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In ready_chunks, reserve capacity based on size_hint #2661

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In ready_chunks, reserve capacity based on size_hint #2661

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7 changes: 6 additions & 1 deletion futures-util/src/stream/stream/ready_chunks.rs
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Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
use crate::stream::Fuse;
use alloc::vec::Vec;
use core::cmp;
use core::pin::Pin;
use futures_core::stream::{FusedStream, Stream};
use futures_core::task::{Context, Poll};
Expand Down Expand Up @@ -49,7 +50,11 @@ impl<St: Stream> Stream for ReadyChunks<St> {
// the full one.
Poll::Ready(Some(item)) => {
if items.is_empty() {
items.reserve(*this.cap);
// Note we reserve capacity here, not when `items` is created,
// because stream may know the remaining size,
// but items might not be readily available.
let size_hint = this.stream.as_mut().size_hint().0.wrapping_add(1);
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Why is this wrapping_add instead of saturating_add?

items.reserve(cmp::min(*this.cap, size_hint));
}
items.push(item);
if items.len() >= *this.cap {
Expand Down