remove invalid unsafe

- add new top level `channels` module
- reimplement existing channels using `async-channel`

Cargo.toml

@@ -49,6 +49,7 @@ std = [
   "wasm-bindgen-futures",
   "futures-channel",
   "async-mutex",
+  "async-channel",
 ]
 alloc = [
   "futures-core/alloc",
@@ -72,10 +73,12 @@ pin-project-lite = { version = "0.1.4", optional = true }
 pin-utils = { version = "0.1.0-alpha.4", optional = true }
 slab = { version = "0.4.2", optional = true }
 futures-timer = { version = "3.0.2", optional = true }
+async-channel = { version = "1.1.1", optional = true }
 
 # Devdepencency, but they are not allowed to be optional :/
 surf = { version = "1.0.3", optional = true }
 
+
 [target.'cfg(not(target_os = "unknown"))'.dependencies]
 smol = { version = "0.1.17", optional = true }
 

src/channel.rs

@@ -0,0 +1,6 @@
+//! Channels
+
+#[cfg(feature = "unstable")]
+#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
+#[doc(inline)]
+pub use async_channel::*;

src/lib.rs

@@ -306,6 +306,7 @@ cfg_unstable! {
     pub mod pin;
     #[cfg(not(target_os = "unknown"))]
     pub mod process;
+    pub mod channel;
 
     mod unit;
     mod vec;

src/sync/channel.rs

@@ -1,21 +1,15 @@
-use std::cell::UnsafeCell;
-use std::error::Error;
-use std::fmt::{self, Debug, Display};
+use std::fmt;
 use std::future::Future;
-use std::isize;
-use std::marker::PhantomData;
-use std::mem;
 use std::pin::Pin;
-use std::process;
-use std::ptr;
-use std::sync::atomic::{self, AtomicUsize, Ordering};
-use std::sync::Arc;
 use std::task::{Context, Poll};
 
-use crossbeam_utils::Backoff;
-
 use crate::stream::Stream;
-use crate::sync::WakerSet;
+use crate::channel::{bounded, Sender as InnerSender, Receiver as InnerReceiver, SendError};
+
+#[cfg(feature = "unstable")]
+#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
+#[doc(inline)]
+pub use crate::channel::{RecvError, TryRecvError, TrySendError};
 
 /// Creates a bounded multi-producer multi-consumer channel.
 ///
@@ -61,14 +55,16 @@ use crate::sync::WakerSet;
 #[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
 pub fn channel<T>(cap: usize) -> (Sender<T>, Receiver<T>) {
-    let channel = Arc::new(Channel::with_capacity(cap));
+    let (sender, receiver) = bounded(cap);
+    
     let s = Sender {
-        channel: channel.clone(),
+        sender,
     };
+
     let r = Receiver {
-        channel,
-        opt_key: None,
+        receiver,
     };
+
     (s, r)
 }
 
@@ -103,8 +99,8 @@ pub fn channel<T>(cap: usize) -> (Sender<T>, Receiver<T>) {
 #[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
 pub struct Sender<T> {
-    /// The inner channel.
-    channel: Arc<Channel<T>>,
+    /// The sender.
+    sender: InnerSender<T>,
 }
 
 impl<T> Sender<T> {
@@ -137,9 +133,7 @@ impl<T> Sender<T> {
     /// ```
     pub async fn send(&self, msg: T) {
         struct SendFuture<'a, T> {
-            channel: &'a Channel<T>,
-            msg: Option<T>,
-            opt_key: Option<usize>,
+            inner: Pin<Box<dyn Future<Output = Result<(), SendError<T>>> + 'a>>,
         }
 
         impl<T> Unpin for SendFuture<'_, T> {}
@@ -148,51 +142,20 @@ impl<T> Sender<T> {
             type Output = ();
 
             fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-                loop {
-                    let msg = self.msg.take().unwrap();
-
-                    // If the current task is in the set, remove it.
-                    if let Some(key) = self.opt_key.take() {
-                        self.channel.send_wakers.remove(key);
-                    }
-
-                    // Try sending the message.
-                    match self.channel.try_send(msg) {
-                        Ok(()) => return Poll::Ready(()),
-                        Err(TrySendError::Disconnected(msg)) => {
-                            self.msg = Some(msg);
-                            return Poll::Pending;
-                        }
-                        Err(TrySendError::Full(msg)) => {
-                            self.msg = Some(msg);
-
-                            // Insert this send operation.
-                            self.opt_key = Some(self.channel.send_wakers.insert(cx));
-
-                            // If the channel is still full and not disconnected, return.
-                            if self.channel.is_full() && !self.channel.is_disconnected() {
-                                return Poll::Pending;
-                            }
-                        }
+                match Pin::new(&mut self.inner).poll(cx) {
+                    Poll::Pending => Poll::Pending,
+                    Poll::Ready(Ok(())) => Poll::Ready(()),
+                    Poll::Ready(Err(_)) => {
+                        log::error!("attempted to send on dropped receiver");
+                        // return pending to preserve old behaviour
+                        Poll::Pending
                     }
                 }
             }
-        }
-
-        impl<T> Drop for SendFuture<'_, T> {
-            fn drop(&mut self) {
-                // If the current task is still in the set, that means it is being cancelled now.
-                // Wake up another task instead.
-                if let Some(key) = self.opt_key {
-                    self.channel.send_wakers.cancel(key);
-                }
-            }
-        }
+        }       
 
         SendFuture {
-            channel: &self.channel,
-            msg: Some(msg),
-            opt_key: None,
+            inner: Box::pin(self.sender.send(msg)),
         }
         .await
     }
@@ -215,7 +178,7 @@ impl<T> Sender<T> {
     /// # })
     /// ```
     pub fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> {
-        self.channel.try_send(msg)
+        self.sender.try_send(msg)
     }
 
     /// Returns the channel capacity.
@@ -229,7 +192,7 @@ impl<T> Sender<T> {
     /// assert_eq!(s.capacity(), 5);
     /// ```
     pub fn capacity(&self) -> usize {
-        self.channel.cap
+        self.sender.capacity().unwrap()
     }
 
     /// Returns `true` if the channel is empty.
@@ -250,7 +213,7 @@ impl<T> Sender<T> {
     /// # })
     /// ```
     pub fn is_empty(&self) -> bool {
-        self.channel.is_empty()
+        self.sender.is_empty()
     }
 
     /// Returns `true` if the channel is full.
@@ -271,7 +234,7 @@ impl<T> Sender<T> {
     /// # })
     /// ```
     pub fn is_full(&self) -> bool {
-        self.channel.is_full()
+        self.sender.is_full()
     }
 
     /// Returns the number of messages in the channel.
@@ -293,30 +256,14 @@ impl<T> Sender<T> {
     /// # })
     /// ```
     pub fn len(&self) -> usize {
-        self.channel.len()
-    }
-}
-
-impl<T> Drop for Sender<T> {
-    fn drop(&mut self) {
-        // Decrement the sender count and disconnect the channel if it drops down to zero.
-        if self.channel.sender_count.fetch_sub(1, Ordering::AcqRel) == 1 {
-            self.channel.disconnect();
-        }
+        self.sender.len()
     }
 }
 
 impl<T> Clone for Sender<T> {
-    fn clone(&self) -> Sender<T> {
-        let count = self.channel.sender_count.fetch_add(1, Ordering::Relaxed);
-
-        // Make sure the count never overflows, even if lots of sender clones are leaked.
-        if count > isize::MAX as usize {
-            process::abort();
-        }
-
+    fn clone(&self) -> Self {
         Sender {
-            channel: self.channel.clone(),
+            sender: self.sender.clone(),
         }
     }
 }
@@ -364,11 +311,8 @@ impl<T> fmt::Debug for Sender<T> {
 #[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
 pub struct Receiver<T> {
-    /// The inner channel.
-    channel: Arc<Channel<T>>,
-
-    /// The key for this receiver in the `channel.stream_wakers` set.
-    opt_key: Option<usize>,
+    /// The inner receiver.
+    receiver: InnerReceiver<T>,
 }
 
 impl<T> Receiver<T> {
@@ -403,38 +347,7 @@ impl<T> Receiver<T> {
     /// # }) }
     /// ```
     pub async fn recv(&self) -> Result<T, RecvError> {
-        struct RecvFuture<'a, T> {
-            channel: &'a Channel<T>,
-            opt_key: Option<usize>,
-        }
-
-        impl<T> Future for RecvFuture<'_, T> {
-            type Output = Result<T, RecvError>;
-
-            fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-                poll_recv(
-                    &self.channel,
-                    &self.channel.recv_wakers,
-                    &mut self.opt_key,
-                    cx,
-                )
-            }
-        }
-
-        impl<T> Drop for RecvFuture<'_, T> {
-            fn drop(&mut self) {
-                // If the current task is still in the set, that means it is being cancelled now.
-                if let Some(key) = self.opt_key {
-                    self.channel.recv_wakers.cancel(key);
-                }
-            }
-        }
-
-        RecvFuture {
-            channel: &self.channel,
-            opt_key: None,
-        }
-        .await
+        self.receiver.recv().await
     }
 
     /// Attempts to receive a message from the channel.
@@ -458,7 +371,7 @@ impl<T> Receiver<T> {
     /// # })
     /// ```
     pub fn try_recv(&self) -> Result<T, TryRecvError> {
-        self.channel.try_recv()
+        self.receiver.try_recv()
     }
 
     /// Returns the channel capacity.
@@ -472,7 +385,7 @@ impl<T> Receiver<T> {
     /// assert_eq!(r.capacity(), 5);
     /// ```
     pub fn capacity(&self) -> usize {
-        self.channel.cap
+        self.receiver.capacity().unwrap()
     }
 
     /// Returns `true` if the channel is empty.
@@ -493,7 +406,7 @@ impl<T> Receiver<T> {
     /// # })
     /// ```
     pub fn is_empty(&self) -> bool {
-        self.channel.is_empty()
+        self.receiver.is_empty()
     }
 
     /// Returns `true` if the channel is full.
@@ -514,7 +427,7 @@ impl<T> Receiver<T> {
     /// # })
     /// ```
     pub fn is_full(&self) -> bool {
-        self.channel.is_full()
+        self.receiver.is_full()
     }
 
     /// Returns the number of messages in the channel.
@@ -536,36 +449,14 @@ impl<T> Receiver<T> {
     /// # })
     /// ```
     pub fn len(&self) -> usize {
-        self.channel.len()
-    }
-}
-
-impl<T> Drop for Receiver<T> {
-    fn drop(&mut self) {
-        // If the current task is still in the stream set, that means it is being cancelled now.
-        if let Some(key) = self.opt_key {
-            self.channel.stream_wakers.cancel(key);
-        }
-
-        // Decrement the receiver count and disconnect the channel if it drops down to zero.
-        if self.channel.receiver_count.fetch_sub(1, Ordering::AcqRel) == 1 {
-            self.channel.disconnect();
-        }
+        self.receiver.len()
     }
 }
 
 impl<T> Clone for Receiver<T> {
-    fn clone(&self) -> Receiver<T> {
-        let count = self.channel.receiver_count.fetch_add(1, Ordering::Relaxed);
-
-        // Make sure the count never overflows, even if lots of receiver clones are leaked.
-        if count > isize::MAX as usize {
-            process::abort();
-        }
-
+    fn clone(&self) -> Self {
         Receiver {
-            channel: self.channel.clone(),
-            opt_key: None,
+            receiver: self.receiver.clone(),
         }
     }
 }
@@ -574,14 +465,7 @@ impl<T> Stream for Receiver<T> {
     type Item = T;
 
     fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
-        let this = &mut *self;
-        let res = futures_core::ready!(poll_recv(
-            &this.channel,
-            &this.channel.stream_wakers,
-            &mut this.opt_key,
-            cx,
-        ));
-        Poll::Ready(res.ok())
+        Pin::new(&mut self.receiver).poll_next(cx)
     }
 }
 
@@ -591,469 +475,5 @@ impl<T> fmt::Debug for Receiver<T> {
     }
 }
 
-/// Polls a receive operation on a channel.
-///
-/// If the receive operation is blocked, the current task will be inserted into `wakers` and its
-/// associated key will then be stored in `opt_key`.
-fn poll_recv<T>(
-    channel: &Channel<T>,
-    wakers: &WakerSet,
-    opt_key: &mut Option<usize>,
-    cx: &mut Context<'_>,
-) -> Poll<Result<T, RecvError>> {
-    loop {
-        // If the current task is in the set, remove it.
-        if let Some(key) = opt_key.take() {
-            wakers.remove(key);
-        }
 
-        // Try receiving a message.
-        match channel.try_recv() {
-            Ok(msg) => return Poll::Ready(Ok(msg)),
-            Err(TryRecvError::Disconnected) => return Poll::Ready(Err(RecvError {})),
-            Err(TryRecvError::Empty) => {
-                // Insert this receive operation.
-                *opt_key = Some(wakers.insert(cx));
 
-                // If the channel is still empty and not disconnected, return.
-                if channel.is_empty() && !channel.is_disconnected() {
-                    return Poll::Pending;
-                }
-            }
-        }
-    }
-}
-
-/// A slot in a channel.
-struct Slot<T> {
-    /// The current stamp.
-    stamp: AtomicUsize,
-
-    /// The message in this slot.
-    msg: UnsafeCell<T>,
-}
-
-/// Bounded channel based on a preallocated array.
-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: 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: AtomicUsize,
-
-    /// The buffer holding slots.
-    buffer: *mut 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 either all senders were dropped or all receivers
-    /// were dropped.
-    mark_bit: usize,
-
-    /// Send operations waiting while the channel is full.
-    send_wakers: WakerSet,
-
-    /// Receive operations waiting while the channel is empty and not disconnected.
-    recv_wakers: WakerSet,
-
-    /// Streams waiting while the channel is empty and not disconnected.
-    stream_wakers: WakerSet,
-
-    /// The number of currently active `Sender`s.
-    sender_count: AtomicUsize,
-
-    /// The number of currently active `Receivers`s.
-    receiver_count: AtomicUsize,
-
-    /// Indicates that dropping a `Channel<T>` may drop values of type `T`.
-    _marker: PhantomData<T>,
-}
-
-unsafe impl<T: Send> Send for Channel<T> {}
-unsafe impl<T: Send> Sync for Channel<T> {}
-impl<T> Unpin for Channel<T> {}
-
-impl<T> Channel<T> {
-    /// Creates a bounded channel of capacity `cap`.
-    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.
-        let buffer = {
-            let mut v = Vec::<Slot<T>>::with_capacity(cap);
-            let ptr = v.as_mut_ptr();
-            mem::forget(v);
-            ptr
-        };
-
-        // Initialize stamps in the slots.
-        for i in 0..cap {
-            unsafe {
-                // Set the stamp to `{ lap: 0, mark: 0, index: i }`.
-                let slot = buffer.add(i);
-                ptr::write(&mut (*slot).stamp, AtomicUsize::new(i));
-            }
-        }
-
-        Channel {
-            buffer,
-            cap,
-            one_lap,
-            mark_bit,
-            head: AtomicUsize::new(head),
-            tail: AtomicUsize::new(tail),
-            send_wakers: WakerSet::new(),
-            recv_wakers: WakerSet::new(),
-            stream_wakers: WakerSet::new(),
-            sender_count: AtomicUsize::new(1),
-            receiver_count: AtomicUsize::new(1),
-            _marker: PhantomData,
-        }
-    }
-
-    /// Attempts to send a message.
-    fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> {
-        let backoff = Backoff::new();
-        let mut tail = self.tail.load(Ordering::Relaxed);
-
-        loop {
-            // Extract mark bit from the tail and unset it.
-            //
-            // If the mark bit was set (which means all receivers have been dropped), we will still
-            // send the message into the channel if there is enough capacity. The message will get
-            // dropped when the channel is dropped (which means when all senders are also dropped).
-            let mark_bit = tail & self.mark_bit;
-            tail ^= mark_bit;
-
-            // Deconstruct the tail.
-            let index = tail & (self.mark_bit - 1);
-            let lap = tail & !(self.one_lap - 1);
-
-            // Inspect the corresponding slot.
-            let slot = unsafe { &*self.buffer.add(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 | mark_bit,
-                    new_tail | mark_bit,
-                    Ordering::SeqCst,
-                    Ordering::Relaxed,
-                ) {
-                    Ok(_) => {
-                        // Write the message into the slot and update the stamp.
-                        unsafe { slot.msg.get().write(msg) };
-                        let stamp = tail + 1;
-                        slot.stamp.store(stamp, Ordering::Release);
-
-                        // Wake a blocked receive operation.
-                        self.recv_wakers.notify_one();
-
-                        // Wake all blocked streams.
-                        self.stream_wakers.notify_all();
-
-                        return Ok(());
-                    }
-                    Err(t) => {
-                        tail = t;
-                        backoff.spin();
-                    }
-                }
-            } 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.
-
-                    // Check if the channel is disconnected.
-                    if mark_bit != 0 {
-                        return Err(TrySendError::Disconnected(msg));
-                    } else {
-                        return Err(TrySendError::Full(msg));
-                    }
-                }
-
-                backoff.spin();
-                tail = self.tail.load(Ordering::Relaxed);
-            } else {
-                // Snooze because we need to wait for the stamp to get updated.
-                backoff.snooze();
-                tail = self.tail.load(Ordering::Relaxed);
-            }
-        }
-    }
-
-    /// Attempts to receive a message.
-    fn try_recv(&self) -> Result<T, TryRecvError> {
-        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.
-            let slot = unsafe { &*self.buffer.add(index) };
-            let stamp = slot.stamp.load(Ordering::Acquire);
-
-            // If the 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(_) => {
-                        // Read the message from the slot and update the stamp.
-                        let msg = unsafe { slot.msg.get().read() };
-                        let stamp = head.wrapping_add(self.one_lap);
-                        slot.stamp.store(stamp, Ordering::Release);
-
-                        // Wake a blocked send operation.
-                        self.send_wakers.notify_one();
-
-                        return Ok(msg);
-                    }
-                    Err(h) => {
-                        head = h;
-                        backoff.spin();
-                    }
-                }
-            } 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 {
-                        return Err(TryRecvError::Disconnected);
-                    } else {
-                        // Otherwise, the receive operation is not ready.
-                        return Err(TryRecvError::Empty);
-                    }
-                }
-
-                backoff.spin();
-                head = self.head.load(Ordering::Relaxed);
-            } else {
-                // Snooze because we need to wait for the stamp to get updated.
-                backoff.snooze();
-                head = self.head.load(Ordering::Relaxed);
-            }
-        }
-    }
-
-    /// Returns the current number of messages inside the channel.
-    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 `true` if the channel is disconnected.
-    pub fn is_disconnected(&self) -> bool {
-        self.tail.load(Ordering::SeqCst) & self.mark_bit != 0
-    }
-
-    /// Returns `true` if the channel is empty.
-    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.
-    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
-    }
-
-    /// Disconnects the channel and wakes up all blocked operations.
-    fn disconnect(&self) {
-        let tail = self.tail.fetch_or(self.mark_bit, Ordering::SeqCst);
-
-        if tail & self.mark_bit == 0 {
-            // Notify everyone blocked on this channel.
-            self.send_wakers.notify_all();
-            self.recv_wakers.notify_all();
-            self.stream_wakers.notify_all();
-        }
-    }
-}
-
-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 {
-                self.buffer.add(index).drop_in_place();
-            }
-        }
-
-        // Finally, deallocate the buffer, but don't run any destructors.
-        unsafe {
-            Vec::from_raw_parts(self.buffer, 0, self.cap);
-        }
-    }
-}
-
-/// An error returned from the `try_send` method.
-#[cfg(feature = "unstable")]
-#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
-#[derive(PartialEq, Eq)]
-pub enum TrySendError<T> {
-    /// The channel is full but not disconnected.
-    Full(T),
-
-    /// The channel is full and disconnected.
-    Disconnected(T),
-}
-
-impl<T> Error for TrySendError<T> {}
-
-impl<T> Debug for TrySendError<T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match self {
-            Self::Full(_) => Debug::fmt("Full<T>", f),
-            Self::Disconnected(_) => Debug::fmt("Disconnected<T>", f),
-        }
-    }
-}
-
-impl<T> Display for TrySendError<T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match self {
-            Self::Full(_) => Display::fmt("The channel is full.", f),
-            Self::Disconnected(_) => Display::fmt("The channel is full and disconnected.", f),
-        }
-    }
-}
-
-/// An error returned from the `try_recv` method.
-#[cfg(feature = "unstable")]
-#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
-#[derive(Debug, PartialEq, Eq)]
-pub enum TryRecvError {
-    /// The channel is empty but not disconnected.
-    Empty,
-
-    /// The channel is empty and disconnected.
-    Disconnected,
-}
-
-impl Error for TryRecvError {}
-
-impl Display for TryRecvError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match self {
-            Self::Empty => Display::fmt("The channel is empty.", f),
-            Self::Disconnected => Display::fmt("The channel is empty and disconnected.", f),
-        }
-    }
-}
-
-/// An error returned from the `recv` method.
-#[cfg(feature = "unstable")]
-#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
-#[derive(Debug, PartialEq, Eq)]
-pub struct RecvError;
-
-impl Error for RecvError {}
-
-impl Display for RecvError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        Display::fmt("The channel is empty.", f)
-    }
-}