1 use core::pin::Pin;
2 use futures_core::ready;
3 use futures_core::stream::{FusedStream, Stream};
4 use futures_core::task::{Context, Poll};
5 #[cfg(feature = "sink")]
6 use futures_sink::Sink;
7 use pin_project_lite::pin_project;
8 
9 pin_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 
21 impl<St, U> Flatten<St, U> {
new(stream: St) -> Self22     pub(super) fn new(stream: St) -> Self {
23         Self { stream, next: None }
24     }
25 
26     delegate_access_inner!(stream, St, ());
27 }
28 
29 impl<St> FusedStream for Flatten<St, St::Item>
30 where
31     St: FusedStream,
32     St::Item: Stream,
33 {
is_terminated(&self) -> bool34     fn is_terminated(&self) -> bool {
35         self.next.is_none() && self.stream.is_terminated()
36     }
37 }
38 
39 impl<St> Stream for Flatten<St, St::Item>
40 where
41     St: Stream,
42     St::Item: Stream,
43 {
44     type Item = <St::Item as Stream>::Item;
45 
poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>>46     fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
47         let mut this = self.project();
48         Poll::Ready(loop {
49             if let Some(s) = this.next.as_mut().as_pin_mut() {
50                 if let Some(item) = ready!(s.poll_next(cx)) {
51                     break Some(item);
52                 } else {
53                     this.next.set(None);
54                 }
55             } else if let Some(s) = 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")]
66 impl<S, Item> Sink<Item> for Flatten<S, S::Item>
67 where
68     S: Stream + Sink<Item>,
69 {
70     type Error = S::Error;
71 
72     delegate_sink!(stream, Item);
73 }
74