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    ///! Reference counter for channels.

    use std::isize;
    use std::ops;
    use std::process;
    use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};

    /// Reference counter internals.
    struct Counter<C> {
        /// The number of senders associated with the channel.
        senders: AtomicUsize,

        /// The number of receivers associated with the channel.
        receivers: AtomicUsize,

        /// If `true`, either the sending or receiving side has been dropped.
        disconnected: AtomicBool,

        /// The internal channel.
        chan: C,
    }

    /// Wraps a channel into the reference counter.
    pub fn new<C>(chan: C) -> (Sender<C>, Receiver<C>) {
        let counter = Box::into_raw(Box::new(Counter {
            senders: AtomicUsize::new(1),
            receivers: AtomicUsize::new(1),
            disconnected: AtomicBool::new(false),
            chan,
        }));
        let s = Sender { counter };
        let r = Receiver { counter };
        (s, r)
    }

    /// The sending side.
    pub struct Sender<C> {
        counter: *mut Counter<C>,
    }

    impl<C> Sender<C> {
        /// Returns the internal `Counter`.
        fn counter(&self) -> &Counter<C> {
            unsafe { &*self.counter }
        }

        /// Acquires another sender reference.
        pub fn acquire(&self) -> Sender<C> {
            let count = self.counter().senders.fetch_add(1, Ordering::Relaxed);

            // Cloning senders and calling `mem::forget` on the clones could potentially overflow the
            // counter. It's very difficult to recover sensibly from such degenerate scenarios so we
            // just abort when the count becomes very large.
            if count > isize::MAX as usize {
                process::abort();
            }

            Sender {
                counter: self.counter,
            }
        }

        /// Releases the sender reference.
        ///
        /// Function `f` will be called if this is the last sender reference.
        pub unsafe fn release<F: FnOnce(&C)>(&self, f: F) {
            if self.counter().senders.fetch_sub(1, Ordering::AcqRel) == 1 {
                f(&self.counter().chan);

                if self.counter().disconnected.swap(true, Ordering::AcqRel) {
                    drop(Box::from_raw(self.counter));
                }
            }
        }
    }

    impl<C> ops::Deref for Sender<C> {
        type Target = C;

        fn deref(&self) -> &C {
            &self.counter().chan
        }
    }

    /// The receiving side.
    pub struct Receiver<C> {
        counter: *mut Counter<C>,
    }

    impl<C> Receiver<C> {
        /// Returns the internal `Counter`.
        fn counter(&self) -> &Counter<C> {
            unsafe { &*self.counter }
        }

        /// Acquires another receiver reference.
        pub fn acquire(&self) -> Receiver<C> {
            let count = self.counter().receivers.fetch_add(1, Ordering::Relaxed);

            // Cloning receivers and calling `mem::forget` on the clones could potentially overflow the
            // counter. It's very difficult to recover sensibly from such degenerate scenarios so we
            // just abort when the count becomes very large.
            if count > isize::MAX as usize {
                process::abort();
            }

            Receiver {
                counter: self.counter,
            }
        }

        /// Releases the receiver reference.
        ///
        /// Function `f` will be called if this is the last receiver reference.
        pub unsafe fn release<F: FnOnce(&C)>(&self, f: F) {
            if self.counter().receivers.fetch_sub(1, Ordering::AcqRel) == 1 {
                f(&self.counter().chan);

                if self.counter().disconnected.swap(true, Ordering::AcqRel) {
                    drop(Box::from_raw(self.counter));
                }
            }
        }
    }

    impl<C> ops::Deref for Receiver<C> {
        type Target = C;

        fn deref(&self) -> &C {
            &self.counter().chan
        }
    }

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