rust: add async yield_now

It gives up execution of the current task but keeps it runnable so that
it will run again as soon as the executor is available again.

Signed-off-by: Wedson Almeida Filho <[email protected]>

Author: wedsonaf

rust/kernel/kasync.rs

@@ -2,5 +2,48 @@
 
 //! Kernel async functionality.
 
+use core::{
+    future::Future,
+    pin::Pin,
+    task::{Context, Poll},
+};
+
 #[cfg(CONFIG_NET)]
 pub mod net;
+
+/// Yields execution of the current task so that other tasks may execute.
+///
+/// The task continues to be in a "runnable" state though, so it will eventually run again.
+///
+/// # Examples
+///
+/// ```
+/// use kernel::kasync::yield_now;
+///
+/// async fn example() {
+///     pr_info!("Before yield\n");
+///     yield_now().await;
+///     pr_info!("After yield\n");
+/// }
+/// ```
+pub fn yield_now() -> impl Future<Output = ()> {
+    struct Yield {
+        first_poll: bool,
+    }
+
+    impl Future for Yield {
+        type Output = ();
+
+        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
+            if !self.first_poll {
+                Poll::Ready(())
+            } else {
+                self.first_poll = false;
+                cx.waker().wake_by_ref();
+                Poll::Pending
+            }
+        }
+    }
+
+    Yield { first_poll: true }
+}