stream: add stream utility function

Allows extending a Vec<T> with a Stream<Item=T>.

Ref: #2842

Author: dungex

tokio/src/stream/extend.rs

@@ -0,0 +1,20 @@
+use crate::stream::{Stream, StreamExt};
+use std::marker::Unpin;
+/// Extends a slice from a Stream
+/// # Example
+/// `rust,no_run`
+/// use tokio::stream;
+///
+/// let mut b = vec![0, 0, 0, 0];
+/// let stream = stream::iter(vec![0, 2, 4, 6]);
+///
+/// stream::extend(&mut buff, s).await;
+/// assert_eq!(vec![0, 2, 4, 6], buff);
+pub async fn extend<T, S>(buff: &mut Vec<T>, mut b: S)
+where
+    S: Stream<Item = T> + Unpin,
+{
+    while let Some(item) = b.next().await {
+        buff.push(item)
+    }
+}

tokio/src/stream/mod.rs

@@ -66,6 +66,8 @@ mod collect;
 use collect::Collect;
 pub use collect::FromStream;
 
+mod extend;
+pub use extend::extend;
 mod empty;
 pub use empty::{empty, Empty};
 

tokio/tests/stream_extend.rs

@@ -0,0 +1,9 @@
+use tokio::stream;
+
+#[tokio::test]
+async fn extend_stream() {
+    let s = stream::iter(vec![0, 2, 4, 6]);
+    let mut buff = vec![-2];
+    stream::extend(&mut buff, s).await;
+    assert_eq!(buff, vec![-2, 0, 2, 4, 6]);
+}