Adding upstream version 86.0.1.upstream/86.0.1upstream

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

274 files changed, 47932 insertions, 0 deletions

diff --git a/third_party/rust/tokio/src/coop.rs b/third_party/rust/tokio/src/coop.rs
new file mode 100644
index 0000000000..1d62459166
--- /dev/null
+++ b/third_party/rust/tokio/src/coop.rs
@@ -0,0 +1,379 @@
+//! Opt-in yield points for improved cooperative scheduling.
+//!
+//! A single call to [`poll`] on a top-level task may potentially do a lot of work before it
+//! returns `Poll::Pending`. If a task runs for a long period of time without yielding back to the
+//! executor, it can starve other tasks waiting on that executor to execute them, or drive
+//! underlying resources. Since Rust does not have a runtime, it is difficult to forcibly preempt a
+//! long-running task. Instead, this module provides an opt-in mechanism for futures to collaborate
+//! with the executor to avoid starvation.
+//!
+//! Consider a future like this one:
+//!
+//! ```
+//! # use tokio::stream::{Stream, StreamExt};
+//! async fn drop_all<I: Stream + Unpin>(mut input: I) {
+//!     while let Some(_) = input.next().await {}
+//! }
+//! ```
+//!
+//! It may look harmless, but consider what happens under heavy load if the input stream is
+//! _always_ ready. If we spawn `drop_all`, the task will never yield, and will starve other tasks
+//! and resources on the same executor. With opt-in yield points, this problem is alleviated:
+//!
+//! ```ignore
+//! # use tokio::stream::{Stream, StreamExt};
+//! async fn drop_all<I: Stream + Unpin>(mut input: I) {
+//!     while let Some(_) = input.next().await {
+//!         tokio::coop::proceed().await;
+//!     }
+//! }
+//! ```
+//!
+//! The `proceed` future will coordinate with the executor to make sure that every so often control
+//! is yielded back to the executor so it can run other tasks.
+//!
+//! # Placing yield points
+//!
+//! Voluntary yield points should be placed _after_ at least some work has been done. If they are
+//! not, a future sufficiently deep in the task hierarchy may end up _never_ getting to run because
+//! of the number of yield points that inevitably appear before it is reached. In general, you will
+//! want yield points to only appear in "leaf" futures -- those that do not themselves poll other
+//! futures. By doing this, you avoid double-counting each iteration of the outer future against
+//! the cooperating budget.
+//!
+//!   [`poll`]: https://doc.rust-lang.org/std/future/trait.Future.html#tymethod.poll
+
+// NOTE: The doctests in this module are ignored since the whole module is (currently) private.
+
+use std::cell::Cell;
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// Constant used to determine how much "work" a task is allowed to do without yielding.
+///
+/// The value itself is chosen somewhat arbitrarily. It needs to be high enough to amortize wakeup
+/// and scheduling costs, but low enough that we do not starve other tasks for too long. The value
+/// also needs to be high enough that particularly deep tasks are able to do at least some useful
+/// work at all.
+///
+/// Note that as more yield points are added in the ecosystem, this value will probably also have
+/// to be raised.
+const BUDGET: usize = 128;
+
+/// Constant used to determine if budgeting has been disabled.
+const UNCONSTRAINED: usize = usize::max_value();
+
+thread_local! {
+    static HITS: Cell<usize> = Cell::new(UNCONSTRAINED);
+}
+
+/// Run the given closure with a cooperative task budget.
+///
+/// Enabling budgeting when it is already enabled is a no-op.
+#[inline(always)]
+pub(crate) fn budget<F, R>(f: F) -> R
+where
+    F: FnOnce() -> R,
+{
+    HITS.with(move |hits| {
+        if hits.get() != UNCONSTRAINED {
+            // We are already being budgeted.
+            //
+            // Arguably this should be an error, but it can happen "correctly"
+            // such as with block_on + LocalSet, so we make it a no-op.
+            return f();
+        }
+
+        struct Guard<'a>(&'a Cell<usize>);
+        impl<'a> Drop for Guard<'a> {
+            fn drop(&mut self) {
+                self.0.set(UNCONSTRAINED);
+            }
+        }
+
+        hits.set(BUDGET);
+        let _guard = Guard(hits);
+        f()
+    })
+}
+
+cfg_rt_threaded! {
+    #[inline(always)]
+    pub(crate) fn has_budget_remaining() -> bool {
+        HITS.with(|hits| hits.get() > 0)
+    }
+}
+
+cfg_blocking_impl! {
+    /// Forcibly remove the budgeting constraints early.
+    pub(crate) fn stop() {
+        HITS.with(|hits| {
+            hits.set(UNCONSTRAINED);
+        });
+    }
+}
+
+/// Invoke `f` with a subset of the remaining budget.
+///
+/// This is useful if you have sub-futures that you need to poll, but that you want to restrict
+/// from using up your entire budget. For example, imagine the following future:
+///
+/// ```rust
+/// # use std::{future::Future, pin::Pin, task::{Context, Poll}};
+/// use futures::stream::FuturesUnordered;
+/// struct MyFuture<F1, F2> {
+///     big: FuturesUnordered<F1>,
+///     small: F2,
+/// }
+///
+/// use tokio::stream::Stream;
+/// impl<F1, F2> Future for MyFuture<F1, F2>
+///   where F1: Future, F2: Future
+/// # , F1: Unpin, F2: Unpin
+/// {
+///     type Output = F2::Output;
+///
+///     // fn poll(...)
+/// # fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<F2::Output> {
+/// #   let this = &mut *self;
+///     let mut big = // something to pin self.big
+/// #                 Pin::new(&mut this.big);
+///     let small = // something to pin self.small
+/// #             Pin::new(&mut this.small);
+///
+///     // see if any of the big futures have finished
+///     while let Some(e) = futures::ready!(big.as_mut().poll_next(cx)) {
+///         // do something with e
+/// #       let _ = e;
+///     }
+///
+///     // see if the small future has finished
+///     small.poll(cx)
+/// }
+/// # }
+/// ```
+///
+/// It could be that every time `poll` gets called, `big` ends up spending the entire budget, and
+/// `small` never gets polled. That would be sad. If you want to stick up for the little future,
+/// that's what `limit` is for. It lets you portion out a smaller part of the yield budget to a
+/// particular segment of your code. In the code above, you would write
+///
+/// ```rust,ignore
+/// # use std::{future::Future, pin::Pin, task::{Context, Poll}};
+/// # use futures::stream::FuturesUnordered;
+/// # struct MyFuture<F1, F2> {
+/// #     big: FuturesUnordered<F1>,
+/// #     small: F2,
+/// # }
+/// #
+/// # use tokio::stream::Stream;
+/// # impl<F1, F2> Future for MyFuture<F1, F2>
+/// #   where F1: Future, F2: Future
+/// # , F1: Unpin, F2: Unpin
+/// # {
+/// # type Output = F2::Output;
+/// # fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<F2::Output> {
+/// #   let this = &mut *self;
+/// #   let mut big = Pin::new(&mut this.big);
+/// #   let small = Pin::new(&mut this.small);
+/// #
+///     // see if any of the big futures have finished
+///     while let Some(e) = futures::ready!(tokio::coop::limit(64, || big.as_mut().poll_next(cx))) {
+/// #       // do something with e
+/// #       let _ = e;
+/// #   }
+/// #   small.poll(cx)
+/// # }
+/// # }
+/// ```
+///
+/// Now, even if `big` spends its entire budget, `small` will likely be left with some budget left
+/// to also do useful work. In particular, if the remaining budget was `N` at the start of `poll`,
+/// `small` will have at least a budget of `N - 64`. It may be more if `big` did not spend its
+/// entire budget.
+///
+/// Note that you cannot _increase_ your budget by calling `limit`. The budget provided to the code
+/// inside the buget is the _minimum_ of the _current_ budget and the bound.
+///
+#[allow(unreachable_pub, dead_code)]
+pub fn limit<R>(bound: usize, f: impl FnOnce() -> R) -> R {
+    HITS.with(|hits| {
+        let budget = hits.get();
+        // with_bound cannot _increase_ the remaining budget
+        let bound = std::cmp::min(budget, bound);
+        // When f() exits, how much should we add to what is left?
+        let floor = budget.saturating_sub(bound);
+        // Make sure we restore the remaining budget even on panic
+        struct RestoreBudget<'a>(&'a Cell<usize>, usize);
+        impl<'a> Drop for RestoreBudget<'a> {
+            fn drop(&mut self) {
+                let left = self.0.get();
+                self.0.set(self.1 + left);
+            }
+        }
+        // Time to restrict!
+        hits.set(bound);
+        let _restore = RestoreBudget(&hits, floor);
+        f()
+    })
+}
+
+/// Returns `Poll::Pending` if the current task has exceeded its budget and should yield.
+#[allow(unreachable_pub, dead_code)]
+#[inline]
+pub fn poll_proceed(cx: &mut Context<'_>) -> Poll<()> {
+    HITS.with(|hits| {
+        let n = hits.get();
+        if n == UNCONSTRAINED {
+            // opted out of budgeting
+            Poll::Ready(())
+        } else if n == 0 {
+            cx.waker().wake_by_ref();
+            Poll::Pending
+        } else {
+            hits.set(n.saturating_sub(1));
+            Poll::Ready(())
+        }
+    })
+}
+
+/// Resolves immediately unless the current task has already exceeded its budget.
+///
+/// This should be placed after at least some work has been done. Otherwise a future sufficiently
+/// deep in the task hierarchy may end up never getting to run because of the number of yield
+/// points that inevitably appear before it is even reached. For example:
+///
+/// ```ignore
+/// # use tokio::stream::{Stream, StreamExt};
+/// async fn drop_all<I: Stream + Unpin>(mut input: I) {
+///     while let Some(_) = input.next().await {
+///         tokio::coop::proceed().await;
+///     }
+/// }
+/// ```
+#[allow(unreachable_pub, dead_code)]
+#[inline]
+pub async fn proceed() {
+    use crate::future::poll_fn;
+    poll_fn(|cx| poll_proceed(cx)).await;
+}
+
+pin_project_lite::pin_project! {
+    /// A future that cooperatively yields to the task scheduler when polling,
+    /// if the task's budget is exhausted.
+    ///
+    /// Internally, this is simply a future combinator which calls
+    /// [`poll_proceed`] in its `poll` implementation before polling the wrapped
+    /// future.
+    ///
+    /// # Examples
+    ///
+    /// ```rust,ignore
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::coop::CoopFutureExt;
+    ///
+    /// async {  /* ... */ }
+    ///     .cooperate()
+    ///     .await;
+    /// # }
+    /// ```
+    ///
+    /// [`poll_proceed`]: fn@poll_proceed
+    #[derive(Debug)]
+    #[allow(unreachable_pub, dead_code)]
+    pub struct CoopFuture<F> {
+        #[pin]
+        future: F,
+    }
+}
+
+impl<F: Future> Future for CoopFuture<F> {
+    type Output = F::Output;
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        ready!(poll_proceed(cx));
+        self.project().future.poll(cx)
+    }
+}
+
+impl<F: Future> CoopFuture<F> {
+    /// Returns a new `CoopFuture` wrapping the given future.
+    ///
+    #[allow(unreachable_pub, dead_code)]
+    pub fn new(future: F) -> Self {
+        Self { future }
+    }
+}
+
+// Currently only used by `tokio::sync`; and if we make this combinator public,
+// it should probably be on the `FutureExt` trait instead.
+cfg_sync! {
+    /// Extension trait providing `Future::cooperate` extension method.
+    ///
+    /// Note: if/when the co-op API becomes public, this method should probably be
+    /// provided by `FutureExt`, instead.
+    pub(crate) trait CoopFutureExt: Future {
+        /// Wrap `self` to cooperatively yield to the scheduler when polling, if the
+        /// task's budget is exhausted.
+        fn cooperate(self) -> CoopFuture<Self>
+        where
+            Self: Sized,
+        {
+            CoopFuture::new(self)
+        }
+    }
+
+    impl<F> CoopFutureExt for F where F: Future {}
+}
+
+#[cfg(all(test, not(loom)))]
+mod test {
+    use super::*;
+
+    fn get() -> usize {
+        HITS.with(|hits| hits.get())
+    }
+
+    #[test]
+    fn bugeting() {
+        use tokio_test::*;
+
+        assert_eq!(get(), UNCONSTRAINED);
+        assert_ready!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
+        assert_eq!(get(), UNCONSTRAINED);
+        budget(|| {
+            assert_eq!(get(), BUDGET);
+            assert_ready!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
+            assert_eq!(get(), BUDGET - 1);
+            assert_ready!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
+            assert_eq!(get(), BUDGET - 2);
+        });
+        assert_eq!(get(), UNCONSTRAINED);
+
+        budget(|| {
+            limit(3, || {
+                assert_eq!(get(), 3);
+                assert_ready!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
+                assert_eq!(get(), 2);
+                limit(4, || {
+                    assert_eq!(get(), 2);
+                    assert_ready!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
+                    assert_eq!(get(), 1);
+                });
+                assert_eq!(get(), 1);
+                assert_ready!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
+                assert_eq!(get(), 0);
+                assert_pending!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
+                assert_eq!(get(), 0);
+                assert_pending!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
+                assert_eq!(get(), 0);
+            });
+            assert_eq!(get(), BUDGET - 3);
+            assert_ready!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
+            assert_eq!(get(), BUDGET - 4);
+            assert_ready!(task::spawn(proceed()).poll());
+            assert_eq!(get(), BUDGET - 5);
+        });
+    }
+}
diff --git a/third_party/rust/tokio/src/fs/canonicalize.rs b/third_party/rust/tokio/src/fs/canonicalize.rs
new file mode 100644
index 0000000000..403662685c
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/canonicalize.rs
@@ -0,0 +1,51 @@
+use crate::fs::asyncify;
+
+use std::io;
+use std::path::{Path, PathBuf};
+
+/// Returns the canonical, absolute form of a path with all intermediate
+/// components normalized and symbolic links resolved.
+///
+/// This is an async version of [`std::fs::canonicalize`][std]
+///
+/// [std]: std::fs::canonicalize
+///
+/// # Platform-specific behavior
+///
+/// This function currently corresponds to the `realpath` function on Unix
+/// and the `CreateFile` and `GetFinalPathNameByHandle` functions on Windows.
+/// Note that, this [may change in the future][changes].
+///
+/// On Windows, this converts the path to use [extended length path][path]
+/// syntax, which allows your program to use longer path names, but means you
+/// can only join backslash-delimited paths to it, and it may be incompatible
+/// with other applications (if passed to the application on the command-line,
+/// or written to a file another application may read).
+///
+/// [changes]: https://doc.rust-lang.org/std/io/index.html#platform-specific-behavior
+/// [path]: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx#maxpath
+///
+/// # Errors
+///
+/// This function will return an error in the following situations, but is not
+/// limited to just these cases:
+///
+/// * `path` does not exist.
+/// * A non-final component in path is not a directory.
+///
+/// # Examples
+///
+/// ```no_run
+/// use tokio::fs;
+/// use std::io;
+///
+/// #[tokio::main]
+/// async fn main() -> io::Result<()> {
+///     let path = fs::canonicalize("../a/../foo.txt").await?;
+///     Ok(())
+/// }
+/// ```
+pub async fn canonicalize(path: impl AsRef<Path>) -> io::Result<PathBuf> {
+    let path = path.as_ref().to_owned();
+    asyncify(move || std::fs::canonicalize(path)).await
+}
diff --git a/third_party/rust/tokio/src/fs/copy.rs b/third_party/rust/tokio/src/fs/copy.rs
new file mode 100644
index 0000000000..d4d4d29c85
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/copy.rs
@@ -0,0 +1,24 @@
+use crate::fs::asyncify;
+use std::path::Path;
+
+/// Copies the contents of one file to another. This function will also copy the permission bits of the original file to the destination file.
+/// This function will overwrite the contents of to.
+///
+/// This is the async equivalent of `std::fs::copy`.
+///
+/// # Examples
+///
+/// ```no_run
+/// use tokio::fs;
+///
+/// # async fn dox() -> std::io::Result<()> {
+/// fs::copy("foo.txt", "bar.txt").await?;
+/// # Ok(())
+/// # }
+/// ```
+
+pub async fn copy<P: AsRef<Path>, Q: AsRef<Path>>(from: P, to: Q) -> Result<u64, std::io::Error> {
+    let from = from.as_ref().to_owned();
+    let to = to.as_ref().to_owned();
+    asyncify(|| std::fs::copy(from, to)).await
+}
diff --git a/third_party/rust/tokio/src/fs/create_dir.rs b/third_party/rust/tokio/src/fs/create_dir.rs
new file mode 100644
index 0000000000..e03b04dc4b
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/create_dir.rs
@@ -0,0 +1,52 @@
+use crate::fs::asyncify;
+
+use std::io;
+use std::path::Path;
+
+/// Creates a new, empty directory at the provided path
+///
+/// This is an async version of [`std::fs::create_dir`][std]
+///
+/// [std]: std::fs::create_dir
+///
+/// # Platform-specific behavior
+///
+/// This function currently corresponds to the `mkdir` function on Unix
+/// and the `CreateDirectory` function on Windows.
+/// Note that, this [may change in the future][changes].
+///
+/// [changes]: https://doc.rust-lang.org/std/io/index.html#platform-specific-behavior
+///
+/// **NOTE**: If a parent of the given path doesn't exist, this function will
+/// return an error. To create a directory and all its missing parents at the
+/// same time, use the [`create_dir_all`] function.
+///
+/// # Errors
+///
+/// This function will return an error in the following situations, but is not
+/// limited to just these cases:
+///
+/// * User lacks permissions to create directory at `path`.
+/// * A parent of the given path doesn't exist. (To create a directory and all
+///   its missing parents at the same time, use the [`create_dir_all`]
+///   function.)
+/// * `path` already exists.
+///
+/// [`create_dir_all`]: super::create_dir_all()
+///
+/// # Examples
+///
+/// ```no_run
+/// use tokio::fs;
+/// use std::io;
+///
+/// #[tokio::main]
+/// async fn main() -> io::Result<()> {
+///     fs::create_dir("/some/dir").await?;
+///     Ok(())
+/// }
+/// ```
+pub async fn create_dir(path: impl AsRef<Path>) -> io::Result<()> {
+    let path = path.as_ref().to_owned();
+    asyncify(move || std::fs::create_dir(path)).await
+}
diff --git a/third_party/rust/tokio/src/fs/create_dir_all.rs b/third_party/rust/tokio/src/fs/create_dir_all.rs
new file mode 100644
index 0000000000..21f0c82d11
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/create_dir_all.rs
@@ -0,0 +1,53 @@
+use crate::fs::asyncify;
+
+use std::io;
+use std::path::Path;
+
+/// Recursively creates a directory and all of its parent components if they
+/// are missing.
+///
+/// This is an async version of [`std::fs::create_dir_all`][std]
+///
+/// [std]: std::fs::create_dir_all
+///
+/// # Platform-specific behavior
+///
+/// This function currently corresponds to the `mkdir` function on Unix
+/// and the `CreateDirectory` function on Windows.
+/// Note that, this [may change in the future][changes].
+///
+/// [changes]: https://doc.rust-lang.org/std/io/index.html#platform-specific-behavior
+///
+/// # Errors
+///
+/// This function will return an error in the following situations, but is not
+/// limited to just these cases:
+///
+/// * If any directory in the path specified by `path` does not already exist
+/// and it could not be created otherwise. The specific error conditions for
+/// when a directory is being created (after it is determined to not exist) are
+/// outlined by [`fs::create_dir`].
+///
+/// Notable exception is made for situations where any of the directories
+/// specified in the `path` could not be created as it was being created concurrently.
+/// Such cases are considered to be successful. That is, calling `create_dir_all`
+/// concurrently from multiple threads or processes is guaranteed not to fail
+/// due to a race condition with itself.
+///
+/// [`fs::create_dir`]: std::fs::create_dir
+///
+/// # Examples
+///
+/// ```no_run
+/// use tokio::fs;
+///
+/// #[tokio::main]
+/// async fn main() -> std::io::Result<()> {
+///     fs::create_dir_all("/some/dir").await?;
+///     Ok(())
+/// }
+/// ```
+pub async fn create_dir_all(path: impl AsRef<Path>) -> io::Result<()> {
+    let path = path.as_ref().to_owned();
+    asyncify(move || std::fs::create_dir_all(path)).await
+}
diff --git a/third_party/rust/tokio/src/fs/file.rs b/third_party/rust/tokio/src/fs/file.rs
new file mode 100644
index 0000000000..a1f22fc9b6
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/file.rs
@@ -0,0 +1,739 @@
+//! Types for working with [`File`].
+//!
+//! [`File`]: File
+
+use self::State::*;
+use crate::fs::{asyncify, sys};
+use crate::io::blocking::Buf;
+use crate::io::{AsyncRead, AsyncSeek, AsyncWrite};
+
+use std::fmt;
+use std::fs::{Metadata, Permissions};
+use std::future::Future;
+use std::io::{self, Seek, SeekFrom};
+use std::path::Path;
+use std::pin::Pin;
+use std::sync::Arc;
+use std::task::Context;
+use std::task::Poll;
+use std::task::Poll::*;
+
+/// A reference to an open file on the filesystem.
+///
+/// This is a specialized version of [`std::fs::File`][std] for usage from the
+/// Tokio runtime.
+///
+/// An instance of a `File` can be read and/or written depending on what options
+/// it was opened with. Files also implement Seek to alter the logical cursor
+/// that the file contains internally.
+///
+/// Files are automatically closed when they go out of scope.
+///
+/// [std]: std::fs::File
+///
+/// # Examples
+///
+/// Create a new file and asynchronously write bytes to it:
+///
+/// ```no_run
+/// use tokio::fs::File;
+/// use tokio::prelude::*;
+///
+/// # async fn dox() -> std::io::Result<()> {
+/// let mut file = File::create("foo.txt").await?;
+/// file.write_all(b"hello, world!").await?;
+/// # Ok(())
+/// # }
+/// ```
+///
+/// Read the contents of a file into a buffer
+///
+/// ```no_run
+/// use tokio::fs::File;
+/// use tokio::prelude::*;
+///
+/// # async fn dox() -> std::io::Result<()> {
+/// let mut file = File::open("foo.txt").await?;
+///
+/// let mut contents = vec![];
+/// file.read_to_end(&mut contents).await?;
+///
+/// println!("len = {}", contents.len());
+/// # Ok(())
+/// # }
+/// ```
+pub struct File {
+    std: Arc<sys::File>,
+    state: State,
+
+    /// Errors from writes/flushes are returned in write/flush calls. If a write
+    /// error is observed while performing a read, it is saved until the next
+    /// write / flush call.
+    last_write_err: Option<io::ErrorKind>,
+}
+
+#[derive(Debug)]
+enum State {
+    Idle(Option<Buf>),
+    Busy(sys::Blocking<(Operation, Buf)>),
+}
+
+#[derive(Debug)]
+enum Operation {
+    Read(io::Result<usize>),
+    Write(io::Result<()>),
+    Seek(io::Result<u64>),
+}
+
+impl File {
+    /// Attempts to open a file in read-only mode.
+    ///
+    /// See [`OpenOptions`] for more details.
+    ///
+    /// [`OpenOptions`]: super::OpenOptions
+    ///
+    /// # Errors
+    ///
+    /// This function will return an error if called from outside of the Tokio
+    /// runtime or if path does not already exist. Other errors may also be
+    /// returned according to OpenOptions::open.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    /// use tokio::prelude::*;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut file = File::open("foo.txt").await?;
+    ///
+    /// let mut contents = vec![];
+    /// file.read_to_end(&mut contents).await?;
+    ///
+    /// println!("len = {}", contents.len());
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn open(path: impl AsRef<Path>) -> io::Result<File> {
+        let path = path.as_ref().to_owned();
+        let std = asyncify(|| sys::File::open(path)).await?;
+
+        Ok(File::from_std(std))
+    }
+
+    /// Opens a file in write-only mode.
+    ///
+    /// This function will create a file if it does not exist, and will truncate
+    /// it if it does.
+    ///
+    /// See [`OpenOptions`] for more details.
+    ///
+    /// [`OpenOptions`]: super::OpenOptions
+    ///
+    /// # Errors
+    ///
+    /// Results in an error if called from outside of the Tokio runtime or if
+    /// the underlying [`create`] call results in an error.
+    ///
+    /// [`create`]: std::fs::File::create
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    /// use tokio::prelude::*;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut file = File::create("foo.txt").await?;
+    /// file.write_all(b"hello, world!").await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn create(path: impl AsRef<Path>) -> io::Result<File> {
+        let path = path.as_ref().to_owned();
+        let std_file = asyncify(move || sys::File::create(path)).await?;
+        Ok(File::from_std(std_file))
+    }
+
+    /// Converts a [`std::fs::File`][std] to a [`tokio::fs::File`][file].
+    ///
+    /// [std]: std::fs::File
+    /// [file]: File
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// // This line could block. It is not recommended to do this on the Tokio
+    /// // runtime.
+    /// let std_file = std::fs::File::open("foo.txt").unwrap();
+    /// let file = tokio::fs::File::from_std(std_file);
+    /// ```
+    pub fn from_std(std: sys::File) -> File {
+        File {
+            std: Arc::new(std),
+            state: State::Idle(Some(Buf::with_capacity(0))),
+            last_write_err: None,
+        }
+    }
+
+    /// Seeks to an offset, in bytes, in a stream.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    /// use tokio::prelude::*;
+    ///
+    /// use std::io::SeekFrom;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut file = File::open("foo.txt").await?;
+    /// file.seek(SeekFrom::Start(6)).await?;
+    ///
+    /// let mut contents = vec![0u8; 10];
+    /// file.read_exact(&mut contents).await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn seek(&mut self, mut pos: SeekFrom) -> io::Result<u64> {
+        self.complete_inflight().await;
+
+        let mut buf = match self.state {
+            Idle(ref mut buf_cell) => buf_cell.take().unwrap(),
+            _ => unreachable!(),
+        };
+
+        // Factor in any unread data from the buf
+        if !buf.is_empty() {
+            let n = buf.discard_read();
+
+            if let SeekFrom::Current(ref mut offset) = pos {
+                *offset += n;
+            }
+        }
+
+        let std = self.std.clone();
+
+        // Start the operation
+        self.state = Busy(sys::run(move || {
+            let res = (&*std).seek(pos);
+            (Operation::Seek(res), buf)
+        }));
+
+        let (op, buf) = match self.state {
+            Idle(_) => unreachable!(),
+            Busy(ref mut rx) => rx.await.unwrap(),
+        };
+
+        self.state = Idle(Some(buf));
+
+        match op {
+            Operation::Seek(res) => res,
+            _ => unreachable!(),
+        }
+    }
+
+    /// Attempts to sync all OS-internal metadata to disk.
+    ///
+    /// This function will attempt to ensure that all in-core data reaches the
+    /// filesystem before returning.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    /// use tokio::prelude::*;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut file = File::create("foo.txt").await?;
+    /// file.write_all(b"hello, world!").await?;
+    /// file.sync_all().await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn sync_all(&mut self) -> io::Result<()> {
+        self.complete_inflight().await;
+
+        let std = self.std.clone();
+        asyncify(move || std.sync_all()).await
+    }
+
+    /// This function is similar to `sync_all`, except that it may not
+    /// synchronize file metadata to the filesystem.
+    ///
+    /// This is intended for use cases that must synchronize content, but don't
+    /// need the metadata on disk. The goal of this method is to reduce disk
+    /// operations.
+    ///
+    /// Note that some platforms may simply implement this in terms of `sync_all`.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    /// use tokio::prelude::*;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut file = File::create("foo.txt").await?;
+    /// file.write_all(b"hello, world!").await?;
+    /// file.sync_data().await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn sync_data(&mut self) -> io::Result<()> {
+        self.complete_inflight().await;
+
+        let std = self.std.clone();
+        asyncify(move || std.sync_data()).await
+    }
+
+    /// Truncates or extends the underlying file, updating the size of this file to become size.
+    ///
+    /// If the size is less than the current file's size, then the file will be
+    /// shrunk. If it is greater than the current file's size, then the file
+    /// will be extended to size and have all of the intermediate data filled in
+    /// with 0s.
+    ///
+    /// # Errors
+    ///
+    /// This function will return an error if the file is not opened for
+    /// writing.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    /// use tokio::prelude::*;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut file = File::create("foo.txt").await?;
+    /// file.write_all(b"hello, world!").await?;
+    /// file.set_len(10).await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn set_len(&mut self, size: u64) -> io::Result<()> {
+        self.complete_inflight().await;
+
+        let mut buf = match self.state {
+            Idle(ref mut buf_cell) => buf_cell.take().unwrap(),
+            _ => unreachable!(),
+        };
+
+        let seek = if !buf.is_empty() {
+            Some(SeekFrom::Current(buf.discard_read()))
+        } else {
+            None
+        };
+
+        let std = self.std.clone();
+
+        self.state = Busy(sys::run(move || {
+            let res = if let Some(seek) = seek {
+                (&*std).seek(seek).and_then(|_| std.set_len(size))
+            } else {
+                std.set_len(size)
+            }
+            .map(|_| 0); // the value is discarded later
+
+            // Return the result as a seek
+            (Operation::Seek(res), buf)
+        }));
+
+        let (op, buf) = match self.state {
+            Idle(_) => unreachable!(),
+            Busy(ref mut rx) => rx.await?,
+        };
+
+        self.state = Idle(Some(buf));
+
+        match op {
+            Operation::Seek(res) => res.map(|_| ()),
+            _ => unreachable!(),
+        }
+    }
+
+    /// Queries metadata about the underlying file.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let file = File::open("foo.txt").await?;
+    /// let metadata = file.metadata().await?;
+    ///
+    /// println!("{:?}", metadata);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn metadata(&self) -> io::Result<Metadata> {
+        let std = self.std.clone();
+        asyncify(move || std.metadata()).await
+    }
+
+    /// Create a new `File` instance that shares the same underlying file handle
+    /// as the existing `File` instance. Reads, writes, and seeks will affect both
+    /// File instances simultaneously.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let file = File::open("foo.txt").await?;
+    /// let file_clone = file.try_clone().await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn try_clone(&self) -> io::Result<File> {
+        let std = self.std.clone();
+        let std_file = asyncify(move || std.try_clone()).await?;
+        Ok(File::from_std(std_file))
+    }
+
+    /// Destructures `File` into a [`std::fs::File`][std]. This function is
+    /// async to allow any in-flight operations to complete.
+    ///
+    /// Use `File::try_into_std` to attempt conversion immediately.
+    ///
+    /// [std]: std::fs::File
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let tokio_file = File::open("foo.txt").await?;
+    /// let std_file = tokio_file.into_std().await;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn into_std(mut self) -> sys::File {
+        self.complete_inflight().await;
+        Arc::try_unwrap(self.std).expect("Arc::try_unwrap failed")
+    }
+
+    /// Tries to immediately destructure `File` into a [`std::fs::File`][std].
+    ///
+    /// [std]: std::fs::File
+    ///
+    /// # Errors
+    ///
+    /// This function will return an error containing the file if some
+    /// operation is in-flight.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let tokio_file = File::open("foo.txt").await?;
+    /// let std_file = tokio_file.try_into_std().unwrap();
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn try_into_std(mut self) -> Result<sys::File, Self> {
+        match Arc::try_unwrap(self.std) {
+            Ok(file) => Ok(file),
+            Err(std_file_arc) => {
+                self.std = std_file_arc;
+                Err(self)
+            }
+        }
+    }
+
+    /// Changes the permissions on the underlying file.
+    ///
+    /// # Platform-specific behavior
+    ///
+    /// This function currently corresponds to the `fchmod` function on Unix and
+    /// the `SetFileInformationByHandle` function on Windows. Note that, this
+    /// [may change in the future][changes].
+    ///
+    /// [changes]: https://doc.rust-lang.org/std/io/index.html#platform-specific-behavior
+    ///
+    /// # Errors
+    ///
+    /// This function will return an error if the user lacks permission change
+    /// attributes on the underlying file. It may also return an error in other
+    /// os-specific unspecified cases.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let file = File::open("foo.txt").await?;
+    /// let mut perms = file.metadata().await?.permissions();
+    /// perms.set_readonly(true);
+    /// file.set_permissions(perms).await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn set_permissions(&self, perm: Permissions) -> io::Result<()> {
+        let std = self.std.clone();
+        asyncify(move || std.set_permissions(perm)).await
+    }
+
+    async fn complete_inflight(&mut self) {
+        use crate::future::poll_fn;
+
+        if let Err(e) = poll_fn(|cx| Pin::new(&mut *self).poll_flush(cx)).await {
+            self.last_write_err = Some(e.kind());
+        }
+    }
+}
+
+impl AsyncRead for File {
+    fn poll_read(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        dst: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        loop {
+            match self.state {
+                Idle(ref mut buf_cell) => {
+                    let mut buf = buf_cell.take().unwrap();
+
+                    if !buf.is_empty() {
+                        let n = buf.copy_to(dst);
+                        *buf_cell = Some(buf);
+                        return Ready(Ok(n));
+                    }
+
+                    buf.ensure_capacity_for(dst);
+                    let std = self.std.clone();
+
+                    self.state = Busy(sys::run(move || {
+                        let res = buf.read_from(&mut &*std);
+                        (Operation::Read(res), buf)
+                    }));
+                }
+                Busy(ref mut rx) => {
+                    let (op, mut buf) = ready!(Pin::new(rx).poll(cx))?;
+
+                    match op {
+                        Operation::Read(Ok(_)) => {
+                            let n = buf.copy_to(dst);
+                            self.state = Idle(Some(buf));
+                            return Ready(Ok(n));
+                        }
+                        Operation::Read(Err(e)) => {
+                            assert!(buf.is_empty());
+
+                            self.state = Idle(Some(buf));
+                            return Ready(Err(e));
+                        }
+                        Operation::Write(Ok(_)) => {
+                            assert!(buf.is_empty());
+                            self.state = Idle(Some(buf));
+                            continue;
+                        }
+                        Operation::Write(Err(e)) => {
+                            assert!(self.last_write_err.is_none());
+                            self.last_write_err = Some(e.kind());
+                            self.state = Idle(Some(buf));
+                        }
+                        Operation::Seek(_) => {
+                            assert!(buf.is_empty());
+                            self.state = Idle(Some(buf));
+                            continue;
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+
+impl AsyncSeek for File {
+    fn start_seek(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        mut pos: SeekFrom,
+    ) -> Poll<io::Result<()>> {
+        loop {
+            match self.state {
+                Idle(ref mut buf_cell) => {
+                    let mut buf = buf_cell.take().unwrap();
+
+                    // Factor in any unread data from the buf
+                    if !buf.is_empty() {
+                        let n = buf.discard_read();
+
+                        if let SeekFrom::Current(ref mut offset) = pos {
+                            *offset += n;
+                        }
+                    }
+
+                    let std = self.std.clone();
+
+                    self.state = Busy(sys::run(move || {
+                        let res = (&*std).seek(pos);
+                        (Operation::Seek(res), buf)
+                    }));
+
+                    return Ready(Ok(()));
+                }
+                Busy(ref mut rx) => {
+                    let (op, buf) = ready!(Pin::new(rx).poll(cx))?;
+                    self.state = Idle(Some(buf));
+
+                    match op {
+                        Operation::Read(_) => {}
+                        Operation::Write(Err(e)) => {
+                            assert!(self.last_write_err.is_none());
+                            self.last_write_err = Some(e.kind());
+                        }
+                        Operation::Write(_) => {}
+                        Operation::Seek(_) => {}
+                    }
+                }
+            }
+        }
+    }
+
+    fn poll_complete(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>> {
+        loop {
+            match self.state {
+                Idle(_) => panic!("must call start_seek before calling poll_complete"),
+                Busy(ref mut rx) => {
+                    let (op, buf) = ready!(Pin::new(rx).poll(cx))?;
+                    self.state = Idle(Some(buf));
+
+                    match op {
+                        Operation::Read(_) => {}
+                        Operation::Write(Err(e)) => {
+                            assert!(self.last_write_err.is_none());
+                            self.last_write_err = Some(e.kind());
+                        }
+                        Operation::Write(_) => {}
+                        Operation::Seek(res) => return Ready(res),
+                    }
+                }
+            }
+        }
+    }
+}
+
+impl AsyncWrite for File {
+    fn poll_write(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        src: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        if let Some(e) = self.last_write_err.take() {
+            return Ready(Err(e.into()));
+        }
+
+        loop {
+            match self.state {
+                Idle(ref mut buf_cell) => {
+                    let mut buf = buf_cell.take().unwrap();
+
+                    let seek = if !buf.is_empty() {
+                        Some(SeekFrom::Current(buf.discard_read()))
+                    } else {
+                        None
+                    };
+
+                    let n = buf.copy_from(src);
+                    let std = self.std.clone();
+
+                    self.state = Busy(sys::run(move || {
+                        let res = if let Some(seek) = seek {
+                            (&*std).seek(seek).and_then(|_| buf.write_to(&mut &*std))
+                        } else {
+                            buf.write_to(&mut &*std)
+                        };
+
+                        (Operation::Write(res), buf)
+                    }));
+
+                    return Ready(Ok(n));
+                }
+                Busy(ref mut rx) => {
+                    let (op, buf) = ready!(Pin::new(rx).poll(cx))?;
+                    self.state = Idle(Some(buf));
+
+                    match op {
+                        Operation::Read(_) => {
+                            // We don't care about the result here. The fact
+                            // that the cursor has advanced will be reflected in
+                            // the next iteration of the loop
+                            continue;
+                        }
+                        Operation::Write(res) => {
+                            // If the previous write was successful, continue.
+                            // Otherwise, error.
+                            res?;
+                            continue;
+                        }
+                        Operation::Seek(_) => {
+                            // Ignore the seek
+                            continue;
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
+        if let Some(e) = self.last_write_err.take() {
+            return Ready(Err(e.into()));
+        }
+
+        let (op, buf) = match self.state {
+            Idle(_) => return Ready(Ok(())),
+            Busy(ref mut rx) => ready!(Pin::new(rx).poll(cx))?,
+        };
+
+        // The buffer is not used here
+        self.state = Idle(Some(buf));
+
+        match op {
+            Operation::Read(_) => Ready(Ok(())),
+            Operation::Write(res) => Ready(res),
+            Operation::Seek(_) => Ready(Ok(())),
+        }
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
+        Poll::Ready(Ok(()))
+    }
+}
+
+impl From<sys::File> for File {
+    fn from(std: sys::File) -> Self {
+        Self::from_std(std)
+    }
+}
+
+impl fmt::Debug for File {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("tokio::fs::File")
+            .field("std", &self.std)
+            .finish()
+    }
+}
+
+#[cfg(unix)]
+impl std::os::unix::io::AsRawFd for File {
+    fn as_raw_fd(&self) -> std::os::unix::io::RawFd {
+        self.std.as_raw_fd()
+    }
+}
+
+#[cfg(windows)]
+impl std::os::windows::io::AsRawHandle for File {
+    fn as_raw_handle(&self) -> std::os::windows::io::RawHandle {
+        self.std.as_raw_handle()
+    }
+}
diff --git a/third_party/rust/tokio/src/fs/hard_link.rs b/third_party/rust/tokio/src/fs/hard_link.rs
new file mode 100644
index 0000000000..50cc17d286
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/hard_link.rs
@@ -0,0 +1,46 @@
+use crate::fs::asyncify;
+
+use std::io;
+use std::path::Path;
+
+/// Creates a new hard link on the filesystem.
+///
+/// This is an async version of [`std::fs::hard_link`][std]
+///
+/// [std]: std::fs::hard_link
+///
+/// The `dst` path will be a link pointing to the `src` path. Note that systems
+/// often require these two paths to both be located on the same filesystem.
+///
+/// # Platform-specific behavior
+///
+/// This function currently corresponds to the `link` function on Unix
+/// and the `CreateHardLink` function on Windows.
+/// Note that, this [may change in the future][changes].
+///
+/// [changes]: https://doc.rust-lang.org/std/io/index.html#platform-specific-behavior
+///
+/// # Errors
+///
+/// This function will return an error in the following situations, but is not
+/// limited to just these cases:
+///
+/// * The `src` path is not a file or doesn't exist.
+///
+/// # Examples
+///
+/// ```no_run
+/// use tokio::fs;
+///
+/// #[tokio::main]
+/// async fn main() -> std::io::Result<()> {
+///     fs::hard_link("a.txt", "b.txt").await?; // Hard link a.txt to b.txt
+///     Ok(())
+/// }
+/// ```
+pub async fn hard_link(src: impl AsRef<Path>, dst: impl AsRef<Path>) -> io::Result<()> {
+    let src = src.as_ref().to_owned();
+    let dst = dst.as_ref().to_owned();
+
+    asyncify(move || std::fs::hard_link(src, dst)).await
+}
diff --git a/third_party/rust/tokio/src/fs/metadata.rs b/third_party/rust/tokio/src/fs/metadata.rs
new file mode 100644
index 0000000000..ff9cded79a
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/metadata.rs
@@ -0,0 +1,47 @@
+use crate::fs::asyncify;
+
+use std::fs::Metadata;
+use std::io;
+use std::path::Path;
+
+/// Given a path, queries the file system to get information about a file,
+/// directory, etc.
+///
+/// This is an async version of [`std::fs::metadata`][std]
+///
+/// This function will traverse symbolic links to query information about the
+/// destination file.
+///
+/// # Platform-specific behavior
+///
+/// This function currently corresponds to the `stat` function on Unix and the
+/// `GetFileAttributesEx` function on Windows.  Note that, this [may change in
+/// the future][changes].
+///
+/// [std]: std::fs::metadata
+/// [changes]: https://doc.rust-lang.org/std/io/index.html#platform-specific-behavior
+///
+/// # Errors
+///
+/// This function will return an error in the following situations, but is not
+/// limited to just these cases:
+///
+/// * The user lacks permissions to perform `metadata` call on `path`.
+/// * `path` does not exist.
+///
+/// # Examples
+///
+/// ```rust,no_run
+/// use tokio::fs;
+///
+/// #[tokio::main]
+/// async fn main() -> std::io::Result<()> {
+///     let attr = fs::metadata("/some/file/path.txt").await?;
+///     // inspect attr ...
+///     Ok(())
+/// }
+/// ```
+pub async fn metadata(path: impl AsRef<Path>) -> io::Result<Metadata> {
+    let path = path.as_ref().to_owned();
+    asyncify(|| std::fs::metadata(path)).await
+}
diff --git a/third_party/rust/tokio/src/fs/mod.rs b/third_party/rust/tokio/src/fs/mod.rs
new file mode 100644
index 0000000000..3eb0376463
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/mod.rs
@@ -0,0 +1,109 @@
+#![cfg(not(loom))]
+
+//! Asynchronous file and standard stream adaptation.
+//!
+//! This module contains utility methods and adapter types for input/output to
+//! files or standard streams (`Stdin`, `Stdout`, `Stderr`), and
+//! filesystem manipulation, for use within (and only within) a Tokio runtime.
+//!
+//! Tasks run by *worker* threads should not block, as this could delay
+//! servicing reactor events. Portable filesystem operations are blocking,
+//! however. This module offers adapters which use a `blocking` annotation
+//! to inform the runtime that a blocking operation is required. When
+//! necessary, this allows the runtime to convert the current thread from a
+//! *worker* to a *backup* thread, where blocking is acceptable.
+//!
+//! ## Usage
+//!
+//! Where possible, users should prefer the provided asynchronous-specific
+//! traits such as [`AsyncRead`], or methods returning a `Future` or `Poll`
+//! type. Adaptions also extend to traits like `std::io::Read` where methods
+//! return `std::io::Result`. Be warned that these adapted methods may return
+//! `std::io::ErrorKind::WouldBlock` if a *worker* thread can not be converted
+//! to a *backup* thread immediately.
+//!
+//! [`AsyncRead`]: https://docs.rs/tokio-io/0.1/tokio_io/trait.AsyncRead.html
+
+mod canonicalize;
+pub use self::canonicalize::canonicalize;
+
+mod create_dir;
+pub use self::create_dir::create_dir;
+
+mod create_dir_all;
+pub use self::create_dir_all::create_dir_all;
+
+mod file;
+pub use self::file::File;
+
+mod hard_link;
+pub use self::hard_link::hard_link;
+
+mod metadata;
+pub use self::metadata::metadata;
+
+mod open_options;
+pub use self::open_options::OpenOptions;
+
+pub mod os;
+
+mod read;
+pub use self::read::read;
+
+mod read_dir;
+pub use self::read_dir::{read_dir, DirEntry, ReadDir};
+
+mod read_link;
+pub use self::read_link::read_link;
+
+mod read_to_string;
+pub use self::read_to_string::read_to_string;
+
+mod remove_dir;
+pub use self::remove_dir::remove_dir;
+
+mod remove_dir_all;
+pub use self::remove_dir_all::remove_dir_all;
+
+mod remove_file;
+pub use self::remove_file::remove_file;
+
+mod rename;
+pub use self::rename::rename;
+
+mod set_permissions;
+pub use self::set_permissions::set_permissions;
+
+mod symlink_metadata;
+pub use self::symlink_metadata::symlink_metadata;
+
+mod write;
+pub use self::write::write;
+
+mod copy;
+pub use self::copy::copy;
+
+use std::io;
+
+pub(crate) async fn asyncify<F, T>(f: F) -> io::Result<T>
+where
+    F: FnOnce() -> io::Result<T> + Send + 'static,
+    T: Send + 'static,
+{
+    match sys::run(f).await {
+        Ok(res) => res,
+        Err(_) => Err(io::Error::new(
+            io::ErrorKind::Other,
+            "background task failed",
+        )),
+    }
+}
+
+/// Types in this module can be mocked out in tests.
+mod sys {
+    pub(crate) use std::fs::File;
+
+    // TODO: don't rename
+    pub(crate) use crate::runtime::spawn_blocking as run;
+    pub(crate) use crate::task::JoinHandle as Blocking;
+}
diff --git a/third_party/rust/tokio/src/fs/open_options.rs b/third_party/rust/tokio/src/fs/open_options.rs
new file mode 100644
index 0000000000..3210f4b7b5
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/open_options.rs
@@ -0,0 +1,397 @@
+use crate::fs::{asyncify, File};
+
+use std::io;
+use std::path::Path;
+
+/// Options and flags which can be used to configure how a file is opened.
+///
+/// This builder exposes the ability to configure how a [`File`] is opened and
+/// what operations are permitted on the open file. The [`File::open`] and
+/// [`File::create`] methods are aliases for commonly used options using this
+/// builder.
+///
+/// Generally speaking, when using `OpenOptions`, you'll first call [`new`],
+/// then chain calls to methods to set each option, then call [`open`], passing
+/// the path of the file you're trying to open. This will give you a
+/// [`io::Result`][result] with a [`File`] inside that you can further operate
+/// on.
+///
+/// This is a specialized version of [`std::fs::OpenOptions`] for usage from
+/// the Tokio runtime.
+///
+/// `From<std::fs::OpenOptions>` is implemented for more advanced configuration
+/// than the methods provided here.
+///
+/// [`new`]: OpenOptions::new
+/// [`open`]: OpenOptions::open
+/// [result]: std::io::Result
+/// [`File`]: File
+/// [`File::open`]: File::open
+/// [`File::create`]: File::create
+/// [`std::fs::OpenOptions`]: std::fs::OpenOptions
+///
+/// # Examples
+///
+/// Opening a file to read:
+///
+/// ```no_run
+/// use tokio::fs::OpenOptions;
+/// use std::io;
+///
+/// #[tokio::main]
+/// async fn main() -> io::Result<()> {
+///     let file = OpenOptions::new()
+///         .read(true)
+///         .open("foo.txt")
+///         .await?;
+///
+///     Ok(())
+/// }
+/// ```
+///
+/// Opening a file for both reading and writing, as well as creating it if it
+/// doesn't exist:
+///
+/// ```no_run
+/// use tokio::fs::OpenOptions;
+/// use std::io;
+///
+/// #[tokio::main]
+/// async fn main() -> io::Result<()> {
+///     let file = OpenOptions::new()
+///         .read(true)
+///         .write(true)
+///         .create(true)
+///         .open("foo.txt")
+///         .await?;
+///
+///     Ok(())
+/// }
+/// ```
+#[derive(Clone, Debug)]
+pub struct OpenOptions(std::fs::OpenOptions);
+
+impl OpenOptions {
+    /// Creates a blank new set of options ready for configuration.
+    ///
+    /// All options are initially set to `false`.
+    ///
+    /// This is an async version of [`std::fs::OpenOptions::new`][std]
+    ///
+    /// [std]: std::fs::OpenOptions::new
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::OpenOptions;
+    ///
+    /// let mut options = OpenOptions::new();
+    /// let future = options.read(true).open("foo.txt");
+    /// ```
+    pub fn new() -> OpenOptions {
+        OpenOptions(std::fs::OpenOptions::new())
+    }
+
+    /// Sets the option for read access.
+    ///
+    /// This option, when true, will indicate that the file should be
+    /// `read`-able if opened.
+    ///
+    /// This is an async version of [`std::fs::OpenOptions::read`][std]
+    ///
+    /// [std]: std::fs::OpenOptions::read
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::OpenOptions;
+    /// use std::io;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let file = OpenOptions::new()
+    ///         .read(true)
+    ///         .open("foo.txt")
+    ///         .await?;
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn read(&mut self, read: bool) -> &mut OpenOptions {
+        self.0.read(read);
+        self
+    }
+
+    /// Sets the option for write access.
+    ///
+    /// This option, when true, will indicate that the file should be
+    /// `write`-able if opened.
+    ///
+    /// This is an async version of [`std::fs::OpenOptions::write`][std]
+    ///
+    /// [std]: std::fs::OpenOptions::write
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::OpenOptions;
+    /// use std::io;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let file = OpenOptions::new()
+    ///         .write(true)
+    ///         .open("foo.txt")
+    ///         .await?;
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn write(&mut self, write: bool) -> &mut OpenOptions {
+        self.0.write(write);
+        self
+    }
+
+    /// Sets the option for the append mode.
+    ///
+    /// This option, when true, means that writes will append to a file instead
+    /// of overwriting previous contents.  Note that setting
+    /// `.write(true).append(true)` has the same effect as setting only
+    /// `.append(true)`.
+    ///
+    /// For most filesystems, the operating system guarantees that all writes are
+    /// atomic: no writes get mangled because another process writes at the same
+    /// time.
+    ///
+    /// One maybe obvious note when using append-mode: make sure that all data
+    /// that belongs together is written to the file in one operation. This
+    /// can be done by concatenating strings before passing them to [`write()`],
+    /// or using a buffered writer (with a buffer of adequate size),
+    /// and calling [`flush()`] when the message is complete.
+    ///
+    /// If a file is opened with both read and append access, beware that after
+    /// opening, and after every write, the position for reading may be set at the
+    /// end of the file. So, before writing, save the current position (using
+    /// [`seek`]`(`[`SeekFrom`]`::`[`Current`]`(0))`), and restore it before the next read.
+    ///
+    /// This is an async version of [`std::fs::OpenOptions::append`][std]
+    ///
+    /// [std]: std::fs::OpenOptions::append
+    ///
+    /// ## Note
+    ///
+    /// This function doesn't create the file if it doesn't exist. Use the [`create`]
+    /// method to do so.
+    ///
+    /// [`write()`]: crate::io::AsyncWriteExt::write
+    /// [`flush()`]: crate::io::AsyncWriteExt::flush
+    /// [`seek`]: crate::io::AsyncSeekExt::seek
+    /// [`SeekFrom`]: std::io::SeekFrom
+    /// [`Current`]: std::io::SeekFrom::Current
+    /// [`create`]: OpenOptions::create
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::OpenOptions;
+    /// use std::io;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let file = OpenOptions::new()
+    ///         .append(true)
+    ///         .open("foo.txt")
+    ///         .await?;
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn append(&mut self, append: bool) -> &mut OpenOptions {
+        self.0.append(append);
+        self
+    }
+
+    /// Sets the option for truncating a previous file.
+    ///
+    /// If a file is successfully opened with this option set it will truncate
+    /// the file to 0 length if it already exists.
+    ///
+    /// The file must be opened with write access for truncate to work.
+    ///
+    /// This is an async version of [`std::fs::OpenOptions::truncate`][std]
+    ///
+    /// [std]: std::fs::OpenOptions::truncate
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::OpenOptions;
+    /// use std::io;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let file = OpenOptions::new()
+    ///         .write(true)
+    ///         .truncate(true)
+    ///         .open("foo.txt")
+    ///         .await?;
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn truncate(&mut self, truncate: bool) -> &mut OpenOptions {
+        self.0.truncate(truncate);
+        self
+    }
+
+    /// Sets the option for creating a new file.
+    ///
+    /// This option indicates whether a new file will be created if the file
+    /// does not yet already exist.
+    ///
+    /// In order for the file to be created, [`write`] or [`append`] access must
+    /// be used.
+    ///
+    /// This is an async version of [`std::fs::OpenOptions::create`][std]
+    ///
+    /// [std]: std::fs::OpenOptions::create
+    /// [`write`]: OpenOptions::write
+    /// [`append`]: OpenOptions::append
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::OpenOptions;
+    /// use std::io;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let file = OpenOptions::new()
+    ///         .write(true)
+    ///         .create(true)
+    ///         .open("foo.txt")
+    ///         .await?;
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn create(&mut self, create: bool) -> &mut OpenOptions {
+        self.0.create(create);
+        self
+    }
+
+    /// Sets the option to always create a new file.
+    ///
+    /// This option indicates whether a new file will be created.  No file is
+    /// allowed to exist at the target location, also no (dangling) symlink.
+    ///
+    /// This option is useful because it is atomic. Otherwise between checking
+    /// whether a file exists and creating a new one, the file may have been
+    /// created by another process (a TOCTOU race condition / attack).
+    ///
+    /// If `.create_new(true)` is set, [`.create()`] and [`.truncate()`] are
+    /// ignored.
+    ///
+    /// The file must be opened with write or append access in order to create a
+    /// new file.
+    ///
+    /// This is an async version of [`std::fs::OpenOptions::create_new`][std]
+    ///
+    /// [std]: std::fs::OpenOptions::create_new
+    /// [`.create()`]: OpenOptions::create
+    /// [`.truncate()`]: OpenOptions::truncate
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::OpenOptions;
+    /// use std::io;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let file = OpenOptions::new()
+    ///         .write(true)
+    ///         .create_new(true)
+    ///         .open("foo.txt")
+    ///         .await?;
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn create_new(&mut self, create_new: bool) -> &mut OpenOptions {
+        self.0.create_new(create_new);
+        self
+    }
+
+    /// Opens a file at `path` with the options specified by `self`.
+    ///
+    /// This is an async version of [`std::fs::OpenOptions::open`][std]
+    ///
+    /// [std]: std::fs::OpenOptions::open
+    ///
+    /// # Errors
+    ///
+    /// This function will return an error under a number of different
+    /// circumstances. Some of these error conditions are listed here, together
+    /// with their [`ErrorKind`]. The mapping to [`ErrorKind`]s is not part of
+    /// the compatibility contract of the function, especially the `Other` kind
+    /// might change to more specific kinds in the future.
+    ///
+    /// * [`NotFound`]: The specified file does not exist and neither `create`
+    ///   or `create_new` is set.
+    /// * [`NotFound`]: One of the directory components of the file path does
+    ///   not exist.
+    /// * [`PermissionDenied`]: The user lacks permission to get the specified
+    ///   access rights for the file.
+    /// * [`PermissionDenied`]: The user lacks permission to open one of the
+    ///   directory components of the specified path.
+    /// * [`AlreadyExists`]: `create_new` was specified and the file already
+    ///   exists.
+    /// * [`InvalidInput`]: Invalid combinations of open options (truncate
+    ///   without write access, no access mode set, etc.).
+    /// * [`Other`]: One of the directory components of the specified file path
+    ///   was not, in fact, a directory.
+    /// * [`Other`]: Filesystem-level errors: full disk, write permission
+    ///   requested on a read-only file system, exceeded disk quota, too many
+    ///   open files, too long filename, too many symbolic links in the
+    ///   specified path (Unix-like systems only), etc.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::OpenOptions;
+    /// use std::io;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let file = OpenOptions::new().open("foo.txt").await?;
+    ///     Ok(())
+    /// }
+    /// ```
+    ///
+    /// [`ErrorKind`]: std::io::ErrorKind
+    /// [`AlreadyExists`]: std::io::ErrorKind::AlreadyExists
+    /// [`InvalidInput`]: std::io::ErrorKind::InvalidInput
+    /// [`NotFound`]: std::io::ErrorKind::NotFound
+    /// [`Other`]: std::io::ErrorKind::Other
+    /// [`PermissionDenied`]: std::io::ErrorKind::PermissionDenied
+    pub async fn open(&self, path: impl AsRef<Path>) -> io::Result<File> {
+        let path = path.as_ref().to_owned();
+        let opts = self.0.clone();
+
+        let std = asyncify(move || opts.open(path)).await?;
+        Ok(File::from_std(std))
+    }
+}
+
+impl From<std::fs::OpenOptions> for OpenOptions {
+    fn from(options: std::fs::OpenOptions) -> OpenOptions {
+        OpenOptions(options)
+    }
+}
+
+impl Default for OpenOptions {
+    fn default() -> Self {
+        Self::new()
+    }
+}
diff --git a/third_party/rust/tokio/src/fs/os/mod.rs b/third_party/rust/tokio/src/fs/os/mod.rs
new file mode 100644
index 0000000000..f4b8bfb617
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/os/mod.rs
@@ -0,0 +1,7 @@
+//! OS-specific functionality.
+
+#[cfg(unix)]
+pub mod unix;
+
+#[cfg(windows)]
+pub mod windows;
diff --git a/third_party/rust/tokio/src/fs/os/unix/mod.rs b/third_party/rust/tokio/src/fs/os/unix/mod.rs
new file mode 100644
index 0000000000..3b0bec38bd
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/os/unix/mod.rs
@@ -0,0 +1,4 @@
+//! Unix-specific extensions to primitives in the `tokio_fs` module.
+
+mod symlink;
+pub use self::symlink::symlink;
diff --git a/third_party/rust/tokio/src/fs/os/unix/symlink.rs b/third_party/rust/tokio/src/fs/os/unix/symlink.rs
new file mode 100644
index 0000000000..22ece7250f
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/os/unix/symlink.rs
@@ -0,0 +1,18 @@
+use crate::fs::asyncify;
+
+use std::io;
+use std::path::Path;
+
+/// Creates a new symbolic link on the filesystem.
+///
+/// The `dst` path will be a symbolic link pointing to the `src` path.
+///
+/// This is an async version of [`std::os::unix::fs::symlink`][std]
+///
+/// [std]: std::os::unix::fs::symlink
+pub async fn symlink(src: impl AsRef<Path>, dst: impl AsRef<Path>) -> io::Result<()> {
+    let src = src.as_ref().to_owned();
+    let dst = dst.as_ref().to_owned();
+
+    asyncify(move || std::os::unix::fs::symlink(src, dst)).await
+}
diff --git a/third_party/rust/tokio/src/fs/os/windows/mod.rs b/third_party/rust/tokio/src/fs/os/windows/mod.rs
new file mode 100644
index 0000000000..42eb7bdb92
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/os/windows/mod.rs
@@ -0,0 +1,7 @@
+//! Windows-specific extensions for the primitives in the `tokio_fs` module.
+
+mod symlink_dir;
+pub use self::symlink_dir::symlink_dir;
+
+mod symlink_file;
+pub use self::symlink_file::symlink_file;
diff --git a/third_party/rust/tokio/src/fs/os/windows/symlink_dir.rs b/third_party/rust/tokio/src/fs/os/windows/symlink_dir.rs
new file mode 100644
index 0000000000..736e762b48
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/os/windows/symlink_dir.rs
@@ -0,0 +1,19 @@
+use crate::fs::asyncify;
+
+use std::io;
+use std::path::Path;
+
+/// Creates a new directory symlink on the filesystem.
+///
+/// The `dst` path will be a directory symbolic link pointing to the `src`
+/// path.
+///
+/// This is an async version of [`std::os::windows::fs::symlink_dir`][std]
+///
+/// [std]: std::os::windows::fs::symlink_dir
+pub async fn symlink_dir(src: impl AsRef<Path>, dst: impl AsRef<Path>) -> io::Result<()> {
+    let src = src.as_ref().to_owned();
+    let dst = dst.as_ref().to_owned();
+
+    asyncify(move || std::os::windows::fs::symlink_dir(src, dst)).await
+}
diff --git a/third_party/rust/tokio/src/fs/os/windows/symlink_file.rs b/third_party/rust/tokio/src/fs/os/windows/symlink_file.rs
new file mode 100644
index 0000000000..07d8e60419
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/os/windows/symlink_file.rs
@@ -0,0 +1,19 @@
+use crate::fs::asyncify;
+
+use std::io;
+use std::path::Path;
+
+/// Creates a new file symbolic link on the filesystem.
+///
+/// The `dst` path will be a file symbolic link pointing to the `src`
+/// path.
+///
+/// This is an async version of [`std::os::windows::fs::symlink_file`][std]
+///
+/// [std]: std::os::windows::fs::symlink_file
+pub async fn symlink_file(src: impl AsRef<Path>, dst: impl AsRef<Path>) -> io::Result<()> {
+    let src = src.as_ref().to_owned();
+    let dst = dst.as_ref().to_owned();
+
+    asyncify(move || std::os::windows::fs::symlink_file(src, dst)).await
+}
diff --git a/third_party/rust/tokio/src/fs/read.rs b/third_party/rust/tokio/src/fs/read.rs
new file mode 100644
index 0000000000..2d80eb5bd3
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/read.rs
@@ -0,0 +1,47 @@
+use crate::fs::asyncify;
+
+use std::{io, path::Path};
+
+/// Reads the entire contents of a file into a bytes vector.
+///
+/// This is an async version of [`std::fs::read`][std]
+///
+/// [std]: std::fs::read
+///
+/// This is a convenience function for using [`File::open`] and [`read_to_end`]
+/// with fewer imports and without an intermediate variable. It pre-allocates a
+/// buffer based on the file size when available, so it is generally faster than
+/// reading into a vector created with `Vec::new()`.
+///
+/// [`File::open`]: super::File::open
+/// [`read_to_end`]: crate::io::AsyncReadExt::read_to_end
+///
+/// # Errors
+///
+/// This function will return an error if `path` does not already exist.
+/// Other errors may also be returned according to [`OpenOptions::open`].
+///
+/// [`OpenOptions::open`]: super::OpenOptions::open
+///
+/// It will also return an error if it encounters while reading an error
+/// of a kind other than [`ErrorKind::Interrupted`].
+///
+/// [`ErrorKind::Interrupted`]: std::io::ErrorKind::Interrupted
+///
+/// # Examples
+///
+/// ```no_run
+/// use tokio::fs;
+/// use std::net::SocketAddr;
+///
+/// #[tokio::main]
+/// async fn main() -> Result<(), Box<dyn std::error::Error + 'static>> {
+///     let contents = fs::read("address.txt").await?;
+///     let foo: SocketAddr = String::from_utf8_lossy(&contents).parse()?;
+///     Ok(())
+/// }
+/// ```
+pub async fn read(path: impl AsRef<Path>) -> io::Result<Vec<u8>> {
+    let path = path.as_ref().to_owned();
+    asyncify(move || std::fs::read(path)).await
+}
diff --git a/third_party/rust/tokio/src/fs/read_dir.rs b/third_party/rust/tokio/src/fs/read_dir.rs
new file mode 100644
index 0000000000..f9b16c66c5
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/read_dir.rs
@@ -0,0 +1,244 @@
+use crate::fs::{asyncify, sys};
+
+use std::ffi::OsString;
+use std::fs::{FileType, Metadata};
+use std::future::Future;
+use std::io;
+#[cfg(unix)]
+use std::os::unix::fs::DirEntryExt;
+use std::path::{Path, PathBuf};
+use std::pin::Pin;
+use std::sync::Arc;
+use std::task::Context;
+use std::task::Poll;
+
+/// Returns a stream over the entries within a directory.
+///
+/// This is an async version of [`std::fs::read_dir`](std::fs::read_dir)
+pub async fn read_dir(path: impl AsRef<Path>) -> io::Result<ReadDir> {
+    let path = path.as_ref().to_owned();
+    let std = asyncify(|| std::fs::read_dir(path)).await?;
+
+    Ok(ReadDir(State::Idle(Some(std))))
+}
+
+/// Stream of the entries in a directory.
+///
+/// This stream is returned from the [`read_dir`] function of this module and
+/// will yield instances of [`DirEntry`]. Through a [`DirEntry`]
+/// information like the entry's path and possibly other metadata can be
+/// learned.
+///
+/// # Errors
+///
+/// This [`Stream`] will return an [`Err`] if there's some sort of intermittent
+/// IO error during iteration.
+///
+/// [`read_dir`]: read_dir
+/// [`DirEntry`]: DirEntry
+/// [`Stream`]: crate::stream::Stream
+/// [`Err`]: std::result::Result::Err
+#[derive(Debug)]
+#[must_use = "streams do nothing unless polled"]
+pub struct ReadDir(State);
+
+#[derive(Debug)]
+enum State {
+    Idle(Option<std::fs::ReadDir>),
+    Pending(sys::Blocking<(Option<io::Result<std::fs::DirEntry>>, std::fs::ReadDir)>),
+}
+
+impl ReadDir {
+    /// Returns the next entry in the directory stream.
+    pub async fn next_entry(&mut self) -> io::Result<Option<DirEntry>> {
+        use crate::future::poll_fn;
+        poll_fn(|cx| self.poll_next_entry(cx)).await
+    }
+
+    #[doc(hidden)]
+    pub fn poll_next_entry(&mut self, cx: &mut Context<'_>) -> Poll<io::Result<Option<DirEntry>>> {
+        loop {
+            match self.0 {
+                State::Idle(ref mut std) => {
+                    let mut std = std.take().unwrap();
+
+                    self.0 = State::Pending(sys::run(move || {
+                        let ret = std.next();
+                        (ret, std)
+                    }));
+                }
+                State::Pending(ref mut rx) => {
+                    let (ret, std) = ready!(Pin::new(rx).poll(cx))?;
+                    self.0 = State::Idle(Some(std));
+
+                    let ret = match ret {
+                        Some(Ok(std)) => Ok(Some(DirEntry(Arc::new(std)))),
+                        Some(Err(e)) => Err(e),
+                        None => Ok(None),
+                    };
+
+                    return Poll::Ready(ret);
+                }
+            }
+        }
+    }
+}
+
+#[cfg(feature = "stream")]
+impl crate::stream::Stream for ReadDir {
+    type Item = io::Result<DirEntry>;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        Poll::Ready(match ready!(self.poll_next_entry(cx)) {
+            Ok(Some(entry)) => Some(Ok(entry)),
+            Ok(None) => None,
+            Err(err) => Some(Err(err)),
+        })
+    }
+}
+
+/// Entries returned by the [`ReadDir`] stream.
+///
+/// [`ReadDir`]: struct@ReadDir
+///
+/// This is a specialized version of [`std::fs::DirEntry`] for usage from the
+/// Tokio runtime.
+///
+/// An instance of `DirEntry` represents an entry inside of a directory on the
+/// filesystem. Each entry can be inspected via methods to learn about the full
+/// path or possibly other metadata through per-platform extension traits.
+#[derive(Debug)]
+pub struct DirEntry(Arc<std::fs::DirEntry>);
+
+impl DirEntry {
+    /// Returns the full path to the file that this entry represents.
+    ///
+    /// The full path is created by joining the original path to `read_dir`
+    /// with the filename of this entry.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut entries = fs::read_dir(".").await?;
+    ///
+    /// while let Some(entry) = entries.next_entry().await? {
+    ///     println!("{:?}", entry.path());
+    /// }
+    /// # Ok(())
+    /// # }
+    /// ```
+    ///
+    /// This prints output like:
+    ///
+    /// ```text
+    /// "./whatever.txt"
+    /// "./foo.html"
+    /// "./hello_world.rs"
+    /// ```
+    ///
+    /// The exact text, of course, depends on what files you have in `.`.
+    pub fn path(&self) -> PathBuf {
+        self.0.path()
+    }
+
+    /// Returns the bare file name of this directory entry without any other
+    /// leading path component.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::fs;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut entries = fs::read_dir(".").await?;
+    ///
+    /// while let Some(entry) = entries.next_entry().await? {
+    ///     println!("{:?}", entry.file_name());
+    /// }
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn file_name(&self) -> OsString {
+        self.0.file_name()
+    }
+
+    /// Returns the metadata for the file that this entry points at.
+    ///
+    /// This function will not traverse symlinks if this entry points at a
+    /// symlink.
+    ///
+    /// # Platform-specific behavior
+    ///
+    /// On Windows this function is cheap to call (no extra system calls
+    /// needed), but on Unix platforms this function is the equivalent of
+    /// calling `symlink_metadata` on the path.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::fs;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut entries = fs::read_dir(".").await?;
+    ///
+    /// while let Some(entry) = entries.next_entry().await? {
+    ///     if let Ok(metadata) = entry.metadata().await {
+    ///         // Now let's show our entry's permissions!
+    ///         println!("{:?}: {:?}", entry.path(), metadata.permissions());
+    ///     } else {
+    ///         println!("Couldn't get file type for {:?}", entry.path());
+    ///     }
+    /// }
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn metadata(&self) -> io::Result<Metadata> {
+        let std = self.0.clone();
+        asyncify(move || std.metadata()).await
+    }
+
+    /// Returns the file type for the file that this entry points at.
+    ///
+    /// This function will not traverse symlinks if this entry points at a
+    /// symlink.
+    ///
+    /// # Platform-specific behavior
+    ///
+    /// On Windows and most Unix platforms this function is free (no extra
+    /// system calls needed), but some Unix platforms may require the equivalent
+    /// call to `symlink_metadata` to learn about the target file type.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::fs;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut entries = fs::read_dir(".").await?;
+    ///
+    /// while let Some(entry) = entries.next_entry().await? {
+    ///     if let Ok(file_type) = entry.file_type().await {
+    ///         // Now let's show our entry's file type!
+    ///         println!("{:?}: {:?}", entry.path(), file_type);
+    ///     } else {
+    ///         println!("Couldn't get file type for {:?}", entry.path());
+    ///     }
+    /// }
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn file_type(&self) -> io::Result<FileType> {
+        let std = self.0.clone();
+        asyncify(move || std.file_type()).await
+    }
+}
+
+#[cfg(unix)]
+impl DirEntryExt for DirEntry {
+    fn ino(&self) -> u64 {
+        self.0.ino()
+    }
+}
diff --git a/third_party/rust/tokio/src/fs/read_link.rs b/third_party/rust/tokio/src/fs/read_link.rs
new file mode 100644
index 0000000000..6c48c5e156
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/read_link.rs
@@ -0,0 +1,14 @@
+use crate::fs::asyncify;
+
+use std::io;
+use std::path::{Path, PathBuf};
+
+/// Reads a symbolic link, returning the file that the link points to.
+///
+/// This is an async version of [`std::fs::read_link`][std]
+///
+/// [std]: std::fs::read_link
+pub async fn read_link(path: impl AsRef<Path>) -> io::Result<PathBuf> {
+    let path = path.as_ref().to_owned();
+    asyncify(move || std::fs::read_link(path)).await
+}
diff --git a/third_party/rust/tokio/src/fs/read_to_string.rs b/third_party/rust/tokio/src/fs/read_to_string.rs
new file mode 100644
index 0000000000..c743bb4ddc
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/read_to_string.rs
@@ -0,0 +1,24 @@
+use crate::fs::asyncify;
+
+use std::{io, path::Path};
+
+/// Creates a future which will open a file for reading and read the entire
+/// contents into a string and return said string.
+///
+/// This is the async equivalent of `std::fs::read_to_string`.
+///
+/// # Examples
+///
+/// ```no_run
+/// use tokio::fs;
+///
+/// # async fn dox() -> std::io::Result<()> {
+/// let contents = fs::read_to_string("foo.txt").await?;
+/// println!("foo.txt contains {} bytes", contents.len());
+/// # Ok(())
+/// # }
+/// ```
+pub async fn read_to_string(path: impl AsRef<Path>) -> io::Result<String> {
+    let path = path.as_ref().to_owned();
+    asyncify(move || std::fs::read_to_string(path)).await
+}
diff --git a/third_party/rust/tokio/src/fs/remove_dir.rs b/third_party/rust/tokio/src/fs/remove_dir.rs
new file mode 100644
index 0000000000..6e7cbd08f6
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/remove_dir.rs
@@ -0,0 +1,12 @@
+use crate::fs::asyncify;
+
+use std::io;
+use std::path::Path;
+
+/// Removes an existing, empty directory.
+///
+/// This is an async version of [`std::fs::remove_dir`](std::fs::remove_dir)
+pub async fn remove_dir(path: impl AsRef<Path>) -> io::Result<()> {
+    let path = path.as_ref().to_owned();
+    asyncify(move || std::fs::remove_dir(path)).await
+}
diff --git a/third_party/rust/tokio/src/fs/remove_dir_all.rs b/third_party/rust/tokio/src/fs/remove_dir_all.rs
new file mode 100644
index 0000000000..3b2b2e0453
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/remove_dir_all.rs
@@ -0,0 +1,14 @@
+use crate::fs::asyncify;
+
+use std::io;
+use std::path::Path;
+
+/// Removes a directory at this path, after removing all its contents. Use carefully!
+///
+/// This is an async version of [`std::fs::remove_dir_all`][std]
+///
+/// [std]: https://doc.rust-lang.org/std/fs/fn.remove_dir_all.html
+pub async fn remove_dir_all(path: impl AsRef<Path>) -> io::Result<()> {
+    let path = path.as_ref().to_owned();
+    asyncify(move || std::fs::remove_dir_all(path)).await
+}
diff --git a/third_party/rust/tokio/src/fs/remove_file.rs b/third_party/rust/tokio/src/fs/remove_file.rs
new file mode 100644
index 0000000000..d22a5bfc88
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/remove_file.rs
@@ -0,0 +1,18 @@
+use crate::fs::asyncify;
+
+use std::io;
+use std::path::Path;
+
+/// Removes a file from the filesystem.
+///
+/// Note that there is no guarantee that the file is immediately deleted (e.g.
+/// depending on platform, other open file descriptors may prevent immediate
+/// removal).
+///
+/// This is an async version of [`std::fs::remove_file`][std]
+///
+/// [std]: std::fs::remove_file
+pub async fn remove_file(path: impl AsRef<Path>) -> io::Result<()> {
+    let path = path.as_ref().to_owned();
+    asyncify(move || std::fs::remove_file(path)).await
+}
diff --git a/third_party/rust/tokio/src/fs/rename.rs b/third_party/rust/tokio/src/fs/rename.rs
new file mode 100644
index 0000000000..4f980821d2
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/rename.rs
@@ -0,0 +1,17 @@
+use crate::fs::asyncify;
+
+use std::io;
+use std::path::Path;
+
+/// Renames a file or directory to a new name, replacing the original file if
+/// `to` already exists.
+///
+/// This will not work if the new name is on a different mount point.
+///
+/// This is an async version of [`std::fs::rename`](std::fs::rename)
+pub async fn rename(from: impl AsRef<Path>, to: impl AsRef<Path>) -> io::Result<()> {
+    let from = from.as_ref().to_owned();
+    let to = to.as_ref().to_owned();
+
+    asyncify(move || std::fs::rename(from, to)).await
+}
diff --git a/third_party/rust/tokio/src/fs/set_permissions.rs b/third_party/rust/tokio/src/fs/set_permissions.rs
new file mode 100644
index 0000000000..b6249d13f0
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/set_permissions.rs
@@ -0,0 +1,15 @@
+use crate::fs::asyncify;
+
+use std::fs::Permissions;
+use std::io;
+use std::path::Path;
+
+/// Changes the permissions found on a file or a directory.
+///
+/// This is an async version of [`std::fs::set_permissions`][std]
+///
+/// [std]: https://doc.rust-lang.org/std/fs/fn.set_permissions.html
+pub async fn set_permissions(path: impl AsRef<Path>, perm: Permissions) -> io::Result<()> {
+    let path = path.as_ref().to_owned();
+    asyncify(|| std::fs::set_permissions(path, perm)).await
+}
diff --git a/third_party/rust/tokio/src/fs/symlink_metadata.rs b/third_party/rust/tokio/src/fs/symlink_metadata.rs
new file mode 100644
index 0000000000..682b43a70e
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/symlink_metadata.rs
@@ -0,0 +1,15 @@
+use crate::fs::asyncify;
+
+use std::fs::Metadata;
+use std::io;
+use std::path::Path;
+
+/// Queries the file system metadata for a path.
+///
+/// This is an async version of [`std::fs::symlink_metadata`][std]
+///
+/// [std]: https://doc.rust-lang.org/std/fs/fn.symlink_metadata.html
+pub async fn symlink_metadata(path: impl AsRef<Path>) -> io::Result<Metadata> {
+    let path = path.as_ref().to_owned();
+    asyncify(|| std::fs::symlink_metadata(path)).await
+}
diff --git a/third_party/rust/tokio/src/fs/write.rs b/third_party/rust/tokio/src/fs/write.rs
new file mode 100644
index 0000000000..0114cab8a8
--- /dev/null
+++ b/third_party/rust/tokio/src/fs/write.rs
@@ -0,0 +1,25 @@
+use crate::fs::asyncify;
+
+use std::{io, path::Path};
+
+/// Creates a future that will open a file for writing and write the entire
+/// contents of `contents` to it.
+///
+/// This is the async equivalent of `std::fs::write`.
+///
+/// # Examples
+///
+/// ```no_run
+/// use tokio::fs;
+///
+/// # async fn dox() -> std::io::Result<()> {
+/// fs::write("foo.txt", b"Hello world!").await?;
+/// # Ok(())
+/// # }
+/// ```
+pub async fn write<C: AsRef<[u8]> + Unpin>(path: impl AsRef<Path>, contents: C) -> io::Result<()> {
+    let path = path.as_ref().to_owned();
+    let contents = contents.as_ref().to_owned();
+
+    asyncify(move || std::fs::write(path, contents)).await
+}
diff --git a/third_party/rust/tokio/src/future/maybe_done.rs b/third_party/rust/tokio/src/future/maybe_done.rs
new file mode 100644
index 0000000000..1e083ad7fd
--- /dev/null
+++ b/third_party/rust/tokio/src/future/maybe_done.rs
@@ -0,0 +1,76 @@
+//! Definition of the MaybeDone combinator
+
+use std::future::Future;
+use std::mem;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// A future that may have completed.
+#[derive(Debug)]
+pub enum MaybeDone<Fut: Future> {
+    /// A not-yet-completed future
+    Future(Fut),
+    /// The output of the completed future
+    Done(Fut::Output),
+    /// The empty variant after the result of a [`MaybeDone`] has been
+    /// taken using the [`take_output`](MaybeDone::take_output) method.
+    Gone,
+}
+
+// Safe because we never generate `Pin<&mut Fut::Output>`
+impl<Fut: Future + Unpin> Unpin for MaybeDone<Fut> {}
+
+/// Wraps a future into a `MaybeDone`
+pub fn maybe_done<Fut: Future>(future: Fut) -> MaybeDone<Fut> {
+    MaybeDone::Future(future)
+}
+
+impl<Fut: Future> MaybeDone<Fut> {
+    /// Returns an [`Option`] containing a mutable reference to the output of the future.
+    /// The output of this method will be [`Some`] if and only if the inner
+    /// future has been completed and [`take_output`](MaybeDone::take_output)
+    /// has not yet been called.
+    pub fn output_mut(self: Pin<&mut Self>) -> Option<&mut Fut::Output> {
+        unsafe {
+            let this = self.get_unchecked_mut();
+            match this {
+                MaybeDone::Done(res) => Some(res),
+                _ => None,
+            }
+        }
+    }
+
+    /// Attempts to take the output of a `MaybeDone` without driving it
+    /// towards completion.
+    #[inline]
+    pub fn take_output(self: Pin<&mut Self>) -> Option<Fut::Output> {
+        unsafe {
+            let this = self.get_unchecked_mut();
+            match this {
+                MaybeDone::Done(_) => {}
+                MaybeDone::Future(_) | MaybeDone::Gone => return None,
+            };
+            if let MaybeDone::Done(output) = mem::replace(this, MaybeDone::Gone) {
+                Some(output)
+            } else {
+                unreachable!()
+            }
+        }
+    }
+}
+
+impl<Fut: Future> Future for MaybeDone<Fut> {
+    type Output = ();
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let res = unsafe {
+            match self.as_mut().get_unchecked_mut() {
+                MaybeDone::Future(a) => ready!(Pin::new_unchecked(a).poll(cx)),
+                MaybeDone::Done(_) => return Poll::Ready(()),
+                MaybeDone::Gone => panic!("MaybeDone polled after value taken"),
+            }
+        };
+        self.set(MaybeDone::Done(res));
+        Poll::Ready(())
+    }
+}
diff --git a/third_party/rust/tokio/src/future/mod.rs b/third_party/rust/tokio/src/future/mod.rs
new file mode 100644
index 0000000000..770753f319
--- /dev/null
+++ b/third_party/rust/tokio/src/future/mod.rs
@@ -0,0 +1,15 @@
+#![allow(unused_imports, dead_code)]
+
+//! Asynchronous values.
+
+mod maybe_done;
+pub use maybe_done::{maybe_done, MaybeDone};
+
+mod poll_fn;
+pub use poll_fn::poll_fn;
+
+mod ready;
+pub(crate) use ready::{ok, Ready};
+
+mod try_join;
+pub(crate) use try_join::try_join3;
diff --git a/third_party/rust/tokio/src/future/pending.rs b/third_party/rust/tokio/src/future/pending.rs
new file mode 100644
index 0000000000..287e836fd3
--- /dev/null
+++ b/third_party/rust/tokio/src/future/pending.rs
@@ -0,0 +1,44 @@
+use sdt::pin::Pin;
+use std::future::Future;
+use std::marker;
+use std::task::{Context, Poll};
+
+/// Future for the [`pending()`] function.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+struct Pending<T> {
+    _data: marker::PhantomData<T>,
+}
+
+/// Creates a future which never resolves, representing a computation that never
+/// finishes.
+///
+/// The returned future will forever return [`Poll::Pending`].
+///
+/// # Examples
+///
+/// ```no_run
+/// use tokio::future;
+///
+/// #[tokio::main]
+/// async fn main {
+///     future::pending().await;
+///     unreachable!();
+/// }
+/// ```
+pub async fn pending() -> ! {
+    Pending {
+        _data: marker::PhantomData,
+    }
+    .await
+}
+
+impl<T> Future for Pending<T> {
+    type Output = !;
+
+    fn poll(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<T> {
+        Poll::Pending
+    }
+}
+
+impl<T> Unpin for Pending<T> {}
diff --git a/third_party/rust/tokio/src/future/poll_fn.rs b/third_party/rust/tokio/src/future/poll_fn.rs
new file mode 100644
index 0000000000..9b3d1370ea
--- /dev/null
+++ b/third_party/rust/tokio/src/future/poll_fn.rs
@@ -0,0 +1,38 @@
+//! Definition of the `PollFn` adapter combinator
+
+use std::fmt;
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// Future for the [`poll_fn`] function.
+pub struct PollFn<F> {
+    f: F,
+}
+
+impl<F> Unpin for PollFn<F> {}
+
+/// Creates a new future wrapping around a function returning [`Poll`].
+pub fn poll_fn<T, F>(f: F) -> PollFn<F>
+where
+    F: FnMut(&mut Context<'_>) -> Poll<T>,
+{
+    PollFn { f }
+}
+
+impl<F> fmt::Debug for PollFn<F> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("PollFn").finish()
+    }
+}
+
+impl<T, F> Future for PollFn<F>
+where
+    F: FnMut(&mut Context<'_>) -> Poll<T>,
+{
+    type Output = T;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
+        (&mut self.f)(cx)
+    }
+}
diff --git a/third_party/rust/tokio/src/future/ready.rs b/third_party/rust/tokio/src/future/ready.rs
new file mode 100644
index 0000000000..d74f999e5d
--- /dev/null
+++ b/third_party/rust/tokio/src/future/ready.rs
@@ -0,0 +1,27 @@
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// Future for the [`ready`](ready()) function.
+///
+/// `pub` in order to use the future as an associated type in a sealed trait.
+#[derive(Debug)]
+// Used as an associated type in a "sealed" trait.
+#[allow(unreachable_pub)]
+pub struct Ready<T>(Option<T>);
+
+impl<T> Unpin for Ready<T> {}
+
+impl<T> Future for Ready<T> {
+    type Output = T;
+
+    #[inline]
+    fn poll(mut self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<T> {
+        Poll::Ready(self.0.take().unwrap())
+    }
+}
+
+/// Creates a future that is immediately ready with a success value.
+pub(crate) fn ok<T, E>(t: T) -> Ready<Result<T, E>> {
+    Ready(Some(Ok(t)))
+}
diff --git a/third_party/rust/tokio/src/future/try_join.rs b/third_party/rust/tokio/src/future/try_join.rs
new file mode 100644
index 0000000000..5bd80dc89a
--- /dev/null
+++ b/third_party/rust/tokio/src/future/try_join.rs
@@ -0,0 +1,82 @@
+use crate::future::{maybe_done, MaybeDone};
+
+use pin_project_lite::pin_project;
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+pub(crate) fn try_join3<T1, F1, T2, F2, T3, F3, E>(
+    future1: F1,
+    future2: F2,
+    future3: F3,
+) -> TryJoin3<F1, F2, F3>
+where
+    F1: Future<Output = Result<T1, E>>,
+    F2: Future<Output = Result<T2, E>>,
+    F3: Future<Output = Result<T3, E>>,
+{
+    TryJoin3 {
+        future1: maybe_done(future1),
+        future2: maybe_done(future2),
+        future3: maybe_done(future3),
+    }
+}
+
+pin_project! {
+    pub(crate) struct TryJoin3<F1, F2, F3>
+    where
+        F1: Future,
+        F2: Future,
+        F3: Future,
+    {
+        #[pin]
+        future1: MaybeDone<F1>,
+        #[pin]
+        future2: MaybeDone<F2>,
+        #[pin]
+        future3: MaybeDone<F3>,
+    }
+}
+
+impl<T1, F1, T2, F2, T3, F3, E> Future for TryJoin3<F1, F2, F3>
+where
+    F1: Future<Output = Result<T1, E>>,
+    F2: Future<Output = Result<T2, E>>,
+    F3: Future<Output = Result<T3, E>>,
+{
+    type Output = Result<(T1, T2, T3), E>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let mut all_done = true;
+
+        let mut me = self.project();
+
+        if me.future1.as_mut().poll(cx).is_pending() {
+            all_done = false;
+        } else if me.future1.as_mut().output_mut().unwrap().is_err() {
+            return Poll::Ready(Err(me.future1.take_output().unwrap().err().unwrap()));
+        }
+
+        if me.future2.as_mut().poll(cx).is_pending() {
+            all_done = false;
+        } else if me.future2.as_mut().output_mut().unwrap().is_err() {
+            return Poll::Ready(Err(me.future2.take_output().unwrap().err().unwrap()));
+        }
+
+        if me.future3.as_mut().poll(cx).is_pending() {
+            all_done = false;
+        } else if me.future3.as_mut().output_mut().unwrap().is_err() {
+            return Poll::Ready(Err(me.future3.take_output().unwrap().err().unwrap()));
+        }
+
+        if all_done {
+            Poll::Ready(Ok((
+                me.future1.take_output().unwrap().ok().unwrap(),
+                me.future2.take_output().unwrap().ok().unwrap(),
+                me.future3.take_output().unwrap().ok().unwrap(),
+            )))
+        } else {
+            Poll::Pending
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/io/async_buf_read.rs b/third_party/rust/tokio/src/io/async_buf_read.rs
new file mode 100644
index 0000000000..1ab73cd9b7
--- /dev/null
+++ b/third_party/rust/tokio/src/io/async_buf_read.rs
@@ -0,0 +1,115 @@
+use crate::io::AsyncRead;
+
+use std::io;
+use std::ops::DerefMut;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// Reads bytes asynchronously.
+///
+/// This trait inherits from [`std::io::BufRead`] and indicates that an I/O object is
+/// **non-blocking**. All non-blocking I/O objects must return an error when
+/// bytes are unavailable instead of blocking the current thread.
+///
+/// Utilities for working with `AsyncBufRead` values are provided by
+/// [`AsyncBufReadExt`].
+///
+/// [`std::io::BufRead`]: std::io::BufRead
+/// [`AsyncBufReadExt`]: crate::io::AsyncBufReadExt
+pub trait AsyncBufRead: AsyncRead {
+    /// Attempts to return the contents of the internal buffer, filling it with more data
+    /// from the inner reader if it is empty.
+    ///
+    /// On success, returns `Poll::Ready(Ok(buf))`.
+    ///
+    /// If no data is available for reading, the method returns
+    /// `Poll::Pending` and arranges for the current task (via
+    /// `cx.waker().wake_by_ref()`) to receive a notification when the object becomes
+    /// readable or is closed.
+    ///
+    /// This function is a lower-level call. It needs to be paired with the
+    /// [`consume`] method to function properly. When calling this
+    /// method, none of the contents will be "read" in the sense that later
+    /// calling [`poll_read`] may return the same contents. As such, [`consume`] must
+    /// be called with the number of bytes that are consumed from this buffer to
+    /// ensure that the bytes are never returned twice.
+    ///
+    /// An empty buffer returned indicates that the stream has reached EOF.
+    ///
+    /// [`poll_read`]: AsyncRead::poll_read
+    /// [`consume`]: AsyncBufRead::consume
+    fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>>;
+
+    /// Tells this buffer that `amt` bytes have been consumed from the buffer,
+    /// so they should no longer be returned in calls to [`poll_read`].
+    ///
+    /// This function is a lower-level call. It needs to be paired with the
+    /// [`poll_fill_buf`] method to function properly. This function does
+    /// not perform any I/O, it simply informs this object that some amount of
+    /// its buffer, returned from [`poll_fill_buf`], has been consumed and should
+    /// no longer be returned. As such, this function may do odd things if
+    /// [`poll_fill_buf`] isn't called before calling it.
+    ///
+    /// The `amt` must be `<=` the number of bytes in the buffer returned by
+    /// [`poll_fill_buf`].
+    ///
+    /// [`poll_read`]: AsyncRead::poll_read
+    /// [`poll_fill_buf`]: AsyncBufRead::poll_fill_buf
+    fn consume(self: Pin<&mut Self>, amt: usize);
+}
+
+macro_rules! deref_async_buf_read {
+    () => {
+        fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>)
+            -> Poll<io::Result<&[u8]>>
+        {
+            Pin::new(&mut **self.get_mut()).poll_fill_buf(cx)
+        }
+
+        fn consume(mut self: Pin<&mut Self>, amt: usize) {
+            Pin::new(&mut **self).consume(amt)
+        }
+    }
+}
+
+impl<T: ?Sized + AsyncBufRead + Unpin> AsyncBufRead for Box<T> {
+    deref_async_buf_read!();
+}
+
+impl<T: ?Sized + AsyncBufRead + Unpin> AsyncBufRead for &mut T {
+    deref_async_buf_read!();
+}
+
+impl<P> AsyncBufRead for Pin<P>
+where
+    P: DerefMut + Unpin,
+    P::Target: AsyncBufRead,
+{
+    fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
+        self.get_mut().as_mut().poll_fill_buf(cx)
+    }
+
+    fn consume(self: Pin<&mut Self>, amt: usize) {
+        self.get_mut().as_mut().consume(amt)
+    }
+}
+
+impl AsyncBufRead for &[u8] {
+    fn poll_fill_buf(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
+        Poll::Ready(Ok(*self))
+    }
+
+    fn consume(mut self: Pin<&mut Self>, amt: usize) {
+        *self = &self[amt..];
+    }
+}
+
+impl<T: AsRef<[u8]> + Unpin> AsyncBufRead for io::Cursor<T> {
+    fn poll_fill_buf(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
+        Poll::Ready(io::BufRead::fill_buf(self.get_mut()))
+    }
+
+    fn consume(self: Pin<&mut Self>, amt: usize) {
+        io::BufRead::consume(self.get_mut(), amt)
+    }
+}
diff --git a/third_party/rust/tokio/src/io/async_read.rs b/third_party/rust/tokio/src/io/async_read.rs
new file mode 100644
index 0000000000..de08d65810
--- /dev/null
+++ b/third_party/rust/tokio/src/io/async_read.rs
@@ -0,0 +1,203 @@
+use bytes::BufMut;
+use std::io;
+use std::mem::MaybeUninit;
+use std::ops::DerefMut;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// Reads bytes from a source.
+///
+/// This trait is analogous to the [`std::io::Read`] trait, but integrates with
+/// the asynchronous task system. In particular, the [`poll_read`] method,
+/// unlike [`Read::read`], will automatically queue the current task for wakeup
+/// and return if data is not yet available, rather than blocking the calling
+/// thread.
+///
+/// Specifically, this means that the `poll_read` function will return one of
+/// the following:
+///
+/// * `Poll::Ready(Ok(n))` means that `n` bytes of data was immediately read
+///   and placed into the output buffer, where `n` == 0 implies that EOF has
+///   been reached.
+///
+/// * `Poll::Pending` means that no data was read into the buffer
+///   provided. The I/O object is not currently readable but may become readable
+///   in the future. Most importantly, **the current future's task is scheduled
+///   to get unparked when the object is readable**. This means that like
+///   `Future::poll` you'll receive a notification when the I/O object is
+///   readable again.
+///
+/// * `Poll::Ready(Err(e))` for other errors are standard I/O errors coming from the
+///   underlying object.
+///
+/// This trait importantly means that the `read` method only works in the
+/// context of a future's task. The object may panic if used outside of a task.
+///
+/// Utilities for working with `AsyncRead` values are provided by
+/// [`AsyncReadExt`].
+///
+/// [`poll_read`]: AsyncRead::poll_read
+/// [`std::io::Read`]: std::io::Read
+/// [`Read::read`]: std::io::Read::read
+/// [`AsyncReadExt`]: crate::io::AsyncReadExt
+pub trait AsyncRead {
+    /// Prepares an uninitialized buffer to be safe to pass to `read`. Returns
+    /// `true` if the supplied buffer was zeroed out.
+    ///
+    /// While it would be highly unusual, implementations of [`io::Read`] are
+    /// able to read data from the buffer passed as an argument. Because of
+    /// this, the buffer passed to [`io::Read`] must be initialized memory. In
+    /// situations where large numbers of buffers are used, constantly having to
+    /// zero out buffers can be expensive.
+    ///
+    /// This function does any necessary work to prepare an uninitialized buffer
+    /// to be safe to pass to `read`. If `read` guarantees to never attempt to
+    /// read data out of the supplied buffer, then `prepare_uninitialized_buffer`
+    /// doesn't need to do any work.
+    ///
+    /// If this function returns `true`, then the memory has been zeroed out.
+    /// This allows implementations of `AsyncRead` which are composed of
+    /// multiple subimplementations to efficiently implement
+    /// `prepare_uninitialized_buffer`.
+    ///
+    /// This function isn't actually `unsafe` to call but `unsafe` to implement.
+    /// The implementer must ensure that either the whole `buf` has been zeroed
+    /// or `poll_read_buf()` overwrites the buffer without reading it and returns
+    /// correct value.
+    ///
+    /// This function is called from [`poll_read_buf`].
+    ///
+    /// # Safety
+    ///
+    /// Implementations that return `false` must never read from data slices
+    /// that they did not write to.
+    ///
+    /// [`io::Read`]: std::io::Read
+    /// [`poll_read_buf`]: #method.poll_read_buf
+    unsafe fn prepare_uninitialized_buffer(&self, buf: &mut [MaybeUninit<u8>]) -> bool {
+        for x in buf {
+            *x.as_mut_ptr() = 0;
+        }
+
+        true
+    }
+
+    /// Attempts to read from the `AsyncRead` into `buf`.
+    ///
+    /// On success, returns `Poll::Ready(Ok(num_bytes_read))`.
+    ///
+    /// If no data is available for reading, the method returns
+    /// `Poll::Pending` and arranges for the current task (via
+    /// `cx.waker()`) to receive a notification when the object becomes
+    /// readable or is closed.
+    fn poll_read(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>>;
+
+    /// Pulls some bytes from this source into the specified `BufMut`, returning
+    /// how many bytes were read.
+    ///
+    /// The `buf` provided will have bytes read into it and the internal cursor
+    /// will be advanced if any bytes were read. Note that this method typically
+    /// will not reallocate the buffer provided.
+    fn poll_read_buf<B: BufMut>(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut B,
+    ) -> Poll<io::Result<usize>>
+    where
+        Self: Sized,
+    {
+        if !buf.has_remaining_mut() {
+            return Poll::Ready(Ok(0));
+        }
+
+        unsafe {
+            let n = {
+                let b = buf.bytes_mut();
+
+                self.prepare_uninitialized_buffer(b);
+
+                // Convert to `&mut [u8]`
+                let b = &mut *(b as *mut [MaybeUninit<u8>] as *mut [u8]);
+
+                let n = ready!(self.poll_read(cx, b))?;
+                assert!(n <= b.len(), "Bad AsyncRead implementation, more bytes were reported as read than the buffer can hold");
+                n
+            };
+
+            buf.advance_mut(n);
+            Poll::Ready(Ok(n))
+        }
+    }
+}
+
+macro_rules! deref_async_read {
+    () => {
+        unsafe fn prepare_uninitialized_buffer(&self, buf: &mut [MaybeUninit<u8>]) -> bool {
+            (**self).prepare_uninitialized_buffer(buf)
+        }
+
+        fn poll_read(mut self: Pin<&mut Self>, cx: &mut Context<'_>, buf: &mut [u8])
+            -> Poll<io::Result<usize>>
+        {
+            Pin::new(&mut **self).poll_read(cx, buf)
+        }
+    }
+}
+
+impl<T: ?Sized + AsyncRead + Unpin> AsyncRead for Box<T> {
+    deref_async_read!();
+}
+
+impl<T: ?Sized + AsyncRead + Unpin> AsyncRead for &mut T {
+    deref_async_read!();
+}
+
+impl<P> AsyncRead for Pin<P>
+where
+    P: DerefMut + Unpin,
+    P::Target: AsyncRead,
+{
+    unsafe fn prepare_uninitialized_buffer(&self, buf: &mut [MaybeUninit<u8>]) -> bool {
+        (**self).prepare_uninitialized_buffer(buf)
+    }
+
+    fn poll_read(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        self.get_mut().as_mut().poll_read(cx, buf)
+    }
+}
+
+impl AsyncRead for &[u8] {
+    unsafe fn prepare_uninitialized_buffer(&self, _buf: &mut [MaybeUninit<u8>]) -> bool {
+        false
+    }
+
+    fn poll_read(
+        self: Pin<&mut Self>,
+        _cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        Poll::Ready(io::Read::read(self.get_mut(), buf))
+    }
+}
+
+impl<T: AsRef<[u8]> + Unpin> AsyncRead for io::Cursor<T> {
+    unsafe fn prepare_uninitialized_buffer(&self, _buf: &mut [MaybeUninit<u8>]) -> bool {
+        false
+    }
+
+    fn poll_read(
+        self: Pin<&mut Self>,
+        _cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        Poll::Ready(io::Read::read(self.get_mut(), buf))
+    }
+}
diff --git a/third_party/rust/tokio/src/io/async_seek.rs b/third_party/rust/tokio/src/io/async_seek.rs
new file mode 100644
index 0000000000..0be9c90d56
--- /dev/null
+++ b/third_party/rust/tokio/src/io/async_seek.rs
@@ -0,0 +1,104 @@
+use std::io::{self, SeekFrom};
+use std::ops::DerefMut;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// Seek bytes asynchronously.
+///
+/// This trait is analogous to the [`std::io::Seek`] trait, but integrates
+/// with the asynchronous task system. In particular, the `start_seek`
+/// method, unlike [`Seek::seek`], will not block the calling thread.
+///
+/// Utilities for working with `AsyncSeek` values are provided by
+/// [`AsyncSeekExt`].
+///
+/// [`std::io::Seek`]: std::io::Seek
+/// [`Seek::seek`]: std::io::Seek::seek()
+/// [`AsyncSeekExt`]: crate::io::AsyncSeekExt
+pub trait AsyncSeek {
+    /// Attempts to seek to an offset, in bytes, in a stream.
+    ///
+    /// A seek beyond the end of a stream is allowed, but behavior is defined
+    /// by the implementation.
+    ///
+    /// If this function returns successfully, then the job has been submitted.
+    /// To find out when it completes, call `poll_complete`.
+    fn start_seek(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        position: SeekFrom,
+    ) -> Poll<io::Result<()>>;
+
+    /// Waits for a seek operation to complete.
+    ///
+    /// If the seek operation completed successfully,
+    /// this method returns the new position from the start of the stream.
+    /// That position can be used later with [`SeekFrom::Start`].
+    ///
+    /// # Errors
+    ///
+    /// Seeking to a negative offset is considered an error.
+    ///
+    /// # Panics
+    ///
+    /// Calling this method without calling `start_seek` first is an error.
+    fn poll_complete(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>>;
+}
+
+macro_rules! deref_async_seek {
+    () => {
+        fn start_seek(
+            mut self: Pin<&mut Self>,
+            cx: &mut Context<'_>,
+            pos: SeekFrom,
+        ) -> Poll<io::Result<()>> {
+            Pin::new(&mut **self).start_seek(cx, pos)
+        }
+
+        fn poll_complete(
+            mut self: Pin<&mut Self>,
+            cx: &mut Context<'_>,
+        ) -> Poll<io::Result<u64>> {
+            Pin::new(&mut **self).poll_complete(cx)
+        }
+    }
+}
+
+impl<T: ?Sized + AsyncSeek + Unpin> AsyncSeek for Box<T> {
+    deref_async_seek!();
+}
+
+impl<T: ?Sized + AsyncSeek + Unpin> AsyncSeek for &mut T {
+    deref_async_seek!();
+}
+
+impl<P> AsyncSeek for Pin<P>
+where
+    P: DerefMut + Unpin,
+    P::Target: AsyncSeek,
+{
+    fn start_seek(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        pos: SeekFrom,
+    ) -> Poll<io::Result<()>> {
+        self.get_mut().as_mut().start_seek(cx, pos)
+    }
+
+    fn poll_complete(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>> {
+        self.get_mut().as_mut().poll_complete(cx)
+    }
+}
+
+impl<T: AsRef<[u8]> + Unpin> AsyncSeek for io::Cursor<T> {
+    fn start_seek(
+        mut self: Pin<&mut Self>,
+        _: &mut Context<'_>,
+        pos: SeekFrom,
+    ) -> Poll<io::Result<()>> {
+        Poll::Ready(io::Seek::seek(&mut *self, pos).map(drop))
+    }
+    fn poll_complete(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<u64>> {
+        Poll::Ready(Ok(self.get_mut().position()))
+    }
+}
diff --git a/third_party/rust/tokio/src/io/async_write.rs b/third_party/rust/tokio/src/io/async_write.rs
new file mode 100644
index 0000000000..0bfed056ef
--- /dev/null
+++ b/third_party/rust/tokio/src/io/async_write.rs
@@ -0,0 +1,291 @@
+use bytes::Buf;
+use std::io;
+use std::ops::DerefMut;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// Writes bytes asynchronously.
+///
+/// The trait inherits from [`std::io::Write`] and indicates that an I/O object is
+/// **nonblocking**. All non-blocking I/O objects must return an error when
+/// bytes cannot be written instead of blocking the current thread.
+///
+/// Specifically, this means that the [`poll_write`] function will return one of
+/// the following:
+///
+/// * `Poll::Ready(Ok(n))` means that `n` bytes of data was immediately
+///   written.
+///
+/// * `Poll::Pending` means that no data was written from the buffer
+///   provided. The I/O object is not currently writable but may become writable
+///   in the future. Most importantly, **the current future's task is scheduled
+///   to get unparked when the object is writable**. This means that like
+///   `Future::poll` you'll receive a notification when the I/O object is
+///   writable again.
+///
+/// * `Poll::Ready(Err(e))` for other errors are standard I/O errors coming from the
+///   underlying object.
+///
+/// This trait importantly means that the [`write`][stdwrite] method only works in
+/// the context of a future's task. The object may panic if used outside of a task.
+///
+/// Note that this trait also represents that the  [`Write::flush`][stdflush] method
+/// works very similarly to the `write` method, notably that `Ok(())` means that the
+/// writer has successfully been flushed, a "would block" error means that the
+/// current task is ready to receive a notification when flushing can make more
+/// progress, and otherwise normal errors can happen as well.
+///
+/// Utilities for working with `AsyncWrite` values are provided by
+/// [`AsyncWriteExt`].
+///
+/// [`std::io::Write`]: std::io::Write
+/// [`poll_write`]: AsyncWrite::poll_write()
+/// [stdwrite]: std::io::Write::write()
+/// [stdflush]: std::io::Write::flush()
+/// [`AsyncWriteExt`]: crate::io::AsyncWriteExt
+pub trait AsyncWrite {
+    /// Attempt to write bytes from `buf` into the object.
+    ///
+    /// On success, returns `Poll::Ready(Ok(num_bytes_written))`.
+    ///
+    /// If the object is not ready for writing, the method returns
+    /// `Poll::Pending` and arranges for the current task (via
+    /// `cx.waker()`) to receive a notification when the object becomes
+    /// readable or is closed.
+    fn poll_write(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<Result<usize, io::Error>>;
+
+    /// Attempts to flush the object, ensuring that any buffered data reach
+    /// their destination.
+    ///
+    /// On success, returns `Poll::Ready(Ok(()))`.
+    ///
+    /// If flushing cannot immediately complete, this method returns
+    /// `Poll::Pending` and arranges for the current task (via
+    /// `cx.waker()`) to receive a notification when the object can make
+    /// progress towards flushing.
+    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>>;
+
+    /// Initiates or attempts to shut down this writer, returning success when
+    /// the I/O connection has completely shut down.
+    ///
+    /// This method is intended to be used for asynchronous shutdown of I/O
+    /// connections. For example this is suitable for implementing shutdown of a
+    /// TLS connection or calling `TcpStream::shutdown` on a proxied connection.
+    /// Protocols sometimes need to flush out final pieces of data or otherwise
+    /// perform a graceful shutdown handshake, reading/writing more data as
+    /// appropriate. This method is the hook for such protocols to implement the
+    /// graceful shutdown logic.
+    ///
+    /// This `shutdown` method is required by implementers of the
+    /// `AsyncWrite` trait. Wrappers typically just want to proxy this call
+    /// through to the wrapped type, and base types will typically implement
+    /// shutdown logic here or just return `Ok(().into())`. Note that if you're
+    /// wrapping an underlying `AsyncWrite` a call to `shutdown` implies that
+    /// transitively the entire stream has been shut down. After your wrapper's
+    /// shutdown logic has been executed you should shut down the underlying
+    /// stream.
+    ///
+    /// Invocation of a `shutdown` implies an invocation of `flush`. Once this
+    /// method returns `Ready` it implies that a flush successfully happened
+    /// before the shutdown happened. That is, callers don't need to call
+    /// `flush` before calling `shutdown`. They can rely that by calling
+    /// `shutdown` any pending buffered data will be written out.
+    ///
+    /// # Return value
+    ///
+    /// This function returns a `Poll<io::Result<()>>` classified as such:
+    ///
+    /// * `Poll::Ready(Ok(()))` - indicates that the connection was
+    ///   successfully shut down and is now safe to deallocate/drop/close
+    ///   resources associated with it. This method means that the current task
+    ///   will no longer receive any notifications due to this method and the
+    ///   I/O object itself is likely no longer usable.
+    ///
+    /// * `Poll::Pending` - indicates that shutdown is initiated but could
+    ///   not complete just yet. This may mean that more I/O needs to happen to
+    ///   continue this shutdown operation. The current task is scheduled to
+    ///   receive a notification when it's otherwise ready to continue the
+    ///   shutdown operation. When woken up this method should be called again.
+    ///
+    /// * `Poll::Ready(Err(e))` - indicates a fatal error has happened with shutdown,
+    ///   indicating that the shutdown operation did not complete successfully.
+    ///   This typically means that the I/O object is no longer usable.
+    ///
+    /// # Errors
+    ///
+    /// This function can return normal I/O errors through `Err`, described
+    /// above. Additionally this method may also render the underlying
+    /// `Write::write` method no longer usable (e.g. will return errors in the
+    /// future). It's recommended that once `shutdown` is called the
+    /// `write` method is no longer called.
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if not called within the context of a future's
+    /// task.
+    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>>;
+
+    /// Writes a `Buf` into this value, returning how many bytes were written.
+    ///
+    /// Note that this method will advance the `buf` provided automatically by
+    /// the number of bytes written.
+    fn poll_write_buf<B: Buf>(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut B,
+    ) -> Poll<Result<usize, io::Error>>
+    where
+        Self: Sized,
+    {
+        if !buf.has_remaining() {
+            return Poll::Ready(Ok(0));
+        }
+
+        let n = ready!(self.poll_write(cx, buf.bytes()))?;
+        buf.advance(n);
+        Poll::Ready(Ok(n))
+    }
+}
+
+macro_rules! deref_async_write {
+    () => {
+        fn poll_write(mut self: Pin<&mut Self>, cx: &mut Context<'_>, buf: &[u8])
+            -> Poll<io::Result<usize>>
+        {
+            Pin::new(&mut **self).poll_write(cx, buf)
+        }
+
+        fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+            Pin::new(&mut **self).poll_flush(cx)
+        }
+
+        fn poll_shutdown(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+            Pin::new(&mut **self).poll_shutdown(cx)
+        }
+    }
+}
+
+impl<T: ?Sized + AsyncWrite + Unpin> AsyncWrite for Box<T> {
+    deref_async_write!();
+}
+
+impl<T: ?Sized + AsyncWrite + Unpin> AsyncWrite for &mut T {
+    deref_async_write!();
+}
+
+impl<P> AsyncWrite for Pin<P>
+where
+    P: DerefMut + Unpin,
+    P::Target: AsyncWrite,
+{
+    fn poll_write(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        self.get_mut().as_mut().poll_write(cx, buf)
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        self.get_mut().as_mut().poll_flush(cx)
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        self.get_mut().as_mut().poll_shutdown(cx)
+    }
+}
+
+impl AsyncWrite for Vec<u8> {
+    fn poll_write(
+        self: Pin<&mut Self>,
+        _cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        self.get_mut().extend_from_slice(buf);
+        Poll::Ready(Ok(buf.len()))
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        Poll::Ready(Ok(()))
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        Poll::Ready(Ok(()))
+    }
+}
+
+impl AsyncWrite for io::Cursor<&mut [u8]> {
+    fn poll_write(
+        mut self: Pin<&mut Self>,
+        _: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        Poll::Ready(io::Write::write(&mut *self, buf))
+    }
+
+    fn poll_flush(mut self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
+        Poll::Ready(io::Write::flush(&mut *self))
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        self.poll_flush(cx)
+    }
+}
+
+impl AsyncWrite for io::Cursor<&mut Vec<u8>> {
+    fn poll_write(
+        mut self: Pin<&mut Self>,
+        _: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        Poll::Ready(io::Write::write(&mut *self, buf))
+    }
+
+    fn poll_flush(mut self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
+        Poll::Ready(io::Write::flush(&mut *self))
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        self.poll_flush(cx)
+    }
+}
+
+impl AsyncWrite for io::Cursor<Vec<u8>> {
+    fn poll_write(
+        mut self: Pin<&mut Self>,
+        _: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        Poll::Ready(io::Write::write(&mut *self, buf))
+    }
+
+    fn poll_flush(mut self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
+        Poll::Ready(io::Write::flush(&mut *self))
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        self.poll_flush(cx)
+    }
+}
+
+impl AsyncWrite for io::Cursor<Box<[u8]>> {
+    fn poll_write(
+        mut self: Pin<&mut Self>,
+        _: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        Poll::Ready(io::Write::write(&mut *self, buf))
+    }
+
+    fn poll_flush(mut self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
+        Poll::Ready(io::Write::flush(&mut *self))
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        self.poll_flush(cx)
+    }
+}
diff --git a/third_party/rust/tokio/src/io/blocking.rs b/third_party/rust/tokio/src/io/blocking.rs
new file mode 100644
index 0000000000..2491039a3f
--- /dev/null
+++ b/third_party/rust/tokio/src/io/blocking.rs
@@ -0,0 +1,279 @@
+use crate::io::sys;
+use crate::io::{AsyncRead, AsyncWrite};
+
+use std::cmp;
+use std::future::Future;
+use std::io;
+use std::io::prelude::*;
+use std::pin::Pin;
+use std::task::Poll::*;
+use std::task::{Context, Poll};
+
+use self::State::*;
+
+/// `T` should not implement _both_ Read and Write.
+#[derive(Debug)]
+pub(crate) struct Blocking<T> {
+    inner: Option<T>,
+    state: State<T>,
+    /// `true` if the lower IO layer needs flushing
+    need_flush: bool,
+}
+
+#[derive(Debug)]
+pub(crate) struct Buf {
+    buf: Vec<u8>,
+    pos: usize,
+}
+
+pub(crate) const MAX_BUF: usize = 16 * 1024;
+
+#[derive(Debug)]
+enum State<T> {
+    Idle(Option<Buf>),
+    Busy(sys::Blocking<(io::Result<usize>, Buf, T)>),
+}
+
+cfg_io_std! {
+    impl<T> Blocking<T> {
+        pub(crate) fn new(inner: T) -> Blocking<T> {
+            Blocking {
+                inner: Some(inner),
+                state: State::Idle(Some(Buf::with_capacity(0))),
+                need_flush: false,
+            }
+        }
+    }
+}
+
+impl<T> AsyncRead for Blocking<T>
+where
+    T: Read + Unpin + Send + 'static,
+{
+    fn poll_read(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        dst: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        loop {
+            match self.state {
+                Idle(ref mut buf_cell) => {
+                    let mut buf = buf_cell.take().unwrap();
+
+                    if !buf.is_empty() {
+                        let n = buf.copy_to(dst);
+                        *buf_cell = Some(buf);
+                        return Ready(Ok(n));
+                    }
+
+                    buf.ensure_capacity_for(dst);
+                    let mut inner = self.inner.take().unwrap();
+
+                    self.state = Busy(sys::run(move || {
+                        let res = buf.read_from(&mut inner);
+                        (res, buf, inner)
+                    }));
+                }
+                Busy(ref mut rx) => {
+                    let (res, mut buf, inner) = ready!(Pin::new(rx).poll(cx))?;
+                    self.inner = Some(inner);
+
+                    match res {
+                        Ok(_) => {
+                            let n = buf.copy_to(dst);
+                            self.state = Idle(Some(buf));
+                            return Ready(Ok(n));
+                        }
+                        Err(e) => {
+                            assert!(buf.is_empty());
+
+                            self.state = Idle(Some(buf));
+                            return Ready(Err(e));
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+
+impl<T> AsyncWrite for Blocking<T>
+where
+    T: Write + Unpin + Send + 'static,
+{
+    fn poll_write(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        src: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        loop {
+            match self.state {
+                Idle(ref mut buf_cell) => {
+                    let mut buf = buf_cell.take().unwrap();
+
+                    assert!(buf.is_empty());
+
+                    let n = buf.copy_from(src);
+                    let mut inner = self.inner.take().unwrap();
+
+                    self.state = Busy(sys::run(move || {
+                        let n = buf.len();
+                        let res = buf.write_to(&mut inner).map(|_| n);
+
+                        (res, buf, inner)
+                    }));
+                    self.need_flush = true;
+
+                    return Ready(Ok(n));
+                }
+                Busy(ref mut rx) => {
+                    let (res, buf, inner) = ready!(Pin::new(rx).poll(cx))?;
+                    self.state = Idle(Some(buf));
+                    self.inner = Some(inner);
+
+                    // If error, return
+                    res?;
+                }
+            }
+        }
+    }
+
+    fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
+        loop {
+            let need_flush = self.need_flush;
+            match self.state {
+                // The buffer is not used here
+                Idle(ref mut buf_cell) => {
+                    if need_flush {
+                        let buf = buf_cell.take().unwrap();
+                        let mut inner = self.inner.take().unwrap();
+
+                        self.state = Busy(sys::run(move || {
+                            let res = inner.flush().map(|_| 0);
+                            (res, buf, inner)
+                        }));
+
+                        self.need_flush = false;
+                    } else {
+                        return Ready(Ok(()));
+                    }
+                }
+                Busy(ref mut rx) => {
+                    let (res, buf, inner) = ready!(Pin::new(rx).poll(cx))?;
+                    self.state = Idle(Some(buf));
+                    self.inner = Some(inner);
+
+                    // If error, return
+                    res?;
+                }
+            }
+        }
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
+        Poll::Ready(Ok(()))
+    }
+}
+
+/// Repeates operations that are interrupted
+macro_rules! uninterruptibly {
+    ($e:expr) => {{
+        loop {
+            match $e {
+                Err(ref e) if e.kind() == io::ErrorKind::Interrupted => {}
+                res => break res,
+            }
+        }
+    }};
+}
+
+impl Buf {
+    pub(crate) fn with_capacity(n: usize) -> Buf {
+        Buf {
+            buf: Vec::with_capacity(n),
+            pos: 0,
+        }
+    }
+
+    pub(crate) fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    pub(crate) fn len(&self) -> usize {
+        self.buf.len() - self.pos
+    }
+
+    pub(crate) fn copy_to(&mut self, dst: &mut [u8]) -> usize {
+        let n = cmp::min(self.len(), dst.len());
+        dst[..n].copy_from_slice(&self.bytes()[..n]);
+        self.pos += n;
+
+        if self.pos == self.buf.len() {
+            self.buf.truncate(0);
+            self.pos = 0;
+        }
+
+        n
+    }
+
+    pub(crate) fn copy_from(&mut self, src: &[u8]) -> usize {
+        assert!(self.is_empty());
+
+        let n = cmp::min(src.len(), MAX_BUF);
+
+        self.buf.extend_from_slice(&src[..n]);
+        n
+    }
+
+    pub(crate) fn bytes(&self) -> &[u8] {
+        &self.buf[self.pos..]
+    }
+
+    pub(crate) fn ensure_capacity_for(&mut self, bytes: &[u8]) {
+        assert!(self.is_empty());
+
+        let len = cmp::min(bytes.len(), MAX_BUF);
+
+        if self.buf.len() < len {
+            self.buf.reserve(len - self.buf.len());
+        }
+
+        unsafe {
+            self.buf.set_len(len);
+        }
+    }
+
+    pub(crate) fn read_from<T: Read>(&mut self, rd: &mut T) -> io::Result<usize> {
+        let res = uninterruptibly!(rd.read(&mut self.buf));
+
+        if let Ok(n) = res {
+            self.buf.truncate(n);
+        } else {
+            self.buf.clear();
+        }
+
+        assert_eq!(self.pos, 0);
+
+        res
+    }
+
+    pub(crate) fn write_to<T: Write>(&mut self, wr: &mut T) -> io::Result<()> {
+        assert_eq!(self.pos, 0);
+
+        // `write_all` already ignores interrupts
+        let res = wr.write_all(&self.buf);
+        self.buf.clear();
+        res
+    }
+}
+
+cfg_fs! {
+    impl Buf {
+        pub(crate) fn discard_read(&mut self) -> i64 {
+            let ret = -(self.bytes().len() as i64);
+            self.pos = 0;
+            self.buf.truncate(0);
+            ret
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/io/driver/mod.rs b/third_party/rust/tokio/src/io/driver/mod.rs
new file mode 100644
index 0000000000..d8535d9ab2
--- /dev/null
+++ b/third_party/rust/tokio/src/io/driver/mod.rs
@@ -0,0 +1,396 @@
+pub(crate) mod platform;
+
+mod scheduled_io;
+pub(crate) use scheduled_io::ScheduledIo; // pub(crate) for tests
+
+use crate::loom::sync::atomic::AtomicUsize;
+use crate::park::{Park, Unpark};
+use crate::runtime::context;
+use crate::util::slab::{Address, Slab};
+
+use mio::event::Evented;
+use std::fmt;
+use std::io;
+use std::sync::atomic::Ordering::SeqCst;
+use std::sync::{Arc, Weak};
+use std::task::Waker;
+use std::time::Duration;
+
+/// I/O driver, backed by Mio
+pub(crate) struct Driver {
+    /// Reuse the `mio::Events` value across calls to poll.
+    events: mio::Events,
+
+    /// State shared between the reactor and the handles.
+    inner: Arc<Inner>,
+
+    _wakeup_registration: mio::Registration,
+}
+
+/// A reference to an I/O driver
+#[derive(Clone)]
+pub(crate) struct Handle {
+    inner: Weak<Inner>,
+}
+
+pub(super) struct Inner {
+    /// The underlying system event queue.
+    io: mio::Poll,
+
+    /// Dispatch slabs for I/O and futures events
+    pub(super) io_dispatch: Slab<ScheduledIo>,
+
+    /// The number of sources in `io_dispatch`.
+    n_sources: AtomicUsize,
+
+    /// Used to wake up the reactor from a call to `turn`
+    wakeup: mio::SetReadiness,
+}
+
+#[derive(Debug, Eq, PartialEq, Clone, Copy)]
+pub(super) enum Direction {
+    Read,
+    Write,
+}
+
+const TOKEN_WAKEUP: mio::Token = mio::Token(Address::NULL);
+
+fn _assert_kinds() {
+    fn _assert<T: Send + Sync>() {}
+
+    _assert::<Handle>();
+}
+
+// ===== impl Driver =====
+
+impl Driver {
+    /// Creates a new event loop, returning any error that happened during the
+    /// creation.
+    pub(crate) fn new() -> io::Result<Driver> {
+        let io = mio::Poll::new()?;
+        let wakeup_pair = mio::Registration::new2();
+
+        io.register(
+            &wakeup_pair.0,
+            TOKEN_WAKEUP,
+            mio::Ready::readable(),
+            mio::PollOpt::level(),
+        )?;
+
+        Ok(Driver {
+            events: mio::Events::with_capacity(1024),
+            _wakeup_registration: wakeup_pair.0,
+            inner: Arc::new(Inner {
+                io,
+                io_dispatch: Slab::new(),
+                n_sources: AtomicUsize::new(0),
+                wakeup: wakeup_pair.1,
+            }),
+        })
+    }
+
+    /// Returns a handle to this event loop which can be sent across threads
+    /// and can be used as a proxy to the event loop itself.
+    ///
+    /// Handles are cloneable and clones always refer to the same event loop.
+    /// This handle is typically passed into functions that create I/O objects
+    /// to bind them to this event loop.
+    pub(crate) fn handle(&self) -> Handle {
+        Handle {
+            inner: Arc::downgrade(&self.inner),
+        }
+    }
+
+    fn turn(&mut self, max_wait: Option<Duration>) -> io::Result<()> {
+        // Block waiting for an event to happen, peeling out how many events
+        // happened.
+        match self.inner.io.poll(&mut self.events, max_wait) {
+            Ok(_) => {}
+            Err(e) => return Err(e),
+        }
+
+        // Process all the events that came in, dispatching appropriately
+
+        for event in self.events.iter() {
+            let token = event.token();
+
+            if token == TOKEN_WAKEUP {
+                self.inner
+                    .wakeup
+                    .set_readiness(mio::Ready::empty())
+                    .unwrap();
+            } else {
+                self.dispatch(token, event.readiness());
+            }
+        }
+
+        Ok(())
+    }
+
+    fn dispatch(&self, token: mio::Token, ready: mio::Ready) {
+        let mut rd = None;
+        let mut wr = None;
+
+        let address = Address::from_usize(token.0);
+
+        let io = match self.inner.io_dispatch.get(address) {
+            Some(io) => io,
+            None => return,
+        };
+
+        if io
+            .set_readiness(address, |curr| curr | ready.as_usize())
+            .is_err()
+        {
+            // token no longer valid!
+            return;
+        }
+
+        if ready.is_writable() || platform::is_hup(ready) || platform::is_error(ready) {
+            wr = io.writer.take_waker();
+        }
+
+        if !(ready & (!mio::Ready::writable())).is_empty() {
+            rd = io.reader.take_waker();
+        }
+
+        if let Some(w) = rd {
+            w.wake();
+        }
+
+        if let Some(w) = wr {
+            w.wake();
+        }
+    }
+}
+
+impl Park for Driver {
+    type Unpark = Handle;
+    type Error = io::Error;
+
+    fn unpark(&self) -> Self::Unpark {
+        self.handle()
+    }
+
+    fn park(&mut self) -> io::Result<()> {
+        self.turn(None)?;
+        Ok(())
+    }
+
+    fn park_timeout(&mut self, duration: Duration) -> io::Result<()> {
+        self.turn(Some(duration))?;
+        Ok(())
+    }
+}
+
+impl fmt::Debug for Driver {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "Driver")
+    }
+}
+
+// ===== impl Handle =====
+
+impl Handle {
+    /// Returns a handle to the current reactor
+    ///
+    /// # Panics
+    ///
+    /// This function panics if there is no current reactor set.
+    pub(super) fn current() -> Self {
+        context::io_handle()
+            .expect("there is no reactor running, must be called from the context of Tokio runtime")
+    }
+
+    /// Forces a reactor blocked in a call to `turn` to wakeup, or otherwise
+    /// makes the next call to `turn` return immediately.
+    ///
+    /// This method is intended to be used in situations where a notification
+    /// needs to otherwise be sent to the main reactor. If the reactor is
+    /// currently blocked inside of `turn` then it will wake up and soon return
+    /// after this method has been called. If the reactor is not currently
+    /// blocked in `turn`, then the next call to `turn` will not block and
+    /// return immediately.
+    fn wakeup(&self) {
+        if let Some(inner) = self.inner() {
+            inner.wakeup.set_readiness(mio::Ready::readable()).unwrap();
+        }
+    }
+
+    pub(super) fn inner(&self) -> Option<Arc<Inner>> {
+        self.inner.upgrade()
+    }
+}
+
+impl Unpark for Handle {
+    fn unpark(&self) {
+        self.wakeup();
+    }
+}
+
+impl fmt::Debug for Handle {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "Handle")
+    }
+}
+
+// ===== impl Inner =====
+
+impl Inner {
+    /// Registers an I/O resource with the reactor.
+    ///
+    /// The registration token is returned.
+    pub(super) fn add_source(&self, source: &dyn Evented) -> io::Result<Address> {
+        let address = self.io_dispatch.alloc().ok_or_else(|| {
+            io::Error::new(
+                io::ErrorKind::Other,
+                "reactor at max registered I/O resources",
+            )
+        })?;
+
+        self.n_sources.fetch_add(1, SeqCst);
+
+        self.io.register(
+            source,
+            mio::Token(address.to_usize()),
+            mio::Ready::all(),
+            mio::PollOpt::edge(),
+        )?;
+
+        Ok(address)
+    }
+
+    /// Deregisters an I/O resource from the reactor.
+    pub(super) fn deregister_source(&self, source: &dyn Evented) -> io::Result<()> {
+        self.io.deregister(source)
+    }
+
+    pub(super) fn drop_source(&self, address: Address) {
+        self.io_dispatch.remove(address);
+        self.n_sources.fetch_sub(1, SeqCst);
+    }
+
+    /// Registers interest in the I/O resource associated with `token`.
+    pub(super) fn register(&self, token: Address, dir: Direction, w: Waker) {
+        let sched = self
+            .io_dispatch
+            .get(token)
+            .unwrap_or_else(|| panic!("IO resource for token {:?} does not exist!", token));
+
+        let waker = match dir {
+            Direction::Read => &sched.reader,
+            Direction::Write => &sched.writer,
+        };
+
+        waker.register(w);
+    }
+}
+
+impl Direction {
+    pub(super) fn mask(self) -> mio::Ready {
+        match self {
+            Direction::Read => {
+                // Everything except writable is signaled through read.
+                mio::Ready::all() - mio::Ready::writable()
+            }
+            Direction::Write => mio::Ready::writable() | platform::hup() | platform::error(),
+        }
+    }
+}
+
+#[cfg(all(test, loom))]
+mod tests {
+    use super::*;
+    use loom::thread;
+
+    // No-op `Evented` impl just so we can have something to pass to `add_source`.
+    struct NotEvented;
+
+    impl Evented for NotEvented {
+        fn register(
+            &self,
+            _: &mio::Poll,
+            _: mio::Token,
+            _: mio::Ready,
+            _: mio::PollOpt,
+        ) -> io::Result<()> {
+            Ok(())
+        }
+
+        fn reregister(
+            &self,
+            _: &mio::Poll,
+            _: mio::Token,
+            _: mio::Ready,
+            _: mio::PollOpt,
+        ) -> io::Result<()> {
+            Ok(())
+        }
+
+        fn deregister(&self, _: &mio::Poll) -> io::Result<()> {
+            Ok(())
+        }
+    }
+
+    #[test]
+    fn tokens_unique_when_dropped() {
+        loom::model(|| {
+            let reactor = Driver::new().unwrap();
+            let inner = reactor.inner;
+            let inner2 = inner.clone();
+
+            let token_1 = inner.add_source(&NotEvented).unwrap();
+            let thread = thread::spawn(move || {
+                inner2.drop_source(token_1);
+            });
+
+            let token_2 = inner.add_source(&NotEvented).unwrap();
+            thread.join().unwrap();
+
+            assert!(token_1 != token_2);
+        })
+    }
+
+    #[test]
+    fn tokens_unique_when_dropped_on_full_page() {
+        loom::model(|| {
+            let reactor = Driver::new().unwrap();
+            let inner = reactor.inner;
+            let inner2 = inner.clone();
+            // add sources to fill up the first page so that the dropped index
+            // may be reused.
+            for _ in 0..31 {
+                inner.add_source(&NotEvented).unwrap();
+            }
+
+            let token_1 = inner.add_source(&NotEvented).unwrap();
+            let thread = thread::spawn(move || {
+                inner2.drop_source(token_1);
+            });
+
+            let token_2 = inner.add_source(&NotEvented).unwrap();
+            thread.join().unwrap();
+
+            assert!(token_1 != token_2);
+        })
+    }
+
+    #[test]
+    fn tokens_unique_concurrent_add() {
+        loom::model(|| {
+            let reactor = Driver::new().unwrap();
+            let inner = reactor.inner;
+            let inner2 = inner.clone();
+
+            let thread = thread::spawn(move || {
+                let token_2 = inner2.add_source(&NotEvented).unwrap();
+                token_2
+            });
+
+            let token_1 = inner.add_source(&NotEvented).unwrap();
+            let token_2 = thread.join().unwrap();
+
+            assert!(token_1 != token_2);
+        })
+    }
+}
diff --git a/third_party/rust/tokio/src/io/driver/platform.rs b/third_party/rust/tokio/src/io/driver/platform.rs
new file mode 100644
index 0000000000..6b27988ce6
--- /dev/null
+++ b/third_party/rust/tokio/src/io/driver/platform.rs
@@ -0,0 +1,44 @@
+pub(crate) use self::sys::*;
+
+#[cfg(unix)]
+mod sys {
+    use mio::unix::UnixReady;
+    use mio::Ready;
+
+    pub(crate) fn hup() -> Ready {
+        UnixReady::hup().into()
+    }
+
+    pub(crate) fn is_hup(ready: Ready) -> bool {
+        UnixReady::from(ready).is_hup()
+    }
+
+    pub(crate) fn error() -> Ready {
+        UnixReady::error().into()
+    }
+
+    pub(crate) fn is_error(ready: Ready) -> bool {
+        UnixReady::from(ready).is_error()
+    }
+}
+
+#[cfg(windows)]
+mod sys {
+    use mio::Ready;
+
+    pub(crate) fn hup() -> Ready {
+        Ready::empty()
+    }
+
+    pub(crate) fn is_hup(_: Ready) -> bool {
+        false
+    }
+
+    pub(crate) fn error() -> Ready {
+        Ready::empty()
+    }
+
+    pub(crate) fn is_error(_: Ready) -> bool {
+        false
+    }
+}
diff --git a/third_party/rust/tokio/src/io/driver/scheduled_io.rs b/third_party/rust/tokio/src/io/driver/scheduled_io.rs
new file mode 100644
index 0000000000..7f6446e3f5
--- /dev/null
+++ b/third_party/rust/tokio/src/io/driver/scheduled_io.rs
@@ -0,0 +1,141 @@
+use crate::loom::future::AtomicWaker;
+use crate::loom::sync::atomic::AtomicUsize;
+use crate::util::bit;
+use crate::util::slab::{Address, Entry, Generation};
+
+use std::sync::atomic::Ordering::{AcqRel, Acquire, SeqCst};
+
+#[derive(Debug)]
+pub(crate) struct ScheduledIo {
+    readiness: AtomicUsize,
+    pub(crate) reader: AtomicWaker,
+    pub(crate) writer: AtomicWaker,
+}
+
+const PACK: bit::Pack = bit::Pack::most_significant(Generation::WIDTH);
+
+impl Entry for ScheduledIo {
+    fn generation(&self) -> Generation {
+        unpack_generation(self.readiness.load(SeqCst))
+    }
+
+    fn reset(&self, generation: Generation) -> bool {
+        let mut current = self.readiness.load(Acquire);
+
+        loop {
+            if unpack_generation(current) != generation {
+                return false;
+            }
+
+            let next = PACK.pack(generation.next().to_usize(), 0);
+
+            match self
+                .readiness
+                .compare_exchange(current, next, AcqRel, Acquire)
+            {
+                Ok(_) => break,
+                Err(actual) => current = actual,
+            }
+        }
+
+        drop(self.reader.take_waker());
+        drop(self.writer.take_waker());
+
+        true
+    }
+}
+
+impl Default for ScheduledIo {
+    fn default() -> ScheduledIo {
+        ScheduledIo {
+            readiness: AtomicUsize::new(0),
+            reader: AtomicWaker::new(),
+            writer: AtomicWaker::new(),
+        }
+    }
+}
+
+impl ScheduledIo {
+    #[cfg(all(test, loom))]
+    /// Returns the current readiness value of this `ScheduledIo`, if the
+    /// provided `token` is still a valid access.
+    ///
+    /// # Returns
+    ///
+    /// If the given token's generation no longer matches the `ScheduledIo`'s
+    /// generation, then the corresponding IO resource has been removed and
+    /// replaced with a new resource. In that case, this method returns `None`.
+    /// Otherwise, this returns the current readiness.
+    pub(crate) fn get_readiness(&self, address: Address) -> Option<usize> {
+        let ready = self.readiness.load(Acquire);
+
+        if unpack_generation(ready) != address.generation() {
+            return None;
+        }
+
+        Some(ready & !PACK.mask())
+    }
+
+    /// Sets the readiness on this `ScheduledIo` by invoking the given closure on
+    /// the current value, returning the previous readiness value.
+    ///
+    /// # Arguments
+    /// - `token`: the token for this `ScheduledIo`.
+    /// - `f`: a closure returning a new readiness value given the previous
+    ///   readiness.
+    ///
+    /// # Returns
+    ///
+    /// If the given token's generation no longer matches the `ScheduledIo`'s
+    /// generation, then the corresponding IO resource has been removed and
+    /// replaced with a new resource. In that case, this method returns `Err`.
+    /// Otherwise, this returns the previous readiness.
+    pub(crate) fn set_readiness(
+        &self,
+        address: Address,
+        f: impl Fn(usize) -> usize,
+    ) -> Result<usize, ()> {
+        let generation = address.generation();
+
+        let mut current = self.readiness.load(Acquire);
+
+        loop {
+            // Check that the generation for this access is still the current
+            // one.
+            if unpack_generation(current) != generation {
+                return Err(());
+            }
+            // Mask out the generation bits so that the modifying function
+            // doesn't see them.
+            let current_readiness = current & mio::Ready::all().as_usize();
+            let new = f(current_readiness);
+
+            debug_assert!(
+                new <= !PACK.max_value(),
+                "new readiness value would overwrite generation bits!"
+            );
+
+            match self.readiness.compare_exchange(
+                current,
+                PACK.pack(generation.to_usize(), new),
+                AcqRel,
+                Acquire,
+            ) {
+                Ok(_) => return Ok(current),
+                // we lost the race, retry!
+                Err(actual) => current = actual,
+            }
+        }
+    }
+}
+
+impl Drop for ScheduledIo {
+    fn drop(&mut self) {
+        self.writer.wake();
+        self.reader.wake();
+    }
+}
+
+fn unpack_generation(src: usize) -> Generation {
+    Generation::new(PACK.unpack(src))
+}
diff --git a/third_party/rust/tokio/src/io/mod.rs b/third_party/rust/tokio/src/io/mod.rs
new file mode 100644
index 0000000000..29d8bc5554
--- /dev/null
+++ b/third_party/rust/tokio/src/io/mod.rs
@@ -0,0 +1,229 @@
+#![cfg_attr(loom, allow(dead_code, unreachable_pub))]
+
+//! Traits, helpers, and type definitions for asynchronous I/O functionality.
+//!
+//! This module is the asynchronous version of `std::io`. Primarily, it
+//! defines two traits, [`AsyncRead`] and [`AsyncWrite`], which are asynchronous
+//! versions of the [`Read`] and [`Write`] traits in the standard library.
+//!
+//! # AsyncRead and AsyncWrite
+//!
+//! Like the standard library's [`Read`] and [`Write`] traits, [`AsyncRead`] and
+//! [`AsyncWrite`] provide the most general interface for reading and writing
+//! input and output. Unlike the standard library's traits, however, they are
+//! _asynchronous_ &mdash; meaning that reading from or writing to a `tokio::io`
+//! type will _yield_ to the Tokio scheduler when IO is not ready, rather than
+//! blocking. This allows other tasks to run while waiting on IO.
+//!
+//! Another difference is that [`AsyncRead`] and [`AsyncWrite`] only contain
+//! core methods needed to provide asynchronous reading and writing
+//! functionality. Instead, utility methods are defined in the [`AsyncReadExt`]
+//! and [`AsyncWriteExt`] extension traits. These traits are automatically
+//! implemented for all values that implement [`AsyncRead`] and [`AsyncWrite`]
+//! respectively.
+//!
+//! End users will rarely interact directly with [`AsyncRead`] and
+//! [`AsyncWrite`]. Instead, they will use the async functions defined in the
+//! extension traits. Library authors are expected to implement [`AsyncRead`]
+//! and [`AsyncWrite`] in order to provide types that behave like byte streams.
+//!
+//! Even with these differences, Tokio's [`AsyncRead`] and [`AsyncWrite`] traits
+//! can be used in almost exactly the same manner as the standard library's
+//! `Read` and `Write`. Most types in the standard library that implement `Read`
+//! and `Write` have asynchronous equivalents in `tokio` that implement
+//! `AsyncRead` and `AsyncWrite`, such as [`File`] and [`TcpStream`].
+//!
+//! For example, the standard library documentation introduces `Read` by
+//! [demonstrating][std_example] reading some bytes from a [`std::fs::File`]. We
+//! can do the same with [`tokio::fs::File`][`File`]:
+//!
+//! ```no_run
+//! use tokio::io::{self, AsyncReadExt};
+//! use tokio::fs::File;
+//!
+//! #[tokio::main]
+//! async fn main() -> io::Result<()> {
+//!     let mut f = File::open("foo.txt").await?;
+//!     let mut buffer = [0; 10];
+//!
+//!     // read up to 10 bytes
+//!     let n = f.read(&mut buffer).await?;
+//!
+//!     println!("The bytes: {:?}", &buffer[..n]);
+//!     Ok(())
+//! }
+//! ```
+//!
+//! [`File`]: crate::fs::File
+//! [`TcpStream`]: crate::net::TcpStream
+//! [`std::fs::File`]: std::fs::File
+//! [std_example]: https://doc.rust-lang.org/std/io/index.html#read-and-write
+//!
+//! ## Buffered Readers and Writers
+//!
+//! Byte-based interfaces are unwieldy and can be inefficient, as we'd need to be
+//! making near-constant calls to the operating system. To help with this,
+//! `std::io` comes with [support for _buffered_ readers and writers][stdbuf],
+//! and therefore, `tokio::io` does as well.
+//!
+//! Tokio provides an async version of the [`std::io::BufRead`] trait,
+//! [`AsyncBufRead`]; and async [`BufReader`] and [`BufWriter`] structs, which
+//! wrap readers and writers. These wrappers use a buffer, reducing the number
+//! of calls and providing nicer methods for accessing exactly what you want.
+//!
+//! For example, [`BufReader`] works with the [`AsyncBufRead`] trait to add
+//! extra methods to any async reader:
+//!
+//! ```no_run
+//! use tokio::io::{self, BufReader, AsyncBufReadExt};
+//! use tokio::fs::File;
+//!
+//! #[tokio::main]
+//! async fn main() -> io::Result<()> {
+//!     let f = File::open("foo.txt").await?;
+//!     let mut reader = BufReader::new(f);
+//!     let mut buffer = String::new();
+//!
+//!     // read a line into buffer
+//!     reader.read_line(&mut buffer).await?;
+//!
+//!     println!("{}", buffer);
+//!     Ok(())
+//! }
+//! ```
+//!
+//! [`BufWriter`] doesn't add any new ways of writing; it just buffers every call
+//! to [`write`](crate::io::AsyncWriteExt::write):
+//!
+//! ```no_run
+//! use tokio::io::{self, BufWriter, AsyncWriteExt};
+//! use tokio::fs::File;
+//!
+//! #[tokio::main]
+//! async fn main() -> io::Result<()> {
+//!     let f = File::create("foo.txt").await?;
+//!     {
+//!         let mut writer = BufWriter::new(f);
+//!
+//!         // write a byte to the buffer
+//!         writer.write(&[42u8]).await?;
+//!
+//!     } // the buffer is flushed once writer goes out of scope
+//!
+//!     Ok(())
+//! }
+//! ```
+//!
+//! [stdbuf]: https://doc.rust-lang.org/std/io/index.html#bufreader-and-bufwriter
+//! [`std::io::BufRead`]: std::io::BufRead
+//! [`AsyncBufRead`]: crate::io::AsyncBufRead
+//! [`BufReader`]: crate::io::BufReader
+//! [`BufWriter`]: crate::io::BufWriter
+//!
+//! ## Implementing AsyncRead and AsyncWrite
+//!
+//! Because they are traits, we can implement `AsyncRead` and `AsyncWrite` for
+//! our own types, as well. Note that these traits must only be implemented for
+//! non-blocking I/O types that integrate with the futures type system. In
+//! other words, these types must never block the thread, and instead the
+//! current task is notified when the I/O resource is ready.
+//!
+//! # Standard input and output
+//!
+//! Tokio provides asynchronous APIs to standard [input], [output], and [error].
+//! These APIs are very similar to the ones provided by `std`, but they also
+//! implement [`AsyncRead`] and [`AsyncWrite`].
+//!
+//! Note that the standard input / output APIs  **must** be used from the
+//! context of the Tokio runtime, as they require Tokio-specific features to
+//! function. Calling these functions outside of a Tokio runtime will panic.
+//!
+//! [input]: fn@stdin
+//! [output]: fn@stdout
+//! [error]: fn@stderr
+//!
+//! # `std` re-exports
+//!
+//! Additionally, [`Error`], [`ErrorKind`], and [`Result`] are re-exported
+//! from `std::io` for ease of use.
+//!
+//! [`AsyncRead`]: trait@AsyncRead
+//! [`AsyncWrite`]: trait@AsyncWrite
+//! [`Error`]: struct@Error
+//! [`ErrorKind`]: enum@ErrorKind
+//! [`Result`]: type@Result
+//! [`Read`]: std::io::Read
+//! [`Write`]: std::io::Write
+cfg_io_blocking! {
+    pub(crate) mod blocking;
+}
+
+mod async_buf_read;
+pub use self::async_buf_read::AsyncBufRead;
+
+mod async_read;
+pub use self::async_read::AsyncRead;
+
+mod async_seek;
+pub use self::async_seek::AsyncSeek;
+
+mod async_write;
+pub use self::async_write::AsyncWrite;
+
+cfg_io_driver! {
+    pub(crate) mod driver;
+
+    mod poll_evented;
+    pub use poll_evented::PollEvented;
+
+    mod registration;
+    pub use registration::Registration;
+}
+
+cfg_io_std! {
+    mod stderr;
+    pub use stderr::{stderr, Stderr};
+
+    mod stdin;
+    pub use stdin::{stdin, Stdin};
+
+    mod stdout;
+    pub use stdout::{stdout, Stdout};
+}
+
+cfg_io_util! {
+    mod split;
+    pub use split::{split, ReadHalf, WriteHalf};
+
+    pub(crate) mod seek;
+    pub use self::seek::Seek;
+
+    pub(crate) mod util;
+    pub use util::{
+        copy, empty, repeat, sink, AsyncBufReadExt, AsyncReadExt, AsyncSeekExt, AsyncWriteExt,
+        BufReader, BufStream, BufWriter, Copy, Empty, Lines, Repeat, Sink, Split, Take,
+    };
+
+    cfg_stream! {
+        pub use util::{stream_reader, StreamReader};
+    }
+
+    // Re-export io::Error so that users don't have to deal with conflicts when
+    // `use`ing `tokio::io` and `std::io`.
+    pub use std::io::{Error, ErrorKind, Result};
+}
+
+cfg_not_io_util! {
+    cfg_process! {
+        pub(crate) mod util;
+    }
+}
+
+cfg_io_blocking! {
+    /// Types in this module can be mocked out in tests.
+    mod sys {
+        // TODO: don't rename
+        pub(crate) use crate::runtime::spawn_blocking as run;
+        pub(crate) use crate::task::JoinHandle as Blocking;
+    }
+}
diff --git a/third_party/rust/tokio/src/io/poll_evented.rs b/third_party/rust/tokio/src/io/poll_evented.rs
new file mode 100644
index 0000000000..298e6e58cf
--- /dev/null
+++ b/third_party/rust/tokio/src/io/poll_evented.rs
@@ -0,0 +1,423 @@
+use crate::io::driver::platform;
+use crate::io::{AsyncRead, AsyncWrite, Registration};
+
+use mio::event::Evented;
+use std::fmt;
+use std::io::{self, Read, Write};
+use std::marker::Unpin;
+use std::pin::Pin;
+use std::sync::atomic::AtomicUsize;
+use std::sync::atomic::Ordering::Relaxed;
+use std::task::{Context, Poll};
+
+cfg_io_driver! {
+    /// Associates an I/O resource that implements the [`std::io::Read`] and/or
+    /// [`std::io::Write`] traits with the reactor that drives it.
+    ///
+    /// `PollEvented` uses [`Registration`] internally to take a type that
+    /// implements [`mio::Evented`] as well as [`std::io::Read`] and or
+    /// [`std::io::Write`] and associate it with a reactor that will drive it.
+    ///
+    /// Once the [`mio::Evented`] type is wrapped by `PollEvented`, it can be
+    /// used from within the future's execution model. As such, the
+    /// `PollEvented` type provides [`AsyncRead`] and [`AsyncWrite`]
+    /// implementations using the underlying I/O resource as well as readiness
+    /// events provided by the reactor.
+    ///
+    /// **Note**: While `PollEvented` is `Sync` (if the underlying I/O type is
+    /// `Sync`), the caller must ensure that there are at most two tasks that
+    /// use a `PollEvented` instance concurrently. One for reading and one for
+    /// writing. While violating this requirement is "safe" from a Rust memory
+    /// model point of view, it will result in unexpected behavior in the form
+    /// of lost notifications and tasks hanging.
+    ///
+    /// ## Readiness events
+    ///
+    /// Besides just providing [`AsyncRead`] and [`AsyncWrite`] implementations,
+    /// this type also supports access to the underlying readiness event stream.
+    /// While similar in function to what [`Registration`] provides, the
+    /// semantics are a bit different.
+    ///
+    /// Two functions are provided to access the readiness events:
+    /// [`poll_read_ready`] and [`poll_write_ready`]. These functions return the
+    /// current readiness state of the `PollEvented` instance. If
+    /// [`poll_read_ready`] indicates read readiness, immediately calling
+    /// [`poll_read_ready`] again will also indicate read readiness.
+    ///
+    /// When the operation is attempted and is unable to succeed due to the I/O
+    /// resource not being ready, the caller must call [`clear_read_ready`] or
+    /// [`clear_write_ready`]. This clears the readiness state until a new
+    /// readiness event is received.
+    ///
+    /// This allows the caller to implement additional functions. For example,
+    /// [`TcpListener`] implements poll_accept by using [`poll_read_ready`] and
+    /// [`clear_read_ready`].
+    ///
+    /// ```rust
+    /// use tokio::io::PollEvented;
+    ///
+    /// use futures::ready;
+    /// use mio::Ready;
+    /// use mio::net::{TcpStream, TcpListener};
+    /// use std::io;
+    /// use std::task::{Context, Poll};
+    ///
+    /// struct MyListener {
+    ///     poll_evented: PollEvented<TcpListener>,
+    /// }
+    ///
+    /// impl MyListener {
+    ///     pub fn poll_accept(&mut self, cx: &mut Context<'_>) -> Poll<Result<TcpStream, io::Error>> {
+    ///         let ready = Ready::readable();
+    ///
+    ///         ready!(self.poll_evented.poll_read_ready(cx, ready))?;
+    ///
+    ///         match self.poll_evented.get_ref().accept() {
+    ///             Ok((socket, _)) => Poll::Ready(Ok(socket)),
+    ///             Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
+    ///                 self.poll_evented.clear_read_ready(cx, ready)?;
+    ///                 Poll::Pending
+    ///             }
+    ///             Err(e) => Poll::Ready(Err(e)),
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// ## Platform-specific events
+    ///
+    /// `PollEvented` also allows receiving platform-specific `mio::Ready` events.
+    /// These events are included as part of the read readiness event stream. The
+    /// write readiness event stream is only for `Ready::writable()` events.
+    ///
+    /// [`std::io::Read`]: https://doc.rust-lang.org/std/io/trait.Read.html
+    /// [`std::io::Write`]: https://doc.rust-lang.org/std/io/trait.Write.html
+    /// [`AsyncRead`]: ../io/trait.AsyncRead.html
+    /// [`AsyncWrite`]: ../io/trait.AsyncWrite.html
+    /// [`mio::Evented`]: https://docs.rs/mio/0.6/mio/trait.Evented.html
+    /// [`Registration`]: struct@Registration
+    /// [`TcpListener`]: ../net/struct.TcpListener.html
+    /// [`clear_read_ready`]: #method.clear_read_ready
+    /// [`clear_write_ready`]: #method.clear_write_ready
+    /// [`poll_read_ready`]: #method.poll_read_ready
+    /// [`poll_write_ready`]: #method.poll_write_ready
+    pub struct PollEvented<E: Evented> {
+        io: Option<E>,
+        inner: Inner,
+    }
+}
+
+struct Inner {
+    registration: Registration,
+
+    /// Currently visible read readiness
+    read_readiness: AtomicUsize,
+
+    /// Currently visible write readiness
+    write_readiness: AtomicUsize,
+}
+
+// ===== impl PollEvented =====
+
+macro_rules! poll_ready {
+    ($me:expr, $mask:expr, $cache:ident, $take:ident, $poll:expr) => {{
+        // Load cached & encoded readiness.
+        let mut cached = $me.inner.$cache.load(Relaxed);
+        let mask = $mask | platform::hup() | platform::error();
+
+        // See if the current readiness matches any bits.
+        let mut ret = mio::Ready::from_usize(cached) & $mask;
+
+        if ret.is_empty() {
+            // Readiness does not match, consume the registration's readiness
+            // stream. This happens in a loop to ensure that the stream gets
+            // drained.
+            loop {
+                let ready = match $poll? {
+                    Poll::Ready(v) => v,
+                    Poll::Pending => return Poll::Pending,
+                };
+                cached |= ready.as_usize();
+
+                // Update the cache store
+                $me.inner.$cache.store(cached, Relaxed);
+
+                ret |= ready & mask;
+
+                if !ret.is_empty() {
+                    return Poll::Ready(Ok(ret));
+                }
+            }
+        } else {
+            // Check what's new with the registration stream. This will not
+            // request to be notified
+            if let Some(ready) = $me.inner.registration.$take()? {
+                cached |= ready.as_usize();
+                $me.inner.$cache.store(cached, Relaxed);
+            }
+
+            Poll::Ready(Ok(mio::Ready::from_usize(cached)))
+        }
+    }};
+}
+
+impl<E> PollEvented<E>
+where
+    E: Evented,
+{
+    /// Creates a new `PollEvented` associated with the default reactor.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if thread-local runtime is not set.
+    ///
+    /// The runtime is usually set implicitly when this function is called
+    /// from a future driven by a tokio runtime, otherwise runtime can be set
+    /// explicitly with [`Handle::enter`](crate::runtime::Handle::enter) function.
+    pub fn new(io: E) -> io::Result<Self> {
+        let registration = Registration::new(&io)?;
+        Ok(Self {
+            io: Some(io),
+            inner: Inner {
+                registration,
+                read_readiness: AtomicUsize::new(0),
+                write_readiness: AtomicUsize::new(0),
+            },
+        })
+    }
+
+    /// Returns a shared reference to the underlying I/O object this readiness
+    /// stream is wrapping.
+    pub fn get_ref(&self) -> &E {
+        self.io.as_ref().unwrap()
+    }
+
+    /// Returns a mutable reference to the underlying I/O object this readiness
+    /// stream is wrapping.
+    pub fn get_mut(&mut self) -> &mut E {
+        self.io.as_mut().unwrap()
+    }
+
+    /// Consumes self, returning the inner I/O object
+    ///
+    /// This function will deregister the I/O resource from the reactor before
+    /// returning. If the deregistration operation fails, an error is returned.
+    ///
+    /// Note that deregistering does not guarantee that the I/O resource can be
+    /// registered with a different reactor. Some I/O resource types can only be
+    /// associated with a single reactor instance for their lifetime.
+    pub fn into_inner(mut self) -> io::Result<E> {
+        let io = self.io.take().unwrap();
+        self.inner.registration.deregister(&io)?;
+        Ok(io)
+    }
+
+    /// Checks the I/O resource's read readiness state.
+    ///
+    /// The mask argument allows specifying what readiness to notify on. This
+    /// can be any value, including platform specific readiness, **except**
+    /// `writable`. HUP is always implicitly included on platforms that support
+    /// it.
+    ///
+    /// If the resource is not ready for a read then `Poll::Pending` is returned
+    /// and the current task is notified once a new event is received.
+    ///
+    /// The I/O resource will remain in a read-ready state until readiness is
+    /// cleared by calling [`clear_read_ready`].
+    ///
+    /// [`clear_read_ready`]: #method.clear_read_ready
+    ///
+    /// # Panics
+    ///
+    /// This function panics if:
+    ///
+    /// * `ready` includes writable.
+    /// * called from outside of a task context.
+    pub fn poll_read_ready(
+        &self,
+        cx: &mut Context<'_>,
+        mask: mio::Ready,
+    ) -> Poll<io::Result<mio::Ready>> {
+        assert!(!mask.is_writable(), "cannot poll for write readiness");
+        poll_ready!(
+            self,
+            mask,
+            read_readiness,
+            take_read_ready,
+            self.inner.registration.poll_read_ready(cx)
+        )
+    }
+
+    /// Clears the I/O resource's read readiness state and registers the current
+    /// task to be notified once a read readiness event is received.
+    ///
+    /// After calling this function, `poll_read_ready` will return
+    /// `Poll::Pending` until a new read readiness event has been received.
+    ///
+    /// The `mask` argument specifies the readiness bits to clear. This may not
+    /// include `writable` or `hup`.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if:
+    ///
+    /// * `ready` includes writable or HUP
+    /// * called from outside of a task context.
+    pub fn clear_read_ready(&self, cx: &mut Context<'_>, ready: mio::Ready) -> io::Result<()> {
+        // Cannot clear write readiness
+        assert!(!ready.is_writable(), "cannot clear write readiness");
+        assert!(!platform::is_hup(ready), "cannot clear HUP readiness");
+
+        self.inner
+            .read_readiness
+            .fetch_and(!ready.as_usize(), Relaxed);
+
+        if self.poll_read_ready(cx, ready)?.is_ready() {
+            // Notify the current task
+            cx.waker().wake_by_ref();
+        }
+
+        Ok(())
+    }
+
+    /// Checks the I/O resource's write readiness state.
+    ///
+    /// This always checks for writable readiness and also checks for HUP
+    /// readiness on platforms that support it.
+    ///
+    /// If the resource is not ready for a write then `Poll::Pending` is
+    /// returned and the current task is notified once a new event is received.
+    ///
+    /// The I/O resource will remain in a write-ready state until readiness is
+    /// cleared by calling [`clear_write_ready`].
+    ///
+    /// [`clear_write_ready`]: #method.clear_write_ready
+    ///
+    /// # Panics
+    ///
+    /// This function panics if:
+    ///
+    /// * `ready` contains bits besides `writable` and `hup`.
+    /// * called from outside of a task context.
+    pub fn poll_write_ready(&self, cx: &mut Context<'_>) -> Poll<io::Result<mio::Ready>> {
+        poll_ready!(
+            self,
+            mio::Ready::writable(),
+            write_readiness,
+            take_write_ready,
+            self.inner.registration.poll_write_ready(cx)
+        )
+    }
+
+    /// Resets the I/O resource's write readiness state and registers the current
+    /// task to be notified once a write readiness event is received.
+    ///
+    /// This only clears writable readiness. HUP (on platforms that support HUP)
+    /// cannot be cleared as it is a final state.
+    ///
+    /// After calling this function, `poll_write_ready(Ready::writable())` will
+    /// return `NotReady` until a new write readiness event has been received.
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if called from outside of a task context.
+    pub fn clear_write_ready(&self, cx: &mut Context<'_>) -> io::Result<()> {
+        let ready = mio::Ready::writable();
+
+        self.inner
+            .write_readiness
+            .fetch_and(!ready.as_usize(), Relaxed);
+
+        if self.poll_write_ready(cx)?.is_ready() {
+            // Notify the current task
+            cx.waker().wake_by_ref();
+        }
+
+        Ok(())
+    }
+}
+
+// ===== Read / Write impls =====
+
+impl<E> AsyncRead for PollEvented<E>
+where
+    E: Evented + Read + Unpin,
+{
+    fn poll_read(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        ready!(self.poll_read_ready(cx, mio::Ready::readable()))?;
+
+        let r = (*self).get_mut().read(buf);
+
+        if is_wouldblock(&r) {
+            self.clear_read_ready(cx, mio::Ready::readable())?;
+            return Poll::Pending;
+        }
+
+        Poll::Ready(r)
+    }
+}
+
+impl<E> AsyncWrite for PollEvented<E>
+where
+    E: Evented + Write + Unpin,
+{
+    fn poll_write(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        ready!(self.poll_write_ready(cx))?;
+
+        let r = (*self).get_mut().write(buf);
+
+        if is_wouldblock(&r) {
+            self.clear_write_ready(cx)?;
+            return Poll::Pending;
+        }
+
+        Poll::Ready(r)
+    }
+
+    fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        ready!(self.poll_write_ready(cx))?;
+
+        let r = (*self).get_mut().flush();
+
+        if is_wouldblock(&r) {
+            self.clear_write_ready(cx)?;
+            return Poll::Pending;
+        }
+
+        Poll::Ready(r)
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        Poll::Ready(Ok(()))
+    }
+}
+
+fn is_wouldblock<T>(r: &io::Result<T>) -> bool {
+    match *r {
+        Ok(_) => false,
+        Err(ref e) => e.kind() == io::ErrorKind::WouldBlock,
+    }
+}
+
+impl<E: Evented + fmt::Debug> fmt::Debug for PollEvented<E> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("PollEvented").field("io", &self.io).finish()
+    }
+}
+
+impl<E: Evented> Drop for PollEvented<E> {
+    fn drop(&mut self) {
+        if let Some(io) = self.io.take() {
+            // Ignore errors
+            let _ = self.inner.registration.deregister(&io);
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/io/registration.rs b/third_party/rust/tokio/src/io/registration.rs
new file mode 100644
index 0000000000..4df11999f5
--- /dev/null
+++ b/third_party/rust/tokio/src/io/registration.rs
@@ -0,0 +1,299 @@
+use crate::io::driver::{platform, Direction, Handle};
+use crate::util::slab::Address;
+
+use mio::{self, Evented};
+use std::io;
+use std::task::{Context, Poll};
+
+cfg_io_driver! {
+    /// Associates an I/O resource with the reactor instance that drives it.
+    ///
+    /// A registration represents an I/O resource registered with a Reactor such
+    /// that it will receive task notifications on readiness. This is the lowest
+    /// level API for integrating with a reactor.
+    ///
+    /// The association between an I/O resource is made by calling [`new`]. Once
+    /// the association is established, it remains established until the
+    /// registration instance is dropped.
+    ///
+    /// A registration instance represents two separate readiness streams. One
+    /// for the read readiness and one for write readiness. These streams are
+    /// independent and can be consumed from separate tasks.
+    ///
+    /// **Note**: while `Registration` is `Sync`, the caller must ensure that
+    /// there are at most two tasks that use a registration instance
+    /// concurrently. One task for [`poll_read_ready`] and one task for
+    /// [`poll_write_ready`]. While violating this requirement is "safe" from a
+    /// Rust memory safety point of view, it will result in unexpected behavior
+    /// in the form of lost notifications and tasks hanging.
+    ///
+    /// ## Platform-specific events
+    ///
+    /// `Registration` also allows receiving platform-specific `mio::Ready`
+    /// events. These events are included as part of the read readiness event
+    /// stream. The write readiness event stream is only for `Ready::writable()`
+    /// events.
+    ///
+    /// [`new`]: #method.new
+    /// [`poll_read_ready`]: #method.poll_read_ready`]
+    /// [`poll_write_ready`]: #method.poll_write_ready`]
+    #[derive(Debug)]
+    pub struct Registration {
+        handle: Handle,
+        address: Address,
+    }
+}
+
+// ===== impl Registration =====
+
+impl Registration {
+    /// Registers the I/O resource with the default reactor.
+    ///
+    /// # Return
+    ///
+    /// - `Ok` if the registration happened successfully
+    /// - `Err` if an error was encountered during registration
+    ///
+    ///
+    /// # Panics
+    ///
+    /// This function panics if thread-local runtime is not set.
+    ///
+    /// The runtime is usually set implicitly when this function is called
+    /// from a future driven by a tokio runtime, otherwise runtime can be set
+    /// explicitly with [`Handle::enter`](crate::runtime::Handle::enter) function.
+    pub fn new<T>(io: &T) -> io::Result<Registration>
+    where
+        T: Evented,
+    {
+        let handle = Handle::current();
+        let address = if let Some(inner) = handle.inner() {
+            inner.add_source(io)?
+        } else {
+            return Err(io::Error::new(
+                io::ErrorKind::Other,
+                "failed to find event loop",
+            ));
+        };
+
+        Ok(Registration { handle, address })
+    }
+
+    /// Deregisters the I/O resource from the reactor it is associated with.
+    ///
+    /// This function must be called before the I/O resource associated with the
+    /// registration is dropped.
+    ///
+    /// Note that deregistering does not guarantee that the I/O resource can be
+    /// registered with a different reactor. Some I/O resource types can only be
+    /// associated with a single reactor instance for their lifetime.
+    ///
+    /// # Return
+    ///
+    /// If the deregistration was successful, `Ok` is returned. Any calls to
+    /// `Reactor::turn` that happen after a successful call to `deregister` will
+    /// no longer result in notifications getting sent for this registration.
+    ///
+    /// `Err` is returned if an error is encountered.
+    pub fn deregister<T>(&mut self, io: &T) -> io::Result<()>
+    where
+        T: Evented,
+    {
+        let inner = match self.handle.inner() {
+            Some(inner) => inner,
+            None => return Err(io::Error::new(io::ErrorKind::Other, "reactor gone")),
+        };
+        inner.deregister_source(io)
+    }
+
+    /// Polls for events on the I/O resource's read readiness stream.
+    ///
+    /// If the I/O resource receives a new read readiness event since the last
+    /// call to `poll_read_ready`, it is returned. If it has not, the current
+    /// task is notified once a new event is received.
+    ///
+    /// All events except `HUP` are [edge-triggered]. Once `HUP` is returned,
+    /// the function will always return `Ready(HUP)`. This should be treated as
+    /// the end of the readiness stream.
+    ///
+    /// Ensure that [`register`] has been called first.
+    ///
+    /// # Return value
+    ///
+    /// There are several possible return values:
+    ///
+    /// * `Poll::Ready(Ok(readiness))` means that the I/O resource has received
+    ///   a new readiness event. The readiness value is included.
+    ///
+    /// * `Poll::Pending` means that no new readiness events have been received
+    ///   since the last call to `poll_read_ready`.
+    ///
+    /// * `Poll::Ready(Err(err))` means that the registration has encountered an
+    ///   error. This error either represents a permanent internal error **or**
+    ///   the fact that [`register`] was not called first.
+    ///
+    /// [`register`]: #method.register
+    /// [edge-triggered]: https://docs.rs/mio/0.6/mio/struct.Poll.html#edge-triggered-and-level-triggered
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if called from outside of a task context.
+    pub fn poll_read_ready(&self, cx: &mut Context<'_>) -> Poll<io::Result<mio::Ready>> {
+        // Keep track of task budget
+        ready!(crate::coop::poll_proceed(cx));
+
+        let v = self.poll_ready(Direction::Read, Some(cx))?;
+        match v {
+            Some(v) => Poll::Ready(Ok(v)),
+            None => Poll::Pending,
+        }
+    }
+
+    /// Consume any pending read readiness event.
+    ///
+    /// This function is identical to [`poll_read_ready`] **except** that it
+    /// will not notify the current task when a new event is received. As such,
+    /// it is safe to call this function from outside of a task context.
+    ///
+    /// [`poll_read_ready`]: #method.poll_read_ready
+    pub fn take_read_ready(&self) -> io::Result<Option<mio::Ready>> {
+        self.poll_ready(Direction::Read, None)
+    }
+
+    /// Polls for events on the I/O resource's write readiness stream.
+    ///
+    /// If the I/O resource receives a new write readiness event since the last
+    /// call to `poll_write_ready`, it is returned. If it has not, the current
+    /// task is notified once a new event is received.
+    ///
+    /// All events except `HUP` are [edge-triggered]. Once `HUP` is returned,
+    /// the function will always return `Ready(HUP)`. This should be treated as
+    /// the end of the readiness stream.
+    ///
+    /// Ensure that [`register`] has been called first.
+    ///
+    /// # Return value
+    ///
+    /// There are several possible return values:
+    ///
+    /// * `Poll::Ready(Ok(readiness))` means that the I/O resource has received
+    ///   a new readiness event. The readiness value is included.
+    ///
+    /// * `Poll::Pending` means that no new readiness events have been received
+    ///   since the last call to `poll_write_ready`.
+    ///
+    /// * `Poll::Ready(Err(err))` means that the registration has encountered an
+    ///   error. This error either represents a permanent internal error **or**
+    ///   the fact that [`register`] was not called first.
+    ///
+    /// [`register`]: #method.register
+    /// [edge-triggered]: https://docs.rs/mio/0.6/mio/struct.Poll.html#edge-triggered-and-level-triggered
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if called from outside of a task context.
+    pub fn poll_write_ready(&self, cx: &mut Context<'_>) -> Poll<io::Result<mio::Ready>> {
+        // Keep track of task budget
+        ready!(crate::coop::poll_proceed(cx));
+
+        let v = self.poll_ready(Direction::Write, Some(cx))?;
+        match v {
+            Some(v) => Poll::Ready(Ok(v)),
+            None => Poll::Pending,
+        }
+    }
+
+    /// Consumes any pending write readiness event.
+    ///
+    /// This function is identical to [`poll_write_ready`] **except** that it
+    /// will not notify the current task when a new event is received. As such,
+    /// it is safe to call this function from outside of a task context.
+    ///
+    /// [`poll_write_ready`]: #method.poll_write_ready
+    pub fn take_write_ready(&self) -> io::Result<Option<mio::Ready>> {
+        self.poll_ready(Direction::Write, None)
+    }
+
+    /// Polls for events on the I/O resource's `direction` readiness stream.
+    ///
+    /// If called with a task context, notify the task when a new event is
+    /// received.
+    fn poll_ready(
+        &self,
+        direction: Direction,
+        cx: Option<&mut Context<'_>>,
+    ) -> io::Result<Option<mio::Ready>> {
+        let inner = match self.handle.inner() {
+            Some(inner) => inner,
+            None => return Err(io::Error::new(io::ErrorKind::Other, "reactor gone")),
+        };
+
+        // If the task should be notified about new events, ensure that it has
+        // been registered
+        if let Some(ref cx) = cx {
+            inner.register(self.address, direction, cx.waker().clone())
+        }
+
+        let mask = direction.mask();
+        let mask_no_hup = (mask - platform::hup() - platform::error()).as_usize();
+
+        let sched = inner.io_dispatch.get(self.address).unwrap();
+
+        // This consumes the current readiness state **except** for HUP and
+        // error. HUP and error are excluded because a) they are final states
+        // and never transitition out and b) both the read AND the write
+        // directions need to be able to obvserve these states.
+        //
+        // # Platform-specific behavior
+        //
+        // HUP and error readiness are platform-specific. On epoll platforms,
+        // HUP has specific conditions that must be met by both peers of a
+        // connection in order to be triggered.
+        //
+        // On epoll platforms, `EPOLLERR` is signaled through
+        // `UnixReady::error()` and is important to be observable by both read
+        // AND write. A specific case that `EPOLLERR` occurs is when the read
+        // end of a pipe is closed. When this occurs, a peer blocked by
+        // writing to the pipe should be notified.
+        let curr_ready = sched
+            .set_readiness(self.address, |curr| curr & (!mask_no_hup))
+            .unwrap_or_else(|_| panic!("address {:?} no longer valid!", self.address));
+
+        let mut ready = mask & mio::Ready::from_usize(curr_ready);
+
+        if ready.is_empty() {
+            if let Some(cx) = cx {
+                // Update the task info
+                match direction {
+                    Direction::Read => sched.reader.register_by_ref(cx.waker()),
+                    Direction::Write => sched.writer.register_by_ref(cx.waker()),
+                }
+
+                // Try again
+                let curr_ready = sched
+                    .set_readiness(self.address, |curr| curr & (!mask_no_hup))
+                    .unwrap_or_else(|_| panic!("address {:?} no longer valid!", self.address));
+                ready = mask & mio::Ready::from_usize(curr_ready);
+            }
+        }
+
+        if ready.is_empty() {
+            Ok(None)
+        } else {
+            Ok(Some(ready))
+        }
+    }
+}
+
+unsafe impl Send for Registration {}
+unsafe impl Sync for Registration {}
+
+impl Drop for Registration {
+    fn drop(&mut self) {
+        let inner = match self.handle.inner() {
+            Some(inner) => inner,
+            None => return,
+        };
+        inner.drop_source(self.address);
+    }
+}
diff --git a/third_party/rust/tokio/src/io/seek.rs b/third_party/rust/tokio/src/io/seek.rs
new file mode 100644
index 0000000000..e3b5bf6b6f
--- /dev/null
+++ b/third_party/rust/tokio/src/io/seek.rs
@@ -0,0 +1,56 @@
+use crate::io::AsyncSeek;
+use std::future::Future;
+use std::io::{self, SeekFrom};
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// Future for the [`seek`](crate::io::AsyncSeekExt::seek) method.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct Seek<'a, S: ?Sized> {
+    seek: &'a mut S,
+    pos: Option<SeekFrom>,
+}
+
+pub(crate) fn seek<S>(seek: &mut S, pos: SeekFrom) -> Seek<'_, S>
+where
+    S: AsyncSeek + ?Sized + Unpin,
+{
+    Seek {
+        seek,
+        pos: Some(pos),
+    }
+}
+
+impl<S> Future for Seek<'_, S>
+where
+    S: AsyncSeek + ?Sized + Unpin,
+{
+    type Output = io::Result<u64>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let me = &mut *self;
+        match me.pos {
+            Some(pos) => match Pin::new(&mut me.seek).start_seek(cx, pos) {
+                Poll::Ready(Ok(())) => {
+                    me.pos = None;
+                    Pin::new(&mut me.seek).poll_complete(cx)
+                }
+                Poll::Ready(Err(e)) => Poll::Ready(Err(e)),
+                Poll::Pending => Poll::Pending,
+            },
+            None => Pin::new(&mut me.seek).poll_complete(cx),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        use std::marker::PhantomPinned;
+        crate::is_unpin::<Seek<'_, PhantomPinned>>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/split.rs b/third_party/rust/tokio/src/io/split.rs
new file mode 100644
index 0000000000..134b937a5f
--- /dev/null
+++ b/third_party/rust/tokio/src/io/split.rs
@@ -0,0 +1,195 @@
+//! Split a single value implementing `AsyncRead + AsyncWrite` into separate
+//! `AsyncRead` and `AsyncWrite` handles.
+//!
+//! To restore this read/write object from its `split::ReadHalf` and
+//! `split::WriteHalf` use `unsplit`.
+
+use crate::io::{AsyncRead, AsyncWrite};
+
+use bytes::{Buf, BufMut};
+use std::cell::UnsafeCell;
+use std::fmt;
+use std::io;
+use std::pin::Pin;
+use std::sync::atomic::AtomicBool;
+use std::sync::atomic::Ordering::{Acquire, Release};
+use std::sync::Arc;
+use std::task::{Context, Poll};
+
+cfg_io_util! {
+    /// The readable half of a value returned from [`split`](split()).
+    pub struct ReadHalf<T> {
+        inner: Arc<Inner<T>>,
+    }
+
+    /// The writable half of a value returned from [`split`](split()).
+    pub struct WriteHalf<T> {
+        inner: Arc<Inner<T>>,
+    }
+
+    /// Splits a single value implementing `AsyncRead + AsyncWrite` into separate
+    /// `AsyncRead` and `AsyncWrite` handles.
+    ///
+    /// To restore this read/write object from its `ReadHalf` and
+    /// `WriteHalf` use [`unsplit`](ReadHalf::unsplit()).
+    pub fn split<T>(stream: T) -> (ReadHalf<T>, WriteHalf<T>)
+    where
+        T: AsyncRead + AsyncWrite,
+    {
+        let inner = Arc::new(Inner {
+            locked: AtomicBool::new(false),
+            stream: UnsafeCell::new(stream),
+        });
+
+        let rd = ReadHalf {
+            inner: inner.clone(),
+        };
+
+        let wr = WriteHalf { inner };
+
+        (rd, wr)
+    }
+}
+
+struct Inner<T> {
+    locked: AtomicBool,
+    stream: UnsafeCell<T>,
+}
+
+struct Guard<'a, T> {
+    inner: &'a Inner<T>,
+}
+
+impl<T> ReadHalf<T> {
+    /// Checks if this `ReadHalf` and some `WriteHalf` were split from the same
+    /// stream.
+    pub fn is_pair_of(&self, other: &WriteHalf<T>) -> bool {
+        other.is_pair_of(&self)
+    }
+
+    /// Reunites with a previously split `WriteHalf`.
+    ///
+    /// # Panics
+    ///
+    /// If this `ReadHalf` and the given `WriteHalf` do not originate from the
+    /// same `split` operation this method will panic.
+    /// This can be checked ahead of time by comparing the stream ID
+    /// of the two halves.
+    pub fn unsplit(self, wr: WriteHalf<T>) -> T {
+        if self.is_pair_of(&wr) {
+            drop(wr);
+
+            let inner = Arc::try_unwrap(self.inner)
+                .ok()
+                .expect("`Arc::try_unwrap` failed");
+
+            inner.stream.into_inner()
+        } else {
+            panic!("Unrelated `split::Write` passed to `split::Read::unsplit`.")
+        }
+    }
+}
+
+impl<T> WriteHalf<T> {
+    /// Check if this `WriteHalf` and some `ReadHalf` were split from the same
+    /// stream.
+    pub fn is_pair_of(&self, other: &ReadHalf<T>) -> bool {
+        Arc::ptr_eq(&self.inner, &other.inner)
+    }
+}
+
+impl<T: AsyncRead> AsyncRead for ReadHalf<T> {
+    fn poll_read(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        let mut inner = ready!(self.inner.poll_lock(cx));
+        inner.stream_pin().poll_read(cx, buf)
+    }
+
+    fn poll_read_buf<B: BufMut>(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut B,
+    ) -> Poll<io::Result<usize>> {
+        let mut inner = ready!(self.inner.poll_lock(cx));
+        inner.stream_pin().poll_read_buf(cx, buf)
+    }
+}
+
+impl<T: AsyncWrite> AsyncWrite for WriteHalf<T> {
+    fn poll_write(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<Result<usize, io::Error>> {
+        let mut inner = ready!(self.inner.poll_lock(cx));
+        inner.stream_pin().poll_write(cx, buf)
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
+        let mut inner = ready!(self.inner.poll_lock(cx));
+        inner.stream_pin().poll_flush(cx)
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
+        let mut inner = ready!(self.inner.poll_lock(cx));
+        inner.stream_pin().poll_shutdown(cx)
+    }
+
+    fn poll_write_buf<B: Buf>(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut B,
+    ) -> Poll<Result<usize, io::Error>> {
+        let mut inner = ready!(self.inner.poll_lock(cx));
+        inner.stream_pin().poll_write_buf(cx, buf)
+    }
+}
+
+impl<T> Inner<T> {
+    fn poll_lock(&self, cx: &mut Context<'_>) -> Poll<Guard<'_, T>> {
+        if !self.locked.compare_and_swap(false, true, Acquire) {
+            Poll::Ready(Guard { inner: self })
+        } else {
+            // Spin... but investigate a better strategy
+
+            std::thread::yield_now();
+            cx.waker().wake_by_ref();
+
+            Poll::Pending
+        }
+    }
+}
+
+impl<T> Guard<'_, T> {
+    fn stream_pin(&mut self) -> Pin<&mut T> {
+        // safety: the stream is pinned in `Arc` and the `Guard` ensures mutual
+        // exclusion.
+        unsafe { Pin::new_unchecked(&mut *self.inner.stream.get()) }
+    }
+}
+
+impl<T> Drop for Guard<'_, T> {
+    fn drop(&mut self) {
+        self.inner.locked.store(false, Release);
+    }
+}
+
+unsafe impl<T: Send> Send for ReadHalf<T> {}
+unsafe impl<T: Send> Send for WriteHalf<T> {}
+unsafe impl<T: Sync> Sync for ReadHalf<T> {}
+unsafe impl<T: Sync> Sync for WriteHalf<T> {}
+
+impl<T: fmt::Debug> fmt::Debug for ReadHalf<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("split::ReadHalf").finish()
+    }
+}
+
+impl<T: fmt::Debug> fmt::Debug for WriteHalf<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("split::WriteHalf").finish()
+    }
+}
diff --git a/third_party/rust/tokio/src/io/stderr.rs b/third_party/rust/tokio/src/io/stderr.rs
new file mode 100644
index 0000000000..99607dc604
--- /dev/null
+++ b/third_party/rust/tokio/src/io/stderr.rs
@@ -0,0 +1,108 @@
+use crate::io::blocking::Blocking;
+use crate::io::AsyncWrite;
+
+use std::io;
+use std::pin::Pin;
+use std::task::Context;
+use std::task::Poll;
+
+cfg_io_std! {
+    /// A handle to the standard error stream of a process.
+    ///
+    /// Concurrent writes to stderr must be executed with care: Only individual
+    /// writes to this [`AsyncWrite`] are guaranteed to be intact. In particular
+    /// you should be aware that writes using [`write_all`] are not guaranteed
+    /// to occur as a single write, so multiple threads writing data with
+    /// [`write_all`] may result in interleaved output.
+    ///
+    /// Created by the [`stderr`] function.
+    ///
+    /// [`stderr`]: stderr()
+    /// [`AsyncWrite`]: AsyncWrite
+    /// [`write_all`]: crate::io::AsyncWriteExt::write_all()
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::io::{self, AsyncWriteExt};
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let mut stderr = io::stdout();
+    ///     stderr.write_all(b"Print some error here.").await?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[derive(Debug)]
+    pub struct Stderr {
+        std: Blocking<std::io::Stderr>,
+    }
+
+    /// Constructs a new handle to the standard error of the current process.
+    ///
+    /// The returned handle allows writing to standard error from the within the
+    /// Tokio runtime.
+    ///
+    /// Concurrent writes to stderr must be executed with care: Only individual
+    /// writes to this [`AsyncWrite`] are guaranteed to be intact. In particular
+    /// you should be aware that writes using [`write_all`] are not guaranteed
+    /// to occur as a single write, so multiple threads writing data with
+    /// [`write_all`] may result in interleaved output.
+    ///
+    /// [`AsyncWrite`]: AsyncWrite
+    /// [`write_all`]: crate::io::AsyncWriteExt::write_all()
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::io::{self, AsyncWriteExt};
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let mut stderr = io::stdout();
+    ///     stderr.write_all(b"Print some error here.").await?;
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn stderr() -> Stderr {
+        let std = io::stderr();
+        Stderr {
+            std: Blocking::new(std),
+        }
+    }
+}
+
+#[cfg(unix)]
+impl std::os::unix::io::AsRawFd for Stderr {
+    fn as_raw_fd(&self) -> std::os::unix::io::RawFd {
+        std::io::stderr().as_raw_fd()
+    }
+}
+
+#[cfg(windows)]
+impl std::os::windows::io::AsRawHandle for Stderr {
+    fn as_raw_handle(&self) -> std::os::windows::io::RawHandle {
+        std::io::stderr().as_raw_handle()
+    }
+}
+
+impl AsyncWrite for Stderr {
+    fn poll_write(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        Pin::new(&mut self.std).poll_write(cx, buf)
+    }
+
+    fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
+        Pin::new(&mut self.std).poll_flush(cx)
+    }
+
+    fn poll_shutdown(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Result<(), io::Error>> {
+        Pin::new(&mut self.std).poll_shutdown(cx)
+    }
+}
diff --git a/third_party/rust/tokio/src/io/stdin.rs b/third_party/rust/tokio/src/io/stdin.rs
new file mode 100644
index 0000000000..214c4d0564
--- /dev/null
+++ b/third_party/rust/tokio/src/io/stdin.rs
@@ -0,0 +1,70 @@
+use crate::io::blocking::Blocking;
+use crate::io::AsyncRead;
+
+use std::io;
+use std::pin::Pin;
+use std::task::Context;
+use std::task::Poll;
+
+cfg_io_std! {
+    /// A handle to the standard input stream of a process.
+    ///
+    /// The handle implements the [`AsyncRead`] trait, but beware that concurrent
+    /// reads of `Stdin` must be executed with care.
+    ///
+    /// As an additional caveat, reading from the handle may block the calling
+    /// future indefinitely if there is not enough data available. This makes this
+    /// handle unsuitable for use in any circumstance where immediate reaction to
+    /// available data is required, e.g. interactive use or when implementing a
+    /// subprocess driven by requests on the standard input.
+    ///
+    /// Created by the [`stdin`] function.
+    ///
+    /// [`stdin`]: fn@stdin
+    /// [`AsyncRead`]: trait@AsyncRead
+    #[derive(Debug)]
+    pub struct Stdin {
+        std: Blocking<std::io::Stdin>,
+    }
+
+    /// Constructs a new handle to the standard input of the current process.
+    ///
+    /// The returned handle allows reading from standard input from the within the
+    /// Tokio runtime.
+    ///
+    /// As an additional caveat, reading from the handle may block the calling
+    /// future indefinitely if there is not enough data available. This makes this
+    /// handle unsuitable for use in any circumstance where immediate reaction to
+    /// available data is required, e.g. interactive use or when implementing a
+    /// subprocess driven by requests on the standard input.
+    pub fn stdin() -> Stdin {
+        let std = io::stdin();
+        Stdin {
+            std: Blocking::new(std),
+        }
+    }
+}
+
+#[cfg(unix)]
+impl std::os::unix::io::AsRawFd for Stdin {
+    fn as_raw_fd(&self) -> std::os::unix::io::RawFd {
+        std::io::stdin().as_raw_fd()
+    }
+}
+
+#[cfg(windows)]
+impl std::os::windows::io::AsRawHandle for Stdin {
+    fn as_raw_handle(&self) -> std::os::windows::io::RawHandle {
+        std::io::stdin().as_raw_handle()
+    }
+}
+
+impl AsyncRead for Stdin {
+    fn poll_read(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        Pin::new(&mut self.std).poll_read(cx, buf)
+    }
+}
diff --git a/third_party/rust/tokio/src/io/stdout.rs b/third_party/rust/tokio/src/io/stdout.rs
new file mode 100644
index 0000000000..5377993a46
--- /dev/null
+++ b/third_party/rust/tokio/src/io/stdout.rs
@@ -0,0 +1,108 @@
+use crate::io::blocking::Blocking;
+use crate::io::AsyncWrite;
+
+use std::io;
+use std::pin::Pin;
+use std::task::Context;
+use std::task::Poll;
+
+cfg_io_std! {
+    /// A handle to the standard output stream of a process.
+    ///
+    /// Concurrent writes to stdout must be executed with care: Only individual
+    /// writes to this [`AsyncWrite`] are guaranteed to be intact. In particular
+    /// you should be aware that writes using [`write_all`] are not guaranteed
+    /// to occur as a single write, so multiple threads writing data with
+    /// [`write_all`] may result in interleaved output.
+    ///
+    /// Created by the [`stdout`] function.
+    ///
+    /// [`stdout`]: stdout()
+    /// [`AsyncWrite`]: AsyncWrite
+    /// [`write_all`]: crate::io::AsyncWriteExt::write_all()
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::io::{self, AsyncWriteExt};
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let mut stdout = io::stdout();
+    ///     stdout.write_all(b"Hello world!").await?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[derive(Debug)]
+    pub struct Stdout {
+        std: Blocking<std::io::Stdout>,
+    }
+
+    /// Constructs a new handle to the standard output of the current process.
+    ///
+    /// The returned handle allows writing to standard out from the within the
+    /// Tokio runtime.
+    ///
+    /// Concurrent writes to stdout must be executed with care: Only individual
+    /// writes to this [`AsyncWrite`] are guaranteed to be intact. In particular
+    /// you should be aware that writes using [`write_all`] are not guaranteed
+    /// to occur as a single write, so multiple threads writing data with
+    /// [`write_all`] may result in interleaved output.
+    ///
+    /// [`AsyncWrite`]: AsyncWrite
+    /// [`write_all`]: crate::io::AsyncWriteExt::write_all()
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::io::{self, AsyncWriteExt};
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let mut stdout = io::stdout();
+    ///     stdout.write_all(b"Hello world!").await?;
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn stdout() -> Stdout {
+        let std = io::stdout();
+        Stdout {
+            std: Blocking::new(std),
+        }
+    }
+}
+
+#[cfg(unix)]
+impl std::os::unix::io::AsRawFd for Stdout {
+    fn as_raw_fd(&self) -> std::os::unix::io::RawFd {
+        std::io::stdout().as_raw_fd()
+    }
+}
+
+#[cfg(windows)]
+impl std::os::windows::io::AsRawHandle for Stdout {
+    fn as_raw_handle(&self) -> std::os::windows::io::RawHandle {
+        std::io::stdout().as_raw_handle()
+    }
+}
+
+impl AsyncWrite for Stdout {
+    fn poll_write(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        Pin::new(&mut self.std).poll_write(cx, buf)
+    }
+
+    fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
+        Pin::new(&mut self.std).poll_flush(cx)
+    }
+
+    fn poll_shutdown(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Result<(), io::Error>> {
+        Pin::new(&mut self.std).poll_shutdown(cx)
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/async_buf_read_ext.rs b/third_party/rust/tokio/src/io/util/async_buf_read_ext.rs
new file mode 100644
index 0000000000..1bfab90220
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/async_buf_read_ext.rs
@@ -0,0 +1,258 @@
+use crate::io::util::lines::{lines, Lines};
+use crate::io::util::read_line::{read_line, ReadLine};
+use crate::io::util::read_until::{read_until, ReadUntil};
+use crate::io::util::split::{split, Split};
+use crate::io::AsyncBufRead;
+
+cfg_io_util! {
+    /// An extension trait which adds utility methods to [`AsyncBufRead`] types.
+    ///
+    /// [`AsyncBufRead`]: crate::io::AsyncBufRead
+    pub trait AsyncBufReadExt: AsyncBufRead {
+        /// Reads all bytes into `buf` until the delimiter `byte` or EOF is reached.
+        ///
+        /// Equivalent to:
+        ///
+        /// ```ignore
+        /// async fn read_until(&mut self, buf: &mut Vec<u8>) -> io::Result<usize>;
+        /// ```
+        ///
+        /// This function will read bytes from the underlying stream until the
+        /// delimiter or EOF is found. Once found, all bytes up to, and including,
+        /// the delimiter (if found) will be appended to `buf`.
+        ///
+        /// If successful, this function will return the total number of bytes read.
+        ///
+        /// # Errors
+        ///
+        /// This function will ignore all instances of [`ErrorKind::Interrupted`] and
+        /// will otherwise return any errors returned by [`fill_buf`].
+        ///
+        /// If an I/O error is encountered then all bytes read so far will be
+        /// present in `buf` and its length will have been adjusted appropriately.
+        ///
+        /// [`fill_buf`]: AsyncBufRead::poll_fill_buf
+        /// [`ErrorKind::Interrupted`]: std::io::ErrorKind::Interrupted
+        ///
+        /// # Examples
+        ///
+        /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
+        /// this example, we use [`Cursor`] to read all the bytes in a byte slice
+        /// in hyphen delimited segments:
+        ///
+        /// [`Cursor`]: std::io::Cursor
+        ///
+        /// ```
+        /// use tokio::io::AsyncBufReadExt;
+        ///
+        /// use std::io::Cursor;
+        ///
+        /// #[tokio::main]
+        /// async fn main() {
+        ///     let mut cursor = Cursor::new(b"lorem-ipsum");
+        ///     let mut buf = vec![];
+        ///
+        ///     // cursor is at 'l'
+        ///     let num_bytes = cursor.read_until(b'-', &mut buf)
+        ///         .await
+        ///         .expect("reading from cursor won't fail");
+        ///
+        ///     assert_eq!(num_bytes, 6);
+        ///     assert_eq!(buf, b"lorem-");
+        ///     buf.clear();
+        ///
+        ///     // cursor is at 'i'
+        ///     let num_bytes = cursor.read_until(b'-', &mut buf)
+        ///         .await
+        ///         .expect("reading from cursor won't fail");
+        ///
+        ///     assert_eq!(num_bytes, 5);
+        ///     assert_eq!(buf, b"ipsum");
+        ///     buf.clear();
+        ///
+        ///     // cursor is at EOF
+        ///     let num_bytes = cursor.read_until(b'-', &mut buf)
+        ///         .await
+        ///         .expect("reading from cursor won't fail");
+        ///     assert_eq!(num_bytes, 0);
+        ///     assert_eq!(buf, b"");
+        /// }
+        /// ```
+        fn read_until<'a>(&'a mut self, byte: u8, buf: &'a mut Vec<u8>) -> ReadUntil<'a, Self>
+        where
+            Self: Unpin,
+        {
+            read_until(self, byte, buf)
+        }
+
+        /// Reads all bytes until a newline (the 0xA byte) is reached, and append
+        /// them to the provided buffer.
+        ///
+        /// Equivalent to:
+        ///
+        /// ```ignore
+        /// async fn read_line(&mut self, buf: &mut String) -> io::Result<usize>;
+        /// ```
+        ///
+        /// This function will read bytes from the underlying stream until the
+        /// newline delimiter (the 0xA byte) or EOF is found. Once found, all bytes
+        /// up to, and including, the delimiter (if found) will be appended to
+        /// `buf`.
+        ///
+        /// If successful, this function will return the total number of bytes read.
+        ///
+        /// If this function returns `Ok(0)`, the stream has reached EOF.
+        ///
+        /// # Errors
+        ///
+        /// This function has the same error semantics as [`read_until`] and will
+        /// also return an error if the read bytes are not valid UTF-8. If an I/O
+        /// error is encountered then `buf` may contain some bytes already read in
+        /// the event that all data read so far was valid UTF-8.
+        ///
+        /// [`read_until`]: AsyncBufReadExt::read_until
+        ///
+        /// # Examples
+        ///
+        /// [`std::io::Cursor`][`Cursor`] is a type that implements
+        /// `AsyncBufRead`. In this example, we use [`Cursor`] to read all the
+        /// lines in a byte slice:
+        ///
+        /// [`Cursor`]: std::io::Cursor
+        ///
+        /// ```
+        /// use tokio::io::AsyncBufReadExt;
+        ///
+        /// use std::io::Cursor;
+        ///
+        /// #[tokio::main]
+        /// async fn main() {
+        ///     let mut cursor = Cursor::new(b"foo\nbar");
+        ///     let mut buf = String::new();
+        ///
+        ///     // cursor is at 'f'
+        ///     let num_bytes = cursor.read_line(&mut buf)
+        ///         .await
+        ///         .expect("reading from cursor won't fail");
+        ///
+        ///     assert_eq!(num_bytes, 4);
+        ///     assert_eq!(buf, "foo\n");
+        ///     buf.clear();
+        ///
+        ///     // cursor is at 'b'
+        ///     let num_bytes = cursor.read_line(&mut buf)
+        ///         .await
+        ///         .expect("reading from cursor won't fail");
+        ///
+        ///     assert_eq!(num_bytes, 3);
+        ///     assert_eq!(buf, "bar");
+        ///     buf.clear();
+        ///
+        ///     // cursor is at EOF
+        ///     let num_bytes = cursor.read_line(&mut buf)
+        ///         .await
+        ///         .expect("reading from cursor won't fail");
+        ///
+        ///     assert_eq!(num_bytes, 0);
+        ///     assert_eq!(buf, "");
+        /// }
+        /// ```
+        fn read_line<'a>(&'a mut self, buf: &'a mut String) -> ReadLine<'a, Self>
+        where
+            Self: Unpin,
+        {
+            read_line(self, buf)
+        }
+
+        /// Returns a stream of the contents of this reader split on the byte
+        /// `byte`.
+        ///
+        /// This method is the asynchronous equivalent to
+        /// [`BufRead::split`](std::io::BufRead::split).
+        ///
+        /// The stream returned from this function will yield instances of
+        /// [`io::Result`]`<`[`Vec<u8>`]`>`. Each vector returned will *not* have
+        /// the delimiter byte at the end.
+        ///
+        /// [`io::Result`]: std::io::Result
+        /// [`Vec<u8>`]: std::vec::Vec
+        ///
+        /// # Errors
+        ///
+        /// Each item of the stream has the same error semantics as
+        /// [`AsyncBufReadExt::read_until`](AsyncBufReadExt::read_until).
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// # use tokio::io::AsyncBufRead;
+        /// use tokio::io::AsyncBufReadExt;
+        ///
+        /// # async fn dox(my_buf_read: impl AsyncBufRead + Unpin) -> std::io::Result<()> {
+        /// let mut segments = my_buf_read.split(b'f');
+        ///
+        /// while let Some(segment) = segments.next_segment().await? {
+        ///     println!("length = {}", segment.len())
+        /// }
+        /// # Ok(())
+        /// # }
+        /// ```
+        fn split(self, byte: u8) -> Split<Self>
+        where
+            Self: Sized + Unpin,
+        {
+            split(self, byte)
+        }
+
+        /// Returns a stream over the lines of this reader.
+        /// This method is the async equivalent to [`BufRead::lines`](std::io::BufRead::lines).
+        ///
+        /// The stream returned from this function will yield instances of
+        /// [`io::Result`]`<`[`String`]`>`. Each string returned will *not* have a newline
+        /// byte (the 0xA byte) or CRLF (0xD, 0xA bytes) at the end.
+        ///
+        /// [`io::Result`]: std::io::Result
+        /// [`String`]: String
+        ///
+        /// # Errors
+        ///
+        /// Each line of the stream has the same error semantics as [`AsyncBufReadExt::read_line`].
+        ///
+        /// # Examples
+        ///
+        /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
+        /// this example, we use [`Cursor`] to iterate over all the lines in a byte
+        /// slice.
+        ///
+        /// [`Cursor`]: std::io::Cursor
+        ///
+        /// ```
+        /// use tokio::io::AsyncBufReadExt;
+        /// use tokio::stream::StreamExt;
+        ///
+        /// use std::io::Cursor;
+        ///
+        /// #[tokio::main]
+        /// async fn main() {
+        ///     let cursor = Cursor::new(b"lorem\nipsum\r\ndolor");
+        ///
+        ///     let mut lines = cursor.lines().map(|res| res.unwrap());
+        ///
+        ///     assert_eq!(lines.next().await, Some(String::from("lorem")));
+        ///     assert_eq!(lines.next().await, Some(String::from("ipsum")));
+        ///     assert_eq!(lines.next().await, Some(String::from("dolor")));
+        ///     assert_eq!(lines.next().await, None);
+        /// }
+        /// ```
+        ///
+        /// [`AsyncBufReadExt::read_line`]: AsyncBufReadExt::read_line
+        fn lines(self) -> Lines<Self>
+        where
+            Self: Sized,
+        {
+            lines(self)
+        }
+    }
+}
+
+impl<R: AsyncBufRead + ?Sized> AsyncBufReadExt for R {}
diff --git a/third_party/rust/tokio/src/io/util/async_read_ext.rs b/third_party/rust/tokio/src/io/util/async_read_ext.rs
new file mode 100644
index 0000000000..d4402db621
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/async_read_ext.rs
@@ -0,0 +1,807 @@
+use crate::io::util::chain::{chain, Chain};
+use crate::io::util::read::{read, Read};
+use crate::io::util::read_buf::{read_buf, ReadBuf};
+use crate::io::util::read_exact::{read_exact, ReadExact};
+use crate::io::util::read_int::{ReadI128, ReadI16, ReadI32, ReadI64, ReadI8};
+use crate::io::util::read_int::{ReadU128, ReadU16, ReadU32, ReadU64, ReadU8};
+use crate::io::util::read_to_end::{read_to_end, ReadToEnd};
+use crate::io::util::read_to_string::{read_to_string, ReadToString};
+use crate::io::util::take::{take, Take};
+use crate::io::AsyncRead;
+
+use bytes::BufMut;
+
+cfg_io_util! {
+    /// Defines numeric reader
+    macro_rules! read_impl {
+        (
+            $(
+                $(#[$outer:meta])*
+                fn $name:ident(&mut self) -> $($fut:ident)*;
+            )*
+        ) => {
+            $(
+                $(#[$outer])*
+                fn $name<'a>(&'a mut self) -> $($fut)*<&'a mut Self> where Self: Unpin {
+                    $($fut)*::new(self)
+                }
+            )*
+        }
+    }
+
+    /// Reads bytes from a source.
+    ///
+    /// Implemented as an extention trait, adding utility methods to all
+    /// [`AsyncRead`] types. Callers will tend to import this trait instead of
+    /// [`AsyncRead`].
+    ///
+    /// As a convenience, this trait may be imported using the [`prelude`]:
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    /// use tokio::prelude::*;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let mut f = File::open("foo.txt").await?;
+    ///     let mut buffer = [0; 10];
+    ///
+    ///     // The `read` method is defined by this trait.
+    ///     let n = f.read(&mut buffer[..]).await?;
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    ///
+    /// See [module][crate::io] documentation for more details.
+    ///
+    /// [`AsyncRead`]: AsyncRead
+    /// [`prelude`]: crate::prelude
+    pub trait AsyncReadExt: AsyncRead {
+        /// Creates a new `AsyncRead` instance that chains this stream with
+        /// `next`.
+        ///
+        /// The returned `AsyncRead` instance will first read all bytes from this object
+        /// until EOF is encountered. Afterwards the output is equivalent to the
+        /// output of `next`.
+        ///
+        /// # Examples
+        ///
+        /// [`File`][crate::fs::File]s implement `AsyncRead`:
+        ///
+        /// ```no_run
+        /// use tokio::fs::File;
+        /// use tokio::io::{self, AsyncReadExt};
+        ///
+        /// #[tokio::main]
+        /// async fn main() -> io::Result<()> {
+        ///     let f1 = File::open("foo.txt").await?;
+        ///     let f2 = File::open("bar.txt").await?;
+        ///
+        ///     let mut handle = f1.chain(f2);
+        ///     let mut buffer = String::new();
+        ///
+        ///     // read the value into a String. We could use any AsyncRead
+        ///     // method here, this is just one example.
+        ///     handle.read_to_string(&mut buffer).await?;
+        ///     Ok(())
+        /// }
+        /// ```
+        fn chain<R>(self, next: R) -> Chain<Self, R>
+        where
+            Self: Sized,
+            R: AsyncRead,
+        {
+            chain(self, next)
+        }
+
+        /// Pulls some bytes from this source into the specified buffer,
+        /// returning how many bytes were read.
+        ///
+        /// Equivalent to:
+        ///
+        /// ```ignore
+        /// async fn read(&mut self, buf: &mut [u8]) -> io::Result<usize>;
+        /// ```
+        ///
+        /// This function does not provide any guarantees about whether it
+        /// completes immediately or asynchronously
+        ///
+        /// If the return value of this method is `Ok(n)`, then it must be
+        /// guaranteed that `0 <= n <= buf.len()`. A nonzero `n` value indicates
+        /// that the buffer `buf` has been filled in with `n` bytes of data from
+        /// this source. If `n` is `0`, then it can indicate one of two
+        /// scenarios:
+        ///
+        /// 1. This reader has reached its "end of file" and will likely no longer
+        ///    be able to produce bytes. Note that this does not mean that the
+        ///    reader will *always* no longer be able to produce bytes.
+        /// 2. The buffer specified was 0 bytes in length.
+        ///
+        /// No guarantees are provided about the contents of `buf` when this
+        /// function is called, implementations cannot rely on any property of the
+        /// contents of `buf` being `true`. It is recommended that *implementations*
+        /// only write data to `buf` instead of reading its contents.
+        ///
+        /// Correspondingly, however, *callers* of this method may not assume
+        /// any guarantees about how the implementation uses `buf`. It is
+        /// possible that the code that's supposed to write to the buffer might
+        /// also read from it. It is your responsibility to make sure that `buf`
+        /// is initialized before calling `read`.
+        ///
+        /// # Errors
+        ///
+        /// If this function encounters any form of I/O or other error, an error
+        /// variant will be returned. If an error is returned then it must be
+        /// guaranteed that no bytes were read.
+        ///
+        /// # Examples
+        ///
+        /// [`File`][crate::fs::File]s implement `Read`:
+        ///
+        /// ```no_run
+        /// use tokio::fs::File;
+        /// use tokio::io::{self, AsyncReadExt};
+        ///
+        /// #[tokio::main]
+        /// async fn main() -> io::Result<()> {
+        ///     let mut f = File::open("foo.txt").await?;
+        ///     let mut buffer = [0; 10];
+        ///
+        ///     // read up to 10 bytes
+        ///     let n = f.read(&mut buffer[..]).await?;
+        ///
+        ///     println!("The bytes: {:?}", &buffer[..n]);
+        ///     Ok(())
+        /// }
+        /// ```
+        fn read<'a>(&'a mut self, buf: &'a mut [u8]) -> Read<'a, Self>
+        where
+            Self: Unpin,
+        {
+            read(self, buf)
+        }
+
+        /// Pulls some bytes from this source into the specified buffer,
+        /// advancing the buffer's internal cursor.
+        ///
+        /// Equivalent to:
+        ///
+        /// ```ignore
+        /// async fn read_buf<B: BufMut>(&mut self, buf: &mut B) -> io::Result<usize>;
+        /// ```
+        ///
+        /// Usually, only a single `read` syscall is issued, even if there is
+        /// more space in the supplied buffer.
+        ///
+        /// This function does not provide any guarantees about whether it
+        /// completes immediately or asynchronously
+        ///
+        /// # Return
+        ///
+        /// On a successful read, the number of read bytes is returned. If the
+        /// supplied buffer is not empty and the function returns `Ok(0)` then
+        /// the source as reached an "end-of-file" event.
+        ///
+        /// # Errors
+        ///
+        /// If this function encounters any form of I/O or other error, an error
+        /// variant will be returned. If an error is returned then it must be
+        /// guaranteed that no bytes were read.
+        ///
+        /// # Examples
+        ///
+        /// [`File`] implements `Read` and [`BytesMut`] implements [`BufMut`]:
+        ///
+        /// [`File`]: crate::fs::File
+        /// [`BytesMut`]: bytes::BytesMut
+        /// [`BufMut`]: bytes::BufMut
+        ///
+        /// ```no_run
+        /// use tokio::fs::File;
+        /// use tokio::io::{self, AsyncReadExt};
+        ///
+        /// use bytes::BytesMut;
+        ///
+        /// #[tokio::main]
+        /// async fn main() -> io::Result<()> {
+        ///     let mut f = File::open("foo.txt").await?;
+        ///     let mut buffer = BytesMut::with_capacity(10);
+        ///
+        ///     assert!(buffer.is_empty());
+        ///
+        ///     // read up to 10 bytes, note that the return value is not needed
+        ///     // to access the data that was read as `buffer`'s internal
+        ///     // cursor is updated.
+        ///     f.read_buf(&mut buffer).await?;
+        ///
+        ///     println!("The bytes: {:?}", &buffer[..]);
+        ///     Ok(())
+        /// }
+        /// ```
+        fn read_buf<'a, B>(&'a mut self, buf: &'a mut B) -> ReadBuf<'a, Self, B>
+        where
+            Self: Sized,
+            B: BufMut,
+        {
+            read_buf(self, buf)
+        }
+
+        /// Reads the exact number of bytes required to fill `buf`.
+        ///
+        /// Equivalent to:
+        ///
+        /// ```ignore
+        /// async fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<usize>;
+        /// ```
+        ///
+        /// This function reads as many bytes as necessary to completely fill
+        /// the specified buffer `buf`.
+        ///
+        /// No guarantees are provided about the contents of `buf` when this
+        /// function is called, implementations cannot rely on any property of
+        /// the contents of `buf` being `true`. It is recommended that
+        /// implementations only write data to `buf` instead of reading its
+        /// contents.
+        ///
+        /// # Errors
+        ///
+        /// If the operation encounters an "end of file" before completely
+        /// filling the buffer, it returns an error of the kind
+        /// [`ErrorKind::UnexpectedEof`]. The contents of `buf` are unspecified
+        /// in this case.
+        ///
+        /// If any other read error is encountered then the operation
+        /// immediately returns. The contents of `buf` are unspecified in this
+        /// case.
+        ///
+        /// If this operation returns an error, it is unspecified how many bytes
+        /// it has read, but it will never read more than would be necessary to
+        /// completely fill the buffer.
+        ///
+        /// # Examples
+        ///
+        /// [`File`][crate::fs::File]s implement `Read`:
+        ///
+        /// ```no_run
+        /// use tokio::fs::File;
+        /// use tokio::io::{self, AsyncReadExt};
+        ///
+        /// #[tokio::main]
+        /// async fn main() -> io::Result<()> {
+        ///     let mut f = File::open("foo.txt").await?;
+        ///     let mut buffer = [0; 10];
+        ///
+        ///     // read exactly 10 bytes
+        ///     f.read_exact(&mut buffer).await?;
+        ///     Ok(())
+        /// }
+        /// ```
+        ///
+        /// [`ErrorKind::UnexpectedEof`]: std::io::ErrorKind::UnexpectedEof
+        fn read_exact<'a>(&'a mut self, buf: &'a mut [u8]) -> ReadExact<'a, Self>
+        where
+            Self: Unpin,
+        {
+            read_exact(self, buf)
+        }
+
+        read_impl! {
+            /// Reads an unsigned 8 bit integer from the underlying reader.
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn read_u8(&mut self) -> io::Result<u8>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered reader to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncReadExt::read_exact`].
+            ///
+            /// [`AsyncReadExt::read_exact`]: AsyncReadExt::read_exact
+            ///
+            /// # Examples
+            ///
+            /// Read unsigned 8 bit integers from an `AsyncRead`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncReadExt};
+            ///
+            /// use std::io::Cursor;
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut reader = Cursor::new(vec![2, 5]);
+            ///
+            ///     assert_eq!(2, reader.read_u8().await?);
+            ///     assert_eq!(5, reader.read_u8().await?);
+            ///
+            ///     Ok(())
+            /// }
+            /// ```
+            fn read_u8(&mut self) -> ReadU8;
+
+            /// Reads a signed 8 bit integer from the underlying reader.
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn read_i8(&mut self) -> io::Result<i8>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered reader to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncReadExt::read_exact`].
+            ///
+            /// [`AsyncReadExt::read_exact`]: AsyncReadExt::read_exact
+            ///
+            /// # Examples
+            ///
+            /// Read unsigned 8 bit integers from an `AsyncRead`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncReadExt};
+            ///
+            /// use std::io::Cursor;
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut reader = Cursor::new(vec![0x02, 0xfb]);
+            ///
+            ///     assert_eq!(2, reader.read_i8().await?);
+            ///     assert_eq!(-5, reader.read_i8().await?);
+            ///
+            ///     Ok(())
+            /// }
+            /// ```
+            fn read_i8(&mut self) -> ReadI8;
+
+            /// Reads an unsigned 16-bit integer in big-endian order from the
+            /// underlying reader.
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn read_u16(&mut self) -> io::Result<u16>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered reader to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncReadExt::read_exact`].
+            ///
+            /// [`AsyncReadExt::read_exact`]: AsyncReadExt::read_exact
+            ///
+            /// # Examples
+            ///
+            /// Read unsigned 16 bit big-endian integers from a `AsyncRead`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncReadExt};
+            ///
+            /// use std::io::Cursor;
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut reader = Cursor::new(vec![2, 5, 3, 0]);
+            ///
+            ///     assert_eq!(517, reader.read_u16().await?);
+            ///     assert_eq!(768, reader.read_u16().await?);
+            ///     Ok(())
+            /// }
+            /// ```
+            fn read_u16(&mut self) -> ReadU16;
+
+            /// Reads a signed 16-bit integer in big-endian order from the
+            /// underlying reader.
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn read_i16(&mut self) -> io::Result<i16>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered reader to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncReadExt::read_exact`].
+            ///
+            /// [`AsyncReadExt::read_exact`]: AsyncReadExt::read_exact
+            ///
+            /// # Examples
+            ///
+            /// Read signed 16 bit big-endian integers from a `AsyncRead`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncReadExt};
+            ///
+            /// use std::io::Cursor;
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut reader = Cursor::new(vec![0x00, 0xc1, 0xff, 0x7c]);
+            ///
+            ///     assert_eq!(193, reader.read_i16().await?);
+            ///     assert_eq!(-132, reader.read_i16().await?);
+            ///     Ok(())
+            /// }
+            /// ```
+            fn read_i16(&mut self) -> ReadI16;
+
+            /// Reads an unsigned 32-bit integer in big-endian order from the
+            /// underlying reader.
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn read_u32(&mut self) -> io::Result<u32>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered reader to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncReadExt::read_exact`].
+            ///
+            /// [`AsyncReadExt::read_exact`]: AsyncReadExt::read_exact
+            ///
+            /// # Examples
+            ///
+            /// Read unsigned 32-bit big-endian integers from a `AsyncRead`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncReadExt};
+            ///
+            /// use std::io::Cursor;
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut reader = Cursor::new(vec![0x00, 0x00, 0x01, 0x0b]);
+            ///
+            ///     assert_eq!(267, reader.read_u32().await?);
+            ///     Ok(())
+            /// }
+            /// ```
+            fn read_u32(&mut self) -> ReadU32;
+
+            /// Reads a signed 32-bit integer in big-endian order from the
+            /// underlying reader.
+            ///
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn read_i32(&mut self) -> io::Result<i32>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered reader to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncReadExt::read_exact`].
+            ///
+            /// [`AsyncReadExt::read_exact`]: AsyncReadExt::read_exact
+            ///
+            /// # Examples
+            ///
+            /// Read signed 32-bit big-endian integers from a `AsyncRead`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncReadExt};
+            ///
+            /// use std::io::Cursor;
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut reader = Cursor::new(vec![0xff, 0xff, 0x7a, 0x33]);
+            ///
+            ///     assert_eq!(-34253, reader.read_i32().await?);
+            ///     Ok(())
+            /// }
+            /// ```
+            fn read_i32(&mut self) -> ReadI32;
+
+            /// Reads an unsigned 64-bit integer in big-endian order from the
+            /// underlying reader.
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn read_u64(&mut self) -> io::Result<u64>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered reader to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncReadExt::read_exact`].
+            ///
+            /// [`AsyncReadExt::read_exact`]: AsyncReadExt::read_exact
+            ///
+            /// # Examples
+            ///
+            /// Read unsigned 64-bit big-endian integers from a `AsyncRead`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncReadExt};
+            ///
+            /// use std::io::Cursor;
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut reader = Cursor::new(vec![
+            ///         0x00, 0x03, 0x43, 0x95, 0x4d, 0x60, 0x86, 0x83
+            ///     ]);
+            ///
+            ///     assert_eq!(918733457491587, reader.read_u64().await?);
+            ///     Ok(())
+            /// }
+            /// ```
+            fn read_u64(&mut self) -> ReadU64;
+
+            /// Reads an signed 64-bit integer in big-endian order from the
+            /// underlying reader.
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn read_i64(&mut self) -> io::Result<i64>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered reader to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncReadExt::read_exact`].
+            ///
+            /// [`AsyncReadExt::read_exact`]: AsyncReadExt::read_exact
+            ///
+            /// # Examples
+            ///
+            /// Read signed 64-bit big-endian integers from a `AsyncRead`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncReadExt};
+            ///
+            /// use std::io::Cursor;
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut reader = Cursor::new(vec![0x80, 0, 0, 0, 0, 0, 0, 0]);
+            ///
+            ///     assert_eq!(i64::min_value(), reader.read_i64().await?);
+            ///     Ok(())
+            /// }
+            /// ```
+            fn read_i64(&mut self) -> ReadI64;
+
+            /// Reads an unsigned 128-bit integer in big-endian order from the
+            /// underlying reader.
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn read_u128(&mut self) -> io::Result<u128>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered reader to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncReadExt::read_exact`].
+            ///
+            /// [`AsyncReadExt::read_exact`]: AsyncReadExt::read_exact
+            ///
+            /// # Examples
+            ///
+            /// Read unsigned 128-bit big-endian integers from a `AsyncRead`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncReadExt};
+            ///
+            /// use std::io::Cursor;
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut reader = Cursor::new(vec![
+            ///         0x00, 0x03, 0x43, 0x95, 0x4d, 0x60, 0x86, 0x83,
+            ///         0x00, 0x03, 0x43, 0x95, 0x4d, 0x60, 0x86, 0x83
+            ///     ]);
+            ///
+            ///     assert_eq!(16947640962301618749969007319746179, reader.read_u128().await?);
+            ///     Ok(())
+            /// }
+            /// ```
+            fn read_u128(&mut self) -> ReadU128;
+
+            /// Reads an signed 128-bit integer in big-endian order from the
+            /// underlying reader.
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn read_i128(&mut self) -> io::Result<i128>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered reader to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncReadExt::read_exact`].
+            ///
+            /// [`AsyncReadExt::read_exact`]: AsyncReadExt::read_exact
+            ///
+            /// # Examples
+            ///
+            /// Read signed 128-bit big-endian integers from a `AsyncRead`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncReadExt};
+            ///
+            /// use std::io::Cursor;
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut reader = Cursor::new(vec![
+            ///         0x80, 0, 0, 0, 0, 0, 0, 0,
+            ///         0, 0, 0, 0, 0, 0, 0, 0
+            ///     ]);
+            ///
+            ///     assert_eq!(i128::min_value(), reader.read_i128().await?);
+            ///     Ok(())
+            /// }
+            /// ```
+            fn read_i128(&mut self) -> ReadI128;
+        }
+
+        /// Reads all bytes until EOF in this source, placing them into `buf`.
+        ///
+        /// Equivalent to:
+        ///
+        /// ```ignore
+        /// async fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize>;
+        /// ```
+        ///
+        /// All bytes read from this source will be appended to the specified
+        /// buffer `buf`. This function will continuously call [`read()`] to
+        /// append more data to `buf` until [`read()`][read] returns `Ok(0)`.
+        ///
+        /// If successful, the total number of bytes read is returned.
+        ///
+        /// # Errors
+        ///
+        /// If a read error is encountered then the `read_to_end` operation
+        /// immediately completes. Any bytes which have already been read will
+        /// be appended to `buf`.
+        ///
+        /// # Examples
+        ///
+        /// [`File`][crate::fs::File]s implement `Read`:
+        ///
+        /// ```no_run
+        /// use tokio::io::{self, AsyncReadExt};
+        /// use tokio::fs::File;
+        ///
+        /// #[tokio::main]
+        /// async fn main() -> io::Result<()> {
+        ///     let mut f = File::open("foo.txt").await?;
+        ///     let mut buffer = Vec::new();
+        ///
+        ///     // read the whole file
+        ///     f.read_to_end(&mut buffer).await?;
+        ///     Ok(())
+        /// }
+        /// ```
+        ///
+        /// (See also the [`tokio::fs::read`] convenience function for reading from a
+        /// file.)
+        ///
+        /// [`tokio::fs::read`]: crate::fs::read::read
+        fn read_to_end<'a>(&'a mut self, buf: &'a mut Vec<u8>) -> ReadToEnd<'a, Self>
+        where
+            Self: Unpin,
+        {
+            read_to_end(self, buf)
+        }
+
+        /// Reads all bytes until EOF in this source, appending them to `buf`.
+        ///
+        /// Equivalent to:
+        ///
+        /// ```ignore
+        /// async fn read_to_string(&mut self, buf: &mut String) -> io::Result<usize>;
+        /// ```
+        ///
+        /// If successful, the number of bytes which were read and appended to
+        /// `buf` is returned.
+        ///
+        /// # Errors
+        ///
+        /// If the data in this stream is *not* valid UTF-8 then an error is
+        /// returned and `buf` is unchanged.
+        ///
+        /// See [`read_to_end`][AsyncReadExt::read_to_end] for other error semantics.
+        ///
+        /// # Examples
+        ///
+        /// [`File`][crate::fs::File]s implement `Read`:
+        ///
+        /// ```no_run
+        /// use tokio::io::{self, AsyncReadExt};
+        /// use tokio::fs::File;
+        ///
+        /// #[tokio::main]
+        /// async fn main() -> io::Result<()> {
+        ///     let mut f = File::open("foo.txt").await?;
+        ///     let mut buffer = String::new();
+        ///
+        ///     f.read_to_string(&mut buffer).await?;
+        ///     Ok(())
+        /// }
+        /// ```
+        ///
+        /// (See also the [`crate::fs::read_to_string`] convenience function for
+        /// reading from a file.)
+        ///
+        /// [`crate::fs::read_to_string`]: crate::fs::read_to_string::read_to_string
+        fn read_to_string<'a>(&'a mut self, dst: &'a mut String) -> ReadToString<'a, Self>
+        where
+            Self: Unpin,
+        {
+            read_to_string(self, dst)
+        }
+
+        /// Creates an adaptor which reads at most `limit` bytes from it.
+        ///
+        /// This function returns a new instance of `AsyncRead` which will read
+        /// at most `limit` bytes, after which it will always return EOF
+        /// (`Ok(0)`). Any read errors will not count towards the number of
+        /// bytes read and future calls to [`read()`][read] may succeed.
+        ///
+        /// # Examples
+        ///
+        /// [`File`][crate::fs::File]s implement `Read`:
+        ///
+        /// ```no_run
+        /// use tokio::io::{self, AsyncReadExt};
+        /// use tokio::fs::File;
+        ///
+        /// #[tokio::main]
+        /// async fn main() -> io::Result<()> {
+        ///     let f = File::open("foo.txt").await?;
+        ///     let mut buffer = [0; 5];
+        ///
+        ///     // read at most five bytes
+        ///     let mut handle = f.take(5);
+        ///
+        ///     handle.read(&mut buffer).await?;
+        ///     Ok(())
+        /// }
+        /// ```
+        fn take(self, limit: u64) -> Take<Self>
+        where
+            Self: Sized,
+        {
+            take(self, limit)
+        }
+    }
+}
+
+impl<R: AsyncRead + ?Sized> AsyncReadExt for R {}
diff --git a/third_party/rust/tokio/src/io/util/async_seek_ext.rs b/third_party/rust/tokio/src/io/util/async_seek_ext.rs
new file mode 100644
index 0000000000..c7243c7f3e
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/async_seek_ext.rs
@@ -0,0 +1,60 @@
+use crate::io::seek::{seek, Seek};
+use crate::io::AsyncSeek;
+use std::io::SeekFrom;
+
+/// An extension trait which adds utility methods to `AsyncSeek` types.
+///
+/// # Examples
+///
+/// ```
+/// use std::io::{Cursor, SeekFrom};
+/// use tokio::prelude::*;
+///
+/// #[tokio::main]
+/// async fn main() -> io::Result<()> {
+///     let mut cursor = Cursor::new(b"abcdefg");
+///
+///     // the `seek` method is defined by this trait
+///     cursor.seek(SeekFrom::Start(3)).await?;
+///
+///     let mut buf = [0; 1];
+///     let n = cursor.read(&mut buf).await?;
+///     assert_eq!(n, 1);
+///     assert_eq!(buf, [b'd']);
+///
+///     Ok(())
+/// }
+/// ```
+pub trait AsyncSeekExt: AsyncSeek {
+    /// Creates a future which will seek an IO object, and then yield the
+    /// new position in the object and the object itself.
+    ///
+    /// In the case of an error the buffer and the object will be discarded, with
+    /// the error yielded.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    /// use tokio::prelude::*;
+    ///
+    /// use std::io::SeekFrom;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut file = File::open("foo.txt").await?;
+    /// file.seek(SeekFrom::Start(6)).await?;
+    ///
+    /// let mut contents = vec![0u8; 10];
+    /// file.read_exact(&mut contents).await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    fn seek(&mut self, pos: SeekFrom) -> Seek<'_, Self>
+    where
+        Self: Unpin,
+    {
+        seek(self, pos)
+    }
+}
+
+impl<S: AsyncSeek + ?Sized> AsyncSeekExt for S {}
diff --git a/third_party/rust/tokio/src/io/util/async_write_ext.rs b/third_party/rust/tokio/src/io/util/async_write_ext.rs
new file mode 100644
index 0000000000..377f4ecaf8
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/async_write_ext.rs
@@ -0,0 +1,689 @@
+use crate::io::util::flush::{flush, Flush};
+use crate::io::util::shutdown::{shutdown, Shutdown};
+use crate::io::util::write::{write, Write};
+use crate::io::util::write_all::{write_all, WriteAll};
+use crate::io::util::write_buf::{write_buf, WriteBuf};
+use crate::io::util::write_int::{WriteI128, WriteI16, WriteI32, WriteI64, WriteI8};
+use crate::io::util::write_int::{WriteU128, WriteU16, WriteU32, WriteU64, WriteU8};
+use crate::io::AsyncWrite;
+
+use bytes::Buf;
+
+cfg_io_util! {
+    /// Defines numeric writer
+    macro_rules! write_impl {
+        (
+            $(
+                $(#[$outer:meta])*
+                fn $name:ident(&mut self, n: $ty:ty) -> $($fut:ident)*;
+            )*
+        ) => {
+            $(
+                $(#[$outer])*
+                fn $name<'a>(&'a mut self, n: $ty) -> $($fut)*<&'a mut Self> where Self: Unpin {
+                    $($fut)*::new(self, n)
+                }
+            )*
+        }
+    }
+
+    /// Writes bytes to a sink.
+    ///
+    /// Implemented as an extention trait, adding utility methods to all
+    /// [`AsyncWrite`] types. Callers will tend to import this trait instead of
+    /// [`AsyncWrite`].
+    ///
+    /// As a convenience, this trait may be imported using the [`prelude`]:
+    ///
+    /// ```no_run
+    /// use tokio::prelude::*;
+    /// use tokio::fs::File;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let data = b"some bytes";
+    ///
+    ///     let mut pos = 0;
+    ///     let mut buffer = File::create("foo.txt").await?;
+    ///
+    ///     while pos < data.len() {
+    ///         let bytes_written = buffer.write(&data[pos..]).await?;
+    ///         pos += bytes_written;
+    ///     }
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    ///
+    /// See [module][crate::io] documentation for more details.
+    ///
+    /// [`AsyncWrite`]: AsyncWrite
+    /// [`prelude`]: crate::prelude
+    pub trait AsyncWriteExt: AsyncWrite {
+        /// Writes a buffer into this writer, returning how many bytes were
+        /// written.
+        ///
+        /// Equivalent to:
+        ///
+        /// ```ignore
+        /// async fn write(&mut self, buf: &[u8]) -> io::Result<usize>;
+        /// ```
+        ///
+        /// This function will attempt to write the entire contents of `buf`, but
+        /// the entire write may not succeed, or the write may also generate an
+        /// error. A call to `write` represents *at most one* attempt to write to
+        /// any wrapped object.
+        ///
+        /// # Return
+        ///
+        /// If the return value is `Ok(n)` then it must be guaranteed that `n <=
+        /// buf.len()`. A return value of `0` typically means that the
+        /// underlying object is no longer able to accept bytes and will likely
+        /// not be able to in the future as well, or that the buffer provided is
+        /// empty.
+        ///
+        /// # Errors
+        ///
+        /// Each call to `write` may generate an I/O error indicating that the
+        /// operation could not be completed. If an error is returned then no bytes
+        /// in the buffer were written to this writer.
+        ///
+        /// It is **not** considered an error if the entire buffer could not be
+        /// written to this writer.
+        ///
+        /// # Examples
+        ///
+        /// ```no_run
+        /// use tokio::io::{self, AsyncWriteExt};
+        /// use tokio::fs::File;
+        ///
+        /// #[tokio::main]
+        /// async fn main() -> io::Result<()> {
+        ///     let mut file = File::create("foo.txt").await?;
+        ///
+        ///     // Writes some prefix of the byte string, not necessarily all of it.
+        ///     file.write(b"some bytes").await?;
+        ///     Ok(())
+        /// }
+        /// ```
+        fn write<'a>(&'a mut self, src: &'a [u8]) -> Write<'a, Self>
+        where
+            Self: Unpin,
+        {
+            write(self, src)
+        }
+
+        /// Writes a buffer into this writer, advancing the buffer's internal
+        /// cursor.
+        ///
+        /// Equivalent to:
+        ///
+        /// ```ignore
+        /// async fn write_buf<B: Buf>(&mut self, buf: &mut B) -> io::Result<usize>;
+        /// ```
+        ///
+        /// This function will attempt to write the entire contents of `buf`, but
+        /// the entire write may not succeed, or the write may also generate an
+        /// error. After the operation completes, the buffer's
+        /// internal cursor is advanced by the number of bytes written. A
+        /// subsequent call to `write_buf` using the **same** `buf` value will
+        /// resume from the point that the first call to `write_buf` completed.
+        /// A call to `write` represents *at most one* attempt to write to any
+        /// wrapped object.
+        ///
+        /// # Return
+        ///
+        /// If the return value is `Ok(n)` then it must be guaranteed that `n <=
+        /// buf.len()`. A return value of `0` typically means that the
+        /// underlying object is no longer able to accept bytes and will likely
+        /// not be able to in the future as well, or that the buffer provided is
+        /// empty.
+        ///
+        /// # Errors
+        ///
+        /// Each call to `write` may generate an I/O error indicating that the
+        /// operation could not be completed. If an error is returned then no bytes
+        /// in the buffer were written to this writer.
+        ///
+        /// It is **not** considered an error if the entire buffer could not be
+        /// written to this writer.
+        ///
+        /// # Examples
+        ///
+        /// [`File`] implements `Read` and [`Cursor<&[u8]>`] implements [`Buf`]:
+        ///
+        /// [`File`]: crate::fs::File
+        /// [`Buf`]: bytes::Buf
+        ///
+        /// ```no_run
+        /// use tokio::io::{self, AsyncWriteExt};
+        /// use tokio::fs::File;
+        ///
+        /// use bytes::Buf;
+        /// use std::io::Cursor;
+        ///
+        /// #[tokio::main]
+        /// async fn main() -> io::Result<()> {
+        ///     let mut file = File::create("foo.txt").await?;
+        ///     let mut buffer = Cursor::new(b"data to write");
+        ///
+        ///     // Loop until the entire contents of the buffer are written to
+        ///     // the file.
+        ///     while buffer.has_remaining() {
+        ///         // Writes some prefix of the byte string, not necessarily
+        ///         // all of it.
+        ///         file.write_buf(&mut buffer).await?;
+        ///     }
+        ///
+        ///     Ok(())
+        /// }
+        /// ```
+        fn write_buf<'a, B>(&'a mut self, src: &'a mut B) -> WriteBuf<'a, Self, B>
+        where
+            Self: Sized,
+            B: Buf,
+        {
+            write_buf(self, src)
+        }
+
+        /// Attempts to write an entire buffer into this writer.
+        ///
+        /// Equivalent to:
+        ///
+        /// ```ignore
+        /// async fn write_all(&mut self, buf: &[u8]) -> io::Result<()>;
+        /// ```
+        ///
+        /// This method will continuously call [`write`] until there is no more data
+        /// to be written. This method will not return until the entire buffer
+        /// has been successfully written or such an error occurs. The first
+        /// error generated from this method will be returned.
+        ///
+        /// # Errors
+        ///
+        /// This function will return the first error that [`write`] returns.
+        ///
+        /// # Examples
+        ///
+        /// ```no_run
+        /// use tokio::io::{self, AsyncWriteExt};
+        /// use tokio::fs::File;
+        ///
+        /// #[tokio::main]
+        /// async fn main() -> io::Result<()> {
+        ///     let mut buffer = File::create("foo.txt").await?;
+        ///
+        ///     buffer.write_all(b"some bytes").await?;
+        ///     Ok(())
+        /// }
+        /// ```
+        ///
+        /// [`write`]: AsyncWriteExt::write
+        fn write_all<'a>(&'a mut self, src: &'a [u8]) -> WriteAll<'a, Self>
+        where
+            Self: Unpin,
+        {
+            write_all(self, src)
+        }
+
+        write_impl! {
+            /// Writes an unsigned 8-bit integer to the underlying writer.
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn write_u8(&mut self, n: u8) -> io::Result<()>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered writer to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncWriteExt::write_all`].
+            ///
+            /// [`AsyncWriteExt::write_all`]: AsyncWriteExt::write_all
+            ///
+            /// # Examples
+            ///
+            /// Write unsigned 8 bit integers to a `AsyncWrite`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncWriteExt};
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut writer = Vec::new();
+            ///
+            ///     writer.write_u8(2).await?;
+            ///     writer.write_u8(5).await?;
+            ///
+            ///     assert_eq!(writer, b"\x02\x05");
+            ///     Ok(())
+            /// }
+            /// ```
+            fn write_u8(&mut self, n: u8) -> WriteU8;
+
+            /// Writes an unsigned 8-bit integer to the underlying writer.
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn write_i8(&mut self, n: i8) -> io::Result<()>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered writer to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncWriteExt::write_all`].
+            ///
+            /// [`AsyncWriteExt::write_all`]: AsyncWriteExt::write_all
+            ///
+            /// # Examples
+            ///
+            /// Write unsigned 8 bit integers to a `AsyncWrite`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncWriteExt};
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut writer = Vec::new();
+            ///
+            ///     writer.write_u8(2).await?;
+            ///     writer.write_u8(5).await?;
+            ///
+            ///     assert_eq!(writer, b"\x02\x05");
+            ///     Ok(())
+            /// }
+            /// ```
+            fn write_i8(&mut self, n: i8) -> WriteI8;
+
+            /// Writes an unsigned 16-bit integer in big-endian order to the
+            /// underlying writer.
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn write_u16(&mut self, n: u16) -> io::Result<()>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered writer to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncWriteExt::write_all`].
+            ///
+            /// [`AsyncWriteExt::write_all`]: AsyncWriteExt::write_all
+            ///
+            /// # Examples
+            ///
+            /// Write unsigned 16-bit integers to a `AsyncWrite`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncWriteExt};
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut writer = Vec::new();
+            ///
+            ///     writer.write_u16(517).await?;
+            ///     writer.write_u16(768).await?;
+            ///
+            ///     assert_eq!(writer, b"\x02\x05\x03\x00");
+            ///     Ok(())
+            /// }
+            /// ```
+            fn write_u16(&mut self, n: u16) -> WriteU16;
+
+            /// Writes a signed 16-bit integer in big-endian order to the
+            /// underlying writer.
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn write_i16(&mut self, n: i16) -> io::Result<()>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered writer to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncWriteExt::write_all`].
+            ///
+            /// [`AsyncWriteExt::write_all`]: AsyncWriteExt::write_all
+            ///
+            /// # Examples
+            ///
+            /// Write signed 16-bit integers to a `AsyncWrite`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncWriteExt};
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut writer = Vec::new();
+            ///
+            ///     writer.write_i16(193).await?;
+            ///     writer.write_i16(-132).await?;
+            ///
+            ///     assert_eq!(writer, b"\x00\xc1\xff\x7c");
+            ///     Ok(())
+            /// }
+            /// ```
+            fn write_i16(&mut self, n: i16) -> WriteI16;
+
+            /// Writes an unsigned 32-bit integer in big-endian order to the
+            /// underlying writer.
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn write_u32(&mut self, n: u32) -> io::Result<()>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered writer to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncWriteExt::write_all`].
+            ///
+            /// [`AsyncWriteExt::write_all`]: AsyncWriteExt::write_all
+            ///
+            /// # Examples
+            ///
+            /// Write unsigned 32-bit integers to a `AsyncWrite`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncWriteExt};
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut writer = Vec::new();
+            ///
+            ///     writer.write_u32(267).await?;
+            ///     writer.write_u32(1205419366).await?;
+            ///
+            ///     assert_eq!(writer, b"\x00\x00\x01\x0b\x47\xd9\x3d\x66");
+            ///     Ok(())
+            /// }
+            /// ```
+            fn write_u32(&mut self, n: u32) -> WriteU32;
+
+            /// Writes a signed 32-bit integer in big-endian order to the
+            /// underlying writer.
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn write_i32(&mut self, n: i32) -> io::Result<()>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered writer to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncWriteExt::write_all`].
+            ///
+            /// [`AsyncWriteExt::write_all`]: AsyncWriteExt::write_all
+            ///
+            /// # Examples
+            ///
+            /// Write signed 32-bit integers to a `AsyncWrite`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncWriteExt};
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut writer = Vec::new();
+            ///
+            ///     writer.write_i32(267).await?;
+            ///     writer.write_i32(1205419366).await?;
+            ///
+            ///     assert_eq!(writer, b"\x00\x00\x01\x0b\x47\xd9\x3d\x66");
+            ///     Ok(())
+            /// }
+            /// ```
+            fn write_i32(&mut self, n: i32) -> WriteI32;
+
+            /// Writes an unsigned 64-bit integer in big-endian order to the
+            /// underlying writer.
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn write_u64(&mut self, n: u64) -> io::Result<()>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered writer to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncWriteExt::write_all`].
+            ///
+            /// [`AsyncWriteExt::write_all`]: AsyncWriteExt::write_all
+            ///
+            /// # Examples
+            ///
+            /// Write unsigned 64-bit integers to a `AsyncWrite`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncWriteExt};
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut writer = Vec::new();
+            ///
+            ///     writer.write_u64(918733457491587).await?;
+            ///     writer.write_u64(143).await?;
+            ///
+            ///     assert_eq!(writer, b"\x00\x03\x43\x95\x4d\x60\x86\x83\x00\x00\x00\x00\x00\x00\x00\x8f");
+            ///     Ok(())
+            /// }
+            /// ```
+            fn write_u64(&mut self, n: u64) -> WriteU64;
+
+            /// Writes an signed 64-bit integer in big-endian order to the
+            /// underlying writer.
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn write_i64(&mut self, n: i64) -> io::Result<()>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered writer to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncWriteExt::write_all`].
+            ///
+            /// [`AsyncWriteExt::write_all`]: AsyncWriteExt::write_all
+            ///
+            /// # Examples
+            ///
+            /// Write signed 64-bit integers to a `AsyncWrite`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncWriteExt};
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut writer = Vec::new();
+            ///
+            ///     writer.write_i64(i64::min_value()).await?;
+            ///     writer.write_i64(i64::max_value()).await?;
+            ///
+            ///     assert_eq!(writer, b"\x80\x00\x00\x00\x00\x00\x00\x00\x7f\xff\xff\xff\xff\xff\xff\xff");
+            ///     Ok(())
+            /// }
+            /// ```
+            fn write_i64(&mut self, n: i64) -> WriteI64;
+
+            /// Writes an unsigned 128-bit integer in big-endian order to the
+            /// underlying writer.
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn write_u128(&mut self, n: u128) -> io::Result<()>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered writer to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncWriteExt::write_all`].
+            ///
+            /// [`AsyncWriteExt::write_all`]: AsyncWriteExt::write_all
+            ///
+            /// # Examples
+            ///
+            /// Write unsigned 128-bit integers to a `AsyncWrite`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncWriteExt};
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut writer = Vec::new();
+            ///
+            ///     writer.write_u128(16947640962301618749969007319746179).await?;
+            ///
+            ///     assert_eq!(writer, vec![
+            ///         0x00, 0x03, 0x43, 0x95, 0x4d, 0x60, 0x86, 0x83,
+            ///         0x00, 0x03, 0x43, 0x95, 0x4d, 0x60, 0x86, 0x83
+            ///     ]);
+            ///     Ok(())
+            /// }
+            /// ```
+            fn write_u128(&mut self, n: u128) -> WriteU128;
+
+            /// Writes an signed 128-bit integer in big-endian order to the
+            /// underlying writer.
+            ///
+            /// Equivalent to:
+            ///
+            /// ```ignore
+            /// async fn write_i128(&mut self, n: i128) -> io::Result<()>;
+            /// ```
+            ///
+            /// It is recommended to use a buffered writer to avoid excessive
+            /// syscalls.
+            ///
+            /// # Errors
+            ///
+            /// This method returns the same errors as [`AsyncWriteExt::write_all`].
+            ///
+            /// [`AsyncWriteExt::write_all`]: AsyncWriteExt::write_all
+            ///
+            /// # Examples
+            ///
+            /// Write signed 128-bit integers to a `AsyncWrite`:
+            ///
+            /// ```rust
+            /// use tokio::io::{self, AsyncWriteExt};
+            ///
+            /// #[tokio::main]
+            /// async fn main() -> io::Result<()> {
+            ///     let mut writer = Vec::new();
+            ///
+            ///     writer.write_i128(i128::min_value()).await?;
+            ///
+            ///     assert_eq!(writer, vec![
+            ///         0x80, 0, 0, 0, 0, 0, 0, 0,
+            ///         0, 0, 0, 0, 0, 0, 0, 0
+            ///     ]);
+            ///     Ok(())
+            /// }
+            /// ```
+            fn write_i128(&mut self, n: i128) -> WriteI128;
+        }
+
+        /// Flushes this output stream, ensuring that all intermediately buffered
+        /// contents reach their destination.
+        ///
+        /// Equivalent to:
+        ///
+        /// ```ignore
+        /// async fn flush(&mut self) -> io::Result<()>;
+        /// ```
+        ///
+        /// # Errors
+        ///
+        /// It is considered an error if not all bytes could be written due to
+        /// I/O errors or EOF being reached.
+        ///
+        /// # Examples
+        ///
+        /// ```no_run
+        /// use tokio::io::{self, BufWriter, AsyncWriteExt};
+        /// use tokio::fs::File;
+        ///
+        /// #[tokio::main]
+        /// async fn main() -> io::Result<()> {
+        ///     let f = File::create("foo.txt").await?;
+        ///     let mut buffer = BufWriter::new(f);
+        ///
+        ///     buffer.write_all(b"some bytes").await?;
+        ///     buffer.flush().await?;
+        ///     Ok(())
+        /// }
+        /// ```
+        fn flush(&mut self) -> Flush<'_, Self>
+        where
+            Self: Unpin,
+        {
+            flush(self)
+        }
+
+        /// Shuts down the output stream, ensuring that the value can be dropped
+        /// cleanly.
+        ///
+        /// Equivalent to:
+        ///
+        /// ```ignore
+        /// async fn shutdown(&mut self) -> io::Result<()>;
+        /// ```
+        ///
+        /// Similar to [`flush`], all intermediately buffered is written to the
+        /// underlying stream. Once the operation completes, the caller should
+        /// no longer attempt to write to the stream. For example, the
+        /// `TcpStream` implementation will issue a `shutdown(Write)` sys call.
+        ///
+        /// # Examples
+        ///
+        /// ```no_run
+        /// use tokio::io::{self, BufWriter, AsyncWriteExt};
+        /// use tokio::fs::File;
+        ///
+        /// #[tokio::main]
+        /// async fn main() -> io::Result<()> {
+        ///     let f = File::create("foo.txt").await?;
+        ///     let mut buffer = BufWriter::new(f);
+        ///
+        ///     buffer.write_all(b"some bytes").await?;
+        ///     buffer.shutdown().await?;
+        ///     Ok(())
+        /// }
+        /// ```
+        fn shutdown(&mut self) -> Shutdown<'_, Self>
+        where
+            Self: Unpin,
+        {
+            shutdown(self)
+        }
+    }
+}
+
+impl<W: AsyncWrite + ?Sized> AsyncWriteExt for W {}
diff --git a/third_party/rust/tokio/src/io/util/buf_reader.rs b/third_party/rust/tokio/src/io/util/buf_reader.rs
new file mode 100644
index 0000000000..0177c0e344
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/buf_reader.rs
@@ -0,0 +1,194 @@
+use crate::io::util::DEFAULT_BUF_SIZE;
+use crate::io::{AsyncBufRead, AsyncRead, AsyncWrite};
+
+use pin_project_lite::pin_project;
+use std::io::{self, Read};
+use std::mem::MaybeUninit;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+use std::{cmp, fmt};
+
+pin_project! {
+    /// The `BufReader` struct adds buffering to any reader.
+    ///
+    /// It can be excessively inefficient to work directly with a [`AsyncRead`]
+    /// instance. A `BufReader` performs large, infrequent reads on the underlying
+    /// [`AsyncRead`] and maintains an in-memory buffer of the results.
+    ///
+    /// `BufReader` can improve the speed of programs that make *small* and
+    /// *repeated* read calls to the same file or network socket. It does not
+    /// help when reading very large amounts at once, or reading just one or a few
+    /// times. It also provides no advantage when reading from a source that is
+    /// already in memory, like a `Vec<u8>`.
+    ///
+    /// When the `BufReader` is dropped, the contents of its buffer will be
+    /// discarded. Creating multiple instances of a `BufReader` on the same
+    /// stream can cause data loss.
+    #[cfg_attr(docsrs, doc(cfg(feature = "io-util")))]
+    pub struct BufReader<R> {
+        #[pin]
+        pub(super) inner: R,
+        pub(super) buf: Box<[u8]>,
+        pub(super) pos: usize,
+        pub(super) cap: usize,
+    }
+}
+
+impl<R: AsyncRead> BufReader<R> {
+    /// Creates a new `BufReader` with a default buffer capacity. The default is currently 8 KB,
+    /// but may change in the future.
+    pub fn new(inner: R) -> Self {
+        Self::with_capacity(DEFAULT_BUF_SIZE, inner)
+    }
+
+    /// Creates a new `BufReader` with the specified buffer capacity.
+    pub fn with_capacity(capacity: usize, inner: R) -> Self {
+        unsafe {
+            let mut buffer = Vec::with_capacity(capacity);
+            buffer.set_len(capacity);
+
+            {
+                // Convert to MaybeUninit
+                let b = &mut *(&mut buffer[..] as *mut [u8] as *mut [MaybeUninit<u8>]);
+                inner.prepare_uninitialized_buffer(b);
+            }
+            Self {
+                inner,
+                buf: buffer.into_boxed_slice(),
+                pos: 0,
+                cap: 0,
+            }
+        }
+    }
+
+    /// Gets a reference to the underlying reader.
+    ///
+    /// It is inadvisable to directly read from the underlying reader.
+    pub fn get_ref(&self) -> &R {
+        &self.inner
+    }
+
+    /// Gets a mutable reference to the underlying reader.
+    ///
+    /// It is inadvisable to directly read from the underlying reader.
+    pub fn get_mut(&mut self) -> &mut R {
+        &mut self.inner
+    }
+
+    /// Gets a pinned mutable reference to the underlying reader.
+    ///
+    /// It is inadvisable to directly read from the underlying reader.
+    pub fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut R> {
+        self.project().inner
+    }
+
+    /// Consumes this `BufWriter`, returning the underlying reader.
+    ///
+    /// Note that any leftover data in the internal buffer is lost.
+    pub fn into_inner(self) -> R {
+        self.inner
+    }
+
+    /// Returns a reference to the internally buffered data.
+    ///
+    /// Unlike `fill_buf`, this will not attempt to fill the buffer if it is empty.
+    pub fn buffer(&self) -> &[u8] {
+        &self.buf[self.pos..self.cap]
+    }
+
+    /// Invalidates all data in the internal buffer.
+    #[inline]
+    fn discard_buffer(self: Pin<&mut Self>) {
+        let me = self.project();
+        *me.pos = 0;
+        *me.cap = 0;
+    }
+}
+
+impl<R: AsyncRead> AsyncRead for BufReader<R> {
+    fn poll_read(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        // If we don't have any buffered data and we're doing a massive read
+        // (larger than our internal buffer), bypass our internal buffer
+        // entirely.
+        if self.pos == self.cap && buf.len() >= self.buf.len() {
+            let res = ready!(self.as_mut().get_pin_mut().poll_read(cx, buf));
+            self.discard_buffer();
+            return Poll::Ready(res);
+        }
+        let mut rem = ready!(self.as_mut().poll_fill_buf(cx))?;
+        let nread = rem.read(buf)?;
+        self.consume(nread);
+        Poll::Ready(Ok(nread))
+    }
+
+    // we can't skip unconditionally because of the large buffer case in read.
+    unsafe fn prepare_uninitialized_buffer(&self, buf: &mut [MaybeUninit<u8>]) -> bool {
+        self.inner.prepare_uninitialized_buffer(buf)
+    }
+}
+
+impl<R: AsyncRead> AsyncBufRead for BufReader<R> {
+    fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
+        let me = self.project();
+
+        // If we've reached the end of our internal buffer then we need to fetch
+        // some more data from the underlying reader.
+        // Branch using `>=` instead of the more correct `==`
+        // to tell the compiler that the pos..cap slice is always valid.
+        if *me.pos >= *me.cap {
+            debug_assert!(*me.pos == *me.cap);
+            *me.cap = ready!(me.inner.poll_read(cx, me.buf))?;
+            *me.pos = 0;
+        }
+        Poll::Ready(Ok(&me.buf[*me.pos..*me.cap]))
+    }
+
+    fn consume(self: Pin<&mut Self>, amt: usize) {
+        let me = self.project();
+        *me.pos = cmp::min(*me.pos + amt, *me.cap);
+    }
+}
+
+impl<R: AsyncRead + AsyncWrite> AsyncWrite for BufReader<R> {
+    fn poll_write(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        self.get_pin_mut().poll_write(cx, buf)
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        self.get_pin_mut().poll_flush(cx)
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        self.get_pin_mut().poll_shutdown(cx)
+    }
+}
+
+impl<R: fmt::Debug> fmt::Debug for BufReader<R> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("BufReader")
+            .field("reader", &self.inner)
+            .field(
+                "buffer",
+                &format_args!("{}/{}", self.cap - self.pos, self.buf.len()),
+            )
+            .finish()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        crate::is_unpin::<BufReader<()>>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/buf_stream.rs b/third_party/rust/tokio/src/io/util/buf_stream.rs
new file mode 100644
index 0000000000..a56a4517fa
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/buf_stream.rs
@@ -0,0 +1,169 @@
+use crate::io::util::{BufReader, BufWriter};
+use crate::io::{AsyncBufRead, AsyncRead, AsyncWrite};
+
+use pin_project_lite::pin_project;
+use std::io;
+use std::mem::MaybeUninit;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+pin_project! {
+    /// Wraps a type that is [`AsyncWrite`] and [`AsyncRead`], and buffers its input and output.
+    ///
+    /// It can be excessively inefficient to work directly with something that implements [`AsyncWrite`]
+    /// and [`AsyncRead`]. For example, every `write`, however small, has to traverse the syscall
+    /// interface, and similarly, every read has to do the same. The [`BufWriter`] and [`BufReader`]
+    /// types aid with these problems respectively, but do so in only one direction. `BufStream` wraps
+    /// one in the other so that both directions are buffered. See their documentation for details.
+    #[derive(Debug)]
+    #[cfg_attr(docsrs, doc(cfg(feature = "io-util")))]
+    pub struct BufStream<RW> {
+        #[pin]
+        inner: BufReader<BufWriter<RW>>,
+    }
+}
+
+impl<RW: AsyncRead + AsyncWrite> BufStream<RW> {
+    /// Wraps a type in both [`BufWriter`] and [`BufReader`].
+    ///
+    /// See the documentation for those types and [`BufStream`] for details.
+    pub fn new(stream: RW) -> BufStream<RW> {
+        BufStream {
+            inner: BufReader::new(BufWriter::new(stream)),
+        }
+    }
+
+    /// Creates a `BufStream` with the specified [`BufReader`] capacity and [`BufWriter`]
+    /// capacity.
+    ///
+    /// See the documentation for those types and [`BufStream`] for details.
+    pub fn with_capacity(
+        reader_capacity: usize,
+        writer_capacity: usize,
+        stream: RW,
+    ) -> BufStream<RW> {
+        BufStream {
+            inner: BufReader::with_capacity(
+                reader_capacity,
+                BufWriter::with_capacity(writer_capacity, stream),
+            ),
+        }
+    }
+
+    /// Gets a reference to the underlying I/O object.
+    ///
+    /// It is inadvisable to directly read from the underlying I/O object.
+    pub fn get_ref(&self) -> &RW {
+        self.inner.get_ref().get_ref()
+    }
+
+    /// Gets a mutable reference to the underlying I/O object.
+    ///
+    /// It is inadvisable to directly read from the underlying I/O object.
+    pub fn get_mut(&mut self) -> &mut RW {
+        self.inner.get_mut().get_mut()
+    }
+
+    /// Gets a pinned mutable reference to the underlying I/O object.
+    ///
+    /// It is inadvisable to directly read from the underlying I/O object.
+    pub fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut RW> {
+        self.project().inner.get_pin_mut().get_pin_mut()
+    }
+
+    /// Consumes this `BufStream`, returning the underlying I/O object.
+    ///
+    /// Note that any leftover data in the internal buffer is lost.
+    pub fn into_inner(self) -> RW {
+        self.inner.into_inner().into_inner()
+    }
+}
+
+impl<RW> From<BufReader<BufWriter<RW>>> for BufStream<RW> {
+    fn from(b: BufReader<BufWriter<RW>>) -> Self {
+        BufStream { inner: b }
+    }
+}
+
+impl<RW> From<BufWriter<BufReader<RW>>> for BufStream<RW> {
+    fn from(b: BufWriter<BufReader<RW>>) -> Self {
+        // we need to "invert" the reader and writer
+        let BufWriter {
+            inner:
+                BufReader {
+                    inner,
+                    buf: rbuf,
+                    pos,
+                    cap,
+                },
+            buf: wbuf,
+            written,
+        } = b;
+
+        BufStream {
+            inner: BufReader {
+                inner: BufWriter {
+                    inner,
+                    buf: wbuf,
+                    written,
+                },
+                buf: rbuf,
+                pos,
+                cap,
+            },
+        }
+    }
+}
+
+impl<RW: AsyncRead + AsyncWrite> AsyncWrite for BufStream<RW> {
+    fn poll_write(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        self.project().inner.poll_write(cx, buf)
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        self.project().inner.poll_flush(cx)
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        self.project().inner.poll_shutdown(cx)
+    }
+}
+
+impl<RW: AsyncRead + AsyncWrite> AsyncRead for BufStream<RW> {
+    fn poll_read(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        self.project().inner.poll_read(cx, buf)
+    }
+
+    // we can't skip unconditionally because of the large buffer case in read.
+    unsafe fn prepare_uninitialized_buffer(&self, buf: &mut [MaybeUninit<u8>]) -> bool {
+        self.inner.prepare_uninitialized_buffer(buf)
+    }
+}
+
+impl<RW: AsyncRead + AsyncWrite> AsyncBufRead for BufStream<RW> {
+    fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
+        self.project().inner.poll_fill_buf(cx)
+    }
+
+    fn consume(self: Pin<&mut Self>, amt: usize) {
+        self.project().inner.consume(amt)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        crate::is_unpin::<BufStream<()>>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/buf_writer.rs b/third_party/rust/tokio/src/io/util/buf_writer.rs
new file mode 100644
index 0000000000..efd053ebac
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/buf_writer.rs
@@ -0,0 +1,192 @@
+use crate::io::util::DEFAULT_BUF_SIZE;
+use crate::io::{AsyncBufRead, AsyncRead, AsyncWrite};
+
+use pin_project_lite::pin_project;
+use std::fmt;
+use std::io::{self, Write};
+use std::mem::MaybeUninit;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+pin_project! {
+    /// Wraps a writer and buffers its output.
+    ///
+    /// It can be excessively inefficient to work directly with something that
+    /// implements [`AsyncWrite`]. A `BufWriter` keeps an in-memory buffer of data and
+    /// writes it to an underlying writer in large, infrequent batches.
+    ///
+    /// `BufWriter` can improve the speed of programs that make *small* and
+    /// *repeated* write calls to the same file or network socket. It does not
+    /// help when writing very large amounts at once, or writing just one or a few
+    /// times. It also provides no advantage when writing to a destination that is
+    /// in memory, like a `Vec<u8>`.
+    ///
+    /// When the `BufWriter` is dropped, the contents of its buffer will be
+    /// discarded. Creating multiple instances of a `BufWriter` on the same
+    /// stream can cause data loss. If you need to write out the contents of its
+    /// buffer, you must manually call flush before the writer is dropped.
+    ///
+    /// [`AsyncWrite`]: AsyncWrite
+    /// [`flush`]: super::AsyncWriteExt::flush
+    ///
+    #[cfg_attr(docsrs, doc(cfg(feature = "io-util")))]
+    pub struct BufWriter<W> {
+        #[pin]
+        pub(super) inner: W,
+        pub(super) buf: Vec<u8>,
+        pub(super) written: usize,
+    }
+}
+
+impl<W: AsyncWrite> BufWriter<W> {
+    /// Creates a new `BufWriter` with a default buffer capacity. The default is currently 8 KB,
+    /// but may change in the future.
+    pub fn new(inner: W) -> Self {
+        Self::with_capacity(DEFAULT_BUF_SIZE, inner)
+    }
+
+    /// Creates a new `BufWriter` with the specified buffer capacity.
+    pub fn with_capacity(cap: usize, inner: W) -> Self {
+        Self {
+            inner,
+            buf: Vec::with_capacity(cap),
+            written: 0,
+        }
+    }
+
+    fn flush_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        let mut me = self.project();
+
+        let len = me.buf.len();
+        let mut ret = Ok(());
+        while *me.written < len {
+            match ready!(me.inner.as_mut().poll_write(cx, &me.buf[*me.written..])) {
+                Ok(0) => {
+                    ret = Err(io::Error::new(
+                        io::ErrorKind::WriteZero,
+                        "failed to write the buffered data",
+                    ));
+                    break;
+                }
+                Ok(n) => *me.written += n,
+                Err(e) => {
+                    ret = Err(e);
+                    break;
+                }
+            }
+        }
+        if *me.written > 0 {
+            me.buf.drain(..*me.written);
+        }
+        *me.written = 0;
+        Poll::Ready(ret)
+    }
+
+    /// Gets a reference to the underlying writer.
+    pub fn get_ref(&self) -> &W {
+        &self.inner
+    }
+
+    /// Gets a mutable reference to the underlying writer.
+    ///
+    /// It is inadvisable to directly write to the underlying writer.
+    pub fn get_mut(&mut self) -> &mut W {
+        &mut self.inner
+    }
+
+    /// Gets a pinned mutable reference to the underlying writer.
+    ///
+    /// It is inadvisable to directly write to the underlying writer.
+    pub fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut W> {
+        self.project().inner
+    }
+
+    /// Consumes this `BufWriter`, returning the underlying writer.
+    ///
+    /// Note that any leftover data in the internal buffer is lost.
+    pub fn into_inner(self) -> W {
+        self.inner
+    }
+
+    /// Returns a reference to the internally buffered data.
+    pub fn buffer(&self) -> &[u8] {
+        &self.buf
+    }
+}
+
+impl<W: AsyncWrite> AsyncWrite for BufWriter<W> {
+    fn poll_write(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        if self.buf.len() + buf.len() > self.buf.capacity() {
+            ready!(self.as_mut().flush_buf(cx))?;
+        }
+
+        let me = self.project();
+        if buf.len() >= me.buf.capacity() {
+            me.inner.poll_write(cx, buf)
+        } else {
+            Poll::Ready(me.buf.write(buf))
+        }
+    }
+
+    fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        ready!(self.as_mut().flush_buf(cx))?;
+        self.get_pin_mut().poll_flush(cx)
+    }
+
+    fn poll_shutdown(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        ready!(self.as_mut().flush_buf(cx))?;
+        self.get_pin_mut().poll_shutdown(cx)
+    }
+}
+
+impl<W: AsyncWrite + AsyncRead> AsyncRead for BufWriter<W> {
+    fn poll_read(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        self.get_pin_mut().poll_read(cx, buf)
+    }
+
+    // we can't skip unconditionally because of the large buffer case in read.
+    unsafe fn prepare_uninitialized_buffer(&self, buf: &mut [MaybeUninit<u8>]) -> bool {
+        self.get_ref().prepare_uninitialized_buffer(buf)
+    }
+}
+
+impl<W: AsyncWrite + AsyncBufRead> AsyncBufRead for BufWriter<W> {
+    fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
+        self.get_pin_mut().poll_fill_buf(cx)
+    }
+
+    fn consume(self: Pin<&mut Self>, amt: usize) {
+        self.get_pin_mut().consume(amt)
+    }
+}
+
+impl<W: fmt::Debug> fmt::Debug for BufWriter<W> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("BufWriter")
+            .field("writer", &self.inner)
+            .field(
+                "buffer",
+                &format_args!("{}/{}", self.buf.len(), self.buf.capacity()),
+            )
+            .field("written", &self.written)
+            .finish()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        crate::is_unpin::<BufWriter<()>>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/chain.rs b/third_party/rust/tokio/src/io/util/chain.rs
new file mode 100644
index 0000000000..bc76af341d
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/chain.rs
@@ -0,0 +1,141 @@
+use crate::io::{AsyncBufRead, AsyncRead};
+
+use pin_project_lite::pin_project;
+use std::fmt;
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+pin_project! {
+    /// Stream for the [`chain`](super::AsyncReadExt::chain) method.
+    #[must_use = "streams do nothing unless polled"]
+    #[cfg_attr(docsrs, doc(cfg(feature = "io-util")))]
+    pub struct Chain<T, U> {
+        #[pin]
+        first: T,
+        #[pin]
+        second: U,
+        done_first: bool,
+    }
+}
+
+pub(super) fn chain<T, U>(first: T, second: U) -> Chain<T, U>
+where
+    T: AsyncRead,
+    U: AsyncRead,
+{
+    Chain {
+        first,
+        second,
+        done_first: false,
+    }
+}
+
+impl<T, U> Chain<T, U>
+where
+    T: AsyncRead,
+    U: AsyncRead,
+{
+    /// Gets references to the underlying readers in this `Chain`.
+    pub fn get_ref(&self) -> (&T, &U) {
+        (&self.first, &self.second)
+    }
+
+    /// Gets mutable references to the underlying readers in this `Chain`.
+    ///
+    /// Care should be taken to avoid modifying the internal I/O state of the
+    /// underlying readers as doing so may corrupt the internal state of this
+    /// `Chain`.
+    pub fn get_mut(&mut self) -> (&mut T, &mut U) {
+        (&mut self.first, &mut self.second)
+    }
+
+    /// Gets pinned mutable references to the underlying readers in this `Chain`.
+    ///
+    /// Care should be taken to avoid modifying the internal I/O state of the
+    /// underlying readers as doing so may corrupt the internal state of this
+    /// `Chain`.
+    pub fn get_pin_mut(self: Pin<&mut Self>) -> (Pin<&mut T>, Pin<&mut U>) {
+        let me = self.project();
+        (me.first, me.second)
+    }
+
+    /// Consumes the `Chain`, returning the wrapped readers.
+    pub fn into_inner(self) -> (T, U) {
+        (self.first, self.second)
+    }
+}
+
+impl<T, U> fmt::Debug for Chain<T, U>
+where
+    T: fmt::Debug,
+    U: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Chain")
+            .field("t", &self.first)
+            .field("u", &self.second)
+            .finish()
+    }
+}
+
+impl<T, U> AsyncRead for Chain<T, U>
+where
+    T: AsyncRead,
+    U: AsyncRead,
+{
+    fn poll_read(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        let me = self.project();
+
+        if !*me.done_first {
+            match ready!(me.first.poll_read(cx, buf)?) {
+                0 if !buf.is_empty() => *me.done_first = true,
+                n => return Poll::Ready(Ok(n)),
+            }
+        }
+        me.second.poll_read(cx, buf)
+    }
+}
+
+impl<T, U> AsyncBufRead for Chain<T, U>
+where
+    T: AsyncBufRead,
+    U: AsyncBufRead,
+{
+    fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
+        let me = self.project();
+
+        if !*me.done_first {
+            match ready!(me.first.poll_fill_buf(cx)?) {
+                buf if buf.is_empty() => {
+                    *me.done_first = true;
+                }
+                buf => return Poll::Ready(Ok(buf)),
+            }
+        }
+        me.second.poll_fill_buf(cx)
+    }
+
+    fn consume(self: Pin<&mut Self>, amt: usize) {
+        let me = self.project();
+        if !*me.done_first {
+            me.first.consume(amt)
+        } else {
+            me.second.consume(amt)
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        crate::is_unpin::<Chain<(), ()>>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/copy.rs b/third_party/rust/tokio/src/io/util/copy.rs
new file mode 100644
index 0000000000..8e0058c1c2
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/copy.rs
@@ -0,0 +1,135 @@
+use crate::io::{AsyncRead, AsyncWrite};
+
+use std::future::Future;
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+cfg_io_util! {
+    /// A future that asynchronously copies the entire contents of a reader into a
+    /// writer.
+    ///
+    /// This struct is generally created by calling [`copy`][copy]. Please
+    /// see the documentation of `copy()` for more details.
+    ///
+    /// [copy]: copy()
+    #[derive(Debug)]
+    #[must_use = "futures do nothing unless you `.await` or poll them"]
+    pub struct Copy<'a, R: ?Sized, W: ?Sized> {
+        reader: &'a mut R,
+        read_done: bool,
+        writer: &'a mut W,
+        pos: usize,
+        cap: usize,
+        amt: u64,
+        buf: Box<[u8]>,
+    }
+
+    /// Asynchronously copies the entire contents of a reader into a writer.
+    ///
+    /// This function returns a future that will continuously read data from
+    /// `reader` and then write it into `writer` in a streaming fashion until
+    /// `reader` returns EOF.
+    ///
+    /// On success, the total number of bytes that were copied from `reader` to
+    /// `writer` is returned.
+    ///
+    /// This is an asynchronous version of [`std::io::copy`][std].
+    ///
+    /// [std]: std::io::copy
+    ///
+    /// # Errors
+    ///
+    /// The returned future will finish with an error will return an error
+    /// immediately if any call to `poll_read` or `poll_write` returns an error.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::io;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut reader: &[u8] = b"hello";
+    /// let mut writer: Vec<u8> = vec![];
+    ///
+    /// io::copy(&mut reader, &mut writer).await?;
+    ///
+    /// assert_eq!(&b"hello"[..], &writer[..]);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn copy<'a, R, W>(reader: &'a mut R, writer: &'a mut W) -> Copy<'a, R, W>
+    where
+        R: AsyncRead + Unpin + ?Sized,
+        W: AsyncWrite + Unpin + ?Sized,
+    {
+        Copy {
+            reader,
+            read_done: false,
+            writer,
+            amt: 0,
+            pos: 0,
+            cap: 0,
+            buf: Box::new([0; 2048]),
+        }
+    }
+}
+
+impl<R, W> Future for Copy<'_, R, W>
+where
+    R: AsyncRead + Unpin + ?Sized,
+    W: AsyncWrite + Unpin + ?Sized,
+{
+    type Output = io::Result<u64>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>> {
+        loop {
+            // If our buffer is empty, then we need to read some data to
+            // continue.
+            if self.pos == self.cap && !self.read_done {
+                let me = &mut *self;
+                let n = ready!(Pin::new(&mut *me.reader).poll_read(cx, &mut me.buf))?;
+                if n == 0 {
+                    self.read_done = true;
+                } else {
+                    self.pos = 0;
+                    self.cap = n;
+                }
+            }
+
+            // If our buffer has some data, let's write it out!
+            while self.pos < self.cap {
+                let me = &mut *self;
+                let i = ready!(Pin::new(&mut *me.writer).poll_write(cx, &me.buf[me.pos..me.cap]))?;
+                if i == 0 {
+                    return Poll::Ready(Err(io::Error::new(
+                        io::ErrorKind::WriteZero,
+                        "write zero byte into writer",
+                    )));
+                } else {
+                    self.pos += i;
+                    self.amt += i as u64;
+                }
+            }
+
+            // If we've written all the data and we've seen EOF, flush out the
+            // data and finish the transfer.
+            if self.pos == self.cap && self.read_done {
+                let me = &mut *self;
+                ready!(Pin::new(&mut *me.writer).poll_flush(cx))?;
+                return Poll::Ready(Ok(self.amt));
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        use std::marker::PhantomPinned;
+        crate::is_unpin::<Copy<'_, PhantomPinned, PhantomPinned>>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/empty.rs b/third_party/rust/tokio/src/io/util/empty.rs
new file mode 100644
index 0000000000..121102c78f
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/empty.rs
@@ -0,0 +1,84 @@
+use crate::io::{AsyncBufRead, AsyncRead};
+
+use std::fmt;
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+cfg_io_util! {
+    /// An async reader which is always at EOF.
+    ///
+    /// This struct is generally created by calling [`empty`]. Please see
+    /// the documentation of [`empty()`][`empty`] for more details.
+    ///
+    /// This is an asynchronous version of [`std::io::empty`][std].
+    ///
+    /// [`empty`]: fn@empty
+    /// [std]: std::io::empty
+    pub struct Empty {
+        _p: (),
+    }
+
+    /// Creates a new empty async reader.
+    ///
+    /// All reads from the returned reader will return `Poll::Ready(Ok(0))`.
+    ///
+    /// This is an asynchronous version of [`std::io::empty`][std].
+    ///
+    /// [std]: std::io::empty
+    ///
+    /// # Examples
+    ///
+    /// A slightly sad example of not reading anything into a buffer:
+    ///
+    /// ```
+    /// use tokio::io::{self, AsyncReadExt};
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let mut buffer = String::new();
+    ///     io::empty().read_to_string(&mut buffer).await.unwrap();
+    ///     assert!(buffer.is_empty());
+    /// }
+    /// ```
+    pub fn empty() -> Empty {
+        Empty { _p: () }
+    }
+}
+
+impl AsyncRead for Empty {
+    #[inline]
+    fn poll_read(
+        self: Pin<&mut Self>,
+        _: &mut Context<'_>,
+        _: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        Poll::Ready(Ok(0))
+    }
+}
+
+impl AsyncBufRead for Empty {
+    #[inline]
+    fn poll_fill_buf(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
+        Poll::Ready(Ok(&[]))
+    }
+
+    #[inline]
+    fn consume(self: Pin<&mut Self>, _: usize) {}
+}
+
+impl fmt::Debug for Empty {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad("Empty { .. }")
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        crate::is_unpin::<Empty>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/flush.rs b/third_party/rust/tokio/src/io/util/flush.rs
new file mode 100644
index 0000000000..1465f30448
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/flush.rs
@@ -0,0 +1,47 @@
+use crate::io::AsyncWrite;
+
+use std::future::Future;
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+cfg_io_util! {
+    /// A future used to fully flush an I/O object.
+    ///
+    /// Created by the [`AsyncWriteExt::flush`] function.
+    #[derive(Debug)]
+    pub struct Flush<'a, A: ?Sized> {
+        a: &'a mut A,
+    }
+}
+
+/// Creates a future which will entirely flush an I/O object.
+pub(super) fn flush<A>(a: &mut A) -> Flush<'_, A>
+where
+    A: AsyncWrite + Unpin + ?Sized,
+{
+    Flush { a }
+}
+
+impl<A> Future for Flush<'_, A>
+where
+    A: AsyncWrite + Unpin + ?Sized,
+{
+    type Output = io::Result<()>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let me = &mut *self;
+        Pin::new(&mut *me.a).poll_flush(cx)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        use std::marker::PhantomPinned;
+        crate::is_unpin::<Flush<'_, PhantomPinned>>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/lines.rs b/third_party/rust/tokio/src/io/util/lines.rs
new file mode 100644
index 0000000000..f0e75de4b1
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/lines.rs
@@ -0,0 +1,114 @@
+use crate::io::util::read_line::read_line_internal;
+use crate::io::AsyncBufRead;
+
+use pin_project_lite::pin_project;
+use std::io;
+use std::mem;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+pin_project! {
+    /// Stream for the [`lines`](crate::io::AsyncBufReadExt::lines) method.
+    #[derive(Debug)]
+    #[must_use = "streams do nothing unless polled"]
+    #[cfg_attr(docsrs, doc(cfg(feature = "io-util")))]
+    pub struct Lines<R> {
+        #[pin]
+        reader: R,
+        buf: String,
+        bytes: Vec<u8>,
+        read: usize,
+    }
+}
+
+pub(crate) fn lines<R>(reader: R) -> Lines<R>
+where
+    R: AsyncBufRead,
+{
+    Lines {
+        reader,
+        buf: String::new(),
+        bytes: Vec::new(),
+        read: 0,
+    }
+}
+
+impl<R> Lines<R>
+where
+    R: AsyncBufRead + Unpin,
+{
+    /// Returns the next line in the stream.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use tokio::io::AsyncBufRead;
+    /// use tokio::io::AsyncBufReadExt;
+    ///
+    /// # async fn dox(my_buf_read: impl AsyncBufRead + Unpin) -> std::io::Result<()> {
+    /// let mut lines = my_buf_read.lines();
+    ///
+    /// while let Some(line) = lines.next_line().await? {
+    ///     println!("length = {}", line.len())
+    /// }
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn next_line(&mut self) -> io::Result<Option<String>> {
+        use crate::future::poll_fn;
+
+        poll_fn(|cx| Pin::new(&mut *self).poll_next_line(cx)).await
+    }
+}
+
+impl<R> Lines<R>
+where
+    R: AsyncBufRead,
+{
+    #[doc(hidden)]
+    pub fn poll_next_line(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<io::Result<Option<String>>> {
+        let me = self.project();
+
+        let n = ready!(read_line_internal(me.reader, cx, me.buf, me.bytes, me.read))?;
+
+        if n == 0 && me.buf.is_empty() {
+            return Poll::Ready(Ok(None));
+        }
+
+        if me.buf.ends_with('\n') {
+            me.buf.pop();
+
+            if me.buf.ends_with('\r') {
+                me.buf.pop();
+            }
+        }
+
+        Poll::Ready(Ok(Some(mem::replace(me.buf, String::new()))))
+    }
+}
+
+#[cfg(feature = "stream")]
+impl<R: AsyncBufRead> crate::stream::Stream for Lines<R> {
+    type Item = io::Result<String>;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        Poll::Ready(match ready!(self.poll_next_line(cx)) {
+            Ok(Some(line)) => Some(Ok(line)),
+            Ok(None) => None,
+            Err(err) => Some(Err(err)),
+        })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        crate::is_unpin::<Lines<()>>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/mod.rs b/third_party/rust/tokio/src/io/util/mod.rs
new file mode 100644
index 0000000000..c4754abf05
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/mod.rs
@@ -0,0 +1,88 @@
+#![allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
+
+cfg_io_util! {
+    mod async_buf_read_ext;
+    pub use async_buf_read_ext::AsyncBufReadExt;
+
+    mod async_read_ext;
+    pub use async_read_ext::AsyncReadExt;
+
+    mod async_seek_ext;
+    pub use async_seek_ext::AsyncSeekExt;
+
+    mod async_write_ext;
+    pub use async_write_ext::AsyncWriteExt;
+
+    mod buf_reader;
+    pub use buf_reader::BufReader;
+
+    mod buf_stream;
+    pub use buf_stream::BufStream;
+
+    mod buf_writer;
+    pub use buf_writer::BufWriter;
+
+    mod chain;
+
+    mod copy;
+    pub use copy::{copy, Copy};
+
+    mod empty;
+    pub use empty::{empty, Empty};
+
+    mod flush;
+
+    mod lines;
+    pub use lines::Lines;
+
+    mod read;
+    mod read_buf;
+    mod read_exact;
+    mod read_int;
+    mod read_line;
+
+    mod read_to_end;
+    cfg_process! {
+        pub(crate) use read_to_end::read_to_end;
+    }
+
+    mod read_to_string;
+    mod read_until;
+
+    mod repeat;
+    pub use repeat::{repeat, Repeat};
+
+    mod shutdown;
+
+    mod sink;
+    pub use sink::{sink, Sink};
+
+    mod split;
+    pub use split::Split;
+
+    cfg_stream! {
+        mod stream_reader;
+        pub use stream_reader::{stream_reader, StreamReader};
+    }
+
+    mod take;
+    pub use take::Take;
+
+    mod write;
+    mod write_all;
+    mod write_buf;
+    mod write_int;
+
+
+    // used by `BufReader` and `BufWriter`
+    // https://github.com/rust-lang/rust/blob/master/src/libstd/sys_common/io.rs#L1
+    const DEFAULT_BUF_SIZE: usize = 8 * 1024;
+}
+
+cfg_not_io_util! {
+    cfg_process! {
+        mod read_to_end;
+        // Used by process
+        pub(crate) use read_to_end::read_to_end;
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/read.rs b/third_party/rust/tokio/src/io/util/read.rs
new file mode 100644
index 0000000000..a8ca370ea8
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/read.rs
@@ -0,0 +1,55 @@
+use crate::io::AsyncRead;
+
+use std::future::Future;
+use std::io;
+use std::marker::Unpin;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// Tries to read some bytes directly into the given `buf` in asynchronous
+/// manner, returning a future type.
+///
+/// The returned future will resolve to both the I/O stream and the buffer
+/// as well as the number of bytes read once the read operation is completed.
+pub(crate) fn read<'a, R>(reader: &'a mut R, buf: &'a mut [u8]) -> Read<'a, R>
+where
+    R: AsyncRead + Unpin + ?Sized,
+{
+    Read { reader, buf }
+}
+
+cfg_io_util! {
+    /// A future which can be used to easily read available number of bytes to fill
+    /// a buffer.
+    ///
+    /// Created by the [`read`] function.
+    #[derive(Debug)]
+    #[must_use = "futures do nothing unless you `.await` or poll them"]
+    pub struct Read<'a, R: ?Sized> {
+        reader: &'a mut R,
+        buf: &'a mut [u8],
+    }
+}
+
+impl<R> Future for Read<'_, R>
+where
+    R: AsyncRead + Unpin + ?Sized,
+{
+    type Output = io::Result<usize>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<usize>> {
+        let me = &mut *self;
+        Pin::new(&mut *me.reader).poll_read(cx, me.buf)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        use std::marker::PhantomPinned;
+        crate::is_unpin::<Read<'_, PhantomPinned>>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/read_buf.rs b/third_party/rust/tokio/src/io/util/read_buf.rs
new file mode 100644
index 0000000000..550499b933
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/read_buf.rs
@@ -0,0 +1,41 @@
+use crate::io::AsyncRead;
+
+use bytes::BufMut;
+use std::future::Future;
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+pub(crate) fn read_buf<'a, R, B>(reader: &'a mut R, buf: &'a mut B) -> ReadBuf<'a, R, B>
+where
+    R: AsyncRead,
+    B: BufMut,
+{
+    ReadBuf { reader, buf }
+}
+
+cfg_io_util! {
+    /// Future returned by [`read_buf`](AsyncReadExt::read_buf).
+    #[derive(Debug)]
+    #[must_use = "futures do nothing unless you `.await` or poll them"]
+    pub struct ReadBuf<'a, R, B> {
+        reader: &'a mut R,
+        buf: &'a mut B,
+    }
+}
+
+impl<R, B> Future for ReadBuf<'_, R, B>
+where
+    R: AsyncRead,
+    B: BufMut,
+{
+    type Output = io::Result<usize>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<usize>> {
+        // safety: no data is moved from self
+        unsafe {
+            let me = self.get_unchecked_mut();
+            Pin::new_unchecked(&mut *me.reader).poll_read_buf(cx, &mut me.buf)
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/read_exact.rs b/third_party/rust/tokio/src/io/util/read_exact.rs
new file mode 100644
index 0000000000..d6983c9953
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/read_exact.rs
@@ -0,0 +1,76 @@
+use crate::io::AsyncRead;
+
+use std::future::Future;
+use std::io;
+use std::marker::Unpin;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// A future which can be used to easily read exactly enough bytes to fill
+/// a buffer.
+///
+/// Created by the [`AsyncRead::read_exact`].
+pub(crate) fn read_exact<'a, A>(reader: &'a mut A, buf: &'a mut [u8]) -> ReadExact<'a, A>
+where
+    A: AsyncRead + Unpin + ?Sized,
+{
+    ReadExact {
+        reader,
+        buf,
+        pos: 0,
+    }
+}
+
+cfg_io_util! {
+    /// Creates a future which will read exactly enough bytes to fill `buf`,
+    /// returning an error if EOF is hit sooner.
+    ///
+    /// On success the number of bytes is returned
+    #[derive(Debug)]
+    #[must_use = "futures do nothing unless you `.await` or poll them"]
+    pub struct ReadExact<'a, A: ?Sized> {
+        reader: &'a mut A,
+        buf: &'a mut [u8],
+        pos: usize,
+    }
+}
+
+fn eof() -> io::Error {
+    io::Error::new(io::ErrorKind::UnexpectedEof, "early eof")
+}
+
+impl<A> Future for ReadExact<'_, A>
+where
+    A: AsyncRead + Unpin + ?Sized,
+{
+    type Output = io::Result<usize>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<usize>> {
+        loop {
+            // if our buffer is empty, then we need to read some data to continue.
+            if self.pos < self.buf.len() {
+                let me = &mut *self;
+                let n = ready!(Pin::new(&mut *me.reader).poll_read(cx, &mut me.buf[me.pos..]))?;
+                me.pos += n;
+                if n == 0 {
+                    return Err(eof()).into();
+                }
+            }
+
+            if self.pos >= self.buf.len() {
+                return Poll::Ready(Ok(self.pos));
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        use std::marker::PhantomPinned;
+        crate::is_unpin::<ReadExact<'_, PhantomPinned>>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/read_int.rs b/third_party/rust/tokio/src/io/util/read_int.rs
new file mode 100644
index 0000000000..9dc4402f88
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/read_int.rs
@@ -0,0 +1,123 @@
+use crate::io::AsyncRead;
+
+use bytes::Buf;
+use pin_project_lite::pin_project;
+use std::future::Future;
+use std::io;
+use std::io::ErrorKind::UnexpectedEof;
+use std::mem::size_of;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+macro_rules! reader {
+    ($name:ident, $ty:ty, $reader:ident) => {
+        reader!($name, $ty, $reader, size_of::<$ty>());
+    };
+    ($name:ident, $ty:ty, $reader:ident, $bytes:expr) => {
+        pin_project! {
+            #[doc(hidden)]
+            pub struct $name<R> {
+                #[pin]
+                src: R,
+                buf: [u8; $bytes],
+                read: u8,
+            }
+        }
+
+        impl<R> $name<R> {
+            pub(crate) fn new(src: R) -> Self {
+                $name {
+                    src,
+                    buf: [0; $bytes],
+                    read: 0,
+                }
+            }
+        }
+
+        impl<R> Future for $name<R>
+        where
+            R: AsyncRead,
+        {
+            type Output = io::Result<$ty>;
+
+            fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+                let mut me = self.project();
+
+                if *me.read == $bytes as u8 {
+                    return Poll::Ready(Ok(Buf::$reader(&mut &me.buf[..])));
+                }
+
+                while *me.read < $bytes as u8 {
+                    *me.read += match me
+                        .src
+                        .as_mut()
+                        .poll_read(cx, &mut me.buf[*me.read as usize..])
+                    {
+                        Poll::Pending => return Poll::Pending,
+                        Poll::Ready(Err(e)) => return Poll::Ready(Err(e.into())),
+                        Poll::Ready(Ok(0)) => {
+                            return Poll::Ready(Err(UnexpectedEof.into()));
+                        }
+                        Poll::Ready(Ok(n)) => n as u8,
+                    };
+                }
+
+                let num = Buf::$reader(&mut &me.buf[..]);
+
+                Poll::Ready(Ok(num))
+            }
+        }
+    };
+}
+
+macro_rules! reader8 {
+    ($name:ident, $ty:ty) => {
+        pin_project! {
+            /// Future returned from `read_u8`
+            #[doc(hidden)]
+            pub struct $name<R> {
+                #[pin]
+                reader: R,
+            }
+        }
+
+        impl<R> $name<R> {
+            pub(crate) fn new(reader: R) -> $name<R> {
+                $name { reader }
+            }
+        }
+
+        impl<R> Future for $name<R>
+        where
+            R: AsyncRead,
+        {
+            type Output = io::Result<$ty>;
+
+            fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+                let me = self.project();
+
+                let mut buf = [0; 1];
+                match me.reader.poll_read(cx, &mut buf[..]) {
+                    Poll::Pending => Poll::Pending,
+                    Poll::Ready(Err(e)) => Poll::Ready(Err(e.into())),
+                    Poll::Ready(Ok(0)) => Poll::Ready(Err(UnexpectedEof.into())),
+                    Poll::Ready(Ok(1)) => Poll::Ready(Ok(buf[0] as $ty)),
+                    Poll::Ready(Ok(_)) => unreachable!(),
+                }
+            }
+        }
+    };
+}
+
+reader8!(ReadU8, u8);
+reader8!(ReadI8, i8);
+
+reader!(ReadU16, u16, get_u16);
+reader!(ReadU32, u32, get_u32);
+reader!(ReadU64, u64, get_u64);
+reader!(ReadU128, u128, get_u128);
+
+reader!(ReadI16, i16, get_i16);
+reader!(ReadI32, i32, get_i32);
+reader!(ReadI64, i64, get_i64);
+reader!(ReadI128, i128, get_i128);
diff --git a/third_party/rust/tokio/src/io/util/read_line.rs b/third_party/rust/tokio/src/io/util/read_line.rs
new file mode 100644
index 0000000000..c5ee597486
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/read_line.rs
@@ -0,0 +1,82 @@
+use crate::io::util::read_until::read_until_internal;
+use crate::io::AsyncBufRead;
+
+use std::future::Future;
+use std::io;
+use std::mem;
+use std::pin::Pin;
+use std::str;
+use std::task::{Context, Poll};
+
+cfg_io_util! {
+    /// Future for the [`read_line`](crate::io::AsyncBufReadExt::read_line) method.
+    #[derive(Debug)]
+    #[must_use = "futures do nothing unless you `.await` or poll them"]
+    pub struct ReadLine<'a, R: ?Sized> {
+        reader: &'a mut R,
+        buf: &'a mut String,
+        bytes: Vec<u8>,
+        read: usize,
+    }
+}
+
+pub(crate) fn read_line<'a, R>(reader: &'a mut R, buf: &'a mut String) -> ReadLine<'a, R>
+where
+    R: AsyncBufRead + ?Sized + Unpin,
+{
+    ReadLine {
+        reader,
+        bytes: unsafe { mem::replace(buf.as_mut_vec(), Vec::new()) },
+        buf,
+        read: 0,
+    }
+}
+
+pub(super) fn read_line_internal<R: AsyncBufRead + ?Sized>(
+    reader: Pin<&mut R>,
+    cx: &mut Context<'_>,
+    buf: &mut String,
+    bytes: &mut Vec<u8>,
+    read: &mut usize,
+) -> Poll<io::Result<usize>> {
+    let ret = ready!(read_until_internal(reader, cx, b'\n', bytes, read));
+    if str::from_utf8(&bytes).is_err() {
+        Poll::Ready(ret.and_then(|_| {
+            Err(io::Error::new(
+                io::ErrorKind::InvalidData,
+                "stream did not contain valid UTF-8",
+            ))
+        }))
+    } else {
+        debug_assert!(buf.is_empty());
+        debug_assert_eq!(*read, 0);
+        // Safety: `bytes` is a valid UTF-8 because `str::from_utf8` returned `Ok`.
+        mem::swap(unsafe { buf.as_mut_vec() }, bytes);
+        Poll::Ready(ret)
+    }
+}
+
+impl<R: AsyncBufRead + ?Sized + Unpin> Future for ReadLine<'_, R> {
+    type Output = io::Result<usize>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let Self {
+            reader,
+            buf,
+            bytes,
+            read,
+        } = &mut *self;
+        read_line_internal(Pin::new(reader), cx, buf, bytes, read)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        use std::marker::PhantomPinned;
+        crate::is_unpin::<ReadLine<'_, PhantomPinned>>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/read_to_end.rs b/third_party/rust/tokio/src/io/util/read_to_end.rs
new file mode 100644
index 0000000000..a2cd99bed0
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/read_to_end.rs
@@ -0,0 +1,113 @@
+use crate::io::AsyncRead;
+
+use std::future::Future;
+use std::io;
+use std::mem::MaybeUninit;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+#[cfg_attr(docsrs, doc(cfg(feature = "io-util")))]
+pub struct ReadToEnd<'a, R: ?Sized> {
+    reader: &'a mut R,
+    buf: &'a mut Vec<u8>,
+    start_len: usize,
+}
+
+pub(crate) fn read_to_end<'a, R>(reader: &'a mut R, buf: &'a mut Vec<u8>) -> ReadToEnd<'a, R>
+where
+    R: AsyncRead + Unpin + ?Sized,
+{
+    let start_len = buf.len();
+    ReadToEnd {
+        reader,
+        buf,
+        start_len,
+    }
+}
+
+struct Guard<'a> {
+    buf: &'a mut Vec<u8>,
+    len: usize,
+}
+
+impl Drop for Guard<'_> {
+    fn drop(&mut self) {
+        unsafe {
+            self.buf.set_len(self.len);
+        }
+    }
+}
+
+// This uses an adaptive system to extend the vector when it fills. We want to
+// avoid paying to allocate and zero a huge chunk of memory if the reader only
+// has 4 bytes while still making large reads if the reader does have a ton
+// of data to return. Simply tacking on an extra DEFAULT_BUF_SIZE space every
+// time is 4,500 times (!) slower than this if the reader has a very small
+// amount of data to return.
+//
+// Because we're extending the buffer with uninitialized data for trusted
+// readers, we need to make sure to truncate that if any of this panics.
+pub(super) fn read_to_end_internal<R: AsyncRead + ?Sized>(
+    mut rd: Pin<&mut R>,
+    cx: &mut Context<'_>,
+    buf: &mut Vec<u8>,
+    start_len: usize,
+) -> Poll<io::Result<usize>> {
+    let mut g = Guard {
+        len: buf.len(),
+        buf,
+    };
+    let ret;
+    loop {
+        if g.len == g.buf.len() {
+            unsafe {
+                g.buf.reserve(32);
+                let capacity = g.buf.capacity();
+                g.buf.set_len(capacity);
+
+                let b = &mut *(&mut g.buf[g.len..] as *mut [u8] as *mut [MaybeUninit<u8>]);
+
+                rd.prepare_uninitialized_buffer(b);
+            }
+        }
+
+        match ready!(rd.as_mut().poll_read(cx, &mut g.buf[g.len..])) {
+            Ok(0) => {
+                ret = Poll::Ready(Ok(g.len - start_len));
+                break;
+            }
+            Ok(n) => g.len += n,
+            Err(e) => {
+                ret = Poll::Ready(Err(e));
+                break;
+            }
+        }
+    }
+
+    ret
+}
+
+impl<A> Future for ReadToEnd<'_, A>
+where
+    A: AsyncRead + ?Sized + Unpin,
+{
+    type Output = io::Result<usize>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let this = &mut *self;
+        read_to_end_internal(Pin::new(&mut this.reader), cx, this.buf, this.start_len)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        use std::marker::PhantomPinned;
+        crate::is_unpin::<ReadToEnd<'_, PhantomPinned>>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/read_to_string.rs b/third_party/rust/tokio/src/io/util/read_to_string.rs
new file mode 100644
index 0000000000..e77d836dee
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/read_to_string.rs
@@ -0,0 +1,83 @@
+use crate::io::util::read_to_end::read_to_end_internal;
+use crate::io::AsyncRead;
+
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+use std::{io, mem, str};
+
+cfg_io_util! {
+    /// Future for the [`read_to_string`](super::AsyncReadExt::read_to_string) method.
+    #[derive(Debug)]
+    #[must_use = "futures do nothing unless you `.await` or poll them"]
+    pub struct ReadToString<'a, R: ?Sized> {
+        reader: &'a mut R,
+        buf: &'a mut String,
+        bytes: Vec<u8>,
+        start_len: usize,
+    }
+}
+
+pub(crate) fn read_to_string<'a, R>(reader: &'a mut R, buf: &'a mut String) -> ReadToString<'a, R>
+where
+    R: AsyncRead + ?Sized + Unpin,
+{
+    let start_len = buf.len();
+    ReadToString {
+        reader,
+        bytes: unsafe { mem::replace(buf.as_mut_vec(), Vec::new()) },
+        buf,
+        start_len,
+    }
+}
+
+fn read_to_string_internal<R: AsyncRead + ?Sized>(
+    reader: Pin<&mut R>,
+    cx: &mut Context<'_>,
+    buf: &mut String,
+    bytes: &mut Vec<u8>,
+    start_len: usize,
+) -> Poll<io::Result<usize>> {
+    let ret = ready!(read_to_end_internal(reader, cx, bytes, start_len));
+    if str::from_utf8(&bytes).is_err() {
+        Poll::Ready(ret.and_then(|_| {
+            Err(io::Error::new(
+                io::ErrorKind::InvalidData,
+                "stream did not contain valid UTF-8",
+            ))
+        }))
+    } else {
+        debug_assert!(buf.is_empty());
+        // Safety: `bytes` is a valid UTF-8 because `str::from_utf8` returned `Ok`.
+        mem::swap(unsafe { buf.as_mut_vec() }, bytes);
+        Poll::Ready(ret)
+    }
+}
+
+impl<A> Future for ReadToString<'_, A>
+where
+    A: AsyncRead + ?Sized + Unpin,
+{
+    type Output = io::Result<usize>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let Self {
+            reader,
+            buf,
+            bytes,
+            start_len,
+        } = &mut *self;
+        read_to_string_internal(Pin::new(reader), cx, buf, bytes, *start_len)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        use std::marker::PhantomPinned;
+        crate::is_unpin::<ReadToString<'_, PhantomPinned>>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/read_until.rs b/third_party/rust/tokio/src/io/util/read_until.rs
new file mode 100644
index 0000000000..1adeda66f0
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/read_until.rs
@@ -0,0 +1,86 @@
+use crate::io::AsyncBufRead;
+
+use std::future::Future;
+use std::io;
+use std::mem;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+cfg_io_util! {
+    /// Future for the [`read_until`](crate::io::AsyncBufReadExt::read_until) method.
+    #[derive(Debug)]
+    #[must_use = "futures do nothing unless you `.await` or poll them"]
+    pub struct ReadUntil<'a, R: ?Sized> {
+        reader: &'a mut R,
+        byte: u8,
+        buf: &'a mut Vec<u8>,
+        read: usize,
+    }
+}
+
+pub(crate) fn read_until<'a, R>(
+    reader: &'a mut R,
+    byte: u8,
+    buf: &'a mut Vec<u8>,
+) -> ReadUntil<'a, R>
+where
+    R: AsyncBufRead + ?Sized + Unpin,
+{
+    ReadUntil {
+        reader,
+        byte,
+        buf,
+        read: 0,
+    }
+}
+
+pub(super) fn read_until_internal<R: AsyncBufRead + ?Sized>(
+    mut reader: Pin<&mut R>,
+    cx: &mut Context<'_>,
+    byte: u8,
+    buf: &mut Vec<u8>,
+    read: &mut usize,
+) -> Poll<io::Result<usize>> {
+    loop {
+        let (done, used) = {
+            let available = ready!(reader.as_mut().poll_fill_buf(cx))?;
+            if let Some(i) = memchr::memchr(byte, available) {
+                buf.extend_from_slice(&available[..=i]);
+                (true, i + 1)
+            } else {
+                buf.extend_from_slice(available);
+                (false, available.len())
+            }
+        };
+        reader.as_mut().consume(used);
+        *read += used;
+        if done || used == 0 {
+            return Poll::Ready(Ok(mem::replace(read, 0)));
+        }
+    }
+}
+
+impl<R: AsyncBufRead + ?Sized + Unpin> Future for ReadUntil<'_, R> {
+    type Output = io::Result<usize>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let Self {
+            reader,
+            byte,
+            buf,
+            read,
+        } = &mut *self;
+        read_until_internal(Pin::new(reader), cx, *byte, buf, read)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        use std::marker::PhantomPinned;
+        crate::is_unpin::<ReadUntil<'_, PhantomPinned>>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/repeat.rs b/third_party/rust/tokio/src/io/util/repeat.rs
new file mode 100644
index 0000000000..6b9067e853
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/repeat.rs
@@ -0,0 +1,71 @@
+use crate::io::AsyncRead;
+
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+cfg_io_util! {
+    /// An async reader which yields one byte over and over and over and over and
+    /// over and...
+    ///
+    /// This struct is generally created by calling [`repeat`][repeat]. Please
+    /// see the documentation of `repeat()` for more details.
+    ///
+    /// This is an asynchronous version of [`std::io::Repeat`][std].
+    ///
+    /// [repeat]: fn@repeat
+    /// [std]: std::io::Repeat
+    #[derive(Debug)]
+    pub struct Repeat {
+        byte: u8,
+    }
+
+    /// Creates an instance of an async reader that infinitely repeats one byte.
+    ///
+    /// All reads from this reader will succeed by filling the specified buffer with
+    /// the given byte.
+    ///
+    /// This is an asynchronous version of [`std::io::repeat`][std].
+    ///
+    /// [std]: std::io::repeat
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::io::{self, AsyncReadExt};
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let mut buffer = [0; 3];
+    ///     io::repeat(0b101).read_exact(&mut buffer).await.unwrap();
+    ///     assert_eq!(buffer, [0b101, 0b101, 0b101]);
+    /// }
+    /// ```
+    pub fn repeat(byte: u8) -> Repeat {
+        Repeat { byte }
+    }
+}
+
+impl AsyncRead for Repeat {
+    #[inline]
+    fn poll_read(
+        self: Pin<&mut Self>,
+        _: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        for byte in &mut *buf {
+            *byte = self.byte;
+        }
+        Poll::Ready(Ok(buf.len()))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        crate::is_unpin::<Repeat>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/shutdown.rs b/third_party/rust/tokio/src/io/util/shutdown.rs
new file mode 100644
index 0000000000..f24e288541
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/shutdown.rs
@@ -0,0 +1,47 @@
+use crate::io::AsyncWrite;
+
+use std::future::Future;
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+cfg_io_util! {
+    /// A future used to shutdown an I/O object.
+    ///
+    /// Created by the [`AsyncWriteExt::shutdown`] function.
+    #[derive(Debug)]
+    pub struct Shutdown<'a, A: ?Sized> {
+        a: &'a mut A,
+    }
+}
+
+/// Creates a future which will shutdown an I/O object.
+pub(super) fn shutdown<A>(a: &mut A) -> Shutdown<'_, A>
+where
+    A: AsyncWrite + Unpin + ?Sized,
+{
+    Shutdown { a }
+}
+
+impl<A> Future for Shutdown<'_, A>
+where
+    A: AsyncWrite + Unpin + ?Sized,
+{
+    type Output = io::Result<()>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let me = &mut *self;
+        Pin::new(&mut *me.a).poll_shutdown(cx)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        use std::marker::PhantomPinned;
+        crate::is_unpin::<Shutdown<'_, PhantomPinned>>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/sink.rs b/third_party/rust/tokio/src/io/util/sink.rs
new file mode 100644
index 0000000000..05ee773fa3
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/sink.rs
@@ -0,0 +1,87 @@
+use crate::io::AsyncWrite;
+
+use std::fmt;
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+cfg_io_util! {
+    /// An async writer which will move data into the void.
+    ///
+    /// This struct is generally created by calling [`sink`][sink]. Please
+    /// see the documentation of `sink()` for more details.
+    ///
+    /// This is an asynchronous version of [`std::io::Sink`][std].
+    ///
+    /// [sink]: sink()
+    /// [std]: std::io::Sink
+    pub struct Sink {
+        _p: (),
+    }
+
+    /// Creates an instance of an async writer which will successfully consume all
+    /// data.
+    ///
+    /// All calls to [`poll_write`] on the returned instance will return
+    /// `Poll::Ready(Ok(buf.len()))` and the contents of the buffer will not be
+    /// inspected.
+    ///
+    /// This is an asynchronous version of [`std::io::sink`][std].
+    ///
+    /// [`poll_write`]: crate::io::AsyncWrite::poll_write()
+    /// [std]: std::io::sink
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::io::{self, AsyncWriteExt};
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let buffer = vec![1, 2, 3, 5, 8];
+    ///     let num_bytes = io::sink().write(&buffer).await?;
+    ///     assert_eq!(num_bytes, 5);
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn sink() -> Sink {
+        Sink { _p: () }
+    }
+}
+
+impl AsyncWrite for Sink {
+    #[inline]
+    fn poll_write(
+        self: Pin<&mut Self>,
+        _: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<Result<usize, io::Error>> {
+        Poll::Ready(Ok(buf.len()))
+    }
+
+    #[inline]
+    fn poll_flush(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
+        Poll::Ready(Ok(()))
+    }
+
+    #[inline]
+    fn poll_shutdown(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
+        Poll::Ready(Ok(()))
+    }
+}
+
+impl fmt::Debug for Sink {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad("Sink { .. }")
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        crate::is_unpin::<Sink>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/split.rs b/third_party/rust/tokio/src/io/util/split.rs
new file mode 100644
index 0000000000..f1ed2fd89d
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/split.rs
@@ -0,0 +1,112 @@
+use crate::io::util::read_until::read_until_internal;
+use crate::io::AsyncBufRead;
+
+use pin_project_lite::pin_project;
+use std::io;
+use std::mem;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+pin_project! {
+    /// Stream for the [`split`](crate::io::AsyncBufReadExt::split) method.
+    #[derive(Debug)]
+    #[must_use = "streams do nothing unless polled"]
+    #[cfg_attr(docsrs, doc(cfg(feature = "io-util")))]
+    pub struct Split<R> {
+        #[pin]
+        reader: R,
+        buf: Vec<u8>,
+        delim: u8,
+        read: usize,
+    }
+}
+
+pub(crate) fn split<R>(reader: R, delim: u8) -> Split<R>
+where
+    R: AsyncBufRead,
+{
+    Split {
+        reader,
+        buf: Vec::new(),
+        delim,
+        read: 0,
+    }
+}
+
+impl<R> Split<R>
+where
+    R: AsyncBufRead + Unpin,
+{
+    /// Returns the next segment in the stream.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use tokio::io::AsyncBufRead;
+    /// use tokio::io::AsyncBufReadExt;
+    ///
+    /// # async fn dox(my_buf_read: impl AsyncBufRead + Unpin) -> std::io::Result<()> {
+    /// let mut segments = my_buf_read.split(b'f');
+    ///
+    /// while let Some(segment) = segments.next_segment().await? {
+    ///     println!("length = {}", segment.len())
+    /// }
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn next_segment(&mut self) -> io::Result<Option<Vec<u8>>> {
+        use crate::future::poll_fn;
+
+        poll_fn(|cx| Pin::new(&mut *self).poll_next_segment(cx)).await
+    }
+}
+
+impl<R> Split<R>
+where
+    R: AsyncBufRead,
+{
+    #[doc(hidden)]
+    pub fn poll_next_segment(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<io::Result<Option<Vec<u8>>>> {
+        let me = self.project();
+
+        let n = ready!(read_until_internal(
+            me.reader, cx, *me.delim, me.buf, me.read,
+        ))?;
+
+        if n == 0 && me.buf.is_empty() {
+            return Poll::Ready(Ok(None));
+        }
+
+        if me.buf.last() == Some(me.delim) {
+            me.buf.pop();
+        }
+
+        Poll::Ready(Ok(Some(mem::replace(me.buf, Vec::new()))))
+    }
+}
+
+#[cfg(feature = "stream")]
+impl<R: AsyncBufRead> crate::stream::Stream for Split<R> {
+    type Item = io::Result<Vec<u8>>;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        Poll::Ready(match ready!(self.poll_next_segment(cx)) {
+            Ok(Some(segment)) => Some(Ok(segment)),
+            Ok(None) => None,
+            Err(err) => Some(Err(err)),
+        })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        crate::is_unpin::<Split<()>>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/stream_reader.rs b/third_party/rust/tokio/src/io/util/stream_reader.rs
new file mode 100644
index 0000000000..b98f8bdfc2
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/stream_reader.rs
@@ -0,0 +1,184 @@
+use crate::io::{AsyncBufRead, AsyncRead};
+use crate::stream::Stream;
+use bytes::{Buf, BufMut};
+use pin_project_lite::pin_project;
+use std::io;
+use std::mem::MaybeUninit;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+pin_project! {
+    /// Convert a stream of byte chunks into an [`AsyncRead`].
+    ///
+    /// This type is usually created using the [`stream_reader`] function.
+    ///
+    /// [`AsyncRead`]: crate::io::AsyncRead
+    /// [`stream_reader`]: crate::io::stream_reader
+    #[derive(Debug)]
+    #[cfg_attr(docsrs, doc(cfg(feature = "stream")))]
+    #[cfg_attr(docsrs, doc(cfg(feature = "io-util")))]
+    pub struct StreamReader<S, B> {
+        #[pin]
+        inner: S,
+        chunk: Option<B>,
+    }
+}
+
+/// Convert a stream of byte chunks into an [`AsyncRead`](crate::io::AsyncRead).
+///
+/// # Example
+///
+/// ```
+/// use bytes::Bytes;
+/// use tokio::io::{stream_reader, AsyncReadExt};
+/// # #[tokio::main]
+/// # async fn main() -> std::io::Result<()> {
+///
+/// // Create a stream from an iterator.
+/// let stream = tokio::stream::iter(vec![
+///     Ok(Bytes::from_static(&[0, 1, 2, 3])),
+///     Ok(Bytes::from_static(&[4, 5, 6, 7])),
+///     Ok(Bytes::from_static(&[8, 9, 10, 11])),
+/// ]);
+///
+/// // Convert it to an AsyncRead.
+/// let mut read = stream_reader(stream);
+///
+/// // Read five bytes from the stream.
+/// let mut buf = [0; 5];
+/// read.read_exact(&mut buf).await?;
+/// assert_eq!(buf, [0, 1, 2, 3, 4]);
+///
+/// // Read the rest of the current chunk.
+/// assert_eq!(read.read(&mut buf).await?, 3);
+/// assert_eq!(&buf[..3], [5, 6, 7]);
+///
+/// // Read the next chunk.
+/// assert_eq!(read.read(&mut buf).await?, 4);
+/// assert_eq!(&buf[..4], [8, 9, 10, 11]);
+///
+/// // We have now reached the end.
+/// assert_eq!(read.read(&mut buf).await?, 0);
+///
+/// # Ok(())
+/// # }
+/// ```
+#[cfg_attr(docsrs, doc(cfg(feature = "stream")))]
+#[cfg_attr(docsrs, doc(cfg(feature = "io-util")))]
+pub fn stream_reader<S, B>(stream: S) -> StreamReader<S, B>
+where
+    S: Stream<Item = Result<B, io::Error>>,
+    B: Buf,
+{
+    StreamReader::new(stream)
+}
+
+impl<S, B> StreamReader<S, B>
+where
+    S: Stream<Item = Result<B, io::Error>>,
+    B: Buf,
+{
+    /// Convert the provided stream into an `AsyncRead`.
+    fn new(stream: S) -> Self {
+        Self {
+            inner: stream,
+            chunk: None,
+        }
+    }
+    /// Do we have a chunk and is it non-empty?
+    fn has_chunk(self: Pin<&mut Self>) -> bool {
+        if let Some(chunk) = self.project().chunk {
+            chunk.remaining() > 0
+        } else {
+            false
+        }
+    }
+}
+
+impl<S, B> AsyncRead for StreamReader<S, B>
+where
+    S: Stream<Item = Result<B, io::Error>>,
+    B: Buf,
+{
+    fn poll_read(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        if buf.is_empty() {
+            return Poll::Ready(Ok(0));
+        }
+
+        let inner_buf = match self.as_mut().poll_fill_buf(cx) {
+            Poll::Ready(Ok(buf)) => buf,
+            Poll::Ready(Err(err)) => return Poll::Ready(Err(err)),
+            Poll::Pending => return Poll::Pending,
+        };
+        let len = std::cmp::min(inner_buf.len(), buf.len());
+        (&mut buf[..len]).copy_from_slice(&inner_buf[..len]);
+
+        self.consume(len);
+        Poll::Ready(Ok(len))
+    }
+    fn poll_read_buf<BM: BufMut>(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut BM,
+    ) -> Poll<io::Result<usize>>
+    where
+        Self: Sized,
+    {
+        if !buf.has_remaining_mut() {
+            return Poll::Ready(Ok(0));
+        }
+
+        let inner_buf = match self.as_mut().poll_fill_buf(cx) {
+            Poll::Ready(Ok(buf)) => buf,
+            Poll::Ready(Err(err)) => return Poll::Ready(Err(err)),
+            Poll::Pending => return Poll::Pending,
+        };
+        let len = std::cmp::min(inner_buf.len(), buf.remaining_mut());
+        buf.put_slice(&inner_buf[..len]);
+
+        self.consume(len);
+        Poll::Ready(Ok(len))
+    }
+    unsafe fn prepare_uninitialized_buffer(&self, _buf: &mut [MaybeUninit<u8>]) -> bool {
+        false
+    }
+}
+
+impl<S, B> AsyncBufRead for StreamReader<S, B>
+where
+    S: Stream<Item = Result<B, io::Error>>,
+    B: Buf,
+{
+    fn poll_fill_buf(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
+        loop {
+            if self.as_mut().has_chunk() {
+                // This unwrap is very sad, but it can't be avoided.
+                let buf = self.project().chunk.as_ref().unwrap().bytes();
+                return Poll::Ready(Ok(buf));
+            } else {
+                match self.as_mut().project().inner.poll_next(cx) {
+                    Poll::Ready(Some(Ok(chunk))) => {
+                        // Go around the loop in case the chunk is empty.
+                        *self.as_mut().project().chunk = Some(chunk);
+                    }
+                    Poll::Ready(Some(Err(err))) => return Poll::Ready(Err(err)),
+                    Poll::Ready(None) => return Poll::Ready(Ok(&[])),
+                    Poll::Pending => return Poll::Pending,
+                }
+            }
+        }
+    }
+    fn consume(self: Pin<&mut Self>, amt: usize) {
+        if amt > 0 {
+            self.project()
+                .chunk
+                .as_mut()
+                .expect("No chunk present")
+                .advance(amt);
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/take.rs b/third_party/rust/tokio/src/io/util/take.rs
new file mode 100644
index 0000000000..5d6bd90aa3
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/take.rs
@@ -0,0 +1,131 @@
+use crate::io::{AsyncBufRead, AsyncRead};
+
+use pin_project_lite::pin_project;
+use std::mem::MaybeUninit;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+use std::{cmp, io};
+
+pin_project! {
+    /// Stream for the [`take`](super::AsyncReadExt::take) method.
+    #[derive(Debug)]
+    #[must_use = "streams do nothing unless you `.await` or poll them"]
+    #[cfg_attr(docsrs, doc(cfg(feature = "io-util")))]
+    pub struct Take<R> {
+        #[pin]
+        inner: R,
+        // Add '_' to avoid conflicts with `limit` method.
+        limit_: u64,
+    }
+}
+
+pub(super) fn take<R: AsyncRead>(inner: R, limit: u64) -> Take<R> {
+    Take {
+        inner,
+        limit_: limit,
+    }
+}
+
+impl<R: AsyncRead> Take<R> {
+    /// Returns the remaining number of bytes that can be
+    /// read before this instance will return EOF.
+    ///
+    /// # Note
+    ///
+    /// This instance may reach `EOF` after reading fewer bytes than indicated by
+    /// this method if the underlying [`AsyncRead`] instance reaches EOF.
+    pub fn limit(&self) -> u64 {
+        self.limit_
+    }
+
+    /// Sets the number of bytes that can be read before this instance will
+    /// return EOF. This is the same as constructing a new `Take` instance, so
+    /// the amount of bytes read and the previous limit value don't matter when
+    /// calling this method.
+    pub fn set_limit(&mut self, limit: u64) {
+        self.limit_ = limit
+    }
+
+    /// Gets a reference to the underlying reader.
+    pub fn get_ref(&self) -> &R {
+        &self.inner
+    }
+
+    /// Gets a mutable reference to the underlying reader.
+    ///
+    /// Care should be taken to avoid modifying the internal I/O state of the
+    /// underlying reader as doing so may corrupt the internal limit of this
+    /// `Take`.
+    pub fn get_mut(&mut self) -> &mut R {
+        &mut self.inner
+    }
+
+    /// Gets a pinned mutable reference to the underlying reader.
+    ///
+    /// Care should be taken to avoid modifying the internal I/O state of the
+    /// underlying reader as doing so may corrupt the internal limit of this
+    /// `Take`.
+    pub fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut R> {
+        self.project().inner
+    }
+
+    /// Consumes the `Take`, returning the wrapped reader.
+    pub fn into_inner(self) -> R {
+        self.inner
+    }
+}
+
+impl<R: AsyncRead> AsyncRead for Take<R> {
+    unsafe fn prepare_uninitialized_buffer(&self, buf: &mut [MaybeUninit<u8>]) -> bool {
+        self.inner.prepare_uninitialized_buffer(buf)
+    }
+
+    fn poll_read(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<Result<usize, io::Error>> {
+        if self.limit_ == 0 {
+            return Poll::Ready(Ok(0));
+        }
+
+        let me = self.project();
+        let max = std::cmp::min(buf.len() as u64, *me.limit_) as usize;
+        let n = ready!(me.inner.poll_read(cx, &mut buf[..max]))?;
+        *me.limit_ -= n as u64;
+        Poll::Ready(Ok(n))
+    }
+}
+
+impl<R: AsyncBufRead> AsyncBufRead for Take<R> {
+    fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
+        let me = self.project();
+
+        // Don't call into inner reader at all at EOF because it may still block
+        if *me.limit_ == 0 {
+            return Poll::Ready(Ok(&[]));
+        }
+
+        let buf = ready!(me.inner.poll_fill_buf(cx)?);
+        let cap = cmp::min(buf.len() as u64, *me.limit_) as usize;
+        Poll::Ready(Ok(&buf[..cap]))
+    }
+
+    fn consume(self: Pin<&mut Self>, amt: usize) {
+        let me = self.project();
+        // Don't let callers reset the limit by passing an overlarge value
+        let amt = cmp::min(amt as u64, *me.limit_) as usize;
+        *me.limit_ -= amt as u64;
+        me.inner.consume(amt);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        crate::is_unpin::<Take<()>>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/write.rs b/third_party/rust/tokio/src/io/util/write.rs
new file mode 100644
index 0000000000..433a421d34
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/write.rs
@@ -0,0 +1,37 @@
+use crate::io::AsyncWrite;
+
+use std::future::Future;
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+cfg_io_util! {
+    /// A future to write some of the buffer to an `AsyncWrite`.
+    #[derive(Debug)]
+    #[must_use = "futures do nothing unless you `.await` or poll them"]
+    pub struct Write<'a, W: ?Sized> {
+        writer: &'a mut W,
+        buf: &'a [u8],
+    }
+}
+
+/// Tries to write some bytes from the given `buf` to the writer in an
+/// asynchronous manner, returning a future.
+pub(crate) fn write<'a, W>(writer: &'a mut W, buf: &'a [u8]) -> Write<'a, W>
+where
+    W: AsyncWrite + Unpin + ?Sized,
+{
+    Write { writer, buf }
+}
+
+impl<W> Future for Write<'_, W>
+where
+    W: AsyncWrite + Unpin + ?Sized,
+{
+    type Output = io::Result<usize>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<usize>> {
+        let me = &mut *self;
+        Pin::new(&mut *me.writer).poll_write(cx, me.buf)
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/write_all.rs b/third_party/rust/tokio/src/io/util/write_all.rs
new file mode 100644
index 0000000000..898006c56c
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/write_all.rs
@@ -0,0 +1,57 @@
+use crate::io::AsyncWrite;
+
+use std::future::Future;
+use std::io;
+use std::mem;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+cfg_io_util! {
+    #[derive(Debug)]
+    #[must_use = "futures do nothing unless you `.await` or poll them"]
+    pub struct WriteAll<'a, W: ?Sized> {
+        writer: &'a mut W,
+        buf: &'a [u8],
+    }
+}
+
+pub(crate) fn write_all<'a, W>(writer: &'a mut W, buf: &'a [u8]) -> WriteAll<'a, W>
+where
+    W: AsyncWrite + Unpin + ?Sized,
+{
+    WriteAll { writer, buf }
+}
+
+impl<W> Future for WriteAll<'_, W>
+where
+    W: AsyncWrite + Unpin + ?Sized,
+{
+    type Output = io::Result<()>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        let me = &mut *self;
+        while !me.buf.is_empty() {
+            let n = ready!(Pin::new(&mut me.writer).poll_write(cx, me.buf))?;
+            {
+                let (_, rest) = mem::replace(&mut me.buf, &[]).split_at(n);
+                me.buf = rest;
+            }
+            if n == 0 {
+                return Poll::Ready(Err(io::ErrorKind::WriteZero.into()));
+            }
+        }
+
+        Poll::Ready(Ok(()))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        use std::marker::PhantomPinned;
+        crate::is_unpin::<WriteAll<'_, PhantomPinned>>();
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/write_buf.rs b/third_party/rust/tokio/src/io/util/write_buf.rs
new file mode 100644
index 0000000000..e49282fe0c
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/write_buf.rs
@@ -0,0 +1,43 @@
+use crate::io::AsyncWrite;
+
+use bytes::Buf;
+use std::future::Future;
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+cfg_io_util! {
+    /// A future to write some of the buffer to an `AsyncWrite`.
+    #[derive(Debug)]
+    #[must_use = "futures do nothing unless you `.await` or poll them"]
+    pub struct WriteBuf<'a, W, B> {
+        writer: &'a mut W,
+        buf: &'a mut B,
+    }
+}
+
+/// Tries to write some bytes from the given `buf` to the writer in an
+/// asynchronous manner, returning a future.
+pub(crate) fn write_buf<'a, W, B>(writer: &'a mut W, buf: &'a mut B) -> WriteBuf<'a, W, B>
+where
+    W: AsyncWrite,
+    B: Buf,
+{
+    WriteBuf { writer, buf }
+}
+
+impl<W, B> Future for WriteBuf<'_, W, B>
+where
+    W: AsyncWrite,
+    B: Buf,
+{
+    type Output = io::Result<usize>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<usize>> {
+        // safety: no data is moved from self
+        unsafe {
+            let me = self.get_unchecked_mut();
+            Pin::new_unchecked(&mut *me.writer).poll_write_buf(cx, &mut me.buf)
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/io/util/write_int.rs b/third_party/rust/tokio/src/io/util/write_int.rs
new file mode 100644
index 0000000000..672c35f076
--- /dev/null
+++ b/third_party/rust/tokio/src/io/util/write_int.rs
@@ -0,0 +1,122 @@
+use crate::io::AsyncWrite;
+
+use bytes::BufMut;
+use pin_project_lite::pin_project;
+use std::future::Future;
+use std::io;
+use std::mem::size_of;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+macro_rules! writer {
+    ($name:ident, $ty:ty, $writer:ident) => {
+        writer!($name, $ty, $writer, size_of::<$ty>());
+    };
+    ($name:ident, $ty:ty, $writer:ident, $bytes:expr) => {
+        pin_project! {
+            #[doc(hidden)]
+            pub struct $name<W> {
+                #[pin]
+                dst: W,
+                buf: [u8; $bytes],
+                written: u8,
+            }
+        }
+
+        impl<W> $name<W> {
+            pub(crate) fn new(w: W, value: $ty) -> Self {
+                let mut writer = $name {
+                    buf: [0; $bytes],
+                    written: 0,
+                    dst: w,
+                };
+                BufMut::$writer(&mut &mut writer.buf[..], value);
+                writer
+            }
+        }
+
+        impl<W> Future for $name<W>
+        where
+            W: AsyncWrite,
+        {
+            type Output = io::Result<()>;
+
+            fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+                let mut me = self.project();
+
+                if *me.written == $bytes as u8 {
+                    return Poll::Ready(Ok(()));
+                }
+
+                while *me.written < $bytes as u8 {
+                    *me.written += match me
+                        .dst
+                        .as_mut()
+                        .poll_write(cx, &me.buf[*me.written as usize..])
+                    {
+                        Poll::Pending => return Poll::Pending,
+                        Poll::Ready(Err(e)) => return Poll::Ready(Err(e.into())),
+                        Poll::Ready(Ok(0)) => {
+                            return Poll::Ready(Err(io::ErrorKind::WriteZero.into()));
+                        }
+                        Poll::Ready(Ok(n)) => n as u8,
+                    };
+                }
+                Poll::Ready(Ok(()))
+            }
+        }
+    };
+}
+
+macro_rules! writer8 {
+    ($name:ident, $ty:ty) => {
+        pin_project! {
+            #[doc(hidden)]
+            pub struct $name<W> {
+                #[pin]
+                dst: W,
+                byte: $ty,
+            }
+        }
+
+        impl<W> $name<W> {
+            pub(crate) fn new(dst: W, byte: $ty) -> Self {
+                Self { dst, byte }
+            }
+        }
+
+        impl<W> Future for $name<W>
+        where
+            W: AsyncWrite,
+        {
+            type Output = io::Result<()>;
+
+            fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+                let me = self.project();
+
+                let buf = [*me.byte as u8];
+
+                match me.dst.poll_write(cx, &buf[..]) {
+                    Poll::Pending => Poll::Pending,
+                    Poll::Ready(Err(e)) => Poll::Ready(Err(e.into())),
+                    Poll::Ready(Ok(0)) => Poll::Ready(Err(io::ErrorKind::WriteZero.into())),
+                    Poll::Ready(Ok(1)) => Poll::Ready(Ok(())),
+                    Poll::Ready(Ok(_)) => unreachable!(),
+                }
+            }
+        }
+    };
+}
+
+writer8!(WriteU8, u8);
+writer8!(WriteI8, i8);
+
+writer!(WriteU16, u16, put_u16);
+writer!(WriteU32, u32, put_u32);
+writer!(WriteU64, u64, put_u64);
+writer!(WriteU128, u128, put_u128);
+
+writer!(WriteI16, i16, put_i16);
+writer!(WriteI32, i32, put_i32);
+writer!(WriteI64, i64, put_i64);
+writer!(WriteI128, i128, put_i128);
diff --git a/third_party/rust/tokio/src/lib.rs b/third_party/rust/tokio/src/lib.rs
new file mode 100644
index 0000000000..8172f40a3b
--- /dev/null
+++ b/third_party/rust/tokio/src/lib.rs
@@ -0,0 +1,390 @@
+#![doc(html_root_url = "https://docs.rs/tokio/0.2.18")]
+#![allow(
+    clippy::cognitive_complexity,
+    clippy::large_enum_variant,
+    clippy::needless_doctest_main
+)]
+#![warn(
+    missing_debug_implementations,
+    missing_docs,
+    rust_2018_idioms,
+    unreachable_pub
+)]
+#![deny(intra_doc_link_resolution_failure)]
+#![doc(test(
+    no_crate_inject,
+    attr(deny(warnings, rust_2018_idioms), allow(dead_code, unused_variables))
+))]
+#![cfg_attr(docsrs, feature(doc_cfg))]
+
+//! A runtime for writing reliable, asynchronous, and slim applications.
+//!
+//! Tokio is an event-driven, non-blocking I/O platform for writing asynchronous
+//! applications with the Rust programming language. At a high level, it
+//! provides a few major components:
+//!
+//! * Tools for [working with asynchronous tasks][tasks], including
+//!   [synchronization primitives and channels][sync] and [timeouts, delays, and
+//!   intervals][time].
+//! * APIs for [performing asynchronous I/O][io], including [TCP and UDP][net] sockets,
+//!   [filesystem][fs] operations, and [process] and [signal] management.
+//! * A [runtime] for executing asynchronous code, including a task scheduler,
+//!   an I/O driver backed by the operating system's event queue (epoll, kqueue,
+//!   IOCP, etc...), and a high performance timer.
+//!
+//! Guide level documentation is found on the [website].
+//!
+//! [tasks]: #working-with-tasks
+//! [sync]: crate::sync
+//! [time]: crate::time
+//! [io]: #asynchronous-io
+//! [net]: crate::net
+//! [fs]: crate::fs
+//! [process]: crate::process
+//! [signal]: crate::signal
+//! [fs]: crate::fs
+//! [runtime]: crate::runtime
+//! [website]: https://tokio.rs/docs/overview/
+//!
+//! # A Tour of Tokio
+//!
+//! Tokio consists of a number of modules that provide a range of functionality
+//! essential for implementing asynchronous applications in Rust. In this
+//! section, we will take a brief tour of Tokio, summarizing the major APIs and
+//! their uses.
+//!
+//! The easiest way to get started is to enable all features. Do this by
+//! enabling the `full` feature flag:
+//!
+//! ```toml
+//! tokio = { version = "0.2", features = ["full"] }
+//! ```
+//!
+//! ## Feature flags
+//!
+//! Tokio uses a set of [feature flags] to reduce the amount of compiled code. It
+//! is possible to just enable certain features over others. By default, Tokio
+//! does not enable any features but allows one to enable a subset for their use
+//! case. Below is a list of the available feature flags. You may also notice
+//! above each function, struct and trait there is listed one or more feature flags
+//! that are required for that item to be used. If you are new to Tokio it is
+//! recommended that you use the `full` feature flag which will enable all public APIs.
+//! Beware though that this will pull in many extra dependencies that you may not
+//! need.
+//!
+//! - `full`: Enables all Tokio public API features listed below.
+//! - `rt-core`: Enables `tokio::spawn` and the basic (single-threaded) scheduler.
+//! - `rt-threaded`: Enables the heavier, multi-threaded, work-stealing scheduler.
+//! - `rt-util`: Enables non-scheduler utilities.
+//! - `io-driver`: Enables the `mio` based IO driver.
+//! - `io-util`: Enables the IO based `Ext` traits.
+//! - `io-std`: Enable `Stdout`, `Stdin` and `Stderr` types.
+//! - `net`: Enables `tokio::net` types such as `TcpStream`, `UnixStream` and `UdpSocket`.
+//! - `tcp`: Enables all `tokio::net::tcp` types.
+//! - `udp`: Enables all `tokio::net::udp` types.
+//! - `uds`: Enables all `tokio::net::unix` types.
+//! - `time`: Enables `tokio::time` types and allows the schedulers to enable
+//! the built in timer.
+//! - `process`: Enables `tokio::process` types.
+//! - `macros`: Enables `#[tokio::main]` and `#[tokio::test]` macros.
+//! - `sync`: Enables all `tokio::sync` types.
+//! - `stream`: Enables optional `Stream` implementations for types within Tokio.
+//! - `signal`: Enables all `tokio::signal` types.
+//! - `fs`: Enables `tokio::fs` types.
+//! - `dns`: Enables async `tokio::net::ToSocketAddrs`.
+//! - `test-util`: Enables testing based infrastructure for the Tokio runtime.
+//! - `blocking`: Enables `block_in_place` and `spawn_blocking`.
+//!
+//! _Note: `AsyncRead` and `AsyncWrite` traits do not require any features and are
+//! always available._
+//!
+//! ### Internal features
+//!
+//! These features do not expose any new API, but influence internal
+//! implementation aspects of Tokio, and can pull in additional
+//! dependencies. They are not included in `full`:
+//!
+//! - `parking_lot`: As a potential optimization, use the _parking_lot_ crate's
+//! synchronization primitives internally. MSRV may increase according to the
+//! _parking_lot_ release in use.
+//!
+//! [feature flags]: https://doc.rust-lang.org/cargo/reference/manifest.html#the-features-section
+//!
+//! ### Authoring applications
+//!
+//! Tokio is great for writing applications and most users in this case shouldn't
+//! worry too much about what features they should pick. If you're unsure, we suggest
+//! going with `full` to ensure that you don't run into any road blocks while you're
+//! building your application.
+//!
+//! #### Example
+//!
+//! This example shows the quickest way to get started with Tokio.
+//!
+//! ```toml
+//! tokio = { version = "0.2", features = ["full"] }
+//! ```
+//!
+//! ### Authoring libraries
+//!
+//! As a library author your goal should be to provide the lighest weight crate
+//! that is based on Tokio. To achieve this you should ensure that you only enable
+//! the features you need. This allows users to pick up your crate without having
+//! to enable unnecessary features.
+//!
+//! #### Example
+//!
+//! This example shows how you may want to import features for a library that just
+//! needs to `tokio::spawn` and use a `TcpStream`.
+//!
+//! ```toml
+//! tokio = { version = "0.2", features = ["rt-core", "tcp"] }
+//! ```
+//!
+//! ## Working With Tasks
+//!
+//! Asynchronous programs in Rust are based around lightweight, non-blocking
+//! units of execution called [_tasks_][tasks]. The [`tokio::task`] module provides
+//! important tools for working with tasks:
+//!
+//! * The [`spawn`] function and [`JoinHandle`] type, for scheduling a new task
+//!   on the Tokio runtime and awaiting the output of a spawned task, respectively,
+//! * Functions for [running blocking operations][blocking] in an asynchronous
+//!   task context.
+//!
+//! The [`tokio::task`] module is present only when the "rt-core" feature flag
+//! is enabled.
+//!
+//! [tasks]: task/index.html#what-are-tasks
+//! [`tokio::task`]: crate::task
+//! [`spawn`]: crate::task::spawn()
+//! [`JoinHandle`]: crate::task::JoinHandle
+//! [blocking]: task/index.html#blocking-and-yielding
+//!
+//! The [`tokio::sync`] module contains synchronization primitives to use when
+//! needing to communicate or share data. These include:
+//!
+//! * channels ([`oneshot`], [`mpsc`], and [`watch`]), for sending values
+//!   between tasks,
+//! * a non-blocking [`Mutex`], for controlling access to a shared, mutable
+//!   value,
+//! * an asynchronous [`Barrier`] type, for multiple tasks to synchronize before
+//!   beginning a computation.
+//!
+//! The `tokio::sync` module is present only when the "sync" feature flag is
+//! enabled.
+//!
+//! [`tokio::sync`]: crate::sync
+//! [`Mutex`]: crate::sync::Mutex
+//! [`Barrier`]: crate::sync::Barrier
+//! [`oneshot`]: crate::sync::oneshot
+//! [`mpsc`]: crate::sync::mpsc
+//! [`watch`]: crate::sync::watch
+//!
+//! The [`tokio::time`] module provides utilities for tracking time and
+//! scheduling work. This includes functions for setting [timeouts][timeout] for
+//! tasks, [delaying][delay] work to run in the future, or [repeating an operation at an
+//! interval][interval].
+//!
+//! In order to use `tokio::time`, the "time" feature flag must be enabled.
+//!
+//! [`tokio::time`]: crate::time
+//! [delay]: crate::time::delay_for()
+//! [interval]: crate::time::interval()
+//! [timeout]: crate::time::timeout()
+//!
+//! Finally, Tokio provides a _runtime_ for executing asynchronous tasks. Most
+//! applications can use the [`#[tokio::main]`][main] macro to run their code on the
+//! Tokio runtime. In use-cases where manual control over the runtime is
+//! required, the [`tokio::runtime`] module provides APIs for configuring and
+//! managing runtimes.
+//!
+//! Using the runtime requires the "rt-core" or "rt-threaded" feature flags, to
+//! enable the basic [single-threaded scheduler][rt-core] and the [thread-pool
+//! scheduler][rt-threaded], respectively. See the [`runtime` module
+//! documentation][rt-features] for details. In addition, the "macros" feature
+//! flag enables the `#[tokio::main]` and `#[tokio::test]` attributes.
+//!
+//! [main]: attr.main.html
+//! [`tokio::runtime`]: crate::runtime
+//! [`Builder`]: crate::runtime::Builder
+//! [`Runtime`]: crate::runtime::Runtime
+//! [rt-core]: runtime/index.html#basic-scheduler
+//! [rt-threaded]: runtime/index.html#threaded-scheduler
+//! [rt-features]: runtime/index.html#runtime-scheduler
+//!
+//! ## Asynchronous IO
+//!
+//! As well as scheduling and running tasks, Tokio provides everything you need
+//! to perform input and output asynchronously.
+//!
+//! The [`tokio::io`] module provides Tokio's asynchronous core I/O primitives,
+//! the [`AsyncRead`], [`AsyncWrite`], and [`AsyncBufRead`] traits. In addition,
+//! when the "io-util" feature flag is enabled, it also provides combinators and
+//! functions for working with these traits, forming as an asynchronous
+//! counterpart to [`std::io`]. When the "io-driver" feature flag is enabled, it
+//! also provides utilities for library authors implementing I/O resources.
+//!
+//! Tokio also includes APIs for performing various kinds of I/O and interacting
+//! with the operating system asynchronously. These include:
+//!
+//! * [`tokio::net`], which contains non-blocking versions of [TCP], [UDP], and
+//!   [Unix Domain Sockets][UDS] (enabled by the "net" feature flag),
+//! * [`tokio::fs`], similar to [`std::fs`] but for performing filesystem I/O
+//!   asynchronously (enabled by the "fs" feature flag),
+//! * [`tokio::signal`], for asynchronously handling Unix and Windows OS signals
+//!   (enabled by the "signal" feature flag),
+//! * [`tokio::process`], for spawning and managing child processes (enabled by
+//!   the "process" feature flag).
+//!
+//! [`tokio::io`]: crate::io
+//! [`AsyncRead`]: crate::io::AsyncRead
+//! [`AsyncWrite`]: crate::io::AsyncWrite
+//! [`AsyncBufRead`]: crate::io::AsyncBufRead
+//! [`std::io`]: std::io
+//! [`tokio::net`]: crate::net
+//! [TCP]: crate::net::tcp
+//! [UDP]: crate::net::udp
+//! [UDS]: crate::net::unix
+//! [`tokio::fs`]: crate::fs
+//! [`std::fs`]: std::fs
+//! [`tokio::signal`]: crate::signal
+//! [`tokio::process`]: crate::process
+//!
+//! # Examples
+//!
+//! A simple TCP echo server:
+//!
+//! ```no_run
+//! use tokio::net::TcpListener;
+//! use tokio::prelude::*;
+//!
+//! #[tokio::main]
+//! async fn main() -> Result<(), Box<dyn std::error::Error>> {
+//!     let mut listener = TcpListener::bind("127.0.0.1:8080").await?;
+//!
+//!     loop {
+//!         let (mut socket, _) = listener.accept().await?;
+//!
+//!         tokio::spawn(async move {
+//!             let mut buf = [0; 1024];
+//!
+//!             // In a loop, read data from the socket and write the data back.
+//!             loop {
+//!                 let n = match socket.read(&mut buf).await {
+//!                     // socket closed
+//!                     Ok(n) if n == 0 => return,
+//!                     Ok(n) => n,
+//!                     Err(e) => {
+//!                         eprintln!("failed to read from socket; err = {:?}", e);
+//!                         return;
+//!                     }
+//!                 };
+//!
+//!                 // Write the data back
+//!                 if let Err(e) = socket.write_all(&buf[0..n]).await {
+//!                     eprintln!("failed to write to socket; err = {:?}", e);
+//!                     return;
+//!                 }
+//!             }
+//!         });
+//!     }
+//! }
+//! ```
+
+// Includes re-exports used by macros.
+//
+// This module is not intended to be part of the public API. In general, any
+// `doc(hidden)` code is not part of Tokio's public and stable API.
+#[macro_use]
+#[doc(hidden)]
+pub mod macros;
+
+cfg_fs! {
+    pub mod fs;
+}
+
+#[doc(hidden)]
+pub mod future;
+
+pub mod io;
+pub mod net;
+
+mod loom;
+mod park;
+
+pub mod prelude;
+
+cfg_process! {
+    pub mod process;
+}
+
+pub mod runtime;
+
+pub(crate) mod coop;
+
+cfg_signal! {
+    pub mod signal;
+}
+
+cfg_stream! {
+    pub mod stream;
+}
+
+cfg_sync! {
+    pub mod sync;
+}
+cfg_not_sync! {
+    mod sync;
+}
+
+cfg_rt_core! {
+    pub mod task;
+    pub use task::spawn;
+}
+
+cfg_time! {
+    pub mod time;
+}
+
+mod util;
+
+cfg_macros! {
+    /// Implementation detail of the `select!` macro. This macro is **not**
+    /// intended to be used as part of the public API and is permitted to
+    /// change.
+    #[doc(hidden)]
+    pub use tokio_macros::select_priv_declare_output_enum;
+
+    doc_rt_core! {
+        cfg_rt_threaded! {
+            // This is the docs.rs case (with all features) so make sure macros
+            // is included in doc(cfg).
+
+            #[cfg(not(test))] // Work around for rust-lang/rust#62127
+            #[cfg_attr(docsrs, doc(cfg(feature = "macros")))]
+            pub use tokio_macros::main_threaded as main;
+
+            #[cfg_attr(docsrs, doc(cfg(feature = "macros")))]
+            pub use tokio_macros::test_threaded as test;
+        }
+
+        cfg_not_rt_threaded! {
+            #[cfg(not(test))] // Work around for rust-lang/rust#62127
+            pub use tokio_macros::main_basic as main;
+            pub use tokio_macros::test_basic as test;
+        }
+    }
+
+    // Maintains old behavior
+    cfg_not_rt_core! {
+        #[cfg(not(test))]
+        pub use tokio_macros::main;
+        pub use tokio_macros::test;
+    }
+}
+
+// TODO: rm
+#[cfg(feature = "io-util")]
+#[cfg(test)]
+fn is_unpin<T: Unpin>() {}
diff --git a/third_party/rust/tokio/src/loom/mocked.rs b/third_party/rust/tokio/src/loom/mocked.rs
new file mode 100644
index 0000000000..7891395225
--- /dev/null
+++ b/third_party/rust/tokio/src/loom/mocked.rs
@@ -0,0 +1,13 @@
+pub(crate) use loom::*;
+
+pub(crate) mod rand {
+    pub(crate) fn seed() -> u64 {
+        1
+    }
+}
+
+pub(crate) mod sys {
+    pub(crate) fn num_cpus() -> usize {
+        2
+    }
+}
diff --git a/third_party/rust/tokio/src/loom/mod.rs b/third_party/rust/tokio/src/loom/mod.rs
new file mode 100644
index 0000000000..56a41f25a0
--- /dev/null
+++ b/third_party/rust/tokio/src/loom/mod.rs
@@ -0,0 +1,12 @@
+//! This module abstracts over `loom` and `std::sync` depending on whether we
+//! are running tests or not.
+
+#[cfg(not(all(test, loom)))]
+mod std;
+#[cfg(not(all(test, loom)))]
+pub(crate) use self::std::*;
+
+#[cfg(all(test, loom))]
+mod mocked;
+#[cfg(all(test, loom))]
+pub(crate) use self::mocked::*;
diff --git a/third_party/rust/tokio/src/loom/std/atomic_ptr.rs b/third_party/rust/tokio/src/loom/std/atomic_ptr.rs
new file mode 100644
index 0000000000..eb8e47557a
--- /dev/null
+++ b/third_party/rust/tokio/src/loom/std/atomic_ptr.rs
@@ -0,0 +1,32 @@
+use std::fmt;
+use std::ops::Deref;
+
+/// `AtomicPtr` providing an additional `load_unsync` function.
+pub(crate) struct AtomicPtr<T> {
+    inner: std::sync::atomic::AtomicPtr<T>,
+}
+
+impl<T> AtomicPtr<T> {
+    pub(crate) fn new(ptr: *mut T) -> AtomicPtr<T> {
+        let inner = std::sync::atomic::AtomicPtr::new(ptr);
+        AtomicPtr { inner }
+    }
+
+    pub(crate) fn with_mut<R>(&mut self, f: impl FnOnce(&mut *mut T) -> R) -> R {
+        f(self.inner.get_mut())
+    }
+}
+
+impl<T> Deref for AtomicPtr<T> {
+    type Target = std::sync::atomic::AtomicPtr<T>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.inner
+    }
+}
+
+impl<T> fmt::Debug for AtomicPtr<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.deref().fmt(fmt)
+    }
+}
diff --git a/third_party/rust/tokio/src/loom/std/atomic_u16.rs b/third_party/rust/tokio/src/loom/std/atomic_u16.rs
new file mode 100644
index 0000000000..70390972b4
--- /dev/null
+++ b/third_party/rust/tokio/src/loom/std/atomic_u16.rs
@@ -0,0 +1,44 @@
+use std::cell::UnsafeCell;
+use std::fmt;
+use std::ops::Deref;
+
+/// `AtomicU16` providing an additional `load_unsync` function.
+pub(crate) struct AtomicU16 {
+    inner: UnsafeCell<std::sync::atomic::AtomicU16>,
+}
+
+unsafe impl Send for AtomicU16 {}
+unsafe impl Sync for AtomicU16 {}
+
+impl AtomicU16 {
+    pub(crate) fn new(val: u16) -> AtomicU16 {
+        let inner = UnsafeCell::new(std::sync::atomic::AtomicU16::new(val));
+        AtomicU16 { inner }
+    }
+
+    /// Performs an unsynchronized load.
+    ///
+    /// # Safety
+    ///
+    /// All mutations must have happened before the unsynchronized load.
+    /// Additionally, there must be no concurrent mutations.
+    pub(crate) unsafe fn unsync_load(&self) -> u16 {
+        *(*self.inner.get()).get_mut()
+    }
+}
+
+impl Deref for AtomicU16 {
+    type Target = std::sync::atomic::AtomicU16;
+
+    fn deref(&self) -> &Self::Target {
+        // safety: it is always safe to access `&self` fns on the inner value as
+        // we never perform unsafe mutations.
+        unsafe { &*self.inner.get() }
+    }
+}
+
+impl fmt::Debug for AtomicU16 {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.deref().fmt(fmt)
+    }
+}
diff --git a/third_party/rust/tokio/src/loom/std/atomic_u32.rs b/third_party/rust/tokio/src/loom/std/atomic_u32.rs
new file mode 100644
index 0000000000..6f786c519f
--- /dev/null
+++ b/third_party/rust/tokio/src/loom/std/atomic_u32.rs
@@ -0,0 +1,34 @@
+use std::cell::UnsafeCell;
+use std::fmt;
+use std::ops::Deref;
+
+/// `AtomicU32` providing an additional `load_unsync` function.
+pub(crate) struct AtomicU32 {
+    inner: UnsafeCell<std::sync::atomic::AtomicU32>,
+}
+
+unsafe impl Send for AtomicU32 {}
+unsafe impl Sync for AtomicU32 {}
+
+impl AtomicU32 {
+    pub(crate) fn new(val: u32) -> AtomicU32 {
+        let inner = UnsafeCell::new(std::sync::atomic::AtomicU32::new(val));
+        AtomicU32 { inner }
+    }
+}
+
+impl Deref for AtomicU32 {
+    type Target = std::sync::atomic::AtomicU32;
+
+    fn deref(&self) -> &Self::Target {
+        // safety: it is always safe to access `&self` fns on the inner value as
+        // we never perform unsafe mutations.
+        unsafe { &*self.inner.get() }
+    }
+}
+
+impl fmt::Debug for AtomicU32 {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.deref().fmt(fmt)
+    }
+}
diff --git a/third_party/rust/tokio/src/loom/std/atomic_u64.rs b/third_party/rust/tokio/src/loom/std/atomic_u64.rs
new file mode 100644
index 0000000000..206954fcc3
--- /dev/null
+++ b/third_party/rust/tokio/src/loom/std/atomic_u64.rs
@@ -0,0 +1,60 @@
+//! Implementation of an atomic u64 cell. On 64 bit platforms, this is a
+//! re-export of `AtomicU64`. On 32 bit platforms, this is implemented using a
+//! `Mutex`.
+
+pub(crate) use self::imp::AtomicU64;
+
+// `AtomicU64` can only be used on targets with `target_has_atomic` is 64 or greater.
+// Once `cfg_target_has_atomic` feature is stable, we can replace it with
+// `#[cfg(target_has_atomic = "64")]`.
+// Refs: https://github.com/rust-lang/rust/tree/master/src/librustc_target
+#[cfg(not(any(target_arch = "arm", target_arch = "mips", target_arch = "powerpc")))]
+mod imp {
+    pub(crate) use std::sync::atomic::AtomicU64;
+}
+
+#[cfg(any(target_arch = "arm", target_arch = "mips", target_arch = "powerpc"))]
+mod imp {
+    use std::sync::atomic::Ordering;
+    use std::sync::Mutex;
+
+    #[derive(Debug)]
+    pub(crate) struct AtomicU64 {
+        inner: Mutex<u64>,
+    }
+
+    impl AtomicU64 {
+        pub(crate) fn new(val: u64) -> AtomicU64 {
+            AtomicU64 {
+                inner: Mutex::new(val),
+            }
+        }
+
+        pub(crate) fn load(&self, _: Ordering) -> u64 {
+            *self.inner.lock().unwrap()
+        }
+
+        pub(crate) fn store(&self, val: u64, _: Ordering) {
+            *self.inner.lock().unwrap() = val;
+        }
+
+        pub(crate) fn fetch_or(&self, val: u64, _: Ordering) -> u64 {
+            let mut lock = self.inner.lock().unwrap();
+            let prev = *lock;
+            *lock = prev | val;
+            prev
+        }
+
+        pub(crate) fn compare_and_swap(&self, old: u64, new: u64, _: Ordering) -> u64 {
+            let mut lock = self.inner.lock().unwrap();
+            let prev = *lock;
+
+            if prev != old {
+                return prev;
+            }
+
+            *lock = new;
+            prev
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/loom/std/atomic_u8.rs b/third_party/rust/tokio/src/loom/std/atomic_u8.rs
new file mode 100644
index 0000000000..4fcd0df3d4
--- /dev/null
+++ b/third_party/rust/tokio/src/loom/std/atomic_u8.rs
@@ -0,0 +1,34 @@
+use std::cell::UnsafeCell;
+use std::fmt;
+use std::ops::Deref;
+
+/// `AtomicU8` providing an additional `load_unsync` function.
+pub(crate) struct AtomicU8 {
+    inner: UnsafeCell<std::sync::atomic::AtomicU8>,
+}
+
+unsafe impl Send for AtomicU8 {}
+unsafe impl Sync for AtomicU8 {}
+
+impl AtomicU8 {
+    pub(crate) fn new(val: u8) -> AtomicU8 {
+        let inner = UnsafeCell::new(std::sync::atomic::AtomicU8::new(val));
+        AtomicU8 { inner }
+    }
+}
+
+impl Deref for AtomicU8 {
+    type Target = std::sync::atomic::AtomicU8;
+
+    fn deref(&self) -> &Self::Target {
+        // safety: it is always safe to access `&self` fns on the inner value as
+        // we never perform unsafe mutations.
+        unsafe { &*self.inner.get() }
+    }
+}
+
+impl fmt::Debug for AtomicU8 {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.deref().fmt(fmt)
+    }
+}
diff --git a/third_party/rust/tokio/src/loom/std/atomic_usize.rs b/third_party/rust/tokio/src/loom/std/atomic_usize.rs
new file mode 100644
index 0000000000..0fe998f1f9
--- /dev/null
+++ b/third_party/rust/tokio/src/loom/std/atomic_usize.rs
@@ -0,0 +1,56 @@
+use std::cell::UnsafeCell;
+use std::fmt;
+use std::ops;
+
+/// `AtomicUsize` providing an additional `load_unsync` function.
+pub(crate) struct AtomicUsize {
+    inner: UnsafeCell<std::sync::atomic::AtomicUsize>,
+}
+
+unsafe impl Send for AtomicUsize {}
+unsafe impl Sync for AtomicUsize {}
+
+impl AtomicUsize {
+    pub(crate) fn new(val: usize) -> AtomicUsize {
+        let inner = UnsafeCell::new(std::sync::atomic::AtomicUsize::new(val));
+        AtomicUsize { inner }
+    }
+
+    /// Performs an unsynchronized load.
+    ///
+    /// # Safety
+    ///
+    /// All mutations must have happened before the unsynchronized load.
+    /// Additionally, there must be no concurrent mutations.
+    pub(crate) unsafe fn unsync_load(&self) -> usize {
+        *(*self.inner.get()).get_mut()
+    }
+
+    pub(crate) fn with_mut<R>(&mut self, f: impl FnOnce(&mut usize) -> R) -> R {
+        // safety: we have mutable access
+        f(unsafe { (*self.inner.get()).get_mut() })
+    }
+}
+
+impl ops::Deref for AtomicUsize {
+    type Target = std::sync::atomic::AtomicUsize;
+
+    fn deref(&self) -> &Self::Target {
+        // safety: it is always safe to access `&self` fns on the inner value as
+        // we never perform unsafe mutations.
+        unsafe { &*self.inner.get() }
+    }
+}
+
+impl ops::DerefMut for AtomicUsize {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        // safety: we hold `&mut self`
+        unsafe { &mut *self.inner.get() }
+    }
+}
+
+impl fmt::Debug for AtomicUsize {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(fmt)
+    }
+}
diff --git a/third_party/rust/tokio/src/loom/std/mod.rs b/third_party/rust/tokio/src/loom/std/mod.rs
new file mode 100644
index 0000000000..595bdf60ed
--- /dev/null
+++ b/third_party/rust/tokio/src/loom/std/mod.rs
@@ -0,0 +1,87 @@
+#![cfg_attr(any(not(feature = "full"), loom), allow(unused_imports, dead_code))]
+
+mod atomic_ptr;
+mod atomic_u16;
+mod atomic_u32;
+mod atomic_u64;
+mod atomic_u8;
+mod atomic_usize;
+mod unsafe_cell;
+
+pub(crate) mod cell {
+    pub(crate) use super::unsafe_cell::UnsafeCell;
+}
+
+#[cfg(any(feature = "sync", feature = "io-driver"))]
+pub(crate) mod future {
+    pub(crate) use crate::sync::AtomicWaker;
+}
+
+pub(crate) mod rand {
+    use std::collections::hash_map::RandomState;
+    use std::hash::{BuildHasher, Hash, Hasher};
+    use std::sync::atomic::AtomicU32;
+    use std::sync::atomic::Ordering::Relaxed;
+
+    static COUNTER: AtomicU32 = AtomicU32::new(1);
+
+    pub(crate) fn seed() -> u64 {
+        let rand_state = RandomState::new();
+
+        let mut hasher = rand_state.build_hasher();
+
+        // Hash some unique-ish data to generate some new state
+        COUNTER.fetch_add(1, Relaxed).hash(&mut hasher);
+
+        // Get the seed
+        hasher.finish()
+    }
+}
+
+pub(crate) mod sync {
+    pub(crate) use std::sync::Arc;
+
+    #[cfg(feature = "parking_lot")]
+    mod pl_wrappers;
+
+    // Below, make sure all the feature-influenced types are exported for
+    // internal use. Note however that some are not _currently_ named by
+    // consuming code.
+
+    #[cfg(feature = "parking_lot")]
+    #[allow(unused_imports)]
+    pub(crate) use pl_wrappers::{Condvar, Mutex};
+
+    #[cfg(feature = "parking_lot")]
+    #[allow(unused_imports)]
+    pub(crate) use parking_lot::{MutexGuard, WaitTimeoutResult};
+
+    #[cfg(not(feature = "parking_lot"))]
+    #[allow(unused_imports)]
+    pub(crate) use std::sync::{Condvar, Mutex, MutexGuard, WaitTimeoutResult};
+
+    pub(crate) mod atomic {
+        pub(crate) use crate::loom::std::atomic_ptr::AtomicPtr;
+        pub(crate) use crate::loom::std::atomic_u16::AtomicU16;
+        pub(crate) use crate::loom::std::atomic_u32::AtomicU32;
+        pub(crate) use crate::loom::std::atomic_u64::AtomicU64;
+        pub(crate) use crate::loom::std::atomic_u8::AtomicU8;
+        pub(crate) use crate::loom::std::atomic_usize::AtomicUsize;
+
+        pub(crate) use std::sync::atomic::{spin_loop_hint, AtomicBool};
+    }
+}
+
+pub(crate) mod sys {
+    #[cfg(feature = "rt-threaded")]
+    pub(crate) fn num_cpus() -> usize {
+        usize::max(1, num_cpus::get())
+    }
+
+    #[cfg(not(feature = "rt-threaded"))]
+    pub(crate) fn num_cpus() -> usize {
+        1
+    }
+}
+
+pub(crate) use std::thread;
diff --git a/third_party/rust/tokio/src/loom/std/sync/pl_wrappers.rs b/third_party/rust/tokio/src/loom/std/sync/pl_wrappers.rs
new file mode 100644
index 0000000000..3be8ba1c10
--- /dev/null
+++ b/third_party/rust/tokio/src/loom/std/sync/pl_wrappers.rs
@@ -0,0 +1,79 @@
+//! A minimal adaption of the `parking_lot` synchronization primitives to the
+//! equivalent `std::sync` types.
+//!
+//! This can be extended to additional types/methods as required.
+
+use std::sync::{LockResult, TryLockError, TryLockResult};
+use std::time::Duration;
+
+use parking_lot as pl;
+
+/// Adapter for `parking_lot::Mutex` to the `std::sync::Mutex` interface.
+#[derive(Debug)]
+pub(crate) struct Mutex<T: ?Sized>(pl::Mutex<T>);
+
+impl<T> Mutex<T> {
+    #[inline]
+    pub(crate) fn new(t: T) -> Mutex<T> {
+        Mutex(pl::Mutex::new(t))
+    }
+
+    #[inline]
+    pub(crate) fn lock(&self) -> LockResult<pl::MutexGuard<'_, T>> {
+        Ok(self.0.lock())
+    }
+
+    #[inline]
+    pub(crate) fn try_lock(&self) -> TryLockResult<pl::MutexGuard<'_, T>> {
+        match self.0.try_lock() {
+            Some(guard) => Ok(guard),
+            None => Err(TryLockError::WouldBlock),
+        }
+    }
+
+    // Note: Additional methods `is_poisoned` and `into_inner`, can be
+    // provided here as needed.
+}
+
+/// Adapter for `parking_lot::Condvar` to the `std::sync::Condvar` interface.
+#[derive(Debug)]
+pub(crate) struct Condvar(pl::Condvar);
+
+impl Condvar {
+    #[inline]
+    pub(crate) fn new() -> Condvar {
+        Condvar(pl::Condvar::new())
+    }
+
+    #[inline]
+    pub(crate) fn notify_one(&self) {
+        self.0.notify_one();
+    }
+
+    #[inline]
+    pub(crate) fn notify_all(&self) {
+        self.0.notify_all();
+    }
+
+    #[inline]
+    pub(crate) fn wait<'a, T>(
+        &self,
+        mut guard: pl::MutexGuard<'a, T>,
+    ) -> LockResult<pl::MutexGuard<'a, T>> {
+        self.0.wait(&mut guard);
+        Ok(guard)
+    }
+
+    #[inline]
+    pub(crate) fn wait_timeout<'a, T>(
+        &self,
+        mut guard: pl::MutexGuard<'a, T>,
+        timeout: Duration,
+    ) -> LockResult<(pl::MutexGuard<'a, T>, pl::WaitTimeoutResult)> {
+        let wtr = self.0.wait_for(&mut guard, timeout);
+        Ok((guard, wtr))
+    }
+
+    // Note: Additional methods `wait_timeout_ms`, `wait_timeout_until`,
+    // `wait_until` can be provided here as needed.
+}
diff --git a/third_party/rust/tokio/src/loom/std/unsafe_cell.rs b/third_party/rust/tokio/src/loom/std/unsafe_cell.rs
new file mode 100644
index 0000000000..f2b03d8dc2
--- /dev/null
+++ b/third_party/rust/tokio/src/loom/std/unsafe_cell.rs
@@ -0,0 +1,16 @@
+#[derive(Debug)]
+pub(crate) struct UnsafeCell<T>(std::cell::UnsafeCell<T>);
+
+impl<T> UnsafeCell<T> {
+    pub(crate) fn new(data: T) -> UnsafeCell<T> {
+        UnsafeCell(std::cell::UnsafeCell::new(data))
+    }
+
+    pub(crate) fn with<R>(&self, f: impl FnOnce(*const T) -> R) -> R {
+        f(self.0.get())
+    }
+
+    pub(crate) fn with_mut<R>(&self, f: impl FnOnce(*mut T) -> R) -> R {
+        f(self.0.get())
+    }
+}
diff --git a/third_party/rust/tokio/src/macros/cfg.rs b/third_party/rust/tokio/src/macros/cfg.rs
new file mode 100644
index 0000000000..18beb1bdbb
--- /dev/null
+++ b/third_party/rust/tokio/src/macros/cfg.rs
@@ -0,0 +1,322 @@
+#![allow(unused_macros)]
+
+macro_rules! cfg_resource_drivers {
+    ($($item:item)*) => {
+        $(
+            #[cfg(any(feature = "io-driver", feature = "time"))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_blocking {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "blocking")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "blocking")))]
+            $item
+        )*
+    }
+}
+
+/// Enables blocking API internals
+macro_rules! cfg_blocking_impl {
+    ($($item:item)*) => {
+        $(
+            #[cfg(any(
+                    feature = "blocking",
+                    feature = "fs",
+                    feature = "dns",
+                    feature = "io-std",
+                    feature = "rt-threaded",
+                    ))]
+            $item
+        )*
+    }
+}
+
+/// Enables blocking API internals
+macro_rules! cfg_not_blocking_impl {
+    ($($item:item)*) => {
+        $(
+            #[cfg(not(any(
+                        feature = "blocking",
+                        feature = "fs",
+                        feature = "dns",
+                        feature = "io-std",
+                        feature = "rt-threaded",
+                        )))]
+            $item
+        )*
+    }
+}
+
+/// Enables internal `AtomicWaker` impl
+macro_rules! cfg_atomic_waker_impl {
+    ($($item:item)*) => {
+        $(
+            #[cfg(any(
+                feature = "io-driver",
+                feature = "time",
+                all(feature = "rt-core", feature = "rt-util")
+            ))]
+            #[cfg(not(loom))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_dns {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "dns")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "dns")))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_fs {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "fs")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "fs")))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_io_blocking {
+    ($($item:item)*) => {
+        $( #[cfg(any(feature = "io-std", feature = "fs"))] $item )*
+    }
+}
+
+macro_rules! cfg_io_driver {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "io-driver")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "io-driver")))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_not_io_driver {
+    ($($item:item)*) => {
+        $(
+            #[cfg(not(feature = "io-driver"))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_io_std {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "io-std")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "io-std")))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_io_util {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "io-util")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "io-util")))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_not_io_util {
+    ($($item:item)*) => {
+        $( #[cfg(not(feature = "io-util"))] $item )*
+    }
+}
+
+macro_rules! cfg_loom {
+    ($($item:item)*) => {
+        $( #[cfg(loom)] $item )*
+    }
+}
+
+macro_rules! cfg_not_loom {
+    ($($item:item)*) => {
+        $( #[cfg(not(loom))] $item )*
+    }
+}
+
+macro_rules! cfg_macros {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "macros")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "macros")))]
+            #[doc(inline)]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_process {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "process")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "process")))]
+            #[cfg(not(loom))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_signal {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "signal")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "signal")))]
+            #[cfg(not(loom))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_stream {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "stream")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "stream")))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_sync {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "sync")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "sync")))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_not_sync {
+    ($($item:item)*) => {
+        $( #[cfg(not(feature = "sync"))] $item )*
+    }
+}
+
+macro_rules! cfg_rt_core {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "rt-core")]
+            $item
+        )*
+    }
+}
+
+macro_rules! doc_rt_core {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "rt-core")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "rt-core")))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_not_rt_core {
+    ($($item:item)*) => {
+        $( #[cfg(not(feature = "rt-core"))] $item )*
+    }
+}
+
+macro_rules! cfg_rt_threaded {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "rt-threaded")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "rt-threaded")))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_rt_util {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "rt-util")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "rt-util")))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_not_rt_threaded {
+    ($($item:item)*) => {
+        $( #[cfg(not(feature = "rt-threaded"))] $item )*
+    }
+}
+
+macro_rules! cfg_tcp {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "tcp")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "tcp")))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_test_util {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "test-util")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "test-util")))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_not_test_util {
+    ($($item:item)*) => {
+        $( #[cfg(not(feature = "test-util"))] $item )*
+    }
+}
+
+macro_rules! cfg_time {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "time")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "time")))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_not_time {
+    ($($item:item)*) => {
+        $( #[cfg(not(feature = "time"))] $item )*
+    }
+}
+
+macro_rules! cfg_udp {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "udp")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "udp")))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_uds {
+    ($($item:item)*) => {
+        $(
+            #[cfg(all(unix, feature = "uds"))]
+            #[cfg_attr(docsrs, doc(cfg(feature = "uds")))]
+            $item
+        )*
+    }
+}
diff --git a/third_party/rust/tokio/src/macros/join.rs b/third_party/rust/tokio/src/macros/join.rs
new file mode 100644
index 0000000000..5f37af510d
--- /dev/null
+++ b/third_party/rust/tokio/src/macros/join.rs
@@ -0,0 +1,119 @@
+/// Wait on multiple concurrent branches, returning when **all** branches
+/// complete.
+///
+/// The `join!` macro must be used inside of async functions, closures, and
+/// blocks.
+///
+/// The `join!` macro takes a list of async expressions and evaluates them
+/// concurrently on the same task. Each async expression evaluates to a future
+/// and the futures from each expression are multiplexed on the current task.
+///
+/// When working with async expressions returning `Result`, `join!` will wait
+/// for **all** branches complete regardless if any complete with `Err`. Use
+/// [`try_join!`] to return early when `Err` is encountered.
+///
+/// [`try_join!`]: macro@try_join
+///
+/// # Notes
+///
+/// The supplied futures are stored inline and does not require allocating a
+/// `Vec`.
+///
+/// ### Runtime characteristics
+///
+/// By running all async expressions on the current task, the expressions are
+/// able to run **concurrently** but not in **parallel**. This means all
+/// expressions are run on the same thread and if one branch blocks the thread,
+/// all other expressions will be unable to continue. If parallelism is
+/// required, spawn each async expression using [`tokio::spawn`] and pass the
+/// join handle to `join!`.
+///
+/// [`tokio::spawn`]: crate::spawn
+///
+/// # Examples
+///
+/// Basic join with two branches
+///
+/// ```
+/// async fn do_stuff_async() {
+///     // async work
+/// }
+///
+/// async fn more_async_work() {
+///     // more here
+/// }
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let (first, second) = tokio::join!(
+///         do_stuff_async(),
+///         more_async_work());
+///
+///     // do something with the values
+/// }
+/// ```
+#[macro_export]
+#[cfg_attr(docsrs, doc(cfg(feature = "macros")))]
+macro_rules! join {
+    (@ {
+        // One `_` for each branch in the `join!` macro. This is not used once
+        // normalization is complete.
+        ( $($count:tt)* )
+
+        // Normalized join! branches
+        $( ( $($skip:tt)* ) $e:expr, )*
+
+    }) => {{
+        use $crate::macros::support::{maybe_done, poll_fn, Future, Pin};
+        use $crate::macros::support::Poll::{Ready, Pending};
+
+        // Safety: nothing must be moved out of `futures`. This is to satisfy
+        // the requirement of `Pin::new_unchecked` called below.
+        let mut futures = ( $( maybe_done($e), )* );
+
+        poll_fn(move |cx| {
+            let mut is_pending = false;
+
+            $(
+                // Extract the future for this branch from the tuple.
+                let ( $($skip,)* fut, .. ) = &mut futures;
+
+                // Safety: future is stored on the stack above
+                // and never moved.
+                let mut fut = unsafe { Pin::new_unchecked(fut) };
+
+                // Try polling
+                if fut.poll(cx).is_pending() {
+                    is_pending = true;
+                }
+            )*
+
+            if is_pending {
+                Pending
+            } else {
+                Ready(($({
+                    // Extract the future for this branch from the tuple.
+                    let ( $($skip,)* fut, .. ) = &mut futures;
+
+                    // Safety: future is stored on the stack above
+                    // and never moved.
+                    let mut fut = unsafe { Pin::new_unchecked(fut) };
+
+                    fut.take_output().expect("expected completed future")
+                },)*))
+            }
+        }).await
+    }};
+
+    // ===== Normalize =====
+
+    (@ { ( $($s:tt)* ) $($t:tt)* } $e:expr, $($r:tt)* ) => {
+        $crate::join!(@{ ($($s)* _) $($t)* ($($s)*) $e, } $($r)*)
+    };
+
+    // ===== Entry point =====
+
+    ( $($e:expr),* $(,)?) => {
+        $crate::join!(@{ () } $($e,)*)
+    };
+}
diff --git a/third_party/rust/tokio/src/macros/loom.rs b/third_party/rust/tokio/src/macros/loom.rs
new file mode 100644
index 0000000000..d57d9fb0f7
--- /dev/null
+++ b/third_party/rust/tokio/src/macros/loom.rs
@@ -0,0 +1,12 @@
+macro_rules! if_loom {
+    ($($t:tt)*) => {{
+        #[cfg(loom)]
+        const LOOM: bool = true;
+        #[cfg(not(loom))]
+        const LOOM: bool = false;
+
+        if LOOM {
+            $($t)*
+        }
+    }}
+}
diff --git a/third_party/rust/tokio/src/macros/mod.rs b/third_party/rust/tokio/src/macros/mod.rs
new file mode 100644
index 0000000000..2643c36018
--- /dev/null
+++ b/third_party/rust/tokio/src/macros/mod.rs
@@ -0,0 +1,35 @@
+#![cfg_attr(not(feature = "full"), allow(unused_macros))]
+
+#[macro_use]
+mod cfg;
+
+#[macro_use]
+mod loom;
+
+#[macro_use]
+mod pin;
+
+#[macro_use]
+mod ready;
+
+#[macro_use]
+mod thread_local;
+
+#[macro_use]
+#[cfg(feature = "rt-core")]
+pub(crate) mod scoped_tls;
+
+cfg_macros! {
+    #[macro_use]
+    mod select;
+
+    #[macro_use]
+    mod join;
+
+    #[macro_use]
+    mod try_join;
+}
+
+// Includes re-exports needed to implement macros
+#[doc(hidden)]
+pub mod support;
diff --git a/third_party/rust/tokio/src/macros/pin.rs b/third_party/rust/tokio/src/macros/pin.rs
new file mode 100644
index 0000000000..33d8499e10
--- /dev/null
+++ b/third_party/rust/tokio/src/macros/pin.rs
@@ -0,0 +1,144 @@
+/// Pins a value on the stack.
+///
+/// Calls to `async fn` return anonymous [`Future`] values that are `!Unpin`.
+/// These values must be pinned before they can be polled. Calling `.await` will
+/// handle this, but consumes the future. If it is required to call `.await` on
+/// a `&mut _` reference, the caller is responsible for pinning the future.
+///
+/// Pinning may be done by allocating with [`Box::pin`] or by using the stack
+/// with the `pin!` macro.
+///
+/// The following will **fail to compile**:
+///
+/// ```compile_fail
+/// async fn my_async_fn() {
+///     // async logic here
+/// }
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let mut future = my_async_fn();
+///     (&mut future).await;
+/// }
+/// ```
+///
+/// To make this work requires pinning:
+///
+/// ```
+/// use tokio::pin;
+///
+/// async fn my_async_fn() {
+///     // async logic here
+/// }
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let future = my_async_fn();
+///     pin!(future);
+///
+///     (&mut future).await;
+/// }
+/// ```
+///
+/// Pinning is useful when using `select!` and stream operators that require `T:
+/// Stream + Unpin`.
+///
+/// [`Future`]: https://doc.rust-lang.org/std/future/trait.Future.html
+/// [`Box::pin`]: #
+///
+/// # Usage
+///
+/// The `pin!` macro takes **identifiers** as arguments. It does **not** work
+/// with expressions.
+///
+/// The following does not compile as an expression is passed to `pin!`.
+///
+/// ```compile_fail
+/// async fn my_async_fn() {
+///     // async logic here
+/// }
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let mut future = pin!(my_async_fn());
+///     (&mut future).await;
+/// }
+/// ```
+///
+/// # Examples
+///
+/// Using with select:
+///
+/// ```
+/// use tokio::{pin, select};
+/// use tokio::stream::{self, StreamExt};
+///
+/// async fn my_async_fn() {
+///     // async logic here
+/// }
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let mut stream = stream::iter(vec![1, 2, 3, 4]);
+///
+///     let future = my_async_fn();
+///     pin!(future);
+///
+///     loop {
+///         select! {
+///             _ = &mut future => {
+///                 // Stop looping `future` will be polled after completion
+///                 break;
+///             }
+///             Some(val) = stream.next() => {
+///                 println!("got value = {}", val);
+///             }
+///         }
+///     }
+/// }
+/// ```
+///
+/// Because assigning to a variable followed by pinning is common, there is also
+/// a variant of the macro that supports doing both in one go.
+///
+/// ```
+/// use tokio::{pin, select};
+///
+/// async fn my_async_fn() {
+///     // async logic here
+/// }
+///
+/// #[tokio::main]
+/// async fn main() {
+///     pin! {
+///         let future1 = my_async_fn();
+///         let future2 = my_async_fn();
+///     }
+///
+///     select! {
+///         _ = &mut future1 => {}
+///         _ = &mut future2 => {}
+///     }
+/// }
+/// ```
+#[macro_export]
+macro_rules! pin {
+    ($($x:ident),*) => { $(
+        // Move the value to ensure that it is owned
+        let mut $x = $x;
+        // Shadow the original binding so that it can't be directly accessed
+        // ever again.
+        #[allow(unused_mut)]
+        let mut $x = unsafe {
+            $crate::macros::support::Pin::new_unchecked(&mut $x)
+        };
+    )* };
+    ($(
+            let $x:ident = $init:expr;
+    )*) => {
+        $(
+            let $x = $init;
+            $crate::pin!($x);
+        )*
+    };
+}
diff --git a/third_party/rust/tokio/src/macros/ready.rs b/third_party/rust/tokio/src/macros/ready.rs
new file mode 100644
index 0000000000..1f48623b80
--- /dev/null
+++ b/third_party/rust/tokio/src/macros/ready.rs
@@ -0,0 +1,8 @@
+macro_rules! ready {
+    ($e:expr $(,)?) => {
+        match $e {
+            std::task::Poll::Ready(t) => t,
+            std::task::Poll::Pending => return std::task::Poll::Pending,
+        }
+    };
+}
diff --git a/third_party/rust/tokio/src/macros/scoped_tls.rs b/third_party/rust/tokio/src/macros/scoped_tls.rs
new file mode 100644
index 0000000000..666f382b28
--- /dev/null
+++ b/third_party/rust/tokio/src/macros/scoped_tls.rs
@@ -0,0 +1,80 @@
+use crate::loom::thread::LocalKey;
+
+use std::cell::Cell;
+use std::marker;
+
+/// Set a reference as a thread-local
+#[macro_export]
+macro_rules! scoped_thread_local {
+    ($(#[$attrs:meta])* $vis:vis static $name:ident: $ty:ty) => (
+        $(#[$attrs])*
+        $vis static $name: $crate::macros::scoped_tls::ScopedKey<$ty>
+            = $crate::macros::scoped_tls::ScopedKey {
+                inner: {
+                    thread_local!(static FOO: ::std::cell::Cell<*const ()> = {
+                        std::cell::Cell::new(::std::ptr::null())
+                    });
+                    &FOO
+                },
+                _marker: ::std::marker::PhantomData,
+            };
+    )
+}
+
+/// Type representing a thread local storage key corresponding to a reference
+/// to the type parameter `T`.
+pub(crate) struct ScopedKey<T> {
+    #[doc(hidden)]
+    pub(crate) inner: &'static LocalKey<Cell<*const ()>>,
+    #[doc(hidden)]
+    pub(crate) _marker: marker::PhantomData<T>,
+}
+
+unsafe impl<T> Sync for ScopedKey<T> {}
+
+impl<T> ScopedKey<T> {
+    /// Inserts a value into this scoped thread local storage slot for a
+    /// duration of a closure.
+    pub(crate) fn set<F, R>(&'static self, t: &T, f: F) -> R
+    where
+        F: FnOnce() -> R,
+    {
+        struct Reset {
+            key: &'static LocalKey<Cell<*const ()>>,
+            val: *const (),
+        }
+
+        impl Drop for Reset {
+            fn drop(&mut self) {
+                self.key.with(|c| c.set(self.val));
+            }
+        }
+
+        let prev = self.inner.with(|c| {
+            let prev = c.get();
+            c.set(t as *const _ as *const ());
+            prev
+        });
+
+        let _reset = Reset {
+            key: self.inner,
+            val: prev,
+        };
+
+        f()
+    }
+
+    /// Gets a value out of this scoped variable.
+    pub(crate) fn with<F, R>(&'static self, f: F) -> R
+    where
+        F: FnOnce(Option<&T>) -> R,
+    {
+        let val = self.inner.with(|c| c.get());
+
+        if val.is_null() {
+            f(None)
+        } else {
+            unsafe { f(Some(&*(val as *const T))) }
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/macros/select.rs b/third_party/rust/tokio/src/macros/select.rs
new file mode 100644
index 0000000000..51b6fcd608
--- /dev/null
+++ b/third_party/rust/tokio/src/macros/select.rs
@@ -0,0 +1,876 @@
+/// Wait on multiple concurrent branches, returning when the **first** branch
+/// completes, cancelling the remaining branches.
+///
+/// The `select!` macro must be used inside of async functions, closures, and
+/// blocks.
+///
+/// The `select!` macro accepts one or more branches with the following pattern:
+///
+/// ```text
+/// <pattern> = <async expression> (, if <precondition>)? => <handler>,
+/// ```
+///
+/// Additionally, the `select!` macro may include a single, optional `else`
+/// branch, which evaluates if none of the other branches match their patterns:
+///
+/// ```text
+/// else <expression>
+/// ```
+///
+/// The macro aggregates all `<async expression>` expressions and runs them
+/// concurrently on the **current** task. Once the **first** expression
+/// completes with a value that matches its `<pattern>`, the `select!` macro
+/// returns the result of evaluating the completed branch's `<handler>`
+/// expression.
+///
+/// Additionally, each branch may include an optional `if` precondition. This
+/// precondition is evaluated **before** the `<async expression>`. If the
+/// precondition returns `false`, the branch is entirely disabled. This
+/// capability is useful when using `select!` within a loop.
+///
+/// The complete lifecycle of a `select!` expression is as follows:
+///
+/// 1. Evaluate all provded `<precondition>` expressions. If the precondition
+///    returns `false`, disable the branch for the remainder of the current call
+///    to `select!`. Re-entering `select!` due to a loop clears the "disabled"
+///    state.
+/// 2. Aggregate the `<async expression>`s from each branch, including the
+///    disabled ones. If the branch is disabled, `<async expression>` is still
+///    evaluated, but the resulting future is not polled.
+/// 3. Concurrently await on the results for all remaining `<async expression>`s.
+/// 4. Once an `<async expression>` returns a value, attempt to apply the value
+///    to the provided `<pattern>`, if the pattern matches, evaluate `<handler>`
+///    and return. If the pattern **does not** match, disable the current branch
+///    and for the remainder of the current call to `select!`. Continue from step 3.
+/// 5. If **all** branches are disabled, evaluate the `else` expression. If none
+///    is provided, panic.
+///
+/// # Notes
+///
+/// ### Runtime characteristics
+///
+/// By running all async expressions on the current task, the expressions are
+/// able to run **concurrently** but not in **parallel**. This means all
+/// expressions are run on the same thread and if one branch blocks the thread,
+/// all other expressions will be unable to continue. If parallelism is
+/// required, spawn each async expression using [`tokio::spawn`] and pass the
+/// join handle to `select!`.
+///
+/// [`tokio::spawn`]: crate::spawn
+///
+/// ### Avoid racy `if` preconditions
+///
+/// Given that `if` preconditions are used to disable `select!` branches, some
+/// caution must be used to avoid missing values.
+///
+/// For example, here is **incorrect** usage of `delay` with `if`. The objective
+/// is to repeatedly run an asynchronous task for up to 50 milliseconds.
+/// However, there is a potential for the `delay` completion to be missed.
+///
+/// ```no_run
+/// use tokio::time::{self, Duration};
+///
+/// async fn some_async_work() {
+///     // do work
+/// }
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let mut delay = time::delay_for(Duration::from_millis(50));
+///
+///     while !delay.is_elapsed() {
+///         tokio::select! {
+///             _ = &mut delay, if !delay.is_elapsed() => {
+///                 println!("operation timed out");
+///             }
+///             _ = some_async_work() => {
+///                 println!("operation completed");
+///             }
+///         }
+///     }
+/// }
+/// ```
+///
+/// In the above example, `delay.is_elapsed()` may return `true` even if
+/// `delay.poll()` never returned `Ready`. This opens up a potential race
+/// condition where `delay` expires between the `while !delay.is_elapsed()`
+/// check and the call to `select!` resulting in the `some_async_work()` call to
+/// run uninterrupted despite the delay having elapsed.
+///
+/// One way to write the above example without the race would be:
+///
+/// ```
+/// use tokio::time::{self, Duration};
+///
+/// async fn some_async_work() {
+/// # time::delay_for(Duration::from_millis(10)).await;
+///     // do work
+/// }
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let mut delay = time::delay_for(Duration::from_millis(50));
+///
+///     loop {
+///         tokio::select! {
+///             _ = &mut delay => {
+///                 println!("operation timed out");
+///                 break;
+///             }
+///             _ = some_async_work() => {
+///                 println!("operation completed");
+///             }
+///         }
+///     }
+/// }
+/// ```
+///
+/// ### Fairness
+///
+/// `select!` randomly picks a branch to check first. This provides some level
+/// of fairness when calling `select!` in a loop with branches that are always
+/// ready.
+///
+/// # Panics
+///
+/// `select!` panics if all branches are disabled **and** there is no provided
+/// `else` branch. A branch is disabled when the provided `if` precondition
+/// returns `false` **or** when the pattern does not match the result of `<async
+/// expression>.
+///
+/// # Examples
+///
+/// Basic select with two branches.
+///
+/// ```
+/// async fn do_stuff_async() {
+///     // async work
+/// }
+///
+/// async fn more_async_work() {
+///     // more here
+/// }
+///
+/// #[tokio::main]
+/// async fn main() {
+///     tokio::select! {
+///         _ = do_stuff_async() => {
+///             println!("do_stuff_async() completed first")
+///         }
+///         _ = more_async_work() => {
+///             println!("more_async_work() completed first")
+///         }
+///     };
+/// }
+/// ```
+///
+/// Basic stream selecting.
+///
+/// ```
+/// use tokio::stream::{self, StreamExt};
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let mut stream1 = stream::iter(vec![1, 2, 3]);
+///     let mut stream2 = stream::iter(vec![4, 5, 6]);
+///
+///     let next = tokio::select! {
+///         v = stream1.next() => v.unwrap(),
+///         v = stream2.next() => v.unwrap(),
+///     };
+///
+///     assert!(next == 1 || next == 4);
+/// }
+/// ```
+///
+/// Collect the contents of two streams. In this example, we rely on pattern
+/// matching and the fact that `stream::iter` is "fused", i.e. once the stream
+/// is complete, all calls to `next()` return `None`.
+///
+/// ```
+/// use tokio::stream::{self, StreamExt};
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let mut stream1 = stream::iter(vec![1, 2, 3]);
+///     let mut stream2 = stream::iter(vec![4, 5, 6]);
+///
+///     let mut values = vec![];
+///
+///     loop {
+///         tokio::select! {
+///             Some(v) = stream1.next() => values.push(v),
+///             Some(v) = stream2.next() => values.push(v),
+///             else => break,
+///         }
+///     }
+///
+///     values.sort();
+///     assert_eq!(&[1, 2, 3, 4, 5, 6], &values[..]);
+/// }
+/// ```
+///
+/// Using the same future in multiple `select!` expressions can be done by passing
+/// a reference to the future. Doing so requires the future to be [`Unpin`]. A
+/// future can be made [`Unpin`] by either using [`Box::pin`] or stack pinning.
+///
+/// [`Unpin`]: std::marker::Unpin
+/// [`Box::pin`]: std::boxed::Box::pin
+///
+/// Here, a stream is consumed for at most 1 second.
+///
+/// ```
+/// use tokio::stream::{self, StreamExt};
+/// use tokio::time::{self, Duration};
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let mut stream = stream::iter(vec![1, 2, 3]);
+///     let mut delay = time::delay_for(Duration::from_secs(1));
+///
+///     loop {
+///         tokio::select! {
+///             maybe_v = stream.next() => {
+///                 if let Some(v) = maybe_v {
+///                     println!("got = {}", v);
+///                 } else {
+///                     break;
+///                 }
+///             }
+///             _ = &mut delay => {
+///                 println!("timeout");
+///                 break;
+///             }
+///         }
+///     }
+/// }
+/// ```
+///
+/// Joining two values using `select!`.
+///
+/// ```
+/// use tokio::sync::oneshot;
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let (tx1, mut rx1) = oneshot::channel();
+///     let (tx2, mut rx2) = oneshot::channel();
+///
+///     tokio::spawn(async move {
+///         tx1.send("first").unwrap();
+///     });
+///
+///     tokio::spawn(async move {
+///         tx2.send("second").unwrap();
+///     });
+///
+///     let mut a = None;
+///     let mut b = None;
+///
+///     while a.is_none() || b.is_none() {
+///         tokio::select! {
+///             v1 = (&mut rx1), if a.is_none() => a = Some(v1.unwrap()),
+///             v2 = (&mut rx2), if b.is_none() => b = Some(v2.unwrap()),
+///         }
+///     }
+///
+///     let res = (a.unwrap(), b.unwrap());
+///
+///     assert_eq!(res.0, "first");
+///     assert_eq!(res.1, "second");
+/// }
+/// ```
+#[macro_export]
+#[cfg_attr(docsrs, doc(cfg(feature = "macros")))]
+macro_rules! select {
+    // Uses a declarative macro to do **most** of the work. While it is possible
+    // to implement fully with a declarative macro, a procedural macro is used
+    // to enable improved error messages.
+    //
+    // The macro is structured as a tt-muncher. All branches are processed and
+    // normalized. Once the input is normalized, it is passed to the top-most
+    // rule. When entering the macro, `@{ }` is inserted at the front. This is
+    // used to collect the normalized input.
+    //
+    // The macro only recurses once per branch. This allows using `select!`
+    // without requiring the user to increase the recursion limit.
+
+    // All input is normalized, now transform.
+    (@ {
+        // One `_` for each branch in the `select!` macro. Passing this to
+        // `count!` converts $skip to an integer.
+        ( $($count:tt)* )
+
+        // Normalized select branches. `( $skip )` is a set of `_` characters.
+        // There is one `_` for each select branch **before** this one. Given
+        // that all input futures are stored in a tuple, $skip is useful for
+        // generating a pattern to reference the future for the current branch.
+        // $skip is also used as an argument to `count!`, returning the index of
+        // the current select branch.
+        $( ( $($skip:tt)* ) $bind:pat = $fut:expr, if $c:expr => $handle:expr, )+
+
+        // Fallback expression used when all select branches have been disabled.
+        ; $else:expr
+
+    }) => {{
+        // Enter a context where stable "function-like" proc macros can be used.
+        //
+        // This module is defined within a scope and should not leak out of this
+        // macro.
+        mod util {
+            // Generate an enum with one variant per select branch
+            $crate::select_priv_declare_output_enum!( ( $($count)* ) );
+        }
+
+        // `tokio::macros::support` is a public, but doc(hidden) module
+        // including a re-export of all types needed by this macro.
+        use $crate::macros::support::Future;
+        use $crate::macros::support::Pin;
+        use $crate::macros::support::Poll::{Ready, Pending};
+
+        const BRANCHES: u32 = $crate::count!( $($count)* );
+
+        let mut disabled: util::Mask = Default::default();
+
+        // First, invoke all the pre-conditions. For any that return true,
+        // set the appropriate bit in `disabled`.
+        $(
+            if !$c {
+                let mask = 1 << $crate::count!( $($skip)* );
+                disabled |= mask;
+            }
+        )*
+
+        // Create a scope to separate polling from handling the output. This
+        // adds borrow checker flexibility when using the macro.
+        let mut output = {
+            // Safety: Nothing must be moved out of `futures`. This is to
+            // satisfy the requirement of `Pin::new_unchecked` called below.
+            let mut futures = ( $( $fut , )+ );
+
+            $crate::macros::support::poll_fn(|cx| {
+                // Track if any branch returns pending. If no branch completes
+                // **or** returns pending, this implies that all branches are
+                // disabled.
+                let mut is_pending = false;
+
+                // Randomly generate a starting point. This makes `select!` a
+                // bit more fair and avoids always polling the first future.
+                let start = $crate::macros::support::thread_rng_n(BRANCHES);
+
+                for i in 0..BRANCHES {
+                    let branch = (start + i) % BRANCHES;
+
+                    match branch {
+                        $(
+                            $crate::count!( $($skip)* ) => {
+                                // First, if the future has previously been
+                                // disabled, do not poll it again. This is done
+                                // by checking the associated bit in the
+                                // `disabled` bit field.
+                                let mask = 1 << branch;
+
+                                if disabled & mask == mask {
+                                    // The future has been disabled.
+                                    continue;
+                                }
+
+                                // Extract the future for this branch from the
+                                // tuple
+                                let ( $($skip,)* fut, .. ) = &mut futures;
+
+                                // Safety: future is stored on the stack above
+                                // and never moved.
+                                let mut fut = unsafe { Pin::new_unchecked(fut) };
+
+                                // Try polling it
+                                let out = match fut.poll(cx) {
+                                    Ready(out) => out,
+                                    Pending => {
+                                        // Track that at least one future is
+                                        // still pending and continue polling.
+                                        is_pending = true;
+                                        continue;
+                                    }
+                                };
+
+                                // Disable the future from future polling.
+                                disabled |= mask;
+
+                                // The future returned a value, check if matches
+                                // the specified pattern.
+                                #[allow(unused_variables)]
+                                match &out {
+                                    $bind => {}
+                                    _ => continue,
+                                }
+
+                                // The select is complete, return the value
+                                return Ready($crate::select_variant!(util::Out, ($($skip)*))(out));
+                            }
+                        )*
+                        _ => unreachable!("reaching this means there probably is an off by one bug"),
+                    }
+                }
+
+                if is_pending {
+                    Pending
+                } else {
+                    // All branches have been disabled.
+                    Ready(util::Out::Disabled)
+                }
+            }).await
+        };
+
+        match output {
+            $(
+                $crate::select_variant!(util::Out, ($($skip)*) ($bind)) => $handle,
+            )*
+            util::Out::Disabled => $else,
+            _ => unreachable!("failed to match bind"),
+        }
+    }};
+
+    // ==== Normalize =====
+
+    // These rules match a single `select!` branch and normalize it for
+    // processing by the first rule.
+
+    (@ { $($t:tt)* } ) => {
+        // No `else` branch
+        $crate::select!(@{ $($t)*; unreachable!() })
+    };
+    (@ { $($t:tt)* } else => $else:expr $(,)?) => {
+        $crate::select!(@{ $($t)*; $else })
+    };
+    (@ { ( $($s:tt)* ) $($t:tt)* } $p:pat = $f:expr, if $c:expr => $h:block, $($r:tt)* ) => {
+        $crate::select!(@{ ($($s)* _) $($t)* ($($s)*) $p = $f, if $c => $h, } $($r)*)
+    };
+    (@ { ( $($s:tt)* ) $($t:tt)* } $p:pat = $f:expr => $h:block, $($r:tt)* ) => {
+        $crate::select!(@{ ($($s)* _) $($t)* ($($s)*) $p = $f, if true => $h, } $($r)*)
+    };
+    (@ { ( $($s:tt)* ) $($t:tt)* } $p:pat = $f:expr, if $c:expr => $h:block $($r:tt)* ) => {
+        $crate::select!(@{ ($($s)* _) $($t)* ($($s)*) $p = $f, if $c => $h, } $($r)*)
+    };
+    (@ { ( $($s:tt)* ) $($t:tt)* } $p:pat = $f:expr => $h:block $($r:tt)* ) => {
+        $crate::select!(@{ ($($s)* _) $($t)* ($($s)*) $p = $f, if true => $h, } $($r)*)
+    };
+    (@ { ( $($s:tt)* ) $($t:tt)* } $p:pat = $f:expr, if $c:expr => $h:expr ) => {
+        $crate::select!(@{ ($($s)* _) $($t)* ($($s)*) $p = $f, if $c => $h, })
+    };
+    (@ { ( $($s:tt)* ) $($t:tt)* } $p:pat = $f:expr => $h:expr ) => {
+        $crate::select!(@{ ($($s)* _) $($t)* ($($s)*) $p = $f, if true => $h, })
+    };
+    (@ { ( $($s:tt)* ) $($t:tt)* } $p:pat = $f:expr, if $c:expr => $h:expr, $($r:tt)* ) => {
+        $crate::select!(@{ ($($s)* _) $($t)* ($($s)*) $p = $f, if $c => $h, } $($r)*)
+    };
+    (@ { ( $($s:tt)* ) $($t:tt)* } $p:pat = $f:expr => $h:expr, $($r:tt)* ) => {
+        $crate::select!(@{ ($($s)* _) $($t)* ($($s)*) $p = $f, if true => $h, } $($r)*)
+    };
+
+    // ===== Entry point =====
+
+    ( $p:pat = $($t:tt)* ) => {
+        $crate::select!(@{ () } $p = $($t)*)
+    };
+    () => {
+        compile_error!("select! requires at least one branch.")
+    };
+}
+
+// And here... we manually list out matches for up to 64 branches... I'm not
+// happy about it either, but this is how we manage to use a declarative macro!
+
+#[macro_export]
+#[doc(hidden)]
+macro_rules! count {
+    () => {
+        0
+    };
+    (_) => {
+        1
+    };
+    (_ _) => {
+        2
+    };
+    (_ _ _) => {
+        3
+    };
+    (_ _ _ _) => {
+        4
+    };
+    (_ _ _ _ _) => {
+        5
+    };
+    (_ _ _ _ _ _) => {
+        6
+    };
+    (_ _ _ _ _ _ _) => {
+        7
+    };
+    (_ _ _ _ _ _ _ _) => {
+        8
+    };
+    (_ _ _ _ _ _ _ _ _) => {
+        9
+    };
+    (_ _ _ _ _ _ _ _ _ _) => {
+        10
+    };
+    (_ _ _ _ _ _ _ _ _ _ _) => {
+        11
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _) => {
+        12
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        13
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        14
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        15
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        16
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        17
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        18
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        19
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        20
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        21
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        22
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        23
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        24
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        25
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        26
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        27
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        28
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        29
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        30
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        31
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        32
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        33
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        34
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        35
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        36
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        37
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        38
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        39
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        40
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        41
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        42
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        43
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        44
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        45
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        46
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        47
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        48
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        49
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        50
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        51
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        52
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        53
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        54
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        55
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        56
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        57
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        58
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        59
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        60
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        61
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        62
+    };
+    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
+        63
+    };
+}
+
+#[macro_export]
+#[doc(hidden)]
+macro_rules! select_variant {
+    ($($p:ident)::*, () $($t:tt)*) => {
+        $($p)::*::_0 $($t)*
+    };
+    ($($p:ident)::*, (_) $($t:tt)*) => {
+        $($p)::*::_1 $($t)*
+    };
+    ($($p:ident)::*, (_ _) $($t:tt)*) => {
+        $($p)::*::_2 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _) $($t:tt)*) => {
+        $($p)::*::_3 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _) $($t:tt)*) => {
+        $($p)::*::_4 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_5 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_6 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_7 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_8 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_9 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_10 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_11 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_12 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_13 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_14 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_15 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_16 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_17 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_18 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_19 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_20 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_21 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_22 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_23 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_24 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_25 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_26 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_27 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_28 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_29 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_30 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_31 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_32 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_33 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_34 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_35 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_36 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_37 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_38 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_39 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_40 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_41 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_42 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_43 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_44 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_45 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_46 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_47 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_48 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_49 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_50 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_51 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_52 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_53 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_54 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_55 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_56 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_57 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_58 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_59 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_60 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_61 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_62 $($t)*
+    };
+    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
+        $($p)::*::_63 $($t)*
+    };
+}
diff --git a/third_party/rust/tokio/src/macros/support.rs b/third_party/rust/tokio/src/macros/support.rs
new file mode 100644
index 0000000000..fc1cdfcfa0
--- /dev/null
+++ b/third_party/rust/tokio/src/macros/support.rs
@@ -0,0 +1,8 @@
+cfg_macros! {
+    pub use crate::future::{maybe_done, poll_fn};
+    pub use crate::util::thread_rng_n;
+}
+
+pub use std::future::Future;
+pub use std::pin::Pin;
+pub use std::task::Poll;
diff --git a/third_party/rust/tokio/src/macros/thread_local.rs b/third_party/rust/tokio/src/macros/thread_local.rs
new file mode 100644
index 0000000000..d848947350
--- /dev/null
+++ b/third_party/rust/tokio/src/macros/thread_local.rs
@@ -0,0 +1,4 @@
+#[cfg(all(loom, test))]
+macro_rules! thread_local {
+    ($($tts:tt)+) => { loom::thread_local!{ $($tts)+ } }
+}
diff --git a/third_party/rust/tokio/src/macros/try_join.rs b/third_party/rust/tokio/src/macros/try_join.rs
new file mode 100644
index 0000000000..fa5850ef0e
--- /dev/null
+++ b/third_party/rust/tokio/src/macros/try_join.rs
@@ -0,0 +1,132 @@
+/// Wait on multiple concurrent branches, returning when **all** branches
+/// complete with `Ok(_)` or on the first `Err(_)`.
+///
+/// The `try_join!` macro must be used inside of async functions, closures, and
+/// blocks.
+///
+/// Similar to [`join!`], the `try_join!` macro takes a list of async
+/// expressions and evaluates them concurrently on the same task. Each async
+/// expression evaluates to a future and the futures from each expression are
+/// multiplexed on the current task. The `try_join!` macro returns when **all**
+/// branches return with `Ok` or when the **first** branch returns with `Err`.
+///
+/// [`join!`]: macro@join
+///
+/// # Notes
+///
+/// The supplied futures are stored inline and does not require allocating a
+/// `Vec`.
+///
+/// ### Runtime characteristics
+///
+/// By running all async expressions on the current task, the expressions are
+/// able to run **concurrently** but not in **parallel**. This means all
+/// expressions are run on the same thread and if one branch blocks the thread,
+/// all other expressions will be unable to continue. If parallelism is
+/// required, spawn each async expression using [`tokio::spawn`] and pass the
+/// join handle to `try_join!`.
+///
+/// [`tokio::spawn`]: crate::spawn
+///
+/// # Examples
+///
+/// Basic try_join with two branches.
+///
+/// ```
+/// async fn do_stuff_async() -> Result<(), &'static str> {
+///     // async work
+/// # Ok(())
+/// }
+///
+/// async fn more_async_work() -> Result<(), &'static str> {
+///     // more here
+/// # Ok(())
+/// }
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let res = tokio::try_join!(
+///         do_stuff_async(),
+///         more_async_work());
+///
+///     match res {
+///          Ok((first, second)) => {
+///              // do something with the values
+///          }
+///          Err(err) => {
+///             println!("processing failed; error = {}", err);
+///          }
+///     }
+/// }
+/// ```
+#[macro_export]
+#[cfg_attr(docsrs, doc(cfg(feature = "macros")))]
+macro_rules! try_join {
+    (@ {
+        // One `_` for each branch in the `try_join!` macro. This is not used once
+        // normalization is complete.
+        ( $($count:tt)* )
+
+        // Normalized try_join! branches
+        $( ( $($skip:tt)* ) $e:expr, )*
+
+    }) => {{
+        use $crate::macros::support::{maybe_done, poll_fn, Future, Pin};
+        use $crate::macros::support::Poll::{Ready, Pending};
+
+        // Safety: nothing must be moved out of `futures`. This is to satisfy
+        // the requirement of `Pin::new_unchecked` called below.
+        let mut futures = ( $( maybe_done($e), )* );
+
+        poll_fn(move |cx| {
+            let mut is_pending = false;
+
+            $(
+                // Extract the future for this branch from the tuple.
+                let ( $($skip,)* fut, .. ) = &mut futures;
+
+                // Safety: future is stored on the stack above
+                // and never moved.
+                let mut fut = unsafe { Pin::new_unchecked(fut) };
+
+                // Try polling
+                if fut.as_mut().poll(cx).is_pending() {
+                    is_pending = true;
+                } else if fut.as_mut().output_mut().expect("expected completed future").is_err() {
+                    return Ready(Err(fut.take_output().expect("expected completed future").err().unwrap()))
+                }
+            )*
+
+            if is_pending {
+                Pending
+            } else {
+                Ready(Ok(($({
+                    // Extract the future for this branch from the tuple.
+                    let ( $($skip,)* fut, .. ) = &mut futures;
+
+                    // Safety: future is stored on the stack above
+                    // and never moved.
+                    let mut fut = unsafe { Pin::new_unchecked(fut) };
+
+                    fut
+                        .take_output()
+                        .expect("expected completed future")
+                        .ok()
+                        .expect("expected Ok(_)")
+                },)*)))
+            }
+        }).await
+    }};
+
+    // ===== Normalize =====
+
+    (@ { ( $($s:tt)* ) $($t:tt)* } $e:expr, $($r:tt)* ) => {
+        $crate::try_join!(@{ ($($s)* _) $($t)* ($($s)*) $e, } $($r)*)
+    };
+
+    // ===== Entry point =====
+
+    ( $($e:expr),* $(,)?) => {
+        $crate::try_join!(@{ () } $($e,)*)
+    };
+}
diff --git a/third_party/rust/tokio/src/net/addr.rs b/third_party/rust/tokio/src/net/addr.rs
new file mode 100644
index 0000000000..343d4e21ff
--- /dev/null
+++ b/third_party/rust/tokio/src/net/addr.rs
@@ -0,0 +1,281 @@
+use crate::future;
+
+use std::io;
+use std::net::{IpAddr, Ipv4Addr, Ipv6Addr, SocketAddr, SocketAddrV4, SocketAddrV6};
+
+/// Converts or resolves without blocking to one or more `SocketAddr` values.
+///
+/// # DNS
+///
+/// Implementations of `ToSocketAddrs` for string types require a DNS lookup.
+/// These implementations are only provided when Tokio is used with the
+/// **`dns`** feature flag.
+///
+/// # Calling
+///
+/// Currently, this trait is only used as an argument to Tokio functions that
+/// need to reference a target socket address. To perform a `SocketAddr`
+/// conversion directly, use [`lookup_host()`](super::lookup_host()).
+///
+/// This trait is sealed and is intended to be opaque. The details of the trait
+/// will change. Stabilization is pending enhancements to the Rust langague.
+pub trait ToSocketAddrs: sealed::ToSocketAddrsPriv {}
+
+type ReadyFuture<T> = future::Ready<io::Result<T>>;
+
+// ===== impl &impl ToSocketAddrs =====
+
+impl<T: ToSocketAddrs + ?Sized> ToSocketAddrs for &T {}
+
+impl<T> sealed::ToSocketAddrsPriv for &T
+where
+    T: sealed::ToSocketAddrsPriv + ?Sized,
+{
+    type Iter = T::Iter;
+    type Future = T::Future;
+
+    fn to_socket_addrs(&self) -> Self::Future {
+        (**self).to_socket_addrs()
+    }
+}
+
+// ===== impl SocketAddr =====
+
+impl ToSocketAddrs for SocketAddr {}
+
+impl sealed::ToSocketAddrsPriv for SocketAddr {
+    type Iter = std::option::IntoIter<SocketAddr>;
+    type Future = ReadyFuture<Self::Iter>;
+
+    fn to_socket_addrs(&self) -> Self::Future {
+        let iter = Some(*self).into_iter();
+        future::ok(iter)
+    }
+}
+
+// ===== impl SocketAddrV4 =====
+
+impl ToSocketAddrs for SocketAddrV4 {}
+
+impl sealed::ToSocketAddrsPriv for SocketAddrV4 {
+    type Iter = std::option::IntoIter<SocketAddr>;
+    type Future = ReadyFuture<Self::Iter>;
+
+    fn to_socket_addrs(&self) -> Self::Future {
+        SocketAddr::V4(*self).to_socket_addrs()
+    }
+}
+
+// ===== impl SocketAddrV6 =====
+
+impl ToSocketAddrs for SocketAddrV6 {}
+
+impl sealed::ToSocketAddrsPriv for SocketAddrV6 {
+    type Iter = std::option::IntoIter<SocketAddr>;
+    type Future = ReadyFuture<Self::Iter>;
+
+    fn to_socket_addrs(&self) -> Self::Future {
+        SocketAddr::V6(*self).to_socket_addrs()
+    }
+}
+
+// ===== impl (IpAddr, u16) =====
+
+impl ToSocketAddrs for (IpAddr, u16) {}
+
+impl sealed::ToSocketAddrsPriv for (IpAddr, u16) {
+    type Iter = std::option::IntoIter<SocketAddr>;
+    type Future = ReadyFuture<Self::Iter>;
+
+    fn to_socket_addrs(&self) -> Self::Future {
+        let iter = Some(SocketAddr::from(*self)).into_iter();
+        future::ok(iter)
+    }
+}
+
+// ===== impl (Ipv4Addr, u16) =====
+
+impl ToSocketAddrs for (Ipv4Addr, u16) {}
+
+impl sealed::ToSocketAddrsPriv for (Ipv4Addr, u16) {
+    type Iter = std::option::IntoIter<SocketAddr>;
+    type Future = ReadyFuture<Self::Iter>;
+
+    fn to_socket_addrs(&self) -> Self::Future {
+        let (ip, port) = *self;
+        SocketAddrV4::new(ip, port).to_socket_addrs()
+    }
+}
+
+// ===== impl (Ipv6Addr, u16) =====
+
+impl ToSocketAddrs for (Ipv6Addr, u16) {}
+
+impl sealed::ToSocketAddrsPriv for (Ipv6Addr, u16) {
+    type Iter = std::option::IntoIter<SocketAddr>;
+    type Future = ReadyFuture<Self::Iter>;
+
+    fn to_socket_addrs(&self) -> Self::Future {
+        let (ip, port) = *self;
+        SocketAddrV6::new(ip, port, 0, 0).to_socket_addrs()
+    }
+}
+
+cfg_dns! {
+    // ===== impl str =====
+
+    impl ToSocketAddrs for str {}
+
+    impl sealed::ToSocketAddrsPriv for str {
+        type Iter = sealed::OneOrMore;
+        type Future = sealed::MaybeReady;
+
+        fn to_socket_addrs(&self) -> Self::Future {
+            use crate::runtime::spawn_blocking;
+            use sealed::MaybeReady;
+
+            // First check if the input parses as a socket address
+            let res: Result<SocketAddr, _> = self.parse();
+
+            if let Ok(addr) = res {
+                return MaybeReady::Ready(Some(addr));
+            }
+
+            // Run DNS lookup on the blocking pool
+            let s = self.to_owned();
+
+            MaybeReady::Blocking(spawn_blocking(move || {
+                std::net::ToSocketAddrs::to_socket_addrs(&s)
+            }))
+        }
+    }
+
+    // ===== impl (&str, u16) =====
+
+    impl ToSocketAddrs for (&str, u16) {}
+
+    impl sealed::ToSocketAddrsPriv for (&str, u16) {
+        type Iter = sealed::OneOrMore;
+        type Future = sealed::MaybeReady;
+
+        fn to_socket_addrs(&self) -> Self::Future {
+            use crate::runtime::spawn_blocking;
+            use sealed::MaybeReady;
+
+            let (host, port) = *self;
+
+            // try to parse the host as a regular IP address first
+            if let Ok(addr) = host.parse::<Ipv4Addr>() {
+                let addr = SocketAddrV4::new(addr, port);
+                let addr = SocketAddr::V4(addr);
+
+                return MaybeReady::Ready(Some(addr));
+            }
+
+            if let Ok(addr) = host.parse::<Ipv6Addr>() {
+                let addr = SocketAddrV6::new(addr, port, 0, 0);
+                let addr = SocketAddr::V6(addr);
+
+                return MaybeReady::Ready(Some(addr));
+            }
+
+            let host = host.to_owned();
+
+            MaybeReady::Blocking(spawn_blocking(move || {
+                std::net::ToSocketAddrs::to_socket_addrs(&(&host[..], port))
+            }))
+        }
+    }
+
+    // ===== impl String =====
+
+    impl ToSocketAddrs for String {}
+
+    impl sealed::ToSocketAddrsPriv for String {
+        type Iter = <str as sealed::ToSocketAddrsPriv>::Iter;
+        type Future = <str as sealed::ToSocketAddrsPriv>::Future;
+
+        fn to_socket_addrs(&self) -> Self::Future {
+            (&self[..]).to_socket_addrs()
+        }
+    }
+}
+
+pub(crate) mod sealed {
+    //! The contents of this trait are intended to remain private and __not__
+    //! part of the `ToSocketAddrs` public API. The details will change over
+    //! time.
+
+    use std::future::Future;
+    use std::io;
+    use std::net::SocketAddr;
+
+    cfg_dns! {
+        use crate::task::JoinHandle;
+
+        use std::option;
+        use std::pin::Pin;
+        use std::task::{Context, Poll};
+        use std::vec;
+    }
+
+    #[doc(hidden)]
+    pub trait ToSocketAddrsPriv {
+        type Iter: Iterator<Item = SocketAddr> + Send + 'static;
+        type Future: Future<Output = io::Result<Self::Iter>> + Send + 'static;
+
+        fn to_socket_addrs(&self) -> Self::Future;
+    }
+
+    cfg_dns! {
+        #[doc(hidden)]
+        #[derive(Debug)]
+        pub enum MaybeReady {
+            Ready(Option<SocketAddr>),
+            Blocking(JoinHandle<io::Result<vec::IntoIter<SocketAddr>>>),
+        }
+
+        #[doc(hidden)]
+        #[derive(Debug)]
+        pub enum OneOrMore {
+            One(option::IntoIter<SocketAddr>),
+            More(vec::IntoIter<SocketAddr>),
+        }
+
+        impl Future for MaybeReady {
+            type Output = io::Result<OneOrMore>;
+
+            fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+                match *self {
+                    MaybeReady::Ready(ref mut i) => {
+                        let iter = OneOrMore::One(i.take().into_iter());
+                        Poll::Ready(Ok(iter))
+                    }
+                    MaybeReady::Blocking(ref mut rx) => {
+                        let res = ready!(Pin::new(rx).poll(cx))?.map(OneOrMore::More);
+
+                        Poll::Ready(res)
+                    }
+                }
+            }
+        }
+
+        impl Iterator for OneOrMore {
+            type Item = SocketAddr;
+
+            fn next(&mut self) -> Option<Self::Item> {
+                match self {
+                    OneOrMore::One(i) => i.next(),
+                    OneOrMore::More(i) => i.next(),
+                }
+            }
+
+            fn size_hint(&self) -> (usize, Option<usize>) {
+                match self {
+                    OneOrMore::One(i) => i.size_hint(),
+                    OneOrMore::More(i) => i.size_hint(),
+                }
+            }
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/net/lookup_host.rs b/third_party/rust/tokio/src/net/lookup_host.rs
new file mode 100644
index 0000000000..3098b463e3
--- /dev/null
+++ b/third_party/rust/tokio/src/net/lookup_host.rs
@@ -0,0 +1,38 @@
+cfg_dns! {
+    use crate::net::addr::ToSocketAddrs;
+
+    use std::io;
+    use std::net::SocketAddr;
+
+    /// Performs a DNS resolution.
+    ///
+    /// The returned iterator may not actually yield any values depending on the
+    /// outcome of any resolution performed.
+    ///
+    /// This API is not intended to cover all DNS use cases. Anything beyond the
+    /// basic use case should be done with a specialized library.
+    ///
+    /// # Examples
+    ///
+    /// To resolve a DNS entry:
+    ///
+    /// ```no_run
+    /// use tokio::net;
+    /// use std::io;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     for addr in net::lookup_host("localhost:3000").await? {
+    ///         println!("socket address is {}", addr);
+    ///     }
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    pub async fn lookup_host<T>(host: T) -> io::Result<impl Iterator<Item = SocketAddr>>
+    where
+        T: ToSocketAddrs
+    {
+        host.to_socket_addrs().await
+    }
+}
diff --git a/third_party/rust/tokio/src/net/mod.rs b/third_party/rust/tokio/src/net/mod.rs
new file mode 100644
index 0000000000..eb24ac0ba5
--- /dev/null
+++ b/third_party/rust/tokio/src/net/mod.rs
@@ -0,0 +1,49 @@
+#![cfg(not(loom))]
+
+//! TCP/UDP/Unix bindings for `tokio`.
+//!
+//! This module contains the TCP/UDP/Unix networking types, similar to the standard
+//! library, which can be used to implement networking protocols.
+//!
+//! # Organization
+//!
+//! * [`TcpListener`] and [`TcpStream`] provide functionality for communication over TCP
+//! * [`UdpSocket`] provides functionality for communication over UDP
+//! * [`UnixListener`] and [`UnixStream`] provide functionality for communication over a
+//! Unix Domain Stream Socket **(available on Unix only)**
+//! * [`UnixDatagram`] provides functionality for communication
+//! over Unix Domain Datagram Socket **(available on Unix only)**
+
+//!
+//! [`TcpListener`]: TcpListener
+//! [`TcpStream`]: TcpStream
+//! [`UdpSocket`]: UdpSocket
+//! [`UnixListener`]: UnixListener
+//! [`UnixStream`]: UnixStream
+//! [`UnixDatagram`]: UnixDatagram
+
+mod addr;
+pub use addr::ToSocketAddrs;
+
+cfg_dns! {
+    mod lookup_host;
+    pub use lookup_host::lookup_host;
+}
+
+cfg_tcp! {
+    pub mod tcp;
+    pub use tcp::listener::TcpListener;
+    pub use tcp::stream::TcpStream;
+}
+
+cfg_udp! {
+    pub mod udp;
+    pub use udp::socket::UdpSocket;
+}
+
+cfg_uds! {
+    pub mod unix;
+    pub use unix::datagram::UnixDatagram;
+    pub use unix::listener::UnixListener;
+    pub use unix::stream::UnixStream;
+}
diff --git a/third_party/rust/tokio/src/net/tcp/incoming.rs b/third_party/rust/tokio/src/net/tcp/incoming.rs
new file mode 100644
index 0000000000..062be1e9cf
--- /dev/null
+++ b/third_party/rust/tokio/src/net/tcp/incoming.rs
@@ -0,0 +1,42 @@
+use crate::net::tcp::{TcpListener, TcpStream};
+
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// Stream returned by the `TcpListener::incoming` function representing the
+/// stream of sockets received from a listener.
+#[must_use = "streams do nothing unless polled"]
+#[derive(Debug)]
+pub struct Incoming<'a> {
+    inner: &'a mut TcpListener,
+}
+
+impl Incoming<'_> {
+    pub(crate) fn new(listener: &mut TcpListener) -> Incoming<'_> {
+        Incoming { inner: listener }
+    }
+
+    /// Attempts to poll `TcpStream` by polling inner `TcpListener` to accept
+    /// connection.
+    ///
+    /// If `TcpListener` isn't ready yet, `Poll::Pending` is returned and
+    /// current task will be notified by a waker.
+    pub fn poll_accept(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<io::Result<TcpStream>> {
+        let (socket, _) = ready!(self.inner.poll_accept(cx))?;
+        Poll::Ready(Ok(socket))
+    }
+}
+
+#[cfg(feature = "stream")]
+impl crate::stream::Stream for Incoming<'_> {
+    type Item = io::Result<TcpStream>;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        let (socket, _) = ready!(self.inner.poll_accept(cx))?;
+        Poll::Ready(Some(Ok(socket)))
+    }
+}
diff --git a/third_party/rust/tokio/src/net/tcp/listener.rs b/third_party/rust/tokio/src/net/tcp/listener.rs
new file mode 100644
index 0000000000..cde22cb636
--- /dev/null
+++ b/third_party/rust/tokio/src/net/tcp/listener.rs
@@ -0,0 +1,441 @@
+use crate::future::poll_fn;
+use crate::io::PollEvented;
+use crate::net::tcp::{Incoming, TcpStream};
+use crate::net::ToSocketAddrs;
+
+use std::convert::TryFrom;
+use std::fmt;
+use std::io;
+use std::net::{self, SocketAddr};
+use std::task::{Context, Poll};
+
+cfg_tcp! {
+    /// A TCP socket server, listening for connections.
+    ///
+    /// You can accept a new connection by using the [`accept`](`TcpListener::accept`) method. Alternatively `TcpListener`
+    /// implements the [`Stream`](`crate::stream::Stream`) trait, which allows you to use the listener in places that want a
+    /// stream. The stream will never return `None` and will also not yield the peer's `SocketAddr` structure.  Iterating over
+    /// it is equivalent to calling accept in a loop.
+    ///
+    /// # Errors
+    ///
+    /// Note that accepting a connection can lead to various errors and not all
+    /// of them are necessarily fatal ‒ for example having too many open file
+    /// descriptors or the other side closing the connection while it waits in
+    /// an accept queue. These would terminate the stream if not handled in any
+    /// way.
+    ///
+    /// # Examples
+    ///
+    /// Using `accept`:
+    /// ```no_run
+    /// use tokio::net::TcpListener;
+    ///
+    /// use std::io;
+    ///
+    /// async fn process_socket<T>(socket: T) {
+    ///     # drop(socket);
+    ///     // do work with socket here
+    /// }
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let mut listener = TcpListener::bind("127.0.0.1:8080").await?;
+    ///
+    ///     loop {
+    ///         let (socket, _) = listener.accept().await?;
+    ///         process_socket(socket).await;
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// Using `impl Stream`:
+    /// ```no_run
+    /// use tokio::{net::TcpListener, stream::StreamExt};
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let mut listener = TcpListener::bind("127.0.0.1:8080").await.unwrap();
+    ///     while let Some(stream) = listener.next().await {
+    ///         match stream {
+    ///             Ok(stream) => {
+    ///                 println!("new client!");
+    ///             }
+    ///             Err(e) => { /* connection failed */ }
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    pub struct TcpListener {
+        io: PollEvented<mio::net::TcpListener>,
+    }
+}
+
+impl TcpListener {
+    /// Creates a new TcpListener which will be bound to the specified address.
+    ///
+    /// The returned listener is ready for accepting connections.
+    ///
+    /// Binding with a port number of 0 will request that the OS assigns a port
+    /// to this listener. The port allocated can be queried via the `local_addr`
+    /// method.
+    ///
+    /// The address type can be any implementor of `ToSocketAddrs` trait.
+    ///
+    /// If `addr` yields multiple addresses, bind will be attempted with each of
+    /// the addresses until one succeeds and returns the listener. If none of
+    /// the addresses succeed in creating a listener, the error returned from
+    /// the last attempt (the last address) is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpListener;
+    ///
+    /// use std::io;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let listener = TcpListener::bind("127.0.0.1:0").await?;
+    ///
+    ///     // use the listener
+    ///
+    ///     # let _ = listener;
+    ///     Ok(())
+    /// }
+    /// ```
+    pub async fn bind<A: ToSocketAddrs>(addr: A) -> io::Result<TcpListener> {
+        let addrs = addr.to_socket_addrs().await?;
+
+        let mut last_err = None;
+
+        for addr in addrs {
+            match TcpListener::bind_addr(addr) {
+                Ok(listener) => return Ok(listener),
+                Err(e) => last_err = Some(e),
+            }
+        }
+
+        Err(last_err.unwrap_or_else(|| {
+            io::Error::new(
+                io::ErrorKind::InvalidInput,
+                "could not resolve to any address",
+            )
+        }))
+    }
+
+    fn bind_addr(addr: SocketAddr) -> io::Result<TcpListener> {
+        let listener = mio::net::TcpListener::bind(&addr)?;
+        TcpListener::new(listener)
+    }
+
+    /// Accepts a new incoming connection from this listener.
+    ///
+    /// This function will yield once a new TCP connection is established. When
+    /// established, the corresponding [`TcpStream`] and the remote peer's
+    /// address will be returned.
+    ///
+    /// [`TcpStream`]: ../struct.TcpStream.html
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpListener;
+    ///
+    /// use std::io;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let mut listener = TcpListener::bind("127.0.0.1:8080").await?;
+    ///
+    ///     match listener.accept().await {
+    ///         Ok((_socket, addr)) => println!("new client: {:?}", addr),
+    ///         Err(e) => println!("couldn't get client: {:?}", e),
+    ///     }
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    pub async fn accept(&mut self) -> io::Result<(TcpStream, SocketAddr)> {
+        poll_fn(|cx| self.poll_accept(cx)).await
+    }
+
+    /// Attempts to poll `SocketAddr` and `TcpStream` bound to this address.
+    ///
+    /// In case if I/O resource isn't ready yet, `Poll::Pending` is returned and
+    /// current task will be notified by a waker.
+    pub fn poll_accept(
+        &mut self,
+        cx: &mut Context<'_>,
+    ) -> Poll<io::Result<(TcpStream, SocketAddr)>> {
+        let (io, addr) = ready!(self.poll_accept_std(cx))?;
+
+        let io = mio::net::TcpStream::from_stream(io)?;
+        let io = TcpStream::new(io)?;
+
+        Poll::Ready(Ok((io, addr)))
+    }
+
+    fn poll_accept_std(
+        &mut self,
+        cx: &mut Context<'_>,
+    ) -> Poll<io::Result<(net::TcpStream, SocketAddr)>> {
+        ready!(self.io.poll_read_ready(cx, mio::Ready::readable()))?;
+
+        match self.io.get_ref().accept_std() {
+            Ok(pair) => Poll::Ready(Ok(pair)),
+            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
+                self.io.clear_read_ready(cx, mio::Ready::readable())?;
+                Poll::Pending
+            }
+            Err(e) => Poll::Ready(Err(e)),
+        }
+    }
+
+    /// Creates a new TCP listener from the standard library's TCP listener.
+    ///
+    /// This method can be used when the `Handle::tcp_listen` method isn't
+    /// sufficient because perhaps some more configuration is needed in terms of
+    /// before the calls to `bind` and `listen`.
+    ///
+    /// This API is typically paired with the `net2` crate and the `TcpBuilder`
+    /// type to build up and customize a listener before it's shipped off to the
+    /// backing event loop. This allows configuration of options like
+    /// `SO_REUSEPORT`, binding to multiple addresses, etc.
+    ///
+    /// The `addr` argument here is one of the addresses that `listener` is
+    /// bound to and the listener will only be guaranteed to accept connections
+    /// of the same address type currently.
+    ///
+    /// The platform specific behavior of this function looks like:
+    ///
+    /// * On Unix, the socket is placed into nonblocking mode and connections
+    ///   can be accepted as normal
+    ///
+    /// * On Windows, the address is stored internally and all future accepts
+    ///   will only be for the same IP version as `addr` specified. That is, if
+    ///   `addr` is an IPv4 address then all sockets accepted will be IPv4 as
+    ///   well (same for IPv6).
+    ///
+    /// # Examples
+    ///
+    /// ```rust,no_run
+    /// use std::error::Error;
+    /// use tokio::net::TcpListener;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> Result<(), Box<dyn Error>> {
+    ///     let std_listener = std::net::TcpListener::bind("127.0.0.1:0")?;
+    ///     let listener = TcpListener::from_std(std_listener)?;
+    ///     Ok(())
+    /// }
+    /// ```
+    ///
+    /// # Panics
+    ///
+    /// This function panics if thread-local runtime is not set.
+    ///
+    /// The runtime is usually set implicitly when this function is called
+    /// from a future driven by a tokio runtime, otherwise runtime can be set
+    /// explicitly with [`Handle::enter`](crate::runtime::Handle::enter) function.
+    pub fn from_std(listener: net::TcpListener) -> io::Result<TcpListener> {
+        let io = mio::net::TcpListener::from_std(listener)?;
+        let io = PollEvented::new(io)?;
+        Ok(TcpListener { io })
+    }
+
+    fn new(listener: mio::net::TcpListener) -> io::Result<TcpListener> {
+        let io = PollEvented::new(listener)?;
+        Ok(TcpListener { io })
+    }
+
+    /// Returns the local address that this listener is bound to.
+    ///
+    /// This can be useful, for example, when binding to port 0 to figure out
+    /// which port was actually bound.
+    ///
+    /// # Examples
+    ///
+    /// ```rust,no_run
+    /// use tokio::net::TcpListener;
+    ///
+    /// use std::io;
+    /// use std::net::{Ipv4Addr, SocketAddr, SocketAddrV4};
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let listener = TcpListener::bind("127.0.0.1:8080").await?;
+    ///
+    ///     assert_eq!(listener.local_addr()?,
+    ///                SocketAddr::V4(SocketAddrV4::new(Ipv4Addr::new(127, 0, 0, 1), 8080)));
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn local_addr(&self) -> io::Result<SocketAddr> {
+        self.io.get_ref().local_addr()
+    }
+
+    /// Returns a stream over the connections being received on this listener.
+    ///
+    /// Note that `TcpListener` also directly implements `Stream`.
+    ///
+    /// The returned stream will never return `None` and will also not yield the
+    /// peer's `SocketAddr` structure. Iterating over it is equivalent to
+    /// calling accept in a loop.
+    ///
+    /// # Errors
+    ///
+    /// Note that accepting a connection can lead to various errors and not all
+    /// of them are necessarily fatal ‒ for example having too many open file
+    /// descriptors or the other side closing the connection while it waits in
+    /// an accept queue. These would terminate the stream if not handled in any
+    /// way.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::{net::TcpListener, stream::StreamExt};
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let mut listener = TcpListener::bind("127.0.0.1:8080").await.unwrap();
+    ///     let mut incoming = listener.incoming();
+    ///
+    ///     while let Some(stream) = incoming.next().await {
+    ///         match stream {
+    ///             Ok(stream) => {
+    ///                 println!("new client!");
+    ///             }
+    ///             Err(e) => { /* connection failed */ }
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    pub fn incoming(&mut self) -> Incoming<'_> {
+        Incoming::new(self)
+    }
+
+    /// Gets the value of the `IP_TTL` option for this socket.
+    ///
+    /// For more information about this option, see [`set_ttl`].
+    ///
+    /// [`set_ttl`]: #method.set_ttl
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpListener;
+    ///
+    /// use std::io;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///    let listener = TcpListener::bind("127.0.0.1:0").await?;
+    ///
+    ///    listener.set_ttl(100).expect("could not set TTL");
+    ///    assert_eq!(listener.ttl()?, 100);
+    ///
+    ///    Ok(())
+    /// }
+    /// ```
+    pub fn ttl(&self) -> io::Result<u32> {
+        self.io.get_ref().ttl()
+    }
+
+    /// Sets the value for the `IP_TTL` option on this socket.
+    ///
+    /// This value sets the time-to-live field that is used in every packet sent
+    /// from this socket.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpListener;
+    ///
+    /// use std::io;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let listener = TcpListener::bind("127.0.0.1:0").await?;
+    ///
+    ///     listener.set_ttl(100).expect("could not set TTL");
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn set_ttl(&self, ttl: u32) -> io::Result<()> {
+        self.io.get_ref().set_ttl(ttl)
+    }
+}
+
+#[cfg(feature = "stream")]
+impl crate::stream::Stream for TcpListener {
+    type Item = io::Result<TcpStream>;
+
+    fn poll_next(
+        mut self: std::pin::Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Option<Self::Item>> {
+        let (socket, _) = ready!(self.poll_accept(cx))?;
+        Poll::Ready(Some(Ok(socket)))
+    }
+}
+
+impl TryFrom<TcpListener> for mio::net::TcpListener {
+    type Error = io::Error;
+
+    /// Consumes value, returning the mio I/O object.
+    ///
+    /// See [`PollEvented::into_inner`] for more details about
+    /// resource deregistration that happens during the call.
+    ///
+    /// [`PollEvented::into_inner`]: crate::io::PollEvented::into_inner
+    fn try_from(value: TcpListener) -> Result<Self, Self::Error> {
+        value.io.into_inner()
+    }
+}
+
+impl TryFrom<net::TcpListener> for TcpListener {
+    type Error = io::Error;
+
+    /// Consumes stream, returning the tokio I/O object.
+    ///
+    /// This is equivalent to
+    /// [`TcpListener::from_std(stream)`](TcpListener::from_std).
+    fn try_from(stream: net::TcpListener) -> Result<Self, Self::Error> {
+        Self::from_std(stream)
+    }
+}
+
+impl fmt::Debug for TcpListener {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.io.get_ref().fmt(f)
+    }
+}
+
+#[cfg(unix)]
+mod sys {
+    use super::TcpListener;
+    use std::os::unix::prelude::*;
+
+    impl AsRawFd for TcpListener {
+        fn as_raw_fd(&self) -> RawFd {
+            self.io.get_ref().as_raw_fd()
+        }
+    }
+}
+
+#[cfg(windows)]
+mod sys {
+    // TODO: let's land these upstream with mio and then we can add them here.
+    //
+    // use std::os::windows::prelude::*;
+    // use super::{TcpListener;
+    //
+    // impl AsRawHandle for TcpListener {
+    //     fn as_raw_handle(&self) -> RawHandle {
+    //         self.listener.io().as_raw_handle()
+    //     }
+    // }
+}
diff --git a/third_party/rust/tokio/src/net/tcp/mod.rs b/third_party/rust/tokio/src/net/tcp/mod.rs
new file mode 100644
index 0000000000..d5354b38d2
--- /dev/null
+++ b/third_party/rust/tokio/src/net/tcp/mod.rs
@@ -0,0 +1,13 @@
+//! TCP utility types
+
+pub(crate) mod listener;
+pub(crate) use listener::TcpListener;
+
+mod incoming;
+pub use incoming::Incoming;
+
+mod split;
+pub use split::{ReadHalf, WriteHalf};
+
+pub(crate) mod stream;
+pub(crate) use stream::TcpStream;
diff --git a/third_party/rust/tokio/src/net/tcp/split.rs b/third_party/rust/tokio/src/net/tcp/split.rs
new file mode 100644
index 0000000000..cce50f6ab3
--- /dev/null
+++ b/third_party/rust/tokio/src/net/tcp/split.rs
@@ -0,0 +1,163 @@
+//! `TcpStream` split support.
+//!
+//! A `TcpStream` can be split into a `ReadHalf` and a
+//! `WriteHalf` with the `TcpStream::split` method. `ReadHalf`
+//! implements `AsyncRead` while `WriteHalf` implements `AsyncWrite`.
+//!
+//! Compared to the generic split of `AsyncRead + AsyncWrite`, this specialized
+//! split has no associated overhead and enforces all invariants at the type
+//! level.
+
+use crate::future::poll_fn;
+use crate::io::{AsyncRead, AsyncWrite};
+use crate::net::TcpStream;
+
+use bytes::Buf;
+use std::io;
+use std::mem::MaybeUninit;
+use std::net::Shutdown;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// Read half of a `TcpStream`.
+#[derive(Debug)]
+pub struct ReadHalf<'a>(&'a TcpStream);
+
+/// Write half of a `TcpStream`.
+///
+/// Note that in the `AsyncWrite` implemenation of `TcpStreamWriteHalf`,
+/// `poll_shutdown` actually shuts down the TCP stream in the write direction.
+#[derive(Debug)]
+pub struct WriteHalf<'a>(&'a TcpStream);
+
+pub(crate) fn split(stream: &mut TcpStream) -> (ReadHalf<'_>, WriteHalf<'_>) {
+    (ReadHalf(&*stream), WriteHalf(&*stream))
+}
+
+impl ReadHalf<'_> {
+    /// Attempt to receive data on the socket, without removing that data from
+    /// the queue, registering the current task for wakeup if data is not yet
+    /// available.
+    ///
+    /// See the [`TcpStream::poll_peek`] level documenation for more details.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::io;
+    /// use tokio::net::TcpStream;
+    ///
+    /// use futures::future::poll_fn;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let mut stream = TcpStream::connect("127.0.0.1:8000").await?;
+    ///     let (mut read_half, _) = stream.split();
+    ///     let mut buf = [0; 10];
+    ///
+    ///     poll_fn(|cx| {
+    ///         read_half.poll_peek(cx, &mut buf)
+    ///     }).await?;
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    ///
+    /// [`TcpStream::poll_peek`]: TcpStream::poll_peek
+    pub fn poll_peek(&mut self, cx: &mut Context<'_>, buf: &mut [u8]) -> Poll<io::Result<usize>> {
+        self.0.poll_peek2(cx, buf)
+    }
+
+    /// Receives data on the socket from the remote address to which it is
+    /// connected, without removing that data from the queue. On success,
+    /// returns the number of bytes peeked.
+    ///
+    /// See the [`TcpStream::peek`] level documenation for more details.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpStream;
+    /// use tokio::prelude::*;
+    /// use std::error::Error;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> Result<(), Box<dyn Error>> {
+    ///     // Connect to a peer
+    ///     let mut stream = TcpStream::connect("127.0.0.1:8080").await?;
+    ///     let (mut read_half, _) = stream.split();
+    ///
+    ///     let mut b1 = [0; 10];
+    ///     let mut b2 = [0; 10];
+    ///
+    ///     // Peek at the data
+    ///     let n = read_half.peek(&mut b1).await?;
+    ///
+    ///     // Read the data
+    ///     assert_eq!(n, read_half.read(&mut b2[..n]).await?);
+    ///     assert_eq!(&b1[..n], &b2[..n]);
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    ///
+    /// [`TcpStream::peek`]: TcpStream::peek
+    pub async fn peek(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        poll_fn(|cx| self.poll_peek(cx, buf)).await
+    }
+}
+
+impl AsyncRead for ReadHalf<'_> {
+    unsafe fn prepare_uninitialized_buffer(&self, _: &mut [MaybeUninit<u8>]) -> bool {
+        false
+    }
+
+    fn poll_read(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        self.0.poll_read_priv(cx, buf)
+    }
+}
+
+impl AsyncWrite for WriteHalf<'_> {
+    fn poll_write(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        self.0.poll_write_priv(cx, buf)
+    }
+
+    fn poll_write_buf<B: Buf>(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut B,
+    ) -> Poll<io::Result<usize>> {
+        self.0.poll_write_buf_priv(cx, buf)
+    }
+
+    #[inline]
+    fn poll_flush(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
+        // tcp flush is a no-op
+        Poll::Ready(Ok(()))
+    }
+
+    // `poll_shutdown` on a write half shutdowns the stream in the "write" direction.
+    fn poll_shutdown(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
+        self.0.shutdown(Shutdown::Write).into()
+    }
+}
+
+impl AsRef<TcpStream> for ReadHalf<'_> {
+    fn as_ref(&self) -> &TcpStream {
+        self.0
+    }
+}
+
+impl AsRef<TcpStream> for WriteHalf<'_> {
+    fn as_ref(&self) -> &TcpStream {
+        self.0
+    }
+}
diff --git a/third_party/rust/tokio/src/net/tcp/stream.rs b/third_party/rust/tokio/src/net/tcp/stream.rs
new file mode 100644
index 0000000000..732c0ca381
--- /dev/null
+++ b/third_party/rust/tokio/src/net/tcp/stream.rs
@@ -0,0 +1,869 @@
+use crate::future::poll_fn;
+use crate::io::{AsyncRead, AsyncWrite, PollEvented};
+use crate::net::tcp::split::{split, ReadHalf, WriteHalf};
+use crate::net::ToSocketAddrs;
+
+use bytes::Buf;
+use iovec::IoVec;
+use std::convert::TryFrom;
+use std::fmt;
+use std::io::{self, Read, Write};
+use std::mem::MaybeUninit;
+use std::net::{self, Shutdown, SocketAddr};
+use std::pin::Pin;
+use std::task::{Context, Poll};
+use std::time::Duration;
+
+cfg_tcp! {
+    /// A TCP stream between a local and a remote socket.
+    ///
+    /// A TCP stream can either be created by connecting to an endpoint, via the
+    /// [`connect`] method, or by [accepting] a connection from a [listener].
+    ///
+    /// [`connect`]: method@TcpStream::connect
+    /// [accepting]: method@super::TcpListener::accept
+    /// [listener]: struct@super::TcpListener
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpStream;
+    /// use tokio::prelude::*;
+    /// use std::error::Error;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> Result<(), Box<dyn Error>> {
+    ///     // Connect to a peer
+    ///     let mut stream = TcpStream::connect("127.0.0.1:8080").await?;
+    ///
+    ///     // Write some data.
+    ///     stream.write_all(b"hello world!").await?;
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    pub struct TcpStream {
+        io: PollEvented<mio::net::TcpStream>,
+    }
+}
+
+impl TcpStream {
+    /// Opens a TCP connection to a remote host.
+    ///
+    /// `addr` is an address of the remote host. Anything which implements
+    /// `ToSocketAddrs` trait can be supplied for the address.
+    ///
+    /// If `addr` yields multiple addresses, connect will be attempted with each
+    /// of the addresses until a connection is successful. If none of the
+    /// addresses result in a successful connection, the error returned from the
+    /// last connection attempt (the last address) is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpStream;
+    /// use tokio::prelude::*;
+    /// use std::error::Error;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> Result<(), Box<dyn Error>> {
+    ///     // Connect to a peer
+    ///     let mut stream = TcpStream::connect("127.0.0.1:8080").await?;
+    ///
+    ///     // Write some data.
+    ///     stream.write_all(b"hello world!").await?;
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    pub async fn connect<A: ToSocketAddrs>(addr: A) -> io::Result<TcpStream> {
+        let addrs = addr.to_socket_addrs().await?;
+
+        let mut last_err = None;
+
+        for addr in addrs {
+            match TcpStream::connect_addr(addr).await {
+                Ok(stream) => return Ok(stream),
+                Err(e) => last_err = Some(e),
+            }
+        }
+
+        Err(last_err.unwrap_or_else(|| {
+            io::Error::new(
+                io::ErrorKind::InvalidInput,
+                "could not resolve to any address",
+            )
+        }))
+    }
+
+    /// Establishes a connection to the specified `addr`.
+    async fn connect_addr(addr: SocketAddr) -> io::Result<TcpStream> {
+        let sys = mio::net::TcpStream::connect(&addr)?;
+        let stream = TcpStream::new(sys)?;
+
+        // Once we've connected, wait for the stream to be writable as
+        // that's when the actual connection has been initiated. Once we're
+        // writable we check for `take_socket_error` to see if the connect
+        // actually hit an error or not.
+        //
+        // If all that succeeded then we ship everything on up.
+        poll_fn(|cx| stream.io.poll_write_ready(cx)).await?;
+
+        if let Some(e) = stream.io.get_ref().take_error()? {
+            return Err(e);
+        }
+
+        Ok(stream)
+    }
+
+    pub(crate) fn new(connected: mio::net::TcpStream) -> io::Result<TcpStream> {
+        let io = PollEvented::new(connected)?;
+        Ok(TcpStream { io })
+    }
+
+    /// Creates new `TcpStream` from a `std::net::TcpStream`.
+    ///
+    /// This function will convert a TCP stream created by the standard library
+    /// to a TCP stream ready to be used with the provided event loop handle.
+    ///
+    /// # Examples
+    ///
+    /// ```rust,no_run
+    /// use std::error::Error;
+    /// use tokio::net::TcpStream;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> Result<(), Box<dyn Error>> {
+    ///     let std_stream = std::net::TcpStream::connect("127.0.0.1:34254")?;
+    ///     let stream = TcpStream::from_std(std_stream)?;
+    ///     Ok(())
+    /// }
+    /// ```
+    ///
+    /// # Panics
+    ///
+    /// This function panics if thread-local runtime is not set.
+    ///
+    /// The runtime is usually set implicitly when this function is called
+    /// from a future driven by a tokio runtime, otherwise runtime can be set
+    /// explicitly with [`Handle::enter`](crate::runtime::Handle::enter) function.
+    pub fn from_std(stream: net::TcpStream) -> io::Result<TcpStream> {
+        let io = mio::net::TcpStream::from_stream(stream)?;
+        let io = PollEvented::new(io)?;
+        Ok(TcpStream { io })
+    }
+
+    // Connects `TcpStream` asynchronously that may be built with a net2 `TcpBuilder`.
+    //
+    // This should be removed in favor of some in-crate TcpSocket builder API.
+    #[doc(hidden)]
+    pub async fn connect_std(stream: net::TcpStream, addr: &SocketAddr) -> io::Result<TcpStream> {
+        let io = mio::net::TcpStream::connect_stream(stream, addr)?;
+        let io = PollEvented::new(io)?;
+        let stream = TcpStream { io };
+
+        // Once we've connected, wait for the stream to be writable as
+        // that's when the actual connection has been initiated. Once we're
+        // writable we check for `take_socket_error` to see if the connect
+        // actually hit an error or not.
+        //
+        // If all that succeeded then we ship everything on up.
+        poll_fn(|cx| stream.io.poll_write_ready(cx)).await?;
+
+        if let Some(e) = stream.io.get_ref().take_error()? {
+            return Err(e);
+        }
+
+        Ok(stream)
+    }
+
+    /// Returns the local address that this stream is bound to.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpStream;
+    ///
+    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
+    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
+    ///
+    /// println!("{:?}", stream.local_addr()?);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn local_addr(&self) -> io::Result<SocketAddr> {
+        self.io.get_ref().local_addr()
+    }
+
+    /// Returns the remote address that this stream is connected to.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpStream;
+    ///
+    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
+    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
+    ///
+    /// println!("{:?}", stream.peer_addr()?);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn peer_addr(&self) -> io::Result<SocketAddr> {
+        self.io.get_ref().peer_addr()
+    }
+
+    /// Attempts to receive data on the socket, without removing that data from
+    /// the queue, registering the current task for wakeup if data is not yet
+    /// available.
+    ///
+    /// # Return value
+    ///
+    /// The function returns:
+    ///
+    /// * `Poll::Pending` if data is not yet available.
+    /// * `Poll::Ready(Ok(n))` if data is available. `n` is the number of bytes peeked.
+    /// * `Poll::Ready(Err(e))` if an error is encountered.
+    ///
+    /// # Errors
+    ///
+    /// This function may encounter any standard I/O error except `WouldBlock`.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::io;
+    /// use tokio::net::TcpStream;
+    ///
+    /// use futures::future::poll_fn;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let mut stream = TcpStream::connect("127.0.0.1:8000").await?;
+    ///     let mut buf = [0; 10];
+    ///
+    ///     poll_fn(|cx| {
+    ///         stream.poll_peek(cx, &mut buf)
+    ///     }).await?;
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn poll_peek(&mut self, cx: &mut Context<'_>, buf: &mut [u8]) -> Poll<io::Result<usize>> {
+        self.poll_peek2(cx, buf)
+    }
+
+    pub(super) fn poll_peek2(
+        &self,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        ready!(self.io.poll_read_ready(cx, mio::Ready::readable()))?;
+
+        match self.io.get_ref().peek(buf) {
+            Ok(ret) => Poll::Ready(Ok(ret)),
+            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
+                self.io.clear_read_ready(cx, mio::Ready::readable())?;
+                Poll::Pending
+            }
+            Err(e) => Poll::Ready(Err(e)),
+        }
+    }
+
+    /// Receives data on the socket from the remote address to which it is
+    /// connected, without removing that data from the queue. On success,
+    /// returns the number of bytes peeked.
+    ///
+    /// Successive calls return the same data. This is accomplished by passing
+    /// `MSG_PEEK` as a flag to the underlying recv system call.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpStream;
+    /// use tokio::prelude::*;
+    /// use std::error::Error;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> Result<(), Box<dyn Error>> {
+    ///     // Connect to a peer
+    ///     let mut stream = TcpStream::connect("127.0.0.1:8080").await?;
+    ///
+    ///     let mut b1 = [0; 10];
+    ///     let mut b2 = [0; 10];
+    ///
+    ///     // Peek at the data
+    ///     let n = stream.peek(&mut b1).await?;
+    ///
+    ///     // Read the data
+    ///     assert_eq!(n, stream.read(&mut b2[..n]).await?);
+    ///     assert_eq!(&b1[..n], &b2[..n]);
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    pub async fn peek(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        poll_fn(|cx| self.poll_peek(cx, buf)).await
+    }
+
+    /// Shuts down the read, write, or both halves of this connection.
+    ///
+    /// This function will cause all pending and future I/O on the specified
+    /// portions to return immediately with an appropriate value (see the
+    /// documentation of `Shutdown`).
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpStream;
+    /// use std::error::Error;
+    /// use std::net::Shutdown;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> Result<(), Box<dyn Error>> {
+    ///     // Connect to a peer
+    ///     let stream = TcpStream::connect("127.0.0.1:8080").await?;
+    ///
+    ///     // Shutdown the stream
+    ///     stream.shutdown(Shutdown::Write)?;
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn shutdown(&self, how: Shutdown) -> io::Result<()> {
+        self.io.get_ref().shutdown(how)
+    }
+
+    /// Gets the value of the `TCP_NODELAY` option on this socket.
+    ///
+    /// For more information about this option, see [`set_nodelay`].
+    ///
+    /// [`set_nodelay`]: TcpStream::set_nodelay
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpStream;
+    ///
+    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
+    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
+    ///
+    /// println!("{:?}", stream.nodelay()?);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn nodelay(&self) -> io::Result<bool> {
+        self.io.get_ref().nodelay()
+    }
+
+    /// Sets the value of the `TCP_NODELAY` option on this socket.
+    ///
+    /// If set, this option disables the Nagle algorithm. This means that
+    /// segments are always sent as soon as possible, even if there is only a
+    /// small amount of data. When not set, data is buffered until there is a
+    /// sufficient amount to send out, thereby avoiding the frequent sending of
+    /// small packets.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpStream;
+    ///
+    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
+    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
+    ///
+    /// stream.set_nodelay(true)?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn set_nodelay(&self, nodelay: bool) -> io::Result<()> {
+        self.io.get_ref().set_nodelay(nodelay)
+    }
+
+    /// Gets the value of the `SO_RCVBUF` option on this socket.
+    ///
+    /// For more information about this option, see [`set_recv_buffer_size`].
+    ///
+    /// [`set_recv_buffer_size`]: TcpStream::set_recv_buffer_size
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpStream;
+    ///
+    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
+    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
+    ///
+    /// println!("{:?}", stream.recv_buffer_size()?);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn recv_buffer_size(&self) -> io::Result<usize> {
+        self.io.get_ref().recv_buffer_size()
+    }
+
+    /// Sets the value of the `SO_RCVBUF` option on this socket.
+    ///
+    /// Changes the size of the operating system's receive buffer associated
+    /// with the socket.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpStream;
+    ///
+    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
+    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
+    ///
+    /// stream.set_recv_buffer_size(100)?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn set_recv_buffer_size(&self, size: usize) -> io::Result<()> {
+        self.io.get_ref().set_recv_buffer_size(size)
+    }
+
+    /// Gets the value of the `SO_SNDBUF` option on this socket.
+    ///
+    /// For more information about this option, see [`set_send_buffer_size`].
+    ///
+    /// [`set_send_buffer_size`]: TcpStream::set_send_buffer_size
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpStream;
+    ///
+    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
+    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
+    ///
+    /// println!("{:?}", stream.send_buffer_size()?);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn send_buffer_size(&self) -> io::Result<usize> {
+        self.io.get_ref().send_buffer_size()
+    }
+
+    /// Sets the value of the `SO_SNDBUF` option on this socket.
+    ///
+    /// Changes the size of the operating system's send buffer associated with
+    /// the socket.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpStream;
+    ///
+    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
+    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
+    ///
+    /// stream.set_send_buffer_size(100)?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn set_send_buffer_size(&self, size: usize) -> io::Result<()> {
+        self.io.get_ref().set_send_buffer_size(size)
+    }
+
+    /// Returns whether keepalive messages are enabled on this socket, and if so
+    /// the duration of time between them.
+    ///
+    /// For more information about this option, see [`set_keepalive`].
+    ///
+    /// [`set_keepalive`]: TcpStream::set_keepalive
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpStream;
+    ///
+    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
+    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
+    ///
+    /// println!("{:?}", stream.keepalive()?);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn keepalive(&self) -> io::Result<Option<Duration>> {
+        self.io.get_ref().keepalive()
+    }
+
+    /// Sets whether keepalive messages are enabled to be sent on this socket.
+    ///
+    /// On Unix, this option will set the `SO_KEEPALIVE` as well as the
+    /// `TCP_KEEPALIVE` or `TCP_KEEPIDLE` option (depending on your platform).
+    /// On Windows, this will set the `SIO_KEEPALIVE_VALS` option.
+    ///
+    /// If `None` is specified then keepalive messages are disabled, otherwise
+    /// the duration specified will be the time to remain idle before sending a
+    /// TCP keepalive probe.
+    ///
+    /// Some platforms specify this value in seconds, so sub-second
+    /// specifications may be omitted.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpStream;
+    ///
+    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
+    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
+    ///
+    /// stream.set_keepalive(None)?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn set_keepalive(&self, keepalive: Option<Duration>) -> io::Result<()> {
+        self.io.get_ref().set_keepalive(keepalive)
+    }
+
+    /// Gets the value of the `IP_TTL` option for this socket.
+    ///
+    /// For more information about this option, see [`set_ttl`].
+    ///
+    /// [`set_ttl`]: TcpStream::set_ttl
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpStream;
+    ///
+    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
+    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
+    ///
+    /// println!("{:?}", stream.ttl()?);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn ttl(&self) -> io::Result<u32> {
+        self.io.get_ref().ttl()
+    }
+
+    /// Sets the value for the `IP_TTL` option on this socket.
+    ///
+    /// This value sets the time-to-live field that is used in every packet sent
+    /// from this socket.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpStream;
+    ///
+    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
+    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
+    ///
+    /// stream.set_ttl(123)?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn set_ttl(&self, ttl: u32) -> io::Result<()> {
+        self.io.get_ref().set_ttl(ttl)
+    }
+
+    /// Reads the linger duration for this socket by getting the `SO_LINGER`
+    /// option.
+    ///
+    /// For more information about this option, see [`set_linger`].
+    ///
+    /// [`set_linger`]: TcpStream::set_linger
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpStream;
+    ///
+    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
+    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
+    ///
+    /// println!("{:?}", stream.linger()?);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn linger(&self) -> io::Result<Option<Duration>> {
+        self.io.get_ref().linger()
+    }
+
+    /// Sets the linger duration of this socket by setting the `SO_LINGER`
+    /// option.
+    ///
+    /// This option controls the action taken when a stream has unsent messages
+    /// and the stream is closed. If `SO_LINGER` is set, the system
+    /// shall block the process until it can transmit the data or until the
+    /// time expires.
+    ///
+    /// If `SO_LINGER` is not specified, and the stream is closed, the system
+    /// handles the call in a way that allows the process to continue as quickly
+    /// as possible.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::TcpStream;
+    ///
+    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
+    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
+    ///
+    /// stream.set_linger(None)?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn set_linger(&self, dur: Option<Duration>) -> io::Result<()> {
+        self.io.get_ref().set_linger(dur)
+    }
+
+    /// Splits a `TcpStream` into a read half and a write half, which can be used
+    /// to read and write the stream concurrently.
+    pub fn split(&mut self) -> (ReadHalf<'_>, WriteHalf<'_>) {
+        split(self)
+    }
+
+    // == Poll IO functions that takes `&self` ==
+    //
+    // They are not public because (taken from the doc of `PollEvented`):
+    //
+    // While `PollEvented` is `Sync` (if the underlying I/O type is `Sync`), the
+    // caller must ensure that there are at most two tasks that use a
+    // `PollEvented` instance concurrently. One for reading and one for writing.
+    // While violating this requirement is "safe" from a Rust memory model point
+    // of view, it will result in unexpected behavior in the form of lost
+    // notifications and tasks hanging.
+
+    pub(crate) fn poll_read_priv(
+        &self,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        ready!(self.io.poll_read_ready(cx, mio::Ready::readable()))?;
+
+        match self.io.get_ref().read(buf) {
+            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
+                self.io.clear_read_ready(cx, mio::Ready::readable())?;
+                Poll::Pending
+            }
+            x => Poll::Ready(x),
+        }
+    }
+
+    pub(super) fn poll_write_priv(
+        &self,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        ready!(self.io.poll_write_ready(cx))?;
+
+        match self.io.get_ref().write(buf) {
+            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
+                self.io.clear_write_ready(cx)?;
+                Poll::Pending
+            }
+            x => Poll::Ready(x),
+        }
+    }
+
+    pub(super) fn poll_write_buf_priv<B: Buf>(
+        &self,
+        cx: &mut Context<'_>,
+        buf: &mut B,
+    ) -> Poll<io::Result<usize>> {
+        use std::io::IoSlice;
+
+        ready!(self.io.poll_write_ready(cx))?;
+
+        // The `IoVec` (v0.1.x) type can't have a zero-length size, so create
+        // a dummy version from a 1-length slice which we'll overwrite with
+        // the `bytes_vectored` method.
+        static S: &[u8] = &[0];
+        const MAX_BUFS: usize = 64;
+
+        // IoSlice isn't Copy, so we must expand this manually ;_;
+        let mut slices: [IoSlice<'_>; MAX_BUFS] = [
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+            IoSlice::new(S),
+        ];
+        let cnt = buf.bytes_vectored(&mut slices);
+
+        let iovec = <&IoVec>::from(S);
+        let mut vecs = [iovec; MAX_BUFS];
+        for i in 0..cnt {
+            vecs[i] = (*slices[i]).into();
+        }
+
+        match self.io.get_ref().write_bufs(&vecs[..cnt]) {
+            Ok(n) => {
+                buf.advance(n);
+                Poll::Ready(Ok(n))
+            }
+            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
+                self.io.clear_write_ready(cx)?;
+                Poll::Pending
+            }
+            Err(e) => Poll::Ready(Err(e)),
+        }
+    }
+}
+
+impl TryFrom<TcpStream> for mio::net::TcpStream {
+    type Error = io::Error;
+
+    /// Consumes value, returning the mio I/O object.
+    ///
+    /// See [`PollEvented::into_inner`] for more details about
+    /// resource deregistration that happens during the call.
+    ///
+    /// [`PollEvented::into_inner`]: crate::io::PollEvented::into_inner
+    fn try_from(value: TcpStream) -> Result<Self, Self::Error> {
+        value.io.into_inner()
+    }
+}
+
+impl TryFrom<net::TcpStream> for TcpStream {
+    type Error = io::Error;
+
+    /// Consumes stream, returning the tokio I/O object.
+    ///
+    /// This is equivalent to
+    /// [`TcpStream::from_std(stream)`](TcpStream::from_std).
+    fn try_from(stream: net::TcpStream) -> Result<Self, Self::Error> {
+        Self::from_std(stream)
+    }
+}
+
+// ===== impl Read / Write =====
+
+impl AsyncRead for TcpStream {
+    unsafe fn prepare_uninitialized_buffer(&self, _: &mut [MaybeUninit<u8>]) -> bool {
+        false
+    }
+
+    fn poll_read(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        self.poll_read_priv(cx, buf)
+    }
+}
+
+impl AsyncWrite for TcpStream {
+    fn poll_write(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        self.poll_write_priv(cx, buf)
+    }
+
+    fn poll_write_buf<B: Buf>(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut B,
+    ) -> Poll<io::Result<usize>> {
+        self.poll_write_buf_priv(cx, buf)
+    }
+
+    #[inline]
+    fn poll_flush(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
+        // tcp flush is a no-op
+        Poll::Ready(Ok(()))
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
+        self.shutdown(std::net::Shutdown::Write)?;
+        Poll::Ready(Ok(()))
+    }
+}
+
+impl fmt::Debug for TcpStream {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.io.get_ref().fmt(f)
+    }
+}
+
+#[cfg(unix)]
+mod sys {
+    use super::TcpStream;
+    use std::os::unix::prelude::*;
+
+    impl AsRawFd for TcpStream {
+        fn as_raw_fd(&self) -> RawFd {
+            self.io.get_ref().as_raw_fd()
+        }
+    }
+}
+
+#[cfg(windows)]
+mod sys {
+    // TODO: let's land these upstream with mio and then we can add them here.
+    //
+    // use std::os::windows::prelude::*;
+    // use super::TcpStream;
+    //
+    // impl AsRawHandle for TcpStream {
+    //     fn as_raw_handle(&self) -> RawHandle {
+    //         self.io.get_ref().as_raw_handle()
+    //     }
+    // }
+}
diff --git a/third_party/rust/tokio/src/net/udp/mod.rs b/third_party/rust/tokio/src/net/udp/mod.rs
new file mode 100644
index 0000000000..d43121a1ca
--- /dev/null
+++ b/third_party/rust/tokio/src/net/udp/mod.rs
@@ -0,0 +1,7 @@
+//! UDP utility types.
+
+pub(crate) mod socket;
+pub(crate) use socket::UdpSocket;
+
+mod split;
+pub use split::{RecvHalf, ReuniteError, SendHalf};
diff --git a/third_party/rust/tokio/src/net/udp/socket.rs b/third_party/rust/tokio/src/net/udp/socket.rs
new file mode 100644
index 0000000000..faf1dca615
--- /dev/null
+++ b/third_party/rust/tokio/src/net/udp/socket.rs
@@ -0,0 +1,425 @@
+use crate::future::poll_fn;
+use crate::io::PollEvented;
+use crate::net::udp::split::{split, RecvHalf, SendHalf};
+use crate::net::ToSocketAddrs;
+
+use std::convert::TryFrom;
+use std::fmt;
+use std::io;
+use std::net::{self, Ipv4Addr, Ipv6Addr, SocketAddr};
+use std::task::{Context, Poll};
+
+cfg_udp! {
+    /// A UDP socket
+    pub struct UdpSocket {
+        io: PollEvented<mio::net::UdpSocket>,
+    }
+}
+
+impl UdpSocket {
+    /// This function will create a new UDP socket and attempt to bind it to
+    /// the `addr` provided.
+    pub async fn bind<A: ToSocketAddrs>(addr: A) -> io::Result<UdpSocket> {
+        let addrs = addr.to_socket_addrs().await?;
+        let mut last_err = None;
+
+        for addr in addrs {
+            match UdpSocket::bind_addr(addr) {
+                Ok(socket) => return Ok(socket),
+                Err(e) => last_err = Some(e),
+            }
+        }
+
+        Err(last_err.unwrap_or_else(|| {
+            io::Error::new(
+                io::ErrorKind::InvalidInput,
+                "could not resolve to any address",
+            )
+        }))
+    }
+
+    fn bind_addr(addr: SocketAddr) -> io::Result<UdpSocket> {
+        let sys = mio::net::UdpSocket::bind(&addr)?;
+        UdpSocket::new(sys)
+    }
+
+    fn new(socket: mio::net::UdpSocket) -> io::Result<UdpSocket> {
+        let io = PollEvented::new(socket)?;
+        Ok(UdpSocket { io })
+    }
+
+    /// Creates a new `UdpSocket` from the previously bound socket provided.
+    ///
+    /// The socket given will be registered with the event loop that `handle`
+    /// is associated with. This function requires that `socket` has previously
+    /// been bound to an address to work correctly.
+    ///
+    /// This can be used in conjunction with net2's `UdpBuilder` interface to
+    /// configure a socket before it's handed off, such as setting options like
+    /// `reuse_address` or binding to multiple addresses.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if thread-local runtime is not set.
+    ///
+    /// The runtime is usually set implicitly when this function is called
+    /// from a future driven by a tokio runtime, otherwise runtime can be set
+    /// explicitly with [`Handle::enter`](crate::runtime::Handle::enter) function.
+    pub fn from_std(socket: net::UdpSocket) -> io::Result<UdpSocket> {
+        let io = mio::net::UdpSocket::from_socket(socket)?;
+        let io = PollEvented::new(io)?;
+        Ok(UdpSocket { io })
+    }
+
+    /// Splits the `UdpSocket` into a receive half and a send half. The two parts
+    /// can be used to receive and send datagrams concurrently, even from two
+    /// different tasks.
+    pub fn split(self) -> (RecvHalf, SendHalf) {
+        split(self)
+    }
+
+    /// Returns the local address that this socket is bound to.
+    pub fn local_addr(&self) -> io::Result<SocketAddr> {
+        self.io.get_ref().local_addr()
+    }
+
+    /// Connects the UDP socket setting the default destination for send() and
+    /// limiting packets that are read via recv from the address specified in
+    /// `addr`.
+    pub async fn connect<A: ToSocketAddrs>(&self, addr: A) -> io::Result<()> {
+        let addrs = addr.to_socket_addrs().await?;
+        let mut last_err = None;
+
+        for addr in addrs {
+            match self.io.get_ref().connect(addr) {
+                Ok(_) => return Ok(()),
+                Err(e) => last_err = Some(e),
+            }
+        }
+
+        Err(last_err.unwrap_or_else(|| {
+            io::Error::new(
+                io::ErrorKind::InvalidInput,
+                "could not resolve to any address",
+            )
+        }))
+    }
+
+    /// Returns a future that sends data on the socket to the remote address to which it is connected.
+    /// On success, the future will resolve to the number of bytes written.
+    ///
+    /// The [`connect`] method will connect this socket to a remote address. The future
+    /// will resolve to an error if the socket is not connected.
+    ///
+    /// [`connect`]: #method.connect
+    pub async fn send(&mut self, buf: &[u8]) -> io::Result<usize> {
+        poll_fn(|cx| self.poll_send(cx, buf)).await
+    }
+
+    // Poll IO functions that takes `&self` are provided for the split API.
+    //
+    // They are not public because (taken from the doc of `PollEvented`):
+    //
+    // While `PollEvented` is `Sync` (if the underlying I/O type is `Sync`), the
+    // caller must ensure that there are at most two tasks that use a
+    // `PollEvented` instance concurrently. One for reading and one for writing.
+    // While violating this requirement is "safe" from a Rust memory model point
+    // of view, it will result in unexpected behavior in the form of lost
+    // notifications and tasks hanging.
+    #[doc(hidden)]
+    pub fn poll_send(&self, cx: &mut Context<'_>, buf: &[u8]) -> Poll<io::Result<usize>> {
+        ready!(self.io.poll_write_ready(cx))?;
+
+        match self.io.get_ref().send(buf) {
+            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
+                self.io.clear_write_ready(cx)?;
+                Poll::Pending
+            }
+            x => Poll::Ready(x),
+        }
+    }
+
+    /// Returns a future that receives a single datagram message on the socket from
+    /// the remote address to which it is connected. On success, the future will resolve
+    /// to the number of bytes read.
+    ///
+    /// The function must be called with valid byte array `buf` of sufficient size to
+    /// hold the message bytes. If a message is too long to fit in the supplied buffer,
+    /// excess bytes may be discarded.
+    ///
+    /// The [`connect`] method will connect this socket to a remote address. The future
+    /// will fail if the socket is not connected.
+    ///
+    /// [`connect`]: #method.connect
+    pub async fn recv(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        poll_fn(|cx| self.poll_recv(cx, buf)).await
+    }
+
+    #[doc(hidden)]
+    pub fn poll_recv(&self, cx: &mut Context<'_>, buf: &mut [u8]) -> Poll<io::Result<usize>> {
+        ready!(self.io.poll_read_ready(cx, mio::Ready::readable()))?;
+
+        match self.io.get_ref().recv(buf) {
+            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
+                self.io.clear_read_ready(cx, mio::Ready::readable())?;
+                Poll::Pending
+            }
+            x => Poll::Ready(x),
+        }
+    }
+
+    /// Returns a future that sends data on the socket to the given address.
+    /// On success, the future will resolve to the number of bytes written.
+    ///
+    /// The future will resolve to an error if the IP version of the socket does
+    /// not match that of `target`.
+    pub async fn send_to<A: ToSocketAddrs>(&mut self, buf: &[u8], target: A) -> io::Result<usize> {
+        let mut addrs = target.to_socket_addrs().await?;
+
+        match addrs.next() {
+            Some(target) => poll_fn(|cx| self.poll_send_to(cx, buf, &target)).await,
+            None => Err(io::Error::new(
+                io::ErrorKind::InvalidInput,
+                "no addresses to send data to",
+            )),
+        }
+    }
+
+    // TODO: Public or not?
+    #[doc(hidden)]
+    pub fn poll_send_to(
+        &self,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+        target: &SocketAddr,
+    ) -> Poll<io::Result<usize>> {
+        ready!(self.io.poll_write_ready(cx))?;
+
+        match self.io.get_ref().send_to(buf, target) {
+            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
+                self.io.clear_write_ready(cx)?;
+                Poll::Pending
+            }
+            x => Poll::Ready(x),
+        }
+    }
+
+    /// Returns a future that receives a single datagram on the socket. On success,
+    /// the future resolves to the number of bytes read and the origin.
+    ///
+    /// The function must be called with valid byte array `buf` of sufficient size
+    /// to hold the message bytes. If a message is too long to fit in the supplied
+    /// buffer, excess bytes may be discarded.
+    pub async fn recv_from(&mut self, buf: &mut [u8]) -> io::Result<(usize, SocketAddr)> {
+        poll_fn(|cx| self.poll_recv_from(cx, buf)).await
+    }
+
+    #[doc(hidden)]
+    pub fn poll_recv_from(
+        &self,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<Result<(usize, SocketAddr), io::Error>> {
+        ready!(self.io.poll_read_ready(cx, mio::Ready::readable()))?;
+
+        match self.io.get_ref().recv_from(buf) {
+            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
+                self.io.clear_read_ready(cx, mio::Ready::readable())?;
+                Poll::Pending
+            }
+            x => Poll::Ready(x),
+        }
+    }
+
+    /// Gets the value of the `SO_BROADCAST` option for this socket.
+    ///
+    /// For more information about this option, see [`set_broadcast`].
+    ///
+    /// [`set_broadcast`]: #method.set_broadcast
+    pub fn broadcast(&self) -> io::Result<bool> {
+        self.io.get_ref().broadcast()
+    }
+
+    /// Sets the value of the `SO_BROADCAST` option for this socket.
+    ///
+    /// When enabled, this socket is allowed to send packets to a broadcast
+    /// address.
+    pub fn set_broadcast(&self, on: bool) -> io::Result<()> {
+        self.io.get_ref().set_broadcast(on)
+    }
+
+    /// Gets the value of the `IP_MULTICAST_LOOP` option for this socket.
+    ///
+    /// For more information about this option, see [`set_multicast_loop_v4`].
+    ///
+    /// [`set_multicast_loop_v4`]: #method.set_multicast_loop_v4
+    pub fn multicast_loop_v4(&self) -> io::Result<bool> {
+        self.io.get_ref().multicast_loop_v4()
+    }
+
+    /// Sets the value of the `IP_MULTICAST_LOOP` option for this socket.
+    ///
+    /// If enabled, multicast packets will be looped back to the local socket.
+    ///
+    /// # Note
+    ///
+    /// This may not have any affect on IPv6 sockets.
+    pub fn set_multicast_loop_v4(&self, on: bool) -> io::Result<()> {
+        self.io.get_ref().set_multicast_loop_v4(on)
+    }
+
+    /// Gets the value of the `IP_MULTICAST_TTL` option for this socket.
+    ///
+    /// For more information about this option, see [`set_multicast_ttl_v4`].
+    ///
+    /// [`set_multicast_ttl_v4`]: #method.set_multicast_ttl_v4
+    pub fn multicast_ttl_v4(&self) -> io::Result<u32> {
+        self.io.get_ref().multicast_ttl_v4()
+    }
+
+    /// Sets the value of the `IP_MULTICAST_TTL` option for this socket.
+    ///
+    /// Indicates the time-to-live value of outgoing multicast packets for
+    /// this socket. The default value is 1 which means that multicast packets
+    /// don't leave the local network unless explicitly requested.
+    ///
+    /// # Note
+    ///
+    /// This may not have any affect on IPv6 sockets.
+    pub fn set_multicast_ttl_v4(&self, ttl: u32) -> io::Result<()> {
+        self.io.get_ref().set_multicast_ttl_v4(ttl)
+    }
+
+    /// Gets the value of the `IPV6_MULTICAST_LOOP` option for this socket.
+    ///
+    /// For more information about this option, see [`set_multicast_loop_v6`].
+    ///
+    /// [`set_multicast_loop_v6`]: #method.set_multicast_loop_v6
+    pub fn multicast_loop_v6(&self) -> io::Result<bool> {
+        self.io.get_ref().multicast_loop_v6()
+    }
+
+    /// Sets the value of the `IPV6_MULTICAST_LOOP` option for this socket.
+    ///
+    /// Controls whether this socket sees the multicast packets it sends itself.
+    ///
+    /// # Note
+    ///
+    /// This may not have any affect on IPv4 sockets.
+    pub fn set_multicast_loop_v6(&self, on: bool) -> io::Result<()> {
+        self.io.get_ref().set_multicast_loop_v6(on)
+    }
+
+    /// Gets the value of the `IP_TTL` option for this socket.
+    ///
+    /// For more information about this option, see [`set_ttl`].
+    ///
+    /// [`set_ttl`]: #method.set_ttl
+    pub fn ttl(&self) -> io::Result<u32> {
+        self.io.get_ref().ttl()
+    }
+
+    /// Sets the value for the `IP_TTL` option on this socket.
+    ///
+    /// This value sets the time-to-live field that is used in every packet sent
+    /// from this socket.
+    pub fn set_ttl(&self, ttl: u32) -> io::Result<()> {
+        self.io.get_ref().set_ttl(ttl)
+    }
+
+    /// Executes an operation of the `IP_ADD_MEMBERSHIP` type.
+    ///
+    /// This function specifies a new multicast group for this socket to join.
+    /// The address must be a valid multicast address, and `interface` is the
+    /// address of the local interface with which the system should join the
+    /// multicast group. If it's equal to `INADDR_ANY` then an appropriate
+    /// interface is chosen by the system.
+    pub fn join_multicast_v4(&self, multiaddr: Ipv4Addr, interface: Ipv4Addr) -> io::Result<()> {
+        self.io.get_ref().join_multicast_v4(&multiaddr, &interface)
+    }
+
+    /// Executes an operation of the `IPV6_ADD_MEMBERSHIP` type.
+    ///
+    /// This function specifies a new multicast group for this socket to join.
+    /// The address must be a valid multicast address, and `interface` is the
+    /// index of the interface to join/leave (or 0 to indicate any interface).
+    pub fn join_multicast_v6(&self, multiaddr: &Ipv6Addr, interface: u32) -> io::Result<()> {
+        self.io.get_ref().join_multicast_v6(multiaddr, interface)
+    }
+
+    /// Executes an operation of the `IP_DROP_MEMBERSHIP` type.
+    ///
+    /// For more information about this option, see [`join_multicast_v4`].
+    ///
+    /// [`join_multicast_v4`]: #method.join_multicast_v4
+    pub fn leave_multicast_v4(&self, multiaddr: Ipv4Addr, interface: Ipv4Addr) -> io::Result<()> {
+        self.io.get_ref().leave_multicast_v4(&multiaddr, &interface)
+    }
+
+    /// Executes an operation of the `IPV6_DROP_MEMBERSHIP` type.
+    ///
+    /// For more information about this option, see [`join_multicast_v6`].
+    ///
+    /// [`join_multicast_v6`]: #method.join_multicast_v6
+    pub fn leave_multicast_v6(&self, multiaddr: &Ipv6Addr, interface: u32) -> io::Result<()> {
+        self.io.get_ref().leave_multicast_v6(multiaddr, interface)
+    }
+}
+
+impl TryFrom<UdpSocket> for mio::net::UdpSocket {
+    type Error = io::Error;
+
+    /// Consumes value, returning the mio I/O object.
+    ///
+    /// See [`PollEvented::into_inner`] for more details about
+    /// resource deregistration that happens during the call.
+    ///
+    /// [`PollEvented::into_inner`]: crate::io::PollEvented::into_inner
+    fn try_from(value: UdpSocket) -> Result<Self, Self::Error> {
+        value.io.into_inner()
+    }
+}
+
+impl TryFrom<net::UdpSocket> for UdpSocket {
+    type Error = io::Error;
+
+    /// Consumes stream, returning the tokio I/O object.
+    ///
+    /// This is equivalent to
+    /// [`UdpSocket::from_std(stream)`](UdpSocket::from_std).
+    fn try_from(stream: net::UdpSocket) -> Result<Self, Self::Error> {
+        Self::from_std(stream)
+    }
+}
+
+impl fmt::Debug for UdpSocket {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.io.get_ref().fmt(f)
+    }
+}
+
+#[cfg(all(unix))]
+mod sys {
+    use super::UdpSocket;
+    use std::os::unix::prelude::*;
+
+    impl AsRawFd for UdpSocket {
+        fn as_raw_fd(&self) -> RawFd {
+            self.io.get_ref().as_raw_fd()
+        }
+    }
+}
+
+#[cfg(windows)]
+mod sys {
+    // TODO: let's land these upstream with mio and then we can add them here.
+    //
+    // use std::os::windows::prelude::*;
+    // use super::UdpSocket;
+    //
+    // impl AsRawHandle for UdpSocket {
+    //     fn as_raw_handle(&self) -> RawHandle {
+    //         self.io.get_ref().as_raw_handle()
+    //     }
+    // }
+}
diff --git a/third_party/rust/tokio/src/net/udp/split.rs b/third_party/rust/tokio/src/net/udp/split.rs
new file mode 100644
index 0000000000..55542cb631
--- /dev/null
+++ b/third_party/rust/tokio/src/net/udp/split.rs
@@ -0,0 +1,148 @@
+//! [`UdpSocket`](../struct.UdpSocket.html) split support.
+//!
+//! The [`split`](../struct.UdpSocket.html#method.split) method splits a
+//! `UdpSocket` into a receive half and a send half, which can be used to
+//! receive and send datagrams concurrently, even from two different tasks.
+//!
+//! The halves provide access to the underlying socket, implementing
+//! `AsRef<UdpSocket>`. This allows you to call `UdpSocket` methods that takes
+//! `&self`, e.g., to get local address, to get and set socket options, to join
+//! or leave multicast groups, etc.
+//!
+//! The halves can be reunited to the original socket with their `reunite`
+//! methods.
+
+use crate::future::poll_fn;
+use crate::net::udp::UdpSocket;
+
+use std::error::Error;
+use std::fmt;
+use std::io;
+use std::net::SocketAddr;
+use std::sync::Arc;
+
+/// The send half after [`split`](super::UdpSocket::split).
+///
+/// Use [`send_to`](#method.send_to) or [`send`](#method.send) to send
+/// datagrams.
+#[derive(Debug)]
+pub struct SendHalf(Arc<UdpSocket>);
+
+/// The recv half after [`split`](super::UdpSocket::split).
+///
+/// Use [`recv_from`](#method.recv_from) or [`recv`](#method.recv) to receive
+/// datagrams.
+#[derive(Debug)]
+pub struct RecvHalf(Arc<UdpSocket>);
+
+pub(crate) fn split(socket: UdpSocket) -> (RecvHalf, SendHalf) {
+    let shared = Arc::new(socket);
+    let send = shared.clone();
+    let recv = shared;
+    (RecvHalf(recv), SendHalf(send))
+}
+
+/// Error indicating two halves were not from the same socket, and thus could
+/// not be `reunite`d.
+#[derive(Debug)]
+pub struct ReuniteError(pub SendHalf, pub RecvHalf);
+
+impl fmt::Display for ReuniteError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "tried to reunite halves that are not from the same socket"
+        )
+    }
+}
+
+impl Error for ReuniteError {}
+
+fn reunite(s: SendHalf, r: RecvHalf) -> Result<UdpSocket, ReuniteError> {
+    if Arc::ptr_eq(&s.0, &r.0) {
+        drop(r);
+        // Only two instances of the `Arc` are ever created, one for the
+        // receiver and one for the sender, and those `Arc`s are never exposed
+        // externally. And so when we drop one here, the other one must be the
+        // only remaining one.
+        Ok(Arc::try_unwrap(s.0).expect("udp: try_unwrap failed in reunite"))
+    } else {
+        Err(ReuniteError(s, r))
+    }
+}
+
+impl RecvHalf {
+    /// Attempts to put the two "halves" of a `UdpSocket` back together and
+    /// recover the original socket. Succeeds only if the two "halves"
+    /// originated from the same call to `UdpSocket::split`.
+    pub fn reunite(self, other: SendHalf) -> Result<UdpSocket, ReuniteError> {
+        reunite(other, self)
+    }
+
+    /// Returns a future that receives a single datagram on the socket. On success,
+    /// the future resolves to the number of bytes read and the origin.
+    ///
+    /// The function must be called with valid byte array `buf` of sufficient size
+    /// to hold the message bytes. If a message is too long to fit in the supplied
+    /// buffer, excess bytes may be discarded.
+    pub async fn recv_from(&mut self, buf: &mut [u8]) -> io::Result<(usize, SocketAddr)> {
+        poll_fn(|cx| self.0.poll_recv_from(cx, buf)).await
+    }
+
+    /// Returns a future that receives a single datagram message on the socket from
+    /// the remote address to which it is connected. On success, the future will resolve
+    /// to the number of bytes read.
+    ///
+    /// The function must be called with valid byte array `buf` of sufficient size to
+    /// hold the message bytes. If a message is too long to fit in the supplied buffer,
+    /// excess bytes may be discarded.
+    ///
+    /// The [`connect`] method will connect this socket to a remote address. The future
+    /// will fail if the socket is not connected.
+    ///
+    /// [`connect`]: super::UdpSocket::connect
+    pub async fn recv(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        poll_fn(|cx| self.0.poll_recv(cx, buf)).await
+    }
+}
+
+impl SendHalf {
+    /// Attempts to put the two "halves" of a `UdpSocket` back together and
+    /// recover the original socket. Succeeds only if the two "halves"
+    /// originated from the same call to `UdpSocket::split`.
+    pub fn reunite(self, other: RecvHalf) -> Result<UdpSocket, ReuniteError> {
+        reunite(self, other)
+    }
+
+    /// Returns a future that sends data on the socket to the given address.
+    /// On success, the future will resolve to the number of bytes written.
+    ///
+    /// The future will resolve to an error if the IP version of the socket does
+    /// not match that of `target`.
+    pub async fn send_to(&mut self, buf: &[u8], target: &SocketAddr) -> io::Result<usize> {
+        poll_fn(|cx| self.0.poll_send_to(cx, buf, target)).await
+    }
+
+    /// Returns a future that sends data on the socket to the remote address to which it is connected.
+    /// On success, the future will resolve to the number of bytes written.
+    ///
+    /// The [`connect`] method will connect this socket to a remote address. The future
+    /// will resolve to an error if the socket is not connected.
+    ///
+    /// [`connect`]: super::UdpSocket::connect
+    pub async fn send(&mut self, buf: &[u8]) -> io::Result<usize> {
+        poll_fn(|cx| self.0.poll_send(cx, buf)).await
+    }
+}
+
+impl AsRef<UdpSocket> for SendHalf {
+    fn as_ref(&self) -> &UdpSocket {
+        &self.0
+    }
+}
+
+impl AsRef<UdpSocket> for RecvHalf {
+    fn as_ref(&self) -> &UdpSocket {
+        &self.0
+    }
+}
diff --git a/third_party/rust/tokio/src/net/unix/datagram.rs b/third_party/rust/tokio/src/net/unix/datagram.rs
new file mode 100644
index 0000000000..ff0f4241d5
--- /dev/null
+++ b/third_party/rust/tokio/src/net/unix/datagram.rs
@@ -0,0 +1,242 @@
+use crate::future::poll_fn;
+use crate::io::PollEvented;
+
+use std::convert::TryFrom;
+use std::fmt;
+use std::io;
+use std::net::Shutdown;
+use std::os::unix::io::{AsRawFd, RawFd};
+use std::os::unix::net::{self, SocketAddr};
+use std::path::Path;
+use std::task::{Context, Poll};
+
+cfg_uds! {
+    /// An I/O object representing a Unix datagram socket.
+    pub struct UnixDatagram {
+        io: PollEvented<mio_uds::UnixDatagram>,
+    }
+}
+
+impl UnixDatagram {
+    /// Creates a new `UnixDatagram` bound to the specified path.
+    pub fn bind<P>(path: P) -> io::Result<UnixDatagram>
+    where
+        P: AsRef<Path>,
+    {
+        let socket = mio_uds::UnixDatagram::bind(path)?;
+        UnixDatagram::new(socket)
+    }
+
+    /// Creates an unnamed pair of connected sockets.
+    ///
+    /// This function will create a pair of interconnected Unix sockets for
+    /// communicating back and forth between one another. Each socket will
+    /// be associated with the default event loop's handle.
+    pub fn pair() -> io::Result<(UnixDatagram, UnixDatagram)> {
+        let (a, b) = mio_uds::UnixDatagram::pair()?;
+        let a = UnixDatagram::new(a)?;
+        let b = UnixDatagram::new(b)?;
+
+        Ok((a, b))
+    }
+
+    /// Consumes a `UnixDatagram` in the standard library and returns a
+    /// nonblocking `UnixDatagram` from this crate.
+    ///
+    /// The returned datagram will be associated with the given event loop
+    /// specified by `handle` and is ready to perform I/O.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if thread-local runtime is not set.
+    ///
+    /// The runtime is usually set implicitly when this function is called
+    /// from a future driven by a tokio runtime, otherwise runtime can be set
+    /// explicitly with [`Handle::enter`](crate::runtime::Handle::enter) function.
+    pub fn from_std(datagram: net::UnixDatagram) -> io::Result<UnixDatagram> {
+        let socket = mio_uds::UnixDatagram::from_datagram(datagram)?;
+        let io = PollEvented::new(socket)?;
+        Ok(UnixDatagram { io })
+    }
+
+    fn new(socket: mio_uds::UnixDatagram) -> io::Result<UnixDatagram> {
+        let io = PollEvented::new(socket)?;
+        Ok(UnixDatagram { io })
+    }
+
+    /// Creates a new `UnixDatagram` which is not bound to any address.
+    pub fn unbound() -> io::Result<UnixDatagram> {
+        let socket = mio_uds::UnixDatagram::unbound()?;
+        UnixDatagram::new(socket)
+    }
+
+    /// Connects the socket to the specified address.
+    ///
+    /// The `send` method may be used to send data to the specified address.
+    /// `recv` and `recv_from` will only receive data from that address.
+    pub fn connect<P: AsRef<Path>>(&self, path: P) -> io::Result<()> {
+        self.io.get_ref().connect(path)
+    }
+
+    /// Sends data on the socket to the socket's peer.
+    pub async fn send(&mut self, buf: &[u8]) -> io::Result<usize> {
+        poll_fn(|cx| self.poll_send_priv(cx, buf)).await
+    }
+
+    // Poll IO functions that takes `&self` are provided for the split API.
+    //
+    // They are not public because (taken from the doc of `PollEvented`):
+    //
+    // While `PollEvented` is `Sync` (if the underlying I/O type is `Sync`), the
+    // caller must ensure that there are at most two tasks that use a
+    // `PollEvented` instance concurrently. One for reading and one for writing.
+    // While violating this requirement is "safe" from a Rust memory model point
+    // of view, it will result in unexpected behavior in the form of lost
+    // notifications and tasks hanging.
+    pub(crate) fn poll_send_priv(
+        &self,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        ready!(self.io.poll_write_ready(cx))?;
+
+        match self.io.get_ref().send(buf) {
+            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
+                self.io.clear_write_ready(cx)?;
+                Poll::Pending
+            }
+            x => Poll::Ready(x),
+        }
+    }
+
+    /// Receives data from the socket.
+    pub async fn recv(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        poll_fn(|cx| self.poll_recv_priv(cx, buf)).await
+    }
+
+    pub(crate) fn poll_recv_priv(
+        &self,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        ready!(self.io.poll_read_ready(cx, mio::Ready::readable()))?;
+
+        match self.io.get_ref().recv(buf) {
+            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
+                self.io.clear_read_ready(cx, mio::Ready::readable())?;
+                Poll::Pending
+            }
+            x => Poll::Ready(x),
+        }
+    }
+
+    /// Sends data on the socket to the specified address.
+    pub async fn send_to<P>(&mut self, buf: &[u8], target: P) -> io::Result<usize>
+    where
+        P: AsRef<Path> + Unpin,
+    {
+        poll_fn(|cx| self.poll_send_to_priv(cx, buf, target.as_ref())).await
+    }
+
+    pub(crate) fn poll_send_to_priv(
+        &self,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+        target: &Path,
+    ) -> Poll<io::Result<usize>> {
+        ready!(self.io.poll_write_ready(cx))?;
+
+        match self.io.get_ref().send_to(buf, target) {
+            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
+                self.io.clear_write_ready(cx)?;
+                Poll::Pending
+            }
+            x => Poll::Ready(x),
+        }
+    }
+
+    /// Receives data from the socket.
+    pub async fn recv_from(&mut self, buf: &mut [u8]) -> io::Result<(usize, SocketAddr)> {
+        poll_fn(|cx| self.poll_recv_from_priv(cx, buf)).await
+    }
+
+    pub(crate) fn poll_recv_from_priv(
+        &self,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<Result<(usize, SocketAddr), io::Error>> {
+        ready!(self.io.poll_read_ready(cx, mio::Ready::readable()))?;
+
+        match self.io.get_ref().recv_from(buf) {
+            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
+                self.io.clear_read_ready(cx, mio::Ready::readable())?;
+                Poll::Pending
+            }
+            x => Poll::Ready(x),
+        }
+    }
+
+    /// Returns the local address that this socket is bound to.
+    pub fn local_addr(&self) -> io::Result<SocketAddr> {
+        self.io.get_ref().local_addr()
+    }
+
+    /// Returns the address of this socket's peer.
+    ///
+    /// The `connect` method will connect the socket to a peer.
+    pub fn peer_addr(&self) -> io::Result<SocketAddr> {
+        self.io.get_ref().peer_addr()
+    }
+
+    /// Returns the value of the `SO_ERROR` option.
+    pub fn take_error(&self) -> io::Result<Option<io::Error>> {
+        self.io.get_ref().take_error()
+    }
+
+    /// Shuts down the read, write, or both halves of this connection.
+    ///
+    /// This function will cause all pending and future I/O calls on the
+    /// specified portions to immediately return with an appropriate value
+    /// (see the documentation of `Shutdown`).
+    pub fn shutdown(&self, how: Shutdown) -> io::Result<()> {
+        self.io.get_ref().shutdown(how)
+    }
+}
+
+impl TryFrom<UnixDatagram> for mio_uds::UnixDatagram {
+    type Error = io::Error;
+
+    /// Consumes value, returning the mio I/O object.
+    ///
+    /// See [`PollEvented::into_inner`] for more details about
+    /// resource deregistration that happens during the call.
+    ///
+    /// [`PollEvented::into_inner`]: crate::io::PollEvented::into_inner
+    fn try_from(value: UnixDatagram) -> Result<Self, Self::Error> {
+        value.io.into_inner()
+    }
+}
+
+impl TryFrom<net::UnixDatagram> for UnixDatagram {
+    type Error = io::Error;
+
+    /// Consumes stream, returning the tokio I/O object.
+    ///
+    /// This is equivalent to
+    /// [`UnixDatagram::from_std(stream)`](UnixDatagram::from_std).
+    fn try_from(stream: net::UnixDatagram) -> Result<Self, Self::Error> {
+        Self::from_std(stream)
+    }
+}
+
+impl fmt::Debug for UnixDatagram {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.io.get_ref().fmt(f)
+    }
+}
+
+impl AsRawFd for UnixDatagram {
+    fn as_raw_fd(&self) -> RawFd {
+        self.io.get_ref().as_raw_fd()
+    }
+}
diff --git a/third_party/rust/tokio/src/net/unix/incoming.rs b/third_party/rust/tokio/src/net/unix/incoming.rs
new file mode 100644
index 0000000000..af49360435
--- /dev/null
+++ b/third_party/rust/tokio/src/net/unix/incoming.rs
@@ -0,0 +1,42 @@
+use crate::net::unix::{UnixListener, UnixStream};
+
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// Stream of listeners
+#[derive(Debug)]
+#[must_use = "streams do nothing unless polled"]
+pub struct Incoming<'a> {
+    inner: &'a mut UnixListener,
+}
+
+impl Incoming<'_> {
+    pub(crate) fn new(listener: &mut UnixListener) -> Incoming<'_> {
+        Incoming { inner: listener }
+    }
+
+    /// Attempts to poll `UnixStream` by polling inner `UnixListener` to accept
+    /// connection.
+    ///
+    /// If `UnixListener` isn't ready yet, `Poll::Pending` is returned and
+    /// current task will be notified by a waker.  Otherwise `Poll::Ready` with
+    /// `Result` containing `UnixStream` will be returned.
+    pub fn poll_accept(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<io::Result<UnixStream>> {
+        let (socket, _) = ready!(self.inner.poll_accept(cx))?;
+        Poll::Ready(Ok(socket))
+    }
+}
+
+#[cfg(feature = "stream")]
+impl crate::stream::Stream for Incoming<'_> {
+    type Item = io::Result<UnixStream>;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        let (socket, _) = ready!(self.inner.poll_accept(cx))?;
+        Poll::Ready(Some(Ok(socket)))
+    }
+}
diff --git a/third_party/rust/tokio/src/net/unix/listener.rs b/third_party/rust/tokio/src/net/unix/listener.rs
new file mode 100644
index 0000000000..5acc1b7e82
--- /dev/null
+++ b/third_party/rust/tokio/src/net/unix/listener.rs
@@ -0,0 +1,229 @@
+use crate::future::poll_fn;
+use crate::io::PollEvented;
+use crate::net::unix::{Incoming, UnixStream};
+
+use mio::Ready;
+use mio_uds;
+use std::convert::TryFrom;
+use std::fmt;
+use std::io;
+use std::os::unix::io::{AsRawFd, RawFd};
+use std::os::unix::net::{self, SocketAddr};
+use std::path::Path;
+use std::task::{Context, Poll};
+
+cfg_uds! {
+    /// A Unix socket which can accept connections from other Unix sockets.
+    ///
+    /// You can accept a new connection by using the [`accept`](`UnixListener::accept`) method. Alternatively `UnixListener`
+    /// implements the [`Stream`](`crate::stream::Stream`) trait, which allows you to use the listener in places that want a
+    /// stream. The stream will never return `None` and will also not yield the peer's `SocketAddr` structure. Iterating over
+    /// it is equivalent to calling accept in a loop.
+    ///
+    /// # Errors
+    ///
+    /// Note that accepting a connection can lead to various errors and not all
+    /// of them are necessarily fatal ‒ for example having too many open file
+    /// descriptors or the other side closing the connection while it waits in
+    /// an accept queue. These would terminate the stream if not handled in any
+    /// way.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::UnixListener;
+    /// use tokio::stream::StreamExt;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let mut listener = UnixListener::bind("/path/to/the/socket").unwrap();
+    ///     while let Some(stream) = listener.next().await {
+    ///         match stream {
+    ///             Ok(stream) => {
+    ///                 println!("new client!");
+    ///             }
+    ///             Err(e) => { /* connection failed */ }
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    pub struct UnixListener {
+        io: PollEvented<mio_uds::UnixListener>,
+    }
+}
+
+impl UnixListener {
+    /// Creates a new `UnixListener` bound to the specified path.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if thread-local runtime is not set.
+    ///
+    /// The runtime is usually set implicitly when this function is called
+    /// from a future driven by a tokio runtime, otherwise runtime can be set
+    /// explicitly with [`Handle::enter`](crate::runtime::Handle::enter) function.
+    pub fn bind<P>(path: P) -> io::Result<UnixListener>
+    where
+        P: AsRef<Path>,
+    {
+        let listener = mio_uds::UnixListener::bind(path)?;
+        let io = PollEvented::new(listener)?;
+        Ok(UnixListener { io })
+    }
+
+    /// Consumes a `UnixListener` in the standard library and returns a
+    /// nonblocking `UnixListener` from this crate.
+    ///
+    /// The returned listener will be associated with the given event loop
+    /// specified by `handle` and is ready to perform I/O.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if thread-local runtime is not set.
+    ///
+    /// The runtime is usually set implicitly when this function is called
+    /// from a future driven by a tokio runtime, otherwise runtime can be set
+    /// explicitly with [`Handle::enter`](crate::runtime::Handle::enter) function.
+    pub fn from_std(listener: net::UnixListener) -> io::Result<UnixListener> {
+        let listener = mio_uds::UnixListener::from_listener(listener)?;
+        let io = PollEvented::new(listener)?;
+        Ok(UnixListener { io })
+    }
+
+    /// Returns the local socket address of this listener.
+    pub fn local_addr(&self) -> io::Result<SocketAddr> {
+        self.io.get_ref().local_addr()
+    }
+
+    /// Returns the value of the `SO_ERROR` option.
+    pub fn take_error(&self) -> io::Result<Option<io::Error>> {
+        self.io.get_ref().take_error()
+    }
+
+    /// Accepts a new incoming connection to this listener.
+    pub async fn accept(&mut self) -> io::Result<(UnixStream, SocketAddr)> {
+        poll_fn(|cx| self.poll_accept(cx)).await
+    }
+
+    pub(crate) fn poll_accept(
+        &mut self,
+        cx: &mut Context<'_>,
+    ) -> Poll<io::Result<(UnixStream, SocketAddr)>> {
+        let (io, addr) = ready!(self.poll_accept_std(cx))?;
+
+        let io = mio_uds::UnixStream::from_stream(io)?;
+        Ok((UnixStream::new(io)?, addr)).into()
+    }
+
+    fn poll_accept_std(
+        &mut self,
+        cx: &mut Context<'_>,
+    ) -> Poll<io::Result<(net::UnixStream, SocketAddr)>> {
+        ready!(self.io.poll_read_ready(cx, Ready::readable()))?;
+
+        match self.io.get_ref().accept_std() {
+            Ok(None) => {
+                self.io.clear_read_ready(cx, Ready::readable())?;
+                Poll::Pending
+            }
+            Ok(Some((sock, addr))) => Ok((sock, addr)).into(),
+            Err(ref err) if err.kind() == io::ErrorKind::WouldBlock => {
+                self.io.clear_read_ready(cx, Ready::readable())?;
+                Poll::Pending
+            }
+            Err(err) => Err(err).into(),
+        }
+    }
+
+    /// Returns a stream over the connections being received on this listener.
+    ///
+    /// Note that `UnixListener` also directly implements `Stream`.
+    ///
+    /// The returned stream will never return `None` and will also not yield the
+    /// peer's `SocketAddr` structure. Iterating over it is equivalent to
+    /// calling accept in a loop.
+    ///
+    /// # Errors
+    ///
+    /// Note that accepting a connection can lead to various errors and not all
+    /// of them are necessarily fatal ‒ for example having too many open file
+    /// descriptors or the other side closing the connection while it waits in
+    /// an accept queue. These would terminate the stream if not handled in any
+    /// way.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::net::UnixListener;
+    /// use tokio::stream::StreamExt;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let mut listener = UnixListener::bind("/path/to/the/socket").unwrap();
+    ///     let mut incoming = listener.incoming();
+    ///
+    ///     while let Some(stream) = incoming.next().await {
+    ///         match stream {
+    ///             Ok(stream) => {
+    ///                 println!("new client!");
+    ///             }
+    ///             Err(e) => { /* connection failed */ }
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    pub fn incoming(&mut self) -> Incoming<'_> {
+        Incoming::new(self)
+    }
+}
+
+#[cfg(feature = "stream")]
+impl crate::stream::Stream for UnixListener {
+    type Item = io::Result<UnixStream>;
+
+    fn poll_next(
+        mut self: std::pin::Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Option<Self::Item>> {
+        let (socket, _) = ready!(self.poll_accept(cx))?;
+        Poll::Ready(Some(Ok(socket)))
+    }
+}
+
+impl TryFrom<UnixListener> for mio_uds::UnixListener {
+    type Error = io::Error;
+
+    /// Consumes value, returning the mio I/O object.
+    ///
+    /// See [`PollEvented::into_inner`] for more details about
+    /// resource deregistration that happens during the call.
+    ///
+    /// [`PollEvented::into_inner`]: crate::io::PollEvented::into_inner
+    fn try_from(value: UnixListener) -> Result<Self, Self::Error> {
+        value.io.into_inner()
+    }
+}
+
+impl TryFrom<net::UnixListener> for UnixListener {
+    type Error = io::Error;
+
+    /// Consumes stream, returning the tokio I/O object.
+    ///
+    /// This is equivalent to
+    /// [`UnixListener::from_std(stream)`](UnixListener::from_std).
+    fn try_from(stream: net::UnixListener) -> io::Result<Self> {
+        Self::from_std(stream)
+    }
+}
+
+impl fmt::Debug for UnixListener {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.io.get_ref().fmt(f)
+    }
+}
+
+impl AsRawFd for UnixListener {
+    fn as_raw_fd(&self) -> RawFd {
+        self.io.get_ref().as_raw_fd()
+    }
+}
diff --git a/third_party/rust/tokio/src/net/unix/mod.rs b/third_party/rust/tokio/src/net/unix/mod.rs
new file mode 100644
index 0000000000..ddba60d10a
--- /dev/null
+++ b/third_party/rust/tokio/src/net/unix/mod.rs
@@ -0,0 +1,18 @@
+//! Unix domain socket utility types
+
+pub(crate) mod datagram;
+
+mod incoming;
+pub use incoming::Incoming;
+
+pub(crate) mod listener;
+pub(crate) use listener::UnixListener;
+
+mod split;
+pub use split::{ReadHalf, WriteHalf};
+
+pub(crate) mod stream;
+pub(crate) use stream::UnixStream;
+
+mod ucred;
+pub use ucred::UCred;
diff --git a/third_party/rust/tokio/src/net/unix/split.rs b/third_party/rust/tokio/src/net/unix/split.rs
new file mode 100644
index 0000000000..9b9fa5ee1d
--- /dev/null
+++ b/third_party/rust/tokio/src/net/unix/split.rs
@@ -0,0 +1,74 @@
+//! `UnixStream` split support.
+//!
+//! A `UnixStream` can be split into a read half and a write half with
+//! `UnixStream::split`. The read half implements `AsyncRead` while the write
+//! half implements `AsyncWrite`.
+//!
+//! Compared to the generic split of `AsyncRead + AsyncWrite`, this specialized
+//! split has no associated overhead and enforces all invariants at the type
+//! level.
+
+use crate::io::{AsyncRead, AsyncWrite};
+use crate::net::UnixStream;
+
+use std::io;
+use std::mem::MaybeUninit;
+use std::net::Shutdown;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// Read half of a `UnixStream`.
+#[derive(Debug)]
+pub struct ReadHalf<'a>(&'a UnixStream);
+
+/// Write half of a `UnixStream`.
+#[derive(Debug)]
+pub struct WriteHalf<'a>(&'a UnixStream);
+
+pub(crate) fn split(stream: &mut UnixStream) -> (ReadHalf<'_>, WriteHalf<'_>) {
+    (ReadHalf(stream), WriteHalf(stream))
+}
+
+impl AsyncRead for ReadHalf<'_> {
+    unsafe fn prepare_uninitialized_buffer(&self, _: &mut [MaybeUninit<u8>]) -> bool {
+        false
+    }
+
+    fn poll_read(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        self.0.poll_read_priv(cx, buf)
+    }
+}
+
+impl AsyncWrite for WriteHalf<'_> {
+    fn poll_write(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        self.0.poll_write_priv(cx, buf)
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
+        Poll::Ready(Ok(()))
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
+        self.0.shutdown(Shutdown::Write).into()
+    }
+}
+
+impl AsRef<UnixStream> for ReadHalf<'_> {
+    fn as_ref(&self) -> &UnixStream {
+        self.0
+    }
+}
+
+impl AsRef<UnixStream> for WriteHalf<'_> {
+    fn as_ref(&self) -> &UnixStream {
+        self.0
+    }
+}
diff --git a/third_party/rust/tokio/src/net/unix/stream.rs b/third_party/rust/tokio/src/net/unix/stream.rs
new file mode 100644
index 0000000000..beae699962
--- /dev/null
+++ b/third_party/rust/tokio/src/net/unix/stream.rs
@@ -0,0 +1,233 @@
+use crate::future::poll_fn;
+use crate::io::{AsyncRead, AsyncWrite, PollEvented};
+use crate::net::unix::split::{split, ReadHalf, WriteHalf};
+use crate::net::unix::ucred::{self, UCred};
+
+use std::convert::TryFrom;
+use std::fmt;
+use std::io::{self, Read, Write};
+use std::mem::MaybeUninit;
+use std::net::Shutdown;
+use std::os::unix::io::{AsRawFd, RawFd};
+use std::os::unix::net::{self, SocketAddr};
+use std::path::Path;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+cfg_uds! {
+    /// A structure representing a connected Unix socket.
+    ///
+    /// This socket can be connected directly with `UnixStream::connect` or accepted
+    /// from a listener with `UnixListener::incoming`. Additionally, a pair of
+    /// anonymous Unix sockets can be created with `UnixStream::pair`.
+    pub struct UnixStream {
+        io: PollEvented<mio_uds::UnixStream>,
+    }
+}
+
+impl UnixStream {
+    /// Connects to the socket named by `path`.
+    ///
+    /// This function will create a new Unix socket and connect to the path
+    /// specified, associating the returned stream with the default event loop's
+    /// handle.
+    pub async fn connect<P>(path: P) -> io::Result<UnixStream>
+    where
+        P: AsRef<Path>,
+    {
+        let stream = mio_uds::UnixStream::connect(path)?;
+        let stream = UnixStream::new(stream)?;
+
+        poll_fn(|cx| stream.io.poll_write_ready(cx)).await?;
+        Ok(stream)
+    }
+
+    /// Consumes a `UnixStream` in the standard library and returns a
+    /// nonblocking `UnixStream` from this crate.
+    ///
+    /// The returned stream will be associated with the given event loop
+    /// specified by `handle` and is ready to perform I/O.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if thread-local runtime is not set.
+    ///
+    /// The runtime is usually set implicitly when this function is called
+    /// from a future driven by a tokio runtime, otherwise runtime can be set
+    /// explicitly with [`Handle::enter`](crate::runtime::Handle::enter) function.
+    pub fn from_std(stream: net::UnixStream) -> io::Result<UnixStream> {
+        let stream = mio_uds::UnixStream::from_stream(stream)?;
+        let io = PollEvented::new(stream)?;
+
+        Ok(UnixStream { io })
+    }
+
+    /// Creates an unnamed pair of connected sockets.
+    ///
+    /// This function will create a pair of interconnected Unix sockets for
+    /// communicating back and forth between one another. Each socket will
+    /// be associated with the default event loop's handle.
+    pub fn pair() -> io::Result<(UnixStream, UnixStream)> {
+        let (a, b) = mio_uds::UnixStream::pair()?;
+        let a = UnixStream::new(a)?;
+        let b = UnixStream::new(b)?;
+
+        Ok((a, b))
+    }
+
+    pub(crate) fn new(stream: mio_uds::UnixStream) -> io::Result<UnixStream> {
+        let io = PollEvented::new(stream)?;
+        Ok(UnixStream { io })
+    }
+
+    /// Returns the socket address of the local half of this connection.
+    pub fn local_addr(&self) -> io::Result<SocketAddr> {
+        self.io.get_ref().local_addr()
+    }
+
+    /// Returns the socket address of the remote half of this connection.
+    pub fn peer_addr(&self) -> io::Result<SocketAddr> {
+        self.io.get_ref().peer_addr()
+    }
+
+    /// Returns effective credentials of the process which called `connect` or `pair`.
+    pub fn peer_cred(&self) -> io::Result<UCred> {
+        ucred::get_peer_cred(self)
+    }
+
+    /// Returns the value of the `SO_ERROR` option.
+    pub fn take_error(&self) -> io::Result<Option<io::Error>> {
+        self.io.get_ref().take_error()
+    }
+
+    /// Shuts down the read, write, or both halves of this connection.
+    ///
+    /// This function will cause all pending and future I/O calls on the
+    /// specified portions to immediately return with an appropriate value
+    /// (see the documentation of `Shutdown`).
+    pub fn shutdown(&self, how: Shutdown) -> io::Result<()> {
+        self.io.get_ref().shutdown(how)
+    }
+
+    /// Split a `UnixStream` into a read half and a write half, which can be used
+    /// to read and write the stream concurrently.
+    pub fn split(&mut self) -> (ReadHalf<'_>, WriteHalf<'_>) {
+        split(self)
+    }
+}
+
+impl TryFrom<UnixStream> for mio_uds::UnixStream {
+    type Error = io::Error;
+
+    /// Consumes value, returning the mio I/O object.
+    ///
+    /// See [`PollEvented::into_inner`] for more details about
+    /// resource deregistration that happens during the call.
+    ///
+    /// [`PollEvented::into_inner`]: crate::io::PollEvented::into_inner
+    fn try_from(value: UnixStream) -> Result<Self, Self::Error> {
+        value.io.into_inner()
+    }
+}
+
+impl TryFrom<net::UnixStream> for UnixStream {
+    type Error = io::Error;
+
+    /// Consumes stream, returning the tokio I/O object.
+    ///
+    /// This is equivalent to
+    /// [`UnixStream::from_std(stream)`](UnixStream::from_std).
+    fn try_from(stream: net::UnixStream) -> io::Result<Self> {
+        Self::from_std(stream)
+    }
+}
+
+impl AsyncRead for UnixStream {
+    unsafe fn prepare_uninitialized_buffer(&self, _: &mut [MaybeUninit<u8>]) -> bool {
+        false
+    }
+
+    fn poll_read(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        self.poll_read_priv(cx, buf)
+    }
+}
+
+impl AsyncWrite for UnixStream {
+    fn poll_write(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        self.poll_write_priv(cx, buf)
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
+        Poll::Ready(Ok(()))
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
+        self.shutdown(std::net::Shutdown::Write)?;
+        Poll::Ready(Ok(()))
+    }
+}
+
+impl UnixStream {
+    // == Poll IO functions that takes `&self` ==
+    //
+    // They are not public because (taken from the doc of `PollEvented`):
+    //
+    // While `PollEvented` is `Sync` (if the underlying I/O type is `Sync`), the
+    // caller must ensure that there are at most two tasks that use a
+    // `PollEvented` instance concurrently. One for reading and one for writing.
+    // While violating this requirement is "safe" from a Rust memory model point
+    // of view, it will result in unexpected behavior in the form of lost
+    // notifications and tasks hanging.
+
+    pub(crate) fn poll_read_priv(
+        &self,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        ready!(self.io.poll_read_ready(cx, mio::Ready::readable()))?;
+
+        match self.io.get_ref().read(buf) {
+            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
+                self.io.clear_read_ready(cx, mio::Ready::readable())?;
+                Poll::Pending
+            }
+            x => Poll::Ready(x),
+        }
+    }
+
+    pub(crate) fn poll_write_priv(
+        &self,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        ready!(self.io.poll_write_ready(cx))?;
+
+        match self.io.get_ref().write(buf) {
+            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
+                self.io.clear_write_ready(cx)?;
+                Poll::Pending
+            }
+            x => Poll::Ready(x),
+        }
+    }
+}
+
+impl fmt::Debug for UnixStream {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.io.get_ref().fmt(f)
+    }
+}
+
+impl AsRawFd for UnixStream {
+    fn as_raw_fd(&self) -> RawFd {
+        self.io.get_ref().as_raw_fd()
+    }
+}
diff --git a/third_party/rust/tokio/src/net/unix/ucred.rs b/third_party/rust/tokio/src/net/unix/ucred.rs
new file mode 100644
index 0000000000..cdd77ea414
--- /dev/null
+++ b/third_party/rust/tokio/src/net/unix/ucred.rs
@@ -0,0 +1,151 @@
+use libc::{gid_t, uid_t};
+
+/// Credentials of a process
+#[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
+pub struct UCred {
+    /// UID (user ID) of the process
+    pub uid: uid_t,
+    /// GID (group ID) of the process
+    pub gid: gid_t,
+}
+
+#[cfg(any(target_os = "linux", target_os = "android"))]
+pub(crate) use self::impl_linux::get_peer_cred;
+
+#[cfg(any(
+    target_os = "dragonfly",
+    target_os = "macos",
+    target_os = "ios",
+    target_os = "freebsd",
+    target_os = "netbsd",
+    target_os = "openbsd"
+))]
+pub(crate) use self::impl_macos::get_peer_cred;
+
+#[cfg(any(target_os = "solaris"))]
+pub(crate) use self::impl_solaris::get_peer_cred;
+
+#[cfg(any(target_os = "linux", target_os = "android"))]
+pub(crate) mod impl_linux {
+    use crate::net::unix::UnixStream;
+
+    use libc::{c_void, getsockopt, socklen_t, SOL_SOCKET, SO_PEERCRED};
+    use std::{io, mem};
+
+    use libc::ucred;
+
+    pub(crate) fn get_peer_cred(sock: &UnixStream) -> io::Result<super::UCred> {
+        use std::os::unix::io::AsRawFd;
+
+        unsafe {
+            let raw_fd = sock.as_raw_fd();
+
+            let mut ucred = ucred {
+                pid: 0,
+                uid: 0,
+                gid: 0,
+            };
+
+            let ucred_size = mem::size_of::<ucred>();
+
+            // These paranoid checks should be optimized-out
+            assert!(mem::size_of::<u32>() <= mem::size_of::<usize>());
+            assert!(ucred_size <= u32::max_value() as usize);
+
+            let mut ucred_size = ucred_size as socklen_t;
+
+            let ret = getsockopt(
+                raw_fd,
+                SOL_SOCKET,
+                SO_PEERCRED,
+                &mut ucred as *mut ucred as *mut c_void,
+                &mut ucred_size,
+            );
+            if ret == 0 && ucred_size as usize == mem::size_of::<ucred>() {
+                Ok(super::UCred {
+                    uid: ucred.uid,
+                    gid: ucred.gid,
+                })
+            } else {
+                Err(io::Error::last_os_error())
+            }
+        }
+    }
+}
+
+#[cfg(any(
+    target_os = "dragonfly",
+    target_os = "macos",
+    target_os = "ios",
+    target_os = "freebsd",
+    target_os = "netbsd",
+    target_os = "openbsd"
+))]
+pub(crate) mod impl_macos {
+    use crate::net::unix::UnixStream;
+
+    use libc::getpeereid;
+    use std::io;
+    use std::mem::MaybeUninit;
+    use std::os::unix::io::AsRawFd;
+
+    pub(crate) fn get_peer_cred(sock: &UnixStream) -> io::Result<super::UCred> {
+        unsafe {
+            let raw_fd = sock.as_raw_fd();
+
+            let mut uid = MaybeUninit::uninit();
+            let mut gid = MaybeUninit::uninit();
+
+            let ret = getpeereid(raw_fd, uid.as_mut_ptr(), gid.as_mut_ptr());
+
+            if ret == 0 {
+                Ok(super::UCred {
+                    uid: uid.assume_init(),
+                    gid: gid.assume_init(),
+                })
+            } else {
+                Err(io::Error::last_os_error())
+            }
+        }
+    }
+}
+
+#[cfg(any(target_os = "solaris"))]
+pub(crate) mod impl_solaris {
+    use crate::net::unix::UnixStream;
+    use std::io;
+    use std::os::unix::io::AsRawFd;
+    use std::ptr;
+
+    #[allow(non_camel_case_types)]
+    enum ucred_t {}
+
+    extern "C" {
+        fn ucred_free(cred: *mut ucred_t);
+        fn ucred_geteuid(cred: *const ucred_t) -> super::uid_t;
+        fn ucred_getegid(cred: *const ucred_t) -> super::gid_t;
+
+        fn getpeerucred(fd: std::os::raw::c_int, cred: *mut *mut ucred_t) -> std::os::raw::c_int;
+    }
+
+    pub(crate) fn get_peer_cred(sock: &UnixStream) -> io::Result<super::UCred> {
+        unsafe {
+            let raw_fd = sock.as_raw_fd();
+
+            let mut cred = ptr::null_mut::<*mut ucred_t>() as *mut ucred_t;
+
+            let ret = getpeerucred(raw_fd, &mut cred);
+
+            if ret == 0 {
+                let uid = ucred_geteuid(cred);
+                let gid = ucred_getegid(cred);
+
+                ucred_free(cred);
+
+                Ok(super::UCred { uid, gid })
+            } else {
+                Err(io::Error::last_os_error())
+            }
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/park/either.rs b/third_party/rust/tokio/src/park/either.rs
new file mode 100644
index 0000000000..67f1e17274
--- /dev/null
+++ b/third_party/rust/tokio/src/park/either.rs
@@ -0,0 +1,65 @@
+use crate::park::{Park, Unpark};
+
+use std::fmt;
+use std::time::Duration;
+
+pub(crate) enum Either<A, B> {
+    A(A),
+    B(B),
+}
+
+impl<A, B> Park for Either<A, B>
+where
+    A: Park,
+    B: Park,
+{
+    type Unpark = Either<A::Unpark, B::Unpark>;
+    type Error = Either<A::Error, B::Error>;
+
+    fn unpark(&self) -> Self::Unpark {
+        match self {
+            Either::A(a) => Either::A(a.unpark()),
+            Either::B(b) => Either::B(b.unpark()),
+        }
+    }
+
+    fn park(&mut self) -> Result<(), Self::Error> {
+        match self {
+            Either::A(a) => a.park().map_err(Either::A),
+            Either::B(b) => b.park().map_err(Either::B),
+        }
+    }
+
+    fn park_timeout(&mut self, duration: Duration) -> Result<(), Self::Error> {
+        match self {
+            Either::A(a) => a.park_timeout(duration).map_err(Either::A),
+            Either::B(b) => b.park_timeout(duration).map_err(Either::B),
+        }
+    }
+}
+
+impl<A, B> Unpark for Either<A, B>
+where
+    A: Unpark,
+    B: Unpark,
+{
+    fn unpark(&self) {
+        match self {
+            Either::A(a) => a.unpark(),
+            Either::B(b) => b.unpark(),
+        }
+    }
+}
+
+impl<A, B> fmt::Debug for Either<A, B>
+where
+    A: fmt::Debug,
+    B: fmt::Debug,
+{
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Either::A(a) => a.fmt(fmt),
+            Either::B(b) => b.fmt(fmt),
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/park/mod.rs b/third_party/rust/tokio/src/park/mod.rs
new file mode 100644
index 0000000000..a3e49bbede
--- /dev/null
+++ b/third_party/rust/tokio/src/park/mod.rs
@@ -0,0 +1,118 @@
+//! Abstraction over blocking and unblocking the current thread.
+//!
+//! Provides an abstraction over blocking the current thread. This is similar to
+//! the park / unpark constructs provided by `std` but made generic. This allows
+//! embedding custom functionality to perform when the thread is blocked.
+//!
+//! A blocked `Park` instance is unblocked by calling `unpark` on its
+//! `Unpark` handle.
+//!
+//! The `ParkThread` struct implements `Park` using `thread::park` to put the
+//! thread to sleep. The Tokio reactor also implements park, but uses
+//! `mio::Poll` to block the thread instead.
+//!
+//! The `Park` trait is composable. A timer implementation might decorate a
+//! `Park` implementation by checking if any timeouts have elapsed after the
+//! inner `Park` implementation unblocks.
+//!
+//! # Model
+//!
+//! Conceptually, each `Park` instance has an associated token, which is
+//! initially not present:
+//!
+//! * The `park` method blocks the current thread unless or until the token is
+//!   available, at which point it atomically consumes the token.
+//! * The `unpark` method atomically makes the token available if it wasn't
+//!   already.
+//!
+//! Some things to note:
+//!
+//! * If `unpark` is called before `park`, the next call to `park` will
+//!   **not** block the thread.
+//! * **Spurious** wakeups are permitted, i.e., the `park` method may unblock
+//!   even if `unpark` was not called.
+//! * `park_timeout` does the same as `park` but allows specifying a maximum
+//!   time to block the thread for.
+
+cfg_resource_drivers! {
+    mod either;
+    pub(crate) use self::either::Either;
+}
+
+mod thread;
+pub(crate) use self::thread::ParkThread;
+
+cfg_blocking_impl! {
+    pub(crate) use self::thread::{CachedParkThread, ParkError};
+}
+
+use std::sync::Arc;
+use std::time::Duration;
+
+/// Block the current thread.
+pub(crate) trait Park {
+    /// Unpark handle type for the `Park` implementation.
+    type Unpark: Unpark;
+
+    /// Error returned by `park`
+    type Error;
+
+    /// Gets a new `Unpark` handle associated with this `Park` instance.
+    fn unpark(&self) -> Self::Unpark;
+
+    /// Blocks the current thread unless or until the token is available.
+    ///
+    /// A call to `park` does not guarantee that the thread will remain blocked
+    /// forever, and callers should be prepared for this possibility. This
+    /// function may wakeup spuriously for any reason.
+    ///
+    /// # Panics
+    ///
+    /// This function **should** not panic, but ultimately, panics are left as
+    /// an implementation detail. Refer to the documentation for the specific
+    /// `Park` implementation
+    fn park(&mut self) -> Result<(), Self::Error>;
+
+    /// Parks the current thread for at most `duration`.
+    ///
+    /// This function is the same as `park` but allows specifying a maximum time
+    /// to block the thread for.
+    ///
+    /// Same as `park`, there is no guarantee that the thread will remain
+    /// blocked for any amount of time. Spurious wakeups are permitted for any
+    /// reason.
+    ///
+    /// # Panics
+    ///
+    /// This function **should** not panic, but ultimately, panics are left as
+    /// an implementation detail. Refer to the documentation for the specific
+    /// `Park` implementation
+    fn park_timeout(&mut self, duration: Duration) -> Result<(), Self::Error>;
+}
+
+/// Unblock a thread blocked by the associated `Park` instance.
+pub(crate) trait Unpark: Sync + Send + 'static {
+    /// Unblocks a thread that is blocked by the associated `Park` handle.
+    ///
+    /// Calling `unpark` atomically makes available the unpark token, if it is
+    /// not already available.
+    ///
+    /// # Panics
+    ///
+    /// This function **should** not panic, but ultimately, panics are left as
+    /// an implementation detail. Refer to the documentation for the specific
+    /// `Unpark` implementation
+    fn unpark(&self);
+}
+
+impl Unpark for Box<dyn Unpark> {
+    fn unpark(&self) {
+        (**self).unpark()
+    }
+}
+
+impl Unpark for Arc<dyn Unpark> {
+    fn unpark(&self) {
+        (**self).unpark()
+    }
+}
diff --git a/third_party/rust/tokio/src/park/thread.rs b/third_party/rust/tokio/src/park/thread.rs
new file mode 100644
index 0000000000..a8cdf1432b
--- /dev/null
+++ b/third_party/rust/tokio/src/park/thread.rs
@@ -0,0 +1,317 @@
+use crate::loom::sync::atomic::AtomicUsize;
+use crate::loom::sync::{Arc, Condvar, Mutex};
+use crate::park::{Park, Unpark};
+
+use std::sync::atomic::Ordering::SeqCst;
+use std::time::Duration;
+
+#[derive(Debug)]
+pub(crate) struct ParkThread {
+    inner: Arc<Inner>,
+}
+
+pub(crate) type ParkError = ();
+
+/// Unblocks a thread that was blocked by `ParkThread`.
+#[derive(Clone, Debug)]
+pub(crate) struct UnparkThread {
+    inner: Arc<Inner>,
+}
+
+#[derive(Debug)]
+struct Inner {
+    state: AtomicUsize,
+    mutex: Mutex<()>,
+    condvar: Condvar,
+}
+
+const EMPTY: usize = 0;
+const PARKED: usize = 1;
+const NOTIFIED: usize = 2;
+
+thread_local! {
+    static CURRENT_PARKER: ParkThread = ParkThread::new();
+}
+
+// ==== impl ParkThread ====
+
+impl ParkThread {
+    pub(crate) fn new() -> Self {
+        Self {
+            inner: Arc::new(Inner {
+                state: AtomicUsize::new(EMPTY),
+                mutex: Mutex::new(()),
+                condvar: Condvar::new(),
+            }),
+        }
+    }
+}
+
+impl Park for ParkThread {
+    type Unpark = UnparkThread;
+    type Error = ParkError;
+
+    fn unpark(&self) -> Self::Unpark {
+        let inner = self.inner.clone();
+        UnparkThread { inner }
+    }
+
+    fn park(&mut self) -> Result<(), Self::Error> {
+        self.inner.park();
+        Ok(())
+    }
+
+    fn park_timeout(&mut self, duration: Duration) -> Result<(), Self::Error> {
+        self.inner.park_timeout(duration);
+        Ok(())
+    }
+}
+
+// ==== impl Inner ====
+
+impl Inner {
+    /// Park the current thread for at most `dur`.
+    fn park(&self) {
+        // If we were previously notified then we consume this notification and
+        // return quickly.
+        if self
+            .state
+            .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
+            .is_ok()
+        {
+            return;
+        }
+
+        // Otherwise we need to coordinate going to sleep
+        let mut m = self.mutex.lock().unwrap();
+
+        match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
+            Ok(_) => {}
+            Err(NOTIFIED) => {
+                // We must read here, even though we know it will be `NOTIFIED`.
+                // This is because `unpark` may have been called again since we read
+                // `NOTIFIED` in the `compare_exchange` above. We must perform an
+                // acquire operation that synchronizes with that `unpark` to observe
+                // any writes it made before the call to unpark. To do that we must
+                // read from the write it made to `state`.
+                let old = self.state.swap(EMPTY, SeqCst);
+                debug_assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
+
+                return;
+            }
+            Err(actual) => panic!("inconsistent park state; actual = {}", actual),
+        }
+
+        loop {
+            m = self.condvar.wait(m).unwrap();
+
+            if self
+                .state
+                .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
+                .is_ok()
+            {
+                // got a notification
+                return;
+            }
+
+            // spurious wakeup, go back to sleep
+        }
+    }
+
+    fn park_timeout(&self, dur: Duration) {
+        // Like `park` above we have a fast path for an already-notified thread,
+        // and afterwards we start coordinating for a sleep. Return quickly.
+        if self
+            .state
+            .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
+            .is_ok()
+        {
+            return;
+        }
+
+        if dur == Duration::from_millis(0) {
+            return;
+        }
+
+        let m = self.mutex.lock().unwrap();
+
+        match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
+            Ok(_) => {}
+            Err(NOTIFIED) => {
+                // We must read again here, see `park`.
+                let old = self.state.swap(EMPTY, SeqCst);
+                debug_assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
+
+                return;
+            }
+            Err(actual) => panic!("inconsistent park_timeout state; actual = {}", actual),
+        }
+
+        // Wait with a timeout, and if we spuriously wake up or otherwise wake up
+        // from a notification, we just want to unconditionally set the state back to
+        // empty, either consuming a notification or un-flagging ourselves as
+        // parked.
+        let (_m, _result) = self.condvar.wait_timeout(m, dur).unwrap();
+
+        match self.state.swap(EMPTY, SeqCst) {
+            NOTIFIED => {} // got a notification, hurray!
+            PARKED => {}   // no notification, alas
+            n => panic!("inconsistent park_timeout state: {}", n),
+        }
+    }
+
+    fn unpark(&self) {
+        // To ensure the unparked thread will observe any writes we made before
+        // this call, we must perform a release operation that `park` can
+        // synchronize with. To do that we must write `NOTIFIED` even if `state`
+        // is already `NOTIFIED`. That is why this must be a swap rather than a
+        // compare-and-swap that returns if it reads `NOTIFIED` on failure.
+        match self.state.swap(NOTIFIED, SeqCst) {
+            EMPTY => return,    // no one was waiting
+            NOTIFIED => return, // already unparked
+            PARKED => {}        // gotta go wake someone up
+            _ => panic!("inconsistent state in unpark"),
+        }
+
+        // There is a period between when the parked thread sets `state` to
+        // `PARKED` (or last checked `state` in the case of a spurious wake
+        // up) and when it actually waits on `cvar`. If we were to notify
+        // during this period it would be ignored and then when the parked
+        // thread went to sleep it would never wake up. Fortunately, it has
+        // `lock` locked at this stage so we can acquire `lock` to wait until
+        // it is ready to receive the notification.
+        //
+        // Releasing `lock` before the call to `notify_one` means that when the
+        // parked thread wakes it doesn't get woken only to have to wait for us
+        // to release `lock`.
+        drop(self.mutex.lock().unwrap());
+
+        self.condvar.notify_one()
+    }
+}
+
+impl Default for ParkThread {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+// ===== impl UnparkThread =====
+
+impl Unpark for UnparkThread {
+    fn unpark(&self) {
+        self.inner.unpark();
+    }
+}
+
+cfg_blocking_impl! {
+    use std::marker::PhantomData;
+    use std::rc::Rc;
+
+    use std::mem;
+    use std::task::{RawWaker, RawWakerVTable, Waker};
+
+    /// Blocks the current thread using a condition variable.
+    #[derive(Debug)]
+    pub(crate) struct CachedParkThread {
+        _anchor: PhantomData<Rc<()>>,
+    }
+
+    impl CachedParkThread {
+        /// Create a new `ParkThread` handle for the current thread.
+        ///
+        /// This type cannot be moved to other threads, so it should be created on
+        /// the thread that the caller intends to park.
+        pub(crate) fn new() -> CachedParkThread {
+            CachedParkThread {
+                _anchor: PhantomData,
+            }
+        }
+
+        pub(crate) fn get_unpark(&self) -> Result<UnparkThread, ParkError> {
+            self.with_current(|park_thread| park_thread.unpark())
+        }
+
+        /// Get a reference to the `ParkThread` handle for this thread.
+        fn with_current<F, R>(&self, f: F) -> Result<R, ParkError>
+        where
+            F: FnOnce(&ParkThread) -> R,
+        {
+            CURRENT_PARKER.try_with(|inner| f(inner))
+                .map_err(|_| ())
+        }
+    }
+
+    impl Park for CachedParkThread {
+        type Unpark = UnparkThread;
+        type Error = ParkError;
+
+        fn unpark(&self) -> Self::Unpark {
+            self.get_unpark().unwrap()
+        }
+
+        fn park(&mut self) -> Result<(), Self::Error> {
+            self.with_current(|park_thread| park_thread.inner.park())?;
+            Ok(())
+        }
+
+        fn park_timeout(&mut self, duration: Duration) -> Result<(), Self::Error> {
+            self.with_current(|park_thread| park_thread.inner.park_timeout(duration))?;
+            Ok(())
+        }
+    }
+
+
+    impl UnparkThread {
+        pub(crate) fn into_waker(self) -> Waker {
+            unsafe {
+                let raw = unparker_to_raw_waker(self.inner);
+                Waker::from_raw(raw)
+            }
+        }
+    }
+
+    impl Inner {
+        #[allow(clippy::wrong_self_convention)]
+        fn into_raw(this: Arc<Inner>) -> *const () {
+            Arc::into_raw(this) as *const ()
+        }
+
+        unsafe fn from_raw(ptr: *const ()) -> Arc<Inner> {
+            Arc::from_raw(ptr as *const Inner)
+        }
+    }
+
+    unsafe fn unparker_to_raw_waker(unparker: Arc<Inner>) -> RawWaker {
+        RawWaker::new(
+            Inner::into_raw(unparker),
+            &RawWakerVTable::new(clone, wake, wake_by_ref, drop_waker),
+        )
+    }
+
+    unsafe fn clone(raw: *const ()) -> RawWaker {
+        let unparker = Inner::from_raw(raw);
+
+        // Increment the ref count
+        mem::forget(unparker.clone());
+
+        unparker_to_raw_waker(unparker)
+    }
+
+    unsafe fn drop_waker(raw: *const ()) {
+        let _ = Inner::from_raw(raw);
+    }
+
+    unsafe fn wake(raw: *const ()) {
+        let unparker = Inner::from_raw(raw);
+        unparker.unpark();
+    }
+
+    unsafe fn wake_by_ref(raw: *const ()) {
+        let unparker = Inner::from_raw(raw);
+        unparker.unpark();
+
+        // We don't actually own a reference to the unparker
+        mem::forget(unparker);
+    }
+}
diff --git a/third_party/rust/tokio/src/prelude.rs b/third_party/rust/tokio/src/prelude.rs
new file mode 100644
index 0000000000..1909f9da6a
--- /dev/null
+++ b/third_party/rust/tokio/src/prelude.rs
@@ -0,0 +1,21 @@
+#![cfg(not(loom))]
+
+//! A "prelude" for users of the `tokio` crate.
+//!
+//! This prelude is similar to the standard library's prelude in that you'll
+//! almost always want to import its entire contents, but unlike the standard
+//! library's prelude you'll have to do so manually:
+//!
+//! ```
+//! # #![allow(warnings)]
+//! use tokio::prelude::*;
+//! ```
+//!
+//! The prelude may grow over time as additional items see ubiquitous use.
+
+pub use crate::io::{self, AsyncBufRead, AsyncRead, AsyncWrite};
+
+cfg_io_util! {
+    #[doc(no_inline)]
+    pub use crate::io::{AsyncBufReadExt as _, AsyncReadExt as _, AsyncSeekExt as _, AsyncWriteExt as _};
+}
diff --git a/third_party/rust/tokio/src/process/kill.rs b/third_party/rust/tokio/src/process/kill.rs
new file mode 100644
index 0000000000..a1f1652281
--- /dev/null
+++ b/third_party/rust/tokio/src/process/kill.rs
@@ -0,0 +1,13 @@
+use std::io;
+
+/// An interface for killing a running process.
+pub(crate) trait Kill {
+    /// Forcefully kills the process.
+    fn kill(&mut self) -> io::Result<()>;
+}
+
+impl<T: Kill> Kill for &mut T {
+    fn kill(&mut self) -> io::Result<()> {
+        (**self).kill()
+    }
+}
diff --git a/third_party/rust/tokio/src/process/mod.rs b/third_party/rust/tokio/src/process/mod.rs
new file mode 100644
index 0000000000..7231511235
--- /dev/null
+++ b/third_party/rust/tokio/src/process/mod.rs
@@ -0,0 +1,1078 @@
+//! An implementation of asynchronous process management for Tokio.
+//!
+//! This module provides a [`Command`] struct that imitates the interface of the
+//! [`std::process::Command`] type in the standard library, but provides asynchronous versions of
+//! functions that create processes. These functions (`spawn`, `status`, `output` and their
+//! variants) return "future aware" types that interoperate with Tokio. The asynchronous process
+//! support is provided through signal handling on Unix and system APIs on Windows.
+//!
+//! # Examples
+//!
+//! Here's an example program which will spawn `echo hello world` and then wait
+//! for it complete.
+//!
+//! ```no_run
+//! use tokio::process::Command;
+//!
+//! #[tokio::main]
+//! async fn main() -> Result<(), Box<dyn std::error::Error>> {
+//!     // The usage is the same as with the standard library's `Command` type, however the value
+//!     // returned from `spawn` is a `Result` containing a `Future`.
+//!     let child = Command::new("echo").arg("hello").arg("world")
+//!                         .spawn();
+//!
+//!     // Make sure our child succeeded in spawning and process the result
+//!     let future = child.expect("failed to spawn");
+//!
+//!     // Await until the future (and the command) completes
+//!     let status = future.await?;
+//!     println!("the command exited with: {}", status);
+//!     Ok(())
+//! }
+//! ```
+//!
+//! Next, let's take a look at an example where we not only spawn `echo hello
+//! world` but we also capture its output.
+//!
+//! ```no_run
+//! use tokio::process::Command;
+//!
+//! #[tokio::main]
+//! async fn main() -> Result<(), Box<dyn std::error::Error>> {
+//!     // Like above, but use `output` which returns a future instead of
+//!     // immediately returning the `Child`.
+//!     let output = Command::new("echo").arg("hello").arg("world")
+//!                         .output();
+//!
+//!     let output = output.await?;
+//!
+//!     assert!(output.status.success());
+//!     assert_eq!(output.stdout, b"hello world\n");
+//!     Ok(())
+//! }
+//! ```
+//!
+//! We can also read input line by line.
+//!
+//! ```no_run
+//! use tokio::io::{BufReader, AsyncBufReadExt};
+//! use tokio::process::Command;
+//!
+//! use std::process::Stdio;
+//!
+//! #[tokio::main]
+//! async fn main() -> Result<(), Box<dyn std::error::Error>> {
+//!     let mut cmd = Command::new("cat");
+//!
+//!     // Specify that we want the command's standard output piped back to us.
+//!     // By default, standard input/output/error will be inherited from the
+//!     // current process (for example, this means that standard input will
+//!     // come from the keyboard and standard output/error will go directly to
+//!     // the terminal if this process is invoked from the command line).
+//!     cmd.stdout(Stdio::piped());
+//!
+//!     let mut child = cmd.spawn()
+//!         .expect("failed to spawn command");
+//!
+//!     let stdout = child.stdout.take()
+//!         .expect("child did not have a handle to stdout");
+//!
+//!     let mut reader = BufReader::new(stdout).lines();
+//!
+//!     // Ensure the child process is spawned in the runtime so it can
+//!     // make progress on its own while we await for any output.
+//!     tokio::spawn(async {
+//!         let status = child.await
+//!             .expect("child process encountered an error");
+//!
+//!         println!("child status was: {}", status);
+//!     });
+//!
+//!     while let Some(line) = reader.next_line().await? {
+//!         println!("Line: {}", line);
+//!     }
+//!
+//!     Ok(())
+//! }
+//! ```
+//!
+//! # Caveats
+//!
+//! Similar to the behavior to the standard library, and unlike the futures
+//! paradigm of dropping-implies-cancellation, a spawned process will, by
+//! default, continue to execute even after the `Child` handle has been dropped.
+//!
+//! The `Command::kill_on_drop` method can be used to modify this behavior
+//! and kill the child process if the `Child` wrapper is dropped before it
+//! has exited.
+//!
+//! [`Command`]: crate::process::Command
+
+#[path = "unix/mod.rs"]
+#[cfg(unix)]
+mod imp;
+
+#[path = "windows.rs"]
+#[cfg(windows)]
+mod imp;
+
+mod kill;
+
+use crate::io::{AsyncRead, AsyncWrite};
+use crate::process::kill::Kill;
+
+use std::ffi::OsStr;
+use std::future::Future;
+use std::io;
+#[cfg(unix)]
+use std::os::unix::process::CommandExt;
+#[cfg(windows)]
+use std::os::windows::process::CommandExt;
+use std::path::Path;
+use std::pin::Pin;
+use std::process::{Command as StdCommand, ExitStatus, Output, Stdio};
+use std::task::Context;
+use std::task::Poll;
+
+/// This structure mimics the API of [`std::process::Command`] found in the standard library, but
+/// replaces functions that create a process with an asynchronous variant. The main provided
+/// asynchronous functions are [spawn](Command::spawn), [status](Command::status), and
+/// [output](Command::output).
+///
+/// `Command` uses asynchronous versions of some `std` types (for example [`Child`]).
+#[derive(Debug)]
+pub struct Command {
+    std: StdCommand,
+    kill_on_drop: bool,
+}
+
+pub(crate) struct SpawnedChild {
+    child: imp::Child,
+    stdin: Option<imp::ChildStdin>,
+    stdout: Option<imp::ChildStdout>,
+    stderr: Option<imp::ChildStderr>,
+}
+
+impl Command {
+    /// Constructs a new `Command` for launching the program at
+    /// path `program`, with the following default configuration:
+    ///
+    /// * No arguments to the program
+    /// * Inherit the current process's environment
+    /// * Inherit the current process's working directory
+    /// * Inherit stdin/stdout/stderr for `spawn` or `status`, but create pipes for `output`
+    ///
+    /// Builder methods are provided to change these defaults and
+    /// otherwise configure the process.
+    ///
+    /// If `program` is not an absolute path, the `PATH` will be searched in
+    /// an OS-defined way.
+    ///
+    /// The search path to be used may be controlled by setting the
+    /// `PATH` environment variable on the Command,
+    /// but this has some implementation limitations on Windows
+    /// (see issue rust-lang/rust#37519).
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```no_run
+    /// use tokio::process::Command;
+    /// let command = Command::new("sh");
+    /// ```
+    pub fn new<S: AsRef<OsStr>>(program: S) -> Command {
+        Self::from(StdCommand::new(program))
+    }
+
+    /// Adds an argument to pass to the program.
+    ///
+    /// Only one argument can be passed per use. So instead of:
+    ///
+    /// ```no_run
+    /// tokio::process::Command::new("sh")
+    ///   .arg("-C /path/to/repo");
+    /// ```
+    ///
+    /// usage would be:
+    ///
+    /// ```no_run
+    /// tokio::process::Command::new("sh")
+    ///   .arg("-C")
+    ///   .arg("/path/to/repo");
+    /// ```
+    ///
+    /// To pass multiple arguments see [`args`].
+    ///
+    /// [`args`]: #method.args
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```no_run
+    /// use tokio::process::Command;
+    ///
+    /// let command = Command::new("ls")
+    ///         .arg("-l")
+    ///         .arg("-a");
+    /// ```
+    pub fn arg<S: AsRef<OsStr>>(&mut self, arg: S) -> &mut Command {
+        self.std.arg(arg);
+        self
+    }
+
+    /// Adds multiple arguments to pass to the program.
+    ///
+    /// To pass a single argument see [`arg`].
+    ///
+    /// [`arg`]: #method.arg
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```no_run
+    /// use tokio::process::Command;
+    ///
+    /// let command = Command::new("ls")
+    ///         .args(&["-l", "-a"]);
+    /// ```
+    pub fn args<I, S>(&mut self, args: I) -> &mut Command
+    where
+        I: IntoIterator<Item = S>,
+        S: AsRef<OsStr>,
+    {
+        self.std.args(args);
+        self
+    }
+
+    /// Inserts or updates an environment variable mapping.
+    ///
+    /// Note that environment variable names are case-insensitive (but case-preserving) on Windows,
+    /// and case-sensitive on all other platforms.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```no_run
+    /// use tokio::process::Command;
+    ///
+    /// let command = Command::new("ls")
+    ///         .env("PATH", "/bin");
+    /// ```
+    pub fn env<K, V>(&mut self, key: K, val: V) -> &mut Command
+    where
+        K: AsRef<OsStr>,
+        V: AsRef<OsStr>,
+    {
+        self.std.env(key, val);
+        self
+    }
+
+    /// Adds or updates multiple environment variable mappings.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```no_run
+    /// use tokio::process::Command;
+    /// use std::process::{Stdio};
+    /// use std::env;
+    /// use std::collections::HashMap;
+    ///
+    /// let filtered_env : HashMap<String, String> =
+    ///     env::vars().filter(|&(ref k, _)|
+    ///         k == "TERM" || k == "TZ" || k == "LANG" || k == "PATH"
+    ///     ).collect();
+    ///
+    /// let command = Command::new("printenv")
+    ///         .stdin(Stdio::null())
+    ///         .stdout(Stdio::inherit())
+    ///         .env_clear()
+    ///         .envs(&filtered_env);
+    /// ```
+    pub fn envs<I, K, V>(&mut self, vars: I) -> &mut Command
+    where
+        I: IntoIterator<Item = (K, V)>,
+        K: AsRef<OsStr>,
+        V: AsRef<OsStr>,
+    {
+        self.std.envs(vars);
+        self
+    }
+
+    /// Removes an environment variable mapping.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```no_run
+    /// use tokio::process::Command;
+    ///
+    /// let command = Command::new("ls")
+    ///         .env_remove("PATH");
+    /// ```
+    pub fn env_remove<K: AsRef<OsStr>>(&mut self, key: K) -> &mut Command {
+        self.std.env_remove(key);
+        self
+    }
+
+    /// Clears the entire environment map for the child process.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```no_run
+    /// use tokio::process::Command;
+    ///
+    /// let command = Command::new("ls")
+    ///         .env_clear();
+    /// ```
+    pub fn env_clear(&mut self) -> &mut Command {
+        self.std.env_clear();
+        self
+    }
+
+    /// Sets the working directory for the child process.
+    ///
+    /// # Platform-specific behavior
+    ///
+    /// If the program path is relative (e.g., `"./script.sh"`), it's ambiguous
+    /// whether it should be interpreted relative to the parent's working
+    /// directory or relative to `current_dir`. The behavior in this case is
+    /// platform specific and unstable, and it's recommended to use
+    /// [`canonicalize`] to get an absolute program path instead.
+    ///
+    /// [`canonicalize`]: crate::fs::canonicalize()
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```no_run
+    /// use tokio::process::Command;
+    ///
+    /// let command = Command::new("ls")
+    ///         .current_dir("/bin");
+    /// ```
+    pub fn current_dir<P: AsRef<Path>>(&mut self, dir: P) -> &mut Command {
+        self.std.current_dir(dir);
+        self
+    }
+
+    /// Sets configuration for the child process's standard input (stdin) handle.
+    ///
+    /// Defaults to [`inherit`] when used with `spawn` or `status`, and
+    /// defaults to [`piped`] when used with `output`.
+    ///
+    /// [`inherit`]: std::process::Stdio::inherit
+    /// [`piped`]: std::process::Stdio::piped
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```no_run
+    /// use std::process::{Stdio};
+    /// use tokio::process::Command;
+    ///
+    /// let command = Command::new("ls")
+    ///         .stdin(Stdio::null());
+    /// ```
+    pub fn stdin<T: Into<Stdio>>(&mut self, cfg: T) -> &mut Command {
+        self.std.stdin(cfg);
+        self
+    }
+
+    /// Sets configuration for the child process's standard output (stdout) handle.
+    ///
+    /// Defaults to [`inherit`] when used with `spawn` or `status`, and
+    /// defaults to [`piped`] when used with `output`.
+    ///
+    /// [`inherit`]: std::process::Stdio::inherit
+    /// [`piped`]: std::process::Stdio::piped
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```no_run
+    /// use tokio::process::Command;;
+    /// use std::process::Stdio;
+    ///
+    /// let command = Command::new("ls")
+    ///         .stdout(Stdio::null());
+    /// ```
+    pub fn stdout<T: Into<Stdio>>(&mut self, cfg: T) -> &mut Command {
+        self.std.stdout(cfg);
+        self
+    }
+
+    /// Sets configuration for the child process's standard error (stderr) handle.
+    ///
+    /// Defaults to [`inherit`] when used with `spawn` or `status`, and
+    /// defaults to [`piped`] when used with `output`.
+    ///
+    /// [`inherit`]: std::process::Stdio::inherit
+    /// [`piped`]: std::process::Stdio::piped
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```no_run
+    /// use tokio::process::Command;;
+    /// use std::process::{Stdio};
+    ///
+    /// let command = Command::new("ls")
+    ///         .stderr(Stdio::null());
+    /// ```
+    pub fn stderr<T: Into<Stdio>>(&mut self, cfg: T) -> &mut Command {
+        self.std.stderr(cfg);
+        self
+    }
+
+    /// Controls whether a `kill` operation should be invoked on a spawned child
+    /// process when its corresponding `Child` handle is dropped.
+    ///
+    /// By default, this value is assumed to be `false`, meaning the next spawned
+    /// process will not be killed on drop, similar to the behavior of the standard
+    /// library.
+    pub fn kill_on_drop(&mut self, kill_on_drop: bool) -> &mut Command {
+        self.kill_on_drop = kill_on_drop;
+        self
+    }
+
+    /// Sets the [process creation flags][1] to be passed to `CreateProcess`.
+    ///
+    /// These will always be ORed with `CREATE_UNICODE_ENVIRONMENT`.
+    ///
+    /// [1]: https://msdn.microsoft.com/en-us/library/windows/desktop/ms684863(v=vs.85).aspx
+    #[cfg(windows)]
+    pub fn creation_flags(&mut self, flags: u32) -> &mut Command {
+        self.std.creation_flags(flags);
+        self
+    }
+
+    /// Sets the child process's user ID. This translates to a
+    /// `setuid` call in the child process. Failure in the `setuid`
+    /// call will cause the spawn to fail.
+    #[cfg(unix)]
+    pub fn uid(&mut self, id: u32) -> &mut Command {
+        self.std.uid(id);
+        self
+    }
+
+    /// Similar to `uid` but sets the group ID of the child process. This has
+    /// the same semantics as the `uid` field.
+    #[cfg(unix)]
+    pub fn gid(&mut self, id: u32) -> &mut Command {
+        self.std.gid(id);
+        self
+    }
+
+    /// Schedules a closure to be run just before the `exec` function is
+    /// invoked.
+    ///
+    /// The closure is allowed to return an I/O error whose OS error code will
+    /// be communicated back to the parent and returned as an error from when
+    /// the spawn was requested.
+    ///
+    /// Multiple closures can be registered and they will be called in order of
+    /// their registration. If a closure returns `Err` then no further closures
+    /// will be called and the spawn operation will immediately return with a
+    /// failure.
+    ///
+    /// # Safety
+    ///
+    /// This closure will be run in the context of the child process after a
+    /// `fork`. This primarily means that any modifications made to memory on
+    /// behalf of this closure will **not** be visible to the parent process.
+    /// This is often a very constrained environment where normal operations
+    /// like `malloc` or acquiring a mutex are not guaranteed to work (due to
+    /// other threads perhaps still running when the `fork` was run).
+    ///
+    /// This also means that all resources such as file descriptors and
+    /// memory-mapped regions got duplicated. It is your responsibility to make
+    /// sure that the closure does not violate library invariants by making
+    /// invalid use of these duplicates.
+    ///
+    /// When this closure is run, aspects such as the stdio file descriptors and
+    /// working directory have successfully been changed, so output to these
+    /// locations may not appear where intended.
+    #[cfg(unix)]
+    pub unsafe fn pre_exec<F>(&mut self, f: F) -> &mut Command
+    where
+        F: FnMut() -> io::Result<()> + Send + Sync + 'static,
+    {
+        self.std.pre_exec(f);
+        self
+    }
+
+    /// Executes the command as a child process, returning a handle to it.
+    ///
+    /// By default, stdin, stdout and stderr are inherited from the parent.
+    ///
+    /// This method will spawn the child process synchronously and return a
+    /// handle to a future-aware child process. The `Child` returned implements
+    /// `Future` itself to acquire the `ExitStatus` of the child, and otherwise
+    /// the `Child` has methods to acquire handles to the stdin, stdout, and
+    /// stderr streams.
+    ///
+    /// All I/O this child does will be associated with the current default
+    /// event loop.
+    ///
+    /// # Caveats
+    ///
+    /// Similar to the behavior to the standard library, and unlike the futures
+    /// paradigm of dropping-implies-cancellation, the spawned process will, by
+    /// default, continue to execute even after the `Child` handle has been dropped.
+    ///
+    /// The `Command::kill_on_drop` method can be used to modify this behavior
+    /// and kill the child process if the `Child` wrapper is dropped before it
+    /// has exited.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```no_run
+    /// use tokio::process::Command;
+    ///
+    /// async fn run_ls() -> std::process::ExitStatus {
+    ///     Command::new("ls")
+    ///         .spawn()
+    ///         .expect("ls command failed to start")
+    ///         .await
+    ///         .expect("ls command failed to run")
+    /// }
+    /// ```
+    pub fn spawn(&mut self) -> io::Result<Child> {
+        imp::spawn_child(&mut self.std).map(|spawned_child| Child {
+            child: ChildDropGuard {
+                inner: spawned_child.child,
+                kill_on_drop: self.kill_on_drop,
+            },
+            stdin: spawned_child.stdin.map(|inner| ChildStdin { inner }),
+            stdout: spawned_child.stdout.map(|inner| ChildStdout { inner }),
+            stderr: spawned_child.stderr.map(|inner| ChildStderr { inner }),
+        })
+    }
+
+    /// Executes the command as a child process, waiting for it to finish and
+    /// collecting its exit status.
+    ///
+    /// By default, stdin, stdout and stderr are inherited from the parent.
+    /// If any input/output handles are set to a pipe then they will be immediately
+    /// closed after the child is spawned.
+    ///
+    /// All I/O this child does will be associated with the current default
+    /// event loop.
+    ///
+    /// If this future is dropped before the future resolves, then
+    /// the child will be killed, if it was spawned.
+    ///
+    /// # Errors
+    ///
+    /// This future will return an error if the child process cannot be spawned
+    /// or if there is an error while awaiting its status.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```no_run
+    /// use tokio::process::Command;
+    ///
+    /// async fn run_ls() -> std::process::ExitStatus {
+    ///     Command::new("ls")
+    ///         .status()
+    ///         .await
+    ///         .expect("ls command failed to run")
+    /// }
+    pub fn status(&mut self) -> impl Future<Output = io::Result<ExitStatus>> {
+        let child = self.spawn();
+
+        async {
+            let mut child = child?;
+
+            // Ensure we close any stdio handles so we can't deadlock
+            // waiting on the child which may be waiting to read/write
+            // to a pipe we're holding.
+            child.stdin.take();
+            child.stdout.take();
+            child.stderr.take();
+
+            child.await
+        }
+    }
+
+    /// Executes the command as a child process, waiting for it to finish and
+    /// collecting all of its output.
+    ///
+    /// > **Note**: this method, unlike the standard library, will
+    /// > unconditionally configure the stdout/stderr handles to be pipes, even
+    /// > if they have been previously configured. If this is not desired then
+    /// > the `spawn` method should be used in combination with the
+    /// > `wait_with_output` method on child.
+    ///
+    /// This method will return a future representing the collection of the
+    /// child process's stdout/stderr. It will resolve to
+    /// the `Output` type in the standard library, containing `stdout` and
+    /// `stderr` as `Vec<u8>` along with an `ExitStatus` representing how the
+    /// process exited.
+    ///
+    /// All I/O this child does will be associated with the current default
+    /// event loop.
+    ///
+    /// If this future is dropped before the future resolves, then
+    /// the child will be killed, if it was spawned.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```no_run
+    /// use tokio::process::Command;
+    ///
+    /// async fn run_ls() {
+    ///     let output: std::process::Output = Command::new("ls")
+    ///         .output()
+    ///         .await
+    ///         .expect("ls command failed to run");
+    ///     println!("stderr of ls: {:?}", output.stderr);
+    /// }
+    pub fn output(&mut self) -> impl Future<Output = io::Result<Output>> {
+        self.std.stdout(Stdio::piped());
+        self.std.stderr(Stdio::piped());
+
+        let child = self.spawn();
+
+        async { child?.wait_with_output().await }
+    }
+}
+
+impl From<StdCommand> for Command {
+    fn from(std: StdCommand) -> Command {
+        Command {
+            std,
+            kill_on_drop: false,
+        }
+    }
+}
+
+/// A drop guard which can ensure the child process is killed on drop if specified.
+#[derive(Debug)]
+struct ChildDropGuard<T: Kill> {
+    inner: T,
+    kill_on_drop: bool,
+}
+
+impl<T: Kill> Kill for ChildDropGuard<T> {
+    fn kill(&mut self) -> io::Result<()> {
+        let ret = self.inner.kill();
+
+        if ret.is_ok() {
+            self.kill_on_drop = false;
+        }
+
+        ret
+    }
+}
+
+impl<T: Kill> Drop for ChildDropGuard<T> {
+    fn drop(&mut self) {
+        if self.kill_on_drop {
+            drop(self.kill());
+        }
+    }
+}
+
+impl<T, E, F> Future for ChildDropGuard<F>
+where
+    F: Future<Output = Result<T, E>> + Kill + Unpin,
+{
+    type Output = Result<T, E>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        // Keep track of task budget
+        ready!(crate::coop::poll_proceed(cx));
+
+        let ret = Pin::new(&mut self.inner).poll(cx);
+
+        if let Poll::Ready(Ok(_)) = ret {
+            // Avoid the overhead of trying to kill a reaped process
+            self.kill_on_drop = false;
+        }
+
+        ret
+    }
+}
+
+/// Representation of a child process spawned onto an event loop.
+///
+/// This type is also a future which will yield the `ExitStatus` of the
+/// underlying child process. A `Child` here also provides access to information
+/// like the OS-assigned identifier and the stdio streams.
+///
+/// # Caveats
+/// Similar to the behavior to the standard library, and unlike the futures
+/// paradigm of dropping-implies-cancellation, a spawned process will, by
+/// default, continue to execute even after the `Child` handle has been dropped.
+///
+/// The `Command::kill_on_drop` method can be used to modify this behavior
+/// and kill the child process if the `Child` wrapper is dropped before it
+/// has exited.
+#[must_use = "futures do nothing unless polled"]
+#[derive(Debug)]
+pub struct Child {
+    child: ChildDropGuard<imp::Child>,
+
+    /// The handle for writing to the child's standard input (stdin), if it has
+    /// been captured.
+    pub stdin: Option<ChildStdin>,
+
+    /// The handle for reading from the child's standard output (stdout), if it
+    /// has been captured.
+    pub stdout: Option<ChildStdout>,
+
+    /// The handle for reading from the child's standard error (stderr), if it
+    /// has been captured.
+    pub stderr: Option<ChildStderr>,
+}
+
+impl Child {
+    /// Returns the OS-assigned process identifier associated with this child.
+    pub fn id(&self) -> u32 {
+        self.child.inner.id()
+    }
+
+    /// Forces the child to exit.
+    ///
+    /// This is equivalent to sending a SIGKILL on unix platforms.
+    pub fn kill(&mut self) -> io::Result<()> {
+        self.child.kill()
+    }
+
+    #[doc(hidden)]
+    #[deprecated(note = "please use `child.stdin` instead")]
+    pub fn stdin(&mut self) -> &mut Option<ChildStdin> {
+        &mut self.stdin
+    }
+
+    #[doc(hidden)]
+    #[deprecated(note = "please use `child.stdout` instead")]
+    pub fn stdout(&mut self) -> &mut Option<ChildStdout> {
+        &mut self.stdout
+    }
+
+    #[doc(hidden)]
+    #[deprecated(note = "please use `child.stderr` instead")]
+    pub fn stderr(&mut self) -> &mut Option<ChildStderr> {
+        &mut self.stderr
+    }
+
+    /// Returns a future that will resolve to an `Output`, containing the exit
+    /// status, stdout, and stderr of the child process.
+    ///
+    /// The returned future will simultaneously waits for the child to exit and
+    /// collect all remaining output on the stdout/stderr handles, returning an
+    /// `Output` instance.
+    ///
+    /// The stdin handle to the child process, if any, will be closed before
+    /// waiting. This helps avoid deadlock: it ensures that the child does not
+    /// block waiting for input from the parent, while the parent waits for the
+    /// child to exit.
+    ///
+    /// By default, stdin, stdout and stderr are inherited from the parent. In
+    /// order to capture the output into this `Output` it is necessary to create
+    /// new pipes between parent and child. Use `stdout(Stdio::piped())` or
+    /// `stderr(Stdio::piped())`, respectively, when creating a `Command`.
+    pub async fn wait_with_output(mut self) -> io::Result<Output> {
+        use crate::future::try_join3;
+
+        async fn read_to_end<A: AsyncRead + Unpin>(io: Option<A>) -> io::Result<Vec<u8>> {
+            let mut vec = Vec::new();
+            if let Some(mut io) = io {
+                crate::io::util::read_to_end(&mut io, &mut vec).await?;
+            }
+            Ok(vec)
+        }
+
+        drop(self.stdin.take());
+        let stdout_fut = read_to_end(self.stdout.take());
+        let stderr_fut = read_to_end(self.stderr.take());
+
+        let (status, stdout, stderr) = try_join3(self, stdout_fut, stderr_fut).await?;
+
+        Ok(Output {
+            status,
+            stdout,
+            stderr,
+        })
+    }
+}
+
+impl Future for Child {
+    type Output = io::Result<ExitStatus>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        Pin::new(&mut self.child).poll(cx)
+    }
+}
+
+/// The standard input stream for spawned children.
+///
+/// This type implements the `AsyncWrite` trait to pass data to the stdin handle of
+/// handle of a child process asynchronously.
+#[derive(Debug)]
+pub struct ChildStdin {
+    inner: imp::ChildStdin,
+}
+
+/// The standard output stream for spawned children.
+///
+/// This type implements the `AsyncRead` trait to read data from the stdout
+/// handle of a child process asynchronously.
+#[derive(Debug)]
+pub struct ChildStdout {
+    inner: imp::ChildStdout,
+}
+
+/// The standard error stream for spawned children.
+///
+/// This type implements the `AsyncRead` trait to read data from the stderr
+/// handle of a child process asynchronously.
+#[derive(Debug)]
+pub struct ChildStderr {
+    inner: imp::ChildStderr,
+}
+
+impl AsyncWrite for ChildStdin {
+    fn poll_write(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        Pin::new(&mut self.inner).poll_write(cx, buf)
+    }
+
+    fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        Pin::new(&mut self.inner).poll_flush(cx)
+    }
+
+    fn poll_shutdown(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+        Pin::new(&mut self.inner).poll_shutdown(cx)
+    }
+}
+
+impl AsyncRead for ChildStdout {
+    fn poll_read(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        Pin::new(&mut self.inner).poll_read(cx, buf)
+    }
+}
+
+impl AsyncRead for ChildStderr {
+    fn poll_read(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        Pin::new(&mut self.inner).poll_read(cx, buf)
+    }
+}
+
+#[cfg(unix)]
+mod sys {
+    use std::os::unix::io::{AsRawFd, RawFd};
+
+    use super::{ChildStderr, ChildStdin, ChildStdout};
+
+    impl AsRawFd for ChildStdin {
+        fn as_raw_fd(&self) -> RawFd {
+            self.inner.get_ref().as_raw_fd()
+        }
+    }
+
+    impl AsRawFd for ChildStdout {
+        fn as_raw_fd(&self) -> RawFd {
+            self.inner.get_ref().as_raw_fd()
+        }
+    }
+
+    impl AsRawFd for ChildStderr {
+        fn as_raw_fd(&self) -> RawFd {
+            self.inner.get_ref().as_raw_fd()
+        }
+    }
+}
+
+#[cfg(windows)]
+mod sys {
+    use std::os::windows::io::{AsRawHandle, RawHandle};
+
+    use super::{ChildStderr, ChildStdin, ChildStdout};
+
+    impl AsRawHandle for ChildStdin {
+        fn as_raw_handle(&self) -> RawHandle {
+            self.inner.get_ref().as_raw_handle()
+        }
+    }
+
+    impl AsRawHandle for ChildStdout {
+        fn as_raw_handle(&self) -> RawHandle {
+            self.inner.get_ref().as_raw_handle()
+        }
+    }
+
+    impl AsRawHandle for ChildStderr {
+        fn as_raw_handle(&self) -> RawHandle {
+            self.inner.get_ref().as_raw_handle()
+        }
+    }
+}
+
+#[cfg(all(test, not(loom)))]
+mod test {
+    use super::kill::Kill;
+    use super::ChildDropGuard;
+
+    use futures::future::FutureExt;
+    use std::future::Future;
+    use std::io;
+    use std::pin::Pin;
+    use std::task::{Context, Poll};
+
+    struct Mock {
+        num_kills: usize,
+        num_polls: usize,
+        poll_result: Poll<Result<(), ()>>,
+    }
+
+    impl Mock {
+        fn new() -> Self {
+            Self::with_result(Poll::Pending)
+        }
+
+        fn with_result(result: Poll<Result<(), ()>>) -> Self {
+            Self {
+                num_kills: 0,
+                num_polls: 0,
+                poll_result: result,
+            }
+        }
+    }
+
+    impl Kill for Mock {
+        fn kill(&mut self) -> io::Result<()> {
+            self.num_kills += 1;
+            Ok(())
+        }
+    }
+
+    impl Future for Mock {
+        type Output = Result<(), ()>;
+
+        fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Self::Output> {
+            let inner = Pin::get_mut(self);
+            inner.num_polls += 1;
+            inner.poll_result
+        }
+    }
+
+    #[test]
+    fn kills_on_drop_if_specified() {
+        let mut mock = Mock::new();
+
+        {
+            let guard = ChildDropGuard {
+                inner: &mut mock,
+                kill_on_drop: true,
+            };
+            drop(guard);
+        }
+
+        assert_eq!(1, mock.num_kills);
+        assert_eq!(0, mock.num_polls);
+    }
+
+    #[test]
+    fn no_kill_on_drop_by_default() {
+        let mut mock = Mock::new();
+
+        {
+            let guard = ChildDropGuard {
+                inner: &mut mock,
+                kill_on_drop: false,
+            };
+            drop(guard);
+        }
+
+        assert_eq!(0, mock.num_kills);
+        assert_eq!(0, mock.num_polls);
+    }
+
+    #[test]
+    fn no_kill_if_already_killed() {
+        let mut mock = Mock::new();
+
+        {
+            let mut guard = ChildDropGuard {
+                inner: &mut mock,
+                kill_on_drop: true,
+            };
+            let _ = guard.kill();
+            drop(guard);
+        }
+
+        assert_eq!(1, mock.num_kills);
+        assert_eq!(0, mock.num_polls);
+    }
+
+    #[test]
+    fn no_kill_if_reaped() {
+        let mut mock_pending = Mock::with_result(Poll::Pending);
+        let mut mock_reaped = Mock::with_result(Poll::Ready(Ok(())));
+        let mut mock_err = Mock::with_result(Poll::Ready(Err(())));
+
+        let waker = futures::task::noop_waker();
+        let mut context = Context::from_waker(&waker);
+        {
+            let mut guard = ChildDropGuard {
+                inner: &mut mock_pending,
+                kill_on_drop: true,
+            };
+            let _ = guard.poll_unpin(&mut context);
+
+            let mut guard = ChildDropGuard {
+                inner: &mut mock_reaped,
+                kill_on_drop: true,
+            };
+            let _ = guard.poll_unpin(&mut context);
+
+            let mut guard = ChildDropGuard {
+                inner: &mut mock_err,
+                kill_on_drop: true,
+            };
+            let _ = guard.poll_unpin(&mut context);
+        }
+
+        assert_eq!(1, mock_pending.num_kills);
+        assert_eq!(1, mock_pending.num_polls);
+
+        assert_eq!(0, mock_reaped.num_kills);
+        assert_eq!(1, mock_reaped.num_polls);
+
+        assert_eq!(1, mock_err.num_kills);
+        assert_eq!(1, mock_err.num_polls);
+    }
+}
diff --git a/third_party/rust/tokio/src/process/unix/mod.rs b/third_party/rust/tokio/src/process/unix/mod.rs
new file mode 100644
index 0000000000..c25d98974a
--- /dev/null
+++ b/third_party/rust/tokio/src/process/unix/mod.rs
@@ -0,0 +1,227 @@
+//! Unix handling of child processes
+//!
+//! Right now the only "fancy" thing about this is how we implement the
+//! `Future` implementation on `Child` to get the exit status. Unix offers
+//! no way to register a child with epoll, and the only real way to get a
+//! notification when a process exits is the SIGCHLD signal.
+//!
+//! Signal handling in general is *super* hairy and complicated, and it's even
+//! more complicated here with the fact that signals are coalesced, so we may
+//! not get a SIGCHLD-per-child.
+//!
+//! Our best approximation here is to check *all spawned processes* for all
+//! SIGCHLD signals received. To do that we create a `Signal`, implemented in
+//! the `tokio-net` crate, which is a stream over signals being received.
+//!
+//! Later when we poll the process's exit status we simply check to see if a
+//! SIGCHLD has happened since we last checked, and while that returns "yes" we
+//! keep trying.
+//!
+//! Note that this means that this isn't really scalable, but then again
+//! processes in general aren't scalable (e.g. millions) so it shouldn't be that
+//! bad in theory...
+
+mod orphan;
+use orphan::{OrphanQueue, OrphanQueueImpl, Wait};
+
+mod reap;
+use reap::Reaper;
+
+use crate::io::PollEvented;
+use crate::process::kill::Kill;
+use crate::process::SpawnedChild;
+use crate::signal::unix::{signal, Signal, SignalKind};
+
+use mio::event::Evented;
+use mio::unix::{EventedFd, UnixReady};
+use mio::{Poll as MioPoll, PollOpt, Ready, Token};
+use std::fmt;
+use std::future::Future;
+use std::io;
+use std::os::unix::io::{AsRawFd, RawFd};
+use std::pin::Pin;
+use std::process::ExitStatus;
+use std::task::Context;
+use std::task::Poll;
+
+impl Wait for std::process::Child {
+    fn id(&self) -> u32 {
+        self.id()
+    }
+
+    fn try_wait(&mut self) -> io::Result<Option<ExitStatus>> {
+        self.try_wait()
+    }
+}
+
+impl Kill for std::process::Child {
+    fn kill(&mut self) -> io::Result<()> {
+        self.kill()
+    }
+}
+
+lazy_static::lazy_static! {
+    static ref ORPHAN_QUEUE: OrphanQueueImpl<std::process::Child> = OrphanQueueImpl::new();
+}
+
+struct GlobalOrphanQueue;
+
+impl fmt::Debug for GlobalOrphanQueue {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        ORPHAN_QUEUE.fmt(fmt)
+    }
+}
+
+impl OrphanQueue<std::process::Child> for GlobalOrphanQueue {
+    fn push_orphan(&self, orphan: std::process::Child) {
+        ORPHAN_QUEUE.push_orphan(orphan)
+    }
+
+    fn reap_orphans(&self) {
+        ORPHAN_QUEUE.reap_orphans()
+    }
+}
+
+#[must_use = "futures do nothing unless polled"]
+pub(crate) struct Child {
+    inner: Reaper<std::process::Child, GlobalOrphanQueue, Signal>,
+}
+
+impl fmt::Debug for Child {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("Child")
+            .field("pid", &self.inner.id())
+            .finish()
+    }
+}
+
+pub(crate) fn spawn_child(cmd: &mut std::process::Command) -> io::Result<SpawnedChild> {
+    let mut child = cmd.spawn()?;
+    let stdin = stdio(child.stdin.take())?;
+    let stdout = stdio(child.stdout.take())?;
+    let stderr = stdio(child.stderr.take())?;
+
+    let signal = signal(SignalKind::child())?;
+
+    Ok(SpawnedChild {
+        child: Child {
+            inner: Reaper::new(child, GlobalOrphanQueue, signal),
+        },
+        stdin,
+        stdout,
+        stderr,
+    })
+}
+
+impl Child {
+    pub(crate) fn id(&self) -> u32 {
+        self.inner.id()
+    }
+}
+
+impl Kill for Child {
+    fn kill(&mut self) -> io::Result<()> {
+        self.inner.kill()
+    }
+}
+
+impl Future for Child {
+    type Output = io::Result<ExitStatus>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        Pin::new(&mut self.inner).poll(cx)
+    }
+}
+
+#[derive(Debug)]
+pub(crate) struct Fd<T> {
+    inner: T,
+}
+
+impl<T> io::Read for Fd<T>
+where
+    T: io::Read,
+{
+    fn read(&mut self, bytes: &mut [u8]) -> io::Result<usize> {
+        self.inner.read(bytes)
+    }
+}
+
+impl<T> io::Write for Fd<T>
+where
+    T: io::Write,
+{
+    fn write(&mut self, bytes: &[u8]) -> io::Result<usize> {
+        self.inner.write(bytes)
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        self.inner.flush()
+    }
+}
+
+impl<T> AsRawFd for Fd<T>
+where
+    T: AsRawFd,
+{
+    fn as_raw_fd(&self) -> RawFd {
+        self.inner.as_raw_fd()
+    }
+}
+
+impl<T> Evented for Fd<T>
+where
+    T: AsRawFd,
+{
+    fn register(
+        &self,
+        poll: &MioPoll,
+        token: Token,
+        interest: Ready,
+        opts: PollOpt,
+    ) -> io::Result<()> {
+        EventedFd(&self.as_raw_fd()).register(poll, token, interest | UnixReady::hup(), opts)
+    }
+
+    fn reregister(
+        &self,
+        poll: &MioPoll,
+        token: Token,
+        interest: Ready,
+        opts: PollOpt,
+    ) -> io::Result<()> {
+        EventedFd(&self.as_raw_fd()).reregister(poll, token, interest | UnixReady::hup(), opts)
+    }
+
+    fn deregister(&self, poll: &MioPoll) -> io::Result<()> {
+        EventedFd(&self.as_raw_fd()).deregister(poll)
+    }
+}
+
+pub(crate) type ChildStdin = PollEvented<Fd<std::process::ChildStdin>>;
+pub(crate) type ChildStdout = PollEvented<Fd<std::process::ChildStdout>>;
+pub(crate) type ChildStderr = PollEvented<Fd<std::process::ChildStderr>>;
+
+fn stdio<T>(option: Option<T>) -> io::Result<Option<PollEvented<Fd<T>>>>
+where
+    T: AsRawFd,
+{
+    let io = match option {
+        Some(io) => io,
+        None => return Ok(None),
+    };
+
+    // Set the fd to nonblocking before we pass it to the event loop
+    unsafe {
+        let fd = io.as_raw_fd();
+        let r = libc::fcntl(fd, libc::F_GETFL);
+        if r == -1 {
+            return Err(io::Error::last_os_error());
+        }
+        let r = libc::fcntl(fd, libc::F_SETFL, r | libc::O_NONBLOCK);
+        if r == -1 {
+            return Err(io::Error::last_os_error());
+        }
+    }
+    Ok(Some(PollEvented::new(Fd { inner: io })?))
+}
diff --git a/third_party/rust/tokio/src/process/unix/orphan.rs b/third_party/rust/tokio/src/process/unix/orphan.rs
new file mode 100644
index 0000000000..6c449a9093
--- /dev/null
+++ b/third_party/rust/tokio/src/process/unix/orphan.rs
@@ -0,0 +1,191 @@
+use std::io;
+use std::process::ExitStatus;
+use std::sync::Mutex;
+
+/// An interface for waiting on a process to exit.
+pub(crate) trait Wait {
+    /// Get the identifier for this process or diagnostics.
+    fn id(&self) -> u32;
+    /// Try waiting for a process to exit in a non-blocking manner.
+    fn try_wait(&mut self) -> io::Result<Option<ExitStatus>>;
+}
+
+impl<T: Wait> Wait for &mut T {
+    fn id(&self) -> u32 {
+        (**self).id()
+    }
+
+    fn try_wait(&mut self) -> io::Result<Option<ExitStatus>> {
+        (**self).try_wait()
+    }
+}
+
+/// An interface for queueing up an orphaned process so that it can be reaped.
+pub(crate) trait OrphanQueue<T> {
+    /// Adds an orphan to the queue.
+    fn push_orphan(&self, orphan: T);
+    /// Attempts to reap every process in the queue, ignoring any errors and
+    /// enqueueing any orphans which have not yet exited.
+    fn reap_orphans(&self);
+}
+
+impl<T, O: OrphanQueue<T>> OrphanQueue<T> for &O {
+    fn push_orphan(&self, orphan: T) {
+        (**self).push_orphan(orphan);
+    }
+
+    fn reap_orphans(&self) {
+        (**self).reap_orphans()
+    }
+}
+
+/// An implementation of `OrphanQueue`.
+#[derive(Debug)]
+pub(crate) struct OrphanQueueImpl<T> {
+    queue: Mutex<Vec<T>>,
+}
+
+impl<T> OrphanQueueImpl<T> {
+    pub(crate) fn new() -> Self {
+        Self {
+            queue: Mutex::new(Vec::new()),
+        }
+    }
+
+    #[cfg(test)]
+    fn len(&self) -> usize {
+        self.queue.lock().unwrap().len()
+    }
+}
+
+impl<T: Wait> OrphanQueue<T> for OrphanQueueImpl<T> {
+    fn push_orphan(&self, orphan: T) {
+        self.queue.lock().unwrap().push(orphan)
+    }
+
+    fn reap_orphans(&self) {
+        let mut queue = self.queue.lock().unwrap();
+        let queue = &mut *queue;
+
+        let mut i = 0;
+        while i < queue.len() {
+            match queue[i].try_wait() {
+                Ok(Some(_)) => {}
+                Err(_) => {
+                    // TODO: bubble up error some how. Is this an internal bug?
+                    // Shoudl we panic? Is it OK for this to be silently
+                    // dropped?
+                }
+                // Still not done yet
+                Ok(None) => {
+                    i += 1;
+                    continue;
+                }
+            }
+
+            queue.remove(i);
+        }
+    }
+}
+
+#[cfg(all(test, not(loom)))]
+mod test {
+    use super::Wait;
+    use super::{OrphanQueue, OrphanQueueImpl};
+    use std::cell::Cell;
+    use std::io;
+    use std::os::unix::process::ExitStatusExt;
+    use std::process::ExitStatus;
+    use std::rc::Rc;
+
+    struct MockWait {
+        total_waits: Rc<Cell<usize>>,
+        num_wait_until_status: usize,
+        return_err: bool,
+    }
+
+    impl MockWait {
+        fn new(num_wait_until_status: usize) -> Self {
+            Self {
+                total_waits: Rc::new(Cell::new(0)),
+                num_wait_until_status,
+                return_err: false,
+            }
+        }
+
+        fn with_err() -> Self {
+            Self {
+                total_waits: Rc::new(Cell::new(0)),
+                num_wait_until_status: 0,
+                return_err: true,
+            }
+        }
+    }
+
+    impl Wait for MockWait {
+        fn id(&self) -> u32 {
+            42
+        }
+
+        fn try_wait(&mut self) -> io::Result<Option<ExitStatus>> {
+            let waits = self.total_waits.get();
+
+            let ret = if self.num_wait_until_status == waits {
+                if self.return_err {
+                    Ok(Some(ExitStatus::from_raw(0)))
+                } else {
+                    Err(io::Error::new(io::ErrorKind::Other, "mock err"))
+                }
+            } else {
+                Ok(None)
+            };
+
+            self.total_waits.set(waits + 1);
+            ret
+        }
+    }
+
+    #[test]
+    fn drain_attempts_a_single_reap_of_all_queued_orphans() {
+        let first_orphan = MockWait::new(0);
+        let second_orphan = MockWait::new(1);
+        let third_orphan = MockWait::new(2);
+        let fourth_orphan = MockWait::with_err();
+
+        let first_waits = first_orphan.total_waits.clone();
+        let second_waits = second_orphan.total_waits.clone();
+        let third_waits = third_orphan.total_waits.clone();
+        let fourth_waits = fourth_orphan.total_waits.clone();
+
+        let orphanage = OrphanQueueImpl::new();
+        orphanage.push_orphan(first_orphan);
+        orphanage.push_orphan(third_orphan);
+        orphanage.push_orphan(second_orphan);
+        orphanage.push_orphan(fourth_orphan);
+
+        assert_eq!(orphanage.len(), 4);
+
+        orphanage.reap_orphans();
+        assert_eq!(orphanage.len(), 2);
+        assert_eq!(first_waits.get(), 1);
+        assert_eq!(second_waits.get(), 1);
+        assert_eq!(third_waits.get(), 1);
+        assert_eq!(fourth_waits.get(), 1);
+
+        orphanage.reap_orphans();
+        assert_eq!(orphanage.len(), 1);
+        assert_eq!(first_waits.get(), 1);
+        assert_eq!(second_waits.get(), 2);
+        assert_eq!(third_waits.get(), 2);
+        assert_eq!(fourth_waits.get(), 1);
+
+        orphanage.reap_orphans();
+        assert_eq!(orphanage.len(), 0);
+        assert_eq!(first_waits.get(), 1);
+        assert_eq!(second_waits.get(), 2);
+        assert_eq!(third_waits.get(), 3);
+        assert_eq!(fourth_waits.get(), 1);
+
+        orphanage.reap_orphans(); // Safe to reap when empty
+    }
+}
diff --git a/third_party/rust/tokio/src/process/unix/reap.rs b/third_party/rust/tokio/src/process/unix/reap.rs
new file mode 100644
index 0000000000..8963805afe
--- /dev/null
+++ b/third_party/rust/tokio/src/process/unix/reap.rs
@@ -0,0 +1,342 @@
+use crate::process::imp::orphan::{OrphanQueue, Wait};
+use crate::process::kill::Kill;
+use crate::signal::unix::Signal;
+
+use std::future::Future;
+use std::io;
+use std::ops::Deref;
+use std::pin::Pin;
+use std::process::ExitStatus;
+use std::task::Context;
+use std::task::Poll;
+
+/// Orchestrates between registering interest for receiving signals when a
+/// child process has exited, and attempting to poll for process completion.
+#[derive(Debug)]
+pub(crate) struct Reaper<W, Q, S>
+where
+    W: Wait + Unpin,
+    Q: OrphanQueue<W>,
+{
+    inner: Option<W>,
+    orphan_queue: Q,
+    signal: S,
+}
+
+// Work around removal of `futures_core` dependency
+pub(crate) trait Stream: Unpin {
+    fn poll_recv(&mut self, cx: &mut Context<'_>) -> Poll<Option<()>>;
+}
+
+impl Stream for Signal {
+    fn poll_recv(&mut self, cx: &mut Context<'_>) -> Poll<Option<()>> {
+        Signal::poll_recv(self, cx)
+    }
+}
+
+impl<W, Q, S> Deref for Reaper<W, Q, S>
+where
+    W: Wait + Unpin,
+    Q: OrphanQueue<W>,
+{
+    type Target = W;
+
+    fn deref(&self) -> &Self::Target {
+        self.inner()
+    }
+}
+
+impl<W, Q, S> Reaper<W, Q, S>
+where
+    W: Wait + Unpin,
+    Q: OrphanQueue<W>,
+{
+    pub(crate) fn new(inner: W, orphan_queue: Q, signal: S) -> Self {
+        Self {
+            inner: Some(inner),
+            orphan_queue,
+            signal,
+        }
+    }
+
+    fn inner(&self) -> &W {
+        self.inner.as_ref().expect("inner has gone away")
+    }
+
+    fn inner_mut(&mut self) -> &mut W {
+        self.inner.as_mut().expect("inner has gone away")
+    }
+}
+
+impl<W, Q, S> Future for Reaper<W, Q, S>
+where
+    W: Wait + Unpin,
+    Q: OrphanQueue<W> + Unpin,
+    S: Stream,
+{
+    type Output = io::Result<ExitStatus>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        loop {
+            // If the child hasn't exited yet, then it's our responsibility to
+            // ensure the current task gets notified when it might be able to
+            // make progress.
+            //
+            // As described in `spawn` above, we just indicate that we can
+            // next make progress once a SIGCHLD is received.
+            //
+            // However, we will register for a notification on the next signal
+            // BEFORE we poll the child. Otherwise it is possible that the child
+            // can exit and the signal can arrive after we last polled the child,
+            // but before we've registered for a notification on the next signal
+            // (this can cause a deadlock if there are no more spawned children
+            // which can generate a different signal for us). A side effect of
+            // pre-registering for signal notifications is that when the child
+            // exits, we will have already registered for an additional
+            // notification we don't need to consume. If another signal arrives,
+            // this future's task will be notified/woken up again. Since the
+            // futures model allows for spurious wake ups this extra wakeup
+            // should not cause significant issues with parent futures.
+            let registered_interest = self.signal.poll_recv(cx).is_pending();
+
+            self.orphan_queue.reap_orphans();
+            if let Some(status) = self.inner_mut().try_wait()? {
+                return Poll::Ready(Ok(status));
+            }
+
+            // If our attempt to poll for the next signal was not ready, then
+            // we've arranged for our task to get notified and we can bail out.
+            if registered_interest {
+                return Poll::Pending;
+            } else {
+                // Otherwise, if the signal stream delivered a signal to us, we
+                // won't get notified at the next signal, so we'll loop and try
+                // again.
+                continue;
+            }
+        }
+    }
+}
+
+impl<W, Q, S> Kill for Reaper<W, Q, S>
+where
+    W: Kill + Wait + Unpin,
+    Q: OrphanQueue<W>,
+{
+    fn kill(&mut self) -> io::Result<()> {
+        self.inner_mut().kill()
+    }
+}
+
+impl<W, Q, S> Drop for Reaper<W, Q, S>
+where
+    W: Wait + Unpin,
+    Q: OrphanQueue<W>,
+{
+    fn drop(&mut self) {
+        if let Ok(Some(_)) = self.inner_mut().try_wait() {
+            return;
+        }
+
+        let orphan = self.inner.take().unwrap();
+        self.orphan_queue.push_orphan(orphan);
+    }
+}
+
+#[cfg(all(test, not(loom)))]
+mod test {
+    use super::*;
+
+    use futures::future::FutureExt;
+    use std::cell::{Cell, RefCell};
+    use std::os::unix::process::ExitStatusExt;
+    use std::process::ExitStatus;
+    use std::task::Context;
+    use std::task::Poll;
+
+    #[derive(Debug)]
+    struct MockWait {
+        total_kills: usize,
+        total_waits: usize,
+        num_wait_until_status: usize,
+        status: ExitStatus,
+    }
+
+    impl MockWait {
+        fn new(status: ExitStatus, num_wait_until_status: usize) -> Self {
+            Self {
+                total_kills: 0,
+                total_waits: 0,
+                num_wait_until_status,
+                status,
+            }
+        }
+    }
+
+    impl Wait for MockWait {
+        fn id(&self) -> u32 {
+            0
+        }
+
+        fn try_wait(&mut self) -> io::Result<Option<ExitStatus>> {
+            let ret = if self.num_wait_until_status == self.total_waits {
+                Some(self.status)
+            } else {
+                None
+            };
+
+            self.total_waits += 1;
+            Ok(ret)
+        }
+    }
+
+    impl Kill for MockWait {
+        fn kill(&mut self) -> io::Result<()> {
+            self.total_kills += 1;
+            Ok(())
+        }
+    }
+
+    struct MockStream {
+        total_polls: usize,
+        values: Vec<Option<()>>,
+    }
+
+    impl MockStream {
+        fn new(values: Vec<Option<()>>) -> Self {
+            Self {
+                total_polls: 0,
+                values,
+            }
+        }
+    }
+
+    impl Stream for MockStream {
+        fn poll_recv(&mut self, _cx: &mut Context<'_>) -> Poll<Option<()>> {
+            self.total_polls += 1;
+            match self.values.remove(0) {
+                Some(()) => Poll::Ready(Some(())),
+                None => Poll::Pending,
+            }
+        }
+    }
+
+    struct MockQueue<W> {
+        all_enqueued: RefCell<Vec<W>>,
+        total_reaps: Cell<usize>,
+    }
+
+    impl<W> MockQueue<W> {
+        fn new() -> Self {
+            Self {
+                all_enqueued: RefCell::new(Vec::new()),
+                total_reaps: Cell::new(0),
+            }
+        }
+    }
+
+    impl<W: Wait> OrphanQueue<W> for MockQueue<W> {
+        fn push_orphan(&self, orphan: W) {
+            self.all_enqueued.borrow_mut().push(orphan);
+        }
+
+        fn reap_orphans(&self) {
+            self.total_reaps.set(self.total_reaps.get() + 1);
+        }
+    }
+
+    #[test]
+    fn reaper() {
+        let exit = ExitStatus::from_raw(0);
+        let mock = MockWait::new(exit, 3);
+        let mut grim = Reaper::new(
+            mock,
+            MockQueue::new(),
+            MockStream::new(vec![None, Some(()), None, None, None]),
+        );
+
+        let waker = futures::task::noop_waker();
+        let mut context = Context::from_waker(&waker);
+
+        // Not yet exited, interest registered
+        assert!(grim.poll_unpin(&mut context).is_pending());
+        assert_eq!(1, grim.signal.total_polls);
+        assert_eq!(1, grim.total_waits);
+        assert_eq!(1, grim.orphan_queue.total_reaps.get());
+        assert!(grim.orphan_queue.all_enqueued.borrow().is_empty());
+
+        // Not yet exited, couldn't register interest the first time
+        // but managed to register interest the second time around
+        assert!(grim.poll_unpin(&mut context).is_pending());
+        assert_eq!(3, grim.signal.total_polls);
+        assert_eq!(3, grim.total_waits);
+        assert_eq!(3, grim.orphan_queue.total_reaps.get());
+        assert!(grim.orphan_queue.all_enqueued.borrow().is_empty());
+
+        // Exited
+        if let Poll::Ready(r) = grim.poll_unpin(&mut context) {
+            assert!(r.is_ok());
+            let exit_code = r.unwrap();
+            assert_eq!(exit_code, exit);
+        } else {
+            unreachable!();
+        }
+        assert_eq!(4, grim.signal.total_polls);
+        assert_eq!(4, grim.total_waits);
+        assert_eq!(4, grim.orphan_queue.total_reaps.get());
+        assert!(grim.orphan_queue.all_enqueued.borrow().is_empty());
+    }
+
+    #[test]
+    fn kill() {
+        let exit = ExitStatus::from_raw(0);
+        let mut grim = Reaper::new(
+            MockWait::new(exit, 0),
+            MockQueue::new(),
+            MockStream::new(vec![None]),
+        );
+
+        grim.kill().unwrap();
+        assert_eq!(1, grim.total_kills);
+        assert_eq!(0, grim.orphan_queue.total_reaps.get());
+        assert!(grim.orphan_queue.all_enqueued.borrow().is_empty());
+    }
+
+    #[test]
+    fn drop_reaps_if_possible() {
+        let exit = ExitStatus::from_raw(0);
+        let mut mock = MockWait::new(exit, 0);
+
+        {
+            let queue = MockQueue::new();
+
+            let grim = Reaper::new(&mut mock, &queue, MockStream::new(vec![]));
+
+            drop(grim);
+
+            assert_eq!(0, queue.total_reaps.get());
+            assert!(queue.all_enqueued.borrow().is_empty());
+        }
+
+        assert_eq!(1, mock.total_waits);
+        assert_eq!(0, mock.total_kills);
+    }
+
+    #[test]
+    fn drop_enqueues_orphan_if_wait_fails() {
+        let exit = ExitStatus::from_raw(0);
+        let mut mock = MockWait::new(exit, 2);
+
+        {
+            let queue = MockQueue::<&mut MockWait>::new();
+            let grim = Reaper::new(&mut mock, &queue, MockStream::new(vec![]));
+            drop(grim);
+
+            assert_eq!(0, queue.total_reaps.get());
+            assert_eq!(1, queue.all_enqueued.borrow().len());
+        }
+
+        assert_eq!(1, mock.total_waits);
+        assert_eq!(0, mock.total_kills);
+    }
+}
diff --git a/third_party/rust/tokio/src/process/windows.rs b/third_party/rust/tokio/src/process/windows.rs
new file mode 100644
index 0000000000..cbe2fa7596
--- /dev/null
+++ b/third_party/rust/tokio/src/process/windows.rs
@@ -0,0 +1,191 @@
+//! Windows asynchronous process handling.
+//!
+//! Like with Unix we don't actually have a way of registering a process with an
+//! IOCP object. As a result we similarly need another mechanism for getting a
+//! signal when a process has exited. For now this is implemented with the
+//! `RegisterWaitForSingleObject` function in the kernel32.dll.
+//!
+//! This strategy is the same that libuv takes and essentially just queues up a
+//! wait for the process in a kernel32-specific thread pool. Once the object is
+//! notified (e.g. the process exits) then we have a callback that basically
+//! just completes a `Oneshot`.
+//!
+//! The `poll_exit` implementation will attempt to wait for the process in a
+//! nonblocking fashion, but failing that it'll fire off a
+//! `RegisterWaitForSingleObject` and then wait on the other end of the oneshot
+//! from then on out.
+
+use crate::io::PollEvented;
+use crate::process::kill::Kill;
+use crate::process::SpawnedChild;
+use crate::sync::oneshot;
+
+use mio_named_pipes::NamedPipe;
+use std::fmt;
+use std::future::Future;
+use std::io;
+use std::os::windows::prelude::*;
+use std::os::windows::process::ExitStatusExt;
+use std::pin::Pin;
+use std::process::{Child as StdChild, Command as StdCommand, ExitStatus};
+use std::ptr;
+use std::task::Context;
+use std::task::Poll;
+use winapi::shared::minwindef::FALSE;
+use winapi::shared::winerror::WAIT_TIMEOUT;
+use winapi::um::handleapi::INVALID_HANDLE_VALUE;
+use winapi::um::processthreadsapi::GetExitCodeProcess;
+use winapi::um::synchapi::WaitForSingleObject;
+use winapi::um::threadpoollegacyapiset::UnregisterWaitEx;
+use winapi::um::winbase::{RegisterWaitForSingleObject, INFINITE, WAIT_OBJECT_0};
+use winapi::um::winnt::{BOOLEAN, HANDLE, PVOID, WT_EXECUTEINWAITTHREAD, WT_EXECUTEONLYONCE};
+
+#[must_use = "futures do nothing unless polled"]
+pub(crate) struct Child {
+    child: StdChild,
+    waiting: Option<Waiting>,
+}
+
+impl fmt::Debug for Child {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("Child")
+            .field("pid", &self.id())
+            .field("child", &self.child)
+            .field("waiting", &"..")
+            .finish()
+    }
+}
+
+struct Waiting {
+    rx: oneshot::Receiver<()>,
+    wait_object: HANDLE,
+    tx: *mut Option<oneshot::Sender<()>>,
+}
+
+unsafe impl Sync for Waiting {}
+unsafe impl Send for Waiting {}
+
+pub(crate) fn spawn_child(cmd: &mut StdCommand) -> io::Result<SpawnedChild> {
+    let mut child = cmd.spawn()?;
+    let stdin = stdio(child.stdin.take());
+    let stdout = stdio(child.stdout.take());
+    let stderr = stdio(child.stderr.take());
+
+    Ok(SpawnedChild {
+        child: Child {
+            child,
+            waiting: None,
+        },
+        stdin,
+        stdout,
+        stderr,
+    })
+}
+
+impl Child {
+    pub(crate) fn id(&self) -> u32 {
+        self.child.id()
+    }
+}
+
+impl Kill for Child {
+    fn kill(&mut self) -> io::Result<()> {
+        self.child.kill()
+    }
+}
+
+impl Future for Child {
+    type Output = io::Result<ExitStatus>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let inner = Pin::get_mut(self);
+        loop {
+            if let Some(ref mut w) = inner.waiting {
+                match Pin::new(&mut w.rx).poll(cx) {
+                    Poll::Ready(Ok(())) => {}
+                    Poll::Ready(Err(_)) => panic!("should not be canceled"),
+                    Poll::Pending => return Poll::Pending,
+                }
+                let status = try_wait(&inner.child)?.expect("not ready yet");
+                return Poll::Ready(Ok(status));
+            }
+
+            if let Some(e) = try_wait(&inner.child)? {
+                return Poll::Ready(Ok(e));
+            }
+            let (tx, rx) = oneshot::channel();
+            let ptr = Box::into_raw(Box::new(Some(tx)));
+            let mut wait_object = ptr::null_mut();
+            let rc = unsafe {
+                RegisterWaitForSingleObject(
+                    &mut wait_object,
+                    inner.child.as_raw_handle(),
+                    Some(callback),
+                    ptr as *mut _,
+                    INFINITE,
+                    WT_EXECUTEINWAITTHREAD | WT_EXECUTEONLYONCE,
+                )
+            };
+            if rc == 0 {
+                let err = io::Error::last_os_error();
+                drop(unsafe { Box::from_raw(ptr) });
+                return Poll::Ready(Err(err));
+            }
+            inner.waiting = Some(Waiting {
+                rx,
+                wait_object,
+                tx: ptr,
+            });
+        }
+    }
+}
+
+impl Drop for Waiting {
+    fn drop(&mut self) {
+        unsafe {
+            let rc = UnregisterWaitEx(self.wait_object, INVALID_HANDLE_VALUE);
+            if rc == 0 {
+                panic!("failed to unregister: {}", io::Error::last_os_error());
+            }
+            drop(Box::from_raw(self.tx));
+        }
+    }
+}
+
+unsafe extern "system" fn callback(ptr: PVOID, _timer_fired: BOOLEAN) {
+    let complete = &mut *(ptr as *mut Option<oneshot::Sender<()>>);
+    let _ = complete.take().unwrap().send(());
+}
+
+pub(crate) fn try_wait(child: &StdChild) -> io::Result<Option<ExitStatus>> {
+    unsafe {
+        match WaitForSingleObject(child.as_raw_handle(), 0) {
+            WAIT_OBJECT_0 => {}
+            WAIT_TIMEOUT => return Ok(None),
+            _ => return Err(io::Error::last_os_error()),
+        }
+        let mut status = 0;
+        let rc = GetExitCodeProcess(child.as_raw_handle(), &mut status);
+        if rc == FALSE {
+            Err(io::Error::last_os_error())
+        } else {
+            Ok(Some(ExitStatus::from_raw(status)))
+        }
+    }
+}
+
+pub(crate) type ChildStdin = PollEvented<NamedPipe>;
+pub(crate) type ChildStdout = PollEvented<NamedPipe>;
+pub(crate) type ChildStderr = PollEvented<NamedPipe>;
+
+fn stdio<T>(option: Option<T>) -> Option<PollEvented<NamedPipe>>
+where
+    T: IntoRawHandle,
+{
+    let io = match option {
+        Some(io) => io,
+        None => return None,
+    };
+    let pipe = unsafe { NamedPipe::from_raw_handle(io.into_raw_handle()) };
+    PollEvented::new(pipe).ok()
+}
diff --git a/third_party/rust/tokio/src/runtime/basic_scheduler.rs b/third_party/rust/tokio/src/runtime/basic_scheduler.rs
new file mode 100644
index 0000000000..301554280f
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/basic_scheduler.rs
@@ -0,0 +1,326 @@
+use crate::park::{Park, Unpark};
+use crate::runtime;
+use crate::runtime::task::{self, JoinHandle, Schedule, Task};
+use crate::util::linked_list::LinkedList;
+use crate::util::{waker_ref, Wake};
+
+use std::cell::RefCell;
+use std::collections::VecDeque;
+use std::fmt;
+use std::future::Future;
+use std::sync::{Arc, Mutex};
+use std::task::Poll::Ready;
+use std::time::Duration;
+
+/// Executes tasks on the current thread
+pub(crate) struct BasicScheduler<P>
+where
+    P: Park,
+{
+    /// Scheduler run queue
+    ///
+    /// When the scheduler is executed, the queue is removed from `self` and
+    /// moved into `Context`.
+    ///
+    /// This indirection is to allow `BasicScheduler` to be `Send`.
+    tasks: Option<Tasks>,
+
+    /// Sendable task spawner
+    spawner: Spawner,
+
+    /// Current tick
+    tick: u8,
+
+    /// Thread park handle
+    park: P,
+}
+
+#[derive(Clone)]
+pub(crate) struct Spawner {
+    shared: Arc<Shared>,
+}
+
+struct Tasks {
+    /// Collection of all active tasks spawned onto this executor.
+    owned: LinkedList<Task<Arc<Shared>>>,
+
+    /// Local run queue.
+    ///
+    /// Tasks notified from the current thread are pushed into this queue.
+    queue: VecDeque<task::Notified<Arc<Shared>>>,
+}
+
+/// Scheduler state shared between threads.
+struct Shared {
+    /// Remote run queue
+    queue: Mutex<VecDeque<task::Notified<Arc<Shared>>>>,
+
+    /// Unpark the blocked thread
+    unpark: Box<dyn Unpark>,
+}
+
+/// Thread-local context
+struct Context {
+    /// Shared scheduler state
+    shared: Arc<Shared>,
+
+    /// Local queue
+    tasks: RefCell<Tasks>,
+}
+
+/// Initial queue capacity
+const INITIAL_CAPACITY: usize = 64;
+
+/// Max number of tasks to poll per tick.
+const MAX_TASKS_PER_TICK: usize = 61;
+
+/// How often ot check the remote queue first
+const REMOTE_FIRST_INTERVAL: u8 = 31;
+
+// Tracks the current BasicScheduler
+scoped_thread_local!(static CURRENT: Context);
+
+impl<P> BasicScheduler<P>
+where
+    P: Park,
+{
+    pub(crate) fn new(park: P) -> BasicScheduler<P> {
+        let unpark = Box::new(park.unpark());
+
+        BasicScheduler {
+            tasks: Some(Tasks {
+                owned: LinkedList::new(),
+                queue: VecDeque::with_capacity(INITIAL_CAPACITY),
+            }),
+            spawner: Spawner {
+                shared: Arc::new(Shared {
+                    queue: Mutex::new(VecDeque::with_capacity(INITIAL_CAPACITY)),
+                    unpark: unpark as Box<dyn Unpark>,
+                }),
+            },
+            tick: 0,
+            park,
+        }
+    }
+
+    pub(crate) fn spawner(&self) -> &Spawner {
+        &self.spawner
+    }
+
+    /// Spawns a future onto the thread pool
+    pub(crate) fn spawn<F>(&self, future: F) -> JoinHandle<F::Output>
+    where
+        F: Future + Send + 'static,
+        F::Output: Send + 'static,
+    {
+        self.spawner.spawn(future)
+    }
+
+    pub(crate) fn block_on<F>(&mut self, future: F) -> F::Output
+    where
+        F: Future,
+    {
+        enter(self, |scheduler, context| {
+            let _enter = runtime::enter();
+            let waker = waker_ref(&scheduler.spawner.shared);
+            let mut cx = std::task::Context::from_waker(&waker);
+
+            pin!(future);
+
+            'outer: loop {
+                if let Ready(v) = crate::coop::budget(|| future.as_mut().poll(&mut cx)) {
+                    return v;
+                }
+
+                for _ in 0..MAX_TASKS_PER_TICK {
+                    // Get and increment the current tick
+                    let tick = scheduler.tick;
+                    scheduler.tick = scheduler.tick.wrapping_add(1);
+
+                    let next = if tick % REMOTE_FIRST_INTERVAL == 0 {
+                        scheduler
+                            .spawner
+                            .pop()
+                            .or_else(|| context.tasks.borrow_mut().queue.pop_front())
+                    } else {
+                        context
+                            .tasks
+                            .borrow_mut()
+                            .queue
+                            .pop_front()
+                            .or_else(|| scheduler.spawner.pop())
+                    };
+
+                    match next {
+                        Some(task) => crate::coop::budget(|| task.run()),
+                        None => {
+                            // Park until the thread is signaled
+                            scheduler.park.park().ok().expect("failed to park");
+
+                            // Try polling the `block_on` future next
+                            continue 'outer;
+                        }
+                    }
+                }
+
+                // Yield to the park, this drives the timer and pulls any pending
+                // I/O events.
+                scheduler
+                    .park
+                    .park_timeout(Duration::from_millis(0))
+                    .ok()
+                    .expect("failed to park");
+            }
+        })
+    }
+}
+
+/// Enter the scheduler context. This sets the queue and other necessary
+/// scheduler state in the thread-local
+fn enter<F, R, P>(scheduler: &mut BasicScheduler<P>, f: F) -> R
+where
+    F: FnOnce(&mut BasicScheduler<P>, &Context) -> R,
+    P: Park,
+{
+    // Ensures the run queue is placed back in the `BasicScheduler` instance
+    // once `block_on` returns.`
+    struct Guard<'a, P: Park> {
+        context: Option<Context>,
+        scheduler: &'a mut BasicScheduler<P>,
+    }
+
+    impl<P: Park> Drop for Guard<'_, P> {
+        fn drop(&mut self) {
+            let Context { tasks, .. } = self.context.take().expect("context missing");
+            self.scheduler.tasks = Some(tasks.into_inner());
+        }
+    }
+
+    // Remove `tasks` from `self` and place it in a `Context`.
+    let tasks = scheduler.tasks.take().expect("invalid state");
+
+    let guard = Guard {
+        context: Some(Context {
+            shared: scheduler.spawner.shared.clone(),
+            tasks: RefCell::new(tasks),
+        }),
+        scheduler,
+    };
+
+    let context = guard.context.as_ref().unwrap();
+    let scheduler = &mut *guard.scheduler;
+
+    CURRENT.set(context, || f(scheduler, context))
+}
+
+impl<P> Drop for BasicScheduler<P>
+where
+    P: Park,
+{
+    fn drop(&mut self) {
+        enter(self, |scheduler, context| {
+            // Loop required here to ensure borrow is dropped between iterations
+            #[allow(clippy::while_let_loop)]
+            loop {
+                let task = match context.tasks.borrow_mut().owned.pop_back() {
+                    Some(task) => task,
+                    None => break,
+                };
+
+                task.shutdown();
+            }
+
+            // Drain local queue
+            for task in context.tasks.borrow_mut().queue.drain(..) {
+                task.shutdown();
+            }
+
+            // Drain remote queue
+            for task in scheduler.spawner.shared.queue.lock().unwrap().drain(..) {
+                task.shutdown();
+            }
+
+            assert!(context.tasks.borrow().owned.is_empty());
+        });
+    }
+}
+
+impl<P: Park> fmt::Debug for BasicScheduler<P> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("BasicScheduler").finish()
+    }
+}
+
+// ===== impl Spawner =====
+
+impl Spawner {
+    /// Spawns a future onto the thread pool
+    pub(crate) fn spawn<F>(&self, future: F) -> JoinHandle<F::Output>
+    where
+        F: Future + Send + 'static,
+        F::Output: Send + 'static,
+    {
+        let (task, handle) = task::joinable(future);
+        self.shared.schedule(task);
+        handle
+    }
+
+    fn pop(&self) -> Option<task::Notified<Arc<Shared>>> {
+        self.shared.queue.lock().unwrap().pop_front()
+    }
+}
+
+impl fmt::Debug for Spawner {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("Spawner").finish()
+    }
+}
+
+// ===== impl Shared =====
+
+impl Schedule for Arc<Shared> {
+    fn bind(task: Task<Self>) -> Arc<Shared> {
+        CURRENT.with(|maybe_cx| {
+            let cx = maybe_cx.expect("scheduler context missing");
+            cx.tasks.borrow_mut().owned.push_front(task);
+            cx.shared.clone()
+        })
+    }
+
+    fn release(&self, task: &Task<Self>) -> Option<Task<Self>> {
+        use std::ptr::NonNull;
+
+        CURRENT.with(|maybe_cx| {
+            let cx = maybe_cx.expect("scheduler context missing");
+
+            // safety: the task is inserted in the list in `bind`.
+            unsafe {
+                let ptr = NonNull::from(task.header());
+                cx.tasks.borrow_mut().owned.remove(ptr)
+            }
+        })
+    }
+
+    fn schedule(&self, task: task::Notified<Self>) {
+        CURRENT.with(|maybe_cx| match maybe_cx {
+            Some(cx) if Arc::ptr_eq(self, &cx.shared) => {
+                cx.tasks.borrow_mut().queue.push_back(task);
+            }
+            _ => {
+                self.queue.lock().unwrap().push_back(task);
+                self.unpark.unpark();
+            }
+        });
+    }
+}
+
+impl Wake for Shared {
+    fn wake(self: Arc<Self>) {
+        Wake::wake_by_ref(&self)
+    }
+
+    /// Wake by reference
+    fn wake_by_ref(arc_self: &Arc<Self>) {
+        arc_self.unpark.unpark();
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/blocking/mod.rs b/third_party/rust/tokio/src/runtime/blocking/mod.rs
new file mode 100644
index 0000000000..5c808335cc
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/blocking/mod.rs
@@ -0,0 +1,43 @@
+//! Abstracts out the APIs necessary to `Runtime` for integrating the blocking
+//! pool. When the `blocking` feature flag is **not** enabled, these APIs are
+//! shells. This isolates the complexity of dealing with conditional
+//! compilation.
+
+cfg_blocking_impl! {
+    mod pool;
+    pub(crate) use pool::{spawn_blocking, try_spawn_blocking, BlockingPool, Spawner};
+
+    mod schedule;
+    mod shutdown;
+    mod task;
+
+    use crate::runtime::Builder;
+
+    pub(crate) fn create_blocking_pool(builder: &Builder, thread_cap: usize) -> BlockingPool {
+        BlockingPool::new(builder, thread_cap)
+
+    }
+}
+
+cfg_not_blocking_impl! {
+    use crate::runtime::Builder;
+    use std::time::Duration;
+
+    #[derive(Debug, Clone)]
+    pub(crate) struct BlockingPool {}
+
+    pub(crate) use BlockingPool as Spawner;
+
+    pub(crate) fn create_blocking_pool(_builder: &Builder, _thread_cap: usize) -> BlockingPool {
+        BlockingPool {}
+    }
+
+    impl BlockingPool {
+        pub(crate) fn spawner(&self) -> &BlockingPool {
+            self
+        }
+
+        pub(crate) fn shutdown(&mut self, _duration: Option<Duration>) {
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/blocking/pool.rs b/third_party/rust/tokio/src/runtime/blocking/pool.rs
new file mode 100644
index 0000000000..a3b208d171
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/blocking/pool.rs
@@ -0,0 +1,307 @@
+//! Thread pool for blocking operations
+
+use crate::loom::sync::{Arc, Condvar, Mutex};
+use crate::loom::thread;
+use crate::runtime::blocking::schedule::NoopSchedule;
+use crate::runtime::blocking::shutdown;
+use crate::runtime::blocking::task::BlockingTask;
+use crate::runtime::task::{self, JoinHandle};
+use crate::runtime::{Builder, Callback, Handle};
+
+use std::collections::VecDeque;
+use std::fmt;
+use std::time::Duration;
+
+pub(crate) struct BlockingPool {
+    spawner: Spawner,
+    shutdown_rx: shutdown::Receiver,
+}
+
+#[derive(Clone)]
+pub(crate) struct Spawner {
+    inner: Arc<Inner>,
+}
+
+struct Inner {
+    /// State shared between worker threads
+    shared: Mutex<Shared>,
+
+    /// Pool threads wait on this.
+    condvar: Condvar,
+
+    /// Spawned threads use this name
+    thread_name: String,
+
+    /// Spawned thread stack size
+    stack_size: Option<usize>,
+
+    /// Call after a thread starts
+    after_start: Option<Callback>,
+
+    /// Call before a thread stops
+    before_stop: Option<Callback>,
+
+    thread_cap: usize,
+}
+
+struct Shared {
+    queue: VecDeque<Task>,
+    num_th: usize,
+    num_idle: u32,
+    num_notify: u32,
+    shutdown: bool,
+    shutdown_tx: Option<shutdown::Sender>,
+}
+
+type Task = task::Notified<NoopSchedule>;
+
+const KEEP_ALIVE: Duration = Duration::from_secs(10);
+
+/// Run the provided function on an executor dedicated to blocking operations.
+pub(crate) fn spawn_blocking<F, R>(func: F) -> JoinHandle<R>
+where
+    F: FnOnce() -> R + Send + 'static,
+{
+    let rt = Handle::current();
+
+    let (task, handle) = task::joinable(BlockingTask::new(func));
+    let _ = rt.blocking_spawner.spawn(task, &rt);
+    handle
+}
+
+#[allow(dead_code)]
+pub(crate) fn try_spawn_blocking<F, R>(func: F) -> Result<(), ()>
+where
+    F: FnOnce() -> R + Send + 'static,
+{
+    let rt = Handle::current();
+
+    let (task, _handle) = task::joinable(BlockingTask::new(func));
+    rt.blocking_spawner.spawn(task, &rt)
+}
+
+// ===== impl BlockingPool =====
+
+impl BlockingPool {
+    pub(crate) fn new(builder: &Builder, thread_cap: usize) -> BlockingPool {
+        let (shutdown_tx, shutdown_rx) = shutdown::channel();
+
+        BlockingPool {
+            spawner: Spawner {
+                inner: Arc::new(Inner {
+                    shared: Mutex::new(Shared {
+                        queue: VecDeque::new(),
+                        num_th: 0,
+                        num_idle: 0,
+                        num_notify: 0,
+                        shutdown: false,
+                        shutdown_tx: Some(shutdown_tx),
+                    }),
+                    condvar: Condvar::new(),
+                    thread_name: builder.thread_name.clone(),
+                    stack_size: builder.thread_stack_size,
+                    after_start: builder.after_start.clone(),
+                    before_stop: builder.before_stop.clone(),
+                    thread_cap,
+                }),
+            },
+            shutdown_rx,
+        }
+    }
+
+    pub(crate) fn spawner(&self) -> &Spawner {
+        &self.spawner
+    }
+
+    pub(crate) fn shutdown(&mut self, timeout: Option<Duration>) {
+        let mut shared = self.spawner.inner.shared.lock().unwrap();
+
+        // The function can be called multiple times. First, by explicitly
+        // calling `shutdown` then by the drop handler calling `shutdown`. This
+        // prevents shutting down twice.
+        if shared.shutdown {
+            return;
+        }
+
+        shared.shutdown = true;
+        shared.shutdown_tx = None;
+        self.spawner.inner.condvar.notify_all();
+
+        drop(shared);
+
+        self.shutdown_rx.wait(timeout);
+    }
+}
+
+impl Drop for BlockingPool {
+    fn drop(&mut self) {
+        self.shutdown(None);
+    }
+}
+
+impl fmt::Debug for BlockingPool {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("BlockingPool").finish()
+    }
+}
+
+// ===== impl Spawner =====
+
+impl Spawner {
+    fn spawn(&self, task: Task, rt: &Handle) -> Result<(), ()> {
+        let shutdown_tx = {
+            let mut shared = self.inner.shared.lock().unwrap();
+
+            if shared.shutdown {
+                // Shutdown the task
+                task.shutdown();
+
+                // no need to even push this task; it would never get picked up
+                return Err(());
+            }
+
+            shared.queue.push_back(task);
+
+            if shared.num_idle == 0 {
+                // No threads are able to process the task.
+
+                if shared.num_th == self.inner.thread_cap {
+                    // At max number of threads
+                    None
+                } else {
+                    shared.num_th += 1;
+                    assert!(shared.shutdown_tx.is_some());
+                    shared.shutdown_tx.clone()
+                }
+            } else {
+                // Notify an idle worker thread. The notification counter
+                // is used to count the needed amount of notifications
+                // exactly. Thread libraries may generate spurious
+                // wakeups, this counter is used to keep us in a
+                // consistent state.
+                shared.num_idle -= 1;
+                shared.num_notify += 1;
+                self.inner.condvar.notify_one();
+                None
+            }
+        };
+
+        if let Some(shutdown_tx) = shutdown_tx {
+            self.spawn_thread(shutdown_tx, rt);
+        }
+
+        Ok(())
+    }
+
+    fn spawn_thread(&self, shutdown_tx: shutdown::Sender, rt: &Handle) {
+        let mut builder = thread::Builder::new().name(self.inner.thread_name.clone());
+
+        if let Some(stack_size) = self.inner.stack_size {
+            builder = builder.stack_size(stack_size);
+        }
+
+        let rt = rt.clone();
+
+        builder
+            .spawn(move || {
+                // Only the reference should be moved into the closure
+                let rt = &rt;
+                rt.enter(move || {
+                    rt.blocking_spawner.inner.run();
+                    drop(shutdown_tx);
+                })
+            })
+            .unwrap();
+    }
+}
+
+impl Inner {
+    fn run(&self) {
+        if let Some(f) = &self.after_start {
+            f()
+        }
+
+        let mut shared = self.shared.lock().unwrap();
+
+        'main: loop {
+            // BUSY
+            while let Some(task) = shared.queue.pop_front() {
+                drop(shared);
+                task.run();
+
+                shared = self.shared.lock().unwrap();
+            }
+
+            // IDLE
+            shared.num_idle += 1;
+
+            while !shared.shutdown {
+                let lock_result = self.condvar.wait_timeout(shared, KEEP_ALIVE).unwrap();
+
+                shared = lock_result.0;
+                let timeout_result = lock_result.1;
+
+                if shared.num_notify != 0 {
+                    // We have received a legitimate wakeup,
+                    // acknowledge it by decrementing the counter
+                    // and transition to the BUSY state.
+                    shared.num_notify -= 1;
+                    break;
+                }
+
+                // Even if the condvar "timed out", if the pool is entering the
+                // shutdown phase, we want to perform the cleanup logic.
+                if !shared.shutdown && timeout_result.timed_out() {
+                    break 'main;
+                }
+
+                // Spurious wakeup detected, go back to sleep.
+            }
+
+            if shared.shutdown {
+                // Drain the queue
+                while let Some(task) = shared.queue.pop_front() {
+                    drop(shared);
+                    task.shutdown();
+
+                    shared = self.shared.lock().unwrap();
+                }
+
+                // Work was produced, and we "took" it (by decrementing num_notify).
+                // This means that num_idle was decremented once for our wakeup.
+                // But, since we are exiting, we need to "undo" that, as we'll stay idle.
+                shared.num_idle += 1;
+                // NOTE: Technically we should also do num_notify++ and notify again,
+                // but since we're shutting down anyway, that won't be necessary.
+                break;
+            }
+        }
+
+        // Thread exit
+        shared.num_th -= 1;
+
+        // num_idle should now be tracked exactly, panic
+        // with a descriptive message if it is not the
+        // case.
+        shared.num_idle = shared
+            .num_idle
+            .checked_sub(1)
+            .expect("num_idle underflowed on thread exit");
+
+        if shared.shutdown && shared.num_th == 0 {
+            self.condvar.notify_one();
+        }
+
+        drop(shared);
+
+        if let Some(f) = &self.before_stop {
+            f()
+        }
+    }
+}
+
+impl fmt::Debug for Spawner {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("blocking::Spawner").finish()
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/blocking/schedule.rs b/third_party/rust/tokio/src/runtime/blocking/schedule.rs
new file mode 100644
index 0000000000..e10778d530
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/blocking/schedule.rs
@@ -0,0 +1,24 @@
+use crate::runtime::task::{self, Task};
+
+/// `task::Schedule` implementation that does nothing. This is unique to the
+/// blocking scheduler as tasks scheduled are not really futures but blocking
+/// operations.
+///
+/// We avoid storing the task by forgetting it in `bind` and re-materializing it
+/// in `release.
+pub(super) struct NoopSchedule;
+
+impl task::Schedule for NoopSchedule {
+    fn bind(_task: Task<Self>) -> NoopSchedule {
+        // Do nothing w/ the task
+        NoopSchedule
+    }
+
+    fn release(&self, _task: &Task<Self>) -> Option<Task<Self>> {
+        None
+    }
+
+    fn schedule(&self, _task: task::Notified<Self>) {
+        unreachable!();
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/blocking/shutdown.rs b/third_party/rust/tokio/src/runtime/blocking/shutdown.rs
new file mode 100644
index 0000000000..5ee8af0fbc
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/blocking/shutdown.rs
@@ -0,0 +1,58 @@
+//! A shutdown channel.
+//!
+//! Each worker holds the `Sender` half. When all the `Sender` halves are
+//! dropped, the `Receiver` receives a notification.
+
+use crate::loom::sync::Arc;
+use crate::sync::oneshot;
+
+use std::time::Duration;
+
+#[derive(Debug, Clone)]
+pub(super) struct Sender {
+    tx: Arc<oneshot::Sender<()>>,
+}
+
+#[derive(Debug)]
+pub(super) struct Receiver {
+    rx: oneshot::Receiver<()>,
+}
+
+pub(super) fn channel() -> (Sender, Receiver) {
+    let (tx, rx) = oneshot::channel();
+    let tx = Sender { tx: Arc::new(tx) };
+    let rx = Receiver { rx };
+
+    (tx, rx)
+}
+
+impl Receiver {
+    /// Blocks the current thread until all `Sender` handles drop.
+    ///
+    /// If `timeout` is `Some`, the thread is blocked for **at most** `timeout`
+    /// duration. If `timeout` is `None`, then the thread is blocked until the
+    /// shutdown signal is received.
+    pub(crate) fn wait(&mut self, timeout: Option<Duration>) {
+        use crate::runtime::enter::{enter, try_enter};
+
+        let mut e = if std::thread::panicking() {
+            match try_enter() {
+                Some(enter) => enter,
+                _ => return,
+            }
+        } else {
+            enter()
+        };
+
+        // The oneshot completes with an Err
+        //
+        // If blocking fails to wait, this indicates a problem parking the
+        // current thread (usually, shutting down a runtime stored in a
+        // thread-local).
+        if let Some(timeout) = timeout {
+            let _ = e.block_on_timeout(&mut self.rx, timeout);
+        } else {
+            let _ = e.block_on(&mut self.rx);
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/blocking/task.rs b/third_party/rust/tokio/src/runtime/blocking/task.rs
new file mode 100644
index 0000000000..f98b85494c
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/blocking/task.rs
@@ -0,0 +1,40 @@
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// Converts a function to a future that completes on poll
+pub(super) struct BlockingTask<T> {
+    func: Option<T>,
+}
+
+impl<T> BlockingTask<T> {
+    /// Initializes a new blocking task from the given function
+    pub(super) fn new(func: T) -> BlockingTask<T> {
+        BlockingTask { func: Some(func) }
+    }
+}
+
+impl<T, R> Future for BlockingTask<T>
+where
+    T: FnOnce() -> R,
+{
+    type Output = R;
+
+    fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<R> {
+        let me = unsafe { self.get_unchecked_mut() };
+        let func = me
+            .func
+            .take()
+            .expect("[internal exception] blocking task ran twice.");
+
+        // This is a little subtle:
+        // For convenience, we'd like _every_ call tokio ever makes to Task::poll() to be budgeted
+        // using coop. However, the way things are currently modeled, even running a blocking task
+        // currently goes through Task::poll(), and so is subject to budgeting. That isn't really
+        // what we want; a blocking task may itself want to run tasks (it might be a Worker!), so
+        // we want it to start without any budgeting.
+        crate::coop::stop();
+
+        Poll::Ready(func())
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/builder.rs b/third_party/rust/tokio/src/runtime/builder.rs
new file mode 100644
index 0000000000..cfde998251
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/builder.rs
@@ -0,0 +1,519 @@
+use crate::runtime::handle::Handle;
+use crate::runtime::shell::Shell;
+use crate::runtime::{blocking, io, time, Callback, Runtime, Spawner};
+
+use std::fmt;
+#[cfg(not(loom))]
+use std::sync::Arc;
+
+/// Builds Tokio Runtime with custom configuration values.
+///
+/// Methods can be chained in order to set the configuration values. The
+/// Runtime is constructed by calling [`build`].
+///
+/// New instances of `Builder` are obtained via [`Builder::new`].
+///
+/// See function level documentation for details on the various configuration
+/// settings.
+///
+/// [`build`]: #method.build
+/// [`Builder::new`]: #method.new
+///
+/// # Examples
+///
+/// ```
+/// use tokio::runtime::Builder;
+///
+/// fn main() {
+///     // build runtime
+///     let runtime = Builder::new()
+///         .threaded_scheduler()
+///         .core_threads(4)
+///         .thread_name("my-custom-name")
+///         .thread_stack_size(3 * 1024 * 1024)
+///         .build()
+///         .unwrap();
+///
+///     // use runtime ...
+/// }
+/// ```
+pub struct Builder {
+    /// The task execution model to use.
+    kind: Kind,
+
+    /// Whether or not to enable the I/O driver
+    enable_io: bool,
+
+    /// Whether or not to enable the time driver
+    enable_time: bool,
+
+    /// The number of worker threads, used by Runtime.
+    ///
+    /// Only used when not using the current-thread executor.
+    core_threads: Option<usize>,
+
+    /// Cap on thread usage.
+    max_threads: usize,
+
+    /// Name used for threads spawned by the runtime.
+    pub(super) thread_name: String,
+
+    /// Stack size used for threads spawned by the runtime.
+    pub(super) thread_stack_size: Option<usize>,
+
+    /// Callback to run after each thread starts.
+    pub(super) after_start: Option<Callback>,
+
+    /// To run before each worker thread stops
+    pub(super) before_stop: Option<Callback>,
+}
+
+#[derive(Debug, Clone, Copy)]
+enum Kind {
+    Shell,
+    #[cfg(feature = "rt-core")]
+    Basic,
+    #[cfg(feature = "rt-threaded")]
+    ThreadPool,
+}
+
+impl Builder {
+    /// Returns a new runtime builder initialized with default configuration
+    /// values.
+    ///
+    /// Configuration methods can be chained on the return value.
+    pub fn new() -> Builder {
+        Builder {
+            // No task execution by default
+            kind: Kind::Shell,
+
+            // I/O defaults to "off"
+            enable_io: false,
+
+            // Time defaults to "off"
+            enable_time: false,
+
+            // Default to lazy auto-detection (one thread per CPU core)
+            core_threads: None,
+
+            max_threads: 512,
+
+            // Default thread name
+            thread_name: "tokio-runtime-worker".into(),
+
+            // Do not set a stack size by default
+            thread_stack_size: None,
+
+            // No worker thread callbacks
+            after_start: None,
+            before_stop: None,
+        }
+    }
+
+    /// Enables both I/O and time drivers.
+    ///
+    /// Doing this is a shorthand for calling `enable_io` and `enable_time`
+    /// individually. If additional components are added to Tokio in the future,
+    /// `enable_all` will include these future components.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::runtime;
+    ///
+    /// let rt = runtime::Builder::new()
+    ///     .threaded_scheduler()
+    ///     .enable_all()
+    ///     .build()
+    ///     .unwrap();
+    /// ```
+    pub fn enable_all(&mut self) -> &mut Self {
+        #[cfg(feature = "io-driver")]
+        self.enable_io();
+        #[cfg(feature = "time")]
+        self.enable_time();
+
+        self
+    }
+
+    #[deprecated(note = "In future will be replaced by core_threads method")]
+    /// Sets the maximum number of worker threads for the `Runtime`'s thread pool.
+    ///
+    /// This must be a number between 1 and 32,768 though it is advised to keep
+    /// this value on the smaller side.
+    ///
+    /// The default value is the number of cores available to the system.
+    pub fn num_threads(&mut self, val: usize) -> &mut Self {
+        self.core_threads = Some(val);
+        self
+    }
+
+    /// Sets the core number of worker threads for the `Runtime`'s thread pool.
+    ///
+    /// This should be a number between 1 and 32,768 though it is advised to keep
+    /// this value on the smaller side.
+    ///
+    /// The default value is the number of cores available to the system.
+    ///
+    /// These threads will be always active and running.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::runtime;
+    ///
+    /// let rt = runtime::Builder::new()
+    ///     .threaded_scheduler()
+    ///     .core_threads(4)
+    ///     .build()
+    ///     .unwrap();
+    /// ```
+    pub fn core_threads(&mut self, val: usize) -> &mut Self {
+        assert_ne!(val, 0, "Core threads cannot be zero");
+        self.core_threads = Some(val);
+        self
+    }
+
+    /// Specifies limit for threads, spawned by the Runtime.
+    ///
+    /// This is number of threads to be used by Runtime, including `core_threads`
+    /// Having `max_threads` less than `core_threads` results in invalid configuration
+    /// when building multi-threaded `Runtime`, which would cause a panic.
+    ///
+    /// Similarly to the `core_threads`, this number should be between 1 and 32,768.
+    ///
+    /// The default value is 512.
+    ///
+    /// When multi-threaded runtime is not used, will act as limit on additional threads.
+    ///
+    /// Otherwise as `core_threads` are always active, it limits additional threads (e.g. for
+    /// blocking annotations) as `max_threads - core_threads`.
+    pub fn max_threads(&mut self, val: usize) -> &mut Self {
+        assert_ne!(val, 0, "Thread limit cannot be zero");
+        self.max_threads = val;
+        self
+    }
+
+    /// Sets name of threads spawned by the `Runtime`'s thread pool.
+    ///
+    /// The default name is "tokio-runtime-worker".
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use tokio::runtime;
+    ///
+    /// # pub fn main() {
+    /// let rt = runtime::Builder::new()
+    ///     .thread_name("my-pool")
+    ///     .build();
+    /// # }
+    /// ```
+    pub fn thread_name(&mut self, val: impl Into<String>) -> &mut Self {
+        self.thread_name = val.into();
+        self
+    }
+
+    /// Sets the stack size (in bytes) for worker threads.
+    ///
+    /// The actual stack size may be greater than this value if the platform
+    /// specifies minimal stack size.
+    ///
+    /// The default stack size for spawned threads is 2 MiB, though this
+    /// particular stack size is subject to change in the future.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use tokio::runtime;
+    ///
+    /// # pub fn main() {
+    /// let rt = runtime::Builder::new()
+    ///     .threaded_scheduler()
+    ///     .thread_stack_size(32 * 1024)
+    ///     .build();
+    /// # }
+    /// ```
+    pub fn thread_stack_size(&mut self, val: usize) -> &mut Self {
+        self.thread_stack_size = Some(val);
+        self
+    }
+
+    /// Executes function `f` after each thread is started but before it starts
+    /// doing work.
+    ///
+    /// This is intended for bookkeeping and monitoring use cases.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use tokio::runtime;
+    ///
+    /// # pub fn main() {
+    /// let runtime = runtime::Builder::new()
+    ///     .threaded_scheduler()
+    ///     .on_thread_start(|| {
+    ///         println!("thread started");
+    ///     })
+    ///     .build();
+    /// # }
+    /// ```
+    #[cfg(not(loom))]
+    pub fn on_thread_start<F>(&mut self, f: F) -> &mut Self
+    where
+        F: Fn() + Send + Sync + 'static,
+    {
+        self.after_start = Some(Arc::new(f));
+        self
+    }
+
+    /// Executes function `f` before each thread stops.
+    ///
+    /// This is intended for bookkeeping and monitoring use cases.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use tokio::runtime;
+    ///
+    /// # pub fn main() {
+    /// let runtime = runtime::Builder::new()
+    ///     .threaded_scheduler()
+    ///     .on_thread_stop(|| {
+    ///         println!("thread stopping");
+    ///     })
+    ///     .build();
+    /// # }
+    /// ```
+    #[cfg(not(loom))]
+    pub fn on_thread_stop<F>(&mut self, f: F) -> &mut Self
+    where
+        F: Fn() + Send + Sync + 'static,
+    {
+        self.before_stop = Some(Arc::new(f));
+        self
+    }
+
+    /// Creates the configured `Runtime`.
+    ///
+    /// The returned `ThreadPool` instance is ready to spawn tasks.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::runtime::Builder;
+    ///
+    /// let mut rt = Builder::new().build().unwrap();
+    ///
+    /// rt.block_on(async {
+    ///     println!("Hello from the Tokio runtime");
+    /// });
+    /// ```
+    pub fn build(&mut self) -> io::Result<Runtime> {
+        match self.kind {
+            Kind::Shell => self.build_shell_runtime(),
+            #[cfg(feature = "rt-core")]
+            Kind::Basic => self.build_basic_runtime(),
+            #[cfg(feature = "rt-threaded")]
+            Kind::ThreadPool => self.build_threaded_runtime(),
+        }
+    }
+
+    fn build_shell_runtime(&mut self) -> io::Result<Runtime> {
+        use crate::runtime::Kind;
+
+        let clock = time::create_clock();
+
+        // Create I/O driver
+        let (io_driver, io_handle) = io::create_driver(self.enable_io)?;
+        let (driver, time_handle) = time::create_driver(self.enable_time, io_driver, clock.clone());
+
+        let spawner = Spawner::Shell;
+
+        let blocking_pool = blocking::create_blocking_pool(self, self.max_threads);
+        let blocking_spawner = blocking_pool.spawner().clone();
+
+        Ok(Runtime {
+            kind: Kind::Shell(Shell::new(driver)),
+            handle: Handle {
+                spawner,
+                io_handle,
+                time_handle,
+                clock,
+                blocking_spawner,
+            },
+            blocking_pool,
+        })
+    }
+}
+
+cfg_io_driver! {
+    impl Builder {
+        /// Enables the I/O driver.
+        ///
+        /// Doing this enables using net, process, signal, and some I/O types on
+        /// the runtime.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use tokio::runtime;
+        ///
+        /// let rt = runtime::Builder::new()
+        ///     .enable_io()
+        ///     .build()
+        ///     .unwrap();
+        /// ```
+        pub fn enable_io(&mut self) -> &mut Self {
+            self.enable_io = true;
+            self
+        }
+    }
+}
+
+cfg_time! {
+    impl Builder {
+        /// Enables the time driver.
+        ///
+        /// Doing this enables using `tokio::time` on the runtime.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use tokio::runtime;
+        ///
+        /// let rt = runtime::Builder::new()
+        ///     .enable_time()
+        ///     .build()
+        ///     .unwrap();
+        /// ```
+        pub fn enable_time(&mut self) -> &mut Self {
+            self.enable_time = true;
+            self
+        }
+    }
+}
+
+cfg_rt_core! {
+    impl Builder {
+        /// Sets runtime to use a simpler scheduler that runs all tasks on the current-thread.
+        ///
+        /// The executor and all necessary drivers will all be run on the current
+        /// thread during `block_on` calls.
+        ///
+        /// See also [the module level documentation][1], which has a section on scheduler
+        /// types.
+        ///
+        /// [1]: index.html#runtime-configurations
+        pub fn basic_scheduler(&mut self) -> &mut Self {
+            self.kind = Kind::Basic;
+            self
+        }
+
+        fn build_basic_runtime(&mut self) -> io::Result<Runtime> {
+            use crate::runtime::{BasicScheduler, Kind};
+
+            let clock = time::create_clock();
+
+            // Create I/O driver
+            let (io_driver, io_handle) = io::create_driver(self.enable_io)?;
+
+            let (driver, time_handle) = time::create_driver(self.enable_time, io_driver, clock.clone());
+
+            // And now put a single-threaded scheduler on top of the timer. When
+            // there are no futures ready to do something, it'll let the timer or
+            // the reactor to generate some new stimuli for the futures to continue
+            // in their life.
+            let scheduler = BasicScheduler::new(driver);
+            let spawner = Spawner::Basic(scheduler.spawner().clone());
+
+            // Blocking pool
+            let blocking_pool = blocking::create_blocking_pool(self, self.max_threads);
+            let blocking_spawner = blocking_pool.spawner().clone();
+
+            Ok(Runtime {
+                kind: Kind::Basic(scheduler),
+                handle: Handle {
+                    spawner,
+                    io_handle,
+                    time_handle,
+                    clock,
+                    blocking_spawner,
+                },
+                blocking_pool,
+            })
+        }
+    }
+}
+
+cfg_rt_threaded! {
+    impl Builder {
+        /// Sets runtime to use a multi-threaded scheduler for executing tasks.
+        ///
+        /// See also [the module level documentation][1], which has a section on scheduler
+        /// types.
+        ///
+        /// [1]: index.html#runtime-configurations
+        pub fn threaded_scheduler(&mut self) -> &mut Self {
+            self.kind = Kind::ThreadPool;
+            self
+        }
+
+        fn build_threaded_runtime(&mut self) -> io::Result<Runtime> {
+            use crate::runtime::{Kind, ThreadPool};
+            use crate::runtime::park::Parker;
+
+            let core_threads = self.core_threads.unwrap_or_else(crate::loom::sys::num_cpus);
+            assert!(core_threads <= self.max_threads, "Core threads number cannot be above max limit");
+
+            let clock = time::create_clock();
+
+            let (io_driver, io_handle) = io::create_driver(self.enable_io)?;
+            let (driver, time_handle) = time::create_driver(self.enable_time, io_driver, clock.clone());
+            let (scheduler, launch) = ThreadPool::new(core_threads, Parker::new(driver));
+            let spawner = Spawner::ThreadPool(scheduler.spawner().clone());
+
+            // Create the blocking pool
+            let blocking_pool = blocking::create_blocking_pool(self, self.max_threads);
+            let blocking_spawner = blocking_pool.spawner().clone();
+
+            // Create the runtime handle
+            let handle = Handle {
+                spawner,
+                io_handle,
+                time_handle,
+                clock,
+                blocking_spawner,
+            };
+
+            // Spawn the thread pool workers
+            handle.enter(|| launch.launch());
+
+            Ok(Runtime {
+                kind: Kind::ThreadPool(scheduler),
+                handle,
+                blocking_pool,
+            })
+        }
+    }
+}
+
+impl Default for Builder {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl fmt::Debug for Builder {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("Builder")
+            .field("kind", &self.kind)
+            .field("core_threads", &self.core_threads)
+            .field("max_threads", &self.max_threads)
+            .field("thread_name", &self.thread_name)
+            .field("thread_stack_size", &self.thread_stack_size)
+            .field("after_start", &self.after_start.as_ref().map(|_| "..."))
+            .field("before_stop", &self.after_start.as_ref().map(|_| "..."))
+            .finish()
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/context.rs b/third_party/rust/tokio/src/runtime/context.rs
new file mode 100644
index 0000000000..4af2df23eb
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/context.rs
@@ -0,0 +1,73 @@
+//! Thread local runtime context
+use crate::runtime::Handle;
+
+use std::cell::RefCell;
+
+thread_local! {
+    static CONTEXT: RefCell<Option<Handle>> = RefCell::new(None)
+}
+
+pub(crate) fn current() -> Option<Handle> {
+    CONTEXT.with(|ctx| ctx.borrow().clone())
+}
+
+cfg_io_driver! {
+    pub(crate) fn io_handle() -> crate::runtime::io::Handle {
+        CONTEXT.with(|ctx| match *ctx.borrow() {
+            Some(ref ctx) => ctx.io_handle.clone(),
+            None => Default::default(),
+        })
+    }
+}
+
+cfg_time! {
+    pub(crate) fn time_handle() -> crate::runtime::time::Handle {
+        CONTEXT.with(|ctx| match *ctx.borrow() {
+            Some(ref ctx) => ctx.time_handle.clone(),
+            None => Default::default(),
+        })
+    }
+
+    cfg_test_util! {
+        pub(crate) fn clock() -> Option<crate::runtime::time::Clock> {
+            CONTEXT.with(|ctx| match *ctx.borrow() {
+                Some(ref ctx) => Some(ctx.clock.clone()),
+                None => None,
+            })
+        }
+    }
+}
+
+cfg_rt_core! {
+    pub(crate) fn spawn_handle() -> Option<crate::runtime::Spawner> {
+        CONTEXT.with(|ctx| match *ctx.borrow() {
+            Some(ref ctx) => Some(ctx.spawner.clone()),
+            None => None,
+        })
+    }
+}
+
+/// Set this [`ThreadContext`] as the current active [`ThreadContext`].
+///
+/// [`ThreadContext`]: struct@ThreadContext
+pub(crate) fn enter<F, R>(new: Handle, f: F) -> R
+where
+    F: FnOnce() -> R,
+{
+    struct DropGuard(Option<Handle>);
+
+    impl Drop for DropGuard {
+        fn drop(&mut self) {
+            CONTEXT.with(|ctx| {
+                *ctx.borrow_mut() = self.0.take();
+            });
+        }
+    }
+
+    let _guard = CONTEXT.with(|ctx| {
+        let old = ctx.borrow_mut().replace(new);
+        DropGuard(old)
+    });
+
+    f()
+}
diff --git a/third_party/rust/tokio/src/runtime/enter.rs b/third_party/rust/tokio/src/runtime/enter.rs
new file mode 100644
index 0000000000..afdb67a3b7
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/enter.rs
@@ -0,0 +1,162 @@
+use std::cell::{Cell, RefCell};
+use std::fmt;
+use std::marker::PhantomData;
+
+thread_local!(static ENTERED: Cell<bool> = Cell::new(false));
+
+/// Represents an executor context.
+pub(crate) struct Enter {
+    _p: PhantomData<RefCell<()>>,
+}
+
+/// Marks the current thread as being within the dynamic extent of an
+/// executor.
+pub(crate) fn enter() -> Enter {
+    if let Some(enter) = try_enter() {
+        return enter;
+    }
+
+    panic!(
+        "Cannot start a runtime from within a runtime. This happens \
+         because a function (like `block_on`) attempted to block the \
+         current thread while the thread is being used to drive \
+         asynchronous tasks."
+    );
+}
+
+/// Tries to enter a runtime context, returns `None` if already in a runtime
+/// context.
+pub(crate) fn try_enter() -> Option<Enter> {
+    ENTERED.with(|c| {
+        if c.get() {
+            None
+        } else {
+            c.set(true);
+            Some(Enter { _p: PhantomData })
+        }
+    })
+}
+
+// Forces the current "entered" state to be cleared while the closure
+// is executed.
+//
+// # Warning
+//
+// This is hidden for a reason. Do not use without fully understanding
+// executors. Misuing can easily cause your program to deadlock.
+#[cfg(all(feature = "rt-threaded", feature = "blocking"))]
+pub(crate) fn exit<F: FnOnce() -> R, R>(f: F) -> R {
+    // Reset in case the closure panics
+    struct Reset;
+    impl Drop for Reset {
+        fn drop(&mut self) {
+            ENTERED.with(|c| {
+                c.set(true);
+            });
+        }
+    }
+
+    ENTERED.with(|c| {
+        debug_assert!(c.get());
+        c.set(false);
+    });
+
+    let reset = Reset;
+    let ret = f();
+    std::mem::forget(reset);
+
+    ENTERED.with(|c| {
+        assert!(!c.get(), "closure claimed permanent executor");
+        c.set(true);
+    });
+
+    ret
+}
+
+cfg_blocking_impl! {
+    use crate::park::ParkError;
+    use std::time::Duration;
+
+    impl Enter {
+        /// Blocks the thread on the specified future, returning the value with
+        /// which that future completes.
+        pub(crate) fn block_on<F>(&mut self, mut f: F) -> Result<F::Output, ParkError>
+        where
+            F: std::future::Future,
+        {
+            use crate::park::{CachedParkThread, Park};
+            use std::pin::Pin;
+            use std::task::Context;
+            use std::task::Poll::Ready;
+
+            let mut park = CachedParkThread::new();
+            let waker = park.get_unpark()?.into_waker();
+            let mut cx = Context::from_waker(&waker);
+
+            // `block_on` takes ownership of `f`. Once it is pinned here, the original `f` binding can
+            // no longer be accessed, making the pinning safe.
+            let mut f = unsafe { Pin::new_unchecked(&mut f) };
+
+            loop {
+                if let Ready(v) = crate::coop::budget(|| f.as_mut().poll(&mut cx)) {
+                    return Ok(v);
+                }
+
+                park.park()?;
+            }
+        }
+
+        /// Blocks the thread on the specified future for **at most** `timeout`
+        ///
+        /// If the future completes before `timeout`, the result is returned. If
+        /// `timeout` elapses, then `Err` is returned.
+        pub(crate) fn block_on_timeout<F>(&mut self, mut f: F, timeout: Duration) -> Result<F::Output, ParkError>
+        where
+            F: std::future::Future,
+        {
+            use crate::park::{CachedParkThread, Park};
+            use std::pin::Pin;
+            use std::task::Context;
+            use std::task::Poll::Ready;
+            use std::time::Instant;
+
+            let mut park = CachedParkThread::new();
+            let waker = park.get_unpark()?.into_waker();
+            let mut cx = Context::from_waker(&waker);
+
+            // `block_on` takes ownership of `f`. Once it is pinned here, the original `f` binding can
+            // no longer be accessed, making the pinning safe.
+            let mut f = unsafe { Pin::new_unchecked(&mut f) };
+            let when = Instant::now() + timeout;
+
+            loop {
+                if let Ready(v) = crate::coop::budget(|| f.as_mut().poll(&mut cx)) {
+                    return Ok(v);
+                }
+
+                let now = Instant::now();
+
+                if now >= when {
+                    return Err(());
+                }
+
+                park.park_timeout(when - now)?;
+            }
+        }
+    }
+}
+
+impl fmt::Debug for Enter {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Enter").finish()
+    }
+}
+
+impl Drop for Enter {
+    fn drop(&mut self) {
+        ENTERED.with(|c| {
+            assert!(c.get());
+            c.set(false);
+        });
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/handle.rs b/third_party/rust/tokio/src/runtime/handle.rs
new file mode 100644
index 0000000000..db53543e85
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/handle.rs
@@ -0,0 +1,140 @@
+use crate::runtime::{blocking, context, io, time, Spawner};
+use std::{error, fmt};
+
+cfg_rt_core! {
+    use crate::task::JoinHandle;
+
+    use std::future::Future;
+}
+
+/// Handle to the runtime.
+///
+/// The handle is internally reference-counted and can be freely cloned. A handle can be
+/// obtained using the [`Runtime::handle`] method.
+///
+/// [`Runtime::handle`]: crate::runtime::Runtime::handle()
+#[derive(Debug, Clone)]
+pub struct Handle {
+    pub(super) spawner: Spawner,
+
+    /// Handles to the I/O drivers
+    pub(super) io_handle: io::Handle,
+
+    /// Handles to the time drivers
+    pub(super) time_handle: time::Handle,
+
+    /// Source of `Instant::now()`
+    pub(super) clock: time::Clock,
+
+    /// Blocking pool spawner
+    pub(super) blocking_spawner: blocking::Spawner,
+}
+
+impl Handle {
+    /// Enter the runtime context.
+    pub fn enter<F, R>(&self, f: F) -> R
+    where
+        F: FnOnce() -> R,
+    {
+        context::enter(self.clone(), f)
+    }
+
+    /// Returns a Handle view over the currently running Runtime
+    ///
+    /// # Panic
+    ///
+    /// This will panic if called outside the context of a Tokio runtime.
+    ///
+    /// # Examples
+    ///
+    /// This can be used to obtain the handle of the surrounding runtime from an async
+    /// block or function running on that runtime.
+    ///
+    /// ```
+    /// # use tokio::runtime::Runtime;
+    /// # fn dox() {
+    /// # let rt = Runtime::new().unwrap();
+    /// # rt.spawn(async {
+    /// use tokio::runtime::Handle;
+    ///
+    /// // Inside an async block or function.
+    /// let handle = Handle::current();
+    /// handle.spawn(async {
+    ///     println!("now running in the existing Runtime");
+    /// })
+    /// # });
+    /// # }
+    /// ```
+    pub fn current() -> Self {
+        context::current().expect("not currently running on the Tokio runtime.")
+    }
+
+    /// Returns a Handle view over the currently running Runtime
+    ///
+    /// Returns an error if no Runtime has been started
+    ///
+    /// Contrary to `current`, this never panics
+    pub fn try_current() -> Result<Self, TryCurrentError> {
+        context::current().ok_or(TryCurrentError(()))
+    }
+}
+
+cfg_rt_core! {
+    impl Handle {
+        /// Spawns a future onto the Tokio runtime.
+        ///
+        /// This spawns the given future onto the runtime's executor, usually a
+        /// thread pool. The thread pool is then responsible for polling the future
+        /// until it completes.
+        ///
+        /// See [module level][mod] documentation for more details.
+        ///
+        /// [mod]: index.html
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use tokio::runtime::Runtime;
+        ///
+        /// # fn dox() {
+        /// // Create the runtime
+        /// let rt = Runtime::new().unwrap();
+        /// let handle = rt.handle();
+        ///
+        /// // Spawn a future onto the runtime
+        /// handle.spawn(async {
+        ///     println!("now running on a worker thread");
+        /// });
+        /// # }
+        /// ```
+        ///
+        /// # Panics
+        ///
+        /// This function panics if the spawn fails. Failure occurs if the executor
+        /// is currently at capacity and is unable to spawn a new future.
+        pub fn spawn<F>(&self, future: F) -> JoinHandle<F::Output>
+        where
+            F: Future + Send + 'static,
+            F::Output: Send + 'static,
+        {
+            self.spawner.spawn(future)
+        }
+    }
+}
+
+/// Error returned by `try_current` when no Runtime has been started
+pub struct TryCurrentError(());
+
+impl fmt::Debug for TryCurrentError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("TryCurrentError").finish()
+    }
+}
+
+impl fmt::Display for TryCurrentError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str("no tokio Runtime has been initialized")
+    }
+}
+
+impl error::Error for TryCurrentError {}
diff --git a/third_party/rust/tokio/src/runtime/io.rs b/third_party/rust/tokio/src/runtime/io.rs
new file mode 100644
index 0000000000..6a0953af85
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/io.rs
@@ -0,0 +1,63 @@
+//! Abstracts out the APIs necessary to `Runtime` for integrating the I/O
+//! driver. When the `time` feature flag is **not** enabled. These APIs are
+//! shells. This isolates the complexity of dealing with conditional
+//! compilation.
+
+/// Re-exported for convenience.
+pub(crate) use std::io::Result;
+
+pub(crate) use variant::*;
+
+#[cfg(feature = "io-driver")]
+mod variant {
+    use crate::io::driver;
+    use crate::park::{Either, ParkThread};
+
+    use std::io;
+
+    /// The driver value the runtime passes to the `timer` layer.
+    ///
+    /// When the `io-driver` feature is enabled, this is the "real" I/O driver
+    /// backed by Mio. Without the `io-driver` feature, this is a thread parker
+    /// backed by a condition variable.
+    pub(crate) type Driver = Either<driver::Driver, ParkThread>;
+
+    /// The handle the runtime stores for future use.
+    ///
+    /// When the `io-driver` feature is **not** enabled, this is `()`.
+    pub(crate) type Handle = Option<driver::Handle>;
+
+    pub(crate) fn create_driver(enable: bool) -> io::Result<(Driver, Handle)> {
+        #[cfg(loom)]
+        assert!(!enable);
+
+        if enable {
+            let driver = driver::Driver::new()?;
+            let handle = driver.handle();
+
+            Ok((Either::A(driver), Some(handle)))
+        } else {
+            let driver = ParkThread::new();
+            Ok((Either::B(driver), None))
+        }
+    }
+}
+
+#[cfg(not(feature = "io-driver"))]
+mod variant {
+    use crate::park::ParkThread;
+
+    use std::io;
+
+    /// I/O is not enabled, use a condition variable based parker
+    pub(crate) type Driver = ParkThread;
+
+    /// There is no handle
+    pub(crate) type Handle = ();
+
+    pub(crate) fn create_driver(_enable: bool) -> io::Result<(Driver, Handle)> {
+        let driver = ParkThread::new();
+
+        Ok((driver, ()))
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/mod.rs b/third_party/rust/tokio/src/runtime/mod.rs
new file mode 100644
index 0000000000..36b2b442ee
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/mod.rs
@@ -0,0 +1,494 @@
+//! The Tokio runtime.
+//!
+//! Unlike other Rust programs, asynchronous applications require
+//! runtime support. In particular, the following runtime services are
+//! necessary:
+//!
+//! * An **I/O event loop**, called the driver, which drives I/O resources and
+//!   dispatches I/O events to tasks that depend on them.
+//! * A **scheduler** to execute [tasks] that use these I/O resources.
+//! * A **timer** for scheduling work to run after a set period of time.
+//!
+//! Tokio's [`Runtime`] bundles all of these services as a single type, allowing
+//! them to be started, shut down, and configured together. However, most
+//! applications won't need to use [`Runtime`] directly. Instead, they can
+//! use the [`tokio::main`] attribute macro, which creates a [`Runtime`] under
+//! the hood.
+//!
+//! # Usage
+//!
+//! Most applications will use the [`tokio::main`] attribute macro.
+//!
+//! ```no_run
+//! use tokio::net::TcpListener;
+//! use tokio::prelude::*;
+//!
+//! #[tokio::main]
+//! async fn main() -> Result<(), Box<dyn std::error::Error>> {
+//!     let mut listener = TcpListener::bind("127.0.0.1:8080").await?;
+//!
+//!     loop {
+//!         let (mut socket, _) = listener.accept().await?;
+//!
+//!         tokio::spawn(async move {
+//!             let mut buf = [0; 1024];
+//!
+//!             // In a loop, read data from the socket and write the data back.
+//!             loop {
+//!                 let n = match socket.read(&mut buf).await {
+//!                     // socket closed
+//!                     Ok(n) if n == 0 => return,
+//!                     Ok(n) => n,
+//!                     Err(e) => {
+//!                         println!("failed to read from socket; err = {:?}", e);
+//!                         return;
+//!                     }
+//!                 };
+//!
+//!                 // Write the data back
+//!                 if let Err(e) = socket.write_all(&buf[0..n]).await {
+//!                     println!("failed to write to socket; err = {:?}", e);
+//!                     return;
+//!                 }
+//!             }
+//!         });
+//!     }
+//! }
+//! ```
+//!
+//! From within the context of the runtime, additional tasks are spawned using
+//! the [`tokio::spawn`] function. Futures spawned using this function will be
+//! executed on the same thread pool used by the [`Runtime`].
+//!
+//! A [`Runtime`] instance can also be used directly.
+//!
+//! ```no_run
+//! use tokio::net::TcpListener;
+//! use tokio::prelude::*;
+//! use tokio::runtime::Runtime;
+//!
+//! fn main() -> Result<(), Box<dyn std::error::Error>> {
+//!     // Create the runtime
+//!     let mut rt = Runtime::new()?;
+//!
+//!     // Spawn the root task
+//!     rt.block_on(async {
+//!         let mut listener = TcpListener::bind("127.0.0.1:8080").await?;
+//!
+//!         loop {
+//!             let (mut socket, _) = listener.accept().await?;
+//!
+//!             tokio::spawn(async move {
+//!                 let mut buf = [0; 1024];
+//!
+//!                 // In a loop, read data from the socket and write the data back.
+//!                 loop {
+//!                     let n = match socket.read(&mut buf).await {
+//!                         // socket closed
+//!                         Ok(n) if n == 0 => return,
+//!                         Ok(n) => n,
+//!                         Err(e) => {
+//!                             println!("failed to read from socket; err = {:?}", e);
+//!                             return;
+//!                         }
+//!                     };
+//!
+//!                     // Write the data back
+//!                     if let Err(e) = socket.write_all(&buf[0..n]).await {
+//!                         println!("failed to write to socket; err = {:?}", e);
+//!                         return;
+//!                     }
+//!                 }
+//!             });
+//!         }
+//!     })
+//! }
+//! ```
+//!
+//! ## Runtime Configurations
+//!
+//! Tokio provides multiple task scheduling strategies, suitable for different
+//! applications. The [runtime builder] or `#[tokio::main]` attribute may be
+//! used to select which scheduler to use.
+//!
+//! #### Basic Scheduler
+//!
+//! The basic scheduler provides a _single-threaded_ future executor. All tasks
+//! will be created and executed on the current thread. The basic scheduler
+//! requires the `rt-core` feature flag, and can be selected using the
+//! [`Builder::basic_scheduler`] method:
+//! ```
+//! use tokio::runtime;
+//!
+//! # fn main() -> Result<(), Box<dyn std::error::Error>> {
+//! let basic_rt = runtime::Builder::new()
+//!     .basic_scheduler()
+//!     .build()?;
+//! # Ok(()) }
+//! ```
+//!
+//! If the `rt-core` feature is enabled and `rt-threaded` is not,
+//! [`Runtime::new`] will return a basic scheduler runtime by default.
+//!
+//! #### Threaded Scheduler
+//!
+//! The threaded scheduler executes futures on a _thread pool_, using a
+//! work-stealing strategy. By default, it will start a worker thread for each
+//! CPU core available on the system. This tends to be the ideal configurations
+//! for most applications. The threaded scheduler requires the `rt-threaded` feature
+//! flag, and can be selected using the  [`Builder::threaded_scheduler`] method:
+//! ```
+//! use tokio::runtime;
+//!
+//! # fn main() -> Result<(), Box<dyn std::error::Error>> {
+//! let threaded_rt = runtime::Builder::new()
+//!     .threaded_scheduler()
+//!     .build()?;
+//! # Ok(()) }
+//! ```
+//!
+//! If the `rt-threaded` feature flag is enabled, [`Runtime::new`] will return a
+//! threaded scheduler runtime by default.
+//!
+//! Most applications should use the threaded scheduler, except in some niche
+//! use-cases, such as when running only a single thread is required.
+//!
+//! #### Resource drivers
+//!
+//! When configuring a runtime by hand, no resource drivers are enabled by
+//! default. In this case, attempting to use networking types or time types will
+//! fail. In order to enable these types, the resource drivers must be enabled.
+//! This is done with [`Builder::enable_io`] and [`Builder::enable_time`]. As a
+//! shorthand, [`Builder::enable_all`] enables both resource drivers.
+//!
+//! ## Lifetime of spawned threads
+//!
+//! The runtime may spawn threads depending on its configuration and usage. The
+//! threaded scheduler spawns threads to schedule tasks and calls to
+//! `spawn_blocking` spawn threads to run blocking operations.
+//!
+//! While the `Runtime` is active, threads may shutdown after periods of being
+//! idle. Once `Runtime` is dropped, all runtime threads are forcibly shutdown.
+//! Any tasks that have not yet completed will be dropped.
+//!
+//! [tasks]: crate::task
+//! [`Runtime`]: Runtime
+//! [`tokio::spawn`]: crate::spawn
+//! [`tokio::main`]: ../attr.main.html
+//! [runtime builder]: crate::runtime::Builder
+//! [`Runtime::new`]: crate::runtime::Runtime::new
+//! [`Builder::basic_scheduler`]: crate::runtime::Builder::basic_scheduler
+//! [`Builder::threaded_scheduler`]: crate::runtime::Builder::threaded_scheduler
+//! [`Builder::enable_io`]: crate::runtime::Builder::enable_io
+//! [`Builder::enable_time`]: crate::runtime::Builder::enable_time
+//! [`Builder::enable_all`]: crate::runtime::Builder::enable_all
+
+// At the top due to macros
+#[cfg(test)]
+#[macro_use]
+mod tests;
+
+pub(crate) mod context;
+
+cfg_rt_core! {
+    mod basic_scheduler;
+    use basic_scheduler::BasicScheduler;
+
+    pub(crate) mod task;
+}
+
+mod blocking;
+use blocking::BlockingPool;
+
+cfg_blocking_impl! {
+    #[allow(unused_imports)]
+    pub(crate) use blocking::{spawn_blocking, try_spawn_blocking};
+}
+
+mod builder;
+pub use self::builder::Builder;
+
+pub(crate) mod enter;
+use self::enter::enter;
+
+mod handle;
+pub use self::handle::{Handle, TryCurrentError};
+
+mod io;
+
+cfg_rt_threaded! {
+    mod park;
+    use park::Parker;
+}
+
+mod shell;
+use self::shell::Shell;
+
+mod spawner;
+use self::spawner::Spawner;
+
+mod time;
+
+cfg_rt_threaded! {
+    mod queue;
+
+    pub(crate) mod thread_pool;
+    use self::thread_pool::ThreadPool;
+}
+
+cfg_rt_core! {
+    use crate::task::JoinHandle;
+}
+
+use std::future::Future;
+use std::time::Duration;
+
+/// The Tokio runtime.
+///
+/// The runtime provides an I/O [driver], task scheduler, [timer], and blocking
+/// pool, necessary for running asynchronous tasks.
+///
+/// Instances of `Runtime` can be created using [`new`] or [`Builder`]. However,
+/// most users will use the `#[tokio::main]` annotation on their entry point instead.
+///
+/// See [module level][mod] documentation for more details.
+///
+/// # Shutdown
+///
+/// Shutting down the runtime is done by dropping the value. The current thread
+/// will block until the shut down operation has completed.
+///
+/// * Drain any scheduled work queues.
+/// * Drop any futures that have not yet completed.
+/// * Drop the reactor.
+///
+/// Once the reactor has dropped, any outstanding I/O resources bound to
+/// that reactor will no longer function. Calling any method on them will
+/// result in an error.
+///
+/// [driver]: crate::io::driver
+/// [timer]: crate::time
+/// [mod]: index.html
+/// [`new`]: #method.new
+/// [`Builder`]: struct@Builder
+/// [`tokio::run`]: fn@run
+#[derive(Debug)]
+pub struct Runtime {
+    /// Task executor
+    kind: Kind,
+
+    /// Handle to runtime, also contains driver handles
+    handle: Handle,
+
+    /// Blocking pool handle, used to signal shutdown
+    blocking_pool: BlockingPool,
+}
+
+/// The runtime executor is either a thread-pool or a current-thread executor.
+#[derive(Debug)]
+enum Kind {
+    /// Not able to execute concurrent tasks. This variant is mostly used to get
+    /// access to the driver handles.
+    Shell(Shell),
+
+    /// Execute all tasks on the current-thread.
+    #[cfg(feature = "rt-core")]
+    Basic(BasicScheduler<time::Driver>),
+
+    /// Execute tasks across multiple threads.
+    #[cfg(feature = "rt-threaded")]
+    ThreadPool(ThreadPool),
+}
+
+/// After thread starts / before thread stops
+type Callback = std::sync::Arc<dyn Fn() + Send + Sync>;
+
+impl Runtime {
+    /// Create a new runtime instance with default configuration values.
+    ///
+    /// This results in a scheduler, I/O driver, and time driver being
+    /// initialized. The type of scheduler used depends on what feature flags
+    /// are enabled: if the `rt-threaded` feature is enabled, the [threaded
+    /// scheduler] is used, while if only the `rt-core` feature is enabled, the
+    /// [basic scheduler] is used instead.
+    ///
+    /// If the threaded scheduler is selected, it will not spawn
+    /// any worker threads until it needs to, i.e. tasks are scheduled to run.
+    ///
+    /// Most applications will not need to call this function directly. Instead,
+    /// they will use the  [`#[tokio::main]` attribute][main]. When more complex
+    /// configuration is necessary, the [runtime builder] may be used.
+    ///
+    /// See [module level][mod] documentation for more details.
+    ///
+    /// # Examples
+    ///
+    /// Creating a new `Runtime` with default configuration values.
+    ///
+    /// ```
+    /// use tokio::runtime::Runtime;
+    ///
+    /// let rt = Runtime::new()
+    ///     .unwrap();
+    ///
+    /// // Use the runtime...
+    /// ```
+    ///
+    /// [mod]: index.html
+    /// [main]: ../../tokio_macros/attr.main.html
+    /// [threaded scheduler]: index.html#threaded-scheduler
+    /// [basic scheduler]: index.html#basic-scheduler
+    /// [runtime builder]: crate::runtime::Builder
+    pub fn new() -> io::Result<Runtime> {
+        #[cfg(feature = "rt-threaded")]
+        let ret = Builder::new().threaded_scheduler().enable_all().build();
+
+        #[cfg(all(not(feature = "rt-threaded"), feature = "rt-core"))]
+        let ret = Builder::new().basic_scheduler().enable_all().build();
+
+        #[cfg(not(feature = "rt-core"))]
+        let ret = Builder::new().enable_all().build();
+
+        ret
+    }
+
+    /// Spawn a future onto the Tokio runtime.
+    ///
+    /// This spawns the given future onto the runtime's executor, usually a
+    /// thread pool. The thread pool is then responsible for polling the future
+    /// until it completes.
+    ///
+    /// See [module level][mod] documentation for more details.
+    ///
+    /// [mod]: index.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::runtime::Runtime;
+    ///
+    /// # fn dox() {
+    /// // Create the runtime
+    /// let rt = Runtime::new().unwrap();
+    ///
+    /// // Spawn a future onto the runtime
+    /// rt.spawn(async {
+    ///     println!("now running on a worker thread");
+    /// });
+    /// # }
+    /// ```
+    ///
+    /// # Panics
+    ///
+    /// This function panics if the spawn fails. Failure occurs if the executor
+    /// is currently at capacity and is unable to spawn a new future.
+    #[cfg(feature = "rt-core")]
+    pub fn spawn<F>(&self, future: F) -> JoinHandle<F::Output>
+    where
+        F: Future + Send + 'static,
+        F::Output: Send + 'static,
+    {
+        match &self.kind {
+            Kind::Shell(_) => panic!("task execution disabled"),
+            #[cfg(feature = "rt-threaded")]
+            Kind::ThreadPool(exec) => exec.spawn(future),
+            Kind::Basic(exec) => exec.spawn(future),
+        }
+    }
+
+    /// Run a future to completion on the Tokio runtime. This is the runtime's
+    /// entry point.
+    ///
+    /// This runs the given future on the runtime, blocking until it is
+    /// complete, and yielding its resolved result. Any tasks or timers which
+    /// the future spawns internally will be executed on the runtime.
+    ///
+    /// This method should not be called from an asynchronous context.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if the executor is at capacity, if the provided
+    /// future panics, or if called within an asynchronous execution context.
+    pub fn block_on<F: Future>(&mut self, future: F) -> F::Output {
+        let kind = &mut self.kind;
+
+        self.handle.enter(|| match kind {
+            Kind::Shell(exec) => exec.block_on(future),
+            #[cfg(feature = "rt-core")]
+            Kind::Basic(exec) => exec.block_on(future),
+            #[cfg(feature = "rt-threaded")]
+            Kind::ThreadPool(exec) => exec.block_on(future),
+        })
+    }
+
+    /// Enter the runtime context.
+    pub fn enter<F, R>(&self, f: F) -> R
+    where
+        F: FnOnce() -> R,
+    {
+        self.handle.enter(f)
+    }
+
+    /// Return a handle to the runtime's spawner.
+    ///
+    /// The returned handle can be used to spawn tasks that run on this runtime, and can
+    /// be cloned to allow moving the `Handle` to other threads.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::runtime::Runtime;
+    ///
+    /// let rt = Runtime::new()
+    ///     .unwrap();
+    ///
+    /// let handle = rt.handle();
+    ///
+    /// handle.spawn(async { println!("hello"); });
+    /// ```
+    pub fn handle(&self) -> &Handle {
+        &self.handle
+    }
+
+    /// Shutdown the runtime, waiting for at most `duration` for all spawned
+    /// task to shutdown.
+    ///
+    /// Usually, dropping a `Runtime` handle is sufficient as tasks are able to
+    /// shutdown in a timely fashion. However, dropping a `Runtime` will wait
+    /// indefinitely for all tasks to terminate, and there are cases where a long
+    /// blocking task has been spawned which can block dropping `Runtime`.
+    ///
+    /// In this case, calling `shutdown_timeout` with an explicit wait timeout
+    /// can work. The `shutdown_timeout` will signal all tasks to shutdown and
+    /// will wait for at most `duration` for all spawned tasks to terminate. If
+    /// `timeout` elapses before all tasks are dropped, the function returns and
+    /// outstanding tasks are potentially leaked.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::runtime::Runtime;
+    /// use tokio::task;
+    ///
+    /// use std::thread;
+    /// use std::time::Duration;
+    ///
+    /// fn main() {
+    ///    let mut runtime = Runtime::new().unwrap();
+    ///
+    ///    runtime.block_on(async move {
+    ///        task::spawn_blocking(move || {
+    ///            thread::sleep(Duration::from_secs(10_000));
+    ///        });
+    ///    });
+    ///
+    ///    runtime.shutdown_timeout(Duration::from_millis(100));
+    /// }
+    /// ```
+    pub fn shutdown_timeout(self, duration: Duration) {
+        let Runtime {
+            mut blocking_pool, ..
+        } = self;
+        blocking_pool.shutdown(Some(duration));
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/park.rs b/third_party/rust/tokio/src/runtime/park.rs
new file mode 100644
index 0000000000..ee437d1d94
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/park.rs
@@ -0,0 +1,245 @@
+//! Parks the runtime.
+//!
+//! A combination of the various resource driver park handles.
+
+use crate::loom::sync::atomic::AtomicUsize;
+use crate::loom::sync::{Arc, Condvar, Mutex};
+use crate::loom::thread;
+use crate::park::{Park, Unpark};
+use crate::runtime::time;
+use crate::util::TryLock;
+
+use std::sync::atomic::Ordering::SeqCst;
+use std::time::Duration;
+
+pub(crate) struct Parker {
+    inner: Arc<Inner>,
+}
+
+pub(crate) struct Unparker {
+    inner: Arc<Inner>,
+}
+
+struct Inner {
+    /// Avoids entering the park if possible
+    state: AtomicUsize,
+
+    /// Used to coordinate access to the driver / condvar
+    mutex: Mutex<()>,
+
+    /// Condvar to block on if the driver is unavailable.
+    condvar: Condvar,
+
+    /// Resource (I/O, time, ...) driver
+    shared: Arc<Shared>,
+}
+
+const EMPTY: usize = 0;
+const PARKED_CONDVAR: usize = 1;
+const PARKED_DRIVER: usize = 2;
+const NOTIFIED: usize = 3;
+
+/// Shared across multiple Parker handles
+struct Shared {
+    /// Shared driver. Only one thread at a time can use this
+    driver: TryLock<time::Driver>,
+
+    /// Unpark handle
+    handle: <time::Driver as Park>::Unpark,
+}
+
+impl Parker {
+    pub(crate) fn new(driver: time::Driver) -> Parker {
+        let handle = driver.unpark();
+
+        Parker {
+            inner: Arc::new(Inner {
+                state: AtomicUsize::new(EMPTY),
+                mutex: Mutex::new(()),
+                condvar: Condvar::new(),
+                shared: Arc::new(Shared {
+                    driver: TryLock::new(driver),
+                    handle,
+                }),
+            }),
+        }
+    }
+}
+
+impl Clone for Parker {
+    fn clone(&self) -> Parker {
+        Parker {
+            inner: Arc::new(Inner {
+                state: AtomicUsize::new(EMPTY),
+                mutex: Mutex::new(()),
+                condvar: Condvar::new(),
+                shared: self.inner.shared.clone(),
+            }),
+        }
+    }
+}
+
+impl Park for Parker {
+    type Unpark = Unparker;
+    type Error = ();
+
+    fn unpark(&self) -> Unparker {
+        Unparker {
+            inner: self.inner.clone(),
+        }
+    }
+
+    fn park(&mut self) -> Result<(), Self::Error> {
+        self.inner.park();
+        Ok(())
+    }
+
+    fn park_timeout(&mut self, duration: Duration) -> Result<(), Self::Error> {
+        // Only parking with zero is supported...
+        assert_eq!(duration, Duration::from_millis(0));
+
+        if let Some(mut driver) = self.inner.shared.driver.try_lock() {
+            driver.park_timeout(duration).map_err(|_| ())
+        } else {
+            Ok(())
+        }
+    }
+}
+
+impl Unpark for Unparker {
+    fn unpark(&self) {
+        self.inner.unpark();
+    }
+}
+
+impl Inner {
+    /// Parks the current thread for at most `dur`.
+    fn park(&self) {
+        for _ in 0..3 {
+            // If we were previously notified then we consume this notification and
+            // return quickly.
+            if self
+                .state
+                .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
+                .is_ok()
+            {
+                return;
+            }
+
+            thread::yield_now();
+        }
+
+        if let Some(mut driver) = self.shared.driver.try_lock() {
+            self.park_driver(&mut driver);
+        } else {
+            self.park_condvar();
+        }
+    }
+
+    fn park_condvar(&self) {
+        // Otherwise we need to coordinate going to sleep
+        let mut m = self.mutex.lock().unwrap();
+
+        match self
+            .state
+            .compare_exchange(EMPTY, PARKED_CONDVAR, SeqCst, SeqCst)
+        {
+            Ok(_) => {}
+            Err(NOTIFIED) => {
+                // We must read here, even though we know it will be `NOTIFIED`.
+                // This is because `unpark` may have been called again since we read
+                // `NOTIFIED` in the `compare_exchange` above. We must perform an
+                // acquire operation that synchronizes with that `unpark` to observe
+                // any writes it made before the call to unpark. To do that we must
+                // read from the write it made to `state`.
+                let old = self.state.swap(EMPTY, SeqCst);
+                debug_assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
+
+                return;
+            }
+            Err(actual) => panic!("inconsistent park state; actual = {}", actual),
+        }
+
+        loop {
+            m = self.condvar.wait(m).unwrap();
+
+            if self
+                .state
+                .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
+                .is_ok()
+            {
+                // got a notification
+                return;
+            }
+
+            // spurious wakeup, go back to sleep
+        }
+    }
+
+    fn park_driver(&self, driver: &mut time::Driver) {
+        match self
+            .state
+            .compare_exchange(EMPTY, PARKED_DRIVER, SeqCst, SeqCst)
+        {
+            Ok(_) => {}
+            Err(NOTIFIED) => {
+                // We must read here, even though we know it will be `NOTIFIED`.
+                // This is because `unpark` may have been called again since we read
+                // `NOTIFIED` in the `compare_exchange` above. We must perform an
+                // acquire operation that synchronizes with that `unpark` to observe
+                // any writes it made before the call to unpark. To do that we must
+                // read from the write it made to `state`.
+                let old = self.state.swap(EMPTY, SeqCst);
+                debug_assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
+
+                return;
+            }
+            Err(actual) => panic!("inconsistent park state; actual = {}", actual),
+        }
+
+        // TODO: don't unwrap
+        driver.park().unwrap();
+
+        match self.state.swap(EMPTY, SeqCst) {
+            NOTIFIED => {}      // got a notification, hurray!
+            PARKED_DRIVER => {} // no notification, alas
+            n => panic!("inconsistent park_timeout state: {}", n),
+        }
+    }
+
+    fn unpark(&self) {
+        // To ensure the unparked thread will observe any writes we made before
+        // this call, we must perform a release operation that `park` can
+        // synchronize with. To do that we must write `NOTIFIED` even if `state`
+        // is already `NOTIFIED`. That is why this must be a swap rather than a
+        // compare-and-swap that returns if it reads `NOTIFIED` on failure.
+        match self.state.swap(NOTIFIED, SeqCst) {
+            EMPTY => {}    // no one was waiting
+            NOTIFIED => {} // already unparked
+            PARKED_CONDVAR => self.unpark_condvar(),
+            PARKED_DRIVER => self.unpark_driver(),
+            actual => panic!("inconsistent state in unpark; actual = {}", actual),
+        }
+    }
+
+    fn unpark_condvar(&self) {
+        // There is a period between when the parked thread sets `state` to
+        // `PARKED` (or last checked `state` in the case of a spurious wake
+        // up) and when it actually waits on `cvar`. If we were to notify
+        // during this period it would be ignored and then when the parked
+        // thread went to sleep it would never wake up. Fortunately, it has
+        // `lock` locked at this stage so we can acquire `lock` to wait until
+        // it is ready to receive the notification.
+        //
+        // Releasing `lock` before the call to `notify_one` means that when the
+        // parked thread wakes it doesn't get woken only to have to wait for us
+        // to release `lock`.
+        drop(self.mutex.lock().unwrap());
+
+        self.condvar.notify_one()
+    }
+
+    fn unpark_driver(&self) {
+        self.shared.handle.unpark();
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/queue.rs b/third_party/rust/tokio/src/runtime/queue.rs
new file mode 100644
index 0000000000..c654514bbc
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/queue.rs
@@ -0,0 +1,630 @@
+//! Run-queue structures to support a work-stealing scheduler
+
+use crate::loom::cell::UnsafeCell;
+use crate::loom::sync::atomic::{AtomicU16, AtomicU32, AtomicUsize};
+use crate::loom::sync::{Arc, Mutex};
+use crate::runtime::task;
+
+use std::marker::PhantomData;
+use std::mem::MaybeUninit;
+use std::ptr::{self, NonNull};
+use std::sync::atomic::Ordering::{AcqRel, Acquire, Release};
+
+/// Producer handle. May only be used from a single thread.
+pub(super) struct Local<T: 'static> {
+    inner: Arc<Inner<T>>,
+}
+
+/// Consumer handle. May be used from many threads.
+pub(super) struct Steal<T: 'static>(Arc<Inner<T>>);
+
+/// Growable, MPMC queue used to inject new tasks into the scheduler and as an
+/// overflow queue when the local, fixed-size, array queue overflows.
+pub(super) struct Inject<T: 'static> {
+    /// Pointers to the head and tail of the queue
+    pointers: Mutex<Pointers>,
+
+    /// Number of pending tasks in the queue. This helps prevent unnecessary
+    /// locking in the hot path.
+    len: AtomicUsize,
+
+    _p: PhantomData<T>,
+}
+
+pub(super) struct Inner<T: 'static> {
+    /// Concurrently updated by many threads.
+    ///
+    /// Contains two `u16` values. The LSB byte is the "real" head of the queue.
+    /// The `u16` in the MSB is set by a stealer in process of stealing values.
+    /// It represents the first value being stolen in the batch. `u16` is used
+    /// in order to distinguish between `head == tail` and `head == tail -
+    /// capacity`.
+    ///
+    /// When both `u16` values are the same, there is no active stealer.
+    ///
+    /// Tracking an in-progress stealer prevents a wrapping scenario.
+    head: AtomicU32,
+
+    /// Only updated by producer thread but read by many threads.
+    tail: AtomicU16,
+
+    /// Elements
+    buffer: Box<[UnsafeCell<MaybeUninit<task::Notified<T>>>]>,
+}
+
+struct Pointers {
+    /// True if the queue is closed
+    is_closed: bool,
+
+    /// Linked-list head
+    head: Option<NonNull<task::Header>>,
+
+    /// Linked-list tail
+    tail: Option<NonNull<task::Header>>,
+}
+
+unsafe impl<T> Send for Inner<T> {}
+unsafe impl<T> Sync for Inner<T> {}
+unsafe impl<T> Send for Inject<T> {}
+unsafe impl<T> Sync for Inject<T> {}
+
+#[cfg(not(loom))]
+const LOCAL_QUEUE_CAPACITY: usize = 256;
+
+// Shrink the size of the local queue when using loom. This shouldn't impact
+// logic, but allows loom to test more edge cases in a reasonable a mount of
+// time.
+#[cfg(loom)]
+const LOCAL_QUEUE_CAPACITY: usize = 4;
+
+const MASK: usize = LOCAL_QUEUE_CAPACITY - 1;
+
+/// Create a new local run-queue
+pub(super) fn local<T: 'static>() -> (Steal<T>, Local<T>) {
+    let mut buffer = Vec::with_capacity(LOCAL_QUEUE_CAPACITY);
+
+    for _ in 0..LOCAL_QUEUE_CAPACITY {
+        buffer.push(UnsafeCell::new(MaybeUninit::uninit()));
+    }
+
+    let inner = Arc::new(Inner {
+        head: AtomicU32::new(0),
+        tail: AtomicU16::new(0),
+        buffer: buffer.into(),
+    });
+
+    let local = Local {
+        inner: inner.clone(),
+    };
+
+    let remote = Steal(inner);
+
+    (remote, local)
+}
+
+impl<T> Local<T> {
+    /// Returns true if the queue has entries that can be stealed.
+    pub(super) fn is_stealable(&self) -> bool {
+        !self.inner.is_empty()
+    }
+
+    /// Pushes a task to the back of the local queue, skipping the LIFO slot.
+    pub(super) fn push_back(&mut self, mut task: task::Notified<T>, inject: &Inject<T>) {
+        let tail = loop {
+            let head = self.inner.head.load(Acquire);
+            let (steal, real) = unpack(head);
+
+            // safety: this is the **only** thread that updates this cell.
+            let tail = unsafe { self.inner.tail.unsync_load() };
+
+            if tail.wrapping_sub(steal) < LOCAL_QUEUE_CAPACITY as u16 {
+                // There is capacity for the task
+                break tail;
+            } else if steal != real {
+                // Concurrently stealing, this will free up capacity, so
+                // only push the new task onto the inject queue
+                inject.push(task);
+                return;
+            } else {
+                // Push the current task and half of the queue into the
+                // inject queue.
+                match self.push_overflow(task, real, tail, inject) {
+                    Ok(_) => return,
+                    // Lost the race, try again
+                    Err(v) => {
+                        task = v;
+                    }
+                }
+            }
+        };
+
+        // Map the position to a slot index.
+        let idx = tail as usize & MASK;
+
+        self.inner.buffer[idx].with_mut(|ptr| {
+            // Write the task to the slot
+            //
+            // Safety: There is only one producer and the above `if`
+            // condition ensures we don't touch a cell if there is a
+            // value, thus no consumer.
+            unsafe {
+                ptr::write((*ptr).as_mut_ptr(), task);
+            }
+        });
+
+        // Make the task available. Synchronizes with a load in
+        // `steal_into2`.
+        self.inner.tail.store(tail.wrapping_add(1), Release);
+    }
+
+    /// Moves a batch of tasks into the inject queue.
+    ///
+    /// This will temporarily make some of the tasks unavailable to stealers.
+    /// Once `push_overflow` is done, a notification is sent out, so if other
+    /// workers "missed" some of the tasks during a steal, they will get
+    /// another opportunity.
+    #[inline(never)]
+    fn push_overflow(
+        &mut self,
+        task: task::Notified<T>,
+        head: u16,
+        tail: u16,
+        inject: &Inject<T>,
+    ) -> Result<(), task::Notified<T>> {
+        const BATCH_LEN: usize = LOCAL_QUEUE_CAPACITY / 2 + 1;
+
+        let n = (LOCAL_QUEUE_CAPACITY / 2) as u16;
+        assert_eq!(
+            tail.wrapping_sub(head) as usize,
+            LOCAL_QUEUE_CAPACITY,
+            "queue is not full; tail = {}; head = {}",
+            tail,
+            head
+        );
+
+        let prev = pack(head, head);
+
+        // Claim a bunch of tasks
+        //
+        // We are claiming the tasks **before** reading them out of the buffer.
+        // This is safe because only the **current** thread is able to push new
+        // tasks.
+        //
+        // There isn't really any need for memory ordering... Relaxed would
+        // work. This is because all tasks are pushed into the queue from the
+        // current thread (or memory has been acquired if the local queue handle
+        // moved).
+        let actual = self.inner.head.compare_and_swap(
+            prev,
+            pack(head.wrapping_add(n), head.wrapping_add(n)),
+            Release,
+        );
+
+        if actual != prev {
+            // We failed to claim the tasks, losing the race. Return out of
+            // this function and try the full `push` routine again. The queue
+            // may not be full anymore.
+            return Err(task);
+        }
+
+        // link the tasks
+        for i in 0..n {
+            let j = i + 1;
+
+            let i_idx = i.wrapping_add(head) as usize & MASK;
+            let j_idx = j.wrapping_add(head) as usize & MASK;
+
+            // Get the next pointer
+            let next = if j == n {
+                // The last task in the local queue being moved
+                task.header().into()
+            } else {
+                // safety: The above CAS prevents a stealer from accessing these
+                // tasks and we are the only producer.
+                self.inner.buffer[j_idx].with(|ptr| unsafe {
+                    let value = (*ptr).as_ptr();
+                    (*value).header().into()
+                })
+            };
+
+            // safety: the above CAS prevents a stealer from accessing these
+            // tasks and we are the only producer.
+            self.inner.buffer[i_idx].with_mut(|ptr| unsafe {
+                let ptr = (*ptr).as_ptr();
+                (*ptr).header().queue_next.with_mut(|ptr| *ptr = Some(next));
+            });
+        }
+
+        // safety: the above CAS prevents a stealer from accessing these tasks
+        // and we are the only producer.
+        let head = self.inner.buffer[head as usize & MASK]
+            .with(|ptr| unsafe { ptr::read((*ptr).as_ptr()) });
+
+        // Push the tasks onto the inject queue
+        inject.push_batch(head, task, BATCH_LEN);
+
+        Ok(())
+    }
+
+    /// Pops a task from the local queue.
+    pub(super) fn pop(&mut self) -> Option<task::Notified<T>> {
+        let mut head = self.inner.head.load(Acquire);
+
+        let idx = loop {
+            let (steal, real) = unpack(head);
+
+            // safety: this is the **only** thread that updates this cell.
+            let tail = unsafe { self.inner.tail.unsync_load() };
+
+            if real == tail {
+                // queue is empty
+                return None;
+            }
+
+            let next_real = real.wrapping_add(1);
+
+            // If `steal == real` there are no concurrent stealers. Both `steal`
+            // and `real` are updated.
+            let next = if steal == real {
+                pack(next_real, next_real)
+            } else {
+                assert_ne!(steal, next_real);
+                pack(steal, next_real)
+            };
+
+            // Attempt to claim a task.
+            let res = self
+                .inner
+                .head
+                .compare_exchange(head, next, AcqRel, Acquire);
+
+            match res {
+                Ok(_) => break real as usize & MASK,
+                Err(actual) => head = actual,
+            }
+        };
+
+        Some(self.inner.buffer[idx].with(|ptr| unsafe { ptr::read(ptr).assume_init() }))
+    }
+}
+
+impl<T> Steal<T> {
+    pub(super) fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    /// Steals half the tasks from self and place them into `dst`.
+    pub(super) fn steal_into(&self, dst: &mut Local<T>) -> Option<task::Notified<T>> {
+        // Safety: the caller is the only thread that mutates `dst.tail` and
+        // holds a mutable reference.
+        let dst_tail = unsafe { dst.inner.tail.unsync_load() };
+
+        // To the caller, `dst` may **look** empty but still have values
+        // contained in the buffer. If another thread is concurrently stealing
+        // from `dst` there may not be enough capacity to steal.
+        let (steal, _) = unpack(dst.inner.head.load(Acquire));
+
+        if dst_tail.wrapping_sub(steal) > LOCAL_QUEUE_CAPACITY as u16 / 2 {
+            // we *could* try to steal less here, but for simplicity, we're just
+            // going to abort.
+            return None;
+        }
+
+        // Steal the tasks into `dst`'s buffer. This does not yet expose the
+        // tasks in `dst`.
+        let mut n = self.steal_into2(dst, dst_tail);
+
+        if n == 0 {
+            // No tasks were stolen
+            return None;
+        }
+
+        // We are returning a task here
+        n -= 1;
+
+        let ret_pos = dst_tail.wrapping_add(n);
+        let ret_idx = ret_pos as usize & MASK;
+
+        // safety: the value was written as part of `steal_into2` and not
+        // exposed to stealers, so no other thread can access it.
+        let ret = dst.inner.buffer[ret_idx].with(|ptr| unsafe { ptr::read((*ptr).as_ptr()) });
+
+        if n == 0 {
+            // The `dst` queue is empty, but a single task was stolen
+            return Some(ret);
+        }
+
+        // Make the stolen items available to consumers
+        dst.inner.tail.store(dst_tail.wrapping_add(n), Release);
+
+        Some(ret)
+    }
+
+    // Steal tasks from `self`, placing them into `dst`. Returns the number of
+    // tasks that were stolen.
+    fn steal_into2(&self, dst: &mut Local<T>, dst_tail: u16) -> u16 {
+        let mut prev_packed = self.0.head.load(Acquire);
+        let mut next_packed;
+
+        let n = loop {
+            let (src_head_steal, src_head_real) = unpack(prev_packed);
+            let src_tail = self.0.tail.load(Acquire);
+
+            // If these two do not match, another thread is concurrently
+            // stealing from the queue.
+            if src_head_steal != src_head_real {
+                return 0;
+            }
+
+            // Number of available tasks to steal
+            let n = src_tail.wrapping_sub(src_head_real);
+            let n = n - n / 2;
+
+            if n == 0 {
+                // No tasks available to steal
+                return 0;
+            }
+
+            // Update the real head index to acquire the tasks.
+            let steal_to = src_head_real.wrapping_add(n);
+            assert_ne!(src_head_steal, steal_to);
+            next_packed = pack(src_head_steal, steal_to);
+
+            // Claim all those tasks. This is done by incrementing the "real"
+            // head but not the steal. By doing this, no other thread is able to
+            // steal from this queue until the current thread completes.
+            let res = self
+                .0
+                .head
+                .compare_exchange(prev_packed, next_packed, AcqRel, Acquire);
+
+            match res {
+                Ok(_) => break n,
+                Err(actual) => prev_packed = actual,
+            }
+        };
+
+        assert!(n <= LOCAL_QUEUE_CAPACITY as u16 / 2, "actual = {}", n);
+
+        let (first, _) = unpack(next_packed);
+
+        // Take all the tasks
+        for i in 0..n {
+            // Compute the positions
+            let src_pos = first.wrapping_add(i);
+            let dst_pos = dst_tail.wrapping_add(i);
+
+            // Map to slots
+            let src_idx = src_pos as usize & MASK;
+            let dst_idx = dst_pos as usize & MASK;
+
+            // Read the task
+            //
+            // safety: We acquired the task with the atomic exchange above.
+            let task = self.0.buffer[src_idx].with(|ptr| unsafe { ptr::read((*ptr).as_ptr()) });
+
+            // Write the task to the new slot
+            //
+            // safety: `dst` queue is empty and we are the only producer to
+            // this queue.
+            dst.inner.buffer[dst_idx]
+                .with_mut(|ptr| unsafe { ptr::write((*ptr).as_mut_ptr(), task) });
+        }
+
+        let mut prev_packed = next_packed;
+
+        // Update `src_head_steal` to match `src_head_real` signalling that the
+        // stealing routine is complete.
+        loop {
+            let head = unpack(prev_packed).1;
+            next_packed = pack(head, head);
+
+            let res = self
+                .0
+                .head
+                .compare_exchange(prev_packed, next_packed, AcqRel, Acquire);
+
+            match res {
+                Ok(_) => return n,
+                Err(actual) => {
+                    let (actual_steal, actual_real) = unpack(actual);
+
+                    assert_ne!(actual_steal, actual_real);
+
+                    prev_packed = actual;
+                }
+            }
+        }
+    }
+}
+
+impl<T> Clone for Steal<T> {
+    fn clone(&self) -> Steal<T> {
+        Steal(self.0.clone())
+    }
+}
+
+impl<T> Drop for Local<T> {
+    fn drop(&mut self) {
+        if !std::thread::panicking() {
+            assert!(self.pop().is_none(), "queue not empty");
+        }
+    }
+}
+
+impl<T> Inner<T> {
+    fn is_empty(&self) -> bool {
+        let (_, head) = unpack(self.head.load(Acquire));
+        let tail = self.tail.load(Acquire);
+
+        head == tail
+    }
+}
+
+impl<T: 'static> Inject<T> {
+    pub(super) fn new() -> Inject<T> {
+        Inject {
+            pointers: Mutex::new(Pointers {
+                is_closed: false,
+                head: None,
+                tail: None,
+            }),
+            len: AtomicUsize::new(0),
+            _p: PhantomData,
+        }
+    }
+
+    pub(super) fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    /// Close the injection queue, returns `true` if the queue is open when the
+    /// transition is made.
+    pub(super) fn close(&self) -> bool {
+        let mut p = self.pointers.lock().unwrap();
+
+        if p.is_closed {
+            return false;
+        }
+
+        p.is_closed = true;
+        true
+    }
+
+    pub(super) fn is_closed(&self) -> bool {
+        self.pointers.lock().unwrap().is_closed
+    }
+
+    pub(super) fn len(&self) -> usize {
+        self.len.load(Acquire)
+    }
+
+    /// Pushes a value into the queue.
+    pub(super) fn push(&self, task: task::Notified<T>) {
+        // Acquire queue lock
+        let mut p = self.pointers.lock().unwrap();
+
+        if p.is_closed {
+            // Drop the mutex to avoid a potential deadlock when
+            // re-entering.
+            drop(p);
+            drop(task);
+            return;
+        }
+
+        // safety: only mutated with the lock held
+        let len = unsafe { self.len.unsync_load() };
+        let task = task.into_raw();
+
+        // The next pointer should already be null
+        debug_assert!(get_next(task).is_none());
+
+        if let Some(tail) = p.tail {
+            set_next(tail, Some(task));
+        } else {
+            p.head = Some(task);
+        }
+
+        p.tail = Some(task);
+
+        self.len.store(len + 1, Release);
+    }
+
+    pub(super) fn push_batch(
+        &self,
+        batch_head: task::Notified<T>,
+        batch_tail: task::Notified<T>,
+        num: usize,
+    ) {
+        let batch_head = batch_head.into_raw();
+        let batch_tail = batch_tail.into_raw();
+
+        debug_assert!(get_next(batch_tail).is_none());
+
+        let mut p = self.pointers.lock().unwrap();
+
+        if let Some(tail) = p.tail {
+            set_next(tail, Some(batch_head));
+        } else {
+            p.head = Some(batch_head);
+        }
+
+        p.tail = Some(batch_tail);
+
+        // Increment the count.
+        //
+        // safety: All updates to the len atomic are guarded by the mutex. As
+        // such, a non-atomic load followed by a store is safe.
+        let len = unsafe { self.len.unsync_load() };
+
+        self.len.store(len + num, Release);
+    }
+
+    pub(super) fn pop(&self) -> Option<task::Notified<T>> {
+        // Fast path, if len == 0, then there are no values
+        if self.is_empty() {
+            return None;
+        }
+
+        let mut p = self.pointers.lock().unwrap();
+
+        // It is possible to hit null here if another thread poped the last
+        // task between us checking `len` and acquiring the lock.
+        let task = p.head?;
+
+        p.head = get_next(task);
+
+        if p.head.is_none() {
+            p.tail = None;
+        }
+
+        set_next(task, None);
+
+        // Decrement the count.
+        //
+        // safety: All updates to the len atomic are guarded by the mutex. As
+        // such, a non-atomic load followed by a store is safe.
+        self.len
+            .store(unsafe { self.len.unsync_load() } - 1, Release);
+
+        // safety: a `Notified` is pushed into the queue and now it is popped!
+        Some(unsafe { task::Notified::from_raw(task) })
+    }
+}
+
+impl<T: 'static> Drop for Inject<T> {
+    fn drop(&mut self) {
+        if !std::thread::panicking() {
+            assert!(self.pop().is_none(), "queue not empty");
+        }
+    }
+}
+
+fn get_next(header: NonNull<task::Header>) -> Option<NonNull<task::Header>> {
+    unsafe { header.as_ref().queue_next.with(|ptr| *ptr) }
+}
+
+fn set_next(header: NonNull<task::Header>, val: Option<NonNull<task::Header>>) {
+    unsafe {
+        header.as_ref().queue_next.with_mut(|ptr| *ptr = val);
+    }
+}
+
+/// Split the head value into the real head and the index a stealer is working
+/// on.
+fn unpack(n: u32) -> (u16, u16) {
+    let real = n & u16::max_value() as u32;
+    let steal = n >> 16;
+
+    (steal as u16, real as u16)
+}
+
+/// Join the two head values
+fn pack(steal: u16, real: u16) -> u32 {
+    (real as u32) | ((steal as u32) << 16)
+}
+
+#[test]
+fn test_local_queue_capacity() {
+    assert!(LOCAL_QUEUE_CAPACITY - 1 <= u8::max_value() as usize);
+}
diff --git a/third_party/rust/tokio/src/runtime/shell.rs b/third_party/rust/tokio/src/runtime/shell.rs
new file mode 100644
index 0000000000..294f2a16d8
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/shell.rs
@@ -0,0 +1,62 @@
+#![allow(clippy::redundant_clone)]
+
+use crate::park::{Park, Unpark};
+use crate::runtime::enter;
+use crate::runtime::time;
+use crate::util::{waker_ref, Wake};
+
+use std::future::Future;
+use std::sync::Arc;
+use std::task::Context;
+use std::task::Poll::Ready;
+
+#[derive(Debug)]
+pub(super) struct Shell {
+    driver: time::Driver,
+
+    /// TODO: don't store this
+    unpark: Arc<Handle>,
+}
+
+#[derive(Debug)]
+struct Handle(<time::Driver as Park>::Unpark);
+
+impl Shell {
+    pub(super) fn new(driver: time::Driver) -> Shell {
+        let unpark = Arc::new(Handle(driver.unpark()));
+
+        Shell { driver, unpark }
+    }
+
+    pub(super) fn block_on<F>(&mut self, f: F) -> F::Output
+    where
+        F: Future,
+    {
+        let _e = enter();
+
+        pin!(f);
+
+        let waker = waker_ref(&self.unpark);
+        let mut cx = Context::from_waker(&waker);
+
+        loop {
+            if let Ready(v) = crate::coop::budget(|| f.as_mut().poll(&mut cx)) {
+                return v;
+            }
+
+            self.driver.park().unwrap();
+        }
+    }
+}
+
+impl Wake for Handle {
+    /// Wake by value
+    fn wake(self: Arc<Self>) {
+        Wake::wake_by_ref(&self);
+    }
+
+    /// Wake by reference
+    fn wake_by_ref(arc_self: &Arc<Self>) {
+        arc_self.0.unpark();
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/spawner.rs b/third_party/rust/tokio/src/runtime/spawner.rs
new file mode 100644
index 0000000000..d136945cdc
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/spawner.rs
@@ -0,0 +1,37 @@
+cfg_rt_core! {
+    use crate::runtime::basic_scheduler;
+    use crate::task::JoinHandle;
+
+    use std::future::Future;
+}
+
+cfg_rt_threaded! {
+    use crate::runtime::thread_pool;
+}
+
+#[derive(Debug, Clone)]
+pub(crate) enum Spawner {
+    Shell,
+    #[cfg(feature = "rt-core")]
+    Basic(basic_scheduler::Spawner),
+    #[cfg(feature = "rt-threaded")]
+    ThreadPool(thread_pool::Spawner),
+}
+
+cfg_rt_core! {
+    impl Spawner {
+        pub(crate) fn spawn<F>(&self, future: F) -> JoinHandle<F::Output>
+        where
+            F: Future + Send + 'static,
+            F::Output: Send + 'static,
+        {
+            match self {
+                Spawner::Shell => panic!("spawning not enabled for runtime"),
+                #[cfg(feature = "rt-core")]
+                Spawner::Basic(spawner) => spawner.spawn(future),
+                #[cfg(feature = "rt-threaded")]
+                Spawner::ThreadPool(spawner) => spawner.spawn(future),
+            }
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/task/core.rs b/third_party/rust/tokio/src/runtime/task/core.rs
new file mode 100644
index 0000000000..573b9f3c9c
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/task/core.rs
@@ -0,0 +1,279 @@
+use crate::loom::cell::UnsafeCell;
+use crate::runtime::task::raw::{self, Vtable};
+use crate::runtime::task::state::State;
+use crate::runtime::task::waker::waker_ref;
+use crate::runtime::task::{Notified, Schedule, Task};
+use crate::util::linked_list;
+
+use std::future::Future;
+use std::pin::Pin;
+use std::ptr::NonNull;
+use std::task::{Context, Poll, Waker};
+
+/// The task cell. Contains the components of the task.
+///
+/// It is critical for `Header` to be the first field as the task structure will
+/// be referenced by both *mut Cell and *mut Header.
+#[repr(C)]
+pub(super) struct Cell<T: Future, S> {
+    /// Hot task state data
+    pub(super) header: Header,
+
+    /// Either the future or output, depending on the execution stage.
+    pub(super) core: Core<T, S>,
+
+    /// Cold data
+    pub(super) trailer: Trailer,
+}
+
+/// The core of the task.
+///
+/// Holds the future or output, depending on the stage of execution.
+pub(super) struct Core<T: Future, S> {
+    /// Scheduler used to drive this future
+    pub(super) scheduler: UnsafeCell<Option<S>>,
+
+    /// Either the future or the output
+    pub(super) stage: UnsafeCell<Stage<T>>,
+}
+
+/// Crate public as this is also needed by the pool.
+#[repr(C)]
+pub(crate) struct Header {
+    /// Task state
+    pub(super) state: State,
+
+    pub(crate) owned: UnsafeCell<linked_list::Pointers<Header>>,
+
+    /// Pointer to next task, used with the injection queue
+    pub(crate) queue_next: UnsafeCell<Option<NonNull<Header>>>,
+
+    /// Pointer to the next task in the transfer stack
+    pub(super) stack_next: UnsafeCell<Option<NonNull<Header>>>,
+
+    /// Table of function pointers for executing actions on the task.
+    pub(super) vtable: &'static Vtable,
+}
+
+unsafe impl Send for Header {}
+unsafe impl Sync for Header {}
+
+/// Cold data is stored after the future.
+pub(super) struct Trailer {
+    /// Consumer task waiting on completion of this task.
+    pub(super) waker: UnsafeCell<Option<Waker>>,
+}
+
+/// Either the future or the output.
+pub(super) enum Stage<T: Future> {
+    Running(T),
+    Finished(super::Result<T::Output>),
+    Consumed,
+}
+
+impl<T: Future, S: Schedule> Cell<T, S> {
+    /// Allocates a new task cell, containing the header, trailer, and core
+    /// structures.
+    pub(super) fn new(future: T, state: State) -> Box<Cell<T, S>> {
+        Box::new(Cell {
+            header: Header {
+                state,
+                owned: UnsafeCell::new(linked_list::Pointers::new()),
+                queue_next: UnsafeCell::new(None),
+                stack_next: UnsafeCell::new(None),
+                vtable: raw::vtable::<T, S>(),
+            },
+            core: Core {
+                scheduler: UnsafeCell::new(None),
+                stage: UnsafeCell::new(Stage::Running(future)),
+            },
+            trailer: Trailer {
+                waker: UnsafeCell::new(None),
+            },
+        })
+    }
+}
+
+impl<T: Future, S: Schedule> Core<T, S> {
+    /// If needed, bind a scheduler to the task.
+    ///
+    /// This only happens on the first poll.
+    pub(super) fn bind_scheduler(&self, task: Task<S>) {
+        use std::mem::ManuallyDrop;
+
+        // TODO: it would be nice to not have to wrap with a ManuallyDrop
+        let task = ManuallyDrop::new(task);
+
+        // This function may be called concurrently, but the __first__ time it
+        // is called, the caller has unique access to this field. All subsequent
+        // concurrent calls will be via the `Waker`, which will "happens after"
+        // the first poll.
+        //
+        // In other words, it is always safe to read the field and it is safe to
+        // write to the field when it is `None`.
+        if self.is_bound() {
+            return;
+        }
+
+        // Bind the task to the scheduler
+        let scheduler = S::bind(ManuallyDrop::into_inner(task));
+
+        // Safety: As `scheduler` is not set, this is the first poll
+        self.scheduler.with_mut(|ptr| unsafe {
+            *ptr = Some(scheduler);
+        });
+    }
+
+    /// Returns true if the task is bound to a scheduler.
+    pub(super) fn is_bound(&self) -> bool {
+        // Safety: never called concurrently w/ a mutation.
+        self.scheduler.with(|ptr| unsafe { (*ptr).is_some() })
+    }
+
+    /// Poll the future
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure it is safe to mutate the `state` field. This
+    /// requires ensuring mutal exclusion between any concurrent thread that
+    /// might modify the future or output field.
+    ///
+    /// The mutual exclusion is implemented by `Harness` and the `Lifecycle`
+    /// component of the task state.
+    ///
+    /// `self` must also be pinned. This is handled by storing the task on the
+    /// heap.
+    pub(super) fn poll(&self, header: &Header) -> Poll<T::Output> {
+        let res = {
+            self.stage.with_mut(|ptr| {
+                // Safety: The caller ensures mutual exclusion to the field.
+                let future = match unsafe { &mut *ptr } {
+                    Stage::Running(future) => future,
+                    _ => unreachable!("unexpected stage"),
+                };
+
+                // Safety: The caller ensures the future is pinned.
+                let future = unsafe { Pin::new_unchecked(future) };
+
+                // The waker passed into the `poll` function does not require a ref
+                // count increment.
+                let waker_ref = waker_ref::<T, S>(header);
+                let mut cx = Context::from_waker(&*waker_ref);
+
+                future.poll(&mut cx)
+            })
+        };
+
+        if res.is_ready() {
+            self.drop_future_or_output();
+        }
+
+        res
+    }
+
+    /// Drop the future
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure it is safe to mutate the `stage` field.
+    pub(super) fn drop_future_or_output(&self) {
+        self.stage.with_mut(|ptr| {
+            // Safety: The caller ensures mutal exclusion to the field.
+            unsafe { *ptr = Stage::Consumed };
+        });
+    }
+
+    /// Store the task output
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure it is safe to mutate the `stage` field.
+    pub(super) fn store_output(&self, output: super::Result<T::Output>) {
+        self.stage.with_mut(|ptr| {
+            // Safety: the caller ensures mutual exclusion to the field.
+            unsafe { *ptr = Stage::Finished(output) };
+        });
+    }
+
+    /// Take the task output
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure it is safe to mutate the `stage` field.
+    pub(super) fn take_output(&self) -> super::Result<T::Output> {
+        use std::mem;
+
+        self.stage.with_mut(|ptr| {
+            // Safety:: the caller ensures mutal exclusion to the field.
+            match mem::replace(unsafe { &mut *ptr }, Stage::Consumed) {
+                Stage::Finished(output) => output,
+                _ => panic!("unexpected task state"),
+            }
+        })
+    }
+
+    /// Schedule the future for execution
+    pub(super) fn schedule(&self, task: Notified<S>) {
+        self.scheduler.with(|ptr| {
+            // Safety: Can only be called after initial `poll`, which is the
+            // only time the field is mutated.
+            match unsafe { &*ptr } {
+                Some(scheduler) => scheduler.schedule(task),
+                None => panic!("no scheduler set"),
+            }
+        });
+    }
+
+    /// Schedule the future for execution in the near future, yielding the
+    /// thread to other tasks.
+    pub(super) fn yield_now(&self, task: Notified<S>) {
+        self.scheduler.with(|ptr| {
+            // Safety: Can only be called after initial `poll`, which is the
+            // only time the field is mutated.
+            match unsafe { &*ptr } {
+                Some(scheduler) => scheduler.yield_now(task),
+                None => panic!("no scheduler set"),
+            }
+        });
+    }
+
+    /// Release the task
+    ///
+    /// If the `Scheduler` implementation is able to, it returns the `Task`
+    /// handle immediately. The caller of this function will batch a ref-dec
+    /// with a state change.
+    pub(super) fn release(&self, task: Task<S>) -> Option<Task<S>> {
+        use std::mem::ManuallyDrop;
+
+        let task = ManuallyDrop::new(task);
+
+        self.scheduler.with(|ptr| {
+            // Safety: Can only be called after initial `poll`, which is the
+            // only time the field is mutated.
+            match unsafe { &*ptr } {
+                Some(scheduler) => scheduler.release(&*task),
+                // Task was never polled
+                None => None,
+            }
+        })
+    }
+}
+
+cfg_rt_threaded! {
+    impl Header {
+        pub(crate) fn shutdown(&self) {
+            use crate::runtime::task::RawTask;
+
+            let task = unsafe { RawTask::from_raw(self.into()) };
+            task.shutdown();
+        }
+    }
+}
+
+#[test]
+#[cfg(not(loom))]
+fn header_lte_cache_line() {
+    use std::mem::size_of;
+
+    assert!(size_of::<Header>() <= 8 * size_of::<*const ()>());
+}
diff --git a/third_party/rust/tokio/src/runtime/task/error.rs b/third_party/rust/tokio/src/runtime/task/error.rs
new file mode 100644
index 0000000000..d5f65a4981
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/task/error.rs
@@ -0,0 +1,163 @@
+use std::any::Any;
+use std::fmt;
+use std::io;
+use std::sync::Mutex;
+
+doc_rt_core! {
+    /// Task failed to execute to completion.
+    pub struct JoinError {
+        repr: Repr,
+    }
+}
+
+enum Repr {
+    Cancelled,
+    Panic(Mutex<Box<dyn Any + Send + 'static>>),
+}
+
+impl JoinError {
+    #[doc(hidden)]
+    #[deprecated]
+    pub fn cancelled() -> JoinError {
+        Self::cancelled2()
+    }
+
+    pub(crate) fn cancelled2() -> JoinError {
+        JoinError {
+            repr: Repr::Cancelled,
+        }
+    }
+
+    #[doc(hidden)]
+    #[deprecated]
+    pub fn panic(err: Box<dyn Any + Send + 'static>) -> JoinError {
+        Self::panic2(err)
+    }
+
+    pub(crate) fn panic2(err: Box<dyn Any + Send + 'static>) -> JoinError {
+        JoinError {
+            repr: Repr::Panic(Mutex::new(err)),
+        }
+    }
+
+    /// Returns true if the error was caused by the task being cancelled
+    pub fn is_cancelled(&self) -> bool {
+        match &self.repr {
+            Repr::Cancelled => true,
+            _ => false,
+        }
+    }
+
+    /// Returns true if the error was caused by the task panicking
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::panic;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let err = tokio::spawn(async {
+    ///         panic!("boom");
+    ///     }).await.unwrap_err();
+    ///
+    ///     assert!(err.is_panic());
+    /// }
+    /// ```
+    pub fn is_panic(&self) -> bool {
+        match &self.repr {
+            Repr::Panic(_) => true,
+            _ => false,
+        }
+    }
+
+    /// Consumes the join error, returning the object with which the task panicked.
+    ///
+    /// # Panics
+    ///
+    /// `into_panic()` panics if the `Error` does not represent the underlying
+    /// task terminating with a panic. Use `is_panic` to check the error reason
+    /// or `try_into_panic` for a variant that does not panic.
+    ///
+    /// # Examples
+    ///
+    /// ```should_panic
+    /// use std::panic;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let err = tokio::spawn(async {
+    ///         panic!("boom");
+    ///     }).await.unwrap_err();
+    ///
+    ///     if err.is_panic() {
+    ///         // Resume the panic on the main task
+    ///         panic::resume_unwind(err.into_panic());
+    ///     }
+    /// }
+    /// ```
+    pub fn into_panic(self) -> Box<dyn Any + Send + 'static> {
+        self.try_into_panic()
+            .expect("`JoinError` reason is not a panic.")
+    }
+
+    /// Consumes the join error, returning the object with which the task
+    /// panicked if the task terminated due to a panic. Otherwise, `self` is
+    /// returned.
+    ///
+    /// # Examples
+    ///
+    /// ```should_panic
+    /// use std::panic;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let err = tokio::spawn(async {
+    ///         panic!("boom");
+    ///     }).await.unwrap_err();
+    ///
+    ///     if let Ok(reason) = err.try_into_panic() {
+    ///         // Resume the panic on the main task
+    ///         panic::resume_unwind(reason);
+    ///     }
+    /// }
+    /// ```
+    pub fn try_into_panic(self) -> Result<Box<dyn Any + Send + 'static>, JoinError> {
+        match self.repr {
+            Repr::Panic(p) => Ok(p.into_inner().expect("Extracting panic from mutex")),
+            _ => Err(self),
+        }
+    }
+}
+
+impl fmt::Display for JoinError {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match &self.repr {
+            Repr::Cancelled => write!(fmt, "cancelled"),
+            Repr::Panic(_) => write!(fmt, "panic"),
+        }
+    }
+}
+
+impl fmt::Debug for JoinError {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match &self.repr {
+            Repr::Cancelled => write!(fmt, "JoinError::Cancelled"),
+            Repr::Panic(_) => write!(fmt, "JoinError::Panic(...)"),
+        }
+    }
+}
+
+impl std::error::Error for JoinError {}
+
+impl From<JoinError> for io::Error {
+    fn from(src: JoinError) -> io::Error {
+        io::Error::new(
+            io::ErrorKind::Other,
+            match src.repr {
+                Repr::Cancelled => "task was cancelled",
+                Repr::Panic(_) => "task panicked",
+            },
+        )
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/task/harness.rs b/third_party/rust/tokio/src/runtime/task/harness.rs
new file mode 100644
index 0000000000..29b231ea88
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/task/harness.rs
@@ -0,0 +1,372 @@
+use crate::runtime::task::core::{Cell, Core, Header, Trailer};
+use crate::runtime::task::state::Snapshot;
+use crate::runtime::task::{JoinError, Notified, Schedule, Task};
+
+use std::future::Future;
+use std::mem;
+use std::panic;
+use std::ptr::NonNull;
+use std::task::{Poll, Waker};
+
+/// Typed raw task handle
+pub(super) struct Harness<T: Future, S: 'static> {
+    cell: NonNull<Cell<T, S>>,
+}
+
+impl<T, S> Harness<T, S>
+where
+    T: Future,
+    S: 'static,
+{
+    pub(super) unsafe fn from_raw(ptr: NonNull<Header>) -> Harness<T, S> {
+        Harness {
+            cell: ptr.cast::<Cell<T, S>>(),
+        }
+    }
+
+    fn header(&self) -> &Header {
+        unsafe { &self.cell.as_ref().header }
+    }
+
+    fn trailer(&self) -> &Trailer {
+        unsafe { &self.cell.as_ref().trailer }
+    }
+
+    fn core(&self) -> &Core<T, S> {
+        unsafe { &self.cell.as_ref().core }
+    }
+}
+
+impl<T, S> Harness<T, S>
+where
+    T: Future,
+    S: Schedule,
+{
+    /// Polls the inner future.
+    ///
+    /// All necessary state checks and transitions are performed.
+    ///
+    /// Panics raised while polling the future are handled.
+    pub(super) fn poll(self) {
+        // If this is the first time the task is polled, the task will be bound
+        // to the scheduler, in which case the task ref count must be
+        // incremented.
+        let ref_inc = !self.core().is_bound();
+
+        // Transition the task to the running state.
+        //
+        // A failure to transition here indicates the task has been cancelled
+        // while in the run queue pending execution.
+        let snapshot = match self.header().state.transition_to_running(ref_inc) {
+            Ok(snapshot) => snapshot,
+            Err(_) => {
+                // The task was shutdown while in the run queue. At this point,
+                // we just hold a ref counted reference. Drop it here.
+                self.drop_reference();
+                return;
+            }
+        };
+
+        // Ensure the task is bound to a scheduler instance. If this is the
+        // first time polling the task, a scheduler instance is pulled from the
+        // local context and assigned to the task.
+        //
+        // The scheduler maintains ownership of the task and responds to `wake`
+        // calls.
+        //
+        // The task reference count has been incremented.
+        self.core().bind_scheduler(self.to_task());
+
+        // The transition to `Running` done above ensures that a lock on the
+        // future has been obtained. This also ensures the `*mut T` pointer
+        // contains the future (as opposed to the output) and is initialized.
+
+        let res = panic::catch_unwind(panic::AssertUnwindSafe(|| {
+            struct Guard<'a, T: Future, S: Schedule> {
+                core: &'a Core<T, S>,
+                polled: bool,
+            }
+
+            impl<T: Future, S: Schedule> Drop for Guard<'_, T, S> {
+                fn drop(&mut self) {
+                    if !self.polled {
+                        self.core.drop_future_or_output();
+                    }
+                }
+            }
+
+            let mut guard = Guard {
+                core: self.core(),
+                polled: false,
+            };
+
+            // If the task is cancelled, avoid polling it, instead signalling it
+            // is complete.
+            if snapshot.is_cancelled() {
+                Poll::Ready(Err(JoinError::cancelled2()))
+            } else {
+                let res = guard.core.poll(self.header());
+
+                // prevent the guard from dropping the future
+                guard.polled = true;
+
+                res.map(Ok)
+            }
+        }));
+
+        match res {
+            Ok(Poll::Ready(out)) => {
+                self.complete(out, snapshot.is_join_interested());
+            }
+            Ok(Poll::Pending) => {
+                match self.header().state.transition_to_idle() {
+                    Ok(snapshot) => {
+                        if snapshot.is_notified() {
+                            // Signal yield
+                            self.core().yield_now(Notified(self.to_task()));
+                            // The ref-count was incremented as part of
+                            // `transition_to_idle`.
+                            self.drop_reference();
+                        }
+                    }
+                    Err(_) => self.cancel_task(),
+                }
+            }
+            Err(err) => {
+                self.complete(Err(JoinError::panic2(err)), snapshot.is_join_interested());
+            }
+        }
+    }
+
+    pub(super) fn dealloc(self) {
+        // Release the join waker, if there is one.
+        self.trailer().waker.with_mut(|_| ());
+
+        // Check causality
+        self.core().stage.with_mut(|_| {});
+        self.core().scheduler.with_mut(|_| {});
+
+        unsafe {
+            drop(Box::from_raw(self.cell.as_ptr()));
+        }
+    }
+
+    // ===== join handle =====
+
+    /// Read the task output into `dst`.
+    pub(super) fn try_read_output(self, dst: &mut Poll<super::Result<T::Output>>, waker: &Waker) {
+        // Load a snapshot of the current task state
+        let snapshot = self.header().state.load();
+
+        debug_assert!(snapshot.is_join_interested());
+
+        if !snapshot.is_complete() {
+            // The waker must be stored in the task struct.
+            let res = if snapshot.has_join_waker() {
+                // There already is a waker stored in the struct. If it matches
+                // the provided waker, then there is no further work to do.
+                // Otherwise, the waker must be swapped.
+                let will_wake = unsafe {
+                    // Safety: when `JOIN_INTEREST` is set, only `JOIN_HANDLE`
+                    // may mutate the `waker` field.
+                    self.trailer()
+                        .waker
+                        .with(|ptr| (*ptr).as_ref().unwrap().will_wake(waker))
+                };
+
+                if will_wake {
+                    // The task is not complete **and** the waker is up to date,
+                    // there is nothing further that needs to be done.
+                    return;
+                }
+
+                // Unset the `JOIN_WAKER` to gain mutable access to the `waker`
+                // field then update the field with the new join worker.
+                //
+                // This requires two atomic operations, unsetting the bit and
+                // then resetting it. If the task transitions to complete
+                // concurrently to either one of those operations, then setting
+                // the join waker fails and we proceed to reading the task
+                // output.
+                self.header()
+                    .state
+                    .unset_waker()
+                    .and_then(|snapshot| self.set_join_waker(waker.clone(), snapshot))
+            } else {
+                self.set_join_waker(waker.clone(), snapshot)
+            };
+
+            match res {
+                Ok(_) => return,
+                Err(snapshot) => {
+                    assert!(snapshot.is_complete());
+                }
+            }
+        }
+
+        *dst = Poll::Ready(self.core().take_output());
+    }
+
+    fn set_join_waker(&self, waker: Waker, snapshot: Snapshot) -> Result<Snapshot, Snapshot> {
+        assert!(snapshot.is_join_interested());
+        assert!(!snapshot.has_join_waker());
+
+        // Safety: Only the `JoinHandle` may set the `waker` field. When
+        // `JOIN_INTEREST` is **not** set, nothing else will touch the field.
+        unsafe {
+            self.trailer().waker.with_mut(|ptr| {
+                *ptr = Some(waker);
+            });
+        }
+
+        // Update the `JoinWaker` state accordingly
+        let res = self.header().state.set_join_waker();
+
+        // If the state could not be updated, then clear the join waker
+        if res.is_err() {
+            unsafe {
+                self.trailer().waker.with_mut(|ptr| {
+                    *ptr = None;
+                });
+            }
+        }
+
+        res
+    }
+
+    pub(super) fn drop_join_handle_slow(self) {
+        // Try to unset `JOIN_INTEREST`. This must be done as a first step in
+        // case the task concurrently completed.
+        if self.header().state.unset_join_interested().is_err() {
+            // It is our responsibility to drop the output. This is critical as
+            // the task output may not be `Send` and as such must remain with
+            // the scheduler or `JoinHandle`. i.e. if the output remains in the
+            // task structure until the task is deallocated, it may be dropped
+            // by a Waker on any arbitrary thread.
+            self.core().drop_future_or_output();
+        }
+
+        // Drop the `JoinHandle` reference, possibly deallocating the task
+        self.drop_reference();
+    }
+
+    // ===== waker behavior =====
+
+    pub(super) fn wake_by_val(self) {
+        self.wake_by_ref();
+        self.drop_reference();
+    }
+
+    pub(super) fn wake_by_ref(&self) {
+        if self.header().state.transition_to_notified() {
+            self.core().schedule(Notified(self.to_task()));
+        }
+    }
+
+    pub(super) fn drop_reference(self) {
+        if self.header().state.ref_dec() {
+            self.dealloc();
+        }
+    }
+
+    /// Forcibly shutdown the task
+    ///
+    /// Attempt to transition to `Running` in order to forcibly shutdown the
+    /// task. If the task is currently running or in a state of completion, then
+    /// there is nothing further to do. When the task completes running, it will
+    /// notice the `CANCELLED` bit and finalize the task.
+    pub(super) fn shutdown(self) {
+        if !self.header().state.transition_to_shutdown() {
+            // The task is concurrently running. No further work needed.
+            return;
+        }
+
+        // By transitioning the lifcycle to `Running`, we have permission to
+        // drop the future.
+        self.cancel_task();
+    }
+
+    // ====== internal ======
+
+    fn cancel_task(self) {
+        // Drop the future from a panic guard.
+        let res = panic::catch_unwind(panic::AssertUnwindSafe(|| {
+            self.core().drop_future_or_output();
+        }));
+
+        if let Err(err) = res {
+            // Dropping the future panicked, complete the join
+            // handle with the panic to avoid dropping the panic
+            // on the ground.
+            self.complete(Err(JoinError::panic2(err)), true);
+        } else {
+            self.complete(Err(JoinError::cancelled2()), true);
+        }
+    }
+
+    fn complete(mut self, output: super::Result<T::Output>, is_join_interested: bool) {
+        if is_join_interested {
+            // Store the output. The future has already been dropped
+            //
+            // Safety: Mutual exclusion is obtained by having transitioned the task
+            // state -> Running
+            self.core().store_output(output);
+
+            // Transition to `Complete`, notifying the `JoinHandle` if necessary.
+            self.transition_to_complete();
+        }
+
+        // The task has completed execution and will no longer be scheduled.
+        //
+        // Attempts to batch a ref-dec with the state transition below.
+        let ref_dec = if self.core().is_bound() {
+            if let Some(task) = self.core().release(self.to_task()) {
+                mem::forget(task);
+                true
+            } else {
+                false
+            }
+        } else {
+            false
+        };
+
+        // This might deallocate
+        let snapshot = self
+            .header()
+            .state
+            .transition_to_terminal(!is_join_interested, ref_dec);
+
+        if snapshot.ref_count() == 0 {
+            self.dealloc()
+        }
+    }
+
+    /// Transitions the task's lifecycle to `Complete`. Notifies the
+    /// `JoinHandle` if it still has interest in the completion.
+    fn transition_to_complete(&mut self) {
+        // Transition the task's lifecycle to `Complete` and get a snapshot of
+        // the task's sate.
+        let snapshot = self.header().state.transition_to_complete();
+
+        if !snapshot.is_join_interested() {
+            // The `JoinHandle` is not interested in the output of this task. It
+            // is our responsibility to drop the output.
+            self.core().drop_future_or_output();
+        } else if snapshot.has_join_waker() {
+            // Notify the join handle. The previous transition obtains the
+            // lock on the waker cell.
+            self.wake_join();
+        }
+    }
+
+    fn wake_join(&self) {
+        self.trailer().waker.with(|ptr| match unsafe { &*ptr } {
+            Some(waker) => waker.wake_by_ref(),
+            None => panic!("waker missing"),
+        });
+    }
+
+    fn to_task(&self) -> Task<S> {
+        unsafe { Task::from_raw(self.header().into()) }
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/task/join.rs b/third_party/rust/tokio/src/runtime/task/join.rs
new file mode 100644
index 0000000000..fdcc346e5c
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/task/join.rs
@@ -0,0 +1,152 @@
+use crate::runtime::task::RawTask;
+
+use std::fmt;
+use std::future::Future;
+use std::marker::PhantomData;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+doc_rt_core! {
+    /// An owned permission to join on a task (await its termination).
+    ///
+    /// This can be thought of as the equivalent of [`std::thread::JoinHandle`] for
+    /// a task rather than a thread.
+    ///
+    /// A `JoinHandle` *detaches* the associated task when it is dropped, which
+    /// means that there is no longer any handle to the task, and no way to `join`
+    /// on it.
+    ///
+    /// This `struct` is created by the [`task::spawn`] and [`task::spawn_blocking`]
+    /// functions.
+    ///
+    /// # Examples
+    ///
+    /// Creation from [`task::spawn`]:
+    ///
+    /// ```
+    /// use tokio::task;
+    ///
+    /// # async fn doc() {
+    /// let join_handle: task::JoinHandle<_> = task::spawn(async {
+    ///     // some work here
+    /// });
+    /// # }
+    /// ```
+    ///
+    /// Creation from [`task::spawn_blocking`]:
+    ///
+    /// ```
+    /// use tokio::task;
+    ///
+    /// # async fn doc() {
+    /// let join_handle: task::JoinHandle<_> = task::spawn_blocking(|| {
+    ///     // some blocking work here
+    /// });
+    /// # }
+    /// ```
+    ///
+    /// Child being detached and outliving its parent:
+    ///
+    /// ```no_run
+    /// use tokio::task;
+    /// use tokio::time;
+    /// use std::time::Duration;
+    ///
+    /// # #[tokio::main] async fn main() {
+    /// let original_task = task::spawn(async {
+    ///     let _detached_task = task::spawn(async {
+    ///         // Here we sleep to make sure that the first task returns before.
+    ///         time::delay_for(Duration::from_millis(10)).await;
+    ///         // This will be called, even though the JoinHandle is dropped.
+    ///         println!("♫ Still alive ♫");
+    ///     });
+    /// });
+    ///
+    /// original_task.await.expect("The task being joined has panicked");
+    /// println!("Original task is joined.");
+    ///
+    /// // We make sure that the new task has time to run, before the main
+    /// // task returns.
+    ///
+    /// time::delay_for(Duration::from_millis(1000)).await;
+    /// # }
+    /// ```
+    ///
+    /// [`task::spawn`]: crate::task::spawn()
+    /// [`task::spawn_blocking`]: crate::task::spawn_blocking
+    /// [`std::thread::JoinHandle`]: std::thread::JoinHandle
+    pub struct JoinHandle<T> {
+        raw: Option<RawTask>,
+        _p: PhantomData<T>,
+    }
+}
+
+unsafe impl<T: Send> Send for JoinHandle<T> {}
+unsafe impl<T: Send> Sync for JoinHandle<T> {}
+
+impl<T> JoinHandle<T> {
+    pub(super) fn new(raw: RawTask) -> JoinHandle<T> {
+        JoinHandle {
+            raw: Some(raw),
+            _p: PhantomData,
+        }
+    }
+}
+
+impl<T> Unpin for JoinHandle<T> {}
+
+impl<T> Future for JoinHandle<T> {
+    type Output = super::Result<T>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let mut ret = Poll::Pending;
+
+        // Keep track of task budget
+        ready!(crate::coop::poll_proceed(cx));
+
+        // Raw should always be set. If it is not, this is due to polling after
+        // completion
+        let raw = self
+            .raw
+            .as_ref()
+            .expect("polling after `JoinHandle` already completed");
+
+        // Try to read the task output. If the task is not yet complete, the
+        // waker is stored and is notified once the task does complete.
+        //
+        // The function must go via the vtable, which requires erasing generic
+        // types. To do this, the function "return" is placed on the stack
+        // **before** calling the function and is passed into the function using
+        // `*mut ()`.
+        //
+        // Safety:
+        //
+        // The type of `T` must match the task's output type.
+        unsafe {
+            raw.try_read_output(&mut ret as *mut _ as *mut (), cx.waker());
+        }
+
+        ret
+    }
+}
+
+impl<T> Drop for JoinHandle<T> {
+    fn drop(&mut self) {
+        if let Some(raw) = self.raw.take() {
+            if raw.header().state.drop_join_handle_fast().is_ok() {
+                return;
+            }
+
+            raw.drop_join_handle_slow();
+        }
+    }
+}
+
+impl<T> fmt::Debug for JoinHandle<T>
+where
+    T: fmt::Debug,
+{
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("JoinHandle").finish()
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/task/mod.rs b/third_party/rust/tokio/src/runtime/task/mod.rs
new file mode 100644
index 0000000000..17b5157e84
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/task/mod.rs
@@ -0,0 +1,220 @@
+mod core;
+use self::core::Cell;
+pub(crate) use self::core::Header;
+
+mod error;
+#[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
+pub use self::error::JoinError;
+
+mod harness;
+use self::harness::Harness;
+
+mod join;
+#[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
+pub use self::join::JoinHandle;
+
+mod raw;
+use self::raw::RawTask;
+
+mod state;
+use self::state::State;
+
+mod waker;
+
+cfg_rt_threaded! {
+    mod stack;
+    pub(crate) use self::stack::TransferStack;
+}
+
+use crate::util::linked_list;
+
+use std::future::Future;
+use std::marker::PhantomData;
+use std::ptr::NonNull;
+use std::{fmt, mem};
+
+/// An owned handle to the task, tracked by ref count
+#[repr(transparent)]
+pub(crate) struct Task<S: 'static> {
+    raw: RawTask,
+    _p: PhantomData<S>,
+}
+
+unsafe impl<S> Send for Task<S> {}
+unsafe impl<S> Sync for Task<S> {}
+
+/// A task was notified
+#[repr(transparent)]
+pub(crate) struct Notified<S: 'static>(Task<S>);
+
+unsafe impl<S: Schedule> Send for Notified<S> {}
+unsafe impl<S: Schedule> Sync for Notified<S> {}
+
+/// Task result sent back
+pub(crate) type Result<T> = std::result::Result<T, JoinError>;
+
+pub(crate) trait Schedule: Sync + Sized + 'static {
+    /// Bind a task to the executor.
+    ///
+    /// Guaranteed to be called from the thread that called `poll` on the task.
+    /// The returned `Schedule` instance is associated with the task and is used
+    /// as `&self` in the other methods on this trait.
+    fn bind(task: Task<Self>) -> Self;
+
+    /// The task has completed work and is ready to be released. The scheduler
+    /// is free to drop it whenever.
+    ///
+    /// If the scheduler can immediately release the task, it should return
+    /// it as part of the function. This enables the task module to batch
+    /// the ref-dec with other options.
+    fn release(&self, task: &Task<Self>) -> Option<Task<Self>>;
+
+    /// Schedule the task
+    fn schedule(&self, task: Notified<Self>);
+
+    /// Schedule the task to run in the near future, yielding the thread to
+    /// other tasks.
+    fn yield_now(&self, task: Notified<Self>) {
+        self.schedule(task);
+    }
+}
+
+/// Create a new task with an associated join handle
+pub(crate) fn joinable<T, S>(task: T) -> (Notified<S>, JoinHandle<T::Output>)
+where
+    T: Future + Send + 'static,
+    S: Schedule,
+{
+    let raw = RawTask::new::<_, S>(task);
+
+    let task = Task {
+        raw,
+        _p: PhantomData,
+    };
+
+    let join = JoinHandle::new(raw);
+
+    (Notified(task), join)
+}
+
+cfg_rt_util! {
+    /// Create a new `!Send` task with an associated join handle
+    pub(crate) unsafe fn joinable_local<T, S>(task: T) -> (Notified<S>, JoinHandle<T::Output>)
+    where
+        T: Future + 'static,
+        S: Schedule,
+    {
+        let raw = RawTask::new::<_, S>(task);
+
+        let task = Task {
+            raw,
+            _p: PhantomData,
+        };
+
+        let join = JoinHandle::new(raw);
+
+        (Notified(task), join)
+    }
+}
+
+impl<S: 'static> Task<S> {
+    pub(crate) unsafe fn from_raw(ptr: NonNull<Header>) -> Task<S> {
+        Task {
+            raw: RawTask::from_raw(ptr),
+            _p: PhantomData,
+        }
+    }
+
+    pub(crate) fn header(&self) -> &Header {
+        self.raw.header()
+    }
+}
+
+cfg_rt_threaded! {
+    impl<S: 'static> Notified<S> {
+        pub(crate) unsafe fn from_raw(ptr: NonNull<Header>) -> Notified<S> {
+            Notified(Task::from_raw(ptr))
+        }
+
+        pub(crate) fn header(&self) -> &Header {
+            self.0.header()
+        }
+    }
+
+    impl<S: 'static> Task<S> {
+        pub(crate) fn into_raw(self) -> NonNull<Header> {
+            let ret = self.header().into();
+            mem::forget(self);
+            ret
+        }
+    }
+
+    impl<S: 'static> Notified<S> {
+        pub(crate) fn into_raw(self) -> NonNull<Header> {
+            self.0.into_raw()
+        }
+    }
+}
+
+impl<S: Schedule> Task<S> {
+    /// Pre-emptively cancel the task as part of the shutdown process.
+    pub(crate) fn shutdown(&self) {
+        self.raw.shutdown();
+    }
+}
+
+impl<S: Schedule> Notified<S> {
+    /// Run the task
+    pub(crate) fn run(self) {
+        self.0.raw.poll();
+        mem::forget(self);
+    }
+
+    /// Pre-emptively cancel the task as part of the shutdown process.
+    pub(crate) fn shutdown(self) {
+        self.0.shutdown();
+    }
+}
+
+impl<S: 'static> Drop for Task<S> {
+    fn drop(&mut self) {
+        // Decrement the ref count
+        if self.header().state.ref_dec() {
+            // Deallocate if this is the final ref count
+            self.raw.dealloc();
+        }
+    }
+}
+
+impl<S> fmt::Debug for Task<S> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(fmt, "Task({:p})", self.header())
+    }
+}
+
+impl<S> fmt::Debug for Notified<S> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(fmt, "task::Notified({:p})", self.0.header())
+    }
+}
+
+/// # Safety
+///
+/// Tasks are pinned
+unsafe impl<S> linked_list::Link for Task<S> {
+    type Handle = Task<S>;
+    type Target = Header;
+
+    fn as_raw(handle: &Task<S>) -> NonNull<Header> {
+        handle.header().into()
+    }
+
+    unsafe fn from_raw(ptr: NonNull<Header>) -> Task<S> {
+        Task::from_raw(ptr)
+    }
+
+    unsafe fn pointers(target: NonNull<Header>) -> NonNull<linked_list::Pointers<Header>> {
+        // Not super great as it avoids some of looms checking...
+        NonNull::from(target.as_ref().owned.with_mut(|ptr| &mut *ptr))
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/task/raw.rs b/third_party/rust/tokio/src/runtime/task/raw.rs
new file mode 100644
index 0000000000..cae56d037d
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/task/raw.rs
@@ -0,0 +1,131 @@
+use crate::runtime::task::{Cell, Harness, Header, Schedule, State};
+
+use std::future::Future;
+use std::ptr::NonNull;
+use std::task::{Poll, Waker};
+
+/// Raw task handle
+pub(super) struct RawTask {
+    ptr: NonNull<Header>,
+}
+
+pub(super) struct Vtable {
+    /// Poll the future
+    pub(super) poll: unsafe fn(NonNull<Header>),
+
+    /// Deallocate the memory
+    pub(super) dealloc: unsafe fn(NonNull<Header>),
+
+    /// Read the task output, if complete
+    pub(super) try_read_output: unsafe fn(NonNull<Header>, *mut (), &Waker),
+
+    /// The join handle has been dropped
+    pub(super) drop_join_handle_slow: unsafe fn(NonNull<Header>),
+
+    /// Scheduler is being shutdown
+    pub(super) shutdown: unsafe fn(NonNull<Header>),
+}
+
+/// Get the vtable for the requested `T` and `S` generics.
+pub(super) fn vtable<T: Future, S: Schedule>() -> &'static Vtable {
+    &Vtable {
+        poll: poll::<T, S>,
+        dealloc: dealloc::<T, S>,
+        try_read_output: try_read_output::<T, S>,
+        drop_join_handle_slow: drop_join_handle_slow::<T, S>,
+        shutdown: shutdown::<T, S>,
+    }
+}
+
+impl RawTask {
+    pub(super) fn new<T, S>(task: T) -> RawTask
+    where
+        T: Future,
+        S: Schedule,
+    {
+        let ptr = Box::into_raw(Cell::<_, S>::new(task, State::new()));
+        let ptr = unsafe { NonNull::new_unchecked(ptr as *mut Header) };
+
+        RawTask { ptr }
+    }
+
+    pub(super) unsafe fn from_raw(ptr: NonNull<Header>) -> RawTask {
+        RawTask { ptr }
+    }
+
+    /// Returns a reference to the task's meta structure.
+    ///
+    /// Safe as `Header` is `Sync`.
+    pub(super) fn header(&self) -> &Header {
+        unsafe { self.ptr.as_ref() }
+    }
+
+    /// Safety: mutual exclusion is required to call this function.
+    pub(super) fn poll(self) {
+        let vtable = self.header().vtable;
+        unsafe { (vtable.poll)(self.ptr) }
+    }
+
+    pub(super) fn dealloc(self) {
+        let vtable = self.header().vtable;
+        unsafe {
+            (vtable.dealloc)(self.ptr);
+        }
+    }
+
+    /// Safety: `dst` must be a `*mut Poll<super::Result<T::Output>>` where `T`
+    /// is the future stored by the task.
+    pub(super) unsafe fn try_read_output(self, dst: *mut (), waker: &Waker) {
+        let vtable = self.header().vtable;
+        (vtable.try_read_output)(self.ptr, dst, waker);
+    }
+
+    pub(super) fn drop_join_handle_slow(self) {
+        let vtable = self.header().vtable;
+        unsafe { (vtable.drop_join_handle_slow)(self.ptr) }
+    }
+
+    pub(super) fn shutdown(self) {
+        let vtable = self.header().vtable;
+        unsafe { (vtable.shutdown)(self.ptr) }
+    }
+}
+
+impl Clone for RawTask {
+    fn clone(&self) -> Self {
+        RawTask { ptr: self.ptr }
+    }
+}
+
+impl Copy for RawTask {}
+
+unsafe fn poll<T: Future, S: Schedule>(ptr: NonNull<Header>) {
+    let harness = Harness::<T, S>::from_raw(ptr);
+    harness.poll();
+}
+
+unsafe fn dealloc<T: Future, S: Schedule>(ptr: NonNull<Header>) {
+    let harness = Harness::<T, S>::from_raw(ptr);
+    harness.dealloc();
+}
+
+unsafe fn try_read_output<T: Future, S: Schedule>(
+    ptr: NonNull<Header>,
+    dst: *mut (),
+    waker: &Waker,
+) {
+    let out = &mut *(dst as *mut Poll<super::Result<T::Output>>);
+
+    let harness = Harness::<T, S>::from_raw(ptr);
+    harness.try_read_output(out, waker);
+}
+
+unsafe fn drop_join_handle_slow<T: Future, S: Schedule>(ptr: NonNull<Header>) {
+    let harness = Harness::<T, S>::from_raw(ptr);
+    harness.drop_join_handle_slow()
+}
+
+unsafe fn shutdown<T: Future, S: Schedule>(ptr: NonNull<Header>) {
+    let harness = Harness::<T, S>::from_raw(ptr);
+    harness.shutdown()
+}
diff --git a/third_party/rust/tokio/src/runtime/task/stack.rs b/third_party/rust/tokio/src/runtime/task/stack.rs
new file mode 100644
index 0000000000..9dd8d3f43f
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/task/stack.rs
@@ -0,0 +1,83 @@
+use crate::loom::sync::atomic::AtomicPtr;
+use crate::runtime::task::{Header, Task};
+
+use std::marker::PhantomData;
+use std::ptr::{self, NonNull};
+use std::sync::atomic::Ordering::{Acquire, Relaxed, Release};
+
+/// Concurrent stack of tasks, used to pass ownership of a task from one worker
+/// to another.
+pub(crate) struct TransferStack<T: 'static> {
+    head: AtomicPtr<Header>,
+    _p: PhantomData<T>,
+}
+
+impl<T: 'static> TransferStack<T> {
+    pub(crate) fn new() -> TransferStack<T> {
+        TransferStack {
+            head: AtomicPtr::new(ptr::null_mut()),
+            _p: PhantomData,
+        }
+    }
+
+    pub(crate) fn push(&self, task: Task<T>) {
+        let task = task.into_raw();
+
+        // We don't care about any memory associated w/ setting the `head`
+        // field, just the current value.
+        //
+        // The compare-exchange creates a release sequence.
+        let mut curr = self.head.load(Relaxed);
+
+        loop {
+            unsafe {
+                task.as_ref()
+                    .stack_next
+                    .with_mut(|ptr| *ptr = NonNull::new(curr))
+            };
+
+            let res = self
+                .head
+                .compare_exchange(curr, task.as_ptr() as *mut _, Release, Relaxed);
+
+            match res {
+                Ok(_) => return,
+                Err(actual) => {
+                    curr = actual;
+                }
+            }
+        }
+    }
+
+    pub(crate) fn drain(&self) -> impl Iterator<Item = Task<T>> {
+        struct Iter<T: 'static>(Option<NonNull<Header>>, PhantomData<T>);
+
+        impl<T: 'static> Iterator for Iter<T> {
+            type Item = Task<T>;
+
+            fn next(&mut self) -> Option<Task<T>> {
+                let task = self.0?;
+
+                // Move the cursor forward
+                self.0 = unsafe { task.as_ref().stack_next.with(|ptr| *ptr) };
+
+                // Return the task
+                unsafe { Some(Task::from_raw(task)) }
+            }
+        }
+
+        impl<T: 'static> Drop for Iter<T> {
+            fn drop(&mut self) {
+                use std::process;
+
+                if self.0.is_some() {
+                    // we have bugs
+                    process::abort();
+                }
+            }
+        }
+
+        let ptr = self.head.swap(ptr::null_mut(), Acquire);
+        Iter(NonNull::new(ptr), PhantomData)
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/task/state.rs b/third_party/rust/tokio/src/runtime/task/state.rs
new file mode 100644
index 0000000000..21e90430db
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/task/state.rs
@@ -0,0 +1,446 @@
+use crate::loom::sync::atomic::AtomicUsize;
+
+use std::fmt;
+use std::sync::atomic::Ordering::{AcqRel, Acquire, Release};
+use std::usize;
+
+pub(super) struct State {
+    val: AtomicUsize,
+}
+
+/// Current state value
+#[derive(Copy, Clone)]
+pub(super) struct Snapshot(usize);
+
+type UpdateResult = Result<Snapshot, Snapshot>;
+
+/// The task is currently being run.
+const RUNNING: usize = 0b0001;
+
+/// The task is complete.
+///
+/// Once this bit is set, it is never unset
+const COMPLETE: usize = 0b0010;
+
+/// Extracts the task's lifecycle value from the state
+const LIFECYCLE_MASK: usize = 0b11;
+
+/// Flag tracking if the task has been pushed into a run queue.
+const NOTIFIED: usize = 0b100;
+
+/// The join handle is still around
+const JOIN_INTEREST: usize = 0b1_000;
+
+/// A join handle waker has been set
+const JOIN_WAKER: usize = 0b10_000;
+
+/// The task has been forcibly cancelled.
+const CANCELLED: usize = 0b100_000;
+
+/// All bits
+const STATE_MASK: usize = LIFECYCLE_MASK | NOTIFIED | JOIN_INTEREST | JOIN_WAKER | CANCELLED;
+
+/// Bits used by the ref count portion of the state.
+const REF_COUNT_MASK: usize = !STATE_MASK;
+
+/// Number of positions to shift the ref count
+const REF_COUNT_SHIFT: usize = REF_COUNT_MASK.count_zeros() as usize;
+
+/// One ref count
+const REF_ONE: usize = 1 << REF_COUNT_SHIFT;
+
+/// State a task is initialized with
+///
+/// A task is initialized with two references: one for the scheduler and one for
+/// the `JoinHandle`. As the task starts with a `JoinHandle`, `JOIN_INTERST` is
+/// set. A new task is immediately pushed into the run queue for execution and
+/// starts with the `NOTIFIED` flag set.
+const INITIAL_STATE: usize = (REF_ONE * 2) | JOIN_INTEREST | NOTIFIED;
+
+/// All transitions are performed via RMW operations. This establishes an
+/// unambiguous modification order.
+impl State {
+    /// Return a task's initial state
+    pub(super) fn new() -> State {
+        // A task is initialized with three references: one for the scheduler,
+        // one for the `JoinHandle`, one for the task handle made available in
+        // release. As the task starts with a `JoinHandle`, `JOIN_INTERST` is
+        // set. A new task is immediately pushed into the run queue for
+        // execution and starts with the `NOTIFIED` flag set.
+        State {
+            val: AtomicUsize::new(INITIAL_STATE),
+        }
+    }
+
+    /// Loads the current state, establishes `Acquire` ordering.
+    pub(super) fn load(&self) -> Snapshot {
+        Snapshot(self.val.load(Acquire))
+    }
+
+    /// Attempt to transition the lifecycle to `Running`.
+    ///
+    /// If `ref_inc` is set, the reference count is also incremented.
+    ///
+    /// The `NOTIFIED` bit is always unset.
+    pub(super) fn transition_to_running(&self, ref_inc: bool) -> UpdateResult {
+        self.fetch_update(|curr| {
+            assert!(curr.is_notified());
+
+            let mut next = curr;
+
+            if !next.is_idle() {
+                return None;
+            }
+
+            if ref_inc {
+                next.ref_inc();
+            }
+
+            next.set_running();
+            next.unset_notified();
+            Some(next)
+        })
+    }
+
+    /// Transitions the task from `Running` -> `Idle`.
+    ///
+    /// Returns `Ok` if the transition to `Idle` is successful, `Err` otherwise.
+    /// In both cases, a snapshot of the state from **after** the transition is
+    /// returned.
+    ///
+    /// The transition to `Idle` fails if the task has been flagged to be
+    /// cancelled.
+    pub(super) fn transition_to_idle(&self) -> UpdateResult {
+        self.fetch_update(|curr| {
+            assert!(curr.is_running());
+
+            if curr.is_cancelled() {
+                return None;
+            }
+
+            let mut next = curr;
+            next.unset_running();
+
+            if next.is_notified() {
+                // The caller needs to schedule the task. To do this, it needs a
+                // waker. The waker requires a ref count.
+                next.ref_inc();
+            }
+
+            Some(next)
+        })
+    }
+
+    /// Transitions the task from `Running` -> `Complete`.
+    pub(super) fn transition_to_complete(&self) -> Snapshot {
+        const DELTA: usize = RUNNING | COMPLETE;
+
+        let prev = Snapshot(self.val.fetch_xor(DELTA, AcqRel));
+        assert!(prev.is_running());
+        assert!(!prev.is_complete());
+
+        Snapshot(prev.0 ^ DELTA)
+    }
+
+    /// Transition from `Complete` -> `Terminal`, decrementing the reference
+    /// count by 1.
+    ///
+    /// When `ref_dec` is set, an additional ref count decrement is performed.
+    /// This is used to batch atomic ops when possible.
+    pub(super) fn transition_to_terminal(&self, complete: bool, ref_dec: bool) -> Snapshot {
+        self.fetch_update(|mut snapshot| {
+            if complete {
+                snapshot.set_complete();
+            } else {
+                assert!(snapshot.is_complete());
+            }
+
+            // Decrement the primary handle
+            snapshot.ref_dec();
+
+            if ref_dec {
+                // Decrement a second time
+                snapshot.ref_dec();
+            }
+
+            Some(snapshot)
+        })
+        .unwrap()
+    }
+
+    /// Transitions the state to `NOTIFIED`.
+    ///
+    /// Returns `true` if the task needs to be submitted to the pool for
+    /// execution
+    pub(super) fn transition_to_notified(&self) -> bool {
+        let prev = Snapshot(self.val.fetch_or(NOTIFIED, AcqRel));
+        prev.will_need_queueing()
+    }
+
+    /// Set the `CANCELLED` bit and attempt to transition to `Running`.
+    ///
+    /// Returns `true` if the transition to `Running` succeeded.
+    pub(super) fn transition_to_shutdown(&self) -> bool {
+        let mut prev = Snapshot(0);
+
+        let _ = self.fetch_update(|mut snapshot| {
+            prev = snapshot;
+
+            if snapshot.is_idle() {
+                snapshot.set_running();
+
+                if snapshot.is_notified() {
+                    // If the task is idle and notified, this indicates the task is
+                    // in the run queue and is considered owned by the scheduler.
+                    // The shutdown operation claims ownership of the task, which
+                    // means we need to assign an additional ref-count to the task
+                    // in the queue.
+                    snapshot.ref_inc();
+                }
+            }
+
+            snapshot.set_cancelled();
+            Some(snapshot)
+        });
+
+        prev.is_idle()
+    }
+
+    /// Optimistically tries to swap the state assuming the join handle is
+    /// __immediately__ dropped on spawn
+    pub(super) fn drop_join_handle_fast(&self) -> Result<(), ()> {
+        use std::sync::atomic::Ordering::Relaxed;
+
+        // Relaxed is acceptable as if this function is called and succeeds,
+        // then nothing has been done w/ the join handle.
+        //
+        // The moment the join handle is used (polled), the `JOIN_WAKER` flag is
+        // set, at which point the CAS will fail.
+        //
+        // Given this, there is no risk if this operation is reordered.
+        self.val
+            .compare_exchange_weak(
+                INITIAL_STATE,
+                (INITIAL_STATE - REF_ONE) & !JOIN_INTEREST,
+                Release,
+                Relaxed,
+            )
+            .map(|_| ())
+            .map_err(|_| ())
+    }
+
+    /// Try to unset the JOIN_INTEREST flag.
+    ///
+    /// Returns `Ok` if the operation happens before the task transitions to a
+    /// completed state, `Err` otherwise.
+    pub(super) fn unset_join_interested(&self) -> UpdateResult {
+        self.fetch_update(|curr| {
+            assert!(curr.is_join_interested());
+
+            if curr.is_complete() {
+                return None;
+            }
+
+            let mut next = curr;
+            next.unset_join_interested();
+
+            Some(next)
+        })
+    }
+
+    /// Set the `JOIN_WAKER` bit.
+    ///
+    /// Returns `Ok` if the bit is set, `Err` otherwise. This operation fails if
+    /// the task has completed.
+    pub(super) fn set_join_waker(&self) -> UpdateResult {
+        self.fetch_update(|curr| {
+            assert!(curr.is_join_interested());
+            assert!(!curr.has_join_waker());
+
+            if curr.is_complete() {
+                return None;
+            }
+
+            let mut next = curr;
+            next.set_join_waker();
+
+            Some(next)
+        })
+    }
+
+    /// Unsets the `JOIN_WAKER` bit.
+    ///
+    /// Returns `Ok` has been unset, `Err` otherwise. This operation fails if
+    /// the task has completed.
+    pub(super) fn unset_waker(&self) -> UpdateResult {
+        self.fetch_update(|curr| {
+            assert!(curr.is_join_interested());
+            assert!(curr.has_join_waker());
+
+            if curr.is_complete() {
+                return None;
+            }
+
+            let mut next = curr;
+            next.unset_join_waker();
+
+            Some(next)
+        })
+    }
+
+    pub(super) fn ref_inc(&self) {
+        use std::process;
+        use std::sync::atomic::Ordering::Relaxed;
+
+        // Using a relaxed ordering is alright here, as knowledge of the
+        // original reference prevents other threads from erroneously deleting
+        // the object.
+        //
+        // As explained in the [Boost documentation][1], Increasing the
+        // reference counter can always be done with memory_order_relaxed: New
+        // references to an object can only be formed from an existing
+        // reference, and passing an existing reference from one thread to
+        // another must already provide any required synchronization.
+        //
+        // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
+        let prev = self.val.fetch_add(REF_ONE, Relaxed);
+
+        // If the reference count overflowed, abort.
+        if prev > isize::max_value() as usize {
+            process::abort();
+        }
+    }
+
+    /// Returns `true` if the task should be released.
+    pub(super) fn ref_dec(&self) -> bool {
+        let prev = Snapshot(self.val.fetch_sub(REF_ONE, AcqRel));
+        prev.ref_count() == 1
+    }
+
+    fn fetch_update<F>(&self, mut f: F) -> Result<Snapshot, Snapshot>
+    where
+        F: FnMut(Snapshot) -> Option<Snapshot>,
+    {
+        let mut curr = self.load();
+
+        loop {
+            let next = match f(curr) {
+                Some(next) => next,
+                None => return Err(curr),
+            };
+
+            let res = self.val.compare_exchange(curr.0, next.0, AcqRel, Acquire);
+
+            match res {
+                Ok(_) => return Ok(next),
+                Err(actual) => curr = Snapshot(actual),
+            }
+        }
+    }
+}
+
+// ===== impl Snapshot =====
+
+impl Snapshot {
+    /// Returns `true` if the task is in an idle state.
+    pub(super) fn is_idle(self) -> bool {
+        self.0 & (RUNNING | COMPLETE) == 0
+    }
+
+    /// Returns `true` if the task has been flagged as notified.
+    pub(super) fn is_notified(self) -> bool {
+        self.0 & NOTIFIED == NOTIFIED
+    }
+
+    fn unset_notified(&mut self) {
+        self.0 &= !NOTIFIED
+    }
+
+    pub(super) fn is_running(self) -> bool {
+        self.0 & RUNNING == RUNNING
+    }
+
+    fn set_running(&mut self) {
+        self.0 |= RUNNING;
+    }
+
+    fn unset_running(&mut self) {
+        self.0 &= !RUNNING;
+    }
+
+    pub(super) fn is_cancelled(self) -> bool {
+        self.0 & CANCELLED == CANCELLED
+    }
+
+    fn set_cancelled(&mut self) {
+        self.0 |= CANCELLED;
+    }
+
+    fn set_complete(&mut self) {
+        self.0 |= COMPLETE;
+    }
+
+    /// Returns `true` if the task's future has completed execution.
+    pub(super) fn is_complete(self) -> bool {
+        self.0 & COMPLETE == COMPLETE
+    }
+
+    pub(super) fn is_join_interested(self) -> bool {
+        self.0 & JOIN_INTEREST == JOIN_INTEREST
+    }
+
+    fn unset_join_interested(&mut self) {
+        self.0 &= !JOIN_INTEREST
+    }
+
+    pub(super) fn has_join_waker(self) -> bool {
+        self.0 & JOIN_WAKER == JOIN_WAKER
+    }
+
+    fn set_join_waker(&mut self) {
+        self.0 |= JOIN_WAKER;
+    }
+
+    fn unset_join_waker(&mut self) {
+        self.0 &= !JOIN_WAKER
+    }
+
+    pub(super) fn ref_count(self) -> usize {
+        (self.0 & REF_COUNT_MASK) >> REF_COUNT_SHIFT
+    }
+
+    fn ref_inc(&mut self) {
+        assert!(self.0 <= isize::max_value() as usize);
+        self.0 += REF_ONE;
+    }
+
+    pub(super) fn ref_dec(&mut self) {
+        assert!(self.ref_count() > 0);
+        self.0 -= REF_ONE
+    }
+
+    fn will_need_queueing(self) -> bool {
+        !self.is_notified() && self.is_idle()
+    }
+}
+
+impl fmt::Debug for State {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let snapshot = self.load();
+        snapshot.fmt(fmt)
+    }
+}
+
+impl fmt::Debug for Snapshot {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("Snapshot")
+            .field("is_running", &self.is_running())
+            .field("is_complete", &self.is_complete())
+            .field("is_notified", &self.is_notified())
+            .field("is_cancelled", &self.is_cancelled())
+            .field("is_join_interested", &self.is_join_interested())
+            .field("has_join_waker", &self.has_join_waker())
+            .field("ref_count", &self.ref_count())
+            .finish()
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/task/waker.rs b/third_party/rust/tokio/src/runtime/task/waker.rs
new file mode 100644
index 0000000000..5c2d478fbb
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/task/waker.rs
@@ -0,0 +1,101 @@
+use crate::runtime::task::harness::Harness;
+use crate::runtime::task::{Header, Schedule};
+
+use std::future::Future;
+use std::marker::PhantomData;
+use std::mem::ManuallyDrop;
+use std::ops;
+use std::ptr::NonNull;
+use std::task::{RawWaker, RawWakerVTable, Waker};
+
+pub(super) struct WakerRef<'a, S: 'static> {
+    waker: ManuallyDrop<Waker>,
+    _p: PhantomData<(&'a Header, S)>,
+}
+
+/// Returns a `WakerRef` which avoids having to pre-emptively increase the
+/// refcount if there is no need to do so.
+pub(super) fn waker_ref<T, S>(header: &Header) -> WakerRef<'_, S>
+where
+    T: Future,
+    S: Schedule,
+{
+    // `Waker::will_wake` uses the VTABLE pointer as part of the check. This
+    // means that `will_wake` will always return false when using the current
+    // task's waker. (discussion at rust-lang/rust#66281).
+    //
+    // To fix this, we use a single vtable. Since we pass in a reference at this
+    // point and not an *owned* waker, we must ensure that `drop` is never
+    // called on this waker instance. This is done by wrapping it with
+    // `ManuallyDrop` and then never calling drop.
+    let waker = unsafe { ManuallyDrop::new(Waker::from_raw(raw_waker::<T, S>(header))) };
+
+    WakerRef {
+        waker,
+        _p: PhantomData,
+    }
+}
+
+impl<S> ops::Deref for WakerRef<'_, S> {
+    type Target = Waker;
+
+    fn deref(&self) -> &Waker {
+        &self.waker
+    }
+}
+
+unsafe fn clone_waker<T, S>(ptr: *const ()) -> RawWaker
+where
+    T: Future,
+    S: Schedule,
+{
+    let header = ptr as *const Header;
+    (*header).state.ref_inc();
+    raw_waker::<T, S>(header)
+}
+
+unsafe fn drop_waker<T, S>(ptr: *const ())
+where
+    T: Future,
+    S: Schedule,
+{
+    let ptr = NonNull::new_unchecked(ptr as *mut Header);
+    let harness = Harness::<T, S>::from_raw(ptr);
+    harness.drop_reference();
+}
+
+unsafe fn wake_by_val<T, S>(ptr: *const ())
+where
+    T: Future,
+    S: Schedule,
+{
+    let ptr = NonNull::new_unchecked(ptr as *mut Header);
+    let harness = Harness::<T, S>::from_raw(ptr);
+    harness.wake_by_val();
+}
+
+// Wake without consuming the waker
+unsafe fn wake_by_ref<T, S>(ptr: *const ())
+where
+    T: Future,
+    S: Schedule,
+{
+    let ptr = NonNull::new_unchecked(ptr as *mut Header);
+    let harness = Harness::<T, S>::from_raw(ptr);
+    harness.wake_by_ref();
+}
+
+fn raw_waker<T, S>(header: *const Header) -> RawWaker
+where
+    T: Future,
+    S: Schedule,
+{
+    let ptr = header as *const ();
+    let vtable = &RawWakerVTable::new(
+        clone_waker::<T, S>,
+        wake_by_val::<T, S>,
+        wake_by_ref::<T, S>,
+        drop_waker::<T, S>,
+    );
+    RawWaker::new(ptr, vtable)
+}
diff --git a/third_party/rust/tokio/src/runtime/tests/loom_blocking.rs b/third_party/rust/tokio/src/runtime/tests/loom_blocking.rs
new file mode 100644
index 0000000000..db7048e3f9
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/tests/loom_blocking.rs
@@ -0,0 +1,31 @@
+use crate::runtime::{self, Runtime};
+
+use std::sync::Arc;
+
+#[test]
+fn blocking_shutdown() {
+    loom::model(|| {
+        let v = Arc::new(());
+
+        let rt = mk_runtime(1);
+        rt.enter(|| {
+            for _ in 0..2 {
+                let v = v.clone();
+                crate::task::spawn_blocking(move || {
+                    assert!(1 < Arc::strong_count(&v));
+                });
+            }
+        });
+
+        drop(rt);
+        assert_eq!(1, Arc::strong_count(&v));
+    });
+}
+
+fn mk_runtime(num_threads: usize) -> Runtime {
+    runtime::Builder::new()
+        .threaded_scheduler()
+        .core_threads(num_threads)
+        .build()
+        .unwrap()
+}
diff --git a/third_party/rust/tokio/src/runtime/tests/loom_oneshot.rs b/third_party/rust/tokio/src/runtime/tests/loom_oneshot.rs
new file mode 100644
index 0000000000..c126fe479a
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/tests/loom_oneshot.rs
@@ -0,0 +1,49 @@
+use loom::sync::Notify;
+
+use std::sync::{Arc, Mutex};
+
+pub(crate) fn channel<T>() -> (Sender<T>, Receiver<T>) {
+    let inner = Arc::new(Inner {
+        notify: Notify::new(),
+        value: Mutex::new(None),
+    });
+
+    let tx = Sender {
+        inner: inner.clone(),
+    };
+    let rx = Receiver { inner };
+
+    (tx, rx)
+}
+
+pub(crate) struct Sender<T> {
+    inner: Arc<Inner<T>>,
+}
+
+pub(crate) struct Receiver<T> {
+    inner: Arc<Inner<T>>,
+}
+
+struct Inner<T> {
+    notify: Notify,
+    value: Mutex<Option<T>>,
+}
+
+impl<T> Sender<T> {
+    pub(crate) fn send(self, value: T) {
+        *self.inner.value.lock().unwrap() = Some(value);
+        self.inner.notify.notify();
+    }
+}
+
+impl<T> Receiver<T> {
+    pub(crate) fn recv(self) -> T {
+        loop {
+            if let Some(v) = self.inner.value.lock().unwrap().take() {
+                return v;
+            }
+
+            self.inner.notify.wait();
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/tests/loom_pool.rs b/third_party/rust/tokio/src/runtime/tests/loom_pool.rs
new file mode 100644
index 0000000000..c08658cde8
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/tests/loom_pool.rs
@@ -0,0 +1,380 @@
+/// Full runtime loom tests. These are heavy tests and take significant time to
+/// run on CI.
+///
+/// Use `LOOM_MAX_PREEMPTIONS=1` to do a "quick" run as a smoke test.
+///
+/// In order to speed up the C
+use crate::future::poll_fn;
+use crate::runtime::tests::loom_oneshot as oneshot;
+use crate::runtime::{self, Runtime};
+use crate::{spawn, task};
+use tokio_test::assert_ok;
+
+use loom::sync::atomic::{AtomicBool, AtomicUsize};
+use loom::sync::{Arc, Mutex};
+
+use pin_project_lite::pin_project;
+use std::future::Future;
+use std::pin::Pin;
+use std::sync::atomic::Ordering::{Relaxed, SeqCst};
+use std::task::{Context, Poll};
+
+/// Tests are divided into groups to make the runs faster on CI.
+mod group_a {
+    use super::*;
+
+    #[test]
+    fn racy_shutdown() {
+        loom::model(|| {
+            let pool = mk_pool(1);
+
+            // here's the case we want to exercise:
+            //
+            // a worker that still has tasks in its local queue gets sent to the blocking pool (due to
+            // block_in_place). the blocking pool is shut down, so drops the worker. the worker's
+            // shutdown method never gets run.
+            //
+            // we do this by spawning two tasks on one worker, the first of which does block_in_place,
+            // and then immediately drop the pool.
+
+            pool.spawn(track(async {
+                crate::task::block_in_place(|| {});
+            }));
+            pool.spawn(track(async {}));
+            drop(pool);
+        });
+    }
+
+    #[test]
+    fn pool_multi_spawn() {
+        loom::model(|| {
+            let pool = mk_pool(2);
+            let c1 = Arc::new(AtomicUsize::new(0));
+
+            let (tx, rx) = oneshot::channel();
+            let tx1 = Arc::new(Mutex::new(Some(tx)));
+
+            // Spawn a task
+            let c2 = c1.clone();
+            let tx2 = tx1.clone();
+            pool.spawn(track(async move {
+                spawn(track(async move {
+                    if 1 == c1.fetch_add(1, Relaxed) {
+                        tx1.lock().unwrap().take().unwrap().send(());
+                    }
+                }));
+            }));
+
+            // Spawn a second task
+            pool.spawn(track(async move {
+                spawn(track(async move {
+                    if 1 == c2.fetch_add(1, Relaxed) {
+                        tx2.lock().unwrap().take().unwrap().send(());
+                    }
+                }));
+            }));
+
+            rx.recv();
+        });
+    }
+
+    fn only_blocking_inner(first_pending: bool) {
+        loom::model(move || {
+            let pool = mk_pool(1);
+            let (block_tx, block_rx) = oneshot::channel();
+
+            pool.spawn(track(async move {
+                crate::task::block_in_place(move || {
+                    block_tx.send(());
+                });
+                if first_pending {
+                    task::yield_now().await
+                }
+            }));
+
+            block_rx.recv();
+            drop(pool);
+        });
+    }
+
+    #[test]
+    fn only_blocking_without_pending() {
+        only_blocking_inner(false)
+    }
+
+    #[test]
+    fn only_blocking_with_pending() {
+        only_blocking_inner(true)
+    }
+}
+
+mod group_b {
+    use super::*;
+
+    fn blocking_and_regular_inner(first_pending: bool) {
+        const NUM: usize = 3;
+        loom::model(move || {
+            let pool = mk_pool(1);
+            let cnt = Arc::new(AtomicUsize::new(0));
+
+            let (block_tx, block_rx) = oneshot::channel();
+            let (done_tx, done_rx) = oneshot::channel();
+            let done_tx = Arc::new(Mutex::new(Some(done_tx)));
+
+            pool.spawn(track(async move {
+                crate::task::block_in_place(move || {
+                    block_tx.send(());
+                });
+                if first_pending {
+                    task::yield_now().await
+                }
+            }));
+
+            for _ in 0..NUM {
+                let cnt = cnt.clone();
+                let done_tx = done_tx.clone();
+
+                pool.spawn(track(async move {
+                    if NUM == cnt.fetch_add(1, Relaxed) + 1 {
+                        done_tx.lock().unwrap().take().unwrap().send(());
+                    }
+                }));
+            }
+
+            done_rx.recv();
+            block_rx.recv();
+
+            drop(pool);
+        });
+    }
+
+    #[test]
+    fn blocking_and_regular() {
+        blocking_and_regular_inner(false);
+    }
+
+    #[test]
+    fn blocking_and_regular_with_pending() {
+        blocking_and_regular_inner(true);
+    }
+
+    #[test]
+    fn pool_shutdown() {
+        loom::model(|| {
+            let pool = mk_pool(2);
+
+            pool.spawn(track(async move {
+                gated2(true).await;
+            }));
+
+            pool.spawn(track(async move {
+                gated2(false).await;
+            }));
+
+            drop(pool);
+        });
+    }
+
+    #[test]
+    fn join_output() {
+        loom::model(|| {
+            let mut rt = mk_pool(1);
+
+            rt.block_on(async {
+                let t = crate::spawn(track(async { "hello" }));
+
+                let out = assert_ok!(t.await);
+                assert_eq!("hello", out.into_inner());
+            });
+        });
+    }
+
+    #[test]
+    fn poll_drop_handle_then_drop() {
+        loom::model(|| {
+            let mut rt = mk_pool(1);
+
+            rt.block_on(async move {
+                let mut t = crate::spawn(track(async { "hello" }));
+
+                poll_fn(|cx| {
+                    let _ = Pin::new(&mut t).poll(cx);
+                    Poll::Ready(())
+                })
+                .await;
+            });
+        })
+    }
+
+    #[test]
+    fn complete_block_on_under_load() {
+        loom::model(|| {
+            let mut pool = mk_pool(1);
+
+            pool.block_on(async {
+                // Trigger a re-schedule
+                crate::spawn(track(async {
+                    for _ in 0..2 {
+                        task::yield_now().await;
+                    }
+                }));
+
+                gated2(true).await
+            });
+        });
+    }
+}
+
+mod group_c {
+    use super::*;
+
+    #[test]
+    fn shutdown_with_notification() {
+        use crate::sync::oneshot;
+
+        loom::model(|| {
+            let rt = mk_pool(2);
+            let (done_tx, done_rx) = oneshot::channel::<()>();
+
+            rt.spawn(track(async move {
+                let (tx, rx) = oneshot::channel::<()>();
+
+                crate::spawn(async move {
+                    crate::task::spawn_blocking(move || {
+                        let _ = tx.send(());
+                    });
+
+                    let _ = done_rx.await;
+                });
+
+                let _ = rx.await;
+
+                let _ = done_tx.send(());
+            }));
+        });
+    }
+}
+
+mod group_d {
+    use super::*;
+
+    #[test]
+    fn pool_multi_notify() {
+        loom::model(|| {
+            let pool = mk_pool(2);
+
+            let c1 = Arc::new(AtomicUsize::new(0));
+
+            let (done_tx, done_rx) = oneshot::channel();
+            let done_tx1 = Arc::new(Mutex::new(Some(done_tx)));
+
+            // Spawn a task
+            let c2 = c1.clone();
+            let done_tx2 = done_tx1.clone();
+            pool.spawn(track(async move {
+                gated().await;
+                gated().await;
+
+                if 1 == c1.fetch_add(1, Relaxed) {
+                    done_tx1.lock().unwrap().take().unwrap().send(());
+                }
+            }));
+
+            // Spawn a second task
+            pool.spawn(track(async move {
+                gated().await;
+                gated().await;
+
+                if 1 == c2.fetch_add(1, Relaxed) {
+                    done_tx2.lock().unwrap().take().unwrap().send(());
+                }
+            }));
+
+            done_rx.recv();
+        });
+    }
+}
+
+fn mk_pool(num_threads: usize) -> Runtime {
+    runtime::Builder::new()
+        .threaded_scheduler()
+        .core_threads(num_threads)
+        .build()
+        .unwrap()
+}
+
+fn gated() -> impl Future<Output = &'static str> {
+    gated2(false)
+}
+
+fn gated2(thread: bool) -> impl Future<Output = &'static str> {
+    use loom::thread;
+    use std::sync::Arc;
+
+    let gate = Arc::new(AtomicBool::new(false));
+    let mut fired = false;
+
+    poll_fn(move |cx| {
+        if !fired {
+            let gate = gate.clone();
+            let waker = cx.waker().clone();
+
+            if thread {
+                thread::spawn(move || {
+                    gate.store(true, SeqCst);
+                    waker.wake_by_ref();
+                });
+            } else {
+                spawn(track(async move {
+                    gate.store(true, SeqCst);
+                    waker.wake_by_ref();
+                }));
+            }
+
+            fired = true;
+
+            return Poll::Pending;
+        }
+
+        if gate.load(SeqCst) {
+            Poll::Ready("hello world")
+        } else {
+            Poll::Pending
+        }
+    })
+}
+
+fn track<T: Future>(f: T) -> Track<T> {
+    Track {
+        inner: f,
+        arc: Arc::new(()),
+    }
+}
+
+pin_project! {
+    struct Track<T> {
+        #[pin]
+        inner: T,
+        // Arc is used to hook into loom's leak tracking.
+        arc: Arc<()>,
+    }
+}
+
+impl<T> Track<T> {
+    fn into_inner(self) -> T {
+        self.inner
+    }
+}
+
+impl<T: Future> Future for Track<T> {
+    type Output = Track<T::Output>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let me = self.project();
+
+        Poll::Ready(Track {
+            inner: ready!(me.inner.poll(cx)),
+            arc: me.arc.clone(),
+        })
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/tests/loom_queue.rs b/third_party/rust/tokio/src/runtime/tests/loom_queue.rs
new file mode 100644
index 0000000000..de02610db0
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/tests/loom_queue.rs
@@ -0,0 +1,216 @@
+use crate::runtime::queue;
+use crate::runtime::task::{self, Schedule, Task};
+
+use loom::thread;
+
+#[test]
+fn basic() {
+    loom::model(|| {
+        let (steal, mut local) = queue::local();
+        let inject = queue::Inject::new();
+
+        let th = thread::spawn(move || {
+            let (_, mut local) = queue::local();
+            let mut n = 0;
+
+            for _ in 0..3 {
+                if steal.steal_into(&mut local).is_some() {
+                    n += 1;
+                }
+
+                while local.pop().is_some() {
+                    n += 1;
+                }
+            }
+
+            n
+        });
+
+        let mut n = 0;
+
+        for _ in 0..2 {
+            for _ in 0..2 {
+                let (task, _) = task::joinable::<_, Runtime>(async {});
+                local.push_back(task, &inject);
+            }
+
+            if local.pop().is_some() {
+                n += 1;
+            }
+
+            // Push another task
+            let (task, _) = task::joinable::<_, Runtime>(async {});
+            local.push_back(task, &inject);
+
+            while local.pop().is_some() {
+                n += 1;
+            }
+        }
+
+        while inject.pop().is_some() {
+            n += 1;
+        }
+
+        n += th.join().unwrap();
+
+        assert_eq!(6, n);
+    });
+}
+
+#[test]
+fn steal_overflow() {
+    loom::model(|| {
+        let (steal, mut local) = queue::local();
+        let inject = queue::Inject::new();
+
+        let th = thread::spawn(move || {
+            let (_, mut local) = queue::local();
+            let mut n = 0;
+
+            if steal.steal_into(&mut local).is_some() {
+                n += 1;
+            }
+
+            while local.pop().is_some() {
+                n += 1;
+            }
+
+            n
+        });
+
+        let mut n = 0;
+
+        // push a task, pop a task
+        let (task, _) = task::joinable::<_, Runtime>(async {});
+        local.push_back(task, &inject);
+
+        if local.pop().is_some() {
+            n += 1;
+        }
+
+        for _ in 0..6 {
+            let (task, _) = task::joinable::<_, Runtime>(async {});
+            local.push_back(task, &inject);
+        }
+
+        n += th.join().unwrap();
+
+        while local.pop().is_some() {
+            n += 1;
+        }
+
+        while inject.pop().is_some() {
+            n += 1;
+        }
+
+        assert_eq!(7, n);
+    });
+}
+
+#[test]
+fn multi_stealer() {
+    const NUM_TASKS: usize = 5;
+
+    fn steal_tasks(steal: queue::Steal<Runtime>) -> usize {
+        let (_, mut local) = queue::local();
+
+        if steal.steal_into(&mut local).is_none() {
+            return 0;
+        }
+
+        let mut n = 1;
+
+        while local.pop().is_some() {
+            n += 1;
+        }
+
+        n
+    }
+
+    loom::model(|| {
+        let (steal, mut local) = queue::local();
+        let inject = queue::Inject::new();
+
+        // Push work
+        for _ in 0..NUM_TASKS {
+            let (task, _) = task::joinable::<_, Runtime>(async {});
+            local.push_back(task, &inject);
+        }
+
+        let th1 = {
+            let steal = steal.clone();
+            thread::spawn(move || steal_tasks(steal))
+        };
+
+        let th2 = thread::spawn(move || steal_tasks(steal));
+
+        let mut n = 0;
+
+        while local.pop().is_some() {
+            n += 1;
+        }
+
+        while inject.pop().is_some() {
+            n += 1;
+        }
+
+        n += th1.join().unwrap();
+        n += th2.join().unwrap();
+
+        assert_eq!(n, NUM_TASKS);
+    });
+}
+
+#[test]
+fn chained_steal() {
+    loom::model(|| {
+        let (s1, mut l1) = queue::local();
+        let (s2, mut l2) = queue::local();
+        let inject = queue::Inject::new();
+
+        // Load up some tasks
+        for _ in 0..4 {
+            let (task, _) = task::joinable::<_, Runtime>(async {});
+            l1.push_back(task, &inject);
+
+            let (task, _) = task::joinable::<_, Runtime>(async {});
+            l2.push_back(task, &inject);
+        }
+
+        // Spawn a task to steal from **our** queue
+        let th = thread::spawn(move || {
+            let (_, mut local) = queue::local();
+            s1.steal_into(&mut local);
+
+            while local.pop().is_some() {}
+        });
+
+        // Drain our tasks, then attempt to steal
+        while l1.pop().is_some() {}
+
+        s2.steal_into(&mut l1);
+
+        th.join().unwrap();
+
+        while l1.pop().is_some() {}
+        while l2.pop().is_some() {}
+        while inject.pop().is_some() {}
+    });
+}
+
+struct Runtime;
+
+impl Schedule for Runtime {
+    fn bind(task: Task<Self>) -> Runtime {
+        std::mem::forget(task);
+        Runtime
+    }
+
+    fn release(&self, _task: &Task<Self>) -> Option<Task<Self>> {
+        None
+    }
+
+    fn schedule(&self, _task: task::Notified<Self>) {
+        unreachable!();
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/tests/mod.rs b/third_party/rust/tokio/src/runtime/tests/mod.rs
new file mode 100644
index 0000000000..123a7e35a3
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/tests/mod.rs
@@ -0,0 +1,13 @@
+cfg_loom! {
+    mod loom_blocking;
+    mod loom_oneshot;
+    mod loom_pool;
+    mod loom_queue;
+}
+
+cfg_not_loom! {
+    mod queue;
+
+    #[cfg(miri)]
+    mod task;
+}
diff --git a/third_party/rust/tokio/src/runtime/tests/queue.rs b/third_party/rust/tokio/src/runtime/tests/queue.rs
new file mode 100644
index 0000000000..d228d5dcc7
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/tests/queue.rs
@@ -0,0 +1,202 @@
+use crate::runtime::queue;
+use crate::runtime::task::{self, Schedule, Task};
+
+use std::thread;
+use std::time::Duration;
+
+#[test]
+fn fits_256() {
+    let (_, mut local) = queue::local();
+    let inject = queue::Inject::new();
+
+    for _ in 0..256 {
+        let (task, _) = task::joinable::<_, Runtime>(async {});
+        local.push_back(task, &inject);
+    }
+
+    assert!(inject.pop().is_none());
+
+    while local.pop().is_some() {}
+}
+
+#[test]
+fn overflow() {
+    let (_, mut local) = queue::local();
+    let inject = queue::Inject::new();
+
+    for _ in 0..257 {
+        let (task, _) = task::joinable::<_, Runtime>(async {});
+        local.push_back(task, &inject);
+    }
+
+    let mut n = 0;
+
+    while inject.pop().is_some() {
+        n += 1;
+    }
+
+    while local.pop().is_some() {
+        n += 1;
+    }
+
+    assert_eq!(n, 257);
+}
+
+#[test]
+fn steal_batch() {
+    let (steal1, mut local1) = queue::local();
+    let (_, mut local2) = queue::local();
+    let inject = queue::Inject::new();
+
+    for _ in 0..4 {
+        let (task, _) = task::joinable::<_, Runtime>(async {});
+        local1.push_back(task, &inject);
+    }
+
+    assert!(steal1.steal_into(&mut local2).is_some());
+
+    for _ in 0..1 {
+        assert!(local2.pop().is_some());
+    }
+
+    assert!(local2.pop().is_none());
+
+    for _ in 0..2 {
+        assert!(local1.pop().is_some());
+    }
+
+    assert!(local1.pop().is_none());
+}
+
+#[test]
+fn stress1() {
+    const NUM_ITER: usize = 1;
+    const NUM_STEAL: usize = 1_000;
+    const NUM_LOCAL: usize = 1_000;
+    const NUM_PUSH: usize = 500;
+    const NUM_POP: usize = 250;
+
+    for _ in 0..NUM_ITER {
+        let (steal, mut local) = queue::local();
+        let inject = queue::Inject::new();
+
+        let th = thread::spawn(move || {
+            let (_, mut local) = queue::local();
+            let mut n = 0;
+
+            for _ in 0..NUM_STEAL {
+                if steal.steal_into(&mut local).is_some() {
+                    n += 1;
+                }
+
+                while local.pop().is_some() {
+                    n += 1;
+                }
+
+                thread::yield_now();
+            }
+
+            n
+        });
+
+        let mut n = 0;
+
+        for _ in 0..NUM_LOCAL {
+            for _ in 0..NUM_PUSH {
+                let (task, _) = task::joinable::<_, Runtime>(async {});
+                local.push_back(task, &inject);
+            }
+
+            for _ in 0..NUM_POP {
+                if local.pop().is_some() {
+                    n += 1;
+                } else {
+                    break;
+                }
+            }
+        }
+
+        while inject.pop().is_some() {
+            n += 1;
+        }
+
+        n += th.join().unwrap();
+
+        assert_eq!(n, NUM_LOCAL * NUM_PUSH);
+    }
+}
+
+#[test]
+fn stress2() {
+    const NUM_ITER: usize = 1;
+    const NUM_TASKS: usize = 1_000_000;
+    const NUM_STEAL: usize = 1_000;
+
+    for _ in 0..NUM_ITER {
+        let (steal, mut local) = queue::local();
+        let inject = queue::Inject::new();
+
+        let th = thread::spawn(move || {
+            let (_, mut local) = queue::local();
+            let mut n = 0;
+
+            for _ in 0..NUM_STEAL {
+                if steal.steal_into(&mut local).is_some() {
+                    n += 1;
+                }
+
+                while local.pop().is_some() {
+                    n += 1;
+                }
+
+                thread::sleep(Duration::from_micros(10));
+            }
+
+            n
+        });
+
+        let mut num_pop = 0;
+
+        for i in 0..NUM_TASKS {
+            let (task, _) = task::joinable::<_, Runtime>(async {});
+            local.push_back(task, &inject);
+
+            if i % 128 == 0 && local.pop().is_some() {
+                num_pop += 1;
+            }
+
+            while inject.pop().is_some() {
+                num_pop += 1;
+            }
+        }
+
+        num_pop += th.join().unwrap();
+
+        while local.pop().is_some() {
+            num_pop += 1;
+        }
+
+        while inject.pop().is_some() {
+            num_pop += 1;
+        }
+
+        assert_eq!(num_pop, NUM_TASKS);
+    }
+}
+
+struct Runtime;
+
+impl Schedule for Runtime {
+    fn bind(task: Task<Self>) -> Runtime {
+        std::mem::forget(task);
+        Runtime
+    }
+
+    fn release(&self, _task: &Task<Self>) -> Option<Task<Self>> {
+        None
+    }
+
+    fn schedule(&self, _task: task::Notified<Self>) {
+        unreachable!();
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/tests/task.rs b/third_party/rust/tokio/src/runtime/tests/task.rs
new file mode 100644
index 0000000000..82315a04ff
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/tests/task.rs
@@ -0,0 +1,159 @@
+use crate::runtime::task::{self, Schedule, Task};
+use crate::util::linked_list::LinkedList;
+use crate::util::TryLock;
+
+use std::collections::VecDeque;
+use std::sync::Arc;
+
+#[test]
+fn create_drop() {
+    let _ = task::joinable::<_, Runtime>(async { unreachable!() });
+}
+
+#[test]
+fn schedule() {
+    with(|rt| {
+        let (task, _) = task::joinable(async {
+            crate::task::yield_now().await;
+        });
+
+        rt.schedule(task);
+
+        assert_eq!(2, rt.tick());
+    })
+}
+
+#[test]
+fn shutdown() {
+    with(|rt| {
+        let (task, _) = task::joinable(async {
+            loop {
+                crate::task::yield_now().await;
+            }
+        });
+
+        rt.schedule(task);
+        rt.tick_max(1);
+
+        rt.shutdown();
+    })
+}
+
+fn with(f: impl FnOnce(Runtime)) {
+    struct Reset;
+
+    impl Drop for Reset {
+        fn drop(&mut self) {
+            let _rt = CURRENT.try_lock().unwrap().take();
+        }
+    }
+
+    let _reset = Reset;
+
+    let rt = Runtime(Arc::new(Inner {
+        released: task::TransferStack::new(),
+        core: TryLock::new(Core {
+            queue: VecDeque::new(),
+            tasks: LinkedList::new(),
+        }),
+    }));
+
+    *CURRENT.try_lock().unwrap() = Some(rt.clone());
+    f(rt)
+}
+
+#[derive(Clone)]
+struct Runtime(Arc<Inner>);
+
+struct Inner {
+    released: task::TransferStack<Runtime>,
+    core: TryLock<Core>,
+}
+
+struct Core {
+    queue: VecDeque<task::Notified<Runtime>>,
+    tasks: LinkedList<Task<Runtime>>,
+}
+
+static CURRENT: TryLock<Option<Runtime>> = TryLock::new(None);
+
+impl Runtime {
+    fn tick(&self) -> usize {
+        self.tick_max(usize::max_value())
+    }
+
+    fn tick_max(&self, max: usize) -> usize {
+        let mut n = 0;
+
+        while !self.is_empty() && n < max {
+            let task = self.next_task();
+            n += 1;
+            task.run();
+        }
+
+        self.0.maintenance();
+
+        n
+    }
+
+    fn is_empty(&self) -> bool {
+        self.0.core.try_lock().unwrap().queue.is_empty()
+    }
+
+    fn next_task(&self) -> task::Notified<Runtime> {
+        self.0.core.try_lock().unwrap().queue.pop_front().unwrap()
+    }
+
+    fn shutdown(&self) {
+        let mut core = self.0.core.try_lock().unwrap();
+
+        for task in core.tasks.iter() {
+            task.shutdown();
+        }
+
+        while let Some(task) = core.queue.pop_back() {
+            task.shutdown();
+        }
+
+        drop(core);
+
+        while !self.0.core.try_lock().unwrap().tasks.is_empty() {
+            self.0.maintenance();
+        }
+    }
+}
+
+impl Inner {
+    fn maintenance(&self) {
+        use std::mem::ManuallyDrop;
+
+        for task in self.released.drain() {
+            let task = ManuallyDrop::new(task);
+
+            // safety: see worker.rs
+            unsafe {
+                let ptr = task.header().into();
+                self.core.try_lock().unwrap().tasks.remove(ptr);
+            }
+        }
+    }
+}
+
+impl Schedule for Runtime {
+    fn bind(task: Task<Self>) -> Runtime {
+        let rt = CURRENT.try_lock().unwrap().as_ref().unwrap().clone();
+        rt.0.core.try_lock().unwrap().tasks.push_front(task);
+        rt
+    }
+
+    fn release(&self, task: &Task<Self>) -> Option<Task<Self>> {
+        // safety: copying worker.rs
+        let task = unsafe { Task::from_raw(task.header().into()) };
+        self.0.released.push(task);
+        None
+    }
+
+    fn schedule(&self, task: task::Notified<Self>) {
+        self.0.core.try_lock().unwrap().queue.push_back(task);
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/thread_pool/atomic_cell.rs b/third_party/rust/tokio/src/runtime/thread_pool/atomic_cell.rs
new file mode 100644
index 0000000000..2bda0fc738
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/thread_pool/atomic_cell.rs
@@ -0,0 +1,52 @@
+use crate::loom::sync::atomic::AtomicPtr;
+
+use std::ptr;
+use std::sync::atomic::Ordering::AcqRel;
+
+pub(super) struct AtomicCell<T> {
+    data: AtomicPtr<T>,
+}
+
+unsafe impl<T: Send> Send for AtomicCell<T> {}
+unsafe impl<T: Send> Sync for AtomicCell<T> {}
+
+impl<T> AtomicCell<T> {
+    pub(super) fn new(data: Option<Box<T>>) -> AtomicCell<T> {
+        AtomicCell {
+            data: AtomicPtr::new(to_raw(data)),
+        }
+    }
+
+    pub(super) fn swap(&self, val: Option<Box<T>>) -> Option<Box<T>> {
+        let old = self.data.swap(to_raw(val), AcqRel);
+        from_raw(old)
+    }
+
+    #[cfg(feature = "blocking")]
+    pub(super) fn set(&self, val: Box<T>) {
+        let _ = self.swap(Some(val));
+    }
+
+    pub(super) fn take(&self) -> Option<Box<T>> {
+        self.swap(None)
+    }
+}
+
+fn to_raw<T>(data: Option<Box<T>>) -> *mut T {
+    data.map(Box::into_raw).unwrap_or(ptr::null_mut())
+}
+
+fn from_raw<T>(val: *mut T) -> Option<Box<T>> {
+    if val.is_null() {
+        None
+    } else {
+        Some(unsafe { Box::from_raw(val) })
+    }
+}
+
+impl<T> Drop for AtomicCell<T> {
+    fn drop(&mut self) {
+        // Free any data still held by the cell
+        let _ = self.take();
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/thread_pool/idle.rs b/third_party/rust/tokio/src/runtime/thread_pool/idle.rs
new file mode 100644
index 0000000000..ae87ca4ba1
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/thread_pool/idle.rs
@@ -0,0 +1,222 @@
+//! Coordinates idling workers
+
+use crate::loom::sync::atomic::AtomicUsize;
+use crate::loom::sync::Mutex;
+
+use std::fmt;
+use std::sync::atomic::Ordering::{self, SeqCst};
+
+pub(super) struct Idle {
+    /// Tracks both the number of searching workers and the number of unparked
+    /// workers.
+    ///
+    /// Used as a fast-path to avoid acquiring the lock when needed.
+    state: AtomicUsize,
+
+    /// Sleeping workers
+    sleepers: Mutex<Vec<usize>>,
+
+    /// Total number of workers.
+    num_workers: usize,
+}
+
+const UNPARK_SHIFT: usize = 16;
+const UNPARK_MASK: usize = !SEARCH_MASK;
+const SEARCH_MASK: usize = (1 << UNPARK_SHIFT) - 1;
+
+#[derive(Copy, Clone)]
+struct State(usize);
+
+impl Idle {
+    pub(super) fn new(num_workers: usize) -> Idle {
+        let init = State::new(num_workers);
+
+        Idle {
+            state: AtomicUsize::new(init.into()),
+            sleepers: Mutex::new(Vec::with_capacity(num_workers)),
+            num_workers,
+        }
+    }
+
+    /// If there are no workers actively searching, returns the index of a
+    /// worker currently sleeping.
+    pub(super) fn worker_to_notify(&self) -> Option<usize> {
+        // If at least one worker is spinning, work being notified will
+        // eventully be found. A searching thread will find **some** work and
+        // notify another worker, eventually leading to our work being found.
+        //
+        // For this to happen, this load must happen before the thread
+        // transitioning `num_searching` to zero. Acquire / Relese does not
+        // provide sufficient guarantees, so this load is done with `SeqCst` and
+        // will pair with the `fetch_sub(1)` when transitioning out of
+        // searching.
+        if !self.notify_should_wakeup() {
+            return None;
+        }
+
+        // Acquire the lock
+        let mut sleepers = self.sleepers.lock().unwrap();
+
+        // Check again, now that the lock is acquired
+        if !self.notify_should_wakeup() {
+            return None;
+        }
+
+        // A worker should be woken up, atomically increment the number of
+        // searching workers as well as the number of unparked workers.
+        State::unpark_one(&self.state);
+
+        // Get the worker to unpark
+        let ret = sleepers.pop();
+        debug_assert!(ret.is_some());
+
+        ret
+    }
+
+    /// Returns `true` if the worker needs to do a final check for submitted
+    /// work.
+    pub(super) fn transition_worker_to_parked(&self, worker: usize, is_searching: bool) -> bool {
+        // Acquire the lock
+        let mut sleepers = self.sleepers.lock().unwrap();
+
+        // Decrement the number of unparked threads
+        let ret = State::dec_num_unparked(&self.state, is_searching);
+
+        // Track the sleeping worker
+        sleepers.push(worker);
+
+        ret
+    }
+
+    pub(super) fn transition_worker_to_searching(&self) -> bool {
+        let state = State::load(&self.state, SeqCst);
+        if 2 * state.num_searching() >= self.num_workers {
+            return false;
+        }
+
+        // It is possible for this routine to allow more than 50% of the workers
+        // to search. That is OK. Limiting searchers is only an optimization to
+        // prevent too much contention.
+        State::inc_num_searching(&self.state, SeqCst);
+        true
+    }
+
+    /// A lightweight transition from searching -> running.
+    ///
+    /// Returns `true` if this is the final searching worker. The caller
+    /// **must** notify a new worker.
+    pub(super) fn transition_worker_from_searching(&self) -> bool {
+        State::dec_num_searching(&self.state)
+    }
+
+    /// Unpark a specific worker. This happens if tasks are submitted from
+    /// within the worker's park routine.
+    pub(super) fn unpark_worker_by_id(&self, worker_id: usize) {
+        let mut sleepers = self.sleepers.lock().unwrap();
+
+        for index in 0..sleepers.len() {
+            if sleepers[index] == worker_id {
+                sleepers.swap_remove(index);
+
+                // Update the state accordingly whle the lock is held.
+                State::unpark_one(&self.state);
+
+                return;
+            }
+        }
+    }
+
+    /// Returns `true` if `worker_id` is contained in the sleep set
+    pub(super) fn is_parked(&self, worker_id: usize) -> bool {
+        let sleepers = self.sleepers.lock().unwrap();
+        sleepers.contains(&worker_id)
+    }
+
+    fn notify_should_wakeup(&self) -> bool {
+        let state = State(self.state.fetch_add(0, SeqCst));
+        state.num_searching() == 0 && state.num_unparked() < self.num_workers
+    }
+}
+
+impl State {
+    fn new(num_workers: usize) -> State {
+        // All workers start in the unparked state
+        let ret = State(num_workers << UNPARK_SHIFT);
+        debug_assert_eq!(num_workers, ret.num_unparked());
+        debug_assert_eq!(0, ret.num_searching());
+        ret
+    }
+
+    fn load(cell: &AtomicUsize, ordering: Ordering) -> State {
+        State(cell.load(ordering))
+    }
+
+    fn unpark_one(cell: &AtomicUsize) {
+        cell.fetch_add(1 | (1 << UNPARK_SHIFT), SeqCst);
+    }
+
+    fn inc_num_searching(cell: &AtomicUsize, ordering: Ordering) {
+        cell.fetch_add(1, ordering);
+    }
+
+    /// Returns `true` if this is the final searching worker
+    fn dec_num_searching(cell: &AtomicUsize) -> bool {
+        let state = State(cell.fetch_sub(1, SeqCst));
+        state.num_searching() == 1
+    }
+
+    /// Track a sleeping worker
+    ///
+    /// Returns `true` if this is the final searching worker.
+    fn dec_num_unparked(cell: &AtomicUsize, is_searching: bool) -> bool {
+        let mut dec = 1 << UNPARK_SHIFT;
+
+        if is_searching {
+            dec += 1;
+        }
+
+        let prev = State(cell.fetch_sub(dec, SeqCst));
+        is_searching && prev.num_searching() == 1
+    }
+
+    /// Number of workers currently searching
+    fn num_searching(self) -> usize {
+        self.0 & SEARCH_MASK
+    }
+
+    /// Number of workers currently unparked
+    fn num_unparked(self) -> usize {
+        (self.0 & UNPARK_MASK) >> UNPARK_SHIFT
+    }
+}
+
+impl From<usize> for State {
+    fn from(src: usize) -> State {
+        State(src)
+    }
+}
+
+impl From<State> for usize {
+    fn from(src: State) -> usize {
+        src.0
+    }
+}
+
+impl fmt::Debug for State {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("worker::State")
+            .field("num_unparked", &self.num_unparked())
+            .field("num_searching", &self.num_searching())
+            .finish()
+    }
+}
+
+#[test]
+fn test_state() {
+    assert_eq!(0, UNPARK_MASK & SEARCH_MASK);
+    assert_eq!(0, !(UNPARK_MASK | SEARCH_MASK));
+
+    let state = State::new(10);
+    assert_eq!(10, state.num_unparked());
+    assert_eq!(0, state.num_searching());
+}
diff --git a/third_party/rust/tokio/src/runtime/thread_pool/mod.rs b/third_party/rust/tokio/src/runtime/thread_pool/mod.rs
new file mode 100644
index 0000000000..82e82d5b30
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/thread_pool/mod.rs
@@ -0,0 +1,117 @@
+//! Threadpool
+
+mod atomic_cell;
+use atomic_cell::AtomicCell;
+
+mod idle;
+use self::idle::Idle;
+
+mod worker;
+pub(crate) use worker::Launch;
+
+cfg_blocking! {
+    pub(crate) use worker::block_in_place;
+}
+
+use crate::loom::sync::Arc;
+use crate::runtime::task::{self, JoinHandle};
+use crate::runtime::Parker;
+
+use std::fmt;
+use std::future::Future;
+
+/// Work-stealing based thread pool for executing futures.
+pub(crate) struct ThreadPool {
+    spawner: Spawner,
+}
+
+/// Submit futures to the associated thread pool for execution.
+///
+/// A `Spawner` instance is a handle to a single thread pool that allows the owner
+/// of the handle to spawn futures onto the thread pool.
+///
+/// The `Spawner` handle is *only* used for spawning new futures. It does not
+/// impact the lifecycle of the thread pool in any way. The thread pool may
+/// shutdown while there are outstanding `Spawner` instances.
+///
+/// `Spawner` instances are obtained by calling [`ThreadPool::spawner`].
+///
+/// [`ThreadPool::spawner`]: method@ThreadPool::spawner
+#[derive(Clone)]
+pub(crate) struct Spawner {
+    shared: Arc<worker::Shared>,
+}
+
+// ===== impl ThreadPool =====
+
+impl ThreadPool {
+    pub(crate) fn new(size: usize, parker: Parker) -> (ThreadPool, Launch) {
+        let (shared, launch) = worker::create(size, parker);
+        let spawner = Spawner { shared };
+        let thread_pool = ThreadPool { spawner };
+
+        (thread_pool, launch)
+    }
+
+    /// Returns reference to `Spawner`.
+    ///
+    /// The `Spawner` handle can be cloned and enables spawning tasks from other
+    /// threads.
+    pub(crate) fn spawner(&self) -> &Spawner {
+        &self.spawner
+    }
+
+    /// Spawns a task
+    pub(crate) fn spawn<F>(&self, future: F) -> JoinHandle<F::Output>
+    where
+        F: Future + Send + 'static,
+        F::Output: Send + 'static,
+    {
+        self.spawner.spawn(future)
+    }
+
+    /// Blocks the current thread waiting for the future to complete.
+    ///
+    /// The future will execute on the current thread, but all spawned tasks
+    /// will be executed on the thread pool.
+    pub(crate) fn block_on<F>(&self, future: F) -> F::Output
+    where
+        F: Future,
+    {
+        let mut enter = crate::runtime::enter();
+        enter.block_on(future).expect("failed to park thread")
+    }
+}
+
+impl fmt::Debug for ThreadPool {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("ThreadPool").finish()
+    }
+}
+
+impl Drop for ThreadPool {
+    fn drop(&mut self) {
+        self.spawner.shared.close();
+    }
+}
+
+// ==== impl Spawner =====
+
+impl Spawner {
+    /// Spawns a future onto the thread pool
+    pub(crate) fn spawn<F>(&self, future: F) -> JoinHandle<F::Output>
+    where
+        F: Future + Send + 'static,
+        F::Output: Send + 'static,
+    {
+        let (task, handle) = task::joinable(future);
+        self.shared.schedule(task, false);
+        handle
+    }
+}
+
+impl fmt::Debug for Spawner {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("Spawner").finish()
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/thread_pool/worker.rs b/third_party/rust/tokio/src/runtime/thread_pool/worker.rs
new file mode 100644
index 0000000000..400e2a938c
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/thread_pool/worker.rs
@@ -0,0 +1,761 @@
+//! A scheduler is initialized with a fixed number of workers. Each worker is
+//! driven by a thread. Each worker has a "core" which contains data such as the
+//! run queue and other state. When `block_in_place` is called, the worker's
+//! "core" is handed off to a new thread allowing the scheduler to continue to
+//! make progress while the originating thread blocks.
+
+use crate::loom::rand::seed;
+use crate::loom::sync::{Arc, Mutex};
+use crate::park::{Park, Unpark};
+use crate::runtime;
+use crate::runtime::park::{Parker, Unparker};
+use crate::runtime::thread_pool::{AtomicCell, Idle};
+use crate::runtime::{queue, task};
+use crate::util::linked_list::LinkedList;
+use crate::util::FastRand;
+
+use std::cell::RefCell;
+use std::time::Duration;
+
+/// A scheduler worker
+pub(super) struct Worker {
+    /// Reference to shared state
+    shared: Arc<Shared>,
+
+    /// Index holding this worker's remote state
+    index: usize,
+
+    /// Used to hand-off a worker's core to another thread.
+    core: AtomicCell<Core>,
+}
+
+/// Core data
+struct Core {
+    /// Used to schedule bookkeeping tasks every so often.
+    tick: u8,
+
+    /// When a task is scheduled from a worker, it is stored in this slot. The
+    /// worker will check this slot for a task **before** checking the run
+    /// queue. This effectively results in the **last** scheduled task to be run
+    /// next (LIFO). This is an optimization for message passing patterns and
+    /// helps to reduce latency.
+    lifo_slot: Option<Notified>,
+
+    /// The worker-local run queue.
+    run_queue: queue::Local<Arc<Worker>>,
+
+    /// True if the worker is currently searching for more work. Searching
+    /// involves attempting to steal from other workers.
+    is_searching: bool,
+
+    /// True if the scheduler is being shutdown
+    is_shutdown: bool,
+
+    /// Tasks owned by the core
+    tasks: LinkedList<Task>,
+
+    /// Parker
+    ///
+    /// Stored in an `Option` as the parker is added / removed to make the
+    /// borrow checker happy.
+    park: Option<Parker>,
+
+    /// Fast random number generator.
+    rand: FastRand,
+}
+
+/// State shared across all workers
+pub(super) struct Shared {
+    /// Per-worker remote state. All other workers have access to this and is
+    /// how they communicate between each other.
+    remotes: Box<[Remote]>,
+
+    /// Submit work to the scheduler while **not** currently on a worker thread.
+    inject: queue::Inject<Arc<Worker>>,
+
+    /// Coordinates idle workers
+    idle: Idle,
+
+    /// Workers have have observed the shutdown signal
+    ///
+    /// The core is **not** placed back in the worker to avoid it from being
+    /// stolen by a thread that was spawned as part of `block_in_place`.
+    shutdown_workers: Mutex<Vec<(Box<Core>, Arc<Worker>)>>,
+}
+
+/// Used to communicate with a worker from other threads.
+struct Remote {
+    /// Steal tasks from this worker.
+    steal: queue::Steal<Arc<Worker>>,
+
+    /// Transfers tasks to be released. Any worker pushes tasks, only the owning
+    /// worker pops.
+    pending_drop: task::TransferStack<Arc<Worker>>,
+
+    /// Unparks the associated worker thread
+    unpark: Unparker,
+}
+
+/// Thread-local context
+struct Context {
+    /// Worker
+    worker: Arc<Worker>,
+
+    /// Core data
+    core: RefCell<Option<Box<Core>>>,
+}
+
+/// Starts the workers
+pub(crate) struct Launch(Vec<Arc<Worker>>);
+
+/// Running a task may consume the core. If the core is still available when
+/// running the task completes, it is returned. Otherwise, the worker will need
+/// to stop processing.
+type RunResult = Result<Box<Core>, ()>;
+
+/// A task handle
+type Task = task::Task<Arc<Worker>>;
+
+/// A notified task handle
+type Notified = task::Notified<Arc<Worker>>;
+
+// Tracks thread-local state
+scoped_thread_local!(static CURRENT: Context);
+
+pub(super) fn create(size: usize, park: Parker) -> (Arc<Shared>, Launch) {
+    let mut cores = vec![];
+    let mut remotes = vec![];
+
+    // Create the local queues
+    for _ in 0..size {
+        let (steal, run_queue) = queue::local();
+
+        let park = park.clone();
+        let unpark = park.unpark();
+
+        cores.push(Box::new(Core {
+            tick: 0,
+            lifo_slot: None,
+            run_queue,
+            is_searching: false,
+            is_shutdown: false,
+            tasks: LinkedList::new(),
+            park: Some(park),
+            rand: FastRand::new(seed()),
+        }));
+
+        remotes.push(Remote {
+            steal,
+            pending_drop: task::TransferStack::new(),
+            unpark,
+        });
+    }
+
+    let shared = Arc::new(Shared {
+        remotes: remotes.into_boxed_slice(),
+        inject: queue::Inject::new(),
+        idle: Idle::new(size),
+        shutdown_workers: Mutex::new(vec![]),
+    });
+
+    let mut launch = Launch(vec![]);
+
+    for (index, core) in cores.drain(..).enumerate() {
+        launch.0.push(Arc::new(Worker {
+            shared: shared.clone(),
+            index,
+            core: AtomicCell::new(Some(core)),
+        }));
+    }
+
+    (shared, launch)
+}
+
+cfg_blocking! {
+    pub(crate) fn block_in_place<F, R>(f: F) -> R
+    where
+        F: FnOnce() -> R,
+    {
+        // Try to steal the worker core back
+        struct Reset;
+
+        impl Drop for Reset {
+            fn drop(&mut self) {
+                CURRENT.with(|maybe_cx| {
+                    if let Some(cx) = maybe_cx {
+                        let core = cx.worker.core.take();
+                        *cx.core.borrow_mut() = core;
+                    }
+                });
+            }
+        }
+
+        CURRENT.with(|maybe_cx| {
+            let cx = maybe_cx.expect("can call blocking only when running in a spawned task");
+
+            // Get the worker core. If none is set, then blocking is fine!
+            let core = match cx.core.borrow_mut().take() {
+                Some(core) => {
+                    // We are effectively leaving the executor, so we need to
+                    // forcibly end budgeting.
+                    crate::coop::stop();
+                    core
+                },
+                None => return,
+            };
+
+            // The parker should be set here
+            assert!(core.park.is_some());
+
+            // In order to block, the core must be sent to another thread for
+            // execution.
+            //
+            // First, move the core back into the worker's shared core slot.
+            cx.worker.core.set(core);
+
+            // Next, clone the worker handle and send it to a new thread for
+            // processing.
+            //
+            // Once the blocking task is done executing, we will attempt to
+            // steal the core back.
+            let worker = cx.worker.clone();
+            runtime::spawn_blocking(move || run(worker));
+        });
+
+        let _reset = Reset;
+
+        f()
+    }
+}
+
+/// After how many ticks is the global queue polled. This helps to ensure
+/// fairness.
+///
+/// The number is fairly arbitrary. I believe this value was copied from golang.
+const GLOBAL_POLL_INTERVAL: u8 = 61;
+
+impl Launch {
+    pub(crate) fn launch(mut self) {
+        for worker in self.0.drain(..) {
+            runtime::spawn_blocking(move || run(worker));
+        }
+    }
+}
+
+fn run(worker: Arc<Worker>) {
+    // Acquire a core. If this fails, then another thread is running this
+    // worker and there is nothing further to do.
+    let core = match worker.core.take() {
+        Some(core) => core,
+        None => return,
+    };
+
+    // Set the worker context.
+    let cx = Context {
+        worker,
+        core: RefCell::new(None),
+    };
+
+    let _enter = crate::runtime::enter();
+
+    CURRENT.set(&cx, || {
+        // This should always be an error. It only returns a `Result` to support
+        // using `?` to short circuit.
+        assert!(cx.run(core).is_err());
+    });
+}
+
+impl Context {
+    fn run(&self, mut core: Box<Core>) -> RunResult {
+        while !core.is_shutdown {
+            // Increment the tick
+            core.tick();
+
+            // Run maintenance, if needed
+            core = self.maintenance(core);
+
+            // First, check work available to the current worker.
+            if let Some(task) = core.next_task(&self.worker) {
+                core = self.run_task(task, core)?;
+                continue;
+            }
+
+            // There is no more **local** work to process, try to steal work
+            // from other workers.
+            if let Some(task) = core.steal_work(&self.worker) {
+                core = self.run_task(task, core)?;
+            } else {
+                // Wait for work
+                core = self.park(core);
+            }
+        }
+
+        // Signal shutdown
+        self.worker.shared.shutdown(core, self.worker.clone());
+        Err(())
+    }
+
+    fn run_task(&self, task: Notified, mut core: Box<Core>) -> RunResult {
+        // Make sure thew orker is not in the **searching** state. This enables
+        // another idle worker to try to steal work.
+        core.transition_from_searching(&self.worker);
+
+        // Make the core available to the runtime context
+        *self.core.borrow_mut() = Some(core);
+
+        // Run the task
+        crate::coop::budget(|| {
+            task.run();
+
+            // As long as there is budget remaining and a task exists in the
+            // `lifo_slot`, then keep running.
+            loop {
+                // Check if we still have the core. If not, the core was stolen
+                // by another worker.
+                let mut core = match self.core.borrow_mut().take() {
+                    Some(core) => core,
+                    None => return Err(()),
+                };
+
+                // Check for a task in the LIFO slot
+                let task = match core.lifo_slot.take() {
+                    Some(task) => task,
+                    None => return Ok(core),
+                };
+
+                if crate::coop::has_budget_remaining() {
+                    // Run the LIFO task, then loop
+                    *self.core.borrow_mut() = Some(core);
+                    task.run();
+                } else {
+                    // Not enough budget left to run the LIFO task, push it to
+                    // the back of the queue and return.
+                    core.run_queue.push_back(task, self.worker.inject());
+                    return Ok(core);
+                }
+            }
+        })
+    }
+
+    fn maintenance(&self, mut core: Box<Core>) -> Box<Core> {
+        if core.tick % GLOBAL_POLL_INTERVAL == 0 {
+            // Call `park` with a 0 timeout. This enables the I/O driver, timer, ...
+            // to run without actually putting the thread to sleep.
+            core = self.park_timeout(core, Some(Duration::from_millis(0)));
+
+            // Run regularly scheduled maintenance
+            core.maintenance(&self.worker);
+        }
+
+        core
+    }
+
+    fn park(&self, mut core: Box<Core>) -> Box<Core> {
+        core.transition_to_parked(&self.worker);
+
+        while !core.is_shutdown {
+            core = self.park_timeout(core, None);
+
+            // Run regularly scheduled maintenance
+            core.maintenance(&self.worker);
+
+            if core.transition_from_parked(&self.worker) {
+                return core;
+            }
+        }
+
+        core
+    }
+
+    fn park_timeout(&self, mut core: Box<Core>, duration: Option<Duration>) -> Box<Core> {
+        // Take the parker out of core
+        let mut park = core.park.take().expect("park missing");
+
+        // Store `core` in context
+        *self.core.borrow_mut() = Some(core);
+
+        // Park thread
+        if let Some(timeout) = duration {
+            park.park_timeout(timeout).expect("park failed");
+        } else {
+            park.park().expect("park failed");
+        }
+
+        // Remove `core` from context
+        core = self.core.borrow_mut().take().expect("core missing");
+
+        // Place `park` back in `core`
+        core.park = Some(park);
+
+        // If there are tasks available to steal, notify a worker
+        if core.run_queue.is_stealable() {
+            self.worker.shared.notify_parked();
+        }
+
+        core
+    }
+}
+
+impl Core {
+    /// Increment the tick
+    fn tick(&mut self) {
+        self.tick = self.tick.wrapping_add(1);
+    }
+
+    /// Return the next notified task available to this worker.
+    fn next_task(&mut self, worker: &Worker) -> Option<Notified> {
+        if self.tick % GLOBAL_POLL_INTERVAL == 0 {
+            worker.inject().pop().or_else(|| self.next_local_task())
+        } else {
+            self.next_local_task().or_else(|| worker.inject().pop())
+        }
+    }
+
+    fn next_local_task(&mut self) -> Option<Notified> {
+        self.lifo_slot.take().or_else(|| self.run_queue.pop())
+    }
+
+    fn steal_work(&mut self, worker: &Worker) -> Option<Notified> {
+        if !self.transition_to_searching(worker) {
+            return None;
+        }
+
+        let num = worker.shared.remotes.len();
+        // Start from a random worker
+        let start = self.rand.fastrand_n(num as u32) as usize;
+
+        for i in 0..num {
+            let i = (start + i) % num;
+
+            // Don't steal from ourself! We know we don't have work.
+            if i == worker.index {
+                continue;
+            }
+
+            let target = &worker.shared.remotes[i];
+            if let Some(task) = target.steal.steal_into(&mut self.run_queue) {
+                return Some(task);
+            }
+        }
+
+        // Fallback on checking the global queue
+        worker.shared.inject.pop()
+    }
+
+    fn transition_to_searching(&mut self, worker: &Worker) -> bool {
+        if !self.is_searching {
+            self.is_searching = worker.shared.idle.transition_worker_to_searching();
+        }
+
+        self.is_searching
+    }
+
+    fn transition_from_searching(&mut self, worker: &Worker) {
+        if !self.is_searching {
+            return;
+        }
+
+        self.is_searching = false;
+        worker.shared.transition_worker_from_searching();
+    }
+
+    /// Prepare the worker state for parking
+    fn transition_to_parked(&mut self, worker: &Worker) {
+        // When the final worker transitions **out** of searching to parked, it
+        // must check all the queues one last time in case work materialized
+        // between the last work scan and transitioning out of searching.
+        let is_last_searcher = worker
+            .shared
+            .idle
+            .transition_worker_to_parked(worker.index, self.is_searching);
+
+        // The worker is no longer searching. Setting this is the local cache
+        // only.
+        self.is_searching = false;
+
+        if is_last_searcher {
+            worker.shared.notify_if_work_pending();
+        }
+    }
+
+    /// Returns `true` if the transition happened.
+    fn transition_from_parked(&mut self, worker: &Worker) -> bool {
+        // If a task is in the lifo slot, then we must unpark regardless of
+        // being notified
+        if self.lifo_slot.is_some() {
+            worker.shared.idle.unpark_worker_by_id(worker.index);
+            self.is_searching = true;
+            return true;
+        }
+
+        if worker.shared.idle.is_parked(worker.index) {
+            return false;
+        }
+
+        // When unparked, the worker is in the searching state.
+        self.is_searching = true;
+        true
+    }
+
+    /// Runs maintenance work such as free pending tasks and check the pool's
+    /// state.
+    fn maintenance(&mut self, worker: &Worker) {
+        self.drain_pending_drop(worker);
+
+        if !self.is_shutdown {
+            // Check if the scheduler has been shutdown
+            self.is_shutdown = worker.inject().is_closed();
+        }
+    }
+
+    // Shutdown the core
+    fn shutdown(&mut self, worker: &Worker) {
+        // Take the core
+        let mut park = self.park.take().expect("park missing");
+
+        // Signal to all tasks to shut down.
+        for header in self.tasks.iter() {
+            header.shutdown();
+        }
+
+        loop {
+            self.drain_pending_drop(worker);
+
+            if self.tasks.is_empty() {
+                break;
+            }
+
+            // Wait until signalled
+            park.park().expect("park failed");
+        }
+
+        // Drain the queue
+        while let Some(_) = self.next_local_task() {}
+    }
+
+    fn drain_pending_drop(&mut self, worker: &Worker) {
+        use std::mem::ManuallyDrop;
+
+        for task in worker.remote().pending_drop.drain() {
+            let task = ManuallyDrop::new(task);
+
+            // safety: tasks are only pushed into the `pending_drop` stacks that
+            // are associated with the list they are inserted into. When a task
+            // is pushed into `pending_drop`, the ref-inc is skipped, so we must
+            // not ref-dec here.
+            //
+            // See `bind` and `release` implementations.
+            unsafe {
+                self.tasks.remove(task.header().into());
+            }
+        }
+    }
+}
+
+impl Worker {
+    /// Returns a reference to the scheduler's injection queue
+    fn inject(&self) -> &queue::Inject<Arc<Worker>> {
+        &self.shared.inject
+    }
+
+    /// Return a reference to this worker's remote data
+    fn remote(&self) -> &Remote {
+        &self.shared.remotes[self.index]
+    }
+
+    fn eq(&self, other: &Worker) -> bool {
+        self.shared.ptr_eq(&other.shared) && self.index == other.index
+    }
+}
+
+impl task::Schedule for Arc<Worker> {
+    fn bind(task: Task) -> Arc<Worker> {
+        CURRENT.with(|maybe_cx| {
+            let cx = maybe_cx.expect("scheduler context missing");
+
+            // Track the task
+            cx.core
+                .borrow_mut()
+                .as_mut()
+                .expect("scheduler core missing")
+                .tasks
+                .push_front(task);
+
+            // Return a clone of the worker
+            cx.worker.clone()
+        })
+    }
+
+    fn release(&self, task: &Task) -> Option<Task> {
+        use std::ptr::NonNull;
+
+        CURRENT.with(|maybe_cx| {
+            let cx = maybe_cx.expect("scheduler context missing");
+
+            if self.eq(&cx.worker) {
+                let mut maybe_core = cx.core.borrow_mut();
+
+                if let Some(core) = &mut *maybe_core {
+                    // Directly remove the task
+                    //
+                    // safety: the task is inserted in the list in `bind`.
+                    unsafe {
+                        let ptr = NonNull::from(task.header());
+                        return core.tasks.remove(ptr);
+                    }
+                }
+            }
+
+            // Track the task to be released by the worker that owns it
+            //
+            // Safety: We get a new handle without incrementing the ref-count.
+            // A ref-count is held by the "owned" linked list and it is only
+            // ever removed from that list as part of the release process: this
+            // method or popping the task from `pending_drop`. Thus, we can rely
+            // on the ref-count held by the linked-list to keep the memory
+            // alive.
+            //
+            // When the task is removed from the stack, it is forgotten instead
+            // of dropped.
+            let task = unsafe { Task::from_raw(task.header().into()) };
+
+            self.remote().pending_drop.push(task);
+
+            if cx.core.borrow().is_some() {
+                return None;
+            }
+
+            // The worker core has been handed off to another thread. In the
+            // event that the scheduler is currently shutting down, the thread
+            // that owns the task may be waiting on the release to complete
+            // shutdown.
+            if self.inject().is_closed() {
+                self.remote().unpark.unpark();
+            }
+
+            None
+        })
+    }
+
+    fn schedule(&self, task: Notified) {
+        self.shared.schedule(task, false);
+    }
+
+    fn yield_now(&self, task: Notified) {
+        self.shared.schedule(task, true);
+    }
+}
+
+impl Shared {
+    pub(super) fn schedule(&self, task: Notified, is_yield: bool) {
+        CURRENT.with(|maybe_cx| {
+            if let Some(cx) = maybe_cx {
+                // Make sure the task is part of the **current** scheduler.
+                if self.ptr_eq(&cx.worker.shared) {
+                    // And the current thread still holds a core
+                    if let Some(core) = cx.core.borrow_mut().as_mut() {
+                        self.schedule_local(core, task, is_yield);
+                        return;
+                    }
+                }
+            }
+
+            // Otherwise, use the inject queue
+            self.inject.push(task);
+            self.notify_parked();
+        });
+    }
+
+    fn schedule_local(&self, core: &mut Core, task: Notified, is_yield: bool) {
+        // Spawning from the worker thread. If scheduling a "yield" then the
+        // task must always be pushed to the back of the queue, enabling other
+        // tasks to be executed. If **not** a yield, then there is more
+        // flexibility and the task may go to the front of the queue.
+        let should_notify = if is_yield {
+            core.run_queue.push_back(task, &self.inject);
+            true
+        } else {
+            // Push to the LIFO slot
+            let prev = core.lifo_slot.take();
+            let ret = prev.is_some();
+
+            if let Some(prev) = prev {
+                core.run_queue.push_back(prev, &self.inject);
+            }
+
+            core.lifo_slot = Some(task);
+
+            ret
+        };
+
+        // Only notify if not currently parked. If `park` is `None`, then the
+        // scheduling is from a resource driver. As notifications often come in
+        // batches, the notification is delayed until the park is complete.
+        if should_notify && core.park.is_some() {
+            self.notify_parked();
+        }
+    }
+
+    pub(super) fn close(&self) {
+        if self.inject.close() {
+            self.notify_all();
+        }
+    }
+
+    fn notify_parked(&self) {
+        if let Some(index) = self.idle.worker_to_notify() {
+            self.remotes[index].unpark.unpark();
+        }
+    }
+
+    fn notify_all(&self) {
+        for remote in &self.remotes[..] {
+            remote.unpark.unpark();
+        }
+    }
+
+    fn notify_if_work_pending(&self) {
+        for remote in &self.remotes[..] {
+            if !remote.steal.is_empty() {
+                self.notify_parked();
+                return;
+            }
+        }
+
+        if !self.inject.is_empty() {
+            self.notify_parked();
+        }
+    }
+
+    fn transition_worker_from_searching(&self) {
+        if self.idle.transition_worker_from_searching() {
+            // We are the final searching worker. Because work was found, we
+            // need to notify another worker.
+            self.notify_parked();
+        }
+    }
+
+    /// Signals that a worker has observed the shutdown signal and has replaced
+    /// its core back into its handle.
+    ///
+    /// If all workers have reached this point, the final cleanup is performed.
+    fn shutdown(&self, core: Box<Core>, worker: Arc<Worker>) {
+        let mut workers = self.shutdown_workers.lock().unwrap();
+        workers.push((core, worker));
+
+        if workers.len() != self.remotes.len() {
+            return;
+        }
+
+        for (mut core, worker) in workers.drain(..) {
+            core.shutdown(&worker);
+        }
+
+        // Drain the injection queue
+        while let Some(_) = self.inject.pop() {}
+    }
+
+    fn ptr_eq(&self, other: &Shared) -> bool {
+        self as *const _ == other as *const _
+    }
+}
diff --git a/third_party/rust/tokio/src/runtime/time.rs b/third_party/rust/tokio/src/runtime/time.rs
new file mode 100644
index 0000000000..c623d9641a
--- /dev/null
+++ b/third_party/rust/tokio/src/runtime/time.rs
@@ -0,0 +1,59 @@
+//! Abstracts out the APIs necessary to `Runtime` for integrating the time
+//! driver. When the `time` feature flag is **not** enabled. These APIs are
+//! shells. This isolates the complexity of dealing with conditional
+//! compilation.
+
+pub(crate) use variant::*;
+
+#[cfg(feature = "time")]
+mod variant {
+    use crate::park::Either;
+    use crate::runtime::io;
+    use crate::time::{self, driver};
+
+    pub(crate) type Clock = time::Clock;
+    pub(crate) type Driver = Either<driver::Driver<io::Driver>, io::Driver>;
+    pub(crate) type Handle = Option<driver::Handle>;
+
+    pub(crate) fn create_clock() -> Clock {
+        Clock::new()
+    }
+
+    /// Create a new timer driver / handle pair
+    pub(crate) fn create_driver(
+        enable: bool,
+        io_driver: io::Driver,
+        clock: Clock,
+    ) -> (Driver, Handle) {
+        if enable {
+            let driver = driver::Driver::new(io_driver, clock);
+            let handle = driver.handle();
+
+            (Either::A(driver), Some(handle))
+        } else {
+            (Either::B(io_driver), None)
+        }
+    }
+}
+
+#[cfg(not(feature = "time"))]
+mod variant {
+    use crate::runtime::io;
+
+    pub(crate) type Clock = ();
+    pub(crate) type Driver = io::Driver;
+    pub(crate) type Handle = ();
+
+    pub(crate) fn create_clock() -> Clock {
+        ()
+    }
+
+    /// Create a new timer driver / handle pair
+    pub(crate) fn create_driver(
+        _enable: bool,
+        io_driver: io::Driver,
+        _clock: Clock,
+    ) -> (Driver, Handle) {
+        (io_driver, ())
+    }
+}
diff --git a/third_party/rust/tokio/src/signal/ctrl_c.rs b/third_party/rust/tokio/src/signal/ctrl_c.rs
new file mode 100644
index 0000000000..1eeeb85aa1
--- /dev/null
+++ b/third_party/rust/tokio/src/signal/ctrl_c.rs
@@ -0,0 +1,53 @@
+#[cfg(unix)]
+use super::unix::{self as os_impl};
+#[cfg(windows)]
+use super::windows::{self as os_impl};
+
+use std::io;
+
+/// Completes when a "ctrl-c" notification is sent to the process.
+///
+/// While signals are handled very differently between Unix and Windows, both
+/// platforms support receiving a signal on "ctrl-c". This function provides a
+/// portable API for receiving this notification.
+///
+/// Once the returned future is polled, a listener is registered. The future
+/// will complete on the first received `ctrl-c` **after** the initial call to
+/// either `Future::poll` or `.await`.
+///
+/// # Caveats
+///
+/// On Unix platforms, the first time that a `Signal` instance is registered for a
+/// particular signal kind, an OS signal-handler is installed which replaces the
+/// default platform behavior when that signal is received, **for the duration of
+/// the entire process**.
+///
+/// For example, Unix systems will terminate a process by default when it
+/// receives a signal generated by "CTRL+C" on the terminal. But, when a
+/// `ctrl_c` stream is created to listen for this signal, the time it arrives,
+/// it will be translated to a stream event, and the process will continue to
+/// execute.  **Even if this `Signal` instance is dropped, subsequent SIGINT
+/// deliveries will end up captured by Tokio, and the default platform behavior
+/// will NOT be reset**.
+///
+/// Thus, applications should take care to ensure the expected signal behavior
+/// occurs as expected after listening for specific signals.
+///
+/// # Examples
+///
+/// ```rust,no_run
+/// use tokio::signal;
+///
+/// #[tokio::main]
+/// async fn main() {
+///     println!("waiting for ctrl-c");
+///
+///     signal::ctrl_c().await.expect("failed to listen for event");
+///
+///     println!("received ctrl-c event");
+/// }
+/// ```
+pub async fn ctrl_c() -> io::Result<()> {
+    os_impl::ctrl_c()?.recv().await;
+    Ok(())
+}
diff --git a/third_party/rust/tokio/src/signal/mod.rs b/third_party/rust/tokio/src/signal/mod.rs
new file mode 100644
index 0000000000..6e5e350df5
--- /dev/null
+++ b/third_party/rust/tokio/src/signal/mod.rs
@@ -0,0 +1,60 @@
+//! Asynchronous signal handling for Tokio
+//!
+//! Note that signal handling is in general a very tricky topic and should be
+//! used with great care. This crate attempts to implement 'best practice' for
+//! signal handling, but it should be evaluated for your own applications' needs
+//! to see if it's suitable.
+//!
+//! The are some fundamental limitations of this crate documented on the OS
+//! specific structures, as well.
+//!
+//! # Examples
+//!
+//! Print on "ctrl-c" notification.
+//!
+//! ```rust,no_run
+//! use tokio::signal;
+//!
+//! #[tokio::main]
+//! async fn main() -> Result<(), Box<dyn std::error::Error>> {
+//!     signal::ctrl_c().await?;
+//!     println!("ctrl-c received!");
+//!     Ok(())
+//! }
+//! ```
+//!
+//! Wait for SIGHUP on Unix
+//!
+//! ```rust,no_run
+//! # #[cfg(unix)] {
+//! use tokio::signal::unix::{signal, SignalKind};
+//!
+//! #[tokio::main]
+//! async fn main() -> Result<(), Box<dyn std::error::Error>> {
+//!     // An infinite stream of hangup signals.
+//!     let mut stream = signal(SignalKind::hangup())?;
+//!
+//!     // Print whenever a HUP signal is received
+//!     loop {
+//!         stream.recv().await;
+//!         println!("got signal HUP");
+//!     }
+//! }
+//! # }
+//! ```
+
+mod ctrl_c;
+pub use ctrl_c::ctrl_c;
+
+mod registry;
+
+mod os {
+    #[cfg(unix)]
+    pub(crate) use super::unix::{OsExtraData, OsStorage};
+
+    #[cfg(windows)]
+    pub(crate) use super::windows::{OsExtraData, OsStorage};
+}
+
+pub mod unix;
+pub mod windows;
diff --git a/third_party/rust/tokio/src/signal/registry.rs b/third_party/rust/tokio/src/signal/registry.rs
new file mode 100644
index 0000000000..50edd2b6c4
--- /dev/null
+++ b/third_party/rust/tokio/src/signal/registry.rs
@@ -0,0 +1,321 @@
+#![allow(clippy::unit_arg)]
+
+use crate::signal::os::{OsExtraData, OsStorage};
+
+use crate::sync::mpsc::Sender;
+
+use lazy_static::lazy_static;
+use std::ops;
+use std::pin::Pin;
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Mutex;
+
+pub(crate) type EventId = usize;
+
+/// State for a specific event, whether a notification is pending delivery,
+/// and what listeners are registered.
+#[derive(Default, Debug)]
+pub(crate) struct EventInfo {
+    pending: AtomicBool,
+    recipients: Mutex<Vec<Sender<()>>>,
+}
+
+/// An interface for retrieving the `EventInfo` for a particular eventId.
+pub(crate) trait Storage {
+    /// Gets the `EventInfo` for `id` if it exists.
+    fn event_info(&self, id: EventId) -> Option<&EventInfo>;
+
+    /// Invokes `f` once for each defined `EventInfo` in this storage.
+    fn for_each<'a, F>(&'a self, f: F)
+    where
+        F: FnMut(&'a EventInfo);
+}
+
+impl Storage for Vec<EventInfo> {
+    fn event_info(&self, id: EventId) -> Option<&EventInfo> {
+        self.get(id)
+    }
+
+    fn for_each<'a, F>(&'a self, f: F)
+    where
+        F: FnMut(&'a EventInfo),
+    {
+        self.iter().for_each(f)
+    }
+}
+
+/// An interface for initializing a type. Useful for situations where we cannot
+/// inject a configured instance in the constructor of another type.
+pub(crate) trait Init {
+    fn init() -> Self;
+}
+
+/// Manages and distributes event notifications to any registered listeners.
+///
+/// Generic over the underlying storage to allow for domain specific
+/// optimizations (e.g. eventIds may or may not be contiguous).
+#[derive(Debug)]
+pub(crate) struct Registry<S> {
+    storage: S,
+}
+
+impl<S> Registry<S> {
+    fn new(storage: S) -> Self {
+        Self { storage }
+    }
+}
+
+impl<S: Storage> Registry<S> {
+    /// Registers a new listener for `event_id`.
+    fn register_listener(&self, event_id: EventId, listener: Sender<()>) {
+        self.storage
+            .event_info(event_id)
+            .unwrap_or_else(|| panic!("invalid event_id: {}", event_id))
+            .recipients
+            .lock()
+            .unwrap()
+            .push(listener);
+    }
+
+    /// Marks `event_id` as having been delivered, without broadcasting it to
+    /// any listeners.
+    fn record_event(&self, event_id: EventId) {
+        if let Some(event_info) = self.storage.event_info(event_id) {
+            event_info.pending.store(true, Ordering::SeqCst)
+        }
+    }
+
+    /// Broadcasts all previously recorded events to their respective listeners.
+    ///
+    /// Returns `true` if an event was delivered to at least one listener.
+    fn broadcast(&self) -> bool {
+        use crate::sync::mpsc::error::TrySendError;
+
+        let mut did_notify = false;
+        self.storage.for_each(|event_info| {
+            // Any signal of this kind arrived since we checked last?
+            if !event_info.pending.swap(false, Ordering::SeqCst) {
+                return;
+            }
+
+            let mut recipients = event_info.recipients.lock().unwrap();
+
+            // Notify all waiters on this signal that the signal has been
+            // received. If we can't push a message into the queue then we don't
+            // worry about it as everything is coalesced anyway. If the channel
+            // has gone away then we can remove that slot.
+            for i in (0..recipients.len()).rev() {
+                match recipients[i].try_send(()) {
+                    Ok(()) => did_notify = true,
+                    Err(TrySendError::Closed(..)) => {
+                        recipients.swap_remove(i);
+                    }
+
+                    // Channel is full, ignore the error since the
+                    // receiver has already been woken up
+                    Err(_) => {}
+                }
+            }
+        });
+
+        did_notify
+    }
+}
+
+pub(crate) struct Globals {
+    extra: OsExtraData,
+    registry: Registry<OsStorage>,
+}
+
+impl ops::Deref for Globals {
+    type Target = OsExtraData;
+
+    fn deref(&self) -> &Self::Target {
+        &self.extra
+    }
+}
+
+impl Globals {
+    /// Registers a new listener for `event_id`.
+    pub(crate) fn register_listener(&self, event_id: EventId, listener: Sender<()>) {
+        self.registry.register_listener(event_id, listener);
+    }
+
+    /// Marks `event_id` as having been delivered, without broadcasting it to
+    /// any listeners.
+    pub(crate) fn record_event(&self, event_id: EventId) {
+        self.registry.record_event(event_id);
+    }
+
+    /// Broadcasts all previously recorded events to their respective listeners.
+    ///
+    /// Returns `true` if an event was delivered to at least one listener.
+    pub(crate) fn broadcast(&self) -> bool {
+        self.registry.broadcast()
+    }
+
+    #[cfg(unix)]
+    pub(crate) fn storage(&self) -> &OsStorage {
+        &self.registry.storage
+    }
+}
+
+pub(crate) fn globals() -> Pin<&'static Globals>
+where
+    OsExtraData: 'static + Send + Sync + Init,
+    OsStorage: 'static + Send + Sync + Init,
+{
+    lazy_static! {
+        static ref GLOBALS: Pin<Box<Globals>> = Box::pin(Globals {
+            extra: OsExtraData::init(),
+            registry: Registry::new(OsStorage::init()),
+        });
+    }
+
+    GLOBALS.as_ref()
+}
+
+#[cfg(all(test, not(loom)))]
+mod tests {
+    use super::*;
+    use crate::runtime::{self, Runtime};
+    use crate::sync::{mpsc, oneshot};
+
+    use futures::future;
+
+    #[test]
+    fn smoke() {
+        let mut rt = rt();
+        rt.block_on(async move {
+            let registry = Registry::new(vec![
+                EventInfo::default(),
+                EventInfo::default(),
+                EventInfo::default(),
+            ]);
+
+            let (first_tx, first_rx) = mpsc::channel(3);
+            let (second_tx, second_rx) = mpsc::channel(3);
+            let (third_tx, third_rx) = mpsc::channel(3);
+
+            registry.register_listener(0, first_tx);
+            registry.register_listener(1, second_tx);
+            registry.register_listener(2, third_tx);
+
+            let (fire, wait) = oneshot::channel();
+
+            crate::spawn(async {
+                wait.await.expect("wait failed");
+
+                // Record some events which should get coalesced
+                registry.record_event(0);
+                registry.record_event(0);
+                registry.record_event(1);
+                registry.record_event(1);
+                registry.broadcast();
+
+                // Send subsequent signal
+                registry.record_event(0);
+                registry.broadcast();
+
+                drop(registry);
+            });
+
+            let _ = fire.send(());
+            let all = future::join3(collect(first_rx), collect(second_rx), collect(third_rx));
+
+            let (first_results, second_results, third_results) = all.await;
+            assert_eq!(2, first_results.len());
+            assert_eq!(1, second_results.len());
+            assert_eq!(0, third_results.len());
+        });
+    }
+
+    #[test]
+    #[should_panic = "invalid event_id: 1"]
+    fn register_panics_on_invalid_input() {
+        let registry = Registry::new(vec![EventInfo::default()]);
+
+        let (tx, _) = mpsc::channel(1);
+        registry.register_listener(1, tx);
+    }
+
+    #[test]
+    fn record_invalid_event_does_nothing() {
+        let registry = Registry::new(vec![EventInfo::default()]);
+        registry.record_event(42);
+    }
+
+    #[test]
+    fn broadcast_cleans_up_disconnected_listeners() {
+        let mut rt = Runtime::new().unwrap();
+
+        rt.block_on(async {
+            let registry = Registry::new(vec![EventInfo::default()]);
+
+            let (first_tx, first_rx) = mpsc::channel(1);
+            let (second_tx, second_rx) = mpsc::channel(1);
+            let (third_tx, third_rx) = mpsc::channel(1);
+
+            registry.register_listener(0, first_tx);
+            registry.register_listener(0, second_tx);
+            registry.register_listener(0, third_tx);
+
+            drop(first_rx);
+            drop(second_rx);
+
+            let (fire, wait) = oneshot::channel();
+
+            crate::spawn(async {
+                wait.await.expect("wait failed");
+
+                registry.record_event(0);
+                registry.broadcast();
+
+                assert_eq!(1, registry.storage[0].recipients.lock().unwrap().len());
+                drop(registry);
+            });
+
+            let _ = fire.send(());
+            let results = collect(third_rx).await;
+
+            assert_eq!(1, results.len());
+        });
+    }
+
+    #[test]
+    fn broadcast_returns_if_at_least_one_event_fired() {
+        let registry = Registry::new(vec![EventInfo::default()]);
+
+        registry.record_event(0);
+        assert_eq!(false, registry.broadcast());
+
+        let (first_tx, first_rx) = mpsc::channel(1);
+        let (second_tx, second_rx) = mpsc::channel(1);
+
+        registry.register_listener(0, first_tx);
+        registry.register_listener(0, second_tx);
+
+        registry.record_event(0);
+        assert_eq!(true, registry.broadcast());
+
+        drop(first_rx);
+        registry.record_event(0);
+        assert_eq!(false, registry.broadcast());
+
+        drop(second_rx);
+    }
+
+    fn rt() -> Runtime {
+        runtime::Builder::new().basic_scheduler().build().unwrap()
+    }
+
+    async fn collect(mut rx: crate::sync::mpsc::Receiver<()>) -> Vec<()> {
+        let mut ret = vec![];
+
+        while let Some(v) = rx.recv().await {
+            ret.push(v);
+        }
+
+        ret
+    }
+}
diff --git a/third_party/rust/tokio/src/signal/unix.rs b/third_party/rust/tokio/src/signal/unix.rs
new file mode 100644
index 0000000000..06f5cf4eba
--- /dev/null
+++ b/third_party/rust/tokio/src/signal/unix.rs
@@ -0,0 +1,513 @@
+//! Unix-specific types for signal handling.
+//!
+//! This module is only defined on Unix platforms and contains the primary
+//! `Signal` type for receiving notifications of signals.
+
+#![cfg(unix)]
+
+use crate::io::{AsyncRead, PollEvented};
+use crate::signal::registry::{globals, EventId, EventInfo, Globals, Init, Storage};
+use crate::sync::mpsc::{channel, Receiver};
+
+use libc::c_int;
+use mio_uds::UnixStream;
+use std::io::{self, Error, ErrorKind, Write};
+use std::pin::Pin;
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Once;
+use std::task::{Context, Poll};
+
+pub(crate) type OsStorage = Vec<SignalInfo>;
+
+// Number of different unix signals
+// (FreeBSD has 33)
+const SIGNUM: usize = 33;
+
+impl Init for OsStorage {
+    fn init() -> Self {
+        (0..SIGNUM).map(|_| SignalInfo::default()).collect()
+    }
+}
+
+impl Storage for OsStorage {
+    fn event_info(&self, id: EventId) -> Option<&EventInfo> {
+        self.get(id).map(|si| &si.event_info)
+    }
+
+    fn for_each<'a, F>(&'a self, f: F)
+    where
+        F: FnMut(&'a EventInfo),
+    {
+        self.iter().map(|si| &si.event_info).for_each(f)
+    }
+}
+
+#[derive(Debug)]
+pub(crate) struct OsExtraData {
+    sender: UnixStream,
+    receiver: UnixStream,
+}
+
+impl Init for OsExtraData {
+    fn init() -> Self {
+        let (receiver, sender) = UnixStream::pair().expect("failed to create UnixStream");
+
+        Self { sender, receiver }
+    }
+}
+
+/// Represents the specific kind of signal to listen for.
+#[derive(Debug, Clone, Copy)]
+pub struct SignalKind(c_int);
+
+impl SignalKind {
+    /// Allows for listening to any valid OS signal.
+    ///
+    /// For example, this can be used for listening for platform-specific
+    /// signals.
+    /// ```rust,no_run
+    /// # use tokio::signal::unix::SignalKind;
+    /// # let signum = -1;
+    /// // let signum = libc::OS_SPECIFIC_SIGNAL;
+    /// let kind = SignalKind::from_raw(signum);
+    /// ```
+    pub fn from_raw(signum: c_int) -> Self {
+        Self(signum)
+    }
+
+    /// Represents the SIGALRM signal.
+    ///
+    /// On Unix systems this signal is sent when a real-time timer has expired.
+    /// By default, the process is terminated by this signal.
+    pub fn alarm() -> Self {
+        Self(libc::SIGALRM)
+    }
+
+    /// Represents the SIGCHLD signal.
+    ///
+    /// On Unix systems this signal is sent when the status of a child process
+    /// has changed. By default, this signal is ignored.
+    pub fn child() -> Self {
+        Self(libc::SIGCHLD)
+    }
+
+    /// Represents the SIGHUP signal.
+    ///
+    /// On Unix systems this signal is sent when the terminal is disconnected.
+    /// By default, the process is terminated by this signal.
+    pub fn hangup() -> Self {
+        Self(libc::SIGHUP)
+    }
+
+    /// Represents the SIGINFO signal.
+    ///
+    /// On Unix systems this signal is sent to request a status update from the
+    /// process. By default, this signal is ignored.
+    #[cfg(any(
+        target_os = "dragonfly",
+        target_os = "freebsd",
+        target_os = "macos",
+        target_os = "netbsd",
+        target_os = "openbsd"
+    ))]
+    pub fn info() -> Self {
+        Self(libc::SIGINFO)
+    }
+
+    /// Represents the SIGINT signal.
+    ///
+    /// On Unix systems this signal is sent to interrupt a program.
+    /// By default, the process is terminated by this signal.
+    pub fn interrupt() -> Self {
+        Self(libc::SIGINT)
+    }
+
+    /// Represents the SIGIO signal.
+    ///
+    /// On Unix systems this signal is sent when I/O operations are possible
+    /// on some file descriptor. By default, this signal is ignored.
+    pub fn io() -> Self {
+        Self(libc::SIGIO)
+    }
+
+    /// Represents the SIGPIPE signal.
+    ///
+    /// On Unix systems this signal is sent when the process attempts to write
+    /// to a pipe which has no reader. By default, the process is terminated by
+    /// this signal.
+    pub fn pipe() -> Self {
+        Self(libc::SIGPIPE)
+    }
+
+    /// Represents the SIGQUIT signal.
+    ///
+    /// On Unix systems this signal is sent to issue a shutdown of the
+    /// process, after which the OS will dump the process core.
+    /// By default, the process is terminated by this signal.
+    pub fn quit() -> Self {
+        Self(libc::SIGQUIT)
+    }
+
+    /// Represents the SIGTERM signal.
+    ///
+    /// On Unix systems this signal is sent to issue a shutdown of the
+    /// process. By default, the process is terminated by this signal.
+    pub fn terminate() -> Self {
+        Self(libc::SIGTERM)
+    }
+
+    /// Represents the SIGUSR1 signal.
+    ///
+    /// On Unix systems this is a user defined signal.
+    /// By default, the process is terminated by this signal.
+    pub fn user_defined1() -> Self {
+        Self(libc::SIGUSR1)
+    }
+
+    /// Represents the SIGUSR2 signal.
+    ///
+    /// On Unix systems this is a user defined signal.
+    /// By default, the process is terminated by this signal.
+    pub fn user_defined2() -> Self {
+        Self(libc::SIGUSR2)
+    }
+
+    /// Represents the SIGWINCH signal.
+    ///
+    /// On Unix systems this signal is sent when the terminal window is resized.
+    /// By default, this signal is ignored.
+    pub fn window_change() -> Self {
+        Self(libc::SIGWINCH)
+    }
+}
+
+pub(crate) struct SignalInfo {
+    event_info: EventInfo,
+    init: Once,
+    initialized: AtomicBool,
+}
+
+impl Default for SignalInfo {
+    fn default() -> SignalInfo {
+        SignalInfo {
+            event_info: Default::default(),
+            init: Once::new(),
+            initialized: AtomicBool::new(false),
+        }
+    }
+}
+
+/// Our global signal handler for all signals registered by this module.
+///
+/// The purpose of this signal handler is to primarily:
+///
+/// 1. Flag that our specific signal was received (e.g. store an atomic flag)
+/// 2. Wake up driver tasks by writing a byte to a pipe
+///
+/// Those two operations shoudl both be async-signal safe.
+fn action(globals: Pin<&'static Globals>, signal: c_int) {
+    globals.record_event(signal as EventId);
+
+    // Send a wakeup, ignore any errors (anything reasonably possible is
+    // full pipe and then it will wake up anyway).
+    let mut sender = &globals.sender;
+    drop(sender.write(&[1]));
+}
+
+/// Enables this module to receive signal notifications for the `signal`
+/// provided.
+///
+/// This will register the signal handler if it hasn't already been registered,
+/// returning any error along the way if that fails.
+fn signal_enable(signal: c_int) -> io::Result<()> {
+    if signal < 0 || signal_hook_registry::FORBIDDEN.contains(&signal) {
+        return Err(Error::new(
+            ErrorKind::Other,
+            format!("Refusing to register signal {}", signal),
+        ));
+    }
+
+    let globals = globals();
+    let siginfo = match globals.storage().get(signal as EventId) {
+        Some(slot) => slot,
+        None => return Err(io::Error::new(io::ErrorKind::Other, "signal too large")),
+    };
+    let mut registered = Ok(());
+    siginfo.init.call_once(|| {
+        registered = unsafe {
+            signal_hook_registry::register(signal, move || action(globals, signal)).map(|_| ())
+        };
+        if registered.is_ok() {
+            siginfo.initialized.store(true, Ordering::Relaxed);
+        }
+    });
+    registered?;
+    // If the call_once failed, it won't be retried on the next attempt to register the signal. In
+    // such case it is not run, registered is still `Ok(())`, initialized is still `false`.
+    if siginfo.initialized.load(Ordering::Relaxed) {
+        Ok(())
+    } else {
+        Err(Error::new(
+            ErrorKind::Other,
+            "Failed to register signal handler",
+        ))
+    }
+}
+
+#[derive(Debug)]
+struct Driver {
+    wakeup: PollEvented<UnixStream>,
+}
+
+impl Driver {
+    fn poll(&mut self, cx: &mut Context<'_>) -> Poll<()> {
+        // Drain the data from the pipe and maintain interest in getting more
+        self.drain(cx);
+        // Broadcast any signals which were received
+        globals().broadcast();
+
+        Poll::Pending
+    }
+}
+
+impl Driver {
+    fn new() -> io::Result<Driver> {
+        // NB: We give each driver a "fresh" reciever file descriptor to avoid
+        // the issues described in alexcrichton/tokio-process#42.
+        //
+        // In the past we would reuse the actual receiver file descriptor and
+        // swallow any errors around double registration of the same descriptor.
+        // I'm not sure if the second (failed) registration simply doesn't end up
+        // receiving wake up notifications, or there could be some race condition
+        // when consuming readiness events, but having distinct descriptors for
+        // distinct PollEvented instances appears to mitigate this.
+        //
+        // Unfortunately we cannot just use a single global PollEvented instance
+        // either, since we can't compare Handles or assume they will always
+        // point to the exact same reactor.
+        let stream = globals().receiver.try_clone()?;
+        let wakeup = PollEvented::new(stream)?;
+
+        Ok(Driver { wakeup })
+    }
+
+    /// Drain all data in the global receiver, ensuring we'll get woken up when
+    /// there is a write on the other end.
+    ///
+    /// We do *NOT* use the existence of any read bytes as evidence a signal was
+    /// received since the `pending` flags would have already been set if that
+    /// was the case. See
+    /// [#38](https://github.com/alexcrichton/tokio-signal/issues/38) for more
+    /// info.
+    fn drain(&mut self, cx: &mut Context<'_>) {
+        loop {
+            match Pin::new(&mut self.wakeup).poll_read(cx, &mut [0; 128]) {
+                Poll::Ready(Ok(0)) => panic!("EOF on self-pipe"),
+                Poll::Ready(Ok(_)) => {}
+                Poll::Ready(Err(e)) => panic!("Bad read on self-pipe: {}", e),
+                Poll::Pending => break,
+            }
+        }
+    }
+}
+
+/// A stream of events for receiving a particular type of OS signal.
+///
+/// In general signal handling on Unix is a pretty tricky topic, and this
+/// structure is no exception! There are some important limitations to keep in
+/// mind when using `Signal` streams:
+///
+/// * Signals handling in Unix already necessitates coalescing signals
+///   together sometimes. This `Signal` stream is also no exception here in
+///   that it will also coalesce signals. That is, even if the signal handler
+///   for this process runs multiple times, the `Signal` stream may only return
+///   one signal notification. Specifically, before `poll` is called, all
+///   signal notifications are coalesced into one item returned from `poll`.
+///   Once `poll` has been called, however, a further signal is guaranteed to
+///   be yielded as an item.
+///
+///   Put another way, any element pulled off the returned stream corresponds to
+///   *at least one* signal, but possibly more.
+///
+/// * Signal handling in general is relatively inefficient. Although some
+///   improvements are possible in this crate, it's recommended to not plan on
+///   having millions of signal channels open.
+///
+/// If you've got any questions about this feel free to open an issue on the
+/// repo! New approaches to alleviate some of these limitations are always
+/// appreciated!
+///
+/// # Caveats
+///
+/// The first time that a `Signal` instance is registered for a particular
+/// signal kind, an OS signal-handler is installed which replaces the default
+/// platform behavior when that signal is received, **for the duration of the
+/// entire process**.
+///
+/// For example, Unix systems will terminate a process by default when it
+/// receives SIGINT. But, when a `Signal` instance is created to listen for
+/// this signal, the next SIGINT that arrives will be translated to a stream
+/// event, and the process will continue to execute. **Even if this `Signal`
+/// instance is dropped, subsequent SIGINT deliveries will end up captured by
+/// Tokio, and the default platform behavior will NOT be reset**.
+///
+/// Thus, applications should take care to ensure the expected signal behavior
+/// occurs as expected after listening for specific signals.
+///
+/// # Examples
+///
+/// Wait for SIGHUP
+///
+/// ```rust,no_run
+/// use tokio::signal::unix::{signal, SignalKind};
+///
+/// #[tokio::main]
+/// async fn main() -> Result<(), Box<dyn std::error::Error>> {
+///     // An infinite stream of hangup signals.
+///     let mut stream = signal(SignalKind::hangup())?;
+///
+///     // Print whenever a HUP signal is received
+///     loop {
+///         stream.recv().await;
+///         println!("got signal HUP");
+///     }
+/// }
+/// ```
+#[must_use = "streams do nothing unless polled"]
+#[derive(Debug)]
+pub struct Signal {
+    driver: Driver,
+    rx: Receiver<()>,
+}
+
+/// Creates a new stream which will receive notifications when the current
+/// process receives the specified signal `kind`.
+///
+/// This function will create a new stream which binds to the default reactor.
+/// The `Signal` stream is an infinite stream which will receive
+/// notifications whenever a signal is received. More documentation can be
+/// found on `Signal` itself, but to reiterate:
+///
+/// * Signals may be coalesced beyond what the kernel already does.
+/// * Once a signal handler is registered with the process the underlying
+///   libc signal handler is never unregistered.
+///
+/// A `Signal` stream can be created for a particular signal number
+/// multiple times. When a signal is received then all the associated
+/// channels will receive the signal notification.
+///
+/// # Errors
+///
+/// * If the lower-level C functions fail for some reason.
+/// * If the previous initialization of this specific signal failed.
+/// * If the signal is one of
+///   [`signal_hook::FORBIDDEN`](https://docs.rs/signal-hook/*/signal_hook/fn.register.html#panics)
+pub fn signal(kind: SignalKind) -> io::Result<Signal> {
+    let signal = kind.0;
+
+    // Turn the signal delivery on once we are ready for it
+    signal_enable(signal)?;
+
+    // Ensure there's a driver for our associated event loop processing
+    // signals.
+    let driver = Driver::new()?;
+
+    // One wakeup in a queue is enough, no need for us to buffer up any
+    // more.
+    let (tx, rx) = channel(1);
+    globals().register_listener(signal as EventId, tx);
+
+    Ok(Signal { driver, rx })
+}
+
+impl Signal {
+    /// Receives the next signal notification event.
+    ///
+    /// `None` is returned if no more events can be received by this stream.
+    ///
+    /// # Examples
+    ///
+    /// Wait for SIGHUP
+    ///
+    /// ```rust,no_run
+    /// use tokio::signal::unix::{signal, SignalKind};
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    ///     // An infinite stream of hangup signals.
+    ///     let mut stream = signal(SignalKind::hangup())?;
+    ///
+    ///     // Print whenever a HUP signal is received
+    ///     loop {
+    ///         stream.recv().await;
+    ///         println!("got signal HUP");
+    ///     }
+    /// }
+    /// ```
+    pub async fn recv(&mut self) -> Option<()> {
+        use crate::future::poll_fn;
+        poll_fn(|cx| self.poll_recv(cx)).await
+    }
+
+    /// Polls to receive the next signal notification event, outside of an
+    /// `async` context.
+    ///
+    /// `None` is returned if no more events can be received by this stream.
+    ///
+    /// # Examples
+    ///
+    /// Polling from a manually implemented future
+    ///
+    /// ```rust,no_run
+    /// use std::pin::Pin;
+    /// use std::future::Future;
+    /// use std::task::{Context, Poll};
+    /// use tokio::signal::unix::Signal;
+    ///
+    /// struct MyFuture {
+    ///     signal: Signal,
+    /// }
+    ///
+    /// impl Future for MyFuture {
+    ///     type Output = Option<()>;
+    ///
+    ///     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+    ///         println!("polling MyFuture");
+    ///         self.signal.poll_recv(cx)
+    ///     }
+    /// }
+    /// ```
+    pub fn poll_recv(&mut self, cx: &mut Context<'_>) -> Poll<Option<()>> {
+        let _ = self.driver.poll(cx);
+        self.rx.poll_recv(cx)
+    }
+}
+
+cfg_stream! {
+    impl crate::stream::Stream for Signal {
+        type Item = ();
+
+        fn poll_next(mut self: std::pin::Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<()>> {
+            self.poll_recv(cx)
+        }
+    }
+}
+
+pub(crate) fn ctrl_c() -> io::Result<Signal> {
+    signal(SignalKind::interrupt())
+}
+
+#[cfg(all(test, not(loom)))]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn signal_enable_error_on_invalid_input() {
+        signal_enable(-1).unwrap_err();
+    }
+
+    #[test]
+    fn signal_enable_error_on_forbidden_input() {
+        signal_enable(signal_hook_registry::FORBIDDEN[0]).unwrap_err();
+    }
+}
diff --git a/third_party/rust/tokio/src/signal/windows.rs b/third_party/rust/tokio/src/signal/windows.rs
new file mode 100644
index 0000000000..f55e504b00
--- /dev/null
+++ b/third_party/rust/tokio/src/signal/windows.rs
@@ -0,0 +1,297 @@
+//! Windows-specific types for signal handling.
+//!
+//! This module is only defined on Windows and contains the primary `Event` type
+//! for receiving notifications of events. These events are listened for via the
+//! `SetConsoleCtrlHandler` function which receives events of the type
+//! `CTRL_C_EVENT` and `CTRL_BREAK_EVENT`
+
+#![cfg(windows)]
+
+use crate::signal::registry::{globals, EventId, EventInfo, Init, Storage};
+use crate::sync::mpsc::{channel, Receiver};
+
+use std::convert::TryFrom;
+use std::io;
+use std::sync::Once;
+use std::task::{Context, Poll};
+use winapi::shared::minwindef::*;
+use winapi::um::consoleapi::SetConsoleCtrlHandler;
+use winapi::um::wincon::*;
+
+#[derive(Debug)]
+pub(crate) struct OsStorage {
+    ctrl_c: EventInfo,
+    ctrl_break: EventInfo,
+}
+
+impl Init for OsStorage {
+    fn init() -> Self {
+        Self {
+            ctrl_c: EventInfo::default(),
+            ctrl_break: EventInfo::default(),
+        }
+    }
+}
+
+impl Storage for OsStorage {
+    fn event_info(&self, id: EventId) -> Option<&EventInfo> {
+        match DWORD::try_from(id) {
+            Ok(CTRL_C_EVENT) => Some(&self.ctrl_c),
+            Ok(CTRL_BREAK_EVENT) => Some(&self.ctrl_break),
+            _ => None,
+        }
+    }
+
+    fn for_each<'a, F>(&'a self, mut f: F)
+    where
+        F: FnMut(&'a EventInfo),
+    {
+        f(&self.ctrl_c);
+        f(&self.ctrl_break);
+    }
+}
+
+#[derive(Debug)]
+pub(crate) struct OsExtraData {}
+
+impl Init for OsExtraData {
+    fn init() -> Self {
+        Self {}
+    }
+}
+
+/// Stream of events discovered via `SetConsoleCtrlHandler`.
+///
+/// This structure can be used to listen for events of the type `CTRL_C_EVENT`
+/// and `CTRL_BREAK_EVENT`. The `Stream` trait is implemented for this struct
+/// and will resolve for each notification received by the process. Note that
+/// there are few limitations with this as well:
+///
+/// * A notification to this process notifies *all* `Event` streams for that
+///   event type.
+/// * Notifications to an `Event` stream **are coalesced** if they aren't
+///   processed quickly enough. This means that if two notifications are
+///   received back-to-back, then the stream may only receive one item about the
+///   two notifications.
+#[must_use = "streams do nothing unless polled"]
+#[derive(Debug)]
+pub(crate) struct Event {
+    rx: Receiver<()>,
+}
+
+pub(crate) fn ctrl_c() -> io::Result<Event> {
+    Event::new(CTRL_C_EVENT)
+}
+
+impl Event {
+    fn new(signum: DWORD) -> io::Result<Self> {
+        global_init()?;
+
+        let (tx, rx) = channel(1);
+        globals().register_listener(signum as EventId, tx);
+
+        Ok(Event { rx })
+    }
+
+    pub(crate) async fn recv(&mut self) -> Option<()> {
+        use crate::future::poll_fn;
+        poll_fn(|cx| self.rx.poll_recv(cx)).await
+    }
+}
+
+fn global_init() -> io::Result<()> {
+    static INIT: Once = Once::new();
+
+    let mut init = None;
+
+    INIT.call_once(|| unsafe {
+        let rc = SetConsoleCtrlHandler(Some(handler), TRUE);
+        let ret = if rc == 0 {
+            Err(io::Error::last_os_error())
+        } else {
+            Ok(())
+        };
+
+        init = Some(ret);
+    });
+
+    init.unwrap_or_else(|| Ok(()))
+}
+
+unsafe extern "system" fn handler(ty: DWORD) -> BOOL {
+    let globals = globals();
+    globals.record_event(ty as EventId);
+
+    // According to https://docs.microsoft.com/en-us/windows/console/handlerroutine
+    // the handler routine is always invoked in a new thread, thus we don't
+    // have the same restrictions as in Unix signal handlers, meaning we can
+    // go ahead and perform the broadcast here.
+    if globals.broadcast() {
+        TRUE
+    } else {
+        // No one is listening for this notification any more
+        // let the OS fire the next (possibly the default) handler.
+        FALSE
+    }
+}
+
+/// Represents a stream which receives "ctrl-break" notifications sent to the process
+/// via `SetConsoleCtrlHandler`.
+///
+/// A notification to this process notifies *all* streams listening for
+/// this event. Moreover, the notifications **are coalesced** if they aren't processed
+/// quickly enough. This means that if two notifications are received back-to-back,
+/// then the stream may only receive one item about the two notifications.
+#[must_use = "streams do nothing unless polled"]
+#[derive(Debug)]
+pub struct CtrlBreak {
+    inner: Event,
+}
+
+impl CtrlBreak {
+    /// Receives the next signal notification event.
+    ///
+    /// `None` is returned if no more events can be received by this stream.
+    ///
+    /// # Examples
+    ///
+    /// ```rust,no_run
+    /// use tokio::signal::windows::ctrl_break;
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    ///     // An infinite stream of CTRL-BREAK events.
+    ///     let mut stream = ctrl_break()?;
+    ///
+    ///     // Print whenever a CTRL-BREAK event is received
+    ///     loop {
+    ///         stream.recv().await;
+    ///         println!("got signal CTRL-BREAK");
+    ///     }
+    /// }
+    /// ```
+    pub async fn recv(&mut self) -> Option<()> {
+        use crate::future::poll_fn;
+        poll_fn(|cx| self.poll_recv(cx)).await
+    }
+
+    /// Polls to receive the next signal notification event, outside of an
+    /// `async` context.
+    ///
+    /// `None` is returned if no more events can be received by this stream.
+    ///
+    /// # Examples
+    ///
+    /// Polling from a manually implemented future
+    ///
+    /// ```rust,no_run
+    /// use std::pin::Pin;
+    /// use std::future::Future;
+    /// use std::task::{Context, Poll};
+    /// use tokio::signal::windows::CtrlBreak;
+    ///
+    /// struct MyFuture {
+    ///     ctrl_break: CtrlBreak,
+    /// }
+    ///
+    /// impl Future for MyFuture {
+    ///     type Output = Option<()>;
+    ///
+    ///     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+    ///         println!("polling MyFuture");
+    ///         self.ctrl_break.poll_recv(cx)
+    ///     }
+    /// }
+    /// ```
+    pub fn poll_recv(&mut self, cx: &mut Context<'_>) -> Poll<Option<()>> {
+        self.inner.rx.poll_recv(cx)
+    }
+}
+
+cfg_stream! {
+    impl crate::stream::Stream for CtrlBreak {
+        type Item = ();
+
+        fn poll_next(mut self: std::pin::Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<()>> {
+            self.poll_recv(cx)
+        }
+    }
+}
+
+/// Creates a new stream which receives "ctrl-break" notifications sent to the
+/// process.
+///
+/// # Examples
+///
+/// ```rust,no_run
+/// use tokio::signal::windows::ctrl_break;
+///
+/// #[tokio::main]
+/// async fn main() -> Result<(), Box<dyn std::error::Error>> {
+///     // An infinite stream of CTRL-BREAK events.
+///     let mut stream = ctrl_break()?;
+///
+///     // Print whenever a CTRL-BREAK event is received
+///     loop {
+///         stream.recv().await;
+///         println!("got signal CTRL-BREAK");
+///     }
+/// }
+/// ```
+pub fn ctrl_break() -> io::Result<CtrlBreak> {
+    Event::new(CTRL_BREAK_EVENT).map(|inner| CtrlBreak { inner })
+}
+
+#[cfg(all(test, not(loom)))]
+mod tests {
+    use super::*;
+    use crate::runtime::Runtime;
+    use crate::stream::StreamExt;
+
+    use tokio_test::{assert_ok, assert_pending, assert_ready_ok, task};
+
+    #[test]
+    fn ctrl_c() {
+        let rt = rt();
+
+        rt.enter(|| {
+            let mut ctrl_c = task::spawn(crate::signal::ctrl_c());
+
+            assert_pending!(ctrl_c.poll());
+
+            // Windows doesn't have a good programmatic way of sending events
+            // like sending signals on Unix, so we'll stub out the actual OS
+            // integration and test that our handling works.
+            unsafe {
+                super::handler(CTRL_C_EVENT);
+            }
+
+            assert_ready_ok!(ctrl_c.poll());
+        });
+    }
+
+    #[test]
+    fn ctrl_break() {
+        let mut rt = rt();
+
+        rt.block_on(async {
+            let mut ctrl_break = assert_ok!(super::ctrl_break());
+
+            // Windows doesn't have a good programmatic way of sending events
+            // like sending signals on Unix, so we'll stub out the actual OS
+            // integration and test that our handling works.
+            unsafe {
+                super::handler(CTRL_BREAK_EVENT);
+            }
+
+            ctrl_break.next().await.unwrap();
+        });
+    }
+
+    fn rt() -> Runtime {
+        crate::runtime::Builder::new()
+            .basic_scheduler()
+            .build()
+            .unwrap()
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/all.rs b/third_party/rust/tokio/src/stream/all.rs
new file mode 100644
index 0000000000..615665d270
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/all.rs
@@ -0,0 +1,45 @@
+use crate::stream::Stream;
+
+use core::future::Future;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+/// Future for the [`all`](super::StreamExt::all) method.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct AllFuture<'a, St: ?Sized, F> {
+    stream: &'a mut St,
+    f: F,
+}
+
+impl<'a, St: ?Sized, F> AllFuture<'a, St, F> {
+    pub(super) fn new(stream: &'a mut St, f: F) -> Self {
+        Self { stream, f }
+    }
+}
+
+impl<St: ?Sized + Unpin, F> Unpin for AllFuture<'_, St, F> {}
+
+impl<St, F> Future for AllFuture<'_, St, F>
+where
+    St: ?Sized + Stream + Unpin,
+    F: FnMut(St::Item) -> bool,
+{
+    type Output = bool;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let next = futures_core::ready!(Pin::new(&mut self.stream).poll_next(cx));
+
+        match next {
+            Some(v) => {
+                if !(&mut self.f)(v) {
+                    Poll::Ready(false)
+                } else {
+                    cx.waker().wake_by_ref();
+                    Poll::Pending
+                }
+            }
+            None => Poll::Ready(true),
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/any.rs b/third_party/rust/tokio/src/stream/any.rs
new file mode 100644
index 0000000000..f2ecad5edb
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/any.rs
@@ -0,0 +1,45 @@
+use crate::stream::Stream;
+
+use core::future::Future;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+/// Future for the [`any`](super::StreamExt::any) method.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct AnyFuture<'a, St: ?Sized, F> {
+    stream: &'a mut St,
+    f: F,
+}
+
+impl<'a, St: ?Sized, F> AnyFuture<'a, St, F> {
+    pub(super) fn new(stream: &'a mut St, f: F) -> Self {
+        Self { stream, f }
+    }
+}
+
+impl<St: ?Sized + Unpin, F> Unpin for AnyFuture<'_, St, F> {}
+
+impl<St, F> Future for AnyFuture<'_, St, F>
+where
+    St: ?Sized + Stream + Unpin,
+    F: FnMut(St::Item) -> bool,
+{
+    type Output = bool;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let next = futures_core::ready!(Pin::new(&mut self.stream).poll_next(cx));
+
+        match next {
+            Some(v) => {
+                if (&mut self.f)(v) {
+                    Poll::Ready(true)
+                } else {
+                    cx.waker().wake_by_ref();
+                    Poll::Pending
+                }
+            }
+            None => Poll::Ready(false),
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/chain.rs b/third_party/rust/tokio/src/stream/chain.rs
new file mode 100644
index 0000000000..5f0324a4b5
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/chain.rs
@@ -0,0 +1,57 @@
+use crate::stream::{Fuse, Stream};
+
+use core::pin::Pin;
+use core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Stream returned by the [`chain`](super::StreamExt::chain) method.
+    pub struct Chain<T, U> {
+        #[pin]
+        a: Fuse<T>,
+        #[pin]
+        b: U,
+    }
+}
+
+impl<T, U> Chain<T, U> {
+    pub(super) fn new(a: T, b: U) -> Chain<T, U>
+    where
+        T: Stream,
+        U: Stream,
+    {
+        Chain { a: Fuse::new(a), b }
+    }
+}
+
+impl<T, U> Stream for Chain<T, U>
+where
+    T: Stream,
+    U: Stream<Item = T::Item>,
+{
+    type Item = T::Item;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T::Item>> {
+        use Poll::Ready;
+
+        let me = self.project();
+
+        if let Some(v) = ready!(me.a.poll_next(cx)) {
+            return Ready(Some(v));
+        }
+
+        me.b.poll_next(cx)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (a_lower, a_upper) = self.a.size_hint();
+        let (b_lower, b_upper) = self.b.size_hint();
+
+        let upper = match (a_upper, b_upper) {
+            (Some(a_upper), Some(b_upper)) => Some(a_upper + b_upper),
+            _ => None,
+        };
+
+        (a_lower + b_lower, upper)
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/collect.rs b/third_party/rust/tokio/src/stream/collect.rs
new file mode 100644
index 0000000000..f44c72b7b3
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/collect.rs
@@ -0,0 +1,246 @@
+use crate::stream::Stream;
+
+use bytes::{Buf, BufMut, Bytes, BytesMut};
+use core::future::Future;
+use core::mem;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+
+// Do not export this struct until `FromStream` can be unsealed.
+pin_project! {
+    /// Future returned by the [`collect`](super::StreamExt::collect) method.
+    #[must_use = "streams do nothing unless polled"]
+    #[derive(Debug)]
+    pub struct Collect<T, U>
+    where
+        T: Stream,
+        U: FromStream<T::Item>,
+    {
+        #[pin]
+        stream: T,
+        collection: U::Collection,
+    }
+}
+
+/// Convert from a [`Stream`](crate::stream::Stream).
+///
+/// This trait is not intended to be used directly. Instead, call
+/// [`StreamExt::collect()`](super::StreamExt::collect).
+///
+/// # Implementing
+///
+/// Currently, this trait may not be implemented by third parties. The trait is
+/// sealed in order to make changes in the future. Stabilization is pending
+/// enhancements to the Rust langague.
+pub trait FromStream<T>: sealed::FromStreamPriv<T> {}
+
+impl<T, U> Collect<T, U>
+where
+    T: Stream,
+    U: FromStream<T::Item>,
+{
+    pub(super) fn new(stream: T) -> Collect<T, U> {
+        let (lower, upper) = stream.size_hint();
+        let collection = U::initialize(lower, upper);
+
+        Collect { stream, collection }
+    }
+}
+
+impl<T, U> Future for Collect<T, U>
+where
+    T: Stream,
+    U: FromStream<T::Item>,
+{
+    type Output = U;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<U> {
+        use Poll::Ready;
+
+        loop {
+            let mut me = self.as_mut().project();
+
+            let item = match ready!(me.stream.poll_next(cx)) {
+                Some(item) => item,
+                None => {
+                    return Ready(U::finalize(&mut me.collection));
+                }
+            };
+
+            if !U::extend(&mut me.collection, item) {
+                return Ready(U::finalize(&mut me.collection));
+            }
+        }
+    }
+}
+
+// ===== FromStream implementations
+
+impl FromStream<()> for () {}
+
+impl sealed::FromStreamPriv<()> for () {
+    type Collection = ();
+
+    fn initialize(_lower: usize, _upper: Option<usize>) {}
+
+    fn extend(_collection: &mut (), _item: ()) -> bool {
+        true
+    }
+
+    fn finalize(_collection: &mut ()) {}
+}
+
+impl<T: AsRef<str>> FromStream<T> for String {}
+
+impl<T: AsRef<str>> sealed::FromStreamPriv<T> for String {
+    type Collection = String;
+
+    fn initialize(_lower: usize, _upper: Option<usize>) -> String {
+        String::new()
+    }
+
+    fn extend(collection: &mut String, item: T) -> bool {
+        collection.push_str(item.as_ref());
+        true
+    }
+
+    fn finalize(collection: &mut String) -> String {
+        mem::replace(collection, String::new())
+    }
+}
+
+impl<T> FromStream<T> for Vec<T> {}
+
+impl<T> sealed::FromStreamPriv<T> for Vec<T> {
+    type Collection = Vec<T>;
+
+    fn initialize(lower: usize, _upper: Option<usize>) -> Vec<T> {
+        Vec::with_capacity(lower)
+    }
+
+    fn extend(collection: &mut Vec<T>, item: T) -> bool {
+        collection.push(item);
+        true
+    }
+
+    fn finalize(collection: &mut Vec<T>) -> Vec<T> {
+        mem::replace(collection, vec![])
+    }
+}
+
+impl<T> FromStream<T> for Box<[T]> {}
+
+impl<T> sealed::FromStreamPriv<T> for Box<[T]> {
+    type Collection = Vec<T>;
+
+    fn initialize(lower: usize, upper: Option<usize>) -> Vec<T> {
+        <Vec<T> as sealed::FromStreamPriv<T>>::initialize(lower, upper)
+    }
+
+    fn extend(collection: &mut Vec<T>, item: T) -> bool {
+        <Vec<T> as sealed::FromStreamPriv<T>>::extend(collection, item)
+    }
+
+    fn finalize(collection: &mut Vec<T>) -> Box<[T]> {
+        <Vec<T> as sealed::FromStreamPriv<T>>::finalize(collection).into_boxed_slice()
+    }
+}
+
+impl<T, U, E> FromStream<Result<T, E>> for Result<U, E> where U: FromStream<T> {}
+
+impl<T, U, E> sealed::FromStreamPriv<Result<T, E>> for Result<U, E>
+where
+    U: FromStream<T>,
+{
+    type Collection = Result<U::Collection, E>;
+
+    fn initialize(lower: usize, upper: Option<usize>) -> Result<U::Collection, E> {
+        Ok(U::initialize(lower, upper))
+    }
+
+    fn extend(collection: &mut Self::Collection, item: Result<T, E>) -> bool {
+        assert!(collection.is_ok());
+        match item {
+            Ok(item) => {
+                let collection = collection.as_mut().ok().expect("invalid state");
+                U::extend(collection, item)
+            }
+            Err(err) => {
+                *collection = Err(err);
+                false
+            }
+        }
+    }
+
+    fn finalize(collection: &mut Self::Collection) -> Result<U, E> {
+        if let Ok(collection) = collection.as_mut() {
+            Ok(U::finalize(collection))
+        } else {
+            let res = mem::replace(collection, Ok(U::initialize(0, Some(0))));
+
+            if let Err(err) = res {
+                Err(err)
+            } else {
+                unreachable!();
+            }
+        }
+    }
+}
+
+impl<T: Buf> FromStream<T> for Bytes {}
+
+impl<T: Buf> sealed::FromStreamPriv<T> for Bytes {
+    type Collection = BytesMut;
+
+    fn initialize(_lower: usize, _upper: Option<usize>) -> BytesMut {
+        BytesMut::new()
+    }
+
+    fn extend(collection: &mut BytesMut, item: T) -> bool {
+        collection.put(item);
+        true
+    }
+
+    fn finalize(collection: &mut BytesMut) -> Bytes {
+        mem::replace(collection, BytesMut::new()).freeze()
+    }
+}
+
+impl<T: Buf> FromStream<T> for BytesMut {}
+
+impl<T: Buf> sealed::FromStreamPriv<T> for BytesMut {
+    type Collection = BytesMut;
+
+    fn initialize(_lower: usize, _upper: Option<usize>) -> BytesMut {
+        BytesMut::new()
+    }
+
+    fn extend(collection: &mut BytesMut, item: T) -> bool {
+        collection.put(item);
+        true
+    }
+
+    fn finalize(collection: &mut BytesMut) -> BytesMut {
+        mem::replace(collection, BytesMut::new())
+    }
+}
+
+pub(crate) mod sealed {
+    #[doc(hidden)]
+    pub trait FromStreamPriv<T> {
+        /// Intermediate type used during collection process
+        type Collection;
+
+        /// Initialize the collection
+        fn initialize(lower: usize, upper: Option<usize>) -> Self::Collection;
+
+        /// Extend the collection with the received item
+        ///
+        /// Return `true` to continue streaming, `false` complete collection.
+        fn extend(collection: &mut Self::Collection, item: T) -> bool;
+
+        /// Finalize collection into target type.
+        fn finalize(collection: &mut Self::Collection) -> Self;
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/empty.rs b/third_party/rust/tokio/src/stream/empty.rs
new file mode 100644
index 0000000000..6118673e50
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/empty.rs
@@ -0,0 +1,50 @@
+use crate::stream::Stream;
+
+use core::marker::PhantomData;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+/// Stream for the [`empty`] function.
+#[derive(Debug)]
+#[must_use = "streams do nothing unless polled"]
+pub struct Empty<T>(PhantomData<T>);
+
+impl<T> Unpin for Empty<T> {}
+unsafe impl<T> Send for Empty<T> {}
+unsafe impl<T> Sync for Empty<T> {}
+
+/// Creates a stream that yields nothing.
+///
+/// The returned stream is immediately ready and returns `None`. Use
+/// [`stream::pending()`](super::pending()) to obtain a stream that is never
+/// ready.
+///
+/// # Examples
+///
+/// Basic usage:
+///
+/// ```
+/// use tokio::stream::{self, StreamExt};
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let mut none = stream::empty::<i32>();
+///
+///     assert_eq!(None, none.next().await);
+/// }
+/// ```
+pub const fn empty<T>() -> Empty<T> {
+    Empty(PhantomData)
+}
+
+impl<T> Stream for Empty<T> {
+    type Item = T;
+
+    fn poll_next(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Option<T>> {
+        Poll::Ready(None)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, Some(0))
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/filter.rs b/third_party/rust/tokio/src/stream/filter.rs
new file mode 100644
index 0000000000..799630b234
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/filter.rs
@@ -0,0 +1,58 @@
+use crate::stream::Stream;
+
+use core::fmt;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Stream returned by the [`filter`](super::StreamExt::filter) method.
+    #[must_use = "streams do nothing unless polled"]
+    pub struct Filter<St, F> {
+        #[pin]
+        stream: St,
+        f: F,
+    }
+}
+
+impl<St, F> fmt::Debug for Filter<St, F>
+where
+    St: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Filter")
+            .field("stream", &self.stream)
+            .finish()
+    }
+}
+
+impl<St, F> Filter<St, F> {
+    pub(super) fn new(stream: St, f: F) -> Self {
+        Self { stream, f }
+    }
+}
+
+impl<St, F> Stream for Filter<St, F>
+where
+    St: Stream,
+    F: FnMut(&St::Item) -> bool,
+{
+    type Item = St::Item;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<St::Item>> {
+        loop {
+            match ready!(self.as_mut().project().stream.poll_next(cx)) {
+                Some(e) => {
+                    if (self.as_mut().project().f)(&e) {
+                        return Poll::Ready(Some(e));
+                    }
+                }
+                None => return Poll::Ready(None),
+            }
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, self.stream.size_hint().1) // can't know a lower bound, due to the predicate
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/filter_map.rs b/third_party/rust/tokio/src/stream/filter_map.rs
new file mode 100644
index 0000000000..8dc05a5460
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/filter_map.rs
@@ -0,0 +1,58 @@
+use crate::stream::Stream;
+
+use core::fmt;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Stream returned by the [`filter_map`](super::StreamExt::filter_map) method.
+    #[must_use = "streams do nothing unless polled"]
+    pub struct FilterMap<St, F> {
+        #[pin]
+        stream: St,
+        f: F,
+    }
+}
+
+impl<St, F> fmt::Debug for FilterMap<St, F>
+where
+    St: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("FilterMap")
+            .field("stream", &self.stream)
+            .finish()
+    }
+}
+
+impl<St, F> FilterMap<St, F> {
+    pub(super) fn new(stream: St, f: F) -> Self {
+        Self { stream, f }
+    }
+}
+
+impl<St, F, T> Stream for FilterMap<St, F>
+where
+    St: Stream,
+    F: FnMut(St::Item) -> Option<T>,
+{
+    type Item = T;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
+        loop {
+            match ready!(self.as_mut().project().stream.poll_next(cx)) {
+                Some(e) => {
+                    if let Some(e) = (self.as_mut().project().f)(e) {
+                        return Poll::Ready(Some(e));
+                    }
+                }
+                None => return Poll::Ready(None),
+            }
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, self.stream.size_hint().1) // can't know a lower bound, due to the predicate
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/fold.rs b/third_party/rust/tokio/src/stream/fold.rs
new file mode 100644
index 0000000000..7b9fead3db
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/fold.rs
@@ -0,0 +1,51 @@
+use crate::stream::Stream;
+
+use core::future::Future;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Future returned by the [`fold`](super::StreamExt::fold) method.
+    #[derive(Debug)]
+    pub struct FoldFuture<St, B, F> {
+        #[pin]
+        stream: St,
+        acc: Option<B>,
+        f: F,
+    }
+}
+
+impl<St, B, F> FoldFuture<St, B, F> {
+    pub(super) fn new(stream: St, init: B, f: F) -> Self {
+        Self {
+            stream,
+            acc: Some(init),
+            f,
+        }
+    }
+}
+
+impl<St, B, F> Future for FoldFuture<St, B, F>
+where
+    St: Stream,
+    F: FnMut(B, St::Item) -> B,
+{
+    type Output = B;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let mut me = self.project();
+        loop {
+            let next = ready!(me.stream.as_mut().poll_next(cx));
+
+            match next {
+                Some(v) => {
+                    let old = me.acc.take().unwrap();
+                    let new = (me.f)(old, v);
+                    *me.acc = Some(new);
+                }
+                None => return Poll::Ready(me.acc.take().unwrap()),
+            }
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/fuse.rs b/third_party/rust/tokio/src/stream/fuse.rs
new file mode 100644
index 0000000000..6c9e02d664
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/fuse.rs
@@ -0,0 +1,53 @@
+use crate::stream::Stream;
+
+use pin_project_lite::pin_project;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+pin_project! {
+    /// Stream returned by [`fuse()`][super::StreamExt::fuse].
+    #[derive(Debug)]
+    pub struct Fuse<T> {
+        #[pin]
+        stream: Option<T>,
+    }
+}
+
+impl<T> Fuse<T>
+where
+    T: Stream,
+{
+    pub(crate) fn new(stream: T) -> Fuse<T> {
+        Fuse {
+            stream: Some(stream),
+        }
+    }
+}
+
+impl<T> Stream for Fuse<T>
+where
+    T: Stream,
+{
+    type Item = T::Item;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T::Item>> {
+        let res = match Option::as_pin_mut(self.as_mut().project().stream) {
+            Some(stream) => ready!(stream.poll_next(cx)),
+            None => return Poll::Ready(None),
+        };
+
+        if res.is_none() {
+            // Do not poll the stream anymore
+            self.as_mut().project().stream.set(None);
+        }
+
+        Poll::Ready(res)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        match self.stream {
+            Some(ref stream) => stream.size_hint(),
+            None => (0, Some(0)),
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/iter.rs b/third_party/rust/tokio/src/stream/iter.rs
new file mode 100644
index 0000000000..36eeb5612f
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/iter.rs
@@ -0,0 +1,55 @@
+use crate::stream::Stream;
+
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+/// Stream for the [`iter`] function.
+#[derive(Debug)]
+#[must_use = "streams do nothing unless polled"]
+pub struct Iter<I> {
+    iter: I,
+}
+
+impl<I> Unpin for Iter<I> {}
+
+/// Converts an `Iterator` into a `Stream` which is always ready
+/// to yield the next value.
+///
+/// Iterators in Rust don't express the ability to block, so this adapter
+/// simply always calls `iter.next()` and returns that.
+///
+/// ```
+/// # async fn dox() {
+/// use tokio::stream::{self, StreamExt};
+///
+/// let mut stream = stream::iter(vec![17, 19]);
+///
+/// assert_eq!(stream.next().await, Some(17));
+/// assert_eq!(stream.next().await, Some(19));
+/// assert_eq!(stream.next().await, None);
+/// # }
+/// ```
+pub fn iter<I>(i: I) -> Iter<I::IntoIter>
+where
+    I: IntoIterator,
+{
+    Iter {
+        iter: i.into_iter(),
+    }
+}
+
+impl<I> Stream for Iter<I>
+where
+    I: Iterator,
+{
+    type Item = I::Item;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<I::Item>> {
+        ready!(crate::coop::poll_proceed(cx));
+        Poll::Ready(self.iter.next())
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/map.rs b/third_party/rust/tokio/src/stream/map.rs
new file mode 100644
index 0000000000..dfac5a2c94
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/map.rs
@@ -0,0 +1,51 @@
+use crate::stream::Stream;
+
+use core::fmt;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Stream for the [`map`](super::StreamExt::map) method.
+    #[must_use = "streams do nothing unless polled"]
+    pub struct Map<St, F> {
+        #[pin]
+        stream: St,
+        f: F,
+    }
+}
+
+impl<St, F> fmt::Debug for Map<St, F>
+where
+    St: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Map").field("stream", &self.stream).finish()
+    }
+}
+
+impl<St, F> Map<St, F> {
+    pub(super) fn new(stream: St, f: F) -> Self {
+        Map { stream, f }
+    }
+}
+
+impl<St, F, T> Stream for Map<St, F>
+where
+    St: Stream,
+    F: FnMut(St::Item) -> T,
+{
+    type Item = T;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
+        self.as_mut()
+            .project()
+            .stream
+            .poll_next(cx)
+            .map(|opt| opt.map(|x| (self.as_mut().project().f)(x)))
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.stream.size_hint()
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/merge.rs b/third_party/rust/tokio/src/stream/merge.rs
new file mode 100644
index 0000000000..4850cd40c7
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/merge.rs
@@ -0,0 +1,97 @@
+use crate::stream::{Fuse, Stream};
+
+use core::pin::Pin;
+use core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Stream returned by the [`merge`](super::StreamExt::merge) method.
+    pub struct Merge<T, U> {
+        #[pin]
+        a: Fuse<T>,
+        #[pin]
+        b: Fuse<U>,
+        // When `true`, poll `a` first, otherwise, `poll` b`.
+        a_first: bool,
+    }
+}
+
+impl<T, U> Merge<T, U> {
+    pub(super) fn new(a: T, b: U) -> Merge<T, U>
+    where
+        T: Stream,
+        U: Stream,
+    {
+        Merge {
+            a: Fuse::new(a),
+            b: Fuse::new(b),
+            a_first: true,
+        }
+    }
+}
+
+impl<T, U> Stream for Merge<T, U>
+where
+    T: Stream,
+    U: Stream<Item = T::Item>,
+{
+    type Item = T::Item;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T::Item>> {
+        let me = self.project();
+        let a_first = *me.a_first;
+
+        // Toggle the flag
+        *me.a_first = !a_first;
+
+        if a_first {
+            poll_next(me.a, me.b, cx)
+        } else {
+            poll_next(me.b, me.a, cx)
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (a_lower, a_upper) = self.a.size_hint();
+        let (b_lower, b_upper) = self.b.size_hint();
+
+        let upper = match (a_upper, b_upper) {
+            (Some(a_upper), Some(b_upper)) => Some(a_upper + b_upper),
+            _ => None,
+        };
+
+        (a_lower + b_lower, upper)
+    }
+}
+
+fn poll_next<T, U>(
+    first: Pin<&mut T>,
+    second: Pin<&mut U>,
+    cx: &mut Context<'_>,
+) -> Poll<Option<T::Item>>
+where
+    T: Stream,
+    U: Stream<Item = T::Item>,
+{
+    use Poll::*;
+
+    let mut done = true;
+
+    match first.poll_next(cx) {
+        Ready(Some(val)) => return Ready(Some(val)),
+        Ready(None) => {}
+        Pending => done = false,
+    }
+
+    match second.poll_next(cx) {
+        Ready(Some(val)) => return Ready(Some(val)),
+        Ready(None) => {}
+        Pending => done = false,
+    }
+
+    if done {
+        Ready(None)
+    } else {
+        Pending
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/mod.rs b/third_party/rust/tokio/src/stream/mod.rs
new file mode 100644
index 0000000000..307ead5fba
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/mod.rs
@@ -0,0 +1,819 @@
+//! Stream utilities for Tokio.
+//!
+//! A `Stream` is an asynchronous sequence of values. It can be thought of as an asynchronous version of the standard library's `Iterator` trait.
+//!
+//! This module provides helpers to work with them.
+
+mod all;
+use all::AllFuture;
+
+mod any;
+use any::AnyFuture;
+
+mod chain;
+use chain::Chain;
+
+mod collect;
+use collect::Collect;
+pub use collect::FromStream;
+
+mod empty;
+pub use empty::{empty, Empty};
+
+mod filter;
+use filter::Filter;
+
+mod filter_map;
+use filter_map::FilterMap;
+
+mod fold;
+use fold::FoldFuture;
+
+mod fuse;
+use fuse::Fuse;
+
+mod iter;
+pub use iter::{iter, Iter};
+
+mod map;
+use map::Map;
+
+mod merge;
+use merge::Merge;
+
+mod next;
+use next::Next;
+
+mod once;
+pub use once::{once, Once};
+
+mod pending;
+pub use pending::{pending, Pending};
+
+mod stream_map;
+pub use stream_map::StreamMap;
+
+mod skip;
+use skip::Skip;
+
+mod skip_while;
+use skip_while::SkipWhile;
+
+mod try_next;
+use try_next::TryNext;
+
+mod take;
+use take::Take;
+
+mod take_while;
+use take_while::TakeWhile;
+
+cfg_time! {
+    mod timeout;
+    use timeout::Timeout;
+    use std::time::Duration;
+}
+
+pub use futures_core::Stream;
+
+/// An extension trait for `Stream`s that provides a variety of convenient
+/// combinator functions.
+pub trait StreamExt: Stream {
+    /// Consumes and returns the next value in the stream or `None` if the
+    /// stream is finished.
+    ///
+    /// Equivalent to:
+    ///
+    /// ```ignore
+    /// async fn next(&mut self) -> Option<Self::Item>;
+    /// ```
+    ///
+    /// Note that because `next` doesn't take ownership over the stream,
+    /// the [`Stream`] type must be [`Unpin`]. If you want to use `next` with a
+    /// [`!Unpin`](Unpin) stream, you'll first have to pin the stream. This can
+    /// be done by boxing the stream using [`Box::pin`] or
+    /// pinning it to the stack using the `pin_mut!` macro from the `pin_utils`
+    /// crate.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::stream::{self, StreamExt};
+    ///
+    /// let mut stream = stream::iter(1..=3);
+    ///
+    /// assert_eq!(stream.next().await, Some(1));
+    /// assert_eq!(stream.next().await, Some(2));
+    /// assert_eq!(stream.next().await, Some(3));
+    /// assert_eq!(stream.next().await, None);
+    /// # }
+    /// ```
+    fn next(&mut self) -> Next<'_, Self>
+    where
+        Self: Unpin,
+    {
+        Next::new(self)
+    }
+
+    /// Consumes and returns the next item in the stream. If an error is
+    /// encountered before the next item, the error is returned instead.
+    ///
+    /// Equivalent to:
+    ///
+    /// ```ignore
+    /// async fn try_next(&mut self) -> Result<Option<T>, E>;
+    /// ```
+    ///
+    /// This is similar to the [`next`](StreamExt::next) combinator,
+    /// but returns a [`Result<Option<T>, E>`](Result) rather than
+    /// an [`Option<Result<T, E>>`](Option), making for easy use
+    /// with the [`?`](std::ops::Try) operator.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::stream::{self, StreamExt};
+    ///
+    /// let mut stream = stream::iter(vec![Ok(1), Ok(2), Err("nope")]);
+    ///
+    /// assert_eq!(stream.try_next().await, Ok(Some(1)));
+    /// assert_eq!(stream.try_next().await, Ok(Some(2)));
+    /// assert_eq!(stream.try_next().await, Err("nope"));
+    /// # }
+    /// ```
+    fn try_next<T, E>(&mut self) -> TryNext<'_, Self>
+    where
+        Self: Stream<Item = Result<T, E>> + Unpin,
+    {
+        TryNext::new(self)
+    }
+
+    /// Maps this stream's items to a different type, returning a new stream of
+    /// the resulting type.
+    ///
+    /// The provided closure is executed over all elements of this stream as
+    /// they are made available. It is executed inline with calls to
+    /// [`poll_next`](Stream::poll_next).
+    ///
+    /// Note that this function consumes the stream passed into it and returns a
+    /// wrapped version of it, similar to the existing `map` methods in the
+    /// standard library.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::stream::{self, StreamExt};
+    ///
+    /// let stream = stream::iter(1..=3);
+    /// let mut stream = stream.map(|x| x + 3);
+    ///
+    /// assert_eq!(stream.next().await, Some(4));
+    /// assert_eq!(stream.next().await, Some(5));
+    /// assert_eq!(stream.next().await, Some(6));
+    /// # }
+    /// ```
+    fn map<T, F>(self, f: F) -> Map<Self, F>
+    where
+        F: FnMut(Self::Item) -> T,
+        Self: Sized,
+    {
+        Map::new(self, f)
+    }
+
+    /// Combine two streams into one by interleaving the output of both as it
+    /// is produced.
+    ///
+    /// Values are produced from the merged stream in the order they arrive from
+    /// the two source streams. If both source streams provide values
+    /// simultaneously, the merge stream alternates between them. This provides
+    /// some level of fairness.
+    ///
+    /// The merged stream completes once **both** source streams complete. When
+    /// one source stream completes before the other, the merge stream
+    /// exclusively polls the remaining stream.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::stream::StreamExt;
+    /// use tokio::sync::mpsc;
+    /// use tokio::time;
+    ///
+    /// use std::time::Duration;
+    ///
+    /// # /*
+    /// #[tokio::main]
+    /// # */
+    /// # #[tokio::main(basic_scheduler)]
+    /// async fn main() {
+    /// # time::pause();
+    ///     let (mut tx1, rx1) = mpsc::channel(10);
+    ///     let (mut tx2, rx2) = mpsc::channel(10);
+    ///
+    ///     let mut rx = rx1.merge(rx2);
+    ///
+    ///     tokio::spawn(async move {
+    ///         // Send some values immediately
+    ///         tx1.send(1).await.unwrap();
+    ///         tx1.send(2).await.unwrap();
+    ///
+    ///         // Let the other task send values
+    ///         time::delay_for(Duration::from_millis(20)).await;
+    ///
+    ///         tx1.send(4).await.unwrap();
+    ///     });
+    ///
+    ///     tokio::spawn(async move {
+    ///         // Wait for the first task to send values
+    ///         time::delay_for(Duration::from_millis(5)).await;
+    ///
+    ///         tx2.send(3).await.unwrap();
+    ///
+    ///         time::delay_for(Duration::from_millis(25)).await;
+    ///
+    ///         // Send the final value
+    ///         tx2.send(5).await.unwrap();
+    ///     });
+    ///
+    ///    assert_eq!(1, rx.next().await.unwrap());
+    ///    assert_eq!(2, rx.next().await.unwrap());
+    ///    assert_eq!(3, rx.next().await.unwrap());
+    ///    assert_eq!(4, rx.next().await.unwrap());
+    ///    assert_eq!(5, rx.next().await.unwrap());
+    ///
+    ///    // The merged stream is consumed
+    ///    assert!(rx.next().await.is_none());
+    /// }
+    /// ```
+    fn merge<U>(self, other: U) -> Merge<Self, U>
+    where
+        U: Stream<Item = Self::Item>,
+        Self: Sized,
+    {
+        Merge::new(self, other)
+    }
+
+    /// Filters the values produced by this stream according to the provided
+    /// predicate.
+    ///
+    /// As values of this stream are made available, the provided predicate `f`
+    /// will be run against them. If the predicate
+    /// resolves to `true`, then the stream will yield the value, but if the
+    /// predicate resolves to `false`, then the value
+    /// will be discarded and the next value will be produced.
+    ///
+    /// Note that this function consumes the stream passed into it and returns a
+    /// wrapped version of it, similar to [`Iterator::filter`] method in the
+    /// standard library.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::stream::{self, StreamExt};
+    ///
+    /// let stream = stream::iter(1..=8);
+    /// let mut evens = stream.filter(|x| x % 2 == 0);
+    ///
+    /// assert_eq!(Some(2), evens.next().await);
+    /// assert_eq!(Some(4), evens.next().await);
+    /// assert_eq!(Some(6), evens.next().await);
+    /// assert_eq!(Some(8), evens.next().await);
+    /// assert_eq!(None, evens.next().await);
+    /// # }
+    /// ```
+    fn filter<F>(self, f: F) -> Filter<Self, F>
+    where
+        F: FnMut(&Self::Item) -> bool,
+        Self: Sized,
+    {
+        Filter::new(self, f)
+    }
+
+    /// Filters the values produced by this stream while simultaneously mapping
+    /// them to a different type according to the provided closure.
+    ///
+    /// As values of this stream are made available, the provided function will
+    /// be run on them. If the predicate `f` resolves to
+    /// [`Some(item)`](Some) then the stream will yield the value `item`, but if
+    /// it resolves to [`None`] then the next value will be produced.
+    ///
+    /// Note that this function consumes the stream passed into it and returns a
+    /// wrapped version of it, similar to [`Iterator::filter_map`] method in the
+    /// standard library.
+    ///
+    /// # Examples
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::stream::{self, StreamExt};
+    ///
+    /// let stream = stream::iter(1..=8);
+    /// let mut evens = stream.filter_map(|x| {
+    ///     if x % 2 == 0 { Some(x + 1) } else { None }
+    /// });
+    ///
+    /// assert_eq!(Some(3), evens.next().await);
+    /// assert_eq!(Some(5), evens.next().await);
+    /// assert_eq!(Some(7), evens.next().await);
+    /// assert_eq!(Some(9), evens.next().await);
+    /// assert_eq!(None, evens.next().await);
+    /// # }
+    /// ```
+    fn filter_map<T, F>(self, f: F) -> FilterMap<Self, F>
+    where
+        F: FnMut(Self::Item) -> Option<T>,
+        Self: Sized,
+    {
+        FilterMap::new(self, f)
+    }
+
+    /// Creates a stream which ends after the first `None`.
+    ///
+    /// After a stream returns `None`, behavior is undefined. Future calls to
+    /// `poll_next` may or may not return `Some(T)` again or they may panic.
+    /// `fuse()` adapts a stream, ensuring that after `None` is given, it will
+    /// return `None` forever.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::stream::{Stream, StreamExt};
+    ///
+    /// use std::pin::Pin;
+    /// use std::task::{Context, Poll};
+    ///
+    /// // a stream which alternates between Some and None
+    /// struct Alternate {
+    ///     state: i32,
+    /// }
+    ///
+    /// impl Stream for Alternate {
+    ///     type Item = i32;
+    ///
+    ///     fn poll_next(mut self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Option<i32>> {
+    ///         let val = self.state;
+    ///         self.state = self.state + 1;
+    ///
+    ///         // if it's even, Some(i32), else None
+    ///         if val % 2 == 0 {
+    ///             Poll::Ready(Some(val))
+    ///         } else {
+    ///             Poll::Ready(None)
+    ///         }
+    ///     }
+    /// }
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let mut stream = Alternate { state: 0 };
+    ///
+    ///     // the stream goes back and forth
+    ///     assert_eq!(stream.next().await, Some(0));
+    ///     assert_eq!(stream.next().await, None);
+    ///     assert_eq!(stream.next().await, Some(2));
+    ///     assert_eq!(stream.next().await, None);
+    ///
+    ///     // however, once it is fused
+    ///     let mut stream = stream.fuse();
+    ///
+    ///     assert_eq!(stream.next().await, Some(4));
+    ///     assert_eq!(stream.next().await, None);
+    ///
+    ///     // it will always return `None` after the first time.
+    ///     assert_eq!(stream.next().await, None);
+    ///     assert_eq!(stream.next().await, None);
+    ///     assert_eq!(stream.next().await, None);
+    /// }
+    /// ```
+    fn fuse(self) -> Fuse<Self>
+    where
+        Self: Sized,
+    {
+        Fuse::new(self)
+    }
+
+    /// Creates a new stream of at most `n` items of the underlying stream.
+    ///
+    /// Once `n` items have been yielded from this stream then it will always
+    /// return that the stream is done.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::stream::{self, StreamExt};
+    ///
+    /// let mut stream = stream::iter(1..=10).take(3);
+    ///
+    /// assert_eq!(Some(1), stream.next().await);
+    /// assert_eq!(Some(2), stream.next().await);
+    /// assert_eq!(Some(3), stream.next().await);
+    /// assert_eq!(None, stream.next().await);
+    /// # }
+    /// ```
+    fn take(self, n: usize) -> Take<Self>
+    where
+        Self: Sized,
+    {
+        Take::new(self, n)
+    }
+
+    /// Take elements from this stream while the provided predicate
+    /// resolves to `true`.
+    ///
+    /// This function, like `Iterator::take_while`, will take elements from the
+    /// stream until the predicate `f` resolves to `false`. Once one element
+    /// returns false it will always return that the stream is done.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::stream::{self, StreamExt};
+    ///
+    /// let mut stream = stream::iter(1..=10).take_while(|x| *x <= 3);
+    ///
+    /// assert_eq!(Some(1), stream.next().await);
+    /// assert_eq!(Some(2), stream.next().await);
+    /// assert_eq!(Some(3), stream.next().await);
+    /// assert_eq!(None, stream.next().await);
+    /// # }
+    /// ```
+    fn take_while<F>(self, f: F) -> TakeWhile<Self, F>
+    where
+        F: FnMut(&Self::Item) -> bool,
+        Self: Sized,
+    {
+        TakeWhile::new(self, f)
+    }
+
+    /// Creates a new stream that will skip the `n` first items of the
+    /// underlying stream.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::stream::{self, StreamExt};
+    ///
+    /// let mut stream = stream::iter(1..=10).skip(7);
+    ///
+    /// assert_eq!(Some(8), stream.next().await);
+    /// assert_eq!(Some(9), stream.next().await);
+    /// assert_eq!(Some(10), stream.next().await);
+    /// assert_eq!(None, stream.next().await);
+    /// # }
+    /// ```
+    fn skip(self, n: usize) -> Skip<Self>
+    where
+        Self: Sized,
+    {
+        Skip::new(self, n)
+    }
+
+    /// Skip elements from the underlying stream while the provided predicate
+    /// resolves to `true`.
+    ///
+    /// This function, like [`Iterator::skip_while`], will ignore elemets from the
+    /// stream until the predicate `f` resolves to `false`. Once one element
+    /// returns false, the rest of the elements will be yielded.
+    ///
+    /// [`Iterator::skip_while`]: std::iter::Iterator::skip_while()
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::stream::{self, StreamExt};
+    /// let mut stream = stream::iter(vec![1,2,3,4,1]).skip_while(|x| *x < 3);
+    ///
+    /// assert_eq!(Some(3), stream.next().await);
+    /// assert_eq!(Some(4), stream.next().await);
+    /// assert_eq!(Some(1), stream.next().await);
+    /// assert_eq!(None, stream.next().await);
+    /// # }
+    /// ```
+    fn skip_while<F>(self, f: F) -> SkipWhile<Self, F>
+    where
+        F: FnMut(&Self::Item) -> bool,
+        Self: Sized,
+    {
+        SkipWhile::new(self, f)
+    }
+
+    /// Tests if every element of the stream matches a predicate.
+    ///
+    /// `all()` takes a closure that returns `true` or `false`. It applies
+    /// this closure to each element of the stream, and if they all return
+    /// `true`, then so does `all`. If any of them return `false`, it
+    /// returns `false`. An empty stream returns `true`.
+    ///
+    /// `all()` is short-circuiting; in other words, it will stop processing
+    /// as soon as it finds a `false`, given that no matter what else happens,
+    /// the result will also be `false`.
+    ///
+    /// An empty stream returns `true`.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::stream::{self, StreamExt};
+    ///
+    /// let a = [1, 2, 3];
+    ///
+    /// assert!(stream::iter(&a).all(|&x| x > 0).await);
+    ///
+    /// assert!(!stream::iter(&a).all(|&x| x > 2).await);
+    /// # }
+    /// ```
+    ///
+    /// Stopping at the first `false`:
+    ///
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::stream::{self, StreamExt};
+    ///
+    /// let a = [1, 2, 3];
+    ///
+    /// let mut iter = stream::iter(&a);
+    ///
+    /// assert!(!iter.all(|&x| x != 2).await);
+    ///
+    /// // we can still use `iter`, as there are more elements.
+    /// assert_eq!(iter.next().await, Some(&3));
+    /// # }
+    /// ```
+    fn all<F>(&mut self, f: F) -> AllFuture<'_, Self, F>
+    where
+        Self: Unpin,
+        F: FnMut(Self::Item) -> bool,
+    {
+        AllFuture::new(self, f)
+    }
+
+    /// Tests if any element of the stream matches a predicate.
+    ///
+    /// `any()` takes a closure that returns `true` or `false`. It applies
+    /// this closure to each element of the stream, and if any of them return
+    /// `true`, then so does `any()`. If they all return `false`, it
+    /// returns `false`.
+    ///
+    /// `any()` is short-circuiting; in other words, it will stop processing
+    /// as soon as it finds a `true`, given that no matter what else happens,
+    /// the result will also be `true`.
+    ///
+    /// An empty stream returns `false`.
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::stream::{self, StreamExt};
+    ///
+    /// let a = [1, 2, 3];
+    ///
+    /// assert!(stream::iter(&a).any(|&x| x > 0).await);
+    ///
+    /// assert!(!stream::iter(&a).any(|&x| x > 5).await);
+    /// # }
+    /// ```
+    ///
+    /// Stopping at the first `true`:
+    ///
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::stream::{self, StreamExt};
+    ///
+    /// let a = [1, 2, 3];
+    ///
+    /// let mut iter = stream::iter(&a);
+    ///
+    /// assert!(iter.any(|&x| x != 2).await);
+    ///
+    /// // we can still use `iter`, as there are more elements.
+    /// assert_eq!(iter.next().await, Some(&2));
+    /// # }
+    /// ```
+    fn any<F>(&mut self, f: F) -> AnyFuture<'_, Self, F>
+    where
+        Self: Unpin,
+        F: FnMut(Self::Item) -> bool,
+    {
+        AnyFuture::new(self, f)
+    }
+
+    /// Combine two streams into one by first returning all values from the
+    /// first stream then all values from the second stream.
+    ///
+    /// As long as `self` still has values to emit, no values from `other` are
+    /// emitted, even if some are ready.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::stream::{self, StreamExt};
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let one = stream::iter(vec![1, 2, 3]);
+    ///     let two = stream::iter(vec![4, 5, 6]);
+    ///
+    ///     let mut stream = one.chain(two);
+    ///
+    ///     assert_eq!(stream.next().await, Some(1));
+    ///     assert_eq!(stream.next().await, Some(2));
+    ///     assert_eq!(stream.next().await, Some(3));
+    ///     assert_eq!(stream.next().await, Some(4));
+    ///     assert_eq!(stream.next().await, Some(5));
+    ///     assert_eq!(stream.next().await, Some(6));
+    ///     assert_eq!(stream.next().await, None);
+    /// }
+    /// ```
+    fn chain<U>(self, other: U) -> Chain<Self, U>
+    where
+        U: Stream<Item = Self::Item>,
+        Self: Sized,
+    {
+        Chain::new(self, other)
+    }
+
+    /// A combinator that applies a function to every element in a stream
+    /// producing a single, final value.
+    ///
+    /// # Examples
+    /// Basic usage:
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::stream::{self, *};
+    ///
+    /// let s = stream::iter(vec![1u8, 2, 3]);
+    /// let sum = s.fold(0, |acc, x| acc + x).await;
+    ///
+    /// assert_eq!(sum, 6);
+    /// # }
+    /// ```
+    fn fold<B, F>(self, init: B, f: F) -> FoldFuture<Self, B, F>
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        FoldFuture::new(self, init, f)
+    }
+
+    /// Drain stream pushing all emitted values into a collection.
+    ///
+    /// `collect` streams all values, awaiting as needed. Values are pushed into
+    /// a collection. A number of different target collection types are
+    /// supported, including [`Vec`](std::vec::Vec),
+    /// [`String`](std::string::String), and [`Bytes`](bytes::Bytes).
+    ///
+    /// # `Result`
+    ///
+    /// `collect()` can also be used with streams of type `Result<T, E>` where
+    /// `T: FromStream<_>`. In this case, `collect()` will stream as long as
+    /// values yielded from the stream are `Ok(_)`. If `Err(_)` is encountered,
+    /// streaming is terminated and `collect()` returns the `Err`.
+    ///
+    /// # Notes
+    ///
+    /// `FromStream` is currently a sealed trait. Stabilization is pending
+    /// enhancements to the Rust langague.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// use tokio::stream::{self, StreamExt};
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let doubled: Vec<i32> =
+    ///         stream::iter(vec![1, 2, 3])
+    ///             .map(|x| x * 2)
+    ///             .collect()
+    ///             .await;
+    ///
+    ///     assert_eq!(vec![2, 4, 6], doubled);
+    /// }
+    /// ```
+    ///
+    /// Collecting a stream of `Result` values
+    ///
+    /// ```
+    /// use tokio::stream::{self, StreamExt};
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     // A stream containing only `Ok` values will be collected
+    ///     let values: Result<Vec<i32>, &str> =
+    ///         stream::iter(vec![Ok(1), Ok(2), Ok(3)])
+    ///             .collect()
+    ///             .await;
+    ///
+    ///     assert_eq!(Ok(vec![1, 2, 3]), values);
+    ///
+    ///     // A stream containing `Err` values will return the first error.
+    ///     let results = vec![Ok(1), Err("no"), Ok(2), Ok(3), Err("nein")];
+    ///
+    ///     let values: Result<Vec<i32>, &str> =
+    ///         stream::iter(results)
+    ///             .collect()
+    ///             .await;
+    ///
+    ///     assert_eq!(Err("no"), values);
+    /// }
+    /// ```
+    fn collect<T>(self) -> Collect<Self, T>
+    where
+        T: FromStream<Self::Item>,
+        Self: Sized,
+    {
+        Collect::new(self)
+    }
+
+    /// Applies a per-item timeout to the passed stream.
+    ///
+    /// `timeout()` takes a `Duration` that represents the maximum amount of
+    /// time each element of the stream has to complete before timing out.
+    ///
+    /// If the wrapped stream yields a value before the deadline is reached, the
+    /// value is returned. Otherwise, an error is returned. The caller may decide
+    /// to continue consuming the stream and will eventually get the next source
+    /// stream value once it becomes available.
+    ///
+    /// # Notes
+    ///
+    /// This function consumes the stream passed into it and returns a
+    /// wrapped version of it.
+    ///
+    /// Polling the returned stream will continue to poll the inner stream even
+    /// if one or more items time out.
+    ///
+    /// # Examples
+    ///
+    /// Suppose we have a stream `int_stream` that yields 3 numbers (1, 2, 3):
+    ///
+    /// ```
+    /// # #[tokio::main]
+    /// # async fn main() {
+    /// use tokio::stream::{self, StreamExt};
+    /// use std::time::Duration;
+    /// # let int_stream = stream::iter(1..=3);
+    ///
+    /// let mut int_stream = int_stream.timeout(Duration::from_secs(1));
+    ///
+    /// // When no items time out, we get the 3 elements in succession:
+    /// assert_eq!(int_stream.try_next().await, Ok(Some(1)));
+    /// assert_eq!(int_stream.try_next().await, Ok(Some(2)));
+    /// assert_eq!(int_stream.try_next().await, Ok(Some(3)));
+    /// assert_eq!(int_stream.try_next().await, Ok(None));
+    ///
+    /// // If the second item times out, we get an error and continue polling the stream:
+    /// # let mut int_stream = stream::iter(vec![Ok(1), Err(()), Ok(2), Ok(3)]);
+    /// assert_eq!(int_stream.try_next().await, Ok(Some(1)));
+    /// assert!(int_stream.try_next().await.is_err());
+    /// assert_eq!(int_stream.try_next().await, Ok(Some(2)));
+    /// assert_eq!(int_stream.try_next().await, Ok(Some(3)));
+    /// assert_eq!(int_stream.try_next().await, Ok(None));
+    ///
+    /// // If we want to stop consuming the source stream the first time an
+    /// // element times out, we can use the `take_while` operator:
+    /// # let int_stream = stream::iter(vec![Ok(1), Err(()), Ok(2), Ok(3)]);
+    /// let mut int_stream = int_stream.take_while(Result::is_ok);
+    ///
+    /// assert_eq!(int_stream.try_next().await, Ok(Some(1)));
+    /// assert_eq!(int_stream.try_next().await, Ok(None));
+    /// # }
+    /// ```
+    #[cfg(all(feature = "time"))]
+    #[cfg_attr(docsrs, doc(cfg(feature = "time")))]
+    fn timeout(self, duration: Duration) -> Timeout<Self>
+    where
+        Self: Sized,
+    {
+        Timeout::new(self, duration)
+    }
+}
+
+impl<St: ?Sized> StreamExt for St where St: Stream {}
diff --git a/third_party/rust/tokio/src/stream/next.rs b/third_party/rust/tokio/src/stream/next.rs
new file mode 100644
index 0000000000..3909c0c233
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/next.rs
@@ -0,0 +1,28 @@
+use crate::stream::Stream;
+
+use core::future::Future;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+/// Future for the [`next`](super::StreamExt::next) method.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct Next<'a, St: ?Sized> {
+    stream: &'a mut St,
+}
+
+impl<St: ?Sized + Unpin> Unpin for Next<'_, St> {}
+
+impl<'a, St: ?Sized> Next<'a, St> {
+    pub(super) fn new(stream: &'a mut St) -> Self {
+        Next { stream }
+    }
+}
+
+impl<St: ?Sized + Stream + Unpin> Future for Next<'_, St> {
+    type Output = Option<St::Item>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        Pin::new(&mut self.stream).poll_next(cx)
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/once.rs b/third_party/rust/tokio/src/stream/once.rs
new file mode 100644
index 0000000000..04a642f309
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/once.rs
@@ -0,0 +1,52 @@
+use crate::stream::{self, Iter, Stream};
+
+use core::option;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+/// Stream for the [`once`] function.
+#[derive(Debug)]
+#[must_use = "streams do nothing unless polled"]
+pub struct Once<T> {
+    iter: Iter<option::IntoIter<T>>,
+}
+
+impl<I> Unpin for Once<I> {}
+
+/// Creates a stream that emits an element exactly once.
+///
+/// The returned stream is immediately ready and emits the provided value once.
+///
+/// # Examples
+///
+/// ```
+/// use tokio::stream::{self, StreamExt};
+///
+/// #[tokio::main]
+/// async fn main() {
+///     // one is the loneliest number
+///     let mut one = stream::once(1);
+///
+///     assert_eq!(Some(1), one.next().await);
+///
+///     // just one, that's all we get
+///     assert_eq!(None, one.next().await);
+/// }
+/// ```
+pub fn once<T>(value: T) -> Once<T> {
+    Once {
+        iter: stream::iter(Some(value).into_iter()),
+    }
+}
+
+impl<T> Stream for Once<T> {
+    type Item = T;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
+        Pin::new(&mut self.iter).poll_next(cx)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/pending.rs b/third_party/rust/tokio/src/stream/pending.rs
new file mode 100644
index 0000000000..2e06a1c261
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/pending.rs
@@ -0,0 +1,54 @@
+use crate::stream::Stream;
+
+use core::marker::PhantomData;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+/// Stream for the [`pending`] function.
+#[derive(Debug)]
+#[must_use = "streams do nothing unless polled"]
+pub struct Pending<T>(PhantomData<T>);
+
+impl<T> Unpin for Pending<T> {}
+unsafe impl<T> Send for Pending<T> {}
+unsafe impl<T> Sync for Pending<T> {}
+
+/// Creates a stream that is never ready
+///
+/// The returned stream is never ready. Attempting to call
+/// [`next()`](crate::stream::StreamExt::next) will never complete. Use
+/// [`stream::empty()`](super::empty()) to obtain a stream that is is
+/// immediately empty but returns no values.
+///
+/// # Examples
+///
+/// Basic usage:
+///
+/// ```no_run
+/// use tokio::stream::{self, StreamExt};
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let mut never = stream::pending::<i32>();
+///
+///     // This will never complete
+///     never.next().await;
+///
+///     unreachable!();
+/// }
+/// ```
+pub const fn pending<T>() -> Pending<T> {
+    Pending(PhantomData)
+}
+
+impl<T> Stream for Pending<T> {
+    type Item = T;
+
+    fn poll_next(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Option<T>> {
+        Poll::Pending
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, None)
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/skip.rs b/third_party/rust/tokio/src/stream/skip.rs
new file mode 100644
index 0000000000..39540cc984
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/skip.rs
@@ -0,0 +1,63 @@
+use crate::stream::Stream;
+
+use core::fmt;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Stream for the [`skip`](super::StreamExt::skip) method.
+    #[must_use = "streams do nothing unless polled"]
+    pub struct Skip<St> {
+        #[pin]
+        stream: St,
+        remaining: usize,
+    }
+}
+
+impl<St> fmt::Debug for Skip<St>
+where
+    St: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Skip")
+            .field("stream", &self.stream)
+            .finish()
+    }
+}
+
+impl<St> Skip<St> {
+    pub(super) fn new(stream: St, remaining: usize) -> Self {
+        Self { stream, remaining }
+    }
+}
+
+impl<St> Stream for Skip<St>
+where
+    St: Stream,
+{
+    type Item = St::Item;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        loop {
+            match ready!(self.as_mut().project().stream.poll_next(cx)) {
+                Some(e) => {
+                    if self.remaining == 0 {
+                        return Poll::Ready(Some(e));
+                    }
+                    *self.as_mut().project().remaining -= 1;
+                }
+                None => return Poll::Ready(None),
+            }
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (lower, upper) = self.stream.size_hint();
+
+        let lower = lower.saturating_sub(self.remaining);
+        let upper = upper.map(|x| x.saturating_sub(self.remaining));
+
+        (lower, upper)
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/skip_while.rs b/third_party/rust/tokio/src/stream/skip_while.rs
new file mode 100644
index 0000000000..4e0500701a
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/skip_while.rs
@@ -0,0 +1,73 @@
+use crate::stream::Stream;
+
+use core::fmt;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Stream for the [`skip_while`](super::StreamExt::skip_while) method.
+    #[must_use = "streams do nothing unless polled"]
+    pub struct SkipWhile<St, F> {
+        #[pin]
+        stream: St,
+        predicate: Option<F>,
+    }
+}
+
+impl<St, F> fmt::Debug for SkipWhile<St, F>
+where
+    St: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("SkipWhile")
+            .field("stream", &self.stream)
+            .finish()
+    }
+}
+
+impl<St, F> SkipWhile<St, F> {
+    pub(super) fn new(stream: St, predicate: F) -> Self {
+        Self {
+            stream,
+            predicate: Some(predicate),
+        }
+    }
+}
+
+impl<St, F> Stream for SkipWhile<St, F>
+where
+    St: Stream,
+    F: FnMut(&St::Item) -> bool,
+{
+    type Item = St::Item;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        let mut this = self.project();
+        if let Some(predicate) = this.predicate {
+            loop {
+                match ready!(this.stream.as_mut().poll_next(cx)) {
+                    Some(item) => {
+                        if !(predicate)(&item) {
+                            *this.predicate = None;
+                            return Poll::Ready(Some(item));
+                        }
+                    }
+                    None => return Poll::Ready(None),
+                }
+            }
+        } else {
+            this.stream.poll_next(cx)
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (lower, upper) = self.stream.size_hint();
+
+        if self.predicate.is_some() {
+            return (0, upper);
+        }
+
+        (lower, upper)
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/stream_map.rs b/third_party/rust/tokio/src/stream/stream_map.rs
new file mode 100644
index 0000000000..2f60ea4dda
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/stream_map.rs
@@ -0,0 +1,503 @@
+use crate::stream::Stream;
+
+use std::borrow::Borrow;
+use std::hash::Hash;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// Combine many streams into one, indexing each source stream with a unique
+/// key.
+///
+/// `StreamMap` is similar to [`StreamExt::merge`] in that it combines source
+/// streams into a single merged stream that yields values in the order that
+/// they arrive from the source streams. However, `StreamMap` has a lot more
+/// flexibility in usage patterns.
+///
+/// `StreamMap` can:
+///
+/// * Merge an arbitrary number of streams.
+/// * Track which source stream the value was received from.
+/// * Handle inserting and removing streams from the set of managed streams at
+///   any point during iteration.
+///
+/// All source streams held by `StreamMap` are indexed using a key. This key is
+/// included with the value when a source stream yields a value. The key is also
+/// used to remove the stream from the `StreamMap` before the stream has
+/// completed streaming.
+///
+/// # `Unpin`
+///
+/// Because the `StreamMap` API moves streams during runtime, both streams and
+/// keys must be `Unpin`. In order to insert a `!Unpin` stream into a
+/// `StreamMap`, use [`pin!`] to pin the stream to the stack or [`Box::pin`] to
+/// pin the stream in the heap.
+///
+/// # Implementation
+///
+/// `StreamMap` is backed by a `Vec<(K, V)>`. There is no guarantee that this
+/// internal implementation detail will persist in future versions, but it is
+/// important to know the runtime implications. In general, `StreamMap` works
+/// best with a "smallish" number of streams as all entries are scanned on
+/// insert, remove, and polling. In cases where a large number of streams need
+/// to be merged, it may be advisable to use tasks sending values on a shared
+/// [`mpsc`] channel.
+///
+/// [`StreamExt::merge`]: crate::stream::StreamExt::merge
+/// [`mpsc`]: crate::sync::mpsc
+/// [`pin!`]: macro@pin
+/// [`Box::pin`]: std::boxed::Box::pin
+///
+/// # Examples
+///
+/// Merging two streams, then remove them after receiving the first value
+///
+/// ```
+/// use tokio::stream::{StreamExt, StreamMap};
+/// use tokio::sync::mpsc;
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let (mut tx1, rx1) = mpsc::channel(10);
+///     let (mut tx2, rx2) = mpsc::channel(10);
+///
+///     tokio::spawn(async move {
+///         tx1.send(1).await.unwrap();
+///
+///         // This value will never be received. The send may or may not return
+///         // `Err` depending on if the remote end closed first or not.
+///         let _ = tx1.send(2).await;
+///     });
+///
+///     tokio::spawn(async move {
+///         tx2.send(3).await.unwrap();
+///         let _ = tx2.send(4).await;
+///     });
+///
+///     let mut map = StreamMap::new();
+///
+///     // Insert both streams
+///     map.insert("one", rx1);
+///     map.insert("two", rx2);
+///
+///     // Read twice
+///     for _ in 0..2 {
+///         let (key, val) = map.next().await.unwrap();
+///
+///         if key == "one" {
+///             assert_eq!(val, 1);
+///         } else {
+///             assert_eq!(val, 3);
+///         }
+///
+///         // Remove the stream to prevent reading the next value
+///         map.remove(key);
+///     }
+/// }
+/// ```
+///
+/// This example models a read-only client to a chat system with channels. The
+/// client sends commands to join and leave channels. `StreamMap` is used to
+/// manage active channel subscriptions.
+///
+/// For simplicity, messages are displayed with `println!`, but they could be
+/// sent to the client over a socket.
+///
+/// ```no_run
+/// use tokio::stream::{Stream, StreamExt, StreamMap};
+///
+/// enum Command {
+///     Join(String),
+///     Leave(String),
+/// }
+///
+/// fn commands() -> impl Stream<Item = Command> {
+///     // Streams in user commands by parsing `stdin`.
+/// # tokio::stream::pending()
+/// }
+///
+/// // Join a channel, returns a stream of messages received on the channel.
+/// fn join(channel: &str) -> impl Stream<Item = String> + Unpin {
+///     // left as an exercise to the reader
+/// # tokio::stream::pending()
+/// }
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let mut channels = StreamMap::new();
+///
+///     // Input commands (join / leave channels).
+///     let cmds = commands();
+///     tokio::pin!(cmds);
+///
+///     loop {
+///         tokio::select! {
+///             Some(cmd) = cmds.next() => {
+///                 match cmd {
+///                     Command::Join(chan) => {
+///                         // Join the channel and add it to the `channels`
+///                         // stream map
+///                         let msgs = join(&chan);
+///                         channels.insert(chan, msgs);
+///                     }
+///                     Command::Leave(chan) => {
+///                         channels.remove(&chan);
+///                     }
+///                 }
+///             }
+///             Some((chan, msg)) = channels.next() => {
+///                 // Received a message, display it on stdout with the channel
+///                 // it originated from.
+///                 println!("{}: {}", chan, msg);
+///             }
+///             // Both the `commands` stream and the `channels` stream are
+///             // complete. There is no more work to do, so leave the loop.
+///             else => break,
+///         }
+///     }
+/// }
+/// ```
+#[derive(Debug, Default)]
+pub struct StreamMap<K, V> {
+    /// Streams stored in the map
+    entries: Vec<(K, V)>,
+}
+
+impl<K, V> StreamMap<K, V> {
+    /// Creates an empty `StreamMap`.
+    ///
+    /// The stream map is initially created with a capacity of `0`, so it will
+    /// not allocate until it is first inserted into.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::stream::{StreamMap, Pending};
+    ///
+    /// let map: StreamMap<&str, Pending<()>> = StreamMap::new();
+    /// ```
+    pub fn new() -> StreamMap<K, V> {
+        StreamMap { entries: vec![] }
+    }
+
+    /// Creates an empty `StreamMap` with the specified capacity.
+    ///
+    /// The stream map will be able to hold at least `capacity` elements without
+    /// reallocating. If `capacity` is 0, the stream map will not allocate.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::stream::{StreamMap, Pending};
+    ///
+    /// let map: StreamMap<&str, Pending<()>> = StreamMap::with_capacity(10);
+    /// ```
+    pub fn with_capacity(capacity: usize) -> StreamMap<K, V> {
+        StreamMap {
+            entries: Vec::with_capacity(capacity),
+        }
+    }
+
+    /// Returns an iterator visiting all keys in arbitrary order.
+    ///
+    /// The iterator element type is &'a K.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::stream::{StreamMap, pending};
+    ///
+    /// let mut map = StreamMap::new();
+    ///
+    /// map.insert("a", pending::<i32>());
+    /// map.insert("b", pending());
+    /// map.insert("c", pending());
+    ///
+    /// for key in map.keys() {
+    ///     println!("{}", key);
+    /// }
+    /// ```
+    pub fn keys(&self) -> impl Iterator<Item = &K> {
+        self.entries.iter().map(|(k, _)| k)
+    }
+
+    /// An iterator visiting all values in arbitrary order.
+    ///
+    /// The iterator element type is &'a V.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::stream::{StreamMap, pending};
+    ///
+    /// let mut map = StreamMap::new();
+    ///
+    /// map.insert("a", pending::<i32>());
+    /// map.insert("b", pending());
+    /// map.insert("c", pending());
+    ///
+    /// for stream in map.values() {
+    ///     println!("{:?}", stream);
+    /// }
+    /// ```
+    pub fn values(&self) -> impl Iterator<Item = &V> {
+        self.entries.iter().map(|(_, v)| v)
+    }
+
+    /// An iterator visiting all values mutably in arbitrary order.
+    ///
+    /// The iterator element type is &'a mut V.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::stream::{StreamMap, pending};
+    ///
+    /// let mut map = StreamMap::new();
+    ///
+    /// map.insert("a", pending::<i32>());
+    /// map.insert("b", pending());
+    /// map.insert("c", pending());
+    ///
+    /// for stream in map.values_mut() {
+    ///     println!("{:?}", stream);
+    /// }
+    /// ```
+    pub fn values_mut(&mut self) -> impl Iterator<Item = &mut V> {
+        self.entries.iter_mut().map(|(_, v)| v)
+    }
+
+    /// Returns the number of streams the map can hold without reallocating.
+    ///
+    /// This number is a lower bound; the `StreamMap` might be able to hold
+    /// more, but is guaranteed to be able to hold at least this many.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::stream::{StreamMap, Pending};
+    ///
+    /// let map: StreamMap<i32, Pending<()>> = StreamMap::with_capacity(100);
+    /// assert!(map.capacity() >= 100);
+    /// ```
+    pub fn capacity(&self) -> usize {
+        self.entries.capacity()
+    }
+
+    /// Returns the number of streams in the map.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::stream::{StreamMap, pending};
+    ///
+    /// let mut a = StreamMap::new();
+    /// assert_eq!(a.len(), 0);
+    /// a.insert(1, pending::<i32>());
+    /// assert_eq!(a.len(), 1);
+    /// ```
+    pub fn len(&self) -> usize {
+        self.entries.len()
+    }
+
+    /// Returns `true` if the map contains no elements.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::HashMap;
+    ///
+    /// let mut a = HashMap::new();
+    /// assert!(a.is_empty());
+    /// a.insert(1, "a");
+    /// assert!(!a.is_empty());
+    /// ```
+    pub fn is_empty(&self) -> bool {
+        self.entries.is_empty()
+    }
+
+    /// Clears the map, removing all key-stream pairs. Keeps the allocated
+    /// memory for reuse.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::stream::{StreamMap, pending};
+    ///
+    /// let mut a = StreamMap::new();
+    /// a.insert(1, pending::<i32>());
+    /// a.clear();
+    /// assert!(a.is_empty());
+    /// ```
+    pub fn clear(&mut self) {
+        self.entries.clear();
+    }
+
+    /// Insert a key-stream pair into the map.
+    ///
+    /// If the map did not have this key present, `None` is returned.
+    ///
+    /// If the map did have this key present, the new `stream` replaces the old
+    /// one and the old stream is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::stream::{StreamMap, pending};
+    ///
+    /// let mut map = StreamMap::new();
+    ///
+    /// assert!(map.insert(37, pending::<i32>()).is_none());
+    /// assert!(!map.is_empty());
+    ///
+    /// map.insert(37, pending());
+    /// assert!(map.insert(37, pending()).is_some());
+    /// ```
+    pub fn insert(&mut self, k: K, stream: V) -> Option<V>
+    where
+        K: Hash + Eq,
+    {
+        let ret = self.remove(&k);
+        self.entries.push((k, stream));
+
+        ret
+    }
+
+    /// Removes a key from the map, returning the stream at the key if the key was previously in the map.
+    ///
+    /// The key may be any borrowed form of the map's key type, but `Hash` and
+    /// `Eq` on the borrowed form must match those for the key type.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::stream::{StreamMap, pending};
+    ///
+    /// let mut map = StreamMap::new();
+    /// map.insert(1, pending::<i32>());
+    /// assert!(map.remove(&1).is_some());
+    /// assert!(map.remove(&1).is_none());
+    /// ```
+    pub fn remove<Q: ?Sized>(&mut self, k: &Q) -> Option<V>
+    where
+        K: Borrow<Q>,
+        Q: Hash + Eq,
+    {
+        for i in 0..self.entries.len() {
+            if self.entries[i].0.borrow() == k {
+                return Some(self.entries.swap_remove(i).1);
+            }
+        }
+
+        None
+    }
+
+    /// Returns `true` if the map contains a stream for the specified key.
+    ///
+    /// The key may be any borrowed form of the map's key type, but `Hash` and
+    /// `Eq` on the borrowed form must match those for the key type.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::stream::{StreamMap, pending};
+    ///
+    /// let mut map = StreamMap::new();
+    /// map.insert(1, pending::<i32>());
+    /// assert_eq!(map.contains_key(&1), true);
+    /// assert_eq!(map.contains_key(&2), false);
+    /// ```
+    pub fn contains_key<Q: ?Sized>(&self, k: &Q) -> bool
+    where
+        K: Borrow<Q>,
+        Q: Hash + Eq,
+    {
+        for i in 0..self.entries.len() {
+            if self.entries[i].0.borrow() == k {
+                return true;
+            }
+        }
+
+        false
+    }
+}
+
+impl<K, V> StreamMap<K, V>
+where
+    K: Unpin,
+    V: Stream + Unpin,
+{
+    /// Polls the next value, includes the vec entry index
+    fn poll_next_entry(&mut self, cx: &mut Context<'_>) -> Poll<Option<(usize, V::Item)>> {
+        use Poll::*;
+
+        let start = crate::util::thread_rng_n(self.entries.len() as u32) as usize;
+        let mut idx = start;
+
+        for _ in 0..self.entries.len() {
+            let (_, stream) = &mut self.entries[idx];
+
+            match Pin::new(stream).poll_next(cx) {
+                Ready(Some(val)) => return Ready(Some((idx, val))),
+                Ready(None) => {
+                    // Remove the entry
+                    self.entries.swap_remove(idx);
+
+                    // Check if this was the last entry, if so the cursor needs
+                    // to wrap
+                    if idx == self.entries.len() {
+                        idx = 0;
+                    } else if idx < start && start <= self.entries.len() {
+                        // The stream being swapped into the current index has
+                        // already been polled, so skip it.
+                        idx = idx.wrapping_add(1) % self.entries.len();
+                    }
+                }
+                Pending => {
+                    idx = idx.wrapping_add(1) % self.entries.len();
+                }
+            }
+        }
+
+        // If the map is empty, then the stream is complete.
+        if self.entries.is_empty() {
+            Ready(None)
+        } else {
+            Pending
+        }
+    }
+}
+
+impl<K, V> Stream for StreamMap<K, V>
+where
+    K: Clone + Unpin,
+    V: Stream + Unpin,
+{
+    type Item = (K, V::Item);
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        if let Some((idx, val)) = ready!(self.poll_next_entry(cx)) {
+            let key = self.entries[idx].0.clone();
+            Poll::Ready(Some((key, val)))
+        } else {
+            Poll::Ready(None)
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let mut ret = (0, Some(0));
+
+        for (_, stream) in &self.entries {
+            let hint = stream.size_hint();
+
+            ret.0 += hint.0;
+
+            match (ret.1, hint.1) {
+                (Some(a), Some(b)) => ret.1 = Some(a + b),
+                (Some(_), None) => ret.1 = None,
+                _ => {}
+            }
+        }
+
+        ret
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/take.rs b/third_party/rust/tokio/src/stream/take.rs
new file mode 100644
index 0000000000..a92430b77c
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/take.rs
@@ -0,0 +1,76 @@
+use crate::stream::Stream;
+
+use core::cmp;
+use core::fmt;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Stream for the [`take`](super::StreamExt::take) method.
+    #[must_use = "streams do nothing unless polled"]
+    pub struct Take<St> {
+        #[pin]
+        stream: St,
+        remaining: usize,
+    }
+}
+
+impl<St> fmt::Debug for Take<St>
+where
+    St: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Take")
+            .field("stream", &self.stream)
+            .finish()
+    }
+}
+
+impl<St> Take<St> {
+    pub(super) fn new(stream: St, remaining: usize) -> Self {
+        Self { stream, remaining }
+    }
+}
+
+impl<St> Stream for Take<St>
+where
+    St: Stream,
+{
+    type Item = St::Item;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        if *self.as_mut().project().remaining > 0 {
+            self.as_mut().project().stream.poll_next(cx).map(|ready| {
+                match &ready {
+                    Some(_) => {
+                        *self.as_mut().project().remaining -= 1;
+                    }
+                    None => {
+                        *self.as_mut().project().remaining = 0;
+                    }
+                }
+                ready
+            })
+        } else {
+            Poll::Ready(None)
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        if self.remaining == 0 {
+            return (0, Some(0));
+        }
+
+        let (lower, upper) = self.stream.size_hint();
+
+        let lower = cmp::min(lower, self.remaining as usize);
+
+        let upper = match upper {
+            Some(x) if x < self.remaining as usize => Some(x),
+            _ => Some(self.remaining as usize),
+        };
+
+        (lower, upper)
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/take_while.rs b/third_party/rust/tokio/src/stream/take_while.rs
new file mode 100644
index 0000000000..cf1e160613
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/take_while.rs
@@ -0,0 +1,79 @@
+use crate::stream::Stream;
+
+use core::fmt;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Stream for the [`take_while`](super::StreamExt::take_while) method.
+    #[must_use = "streams do nothing unless polled"]
+    pub struct TakeWhile<St, F> {
+        #[pin]
+        stream: St,
+        predicate: F,
+        done: bool,
+    }
+}
+
+impl<St, F> fmt::Debug for TakeWhile<St, F>
+where
+    St: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("TakeWhile")
+            .field("stream", &self.stream)
+            .field("done", &self.done)
+            .finish()
+    }
+}
+
+impl<St, F> TakeWhile<St, F> {
+    pub(super) fn new(stream: St, predicate: F) -> Self {
+        Self {
+            stream,
+            predicate,
+            done: false,
+        }
+    }
+}
+
+impl<St, F> Stream for TakeWhile<St, F>
+where
+    St: Stream,
+    F: FnMut(&St::Item) -> bool,
+{
+    type Item = St::Item;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        if !*self.as_mut().project().done {
+            self.as_mut().project().stream.poll_next(cx).map(|ready| {
+                let ready = ready.and_then(|item| {
+                    if !(self.as_mut().project().predicate)(&item) {
+                        None
+                    } else {
+                        Some(item)
+                    }
+                });
+
+                if ready.is_none() {
+                    *self.as_mut().project().done = true;
+                }
+
+                ready
+            })
+        } else {
+            Poll::Ready(None)
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        if self.done {
+            return (0, Some(0));
+        }
+
+        let (_, upper) = self.stream.size_hint();
+
+        (0, upper)
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/timeout.rs b/third_party/rust/tokio/src/stream/timeout.rs
new file mode 100644
index 0000000000..b8a2024f6a
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/timeout.rs
@@ -0,0 +1,65 @@
+use crate::stream::{Fuse, Stream};
+use crate::time::{Delay, Elapsed, Instant};
+
+use core::future::Future;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+use std::time::Duration;
+
+pin_project! {
+    /// Stream returned by the [`timeout`](super::StreamExt::timeout) method.
+    #[must_use = "streams do nothing unless polled"]
+    #[derive(Debug)]
+    pub struct Timeout<S> {
+        #[pin]
+        stream: Fuse<S>,
+        deadline: Delay,
+        duration: Duration,
+        poll_deadline: bool,
+    }
+}
+
+impl<S: Stream> Timeout<S> {
+    pub(super) fn new(stream: S, duration: Duration) -> Self {
+        let next = Instant::now() + duration;
+        let deadline = Delay::new_timeout(next, duration);
+
+        Timeout {
+            stream: Fuse::new(stream),
+            deadline,
+            duration,
+            poll_deadline: true,
+        }
+    }
+}
+
+impl<S: Stream> Stream for Timeout<S> {
+    type Item = Result<S::Item, Elapsed>;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        match self.as_mut().project().stream.poll_next(cx) {
+            Poll::Ready(v) => {
+                if v.is_some() {
+                    let next = Instant::now() + self.duration;
+                    self.as_mut().project().deadline.reset(next);
+                    *self.as_mut().project().poll_deadline = true;
+                }
+                return Poll::Ready(v.map(Ok));
+            }
+            Poll::Pending => {}
+        };
+
+        if self.poll_deadline {
+            ready!(Pin::new(self.as_mut().project().deadline).poll(cx));
+            *self.as_mut().project().poll_deadline = false;
+            return Poll::Ready(Some(Err(Elapsed::new())));
+        }
+
+        Poll::Pending
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.stream.size_hint()
+    }
+}
diff --git a/third_party/rust/tokio/src/stream/try_next.rs b/third_party/rust/tokio/src/stream/try_next.rs
new file mode 100644
index 0000000000..59e0eb1a41
--- /dev/null
+++ b/third_party/rust/tokio/src/stream/try_next.rs
@@ -0,0 +1,30 @@
+use crate::stream::{Next, Stream};
+
+use core::future::Future;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+/// Future for the [`try_next`](super::StreamExt::try_next) method.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct TryNext<'a, St: ?Sized> {
+    inner: Next<'a, St>,
+}
+
+impl<St: ?Sized + Unpin> Unpin for TryNext<'_, St> {}
+
+impl<'a, St: ?Sized> TryNext<'a, St> {
+    pub(super) fn new(stream: &'a mut St) -> Self {
+        Self {
+            inner: Next::new(stream),
+        }
+    }
+}
+
+impl<T, E, St: ?Sized + Stream<Item = Result<T, E>> + Unpin> Future for TryNext<'_, St> {
+    type Output = Result<Option<T>, E>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        Pin::new(&mut self.inner).poll(cx).map(Option::transpose)
+    }
+}
diff --git a/third_party/rust/tokio/src/sync/barrier.rs b/third_party/rust/tokio/src/sync/barrier.rs
new file mode 100644
index 0000000000..628633493a
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/barrier.rs
@@ -0,0 +1,136 @@
+use crate::sync::watch;
+
+use std::sync::Mutex;
+
+/// A barrier enables multiple threads to synchronize the beginning of some computation.
+///
+/// ```
+/// # #[tokio::main]
+/// # async fn main() {
+/// use tokio::sync::Barrier;
+///
+/// use futures::future::join_all;
+/// use std::sync::Arc;
+///
+/// let mut handles = Vec::with_capacity(10);
+/// let barrier = Arc::new(Barrier::new(10));
+/// for _ in 0..10 {
+///     let c = barrier.clone();
+///     // The same messages will be printed together.
+///     // You will NOT see any interleaving.
+///     handles.push(async move {
+///         println!("before wait");
+///         let wr = c.wait().await;
+///         println!("after wait");
+///         wr
+///     });
+/// }
+/// // Will not resolve until all "before wait" messages have been printed
+/// let wrs = join_all(handles).await;
+/// // Exactly one barrier will resolve as the "leader"
+/// assert_eq!(wrs.into_iter().filter(|wr| wr.is_leader()).count(), 1);
+/// # }
+/// ```
+#[derive(Debug)]
+pub struct Barrier {
+    state: Mutex<BarrierState>,
+    wait: watch::Receiver<usize>,
+    n: usize,
+}
+
+#[derive(Debug)]
+struct BarrierState {
+    waker: watch::Sender<usize>,
+    arrived: usize,
+    generation: usize,
+}
+
+impl Barrier {
+    /// Creates a new barrier that can block a given number of threads.
+    ///
+    /// A barrier will block `n`-1 threads which call [`Barrier::wait`] and then wake up all
+    /// threads at once when the `n`th thread calls `wait`.
+    pub fn new(mut n: usize) -> Barrier {
+        let (waker, wait) = crate::sync::watch::channel(0);
+
+        if n == 0 {
+            // if n is 0, it's not clear what behavior the user wants.
+            // in std::sync::Barrier, an n of 0 exhibits the same behavior as n == 1, where every
+            // .wait() immediately unblocks, so we adopt that here as well.
+            n = 1;
+        }
+
+        Barrier {
+            state: Mutex::new(BarrierState {
+                waker,
+                arrived: 0,
+                generation: 1,
+            }),
+            n,
+            wait,
+        }
+    }
+
+    /// Does not resolve until all tasks have rendezvoused here.
+    ///
+    /// Barriers are re-usable after all threads have rendezvoused once, and can
+    /// be used continuously.
+    ///
+    /// A single (arbitrary) future will receive a [`BarrierWaitResult`] that returns `true` from
+    /// [`BarrierWaitResult::is_leader`] when returning from this function, and all other threads
+    /// will receive a result that will return `false` from `is_leader`.
+    pub async fn wait(&self) -> BarrierWaitResult {
+        // NOTE: we are taking a _synchronous_ lock here.
+        // It is okay to do so because the critical section is fast and never yields, so it cannot
+        // deadlock even if another future is concurrently holding the lock.
+        // It is _desireable_ to do so as synchronous Mutexes are, at least in theory, faster than
+        // the asynchronous counter-parts, so we should use them where possible [citation needed].
+        // NOTE: the extra scope here is so that the compiler doesn't think `state` is held across
+        // a yield point, and thus marks the returned future as !Send.
+        let generation = {
+            let mut state = self.state.lock().unwrap();
+            let generation = state.generation;
+            state.arrived += 1;
+            if state.arrived == self.n {
+                // we are the leader for this generation
+                // wake everyone, increment the generation, and return
+                state
+                    .waker
+                    .broadcast(state.generation)
+                    .expect("there is at least one receiver");
+                state.arrived = 0;
+                state.generation += 1;
+                return BarrierWaitResult(true);
+            }
+
+            generation
+        };
+
+        // we're going to have to wait for the last of the generation to arrive
+        let mut wait = self.wait.clone();
+
+        loop {
+            // note that the first time through the loop, this _will_ yield a generation
+            // immediately, since we cloned a receiver that has never seen any values.
+            if wait.recv().await.expect("sender hasn't been closed") >= generation {
+                break;
+            }
+        }
+
+        BarrierWaitResult(false)
+    }
+}
+
+/// A `BarrierWaitResult` is returned by `wait` when all threads in the `Barrier` have rendezvoused.
+#[derive(Debug, Clone)]
+pub struct BarrierWaitResult(bool);
+
+impl BarrierWaitResult {
+    /// Returns `true` if this thread from wait is the "leader thread".
+    ///
+    /// Only one thread will have `true` returned from their result, all other threads will have
+    /// `false` returned.
+    pub fn is_leader(&self) -> bool {
+        self.0
+    }
+}
diff --git a/third_party/rust/tokio/src/sync/batch_semaphore.rs b/third_party/rust/tokio/src/sync/batch_semaphore.rs
new file mode 100644
index 0000000000..436737a670
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/batch_semaphore.rs
@@ -0,0 +1,547 @@
+//! # Implementation Details
+//!
+//! The semaphore is implemented using an intrusive linked list of waiters. An
+//! atomic counter tracks the number of available permits. If the semaphore does
+//! not contain the required number of permits, the task attempting to acquire
+//! permits places its waker at the end of a queue. When new permits are made
+//! available (such as by releasing an initial acquisition), they are assigned
+//! to the task at the front of the queue, waking that task if its requested
+//! number of permits is met.
+//!
+//! Because waiters are enqueued at the back of the linked list and dequeued
+//! from the front, the semaphore is fair. Tasks trying to acquire large numbers
+//! of permits at a time will always be woken eventually, even if many other
+//! tasks are acquiring smaller numbers of permits. This means that in a
+//! use-case like tokio's read-write lock, writers will not be starved by
+//! readers.
+use crate::loom::cell::UnsafeCell;
+use crate::loom::sync::atomic::AtomicUsize;
+use crate::loom::sync::{Mutex, MutexGuard};
+use crate::util::linked_list::{self, LinkedList};
+
+use std::future::Future;
+use std::marker::PhantomPinned;
+use std::pin::Pin;
+use std::ptr::NonNull;
+use std::sync::atomic::Ordering::*;
+use std::task::Poll::*;
+use std::task::{Context, Poll, Waker};
+use std::{cmp, fmt};
+
+/// An asynchronous counting semaphore which permits waiting on multiple permits at once.
+pub(crate) struct Semaphore {
+    waiters: Mutex<Waitlist>,
+    /// The current number of available permits in the semaphore.
+    permits: AtomicUsize,
+}
+
+struct Waitlist {
+    queue: LinkedList<Waiter>,
+    closed: bool,
+}
+
+/// Error returned by `Semaphore::try_acquire`.
+#[derive(Debug)]
+pub(crate) enum TryAcquireError {
+    Closed,
+    NoPermits,
+}
+/// Error returned by `Semaphore::acquire`.
+#[derive(Debug)]
+pub(crate) struct AcquireError(());
+
+pub(crate) struct Acquire<'a> {
+    node: Waiter,
+    semaphore: &'a Semaphore,
+    num_permits: u16,
+    queued: bool,
+}
+
+/// An entry in the wait queue.
+struct Waiter {
+    /// The current state of the waiter.
+    ///
+    /// This is either the number of remaining permits required by
+    /// the waiter, or a flag indicating that the waiter is not yet queued.
+    state: AtomicUsize,
+
+    /// The waker to notify the task awaiting permits.
+    ///
+    /// # Safety
+    ///
+    /// This may only be accessed while the wait queue is locked.
+    waker: UnsafeCell<Option<Waker>>,
+
+    /// Intrusive linked-list pointers.
+    ///
+    /// # Safety
+    ///
+    /// This may only be accessed while the wait queue is locked.
+    ///
+    /// TODO: Ideally, we would be able to use loom to enforce that
+    /// this isn't accessed concurrently. However, it is difficult to
+    /// use a `UnsafeCell` here, since the `Link` trait requires _returning_
+    /// references to `Pointers`, and `UnsafeCell` requires that checked access
+    /// take place inside a closure. We should consider changing `Pointers` to
+    /// use `UnsafeCell` internally.
+    pointers: linked_list::Pointers<Waiter>,
+
+    /// Should not be `Unpin`.
+    _p: PhantomPinned,
+}
+
+impl Semaphore {
+    /// The maximum number of permits which a semaphore can hold.
+    ///
+    /// Note that this reserves three bits of flags in the permit counter, but
+    /// we only actually use one of them. However, the previous semaphore
+    /// implementation used three bits, so we will continue to reserve them to
+    /// avoid a breaking change if additional flags need to be aadded in the
+    /// future.
+    pub(crate) const MAX_PERMITS: usize = std::usize::MAX >> 3;
+    const CLOSED: usize = 1;
+    const PERMIT_SHIFT: usize = 1;
+
+    /// Creates a new semaphore with the initial number of permits
+    pub(crate) fn new(permits: usize) -> Self {
+        assert!(
+            permits <= Self::MAX_PERMITS,
+            "a semaphore may not have more than MAX_PERMITS permits ({})",
+            Self::MAX_PERMITS
+        );
+        Self {
+            permits: AtomicUsize::new(permits << Self::PERMIT_SHIFT),
+            waiters: Mutex::new(Waitlist {
+                queue: LinkedList::new(),
+                closed: false,
+            }),
+        }
+    }
+
+    /// Returns the current number of available permits
+    pub(crate) fn available_permits(&self) -> usize {
+        self.permits.load(Acquire) >> Self::PERMIT_SHIFT
+    }
+
+    /// Adds `n` new permits to the semaphore.
+    pub(crate) fn release(&self, added: usize) {
+        if added == 0 {
+            return;
+        }
+
+        // Assign permits to the wait queue
+        self.add_permits_locked(added, self.waiters.lock().unwrap());
+    }
+
+    /// Closes the semaphore. This prevents the semaphore from issuing new
+    /// permits and notifies all pending waiters.
+    // This will be used once the bounded MPSC is updated to use the new
+    // semaphore implementation.
+    #[allow(dead_code)]
+    pub(crate) fn close(&self) {
+        let mut waiters = self.waiters.lock().unwrap();
+        // If the semaphore's permits counter has enough permits for an
+        // unqueued waiter to acquire all the permits it needs immediately,
+        // it won't touch the wait list. Therefore, we have to set a bit on
+        // the permit counter as well. However, we must do this while
+        // holding the lock --- otherwise, if we set the bit and then wait
+        // to acquire the lock we'll enter an inconsistent state where the
+        // permit counter is closed, but the wait list is not.
+        self.permits.fetch_or(Self::CLOSED, Release);
+        waiters.closed = true;
+        while let Some(mut waiter) = waiters.queue.pop_back() {
+            let waker = unsafe { waiter.as_mut().waker.with_mut(|waker| (*waker).take()) };
+            if let Some(waker) = waker {
+                waker.wake();
+            }
+        }
+    }
+
+    pub(crate) fn try_acquire(&self, num_permits: u16) -> Result<(), TryAcquireError> {
+        let mut curr = self.permits.load(Acquire);
+        let num_permits = (num_permits as usize) << Self::PERMIT_SHIFT;
+        loop {
+            // Has the semaphore closed?git
+            if curr & Self::CLOSED > 0 {
+                return Err(TryAcquireError::Closed);
+            }
+
+            // Are there enough permits remaining?
+            if curr < num_permits {
+                return Err(TryAcquireError::NoPermits);
+            }
+
+            let next = curr - num_permits;
+
+            match self.permits.compare_exchange(curr, next, AcqRel, Acquire) {
+                Ok(_) => return Ok(()),
+                Err(actual) => curr = actual,
+            }
+        }
+    }
+
+    pub(crate) fn acquire(&self, num_permits: u16) -> Acquire<'_> {
+        Acquire::new(self, num_permits)
+    }
+
+    /// Release `rem` permits to the semaphore's wait list, starting from the
+    /// end of the queue.
+    ///  
+    /// If `rem` exceeds the number of permits needed by the wait list, the
+    /// remainder are assigned back to the semaphore.
+    fn add_permits_locked(&self, mut rem: usize, waiters: MutexGuard<'_, Waitlist>) {
+        let mut wakers: [Option<Waker>; 8] = Default::default();
+        let mut lock = Some(waiters);
+        let mut is_empty = false;
+        while rem > 0 {
+            let mut waiters = lock.take().unwrap_or_else(|| self.waiters.lock().unwrap());
+            'inner: for slot in &mut wakers[..] {
+                // Was the waiter assigned enough permits to wake it?
+                match waiters.queue.last() {
+                    Some(waiter) => {
+                        if !waiter.assign_permits(&mut rem) {
+                            break 'inner;
+                        }
+                    }
+                    None => {
+                        is_empty = true;
+                        // If we assigned permits to all the waiters in the queue, and there are
+                        // still permits left over, assign them back to the semaphore.
+                        break 'inner;
+                    }
+                };
+                let mut waiter = waiters.queue.pop_back().unwrap();
+                *slot = unsafe { waiter.as_mut().waker.with_mut(|waker| (*waker).take()) };
+            }
+
+            if rem > 0 && is_empty {
+                let permits = rem << Self::PERMIT_SHIFT;
+                assert!(
+                    permits < Self::MAX_PERMITS,
+                    "cannot add more than MAX_PERMITS permits ({})",
+                    Self::MAX_PERMITS
+                );
+                let prev = self.permits.fetch_add(rem << Self::PERMIT_SHIFT, Release);
+                assert!(
+                    prev + permits <= Self::MAX_PERMITS,
+                    "number of added permits ({}) would overflow MAX_PERMITS ({})",
+                    rem,
+                    Self::MAX_PERMITS
+                );
+                rem = 0;
+            }
+
+            drop(waiters); // release the lock
+
+            wakers
+                .iter_mut()
+                .filter_map(Option::take)
+                .for_each(Waker::wake);
+        }
+
+        assert_eq!(rem, 0);
+    }
+
+    fn poll_acquire(
+        &self,
+        cx: &mut Context<'_>,
+        num_permits: u16,
+        node: Pin<&mut Waiter>,
+        queued: bool,
+    ) -> Poll<Result<(), AcquireError>> {
+        let mut acquired = 0;
+
+        let needed = if queued {
+            node.state.load(Acquire) << Self::PERMIT_SHIFT
+        } else {
+            (num_permits as usize) << Self::PERMIT_SHIFT
+        };
+
+        let mut lock = None;
+        // First, try to take the requested number of permits from the
+        // semaphore.
+        let mut curr = self.permits.load(Acquire);
+        let mut waiters = loop {
+            // Has the semaphore closed?
+            if curr & Self::CLOSED > 0 {
+                return Ready(Err(AcquireError::closed()));
+            }
+
+            let mut remaining = 0;
+            let total = curr
+                .checked_add(acquired)
+                .expect("number of permits must not overflow");
+            let (next, acq) = if total >= needed {
+                let next = curr - (needed - acquired);
+                (next, needed >> Self::PERMIT_SHIFT)
+            } else {
+                remaining = (needed - acquired) - curr;
+                (0, curr >> Self::PERMIT_SHIFT)
+            };
+
+            if remaining > 0 && lock.is_none() {
+                // No permits were immediately available, so this permit will
+                // (probably) need to wait. We'll need to acquire a lock on the
+                // wait queue before continuing. We need to do this _before_ the
+                // CAS that sets the new value of the semaphore's `permits`
+                // counter. Otherwise, if we subtract the permits and then
+                // acquire the lock, we might miss additional permits being
+                // added while waiting for the lock.
+                lock = Some(self.waiters.lock().unwrap());
+            }
+
+            match self.permits.compare_exchange(curr, next, AcqRel, Acquire) {
+                Ok(_) => {
+                    acquired += acq;
+                    if remaining == 0 {
+                        if !queued {
+                            return Ready(Ok(()));
+                        } else if lock.is_none() {
+                            break self.waiters.lock().unwrap();
+                        }
+                    }
+                    break lock.expect("lock must be acquired before waiting");
+                }
+                Err(actual) => curr = actual,
+            }
+        };
+
+        if waiters.closed {
+            return Ready(Err(AcquireError::closed()));
+        }
+
+        if node.assign_permits(&mut acquired) {
+            self.add_permits_locked(acquired, waiters);
+            return Ready(Ok(()));
+        }
+
+        assert_eq!(acquired, 0);
+
+        // Otherwise, register the waker & enqueue the node.
+        node.waker.with_mut(|waker| {
+            // Safety: the wait list is locked, so we may modify the waker.
+            let waker = unsafe { &mut *waker };
+            // Do we need to register the new waker?
+            if waker
+                .as_ref()
+                .map(|waker| !waker.will_wake(cx.waker()))
+                .unwrap_or(true)
+            {
+                *waker = Some(cx.waker().clone());
+            }
+        });
+
+        // If the waiter is not already in the wait queue, enqueue it.
+        if !queued {
+            let node = unsafe {
+                let node = Pin::into_inner_unchecked(node) as *mut _;
+                NonNull::new_unchecked(node)
+            };
+
+            waiters.queue.push_front(node);
+        }
+
+        Pending
+    }
+}
+
+impl fmt::Debug for Semaphore {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("Semaphore")
+            .field("permits", &self.permits.load(Relaxed))
+            .finish()
+    }
+}
+
+impl Waiter {
+    fn new(num_permits: u16) -> Self {
+        Waiter {
+            waker: UnsafeCell::new(None),
+            state: AtomicUsize::new(num_permits as usize),
+            pointers: linked_list::Pointers::new(),
+            _p: PhantomPinned,
+        }
+    }
+
+    /// Assign permits to the waiter.
+    ///
+    /// Returns `true` if the waiter should be removed from the queue
+    fn assign_permits(&self, n: &mut usize) -> bool {
+        let mut curr = self.state.load(Acquire);
+        loop {
+            let assign = cmp::min(curr, *n);
+            let next = curr - assign;
+            match self.state.compare_exchange(curr, next, AcqRel, Acquire) {
+                Ok(_) => {
+                    *n -= assign;
+                    return next == 0;
+                }
+                Err(actual) => curr = actual,
+            }
+        }
+    }
+}
+
+impl Future for Acquire<'_> {
+    type Output = Result<(), AcquireError>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let (node, semaphore, needed, queued) = self.project();
+        match semaphore.poll_acquire(cx, needed, node, *queued) {
+            Pending => {
+                *queued = true;
+                Pending
+            }
+            Ready(r) => {
+                r?;
+                *queued = false;
+                Ready(Ok(()))
+            }
+        }
+    }
+}
+
+impl<'a> Acquire<'a> {
+    fn new(semaphore: &'a Semaphore, num_permits: u16) -> Self {
+        Self {
+            node: Waiter::new(num_permits),
+            semaphore,
+            num_permits,
+            queued: false,
+        }
+    }
+
+    fn project(self: Pin<&mut Self>) -> (Pin<&mut Waiter>, &Semaphore, u16, &mut bool) {
+        fn is_unpin<T: Unpin>() {}
+        unsafe {
+            // Safety: all fields other than `node` are `Unpin`
+
+            is_unpin::<&Semaphore>();
+            is_unpin::<&mut bool>();
+            is_unpin::<u16>();
+
+            let this = self.get_unchecked_mut();
+            (
+                Pin::new_unchecked(&mut this.node),
+                &this.semaphore,
+                this.num_permits,
+                &mut this.queued,
+            )
+        }
+    }
+}
+
+impl Drop for Acquire<'_> {
+    fn drop(&mut self) {
+        // If the future is completed, there is no node in the wait list, so we
+        // can skip acquiring the lock.
+        if !self.queued {
+            return;
+        }
+
+        // This is where we ensure safety. The future is being dropped,
+        // which means we must ensure that the waiter entry is no longer stored
+        // in the linked list.
+        let mut waiters = match self.semaphore.waiters.lock() {
+            Ok(lock) => lock,
+            // Removing the node from the linked list is necessary to ensure
+            // safety. Even if the lock was poisoned, we need to make sure it is
+            // removed from the linked list before dropping it --- otherwise,
+            // the list will contain a dangling pointer to this node.
+            Err(e) => e.into_inner(),
+        };
+
+        // remove the entry from the list
+        let node = NonNull::from(&mut self.node);
+        // Safety: we have locked the wait list.
+        unsafe { waiters.queue.remove(node) };
+
+        let acquired_permits = self.num_permits as usize - self.node.state.load(Acquire);
+        if acquired_permits > 0 {
+            self.semaphore.add_permits_locked(acquired_permits, waiters);
+        }
+    }
+}
+
+// Safety: the `Acquire` future is not `Sync` automatically because it contains
+// a `Waiter`, which, in turn, contains an `UnsafeCell`. However, the
+// `UnsafeCell` is only accessed when the future is borrowed mutably (either in
+// `poll` or in `drop`). Therefore, it is safe (although not particularly
+// _useful_) for the future to be borrowed immutably across threads.
+unsafe impl Sync for Acquire<'_> {}
+
+// ===== impl AcquireError ====
+
+impl AcquireError {
+    fn closed() -> AcquireError {
+        AcquireError(())
+    }
+}
+
+impl fmt::Display for AcquireError {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(fmt, "semaphore closed")
+    }
+}
+
+impl std::error::Error for AcquireError {}
+
+// ===== impl TryAcquireError =====
+
+impl TryAcquireError {
+    /// Returns `true` if the error was caused by a closed semaphore.
+    #[allow(dead_code)] // may be used later!
+    pub(crate) fn is_closed(&self) -> bool {
+        match self {
+            TryAcquireError::Closed => true,
+            _ => false,
+        }
+    }
+
+    /// Returns `true` if the error was caused by calling `try_acquire` on a
+    /// semaphore with no available permits.
+    #[allow(dead_code)] // may be used later!
+    pub(crate) fn is_no_permits(&self) -> bool {
+        match self {
+            TryAcquireError::NoPermits => true,
+            _ => false,
+        }
+    }
+}
+
+impl fmt::Display for TryAcquireError {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            TryAcquireError::Closed => write!(fmt, "{}", "semaphore closed"),
+            TryAcquireError::NoPermits => write!(fmt, "{}", "no permits available"),
+        }
+    }
+}
+
+impl std::error::Error for TryAcquireError {}
+
+/// # Safety
+///
+/// `Waiter` is forced to be !Unpin.
+unsafe impl linked_list::Link for Waiter {
+    // XXX: ideally, we would be able to use `Pin` here, to enforce the
+    // invariant that list entries may not move while in the list. However, we
+    // can't do this currently, as using `Pin<&'a mut Waiter>` as the `Handle`
+    // type would require `Semaphore` to be generic over a lifetime. We can't
+    // use `Pin<*mut Waiter>`, as raw pointers are `Unpin` regardless of whether
+    // or not they dereference to an `!Unpin` target.
+    type Handle = NonNull<Waiter>;
+    type Target = Waiter;
+
+    fn as_raw(handle: &Self::Handle) -> NonNull<Waiter> {
+        *handle
+    }
+
+    unsafe fn from_raw(ptr: NonNull<Waiter>) -> NonNull<Waiter> {
+        ptr
+    }
+
+    unsafe fn pointers(mut target: NonNull<Waiter>) -> NonNull<linked_list::Pointers<Waiter>> {
+        NonNull::from(&mut target.as_mut().pointers)
+    }
+}
diff --git a/third_party/rust/tokio/src/sync/broadcast.rs b/third_party/rust/tokio/src/sync/broadcast.rs
new file mode 100644
index 0000000000..05a58070ee
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/broadcast.rs
@@ -0,0 +1,1046 @@
+//! A multi-producer, multi-consumer broadcast queue. Each sent value is seen by
+//! all consumers.
+//!
+//! A [`Sender`] is used to broadcast values to **all** connected [`Receiver`]
+//! values. [`Sender`] handles are clone-able, allowing concurrent send and
+//! receive actions. [`Sender`] and [`Receiver`] are both `Send` and `Sync` as
+//! long as `T` is also `Send` or `Sync` respectively.
+//!
+//! When a value is sent, **all** [`Receiver`] handles are notified and will
+//! receive the value. The value is stored once inside the channel and cloned on
+//! demand for each receiver. Once all receivers have received a clone of the
+//! value, the value is released from the channel.
+//!
+//! A channel is created by calling [`channel`], specifying the maximum number
+//! of messages the channel can retain at any given time.
+//!
+//! New [`Receiver`] handles are created by calling [`Sender::subscribe`]. The
+//! returned [`Receiver`] will receive values sent **after** the call to
+//! `subscribe`.
+//!
+//! ## Lagging
+//!
+//! As sent messages must be retained until **all** [`Receiver`] handles receive
+//! a clone, broadcast channels are suspectible to the "slow receiver" problem.
+//! In this case, all but one receiver are able to receive values at the rate
+//! they are sent. Because one receiver is stalled, the channel starts to fill
+//! up.
+//!
+//! This broadcast channel implementation handles this case by setting a hard
+//! upper bound on the number of values the channel may retain at any given
+//! time. This upper bound is passed to the [`channel`] function as an argument.
+//!
+//! If a value is sent when the channel is at capacity, the oldest value
+//! currently held by the channel is released. This frees up space for the new
+//! value. Any receiver that has not yet seen the released value will return
+//! [`RecvError::Lagged`] the next time [`recv`] is called.
+//!
+//! Once [`RecvError::Lagged`] is returned, the lagging receiver's position is
+//! updated to the oldest value contained by the channel. The next call to
+//! [`recv`] will return this value.
+//!
+//! This behavior enables a receiver to detect when it has lagged so far behind
+//! that data has been dropped. The caller may decide how to respond to this:
+//! either by aborting its task or by tolerating lost messages and resuming
+//! consumption of the channel.
+//!
+//! ## Closing
+//!
+//! When **all** [`Sender`] handles have been dropped, no new values may be
+//! sent. At this point, the channel is "closed". Once a receiver has received
+//! all values retained by the channel, the next call to [`recv`] will return
+//! with [`RecvError::Closed`].
+//!
+//! [`Sender`]: crate::sync::broadcast::Sender
+//! [`Sender::subscribe`]: crate::sync::broadcast::Sender::subscribe
+//! [`Receiver`]: crate::sync::broadcast::Receiver
+//! [`channel`]: crate::sync::broadcast::channel
+//! [`RecvError::Lagged`]: crate::sync::broadcast::RecvError::Lagged
+//! [`RecvError::Closed`]: crate::sync::broadcast::RecvError::Closed
+//! [`recv`]: crate::sync::broadcast::Receiver::recv
+//!
+//! # Examples
+//!
+//! Basic usage
+//!
+//! ```
+//! use tokio::sync::broadcast;
+//!
+//! #[tokio::main]
+//! async fn main() {
+//!     let (tx, mut rx1) = broadcast::channel(16);
+//!     let mut rx2 = tx.subscribe();
+//!
+//!     tokio::spawn(async move {
+//!         assert_eq!(rx1.recv().await.unwrap(), 10);
+//!         assert_eq!(rx1.recv().await.unwrap(), 20);
+//!     });
+//!
+//!     tokio::spawn(async move {
+//!         assert_eq!(rx2.recv().await.unwrap(), 10);
+//!         assert_eq!(rx2.recv().await.unwrap(), 20);
+//!     });
+//!
+//!     tx.send(10).unwrap();
+//!     tx.send(20).unwrap();
+//! }
+//! ```
+//!
+//! Handling lag
+//!
+//! ```
+//! use tokio::sync::broadcast;
+//!
+//! #[tokio::main]
+//! async fn main() {
+//!     let (tx, mut rx) = broadcast::channel(2);
+//!
+//!     tx.send(10).unwrap();
+//!     tx.send(20).unwrap();
+//!     tx.send(30).unwrap();
+//!
+//!     // The receiver lagged behind
+//!     assert!(rx.recv().await.is_err());
+//!
+//!     // At this point, we can abort or continue with lost messages
+//!
+//!     assert_eq!(20, rx.recv().await.unwrap());
+//!     assert_eq!(30, rx.recv().await.unwrap());
+//! }
+
+use crate::loom::cell::UnsafeCell;
+use crate::loom::future::AtomicWaker;
+use crate::loom::sync::atomic::{spin_loop_hint, AtomicBool, AtomicPtr, AtomicUsize};
+use crate::loom::sync::{Arc, Condvar, Mutex};
+
+use std::fmt;
+use std::mem;
+use std::ptr;
+use std::sync::atomic::Ordering::SeqCst;
+use std::task::{Context, Poll, Waker};
+use std::usize;
+
+/// Sending-half of the [`broadcast`] channel.
+///
+/// May be used from many threads. Messages can be sent with
+/// [`send`][Sender::send].
+///
+/// # Examples
+///
+/// ```
+/// use tokio::sync::broadcast;
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let (tx, mut rx1) = broadcast::channel(16);
+///     let mut rx2 = tx.subscribe();
+///
+///     tokio::spawn(async move {
+///         assert_eq!(rx1.recv().await.unwrap(), 10);
+///         assert_eq!(rx1.recv().await.unwrap(), 20);
+///     });
+///
+///     tokio::spawn(async move {
+///         assert_eq!(rx2.recv().await.unwrap(), 10);
+///         assert_eq!(rx2.recv().await.unwrap(), 20);
+///     });
+///
+///     tx.send(10).unwrap();
+///     tx.send(20).unwrap();
+/// }
+/// ```
+///
+/// [`broadcast`]: crate::sync::broadcast
+pub struct Sender<T> {
+    shared: Arc<Shared<T>>,
+}
+
+/// Receiving-half of the [`broadcast`] channel.
+///
+/// Must not be used concurrently. Messages may be retrieved using
+/// [`recv`][Receiver::recv].
+///
+/// # Examples
+///
+/// ```
+/// use tokio::sync::broadcast;
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let (tx, mut rx1) = broadcast::channel(16);
+///     let mut rx2 = tx.subscribe();
+///
+///     tokio::spawn(async move {
+///         assert_eq!(rx1.recv().await.unwrap(), 10);
+///         assert_eq!(rx1.recv().await.unwrap(), 20);
+///     });
+///
+///     tokio::spawn(async move {
+///         assert_eq!(rx2.recv().await.unwrap(), 10);
+///         assert_eq!(rx2.recv().await.unwrap(), 20);
+///     });
+///
+///     tx.send(10).unwrap();
+///     tx.send(20).unwrap();
+/// }
+/// ```
+///
+/// [`broadcast`]: crate::sync::broadcast
+pub struct Receiver<T> {
+    /// State shared with all receivers and senders.
+    shared: Arc<Shared<T>>,
+
+    /// Next position to read from
+    next: u64,
+
+    /// Waiter state
+    wait: Arc<WaitNode>,
+}
+
+/// Error returned by [`Sender::send`][Sender::send].
+///
+/// A **send** operation can only fail if there are no active receivers,
+/// implying that the message could never be received. The error contains the
+/// message being sent as a payload so it can be recovered.
+#[derive(Debug)]
+pub struct SendError<T>(pub T);
+
+/// An error returned from the [`recv`] function on a [`Receiver`].
+///
+/// [`recv`]: crate::sync::broadcast::Receiver::recv
+/// [`Receiver`]: crate::sync::broadcast::Receiver
+#[derive(Debug, PartialEq)]
+pub enum RecvError {
+    /// There are no more active senders implying no further messages will ever
+    /// be sent.
+    Closed,
+
+    /// The receiver lagged too far behind. Attempting to receive again will
+    /// return the oldest message still retained by the channel.
+    ///
+    /// Includes the number of skipped messages.
+    Lagged(u64),
+}
+
+/// An error returned from the [`try_recv`] function on a [`Receiver`].
+///
+/// [`try_recv`]: crate::sync::broadcast::Receiver::try_recv
+/// [`Receiver`]: crate::sync::broadcast::Receiver
+#[derive(Debug, PartialEq)]
+pub enum TryRecvError {
+    /// The channel is currently empty. There are still active
+    /// [`Sender`][Sender] handles, so data may yet become available.
+    Empty,
+
+    /// There are no more active senders implying no further messages will ever
+    /// be sent.
+    Closed,
+
+    /// The receiver lagged too far behind and has been forcibly disconnected.
+    /// Attempting to receive again will return the oldest message still
+    /// retained by the channel.
+    ///
+    /// Includes the number of skipped messages.
+    Lagged(u64),
+}
+
+/// Data shared between senders and receivers
+struct Shared<T> {
+    /// slots in the channel
+    buffer: Box<[Slot<T>]>,
+
+    /// Mask a position -> index
+    mask: usize,
+
+    /// Tail of the queue
+    tail: Mutex<Tail>,
+
+    /// Notifies a sender that the slot is unlocked
+    condvar: Condvar,
+
+    /// Stack of pending waiters
+    wait_stack: AtomicPtr<WaitNode>,
+
+    /// Number of outstanding Sender handles
+    num_tx: AtomicUsize,
+}
+
+/// Next position to write a value
+struct Tail {
+    /// Next position to write to
+    pos: u64,
+
+    /// Number of active receivers
+    rx_cnt: usize,
+}
+
+/// Slot in the buffer
+struct Slot<T> {
+    /// Remaining number of receivers that are expected to see this value.
+    ///
+    /// When this goes to zero, the value is released.
+    rem: AtomicUsize,
+
+    /// Used to lock the `write` field.
+    lock: AtomicUsize,
+
+    /// The value being broadcast
+    ///
+    /// Synchronized by `state`
+    write: Write<T>,
+}
+
+/// A write in the buffer
+struct Write<T> {
+    /// Uniquely identifies this write
+    pos: UnsafeCell<u64>,
+
+    /// The written value
+    val: UnsafeCell<Option<T>>,
+}
+
+/// Tracks a waiting receiver
+#[derive(Debug)]
+struct WaitNode {
+    /// `true` if queued
+    queued: AtomicBool,
+
+    /// Task to wake when a permit is made available.
+    waker: AtomicWaker,
+
+    /// Next pointer in the stack of waiting senders.
+    next: UnsafeCell<*const WaitNode>,
+}
+
+struct RecvGuard<'a, T> {
+    slot: &'a Slot<T>,
+    tail: &'a Mutex<Tail>,
+    condvar: &'a Condvar,
+}
+
+/// Max number of receivers. Reserve space to lock.
+const MAX_RECEIVERS: usize = usize::MAX >> 1;
+
+/// Create a bounded, multi-producer, multi-consumer channel where each sent
+/// value is broadcasted to all active receivers.
+///
+/// All data sent on [`Sender`] will become available on every active
+/// [`Receiver`] in the same order as it was sent.
+///
+/// The `Sender` can be cloned to `send` to the same channel from multiple
+/// points in the process or it can be used concurrently from an `Arc`. New
+/// `Receiver` handles are created by calling [`Sender::subscribe`].
+///
+/// If all [`Receiver`] handles are dropped, the `send` method will return a
+/// [`SendError`]. Similarly, if all [`Sender`] handles are dropped, the [`recv`]
+/// method will return a [`RecvError`].
+///
+/// [`Sender`]: crate::sync::broadcast::Sender
+/// [`Sender::subscribe`]: crate::sync::broadcast::Sender::subscribe
+/// [`Receiver`]: crate::sync::broadcast::Receiver
+/// [`recv`]: crate::sync::broadcast::Receiver::recv
+/// [`SendError`]: crate::sync::broadcast::SendError
+/// [`RecvError`]: crate::sync::broadcast::RecvError
+///
+/// # Examples
+///
+/// ```
+/// use tokio::sync::broadcast;
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let (tx, mut rx1) = broadcast::channel(16);
+///     let mut rx2 = tx.subscribe();
+///
+///     tokio::spawn(async move {
+///         assert_eq!(rx1.recv().await.unwrap(), 10);
+///         assert_eq!(rx1.recv().await.unwrap(), 20);
+///     });
+///
+///     tokio::spawn(async move {
+///         assert_eq!(rx2.recv().await.unwrap(), 10);
+///         assert_eq!(rx2.recv().await.unwrap(), 20);
+///     });
+///
+///     tx.send(10).unwrap();
+///     tx.send(20).unwrap();
+/// }
+/// ```
+pub fn channel<T>(mut capacity: usize) -> (Sender<T>, Receiver<T>) {
+    assert!(capacity > 0, "capacity is empty");
+    assert!(capacity <= usize::MAX >> 1, "requested capacity too large");
+
+    // Round to a power of two
+    capacity = capacity.next_power_of_two();
+
+    let mut buffer = Vec::with_capacity(capacity);
+
+    for i in 0..capacity {
+        buffer.push(Slot {
+            rem: AtomicUsize::new(0),
+            lock: AtomicUsize::new(0),
+            write: Write {
+                pos: UnsafeCell::new((i as u64).wrapping_sub(capacity as u64)),
+                val: UnsafeCell::new(None),
+            },
+        });
+    }
+
+    let shared = Arc::new(Shared {
+        buffer: buffer.into_boxed_slice(),
+        mask: capacity - 1,
+        tail: Mutex::new(Tail { pos: 0, rx_cnt: 1 }),
+        condvar: Condvar::new(),
+        wait_stack: AtomicPtr::new(ptr::null_mut()),
+        num_tx: AtomicUsize::new(1),
+    });
+
+    let rx = Receiver {
+        shared: shared.clone(),
+        next: 0,
+        wait: Arc::new(WaitNode {
+            queued: AtomicBool::new(false),
+            waker: AtomicWaker::new(),
+            next: UnsafeCell::new(ptr::null()),
+        }),
+    };
+
+    let tx = Sender { shared };
+
+    (tx, rx)
+}
+
+unsafe impl<T: Send> Send for Sender<T> {}
+unsafe impl<T: Send> Sync for Sender<T> {}
+
+unsafe impl<T: Send> Send for Receiver<T> {}
+unsafe impl<T: Send> Sync for Receiver<T> {}
+
+impl<T> Sender<T> {
+    /// Attempts to send a value to all active [`Receiver`] handles, returning
+    /// it back if it could not be sent.
+    ///
+    /// A successful send occurs when there is at least one active [`Receiver`]
+    /// handle. An unsuccessful send would be one where all associated
+    /// [`Receiver`] handles have already been dropped.
+    ///
+    /// # Return
+    ///
+    /// On success, the number of subscribed [`Receiver`] handles is returned.
+    /// This does not mean that this number of receivers will see the message as
+    /// a receiver may drop before receiving the message.
+    ///
+    /// # Note
+    ///
+    /// A return value of `Ok` **does not** mean that the sent value will be
+    /// observed by all or any of the active [`Receiver`] handles. [`Receiver`]
+    /// handles may be dropped before receiving the sent message.
+    ///
+    /// A return value of `Err` **does not** mean that future calls to `send`
+    /// will fail. New [`Receiver`] handles may be created by calling
+    /// [`subscribe`].
+    ///
+    /// [`Receiver`]: crate::sync::broadcast::Receiver
+    /// [`subscribe`]: crate::sync::broadcast::Sender::subscribe
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::broadcast;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, mut rx1) = broadcast::channel(16);
+    ///     let mut rx2 = tx.subscribe();
+    ///
+    ///     tokio::spawn(async move {
+    ///         assert_eq!(rx1.recv().await.unwrap(), 10);
+    ///         assert_eq!(rx1.recv().await.unwrap(), 20);
+    ///     });
+    ///
+    ///     tokio::spawn(async move {
+    ///         assert_eq!(rx2.recv().await.unwrap(), 10);
+    ///         assert_eq!(rx2.recv().await.unwrap(), 20);
+    ///     });
+    ///
+    ///     tx.send(10).unwrap();
+    ///     tx.send(20).unwrap();
+    /// }
+    /// ```
+    pub fn send(&self, value: T) -> Result<usize, SendError<T>> {
+        self.send2(Some(value))
+            .map_err(|SendError(maybe_v)| SendError(maybe_v.unwrap()))
+    }
+
+    /// Creates a new [`Receiver`] handle that will receive values sent **after**
+    /// this call to `subscribe`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::broadcast;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, _rx) = broadcast::channel(16);
+    ///
+    ///     // Will not be seen
+    ///     tx.send(10).unwrap();
+    ///
+    ///     let mut rx = tx.subscribe();
+    ///
+    ///     tx.send(20).unwrap();
+    ///
+    ///     let value = rx.recv().await.unwrap();
+    ///     assert_eq!(20, value);
+    /// }
+    /// ```
+    pub fn subscribe(&self) -> Receiver<T> {
+        let shared = self.shared.clone();
+
+        let mut tail = shared.tail.lock().unwrap();
+
+        if tail.rx_cnt == MAX_RECEIVERS {
+            panic!("max receivers");
+        }
+
+        tail.rx_cnt = tail.rx_cnt.checked_add(1).expect("overflow");
+        let next = tail.pos;
+
+        drop(tail);
+
+        Receiver {
+            shared,
+            next,
+            wait: Arc::new(WaitNode {
+                queued: AtomicBool::new(false),
+                waker: AtomicWaker::new(),
+                next: UnsafeCell::new(ptr::null()),
+            }),
+        }
+    }
+
+    /// Returns the number of active receivers
+    ///
+    /// An active receiver is a [`Receiver`] handle returned from [`channel`] or
+    /// [`subscribe`]. These are the handles that will receive values sent on
+    /// this [`Sender`].
+    ///
+    /// # Note
+    ///
+    /// It is not guaranteed that a sent message will reach this number of
+    /// receivers. Active receivers may never call [`recv`] again before
+    /// dropping.
+    ///
+    /// [`recv`]: crate::sync::broadcast::Receiver::recv
+    /// [`Receiver`]: crate::sync::broadcast::Receiver
+    /// [`Sender`]: crate::sync::broadcast::Sender
+    /// [`subscribe`]: crate::sync::broadcast::Sender::subscribe
+    /// [`channel`]: crate::sync::broadcast::channel
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::broadcast;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, _rx1) = broadcast::channel(16);
+    ///
+    ///     assert_eq!(1, tx.receiver_count());
+    ///
+    ///     let mut _rx2 = tx.subscribe();
+    ///
+    ///     assert_eq!(2, tx.receiver_count());
+    ///
+    ///     tx.send(10).unwrap();
+    /// }
+    /// ```
+    pub fn receiver_count(&self) -> usize {
+        let tail = self.shared.tail.lock().unwrap();
+        tail.rx_cnt
+    }
+
+    fn send2(&self, value: Option<T>) -> Result<usize, SendError<Option<T>>> {
+        let mut tail = self.shared.tail.lock().unwrap();
+
+        if tail.rx_cnt == 0 {
+            return Err(SendError(value));
+        }
+
+        // Position to write into
+        let pos = tail.pos;
+        let rem = tail.rx_cnt;
+        let idx = (pos & self.shared.mask as u64) as usize;
+
+        // Update the tail position
+        tail.pos = tail.pos.wrapping_add(1);
+
+        // Get the slot
+        let slot = &self.shared.buffer[idx];
+
+        // Acquire the write lock
+        let mut prev = slot.lock.fetch_or(1, SeqCst);
+
+        while prev & !1 != 0 {
+            // Concurrent readers, we must go to sleep
+            tail = self.shared.condvar.wait(tail).unwrap();
+
+            prev = slot.lock.load(SeqCst);
+
+            if prev & 1 == 0 {
+                // The writer lock bit was cleared while this thread was
+                // sleeping. This can only happen if a newer write happened on
+                // this slot by another thread. Bail early as an optimization,
+                // there is nothing left to do.
+                return Ok(rem);
+            }
+        }
+
+        if tail.pos.wrapping_sub(pos) > self.shared.buffer.len() as u64 {
+            // There is a newer pending write to the same slot.
+            return Ok(rem);
+        }
+
+        // Slot lock acquired
+        slot.write.pos.with_mut(|ptr| unsafe { *ptr = pos });
+        slot.write.val.with_mut(|ptr| unsafe { *ptr = value });
+
+        // Set remaining receivers
+        slot.rem.store(rem, SeqCst);
+
+        // Release the slot lock
+        slot.lock.store(0, SeqCst);
+
+        // Release the mutex. This must happen after the slot lock is released,
+        // otherwise the writer lock bit could be cleared while another thread
+        // is in the critical section.
+        drop(tail);
+
+        // Notify waiting receivers
+        self.notify_rx();
+
+        Ok(rem)
+    }
+
+    fn notify_rx(&self) {
+        let mut curr = self.shared.wait_stack.swap(ptr::null_mut(), SeqCst) as *const WaitNode;
+
+        while !curr.is_null() {
+            let waiter = unsafe { Arc::from_raw(curr) };
+
+            // Update `curr` before toggling `queued` and waking
+            curr = waiter.next.with(|ptr| unsafe { *ptr });
+
+            // Unset queued
+            waiter.queued.store(false, SeqCst);
+
+            // Wake
+            waiter.waker.wake();
+        }
+    }
+}
+
+impl<T> Clone for Sender<T> {
+    fn clone(&self) -> Sender<T> {
+        let shared = self.shared.clone();
+        shared.num_tx.fetch_add(1, SeqCst);
+
+        Sender { shared }
+    }
+}
+
+impl<T> Drop for Sender<T> {
+    fn drop(&mut self) {
+        if 1 == self.shared.num_tx.fetch_sub(1, SeqCst) {
+            let _ = self.send2(None);
+        }
+    }
+}
+
+impl<T> Receiver<T> {
+    /// Locks the next value if there is one.
+    ///
+    /// The caller is responsible for unlocking
+    fn recv_ref(&mut self, spin: bool) -> Result<RecvGuard<'_, T>, TryRecvError> {
+        let idx = (self.next & self.shared.mask as u64) as usize;
+
+        // The slot holding the next value to read
+        let slot = &self.shared.buffer[idx];
+
+        // Lock the slot
+        if !slot.try_rx_lock() {
+            if spin {
+                while !slot.try_rx_lock() {
+                    spin_loop_hint();
+                }
+            } else {
+                return Err(TryRecvError::Empty);
+            }
+        }
+
+        let guard = RecvGuard {
+            slot,
+            tail: &self.shared.tail,
+            condvar: &self.shared.condvar,
+        };
+
+        if guard.pos() != self.next {
+            let pos = guard.pos();
+
+            guard.drop_no_rem_dec();
+
+            if pos.wrapping_add(self.shared.buffer.len() as u64) == self.next {
+                return Err(TryRecvError::Empty);
+            } else {
+                let tail = self.shared.tail.lock().unwrap();
+
+                // `tail.pos` points to the slot the **next** send writes to.
+                // Because a receiver is lagging, this slot also holds the
+                // oldest value. To make the positions match, we subtract the
+                // capacity.
+                let next = tail.pos.wrapping_sub(self.shared.buffer.len() as u64);
+                let missed = next.wrapping_sub(self.next);
+
+                self.next = next;
+
+                return Err(TryRecvError::Lagged(missed));
+            }
+        }
+
+        self.next = self.next.wrapping_add(1);
+
+        Ok(guard)
+    }
+}
+
+impl<T> Receiver<T>
+where
+    T: Clone,
+{
+    /// Attempts to return a pending value on this receiver without awaiting.
+    ///
+    /// This is useful for a flavor of "optimistic check" before deciding to
+    /// await on a receiver.
+    ///
+    /// Compared with [`recv`], this function has three failure cases instead of one
+    /// (one for closed, one for an empty buffer, one for a lagging receiver).
+    ///
+    /// `Err(TryRecvError::Closed)` is returned when all `Sender` halves have
+    /// dropped, indicating that no further values can be sent on the channel.
+    ///
+    /// If the [`Receiver`] handle falls behind, once the channel is full, newly
+    /// sent values will overwrite old values. At this point, a call to [`recv`]
+    /// will return with `Err(TryRecvError::Lagged)` and the [`Receiver`]'s
+    /// internal cursor is updated to point to the oldest value still held by
+    /// the channel. A subsequent call to [`try_recv`] will return this value
+    /// **unless** it has been since overwritten. If there are no values to
+    /// receive, `Err(TryRecvError::Empty)` is returned.
+    ///
+    /// [`recv`]: crate::sync::broadcast::Receiver::recv
+    /// [`Receiver`]: crate::sync::broadcast::Receiver
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::broadcast;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, mut rx) = broadcast::channel(16);
+    ///
+    ///     assert!(rx.try_recv().is_err());
+    ///
+    ///     tx.send(10).unwrap();
+    ///
+    ///     let value = rx.try_recv().unwrap();
+    ///     assert_eq!(10, value);
+    /// }
+    /// ```
+    pub fn try_recv(&mut self) -> Result<T, TryRecvError> {
+        let guard = self.recv_ref(false)?;
+        guard.clone_value().ok_or(TryRecvError::Closed)
+    }
+
+    #[doc(hidden)] // TODO: document
+    pub fn poll_recv(&mut self, cx: &mut Context<'_>) -> Poll<Result<T, RecvError>> {
+        if let Some(value) = ok_empty(self.try_recv())? {
+            return Poll::Ready(Ok(value));
+        }
+
+        self.register_waker(cx.waker());
+
+        if let Some(value) = ok_empty(self.try_recv())? {
+            Poll::Ready(Ok(value))
+        } else {
+            Poll::Pending
+        }
+    }
+
+    /// Receives the next value for this receiver.
+    ///
+    /// Each [`Receiver`] handle will receive a clone of all values sent
+    /// **after** it has subscribed.
+    ///
+    /// `Err(RecvError::Closed)` is returned when all `Sender` halves have
+    /// dropped, indicating that no further values can be sent on the channel.
+    ///
+    /// If the [`Receiver`] handle falls behind, once the channel is full, newly
+    /// sent values will overwrite old values. At this point, a call to [`recv`]
+    /// will return with `Err(RecvError::Lagged)` and the [`Receiver`]'s
+    /// internal cursor is updated to point to the oldest value still held by
+    /// the channel. A subsequent call to [`recv`] will return this value
+    /// **unless** it has been since overwritten.
+    ///
+    /// [`Receiver`]: crate::sync::broadcast::Receiver
+    /// [`recv`]: crate::sync::broadcast::Receiver::recv
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::broadcast;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, mut rx1) = broadcast::channel(16);
+    ///     let mut rx2 = tx.subscribe();
+    ///
+    ///     tokio::spawn(async move {
+    ///         assert_eq!(rx1.recv().await.unwrap(), 10);
+    ///         assert_eq!(rx1.recv().await.unwrap(), 20);
+    ///     });
+    ///
+    ///     tokio::spawn(async move {
+    ///         assert_eq!(rx2.recv().await.unwrap(), 10);
+    ///         assert_eq!(rx2.recv().await.unwrap(), 20);
+    ///     });
+    ///
+    ///     tx.send(10).unwrap();
+    ///     tx.send(20).unwrap();
+    /// }
+    /// ```
+    ///
+    /// Handling lag
+    ///
+    /// ```
+    /// use tokio::sync::broadcast;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, mut rx) = broadcast::channel(2);
+    ///
+    ///     tx.send(10).unwrap();
+    ///     tx.send(20).unwrap();
+    ///     tx.send(30).unwrap();
+    ///
+    ///     // The receiver lagged behind
+    ///     assert!(rx.recv().await.is_err());
+    ///
+    ///     // At this point, we can abort or continue with lost messages
+    ///
+    ///     assert_eq!(20, rx.recv().await.unwrap());
+    ///     assert_eq!(30, rx.recv().await.unwrap());
+    /// }
+    pub async fn recv(&mut self) -> Result<T, RecvError> {
+        use crate::future::poll_fn;
+
+        poll_fn(|cx| self.poll_recv(cx)).await
+    }
+
+    fn register_waker(&self, cx: &Waker) {
+        self.wait.waker.register_by_ref(cx);
+
+        if !self.wait.queued.load(SeqCst) {
+            // Set `queued` before queuing.
+            self.wait.queued.store(true, SeqCst);
+
+            let mut curr = self.shared.wait_stack.load(SeqCst);
+
+            // The ref count is decremented in `notify_rx` when all nodes are
+            // removed from the waiter stack.
+            let node = Arc::into_raw(self.wait.clone()) as *mut _;
+
+            loop {
+                // Safety: `queued == false` means the caller has exclusive
+                // access to `self.wait.next`.
+                self.wait.next.with_mut(|ptr| unsafe { *ptr = curr });
+
+                let res = self
+                    .shared
+                    .wait_stack
+                    .compare_exchange(curr, node, SeqCst, SeqCst);
+
+                match res {
+                    Ok(_) => return,
+                    Err(actual) => curr = actual,
+                }
+            }
+        }
+    }
+}
+
+#[cfg(feature = "stream")]
+impl<T> crate::stream::Stream for Receiver<T>
+where
+    T: Clone,
+{
+    type Item = Result<T, RecvError>;
+
+    fn poll_next(
+        mut self: std::pin::Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Option<Result<T, RecvError>>> {
+        self.poll_recv(cx).map(|v| match v {
+            Ok(v) => Some(Ok(v)),
+            lag @ Err(RecvError::Lagged(_)) => Some(lag),
+            Err(RecvError::Closed) => None,
+        })
+    }
+}
+
+impl<T> Drop for Receiver<T> {
+    fn drop(&mut self) {
+        let mut tail = self.shared.tail.lock().unwrap();
+
+        tail.rx_cnt -= 1;
+        let until = tail.pos;
+
+        drop(tail);
+
+        while self.next != until {
+            match self.recv_ref(true) {
+                // Ignore the value
+                Ok(_) => {}
+                // The channel is closed
+                Err(TryRecvError::Closed) => break,
+                // Ignore lagging, we will catch up
+                Err(TryRecvError::Lagged(..)) => {}
+                // Can't be empty
+                Err(TryRecvError::Empty) => panic!("unexpected empty broadcast channel"),
+            }
+        }
+    }
+}
+
+impl<T> Drop for Shared<T> {
+    fn drop(&mut self) {
+        // Clear the wait stack
+        let mut curr = self.wait_stack.with_mut(|ptr| *ptr as *const WaitNode);
+
+        while !curr.is_null() {
+            let waiter = unsafe { Arc::from_raw(curr) };
+            curr = waiter.next.with(|ptr| unsafe { *ptr });
+        }
+    }
+}
+
+impl<T> fmt::Debug for Sender<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(fmt, "broadcast::Sender")
+    }
+}
+
+impl<T> fmt::Debug for Receiver<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(fmt, "broadcast::Receiver")
+    }
+}
+
+impl<T> Slot<T> {
+    /// Tries to lock the slot for a receiver. If `false`, then a sender holds the
+    /// lock and the calling task will be notified once the sender has released
+    /// the lock.
+    fn try_rx_lock(&self) -> bool {
+        let mut curr = self.lock.load(SeqCst);
+
+        loop {
+            if curr & 1 == 1 {
+                // Locked by sender
+                return false;
+            }
+
+            // Only increment (by 2) if the LSB "lock" bit is not set.
+            let res = self.lock.compare_exchange(curr, curr + 2, SeqCst, SeqCst);
+
+            match res {
+                Ok(_) => return true,
+                Err(actual) => curr = actual,
+            }
+        }
+    }
+
+    fn rx_unlock(&self, tail: &Mutex<Tail>, condvar: &Condvar, rem_dec: bool) {
+        if rem_dec {
+            // Decrement the remaining counter
+            if 1 == self.rem.fetch_sub(1, SeqCst) {
+                // Last receiver, drop the value
+                self.write.val.with_mut(|ptr| unsafe { *ptr = None });
+            }
+        }
+
+        if 1 == self.lock.fetch_sub(2, SeqCst) - 2 {
+            // First acquire the lock to make sure our sender is waiting on the
+            // condition variable, otherwise the notification could be lost.
+            mem::drop(tail.lock().unwrap());
+            // Wake up senders
+            condvar.notify_all();
+        }
+    }
+}
+
+impl<'a, T> RecvGuard<'a, T> {
+    fn pos(&self) -> u64 {
+        self.slot.write.pos.with(|ptr| unsafe { *ptr })
+    }
+
+    fn clone_value(&self) -> Option<T>
+    where
+        T: Clone,
+    {
+        self.slot.write.val.with(|ptr| unsafe { (*ptr).clone() })
+    }
+
+    fn drop_no_rem_dec(self) {
+        self.slot.rx_unlock(self.tail, self.condvar, false);
+
+        mem::forget(self);
+    }
+}
+
+impl<'a, T> Drop for RecvGuard<'a, T> {
+    fn drop(&mut self) {
+        self.slot.rx_unlock(self.tail, self.condvar, true)
+    }
+}
+
+fn ok_empty<T>(res: Result<T, TryRecvError>) -> Result<Option<T>, RecvError> {
+    match res {
+        Ok(value) => Ok(Some(value)),
+        Err(TryRecvError::Empty) => Ok(None),
+        Err(TryRecvError::Lagged(n)) => Err(RecvError::Lagged(n)),
+        Err(TryRecvError::Closed) => Err(RecvError::Closed),
+    }
+}
+
+impl fmt::Display for RecvError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            RecvError::Closed => write!(f, "channel closed"),
+            RecvError::Lagged(amt) => write!(f, "channel lagged by {}", amt),
+        }
+    }
+}
+
+impl std::error::Error for RecvError {}
+
+impl fmt::Display for TryRecvError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            TryRecvError::Empty => write!(f, "channel empty"),
+            TryRecvError::Closed => write!(f, "channel closed"),
+            TryRecvError::Lagged(amt) => write!(f, "channel lagged by {}", amt),
+        }
+    }
+}
+
+impl std::error::Error for TryRecvError {}
diff --git a/third_party/rust/tokio/src/sync/mod.rs b/third_party/rust/tokio/src/sync/mod.rs
new file mode 100644
index 0000000000..0607f78ad4
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/mod.rs
@@ -0,0 +1,472 @@
+#![cfg_attr(loom, allow(dead_code, unreachable_pub, unused_imports))]
+
+//! Synchronization primitives for use in asynchronous contexts.
+//!
+//! Tokio programs tend to be organized as a set of [tasks] where each task
+//! operates independently and may be executed on separate physical threads. The
+//! synchronization primitives provided in this module permit these independent
+//! tasks to communicate together.
+//!
+//! [tasks]: crate::task
+//!
+//! # Message passing
+//!
+//! The most common form of synchronization in a Tokio program is message
+//! passing. Two tasks operate independently and send messages to each other to
+//! synchronize. Doing so has the advantage of avoiding shared state.
+//!
+//! Message passing is implemented using channels. A channel supports sending a
+//! message from one producer task to one or more consumer tasks. There are a
+//! few flavors of channels provided by Tokio. Each channel flavor supports
+//! different message passing patterns. When a channel supports multiple
+//! producers, many separate tasks may **send** messages. When a channel
+//! supports muliple consumers, many different separate tasks may **receive**
+//! messages.
+//!
+//! Tokio provides many different channel flavors as different message passing
+//! patterns are best handled with different implementations.
+//!
+//! ## `oneshot` channel
+//!
+//! The [`oneshot` channel][oneshot] supports sending a **single** value from a
+//! single producer to a single consumer. This channel is usually used to send
+//! the result of a computation to a waiter.
+//!
+//! **Example:** using a `oneshot` channel to receive the result of a
+//! computation.
+//!
+//! ```
+//! use tokio::sync::oneshot;
+//!
+//! async fn some_computation() -> String {
+//!     "represents the result of the computation".to_string()
+//! }
+//!
+//! #[tokio::main]
+//! async fn main() {
+//!     let (tx, rx) = oneshot::channel();
+//!
+//!     tokio::spawn(async move {
+//!         let res = some_computation().await;
+//!         tx.send(res).unwrap();
+//!     });
+//!
+//!     // Do other work while the computation is happening in the background
+//!
+//!     // Wait for the computation result
+//!     let res = rx.await.unwrap();
+//! }
+//! ```
+//!
+//! Note, if the task produces the the computation result as its final action
+//! before terminating, the [`JoinHandle`] can be used to receive the
+//! computation result instead of allocating resources for the `oneshot`
+//! channel. Awaiting on [`JoinHandle`] returns `Result`. If the task panics,
+//! the `Joinhandle` yields `Err` with the panic cause.
+//!
+//! **Example:**
+//!
+//! ```
+//! async fn some_computation() -> String {
+//!     "the result of the computation".to_string()
+//! }
+//!
+//! #[tokio::main]
+//! async fn main() {
+//!     let join_handle = tokio::spawn(async move {
+//!         some_computation().await
+//!     });
+//!
+//!     // Do other work while the computation is happening in the background
+//!
+//!     // Wait for the computation result
+//!     let res = join_handle.await.unwrap();
+//! }
+//! ```
+//!
+//! [`JoinHandle`]: crate::task::JoinHandle
+//!
+//! ## `mpsc` channel
+//!
+//! The [`mpsc` channel][mpsc] supports sending **many** values from **many**
+//! producers to a single consumer. This channel is often used to send work to a
+//! task or to receive the result of many computations.
+//!
+//! **Example:** using an mpsc to incrementally stream the results of a series
+//! of computations.
+//!
+//! ```
+//! use tokio::sync::mpsc;
+//!
+//! async fn some_computation(input: u32) -> String {
+//!     format!("the result of computation {}", input)
+//! }
+//!
+//! #[tokio::main]
+//! async fn main() {
+//!     let (mut tx, mut rx) = mpsc::channel(100);
+//!
+//!     tokio::spawn(async move {
+//!         for i in 0..10 {
+//!             let res = some_computation(i).await;
+//!             tx.send(res).await.unwrap();
+//!         }
+//!     });
+//!
+//!     while let Some(res) = rx.recv().await {
+//!         println!("got = {}", res);
+//!     }
+//! }
+//! ```
+//!
+//! The argument to `mpsc::channel` is the channel capacity. This is the maximum
+//! number of values that can be stored in the channel pending receipt at any
+//! given time. Properly setting this value is key in implementing robust
+//! programs as the channel capacity plays a critical part in handling back
+//! pressure.
+//!
+//! A common concurrency pattern for resource management is to spawn a task
+//! dedicated to managing that resource and using message passing betwen other
+//! tasks to interact with the resource. The resource may be anything that may
+//! not be concurrently used. Some examples include a socket and program state.
+//! For example, if multiple tasks need to send data over a single socket, spawn
+//! a task to manage the socket and use a channel to synchronize.
+//!
+//! **Example:** sending data from many tasks over a single socket using message
+//! passing.
+//!
+//! ```no_run
+//! use tokio::io::{self, AsyncWriteExt};
+//! use tokio::net::TcpStream;
+//! use tokio::sync::mpsc;
+//!
+//! #[tokio::main]
+//! async fn main() -> io::Result<()> {
+//!     let mut socket = TcpStream::connect("www.example.com:1234").await?;
+//!     let (tx, mut rx) = mpsc::channel(100);
+//!
+//!     for _ in 0..10 {
+//!         // Each task needs its own `tx` handle. This is done by cloning the
+//!         // original handle.
+//!         let mut tx = tx.clone();
+//!
+//!         tokio::spawn(async move {
+//!             tx.send(&b"data to write"[..]).await.unwrap();
+//!         });
+//!     }
+//!
+//!     // The `rx` half of the channel returns `None` once **all** `tx` clones
+//!     // drop. To ensure `None` is returned, drop the handle owned by the
+//!     // current task. If this `tx` handle is not dropped, there will always
+//!     // be a single outstanding `tx` handle.
+//!     drop(tx);
+//!
+//!     while let Some(res) = rx.recv().await {
+//!         socket.write_all(res).await?;
+//!     }
+//!
+//!     Ok(())
+//! }
+//! ```
+//!
+//! The [`mpsc`][mpsc] and [`oneshot`][oneshot] channels can be combined to
+//! provide a request / response type synchronization pattern with a shared
+//! resource. A task is spawned to synchronize a resource and waits on commands
+//! received on a [`mpsc`][mpsc] channel. Each command includes a
+//! [`oneshot`][oneshot] `Sender` on which the result of the command is sent.
+//!
+//! **Example:** use a task to synchronize a `u64` counter. Each task sends an
+//! "fetch and increment" command. The counter value **before** the increment is
+//! sent over the provided `oneshot` channel.
+//!
+//! ```
+//! use tokio::sync::{oneshot, mpsc};
+//! use Command::Increment;
+//!
+//! enum Command {
+//!     Increment,
+//!     // Other commands can be added here
+//! }
+//!
+//! #[tokio::main]
+//! async fn main() {
+//!     let (cmd_tx, mut cmd_rx) = mpsc::channel::<(Command, oneshot::Sender<u64>)>(100);
+//!
+//!     // Spawn a task to manage the counter
+//!     tokio::spawn(async move {
+//!         let mut counter: u64 = 0;
+//!
+//!         while let Some((cmd, response)) = cmd_rx.recv().await {
+//!             match cmd {
+//!                 Increment => {
+//!                     let prev = counter;
+//!                     counter += 1;
+//!                     response.send(prev).unwrap();
+//!                 }
+//!             }
+//!         }
+//!     });
+//!
+//!     let mut join_handles = vec![];
+//!
+//!     // Spawn tasks that will send the increment command.
+//!     for _ in 0..10 {
+//!         let mut cmd_tx = cmd_tx.clone();
+//!
+//!         join_handles.push(tokio::spawn(async move {
+//!             let (resp_tx, resp_rx) = oneshot::channel();
+//!
+//!             cmd_tx.send((Increment, resp_tx)).await.ok().unwrap();
+//!             let res = resp_rx.await.unwrap();
+//!
+//!             println!("previous value = {}", res);
+//!         }));
+//!     }
+//!
+//!     // Wait for all tasks to complete
+//!     for join_handle in join_handles.drain(..) {
+//!         join_handle.await.unwrap();
+//!     }
+//! }
+//! ```
+//!
+//! ## `broadcast` channel
+//!
+//! The [`broadcast` channel][broadcast] supports sending **many** values from
+//! **many** producers to **many** consumers. Each consumer will receive
+//! **each** value. This channel can be used to implement "fan out" style
+//! patterns common with pub / sub or "chat" systems.
+//!
+//! This channel tends to be used less often than `oneshot` and `mpsc` but still
+//! has its use cases.
+//!
+//! Basic usage
+//!
+//! ```
+//! use tokio::sync::broadcast;
+//!
+//! #[tokio::main]
+//! async fn main() {
+//!     let (tx, mut rx1) = broadcast::channel(16);
+//!     let mut rx2 = tx.subscribe();
+//!
+//!     tokio::spawn(async move {
+//!         assert_eq!(rx1.recv().await.unwrap(), 10);
+//!         assert_eq!(rx1.recv().await.unwrap(), 20);
+//!     });
+//!
+//!     tokio::spawn(async move {
+//!         assert_eq!(rx2.recv().await.unwrap(), 10);
+//!         assert_eq!(rx2.recv().await.unwrap(), 20);
+//!     });
+//!
+//!     tx.send(10).unwrap();
+//!     tx.send(20).unwrap();
+//! }
+//! ```
+//!
+//! ## `watch` channel
+//!
+//! The [`watch` channel][watch] supports sending **many** values from a
+//! **single** producer to **many** consumers. However, only the **most recent**
+//! value is stored in the channel. Consumers are notified when a new value is
+//! sent, but there is no guarantee that consumers will see **all** values.
+//!
+//! The [`watch` channel] is similar to a [`broadcast` channel] with capacity 1.
+//!
+//! Use cases for the [`watch` channel] include broadcasting configuration
+//! changes or signalling program state changes, such as transitioning to
+//! shutdown.
+//!
+//! **Example:** use a `watch` channel to notify tasks of configuration changes.
+//! In this example, a configuration file is checked periodically. When the file
+//! changes, the configuration changes are signalled to consumers.
+//!
+//! ```
+//! use tokio::sync::watch;
+//! use tokio::time::{self, Duration, Instant};
+//!
+//! use std::io;
+//!
+//! #[derive(Debug, Clone, Eq, PartialEq)]
+//! struct Config {
+//!     timeout: Duration,
+//! }
+//!
+//! impl Config {
+//!     async fn load_from_file() -> io::Result<Config> {
+//!         // file loading and deserialization logic here
+//! # Ok(Config { timeout: Duration::from_secs(1) })
+//!     }
+//! }
+//!
+//! async fn my_async_operation() {
+//!     // Do something here
+//! }
+//!
+//! #[tokio::main]
+//! async fn main() {
+//!     // Load initial configuration value
+//!     let mut config = Config::load_from_file().await.unwrap();
+//!
+//!     // Create the watch channel, initialized with the loaded configuration
+//!     let (tx, rx) = watch::channel(config.clone());
+//!
+//!     // Spawn a task to monitor the file.
+//!     tokio::spawn(async move {
+//!         loop {
+//!             // Wait 10 seconds between checks
+//!             time::delay_for(Duration::from_secs(10)).await;
+//!
+//!             // Load the configuration file
+//!             let new_config = Config::load_from_file().await.unwrap();
+//!
+//!             // If the configuration changed, send the new config value
+//!             // on the watch channel.
+//!             if new_config != config {
+//!                 tx.broadcast(new_config.clone()).unwrap();
+//!                 config = new_config;
+//!             }
+//!         }
+//!     });
+//!
+//!     let mut handles = vec![];
+//!
+//!     // Spawn tasks that runs the async operation for at most `timeout`. If
+//!     // the timeout elapses, restart the operation.
+//!     //
+//!     // The task simultaneously watches the `Config` for changes. When the
+//!     // timeout duration changes, the timeout is updated without restarting
+//!     // the in-flight operation.
+//!     for _ in 0..5 {
+//!         // Clone a config watch handle for use in this task
+//!         let mut rx = rx.clone();
+//!
+//!         let handle = tokio::spawn(async move {
+//!             // Start the initial operation and pin the future to the stack.
+//!             // Pinning to the stack is required to resume the operation
+//!             // across multiple calls to `select!`
+//!             let op = my_async_operation();
+//!             tokio::pin!(op);
+//!
+//!             // Receive the **initial** configuration value. As this is the
+//!             // first time the config is received from the watch, it will
+//!             // always complete immediatedly.
+//!             let mut conf = rx.recv().await.unwrap();
+//!
+//!             let mut op_start = Instant::now();
+//!             let mut delay = time::delay_until(op_start + conf.timeout);
+//!
+//!             loop {
+//!                 tokio::select! {
+//!                     _ = &mut delay => {
+//!                         // The operation elapsed. Restart it
+//!                         op.set(my_async_operation());
+//!
+//!                         // Track the new start time
+//!                         op_start = Instant::now();
+//!
+//!                         // Restart the timeout
+//!                         delay = time::delay_until(op_start + conf.timeout);
+//!                     }
+//!                     new_conf = rx.recv() => {
+//!                         conf = new_conf.unwrap();
+//!
+//!                         // The configuration has been updated. Update the
+//!                         // `delay` using the new `timeout` value.
+//!                         delay.reset(op_start + conf.timeout);
+//!                     }
+//!                     _ = &mut op => {
+//!                         // The operation completed!
+//!                         return
+//!                     }
+//!                 }
+//!             }
+//!         });
+//!
+//!         handles.push(handle);
+//!     }
+//!
+//!     for handle in handles.drain(..) {
+//!         handle.await.unwrap();
+//!     }
+//! }
+//! ```
+//!
+//! # State synchronization
+//!
+//! The remaining synchronization primitives focus on synchronizing state.
+//! These are asynchronous equivalents to versions provided by `std`. They
+//! operate in a similar way as their `std` counterparts parts but will wait
+//! asynchronously instead of blocking the thread.
+//!
+//! * [`Barrier`][Barrier] Ensures multiple tasks will wait for each other to
+//!   reach a point in the program, before continuing execution all together.
+//!
+//! * [`Mutex`][Mutex] Mutual Exclusion mechanism, which ensures that at most
+//!   one thread at a time is able to access some data.
+//!
+//! * [`Notify`][Notify] Basic task notification. `Notify` supports notifying a
+//!   receiving task without sending data. In this case, the task wakes up and
+//!   resumes processing.
+//!
+//! * [`RwLock`][RwLock] Provides a mutual exclusion mechanism which allows
+//!   multiple readers at the same time, while allowing only one writer at a
+//!   time. In some cases, this can be more efficient than a mutex.
+//!
+//! * [`Semaphore`][Semaphore] Limits the amount of concurrency. A semaphore
+//!   holds a number of permits, which tasks may request in order to enter a
+//!   critical section. Semaphores are useful for implementing limiting of
+//!   bounding of any kind.
+
+cfg_sync! {
+    mod barrier;
+    pub use barrier::{Barrier, BarrierWaitResult};
+
+    pub mod broadcast;
+
+    pub mod mpsc;
+
+    mod mutex;
+    pub use mutex::{Mutex, MutexGuard};
+
+    mod notify;
+    pub use notify::Notify;
+
+    pub mod oneshot;
+
+    pub(crate) mod batch_semaphore;
+    pub(crate) mod semaphore_ll;
+    mod semaphore;
+    pub use semaphore::{Semaphore, SemaphorePermit};
+
+    mod rwlock;
+    pub use rwlock::{RwLock, RwLockReadGuard, RwLockWriteGuard};
+
+    mod task;
+    pub(crate) use task::AtomicWaker;
+
+    pub mod watch;
+}
+
+cfg_not_sync! {
+    cfg_atomic_waker_impl! {
+        mod task;
+        pub(crate) use task::AtomicWaker;
+    }
+
+    #[cfg(any(
+            feature = "rt-core",
+            feature = "process",
+            feature = "signal"))]
+    pub(crate) mod oneshot;
+
+    cfg_signal! {
+        pub(crate) mod mpsc;
+        pub(crate) mod semaphore_ll;
+    }
+}
+
+/// Unit tests
+#[cfg(test)]
+mod tests;
diff --git a/third_party/rust/tokio/src/sync/mpsc/block.rs b/third_party/rust/tokio/src/sync/mpsc/block.rs
new file mode 100644
index 0000000000..7bf161967b
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/mpsc/block.rs
@@ -0,0 +1,387 @@
+use crate::loom::{
+    cell::UnsafeCell,
+    sync::atomic::{AtomicPtr, AtomicUsize},
+    thread,
+};
+
+use std::mem::MaybeUninit;
+use std::ops;
+use std::ptr::{self, NonNull};
+use std::sync::atomic::Ordering::{self, AcqRel, Acquire, Release};
+
+/// A block in a linked list.
+///
+/// Each block in the list can hold up to `BLOCK_CAP` messages.
+pub(crate) struct Block<T> {
+    /// The start index of this block.
+    ///
+    /// Slots in this block have indices in `start_index .. start_index + BLOCK_CAP`.
+    start_index: usize,
+
+    /// The next block in the linked list.
+    next: AtomicPtr<Block<T>>,
+
+    /// Bitfield tracking slots that are ready to have their values consumed.
+    ready_slots: AtomicUsize,
+
+    /// The observed `tail_position` value *after* the block has been passed by
+    /// `block_tail`.
+    observed_tail_position: UnsafeCell<usize>,
+
+    /// Array containing values pushed into the block. Values are stored in a
+    /// continuous array in order to improve cache line behavior when reading.
+    /// The values must be manually dropped.
+    values: Values<T>,
+}
+
+pub(crate) enum Read<T> {
+    Value(T),
+    Closed,
+}
+
+struct Values<T>([UnsafeCell<MaybeUninit<T>>; BLOCK_CAP]);
+
+use super::BLOCK_CAP;
+
+/// Masks an index to get the block identifier
+const BLOCK_MASK: usize = !(BLOCK_CAP - 1);
+
+/// Masks an index to get the value offset in a block.
+const SLOT_MASK: usize = BLOCK_CAP - 1;
+
+/// Flag tracking that a block has gone through the sender's release routine.
+///
+/// When this is set, the receiver may consider freeing the block.
+const RELEASED: usize = 1 << BLOCK_CAP;
+
+/// Flag tracking all senders dropped.
+///
+/// When this flag is set, the send half of the channel has closed.
+const TX_CLOSED: usize = RELEASED << 1;
+
+/// Mask covering all bits used to track slot readiness.
+const READY_MASK: usize = RELEASED - 1;
+
+/// Returns the index of the first slot in the block referenced by `slot_index`.
+#[inline(always)]
+pub(crate) fn start_index(slot_index: usize) -> usize {
+    BLOCK_MASK & slot_index
+}
+
+/// Returns the offset into the block referenced by `slot_index`.
+#[inline(always)]
+pub(crate) fn offset(slot_index: usize) -> usize {
+    SLOT_MASK & slot_index
+}
+
+impl<T> Block<T> {
+    pub(crate) fn new(start_index: usize) -> Block<T> {
+        Block {
+            // The absolute index in the channel of the first slot in the block.
+            start_index,
+
+            // Pointer to the next block in the linked list.
+            next: AtomicPtr::new(ptr::null_mut()),
+
+            ready_slots: AtomicUsize::new(0),
+
+            observed_tail_position: UnsafeCell::new(0),
+
+            // Value storage
+            values: unsafe { Values::uninitialized() },
+        }
+    }
+
+    /// Returns `true` if the block matches the given index
+    pub(crate) fn is_at_index(&self, index: usize) -> bool {
+        debug_assert!(offset(index) == 0);
+        self.start_index == index
+    }
+
+    /// Returns the number of blocks between `self` and the block at the
+    /// specified index.
+    ///
+    /// `start_index` must represent a block *after* `self`.
+    pub(crate) fn distance(&self, other_index: usize) -> usize {
+        debug_assert!(offset(other_index) == 0);
+        other_index.wrapping_sub(self.start_index) / BLOCK_CAP
+    }
+
+    /// Reads the value at the given offset.
+    ///
+    /// Returns `None` if the slot is empty.
+    ///
+    /// # Safety
+    ///
+    /// To maintain safety, the caller must ensure:
+    ///
+    /// * No concurrent access to the slot.
+    pub(crate) unsafe fn read(&self, slot_index: usize) -> Option<Read<T>> {
+        let offset = offset(slot_index);
+
+        let ready_bits = self.ready_slots.load(Acquire);
+
+        if !is_ready(ready_bits, offset) {
+            if is_tx_closed(ready_bits) {
+                return Some(Read::Closed);
+            }
+
+            return None;
+        }
+
+        // Get the value
+        let value = self.values[offset].with(|ptr| ptr::read(ptr));
+
+        Some(Read::Value(value.assume_init()))
+    }
+
+    /// Writes a value to the block at the given offset.
+    ///
+    /// # Safety
+    ///
+    /// To maintain safety, the caller must ensure:
+    ///
+    /// * The slot is empty.
+    /// * No concurrent access to the slot.
+    pub(crate) unsafe fn write(&self, slot_index: usize, value: T) {
+        // Get the offset into the block
+        let slot_offset = offset(slot_index);
+
+        self.values[slot_offset].with_mut(|ptr| {
+            ptr::write(ptr, MaybeUninit::new(value));
+        });
+
+        // Release the value. After this point, the slot ref may no longer
+        // be used. It is possible for the receiver to free the memory at
+        // any point.
+        self.set_ready(slot_offset);
+    }
+
+    /// Signal to the receiver that the sender half of the list is closed.
+    pub(crate) unsafe fn tx_close(&self) {
+        self.ready_slots.fetch_or(TX_CLOSED, Release);
+    }
+
+    /// Resets the block to a blank state. This enables reusing blocks in the
+    /// channel.
+    ///
+    /// # Safety
+    ///
+    /// To maintain safety, the caller must ensure:
+    ///
+    /// * All slots are empty.
+    /// * The caller holds a unique pointer to the block.
+    pub(crate) unsafe fn reclaim(&mut self) {
+        self.start_index = 0;
+        self.next = AtomicPtr::new(ptr::null_mut());
+        self.ready_slots = AtomicUsize::new(0);
+    }
+
+    /// Releases the block to the rx half for freeing.
+    ///
+    /// This function is called by the tx half once it can be guaranteed that no
+    /// more senders will attempt to access the block.
+    ///
+    /// # Safety
+    ///
+    /// To maintain safety, the caller must ensure:
+    ///
+    /// * The block will no longer be accessed by any sender.
+    pub(crate) unsafe fn tx_release(&self, tail_position: usize) {
+        // Track the observed tail_position. Any sender targetting a greater
+        // tail_position is guaranteed to not access this block.
+        self.observed_tail_position
+            .with_mut(|ptr| *ptr = tail_position);
+
+        // Set the released bit, signalling to the receiver that it is safe to
+        // free the block's memory as soon as all slots **prior** to
+        // `observed_tail_position` have been filled.
+        self.ready_slots.fetch_or(RELEASED, Release);
+    }
+
+    /// Mark a slot as ready
+    fn set_ready(&self, slot: usize) {
+        let mask = 1 << slot;
+        self.ready_slots.fetch_or(mask, Release);
+    }
+
+    /// Returns `true` when all slots have their `ready` bits set.
+    ///
+    /// This indicates that the block is in its final state and will no longer
+    /// be mutated.
+    ///
+    /// # Implementation
+    ///
+    /// The implementation walks each slot checking the `ready` flag. It might
+    /// be that it would make more sense to coalesce ready flags as bits in a
+    /// single atomic cell. However, this could have negative impact on cache
+    /// behavior as there would be many more mutations to a single slot.
+    pub(crate) fn is_final(&self) -> bool {
+        self.ready_slots.load(Acquire) & READY_MASK == READY_MASK
+    }
+
+    /// Returns the `observed_tail_position` value, if set
+    pub(crate) fn observed_tail_position(&self) -> Option<usize> {
+        if 0 == RELEASED & self.ready_slots.load(Acquire) {
+            None
+        } else {
+            Some(self.observed_tail_position.with(|ptr| unsafe { *ptr }))
+        }
+    }
+
+    /// Loads the next block
+    pub(crate) fn load_next(&self, ordering: Ordering) -> Option<NonNull<Block<T>>> {
+        let ret = NonNull::new(self.next.load(ordering));
+
+        debug_assert!(unsafe {
+            ret.map(|block| block.as_ref().start_index == self.start_index.wrapping_add(BLOCK_CAP))
+                .unwrap_or(true)
+        });
+
+        ret
+    }
+
+    /// Pushes `block` as the next block in the link.
+    ///
+    /// Returns Ok if successful, otherwise, a pointer to the next block in
+    /// the list is returned.
+    ///
+    /// This requires that the next pointer is null.
+    ///
+    /// # Ordering
+    ///
+    /// This performs a compare-and-swap on `next` using AcqRel ordering.
+    ///
+    /// # Safety
+    ///
+    /// To maintain safety, the caller must ensure:
+    ///
+    /// * `block` is not freed until it has been removed from the list.
+    pub(crate) unsafe fn try_push(
+        &self,
+        block: &mut NonNull<Block<T>>,
+        ordering: Ordering,
+    ) -> Result<(), NonNull<Block<T>>> {
+        block.as_mut().start_index = self.start_index.wrapping_add(BLOCK_CAP);
+
+        let next_ptr = self
+            .next
+            .compare_and_swap(ptr::null_mut(), block.as_ptr(), ordering);
+
+        match NonNull::new(next_ptr) {
+            Some(next_ptr) => Err(next_ptr),
+            None => Ok(()),
+        }
+    }
+
+    /// Grows the `Block` linked list by allocating and appending a new block.
+    ///
+    /// The next block in the linked list is returned. This may or may not be
+    /// the one allocated by the function call.
+    ///
+    /// # Implementation
+    ///
+    /// It is assumed that `self.next` is null. A new block is allocated with
+    /// `start_index` set to be the next block. A compare-and-swap is performed
+    /// with AcqRel memory ordering. If the compare-and-swap is successful, the
+    /// newly allocated block is released to other threads walking the block
+    /// linked list. If the compare-and-swap fails, the current thread acquires
+    /// the next block in the linked list, allowing the current thread to access
+    /// the slots.
+    pub(crate) fn grow(&self) -> NonNull<Block<T>> {
+        // Create the new block. It is assumed that the block will become the
+        // next one after `&self`. If this turns out to not be the case,
+        // `start_index` is updated accordingly.
+        let new_block = Box::new(Block::new(self.start_index + BLOCK_CAP));
+
+        let mut new_block = unsafe { NonNull::new_unchecked(Box::into_raw(new_block)) };
+
+        // Attempt to store the block. The first compare-and-swap attempt is
+        // "unrolled" due to minor differences in logic
+        //
+        // `AcqRel` is used as the ordering **only** when attempting the
+        // compare-and-swap on self.next.
+        //
+        // If the compare-and-swap fails, then the actual value of the cell is
+        // returned from this function and accessed by the caller. Given this,
+        // the memory must be acquired.
+        //
+        // `Release` ensures that the newly allocated block is available to
+        // other threads acquiring the next pointer.
+        let next = NonNull::new(self.next.compare_and_swap(
+            ptr::null_mut(),
+            new_block.as_ptr(),
+            AcqRel,
+        ));
+
+        let next = match next {
+            Some(next) => next,
+            None => {
+                // The compare-and-swap succeeded and the newly allocated block
+                // is successfully pushed.
+                return new_block;
+            }
+        };
+
+        // There already is a next block in the linked list. The newly allocated
+        // block could be dropped and the discovered next block returned;
+        // however, that would be wasteful. Instead, the linked list is walked
+        // by repeatedly attempting to compare-and-swap the pointer into the
+        // `next` register until the compare-and-swap succeed.
+        //
+        // Care is taken to update new_block's start_index field as appropriate.
+
+        let mut curr = next;
+
+        // TODO: Should this iteration be capped?
+        loop {
+            let actual = unsafe { curr.as_ref().try_push(&mut new_block, AcqRel) };
+
+            curr = match actual {
+                Ok(_) => {
+                    return next;
+                }
+                Err(curr) => curr,
+            };
+
+            // When running outside of loom, this calls `spin_loop_hint`.
+            thread::yield_now();
+        }
+    }
+}
+
+/// Returns `true` if the specificed slot has a value ready to be consumed.
+fn is_ready(bits: usize, slot: usize) -> bool {
+    let mask = 1 << slot;
+    mask == mask & bits
+}
+
+/// Returns `true` if the closed flag has been set.
+fn is_tx_closed(bits: usize) -> bool {
+    TX_CLOSED == bits & TX_CLOSED
+}
+
+impl<T> Values<T> {
+    unsafe fn uninitialized() -> Values<T> {
+        let mut vals = MaybeUninit::uninit();
+
+        // When fuzzing, `UnsafeCell` needs to be initialized.
+        if_loom! {
+            let p = vals.as_mut_ptr() as *mut UnsafeCell<MaybeUninit<T>>;
+            for i in 0..BLOCK_CAP {
+                p.add(i)
+                    .write(UnsafeCell::new(MaybeUninit::uninit()));
+            }
+        }
+
+        Values(vals.assume_init())
+    }
+}
+
+impl<T> ops::Index<usize> for Values<T> {
+    type Output = UnsafeCell<MaybeUninit<T>>;
+
+    fn index(&self, index: usize) -> &Self::Output {
+        self.0.index(index)
+    }
+}
diff --git a/third_party/rust/tokio/src/sync/mpsc/bounded.rs b/third_party/rust/tokio/src/sync/mpsc/bounded.rs
new file mode 100644
index 0000000000..afca8c524d
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/mpsc/bounded.rs
@@ -0,0 +1,479 @@
+use crate::sync::mpsc::chan;
+use crate::sync::mpsc::error::{ClosedError, SendError, TryRecvError, TrySendError};
+use crate::sync::semaphore_ll as semaphore;
+
+cfg_time! {
+    use crate::sync::mpsc::error::SendTimeoutError;
+    use crate::time::Duration;
+}
+
+use std::fmt;
+use std::task::{Context, Poll};
+
+/// Send values to the associated `Receiver`.
+///
+/// Instances are created by the [`channel`](channel) function.
+pub struct Sender<T> {
+    chan: chan::Tx<T, Semaphore>,
+}
+
+impl<T> Clone for Sender<T> {
+    fn clone(&self) -> Self {
+        Sender {
+            chan: self.chan.clone(),
+        }
+    }
+}
+
+impl<T> fmt::Debug for Sender<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("Sender")
+            .field("chan", &self.chan)
+            .finish()
+    }
+}
+
+/// Receive values from the associated `Sender`.
+///
+/// Instances are created by the [`channel`](channel) function.
+pub struct Receiver<T> {
+    /// The channel receiver
+    chan: chan::Rx<T, Semaphore>,
+}
+
+impl<T> fmt::Debug for Receiver<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("Receiver")
+            .field("chan", &self.chan)
+            .finish()
+    }
+}
+
+/// Creates a bounded mpsc channel for communicating between asynchronous tasks,
+/// returning the sender/receiver halves.
+///
+/// All data sent on `Sender` will become available on `Receiver` in the same
+/// order as it was sent.
+///
+/// The `Sender` can be cloned to `send` to the same channel from multiple code
+/// locations. Only one `Receiver` is supported.
+///
+/// If the `Receiver` is disconnected while trying to `send`, the `send` method
+/// will return a `SendError`. Similarly, if `Sender` is disconnected while
+/// trying to `recv`, the `recv` method will return a `RecvError`.
+///
+/// # Examples
+///
+/// ```rust
+/// use tokio::sync::mpsc;
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let (mut tx, mut rx) = mpsc::channel(100);
+///
+///     tokio::spawn(async move {
+///         for i in 0..10 {
+///             if let Err(_) = tx.send(i).await {
+///                 println!("receiver dropped");
+///                 return;
+///             }
+///         }
+///     });
+///
+///     while let Some(i) = rx.recv().await {
+///         println!("got = {}", i);
+///     }
+/// }
+/// ```
+pub fn channel<T>(buffer: usize) -> (Sender<T>, Receiver<T>) {
+    assert!(buffer > 0, "mpsc bounded channel requires buffer > 0");
+    let semaphore = (semaphore::Semaphore::new(buffer), buffer);
+    let (tx, rx) = chan::channel(semaphore);
+
+    let tx = Sender::new(tx);
+    let rx = Receiver::new(rx);
+
+    (tx, rx)
+}
+
+/// Channel semaphore is a tuple of the semaphore implementation and a `usize`
+/// representing the channel bound.
+type Semaphore = (semaphore::Semaphore, usize);
+
+impl<T> Receiver<T> {
+    pub(crate) fn new(chan: chan::Rx<T, Semaphore>) -> Receiver<T> {
+        Receiver { chan }
+    }
+
+    /// Receives the next value for this receiver.
+    ///
+    /// `None` is returned when all `Sender` halves have dropped, indicating
+    /// that no further values can be sent on the channel.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::mpsc;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (mut tx, mut rx) = mpsc::channel(100);
+    ///
+    ///     tokio::spawn(async move {
+    ///         tx.send("hello").await.unwrap();
+    ///     });
+    ///
+    ///     assert_eq!(Some("hello"), rx.recv().await);
+    ///     assert_eq!(None, rx.recv().await);
+    /// }
+    /// ```
+    ///
+    /// Values are buffered:
+    ///
+    /// ```
+    /// use tokio::sync::mpsc;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (mut tx, mut rx) = mpsc::channel(100);
+    ///
+    ///     tx.send("hello").await.unwrap();
+    ///     tx.send("world").await.unwrap();
+    ///
+    ///     assert_eq!(Some("hello"), rx.recv().await);
+    ///     assert_eq!(Some("world"), rx.recv().await);
+    /// }
+    /// ```
+    pub async fn recv(&mut self) -> Option<T> {
+        use crate::future::poll_fn;
+
+        poll_fn(|cx| self.poll_recv(cx)).await
+    }
+
+    #[doc(hidden)] // TODO: document
+    pub fn poll_recv(&mut self, cx: &mut Context<'_>) -> Poll<Option<T>> {
+        self.chan.recv(cx)
+    }
+
+    /// Attempts to return a pending value on this receiver without blocking.
+    ///
+    /// This method will never block the caller in order to wait for data to
+    /// become available. Instead, this will always return immediately with
+    /// a possible option of pending data on the channel.
+    ///
+    /// This is useful for a flavor of "optimistic check" before deciding to
+    /// block on a receiver.
+    ///
+    /// Compared with recv, this function has two failure cases instead of
+    /// one (one for disconnection, one for an empty buffer).
+    pub fn try_recv(&mut self) -> Result<T, TryRecvError> {
+        self.chan.try_recv()
+    }
+
+    /// Closes the receiving half of a channel, without dropping it.
+    ///
+    /// This prevents any further messages from being sent on the channel while
+    /// still enabling the receiver to drain messages that are buffered.
+    pub fn close(&mut self) {
+        self.chan.close();
+    }
+}
+
+impl<T> Unpin for Receiver<T> {}
+
+cfg_stream! {
+    impl<T> crate::stream::Stream for Receiver<T> {
+        type Item = T;
+
+        fn poll_next(mut self: std::pin::Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
+            self.poll_recv(cx)
+        }
+    }
+}
+
+impl<T> Sender<T> {
+    pub(crate) fn new(chan: chan::Tx<T, Semaphore>) -> Sender<T> {
+        Sender { chan }
+    }
+
+    /// Sends a value, waiting until there is capacity.
+    ///
+    /// A successful send occurs when it is determined that the other end of the
+    /// channel has not hung up already. An unsuccessful send would be one where
+    /// the corresponding receiver has already been closed. Note that a return
+    /// value of `Err` means that the data will never be received, but a return
+    /// value of `Ok` does not mean that the data will be received. It is
+    /// possible for the corresponding receiver to hang up immediately after
+    /// this function returns `Ok`.
+    ///
+    /// # Errors
+    ///
+    /// If the receive half of the channel is closed, either due to [`close`]
+    /// being called or the [`Receiver`] handle dropping, the function returns
+    /// an error. The error includes the value passed to `send`.
+    ///
+    /// [`close`]: Receiver::close
+    /// [`Receiver`]: Receiver
+    ///
+    /// # Examples
+    ///
+    /// In the following example, each call to `send` will block until the
+    /// previously sent value was received.
+    ///
+    /// ```rust
+    /// use tokio::sync::mpsc;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (mut tx, mut rx) = mpsc::channel(1);
+    ///
+    ///     tokio::spawn(async move {
+    ///         for i in 0..10 {
+    ///             if let Err(_) = tx.send(i).await {
+    ///                 println!("receiver dropped");
+    ///                 return;
+    ///             }
+    ///         }
+    ///     });
+    ///
+    ///     while let Some(i) = rx.recv().await {
+    ///         println!("got = {}", i);
+    ///     }
+    /// }
+    /// ```
+    pub async fn send(&mut self, value: T) -> Result<(), SendError<T>> {
+        use crate::future::poll_fn;
+
+        if poll_fn(|cx| self.poll_ready(cx)).await.is_err() {
+            return Err(SendError(value));
+        }
+
+        match self.try_send(value) {
+            Ok(()) => Ok(()),
+            Err(TrySendError::Full(_)) => unreachable!(),
+            Err(TrySendError::Closed(value)) => Err(SendError(value)),
+        }
+    }
+
+    /// Attempts to immediately send a message on this `Sender`
+    ///
+    /// This method differs from [`send`] by returning immediately if the channel's
+    /// buffer is full or no receiver is waiting to acquire some data. Compared
+    /// with [`send`], this function has two failure cases instead of one (one for
+    /// disconnection, one for a full buffer).
+    ///
+    /// This function may be paired with [`poll_ready`] in order to wait for
+    /// channel capacity before trying to send a value.
+    ///
+    /// # Errors
+    ///
+    /// If the channel capacity has been reached, i.e., the channel has `n`
+    /// buffered values where `n` is the argument passed to [`channel`], then an
+    /// error is returned.
+    ///
+    /// If the receive half of the channel is closed, either due to [`close`]
+    /// being called or the [`Receiver`] handle dropping, the function returns
+    /// an error. The error includes the value passed to `send`.
+    ///
+    /// [`send`]: Sender::send
+    /// [`poll_ready`]: Sender::poll_ready
+    /// [`channel`]: channel
+    /// [`close`]: Receiver::close
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::mpsc;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     // Create a channel with buffer size 1
+    ///     let (mut tx1, mut rx) = mpsc::channel(1);
+    ///     let mut tx2 = tx1.clone();
+    ///
+    ///     tokio::spawn(async move {
+    ///         tx1.send(1).await.unwrap();
+    ///         tx1.send(2).await.unwrap();
+    ///         // task waits until the receiver receives a value.
+    ///     });
+    ///
+    ///     tokio::spawn(async move {
+    ///         // This will return an error and send
+    ///         // no message if the buffer is full
+    ///         let _ = tx2.try_send(3);
+    ///     });
+    ///
+    ///     let mut msg;
+    ///     msg = rx.recv().await.unwrap();
+    ///     println!("message {} received", msg);
+    ///
+    ///     msg = rx.recv().await.unwrap();
+    ///     println!("message {} received", msg);
+    ///
+    ///     // Third message may have never been sent
+    ///     match rx.recv().await {
+    ///         Some(msg) => println!("message {} received", msg),
+    ///         None => println!("the third message was never sent"),
+    ///     }
+    /// }
+    /// ```
+    pub fn try_send(&mut self, message: T) -> Result<(), TrySendError<T>> {
+        self.chan.try_send(message)?;
+        Ok(())
+    }
+
+    /// Sends a value, waiting until there is capacity, but only for a limited time.
+    ///
+    /// Shares the same success and error conditions as [`send`], adding one more
+    /// condition for an unsuccessful send, which is when the provided timeout has
+    /// elapsed, and there is no capacity available.
+    ///
+    /// [`send`]: Sender::send
+    ///
+    /// # Errors
+    ///
+    /// If the receive half of the channel is closed, either due to [`close`]
+    /// being called or the [`Receiver`] having been dropped,
+    /// the function returns an error. The error includes the value passed to `send`.
+    ///
+    /// [`close`]: Receiver::close
+    /// [`Receiver`]: Receiver
+    ///
+    /// # Examples
+    ///
+    /// In the following example, each call to `send_timeout` will block until the
+    /// previously sent value was received, unless the timeout has elapsed.
+    ///
+    /// ```rust
+    /// use tokio::sync::mpsc;
+    /// use tokio::time::{delay_for, Duration};
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (mut tx, mut rx) = mpsc::channel(1);
+    ///
+    ///     tokio::spawn(async move {
+    ///         for i in 0..10 {
+    ///             if let Err(e) = tx.send_timeout(i, Duration::from_millis(100)).await {
+    ///                 println!("send error: #{:?}", e);
+    ///                 return;
+    ///             }
+    ///         }
+    ///     });
+    ///
+    ///     while let Some(i) = rx.recv().await {
+    ///         println!("got = {}", i);
+    ///         delay_for(Duration::from_millis(200)).await;
+    ///     }
+    /// }
+    /// ```
+    #[cfg(feature = "time")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "time")))]
+    pub async fn send_timeout(
+        &mut self,
+        value: T,
+        timeout: Duration,
+    ) -> Result<(), SendTimeoutError<T>> {
+        use crate::future::poll_fn;
+
+        match crate::time::timeout(timeout, poll_fn(|cx| self.poll_ready(cx))).await {
+            Err(_) => {
+                return Err(SendTimeoutError::Timeout(value));
+            }
+            Ok(Err(_)) => {
+                return Err(SendTimeoutError::Closed(value));
+            }
+            Ok(_) => {}
+        }
+
+        match self.try_send(value) {
+            Ok(()) => Ok(()),
+            Err(TrySendError::Full(_)) => unreachable!(),
+            Err(TrySendError::Closed(value)) => Err(SendTimeoutError::Closed(value)),
+        }
+    }
+
+    /// Returns `Poll::Ready(Ok(()))` when the channel is able to accept another item.
+    ///
+    /// If the channel is full, then `Poll::Pending` is returned and the task is notified when a
+    /// slot becomes available.
+    ///
+    /// Once `poll_ready` returns `Poll::Ready(Ok(()))`, a call to `try_send` will succeed unless
+    /// the channel has since been closed. To provide this guarantee, the channel reserves one slot
+    /// in the channel for the coming send. This reserved slot is not available to other `Sender`
+    /// instances, so you need to be careful to not end up with deadlocks by blocking after calling
+    /// `poll_ready` but before sending an element.
+    ///
+    /// If, after `poll_ready` succeeds, you decide you do not wish to send an item after all, you
+    /// can use [`disarm`](Sender::disarm) to release the reserved slot.
+    ///
+    /// Until an item is sent or [`disarm`](Sender::disarm) is called, repeated calls to
+    /// `poll_ready` will return either `Poll::Ready(Ok(()))` or `Poll::Ready(Err(_))` if channel
+    /// is closed.
+    pub fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), ClosedError>> {
+        self.chan.poll_ready(cx).map_err(|_| ClosedError::new())
+    }
+
+    /// Undo a successful call to `poll_ready`.
+    ///
+    /// Once a call to `poll_ready` returns `Poll::Ready(Ok(()))`, it holds up one slot in the
+    /// channel to make room for the coming send. `disarm` allows you to give up that slot if you
+    /// decide you do not wish to send an item after all. After calling `disarm`, you must call
+    /// `poll_ready` until it returns `Poll::Ready(Ok(()))` before attempting to send again.
+    ///
+    /// Returns `false` if no slot is reserved for this sender (usually because `poll_ready` was
+    /// not previously called, or did not succeed).
+    ///
+    /// # Motivation
+    ///
+    /// Since `poll_ready` takes up one of the finite number of slots in a bounded channel, callers
+    /// need to send an item shortly after `poll_ready` succeeds. If they do not, idle senders may
+    /// take up all the slots of the channel, and prevent active senders from getting any requests
+    /// through. Consider this code that forwards from one channel to another:
+    ///
+    /// ```rust,ignore
+    /// loop {
+    ///   ready!(tx.poll_ready(cx))?;
+    ///   if let Some(item) = ready!(rx.poll_recv(cx)) {
+    ///     tx.try_send(item)?;
+    ///   } else {
+    ///     break;
+    ///   }
+    /// }
+    /// ```
+    ///
+    /// If many such forwarders exist, and they all forward into a single (cloned) `Sender`, then
+    /// any number of forwarders may be waiting for `rx.poll_recv` at the same time. While they do,
+    /// they are effectively each reducing the channel's capacity by 1. If enough of these
+    /// forwarders are idle, forwarders whose `rx` _do_ have elements will be unable to find a spot
+    /// for them through `poll_ready`, and the system will deadlock.
+    ///
+    /// `disarm` solves this problem by allowing you to give up the reserved slot if you find that
+    /// you have to block. We can then fix the code above by writing:
+    ///
+    /// ```rust,ignore
+    /// loop {
+    ///   ready!(tx.poll_ready(cx))?;
+    ///   let item = rx.poll_recv(cx);
+    ///   if let Poll::Ready(Ok(_)) = item {
+    ///     // we're going to send the item below, so don't disarm
+    ///   } else {
+    ///     // give up our send slot, we won't need it for a while
+    ///     tx.disarm();
+    ///   }
+    ///   if let Some(item) = ready!(item) {
+    ///     tx.try_send(item)?;
+    ///   } else {
+    ///     break;
+    ///   }
+    /// }
+    /// ```
+    pub fn disarm(&mut self) -> bool {
+        if self.chan.is_ready() {
+            self.chan.disarm();
+            true
+        } else {
+            false
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/sync/mpsc/chan.rs b/third_party/rust/tokio/src/sync/mpsc/chan.rs
new file mode 100644
index 0000000000..3466395788
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/mpsc/chan.rs
@@ -0,0 +1,524 @@
+use crate::loom::cell::UnsafeCell;
+use crate::loom::future::AtomicWaker;
+use crate::loom::sync::atomic::AtomicUsize;
+use crate::loom::sync::Arc;
+use crate::sync::mpsc::error::{ClosedError, TryRecvError};
+use crate::sync::mpsc::{error, list};
+
+use std::fmt;
+use std::process;
+use std::sync::atomic::Ordering::{AcqRel, Relaxed};
+use std::task::Poll::{Pending, Ready};
+use std::task::{Context, Poll};
+
+/// Channel sender
+pub(crate) struct Tx<T, S: Semaphore> {
+    inner: Arc<Chan<T, S>>,
+    permit: S::Permit,
+}
+
+impl<T, S: Semaphore> fmt::Debug for Tx<T, S>
+where
+    S::Permit: fmt::Debug,
+    S: fmt::Debug,
+{
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("Tx")
+            .field("inner", &self.inner)
+            .field("permit", &self.permit)
+            .finish()
+    }
+}
+
+/// Channel receiver
+pub(crate) struct Rx<T, S: Semaphore> {
+    inner: Arc<Chan<T, S>>,
+}
+
+impl<T, S: Semaphore> fmt::Debug for Rx<T, S>
+where
+    S: fmt::Debug,
+{
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("Rx").field("inner", &self.inner).finish()
+    }
+}
+
+#[derive(Debug, Eq, PartialEq)]
+pub(crate) enum TrySendError {
+    Closed,
+    Full,
+}
+
+impl<T> From<(T, TrySendError)> for error::SendError<T> {
+    fn from(src: (T, TrySendError)) -> error::SendError<T> {
+        match src.1 {
+            TrySendError::Closed => error::SendError(src.0),
+            TrySendError::Full => unreachable!(),
+        }
+    }
+}
+
+impl<T> From<(T, TrySendError)> for error::TrySendError<T> {
+    fn from(src: (T, TrySendError)) -> error::TrySendError<T> {
+        match src.1 {
+            TrySendError::Closed => error::TrySendError::Closed(src.0),
+            TrySendError::Full => error::TrySendError::Full(src.0),
+        }
+    }
+}
+
+pub(crate) trait Semaphore {
+    type Permit;
+
+    fn new_permit() -> Self::Permit;
+
+    /// The permit is dropped without a value being sent. In this case, the
+    /// permit must be returned to the semaphore.
+    fn drop_permit(&self, permit: &mut Self::Permit);
+
+    fn is_idle(&self) -> bool;
+
+    fn add_permit(&self);
+
+    fn poll_acquire(
+        &self,
+        cx: &mut Context<'_>,
+        permit: &mut Self::Permit,
+    ) -> Poll<Result<(), ClosedError>>;
+
+    fn try_acquire(&self, permit: &mut Self::Permit) -> Result<(), TrySendError>;
+
+    /// A value was sent into the channel and the permit held by `tx` is
+    /// dropped. In this case, the permit should not immeditely be returned to
+    /// the semaphore. Instead, the permit is returnred to the semaphore once
+    /// the sent value is read by the rx handle.
+    fn forget(&self, permit: &mut Self::Permit);
+
+    fn close(&self);
+}
+
+struct Chan<T, S> {
+    /// Handle to the push half of the lock-free list.
+    tx: list::Tx<T>,
+
+    /// Coordinates access to channel's capacity.
+    semaphore: S,
+
+    /// Receiver waker. Notified when a value is pushed into the channel.
+    rx_waker: AtomicWaker,
+
+    /// Tracks the number of outstanding sender handles.
+    ///
+    /// When this drops to zero, the send half of the channel is closed.
+    tx_count: AtomicUsize,
+
+    /// Only accessed by `Rx` handle.
+    rx_fields: UnsafeCell<RxFields<T>>,
+}
+
+impl<T, S> fmt::Debug for Chan<T, S>
+where
+    S: fmt::Debug,
+{
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("Chan")
+            .field("tx", &self.tx)
+            .field("semaphore", &self.semaphore)
+            .field("rx_waker", &self.rx_waker)
+            .field("tx_count", &self.tx_count)
+            .field("rx_fields", &"...")
+            .finish()
+    }
+}
+
+/// Fields only accessed by `Rx` handle.
+struct RxFields<T> {
+    /// Channel receiver. This field is only accessed by the `Receiver` type.
+    list: list::Rx<T>,
+
+    /// `true` if `Rx::close` is called.
+    rx_closed: bool,
+}
+
+impl<T> fmt::Debug for RxFields<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("RxFields")
+            .field("list", &self.list)
+            .field("rx_closed", &self.rx_closed)
+            .finish()
+    }
+}
+
+unsafe impl<T: Send, S: Send> Send for Chan<T, S> {}
+unsafe impl<T: Send, S: Sync> Sync for Chan<T, S> {}
+
+pub(crate) fn channel<T, S>(semaphore: S) -> (Tx<T, S>, Rx<T, S>)
+where
+    S: Semaphore,
+{
+    let (tx, rx) = list::channel();
+
+    let chan = Arc::new(Chan {
+        tx,
+        semaphore,
+        rx_waker: AtomicWaker::new(),
+        tx_count: AtomicUsize::new(1),
+        rx_fields: UnsafeCell::new(RxFields {
+            list: rx,
+            rx_closed: false,
+        }),
+    });
+
+    (Tx::new(chan.clone()), Rx::new(chan))
+}
+
+// ===== impl Tx =====
+
+impl<T, S> Tx<T, S>
+where
+    S: Semaphore,
+{
+    fn new(chan: Arc<Chan<T, S>>) -> Tx<T, S> {
+        Tx {
+            inner: chan,
+            permit: S::new_permit(),
+        }
+    }
+
+    pub(crate) fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), ClosedError>> {
+        self.inner.semaphore.poll_acquire(cx, &mut self.permit)
+    }
+
+    pub(crate) fn disarm(&mut self) {
+        // TODO: should this error if not acquired?
+        self.inner.semaphore.drop_permit(&mut self.permit)
+    }
+
+    /// Send a message and notify the receiver.
+    pub(crate) fn try_send(&mut self, value: T) -> Result<(), (T, TrySendError)> {
+        self.inner.try_send(value, &mut self.permit)
+    }
+}
+
+impl<T> Tx<T, (crate::sync::semaphore_ll::Semaphore, usize)> {
+    pub(crate) fn is_ready(&self) -> bool {
+        self.permit.is_acquired()
+    }
+}
+
+impl<T> Tx<T, AtomicUsize> {
+    pub(crate) fn send_unbounded(&self, value: T) -> Result<(), (T, TrySendError)> {
+        self.inner.try_send(value, &mut ())
+    }
+}
+
+impl<T, S> Clone for Tx<T, S>
+where
+    S: Semaphore,
+{
+    fn clone(&self) -> Tx<T, S> {
+        // Using a Relaxed ordering here is sufficient as the caller holds a
+        // strong ref to `self`, preventing a concurrent decrement to zero.
+        self.inner.tx_count.fetch_add(1, Relaxed);
+
+        Tx {
+            inner: self.inner.clone(),
+            permit: S::new_permit(),
+        }
+    }
+}
+
+impl<T, S> Drop for Tx<T, S>
+where
+    S: Semaphore,
+{
+    fn drop(&mut self) {
+        self.inner.semaphore.drop_permit(&mut self.permit);
+
+        if self.inner.tx_count.fetch_sub(1, AcqRel) != 1 {
+            return;
+        }
+
+        // Close the list, which sends a `Close` message
+        self.inner.tx.close();
+
+        // Notify the receiver
+        self.inner.rx_waker.wake();
+    }
+}
+
+// ===== impl Rx =====
+
+impl<T, S> Rx<T, S>
+where
+    S: Semaphore,
+{
+    fn new(chan: Arc<Chan<T, S>>) -> Rx<T, S> {
+        Rx { inner: chan }
+    }
+
+    pub(crate) fn close(&mut self) {
+        self.inner.rx_fields.with_mut(|rx_fields_ptr| {
+            let rx_fields = unsafe { &mut *rx_fields_ptr };
+
+            if rx_fields.rx_closed {
+                return;
+            }
+
+            rx_fields.rx_closed = true;
+        });
+
+        self.inner.semaphore.close();
+    }
+
+    /// Receive the next value
+    pub(crate) fn recv(&mut self, cx: &mut Context<'_>) -> Poll<Option<T>> {
+        use super::block::Read::*;
+
+        // Keep track of task budget
+        ready!(crate::coop::poll_proceed(cx));
+
+        self.inner.rx_fields.with_mut(|rx_fields_ptr| {
+            let rx_fields = unsafe { &mut *rx_fields_ptr };
+
+            macro_rules! try_recv {
+                () => {
+                    match rx_fields.list.pop(&self.inner.tx) {
+                        Some(Value(value)) => {
+                            self.inner.semaphore.add_permit();
+                            return Ready(Some(value));
+                        }
+                        Some(Closed) => {
+                            // TODO: This check may not be required as it most
+                            // likely can only return `true` at this point. A
+                            // channel is closed when all tx handles are
+                            // dropped. Dropping a tx handle releases memory,
+                            // which ensures that if dropping the tx handle is
+                            // visible, then all messages sent are also visible.
+                            assert!(self.inner.semaphore.is_idle());
+                            return Ready(None);
+                        }
+                        None => {} // fall through
+                    }
+                };
+            }
+
+            try_recv!();
+
+            self.inner.rx_waker.register_by_ref(cx.waker());
+
+            // It is possible that a value was pushed between attempting to read
+            // and registering the task, so we have to check the channel a
+            // second time here.
+            try_recv!();
+
+            if rx_fields.rx_closed && self.inner.semaphore.is_idle() {
+                Ready(None)
+            } else {
+                Pending
+            }
+        })
+    }
+
+    /// Receives the next value without blocking
+    pub(crate) fn try_recv(&mut self) -> Result<T, TryRecvError> {
+        use super::block::Read::*;
+        self.inner.rx_fields.with_mut(|rx_fields_ptr| {
+            let rx_fields = unsafe { &mut *rx_fields_ptr };
+            match rx_fields.list.pop(&self.inner.tx) {
+                Some(Value(value)) => {
+                    self.inner.semaphore.add_permit();
+                    Ok(value)
+                }
+                Some(Closed) => Err(TryRecvError::Closed),
+                None => Err(TryRecvError::Empty),
+            }
+        })
+    }
+}
+
+impl<T, S> Drop for Rx<T, S>
+where
+    S: Semaphore,
+{
+    fn drop(&mut self) {
+        use super::block::Read::Value;
+
+        self.close();
+
+        self.inner.rx_fields.with_mut(|rx_fields_ptr| {
+            let rx_fields = unsafe { &mut *rx_fields_ptr };
+
+            while let Some(Value(_)) = rx_fields.list.pop(&self.inner.tx) {
+                self.inner.semaphore.add_permit();
+            }
+        })
+    }
+}
+
+// ===== impl Chan =====
+
+impl<T, S> Chan<T, S>
+where
+    S: Semaphore,
+{
+    fn try_send(&self, value: T, permit: &mut S::Permit) -> Result<(), (T, TrySendError)> {
+        if let Err(e) = self.semaphore.try_acquire(permit) {
+            return Err((value, e));
+        }
+
+        // Push the value
+        self.tx.push(value);
+
+        // Notify the rx task
+        self.rx_waker.wake();
+
+        // Release the permit
+        self.semaphore.forget(permit);
+
+        Ok(())
+    }
+}
+
+impl<T, S> Drop for Chan<T, S> {
+    fn drop(&mut self) {
+        use super::block::Read::Value;
+
+        // Safety: the only owner of the rx fields is Chan, and eing
+        // inside its own Drop means we're the last ones to touch it.
+        self.rx_fields.with_mut(|rx_fields_ptr| {
+            let rx_fields = unsafe { &mut *rx_fields_ptr };
+
+            while let Some(Value(_)) = rx_fields.list.pop(&self.tx) {}
+            unsafe { rx_fields.list.free_blocks() };
+        });
+    }
+}
+
+use crate::sync::semaphore_ll::TryAcquireError;
+
+impl From<TryAcquireError> for TrySendError {
+    fn from(src: TryAcquireError) -> TrySendError {
+        if src.is_closed() {
+            TrySendError::Closed
+        } else if src.is_no_permits() {
+            TrySendError::Full
+        } else {
+            unreachable!();
+        }
+    }
+}
+
+// ===== impl Semaphore for (::Semaphore, capacity) =====
+
+use crate::sync::semaphore_ll::Permit;
+
+impl Semaphore for (crate::sync::semaphore_ll::Semaphore, usize) {
+    type Permit = Permit;
+
+    fn new_permit() -> Permit {
+        Permit::new()
+    }
+
+    fn drop_permit(&self, permit: &mut Permit) {
+        permit.release(1, &self.0);
+    }
+
+    fn add_permit(&self) {
+        self.0.add_permits(1)
+    }
+
+    fn is_idle(&self) -> bool {
+        self.0.available_permits() == self.1
+    }
+
+    fn poll_acquire(
+        &self,
+        cx: &mut Context<'_>,
+        permit: &mut Permit,
+    ) -> Poll<Result<(), ClosedError>> {
+        // Keep track of task budget
+        ready!(crate::coop::poll_proceed(cx));
+
+        permit
+            .poll_acquire(cx, 1, &self.0)
+            .map_err(|_| ClosedError::new())
+    }
+
+    fn try_acquire(&self, permit: &mut Permit) -> Result<(), TrySendError> {
+        permit.try_acquire(1, &self.0)?;
+        Ok(())
+    }
+
+    fn forget(&self, permit: &mut Self::Permit) {
+        permit.forget(1);
+    }
+
+    fn close(&self) {
+        self.0.close();
+    }
+}
+
+// ===== impl Semaphore for AtomicUsize =====
+
+use std::sync::atomic::Ordering::{Acquire, Release};
+use std::usize;
+
+impl Semaphore for AtomicUsize {
+    type Permit = ();
+
+    fn new_permit() {}
+
+    fn drop_permit(&self, _permit: &mut ()) {}
+
+    fn add_permit(&self) {
+        let prev = self.fetch_sub(2, Release);
+
+        if prev >> 1 == 0 {
+            // Something went wrong
+            process::abort();
+        }
+    }
+
+    fn is_idle(&self) -> bool {
+        self.load(Acquire) >> 1 == 0
+    }
+
+    fn poll_acquire(
+        &self,
+        _cx: &mut Context<'_>,
+        permit: &mut (),
+    ) -> Poll<Result<(), ClosedError>> {
+        Ready(self.try_acquire(permit).map_err(|_| ClosedError::new()))
+    }
+
+    fn try_acquire(&self, _permit: &mut ()) -> Result<(), TrySendError> {
+        let mut curr = self.load(Acquire);
+
+        loop {
+            if curr & 1 == 1 {
+                return Err(TrySendError::Closed);
+            }
+
+            if curr == usize::MAX ^ 1 {
+                // Overflowed the ref count. There is no safe way to recover, so
+                // abort the process. In practice, this should never happen.
+                process::abort()
+            }
+
+            match self.compare_exchange(curr, curr + 2, AcqRel, Acquire) {
+                Ok(_) => return Ok(()),
+                Err(actual) => {
+                    curr = actual;
+                }
+            }
+        }
+    }
+
+    fn forget(&self, _permit: &mut ()) {}
+
+    fn close(&self) {
+        self.fetch_or(1, Release);
+    }
+}
diff --git a/third_party/rust/tokio/src/sync/mpsc/error.rs b/third_party/rust/tokio/src/sync/mpsc/error.rs
new file mode 100644
index 0000000000..72c42aa53e
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/mpsc/error.rs
@@ -0,0 +1,146 @@
+//! Channel error types
+
+use std::error::Error;
+use std::fmt;
+
+/// Error returned by the `Sender`.
+#[derive(Debug)]
+pub struct SendError<T>(pub T);
+
+impl<T> fmt::Display for SendError<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(fmt, "channel closed")
+    }
+}
+
+impl<T: fmt::Debug> std::error::Error for SendError<T> {}
+
+// ===== TrySendError =====
+
+/// This enumeration is the list of the possible error outcomes for the
+/// [try_send](super::Sender::try_send) method.
+#[derive(Debug)]
+pub enum TrySendError<T> {
+    /// The data could not be sent on the channel because the channel is
+    /// currently full and sending would require blocking.
+    Full(T),
+
+    /// The receive half of the channel was explicitly closed or has been
+    /// dropped.
+    Closed(T),
+}
+
+impl<T: fmt::Debug> Error for TrySendError<T> {}
+
+impl<T> fmt::Display for TrySendError<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            fmt,
+            "{}",
+            match self {
+                TrySendError::Full(..) => "no available capacity",
+                TrySendError::Closed(..) => "channel closed",
+            }
+        )
+    }
+}
+
+impl<T> From<SendError<T>> for TrySendError<T> {
+    fn from(src: SendError<T>) -> TrySendError<T> {
+        TrySendError::Closed(src.0)
+    }
+}
+
+// ===== RecvError =====
+
+/// Error returned by `Receiver`.
+#[derive(Debug)]
+pub struct RecvError(());
+
+impl fmt::Display for RecvError {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(fmt, "channel closed")
+    }
+}
+
+impl Error for RecvError {}
+
+// ===== TryRecvError =====
+
+/// This enumeration is the list of the possible reasons that try_recv
+/// could not return data when called.
+#[derive(Debug, PartialEq)]
+pub enum TryRecvError {
+    /// This channel is currently empty, but the Sender(s) have not yet
+    /// disconnected, so data may yet become available.
+    Empty,
+    /// The channel's sending half has been closed, and there will
+    /// never be any more data received on it.
+    Closed,
+}
+
+impl fmt::Display for TryRecvError {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            fmt,
+            "{}",
+            match self {
+                TryRecvError::Empty => "channel empty",
+                TryRecvError::Closed => "channel closed",
+            }
+        )
+    }
+}
+
+impl Error for TryRecvError {}
+
+// ===== ClosedError =====
+
+/// Error returned by [`Sender::poll_ready`](super::Sender::poll_ready).
+#[derive(Debug)]
+pub struct ClosedError(());
+
+impl ClosedError {
+    pub(crate) fn new() -> ClosedError {
+        ClosedError(())
+    }
+}
+
+impl fmt::Display for ClosedError {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(fmt, "channel closed")
+    }
+}
+
+impl Error for ClosedError {}
+
+cfg_time! {
+    // ===== SendTimeoutError =====
+
+    #[derive(Debug)]
+    /// Error returned by [`Sender::send_timeout`](super::Sender::send_timeout)].
+    pub enum SendTimeoutError<T> {
+        /// The data could not be sent on the channel because the channel is
+        /// full, and the timeout to send has elapsed.
+        Timeout(T),
+
+        /// The receive half of the channel was explicitly closed or has been
+        /// dropped.
+        Closed(T),
+    }
+
+    impl<T: fmt::Debug> Error for SendTimeoutError<T> {}
+
+    impl<T> fmt::Display for SendTimeoutError<T> {
+        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+            write!(
+                fmt,
+                "{}",
+                match self {
+                    SendTimeoutError::Timeout(..) => "timed out waiting on send operation",
+                    SendTimeoutError::Closed(..) => "channel closed",
+                }
+            )
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/sync/mpsc/list.rs b/third_party/rust/tokio/src/sync/mpsc/list.rs
new file mode 100644
index 0000000000..53f82a25ef
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/mpsc/list.rs
@@ -0,0 +1,341 @@
+//! A concurrent, lock-free, FIFO list.
+
+use crate::loom::{
+    sync::atomic::{AtomicPtr, AtomicUsize},
+    thread,
+};
+use crate::sync::mpsc::block::{self, Block};
+
+use std::fmt;
+use std::ptr::NonNull;
+use std::sync::atomic::Ordering::{AcqRel, Acquire, Relaxed, Release};
+
+/// List queue transmit handle
+pub(crate) struct Tx<T> {
+    /// Tail in the `Block` mpmc list.
+    block_tail: AtomicPtr<Block<T>>,
+
+    /// Position to push the next message. This reference a block and offset
+    /// into the block.
+    tail_position: AtomicUsize,
+}
+
+/// List queue receive handle
+pub(crate) struct Rx<T> {
+    /// Pointer to the block being processed
+    head: NonNull<Block<T>>,
+
+    /// Next slot index to process
+    index: usize,
+
+    /// Pointer to the next block pending release
+    free_head: NonNull<Block<T>>,
+}
+
+pub(crate) fn channel<T>() -> (Tx<T>, Rx<T>) {
+    // Create the initial block shared between the tx and rx halves.
+    let initial_block = Box::new(Block::new(0));
+    let initial_block_ptr = Box::into_raw(initial_block);
+
+    let tx = Tx {
+        block_tail: AtomicPtr::new(initial_block_ptr),
+        tail_position: AtomicUsize::new(0),
+    };
+
+    let head = NonNull::new(initial_block_ptr).unwrap();
+
+    let rx = Rx {
+        head,
+        index: 0,
+        free_head: head,
+    };
+
+    (tx, rx)
+}
+
+impl<T> Tx<T> {
+    /// Pushes a value into the list.
+    pub(crate) fn push(&self, value: T) {
+        // First, claim a slot for the value. `Acquire` is used here to
+        // synchronize with the `fetch_add` in `reclaim_blocks`.
+        let slot_index = self.tail_position.fetch_add(1, Acquire);
+
+        // Load the current block and write the value
+        let block = self.find_block(slot_index);
+
+        unsafe {
+            // Write the value to the block
+            block.as_ref().write(slot_index, value);
+        }
+    }
+
+    /// Closes the send half of the list
+    ///
+    /// Similar process as pushing a value, but instead of writing the value &
+    /// setting the ready flag, the TX_CLOSED flag is set on the block.
+    pub(crate) fn close(&self) {
+        // First, claim a slot for the value. This is the last slot that will be
+        // claimed.
+        let slot_index = self.tail_position.fetch_add(1, Acquire);
+
+        let block = self.find_block(slot_index);
+
+        unsafe { block.as_ref().tx_close() }
+    }
+
+    fn find_block(&self, slot_index: usize) -> NonNull<Block<T>> {
+        // The start index of the block that contains `index`.
+        let start_index = block::start_index(slot_index);
+
+        // The index offset into the block
+        let offset = block::offset(slot_index);
+
+        // Load the current head of the block
+        let mut block_ptr = self.block_tail.load(Acquire);
+
+        let block = unsafe { &*block_ptr };
+
+        // Calculate the distance between the tail ptr and the target block
+        let distance = block.distance(start_index);
+
+        // Decide if this call to `find_block` should attempt to update the
+        // `block_tail` pointer.
+        //
+        // Updating `block_tail` is not always performed in order to reduce
+        // contention.
+        //
+        // When set, as the routine walks the linked list, it attempts to update
+        // `block_tail`. If the update cannot be performed, `try_updating_tail`
+        // is unset.
+        let mut try_updating_tail = distance > offset;
+
+        // Walk the linked list of blocks until the block with `start_index` is
+        // found.
+        loop {
+            let block = unsafe { &(*block_ptr) };
+
+            if block.is_at_index(start_index) {
+                return unsafe { NonNull::new_unchecked(block_ptr) };
+            }
+
+            let next_block = block
+                .load_next(Acquire)
+                // There is no allocated next block, grow the linked list.
+                .unwrap_or_else(|| block.grow());
+
+            // If the block is **not** final, then the tail pointer cannot be
+            // advanced any more.
+            try_updating_tail &= block.is_final();
+
+            if try_updating_tail {
+                // Advancing `block_tail` must happen when walking the linked
+                // list. `block_tail` may not advance passed any blocks that are
+                // not "final". At the point a block is finalized, it is unknown
+                // if there are any prior blocks that are unfinalized, which
+                // makes it impossible to advance `block_tail`.
+                //
+                // While walking the linked list, `block_tail` can be advanced
+                // as long as finalized blocks are traversed.
+                //
+                // Release ordering is used to ensure that any subsequent reads
+                // are able to see the memory pointed to by `block_tail`.
+                //
+                // Acquire is not needed as any "actual" value is not accessed.
+                // At this point, the linked list is walked to acquire blocks.
+                let actual =
+                    self.block_tail
+                        .compare_and_swap(block_ptr, next_block.as_ptr(), Release);
+
+                if actual == block_ptr {
+                    // Synchronize with any senders
+                    let tail_position = self.tail_position.fetch_add(0, Release);
+
+                    unsafe {
+                        block.tx_release(tail_position);
+                    }
+                } else {
+                    // A concurrent sender is also working on advancing
+                    // `block_tail` and this thread is falling behind.
+                    //
+                    // Stop trying to advance the tail pointer
+                    try_updating_tail = false;
+                }
+            }
+
+            block_ptr = next_block.as_ptr();
+
+            thread::yield_now();
+        }
+    }
+
+    pub(crate) unsafe fn reclaim_block(&self, mut block: NonNull<Block<T>>) {
+        // The block has been removed from the linked list and ownership
+        // is reclaimed.
+        //
+        // Before dropping the block, see if it can be reused by
+        // inserting it back at the end of the linked list.
+        //
+        // First, reset the data
+        block.as_mut().reclaim();
+
+        let mut reused = false;
+
+        // Attempt to insert the block at the end
+        //
+        // Walk at most three times
+        //
+        let curr_ptr = self.block_tail.load(Acquire);
+
+        // The pointer can never be null
+        debug_assert!(!curr_ptr.is_null());
+
+        let mut curr = NonNull::new_unchecked(curr_ptr);
+
+        // TODO: Unify this logic with Block::grow
+        for _ in 0..3 {
+            match curr.as_ref().try_push(&mut block, AcqRel) {
+                Ok(_) => {
+                    reused = true;
+                    break;
+                }
+                Err(next) => {
+                    curr = next;
+                }
+            }
+        }
+
+        if !reused {
+            let _ = Box::from_raw(block.as_ptr());
+        }
+    }
+}
+
+impl<T> fmt::Debug for Tx<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("Tx")
+            .field("block_tail", &self.block_tail.load(Relaxed))
+            .field("tail_position", &self.tail_position.load(Relaxed))
+            .finish()
+    }
+}
+
+impl<T> Rx<T> {
+    /// Pops the next value off the queue
+    pub(crate) fn pop(&mut self, tx: &Tx<T>) -> Option<block::Read<T>> {
+        // Advance `head`, if needed
+        if !self.try_advancing_head() {
+            return None;
+        }
+
+        self.reclaim_blocks(tx);
+
+        unsafe {
+            let block = self.head.as_ref();
+
+            let ret = block.read(self.index);
+
+            if let Some(block::Read::Value(..)) = ret {
+                self.index = self.index.wrapping_add(1);
+            }
+
+            ret
+        }
+    }
+
+    /// Tries advancing the block pointer to the block referenced by `self.index`.
+    ///
+    /// Returns `true` if successful, `false` if there is no next block to load.
+    fn try_advancing_head(&mut self) -> bool {
+        let block_index = block::start_index(self.index);
+
+        loop {
+            let next_block = {
+                let block = unsafe { self.head.as_ref() };
+
+                if block.is_at_index(block_index) {
+                    return true;
+                }
+
+                block.load_next(Acquire)
+            };
+
+            let next_block = match next_block {
+                Some(next_block) => next_block,
+                None => {
+                    return false;
+                }
+            };
+
+            self.head = next_block;
+
+            thread::yield_now();
+        }
+    }
+
+    fn reclaim_blocks(&mut self, tx: &Tx<T>) {
+        while self.free_head != self.head {
+            unsafe {
+                // Get a handle to the block that will be freed and update
+                // `free_head` to point to the next block.
+                let block = self.free_head;
+
+                let observed_tail_position = block.as_ref().observed_tail_position();
+
+                let required_index = match observed_tail_position {
+                    Some(i) => i,
+                    None => return,
+                };
+
+                if required_index > self.index {
+                    return;
+                }
+
+                // We may read the next pointer with `Relaxed` ordering as it is
+                // guaranteed that the `reclaim_blocks` routine trails the `recv`
+                // routine. Any memory accessed by `reclaim_blocks` has already
+                // been acquired by `recv`.
+                let next_block = block.as_ref().load_next(Relaxed);
+
+                // Update the free list head
+                self.free_head = next_block.unwrap();
+
+                // Push the emptied block onto the back of the queue, making it
+                // available to senders.
+                tx.reclaim_block(block);
+            }
+
+            thread::yield_now();
+        }
+    }
+
+    /// Effectively `Drop` all the blocks. Should only be called once, when
+    /// the list is dropping.
+    pub(super) unsafe fn free_blocks(&mut self) {
+        debug_assert_ne!(self.free_head, NonNull::dangling());
+
+        let mut cur = Some(self.free_head);
+
+        #[cfg(debug_assertions)]
+        {
+            // to trigger the debug assert above so as to catch that we
+            // don't call `free_blocks` more than once.
+            self.free_head = NonNull::dangling();
+            self.head = NonNull::dangling();
+        }
+
+        while let Some(block) = cur {
+            cur = block.as_ref().load_next(Relaxed);
+            drop(Box::from_raw(block.as_ptr()));
+        }
+    }
+}
+
+impl<T> fmt::Debug for Rx<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("Rx")
+            .field("head", &self.head)
+            .field("index", &self.index)
+            .field("free_head", &self.free_head)
+            .finish()
+    }
+}
diff --git a/third_party/rust/tokio/src/sync/mpsc/mod.rs b/third_party/rust/tokio/src/sync/mpsc/mod.rs
new file mode 100644
index 0000000000..4cfd6150f3
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/mpsc/mod.rs
@@ -0,0 +1,64 @@
+#![cfg_attr(not(feature = "sync"), allow(dead_code, unreachable_pub))]
+
+//! A multi-producer, single-consumer queue for sending values across
+//! asynchronous tasks.
+//!
+//! Similar to `std`, channel creation provides [`Receiver`] and [`Sender`]
+//! handles. [`Receiver`] implements `Stream` and allows a task to read values
+//! out of the channel. If there is no message to read, the current task will be
+//! notified when a new value is sent.  [`Sender`] implements the `Sink` trait
+//! and allows sending messages into the channel. If the channel is at capacity,
+//! the send is rejected and the task will be notified when additional capacity
+//! is available. In other words, the channel provides backpressure.
+//!
+//! Unbounded channels are also available using the `unbounded_channel`
+//! constructor.
+//!
+//! # Disconnection
+//!
+//! When all [`Sender`] handles have been dropped, it is no longer
+//! possible to send values into the channel. This is considered the termination
+//! event of the stream. As such, `Receiver::poll` returns `Ok(Ready(None))`.
+//!
+//! If the [`Receiver`] handle is dropped, then messages can no longer
+//! be read out of the channel. In this case, all further attempts to send will
+//! result in an error.
+//!
+//! # Clean Shutdown
+//!
+//! When the [`Receiver`] is dropped, it is possible for unprocessed messages to
+//! remain in the channel. Instead, it is usually desirable to perform a "clean"
+//! shutdown. To do this, the receiver first calls `close`, which will prevent
+//! any further messages to be sent into the channel. Then, the receiver
+//! consumes the channel to completion, at which point the receiver can be
+//! dropped.
+//!
+//! [`Sender`]: crate::sync::mpsc::Sender
+//! [`Receiver`]: crate::sync::mpsc::Receiver
+
+pub(super) mod block;
+
+mod bounded;
+pub use self::bounded::{channel, Receiver, Sender};
+
+mod chan;
+
+pub(super) mod list;
+
+mod unbounded;
+pub use self::unbounded::{unbounded_channel, UnboundedReceiver, UnboundedSender};
+
+pub mod error;
+
+/// The number of values a block can contain.
+///
+/// This value must be a power of 2. It also must be smaller than the number of
+/// bits in `usize`.
+#[cfg(all(target_pointer_width = "64", not(loom)))]
+const BLOCK_CAP: usize = 32;
+
+#[cfg(all(not(target_pointer_width = "64"), not(loom)))]
+const BLOCK_CAP: usize = 16;
+
+#[cfg(loom)]
+const BLOCK_CAP: usize = 2;
diff --git a/third_party/rust/tokio/src/sync/mpsc/unbounded.rs b/third_party/rust/tokio/src/sync/mpsc/unbounded.rs
new file mode 100644
index 0000000000..ba543fe4c8
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/mpsc/unbounded.rs
@@ -0,0 +1,176 @@
+use crate::loom::sync::atomic::AtomicUsize;
+use crate::sync::mpsc::chan;
+use crate::sync::mpsc::error::{SendError, TryRecvError};
+
+use std::fmt;
+use std::task::{Context, Poll};
+
+/// Send values to the associated `UnboundedReceiver`.
+///
+/// Instances are created by the
+/// [`unbounded_channel`](unbounded_channel) function.
+pub struct UnboundedSender<T> {
+    chan: chan::Tx<T, Semaphore>,
+}
+
+impl<T> Clone for UnboundedSender<T> {
+    fn clone(&self) -> Self {
+        UnboundedSender {
+            chan: self.chan.clone(),
+        }
+    }
+}
+
+impl<T> fmt::Debug for UnboundedSender<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("UnboundedSender")
+            .field("chan", &self.chan)
+            .finish()
+    }
+}
+
+/// Receive values from the associated `UnboundedSender`.
+///
+/// Instances are created by the
+/// [`unbounded_channel`](unbounded_channel) function.
+pub struct UnboundedReceiver<T> {
+    /// The channel receiver
+    chan: chan::Rx<T, Semaphore>,
+}
+
+impl<T> fmt::Debug for UnboundedReceiver<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("UnboundedReceiver")
+            .field("chan", &self.chan)
+            .finish()
+    }
+}
+
+/// Creates an unbounded mpsc channel for communicating between asynchronous
+/// tasks.
+///
+/// A `send` on this channel will always succeed as long as the receive half has
+/// not been closed. If the receiver falls behind, messages will be arbitrarily
+/// buffered.
+///
+/// **Note** that the amount of available system memory is an implicit bound to
+/// the channel. Using an `unbounded` channel has the ability of causing the
+/// process to run out of memory. In this case, the process will be aborted.
+pub fn unbounded_channel<T>() -> (UnboundedSender<T>, UnboundedReceiver<T>) {
+    let (tx, rx) = chan::channel(AtomicUsize::new(0));
+
+    let tx = UnboundedSender::new(tx);
+    let rx = UnboundedReceiver::new(rx);
+
+    (tx, rx)
+}
+
+/// No capacity
+type Semaphore = AtomicUsize;
+
+impl<T> UnboundedReceiver<T> {
+    pub(crate) fn new(chan: chan::Rx<T, Semaphore>) -> UnboundedReceiver<T> {
+        UnboundedReceiver { chan }
+    }
+
+    #[doc(hidden)] // TODO: doc
+    pub fn poll_recv(&mut self, cx: &mut Context<'_>) -> Poll<Option<T>> {
+        self.chan.recv(cx)
+    }
+
+    /// Receives the next value for this receiver.
+    ///
+    /// `None` is returned when all `Sender` halves have dropped, indicating
+    /// that no further values can be sent on the channel.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::mpsc;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, mut rx) = mpsc::unbounded_channel();
+    ///
+    ///     tokio::spawn(async move {
+    ///         tx.send("hello").unwrap();
+    ///     });
+    ///
+    ///     assert_eq!(Some("hello"), rx.recv().await);
+    ///     assert_eq!(None, rx.recv().await);
+    /// }
+    /// ```
+    ///
+    /// Values are buffered:
+    ///
+    /// ```
+    /// use tokio::sync::mpsc;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, mut rx) = mpsc::unbounded_channel();
+    ///
+    ///     tx.send("hello").unwrap();
+    ///     tx.send("world").unwrap();
+    ///
+    ///     assert_eq!(Some("hello"), rx.recv().await);
+    ///     assert_eq!(Some("world"), rx.recv().await);
+    /// }
+    /// ```
+    pub async fn recv(&mut self) -> Option<T> {
+        use crate::future::poll_fn;
+
+        poll_fn(|cx| self.poll_recv(cx)).await
+    }
+
+    /// Attempts to return a pending value on this receiver without blocking.
+    ///
+    /// This method will never block the caller in order to wait for data to
+    /// become available. Instead, this will always return immediately with
+    /// a possible option of pending data on the channel.
+    ///
+    /// This is useful for a flavor of "optimistic check" before deciding to
+    /// block on a receiver.
+    ///
+    /// Compared with recv, this function has two failure cases instead of
+    /// one (one for disconnection, one for an empty buffer).
+    pub fn try_recv(&mut self) -> Result<T, TryRecvError> {
+        self.chan.try_recv()
+    }
+
+    /// Closes the receiving half of a channel, without dropping it.
+    ///
+    /// This prevents any further messages from being sent on the channel while
+    /// still enabling the receiver to drain messages that are buffered.
+    pub fn close(&mut self) {
+        self.chan.close();
+    }
+}
+
+#[cfg(feature = "stream")]
+impl<T> crate::stream::Stream for UnboundedReceiver<T> {
+    type Item = T;
+
+    fn poll_next(mut self: std::pin::Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
+        self.poll_recv(cx)
+    }
+}
+
+impl<T> UnboundedSender<T> {
+    pub(crate) fn new(chan: chan::Tx<T, Semaphore>) -> UnboundedSender<T> {
+        UnboundedSender { chan }
+    }
+
+    /// Attempts to send a message on this `UnboundedSender` without blocking.
+    ///
+    /// If the receive half of the channel is closed, either due to [`close`]
+    /// being called or the [`UnboundedReceiver`] having been dropped,
+    /// the function returns an error. The error includes the value passed to `send`.
+    ///
+    /// [`close`]: UnboundedReceiver::close
+    /// [`UnboundedReceiver`]: UnboundedReceiver
+    pub fn send(&self, message: T) -> Result<(), SendError<T>> {
+        self.chan.send_unbounded(message)?;
+        Ok(())
+    }
+}
diff --git a/third_party/rust/tokio/src/sync/mutex.rs b/third_party/rust/tokio/src/sync/mutex.rs
new file mode 100644
index 0000000000..7167906de1
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/mutex.rs
@@ -0,0 +1,228 @@
+//! An asynchronous `Mutex`-like type.
+//!
+//! This module provides [`Mutex`], a type that acts similarly to an asynchronous `Mutex`, with one
+//! major difference: the [`MutexGuard`] returned by `lock` is not tied to the lifetime of the
+//! `Mutex`. This enables you to acquire a lock, and then pass that guard into a future, and then
+//! release it at some later point in time.
+//!
+//! This allows you to do something along the lines of:
+//!
+//! ```rust,no_run
+//! use tokio::sync::Mutex;
+//! use std::sync::Arc;
+//!
+//! #[tokio::main]
+//! async fn main() {
+//!     let data1 = Arc::new(Mutex::new(0));
+//!     let data2 = Arc::clone(&data1);
+//!
+//!     tokio::spawn(async move {
+//!         let mut lock = data2.lock().await;
+//!         *lock += 1;
+//!     });
+//!
+//!     let mut lock = data1.lock().await;
+//!     *lock += 1;
+//! }
+//! ```
+//!
+//! Another example
+//! ```rust,no_run
+//! #![warn(rust_2018_idioms)]
+//!
+//! use tokio::sync::Mutex;
+//! use std::sync::Arc;
+//!
+//!
+//! #[tokio::main]
+//! async fn main() {
+//!    let count = Arc::new(Mutex::new(0));
+//!
+//!    for _ in 0..5 {
+//!        let my_count = Arc::clone(&count);
+//!        tokio::spawn(async move {
+//!            for _ in 0..10 {
+//!                let mut lock = my_count.lock().await;
+//!                *lock += 1;
+//!                println!("{}", lock);
+//!            }
+//!        });
+//!    }
+//!
+//!    loop {
+//!        if *count.lock().await >= 50 {
+//!            break;
+//!        }
+//!    }
+//!   println!("Count hit 50.");
+//! }
+//! ```
+//! There are a few things of note here to pay attention to in this example.
+//! 1. The mutex is wrapped in an [`std::sync::Arc`] to allow it to be shared across threads.
+//! 2. Each spawned task obtains a lock and releases it on every iteration.
+//! 3. Mutation of the data the Mutex is protecting is done by de-referencing the the obtained lock
+//!    as seen on lines 23 and 30.
+//!
+//! Tokio's Mutex works in a simple FIFO (first in, first out) style where as requests for a lock are
+//! made Tokio will queue them up and provide a lock when it is that requester's turn. In that way
+//! the Mutex is "fair" and predictable in how it distributes the locks to inner data. This is why
+//! the output of this program is an in-order count to 50. Locks are released and reacquired
+//! after every iteration, so basically, each thread goes to the back of the line after it increments
+//! the value once. Also, since there is only a single valid lock at any given time there is no
+//! possibility of a race condition when mutating the inner value.
+//!
+//! Note that in contrast to `std::sync::Mutex`, this implementation does not
+//! poison the mutex when a thread holding the `MutexGuard` panics. In such a
+//! case, the mutex will be unlocked. If the panic is caught, this might leave
+//! the data protected by the mutex in an inconsistent state.
+//!
+//! [`Mutex`]: struct@Mutex
+//! [`MutexGuard`]: struct@MutexGuard
+use crate::coop::CoopFutureExt;
+use crate::sync::batch_semaphore as semaphore;
+
+use std::cell::UnsafeCell;
+use std::error::Error;
+use std::fmt;
+use std::ops::{Deref, DerefMut};
+
+/// An asynchronous mutual exclusion primitive useful for protecting shared data
+///
+/// Each mutex has a type parameter (`T`) which represents the data that it is protecting. The data
+/// can only be accessed through the RAII guards returned from `lock`, which
+/// guarantees that the data is only ever accessed when the mutex is locked.
+#[derive(Debug)]
+pub struct Mutex<T> {
+    c: UnsafeCell<T>,
+    s: semaphore::Semaphore,
+}
+
+/// A handle to a held `Mutex`.
+///
+/// As long as you have this guard, you have exclusive access to the underlying `T`. The guard
+/// internally keeps a reference-couned pointer to the original `Mutex`, so even if the lock goes
+/// away, the guard remains valid.
+///
+/// The lock is automatically released whenever the guard is dropped, at which point `lock`
+/// will succeed yet again.
+pub struct MutexGuard<'a, T> {
+    lock: &'a Mutex<T>,
+}
+
+// As long as T: Send, it's fine to send and share Mutex<T> between threads.
+// If T was not Send, sending and sharing a Mutex<T> would be bad, since you can access T through
+// Mutex<T>.
+unsafe impl<T> Send for Mutex<T> where T: Send {}
+unsafe impl<T> Sync for Mutex<T> where T: Send {}
+unsafe impl<'a, T> Sync for MutexGuard<'a, T> where T: Send + Sync {}
+
+/// Error returned from the [`Mutex::try_lock`] function.
+///
+/// A `try_lock` operation can only fail if the mutex is already locked.
+///
+/// [`Mutex::try_lock`]: Mutex::try_lock
+#[derive(Debug)]
+pub struct TryLockError(());
+
+impl fmt::Display for TryLockError {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(fmt, "{}", "operation would block")
+    }
+}
+
+impl Error for TryLockError {}
+
+#[test]
+#[cfg(not(loom))]
+fn bounds() {
+    fn check_send<T: Send>() {}
+    fn check_unpin<T: Unpin>() {}
+    // This has to take a value, since the async fn's return type is unnameable.
+    fn check_send_sync_val<T: Send + Sync>(_t: T) {}
+    fn check_send_sync<T: Send + Sync>() {}
+    check_send::<MutexGuard<'_, u32>>();
+    check_unpin::<Mutex<u32>>();
+    check_send_sync::<Mutex<u32>>();
+
+    let mutex = Mutex::new(1);
+    check_send_sync_val(mutex.lock());
+}
+
+impl<T> Mutex<T> {
+    /// Creates a new lock in an unlocked state ready for use.
+    pub fn new(t: T) -> Self {
+        Self {
+            c: UnsafeCell::new(t),
+            s: semaphore::Semaphore::new(1),
+        }
+    }
+
+    /// A future that resolves on acquiring the lock and returns the `MutexGuard`.
+    pub async fn lock(&self) -> MutexGuard<'_, T> {
+        self.s.acquire(1).cooperate().await.unwrap_or_else(|_| {
+            // The semaphore was closed. but, we never explicitly close it, and we have a
+            // handle to it through the Arc, which means that this can never happen.
+            unreachable!()
+        });
+        MutexGuard { lock: self }
+    }
+
+    /// Tries to acquire the lock
+    pub fn try_lock(&self) -> Result<MutexGuard<'_, T>, TryLockError> {
+        match self.s.try_acquire(1) {
+            Ok(_) => Ok(MutexGuard { lock: self }),
+            Err(_) => Err(TryLockError(())),
+        }
+    }
+
+    /// Consumes the mutex, returning the underlying data.
+    pub fn into_inner(self) -> T {
+        self.c.into_inner()
+    }
+}
+
+impl<'a, T> Drop for MutexGuard<'a, T> {
+    fn drop(&mut self) {
+        self.lock.s.release(1)
+    }
+}
+
+impl<T> From<T> for Mutex<T> {
+    fn from(s: T) -> Self {
+        Self::new(s)
+    }
+}
+
+impl<T> Default for Mutex<T>
+where
+    T: Default,
+{
+    fn default() -> Self {
+        Self::new(T::default())
+    }
+}
+
+impl<'a, T> Deref for MutexGuard<'a, T> {
+    type Target = T;
+    fn deref(&self) -> &Self::Target {
+        unsafe { &*self.lock.c.get() }
+    }
+}
+
+impl<'a, T> DerefMut for MutexGuard<'a, T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        unsafe { &mut *self.lock.c.get() }
+    }
+}
+
+impl<'a, T: fmt::Debug> fmt::Debug for MutexGuard<'a, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&**self, f)
+    }
+}
+
+impl<'a, T: fmt::Display> fmt::Display for MutexGuard<'a, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(&**self, f)
+    }
+}
diff --git a/third_party/rust/tokio/src/sync/notify.rs b/third_party/rust/tokio/src/sync/notify.rs
new file mode 100644
index 0000000000..5cb41e89ea
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/notify.rs
@@ -0,0 +1,556 @@
+use crate::loom::sync::atomic::AtomicU8;
+use crate::loom::sync::Mutex;
+use crate::util::linked_list::{self, LinkedList};
+
+use std::cell::UnsafeCell;
+use std::future::Future;
+use std::marker::PhantomPinned;
+use std::pin::Pin;
+use std::ptr::NonNull;
+use std::sync::atomic::Ordering::SeqCst;
+use std::task::{Context, Poll, Waker};
+
+/// Notify a single task to wake up.
+///
+/// `Notify` provides a basic mechanism to notify a single task of an event.
+/// `Notify` itself does not carry any data. Instead, it is to be used to signal
+/// another task to perform an operation.
+///
+/// `Notify` can be thought of as a [`Semaphore`] starting with 0 permits.
+/// [`notified().await`] waits for a permit to become available, and [`notify()`]
+/// sets a permit **if there currently are no available permits**.
+///
+/// The synchronization details of `Notify` are similar to
+/// [`thread::park`][park] and [`Thread::unpark`][unpark] from std. A [`Notify`]
+/// value contains a single permit. [`notified().await`] waits for the permit to
+/// be made available, consumes the permit, and resumes.  [`notify()`] sets the
+/// permit, waking a pending task if there is one.
+///
+/// If `notify()` is called **before** `notfied().await`, then the next call to
+/// `notified().await` will complete immediately, consuming the permit. Any
+/// subsequent calls to `notified().await` will wait for a new permit.
+///
+/// If `notify()` is called **multiple** times before `notified().await`, only a
+/// **single** permit is stored. The next call to `notified().await` will
+/// complete immediately, but the one after will wait for a new permit.
+///
+/// # Examples
+///
+/// Basic usage.
+///
+/// ```
+/// use tokio::sync::Notify;
+/// use std::sync::Arc;
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let notify = Arc::new(Notify::new());
+///     let notify2 = notify.clone();
+///
+///     tokio::spawn(async move {
+///         notify2.notified().await;
+///         println!("received notification");
+///     });
+///
+///     println!("sending notification");
+///     notify.notify();
+/// }
+/// ```
+///
+/// Unbound mpsc channel.
+///
+/// ```
+/// use tokio::sync::Notify;
+///
+/// use std::collections::VecDeque;
+/// use std::sync::Mutex;
+///
+/// struct Channel<T> {
+///     values: Mutex<VecDeque<T>>,
+///     notify: Notify,
+/// }
+///
+/// impl<T> Channel<T> {
+///     pub fn send(&self, value: T) {
+///         self.values.lock().unwrap()
+///             .push_back(value);
+///
+///         // Notify the consumer a value is available
+///         self.notify.notify();
+///     }
+///
+///     pub async fn recv(&self) -> T {
+///         loop {
+///             // Drain values
+///             if let Some(value) = self.values.lock().unwrap().pop_front() {
+///                 return value;
+///             }
+///
+///             // Wait for values to be available
+///             self.notify.notified().await;
+///         }
+///     }
+/// }
+/// ```
+///
+/// [park]: std::thread::park
+/// [unpark]: std::thread::Thread::unpark
+/// [`notified().await`]: Notify::notified()
+/// [`notify()`]: Notify::notify()
+/// [`Semaphore`]: crate::sync::Semaphore
+#[derive(Debug)]
+pub struct Notify {
+    state: AtomicU8,
+    waiters: Mutex<LinkedList<Waiter>>,
+}
+
+#[derive(Debug)]
+struct Waiter {
+    /// Intrusive linked-list pointers
+    pointers: linked_list::Pointers<Waiter>,
+
+    /// Waiting task's waker
+    waker: Option<Waker>,
+
+    /// `true` if the notification has been assigned to this waiter.
+    notified: bool,
+
+    /// Should not be `Unpin`.
+    _p: PhantomPinned,
+}
+
+/// Future returned from `notified()`
+#[derive(Debug)]
+struct Notified<'a> {
+    /// The `Notify` being received on.
+    notify: &'a Notify,
+
+    /// The current state of the receiving process.
+    state: State,
+
+    /// Entry in the waiter `LinkedList`.
+    waiter: UnsafeCell<Waiter>,
+}
+
+unsafe impl<'a> Send for Notified<'a> {}
+unsafe impl<'a> Sync for Notified<'a> {}
+
+#[derive(Debug)]
+enum State {
+    Init,
+    Waiting,
+    Done,
+}
+
+/// Initial "idle" state
+const EMPTY: u8 = 0;
+
+/// One or more threads are currently waiting to be notified.
+const WAITING: u8 = 1;
+
+/// Pending notification
+const NOTIFIED: u8 = 2;
+
+impl Notify {
+    /// Create a new `Notify`, initialized without a permit.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::Notify;
+    ///
+    /// let notify = Notify::new();
+    /// ```
+    pub fn new() -> Notify {
+        Notify {
+            state: AtomicU8::new(0),
+            waiters: Mutex::new(LinkedList::new()),
+        }
+    }
+
+    /// Wait for a notification.
+    ///
+    /// Each `Notify` value holds a single permit. If a permit is available from
+    /// an earlier call to [`notify()`], then `notified().await` will complete
+    /// immediately, consuming that permit. Otherwise, `notified().await` waits
+    /// for a permit to be made available by the next call to `notify()`.
+    ///
+    /// [`notify()`]: Notify::notify
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::Notify;
+    /// use std::sync::Arc;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let notify = Arc::new(Notify::new());
+    ///     let notify2 = notify.clone();
+    ///
+    ///     tokio::spawn(async move {
+    ///         notify2.notified().await;
+    ///         println!("received notification");
+    ///     });
+    ///
+    ///     println!("sending notification");
+    ///     notify.notify();
+    /// }
+    /// ```
+    pub async fn notified(&self) {
+        Notified {
+            notify: self,
+            state: State::Init,
+            waiter: UnsafeCell::new(Waiter {
+                pointers: linked_list::Pointers::new(),
+                waker: None,
+                notified: false,
+                _p: PhantomPinned,
+            }),
+        }
+        .await
+    }
+
+    /// Notifies a waiting task
+    ///
+    /// If a task is currently waiting, that task is notified. Otherwise, a
+    /// permit is stored in this `Notify` value and the **next** call to
+    /// [`notified().await`] will complete immediately consuming the permit made
+    /// available by this call to `notify()`.
+    ///
+    /// At most one permit may be stored by `Notify`. Many sequential calls to
+    /// `notify` will result in a single permit being stored. The next call to
+    /// `notified().await` will complete immediately, but the one after that
+    /// will wait.
+    ///
+    /// [`notified().await`]: Notify::notified()
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::Notify;
+    /// use std::sync::Arc;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let notify = Arc::new(Notify::new());
+    ///     let notify2 = notify.clone();
+    ///
+    ///     tokio::spawn(async move {
+    ///         notify2.notified().await;
+    ///         println!("received notification");
+    ///     });
+    ///
+    ///     println!("sending notification");
+    ///     notify.notify();
+    /// }
+    /// ```
+    pub fn notify(&self) {
+        // Load the current state
+        let mut curr = self.state.load(SeqCst);
+
+        // If the state is `EMPTY`, transition to `NOTIFIED` and return.
+        while let EMPTY | NOTIFIED = curr {
+            // The compare-exchange from `NOTIFIED` -> `NOTIFIED` is intended. A
+            // happens-before synchronization must happen between this atomic
+            // operation and a task calling `notified().await`.
+            let res = self.state.compare_exchange(curr, NOTIFIED, SeqCst, SeqCst);
+
+            match res {
+                // No waiters, no further work to do
+                Ok(_) => return,
+                Err(actual) => {
+                    curr = actual;
+                }
+            }
+        }
+
+        // There are waiters, the lock must be acquired to notify.
+        let mut waiters = self.waiters.lock().unwrap();
+
+        // The state must be reloaded while the lock is held. The state may only
+        // transition out of WAITING while the lock is held.
+        curr = self.state.load(SeqCst);
+
+        if let Some(waker) = notify_locked(&mut waiters, &self.state, curr) {
+            drop(waiters);
+            waker.wake();
+        }
+    }
+}
+
+impl Default for Notify {
+    fn default() -> Notify {
+        Notify::new()
+    }
+}
+
+fn notify_locked(waiters: &mut LinkedList<Waiter>, state: &AtomicU8, curr: u8) -> Option<Waker> {
+    loop {
+        match curr {
+            EMPTY | NOTIFIED => {
+                let res = state.compare_exchange(curr, NOTIFIED, SeqCst, SeqCst);
+
+                match res {
+                    Ok(_) => return None,
+                    Err(actual) => {
+                        assert!(actual == EMPTY || actual == NOTIFIED);
+                        state.store(NOTIFIED, SeqCst);
+                        return None;
+                    }
+                }
+            }
+            WAITING => {
+                // At this point, it is guaranteed that the state will not
+                // concurrently change as holding the lock is required to
+                // transition **out** of `WAITING`.
+                //
+                // Get a pending waiter
+                let mut waiter = waiters.pop_back().unwrap();
+
+                // Safety: `waiters` lock is still held.
+                let waiter = unsafe { waiter.as_mut() };
+
+                assert!(!waiter.notified);
+
+                waiter.notified = true;
+                let waker = waiter.waker.take();
+
+                if waiters.is_empty() {
+                    // As this the **final** waiter in the list, the state
+                    // must be transitioned to `EMPTY`. As transitioning
+                    // **from** `WAITING` requires the lock to be held, a
+                    // `store` is sufficient.
+                    state.store(EMPTY, SeqCst);
+                }
+
+                return waker;
+            }
+            _ => unreachable!(),
+        }
+    }
+}
+
+// ===== impl Notified =====
+
+impl Notified<'_> {
+    /// A custom `project` implementation is used in place of `pin-project-lite`
+    /// as a custom drop implementation is needed.
+    fn project(self: Pin<&mut Self>) -> (&Notify, &mut State, &UnsafeCell<Waiter>) {
+        unsafe {
+            // Safety: both `notify` and `state` are `Unpin`.
+
+            is_unpin::<&Notify>();
+            is_unpin::<AtomicU8>();
+
+            let me = self.get_unchecked_mut();
+            (&me.notify, &mut me.state, &me.waiter)
+        }
+    }
+}
+
+impl Future for Notified<'_> {
+    type Output = ();
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
+        use State::*;
+
+        let (notify, state, waiter) = self.project();
+
+        loop {
+            match *state {
+                Init => {
+                    // Optimistically try acquiring a pending notification
+                    let res = notify
+                        .state
+                        .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst);
+
+                    if res.is_ok() {
+                        // Acquired the notification
+                        *state = Done;
+                        return Poll::Ready(());
+                    }
+
+                    // Acquire the lock and attempt to transition to the waiting
+                    // state.
+                    let mut waiters = notify.waiters.lock().unwrap();
+
+                    // Reload the state with the lock held
+                    let mut curr = notify.state.load(SeqCst);
+
+                    // Transition the state to WAITING.
+                    loop {
+                        match curr {
+                            EMPTY => {
+                                // Transition to WAITING
+                                let res = notify
+                                    .state
+                                    .compare_exchange(EMPTY, WAITING, SeqCst, SeqCst);
+
+                                if let Err(actual) = res {
+                                    assert_eq!(actual, NOTIFIED);
+                                    curr = actual;
+                                } else {
+                                    break;
+                                }
+                            }
+                            WAITING => break,
+                            NOTIFIED => {
+                                // Try consuming the notification
+                                let res = notify
+                                    .state
+                                    .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst);
+
+                                match res {
+                                    Ok(_) => {
+                                        // Acquired the notification
+                                        *state = Done;
+                                        return Poll::Ready(());
+                                    }
+                                    Err(actual) => {
+                                        assert_eq!(actual, EMPTY);
+                                        curr = actual;
+                                    }
+                                }
+                            }
+                            _ => unreachable!(),
+                        }
+                    }
+
+                    // Safety: called while locked.
+                    unsafe {
+                        (*waiter.get()).waker = Some(cx.waker().clone());
+                    }
+
+                    // Insert the waiter into the linked list
+                    //
+                    // safety: pointers from `UnsafeCell` are never null.
+                    waiters.push_front(unsafe { NonNull::new_unchecked(waiter.get()) });
+
+                    *state = Waiting;
+                }
+                Waiting => {
+                    // Currently in the "Waiting" state, implying the caller has
+                    // a waiter stored in the waiter list (guarded by
+                    // `notify.waiters`). In order to access the waker fields,
+                    // we must hold the lock.
+
+                    let waiters = notify.waiters.lock().unwrap();
+
+                    // Safety: called while locked
+                    let w = unsafe { &mut *waiter.get() };
+
+                    if w.notified {
+                        // Our waker has been notified. Reset the fields and
+                        // remove it from the list.
+                        w.waker = None;
+                        w.notified = false;
+
+                        *state = Done;
+                    } else {
+                        // Update the waker, if necessary.
+                        if !w.waker.as_ref().unwrap().will_wake(cx.waker()) {
+                            w.waker = Some(cx.waker().clone());
+                        }
+
+                        return Poll::Pending;
+                    }
+
+                    // Explicit drop of the lock to indicate the scope that the
+                    // lock is held. Because holding the lock is required to
+                    // ensure safe access to fields not held within the lock, it
+                    // is helpful to visualize the scope of the critical
+                    // section.
+                    drop(waiters);
+                }
+                Done => {
+                    return Poll::Ready(());
+                }
+            }
+        }
+    }
+}
+
+impl Drop for Notified<'_> {
+    fn drop(&mut self) {
+        use State::*;
+
+        // Safety: The type only transitions to a "Waiting" state when pinned.
+        let (notify, state, waiter) = unsafe { Pin::new_unchecked(self).project() };
+
+        // This is where we ensure safety. The `Notified` value is being
+        // dropped, which means we must ensure that the waiter entry is no
+        // longer stored in the linked list.
+        if let Waiting = *state {
+            let mut notify_state = WAITING;
+            let mut waiters = notify.waiters.lock().unwrap();
+
+            // `Notify.state` may be in any of the three states (Empty, Waiting,
+            // Notified). It doesn't actually matter what the atomic is set to
+            // at this point. We hold the lock and will ensure the atomic is in
+            // the correct state once th elock is dropped.
+            //
+            // Because the atomic state is not checked, at first glance, it may
+            // seem like this routine does not handle the case where the
+            // receiver is notified but has not yet observed the notification.
+            // If this happens, no matter how many notifications happen between
+            // this receiver being notified and the receive future dropping, all
+            // we need to do is ensure that one notification is returned back to
+            // the `Notify`. This is done by calling `notify_locked` if `self`
+            // has the `notified` flag set.
+
+            // remove the entry from the list
+            //
+            // safety: the waiter is only added to `waiters` by virtue of it
+            // being the only `LinkedList` available to the type.
+            unsafe { waiters.remove(NonNull::new_unchecked(waiter.get())) };
+
+            if waiters.is_empty() {
+                notify_state = EMPTY;
+                // If the state *should* be `NOTIFIED`, the call to
+                // `notify_locked` below will end up doing the
+                // `store(NOTIFIED)`. If a concurrent receiver races and
+                // observes the incorrect `EMPTY` state, it will then obtain the
+                // lock and block until `notify.state` is in the correct final
+                // state.
+                notify.state.store(EMPTY, SeqCst);
+            }
+
+            // See if the node was notified but not received. In this case, the
+            // notification must be sent to another waiter.
+            //
+            // Safety: with the entry removed from the linked list, there can be
+            // no concurrent access to the entry
+            let notified = unsafe { (*waiter.get()).notified };
+
+            if notified {
+                if let Some(waker) = notify_locked(&mut waiters, &notify.state, notify_state) {
+                    drop(waiters);
+                    waker.wake();
+                }
+            }
+        }
+    }
+}
+
+/// # Safety
+///
+/// `Waiter` is forced to be !Unpin.
+unsafe impl linked_list::Link for Waiter {
+    type Handle = NonNull<Waiter>;
+    type Target = Waiter;
+
+    fn as_raw(handle: &NonNull<Waiter>) -> NonNull<Waiter> {
+        *handle
+    }
+
+    unsafe fn from_raw(ptr: NonNull<Waiter>) -> NonNull<Waiter> {
+        ptr
+    }
+
+    unsafe fn pointers(mut target: NonNull<Waiter>) -> NonNull<linked_list::Pointers<Waiter>> {
+        NonNull::from(&mut target.as_mut().pointers)
+    }
+}
+
+fn is_unpin<T: Unpin>() {}
diff --git a/third_party/rust/tokio/src/sync/oneshot.rs b/third_party/rust/tokio/src/sync/oneshot.rs
new file mode 100644
index 0000000000..62ad484eec
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/oneshot.rs
@@ -0,0 +1,784 @@
+#![cfg_attr(not(feature = "sync"), allow(dead_code, unreachable_pub))]
+
+//! A channel for sending a single message between asynchronous tasks.
+
+use crate::loom::cell::UnsafeCell;
+use crate::loom::sync::atomic::AtomicUsize;
+use crate::loom::sync::Arc;
+
+use std::fmt;
+use std::future::Future;
+use std::mem::MaybeUninit;
+use std::pin::Pin;
+use std::sync::atomic::Ordering::{self, AcqRel, Acquire};
+use std::task::Poll::{Pending, Ready};
+use std::task::{Context, Poll, Waker};
+
+/// Sends a value to the associated `Receiver`.
+///
+/// Instances are created by the [`channel`](fn@channel) function.
+#[derive(Debug)]
+pub struct Sender<T> {
+    inner: Option<Arc<Inner<T>>>,
+}
+
+/// Receive a value from the associated `Sender`.
+///
+/// Instances are created by the [`channel`](fn@channel) function.
+#[derive(Debug)]
+pub struct Receiver<T> {
+    inner: Option<Arc<Inner<T>>>,
+}
+
+pub mod error {
+    //! Oneshot error types
+
+    use std::fmt;
+
+    /// Error returned by the `Future` implementation for `Receiver`.
+    #[derive(Debug, Eq, PartialEq)]
+    pub struct RecvError(pub(super) ());
+
+    /// Error returned by the `try_recv` function on `Receiver`.
+    #[derive(Debug, Eq, PartialEq)]
+    pub enum TryRecvError {
+        /// The send half of the channel has not yet sent a value.
+        Empty,
+
+        /// The send half of the channel was dropped without sending a value.
+        Closed,
+    }
+
+    // ===== impl RecvError =====
+
+    impl fmt::Display for RecvError {
+        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+            write!(fmt, "channel closed")
+        }
+    }
+
+    impl std::error::Error for RecvError {}
+
+    // ===== impl TryRecvError =====
+
+    impl fmt::Display for TryRecvError {
+        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+            match self {
+                TryRecvError::Empty => write!(fmt, "channel empty"),
+                TryRecvError::Closed => write!(fmt, "channel closed"),
+            }
+        }
+    }
+
+    impl std::error::Error for TryRecvError {}
+}
+
+use self::error::*;
+
+struct Inner<T> {
+    /// Manages the state of the inner cell
+    state: AtomicUsize,
+
+    /// The value. This is set by `Sender` and read by `Receiver`. The state of
+    /// the cell is tracked by `state`.
+    value: UnsafeCell<Option<T>>,
+
+    /// The task to notify when the receiver drops without consuming the value.
+    tx_task: UnsafeCell<MaybeUninit<Waker>>,
+
+    /// The task to notify when the value is sent.
+    rx_task: UnsafeCell<MaybeUninit<Waker>>,
+}
+
+#[derive(Clone, Copy)]
+struct State(usize);
+
+/// Create a new one-shot channel for sending single values across asynchronous
+/// tasks.
+///
+/// The function returns separate "send" and "receive" handles. The `Sender`
+/// handle is used by the producer to send the value. The `Receiver` handle is
+/// used by the consumer to receive the value.
+///
+/// Each handle can be used on separate tasks.
+///
+/// # Examples
+///
+/// ```
+/// use tokio::sync::oneshot;
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let (tx, rx) = oneshot::channel();
+///
+///     tokio::spawn(async move {
+///         if let Err(_) = tx.send(3) {
+///             println!("the receiver dropped");
+///         }
+///     });
+///
+///     match rx.await {
+///         Ok(v) => println!("got = {:?}", v),
+///         Err(_) => println!("the sender dropped"),
+///     }
+/// }
+/// ```
+pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
+    #[allow(deprecated)]
+    let inner = Arc::new(Inner {
+        state: AtomicUsize::new(State::new().as_usize()),
+        value: UnsafeCell::new(None),
+        tx_task: UnsafeCell::new(MaybeUninit::uninit()),
+        rx_task: UnsafeCell::new(MaybeUninit::uninit()),
+    });
+
+    let tx = Sender {
+        inner: Some(inner.clone()),
+    };
+    let rx = Receiver { inner: Some(inner) };
+
+    (tx, rx)
+}
+
+impl<T> Sender<T> {
+    /// Attempts to send a value on this channel, returning it back if it could
+    /// not be sent.
+    ///
+    /// The function consumes `self` as only one value may ever be sent on a
+    /// one-shot channel.
+    ///
+    /// A successful send occurs when it is determined that the other end of the
+    /// channel has not hung up already. An unsuccessful send would be one where
+    /// the corresponding receiver has already been deallocated. Note that a
+    /// return value of `Err` means that the data will never be received, but
+    /// a return value of `Ok` does *not* mean that the data will be received.
+    /// It is possible for the corresponding receiver to hang up immediately
+    /// after this function returns `Ok`.
+    ///
+    /// # Examples
+    ///
+    /// Send a value to another task
+    ///
+    /// ```
+    /// use tokio::sync::oneshot;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, rx) = oneshot::channel();
+    ///
+    ///     tokio::spawn(async move {
+    ///         if let Err(_) = tx.send(3) {
+    ///             println!("the receiver dropped");
+    ///         }
+    ///     });
+    ///
+    ///     match rx.await {
+    ///         Ok(v) => println!("got = {:?}", v),
+    ///         Err(_) => println!("the sender dropped"),
+    ///     }
+    /// }
+    /// ```
+    pub fn send(mut self, t: T) -> Result<(), T> {
+        let inner = self.inner.take().unwrap();
+
+        inner.value.with_mut(|ptr| unsafe {
+            *ptr = Some(t);
+        });
+
+        if !inner.complete() {
+            return Err(inner
+                .value
+                .with_mut(|ptr| unsafe { (*ptr).take() }.unwrap()));
+        }
+
+        Ok(())
+    }
+
+    #[doc(hidden)] // TODO: remove
+    pub fn poll_closed(&mut self, cx: &mut Context<'_>) -> Poll<()> {
+        // Keep track of task budget
+        ready!(crate::coop::poll_proceed(cx));
+
+        let inner = self.inner.as_ref().unwrap();
+
+        let mut state = State::load(&inner.state, Acquire);
+
+        if state.is_closed() {
+            return Poll::Ready(());
+        }
+
+        if state.is_tx_task_set() {
+            let will_notify = unsafe { inner.with_tx_task(|w| w.will_wake(cx.waker())) };
+
+            if !will_notify {
+                state = State::unset_tx_task(&inner.state);
+
+                if state.is_closed() {
+                    // Set the flag again so that the waker is released in drop
+                    State::set_tx_task(&inner.state);
+                    return Ready(());
+                } else {
+                    unsafe { inner.drop_tx_task() };
+                }
+            }
+        }
+
+        if !state.is_tx_task_set() {
+            // Attempt to set the task
+            unsafe {
+                inner.set_tx_task(cx);
+            }
+
+            // Update the state
+            state = State::set_tx_task(&inner.state);
+
+            if state.is_closed() {
+                return Ready(());
+            }
+        }
+
+        Pending
+    }
+
+    /// Waits for the associated [`Receiver`] handle to close.
+    ///
+    /// A [`Receiver`] is closed by either calling [`close`] explicitly or the
+    /// [`Receiver`] value is dropped.
+    ///
+    /// This function is useful when paired with `select!` to abort a
+    /// computation when the receiver is no longer interested in the result.
+    ///
+    /// # Return
+    ///
+    /// Returns a `Future` which must be awaited on.
+    ///
+    /// [`Receiver`]: Receiver
+    /// [`close`]: Receiver::close
+    ///
+    /// # Examples
+    ///
+    /// Basic usage
+    ///
+    /// ```
+    /// use tokio::sync::oneshot;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (mut tx, rx) = oneshot::channel::<()>();
+    ///
+    ///     tokio::spawn(async move {
+    ///         drop(rx);
+    ///     });
+    ///
+    ///     tx.closed().await;
+    ///     println!("the receiver dropped");
+    /// }
+    /// ```
+    ///
+    /// Paired with select
+    ///
+    /// ```
+    /// use tokio::sync::oneshot;
+    /// use tokio::time::{self, Duration};
+    ///
+    /// use futures::{select, FutureExt};
+    ///
+    /// async fn compute() -> String {
+    ///     // Complex computation returning a `String`
+    /// # "hello".to_string()
+    /// }
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (mut tx, rx) = oneshot::channel();
+    ///
+    ///     tokio::spawn(async move {
+    ///         select! {
+    ///             _ = tx.closed().fuse() => {
+    ///                 // The receiver dropped, no need to do any further work
+    ///             }
+    ///             value = compute().fuse() => {
+    ///                 tx.send(value).unwrap()
+    ///             }
+    ///         }
+    ///     });
+    ///
+    ///     // Wait for up to 10 seconds
+    ///     let _ = time::timeout(Duration::from_secs(10), rx).await;
+    /// }
+    /// ```
+    pub async fn closed(&mut self) {
+        use crate::future::poll_fn;
+
+        poll_fn(|cx| self.poll_closed(cx)).await
+    }
+
+    /// Returns `true` if the associated [`Receiver`] handle has been dropped.
+    ///
+    /// A [`Receiver`] is closed by either calling [`close`] explicitly or the
+    /// [`Receiver`] value is dropped.
+    ///
+    /// If `true` is returned, a call to `send` will always result in an error.
+    ///
+    /// [`Receiver`]: Receiver
+    /// [`close`]: Receiver::close
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::oneshot;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, rx) = oneshot::channel();
+    ///
+    ///     assert!(!tx.is_closed());
+    ///
+    ///     drop(rx);
+    ///
+    ///     assert!(tx.is_closed());
+    ///     assert!(tx.send("never received").is_err());
+    /// }
+    /// ```
+    pub fn is_closed(&self) -> bool {
+        let inner = self.inner.as_ref().unwrap();
+
+        let state = State::load(&inner.state, Acquire);
+        state.is_closed()
+    }
+}
+
+impl<T> Drop for Sender<T> {
+    fn drop(&mut self) {
+        if let Some(inner) = self.inner.as_ref() {
+            inner.complete();
+        }
+    }
+}
+
+impl<T> Receiver<T> {
+    /// Prevents the associated [`Sender`] handle from sending a value.
+    ///
+    /// Any `send` operation which happens after calling `close` is guaranteed
+    /// to fail. After calling `close`, `Receiver::poll`] should be called to
+    /// receive a value if one was sent **before** the call to `close`
+    /// completed.
+    ///
+    /// This function is useful to perform a graceful shutdown and ensure that a
+    /// value will not be sent into the channel and never received.
+    ///
+    /// [`Sender`]: Sender
+    ///
+    /// # Examples
+    ///
+    /// Prevent a value from being sent
+    ///
+    /// ```
+    /// use tokio::sync::oneshot;
+    /// use tokio::sync::oneshot::error::TryRecvError;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, mut rx) = oneshot::channel();
+    ///
+    ///     assert!(!tx.is_closed());
+    ///
+    ///     rx.close();
+    ///
+    ///     assert!(tx.is_closed());
+    ///     assert!(tx.send("never received").is_err());
+    ///
+    ///     match rx.try_recv() {
+    ///         Err(TryRecvError::Closed) => {}
+    ///         _ => unreachable!(),
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// Receive a value sent **before** calling `close`
+    ///
+    /// ```
+    /// use tokio::sync::oneshot;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, mut rx) = oneshot::channel();
+    ///
+    ///     assert!(tx.send("will receive").is_ok());
+    ///
+    ///     rx.close();
+    ///
+    ///     let msg = rx.try_recv().unwrap();
+    ///     assert_eq!(msg, "will receive");
+    /// }
+    /// ```
+    pub fn close(&mut self) {
+        let inner = self.inner.as_ref().unwrap();
+        inner.close();
+    }
+
+    /// Attempts to receive a value.
+    ///
+    /// If a pending value exists in the channel, it is returned. If no value
+    /// has been sent, the current task **will not** be registered for
+    /// future notification.
+    ///
+    /// This function is useful to call from outside the context of an
+    /// asynchronous task.
+    ///
+    /// # Return
+    ///
+    /// - `Ok(T)` if a value is pending in the channel.
+    /// - `Err(TryRecvError::Empty)` if no value has been sent yet.
+    /// - `Err(TryRecvError::Closed)` if the sender has dropped without sending
+    ///   a value.
+    ///
+    /// # Examples
+    ///
+    /// `try_recv` before a value is sent, then after.
+    ///
+    /// ```
+    /// use tokio::sync::oneshot;
+    /// use tokio::sync::oneshot::error::TryRecvError;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, mut rx) = oneshot::channel();
+    ///
+    ///     match rx.try_recv() {
+    ///         // The channel is currently empty
+    ///         Err(TryRecvError::Empty) => {}
+    ///         _ => unreachable!(),
+    ///     }
+    ///
+    ///     // Send a value
+    ///     tx.send("hello").unwrap();
+    ///
+    ///     match rx.try_recv() {
+    ///         Ok(value) => assert_eq!(value, "hello"),
+    ///         _ => unreachable!(),
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// `try_recv` when the sender dropped before sending a value
+    ///
+    /// ```
+    /// use tokio::sync::oneshot;
+    /// use tokio::sync::oneshot::error::TryRecvError;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, mut rx) = oneshot::channel::<()>();
+    ///
+    ///     drop(tx);
+    ///
+    ///     match rx.try_recv() {
+    ///         // The channel will never receive a value.
+    ///         Err(TryRecvError::Closed) => {}
+    ///         _ => unreachable!(),
+    ///     }
+    /// }
+    /// ```
+    pub fn try_recv(&mut self) -> Result<T, TryRecvError> {
+        let result = if let Some(inner) = self.inner.as_ref() {
+            let state = State::load(&inner.state, Acquire);
+
+            if state.is_complete() {
+                match unsafe { inner.consume_value() } {
+                    Some(value) => Ok(value),
+                    None => Err(TryRecvError::Closed),
+                }
+            } else if state.is_closed() {
+                Err(TryRecvError::Closed)
+            } else {
+                // Not ready, this does not clear `inner`
+                return Err(TryRecvError::Empty);
+            }
+        } else {
+            panic!("called after complete");
+        };
+
+        self.inner = None;
+        result
+    }
+}
+
+impl<T> Drop for Receiver<T> {
+    fn drop(&mut self) {
+        if let Some(inner) = self.inner.as_ref() {
+            inner.close();
+        }
+    }
+}
+
+impl<T> Future for Receiver<T> {
+    type Output = Result<T, RecvError>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        // If `inner` is `None`, then `poll()` has already completed.
+        let ret = if let Some(inner) = self.as_ref().get_ref().inner.as_ref() {
+            ready!(inner.poll_recv(cx))?
+        } else {
+            panic!("called after complete");
+        };
+
+        self.inner = None;
+        Ready(Ok(ret))
+    }
+}
+
+impl<T> Inner<T> {
+    fn complete(&self) -> bool {
+        let prev = State::set_complete(&self.state);
+
+        if prev.is_closed() {
+            return false;
+        }
+
+        if prev.is_rx_task_set() {
+            // TODO: Consume waker?
+            unsafe {
+                self.with_rx_task(Waker::wake_by_ref);
+            }
+        }
+
+        true
+    }
+
+    fn poll_recv(&self, cx: &mut Context<'_>) -> Poll<Result<T, RecvError>> {
+        // Keep track of task budget
+        ready!(crate::coop::poll_proceed(cx));
+
+        // Load the state
+        let mut state = State::load(&self.state, Acquire);
+
+        if state.is_complete() {
+            match unsafe { self.consume_value() } {
+                Some(value) => Ready(Ok(value)),
+                None => Ready(Err(RecvError(()))),
+            }
+        } else if state.is_closed() {
+            Ready(Err(RecvError(())))
+        } else {
+            if state.is_rx_task_set() {
+                let will_notify = unsafe { self.with_rx_task(|w| w.will_wake(cx.waker())) };
+
+                // Check if the task is still the same
+                if !will_notify {
+                    // Unset the task
+                    state = State::unset_rx_task(&self.state);
+                    if state.is_complete() {
+                        // Set the flag again so that the waker is released in drop
+                        State::set_rx_task(&self.state);
+
+                        return match unsafe { self.consume_value() } {
+                            Some(value) => Ready(Ok(value)),
+                            None => Ready(Err(RecvError(()))),
+                        };
+                    } else {
+                        unsafe { self.drop_rx_task() };
+                    }
+                }
+            }
+
+            if !state.is_rx_task_set() {
+                // Attempt to set the task
+                unsafe {
+                    self.set_rx_task(cx);
+                }
+
+                // Update the state
+                state = State::set_rx_task(&self.state);
+
+                if state.is_complete() {
+                    match unsafe { self.consume_value() } {
+                        Some(value) => Ready(Ok(value)),
+                        None => Ready(Err(RecvError(()))),
+                    }
+                } else {
+                    Pending
+                }
+            } else {
+                Pending
+            }
+        }
+    }
+
+    /// Called by `Receiver` to indicate that the value will never be received.
+    fn close(&self) {
+        let prev = State::set_closed(&self.state);
+
+        if prev.is_tx_task_set() && !prev.is_complete() {
+            unsafe {
+                self.with_tx_task(Waker::wake_by_ref);
+            }
+        }
+    }
+
+    /// Consumes the value. This function does not check `state`.
+    unsafe fn consume_value(&self) -> Option<T> {
+        self.value.with_mut(|ptr| (*ptr).take())
+    }
+
+    unsafe fn with_rx_task<F, R>(&self, f: F) -> R
+    where
+        F: FnOnce(&Waker) -> R,
+    {
+        self.rx_task.with(|ptr| {
+            let waker: *const Waker = (&*ptr).as_ptr();
+            f(&*waker)
+        })
+    }
+
+    unsafe fn with_tx_task<F, R>(&self, f: F) -> R
+    where
+        F: FnOnce(&Waker) -> R,
+    {
+        self.tx_task.with(|ptr| {
+            let waker: *const Waker = (&*ptr).as_ptr();
+            f(&*waker)
+        })
+    }
+
+    unsafe fn drop_rx_task(&self) {
+        self.rx_task.with_mut(|ptr| {
+            let ptr: *mut Waker = (&mut *ptr).as_mut_ptr();
+            ptr.drop_in_place();
+        });
+    }
+
+    unsafe fn drop_tx_task(&self) {
+        self.tx_task.with_mut(|ptr| {
+            let ptr: *mut Waker = (&mut *ptr).as_mut_ptr();
+            ptr.drop_in_place();
+        });
+    }
+
+    unsafe fn set_rx_task(&self, cx: &mut Context<'_>) {
+        self.rx_task.with_mut(|ptr| {
+            let ptr: *mut Waker = (&mut *ptr).as_mut_ptr();
+            ptr.write(cx.waker().clone());
+        });
+    }
+
+    unsafe fn set_tx_task(&self, cx: &mut Context<'_>) {
+        self.tx_task.with_mut(|ptr| {
+            let ptr: *mut Waker = (&mut *ptr).as_mut_ptr();
+            ptr.write(cx.waker().clone());
+        });
+    }
+}
+
+unsafe impl<T: Send> Send for Inner<T> {}
+unsafe impl<T: Send> Sync for Inner<T> {}
+
+impl<T> Drop for Inner<T> {
+    fn drop(&mut self) {
+        let state = State(self.state.with_mut(|v| *v));
+
+        if state.is_rx_task_set() {
+            unsafe {
+                self.drop_rx_task();
+            }
+        }
+
+        if state.is_tx_task_set() {
+            unsafe {
+                self.drop_tx_task();
+            }
+        }
+    }
+}
+
+impl<T: fmt::Debug> fmt::Debug for Inner<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        use std::sync::atomic::Ordering::Relaxed;
+
+        fmt.debug_struct("Inner")
+            .field("state", &State::load(&self.state, Relaxed))
+            .finish()
+    }
+}
+
+const RX_TASK_SET: usize = 0b00001;
+const VALUE_SENT: usize = 0b00010;
+const CLOSED: usize = 0b00100;
+const TX_TASK_SET: usize = 0b01000;
+
+impl State {
+    fn new() -> State {
+        State(0)
+    }
+
+    fn is_complete(self) -> bool {
+        self.0 & VALUE_SENT == VALUE_SENT
+    }
+
+    fn set_complete(cell: &AtomicUsize) -> State {
+        // TODO: This could be `Release`, followed by an `Acquire` fence *if*
+        // the `RX_TASK_SET` flag is set. However, `loom` does not support
+        // fences yet.
+        let val = cell.fetch_or(VALUE_SENT, AcqRel);
+        State(val)
+    }
+
+    fn is_rx_task_set(self) -> bool {
+        self.0 & RX_TASK_SET == RX_TASK_SET
+    }
+
+    fn set_rx_task(cell: &AtomicUsize) -> State {
+        let val = cell.fetch_or(RX_TASK_SET, AcqRel);
+        State(val | RX_TASK_SET)
+    }
+
+    fn unset_rx_task(cell: &AtomicUsize) -> State {
+        let val = cell.fetch_and(!RX_TASK_SET, AcqRel);
+        State(val & !RX_TASK_SET)
+    }
+
+    fn is_closed(self) -> bool {
+        self.0 & CLOSED == CLOSED
+    }
+
+    fn set_closed(cell: &AtomicUsize) -> State {
+        // Acquire because we want all later writes (attempting to poll) to be
+        // ordered after this.
+        let val = cell.fetch_or(CLOSED, Acquire);
+        State(val)
+    }
+
+    fn set_tx_task(cell: &AtomicUsize) -> State {
+        let val = cell.fetch_or(TX_TASK_SET, AcqRel);
+        State(val | TX_TASK_SET)
+    }
+
+    fn unset_tx_task(cell: &AtomicUsize) -> State {
+        let val = cell.fetch_and(!TX_TASK_SET, AcqRel);
+        State(val & !TX_TASK_SET)
+    }
+
+    fn is_tx_task_set(self) -> bool {
+        self.0 & TX_TASK_SET == TX_TASK_SET
+    }
+
+    fn as_usize(self) -> usize {
+        self.0
+    }
+
+    fn load(cell: &AtomicUsize, order: Ordering) -> State {
+        let val = cell.load(order);
+        State(val)
+    }
+}
+
+impl fmt::Debug for State {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("State")
+            .field("is_complete", &self.is_complete())
+            .field("is_closed", &self.is_closed())
+            .field("is_rx_task_set", &self.is_rx_task_set())
+            .field("is_tx_task_set", &self.is_tx_task_set())
+            .finish()
+    }
+}
diff --git a/third_party/rust/tokio/src/sync/rwlock.rs b/third_party/rust/tokio/src/sync/rwlock.rs
new file mode 100644
index 0000000000..68cf710e84
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/rwlock.rs
@@ -0,0 +1,294 @@
+use crate::coop::CoopFutureExt;
+use crate::sync::batch_semaphore::{AcquireError, Semaphore};
+use std::cell::UnsafeCell;
+use std::ops;
+
+#[cfg(not(loom))]
+const MAX_READS: usize = 32;
+
+#[cfg(loom)]
+const MAX_READS: usize = 10;
+
+/// An asynchronous reader-writer lock
+///
+/// This type of lock allows a number of readers or at most one writer at any
+/// point in time. The write portion of this lock typically allows modification
+/// of the underlying data (exclusive access) and the read portion of this lock
+/// typically allows for read-only access (shared access).
+///
+/// In comparison, a [`Mutex`] does not distinguish between readers or writers
+/// that acquire the lock, therefore causing any tasks waiting for the lock to
+/// become available to yield. An `RwLock` will allow any number of readers to
+/// acquire the lock as long as a writer is not holding the lock.
+///
+/// The priority policy of Tokio's read-write lock is _fair_ (or
+/// [_write-preferring_]), in order to ensure that readers cannot starve
+/// writers. Fairness is ensured using a first-in, first-out queue for the tasks
+/// awaiting the lock; if a task that wishes to acquire the write lock is at the
+/// head of the queue, read locks will not be given out until the write lock has
+/// been released. This is in contrast to the Rust standard library's
+/// `std::sync::RwLock`, where the priority policy is dependent on the
+/// operating system's implementation.
+///
+/// The type parameter `T` represents the data that this lock protects. It is
+/// required that `T` satisfies [`Send`] to be shared across threads. The RAII guards
+/// returned from the locking methods implement [`Deref`](https://doc.rust-lang.org/std/ops/trait.Deref.html)
+/// (and [`DerefMut`](https://doc.rust-lang.org/std/ops/trait.DerefMut.html)
+/// for the `write` methods) to allow access to the content of the lock.
+///
+/// # Examples
+///
+/// ```
+/// use tokio::sync::RwLock;
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let lock = RwLock::new(5);
+///
+///     // many reader locks can be held at once
+///     {
+///         let r1 = lock.read().await;
+///         let r2 = lock.read().await;
+///         assert_eq!(*r1, 5);
+///         assert_eq!(*r2, 5);
+///     } // read locks are dropped at this point
+///
+///     // only one write lock may be held, however
+///     {
+///         let mut w = lock.write().await;
+///         *w += 1;
+///         assert_eq!(*w, 6);
+///     } // write lock is dropped here
+/// }
+/// ```
+///
+/// [`Mutex`]: struct@super::Mutex
+/// [`RwLock`]: struct@RwLock
+/// [`RwLockReadGuard`]: struct@RwLockReadGuard
+/// [`RwLockWriteGuard`]: struct@RwLockWriteGuard
+/// [`Send`]: https://doc.rust-lang.org/std/marker/trait.Send.html
+/// [_write-preferring_]: https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock#Priority_policies
+#[derive(Debug)]
+pub struct RwLock<T> {
+    //semaphore to coordinate read and write access to T
+    s: Semaphore,
+
+    //inner data T
+    c: UnsafeCell<T>,
+}
+
+/// RAII structure used to release the shared read access of a lock when
+/// dropped.
+///
+/// This structure is created by the [`read`] method on
+/// [`RwLock`].
+///
+/// [`read`]: method@RwLock::read
+#[derive(Debug)]
+pub struct RwLockReadGuard<'a, T> {
+    permit: ReleasingPermit<'a, T>,
+    lock: &'a RwLock<T>,
+}
+
+/// RAII structure used to release the exclusive write access of a lock when
+/// dropped.
+///
+/// This structure is created by the [`write`] and method
+/// on [`RwLock`].
+///
+/// [`write`]: method@RwLock::write
+/// [`RwLock`]: struct@RwLock
+#[derive(Debug)]
+pub struct RwLockWriteGuard<'a, T> {
+    permit: ReleasingPermit<'a, T>,
+    lock: &'a RwLock<T>,
+}
+
+// Wrapper arround Permit that releases on Drop
+#[derive(Debug)]
+struct ReleasingPermit<'a, T> {
+    num_permits: u16,
+    lock: &'a RwLock<T>,
+}
+
+impl<'a, T> ReleasingPermit<'a, T> {
+    async fn acquire(
+        lock: &'a RwLock<T>,
+        num_permits: u16,
+    ) -> Result<ReleasingPermit<'a, T>, AcquireError> {
+        lock.s.acquire(num_permits).cooperate().await?;
+        Ok(Self { num_permits, lock })
+    }
+}
+
+impl<'a, T> Drop for ReleasingPermit<'a, T> {
+    fn drop(&mut self) {
+        self.lock.s.release(self.num_permits as usize);
+    }
+}
+
+#[test]
+#[cfg(not(loom))]
+fn bounds() {
+    fn check_send<T: Send>() {}
+    fn check_sync<T: Sync>() {}
+    fn check_unpin<T: Unpin>() {}
+    // This has to take a value, since the async fn's return type is unnameable.
+    fn check_send_sync_val<T: Send + Sync>(_t: T) {}
+
+    check_send::<RwLock<u32>>();
+    check_sync::<RwLock<u32>>();
+    check_unpin::<RwLock<u32>>();
+
+    check_sync::<RwLockReadGuard<'_, u32>>();
+    check_unpin::<RwLockReadGuard<'_, u32>>();
+
+    check_sync::<RwLockWriteGuard<'_, u32>>();
+    check_unpin::<RwLockWriteGuard<'_, u32>>();
+
+    let rwlock = RwLock::new(0);
+    check_send_sync_val(rwlock.read());
+    check_send_sync_val(rwlock.write());
+}
+
+// As long as T: Send + Sync, it's fine to send and share RwLock<T> between threads.
+// If T were not Send, sending and sharing a RwLock<T> would be bad, since you can access T through
+// RwLock<T>.
+unsafe impl<T> Send for RwLock<T> where T: Send {}
+unsafe impl<T> Sync for RwLock<T> where T: Send + Sync {}
+unsafe impl<'a, T> Sync for RwLockReadGuard<'a, T> where T: Send + Sync {}
+unsafe impl<'a, T> Sync for RwLockWriteGuard<'a, T> where T: Send + Sync {}
+
+impl<T> RwLock<T> {
+    /// Creates a new instance of an `RwLock<T>` which is unlocked.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::RwLock;
+    ///
+    /// let lock = RwLock::new(5);
+    /// ```
+    pub fn new(value: T) -> RwLock<T> {
+        RwLock {
+            c: UnsafeCell::new(value),
+            s: Semaphore::new(MAX_READS),
+        }
+    }
+
+    /// Locks this rwlock with shared read access, causing the current task
+    /// to yield until the lock has been acquired.
+    ///
+    /// The calling task will yield until there are no more writers which
+    /// hold the lock. There may be other readers currently inside the lock when
+    /// this method returns.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::sync::Arc;
+    /// use tokio::sync::RwLock;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let lock = Arc::new(RwLock::new(1));
+    ///     let c_lock = lock.clone();
+    ///
+    ///     let n = lock.read().await;
+    ///     assert_eq!(*n, 1);
+    ///
+    ///     tokio::spawn(async move {
+    ///         // While main has an active read lock, we acquire one too.
+    ///         let r = c_lock.read().await;
+    ///         assert_eq!(*r, 1);
+    ///     }).await.expect("The spawned task has paniced");
+    ///
+    ///     // Drop the guard after the spawned task finishes.
+    ///     drop(n);
+    ///}
+    /// ```
+    pub async fn read(&self) -> RwLockReadGuard<'_, T> {
+        let permit = ReleasingPermit::acquire(self, 1).await.unwrap_or_else(|_| {
+            // The semaphore was closed. but, we never explicitly close it, and we have a
+            // handle to it through the Arc, which means that this can never happen.
+            unreachable!()
+        });
+        RwLockReadGuard { lock: self, permit }
+    }
+
+    /// Locks this rwlock with exclusive write access, causing the current task
+    /// to yield until the lock has been acquired.
+    ///
+    /// This function will not return while other writers or other readers
+    /// currently have access to the lock.
+    ///
+    /// Returns an RAII guard which will drop the write access of this rwlock
+    /// when dropped.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::RwLock;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///   let lock = RwLock::new(1);
+    ///
+    ///   let mut n = lock.write().await;
+    ///   *n = 2;
+    ///}
+    /// ```
+    pub async fn write(&self) -> RwLockWriteGuard<'_, T> {
+        let permit = ReleasingPermit::acquire(self, MAX_READS as u16)
+            .await
+            .unwrap_or_else(|_| {
+                // The semaphore was closed. but, we never explicitly close it, and we have a
+                // handle to it through the Arc, which means that this can never happen.
+                unreachable!()
+            });
+
+        RwLockWriteGuard { lock: self, permit }
+    }
+
+    /// Consumes the lock, returning the underlying data.
+    pub fn into_inner(self) -> T {
+        self.c.into_inner()
+    }
+}
+
+impl<T> ops::Deref for RwLockReadGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        unsafe { &*self.lock.c.get() }
+    }
+}
+
+impl<T> ops::Deref for RwLockWriteGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        unsafe { &*self.lock.c.get() }
+    }
+}
+
+impl<T> ops::DerefMut for RwLockWriteGuard<'_, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        unsafe { &mut *self.lock.c.get() }
+    }
+}
+
+impl<T> From<T> for RwLock<T> {
+    fn from(s: T) -> Self {
+        Self::new(s)
+    }
+}
+
+impl<T> Default for RwLock<T>
+where
+    T: Default,
+{
+    fn default() -> Self {
+        Self::new(T::default())
+    }
+}
diff --git a/third_party/rust/tokio/src/sync/semaphore.rs b/third_party/rust/tokio/src/sync/semaphore.rs
new file mode 100644
index 0000000000..4cce7e8f5b
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/semaphore.rs
@@ -0,0 +1,105 @@
+use super::batch_semaphore as ll; // low level implementation
+use crate::coop::CoopFutureExt;
+
+/// Counting semaphore performing asynchronous permit aquisition.
+///
+/// A semaphore maintains a set of permits. Permits are used to synchronize
+/// access to a shared resource. A semaphore differs from a mutex in that it
+/// can allow more than one concurrent caller to access the shared resource at a
+/// time.
+///
+/// When `acquire` is called and the semaphore has remaining permits, the
+/// function immediately returns a permit. However, if no remaining permits are
+/// available, `acquire` (asynchronously) waits until an outstanding permit is
+/// dropped. At this point, the freed permit is assigned to the caller.
+#[derive(Debug)]
+pub struct Semaphore {
+    /// The low level semaphore
+    ll_sem: ll::Semaphore,
+}
+
+/// A permit from the semaphore
+#[must_use]
+#[derive(Debug)]
+pub struct SemaphorePermit<'a> {
+    sem: &'a Semaphore,
+    permits: u16,
+}
+
+/// Error returned from the [`Semaphore::try_acquire`] function.
+///
+/// A `try_acquire` operation can only fail if the semaphore has no available
+/// permits.
+///
+/// [`Semaphore::try_acquire`]: Semaphore::try_acquire
+#[derive(Debug)]
+pub struct TryAcquireError(());
+
+#[test]
+#[cfg(not(loom))]
+fn bounds() {
+    fn check_unpin<T: Unpin>() {}
+    // This has to take a value, since the async fn's return type is unnameable.
+    fn check_send_sync_val<T: Send + Sync>(_t: T) {}
+    fn check_send_sync<T: Send + Sync>() {}
+    check_unpin::<Semaphore>();
+    check_unpin::<SemaphorePermit<'_>>();
+    check_send_sync::<Semaphore>();
+
+    let semaphore = Semaphore::new(0);
+    check_send_sync_val(semaphore.acquire());
+}
+
+impl Semaphore {
+    /// Creates a new semaphore with the initial number of permits
+    pub fn new(permits: usize) -> Self {
+        Self {
+            ll_sem: ll::Semaphore::new(permits),
+        }
+    }
+
+    /// Returns the current number of available permits
+    pub fn available_permits(&self) -> usize {
+        self.ll_sem.available_permits()
+    }
+
+    /// Adds `n` new permits to the semaphore.
+    pub fn add_permits(&self, n: usize) {
+        self.ll_sem.release(n);
+    }
+
+    /// Acquires permit from the semaphore
+    pub async fn acquire(&self) -> SemaphorePermit<'_> {
+        self.ll_sem.acquire(1).cooperate().await.unwrap();
+        SemaphorePermit {
+            sem: &self,
+            permits: 1,
+        }
+    }
+
+    /// Tries to acquire a permit form the semaphore
+    pub fn try_acquire(&self) -> Result<SemaphorePermit<'_>, TryAcquireError> {
+        match self.ll_sem.try_acquire(1) {
+            Ok(_) => Ok(SemaphorePermit {
+                sem: self,
+                permits: 1,
+            }),
+            Err(_) => Err(TryAcquireError(())),
+        }
+    }
+}
+
+impl<'a> SemaphorePermit<'a> {
+    /// Forgets the permit **without** releasing it back to the semaphore.
+    /// This can be used to reduce the amount of permits available from a
+    /// semaphore.
+    pub fn forget(mut self) {
+        self.permits = 0;
+    }
+}
+
+impl<'a> Drop for SemaphorePermit<'_> {
+    fn drop(&mut self) {
+        self.sem.add_permits(self.permits as usize);
+    }
+}
diff --git a/third_party/rust/tokio/src/sync/semaphore_ll.rs b/third_party/rust/tokio/src/sync/semaphore_ll.rs
new file mode 100644
index 0000000000..0bdc4e2761
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/semaphore_ll.rs
@@ -0,0 +1,1220 @@
+#![cfg_attr(not(feature = "sync"), allow(dead_code, unreachable_pub))]
+
+//! Thread-safe, asynchronous counting semaphore.
+//!
+//! A `Semaphore` instance holds a set of permits. Permits are used to
+//! synchronize access to a shared resource.
+//!
+//! Before accessing the shared resource, callers acquire a permit from the
+//! semaphore. Once the permit is acquired, the caller then enters the critical
+//! section. If no permits are available, then acquiring the semaphore returns
+//! `Pending`. The task is woken once a permit becomes available.
+
+use crate::loom::cell::UnsafeCell;
+use crate::loom::future::AtomicWaker;
+use crate::loom::sync::atomic::{AtomicPtr, AtomicUsize};
+use crate::loom::thread;
+
+use std::cmp;
+use std::fmt;
+use std::ptr::{self, NonNull};
+use std::sync::atomic::Ordering::{self, AcqRel, Acquire, Relaxed, Release};
+use std::task::Poll::{Pending, Ready};
+use std::task::{Context, Poll};
+use std::usize;
+
+/// Futures-aware semaphore.
+pub(crate) struct Semaphore {
+    /// Tracks both the waiter queue tail pointer and the number of remaining
+    /// permits.
+    state: AtomicUsize,
+
+    /// waiter queue head pointer.
+    head: UnsafeCell<NonNull<Waiter>>,
+
+    /// Coordinates access to the queue head.
+    rx_lock: AtomicUsize,
+
+    /// Stub waiter node used as part of the MPSC channel algorithm.
+    stub: Box<Waiter>,
+}
+
+/// A semaphore permit
+///
+/// Tracks the lifecycle of a semaphore permit.
+///
+/// An instance of `Permit` is intended to be used with a **single** instance of
+/// `Semaphore`. Using a single instance of `Permit` with multiple semaphore
+/// instances will result in unexpected behavior.
+///
+/// `Permit` does **not** release the permit back to the semaphore on drop. It
+/// is the user's responsibility to ensure that `Permit::release` is called
+/// before dropping the permit.
+#[derive(Debug)]
+pub(crate) struct Permit {
+    waiter: Option<Box<Waiter>>,
+    state: PermitState,
+}
+
+/// Error returned by `Permit::poll_acquire`.
+#[derive(Debug)]
+pub(crate) struct AcquireError(());
+
+/// Error returned by `Permit::try_acquire`.
+#[derive(Debug)]
+pub(crate) enum TryAcquireError {
+    Closed,
+    NoPermits,
+}
+
+/// Node used to notify the semaphore waiter when permit is available.
+#[derive(Debug)]
+struct Waiter {
+    /// Stores waiter state.
+    ///
+    /// See `WaiterState` for more details.
+    state: AtomicUsize,
+
+    /// Task to wake when a permit is made available.
+    waker: AtomicWaker,
+
+    /// Next pointer in the queue of waiting senders.
+    next: AtomicPtr<Waiter>,
+}
+
+/// Semaphore state
+///
+/// The 2 low bits track the modes.
+///
+/// - Closed
+/// - Full
+///
+/// When not full, the rest of the `usize` tracks the total number of messages
+/// in the channel. When full, the rest of the `usize` is a pointer to the tail
+/// of the "waiting senders" queue.
+#[derive(Copy, Clone)]
+struct SemState(usize);
+
+/// Permit state
+#[derive(Debug, Copy, Clone)]
+enum PermitState {
+    /// Currently waiting for permits to be made available and assigned to the
+    /// waiter.
+    Waiting(u16),
+
+    /// The number of acquired permits
+    Acquired(u16),
+}
+
+/// State for an individual waker node
+#[derive(Debug, Copy, Clone)]
+struct WaiterState(usize);
+
+/// Waiter node is in the semaphore queue
+const QUEUED: usize = 0b001;
+
+/// Semaphore has been closed, no more permits will be issued.
+const CLOSED: usize = 0b10;
+
+/// The permit that owns the `Waiter` dropped.
+const DROPPED: usize = 0b100;
+
+/// Represents "one requested permit" in the waiter state
+const PERMIT_ONE: usize = 0b1000;
+
+/// Masks the waiter state to only contain bits tracking number of requested
+/// permits.
+const PERMIT_MASK: usize = usize::MAX - (PERMIT_ONE - 1);
+
+/// How much to shift a permit count to pack it into the waker state
+const PERMIT_SHIFT: u32 = PERMIT_ONE.trailing_zeros();
+
+/// Flag differentiating between available permits and waiter pointers.
+///
+/// If we assume pointers are properly aligned, then the least significant bit
+/// will always be zero. So, we use that bit to track if the value represents a
+/// number.
+const NUM_FLAG: usize = 0b01;
+
+/// Signal the semaphore is closed
+const CLOSED_FLAG: usize = 0b10;
+
+/// Maximum number of permits a semaphore can manage
+const MAX_PERMITS: usize = usize::MAX >> NUM_SHIFT;
+
+/// When representing "numbers", the state has to be shifted this much (to get
+/// rid of the flag bit).
+const NUM_SHIFT: usize = 2;
+
+// ===== impl Semaphore =====
+
+impl Semaphore {
+    /// Creates a new semaphore with the initial number of permits
+    ///
+    /// # Panics
+    ///
+    /// Panics if `permits` is zero.
+    pub(crate) fn new(permits: usize) -> Semaphore {
+        let stub = Box::new(Waiter::new());
+        let ptr = NonNull::from(&*stub);
+
+        // Allocations are aligned
+        debug_assert!(ptr.as_ptr() as usize & NUM_FLAG == 0);
+
+        let state = SemState::new(permits, &stub);
+
+        Semaphore {
+            state: AtomicUsize::new(state.to_usize()),
+            head: UnsafeCell::new(ptr),
+            rx_lock: AtomicUsize::new(0),
+            stub,
+        }
+    }
+
+    /// Returns the current number of available permits
+    pub(crate) fn available_permits(&self) -> usize {
+        let curr = SemState(self.state.load(Acquire));
+        curr.available_permits()
+    }
+
+    /// Tries to acquire the requested number of permits, registering the waiter
+    /// if not enough permits are available.
+    fn poll_acquire(
+        &self,
+        cx: &mut Context<'_>,
+        num_permits: u16,
+        permit: &mut Permit,
+    ) -> Poll<Result<(), AcquireError>> {
+        self.poll_acquire2(num_permits, || {
+            let waiter = permit.waiter.get_or_insert_with(|| Box::new(Waiter::new()));
+
+            waiter.waker.register_by_ref(cx.waker());
+
+            Some(NonNull::from(&**waiter))
+        })
+    }
+
+    fn try_acquire(&self, num_permits: u16) -> Result<(), TryAcquireError> {
+        match self.poll_acquire2(num_permits, || None) {
+            Poll::Ready(res) => res.map_err(to_try_acquire),
+            Poll::Pending => Err(TryAcquireError::NoPermits),
+        }
+    }
+
+    /// Polls for a permit
+    ///
+    /// Tries to acquire available permits first. If unable to acquire a
+    /// sufficient number of permits, the caller's waiter is pushed onto the
+    /// semaphore's wait queue.
+    fn poll_acquire2<F>(
+        &self,
+        num_permits: u16,
+        mut get_waiter: F,
+    ) -> Poll<Result<(), AcquireError>>
+    where
+        F: FnMut() -> Option<NonNull<Waiter>>,
+    {
+        let num_permits = num_permits as usize;
+
+        // Load the current state
+        let mut curr = SemState(self.state.load(Acquire));
+
+        // Saves a ref to the waiter node
+        let mut maybe_waiter: Option<NonNull<Waiter>> = None;
+
+        /// Used in branches where we attempt to push the waiter into the wait
+        /// queue but fail due to permits becoming available or the wait queue
+        /// transitioning to "closed". In this case, the waiter must be
+        /// transitioned back to the "idle" state.
+        macro_rules! revert_to_idle {
+            () => {
+                if let Some(waiter) = maybe_waiter {
+                    unsafe { waiter.as_ref() }.revert_to_idle();
+                }
+            };
+        }
+
+        loop {
+            let mut next = curr;
+
+            if curr.is_closed() {
+                revert_to_idle!();
+                return Ready(Err(AcquireError::closed()));
+            }
+
+            let acquired = next.acquire_permits(num_permits, &self.stub);
+
+            if !acquired {
+                // There are not enough available permits to satisfy the
+                // request. The permit transitions to a waiting state.
+                debug_assert!(curr.waiter().is_some() || curr.available_permits() < num_permits);
+
+                if let Some(waiter) = maybe_waiter.as_ref() {
+                    // Safety: the caller owns the waiter.
+                    let w = unsafe { waiter.as_ref() };
+                    w.set_permits_to_acquire(num_permits - curr.available_permits());
+                } else {
+                    // Get the waiter for the permit.
+                    if let Some(waiter) = get_waiter() {
+                        // Safety: the caller owns the waiter.
+                        let w = unsafe { waiter.as_ref() };
+
+                        // If there are any currently available permits, the
+                        // waiter acquires those immediately and waits for the
+                        // remaining permits to become available.
+                        if !w.to_queued(num_permits - curr.available_permits()) {
+                            // The node is alrady queued, there is no further work
+                            // to do.
+                            return Pending;
+                        }
+
+                        maybe_waiter = Some(waiter);
+                    } else {
+                        // No waiter, this indicates the caller does not wish to
+                        // "wait", so there is nothing left to do.
+                        return Pending;
+                    }
+                }
+
+                next.set_waiter(maybe_waiter.unwrap());
+            }
+
+            debug_assert_ne!(curr.0, 0);
+            debug_assert_ne!(next.0, 0);
+
+            match self.state.compare_exchange(curr.0, next.0, AcqRel, Acquire) {
+                Ok(_) => {
+                    if acquired {
+                        // Successfully acquire permits **without** queuing the
+                        // waiter node. The waiter node is not currently in the
+                        // queue.
+                        revert_to_idle!();
+                        return Ready(Ok(()));
+                    } else {
+                        // The node is pushed into the queue, the final step is
+                        // to set the node's "next" pointer to return the wait
+                        // queue into a consistent state.
+
+                        let prev_waiter =
+                            curr.waiter().unwrap_or_else(|| NonNull::from(&*self.stub));
+
+                        let waiter = maybe_waiter.unwrap();
+
+                        // Link the nodes.
+                        //
+                        // Safety: the mpsc algorithm guarantees the old tail of
+                        // the queue is not removed from the queue during the
+                        // push process.
+                        unsafe {
+                            prev_waiter.as_ref().store_next(waiter);
+                        }
+
+                        return Pending;
+                    }
+                }
+                Err(actual) => {
+                    curr = SemState(actual);
+                }
+            }
+        }
+    }
+
+    /// Closes the semaphore. This prevents the semaphore from issuing new
+    /// permits and notifies all pending waiters.
+    pub(crate) fn close(&self) {
+        // Acquire the `rx_lock`, setting the "closed" flag on the lock.
+        let prev = self.rx_lock.fetch_or(1, AcqRel);
+
+        if prev != 0 {
+            // Another thread has the lock and will be responsible for notifying
+            // pending waiters.
+            return;
+        }
+
+        self.add_permits_locked(0, true);
+    }
+
+    /// Adds `n` new permits to the semaphore.
+    pub(crate) fn add_permits(&self, n: usize) {
+        if n == 0 {
+            return;
+        }
+
+        // TODO: Handle overflow. A panic is not sufficient, the process must
+        // abort.
+        let prev = self.rx_lock.fetch_add(n << 1, AcqRel);
+
+        if prev != 0 {
+            // Another thread has the lock and will be responsible for notifying
+            // pending waiters.
+            return;
+        }
+
+        self.add_permits_locked(n, false);
+    }
+
+    fn add_permits_locked(&self, mut rem: usize, mut closed: bool) {
+        while rem > 0 || closed {
+            if closed {
+                SemState::fetch_set_closed(&self.state, AcqRel);
+            }
+
+            // Release the permits and notify
+            self.add_permits_locked2(rem, closed);
+
+            let n = rem << 1;
+
+            let actual = if closed {
+                let actual = self.rx_lock.fetch_sub(n | 1, AcqRel);
+                closed = false;
+                actual
+            } else {
+                let actual = self.rx_lock.fetch_sub(n, AcqRel);
+                closed = actual & 1 == 1;
+                actual
+            };
+
+            rem = (actual >> 1) - rem;
+        }
+    }
+
+    /// Releases a specific amount of permits to the semaphore
+    ///
+    /// This function is called by `add_permits` after the add lock has been
+    /// acquired.
+    fn add_permits_locked2(&self, mut n: usize, closed: bool) {
+        // If closing the semaphore, we want to drain the entire queue. The
+        // number of permits being assigned doesn't matter.
+        if closed {
+            n = usize::MAX;
+        }
+
+        'outer: while n > 0 {
+            unsafe {
+                let mut head = self.head.with(|head| *head);
+                let mut next_ptr = head.as_ref().next.load(Acquire);
+
+                let stub = self.stub();
+
+                if head == stub {
+                    // The stub node indicates an empty queue. Any remaining
+                    // permits get assigned back to the semaphore.
+                    let next = match NonNull::new(next_ptr) {
+                        Some(next) => next,
+                        None => {
+                            // This loop is not part of the standard intrusive mpsc
+                            // channel algorithm. This is where we atomically pop
+                            // the last task and add `n` to the remaining capacity.
+                            //
+                            // This modification to the pop algorithm works because,
+                            // at this point, we have not done any work (only done
+                            // reading). We have a *pretty* good idea that there is
+                            // no concurrent pusher.
+                            //
+                            // The capacity is then atomically added by doing an
+                            // AcqRel CAS on `state`. The `state` cell is the
+                            // linchpin of the algorithm.
+                            //
+                            // By successfully CASing `head` w/ AcqRel, we ensure
+                            // that, if any thread was racing and entered a push, we
+                            // see that and abort pop, retrying as it is
+                            // "inconsistent".
+                            let mut curr = SemState::load(&self.state, Acquire);
+
+                            loop {
+                                if curr.has_waiter(&self.stub) {
+                                    // A waiter is being added concurrently.
+                                    // This is the MPSC queue's "inconsistent"
+                                    // state and we must loop and try again.
+                                    thread::yield_now();
+                                    continue 'outer;
+                                }
+
+                                // If closing, nothing more to do.
+                                if closed {
+                                    debug_assert!(curr.is_closed(), "state = {:?}", curr);
+                                    return;
+                                }
+
+                                let mut next = curr;
+                                next.release_permits(n, &self.stub);
+
+                                match self.state.compare_exchange(curr.0, next.0, AcqRel, Acquire) {
+                                    Ok(_) => return,
+                                    Err(actual) => {
+                                        curr = SemState(actual);
+                                    }
+                                }
+                            }
+                        }
+                    };
+
+                    self.head.with_mut(|head| *head = next);
+                    head = next;
+                    next_ptr = next.as_ref().next.load(Acquire);
+                }
+
+                // `head` points to a waiter assign permits to the waiter. If
+                // all requested permits are satisfied, then we can continue,
+                // otherwise the node stays in the wait queue.
+                if !head.as_ref().assign_permits(&mut n, closed) {
+                    assert_eq!(n, 0);
+                    return;
+                }
+
+                if let Some(next) = NonNull::new(next_ptr) {
+                    self.head.with_mut(|head| *head = next);
+
+                    self.remove_queued(head, closed);
+                    continue 'outer;
+                }
+
+                let state = SemState::load(&self.state, Acquire);
+
+                // This must always be a pointer as the wait list is not empty.
+                let tail = state.waiter().unwrap();
+
+                if tail != head {
+                    // Inconsistent
+                    thread::yield_now();
+                    continue 'outer;
+                }
+
+                self.push_stub(closed);
+
+                next_ptr = head.as_ref().next.load(Acquire);
+
+                if let Some(next) = NonNull::new(next_ptr) {
+                    self.head.with_mut(|head| *head = next);
+
+                    self.remove_queued(head, closed);
+                    continue 'outer;
+                }
+
+                // Inconsistent state, loop
+                thread::yield_now();
+            }
+        }
+    }
+
+    /// The wait node has had all of its permits assigned and has been removed
+    /// from the wait queue.
+    ///
+    /// Attempt to remove the QUEUED bit from the node. If additional permits
+    /// are concurrently requested, the node must be pushed back into the wait
+    /// queued.
+    fn remove_queued(&self, waiter: NonNull<Waiter>, closed: bool) {
+        let mut curr = WaiterState(unsafe { waiter.as_ref() }.state.load(Acquire));
+
+        loop {
+            if curr.is_dropped() {
+                // The Permit dropped, it is on us to release the memory
+                let _ = unsafe { Box::from_raw(waiter.as_ptr()) };
+                return;
+            }
+
+            // The node is removed from the queue. We attempt to unset the
+            // queued bit, but concurrently the waiter has requested more
+            // permits. When the waiter requested more permits, it saw the
+            // queued bit set so took no further action. This requires us to
+            // push the node back into the queue.
+            if curr.permits_to_acquire() > 0 {
+                // More permits are requested. The waiter must be re-queued
+                unsafe {
+                    self.push_waiter(waiter, closed);
+                }
+                return;
+            }
+
+            let mut next = curr;
+            next.unset_queued();
+
+            let w = unsafe { waiter.as_ref() };
+
+            match w.state.compare_exchange(curr.0, next.0, AcqRel, Acquire) {
+                Ok(_) => return,
+                Err(actual) => {
+                    curr = WaiterState(actual);
+                }
+            }
+        }
+    }
+
+    unsafe fn push_stub(&self, closed: bool) {
+        self.push_waiter(self.stub(), closed);
+    }
+
+    unsafe fn push_waiter(&self, waiter: NonNull<Waiter>, closed: bool) {
+        // Set the next pointer. This does not require an atomic operation as
+        // this node is not accessible. The write will be flushed with the next
+        // operation
+        waiter.as_ref().next.store(ptr::null_mut(), Relaxed);
+
+        // Update the tail to point to the new node. We need to see the previous
+        // node in order to update the next pointer as well as release `task`
+        // to any other threads calling `push`.
+        let next = SemState::new_ptr(waiter, closed);
+        let prev = SemState(self.state.swap(next.0, AcqRel));
+
+        debug_assert_eq!(closed, prev.is_closed());
+
+        // This function is only called when there are pending tasks. Because of
+        // this, the state must *always* be in pointer mode.
+        let prev = prev.waiter().unwrap();
+
+        // No cycles plz
+        debug_assert_ne!(prev, waiter);
+
+        // Release `task` to the consume end.
+        prev.as_ref().next.store(waiter.as_ptr(), Release);
+    }
+
+    fn stub(&self) -> NonNull<Waiter> {
+        unsafe { NonNull::new_unchecked(&*self.stub as *const _ as *mut _) }
+    }
+}
+
+impl Drop for Semaphore {
+    fn drop(&mut self) {
+        self.close();
+    }
+}
+
+impl fmt::Debug for Semaphore {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("Semaphore")
+            .field("state", &SemState::load(&self.state, Relaxed))
+            .field("head", &self.head.with(|ptr| ptr))
+            .field("rx_lock", &self.rx_lock.load(Relaxed))
+            .field("stub", &self.stub)
+            .finish()
+    }
+}
+
+unsafe impl Send for Semaphore {}
+unsafe impl Sync for Semaphore {}
+
+// ===== impl Permit =====
+
+impl Permit {
+    /// Creates a new `Permit`.
+    ///
+    /// The permit begins in the "unacquired" state.
+    pub(crate) fn new() -> Permit {
+        use PermitState::Acquired;
+
+        Permit {
+            waiter: None,
+            state: Acquired(0),
+        }
+    }
+
+    /// Returns `true` if the permit has been acquired
+    #[allow(dead_code)] // may be used later
+    pub(crate) fn is_acquired(&self) -> bool {
+        match self.state {
+            PermitState::Acquired(num) if num > 0 => true,
+            _ => false,
+        }
+    }
+
+    /// Tries to acquire the permit. If no permits are available, the current task
+    /// is notified once a new permit becomes available.
+    pub(crate) fn poll_acquire(
+        &mut self,
+        cx: &mut Context<'_>,
+        num_permits: u16,
+        semaphore: &Semaphore,
+    ) -> Poll<Result<(), AcquireError>> {
+        use std::cmp::Ordering::*;
+        use PermitState::*;
+
+        match self.state {
+            Waiting(requested) => {
+                // There must be a waiter
+                let waiter = self.waiter.as_ref().unwrap();
+
+                match requested.cmp(&num_permits) {
+                    Less => {
+                        let delta = num_permits - requested;
+
+                        // Request additional permits. If the waiter has been
+                        // dequeued, it must be re-queued.
+                        if !waiter.try_inc_permits_to_acquire(delta as usize) {
+                            let waiter = NonNull::from(&**waiter);
+
+                            // Ignore the result. The check for
+                            // `permits_to_acquire()` will converge the state as
+                            // needed
+                            let _ = semaphore.poll_acquire2(delta, || Some(waiter))?;
+                        }
+
+                        self.state = Waiting(num_permits);
+                    }
+                    Greater => {
+                        let delta = requested - num_permits;
+                        let to_release = waiter.try_dec_permits_to_acquire(delta as usize);
+
+                        semaphore.add_permits(to_release);
+                        self.state = Waiting(num_permits);
+                    }
+                    Equal => {}
+                }
+
+                if waiter.permits_to_acquire()? == 0 {
+                    self.state = Acquired(requested);
+                    return Ready(Ok(()));
+                }
+
+                waiter.waker.register_by_ref(cx.waker());
+
+                if waiter.permits_to_acquire()? == 0 {
+                    self.state = Acquired(requested);
+                    return Ready(Ok(()));
+                }
+
+                Pending
+            }
+            Acquired(acquired) => {
+                if acquired >= num_permits {
+                    Ready(Ok(()))
+                } else {
+                    match semaphore.poll_acquire(cx, num_permits - acquired, self)? {
+                        Ready(()) => {
+                            self.state = Acquired(num_permits);
+                            Ready(Ok(()))
+                        }
+                        Pending => {
+                            self.state = Waiting(num_permits);
+                            Pending
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    /// Tries to acquire the permit.
+    pub(crate) fn try_acquire(
+        &mut self,
+        num_permits: u16,
+        semaphore: &Semaphore,
+    ) -> Result<(), TryAcquireError> {
+        use PermitState::*;
+
+        match self.state {
+            Waiting(requested) => {
+                // There must be a waiter
+                let waiter = self.waiter.as_ref().unwrap();
+
+                if requested > num_permits {
+                    let delta = requested - num_permits;
+                    let to_release = waiter.try_dec_permits_to_acquire(delta as usize);
+
+                    semaphore.add_permits(to_release);
+                    self.state = Waiting(num_permits);
+                }
+
+                let res = waiter.permits_to_acquire().map_err(to_try_acquire)?;
+
+                if res == 0 {
+                    if requested < num_permits {
+                        // Try to acquire the additional permits
+                        semaphore.try_acquire(num_permits - requested)?;
+                    }
+
+                    self.state = Acquired(num_permits);
+                    Ok(())
+                } else {
+                    Err(TryAcquireError::NoPermits)
+                }
+            }
+            Acquired(acquired) => {
+                if acquired < num_permits {
+                    semaphore.try_acquire(num_permits - acquired)?;
+                    self.state = Acquired(num_permits);
+                }
+
+                Ok(())
+            }
+        }
+    }
+
+    /// Releases a permit back to the semaphore
+    pub(crate) fn release(&mut self, n: u16, semaphore: &Semaphore) {
+        let n = self.forget(n);
+        semaphore.add_permits(n as usize);
+    }
+
+    /// Forgets the permit **without** releasing it back to the semaphore.
+    ///
+    /// After calling `forget`, `poll_acquire` is able to acquire new permit
+    /// from the sempahore.
+    ///
+    /// Repeatedly calling `forget` without associated calls to `add_permit`
+    /// will result in the semaphore losing all permits.
+    ///
+    /// Will forget **at most** the number of acquired permits. This number is
+    /// returned.
+    pub(crate) fn forget(&mut self, n: u16) -> u16 {
+        use PermitState::*;
+
+        match self.state {
+            Waiting(requested) => {
+                let n = cmp::min(n, requested);
+
+                // Decrement
+                let acquired = self
+                    .waiter
+                    .as_ref()
+                    .unwrap()
+                    .try_dec_permits_to_acquire(n as usize) as u16;
+
+                if n == requested {
+                    self.state = Acquired(0);
+                } else if acquired == requested - n {
+                    self.state = Waiting(acquired);
+                } else {
+                    self.state = Waiting(requested - n);
+                }
+
+                acquired
+            }
+            Acquired(acquired) => {
+                let n = cmp::min(n, acquired);
+                self.state = Acquired(acquired - n);
+                n
+            }
+        }
+    }
+}
+
+impl Default for Permit {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl Drop for Permit {
+    fn drop(&mut self) {
+        if let Some(waiter) = self.waiter.take() {
+            // Set the dropped flag
+            let state = WaiterState(waiter.state.fetch_or(DROPPED, AcqRel));
+
+            if state.is_queued() {
+                // The waiter is stored in the queue. The semaphore will drop it
+                std::mem::forget(waiter);
+            }
+        }
+    }
+}
+
+// ===== impl AcquireError ====
+
+impl AcquireError {
+    fn closed() -> AcquireError {
+        AcquireError(())
+    }
+}
+
+fn to_try_acquire(_: AcquireError) -> TryAcquireError {
+    TryAcquireError::Closed
+}
+
+impl fmt::Display for AcquireError {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(fmt, "semaphore closed")
+    }
+}
+
+impl std::error::Error for AcquireError {}
+
+// ===== impl TryAcquireError =====
+
+impl TryAcquireError {
+    /// Returns `true` if the error was caused by a closed semaphore.
+    pub(crate) fn is_closed(&self) -> bool {
+        match self {
+            TryAcquireError::Closed => true,
+            _ => false,
+        }
+    }
+
+    /// Returns `true` if the error was caused by calling `try_acquire` on a
+    /// semaphore with no available permits.
+    pub(crate) fn is_no_permits(&self) -> bool {
+        match self {
+            TryAcquireError::NoPermits => true,
+            _ => false,
+        }
+    }
+}
+
+impl fmt::Display for TryAcquireError {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            TryAcquireError::Closed => write!(fmt, "{}", "semaphore closed"),
+            TryAcquireError::NoPermits => write!(fmt, "{}", "no permits available"),
+        }
+    }
+}
+
+impl std::error::Error for TryAcquireError {}
+
+// ===== impl Waiter =====
+
+impl Waiter {
+    fn new() -> Waiter {
+        Waiter {
+            state: AtomicUsize::new(0),
+            waker: AtomicWaker::new(),
+            next: AtomicPtr::new(ptr::null_mut()),
+        }
+    }
+
+    fn permits_to_acquire(&self) -> Result<usize, AcquireError> {
+        let state = WaiterState(self.state.load(Acquire));
+
+        if state.is_closed() {
+            Err(AcquireError(()))
+        } else {
+            Ok(state.permits_to_acquire())
+        }
+    }
+
+    /// Only increments the number of permits *if* the waiter is currently
+    /// queued.
+    ///
+    /// # Returns
+    ///
+    /// `true` if the number of permits to acquire has been incremented. `false`
+    /// otherwise. On `false`, the caller should use `Semaphore::poll_acquire`.
+    fn try_inc_permits_to_acquire(&self, n: usize) -> bool {
+        let mut curr = WaiterState(self.state.load(Acquire));
+
+        loop {
+            if !curr.is_queued() {
+                assert_eq!(0, curr.permits_to_acquire());
+                return false;
+            }
+
+            let mut next = curr;
+            next.set_permits_to_acquire(n + curr.permits_to_acquire());
+
+            match self.state.compare_exchange(curr.0, next.0, AcqRel, Acquire) {
+                Ok(_) => return true,
+                Err(actual) => curr = WaiterState(actual),
+            }
+        }
+    }
+
+    /// Try to decrement the number of permits to acquire. This returns the
+    /// actual number of permits that were decremented. The delta betweeen `n`
+    /// and the return has been assigned to the permit and the caller must
+    /// assign these back to the semaphore.
+    fn try_dec_permits_to_acquire(&self, n: usize) -> usize {
+        let mut curr = WaiterState(self.state.load(Acquire));
+
+        loop {
+            if !curr.is_queued() {
+                assert_eq!(0, curr.permits_to_acquire());
+            }
+
+            let delta = cmp::min(n, curr.permits_to_acquire());
+            let rem = curr.permits_to_acquire() - delta;
+
+            let mut next = curr;
+            next.set_permits_to_acquire(rem);
+
+            match self.state.compare_exchange(curr.0, next.0, AcqRel, Acquire) {
+                Ok(_) => return n - delta,
+                Err(actual) => curr = WaiterState(actual),
+            }
+        }
+    }
+
+    /// Store the number of remaining permits needed to satisfy the waiter and
+    /// transition to the "QUEUED" state.
+    ///
+    /// # Returns
+    ///
+    /// `true` if the `QUEUED` bit was set as part of the transition.
+    fn to_queued(&self, num_permits: usize) -> bool {
+        let mut curr = WaiterState(self.state.load(Acquire));
+
+        // The waiter should **not** be waiting for any permits.
+        debug_assert_eq!(curr.permits_to_acquire(), 0);
+
+        loop {
+            let mut next = curr;
+            next.set_permits_to_acquire(num_permits);
+            next.set_queued();
+
+            match self.state.compare_exchange(curr.0, next.0, AcqRel, Acquire) {
+                Ok(_) => {
+                    if curr.is_queued() {
+                        return false;
+                    } else {
+                        // Make sure the next pointer is null
+                        self.next.store(ptr::null_mut(), Relaxed);
+                        return true;
+                    }
+                }
+                Err(actual) => curr = WaiterState(actual),
+            }
+        }
+    }
+
+    /// Set the number of permits to acquire.
+    ///
+    /// This function is only called when the waiter is being inserted into the
+    /// wait queue. Because of this, there are no concurrent threads that can
+    /// modify the state and using `store` is safe.
+    fn set_permits_to_acquire(&self, num_permits: usize) {
+        debug_assert!(WaiterState(self.state.load(Acquire)).is_queued());
+
+        let mut state = WaiterState(QUEUED);
+        state.set_permits_to_acquire(num_permits);
+
+        self.state.store(state.0, Release);
+    }
+
+    /// Assign permits to the waiter.
+    ///
+    /// Returns `true` if the waiter should be removed from the queue
+    fn assign_permits(&self, n: &mut usize, closed: bool) -> bool {
+        let mut curr = WaiterState(self.state.load(Acquire));
+
+        loop {
+            let mut next = curr;
+
+            // Number of permits to assign to this waiter
+            let assign = cmp::min(curr.permits_to_acquire(), *n);
+
+            // Assign the permits
+            next.set_permits_to_acquire(curr.permits_to_acquire() - assign);
+
+            if closed {
+                next.set_closed();
+            }
+
+            match self.state.compare_exchange(curr.0, next.0, AcqRel, Acquire) {
+                Ok(_) => {
+                    // Update `n`
+                    *n -= assign;
+
+                    if next.permits_to_acquire() == 0 {
+                        if curr.permits_to_acquire() > 0 {
+                            self.waker.wake();
+                        }
+
+                        return true;
+                    } else {
+                        return false;
+                    }
+                }
+                Err(actual) => curr = WaiterState(actual),
+            }
+        }
+    }
+
+    fn revert_to_idle(&self) {
+        // An idle node is not waiting on any permits
+        self.state.store(0, Relaxed);
+    }
+
+    fn store_next(&self, next: NonNull<Waiter>) {
+        self.next.store(next.as_ptr(), Release);
+    }
+}
+
+// ===== impl SemState =====
+
+impl SemState {
+    /// Returns a new default `State` value.
+    fn new(permits: usize, stub: &Waiter) -> SemState {
+        assert!(permits <= MAX_PERMITS);
+
+        if permits > 0 {
+            SemState((permits << NUM_SHIFT) | NUM_FLAG)
+        } else {
+            SemState(stub as *const _ as usize)
+        }
+    }
+
+    /// Returns a `State` tracking `ptr` as the tail of the queue.
+    fn new_ptr(tail: NonNull<Waiter>, closed: bool) -> SemState {
+        let mut val = tail.as_ptr() as usize;
+
+        if closed {
+            val |= CLOSED_FLAG;
+        }
+
+        SemState(val)
+    }
+
+    /// Returns the amount of remaining capacity
+    fn available_permits(self) -> usize {
+        if !self.has_available_permits() {
+            return 0;
+        }
+
+        self.0 >> NUM_SHIFT
+    }
+
+    /// Returns `true` if the state has permits that can be claimed by a waiter.
+    fn has_available_permits(self) -> bool {
+        self.0 & NUM_FLAG == NUM_FLAG
+    }
+
+    fn has_waiter(self, stub: &Waiter) -> bool {
+        !self.has_available_permits() && !self.is_stub(stub)
+    }
+
+    /// Tries to atomically acquire specified number of permits.
+    ///
+    /// # Return
+    ///
+    /// Returns `true` if the specified number of permits were acquired, `false`
+    /// otherwise. Returning false does not mean that there are no more
+    /// available permits.
+    fn acquire_permits(&mut self, num: usize, stub: &Waiter) -> bool {
+        debug_assert!(num > 0);
+
+        if self.available_permits() < num {
+            return false;
+        }
+
+        debug_assert!(self.waiter().is_none());
+
+        self.0 -= num << NUM_SHIFT;
+
+        if self.0 == NUM_FLAG {
+            // Set the state to the stub pointer.
+            self.0 = stub as *const _ as usize;
+        }
+
+        true
+    }
+
+    /// Releases permits
+    ///
+    /// Returns `true` if the permits were accepted.
+    fn release_permits(&mut self, permits: usize, stub: &Waiter) {
+        debug_assert!(permits > 0);
+
+        if self.is_stub(stub) {
+            self.0 = (permits << NUM_SHIFT) | NUM_FLAG | (self.0 & CLOSED_FLAG);
+            return;
+        }
+
+        debug_assert!(self.has_available_permits());
+
+        self.0 += permits << NUM_SHIFT;
+    }
+
+    fn is_waiter(self) -> bool {
+        self.0 & NUM_FLAG == 0
+    }
+
+    /// Returns the waiter, if one is set.
+    fn waiter(self) -> Option<NonNull<Waiter>> {
+        if self.is_waiter() {
+            let waiter = NonNull::new(self.as_ptr()).expect("null pointer stored");
+
+            Some(waiter)
+        } else {
+            None
+        }
+    }
+
+    /// Assumes `self` represents a pointer
+    fn as_ptr(self) -> *mut Waiter {
+        (self.0 & !CLOSED_FLAG) as *mut Waiter
+    }
+
+    /// Sets to a pointer to a waiter.
+    ///
+    /// This can only be done from the full state.
+    fn set_waiter(&mut self, waiter: NonNull<Waiter>) {
+        let waiter = waiter.as_ptr() as usize;
+        debug_assert!(!self.is_closed());
+
+        self.0 = waiter;
+    }
+
+    fn is_stub(self, stub: &Waiter) -> bool {
+        self.as_ptr() as usize == stub as *const _ as usize
+    }
+
+    /// Loads the state from an AtomicUsize.
+    fn load(cell: &AtomicUsize, ordering: Ordering) -> SemState {
+        let value = cell.load(ordering);
+        SemState(value)
+    }
+
+    fn fetch_set_closed(cell: &AtomicUsize, ordering: Ordering) -> SemState {
+        let value = cell.fetch_or(CLOSED_FLAG, ordering);
+        SemState(value)
+    }
+
+    fn is_closed(self) -> bool {
+        self.0 & CLOSED_FLAG == CLOSED_FLAG
+    }
+
+    /// Converts the state into a `usize` representation.
+    fn to_usize(self) -> usize {
+        self.0
+    }
+}
+
+impl fmt::Debug for SemState {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut fmt = fmt.debug_struct("SemState");
+
+        if self.is_waiter() {
+            fmt.field("state", &"<waiter>");
+        } else {
+            fmt.field("permits", &self.available_permits());
+        }
+
+        fmt.finish()
+    }
+}
+
+// ===== impl WaiterState =====
+
+impl WaiterState {
+    fn permits_to_acquire(self) -> usize {
+        self.0 >> PERMIT_SHIFT
+    }
+
+    fn set_permits_to_acquire(&mut self, val: usize) {
+        self.0 = (val << PERMIT_SHIFT) | (self.0 & !PERMIT_MASK)
+    }
+
+    fn is_queued(self) -> bool {
+        self.0 & QUEUED == QUEUED
+    }
+
+    fn set_queued(&mut self) {
+        self.0 |= QUEUED;
+    }
+
+    fn is_closed(self) -> bool {
+        self.0 & CLOSED == CLOSED
+    }
+
+    fn set_closed(&mut self) {
+        self.0 |= CLOSED;
+    }
+
+    fn unset_queued(&mut self) {
+        assert!(self.is_queued());
+        self.0 -= QUEUED;
+    }
+
+    fn is_dropped(self) -> bool {
+        self.0 & DROPPED == DROPPED
+    }
+}
diff --git a/third_party/rust/tokio/src/sync/task/atomic_waker.rs b/third_party/rust/tokio/src/sync/task/atomic_waker.rs
new file mode 100644
index 0000000000..73b1745f1a
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/task/atomic_waker.rs
@@ -0,0 +1,318 @@
+#![cfg_attr(any(loom, not(feature = "sync")), allow(dead_code, unreachable_pub))]
+
+use crate::loom::cell::UnsafeCell;
+use crate::loom::sync::atomic::{self, AtomicUsize};
+
+use std::fmt;
+use std::sync::atomic::Ordering::{AcqRel, Acquire, Release};
+use std::task::Waker;
+
+/// A synchronization primitive for task waking.
+///
+/// `AtomicWaker` will coordinate concurrent wakes with the consumer
+/// potentially "waking" the underlying task. This is useful in scenarios
+/// where a computation completes in another thread and wants to wake the
+/// consumer, but the consumer is in the process of being migrated to a new
+/// logical task.
+///
+/// Consumers should call `register` before checking the result of a computation
+/// and producers should call `wake` after producing the computation (this
+/// differs from the usual `thread::park` pattern). It is also permitted for
+/// `wake` to be called **before** `register`. This results in a no-op.
+///
+/// A single `AtomicWaker` may be reused for any number of calls to `register` or
+/// `wake`.
+pub(crate) struct AtomicWaker {
+    state: AtomicUsize,
+    waker: UnsafeCell<Option<Waker>>,
+}
+
+// `AtomicWaker` is a multi-consumer, single-producer transfer cell. The cell
+// stores a `Waker` value produced by calls to `register` and many threads can
+// race to take the waker by calling `wake.
+//
+// If a new `Waker` instance is produced by calling `register` before an existing
+// one is consumed, then the existing one is overwritten.
+//
+// While `AtomicWaker` is single-producer, the implementation ensures memory
+// safety. In the event of concurrent calls to `register`, there will be a
+// single winner whose waker will get stored in the cell. The losers will not
+// have their tasks woken. As such, callers should ensure to add synchronization
+// to calls to `register`.
+//
+// The implementation uses a single `AtomicUsize` value to coordinate access to
+// the `Waker` cell. There are two bits that are operated on independently. These
+// are represented by `REGISTERING` and `WAKING`.
+//
+// The `REGISTERING` bit is set when a producer enters the critical section. The
+// `WAKING` bit is set when a consumer enters the critical section. Neither
+// bit being set is represented by `WAITING`.
+//
+// A thread obtains an exclusive lock on the waker cell by transitioning the
+// state from `WAITING` to `REGISTERING` or `WAKING`, depending on the
+// operation the thread wishes to perform. When this transition is made, it is
+// guaranteed that no other thread will access the waker cell.
+//
+// # Registering
+//
+// On a call to `register`, an attempt to transition the state from WAITING to
+// REGISTERING is made. On success, the caller obtains a lock on the waker cell.
+//
+// If the lock is obtained, then the thread sets the waker cell to the waker
+// provided as an argument. Then it attempts to transition the state back from
+// `REGISTERING` -> `WAITING`.
+//
+// If this transition is successful, then the registering process is complete
+// and the next call to `wake` will observe the waker.
+//
+// If the transition fails, then there was a concurrent call to `wake` that
+// was unable to access the waker cell (due to the registering thread holding the
+// lock). To handle this, the registering thread removes the waker it just set
+// from the cell and calls `wake` on it. This call to wake represents the
+// attempt to wake by the other thread (that set the `WAKING` bit). The
+// state is then transitioned from `REGISTERING | WAKING` back to `WAITING`.
+// This transition must succeed because, at this point, the state cannot be
+// transitioned by another thread.
+//
+// # Waking
+//
+// On a call to `wake`, an attempt to transition the state from `WAITING` to
+// `WAKING` is made. On success, the caller obtains a lock on the waker cell.
+//
+// If the lock is obtained, then the thread takes ownership of the current value
+// in the waker cell, and calls `wake` on it. The state is then transitioned
+// back to `WAITING`. This transition must succeed as, at this point, the state
+// cannot be transitioned by another thread.
+//
+// If the thread is unable to obtain the lock, the `WAKING` bit is still.
+// This is because it has either been set by the current thread but the previous
+// value included the `REGISTERING` bit **or** a concurrent thread is in the
+// `WAKING` critical section. Either way, no action must be taken.
+//
+// If the current thread is the only concurrent call to `wake` and another
+// thread is in the `register` critical section, when the other thread **exits**
+// the `register` critical section, it will observe the `WAKING` bit and
+// handle the waker itself.
+//
+// If another thread is in the `waker` critical section, then it will handle
+// waking the caller task.
+//
+// # A potential race (is safely handled).
+//
+// Imagine the following situation:
+//
+// * Thread A obtains the `wake` lock and wakes a task.
+//
+// * Before thread A releases the `wake` lock, the woken task is scheduled.
+//
+// * Thread B attempts to wake the task. In theory this should result in the
+//   task being woken, but it cannot because thread A still holds the wake
+//   lock.
+//
+// This case is handled by requiring users of `AtomicWaker` to call `register`
+// **before** attempting to observe the application state change that resulted
+// in the task being woken. The wakers also change the application state
+// before calling wake.
+//
+// Because of this, the task will do one of two things.
+//
+// 1) Observe the application state change that Thread B is waking on. In
+//    this case, it is OK for Thread B's wake to be lost.
+//
+// 2) Call register before attempting to observe the application state. Since
+//    Thread A still holds the `wake` lock, the call to `register` will result
+//    in the task waking itself and get scheduled again.
+
+/// Idle state
+const WAITING: usize = 0;
+
+/// A new waker value is being registered with the `AtomicWaker` cell.
+const REGISTERING: usize = 0b01;
+
+/// The task currently registered with the `AtomicWaker` cell is being woken.
+const WAKING: usize = 0b10;
+
+impl AtomicWaker {
+    /// Create an `AtomicWaker`
+    pub(crate) fn new() -> AtomicWaker {
+        AtomicWaker {
+            state: AtomicUsize::new(WAITING),
+            waker: UnsafeCell::new(None),
+        }
+    }
+
+    /// Registers the current waker to be notified on calls to `wake`.
+    ///
+    /// This is the same as calling `register_task` with `task::current()`.
+    #[cfg(feature = "io-driver")]
+    pub(crate) fn register(&self, waker: Waker) {
+        self.do_register(waker);
+    }
+
+    /// Registers the provided waker to be notified on calls to `wake`.
+    ///
+    /// The new waker will take place of any previous wakers that were registered
+    /// by previous calls to `register`. Any calls to `wake` that happen after
+    /// a call to `register` (as defined by the memory ordering rules), will
+    /// wake the `register` caller's task.
+    ///
+    /// It is safe to call `register` with multiple other threads concurrently
+    /// calling `wake`. This will result in the `register` caller's current
+    /// task being woken once.
+    ///
+    /// This function is safe to call concurrently, but this is generally a bad
+    /// idea. Concurrent calls to `register` will attempt to register different
+    /// tasks to be woken. One of the callers will win and have its task set,
+    /// but there is no guarantee as to which caller will succeed.
+    pub(crate) fn register_by_ref(&self, waker: &Waker) {
+        self.do_register(waker);
+    }
+
+    fn do_register<W>(&self, waker: W)
+    where
+        W: WakerRef,
+    {
+        match self.state.compare_and_swap(WAITING, REGISTERING, Acquire) {
+            WAITING => {
+                unsafe {
+                    // Locked acquired, update the waker cell
+                    self.waker.with_mut(|t| *t = Some(waker.into_waker()));
+
+                    // Release the lock. If the state transitioned to include
+                    // the `WAKING` bit, this means that a wake has been
+                    // called concurrently, so we have to remove the waker and
+                    // wake it.`
+                    //
+                    // Start by assuming that the state is `REGISTERING` as this
+                    // is what we jut set it to.
+                    let res = self
+                        .state
+                        .compare_exchange(REGISTERING, WAITING, AcqRel, Acquire);
+
+                    match res {
+                        Ok(_) => {}
+                        Err(actual) => {
+                            // This branch can only be reached if a
+                            // concurrent thread called `wake`. In this
+                            // case, `actual` **must** be `REGISTERING |
+                            // `WAKING`.
+                            debug_assert_eq!(actual, REGISTERING | WAKING);
+
+                            // Take the waker to wake once the atomic operation has
+                            // completed.
+                            let waker = self.waker.with_mut(|t| (*t).take()).unwrap();
+
+                            // Just swap, because no one could change state
+                            // while state == `Registering | `Waking`
+                            self.state.swap(WAITING, AcqRel);
+
+                            // The atomic swap was complete, now
+                            // wake the waker and return.
+                            waker.wake();
+                        }
+                    }
+                }
+            }
+            WAKING => {
+                // Currently in the process of waking the task, i.e.,
+                // `wake` is currently being called on the old waker.
+                // So, we call wake on the new waker.
+                waker.wake();
+
+                // This is equivalent to a spin lock, so use a spin hint.
+                atomic::spin_loop_hint();
+            }
+            state => {
+                // In this case, a concurrent thread is holding the
+                // "registering" lock. This probably indicates a bug in the
+                // caller's code as racing to call `register` doesn't make much
+                // sense.
+                //
+                // We just want to maintain memory safety. It is ok to drop the
+                // call to `register`.
+                debug_assert!(state == REGISTERING || state == REGISTERING | WAKING);
+            }
+        }
+    }
+
+    /// Wakes the task that last called `register`.
+    ///
+    /// If `register` has not been called yet, then this does nothing.
+    pub(crate) fn wake(&self) {
+        if let Some(waker) = self.take_waker() {
+            waker.wake();
+        }
+    }
+
+    /// Attempts to take the `Waker` value out of the `AtomicWaker` with the
+    /// intention that the caller will wake the task later.
+    pub(crate) fn take_waker(&self) -> Option<Waker> {
+        // AcqRel ordering is used in order to acquire the value of the `waker`
+        // cell as well as to establish a `release` ordering with whatever
+        // memory the `AtomicWaker` is associated with.
+        match self.state.fetch_or(WAKING, AcqRel) {
+            WAITING => {
+                // The waking lock has been acquired.
+                let waker = unsafe { self.waker.with_mut(|t| (*t).take()) };
+
+                // Release the lock
+                self.state.fetch_and(!WAKING, Release);
+
+                waker
+            }
+            state => {
+                // There is a concurrent thread currently updating the
+                // associated waker.
+                //
+                // Nothing more to do as the `WAKING` bit has been set. It
+                // doesn't matter if there are concurrent registering threads or
+                // not.
+                //
+                debug_assert!(
+                    state == REGISTERING || state == REGISTERING | WAKING || state == WAKING
+                );
+                None
+            }
+        }
+    }
+}
+
+impl Default for AtomicWaker {
+    fn default() -> Self {
+        AtomicWaker::new()
+    }
+}
+
+impl fmt::Debug for AtomicWaker {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(fmt, "AtomicWaker")
+    }
+}
+
+unsafe impl Send for AtomicWaker {}
+unsafe impl Sync for AtomicWaker {}
+
+trait WakerRef {
+    fn wake(self);
+    fn into_waker(self) -> Waker;
+}
+
+impl WakerRef for Waker {
+    fn wake(self) {
+        self.wake()
+    }
+
+    fn into_waker(self) -> Waker {
+        self
+    }
+}
+
+impl WakerRef for &Waker {
+    fn wake(self) {
+        self.wake_by_ref()
+    }
+
+    fn into_waker(self) -> Waker {
+        self.clone()
+    }
+}
diff --git a/third_party/rust/tokio/src/sync/task/mod.rs b/third_party/rust/tokio/src/sync/task/mod.rs
new file mode 100644
index 0000000000..a6bc6ed06e
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/task/mod.rs
@@ -0,0 +1,4 @@
+//! Thread-safe task notification primitives.
+
+mod atomic_waker;
+pub(crate) use self::atomic_waker::AtomicWaker;
diff --git a/third_party/rust/tokio/src/sync/tests/atomic_waker.rs b/third_party/rust/tokio/src/sync/tests/atomic_waker.rs
new file mode 100644
index 0000000000..c832d62e9a
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/tests/atomic_waker.rs
@@ -0,0 +1,34 @@
+use crate::sync::AtomicWaker;
+use tokio_test::task;
+
+use std::task::Waker;
+
+trait AssertSend: Send {}
+trait AssertSync: Send {}
+
+impl AssertSend for AtomicWaker {}
+impl AssertSync for AtomicWaker {}
+
+impl AssertSend for Waker {}
+impl AssertSync for Waker {}
+
+#[test]
+fn basic_usage() {
+    let mut waker = task::spawn(AtomicWaker::new());
+
+    waker.enter(|cx, waker| waker.register_by_ref(cx.waker()));
+    waker.wake();
+
+    assert!(waker.is_woken());
+}
+
+#[test]
+fn wake_without_register() {
+    let mut waker = task::spawn(AtomicWaker::new());
+    waker.wake();
+
+    // Registering should not result in a notification
+    waker.enter(|cx, waker| waker.register_by_ref(cx.waker()));
+
+    assert!(!waker.is_woken());
+}
diff --git a/third_party/rust/tokio/src/sync/tests/loom_atomic_waker.rs b/third_party/rust/tokio/src/sync/tests/loom_atomic_waker.rs
new file mode 100644
index 0000000000..c148bcbe11
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/tests/loom_atomic_waker.rs
@@ -0,0 +1,45 @@
+use crate::sync::task::AtomicWaker;
+
+use futures::future::poll_fn;
+use loom::future::block_on;
+use loom::sync::atomic::AtomicUsize;
+use loom::thread;
+use std::sync::atomic::Ordering::Relaxed;
+use std::sync::Arc;
+use std::task::Poll::{Pending, Ready};
+
+struct Chan {
+    num: AtomicUsize,
+    task: AtomicWaker,
+}
+
+#[test]
+fn basic_notification() {
+    const NUM_NOTIFY: usize = 2;
+
+    loom::model(|| {
+        let chan = Arc::new(Chan {
+            num: AtomicUsize::new(0),
+            task: AtomicWaker::new(),
+        });
+
+        for _ in 0..NUM_NOTIFY {
+            let chan = chan.clone();
+
+            thread::spawn(move || {
+                chan.num.fetch_add(1, Relaxed);
+                chan.task.wake();
+            });
+        }
+
+        block_on(poll_fn(move |cx| {
+            chan.task.register_by_ref(cx.waker());
+
+            if NUM_NOTIFY == chan.num.load(Relaxed) {
+                return Ready(());
+            }
+
+            Pending
+        }));
+    });
+}
diff --git a/third_party/rust/tokio/src/sync/tests/loom_broadcast.rs b/third_party/rust/tokio/src/sync/tests/loom_broadcast.rs
new file mode 100644
index 0000000000..da12fb9ff0
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/tests/loom_broadcast.rs
@@ -0,0 +1,180 @@
+use crate::sync::broadcast;
+use crate::sync::broadcast::RecvError::{Closed, Lagged};
+
+use loom::future::block_on;
+use loom::sync::Arc;
+use loom::thread;
+use tokio_test::{assert_err, assert_ok};
+
+#[test]
+fn broadcast_send() {
+    loom::model(|| {
+        let (tx1, mut rx) = broadcast::channel(2);
+        let tx1 = Arc::new(tx1);
+        let tx2 = tx1.clone();
+
+        let th1 = thread::spawn(move || {
+            block_on(async {
+                assert_ok!(tx1.send("one"));
+                assert_ok!(tx1.send("two"));
+                assert_ok!(tx1.send("three"));
+            });
+        });
+
+        let th2 = thread::spawn(move || {
+            block_on(async {
+                assert_ok!(tx2.send("eins"));
+                assert_ok!(tx2.send("zwei"));
+                assert_ok!(tx2.send("drei"));
+            });
+        });
+
+        block_on(async {
+            let mut num = 0;
+            loop {
+                match rx.recv().await {
+                    Ok(_) => num += 1,
+                    Err(Closed) => break,
+                    Err(Lagged(n)) => num += n as usize,
+                }
+            }
+            assert_eq!(num, 6);
+        });
+
+        assert_ok!(th1.join());
+        assert_ok!(th2.join());
+    });
+}
+
+// An `Arc` is used as the value in order to detect memory leaks.
+#[test]
+fn broadcast_two() {
+    loom::model(|| {
+        let (tx, mut rx1) = broadcast::channel::<Arc<&'static str>>(16);
+        let mut rx2 = tx.subscribe();
+
+        let th1 = thread::spawn(move || {
+            block_on(async {
+                let v = assert_ok!(rx1.recv().await);
+                assert_eq!(*v, "hello");
+
+                let v = assert_ok!(rx1.recv().await);
+                assert_eq!(*v, "world");
+
+                match assert_err!(rx1.recv().await) {
+                    Closed => {}
+                    _ => panic!(),
+                }
+            });
+        });
+
+        let th2 = thread::spawn(move || {
+            block_on(async {
+                let v = assert_ok!(rx2.recv().await);
+                assert_eq!(*v, "hello");
+
+                let v = assert_ok!(rx2.recv().await);
+                assert_eq!(*v, "world");
+
+                match assert_err!(rx2.recv().await) {
+                    Closed => {}
+                    _ => panic!(),
+                }
+            });
+        });
+
+        assert_ok!(tx.send(Arc::new("hello")));
+        assert_ok!(tx.send(Arc::new("world")));
+        drop(tx);
+
+        assert_ok!(th1.join());
+        assert_ok!(th2.join());
+    });
+}
+
+#[test]
+fn broadcast_wrap() {
+    loom::model(|| {
+        let (tx, mut rx1) = broadcast::channel(2);
+        let mut rx2 = tx.subscribe();
+
+        let th1 = thread::spawn(move || {
+            block_on(async {
+                let mut num = 0;
+
+                loop {
+                    match rx1.recv().await {
+                        Ok(_) => num += 1,
+                        Err(Closed) => break,
+                        Err(Lagged(n)) => num += n as usize,
+                    }
+                }
+
+                assert_eq!(num, 3);
+            });
+        });
+
+        let th2 = thread::spawn(move || {
+            block_on(async {
+                let mut num = 0;
+
+                loop {
+                    match rx2.recv().await {
+                        Ok(_) => num += 1,
+                        Err(Closed) => break,
+                        Err(Lagged(n)) => num += n as usize,
+                    }
+                }
+
+                assert_eq!(num, 3);
+            });
+        });
+
+        assert_ok!(tx.send("one"));
+        assert_ok!(tx.send("two"));
+        assert_ok!(tx.send("three"));
+
+        drop(tx);
+
+        assert_ok!(th1.join());
+        assert_ok!(th2.join());
+    });
+}
+
+#[test]
+fn drop_rx() {
+    loom::model(|| {
+        let (tx, mut rx1) = broadcast::channel(16);
+        let rx2 = tx.subscribe();
+
+        let th1 = thread::spawn(move || {
+            block_on(async {
+                let v = assert_ok!(rx1.recv().await);
+                assert_eq!(v, "one");
+
+                let v = assert_ok!(rx1.recv().await);
+                assert_eq!(v, "two");
+
+                let v = assert_ok!(rx1.recv().await);
+                assert_eq!(v, "three");
+
+                match assert_err!(rx1.recv().await) {
+                    Closed => {}
+                    _ => panic!(),
+                }
+            });
+        });
+
+        let th2 = thread::spawn(move || {
+            drop(rx2);
+        });
+
+        assert_ok!(tx.send("one"));
+        assert_ok!(tx.send("two"));
+        assert_ok!(tx.send("three"));
+        drop(tx);
+
+        assert_ok!(th1.join());
+        assert_ok!(th2.join());
+    });
+}
diff --git a/third_party/rust/tokio/src/sync/tests/loom_list.rs b/third_party/rust/tokio/src/sync/tests/loom_list.rs
new file mode 100644
index 0000000000..4067f865ce
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/tests/loom_list.rs
@@ -0,0 +1,48 @@
+use crate::sync::mpsc::list;
+
+use loom::thread;
+use std::sync::Arc;
+
+#[test]
+fn smoke() {
+    use crate::sync::mpsc::block::Read::*;
+
+    const NUM_TX: usize = 2;
+    const NUM_MSG: usize = 2;
+
+    loom::model(|| {
+        let (tx, mut rx) = list::channel();
+        let tx = Arc::new(tx);
+
+        for th in 0..NUM_TX {
+            let tx = tx.clone();
+
+            thread::spawn(move || {
+                for i in 0..NUM_MSG {
+                    tx.push((th, i));
+                }
+            });
+        }
+
+        let mut next = vec![0; NUM_TX];
+
+        loop {
+            match rx.pop(&tx) {
+                Some(Value((th, v))) => {
+                    assert_eq!(v, next[th]);
+                    next[th] += 1;
+
+                    if next.iter().all(|&i| i == NUM_MSG) {
+                        break;
+                    }
+                }
+                Some(Closed) => {
+                    panic!();
+                }
+                None => {
+                    thread::yield_now();
+                }
+            }
+        }
+    });
+}
diff --git a/third_party/rust/tokio/src/sync/tests/loom_mpsc.rs b/third_party/rust/tokio/src/sync/tests/loom_mpsc.rs
new file mode 100644
index 0000000000..6a1a6abedd
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/tests/loom_mpsc.rs
@@ -0,0 +1,77 @@
+use crate::sync::mpsc;
+
+use futures::future::poll_fn;
+use loom::future::block_on;
+use loom::thread;
+
+#[test]
+fn closing_tx() {
+    loom::model(|| {
+        let (mut tx, mut rx) = mpsc::channel(16);
+
+        thread::spawn(move || {
+            tx.try_send(()).unwrap();
+            drop(tx);
+        });
+
+        let v = block_on(poll_fn(|cx| rx.poll_recv(cx)));
+        assert!(v.is_some());
+
+        let v = block_on(poll_fn(|cx| rx.poll_recv(cx)));
+        assert!(v.is_none());
+    });
+}
+
+#[test]
+fn closing_unbounded_tx() {
+    loom::model(|| {
+        let (tx, mut rx) = mpsc::unbounded_channel();
+
+        thread::spawn(move || {
+            tx.send(()).unwrap();
+            drop(tx);
+        });
+
+        let v = block_on(poll_fn(|cx| rx.poll_recv(cx)));
+        assert!(v.is_some());
+
+        let v = block_on(poll_fn(|cx| rx.poll_recv(cx)));
+        assert!(v.is_none());
+    });
+}
+
+#[test]
+fn dropping_tx() {
+    loom::model(|| {
+        let (tx, mut rx) = mpsc::channel::<()>(16);
+
+        for _ in 0..2 {
+            let tx = tx.clone();
+            thread::spawn(move || {
+                drop(tx);
+            });
+        }
+        drop(tx);
+
+        let v = block_on(poll_fn(|cx| rx.poll_recv(cx)));
+        assert!(v.is_none());
+    });
+}
+
+#[test]
+fn dropping_unbounded_tx() {
+    loom::model(|| {
+        let (tx, mut rx) = mpsc::unbounded_channel::<()>();
+
+        for _ in 0..2 {
+            let tx = tx.clone();
+            thread::spawn(move || {
+                drop(tx);
+            });
+        }
+        drop(tx);
+
+        let v = block_on(poll_fn(|cx| rx.poll_recv(cx)));
+        assert!(v.is_none());
+    });
+}
diff --git a/third_party/rust/tokio/src/sync/tests/loom_notify.rs b/third_party/rust/tokio/src/sync/tests/loom_notify.rs
new file mode 100644
index 0000000000..60981d4669
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/tests/loom_notify.rs
@@ -0,0 +1,90 @@
+use crate::sync::Notify;
+
+use loom::future::block_on;
+use loom::sync::Arc;
+use loom::thread;
+
+#[test]
+fn notify_one() {
+    loom::model(|| {
+        let tx = Arc::new(Notify::new());
+        let rx = tx.clone();
+
+        let th = thread::spawn(move || {
+            block_on(async {
+                rx.notified().await;
+            });
+        });
+
+        tx.notify();
+        th.join().unwrap();
+    });
+}
+
+#[test]
+fn notify_multi() {
+    loom::model(|| {
+        let notify = Arc::new(Notify::new());
+
+        let mut ths = vec![];
+
+        for _ in 0..2 {
+            let notify = notify.clone();
+
+            ths.push(thread::spawn(move || {
+                block_on(async {
+                    notify.notified().await;
+                    notify.notify();
+                })
+            }));
+        }
+
+        notify.notify();
+
+        for th in ths.drain(..) {
+            th.join().unwrap();
+        }
+
+        block_on(async {
+            notify.notified().await;
+        });
+    });
+}
+
+#[test]
+fn notify_drop() {
+    use crate::future::poll_fn;
+    use std::future::Future;
+    use std::task::Poll;
+
+    loom::model(|| {
+        let notify = Arc::new(Notify::new());
+        let rx1 = notify.clone();
+        let rx2 = notify.clone();
+
+        let th1 = thread::spawn(move || {
+            let mut recv = Box::pin(rx1.notified());
+
+            block_on(poll_fn(|cx| {
+                if recv.as_mut().poll(cx).is_ready() {
+                    rx1.notify();
+                }
+                Poll::Ready(())
+            }));
+        });
+
+        let th2 = thread::spawn(move || {
+            block_on(async {
+                rx2.notified().await;
+                // Trigger second notification
+                rx2.notify();
+                rx2.notified().await;
+            });
+        });
+
+        notify.notify();
+
+        th1.join().unwrap();
+        th2.join().unwrap();
+    });
+}
diff --git a/third_party/rust/tokio/src/sync/tests/loom_oneshot.rs b/third_party/rust/tokio/src/sync/tests/loom_oneshot.rs
new file mode 100644
index 0000000000..dfa7459da7
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/tests/loom_oneshot.rs
@@ -0,0 +1,109 @@
+use crate::sync::oneshot;
+
+use futures::future::poll_fn;
+use loom::future::block_on;
+use loom::thread;
+use std::task::Poll::{Pending, Ready};
+
+#[test]
+fn smoke() {
+    loom::model(|| {
+        let (tx, rx) = oneshot::channel();
+
+        thread::spawn(move || {
+            tx.send(1).unwrap();
+        });
+
+        let value = block_on(rx).unwrap();
+        assert_eq!(1, value);
+    });
+}
+
+#[test]
+fn changing_rx_task() {
+    loom::model(|| {
+        let (tx, mut rx) = oneshot::channel();
+
+        thread::spawn(move || {
+            tx.send(1).unwrap();
+        });
+
+        let rx = thread::spawn(move || {
+            let ready = block_on(poll_fn(|cx| match Pin::new(&mut rx).poll(cx) {
+                Ready(Ok(value)) => {
+                    assert_eq!(1, value);
+                    Ready(true)
+                }
+                Ready(Err(_)) => unimplemented!(),
+                Pending => Ready(false),
+            }));
+
+            if ready {
+                None
+            } else {
+                Some(rx)
+            }
+        })
+        .join()
+        .unwrap();
+
+        if let Some(rx) = rx {
+            // Previous task parked, use a new task...
+            let value = block_on(rx).unwrap();
+            assert_eq!(1, value);
+        }
+    });
+}
+
+// TODO: Move this into `oneshot` proper.
+
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+struct OnClose<'a> {
+    tx: &'a mut oneshot::Sender<i32>,
+}
+
+impl<'a> OnClose<'a> {
+    fn new(tx: &'a mut oneshot::Sender<i32>) -> Self {
+        OnClose { tx }
+    }
+}
+
+impl Future for OnClose<'_> {
+    type Output = bool;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<bool> {
+        let res = self.get_mut().tx.poll_closed(cx);
+        Ready(res.is_ready())
+    }
+}
+
+#[test]
+fn changing_tx_task() {
+    loom::model(|| {
+        let (mut tx, rx) = oneshot::channel::<i32>();
+
+        thread::spawn(move || {
+            drop(rx);
+        });
+
+        let tx = thread::spawn(move || {
+            let t1 = block_on(OnClose::new(&mut tx));
+
+            if t1 {
+                None
+            } else {
+                Some(tx)
+            }
+        })
+        .join()
+        .unwrap();
+
+        if let Some(mut tx) = tx {
+            // Previous task parked, use a new task...
+            block_on(OnClose::new(&mut tx));
+        }
+    });
+}
diff --git a/third_party/rust/tokio/src/sync/tests/loom_rwlock.rs b/third_party/rust/tokio/src/sync/tests/loom_rwlock.rs
new file mode 100644
index 0000000000..48d06e1d5f
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/tests/loom_rwlock.rs
@@ -0,0 +1,78 @@
+use crate::sync::rwlock::*;
+
+use loom::future::block_on;
+use loom::thread;
+use std::sync::Arc;
+
+#[test]
+fn concurrent_write() {
+    let mut b = loom::model::Builder::new();
+
+    b.check(|| {
+        let rwlock = Arc::new(RwLock::<u32>::new(0));
+
+        let rwclone = rwlock.clone();
+        let t1 = thread::spawn(move || {
+            block_on(async {
+                let mut guard = rwclone.write().await;
+                *guard += 5;
+            });
+        });
+
+        let rwclone = rwlock.clone();
+        let t2 = thread::spawn(move || {
+            block_on(async {
+                let mut guard = rwclone.write().await;
+                *guard += 5;
+            });
+        });
+
+        t1.join().expect("thread 1 write should not panic");
+        t2.join().expect("thread 2 write should not panic");
+        //when all threads have finished the value on the lock should be 10
+        let guard = block_on(rwlock.read());
+        assert_eq!(10, *guard);
+    });
+}
+
+#[test]
+fn concurrent_read_write() {
+    let mut b = loom::model::Builder::new();
+
+    b.check(|| {
+        let rwlock = Arc::new(RwLock::<u32>::new(0));
+
+        let rwclone = rwlock.clone();
+        let t1 = thread::spawn(move || {
+            block_on(async {
+                let mut guard = rwclone.write().await;
+                *guard += 5;
+            });
+        });
+
+        let rwclone = rwlock.clone();
+        let t2 = thread::spawn(move || {
+            block_on(async {
+                let mut guard = rwclone.write().await;
+                *guard += 5;
+            });
+        });
+
+        let rwclone = rwlock.clone();
+        let t3 = thread::spawn(move || {
+            block_on(async {
+                let guard = rwclone.read().await;
+                //at this state the value on the lock may either be 0, 5, or 10
+                assert!(*guard == 0 || *guard == 5 || *guard == 10);
+            });
+        });
+
+        t1.join().expect("thread 1 write should not panic");
+        t2.join().expect("thread 2 write should not panic");
+        t3.join().expect("thread 3 read should not panic");
+
+        let guard = block_on(rwlock.read());
+        //when all threads have finished the value on the lock should be 10
+        assert_eq!(10, *guard);
+    });
+}
diff --git a/third_party/rust/tokio/src/sync/tests/loom_semaphore_batch.rs b/third_party/rust/tokio/src/sync/tests/loom_semaphore_batch.rs
new file mode 100644
index 0000000000..76a1bc0062
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/tests/loom_semaphore_batch.rs
@@ -0,0 +1,215 @@
+use crate::sync::batch_semaphore::*;
+
+use futures::future::poll_fn;
+use loom::future::block_on;
+use loom::sync::atomic::AtomicUsize;
+use loom::thread;
+use std::future::Future;
+use std::pin::Pin;
+use std::sync::atomic::Ordering::SeqCst;
+use std::sync::Arc;
+use std::task::Poll::Ready;
+use std::task::{Context, Poll};
+
+#[test]
+fn basic_usage() {
+    const NUM: usize = 2;
+
+    struct Shared {
+        semaphore: Semaphore,
+        active: AtomicUsize,
+    }
+
+    async fn actor(shared: Arc<Shared>) {
+        shared.semaphore.acquire(1).await.unwrap();
+        let actual = shared.active.fetch_add(1, SeqCst);
+        assert!(actual <= NUM - 1);
+
+        let actual = shared.active.fetch_sub(1, SeqCst);
+        assert!(actual <= NUM);
+        shared.semaphore.release(1);
+    }
+
+    loom::model(|| {
+        let shared = Arc::new(Shared {
+            semaphore: Semaphore::new(NUM),
+            active: AtomicUsize::new(0),
+        });
+
+        for _ in 0..NUM {
+            let shared = shared.clone();
+
+            thread::spawn(move || {
+                block_on(actor(shared));
+            });
+        }
+
+        block_on(actor(shared));
+    });
+}
+
+#[test]
+fn release() {
+    loom::model(|| {
+        let semaphore = Arc::new(Semaphore::new(1));
+
+        {
+            let semaphore = semaphore.clone();
+            thread::spawn(move || {
+                block_on(semaphore.acquire(1)).unwrap();
+                semaphore.release(1);
+            });
+        }
+
+        block_on(semaphore.acquire(1)).unwrap();
+
+        semaphore.release(1);
+    });
+}
+
+#[test]
+fn basic_closing() {
+    const NUM: usize = 2;
+
+    loom::model(|| {
+        let semaphore = Arc::new(Semaphore::new(1));
+
+        for _ in 0..NUM {
+            let semaphore = semaphore.clone();
+
+            thread::spawn(move || {
+                for _ in 0..2 {
+                    block_on(semaphore.acquire(1)).map_err(|_| ())?;
+
+                    semaphore.release(1);
+                }
+
+                Ok::<(), ()>(())
+            });
+        }
+
+        semaphore.close();
+    });
+}
+
+#[test]
+fn concurrent_close() {
+    const NUM: usize = 3;
+
+    loom::model(|| {
+        let semaphore = Arc::new(Semaphore::new(1));
+
+        for _ in 0..NUM {
+            let semaphore = semaphore.clone();
+
+            thread::spawn(move || {
+                block_on(semaphore.acquire(1)).map_err(|_| ())?;
+                semaphore.release(1);
+                semaphore.close();
+
+                Ok::<(), ()>(())
+            });
+        }
+    });
+}
+
+#[test]
+fn concurrent_cancel() {
+    async fn poll_and_cancel(semaphore: Arc<Semaphore>) {
+        let mut acquire1 = Some(semaphore.acquire(1));
+        let mut acquire2 = Some(semaphore.acquire(1));
+        poll_fn(|cx| {
+            // poll the acquire future once, and then immediately throw
+            // it away. this simulates a situation where a future is
+            // polled and then cancelled, such as by a timeout.
+            if let Some(acquire) = acquire1.take() {
+                pin!(acquire);
+                let _ = acquire.poll(cx);
+            }
+            if let Some(acquire) = acquire2.take() {
+                pin!(acquire);
+                let _ = acquire.poll(cx);
+            }
+            Poll::Ready(())
+        })
+        .await
+    }
+
+    loom::model(|| {
+        let semaphore = Arc::new(Semaphore::new(0));
+        let t1 = {
+            let semaphore = semaphore.clone();
+            thread::spawn(move || block_on(poll_and_cancel(semaphore)))
+        };
+        let t2 = {
+            let semaphore = semaphore.clone();
+            thread::spawn(move || block_on(poll_and_cancel(semaphore)))
+        };
+        let t3 = {
+            let semaphore = semaphore.clone();
+            thread::spawn(move || block_on(poll_and_cancel(semaphore)))
+        };
+
+        t1.join().unwrap();
+        semaphore.release(10);
+        t2.join().unwrap();
+        t3.join().unwrap();
+    });
+}
+
+#[test]
+fn batch() {
+    let mut b = loom::model::Builder::new();
+    b.preemption_bound = Some(1);
+
+    b.check(|| {
+        let semaphore = Arc::new(Semaphore::new(10));
+        let active = Arc::new(AtomicUsize::new(0));
+        let mut ths = vec![];
+
+        for _ in 0..2 {
+            let semaphore = semaphore.clone();
+            let active = active.clone();
+
+            ths.push(thread::spawn(move || {
+                for n in &[4, 10, 8] {
+                    block_on(semaphore.acquire(*n)).unwrap();
+
+                    active.fetch_add(*n as usize, SeqCst);
+
+                    let num_active = active.load(SeqCst);
+                    assert!(num_active <= 10);
+
+                    thread::yield_now();
+
+                    active.fetch_sub(*n as usize, SeqCst);
+
+                    semaphore.release(*n as usize);
+                }
+            }));
+        }
+
+        for th in ths.into_iter() {
+            th.join().unwrap();
+        }
+
+        assert_eq!(10, semaphore.available_permits());
+    });
+}
+
+#[test]
+fn release_during_acquire() {
+    loom::model(|| {
+        let semaphore = Arc::new(Semaphore::new(10));
+        semaphore
+            .try_acquire(8)
+            .expect("try_acquire should succeed; semaphore uncontended");
+        let semaphore2 = semaphore.clone();
+        let thread = thread::spawn(move || block_on(semaphore2.acquire(4)).unwrap());
+
+        semaphore.release(8);
+        thread.join().unwrap();
+        semaphore.release(4);
+        assert_eq!(10, semaphore.available_permits());
+    })
+}
diff --git a/third_party/rust/tokio/src/sync/tests/loom_semaphore_ll.rs b/third_party/rust/tokio/src/sync/tests/loom_semaphore_ll.rs
new file mode 100644
index 0000000000..b5e5efba82
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/tests/loom_semaphore_ll.rs
@@ -0,0 +1,192 @@
+use crate::sync::semaphore_ll::*;
+
+use futures::future::poll_fn;
+use loom::future::block_on;
+use loom::thread;
+use std::future::Future;
+use std::pin::Pin;
+use std::sync::atomic::AtomicUsize;
+use std::sync::atomic::Ordering::SeqCst;
+use std::sync::Arc;
+use std::task::Poll::Ready;
+use std::task::{Context, Poll};
+
+#[test]
+fn basic_usage() {
+    const NUM: usize = 2;
+
+    struct Actor {
+        waiter: Permit,
+        shared: Arc<Shared>,
+    }
+
+    struct Shared {
+        semaphore: Semaphore,
+        active: AtomicUsize,
+    }
+
+    impl Future for Actor {
+        type Output = ();
+
+        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
+            let me = &mut *self;
+
+            ready!(me.waiter.poll_acquire(cx, 1, &me.shared.semaphore)).unwrap();
+
+            let actual = me.shared.active.fetch_add(1, SeqCst);
+            assert!(actual <= NUM - 1);
+
+            let actual = me.shared.active.fetch_sub(1, SeqCst);
+            assert!(actual <= NUM);
+
+            me.waiter.release(1, &me.shared.semaphore);
+
+            Ready(())
+        }
+    }
+
+    loom::model(|| {
+        let shared = Arc::new(Shared {
+            semaphore: Semaphore::new(NUM),
+            active: AtomicUsize::new(0),
+        });
+
+        for _ in 0..NUM {
+            let shared = shared.clone();
+
+            thread::spawn(move || {
+                block_on(Actor {
+                    waiter: Permit::new(),
+                    shared,
+                });
+            });
+        }
+
+        block_on(Actor {
+            waiter: Permit::new(),
+            shared,
+        });
+    });
+}
+
+#[test]
+fn release() {
+    loom::model(|| {
+        let semaphore = Arc::new(Semaphore::new(1));
+
+        {
+            let semaphore = semaphore.clone();
+            thread::spawn(move || {
+                let mut permit = Permit::new();
+
+                block_on(poll_fn(|cx| permit.poll_acquire(cx, 1, &semaphore))).unwrap();
+
+                permit.release(1, &semaphore);
+            });
+        }
+
+        let mut permit = Permit::new();
+
+        block_on(poll_fn(|cx| permit.poll_acquire(cx, 1, &semaphore))).unwrap();
+
+        permit.release(1, &semaphore);
+    });
+}
+
+#[test]
+fn basic_closing() {
+    const NUM: usize = 2;
+
+    loom::model(|| {
+        let semaphore = Arc::new(Semaphore::new(1));
+
+        for _ in 0..NUM {
+            let semaphore = semaphore.clone();
+
+            thread::spawn(move || {
+                let mut permit = Permit::new();
+
+                for _ in 0..2 {
+                    block_on(poll_fn(|cx| {
+                        permit.poll_acquire(cx, 1, &semaphore).map_err(|_| ())
+                    }))?;
+
+                    permit.release(1, &semaphore);
+                }
+
+                Ok::<(), ()>(())
+            });
+        }
+
+        semaphore.close();
+    });
+}
+
+#[test]
+fn concurrent_close() {
+    const NUM: usize = 3;
+
+    loom::model(|| {
+        let semaphore = Arc::new(Semaphore::new(1));
+
+        for _ in 0..NUM {
+            let semaphore = semaphore.clone();
+
+            thread::spawn(move || {
+                let mut permit = Permit::new();
+
+                block_on(poll_fn(|cx| {
+                    permit.poll_acquire(cx, 1, &semaphore).map_err(|_| ())
+                }))?;
+
+                permit.release(1, &semaphore);
+
+                semaphore.close();
+
+                Ok::<(), ()>(())
+            });
+        }
+    });
+}
+
+#[test]
+fn batch() {
+    let mut b = loom::model::Builder::new();
+    b.preemption_bound = Some(1);
+
+    b.check(|| {
+        let semaphore = Arc::new(Semaphore::new(10));
+        let active = Arc::new(AtomicUsize::new(0));
+        let mut ths = vec![];
+
+        for _ in 0..2 {
+            let semaphore = semaphore.clone();
+            let active = active.clone();
+
+            ths.push(thread::spawn(move || {
+                let mut permit = Permit::new();
+
+                for n in &[4, 10, 8] {
+                    block_on(poll_fn(|cx| permit.poll_acquire(cx, *n, &semaphore))).unwrap();
+
+                    active.fetch_add(*n as usize, SeqCst);
+
+                    let num_active = active.load(SeqCst);
+                    assert!(num_active <= 10);
+
+                    thread::yield_now();
+
+                    active.fetch_sub(*n as usize, SeqCst);
+
+                    permit.release(*n, &semaphore);
+                }
+            }));
+        }
+
+        for th in ths.into_iter() {
+            th.join().unwrap();
+        }
+
+        assert_eq!(10, semaphore.available_permits());
+    });
+}
diff --git a/third_party/rust/tokio/src/sync/tests/mod.rs b/third_party/rust/tokio/src/sync/tests/mod.rs
new file mode 100644
index 0000000000..d571754c01
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/tests/mod.rs
@@ -0,0 +1,16 @@
+cfg_not_loom! {
+    mod atomic_waker;
+    mod semaphore_ll;
+    mod semaphore_batch;
+}
+
+cfg_loom! {
+    mod loom_atomic_waker;
+    mod loom_broadcast;
+    mod loom_list;
+    mod loom_mpsc;
+    mod loom_notify;
+    mod loom_oneshot;
+    mod loom_semaphore_batch;
+    mod loom_semaphore_ll;
+}
diff --git a/third_party/rust/tokio/src/sync/tests/semaphore_batch.rs b/third_party/rust/tokio/src/sync/tests/semaphore_batch.rs
new file mode 100644
index 0000000000..60f3f231e7
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/tests/semaphore_batch.rs
@@ -0,0 +1,250 @@
+use crate::sync::batch_semaphore::Semaphore;
+use tokio_test::*;
+
+#[test]
+fn poll_acquire_one_available() {
+    let s = Semaphore::new(100);
+    assert_eq!(s.available_permits(), 100);
+
+    // Polling for a permit succeeds immediately
+    assert_ready_ok!(task::spawn(s.acquire(1)).poll());
+    assert_eq!(s.available_permits(), 99);
+}
+
+#[test]
+fn poll_acquire_many_available() {
+    let s = Semaphore::new(100);
+    assert_eq!(s.available_permits(), 100);
+
+    // Polling for a permit succeeds immediately
+    assert_ready_ok!(task::spawn(s.acquire(5)).poll());
+    assert_eq!(s.available_permits(), 95);
+
+    assert_ready_ok!(task::spawn(s.acquire(5)).poll());
+    assert_eq!(s.available_permits(), 90);
+}
+
+#[test]
+fn try_acquire_one_available() {
+    let s = Semaphore::new(100);
+    assert_eq!(s.available_permits(), 100);
+
+    assert_ok!(s.try_acquire(1));
+    assert_eq!(s.available_permits(), 99);
+
+    assert_ok!(s.try_acquire(1));
+    assert_eq!(s.available_permits(), 98);
+}
+
+#[test]
+fn try_acquire_many_available() {
+    let s = Semaphore::new(100);
+    assert_eq!(s.available_permits(), 100);
+
+    assert_ok!(s.try_acquire(5));
+    assert_eq!(s.available_permits(), 95);
+
+    assert_ok!(s.try_acquire(5));
+    assert_eq!(s.available_permits(), 90);
+}
+
+#[test]
+fn poll_acquire_one_unavailable() {
+    let s = Semaphore::new(1);
+
+    // Acquire the first permit
+    assert_ready_ok!(task::spawn(s.acquire(1)).poll());
+    assert_eq!(s.available_permits(), 0);
+
+    let mut acquire_2 = task::spawn(s.acquire(1));
+    // Try to acquire the second permit
+    assert_pending!(acquire_2.poll());
+    assert_eq!(s.available_permits(), 0);
+
+    s.release(1);
+
+    assert_eq!(s.available_permits(), 0);
+    assert!(acquire_2.is_woken());
+    assert_ready_ok!(acquire_2.poll());
+    assert_eq!(s.available_permits(), 0);
+
+    s.release(1);
+    assert_eq!(s.available_permits(), 1);
+}
+
+#[test]
+fn poll_acquire_many_unavailable() {
+    let s = Semaphore::new(5);
+
+    // Acquire the first permit
+    assert_ready_ok!(task::spawn(s.acquire(1)).poll());
+    assert_eq!(s.available_permits(), 4);
+
+    // Try to acquire the second permit
+    let mut acquire_2 = task::spawn(s.acquire(5));
+    assert_pending!(acquire_2.poll());
+    assert_eq!(s.available_permits(), 0);
+
+    // Try to acquire the third permit
+    let mut acquire_3 = task::spawn(s.acquire(3));
+    assert_pending!(acquire_3.poll());
+    assert_eq!(s.available_permits(), 0);
+
+    s.release(1);
+
+    assert_eq!(s.available_permits(), 0);
+    assert!(acquire_2.is_woken());
+    assert_ready_ok!(acquire_2.poll());
+
+    assert!(!acquire_3.is_woken());
+    assert_eq!(s.available_permits(), 0);
+
+    s.release(1);
+    assert!(!acquire_3.is_woken());
+    assert_eq!(s.available_permits(), 0);
+
+    s.release(2);
+    assert!(acquire_3.is_woken());
+
+    assert_ready_ok!(acquire_3.poll());
+}
+
+#[test]
+fn try_acquire_one_unavailable() {
+    let s = Semaphore::new(1);
+
+    // Acquire the first permit
+    assert_ok!(s.try_acquire(1));
+    assert_eq!(s.available_permits(), 0);
+
+    assert_err!(s.try_acquire(1));
+
+    s.release(1);
+
+    assert_eq!(s.available_permits(), 1);
+    assert_ok!(s.try_acquire(1));
+
+    s.release(1);
+    assert_eq!(s.available_permits(), 1);
+}
+
+#[test]
+fn try_acquire_many_unavailable() {
+    let s = Semaphore::new(5);
+
+    // Acquire the first permit
+    assert_ok!(s.try_acquire(1));
+    assert_eq!(s.available_permits(), 4);
+
+    assert_err!(s.try_acquire(5));
+
+    s.release(1);
+    assert_eq!(s.available_permits(), 5);
+
+    assert_ok!(s.try_acquire(5));
+
+    s.release(1);
+    assert_eq!(s.available_permits(), 1);
+
+    s.release(1);
+    assert_eq!(s.available_permits(), 2);
+}
+
+#[test]
+fn poll_acquire_one_zero_permits() {
+    let s = Semaphore::new(0);
+    assert_eq!(s.available_permits(), 0);
+
+    // Try to acquire the permit
+    let mut acquire = task::spawn(s.acquire(1));
+    assert_pending!(acquire.poll());
+
+    s.release(1);
+
+    assert!(acquire.is_woken());
+    assert_ready_ok!(acquire.poll());
+}
+
+#[test]
+#[should_panic]
+fn validates_max_permits() {
+    use std::usize;
+    Semaphore::new((usize::MAX >> 2) + 1);
+}
+
+#[test]
+fn close_semaphore_prevents_acquire() {
+    let s = Semaphore::new(5);
+    s.close();
+
+    assert_eq!(5, s.available_permits());
+
+    assert_ready_err!(task::spawn(s.acquire(1)).poll());
+    assert_eq!(5, s.available_permits());
+
+    assert_ready_err!(task::spawn(s.acquire(1)).poll());
+    assert_eq!(5, s.available_permits());
+}
+
+#[test]
+fn close_semaphore_notifies_permit1() {
+    let s = Semaphore::new(0);
+    let mut acquire = task::spawn(s.acquire(1));
+
+    assert_pending!(acquire.poll());
+
+    s.close();
+
+    assert!(acquire.is_woken());
+    assert_ready_err!(acquire.poll());
+}
+
+#[test]
+fn close_semaphore_notifies_permit2() {
+    let s = Semaphore::new(2);
+
+    // Acquire a couple of permits
+    assert_ready_ok!(task::spawn(s.acquire(1)).poll());
+    assert_ready_ok!(task::spawn(s.acquire(1)).poll());
+
+    let mut acquire3 = task::spawn(s.acquire(1));
+    let mut acquire4 = task::spawn(s.acquire(1));
+    assert_pending!(acquire3.poll());
+    assert_pending!(acquire4.poll());
+
+    s.close();
+
+    assert!(acquire3.is_woken());
+    assert!(acquire4.is_woken());
+
+    assert_ready_err!(acquire3.poll());
+    assert_ready_err!(acquire4.poll());
+
+    assert_eq!(0, s.available_permits());
+
+    s.release(1);
+
+    assert_eq!(1, s.available_permits());
+
+    assert_ready_err!(task::spawn(s.acquire(1)).poll());
+
+    s.release(1);
+
+    assert_eq!(2, s.available_permits());
+}
+
+#[test]
+fn cancel_acquire_releases_permits() {
+    let s = Semaphore::new(10);
+    let _permit1 = s.try_acquire(4).expect("uncontended try_acquire succeeds");
+    assert_eq!(6, s.available_permits());
+
+    let mut acquire = task::spawn(s.acquire(8));
+    assert_pending!(acquire.poll());
+
+    assert_eq!(0, s.available_permits());
+    drop(acquire);
+
+    assert_eq!(6, s.available_permits());
+    assert_ok!(s.try_acquire(6));
+}
diff --git a/third_party/rust/tokio/src/sync/tests/semaphore_ll.rs b/third_party/rust/tokio/src/sync/tests/semaphore_ll.rs
new file mode 100644
index 0000000000..bfb075780b
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/tests/semaphore_ll.rs
@@ -0,0 +1,470 @@
+use crate::sync::semaphore_ll::{Permit, Semaphore};
+use tokio_test::*;
+
+#[test]
+fn poll_acquire_one_available() {
+    let s = Semaphore::new(100);
+    assert_eq!(s.available_permits(), 100);
+
+    // Polling for a permit succeeds immediately
+    let mut permit = task::spawn(Permit::new());
+    assert!(!permit.is_acquired());
+
+    assert_ready_ok!(permit.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+    assert_eq!(s.available_permits(), 99);
+    assert!(permit.is_acquired());
+
+    // Polling again on the same waiter does not claim a new permit
+    assert_ready_ok!(permit.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+    assert_eq!(s.available_permits(), 99);
+    assert!(permit.is_acquired());
+}
+
+#[test]
+fn poll_acquire_many_available() {
+    let s = Semaphore::new(100);
+    assert_eq!(s.available_permits(), 100);
+
+    // Polling for a permit succeeds immediately
+    let mut permit = task::spawn(Permit::new());
+    assert!(!permit.is_acquired());
+
+    assert_ready_ok!(permit.enter(|cx, mut p| p.poll_acquire(cx, 5, &s)));
+    assert_eq!(s.available_permits(), 95);
+    assert!(permit.is_acquired());
+
+    // Polling again on the same waiter does not claim a new permit
+    assert_ready_ok!(permit.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+    assert_eq!(s.available_permits(), 95);
+    assert!(permit.is_acquired());
+
+    assert_ready_ok!(permit.enter(|cx, mut p| p.poll_acquire(cx, 5, &s)));
+    assert_eq!(s.available_permits(), 95);
+    assert!(permit.is_acquired());
+
+    // Polling for a larger number of permits acquires more
+    assert_ready_ok!(permit.enter(|cx, mut p| p.poll_acquire(cx, 8, &s)));
+    assert_eq!(s.available_permits(), 92);
+    assert!(permit.is_acquired());
+}
+
+#[test]
+fn try_acquire_one_available() {
+    let s = Semaphore::new(100);
+    assert_eq!(s.available_permits(), 100);
+
+    // Polling for a permit succeeds immediately
+    let mut permit = Permit::new();
+    assert!(!permit.is_acquired());
+
+    assert_ok!(permit.try_acquire(1, &s));
+    assert_eq!(s.available_permits(), 99);
+    assert!(permit.is_acquired());
+
+    // Polling again on the same waiter does not claim a new permit
+    assert_ok!(permit.try_acquire(1, &s));
+    assert_eq!(s.available_permits(), 99);
+    assert!(permit.is_acquired());
+}
+
+#[test]
+fn try_acquire_many_available() {
+    let s = Semaphore::new(100);
+    assert_eq!(s.available_permits(), 100);
+
+    // Polling for a permit succeeds immediately
+    let mut permit = Permit::new();
+    assert!(!permit.is_acquired());
+
+    assert_ok!(permit.try_acquire(5, &s));
+    assert_eq!(s.available_permits(), 95);
+    assert!(permit.is_acquired());
+
+    // Polling again on the same waiter does not claim a new permit
+    assert_ok!(permit.try_acquire(5, &s));
+    assert_eq!(s.available_permits(), 95);
+    assert!(permit.is_acquired());
+}
+
+#[test]
+fn poll_acquire_one_unavailable() {
+    let s = Semaphore::new(1);
+
+    let mut permit_1 = task::spawn(Permit::new());
+    let mut permit_2 = task::spawn(Permit::new());
+
+    // Acquire the first permit
+    assert_ready_ok!(permit_1.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+    assert_eq!(s.available_permits(), 0);
+
+    permit_2.enter(|cx, mut p| {
+        // Try to acquire the second permit
+        assert_pending!(p.poll_acquire(cx, 1, &s));
+    });
+
+    permit_1.release(1, &s);
+
+    assert_eq!(s.available_permits(), 0);
+    assert!(permit_2.is_woken());
+    assert_ready_ok!(permit_2.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+
+    permit_2.release(1, &s);
+    assert_eq!(s.available_permits(), 1);
+}
+
+#[test]
+fn forget_acquired() {
+    let s = Semaphore::new(1);
+
+    // Polling for a permit succeeds immediately
+    let mut permit = task::spawn(Permit::new());
+
+    assert_ready_ok!(permit.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+
+    assert_eq!(s.available_permits(), 0);
+
+    permit.forget(1);
+    assert_eq!(s.available_permits(), 0);
+}
+
+#[test]
+fn forget_waiting() {
+    let s = Semaphore::new(0);
+
+    // Polling for a permit succeeds immediately
+    let mut permit = task::spawn(Permit::new());
+
+    assert_pending!(permit.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+
+    assert_eq!(s.available_permits(), 0);
+
+    permit.forget(1);
+
+    s.add_permits(1);
+
+    assert!(!permit.is_woken());
+    assert_eq!(s.available_permits(), 1);
+}
+
+#[test]
+fn poll_acquire_many_unavailable() {
+    let s = Semaphore::new(5);
+
+    let mut permit_1 = task::spawn(Permit::new());
+    let mut permit_2 = task::spawn(Permit::new());
+    let mut permit_3 = task::spawn(Permit::new());
+
+    // Acquire the first permit
+    assert_ready_ok!(permit_1.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+    assert_eq!(s.available_permits(), 4);
+
+    permit_2.enter(|cx, mut p| {
+        // Try to acquire the second permit
+        assert_pending!(p.poll_acquire(cx, 5, &s));
+    });
+
+    assert_eq!(s.available_permits(), 0);
+
+    permit_3.enter(|cx, mut p| {
+        // Try to acquire the third permit
+        assert_pending!(p.poll_acquire(cx, 3, &s));
+    });
+
+    permit_1.release(1, &s);
+
+    assert_eq!(s.available_permits(), 0);
+    assert!(permit_2.is_woken());
+    assert_ready_ok!(permit_2.enter(|cx, mut p| p.poll_acquire(cx, 5, &s)));
+
+    assert!(!permit_3.is_woken());
+    assert_eq!(s.available_permits(), 0);
+
+    permit_2.release(1, &s);
+    assert!(!permit_3.is_woken());
+    assert_eq!(s.available_permits(), 0);
+
+    permit_2.release(2, &s);
+    assert!(permit_3.is_woken());
+
+    assert_ready_ok!(permit_3.enter(|cx, mut p| p.poll_acquire(cx, 3, &s)));
+}
+
+#[test]
+fn try_acquire_one_unavailable() {
+    let s = Semaphore::new(1);
+
+    let mut permit_1 = Permit::new();
+    let mut permit_2 = Permit::new();
+
+    // Acquire the first permit
+    assert_ok!(permit_1.try_acquire(1, &s));
+    assert_eq!(s.available_permits(), 0);
+
+    assert_err!(permit_2.try_acquire(1, &s));
+
+    permit_1.release(1, &s);
+
+    assert_eq!(s.available_permits(), 1);
+    assert_ok!(permit_2.try_acquire(1, &s));
+
+    permit_2.release(1, &s);
+    assert_eq!(s.available_permits(), 1);
+}
+
+#[test]
+fn try_acquire_many_unavailable() {
+    let s = Semaphore::new(5);
+
+    let mut permit_1 = Permit::new();
+    let mut permit_2 = Permit::new();
+
+    // Acquire the first permit
+    assert_ok!(permit_1.try_acquire(1, &s));
+    assert_eq!(s.available_permits(), 4);
+
+    assert_err!(permit_2.try_acquire(5, &s));
+
+    permit_1.release(1, &s);
+    assert_eq!(s.available_permits(), 5);
+
+    assert_ok!(permit_2.try_acquire(5, &s));
+
+    permit_2.release(1, &s);
+    assert_eq!(s.available_permits(), 1);
+
+    permit_2.release(1, &s);
+    assert_eq!(s.available_permits(), 2);
+}
+
+#[test]
+fn poll_acquire_one_zero_permits() {
+    let s = Semaphore::new(0);
+    assert_eq!(s.available_permits(), 0);
+
+    let mut permit = task::spawn(Permit::new());
+
+    // Try to acquire the permit
+    permit.enter(|cx, mut p| {
+        assert_pending!(p.poll_acquire(cx, 1, &s));
+    });
+
+    s.add_permits(1);
+
+    assert!(permit.is_woken());
+    assert_ready_ok!(permit.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+}
+
+#[test]
+#[should_panic]
+fn validates_max_permits() {
+    use std::usize;
+    Semaphore::new((usize::MAX >> 2) + 1);
+}
+
+#[test]
+fn close_semaphore_prevents_acquire() {
+    let s = Semaphore::new(5);
+    s.close();
+
+    assert_eq!(5, s.available_permits());
+
+    let mut permit_1 = task::spawn(Permit::new());
+    let mut permit_2 = task::spawn(Permit::new());
+
+    assert_ready_err!(permit_1.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+    assert_eq!(5, s.available_permits());
+
+    assert_ready_err!(permit_2.enter(|cx, mut p| p.poll_acquire(cx, 2, &s)));
+    assert_eq!(5, s.available_permits());
+}
+
+#[test]
+fn close_semaphore_notifies_permit1() {
+    let s = Semaphore::new(0);
+    let mut permit = task::spawn(Permit::new());
+
+    assert_pending!(permit.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+
+    s.close();
+
+    assert!(permit.is_woken());
+    assert_ready_err!(permit.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+}
+
+#[test]
+fn close_semaphore_notifies_permit2() {
+    let s = Semaphore::new(2);
+
+    let mut permit1 = task::spawn(Permit::new());
+    let mut permit2 = task::spawn(Permit::new());
+    let mut permit3 = task::spawn(Permit::new());
+    let mut permit4 = task::spawn(Permit::new());
+
+    // Acquire a couple of permits
+    assert_ready_ok!(permit1.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+    assert_ready_ok!(permit2.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+
+    assert_pending!(permit3.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+    assert_pending!(permit4.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+
+    s.close();
+
+    assert!(permit3.is_woken());
+    assert!(permit4.is_woken());
+
+    assert_ready_err!(permit3.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+    assert_ready_err!(permit4.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+
+    assert_eq!(0, s.available_permits());
+
+    permit1.release(1, &s);
+
+    assert_eq!(1, s.available_permits());
+
+    assert_ready_err!(permit1.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+
+    permit2.release(1, &s);
+
+    assert_eq!(2, s.available_permits());
+}
+
+#[test]
+fn poll_acquire_additional_permits_while_waiting_before_assigned() {
+    let s = Semaphore::new(1);
+
+    let mut permit = task::spawn(Permit::new());
+
+    assert_pending!(permit.enter(|cx, mut p| p.poll_acquire(cx, 2, &s)));
+    assert_pending!(permit.enter(|cx, mut p| p.poll_acquire(cx, 3, &s)));
+
+    s.add_permits(1);
+    assert!(!permit.is_woken());
+
+    s.add_permits(1);
+    assert!(permit.is_woken());
+
+    assert_ready_ok!(permit.enter(|cx, mut p| p.poll_acquire(cx, 3, &s)));
+}
+
+#[test]
+fn try_acquire_additional_permits_while_waiting_before_assigned() {
+    let s = Semaphore::new(1);
+
+    let mut permit = task::spawn(Permit::new());
+
+    assert_pending!(permit.enter(|cx, mut p| p.poll_acquire(cx, 2, &s)));
+
+    assert_err!(permit.enter(|_, mut p| p.try_acquire(3, &s)));
+
+    s.add_permits(1);
+    assert!(permit.is_woken());
+
+    assert_ok!(permit.enter(|_, mut p| p.try_acquire(2, &s)));
+}
+
+#[test]
+fn poll_acquire_additional_permits_while_waiting_after_assigned_success() {
+    let s = Semaphore::new(1);
+
+    let mut permit = task::spawn(Permit::new());
+
+    assert_pending!(permit.enter(|cx, mut p| p.poll_acquire(cx, 2, &s)));
+
+    s.add_permits(2);
+
+    assert!(permit.is_woken());
+    assert_ready_ok!(permit.enter(|cx, mut p| p.poll_acquire(cx, 3, &s)));
+}
+
+#[test]
+fn poll_acquire_additional_permits_while_waiting_after_assigned_requeue() {
+    let s = Semaphore::new(1);
+
+    let mut permit = task::spawn(Permit::new());
+
+    assert_pending!(permit.enter(|cx, mut p| p.poll_acquire(cx, 2, &s)));
+
+    s.add_permits(2);
+
+    assert!(permit.is_woken());
+    assert_pending!(permit.enter(|cx, mut p| p.poll_acquire(cx, 4, &s)));
+
+    s.add_permits(1);
+
+    assert!(permit.is_woken());
+    assert_ready_ok!(permit.enter(|cx, mut p| p.poll_acquire(cx, 4, &s)));
+}
+
+#[test]
+fn poll_acquire_fewer_permits_while_waiting() {
+    let s = Semaphore::new(1);
+
+    let mut permit = task::spawn(Permit::new());
+
+    assert_pending!(permit.enter(|cx, mut p| p.poll_acquire(cx, 2, &s)));
+    assert_eq!(s.available_permits(), 0);
+
+    assert_ready_ok!(permit.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+    assert_eq!(s.available_permits(), 0);
+}
+
+#[test]
+fn poll_acquire_fewer_permits_after_assigned() {
+    let s = Semaphore::new(1);
+
+    let mut permit1 = task::spawn(Permit::new());
+    let mut permit2 = task::spawn(Permit::new());
+
+    assert_pending!(permit1.enter(|cx, mut p| p.poll_acquire(cx, 5, &s)));
+    assert_eq!(s.available_permits(), 0);
+
+    assert_pending!(permit2.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+
+    s.add_permits(4);
+    assert!(permit1.is_woken());
+    assert!(!permit2.is_woken());
+
+    assert_ready_ok!(permit1.enter(|cx, mut p| p.poll_acquire(cx, 3, &s)));
+
+    assert!(permit2.is_woken());
+    assert_eq!(s.available_permits(), 1);
+
+    assert_ready_ok!(permit2.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+}
+
+#[test]
+fn forget_partial_1() {
+    let s = Semaphore::new(0);
+
+    let mut permit = task::spawn(Permit::new());
+
+    assert_pending!(permit.enter(|cx, mut p| p.poll_acquire(cx, 2, &s)));
+    s.add_permits(1);
+
+    assert_eq!(0, s.available_permits());
+
+    permit.release(1, &s);
+
+    assert_ready_ok!(permit.enter(|cx, mut p| p.poll_acquire(cx, 1, &s)));
+
+    assert_eq!(s.available_permits(), 0);
+}
+
+#[test]
+fn forget_partial_2() {
+    let s = Semaphore::new(0);
+
+    let mut permit = task::spawn(Permit::new());
+
+    assert_pending!(permit.enter(|cx, mut p| p.poll_acquire(cx, 2, &s)));
+    s.add_permits(1);
+
+    assert_eq!(0, s.available_permits());
+
+    permit.release(1, &s);
+
+    s.add_permits(1);
+
+    assert_ready_ok!(permit.enter(|cx, mut p| p.poll_acquire(cx, 2, &s)));
+    assert_eq!(s.available_permits(), 0);
+}
diff --git a/third_party/rust/tokio/src/sync/watch.rs b/third_party/rust/tokio/src/sync/watch.rs
new file mode 100644
index 0000000000..ba609a8c6d
--- /dev/null
+++ b/third_party/rust/tokio/src/sync/watch.rs
@@ -0,0 +1,432 @@
+//! A single-producer, multi-consumer channel that only retains the *last* sent
+//! value.
+//!
+//! This channel is useful for watching for changes to a value from multiple
+//! points in the code base, for example, changes to configuration values.
+//!
+//! # Usage
+//!
+//! [`channel`] returns a [`Sender`] / [`Receiver`] pair. These are
+//! the producer and sender halves of the channel. The channel is
+//! created with an initial value. [`Receiver::recv`] will always
+//! be ready upon creation and will yield either this initial value or
+//! the latest value that has been sent by `Sender`.
+//!
+//! Calls to [`Receiver::recv`] will always yield the latest value.
+//!
+//! # Examples
+//!
+//! ```
+//! use tokio::sync::watch;
+//!
+//! # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
+//!     let (tx, mut rx) = watch::channel("hello");
+//!
+//!     tokio::spawn(async move {
+//!         while let Some(value) = rx.recv().await {
+//!             println!("received = {:?}", value);
+//!         }
+//!     });
+//!
+//!     tx.broadcast("world")?;
+//! # Ok(())
+//! # }
+//! ```
+//!
+//! # Closing
+//!
+//! [`Sender::closed`] allows the producer to detect when all [`Receiver`]
+//! handles have been dropped. This indicates that there is no further interest
+//! in the values being produced and work can be stopped.
+//!
+//! # Thread safety
+//!
+//! Both [`Sender`] and [`Receiver`] are thread safe. They can be moved to other
+//! threads and can be used in a concurrent environment. Clones of [`Receiver`]
+//! handles may be moved to separate threads and also used concurrently.
+//!
+//! [`Sender`]: crate::sync::watch::Sender
+//! [`Receiver`]: crate::sync::watch::Receiver
+//! [`Receiver::recv`]: crate::sync::watch::Receiver::recv
+//! [`channel`]: crate::sync::watch::channel
+//! [`Sender::closed`]: crate::sync::watch::Sender::closed
+
+use crate::future::poll_fn;
+use crate::sync::task::AtomicWaker;
+
+use fnv::FnvHashSet;
+use std::ops;
+use std::sync::atomic::AtomicUsize;
+use std::sync::atomic::Ordering::{Relaxed, SeqCst};
+use std::sync::{Arc, Mutex, RwLock, RwLockReadGuard, Weak};
+use std::task::Poll::{Pending, Ready};
+use std::task::{Context, Poll};
+
+/// Receives values from the associated [`Sender`](struct@Sender).
+///
+/// Instances are created by the [`channel`](fn@channel) function.
+#[derive(Debug)]
+pub struct Receiver<T> {
+    /// Pointer to the shared state
+    shared: Arc<Shared<T>>,
+
+    /// Pointer to the watcher's internal state
+    inner: Watcher,
+}
+
+/// Sends values to the associated [`Receiver`](struct@Receiver).
+///
+/// Instances are created by the [`channel`](fn@channel) function.
+#[derive(Debug)]
+pub struct Sender<T> {
+    shared: Weak<Shared<T>>,
+}
+
+/// Returns a reference to the inner value
+///
+/// Outstanding borrows hold a read lock on the inner value. This means that
+/// long lived borrows could cause the produce half to block. It is recommended
+/// to keep the borrow as short lived as possible.
+#[derive(Debug)]
+pub struct Ref<'a, T> {
+    inner: RwLockReadGuard<'a, T>,
+}
+
+pub mod error {
+    //! Watch error types
+
+    use std::fmt;
+
+    /// Error produced when sending a value fails.
+    #[derive(Debug)]
+    pub struct SendError<T> {
+        pub(crate) inner: T,
+    }
+
+    // ===== impl SendError =====
+
+    impl<T: fmt::Debug> fmt::Display for SendError<T> {
+        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+            write!(fmt, "channel closed")
+        }
+    }
+
+    impl<T: fmt::Debug> std::error::Error for SendError<T> {}
+}
+
+#[derive(Debug)]
+struct Shared<T> {
+    /// The most recent value
+    value: RwLock<T>,
+
+    /// The current version
+    ///
+    /// The lowest bit represents a "closed" state. The rest of the bits
+    /// represent the current version.
+    version: AtomicUsize,
+
+    /// All watchers
+    watchers: Mutex<Watchers>,
+
+    /// Task to notify when all watchers drop
+    cancel: AtomicWaker,
+}
+
+type Watchers = FnvHashSet<Watcher>;
+
+/// The watcher's ID is based on the Arc's pointer.
+#[derive(Clone, Debug)]
+struct Watcher(Arc<WatchInner>);
+
+#[derive(Debug)]
+struct WatchInner {
+    /// Last observed version
+    version: AtomicUsize,
+    waker: AtomicWaker,
+}
+
+const CLOSED: usize = 1;
+
+/// Creates a new watch channel, returning the "send" and "receive" handles.
+///
+/// All values sent by [`Sender`] will become visible to the [`Receiver`] handles.
+/// Only the last value sent is made available to the [`Receiver`] half. All
+/// intermediate values are dropped.
+///
+/// # Examples
+///
+/// ```
+/// use tokio::sync::watch;
+///
+/// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
+///     let (tx, mut rx) = watch::channel("hello");
+///
+///     tokio::spawn(async move {
+///         while let Some(value) = rx.recv().await {
+///             println!("received = {:?}", value);
+///         }
+///     });
+///
+///     tx.broadcast("world")?;
+/// # Ok(())
+/// # }
+/// ```
+///
+/// [`Sender`]: struct@Sender
+/// [`Receiver`]: struct@Receiver
+pub fn channel<T: Clone>(init: T) -> (Sender<T>, Receiver<T>) {
+    const VERSION_0: usize = 0;
+    const VERSION_1: usize = 2;
+
+    // We don't start knowing VERSION_1
+    let inner = Watcher::new_version(VERSION_0);
+
+    // Insert the watcher
+    let mut watchers = FnvHashSet::with_capacity_and_hasher(0, Default::default());
+    watchers.insert(inner.clone());
+
+    let shared = Arc::new(Shared {
+        value: RwLock::new(init),
+        version: AtomicUsize::new(VERSION_1),
+        watchers: Mutex::new(watchers),
+        cancel: AtomicWaker::new(),
+    });
+
+    let tx = Sender {
+        shared: Arc::downgrade(&shared),
+    };
+
+    let rx = Receiver { shared, inner };
+
+    (tx, rx)
+}
+
+impl<T> Receiver<T> {
+    /// Returns a reference to the most recently sent value
+    ///
+    /// Outstanding borrows hold a read lock. This means that long lived borrows
+    /// could cause the send half to block. It is recommended to keep the borrow
+    /// as short lived as possible.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::watch;
+    ///
+    /// let (_, rx) = watch::channel("hello");
+    /// assert_eq!(*rx.borrow(), "hello");
+    /// ```
+    pub fn borrow(&self) -> Ref<'_, T> {
+        let inner = self.shared.value.read().unwrap();
+        Ref { inner }
+    }
+
+    // TODO: document
+    #[doc(hidden)]
+    pub fn poll_recv_ref<'a>(&'a mut self, cx: &mut Context<'_>) -> Poll<Option<Ref<'a, T>>> {
+        // Make sure the task is up to date
+        self.inner.waker.register_by_ref(cx.waker());
+
+        let state = self.shared.version.load(SeqCst);
+        let version = state & !CLOSED;
+
+        if self.inner.version.swap(version, Relaxed) != version {
+            let inner = self.shared.value.read().unwrap();
+
+            return Ready(Some(Ref { inner }));
+        }
+
+        if CLOSED == state & CLOSED {
+            // The `Store` handle has been dropped.
+            return Ready(None);
+        }
+
+        Pending
+    }
+}
+
+impl<T: Clone> Receiver<T> {
+    /// Attempts to clone the latest value sent via the channel.
+    ///
+    /// If this is the first time the function is called on a `Receiver`
+    /// instance, then the function completes immediately with the **current**
+    /// value held by the channel. On the next call, the function waits until
+    /// a new value is sent in the channel.
+    ///
+    /// `None` is returned if the `Sender` half is dropped.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::watch;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, mut rx) = watch::channel("hello");
+    ///
+    ///     let v = rx.recv().await.unwrap();
+    ///     assert_eq!(v, "hello");
+    ///
+    ///     tokio::spawn(async move {
+    ///         tx.broadcast("goodbye").unwrap();
+    ///     });
+    ///
+    ///     // Waits for the new task to spawn and send the value.
+    ///     let v = rx.recv().await.unwrap();
+    ///     assert_eq!(v, "goodbye");
+    ///
+    ///     let v = rx.recv().await;
+    ///     assert!(v.is_none());
+    /// }
+    /// ```
+    pub async fn recv(&mut self) -> Option<T> {
+        poll_fn(|cx| {
+            let v_ref = ready!(self.poll_recv_ref(cx));
+            Poll::Ready(v_ref.map(|v_ref| (*v_ref).clone()))
+        })
+        .await
+    }
+}
+
+#[cfg(feature = "stream")]
+impl<T: Clone> crate::stream::Stream for Receiver<T> {
+    type Item = T;
+
+    fn poll_next(mut self: std::pin::Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
+        let v_ref = ready!(self.poll_recv_ref(cx));
+
+        Poll::Ready(v_ref.map(|v_ref| (*v_ref).clone()))
+    }
+}
+
+impl<T> Clone for Receiver<T> {
+    fn clone(&self) -> Self {
+        let ver = self.inner.version.load(Relaxed);
+        let inner = Watcher::new_version(ver);
+        let shared = self.shared.clone();
+
+        shared.watchers.lock().unwrap().insert(inner.clone());
+
+        Receiver { shared, inner }
+    }
+}
+
+impl<T> Drop for Receiver<T> {
+    fn drop(&mut self) {
+        self.shared.watchers.lock().unwrap().remove(&self.inner);
+    }
+}
+
+impl<T> Sender<T> {
+    /// Broadcasts a new value via the channel, notifying all receivers.
+    pub fn broadcast(&self, value: T) -> Result<(), error::SendError<T>> {
+        let shared = match self.shared.upgrade() {
+            Some(shared) => shared,
+            // All `Watch` handles have been canceled
+            None => return Err(error::SendError { inner: value }),
+        };
+
+        // Replace the value
+        {
+            let mut lock = shared.value.write().unwrap();
+            *lock = value;
+        }
+
+        // Update the version. 2 is used so that the CLOSED bit is not set.
+        shared.version.fetch_add(2, SeqCst);
+
+        // Notify all watchers
+        notify_all(&*shared);
+
+        // Return the old value
+        Ok(())
+    }
+
+    /// Completes when all receivers have dropped.
+    ///
+    /// This allows the producer to get notified when interest in the produced
+    /// values is canceled and immediately stop doing work.
+    pub async fn closed(&mut self) {
+        poll_fn(|cx| self.poll_close(cx)).await
+    }
+
+    fn poll_close(&mut self, cx: &mut Context<'_>) -> Poll<()> {
+        match self.shared.upgrade() {
+            Some(shared) => {
+                shared.cancel.register_by_ref(cx.waker());
+                Pending
+            }
+            None => Ready(()),
+        }
+    }
+}
+
+/// Notifies all watchers of a change
+fn notify_all<T>(shared: &Shared<T>) {
+    let watchers = shared.watchers.lock().unwrap();
+
+    for watcher in watchers.iter() {
+        // Notify the task
+        watcher.waker.wake();
+    }
+}
+
+impl<T> Drop for Sender<T> {
+    fn drop(&mut self) {
+        if let Some(shared) = self.shared.upgrade() {
+            shared.version.fetch_or(CLOSED, SeqCst);
+            notify_all(&*shared);
+        }
+    }
+}
+
+// ===== impl Ref =====
+
+impl<T> ops::Deref for Ref<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        self.inner.deref()
+    }
+}
+
+// ===== impl Shared =====
+
+impl<T> Drop for Shared<T> {
+    fn drop(&mut self) {
+        self.cancel.wake();
+    }
+}
+
+// ===== impl Watcher =====
+
+impl Watcher {
+    fn new_version(version: usize) -> Self {
+        Watcher(Arc::new(WatchInner {
+            version: AtomicUsize::new(version),
+            waker: AtomicWaker::new(),
+        }))
+    }
+}
+
+impl std::cmp::PartialEq for Watcher {
+    fn eq(&self, other: &Watcher) -> bool {
+        Arc::ptr_eq(&self.0, &other.0)
+    }
+}
+
+impl std::cmp::Eq for Watcher {}
+
+impl std::hash::Hash for Watcher {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        (&*self.0 as *const WatchInner).hash(state)
+    }
+}
+
+impl std::ops::Deref for Watcher {
+    type Target = WatchInner;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
diff --git a/third_party/rust/tokio/src/task/blocking.rs b/third_party/rust/tokio/src/task/blocking.rs
new file mode 100644
index 0000000000..0069b10ada
--- /dev/null
+++ b/third_party/rust/tokio/src/task/blocking.rs
@@ -0,0 +1,71 @@
+use crate::task::JoinHandle;
+
+cfg_rt_threaded! {
+    /// Runs the provided blocking function without blocking the executor.
+    ///
+    /// In general, issuing a blocking call or performing a lot of compute in a
+    /// future without yielding is not okay, as it may prevent the executor from
+    /// driving other futures forward. If you run a closure through this method,
+    /// the current executor thread will relegate all its executor duties to another
+    /// (possibly new) thread, and only then poll the task. Note that this requires
+    /// additional synchronization.
+    ///
+    /// # Note
+    ///
+    /// This function can only be called from a spawned task when working with
+    /// the [threaded scheduler](https://docs.rs/tokio/0.2.10/tokio/runtime/index.html#threaded-scheduler).
+    /// Consider using [tokio::task::spawn_blocking](https://docs.rs/tokio/0.2.10/tokio/task/fn.spawn_blocking.html).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::task;
+    ///
+    /// # async fn docs() {
+    /// task::block_in_place(move || {
+    ///     // do some compute-heavy work or call synchronous code
+    /// });
+    /// # }
+    /// ```
+    #[cfg_attr(docsrs, doc(cfg(feature = "blocking")))]
+    pub fn block_in_place<F, R>(f: F) -> R
+    where
+        F: FnOnce() -> R,
+    {
+        use crate::runtime::{enter, thread_pool};
+
+        enter::exit(|| thread_pool::block_in_place(f))
+    }
+}
+
+cfg_blocking! {
+    /// Runs the provided closure on a thread where blocking is acceptable.
+    ///
+    /// In general, issuing a blocking call or performing a lot of compute in a future without
+    /// yielding is not okay, as it may prevent the executor from driving other futures forward.
+    /// A closure that is run through this method will instead be run on a dedicated thread pool for
+    /// such blocking tasks without holding up the main futures executor.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::task;
+    ///
+    /// # async fn docs() -> Result<(), Box<dyn std::error::Error>>{
+    /// let res = task::spawn_blocking(move || {
+    ///     // do some compute-heavy work or call synchronous code
+    ///     "done computing"
+    /// }).await?;
+    ///
+    /// assert_eq!(res, "done computing");
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn spawn_blocking<F, R>(f: F) -> JoinHandle<R>
+    where
+        F: FnOnce() -> R + Send + 'static,
+        R: Send + 'static,
+    {
+        crate::runtime::spawn_blocking(f)
+    }
+}
diff --git a/third_party/rust/tokio/src/task/local.rs b/third_party/rust/tokio/src/task/local.rs
new file mode 100644
index 0000000000..fce467079f
--- /dev/null
+++ b/third_party/rust/tokio/src/task/local.rs
@@ -0,0 +1,584 @@
+//! Runs `!Send` futures on the current thread.
+use crate::runtime::task::{self, JoinHandle, Task};
+use crate::sync::AtomicWaker;
+use crate::util::linked_list::LinkedList;
+
+use std::cell::{Cell, RefCell};
+use std::collections::VecDeque;
+use std::fmt;
+use std::future::Future;
+use std::marker::PhantomData;
+use std::pin::Pin;
+use std::sync::{Arc, Mutex};
+use std::task::Poll;
+
+use pin_project_lite::pin_project;
+
+cfg_rt_util! {
+    /// A set of tasks which are executed on the same thread.
+    ///
+    /// In some cases, it is necessary to run one or more futures that do not
+    /// implement [`Send`] and thus are unsafe to send between threads. In these
+    /// cases, a [local task set] may be used to schedule one or more `!Send`
+    /// futures to run together on the same thread.
+    ///
+    /// For example, the following code will not compile:
+    ///
+    /// ```rust,compile_fail
+    /// use std::rc::Rc;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     // `Rc` does not implement `Send`, and thus may not be sent between
+    ///     // threads safely.
+    ///     let unsend_data = Rc::new("my unsend data...");
+    ///
+    ///     let unsend_data = unsend_data.clone();
+    ///     // Because the `async` block here moves `unsend_data`, the future is `!Send`.
+    ///     // Since `tokio::spawn` requires the spawned future to implement `Send`, this
+    ///     // will not compile.
+    ///     tokio::spawn(async move {
+    ///         println!("{}", unsend_data);
+    ///         // ...
+    ///     }).await.unwrap();
+    /// }
+    /// ```
+    /// In order to spawn `!Send` futures, we can use a local task set to
+    /// schedule them on the thread calling [`Runtime::block_on`]. When running
+    /// inside of the local task set, we can use [`task::spawn_local`], which can
+    /// spawn `!Send` futures. For example:
+    ///
+    /// ```rust
+    /// use std::rc::Rc;
+    /// use tokio::task;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let unsend_data = Rc::new("my unsend data...");
+    ///
+    ///     // Construct a local task set that can run `!Send` futures.
+    ///     let local = task::LocalSet::new();
+    ///
+    ///     // Run the local task set.
+    ///     local.run_until(async move {
+    ///         let unsend_data = unsend_data.clone();
+    ///         // `spawn_local` ensures that the future is spawned on the local
+    ///         // task set.
+    ///         task::spawn_local(async move {
+    ///             println!("{}", unsend_data);
+    ///             // ...
+    ///         }).await.unwrap();
+    ///     }).await;
+    /// }
+    /// ```
+    ///
+    /// ## Awaiting a `LocalSet`
+    ///
+    /// Additionally, a `LocalSet` itself implements `Future`, completing when
+    /// *all* tasks spawned on the `LocalSet` complete. This can be used to run
+    /// several futures on a `LocalSet` and drive the whole set until they
+    /// complete. For example,
+    ///
+    /// ```rust
+    /// use tokio::{task, time};
+    /// use std::rc::Rc;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let unsend_data = Rc::new("world");
+    ///     let local = task::LocalSet::new();
+    ///
+    ///     let unsend_data2 = unsend_data.clone();
+    ///     local.spawn_local(async move {
+    ///         // ...
+    ///         println!("hello {}", unsend_data2)
+    ///     });
+    ///
+    ///     local.spawn_local(async move {
+    ///         time::delay_for(time::Duration::from_millis(100)).await;
+    ///         println!("goodbye {}", unsend_data)
+    ///     });
+    ///
+    ///     // ...
+    ///
+    ///     local.await;
+    /// }
+    /// ```
+    ///
+    /// [`Send`]: https://doc.rust-lang.org/std/marker/trait.Send.html
+    /// [local task set]: struct@LocalSet
+    /// [`Runtime::block_on`]: ../struct.Runtime.html#method.block_on
+    /// [`task::spawn_local`]: fn@spawn_local
+    pub struct LocalSet {
+        /// Current scheduler tick
+        tick: Cell<u8>,
+
+        /// State available from thread-local
+        context: Context,
+
+        /// This type should not be Send.
+        _not_send: PhantomData<*const ()>,
+    }
+}
+
+/// State available from the thread-local
+struct Context {
+    /// Owned task set and local run queue
+    tasks: RefCell<Tasks>,
+
+    /// State shared between threads.
+    shared: Arc<Shared>,
+}
+
+struct Tasks {
+    /// Collection of all active tasks spawned onto this executor.
+    owned: LinkedList<Task<Arc<Shared>>>,
+
+    /// Local run queue sender and receiver.
+    queue: VecDeque<task::Notified<Arc<Shared>>>,
+}
+
+/// LocalSet state shared between threads.
+struct Shared {
+    /// Remote run queue sender
+    queue: Mutex<VecDeque<task::Notified<Arc<Shared>>>>,
+
+    /// Wake the `LocalSet` task
+    waker: AtomicWaker,
+}
+
+pin_project! {
+    #[derive(Debug)]
+    struct RunUntil<'a, F> {
+        local_set: &'a LocalSet,
+        #[pin]
+        future: F,
+    }
+}
+
+scoped_thread_local!(static CURRENT: Context);
+
+cfg_rt_util! {
+    /// Spawns a `!Send` future on the local task set.
+    ///
+    /// The spawned future will be run on the same thread that called `spawn_local.`
+    /// This may only be called from the context of a local task set.
+    ///
+    /// # Panics
+    ///
+    /// - This function panics if called outside of a local task set.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use std::rc::Rc;
+    /// use tokio::task;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let unsend_data = Rc::new("my unsend data...");
+    ///
+    ///     let local = task::LocalSet::new();
+    ///
+    ///     // Run the local task set.
+    ///     local.run_until(async move {
+    ///         let unsend_data = unsend_data.clone();
+    ///         task::spawn_local(async move {
+    ///             println!("{}", unsend_data);
+    ///             // ...
+    ///         }).await.unwrap();
+    ///     }).await;
+    /// }
+    /// ```
+    pub fn spawn_local<F>(future: F) -> JoinHandle<F::Output>
+    where
+        F: Future + 'static,
+        F::Output: 'static,
+    {
+        CURRENT.with(|maybe_cx| {
+            let cx = maybe_cx
+                .expect("`spawn_local` called from outside of a `task::LocalSet`");
+
+            // Safety: Tasks are only polled and dropped from the thread that
+            // spawns them.
+            let (task, handle) = unsafe { task::joinable_local(future) };
+            cx.tasks.borrow_mut().queue.push_back(task);
+            handle
+        })
+    }
+}
+
+/// Initial queue capacity
+const INITIAL_CAPACITY: usize = 64;
+
+/// Max number of tasks to poll per tick.
+const MAX_TASKS_PER_TICK: usize = 61;
+
+/// How often it check the remote queue first
+const REMOTE_FIRST_INTERVAL: u8 = 31;
+
+impl LocalSet {
+    /// Returns a new local task set.
+    pub fn new() -> LocalSet {
+        LocalSet {
+            tick: Cell::new(0),
+            context: Context {
+                tasks: RefCell::new(Tasks {
+                    owned: LinkedList::new(),
+                    queue: VecDeque::with_capacity(INITIAL_CAPACITY),
+                }),
+                shared: Arc::new(Shared {
+                    queue: Mutex::new(VecDeque::with_capacity(INITIAL_CAPACITY)),
+                    waker: AtomicWaker::new(),
+                }),
+            },
+            _not_send: PhantomData,
+        }
+    }
+
+    /// Spawns a `!Send` task onto the local task set.
+    ///
+    /// This task is guaranteed to be run on the current thread.
+    ///
+    /// Unlike the free function [`spawn_local`], this method may be used to
+    /// spawn local tasks when the task set is _not_ running. For example:
+    /// ```rust
+    /// use tokio::task;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let local = task::LocalSet::new();
+    ///
+    ///     // Spawn a future on the local set. This future will be run when
+    ///     // we call `run_until` to drive the task set.
+    ///     local.spawn_local(async {
+    ///        // ...
+    ///     });
+    ///
+    ///     // Run the local task set.
+    ///     local.run_until(async move {
+    ///         // ...
+    ///     }).await;
+    ///
+    ///     // When `run` finishes, we can spawn _more_ futures, which will
+    ///     // run in subsequent calls to `run_until`.
+    ///     local.spawn_local(async {
+    ///        // ...
+    ///     });
+    ///
+    ///     local.run_until(async move {
+    ///         // ...
+    ///     }).await;
+    /// }
+    /// ```
+    /// [`spawn_local`]: fn@spawn_local
+    pub fn spawn_local<F>(&self, future: F) -> JoinHandle<F::Output>
+    where
+        F: Future + 'static,
+        F::Output: 'static,
+    {
+        let (task, handle) = unsafe { task::joinable_local(future) };
+        self.context.tasks.borrow_mut().queue.push_back(task);
+        handle
+    }
+
+    /// Runs a future to completion on the provided runtime, driving any local
+    /// futures spawned on this task set on the current thread.
+    ///
+    /// This runs the given future on the runtime, blocking until it is
+    /// complete, and yielding its resolved result. Any tasks or timers which
+    /// the future spawns internally will be executed on the runtime. The future
+    /// may also call [`spawn_local`] to spawn_local additional local futures on the
+    /// current thread.
+    ///
+    /// This method should not be called from an asynchronous context.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if the executor is at capacity, if the provided
+    /// future panics, or if called within an asynchronous execution context.
+    ///
+    /// # Notes
+    ///
+    /// Since this function internally calls [`Runtime::block_on`], and drives
+    /// futures in the local task set inside that call to `block_on`, the local
+    /// futures may not use [in-place blocking]. If a blocking call needs to be
+    /// issued from a local task, the [`spawn_blocking`] API may be used instead.
+    ///
+    /// For example, this will panic:
+    /// ```should_panic
+    /// use tokio::runtime::Runtime;
+    /// use tokio::task;
+    ///
+    /// let mut rt = Runtime::new().unwrap();
+    /// let local = task::LocalSet::new();
+    /// local.block_on(&mut rt, async {
+    ///     let join = task::spawn_local(async {
+    ///         let blocking_result = task::block_in_place(|| {
+    ///             // ...
+    ///         });
+    ///         // ...
+    ///     });
+    ///     join.await.unwrap();
+    /// })
+    /// ```
+    /// This, however, will not panic:
+    /// ```
+    /// use tokio::runtime::Runtime;
+    /// use tokio::task;
+    ///
+    /// let mut rt = Runtime::new().unwrap();
+    /// let local = task::LocalSet::new();
+    /// local.block_on(&mut rt, async {
+    ///     let join = task::spawn_local(async {
+    ///         let blocking_result = task::spawn_blocking(|| {
+    ///             // ...
+    ///         }).await;
+    ///         // ...
+    ///     });
+    ///     join.await.unwrap();
+    /// })
+    /// ```
+    ///
+    /// [`spawn_local`]: fn@spawn_local
+    /// [`Runtime::block_on`]: ../struct.Runtime.html#method.block_on
+    /// [in-place blocking]: ../blocking/fn.in_place.html
+    /// [`spawn_blocking`]: ../blocking/fn.spawn_blocking.html
+    pub fn block_on<F>(&self, rt: &mut crate::runtime::Runtime, future: F) -> F::Output
+    where
+        F: Future,
+    {
+        rt.block_on(self.run_until(future))
+    }
+
+    /// Run a future to completion on the local set, returning its output.
+    ///
+    /// This returns a future that runs the given future with a local set,
+    /// allowing it to call [`spawn_local`] to spawn additional `!Send` futures.
+    /// Any local futures spawned on the local set will be driven in the
+    /// background until the future passed to `run_until` completes. When the future
+    /// passed to `run` finishes, any local futures which have not completed
+    /// will remain on the local set, and will be driven on subsequent calls to
+    /// `run_until` or when [awaiting the local set] itself.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use tokio::task;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     task::LocalSet::new().run_until(async {
+    ///         task::spawn_local(async move {
+    ///             // ...
+    ///         }).await.unwrap();
+    ///         // ...
+    ///     }).await;
+    /// }
+    /// ```
+    ///
+    /// [`spawn_local`]: fn@spawn_local
+    /// [awaiting the local set]: #awaiting-a-localset
+    pub async fn run_until<F>(&self, future: F) -> F::Output
+    where
+        F: Future,
+    {
+        let run_until = RunUntil {
+            future,
+            local_set: self,
+        };
+        run_until.await
+    }
+
+    /// Tick the scheduler, returning whether the local future needs to be
+    /// notified again.
+    fn tick(&self) -> bool {
+        for _ in 0..MAX_TASKS_PER_TICK {
+            match self.next_task() {
+                // Run the task
+                //
+                // Safety: As spawned tasks are `!Send`, `run_unchecked` must be
+                // used. We are responsible for maintaining the invariant that
+                // `run_unchecked` is only called on threads that spawned the
+                // task initially. Because `LocalSet` itself is `!Send`, and
+                // `spawn_local` spawns into the `LocalSet` on the current
+                // thread, the invariant is maintained.
+                Some(task) => crate::coop::budget(|| task.run()),
+                // We have fully drained the queue of notified tasks, so the
+                // local future doesn't need to be notified again — it can wait
+                // until something else wakes a task in the local set.
+                None => return false,
+            }
+        }
+
+        true
+    }
+
+    fn next_task(&self) -> Option<task::Notified<Arc<Shared>>> {
+        let tick = self.tick.get();
+        self.tick.set(tick.wrapping_add(1));
+
+        if tick % REMOTE_FIRST_INTERVAL == 0 {
+            self.context
+                .shared
+                .queue
+                .lock()
+                .unwrap()
+                .pop_front()
+                .or_else(|| self.context.tasks.borrow_mut().queue.pop_front())
+        } else {
+            self.context
+                .tasks
+                .borrow_mut()
+                .queue
+                .pop_front()
+                .or_else(|| self.context.shared.queue.lock().unwrap().pop_front())
+        }
+    }
+
+    fn with<T>(&self, f: impl FnOnce() -> T) -> T {
+        CURRENT.set(&self.context, f)
+    }
+}
+
+impl fmt::Debug for LocalSet {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("LocalSet").finish()
+    }
+}
+
+impl Future for LocalSet {
+    type Output = ();
+
+    fn poll(self: Pin<&mut Self>, cx: &mut std::task::Context<'_>) -> Poll<Self::Output> {
+        // Register the waker before starting to work
+        self.context.shared.waker.register_by_ref(cx.waker());
+
+        if self.with(|| self.tick()) {
+            // If `tick` returns true, we need to notify the local future again:
+            // there are still tasks remaining in the run queue.
+            cx.waker().wake_by_ref();
+            Poll::Pending
+        } else if self.context.tasks.borrow().owned.is_empty() {
+            // If the scheduler has no remaining futures, we're done!
+            Poll::Ready(())
+        } else {
+            // There are still futures in the local set, but we've polled all the
+            // futures in the run queue. Therefore, we can just return Pending
+            // since the remaining futures will be woken from somewhere else.
+            Poll::Pending
+        }
+    }
+}
+
+impl Default for LocalSet {
+    fn default() -> LocalSet {
+        LocalSet::new()
+    }
+}
+
+impl Drop for LocalSet {
+    fn drop(&mut self) {
+        self.with(|| {
+            // Loop required here to ensure borrow is dropped between iterations
+            #[allow(clippy::while_let_loop)]
+            loop {
+                let task = match self.context.tasks.borrow_mut().owned.pop_back() {
+                    Some(task) => task,
+                    None => break,
+                };
+
+                // Safety: same as `run_unchecked`.
+                task.shutdown();
+            }
+
+            for task in self.context.tasks.borrow_mut().queue.drain(..) {
+                task.shutdown();
+            }
+
+            for task in self.context.shared.queue.lock().unwrap().drain(..) {
+                task.shutdown();
+            }
+
+            assert!(self.context.tasks.borrow().owned.is_empty());
+        });
+    }
+}
+
+// === impl LocalFuture ===
+
+impl<T: Future> Future for RunUntil<'_, T> {
+    type Output = T::Output;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut std::task::Context<'_>) -> Poll<Self::Output> {
+        let me = self.project();
+
+        me.local_set.with(|| {
+            me.local_set
+                .context
+                .shared
+                .waker
+                .register_by_ref(cx.waker());
+
+            if let Poll::Ready(output) = me.future.poll(cx) {
+                return Poll::Ready(output);
+            }
+
+            if me.local_set.tick() {
+                // If `tick` returns `true`, we need to notify the local future again:
+                // there are still tasks remaining in the run queue.
+                cx.waker().wake_by_ref();
+            }
+
+            Poll::Pending
+        })
+    }
+}
+
+impl Shared {
+    /// Schedule the provided task on the scheduler.
+    fn schedule(&self, task: task::Notified<Arc<Self>>) {
+        CURRENT.with(|maybe_cx| match maybe_cx {
+            Some(cx) if cx.shared.ptr_eq(self) => {
+                cx.tasks.borrow_mut().queue.push_back(task);
+            }
+            _ => {
+                self.queue.lock().unwrap().push_back(task);
+                self.waker.wake();
+            }
+        });
+    }
+
+    fn ptr_eq(&self, other: &Shared) -> bool {
+        self as *const _ == other as *const _
+    }
+}
+
+impl task::Schedule for Arc<Shared> {
+    fn bind(task: Task<Self>) -> Arc<Shared> {
+        CURRENT.with(|maybe_cx| {
+            let cx = maybe_cx.expect("scheduler context missing");
+            cx.tasks.borrow_mut().owned.push_front(task);
+            cx.shared.clone()
+        })
+    }
+
+    fn release(&self, task: &Task<Self>) -> Option<Task<Self>> {
+        use std::ptr::NonNull;
+
+        CURRENT.with(|maybe_cx| {
+            let cx = maybe_cx.expect("scheduler context missing");
+
+            assert!(cx.shared.ptr_eq(self));
+
+            let ptr = NonNull::from(task.header());
+            // safety: task must be contained by list. It is inserted into the
+            // list in `bind`.
+            unsafe { cx.tasks.borrow_mut().owned.remove(ptr) }
+        })
+    }
+
+    fn schedule(&self, task: task::Notified<Self>) {
+        Shared::schedule(self, task);
+    }
+}
diff --git a/third_party/rust/tokio/src/task/mod.rs b/third_party/rust/tokio/src/task/mod.rs
new file mode 100644
index 0000000000..5c89393a5e
--- /dev/null
+++ b/third_party/rust/tokio/src/task/mod.rs
@@ -0,0 +1,242 @@
+//! Asynchronous green-threads.
+//!
+//! ## What are Tasks?
+//!
+//! A _task_ is a light weight, non-blocking unit of execution. A task is similar
+//! to an OS thread, but rather than being managed by the OS scheduler, they are
+//! managed by the [Tokio runtime][rt]. Another name for this general pattern is
+//! [green threads]. If you are familiar with [Go's goroutines], [Kotlin's
+//! coroutines], or [Erlang's processes], you can think of Tokio's tasks as
+//! something similar.
+//!
+//! Key points about tasks include:
+//!
+//! * Tasks are **light weight**. Because tasks are scheduled by the Tokio
+//!   runtime rather than the operating system, creating new tasks or switching
+//!   between tasks does not require a context switch and has fairly low
+//!   overhead. Creating, running, and destroying large numbers of tasks is
+//!   quite cheap, especially compared to OS threads.
+//!
+//! * Tasks are scheduled **cooperatively**. Most operating systems implement
+//!   _preemptive multitasking_. This is a scheduling technique where the
+//!   operating system allows each thread to run for a period of time, and then
+//!   _preempts_ it, temporarily pausing that thread and switching to another.
+//!   Tasks, on the other hand, implement _cooperative multitasking_. In
+//!   cooperative multitasking, a task is allowed to run until it _yields_,
+//!   indicating to the Tokio runtime's scheduler that it cannot currently
+//!   continue executing. When a task yields, the Tokio runtime switches to
+//!   executing the next task.
+//!
+//! * Tasks are **non-blocking**. Typically, when an OS thread performs I/O or
+//!   must synchronize with another thread, it _blocks_, allowing the OS to
+//!   schedule another thread. When a task cannot continue executing, it must
+//!   yield instead, allowing the Tokio runtime to schedule another task. Tasks
+//!   should generally not perform system calls or other operations that could
+//!   block a thread, as this would prevent other tasks running on the same
+//!   thread from executing as well. Instead, this module provides APIs for
+//!   running blocking operations in an asynchronous context.
+//!
+//! [rt]: crate::runtime
+//! [green threads]: https://en.wikipedia.org/wiki/Green_threads
+//! [Go's goroutines]: https://tour.golang.org/concurrency/1
+//! [Kotlin's coroutines]: https://kotlinlang.org/docs/reference/coroutines-overview.html
+//! [Erlang's processes]: http://erlang.org/doc/getting_started/conc_prog.html#processes
+//!
+//! ## Working with Tasks
+//!
+//! This module provides the following APIs for working with tasks:
+//!
+//! ### Spawning
+//!
+//! Perhaps the most important function in this module is [`task::spawn`]. This
+//! function can be thought of as an async equivalent to the standard library's
+//! [`thread::spawn`][`std::thread::spawn`]. It takes an `async` block or other
+//! [future], and creates a new task to run that work concurrently:
+//!
+//! ```
+//! use tokio::task;
+//!
+//! # async fn doc() {
+//! task::spawn(async {
+//!     // perform some work here...
+//! });
+//! # }
+//! ```
+//!
+//! Like [`std::thread::spawn`], `task::spawn` returns a [`JoinHandle`] struct.
+//! A `JoinHandle` is itself a future which may be used to await the output of
+//! the spawned task. For example:
+//!
+//! ```
+//! use tokio::task;
+//!
+//! # #[tokio::main] async fn main() -> Result<(), Box<dyn std::error::Error>> {
+//! let join = task::spawn(async {
+//!     // ...
+//!     "hello world!"
+//! });
+//!
+//! // ...
+//!
+//! // Await the result of the spawned task.
+//! let result = join.await?;
+//! assert_eq!(result, "hello world!");
+//! # Ok(())
+//! # }
+//! ```
+//!
+//! Again, like `std::thread`'s [`JoinHandle` type][thread_join], if the spawned
+//! task panics, awaiting its `JoinHandle` will return a [`JoinError`]`. For
+//! example:
+//!
+//! ```
+//! use tokio::task;
+//!
+//! # #[tokio::main] async fn main() {
+//! let join = task::spawn(async {
+//!     panic!("something bad happened!")
+//! });
+//!
+//! // The returned result indicates that the task failed.
+//! assert!(join.await.is_err());
+//! # }
+//! ```
+//!
+//! `spawn`, `JoinHandle`, and `JoinError` are present when the "rt-core"
+//! feature flag is enabled.
+//!
+//! [`task::spawn`]: crate::task::spawn()
+//! [future]: std::future::Future
+//! [`std::thread::spawn`]: std::thread::spawn
+//! [`JoinHandle`]: crate::task::JoinHandle
+//! [thread_join]: std::thread::JoinHandle
+//! [`JoinError`]: crate::task::JoinError
+//!
+//! ### Blocking and Yielding
+//!
+//! As we discussed above, code running in asynchronous tasks should not perform
+//! operations that can block. A blocking operation performed in a task running
+//! on a thread that is also running other tasks would block the entire thread,
+//! preventing other tasks from running.
+//!
+//! Instead, Tokio provides two APIs for running blocking operations in an
+//! asynchronous context: [`task::spawn_blocking`] and [`task::block_in_place`].
+//!
+//! #### spawn_blocking
+//!
+//! The `task::spawn_blocking` function is similar to the `task::spawn` function
+//! discussed in the previous section, but rather than spawning an
+//! _non-blocking_ future on the Tokio runtime, it instead spawns a
+//! _blocking_ function on a dedicated thread pool for blocking tasks. For
+//! example:
+//!
+//! ```
+//! use tokio::task;
+//!
+//! # async fn docs() {
+//! task::spawn_blocking(|| {
+//!     // do some compute-heavy work or call synchronous code
+//! });
+//! # }
+//! ```
+//!
+//! Just like `task::spawn`, `task::spawn_blocking` returns a `JoinHandle`
+//! which we can use to await the result of the blocking operation:
+//!
+//! ```rust
+//! # use tokio::task;
+//! # async fn docs() -> Result<(), Box<dyn std::error::Error>>{
+//! let join = task::spawn_blocking(|| {
+//!     // do some compute-heavy work or call synchronous code
+//!     "blocking completed"
+//! });
+//!
+//! let result = join.await?;
+//! assert_eq!(result, "blocking completed");
+//! # Ok(())
+//! # }
+//! ```
+//!
+//! #### block_in_place
+//!
+//! When using the [threaded runtime][rt-threaded], the [`task::block_in_place`]
+//! function is also available. Like `task::spawn_blocking`, this function
+//! allows running a blocking operation from an asynchronous context. Unlike
+//! `spawn_blocking`, however, `block_in_place` works by transitioning the
+//! _current_ worker thread to a blocking thread, moving other tasks running on
+//! that thread to another worker thread. This can improve performance by avoiding
+//! context switches.
+//!
+//! For example:
+//!
+//! ```
+//! use tokio::task;
+//!
+//! # async fn docs() {
+//! let result = task::block_in_place(|| {
+//!     // do some compute-heavy work or call synchronous code
+//!     "blocking completed"
+//! });
+//!
+//! assert_eq!(result, "blocking completed");
+//! # }
+//! ```
+//!
+//! #### yield_now
+//!
+//! In addition, this module provides a [`task::yield_now`] async function
+//! that is analogous to the standard library's [`thread::yield_now`]. Calling
+//! and `await`ing this function will cause the current task to yield to the
+//! Tokio runtime's scheduler, allowing other tasks to be
+//! scheduled. Eventually, the yielding task will be polled again, allowing it
+//! to execute. For example:
+//!
+//! ```rust
+//! use tokio::task;
+//!
+//! # #[tokio::main] async fn main() {
+//! async {
+//!     task::spawn(async {
+//!         // ...
+//!         println!("spawned task done!")
+//!     });
+//!
+//!     // Yield, allowing the newly-spawned task to execute first.
+//!     task::yield_now().await;
+//!     println!("main task done!");
+//! }
+//! # .await;
+//! # }
+//! ```
+//!
+//! [`task::spawn_blocking`]: crate::task::spawn_blocking
+//! [`task::block_in_place`]: crate::task::block_in_place
+//! [rt-threaded]: ../runtime/index.html#threaded-scheduler
+//! [`task::yield_now`]: crate::task::yield_now()
+//! [`thread::yield_now`]: std::thread::yield_now
+cfg_blocking! {
+    mod blocking;
+    pub use blocking::spawn_blocking;
+
+    cfg_rt_threaded! {
+        pub use blocking::block_in_place;
+    }
+}
+
+cfg_rt_core! {
+    pub use crate::runtime::task::{JoinError, JoinHandle};
+
+    mod spawn;
+    pub use spawn::spawn;
+
+    mod yield_now;
+    pub use yield_now::yield_now;
+}
+
+cfg_rt_util! {
+    mod local;
+    pub use local::{spawn_local, LocalSet};
+
+    mod task_local;
+    pub use task_local::LocalKey;
+}
diff --git a/third_party/rust/tokio/src/task/spawn.rs b/third_party/rust/tokio/src/task/spawn.rs
new file mode 100644
index 0000000000..fa5ff13b01
--- /dev/null
+++ b/third_party/rust/tokio/src/task/spawn.rs
@@ -0,0 +1,134 @@
+use crate::runtime;
+use crate::task::JoinHandle;
+
+use std::future::Future;
+
+doc_rt_core! {
+    /// Spawns a new asynchronous task, returning a
+    /// [`JoinHandle`](super::JoinHandle) for it.
+    ///
+    /// Spawning a task enables the task to execute concurrently to other tasks. The
+    /// spawned task may execute on the current thread, or it may be sent to a
+    /// different thread to be executed. The specifics depend on the current
+    /// [`Runtime`](crate::runtime::Runtime) configuration.
+    ///
+    /// There is no guarantee that a spawned task will execute to completion.
+    /// When a runtime is shutdown, all outstanding tasks are dropped,
+    /// regardless of the lifecycle of that task.
+    ///
+    /// This function must be called from the context of a Tokio runtime. Tasks running on
+    /// the Tokio runtime are always inside its context, but you can also enter the context
+    /// using the [`Handle::enter`](crate::runtime::Handle::enter()) method.
+    ///
+    /// # Examples
+    ///
+    /// In this example, a server is started and `spawn` is used to start a new task
+    /// that processes each received connection.
+    ///
+    /// ```no_run
+    /// use tokio::net::{TcpListener, TcpStream};
+    ///
+    /// use std::io;
+    ///
+    /// async fn process(socket: TcpStream) {
+    ///     // ...
+    /// # drop(socket);
+    /// }
+    ///
+    /// #[tokio::main]
+    /// async fn main() -> io::Result<()> {
+    ///     let mut listener = TcpListener::bind("127.0.0.1:8080").await?;
+    ///
+    ///     loop {
+    ///         let (socket, _) = listener.accept().await?;
+    ///
+    ///         tokio::spawn(async move {
+    ///             // Process each socket concurrently.
+    ///             process(socket).await
+    ///         });
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// # Panics
+    ///
+    /// Panics if called from **outside** of the Tokio runtime.
+    ///
+    /// # Using `!Send` values from a task
+    ///
+    /// The task supplied to `spawn` must implement `Send`. However, it is
+    /// possible to **use** `!Send` values from the task as long as they only
+    /// exist between calls to `.await`.
+    ///
+    /// For example, this will work:
+    ///
+    /// ```
+    /// use tokio::task;
+    ///
+    /// use std::rc::Rc;
+    ///
+    /// fn use_rc(rc: Rc<()>) {
+    ///     // Do stuff w/ rc
+    /// # drop(rc);
+    /// }
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     tokio::spawn(async {
+    ///         // Force the `Rc` to stay in a scope with no `.await`
+    ///         {
+    ///             let rc = Rc::new(());
+    ///             use_rc(rc.clone());
+    ///         }
+    ///
+    ///         task::yield_now().await;
+    ///     }).await.unwrap();
+    /// }
+    /// ```
+    ///
+    /// This will **not** work:
+    ///
+    /// ```compile_fail
+    /// use tokio::task;
+    ///
+    /// use std::rc::Rc;
+    ///
+    /// fn use_rc(rc: Rc<()>) {
+    ///     // Do stuff w/ rc
+    /// # drop(rc);
+    /// }
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     tokio::spawn(async {
+    ///         let rc = Rc::new(());
+    ///
+    ///         task::yield_now().await;
+    ///
+    ///         use_rc(rc.clone());
+    ///     }).await.unwrap();
+    /// }
+    /// ```
+    ///
+    /// Holding on to a `!Send` value across calls to `.await` will result in
+    /// an unfriendly compile error message similar to:
+    ///
+    /// ```text
+    /// `[... some type ...]` cannot be sent between threads safely
+    /// ```
+    ///
+    /// or:
+    ///
+    /// ```text
+    /// error[E0391]: cycle detected when processing `main`
+    /// ```
+    pub fn spawn<T>(task: T) -> JoinHandle<T::Output>
+    where
+        T: Future + Send + 'static,
+        T::Output: Send + 'static,
+    {
+        let spawn_handle = runtime::context::spawn_handle()
+        .expect("must be called from the context of Tokio runtime configured with either `basic_scheduler` or `threaded_scheduler`");
+        spawn_handle.spawn(task)
+    }
+}
diff --git a/third_party/rust/tokio/src/task/task_local.rs b/third_party/rust/tokio/src/task/task_local.rs
new file mode 100644
index 0000000000..f3341b6a7e
--- /dev/null
+++ b/third_party/rust/tokio/src/task/task_local.rs
@@ -0,0 +1,240 @@
+use pin_project_lite::pin_project;
+use std::cell::RefCell;
+use std::error::Error;
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+use std::{fmt, thread};
+
+/// Declares a new task-local key of type [`tokio::task::LocalKey`].
+///
+/// # Syntax
+///
+/// The macro wraps any number of static declarations and makes them local to the current task.
+/// Publicity and attributes for each static is preserved. For example:
+///
+/// # Examples
+///
+/// ```
+/// # use tokio::task_local;
+/// task_local! {
+///     pub static ONE: u32;
+///
+///     #[allow(unused)]
+///     static TWO: f32;
+/// }
+/// # fn main() {}
+/// ```
+///
+/// See [LocalKey documentation][`tokio::task::LocalKey`] for more
+/// information.
+///
+/// [`tokio::task::LocalKey`]: ../tokio/task/struct.LocalKey.html
+#[macro_export]
+macro_rules! task_local {
+     // empty (base case for the recursion)
+    () => {};
+
+    ($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty; $($rest:tt)*) => {
+        $crate::__task_local_inner!($(#[$attr])* $vis $name, $t);
+        $crate::task_local!($($rest)*);
+    };
+
+    ($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty) => {
+        $crate::__task_local_inner!($(#[$attr])* $vis $name, $t);
+    }
+}
+
+#[doc(hidden)]
+#[macro_export]
+macro_rules! __task_local_inner {
+    ($(#[$attr:meta])* $vis:vis $name:ident, $t:ty) => {
+        static $name: $crate::task::LocalKey<$t> = {
+            std::thread_local! {
+                static __KEY: std::cell::RefCell<Option<$t>> = std::cell::RefCell::new(None);
+            }
+
+            $crate::task::LocalKey { inner: __KEY }
+        };
+    };
+}
+
+/// A key for task-local data.
+///
+/// This type is generated by the `task_local!` macro.
+///
+/// Unlike [`std::thread::LocalKey`], `tokio::task::LocalKey` will
+/// _not_ lazily initialize the value on first access. Instead, the
+/// value is first initialized when the future containing
+/// the task-local is first polled by a futures executor, like Tokio.
+///
+/// # Examples
+///
+/// ```
+/// # async fn dox() {
+/// tokio::task_local! {
+///     static NUMBER: u32;
+/// }
+///
+/// NUMBER.scope(1, async move {
+///     assert_eq!(NUMBER.get(), 1);
+/// }).await;
+///
+/// NUMBER.scope(2, async move {
+///     assert_eq!(NUMBER.get(), 2);
+///
+///     NUMBER.scope(3, async move {
+///         assert_eq!(NUMBER.get(), 3);
+///     }).await;
+/// }).await;
+/// # }
+/// ```
+/// [`std::thread::LocalKey`]: https://doc.rust-lang.org/std/thread/struct.LocalKey.html
+pub struct LocalKey<T: 'static> {
+    #[doc(hidden)]
+    pub inner: thread::LocalKey<RefCell<Option<T>>>,
+}
+
+impl<T: 'static> LocalKey<T> {
+    /// Sets a value `T` as the task-local value for the future `F`.
+    ///
+    /// On completion of `scope`, the task-local will be dropped.
+    ///
+    /// ### Examples
+    ///
+    /// ```
+    /// # async fn dox() {
+    /// tokio::task_local! {
+    ///     static NUMBER: u32;
+    /// }
+    ///
+    /// NUMBER.scope(1, async move {
+    ///     println!("task local value: {}", NUMBER.get());
+    /// }).await;
+    /// # }
+    /// ```
+    pub async fn scope<F>(&'static self, value: T, f: F) -> F::Output
+    where
+        F: Future,
+    {
+        TaskLocalFuture {
+            local: &self,
+            slot: Some(value),
+            future: f,
+        }
+        .await
+    }
+
+    /// Accesses the current task-local and runs the provided closure.
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if not called within the context
+    /// of a future containing a task-local with the corresponding key.
+    pub fn with<F, R>(&'static self, f: F) -> R
+    where
+        F: FnOnce(&T) -> R,
+    {
+        self.try_with(f).expect(
+            "cannot access a Task Local Storage value \
+             without setting it via `LocalKey::set`",
+        )
+    }
+
+    /// Accesses the current task-local and runs the provided closure.
+    ///
+    /// If the task-local with the accociated key is not present, this
+    /// method will return an `AccessError`. For a panicking variant,
+    /// see `with`.
+    pub fn try_with<F, R>(&'static self, f: F) -> Result<R, AccessError>
+    where
+        F: FnOnce(&T) -> R,
+    {
+        self.inner.with(|v| {
+            if let Some(val) = v.borrow().as_ref() {
+                Ok(f(val))
+            } else {
+                Err(AccessError { _private: () })
+            }
+        })
+    }
+}
+
+impl<T: Copy + 'static> LocalKey<T> {
+    /// Returns a copy of the task-local value
+    /// if the task-local value implements `Copy`.
+    pub fn get(&'static self) -> T {
+        self.with(|v| *v)
+    }
+}
+
+impl<T: 'static> fmt::Debug for LocalKey<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad("LocalKey { .. }")
+    }
+}
+
+pin_project! {
+    struct TaskLocalFuture<T: StaticLifetime, F> {
+        local: &'static LocalKey<T>,
+        slot: Option<T>,
+        #[pin]
+        future: F,
+    }
+}
+
+impl<T: 'static, F: Future> Future for TaskLocalFuture<T, F> {
+    type Output = F::Output;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        struct Guard<'a, T: 'static> {
+            local: &'static LocalKey<T>,
+            slot: &'a mut Option<T>,
+            prev: Option<T>,
+        }
+
+        impl<T> Drop for Guard<'_, T> {
+            fn drop(&mut self) {
+                let value = self.local.inner.with(|c| c.replace(self.prev.take()));
+                *self.slot = value;
+            }
+        }
+
+        let mut project = self.project();
+        let val = project.slot.take();
+
+        let prev = project.local.inner.with(|c| c.replace(val));
+
+        let _guard = Guard {
+            prev,
+            slot: &mut project.slot,
+            local: *project.local,
+        };
+
+        project.future.poll(cx)
+    }
+}
+
+// Required to make `pin_project` happy.
+trait StaticLifetime: 'static {}
+impl<T: 'static> StaticLifetime for T {}
+
+/// An error returned by [`LocalKey::try_with`](method@LocalKey::try_with).
+#[derive(Clone, Copy, Eq, PartialEq)]
+pub struct AccessError {
+    _private: (),
+}
+
+impl fmt::Debug for AccessError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("AccessError").finish()
+    }
+}
+
+impl fmt::Display for AccessError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt("task-local value not set", f)
+    }
+}
+
+impl Error for AccessError {}
diff --git a/third_party/rust/tokio/src/task/yield_now.rs b/third_party/rust/tokio/src/task/yield_now.rs
new file mode 100644
index 0000000000..e0e20841c9
--- /dev/null
+++ b/third_party/rust/tokio/src/task/yield_now.rs
@@ -0,0 +1,38 @@
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+doc_rt_core! {
+    /// Yields execution back to the Tokio runtime.
+    ///
+    /// A task yields by awaiting on `yield_now()`, and may resume when that
+    /// future completes (with no output.) The current task will be re-added as
+    /// a pending task at the _back_ of the pending queue. Any other pending
+    /// tasks will be scheduled. No other waking is required for the task to
+    /// continue.
+    ///
+    /// See also the usage example in the [task module](index.html#yield_now).
+    #[must_use = "yield_now does nothing unless polled/`await`-ed"]
+    pub async fn yield_now() {
+        /// Yield implementation
+        struct YieldNow {
+            yielded: bool,
+        }
+
+        impl Future for YieldNow {
+            type Output = ();
+
+            fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
+                if self.yielded {
+                    return Poll::Ready(());
+                }
+
+                self.yielded = true;
+                cx.waker().wake_by_ref();
+                Poll::Pending
+            }
+        }
+
+        YieldNow { yielded: false }.await
+    }
+}
diff --git a/third_party/rust/tokio/src/time/clock.rs b/third_party/rust/tokio/src/time/clock.rs
new file mode 100644
index 0000000000..4ac24af3d0
--- /dev/null
+++ b/third_party/rust/tokio/src/time/clock.rs
@@ -0,0 +1,164 @@
+//! Source of time abstraction.
+//!
+//! By default, `std::time::Instant::now()` is used. However, when the
+//! `test-util` feature flag is enabled, the values returned for `now()` are
+//! configurable.
+
+cfg_not_test_util! {
+    use crate::time::{Duration, Instant};
+
+    #[derive(Debug, Clone)]
+    pub(crate) struct Clock {}
+
+    pub(crate) fn now() -> Instant {
+        Instant::from_std(std::time::Instant::now())
+    }
+
+    impl Clock {
+        pub(crate) fn new() -> Clock {
+            Clock {}
+        }
+
+        pub(crate) fn now(&self) -> Instant {
+            now()
+        }
+
+        pub(crate) fn is_paused(&self) -> bool {
+            false
+        }
+
+        pub(crate) fn advance(&self, _dur: Duration) {
+            unreachable!();
+        }
+    }
+}
+
+cfg_test_util! {
+    use crate::time::{Duration, Instant};
+    use std::sync::{Arc, Mutex};
+    use crate::runtime::context;
+
+    /// A handle to a source of time.
+    #[derive(Debug, Clone)]
+    pub(crate) struct Clock {
+        inner: Arc<Mutex<Inner>>,
+    }
+
+    #[derive(Debug)]
+    struct Inner {
+        /// Instant to use as the clock's base instant.
+        base: std::time::Instant,
+
+        /// Instant at which the clock was last unfrozen
+        unfrozen: Option<std::time::Instant>,
+    }
+
+    /// Pause time
+    ///
+    /// The current value of `Instant::now()` is saved and all subsequent calls
+    /// to `Instant::now()` will return the saved value. This is useful for
+    /// running tests that are dependent on time.
+    ///
+    /// # Panics
+    ///
+    /// Panics if time is already frozen or if called from outside of the Tokio
+    /// runtime.
+    pub fn pause() {
+        let clock = context::clock().expect("time cannot be frozen from outside the Tokio runtime");
+        clock.pause();
+    }
+
+    /// Resume time
+    ///
+    /// Clears the saved `Instant::now()` value. Subsequent calls to
+    /// `Instant::now()` will return the value returned by the system call.
+    ///
+    /// # Panics
+    ///
+    /// Panics if time is not frozen or if called from outside of the Tokio
+    /// runtime.
+    pub fn resume() {
+        let clock = context::clock().expect("time cannot be frozen from outside the Tokio runtime");
+        let mut inner = clock.inner.lock().unwrap();
+
+        if inner.unfrozen.is_some() {
+            panic!("time is not frozen");
+        }
+
+        inner.unfrozen = Some(std::time::Instant::now());
+    }
+
+    /// Advance time
+    ///
+    /// Increments the saved `Instant::now()` value by `duration`. Subsequent
+    /// calls to `Instant::now()` will return the result of the increment.
+    ///
+    /// # Panics
+    ///
+    /// Panics if time is not frozen or if called from outside of the Tokio
+    /// runtime.
+    pub async fn advance(duration: Duration) {
+        let clock = context::clock().expect("time cannot be frozen from outside the Tokio runtime");
+        clock.advance(duration);
+        crate::task::yield_now().await;
+    }
+
+    /// Return the current instant, factoring in frozen time.
+    pub(crate) fn now() -> Instant {
+        if let Some(clock) = context::clock() {
+            clock.now()
+        } else {
+            Instant::from_std(std::time::Instant::now())
+        }
+    }
+
+    impl Clock {
+        /// Return a new `Clock` instance that uses the current execution context's
+        /// source of time.
+        pub(crate) fn new() -> Clock {
+            let now = std::time::Instant::now();
+
+            Clock {
+                inner: Arc::new(Mutex::new(Inner {
+                    base: now,
+                    unfrozen: Some(now),
+                })),
+            }
+        }
+
+        pub(crate) fn pause(&self) {
+            let mut inner = self.inner.lock().unwrap();
+
+            let elapsed = inner.unfrozen.as_ref().expect("time is already frozen").elapsed();
+            inner.base += elapsed;
+            inner.unfrozen = None;
+        }
+
+        pub(crate) fn is_paused(&self) -> bool {
+            let inner = self.inner.lock().unwrap();
+            inner.unfrozen.is_none()
+        }
+
+        pub(crate) fn advance(&self, duration: Duration) {
+            let mut inner = self.inner.lock().unwrap();
+
+            if inner.unfrozen.is_some() {
+                panic!("time is not frozen");
+            }
+
+            inner.base += duration;
+        }
+
+        pub(crate) fn now(&self) -> Instant {
+            let inner = self.inner.lock().unwrap();
+
+            let mut ret = inner.base;
+
+            if let Some(unfrozen) = inner.unfrozen {
+                ret += unfrozen.elapsed();
+            }
+
+            Instant::from_std(ret)
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/time/delay.rs b/third_party/rust/tokio/src/time/delay.rs
new file mode 100644
index 0000000000..8088c9955c
--- /dev/null
+++ b/third_party/rust/tokio/src/time/delay.rs
@@ -0,0 +1,99 @@
+use crate::time::driver::Registration;
+use crate::time::{Duration, Instant};
+
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{self, Poll};
+
+/// Waits until `deadline` is reached.
+///
+/// No work is performed while awaiting on the delay to complete. The delay
+/// operates at millisecond granularity and should not be used for tasks that
+/// require high-resolution timers.
+///
+/// # Cancellation
+///
+/// Canceling a delay is done by dropping the returned future. No additional
+/// cleanup work is required.
+pub fn delay_until(deadline: Instant) -> Delay {
+    let registration = Registration::new(deadline, Duration::from_millis(0));
+    Delay { registration }
+}
+
+/// Waits until `duration` has elapsed.
+///
+/// Equivalent to `delay_until(Instant::now() + duration)`. An asynchronous
+/// analog to `std::thread::sleep`.
+///
+/// No work is performed while awaiting on the delay to complete. The delay
+/// operates at millisecond granularity and should not be used for tasks that
+/// require high-resolution timers.
+///
+/// # Cancellation
+///
+/// Canceling a delay is done by dropping the returned future. No additional
+/// cleanup work is required.
+pub fn delay_for(duration: Duration) -> Delay {
+    delay_until(Instant::now() + duration)
+}
+
+/// Future returned by [`delay_until`](delay_until) and
+/// [`delay_for`](delay_for).
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct Delay {
+    /// The link between the `Delay` instance and the timer that drives it.
+    ///
+    /// This also stores the `deadline` value.
+    registration: Registration,
+}
+
+impl Delay {
+    pub(crate) fn new_timeout(deadline: Instant, duration: Duration) -> Delay {
+        let registration = Registration::new(deadline, duration);
+        Delay { registration }
+    }
+
+    /// Returns the instant at which the future will complete.
+    pub fn deadline(&self) -> Instant {
+        self.registration.deadline()
+    }
+
+    /// Returns `true` if the `Delay` has elapsed
+    ///
+    /// A `Delay` is elapsed when the requested duration has elapsed.
+    pub fn is_elapsed(&self) -> bool {
+        self.registration.is_elapsed()
+    }
+
+    /// Resets the `Delay` instance to a new deadline.
+    ///
+    /// Calling this function allows changing the instant at which the `Delay`
+    /// future completes without having to create new associated state.
+    ///
+    /// This function can be called both before and after the future has
+    /// completed.
+    pub fn reset(&mut self, deadline: Instant) {
+        self.registration.reset(deadline);
+    }
+}
+
+impl Future for Delay {
+    type Output = ();
+
+    fn poll(self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
+        // `poll_elapsed` can return an error in two cases:
+        //
+        // - AtCapacity: this is a pathlogical case where far too many
+        //   delays have been scheduled.
+        // - Shutdown: No timer has been setup, which is a mis-use error.
+        //
+        // Both cases are extremely rare, and pretty accurately fit into
+        // "logic errors", so we just panic in this case. A user couldn't
+        // really do much better if we passed the error onwards.
+        match ready!(self.registration.poll_elapsed(cx)) {
+            Ok(()) => Poll::Ready(()),
+            Err(e) => panic!("timer error: {}", e),
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/time/delay_queue.rs b/third_party/rust/tokio/src/time/delay_queue.rs
new file mode 100644
index 0000000000..59f901a95d
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+++ b/third_party/rust/tokio/src/time/delay_queue.rs
@@ -0,0 +1,887 @@
+//! A queue of delayed elements.
+//!
+//! See [`DelayQueue`] for more details.
+//!
+//! [`DelayQueue`]: struct@DelayQueue
+
+use crate::time::wheel::{self, Wheel};
+use crate::time::{delay_until, Delay, Duration, Error, Instant};
+
+use slab::Slab;
+use std::cmp;
+use std::future::Future;
+use std::marker::PhantomData;
+use std::pin::Pin;
+use std::task::{self, Poll};
+
+/// A queue of delayed elements.
+///
+/// Once an element is inserted into the `DelayQueue`, it is yielded once the
+/// specified deadline has been reached.
+///
+/// # Usage
+///
+/// Elements are inserted into `DelayQueue` using the [`insert`] or
+/// [`insert_at`] methods. A deadline is provided with the item and a [`Key`] is
+/// returned. The key is used to remove the entry or to change the deadline at
+/// which it should be yielded back.
+///
+/// Once delays have been configured, the `DelayQueue` is used via its
+/// [`Stream`] implementation. [`poll`] is called. If an entry has reached its
+/// deadline, it is returned. If not, `Poll::Pending` indicating that the
+/// current task will be notified once the deadline has been reached.
+///
+/// # `Stream` implementation
+///
+/// Items are retrieved from the queue via [`Stream::poll`]. If no delays have
+/// expired, no items are returned. In this case, `NotReady` is returned and the
+/// current task is registered to be notified once the next item's delay has
+/// expired.
+///
+/// If no items are in the queue, i.e. `is_empty()` returns `true`, then `poll`
+/// returns `Ready(None)`. This indicates that the stream has reached an end.
+/// However, if a new item is inserted *after*, `poll` will once again start
+/// returning items or `NotReady.
+///
+/// Items are returned ordered by their expirations. Items that are configured
+/// to expire first will be returned first. There are no ordering guarantees
+/// for items configured to expire the same instant. Also note that delays are
+/// rounded to the closest millisecond.
+///
+/// # Implementation
+///
+/// The `DelayQueue` is backed by the same hashed timing wheel implementation as
+/// [`Timer`] as such, it offers the same performance benefits. See [`Timer`]
+/// for further implementation notes.
+///
+/// State associated with each entry is stored in a [`slab`]. This allows
+/// amortizing the cost of allocation. Space created for expired entries is
+/// reused when inserting new entries.
+///
+/// Capacity can be checked using [`capacity`] and allocated preemptively by using
+/// the [`reserve`] method.
+///
+/// # Usage
+///
+/// Using `DelayQueue` to manage cache entries.
+///
+/// ```rust,no_run
+/// use tokio::time::{delay_queue, DelayQueue, Error};
+///
+/// use futures::ready;
+/// use std::collections::HashMap;
+/// use std::task::{Context, Poll};
+/// use std::time::Duration;
+/// # type CacheKey = String;
+/// # type Value = String;
+///
+/// struct Cache {
+///     entries: HashMap<CacheKey, (Value, delay_queue::Key)>,
+///     expirations: DelayQueue<CacheKey>,
+/// }
+///
+/// const TTL_SECS: u64 = 30;
+///
+/// impl Cache {
+///     fn insert(&mut self, key: CacheKey, value: Value) {
+///         let delay = self.expirations
+///             .insert(key.clone(), Duration::from_secs(TTL_SECS));
+///
+///         self.entries.insert(key, (value, delay));
+///     }
+///
+///     fn get(&self, key: &CacheKey) -> Option<&Value> {
+///         self.entries.get(key)
+///             .map(|&(ref v, _)| v)
+///     }
+///
+///     fn remove(&mut self, key: &CacheKey) {
+///         if let Some((_, cache_key)) = self.entries.remove(key) {
+///             self.expirations.remove(&cache_key);
+///         }
+///     }
+///
+///     fn poll_purge(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Error>> {
+///         while let Some(res) = ready!(self.expirations.poll_expired(cx)) {
+///             let entry = res?;
+///             self.entries.remove(entry.get_ref());
+///         }
+///
+///         Poll::Ready(Ok(()))
+///     }
+/// }
+/// ```
+///
+/// [`insert`]: #method.insert
+/// [`insert_at`]: #method.insert_at
+/// [`Key`]: struct@Key
+/// [`Stream`]: https://docs.rs/futures/0.1/futures/stream/trait.Stream.html
+/// [`poll`]: #method.poll
+/// [`Stream::poll`]: #method.poll
+/// [`Timer`]: ../struct.Timer.html
+/// [`slab`]: https://docs.rs/slab
+/// [`capacity`]: #method.capacity
+/// [`reserve`]: #method.reserve
+#[derive(Debug)]
+pub struct DelayQueue<T> {
+    /// Stores data associated with entries
+    slab: Slab<Data<T>>,
+
+    /// Lookup structure tracking all delays in the queue
+    wheel: Wheel<Stack<T>>,
+
+    /// Delays that were inserted when already expired. These cannot be stored
+    /// in the wheel
+    expired: Stack<T>,
+
+    /// Delay expiring when the *first* item in the queue expires
+    delay: Option<Delay>,
+
+    /// Wheel polling state
+    poll: wheel::Poll,
+
+    /// Instant at which the timer starts
+    start: Instant,
+}
+
+/// An entry in `DelayQueue` that has expired and removed.
+///
+/// Values are returned by [`DelayQueue::poll`].
+///
+/// [`DelayQueue::poll`]: method@DelayQueue::poll
+#[derive(Debug)]
+pub struct Expired<T> {
+    /// The data stored in the queue
+    data: T,
+
+    /// The expiration time
+    deadline: Instant,
+
+    /// The key associated with the entry
+    key: Key,
+}
+
+/// Token to a value stored in a `DelayQueue`.
+///
+/// Instances of `Key` are returned by [`DelayQueue::insert`]. See [`DelayQueue`]
+/// documentation for more details.
+///
+/// [`DelayQueue`]: struct@DelayQueue
+/// [`DelayQueue::insert`]: method@DelayQueue::insert
+#[derive(Debug, Clone)]
+pub struct Key {
+    index: usize,
+}
+
+#[derive(Debug)]
+struct Stack<T> {
+    /// Head of the stack
+    head: Option<usize>,
+    _p: PhantomData<fn() -> T>,
+}
+
+#[derive(Debug)]
+struct Data<T> {
+    /// The data being stored in the queue and will be returned at the requested
+    /// instant.
+    inner: T,
+
+    /// The instant at which the item is returned.
+    when: u64,
+
+    /// Set to true when stored in the `expired` queue
+    expired: bool,
+
+    /// Next entry in the stack
+    next: Option<usize>,
+
+    /// Previous entry in the stack
+    prev: Option<usize>,
+}
+
+/// Maximum number of entries the queue can handle
+const MAX_ENTRIES: usize = (1 << 30) - 1;
+
+impl<T> DelayQueue<T> {
+    /// Creates a new, empty, `DelayQueue`
+    ///
+    /// The queue will not allocate storage until items are inserted into it.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # use tokio::time::DelayQueue;
+    /// let delay_queue: DelayQueue<u32> = DelayQueue::new();
+    /// ```
+    pub fn new() -> DelayQueue<T> {
+        DelayQueue::with_capacity(0)
+    }
+
+    /// Creates a new, empty, `DelayQueue` with the specified capacity.
+    ///
+    /// The queue will be able to hold at least `capacity` elements without
+    /// reallocating. If `capacity` is 0, the queue will not allocate for
+    /// storage.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # use tokio::time::DelayQueue;
+    /// # use std::time::Duration;
+    ///
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///     let mut delay_queue = DelayQueue::with_capacity(10);
+    ///
+    ///     // These insertions are done without further allocation
+    ///     for i in 0..10 {
+    ///         delay_queue.insert(i, Duration::from_secs(i));
+    ///     }
+    ///
+    ///     // This will make the queue allocate additional storage
+    ///     delay_queue.insert(11, Duration::from_secs(11));
+    /// # }
+    /// ```
+    pub fn with_capacity(capacity: usize) -> DelayQueue<T> {
+        DelayQueue {
+            wheel: Wheel::new(),
+            slab: Slab::with_capacity(capacity),
+            expired: Stack::default(),
+            delay: None,
+            poll: wheel::Poll::new(0),
+            start: Instant::now(),
+        }
+    }
+
+    /// Inserts `value` into the queue set to expire at a specific instant in
+    /// time.
+    ///
+    /// This function is identical to `insert`, but takes an `Instant` instead
+    /// of a `Duration`.
+    ///
+    /// `value` is stored in the queue until `when` is reached. At which point,
+    /// `value` will be returned from [`poll`]. If `when` has already been
+    /// reached, then `value` is immediately made available to poll.
+    ///
+    /// The return value represents the insertion and is used at an argument to
+    /// [`remove`] and [`reset`]. Note that [`Key`] is token and is reused once
+    /// `value` is removed from the queue either by calling [`poll`] after
+    /// `when` is reached or by calling [`remove`]. At this point, the caller
+    /// must take care to not use the returned [`Key`] again as it may reference
+    /// a different item in the queue.
+    ///
+    /// See [type] level documentation for more details.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if `when` is too far in the future.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage
+    ///
+    /// ```rust
+    /// use tokio::time::{DelayQueue, Duration, Instant};
+    ///
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///     let mut delay_queue = DelayQueue::new();
+    ///     let key = delay_queue.insert_at(
+    ///         "foo", Instant::now() + Duration::from_secs(5));
+    ///
+    ///     // Remove the entry
+    ///     let item = delay_queue.remove(&key);
+    ///     assert_eq!(*item.get_ref(), "foo");
+    /// # }
+    /// ```
+    ///
+    /// [`poll`]: #method.poll
+    /// [`remove`]: #method.remove
+    /// [`reset`]: #method.reset
+    /// [`Key`]: struct@Key
+    /// [type]: #
+    pub fn insert_at(&mut self, value: T, when: Instant) -> Key {
+        assert!(self.slab.len() < MAX_ENTRIES, "max entries exceeded");
+
+        // Normalize the deadline. Values cannot be set to expire in the past.
+        let when = self.normalize_deadline(when);
+
+        // Insert the value in the store
+        let key = self.slab.insert(Data {
+            inner: value,
+            when,
+            expired: false,
+            next: None,
+            prev: None,
+        });
+
+        self.insert_idx(when, key);
+
+        // Set a new delay if the current's deadline is later than the one of the new item
+        let should_set_delay = if let Some(ref delay) = self.delay {
+            let current_exp = self.normalize_deadline(delay.deadline());
+            current_exp > when
+        } else {
+            true
+        };
+
+        if should_set_delay {
+            let delay_time = self.start + Duration::from_millis(when);
+            if let Some(ref mut delay) = &mut self.delay {
+                delay.reset(delay_time);
+            } else {
+                self.delay = Some(delay_until(delay_time));
+            }
+        }
+
+        Key::new(key)
+    }
+
+    /// Attempts to pull out the next value of the delay queue, registering the
+    /// current task for wakeup if the value is not yet available, and returning
+    /// None if the queue is exhausted.
+    pub fn poll_expired(
+        &mut self,
+        cx: &mut task::Context<'_>,
+    ) -> Poll<Option<Result<Expired<T>, Error>>> {
+        let item = ready!(self.poll_idx(cx));
+        Poll::Ready(item.map(|result| {
+            result.map(|idx| {
+                let data = self.slab.remove(idx);
+                debug_assert!(data.next.is_none());
+                debug_assert!(data.prev.is_none());
+
+                Expired {
+                    key: Key::new(idx),
+                    data: data.inner,
+                    deadline: self.start + Duration::from_millis(data.when),
+                }
+            })
+        }))
+    }
+
+    /// Inserts `value` into the queue set to expire after the requested duration
+    /// elapses.
+    ///
+    /// This function is identical to `insert_at`, but takes a `Duration`
+    /// instead of an `Instant`.
+    ///
+    /// `value` is stored in the queue until `when` is reached. At which point,
+    /// `value` will be returned from [`poll`]. If `when` has already been
+    /// reached, then `value` is immediately made available to poll.
+    ///
+    /// The return value represents the insertion and is used at an argument to
+    /// [`remove`] and [`reset`]. Note that [`Key`] is token and is reused once
+    /// `value` is removed from the queue either by calling [`poll`] after
+    /// `when` is reached or by calling [`remove`]. At this point, the caller
+    /// must take care to not use the returned [`Key`] again as it may reference
+    /// a different item in the queue.
+    ///
+    /// See [type] level documentation for more details.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if `timeout` is greater than the maximum supported
+    /// duration.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage
+    ///
+    /// ```rust
+    /// use tokio::time::DelayQueue;
+    /// use std::time::Duration;
+    ///
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///     let mut delay_queue = DelayQueue::new();
+    ///     let key = delay_queue.insert("foo", Duration::from_secs(5));
+    ///
+    ///     // Remove the entry
+    ///     let item = delay_queue.remove(&key);
+    ///     assert_eq!(*item.get_ref(), "foo");
+    /// # }
+    /// ```
+    ///
+    /// [`poll`]: #method.poll
+    /// [`remove`]: #method.remove
+    /// [`reset`]: #method.reset
+    /// [`Key`]: struct@Key
+    /// [type]: #
+    pub fn insert(&mut self, value: T, timeout: Duration) -> Key {
+        self.insert_at(value, Instant::now() + timeout)
+    }
+
+    fn insert_idx(&mut self, when: u64, key: usize) {
+        use self::wheel::{InsertError, Stack};
+
+        // Register the deadline with the timer wheel
+        match self.wheel.insert(when, key, &mut self.slab) {
+            Ok(_) => {}
+            Err((_, InsertError::Elapsed)) => {
+                self.slab[key].expired = true;
+                // The delay is already expired, store it in the expired queue
+                self.expired.push(key, &mut self.slab);
+            }
+            Err((_, err)) => panic!("invalid deadline; err={:?}", err),
+        }
+    }
+
+    /// Removes the item associated with `key` from the queue.
+    ///
+    /// There must be an item associated with `key`. The function returns the
+    /// removed item as well as the `Instant` at which it will the delay will
+    /// have expired.
+    ///
+    /// # Panics
+    ///
+    /// The function panics if `key` is not contained by the queue.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage
+    ///
+    /// ```rust
+    /// use tokio::time::DelayQueue;
+    /// use std::time::Duration;
+    ///
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///     let mut delay_queue = DelayQueue::new();
+    ///     let key = delay_queue.insert("foo", Duration::from_secs(5));
+    ///
+    ///     // Remove the entry
+    ///     let item = delay_queue.remove(&key);
+    ///     assert_eq!(*item.get_ref(), "foo");
+    /// # }
+    /// ```
+    pub fn remove(&mut self, key: &Key) -> Expired<T> {
+        use crate::time::wheel::Stack;
+
+        // Special case the `expired` queue
+        if self.slab[key.index].expired {
+            self.expired.remove(&key.index, &mut self.slab);
+        } else {
+            self.wheel.remove(&key.index, &mut self.slab);
+        }
+
+        let data = self.slab.remove(key.index);
+
+        Expired {
+            key: Key::new(key.index),
+            data: data.inner,
+            deadline: self.start + Duration::from_millis(data.when),
+        }
+    }
+
+    /// Sets the delay of the item associated with `key` to expire at `when`.
+    ///
+    /// This function is identical to `reset` but takes an `Instant` instead of
+    /// a `Duration`.
+    ///
+    /// The item remains in the queue but the delay is set to expire at `when`.
+    /// If `when` is in the past, then the item is immediately made available to
+    /// the caller.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if `when` is too far in the future or if `key` is
+    /// not contained by the queue.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage
+    ///
+    /// ```rust
+    /// use tokio::time::{DelayQueue, Duration, Instant};
+    ///
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///     let mut delay_queue = DelayQueue::new();
+    ///     let key = delay_queue.insert("foo", Duration::from_secs(5));
+    ///
+    ///     // "foo" is scheduled to be returned in 5 seconds
+    ///
+    ///     delay_queue.reset_at(&key, Instant::now() + Duration::from_secs(10));
+    ///
+    ///     // "foo"is now scheduled to be returned in 10 seconds
+    /// # }
+    /// ```
+    pub fn reset_at(&mut self, key: &Key, when: Instant) {
+        self.wheel.remove(&key.index, &mut self.slab);
+
+        // Normalize the deadline. Values cannot be set to expire in the past.
+        let when = self.normalize_deadline(when);
+
+        self.slab[key.index].when = when;
+        self.insert_idx(when, key.index);
+
+        let next_deadline = self.next_deadline();
+        if let (Some(ref mut delay), Some(deadline)) = (&mut self.delay, next_deadline) {
+            delay.reset(deadline);
+        }
+    }
+
+    /// Returns the next time poll as determined by the wheel
+    fn next_deadline(&mut self) -> Option<Instant> {
+        self.wheel
+            .poll_at()
+            .map(|poll_at| self.start + Duration::from_millis(poll_at))
+    }
+
+    /// Sets the delay of the item associated with `key` to expire after
+    /// `timeout`.
+    ///
+    /// This function is identical to `reset_at` but takes a `Duration` instead
+    /// of an `Instant`.
+    ///
+    /// The item remains in the queue but the delay is set to expire after
+    /// `timeout`. If `timeout` is zero, then the item is immediately made
+    /// available to the caller.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if `timeout` is greater than the maximum supported
+    /// duration or if `key` is not contained by the queue.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage
+    ///
+    /// ```rust
+    /// use tokio::time::DelayQueue;
+    /// use std::time::Duration;
+    ///
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///     let mut delay_queue = DelayQueue::new();
+    ///     let key = delay_queue.insert("foo", Duration::from_secs(5));
+    ///
+    ///     // "foo" is scheduled to be returned in 5 seconds
+    ///
+    ///     delay_queue.reset(&key, Duration::from_secs(10));
+    ///
+    ///     // "foo"is now scheduled to be returned in 10 seconds
+    /// # }
+    /// ```
+    pub fn reset(&mut self, key: &Key, timeout: Duration) {
+        self.reset_at(key, Instant::now() + timeout);
+    }
+
+    /// Clears the queue, removing all items.
+    ///
+    /// After calling `clear`, [`poll`] will return `Ok(Ready(None))`.
+    ///
+    /// Note that this method has no effect on the allocated capacity.
+    ///
+    /// [`poll`]: #method.poll
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use tokio::time::DelayQueue;
+    /// use std::time::Duration;
+    ///
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///     let mut delay_queue = DelayQueue::new();
+    ///
+    ///     delay_queue.insert("foo", Duration::from_secs(5));
+    ///
+    ///     assert!(!delay_queue.is_empty());
+    ///
+    ///     delay_queue.clear();
+    ///
+    ///     assert!(delay_queue.is_empty());
+    /// # }
+    /// ```
+    pub fn clear(&mut self) {
+        self.slab.clear();
+        self.expired = Stack::default();
+        self.wheel = Wheel::new();
+        self.delay = None;
+    }
+
+    /// Returns the number of elements the queue can hold without reallocating.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use tokio::time::DelayQueue;
+    ///
+    /// let delay_queue: DelayQueue<i32> = DelayQueue::with_capacity(10);
+    /// assert_eq!(delay_queue.capacity(), 10);
+    /// ```
+    pub fn capacity(&self) -> usize {
+        self.slab.capacity()
+    }
+
+    /// Returns the number of elements currently in the queue.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use tokio::time::DelayQueue;
+    /// use std::time::Duration;
+    ///
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///     let mut delay_queue: DelayQueue<i32> = DelayQueue::with_capacity(10);
+    ///     assert_eq!(delay_queue.len(), 0);
+    ///     delay_queue.insert(3, Duration::from_secs(5));
+    ///     assert_eq!(delay_queue.len(), 1);
+    /// # }
+    /// ```
+    pub fn len(&self) -> usize {
+        self.slab.len()
+    }
+
+    /// Reserves capacity for at least `additional` more items to be queued
+    /// without allocating.
+    ///
+    /// `reserve` does nothing if the queue already has sufficient capacity for
+    /// `additional` more values. If more capacity is required, a new segment of
+    /// memory will be allocated and all existing values will be copied into it.
+    /// As such, if the queue is already very large, a call to `reserve` can end
+    /// up being expensive.
+    ///
+    /// The queue may reserve more than `additional` extra space in order to
+    /// avoid frequent reallocations.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the new capacity exceeds the maximum number of entries the
+    /// queue can contain.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::time::DelayQueue;
+    /// use std::time::Duration;
+    ///
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///     let mut delay_queue = DelayQueue::new();
+    ///
+    ///     delay_queue.insert("hello", Duration::from_secs(10));
+    ///     delay_queue.reserve(10);
+    ///
+    ///     assert!(delay_queue.capacity() >= 11);
+    /// # }
+    /// ```
+    pub fn reserve(&mut self, additional: usize) {
+        self.slab.reserve(additional);
+    }
+
+    /// Returns `true` if there are no items in the queue.
+    ///
+    /// Note that this function returns `false` even if all items have not yet
+    /// expired and a call to `poll` will return `NotReady`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::time::DelayQueue;
+    /// use std::time::Duration;
+    ///
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///     let mut delay_queue = DelayQueue::new();
+    ///     assert!(delay_queue.is_empty());
+    ///
+    ///     delay_queue.insert("hello", Duration::from_secs(5));
+    ///     assert!(!delay_queue.is_empty());
+    /// # }
+    /// ```
+    pub fn is_empty(&self) -> bool {
+        self.slab.is_empty()
+    }
+
+    /// Polls the queue, returning the index of the next slot in the slab that
+    /// should be returned.
+    ///
+    /// A slot should be returned when the associated deadline has been reached.
+    fn poll_idx(&mut self, cx: &mut task::Context<'_>) -> Poll<Option<Result<usize, Error>>> {
+        use self::wheel::Stack;
+
+        let expired = self.expired.pop(&mut self.slab);
+
+        if expired.is_some() {
+            return Poll::Ready(expired.map(Ok));
+        }
+
+        loop {
+            if let Some(ref mut delay) = self.delay {
+                if !delay.is_elapsed() {
+                    ready!(Pin::new(&mut *delay).poll(cx));
+                }
+
+                let now = crate::time::ms(delay.deadline() - self.start, crate::time::Round::Down);
+
+                self.poll = wheel::Poll::new(now);
+            }
+
+            // We poll the wheel to get the next value out before finding the next deadline.
+            let wheel_idx = self.wheel.poll(&mut self.poll, &mut self.slab);
+
+            self.delay = self.next_deadline().map(delay_until);
+
+            if let Some(idx) = wheel_idx {
+                return Poll::Ready(Some(Ok(idx)));
+            }
+
+            if self.delay.is_none() {
+                return Poll::Ready(None);
+            }
+        }
+    }
+
+    fn normalize_deadline(&self, when: Instant) -> u64 {
+        let when = if when < self.start {
+            0
+        } else {
+            crate::time::ms(when - self.start, crate::time::Round::Up)
+        };
+
+        cmp::max(when, self.wheel.elapsed())
+    }
+}
+
+// We never put `T` in a `Pin`...
+impl<T> Unpin for DelayQueue<T> {}
+
+impl<T> Default for DelayQueue<T> {
+    fn default() -> DelayQueue<T> {
+        DelayQueue::new()
+    }
+}
+
+#[cfg(feature = "stream")]
+impl<T> futures_core::Stream for DelayQueue<T> {
+    // DelayQueue seems much more specific, where a user may care that it
+    // has reached capacity, so return those errors instead of panicking.
+    type Item = Result<Expired<T>, Error>;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Option<Self::Item>> {
+        DelayQueue::poll_expired(self.get_mut(), cx)
+    }
+}
+
+impl<T> wheel::Stack for Stack<T> {
+    type Owned = usize;
+    type Borrowed = usize;
+    type Store = Slab<Data<T>>;
+
+    fn is_empty(&self) -> bool {
+        self.head.is_none()
+    }
+
+    fn push(&mut self, item: Self::Owned, store: &mut Self::Store) {
+        // Ensure the entry is not already in a stack.
+        debug_assert!(store[item].next.is_none());
+        debug_assert!(store[item].prev.is_none());
+
+        // Remove the old head entry
+        let old = self.head.take();
+
+        if let Some(idx) = old {
+            store[idx].prev = Some(item);
+        }
+
+        store[item].next = old;
+        self.head = Some(item)
+    }
+
+    fn pop(&mut self, store: &mut Self::Store) -> Option<Self::Owned> {
+        if let Some(idx) = self.head {
+            self.head = store[idx].next;
+
+            if let Some(idx) = self.head {
+                store[idx].prev = None;
+            }
+
+            store[idx].next = None;
+            debug_assert!(store[idx].prev.is_none());
+
+            Some(idx)
+        } else {
+            None
+        }
+    }
+
+    fn remove(&mut self, item: &Self::Borrowed, store: &mut Self::Store) {
+        assert!(store.contains(*item));
+
+        // Ensure that the entry is in fact contained by the stack
+        debug_assert!({
+            // This walks the full linked list even if an entry is found.
+            let mut next = self.head;
+            let mut contains = false;
+
+            while let Some(idx) = next {
+                if idx == *item {
+                    debug_assert!(!contains);
+                    contains = true;
+                }
+
+                next = store[idx].next;
+            }
+
+            contains
+        });
+
+        if let Some(next) = store[*item].next {
+            store[next].prev = store[*item].prev;
+        }
+
+        if let Some(prev) = store[*item].prev {
+            store[prev].next = store[*item].next;
+        } else {
+            self.head = store[*item].next;
+        }
+
+        store[*item].next = None;
+        store[*item].prev = None;
+    }
+
+    fn when(item: &Self::Borrowed, store: &Self::Store) -> u64 {
+        store[*item].when
+    }
+}
+
+impl<T> Default for Stack<T> {
+    fn default() -> Stack<T> {
+        Stack {
+            head: None,
+            _p: PhantomData,
+        }
+    }
+}
+
+impl Key {
+    pub(crate) fn new(index: usize) -> Key {
+        Key { index }
+    }
+}
+
+impl<T> Expired<T> {
+    /// Returns a reference to the inner value.
+    pub fn get_ref(&self) -> &T {
+        &self.data
+    }
+
+    /// Returns a mutable reference to the inner value.
+    pub fn get_mut(&mut self) -> &mut T {
+        &mut self.data
+    }
+
+    /// Consumes `self` and returns the inner value.
+    pub fn into_inner(self) -> T {
+        self.data
+    }
+
+    /// Returns the deadline that the expiration was set to.
+    pub fn deadline(&self) -> Instant {
+        self.deadline
+    }
+}
diff --git a/third_party/rust/tokio/src/time/driver/atomic_stack.rs b/third_party/rust/tokio/src/time/driver/atomic_stack.rs
new file mode 100644
index 0000000000..7e5a83fa52
--- /dev/null
+++ b/third_party/rust/tokio/src/time/driver/atomic_stack.rs
@@ -0,0 +1,124 @@
+use crate::time::driver::Entry;
+use crate::time::Error;
+
+use std::ptr;
+use std::sync::atomic::AtomicPtr;
+use std::sync::atomic::Ordering::SeqCst;
+use std::sync::Arc;
+
+/// A stack of `Entry` nodes
+#[derive(Debug)]
+pub(crate) struct AtomicStack {
+    /// Stack head
+    head: AtomicPtr<Entry>,
+}
+
+/// Entries that were removed from the stack
+#[derive(Debug)]
+pub(crate) struct AtomicStackEntries {
+    ptr: *mut Entry,
+}
+
+/// Used to indicate that the timer has shutdown.
+const SHUTDOWN: *mut Entry = 1 as *mut _;
+
+impl AtomicStack {
+    pub(crate) fn new() -> AtomicStack {
+        AtomicStack {
+            head: AtomicPtr::new(ptr::null_mut()),
+        }
+    }
+
+    /// Pushes an entry onto the stack.
+    ///
+    /// Returns `true` if the entry was pushed, `false` if the entry is already
+    /// on the stack, `Err` if the timer is shutdown.
+    pub(crate) fn push(&self, entry: &Arc<Entry>) -> Result<bool, Error> {
+        // First, set the queued bit on the entry
+        let queued = entry.queued.fetch_or(true, SeqCst);
+
+        if queued {
+            // Already queued, nothing more to do
+            return Ok(false);
+        }
+
+        let ptr = Arc::into_raw(entry.clone()) as *mut _;
+
+        let mut curr = self.head.load(SeqCst);
+
+        loop {
+            if curr == SHUTDOWN {
+                // Don't leak the entry node
+                let _ = unsafe { Arc::from_raw(ptr) };
+
+                return Err(Error::shutdown());
+            }
+
+            // Update the `next` pointer. This is safe because setting the queued
+            // bit is a "lock" on this field.
+            unsafe {
+                *(entry.next_atomic.get()) = curr;
+            }
+
+            let actual = self.head.compare_and_swap(curr, ptr, SeqCst);
+
+            if actual == curr {
+                break;
+            }
+
+            curr = actual;
+        }
+
+        Ok(true)
+    }
+
+    /// Takes all entries from the stack
+    pub(crate) fn take(&self) -> AtomicStackEntries {
+        let ptr = self.head.swap(ptr::null_mut(), SeqCst);
+        AtomicStackEntries { ptr }
+    }
+
+    /// Drains all remaining nodes in the stack and prevent any new nodes from
+    /// being pushed onto the stack.
+    pub(crate) fn shutdown(&self) {
+        // Shutdown the processing queue
+        let ptr = self.head.swap(SHUTDOWN, SeqCst);
+
+        // Let the drop fn of `AtomicStackEntries` handle draining the stack
+        drop(AtomicStackEntries { ptr });
+    }
+}
+
+// ===== impl AtomicStackEntries =====
+
+impl Iterator for AtomicStackEntries {
+    type Item = Arc<Entry>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.ptr.is_null() {
+            return None;
+        }
+
+        // Convert the pointer to an `Arc<Entry>`
+        let entry = unsafe { Arc::from_raw(self.ptr) };
+
+        // Update `self.ptr` to point to the next element of the stack
+        self.ptr = unsafe { *entry.next_atomic.get() };
+
+        // Unset the queued flag
+        let res = entry.queued.fetch_and(false, SeqCst);
+        debug_assert!(res);
+
+        // Return the entry
+        Some(entry)
+    }
+}
+
+impl Drop for AtomicStackEntries {
+    fn drop(&mut self) {
+        for entry in self {
+            // Flag the entry as errored
+            entry.error();
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/time/driver/entry.rs b/third_party/rust/tokio/src/time/driver/entry.rs
new file mode 100644
index 0000000000..20cc824019
--- /dev/null
+++ b/third_party/rust/tokio/src/time/driver/entry.rs
@@ -0,0 +1,345 @@
+use crate::loom::sync::atomic::AtomicU64;
+use crate::sync::AtomicWaker;
+use crate::time::driver::{Handle, Inner};
+use crate::time::{Duration, Error, Instant};
+
+use std::cell::UnsafeCell;
+use std::ptr;
+use std::sync::atomic::AtomicBool;
+use std::sync::atomic::Ordering::SeqCst;
+use std::sync::{Arc, Weak};
+use std::task::{self, Poll};
+use std::u64;
+
+/// Internal state shared between a `Delay` instance and the timer.
+///
+/// This struct is used as a node in two intrusive data structures:
+///
+/// * An atomic stack used to signal to the timer thread that the entry state
+///   has changed. The timer thread will observe the entry on this stack and
+///   perform any actions as necessary.
+///
+/// * A doubly linked list used **only** by the timer thread. Each slot in the
+///   timer wheel is a head pointer to the list of entries that must be
+///   processed during that timer tick.
+#[derive(Debug)]
+pub(crate) struct Entry {
+    /// Only accessed from `Registration`.
+    time: CachePadded<UnsafeCell<Time>>,
+
+    /// Timer internals. Using a weak pointer allows the timer to shutdown
+    /// without all `Delay` instances having completed.
+    ///
+    /// When `None`, the entry has not yet been linked with a timer instance.
+    inner: Weak<Inner>,
+
+    /// Tracks the entry state. This value contains the following information:
+    ///
+    /// * The deadline at which the entry must be "fired".
+    /// * A flag indicating if the entry has already been fired.
+    /// * Whether or not the entry transitioned to the error state.
+    ///
+    /// When an `Entry` is created, `state` is initialized to the instant at
+    /// which the entry must be fired. When a timer is reset to a different
+    /// instant, this value is changed.
+    state: AtomicU64,
+
+    /// Task to notify once the deadline is reached.
+    waker: AtomicWaker,
+
+    /// True when the entry is queued in the "process" stack. This value
+    /// is set before pushing the value and unset after popping the value.
+    ///
+    /// TODO: This could possibly be rolled up into `state`.
+    pub(super) queued: AtomicBool,
+
+    /// Next entry in the "process" linked list.
+    ///
+    /// Access to this field is coordinated by the `queued` flag.
+    ///
+    /// Represents a strong Arc ref.
+    pub(super) next_atomic: UnsafeCell<*mut Entry>,
+
+    /// When the entry expires, relative to the `start` of the timer
+    /// (Inner::start). This is only used by the timer.
+    ///
+    /// A `Delay` instance can be reset to a different deadline by the thread
+    /// that owns the `Delay` instance. In this case, the timer thread will not
+    /// immediately know that this has happened. The timer thread must know the
+    /// last deadline that it saw as it uses this value to locate the entry in
+    /// its wheel.
+    ///
+    /// Once the timer thread observes that the instant has changed, it updates
+    /// the wheel and sets this value. The idea is that this value eventually
+    /// converges to the value of `state` as the timer thread makes updates.
+    when: UnsafeCell<Option<u64>>,
+
+    /// Next entry in the State's linked list.
+    ///
+    /// This is only accessed by the timer
+    pub(super) next_stack: UnsafeCell<Option<Arc<Entry>>>,
+
+    /// Previous entry in the State's linked list.
+    ///
+    /// This is only accessed by the timer and is used to unlink a canceled
+    /// entry.
+    ///
+    /// This is a weak reference.
+    pub(super) prev_stack: UnsafeCell<*const Entry>,
+}
+
+/// Stores the info for `Delay`.
+#[derive(Debug)]
+pub(crate) struct Time {
+    pub(crate) deadline: Instant,
+    pub(crate) duration: Duration,
+}
+
+/// Flag indicating a timer entry has elapsed
+const ELAPSED: u64 = 1 << 63;
+
+/// Flag indicating a timer entry has reached an error state
+const ERROR: u64 = u64::MAX;
+
+// ===== impl Entry =====
+
+impl Entry {
+    pub(crate) fn new(handle: &Handle, deadline: Instant, duration: Duration) -> Arc<Entry> {
+        let inner = handle.inner().unwrap();
+        let entry: Entry;
+
+        // Increment the number of active timeouts
+        if inner.increment().is_err() {
+            entry = Entry::new2(deadline, duration, Weak::new(), ERROR)
+        } else {
+            let when = inner.normalize_deadline(deadline);
+            let state = if when <= inner.elapsed() {
+                ELAPSED
+            } else {
+                when
+            };
+            entry = Entry::new2(deadline, duration, Arc::downgrade(&inner), state);
+        }
+
+        let entry = Arc::new(entry);
+        if inner.queue(&entry).is_err() {
+            entry.error();
+        }
+
+        entry
+    }
+
+    /// Only called by `Registration`
+    pub(crate) fn time_ref(&self) -> &Time {
+        unsafe { &*self.time.0.get() }
+    }
+
+    /// Only called by `Registration`
+    #[allow(clippy::mut_from_ref)] // https://github.com/rust-lang/rust-clippy/issues/4281
+    pub(crate) unsafe fn time_mut(&self) -> &mut Time {
+        &mut *self.time.0.get()
+    }
+
+    /// The current entry state as known by the timer. This is not the value of
+    /// `state`, but lets the timer know how to converge its state to `state`.
+    pub(crate) fn when_internal(&self) -> Option<u64> {
+        unsafe { *self.when.get() }
+    }
+
+    pub(crate) fn set_when_internal(&self, when: Option<u64>) {
+        unsafe {
+            *self.when.get() = when;
+        }
+    }
+
+    /// Called by `Timer` to load the current value of `state` for processing
+    pub(crate) fn load_state(&self) -> Option<u64> {
+        let state = self.state.load(SeqCst);
+
+        if is_elapsed(state) {
+            None
+        } else {
+            Some(state)
+        }
+    }
+
+    pub(crate) fn is_elapsed(&self) -> bool {
+        let state = self.state.load(SeqCst);
+        is_elapsed(state)
+    }
+
+    pub(crate) fn fire(&self, when: u64) {
+        let mut curr = self.state.load(SeqCst);
+
+        loop {
+            if is_elapsed(curr) || curr > when {
+                return;
+            }
+
+            let next = ELAPSED | curr;
+            let actual = self.state.compare_and_swap(curr, next, SeqCst);
+
+            if curr == actual {
+                break;
+            }
+
+            curr = actual;
+        }
+
+        self.waker.wake();
+    }
+
+    pub(crate) fn error(&self) {
+        // Only transition to the error state if not currently elapsed
+        let mut curr = self.state.load(SeqCst);
+
+        loop {
+            if is_elapsed(curr) {
+                return;
+            }
+
+            let next = ERROR;
+
+            let actual = self.state.compare_and_swap(curr, next, SeqCst);
+
+            if curr == actual {
+                break;
+            }
+
+            curr = actual;
+        }
+
+        self.waker.wake();
+    }
+
+    pub(crate) fn cancel(entry: &Arc<Entry>) {
+        let state = entry.state.fetch_or(ELAPSED, SeqCst);
+
+        if is_elapsed(state) {
+            // Nothing more to do
+            return;
+        }
+
+        // If registered with a timer instance, try to upgrade the Arc.
+        let inner = match entry.upgrade_inner() {
+            Some(inner) => inner,
+            None => return,
+        };
+
+        let _ = inner.queue(entry);
+    }
+
+    pub(crate) fn poll_elapsed(&self, cx: &mut task::Context<'_>) -> Poll<Result<(), Error>> {
+        let mut curr = self.state.load(SeqCst);
+
+        if is_elapsed(curr) {
+            return Poll::Ready(if curr == ERROR {
+                Err(Error::shutdown())
+            } else {
+                Ok(())
+            });
+        }
+
+        self.waker.register_by_ref(cx.waker());
+
+        curr = self.state.load(SeqCst);
+
+        if is_elapsed(curr) {
+            return Poll::Ready(if curr == ERROR {
+                Err(Error::shutdown())
+            } else {
+                Ok(())
+            });
+        }
+
+        Poll::Pending
+    }
+
+    /// Only called by `Registration`
+    pub(crate) fn reset(entry: &mut Arc<Entry>) {
+        let inner = match entry.upgrade_inner() {
+            Some(inner) => inner,
+            None => return,
+        };
+
+        let deadline = entry.time_ref().deadline;
+        let when = inner.normalize_deadline(deadline);
+        let elapsed = inner.elapsed();
+
+        let mut curr = entry.state.load(SeqCst);
+        let mut notify;
+
+        loop {
+            // In these two cases, there is no work to do when resetting the
+            // timer. If the `Entry` is in an error state, then it cannot be
+            // used anymore. If resetting the entry to the current value, then
+            // the reset is a noop.
+            if curr == ERROR || curr == when {
+                return;
+            }
+
+            let next;
+
+            if when <= elapsed {
+                next = ELAPSED;
+                notify = !is_elapsed(curr);
+            } else {
+                next = when;
+                notify = true;
+            }
+
+            let actual = entry.state.compare_and_swap(curr, next, SeqCst);
+
+            if curr == actual {
+                break;
+            }
+
+            curr = actual;
+        }
+
+        if notify {
+            let _ = inner.queue(entry);
+        }
+    }
+
+    fn new2(deadline: Instant, duration: Duration, inner: Weak<Inner>, state: u64) -> Self {
+        Self {
+            time: CachePadded(UnsafeCell::new(Time { deadline, duration })),
+            inner,
+            waker: AtomicWaker::new(),
+            state: AtomicU64::new(state),
+            queued: AtomicBool::new(false),
+            next_atomic: UnsafeCell::new(ptr::null_mut()),
+            when: UnsafeCell::new(None),
+            next_stack: UnsafeCell::new(None),
+            prev_stack: UnsafeCell::new(ptr::null_mut()),
+        }
+    }
+
+    fn upgrade_inner(&self) -> Option<Arc<Inner>> {
+        self.inner.upgrade()
+    }
+}
+
+fn is_elapsed(state: u64) -> bool {
+    state & ELAPSED == ELAPSED
+}
+
+impl Drop for Entry {
+    fn drop(&mut self) {
+        let inner = match self.upgrade_inner() {
+            Some(inner) => inner,
+            None => return,
+        };
+
+        inner.decrement();
+    }
+}
+
+unsafe impl Send for Entry {}
+unsafe impl Sync for Entry {}
+
+#[cfg_attr(target_arch = "x86_64", repr(align(128)))]
+#[cfg_attr(not(target_arch = "x86_64"), repr(align(64)))]
+#[derive(Debug)]
+struct CachePadded<T>(T);
diff --git a/third_party/rust/tokio/src/time/driver/handle.rs b/third_party/rust/tokio/src/time/driver/handle.rs
new file mode 100644
index 0000000000..38b1761c8e
--- /dev/null
+++ b/third_party/rust/tokio/src/time/driver/handle.rs
@@ -0,0 +1,38 @@
+use crate::runtime::context;
+use crate::time::driver::Inner;
+use std::fmt;
+use std::sync::{Arc, Weak};
+
+/// Handle to time driver instance.
+#[derive(Clone)]
+pub(crate) struct Handle {
+    inner: Weak<Inner>,
+}
+
+impl Handle {
+    /// Creates a new timer `Handle` from a shared `Inner` timer state.
+    pub(crate) fn new(inner: Weak<Inner>) -> Self {
+        Handle { inner }
+    }
+
+    /// Tries to get a handle to the current timer.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if there is no current timer set.
+    pub(crate) fn current() -> Self {
+        context::time_handle()
+            .expect("there is no timer running, must be called from the context of Tokio runtime")
+    }
+
+    /// Tries to return a strong ref to the inner
+    pub(crate) fn inner(&self) -> Option<Arc<Inner>> {
+        self.inner.upgrade()
+    }
+}
+
+impl fmt::Debug for Handle {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "Handle")
+    }
+}
diff --git a/third_party/rust/tokio/src/time/driver/mod.rs b/third_party/rust/tokio/src/time/driver/mod.rs
new file mode 100644
index 0000000000..4616816f3f
--- /dev/null
+++ b/third_party/rust/tokio/src/time/driver/mod.rs
@@ -0,0 +1,391 @@
+//! Time driver
+
+mod atomic_stack;
+use self::atomic_stack::AtomicStack;
+
+mod entry;
+pub(super) use self::entry::Entry;
+
+mod handle;
+pub(crate) use self::handle::Handle;
+
+mod registration;
+pub(crate) use self::registration::Registration;
+
+mod stack;
+use self::stack::Stack;
+
+use crate::loom::sync::atomic::{AtomicU64, AtomicUsize};
+use crate::park::{Park, Unpark};
+use crate::time::{wheel, Error};
+use crate::time::{Clock, Duration, Instant};
+
+use std::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
+
+use std::sync::Arc;
+use std::usize;
+use std::{cmp, fmt};
+
+/// Time implementation that drives [`Delay`], [`Interval`], and [`Timeout`].
+///
+/// A `Driver` instance tracks the state necessary for managing time and
+/// notifying the [`Delay`] instances once their deadlines are reached.
+///
+/// It is expected that a single instance manages many individual [`Delay`]
+/// instances. The `Driver` implementation is thread-safe and, as such, is able
+/// to handle callers from across threads.
+///
+/// After creating the `Driver` instance, the caller must repeatedly call
+/// [`turn`]. The time driver will perform no work unless [`turn`] is called
+/// repeatedly.
+///
+/// The driver has a resolution of one millisecond. Any unit of time that falls
+/// between milliseconds are rounded up to the next millisecond.
+///
+/// When an instance is dropped, any outstanding [`Delay`] instance that has not
+/// elapsed will be notified with an error. At this point, calling `poll` on the
+/// [`Delay`] instance will result in `Err` being returned.
+///
+/// # Implementation
+///
+/// THe time driver is based on the [paper by Varghese and Lauck][paper].
+///
+/// A hashed timing wheel is a vector of slots, where each slot handles a time
+/// slice. As time progresses, the timer walks over the slot for the current
+/// instant, and processes each entry for that slot. When the timer reaches the
+/// end of the wheel, it starts again at the beginning.
+///
+/// The implementation maintains six wheels arranged in a set of levels. As the
+/// levels go up, the slots of the associated wheel represent larger intervals
+/// of time. At each level, the wheel has 64 slots. Each slot covers a range of
+/// time equal to the wheel at the lower level. At level zero, each slot
+/// represents one millisecond of time.
+///
+/// The wheels are:
+///
+/// * Level 0: 64 x 1 millisecond slots.
+/// * Level 1: 64 x 64 millisecond slots.
+/// * Level 2: 64 x ~4 second slots.
+/// * Level 3: 64 x ~4 minute slots.
+/// * Level 4: 64 x ~4 hour slots.
+/// * Level 5: 64 x ~12 day slots.
+///
+/// When the timer processes entries at level zero, it will notify all the
+/// `Delay` instances as their deadlines have been reached. For all higher
+/// levels, all entries will be redistributed across the wheel at the next level
+/// down. Eventually, as time progresses, entries will [`Delay`] instances will
+/// either be canceled (dropped) or their associated entries will reach level
+/// zero and be notified.
+#[derive(Debug)]
+pub(crate) struct Driver<T> {
+    /// Shared state
+    inner: Arc<Inner>,
+
+    /// Timer wheel
+    wheel: wheel::Wheel<Stack>,
+
+    /// Thread parker. The `Driver` park implementation delegates to this.
+    park: T,
+
+    /// Source of "now" instances
+    clock: Clock,
+}
+
+/// Timer state shared between `Driver`, `Handle`, and `Registration`.
+pub(crate) struct Inner {
+    /// The instant at which the timer started running.
+    start: Instant,
+
+    /// The last published timer `elapsed` value.
+    elapsed: AtomicU64,
+
+    /// Number of active timeouts
+    num: AtomicUsize,
+
+    /// Head of the "process" linked list.
+    process: AtomicStack,
+
+    /// Unparks the timer thread.
+    unpark: Box<dyn Unpark>,
+}
+
+/// Maximum number of timeouts the system can handle concurrently.
+const MAX_TIMEOUTS: usize = usize::MAX >> 1;
+
+// ===== impl Driver =====
+
+impl<T> Driver<T>
+where
+    T: Park,
+{
+    /// Creates a new `Driver` instance that uses `park` to block the current
+    /// thread and `now` to get the current `Instant`.
+    ///
+    /// Specifying the source of time is useful when testing.
+    pub(crate) fn new(park: T, clock: Clock) -> Driver<T> {
+        let unpark = Box::new(park.unpark());
+
+        Driver {
+            inner: Arc::new(Inner::new(clock.now(), unpark)),
+            wheel: wheel::Wheel::new(),
+            park,
+            clock,
+        }
+    }
+
+    /// Returns a handle to the timer.
+    ///
+    /// The `Handle` is how `Delay` instances are created. The `Delay` instances
+    /// can either be created directly or the `Handle` instance can be passed to
+    /// `with_default`, setting the timer as the default timer for the execution
+    /// context.
+    pub(crate) fn handle(&self) -> Handle {
+        Handle::new(Arc::downgrade(&self.inner))
+    }
+
+    /// Converts an `Expiration` to an `Instant`.
+    fn expiration_instant(&self, when: u64) -> Instant {
+        self.inner.start + Duration::from_millis(when)
+    }
+
+    /// Runs timer related logic
+    fn process(&mut self) {
+        let now = crate::time::ms(
+            self.clock.now() - self.inner.start,
+            crate::time::Round::Down,
+        );
+        let mut poll = wheel::Poll::new(now);
+
+        while let Some(entry) = self.wheel.poll(&mut poll, &mut ()) {
+            let when = entry.when_internal().expect("invalid internal entry state");
+
+            // Fire the entry
+            entry.fire(when);
+
+            // Track that the entry has been fired
+            entry.set_when_internal(None);
+        }
+
+        // Update the elapsed cache
+        self.inner.elapsed.store(self.wheel.elapsed(), SeqCst);
+    }
+
+    /// Processes the entry queue
+    ///
+    /// This handles adding and canceling timeouts.
+    fn process_queue(&mut self) {
+        for entry in self.inner.process.take() {
+            match (entry.when_internal(), entry.load_state()) {
+                (None, None) => {
+                    // Nothing to do
+                }
+                (Some(_), None) => {
+                    // Remove the entry
+                    self.clear_entry(&entry);
+                }
+                (None, Some(when)) => {
+                    // Queue the entry
+                    self.add_entry(entry, when);
+                }
+                (Some(_), Some(next)) => {
+                    self.clear_entry(&entry);
+                    self.add_entry(entry, next);
+                }
+            }
+        }
+    }
+
+    fn clear_entry(&mut self, entry: &Arc<Entry>) {
+        self.wheel.remove(entry, &mut ());
+        entry.set_when_internal(None);
+    }
+
+    /// Fires the entry if it needs to, otherwise queue it to be processed later.
+    ///
+    /// Returns `None` if the entry was fired.
+    fn add_entry(&mut self, entry: Arc<Entry>, when: u64) {
+        use crate::time::wheel::InsertError;
+
+        entry.set_when_internal(Some(when));
+
+        match self.wheel.insert(when, entry, &mut ()) {
+            Ok(_) => {}
+            Err((entry, InsertError::Elapsed)) => {
+                // The entry's deadline has elapsed, so fire it and update the
+                // internal state accordingly.
+                entry.set_when_internal(None);
+                entry.fire(when);
+            }
+            Err((entry, InsertError::Invalid)) => {
+                // The entry's deadline is invalid, so error it and update the
+                // internal state accordingly.
+                entry.set_when_internal(None);
+                entry.error();
+            }
+        }
+    }
+}
+
+impl<T> Park for Driver<T>
+where
+    T: Park,
+{
+    type Unpark = T::Unpark;
+    type Error = T::Error;
+
+    fn unpark(&self) -> Self::Unpark {
+        self.park.unpark()
+    }
+
+    fn park(&mut self) -> Result<(), Self::Error> {
+        self.process_queue();
+
+        match self.wheel.poll_at() {
+            Some(when) => {
+                let now = self.clock.now();
+                let deadline = self.expiration_instant(when);
+
+                if deadline > now {
+                    let dur = deadline - now;
+
+                    if self.clock.is_paused() {
+                        self.park.park_timeout(Duration::from_secs(0))?;
+                        self.clock.advance(dur);
+                    } else {
+                        self.park.park_timeout(dur)?;
+                    }
+                } else {
+                    self.park.park_timeout(Duration::from_secs(0))?;
+                }
+            }
+            None => {
+                self.park.park()?;
+            }
+        }
+
+        self.process();
+
+        Ok(())
+    }
+
+    fn park_timeout(&mut self, duration: Duration) -> Result<(), Self::Error> {
+        self.process_queue();
+
+        match self.wheel.poll_at() {
+            Some(when) => {
+                let now = self.clock.now();
+                let deadline = self.expiration_instant(when);
+
+                if deadline > now {
+                    let duration = cmp::min(deadline - now, duration);
+
+                    if self.clock.is_paused() {
+                        self.park.park_timeout(Duration::from_secs(0))?;
+                        self.clock.advance(duration);
+                    } else {
+                        self.park.park_timeout(duration)?;
+                    }
+                } else {
+                    self.park.park_timeout(Duration::from_secs(0))?;
+                }
+            }
+            None => {
+                self.park.park_timeout(duration)?;
+            }
+        }
+
+        self.process();
+
+        Ok(())
+    }
+}
+
+impl<T> Drop for Driver<T> {
+    fn drop(&mut self) {
+        use std::u64;
+
+        // Shutdown the stack of entries to process, preventing any new entries
+        // from being pushed.
+        self.inner.process.shutdown();
+
+        // Clear the wheel, using u64::MAX allows us to drain everything
+        let mut poll = wheel::Poll::new(u64::MAX);
+
+        while let Some(entry) = self.wheel.poll(&mut poll, &mut ()) {
+            entry.error();
+        }
+    }
+}
+
+// ===== impl Inner =====
+
+impl Inner {
+    fn new(start: Instant, unpark: Box<dyn Unpark>) -> Inner {
+        Inner {
+            num: AtomicUsize::new(0),
+            elapsed: AtomicU64::new(0),
+            process: AtomicStack::new(),
+            start,
+            unpark,
+        }
+    }
+
+    fn elapsed(&self) -> u64 {
+        self.elapsed.load(SeqCst)
+    }
+
+    #[cfg(all(test, loom))]
+    fn num(&self, ordering: std::sync::atomic::Ordering) -> usize {
+        self.num.load(ordering)
+    }
+
+    /// Increments the number of active timeouts
+    fn increment(&self) -> Result<(), Error> {
+        let mut curr = self.num.load(Relaxed);
+        loop {
+            if curr == MAX_TIMEOUTS {
+                return Err(Error::at_capacity());
+            }
+
+            match self
+                .num
+                .compare_exchange_weak(curr, curr + 1, Release, Relaxed)
+            {
+                Ok(_) => return Ok(()),
+                Err(next) => curr = next,
+            }
+        }
+    }
+
+    /// Decrements the number of active timeouts
+    fn decrement(&self) {
+        let prev = self.num.fetch_sub(1, Acquire);
+        debug_assert!(prev <= MAX_TIMEOUTS);
+    }
+
+    fn queue(&self, entry: &Arc<Entry>) -> Result<(), Error> {
+        if self.process.push(entry)? {
+            // The timer is notified so that it can process the timeout
+            self.unpark.unpark();
+        }
+
+        Ok(())
+    }
+
+    fn normalize_deadline(&self, deadline: Instant) -> u64 {
+        if deadline < self.start {
+            return 0;
+        }
+
+        crate::time::ms(deadline - self.start, crate::time::Round::Up)
+    }
+}
+
+impl fmt::Debug for Inner {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("Inner").finish()
+    }
+}
+
+#[cfg(all(test, loom))]
+mod tests;
diff --git a/third_party/rust/tokio/src/time/driver/registration.rs b/third_party/rust/tokio/src/time/driver/registration.rs
new file mode 100644
index 0000000000..b77357e735
--- /dev/null
+++ b/third_party/rust/tokio/src/time/driver/registration.rs
@@ -0,0 +1,53 @@
+use crate::time::driver::{Entry, Handle};
+use crate::time::{Duration, Error, Instant};
+
+use std::sync::Arc;
+use std::task::{self, Poll};
+
+/// Registration with a timer.
+///
+/// The association between a `Delay` instance and a timer is done lazily in
+/// `poll`
+#[derive(Debug)]
+pub(crate) struct Registration {
+    entry: Arc<Entry>,
+}
+
+impl Registration {
+    pub(crate) fn new(deadline: Instant, duration: Duration) -> Registration {
+        let handle = Handle::current();
+
+        Registration {
+            entry: Entry::new(&handle, deadline, duration),
+        }
+    }
+
+    pub(crate) fn deadline(&self) -> Instant {
+        self.entry.time_ref().deadline
+    }
+
+    pub(crate) fn reset(&mut self, deadline: Instant) {
+        unsafe {
+            self.entry.time_mut().deadline = deadline;
+        }
+
+        Entry::reset(&mut self.entry);
+    }
+
+    pub(crate) fn is_elapsed(&self) -> bool {
+        self.entry.is_elapsed()
+    }
+
+    pub(crate) fn poll_elapsed(&self, cx: &mut task::Context<'_>) -> Poll<Result<(), Error>> {
+        // Keep track of task budget
+        ready!(crate::coop::poll_proceed(cx));
+
+        self.entry.poll_elapsed(cx)
+    }
+}
+
+impl Drop for Registration {
+    fn drop(&mut self) {
+        Entry::cancel(&self.entry);
+    }
+}
diff --git a/third_party/rust/tokio/src/time/driver/stack.rs b/third_party/rust/tokio/src/time/driver/stack.rs
new file mode 100644
index 0000000000..3e2924f265
--- /dev/null
+++ b/third_party/rust/tokio/src/time/driver/stack.rs
@@ -0,0 +1,121 @@
+use crate::time::driver::Entry;
+use crate::time::wheel;
+
+use std::ptr;
+use std::sync::Arc;
+
+/// A doubly linked stack
+#[derive(Debug)]
+pub(crate) struct Stack {
+    head: Option<Arc<Entry>>,
+}
+
+impl Default for Stack {
+    fn default() -> Stack {
+        Stack { head: None }
+    }
+}
+
+impl wheel::Stack for Stack {
+    type Owned = Arc<Entry>;
+    type Borrowed = Entry;
+    type Store = ();
+
+    fn is_empty(&self) -> bool {
+        self.head.is_none()
+    }
+
+    fn push(&mut self, entry: Self::Owned, _: &mut Self::Store) {
+        // Get a pointer to the entry to for the prev link
+        let ptr: *const Entry = &*entry as *const _;
+
+        // Remove the old head entry
+        let old = self.head.take();
+
+        unsafe {
+            // Ensure the entry is not already in a stack.
+            debug_assert!((*entry.next_stack.get()).is_none());
+            debug_assert!((*entry.prev_stack.get()).is_null());
+
+            if let Some(ref entry) = old.as_ref() {
+                debug_assert!({
+                    // The head is not already set to the entry
+                    ptr != &***entry as *const _
+                });
+
+                // Set the previous link on the old head
+                *entry.prev_stack.get() = ptr;
+            }
+
+            // Set this entry's next pointer
+            *entry.next_stack.get() = old;
+        }
+
+        // Update the head pointer
+        self.head = Some(entry);
+    }
+
+    /// Pops an item from the stack
+    fn pop(&mut self, _: &mut ()) -> Option<Arc<Entry>> {
+        let entry = self.head.take();
+
+        unsafe {
+            if let Some(entry) = entry.as_ref() {
+                self.head = (*entry.next_stack.get()).take();
+
+                if let Some(entry) = self.head.as_ref() {
+                    *entry.prev_stack.get() = ptr::null();
+                }
+
+                *entry.prev_stack.get() = ptr::null();
+            }
+        }
+
+        entry
+    }
+
+    fn remove(&mut self, entry: &Entry, _: &mut ()) {
+        unsafe {
+            // Ensure that the entry is in fact contained by the stack
+            debug_assert!({
+                // This walks the full linked list even if an entry is found.
+                let mut next = self.head.as_ref();
+                let mut contains = false;
+
+                while let Some(n) = next {
+                    if entry as *const _ == &**n as *const _ {
+                        debug_assert!(!contains);
+                        contains = true;
+                    }
+
+                    next = (*n.next_stack.get()).as_ref();
+                }
+
+                contains
+            });
+
+            // Unlink `entry` from the next node
+            let next = (*entry.next_stack.get()).take();
+
+            if let Some(next) = next.as_ref() {
+                (*next.prev_stack.get()) = *entry.prev_stack.get();
+            }
+
+            // Unlink `entry` from the prev node
+
+            if let Some(prev) = (*entry.prev_stack.get()).as_ref() {
+                *prev.next_stack.get() = next;
+            } else {
+                // It is the head
+                self.head = next;
+            }
+
+            // Unset the prev pointer
+            *entry.prev_stack.get() = ptr::null();
+        }
+    }
+
+    fn when(item: &Entry, _: &()) -> u64 {
+        item.when_internal().expect("invalid internal state")
+    }
+}
diff --git a/third_party/rust/tokio/src/time/driver/tests/mod.rs b/third_party/rust/tokio/src/time/driver/tests/mod.rs
new file mode 100644
index 0000000000..88ff5525da
--- /dev/null
+++ b/third_party/rust/tokio/src/time/driver/tests/mod.rs
@@ -0,0 +1,55 @@
+use crate::park::Unpark;
+use crate::time::driver::Inner;
+use crate::time::Instant;
+
+use loom::thread;
+
+use std::sync::atomic::Ordering;
+use std::sync::Arc;
+
+struct MockUnpark;
+
+impl Unpark for MockUnpark {
+    fn unpark(&self) {}
+}
+
+#[test]
+fn balanced_incr_and_decr() {
+    const OPS: usize = 5;
+
+    fn incr(inner: Arc<Inner>) {
+        for _ in 0..OPS {
+            inner.increment().expect("increment should not have failed");
+            thread::yield_now();
+        }
+    }
+
+    fn decr(inner: Arc<Inner>) {
+        let mut ops_performed = 0;
+        while ops_performed < OPS {
+            if inner.num(Ordering::Relaxed) > 0 {
+                ops_performed += 1;
+                inner.decrement();
+            }
+            thread::yield_now();
+        }
+    }
+
+    loom::model(|| {
+        let unpark = Box::new(MockUnpark);
+        let instant = Instant::now();
+
+        let inner = Arc::new(Inner::new(instant, unpark));
+
+        let incr_inner = inner.clone();
+        let decr_inner = inner.clone();
+
+        let incr_hndle = thread::spawn(move || incr(incr_inner));
+        let decr_hndle = thread::spawn(move || decr(decr_inner));
+
+        incr_hndle.join().expect("should never fail");
+        decr_hndle.join().expect("should never fail");
+
+        assert_eq!(inner.num(Ordering::SeqCst), 0);
+    })
+}
diff --git a/third_party/rust/tokio/src/time/error.rs b/third_party/rust/tokio/src/time/error.rs
new file mode 100644
index 0000000000..0667b97ac1
--- /dev/null
+++ b/third_party/rust/tokio/src/time/error.rs
@@ -0,0 +1,72 @@
+use self::Kind::*;
+use std::error;
+use std::fmt;
+
+/// Errors encountered by the timer implementation.
+///
+/// Currently, there are two different errors that can occur:
+///
+/// * `shutdown` occurs when a timer operation is attempted, but the timer
+///   instance has been dropped. In this case, the operation will never be able
+///   to complete and the `shutdown` error is returned. This is a permanent
+///   error, i.e., once this error is observed, timer operations will never
+///   succeed in the future.
+///
+/// * `at_capacity` occurs when a timer operation is attempted, but the timer
+///   instance is currently handling its maximum number of outstanding delays.
+///   In this case, the operation is not able to be performed at the current
+///   moment, and `at_capacity` is returned. This is a transient error, i.e., at
+///   some point in the future, if the operation is attempted again, it might
+///   succeed. Callers that observe this error should attempt to [shed load]. One
+///   way to do this would be dropping the future that issued the timer operation.
+///
+/// [shed load]: https://en.wikipedia.org/wiki/Load_Shedding
+#[derive(Debug)]
+pub struct Error(Kind);
+
+#[derive(Debug)]
+enum Kind {
+    Shutdown,
+    AtCapacity,
+}
+
+impl Error {
+    /// Creates an error representing a shutdown timer.
+    pub fn shutdown() -> Error {
+        Error(Shutdown)
+    }
+
+    /// Returns `true` if the error was caused by the timer being shutdown.
+    pub fn is_shutdown(&self) -> bool {
+        match self.0 {
+            Kind::Shutdown => true,
+            _ => false,
+        }
+    }
+
+    /// Creates an error representing a timer at capacity.
+    pub fn at_capacity() -> Error {
+        Error(AtCapacity)
+    }
+
+    /// Returns `true` if the error was caused by the timer being at capacity.
+    pub fn is_at_capacity(&self) -> bool {
+        match self.0 {
+            Kind::AtCapacity => true,
+            _ => false,
+        }
+    }
+}
+
+impl error::Error for Error {}
+
+impl fmt::Display for Error {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        use self::Kind::*;
+        let descr = match self.0 {
+            Shutdown => "the timer is shutdown, must be called from the context of Tokio runtime",
+            AtCapacity => "timer is at capacity and cannot create a new entry",
+        };
+        write!(fmt, "{}", descr)
+    }
+}
diff --git a/third_party/rust/tokio/src/time/instant.rs b/third_party/rust/tokio/src/time/instant.rs
new file mode 100644
index 0000000000..f2cb4bc97d
--- /dev/null
+++ b/third_party/rust/tokio/src/time/instant.rs
@@ -0,0 +1,199 @@
+#![allow(clippy::trivially_copy_pass_by_ref)]
+
+use std::fmt;
+use std::ops;
+use std::time::Duration;
+
+/// A measurement of the system clock, useful for talking to
+/// external entities like the file system or other processes.
+#[derive(Clone, Copy, Eq, PartialEq, PartialOrd, Ord, Hash)]
+pub struct Instant {
+    std: std::time::Instant,
+}
+
+impl Instant {
+    /// Returns an instant corresponding to "now".
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::time::Instant;
+    ///
+    /// let now = Instant::now();
+    /// ```
+    pub fn now() -> Instant {
+        variant::now()
+    }
+
+    /// Create a `tokio::time::Instant` from a `std::time::Instant`.
+    pub fn from_std(std: std::time::Instant) -> Instant {
+        Instant { std }
+    }
+
+    /// Convert the value into a `std::time::Instant`.
+    pub fn into_std(self) -> std::time::Instant {
+        self.std
+    }
+
+    /// Returns the amount of time elapsed from another instant to this one.
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if `earlier` is later than `self`.
+    pub fn duration_since(&self, earlier: Instant) -> Duration {
+        self.std.duration_since(earlier.std)
+    }
+
+    /// Returns the amount of time elapsed from another instant to this one, or
+    /// None if that instant is later than this one.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::time::{Duration, Instant, delay_for};
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let now = Instant::now();
+    ///     delay_for(Duration::new(1, 0)).await;
+    ///     let new_now = Instant::now();
+    ///     println!("{:?}", new_now.checked_duration_since(now));
+    ///     println!("{:?}", now.checked_duration_since(new_now)); // None
+    /// }
+    /// ```
+    pub fn checked_duration_since(&self, earlier: Instant) -> Option<Duration> {
+        self.std.checked_duration_since(earlier.std)
+    }
+
+    /// Returns the amount of time elapsed from another instant to this one, or
+    /// zero duration if that instant is earlier than this one.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::time::{Duration, Instant, delay_for};
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let now = Instant::now();
+    ///     delay_for(Duration::new(1, 0)).await;
+    ///     let new_now = Instant::now();
+    ///     println!("{:?}", new_now.saturating_duration_since(now));
+    ///     println!("{:?}", now.saturating_duration_since(new_now)); // 0ns
+    /// }
+    /// ```
+    pub fn saturating_duration_since(&self, earlier: Instant) -> Duration {
+        self.std.saturating_duration_since(earlier.std)
+    }
+
+    /// Returns the amount of time elapsed since this instant was created.
+    ///
+    /// # Panics
+    ///
+    /// This function may panic if the current time is earlier than this
+    /// instant, which is something that can happen if an `Instant` is
+    /// produced synthetically.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::time::{Duration, Instant, delay_for};
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let instant = Instant::now();
+    ///     let three_secs = Duration::from_secs(3);
+    ///     delay_for(three_secs).await;
+    ///     assert!(instant.elapsed() >= three_secs);
+    /// }
+    /// ```
+    pub fn elapsed(&self) -> Duration {
+        Instant::now() - *self
+    }
+
+    /// Returns `Some(t)` where `t` is the time `self + duration` if `t` can be
+    /// represented as `Instant` (which means it's inside the bounds of the
+    /// underlying data structure), `None` otherwise.
+    pub fn checked_add(&self, duration: Duration) -> Option<Instant> {
+        self.std.checked_add(duration).map(Instant::from_std)
+    }
+
+    /// Returns `Some(t)` where `t` is the time `self - duration` if `t` can be
+    /// represented as `Instant` (which means it's inside the bounds of the
+    /// underlying data structure), `None` otherwise.
+    pub fn checked_sub(&self, duration: Duration) -> Option<Instant> {
+        self.std.checked_sub(duration).map(Instant::from_std)
+    }
+}
+
+impl From<std::time::Instant> for Instant {
+    fn from(time: std::time::Instant) -> Instant {
+        Instant::from_std(time)
+    }
+}
+
+impl From<Instant> for std::time::Instant {
+    fn from(time: Instant) -> std::time::Instant {
+        time.into_std()
+    }
+}
+
+impl ops::Add<Duration> for Instant {
+    type Output = Instant;
+
+    fn add(self, other: Duration) -> Instant {
+        Instant::from_std(self.std + other)
+    }
+}
+
+impl ops::AddAssign<Duration> for Instant {
+    fn add_assign(&mut self, rhs: Duration) {
+        *self = *self + rhs;
+    }
+}
+
+impl ops::Sub for Instant {
+    type Output = Duration;
+
+    fn sub(self, rhs: Instant) -> Duration {
+        self.std - rhs.std
+    }
+}
+
+impl ops::Sub<Duration> for Instant {
+    type Output = Instant;
+
+    fn sub(self, rhs: Duration) -> Instant {
+        Instant::from_std(self.std - rhs)
+    }
+}
+
+impl ops::SubAssign<Duration> for Instant {
+    fn sub_assign(&mut self, rhs: Duration) {
+        *self = *self - rhs;
+    }
+}
+
+impl fmt::Debug for Instant {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.std.fmt(fmt)
+    }
+}
+
+#[cfg(not(feature = "test-util"))]
+mod variant {
+    use super::Instant;
+
+    pub(super) fn now() -> Instant {
+        Instant::from_std(std::time::Instant::now())
+    }
+}
+
+#[cfg(feature = "test-util")]
+mod variant {
+    use super::Instant;
+
+    pub(super) fn now() -> Instant {
+        crate::time::clock::now()
+    }
+}
diff --git a/third_party/rust/tokio/src/time/interval.rs b/third_party/rust/tokio/src/time/interval.rs
new file mode 100644
index 0000000000..090e2d1f05
--- /dev/null
+++ b/third_party/rust/tokio/src/time/interval.rs
@@ -0,0 +1,139 @@
+use crate::future::poll_fn;
+use crate::time::{delay_until, Delay, Duration, Instant};
+
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// Creates new `Interval` that yields with interval of `duration`. The first
+/// tick completes immediately.
+///
+/// An interval will tick indefinitely. At any time, the `Interval` value can be
+/// dropped. This cancels the interval.
+///
+/// This function is equivalent to `interval_at(Instant::now(), period)`.
+///
+/// # Panics
+///
+/// This function panics if `period` is zero.
+///
+/// # Examples
+///
+/// ```
+/// use tokio::time::{self, Duration};
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let mut interval = time::interval(Duration::from_millis(10));
+///
+///     interval.tick().await;
+///     interval.tick().await;
+///     interval.tick().await;
+///
+///     // approximately 20ms have elapsed.
+/// }
+/// ```
+pub fn interval(period: Duration) -> Interval {
+    assert!(period > Duration::new(0, 0), "`period` must be non-zero.");
+
+    interval_at(Instant::now(), period)
+}
+
+/// Creates new `Interval` that yields with interval of `period` with the
+/// first tick completing at `at`.
+///
+/// An interval will tick indefinitely. At any time, the `Interval` value can be
+/// dropped. This cancels the interval.
+///
+/// # Panics
+///
+/// This function panics if `period` is zero.
+///
+/// # Examples
+///
+/// ```
+/// use tokio::time::{interval_at, Duration, Instant};
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let start = Instant::now() + Duration::from_millis(50);
+///     let mut interval = interval_at(start, Duration::from_millis(10));
+///
+///     interval.tick().await;
+///     interval.tick().await;
+///     interval.tick().await;
+///
+///     // approximately 70ms have elapsed.
+/// }
+/// ```
+pub fn interval_at(start: Instant, period: Duration) -> Interval {
+    assert!(period > Duration::new(0, 0), "`period` must be non-zero.");
+
+    Interval {
+        delay: delay_until(start),
+        period,
+    }
+}
+
+/// Stream returned by [`interval`](interval) and [`interval_at`](interval_at).
+#[derive(Debug)]
+pub struct Interval {
+    /// Future that completes the next time the `Interval` yields a value.
+    delay: Delay,
+
+    /// The duration between values yielded by `Interval`.
+    period: Duration,
+}
+
+impl Interval {
+    #[doc(hidden)] // TODO: document
+    pub fn poll_tick(&mut self, cx: &mut Context<'_>) -> Poll<Instant> {
+        // Wait for the delay to be done
+        ready!(Pin::new(&mut self.delay).poll(cx));
+
+        // Get the `now` by looking at the `delay` deadline
+        let now = self.delay.deadline();
+
+        // The next interval value is `duration` after the one that just
+        // yielded.
+        let next = now + self.period;
+        self.delay.reset(next);
+
+        // Return the current instant
+        Poll::Ready(now)
+    }
+
+    /// Completes when the next instant in the interval has been reached.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::time;
+    ///
+    /// use std::time::Duration;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let mut interval = time::interval(Duration::from_millis(10));
+    ///
+    ///     interval.tick().await;
+    ///     interval.tick().await;
+    ///     interval.tick().await;
+    ///
+    ///     // approximately 20ms have elapsed.
+    /// }
+    /// ```
+    #[allow(clippy::should_implement_trait)] // TODO: rename (tokio-rs/tokio#1261)
+    pub async fn tick(&mut self) -> Instant {
+        poll_fn(|cx| self.poll_tick(cx)).await
+    }
+}
+
+#[cfg(feature = "stream")]
+impl crate::stream::Stream for Interval {
+    type Item = Instant;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Instant>> {
+        Poll::Ready(Some(ready!(self.poll_tick(cx))))
+    }
+}
diff --git a/third_party/rust/tokio/src/time/mod.rs b/third_party/rust/tokio/src/time/mod.rs
new file mode 100644
index 0000000000..7070d6b257
--- /dev/null
+++ b/third_party/rust/tokio/src/time/mod.rs
@@ -0,0 +1,130 @@
+//! Utilities for tracking time.
+//!
+//! This module provides a number of types for executing code after a set period
+//! of time.
+//!
+//! * `Delay` is a future that does no work and completes at a specific `Instant`
+//!   in time.
+//!
+//! * `Interval` is a stream yielding a value at a fixed period. It is
+//!   initialized with a `Duration` and repeatedly yields each time the duration
+//!   elapses.
+//!
+//! * `Timeout`: Wraps a future or stream, setting an upper bound to the amount
+//!   of time it is allowed to execute. If the future or stream does not
+//!   complete in time, then it is canceled and an error is returned.
+//!
+//! * `DelayQueue`: A queue where items are returned once the requested delay
+//!   has expired.
+//!
+//! These types are sufficient for handling a large number of scenarios
+//! involving time.
+//!
+//! These types must be used from within the context of the `Runtime`.
+//!
+//! # Examples
+//!
+//! Wait 100ms and print "Hello World!"
+//!
+//! ```
+//! use tokio::time::delay_for;
+//!
+//! use std::time::Duration;
+//!
+//!
+//! #[tokio::main]
+//! async fn main() {
+//!     delay_for(Duration::from_millis(100)).await;
+//!     println!("100 ms have elapsed");
+//! }
+//! ```
+//!
+//! Require that an operation takes no more than 300ms. Note that this uses the
+//! `timeout` function on the `FutureExt` trait. This trait is included in the
+//! prelude.
+//!
+//! ```
+//! use tokio::time::{timeout, Duration};
+//!
+//! async fn long_future() {
+//!     // do work here
+//! }
+//!
+//! # async fn dox() {
+//! let res = timeout(Duration::from_secs(1), long_future()).await;
+//!
+//! if res.is_err() {
+//!     println!("operation timed out");
+//! }
+//! # }
+//! ```
+
+mod clock;
+pub(crate) use self::clock::Clock;
+#[cfg(feature = "test-util")]
+pub use clock::{advance, pause, resume};
+
+pub mod delay_queue;
+#[doc(inline)]
+pub use delay_queue::DelayQueue;
+
+mod delay;
+pub use delay::{delay_for, delay_until, Delay};
+
+pub(crate) mod driver;
+
+mod error;
+pub use error::Error;
+
+mod instant;
+pub use self::instant::Instant;
+
+mod interval;
+pub use interval::{interval, interval_at, Interval};
+
+mod timeout;
+#[doc(inline)]
+pub use timeout::{timeout, timeout_at, Elapsed, Timeout};
+
+cfg_stream! {
+    mod throttle;
+    pub use throttle::{throttle, Throttle};
+}
+
+mod wheel;
+
+#[cfg(test)]
+#[cfg(not(loom))]
+mod tests;
+
+// Re-export for convenience
+pub use std::time::Duration;
+
+// ===== Internal utils =====
+
+enum Round {
+    Up,
+    Down,
+}
+
+/// Convert a `Duration` to milliseconds, rounding up and saturating at
+/// `u64::MAX`.
+///
+/// The saturating is fine because `u64::MAX` milliseconds are still many
+/// million years.
+#[inline]
+fn ms(duration: Duration, round: Round) -> u64 {
+    const NANOS_PER_MILLI: u32 = 1_000_000;
+    const MILLIS_PER_SEC: u64 = 1_000;
+
+    // Round up.
+    let millis = match round {
+        Round::Up => (duration.subsec_nanos() + NANOS_PER_MILLI - 1) / NANOS_PER_MILLI,
+        Round::Down => duration.subsec_millis(),
+    };
+
+    duration
+        .as_secs()
+        .saturating_mul(MILLIS_PER_SEC)
+        .saturating_add(u64::from(millis))
+}
diff --git a/third_party/rust/tokio/src/time/tests/mod.rs b/third_party/rust/tokio/src/time/tests/mod.rs
new file mode 100644
index 0000000000..4710d470f7
--- /dev/null
+++ b/third_party/rust/tokio/src/time/tests/mod.rs
@@ -0,0 +1,22 @@
+mod test_delay;
+
+use crate::time::{self, Instant};
+use std::time::Duration;
+
+fn assert_send<T: Send>() {}
+fn assert_sync<T: Sync>() {}
+
+#[test]
+fn registration_is_send_and_sync() {
+    use crate::time::driver::Registration;
+
+    assert_send::<Registration>();
+    assert_sync::<Registration>();
+}
+
+#[test]
+#[should_panic]
+fn delay_is_eager() {
+    let when = Instant::now() + Duration::from_millis(100);
+    let _ = time::delay_until(when);
+}
diff --git a/third_party/rust/tokio/src/time/tests/test_delay.rs b/third_party/rust/tokio/src/time/tests/test_delay.rs
new file mode 100644
index 0000000000..f843434be4
--- /dev/null
+++ b/third_party/rust/tokio/src/time/tests/test_delay.rs
@@ -0,0 +1,447 @@
+#![warn(rust_2018_idioms)]
+
+use crate::park::{Park, Unpark};
+use crate::time::driver::{Driver, Entry, Handle};
+use crate::time::Clock;
+use crate::time::{Duration, Instant};
+
+use tokio_test::task;
+use tokio_test::{assert_ok, assert_pending, assert_ready_ok};
+
+use std::sync::Arc;
+
+macro_rules! poll {
+    ($e:expr) => {
+        $e.enter(|cx, e| e.poll_elapsed(cx))
+    };
+}
+
+#[test]
+fn frozen_utility_returns_correct_advanced_duration() {
+    let clock = Clock::new();
+    clock.pause();
+    let start = clock.now();
+
+    clock.advance(ms(10));
+    assert_eq!(clock.now() - start, ms(10));
+}
+
+#[test]
+fn immediate_delay() {
+    let (mut driver, clock, handle) = setup();
+    let start = clock.now();
+
+    let when = clock.now();
+    let mut e = task::spawn(delay_until(&handle, when));
+
+    assert_ready_ok!(poll!(e));
+
+    assert_ok!(driver.park_timeout(Duration::from_millis(1000)));
+
+    // The time has not advanced. The `turn` completed immediately.
+    assert_eq!(clock.now() - start, ms(1000));
+}
+
+#[test]
+fn delayed_delay_level_0() {
+    let (mut driver, clock, handle) = setup();
+    let start = clock.now();
+
+    for &i in &[1, 10, 60] {
+        // Create a `Delay` that elapses in the future
+        let mut e = task::spawn(delay_until(&handle, start + ms(i)));
+
+        // The delay has not elapsed.
+        assert_pending!(poll!(e));
+
+        assert_ok!(driver.park());
+        assert_eq!(clock.now() - start, ms(i));
+
+        assert_ready_ok!(poll!(e));
+    }
+}
+
+#[test]
+fn sub_ms_delayed_delay() {
+    let (mut driver, clock, handle) = setup();
+
+    for _ in 0..5 {
+        let deadline = clock.now() + ms(1) + Duration::new(0, 1);
+
+        let mut e = task::spawn(delay_until(&handle, deadline));
+
+        assert_pending!(poll!(e));
+
+        assert_ok!(driver.park());
+        assert_ready_ok!(poll!(e));
+
+        assert!(clock.now() >= deadline);
+
+        clock.advance(Duration::new(0, 1));
+    }
+}
+
+#[test]
+fn delayed_delay_wrapping_level_0() {
+    let (mut driver, clock, handle) = setup();
+    let start = clock.now();
+
+    assert_ok!(driver.park_timeout(ms(5)));
+    assert_eq!(clock.now() - start, ms(5));
+
+    let mut e = task::spawn(delay_until(&handle, clock.now() + ms(60)));
+
+    assert_pending!(poll!(e));
+
+    assert_ok!(driver.park());
+    assert_eq!(clock.now() - start, ms(64));
+    assert_pending!(poll!(e));
+
+    assert_ok!(driver.park());
+    assert_eq!(clock.now() - start, ms(65));
+
+    assert_ready_ok!(poll!(e));
+}
+
+#[test]
+fn timer_wrapping_with_higher_levels() {
+    let (mut driver, clock, handle) = setup();
+    let start = clock.now();
+
+    // Set delay to hit level 1
+    let mut e1 = task::spawn(delay_until(&handle, clock.now() + ms(64)));
+    assert_pending!(poll!(e1));
+
+    // Turn a bit
+    assert_ok!(driver.park_timeout(ms(5)));
+
+    // Set timeout such that it will hit level 0, but wrap
+    let mut e2 = task::spawn(delay_until(&handle, clock.now() + ms(60)));
+    assert_pending!(poll!(e2));
+
+    // This should result in s1 firing
+    assert_ok!(driver.park());
+    assert_eq!(clock.now() - start, ms(64));
+
+    assert_ready_ok!(poll!(e1));
+    assert_pending!(poll!(e2));
+
+    assert_ok!(driver.park());
+    assert_eq!(clock.now() - start, ms(65));
+
+    assert_ready_ok!(poll!(e1));
+}
+
+#[test]
+fn delay_with_deadline_in_past() {
+    let (mut driver, clock, handle) = setup();
+    let start = clock.now();
+
+    // Create `Delay` that elapsed immediately.
+    let mut e = task::spawn(delay_until(&handle, clock.now() - ms(100)));
+
+    // Even though the delay expires in the past, it is not ready yet
+    // because the timer must observe it.
+    assert_ready_ok!(poll!(e));
+
+    // Turn the timer, it runs for the elapsed time
+    assert_ok!(driver.park_timeout(ms(1000)));
+
+    // The time has not advanced. The `turn` completed immediately.
+    assert_eq!(clock.now() - start, ms(1000));
+}
+
+#[test]
+fn delayed_delay_level_1() {
+    let (mut driver, clock, handle) = setup();
+    let start = clock.now();
+
+    // Create a `Delay` that elapses in the future
+    let mut e = task::spawn(delay_until(&handle, clock.now() + ms(234)));
+
+    // The delay has not elapsed.
+    assert_pending!(poll!(e));
+
+    // Turn the timer, this will wake up to cascade the timer down.
+    assert_ok!(driver.park_timeout(ms(1000)));
+    assert_eq!(clock.now() - start, ms(192));
+
+    // The delay has not elapsed.
+    assert_pending!(poll!(e));
+
+    // Turn the timer again
+    assert_ok!(driver.park_timeout(ms(1000)));
+    assert_eq!(clock.now() - start, ms(234));
+
+    // The delay has elapsed.
+    assert_ready_ok!(poll!(e));
+
+    let (mut driver, clock, handle) = setup();
+    let start = clock.now();
+
+    // Create a `Delay` that elapses in the future
+    let mut e = task::spawn(delay_until(&handle, clock.now() + ms(234)));
+
+    // The delay has not elapsed.
+    assert_pending!(poll!(e));
+
+    // Turn the timer with a smaller timeout than the cascade.
+    assert_ok!(driver.park_timeout(ms(100)));
+    assert_eq!(clock.now() - start, ms(100));
+
+    assert_pending!(poll!(e));
+
+    // Turn the timer, this will wake up to cascade the timer down.
+    assert_ok!(driver.park_timeout(ms(1000)));
+    assert_eq!(clock.now() - start, ms(192));
+
+    // The delay has not elapsed.
+    assert_pending!(poll!(e));
+
+    // Turn the timer again
+    assert_ok!(driver.park_timeout(ms(1000)));
+    assert_eq!(clock.now() - start, ms(234));
+
+    // The delay has elapsed.
+    assert_ready_ok!(poll!(e));
+}
+
+#[test]
+fn concurrently_set_two_timers_second_one_shorter() {
+    let (mut driver, clock, handle) = setup();
+    let start = clock.now();
+
+    let mut e1 = task::spawn(delay_until(&handle, clock.now() + ms(500)));
+    let mut e2 = task::spawn(delay_until(&handle, clock.now() + ms(200)));
+
+    // The delay has not elapsed
+    assert_pending!(poll!(e1));
+    assert_pending!(poll!(e2));
+
+    // Delay until a cascade
+    assert_ok!(driver.park());
+    assert_eq!(clock.now() - start, ms(192));
+
+    // Delay until the second timer.
+    assert_ok!(driver.park());
+    assert_eq!(clock.now() - start, ms(200));
+
+    // The shorter delay fires
+    assert_ready_ok!(poll!(e2));
+    assert_pending!(poll!(e1));
+
+    assert_ok!(driver.park());
+    assert_eq!(clock.now() - start, ms(448));
+
+    assert_pending!(poll!(e1));
+
+    // Turn again, this time the time will advance to the second delay
+    assert_ok!(driver.park());
+    assert_eq!(clock.now() - start, ms(500));
+
+    assert_ready_ok!(poll!(e1));
+}
+
+#[test]
+fn short_delay() {
+    let (mut driver, clock, handle) = setup();
+    let start = clock.now();
+
+    // Create a `Delay` that elapses in the future
+    let mut e = task::spawn(delay_until(&handle, clock.now() + ms(1)));
+
+    // The delay has not elapsed.
+    assert_pending!(poll!(e));
+
+    // Turn the timer, but not enough time will go by.
+    assert_ok!(driver.park());
+
+    // The delay has elapsed.
+    assert_ready_ok!(poll!(e));
+
+    // The time has advanced to the point of the delay elapsing.
+    assert_eq!(clock.now() - start, ms(1));
+}
+
+#[test]
+fn sorta_long_delay_until() {
+    const MIN_5: u64 = 5 * 60 * 1000;
+
+    let (mut driver, clock, handle) = setup();
+    let start = clock.now();
+
+    // Create a `Delay` that elapses in the future
+    let mut e = task::spawn(delay_until(&handle, clock.now() + ms(MIN_5)));
+
+    // The delay has not elapsed.
+    assert_pending!(poll!(e));
+
+    let cascades = &[262_144, 262_144 + 9 * 4096, 262_144 + 9 * 4096 + 15 * 64];
+
+    for &elapsed in cascades {
+        assert_ok!(driver.park());
+        assert_eq!(clock.now() - start, ms(elapsed));
+
+        assert_pending!(poll!(e));
+    }
+
+    assert_ok!(driver.park());
+    assert_eq!(clock.now() - start, ms(MIN_5));
+
+    // The delay has elapsed.
+    assert_ready_ok!(poll!(e));
+}
+
+#[test]
+fn very_long_delay() {
+    const MO_5: u64 = 5 * 30 * 24 * 60 * 60 * 1000;
+
+    let (mut driver, clock, handle) = setup();
+    let start = clock.now();
+
+    // Create a `Delay` that elapses in the future
+    let mut e = task::spawn(delay_until(&handle, clock.now() + ms(MO_5)));
+
+    // The delay has not elapsed.
+    assert_pending!(poll!(e));
+
+    let cascades = &[
+        12_884_901_888,
+        12_952_010_752,
+        12_959_875_072,
+        12_959_997_952,
+    ];
+
+    for &elapsed in cascades {
+        assert_ok!(driver.park());
+        assert_eq!(clock.now() - start, ms(elapsed));
+
+        assert_pending!(poll!(e));
+    }
+
+    // Turn the timer, but not enough time will go by.
+    assert_ok!(driver.park());
+
+    // The time has advanced to the point of the delay elapsing.
+    assert_eq!(clock.now() - start, ms(MO_5));
+
+    // The delay has elapsed.
+    assert_ready_ok!(poll!(e));
+}
+
+#[test]
+fn unpark_is_delayed() {
+    // A special park that will take much longer than the requested duration
+    struct MockPark(Clock);
+
+    struct MockUnpark;
+
+    impl Park for MockPark {
+        type Unpark = MockUnpark;
+        type Error = ();
+
+        fn unpark(&self) -> Self::Unpark {
+            MockUnpark
+        }
+
+        fn park(&mut self) -> Result<(), Self::Error> {
+            panic!("parking forever");
+        }
+
+        fn park_timeout(&mut self, duration: Duration) -> Result<(), Self::Error> {
+            assert_eq!(duration, ms(0));
+            self.0.advance(ms(436));
+            Ok(())
+        }
+    }
+
+    impl Unpark for MockUnpark {
+        fn unpark(&self) {}
+    }
+
+    let clock = Clock::new();
+    clock.pause();
+    let start = clock.now();
+    let mut driver = Driver::new(MockPark(clock.clone()), clock.clone());
+    let handle = driver.handle();
+
+    let mut e1 = task::spawn(delay_until(&handle, clock.now() + ms(100)));
+    let mut e2 = task::spawn(delay_until(&handle, clock.now() + ms(101)));
+    let mut e3 = task::spawn(delay_until(&handle, clock.now() + ms(200)));
+
+    assert_pending!(poll!(e1));
+    assert_pending!(poll!(e2));
+    assert_pending!(poll!(e3));
+
+    assert_ok!(driver.park());
+
+    assert_eq!(clock.now() - start, ms(500));
+
+    assert_ready_ok!(poll!(e1));
+    assert_ready_ok!(poll!(e2));
+    assert_ready_ok!(poll!(e3));
+}
+
+#[test]
+fn set_timeout_at_deadline_greater_than_max_timer() {
+    const YR_1: u64 = 365 * 24 * 60 * 60 * 1000;
+    const YR_5: u64 = 5 * YR_1;
+
+    let (mut driver, clock, handle) = setup();
+    let start = clock.now();
+
+    for _ in 0..5 {
+        assert_ok!(driver.park_timeout(ms(YR_1)));
+    }
+
+    let mut e = task::spawn(delay_until(&handle, clock.now() + ms(1)));
+    assert_pending!(poll!(e));
+
+    assert_ok!(driver.park_timeout(ms(1000)));
+    assert_eq!(clock.now() - start, ms(YR_5) + ms(1));
+
+    assert_ready_ok!(poll!(e));
+}
+
+fn setup() -> (Driver<MockPark>, Clock, Handle) {
+    let clock = Clock::new();
+    clock.pause();
+    let driver = Driver::new(MockPark(clock.clone()), clock.clone());
+    let handle = driver.handle();
+
+    (driver, clock, handle)
+}
+
+fn delay_until(handle: &Handle, when: Instant) -> Arc<Entry> {
+    Entry::new(&handle, when, ms(0))
+}
+
+struct MockPark(Clock);
+
+struct MockUnpark;
+
+impl Park for MockPark {
+    type Unpark = MockUnpark;
+    type Error = ();
+
+    fn unpark(&self) -> Self::Unpark {
+        MockUnpark
+    }
+
+    fn park(&mut self) -> Result<(), Self::Error> {
+        panic!("parking forever");
+    }
+
+    fn park_timeout(&mut self, duration: Duration) -> Result<(), Self::Error> {
+        self.0.advance(duration);
+        Ok(())
+    }
+}
+
+impl Unpark for MockUnpark {
+    fn unpark(&self) {}
+}
+
+fn ms(n: u64) -> Duration {
+    Duration::from_millis(n)
+}
diff --git a/third_party/rust/tokio/src/time/throttle.rs b/third_party/rust/tokio/src/time/throttle.rs
new file mode 100644
index 0000000000..435bef6381
--- /dev/null
+++ b/third_party/rust/tokio/src/time/throttle.rs
@@ -0,0 +1,117 @@
+//! Slow down a stream by enforcing a delay between items.
+
+use crate::stream::Stream;
+use crate::time::{Delay, Duration, Instant};
+
+use std::future::Future;
+use std::marker::Unpin;
+use std::pin::Pin;
+use std::task::{self, Poll};
+
+use pin_project_lite::pin_project;
+
+/// Slows down a stream by enforcing a delay between items.
+/// They will be produced not more often than the specified interval.
+///
+/// # Example
+///
+/// Create a throttled stream.
+/// ```rust,norun
+/// use std::time::Duration;
+/// use tokio::stream::StreamExt;
+/// use tokio::time::throttle;
+///
+/// # async fn dox() {
+/// let mut item_stream = throttle(Duration::from_secs(2), futures::stream::repeat("one"));
+///
+/// loop {
+///     // The string will be produced at most every 2 seconds
+///     println!("{:?}", item_stream.next().await);
+/// }
+/// # }
+/// ```
+pub fn throttle<T>(duration: Duration, stream: T) -> Throttle<T>
+where
+    T: Stream,
+{
+    let delay = if duration == Duration::from_millis(0) {
+        None
+    } else {
+        Some(Delay::new_timeout(Instant::now() + duration, duration))
+    };
+
+    Throttle {
+        delay,
+        duration,
+        has_delayed: true,
+        stream,
+    }
+}
+
+pin_project! {
+    /// Stream for the [`throttle`](throttle) function.
+    #[derive(Debug)]
+    #[must_use = "streams do nothing unless polled"]
+    pub struct Throttle<T> {
+        // `None` when duration is zero.
+        delay: Option<Delay>,
+        duration: Duration,
+
+        // Set to true when `delay` has returned ready, but `stream` hasn't.
+        has_delayed: bool,
+
+        // The stream to throttle
+        #[pin]
+        stream: T,
+    }
+}
+
+// XXX: are these safe if `T: !Unpin`?
+impl<T: Unpin> Throttle<T> {
+    /// Acquires a reference to the underlying stream that this combinator is
+    /// pulling from.
+    pub fn get_ref(&self) -> &T {
+        &self.stream
+    }
+
+    /// Acquires a mutable reference to the underlying stream that this combinator
+    /// is pulling from.
+    ///
+    /// Note that care must be taken to avoid tampering with the state of the stream
+    /// which may otherwise confuse this combinator.
+    pub fn get_mut(&mut self) -> &mut T {
+        &mut self.stream
+    }
+
+    /// Consumes this combinator, returning the underlying stream.
+    ///
+    /// Note that this may discard intermediate state of this combinator, so care
+    /// should be taken to avoid losing resources when this is called.
+    pub fn into_inner(self) -> T {
+        self.stream
+    }
+}
+
+impl<T: Stream> Stream for Throttle<T> {
+    type Item = T::Item;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Option<Self::Item>> {
+        if !self.has_delayed && self.delay.is_some() {
+            ready!(Pin::new(self.as_mut().project().delay.as_mut().unwrap()).poll(cx));
+            *self.as_mut().project().has_delayed = true;
+        }
+
+        let value = ready!(self.as_mut().project().stream.poll_next(cx));
+
+        if value.is_some() {
+            let dur = self.duration;
+            if let Some(ref mut delay) = self.as_mut().project().delay {
+                delay.reset(Instant::now() + dur);
+            }
+
+            *self.as_mut().project().has_delayed = false;
+        }
+
+        Poll::Ready(value)
+    }
+}
diff --git a/third_party/rust/tokio/src/time/timeout.rs b/third_party/rust/tokio/src/time/timeout.rs
new file mode 100644
index 0000000000..401856a881
--- /dev/null
+++ b/third_party/rust/tokio/src/time/timeout.rs
@@ -0,0 +1,185 @@
+//! Allows a future to execute for a maximum amount of time.
+//!
+//! See [`Timeout`] documentation for more details.
+//!
+//! [`Timeout`]: struct@Timeout
+
+use crate::time::{delay_until, Delay, Duration, Instant};
+
+use std::fmt;
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{self, Poll};
+
+/// Require a `Future` to complete before the specified duration has elapsed.
+///
+/// If the future completes before the duration has elapsed, then the completed
+/// value is returned. Otherwise, an error is returned and the future is
+/// canceled.
+///
+/// # Cancelation
+///
+/// Cancelling a timeout is done by dropping the future. No additional cleanup
+/// or other work is required.
+///
+/// The original future may be obtained by calling [`Timeout::into_inner`]. This
+/// consumes the `Timeout`.
+///
+/// # Examples
+///
+/// Create a new `Timeout` set to expire in 10 milliseconds.
+///
+/// ```rust
+/// use tokio::time::timeout;
+/// use tokio::sync::oneshot;
+///
+/// use std::time::Duration;
+///
+/// # async fn dox() {
+/// let (tx, rx) = oneshot::channel();
+/// # tx.send(()).unwrap();
+///
+/// // Wrap the future with a `Timeout` set to expire in 10 milliseconds.
+/// if let Err(_) = timeout(Duration::from_millis(10), rx).await {
+///     println!("did not receive value within 10 ms");
+/// }
+/// # }
+/// ```
+pub fn timeout<T>(duration: Duration, future: T) -> Timeout<T>
+where
+    T: Future,
+{
+    let delay = Delay::new_timeout(Instant::now() + duration, duration);
+    Timeout::new_with_delay(future, delay)
+}
+
+/// Require a `Future` to complete before the specified instant in time.
+///
+/// If the future completes before the instant is reached, then the completed
+/// value is returned. Otherwise, an error is returned.
+///
+/// # Cancelation
+///
+/// Cancelling a timeout is done by dropping the future. No additional cleanup
+/// or other work is required.
+///
+/// The original future may be obtained by calling [`Timeout::into_inner`]. This
+/// consumes the `Timeout`.
+///
+/// # Examples
+///
+/// Create a new `Timeout` set to expire in 10 milliseconds.
+///
+/// ```rust
+/// use tokio::time::{Instant, timeout_at};
+/// use tokio::sync::oneshot;
+///
+/// use std::time::Duration;
+///
+/// # async fn dox() {
+/// let (tx, rx) = oneshot::channel();
+/// # tx.send(()).unwrap();
+///
+/// // Wrap the future with a `Timeout` set to expire 10 milliseconds into the
+/// // future.
+/// if let Err(_) = timeout_at(Instant::now() + Duration::from_millis(10), rx).await {
+///     println!("did not receive value within 10 ms");
+/// }
+/// # }
+/// ```
+pub fn timeout_at<T>(deadline: Instant, future: T) -> Timeout<T>
+where
+    T: Future,
+{
+    let delay = delay_until(deadline);
+
+    Timeout {
+        value: future,
+        delay,
+    }
+}
+
+/// Future returned by [`timeout`](timeout) and [`timeout_at`](timeout_at).
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+#[derive(Debug)]
+pub struct Timeout<T> {
+    value: T,
+    delay: Delay,
+}
+
+/// Error returned by `Timeout`.
+#[derive(Debug, PartialEq)]
+pub struct Elapsed(());
+
+impl Elapsed {
+    // Used on StreamExt::timeout
+    #[allow(unused)]
+    pub(crate) fn new() -> Self {
+        Elapsed(())
+    }
+}
+
+impl<T> Timeout<T> {
+    pub(crate) fn new_with_delay(value: T, delay: Delay) -> Timeout<T> {
+        Timeout { value, delay }
+    }
+
+    /// Gets a reference to the underlying value in this timeout.
+    pub fn get_ref(&self) -> &T {
+        &self.value
+    }
+
+    /// Gets a mutable reference to the underlying value in this timeout.
+    pub fn get_mut(&mut self) -> &mut T {
+        &mut self.value
+    }
+
+    /// Consumes this timeout, returning the underlying value.
+    pub fn into_inner(self) -> T {
+        self.value
+    }
+}
+
+impl<T> Future for Timeout<T>
+where
+    T: Future,
+{
+    type Output = Result<T::Output, Elapsed>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
+        // First, try polling the future
+
+        // Safety: we never move `self.value`
+        unsafe {
+            let p = self.as_mut().map_unchecked_mut(|me| &mut me.value);
+            if let Poll::Ready(v) = p.poll(cx) {
+                return Poll::Ready(Ok(v));
+            }
+        }
+
+        // Now check the timer
+        // Safety: X_X!
+        unsafe {
+            match self.map_unchecked_mut(|me| &mut me.delay).poll(cx) {
+                Poll::Ready(()) => Poll::Ready(Err(Elapsed(()))),
+                Poll::Pending => Poll::Pending,
+            }
+        }
+    }
+}
+
+// ===== impl Elapsed =====
+
+impl fmt::Display for Elapsed {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        "deadline has elapsed".fmt(fmt)
+    }
+}
+
+impl std::error::Error for Elapsed {}
+
+impl From<Elapsed> for std::io::Error {
+    fn from(_err: Elapsed) -> std::io::Error {
+        std::io::ErrorKind::TimedOut.into()
+    }
+}
diff --git a/third_party/rust/tokio/src/time/wheel/level.rs b/third_party/rust/tokio/src/time/wheel/level.rs
new file mode 100644
index 0000000000..49f9bfb9cf
--- /dev/null
+++ b/third_party/rust/tokio/src/time/wheel/level.rs
@@ -0,0 +1,255 @@
+use crate::time::wheel::Stack;
+
+use std::fmt;
+
+/// Wheel for a single level in the timer. This wheel contains 64 slots.
+pub(crate) struct Level<T> {
+    level: usize,
+
+    /// Bit field tracking which slots currently contain entries.
+    ///
+    /// Using a bit field to track slots that contain entries allows avoiding a
+    /// scan to find entries. This field is updated when entries are added or
+    /// removed from a slot.
+    ///
+    /// The least-significant bit represents slot zero.
+    occupied: u64,
+
+    /// Slots
+    slot: [T; LEVEL_MULT],
+}
+
+/// Indicates when a slot must be processed next.
+#[derive(Debug)]
+pub(crate) struct Expiration {
+    /// The level containing the slot.
+    pub(crate) level: usize,
+
+    /// The slot index.
+    pub(crate) slot: usize,
+
+    /// The instant at which the slot needs to be processed.
+    pub(crate) deadline: u64,
+}
+
+/// Level multiplier.
+///
+/// Being a power of 2 is very important.
+const LEVEL_MULT: usize = 64;
+
+impl<T: Stack> Level<T> {
+    pub(crate) fn new(level: usize) -> Level<T> {
+        // Rust's derived implementations for arrays require that the value
+        // contained by the array be `Copy`. So, here we have to manually
+        // initialize every single slot.
+        macro_rules! s {
+            () => {
+                T::default()
+            };
+        };
+
+        Level {
+            level,
+            occupied: 0,
+            slot: [
+                // It does not look like the necessary traits are
+                // derived for [T; 64].
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+                s!(),
+            ],
+        }
+    }
+
+    /// Finds the slot that needs to be processed next and returns the slot and
+    /// `Instant` at which this slot must be processed.
+    pub(crate) fn next_expiration(&self, now: u64) -> Option<Expiration> {
+        // Use the `occupied` bit field to get the index of the next slot that
+        // needs to be processed.
+        let slot = match self.next_occupied_slot(now) {
+            Some(slot) => slot,
+            None => return None,
+        };
+
+        // From the slot index, calculate the `Instant` at which it needs to be
+        // processed. This value *must* be in the future with respect to `now`.
+
+        let level_range = level_range(self.level);
+        let slot_range = slot_range(self.level);
+
+        // TODO: This can probably be simplified w/ power of 2 math
+        let level_start = now - (now % level_range);
+        let deadline = level_start + slot as u64 * slot_range;
+
+        debug_assert!(
+            deadline >= now,
+            "deadline={}; now={}; level={}; slot={}; occupied={:b}",
+            deadline,
+            now,
+            self.level,
+            slot,
+            self.occupied
+        );
+
+        Some(Expiration {
+            level: self.level,
+            slot,
+            deadline,
+        })
+    }
+
+    fn next_occupied_slot(&self, now: u64) -> Option<usize> {
+        if self.occupied == 0 {
+            return None;
+        }
+
+        // Get the slot for now using Maths
+        let now_slot = (now / slot_range(self.level)) as usize;
+        let occupied = self.occupied.rotate_right(now_slot as u32);
+        let zeros = occupied.trailing_zeros() as usize;
+        let slot = (zeros + now_slot) % 64;
+
+        Some(slot)
+    }
+
+    pub(crate) fn add_entry(&mut self, when: u64, item: T::Owned, store: &mut T::Store) {
+        let slot = slot_for(when, self.level);
+
+        self.slot[slot].push(item, store);
+        self.occupied |= occupied_bit(slot);
+    }
+
+    pub(crate) fn remove_entry(&mut self, when: u64, item: &T::Borrowed, store: &mut T::Store) {
+        let slot = slot_for(when, self.level);
+
+        self.slot[slot].remove(item, store);
+
+        if self.slot[slot].is_empty() {
+            // The bit is currently set
+            debug_assert!(self.occupied & occupied_bit(slot) != 0);
+
+            // Unset the bit
+            self.occupied ^= occupied_bit(slot);
+        }
+    }
+
+    pub(crate) fn pop_entry_slot(&mut self, slot: usize, store: &mut T::Store) -> Option<T::Owned> {
+        let ret = self.slot[slot].pop(store);
+
+        if ret.is_some() && self.slot[slot].is_empty() {
+            // The bit is currently set
+            debug_assert!(self.occupied & occupied_bit(slot) != 0);
+
+            self.occupied ^= occupied_bit(slot);
+        }
+
+        ret
+    }
+}
+
+impl<T> fmt::Debug for Level<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("Level")
+            .field("occupied", &self.occupied)
+            .finish()
+    }
+}
+
+fn occupied_bit(slot: usize) -> u64 {
+    1 << slot
+}
+
+fn slot_range(level: usize) -> u64 {
+    LEVEL_MULT.pow(level as u32) as u64
+}
+
+fn level_range(level: usize) -> u64 {
+    LEVEL_MULT as u64 * slot_range(level)
+}
+
+/// Convert a duration (milliseconds) and a level to a slot position
+fn slot_for(duration: u64, level: usize) -> usize {
+    ((duration >> (level * 6)) % LEVEL_MULT as u64) as usize
+}
+
+/*
+#[cfg(all(test, not(loom)))]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_slot_for() {
+        for pos in 1..64 {
+            assert_eq!(pos as usize, slot_for(pos, 0));
+        }
+
+        for level in 1..5 {
+            for pos in level..64 {
+                let a = pos * 64_usize.pow(level as u32);
+                assert_eq!(pos as usize, slot_for(a as u64, level));
+            }
+        }
+    }
+}
+*/
diff --git a/third_party/rust/tokio/src/time/wheel/mod.rs b/third_party/rust/tokio/src/time/wheel/mod.rs
new file mode 100644
index 0000000000..a2ef27fc6c
--- /dev/null
+++ b/third_party/rust/tokio/src/time/wheel/mod.rs
@@ -0,0 +1,314 @@
+mod level;
+pub(crate) use self::level::Expiration;
+use self::level::Level;
+
+mod stack;
+pub(crate) use self::stack::Stack;
+
+use std::borrow::Borrow;
+use std::usize;
+
+/// Timing wheel implementation.
+///
+/// This type provides the hashed timing wheel implementation that backs `Timer`
+/// and `DelayQueue`.
+///
+/// The structure is generic over `T: Stack`. This allows handling timeout data
+/// being stored on the heap or in a slab. In order to support the latter case,
+/// the slab must be passed into each function allowing the implementation to
+/// lookup timer entries.
+///
+/// See `Timer` documentation for some implementation notes.
+#[derive(Debug)]
+pub(crate) struct Wheel<T> {
+    /// The number of milliseconds elapsed since the wheel started.
+    elapsed: u64,
+
+    /// Timer wheel.
+    ///
+    /// Levels:
+    ///
+    /// * 1 ms slots / 64 ms range
+    /// * 64 ms slots / ~ 4 sec range
+    /// * ~ 4 sec slots / ~ 4 min range
+    /// * ~ 4 min slots / ~ 4 hr range
+    /// * ~ 4 hr slots / ~ 12 day range
+    /// * ~ 12 day slots / ~ 2 yr range
+    levels: Vec<Level<T>>,
+}
+
+/// Number of levels. Each level has 64 slots. By using 6 levels with 64 slots
+/// each, the timer is able to track time up to 2 years into the future with a
+/// precision of 1 millisecond.
+const NUM_LEVELS: usize = 6;
+
+/// The maximum duration of a delay
+const MAX_DURATION: u64 = (1 << (6 * NUM_LEVELS)) - 1;
+
+#[derive(Debug)]
+pub(crate) enum InsertError {
+    Elapsed,
+    Invalid,
+}
+
+/// Poll expirations from the wheel
+#[derive(Debug, Default)]
+pub(crate) struct Poll {
+    now: u64,
+    expiration: Option<Expiration>,
+}
+
+impl<T> Wheel<T>
+where
+    T: Stack,
+{
+    /// Create a new timing wheel
+    pub(crate) fn new() -> Wheel<T> {
+        let levels = (0..NUM_LEVELS).map(Level::new).collect();
+
+        Wheel { elapsed: 0, levels }
+    }
+
+    /// Return the number of milliseconds that have elapsed since the timing
+    /// wheel's creation.
+    pub(crate) fn elapsed(&self) -> u64 {
+        self.elapsed
+    }
+
+    /// Insert an entry into the timing wheel.
+    ///
+    /// # Arguments
+    ///
+    /// * `when`: is the instant at which the entry should be fired. It is
+    ///           represented as the number of milliseconds since the creation
+    ///           of the timing wheel.
+    ///
+    /// * `item`: The item to insert into the wheel.
+    ///
+    /// * `store`: The slab or `()` when using heap storage.
+    ///
+    /// # Return
+    ///
+    /// Returns `Ok` when the item is successfully inserted, `Err` otherwise.
+    ///
+    /// `Err(Elapsed)` indicates that `when` represents an instant that has
+    /// already passed. In this case, the caller should fire the timeout
+    /// immediately.
+    ///
+    /// `Err(Invalid)` indicates an invalid `when` argument as been supplied.
+    pub(crate) fn insert(
+        &mut self,
+        when: u64,
+        item: T::Owned,
+        store: &mut T::Store,
+    ) -> Result<(), (T::Owned, InsertError)> {
+        if when <= self.elapsed {
+            return Err((item, InsertError::Elapsed));
+        } else if when - self.elapsed > MAX_DURATION {
+            return Err((item, InsertError::Invalid));
+        }
+
+        // Get the level at which the entry should be stored
+        let level = self.level_for(when);
+
+        self.levels[level].add_entry(when, item, store);
+
+        debug_assert!({
+            self.levels[level]
+                .next_expiration(self.elapsed)
+                .map(|e| e.deadline >= self.elapsed)
+                .unwrap_or(true)
+        });
+
+        Ok(())
+    }
+
+    /// Remove `item` from thee timing wheel.
+    pub(crate) fn remove(&mut self, item: &T::Borrowed, store: &mut T::Store) {
+        let when = T::when(item, store);
+        let level = self.level_for(when);
+
+        self.levels[level].remove_entry(when, item, store);
+    }
+
+    /// Instant at which to poll
+    pub(crate) fn poll_at(&self) -> Option<u64> {
+        self.next_expiration().map(|expiration| expiration.deadline)
+    }
+
+    pub(crate) fn poll(&mut self, poll: &mut Poll, store: &mut T::Store) -> Option<T::Owned> {
+        loop {
+            if poll.expiration.is_none() {
+                poll.expiration = self.next_expiration().and_then(|expiration| {
+                    if expiration.deadline > poll.now {
+                        None
+                    } else {
+                        Some(expiration)
+                    }
+                });
+            }
+
+            match poll.expiration {
+                Some(ref expiration) => {
+                    if let Some(item) = self.poll_expiration(expiration, store) {
+                        return Some(item);
+                    }
+
+                    self.set_elapsed(expiration.deadline);
+                }
+                None => {
+                    self.set_elapsed(poll.now);
+                    return None;
+                }
+            }
+
+            poll.expiration = None;
+        }
+    }
+
+    /// Returns the instant at which the next timeout expires.
+    fn next_expiration(&self) -> Option<Expiration> {
+        // Check all levels
+        for level in 0..NUM_LEVELS {
+            if let Some(expiration) = self.levels[level].next_expiration(self.elapsed) {
+                // There cannot be any expirations at a higher level that happen
+                // before this one.
+                debug_assert!(self.no_expirations_before(level + 1, expiration.deadline));
+
+                return Some(expiration);
+            }
+        }
+
+        None
+    }
+
+    /// Used for debug assertions
+    fn no_expirations_before(&self, start_level: usize, before: u64) -> bool {
+        let mut res = true;
+
+        for l2 in start_level..NUM_LEVELS {
+            if let Some(e2) = self.levels[l2].next_expiration(self.elapsed) {
+                if e2.deadline < before {
+                    res = false;
+                }
+            }
+        }
+
+        res
+    }
+
+    pub(crate) fn poll_expiration(
+        &mut self,
+        expiration: &Expiration,
+        store: &mut T::Store,
+    ) -> Option<T::Owned> {
+        while let Some(item) = self.pop_entry(expiration, store) {
+            if expiration.level == 0 {
+                debug_assert_eq!(T::when(item.borrow(), store), expiration.deadline);
+
+                return Some(item);
+            } else {
+                let when = T::when(item.borrow(), store);
+
+                let next_level = expiration.level - 1;
+
+                self.levels[next_level].add_entry(when, item, store);
+            }
+        }
+
+        None
+    }
+
+    fn set_elapsed(&mut self, when: u64) {
+        assert!(
+            self.elapsed <= when,
+            "elapsed={:?}; when={:?}",
+            self.elapsed,
+            when
+        );
+
+        if when > self.elapsed {
+            self.elapsed = when;
+        }
+    }
+
+    fn pop_entry(&mut self, expiration: &Expiration, store: &mut T::Store) -> Option<T::Owned> {
+        self.levels[expiration.level].pop_entry_slot(expiration.slot, store)
+    }
+
+    fn level_for(&self, when: u64) -> usize {
+        level_for(self.elapsed, when)
+    }
+}
+
+fn level_for(elapsed: u64, when: u64) -> usize {
+    let masked = elapsed ^ when;
+
+    assert!(masked != 0, "elapsed={}; when={}", elapsed, when);
+
+    let leading_zeros = masked.leading_zeros() as usize;
+    let significant = 63 - leading_zeros;
+    significant / 6
+}
+
+impl Poll {
+    pub(crate) fn new(now: u64) -> Poll {
+        Poll {
+            now,
+            expiration: None,
+        }
+    }
+}
+
+#[cfg(all(test, not(loom)))]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_level_for() {
+        for pos in 1..64 {
+            assert_eq!(
+                0,
+                level_for(0, pos),
+                "level_for({}) -- binary = {:b}",
+                pos,
+                pos
+            );
+        }
+
+        for level in 1..5 {
+            for pos in level..64 {
+                let a = pos * 64_usize.pow(level as u32);
+                assert_eq!(
+                    level,
+                    level_for(0, a as u64),
+                    "level_for({}) -- binary = {:b}",
+                    a,
+                    a
+                );
+
+                if pos > level {
+                    let a = a - 1;
+                    assert_eq!(
+                        level,
+                        level_for(0, a as u64),
+                        "level_for({}) -- binary = {:b}",
+                        a,
+                        a
+                    );
+                }
+
+                if pos < 64 {
+                    let a = a + 1;
+                    assert_eq!(
+                        level,
+                        level_for(0, a as u64),
+                        "level_for({}) -- binary = {:b}",
+                        a,
+                        a
+                    );
+                }
+            }
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/time/wheel/stack.rs b/third_party/rust/tokio/src/time/wheel/stack.rs
new file mode 100644
index 0000000000..6e55c38ccd
--- /dev/null
+++ b/third_party/rust/tokio/src/time/wheel/stack.rs
@@ -0,0 +1,26 @@
+use std::borrow::Borrow;
+
+/// Abstracts the stack operations needed to track timeouts.
+pub(crate) trait Stack: Default {
+    /// Type of the item stored in the stack
+    type Owned: Borrow<Self::Borrowed>;
+
+    /// Borrowed item
+    type Borrowed;
+
+    /// Item storage, this allows a slab to be used instead of just the heap
+    type Store;
+
+    /// Returns `true` if the stack is empty
+    fn is_empty(&self) -> bool;
+
+    /// Push an item onto the stack
+    fn push(&mut self, item: Self::Owned, store: &mut Self::Store);
+
+    /// Pop an item from the stack
+    fn pop(&mut self, store: &mut Self::Store) -> Option<Self::Owned>;
+
+    fn remove(&mut self, item: &Self::Borrowed, store: &mut Self::Store);
+
+    fn when(item: &Self::Borrowed, store: &Self::Store) -> u64;
+}
diff --git a/third_party/rust/tokio/src/util/bit.rs b/third_party/rust/tokio/src/util/bit.rs
new file mode 100644
index 0000000000..e61ac2165a
--- /dev/null
+++ b/third_party/rust/tokio/src/util/bit.rs
@@ -0,0 +1,85 @@
+use std::fmt;
+
+#[derive(Clone, Copy)]
+pub(crate) struct Pack {
+    mask: usize,
+    shift: u32,
+}
+
+impl Pack {
+    /// Value is packed in the `width` most-significant bits.
+    pub(crate) const fn most_significant(width: u32) -> Pack {
+        let mask = mask_for(width).reverse_bits();
+
+        Pack {
+            mask,
+            shift: mask.trailing_zeros(),
+        }
+    }
+
+    /// Value is packed in the `width` least-significant bits.
+    pub(crate) const fn least_significant(width: u32) -> Pack {
+        let mask = mask_for(width);
+
+        Pack { mask, shift: 0 }
+    }
+
+    /// Value is packed in the `width` more-significant bits.
+    pub(crate) const fn then(&self, width: u32) -> Pack {
+        let shift = pointer_width() - self.mask.leading_zeros();
+        let mask = mask_for(width) << shift;
+
+        Pack { mask, shift }
+    }
+
+    /// Mask used to unpack value
+    #[cfg(all(test, loom))]
+    pub(crate) const fn mask(&self) -> usize {
+        self.mask
+    }
+
+    /// Width, in bits, dedicated to storing the value.
+    pub(crate) const fn width(&self) -> u32 {
+        pointer_width() - (self.mask >> self.shift).leading_zeros()
+    }
+
+    /// Max representable value
+    pub(crate) const fn max_value(&self) -> usize {
+        (1 << self.width()) - 1
+    }
+
+    pub(crate) fn pack(&self, value: usize, base: usize) -> usize {
+        assert!(value <= self.max_value());
+        (base & !self.mask) | (value << self.shift)
+    }
+
+    pub(crate) fn unpack(&self, src: usize) -> usize {
+        unpack(src, self.mask, self.shift)
+    }
+}
+
+impl fmt::Debug for Pack {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            fmt,
+            "Pack {{ mask: {:b}, shift: {} }}",
+            self.mask, self.shift
+        )
+    }
+}
+
+/// Returns the width of a pointer in bits
+pub(crate) const fn pointer_width() -> u32 {
+    std::mem::size_of::<usize>() as u32 * 8
+}
+
+/// Returns a `usize` with the right-most `n` bits set.
+pub(crate) const fn mask_for(n: u32) -> usize {
+    let shift = 1usize.wrapping_shl(n - 1);
+    shift | (shift - 1)
+}
+
+/// Unpack a value using a mask & shift
+pub(crate) const fn unpack(src: usize, mask: usize, shift: u32) -> usize {
+    (src & mask) >> shift
+}
diff --git a/third_party/rust/tokio/src/util/linked_list.rs b/third_party/rust/tokio/src/util/linked_list.rs
new file mode 100644
index 0000000000..aa3ce77188
--- /dev/null
+++ b/third_party/rust/tokio/src/util/linked_list.rs
@@ -0,0 +1,585 @@
+//! An intrusive double linked list of data
+//!
+//! The data structure supports tracking pinned nodes. Most of the data
+//! structure's APIs are `unsafe` as they require the caller to ensure the
+//! specified node is actually contained by the list.
+
+use core::fmt;
+use core::mem::ManuallyDrop;
+use core::ptr::NonNull;
+
+/// An intrusive linked list.
+///
+/// Currently, the list is not emptied on drop. It is the caller's
+/// responsibility to ensure the list is empty before dropping it.
+pub(crate) struct LinkedList<T: Link> {
+    /// Linked list head
+    head: Option<NonNull<T::Target>>,
+
+    /// Linked list tail
+    tail: Option<NonNull<T::Target>>,
+}
+
+unsafe impl<T: Link> Send for LinkedList<T> where T::Target: Send {}
+unsafe impl<T: Link> Sync for LinkedList<T> where T::Target: Sync {}
+
+/// Defines how a type is tracked within a linked list.
+///
+/// In order to support storing a single type within multiple lists, accessing
+/// the list pointers is decoupled from the entry type.
+///
+/// # Safety
+///
+/// Implementations must guarantee that `Target` types are pinned in memory. In
+/// other words, when a node is inserted, the value will not be moved as long as
+/// it is stored in the list.
+pub(crate) unsafe trait Link {
+    /// Handle to the list entry.
+    ///
+    /// This is usually a pointer-ish type.
+    type Handle;
+
+    /// Node type
+    type Target;
+
+    /// Convert the handle to a raw pointer without consuming the handle
+    fn as_raw(handle: &Self::Handle) -> NonNull<Self::Target>;
+
+    /// Convert the raw pointer to a handle
+    unsafe fn from_raw(ptr: NonNull<Self::Target>) -> Self::Handle;
+
+    /// Return the pointers for a node
+    unsafe fn pointers(target: NonNull<Self::Target>) -> NonNull<Pointers<Self::Target>>;
+}
+
+/// Previous / next pointers
+pub(crate) struct Pointers<T> {
+    /// The previous node in the list. null if there is no previous node.
+    prev: Option<NonNull<T>>,
+
+    /// The next node in the list. null if there is no previous node.
+    next: Option<NonNull<T>>,
+}
+
+unsafe impl<T: Send> Send for Pointers<T> {}
+unsafe impl<T: Sync> Sync for Pointers<T> {}
+
+// ===== impl LinkedList =====
+
+impl<T: Link> LinkedList<T> {
+    /// Creates an empty linked list
+    pub(crate) fn new() -> LinkedList<T> {
+        LinkedList {
+            head: None,
+            tail: None,
+        }
+    }
+
+    /// Adds an element first in the list.
+    pub(crate) fn push_front(&mut self, val: T::Handle) {
+        // The value should not be dropped, it is being inserted into the list
+        let val = ManuallyDrop::new(val);
+        let ptr = T::as_raw(&*val);
+        assert_ne!(self.head, Some(ptr));
+        unsafe {
+            T::pointers(ptr).as_mut().next = self.head;
+            T::pointers(ptr).as_mut().prev = None;
+
+            if let Some(head) = self.head {
+                T::pointers(head).as_mut().prev = Some(ptr);
+            }
+
+            self.head = Some(ptr);
+
+            if self.tail.is_none() {
+                self.tail = Some(ptr);
+            }
+        }
+    }
+
+    /// Removes the last element from a list and returns it, or None if it is
+    /// empty.
+    pub(crate) fn pop_back(&mut self) -> Option<T::Handle> {
+        unsafe {
+            let last = self.tail?;
+            self.tail = T::pointers(last).as_ref().prev;
+
+            if let Some(prev) = T::pointers(last).as_ref().prev {
+                T::pointers(prev).as_mut().next = None;
+            } else {
+                self.head = None
+            }
+
+            T::pointers(last).as_mut().prev = None;
+            T::pointers(last).as_mut().next = None;
+
+            Some(T::from_raw(last))
+        }
+    }
+
+    /// Returns whether the linked list doesn not contain any node
+    pub(crate) fn is_empty(&self) -> bool {
+        if self.head.is_some() {
+            return false;
+        }
+
+        assert!(self.tail.is_none());
+        true
+    }
+
+    /// Removes the specified node from the list
+    ///
+    /// # Safety
+    ///
+    /// The caller **must** ensure that `node` is currently contained by
+    /// `self` or not contained by any other list.
+    pub(crate) unsafe fn remove(&mut self, node: NonNull<T::Target>) -> Option<T::Handle> {
+        if let Some(prev) = T::pointers(node).as_ref().prev {
+            debug_assert_eq!(T::pointers(prev).as_ref().next, Some(node));
+            T::pointers(prev).as_mut().next = T::pointers(node).as_ref().next;
+        } else {
+            if self.head != Some(node) {
+                return None;
+            }
+
+            self.head = T::pointers(node).as_ref().next;
+        }
+
+        if let Some(next) = T::pointers(node).as_ref().next {
+            debug_assert_eq!(T::pointers(next).as_ref().prev, Some(node));
+            T::pointers(next).as_mut().prev = T::pointers(node).as_ref().prev;
+        } else {
+            // This might be the last item in the list
+            if self.tail != Some(node) {
+                return None;
+            }
+
+            self.tail = T::pointers(node).as_ref().prev;
+        }
+
+        T::pointers(node).as_mut().next = None;
+        T::pointers(node).as_mut().prev = None;
+
+        Some(T::from_raw(node))
+    }
+}
+
+impl<T: Link> fmt::Debug for LinkedList<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("LinkedList")
+            .field("head", &self.head)
+            .field("tail", &self.tail)
+            .finish()
+    }
+}
+
+cfg_sync! {
+    impl<T: Link> LinkedList<T> {
+        pub(crate) fn last(&self) -> Option<&T::Target> {
+            let tail = self.tail.as_ref()?;
+            unsafe {
+                Some(&*tail.as_ptr())
+            }
+        }
+    }
+}
+
+// ===== impl Iter =====
+
+cfg_rt_threaded! {
+    pub(crate) struct Iter<'a, T: Link> {
+        curr: Option<NonNull<T::Target>>,
+        _p: core::marker::PhantomData<&'a T>,
+    }
+
+    impl<T: Link> LinkedList<T> {
+        pub(crate) fn iter(&self) -> Iter<'_, T> {
+            Iter {
+                curr: self.head,
+                _p: core::marker::PhantomData,
+            }
+        }
+    }
+
+    impl<'a, T: Link> Iterator for Iter<'a, T> {
+        type Item = &'a T::Target;
+
+        fn next(&mut self) -> Option<&'a T::Target> {
+            let curr = self.curr?;
+            // safety: the pointer references data contained by the list
+            self.curr = unsafe { T::pointers(curr).as_ref() }.next;
+
+            // safety: the value is still owned by the linked list.
+            Some(unsafe { &*curr.as_ptr() })
+        }
+    }
+}
+
+// ===== impl Pointers =====
+
+impl<T> Pointers<T> {
+    /// Create a new set of empty pointers
+    pub(crate) fn new() -> Pointers<T> {
+        Pointers {
+            prev: None,
+            next: None,
+        }
+    }
+}
+
+impl<T> fmt::Debug for Pointers<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Pointers")
+            .field("prev", &self.prev)
+            .field("next", &self.next)
+            .finish()
+    }
+}
+
+#[cfg(test)]
+#[cfg(not(loom))]
+mod tests {
+    use super::*;
+
+    use std::pin::Pin;
+
+    #[derive(Debug)]
+    struct Entry {
+        pointers: Pointers<Entry>,
+        val: i32,
+    }
+
+    unsafe impl<'a> Link for &'a Entry {
+        type Handle = Pin<&'a Entry>;
+        type Target = Entry;
+
+        fn as_raw(handle: &Pin<&'_ Entry>) -> NonNull<Entry> {
+            NonNull::from(handle.get_ref())
+        }
+
+        unsafe fn from_raw(ptr: NonNull<Entry>) -> Pin<&'a Entry> {
+            Pin::new(&*ptr.as_ptr())
+        }
+
+        unsafe fn pointers(mut target: NonNull<Entry>) -> NonNull<Pointers<Entry>> {
+            NonNull::from(&mut target.as_mut().pointers)
+        }
+    }
+
+    fn entry(val: i32) -> Pin<Box<Entry>> {
+        Box::pin(Entry {
+            pointers: Pointers::new(),
+            val,
+        })
+    }
+
+    fn ptr(r: &Pin<Box<Entry>>) -> NonNull<Entry> {
+        r.as_ref().get_ref().into()
+    }
+
+    fn collect_list(list: &mut LinkedList<&'_ Entry>) -> Vec<i32> {
+        let mut ret = vec![];
+
+        while let Some(entry) = list.pop_back() {
+            ret.push(entry.val);
+        }
+
+        ret
+    }
+
+    fn push_all<'a>(list: &mut LinkedList<&'a Entry>, entries: &[Pin<&'a Entry>]) {
+        for entry in entries.iter() {
+            list.push_front(*entry);
+        }
+    }
+
+    macro_rules! assert_clean {
+        ($e:ident) => {{
+            assert!($e.pointers.next.is_none());
+            assert!($e.pointers.prev.is_none());
+        }};
+    }
+
+    macro_rules! assert_ptr_eq {
+        ($a:expr, $b:expr) => {{
+            // Deal with mapping a Pin<&mut T> -> Option<NonNull<T>>
+            assert_eq!(Some($a.as_ref().get_ref().into()), $b)
+        }};
+    }
+
+    #[test]
+    fn push_and_drain() {
+        let a = entry(5);
+        let b = entry(7);
+        let c = entry(31);
+
+        let mut list = LinkedList::new();
+        assert!(list.is_empty());
+
+        list.push_front(a.as_ref());
+        assert!(!list.is_empty());
+        list.push_front(b.as_ref());
+        list.push_front(c.as_ref());
+
+        let items: Vec<i32> = collect_list(&mut list);
+        assert_eq!([5, 7, 31].to_vec(), items);
+
+        assert!(list.is_empty());
+    }
+
+    #[test]
+    fn push_pop_push_pop() {
+        let a = entry(5);
+        let b = entry(7);
+
+        let mut list = LinkedList::<&Entry>::new();
+
+        list.push_front(a.as_ref());
+
+        let entry = list.pop_back().unwrap();
+        assert_eq!(5, entry.val);
+        assert!(list.is_empty());
+
+        list.push_front(b.as_ref());
+
+        let entry = list.pop_back().unwrap();
+        assert_eq!(7, entry.val);
+
+        assert!(list.is_empty());
+        assert!(list.pop_back().is_none());
+    }
+
+    #[test]
+    fn remove_by_address() {
+        let a = entry(5);
+        let b = entry(7);
+        let c = entry(31);
+
+        unsafe {
+            // Remove first
+            let mut list = LinkedList::new();
+
+            push_all(&mut list, &[c.as_ref(), b.as_ref(), a.as_ref()]);
+            assert!(list.remove(ptr(&a)).is_some());
+            assert_clean!(a);
+            // `a` should be no longer there and can't be removed twice
+            assert!(list.remove(ptr(&a)).is_none());
+            assert!(!list.is_empty());
+
+            assert!(list.remove(ptr(&b)).is_some());
+            assert_clean!(b);
+            // `b` should be no longer there and can't be removed twice
+            assert!(list.remove(ptr(&b)).is_none());
+            assert!(!list.is_empty());
+
+            assert!(list.remove(ptr(&c)).is_some());
+            assert_clean!(c);
+            // `b` should be no longer there and can't be removed twice
+            assert!(list.remove(ptr(&c)).is_none());
+            assert!(list.is_empty());
+        }
+
+        unsafe {
+            // Remove middle
+            let mut list = LinkedList::new();
+
+            push_all(&mut list, &[c.as_ref(), b.as_ref(), a.as_ref()]);
+
+            assert!(list.remove(ptr(&a)).is_some());
+            assert_clean!(a);
+
+            assert_ptr_eq!(b, list.head);
+            assert_ptr_eq!(c, b.pointers.next);
+            assert_ptr_eq!(b, c.pointers.prev);
+
+            let items = collect_list(&mut list);
+            assert_eq!([31, 7].to_vec(), items);
+        }
+
+        unsafe {
+            // Remove middle
+            let mut list = LinkedList::new();
+
+            push_all(&mut list, &[c.as_ref(), b.as_ref(), a.as_ref()]);
+
+            assert!(list.remove(ptr(&b)).is_some());
+            assert_clean!(b);
+
+            assert_ptr_eq!(c, a.pointers.next);
+            assert_ptr_eq!(a, c.pointers.prev);
+
+            let items = collect_list(&mut list);
+            assert_eq!([31, 5].to_vec(), items);
+        }
+
+        unsafe {
+            // Remove last
+            // Remove middle
+            let mut list = LinkedList::new();
+
+            push_all(&mut list, &[c.as_ref(), b.as_ref(), a.as_ref()]);
+
+            assert!(list.remove(ptr(&c)).is_some());
+            assert_clean!(c);
+
+            assert!(b.pointers.next.is_none());
+            assert_ptr_eq!(b, list.tail);
+
+            let items = collect_list(&mut list);
+            assert_eq!([7, 5].to_vec(), items);
+        }
+
+        unsafe {
+            // Remove first of two
+            let mut list = LinkedList::new();
+
+            push_all(&mut list, &[b.as_ref(), a.as_ref()]);
+
+            assert!(list.remove(ptr(&a)).is_some());
+
+            assert_clean!(a);
+
+            // a should be no longer there and can't be removed twice
+            assert!(list.remove(ptr(&a)).is_none());
+
+            assert_ptr_eq!(b, list.head);
+            assert_ptr_eq!(b, list.tail);
+
+            assert!(b.pointers.next.is_none());
+            assert!(b.pointers.prev.is_none());
+
+            let items = collect_list(&mut list);
+            assert_eq!([7].to_vec(), items);
+        }
+
+        unsafe {
+            // Remove last of two
+            let mut list = LinkedList::new();
+
+            push_all(&mut list, &[b.as_ref(), a.as_ref()]);
+
+            assert!(list.remove(ptr(&b)).is_some());
+
+            assert_clean!(b);
+
+            assert_ptr_eq!(a, list.head);
+            assert_ptr_eq!(a, list.tail);
+
+            assert!(a.pointers.next.is_none());
+            assert!(a.pointers.prev.is_none());
+
+            let items = collect_list(&mut list);
+            assert_eq!([5].to_vec(), items);
+        }
+
+        unsafe {
+            // Remove last item
+            let mut list = LinkedList::new();
+
+            push_all(&mut list, &[a.as_ref()]);
+
+            assert!(list.remove(ptr(&a)).is_some());
+            assert_clean!(a);
+
+            assert!(list.head.is_none());
+            assert!(list.tail.is_none());
+            let items = collect_list(&mut list);
+            assert!(items.is_empty());
+        }
+
+        unsafe {
+            // Remove missing
+            let mut list = LinkedList::<&Entry>::new();
+
+            list.push_front(b.as_ref());
+            list.push_front(a.as_ref());
+
+            assert!(list.remove(ptr(&c)).is_none());
+        }
+    }
+
+    #[test]
+    fn iter() {
+        let a = entry(5);
+        let b = entry(7);
+
+        let mut list = LinkedList::<&Entry>::new();
+
+        assert_eq!(0, list.iter().count());
+
+        list.push_front(a.as_ref());
+        list.push_front(b.as_ref());
+
+        let mut i = list.iter();
+        assert_eq!(7, i.next().unwrap().val);
+        assert_eq!(5, i.next().unwrap().val);
+        assert!(i.next().is_none());
+    }
+
+    proptest::proptest! {
+        #[test]
+        fn fuzz_linked_list(ops: Vec<usize>) {
+            run_fuzz(ops);
+        }
+    }
+
+    fn run_fuzz(ops: Vec<usize>) {
+        use std::collections::VecDeque;
+
+        #[derive(Debug)]
+        enum Op {
+            Push,
+            Pop,
+            Remove(usize),
+        }
+
+        let ops = ops
+            .iter()
+            .map(|i| match i % 3 {
+                0 => Op::Push,
+                1 => Op::Pop,
+                2 => Op::Remove(i / 3),
+                _ => unreachable!(),
+            })
+            .collect::<Vec<_>>();
+
+        let mut ll = LinkedList::<&Entry>::new();
+        let mut reference = VecDeque::new();
+
+        let entries: Vec<_> = (0..ops.len()).map(|i| entry(i as i32)).collect();
+
+        for (i, op) in ops.iter().enumerate() {
+            match op {
+                Op::Push => {
+                    reference.push_front(i as i32);
+                    assert_eq!(entries[i].val, i as i32);
+
+                    ll.push_front(entries[i].as_ref());
+                }
+                Op::Pop => {
+                    if reference.is_empty() {
+                        assert!(ll.is_empty());
+                        continue;
+                    }
+
+                    let v = reference.pop_back();
+                    assert_eq!(v, ll.pop_back().map(|v| v.val));
+                }
+                Op::Remove(n) => {
+                    if reference.is_empty() {
+                        assert!(ll.is_empty());
+                        continue;
+                    }
+
+                    let idx = n % reference.len();
+                    let expect = reference.remove(idx).unwrap();
+
+                    unsafe {
+                        let entry = ll.remove(ptr(&entries[expect as usize])).unwrap();
+                        assert_eq!(expect, entry.val);
+                    }
+                }
+            }
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/util/mod.rs b/third_party/rust/tokio/src/util/mod.rs
new file mode 100644
index 0000000000..a093395c02
--- /dev/null
+++ b/third_party/rust/tokio/src/util/mod.rs
@@ -0,0 +1,24 @@
+cfg_io_driver! {
+    pub(crate) mod bit;
+    pub(crate) mod slab;
+}
+
+#[cfg(any(feature = "sync", feature = "rt-core"))]
+pub(crate) mod linked_list;
+
+#[cfg(any(feature = "rt-threaded", feature = "macros", feature = "stream"))]
+mod rand;
+
+mod wake;
+pub(crate) use wake::{waker_ref, Wake};
+
+cfg_rt_threaded! {
+    pub(crate) use rand::FastRand;
+
+    mod try_lock;
+    pub(crate) use try_lock::TryLock;
+}
+
+#[cfg(any(feature = "macros", feature = "stream"))]
+#[cfg_attr(not(feature = "macros"), allow(unreachable_pub))]
+pub use rand::thread_rng_n;
diff --git a/third_party/rust/tokio/src/util/pad.rs b/third_party/rust/tokio/src/util/pad.rs
new file mode 100644
index 0000000000..bf0913ca85
--- /dev/null
+++ b/third_party/rust/tokio/src/util/pad.rs
@@ -0,0 +1,52 @@
+use core::fmt;
+use core::ops::{Deref, DerefMut};
+
+#[derive(Clone, Copy, Default, Hash, PartialEq, Eq)]
+// Starting from Intel's Sandy Bridge, spatial prefetcher is now pulling pairs of 64-byte cache
+// lines at a time, so we have to align to 128 bytes rather than 64.
+//
+// Sources:
+// - https://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-optimization-manual.pdf
+// - https://github.com/facebook/folly/blob/1b5288e6eea6df074758f877c849b6e73bbb9fbb/folly/lang/Align.h#L107
+#[cfg_attr(target_arch = "x86_64", repr(align(128)))]
+#[cfg_attr(not(target_arch = "x86_64"), repr(align(64)))]
+pub(crate) struct CachePadded<T> {
+    value: T,
+}
+
+unsafe impl<T: Send> Send for CachePadded<T> {}
+unsafe impl<T: Sync> Sync for CachePadded<T> {}
+
+impl<T> CachePadded<T> {
+    pub(crate) fn new(t: T) -> CachePadded<T> {
+        CachePadded::<T> { value: t }
+    }
+}
+
+impl<T> Deref for CachePadded<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        &self.value
+    }
+}
+
+impl<T> DerefMut for CachePadded<T> {
+    fn deref_mut(&mut self) -> &mut T {
+        &mut self.value
+    }
+}
+
+impl<T: fmt::Debug> fmt::Debug for CachePadded<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("CachePadded")
+            .field("value", &self.value)
+            .finish()
+    }
+}
+
+impl<T> From<T> for CachePadded<T> {
+    fn from(t: T) -> Self {
+        CachePadded::new(t)
+    }
+}
diff --git a/third_party/rust/tokio/src/util/rand.rs b/third_party/rust/tokio/src/util/rand.rs
new file mode 100644
index 0000000000..4b72b4b110
--- /dev/null
+++ b/third_party/rust/tokio/src/util/rand.rs
@@ -0,0 +1,64 @@
+use std::cell::Cell;
+
+/// Fast random number generate
+///
+/// Implement xorshift64+: 2 32-bit xorshift sequences added together.
+/// Shift triplet [17,7,16] was calculated as indicated in Marsaglia's
+/// Xorshift paper: https://www.jstatsoft.org/article/view/v008i14/xorshift.pdf
+/// This generator passes the SmallCrush suite, part of TestU01 framework:
+/// http://simul.iro.umontreal.ca/testu01/tu01.html
+#[derive(Debug)]
+pub(crate) struct FastRand {
+    one: Cell<u32>,
+    two: Cell<u32>,
+}
+
+impl FastRand {
+    /// Initialize a new, thread-local, fast random number generator.
+    pub(crate) fn new(seed: u64) -> FastRand {
+        let one = (seed >> 32) as u32;
+        let mut two = seed as u32;
+
+        if two == 0 {
+            // This value cannot be zero
+            two = 1;
+        }
+
+        FastRand {
+            one: Cell::new(one),
+            two: Cell::new(two),
+        }
+    }
+
+    pub(crate) fn fastrand_n(&self, n: u32) -> u32 {
+        // This is similar to fastrand() % n, but faster.
+        // See https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
+        let mul = (self.fastrand() as u64).wrapping_mul(n as u64);
+        (mul >> 32) as u32
+    }
+
+    fn fastrand(&self) -> u32 {
+        let mut s1 = self.one.get();
+        let s0 = self.two.get();
+
+        s1 ^= s1 << 17;
+        s1 = s1 ^ s0 ^ s1 >> 7 ^ s0 >> 16;
+
+        self.one.set(s0);
+        self.two.set(s1);
+
+        s0.wrapping_add(s1)
+    }
+}
+
+// Used by the select macro and `StreamMap`
+#[cfg(any(feature = "macros", feature = "stream"))]
+#[doc(hidden)]
+#[cfg_attr(not(feature = "macros"), allow(unreachable_pub))]
+pub fn thread_rng_n(n: u32) -> u32 {
+    thread_local! {
+        static THREAD_RNG: FastRand = FastRand::new(crate::loom::rand::seed());
+    }
+
+    THREAD_RNG.with(|rng| rng.fastrand_n(n))
+}
diff --git a/third_party/rust/tokio/src/util/slab/addr.rs b/third_party/rust/tokio/src/util/slab/addr.rs
new file mode 100644
index 0000000000..c14e32e909
--- /dev/null
+++ b/third_party/rust/tokio/src/util/slab/addr.rs
@@ -0,0 +1,154 @@
+//! Tracks the location of an entry in a slab.
+//!
+//! # Index packing
+//!
+//! A slab index consists of multiple indices packed into a single `usize` value
+//! that correspond to different parts of the slab.
+//!
+//! The least significant `MAX_PAGES + INITIAL_PAGE_SIZE.trailing_zeros() + 1`
+//! bits store the address within a shard, starting at 0 for the first slot on
+//! the first page. To index a slot within a shard, we first find the index of
+//! the page that the address falls on, and then the offset of the slot within
+//! that page.
+//!
+//! Since every page is twice as large as the previous page, and all page sizes
+//! are powers of two, we can determine the page index that contains a given
+//! address by shifting the address down by the smallest page size and looking
+//! at how many twos places necessary to represent that number, telling us what
+//! power of two page size it fits inside of. We can determine the number of
+//! twos places by counting the number of leading zeros (unused twos places) in
+//! the number's binary representation, and subtracting that count from the
+//! total number of bits in a word.
+//!
+//! Once we know what page contains an address, we can subtract the size of all
+//! previous pages from the address to determine the offset within the page.
+//!
+//! After the page address, the next `MAX_THREADS.trailing_zeros() + 1` least
+//! significant bits are the thread ID. These are used to index the array of
+//! shards to find which shard a slot belongs to. If an entry is being removed
+//! and the thread ID of its index matches that of the current thread, we can
+//! use the `remove_local` fast path; otherwise, we have to use the synchronized
+//! `remove_remote` path.
+//!
+//! Finally, a generation value is packed into the index. The `RESERVED_BITS`
+//! most significant bits are left unused, and the remaining bits between the
+//! last bit of the thread ID and the first reserved bit are used to store the
+//! generation. The generation is used as part of an atomic read-modify-write
+//! loop every time a `ScheduledIo`'s readiness is modified, or when the
+//! resource is removed, to guard against the ABA problem.
+//!
+//! Visualized:
+//!
+//! ```text
+//!     ┌──────────┬───────────────┬──────────────────┬──────────────────────────┐
+//!     │ reserved │  generation   │    thread ID     │         address          │
+//!     └▲─────────┴▲──────────────┴▲─────────────────┴▲────────────────────────▲┘
+//!      │          │               │                  │                        │
+//! bits(usize)     │       bits(MAX_THREADS)          │                        0
+//!                 │                                  │
+//!      bits(usize) - RESERVED       MAX_PAGES + bits(INITIAL_PAGE_SIZE)
+//! ```
+
+use crate::util::bit;
+use crate::util::slab::{Generation, INITIAL_PAGE_SIZE, MAX_PAGES, MAX_THREADS};
+
+use std::usize;
+
+/// References the location at which an entry is stored in a slab.
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+pub(crate) struct Address(usize);
+
+const PAGE_INDEX_SHIFT: u32 = INITIAL_PAGE_SIZE.trailing_zeros() + 1;
+
+/// Address in the shard
+const SLOT: bit::Pack = bit::Pack::least_significant(MAX_PAGES as u32 + PAGE_INDEX_SHIFT);
+
+/// Masks the thread identifier
+const THREAD: bit::Pack = SLOT.then(MAX_THREADS.trailing_zeros() + 1);
+
+/// Masks the generation
+const GENERATION: bit::Pack = THREAD
+    .then(bit::pointer_width().wrapping_sub(RESERVED.width() + THREAD.width() + SLOT.width()));
+
+// Chosen arbitrarily
+const RESERVED: bit::Pack = bit::Pack::most_significant(5);
+
+impl Address {
+    /// Represents no entry, picked to avoid collision with Mio's internals.
+    /// This value should not be passed to mio.
+    pub(crate) const NULL: usize = usize::MAX >> 1;
+
+    /// Re-exported by `Generation`.
+    pub(super) const GENERATION_WIDTH: u32 = GENERATION.width();
+
+    pub(super) fn new(shard_index: usize, generation: Generation) -> Address {
+        let mut repr = 0;
+
+        repr = SLOT.pack(shard_index, repr);
+        repr = GENERATION.pack(generation.to_usize(), repr);
+
+        Address(repr)
+    }
+
+    /// Convert from a `usize` representation.
+    pub(crate) fn from_usize(src: usize) -> Address {
+        assert_ne!(src, Self::NULL);
+
+        Address(src)
+    }
+
+    /// Convert to a `usize` representation
+    pub(crate) fn to_usize(self) -> usize {
+        self.0
+    }
+
+    pub(crate) fn generation(self) -> Generation {
+        Generation::new(GENERATION.unpack(self.0))
+    }
+
+    /// Returns the page index
+    pub(super) fn page(self) -> usize {
+        // Since every page is twice as large as the previous page, and all page
+        // sizes are powers of two, we can determine the page index that
+        // contains a given address by shifting the address down by the smallest
+        // page size and looking at how many twos places necessary to represent
+        // that number, telling us what power of two page size it fits inside
+        // of. We can determine the number of twos places by counting the number
+        // of leading zeros (unused twos places) in the number's binary
+        // representation, and subtracting that count from the total number of
+        // bits in a word.
+        let slot_shifted = (self.slot() + INITIAL_PAGE_SIZE) >> PAGE_INDEX_SHIFT;
+        (bit::pointer_width() - slot_shifted.leading_zeros()) as usize
+    }
+
+    /// Returns the slot index
+    pub(super) fn slot(self) -> usize {
+        SLOT.unpack(self.0)
+    }
+}
+
+#[cfg(test)]
+cfg_not_loom! {
+    use proptest::proptest;
+
+    #[test]
+    fn test_pack_format() {
+        assert_eq!(5, RESERVED.width());
+        assert_eq!(0b11111, RESERVED.max_value());
+    }
+
+    proptest! {
+        #[test]
+        fn address_roundtrips(
+            slot in 0usize..SLOT.max_value(),
+            generation in 0usize..Generation::MAX,
+        ) {
+            let address = Address::new(slot, Generation::new(generation));
+            // Round trip
+            let address = Address::from_usize(address.to_usize());
+
+            assert_eq!(address.slot(), slot);
+            assert_eq!(address.generation().to_usize(), generation);
+        }
+    }
+}
diff --git a/third_party/rust/tokio/src/util/slab/entry.rs b/third_party/rust/tokio/src/util/slab/entry.rs
new file mode 100644
index 0000000000..2e0b10b0fd
--- /dev/null
+++ b/third_party/rust/tokio/src/util/slab/entry.rs
@@ -0,0 +1,7 @@
+use crate::util::slab::Generation;
+
+pub(crate) trait Entry: Default {
+    fn generation(&self) -> Generation;
+
+    fn reset(&self, generation: Generation) -> bool;
+}
diff --git a/third_party/rust/tokio/src/util/slab/generation.rs b/third_party/rust/tokio/src/util/slab/generation.rs
new file mode 100644
index 0000000000..4b16b2caf6
--- /dev/null
+++ b/third_party/rust/tokio/src/util/slab/generation.rs
@@ -0,0 +1,32 @@
+use crate::util::bit;
+use crate::util::slab::Address;
+
+/// An mutation identifier for a slot in the slab. The generation helps prevent
+/// accessing an entry with an outdated token.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Ord, PartialOrd)]
+pub(crate) struct Generation(usize);
+
+impl Generation {
+    pub(crate) const WIDTH: u32 = Address::GENERATION_WIDTH;
+
+    pub(super) const MAX: usize = bit::mask_for(Address::GENERATION_WIDTH);
+
+    /// Create a new generation
+    ///
+    /// # Panics
+    ///
+    /// Panics if `value` is greater than max generation.
+    pub(crate) fn new(value: usize) -> Generation {
+        assert!(value <= Self::MAX);
+        Generation(value)
+    }
+
+    /// Returns the next generation value
+    pub(crate) fn next(self) -> Generation {
+        Generation((self.0 + 1) & Self::MAX)
+    }
+
+    pub(crate) fn to_usize(self) -> usize {
+        self.0
+    }
+}
diff --git a/third_party/rust/tokio/src/util/slab/mod.rs b/third_party/rust/tokio/src/util/slab/mod.rs
new file mode 100644
index 0000000000..5082970507
--- /dev/null
+++ b/third_party/rust/tokio/src/util/slab/mod.rs
@@ -0,0 +1,107 @@
+//! A lock-free concurrent slab.
+
+mod addr;
+pub(crate) use addr::Address;
+
+mod entry;
+pub(crate) use entry::Entry;
+
+mod generation;
+pub(crate) use generation::Generation;
+
+mod page;
+
+mod shard;
+use shard::Shard;
+
+mod slot;
+use slot::Slot;
+
+mod stack;
+use stack::TransferStack;
+
+#[cfg(all(loom, test))]
+mod tests;
+
+use crate::loom::sync::Mutex;
+use crate::util::bit;
+
+use std::fmt;
+
+#[cfg(target_pointer_width = "64")]
+const MAX_THREADS: usize = 4096;
+
+#[cfg(target_pointer_width = "32")]
+const MAX_THREADS: usize = 2048;
+
+/// Max number of pages per slab
+const MAX_PAGES: usize = bit::pointer_width() as usize / 4;
+
+cfg_not_loom! {
+    /// Size of first page
+    const INITIAL_PAGE_SIZE: usize = 32;
+}
+
+cfg_loom! {
+    const INITIAL_PAGE_SIZE: usize = 2;
+}
+
+/// A sharded slab.
+pub(crate) struct Slab<T> {
+    // Signal shard for now. Eventually there will be more.
+    shard: Shard<T>,
+    local: Mutex<()>,
+}
+
+unsafe impl<T: Send> Send for Slab<T> {}
+unsafe impl<T: Sync> Sync for Slab<T> {}
+
+impl<T: Entry> Slab<T> {
+    /// Returns a new slab with the default configuration parameters.
+    pub(crate) fn new() -> Slab<T> {
+        Slab {
+            shard: Shard::new(),
+            local: Mutex::new(()),
+        }
+    }
+
+    /// allocs a value into the slab, returning a key that can be used to
+    /// access it.
+    ///
+    /// If this function returns `None`, then the shard for the current thread
+    /// is full and no items can be added until some are removed, or the maximum
+    /// number of shards has been reached.
+    pub(crate) fn alloc(&self) -> Option<Address> {
+        // we must lock the slab to alloc an item.
+        let _local = self.local.lock().unwrap();
+        self.shard.alloc()
+    }
+
+    /// Removes the value associated with the given key from the slab.
+    pub(crate) fn remove(&self, idx: Address) {
+        // try to lock the slab so that we can use `remove_local`.
+        let lock = self.local.try_lock();
+
+        // if we were able to lock the slab, we are "local" and can use the fast
+        // path; otherwise, we will use `remove_remote`.
+        if lock.is_ok() {
+            self.shard.remove_local(idx)
+        } else {
+            self.shard.remove_remote(idx)
+        }
+    }
+
+    /// Return a reference to the value associated with the given key.
+    ///
+    /// If the slab does not contain a value for the given key, `None` is
+    /// returned instead.
+    pub(crate) fn get(&self, token: Address) -> Option<&T> {
+        self.shard.get(token)
+    }
+}
+
+impl<T> fmt::Debug for Slab<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Slab").field("shard", &self.shard).finish()
+    }
+}
diff --git a/third_party/rust/tokio/src/util/slab/page.rs b/third_party/rust/tokio/src/util/slab/page.rs
new file mode 100644
index 0000000000..0000e934de
--- /dev/null
+++ b/third_party/rust/tokio/src/util/slab/page.rs
@@ -0,0 +1,187 @@
+use crate::loom::cell::UnsafeCell;
+use crate::util::slab::{Address, Entry, Slot, TransferStack, INITIAL_PAGE_SIZE};
+
+use std::fmt;
+
+/// Data accessed only by the thread that owns the shard.
+pub(crate) struct Local {
+    head: UnsafeCell<usize>,
+}
+
+/// Data accessed by any thread.
+pub(crate) struct Shared<T> {
+    remote: TransferStack,
+    size: usize,
+    prev_sz: usize,
+    slab: UnsafeCell<Option<Box<[Slot<T>]>>>,
+}
+
+/// Returns the size of the page at index `n`
+pub(super) fn size(n: usize) -> usize {
+    INITIAL_PAGE_SIZE << n
+}
+
+impl Local {
+    pub(crate) fn new() -> Self {
+        Self {
+            head: UnsafeCell::new(0),
+        }
+    }
+
+    fn head(&self) -> usize {
+        self.head.with(|head| unsafe { *head })
+    }
+
+    fn set_head(&self, new_head: usize) {
+        self.head.with_mut(|head| unsafe {
+            *head = new_head;
+        })
+    }
+}
+
+impl<T: Entry> Shared<T> {
+    pub(crate) fn new(size: usize, prev_sz: usize) -> Shared<T> {
+        Self {
+            prev_sz,
+            size,
+            remote: TransferStack::new(),
+            slab: UnsafeCell::new(None),
+        }
+    }
+
+    /// Allocates storage for this page if it does not allready exist.
+    ///
+    /// This requires unique access to the page (e.g. it is called from the
+    /// thread that owns the page, or, in the case of `SingleShard`, while the
+    /// lock is held). In order to indicate this, a reference to the page's
+    /// `Local` data is taken by this function; the `Local` argument is not
+    /// actually used, but requiring it ensures that this is only called when
+    /// local access is held.
+    #[cold]
+    fn alloc_page(&self, _: &Local) {
+        debug_assert!(self.slab.with(|s| unsafe { (*s).is_none() }));
+
+        let mut slab = Vec::with_capacity(self.size);
+        slab.extend((1..self.size).map(Slot::new));
+        slab.push(Slot::new(Address::NULL));
+
+        self.slab.with_mut(|s| {
+            // this mut access is safe — it only occurs to initially
+            // allocate the page, which only happens on this thread; if the
+            // page has not yet been allocated, other threads will not try
+            // to access it yet.
+            unsafe {
+                *s = Some(slab.into_boxed_slice());
+            }
+        });
+    }
+
+    pub(crate) fn alloc(&self, local: &Local) -> Option<Address> {
+        let head = local.head();
+
+        // are there any items on the local free list? (fast path)
+        let head = if head < self.size {
+            head
+        } else {
+            // if the local free list is empty, pop all the items on the remote
+            // free list onto the local free list.
+            self.remote.pop_all()?
+        };
+
+        // if the head is still null, both the local and remote free lists are
+        // empty --- we can't fit any more items on this page.
+        if head == Address::NULL {
+            return None;
+        }
+
+        // do we need to allocate storage for this page?
+        let page_needs_alloc = self.slab.with(|s| unsafe { (*s).is_none() });
+        if page_needs_alloc {
+            self.alloc_page(local);
+        }
+
+        let gen = self.slab.with(|slab| {
+            let slab = unsafe { &*(slab) }
+                .as_ref()
+                .expect("page must have been allocated to alloc!");
+
+            let slot = &slab[head];
+
+            local.set_head(slot.next());
+            slot.generation()
+        });
+
+        let index = head + self.prev_sz;
+
+        Some(Address::new(index, gen))
+    }
+
+    pub(crate) fn get(&self, addr: Address) -> Option<&T> {
+        let page_offset = addr.slot() - self.prev_sz;
+
+        self.slab
+            .with(|slab| unsafe { &*slab }.as_ref()?.get(page_offset))
+            .map(|slot| slot.get())
+    }
+
+    pub(crate) fn remove_local(&self, local: &Local, addr: Address) {
+        let offset = addr.slot() - self.prev_sz;
+
+        self.slab.with(|slab| {
+            let slab = unsafe { &*slab }.as_ref();
+
+            let slot = if let Some(slot) = slab.and_then(|slab| slab.get(offset)) {
+                slot
+            } else {
+                return;
+            };
+
+            if slot.reset(addr.generation()) {
+                slot.set_next(local.head());
+                local.set_head(offset);
+            }
+        })
+    }
+
+    pub(crate) fn remove_remote(&self, addr: Address) {
+        let offset = addr.slot() - self.prev_sz;
+
+        self.slab.with(|slab| {
+            let slab = unsafe { &*slab }.as_ref();
+
+            let slot = if let Some(slot) = slab.and_then(|slab| slab.get(offset)) {
+                slot
+            } else {
+                return;
+            };
+
+            if !slot.reset(addr.generation()) {
+                return;
+            }
+
+            self.remote.push(offset, |next| slot.set_next(next));
+        })
+    }
+}
+
+impl fmt::Debug for Local {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.head.with(|head| {
+            let head = unsafe { *head };
+            f.debug_struct("Local")
+                .field("head", &format_args!("{:#0x}", head))
+                .finish()
+        })
+    }
+}
+
+impl<T> fmt::Debug for Shared<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Shared")
+            .field("remote", &self.remote)
+            .field("prev_sz", &self.prev_sz)
+            .field("size", &self.size)
+            // .field("slab", &self.slab)
+            .finish()
+    }
+}
diff --git a/third_party/rust/tokio/src/util/slab/shard.rs b/third_party/rust/tokio/src/util/slab/shard.rs
new file mode 100644
index 0000000000..eaca6f656a
--- /dev/null
+++ b/third_party/rust/tokio/src/util/slab/shard.rs
@@ -0,0 +1,105 @@
+use crate::util::slab::{page, Address, Entry, MAX_PAGES};
+
+use std::fmt;
+
+// ┌─────────────┐      ┌────────┐
+// │ page 1      │      │        │
+// ├─────────────┤ ┌───▶│  next──┼─┐
+// │ page 2      │ │    ├────────┤ │
+// │             │ │    │XXXXXXXX│ │
+// │ local_free──┼─┘    ├────────┤ │
+// │ global_free─┼─┐    │        │◀┘
+// ├─────────────┤ └───▶│  next──┼─┐
+// │   page 3    │      ├────────┤ │
+// └─────────────┘      │XXXXXXXX│ │
+//       ...            ├────────┤ │
+// ┌─────────────┐      │XXXXXXXX│ │
+// │ page n      │      ├────────┤ │
+// └─────────────┘      │        │◀┘
+//                      │  next──┼───▶
+//                      ├────────┤
+//                      │XXXXXXXX│
+//                      └────────┘
+//                         ...
+pub(super) struct Shard<T> {
+    /// The local free list for each page.
+    ///
+    /// These are only ever accessed from this shard's thread, so they are
+    /// stored separately from the shared state for the page that can be
+    /// accessed concurrently, to minimize false sharing.
+    local: Box<[page::Local]>,
+    /// The shared state for each page in this shard.
+    ///
+    /// This consists of the page's metadata (size, previous size), remote free
+    /// list, and a pointer to the actual array backing that page.
+    shared: Box<[page::Shared<T>]>,
+}
+
+impl<T: Entry> Shard<T> {
+    pub(super) fn new() -> Shard<T> {
+        let mut total_sz = 0;
+        let shared = (0..MAX_PAGES)
+            .map(|page_num| {
+                let sz = page::size(page_num);
+                let prev_sz = total_sz;
+                total_sz += sz;
+                page::Shared::new(sz, prev_sz)
+            })
+            .collect();
+
+        let local = (0..MAX_PAGES).map(|_| page::Local::new()).collect();
+
+        Shard { local, shared }
+    }
+
+    pub(super) fn alloc(&self) -> Option<Address> {
+        // Can we fit the value into an existing page?
+        for (page_idx, page) in self.shared.iter().enumerate() {
+            let local = self.local(page_idx);
+
+            if let Some(page_offset) = page.alloc(local) {
+                return Some(page_offset);
+            }
+        }
+
+        None
+    }
+
+    pub(super) fn get(&self, addr: Address) -> Option<&T> {
+        let page_idx = addr.page();
+
+        if page_idx > self.shared.len() {
+            return None;
+        }
+
+        self.shared[page_idx].get(addr)
+    }
+
+    /// Remove an item on the shard's local thread.
+    pub(super) fn remove_local(&self, addr: Address) {
+        let page_idx = addr.page();
+
+        if let Some(page) = self.shared.get(page_idx) {
+            page.remove_local(self.local(page_idx), addr);
+        }
+    }
+
+    /// Remove an item, while on a different thread from the shard's local thread.
+    pub(super) fn remove_remote(&self, addr: Address) {
+        if let Some(page) = self.shared.get(addr.page()) {
+            page.remove_remote(addr);
+        }
+    }
+
+    fn local(&self, i: usize) -> &page::Local {
+        &self.local[i]
+    }
+}
+
+impl<T> fmt::Debug for Shard<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Shard")
+            .field("shared", &self.shared)
+            .finish()
+    }
+}
diff --git a/third_party/rust/tokio/src/util/slab/slot.rs b/third_party/rust/tokio/src/util/slab/slot.rs
new file mode 100644
index 0000000000..0608b26189
--- /dev/null
+++ b/third_party/rust/tokio/src/util/slab/slot.rs
@@ -0,0 +1,42 @@
+use crate::loom::cell::UnsafeCell;
+use crate::util::slab::{Entry, Generation};
+
+/// Stores an entry in the slab.
+pub(super) struct Slot<T> {
+    next: UnsafeCell<usize>,
+    entry: T,
+}
+
+impl<T: Entry> Slot<T> {
+    /// Initialize a new `Slot` linked to `next`.
+    ///
+    /// The entry is initialized to a default value.
+    pub(super) fn new(next: usize) -> Slot<T> {
+        Slot {
+            next: UnsafeCell::new(next),
+            entry: T::default(),
+        }
+    }
+
+    pub(super) fn get(&self) -> &T {
+        &self.entry
+    }
+
+    pub(super) fn generation(&self) -> Generation {
+        self.entry.generation()
+    }
+
+    pub(super) fn reset(&self, generation: Generation) -> bool {
+        self.entry.reset(generation)
+    }
+
+    pub(super) fn next(&self) -> usize {
+        self.next.with(|next| unsafe { *next })
+    }
+
+    pub(super) fn set_next(&self, next: usize) {
+        self.next.with_mut(|n| unsafe {
+            (*n) = next;
+        })
+    }
+}
diff --git a/third_party/rust/tokio/src/util/slab/stack.rs b/third_party/rust/tokio/src/util/slab/stack.rs
new file mode 100644
index 0000000000..0ae0d71006
--- /dev/null
+++ b/third_party/rust/tokio/src/util/slab/stack.rs
@@ -0,0 +1,58 @@
+use crate::loom::sync::atomic::AtomicUsize;
+use crate::util::slab::Address;
+
+use std::fmt;
+use std::sync::atomic::Ordering;
+use std::usize;
+
+pub(super) struct TransferStack {
+    head: AtomicUsize,
+}
+
+impl TransferStack {
+    pub(super) fn new() -> Self {
+        Self {
+            head: AtomicUsize::new(Address::NULL),
+        }
+    }
+
+    pub(super) fn pop_all(&self) -> Option<usize> {
+        let val = self.head.swap(Address::NULL, Ordering::Acquire);
+
+        if val == Address::NULL {
+            None
+        } else {
+            Some(val)
+        }
+    }
+
+    pub(super) fn push(&self, value: usize, before: impl Fn(usize)) {
+        let mut next = self.head.load(Ordering::Relaxed);
+
+        loop {
+            before(next);
+
+            match self
+                .head
+                .compare_exchange(next, value, Ordering::AcqRel, Ordering::Acquire)
+            {
+                // lost the race!
+                Err(actual) => next = actual,
+                Ok(_) => return,
+            }
+        }
+    }
+}
+
+impl fmt::Debug for TransferStack {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // Loom likes to dump all its internal state in `fmt::Debug` impls, so
+        // we override this to just print the current value in tests.
+        f.debug_struct("TransferStack")
+            .field(
+                "head",
+                &format_args!("{:#x}", self.head.load(Ordering::Relaxed)),
+            )
+            .finish()
+    }
+}
diff --git a/third_party/rust/tokio/src/util/slab/tests/loom_slab.rs b/third_party/rust/tokio/src/util/slab/tests/loom_slab.rs
new file mode 100644
index 0000000000..48e94f0034
--- /dev/null
+++ b/third_party/rust/tokio/src/util/slab/tests/loom_slab.rs
@@ -0,0 +1,327 @@
+use crate::io::driver::ScheduledIo;
+use crate::util::slab::{Address, Slab};
+
+use loom::sync::{Arc, Condvar, Mutex};
+use loom::thread;
+
+#[test]
+fn local_remove() {
+    loom::model(|| {
+        let slab = Arc::new(Slab::new());
+
+        let s = slab.clone();
+        let t1 = thread::spawn(move || {
+            let idx = store_val(&s, 1);
+            assert_eq!(get_val(&s, idx), Some(1));
+            s.remove(idx);
+            assert_eq!(get_val(&s, idx), None);
+            let idx = store_val(&s, 2);
+            assert_eq!(get_val(&s, idx), Some(2));
+            s.remove(idx);
+            assert_eq!(get_val(&s, idx), None);
+        });
+
+        let s = slab.clone();
+        let t2 = thread::spawn(move || {
+            let idx = store_val(&s, 3);
+            assert_eq!(get_val(&s, idx), Some(3));
+            s.remove(idx);
+            assert_eq!(get_val(&s, idx), None);
+            let idx = store_val(&s, 4);
+            s.remove(idx);
+            assert_eq!(get_val(&s, idx), None);
+        });
+
+        let s = slab;
+        let idx1 = store_val(&s, 5);
+        assert_eq!(get_val(&s, idx1), Some(5));
+        let idx2 = store_val(&s, 6);
+        assert_eq!(get_val(&s, idx2), Some(6));
+        s.remove(idx1);
+        assert_eq!(get_val(&s, idx1), None);
+        assert_eq!(get_val(&s, idx2), Some(6));
+        s.remove(idx2);
+        assert_eq!(get_val(&s, idx2), None);
+
+        t1.join().expect("thread 1 should not panic");
+        t2.join().expect("thread 2 should not panic");
+    });
+}
+
+#[test]
+fn remove_remote() {
+    loom::model(|| {
+        let slab = Arc::new(Slab::new());
+
+        let idx1 = store_val(&slab, 1);
+        assert_eq!(get_val(&slab, idx1), Some(1));
+
+        let idx2 = store_val(&slab, 2);
+        assert_eq!(get_val(&slab, idx2), Some(2));
+
+        let idx3 = store_val(&slab, 3);
+        assert_eq!(get_val(&slab, idx3), Some(3));
+
+        let s = slab.clone();
+        let t1 = thread::spawn(move || {
+            assert_eq!(get_val(&s, idx2), Some(2));
+            s.remove(idx2);
+            assert_eq!(get_val(&s, idx2), None);
+        });
+
+        let s = slab.clone();
+        let t2 = thread::spawn(move || {
+            assert_eq!(get_val(&s, idx3), Some(3));
+            s.remove(idx3);
+            assert_eq!(get_val(&s, idx3), None);
+        });
+
+        t1.join().expect("thread 1 should not panic");
+        t2.join().expect("thread 2 should not panic");
+
+        assert_eq!(get_val(&slab, idx1), Some(1));
+        assert_eq!(get_val(&slab, idx2), None);
+        assert_eq!(get_val(&slab, idx3), None);
+    });
+}
+
+#[test]
+fn remove_remote_and_reuse() {
+    loom::model(|| {
+        let slab = Arc::new(Slab::new());
+
+        let idx1 = store_val(&slab, 1);
+        let idx2 = store_val(&slab, 2);
+
+        assert_eq!(get_val(&slab, idx1), Some(1));
+        assert_eq!(get_val(&slab, idx2), Some(2));
+
+        let s = slab.clone();
+        let t1 = thread::spawn(move || {
+            s.remove(idx1);
+            let value = get_val(&s, idx1);
+
+            // We may or may not see the new value yet, depending on when
+            // this occurs, but we must either see the new value or `None`;
+            // the old value has been removed!
+            assert!(value == None || value == Some(3));
+        });
+
+        let idx3 = store_when_free(&slab, 3);
+        t1.join().expect("thread 1 should not panic");
+
+        assert_eq!(get_val(&slab, idx3), Some(3));
+        assert_eq!(get_val(&slab, idx2), Some(2));
+    });
+}
+
+#[test]
+fn concurrent_alloc_remove() {
+    loom::model(|| {
+        let slab = Arc::new(Slab::new());
+        let pair = Arc::new((Mutex::new(None), Condvar::new()));
+
+        let slab2 = slab.clone();
+        let pair2 = pair.clone();
+        let remover = thread::spawn(move || {
+            let (lock, cvar) = &*pair2;
+            for _ in 0..2 {
+                let mut next = lock.lock().unwrap();
+                while next.is_none() {
+                    next = cvar.wait(next).unwrap();
+                }
+                let key = next.take().unwrap();
+                slab2.remove(key);
+                assert_eq!(get_val(&slab2, key), None);
+                cvar.notify_one();
+            }
+        });
+
+        let (lock, cvar) = &*pair;
+        for i in 0..2 {
+            let key = store_val(&slab, i);
+
+            let mut next = lock.lock().unwrap();
+            *next = Some(key);
+            cvar.notify_one();
+
+            // Wait for the item to be removed.
+            while next.is_some() {
+                next = cvar.wait(next).unwrap();
+            }
+
+            assert_eq!(get_val(&slab, key), None);
+        }
+
+        remover.join().unwrap();
+    })
+}
+
+#[test]
+fn concurrent_remove_remote_and_reuse() {
+    loom::model(|| {
+        let slab = Arc::new(Slab::new());
+
+        let idx1 = store_val(&slab, 1);
+        let idx2 = store_val(&slab, 2);
+
+        assert_eq!(get_val(&slab, idx1), Some(1));
+        assert_eq!(get_val(&slab, idx2), Some(2));
+
+        let s = slab.clone();
+        let s2 = slab.clone();
+        let t1 = thread::spawn(move || {
+            s.remove(idx1);
+        });
+
+        let t2 = thread::spawn(move || {
+            s2.remove(idx2);
+        });
+
+        let idx3 = store_when_free(&slab, 3);
+        t1.join().expect("thread 1 should not panic");
+        t2.join().expect("thread 1 should not panic");
+
+        assert!(get_val(&slab, idx1).is_none());
+        assert!(get_val(&slab, idx2).is_none());
+        assert_eq!(get_val(&slab, idx3), Some(3));
+    });
+}
+
+#[test]
+fn alloc_remove_get() {
+    loom::model(|| {
+        let slab = Arc::new(Slab::new());
+        let pair = Arc::new((Mutex::new(None), Condvar::new()));
+
+        let slab2 = slab.clone();
+        let pair2 = pair.clone();
+        let t1 = thread::spawn(move || {
+            let slab = slab2;
+            let (lock, cvar) = &*pair2;
+            // allocate one entry just so that we have to use the final one for
+            // all future allocations.
+            let _key0 = store_val(&slab, 0);
+            let key = store_val(&slab, 1);
+
+            let mut next = lock.lock().unwrap();
+            *next = Some(key);
+            cvar.notify_one();
+            // remove the second entry
+            slab.remove(key);
+            // store a new readiness at the same location (since the slab
+            // already has an entry in slot 0)
+            store_val(&slab, 2);
+        });
+
+        let (lock, cvar) = &*pair;
+        // wait for the second entry to be stored...
+        let mut next = lock.lock().unwrap();
+        while next.is_none() {
+            next = cvar.wait(next).unwrap();
+        }
+        let key = next.unwrap();
+
+        // our generation will be stale when the second store occurs at that
+        // index, we must not see the value of that store.
+        let val = get_val(&slab, key);
+        assert_ne!(val, Some(2), "generation must have advanced!");
+
+        t1.join().unwrap();
+    })
+}
+
+#[test]
+fn alloc_remove_set() {
+    loom::model(|| {
+        let slab = Arc::new(Slab::new());
+        let pair = Arc::new((Mutex::new(None), Condvar::new()));
+
+        let slab2 = slab.clone();
+        let pair2 = pair.clone();
+        let t1 = thread::spawn(move || {
+            let slab = slab2;
+            let (lock, cvar) = &*pair2;
+            // allocate one entry just so that we have to use the final one for
+            // all future allocations.
+            let _key0 = store_val(&slab, 0);
+            let key = store_val(&slab, 1);
+
+            let mut next = lock.lock().unwrap();
+            *next = Some(key);
+            cvar.notify_one();
+
+            slab.remove(key);
+            // remove the old entry and insert a new one, with a new generation.
+            let key2 = slab.alloc().expect("store key 2");
+            // after the remove, we must not see the value written with the
+            // stale index.
+            assert_eq!(
+                get_val(&slab, key),
+                None,
+                "stale set must no longer be visible"
+            );
+            assert_eq!(get_val(&slab, key2), Some(0));
+            key2
+        });
+
+        let (lock, cvar) = &*pair;
+
+        // wait for the second entry to be stored. the index we get from the
+        // other thread may become stale after a write.
+        let mut next = lock.lock().unwrap();
+        while next.is_none() {
+            next = cvar.wait(next).unwrap();
+        }
+        let key = next.unwrap();
+
+        // try to write to the index with our generation
+        slab.get(key).map(|val| val.set_readiness(key, |_| 2));
+
+        let key2 = t1.join().unwrap();
+        // after the remove, we must not see the value written with the
+        // stale index either.
+        assert_eq!(
+            get_val(&slab, key),
+            None,
+            "stale set must no longer be visible"
+        );
+        assert_eq!(get_val(&slab, key2), Some(0));
+    });
+}
+
+fn get_val(slab: &Arc<Slab<ScheduledIo>>, address: Address) -> Option<usize> {
+    slab.get(address).and_then(|s| s.get_readiness(address))
+}
+
+fn store_val(slab: &Arc<Slab<ScheduledIo>>, readiness: usize) -> Address {
+    let key = slab.alloc().expect("allocate slot");
+
+    if let Some(slot) = slab.get(key) {
+        slot.set_readiness(key, |_| readiness)
+            .expect("generation should still be valid!");
+    } else {
+        panic!("slab did not contain a value for {:?}", key);
+    }
+
+    key
+}
+
+fn store_when_free(slab: &Arc<Slab<ScheduledIo>>, readiness: usize) -> Address {
+    let key = loop {
+        if let Some(key) = slab.alloc() {
+            break key;
+        }
+
+        thread::yield_now();
+    };
+
+    if let Some(slot) = slab.get(key) {
+        slot.set_readiness(key, |_| readiness)
+            .expect("generation should still be valid!");
+    } else {
+        panic!("slab did not contain a value for {:?}", key);
+    }
+
+    key
+}
diff --git a/third_party/rust/tokio/src/util/slab/tests/loom_stack.rs b/third_party/rust/tokio/src/util/slab/tests/loom_stack.rs
new file mode 100644
index 0000000000..47ad46d3a1
--- /dev/null
+++ b/third_party/rust/tokio/src/util/slab/tests/loom_stack.rs
@@ -0,0 +1,88 @@
+use crate::util::slab::TransferStack;
+
+use loom::cell::UnsafeCell;
+use loom::sync::Arc;
+use loom::thread;
+
+#[test]
+fn transfer_stack() {
+    loom::model(|| {
+        let causalities = [UnsafeCell::new(None), UnsafeCell::new(None)];
+        let shared = Arc::new((causalities, TransferStack::new()));
+        let shared1 = shared.clone();
+        let shared2 = shared.clone();
+
+        // Spawn two threads that both try to push to the stack.
+        let t1 = thread::spawn(move || {
+            let (causalities, stack) = &*shared1;
+            stack.push(0, |prev| {
+                causalities[0].with_mut(|c| unsafe {
+                    *c = Some(prev);
+                });
+            });
+        });
+
+        let t2 = thread::spawn(move || {
+            let (causalities, stack) = &*shared2;
+            stack.push(1, |prev| {
+                causalities[1].with_mut(|c| unsafe {
+                    *c = Some(prev);
+                });
+            });
+        });
+
+        let (causalities, stack) = &*shared;
+
+        // Try to pop from the stack...
+        let mut idx = stack.pop_all();
+        while idx == None {
+            idx = stack.pop_all();
+            thread::yield_now();
+        }
+        let idx = idx.unwrap();
+
+        let saw_both = causalities[idx].with(|val| {
+            let val = unsafe { *val };
+            assert!(
+                val.is_some(),
+                "UnsafeCell write must happen-before index is pushed to the stack!",
+            );
+            // were there two entries in the stack? if so, check that
+            // both saw a write.
+            if let Some(c) = causalities.get(val.unwrap()) {
+                c.with(|val| {
+                    let val = unsafe { *val };
+                    assert!(
+                        val.is_some(),
+                        "UnsafeCell write must happen-before index is pushed to the stack!",
+                    );
+                });
+                true
+            } else {
+                false
+            }
+        });
+
+        // We only saw one push. Ensure that the other push happens too.
+        if !saw_both {
+            // Try to pop from the stack...
+            let mut idx = stack.pop_all();
+            while idx == None {
+                idx = stack.pop_all();
+                thread::yield_now();
+            }
+            let idx = idx.unwrap();
+
+            causalities[idx].with(|val| {
+                let val = unsafe { *val };
+                assert!(
+                    val.is_some(),
+                    "UnsafeCell write must happen-before index is pushed to the stack!",
+                );
+            });
+        }
+
+        t1.join().unwrap();
+        t2.join().unwrap();
+    });
+}
diff --git a/third_party/rust/tokio/src/util/slab/tests/mod.rs b/third_party/rust/tokio/src/util/slab/tests/mod.rs
new file mode 100644
index 0000000000..7f79354466
--- /dev/null
+++ b/third_party/rust/tokio/src/util/slab/tests/mod.rs
@@ -0,0 +1,2 @@
+mod loom_slab;
+mod loom_stack;
diff --git a/third_party/rust/tokio/src/util/try_lock.rs b/third_party/rust/tokio/src/util/try_lock.rs
new file mode 100644
index 0000000000..8b0edb4a87
--- /dev/null
+++ b/third_party/rust/tokio/src/util/try_lock.rs
@@ -0,0 +1,80 @@
+use crate::loom::sync::atomic::AtomicBool;
+
+use std::cell::UnsafeCell;
+use std::marker::PhantomData;
+use std::ops::{Deref, DerefMut};
+use std::sync::atomic::Ordering::SeqCst;
+
+pub(crate) struct TryLock<T> {
+    locked: AtomicBool,
+    data: UnsafeCell<T>,
+}
+
+pub(crate) struct LockGuard<'a, T> {
+    lock: &'a TryLock<T>,
+    _p: PhantomData<std::rc::Rc<()>>,
+}
+
+unsafe impl<T: Send> Send for TryLock<T> {}
+unsafe impl<T: Send> Sync for TryLock<T> {}
+
+unsafe impl<T: Sync> Sync for LockGuard<'_, T> {}
+
+macro_rules! new {
+    ($data:ident) => {
+        TryLock {
+            locked: AtomicBool::new(false),
+            data: UnsafeCell::new($data),
+        }
+    };
+}
+
+impl<T> TryLock<T> {
+    #[cfg(not(loom))]
+    /// Create a new `TryLock`
+    pub(crate) const fn new(data: T) -> TryLock<T> {
+        new!(data)
+    }
+
+    #[cfg(loom)]
+    /// Create a new `TryLock`
+    pub(crate) fn new(data: T) -> TryLock<T> {
+        new!(data)
+    }
+
+    /// Attempt to acquire lock
+    pub(crate) fn try_lock(&self) -> Option<LockGuard<'_, T>> {
+        if self
+            .locked
+            .compare_exchange(false, true, SeqCst, SeqCst)
+            .is_err()
+        {
+            return None;
+        }
+
+        Some(LockGuard {
+            lock: self,
+            _p: PhantomData,
+        })
+    }
+}
+
+impl<T> Deref for LockGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        unsafe { &*self.lock.data.get() }
+    }
+}
+
+impl<T> DerefMut for LockGuard<'_, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        unsafe { &mut *self.lock.data.get() }
+    }
+}
+
+impl<T> Drop for LockGuard<'_, T> {
+    fn drop(&mut self) {
+        self.lock.locked.store(false, SeqCst);
+    }
+}
diff --git a/third_party/rust/tokio/src/util/wake.rs b/third_party/rust/tokio/src/util/wake.rs
new file mode 100644
index 0000000000..e49f1e895d
--- /dev/null
+++ b/third_party/rust/tokio/src/util/wake.rs
@@ -0,0 +1,83 @@
+use std::marker::PhantomData;
+use std::mem::ManuallyDrop;
+use std::ops::Deref;
+use std::sync::Arc;
+use std::task::{RawWaker, RawWakerVTable, Waker};
+
+/// Simplfied waking interface based on Arcs
+pub(crate) trait Wake: Send + Sync {
+    /// Wake by value
+    fn wake(self: Arc<Self>);
+
+    /// Wake by reference
+    fn wake_by_ref(arc_self: &Arc<Self>);
+}
+
+/// A `Waker` that is only valid for a given lifetime.
+#[derive(Debug)]
+pub(crate) struct WakerRef<'a> {
+    waker: ManuallyDrop<Waker>,
+    _p: PhantomData<&'a ()>,
+}
+
+impl Deref for WakerRef<'_> {
+    type Target = Waker;
+
+    fn deref(&self) -> &Waker {
+        &self.waker
+    }
+}
+
+/// Creates a reference to a `Waker` from a reference to `Arc<impl Wake>`.
+pub(crate) fn waker_ref<W: Wake>(wake: &Arc<W>) -> WakerRef<'_> {
+    let ptr = &**wake as *const _ as *const ();
+
+    let waker = unsafe { Waker::from_raw(RawWaker::new(ptr, waker_vtable::<W>())) };
+
+    WakerRef {
+        waker: ManuallyDrop::new(waker),
+        _p: PhantomData,
+    }
+}
+
+fn waker_vtable<W: Wake>() -> &'static RawWakerVTable {
+    &RawWakerVTable::new(
+        clone_arc_raw::<W>,
+        wake_arc_raw::<W>,
+        wake_by_ref_arc_raw::<W>,
+        drop_arc_raw::<W>,
+    )
+}
+
+unsafe fn inc_ref_count<T: Wake>(data: *const ()) {
+    // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
+    let arc = ManuallyDrop::new(Arc::<T>::from_raw(data as *const T));
+
+    // Now increase refcount, but don't drop new refcount either
+    let arc_clone: ManuallyDrop<_> = arc.clone();
+
+    // Drop explicitly to avoid clippy warnings
+    drop(arc);
+    drop(arc_clone);
+}
+
+unsafe fn clone_arc_raw<T: Wake>(data: *const ()) -> RawWaker {
+    inc_ref_count::<T>(data);
+    RawWaker::new(data, waker_vtable::<T>())
+}
+
+unsafe fn wake_arc_raw<T: Wake>(data: *const ()) {
+    let arc: Arc<T> = Arc::from_raw(data as *const T);
+    Wake::wake(arc);
+}
+
+// used by `waker_ref`
+unsafe fn wake_by_ref_arc_raw<T: Wake>(data: *const ()) {
+    // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
+    let arc = ManuallyDrop::new(Arc::<T>::from_raw(data as *const T));
+    Wake::wake_by_ref(&arc);
+}
+
+unsafe fn drop_arc_raw<T: Wake>(data: *const ()) {
+    drop(Arc::<T>::from_raw(data as *const T))
+}