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 <!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source of the Rust file `/root/.cargo/registry/src/github.com-1ecc6299db9ec823/oneshot-0.1.5/src/lib.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>lib.rs - source</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../normalize.css"><link rel="stylesheet" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../ayu.css" disabled><link rel="stylesheet" href="../../dark.css" disabled><link rel="stylesheet" href="../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../storage.js"></script><script defer src="../../source-script.js"></script><script defer src="../../source-files.js"></script><script defer src="../../main.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../favicon.svg"></head><body class="rustdoc source"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><a class="sidebar-logo" href="../../oneshot/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div></a></nav><main><div class="width-limiter"><nav class="sub"><a class="sub-logo-container" href="../../oneshot/index.html"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></a><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="example-wrap"><pre class="src-line-numbers"><span id="1">1</span>
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 </pre><pre class="rust"><code><span class="doccomment">//! 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&#39;t want to care about when only sending a
 //! single message on a channel does not exist. For example: The sender can&#39;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&#39;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() -&gt; mpsc::Sender&lt;(Request, oneshot::Sender&lt;usize&gt;)&gt; {
 //!     let (request_sender, request_receiver) = mpsc::channel::&lt;(Request, oneshot::Sender&lt;usize&gt;)&gt;();
 //!     thread::spawn(move || {
 //!         for (request_data, response_sender) in request_receiver.iter() {
 //!             let compute_operation = || request_data.len();
 //!             let _ = response_sender.send(compute_operation()); // &lt;- 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 = &quot;std&quot;)] {
 //!     let (response_sender, response_receiver) = oneshot::channel();
 //!     let request = Request::from(&quot;data from sync thread&quot;);
 //!
 //!     processor.send((request, response_sender)).expect(&quot;Processor down&quot;);
 //!     match response_receiver.recv_timeout(Duration::from_secs(1)) { // &lt;- Receive on the oneshot channel
 //!         Ok(result) =&gt; println!(&quot;Processor returned {}&quot;, result),
 //!         Err(oneshot::RecvTimeoutError::Timeout) =&gt; eprintln!(&quot;Processor was too slow&quot;),
 //!         Err(oneshot::RecvTimeoutError::Disconnected) =&gt; panic!(&quot;Processor exited&quot;),
 //!     }
 //! }
 //!
 //! // If compiled with the `async` feature, the `Receiver` can be awaited in an async context
 //! #[cfg(feature = &quot;async&quot;)] {
 //!     tokio::runtime::Runtime::new()
 //!         .unwrap()
 //!         .block_on(async move {
 //!             let (response_sender, response_receiver) = oneshot::channel();
 //!             let request = Request::from(&quot;data from sync thread&quot;);
 //!
 //!             processor.send((request, response_sender)).expect(&quot;Processor down&quot;);
 //!             match response_receiver.await { // &lt;- Receive on the oneshot channel asynchronously
 //!                 Ok(result) =&gt; println!(&quot;Processor returned {}&quot;, result),
 //!                 Err(_e) =&gt; panic!(&quot;Processor exited&quot;),
 //!             }
 //!         });
 //! }
 //! ```
 //!
 //! # 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&#39;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.
 //!

 </span><span class="comment">// # 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.

 </span><span class="attribute">#![deny(rust_2018_idioms)]
 #![cfg_attr(not(feature = <span class="string">&quot;std&quot;</span>), no_std)]

 #[cfg(not(loom))]
 </span><span class="kw">extern crate </span>alloc;

 <span class="kw">use </span>core::{
     marker::PhantomData,
     mem::{<span class="self">self</span>, MaybeUninit},
     ptr::{<span class="self">self</span>, NonNull},
 };

 <span class="attribute">#[cfg(not(loom))]
 </span><span class="kw">use </span>core::{
     cell::UnsafeCell,
     sync::atomic::{fence, AtomicU8, Ordering::<span class="kw-2">*</span>},
 };
 <span class="attribute">#[cfg(loom)]
 </span><span class="kw">use </span>loom::{
     cell::UnsafeCell,
     sync::atomic::{fence, AtomicU8, Ordering::<span class="kw-2">*</span>},
 };

 <span class="attribute">#[cfg(all(feature = <span class="string">&quot;async&quot;</span>, not(loom)))]
 </span><span class="kw">use </span>core::hint;
 <span class="attribute">#[cfg(all(feature = <span class="string">&quot;async&quot;</span>, loom))]
 </span><span class="kw">use </span>loom::hint;

 <span class="attribute">#[cfg(feature = <span class="string">&quot;async&quot;</span>)]
 </span><span class="kw">use </span>core::{
     pin::Pin,
     task::{<span class="self">self</span>, Poll},
 };
 <span class="attribute">#[cfg(feature = <span class="string">&quot;std&quot;</span>)]
 </span><span class="kw">use </span>std::time::{Duration, Instant};

 <span class="attribute">#[cfg(feature = <span class="string">&quot;std&quot;</span>)]
 </span><span class="kw">mod </span>thread {
     <span class="attribute">#[cfg(not(loom))]
     </span><span class="kw">pub use </span>std::thread::{current, park, park_timeout, yield_now, Thread};

     <span class="attribute">#[cfg(loom)]
     </span><span class="kw">pub use </span>loom::thread::{current, park, yield_now, Thread};

     <span class="comment">// loom does not support parking with a timeout. So we just
     // yield. This means that the &quot;park&quot; will &quot;spuriously&quot; 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.
     </span><span class="attribute">#[cfg(loom)]
     </span><span class="kw">pub fn </span>park_timeout(_timeout: std::time::Duration) {
         loom::thread::yield_now()
     }
 }

 <span class="attribute">#[cfg(loom)]
 </span><span class="kw">mod </span>loombox;
 <span class="attribute">#[cfg(not(loom))]
 </span><span class="kw">use </span>alloc::boxed::Box;
 <span class="attribute">#[cfg(loom)]
 </span><span class="kw">use </span>loombox::Box;

 <span class="kw">mod </span>errors;
 <span class="kw">pub use </span>errors::{RecvError, RecvTimeoutError, SendError, TryRecvError};

 <span class="doccomment">/// Creates a new oneshot channel and returns the two endpoints, [`Sender`] and [`Receiver`].
 </span><span class="kw">pub fn </span>channel&lt;T&gt;() -&gt; (Sender&lt;T&gt;, Receiver&lt;T&gt;) {
     <span class="comment">// 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.

     </span><span class="kw">let </span>channel_ptr = Box::into_raw(Box::new(Channel::new()));

     <span class="comment">// SAFETY: `channel_ptr` came from a Box and thus is not null
     </span><span class="kw">let </span>channel_ptr = <span class="kw">unsafe </span>{ NonNull::new_unchecked(channel_ptr) };

     (
         Sender {
             channel_ptr,
             _invariant: PhantomData,
         },
         Receiver { channel_ptr },
     )
 }

 <span class="attribute">#[derive(Debug)]
 </span><span class="kw">pub struct </span>Sender&lt;T&gt; {
     channel_ptr: NonNull&lt;Channel&lt;T&gt;&gt;,
     <span class="comment">// In reality we want contravariance, however we can&#39;t obtain that.
     //
     // Consider the following scenario:
     // ```
     // let (mut tx, rx) = channel::&lt;&amp;&#39;short u8&gt;();
     // let (tx2, rx2) = channel::&lt;&amp;&#39;long u8&gt;();
     //
     // tx = tx2;
     //
     // // Pretend short_ref is some &amp;&#39;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.
     </span>_invariant: PhantomData&lt;<span class="kw">fn</span>(T) -&gt; T&gt;,
 }

 <span class="attribute">#[derive(Debug)]
 </span><span class="kw">pub struct </span>Receiver&lt;T&gt; {
     <span class="comment">// Covariance is the right choice here. Consider the example presented in Sender, and you&#39;ll
     // see that if we replaced `rx` instead then we would get the expected behavior
     </span>channel_ptr: NonNull&lt;Channel&lt;T&gt;&gt;,
 }

 <span class="kw">unsafe impl</span>&lt;T: Send&gt; Send <span class="kw">for </span>Sender&lt;T&gt; {}
 <span class="kw">unsafe impl</span>&lt;T: Send&gt; Send <span class="kw">for </span>Receiver&lt;T&gt; {}
 <span class="kw">impl</span>&lt;T&gt; Unpin <span class="kw">for </span>Receiver&lt;T&gt; {}

 <span class="kw">impl</span>&lt;T&gt; Sender&lt;T&gt; {
     <span class="doccomment">/// 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&#39;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&#39;s heap allocation, which might or might not be lock-free.
     </span><span class="kw">pub fn </span>send(<span class="self">self</span>, message: T) -&gt; <span class="prelude-ty">Result</span>&lt;(), SendError&lt;T&gt;&gt; {
         <span class="kw">let </span>channel_ptr = <span class="self">self</span>.channel_ptr;

         <span class="comment">// Don&#39;t run our Drop implementation if send was called, any cleanup now happens here
         </span>mem::forget(<span class="self">self</span>);

         <span class="comment">// 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.
         </span><span class="kw">let </span>channel = <span class="kw">unsafe </span>{ channel_ptr.as_ref() };

         <span class="comment">// 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&#39;re responsible for setting that state, we can guarantee that we have
         // exclusive access to this memory location to perform this write.
         </span><span class="kw">unsafe </span>{ channel.write_message(message) };

         <span class="comment">// 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
         </span><span class="kw">match </span>channel.state.fetch_add(<span class="number">1</span>, Release) {
             <span class="comment">// The receiver is alive and has not started waiting. Send done.
             </span>EMPTY =&gt; <span class="prelude-val">Ok</span>(()),
             <span class="comment">// The receiver is waiting. Wake it up so it can return the message.
             </span>RECEIVING =&gt; {
                 <span class="comment">// ORDERING: Synchronizes with the write of the waker to memory, and prevents the
                 // taking of the waker from being ordered before this operation.
                 </span>fence(Acquire);

                 <span class="comment">// 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.
                 </span><span class="kw">let </span>waker = <span class="kw">unsafe </span>{ channel.take_waker() };

                 <span class="comment">// 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.
                 </span>channel.state.swap(MESSAGE, AcqRel);

                 <span class="comment">// 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.
                 </span>waker.unpark();

                 <span class="prelude-val">Ok</span>(())
             }
             <span class="comment">// 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&#39;s resources to the error.
             // Moreover, since we just placed the message in the channel, the channel contains a
             // valid message.
             </span>DISCONNECTED =&gt; <span class="prelude-val">Err</span>(<span class="kw">unsafe </span>{ SendError::new(channel_ptr) }),
             <span class="kw">_ </span>=&gt; <span class="macro">unreachable!</span>(),
         }
     }
 }

 <span class="kw">impl</span>&lt;T&gt; Drop <span class="kw">for </span>Sender&lt;T&gt; {
     <span class="kw">fn </span>drop(<span class="kw-2">&amp;mut </span><span class="self">self</span>) {
         <span class="comment">// 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&#39;t
         // free the channel.
         </span><span class="kw">let </span>channel = <span class="kw">unsafe </span>{ <span class="self">self</span>.channel_ptr.as_ref() };

         <span class="comment">// Set the channel state to disconnected and read what state the receiver was in
         // ORDERING: we don&#39;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
         </span><span class="kw">match </span>channel.state.fetch_xor(<span class="number">0b001</span>, Relaxed) {
             <span class="comment">// The receiver has not started waiting, nor is it dropped.
             </span>EMPTY =&gt; (),
             <span class="comment">// The receiver is waiting. Wake it up so it can detect that the channel disconnected.
             </span>RECEIVING =&gt; {
                 <span class="comment">// See comments in Sender::send

                 </span>fence(Acquire);

                 <span class="kw">let </span>waker = <span class="kw">unsafe </span>{ channel.take_waker() };

                 <span class="comment">// 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.
                 </span>channel.state.swap(DISCONNECTED, AcqRel);

                 <span class="comment">// The Acquire ordering above ensures that the write of the DISCONNECTED state
                 // happens-before unparking the receiver.
                 </span>waker.unpark();
             }
             <span class="comment">// The receiver was already dropped. We are responsible for freeing the channel.
             </span>DISCONNECTED =&gt; {
                 <span class="comment">// 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&#39;re responsible for
                 // freeing the channel allocation.
                 </span><span class="kw">unsafe </span>{ dealloc(<span class="self">self</span>.channel_ptr) };
             }
             <span class="kw">_ </span>=&gt; <span class="macro">unreachable!</span>(),
         }
     }
 }

 <span class="kw">impl</span>&lt;T&gt; Receiver&lt;T&gt; {
     <span class="doccomment">/// 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.
     </span><span class="kw">pub fn </span>try_recv(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="prelude-ty">Result</span>&lt;T, TryRecvError&gt; {
         <span class="comment">// SAFETY: The channel will not be freed while this method is still running.
         </span><span class="kw">let </span>channel = <span class="kw">unsafe </span>{ <span class="self">self</span>.channel_ptr.as_ref() };

         <span class="comment">// ORDERING: we use acquire ordering to synchronize with the store of the message.
         </span><span class="kw">match </span>channel.state.load(Acquire) {
             MESSAGE =&gt; {
                 <span class="comment">// It&#39;s okay to break up the load and store since once we&#39;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&#39;t need to make any side effects visible to it
                 </span>channel.state.store(DISCONNECTED, Relaxed);

                 <span class="comment">// SAFETY: we are in the MESSAGE state so the message is present
                 </span><span class="prelude-val">Ok</span>(<span class="kw">unsafe </span>{ channel.take_message() })
             }
             EMPTY =&gt; <span class="prelude-val">Err</span>(TryRecvError::Empty),
             DISCONNECTED =&gt; <span class="prelude-val">Err</span>(TryRecvError::Disconnected),
             <span class="attribute">#[cfg(feature = <span class="string">&quot;async&quot;</span>)]
             </span>RECEIVING | UNPARKING =&gt; <span class="prelude-val">Err</span>(TryRecvError::Empty),
             <span class="kw">_ </span>=&gt; <span class="macro">unreachable!</span>(),
         }
     }

     <span class="doccomment">/// 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.
     </span><span class="attribute">#[cfg(feature = <span class="string">&quot;std&quot;</span>)]
     </span><span class="kw">pub fn </span>recv(<span class="self">self</span>) -&gt; <span class="prelude-ty">Result</span>&lt;T, RecvError&gt; {
         <span class="comment">// Note that we don&#39;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.

         </span><span class="kw">let </span>channel_ptr = <span class="self">self</span>.channel_ptr;

         <span class="comment">// Don&#39;t run our Drop implementation if we are receiving consuming ourselves.
         </span>mem::forget(<span class="self">self</span>);

         <span class="comment">// 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&#39;re still alive and left the responsibility of deallocating the
         // channel to us, so channel_ptr is valid
         </span><span class="kw">let </span>channel = <span class="kw">unsafe </span>{ channel_ptr.as_ref() };

         <span class="comment">// ORDERING: we use acquire ordering to synchronize with the write of the message in the
         // case that it&#39;s available
         </span><span class="kw">match </span>channel.state.load(Acquire) {
             <span class="comment">// The sender is alive but has not sent anything yet. We prepare to park.
             </span>EMPTY =&gt; {
                 <span class="comment">// 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.
                 </span><span class="attribute">#[cfg(oneshot_test_delay)]
                 </span>std::thread::sleep(std::time::Duration::from_millis(<span class="number">10</span>));

                 <span class="comment">// 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
                 </span><span class="kw">unsafe </span>{ channel.write_waker(ReceiverWaker::current_thread()) };

                 <span class="comment">// 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&#39;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
                 </span><span class="kw">match </span>channel.state.swap(RECEIVING, Release) {
                     <span class="comment">// We stored our waker, now we park until the sender has changed the state
                     </span>EMPTY =&gt; <span class="kw">loop </span>{
                         thread::park();

                         <span class="comment">// ORDERING: synchronize with the write of the message
                         </span><span class="kw">match </span>channel.state.load(Acquire) {
                             <span class="comment">// The sender sent the message while we were parked.
                             </span>MESSAGE =&gt; {
                                 <span class="comment">// SAFETY: we are in the message state so the message is valid
                                 </span><span class="kw">let </span>message = <span class="kw">unsafe </span>{ channel.take_message() };

                                 <span class="comment">// SAFETY: the Sender delegates the responsibility of deallocating
                                 // the channel to us upon sending the message
                                 </span><span class="kw">unsafe </span>{ dealloc(channel_ptr) };

                                 <span class="kw">break </span><span class="prelude-val">Ok</span>(message);
                             }
                             <span class="comment">// The sender was dropped while we were parked.
                             </span>DISCONNECTED =&gt; {
                                 <span class="comment">// SAFETY: the Sender doesn&#39;t deallocate the channel allocation in
                                 // its drop implementation if we&#39;re receiving
                                 </span><span class="kw">unsafe </span>{ dealloc(channel_ptr) };

                                 <span class="kw">break </span><span class="prelude-val">Err</span>(RecvError);
                             }
                             <span class="comment">// State did not change, spurious wakeup, park again.
                             </span>RECEIVING | UNPARKING =&gt; (),
                             <span class="kw">_ </span>=&gt; <span class="macro">unreachable!</span>(),
                         }
                     },
                     <span class="comment">// The sender sent the message while we prepared to park.
                     </span>MESSAGE =&gt; {
                         <span class="comment">// ORDERING: Synchronize with the write of the message. This branch is
                         // unlikely to be taken, so it&#39;s likely more efficient to use a fence here
                         // instead of AcqRel ordering on the RMW operation
                         </span>fence(Acquire);

                         <span class="comment">// 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&#39;re
                         // responsible for dropping it.
                         </span><span class="kw">unsafe </span>{ channel.drop_waker() };

                         <span class="comment">// SAFETY: we are in the message state so the message is valid
                         </span><span class="kw">let </span>message = <span class="kw">unsafe </span>{ channel.take_message() };

                         <span class="comment">// SAFETY: the Sender delegates the responsibility of deallocating the
                         // channel to us upon sending the message
                         </span><span class="kw">unsafe </span>{ dealloc(channel_ptr) };

                         <span class="prelude-val">Ok</span>(message)
                     }
                     <span class="comment">// The sender was dropped before sending anything while we prepared to park.
                     </span>DISCONNECTED =&gt; {
                         <span class="comment">// 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.
                         </span><span class="kw">unsafe </span>{ channel.drop_waker() };

                         <span class="comment">// SAFETY: the sender does not deallocate the channel if it switches from
                         // empty to disconnected so we need to free the allocation
                         </span><span class="kw">unsafe </span>{ dealloc(channel_ptr) };

                         <span class="prelude-val">Err</span>(RecvError)
                     }
                     <span class="kw">_ </span>=&gt; <span class="macro">unreachable!</span>(),
                 }
             }
             <span class="comment">// The sender already sent the message.
             </span>MESSAGE =&gt; {
                 <span class="comment">// SAFETY: we are in the message state so the message is valid
                 </span><span class="kw">let </span>message = <span class="kw">unsafe </span>{ channel.take_message() };

                 <span class="comment">// SAFETY: we are already in the message state so the sender has been forgotten
                 // and it&#39;s our job to clean up resources
                 </span><span class="kw">unsafe </span>{ dealloc(channel_ptr) };

                 <span class="prelude-val">Ok</span>(message)
             }
             <span class="comment">// The sender was dropped before sending anything, or we already received the message.
             </span>DISCONNECTED =&gt; {
                 <span class="comment">// SAFETY: the sender does not deallocate the channel if it switches from empty to
                 // disconnected so we need to free the allocation
                 </span><span class="kw">unsafe </span>{ dealloc(channel_ptr) };

                 <span class="prelude-val">Err</span>(RecvError)
             }
             <span class="comment">// The receiver must have been `Future::poll`ed prior to this call.
             </span><span class="attribute">#[cfg(feature = <span class="string">&quot;async&quot;</span>)]
             </span>RECEIVING | UNPARKING =&gt; <span class="macro">panic!</span>(<span class="string">&quot;{}&quot;</span>, RECEIVER_USED_SYNC_AND_ASYNC_ERROR),
             <span class="kw">_ </span>=&gt; <span class="macro">unreachable!</span>(),
         }
     }

     <span class="doccomment">/// 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.
     </span><span class="attribute">#[cfg(feature = <span class="string">&quot;std&quot;</span>)]
     </span><span class="kw">pub fn </span>recv_ref(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="prelude-ty">Result</span>&lt;T, RecvError&gt; {
         <span class="self">self</span>.start_recv_ref(RecvError, |channel| {
             <span class="kw">loop </span>{
                 thread::park();

                 <span class="comment">// ORDERING: we use acquire ordering to synchronize with the write of the message
                 </span><span class="kw">match </span>channel.state.load(Acquire) {
                     <span class="comment">// The sender sent the message while we were parked.
                     // We take the message and mark the channel disconnected.
                     </span>MESSAGE =&gt; {
                         <span class="comment">// ORDERING: the sender is inactive at this point so we don&#39;t need to make
                         // any reads or writes visible to the sending thread
                         </span>channel.state.store(DISCONNECTED, Relaxed);

                         <span class="comment">// SAFETY: we were just in the message state so the message is valid
                         </span><span class="kw">break </span><span class="prelude-val">Ok</span>(<span class="kw">unsafe </span>{ channel.take_message() });
                     }
                     <span class="comment">// The sender was dropped while we were parked.
                     </span>DISCONNECTED =&gt; <span class="kw">break </span><span class="prelude-val">Err</span>(RecvError),
                     <span class="comment">// State did not change, spurious wakeup, park again.
                     </span>RECEIVING | UNPARKING =&gt; (),
                     <span class="kw">_ </span>=&gt; <span class="macro">unreachable!</span>(),
                 }
             }
         })
     }

     <span class="doccomment">/// 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&#39;s `Instant` type can&#39;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.
     </span><span class="attribute">#[cfg(feature = <span class="string">&quot;std&quot;</span>)]
     </span><span class="kw">pub fn </span>recv_timeout(<span class="kw-2">&amp;</span><span class="self">self</span>, timeout: Duration) -&gt; <span class="prelude-ty">Result</span>&lt;T, RecvTimeoutError&gt; {
         <span class="kw">match </span>Instant::now().checked_add(timeout) {
             <span class="prelude-val">Some</span>(deadline) =&gt; <span class="self">self</span>.recv_deadline(deadline),
             <span class="prelude-val">None </span>=&gt; <span class="self">self</span>.recv_ref().map_err(|<span class="kw">_</span>| RecvTimeoutError::Disconnected),
         }
     }

     <span class="doccomment">/// 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.
     </span><span class="attribute">#[cfg(feature = <span class="string">&quot;std&quot;</span>)]
     </span><span class="kw">pub fn </span>recv_deadline(<span class="kw-2">&amp;</span><span class="self">self</span>, deadline: Instant) -&gt; <span class="prelude-ty">Result</span>&lt;T, RecvTimeoutError&gt; {
         <span class="doccomment">/// # 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
         </span><span class="attribute">#[cold]
         </span><span class="kw">unsafe fn </span>wait_for_unpark&lt;T&gt;(channel: <span class="kw-2">&amp;</span>Channel&lt;T&gt;) -&gt; <span class="prelude-ty">Result</span>&lt;T, RecvTimeoutError&gt; {
             <span class="kw">loop </span>{
                 thread::park();

                 <span class="comment">// ORDERING: The callee has already synchronized with any message write
                 </span><span class="kw">match </span>channel.state.load(Relaxed) {
                     MESSAGE =&gt; {
                         <span class="comment">// ORDERING: the sender has been dropped, so this update only
                         // needs to be visible to us
                         </span>channel.state.store(DISCONNECTED, Relaxed);
                         <span class="kw">break </span><span class="prelude-val">Ok</span>(channel.take_message());
                     }
                     DISCONNECTED =&gt; <span class="kw">break </span><span class="prelude-val">Err</span>(RecvTimeoutError::Disconnected),
                     <span class="comment">// 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.
                     </span>EMPTY =&gt; (),
                     <span class="kw">_ </span>=&gt; <span class="macro">unreachable!</span>(),
                 }
             }
         }

         <span class="self">self</span>.start_recv_ref(RecvTimeoutError::Disconnected, |channel| {
             <span class="kw">loop </span>{
                 <span class="kw">match </span>deadline.checked_duration_since(Instant::now()) {
                     <span class="prelude-val">Some</span>(timeout) =&gt; {
                         thread::park_timeout(timeout);

                         <span class="comment">// ORDERING: synchronize with the write of the message
                         </span><span class="kw">match </span>channel.state.load(Acquire) {
                             <span class="comment">// The sender sent the message while we were parked.
                             </span>MESSAGE =&gt; {
                                 <span class="comment">// ORDERING: the sender has been `mem::forget`-ed so this update
                                 // only needs to be visible to us.
                                 </span>channel.state.store(DISCONNECTED, Relaxed);

                                 <span class="comment">// SAFETY: we either are in the message state or were just in the
                                 // message state
                                 </span><span class="kw">break </span><span class="prelude-val">Ok</span>(<span class="kw">unsafe </span>{ channel.take_message() });
                             }
                             <span class="comment">// The sender was dropped while we were parked.
                             </span>DISCONNECTED =&gt; <span class="kw">break </span><span class="prelude-val">Err</span>(RecvTimeoutError::Disconnected),
                             <span class="comment">// State did not change, spurious wakeup, park again.
                             </span>RECEIVING | UNPARKING =&gt; (),
                             <span class="kw">_ </span>=&gt; <span class="macro">unreachable!</span>(),
                         }
                     }
                     <span class="prelude-val">None </span>=&gt; {
                         <span class="comment">// ORDERING: synchronize with the write of the message
                         </span><span class="kw">match </span>channel.state.swap(EMPTY, Acquire) {
                             <span class="comment">// We reached the end of the timeout without receiving a message
                             </span>RECEIVING =&gt; {
                                 <span class="comment">// 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.
                                 </span><span class="kw">unsafe </span>{ channel.drop_waker() };

                                 <span class="kw">break </span><span class="prelude-val">Err</span>(RecvTimeoutError::Timeout);
                             }
                             <span class="comment">// The sender sent the message while we were parked.
                             </span>MESSAGE =&gt; {
                                 <span class="comment">// Same safety and ordering as the Some branch

                                 </span>channel.state.store(DISCONNECTED, Relaxed);
                                 <span class="kw">break </span><span class="prelude-val">Ok</span>(<span class="kw">unsafe </span>{ channel.take_message() });
                             }
                             <span class="comment">// The sender was dropped while we were parked.
                             </span>DISCONNECTED =&gt; {
                                 <span class="comment">// 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
                                 </span>channel.state.store(DISCONNECTED, Relaxed);

                                 <span class="kw">break </span><span class="prelude-val">Err</span>(RecvTimeoutError::Disconnected);
                             }
                             <span class="comment">// The sender sent the message and started unparking us
                             </span>UNPARKING =&gt; {
                                 <span class="comment">// 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.
                                 </span><span class="kw">break unsafe </span>{ wait_for_unpark(channel) };
                             }
                             <span class="kw">_ </span>=&gt; <span class="macro">unreachable!</span>(),
                         }
                     }
                 }
             }
         })
     }

     <span class="doccomment">/// 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&#39;s resources appropriately before it returns,
     /// such as destroying the waker, for instance.
     </span><span class="attribute">#[cfg(feature = <span class="string">&quot;std&quot;</span>)]
     #[inline]
     </span><span class="kw">fn </span>start_recv_ref&lt;E&gt;(
         <span class="kw-2">&amp;</span><span class="self">self</span>,
         disconnected_error: E,
         finish: <span class="kw">impl </span>FnOnce(<span class="kw-2">&amp;</span>Channel&lt;T&gt;) -&gt; <span class="prelude-ty">Result</span>&lt;T, E&gt;,
     ) -&gt; <span class="prelude-ty">Result</span>&lt;T, E&gt; {
         <span class="comment">// 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&#39;re still alive and left the responsibility of deallocating the
         // channel to us, so `self.channel` is valid
         </span><span class="kw">let </span>channel = <span class="kw">unsafe </span>{ <span class="self">self</span>.channel_ptr.as_ref() };

         <span class="comment">// ORDERING: synchronize with the write of the message
         </span><span class="kw">match </span>channel.state.load(Acquire) {
             <span class="comment">// The sender is alive but has not sent anything yet. We prepare to park.
             </span>EMPTY =&gt; {
                 <span class="comment">// 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.
                 </span><span class="attribute">#[cfg(oneshot_test_delay)]
                 </span>std::thread::sleep(std::time::Duration::from_millis(<span class="number">10</span>));

                 <span class="comment">// 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
                 </span><span class="kw">unsafe </span>{ channel.write_waker(ReceiverWaker::current_thread()) };

                 <span class="comment">// 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
                 </span><span class="kw">match </span>channel
                     .state
                     .compare_exchange(EMPTY, RECEIVING, Release, Relaxed)
                 {
                     <span class="comment">// We stored our waker, now we delegate to the callback to finish the receive
                     // operation
                     </span><span class="prelude-val">Ok</span>(<span class="kw">_</span>) =&gt; finish(channel),
                     <span class="comment">// The sender sent the message while we prepared to finish
                     </span><span class="prelude-val">Err</span>(MESSAGE) =&gt; {
                         <span class="comment">// See comments in `recv` for ordering and safety

                         </span>fence(Acquire);

                         <span class="kw">unsafe </span>{ channel.drop_waker() };

                         <span class="comment">// ORDERING: the sender has been `mem::forget`-ed so this update only
                         // needs to be visible to us
                         </span>channel.state.store(DISCONNECTED, Relaxed);

                         <span class="comment">// SAFETY: The MESSAGE state tells us there is a correctly initialized
                         // message
                         </span><span class="prelude-val">Ok</span>(<span class="kw">unsafe </span>{ channel.take_message() })
                     }
                     <span class="comment">// The sender was dropped before sending anything while we prepared to park.
                     </span><span class="prelude-val">Err</span>(DISCONNECTED) =&gt; {
                         <span class="comment">// See comments in `recv` for safety
                         </span><span class="kw">unsafe </span>{ channel.drop_waker() };
                         <span class="prelude-val">Err</span>(disconnected_error)
                     }
                     <span class="kw">_ </span>=&gt; <span class="macro">unreachable!</span>(),
                 }
             }
             <span class="comment">// The sender sent the message. We take the message and mark the channel disconnected.
             </span>MESSAGE =&gt; {
                 <span class="comment">// ORDERING: the sender has been `mem::forget`-ed so this update only needs to be
                 // visible to us
                 </span>channel.state.store(DISCONNECTED, Relaxed);

                 <span class="comment">// SAFETY: we are in the message state so the message is valid
                 </span><span class="prelude-val">Ok</span>(<span class="kw">unsafe </span>{ channel.take_message() })
             }
             <span class="comment">// The sender was dropped before sending anything, or we already received the message.
             </span>DISCONNECTED =&gt; <span class="prelude-val">Err</span>(disconnected_error),
             <span class="comment">// The receiver must have been `Future::poll`ed prior to this call.
             </span><span class="attribute">#[cfg(feature = <span class="string">&quot;async&quot;</span>)]
             </span>RECEIVING | UNPARKING =&gt; <span class="macro">panic!</span>(<span class="string">&quot;{}&quot;</span>, RECEIVER_USED_SYNC_AND_ASYNC_ERROR),
             <span class="kw">_ </span>=&gt; <span class="macro">unreachable!</span>(),
         }
     }
 }

 <span class="attribute">#[cfg(feature = <span class="string">&quot;async&quot;</span>)]
 </span><span class="kw">impl</span>&lt;T&gt; core::future::Future <span class="kw">for </span>Receiver&lt;T&gt; {
     <span class="kw">type </span>Output = <span class="prelude-ty">Result</span>&lt;T, RecvError&gt;;

     <span class="kw">fn </span>poll(<span class="self">self</span>: Pin&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>&gt;, cx: <span class="kw-2">&amp;mut </span>task::Context&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; Poll&lt;<span class="self">Self</span>::Output&gt; {
         <span class="comment">// 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&#39;re still alive and left the responsibility of deallocating the
         // channel to us, so `self.channel` is valid
         </span><span class="kw">let </span>channel = <span class="kw">unsafe </span>{ <span class="self">self</span>.channel_ptr.as_ref() };

         <span class="comment">// ORDERING: we use acquire ordering to synchronize with the store of the message.
         </span><span class="kw">match </span>channel.state.load(Acquire) {
             <span class="comment">// The sender is alive but has not sent anything yet.
             </span>EMPTY =&gt; {
                 <span class="comment">// SAFETY: We can&#39;t be in the forbidden states, and no waker in the channel.
                 </span><span class="kw">unsafe </span>{ channel.write_async_waker(cx) }
             }
             <span class="comment">// We were polled again while waiting for the sender. Replace the waker with the new one.
             </span>RECEIVING =&gt; {
                 <span class="comment">// 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.
                 </span><span class="kw">match </span>channel
                     .state
                     .compare_exchange(RECEIVING, EMPTY, Relaxed, Relaxed)
                 {
                     <span class="comment">// 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&#39;t use any
                     // memory barriers in the compare_exchange above.
                     </span><span class="prelude-val">Ok</span>(<span class="kw">_</span>) =&gt; {
                         <span class="comment">// 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.
                         </span><span class="kw">unsafe </span>{ channel.drop_waker() };
                         <span class="comment">// SAFETY: We can&#39;t be in the forbidden states, and no waker in the channel.
                         </span><span class="kw">unsafe </span>{ channel.write_async_waker(cx) }
                     }
                     <span class="comment">// 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.
                     </span><span class="prelude-val">Err</span>(MESSAGE) =&gt; {
                         <span class="comment">// ORDERING: Synchronize with the write of the message. This branch is
                         // unlikely to be taken.
                         </span>channel.state.swap(DISCONNECTED, Acquire);
                         <span class="comment">// SAFETY: The state tells us the sender has initialized the message.
                         </span>Poll::Ready(<span class="prelude-val">Ok</span>(<span class="kw">unsafe </span>{ channel.take_message() }))
                     }
                     <span class="comment">// The sender was dropped before sending anything while we prepared to park.
                     // The sender has taken the waker already.
                     </span><span class="prelude-val">Err</span>(DISCONNECTED) =&gt; Poll::Ready(<span class="prelude-val">Err</span>(RecvError)),
                     <span class="comment">// The sender is currently waking us up.
                     </span><span class="prelude-val">Err</span>(UNPARKING) =&gt; {
                         <span class="comment">// We can&#39;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.
                         </span>cx.waker().wake_by_ref();
                         Poll::Pending
                     }
                     <span class="kw">_ </span>=&gt; <span class="macro">unreachable!</span>(),
                 }
             }
             <span class="comment">// The sender sent the message.
             </span>MESSAGE =&gt; {
                 <span class="comment">// ORDERING: the sender has been dropped so this update only needs to be
                 // visible to us
                 </span>channel.state.store(DISCONNECTED, Relaxed);
                 Poll::Ready(<span class="prelude-val">Ok</span>(<span class="kw">unsafe </span>{ channel.take_message() }))
             }
             <span class="comment">// The sender was dropped before sending anything, or we already received the message.
             </span>DISCONNECTED =&gt; Poll::Ready(<span class="prelude-val">Err</span>(RecvError)),
             <span class="comment">// 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.
             </span>UNPARKING =&gt; <span class="kw">loop </span>{
                 hint::spin_loop();
                 <span class="comment">// ORDERING: The load above has already synchronized with the write of the message.
                 </span><span class="kw">match </span>channel.state.load(Relaxed) {
                     MESSAGE =&gt; {
                         <span class="comment">// ORDERING: the sender has been dropped, so this update only
                         // needs to be visible to us
                         </span>channel.state.store(DISCONNECTED, Relaxed);
                         <span class="comment">// SAFETY: We observed the MESSAGE state
                         </span><span class="kw">break </span>Poll::Ready(<span class="prelude-val">Ok</span>(<span class="kw">unsafe </span>{ channel.take_message() }));
                     }
                     DISCONNECTED =&gt; <span class="kw">break </span>Poll::Ready(<span class="prelude-val">Err</span>(RecvError)),
                     UNPARKING =&gt; (),
                     <span class="kw">_ </span>=&gt; <span class="macro">unreachable!</span>(),
                 }
             },
             <span class="kw">_ </span>=&gt; <span class="macro">unreachable!</span>(),
         }
     }
 }

 <span class="kw">impl</span>&lt;T&gt; Drop <span class="kw">for </span>Receiver&lt;T&gt; {
     <span class="kw">fn </span>drop(<span class="kw-2">&amp;mut </span><span class="self">self</span>) {
         <span class="comment">// SAFETY: since the receiving side is still alive the sender would have observed that and
         // left deallocating the channel allocation to us.
         </span><span class="kw">let </span>channel = <span class="kw">unsafe </span>{ <span class="self">self</span>.channel_ptr.as_ref() };

         <span class="comment">// Set the channel state to disconnected and read what state the receiver was in
         </span><span class="kw">match </span>channel.state.swap(DISCONNECTED, Acquire) {
             <span class="comment">// The sender has not sent anything, nor is it dropped.
             </span>EMPTY =&gt; (),
             <span class="comment">// The sender already sent something. We must drop it, and free the channel.
             </span>MESSAGE =&gt; {
                 <span class="comment">// SAFETY: we are in the message state so the message is initialized
                 </span><span class="kw">unsafe </span>{ channel.drop_message() };

                 <span class="comment">// SAFETY: see safety comment at top of function
                 </span><span class="kw">unsafe </span>{ dealloc(<span class="self">self</span>.channel_ptr) };
             }
             <span class="comment">// The receiver has been polled.
             </span><span class="attribute">#[cfg(feature = <span class="string">&quot;async&quot;</span>)]
             </span>RECEIVING =&gt; {
                 <span class="comment">// TODO: figure this out when async is fixed
                 </span><span class="kw">unsafe </span>{ channel.drop_waker() };
             }
             <span class="comment">// The sender was already dropped. We are responsible for freeing the channel.
             </span>DISCONNECTED =&gt; {
                 <span class="comment">// SAFETY: see safety comment at top of function
                 </span><span class="kw">unsafe </span>{ dealloc(<span class="self">self</span>.channel_ptr) };
             }
             <span class="kw">_ </span>=&gt; <span class="macro">unreachable!</span>(),
         }
     }
 }

 <span class="doccomment">/// All the values that the `Channel::state` field can have during the lifetime of a channel.
 </span><span class="kw">mod </span>states {
     <span class="comment">// These values are very explicitly chosen so that we can replace some cmpxchg calls with
     // fetch_* calls.

     </span><span class="doccomment">/// The initial channel state. Active while both endpoints are still alive, no message has been
     /// sent, and the receiver is not receiving.
     </span><span class="kw">pub const </span>EMPTY: u8 = <span class="number">0b011</span>;
     <span class="doccomment">/// A message has been sent to the channel, but the receiver has not yet read it.
     </span><span class="kw">pub const </span>MESSAGE: u8 = <span class="number">0b100</span>;
     <span class="doccomment">/// No message has yet been sent on the channel, but the receiver is currently receiving.
     </span><span class="kw">pub const </span>RECEIVING: u8 = <span class="number">0b000</span>;
     <span class="attribute">#[cfg(any(feature = <span class="string">&quot;std&quot;</span>, feature = <span class="string">&quot;async&quot;</span>))]
     </span><span class="kw">pub const </span>UNPARKING: u8 = <span class="number">0b001</span>;
     <span class="doccomment">/// 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.
     </span><span class="kw">pub const </span>DISCONNECTED: u8 = <span class="number">0b010</span>;
 }
 <span class="kw">use </span>states::<span class="kw-2">*</span>;

 <span class="doccomment">/// 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.
 </span><span class="kw">struct </span>Channel&lt;T&gt; {
     state: AtomicU8,
     message: UnsafeCell&lt;MaybeUninit&lt;T&gt;&gt;,
     waker: UnsafeCell&lt;MaybeUninit&lt;ReceiverWaker&gt;&gt;,
 }

 <span class="kw">impl</span>&lt;T&gt; Channel&lt;T&gt; {
     <span class="kw">pub fn </span>new() -&gt; <span class="self">Self </span>{
         <span class="self">Self </span>{
             state: AtomicU8::new(EMPTY),
             message: UnsafeCell::new(MaybeUninit::uninit()),
             waker: UnsafeCell::new(MaybeUninit::uninit()),
         }
     }

     <span class="attribute">#[inline(always)]
     </span><span class="kw">unsafe fn </span>message(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;</span>MaybeUninit&lt;T&gt; {
         <span class="attribute">#[cfg(loom)]
         </span>{
             <span class="self">self</span>.message.with(|ptr| <span class="kw-2">&amp;*</span>ptr)
         }

         <span class="attribute">#[cfg(not(loom))]
         </span>{
             <span class="kw-2">&amp;*</span><span class="self">self</span>.message.get()
         }
     }

     <span class="attribute">#[inline(always)]
     </span><span class="kw">unsafe fn </span>with_message_mut&lt;F&gt;(<span class="kw-2">&amp;</span><span class="self">self</span>, op: F)
     <span class="kw">where
         </span>F: FnOnce(<span class="kw-2">&amp;mut </span>MaybeUninit&lt;T&gt;),
     {
         <span class="attribute">#[cfg(loom)]
         </span>{
             <span class="self">self</span>.message.with_mut(|ptr| op(<span class="kw-2">&amp;mut *</span>ptr))
         }

         <span class="attribute">#[cfg(not(loom))]
         </span>{
             op(<span class="kw-2">&amp;mut *</span><span class="self">self</span>.message.get())
         }
     }

     <span class="attribute">#[inline(always)]
     #[cfg(any(feature = <span class="string">&quot;std&quot;</span>, feature = <span class="string">&quot;async&quot;</span>))]
     </span><span class="kw">unsafe fn </span>with_waker_mut&lt;F&gt;(<span class="kw-2">&amp;</span><span class="self">self</span>, op: F)
     <span class="kw">where
         </span>F: FnOnce(<span class="kw-2">&amp;mut </span>MaybeUninit&lt;ReceiverWaker&gt;),
     {
         <span class="attribute">#[cfg(loom)]
         </span>{
             <span class="self">self</span>.waker.with_mut(|ptr| op(<span class="kw-2">&amp;mut *</span>ptr))
         }

         <span class="attribute">#[cfg(not(loom))]
         </span>{
             op(<span class="kw-2">&amp;mut *</span><span class="self">self</span>.waker.get())
         }
     }

     <span class="attribute">#[inline(always)]
     </span><span class="kw">unsafe fn </span>write_message(<span class="kw-2">&amp;</span><span class="self">self</span>, message: T) {
         <span class="self">self</span>.with_message_mut(|slot| slot.as_mut_ptr().write(message));
     }

     <span class="attribute">#[inline(always)]
     </span><span class="kw">unsafe fn </span>take_message(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; T {
         <span class="attribute">#[cfg(loom)]
         </span>{
             <span class="self">self</span>.message.with(|ptr| ptr::read(ptr)).assume_init()
         }

         <span class="attribute">#[cfg(not(loom))]
         </span>{
             ptr::read(<span class="self">self</span>.message.get()).assume_init()
         }
     }

     <span class="attribute">#[inline(always)]
     </span><span class="kw">unsafe fn </span>drop_message(<span class="kw-2">&amp;</span><span class="self">self</span>) {
         <span class="self">self</span>.with_message_mut(|slot| slot.assume_init_drop());
     }

     <span class="attribute">#[cfg(any(feature = <span class="string">&quot;std&quot;</span>, feature = <span class="string">&quot;async&quot;</span>))]
     #[inline(always)]
     </span><span class="kw">unsafe fn </span>write_waker(<span class="kw-2">&amp;</span><span class="self">self</span>, waker: ReceiverWaker) {
         <span class="self">self</span>.with_waker_mut(|slot| slot.as_mut_ptr().write(waker));
     }

     <span class="attribute">#[inline(always)]
     </span><span class="kw">unsafe fn </span>take_waker(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; ReceiverWaker {
         <span class="attribute">#[cfg(loom)]
         </span>{
             <span class="self">self</span>.waker.with(|ptr| ptr::read(ptr)).assume_init()
         }

         <span class="attribute">#[cfg(not(loom))]
         </span>{
             ptr::read(<span class="self">self</span>.waker.get()).assume_init()
         }
     }

     <span class="attribute">#[cfg(any(feature = <span class="string">&quot;std&quot;</span>, feature = <span class="string">&quot;async&quot;</span>))]
     #[inline(always)]
     </span><span class="kw">unsafe fn </span>drop_waker(<span class="kw-2">&amp;</span><span class="self">self</span>) {
         <span class="self">self</span>.with_waker_mut(|slot| slot.assume_init_drop());
     }

     <span class="doccomment">/// # 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.
     </span><span class="attribute">#[cfg(feature = <span class="string">&quot;async&quot;</span>)]
     </span><span class="kw">unsafe fn </span>write_async_waker(<span class="kw-2">&amp;</span><span class="self">self</span>, cx: <span class="kw-2">&amp;mut </span>task::Context&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; Poll&lt;<span class="prelude-ty">Result</span>&lt;T, RecvError&gt;&gt; {
         <span class="comment">// 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
         </span><span class="self">self</span>.write_waker(ReceiverWaker::task_waker(cx));

         <span class="comment">// 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
         </span><span class="kw">match </span><span class="self">self
             </span>.state
             .compare_exchange(EMPTY, RECEIVING, Release, Relaxed)
         {
             <span class="comment">// We stored our waker, now we return and let the sender wake us up
             </span><span class="prelude-val">Ok</span>(<span class="kw">_</span>) =&gt; Poll::Pending,
             <span class="comment">// The sender sent the message while we prepared to park.
             // We take the message and mark the channel disconnected.
             </span><span class="prelude-val">Err</span>(MESSAGE) =&gt; {
                 <span class="comment">// ORDERING: Synchronize with the write of the message. This branch is
                 // unlikely to be taken, so it&#39;s likely more efficient to use a fence here
                 // instead of AcqRel ordering on the compare_exchange operation
                 </span>fence(Acquire);

                 <span class="comment">// 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&#39;re
                 // responsible for dropping it.
                 </span><span class="self">self</span>.drop_waker();

                 <span class="comment">// ORDERING: sender does not exist, so this update only needs to be visible to us
                 </span><span class="self">self</span>.state.store(DISCONNECTED, Relaxed);

                 <span class="comment">// SAFETY: The MESSAGE state tells us there is a correctly initialized message
                 </span>Poll::Ready(<span class="prelude-val">Ok</span>(<span class="self">self</span>.take_message()))
             }
             <span class="comment">// The sender was dropped before sending anything while we prepared to park.
             </span><span class="prelude-val">Err</span>(DISCONNECTED) =&gt; {
                 <span class="comment">// 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&#39;re
                 // responsible for dropping it.
                 </span><span class="self">self</span>.drop_waker();
                 Poll::Ready(<span class="prelude-val">Err</span>(RecvError))
             }
             <span class="kw">_ </span>=&gt; <span class="macro">unreachable!</span>(),
         }
     }
 }

 <span class="kw">enum </span>ReceiverWaker {
     <span class="doccomment">/// The receiver is waiting synchronously. Its thread is parked.
     </span><span class="attribute">#[cfg(feature = <span class="string">&quot;std&quot;</span>)]
     </span>Thread(thread::Thread),
     <span class="doccomment">/// The receiver is waiting asynchronously. Its task can be woken up with this `Waker`.
     </span><span class="attribute">#[cfg(feature = <span class="string">&quot;async&quot;</span>)]
     </span>Task(task::Waker),
     <span class="doccomment">/// A little hack to not make this enum an uninhibitable type when no features are enabled.
     </span><span class="attribute">#[cfg(not(any(feature = <span class="string">&quot;async&quot;</span>, feature = <span class="string">&quot;std&quot;</span>)))]
     </span>_Uninhabited,
 }

 <span class="kw">impl </span>ReceiverWaker {
     <span class="attribute">#[cfg(feature = <span class="string">&quot;std&quot;</span>)]
     </span><span class="kw">pub fn </span>current_thread() -&gt; <span class="self">Self </span>{
         <span class="self">Self</span>::Thread(thread::current())
     }

     <span class="attribute">#[cfg(feature = <span class="string">&quot;async&quot;</span>)]
     </span><span class="kw">pub fn </span>task_waker(cx: <span class="kw-2">&amp;</span>task::Context&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; <span class="self">Self </span>{
         <span class="self">Self</span>::Task(cx.waker().clone())
     }

     <span class="kw">pub fn </span>unpark(<span class="self">self</span>) {
         <span class="kw">match </span><span class="self">self </span>{
             <span class="attribute">#[cfg(feature = <span class="string">&quot;std&quot;</span>)]
             </span>ReceiverWaker::Thread(thread) =&gt; thread.unpark(),
             <span class="attribute">#[cfg(feature = <span class="string">&quot;async&quot;</span>)]
             </span>ReceiverWaker::Task(waker) =&gt; waker.wake(),
             <span class="attribute">#[cfg(not(any(feature = <span class="string">&quot;async&quot;</span>, feature = <span class="string">&quot;std&quot;</span>)))]
             </span>ReceiverWaker::_Uninhabited =&gt; <span class="macro">unreachable!</span>(),
         }
     }
 }

 <span class="attribute">#[cfg(not(loom))]
 #[test]
 </span><span class="kw">fn </span>receiver_waker_size() {
     <span class="kw">let </span>expected: usize = <span class="kw">match </span>(<span class="macro">cfg!</span>(feature = <span class="string">&quot;std&quot;</span>), <span class="macro">cfg!</span>(feature = <span class="string">&quot;async&quot;</span>)) {
         (<span class="bool-val">false</span>, <span class="bool-val">false</span>) =&gt; <span class="number">0</span>,
         (<span class="bool-val">false</span>, <span class="bool-val">true</span>) =&gt; <span class="number">16</span>,
         (<span class="bool-val">true</span>, <span class="bool-val">false</span>) =&gt; <span class="number">8</span>,
         (<span class="bool-val">true</span>, <span class="bool-val">true</span>) =&gt; <span class="number">24</span>,
     };
     <span class="macro">assert_eq!</span>(mem::size_of::&lt;ReceiverWaker&gt;(), expected);
 }

 <span class="attribute">#[cfg(all(feature = <span class="string">&quot;std&quot;</span>, feature = <span class="string">&quot;async&quot;</span>))]
 </span><span class="kw">const </span>RECEIVER_USED_SYNC_AND_ASYNC_ERROR: <span class="kw-2">&amp;</span>str =
     <span class="string">&quot;Invalid to call a blocking receive method on oneshot::Receiver after it has been polled&quot;</span>;

 <span class="attribute">#[inline]
 </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">unsafe fn </span>dealloc&lt;T&gt;(channel: NonNull&lt;Channel&lt;T&gt;&gt;) {
     drop(Box::from_raw(channel.as_ptr()))
 }
 </code></pre></div>
 </section></div></main><div id="rustdoc-vars" data-root-path="../../" data-current-crate="oneshot" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html>