stream: add `next` and `map` utility fn (#1962)

Introduces `StreamExt` trait. This trait will be used to add utility functions
to make working with streams easier. This patch includes two functions:

* `next`: a future returning the item in the stream.
* `map`: transform each item in the stream.

42 files changed, 295 insertions, 57 deletions

diff --git a/examples/chat.rs b/examples/chat.rs
index 91589072..e1da5f32 100644
--- a/examples/chat.rs
+++ b/examples/chat.rs
@@ -27,10 +27,11 @@
 #![warn(rust_2018_idioms)]
 
 use tokio::net::{TcpListener, TcpStream};
+use tokio::stream::{Stream, StreamExt};
 use tokio::sync::{mpsc, Mutex};
 use tokio_util::codec::{Framed, LinesCodec, LinesCodecError};
 
-use futures::{SinkExt, Stream, StreamExt};
+use futures::SinkExt;
 use std::collections::HashMap;
 use std::env;
 use std::error::Error;
@@ -163,12 +164,12 @@ impl Stream for Peer {
     fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
         // First poll the `UnboundedReceiver`.
 
-        if let Poll::Ready(Some(v)) = self.rx.poll_next_unpin(cx) {
+        if let Poll::Ready(Some(v)) = Pin::new(&mut self.rx).poll_next(cx) {
             return Poll::Ready(Some(Ok(Message::Received(v))));
         }
 
         // Secondly poll the `Framed` stream.
-        let result: Option<_> = futures::ready!(self.lines.poll_next_unpin(cx));
+        let result: Option<_> = futures::ready!(Pin::new(&mut self.lines).poll_next(cx));
 
         Poll::Ready(match result {
             // We've received a message we should broadcast to others.
diff --git a/examples/connect.rs b/examples/connect.rs
index d51af88c..75640c62 100644
--- a/examples/connect.rs
+++ b/examples/connect.rs
@@ -55,19 +55,21 @@ async fn main() -> Result<(), Box<dyn Error>> {
 
 mod tcp {
     use super::codec;
-    use futures::{future, Sink, SinkExt, Stream, StreamExt};
+    use futures::StreamExt;
+    use futures::{future, Sink, SinkExt};
     use std::{error::Error, io, net::SocketAddr};
     use tokio::net::TcpStream;
+    use tokio::stream::Stream;
     use tokio_util::codec::{FramedRead, FramedWrite};
 
     pub async fn connect(
         addr: &SocketAddr,
-        stdin: impl Stream<Item = Result<Vec<u8>, io::Error>> + Unpin,
+        mut stdin: impl Stream<Item = Result<Vec<u8>, io::Error>> + Unpin,
         mut stdout: impl Sink<Vec<u8>, Error = io::Error> + Unpin,
     ) -> Result<(), Box<dyn Error>> {
         let mut stream = TcpStream::connect(addr).await?;
         let (r, w) = stream.split();
-        let sink = FramedWrite::new(w, codec::Bytes);
+        let mut sink = FramedWrite::new(w, codec::Bytes);
         let mut stream = FramedRead::new(r, codec::Bytes)
             .filter_map(|i| match i {
                 Ok(i) => future::ready(Some(i)),
@@ -78,7 +80,7 @@ mod tcp {
             })
             .map(Ok);
 
-        match future::join(stdin.forward(sink), stdout.send_all(&mut stream)).await {
+        match future::join(sink.send_all(&mut stdin), stdout.send_all(&mut stream)).await {
             (Err(e), _) | (_, Err(e)) => Err(e.into()),
             _ => Ok(()),
         }
@@ -88,8 +90,9 @@ mod tcp {
 mod udp {
     use tokio::net::udp::{RecvHalf, SendHalf};
     use tokio::net::UdpSocket;
+    use tokio::stream::{Stream, StreamExt};
 
-    use futures::{future, Sink, SinkExt, Stream, StreamExt};
+    use futures::{future, Sink, SinkExt};
     use std::error::Error;
     use std::io;
     use std::net::SocketAddr;
diff --git a/examples/print_each_packet.rs b/examples/print_each_packet.rs
index 4604139b..d650b5bd 100644
--- a/examples/print_each_packet.rs
+++ b/examples/print_each_packet.rs
@@ -55,9 +55,9 @@
 #![warn(rust_2018_idioms)]
 
 use tokio::net::TcpListener;
+use tokio::stream::StreamExt;
 use tokio_util::codec::{BytesCodec, Decoder};
 
-use futures::StreamExt;
 use std::env;
 
 #[tokio::main]
diff --git a/examples/tinydb.rs b/examples/tinydb.rs
index cf867a0a..7c71dedf 100644
--- a/examples/tinydb.rs
+++ b/examples/tinydb.rs
@@ -42,9 +42,10 @@
 #![warn(rust_2018_idioms)]
 
 use tokio::net::TcpListener;
+use tokio::stream::StreamExt;
 use tokio_util::codec::{Framed, LinesCodec};
 
-use futures::{SinkExt, StreamExt};
+use futures::SinkExt;
 use std::collections::HashMap;
 use std::env;
 use std::error::Error;
diff --git a/examples/tinyhttp.rs b/examples/tinyhttp.rs
index 5ddf0d48..9ac2806e 100644
--- a/examples/tinyhttp.rs
+++ b/examples/tinyhttp.rs
@@ -14,13 +14,14 @@
 #![warn(rust_2018_idioms)]
 
 use bytes::BytesMut;
-use futures::{SinkExt, StreamExt};
+use futures::SinkExt;
 use http::{header::HeaderValue, Request, Response, StatusCode};
 #[macro_use]
 extern crate serde_derive;
 use serde_json;
 use std::{env, error::Error, fmt, io};
 use tokio::net::{TcpListener, TcpStream};
+use tokio::stream::StreamExt;
 use tokio_util::codec::{Decoder, Encoder, Framed};
 
 #[tokio::main]
diff --git a/examples/udp-codec.rs b/examples/udp-codec.rs
index 6b3f84a0..dc30394f 100644
--- a/examples/udp-codec.rs
+++ b/examples/udp-codec.rs
@@ -9,12 +9,13 @@
 #![warn(rust_2018_idioms)]
 
 use tokio::net::UdpSocket;
+use tokio::stream::StreamExt;
 use tokio::{io, time};
 use tokio_util::codec::BytesCodec;
 use tokio_util::udp::UdpFramed;
 
 use bytes::Bytes;
-use futures::{FutureExt, SinkExt, StreamExt};
+use futures::{FutureExt, SinkExt};
 use std::env;
 use std::error::Error;
 use std::net::SocketAddr;
diff --git a/tokio-test/Cargo.toml b/tokio-test/Cargo.toml
index a2902dc5..a1e60500 100644
--- a/tokio-test/Cargo.toml
+++ b/tokio-test/Cargo.toml
@@ -20,7 +20,7 @@ Testing utilities for Tokio- and futures-based code
 categories = ["asynchronous", "testing"]
 
 [dependencies]
-tokio = { version = "0.2.0", path = "../tokio", features = ["rt-core", "sync", "time", "test-util"] }
+tokio = { version = "0.2.0", path = "../tokio", features = ["rt-core", "stream", "sync", "time", "test-util"] }
 
 bytes = "0.5.0"
 futures-core = "0.3.0"
diff --git a/tokio-test/src/task.rs b/tokio-test/src/task.rs
index 4790de54..71ebe7b4 100644
--- a/tokio-test/src/task.rs
+++ b/tokio-test/src/task.rs
@@ -2,7 +2,6 @@
 
 #![allow(clippy::mutex_atomic)]
 
-use futures_core::Stream;
 use std::future::Future;
 use std::mem;
 use std::ops;
@@ -10,6 +9,8 @@ use std::pin::Pin;
 use std::sync::{Arc, Condvar, Mutex};
 use std::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};
 
+use tokio::stream::Stream;
+
 /// TOOD: dox
 pub fn spawn<T>(task: T) -> Spawn<T> {
     Spawn {
diff --git a/tokio-tls/tests/smoke.rs b/tokio-tls/tests/smoke.rs
index 64dda6ab..a575ba85 100644
--- a/tokio-tls/tests/smoke.rs
+++ b/tokio-tls/tests/smoke.rs
@@ -3,7 +3,6 @@
 use cfg_if::cfg_if;
 use env_logger;
 use futures::join;
-use futures::stream::StreamExt;
 use native_tls;
 use native_tls::{Identity, TlsAcceptor, TlsConnector};
 use std::io::Write;
@@ -12,6 +11,7 @@ use std::process::Command;
 use std::ptr;
 use tokio::io::{AsyncReadExt, AsyncWrite, AsyncWriteExt, Error, ErrorKind};
 use tokio::net::{TcpListener, TcpStream};
+use tokio::stream::StreamExt;
 use tokio_tls;
 
 macro_rules! t {
diff --git a/tokio-util/Cargo.toml b/tokio-util/Cargo.toml
index 7f40660a..c2b83565 100644
--- a/tokio-util/Cargo.toml
+++ b/tokio-util/Cargo.toml
@@ -26,7 +26,7 @@ default = []
 # Shorthand for enabling everything
 full = ["codec", "udp"]
 
-codec = []
+codec = ["tokio/stream"]
 udp = ["tokio/udp"]
 
 [dependencies]
diff --git a/tokio-util/src/codec/framed.rs b/tokio-util/src/codec/framed.rs
index 0c5ef9f6..a3715c24 100644
--- a/tokio-util/src/codec/framed.rs
+++ b/tokio-util/src/codec/framed.rs
@@ -3,10 +3,9 @@ use crate::codec::encoder::Encoder;
 use crate::codec::framed_read::{framed_read2, framed_read2_with_buffer, FramedRead2};
 use crate::codec::framed_write::{framed_write2, framed_write2_with_buffer, FramedWrite2};
 
-use tokio::io::{AsyncBufRead, AsyncRead, AsyncWrite};
+use tokio::{io::{AsyncBufRead, AsyncRead, AsyncWrite}, stream::Stream};
 
 use bytes::BytesMut;
-use futures_core::Stream;
 use futures_sink::Sink;
 use pin_project_lite::pin_project;
 use std::fmt;
diff --git a/tokio-util/src/codec/framed_read.rs b/tokio-util/src/codec/framed_read.rs
index bd1f625b..ca9464b5 100644
--- a/tokio-util/src/codec/framed_read.rs
+++ b/tokio-util/src/codec/framed_read.rs
@@ -1,10 +1,9 @@
 use crate::codec::framed::{Fuse, ProjectFuse};
 use crate::codec::Decoder;
 
-use tokio::io::AsyncRead;
+use tokio::{io::AsyncRead, stream::Stream};
 
 use bytes::BytesMut;
-use futures_core::Stream;
 use futures_sink::Sink;
 use log::trace;
 use pin_project_lite::pin_project;
diff --git a/tokio-util/src/codec/framed_write.rs b/tokio-util/src/codec/framed_write.rs
index 9aed7ea3..2ef91c7c 100644
--- a/tokio-util/src/codec/framed_write.rs
+++ b/tokio-util/src/codec/framed_write.rs
@@ -2,10 +2,10 @@ use crate::codec::decoder::Decoder;
 use crate::codec::encoder::Encoder;
 use crate::codec::framed::{Fuse, ProjectFuse};
 
-use tokio::io::{AsyncBufRead, AsyncRead, AsyncWrite};
+use tokio::{io::{AsyncBufRead, AsyncRead, AsyncWrite}, stream::Stream};
 
 use bytes::BytesMut;
-use futures_core::{ready, Stream};
+use futures_core::ready;
 use futures_sink::Sink;
 use log::trace;
 use pin_project_lite::pin_project;
diff --git a/tokio-util/src/codec/mod.rs b/tokio-util/src/codec/mod.rs
index b162dd3a..4b1b86fb 100644
--- a/tokio-util/src/codec/mod.rs
+++ b/tokio-util/src/codec/mod.rs
@@ -6,8 +6,8 @@
 //!
 //! [`AsyncRead`]: https://docs.rs/tokio/*/tokio/io/trait.AsyncRead.html
 //! [`AsyncWrite`]: https://docs.rs/tokio/*/tokio/io/trait.AsyncWrite.html
+//! [`Stream`]: https://docs.rs/tokio/*/tokio/stream/trait.Stream.html
 //! [`Sink`]: https://docs.rs/futures-sink/*/futures_sink/trait.Sink.html
-//! [`Stream`]: https://docs.rs/futures-core/*/futures_core/stream/trait.Stream.html
 
 mod bytes_codec;
 pub use self::bytes_codec::BytesCodec;
diff --git a/tokio-util/src/udp/frame.rs b/tokio-util/src/udp/frame.rs
index a6c6f220..18ee106e 100644
--- a/tokio-util/src/udp/frame.rs
+++ b/tokio-util/src/udp/frame.rs
@@ -1,9 +1,9 @@
 use crate::codec::{Decoder, Encoder};
 
-use tokio::net::UdpSocket;
+use tokio::{net::UdpSocket, stream::Stream};
 
 use bytes::{BufMut, BytesMut};
-use futures_core::{ready, Stream};
+use futures_core::ready;
 use futures_sink::Sink;
 use std::io;
 use std::net::{Ipv4Addr, SocketAddr, SocketAddrV4};
diff --git a/tokio-util/tests/framed.rs b/tokio-util/tests/framed.rs
index b98df736..cb82f8df 100644
--- a/tokio-util/tests/framed.rs
+++ b/tokio-util/tests/framed.rs
@@ -1,11 +1,10 @@
 #![warn(rust_2018_idioms)]
 
-use tokio::prelude::*;
+use tokio::{prelude::*, stream::StreamExt};
 use tokio_test::assert_ok;
 use tokio_util::codec::{Decoder, Encoder, Framed, FramedParts};
 
 use bytes::{Buf, BufMut, BytesMut};
-use futures::StreamExt;
 use std::io::{self, Read};
 use std::pin::Pin;
 use std::task::{Context, Poll};
diff --git a/tokio-util/tests/udp.rs b/tokio-util/tests/udp.rs
index af8002bd..51c65c63 100644
--- a/tokio-util/tests/udp.rs
+++ b/tokio-util/tests/udp.rs
@@ -1,4 +1,4 @@
-use tokio::net::UdpSocket;
+use tokio::{net::UdpSocket, stream::StreamExt};
 use tokio_util::codec::{Decoder, Encoder};
 use tokio_util::udp::UdpFramed;
 
@@ -6,7 +6,6 @@ use bytes::{BufMut, BytesMut};
 use futures::future::try_join;
 use futures::future::FutureExt;
 use futures::sink::SinkExt;
-use futures::stream::StreamExt;
 use std::io;
 
 #[tokio::test]
diff --git a/tokio/src/fs/read_dir.rs b/tokio/src/fs/read_dir.rs
index d252eabb..06eed384 100644
--- a/tokio/src/fs/read_dir.rs
+++ b/tokio/src/fs/read_dir.rs
@@ -36,7 +36,7 @@ pub async fn read_dir(path: impl AsRef<Path>) -> io::Result<ReadDir> {
 ///
 /// [`read_dir`]: read_dir
 /// [`DirEntry`]: DirEntry
-/// [`Stream`]: futures_core::Stream
+/// [`Stream`]: crate::stream::Stream
 /// [`Err`]: std::result::Result::Err
 #[derive(Debug)]
 #[must_use = "streams do nothing unless polled"]
@@ -85,7 +85,7 @@ impl ReadDir {
 }
 
 #[cfg(feature = "stream")]
-impl futures_core::Stream for ReadDir {
+impl crate::stream::Stream for ReadDir {
     type Item = io::Result<DirEntry>;
 
     fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
diff --git a/tokio/src/io/util/async_buf_read_ext.rs b/tokio/src/io/util/async_buf_read_ext.rs
index 26ce28ca..b2930652 100644
--- a/tokio/src/io/util/async_buf_read_ext.rs
+++ b/tokio/src/io/util/async_buf_read_ext.rs
@@ -226,8 +226,8 @@ cfg_io_util! {
         ///
         /// ```
         /// use tokio::io::AsyncBufReadExt;
+        /// use tokio::stream::StreamExt;
         ///
-        /// use futures::{StreamExt};
         /// use std::io::Cursor;
         ///
         /// #[tokio::main]
diff --git a/tokio/src/io/util/lines.rs b/tokio/src/io/util/lines.rs
index 0f1d946a..f0e75de4 100644
--- a/tokio/src/io/util/lines.rs
+++ b/tokio/src/io/util/lines.rs
@@ -91,7 +91,7 @@ where
 }
 
 #[cfg(feature = "stream")]
-impl<R: AsyncBufRead> futures_core::Stream for Lines<R> {
+impl<R: AsyncBufRead> crate::stream::Stream for Lines<R> {
     type Item = io::Result<String>;
 
     fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
diff --git a/tokio/src/io/util/split.rs b/tokio/src/io/util/split.rs
index a2e168de..f1ed2fd8 100644
--- a/tokio/src/io/util/split.rs
+++ b/tokio/src/io/util/split.rs
@@ -89,7 +89,7 @@ where
 }
 
 #[cfg(feature = "stream")]
-impl<R: AsyncBufRead> futures_core::Stream for Split<R> {
+impl<R: AsyncBufRead> crate::stream::Stream for Split<R> {
     type Item = io::Result<Vec<u8>>;
 
     fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
diff --git a/tokio/src/lib.rs b/tokio/src/lib.rs
index 66fd7975..831a5241 100644
--- a/tokio/src/lib.rs
+++ b/tokio/src/lib.rs
@@ -241,6 +241,10 @@ cfg_signal! {
     pub mod signal;
 }
 
+cfg_stream! {
+    pub mod stream;
+}
+
 cfg_sync! {
     pub mod sync;
 }
diff --git a/tokio/src/net/tcp/incoming.rs b/tokio/src/net/tcp/incoming.rs
index 3033aefa..0abe047d 100644
--- a/tokio/src/net/tcp/incoming.rs
+++ b/tokio/src/net/tcp/incoming.rs
@@ -28,7 +28,7 @@ impl Incoming<'_> {
 }
 
 #[cfg(feature = "stream")]
-impl futures_core::Stream for Incoming<'_> {
+impl crate::stream::Stream for Incoming<'_> {
     type Item = io::Result<TcpStream>;
 
     fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
diff --git a/tokio/src/net/tcp/listener.rs b/tokio/src/net/tcp/listener.rs
index 2cddf24b..3e0e0f7d 100644
--- a/tokio/src/net/tcp/listener.rs
+++ b/tokio/src/net/tcp/listener.rs
@@ -250,9 +250,7 @@ impl TcpListener {
     /// # Examples
     ///
     /// ```no_run
-    /// use tokio::net::TcpListener;
-    ///
-    /// use futures::StreamExt;
+    /// use tokio::{net::TcpListener, stream::StreamExt};
     ///
     /// #[tokio::main]
     /// async fn main() {
diff --git a/tokio/src/net/unix/incoming.rs b/tokio/src/net/unix/incoming.rs
index dbe964a8..bede96dd 100644
--- a/tokio/src/net/unix/incoming.rs
+++ b/tokio/src/net/unix/incoming.rs
@@ -27,7 +27,7 @@ impl Incoming<'_> {
 }
 
 #[cfg(feature = "stream")]
-impl futures_core::Stream for Incoming<'_> {
+impl crate::stream::Stream for Incoming<'_> {
     type Item = io::Result<UnixStream>;
 
     fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
diff --git a/tokio/src/net/unix/listener.rs b/tokio/src/net/unix/listener.rs
index 311bae2c..4b93bf82 100644
--- a/tokio/src/net/unix/listener.rs
+++ b/tokio/src/net/unix/listener.rs
@@ -104,8 +104,7 @@ impl UnixListener {
     ///
     /// ```no_run
     /// use tokio::net::UnixListener;
-    ///
-    /// use futures::StreamExt;
+    /// use tokio::stream::StreamExt;
     ///
     /// #[tokio::main]
     /// async fn main() {
diff --git a/tokio/src/signal/unix.rs b/tokio/src/signal/unix.rs
index cd326424..bfa2e309 100644
--- a/tokio/src/signal/unix.rs
+++ b/tokio/src/signal/unix.rs
@@ -482,7 +482,7 @@ impl Signal {
 }
 
 cfg_stream! {
-    impl futures_core::Stream for Signal {
+    impl crate::stream::Stream for Signal {
         type Item = ();
 
         fn poll_next(mut self: std::pin::Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<()>> {
diff --git a/tokio/src/signal/windows.rs b/tokio/src/signal/windows.rs
index de35643e..def1a1d7 100644
--- a/tokio/src/signal/windows.rs
+++ b/tokio/src/signal/windows.rs
@@ -209,7 +209,7 @@ impl CtrlBreak {
 }
 
 cfg_stream! {
-    impl futures_core::Stream for CtrlBreak {
+    impl crate::stream::Stream for CtrlBreak {
         type Item = ();
 
         fn poll_next(mut self: std::pin::Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<()>> {
@@ -246,9 +246,9 @@ pub fn ctrl_break() -> io::Result<CtrlBreak> {
 mod tests {
     use super::*;
     use crate::runtime::Runtime;
-    use tokio_test::{assert_ok, assert_pending, assert_ready_ok, task};
+    use crate::stream::StreamExt;
 
-    use futures::stream::StreamExt;
+    use tokio_test::{assert_ok, assert_pending, assert_ready_ok, task};
 
     #[test]
     fn ctrl_c() {
diff --git a/tokio/src/stream/iter.rs b/tokio/src/stream/iter.rs
new file mode 100644
index 00000000..dc06495e
--- /dev/null
+++ b/tokio/src/stream/iter.rs
@@ -0,0 +1,52 @@
+use crate::stream::Stream;
+
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+/// Stream for the [`iter`] function.
+#[derive(Debug)]
+#[must_use = "streams do nothing unless polled"]
+pub struct Iter<I> {
+    iter: I,
+}
+
+impl<I> Unpin for Iter<I> {}
+
+/// Converts an `Iterator` into a `Stream` which is always ready
+/// to yield the next value.
+///
+/// Iterators in Rust don't express the ability to block, so this adapter
+/// simply always calls `iter.next()` and returns that.
+///
+/// ```
+/// # async fn dox() {
+/// use tokio::stream::{self, StreamExt};
+///
+/// let mut stream = stream::iter(vec![17, 19]);
+///
+/// assert_eq!(stream.next().await, Some(17));
+/// assert_eq!(stream.next().await, Some(19));
+/// assert_eq!(stream.next().await, None);
+/// # }
+/// ```
+pub fn iter<I>(i: I) -> Iter<I::IntoIter>
+    where I: IntoIterator,
+{
+    Iter {
+        iter: i.into_iter(),
+    }
+}
+
+impl<I> Stream for Iter<I>
+    where I: Iterator,
+{
+    type Item = I::Item;
+
+    fn poll_next(mut self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Option<I::Item>> {
+        Poll::Ready(self.iter.next())
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
+}
diff --git a/tokio/src/stream/map.rs b/tokio/src/stream/map.rs
new file mode 100644
index 00000000..a89769de
--- /dev/null
+++ b/tokio/src/stream/map.rs
@@ -0,0 +1,57 @@
+use crate::stream::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, T, F> Map<St, F>
+    where St: Stream,
+          F: FnMut(St::Item) -> T,
+{
+    pub(super) fn new(stream: St, f: F) -> Map<St, F> {
+        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()
+    }
+}
diff --git a/tokio/src/stream/mod.rs b/tokio/src/stream/mod.rs
new file mode 100644
index 00000000..04e6dc06
--- /dev/null
+++ b/tokio/src/stream/mod.rs
@@ -0,0 +1,93 @@
+//! Stream utilities for Tokio.
+//!
+//! `Stream`s are an asynchoronous version of standard library's Iterator.
+//!
+//! This module provides helpers to work with them.
+
+mod iter;
+pub use iter::{iter, Iter};
+
+mod map;
+use map::Map;
+
+mod next;
+use next::Next;
+
+pub use futures_core::Stream;
+
+/// An extension trait for `Stream`s that provides a variety of convenient
+/// combinator functions.
+pub trait StreamExt: Stream {
+    /// Creates a future that resolves to the next item in the stream.
+    ///
+    /// Equivalent to:
+    ///
+    /// ```ignore
+    /// async fn next(&mut self) -> Option<Self::Item>;
+    /// ```
+    ///
+    /// Note that because `next` doesn't take ownership over the stream,
+    /// the [`Stream`] type must be [`Unpin`]. If you want to use `next` with a
+    /// [`!Unpin`](Unpin) stream, you'll first have to pin the stream. This can
+    /// be done by boxing the stream using [`Box::pin`] or
+    /// pinning it to the stack using the `pin_mut!` macro from the `pin_utils`
+    /// crate.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::stream::{self, StreamExt};
+    ///
+    /// let mut stream = stream::iter(1..=3);
+    ///
+    /// assert_eq!(stream.next().await, Some(1));
+    /// assert_eq!(stream.next().await, Some(2));
+    /// assert_eq!(stream.next().await, Some(3));
+    /// assert_eq!(stream.next().await, None);
+    /// # }
+    /// ```
+    fn next(&mut self) -> Next<'_, Self>
+    where
+        Self: Unpin,
+    {
+        Next::new(self)
+    }
+
+    /// Maps this stream's items to a different type, returning a new stream of
+    /// the resulting type.
+    ///
+    /// The provided closure is executed over all elements of this stream as
+    /// they are made available. It is executed inline with calls to
+    /// [`poll_next`](Stream::poll_next).
+    ///
+    /// Note that this function consumes the stream passed into it and returns a
+    /// wrapped version of it, similar to the existing `map` methods in the
+    /// standard library.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::stream::{self, StreamExt};
+    ///
+    /// let stream = stream::iter(1..=3);
+    /// let mut stream = stream.map(|x| x + 3);
+    ///
+    /// assert_eq!(stream.next().await, Some(4));
+    /// assert_eq!(stream.next().await, Some(5));
+    /// assert_eq!(stream.next().await, Some(6));
+    /// # }
+    /// ```
+    fn map<T, F>(