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    // Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! Multi-producer, single-consumer FIFO queue communication primitives.
    //!
    //! This module provides message-based communication over channels, concretely
    //! defined among three types:
    //!
    //! * `Sender`
    //! * `SyncSender`
    //! * `Receiver`
    //!
    //! A `Sender` or `SyncSender` is used to send data to a `Receiver`. Both
    //! senders are clone-able (multi-producer) such that many threads can send
    //! simultaneously to one receiver (single-consumer).
    //!
    //! These channels come in two flavors:
    //!
    //! 1. An asynchronous, infinitely buffered channel. The `channel()` function
    //!    will return a `(Sender, Receiver)` tuple where all sends will be
    //!    **asynchronous** (they never block). The channel conceptually has an
    //!    infinite buffer.
    //!
    //! 2. A synchronous, bounded channel. The `sync_channel()` function will return
    //!    a `(SyncSender, Receiver)` tuple where the storage for pending messages
    //!    is a pre-allocated buffer of a fixed size. All sends will be
    //!    **synchronous** by blocking until there is buffer space available. Note
    //!    that a bound of 0 is allowed, causing the channel to become a
    //!    "rendezvous" channel where each sender atomically hands off a message to
    //!    a receiver.
    //!
    //! ## Disconnection
    //!
    //! The send and receive operations on channels will all return a `Result`
    //! indicating whether the operation succeeded or not. An unsuccessful operation
    //! is normally indicative of the other half of a channel having "hung up" by
    //! being dropped in its corresponding thread.
    //!
    //! Once half of a channel has been deallocated, most operations can no longer
    //! continue to make progress, so `Err` will be returned. Many applications will
    //! continue to `unwrap()` the results returned from this module, instigating a
    //! propagation of failure among threads if one unexpectedly dies.
    //!
    //! # Examples
    //!
    //! Simple usage:
    //!
    //! ```
    //! use std::thread;
    //! use std::sync::mpsc::channel;
    //!
    //! // Create a simple streaming channel
    //! let (tx, rx) = channel();
    //! thread::spawn(move|| {
    //!     tx.send(10).unwrap();
    //! });
    //! assert_eq!(rx.recv().unwrap(), 10);
    //! ```
    //!
    //! Shared usage:
    //!
    //! ```
    //! use std::thread;
    //! use std::sync::mpsc::channel;
    //!
    //! // Create a shared channel that can be sent along from many threads
    //! // where tx is the sending half (tx for transmission), and rx is the receiving
    //! // half (rx for receiving).
    //! let (tx, rx) = channel();
    //! for i in 0..10 {
    //!     let tx = tx.clone();
    //!     thread::spawn(move|| {
    //!         tx.send(i).unwrap();
    //!     });
    //! }
    //!
    //! for _ in 0..10 {
    //!     let j = rx.recv().unwrap();
    //!     assert!(0 <= j && j < 10);
    //! }
    //! ```
    //!
    //! Propagating panics:
    //!
    //! ```
    //! use std::sync::mpsc::channel;
    //!
    //! // The call to recv() will return an error because the channel has already
    //! // hung up (or been deallocated)
    //! let (tx, rx) = channel::<i32>();
    //! drop(tx);
    //! assert!(rx.recv().is_err());
    //! ```
    //!
    //! Synchronous channels:
    //!
    //! ```
    //! use std::thread;
    //! use std::sync::mpsc::sync_channel;
    //!
    //! let (tx, rx) = sync_channel::<i32>(0);
    //! thread::spawn(move|| {
    //!     // This will wait for the parent thread to start receiving
    //!     tx.send(53).unwrap();
    //! });
    //! rx.recv().unwrap();
    //! ```

    #![stable(feature = "rust1", since = "1.0.0")]

    // A description of how Rust's channel implementation works
    //
    // Channels are supposed to be the basic building block for all other
    // concurrent primitives that are used in Rust. As a result, the channel type
    // needs to be highly optimized, flexible, and broad enough for use everywhere.
    //
    // The choice of implementation of all channels is to be built on lock-free data
    // structures. The channels themselves are then consequently also lock-free data
    // structures. As always with lock-free code, this is a very "here be dragons"
    // territory, especially because I'm unaware of any academic papers that have
    // gone into great length about channels of these flavors.
    //
    // ## Flavors of channels
    //
    // From the perspective of a consumer of this library, there is only one flavor
    // of channel. This channel can be used as a stream and cloned to allow multiple
    // senders. Under the hood, however, there are actually three flavors of
    // channels in play.
    //
    // * Flavor::Oneshots - these channels are highly optimized for the one-send use case.
    //              They contain as few atomics as possible and involve one and
    //              exactly one allocation.
    // * Streams - these channels are optimized for the non-shared use case. They
    //             use a different concurrent queue that is more tailored for this
    //             use case. The initial allocation of this flavor of channel is not
    //             optimized.
    // * Shared - this is the most general form of channel that this module offers,
    //            a channel with multiple senders. This type is as optimized as it
    //            can be, but the previous two types mentioned are much faster for
    //            their use-cases.
    //
    // ## Concurrent queues
    //
    // The basic idea of Rust's Sender/Receiver types is that send() never blocks, but
    // recv() obviously blocks. This means that under the hood there must be some
    // shared and concurrent queue holding all of the actual data.
    //
    // With two flavors of channels, two flavors of queues are also used. We have
    // chosen to use queues from a well-known author that are abbreviated as SPSC
    // and MPSC (single producer, single consumer and multiple producer, single
    // consumer). SPSC queues are used for streams while MPSC queues are used for
    // shared channels.
    //
    // ### SPSC optimizations
    //
    // The SPSC queue found online is essentially a linked list of nodes where one
    // half of the nodes are the "queue of data" and the other half of nodes are a
    // cache of unused nodes. The unused nodes are used such that an allocation is
    // not required on every push() and a free doesn't need to happen on every
    // pop().
    //
    // As found online, however, the cache of nodes is of an infinite size. This
    // means that if a channel at one point in its life had 50k items in the queue,
    // then the queue will always have the capacity for 50k items. I believed that
    // this was an unnecessary limitation of the implementation, so I have altered
    // the queue to optionally have a bound on the cache size.
    //
    // By default, streams will have an unbounded SPSC queue with a small-ish cache
    // size. The hope is that the cache is still large enough to have very fast
    // send() operations while not too large such that millions of channels can
    // coexist at once.
    //
    // ### MPSC optimizations
    //
    // Right now the MPSC queue has not been optimized. Like the SPSC queue, it uses
    // a linked list under the hood to earn its unboundedness, but I have not put
    // forth much effort into having a cache of nodes similar to the SPSC queue.
    //
    // For now, I believe that this is "ok" because shared channels are not the most
    // common type, but soon we may wish to revisit this queue choice and determine
    // another candidate for backend storage of shared channels.
    //
    // ## Overview of the Implementation
    //
    // Now that there's a little background on the concurrent queues used, it's
    // worth going into much more detail about the channels themselves. The basic
    // pseudocode for a send/recv are:
    //
    //
    //      send(t)                             recv()
    //        queue.push(t)                       return if queue.pop()
    //        if increment() == -1                deschedule {
    //          wakeup()                            if decrement() > 0
    //                                                cancel_deschedule()
    //                                            }
    //                                            queue.pop()
    //
    // As mentioned before, there are no locks in this implementation, only atomic
    // instructions are used.
    //
    // ### The internal atomic counter
    //
    // Every channel has a shared counter with each half to keep track of the size
    // of the queue. This counter is used to abort descheduling by the receiver and
    // to know when to wake up on the sending side.
    //
    // As seen in the pseudocode, senders will increment this count and receivers
    // will decrement the count. The theory behind this is that if a sender sees a
    // -1 count, it will wake up the receiver, and if the receiver sees a 1+ count,
    // then it doesn't need to block.
    //
    // The recv() method has a beginning call to pop(), and if successful, it needs
    // to decrement the count. It is a crucial implementation detail that this
    // decrement does *not* happen to the shared counter. If this were the case,
    // then it would be possible for the counter to be very negative when there were
    // no receivers waiting, in which case the senders would have to determine when
    // it was actually appropriate to wake up a receiver.
    //
    // Instead, the "steal count" is kept track of separately (not atomically
    // because it's only used by receivers), and then the decrement() call when
    // descheduling will lump in all of the recent steals into one large decrement.
    //
    // The implication of this is that if a sender sees a -1 count, then there's
    // guaranteed to be a waiter waiting!
    //
    // ## Native Implementation
    //
    // A major goal of these channels is to work seamlessly on and off the runtime.
    // All of the previous race conditions have been worded in terms of
    // scheduler-isms (which is obviously not available without the runtime).
    //
    // For now, native usage of channels (off the runtime) will fall back onto
    // mutexes/cond vars for descheduling/atomic decisions. The no-contention path
    // is still entirely lock-free, the "deschedule" blocks above are surrounded by
    // a mutex and the "wakeup" blocks involve grabbing a mutex and signaling on a
    // condition variable.
    //
    // ## Select
    //
    // Being able to support selection over channels has greatly influenced this
    // design, and not only does selection need to work inside the runtime, but also
    // outside the runtime.
    //
    // The implementation is fairly straightforward. The goal of select() is not to
    // return some data, but only to return which channel can receive data without
    // blocking. The implementation is essentially the entire blocking procedure
    // followed by an increment as soon as its woken up. The cancellation procedure
    // involves an increment and swapping out of to_wake to acquire ownership of the
    // thread to unblock.
    //
    // Sadly this current implementation requires multiple allocations, so I have
    // seen the throughput of select() be much worse than it should be. I do not
    // believe that there is anything fundamental that needs to change about these
    // channels, however, in order to support a more efficient select().
    //
    // # Conclusion
    //
    // And now that you've seen all the races that I found and attempted to fix,
    // here's the code for you to find some more!

    use sync::Arc;
    use error;
    use fmt;
    use mem;
    use cell::UnsafeCell;
    use marker::Reflect;

    #[unstable(feature = "mpsc_select", issue = "27800")]
    pub use self::select::{Select, Handle};
    use self::select::StartResult;
    use self::select::StartResult::*;
    use self::blocking::SignalToken;

    mod blocking;
    mod oneshot;
    mod select;
    mod shared;
    mod stream;
    mod sync;
    mod mpsc_queue;
    mod spsc_queue;

    /// The receiving-half of Rust's channel type. This half can only be owned by
    /// one thread
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Receiver<T> {
        inner: UnsafeCell<Flavor<T>>,
    }

    // The receiver port can be sent from place to place, so long as it
    // is not used to receive non-sendable things.
    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<T: Send> Send for Receiver<T> { }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> !Sync for Receiver<T> { }

    /// An iterator over messages on a receiver, this iterator will block
    /// whenever `next` is called, waiting for a new message, and `None` will be
    /// returned when the corresponding channel has hung up.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Iter<'a, T: 'a> {
        rx: &'a Receiver<T>
    }

    /// An owning iterator over messages on a receiver, this iterator will block
    /// whenever `next` is called, waiting for a new message, and `None` will be
    /// returned when the corresponding channel has hung up.
    #[stable(feature = "receiver_into_iter", since = "1.1.0")]
    pub struct IntoIter<T> {
        rx: Receiver<T>
    }

    /// The sending-half of Rust's asynchronous channel type. This half can only be
    /// owned by one thread, but it can be cloned to send to other threads.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Sender<T> {
        inner: UnsafeCell<Flavor<T>>,
    }

    // The send port can be sent from place to place, so long as it
    // is not used to send non-sendable things.
    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<T: Send> Send for Sender<T> { }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> !Sync for Sender<T> { }

    /// The sending-half of Rust's synchronous channel type. This half can only be
    /// owned by one thread, but it can be cloned to send to other threads.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct SyncSender<T> {
        inner: Arc<UnsafeCell<sync::Packet<T>>>,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<T: Send> Send for SyncSender<T> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> !Sync for SyncSender<T> {}

    /// An error returned from the `send` function on channels.
    ///
    /// A `send` operation can only fail if the receiving end of a channel is
    /// disconnected, implying that the data could never be received. The error
    /// contains the data being sent as a payload so it can be recovered.
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(PartialEq, Eq, Clone, Copy)]
    pub struct SendError<T>(#[stable(feature = "rust1", since = "1.0.0")] pub T);

    /// An error returned from the `recv` function on a `Receiver`.
    ///
    /// The `recv` operation can only fail if the sending half of a channel is
    /// disconnected, implying that no further messages will ever be received.
    #[derive(PartialEq, Eq, Clone, Copy, Debug)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct RecvError;

    /// This enumeration is the list of the possible reasons that `try_recv` could
    /// not return data when called.
    #[derive(PartialEq, Eq, Clone, Copy, Debug)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub enum TryRecvError {
        /// This channel is currently empty, but the sender(s) have not yet
        /// disconnected, so data may yet become available.
        #[stable(feature = "rust1", since = "1.0.0")]
        Empty,

        /// This channel's sending half has become disconnected, and there will
        /// never be any more data received on this channel
        #[stable(feature = "rust1", since = "1.0.0")]
        Disconnected,
    }

    /// This enumeration is the list of the possible error outcomes for the
    /// `SyncSender::try_send` method.
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(PartialEq, Eq, Clone, Copy)]
    pub enum TrySendError<T> {
        /// The data could not be sent on the channel because it would require that
        /// the callee block to send the data.
        ///
        /// If this is a buffered channel, then the buffer is full at this time. If
        /// this is not a buffered channel, then there is no receiver available to
        /// acquire the data.
        #[stable(feature = "rust1", since = "1.0.0")]
        Full(#[stable(feature = "rust1", since = "1.0.0")] T),

        /// This channel's receiving half has disconnected, so the data could not be
        /// sent. The data is returned back to the callee in this case.
        #[stable(feature = "rust1", since = "1.0.0")]
        Disconnected(#[stable(feature = "rust1", since = "1.0.0")] T),
    }

    enum Flavor<T> {
        Oneshot(Arc<UnsafeCell<oneshot::Packet<T>>>),
        Stream(Arc<UnsafeCell<stream::Packet<T>>>),
        Shared(Arc<UnsafeCell<shared::Packet<T>>>),
        Sync(Arc<UnsafeCell<sync::Packet<T>>>),
    }

    #[doc(hidden)]
    trait UnsafeFlavor<T> {
        fn inner_unsafe(&self) -> &UnsafeCell<Flavor<T>>;
        unsafe fn inner_mut(&self) -> &mut Flavor<T> {
            &mut *self.inner_unsafe().get()
        }
        unsafe fn inner(&self) -> &Flavor<T> {
            &*self.inner_unsafe().get()
        }
    }
    impl<T> UnsafeFlavor<T> for Sender<T> {
        fn inner_unsafe(&self) -> &UnsafeCell<Flavor<T>> {
            &self.inner
        }
    }
    impl<T> UnsafeFlavor<T> for Receiver<T> {
        fn inner_unsafe(&self) -> &UnsafeCell<Flavor<T>> {
            &self.inner
        }
    }

    /// Creates a new asynchronous channel, returning the sender/receiver halves.
    ///
    /// All data sent on the sender will become available on the receiver, and no
    /// send will block the calling thread (this channel has an "infinite buffer").
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::mpsc::channel;
    /// use std::thread;
    ///
    /// // tx is the sending half (tx for transmission), and rx is the receiving
    /// // half (rx for receiving).
    /// let (tx, rx) = channel();
    ///
    /// // Spawn off an expensive computation
    /// thread::spawn(move|| {
    /// #   fn expensive_computation() {}
    ///     tx.send(expensive_computation()).unwrap();
    /// });
    ///
    /// // Do some useful work for awhile
    ///
    /// // Let's see what that answer was
    /// println!("{:?}", rx.recv().unwrap());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
        let a = Arc::new(UnsafeCell::new(oneshot::Packet::new()));
        (Sender::new(Flavor::Oneshot(a.clone())), Receiver::new(Flavor::Oneshot(a)))
    }

    /// Creates a new synchronous, bounded channel.
    ///
    /// Like asynchronous channels, the `Receiver` will block until a message
    /// becomes available. These channels differ greatly in the semantics of the
    /// sender from asynchronous channels, however.
    ///
    /// This channel has an internal buffer on which messages will be queued. When
    /// the internal buffer becomes full, future sends will *block* waiting for the
    /// buffer to open up. Note that a buffer size of 0 is valid, in which case this
    /// becomes  "rendezvous channel" where each send will not return until a recv
    /// is paired with it.
    ///
    /// As with asynchronous channels, all senders will panic in `send` if the
    /// `Receiver` has been destroyed.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::mpsc::sync_channel;
    /// use std::thread;
    ///
    /// let (tx, rx) = sync_channel(1);
    ///
    /// // this returns immediately
    /// tx.send(1).unwrap();
    ///
    /// thread::spawn(move|| {
    ///     // this will block until the previous message has been received
    ///     tx.send(2).unwrap();
    /// });
    ///
    /// assert_eq!(rx.recv().unwrap(), 1);
    /// assert_eq!(rx.recv().unwrap(), 2);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn sync_channel<T>(bound: usize) -> (SyncSender<T>, Receiver<T>) {
        let a = Arc::new(UnsafeCell::new(sync::Packet::new(bound)));
        (SyncSender::new(a.clone()), Receiver::new(Flavor::Sync(a)))
    }

    ////////////////////////////////////////////////////////////////////////////////
    // Sender
    ////////////////////////////////////////////////////////////////////////////////

    impl<T> Sender<T> {
        fn new(inner: Flavor<T>) -> Sender<T> {
            Sender {
                inner: UnsafeCell::new(inner),
            }
        }

        /// Attempts to send a value on this channel, returning it back if it could
        /// not be sent.
        ///
        /// 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`.
        ///
        /// This method will never block the current thread.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::mpsc::channel;
        ///
        /// let (tx, rx) = channel();
        ///
        /// // This send is always successful
        /// tx.send(1).unwrap();
        ///
        /// // This send will fail because the receiver is gone
        /// drop(rx);
        /// assert_eq!(tx.send(1).err().unwrap().0, 1);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn send(&self, t: T) -> Result<(), SendError<T>> {
            let (new_inner, ret) = match *unsafe { self.inner() } {
                Flavor::Oneshot(ref p) => {
                    unsafe {
                        let p = p.get();
                        if !(*p).sent() {
                            return (*p).send(t).map_err(SendError);
                        } else {
                            let a =
                                Arc::new(UnsafeCell::new(stream::Packet::new()));
                            let rx = Receiver::new(Flavor::Stream(a.clone()));
                            match (*p).upgrade(rx) {
                                oneshot::UpSuccess => {
                                    let ret = (*a.get()).send(t);
                                    (a, ret)
                                }
                                oneshot::UpDisconnected => (a, Err(t)),
                                oneshot::UpWoke(token) => {
                                    // This send cannot panic because the thread is
                                    // asleep (we're looking at it), so the receiver
                                    // can't go away.
                                    (*a.get()).send(t).ok().unwrap();
                                    token.signal();
                                    (a, Ok(()))
                                }
                            }
                        }
                    }
                }
                Flavor::Stream(ref p) => return unsafe {
                    (*p.get()).send(t).map_err(SendError)
                },
                Flavor::Shared(ref p) => return unsafe {
                    (*p.get()).send(t).map_err(SendError)
                },
                Flavor::Sync(..) => unreachable!(),
            };

            unsafe {
                let tmp = Sender::new(Flavor::Stream(new_inner));
                mem::swap(self.inner_mut(), tmp.inner_mut());
            }
            ret.map_err(SendError)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Clone for Sender<T> {
        fn clone(&self) -> Sender<T> {
            let (packet, sleeper, guard) = match *unsafe { self.inner() } {
                Flavor::Oneshot(ref p) => {
                    let a = Arc::new(UnsafeCell::new(shared::Packet::new()));
                    unsafe {
                        let guard = (*a.get()).postinit_lock();
                        let rx = Receiver::new(Flavor::Shared(a.clone()));
                        match (*p.get()).upgrade(rx) {
                            oneshot::UpSuccess |
                            oneshot::UpDisconnected => (a, None, guard),
                            oneshot::UpWoke(task) => (a, Some(task), guard)
                        }
                    }
                }
                Flavor::Stream(ref p) => {
                    let a = Arc::new(UnsafeCell::new(shared::Packet::new()));
                    unsafe {
                        let guard = (*a.get()).postinit_lock();
                        let rx = Receiver::new(Flavor::Shared(a.clone()));
                        match (*p.get()).upgrade(rx) {
                            stream::UpSuccess |
                            stream::UpDisconnected => (a, None, guard),
                            stream::UpWoke(task) => (a, Some(task), guard),
                        }
                    }
                }
                Flavor::Shared(ref p) => {
                    unsafe { (*p.get()).clone_chan(); }
                    return Sender::new(Flavor::Shared(p.clone()));
                }
                Flavor::Sync(..) => unreachable!(),
            };

            unsafe {
                (*packet.get()).inherit_blocker(sleeper, guard);

                let tmp = Sender::new(Flavor::Shared(packet.clone()));
                mem::swap(self.inner_mut(), tmp.inner_mut());
            }
            Sender::new(Flavor::Shared(packet))
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Drop for Sender<T> {
        fn drop(&mut self) {
            match *unsafe { self.inner_mut() } {
                Flavor::Oneshot(ref mut p) => unsafe { (*p.get()).drop_chan(); },
                Flavor::Stream(ref mut p) => unsafe { (*p.get()).drop_chan(); },
                Flavor::Shared(ref mut p) => unsafe { (*p.get()).drop_chan(); },
                Flavor::Sync(..) => unreachable!(),
            }
        }
    }

    #[stable(feature = "mpsc_debug", since = "1.7.0")]
    impl<T> fmt::Debug for Sender<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "Sender {{ .. }}")
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // SyncSender
    ////////////////////////////////////////////////////////////////////////////////

    impl<T> SyncSender<T> {
        fn new(inner: Arc<UnsafeCell<sync::Packet<T>>>) -> SyncSender<T> {
            SyncSender { inner: inner }
        }

        /// Sends a value on this synchronous channel.
        ///
        /// This function will *block* until space in the internal buffer becomes
        /// available or a receiver is available to hand off the message to.
        ///
        /// Note that a successful send does *not* guarantee that the receiver will
        /// ever see the data if there is a buffer on this channel. Items may be
        /// enqueued in the internal buffer for the receiver to receive at a later
        /// time. If the buffer size is 0, however, it can be guaranteed that the
        /// receiver has indeed received the data if this function returns success.
        ///
        /// This function will never panic, but it may return `Err` if the
        /// `Receiver` has disconnected and is no longer able to receive
        /// information.
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn send(&self, t: T) -> Result<(), SendError<T>> {
            unsafe { (*self.inner.get()).send(t).map_err(SendError) }
        }

        /// Attempts to send a value on this channel without blocking.
        ///
        /// This method differs from `send` by returning immediately if the
        /// channel's buffer is full or no receiver is waiting to acquire some
        /// data. Compared with `send`, this function has two failure cases
        /// instead of one (one for disconnection, one for a full buffer).
        ///
        /// See `SyncSender::send` for notes about guarantees of whether the
        /// receiver has received the data or not if this function is successful.
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn try_send(&self, t: T) -> Result<(), TrySendError<T>> {
            unsafe { (*self.inner.get()).try_send(t) }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Clone for SyncSender<T> {
        fn clone(&self) -> SyncSender<T> {
            unsafe { (*self.inner.get()).clone_chan(); }
            SyncSender::new(self.inner.clone())
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Drop for SyncSender<T> {
        fn drop(&mut self) {
            unsafe { (*self.inner.get()).drop_chan(); }
        }
    }

    #[stable(feature = "mpsc_debug", since = "1.7.0")]
    impl<T> fmt::Debug for SyncSender<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "SyncSender {{ .. }}")
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // Receiver
    ////////////////////////////////////////////////////////////////////////////////

    impl<T> Receiver<T> {
        fn new(inner: Flavor<T>) -> Receiver<T> {
            Receiver { inner: UnsafeCell::new(inner) }
        }

        /// Attempts to return a pending value on this receiver without blocking
        ///
        /// This method will never block the caller in order to wait for data to
        /// become available. Instead, this will always return immediately with a
        /// possible option of pending data on the channel.
        ///
        /// This is useful for a flavor of "optimistic check" before deciding to
        /// block on a receiver.
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn try_recv(&self) -> Result<T, TryRecvError> {
            loop {
                let new_port = match *unsafe { self.inner() } {
                    Flavor::Oneshot(ref p) => {
                        match unsafe { (*p.get()).try_recv() } {
                            Ok(t) => return Ok(t),
                            Err(oneshot::Empty) => return Err(TryRecvError::Empty),
                            Err(oneshot::Disconnected) => {
                                return Err(TryRecvError::Disconnected)
                            }
                            Err(oneshot::Upgraded(rx)) => rx,
                        }
                    }
                    Flavor::Stream(ref p) => {
                        match unsafe { (*p.get()).try_recv() } {
                            Ok(t) => return Ok(t),
                            Err(stream::Empty) => return Err(TryRecvError::Empty),
                            Err(stream::Disconnected) => {
                                return Err(TryRecvError::Disconnected)
                            }
                            Err(stream::Upgraded(rx)) => rx,
                        }
                    }
                    Flavor::Shared(ref p) => {
                        match unsafe { (*p.get()).try_recv() } {
                            Ok(t) => return Ok(t),
                            Err(shared::Empty) => return Err(TryRecvError::Empty),
                            Err(shared::Disconnected) => {
                                return Err(TryRecvError::Disconnected)
                            }
                        }
                    }
                    Flavor::Sync(ref p) => {
                        match unsafe { (*p.get()).try_recv() } {
                            Ok(t) => return Ok(t),
                            Err(sync::Empty) => return Err(TryRecvError::Empty),
                            Err(sync::Disconnected) => {
                                return Err(TryRecvError::Disconnected)
                            }
                        }
                    }
                };
                unsafe {
                    mem::swap(self.inner_mut(),
                              new_port.inner_mut());
                }
            }
        }

        /// Attempts to wait for a value on this receiver, returning an error if the
        /// corresponding channel has hung up.
        ///
        /// This function will always block the current thread if there is no data
        /// available and it's possible for more data to be sent. Once a message is
        /// sent to the corresponding `Sender`, then this receiver will wake up and
        /// return that message.
        ///
        /// If the corresponding `Sender` has disconnected, or it disconnects while
        /// this call is blocking, this call will wake up and return `Err` to
        /// indicate that no more messages can ever be received on this channel.
        /// However, since channels are buffered, messages sent before the disconnect
        /// will still be properly received.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::mpsc;
        /// use std::thread;
        ///
        /// let (send, recv) = mpsc::channel();
        /// let handle = thread::spawn(move || {
        ///     send.send(1u8).unwrap();
        /// });
        ///
        /// handle.join().unwrap();
        ///
        /// assert_eq!(Ok(1), recv.recv());
        /// ```
        ///
        /// Buffering behavior:
        ///
        /// ```
        /// use std::sync::mpsc;
        /// use std::thread;
        /// use std::sync::mpsc::RecvError;
        ///
        /// let (send, recv) = mpsc::channel();
        /// let handle = thread::spawn(move || {
        ///     send.send(1u8).unwrap();
        ///     send.send(2).unwrap();
        ///     send.send(3).unwrap();
        ///     drop(send);
        /// });
        ///
        /// // wait for the thread to join so we ensure the sender is dropped
        /// handle.join().unwrap();
        ///
        /// assert_eq!(Ok(1), recv.recv());
        /// assert_eq!(Ok(2), recv.recv());
        /// assert_eq!(Ok(3), recv.recv());
        /// assert_eq!(Err(RecvError), recv.recv());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn recv(&self) -> Result<T, RecvError> {
            loop {
                let new_port = match *unsafe { self.inner() } {
                    Flavor::Oneshot(ref p) => {
                        match unsafe { (*p.get()).recv() } {
                            Ok(t) => return Ok(t),
                            Err(oneshot::Empty) => return unreachable!(),
                            Err(oneshot::Disconnected) => return Err(RecvError),
                            Err(oneshot::Upgraded(rx)) => rx,
                        }
                    }
                    Flavor::Stream(ref p) => {
                        match unsafe { (*p.get()).recv() } {
                            Ok(t) => return Ok(t),
                            Err(stream::Empty) => return unreachable!(),
                            Err(stream::Disconnected) => return Err(RecvError),
                            Err(stream::Upgraded(rx)) => rx,
                        }
                    }
                    Flavor::Shared(ref p) => {
                        match unsafe { (*p.get()).recv() } {
                            Ok(t) => return Ok(t),
                            Err(shared::Empty) => return unreachable!(),
                            Err(shared::Disconnected) => return Err(RecvError),
                        }
                    }
                    Flavor::Sync(ref p) => return unsafe {
                        (*p.get()).recv().map_err(|()| RecvError)
                    }
                };
                unsafe {
                    mem::swap(self.inner_mut(), new_port.inner_mut());
                }
            }
        }

        /// Returns an iterator that will block waiting for messages, but never
        /// `panic!`. It will return `None` when the channel has hung up.
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn iter(&self) -> Iter<T> {
            Iter { rx: self }
        }
    }

    impl<T> select::Packet for Receiver<T> {
        fn can_recv(&self) -> bool {
            loop {
                let new_port = match *unsafe { self.inner() } {
                    Flavor::Oneshot(ref p) => {
                        match unsafe { (*p.get()).can_recv() } {
                            Ok(ret) => return ret,
                            Err(upgrade) => upgrade,
                        }
                    }
                    Flavor::Stream(ref p) => {
                        match unsafe { (*p.get()).can_recv() } {
                            Ok(ret) => return ret,
                            Err(upgrade) => upgrade,
                        }
                    }
                    Flavor::Shared(ref p) => {
                        return unsafe { (*p.get()).can_recv() };
                    }
                    Flavor::Sync(ref p) => {
                        return unsafe { (*p.get()).can_recv() };
                    }
                };
                unsafe {
                    mem::swap(self.inner_mut(),
                              new_port.inner_mut());
                }
            }
        }

        fn start_selection(&self, mut token: SignalToken) -> StartResult {
            loop {
                let (t, new_port) = match *unsafe { self.inner() } {
                    Flavor::Oneshot(ref p) => {
                        match unsafe { (*p.get()).start_selection(token) } {
                            oneshot::SelSuccess => return Installed,
                            oneshot::SelCanceled => return Abort,
                            oneshot::SelUpgraded(t, rx) => (t, rx),
                        }
                    }
                    Flavor::Stream(ref p) => {
                        match unsafe { (*p.get()).start_selection(token) } {
                            stream::SelSuccess => return Installed,
                            stream::SelCanceled => return Abort,
                            stream::SelUpgraded(t, rx) => (t, rx),
                        }
                    }
                    Flavor::Shared(ref p) => {
                        return unsafe { (*p.get()).start_selection(token) };
                    }
                    Flavor::Sync(ref p) => {
                        return unsafe { (*p.get()).start_selection(token) };
                    }
                };
                token = t;
                unsafe {
                    mem::swap(self.inner_mut(), new_port.inner_mut());
                }
            }
        }

        fn abort_selection(&self) -> bool {
            let mut was_upgrade = false;
            loop {
                let result = match *unsafe { self.inner() } {
                    Flavor::Oneshot(ref p) => unsafe { (*p.get()).abort_selection() },
                    Flavor::Stream(ref p) => unsafe {
                        (*p.get()).abort_selection(was_upgrade)
                    },
                    Flavor::Shared(ref p) => return unsafe {
                        (*p.get()).abort_selection(was_upgrade)
                    },
                    Flavor::Sync(ref p) => return unsafe {
                        (*p.get()).abort_selection()
                    },
                };
                let new_port = match result { Ok(b) => return b, Err(p) => p };
                was_upgrade = true;
                unsafe {
                    mem::swap(self.inner_mut(),
                              new_port.inner_mut());
                }
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> Iterator for Iter<'a, T> {
        type Item = T;

        fn next(&mut self) -> Option<T> { self.rx.recv().ok() }
    }

    #[stable(feature = "receiver_into_iter", since = "1.1.0")]
    impl<'a, T> IntoIterator for &'a Receiver<T> {
        type Item = T;
        type IntoIter = Iter<'a, T>;

        fn into_iter(self) -> Iter<'a, T> { self.iter() }
    }

    #[stable(feature = "receiver_into_iter", since = "1.1.0")]
    impl<T> Iterator for IntoIter<T> {
        type Item = T;
        fn next(&mut self) -> Option<T> { self.rx.recv().ok() }
    }

    #[stable(feature = "receiver_into_iter", since = "1.1.0")]
    impl <T> IntoIterator for Receiver<T> {
        type Item = T;
        type IntoIter = IntoIter<T>;

        fn into_iter(self) -> IntoIter<T> {
            IntoIter { rx: self }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Drop for Receiver<T> {
        fn drop(&mut self) {
            match *unsafe { self.inner_mut() } {
                Flavor::Oneshot(ref mut p) => unsafe { (*p.get()).drop_port(); },
                Flavor::Stream(ref mut p) => unsafe { (*p.get()).drop_port(); },
                Flavor::Shared(ref mut p) => unsafe { (*p.get()).drop_port(); },
                Flavor::Sync(ref mut p) => unsafe { (*p.get()).drop_port(); },
            }
        }
    }

    #[stable(feature = "mpsc_debug", since = "1.7.0")]
    impl<T> fmt::Debug for Receiver<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "Receiver {{ .. }}")
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> fmt::Debug for SendError<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            "SendError(..)".fmt(f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> fmt::Display for SendError<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            "sending on a closed channel".fmt(f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Send + Reflect> error::Error for SendError<T> {
        fn description(&self) -> &str {
            "sending on a closed channel"
        }

        fn cause(&self) -> Option<&error::Error> {
            None
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> fmt::Debug for TrySendError<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            match *self {
                TrySendError::Full(..) => "Full(..)".fmt(f),
                TrySendError::Disconnected(..) => "Disconnected(..)".fmt(f),
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> fmt::Display for TrySendError<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            match *self {
                TrySendError::Full(..) => {
                    "sending on a full channel".fmt(f)
                }
                TrySendError::Disconnected(..) => {
                    "sending on a closed channel".fmt(f)
                }
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Send + Reflect> error::Error for TrySendError<T> {

        fn description(&self) -> &str {
            match *self {
                TrySendError::Full(..) => {
                    "sending on a full channel"
                }
                TrySendError::Disconnected(..) => {
                    "sending on a closed channel"
                }
            }
        }

        fn cause(&self) -> Option<&error::Error> {
            None
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl fmt::Display for RecvError {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            "receiving on a closed channel".fmt(f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl error::Error for RecvError {

        fn description(&self) -> &str {
            "receiving on a closed channel"
        }

        fn cause(&self) -> Option<&error::Error> {
            None
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl fmt::Display for TryRecvError {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            match *self {
                TryRecvError::Empty => {
                    "receiving on an empty channel".fmt(f)
                }
                TryRecvError::Disconnected => {
                    "receiving on a closed channel".fmt(f)
                }
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl error::Error for TryRecvError {

        fn description(&self) -> &str {
            match *self {
                TryRecvError::Empty => {
                    "receiving on an empty channel"
                }
                TryRecvError::Disconnected => {
                    "receiving on a closed channel"
                }
            }
        }

        fn cause(&self) -> Option<&error::Error> {
            None
        }
    }

    #[cfg(test)]
    mod tests {
        use prelude::v1::*;

        use env;
        use super::*;
        use thread;

        pub fn stress_factor() -> usize {
            match env::var("RUST_TEST_STRESS") {
                Ok(val) => val.parse().unwrap(),
                Err(..) => 1,
            }
        }

        #[test]
        fn smoke() {
            let (tx, rx) = channel::<i32>();
            tx.send(1).unwrap();
            assert_eq!(rx.recv().unwrap(), 1);
        }

        #[test]
        fn drop_full() {
            let (tx, _rx) = channel::<Box<isize>>();
            tx.send(box 1).unwrap();
        }

        #[test]
        fn drop_full_shared() {
            let (tx, _rx) = channel::<Box<isize>>();
            drop(tx.clone());
            drop(tx.clone());
            tx.send(box 1).unwrap();
        }

        #[test]
        fn smoke_shared() {
            let (tx, rx) = channel::<i32>();
            tx.send(1).unwrap();
            assert_eq!(rx.recv().unwrap(), 1);
            let tx = tx.clone();
            tx.send(1).unwrap();
            assert_eq!(rx.recv().unwrap(), 1);
        }

        #[test]
        fn smoke_threads() {
            let (tx, rx) = channel::<i32>();
            let _t = thread::spawn(move|| {
                tx.send(1).unwrap();
            });
            assert_eq!(rx.recv().unwrap(), 1);
        }

        #[test]
        fn smoke_port_gone() {
            let (tx, rx) = channel::<i32>();
            drop(rx);
            assert!(tx.send(1).is_err());
        }

        #[test]
        fn smoke_shared_port_gone() {
            let (tx, rx) = channel::<i32>();
            drop(rx);
            assert!(tx.send(1).is_err())
        }

        #[test]
        fn smoke_shared_port_gone2() {
            let (tx, rx) = channel::<i32>();
            drop(rx);
            let tx2 = tx.clone();
            drop(tx);
            assert!(tx2.send(1).is_err());
        }

        #[test]
        fn port_gone_concurrent() {
            let (tx, rx) = channel::<i32>();
            let _t = thread::spawn(move|| {
                rx.recv().unwrap();
            });
            while tx.send(1).is_ok() {}
        }

        #[test]
        fn port_gone_concurrent_shared() {
            let (tx, rx) = channel::<i32>();
            let tx2 = tx.clone();
            let _t = thread::spawn(move|| {
                rx.recv().unwrap();
            });
            while tx.send(1).is_ok() && tx2.send(1).is_ok() {}
        }

        #[test]
        fn smoke_chan_gone() {
            let (tx, rx) = channel::<i32>();
            drop(tx);
            assert!(rx.recv().is_err());
        }

        #[test]
        fn smoke_chan_gone_shared() {
            let (tx, rx) = channel::<()>();
            let tx2 = tx.clone();
            drop(tx);
            drop(tx2);
            assert!(rx.recv().is_err());
        }

        #[test]
        fn chan_gone_concurrent() {
            let (tx, rx) = channel::<i32>();
            let _t = thread::spawn(move|| {
                tx.send(1).unwrap();
                tx.send(1).unwrap();
            });
            while rx.recv().is_ok() {}
        }

        #[test]
        fn stress() {
            let (tx, rx) = channel::<i32>();
            let t = thread::spawn(move|| {
                for _ in 0..10000 { tx.send(1).unwrap(); }
            });
            for _ in 0..10000 {
                assert_eq!(rx.recv().unwrap(), 1);
            }
            t.join().ok().unwrap();
        }

        #[test]
        fn stress_shared() {
            const AMT: u32 = 10000;
            const NTHREADS: u32 = 8;
            let (tx, rx) = channel::<i32>();

            let t = thread::spawn(move|| {
                for _ in 0..AMT * NTHREADS {
                    assert_eq!(rx.recv().unwrap(), 1);
                }
                match rx.try_recv() {
                    Ok(..) => panic!(),
                    _ => {}
                }
            });

            for _ in 0..NTHREADS {
                let tx = tx.clone();
                thread::spawn(move|| {
                    for _ in 0..AMT { tx.send(1).unwrap(); }
                });
            }
            drop(tx);
            t.join().ok().unwrap();
        }

        #[test]
        fn send_from_outside_runtime() {
            let (tx1, rx1) = channel::<()>();
            let (tx2, rx2) = channel::<i32>();
            let t1 = thread::spawn(move|| {
                tx1.send(()).unwrap();
                for _ in 0..40 {
                    assert_eq!(rx2.recv().unwrap(), 1);
                }
            });
            rx1.recv().unwrap();
            let t2 = thread::spawn(move|| {
                for _ in 0..40 {
                    tx2.send(1).unwrap();
                }
            });
            t1.join().ok().unwrap();
            t2.join().ok().unwrap();
        }

        #[test]
        fn recv_from_outside_runtime() {
            let (tx, rx) = channel::<i32>();
            let t = thread::spawn(move|| {
                for _ in 0..40 {
                    assert_eq!(rx.recv().unwrap(), 1);
                }
            });
            for _ in 0..40 {
                tx.send(1).unwrap();
            }
            t.join().ok().unwrap();
        }

        #[test]
        fn no_runtime() {
            let (tx1, rx1) = channel::<i32>();
            let (tx2, rx2) = channel::<i32>();
            let t1 = thread::spawn(move|| {
                assert_eq!(rx1.recv().unwrap(), 1);
                tx2.send(2).unwrap();
            });
            let t2 = thread::spawn(move|| {
                tx1.send(1).unwrap();
                assert_eq!(rx2.recv().unwrap(), 2);
            });
            t1.join().ok().unwrap();
            t2.join().ok().unwrap();
        }

        #[test]
        fn oneshot_single_thread_close_port_first() {
            // Simple test of closing without sending
            let (_tx, rx) = channel::<i32>();
            drop(rx);
        }

        #[test]
        fn oneshot_single_thread_close_chan_first() {
            // Simple test of closing without sending
            let (tx, _rx) = channel::<i32>();
            drop(tx);
        }

        #[test]
        fn oneshot_single_thread_send_port_close() {
            // Testing that the sender cleans up the payload if receiver is closed
            let (tx, rx) = channel::<Box<i32>>();
            drop(rx);
            assert!(tx.send(box 0).is_err());
        }

        #[test]
        fn oneshot_single_thread_recv_chan_close() {
            // Receiving on a closed chan will panic
            let res = thread::spawn(move|| {
                let (tx, rx) = channel::<i32>();
                drop(tx);
                rx.recv().unwrap();
            }).join();
            // What is our res?
            assert!(res.is_err());
        }

        #[test]
        fn oneshot_single_thread_send_then_recv() {
            let (tx, rx) = channel::<Box<i32>>();
            tx.send(box 10).unwrap();
            assert!(rx.recv().unwrap() == box 10);
        }

        #[test]
        fn oneshot_single_thread_try_send_open() {
            let (tx, rx) = channel::<i32>();
            assert!(tx.send(10).is_ok());
            assert!(rx.recv().unwrap() == 10);
        }

        #[test]
        fn oneshot_single_thread_try_send_closed() {
            let (tx, rx) = channel::<i32>();
            drop(rx);
            assert!(tx.send(10).is_err());
        }

        #[test]
        fn oneshot_single_thread_try_recv_open() {
            let (tx, rx) = channel::<i32>();
            tx.send(10).unwrap();
            assert!(rx.recv() == Ok(10));
        }

        #[test]
        fn oneshot_single_thread_try_recv_closed() {
            let (tx, rx) = channel::<i32>();
            drop(tx);
            assert!(rx.recv().is_err());
        }

        #[test]
        fn oneshot_single_thread_peek_data() {
            let (tx, rx) = channel::<i32>();
            assert_eq!(rx.try_recv(), Err(TryRecvError::Empty));
            tx.send(10).unwrap();
            assert_eq!(rx.try_recv(), Ok(10));
        }

        #[test]
        fn oneshot_single_thread_peek_close() {
            let (tx, rx) = channel::<i32>();
            drop(tx);
            assert_eq!(rx.try_recv(), Err(TryRecvError::Disconnected));
            assert_eq!(rx.try_recv(), Err(TryRecvError::Disconnected));
        }

        #[test]
        fn oneshot_single_thread_peek_open() {
            let (_tx, rx) = channel::<i32>();
            assert_eq!(rx.try_recv(), Err(TryRecvError::Empty));
        }

        #[test]
        fn oneshot_multi_task_recv_then_send() {
            let (tx, rx) = channel::<Box<i32>>();
            let _t = thread::spawn(move|| {
                assert!(rx.recv().unwrap() == box 10);
            });

            tx.send(box 10).unwrap();
        }

        #[test]
        fn oneshot_multi_task_recv_then_close() {
            let (tx, rx) = channel::<Box<i32>>();
            let _t = thread::spawn(move|| {
                drop(tx);
            });
            let res = thread::spawn(move|| {
                assert!(rx.recv().unwrap() == box 10);
            }).join();
            assert!(res.is_err());
        }

        #[test]
        fn oneshot_multi_thread_close_stress() {
            for _ in 0..stress_factor() {
                let (tx, rx) = channel::<i32>();
                let _t = thread::spawn(move|| {
                    drop(rx);
                });
                drop(tx);
            }
        }

        #[test]
        fn oneshot_multi_thread_send_close_stress() {
            for _ in 0..stress_factor() {
                let (tx, rx) = channel::<i32>();
                let _t = thread::spawn(move|| {
                    drop(rx);
                });
                let _ = thread::spawn(move|| {
                    tx.send(1).unwrap();
                }).join();
            }
        }

        #[test]
        fn oneshot_multi_thread_recv_close_stress() {
            for _ in 0..stress_factor() {
                let (tx, rx) = channel::<i32>();
                thread::spawn(move|| {
                    let res = thread::spawn(move|| {
                        rx.recv().unwrap();
                    }).join();
                    assert!(res.is_err());
                });
                let _t = thread::spawn(move|| {
                    thread::spawn(move|| {
                        drop(tx);
                    });
                });
            }
        }

        #[test]
        fn oneshot_multi_thread_send_recv_stress() {
            for _ in 0..stress_factor() {
                let (tx, rx) = channel::<Box<isize>>();
                let _t = thread::spawn(move|| {
                    tx.send(box 10).unwrap();
                });
                assert!(rx.recv().unwrap() == box 10);
            }
        }

        #[test]
        fn stream_send_recv_stress() {
            for _ in 0..stress_factor() {
                let (tx, rx) = channel();

                send(tx, 0);
                recv(rx, 0);

                fn send(tx: Sender<Box<i32>>, i: i32) {
                    if i == 10 { return }

                    thread::spawn(move|| {
                        tx.send(box i).unwrap();
                        send(tx, i + 1);
                    });
                }

                fn recv(rx: Receiver<Box<i32>>, i: i32) {
                    if i == 10 { return }

                    thread::spawn(move|| {
                        assert!(rx.recv().unwrap() == box i);
                        recv(rx, i + 1);
                    });
                }
            }
        }

        #[test]
        fn recv_a_lot() {
            // Regression test that we don't run out of stack in scheduler context
            let (tx, rx) = channel();
            for _ in 0..10000 { tx.send(()).unwrap(); }
            for _ in 0..10000 { rx.recv().unwrap(); }
        }

        #[test]
        fn shared_chan_stress() {
            let (tx, rx) = channel();
            let total = stress_factor() + 100;
            for _ in 0..total {
                let tx = tx.clone();
                thread::spawn(move|| {
                    tx.send(()).unwrap();
                });
            }

            for _ in 0..total {
                rx.recv().unwrap();
            }
        }

        #[test]
        fn test_nested_recv_iter() {
            let (tx, rx) = channel::<i32>();
            let (total_tx, total_rx) = channel::<i32>();

            let _t = thread::spawn(move|| {
                let mut acc = 0;
                for x in rx.iter() {
                    acc += x;
                }
                total_tx.send(acc).unwrap();
            });

            tx.send(3).unwrap();
            tx.send(1).unwrap();
            tx.send(2).unwrap();
            drop(tx);
            assert_eq!(total_rx.recv().unwrap(), 6);
        }

        #[test]
        fn test_recv_iter_break() {
            let (tx, rx) = channel::<i32>();
            let (count_tx, count_rx) = channel();

            let _t = thread::spawn(move|| {
                let mut count = 0;
                for x in rx.iter() {
                    if count >= 3 {
                        break;
                    } else {
                        count += x;
                    }
                }
                count_tx.send(count).unwrap();
            });

            tx.send(2).unwrap();
            tx.send(2).unwrap();
            tx.send(2).unwrap();
            let _ = tx.send(2);
            drop(tx);
            assert_eq!(count_rx.recv().unwrap(), 4);
        }

        #[test]
        fn test_recv_into_iter_owned() {
            let mut iter = {
              let (tx, rx) = channel::<i32>();
              tx.send(1).unwrap();
              tx.send(2).unwrap();

              rx.into_iter()
            };
            assert_eq!(iter.next().unwrap(), 1);
            assert_eq!(iter.next().unwrap(), 2);
            assert_eq!(iter.next().is_none(), true);
        }

        #[test]
        fn test_recv_into_iter_borrowed() {
            let (tx, rx) = channel::<i32>();
            tx.send(1).unwrap();
            tx.send(2).unwrap();
            drop(tx);
            let mut iter = (&rx).into_iter();
            assert_eq!(iter.next().unwrap(), 1);
            assert_eq!(iter.next().unwrap(), 2);
            assert_eq!(iter.next().is_none(), true);
        }

        #[test]
        fn try_recv_states() {
            let (tx1, rx1) = channel::<i32>();
            let (tx2, rx2) = channel::<()>();
            let (tx3, rx3) = channel::<()>();
            let _t = thread::spawn(move|| {
                rx2.recv().unwrap();
                tx1.send(1).unwrap();
                tx3.send(()).unwrap();
                rx2.recv().unwrap();
                drop(tx1);
                tx3.send(()).unwrap();
            });

            assert_eq!(rx1.try_recv(), Err(TryRecvError::Empty));
            tx2.send(()).unwrap();
            rx3.recv().unwrap();
            assert_eq!(rx1.try_recv(), Ok(1));
            assert_eq!(rx1.try_recv(), Err(TryRecvError::Empty));
            tx2.send(()).unwrap();
            rx3.recv().unwrap();
            assert_eq!(rx1.try_recv(), Err(TryRecvError::Disconnected));
        }

        // This bug used to end up in a livelock inside of the Receiver destructor
        // because the internal state of the Shared packet was corrupted
        #[test]
        fn destroy_upgraded_shared_port_when_sender_still_active() {
            let (tx, rx) = channel();
            let (tx2, rx2) = channel();
            let _t = thread::spawn(move|| {
                rx.recv().unwrap(); // wait on a oneshot
                drop(rx);  // destroy a shared
                tx2.send(()).unwrap();
            });
            // make sure the other thread has gone to sleep
            for _ in 0..5000 { thread::yield_now(); }

            // upgrade to a shared chan and send a message
            let t = tx.clone();
            drop(tx);
            t.send(()).unwrap();

            // wait for the child thread to exit before we exit
            rx2.recv().unwrap();
        }
    }

    #[cfg(test)]
    mod sync_tests {
        use prelude::v1::*;

        use env;
        use thread;
        use super::*;

        pub fn stress_factor() -> usize {
            match env::var("RUST_TEST_STRESS") {
                Ok(val) => val.parse().unwrap(),
                Err(..) => 1,
            }
        }

        #[test]
        fn smoke() {
            let (tx, rx) = sync_channel::<i32>(1);
            tx.send(1).unwrap();
            assert_eq!(rx.recv().unwrap(), 1);
        }

        #[test]
        fn drop_full() {
            let (tx, _rx) = sync_channel::<Box<isize>>(1);
            tx.send(box 1).unwrap();
        }

        #[test]
        fn smoke_shared() {
            let (tx, rx) = sync_channel::<i32>(1);
            tx.send(1).unwrap();
            assert_eq!(rx.recv().unwrap(), 1);
            let tx = tx.clone();
            tx.send(1).unwrap();
            assert_eq!(rx.recv().unwrap(), 1);
        }

        #[test]
        fn smoke_threads() {
            let (tx, rx) = sync_channel::<i32>(0);
            let _t = thread::spawn(move|| {
                tx.send(1).unwrap();
            });
            assert_eq!(rx.recv().unwrap(), 1);
        }

        #[test]
        fn smoke_port_gone() {
            let (tx, rx) = sync_channel::<i32>(0);
            drop(rx);
            assert!(tx.send(1).is_err());
        }

        #[test]
        fn smoke_shared_port_gone2() {
            let (tx, rx) = sync_channel::<i32>(0);
            drop(rx);
            let tx2 = tx.clone();
            drop(tx);
            assert!(tx2.send(1).is_err());
        }

        #[test]
        fn port_gone_concurrent() {
            let (tx, rx) = sync_channel::<i32>(0);
            let _t = thread::spawn(move|| {
                rx.recv().unwrap();
            });
            while tx.send(1).is_ok() {}
        }

        #[test]
        fn port_gone_concurrent_shared() {
            let (tx, rx) = sync_channel::<i32>(0);
            let tx2 = tx.clone();
            let _t = thread::spawn(move|| {
                rx.recv().unwrap();
            });
            while tx.send(1).is_ok() && tx2.send(1).is_ok() {}
        }

        #[test]
        fn smoke_chan_gone() {
            let (tx, rx) = sync_channel::<i32>(0);
            drop(tx);
            assert!(rx.recv().is_err());
        }

        #[test]
        fn smoke_chan_gone_shared() {
            let (tx, rx) = sync_channel::<()>(0);
            let tx2 = tx.clone();
            drop(tx);
            drop(tx2);
            assert!(rx.recv().is_err());
        }

        #[test]
        fn chan_gone_concurrent() {
            let (tx, rx) = sync_channel::<i32>(0);
            thread::spawn(move|| {
                tx.send(1).unwrap();
                tx.send(1).unwrap();
            });
            while rx.recv().is_ok() {}
        }

        #[test]
        fn stress() {
            let (tx, rx) = sync_channel::<i32>(0);
            thread::spawn(move|| {
                for _ in 0..10000 { tx.send(1).unwrap(); }
            });
            for _ in 0..10000 {
                assert_eq!(rx.recv().unwrap(), 1);
            }
        }

        #[test]
        fn stress_shared() {
            const AMT: u32 = 1000;
            const NTHREADS: u32 = 8;
            let (tx, rx) = sync_channel::<i32>(0);
            let (dtx, drx) = sync_channel::<()>(0);

            thread::spawn(move|| {
                for _ in 0..AMT * NTHREADS {
                    assert_eq!(rx.recv().unwrap(), 1);
                }
                match rx.try_recv() {
                    Ok(..) => panic!(),
                    _ => {}
                }
                dtx.send(()).unwrap();
            });

            for _ in 0..NTHREADS {
                let tx = tx.clone();
                thread::spawn(move|| {
                    for _ in 0..AMT { tx.send(1).unwrap(); }
                });
            }
            drop(tx);
            drx.recv().unwrap();
        }

        #[test]
        fn oneshot_single_thread_close_port_first() {
            // Simple test of closing without sending
            let (_tx, rx) = sync_channel::<i32>(0);
            drop(rx);
        }

        #[test]
        fn oneshot_single_thread_close_chan_first() {
            // Simple test of closing without sending
            let (tx, _rx) = sync_channel::<i32>(0);
            drop(tx);
        }

        #[test]
        fn oneshot_single_thread_send_port_close() {
            // Testing that the sender cleans up the payload if receiver is closed
            let (tx, rx) = sync_channel::<Box<i32>>(0);
            drop(rx);
            assert!(tx.send(box 0).is_err());
        }

        #[test]
        fn oneshot_single_thread_recv_chan_close() {
            // Receiving on a closed chan will panic
            let res = thread::spawn(move|| {
                let (tx, rx) = sync_channel::<i32>(0);
                drop(tx);
                rx.recv().unwrap();
            }).join();
            // What is our res?
            assert!(res.is_err());
        }

        #[test]
        fn oneshot_single_thread_send_then_recv() {
            let (tx, rx) = sync_channel::<Box<i32>>(1);
            tx.send(box 10).unwrap();
            assert!(rx.recv().unwrap() == box 10);
        }

        #[test]
        fn oneshot_single_thread_try_send_open() {
            let (tx, rx) = sync_channel::<i32>(1);
            assert_eq!(tx.try_send(10), Ok(()));
            assert!(rx.recv().unwrap() == 10);
        }

        #[test]
        fn oneshot_single_thread_try_send_closed() {
            let (tx, rx) = sync_channel::<i32>(0);
            drop(rx);
            assert_eq!(tx.try_send(10), Err(TrySendError::Disconnected(10)));
        }

        #[test]
        fn oneshot_single_thread_try_send_closed2() {
            let (tx, _rx) = sync_channel::<i32>(0);
            assert_eq!(tx.try_send(10), Err(TrySendError::Full(10)));
        }

        #[test]
        fn oneshot_single_thread_try_recv_open() {
            let (tx, rx) = sync_channel::<i32>(1);
            tx.send(10).unwrap();
            assert!(rx.recv() == Ok(10));
        }

        #[test]
        fn oneshot_single_thread_try_recv_closed() {
            let (tx, rx) = sync_channel::<i32>(0);
            drop(tx);
            assert!(rx.recv().is_err());
        }

        #[test]
        fn oneshot_single_thread_peek_data() {
            let (tx, rx) = sync_channel::<i32>(1);
            assert_eq!(rx.try_recv(), Err(TryRecvError::Empty));
            tx.send(10).unwrap();
            assert_eq!(rx.try_recv(), Ok(10));
        }

        #[test]
        fn oneshot_single_thread_peek_close() {
            let (tx, rx) = sync_channel::<i32>(0);
            drop(tx);
            assert_eq!(rx.try_recv(), Err(TryRecvError::Disconnected));
            assert_eq!(rx.try_recv(), Err(TryRecvError::Disconnected));
        }

        #[test]
        fn oneshot_single_thread_peek_open() {
            let (_tx, rx) = sync_channel::<i32>(0);
            assert_eq!(rx.try_recv(), Err(TryRecvError::Empty));
        }

        #[test]
        fn oneshot_multi_task_recv_then_send() {
            let (tx, rx) = sync_channel::<Box<i32>>(0);
            let _t = thread::spawn(move|| {
                assert!(rx.recv().unwrap() == box 10);
            });

            tx.send(box 10).unwrap();
        }

        #[test]
        fn oneshot_multi_task_recv_then_close() {
            let (tx, rx) = sync_channel::<Box<i32>>(0);
            let _t = thread::spawn(move|| {
                drop(tx);
            });
            let res = thread::spawn(move|| {
                assert!(rx.recv().unwrap() == box 10);
            }).join();
            assert!(res.is_err());
        }

        #[test]
        fn oneshot_multi_thread_close_stress() {
            for _ in 0..stress_factor() {
                let (tx, rx) = sync_channel::<i32>(0);
                let _t = thread::spawn(move|| {
                    drop(rx);
                });
                drop(tx);
            }
        }

        #[test]
        fn oneshot_multi_thread_send_close_stress() {
            for _ in 0..stress_factor() {
                let (tx, rx) = sync_channel::<i32>(0);
                let _t = thread::spawn(move|| {
                    drop(rx);
                });
                let _ = thread::spawn(move || {
                    tx.send(1).unwrap();
                }).join();
            }
        }

        #[test]
        fn oneshot_multi_thread_recv_close_stress() {
            for _ in 0..stress_factor() {
                let (tx, rx) = sync_channel::<i32>(0);
                let _t = thread::spawn(move|| {
                    let res = thread::spawn(move|| {
                        rx.recv().unwrap();
                    }).join();
                    assert!(res.is_err());
                });
                let _t = thread::spawn(move|| {
                    thread::spawn(move|| {
                        drop(tx);
                    });
                });
            }
        }

        #[test]
        fn oneshot_multi_thread_send_recv_stress() {
            for _ in 0..stress_factor() {
                let (tx, rx) = sync_channel::<Box<i32>>(0);
                let _t = thread::spawn(move|| {
                    tx.send(box 10).unwrap();
                });
                assert!(rx.recv().unwrap() == box 10);
            }
        }

        #[test]
        fn stream_send_recv_stress() {
            for _ in 0..stress_factor() {
                let (tx, rx) = sync_channel::<Box<i32>>(0);

                send(tx, 0);
                recv(rx, 0);

                fn send(tx: SyncSender<Box<i32>>, i: i32) {
                    if i == 10 { return }

                    thread::spawn(move|| {
                        tx.send(box i).unwrap();
                        send(tx, i + 1);
                    });
                }

                fn recv(rx: Receiver<Box<i32>>, i: i32) {
                    if i == 10 { return }

                    thread::spawn(move|| {
                        assert!(rx.recv().unwrap() == box i);
                        recv(rx, i + 1);
                    });
                }
            }
        }

        #[test]
        fn recv_a_lot() {
            // Regression test that we don't run out of stack in scheduler context
            let (tx, rx) = sync_channel(10000);
            for _ in 0..10000 { tx.send(()).unwrap(); }
            for _ in 0..10000 { rx.recv().unwrap(); }
        }

        #[test]
        fn shared_chan_stress() {
            let (tx, rx) = sync_channel(0);
            let total = stress_factor() + 100;
            for _ in 0..total {
                let tx = tx.clone();
                thread::spawn(move|| {
                    tx.send(()).unwrap();
                });
            }

            for _ in 0..total {
                rx.recv().unwrap();
            }
        }

        #[test]
        fn test_nested_recv_iter() {
            let (tx, rx) = sync_channel::<i32>(0);
            let (total_tx, total_rx) = sync_channel::<i32>(0);

            let _t = thread::spawn(move|| {
                let mut acc = 0;
                for x in rx.iter() {
                    acc += x;
                }
                total_tx.send(acc).unwrap();
            });

            tx.send(3).unwrap();
            tx.send(1).unwrap();
            tx.send(2).unwrap();
            drop(tx);
            assert_eq!(total_rx.recv().unwrap(), 6);
        }

        #[test]
        fn test_recv_iter_break() {
            let (tx, rx) = sync_channel::<i32>(0);
            let (count_tx, count_rx) = sync_channel(0);

            let _t = thread::spawn(move|| {
                let mut count = 0;
                for x in rx.iter() {
                    if count >= 3 {
                        break;
                    } else {
                        count += x;
                    }
                }
                count_tx.send(count).unwrap();
            });

            tx.send(2).unwrap();
            tx.send(2).unwrap();
            tx.send(2).unwrap();
            let _ = tx.try_send(2);
            drop(tx);
            assert_eq!(count_rx.recv().unwrap(), 4);
        }

        #[test]
        fn try_recv_states() {
            let (tx1, rx1) = sync_channel::<i32>(1);
            let (tx2, rx2) = sync_channel::<()>(1);
            let (tx3, rx3) = sync_channel::<()>(1);
            let _t = thread::spawn(move|| {
                rx2.recv().unwrap();
                tx1.send(1).unwrap();
                tx3.send(()).unwrap();
                rx2.recv().unwrap();
                drop(tx1);
                tx3.send(()).unwrap();
            });

            assert_eq!(rx1.try_recv(), Err(TryRecvError::Empty));
            tx2.send(()).unwrap();
            rx3.recv().unwrap();
            assert_eq!(rx1.try_recv(), Ok(1));
            assert_eq!(rx1.try_recv(), Err(TryRecvError::Empty));
            tx2.send(()).unwrap();
            rx3.recv().unwrap();
            assert_eq!(rx1.try_recv(), Err(TryRecvError::Disconnected));
        }

        // This bug used to end up in a livelock inside of the Receiver destructor
        // because the internal state of the Shared packet was corrupted
        #[test]
        fn destroy_upgraded_shared_port_when_sender_still_active() {
            let (tx, rx) = sync_channel::<()>(0);
            let (tx2, rx2) = sync_channel::<()>(0);
            let _t = thread::spawn(move|| {
                rx.recv().unwrap(); // wait on a oneshot
                drop(rx);  // destroy a shared
                tx2.send(()).unwrap();
            });
            // make sure the other thread has gone to sleep
            for _ in 0..5000 { thread::yield_now(); }

            // upgrade to a shared chan and send a message
            let t = tx.clone();
            drop(tx);
            t.send(()).unwrap();

            // wait for the child thread to exit before we exit
            rx2.recv().unwrap();
        }

        #[test]
        fn send1() {
            let (tx, rx) = sync_channel::<i32>(0);
            let _t = thread::spawn(move|| { rx.recv().unwrap(); });
            assert_eq!(tx.send(1), Ok(()));
        }

        #[test]
        fn send2() {
            let (tx, rx) = sync_channel::<i32>(0);
            let _t = thread::spawn(move|| { drop(rx); });
            assert!(tx.send(1).is_err());
        }

        #[test]
        fn send3() {
            let (tx, rx) = sync_channel::<i32>(1);
            assert_eq!(tx.send(1), Ok(()));
            let _t =thread::spawn(move|| { drop(rx); });
            assert!(tx.send(1).is_err());
        }

        #[test]
        fn send4() {
            let (tx, rx) = sync_channel::<i32>(0);
            let tx2 = tx.clone();
            let (done, donerx) = channel();
            let done2 = done.clone();
            let _t = thread::spawn(move|| {
                assert!(tx.send(1).is_err());
                done.send(()).unwrap();
            });
            let _t = thread::spawn(move|| {
                assert!(tx2.send(2).is_err());
                done2.send(()).unwrap();
            });
            drop(rx);
            donerx.recv().unwrap();
            donerx.recv().unwrap();
        }

        #[test]
        fn try_send1() {
            let (tx, _rx) = sync_channel::<i32>(0);
            assert_eq!(tx.try_send(1), Err(TrySendError::Full(1)));
        }

        #[test]
        fn try_send2() {
            let (tx, _rx) = sync_channel::<i32>(1);
            assert_eq!(tx.try_send(1), Ok(()));
            assert_eq!(tx.try_send(1), Err(TrySendError::Full(1)));
        }

        #[test]
        fn try_send3() {
            let (tx, rx) = sync_channel::<i32>(1);
            assert_eq!(tx.try_send(1), Ok(()));
            drop(rx);
            assert_eq!(tx.try_send(1), Err(TrySendError::Disconnected(1)));
        }

        #[test]
        fn issue_15761() {
            fn repro() {
                let (tx1, rx1) = sync_channel::<()>(3);
                let (tx2, rx2) = sync_channel::<()>(3);

                let _t = thread::spawn(move|| {
                    rx1.recv().unwrap();
                    tx2.try_send(()).unwrap();
                });

                tx1.try_send(()).unwrap();
                rx2.recv().unwrap();
            }

            for _ in 0..100 {
                repro()
            }
        }

        #[test]
        fn fmt_debug_sender() {
            let (tx, _) = channel::<i32>();
            assert_eq!(format!("{:?}", tx), "Sender { .. }");
        }

        #[test]
        fn fmt_debug_recv() {
            let (_, rx) = channel::<i32>();
            assert_eq!(format!("{:?}", rx), "Receiver { .. }");
        }

        #[test]
        fn fmt_debug_sync_sender() {
            let (tx, _) = sync_channel::<i32>(1);
            assert_eq!(format!("{:?}", tx), "SyncSender { .. }");
        }
    }

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