add stream::ExactSizeStream as unstable (#330)

Signed-off-by: Yoshua Wuyts <yoshuawuyts@gmail.com>

src/prelude.rs

@@ -11,6 +11,8 @@
 //! use async_std::prelude::*;
 //! ```
 
+use cfg_if::cfg_if;
+
 #[doc(no_inline)]
 pub use crate::future::Future;
 #[doc(no_inline)]
@@ -36,3 +38,13 @@ pub use crate::io::seek::SeekExt as _;
 pub use crate::io::write::WriteExt as _;
 #[doc(hidden)]
 pub use crate::stream::stream::StreamExt as _;
+
+cfg_if! {
+    if #[cfg(any(feature = "unstable", feature = "docs"))] {
+        #[doc(no_inline)]
+        pub use crate::stream::DoubleEndedStream;
+
+        #[doc(no_inline)]
+        pub use crate::stream::ExactSizeStream;
+    }
+}

src/stream/exact_size_stream.rs

@@ -0,0 +1,120 @@
+pub use crate::stream::Stream;
+
+/// A stream that knows its exact length.
+///
+/// Many [`Stream`]s don't know how many times they will iterate, but some do.
+/// If a stream knows how many times it can iterate, providing access to
+/// that information can be useful. For example, if you want to iterate
+/// backwards, a good start is to know where the end is.
+///
+/// When implementing an `ExactSizeStream`, you must also implement
+/// [`Stream`]. When doing so, the implementation of [`size_hint`] *must*
+/// return the exact size of the stream.
+///
+/// [`Stream`]: trait.Stream.html
+/// [`size_hint`]: trait.Stream.html#method.size_hint
+///
+/// The [`len`] method has a default implementation, so you usually shouldn't
+/// implement it. However, you may be able to provide a more performant
+/// implementation than the default, so overriding it in this case makes sense.
+///
+/// [`len`]: #method.len
+///
+/// # Examples
+///
+/// Basic usage:
+///
+/// ```
+/// // a finite range knows exactly how many times it will iterate
+/// let five = 0..5;
+///
+/// assert_eq!(5, five.len());
+/// ```
+///
+/// In the [module level docs][moddocs], we implemented an [`Stream`],
+/// `Counter`. Let's implement `ExactSizeStream` for it as well:
+///
+/// [moddocs]: index.html
+///
+/// ```
+/// # use std::task::{Context, Poll};
+/// # use std::pin::Pin;
+/// # use async_std::prelude::*;
+/// # struct Counter {
+/// #     count: usize,
+/// # }
+/// # impl Counter {
+/// #     fn new() -> Counter {
+/// #         Counter { count: 0 }
+/// #     }
+/// # }
+/// # impl Stream for Counter {
+/// #     type Item = usize;
+/// #     fn poll_next(mut self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+/// #         self.count += 1;
+/// #         if self.count < 6 {
+/// #             Poll::Ready(Some(self.count))
+/// #         } else {
+/// #             Poll::Ready(None)
+/// #         }
+/// #     }
+/// # }
+/// # fn main() { async_std::task::block_on(async {
+/// #
+/// impl ExactSizeStream for Counter {
+///     // We can easily calculate the remaining number of iterations.
+///     fn len(&self) -> usize {
+///         5 - self.count
+///     }
+/// }
+///
+/// // And now we can use it!
+///
+/// let counter = Counter::new();
+///
+/// assert_eq!(5, counter.len());
+/// # });
+/// # }
+/// ```
+#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
+#[cfg(any(feature = "unstable", feature = "docs"))]
+pub trait ExactSizeStream: Stream {
+    /// Returns the exact number of times the stream will iterate.
+    ///
+    /// This method has a default implementation, so you usually should not
+    /// implement it directly. However, if you can provide a more efficient
+    /// implementation, you can do so. See the [trait-level] docs for an
+    /// example.
+    ///
+    /// This function has the same safety guarantees as the [`size_hint`]
+    /// function.
+    ///
+    /// [trait-level]: trait.ExactSizeStream.html
+    /// [`size_hint`]: trait.Stream.html#method.size_hint
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// // a finite range knows exactly how many times it will iterate
+    /// let five = 0..5;
+    ///
+    /// assert_eq!(5, five.len());
+    /// ```
+    fn len(&self) -> usize {
+        let (lower, upper) = self.size_hint();
+        // Note: This assertion is overly defensive, but it checks the invariant
+        // guaranteed by the trait. If this trait were rust-internal,
+        // we could use debug_assert!; assert_eq! will check all Rust user
+        // implementations too.
+        assert_eq!(upper, Some(lower));
+        lower
+    }
+}
+
+impl<I: ExactSizeStream + ?Sized + Unpin> ExactSizeStream for &mut I {
+    fn len(&self) -> usize {
+        (**self).len()
+    }
+}

src/stream/mod.rs

@@ -39,12 +39,14 @@ mod repeat;
 cfg_if! {
     if #[cfg(any(feature = "unstable", feature = "docs"))] {
         mod double_ended_stream;
+        mod exact_size_stream;
         mod fused_stream;
         mod extend;
         mod from_stream;
         mod into_stream;
 
         pub use double_ended_stream::DoubleEndedStream;
+        pub use exact_size_stream::ExactSizeStream;
         pub use extend::Extend;
         pub use from_stream::FromStream;
         pub use into_stream::IntoStream;