is not changed, early return.
return Poll::Pending
},
Poll::Ready(stream) => {
// Future resolved to stream.
// We do not return, but poll that
// stream in the next loop iteration.
stream
}
}
}
State::Stream(s) => {
// safety: the stream we're repinning here will never be moved;
// it will just be polled, then dropped in place
return unsafe { Pin::new_unchecked(s) }.poll_next(lw);
}
};
unsafe {
// safety: we use the &mut only for an assignment, which causes
// only an in-place drop
Pin::get_unchecked_mut(self.as_mut()).state = State::Stream(stream);
}
}
}
}