1use core::pin::Pin;
2use futures_core::ready;
3use futures_core::stream::{FusedStream, Stream};
4use futures_core::task::{Context, Poll};
5#[cfg(feature = "sink")]
6use futures_sink::Sink;
7use pin_project_lite::pin_project;
8
9pin_project! {
10 /// Stream for the [`flatten`](super::StreamExt::flatten) method.
11 #[derive(Debug)]
12 #[must_use = "streams do nothing unless polled"]
13 pub struct Flatten<St, U> {
14 #[pin]
15 stream: St,
16 #[pin]
17 next: Option<U>,
18 }
19}
20
21impl<St, U> Flatten<St, U> {
22 pub(super) fn new(stream: St) -> Self {
23 Self { stream, next: None }
24 }
25
26 delegate_access_inner!(stream, St, ());
27}
28
29impl<St> FusedStream for Flatten<St, St::Item>
30where
31 St: FusedStream,
32 St::Item: Stream,
33{
34 fn is_terminated(&self) -> bool {
35 self.next.is_none() && self.stream.is_terminated()
36 }
37}
38
39impl<St> Stream for Flatten<St, St::Item>
40where
41 St: Stream,
42 St::Item: Stream,
43{
44 type Item = <St::Item as Stream>::Item;
45
46 fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
47 let mut this: Projection<'_, St, impl Stream> = self.project();
48 Poll::Ready(loop {
49 if let Some(s: Pin<&mut impl Stream>) = this.next.as_mut().as_pin_mut() {
50 if let Some(item: ::Item) = ready!(s.poll_next(cx)) {
51 break Some(item);
52 } else {
53 this.next.set(None);
54 }
55 } else if let Some(s: impl Stream) = ready!(this.stream.as_mut().poll_next(cx)) {
56 this.next.set(Some(s));
57 } else {
58 break None;
59 }
60 })
61 }
62}
63
64// Forwarding impl of Sink from the underlying stream
65#[cfg(feature = "sink")]
66impl<S, Item> Sink<Item> for Flatten<S, S::Item>
67where
68 S: Stream + Sink<Item>,
69{
70 type Error = S::Error;
71
72 delegate_sink!(stream, Item);
73}
74