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1 use core::fmt;
2 use core::pin::Pin;
3 use futures_core::future::{FusedFuture, Future, TryFuture};
4 use futures_core::ready;
5 use futures_core::stream::TryStream;
6 use futures_core::task::{Context, Poll};
7 use pin_project_lite::pin_project;
8 
9 pin_project! {
10     /// Future for the [`try_fold`](super::TryStreamExt::try_fold) method.
11     #[must_use = "futures do nothing unless you `.await` or poll them"]
12     pub struct TryFold<St, Fut, T, F> {
13         #[pin]
14         stream: St,
15         f: F,
16         accum: Option<T>,
17         #[pin]
18         future: Option<Fut>,
19     }
20 }
21 
22 impl<St, Fut, T, F> fmt::Debug for TryFold<St, Fut, T, F>
23 where
24     St: fmt::Debug,
25     Fut: fmt::Debug,
26     T: fmt::Debug,
27 {
fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result28     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
29         f.debug_struct("TryFold")
30             .field("stream", &self.stream)
31             .field("accum", &self.accum)
32             .field("future", &self.future)
33             .finish()
34     }
35 }
36 
37 impl<St, Fut, T, F> TryFold<St, Fut, T, F>
38 where St: TryStream,
39       F: FnMut(T, St::Ok) -> Fut,
40       Fut: TryFuture<Ok = T, Error = St::Error>,
41 {
new(stream: St, f: F, t: T) -> Self42     pub(super) fn new(stream: St, f: F, t: T) -> Self {
43         Self {
44             stream,
45             f,
46             accum: Some(t),
47             future: None,
48         }
49     }
50 }
51 
52 impl<St, Fut, T, F> FusedFuture for TryFold<St, Fut, T, F>
53     where St: TryStream,
54           F: FnMut(T, St::Ok) -> Fut,
55           Fut: TryFuture<Ok = T, Error = St::Error>,
56 {
is_terminated(&self) -> bool57     fn is_terminated(&self) -> bool {
58         self.accum.is_none() && self.future.is_none()
59     }
60 }
61 
62 impl<St, Fut, T, F> Future for TryFold<St, Fut, T, F>
63     where St: TryStream,
64           F: FnMut(T, St::Ok) -> Fut,
65           Fut: TryFuture<Ok = T, Error = St::Error>,
66 {
67     type Output = Result<T, St::Error>;
68 
poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>69     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
70         let mut this = self.project();
71 
72         Poll::Ready(loop {
73             if let Some(fut) = this.future.as_mut().as_pin_mut() {
74                 // we're currently processing a future to produce a new accum value
75                 let res = ready!(fut.try_poll(cx));
76                 this.future.set(None);
77                 match res {
78                     Ok(a) => *this.accum = Some(a),
79                     Err(e) => break Err(e),
80                 }
81             } else if this.accum.is_some() {
82                 // we're waiting on a new item from the stream
83                 let res = ready!(this.stream.as_mut().try_poll_next(cx));
84                 let a = this.accum.take().unwrap();
85                 match res {
86                     Some(Ok(item)) => this.future.set(Some((this.f)(a, item))),
87                     Some(Err(e)) => break Err(e),
88                     None => break Ok(a),
89                 }
90             } else {
91                 panic!("Fold polled after completion")
92             }
93         })
94     }
95 }
96