feat: Add future::delay

Author: k-nasa

src/future/delay.rs

@@ -0,0 +1,61 @@
+use std::pin::Pin;
+use std::time::Duration;
+
+use futures_timer::Delay;
+
+use crate::future::Future;
+use crate::task::{Context, Poll};
+
+/// Creates a future that is delayed before it starts yielding items.
+///
+/// # Examples
+///
+/// ```
+/// # async_std::task::block_on(async {
+/// use async_std::future;
+/// use std::time::Duration;
+
+/// let a = future::delay(future::ready(1) ,Duration::from_millis(2000));
+/// dbg!(a.await);
+/// # })
+/// ```
+#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
+#[cfg(any(feature = "unstable", feature = "docs"))]
+pub fn delay<F>(f: F, dur: Duration) -> DelayFuture<F>
+where
+    F: Future,
+{
+    DelayFuture::new(f, dur)
+}
+
+#[doc(hidden)]
+#[derive(Debug)]
+pub struct DelayFuture<F> {
+    future: F,
+    delay: Delay,
+}
+
+impl<F> DelayFuture<F> {
+    pin_utils::unsafe_pinned!(future: F);
+    pin_utils::unsafe_pinned!(delay: Delay);
+
+    pub fn new(future: F, dur: Duration) -> DelayFuture<F> {
+        let delay = Delay::new(dur);
+
+        DelayFuture { future, delay }
+    }
+}
+
+impl<F: Future> Future for DelayFuture<F> {
+    type Output = F::Output;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        match self.as_mut().delay().poll(cx) {
+            Poll::Pending => Poll::Pending,
+            Poll::Ready(_) => match self.future().poll(cx) {
+                Poll::Ready(v) => Poll::Ready(v),
+                Poll::Pending => Poll::Pending,
+            },
+        }
+    }
+}

src/future/mod.rs

@@ -65,7 +65,9 @@ mod timeout;
 cfg_if! {
     if #[cfg(any(feature = "unstable", feature = "docs"))] {
         mod into_future;
+        mod delay;
 
         pub use into_future::IntoFuture;
+        pub use delay::delay;
     }
 }