Sketch out nth_back

src/stream/double_ended/mod.rs

@@ -1,9 +1,14 @@
+mod nth_back;
+
+use nth_back::NthBackFuture;
+
 extension_trait! {
     use crate::stream::Stream;
 
     use std::pin::Pin;
     use std::task::{Context, Poll};
 
+
     #[doc = r#"
     Something fancy
     "#]
@@ -17,6 +22,36 @@ extension_trait! {
         Something else
     "#]
     pub trait DoubleEndedStreamExt: crate::stream::DoubleEndedStream {
+
+        #[doc = r#"
+            Returns the nth element from the back of the stream.
+
+            # Examples
+
+            Basic usage:
+
+            ```
+            # fn main() { async_std::task::block_on(async {
+            #
+            use async_std::stream::double_ended::DoubleEndedStreamExt;
+            use async_std::stream;
+
+            let mut s = stream::from_iter(vec![1u8, 2, 3, 4, 5]);
+
+            let second = s.nth_back(1).await;
+            assert_eq!(second, Some(4));
+            #
+            # }) }
+            ```
+        "#]
+        fn nth_back(
+            &mut self,
+            n: usize,
+        ) -> impl Future<Output = Option<Self::Item>> + '_ [NthBackFuture<'_, Self>]
+        where
+            Self: Unpin + Sized,
+        {
+            NthBackFuture::new(self, n)
+        }
     }
 }
-

src/stream/double_ended/nth_back.rs

@@ -0,0 +1,39 @@
+use std::pin::Pin;
+use std::task::{Context, Poll};
+use std::future::Future;
+
+use crate::stream::DoubleEndedStream;
+
+pub struct NthBackFuture<'a, S> {
+    stream: &'a mut S,
+    n: usize,
+}
+
+impl<'a, S> NthBackFuture<'a, S> {
+    pub(crate) fn new(stream: &'a mut S, n: usize) -> Self {
+        NthBackFuture { stream, n }
+    }
+}
+
+impl<'a, S> Future for NthBackFuture<'a, S>
+where
+    S: DoubleEndedStream + Sized + Unpin,
+{
+    type Output = Option<S::Item>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let next = futures_core::ready!(Pin::new(&mut *self.stream).poll_next_back(cx));
+        match next {
+            Some(v) => match self.n {
+                0 => Poll::Ready(Some(v)),
+                _ => {
+                    self.n -= 1;
+                    cx.waker().wake_by_ref();
+                    Poll::Pending
+                }
+            },
+            None => Poll::Ready(None),
+        }
+    }
+}
+

src/stream/mod.rs

@@ -318,7 +318,7 @@ mod repeat;
 mod repeat_with;
 
 cfg_unstable! {
-    mod double_ended;
+    pub mod double_ended;
     mod double_ended_stream;
     mod exact_size_stream;
     mod extend;