Merging upstream version 1.67.1+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

10 files changed, 2655 insertions, 0 deletions

diff --git a/library/std/src/sync/mpmc/array.rs b/library/std/src/sync/mpmc/array.rs
new file mode 100644
index 000000000..c1e3e48b0
--- /dev/null
+++ b/library/std/src/sync/mpmc/array.rs
@@ -0,0 +1,513 @@
+//! Bounded channel based on a preallocated array.
+//!
+//! This flavor has a fixed, positive capacity.
+//!
+//! The implementation is based on Dmitry Vyukov's bounded MPMC queue.
+//!
+//! Source:
+//!   - <http://www.1024cores.net/home/lock-free-algorithms/queues/bounded-mpmc-queue>
+//!   - <https://docs.google.com/document/d/1yIAYmbvL3JxOKOjuCyon7JhW4cSv1wy5hC0ApeGMV9s/pub>
+
+use super::context::Context;
+use super::error::*;
+use super::select::{Operation, Selected, Token};
+use super::utils::{Backoff, CachePadded};
+use super::waker::SyncWaker;
+
+use crate::cell::UnsafeCell;
+use crate::mem::MaybeUninit;
+use crate::ptr;
+use crate::sync::atomic::{self, AtomicUsize, Ordering};
+use crate::time::Instant;
+
+/// A slot in a channel.
+struct Slot<T> {
+    /// The current stamp.
+    stamp: AtomicUsize,
+
+    /// The message in this slot.
+    msg: UnsafeCell<MaybeUninit<T>>,
+}
+
+/// The token type for the array flavor.
+#[derive(Debug)]
+pub(crate) struct ArrayToken {
+    /// Slot to read from or write to.
+    slot: *const u8,
+
+    /// Stamp to store into the slot after reading or writing.
+    stamp: usize,
+}
+
+impl Default for ArrayToken {
+    #[inline]
+    fn default() -> Self {
+        ArrayToken { slot: ptr::null(), stamp: 0 }
+    }
+}
+
+/// Bounded channel based on a preallocated array.
+pub(crate) struct Channel<T> {
+    /// The head of the channel.
+    ///
+    /// This value is a "stamp" consisting of an index into the buffer, a mark bit, and a lap, but
+    /// packed into a single `usize`. The lower bits represent the index, while the upper bits
+    /// represent the lap. The mark bit in the head is always zero.
+    ///
+    /// Messages are popped from the head of the channel.
+    head: CachePadded<AtomicUsize>,
+
+    /// The tail of the channel.
+    ///
+    /// This value is a "stamp" consisting of an index into the buffer, a mark bit, and a lap, but
+    /// packed into a single `usize`. The lower bits represent the index, while the upper bits
+    /// represent the lap. The mark bit indicates that the channel is disconnected.
+    ///
+    /// Messages are pushed into the tail of the channel.
+    tail: CachePadded<AtomicUsize>,
+
+    /// The buffer holding slots.
+    buffer: Box<[Slot<T>]>,
+
+    /// The channel capacity.
+    cap: usize,
+
+    /// A stamp with the value of `{ lap: 1, mark: 0, index: 0 }`.
+    one_lap: usize,
+
+    /// If this bit is set in the tail, that means the channel is disconnected.
+    mark_bit: usize,
+
+    /// Senders waiting while the channel is full.
+    senders: SyncWaker,
+
+    /// Receivers waiting while the channel is empty and not disconnected.
+    receivers: SyncWaker,
+}
+
+impl<T> Channel<T> {
+    /// Creates a bounded channel of capacity `cap`.
+    pub(crate) fn with_capacity(cap: usize) -> Self {
+        assert!(cap > 0, "capacity must be positive");
+
+        // Compute constants `mark_bit` and `one_lap`.
+        let mark_bit = (cap + 1).next_power_of_two();
+        let one_lap = mark_bit * 2;
+
+        // Head is initialized to `{ lap: 0, mark: 0, index: 0 }`.
+        let head = 0;
+        // Tail is initialized to `{ lap: 0, mark: 0, index: 0 }`.
+        let tail = 0;
+
+        // Allocate a buffer of `cap` slots initialized
+        // with stamps.
+        let buffer: Box<[Slot<T>]> = (0..cap)
+            .map(|i| {
+                // Set the stamp to `{ lap: 0, mark: 0, index: i }`.
+                Slot { stamp: AtomicUsize::new(i), msg: UnsafeCell::new(MaybeUninit::uninit()) }
+            })
+            .collect();
+
+        Channel {
+            buffer,
+            cap,
+            one_lap,
+            mark_bit,
+            head: CachePadded::new(AtomicUsize::new(head)),
+            tail: CachePadded::new(AtomicUsize::new(tail)),
+            senders: SyncWaker::new(),
+            receivers: SyncWaker::new(),
+        }
+    }
+
+    /// Attempts to reserve a slot for sending a message.
+    fn start_send(&self, token: &mut Token) -> bool {
+        let backoff = Backoff::new();
+        let mut tail = self.tail.load(Ordering::Relaxed);
+
+        loop {
+            // Check if the channel is disconnected.
+            if tail & self.mark_bit != 0 {
+                token.array.slot = ptr::null();
+                token.array.stamp = 0;
+                return true;
+            }
+
+            // Deconstruct the tail.
+            let index = tail & (self.mark_bit - 1);
+            let lap = tail & !(self.one_lap - 1);
+
+            // Inspect the corresponding slot.
+            debug_assert!(index < self.buffer.len());
+            let slot = unsafe { self.buffer.get_unchecked(index) };
+            let stamp = slot.stamp.load(Ordering::Acquire);
+
+            // If the tail and the stamp match, we may attempt to push.
+            if tail == stamp {
+                let new_tail = if index + 1 < self.cap {
+                    // Same lap, incremented index.
+                    // Set to `{ lap: lap, mark: 0, index: index + 1 }`.
+                    tail + 1
+                } else {
+                    // One lap forward, index wraps around to zero.
+                    // Set to `{ lap: lap.wrapping_add(1), mark: 0, index: 0 }`.
+                    lap.wrapping_add(self.one_lap)
+                };
+
+                // Try moving the tail.
+                match self.tail.compare_exchange_weak(
+                    tail,
+                    new_tail,
+                    Ordering::SeqCst,
+                    Ordering::Relaxed,
+                ) {
+                    Ok(_) => {
+                        // Prepare the token for the follow-up call to `write`.
+                        token.array.slot = slot as *const Slot<T> as *const u8;
+                        token.array.stamp = tail + 1;
+                        return true;
+                    }
+                    Err(_) => {
+                        backoff.spin_light();
+                        tail = self.tail.load(Ordering::Relaxed);
+                    }
+                }
+            } else if stamp.wrapping_add(self.one_lap) == tail + 1 {
+                atomic::fence(Ordering::SeqCst);
+                let head = self.head.load(Ordering::Relaxed);
+
+                // If the head lags one lap behind the tail as well...
+                if head.wrapping_add(self.one_lap) == tail {
+                    // ...then the channel is full.
+                    return false;
+                }
+
+                backoff.spin_light();
+                tail = self.tail.load(Ordering::Relaxed);
+            } else {
+                // Snooze because we need to wait for the stamp to get updated.
+                backoff.spin_heavy();
+                tail = self.tail.load(Ordering::Relaxed);
+            }
+        }
+    }
+
+    /// Writes a message into the channel.
+    pub(crate) unsafe fn write(&self, token: &mut Token, msg: T) -> Result<(), T> {
+        // If there is no slot, the channel is disconnected.
+        if token.array.slot.is_null() {
+            return Err(msg);
+        }
+
+        let slot: &Slot<T> = &*(token.array.slot as *const Slot<T>);
+
+        // Write the message into the slot and update the stamp.
+        slot.msg.get().write(MaybeUninit::new(msg));
+        slot.stamp.store(token.array.stamp, Ordering::Release);
+
+        // Wake a sleeping receiver.
+        self.receivers.notify();
+        Ok(())
+    }
+
+    /// Attempts to reserve a slot for receiving a message.
+    fn start_recv(&self, token: &mut Token) -> bool {
+        let backoff = Backoff::new();
+        let mut head = self.head.load(Ordering::Relaxed);
+
+        loop {
+            // Deconstruct the head.
+            let index = head & (self.mark_bit - 1);
+            let lap = head & !(self.one_lap - 1);
+
+            // Inspect the corresponding slot.
+            debug_assert!(index < self.buffer.len());
+            let slot = unsafe { self.buffer.get_unchecked(index) };
+            let stamp = slot.stamp.load(Ordering::Acquire);
+
+            // If the stamp is ahead of the head by 1, we may attempt to pop.
+            if head + 1 == stamp {
+                let new = if index + 1 < self.cap {
+                    // Same lap, incremented index.
+                    // Set to `{ lap: lap, mark: 0, index: index + 1 }`.
+                    head + 1
+                } else {
+                    // One lap forward, index wraps around to zero.
+                    // Set to `{ lap: lap.wrapping_add(1), mark: 0, index: 0 }`.
+                    lap.wrapping_add(self.one_lap)
+                };
+
+                // Try moving the head.
+                match self.head.compare_exchange_weak(
+                    head,
+                    new,
+                    Ordering::SeqCst,
+                    Ordering::Relaxed,
+                ) {
+                    Ok(_) => {
+                        // Prepare the token for the follow-up call to `read`.
+                        token.array.slot = slot as *const Slot<T> as *const u8;
+                        token.array.stamp = head.wrapping_add(self.one_lap);
+                        return true;
+                    }
+                    Err(_) => {
+                        backoff.spin_light();
+                        head = self.head.load(Ordering::Relaxed);
+                    }
+                }
+            } else if stamp == head {
+                atomic::fence(Ordering::SeqCst);
+                let tail = self.tail.load(Ordering::Relaxed);
+
+                // If the tail equals the head, that means the channel is empty.
+                if (tail & !self.mark_bit) == head {
+                    // If the channel is disconnected...
+                    if tail & self.mark_bit != 0 {
+                        // ...then receive an error.
+                        token.array.slot = ptr::null();
+                        token.array.stamp = 0;
+                        return true;
+                    } else {
+                        // Otherwise, the receive operation is not ready.
+                        return false;
+                    }
+                }
+
+                backoff.spin_light();
+                head = self.head.load(Ordering::Relaxed);
+            } else {
+                // Snooze because we need to wait for the stamp to get updated.
+                backoff.spin_heavy();
+                head = self.head.load(Ordering::Relaxed);
+            }
+        }
+    }
+
+    /// Reads a message from the channel.
+    pub(crate) unsafe fn read(&self, token: &mut Token) -> Result<T, ()> {
+        if token.array.slot.is_null() {
+            // The channel is disconnected.
+            return Err(());
+        }
+
+        let slot: &Slot<T> = &*(token.array.slot as *const Slot<T>);
+
+        // Read the message from the slot and update the stamp.
+        let msg = slot.msg.get().read().assume_init();
+        slot.stamp.store(token.array.stamp, Ordering::Release);
+
+        // Wake a sleeping sender.
+        self.senders.notify();
+        Ok(msg)
+    }
+
+    /// Attempts to send a message into the channel.
+    pub(crate) fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> {
+        let token = &mut Token::default();
+        if self.start_send(token) {
+            unsafe { self.write(token, msg).map_err(TrySendError::Disconnected) }
+        } else {
+            Err(TrySendError::Full(msg))
+        }
+    }
+
+    /// Sends a message into the channel.
+    pub(crate) fn send(
+        &self,
+        msg: T,
+        deadline: Option<Instant>,
+    ) -> Result<(), SendTimeoutError<T>> {
+        let token = &mut Token::default();
+        loop {
+            // Try sending a message several times.
+            let backoff = Backoff::new();
+            loop {
+                if self.start_send(token) {
+                    let res = unsafe { self.write(token, msg) };
+                    return res.map_err(SendTimeoutError::Disconnected);
+                }
+
+                if backoff.is_completed() {
+                    break;
+                } else {
+                    backoff.spin_light();
+                }
+            }
+
+            if let Some(d) = deadline {
+                if Instant::now() >= d {
+                    return Err(SendTimeoutError::Timeout(msg));
+                }
+            }
+
+            Context::with(|cx| {
+                // Prepare for blocking until a receiver wakes us up.
+                let oper = Operation::hook(token);
+                self.senders.register(oper, cx);
+
+                // Has the channel become ready just now?
+                if !self.is_full() || self.is_disconnected() {
+                    let _ = cx.try_select(Selected::Aborted);
+                }
+
+                // Block the current thread.
+                let sel = cx.wait_until(deadline);
+
+                match sel {
+                    Selected::Waiting => unreachable!(),
+                    Selected::Aborted | Selected::Disconnected => {
+                        self.senders.unregister(oper).unwrap();
+                    }
+                    Selected::Operation(_) => {}
+                }
+            });
+        }
+    }
+
+    /// Attempts to receive a message without blocking.
+    pub(crate) fn try_recv(&self) -> Result<T, TryRecvError> {
+        let token = &mut Token::default();
+
+        if self.start_recv(token) {
+            unsafe { self.read(token).map_err(|_| TryRecvError::Disconnected) }
+        } else {
+            Err(TryRecvError::Empty)
+        }
+    }
+
+    /// Receives a message from the channel.
+    pub(crate) fn recv(&self, deadline: Option<Instant>) -> Result<T, RecvTimeoutError> {
+        let token = &mut Token::default();
+        loop {
+            if self.start_recv(token) {
+                let res = unsafe { self.read(token) };
+                return res.map_err(|_| RecvTimeoutError::Disconnected);
+            }
+
+            if let Some(d) = deadline {
+                if Instant::now() >= d {
+                    return Err(RecvTimeoutError::Timeout);
+                }
+            }
+
+            Context::with(|cx| {
+                // Prepare for blocking until a sender wakes us up.
+                let oper = Operation::hook(token);
+                self.receivers.register(oper, cx);
+
+                // Has the channel become ready just now?
+                if !self.is_empty() || self.is_disconnected() {
+                    let _ = cx.try_select(Selected::Aborted);
+                }
+
+                // Block the current thread.
+                let sel = cx.wait_until(deadline);
+
+                match sel {
+                    Selected::Waiting => unreachable!(),
+                    Selected::Aborted | Selected::Disconnected => {
+                        self.receivers.unregister(oper).unwrap();
+                        // If the channel was disconnected, we still have to check for remaining
+                        // messages.
+                    }
+                    Selected::Operation(_) => {}
+                }
+            });
+        }
+    }
+
+    /// Returns the current number of messages inside the channel.
+    pub(crate) fn len(&self) -> usize {
+        loop {
+            // Load the tail, then load the head.
+            let tail = self.tail.load(Ordering::SeqCst);
+            let head = self.head.load(Ordering::SeqCst);
+
+            // If the tail didn't change, we've got consistent values to work with.
+            if self.tail.load(Ordering::SeqCst) == tail {
+                let hix = head & (self.mark_bit - 1);
+                let tix = tail & (self.mark_bit - 1);
+
+                return if hix < tix {
+                    tix - hix
+                } else if hix > tix {
+                    self.cap - hix + tix
+                } else if (tail & !self.mark_bit) == head {
+                    0
+                } else {
+                    self.cap
+                };
+            }
+        }
+    }
+
+    /// Returns the capacity of the channel.
+    #[allow(clippy::unnecessary_wraps)] // This is intentional.
+    pub(crate) fn capacity(&self) -> Option<usize> {
+        Some(self.cap)
+    }
+
+    /// Disconnects the channel and wakes up all blocked senders and receivers.
+    ///
+    /// Returns `true` if this call disconnected the channel.
+    pub(crate) fn disconnect(&self) -> bool {
+        let tail = self.tail.fetch_or(self.mark_bit, Ordering::SeqCst);
+
+        if tail & self.mark_bit == 0 {
+            self.senders.disconnect();
+            self.receivers.disconnect();
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Returns `true` if the channel is disconnected.
+    pub(crate) fn is_disconnected(&self) -> bool {
+        self.tail.load(Ordering::SeqCst) & self.mark_bit != 0
+    }
+
+    /// Returns `true` if the channel is empty.
+    pub(crate) fn is_empty(&self) -> bool {
+        let head = self.head.load(Ordering::SeqCst);
+        let tail = self.tail.load(Ordering::SeqCst);
+
+        // Is the tail equal to the head?
+        //
+        // Note: If the head changes just before we load the tail, that means there was a moment
+        // when the channel was not empty, so it is safe to just return `false`.
+        (tail & !self.mark_bit) == head
+    }
+
+    /// Returns `true` if the channel is full.
+    pub(crate) fn is_full(&self) -> bool {
+        let tail = self.tail.load(Ordering::SeqCst);
+        let head = self.head.load(Ordering::SeqCst);
+
+        // Is the head lagging one lap behind tail?
+        //
+        // Note: If the tail changes just before we load the head, that means there was a moment
+        // when the channel was not full, so it is safe to just return `false`.
+        head.wrapping_add(self.one_lap) == tail & !self.mark_bit
+    }
+}
+
+impl<T> Drop for Channel<T> {
+    fn drop(&mut self) {
+        // Get the index of the head.
+        let hix = self.head.load(Ordering::Relaxed) & (self.mark_bit - 1);
+
+        // Loop over all slots that hold a message and drop them.
+        for i in 0..self.len() {
+            // Compute the index of the next slot holding a message.
+            let index = if hix + i < self.cap { hix + i } else { hix + i - self.cap };
+
+            unsafe {
+                debug_assert!(index < self.buffer.len());
+                let slot = self.buffer.get_unchecked_mut(index);
+                let msg = &mut *slot.msg.get();
+                msg.as_mut_ptr().drop_in_place();
+            }
+        }
+    }
+}
diff --git a/library/std/src/sync/mpmc/context.rs b/library/std/src/sync/mpmc/context.rs
new file mode 100644
index 000000000..bbfc6ce00
--- /dev/null
+++ b/library/std/src/sync/mpmc/context.rs
@@ -0,0 +1,155 @@
+//! Thread-local channel context.
+
+use super::select::Selected;
+use super::waker::current_thread_id;
+
+use crate::cell::Cell;
+use crate::ptr;
+use crate::sync::atomic::{AtomicPtr, AtomicUsize, Ordering};
+use crate::sync::Arc;
+use crate::thread::{self, Thread};
+use crate::time::Instant;
+
+/// Thread-local context.
+#[derive(Debug, Clone)]
+pub struct Context {
+    inner: Arc<Inner>,
+}
+
+/// Inner representation of `Context`.
+#[derive(Debug)]
+struct Inner {
+    /// Selected operation.
+    select: AtomicUsize,
+
+    /// A slot into which another thread may store a pointer to its `Packet`.
+    packet: AtomicPtr<()>,
+
+    /// Thread handle.
+    thread: Thread,
+
+    /// Thread id.
+    thread_id: usize,
+}
+
+impl Context {
+    /// Creates a new context for the duration of the closure.
+    #[inline]
+    pub fn with<F, R>(f: F) -> R
+    where
+        F: FnOnce(&Context) -> R,
+    {
+        thread_local! {
+            /// Cached thread-local context.
+            static CONTEXT: Cell<Option<Context>> = Cell::new(Some(Context::new()));
+        }
+
+        let mut f = Some(f);
+        let mut f = |cx: &Context| -> R {
+            let f = f.take().unwrap();
+            f(cx)
+        };
+
+        CONTEXT
+            .try_with(|cell| match cell.take() {
+                None => f(&Context::new()),
+                Some(cx) => {
+                    cx.reset();
+                    let res = f(&cx);
+                    cell.set(Some(cx));
+                    res
+                }
+            })
+            .unwrap_or_else(|_| f(&Context::new()))
+    }
+
+    /// Creates a new `Context`.
+    #[cold]
+    fn new() -> Context {
+        Context {
+            inner: Arc::new(Inner {
+                select: AtomicUsize::new(Selected::Waiting.into()),
+                packet: AtomicPtr::new(ptr::null_mut()),
+                thread: thread::current(),
+                thread_id: current_thread_id(),
+            }),
+        }
+    }
+
+    /// Resets `select` and `packet`.
+    #[inline]
+    fn reset(&self) {
+        self.inner.select.store(Selected::Waiting.into(), Ordering::Release);
+        self.inner.packet.store(ptr::null_mut(), Ordering::Release);
+    }
+
+    /// Attempts to select an operation.
+    ///
+    /// On failure, the previously selected operation is returned.
+    #[inline]
+    pub fn try_select(&self, select: Selected) -> Result<(), Selected> {
+        self.inner
+            .select
+            .compare_exchange(
+                Selected::Waiting.into(),
+                select.into(),
+                Ordering::AcqRel,
+                Ordering::Acquire,
+            )
+            .map(|_| ())
+            .map_err(|e| e.into())
+    }
+
+    /// Stores a packet.
+    ///
+    /// This method must be called after `try_select` succeeds and there is a packet to provide.
+    #[inline]
+    pub fn store_packet(&self, packet: *mut ()) {
+        if !packet.is_null() {
+            self.inner.packet.store(packet, Ordering::Release);
+        }
+    }
+
+    /// Waits until an operation is selected and returns it.
+    ///
+    /// If the deadline is reached, `Selected::Aborted` will be selected.
+    #[inline]
+    pub fn wait_until(&self, deadline: Option<Instant>) -> Selected {
+        loop {
+            // Check whether an operation has been selected.
+            let sel = Selected::from(self.inner.select.load(Ordering::Acquire));
+            if sel != Selected::Waiting {
+                return sel;
+            }
+
+            // If there's a deadline, park the current thread until the deadline is reached.
+            if let Some(end) = deadline {
+                let now = Instant::now();
+
+                if now < end {
+                    thread::park_timeout(end - now);
+                } else {
+                    // The deadline has been reached. Try aborting select.
+                    return match self.try_select(Selected::Aborted) {
+                        Ok(()) => Selected::Aborted,
+                        Err(s) => s,
+                    };
+                }
+            } else {
+                thread::park();
+            }
+        }
+    }
+
+    /// Unparks the thread this context belongs to.
+    #[inline]
+    pub fn unpark(&self) {
+        self.inner.thread.unpark();
+    }
+
+    /// Returns the id of the thread this context belongs to.
+    #[inline]
+    pub fn thread_id(&self) -> usize {
+        self.inner.thread_id
+    }
+}
diff --git a/library/std/src/sync/mpmc/counter.rs b/library/std/src/sync/mpmc/counter.rs
new file mode 100644
index 000000000..a5a6bdc67
--- /dev/null
+++ b/library/std/src/sync/mpmc/counter.rs
@@ -0,0 +1,137 @@
+use crate::ops;
+use crate::process;
+use crate::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
+
+/// Reference counter internals.
+struct Counter<C> {
+    /// The number of senders associated with the channel.
+    senders: AtomicUsize,
+
+    /// The number of receivers associated with the channel.
+    receivers: AtomicUsize,
+
+    /// Set to `true` if the last sender or the last receiver reference deallocates the channel.
+    destroy: AtomicBool,
+
+    /// The internal channel.
+    chan: C,
+}
+
+/// Wraps a channel into the reference counter.
+pub(crate) fn new<C>(chan: C) -> (Sender<C>, Receiver<C>) {
+    let counter = Box::into_raw(Box::new(Counter {
+        senders: AtomicUsize::new(1),
+        receivers: AtomicUsize::new(1),
+        destroy: AtomicBool::new(false),
+        chan,
+    }));
+    let s = Sender { counter };
+    let r = Receiver { counter };
+    (s, r)
+}
+
+/// The sending side.
+pub(crate) struct Sender<C> {
+    counter: *mut Counter<C>,
+}
+
+impl<C> Sender<C> {
+    /// Returns the internal `Counter`.
+    fn counter(&self) -> &Counter<C> {
+        unsafe { &*self.counter }
+    }
+
+    /// Acquires another sender reference.
+    pub(crate) fn acquire(&self) -> Sender<C> {
+        let count = self.counter().senders.fetch_add(1, Ordering::Relaxed);
+
+        // Cloning senders and calling `mem::forget` on the clones could potentially overflow the
+        // counter. It's very difficult to recover sensibly from such degenerate scenarios so we
+        // just abort when the count becomes very large.
+        if count > isize::MAX as usize {
+            process::abort();
+        }
+
+        Sender { counter: self.counter }
+    }
+
+    /// Releases the sender reference.
+    ///
+    /// Function `disconnect` will be called if this is the last sender reference.
+    pub(crate) unsafe fn release<F: FnOnce(&C) -> bool>(&self, disconnect: F) {
+        if self.counter().senders.fetch_sub(1, Ordering::AcqRel) == 1 {
+            disconnect(&self.counter().chan);
+
+            if self.counter().destroy.swap(true, Ordering::AcqRel) {
+                drop(Box::from_raw(self.counter));
+            }
+        }
+    }
+}
+
+impl<C> ops::Deref for Sender<C> {
+    type Target = C;
+
+    fn deref(&self) -> &C {
+        &self.counter().chan
+    }
+}
+
+impl<C> PartialEq for Sender<C> {
+    fn eq(&self, other: &Sender<C>) -> bool {
+        self.counter == other.counter
+    }
+}
+
+/// The receiving side.
+pub(crate) struct Receiver<C> {
+    counter: *mut Counter<C>,
+}
+
+impl<C> Receiver<C> {
+    /// Returns the internal `Counter`.
+    fn counter(&self) -> &Counter<C> {
+        unsafe { &*self.counter }
+    }
+
+    /// Acquires another receiver reference.
+    pub(crate) fn acquire(&self) -> Receiver<C> {
+        let count = self.counter().receivers.fetch_add(1, Ordering::Relaxed);
+
+        // Cloning receivers and calling `mem::forget` on the clones could potentially overflow the
+        // counter. It's very difficult to recover sensibly from such degenerate scenarios so we
+        // just abort when the count becomes very large.
+        if count > isize::MAX as usize {
+            process::abort();
+        }
+
+        Receiver { counter: self.counter }
+    }
+
+    /// Releases the receiver reference.
+    ///
+    /// Function `disconnect` will be called if this is the last receiver reference.
+    pub(crate) unsafe fn release<F: FnOnce(&C) -> bool>(&self, disconnect: F) {
+        if self.counter().receivers.fetch_sub(1, Ordering::AcqRel) == 1 {
+            disconnect(&self.counter().chan);
+
+            if self.counter().destroy.swap(true, Ordering::AcqRel) {
+                drop(Box::from_raw(self.counter));
+            }
+        }
+    }
+}
+
+impl<C> ops::Deref for Receiver<C> {
+    type Target = C;
+
+    fn deref(&self) -> &C {
+        &self.counter().chan
+    }
+}
+
+impl<C> PartialEq for Receiver<C> {
+    fn eq(&self, other: &Receiver<C>) -> bool {
+        self.counter == other.counter
+    }
+}
diff --git a/library/std/src/sync/mpmc/error.rs b/library/std/src/sync/mpmc/error.rs
new file mode 100644
index 000000000..1b8a1f387
--- /dev/null
+++ b/library/std/src/sync/mpmc/error.rs
@@ -0,0 +1,46 @@
+use crate::error;
+use crate::fmt;
+
+pub use crate::sync::mpsc::{RecvError, RecvTimeoutError, SendError, TryRecvError, TrySendError};
+
+/// An error returned from the [`send_timeout`] method.
+///
+/// The error contains the message being sent so it can be recovered.
+///
+/// [`send_timeout`]: super::Sender::send_timeout
+#[derive(PartialEq, Eq, Clone, Copy)]
+pub enum SendTimeoutError<T> {
+    /// The message could not be sent because the channel is full and the operation timed out.
+    ///
+    /// If this is a zero-capacity channel, then the error indicates that there was no receiver
+    /// available to receive the message and the operation timed out.
+    Timeout(T),
+
+    /// The message could not be sent because the channel is disconnected.
+    Disconnected(T),
+}
+
+impl<T> fmt::Debug for SendTimeoutError<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        "SendTimeoutError(..)".fmt(f)
+    }
+}
+
+impl<T> fmt::Display for SendTimeoutError<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {
+            SendTimeoutError::Timeout(..) => "timed out waiting on send operation".fmt(f),
+            SendTimeoutError::Disconnected(..) => "sending on a disconnected channel".fmt(f),
+        }
+    }
+}
+
+impl<T: Send> error::Error for SendTimeoutError<T> {}
+
+impl<T> From<SendError<T>> for SendTimeoutError<T> {
+    fn from(err: SendError<T>) -> SendTimeoutError<T> {
+        match err {
+            SendError(e) => SendTimeoutError::Disconnected(e),
+        }
+    }
+}
diff --git a/library/std/src/sync/mpmc/list.rs b/library/std/src/sync/mpmc/list.rs
new file mode 100644
index 000000000..ec6c0726a
--- /dev/null
+++ b/library/std/src/sync/mpmc/list.rs
@@ -0,0 +1,638 @@
+//! Unbounded channel implemented as a linked list.
+
+use super::context::Context;
+use super::error::*;
+use super::select::{Operation, Selected, Token};
+use super::utils::{Backoff, CachePadded};
+use super::waker::SyncWaker;
+
+use crate::cell::UnsafeCell;
+use crate::marker::PhantomData;
+use crate::mem::MaybeUninit;
+use crate::ptr;
+use crate::sync::atomic::{self, AtomicPtr, AtomicUsize, Ordering};
+use crate::time::Instant;
+
+// Bits indicating the state of a slot:
+// * If a message has been written into the slot, `WRITE` is set.
+// * If a message has been read from the slot, `READ` is set.
+// * If the block is being destroyed, `DESTROY` is set.
+const WRITE: usize = 1;
+const READ: usize = 2;
+const DESTROY: usize = 4;
+
+// Each block covers one "lap" of indices.
+const LAP: usize = 32;
+// The maximum number of messages a block can hold.
+const BLOCK_CAP: usize = LAP - 1;
+// How many lower bits are reserved for metadata.
+const SHIFT: usize = 1;
+// Has two different purposes:
+// * If set in head, indicates that the block is not the last one.
+// * If set in tail, indicates that the channel is disconnected.
+const MARK_BIT: usize = 1;
+
+/// A slot in a block.
+struct Slot<T> {
+    /// The message.
+    msg: UnsafeCell<MaybeUninit<T>>,
+
+    /// The state of the slot.
+    state: AtomicUsize,
+}
+
+impl<T> Slot<T> {
+    /// Waits until a message is written into the slot.
+    fn wait_write(&self) {
+        let backoff = Backoff::new();
+        while self.state.load(Ordering::Acquire) & WRITE == 0 {
+            backoff.spin_heavy();
+        }
+    }
+}
+
+/// A block in a linked list.
+///
+/// Each block in the list can hold up to `BLOCK_CAP` messages.
+struct Block<T> {
+    /// The next block in the linked list.
+    next: AtomicPtr<Block<T>>,
+
+    /// Slots for messages.
+    slots: [Slot<T>; BLOCK_CAP],
+}
+
+impl<T> Block<T> {
+    /// Creates an empty block.
+    fn new() -> Block<T> {
+        // SAFETY: This is safe because:
+        //  [1] `Block::next` (AtomicPtr) may be safely zero initialized.
+        //  [2] `Block::slots` (Array) may be safely zero initialized because of [3, 4].
+        //  [3] `Slot::msg` (UnsafeCell) may be safely zero initialized because it
+        //       holds a MaybeUninit.
+        //  [4] `Slot::state` (AtomicUsize) may be safely zero initialized.
+        unsafe { MaybeUninit::zeroed().assume_init() }
+    }
+
+    /// Waits until the next pointer is set.
+    fn wait_next(&self) -> *mut Block<T> {
+        let backoff = Backoff::new();
+        loop {
+            let next = self.next.load(Ordering::Acquire);
+            if !next.is_null() {
+                return next;
+            }
+            backoff.spin_heavy();
+        }
+    }
+
+    /// Sets the `DESTROY` bit in slots starting from `start` and destroys the block.
+    unsafe fn destroy(this: *mut Block<T>, start: usize) {
+        // It is not necessary to set the `DESTROY` bit in the last slot because that slot has
+        // begun destruction of the block.
+        for i in start..BLOCK_CAP - 1 {
+            let slot = (*this).slots.get_unchecked(i);
+
+            // Mark the `DESTROY` bit if a thread is still using the slot.
+            if slot.state.load(Ordering::Acquire) & READ == 0
+                && slot.state.fetch_or(DESTROY, Ordering::AcqRel) & READ == 0
+            {
+                // If a thread is still using the slot, it will continue destruction of the block.
+                return;
+            }
+        }
+
+        // No thread is using the block, now it is safe to destroy it.
+        drop(Box::from_raw(this));
+    }
+}
+
+/// A position in a channel.
+#[derive(Debug)]
+struct Position<T> {
+    /// The index in the channel.
+    index: AtomicUsize,
+
+    /// The block in the linked list.
+    block: AtomicPtr<Block<T>>,
+}
+
+/// The token type for the list flavor.
+#[derive(Debug)]
+pub(crate) struct ListToken {
+    /// The block of slots.
+    block: *const u8,
+
+    /// The offset into the block.
+    offset: usize,
+}
+
+impl Default for ListToken {
+    #[inline]
+    fn default() -> Self {
+        ListToken { block: ptr::null(), offset: 0 }
+    }
+}
+
+/// Unbounded channel implemented as a linked list.
+///
+/// Each message sent into the channel is assigned a sequence number, i.e. an index. Indices are
+/// represented as numbers of type `usize` and wrap on overflow.
+///
+/// Consecutive messages are grouped into blocks in order to put less pressure on the allocator and
+/// improve cache efficiency.
+pub(crate) struct Channel<T> {
+    /// The head of the channel.
+    head: CachePadded<Position<T>>,
+
+    /// The tail of the channel.
+    tail: CachePadded<Position<T>>,
+
+    /// Receivers waiting while the channel is empty and not disconnected.
+    receivers: SyncWaker,
+
+    /// Indicates that dropping a `Channel<T>` may drop messages of type `T`.
+    _marker: PhantomData<T>,
+}
+
+impl<T> Channel<T> {
+    /// Creates a new unbounded channel.
+    pub(crate) fn new() -> Self {
+        Channel {
+            head: CachePadded::new(Position {
+                block: AtomicPtr::new(ptr::null_mut()),
+                index: AtomicUsize::new(0),
+            }),
+            tail: CachePadded::new(Position {
+                block: AtomicPtr::new(ptr::null_mut()),
+                index: AtomicUsize::new(0),
+            }),
+            receivers: SyncWaker::new(),
+            _marker: PhantomData,
+        }
+    }
+
+    /// Attempts to reserve a slot for sending a message.
+    fn start_send(&self, token: &mut Token) -> bool {
+        let backoff = Backoff::new();
+        let mut tail = self.tail.index.load(Ordering::Acquire);
+        let mut block = self.tail.block.load(Ordering::Acquire);
+        let mut next_block = None;
+
+        loop {
+            // Check if the channel is disconnected.
+            if tail & MARK_BIT != 0 {
+                token.list.block = ptr::null();
+                return true;
+            }
+
+            // Calculate the offset of the index into the block.
+            let offset = (tail >> SHIFT) % LAP;
+
+            // If we reached the end of the block, wait until the next one is installed.
+            if offset == BLOCK_CAP {
+                backoff.spin_heavy();
+                tail = self.tail.index.load(Ordering::Acquire);
+                block = self.tail.block.load(Ordering::Acquire);
+                continue;
+            }
+
+            // If we're going to have to install the next block, allocate it in advance in order to
+            // make the wait for other threads as short as possible.
+            if offset + 1 == BLOCK_CAP && next_block.is_none() {
+                next_block = Some(Box::new(Block::<T>::new()));
+            }
+
+            // If this is the first message to be sent into the channel, we need to allocate the
+            // first block and install it.
+            if block.is_null() {
+                let new = Box::into_raw(Box::new(Block::<T>::new()));
+
+                if self
+                    .tail
+                    .block
+                    .compare_exchange(block, new, Ordering::Release, Ordering::Relaxed)
+                    .is_ok()
+                {
+                    self.head.block.store(new, Ordering::Release);
+                    block = new;
+                } else {
+                    next_block = unsafe { Some(Box::from_raw(new)) };
+                    tail = self.tail.index.load(Ordering::Acquire);
+                    block = self.tail.block.load(Ordering::Acquire);
+                    continue;
+                }
+            }
+
+            let new_tail = tail + (1 << SHIFT);
+
+            // Try advancing the tail forward.
+            match self.tail.index.compare_exchange_weak(
+                tail,
+                new_tail,
+                Ordering::SeqCst,
+                Ordering::Acquire,
+            ) {
+                Ok(_) => unsafe {
+                    // If we've reached the end of the block, install the next one.
+                    if offset + 1 == BLOCK_CAP {
+                        let next_block = Box::into_raw(next_block.unwrap());
+                        self.tail.block.store(next_block, Ordering::Release);
+                        self.tail.index.fetch_add(1 << SHIFT, Ordering::Release);
+                        (*block).next.store(next_block, Ordering::Release);
+                    }
+
+                    token.list.block = block as *const u8;
+                    token.list.offset = offset;
+                    return true;
+                },
+                Err(_) => {
+                    backoff.spin_light();
+                    tail = self.tail.index.load(Ordering::Acquire);
+                    block = self.tail.block.load(Ordering::Acquire);
+                }
+            }
+        }
+    }
+
+    /// Writes a message into the channel.
+    pub(crate) unsafe fn write(&self, token: &mut Token, msg: T) -> Result<(), T> {
+        // If there is no slot, the channel is disconnected.
+        if token.list.block.is_null() {
+            return Err(msg);
+        }
+
+        // Write the message into the slot.
+        let block = token.list.block as *mut Block<T>;
+        let offset = token.list.offset;
+        let slot = (*block).slots.get_unchecked(offset);
+        slot.msg.get().write(MaybeUninit::new(msg));
+        slot.state.fetch_or(WRITE, Ordering::Release);
+
+        // Wake a sleeping receiver.
+        self.receivers.notify();
+        Ok(())
+    }
+
+    /// Attempts to reserve a slot for receiving a message.
+    fn start_recv(&self, token: &mut Token) -> bool {
+        let backoff = Backoff::new();
+        let mut head = self.head.index.load(Ordering::Acquire);
+        let mut block = self.head.block.load(Ordering::Acquire);
+
+        loop {
+            // Calculate the offset of the index into the block.
+            let offset = (head >> SHIFT) % LAP;
+
+            // If we reached the end of the block, wait until the next one is installed.
+            if offset == BLOCK_CAP {
+                backoff.spin_heavy();
+                head = self.head.index.load(Ordering::Acquire);
+                block = self.head.block.load(Ordering::Acquire);
+                continue;
+            }
+
+            let mut new_head = head + (1 << SHIFT);
+
+            if new_head & MARK_BIT == 0 {
+                atomic::fence(Ordering::SeqCst);
+                let tail = self.tail.index.load(Ordering::Relaxed);
+
+                // If the tail equals the head, that means the channel is empty.
+                if head >> SHIFT == tail >> SHIFT {
+                    // If the channel is disconnected...
+                    if tail & MARK_BIT != 0 {
+                        // ...then receive an error.
+                        token.list.block = ptr::null();
+                        return true;
+                    } else {
+                        // Otherwise, the receive operation is not ready.
+                        return false;
+                    }
+                }
+
+                // If head and tail are not in the same block, set `MARK_BIT` in head.
+                if (head >> SHIFT) / LAP != (tail >> SHIFT) / LAP {
+                    new_head |= MARK_BIT;
+                }
+            }
+
+            // The block can be null here only if the first message is being sent into the channel.
+            // In that case, just wait until it gets initialized.
+            if block.is_null() {
+                backoff.spin_heavy();
+                head = self.head.index.load(Ordering::Acquire);
+                block = self.head.block.load(Ordering::Acquire);
+                continue;
+            }
+
+            // Try moving the head index forward.
+            match self.head.index.compare_exchange_weak(
+                head,
+                new_head,
+                Ordering::SeqCst,
+                Ordering::Acquire,
+            ) {
+                Ok(_) => unsafe {
+                    // If we've reached the end of the block, move to the next one.
+                    if offset + 1 == BLOCK_CAP {
+                        let next = (*block).wait_next();
+                        let mut next_index = (new_head & !MARK_BIT).wrapping_add(1 << SHIFT);
+                        if !(*next).next.load(Ordering::Relaxed).is_null() {
+                            next_index |= MARK_BIT;
+                        }
+
+                        self.head.block.store(next, Ordering::Release);
+                        self.head.index.store(next_index, Ordering::Release);
+                    }
+
+                    token.list.block = block as *const u8;
+                    token.list.offset = offset;
+                    return true;
+                },
+                Err(_) => {
+                    backoff.spin_light();
+                    head = self.head.index.load(Ordering::Acquire);
+                    block = self.head.block.load(Ordering::Acquire);
+                }
+            }
+        }
+    }
+
+    /// Reads a message from the channel.
+    pub(crate) unsafe fn read(&self, token: &mut Token) -> Result<T, ()> {
+        if token.list.block.is_null() {
+            // The channel is disconnected.
+            return Err(());
+        }
+
+        // Read the message.
+        let block = token.list.block as *mut Block<T>;
+        let offset = token.list.offset;
+        let slot = (*block).slots.get_unchecked(offset);
+        slot.wait_write();
+        let msg = slot.msg.get().read().assume_init();
+
+        // Destroy the block if we've reached the end, or if another thread wanted to destroy but
+        // couldn't because we were busy reading from the slot.
+        if offset + 1 == BLOCK_CAP {
+            Block::destroy(block, 0);
+        } else if slot.state.fetch_or(READ, Ordering::AcqRel) & DESTROY != 0 {
+            Block::destroy(block, offset + 1);
+        }
+
+        Ok(msg)
+    }
+
+    /// Attempts to send a message into the channel.
+    pub(crate) fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> {
+        self.send(msg, None).map_err(|err| match err {
+            SendTimeoutError::Disconnected(msg) => TrySendError::Disconnected(msg),
+            SendTimeoutError::Timeout(_) => unreachable!(),
+        })
+    }
+
+    /// Sends a message into the channel.
+    pub(crate) fn send(
+        &self,
+        msg: T,
+        _deadline: Option<Instant>,
+    ) -> Result<(), SendTimeoutError<T>> {
+        let token = &mut Token::default();
+        assert!(self.start_send(token));
+        unsafe { self.write(token, msg).map_err(SendTimeoutError::Disconnected) }
+    }
+
+    /// Attempts to receive a message without blocking.
+    pub(crate) fn try_recv(&self) -> Result<T, TryRecvError> {
+        let token = &mut Token::default();
+
+        if self.start_recv(token) {
+            unsafe { self.read(token).map_err(|_| TryRecvError::Disconnected) }
+        } else {
+            Err(TryRecvError::Empty)
+        }
+    }
+
+    /// Receives a message from the channel.
+    pub(crate) fn recv(&self, deadline: Option<Instant>) -> Result<T, RecvTimeoutError> {
+        let token = &mut Token::default();
+        loop {
+            if self.start_recv(token) {
+                unsafe {
+                    return self.read(token).map_err(|_| RecvTimeoutError::Disconnected);
+                }
+            }
+
+            if let Some(d) = deadline {
+                if Instant::now() >= d {
+                    return Err(RecvTimeoutError::Timeout);
+                }
+            }
+
+            // Prepare for blocking until a sender wakes us up.
+            Context::with(|cx| {
+                let oper = Operation::hook(token);
+                self.receivers.register(oper, cx);
+
+                // Has the channel become ready just now?
+                if !self.is_empty() || self.is_disconnected() {
+                    let _ = cx.try_select(Selected::Aborted);
+                }
+
+                // Block the current thread.
+                let sel = cx.wait_until(deadline);
+
+                match sel {
+                    Selected::Waiting => unreachable!(),
+                    Selected::Aborted | Selected::Disconnected => {
+                        self.receivers.unregister(oper).unwrap();
+                        // If the channel was disconnected, we still have to check for remaining
+                        // messages.
+                    }
+                    Selected::Operation(_) => {}
+                }
+            });
+        }
+    }
+
+    /// Returns the current number of messages inside the channel.
+    pub(crate) fn len(&self) -> usize {
+        loop {
+            // Load the tail index, then load the head index.
+            let mut tail = self.tail.index.load(Ordering::SeqCst);
+            let mut head = self.head.index.load(Ordering::SeqCst);
+
+            // If the tail index didn't change, we've got consistent indices to work with.
+            if self.tail.index.load(Ordering::SeqCst) == tail {
+                // Erase the lower bits.
+                tail &= !((1 << SHIFT) - 1);
+                head &= !((1 << SHIFT) - 1);
+
+                // Fix up indices if they fall onto block ends.
+                if (tail >> SHIFT) & (LAP - 1) == LAP - 1 {
+                    tail = tail.wrapping_add(1 << SHIFT);
+                }
+                if (head >> SHIFT) & (LAP - 1) == LAP - 1 {
+                    head = head.wrapping_add(1 << SHIFT);
+                }
+
+                // Rotate indices so that head falls into the first block.
+                let lap = (head >> SHIFT) / LAP;
+                tail = tail.wrapping_sub((lap * LAP) << SHIFT);
+                head = head.wrapping_sub((lap * LAP) << SHIFT);
+
+                // Remove the lower bits.
+                tail >>= SHIFT;
+                head >>= SHIFT;
+
+                // Return the difference minus the number of blocks between tail and head.
+                return tail - head - tail / LAP;
+            }
+        }
+    }
+
+    /// Returns the capacity of the channel.
+    pub(crate) fn capacity(&self) -> Option<usize> {
+        None
+    }
+
+    /// Disconnects senders and wakes up all blocked receivers.
+    ///
+    /// Returns `true` if this call disconnected the channel.
+    pub(crate) fn disconnect_senders(&self) -> bool {
+        let tail = self.tail.index.fetch_or(MARK_BIT, Ordering::SeqCst);
+
+        if tail & MARK_BIT == 0 {
+            self.receivers.disconnect();
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Disconnects receivers.
+    ///
+    /// Returns `true` if this call disconnected the channel.
+    pub(crate) fn disconnect_receivers(&self) -> bool {
+        let tail = self.tail.index.fetch_or(MARK_BIT, Ordering::SeqCst);
+
+        if tail & MARK_BIT == 0 {
+            // If receivers are dropped first, discard all messages to free
+            // memory eagerly.
+            self.discard_all_messages();
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Discards all messages.
+    ///
+    /// This method should only be called when all receivers are dropped.
+    fn discard_all_messages(&self) {
+        let backoff = Backoff::new();
+        let mut tail = self.tail.index.load(Ordering::Acquire);
+        loop {
+            let offset = (tail >> SHIFT) % LAP;
+            if offset != BLOCK_CAP {
+                break;
+            }
+
+            // New updates to tail will be rejected by MARK_BIT and aborted unless it's
+            // at boundary. We need to wait for the updates take affect otherwise there
+            // can be memory leaks.
+            backoff.spin_heavy();
+            tail = self.tail.index.load(Ordering::Acquire);
+        }
+
+        let mut head = self.head.index.load(Ordering::Acquire);
+        let mut block = self.head.block.load(Ordering::Acquire);
+
+        unsafe {
+            // Drop all messages between head and tail and deallocate the heap-allocated blocks.
+            while head >> SHIFT != tail >> SHIFT {
+                let offset = (head >> SHIFT) % LAP;
+
+                if offset < BLOCK_CAP {
+                    // Drop the message in the slot.
+                    let slot = (*block).slots.get_unchecked(offset);
+                    slot.wait_write();
+                    let p = &mut *slot.msg.get();
+                    p.as_mut_ptr().drop_in_place();
+                } else {
+                    (*block).wait_next();
+                    // Deallocate the block and move to the next one.
+                    let next = (*block).next.load(Ordering::Acquire);
+                    drop(Box::from_raw(block));
+                    block = next;
+                }
+
+                head = head.wrapping_add(1 << SHIFT);
+            }
+
+            // Deallocate the last remaining block.
+            if !block.is_null() {
+                drop(Box::from_raw(block));
+            }
+        }
+        head &= !MARK_BIT;
+        self.head.block.store(ptr::null_mut(), Ordering::Release);
+        self.head.index.store(head, Ordering::Release);
+    }
+
+    /// Returns `true` if the channel is disconnected.
+    pub(crate) fn is_disconnected(&self) -> bool {
+        self.tail.index.load(Ordering::SeqCst) & MARK_BIT != 0
+    }
+
+    /// Returns `true` if the channel is empty.
+    pub(crate) fn is_empty(&self) -> bool {
+        let head = self.head.index.load(Ordering::SeqCst);
+        let tail = self.tail.index.load(Ordering::SeqCst);
+        head >> SHIFT == tail >> SHIFT
+    }
+
+    /// Returns `true` if the channel is full.
+    pub(crate) fn is_full(&self) -> bool {
+        false
+    }
+}
+
+impl<T> Drop for Channel<T> {
+    fn drop(&mut self) {
+        let mut head = self.head.index.load(Ordering::Relaxed);
+        let mut tail = self.tail.index.load(Ordering::Relaxed);
+        let mut block = self.head.block.load(Ordering::Relaxed);
+
+        // Erase the lower bits.
+        head &= !((1 << SHIFT) - 1);
+        tail &= !((1 << SHIFT) - 1);
+
+        unsafe {
+            // Drop all messages between head and tail and deallocate the heap-allocated blocks.
+            while head != tail {
+                let offset = (head >> SHIFT) % LAP;
+
+                if offset < BLOCK_CAP {
+                    // Drop the message in the slot.
+                    let slot = (*block).slots.get_unchecked(offset);
+                    let p = &mut *slot.msg.get();
+                    p.as_mut_ptr().drop_in_place();
+                } else {
+                    // Deallocate the block and move to the next one.
+                    let next = (*block).next.load(Ordering::Relaxed);
+                    drop(Box::from_raw(block));
+                    block = next;
+                }
+
+                head = head.wrapping_add(1 << SHIFT);
+            }
+
+            // Deallocate the last remaining block.
+            if !block.is_null() {
+                drop(Box::from_raw(block));
+            }
+        }
+    }
+}
diff --git a/library/std/src/sync/mpmc/mod.rs b/library/std/src/sync/mpmc/mod.rs
new file mode 100644
index 000000000..7a602cecd
--- /dev/null
+++ b/library/std/src/sync/mpmc/mod.rs
@@ -0,0 +1,430 @@
+//! Multi-producer multi-consumer channels.
+
+// This module is not currently exposed publicly, but is used
+// as the implementation for the channels in `sync::mpsc`. The
+// implementation comes from the crossbeam-channel crate:
+//
+// Copyright (c) 2019 The Crossbeam Project Developers
+//
+// Permission is hereby granted, free of charge, to any
+// person obtaining a copy of this software and associated
+// documentation files (the "Software"), to deal in the
+// Software without restriction, including without
+// limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of
+// the Software, and to permit persons to whom the Software
+// is furnished to do so, subject to the following
+// conditions:
+//
+// The above copyright notice and this permission notice
+// shall be included in all copies or substantial portions
+// of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
+// ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
+// TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
+// PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
+// SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+// CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
+// IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+mod array;
+mod context;
+mod counter;
+mod error;
+mod list;
+mod select;
+mod utils;
+mod waker;
+mod zero;
+
+use crate::fmt;
+use crate::panic::{RefUnwindSafe, UnwindSafe};
+use crate::time::{Duration, Instant};
+pub use error::*;
+
+/// Creates a channel of unbounded capacity.
+///
+/// This channel has a growable buffer that can hold any number of messages at a time.
+pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
+    let (s, r) = counter::new(list::Channel::new());
+    let s = Sender { flavor: SenderFlavor::List(s) };
+    let r = Receiver { flavor: ReceiverFlavor::List(r) };
+    (s, r)
+}
+
+/// Creates a channel of bounded capacity.
+///
+/// This channel has a buffer that can hold at most `cap` messages at a time.
+///
+/// A special case is zero-capacity channel, which cannot hold any messages. Instead, send and
+/// receive operations must appear at the same time in order to pair up and pass the message over.
+pub fn sync_channel<T>(cap: usize) -> (Sender<T>, Receiver<T>) {
+    if cap == 0 {
+        let (s, r) = counter::new(zero::Channel::new());
+        let s = Sender { flavor: SenderFlavor::Zero(s) };
+        let r = Receiver { flavor: ReceiverFlavor::Zero(r) };
+        (s, r)
+    } else {
+        let (s, r) = counter::new(array::Channel::with_capacity(cap));
+        let s = Sender { flavor: SenderFlavor::Array(s) };
+        let r = Receiver { flavor: ReceiverFlavor::Array(r) };
+        (s, r)
+    }
+}
+
+/// The sending side of a channel.
+pub struct Sender<T> {
+    flavor: SenderFlavor<T>,
+}
+
+/// Sender flavors.
+enum SenderFlavor<T> {
+    /// Bounded channel based on a preallocated array.
+    Array(counter::Sender<array::Channel<T>>),
+
+    /// Unbounded channel implemented as a linked list.
+    List(counter::Sender<list::Channel<T>>),
+
+    /// Zero-capacity channel.
+    Zero(counter::Sender<zero::Channel<T>>),
+}
+
+unsafe impl<T: Send> Send for Sender<T> {}
+unsafe impl<T: Send> Sync for Sender<T> {}
+
+impl<T> UnwindSafe for Sender<T> {}
+impl<T> RefUnwindSafe for Sender<T> {}
+
+impl<T> Sender<T> {
+    /// Attempts to send a message into the channel without blocking.
+    ///
+    /// This method will either send a message into the channel immediately or return an error if
+    /// the channel is full or disconnected. The returned error contains the original message.
+    ///
+    /// If called on a zero-capacity channel, this method will send the message only if there
+    /// happens to be a receive operation on the other side of the channel at the same time.
+    pub fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> {
+        match &self.flavor {
+            SenderFlavor::Array(chan) => chan.try_send(msg),
+            SenderFlavor::List(chan) => chan.try_send(msg),
+            SenderFlavor::Zero(chan) => chan.try_send(msg),
+        }
+    }
+
+    /// Blocks the current thread until a message is sent or the channel is disconnected.
+    ///
+    /// If the channel is full and not disconnected, this call will block until the send operation
+    /// can proceed. If the channel becomes disconnected, this call will wake up and return an
+    /// error. The returned error contains the original message.
+    ///
+    /// If called on a zero-capacity channel, this method will wait for a receive operation to
+    /// appear on the other side of the channel.
+    pub fn send(&self, msg: T) -> Result<(), SendError<T>> {
+        match &self.flavor {
+            SenderFlavor::Array(chan) => chan.send(msg, None),
+            SenderFlavor::List(chan) => chan.send(msg, None),
+            SenderFlavor::Zero(chan) => chan.send(msg, None),
+        }
+        .map_err(|err| match err {
+            SendTimeoutError::Disconnected(msg) => SendError(msg),
+            SendTimeoutError::Timeout(_) => unreachable!(),
+        })
+    }
+}
+
+// The methods below are not used by `sync::mpsc`, but
+// are useful and we'll likely want to expose them
+// eventually
+#[allow(unused)]
+impl<T> Sender<T> {
+    /// Waits for a message to be sent into the channel, but only for a limited time.
+    ///
+    /// If the channel is full and not disconnected, this call will block until the send operation
+    /// can proceed or the operation times out. If the channel becomes disconnected, this call will
+    /// wake up and return an error. The returned error contains the original message.
+    ///
+    /// If called on a zero-capacity channel, this method will wait for a receive operation to
+    /// appear on the other side of the channel.
+    pub fn send_timeout(&self, msg: T, timeout: Duration) -> Result<(), SendTimeoutError<T>> {
+        match Instant::now().checked_add(timeout) {
+            Some(deadline) => self.send_deadline(msg, deadline),
+            // So far in the future that it's practically the same as waiting indefinitely.
+            None => self.send(msg).map_err(SendTimeoutError::from),
+        }
+    }
+
+    /// Waits for a message to be sent into the channel, but only until a given deadline.
+    ///
+    /// If the channel is full and not disconnected, this call will block until the send operation
+    /// can proceed or the operation times out. If the channel becomes disconnected, this call will
+    /// wake up and return an error. The returned error contains the original message.
+    ///
+    /// If called on a zero-capacity channel, this method will wait for a receive operation to
+    /// appear on the other side of the channel.
+    pub fn send_deadline(&self, msg: T, deadline: Instant) -> Result<(), SendTimeoutError<T>> {
+        match &self.flavor {
+            SenderFlavor::Array(chan) => chan.send(msg, Some(deadline)),
+            SenderFlavor::List(chan) => chan.send(msg, Some(deadline)),
+            SenderFlavor::Zero(chan) => chan.send(msg, Some(deadline)),
+        }
+    }
+
+    /// Returns `true` if the channel is empty.
+    ///
+    /// Note: Zero-capacity channels are always empty.
+    pub fn is_empty(&self) -> bool {
+        match &self.flavor {
+            SenderFlavor::Array(chan) => chan.is_empty(),
+            SenderFlavor::List(chan) => chan.is_empty(),
+            SenderFlavor::Zero(chan) => chan.is_empty(),
+        }
+    }
+
+    /// Returns `true` if the channel is full.
+    ///
+    /// Note: Zero-capacity channels are always full.
+    pub fn is_full(&self) -> bool {
+        match &self.flavor {
+            SenderFlavor::Array(chan) => chan.is_full(),
+            SenderFlavor::List(chan) => chan.is_full(),
+            SenderFlavor::Zero(chan) => chan.is_full(),
+        }
+    }
+
+    /// Returns the number of messages in the channel.
+    pub fn len(&self) -> usize {
+        match &self.flavor {
+            SenderFlavor::Array(chan) => chan.len(),
+            SenderFlavor::List(chan) => chan.len(),
+            SenderFlavor::Zero(chan) => chan.len(),
+        }
+    }
+
+    /// If the channel is bounded, returns its capacity.
+    pub fn capacity(&self) -> Option<usize> {
+        match &self.flavor {
+            SenderFlavor::Array(chan) => chan.capacity(),
+            SenderFlavor::List(chan) => chan.capacity(),
+            SenderFlavor::Zero(chan) => chan.capacity(),
+        }
+    }
+
+    /// Returns `true` if senders belong to the same channel.
+    pub fn same_channel(&self, other: &Sender<T>) -> bool {
+        match (&self.flavor, &other.flavor) {
+            (SenderFlavor::Array(ref a), SenderFlavor::Array(ref b)) => a == b,
+            (SenderFlavor::List(ref a), SenderFlavor::List(ref b)) => a == b,
+            (SenderFlavor::Zero(ref a), SenderFlavor::Zero(ref b)) => a == b,
+            _ => false,
+        }
+    }
+}
+
+impl<T> Drop for Sender<T> {
+    fn drop(&mut self) {
+        unsafe {
+            match &self.flavor {
+                SenderFlavor::Array(chan) => chan.release(|c| c.disconnect()),
+                SenderFlavor::List(chan) => chan.release(|c| c.disconnect_senders()),
+                SenderFlavor::Zero(chan) => chan.release(|c| c.disconnect()),
+            }
+        }
+    }
+}
+
+impl<T> Clone for Sender<T> {
+    fn clone(&self) -> Self {
+        let flavor = match &self.flavor {
+            SenderFlavor::Array(chan) => SenderFlavor::Array(chan.acquire()),
+            SenderFlavor::List(chan) => SenderFlavor::List(chan.acquire()),
+            SenderFlavor::Zero(chan) => SenderFlavor::Zero(chan.acquire()),
+        };
+
+        Sender { flavor }
+    }
+}
+
+impl<T> fmt::Debug for Sender<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad("Sender { .. }")
+    }
+}
+
+/// The receiving side of a channel.
+pub struct Receiver<T> {
+    flavor: ReceiverFlavor<T>,
+}
+
+/// Receiver flavors.
+enum ReceiverFlavor<T> {
+    /// Bounded channel based on a preallocated array.
+    Array(counter::Receiver<array::Channel<T>>),
+
+    /// Unbounded channel implemented as a linked list.
+    List(counter::Receiver<list::Channel<T>>),
+
+    /// Zero-capacity channel.
+    Zero(counter::Receiver<zero::Channel<T>>),
+}
+
+unsafe impl<T: Send> Send for Receiver<T> {}
+unsafe impl<T: Send> Sync for Receiver<T> {}
+
+impl<T> UnwindSafe for Receiver<T> {}
+impl<T> RefUnwindSafe for Receiver<T> {}
+
+impl<T> Receiver<T> {
+    /// Attempts to receive a message from the channel without blocking.
+    ///
+    /// This method will either receive a message from the channel immediately or return an error
+    /// if the channel is empty.
+    ///
+    /// If called on a zero-capacity channel, this method will receive a message only if there
+    /// happens to be a send operation on the other side of the channel at the same time.
+    pub fn try_recv(&self) -> Result<T, TryRecvError> {
+        match &self.flavor {
+            ReceiverFlavor::Array(chan) => chan.try_recv(),
+            ReceiverFlavor::List(chan) => chan.try_recv(),
+            ReceiverFlavor::Zero(chan) => chan.try_recv(),
+        }
+    }
+
+    /// Blocks the current thread until a message is received or the channel is empty and
+    /// disconnected.
+    ///
+    /// If the channel is empty and not disconnected, this call will block until the receive
+    /// operation can proceed. If the channel is empty and becomes disconnected, this call will
+    /// wake up and return an error.
+    ///
+    /// If called on a zero-capacity channel, this method will wait for a send operation to appear
+    /// on the other side of the channel.
+    pub fn recv(&self) -> Result<T, RecvError> {
+        match &self.flavor {
+            ReceiverFlavor::Array(chan) => chan.recv(None),
+            ReceiverFlavor::List(chan) => chan.recv(None),
+            ReceiverFlavor::Zero(chan) => chan.recv(None),
+        }
+        .map_err(|_| RecvError)
+    }
+
+    /// Waits for a message to be received from the channel, but only for a limited time.
+    ///
+    /// If the channel is empty and not disconnected, this call will block until the receive
+    /// operation can proceed or the operation times out. If the channel is empty and becomes
+    /// disconnected, this call will wake up and return an error.
+    ///
+    /// If called on a zero-capacity channel, this method will wait for a send operation to appear
+    /// on the other side of the channel.
+    pub fn recv_timeout(&self, timeout: Duration) -> Result<T, RecvTimeoutError> {
+        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),
+        }
+    }
+
+    /// Waits for a message to be received from the channel, but only for a limited time.
+    ///
+    /// If the channel is empty and not disconnected, this call will block until the receive
+    /// operation can proceed or the operation times out. If the channel is empty and becomes
+    /// disconnected, this call will wake up and return an error.
+    ///
+    /// If called on a zero-capacity channel, this method will wait for a send operation to appear
+    /// on the other side of the channel.
+    pub fn recv_deadline(&self, deadline: Instant) -> Result<T, RecvTimeoutError> {
+        match &self.flavor {
+            ReceiverFlavor::Array(chan) => chan.recv(Some(deadline)),
+            ReceiverFlavor::List(chan) => chan.recv(Some(deadline)),
+            ReceiverFlavor::Zero(chan) => chan.recv(Some(deadline)),
+        }
+    }
+}
+
+// The methods below are not used by `sync::mpsc`, but
+// are useful and we'll likely want to expose them
+// eventually
+#[allow(unused)]
+impl<T> Receiver<T> {
+    /// Returns `true` if the channel is empty.
+    ///
+    /// Note: Zero-capacity channels are always empty.
+    pub fn is_empty(&self) -> bool {
+        match &self.flavor {
+            ReceiverFlavor::Array(chan) => chan.is_empty(),
+            ReceiverFlavor::List(chan) => chan.is_empty(),
+            ReceiverFlavor::Zero(chan) => chan.is_empty(),
+        }
+    }
+
+    /// Returns `true` if the channel is full.
+    ///
+    /// Note: Zero-capacity channels are always full.
+    pub fn is_full(&self) -> bool {
+        match &self.flavor {
+            ReceiverFlavor::Array(chan) => chan.is_full(),
+            ReceiverFlavor::List(chan) => chan.is_full(),
+            ReceiverFlavor::Zero(chan) => chan.is_full(),
+        }
+    }
+
+    /// Returns the number of messages in the channel.
+    pub fn len(&self) -> usize {
+        match &self.flavor {
+            ReceiverFlavor::Array(chan) => chan.len(),
+            ReceiverFlavor::List(chan) => chan.len(),
+            ReceiverFlavor::Zero(chan) => chan.len(),
+        }
+    }
+
+    /// If the channel is bounded, returns its capacity.
+    pub fn capacity(&self) -> Option<usize> {
+        match &self.flavor {
+            ReceiverFlavor::Array(chan) => chan.capacity(),
+            ReceiverFlavor::List(chan) => chan.capacity(),
+            ReceiverFlavor::Zero(chan) => chan.capacity(),
+        }
+    }
+
+    /// Returns `true` if receivers belong to the same channel.
+    pub fn same_channel(&self, other: &Receiver<T>) -> bool {
+        match (&self.flavor, &other.flavor) {
+            (ReceiverFlavor::Array(a), ReceiverFlavor::Array(b)) => a == b,
+            (ReceiverFlavor::List(a), ReceiverFlavor::List(b)) => a == b,
+            (ReceiverFlavor::Zero(a), ReceiverFlavor::Zero(b)) => a == b,
+            _ => false,
+        }
+    }
+}
+
+impl<T> Drop for Receiver<T> {
+    fn drop(&mut self) {
+        unsafe {
+            match &self.flavor {
+                ReceiverFlavor::Array(chan) => chan.release(|c| c.disconnect()),
+                ReceiverFlavor::List(chan) => chan.release(|c| c.disconnect_receivers()),
+                ReceiverFlavor::Zero(chan) => chan.release(|c| c.disconnect()),
+            }
+        }
+    }
+}
+
+impl<T> Clone for Receiver<T> {
+    fn clone(&self) -> Self {
+        let flavor = match &self.flavor {
+            ReceiverFlavor::Array(chan) => ReceiverFlavor::Array(chan.acquire()),
+            ReceiverFlavor::List(chan) => ReceiverFlavor::List(chan.acquire()),
+            ReceiverFlavor::Zero(chan) => ReceiverFlavor::Zero(chan.acquire()),
+        };
+
+        Receiver { flavor }
+    }
+}
+
+impl<T> fmt::Debug for Receiver<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad("Receiver { .. }")
+    }
+}
diff --git a/library/std/src/sync/mpmc/select.rs b/library/std/src/sync/mpmc/select.rs
new file mode 100644
index 000000000..56a83fee2
--- /dev/null
+++ b/library/std/src/sync/mpmc/select.rs
@@ -0,0 +1,71 @@
+/// Temporary data that gets initialized during a blocking operation, and is consumed by
+/// `read` or `write`.
+///
+/// Each field contains data associated with a specific channel flavor.
+#[derive(Debug, Default)]
+pub struct Token {
+    pub(crate) array: super::array::ArrayToken,
+    pub(crate) list: super::list::ListToken,
+    #[allow(dead_code)]
+    pub(crate) zero: super::zero::ZeroToken,
+}
+
+/// Identifier associated with an operation by a specific thread on a specific channel.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct Operation(usize);
+
+impl Operation {
+    /// Creates an operation identifier from a mutable reference.
+    ///
+    /// This function essentially just turns the address of the reference into a number. The
+    /// reference should point to a variable that is specific to the thread and the operation,
+    /// and is alive for the entire duration of a blocking operation.
+    #[inline]
+    pub fn hook<T>(r: &mut T) -> Operation {
+        let val = r as *mut T as usize;
+        // Make sure that the pointer address doesn't equal the numerical representation of
+        // `Selected::{Waiting, Aborted, Disconnected}`.
+        assert!(val > 2);
+        Operation(val)
+    }
+}
+
+/// Current state of a blocking operation.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Selected {
+    /// Still waiting for an operation.
+    Waiting,
+
+    /// The attempt to block the current thread has been aborted.
+    Aborted,
+
+    /// An operation became ready because a channel is disconnected.
+    Disconnected,
+
+    /// An operation became ready because a message can be sent or received.
+    Operation(Operation),
+}
+
+impl From<usize> for Selected {
+    #[inline]
+    fn from(val: usize) -> Selected {
+        match val {
+            0 => Selected::Waiting,
+            1 => Selected::Aborted,
+            2 => Selected::Disconnected,
+            oper => Selected::Operation(Operation(oper)),
+        }
+    }
+}
+
+impl Into<usize> for Selected {
+    #[inline]
+    fn into(self) -> usize {
+        match self {
+            Selected::Waiting => 0,
+            Selected::Aborted => 1,
+            Selected::Disconnected => 2,
+            Selected::Operation(Operation(val)) => val,
+        }
+    }
+}
diff --git a/library/std/src/sync/mpmc/utils.rs b/library/std/src/sync/mpmc/utils.rs
new file mode 100644
index 000000000..cfe42750d
--- /dev/null
+++ b/library/std/src/sync/mpmc/utils.rs
@@ -0,0 +1,143 @@
+use crate::cell::Cell;
+use crate::ops::{Deref, DerefMut};
+
+/// Pads and aligns a value to the length of a cache line.
+#[derive(Clone, Copy, Default, Hash, PartialEq, Eq)]
+// Starting from Intel's Sandy Bridge, spatial prefetcher is now pulling pairs of 64-byte cache
+// lines at a time, so we have to align to 128 bytes rather than 64.
+//
+// Sources:
+// - https://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-optimization-manual.pdf
+// - https://github.com/facebook/folly/blob/1b5288e6eea6df074758f877c849b6e73bbb9fbb/folly/lang/Align.h#L107
+//
+// ARM's big.LITTLE architecture has asymmetric cores and "big" cores have 128-byte cache line size.
+//
+// Sources:
+// - https://www.mono-project.com/news/2016/09/12/arm64-icache/
+//
+// powerpc64 has 128-byte cache line size.
+//
+// Sources:
+// - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_ppc64x.go#L9
+#[cfg_attr(
+    any(target_arch = "x86_64", target_arch = "aarch64", target_arch = "powerpc64",),
+    repr(align(128))
+)]
+// arm, mips, mips64, and riscv64 have 32-byte cache line size.
+//
+// Sources:
+// - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_arm.go#L7
+// - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_mips.go#L7
+// - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_mipsle.go#L7
+// - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_mips64x.go#L9
+// - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_riscv64.go#L7
+#[cfg_attr(
+    any(
+        target_arch = "arm",
+        target_arch = "mips",
+        target_arch = "mips64",
+        target_arch = "riscv64",
+    ),
+    repr(align(32))
+)]
+// s390x has 256-byte cache line size.
+//
+// Sources:
+// - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_s390x.go#L7
+#[cfg_attr(target_arch = "s390x", repr(align(256)))]
+// x86 and wasm have 64-byte cache line size.
+//
+// Sources:
+// - https://github.com/golang/go/blob/dda2991c2ea0c5914714469c4defc2562a907230/src/internal/cpu/cpu_x86.go#L9
+// - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_wasm.go#L7
+//
+// All others are assumed to have 64-byte cache line size.
+#[cfg_attr(
+    not(any(
+        target_arch = "x86_64",
+        target_arch = "aarch64",
+        target_arch = "powerpc64",
+        target_arch = "arm",
+        target_arch = "mips",
+        target_arch = "mips64",
+        target_arch = "riscv64",
+        target_arch = "s390x",
+    )),
+    repr(align(64))
+)]
+pub struct CachePadded<T> {
+    value: T,
+}
+
+impl<T> CachePadded<T> {
+    /// Pads and aligns a value to the length of a cache line.
+    pub fn new(value: T) -> CachePadded<T> {
+        CachePadded::<T> { value }
+    }
+}
+
+impl<T> Deref for CachePadded<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        &self.value
+    }
+}
+
+impl<T> DerefMut for CachePadded<T> {
+    fn deref_mut(&mut self) -> &mut T {
+        &mut self.value
+    }
+}
+
+const SPIN_LIMIT: u32 = 6;
+
+/// Performs quadratic backoff in spin loops.
+pub struct Backoff {
+    step: Cell<u32>,
+}
+
+impl Backoff {
+    /// Creates a new `Backoff`.
+    pub fn new() -> Self {
+        Backoff { step: Cell::new(0) }
+    }
+
+    /// Backs off using lightweight spinning.
+    ///
+    /// This method should be used for:
+    ///     - Retrying an operation because another thread made progress. i.e. on CAS failure.
+    ///     - Waiting for an operation to complete by spinning optimistically for a few iterations
+    ///     before falling back to parking the thread (see `Backoff::is_completed`).
+    #[inline]
+    pub fn spin_light(&self) {
+        let step = self.step.get().min(SPIN_LIMIT);
+        for _ in 0..step.pow(2) {
+            crate::hint::spin_loop();
+        }
+
+        self.step.set(self.step.get() + 1);
+    }
+
+    /// Backs off using heavyweight spinning.
+    ///
+    /// This method should be used in blocking loops where parking the thread is not an option.
+    #[inline]
+    pub fn spin_heavy(&self) {
+        if self.step.get() <= SPIN_LIMIT {
+            for _ in 0..self.step.get().pow(2) {
+                crate::hint::spin_loop()
+            }
+        } else {
+            crate::thread::yield_now();
+        }
+
+        self.step.set(self.step.get() + 1);
+    }
+
+    /// Returns `true` if quadratic backoff has completed and parking the thread is advised.
+    #[inline]
+    pub fn is_completed(&self) -> bool {
+        self.step.get() > SPIN_LIMIT
+    }
+}
diff --git a/library/std/src/sync/mpmc/waker.rs b/library/std/src/sync/mpmc/waker.rs
new file mode 100644
index 000000000..4912ca4f8
--- /dev/null
+++ b/library/std/src/sync/mpmc/waker.rs
@@ -0,0 +1,204 @@
+//! Waking mechanism for threads blocked on channel operations.
+
+use super::context::Context;
+use super::select::{Operation, Selected};
+
+use crate::ptr;
+use crate::sync::atomic::{AtomicBool, Ordering};
+use crate::sync::Mutex;
+
+/// Represents a thread blocked on a specific channel operation.
+pub(crate) struct Entry {
+    /// The operation.
+    pub(crate) oper: Operation,
+
+    /// Optional packet.
+    pub(crate) packet: *mut (),
+
+    /// Context associated with the thread owning this operation.
+    pub(crate) cx: Context,
+}
+
+/// A queue of threads blocked on channel operations.
+///
+/// This data structure is used by threads to register blocking operations and get woken up once
+/// an operation becomes ready.
+pub(crate) struct Waker {
+    /// A list of select operations.
+    selectors: Vec<Entry>,
+
+    /// A list of operations waiting to be ready.
+    observers: Vec<Entry>,
+}
+
+impl Waker {
+    /// Creates a new `Waker`.
+    #[inline]
+    pub(crate) fn new() -> Self {
+        Waker { selectors: Vec::new(), observers: Vec::new() }
+    }
+
+    /// Registers a select operation.
+    #[inline]
+    pub(crate) fn register(&mut self, oper: Operation, cx: &Context) {
+        self.register_with_packet(oper, ptr::null_mut(), cx);
+    }
+
+    /// Registers a select operation and a packet.
+    #[inline]
+    pub(crate) fn register_with_packet(&mut self, oper: Operation, packet: *mut (), cx: &Context) {
+        self.selectors.push(Entry { oper, packet, cx: cx.clone() });
+    }
+
+    /// Unregisters a select operation.
+    #[inline]
+    pub(crate) fn unregister(&mut self, oper: Operation) -> Option<Entry> {
+        if let Some((i, _)) =
+            self.selectors.iter().enumerate().find(|&(_, entry)| entry.oper == oper)
+        {
+            let entry = self.selectors.remove(i);
+            Some(entry)
+        } else {
+            None
+        }
+    }
+
+    /// Attempts to find another thread's entry, select the operation, and wake it up.
+    #[inline]
+    pub(crate) fn try_select(&mut self) -> Option<Entry> {
+        self.selectors
+            .iter()
+            .position(|selector| {
+                // Does the entry belong to a different thread?
+                selector.cx.thread_id() != current_thread_id()
+                    && selector // Try selecting this operation.
+                        .cx
+                        .try_select(Selected::Operation(selector.oper))
+                        .is_ok()
+                    && {
+                        // Provide the packet.
+                        selector.cx.store_packet(selector.packet);
+                        // Wake the thread up.
+                        selector.cx.unpark();
+                        true
+                    }
+            })
+            // Remove the entry from the queue to keep it clean and improve
+            // performance.
+            .map(|pos| self.selectors.remove(pos))
+    }
+
+    /// Notifies all operations waiting to be ready.
+    #[inline]
+    pub(crate) fn notify(&mut self) {
+        for entry in self.observers.drain(..) {
+            if entry.cx.try_select(Selected::Operation(entry.oper)).is_ok() {
+                entry.cx.unpark();
+            }
+        }
+    }
+
+    /// Notifies all registered operations that the channel is disconnected.
+    #[inline]
+    pub(crate) fn disconnect(&mut self) {
+        for entry in self.selectors.iter() {
+            if entry.cx.try_select(Selected::Disconnected).is_ok() {
+                // Wake the thread up.
+                //
+                // Here we don't remove the entry from the queue. Registered threads must
+                // unregister from the waker by themselves. They might also want to recover the
+                // packet value and destroy it, if necessary.
+                entry.cx.unpark();
+            }
+        }
+
+        self.notify();
+    }
+}
+
+impl Drop for Waker {
+    #[inline]
+    fn drop(&mut self) {
+        debug_assert_eq!(self.selectors.len(), 0);
+        debug_assert_eq!(self.observers.len(), 0);
+    }
+}
+
+/// A waker that can be shared among threads without locking.
+///
+/// This is a simple wrapper around `Waker` that internally uses a mutex for synchronization.
+pub(crate) struct SyncWaker {
+    /// The inner `Waker`.
+    inner: Mutex<Waker>,
+
+    /// `true` if the waker is empty.
+    is_empty: AtomicBool,
+}
+
+impl SyncWaker {
+    /// Creates a new `SyncWaker`.
+    #[inline]
+    pub(crate) fn new() -> Self {
+        SyncWaker { inner: Mutex::new(Waker::new()), is_empty: AtomicBool::new(true) }
+    }
+
+    /// Registers the current thread with an operation.
+    #[inline]
+    pub(crate) fn register(&self, oper: Operation, cx: &Context) {
+        let mut inner = self.inner.lock().unwrap();
+        inner.register(oper, cx);
+        self.is_empty
+            .store(inner.selectors.is_empty() && inner.observers.is_empty(), Ordering::SeqCst);
+    }
+
+    /// Unregisters an operation previously registered by the current thread.
+    #[inline]
+    pub(crate) fn unregister(&self, oper: Operation) -> Option<Entry> {
+        let mut inner = self.inner.lock().unwrap();
+        let entry = inner.unregister(oper);
+        self.is_empty
+            .store(inner.selectors.is_empty() && inner.observers.is_empty(), Ordering::SeqCst);
+        entry
+    }
+
+    /// Attempts to find one thread (not the current one), select its operation, and wake it up.
+    #[inline]
+    pub(crate) fn notify(&self) {
+        if !self.is_empty.load(Ordering::SeqCst) {
+            let mut inner = self.inner.lock().unwrap();
+            if !self.is_empty.load(Ordering::SeqCst) {
+                inner.try_select();
+                inner.notify();
+                self.is_empty.store(
+                    inner.selectors.is_empty() && inner.observers.is_empty(),
+                    Ordering::SeqCst,
+                );
+            }
+        }
+    }
+
+    /// Notifies all threads that the channel is disconnected.
+    #[inline]
+    pub(crate) fn disconnect(&self) {
+        let mut inner = self.inner.lock().unwrap();
+        inner.disconnect();
+        self.is_empty
+            .store(inner.selectors.is_empty() && inner.observers.is_empty(), Ordering::SeqCst);
+    }
+}
+
+impl Drop for SyncWaker {
+    #[inline]
+    fn drop(&mut self) {
+        debug_assert!(self.is_empty.load(Ordering::SeqCst));
+    }
+}
+
+/// Returns a unique id for the current thread.
+#[inline]
+pub fn current_thread_id() -> usize {
+    // `u8` is not drop so this variable will be available during thread destruction,
+    // whereas `thread::current()` would not be
+    thread_local! { static DUMMY: u8 = 0 }
+    DUMMY.with(|x| (x as *const u8).addr())
+}
diff --git a/library/std/src/sync/mpmc/zero.rs b/library/std/src/sync/mpmc/zero.rs
new file mode 100644
index 000000000..33f768dcb
--- /dev/null
+++ b/library/std/src/sync/mpmc/zero.rs
@@ -0,0 +1,318 @@
+//! Zero-capacity channel.
+//!
+//! This kind of channel is also known as *rendezvous* channel.
+
+use super::context::Context;
+use super::error::*;
+use super::select::{Operation, Selected, Token};
+use super::utils::Backoff;
+use super::waker::Waker;
+
+use crate::cell::UnsafeCell;
+use crate::marker::PhantomData;
+use crate::sync::atomic::{AtomicBool, Ordering};
+use crate::sync::Mutex;
+use crate::time::Instant;
+use crate::{fmt, ptr};
+
+/// A pointer to a packet.
+pub(crate) struct ZeroToken(*mut ());
+
+impl Default for ZeroToken {
+    fn default() -> Self {
+        Self(ptr::null_mut())
+    }
+}
+
+impl fmt::Debug for ZeroToken {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&(self.0 as usize), f)
+    }
+}
+
+/// A slot for passing one message from a sender to a receiver.
+struct Packet<T> {
+    /// Equals `true` if the packet is allocated on the stack.
+    on_stack: bool,
+
+    /// Equals `true` once the packet is ready for reading or writing.
+    ready: AtomicBool,
+
+    /// The message.
+    msg: UnsafeCell<Option<T>>,
+}
+
+impl<T> Packet<T> {
+    /// Creates an empty packet on the stack.
+    fn empty_on_stack() -> Packet<T> {
+        Packet { on_stack: true, ready: AtomicBool::new(false), msg: UnsafeCell::new(None) }
+    }
+
+    /// Creates a packet on the stack, containing a message.
+    fn message_on_stack(msg: T) -> Packet<T> {
+        Packet { on_stack: true, ready: AtomicBool::new(false), msg: UnsafeCell::new(Some(msg)) }
+    }
+
+    /// Waits until the packet becomes ready for reading or writing.
+    fn wait_ready(&self) {
+        let backoff = Backoff::new();
+        while !self.ready.load(Ordering::Acquire) {
+            backoff.spin_heavy();
+        }
+    }
+}
+
+/// Inner representation of a zero-capacity channel.
+struct Inner {
+    /// Senders waiting to pair up with a receive operation.
+    senders: Waker,
+
+    /// Receivers waiting to pair up with a send operation.
+    receivers: Waker,
+
+    /// Equals `true` when the channel is disconnected.
+    is_disconnected: bool,
+}
+
+/// Zero-capacity channel.
+pub(crate) struct Channel<T> {
+    /// Inner representation of the channel.
+    inner: Mutex<Inner>,
+
+    /// Indicates that dropping a `Channel<T>` may drop values of type `T`.
+    _marker: PhantomData<T>,
+}
+
+impl<T> Channel<T> {
+    /// Constructs a new zero-capacity channel.
+    pub(crate) fn new() -> Self {
+        Channel {
+            inner: Mutex::new(Inner {
+                senders: Waker::new(),
+                receivers: Waker::new(),
+                is_disconnected: false,
+            }),
+            _marker: PhantomData,
+        }
+    }
+
+    /// Writes a message into the packet.
+    pub(crate) unsafe fn write(&self, token: &mut Token, msg: T) -> Result<(), T> {
+        // If there is no packet, the channel is disconnected.
+        if token.zero.0.is_null() {
+            return Err(msg);
+        }
+
+        let packet = &*(token.zero.0 as *const Packet<T>);
+        packet.msg.get().write(Some(msg));
+        packet.ready.store(true, Ordering::Release);
+        Ok(())
+    }
+
+    /// Reads a message from the packet.
+    pub(crate) unsafe fn read(&self, token: &mut Token) -> Result<T, ()> {
+        // If there is no packet, the channel is disconnected.
+        if token.zero.0.is_null() {
+            return Err(());
+        }
+
+        let packet = &*(token.zero.0 as *const Packet<T>);
+
+        if packet.on_stack {
+            // The message has been in the packet from the beginning, so there is no need to wait
+            // for it. However, after reading the message, we need to set `ready` to `true` in
+            // order to signal that the packet can be destroyed.
+            let msg = packet.msg.get().replace(None).unwrap();
+            packet.ready.store(true, Ordering::Release);
+            Ok(msg)
+        } else {
+            // Wait until the message becomes available, then read it and destroy the
+            // heap-allocated packet.
+            packet.wait_ready();
+            let msg = packet.msg.get().replace(None).unwrap();
+            drop(Box::from_raw(token.zero.0 as *mut Packet<T>));
+            Ok(msg)
+        }
+    }
+
+    /// Attempts to send a message into the channel.
+    pub(crate) fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> {
+        let token = &mut Token::default();
+        let mut inner = self.inner.lock().unwrap();
+
+        // If there's a waiting receiver, pair up with it.
+        if let Some(operation) = inner.receivers.try_select() {
+            token.zero.0 = operation.packet;
+            drop(inner);
+            unsafe {
+                self.write(token, msg).ok().unwrap();
+            }
+            Ok(())
+        } else if inner.is_disconnected {
+            Err(TrySendError::Disconnected(msg))
+        } else {
+            Err(TrySendError::Full(msg))
+        }
+    }
+
+    /// Sends a message into the channel.
+    pub(crate) fn send(
+        &self,
+        msg: T,
+        deadline: Option<Instant>,
+    ) -> Result<(), SendTimeoutError<T>> {
+        let token = &mut Token::default();
+        let mut inner = self.inner.lock().unwrap();
+
+        // If there's a waiting receiver, pair up with it.
+        if let Some(operation) = inner.receivers.try_select() {
+            token.zero.0 = operation.packet;
+            drop(inner);
+            unsafe {
+                self.write(token, msg).ok().unwrap();
+            }
+            return Ok(());
+        }
+
+        if inner.is_disconnected {
+            return Err(SendTimeoutError::Disconnected(msg));
+        }
+
+        Context::with(|cx| {
+            // Prepare for blocking until a receiver wakes us up.
+            let oper = Operation::hook(token);
+            let mut packet = Packet::<T>::message_on_stack(msg);
+            inner.senders.register_with_packet(oper, &mut packet as *mut Packet<T> as *mut (), cx);
+            inner.receivers.notify();
+            drop(inner);
+
+            // Block the current thread.
+            let sel = cx.wait_until(deadline);
+
+            match sel {
+                Selected::Waiting => unreachable!(),
+                Selected::Aborted => {
+                    self.inner.lock().unwrap().senders.unregister(oper).unwrap();
+                    let msg = unsafe { packet.msg.get().replace(None).unwrap() };
+                    Err(SendTimeoutError::Timeout(msg))
+                }
+                Selected::Disconnected => {
+                    self.inner.lock().unwrap().senders.unregister(oper).unwrap();
+                    let msg = unsafe { packet.msg.get().replace(None).unwrap() };
+                    Err(SendTimeoutError::Disconnected(msg))
+                }
+                Selected::Operation(_) => {
+                    // Wait until the message is read, then drop the packet.
+                    packet.wait_ready();
+                    Ok(())
+                }
+            }
+        })
+    }
+
+    /// Attempts to receive a message without blocking.
+    pub(crate) fn try_recv(&self) -> Result<T, TryRecvError> {
+        let token = &mut Token::default();
+        let mut inner = self.inner.lock().unwrap();
+
+        // If there's a waiting sender, pair up with it.
+        if let Some(operation) = inner.senders.try_select() {
+            token.zero.0 = operation.packet;
+            drop(inner);
+            unsafe { self.read(token).map_err(|_| TryRecvError::Disconnected) }
+        } else if inner.is_disconnected {
+            Err(TryRecvError::Disconnected)
+        } else {
+            Err(TryRecvError::Empty)
+        }
+    }
+
+    /// Receives a message from the channel.
+    pub(crate) fn recv(&self, deadline: Option<Instant>) -> Result<T, RecvTimeoutError> {
+        let token = &mut Token::default();
+        let mut inner = self.inner.lock().unwrap();
+
+        // If there's a waiting sender, pair up with it.
+        if let Some(operation) = inner.senders.try_select() {
+            token.zero.0 = operation.packet;
+            drop(inner);
+            unsafe {
+                return self.read(token).map_err(|_| RecvTimeoutError::Disconnected);
+            }
+        }
+
+        if inner.is_disconnected {
+            return Err(RecvTimeoutError::Disconnected);
+        }
+
+        Context::with(|cx| {
+            // Prepare for blocking until a sender wakes us up.
+            let oper = Operation::hook(token);
+            let mut packet = Packet::<T>::empty_on_stack();
+            inner.receivers.register_with_packet(
+                oper,
+                &mut packet as *mut Packet<T> as *mut (),
+                cx,
+            );
+            inner.senders.notify();
+            drop(inner);
+
+            // Block the current thread.
+            let sel = cx.wait_until(deadline);
+
+            match sel {
+                Selected::Waiting => unreachable!(),
+                Selected::Aborted => {
+                    self.inner.lock().unwrap().receivers.unregister(oper).unwrap();
+                    Err(RecvTimeoutError::Timeout)
+                }
+                Selected::Disconnected => {
+                    self.inner.lock().unwrap().receivers.unregister(oper).unwrap();
+                    Err(RecvTimeoutError::Disconnected)
+                }
+                Selected::Operation(_) => {
+                    // Wait until the message is provided, then read it.
+                    packet.wait_ready();
+                    unsafe { Ok(packet.msg.get().replace(None).unwrap()) }
+                }
+            }
+        })
+    }
+
+    /// Disconnects the channel and wakes up all blocked senders and receivers.
+    ///
+    /// Returns `true` if this call disconnected the channel.
+    pub(crate) fn disconnect(&self) -> bool {
+        let mut inner = self.inner.lock().unwrap();
+
+        if !inner.is_disconnected {
+            inner.is_disconnected = true;
+            inner.senders.disconnect();
+            inner.receivers.disconnect();
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Returns the current number of messages inside the channel.
+    pub(crate) fn len(&self) -> usize {
+        0
+    }
+
+    /// Returns the capacity of the channel.
+    #[allow(clippy::unnecessary_wraps)] // This is intentional.
+    pub(crate) fn capacity(&self) -> Option<usize> {
+        Some(0)
+    }
+
+    /// Returns `true` if the channel is empty.
+    pub(crate) fn is_empty(&self) -> bool {
+        true
+    }
+
+    /// Returns `true` if the channel is full.
+    pub(crate) fn is_full(&self) -> bool {
+        true
+    }
+}