chore: create stream_ext module (#3342)

tokio-stream/src/stream_ext/chain.rs

@@ -0,0 +1,50 @@
+use crate::stream_ext::Fuse;
+use crate::Stream;
+
+use core::pin::Pin;
+use core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Stream returned by the [`chain`](super::StreamExt::chain) method.
+    pub struct Chain<T, U> {
+        #[pin]
+        a: Fuse<T>,
+        #[pin]
+        b: U,
+    }
+}
+
+impl<T, U> Chain<T, U> {
+    pub(super) fn new(a: T, b: U) -> Chain<T, U>
+    where
+        T: Stream,
+        U: Stream,
+    {
+        Chain { a: Fuse::new(a), b }
+    }
+}
+
+impl<T, U> Stream for Chain<T, U>
+where
+    T: Stream,
+    U: Stream<Item = T::Item>,
+{
+    type Item = T::Item;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T::Item>> {
+        use Poll::Ready;
+
+        let me = self.project();
+
+        if let Some(v) = ready!(me.a.poll_next(cx)) {
+            return Ready(Some(v));
+        }
+
+        me.b.poll_next(cx)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        super::merge_size_hints(self.a.size_hint(), self.b.size_hint())
+    }
+}