chore: create stream_ext module (#3342)

tokio-stream/src/stream_ext/map.rs

@@ -0,0 +1,51 @@
+use crate::Stream;
+
+use core::fmt;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Stream for the [`map`](super::StreamExt::map) method.
+    #[must_use = "streams do nothing unless polled"]
+    pub struct Map<St, F> {
+        #[pin]
+        stream: St,
+        f: F,
+    }
+}
+
+impl<St, F> fmt::Debug for Map<St, F>
+where
+    St: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Map").field("stream", &self.stream).finish()
+    }
+}
+
+impl<St, F> Map<St, F> {
+    pub(super) fn new(stream: St, f: F) -> Self {
+        Map { stream, f }
+    }
+}
+
+impl<St, F, T> Stream for Map<St, F>
+where
+    St: Stream,
+    F: FnMut(St::Item) -> T,
+{
+    type Item = T;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
+        self.as_mut()
+            .project()
+            .stream
+            .poll_next(cx)
+            .map(|opt| opt.map(|x| (self.as_mut().project().f)(x)))
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.stream.size_hint()
+    }
+}