Add stream position

src/stream/stream/mod.rs

@@ -46,6 +46,7 @@ mod min_by_key;
 mod next;
 mod nth;
 mod partial_cmp;
+mod position;
 mod scan;
 mod skip;
 mod skip_while;
@@ -76,6 +77,7 @@ use min_by_key::MinByKeyFuture;
 use next::NextFuture;
 use nth::NthFuture;
 use partial_cmp::PartialCmpFuture;
+use position::PositionFuture;
 use try_fold::TryFoldFuture;
 use try_for_each::TryForEeachFuture;
 
@@ -1548,6 +1550,45 @@ extension_trait! {
             PartialCmpFuture::new(self, other)
         }
 
+        #[doc = r#"
+            Searches for an element in a Stream that satisfies a predicate, returning 
+            its index.
+
+            # Examples
+
+            ```
+            # fn main() { async_std::task::block_on(async {
+            #
+            use async_std::prelude::*;
+            use std::collections::VecDeque;
+
+            let s: VecDeque<usize> = vec![1, 2, 3].into_iter().collect();
+            let res = s.clone().position(|x| *x == 1).await;
+            assert_eq!(res, Some(0));
+
+            let res = s.clone().position(|x| *x == 2).await;
+            assert_eq!(res, Some(1));
+
+            let res = s.clone().position(|x| *x == 3).await;
+            assert_eq!(res, Some(2));
+
+            let res = s.clone().position(|x| *x == 4).await;
+            assert_eq!(res, None);
+            #
+            # }) }
+            ```
+        "#]
+        fn position<P>(
+           self,
+           predicate: P
+        ) -> impl Future<Output = Option<usize>>  [PositionFuture<Self, P>]
+        where
+            Self: Sized,
+            P: FnMut(&Self::Item) -> bool,
+        {
+            PositionFuture::new(self, predicate)
+        }
+
         #[doc = r#"
             Lexicographically compares the elements of this `Stream` with those
             of another using 'Ord'.

src/stream/stream/position.rs

@@ -0,0 +1,51 @@
+use std::pin::Pin;
+
+use pin_project_lite::pin_project;
+
+use crate::future::Future;
+use crate::stream::Stream;
+use crate::task::{Context, Poll};
+
+pin_project! {
+    #[doc(hidden)]
+    #[allow(missing_debug_implementations)]
+    pub struct PositionFuture<S, P> {
+        #[pin]
+        stream: S,
+        predicate: P,
+        index:usize,
+    }
+}
+
+impl<S, P> PositionFuture<S, P> {
+    pub(super) fn new(stream: S, predicate: P) -> Self {
+        PositionFuture {
+            stream,
+            predicate,
+            index: 0,
+        }
+    }
+}
+
+impl<S, P> Future for PositionFuture<S, P>
+where
+    S: Stream,
+    P: FnMut(&S::Item) -> bool,
+{
+    type Output = Option<usize>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let this = self.project();
+        let next = futures_core::ready!(this.stream.poll_next(cx));
+
+        match next {
+            Some(v) if (this.predicate)(&v) => Poll::Ready(Some(*this.index)),
+            Some(_) => {
+                cx.waker().wake_by_ref();
+                *this.index += 1;
+                Poll::Pending
+            }
+            None => Poll::Ready(None),
+        }
+    }
+}