stream: add StreamExt::all (#2035)

tokio/src/stream/all.rs

@@ -0,0 +1,45 @@
+use crate::stream::Stream;
+
+use core::future::Future;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+/// Future for the [`all`](super::StreamExt::all) method.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct AllFuture<'a, St: ?Sized, F> {
+    stream: &'a mut St,
+    f: F,
+}
+
+impl<'a, St: ?Sized, F> AllFuture<'a, St, F> {
+    pub(super) fn new(stream: &'a mut St, f: F) -> Self {
+        Self { stream, f }
+    }
+}
+
+impl<St: ?Sized + Unpin, F> Unpin for AllFuture<'_, St, F> {}
+
+impl<St, F> Future for AllFuture<'_, St, F>
+where
+    St: ?Sized + Stream + Unpin,
+    F: FnMut(St::Item) -> bool,
+{
+    type Output = bool;
+
+    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(cx));
+
+        match next {
+            Some(v) => {
+                if !(&mut self.f)(v) {
+                    Poll::Ready(false)
+                } else {
+                    cx.waker().wake_by_ref();
+                    Poll::Pending
+                }
+            }
+            None => Poll::Ready(true),
+        }
+    }
+}

tokio/src/stream/mod.rs

@@ -4,6 +4,9 @@
 //!
 //! This module provides helpers to work with them.
 
+mod all;
+use all::AllFuture;
+
 mod filter;
 use filter::Filter;
 
@@ -265,6 +268,59 @@ pub trait StreamExt: Stream {
     {
         TakeWhile::new(self, f)
     }
+
+    /// Tests if every element of the stream matches a predicate.
+    /// `all()` takes a closure that returns `true` or `false`. It applies
+    /// this closure to each element of the stream, and if they all return
+    /// `true`, then so does `all`. If any of them return `false`, it
+    /// returns `false`. An empty stream returns `true`.
+    ///
+    /// `all()` is short-circuiting; in other words, it will stop processing
+    /// as soon as it finds a `false`, given that no matter what else happens,
+    /// the result will also be `false`.
+    ///
+    /// An empty stream returns `true`.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::stream::{self, StreamExt};
+    ///
+    /// let a = [1, 2, 3];
+    ///
+    /// assert!(stream::iter(&a).all(|&x| x > 0).await);
+    ///
+    /// assert!(!stream::iter(&a).all(|&x| x > 2).await);
+    /// # }
+    /// ```
+    ///
+    /// Stopping at the first `false`:
+    ///
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::stream::{self, StreamExt};
+    ///
+    /// let a = [1, 2, 3];
+    ///
+    /// let mut iter = stream::iter(&a);
+    ///
+    /// assert!(!iter.all(|&x| x != 2).await);
+    ///
+    /// // we can still use `iter`, as there are more elements.
+    /// assert_eq!(iter.next().await, Some(&3));
+    /// # }
+    /// ```
+    fn all<F>(&mut self, f: F) -> AllFuture<'_, Self, F>
+    where
+        Self: Unpin,
+    {
+        AllFuture::new(self, f)
+    }
 }
 
 impl<St: ?Sized> StreamExt for St where St: Stream {}