Merge #178

178: adds stream::enumerate combinator r=stjepang a=montekki

enumerate might be handy.
---
Stdlib: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.enumerate
Ref: #129 

Co-authored-by: Fedor Sakharov <fedor.sakharov@gmail.com>

src/stream/stream/enumerate.rs

@@ -0,0 +1,40 @@
+use crate::task::{Context, Poll};
+use std::pin::Pin;
+
+use crate::stream::Stream;
+
+#[doc(hidden)]
+#[allow(missing_debug_implementations)]
+pub struct Enumerate<S> {
+    stream: S,
+    i: usize,
+}
+
+impl<S> Enumerate<S> {
+    pin_utils::unsafe_pinned!(stream: S);
+    pin_utils::unsafe_unpinned!(i: usize);
+
+    pub(super) fn new(stream: S) -> Self {
+        Enumerate { stream, i: 0 }
+    }
+}
+
+impl<S> futures_core::stream::Stream for Enumerate<S>
+where
+    S: Stream,
+{
+    type Item = (usize, S::Item);
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        let next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+
+        match next {
+            Some(v) => {
+                let ret = (self.i, v);
+                *self.as_mut().i() += 1;
+                Poll::Ready(Some(ret))
+            }
+            None => Poll::Ready(None),
+        }
+    }
+}

src/stream/stream/mod.rs

@@ -23,6 +23,7 @@
 
 mod all;
 mod any;
+mod enumerate;
 mod filter_map;
 mod find;
 mod find_map;
@@ -39,6 +40,7 @@ pub use zip::Zip;
 
 use all::AllFuture;
 use any::AnyFuture;
+use enumerate::Enumerate;
 use filter_map::FilterMap;
 use find::FindFuture;
 use find_map::FindMapFuture;
@@ -197,6 +199,36 @@ pub trait Stream {
         }
     }
 
+    /// Creates a stream that gives the current element's count as well as the next value.
+    ///
+    /// # Overflow behaviour.
+    ///
+    /// This combinator does no guarding against overflows.
+    ///
+    /// # Examples
+    /// ```
+    /// # fn main() { async_std::task::block_on(async {
+    /// #
+    /// use async_std::prelude::*;
+    /// use std::collections::VecDeque;
+    ///
+    /// let s: VecDeque<_> = vec!['a', 'b', 'c'].into_iter().collect();
+    /// let mut s = s.enumerate();
+    ///
+    /// assert_eq!(s.next().await, Some((0, 'a')));
+    /// assert_eq!(s.next().await, Some((1, 'b')));
+    /// assert_eq!(s.next().await, Some((2, 'c')));
+    /// assert_eq!(s.next().await, None);
+    ///
+    /// #
+    /// # }) }
+    fn enumerate(self) -> Enumerate<Self>
+    where
+        Self: Sized,
+    {
+        Enumerate::new(self)
+    }
+
     /// Both filters and maps a stream.
     ///
     /// # Examples