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.

1 files changed, 93 insertions, 0 deletions

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>(self, f: F) -> Map<Self, F>
+    where
+        F: FnMut(Self::Item) -> T,
+        Self: Sized,
+    {
+        Map::new(self, f)
+    }
+}
+
+impl<T: ?Sized> StreamExt for T where T: Stream {}