# mod.rs - source
Source of the Rust file `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-channel-0.3.14/src/mpsc/mod.rs`.
Keywords: rust, rustlang, rust-lang

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    //! A multi-producer, single-consumer queue for sending values across
    //! asynchronous tasks.
    //!
    //! Similarly to the `std`, channel creation provides [`Receiver`] and
    //! [`Sender`] handles. [`Receiver`] implements [`Stream`] and allows a task to
    //! read values out of the channel. If there is no message to read from the
    //! channel, the current task will be notified when a new value is sent.
    //! [`Sender`] implements the `Sink` trait and allows a task to send messages into
    //! the channel. If the channel is at capacity, the send will be rejected and
    //! the task will be notified when additional capacity is available. In other
    //! words, the channel provides backpressure.
    //!
    //! Unbounded channels are also available using the `unbounded` constructor.
    //!
    //! # Disconnection
    //!
    //! When all [`Sender`] handles have been dropped, it is no longer
    //! possible to send values into the channel. This is considered the termination
    //! event of the stream. As such, [`Receiver::poll_next`]
    //! will return `Ok(Ready(None))`.
    //!
    //! If the [`Receiver`] handle is dropped, then messages can no longer
    //! be read out of the channel. In this case, all further attempts to send will
    //! result in an error.
    //!
    //! # Clean Shutdown
    //!
    //! If the [`Receiver`] is simply dropped, then it is possible for
    //! there to be messages still in the channel that will not be processed. As
    //! such, it is usually desirable to perform a "clean" shutdown. To do this, the
    //! receiver will first call `close`, which will prevent any further messages to
    //! be sent into the channel. Then, the receiver consumes the channel to
    //! completion, at which point the receiver can be dropped.
    //!
    //! [`Sender`]: struct.Sender.html
    //! [`Receiver`]: struct.Receiver.html
    //! [`Stream`]: ../../futures_core/stream/trait.Stream.html
    //! [`Receiver::poll_next`]:
    //!     ../../futures_core/stream/trait.Stream.html#tymethod.poll_next

    // At the core, the channel uses an atomic FIFO queue for message passing. This
    // queue is used as the primary coordination primitive. In order to enforce
    // capacity limits and handle back pressure, a secondary FIFO queue is used to
    // send parked task handles.
    //
    // The general idea is that the channel is created with a `buffer` size of `n`.
    // The channel capacity is `n + num-senders`. Each sender gets one "guaranteed"
    // slot to hold a message. This allows `Sender` to know for a fact that a send
    // will succeed *before* starting to do the actual work of sending the value.
    // Since most of this work is lock-free, once the work starts, it is impossible
    // to safely revert.
    //
    // If the sender is unable to process a send operation, then the current
    // task is parked and the handle is sent on the parked task queue.
    //
    // Note that the implementation guarantees that the channel capacity will never
    // exceed the configured limit, however there is no *strict* guarantee that the
    // receiver will wake up a parked task *immediately* when a slot becomes
    // available. However, it will almost always unpark a task when a slot becomes
    // available and it is *guaranteed* that a sender will be unparked when the
    // message that caused the sender to become parked is read out of the channel.
    //
    // The steps for sending a message are roughly:
    //
    // 1) Increment the channel message count
    // 2) If the channel is at capacity, push the task handle onto the wait queue
    // 3) Push the message onto the message queue.
    //
    // The steps for receiving a message are roughly:
    //
    // 1) Pop a message from the message queue
    // 2) Pop a task handle from the wait queue
    // 3) Decrement the channel message count.
    //
    // It's important for the order of operations on lock-free structures to happen
    // in reverse order between the sender and receiver. This makes the message
    // queue the primary coordination structure and establishes the necessary
    // happens-before semantics required for the acquire / release semantics used
    // by the queue structure.

    use futures_core::stream::{FusedStream, Stream};
    use futures_core::task::{Context, Poll, Waker};
    use futures_core::task::__internal::AtomicWaker;
    use std::fmt;
    use std::pin::Pin;
    use std::sync::{Arc, Mutex};
    use std::sync::atomic::AtomicUsize;
    use std::sync::atomic::Ordering::SeqCst;
    use std::thread;

    use crate::mpsc::queue::Queue;

    mod queue;
    #[cfg(feature = "sink")]
    mod sink_impl;

    #[derive(Debug)]
    struct UnboundedSenderInner<T> {
        // Channel state shared between the sender and receiver.
        inner: Arc<UnboundedInner<T>>,
    }

    #[derive(Debug)]
    struct BoundedSenderInner<T> {
        // Channel state shared between the sender and receiver.
        inner: Arc<BoundedInner<T>>,

        // Handle to the task that is blocked on this sender. This handle is sent
        // to the receiver half in order to be notified when the sender becomes
        // unblocked.
        sender_task: Arc<Mutex<SenderTask>>,

        // `true` if the sender might be blocked. This is an optimization to avoid
        // having to lock the mutex most of the time.
        maybe_parked: bool,
    }

    // We never project Pin<&mut SenderInner> to `Pin<&mut T>`
    impl<T> Unpin for UnboundedSenderInner<T> {}
    impl<T> Unpin for BoundedSenderInner<T> {}

    /// The transmission end of a bounded mpsc channel.
    ///
    /// This value is created by the [`channel`](channel) function.
    #[derive(Debug)]
    pub struct Sender<T>(Option<BoundedSenderInner<T>>);

    /// The transmission end of an unbounded mpsc channel.
    ///
    /// This value is created by the [`unbounded`](unbounded) function.
    #[derive(Debug)]
    pub struct UnboundedSender<T>(Option<UnboundedSenderInner<T>>);

    trait AssertKinds: Send + Sync + Clone {}
    impl AssertKinds for UnboundedSender<u32> {}

    /// The receiving end of a bounded mpsc channel.
    ///
    /// This value is created by the [`channel`](channel) function.
    #[derive(Debug)]
    pub struct Receiver<T> {
        inner: Option<Arc<BoundedInner<T>>>,
    }

    /// The receiving end of an unbounded mpsc channel.
    ///
    /// This value is created by the [`unbounded`](unbounded) function.
    #[derive(Debug)]
    pub struct UnboundedReceiver<T> {
        inner: Option<Arc<UnboundedInner<T>>>,
    }

    // `Pin<&mut UnboundedReceiver<T>>` is never projected to `Pin<&mut T>`
    impl<T> Unpin for UnboundedReceiver<T> {}

    /// The error type for [`Sender`s](Sender) used as `Sink`s.
    #[derive(Clone, Debug, PartialEq, Eq)]
    pub struct SendError {
        kind: SendErrorKind,
    }

    /// The error type returned from [`try_send`](Sender::try_send).
    #[derive(Clone, PartialEq, Eq)]
    pub struct TrySendError<T> {
        err: SendError,
        val: T,
    }

    #[derive(Clone, Debug, PartialEq, Eq)]
    enum SendErrorKind {
        Full,
        Disconnected,
    }

    /// The error type returned from [`try_next`](Receiver::try_next).
    pub struct TryRecvError {
        _priv: (),
    }

    impl fmt::Display for SendError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            if self.is_full() {
                write!(f, "send failed because channel is full")
            } else {
                write!(f, "send failed because receiver is gone")
            }
        }
    }

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

    impl SendError {
        /// Returns `true` if this error is a result of the channel being full.
        pub fn is_full(&self) -> bool {
            match self.kind {
                SendErrorKind::Full => true,
                _ => false,
            }
        }

        /// Returns `true` if this error is a result of the receiver being dropped.
        pub fn is_disconnected(&self) -> bool {
            match self.kind {
                SendErrorKind::Disconnected => true,
                _ => false,
            }
        }
    }

    impl<T> fmt::Debug for TrySendError<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("TrySendError")
                .field("kind", &self.err.kind)
                .finish()
        }
    }

    impl<T> fmt::Display for TrySendError<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            if self.is_full() {
                write!(f, "send failed because channel is full")
            } else {
                write!(f, "send failed because receiver is gone")
            }
        }
    }

    impl<T: core::any::Any> std::error::Error for TrySendError<T> {}

    impl<T> TrySendError<T> {
        /// Returns `true` if this error is a result of the channel being full.
        pub fn is_full(&self) -> bool {
            self.err.is_full()
        }

        /// Returns `true` if this error is a result of the receiver being dropped.
        pub fn is_disconnected(&self) -> bool {
            self.err.is_disconnected()
        }

        /// Returns the message that was attempted to be sent but failed.
        pub fn into_inner(self) -> T {
            self.val
        }

        /// Drops the message and converts into a `SendError`.
        pub fn into_send_error(self) -> SendError {
            self.err
        }
    }

    impl fmt::Debug for TryRecvError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_tuple("TryRecvError")
                .finish()
        }
    }

    impl fmt::Display for TryRecvError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(f, "receiver channel is empty")
        }
    }

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

    #[derive(Debug)]
    struct UnboundedInner<T> {
        // Internal channel state. Consists of the number of messages stored in the
        // channel as well as a flag signalling that the channel is closed.
        state: AtomicUsize,

        // Atomic, FIFO queue used to send messages to the receiver
        message_queue: Queue<T>,

        // Number of senders in existence
        num_senders: AtomicUsize,

        // Handle to the receiver's task.
        recv_task: AtomicWaker,
    }

    #[derive(Debug)]
    struct BoundedInner<T> {
        // Max buffer size of the channel. If `None` then the channel is unbounded.
        buffer: usize,

        // Internal channel state. Consists of the number of messages stored in the
        // channel as well as a flag signalling that the channel is closed.
        state: AtomicUsize,

        // Atomic, FIFO queue used to send messages to the receiver
        message_queue: Queue<T>,

        // Atomic, FIFO queue used to send parked task handles to the receiver.
        parked_queue: Queue<Arc<Mutex<SenderTask>>>,

        // Number of senders in existence
        num_senders: AtomicUsize,

        // Handle to the receiver's task.
        recv_task: AtomicWaker,
    }

    // Struct representation of `Inner::state`.
    #[derive(Debug, Clone, Copy)]
    struct State {
        // `true` when the channel is open
        is_open: bool,

        // Number of messages in the channel
        num_messages: usize,
    }

    // The `is_open` flag is stored in the left-most bit of `Inner::state`
    const OPEN_MASK: usize = usize::max_value() - (usize::max_value() >> 1);

    // When a new channel is created, it is created in the open state with no
    // pending messages.
    const INIT_STATE: usize = OPEN_MASK;

    // The maximum number of messages that a channel can track is `usize::max_value() >> 1`
    const MAX_CAPACITY: usize = !(OPEN_MASK);

    // The maximum requested buffer size must be less than the maximum capacity of
    // a channel. This is because each sender gets a guaranteed slot.
    const MAX_BUFFER: usize = MAX_CAPACITY >> 1;

    // Sent to the consumer to wake up blocked producers
    #[derive(Debug)]
    struct SenderTask {
        task: Option<Waker>,
        is_parked: bool,
    }

    impl SenderTask {
        fn new() -> Self {
            Self {
                task: None,
                is_parked: false,
            }
        }

        fn notify(&mut self) {
            self.is_parked = false;

            if let Some(task) = self.task.take() {
                task.wake();
            }
        }
    }

    /// Creates a bounded mpsc channel for communicating between asynchronous tasks.
    ///
    /// Being bounded, this channel provides backpressure to ensure that the sender
    /// outpaces the receiver by only a limited amount. The channel's capacity is
    /// equal to `buffer + num-senders`. In other words, each sender gets a
    /// guaranteed slot in the channel capacity, and on top of that there are
    /// `buffer` "first come, first serve" slots available to all senders.
    ///
    /// The [`Receiver`](Receiver) returned implements the
    /// [`Stream`](futures_core::stream::Stream) trait, while [`Sender`](Sender) implements
    /// `Sink`.
    pub fn channel<T>(buffer: usize) -> (Sender<T>, Receiver<T>) {
        // Check that the requested buffer size does not exceed the maximum buffer
        // size permitted by the system.
        assert!(buffer < MAX_BUFFER, "requested buffer size too large");

        let inner = Arc::new(BoundedInner {
            buffer,
            state: AtomicUsize::new(INIT_STATE),
            message_queue: Queue::new(),
            parked_queue: Queue::new(),
            num_senders: AtomicUsize::new(1),
            recv_task: AtomicWaker::new(),
        });

        let tx = BoundedSenderInner {
            inner: inner.clone(),
            sender_task: Arc::new(Mutex::new(SenderTask::new())),
            maybe_parked: false,
        };

        let rx = Receiver {
            inner: Some(inner),
        };

        (Sender(Some(tx)), rx)
    }

    /// Creates an unbounded mpsc channel for communicating between asynchronous
    /// tasks.
    ///
    /// A `send` on this channel will always succeed as long as the receive half has
    /// not been closed. If the receiver falls behind, messages will be arbitrarily
    /// buffered.
    ///
    /// **Note** that the amount of available system memory is an implicit bound to
    /// the channel. Using an `unbounded` channel has the ability of causing the
    /// process to run out of memory. In this case, the process will be aborted.
    pub fn unbounded<T>() -> (UnboundedSender<T>, UnboundedReceiver<T>) {

        let inner = Arc::new(UnboundedInner {
            state: AtomicUsize::new(INIT_STATE),
            message_queue: Queue::new(),
            num_senders: AtomicUsize::new(1),
            recv_task: AtomicWaker::new(),
        });

        let tx = UnboundedSenderInner {
            inner: inner.clone(),
        };

        let rx = UnboundedReceiver {
            inner: Some(inner),
        };

        (UnboundedSender(Some(tx)), rx)
    }

    /*
     *
     * ===== impl Sender =====
     *
     */

    impl<T> UnboundedSenderInner<T> {
        fn poll_ready_nb(&self) -> Poll<Result<(), SendError>> {
            let state = decode_state(self.inner.state.load(SeqCst));
            if state.is_open {
                Poll::Ready(Ok(()))
            } else {
                Poll::Ready(Err(SendError {
                    kind: SendErrorKind::Disconnected,
                }))
            }
        }

        // Push message to the queue and signal to the receiver
        fn queue_push_and_signal(&self, msg: T) {
            // Push the message onto the message queue
            self.inner.message_queue.push(msg);

            // Signal to the receiver that a message has been enqueued. If the
            // receiver is parked, this will unpark the task.
            self.inner.recv_task.wake();
        }

        // Increment the number of queued messages. Returns the resulting number.
        fn inc_num_messages(&self) -> Option<usize> {
            let mut curr = self.inner.state.load(SeqCst);

            loop {
                let mut state = decode_state(curr);

                // The receiver end closed the channel.
                if !state.is_open {
                    return None;
                }

                // This probably is never hit? Odds are the process will run out of
                // memory first. It may be worth to return something else in this
                // case?
                assert!(state.num_messages < MAX_CAPACITY, "buffer space \
                        exhausted; sending this messages would overflow the state");

                state.num_messages += 1;

                let next = encode_state(&state);
                match self.inner.state.compare_exchange(curr, next, SeqCst, SeqCst) {
                    Ok(_) => {
                        return Some(state.num_messages)
                    }
                    Err(actual) => curr = actual,
                }
            }
        }

        /// Returns whether the senders send to the same receiver.
        fn same_receiver(&self, other: &Self) -> bool {
            Arc::ptr_eq(&self.inner, &other.inner)
        }

        /// Returns whether the sender send to this receiver.
        fn is_connected_to(&self, inner: &Arc<UnboundedInner<T>>) -> bool {
            Arc::ptr_eq(&self.inner, inner)
        }

        /// Returns pointer to the Arc containing sender
        ///
        /// The returned pointer is not referenced and should be only used for hashing!
        fn ptr(&self) -> *const UnboundedInner<T> {
            &*self.inner
        }

        /// Returns whether this channel is closed without needing a context.
        fn is_closed(&self) -> bool {
            !decode_state(self.inner.state.load(SeqCst)).is_open
        }

        /// Closes this channel from the sender side, preventing any new messages.
        fn close_channel(&self) {
            // There's no need to park this sender, its dropping,
            // and we don't want to check for capacity, so skip
            // that stuff from `do_send`.

            self.inner.set_closed();
            self.inner.recv_task.wake();
        }
    }

    impl<T> BoundedSenderInner<T> {
        /// Attempts to send a message on this `Sender`, returning the message
        /// if there was an error.
        fn try_send(&mut self, msg: T) -> Result<(), TrySendError<T>> {
            // If the sender is currently blocked, reject the message
            if !self.poll_unparked(None).is_ready() {
                return Err(TrySendError {
                    err: SendError {
                        kind: SendErrorKind::Full,
                    },
                    val: msg,
                });
            }

            // The channel has capacity to accept the message, so send it
            self.do_send_b(msg)
        }

        // Do the send without failing.
        // Can be called only by bounded sender.
        #[allow(clippy::debug_assert_with_mut_call)]
        fn do_send_b(&mut self, msg: T)
            -> Result<(), TrySendError<T>>
        {
            // Anyone callig do_send *should* make sure there is room first,
            // but assert here for tests as a sanity check.
            debug_assert!(self.poll_unparked(None).is_ready());

            // First, increment the number of messages contained by the channel.
            // This operation will also atomically determine if the sender task
            // should be parked.
            //
            // `None` is returned in the case that the channel has been closed by the
            // receiver. This happens when `Receiver::close` is called or the
            // receiver is dropped.
            let park_self = match self.inc_num_messages() {
                Some(num_messages) => {
                    // Block if the current number of pending messages has exceeded
                    // the configured buffer size
                    num_messages > self.inner.buffer
                }
                None => return Err(TrySendError {
                    err: SendError {
                        kind: SendErrorKind::Disconnected,
                    },
                    val: msg,
                }),
            };

            // If the channel has reached capacity, then the sender task needs to
            // be parked. This will send the task handle on the parked task queue.
            //
            // However, when `do_send` is called while dropping the `Sender`,
            // `task::current()` can't be called safely. In this case, in order to
            // maintain internal consistency, a blank message is pushed onto the
            // parked task queue.
            if park_self {
                self.park();
            }

            self.queue_push_and_signal(msg);

            Ok(())
        }

        // Push message to the queue and signal to the receiver
        fn queue_push_and_signal(&self, msg: T) {
            // Push the message onto the message queue
            self.inner.message_queue.push(msg);

            // Signal to the receiver that a message has been enqueued. If the
            // receiver is parked, this will unpark the task.
            self.inner.recv_task.wake();
        }

        // Increment the number of queued messages. Returns the resulting number.
        fn inc_num_messages(&self) -> Option<usize> {
            let mut curr = self.inner.state.load(SeqCst);

            loop {
                let mut state = decode_state(curr);

                // The receiver end closed the channel.
                if !state.is_open {
                    return None;
                }

                // This probably is never hit? Odds are the process will run out of
                // memory first. It may be worth to return something else in this
                // case?
                assert!(state.num_messages < MAX_CAPACITY, "buffer space \
                        exhausted; sending this messages would overflow the state");

                state.num_messages += 1;

                let next = encode_state(&state);
                match self.inner.state.compare_exchange(curr, next, SeqCst, SeqCst) {
                    Ok(_) => {
                        return Some(state.num_messages)
                    }
                    Err(actual) => curr = actual,
                }
            }
        }

        fn park(&mut self) {
            {
                let mut sender = self.sender_task.lock().unwrap();
                sender.task = None;
                sender.is_parked = true;
            }

            // Send handle over queue
            let t = self.sender_task.clone();
            self.inner.parked_queue.push(t);

            // Check to make sure we weren't closed after we sent our task on the
            // queue
            let state = decode_state(self.inner.state.load(SeqCst));
            self.maybe_parked = state.is_open;
        }

        /// Polls the channel to determine if there is guaranteed capacity to send
        /// at least one item without waiting.
        ///
        /// # Return value
        ///
        /// This method returns:
        ///
        /// - `Poll::Ready(Ok(_))` if there is sufficient capacity;
        /// - `Poll::Pending` if the channel may not have
        ///   capacity, in which case the current task is queued to be notified once
        ///   capacity is available;
        /// - `Poll::Ready(Err(SendError))` if the receiver has been dropped.
        fn poll_ready(
            &mut self,
            cx: &mut Context<'_>,
        ) -> Poll<Result<(), SendError>> {
            let state = decode_state(self.inner.state.load(SeqCst));
            if !state.is_open {
                return Poll::Ready(Err(SendError {
                    kind: SendErrorKind::Disconnected,
                }));
            }

            self.poll_unparked(Some(cx)).map(Ok)
        }

        /// Returns whether the senders send to the same receiver.
        fn same_receiver(&self, other: &Self) -> bool {
            Arc::ptr_eq(&self.inner, &other.inner)
        }

        /// Returns whether the sender send to this receiver.
        fn is_connected_to(&self, receiver: &Arc<BoundedInner<T>>) -> bool {
            Arc::ptr_eq(&self.inner, receiver)
        }

        /// Returns pointer to the Arc containing sender
        ///
        /// The returned pointer is not referenced and should be only used for hashing!
        fn ptr(&self) -> *const BoundedInner<T> {
            &*self.inner
        }

        /// Returns whether this channel is closed without needing a context.
        fn is_closed(&self) -> bool {
            !decode_state(self.inner.state.load(SeqCst)).is_open
        }

        /// Closes this channel from the sender side, preventing any new messages.
        fn close_channel(&self) {
            // There's no need to park this sender, its dropping,
            // and we don't want to check for capacity, so skip
            // that stuff from `do_send`.

            self.inner.set_closed();
            self.inner.recv_task.wake();
        }

        fn poll_unparked(&mut self, cx: Option<&mut Context<'_>>) -> Poll<()> {
            // First check the `maybe_parked` variable. This avoids acquiring the
            // lock in most cases
            if self.maybe_parked {
                // Get a lock on the task handle
                let mut task = self.sender_task.lock().unwrap();

                if !task.is_parked {
                    self.maybe_parked = false;
                    return Poll::Ready(())
                }

                // At this point, an unpark request is pending, so there will be an
                // unpark sometime in the future. We just need to make sure that
                // the correct task will be notified.
                //
                // Update the task in case the `Sender` has been moved to another
                // task
                task.task = cx.map(|cx| cx.waker().clone());

                Poll::Pending
            } else {
                Poll::Ready(())
            }
        }
    }

    impl<T> Sender<T> {
        /// Attempts to send a message on this `Sender`, returning the message
        /// if there was an error.
        pub fn try_send(&mut self, msg: T) -> Result<(), TrySendError<T>> {
            if let Some(inner) = &mut self.0 {
                inner.try_send(msg)
            } else {
                Err(TrySendError {
                    err: SendError {
                        kind: SendErrorKind::Disconnected,
                    },
                    val: msg,
                })
            }
        }

        /// Send a message on the channel.
        ///
        /// This function should only be called after
        /// [`poll_ready`](Sender::poll_ready) has reported that the channel is
        /// ready to receive a message.
        pub fn start_send(&mut self, msg: T) -> Result<(), SendError> {
            self.try_send(msg)
                .map_err(|e| e.err)
        }

        /// Polls the channel to determine if there is guaranteed capacity to send
        /// at least one item without waiting.
        ///
        /// # Return value
        ///
        /// This method returns:
        ///
        /// - `Poll::Ready(Ok(_))` if there is sufficient capacity;
        /// - `Poll::Pending` if the channel may not have
        ///   capacity, in which case the current task is queued to be notified once
        ///   capacity is available;
        /// - `Poll::Ready(Err(SendError))` if the receiver has been dropped.
        pub fn poll_ready(
            &mut self,
            cx: &mut Context<'_>,
        ) -> Poll<Result<(), SendError>> {
            let inner = self.0.as_mut().ok_or(SendError {
                kind: SendErrorKind::Disconnected,
            })?;
            inner.poll_ready(cx)
        }

        /// Returns whether this channel is closed without needing a context.
        pub fn is_closed(&self) -> bool {
            self.0.as_ref().map(BoundedSenderInner::is_closed).unwrap_or(true)
        }

        /// Closes this channel from the sender side, preventing any new messages.
        pub fn close_channel(&mut self) {
            if let Some(inner) = &mut self.0 {
                inner.close_channel();
            }
        }

        /// Disconnects this sender from the channel, closing it if there are no more senders left.
        pub fn disconnect(&mut self) {
            self.0 = None;
        }

        /// Returns whether the senders send to the same receiver.
        pub fn same_receiver(&self, other: &Self) -> bool {
            match (&self.0, &other.0) {
                (Some(inner), Some(other)) => inner.same_receiver(other),
                _ => false,
            }
        }

        /// Returns whether the sender send to this receiver.
        pub fn is_connected_to(&self, receiver: &Receiver<T>) -> bool {
            match (&self.0, &receiver.inner) {
                (Some(inner), Some(receiver)) => inner.is_connected_to(receiver),
                _ => false,
            }
        }

        /// Hashes the receiver into the provided hasher
        pub fn hash_receiver<H>(&self, hasher: &mut H) where H: std::hash::Hasher {
            use std::hash::Hash;

            let ptr = self.0.as_ref().map(|inner| inner.ptr());
            ptr.hash(hasher);
        }
    }

    impl<T> UnboundedSender<T> {
        /// Check if the channel is ready to receive a message.
        pub fn poll_ready(
            &self,
            _: &mut Context<'_>,
        ) -> Poll<Result<(), SendError>> {
            let inner = self.0.as_ref().ok_or(SendError {
                kind: SendErrorKind::Disconnected,
            })?;
            inner.poll_ready_nb()
        }

        /// Returns whether this channel is closed without needing a context.
        pub fn is_closed(&self) -> bool {
            self.0.as_ref().map(UnboundedSenderInner::is_closed).unwrap_or(true)
        }

        /// Closes this channel from the sender side, preventing any new messages.
        pub fn close_channel(&self) {
            if let Some(inner) = &self.0 {
                inner.close_channel();
            }
        }

        /// Disconnects this sender from the channel, closing it if there are no more senders left.
        pub fn disconnect(&mut self) {
            self.0 = None;
        }

        // Do the send without parking current task.
        fn do_send_nb(&self, msg: T) -> Result<(), TrySendError<T>> {
            if let Some(inner) = &self.0 {
                if inner.inc_num_messages().is_some() {
                    inner.queue_push_and_signal(msg);
                    return Ok(());
                }
            }

            Err(TrySendError {
                err: SendError {
                    kind: SendErrorKind::Disconnected,
                },
                val: msg,
            })
        }

        /// Send a message on the channel.
        ///
        /// This method should only be called after `poll_ready` has been used to
        /// verify that the channel is ready to receive a message.
        pub fn start_send(&mut self, msg: T) -> Result<(), SendError> {
            self.do_send_nb(msg)
                .map_err(|e| e.err)
        }

        /// Sends a message along this channel.
        ///
        /// This is an unbounded sender, so this function differs from `Sink::send`
        /// by ensuring the return type reflects that the channel is always ready to
        /// receive messages.
        pub fn unbounded_send(&self, msg: T) -> Result<(), TrySendError<T>> {
            self.do_send_nb(msg)
        }

        /// Returns whether the senders send to the same receiver.
        pub fn same_receiver(&self, other: &Self) -> bool {
            match (&self.0, &other.0) {
                (Some(inner), Some(other)) => inner.same_receiver(other),
                _ => false,
            }
        }

        /// Returns whether the sender send to this receiver.
        pub fn is_connected_to(&self, receiver: &UnboundedReceiver<T>) -> bool {
            match (&self.0, &receiver.inner) {
                (Some(inner), Some(receiver)) => inner.is_connected_to(receiver),
                _ => false,
            }
        }

        /// Hashes the receiver into the provided hasher
        pub fn hash_receiver<H>(&self, hasher: &mut H) where H: std::hash::Hasher {
            use std::hash::Hash;

            let ptr = self.0.as_ref().map(|inner| inner.ptr());
            ptr.hash(hasher);
        }
    }

    impl<T> Clone for Sender<T> {
        fn clone(&self) -> Self {
            Self(self.0.clone())
        }
    }

    impl<T> Clone for UnboundedSender<T> {
        fn clone(&self) -> Self {
            Self(self.0.clone())
        }
    }

    impl<T> Clone for UnboundedSenderInner<T> {
        fn clone(&self) -> Self {
            // Since this atomic op isn't actually guarding any memory and we don't
            // care about any orderings besides the ordering on the single atomic
            // variable, a relaxed ordering is acceptable.
            let mut curr = self.inner.num_senders.load(SeqCst);

            loop {
                // If the maximum number of senders has been reached, then fail
                if curr == MAX_BUFFER {
                    panic!("cannot clone `Sender` -- too many outstanding senders");
                }

                debug_assert!(curr < MAX_BUFFER);

                let next = curr + 1;
                match self.inner.num_senders.compare_exchange(curr, next, SeqCst, SeqCst) {
                    Ok(_) => {
                        // The ABA problem doesn't matter here. We only care that the
                        // number of senders never exceeds the maximum.
                        return Self {
                            inner: self.inner.clone(),
                        };
                    }
                    Err(actual) => curr = actual,
                }
            }
        }
    }

    impl<T> Clone for BoundedSenderInner<T> {
        fn clone(&self) -> Self {
            // Since this atomic op isn't actually guarding any memory and we don't
            // care about any orderings besides the ordering on the single atomic
            // variable, a relaxed ordering is acceptable.
            let mut curr = self.inner.num_senders.load(SeqCst);

            loop {
                // If the maximum number of senders has been reached, then fail
                if curr == self.inner.max_senders() {
                    panic!("cannot clone `Sender` -- too many outstanding senders");
                }

                debug_assert!(curr < self.inner.max_senders());

                let next = curr + 1;
                match self.inner.num_senders.compare_exchange(curr, next, SeqCst, SeqCst) {
                    Ok(_) => {
                        // The ABA problem doesn't matter here. We only care that the
                        // number of senders never exceeds the maximum.
                        return Self {
                            inner: self.inner.clone(),
                            sender_task: Arc::new(Mutex::new(SenderTask::new())),
                            maybe_parked: false,
                        };
                    }
                    Err(actual) => curr = actual,
                }
            }
        }
    }

    impl<T> Drop for UnboundedSenderInner<T> {
        fn drop(&mut self) {
            // Ordering between variables don't matter here
            let prev = self.inner.num_senders.fetch_sub(1, SeqCst);

            if prev == 1 {
                self.close_channel();
            }
        }
    }

    impl<T> Drop for BoundedSenderInner<T> {
        fn drop(&mut self) {
            // Ordering between variables don't matter here
            let prev = self.inner.num_senders.fetch_sub(1, SeqCst);

            if prev == 1 {
                self.close_channel();
            }
        }
    }

    /*
     *
     * ===== impl Receiver =====
     *
     */

    impl<T> Receiver<T> {
        /// Closes the receiving half of a channel, without dropping it.
        ///
        /// This prevents any further messages from being sent on the channel while
        /// still enabling the receiver to drain messages that are buffered.
        pub fn close(&mut self) {
            if let Some(inner) = &mut self.inner {
                inner.set_closed();

                // Wake up any threads waiting as they'll see that we've closed the
                // channel and will continue on their merry way.
                while let Some(task) = unsafe { inner.parked_queue.pop_spin() } {
                    task.lock().unwrap().notify();
                }
            }
        }

        /// Tries to receive the next message without notifying a context if empty.
        ///
        /// It is not recommended to call this function from inside of a future,
        /// only when you've otherwise arranged to be notified when the channel is
        /// no longer empty.
        ///
        /// This function returns:
        /// * `Ok(Some(t))` when message is fetched
        /// * `Ok(None)` when channel is closed and no messages left in the queue
        /// * `Err(e)` when there are no messages available, but channel is not yet closed
        pub fn try_next(&mut self) -> Result<Option<T>, TryRecvError> {
            match self.next_message() {
                Poll::Ready(msg) => {
                    Ok(msg)
                },
                Poll::Pending => Err(TryRecvError { _priv: () }),
            }
        }

        fn next_message(&mut self) -> Poll<Option<T>> {
            let inner = match self.inner.as_mut() {
                None => return Poll::Ready(None),
                Some(inner) => inner,
            };
            // Pop off a message
            match unsafe { inner.message_queue.pop_spin() } {
                Some(msg) => {
                    // If there are any parked task handles in the parked queue,
                    // pop one and unpark it.
                    self.unpark_one();

                    // Decrement number of messages
                    self.dec_num_messages();

                    Poll::Ready(Some(msg))
                }
                None => {
                    let state = decode_state(inner.state.load(SeqCst));
                    if state.is_closed() {
                        // If closed flag is set AND there are no pending messages
                        // it means end of stream
                        self.inner = None;
                        Poll::Ready(None)
                    } else {
                        // If queue is open, we need to return Pending
                        // to be woken up when new messages arrive.
                        // If queue is closed but num_messages is non-zero,
                        // it means that senders updated the state,
                        // but didn't put message to queue yet,
                        // so we need to park until sender unparks the task
                        // after queueing the message.
                        Poll::Pending
                    }
                }
            }
        }

        // Unpark a single task handle if there is one pending in the parked queue
        fn unpark_one(&mut self) {
            if let Some(inner) = &mut self.inner {
                if let Some(task) = unsafe { inner.parked_queue.pop_spin() } {
                    task.lock().unwrap().notify();
                }
            }
        }

        fn dec_num_messages(&self) {
            if let Some(inner) = &self.inner {
                // OPEN_MASK is highest bit, so it's unaffected by subtraction
                // unless there's underflow, and we know there's no underflow
                // because number of messages at this point is always > 0.
                inner.state.fetch_sub(1, SeqCst);
            }
        }
    }

    // The receiver does not ever take a Pin to the inner T
    impl<T> Unpin for Receiver<T> {}

    impl<T> FusedStream for Receiver<T> {
        fn is_terminated(&self) -> bool {
            self.inner.is_none()
        }
    }

    impl<T> Stream for Receiver<T> {
        type Item = T;

        fn poll_next(
            mut self: Pin<&mut Self>,
            cx: &mut Context<'_>,
        ) -> Poll<Option<T>> {
                // Try to read a message off of the message queue.
            match self.next_message() {
                Poll::Ready(msg) => {
                    if msg.is_none() {
                        self.inner = None;
                    }
                    Poll::Ready(msg)
                },
                Poll::Pending => {
                    // There are no messages to read, in this case, park.
                    self.inner.as_ref().unwrap().recv_task.register(cx.waker());
                    // Check queue again after parking to prevent race condition:
                    // a message could be added to the queue after previous `next_message`
                    // before `register` call.
                    self.next_message()
                }
            }
        }
    }

    impl<T> Drop for Receiver<T> {
        fn drop(&mut self) {
            // Drain the channel of all pending messages
            self.close();
            if self.inner.is_some() {
                loop {
                    match self.next_message() {
                        Poll::Ready(Some(_)) => {}
                        Poll::Ready(None) => break,
                        Poll::Pending => {
                            let state = decode_state(self.inner.as_ref().unwrap().state.load(SeqCst));

                            // If the channel is closed, then there is no need to park.
                            if state.is_closed() {
                                break;
                            }

                            // TODO: Spinning isn't ideal, it might be worth
                            // investigating using a condvar or some other strategy
                            // here. That said, if this case is hit, then another thread
                            // is about to push the value into the queue and this isn't
                            // the only spinlock in the impl right now.
                            thread::yield_now();
                        }
                    }
                }
            }
        }
    }

    impl<T> UnboundedReceiver<T> {
        /// Closes the receiving half of a channel, without dropping it.
        ///
        /// This prevents any further messages from being sent on the channel while
        /// still enabling the receiver to drain messages that are buffered.
        pub fn close(&mut self) {
            if let Some(inner) = &mut self.inner {
                inner.set_closed();
            }
        }

        /// Tries to receive the next message without notifying a context if empty.
        ///
        /// It is not recommended to call this function from inside of a future,
        /// only when you've otherwise arranged to be notified when the channel is
        /// no longer empty.
        ///
        /// This function returns:
        /// * `Ok(Some(t))` when message is fetched
        /// * `Ok(None)` when channel is closed and no messages left in the queue
        /// * `Err(e)` when there are no messages available, but channel is not yet closed
        pub fn try_next(&mut self) -> Result<Option<T>, TryRecvError> {
            match self.next_message() {
                Poll::Ready(msg) => {
                    Ok(msg)
                },
                Poll::Pending => Err(TryRecvError { _priv: () }),
            }
        }

        fn next_message(&mut self) -> Poll<Option<T>> {
            let inner = match self.inner.as_mut() {
                None => return Poll::Ready(None),
                Some(inner) => inner,
            };
            // Pop off a message
            match unsafe { inner.message_queue.pop_spin() } {
                Some(msg) => {
                    // Decrement number of messages
                    self.dec_num_messages();

                    Poll::Ready(Some(msg))
                }
                None => {
                    let state = decode_state(inner.state.load(SeqCst));
                    if state.is_closed() {
                        // If closed flag is set AND there are no pending messages
                        // it means end of stream
                        self.inner = None;
                        Poll::Ready(None)
                    } else {
                        // If queue is open, we need to return Pending
                        // to be woken up when new messages arrive.
                        // If queue is closed but num_messages is non-zero,
                        // it means that senders updated the state,
                        // but didn't put message to queue yet,
                        // so we need to park until sender unparks the task
                        // after queueing the message.
                        Poll::Pending
                    }
                }
            }
        }

        fn dec_num_messages(&self) {
            if let Some(inner) = &self.inner {
                // OPEN_MASK is highest bit, so it's unaffected by subtraction
                // unless there's underflow, and we know there's no underflow
                // because number of messages at this point is always > 0.
                inner.state.fetch_sub(1, SeqCst);
            }
        }
    }

    impl<T> FusedStream for UnboundedReceiver<T> {
        fn is_terminated(&self) -> bool {
            self.inner.is_none()
        }
    }

    impl<T> Stream for UnboundedReceiver<T> {
        type Item = T;

        fn poll_next(
            mut self: Pin<&mut Self>,
            cx: &mut Context<'_>,
        ) -> Poll<Option<T>> {
            // Try to read a message off of the message queue.
            match self.next_message() {
                Poll::Ready(msg) => {
                    if msg.is_none() {
                        self.inner = None;
                    }
                    Poll::Ready(msg)
                },
                Poll::Pending => {
                    // There are no messages to read, in this case, park.
                    self.inner.as_ref().unwrap().recv_task.register(cx.waker());
                    // Check queue again after parking to prevent race condition:
                    // a message could be added to the queue after previous `next_message`
                    // before `register` call.
                    self.next_message()
                }
            }
        }
    }

    impl<T> Drop for UnboundedReceiver<T> {
        fn drop(&mut self) {
            // Drain the channel of all pending messages
            self.close();
            if self.inner.is_some() {
                loop {
                    match self.next_message() {
                        Poll::Ready(Some(_)) => {}
                        Poll::Ready(None) => break,
                        Poll::Pending => {
                            let state = decode_state(self.inner.as_ref().unwrap().state.load(SeqCst));

                            // If the channel is closed, then there is no need to park.
                            if state.is_closed() {
                                break;
                            }

                            // TODO: Spinning isn't ideal, it might be worth
                            // investigating using a condvar or some other strategy
                            // here. That said, if this case is hit, then another thread
                            // is about to push the value into the queue and this isn't
                            // the only spinlock in the impl right now.
                            thread::yield_now();
                        }
                    }
                }
            }
        }
    }

    /*
     *
     * ===== impl Inner =====
     *
     */

    impl<T> UnboundedInner<T> {
        // Clear `open` flag in the state, keep `num_messages` intact.
        fn set_closed(&self) {
            let curr = self.state.load(SeqCst);
            if !decode_state(curr).is_open {
                return;
            }

            self.state.fetch_and(!OPEN_MASK, SeqCst);
        }
    }

    impl<T> BoundedInner<T> {
        // The return value is such that the total number of messages that can be
        // enqueued into the channel will never exceed MAX_CAPACITY
        fn max_senders(&self) -> usize {
            MAX_CAPACITY - self.buffer
        }

        // Clear `open` flag in the state, keep `num_messages` intact.
        fn set_closed(&self) {
            let curr = self.state.load(SeqCst);
            if !decode_state(curr).is_open {
                return;
            }

            self.state.fetch_and(!OPEN_MASK, SeqCst);
        }
    }

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

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

    impl State {
        fn is_closed(&self) -> bool {
            !self.is_open && self.num_messages == 0
        }
    }

    /*
     *
     * ===== Helpers =====
     *
     */

    fn decode_state(num: usize) -> State {
        State {
            is_open: num & OPEN_MASK == OPEN_MASK,
            num_messages: num & MAX_CAPACITY,
        }
    }

    fn encode_state(state: &State) -> usize {
        let mut num = state.num_messages;

        if state.is_open {
            num |= OPEN_MASK;
        }

        num
    }