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add Stream::last (#347)
* add stream::LastFuture (not compiling) Struggling with the associated type, pinning and how to move/copy LastFuture.last. * fix type signature -> still cannot assign still problems assigning the new value to self.last * remove unused bound * add doctest * unpin LastFuture.last * RustFmt * add static lifetime * remove redundant lifetime
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42
src/stream/stream/last.rs
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42
src/stream/stream/last.rs
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@ -0,0 +1,42 @@
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use std::pin::Pin;
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use crate::future::Future;
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use crate::stream::Stream;
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use crate::task::{Context, Poll};
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#[doc(hidden)]
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#[allow(missing_debug_implementations)]
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pub struct LastFuture<S, T> {
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stream: S,
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last: Option<T>,
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}
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impl<S, T> LastFuture<S, T> {
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pin_utils::unsafe_pinned!(stream: S);
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pin_utils::unsafe_unpinned!(last: Option<T>);
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pub(crate) fn new(stream: S) -> Self {
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LastFuture { stream, last: None }
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}
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}
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impl<S> Future for LastFuture<S, S::Item>
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where
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S: Stream + Unpin + Sized,
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S::Item: Copy,
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{
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type Output = Option<S::Item>;
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fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
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let next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
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match next {
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Some(new) => {
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cx.waker().wake_by_ref();
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*self.as_mut().last() = Some(new);
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Poll::Pending
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}
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None => Poll::Ready(self.last),
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}
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}
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}
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@ -36,6 +36,7 @@ mod fuse;
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mod ge;
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mod gt;
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mod inspect;
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mod last;
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mod le;
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mod lt;
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mod map;
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@ -64,6 +65,7 @@ use fold::FoldFuture;
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use for_each::ForEachFuture;
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use ge::GeFuture;
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use gt::GtFuture;
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use last::LastFuture;
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use le::LeFuture;
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use lt::LtFuture;
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use min_by::MinByFuture;
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@ -456,6 +458,54 @@ extension_trait! {
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Inspect::new(self, f)
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}
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#[doc = r#"
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Returns the last element of the stream.
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# Examples
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Basic usage:
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```
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# fn main() { async_std::task::block_on(async {
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#
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use std::collections::VecDeque;
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use async_std::prelude::*;
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let s: VecDeque<usize> = vec![1, 2, 3].into_iter().collect();
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let last = s.last().await;
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assert_eq!(last, Some(3));
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#
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# }) }
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```
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An empty stream will return `None:
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```
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# fn main() { async_std::task::block_on(async {
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#
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use std::collections::VecDeque;
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use async_std::prelude::*;
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let s: VecDeque<usize> = vec![].into_iter().collect();
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let last = s.last().await;
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assert_eq!(last, None);
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#
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# }) }
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```
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"#]
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fn last(
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self,
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) -> impl Future<Output = Option<Self::Item>> [LastFuture<Self, Self::Item>]
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where
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Self: Sized,
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{
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LastFuture::new(self)
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}
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#[doc = r#"
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Transforms this `Stream` into a "fused" `Stream` such that after the first time
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`poll` returns `Poll::Ready(None)`, all future calls to `poll` will also return
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