Merge #179

179: adds stream::find combinator r=yoshuawuyts a=montekki

A find combinator
---
Stdlib: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.find
Ref: #129 

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

src/stream/stream/find.rs

@@ -0,0 +1,49 @@
+use crate::task::{Context, Poll};
+use std::marker::PhantomData;
+use std::pin::Pin;
+
+#[doc(hidden)]
+#[allow(missing_debug_implementations)]
+pub struct FindFuture<'a, S, P, T> {
+    stream: &'a mut S,
+    p: P,
+    __t: PhantomData<T>,
+}
+
+impl<'a, S, P, T> FindFuture<'a, S, P, T> {
+    pin_utils::unsafe_pinned!(stream: &'a mut S);
+    pin_utils::unsafe_unpinned!(p: P);
+
+    pub(super) fn new(stream: &'a mut S, p: P) -> Self {
+        FindFuture {
+            stream,
+            p,
+            __t: PhantomData,
+        }
+    }
+}
+
+impl<'a, S, P> futures_core::future::Future for FindFuture<'a, S, P, S::Item>
+where
+    S: futures_core::stream::Stream + Unpin + Sized,
+    P: FnMut(&S::Item) -> bool,
+{
+    type Output = Option<S::Item>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        use futures_core::stream::Stream;
+
+        let item = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+
+        match item {
+            Some(v) => match (self.as_mut().p())(&v) {
+                true => Poll::Ready(Some(v)),
+                false => {
+                    cx.waker().wake_by_ref();
+                    Poll::Pending
+                }
+            },
+            None => Poll::Ready(None),
+        }
+    }
+}

src/stream/stream/mod.rs

@@ -24,6 +24,7 @@
 mod all;
 mod any;
 mod filter_map;
+mod find;
 mod find_map;
 mod min_by;
 mod next;
@@ -35,6 +36,7 @@ pub use take::Take;
 use all::AllFuture;
 use any::AnyFuture;
 use filter_map::FilterMap;
+use find::FindFuture;
 use find_map::FindMapFuture;
 use min_by::MinByFuture;
 use next::NextFuture;
@@ -321,6 +323,50 @@ pub trait Stream {
         }
     }
 
+    /// Searches for an element in a stream that satisfies a predicate.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// # fn main() { async_std::task::block_on(async {
+    /// #
+    /// use async_std::prelude::*;
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut s: VecDeque<usize> = vec![1, 2, 3].into_iter().collect();
+    /// let res = s.find(|x| *x == 2).await;
+    /// assert_eq!(res, Some(2));
+    /// #
+    /// # }) }
+    /// ```
+    ///
+    /// Resuming after a first find:
+    ///
+    /// ```
+    /// # fn main() { async_std::task::block_on(async {
+    /// #
+    /// use async_std::prelude::*;
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut s: VecDeque<usize> = vec![1, 2, 3].into_iter().collect();
+    /// let res = s.find(|x| *x == 2).await;
+    /// assert_eq!(res, Some(2));
+    ///
+    /// let next = s.next().await;
+    /// assert_eq!(next, Some(3));
+    /// #
+    /// # }) }
+    /// ```
+    fn find<P>(&mut self, p: P) -> ret!('_, FindFuture, Option<Self::Item>, P, Self::Item)
+    where
+        Self: Sized,
+        P: FnMut(&Self::Item) -> bool,
+    {
+        FindFuture::new(self, p)
+    }
+
     /// Applies function to the elements of stream and returns the first non-none result.
     ///
     /// ```