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use crate::Stream;
use core::fmt;
use core::future::Future;
use core::pin::Pin;
use core::task::{Context, Poll};
use pin_project_lite::pin_project;
pin_project! {
/// Stream for the [`then`](super::StreamExt::then) method.
#[must_use = "streams do nothing unless polled"]
pub struct Then<St, Fut, F> {
#[pin]
stream: St,
#[pin]
future: Option<Fut>,
f: F,
}
}
impl<St, Fut, F> fmt::Debug for Then<St, Fut, F>
where
St: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Then")
.field("stream", &self.stream)
.finish()
}
}
impl<St, Fut, F> Then<St, Fut, F> {
pub(super) fn new(stream: St, f: F) -> Self {
Then {
stream,
future: None,
f,
}
}
}
impl<St, F, Fut> Stream for Then<St, Fut, F>
where
St: Stream,
Fut: Future,
F: FnMut(St::Item) -> Fut,
{
type Item = Fut::Output;
fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Fut::Output>> {
let mut me = self.project();
loop {
if let Some(future) = me.future.as_mut().as_pin_mut() {
match future.poll(cx) {
Poll::Ready(item) => {
me.future.set(None);
return Poll::Ready(Some(item));
}
Poll::Pending => return Poll::Pending,
}
}
match me.stream.as_mut().poll_next(cx) {
Poll::Ready(Some(item)) => {
me.future.set(Some((me.f)(item)));
}
Poll::Ready(None) => return Poll::Ready(None),
Poll::Pending => return Poll::Pending,
}
}
}
fn size_hint(&self) -> (usize, Option<usize>) {
let future_len = if self.future.is_some() { 1 } else { 0 };
let (lower, upper) = self.stream.size_hint();
let lower = lower.saturating_add(future_len);
let upper = upper.and_then(|upper| upper.checked_add(future_len));
(lower, upper)
}
}
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