     1
     2
     3
     4
     5
     6
     7
     8
     9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    20
    21
    22
    23
    24
    25
    26
    27
    28
    29
    30
    31
    32
    33
    34
    35
    36
    37
    38

    use std::future::Future;
    use std::pin::Pin;
    use std::task::{Context, Poll};

    cfg_rt! {
        /// Yields execution back to the Tokio runtime.
        ///
        /// A task yields by awaiting on `yield_now()`, and may resume when that
        /// future completes (with no output.) The current task will be re-added as
        /// a pending task at the _back_ of the pending queue. Any other pending
        /// tasks will be scheduled. No other waking is required for the task to
        /// continue.
        ///
        /// See also the usage example in the [task module](index.html#yield_now).
        #[must_use = "yield_now does nothing unless polled/`await`-ed"]
        pub async fn yield_now() {
            /// Yield implementation
            struct YieldNow {
                yielded: bool,
            }

            impl Future for YieldNow {
                type Output = ();

                fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
                    if self.yielded {
                        return Poll::Ready(());
                    }

                    self.yielded = true;
                    cx.waker().wake_by_ref();
                    Poll::Pending
                }
            }

            YieldNow { yielded: false }.await
        }
    }