Randomize Stream::merge to improve the throughput.

src/stream/stream/merge.rs

@@ -5,6 +5,7 @@ use pin_project_lite::pin_project;
 
 use crate::prelude::*;
 use crate::stream::Fuse;
+use crate::utils;
 
 pin_project! {
     /// A stream that merges two other streams into a single stream.
@@ -27,7 +28,10 @@ pin_project! {
 
 impl<L: Stream, R: Stream> Merge<L, R> {
     pub(crate) fn new(left: L, right: R) -> Self {
-        Self { left: left.fuse(), right: right.fuse() }
+        Self {
+            left: left.fuse(),
+            right: right.fuse(),
+        }
     }
 }
 
@@ -40,14 +44,29 @@ where
 
     fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
         let this = self.project();
-        match this.left.poll_next(cx) {
-            Poll::Ready(Some(item)) => Poll::Ready(Some(item)),
-            Poll::Ready(None) => this.right.poll_next(cx),
-            Poll::Pending => match this.right.poll_next(cx) {
-                Poll::Ready(Some(item)) => Poll::Ready(Some(item)),
-                Poll::Ready(None) => Poll::Pending,
-                Poll::Pending => Poll::Pending,
-            }
+        if utils::random(1) == 1 {
+            poll_next_in_order(this.left, this.right, cx)
+        } else {
+            poll_next_in_order(this.right, this.left, cx)
         }
     }
 }
+
+fn poll_next_in_order<F, S, I>(
+    first: Pin<&mut F>,
+    second: Pin<&mut S>,
+    cx: &mut Context<'_>,
+) -> Poll<Option<I>>
+where
+    F: Stream<Item = I>,
+    S: Stream<Item = I>,
+{
+    match first.poll_next(cx) {
+        Poll::Ready(None) => second.poll_next(cx),
+        Poll::Ready(item) => Poll::Ready(item),
+        Poll::Pending => match second.poll_next(cx) {
+            Poll::Ready(None) | Poll::Pending => Poll::Pending,
+            Poll::Ready(item) => Poll::Ready(item),
+        },
+    }
+}

src/stream/stream/mod.rs

@@ -1663,18 +1663,17 @@ extension_trait! {
             ```
             # async_std::task::block_on(async {
             use async_std::prelude::*;
-            use async_std::stream;
+            use async_std::stream::{self, FromStream};
 
             let a = stream::once(1u8);
             let b = stream::once(2u8);
             let c = stream::once(3u8);
 
-            let mut s = a.merge(b).merge(c);
+            let s = a.merge(b).merge(c);
+            let mut lst = Vec::from_stream(s).await;
 
-            assert_eq!(s.next().await, Some(1u8));
-            assert_eq!(s.next().await, Some(2u8));
-            assert_eq!(s.next().await, Some(3u8));
-            assert_eq!(s.next().await, None);
+            lst.sort_unstable();
+            assert_eq!(&lst, &[1u8, 2u8, 3u8]);
             # });
             ```
         "#]