wip: working on async diff

packages/core/src/yield_now.rs

@@ -0,0 +1,52 @@
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+// use crate::task::{Context, Poll};
+
+/// Cooperatively gives up a timeslice to the task scheduler.
+///
+/// Calling this function will move the currently executing future to the back
+/// of the execution queue, making room for other futures to execute. This is
+/// especially useful after running CPU-intensive operations inside a future.
+///
+/// See also [`task::spawn_blocking`].
+///
+/// [`task::spawn_blocking`]: fn.spawn_blocking.html
+///
+/// # Examples
+///
+/// Basic usage:
+///
+/// ```
+/// # async_std::task::block_on(async {
+/// #
+/// use async_std::task;
+///
+/// task::yield_now().await;
+/// #
+/// # })
+/// ```
+#[inline]
+pub async fn yield_now() {
+    YieldNow(false).await
+}
+
+struct YieldNow(bool);
+
+impl Future for YieldNow {
+    type Output = ();
+
+    // The futures executor is implemented as a FIFO queue, so all this future
+    // does is re-schedule the future back to the end of the queue, giving room
+    // for other futures to progress.
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        if !self.0 {
+            self.0 = true;
+            cx.waker().wake_by_ref();
+            Poll::Pending
+        } else {
+            Poll::Ready(())
+        }
+    }
+}