# oneshot.rs - source
Source of the Rust file `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.4.0/src/sync/oneshot.rs`.
Keywords: rust, rustlang, rust-lang

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    #![cfg_attr(not(feature = "sync"), allow(dead_code, unreachable_pub))]

    //! A one-shot channel is used for sending a single message between
    //! asynchronous tasks. The [`channel`] function is used to create a
    //! [`Sender`] and [`Receiver`] handle pair that form the channel.
    //!
    //! The `Sender` handle is used by the producer to send the value.
    //! The `Receiver` handle is used by the consumer to receive the value.
    //!
    //! Each handle can be used on separate tasks.
    //!
    //! # Examples
    //!
    //! ```
    //! use tokio::sync::oneshot;
    //!
    //! #[tokio::main]
    //! async fn main() {
    //!     let (tx, rx) = oneshot::channel();
    //!
    //!     tokio::spawn(async move {
    //!         if let Err(_) = tx.send(3) {
    //!             println!("the receiver dropped");
    //!         }
    //!     });
    //!
    //!     match rx.await {
    //!         Ok(v) => println!("got = {:?}", v),
    //!         Err(_) => println!("the sender dropped"),
    //!     }
    //! }
    //! ```
    //!
    //! If the sender is dropped without sending, the receiver will fail with
    //! [`error::RecvError`]:
    //!
    //! ```
    //! use tokio::sync::oneshot;
    //!
    //! #[tokio::main]
    //! async fn main() {
    //!     let (tx, rx) = oneshot::channel::<u32>();
    //!
    //!     tokio::spawn(async move {
    //!         drop(tx);
    //!     });
    //!
    //!     match rx.await {
    //!         Ok(_) => panic!("This doesn't happen"),
    //!         Err(_) => println!("the sender dropped"),
    //!     }
    //! }
    //! ```

    use crate::loom::cell::UnsafeCell;
    use crate::loom::sync::atomic::AtomicUsize;
    use crate::loom::sync::Arc;

    use std::fmt;
    use std::future::Future;
    use std::mem::MaybeUninit;
    use std::pin::Pin;
    use std::sync::atomic::Ordering::{self, AcqRel, Acquire};
    use std::task::Poll::{Pending, Ready};
    use std::task::{Context, Poll, Waker};

    /// Sends a value to the associated [`Receiver`].
    ///
    /// A pair of both a [`Sender`] and a [`Receiver`]  are created by the
    /// [`channel`](fn@channel) function.
    #[derive(Debug)]
    pub struct Sender<T> {
        inner: Option<Arc<Inner<T>>>,
    }

    /// Receive a value from the associated [`Sender`].
    ///
    /// A pair of both a [`Sender`] and a [`Receiver`]  are created by the
    /// [`channel`](fn@channel) function.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::oneshot;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, rx) = oneshot::channel();
    ///
    ///     tokio::spawn(async move {
    ///         if let Err(_) = tx.send(3) {
    ///             println!("the receiver dropped");
    ///         }
    ///     });
    ///
    ///     match rx.await {
    ///         Ok(v) => println!("got = {:?}", v),
    ///         Err(_) => println!("the sender dropped"),
    ///     }
    /// }
    /// ```
    ///
    /// If the sender is dropped without sending, the receiver will fail with
    /// [`error::RecvError`]:
    ///
    /// ```
    /// use tokio::sync::oneshot;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, rx) = oneshot::channel::<u32>();
    ///
    ///     tokio::spawn(async move {
    ///         drop(tx);
    ///     });
    ///
    ///     match rx.await {
    ///         Ok(_) => panic!("This doesn't happen"),
    ///         Err(_) => println!("the sender dropped"),
    ///     }
    /// }
    /// ```
    #[derive(Debug)]
    pub struct Receiver<T> {
        inner: Option<Arc<Inner<T>>>,
    }

    pub mod error {
        //! Oneshot error types

        use std::fmt;

        /// Error returned by the `Future` implementation for `Receiver`.
        #[derive(Debug, Eq, PartialEq)]
        pub struct RecvError(pub(super) ());

        /// Error returned by the `try_recv` function on `Receiver`.
        #[derive(Debug, Eq, PartialEq)]
        pub enum TryRecvError {
            /// The send half of the channel has not yet sent a value.
            Empty,

            /// The send half of the channel was dropped without sending a value.
            Closed,
        }

        // ===== impl RecvError =====

        impl fmt::Display for RecvError {
            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
                write!(fmt, "channel closed")
            }
        }

        impl std::error::Error for RecvError {}

        // ===== impl TryRecvError =====

        impl fmt::Display for TryRecvError {
            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
                match self {
                    TryRecvError::Empty => write!(fmt, "channel empty"),
                    TryRecvError::Closed => write!(fmt, "channel closed"),
                }
            }
        }

        impl std::error::Error for TryRecvError {}
    }

    use self::error::*;

    struct Inner<T> {
        /// Manages the state of the inner cell
        state: AtomicUsize,

        /// The value. This is set by `Sender` and read by `Receiver`. The state of
        /// the cell is tracked by `state`.
        value: UnsafeCell<Option<T>>,

        /// The task to notify when the receiver drops without consuming the value.
        tx_task: Task,

        /// The task to notify when the value is sent.
        rx_task: Task,
    }

    struct Task(UnsafeCell<MaybeUninit<Waker>>);

    impl Task {
        unsafe fn will_wake(&self, cx: &mut Context<'_>) -> bool {
            self.with_task(|w| w.will_wake(cx.waker()))
        }

        unsafe fn with_task<F, R>(&self, f: F) -> R
        where
            F: FnOnce(&Waker) -> R,
        {
            self.0.with(|ptr| {
                let waker: *const Waker = (&*ptr).as_ptr();
                f(&*waker)
            })
        }

        unsafe fn drop_task(&self) {
            self.0.with_mut(|ptr| {
                let ptr: *mut Waker = (&mut *ptr).as_mut_ptr();
                ptr.drop_in_place();
            });
        }

        unsafe fn set_task(&self, cx: &mut Context<'_>) {
            self.0.with_mut(|ptr| {
                let ptr: *mut Waker = (&mut *ptr).as_mut_ptr();
                ptr.write(cx.waker().clone());
            });
        }
    }

    #[derive(Clone, Copy)]
    struct State(usize);

    /// Create a new one-shot channel for sending single values across asynchronous
    /// tasks.
    ///
    /// The function returns separate "send" and "receive" handles. The `Sender`
    /// handle is used by the producer to send the value. The `Receiver` handle is
    /// used by the consumer to receive the value.
    ///
    /// Each handle can be used on separate tasks.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::oneshot;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, rx) = oneshot::channel();
    ///
    ///     tokio::spawn(async move {
    ///         if let Err(_) = tx.send(3) {
    ///             println!("the receiver dropped");
    ///         }
    ///     });
    ///
    ///     match rx.await {
    ///         Ok(v) => println!("got = {:?}", v),
    ///         Err(_) => println!("the sender dropped"),
    ///     }
    /// }
    /// ```
    pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
        let inner = Arc::new(Inner {
            state: AtomicUsize::new(State::new().as_usize()),
            value: UnsafeCell::new(None),
            tx_task: Task(UnsafeCell::new(MaybeUninit::uninit())),
            rx_task: Task(UnsafeCell::new(MaybeUninit::uninit())),
        });

        let tx = Sender {
            inner: Some(inner.clone()),
        };
        let rx = Receiver { inner: Some(inner) };

        (tx, rx)
    }

    impl<T> Sender<T> {
        /// Attempts to send a value on this channel, returning it back if it could
        /// not be sent.
        ///
        /// This method consumes `self` as only one value may ever be sent on a oneshot
        /// channel. It is not marked async because sending a message to an oneshot
        /// channel never requires any form of waiting.  Because of this, the `send`
        /// method can be used in both synchronous and asynchronous code without
        /// problems.
        ///
        /// A successful send occurs when it is determined that the other end of the
        /// channel has not hung up already. An unsuccessful send would be one where
        /// the corresponding receiver has already been deallocated. Note that a
        /// return value of `Err` means that the data will never be received, but
        /// a return value of `Ok` does *not* mean that the data will be received.
        /// It is possible for the corresponding receiver to hang up immediately
        /// after this function returns `Ok`.
        ///
        /// # Examples
        ///
        /// Send a value to another task
        ///
        /// ```
        /// use tokio::sync::oneshot;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let (tx, rx) = oneshot::channel();
        ///
        ///     tokio::spawn(async move {
        ///         if let Err(_) = tx.send(3) {
        ///             println!("the receiver dropped");
        ///         }
        ///     });
        ///
        ///     match rx.await {
        ///         Ok(v) => println!("got = {:?}", v),
        ///         Err(_) => println!("the sender dropped"),
        ///     }
        /// }
        /// ```
        pub fn send(mut self, t: T) -> Result<(), T> {
            let inner = self.inner.take().unwrap();

            inner.value.with_mut(|ptr| unsafe {
                *ptr = Some(t);
            });

            if !inner.complete() {
                unsafe {
                    return Err(inner.consume_value().unwrap());
                }
            }

            Ok(())
        }

        /// Waits for the associated [`Receiver`] handle to close.
        ///
        /// A [`Receiver`] is closed by either calling [`close`] explicitly or the
        /// [`Receiver`] value is dropped.
        ///
        /// This function is useful when paired with `select!` to abort a
        /// computation when the receiver is no longer interested in the result.
        ///
        /// # Return
        ///
        /// Returns a `Future` which must be awaited on.
        ///
        /// [`Receiver`]: Receiver
        /// [`close`]: Receiver::close
        ///
        /// # Examples
        ///
        /// Basic usage
        ///
        /// ```
        /// use tokio::sync::oneshot;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let (mut tx, rx) = oneshot::channel::<()>();
        ///
        ///     tokio::spawn(async move {
        ///         drop(rx);
        ///     });
        ///
        ///     tx.closed().await;
        ///     println!("the receiver dropped");
        /// }
        /// ```
        ///
        /// Paired with select
        ///
        /// ```
        /// use tokio::sync::oneshot;
        /// use tokio::time::{self, Duration};
        ///
        /// async fn compute() -> String {
        ///     // Complex computation returning a `String`
        /// # "hello".to_string()
        /// }
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let (mut tx, rx) = oneshot::channel();
        ///
        ///     tokio::spawn(async move {
        ///         tokio::select! {
        ///             _ = tx.closed() => {
        ///                 // The receiver dropped, no need to do any further work
        ///             }
        ///             value = compute() => {
        ///                 // The send can fail if the channel was closed at the exact same
        ///                 // time as when compute() finished, so just ignore the failure.
        ///                 let _ = tx.send(value);
        ///             }
        ///         }
        ///     });
        ///
        ///     // Wait for up to 10 seconds
        ///     let _ = time::timeout(Duration::from_secs(10), rx).await;
        /// }
        /// ```
        pub async fn closed(&mut self) {
            use crate::future::poll_fn;

            poll_fn(|cx| self.poll_closed(cx)).await
        }

        /// Returns `true` if the associated [`Receiver`] handle has been dropped.
        ///
        /// A [`Receiver`] is closed by either calling [`close`] explicitly or the
        /// [`Receiver`] value is dropped.
        ///
        /// If `true` is returned, a call to `send` will always result in an error.
        ///
        /// [`Receiver`]: Receiver
        /// [`close`]: Receiver::close
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::oneshot;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let (tx, rx) = oneshot::channel();
        ///
        ///     assert!(!tx.is_closed());
        ///
        ///     drop(rx);
        ///
        ///     assert!(tx.is_closed());
        ///     assert!(tx.send("never received").is_err());
        /// }
        /// ```
        pub fn is_closed(&self) -> bool {
            let inner = self.inner.as_ref().unwrap();

            let state = State::load(&inner.state, Acquire);
            state.is_closed()
        }

        /// Check whether the oneshot channel has been closed, and if not, schedules the
        /// `Waker` in the provided `Context` to receive a notification when the channel is
        /// closed.
        ///
        /// A [`Receiver`] is closed by either calling [`close`] explicitly, or when the
        /// [`Receiver`] value is dropped.
        ///
        /// Note that on multiple calls to poll, only the `Waker` from the `Context` passed
        /// to the most recent call will be scheduled to receive a wakeup.
        ///
        /// [`Receiver`]: struct@crate::sync::oneshot::Receiver
        /// [`close`]: fn@crate::sync::oneshot::Receiver::close
        ///
        /// # Return value
        ///
        /// This function returns:
        ///
        ///  * `Poll::Pending` if the channel is still open.
        ///  * `Poll::Ready(())` if the channel is closed.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::oneshot;
        ///
        /// use futures::future::poll_fn;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let (mut tx, mut rx) = oneshot::channel::<()>();
        ///
        ///     tokio::spawn(async move {
        ///         rx.close();
        ///     });
        ///
        ///     poll_fn(|cx| tx.poll_closed(cx)).await;
        ///
        ///     println!("the receiver dropped");
        /// }
        /// ```
        pub fn poll_closed(&mut self, cx: &mut Context<'_>) -> Poll<()> {
            // Keep track of task budget
            let coop = ready!(crate::coop::poll_proceed(cx));

            let inner = self.inner.as_ref().unwrap();

            let mut state = State::load(&inner.state, Acquire);

            if state.is_closed() {
                coop.made_progress();
                return Poll::Ready(());
            }

            if state.is_tx_task_set() {
                let will_notify = unsafe { inner.tx_task.will_wake(cx) };

                if !will_notify {
                    state = State::unset_tx_task(&inner.state);

                    if state.is_closed() {
                        // Set the flag again so that the waker is released in drop
                        State::set_tx_task(&inner.state);
                        coop.made_progress();
                        return Ready(());
                    } else {
                        unsafe { inner.tx_task.drop_task() };
                    }
                }
            }

            if !state.is_tx_task_set() {
                // Attempt to set the task
                unsafe {
                    inner.tx_task.set_task(cx);
                }

                // Update the state
                state = State::set_tx_task(&inner.state);

                if state.is_closed() {
                    coop.made_progress();
                    return Ready(());
                }
            }

            Pending
        }
    }

    impl<T> Drop for Sender<T> {
        fn drop(&mut self) {
            if let Some(inner) = self.inner.as_ref() {
                inner.complete();
            }
        }
    }

    impl<T> Receiver<T> {
        /// Prevents the associated [`Sender`] handle from sending a value.
        ///
        /// Any `send` operation which happens after calling `close` is guaranteed
        /// to fail. After calling `close`, [`try_recv`] should be called to
        /// receive a value if one was sent **before** the call to `close`
        /// completed.
        ///
        /// This function is useful to perform a graceful shutdown and ensure that a
        /// value will not be sent into the channel and never received.
        ///
        /// `close` is no-op if a message is already received or the channel
        /// is already closed.
        ///
        /// [`Sender`]: Sender
        /// [`try_recv`]: Receiver::try_recv
        ///
        /// # Examples
        ///
        /// Prevent a value from being sent
        ///
        /// ```
        /// use tokio::sync::oneshot;
        /// use tokio::sync::oneshot::error::TryRecvError;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let (tx, mut rx) = oneshot::channel();
        ///
        ///     assert!(!tx.is_closed());
        ///
        ///     rx.close();
        ///
        ///     assert!(tx.is_closed());
        ///     assert!(tx.send("never received").is_err());
        ///
        ///     match rx.try_recv() {
        ///         Err(TryRecvError::Closed) => {}
        ///         _ => unreachable!(),
        ///     }
        /// }
        /// ```
        ///
        /// Receive a value sent **before** calling `close`
        ///
        /// ```
        /// use tokio::sync::oneshot;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let (tx, mut rx) = oneshot::channel();
        ///
        ///     assert!(tx.send("will receive").is_ok());
        ///
        ///     rx.close();
        ///
        ///     let msg = rx.try_recv().unwrap();
        ///     assert_eq!(msg, "will receive");
        /// }
        /// ```
        pub fn close(&mut self) {
            if let Some(inner) = self.inner.as_ref() {
                inner.close();
            }
        }

        /// Attempts to receive a value.
        ///
        /// If a pending value exists in the channel, it is returned. If no value
        /// has been sent, the current task **will not** be registered for
        /// future notification.
        ///
        /// This function is useful to call from outside the context of an
        /// asynchronous task.
        ///
        /// # Return
        ///
        /// - `Ok(T)` if a value is pending in the channel.
        /// - `Err(TryRecvError::Empty)` if no value has been sent yet.
        /// - `Err(TryRecvError::Closed)` if the sender has dropped without sending
        ///   a value.
        ///
        /// # Examples
        ///
        /// `try_recv` before a value is sent, then after.
        ///
        /// ```
        /// use tokio::sync::oneshot;
        /// use tokio::sync::oneshot::error::TryRecvError;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let (tx, mut rx) = oneshot::channel();
        ///
        ///     match rx.try_recv() {
        ///         // The channel is currently empty
        ///         Err(TryRecvError::Empty) => {}
        ///         _ => unreachable!(),
        ///     }
        ///
        ///     // Send a value
        ///     tx.send("hello").unwrap();
        ///
        ///     match rx.try_recv() {
        ///         Ok(value) => assert_eq!(value, "hello"),
        ///         _ => unreachable!(),
        ///     }
        /// }
        /// ```
        ///
        /// `try_recv` when the sender dropped before sending a value
        ///
        /// ```
        /// use tokio::sync::oneshot;
        /// use tokio::sync::oneshot::error::TryRecvError;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let (tx, mut rx) = oneshot::channel::<()>();
        ///
        ///     drop(tx);
        ///
        ///     match rx.try_recv() {
        ///         // The channel will never receive a value.
        ///         Err(TryRecvError::Closed) => {}
        ///         _ => unreachable!(),
        ///     }
        /// }
        /// ```
        pub fn try_recv(&mut self) -> Result<T, TryRecvError> {
            let result = if let Some(inner) = self.inner.as_ref() {
                let state = State::load(&inner.state, Acquire);

                if state.is_complete() {
                    match unsafe { inner.consume_value() } {
                        Some(value) => Ok(value),
                        None => Err(TryRecvError::Closed),
                    }
                } else if state.is_closed() {
                    Err(TryRecvError::Closed)
                } else {
                    // Not ready, this does not clear `inner`
                    return Err(TryRecvError::Empty);
                }
            } else {
                panic!("called after complete");
            };

            self.inner = None;
            result
        }
    }

    impl<T> Drop for Receiver<T> {
        fn drop(&mut self) {
            if let Some(inner) = self.inner.as_ref() {
                inner.close();
            }
        }
    }

    impl<T> Future for Receiver<T> {
        type Output = Result<T, RecvError>;

        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            // If `inner` is `None`, then `poll()` has already completed.
            let ret = if let Some(inner) = self.as_ref().get_ref().inner.as_ref() {
                ready!(inner.poll_recv(cx))?
            } else {
                panic!("called after complete");
            };

            self.inner = None;
            Ready(Ok(ret))
        }
    }

    impl<T> Inner<T> {
        fn complete(&self) -> bool {
            let prev = State::set_complete(&self.state);

            if prev.is_closed() {
                return false;
            }

            if prev.is_rx_task_set() {
                // TODO: Consume waker?
                unsafe {
                    self.rx_task.with_task(Waker::wake_by_ref);
                }
            }

            true
        }

        fn poll_recv(&self, cx: &mut Context<'_>) -> Poll<Result<T, RecvError>> {
            // Keep track of task budget
            let coop = ready!(crate::coop::poll_proceed(cx));

            // Load the state
            let mut state = State::load(&self.state, Acquire);

            if state.is_complete() {
                coop.made_progress();
                match unsafe { self.consume_value() } {
                    Some(value) => Ready(Ok(value)),
                    None => Ready(Err(RecvError(()))),
                }
            } else if state.is_closed() {
                coop.made_progress();
                Ready(Err(RecvError(())))
            } else {
                if state.is_rx_task_set() {
                    let will_notify = unsafe { self.rx_task.will_wake(cx) };

                    // Check if the task is still the same
                    if !will_notify {
                        // Unset the task
                        state = State::unset_rx_task(&self.state);
                        if state.is_complete() {
                            // Set the flag again so that the waker is released in drop
                            State::set_rx_task(&self.state);

                            coop.made_progress();
                            return match unsafe { self.consume_value() } {
                                Some(value) => Ready(Ok(value)),
                                None => Ready(Err(RecvError(()))),
                            };
                        } else {
                            unsafe { self.rx_task.drop_task() };
                        }
                    }
                }

                if !state.is_rx_task_set() {
                    // Attempt to set the task
                    unsafe {
                        self.rx_task.set_task(cx);
                    }

                    // Update the state
                    state = State::set_rx_task(&self.state);

                    if state.is_complete() {
                        coop.made_progress();
                        match unsafe { self.consume_value() } {
                            Some(value) => Ready(Ok(value)),
                            None => Ready(Err(RecvError(()))),
                        }
                    } else {
                        Pending
                    }
                } else {
                    Pending
                }
            }
        }

        /// Called by `Receiver` to indicate that the value will never be received.
        fn close(&self) {
            let prev = State::set_closed(&self.state);

            if prev.is_tx_task_set() && !prev.is_complete() {
                unsafe {
                    self.tx_task.with_task(Waker::wake_by_ref);
                }
            }
        }

        /// Consumes the value. This function does not check `state`.
        unsafe fn consume_value(&self) -> Option<T> {
            self.value.with_mut(|ptr| (*ptr).take())
        }
    }

    unsafe impl<T: Send> Send for Inner<T> {}
    unsafe impl<T: Send> Sync for Inner<T> {}

    fn mut_load(this: &mut AtomicUsize) -> usize {
        this.with_mut(|v| *v)
    }

    impl<T> Drop for Inner<T> {
        fn drop(&mut self) {
            let state = State(mut_load(&mut self.state));

            if state.is_rx_task_set() {
                unsafe {
                    self.rx_task.drop_task();
                }
            }

            if state.is_tx_task_set() {
                unsafe {
                    self.tx_task.drop_task();
                }
            }
        }
    }

    impl<T: fmt::Debug> fmt::Debug for Inner<T> {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            use std::sync::atomic::Ordering::Relaxed;

            fmt.debug_struct("Inner")
                .field("state", &State::load(&self.state, Relaxed))
                .finish()
        }
    }

    const RX_TASK_SET: usize = 0b00001;
    const VALUE_SENT: usize = 0b00010;
    const CLOSED: usize = 0b00100;
    const TX_TASK_SET: usize = 0b01000;

    impl State {
        fn new() -> State {
            State(0)
        }

        fn is_complete(self) -> bool {
            self.0 & VALUE_SENT == VALUE_SENT
        }

        fn set_complete(cell: &AtomicUsize) -> State {
            // TODO: This could be `Release`, followed by an `Acquire` fence *if*
            // the `RX_TASK_SET` flag is set. However, `loom` does not support
            // fences yet.
            let val = cell.fetch_or(VALUE_SENT, AcqRel);
            State(val)
        }

        fn is_rx_task_set(self) -> bool {
            self.0 & RX_TASK_SET == RX_TASK_SET
        }

        fn set_rx_task(cell: &AtomicUsize) -> State {
            let val = cell.fetch_or(RX_TASK_SET, AcqRel);
            State(val | RX_TASK_SET)
        }

        fn unset_rx_task(cell: &AtomicUsize) -> State {
            let val = cell.fetch_and(!RX_TASK_SET, AcqRel);
            State(val & !RX_TASK_SET)
        }

        fn is_closed(self) -> bool {
            self.0 & CLOSED == CLOSED
        }

        fn set_closed(cell: &AtomicUsize) -> State {
            // Acquire because we want all later writes (attempting to poll) to be
            // ordered after this.
            let val = cell.fetch_or(CLOSED, Acquire);
            State(val)
        }

        fn set_tx_task(cell: &AtomicUsize) -> State {
            let val = cell.fetch_or(TX_TASK_SET, AcqRel);
            State(val | TX_TASK_SET)
        }

        fn unset_tx_task(cell: &AtomicUsize) -> State {
            let val = cell.fetch_and(!TX_TASK_SET, AcqRel);
            State(val & !TX_TASK_SET)
        }

        fn is_tx_task_set(self) -> bool {
            self.0 & TX_TASK_SET == TX_TASK_SET
        }

        fn as_usize(self) -> usize {
            self.0
        }

        fn load(cell: &AtomicUsize, order: Ordering) -> State {
            let val = cell.load(order);
            State(val)
        }
    }

    impl fmt::Debug for State {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("State")
                .field("is_complete", &self.is_complete())
                .field("is_closed", &self.is_closed())
                .field("is_rx_task_set", &self.is_rx_task_set())
                .field("is_tx_task_set", &self.is_tx_task_set())
                .finish()
        }
    }