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    use crate::loom::cell::UnsafeCell;
    use crate::loom::future::AtomicWaker;
    use crate::loom::sync::atomic::AtomicUsize;
    use crate::loom::sync::Arc;
    use crate::sync::mpsc::list;
    use crate::sync::notify::Notify;

    use std::fmt;
    use std::process;
    use std::sync::atomic::Ordering::{AcqRel, Relaxed};
    use std::task::Poll::{Pending, Ready};
    use std::task::{Context, Poll};

    /// Channel sender
    pub(crate) struct Tx<T, S> {
        inner: Arc<Chan<T, S>>,
    }

    impl<T, S: fmt::Debug> fmt::Debug for Tx<T, S> {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("Tx").field("inner", &self.inner).finish()
        }
    }

    /// Channel receiver
    pub(crate) struct Rx<T, S: Semaphore> {
        inner: Arc<Chan<T, S>>,
    }

    impl<T, S: Semaphore + fmt::Debug> fmt::Debug for Rx<T, S> {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("Rx").field("inner", &self.inner).finish()
        }
    }

    pub(crate) trait Semaphore {
        fn is_idle(&self) -> bool;

        fn add_permit(&self);

        fn close(&self);

        fn is_closed(&self) -> bool;
    }

    struct Chan<T, S> {
        /// Notifies all tasks listening for the receiver being dropped
        notify_rx_closed: Notify,

        /// Handle to the push half of the lock-free list.
        tx: list::Tx<T>,

        /// Coordinates access to channel's capacity.
        semaphore: S,

        /// Receiver waker. Notified when a value is pushed into the channel.
        rx_waker: AtomicWaker,

        /// Tracks the number of outstanding sender handles.
        ///
        /// When this drops to zero, the send half of the channel is closed.
        tx_count: AtomicUsize,

        /// Only accessed by `Rx` handle.
        rx_fields: UnsafeCell<RxFields<T>>,
    }

    impl<T, S> fmt::Debug for Chan<T, S>
    where
        S: fmt::Debug,
    {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("Chan")
                .field("tx", &self.tx)
                .field("semaphore", &self.semaphore)
                .field("rx_waker", &self.rx_waker)
                .field("tx_count", &self.tx_count)
                .field("rx_fields", &"...")
                .finish()
        }
    }

    /// Fields only accessed by `Rx` handle.
    struct RxFields<T> {
        /// Channel receiver. This field is only accessed by the `Receiver` type.
        list: list::Rx<T>,

        /// `true` if `Rx::close` is called.
        rx_closed: bool,
    }

    impl<T> fmt::Debug for RxFields<T> {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("RxFields")
                .field("list", &self.list)
                .field("rx_closed", &self.rx_closed)
                .finish()
        }
    }

    unsafe impl<T: Send, S: Send> Send for Chan<T, S> {}
    unsafe impl<T: Send, S: Sync> Sync for Chan<T, S> {}

    pub(crate) fn channel<T, S: Semaphore>(semaphore: S) -> (Tx<T, S>, Rx<T, S>) {
        let (tx, rx) = list::channel();

        let chan = Arc::new(Chan {
            notify_rx_closed: Notify::new(),
            tx,
            semaphore,
            rx_waker: AtomicWaker::new(),
            tx_count: AtomicUsize::new(1),
            rx_fields: UnsafeCell::new(RxFields {
                list: rx,
                rx_closed: false,
            }),
        });

        (Tx::new(chan.clone()), Rx::new(chan))
    }

    // ===== impl Tx =====

    impl<T, S> Tx<T, S> {
        fn new(chan: Arc<Chan<T, S>>) -> Tx<T, S> {
            Tx { inner: chan }
        }

        pub(super) fn semaphore(&self) -> &S {
            &self.inner.semaphore
        }

        /// Send a message and notify the receiver.
        pub(crate) fn send(&self, value: T) {
            self.inner.send(value);
        }

        /// Wake the receive half
        pub(crate) fn wake_rx(&self) {
            self.inner.rx_waker.wake();
        }

        /// Returns `true` if senders belong to the same channel.
        pub(crate) fn same_channel(&self, other: &Self) -> bool {
            Arc::ptr_eq(&self.inner, &other.inner)
        }
    }

    impl<T, S: Semaphore> Tx<T, S> {
        pub(crate) fn is_closed(&self) -> bool {
            self.inner.semaphore.is_closed()
        }

        pub(crate) async fn closed(&self) {
            // In order to avoid a race condition, we first request a notification,
            // **then** check whether the semaphore is closed. If the semaphore is
            // closed the notification request is dropped.
            let notified = self.inner.notify_rx_closed.notified();

            if self.inner.semaphore.is_closed() {
                return;
            }
            notified.await;
        }
    }

    impl<T, S> Clone for Tx<T, S> {
        fn clone(&self) -> Tx<T, S> {
            // Using a Relaxed ordering here is sufficient as the caller holds a
            // strong ref to `self`, preventing a concurrent decrement to zero.
            self.inner.tx_count.fetch_add(1, Relaxed);

            Tx {
                inner: self.inner.clone(),
            }
        }
    }

    impl<T, S> Drop for Tx<T, S> {
        fn drop(&mut self) {
            if self.inner.tx_count.fetch_sub(1, AcqRel) != 1 {
                return;
            }

            // Close the list, which sends a `Close` message
            self.inner.tx.close();

            // Notify the receiver
            self.wake_rx();
        }
    }

    // ===== impl Rx =====

    impl<T, S: Semaphore> Rx<T, S> {
        fn new(chan: Arc<Chan<T, S>>) -> Rx<T, S> {
            Rx { inner: chan }
        }

        pub(crate) fn close(&mut self) {
            self.inner.rx_fields.with_mut(|rx_fields_ptr| {
                let rx_fields = unsafe { &mut *rx_fields_ptr };

                if rx_fields.rx_closed {
                    return;
                }

                rx_fields.rx_closed = true;
            });

            self.inner.semaphore.close();
            self.inner.notify_rx_closed.notify_waiters();
        }

        /// Receive the next value
        pub(crate) fn recv(&mut self, cx: &mut Context<'_>) -> Poll<Option<T>> {
            use super::block::Read::*;

            // Keep track of task budget
            let coop = ready!(crate::coop::poll_proceed(cx));

            self.inner.rx_fields.with_mut(|rx_fields_ptr| {
                let rx_fields = unsafe { &mut *rx_fields_ptr };

                macro_rules! try_recv {
                    () => {
                        match rx_fields.list.pop(&self.inner.tx) {
                            Some(Value(value)) => {
                                self.inner.semaphore.add_permit();
                                coop.made_progress();
                                return Ready(Some(value));
                            }
                            Some(Closed) => {
                                // TODO: This check may not be required as it most
                                // likely can only return `true` at this point. A
                                // channel is closed when all tx handles are
                                // dropped. Dropping a tx handle releases memory,
                                // which ensures that if dropping the tx handle is
                                // visible, then all messages sent are also visible.
                                assert!(self.inner.semaphore.is_idle());
                                coop.made_progress();
                                return Ready(None);
                            }
                            None => {} // fall through
                        }
                    };
                }

                try_recv!();

                self.inner.rx_waker.register_by_ref(cx.waker());

                // It is possible that a value was pushed between attempting to read
                // and registering the task, so we have to check the channel a
                // second time here.
                try_recv!();

                if rx_fields.rx_closed && self.inner.semaphore.is_idle() {
                    coop.made_progress();
                    Ready(None)
                } else {
                    Pending
                }
            })
        }
    }

    feature! {
        #![all(unix, any(feature = "signal", feature = "process"))]

        use crate::sync::mpsc::error::TryRecvError;

        impl<T, S: Semaphore> Rx<T, S> {
            /// Receives the next value without blocking
            pub(crate) fn try_recv(&mut self) -> Result<T, TryRecvError> {
                use super::block::Read::*;
                self.inner.rx_fields.with_mut(|rx_fields_ptr| {
                    let rx_fields = unsafe { &mut *rx_fields_ptr };
                    match rx_fields.list.pop(&self.inner.tx) {
                        Some(Value(value)) => {
                            self.inner.semaphore.add_permit();
                            Ok(value)
                        }
                        Some(Closed) => Err(TryRecvError::Closed),
                        None => Err(TryRecvError::Empty),
                    }
                })
            }
        }
    }

    impl<T, S: Semaphore> Drop for Rx<T, S> {
        fn drop(&mut self) {
            use super::block::Read::Value;

            self.close();

            self.inner.rx_fields.with_mut(|rx_fields_ptr| {
                let rx_fields = unsafe { &mut *rx_fields_ptr };

                while let Some(Value(_)) = rx_fields.list.pop(&self.inner.tx) {
                    self.inner.semaphore.add_permit();
                }
            })
        }
    }

    // ===== impl Chan =====

    impl<T, S> Chan<T, S> {
        fn send(&self, value: T) {
            // Push the value
            self.tx.push(value);

            // Notify the rx task
            self.rx_waker.wake();
        }
    }

    impl<T, S> Drop for Chan<T, S> {
        fn drop(&mut self) {
            use super::block::Read::Value;

            // Safety: the only owner of the rx fields is Chan, and eing
            // inside its own Drop means we're the last ones to touch it.
            self.rx_fields.with_mut(|rx_fields_ptr| {
                let rx_fields = unsafe { &mut *rx_fields_ptr };

                while let Some(Value(_)) = rx_fields.list.pop(&self.tx) {}
                unsafe { rx_fields.list.free_blocks() };
            });
        }
    }

    // ===== impl Semaphore for (::Semaphore, capacity) =====

    impl Semaphore for (crate::sync::batch_semaphore::Semaphore, usize) {
        fn add_permit(&self) {
            self.0.release(1)
        }

        fn is_idle(&self) -> bool {
            self.0.available_permits() == self.1
        }

        fn close(&self) {
            self.0.close();
        }

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

    // ===== impl Semaphore for AtomicUsize =====

    use std::sync::atomic::Ordering::{Acquire, Release};
    use std::usize;

    impl Semaphore for AtomicUsize {
        fn add_permit(&self) {
            let prev = self.fetch_sub(2, Release);

            if prev >> 1 == 0 {
                // Something went wrong
                process::abort();
            }
        }

        fn is_idle(&self) -> bool {
            self.load(Acquire) >> 1 == 0
        }

        fn close(&self) {
            self.fetch_or(1, Release);
        }

        fn is_closed(&self) -> bool {
            self.load(Acquire) & 1 == 1
        }
    }