adds stream::filter combinator

src/stream/stream/filter.rs

@@ -0,0 +1,49 @@
+use std::marker::PhantomData;
+use std::pin::Pin;
+
+use crate::stream::Stream;
+use crate::task::{Context, Poll};
+
+#[doc(hidden)]
+#[allow(missing_debug_implementations)]
+pub struct Filter<S, P, T> {
+    stream: S,
+    predicate: P,
+    __t: PhantomData<T>,
+}
+
+impl<S, P, T> Filter<S, P, T> {
+    pin_utils::unsafe_pinned!(stream: S);
+    pin_utils::unsafe_unpinned!(predicate: P);
+
+    pub(super) fn new(stream: S, predicate: P) -> Self {
+        Filter {
+            stream,
+            predicate,
+            __t: PhantomData,
+        }
+    }
+}
+
+impl<S, P> futures_core::stream::Stream for Filter<S, P, S::Item>
+where
+    S: Stream,
+    P: FnMut(&S::Item) -> bool,
+{
+    type Item = 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) => match (self.as_mut().predicate())(&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 enumerate;
+mod filter;
 mod filter_map;
 mod find;
 mod find_map;
@@ -44,6 +45,7 @@ pub use zip::Zip;
 use all::AllFuture;
 use any::AnyFuture;
 use enumerate::Enumerate;
+use filter::Filter;
 use filter_map::FilterMap;
 use find::FindFuture;
 use find_map::FindMapFuture;
@@ -282,6 +284,34 @@ pub trait Stream {
             done: false,
         }
     }
+    /// Creates a stream that uses a predicate to determine if an element
+    /// should be yeilded.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    ///```
+    /// # fn main() { async_std::task::block_on(async {
+    /// #
+    /// use std::collections::VecDeque;
+    /// use async_std::stream::Stream;
+    ///
+    /// let s: VecDeque<usize> = vec![1, 2, 3, 4].into_iter().collect();
+    /// let mut s = s.filter(|i| i % 2 == 0);
+    ///
+    /// assert_eq!(s.next().await, Some(2));
+    /// assert_eq!(s.next().await, Some(4));
+    /// assert_eq!(s.next().await, None);
+    /// #
+    /// # }) }
+    fn filter<P>(self, predicate: P) -> Filter<Self, P, Self::Item>
+    where
+        Self: Sized,
+        P: FnMut(&Self::Item) -> bool,
+    {
+        Filter::new(self, predicate)
+    }
 
     /// Both filters and maps a stream.
     ///