// ignore-tidy-filelength

//! 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.
//!
//! [`Sender`]: ../../../std/sync/mpsc/struct.Sender.html
//! [`SyncSender`]: ../../../std/sync/mpsc/struct.SyncSender.html
//! [`Receiver`]: ../../../std/sync/mpsc/struct.Receiver.html
//! [`send`]: ../../../std/sync/mpsc/struct.Sender.html#method.send
//! [`channel`]: ../../../std/sync/mpsc/fn.channel.html
//! [`sync_channel`]: ../../../std/sync/mpsc/fn.sync_channel.html
//!
//! ## 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.
//!
//! [`Result`]: ../../../std/result/enum.Result.html
//! [`Err`]: ../../../std/result/enum.Result.html#variant.Err
//! [`unwrap`]: ../../../std/result/enum.Result.html#method.unwrap
//!
//! # 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().
//
// FIXME: Select is now removed, so these factors are ready to be cleaned up!
//
// # 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 crate::cell::UnsafeCell;
use crate::error;
use crate::fmt;
use crate::mem;
use crate::sync::Arc;
use crate::time::{Duration, Instant};

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

mod cache_aligned;

/// The receiving half of Rust's [`channel`][] (or [`sync_channel`]) type.
/// This half can only be owned by one thread.
///
/// Messages sent to the channel can be retrieved using [`recv`].
///
/// [`channel`]: fn.channel.html
/// [`sync_channel`]: fn.sync_channel.html
/// [`recv`]: struct.Receiver.html#method.recv
///
/// # Examples
///
/// ```rust
/// use std::sync::mpsc::channel;
/// use std::thread;
/// use std::time::Duration;
///
/// let (send, recv) = channel();
///
/// thread::spawn(move || {
///     send.send("Hello world!").unwrap();
///     thread::sleep(Duration::from_secs(2)); // block for two seconds
///     send.send("Delayed for 2 seconds").unwrap();
/// });
///
/// println!("{}", recv.recv().unwrap()); // Received immediately
/// println!("Waiting...");
/// println!("{}", recv.recv().unwrap()); // Received after 2 seconds
/// ```
#[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`], created by [`iter`].
///
/// 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.
///
/// [`iter`]: struct.Receiver.html#method.iter
/// [`Receiver`]: struct.Receiver.html
/// [`next`]: ../../../std/iter/trait.Iterator.html#tymethod.next
/// [`None`]: ../../../std/option/enum.Option.html#variant.None
///
/// # Examples
///
/// ```rust
/// use std::sync::mpsc::channel;
/// use std::thread;
///
/// let (send, recv) = channel();
///
/// thread::spawn(move || {
///     send.send(1u8).unwrap();
///     send.send(2u8).unwrap();
///     send.send(3u8).unwrap();
/// });
///
/// for x in recv.iter() {
///     println!("Got: {}", x);
/// }
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Debug)]
pub struct Iter<'a, T: 'a> {
    rx: &'a Receiver<T>,
}

/// An iterator that attempts to yield all pending values for a [`Receiver`],
/// created by [`try_iter`].
///
/// [`None`] will be returned when there are no pending values remaining or
/// if the corresponding channel has hung up.
///
/// This iterator will never block the caller in order to wait for data to
/// become available. Instead, it will return [`None`].
///
/// [`Receiver`]: struct.Receiver.html
/// [`try_iter`]: struct.Receiver.html#method.try_iter
/// [`None`]: ../../../std/option/enum.Option.html#variant.None
///
/// # Examples
///
/// ```rust
/// use std::sync::mpsc::channel;
/// use std::thread;
/// use std::time::Duration;
///
/// let (sender, receiver) = channel();
///
/// // Nothing is in the buffer yet
/// assert!(receiver.try_iter().next().is_none());
/// println!("Nothing in the buffer...");
///
/// thread::spawn(move || {
///     sender.send(1).unwrap();
///     sender.send(2).unwrap();
///     sender.send(3).unwrap();
/// });
///
/// println!("Going to sleep...");
/// thread::sleep(Duration::from_secs(2)); // block for two seconds
///
/// for x in receiver.try_iter() {
///     println!("Got: {}", x);
/// }
/// ```
#[stable(feature = "receiver_try_iter", since = "1.15.0")]
#[derive(Debug)]
pub struct TryIter<'a, T: 'a> {
    rx: &'a Receiver<T>,
}

/// An owning iterator over messages on a [`Receiver`],
/// created by **Receiver::into_iter**.
///
/// This iterator will block whenever [`next`]
/// is called, waiting for a new message, and [`None`] will be
/// returned if the corresponding channel has hung up.
///
/// [`Receiver`]: struct.Receiver.html
/// [`next`]: ../../../std/iter/trait.Iterator.html#tymethod.next
/// [`None`]: ../../../std/option/enum.Option.html#variant.None
///
/// # Examples
///
/// ```rust
/// use std::sync::mpsc::channel;
/// use std::thread;
///
/// let (send, recv) = channel();
///
/// thread::spawn(move || {
///     send.send(1u8).unwrap();
///     send.send(2u8).unwrap();
///     send.send(3u8).unwrap();
/// });
///
/// for x in recv.into_iter() {
///     println!("Got: {}", x);
/// }
/// ```
#[stable(feature = "receiver_into_iter", since = "1.1.0")]
#[derive(Debug)]
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.
///
/// Messages can be sent through this channel with [`send`].
///
/// [`channel`]: fn.channel.html
/// [`send`]: struct.Sender.html#method.send
///
/// # Examples
///
/// ```rust
/// use std::sync::mpsc::channel;
/// use std::thread;
///
/// let (sender, receiver) = channel();
/// let sender2 = sender.clone();
///
/// // First thread owns sender
/// thread::spawn(move || {
///     sender.send(1).unwrap();
/// });
///
/// // Second thread owns sender2
/// thread::spawn(move || {
///     sender2.send(2).unwrap();
/// });
///
/// let msg = receiver.recv().unwrap();
/// let msg2 = receiver.recv().unwrap();
///
/// assert_eq!(3, msg + msg2);
/// ```
#[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 [`sync_channel`] type.
///
/// Messages can be sent through this channel with [`send`] or [`try_send`].
///
/// [`send`] will block if there is no space in the internal buffer.
///
/// [`sync_channel`]: fn.sync_channel.html
/// [`send`]: struct.SyncSender.html#method.send
/// [`try_send`]: struct.SyncSender.html#method.try_send
///
/// # Examples
///
/// ```rust
/// use std::sync::mpsc::sync_channel;
/// use std::thread;
///
/// // Create a sync_channel with buffer size 2
/// let (sync_sender, receiver) = sync_channel(2);
/// let sync_sender2 = sync_sender.clone();
///
/// // First thread owns sync_sender
/// thread::spawn(move || {
///     sync_sender.send(1).unwrap();
///     sync_sender.send(2).unwrap();
/// });
///
/// // Second thread owns sync_sender2
/// thread::spawn(move || {
///     sync_sender2.send(3).unwrap();
///     // thread will now block since the buffer is full
///     println!("Thread unblocked!");
/// });
///
/// let mut msg;
///
/// msg = receiver.recv().unwrap();
/// println!("message {} received", msg);
///
/// // "Thread unblocked!" will be printed now
///
/// msg = receiver.recv().unwrap();
/// println!("message {} received", msg);
///
/// msg = receiver.recv().unwrap();
///
/// println!("message {} received", msg);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub struct SyncSender<T> {
    inner: Arc<sync::Packet<T>>,
}

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

/// An error returned from the [`Sender::send`] or [`SyncSender::send`]
/// function on **channel**s.
///
/// 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.
///
/// [`Sender::send`]: struct.Sender.html#method.send
/// [`SyncSender::send`]: struct.SyncSender.html#method.send
#[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`][`channel`] (or [`sync_channel`]) is disconnected, implying that no further
/// messages will ever be received.
///
/// [`recv`]: struct.Receiver.html#method.recv
/// [`Receiver`]: struct.Receiver.html
/// [`channel`]: fn.channel.html
/// [`sync_channel`]: fn.sync_channel.html
#[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. This can occur with both a [`channel`] and
/// a [`sync_channel`].
///
/// [`try_recv`]: struct.Receiver.html#method.try_recv
/// [`channel`]: fn.channel.html
/// [`sync_channel`]: fn.sync_channel.html
#[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,

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

/// This enumeration is the list of possible errors that made [`recv_timeout`]
/// unable to return data when called. This can occur with both a [`channel`] and
/// a [`sync_channel`].
///
/// [`recv_timeout`]: struct.Receiver.html#method.recv_timeout
/// [`channel`]: fn.channel.html
/// [`sync_channel`]: fn.sync_channel.html
#[derive(PartialEq, Eq, Clone, Copy, Debug)]
#[stable(feature = "mpsc_recv_timeout", since = "1.12.0")]
pub enum RecvTimeoutError {
    /// This **channel** is currently empty, but the **Sender**(s) have not yet
    /// disconnected, so data may yet become available.
    #[stable(feature = "mpsc_recv_timeout", since = "1.12.0")]
    Timeout,
    /// The **channel**'s sending half has become disconnected, and there will
    /// never be any more data received on it.
    #[stable(feature = "mpsc_recv_timeout", since = "1.12.0")]
    Disconnected,
}

/// This enumeration is the list of the possible error outcomes for the
/// [`try_send`] method.
///
/// [`try_send`]: struct.SyncSender.html#method.try_send
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(PartialEq, Eq, Clone, Copy)]
pub enum TrySendError<T> {
    /// The data could not be sent on the [`sync_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.
    ///
    /// [`sync_channel`]: fn.sync_channel.html
    /// [`Receiver`]: struct.Receiver.html
    #[stable(feature = "rust1", since = "1.0.0")]
    Full(#[stable(feature = "rust1", since = "1.0.0")] T),

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

enum Flavor<T> {
    Oneshot(Arc<oneshot::Packet<T>>),
    Stream(Arc<stream::Packet<T>>),
    Shared(Arc<shared::Packet<T>>),
    Sync(Arc<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`] in
/// the same order as it was sent, and no [`send`] will block the calling thread
/// (this channel has an "infinite buffer", unlike [`sync_channel`], which will
/// block after its buffer limit is reached). [`recv`] will block until a message
/// is available.
///
/// The [`Sender`] can be cloned to [`send`] to the same channel multiple times, but
/// only one [`Receiver`] is supported.
///
/// If the [`Receiver`] is disconnected while trying to [`send`] with the
/// [`Sender`], the [`send`] method will return a [`SendError`]. Similarly, if the
/// [`Sender`] is disconnected while trying to [`recv`], the [`recv`] method will
/// return a [`RecvError`].
///
/// [`send`]: struct.Sender.html#method.send
/// [`recv`]: struct.Receiver.html#method.recv
/// [`Sender`]: struct.Sender.html
/// [`Receiver`]: struct.Receiver.html
/// [`sync_channel`]: fn.sync_channel.html
/// [`SendError`]: struct.SendError.html
/// [`RecvError`]: struct.RecvError.html
///
/// # Examples
///
/// ```
/// use std::sync::mpsc::channel;
/// use std::thread;
///
/// let (sender, receiver) = channel();
///
/// // Spawn off an expensive computation
/// thread::spawn(move|| {
/// #   fn expensive_computation() {}
///     sender.send(expensive_computation()).unwrap();
/// });
///
/// // Do some useful work for awhile
///
/// // Let's see what that answer was
/// println!("{:?}", receiver.recv().unwrap());
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
    let a = Arc::new(oneshot::Packet::new());
    (Sender::new(Flavor::Oneshot(a.clone())), Receiver::new(Flavor::Oneshot(a)))
}

/// Creates a new synchronous, bounded channel.
/// All data sent on the [`SyncSender`] will become available on the [`Receiver`]
/// in the same order as it was sent. Like asynchronous [`channel`]s, the
/// [`Receiver`] will block until a message becomes available. `sync_channel`
/// differs greatly in the semantics of the sender, however.
///
/// This channel has an internal buffer on which messages will be queued.
/// `bound` specifies the buffer size. 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.
///
/// The [`SyncSender`] can be cloned to [`send`] to the same channel multiple
/// times, but only one [`Receiver`] is supported.
///
/// Like asynchronous channels, if the [`Receiver`] is disconnected while trying
/// to [`send`] with the [`SyncSender`], the [`send`] method will return a
/// [`SendError`]. Similarly, If the [`SyncSender`] is disconnected while trying
/// to [`recv`], the [`recv`] method will return a [`RecvError`].
///
/// [`channel`]: fn.channel.html
/// [`send`]: struct.SyncSender.html#method.send
/// [`recv`]: struct.Receiver.html#method.recv
/// [`SyncSender`]: struct.SyncSender.html
/// [`Receiver`]: struct.Receiver.html
/// [`SendError`]: struct.SendError.html
/// [`RecvError`]: struct.RecvError.html
///
/// # Examples
///
/// ```
/// use std::sync::mpsc::sync_channel;
/// use std::thread;
///
/// let (sender, receiver) = sync_channel(1);
///
/// // this returns immediately
/// sender.send(1).unwrap();
///
/// thread::spawn(move|| {
///     // this will block until the previous message has been received
///     sender.send(2).unwrap();
/// });
///
/// assert_eq!(receiver.recv().unwrap(), 1);
/// assert_eq!(receiver.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(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`].
    ///
    /// [`Err`]: ../../../std/result/enum.Result.html#variant.Err
    /// [`Ok`]: ../../../std/result/enum.Result.html#variant.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).unwrap_err().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) => {
                if !p.sent() {
                    return p.send(t).map_err(SendError);
                } else {
                    let a = Arc::new(stream::Packet::new());
                    let rx = Receiver::new(Flavor::Stream(a.clone()));
                    match p.upgrade(rx) {
                        oneshot::UpSuccess => {
                            let ret = a.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.send(t).ok().unwrap();
                            token.signal();
                            (a, Ok(()))
                        }
                    }
                }
            }
            Flavor::Stream(ref p) => return p.send(t).map_err(SendError),
            Flavor::Shared(ref p) => return p.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 = match *unsafe { self.inner() } {
            Flavor::Oneshot(ref p) => {
                let a = Arc::new(shared::Packet::new());
                {
                    let guard = a.postinit_lock();
                    let rx = Receiver::new(Flavor::Shared(a.clone()));
                    let sleeper = match p.upgrade(rx) {
                        oneshot::UpSuccess | oneshot::UpDisconnected => None,
                        oneshot::UpWoke(task) => Some(task),
                    };
                    a.inherit_blocker(sleeper, guard);
                }
                a
            }
            Flavor::Stream(ref p) => {
                let a = Arc::new(shared::Packet::new());
                {
                    let guard = a.postinit_lock();
                    let rx = Receiver::new(Flavor::Shared(a.clone()));
                    let sleeper = match p.upgrade(rx) {
                        stream::UpSuccess | stream::UpDisconnected => None,
                        stream::UpWoke(task) => Some(task),
                    };
                    a.inherit_blocker(sleeper, guard);
                }
                a
            }
            Flavor::Shared(ref p) => {
                p.clone_chan();
                return Sender::new(Flavor::Shared(p.clone()));
            }
            Flavor::Sync(..) => unreachable!(),
        };

        unsafe {
            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() } {
            Flavor::Oneshot(ref p) => p.drop_chan(),
            Flavor::Stream(ref p) => p.drop_chan(),
            Flavor::Shared(ref p) => p.drop_chan(),
            Flavor::Sync(..) => unreachable!(),
        }
    }
}

#[stable(feature = "mpsc_debug", since = "1.8.0")]
impl<T> fmt::Debug for Sender<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Sender").finish()
    }
}

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

impl<T> SyncSender<T> {
    fn new(inner: Arc<sync::Packet<T>>) -> SyncSender<T> {
        SyncSender { 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, the channel becomes a rendezvous
    /// channel and it guarantees 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.
    ///
    /// [`Err`]: ../../../std/result/enum.Result.html#variant.Err
    /// [`Receiver`]: ../../../std/sync/mpsc/struct.Receiver.html
    ///
    /// # Examples
    ///
    /// ```rust
    /// use std::sync::mpsc::sync_channel;
    /// use std::thread;
    ///
    /// // Create a rendezvous sync_channel with buffer size 0
    /// let (sync_sender, receiver) = sync_channel(0);
    ///
    /// thread::spawn(move || {
    ///    println!("sending message...");
    ///    sync_sender.send(1).unwrap();
    ///    // Thread is now blocked until the message is received
    ///
    ///    println!("...message received!");
    /// });
    ///
    /// let msg = receiver.recv().unwrap();
    /// assert_eq!(1, msg);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn send(&self, t: T) -> Result<(), SendError<T>> {
        self.inner.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 [`send`] for notes about guarantees of whether the
    /// receiver has received the data or not if this function is successful.
    ///
    /// [`send`]: ../../../std/sync/mpsc/struct.SyncSender.html#method.send
    ///
    /// # Examples
    ///
    /// ```rust
    /// use std::sync::mpsc::sync_channel;
    /// use std::thread;
    ///
    /// // Create a sync_channel with buffer size 1
    /// let (sync_sender, receiver) = sync_channel(1);
    /// let sync_sender2 = sync_sender.clone();
    ///
    /// // First thread owns sync_sender
    /// thread::spawn(move || {
    ///     sync_sender.send(1).unwrap();
    ///     sync_sender.send(2).unwrap();
    ///     // Thread blocked
    /// });
    ///
    /// // Second thread owns sync_sender2
    /// thread::spawn(move || {
    ///     // This will return an error and send
    ///     // no message if the buffer is full
    ///     let _ = sync_sender2.try_send(3);
    /// });
    ///
    /// let mut msg;
    /// msg = receiver.recv().unwrap();
    /// println!("message {} received", msg);
    ///
    /// msg = receiver.recv().unwrap();
    /// println!("message {} received", msg);
    ///
    /// // Third message may have never been sent
    /// match receiver.try_recv() {
    ///     Ok(msg) => println!("message {} received", msg),
    ///     Err(_) => println!("the third message was never sent"),
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn try_send(&self, t: T) -> Result<(), TrySendError<T>> {
        self.inner.try_send(t)
    }
}

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

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

#[stable(feature = "mpsc_debug", since = "1.8.0")]
impl<T> fmt::Debug for SyncSender<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("SyncSender").finish()
    }
}

////////////////////////////////////////////////////////////////////////////////
// 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.
    ///
    /// Compared with [`recv`], this function has two failure cases instead of one
    /// (one for disconnection, one for an empty buffer).
    ///
    /// [`recv`]: struct.Receiver.html#method.recv
    ///
    /// # Examples
    ///
    /// ```rust
    /// use std::sync::mpsc::{Receiver, channel};
    ///
    /// let (_, receiver): (_, Receiver<i32>) = channel();
    ///
    /// assert!(receiver.try_recv().is_err());
    /// ```
    #[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 p.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 p.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 p.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 p.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`][] (or [`SyncSender`]), 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.
    ///
    /// [`Sender`]: struct.Sender.html
    /// [`SyncSender`]: struct.SyncSender.html
    /// [`Err`]: ../../../std/result/enum.Result.html#variant.Err
    ///
    /// # 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 p.recv(None) {
                    Ok(t) => return Ok(t),
                    Err(oneshot::Disconnected) => return Err(RecvError),
                    Err(oneshot::Upgraded(rx)) => rx,
                    Err(oneshot::Empty) => unreachable!(),
                },
                Flavor::Stream(ref p) => match p.recv(None) {
                    Ok(t) => return Ok(t),
                    Err(stream::Disconnected) => return Err(RecvError),
                    Err(stream::Upgraded(rx)) => rx,
                    Err(stream::Empty) => unreachable!(),
                },
                Flavor::Shared(ref p) => match p.recv(None) {
                    Ok(t) => return Ok(t),
                    Err(shared::Disconnected) => return Err(RecvError),
                    Err(shared::Empty) => unreachable!(),
                },
                Flavor::Sync(ref p) => return p.recv(None).map_err(|_| RecvError),
            };
            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, or if it waits more than `timeout`.
    ///
    /// 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`][] (or [`SyncSender`]), 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.
    ///
    /// [`Sender`]: struct.Sender.html
    /// [`SyncSender`]: struct.SyncSender.html
    /// [`Err`]: ../../../std/result/enum.Result.html#variant.Err
    ///
    /// # Known Issues
    ///
    /// There is currently a known issue (see [`#39364`]) that causes `recv_timeout`
    /// to panic unexpectedly with the following example:
    ///
    /// ```no_run
    /// use std::sync::mpsc::channel;
    /// use std::thread;
    /// use std::time::Duration;
    ///
    /// let (tx, rx) = channel::<String>();
    ///
    /// thread::spawn(move || {
    ///     let d = Duration::from_millis(10);
    ///     loop {
    ///         println!("recv");
    ///         let _r = rx.recv_timeout(d);
    ///     }
    /// });
    ///
    /// thread::sleep(Duration::from_millis(100));
    /// let _c1 = tx.clone();
    ///
    /// thread::sleep(Duration::from_secs(1));
    /// ```
    ///
    /// [`#39364`]: https://github.com/rust-lang/rust/issues/39364
    ///
    /// # Examples
    ///
    /// Successfully receiving value before encountering timeout:
    ///
    /// ```no_run
    /// use std::thread;
    /// use std::time::Duration;
    /// use std::sync::mpsc;
    ///
    /// let (send, recv) = mpsc::channel();
    ///
    /// thread::spawn(move || {
    ///     send.send('a').unwrap();
    /// });
    ///
    /// assert_eq!(
    ///     recv.recv_timeout(Duration::from_millis(400)),
    ///     Ok('a')
    /// );
    /// ```
    ///
    /// Receiving an error upon reaching timeout:
    ///
    /// ```no_run
    /// use std::thread;
    /// use std::time::Duration;
    /// use std::sync::mpsc;
    ///
    /// let (send, recv) = mpsc::channel();
    ///
    /// thread::spawn(move || {
    ///     thread::sleep(Duration::from_millis(800));
    ///     send.send('a').unwrap();
    /// });
    ///
    /// assert_eq!(
    ///     recv.recv_timeout(Duration::from_millis(400)),
    ///     Err(mpsc::RecvTimeoutError::Timeout)
    /// );
    /// ```
    #[stable(feature = "mpsc_recv_timeout", since = "1.12.0")]
    pub fn recv_timeout(&self, timeout: Duration) -> Result<T, RecvTimeoutError> {
        // Do an optimistic try_recv to avoid the performance impact of
        // Instant::now() in the full-channel case.
        match self.try_recv() {
            Ok(result) => Ok(result),
            Err(TryRecvError::Disconnected) => Err(RecvTimeoutError::Disconnected),
            Err(TryRecvError::Empty) => match Instant::now().checked_add(timeout) {
                Some(deadline) => self.recv_deadline(deadline),
                // So far in the future that it's practically the same as waiting indefinitely.
                None => self.recv().map_err(RecvTimeoutError::from),
            },
        }
    }

    /// Attempts to wait for a value on this receiver, returning an error if the
    /// corresponding channel has hung up, or if `deadline` is reached.
    ///
    /// 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`][] (or [`SyncSender`]), 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.
    ///
    /// [`Sender`]: struct.Sender.html
    /// [`SyncSender`]: struct.SyncSender.html
    /// [`Err`]: ../../../std/result/enum.Result.html#variant.Err
    ///
    /// # Examples
    ///
    /// Successfully receiving value before reaching deadline:
    ///
    /// ```no_run
    /// #![feature(deadline_api)]
    /// use std::thread;
    /// use std::time::{Duration, Instant};
    /// use std::sync::mpsc;
    ///
    /// let (send, recv) = mpsc::channel();
    ///
    /// thread::spawn(move || {
    ///     send.send('a').unwrap();
    /// });
    ///
    /// assert_eq!(
    ///     recv.recv_deadline(Instant::now() + Duration::from_millis(400)),
    ///     Ok('a')
    /// );
    /// ```
    ///
    /// Receiving an error upon reaching deadline:
    ///
    /// ```no_run
    /// #![feature(deadline_api)]
    /// use std::thread;
    /// use std::time::{Duration, Instant};
    /// use std::sync::mpsc;
    ///
    /// let (send, recv) = mpsc::channel();
    ///
    /// thread::spawn(move || {
    ///     thread::sleep(Duration::from_millis(800));
    ///     send.send('a').unwrap();
    /// });
    ///
    /// assert_eq!(
    ///     recv.recv_deadline(Instant::now() + Duration::from_millis(400)),
    ///     Err(mpsc::RecvTimeoutError::Timeout)
    /// );
    /// ```
    #[unstable(feature = "deadline_api", issue = "46316")]
    pub fn recv_deadline(&self, deadline: Instant) -> Result<T, RecvTimeoutError> {
        use self::RecvTimeoutError::*;

        loop {
            let port_or_empty = match *unsafe { self.inner() } {
                Flavor::Oneshot(ref p) => match p.recv(Some(deadline)) {
                    Ok(t) => return Ok(t),
                    Err(oneshot::Disconnected) => return Err(Disconnected),
                    Err(oneshot::Upgraded(rx)) => Some(rx),
                    Err(oneshot::Empty) => None,
                },
                Flavor::Stream(ref p) => match p.recv(Some(deadline)) {
                    Ok(t) => return Ok(t),
                    Err(stream::Disconnected) => return Err(Disconnected),
                    Err(stream::Upgraded(rx)) => Some(rx),
                    Err(stream::Empty) => None,
                },
                Flavor::Shared(ref p) => match p.recv(Some(deadline)) {
                    Ok(t) => return Ok(t),
                    Err(shared::Disconnected) => return Err(Disconnected),
                    Err(shared::Empty) => None,
                },
                Flavor::Sync(ref p) => match p.recv(Some(deadline)) {
                    Ok(t) => return Ok(t),
                    Err(sync::Disconnected) => return Err(Disconnected),
                    Err(sync::Empty) => None,
                },
            };

            if let Some(new_port) = port_or_empty {
                unsafe {
                    mem::swap(self.inner_mut(), new_port.inner_mut());
                }
            }

            // If we're already passed the deadline, and we're here without
            // data, return a timeout, else try again.
            if Instant::now() >= deadline {
                return Err(Timeout);
            }
        }
    }

    /// Returns an iterator that will block waiting for messages, but never
    /// [`panic!`]. It will return [`None`] when the channel has hung up.
    ///
    /// [`panic!`]: ../../../std/macro.panic.html
    /// [`None`]: ../../../std/option/enum.Option.html#variant.None
    ///
    /// # Examples
    ///
    /// ```rust
    /// use std::sync::mpsc::channel;
    /// use std::thread;
    ///
    /// let (send, recv) = channel();
    ///
    /// thread::spawn(move || {
    ///     send.send(1).unwrap();
    ///     send.send(2).unwrap();
    ///     send.send(3).unwrap();
    /// });
    ///
    /// let mut iter = recv.iter();
    /// assert_eq!(iter.next(), Some(1));
    /// assert_eq!(iter.next(), Some(2));
    /// assert_eq!(iter.next(), Some(3));
    /// assert_eq!(iter.next(), None);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn iter(&self) -> Iter<'_, T> {
        Iter { rx: self }
    }

    /// Returns an iterator that will attempt to yield all pending values.
    /// It will return `None` if there are no more pending values or if the
    /// channel has hung up. The iterator will never [`panic!`] or block the
    /// user by waiting for values.
    ///
    /// [`panic!`]: ../../../std/macro.panic.html
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::sync::mpsc::channel;
    /// use std::thread;
    /// use std::time::Duration;
    ///
    /// let (sender, receiver) = channel();
    ///
    /// // nothing is in the buffer yet
    /// assert!(receiver.try_iter().next().is_none());
    ///
    /// thread::spawn(move || {
    ///     thread::sleep(Duration::from_secs(1));
    ///     sender.send(1).unwrap();
    ///     sender.send(2).unwrap();
    ///     sender.send(3).unwrap();
    /// });
    ///
    /// // nothing is in the buffer yet
    /// assert!(receiver.try_iter().next().is_none());
    ///
    /// // block for two seconds
    /// thread::sleep(Duration::from_secs(2));
    ///
    /// let mut iter = receiver.try_iter();
    /// assert_eq!(iter.next(), Some(1));
    /// assert_eq!(iter.next(), Some(2));
    /// assert_eq!(iter.next(), Some(3));
    /// assert_eq!(iter.next(), None);
    /// ```
    #[stable(feature = "receiver_try_iter", since = "1.15.0")]
    pub fn try_iter(&self) -> TryIter<'_, T> {
        TryIter { rx: self }
    }
}

#[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_try_iter", since = "1.15.0")]
impl<'a, T> Iterator for TryIter<'a, T> {
    type Item = T;

    fn next(&mut self) -> Option<T> {
        self.rx.try_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() } {
            Flavor::Oneshot(ref p) => p.drop_port(),
            Flavor::Stream(ref p) => p.drop_port(),
            Flavor::Shared(ref p) => p.drop_port(),
            Flavor::Sync(ref p) => p.drop_port(),
        }
    }
}

#[stable(feature = "mpsc_debug", since = "1.8.0")]
impl<T> fmt::Debug for Receiver<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Receiver").finish()
    }
}

#[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> error::Error for SendError<T> {
    fn description(&self) -> &str {
        "sending on a closed channel"
    }
}

#[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> 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",
        }
    }
}

#[stable(feature = "mpsc_error_conversions", since = "1.24.0")]
impl<T> From<SendError<T>> for TrySendError<T> {
    fn from(err: SendError<T>) -> TrySendError<T> {
        match err {
            SendError(t) => TrySendError::Disconnected(t),
        }
    }
}

#[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"
    }
}

#[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",
        }
    }
}

#[stable(feature = "mpsc_error_conversions", since = "1.24.0")]
impl From<RecvError> for TryRecvError {
    fn from(err: RecvError) -> TryRecvError {
        match err {
            RecvError => TryRecvError::Disconnected,
        }
    }
}

#[stable(feature = "mpsc_recv_timeout_error", since = "1.15.0")]
impl fmt::Display for RecvTimeoutError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            RecvTimeoutError::Timeout => "timed out waiting on channel".fmt(f),
            RecvTimeoutError::Disconnected => "channel is empty and sending half is closed".fmt(f),
        }
    }
}

#[stable(feature = "mpsc_recv_timeout_error", since = "1.15.0")]
impl error::Error for RecvTimeoutError {
    fn description(&self) -> &str {
        match *self {
            RecvTimeoutError::Timeout => "timed out waiting on channel",
            RecvTimeoutError::Disconnected => "channel is empty and sending half is closed",
        }
    }
}

#[stable(feature = "mpsc_error_conversions", since = "1.24.0")]
impl From<RecvError> for RecvTimeoutError {
    fn from(err: RecvError) -> RecvTimeoutError {
        match err {
            RecvError => RecvTimeoutError::Disconnected,
        }
    }
}

#[cfg(all(test, not(target_os = "emscripten")))]
mod tests {
    use super::*;
    use crate::env;
    use crate::thread;
    use crate::time::{Duration, Instant};

    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().expect("thread panicked");
    }

    #[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().expect("thread panicked");
    }

    #[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().expect("thread panicked");
        t2.join().ok().expect("thread panicked");
    }

    #[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().expect("thread panicked");
    }

    #[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().expect("thread panicked");
        t2.join().ok().expect("thread panicked");
    }

    #[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() == 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() == 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() == 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() == 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() == i);
                    recv(rx, i + 1);
                });
            }
        }
    }

    #[test]
    #[cfg_attr(target_env = "sgx", ignore)] // FIXME: https://github.com/fortanix/rust-sgx/issues/31
    fn oneshot_single_thread_recv_timeout() {
        let (tx, rx) = channel();
        tx.send(()).unwrap();
        assert_eq!(rx.recv_timeout(Duration::from_millis(1)), Ok(()));
        assert_eq!(rx.recv_timeout(Duration::from_millis(1)), Err(RecvTimeoutError::Timeout));
        tx.send(()).unwrap();
        assert_eq!(rx.recv_timeout(Duration::from_millis(1)), Ok(()));
    }

    #[test]
    #[cfg_attr(target_env = "sgx", ignore)] // FIXME: https://github.com/fortanix/rust-sgx/issues/31
    fn stress_recv_timeout_two_threads() {
        let (tx, rx) = channel();
        let stress = stress_factor() + 100;
        let timeout = Duration::from_millis(100);

        thread::spawn(move || {
            for i in 0..stress {
                if i % 2 == 0 {
                    thread::sleep(timeout * 2);
                }
                tx.send(1usize).unwrap();
            }
        });

        let mut recv_count = 0;
        loop {
            match rx.recv_timeout(timeout) {
                Ok(n) => {
                    assert_eq!(n, 1usize);
                    recv_count += 1;
                }
                Err(RecvTimeoutError::Timeout) => continue,
                Err(RecvTimeoutError::Disconnected) => break,
            }
        }

        assert_eq!(recv_count, stress);
    }

    #[test]
    #[cfg_attr(target_env = "sgx", ignore)] // FIXME: https://github.com/fortanix/rust-sgx/issues/31
    fn recv_timeout_upgrade() {
        let (tx, rx) = channel::<()>();
        let timeout = Duration::from_millis(1);
        let _tx_clone = tx.clone();

        let start = Instant::now();
        assert_eq!(rx.recv_timeout(timeout), Err(RecvTimeoutError::Timeout));
        assert!(Instant::now() >= start + timeout);
    }

    #[test]
    #[cfg_attr(target_env = "sgx", ignore)] // FIXME: https://github.com/fortanix/rust-sgx/issues/31
    fn stress_recv_timeout_shared() {
        let (tx, rx) = channel();
        let stress = stress_factor() + 100;

        for i in 0..stress {
            let tx = tx.clone();
            thread::spawn(move || {
                thread::sleep(Duration::from_millis(i as u64 * 10));
                tx.send(1usize).unwrap();
            });
        }

        drop(tx);

        let mut recv_count = 0;
        loop {
            match rx.recv_timeout(Duration::from_millis(10)) {
                Ok(n) => {
                    assert_eq!(n, 1usize);
                    recv_count += 1;
                }
                Err(RecvTimeoutError::Timeout) => continue,
                Err(RecvTimeoutError::Disconnected) => break,
            }
        }

        assert_eq!(recv_count, stress);
    }

    #[test]
    #[cfg_attr(target_env = "sgx", ignore)] // FIXME: https://github.com/fortanix/rust-sgx/issues/31
    fn very_long_recv_timeout_wont_panic() {
        let (tx, rx) = channel::<()>();
        let join_handle =
            thread::spawn(move || rx.recv_timeout(Duration::from_secs(u64::max_value())));
        thread::sleep(Duration::from_secs(1));
        assert!(tx.send(()).is_ok());
        assert_eq!(join_handle.join().unwrap(), Ok(()));
    }

    #[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]
    #[cfg_attr(target_env = "sgx", ignore)] // FIXME: https://github.com/fortanix/rust-sgx/issues/31
    fn shared_recv_timeout() {
        let (tx, rx) = channel();
        let total = 5;
        for _ in 0..total {
            let tx = tx.clone();
            thread::spawn(move || {
                tx.send(()).unwrap();
            });
        }

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

        assert_eq!(rx.recv_timeout(Duration::from_millis(1)), Err(RecvTimeoutError::Timeout));
        tx.send(()).unwrap();
        assert_eq!(rx.recv_timeout(Duration::from_millis(1)), Ok(()));
    }

    #[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_try_iter() {
        let (request_tx, request_rx) = channel();
        let (response_tx, response_rx) = channel();

        // Request `x`s until we have `6`.
        let t = thread::spawn(move || {
            let mut count = 0;
            loop {
                for x in response_rx.try_iter() {
                    count += x;
                    if count == 6 {
                        return count;
                    }
                }
                request_tx.send(()).unwrap();
            }
        });

        for _ in request_rx.iter() {
            if response_tx.send(2).is_err() {
                break;
            }
        }

        assert_eq!(t.join().unwrap(), 6);
    }

    #[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();
    }

    #[test]
    fn issue_32114() {
        let (tx, _) = channel();
        let _ = tx.send(123);
        assert_eq!(tx.send(123), Err(SendError(123)));
    }
}

#[cfg(all(test, not(target_os = "emscripten")))]
mod sync_tests {
    use super::*;
    use crate::env;
    use crate::thread;
    use crate::time::Duration;

    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]
    #[cfg_attr(target_env = "sgx", ignore)] // FIXME: https://github.com/fortanix/rust-sgx/issues/31
    fn recv_timeout() {
        let (tx, rx) = sync_channel::<i32>(1);
        assert_eq!(rx.recv_timeout(Duration::from_millis(1)), Err(RecvTimeoutError::Timeout));
        tx.send(1).unwrap();
        assert_eq!(rx.recv_timeout(Duration::from_millis(1)), Ok(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]
    #[cfg_attr(target_env = "sgx", ignore)] // FIXME: https://github.com/fortanix/rust-sgx/issues/31
    fn stress_recv_timeout_two_threads() {
        let (tx, rx) = sync_channel::<i32>(0);

        thread::spawn(move || {
            for _ in 0..10000 {
                tx.send(1).unwrap();
            }
        });

        let mut recv_count = 0;
        loop {
            match rx.recv_timeout(Duration::from_millis(1)) {
                Ok(v) => {
                    assert_eq!(v, 1);
                    recv_count += 1;
                }
                Err(RecvTimeoutError::Timeout) => continue,
                Err(RecvTimeoutError::Disconnected) => break,
            }
        }

        assert_eq!(recv_count, 10000);
    }

    #[test]
    #[cfg_attr(target_env = "sgx", ignore)] // FIXME: https://github.com/fortanix/rust-sgx/issues/31
    fn stress_recv_timeout_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 || {
            let mut recv_count = 0;
            loop {
                match rx.recv_timeout(Duration::from_millis(10)) {
                    Ok(v) => {
                        assert_eq!(v, 1);
                        recv_count += 1;
                    }
                    Err(RecvTimeoutError::Timeout) => continue,
                    Err(RecvTimeoutError::Disconnected) => break,
                }
            }

            assert_eq!(recv_count, AMT * NTHREADS);
            assert!(rx.try_recv().is_err());

            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 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() == 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_try_recv_closed_with_data() {
        let (tx, rx) = sync_channel::<i32>(1);
        tx.send(10).unwrap();
        drop(tx);
        assert_eq!(rx.try_recv(), Ok(10));
        assert_eq!(rx.try_recv(), Err(TryRecvError::Disconnected));
    }

    #[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() == 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() == 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() == 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() == 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()
        }
    }
}