Set to None only if necessary (#2683)

EFanZh · taiki-e · commit d8beb28d04e3 · 2023-01-31T01:02:07.000+09:00
diff --git a/futures-util/src/stream/stream/chain.rs b/futures-util/src/stream/stream/chain.rs
@@ -50,8 +50,9 @@ where
             if let Some(item) = ready!(first.poll_next(cx)) {
                 return Poll::Ready(Some(item));
             }
+
+            this.first.set(None);
         }
-        this.first.set(None);
         this.second.poll_next(cx)
     }
 

Original file line number	Diff line number	Diff line change
`@@ -50,8 +50,9 @@ where`
`50`	`50`	`if let Some(item) = ready!(first.poll_next(cx)) {`
`51`	`51`	`return Poll::Ready(Some(item));`
`52`	`52`	`}`
	`53`	`+`
	`54`	`+ this.first.set(None);`
`53`	`55`	`}`
`54`		`- this.first.set(None);`
`55`	`56`	`this.second.poll_next(cx)`
`56`	`57`	`}`
`57`	`58`