1 use crate::Stream; 2 3 use core::future::Future; 4 use core::marker::PhantomPinned; 5 use core::pin::Pin; 6 use core::task::{Context, Poll}; 7 use pin_project_lite::pin_project; 8 9 pin_project! { 10 /// Future returned by the [`fold`](super::StreamExt::fold) method. 11 #[derive(Debug)] 12 #[must_use = "futures do nothing unless you `.await` or poll them"] 13 pub struct FoldFuture<St, B, F> { 14 #[pin] 15 stream: St, 16 acc: Option<B>, 17 f: F, 18 // Make this future `!Unpin` for compatibility with async trait methods. 19 #[pin] 20 _pin: PhantomPinned, 21 } 22 } 23 24 impl<St, B, F> FoldFuture<St, B, F> { new(stream: St, init: B, f: F) -> Self25 pub(super) fn new(stream: St, init: B, f: F) -> Self { 26 Self { 27 stream, 28 acc: Some(init), 29 f, 30 _pin: PhantomPinned, 31 } 32 } 33 } 34 35 impl<St, B, F> Future for FoldFuture<St, B, F> 36 where 37 St: Stream, 38 F: FnMut(B, St::Item) -> B, 39 { 40 type Output = B; 41 poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>42 fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> { 43 let mut me = self.project(); 44 loop { 45 let next = ready!(me.stream.as_mut().poll_next(cx)); 46 47 match next { 48 Some(v) => { 49 let old = me.acc.take().unwrap(); 50 let new = (me.f)(old, v); 51 *me.acc = Some(new); 52 } 53 None => return Poll::Ready(me.acc.take().unwrap()), 54 } 55 } 56 } 57 } 58