Do not store items field in ReadyChunks (#2656)

`items` is always empty when `poll_ready` exists, so there's no
reason to store it inside `ReadyChunks`.

This makes code a bit more efficient, but also makes code easier
to understand.

futures-util/src/stream/stream/ready_chunks.rs

@@ -1,6 +1,5 @@
 use crate::stream::Fuse;
 use alloc::vec::Vec;
-use core::mem;
 use core::pin::Pin;
 use futures_core::stream::{FusedStream, Stream};
 use futures_core::task::{Context, Poll};
@@ -15,7 +14,6 @@ pin_project! {
     pub struct ReadyChunks<St: Stream> {
         #[pin]
         stream: Fuse<St>,
-        items: Vec<St::Item>,
         cap: usize, // https://github.com/rust-lang/futures-rs/issues/1475
     }
 }
@@ -24,11 +22,7 @@ impl<St: Stream> ReadyChunks<St> {
     pub(super) fn new(stream: St, capacity: usize) -> Self {
         assert!(capacity > 0);
 
-        Self {
-            stream: super::Fuse::new(stream),
-            items: Vec::with_capacity(capacity),
-            cap: capacity,
-        }
+        Self { stream: super::Fuse::new(stream), cap: capacity }
     }
 
     delegate_access_inner!(stream, St, (.));
@@ -40,40 +34,33 @@ impl<St: Stream> Stream for ReadyChunks<St> {
     fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
         let mut this = self.project();
 
+        let mut items: Vec<St::Item> = Vec::new();
+
         loop {
             match this.stream.as_mut().poll_next(cx) {
                 // Flush all collected data if underlying stream doesn't contain
                 // more ready values
                 Poll::Pending => {
-                    return if this.items.is_empty() {
-                        Poll::Pending
-                    } else {
-                        Poll::Ready(Some(mem::replace(this.items, Vec::with_capacity(*this.cap))))
-                    }
+                    return if items.is_empty() { Poll::Pending } else { Poll::Ready(Some(items)) }
                 }
 
                 // Push the ready item into the buffer and check whether it is full.
                 // If so, replace our buffer with a new and empty one and return
                 // the full one.
                 Poll::Ready(Some(item)) => {
-                    this.items.push(item);
-                    if this.items.len() >= *this.cap {
-                        return Poll::Ready(Some(mem::replace(
-                            this.items,
-                            Vec::with_capacity(*this.cap),
-                        )));
+                    if items.is_empty() {
+                        items.reserve(*this.cap);
+                    }
+                    items.push(item);
+                    if items.len() >= *this.cap {
+                        return Poll::Ready(Some(items));
                     }
                 }
 
                 // Since the underlying stream ran out of values, return what we
                 // have buffered, if we have anything.
                 Poll::Ready(None) => {
-                    let last = if this.items.is_empty() {
-                        None
-                    } else {
-                        let full_buf = mem::take(this.items);
-                        Some(full_buf)
-                    };
+                    let last = if items.is_empty() { None } else { Some(items) };
 
                     return Poll::Ready(last);
                 }
@@ -82,20 +69,15 @@ impl<St: Stream> Stream for ReadyChunks<St> {
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let chunk_len = usize::from(!self.items.is_empty());
         let (lower, upper) = self.stream.size_hint();
-        let lower = (lower / self.cap).saturating_add(chunk_len);
-        let upper = match upper {
-            Some(x) => x.checked_add(chunk_len),
-            None => None,
-        };
+        let lower = lower / self.cap;
         (lower, upper)
     }
 }
 
 impl<St: FusedStream> FusedStream for ReadyChunks<St> {
     fn is_terminated(&self) -> bool {
-        self.stream.is_terminated() && self.items.is_empty()
+        self.stream.is_terminated()
     }
 }
 

futures/tests/auto_traits.rs

@@ -1492,10 +1492,10 @@ pub mod stream {
     assert_not_impl!(PollImmediate<PinnedStream>: Unpin);
 
     assert_impl!(ReadyChunks<SendStream<()>>: Send);
-    assert_not_impl!(ReadyChunks<SendStream>: Send);
+    assert_impl!(ReadyChunks<SendStream>: Send);
     assert_not_impl!(ReadyChunks<LocalStream>: Send);
     assert_impl!(ReadyChunks<SyncStream<()>>: Sync);
-    assert_not_impl!(ReadyChunks<SyncStream>: Sync);
+    assert_impl!(ReadyChunks<SyncStream>: Sync);
     assert_not_impl!(ReadyChunks<LocalStream>: Sync);
     assert_impl!(ReadyChunks<UnpinStream>: Unpin);
     assert_not_impl!(ReadyChunks<PinnedStream>: Unpin);