Adding upstream version 124.0.1.upstream/124.0.1

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

3 files changed, 1488 insertions, 0 deletions

diff --git a/third_party/rust/oneshot-uniffi/src/errors.rs b/third_party/rust/oneshot-uniffi/src/errors.rs
new file mode 100644
index 0000000000..afc48acd03
--- /dev/null
+++ b/third_party/rust/oneshot-uniffi/src/errors.rs
@@ -0,0 +1,144 @@
+use super::{dealloc, Channel};
+use core::fmt;
+use core::mem;
+use core::ptr::NonNull;
+
+/// An error returned when trying to send on a closed channel. Returned from
+/// [`Sender::send`] if the corresponding [`Receiver`] has already been dropped.
+///
+/// The message that could not be sent can be retreived again with [`SendError::into_inner`].
+pub struct SendError<T> {
+    channel_ptr: NonNull<Channel<T>>,
+}
+
+unsafe impl<T: Send> Send for SendError<T> {}
+unsafe impl<T: Sync> Sync for SendError<T> {}
+
+impl<T> SendError<T> {
+    /// # Safety
+    ///
+    /// By calling this function, the caller semantically transfers ownership of the
+    /// channel's resources to the created `SendError`. Thus the caller must ensure that the
+    /// pointer is not used in a way which would violate this ownership transfer. Moreover,
+    /// the caller must assert that the channel contains a valid, initialized message.
+    pub(crate) const unsafe fn new(channel_ptr: NonNull<Channel<T>>) -> Self {
+        Self { channel_ptr }
+    }
+
+    /// Consumes the error and returns the message that failed to be sent.
+    #[inline]
+    pub fn into_inner(self) -> T {
+        let channel_ptr = self.channel_ptr;
+
+        // Don't run destructor if we consumed ourselves. Freeing happens here.
+        mem::forget(self);
+
+        // SAFETY: we have ownership of the channel
+        let channel: &Channel<T> = unsafe { channel_ptr.as_ref() };
+
+        // SAFETY: we know that the message is initialized according to the safety requirements of
+        // `new`
+        let message = unsafe { channel.take_message() };
+
+        // SAFETY: we own the channel
+        unsafe { dealloc(channel_ptr) };
+
+        message
+    }
+
+    /// Get a reference to the message that failed to be sent.
+    #[inline]
+    pub fn as_inner(&self) -> &T {
+        unsafe { self.channel_ptr.as_ref().message().assume_init_ref() }
+    }
+}
+
+impl<T> Drop for SendError<T> {
+    fn drop(&mut self) {
+        // SAFETY: we have ownership of the channel and require that the message is initialized
+        // upon construction
+        unsafe {
+            self.channel_ptr.as_ref().drop_message();
+            dealloc(self.channel_ptr);
+        }
+    }
+}
+
+impl<T> fmt::Display for SendError<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        "sending on a closed channel".fmt(f)
+    }
+}
+
+impl<T> fmt::Debug for SendError<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "SendError<{}>(_)", stringify!(T))
+    }
+}
+
+#[cfg(feature = "std")]
+impl<T> std::error::Error for SendError<T> {}
+
+/// An error returned from the indefinitely blocking recv functions on a [`Receiver`].
+///
+/// The recv operation can only fail if the corresponding [`Sender`] was dropped before sending
+/// any message. Or if a message has already been sent and received on the channel.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
+pub struct RecvError;
+
+impl fmt::Display for RecvError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        "receiving on a closed channel".fmt(f)
+    }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for RecvError {}
+
+/// An error returned when trying a non blocking receive on a [`Receiver`].
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
+pub enum TryRecvError {
+    /// The channel is still open, but there was no message present in it.
+    Empty,
+
+    /// The channel is closed. Either the sender was dropped before sending any message, or the
+    /// message has already been extracted from the receiver.
+    Disconnected,
+}
+
+impl fmt::Display for TryRecvError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let msg = match self {
+            TryRecvError::Empty => "receiving on an empty channel",
+            TryRecvError::Disconnected => "receiving on a closed channel",
+        };
+        msg.fmt(f)
+    }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for TryRecvError {}
+
+/// An error returned when trying a time limited blocking receive on a [`Receiver`].
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
+pub enum RecvTimeoutError {
+    /// No message arrived on the channel before the timeout was reached. The channel is still open.
+    Timeout,
+
+    /// The channel is closed. Either the sender was dropped before sending any message, or the
+    /// message has already been extracted from the receiver.
+    Disconnected,
+}
+
+impl fmt::Display for RecvTimeoutError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let msg = match self {
+            RecvTimeoutError::Timeout => "timed out waiting on channel",
+            RecvTimeoutError::Disconnected => "channel is empty and sending half is closed",
+        };
+        msg.fmt(f)
+    }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for RecvTimeoutError {}
diff --git a/third_party/rust/oneshot-uniffi/src/lib.rs b/third_party/rust/oneshot-uniffi/src/lib.rs
new file mode 100644
index 0000000000..94bb35d12a
--- /dev/null
+++ b/third_party/rust/oneshot-uniffi/src/lib.rs
@@ -0,0 +1,1193 @@
+//! Oneshot spsc (single producer, single consumer) channel. Meaning each channel instance
+//! can only transport a single message. This has a few nice outcomes. One thing is that
+//! the implementation can be very efficient, utilizing the knowledge that there will
+//! only be one message. But more importantly, it allows the API to be expressed in such
+//! a way that certain edge cases that you don't want to care about when only sending a
+//! single message on a channel does not exist. For example: The sender can't be copied
+//! or cloned, and the send method takes ownership and consumes the sender.
+//! So you are guaranteed, at the type level, that there can only be one message sent.
+//!
+//! The sender's send method is non-blocking, and potentially lock- and wait-free.
+//! See documentation on [Sender::send] for situations where it might not be fully wait-free.
+//! The receiver supports both lock- and wait-free `try_recv` as well as indefinite and time
+//! limited thread blocking receive operations. The receiver also implements `Future` and
+//! supports asynchronously awaiting the message.
+//!
+//!
+//! # Examples
+//!
+//! This example sets up a background worker that processes requests coming in on a standard
+//! mpsc channel and replies on a oneshot channel provided with each request. The worker can
+//! be interacted with both from sync and async contexts since the oneshot receiver
+//! can receive both blocking and async.
+//!
+//! ```rust
+//! use std::sync::mpsc;
+//! use std::thread;
+//! use std::time::Duration;
+//!
+//! type Request = String;
+//!
+//! // Starts a background thread performing some computation on requests sent to it.
+//! // Delivers the response back over a oneshot channel.
+//! fn spawn_processing_thread() -> mpsc::Sender<(Request, oneshot::Sender<usize>)> {
+//!     let (request_sender, request_receiver) = mpsc::channel::<(Request, oneshot::Sender<usize>)>();
+//!     thread::spawn(move || {
+//!         for (request_data, response_sender) in request_receiver.iter() {
+//!             let compute_operation = || request_data.len();
+//!             let _ = response_sender.send(compute_operation()); // <- Send on the oneshot channel
+//!         }
+//!     });
+//!     request_sender
+//! }
+//!
+//! let processor = spawn_processing_thread();
+//!
+//! // If compiled with `std` the library can receive messages with timeout on regular threads
+//! #[cfg(feature = "std")] {
+//!     let (response_sender, response_receiver) = oneshot::channel();
+//!     let request = Request::from("data from sync thread");
+//!
+//!     processor.send((request, response_sender)).expect("Processor down");
+//!     match response_receiver.recv_timeout(Duration::from_secs(1)) { // <- Receive on the oneshot channel
+//!         Ok(result) => println!("Processor returned {}", result),
+//!         Err(oneshot::RecvTimeoutError::Timeout) => eprintln!("Processor was too slow"),
+//!         Err(oneshot::RecvTimeoutError::Disconnected) => panic!("Processor exited"),
+//!     }
+//! }
+//!
+//! // If compiled with the `async` feature, the `Receiver` can be awaited in an async context
+//! #[cfg(feature = "async")] {
+//!     tokio::runtime::Runtime::new()
+//!         .unwrap()
+//!         .block_on(async move {
+//!             let (response_sender, response_receiver) = oneshot::channel();
+//!             let request = Request::from("data from sync thread");
+//!
+//!             processor.send((request, response_sender)).expect("Processor down");
+//!             match response_receiver.await { // <- Receive on the oneshot channel asynchronously
+//!                 Ok(result) => println!("Processor returned {}", result),
+//!                 Err(_e) => panic!("Processor exited"),
+//!             }
+//!         });
+//! }
+//! ```
+//!
+//! # Sync vs async
+//!
+//! The main motivation for writing this library was that there were no (known to me) channel
+//! implementations allowing you to seamlessly send messages between a normal thread and an async
+//! task, or the other way around. If message passing is the way you are communicating, of course
+//! that should work smoothly between the sync and async parts of the program!
+//!
+//! This library achieves that by having a fast and cheap send operation that can
+//! be used in both sync threads and async tasks. The receiver has both thread blocking
+//! receive methods for synchronous usage, and implements `Future` for asynchronous usage.
+//!
+//! The receiving endpoint of this channel implements Rust's `Future` trait and can be waited on
+//! in an asynchronous task. This implementation is completely executor/runtime agnostic. It should
+//! be possible to use this library with any executor.
+//!
+
+// # Implementation description
+//
+// When a channel is created via the channel function, it creates a single heap allocation
+// containing:
+// * A one byte atomic integer that represents the current channel state,
+// * Uninitialized memory to fit the message,
+// * Uninitialized memory to fit the waker that can wake the receiving task or thread up.
+//
+// The size of the waker depends on which features are activated, it ranges from 0 to 24 bytes[1].
+// So with all features enabled (the default) each channel allocates 25 bytes plus the size of the
+// message, plus any padding needed to get correct memory alignment.
+//
+// The Sender and Receiver only holds a raw pointer to the heap channel object. The last endpoint
+// to be consumed or dropped is responsible for freeing the heap memory. The first endpoint to
+// be consumed or dropped signal via the state that it is gone. And the second one see this and
+// frees the memory.
+//
+// ## Footnotes
+//
+// [1]: Mind that the waker only takes zero bytes when all features are disabled, making it
+//      impossible to *wait* for the message. `try_recv` the only available method in this scenario.
+
+#![deny(rust_2018_idioms)]
+#![cfg_attr(not(feature = "std"), no_std)]
+
+#[cfg(not(loom))]
+extern crate alloc;
+
+use core::{
+    marker::PhantomData,
+    mem::{self, MaybeUninit},
+    ptr::{self, NonNull},
+};
+
+#[cfg(not(loom))]
+use core::{
+    cell::UnsafeCell,
+    sync::atomic::{fence, AtomicU8, Ordering::*},
+};
+#[cfg(loom)]
+use loom::{
+    cell::UnsafeCell,
+    sync::atomic::{fence, AtomicU8, Ordering::*},
+};
+
+#[cfg(all(feature = "async", not(loom)))]
+use core::hint;
+#[cfg(all(feature = "async", loom))]
+use loom::hint;
+
+#[cfg(feature = "async")]
+use core::{
+    pin::Pin,
+    task::{self, Poll},
+};
+#[cfg(feature = "std")]
+use std::time::{Duration, Instant};
+
+#[cfg(feature = "std")]
+mod thread {
+    #[cfg(not(loom))]
+    pub use std::thread::{current, park, park_timeout, yield_now, Thread};
+
+    #[cfg(loom)]
+    pub use loom::thread::{current, park, yield_now, Thread};
+
+    // loom does not support parking with a timeout. So we just
+    // yield. This means that the "park" will "spuriously" wake up
+    // way too early. But the code should properly handle this.
+    // One thing to note is that very short timeouts are needed
+    // when using loom, since otherwise the looping will cause
+    // an overflow in loom.
+    #[cfg(loom)]
+    pub fn park_timeout(_timeout: std::time::Duration) {
+        loom::thread::yield_now()
+    }
+}
+
+#[cfg(loom)]
+mod loombox;
+#[cfg(not(loom))]
+use alloc::boxed::Box;
+#[cfg(loom)]
+use loombox::Box;
+
+mod errors;
+pub use errors::{RecvError, RecvTimeoutError, SendError, TryRecvError};
+
+/// Creates a new oneshot channel and returns the two endpoints, [`Sender`] and [`Receiver`].
+pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
+    // Allocate the channel on the heap and get the pointer.
+    // The last endpoint of the channel to be alive is responsible for freeing the channel
+    // and dropping any object that might have been written to it.
+
+    let channel_ptr = Box::into_raw(Box::new(Channel::new()));
+
+    // SAFETY: `channel_ptr` came from a Box and thus is not null
+    let channel_ptr = unsafe { NonNull::new_unchecked(channel_ptr) };
+
+    (
+        Sender {
+            channel_ptr,
+            _invariant: PhantomData,
+        },
+        Receiver { channel_ptr },
+    )
+}
+
+#[derive(Debug)]
+pub struct Sender<T> {
+    channel_ptr: NonNull<Channel<T>>,
+    // In reality we want contravariance, however we can't obtain that.
+    //
+    // Consider the following scenario:
+    // ```
+    // let (mut tx, rx) = channel::<&'short u8>();
+    // let (tx2, rx2) = channel::<&'long u8>();
+    //
+    // tx = tx2;
+    //
+    // // Pretend short_ref is some &'short u8
+    // tx.send(short_ref).unwrap();
+    // let long_ref = rx2.recv().unwrap();
+    // ```
+    //
+    // If this type were covariant then we could safely extend lifetimes, which is not okay.
+    // Hence, we enforce invariance.
+    _invariant: PhantomData<fn(T) -> T>,
+}
+
+#[derive(Debug)]
+pub struct Receiver<T> {
+    // Covariance is the right choice here. Consider the example presented in Sender, and you'll
+    // see that if we replaced `rx` instead then we would get the expected behavior
+    channel_ptr: NonNull<Channel<T>>,
+}
+
+unsafe impl<T: Send> Send for Sender<T> {}
+unsafe impl<T: Send> Send for Receiver<T> {}
+impl<T> Unpin for Receiver<T> {}
+
+impl<T> Sender<T> {
+    /// Sends `message` over the channel to the corresponding [`Receiver`].
+    ///
+    /// Returns an error if the receiver has already been dropped. The message can
+    /// be extracted from the error.
+    ///
+    /// This method is lock-free and wait-free when sending on a channel that the
+    /// receiver is currently not receiving on. If the receiver is receiving during the send
+    /// operation this method includes waking up the thread/task. Unparking a thread involves
+    /// a mutex in Rust's standard library at the time of writing this.
+    /// How lock-free waking up an async task is
+    /// depends on your executor. If this method returns a `SendError`, please mind that dropping
+    /// the error involves running any drop implementation on the message type, and freeing the
+    /// channel's heap allocation, which might or might not be lock-free.
+    pub fn send(self, message: T) -> Result<(), SendError<T>> {
+        let channel_ptr = self.channel_ptr;
+
+        // Don't run our Drop implementation if send was called, any cleanup now happens here
+        mem::forget(self);
+
+        // SAFETY: The channel exists on the heap for the entire duration of this method and we
+        // only ever acquire shared references to it. Note that if the receiver disconnects it
+        // does not free the channel.
+        let channel = unsafe { channel_ptr.as_ref() };
+
+        // Write the message into the channel on the heap.
+        // SAFETY: The receiver only ever accesses this memory location if we are in the MESSAGE
+        // state, and since we're responsible for setting that state, we can guarantee that we have
+        // exclusive access to this memory location to perform this write.
+        unsafe { channel.write_message(message) };
+
+        // Set the state to signal there is a message on the channel.
+        // ORDERING: we use release ordering to ensure the write of the message is visible to the
+        // receiving thread. The EMPTY and DISCONNECTED branches do not observe any shared state,
+        // and thus we do not need acquire orderng. The RECEIVING branch manages synchronization
+        // independent of this operation.
+        //
+        // EMPTY + 1 = MESSAGE
+        // RECEIVING + 1 = UNPARKING
+        // DISCONNECTED + 1 = invalid, however this state is never observed
+        match channel.state.fetch_add(1, Release) {
+            // The receiver is alive and has not started waiting. Send done.
+            EMPTY => Ok(()),
+            // The receiver is waiting. Wake it up so it can return the message.
+            RECEIVING => {
+                // ORDERING: Synchronizes with the write of the waker to memory, and prevents the
+                // taking of the waker from being ordered before this operation.
+                fence(Acquire);
+
+                // Take the waker, but critically do not unpark it. If we unparked now, then the
+                // receiving thread could still observe the UNPARKING state and re-park, meaning
+                // that after we change to the MESSAGE state, it would remain parked indefinitely
+                // or until a spurious wakeup.
+                // SAFETY: at this point we are in the UNPARKING state, and the receiving thread
+                // does not access the waker while in this state, nor does it free the channel
+                // allocation in this state.
+                let waker = unsafe { channel.take_waker() };
+
+                // ORDERING: this ordering serves two-fold: it synchronizes with the acquire load
+                // in the receiving thread, ensuring that both our read of the waker and write of
+                // the message happen-before the taking of the message and freeing of the channel.
+                // Furthermore, we need acquire ordering to ensure the unparking of the receiver
+                // happens after the channel state is updated.
+                channel.state.swap(MESSAGE, AcqRel);
+
+                // Note: it is possible that between the store above and this statement that
+                // the receiving thread is spuriously unparked, takes the message, and frees
+                // the channel allocation. However, we took ownership of the channel out of
+                // that allocation, and freeing the channel does not drop the waker since the
+                // waker is wrapped in MaybeUninit. Therefore this data is valid regardless of
+                // whether or not the receive has completed by this point.
+                waker.unpark();
+
+                Ok(())
+            }
+            // The receiver was already dropped. The error is responsible for freeing the channel.
+            // SAFETY: since the receiver disconnected it will no longer access `channel_ptr`, so
+            // we can transfer exclusive ownership of the channel's resources to the error.
+            // Moreover, since we just placed the message in the channel, the channel contains a
+            // valid message.
+            DISCONNECTED => Err(unsafe { SendError::new(channel_ptr) }),
+            _ => unreachable!(),
+        }
+    }
+}
+
+impl<T> Drop for Sender<T> {
+    fn drop(&mut self) {
+        // SAFETY: The receiver only ever frees the channel if we are in the MESSAGE or
+        // DISCONNECTED states. If we are in the MESSAGE state, then we called
+        // mem::forget(self), so we should not be in this function call. If we are in the
+        // DISCONNECTED state, then the receiver either received a MESSAGE so this statement is
+        // unreachable, or was dropped and observed that our side was still alive, and thus didn't
+        // free the channel.
+        let channel = unsafe { self.channel_ptr.as_ref() };
+
+        // Set the channel state to disconnected and read what state the receiver was in
+        // ORDERING: we don't need release ordering here since there are no modifications we
+        // need to make visible to other thread, and the Err(RECEIVING) branch handles
+        // synchronization independent of this cmpxchg
+        //
+        // EMPTY ^ 001 = DISCONNECTED
+        // RECEIVING ^ 001 = UNPARKING
+        // DISCONNECTED ^ 001 = EMPTY (invalid), but this state is never observed
+        match channel.state.fetch_xor(0b001, Relaxed) {
+            // The receiver has not started waiting, nor is it dropped.
+            EMPTY => (),
+            // The receiver is waiting. Wake it up so it can detect that the channel disconnected.
+            RECEIVING => {
+                // See comments in Sender::send
+
+                fence(Acquire);
+
+                let waker = unsafe { channel.take_waker() };
+
+                // We still need release ordering here to make sure our read of the waker happens
+                // before this, and acquire ordering to ensure the unparking of the receiver
+                // happens after this.
+                channel.state.swap(DISCONNECTED, AcqRel);
+
+                // The Acquire ordering above ensures that the write of the DISCONNECTED state
+                // happens-before unparking the receiver.
+                waker.unpark();
+            }
+            // The receiver was already dropped. We are responsible for freeing the channel.
+            DISCONNECTED => {
+                // SAFETY: when the receiver switches the state to DISCONNECTED they have received
+                // the message or will no longer be trying to receive the message, and have
+                // observed that the sender is still alive, meaning that we're responsible for
+                // freeing the channel allocation.
+                unsafe { dealloc(self.channel_ptr) };
+            }
+            _ => unreachable!(),
+        }
+    }
+}
+
+impl<T> Receiver<T> {
+    /// Checks if there is a message in the channel without blocking. Returns:
+    ///  * `Ok(message)` if there was a message in the channel.
+    ///  * `Err(Empty)` if the [`Sender`] is alive, but has not yet sent a message.
+    ///  * `Err(Disconnected)` if the [`Sender`] was dropped before sending anything or if the
+    ///    message has already been extracted by a previous receive call.
+    ///
+    /// If a message is returned, the channel is disconnected and any subsequent receive operation
+    /// using this receiver will return an error.
+    ///
+    /// This method is completely lock-free and wait-free. The only thing it does is an atomic
+    /// integer load of the channel state. And if there is a message in the channel it additionally
+    /// performs one atomic integer store and copies the message from the heap to the stack for
+    /// returning it.
+    pub fn try_recv(&self) -> Result<T, TryRecvError> {
+        // SAFETY: The channel will not be freed while this method is still running.
+        let channel = unsafe { self.channel_ptr.as_ref() };
+
+        // ORDERING: we use acquire ordering to synchronize with the store of the message.
+        match channel.state.load(Acquire) {
+            MESSAGE => {
+                // It's okay to break up the load and store since once we're in the message state
+                // the sender no longer modifies the state
+                // ORDERING: at this point the sender has done its job and is no longer active, so
+                // we don't need to make any side effects visible to it
+                channel.state.store(DISCONNECTED, Relaxed);
+
+                // SAFETY: we are in the MESSAGE state so the message is present
+                Ok(unsafe { channel.take_message() })
+            }
+            EMPTY => Err(TryRecvError::Empty),
+            DISCONNECTED => Err(TryRecvError::Disconnected),
+            #[cfg(feature = "async")]
+            RECEIVING | UNPARKING => Err(TryRecvError::Empty),
+            _ => unreachable!(),
+        }
+    }
+
+    /// Attempts to wait for a message from the [`Sender`], returning an error if the channel is
+    /// disconnected.
+    ///
+    /// This method will always block the current thread if there is no data available and it is
+    /// still possible for the message to be sent. Once the message is sent to the corresponding
+    /// [`Sender`], then this receiver will wake up and return that message.
+    ///
+    /// If the corresponding [`Sender`] has disconnected (been dropped), or it disconnects while
+    /// this call is blocking, this call will wake up and return `Err` to indicate that the message
+    /// can never be received on this channel.
+    ///
+    /// If a sent message has already been extracted from this channel this method will return an
+    /// error.
+    ///
+    /// # Panics
+    ///
+    /// Panics if called after this receiver has been polled asynchronously.
+    #[cfg(feature = "std")]
+    pub fn recv(self) -> Result<T, RecvError> {
+        // Note that we don't need to worry about changing the state to disconnected or setting the
+        // state to an invalid value at any point in this function because we take ownership of
+        // self, and this function does not exit until the message has been received or both side
+        // of the channel are inactive and cleaned up.
+
+        let channel_ptr = self.channel_ptr;
+
+        // Don't run our Drop implementation if we are receiving consuming ourselves.
+        mem::forget(self);
+
+        // SAFETY: the existence of the `self` parameter serves as a certificate that the receiver
+        // is still alive, meaning that even if the sender was dropped then it would have observed
+        // the fact that we're still alive and left the responsibility of deallocating the
+        // channel to us, so channel_ptr is valid
+        let channel = unsafe { channel_ptr.as_ref() };
+
+        // ORDERING: we use acquire ordering to synchronize with the write of the message in the
+        // case that it's available
+        match channel.state.load(Acquire) {
+            // The sender is alive but has not sent anything yet. We prepare to park.
+            EMPTY => {
+                // Conditionally add a delay here to help the tests trigger the edge cases where
+                // the sender manages to be dropped or send something before we are able to store
+                // our waker object in the channel.
+                #[cfg(oneshot_test_delay)]
+                std::thread::sleep(std::time::Duration::from_millis(10));
+
+                // Write our waker instance to the channel.
+                // SAFETY: we are not yet in the RECEIVING state, meaning that the sender will not
+                // try to access the waker until it sees the state set to RECEIVING below
+                unsafe { channel.write_waker(ReceiverWaker::current_thread()) };
+
+                // Switch the state to RECEIVING. We need to do this in one atomic step in case the
+                // sender disconnected or sent the message while we wrote the waker to memory. We
+                // don't need to do a compare exchange here however because if the original state
+                // was not EMPTY, then the sender has either finished sending the message or is
+                // being dropped, so the RECEIVING state will never be observed after we return.
+                // ORDERING: we use release ordering so the sender can synchronize with our writing
+                // of the waker to memory. The individual branches handle any additional
+                // synchronizaton
+                match channel.state.swap(RECEIVING, Release) {
+                    // We stored our waker, now we park until the sender has changed the state
+                    EMPTY => loop {
+                        thread::park();
+
+                        // ORDERING: synchronize with the write of the message
+                        match channel.state.load(Acquire) {
+                            // The sender sent the message while we were parked.
+                            MESSAGE => {
+                                // SAFETY: we are in the message state so the message is valid
+                                let message = unsafe { channel.take_message() };
+
+                                // SAFETY: the Sender delegates the responsibility of deallocating
+                                // the channel to us upon sending the message
+                                unsafe { dealloc(channel_ptr) };
+
+                                break Ok(message);
+                            }
+                            // The sender was dropped while we were parked.
+                            DISCONNECTED => {
+                                // SAFETY: the Sender doesn't deallocate the channel allocation in
+                                // its drop implementation if we're receiving
+                                unsafe { dealloc(channel_ptr) };
+
+                                break Err(RecvError);
+                            }
+                            // State did not change, spurious wakeup, park again.
+                            RECEIVING | UNPARKING => (),
+                            _ => unreachable!(),
+                        }
+                    },
+                    // The sender sent the message while we prepared to park.
+                    MESSAGE => {
+                        // ORDERING: Synchronize with the write of the message. This branch is
+                        // unlikely to be taken, so it's likely more efficient to use a fence here
+                        // instead of AcqRel ordering on the RMW operation
+                        fence(Acquire);
+
+                        // SAFETY: we started in the empty state and the sender switched us to the
+                        // message state. This means that it did not take the waker, so we're
+                        // responsible for dropping it.
+                        unsafe { channel.drop_waker() };
+
+                        // SAFETY: we are in the message state so the message is valid
+                        let message = unsafe { channel.take_message() };
+
+                        // SAFETY: the Sender delegates the responsibility of deallocating the
+                        // channel to us upon sending the message
+                        unsafe { dealloc(channel_ptr) };
+
+                        Ok(message)
+                    }
+                    // The sender was dropped before sending anything while we prepared to park.
+                    DISCONNECTED => {
+                        // SAFETY: we started in the empty state and the sender switched us to the
+                        // disconnected state. It does not take the waker when it does this so we
+                        // need to drop it.
+                        unsafe { channel.drop_waker() };
+
+                        // SAFETY: the sender does not deallocate the channel if it switches from
+                        // empty to disconnected so we need to free the allocation
+                        unsafe { dealloc(channel_ptr) };
+
+                        Err(RecvError)
+                    }
+                    _ => unreachable!(),
+                }
+            }
+            // The sender already sent the message.
+            MESSAGE => {
+                // SAFETY: we are in the message state so the message is valid
+                let message = unsafe { channel.take_message() };
+
+                // SAFETY: we are already in the message state so the sender has been forgotten
+                // and it's our job to clean up resources
+                unsafe { dealloc(channel_ptr) };
+
+                Ok(message)
+            }
+            // The sender was dropped before sending anything, or we already received the message.
+            DISCONNECTED => {
+                // SAFETY: the sender does not deallocate the channel if it switches from empty to
+                // disconnected so we need to free the allocation
+                unsafe { dealloc(channel_ptr) };
+
+                Err(RecvError)
+            }
+            // The receiver must have been `Future::poll`ed prior to this call.
+            #[cfg(feature = "async")]
+            RECEIVING | UNPARKING => panic!("{}", RECEIVER_USED_SYNC_AND_ASYNC_ERROR),
+            _ => unreachable!(),
+        }
+    }
+
+    /// Attempts to wait for a message from the [`Sender`], returning an error if the channel is
+    /// disconnected. This is a non consuming version of [`Receiver::recv`], but with a bit
+    /// worse performance. Prefer `[`Receiver::recv`]` if your code allows consuming the receiver.
+    ///
+    /// If a message is returned, the channel is disconnected and any subsequent receive operation
+    /// using this receiver will return an error.
+    ///
+    /// # Panics
+    ///
+    /// Panics if called after this receiver has been polled asynchronously.
+    #[cfg(feature = "std")]
+    pub fn recv_ref(&self) -> Result<T, RecvError> {
+        self.start_recv_ref(RecvError, |channel| {
+            loop {
+                thread::park();
+
+                // ORDERING: we use acquire ordering to synchronize with the write of the message
+                match channel.state.load(Acquire) {
+                    // The sender sent the message while we were parked.
+                    // We take the message and mark the channel disconnected.
+                    MESSAGE => {
+                        // ORDERING: the sender is inactive at this point so we don't need to make
+                        // any reads or writes visible to the sending thread
+                        channel.state.store(DISCONNECTED, Relaxed);
+
+                        // SAFETY: we were just in the message state so the message is valid
+                        break Ok(unsafe { channel.take_message() });
+                    }
+                    // The sender was dropped while we were parked.
+                    DISCONNECTED => break Err(RecvError),
+                    // State did not change, spurious wakeup, park again.
+                    RECEIVING | UNPARKING => (),
+                    _ => unreachable!(),
+                }
+            }
+        })
+    }
+
+    /// Like [`Receiver::recv`], but will not block longer than `timeout`. Returns:
+    ///  * `Ok(message)` if there was a message in the channel before the timeout was reached.
+    ///  * `Err(Timeout)` if no message arrived on the channel before the timeout was reached.
+    ///  * `Err(Disconnected)` if the sender was dropped before sending anything or if the message
+    ///    has already been extracted by a previous receive call.
+    ///
+    /// If a message is returned, the channel is disconnected and any subsequent receive operation
+    /// using this receiver will return an error.
+    ///
+    /// If the supplied `timeout` is so large that Rust's `Instant` type can't represent this point
+    /// in the future this falls back to an indefinitely blocking receive operation.
+    ///
+    /// # Panics
+    ///
+    /// Panics if called after this receiver has been polled asynchronously.
+    #[cfg(feature = "std")]
+    pub fn recv_timeout(&self, timeout: Duration) -> Result<T, RecvTimeoutError> {
+        match Instant::now().checked_add(timeout) {
+            Some(deadline) => self.recv_deadline(deadline),
+            None => self.recv_ref().map_err(|_| RecvTimeoutError::Disconnected),
+        }
+    }
+
+    /// Like [`Receiver::recv`], but will not block longer than until `deadline`. Returns:
+    ///  * `Ok(message)` if there was a message in the channel before the deadline was reached.
+    ///  * `Err(Timeout)` if no message arrived on the channel before the deadline was reached.
+    ///  * `Err(Disconnected)` if the sender was dropped before sending anything or if the message
+    ///    has already been extracted by a previous receive call.
+    ///
+    /// If a message is returned, the channel is disconnected and any subsequent receive operation
+    /// using this receiver will return an error.
+    ///
+    /// # Panics
+    ///
+    /// Panics if called after this receiver has been polled asynchronously.
+    #[cfg(feature = "std")]
+    pub fn recv_deadline(&self, deadline: Instant) -> Result<T, RecvTimeoutError> {
+        /// # Safety
+        ///
+        /// If the sender is unparking us after a message send, the message must already have been
+        /// written to the channel and an acquire memory barrier issued before calling this function
+        #[cold]
+        unsafe fn wait_for_unpark<T>(channel: &Channel<T>) -> Result<T, RecvTimeoutError> {
+            loop {
+                thread::park();
+
+                // ORDERING: The callee has already synchronized with any message write
+                match channel.state.load(Relaxed) {
+                    MESSAGE => {
+                        // ORDERING: the sender has been dropped, so this update only
+                        // needs to be visible to us
+                        channel.state.store(DISCONNECTED, Relaxed);
+                        break Ok(channel.take_message());
+                    }
+                    DISCONNECTED => break Err(RecvTimeoutError::Disconnected),
+                    // The sender is still unparking us. We continue on the empty state here since
+                    // the current implementation eagerly sets the state to EMPTY upon timeout.
+                    EMPTY => (),
+                    _ => unreachable!(),
+                }
+            }
+        }
+
+        self.start_recv_ref(RecvTimeoutError::Disconnected, |channel| {
+            loop {
+                match deadline.checked_duration_since(Instant::now()) {
+                    Some(timeout) => {
+                        thread::park_timeout(timeout);
+
+                        // ORDERING: synchronize with the write of the message
+                        match channel.state.load(Acquire) {
+                            // The sender sent the message while we were parked.
+                            MESSAGE => {
+                                // ORDERING: the sender has been `mem::forget`-ed so this update
+                                // only needs to be visible to us.
+                                channel.state.store(DISCONNECTED, Relaxed);
+
+                                // SAFETY: we either are in the message state or were just in the
+                                // message state
+                                break Ok(unsafe { channel.take_message() });
+                            }
+                            // The sender was dropped while we were parked.
+                            DISCONNECTED => break Err(RecvTimeoutError::Disconnected),
+                            // State did not change, spurious wakeup, park again.
+                            RECEIVING | UNPARKING => (),
+                            _ => unreachable!(),
+                        }
+                    }
+                    None => {
+                        // ORDERING: synchronize with the write of the message
+                        match channel.state.swap(EMPTY, Acquire) {
+                            // We reached the end of the timeout without receiving a message
+                            RECEIVING => {
+                                // SAFETY: we were in the receiving state and are now in the empty
+                                // state, so the sender has not and will not try to read the waker,
+                                // so we have exclusive access to drop it.
+                                unsafe { channel.drop_waker() };
+
+                                break Err(RecvTimeoutError::Timeout);
+                            }
+                            // The sender sent the message while we were parked.
+                            MESSAGE => {
+                                // Same safety and ordering as the Some branch
+
+                                channel.state.store(DISCONNECTED, Relaxed);
+                                break Ok(unsafe { channel.take_message() });
+                            }
+                            // The sender was dropped while we were parked.
+                            DISCONNECTED => {
+                                // ORDERING: we were originally in the disconnected state meaning
+                                // that the sender is inactive and no longer observing the state,
+                                // so we only need to change it back to DISCONNECTED for if the
+                                // receiver is dropped or a recv* method is called again
+                                channel.state.store(DISCONNECTED, Relaxed);
+
+                                break Err(RecvTimeoutError::Disconnected);
+                            }
+                            // The sender sent the message and started unparking us
+                            UNPARKING => {
+                                // We were in the UNPARKING state and are now in the EMPTY state.
+                                // We wait to be properly unparked and to observe if the sender
+                                // sets MESSAGE or DISCONNECTED state.
+                                // SAFETY: The load above has synchronized with any message write.
+                                break unsafe { wait_for_unpark(channel) };
+                            }
+                            _ => unreachable!(),
+                        }
+                    }
+                }
+            }
+        })
+    }
+
+    /// Begins the process of receiving on the channel by reference. If the message is already
+    /// ready, or the sender has disconnected, then this function will return the appropriate
+    /// Result immediately. Otherwise, it will write the waker to memory, check to see if the
+    /// sender has finished or disconnected again, and then will call `finish`. `finish` is
+    /// thus responsible for cleaning up the channel's resources appropriately before it returns,
+    /// such as destroying the waker, for instance.
+    #[cfg(feature = "std")]
+    #[inline]
+    fn start_recv_ref<E>(
+        &self,
+        disconnected_error: E,
+        finish: impl FnOnce(&Channel<T>) -> Result<T, E>,
+    ) -> Result<T, E> {
+        // SAFETY: the existence of the `self` parameter serves as a certificate that the receiver
+        // is still alive, meaning that even if the sender was dropped then it would have observed
+        // the fact that we're still alive and left the responsibility of deallocating the
+        // channel to us, so `self.channel` is valid
+        let channel = unsafe { self.channel_ptr.as_ref() };
+
+        // ORDERING: synchronize with the write of the message
+        match channel.state.load(Acquire) {
+            // The sender is alive but has not sent anything yet. We prepare to park.
+            EMPTY => {
+                // Conditionally add a delay here to help the tests trigger the edge cases where
+                // the sender manages to be dropped or send something before we are able to store
+                // our waker object in the channel.
+                #[cfg(oneshot_test_delay)]
+                std::thread::sleep(std::time::Duration::from_millis(10));
+
+                // Write our waker instance to the channel.
+                // SAFETY: we are not yet in the RECEIVING state, meaning that the sender will not
+                // try to access the waker until it sees the state set to RECEIVING below
+                unsafe { channel.write_waker(ReceiverWaker::current_thread()) };
+
+                // ORDERING: we use release ordering on success so the sender can synchronize with
+                // our write of the waker. We use relaxed ordering on failure since the sender does
+                // not need to synchronize with our write and the individual match arms handle any
+                // additional synchronization
+                match channel
+                    .state
+                    .compare_exchange(EMPTY, RECEIVING, Release, Relaxed)
+                {
+                    // We stored our waker, now we delegate to the callback to finish the receive
+                    // operation
+                    Ok(_) => finish(channel),
+                    // The sender sent the message while we prepared to finish
+                    Err(MESSAGE) => {
+                        // See comments in `recv` for ordering and safety
+
+                        fence(Acquire);
+
+                        unsafe { channel.drop_waker() };
+
+                        // ORDERING: the sender has been `mem::forget`-ed so this update only
+                        // needs to be visible to us
+                        channel.state.store(DISCONNECTED, Relaxed);
+
+                        // SAFETY: The MESSAGE state tells us there is a correctly initialized
+                        // message
+                        Ok(unsafe { channel.take_message() })
+                    }
+                    // The sender was dropped before sending anything while we prepared to park.
+                    Err(DISCONNECTED) => {
+                        // See comments in `recv` for safety
+                        unsafe { channel.drop_waker() };
+                        Err(disconnected_error)
+                    }
+                    _ => unreachable!(),
+                }
+            }
+            // The sender sent the message. We take the message and mark the channel disconnected.
+            MESSAGE => {
+                // ORDERING: the sender has been `mem::forget`-ed so this update only needs to be
+                // visible to us
+                channel.state.store(DISCONNECTED, Relaxed);
+
+                // SAFETY: we are in the message state so the message is valid
+                Ok(unsafe { channel.take_message() })
+            }
+            // The sender was dropped before sending anything, or we already received the message.
+            DISCONNECTED => Err(disconnected_error),
+            // The receiver must have been `Future::poll`ed prior to this call.
+            #[cfg(feature = "async")]
+            RECEIVING | UNPARKING => panic!("{}", RECEIVER_USED_SYNC_AND_ASYNC_ERROR),
+            _ => unreachable!(),
+        }
+    }
+}
+
+#[cfg(feature = "async")]
+impl<T> core::future::Future for Receiver<T> {
+    type Output = Result<T, RecvError>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
+        // SAFETY: the existence of the `self` parameter serves as a certificate that the receiver
+        // is still alive, meaning that even if the sender was dropped then it would have observed
+        // the fact that we're still alive and left the responsibility of deallocating the
+        // channel to us, so `self.channel` is valid
+        let channel = unsafe { self.channel_ptr.as_ref() };
+
+        // ORDERING: we use acquire ordering to synchronize with the store of the message.
+        match channel.state.load(Acquire) {
+            // The sender is alive but has not sent anything yet.
+            EMPTY => {
+                // SAFETY: We can't be in the forbidden states, and no waker in the channel.
+                unsafe { channel.write_async_waker(cx) }
+            }
+            // We were polled again while waiting for the sender. Replace the waker with the new one.
+            RECEIVING => {
+                // ORDERING: We use relaxed ordering on both success and failure since we have not
+                // written anything above that must be released, and the individual match arms
+                // handle any additional synchronization.
+                match channel
+                    .state
+                    .compare_exchange(RECEIVING, EMPTY, Relaxed, Relaxed)
+                {
+                    // We successfully changed the state back to EMPTY. Replace the waker.
+                    // This is the most likely branch to be taken, which is why we don't use any
+                    // memory barriers in the compare_exchange above.
+                    Ok(_) => {
+                        // SAFETY: We wrote the waker in a previous call to poll. We do not need
+                        // a memory barrier since the previous write here was by ourselves.
+                        unsafe { channel.drop_waker() };
+                        // SAFETY: We can't be in the forbidden states, and no waker in the channel.
+                        unsafe { channel.write_async_waker(cx) }
+                    }
+                    // The sender sent the message while we prepared to replace the waker.
+                    // We take the message and mark the channel disconnected.
+                    // The sender has already taken the waker.
+                    Err(MESSAGE) => {
+                        // ORDERING: Synchronize with the write of the message. This branch is
+                        // unlikely to be taken.
+                        channel.state.swap(DISCONNECTED, Acquire);
+                        // SAFETY: The state tells us the sender has initialized the message.
+                        Poll::Ready(Ok(unsafe { channel.take_message() }))
+                    }
+                    // The sender was dropped before sending anything while we prepared to park.
+                    // The sender has taken the waker already.
+                    Err(DISCONNECTED) => Poll::Ready(Err(RecvError)),
+                    // The sender is currently waking us up.
+                    Err(UNPARKING) => {
+                        // We can't trust that the old waker that the sender has access to
+                        // is honored by the async runtime at this point. So we wake ourselves
+                        // up to get polled instantly again.
+                        cx.waker().wake_by_ref();
+                        Poll::Pending
+                    }
+                    _ => unreachable!(),
+                }
+            }
+            // The sender sent the message.
+            MESSAGE => {
+                // ORDERING: the sender has been dropped so this update only needs to be
+                // visible to us
+                channel.state.store(DISCONNECTED, Relaxed);
+                Poll::Ready(Ok(unsafe { channel.take_message() }))
+            }
+            // The sender was dropped before sending anything, or we already received the message.
+            DISCONNECTED => Poll::Ready(Err(RecvError)),
+            // The sender has observed the RECEIVING state and is currently reading the waker from
+            // a previous poll. We need to loop here until we observe the MESSAGE or DISCONNECTED
+            // state. We busy loop here since we know the sender is done very soon.
+            UNPARKING => loop {
+                hint::spin_loop();
+                // ORDERING: The load above has already synchronized with the write of the message.
+                match channel.state.load(Relaxed) {
+                    MESSAGE => {
+                        // ORDERING: the sender has been dropped, so this update only
+                        // needs to be visible to us
+                        channel.state.store(DISCONNECTED, Relaxed);
+                        // SAFETY: We observed the MESSAGE state
+                        break Poll::Ready(Ok(unsafe { channel.take_message() }));
+                    }
+                    DISCONNECTED => break Poll::Ready(Err(RecvError)),
+                    UNPARKING => (),
+                    _ => unreachable!(),
+                }
+            },
+            _ => unreachable!(),
+        }
+    }
+}
+
+impl<T> Drop for Receiver<T> {
+    fn drop(&mut self) {
+        // SAFETY: since the receiving side is still alive the sender would have observed that and
+        // left deallocating the channel allocation to us.
+        let channel = unsafe { self.channel_ptr.as_ref() };
+
+        // Set the channel state to disconnected and read what state the receiver was in
+        match channel.state.swap(DISCONNECTED, Acquire) {
+            // The sender has not sent anything, nor is it dropped.
+            EMPTY => (),
+            // The sender already sent something. We must drop it, and free the channel.
+            MESSAGE => {
+                // SAFETY: we are in the message state so the message is initialized
+                unsafe { channel.drop_message() };
+
+                // SAFETY: see safety comment at top of function
+                unsafe { dealloc(self.channel_ptr) };
+            }
+            // The receiver has been polled.
+            #[cfg(feature = "async")]
+            RECEIVING => {
+                // TODO: figure this out when async is fixed
+                unsafe { channel.drop_waker() };
+            }
+            // The sender was already dropped. We are responsible for freeing the channel.
+            DISCONNECTED => {
+                // SAFETY: see safety comment at top of function
+                unsafe { dealloc(self.channel_ptr) };
+            }
+            _ => unreachable!(),
+        }
+    }
+}
+
+/// All the values that the `Channel::state` field can have during the lifetime of a channel.
+mod states {
+    // These values are very explicitly chosen so that we can replace some cmpxchg calls with
+    // fetch_* calls.
+
+    /// The initial channel state. Active while both endpoints are still alive, no message has been
+    /// sent, and the receiver is not receiving.
+    pub const EMPTY: u8 = 0b011;
+    /// A message has been sent to the channel, but the receiver has not yet read it.
+    pub const MESSAGE: u8 = 0b100;
+    /// No message has yet been sent on the channel, but the receiver is currently receiving.
+    pub const RECEIVING: u8 = 0b000;
+    #[cfg(any(feature = "std", feature = "async"))]
+    pub const UNPARKING: u8 = 0b001;
+    /// The channel has been closed. This means that either the sender or receiver has been dropped,
+    /// or the message sent to the channel has already been received. Since this is a oneshot
+    /// channel, it is disconnected after the one message it is supposed to hold has been
+    /// transmitted.
+    pub const DISCONNECTED: u8 = 0b010;
+}
+use states::*;
+
+/// Internal channel data structure structure. the `channel` method allocates and puts one instance
+/// of this struct on the heap for each oneshot channel instance. The struct holds:
+/// * The current state of the channel.
+/// * The message in the channel. This memory is uninitialized until the message is sent.
+/// * The waker instance for the thread or task that is currently receiving on this channel.
+///   This memory is uninitialized until the receiver starts receiving.
+struct Channel<T> {
+    state: AtomicU8,
+    message: UnsafeCell<MaybeUninit<T>>,
+    waker: UnsafeCell<MaybeUninit<ReceiverWaker>>,
+}
+
+impl<T> Channel<T> {
+    pub fn new() -> Self {
+        Self {
+            state: AtomicU8::new(EMPTY),
+            message: UnsafeCell::new(MaybeUninit::uninit()),
+            waker: UnsafeCell::new(MaybeUninit::uninit()),
+        }
+    }
+
+    #[inline(always)]
+    unsafe fn message(&self) -> &MaybeUninit<T> {
+        #[cfg(loom)]
+        {
+            self.message.with(|ptr| &*ptr)
+        }
+
+        #[cfg(not(loom))]
+        {
+            &*self.message.get()
+        }
+    }
+
+    #[inline(always)]
+    unsafe fn with_message_mut<F>(&self, op: F)
+    where
+        F: FnOnce(&mut MaybeUninit<T>),
+    {
+        #[cfg(loom)]
+        {
+            self.message.with_mut(|ptr| op(&mut *ptr))
+        }
+
+        #[cfg(not(loom))]
+        {
+            op(&mut *self.message.get())
+        }
+    }
+
+    #[inline(always)]
+    #[cfg(any(feature = "std", feature = "async"))]
+    unsafe fn with_waker_mut<F>(&self, op: F)
+    where
+        F: FnOnce(&mut MaybeUninit<ReceiverWaker>),
+    {
+        #[cfg(loom)]
+        {
+            self.waker.with_mut(|ptr| op(&mut *ptr))
+        }
+
+        #[cfg(not(loom))]
+        {
+            op(&mut *self.waker.get())
+        }
+    }
+
+    #[inline(always)]
+    unsafe fn write_message(&self, message: T) {
+        self.with_message_mut(|slot| slot.as_mut_ptr().write(message));
+    }
+
+    #[inline(always)]
+    unsafe fn take_message(&self) -> T {
+        #[cfg(loom)]
+        {
+            self.message.with(|ptr| ptr::read(ptr)).assume_init()
+        }
+
+        #[cfg(not(loom))]
+        {
+            ptr::read(self.message.get()).assume_init()
+        }
+    }
+
+    #[inline(always)]
+    unsafe fn drop_message(&self) {
+        self.with_message_mut(|slot| slot.assume_init_drop());
+    }
+
+    #[cfg(any(feature = "std", feature = "async"))]
+    #[inline(always)]
+    unsafe fn write_waker(&self, waker: ReceiverWaker) {
+        self.with_waker_mut(|slot| slot.as_mut_ptr().write(waker));
+    }
+
+    #[inline(always)]
+    unsafe fn take_waker(&self) -> ReceiverWaker {
+        #[cfg(loom)]
+        {
+            self.waker.with(|ptr| ptr::read(ptr)).assume_init()
+        }
+
+        #[cfg(not(loom))]
+        {
+            ptr::read(self.waker.get()).assume_init()
+        }
+    }
+
+    #[cfg(any(feature = "std", feature = "async"))]
+    #[inline(always)]
+    unsafe fn drop_waker(&self) {
+        self.with_waker_mut(|slot| slot.assume_init_drop());
+    }
+
+    /// # Safety
+    ///
+    /// * `Channel::waker` must not have a waker stored in it when calling this method.
+    /// * Channel state must not be RECEIVING or UNPARKING when calling this method.
+    #[cfg(feature = "async")]
+    unsafe fn write_async_waker(&self, cx: &mut task::Context<'_>) -> Poll<Result<T, RecvError>> {
+        // Write our thread instance to the channel.
+        // SAFETY: we are not yet in the RECEIVING state, meaning that the sender will not
+        // try to access the waker until it sees the state set to RECEIVING below
+        self.write_waker(ReceiverWaker::task_waker(cx));
+
+        // ORDERING: we use release ordering on success so the sender can synchronize with
+        // our write of the waker. We use relaxed ordering on failure since the sender does
+        // not need to synchronize with our write and the individual match arms handle any
+        // additional synchronization
+        match self
+            .state
+            .compare_exchange(EMPTY, RECEIVING, Release, Relaxed)
+        {
+            // We stored our waker, now we return and let the sender wake us up
+            Ok(_) => Poll::Pending,
+            // The sender sent the message while we prepared to park.
+            // We take the message and mark the channel disconnected.
+            Err(MESSAGE) => {
+                // ORDERING: Synchronize with the write of the message. This branch is
+                // unlikely to be taken, so it's likely more efficient to use a fence here
+                // instead of AcqRel ordering on the compare_exchange operation
+                fence(Acquire);
+
+                // SAFETY: we started in the EMPTY state and the sender switched us to the
+                // MESSAGE state. This means that it did not take the waker, so we're
+                // responsible for dropping it.
+                self.drop_waker();
+
+                // ORDERING: sender does not exist, so this update only needs to be visible to us
+                self.state.store(DISCONNECTED, Relaxed);
+
+                // SAFETY: The MESSAGE state tells us there is a correctly initialized message
+                Poll::Ready(Ok(self.take_message()))
+            }
+            // The sender was dropped before sending anything while we prepared to park.
+            Err(DISCONNECTED) => {
+                // SAFETY: we started in the EMPTY state and the sender switched us to the
+                // DISCONNECTED state. This means that it did not take the waker, so we're
+                // responsible for dropping it.
+                self.drop_waker();
+                Poll::Ready(Err(RecvError))
+            }
+            _ => unreachable!(),
+        }
+    }
+}
+
+enum ReceiverWaker {
+    /// The receiver is waiting synchronously. Its thread is parked.
+    #[cfg(feature = "std")]
+    Thread(thread::Thread),
+    /// The receiver is waiting asynchronously. Its task can be woken up with this `Waker`.
+    #[cfg(feature = "async")]
+    Task(task::Waker),
+    /// A little hack to not make this enum an uninhibitable type when no features are enabled.
+    #[cfg(not(any(feature = "async", feature = "std")))]
+    _Uninhabited,
+}
+
+impl ReceiverWaker {
+    #[cfg(feature = "std")]
+    pub fn current_thread() -> Self {
+        Self::Thread(thread::current())
+    }
+
+    #[cfg(feature = "async")]
+    pub fn task_waker(cx: &task::Context<'_>) -> Self {
+        Self::Task(cx.waker().clone())
+    }
+
+    pub fn unpark(self) {
+        match self {
+            #[cfg(feature = "std")]
+            ReceiverWaker::Thread(thread) => thread.unpark(),
+            #[cfg(feature = "async")]
+            ReceiverWaker::Task(waker) => waker.wake(),
+            #[cfg(not(any(feature = "async", feature = "std")))]
+            ReceiverWaker::_Uninhabited => unreachable!(),
+        }
+    }
+}
+
+#[cfg(not(loom))]
+#[test]
+fn receiver_waker_size() {
+    let expected: usize = match (cfg!(feature = "std"), cfg!(feature = "async")) {
+        (false, false) => 0,
+        (false, true) => 16,
+        (true, false) => 8,
+        (true, true) => 24,
+    };
+    assert_eq!(mem::size_of::<ReceiverWaker>(), expected);
+}
+
+#[cfg(all(feature = "std", feature = "async"))]
+const RECEIVER_USED_SYNC_AND_ASYNC_ERROR: &str =
+    "Invalid to call a blocking receive method on oneshot::Receiver after it has been polled";
+
+#[inline]
+pub(crate) unsafe fn dealloc<T>(channel: NonNull<Channel<T>>) {
+    drop(Box::from_raw(channel.as_ptr()))
+}
diff --git a/third_party/rust/oneshot-uniffi/src/loombox.rs b/third_party/rust/oneshot-uniffi/src/loombox.rs
new file mode 100644
index 0000000000..615db30174
--- /dev/null
+++ b/third_party/rust/oneshot-uniffi/src/loombox.rs
@@ -0,0 +1,151 @@
+use core::{borrow, fmt, hash, mem, ptr};
+use loom::alloc;
+
+pub struct Box<T: ?Sized> {
+    ptr: *mut T,
+}
+
+impl<T> Box<T> {
+    pub fn new(value: T) -> Self {
+        let layout = alloc::Layout::new::<T>();
+        let ptr = unsafe { alloc::alloc(layout) } as *mut T;
+        unsafe { ptr::write(ptr, value) };
+        Self { ptr }
+    }
+}
+
+impl<T: ?Sized> Box<T> {
+    #[inline]
+    pub fn into_raw(b: Box<T>) -> *mut T {
+        let ptr = b.ptr;
+        mem::forget(b);
+        ptr
+    }
+
+    pub const unsafe fn from_raw(ptr: *mut T) -> Box<T> {
+        Self { ptr }
+    }
+}
+
+impl<T: ?Sized> Drop for Box<T> {
+    fn drop(&mut self) {
+        unsafe {
+            let size = mem::size_of_val(&*self.ptr);
+            let align = mem::align_of_val(&*self.ptr);
+            let layout = alloc::Layout::from_size_align(size, align).unwrap();
+            ptr::drop_in_place(self.ptr);
+            alloc::dealloc(self.ptr as *mut u8, layout);
+        }
+    }
+}
+
+unsafe impl<T: Send> Send for Box<T> {}
+unsafe impl<T: Sync> Sync for Box<T> {}
+
+impl<T: ?Sized> core::ops::Deref for Box<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        unsafe { &*self.ptr }
+    }
+}
+
+impl<T: ?Sized> core::ops::DerefMut for Box<T> {
+    fn deref_mut(&mut self) -> &mut T {
+        unsafe { &mut *self.ptr }
+    }
+}
+
+impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
+    fn borrow(&self) -> &T {
+        &**self
+    }
+}
+
+impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
+    fn borrow_mut(&mut self) -> &mut T {
+        &mut **self
+    }
+}
+
+impl<T: ?Sized> AsRef<T> for Box<T> {
+    fn as_ref(&self) -> &T {
+        &**self
+    }
+}
+
+impl<T: ?Sized> AsMut<T> for Box<T> {
+    fn as_mut(&mut self) -> &mut T {
+        &mut **self
+    }
+}
+
+impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(&**self, f)
+    }
+}
+
+impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&**self, f)
+    }
+}
+
+impl<T: Clone> Clone for Box<T> {
+    #[inline]
+    fn clone(&self) -> Box<T> {
+        Self::new(self.as_ref().clone())
+    }
+}
+
+impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
+    #[inline]
+    fn eq(&self, other: &Box<T>) -> bool {
+        PartialEq::eq(&**self, &**other)
+    }
+
+    #[allow(clippy::partialeq_ne_impl)]
+    #[inline]
+    fn ne(&self, other: &Box<T>) -> bool {
+        PartialEq::ne(&**self, &**other)
+    }
+}
+
+impl<T: ?Sized + Eq> Eq for Box<T> {}
+
+impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
+    #[inline]
+    fn partial_cmp(&self, other: &Box<T>) -> Option<core::cmp::Ordering> {
+        PartialOrd::partial_cmp(&**self, &**other)
+    }
+    #[inline]
+    fn lt(&self, other: &Box<T>) -> bool {
+        PartialOrd::lt(&**self, &**other)
+    }
+    #[inline]
+    fn le(&self, other: &Box<T>) -> bool {
+        PartialOrd::le(&**self, &**other)
+    }
+    #[inline]
+    fn ge(&self, other: &Box<T>) -> bool {
+        PartialOrd::ge(&**self, &**other)
+    }
+    #[inline]
+    fn gt(&self, other: &Box<T>) -> bool {
+        PartialOrd::gt(&**self, &**other)
+    }
+}
+
+impl<T: ?Sized + Ord> Ord for Box<T> {
+    #[inline]
+    fn cmp(&self, other: &Box<T>) -> core::cmp::Ordering {
+        Ord::cmp(&**self, &**other)
+    }
+}
+
+impl<T: ?Sized + hash::Hash> hash::Hash for Box<T> {
+    fn hash<H: hash::Hasher>(&self, state: &mut H) {
+        (**self).hash(state);
+    }
+}