Adding upstream version 110.0.1.upstream/110.0.1upstream

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

12 files changed, 4908 insertions, 0 deletions

diff --git a/third_party/rust/crossbeam-epoch/src/atomic.rs b/third_party/rust/crossbeam-epoch/src/atomic.rs
new file mode 100644
index 0000000000..19bab47296
--- /dev/null
+++ b/third_party/rust/crossbeam-epoch/src/atomic.rs
@@ -0,0 +1,1718 @@
+use core::borrow::{Borrow, BorrowMut};
+use core::cmp;
+use core::fmt;
+use core::marker::PhantomData;
+use core::mem::{self, MaybeUninit};
+use core::ops::{Deref, DerefMut};
+use core::slice;
+use core::sync::atomic::Ordering;
+
+use crate::alloc::alloc;
+use crate::alloc::boxed::Box;
+use crate::guard::Guard;
+use crate::primitive::sync::atomic::AtomicUsize;
+use crossbeam_utils::atomic::AtomicConsume;
+
+/// Given ordering for the success case in a compare-exchange operation, returns the strongest
+/// appropriate ordering for the failure case.
+#[inline]
+fn strongest_failure_ordering(ord: Ordering) -> Ordering {
+    use self::Ordering::*;
+    match ord {
+        Relaxed | Release => Relaxed,
+        Acquire | AcqRel => Acquire,
+        _ => SeqCst,
+    }
+}
+
+/// The error returned on failed compare-and-set operation.
+// TODO: remove in the next major version.
+#[deprecated(note = "Use `CompareExchangeError` instead")]
+pub type CompareAndSetError<'g, T, P> = CompareExchangeError<'g, T, P>;
+
+/// The error returned on failed compare-and-swap operation.
+pub struct CompareExchangeError<'g, T: ?Sized + Pointable, P: Pointer<T>> {
+    /// The value in the atomic pointer at the time of the failed operation.
+    pub current: Shared<'g, T>,
+
+    /// The new value, which the operation failed to store.
+    pub new: P,
+}
+
+impl<T, P: Pointer<T> + fmt::Debug> fmt::Debug for CompareExchangeError<'_, T, P> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("CompareExchangeError")
+            .field("current", &self.current)
+            .field("new", &self.new)
+            .finish()
+    }
+}
+
+/// Memory orderings for compare-and-set operations.
+///
+/// A compare-and-set operation can have different memory orderings depending on whether it
+/// succeeds or fails. This trait generalizes different ways of specifying memory orderings.
+///
+/// The two ways of specifying orderings for compare-and-set are:
+///
+/// 1. Just one `Ordering` for the success case. In case of failure, the strongest appropriate
+///    ordering is chosen.
+/// 2. A pair of `Ordering`s. The first one is for the success case, while the second one is
+///    for the failure case.
+// TODO: remove in the next major version.
+#[deprecated(
+    note = "`compare_and_set` and `compare_and_set_weak` that use this trait are deprecated, \
+            use `compare_exchange` or `compare_exchange_weak instead`"
+)]
+pub trait CompareAndSetOrdering {
+    /// The ordering of the operation when it succeeds.
+    fn success(&self) -> Ordering;
+
+    /// The ordering of the operation when it fails.
+    ///
+    /// The failure ordering can't be `Release` or `AcqRel` and must be equivalent or weaker than
+    /// the success ordering.
+    fn failure(&self) -> Ordering;
+}
+
+#[allow(deprecated)]
+impl CompareAndSetOrdering for Ordering {
+    #[inline]
+    fn success(&self) -> Ordering {
+        *self
+    }
+
+    #[inline]
+    fn failure(&self) -> Ordering {
+        strongest_failure_ordering(*self)
+    }
+}
+
+#[allow(deprecated)]
+impl CompareAndSetOrdering for (Ordering, Ordering) {
+    #[inline]
+    fn success(&self) -> Ordering {
+        self.0
+    }
+
+    #[inline]
+    fn failure(&self) -> Ordering {
+        self.1
+    }
+}
+
+/// Returns a bitmask containing the unused least significant bits of an aligned pointer to `T`.
+#[inline]
+fn low_bits<T: ?Sized + Pointable>() -> usize {
+    (1 << T::ALIGN.trailing_zeros()) - 1
+}
+
+/// Panics if the pointer is not properly unaligned.
+#[inline]
+fn ensure_aligned<T: ?Sized + Pointable>(raw: usize) {
+    assert_eq!(raw & low_bits::<T>(), 0, "unaligned pointer");
+}
+
+/// Given a tagged pointer `data`, returns the same pointer, but tagged with `tag`.
+///
+/// `tag` is truncated to fit into the unused bits of the pointer to `T`.
+#[inline]
+fn compose_tag<T: ?Sized + Pointable>(data: usize, tag: usize) -> usize {
+    (data & !low_bits::<T>()) | (tag & low_bits::<T>())
+}
+
+/// Decomposes a tagged pointer `data` into the pointer and the tag.
+#[inline]
+fn decompose_tag<T: ?Sized + Pointable>(data: usize) -> (usize, usize) {
+    (data & !low_bits::<T>(), data & low_bits::<T>())
+}
+
+/// Types that are pointed to by a single word.
+///
+/// In concurrent programming, it is necessary to represent an object within a word because atomic
+/// operations (e.g., reads, writes, read-modify-writes) support only single words.  This trait
+/// qualifies such types that are pointed to by a single word.
+///
+/// The trait generalizes `Box<T>` for a sized type `T`.  In a box, an object of type `T` is
+/// allocated in heap and it is owned by a single-word pointer.  This trait is also implemented for
+/// `[MaybeUninit<T>]` by storing its size along with its elements and pointing to the pair of array
+/// size and elements.
+///
+/// Pointers to `Pointable` types can be stored in [`Atomic`], [`Owned`], and [`Shared`].  In
+/// particular, Crossbeam supports dynamically sized slices as follows.
+///
+/// ```
+/// use std::mem::MaybeUninit;
+/// use crossbeam_epoch::Owned;
+///
+/// let o = Owned::<[MaybeUninit<i32>]>::init(10); // allocating [i32; 10]
+/// ```
+pub trait Pointable {
+    /// The alignment of pointer.
+    const ALIGN: usize;
+
+    /// The type for initializers.
+    type Init;
+
+    /// Initializes a with the given initializer.
+    ///
+    /// # Safety
+    ///
+    /// The result should be a multiple of `ALIGN`.
+    unsafe fn init(init: Self::Init) -> usize;
+
+    /// Dereferences the given pointer.
+    ///
+    /// # Safety
+    ///
+    /// - The given `ptr` should have been initialized with [`Pointable::init`].
+    /// - `ptr` should not have yet been dropped by [`Pointable::drop`].
+    /// - `ptr` should not be mutably dereferenced by [`Pointable::deref_mut`] concurrently.
+    unsafe fn deref<'a>(ptr: usize) -> &'a Self;
+
+    /// Mutably dereferences the given pointer.
+    ///
+    /// # Safety
+    ///
+    /// - The given `ptr` should have been initialized with [`Pointable::init`].
+    /// - `ptr` should not have yet been dropped by [`Pointable::drop`].
+    /// - `ptr` should not be dereferenced by [`Pointable::deref`] or [`Pointable::deref_mut`]
+    ///   concurrently.
+    unsafe fn deref_mut<'a>(ptr: usize) -> &'a mut Self;
+
+    /// Drops the object pointed to by the given pointer.
+    ///
+    /// # Safety
+    ///
+    /// - The given `ptr` should have been initialized with [`Pointable::init`].
+    /// - `ptr` should not have yet been dropped by [`Pointable::drop`].
+    /// - `ptr` should not be dereferenced by [`Pointable::deref`] or [`Pointable::deref_mut`]
+    ///   concurrently.
+    unsafe fn drop(ptr: usize);
+}
+
+impl<T> Pointable for T {
+    const ALIGN: usize = mem::align_of::<T>();
+
+    type Init = T;
+
+    unsafe fn init(init: Self::Init) -> usize {
+        Box::into_raw(Box::new(init)) as usize
+    }
+
+    unsafe fn deref<'a>(ptr: usize) -> &'a Self {
+        &*(ptr as *const T)
+    }
+
+    unsafe fn deref_mut<'a>(ptr: usize) -> &'a mut Self {
+        &mut *(ptr as *mut T)
+    }
+
+    unsafe fn drop(ptr: usize) {
+        drop(Box::from_raw(ptr as *mut T));
+    }
+}
+
+/// Array with size.
+///
+/// # Memory layout
+///
+/// An array consisting of size and elements:
+///
+/// ```text
+///          elements
+///          |
+///          |
+/// ------------------------------------
+/// | size | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
+/// ------------------------------------
+/// ```
+///
+/// Its memory layout is different from that of `Box<[T]>` in that size is in the allocation (not
+/// along with pointer as in `Box<[T]>`).
+///
+/// Elements are not present in the type, but they will be in the allocation.
+/// ```
+///
+// TODO(@jeehoonkang): once we bump the minimum required Rust version to 1.44 or newer, use
+// [`alloc::alloc::Layout::extend`] instead.
+#[repr(C)]
+struct Array<T> {
+    /// The number of elements (not the number of bytes).
+    len: usize,
+    elements: [MaybeUninit<T>; 0],
+}
+
+impl<T> Pointable for [MaybeUninit<T>] {
+    const ALIGN: usize = mem::align_of::<Array<T>>();
+
+    type Init = usize;
+
+    unsafe fn init(len: Self::Init) -> usize {
+        let size = mem::size_of::<Array<T>>() + mem::size_of::<MaybeUninit<T>>() * len;
+        let align = mem::align_of::<Array<T>>();
+        let layout = alloc::Layout::from_size_align(size, align).unwrap();
+        let ptr = alloc::alloc(layout).cast::<Array<T>>();
+        if ptr.is_null() {
+            alloc::handle_alloc_error(layout);
+        }
+        (*ptr).len = len;
+        ptr as usize
+    }
+
+    unsafe fn deref<'a>(ptr: usize) -> &'a Self {
+        let array = &*(ptr as *const Array<T>);
+        slice::from_raw_parts(array.elements.as_ptr() as *const _, array.len)
+    }
+
+    unsafe fn deref_mut<'a>(ptr: usize) -> &'a mut Self {
+        let array = &*(ptr as *mut Array<T>);
+        slice::from_raw_parts_mut(array.elements.as_ptr() as *mut _, array.len)
+    }
+
+    unsafe fn drop(ptr: usize) {
+        let array = &*(ptr as *mut Array<T>);
+        let size = mem::size_of::<Array<T>>() + mem::size_of::<MaybeUninit<T>>() * array.len;
+        let align = mem::align_of::<Array<T>>();
+        let layout = alloc::Layout::from_size_align(size, align).unwrap();
+        alloc::dealloc(ptr as *mut u8, layout);
+    }
+}
+
+/// An atomic pointer that can be safely shared between threads.
+///
+/// The pointer must be properly aligned. Since it is aligned, a tag can be stored into the unused
+/// least significant bits of the address. For example, the tag for a pointer to a sized type `T`
+/// should be less than `(1 << mem::align_of::<T>().trailing_zeros())`.
+///
+/// Any method that loads the pointer must be passed a reference to a [`Guard`].
+///
+/// Crossbeam supports dynamically sized types.  See [`Pointable`] for details.
+pub struct Atomic<T: ?Sized + Pointable> {
+    data: AtomicUsize,
+    _marker: PhantomData<*mut T>,
+}
+
+unsafe impl<T: ?Sized + Pointable + Send + Sync> Send for Atomic<T> {}
+unsafe impl<T: ?Sized + Pointable + Send + Sync> Sync for Atomic<T> {}
+
+impl<T> Atomic<T> {
+    /// Allocates `value` on the heap and returns a new atomic pointer pointing to it.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::Atomic;
+    ///
+    /// let a = Atomic::new(1234);
+    /// # unsafe { drop(a.into_owned()); } // avoid leak
+    /// ```
+    pub fn new(init: T) -> Atomic<T> {
+        Self::init(init)
+    }
+}
+
+impl<T: ?Sized + Pointable> Atomic<T> {
+    /// Allocates `value` on the heap and returns a new atomic pointer pointing to it.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::Atomic;
+    ///
+    /// let a = Atomic::<i32>::init(1234);
+    /// # unsafe { drop(a.into_owned()); } // avoid leak
+    /// ```
+    pub fn init(init: T::Init) -> Atomic<T> {
+        Self::from(Owned::init(init))
+    }
+
+    /// Returns a new atomic pointer pointing to the tagged pointer `data`.
+    fn from_usize(data: usize) -> Self {
+        Self {
+            data: AtomicUsize::new(data),
+            _marker: PhantomData,
+        }
+    }
+
+    /// Returns a new null atomic pointer.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::Atomic;
+    ///
+    /// let a = Atomic::<i32>::null();
+    /// ```
+    #[cfg(all(not(crossbeam_no_const_fn_trait_bound), not(crossbeam_loom)))]
+    pub const fn null() -> Atomic<T> {
+        Self {
+            data: AtomicUsize::new(0),
+            _marker: PhantomData,
+        }
+    }
+
+    /// Returns a new null atomic pointer.
+    #[cfg(not(all(not(crossbeam_no_const_fn_trait_bound), not(crossbeam_loom))))]
+    pub fn null() -> Atomic<T> {
+        Self {
+            data: AtomicUsize::new(0),
+            _marker: PhantomData,
+        }
+    }
+
+    /// Loads a `Shared` from the atomic pointer.
+    ///
+    /// This method takes an [`Ordering`] argument which describes the memory ordering of this
+    /// operation.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::new(1234);
+    /// let guard = &epoch::pin();
+    /// let p = a.load(SeqCst, guard);
+    /// # unsafe { drop(a.into_owned()); } // avoid leak
+    /// ```
+    pub fn load<'g>(&self, ord: Ordering, _: &'g Guard) -> Shared<'g, T> {
+        unsafe { Shared::from_usize(self.data.load(ord)) }
+    }
+
+    /// Loads a `Shared` from the atomic pointer using a "consume" memory ordering.
+    ///
+    /// This is similar to the "acquire" ordering, except that an ordering is
+    /// only guaranteed with operations that "depend on" the result of the load.
+    /// However consume loads are usually much faster than acquire loads on
+    /// architectures with a weak memory model since they don't require memory
+    /// fence instructions.
+    ///
+    /// The exact definition of "depend on" is a bit vague, but it works as you
+    /// would expect in practice since a lot of software, especially the Linux
+    /// kernel, rely on this behavior.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic};
+    ///
+    /// let a = Atomic::new(1234);
+    /// let guard = &epoch::pin();
+    /// let p = a.load_consume(guard);
+    /// # unsafe { drop(a.into_owned()); } // avoid leak
+    /// ```
+    pub fn load_consume<'g>(&self, _: &'g Guard) -> Shared<'g, T> {
+        unsafe { Shared::from_usize(self.data.load_consume()) }
+    }
+
+    /// Stores a `Shared` or `Owned` pointer into the atomic pointer.
+    ///
+    /// This method takes an [`Ordering`] argument which describes the memory ordering of this
+    /// operation.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{Atomic, Owned, Shared};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::new(1234);
+    /// # unsafe { drop(a.load(SeqCst, &crossbeam_epoch::pin()).into_owned()); } // avoid leak
+    /// a.store(Shared::null(), SeqCst);
+    /// a.store(Owned::new(1234), SeqCst);
+    /// # unsafe { drop(a.into_owned()); } // avoid leak
+    /// ```
+    pub fn store<P: Pointer<T>>(&self, new: P, ord: Ordering) {
+        self.data.store(new.into_usize(), ord);
+    }
+
+    /// Stores a `Shared` or `Owned` pointer into the atomic pointer, returning the previous
+    /// `Shared`.
+    ///
+    /// This method takes an [`Ordering`] argument which describes the memory ordering of this
+    /// operation.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic, Shared};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::new(1234);
+    /// let guard = &epoch::pin();
+    /// let p = a.swap(Shared::null(), SeqCst, guard);
+    /// # unsafe { drop(p.into_owned()); } // avoid leak
+    /// ```
+    pub fn swap<'g, P: Pointer<T>>(&self, new: P, ord: Ordering, _: &'g Guard) -> Shared<'g, T> {
+        unsafe { Shared::from_usize(self.data.swap(new.into_usize(), ord)) }
+    }
+
+    /// Stores the pointer `new` (either `Shared` or `Owned`) into the atomic pointer if the current
+    /// value is the same as `current`. The tag is also taken into account, so two pointers to the
+    /// same object, but with different tags, will not be considered equal.
+    ///
+    /// The return value is a result indicating whether the new pointer was written. On success the
+    /// pointer that was written is returned. On failure the actual current value and `new` are
+    /// returned.
+    ///
+    /// This method takes two `Ordering` arguments to describe the memory
+    /// ordering of this operation. `success` describes the required ordering for the
+    /// read-modify-write operation that takes place if the comparison with `current` succeeds.
+    /// `failure` describes the required ordering for the load operation that takes place when
+    /// the comparison fails. Using `Acquire` as success ordering makes the store part
+    /// of this operation `Relaxed`, and using `Release` makes the successful load
+    /// `Relaxed`. The failure ordering can only be `SeqCst`, `Acquire` or `Relaxed`
+    /// and must be equivalent to or weaker than the success ordering.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic, Owned, Shared};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::new(1234);
+    ///
+    /// let guard = &epoch::pin();
+    /// let curr = a.load(SeqCst, guard);
+    /// let res1 = a.compare_exchange(curr, Shared::null(), SeqCst, SeqCst, guard);
+    /// let res2 = a.compare_exchange(curr, Owned::new(5678), SeqCst, SeqCst, guard);
+    /// # unsafe { drop(curr.into_owned()); } // avoid leak
+    /// ```
+    pub fn compare_exchange<'g, P>(
+        &self,
+        current: Shared<'_, T>,
+        new: P,
+        success: Ordering,
+        failure: Ordering,
+        _: &'g Guard,
+    ) -> Result<Shared<'g, T>, CompareExchangeError<'g, T, P>>
+    where
+        P: Pointer<T>,
+    {
+        let new = new.into_usize();
+        self.data
+            .compare_exchange(current.into_usize(), new, success, failure)
+            .map(|_| unsafe { Shared::from_usize(new) })
+            .map_err(|current| unsafe {
+                CompareExchangeError {
+                    current: Shared::from_usize(current),
+                    new: P::from_usize(new),
+                }
+            })
+    }
+
+    /// Stores the pointer `new` (either `Shared` or `Owned`) into the atomic pointer if the current
+    /// value is the same as `current`. The tag is also taken into account, so two pointers to the
+    /// same object, but with different tags, will not be considered equal.
+    ///
+    /// Unlike [`compare_exchange`], this method is allowed to spuriously fail even when comparison
+    /// succeeds, which can result in more efficient code on some platforms.  The return value is a
+    /// result indicating whether the new pointer was written. On success the pointer that was
+    /// written is returned. On failure the actual current value and `new` are returned.
+    ///
+    /// This method takes two `Ordering` arguments to describe the memory
+    /// ordering of this operation. `success` describes the required ordering for the
+    /// read-modify-write operation that takes place if the comparison with `current` succeeds.
+    /// `failure` describes the required ordering for the load operation that takes place when
+    /// the comparison fails. Using `Acquire` as success ordering makes the store part
+    /// of this operation `Relaxed`, and using `Release` makes the successful load
+    /// `Relaxed`. The failure ordering can only be `SeqCst`, `Acquire` or `Relaxed`
+    /// and must be equivalent to or weaker than the success ordering.
+    ///
+    /// [`compare_exchange`]: Atomic::compare_exchange
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic, Owned, Shared};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::new(1234);
+    /// let guard = &epoch::pin();
+    ///
+    /// let mut new = Owned::new(5678);
+    /// let mut ptr = a.load(SeqCst, guard);
+    /// # unsafe { drop(a.load(SeqCst, guard).into_owned()); } // avoid leak
+    /// loop {
+    ///     match a.compare_exchange_weak(ptr, new, SeqCst, SeqCst, guard) {
+    ///         Ok(p) => {
+    ///             ptr = p;
+    ///             break;
+    ///         }
+    ///         Err(err) => {
+    ///             ptr = err.current;
+    ///             new = err.new;
+    ///         }
+    ///     }
+    /// }
+    ///
+    /// let mut curr = a.load(SeqCst, guard);
+    /// loop {
+    ///     match a.compare_exchange_weak(curr, Shared::null(), SeqCst, SeqCst, guard) {
+    ///         Ok(_) => break,
+    ///         Err(err) => curr = err.current,
+    ///     }
+    /// }
+    /// # unsafe { drop(curr.into_owned()); } // avoid leak
+    /// ```
+    pub fn compare_exchange_weak<'g, P>(
+        &self,
+        current: Shared<'_, T>,
+        new: P,
+        success: Ordering,
+        failure: Ordering,
+        _: &'g Guard,
+    ) -> Result<Shared<'g, T>, CompareExchangeError<'g, T, P>>
+    where
+        P: Pointer<T>,
+    {
+        let new = new.into_usize();
+        self.data
+            .compare_exchange_weak(current.into_usize(), new, success, failure)
+            .map(|_| unsafe { Shared::from_usize(new) })
+            .map_err(|current| unsafe {
+                CompareExchangeError {
+                    current: Shared::from_usize(current),
+                    new: P::from_usize(new),
+                }
+            })
+    }
+
+    /// Fetches the pointer, and then applies a function to it that returns a new value.
+    /// Returns a `Result` of `Ok(previous_value)` if the function returned `Some`, else `Err(_)`.
+    ///
+    /// Note that the given function may be called multiple times if the value has been changed by
+    /// other threads in the meantime, as long as the function returns `Some(_)`, but the function
+    /// will have been applied only once to the stored value.
+    ///
+    /// `fetch_update` takes two [`Ordering`] arguments to describe the memory
+    /// ordering of this operation. The first describes the required ordering for
+    /// when the operation finally succeeds while the second describes the
+    /// required ordering for loads. These correspond to the success and failure
+    /// orderings of [`Atomic::compare_exchange`] respectively.
+    ///
+    /// Using [`Acquire`] as success ordering makes the store part of this
+    /// operation [`Relaxed`], and using [`Release`] makes the final successful
+    /// load [`Relaxed`]. The (failed) load ordering can only be [`SeqCst`],
+    /// [`Acquire`] or [`Relaxed`] and must be equivalent to or weaker than the
+    /// success ordering.
+    ///
+    /// [`Relaxed`]: Ordering::Relaxed
+    /// [`Acquire`]: Ordering::Acquire
+    /// [`Release`]: Ordering::Release
+    /// [`SeqCst`]: Ordering::SeqCst
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::new(1234);
+    /// let guard = &epoch::pin();
+    ///
+    /// let res1 = a.fetch_update(SeqCst, SeqCst, guard, |x| Some(x.with_tag(1)));
+    /// assert!(res1.is_ok());
+    ///
+    /// let res2 = a.fetch_update(SeqCst, SeqCst, guard, |x| None);
+    /// assert!(res2.is_err());
+    /// # unsafe { drop(a.into_owned()); } // avoid leak
+    /// ```
+    pub fn fetch_update<'g, F>(
+        &self,
+        set_order: Ordering,
+        fail_order: Ordering,
+        guard: &'g Guard,
+        mut func: F,
+    ) -> Result<Shared<'g, T>, Shared<'g, T>>
+    where
+        F: FnMut(Shared<'g, T>) -> Option<Shared<'g, T>>,
+    {
+        let mut prev = self.load(fail_order, guard);
+        while let Some(next) = func(prev) {
+            match self.compare_exchange_weak(prev, next, set_order, fail_order, guard) {
+                Ok(shared) => return Ok(shared),
+                Err(next_prev) => prev = next_prev.current,
+            }
+        }
+        Err(prev)
+    }
+
+    /// Stores the pointer `new` (either `Shared` or `Owned`) into the atomic pointer if the current
+    /// value is the same as `current`. The tag is also taken into account, so two pointers to the
+    /// same object, but with different tags, will not be considered equal.
+    ///
+    /// The return value is a result indicating whether the new pointer was written. On success the
+    /// pointer that was written is returned. On failure the actual current value and `new` are
+    /// returned.
+    ///
+    /// This method takes a [`CompareAndSetOrdering`] argument which describes the memory
+    /// ordering of this operation.
+    ///
+    /// # Migrating to `compare_exchange`
+    ///
+    /// `compare_and_set` is equivalent to `compare_exchange` with the following mapping for
+    /// memory orderings:
+    ///
+    /// Original | Success | Failure
+    /// -------- | ------- | -------
+    /// Relaxed  | Relaxed | Relaxed
+    /// Acquire  | Acquire | Acquire
+    /// Release  | Release | Relaxed
+    /// AcqRel   | AcqRel  | Acquire
+    /// SeqCst   | SeqCst  | SeqCst
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![allow(deprecated)]
+    /// use crossbeam_epoch::{self as epoch, Atomic, Owned, Shared};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::new(1234);
+    ///
+    /// let guard = &epoch::pin();
+    /// let curr = a.load(SeqCst, guard);
+    /// let res1 = a.compare_and_set(curr, Shared::null(), SeqCst, guard);
+    /// let res2 = a.compare_and_set(curr, Owned::new(5678), SeqCst, guard);
+    /// # unsafe { drop(curr.into_owned()); } // avoid leak
+    /// ```
+    // TODO: remove in the next major version.
+    #[allow(deprecated)]
+    #[deprecated(note = "Use `compare_exchange` instead")]
+    pub fn compare_and_set<'g, O, P>(
+        &self,
+        current: Shared<'_, T>,
+        new: P,
+        ord: O,
+        guard: &'g Guard,
+    ) -> Result<Shared<'g, T>, CompareAndSetError<'g, T, P>>
+    where
+        O: CompareAndSetOrdering,
+        P: Pointer<T>,
+    {
+        self.compare_exchange(current, new, ord.success(), ord.failure(), guard)
+    }
+
+    /// Stores the pointer `new` (either `Shared` or `Owned`) into the atomic pointer if the current
+    /// value is the same as `current`. The tag is also taken into account, so two pointers to the
+    /// same object, but with different tags, will not be considered equal.
+    ///
+    /// Unlike [`compare_and_set`], this method is allowed to spuriously fail even when comparison
+    /// succeeds, which can result in more efficient code on some platforms.  The return value is a
+    /// result indicating whether the new pointer was written. On success the pointer that was
+    /// written is returned. On failure the actual current value and `new` are returned.
+    ///
+    /// This method takes a [`CompareAndSetOrdering`] argument which describes the memory
+    /// ordering of this operation.
+    ///
+    /// [`compare_and_set`]: Atomic::compare_and_set
+    ///
+    /// # Migrating to `compare_exchange_weak`
+    ///
+    /// `compare_and_set_weak` is equivalent to `compare_exchange_weak` with the following mapping for
+    /// memory orderings:
+    ///
+    /// Original | Success | Failure
+    /// -------- | ------- | -------
+    /// Relaxed  | Relaxed | Relaxed
+    /// Acquire  | Acquire | Acquire
+    /// Release  | Release | Relaxed
+    /// AcqRel   | AcqRel  | Acquire
+    /// SeqCst   | SeqCst  | SeqCst
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![allow(deprecated)]
+    /// use crossbeam_epoch::{self as epoch, Atomic, Owned, Shared};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::new(1234);
+    /// let guard = &epoch::pin();
+    ///
+    /// let mut new = Owned::new(5678);
+    /// let mut ptr = a.load(SeqCst, guard);
+    /// # unsafe { drop(a.load(SeqCst, guard).into_owned()); } // avoid leak
+    /// loop {
+    ///     match a.compare_and_set_weak(ptr, new, SeqCst, guard) {
+    ///         Ok(p) => {
+    ///             ptr = p;
+    ///             break;
+    ///         }
+    ///         Err(err) => {
+    ///             ptr = err.current;
+    ///             new = err.new;
+    ///         }
+    ///     }
+    /// }
+    ///
+    /// let mut curr = a.load(SeqCst, guard);
+    /// loop {
+    ///     match a.compare_and_set_weak(curr, Shared::null(), SeqCst, guard) {
+    ///         Ok(_) => break,
+    ///         Err(err) => curr = err.current,
+    ///     }
+    /// }
+    /// # unsafe { drop(curr.into_owned()); } // avoid leak
+    /// ```
+    // TODO: remove in the next major version.
+    #[allow(deprecated)]
+    #[deprecated(note = "Use `compare_exchange_weak` instead")]
+    pub fn compare_and_set_weak<'g, O, P>(
+        &self,
+        current: Shared<'_, T>,
+        new: P,
+        ord: O,
+        guard: &'g Guard,
+    ) -> Result<Shared<'g, T>, CompareAndSetError<'g, T, P>>
+    where
+        O: CompareAndSetOrdering,
+        P: Pointer<T>,
+    {
+        self.compare_exchange_weak(current, new, ord.success(), ord.failure(), guard)
+    }
+
+    /// Bitwise "and" with the current tag.
+    ///
+    /// Performs a bitwise "and" operation on the current tag and the argument `val`, and sets the
+    /// new tag to the result. Returns the previous pointer.
+    ///
+    /// This method takes an [`Ordering`] argument which describes the memory ordering of this
+    /// operation.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic, Shared};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::<i32>::from(Shared::null().with_tag(3));
+    /// let guard = &epoch::pin();
+    /// assert_eq!(a.fetch_and(2, SeqCst, guard).tag(), 3);
+    /// assert_eq!(a.load(SeqCst, guard).tag(), 2);
+    /// ```
+    pub fn fetch_and<'g>(&self, val: usize, ord: Ordering, _: &'g Guard) -> Shared<'g, T> {
+        unsafe { Shared::from_usize(self.data.fetch_and(val | !low_bits::<T>(), ord)) }
+    }
+
+    /// Bitwise "or" with the current tag.
+    ///
+    /// Performs a bitwise "or" operation on the current tag and the argument `val`, and sets the
+    /// new tag to the result. Returns the previous pointer.
+    ///
+    /// This method takes an [`Ordering`] argument which describes the memory ordering of this
+    /// operation.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic, Shared};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::<i32>::from(Shared::null().with_tag(1));
+    /// let guard = &epoch::pin();
+    /// assert_eq!(a.fetch_or(2, SeqCst, guard).tag(), 1);
+    /// assert_eq!(a.load(SeqCst, guard).tag(), 3);
+    /// ```
+    pub fn fetch_or<'g>(&self, val: usize, ord: Ordering, _: &'g Guard) -> Shared<'g, T> {
+        unsafe { Shared::from_usize(self.data.fetch_or(val & low_bits::<T>(), ord)) }
+    }
+
+    /// Bitwise "xor" with the current tag.
+    ///
+    /// Performs a bitwise "xor" operation on the current tag and the argument `val`, and sets the
+    /// new tag to the result. Returns the previous pointer.
+    ///
+    /// This method takes an [`Ordering`] argument which describes the memory ordering of this
+    /// operation.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic, Shared};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::<i32>::from(Shared::null().with_tag(1));
+    /// let guard = &epoch::pin();
+    /// assert_eq!(a.fetch_xor(3, SeqCst, guard).tag(), 1);
+    /// assert_eq!(a.load(SeqCst, guard).tag(), 2);
+    /// ```
+    pub fn fetch_xor<'g>(&self, val: usize, ord: Ordering, _: &'g Guard) -> Shared<'g, T> {
+        unsafe { Shared::from_usize(self.data.fetch_xor(val & low_bits::<T>(), ord)) }
+    }
+
+    /// Takes ownership of the pointee.
+    ///
+    /// This consumes the atomic and converts it into [`Owned`]. As [`Atomic`] doesn't have a
+    /// destructor and doesn't drop the pointee while [`Owned`] does, this is suitable for
+    /// destructors of data structures.
+    ///
+    /// # Panics
+    ///
+    /// Panics if this pointer is null, but only in debug mode.
+    ///
+    /// # Safety
+    ///
+    /// This method may be called only if the pointer is valid and nobody else is holding a
+    /// reference to the same object.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # use std::mem;
+    /// # use crossbeam_epoch::Atomic;
+    /// struct DataStructure {
+    ///     ptr: Atomic<usize>,
+    /// }
+    ///
+    /// impl Drop for DataStructure {
+    ///     fn drop(&mut self) {
+    ///         // By now the DataStructure lives only in our thread and we are sure we don't hold
+    ///         // any Shared or & to it ourselves.
+    ///         unsafe {
+    ///             drop(mem::replace(&mut self.ptr, Atomic::null()).into_owned());
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    pub unsafe fn into_owned(self) -> Owned<T> {
+        #[cfg(crossbeam_loom)]
+        {
+            // FIXME: loom does not yet support into_inner, so we use unsync_load for now,
+            // which should have the same synchronization properties:
+            // https://github.com/tokio-rs/loom/issues/117
+            Owned::from_usize(self.data.unsync_load())
+        }
+        #[cfg(not(crossbeam_loom))]
+        {
+            Owned::from_usize(self.data.into_inner())
+        }
+    }
+
+    /// Takes ownership of the pointee if it is non-null.
+    ///
+    /// This consumes the atomic and converts it into [`Owned`]. As [`Atomic`] doesn't have a
+    /// destructor and doesn't drop the pointee while [`Owned`] does, this is suitable for
+    /// destructors of data structures.
+    ///
+    /// # Safety
+    ///
+    /// This method may be called only if the pointer is valid and nobody else is holding a
+    /// reference to the same object, or the pointer is null.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # use std::mem;
+    /// # use crossbeam_epoch::Atomic;
+    /// struct DataStructure {
+    ///     ptr: Atomic<usize>,
+    /// }
+    ///
+    /// impl Drop for DataStructure {
+    ///     fn drop(&mut self) {
+    ///         // By now the DataStructure lives only in our thread and we are sure we don't hold
+    ///         // any Shared or & to it ourselves, but it may be null, so we have to be careful.
+    ///         let old = mem::replace(&mut self.ptr, Atomic::null());
+    ///         unsafe {
+    ///             if let Some(x) = old.try_into_owned() {
+    ///                 drop(x)
+    ///             }
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    pub unsafe fn try_into_owned(self) -> Option<Owned<T>> {
+        // FIXME: See self.into_owned()
+        #[cfg(crossbeam_loom)]
+        let data = self.data.unsync_load();
+        #[cfg(not(crossbeam_loom))]
+        let data = self.data.into_inner();
+        if decompose_tag::<T>(data).0 == 0 {
+            None
+        } else {
+            Some(Owned::from_usize(data))
+        }
+    }
+}
+
+impl<T: ?Sized + Pointable> fmt::Debug for Atomic<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let data = self.data.load(Ordering::SeqCst);
+        let (raw, tag) = decompose_tag::<T>(data);
+
+        f.debug_struct("Atomic")
+            .field("raw", &raw)
+            .field("tag", &tag)
+            .finish()
+    }
+}
+
+impl<T: ?Sized + Pointable> fmt::Pointer for Atomic<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let data = self.data.load(Ordering::SeqCst);
+        let (raw, _) = decompose_tag::<T>(data);
+        fmt::Pointer::fmt(&(unsafe { T::deref(raw) as *const _ }), f)
+    }
+}
+
+impl<T: ?Sized + Pointable> Clone for Atomic<T> {
+    /// Returns a copy of the atomic value.
+    ///
+    /// Note that a `Relaxed` load is used here. If you need synchronization, use it with other
+    /// atomics or fences.
+    fn clone(&self) -> Self {
+        let data = self.data.load(Ordering::Relaxed);
+        Atomic::from_usize(data)
+    }
+}
+
+impl<T: ?Sized + Pointable> Default for Atomic<T> {
+    fn default() -> Self {
+        Atomic::null()
+    }
+}
+
+impl<T: ?Sized + Pointable> From<Owned<T>> for Atomic<T> {
+    /// Returns a new atomic pointer pointing to `owned`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{Atomic, Owned};
+    ///
+    /// let a = Atomic::<i32>::from(Owned::new(1234));
+    /// # unsafe { drop(a.into_owned()); } // avoid leak
+    /// ```
+    fn from(owned: Owned<T>) -> Self {
+        let data = owned.data;
+        mem::forget(owned);
+        Self::from_usize(data)
+    }
+}
+
+impl<T> From<Box<T>> for Atomic<T> {
+    fn from(b: Box<T>) -> Self {
+        Self::from(Owned::from(b))
+    }
+}
+
+impl<T> From<T> for Atomic<T> {
+    fn from(t: T) -> Self {
+        Self::new(t)
+    }
+}
+
+impl<'g, T: ?Sized + Pointable> From<Shared<'g, T>> for Atomic<T> {
+    /// Returns a new atomic pointer pointing to `ptr`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{Atomic, Shared};
+    ///
+    /// let a = Atomic::<i32>::from(Shared::<i32>::null());
+    /// ```
+    fn from(ptr: Shared<'g, T>) -> Self {
+        Self::from_usize(ptr.data)
+    }
+}
+
+impl<T> From<*const T> for Atomic<T> {
+    /// Returns a new atomic pointer pointing to `raw`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ptr;
+    /// use crossbeam_epoch::Atomic;
+    ///
+    /// let a = Atomic::<i32>::from(ptr::null::<i32>());
+    /// ```
+    fn from(raw: *const T) -> Self {
+        Self::from_usize(raw as usize)
+    }
+}
+
+/// A trait for either `Owned` or `Shared` pointers.
+pub trait Pointer<T: ?Sized + Pointable> {
+    /// Returns the machine representation of the pointer.
+    fn into_usize(self) -> usize;
+
+    /// Returns a new pointer pointing to the tagged pointer `data`.
+    ///
+    /// # Safety
+    ///
+    /// The given `data` should have been created by `Pointer::into_usize()`, and one `data` should
+    /// not be converted back by `Pointer::from_usize()` multiple times.
+    unsafe fn from_usize(data: usize) -> Self;
+}
+
+/// An owned heap-allocated object.
+///
+/// This type is very similar to `Box<T>`.
+///
+/// The pointer must be properly aligned. Since it is aligned, a tag can be stored into the unused
+/// least significant bits of the address.
+pub struct Owned<T: ?Sized + Pointable> {
+    data: usize,
+    _marker: PhantomData<Box<T>>,
+}
+
+impl<T: ?Sized + Pointable> Pointer<T> for Owned<T> {
+    #[inline]
+    fn into_usize(self) -> usize {
+        let data = self.data;
+        mem::forget(self);
+        data
+    }
+
+    /// Returns a new pointer pointing to the tagged pointer `data`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the data is zero in debug mode.
+    #[inline]
+    unsafe fn from_usize(data: usize) -> Self {
+        debug_assert!(data != 0, "converting zero into `Owned`");
+        Owned {
+            data,
+            _marker: PhantomData,
+        }
+    }
+}
+
+impl<T> Owned<T> {
+    /// Returns a new owned pointer pointing to `raw`.
+    ///
+    /// This function is unsafe because improper use may lead to memory problems. Argument `raw`
+    /// must be a valid pointer. Also, a double-free may occur if the function is called twice on
+    /// the same raw pointer.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `raw` is not properly aligned.
+    ///
+    /// # Safety
+    ///
+    /// The given `raw` should have been derived from `Owned`, and one `raw` should not be converted
+    /// back by `Owned::from_raw()` multiple times.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::Owned;
+    ///
+    /// let o = unsafe { Owned::from_raw(Box::into_raw(Box::new(1234))) };
+    /// ```
+    pub unsafe fn from_raw(raw: *mut T) -> Owned<T> {
+        let raw = raw as usize;
+        ensure_aligned::<T>(raw);
+        Self::from_usize(raw)
+    }
+
+    /// Converts the owned pointer into a `Box`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::Owned;
+    ///
+    /// let o = Owned::new(1234);
+    /// let b: Box<i32> = o.into_box();
+    /// assert_eq!(*b, 1234);
+    /// ```
+    pub fn into_box(self) -> Box<T> {
+        let (raw, _) = decompose_tag::<T>(self.data);
+        mem::forget(self);
+        unsafe { Box::from_raw(raw as *mut _) }
+    }
+
+    /// Allocates `value` on the heap and returns a new owned pointer pointing to it.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::Owned;
+    ///
+    /// let o = Owned::new(1234);
+    /// ```
+    pub fn new(init: T) -> Owned<T> {
+        Self::init(init)
+    }
+}
+
+impl<T: ?Sized + Pointable> Owned<T> {
+    /// Allocates `value` on the heap and returns a new owned pointer pointing to it.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::Owned;
+    ///
+    /// let o = Owned::<i32>::init(1234);
+    /// ```
+    pub fn init(init: T::Init) -> Owned<T> {
+        unsafe { Self::from_usize(T::init(init)) }
+    }
+
+    /// Converts the owned pointer into a [`Shared`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Owned};
+    ///
+    /// let o = Owned::new(1234);
+    /// let guard = &epoch::pin();
+    /// let p = o.into_shared(guard);
+    /// # unsafe { drop(p.into_owned()); } // avoid leak
+    /// ```
+    #[allow(clippy::needless_lifetimes)]
+    pub fn into_shared<'g>(self, _: &'g Guard) -> Shared<'g, T> {
+        unsafe { Shared::from_usize(self.into_usize()) }
+    }
+
+    /// Returns the tag stored within the pointer.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::Owned;
+    ///
+    /// assert_eq!(Owned::new(1234).tag(), 0);
+    /// ```
+    pub fn tag(&self) -> usize {
+        let (_, tag) = decompose_tag::<T>(self.data);
+        tag
+    }
+
+    /// Returns the same pointer, but tagged with `tag`. `tag` is truncated to be fit into the
+    /// unused bits of the pointer to `T`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::Owned;
+    ///
+    /// let o = Owned::new(0u64);
+    /// assert_eq!(o.tag(), 0);
+    /// let o = o.with_tag(2);
+    /// assert_eq!(o.tag(), 2);
+    /// ```
+    pub fn with_tag(self, tag: usize) -> Owned<T> {
+        let data = self.into_usize();
+        unsafe { Self::from_usize(compose_tag::<T>(data, tag)) }
+    }
+}
+
+impl<T: ?Sized + Pointable> Drop for Owned<T> {
+    fn drop(&mut self) {
+        let (raw, _) = decompose_tag::<T>(self.data);
+        unsafe {
+            T::drop(raw);
+        }
+    }
+}
+
+impl<T: ?Sized + Pointable> fmt::Debug for Owned<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let (raw, tag) = decompose_tag::<T>(self.data);
+
+        f.debug_struct("Owned")
+            .field("raw", &raw)
+            .field("tag", &tag)
+            .finish()
+    }
+}
+
+impl<T: Clone> Clone for Owned<T> {
+    fn clone(&self) -> Self {
+        Owned::new((**self).clone()).with_tag(self.tag())
+    }
+}
+
+impl<T: ?Sized + Pointable> Deref for Owned<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        let (raw, _) = decompose_tag::<T>(self.data);
+        unsafe { T::deref(raw) }
+    }
+}
+
+impl<T: ?Sized + Pointable> DerefMut for Owned<T> {
+    fn deref_mut(&mut self) -> &mut T {
+        let (raw, _) = decompose_tag::<T>(self.data);
+        unsafe { T::deref_mut(raw) }
+    }
+}
+
+impl<T> From<T> for Owned<T> {
+    fn from(t: T) -> Self {
+        Owned::new(t)
+    }
+}
+
+impl<T> From<Box<T>> for Owned<T> {
+    /// Returns a new owned pointer pointing to `b`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the pointer (the `Box`) is not properly aligned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::Owned;
+    ///
+    /// let o = unsafe { Owned::from_raw(Box::into_raw(Box::new(1234))) };
+    /// ```
+    fn from(b: Box<T>) -> Self {
+        unsafe { Self::from_raw(Box::into_raw(b)) }
+    }
+}
+
+impl<T: ?Sized + Pointable> Borrow<T> for Owned<T> {
+    fn borrow(&self) -> &T {
+        self.deref()
+    }
+}
+
+impl<T: ?Sized + Pointable> BorrowMut<T> for Owned<T> {
+    fn borrow_mut(&mut self) -> &mut T {
+        self.deref_mut()
+    }
+}
+
+impl<T: ?Sized + Pointable> AsRef<T> for Owned<T> {
+    fn as_ref(&self) -> &T {
+        self.deref()
+    }
+}
+
+impl<T: ?Sized + Pointable> AsMut<T> for Owned<T> {
+    fn as_mut(&mut self) -> &mut T {
+        self.deref_mut()
+    }
+}
+
+/// A pointer to an object protected by the epoch GC.
+///
+/// The pointer is valid for use only during the lifetime `'g`.
+///
+/// The pointer must be properly aligned. Since it is aligned, a tag can be stored into the unused
+/// least significant bits of the address.
+pub struct Shared<'g, T: 'g + ?Sized + Pointable> {
+    data: usize,
+    _marker: PhantomData<(&'g (), *const T)>,
+}
+
+impl<T: ?Sized + Pointable> Clone for Shared<'_, T> {
+    fn clone(&self) -> Self {
+        Self {
+            data: self.data,
+            _marker: PhantomData,
+        }
+    }
+}
+
+impl<T: ?Sized + Pointable> Copy for Shared<'_, T> {}
+
+impl<T: ?Sized + Pointable> Pointer<T> for Shared<'_, T> {
+    #[inline]
+    fn into_usize(self) -> usize {
+        self.data
+    }
+
+    #[inline]
+    unsafe fn from_usize(data: usize) -> Self {
+        Shared {
+            data,
+            _marker: PhantomData,
+        }
+    }
+}
+
+impl<'g, T> Shared<'g, T> {
+    /// Converts the pointer to a raw pointer (without the tag).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic, Owned};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let o = Owned::new(1234);
+    /// let raw = &*o as *const _;
+    /// let a = Atomic::from(o);
+    ///
+    /// let guard = &epoch::pin();
+    /// let p = a.load(SeqCst, guard);
+    /// assert_eq!(p.as_raw(), raw);
+    /// # unsafe { drop(a.into_owned()); } // avoid leak
+    /// ```
+    pub fn as_raw(&self) -> *const T {
+        let (raw, _) = decompose_tag::<T>(self.data);
+        raw as *const _
+    }
+}
+
+impl<'g, T: ?Sized + Pointable> Shared<'g, T> {
+    /// Returns a new null pointer.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::Shared;
+    ///
+    /// let p = Shared::<i32>::null();
+    /// assert!(p.is_null());
+    /// ```
+    pub fn null() -> Shared<'g, T> {
+        Shared {
+            data: 0,
+            _marker: PhantomData,
+        }
+    }
+
+    /// Returns `true` if the pointer is null.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic, Owned};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::null();
+    /// let guard = &epoch::pin();
+    /// assert!(a.load(SeqCst, guard).is_null());
+    /// a.store(Owned::new(1234), SeqCst);
+    /// assert!(!a.load(SeqCst, guard).is_null());
+    /// # unsafe { drop(a.into_owned()); } // avoid leak
+    /// ```
+    pub fn is_null(&self) -> bool {
+        let (raw, _) = decompose_tag::<T>(self.data);
+        raw == 0
+    }
+
+    /// Dereferences the pointer.
+    ///
+    /// Returns a reference to the pointee that is valid during the lifetime `'g`.
+    ///
+    /// # Safety
+    ///
+    /// Dereferencing a pointer is unsafe because it could be pointing to invalid memory.
+    ///
+    /// Another concern is the possibility of data races due to lack of proper synchronization.
+    /// For example, consider the following scenario:
+    ///
+    /// 1. A thread creates a new object: `a.store(Owned::new(10), Relaxed)`
+    /// 2. Another thread reads it: `*a.load(Relaxed, guard).as_ref().unwrap()`
+    ///
+    /// The problem is that relaxed orderings don't synchronize initialization of the object with
+    /// the read from the second thread. This is a data race. A possible solution would be to use
+    /// `Release` and `Acquire` orderings.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::new(1234);
+    /// let guard = &epoch::pin();
+    /// let p = a.load(SeqCst, guard);
+    /// unsafe {
+    ///     assert_eq!(p.deref(), &1234);
+    /// }
+    /// # unsafe { drop(a.into_owned()); } // avoid leak
+    /// ```
+    pub unsafe fn deref(&self) -> &'g T {
+        let (raw, _) = decompose_tag::<T>(self.data);
+        T::deref(raw)
+    }
+
+    /// Dereferences the pointer.
+    ///
+    /// Returns a mutable reference to the pointee that is valid during the lifetime `'g`.
+    ///
+    /// # Safety
+    ///
+    /// * There is no guarantee that there are no more threads attempting to read/write from/to the
+    ///   actual object at the same time.
+    ///
+    ///   The user must know that there are no concurrent accesses towards the object itself.
+    ///
+    /// * Other than the above, all safety concerns of `deref()` applies here.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::new(vec![1, 2, 3, 4]);
+    /// let guard = &epoch::pin();
+    ///
+    /// let mut p = a.load(SeqCst, guard);
+    /// unsafe {
+    ///     assert!(!p.is_null());
+    ///     let b = p.deref_mut();
+    ///     assert_eq!(b, &vec![1, 2, 3, 4]);
+    ///     b.push(5);
+    ///     assert_eq!(b, &vec![1, 2, 3, 4, 5]);
+    /// }
+    ///
+    /// let p = a.load(SeqCst, guard);
+    /// unsafe {
+    ///     assert_eq!(p.deref(), &vec![1, 2, 3, 4, 5]);
+    /// }
+    /// # unsafe { drop(a.into_owned()); } // avoid leak
+    /// ```
+    pub unsafe fn deref_mut(&mut self) -> &'g mut T {
+        let (raw, _) = decompose_tag::<T>(self.data);
+        T::deref_mut(raw)
+    }
+
+    /// Converts the pointer to a reference.
+    ///
+    /// Returns `None` if the pointer is null, or else a reference to the object wrapped in `Some`.
+    ///
+    /// # Safety
+    ///
+    /// Dereferencing a pointer is unsafe because it could be pointing to invalid memory.
+    ///
+    /// Another concern is the possibility of data races due to lack of proper synchronization.
+    /// For example, consider the following scenario:
+    ///
+    /// 1. A thread creates a new object: `a.store(Owned::new(10), Relaxed)`
+    /// 2. Another thread reads it: `*a.load(Relaxed, guard).as_ref().unwrap()`
+    ///
+    /// The problem is that relaxed orderings don't synchronize initialization of the object with
+    /// the read from the second thread. This is a data race. A possible solution would be to use
+    /// `Release` and `Acquire` orderings.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::new(1234);
+    /// let guard = &epoch::pin();
+    /// let p = a.load(SeqCst, guard);
+    /// unsafe {
+    ///     assert_eq!(p.as_ref(), Some(&1234));
+    /// }
+    /// # unsafe { drop(a.into_owned()); } // avoid leak
+    /// ```
+    pub unsafe fn as_ref(&self) -> Option<&'g T> {
+        let (raw, _) = decompose_tag::<T>(self.data);
+        if raw == 0 {
+            None
+        } else {
+            Some(T::deref(raw))
+        }
+    }
+
+    /// Takes ownership of the pointee.
+    ///
+    /// # Panics
+    ///
+    /// Panics if this pointer is null, but only in debug mode.
+    ///
+    /// # Safety
+    ///
+    /// This method may be called only if the pointer is valid and nobody else is holding a
+    /// reference to the same object.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::new(1234);
+    /// unsafe {
+    ///     let guard = &epoch::unprotected();
+    ///     let p = a.load(SeqCst, guard);
+    ///     drop(p.into_owned());
+    /// }
+    /// ```
+    pub unsafe fn into_owned(self) -> Owned<T> {
+        debug_assert!(!self.is_null(), "converting a null `Shared` into `Owned`");
+        Owned::from_usize(self.data)
+    }
+
+    /// Takes ownership of the pointee if it is not null.
+    ///
+    /// # Safety
+    ///
+    /// This method may be called only if the pointer is valid and nobody else is holding a
+    /// reference to the same object, or if the pointer is null.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::new(1234);
+    /// unsafe {
+    ///     let guard = &epoch::unprotected();
+    ///     let p = a.load(SeqCst, guard);
+    ///     if let Some(x) = p.try_into_owned() {
+    ///         drop(x);
+    ///     }
+    /// }
+    /// ```
+    pub unsafe fn try_into_owned(self) -> Option<Owned<T>> {
+        if self.is_null() {
+            None
+        } else {
+            Some(Owned::from_usize(self.data))
+        }
+    }
+
+    /// Returns the tag stored within the pointer.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic, Owned};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::<u64>::from(Owned::new(0u64).with_tag(2));
+    /// let guard = &epoch::pin();
+    /// let p = a.load(SeqCst, guard);
+    /// assert_eq!(p.tag(), 2);
+    /// # unsafe { drop(a.into_owned()); } // avoid leak
+    /// ```
+    pub fn tag(&self) -> usize {
+        let (_, tag) = decompose_tag::<T>(self.data);
+        tag
+    }
+
+    /// Returns the same pointer, but tagged with `tag`. `tag` is truncated to be fit into the
+    /// unused bits of the pointer to `T`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::new(0u64);
+    /// let guard = &epoch::pin();
+    /// let p1 = a.load(SeqCst, guard);
+    /// let p2 = p1.with_tag(2);
+    ///
+    /// assert_eq!(p1.tag(), 0);
+    /// assert_eq!(p2.tag(), 2);
+    /// assert_eq!(p1.as_raw(), p2.as_raw());
+    /// # unsafe { drop(a.into_owned()); } // avoid leak
+    /// ```
+    pub fn with_tag(&self, tag: usize) -> Shared<'g, T> {
+        unsafe { Self::from_usize(compose_tag::<T>(self.data, tag)) }
+    }
+}
+
+impl<T> From<*const T> for Shared<'_, T> {
+    /// Returns a new pointer pointing to `raw`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `raw` is not properly aligned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::Shared;
+    ///
+    /// let p = Shared::from(Box::into_raw(Box::new(1234)) as *const _);
+    /// assert!(!p.is_null());
+    /// # unsafe { drop(p.into_owned()); } // avoid leak
+    /// ```
+    fn from(raw: *const T) -> Self {
+        let raw = raw as usize;
+        ensure_aligned::<T>(raw);
+        unsafe { Self::from_usize(raw) }
+    }
+}
+
+impl<'g, T: ?Sized + Pointable> PartialEq<Shared<'g, T>> for Shared<'g, T> {
+    fn eq(&self, other: &Self) -> bool {
+        self.data == other.data
+    }
+}
+
+impl<T: ?Sized + Pointable> Eq for Shared<'_, T> {}
+
+impl<'g, T: ?Sized + Pointable> PartialOrd<Shared<'g, T>> for Shared<'g, T> {
+    fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> {
+        self.data.partial_cmp(&other.data)
+    }
+}
+
+impl<T: ?Sized + Pointable> Ord for Shared<'_, T> {
+    fn cmp(&self, other: &Self) -> cmp::Ordering {
+        self.data.cmp(&other.data)
+    }
+}
+
+impl<T: ?Sized + Pointable> fmt::Debug for Shared<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let (raw, tag) = decompose_tag::<T>(self.data);
+
+        f.debug_struct("Shared")
+            .field("raw", &raw)
+            .field("tag", &tag)
+            .finish()
+    }
+}
+
+impl<T: ?Sized + Pointable> fmt::Pointer for Shared<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Pointer::fmt(&(unsafe { self.deref() as *const _ }), f)
+    }
+}
+
+impl<T: ?Sized + Pointable> Default for Shared<'_, T> {
+    fn default() -> Self {
+        Shared::null()
+    }
+}
+
+#[cfg(all(test, not(crossbeam_loom)))]
+mod tests {
+    use super::{Owned, Shared};
+    use std::mem::MaybeUninit;
+
+    #[test]
+    fn valid_tag_i8() {
+        Shared::<i8>::null().with_tag(0);
+    }
+
+    #[test]
+    fn valid_tag_i64() {
+        Shared::<i64>::null().with_tag(7);
+    }
+
+    #[rustversion::since(1.61)]
+    #[test]
+    fn const_atomic_null() {
+        use super::Atomic;
+        static _U: Atomic<u8> = Atomic::<u8>::null();
+    }
+
+    #[test]
+    fn array_init() {
+        let owned = Owned::<[MaybeUninit<usize>]>::init(10);
+        let arr: &[MaybeUninit<usize>] = &owned;
+        assert_eq!(arr.len(), 10);
+    }
+}
diff --git a/third_party/rust/crossbeam-epoch/src/collector.rs b/third_party/rust/crossbeam-epoch/src/collector.rs
new file mode 100644
index 0000000000..5b08511845
--- /dev/null
+++ b/third_party/rust/crossbeam-epoch/src/collector.rs
@@ -0,0 +1,463 @@
+/// Epoch-based garbage collector.
+///
+/// # Examples
+///
+/// ```
+/// use crossbeam_epoch::Collector;
+///
+/// let collector = Collector::new();
+///
+/// let handle = collector.register();
+/// drop(collector); // `handle` still works after dropping `collector`
+///
+/// handle.pin().flush();
+/// ```
+use core::fmt;
+
+use crate::guard::Guard;
+use crate::internal::{Global, Local};
+use crate::primitive::sync::Arc;
+
+/// An epoch-based garbage collector.
+pub struct Collector {
+    pub(crate) global: Arc<Global>,
+}
+
+unsafe impl Send for Collector {}
+unsafe impl Sync for Collector {}
+
+impl Default for Collector {
+    fn default() -> Self {
+        Self {
+            global: Arc::new(Global::new()),
+        }
+    }
+}
+
+impl Collector {
+    /// Creates a new collector.
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Registers a new handle for the collector.
+    pub fn register(&self) -> LocalHandle {
+        Local::register(self)
+    }
+}
+
+impl Clone for Collector {
+    /// Creates another reference to the same garbage collector.
+    fn clone(&self) -> Self {
+        Collector {
+            global: self.global.clone(),
+        }
+    }
+}
+
+impl fmt::Debug for Collector {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad("Collector { .. }")
+    }
+}
+
+impl PartialEq for Collector {
+    /// Checks if both handles point to the same collector.
+    fn eq(&self, rhs: &Collector) -> bool {
+        Arc::ptr_eq(&self.global, &rhs.global)
+    }
+}
+impl Eq for Collector {}
+
+/// A handle to a garbage collector.
+pub struct LocalHandle {
+    pub(crate) local: *const Local,
+}
+
+impl LocalHandle {
+    /// Pins the handle.
+    #[inline]
+    pub fn pin(&self) -> Guard {
+        unsafe { (*self.local).pin() }
+    }
+
+    /// Returns `true` if the handle is pinned.
+    #[inline]
+    pub fn is_pinned(&self) -> bool {
+        unsafe { (*self.local).is_pinned() }
+    }
+
+    /// Returns the `Collector` associated with this handle.
+    #[inline]
+    pub fn collector(&self) -> &Collector {
+        unsafe { (*self.local).collector() }
+    }
+}
+
+impl Drop for LocalHandle {
+    #[inline]
+    fn drop(&mut self) {
+        unsafe {
+            Local::release_handle(&*self.local);
+        }
+    }
+}
+
+impl fmt::Debug for LocalHandle {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad("LocalHandle { .. }")
+    }
+}
+
+#[cfg(all(test, not(crossbeam_loom)))]
+mod tests {
+    use std::mem::ManuallyDrop;
+    use std::sync::atomic::{AtomicUsize, Ordering};
+
+    use crossbeam_utils::thread;
+
+    use crate::{Collector, Owned};
+
+    const NUM_THREADS: usize = 8;
+
+    #[test]
+    fn pin_reentrant() {
+        let collector = Collector::new();
+        let handle = collector.register();
+        drop(collector);
+
+        assert!(!handle.is_pinned());
+        {
+            let _guard = &handle.pin();
+            assert!(handle.is_pinned());
+            {
+                let _guard = &handle.pin();
+                assert!(handle.is_pinned());
+            }
+            assert!(handle.is_pinned());
+        }
+        assert!(!handle.is_pinned());
+    }
+
+    #[test]
+    fn flush_local_bag() {
+        let collector = Collector::new();
+        let handle = collector.register();
+        drop(collector);
+
+        for _ in 0..100 {
+            let guard = &handle.pin();
+            unsafe {
+                let a = Owned::new(7).into_shared(guard);
+                guard.defer_destroy(a);
+
+                assert!(!(*guard.local).bag.with(|b| (*b).is_empty()));
+
+                while !(*guard.local).bag.with(|b| (*b).is_empty()) {
+                    guard.flush();
+                }
+            }
+        }
+    }
+
+    #[test]
+    fn garbage_buffering() {
+        let collector = Collector::new();
+        let handle = collector.register();
+        drop(collector);
+
+        let guard = &handle.pin();
+        unsafe {
+            for _ in 0..10 {
+                let a = Owned::new(7).into_shared(guard);
+                guard.defer_destroy(a);
+            }
+            assert!(!(*guard.local).bag.with(|b| (*b).is_empty()));
+        }
+    }
+
+    #[test]
+    fn pin_holds_advance() {
+        #[cfg(miri)]
+        const N: usize = 500;
+        #[cfg(not(miri))]
+        const N: usize = 500_000;
+
+        let collector = Collector::new();
+
+        thread::scope(|scope| {
+            for _ in 0..NUM_THREADS {
+                scope.spawn(|_| {
+                    let handle = collector.register();
+                    for _ in 0..N {
+                        let guard = &handle.pin();
+
+                        let before = collector.global.epoch.load(Ordering::Relaxed);
+                        collector.global.collect(guard);
+                        let after = collector.global.epoch.load(Ordering::Relaxed);
+
+                        assert!(after.wrapping_sub(before) <= 2);
+                    }
+                });
+            }
+        })
+        .unwrap();
+    }
+
+    #[cfg(not(crossbeam_sanitize))] // TODO: assertions failed due to `cfg(crossbeam_sanitize)` reduce `internal::MAX_OBJECTS`
+    #[test]
+    fn incremental() {
+        #[cfg(miri)]
+        const COUNT: usize = 500;
+        #[cfg(not(miri))]
+        const COUNT: usize = 100_000;
+        static DESTROYS: AtomicUsize = AtomicUsize::new(0);
+
+        let collector = Collector::new();
+        let handle = collector.register();
+
+        unsafe {
+            let guard = &handle.pin();
+            for _ in 0..COUNT {
+                let a = Owned::new(7i32).into_shared(guard);
+                guard.defer_unchecked(move || {
+                    drop(a.into_owned());
+                    DESTROYS.fetch_add(1, Ordering::Relaxed);
+                });
+            }
+            guard.flush();
+        }
+
+        let mut last = 0;
+
+        while last < COUNT {
+            let curr = DESTROYS.load(Ordering::Relaxed);
+            assert!(curr - last <= 1024);
+            last = curr;
+
+            let guard = &handle.pin();
+            collector.global.collect(guard);
+        }
+        assert!(DESTROYS.load(Ordering::Relaxed) == COUNT);
+    }
+
+    #[test]
+    fn buffering() {
+        const COUNT: usize = 10;
+        #[cfg(miri)]
+        const N: usize = 500;
+        #[cfg(not(miri))]
+        const N: usize = 100_000;
+        static DESTROYS: AtomicUsize = AtomicUsize::new(0);
+
+        let collector = Collector::new();
+        let handle = collector.register();
+
+        unsafe {
+            let guard = &handle.pin();
+            for _ in 0..COUNT {
+                let a = Owned::new(7i32).into_shared(guard);
+                guard.defer_unchecked(move || {
+                    drop(a.into_owned());
+                    DESTROYS.fetch_add(1, Ordering::Relaxed);
+                });
+            }
+        }
+
+        for _ in 0..N {
+            collector.global.collect(&handle.pin());
+        }
+        assert!(DESTROYS.load(Ordering::Relaxed) < COUNT);
+
+        handle.pin().flush();
+
+        while DESTROYS.load(Ordering::Relaxed) < COUNT {
+            let guard = &handle.pin();
+            collector.global.collect(guard);
+        }
+        assert_eq!(DESTROYS.load(Ordering::Relaxed), COUNT);
+    }
+
+    #[test]
+    fn count_drops() {
+        #[cfg(miri)]
+        const COUNT: usize = 500;
+        #[cfg(not(miri))]
+        const COUNT: usize = 100_000;
+        static DROPS: AtomicUsize = AtomicUsize::new(0);
+
+        struct Elem(i32);
+
+        impl Drop for Elem {
+            fn drop(&mut self) {
+                DROPS.fetch_add(1, Ordering::Relaxed);
+            }
+        }
+
+        let collector = Collector::new();
+        let handle = collector.register();
+
+        unsafe {
+            let guard = &handle.pin();
+
+            for _ in 0..COUNT {
+                let a = Owned::new(Elem(7i32)).into_shared(guard);
+                guard.defer_destroy(a);
+            }
+            guard.flush();
+        }
+
+        while DROPS.load(Ordering::Relaxed) < COUNT {
+            let guard = &handle.pin();
+            collector.global.collect(guard);
+        }
+        assert_eq!(DROPS.load(Ordering::Relaxed), COUNT);
+    }
+
+    #[test]
+    fn count_destroy() {
+        #[cfg(miri)]
+        const COUNT: usize = 500;
+        #[cfg(not(miri))]
+        const COUNT: usize = 100_000;
+        static DESTROYS: AtomicUsize = AtomicUsize::new(0);
+
+        let collector = Collector::new();
+        let handle = collector.register();
+
+        unsafe {
+            let guard = &handle.pin();
+
+            for _ in 0..COUNT {
+                let a = Owned::new(7i32).into_shared(guard);
+                guard.defer_unchecked(move || {
+                    drop(a.into_owned());
+                    DESTROYS.fetch_add(1, Ordering::Relaxed);
+                });
+            }
+            guard.flush();
+        }
+
+        while DESTROYS.load(Ordering::Relaxed) < COUNT {
+            let guard = &handle.pin();
+            collector.global.collect(guard);
+        }
+        assert_eq!(DESTROYS.load(Ordering::Relaxed), COUNT);
+    }
+
+    #[test]
+    fn drop_array() {
+        const COUNT: usize = 700;
+        static DROPS: AtomicUsize = AtomicUsize::new(0);
+
+        struct Elem(i32);
+
+        impl Drop for Elem {
+            fn drop(&mut self) {
+                DROPS.fetch_add(1, Ordering::Relaxed);
+            }
+        }
+
+        let collector = Collector::new();
+        let handle = collector.register();
+
+        let mut guard = handle.pin();
+
+        let mut v = Vec::with_capacity(COUNT);
+        for i in 0..COUNT {
+            v.push(Elem(i as i32));
+        }
+
+        {
+            let a = Owned::new(v).into_shared(&guard);
+            unsafe {
+                guard.defer_destroy(a);
+            }
+            guard.flush();
+        }
+
+        while DROPS.load(Ordering::Relaxed) < COUNT {
+            guard.repin();
+            collector.global.collect(&guard);
+        }
+        assert_eq!(DROPS.load(Ordering::Relaxed), COUNT);
+    }
+
+    #[test]
+    fn destroy_array() {
+        #[cfg(miri)]
+        const COUNT: usize = 500;
+        #[cfg(not(miri))]
+        const COUNT: usize = 100_000;
+        static DESTROYS: AtomicUsize = AtomicUsize::new(0);
+
+        let collector = Collector::new();
+        let handle = collector.register();
+
+        unsafe {
+            let guard = &handle.pin();
+
+            let mut v = Vec::with_capacity(COUNT);
+            for i in 0..COUNT {
+                v.push(i as i32);
+            }
+
+            let len = v.len();
+            let ptr = ManuallyDrop::new(v).as_mut_ptr() as usize;
+            guard.defer_unchecked(move || {
+                drop(Vec::from_raw_parts(ptr as *const i32 as *mut i32, len, len));
+                DESTROYS.fetch_add(len, Ordering::Relaxed);
+            });
+            guard.flush();
+        }
+
+        while DESTROYS.load(Ordering::Relaxed) < COUNT {
+            let guard = &handle.pin();
+            collector.global.collect(guard);
+        }
+        assert_eq!(DESTROYS.load(Ordering::Relaxed), COUNT);
+    }
+
+    #[test]
+    fn stress() {
+        const THREADS: usize = 8;
+        #[cfg(miri)]
+        const COUNT: usize = 500;
+        #[cfg(not(miri))]
+        const COUNT: usize = 100_000;
+        static DROPS: AtomicUsize = AtomicUsize::new(0);
+
+        struct Elem(i32);
+
+        impl Drop for Elem {
+            fn drop(&mut self) {
+                DROPS.fetch_add(1, Ordering::Relaxed);
+            }
+        }
+
+        let collector = Collector::new();
+
+        thread::scope(|scope| {
+            for _ in 0..THREADS {
+                scope.spawn(|_| {
+                    let handle = collector.register();
+                    for _ in 0..COUNT {
+                        let guard = &handle.pin();
+                        unsafe {
+                            let a = Owned::new(Elem(7i32)).into_shared(guard);
+                            guard.defer_destroy(a);
+                        }
+                    }
+                });
+            }
+        })
+        .unwrap();
+
+        let handle = collector.register();
+        while DROPS.load(Ordering::Relaxed) < COUNT * THREADS {
+            let guard = &handle.pin();
+            collector.global.collect(guard);
+        }
+        assert_eq!(DROPS.load(Ordering::Relaxed), COUNT * THREADS);
+    }
+}
diff --git a/third_party/rust/crossbeam-epoch/src/default.rs b/third_party/rust/crossbeam-epoch/src/default.rs
new file mode 100644
index 0000000000..b42c1c7a7f
--- /dev/null
+++ b/third_party/rust/crossbeam-epoch/src/default.rs
@@ -0,0 +1,93 @@
+//! The default garbage collector.
+//!
+//! For each thread, a participant is lazily initialized on its first use, when the current thread
+//! is registered in the default collector.  If initialized, the thread's participant will get
+//! destructed on thread exit, which in turn unregisters the thread.
+
+use crate::collector::{Collector, LocalHandle};
+use crate::guard::Guard;
+use crate::primitive::thread_local;
+#[cfg(not(crossbeam_loom))]
+use crate::sync::once_lock::OnceLock;
+
+fn collector() -> &'static Collector {
+    #[cfg(not(crossbeam_loom))]
+    {
+        /// The global data for the default garbage collector.
+        static COLLECTOR: OnceLock<Collector> = OnceLock::new();
+        COLLECTOR.get_or_init(Collector::new)
+    }
+    // FIXME: loom does not currently provide the equivalent of Lazy:
+    // https://github.com/tokio-rs/loom/issues/263
+    #[cfg(crossbeam_loom)]
+    {
+        loom::lazy_static! {
+            /// The global data for the default garbage collector.
+            static ref COLLECTOR: Collector = Collector::new();
+        }
+        &COLLECTOR
+    }
+}
+
+thread_local! {
+    /// The per-thread participant for the default garbage collector.
+    static HANDLE: LocalHandle = collector().register();
+}
+
+/// Pins the current thread.
+#[inline]
+pub fn pin() -> Guard {
+    with_handle(|handle| handle.pin())
+}
+
+/// Returns `true` if the current thread is pinned.
+#[inline]
+pub fn is_pinned() -> bool {
+    with_handle(|handle| handle.is_pinned())
+}
+
+/// Returns the default global collector.
+pub fn default_collector() -> &'static Collector {
+    collector()
+}
+
+#[inline]
+fn with_handle<F, R>(mut f: F) -> R
+where
+    F: FnMut(&LocalHandle) -> R,
+{
+    HANDLE
+        .try_with(|h| f(h))
+        .unwrap_or_else(|_| f(&collector().register()))
+}
+
+#[cfg(all(test, not(crossbeam_loom)))]
+mod tests {
+    use crossbeam_utils::thread;
+
+    #[test]
+    fn pin_while_exiting() {
+        struct Foo;
+
+        impl Drop for Foo {
+            fn drop(&mut self) {
+                // Pin after `HANDLE` has been dropped. This must not panic.
+                super::pin();
+            }
+        }
+
+        thread_local! {
+            static FOO: Foo = Foo;
+        }
+
+        thread::scope(|scope| {
+            scope.spawn(|_| {
+                // Initialize `FOO` and then `HANDLE`.
+                FOO.with(|_| ());
+                super::pin();
+                // At thread exit, `HANDLE` gets dropped first and `FOO` second.
+            });
+        })
+        .unwrap();
+    }
+}
diff --git a/third_party/rust/crossbeam-epoch/src/deferred.rs b/third_party/rust/crossbeam-epoch/src/deferred.rs
new file mode 100644
index 0000000000..2f3d79fdf1
--- /dev/null
+++ b/third_party/rust/crossbeam-epoch/src/deferred.rs
@@ -0,0 +1,147 @@
+use alloc::boxed::Box;
+use core::fmt;
+use core::marker::PhantomData;
+use core::mem::{self, MaybeUninit};
+use core::ptr;
+
+/// Number of words a piece of `Data` can hold.
+///
+/// Three words should be enough for the majority of cases. For example, you can fit inside it the
+/// function pointer together with a fat pointer representing an object that needs to be destroyed.
+const DATA_WORDS: usize = 3;
+
+/// Some space to keep a `FnOnce()` object on the stack.
+type Data = [usize; DATA_WORDS];
+
+/// A `FnOnce()` that is stored inline if small, or otherwise boxed on the heap.
+///
+/// This is a handy way of keeping an unsized `FnOnce()` within a sized structure.
+pub(crate) struct Deferred {
+    call: unsafe fn(*mut u8),
+    data: MaybeUninit<Data>,
+    _marker: PhantomData<*mut ()>, // !Send + !Sync
+}
+
+impl fmt::Debug for Deferred {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
+        f.pad("Deferred { .. }")
+    }
+}
+
+impl Deferred {
+    pub(crate) const NO_OP: Self = {
+        fn no_op_call(_raw: *mut u8) {}
+        Self {
+            call: no_op_call,
+            data: MaybeUninit::uninit(),
+            _marker: PhantomData,
+        }
+    };
+
+    /// Constructs a new `Deferred` from a `FnOnce()`.
+    pub(crate) fn new<F: FnOnce()>(f: F) -> Self {
+        let size = mem::size_of::<F>();
+        let align = mem::align_of::<F>();
+
+        unsafe {
+            if size <= mem::size_of::<Data>() && align <= mem::align_of::<Data>() {
+                let mut data = MaybeUninit::<Data>::uninit();
+                ptr::write(data.as_mut_ptr().cast::<F>(), f);
+
+                unsafe fn call<F: FnOnce()>(raw: *mut u8) {
+                    let f: F = ptr::read(raw.cast::<F>());
+                    f();
+                }
+
+                Deferred {
+                    call: call::<F>,
+                    data,
+                    _marker: PhantomData,
+                }
+            } else {
+                let b: Box<F> = Box::new(f);
+                let mut data = MaybeUninit::<Data>::uninit();
+                ptr::write(data.as_mut_ptr().cast::<Box<F>>(), b);
+
+                unsafe fn call<F: FnOnce()>(raw: *mut u8) {
+                    // It's safe to cast `raw` from `*mut u8` to `*mut Box<F>`, because `raw` is
+                    // originally derived from `*mut Box<F>`.
+                    let b: Box<F> = ptr::read(raw.cast::<Box<F>>());
+                    (*b)();
+                }
+
+                Deferred {
+                    call: call::<F>,
+                    data,
+                    _marker: PhantomData,
+                }
+            }
+        }
+    }
+
+    /// Calls the function.
+    #[inline]
+    pub(crate) fn call(mut self) {
+        let call = self.call;
+        unsafe { call(self.data.as_mut_ptr().cast::<u8>()) };
+    }
+}
+
+#[cfg(all(test, not(crossbeam_loom)))]
+mod tests {
+    #![allow(clippy::drop_copy)]
+
+    use super::Deferred;
+    use std::cell::Cell;
+
+    #[test]
+    fn on_stack() {
+        let fired = &Cell::new(false);
+        let a = [0usize; 1];
+
+        let d = Deferred::new(move || {
+            drop(a);
+            fired.set(true);
+        });
+
+        assert!(!fired.get());
+        d.call();
+        assert!(fired.get());
+    }
+
+    #[test]
+    fn on_heap() {
+        let fired = &Cell::new(false);
+        let a = [0usize; 10];
+
+        let d = Deferred::new(move || {
+            drop(a);
+            fired.set(true);
+        });
+
+        assert!(!fired.get());
+        d.call();
+        assert!(fired.get());
+    }
+
+    #[test]
+    fn string() {
+        let a = "hello".to_string();
+        let d = Deferred::new(move || assert_eq!(a, "hello"));
+        d.call();
+    }
+
+    #[test]
+    fn boxed_slice_i32() {
+        let a: Box<[i32]> = vec![2, 3, 5, 7].into_boxed_slice();
+        let d = Deferred::new(move || assert_eq!(*a, [2, 3, 5, 7]));
+        d.call();
+    }
+
+    #[test]
+    fn long_slice_usize() {
+        let a: [usize; 5] = [2, 3, 5, 7, 11];
+        let d = Deferred::new(move || assert_eq!(a, [2, 3, 5, 7, 11]));
+        d.call();
+    }
+}
diff --git a/third_party/rust/crossbeam-epoch/src/epoch.rs b/third_party/rust/crossbeam-epoch/src/epoch.rs
new file mode 100644
index 0000000000..663508bd7b
--- /dev/null
+++ b/third_party/rust/crossbeam-epoch/src/epoch.rs
@@ -0,0 +1,133 @@
+//! The global epoch
+//!
+//! The last bit in this number is unused and is always zero. Every so often the global epoch is
+//! incremented, i.e. we say it "advances". A pinned participant may advance the global epoch only
+//! if all currently pinned participants have been pinned in the current epoch.
+//!
+//! If an object became garbage in some epoch, then we can be sure that after two advancements no
+//! participant will hold a reference to it. That is the crux of safe memory reclamation.
+
+use crate::primitive::sync::atomic::AtomicUsize;
+use core::sync::atomic::Ordering;
+
+/// An epoch that can be marked as pinned or unpinned.
+///
+/// Internally, the epoch is represented as an integer that wraps around at some unspecified point
+/// and a flag that represents whether it is pinned or unpinned.
+#[derive(Copy, Clone, Default, Debug, Eq, PartialEq)]
+pub(crate) struct Epoch {
+    /// The least significant bit is set if pinned. The rest of the bits hold the epoch.
+    data: usize,
+}
+
+impl Epoch {
+    /// Returns the starting epoch in unpinned state.
+    #[inline]
+    pub(crate) fn starting() -> Self {
+        Self::default()
+    }
+
+    /// Returns the number of epochs `self` is ahead of `rhs`.
+    ///
+    /// Internally, epochs are represented as numbers in the range `(isize::MIN / 2) .. (isize::MAX
+    /// / 2)`, so the returned distance will be in the same interval.
+    pub(crate) fn wrapping_sub(self, rhs: Self) -> isize {
+        // The result is the same with `(self.data & !1).wrapping_sub(rhs.data & !1) as isize >> 1`,
+        // because the possible difference of LSB in `(self.data & !1).wrapping_sub(rhs.data & !1)`
+        // will be ignored in the shift operation.
+        self.data.wrapping_sub(rhs.data & !1) as isize >> 1
+    }
+
+    /// Returns `true` if the epoch is marked as pinned.
+    #[inline]
+    pub(crate) fn is_pinned(self) -> bool {
+        (self.data & 1) == 1
+    }
+
+    /// Returns the same epoch, but marked as pinned.
+    #[inline]
+    pub(crate) fn pinned(self) -> Epoch {
+        Epoch {
+            data: self.data | 1,
+        }
+    }
+
+    /// Returns the same epoch, but marked as unpinned.
+    #[inline]
+    pub(crate) fn unpinned(self) -> Epoch {
+        Epoch {
+            data: self.data & !1,
+        }
+    }
+
+    /// Returns the successor epoch.
+    ///
+    /// The returned epoch will be marked as pinned only if the previous one was as well.
+    #[inline]
+    pub(crate) fn successor(self) -> Epoch {
+        Epoch {
+            data: self.data.wrapping_add(2),
+        }
+    }
+}
+
+/// An atomic value that holds an `Epoch`.
+#[derive(Default, Debug)]
+pub(crate) struct AtomicEpoch {
+    /// Since `Epoch` is just a wrapper around `usize`, an `AtomicEpoch` is similarly represented
+    /// using an `AtomicUsize`.
+    data: AtomicUsize,
+}
+
+impl AtomicEpoch {
+    /// Creates a new atomic epoch.
+    #[inline]
+    pub(crate) fn new(epoch: Epoch) -> Self {
+        let data = AtomicUsize::new(epoch.data);
+        AtomicEpoch { data }
+    }
+
+    /// Loads a value from the atomic epoch.
+    #[inline]
+    pub(crate) fn load(&self, ord: Ordering) -> Epoch {
+        Epoch {
+            data: self.data.load(ord),
+        }
+    }
+
+    /// Stores a value into the atomic epoch.
+    #[inline]
+    pub(crate) fn store(&self, epoch: Epoch, ord: Ordering) {
+        self.data.store(epoch.data, ord);
+    }
+
+    /// Stores a value into the atomic epoch if the current value is the same as `current`.
+    ///
+    /// The return value is a result indicating whether the new value was written and containing
+    /// the previous value. On success this value is guaranteed to be equal to `current`.
+    ///
+    /// This method takes two `Ordering` arguments to describe the memory
+    /// ordering of this operation. `success` describes the required ordering for the
+    /// read-modify-write operation that takes place if the comparison with `current` succeeds.
+    /// `failure` describes the required ordering for the load operation that takes place when
+    /// the comparison fails. Using `Acquire` as success ordering makes the store part
+    /// of this operation `Relaxed`, and using `Release` makes the successful load
+    /// `Relaxed`. The failure ordering can only be `SeqCst`, `Acquire` or `Relaxed`
+    /// and must be equivalent to or weaker than the success ordering.
+    #[inline]
+    pub(crate) fn compare_exchange(
+        &self,
+        current: Epoch,
+        new: Epoch,
+        success: Ordering,
+        failure: Ordering,
+    ) -> Result<Epoch, Epoch> {
+        match self
+            .data
+            .compare_exchange(current.data, new.data, success, failure)
+        {
+            Ok(data) => Ok(Epoch { data }),
+            Err(data) => Err(Epoch { data }),
+        }
+    }
+}
diff --git a/third_party/rust/crossbeam-epoch/src/guard.rs b/third_party/rust/crossbeam-epoch/src/guard.rs
new file mode 100644
index 0000000000..ba7fe1b119
--- /dev/null
+++ b/third_party/rust/crossbeam-epoch/src/guard.rs
@@ -0,0 +1,520 @@
+use core::fmt;
+use core::mem;
+
+use scopeguard::defer;
+
+use crate::atomic::Shared;
+use crate::collector::Collector;
+use crate::deferred::Deferred;
+use crate::internal::Local;
+
+/// A guard that keeps the current thread pinned.
+///
+/// # Pinning
+///
+/// The current thread is pinned by calling [`pin`], which returns a new guard:
+///
+/// ```
+/// use crossbeam_epoch as epoch;
+///
+/// // It is often convenient to prefix a call to `pin` with a `&` in order to create a reference.
+/// // This is not really necessary, but makes passing references to the guard a bit easier.
+/// let guard = &epoch::pin();
+/// ```
+///
+/// When a guard gets dropped, the current thread is automatically unpinned.
+///
+/// # Pointers on the stack
+///
+/// Having a guard allows us to create pointers on the stack to heap-allocated objects.
+/// For example:
+///
+/// ```
+/// use crossbeam_epoch::{self as epoch, Atomic};
+/// use std::sync::atomic::Ordering::SeqCst;
+///
+/// // Create a heap-allocated number.
+/// let a = Atomic::new(777);
+///
+/// // Pin the current thread.
+/// let guard = &epoch::pin();
+///
+/// // Load the heap-allocated object and create pointer `p` on the stack.
+/// let p = a.load(SeqCst, guard);
+///
+/// // Dereference the pointer and print the value:
+/// if let Some(num) = unsafe { p.as_ref() } {
+///     println!("The number is {}.", num);
+/// }
+/// # unsafe { drop(a.into_owned()); } // avoid leak
+/// ```
+///
+/// # Multiple guards
+///
+/// Pinning is reentrant and it is perfectly legal to create multiple guards. In that case, the
+/// thread will actually be pinned only when the first guard is created and unpinned when the last
+/// one is dropped:
+///
+/// ```
+/// use crossbeam_epoch as epoch;
+///
+/// let guard1 = epoch::pin();
+/// let guard2 = epoch::pin();
+/// assert!(epoch::is_pinned());
+/// drop(guard1);
+/// assert!(epoch::is_pinned());
+/// drop(guard2);
+/// assert!(!epoch::is_pinned());
+/// ```
+///
+/// [`pin`]: super::pin
+pub struct Guard {
+    pub(crate) local: *const Local,
+}
+
+impl Guard {
+    /// Stores a function so that it can be executed at some point after all currently pinned
+    /// threads get unpinned.
+    ///
+    /// This method first stores `f` into the thread-local (or handle-local) cache. If this cache
+    /// becomes full, some functions are moved into the global cache. At the same time, some
+    /// functions from both local and global caches may get executed in order to incrementally
+    /// clean up the caches as they fill up.
+    ///
+    /// There is no guarantee when exactly `f` will be executed. The only guarantee is that it
+    /// won't be executed until all currently pinned threads get unpinned. In theory, `f` might
+    /// never run, but the epoch-based garbage collection will make an effort to execute it
+    /// reasonably soon.
+    ///
+    /// If this method is called from an [`unprotected`] guard, the function will simply be
+    /// executed immediately.
+    pub fn defer<F, R>(&self, f: F)
+    where
+        F: FnOnce() -> R,
+        F: Send + 'static,
+    {
+        unsafe {
+            self.defer_unchecked(f);
+        }
+    }
+
+    /// Stores a function so that it can be executed at some point after all currently pinned
+    /// threads get unpinned.
+    ///
+    /// This method first stores `f` into the thread-local (or handle-local) cache. If this cache
+    /// becomes full, some functions are moved into the global cache. At the same time, some
+    /// functions from both local and global caches may get executed in order to incrementally
+    /// clean up the caches as they fill up.
+    ///
+    /// There is no guarantee when exactly `f` will be executed. The only guarantee is that it
+    /// won't be executed until all currently pinned threads get unpinned. In theory, `f` might
+    /// never run, but the epoch-based garbage collection will make an effort to execute it
+    /// reasonably soon.
+    ///
+    /// If this method is called from an [`unprotected`] guard, the function will simply be
+    /// executed immediately.
+    ///
+    /// # Safety
+    ///
+    /// The given function must not hold reference onto the stack. It is highly recommended that
+    /// the passed function is **always** marked with `move` in order to prevent accidental
+    /// borrows.
+    ///
+    /// ```
+    /// use crossbeam_epoch as epoch;
+    ///
+    /// let guard = &epoch::pin();
+    /// let message = "Hello!";
+    /// unsafe {
+    ///     // ALWAYS use `move` when sending a closure into `defer_unchecked`.
+    ///     guard.defer_unchecked(move || {
+    ///         println!("{}", message);
+    ///     });
+    /// }
+    /// ```
+    ///
+    /// Apart from that, keep in mind that another thread may execute `f`, so anything accessed by
+    /// the closure must be `Send`.
+    ///
+    /// We intentionally didn't require `F: Send`, because Rust's type systems usually cannot prove
+    /// `F: Send` for typical use cases. For example, consider the following code snippet, which
+    /// exemplifies the typical use case of deferring the deallocation of a shared reference:
+    ///
+    /// ```ignore
+    /// let shared = Owned::new(7i32).into_shared(guard);
+    /// guard.defer_unchecked(move || shared.into_owned()); // `Shared` is not `Send`!
+    /// ```
+    ///
+    /// While `Shared` is not `Send`, it's safe for another thread to call the deferred function,
+    /// because it's called only after the grace period and `shared` is no longer shared with other
+    /// threads. But we don't expect type systems to prove this.
+    ///
+    /// # Examples
+    ///
+    /// When a heap-allocated object in a data structure becomes unreachable, it has to be
+    /// deallocated. However, the current thread and other threads may be still holding references
+    /// on the stack to that same object. Therefore it cannot be deallocated before those references
+    /// get dropped. This method can defer deallocation until all those threads get unpinned and
+    /// consequently drop all their references on the stack.
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic, Owned};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::new("foo");
+    ///
+    /// // Now suppose that `a` is shared among multiple threads and concurrently
+    /// // accessed and modified...
+    ///
+    /// // Pin the current thread.
+    /// let guard = &epoch::pin();
+    ///
+    /// // Steal the object currently stored in `a` and swap it with another one.
+    /// let p = a.swap(Owned::new("bar").into_shared(guard), SeqCst, guard);
+    ///
+    /// if !p.is_null() {
+    ///     // The object `p` is pointing to is now unreachable.
+    ///     // Defer its deallocation until all currently pinned threads get unpinned.
+    ///     unsafe {
+    ///         // ALWAYS use `move` when sending a closure into `defer_unchecked`.
+    ///         guard.defer_unchecked(move || {
+    ///             println!("{} is now being deallocated.", p.deref());
+    ///             // Now we have unique access to the object pointed to by `p` and can turn it
+    ///             // into an `Owned`. Dropping the `Owned` will deallocate the object.
+    ///             drop(p.into_owned());
+    ///         });
+    ///     }
+    /// }
+    /// # unsafe { drop(a.into_owned()); } // avoid leak
+    /// ```
+    pub unsafe fn defer_unchecked<F, R>(&self, f: F)
+    where
+        F: FnOnce() -> R,
+    {
+        if let Some(local) = self.local.as_ref() {
+            local.defer(Deferred::new(move || drop(f())), self);
+        } else {
+            drop(f());
+        }
+    }
+
+    /// Stores a destructor for an object so that it can be deallocated and dropped at some point
+    /// after all currently pinned threads get unpinned.
+    ///
+    /// This method first stores the destructor into the thread-local (or handle-local) cache. If
+    /// this cache becomes full, some destructors are moved into the global cache. At the same
+    /// time, some destructors from both local and global caches may get executed in order to
+    /// incrementally clean up the caches as they fill up.
+    ///
+    /// There is no guarantee when exactly the destructor will be executed. The only guarantee is
+    /// that it won't be executed until all currently pinned threads get unpinned. In theory, the
+    /// destructor might never run, but the epoch-based garbage collection will make an effort to
+    /// execute it reasonably soon.
+    ///
+    /// If this method is called from an [`unprotected`] guard, the destructor will simply be
+    /// executed immediately.
+    ///
+    /// # Safety
+    ///
+    /// The object must not be reachable by other threads anymore, otherwise it might be still in
+    /// use when the destructor runs.
+    ///
+    /// Apart from that, keep in mind that another thread may execute the destructor, so the object
+    /// must be sendable to other threads.
+    ///
+    /// We intentionally didn't require `T: Send`, because Rust's type systems usually cannot prove
+    /// `T: Send` for typical use cases. For example, consider the following code snippet, which
+    /// exemplifies the typical use case of deferring the deallocation of a shared reference:
+    ///
+    /// ```ignore
+    /// let shared = Owned::new(7i32).into_shared(guard);
+    /// guard.defer_destroy(shared); // `Shared` is not `Send`!
+    /// ```
+    ///
+    /// While `Shared` is not `Send`, it's safe for another thread to call the destructor, because
+    /// it's called only after the grace period and `shared` is no longer shared with other
+    /// threads. But we don't expect type systems to prove this.
+    ///
+    /// # Examples
+    ///
+    /// When a heap-allocated object in a data structure becomes unreachable, it has to be
+    /// deallocated. However, the current thread and other threads may be still holding references
+    /// on the stack to that same object. Therefore it cannot be deallocated before those references
+    /// get dropped. This method can defer deallocation until all those threads get unpinned and
+    /// consequently drop all their references on the stack.
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic, Owned};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::new("foo");
+    ///
+    /// // Now suppose that `a` is shared among multiple threads and concurrently
+    /// // accessed and modified...
+    ///
+    /// // Pin the current thread.
+    /// let guard = &epoch::pin();
+    ///
+    /// // Steal the object currently stored in `a` and swap it with another one.
+    /// let p = a.swap(Owned::new("bar").into_shared(guard), SeqCst, guard);
+    ///
+    /// if !p.is_null() {
+    ///     // The object `p` is pointing to is now unreachable.
+    ///     // Defer its deallocation until all currently pinned threads get unpinned.
+    ///     unsafe {
+    ///         guard.defer_destroy(p);
+    ///     }
+    /// }
+    /// # unsafe { drop(a.into_owned()); } // avoid leak
+    /// ```
+    pub unsafe fn defer_destroy<T>(&self, ptr: Shared<'_, T>) {
+        self.defer_unchecked(move || ptr.into_owned());
+    }
+
+    /// Clears up the thread-local cache of deferred functions by executing them or moving into the
+    /// global cache.
+    ///
+    /// Call this method after deferring execution of a function if you want to get it executed as
+    /// soon as possible. Flushing will make sure it is residing in in the global cache, so that
+    /// any thread has a chance of taking the function and executing it.
+    ///
+    /// If this method is called from an [`unprotected`] guard, it is a no-op (nothing happens).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch as epoch;
+    ///
+    /// let guard = &epoch::pin();
+    /// guard.defer(move || {
+    ///     println!("This better be printed as soon as possible!");
+    /// });
+    /// guard.flush();
+    /// ```
+    pub fn flush(&self) {
+        if let Some(local) = unsafe { self.local.as_ref() } {
+            local.flush(self);
+        }
+    }
+
+    /// Unpins and then immediately re-pins the thread.
+    ///
+    /// This method is useful when you don't want delay the advancement of the global epoch by
+    /// holding an old epoch. For safety, you should not maintain any guard-based reference across
+    /// the call (the latter is enforced by `&mut self`). The thread will only be repinned if this
+    /// is the only active guard for the current thread.
+    ///
+    /// If this method is called from an [`unprotected`] guard, then the call will be just no-op.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::new(777);
+    /// let mut guard = epoch::pin();
+    /// {
+    ///     let p = a.load(SeqCst, &guard);
+    ///     assert_eq!(unsafe { p.as_ref() }, Some(&777));
+    /// }
+    /// guard.repin();
+    /// {
+    ///     let p = a.load(SeqCst, &guard);
+    ///     assert_eq!(unsafe { p.as_ref() }, Some(&777));
+    /// }
+    /// # unsafe { drop(a.into_owned()); } // avoid leak
+    /// ```
+    pub fn repin(&mut self) {
+        if let Some(local) = unsafe { self.local.as_ref() } {
+            local.repin();
+        }
+    }
+
+    /// Temporarily unpins the thread, executes the given function and then re-pins the thread.
+    ///
+    /// This method is useful when you need to perform a long-running operation (e.g. sleeping)
+    /// and don't need to maintain any guard-based reference across the call (the latter is enforced
+    /// by `&mut self`). The thread will only be unpinned if this is the only active guard for the
+    /// current thread.
+    ///
+    /// If this method is called from an [`unprotected`] guard, then the passed function is called
+    /// directly without unpinning the thread.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    /// use std::thread;
+    /// use std::time::Duration;
+    ///
+    /// let a = Atomic::new(777);
+    /// let mut guard = epoch::pin();
+    /// {
+    ///     let p = a.load(SeqCst, &guard);
+    ///     assert_eq!(unsafe { p.as_ref() }, Some(&777));
+    /// }
+    /// guard.repin_after(|| thread::sleep(Duration::from_millis(50)));
+    /// {
+    ///     let p = a.load(SeqCst, &guard);
+    ///     assert_eq!(unsafe { p.as_ref() }, Some(&777));
+    /// }
+    /// # unsafe { drop(a.into_owned()); } // avoid leak
+    /// ```
+    pub fn repin_after<F, R>(&mut self, f: F) -> R
+    where
+        F: FnOnce() -> R,
+    {
+        if let Some(local) = unsafe { self.local.as_ref() } {
+            // We need to acquire a handle here to ensure the Local doesn't
+            // disappear from under us.
+            local.acquire_handle();
+            local.unpin();
+        }
+
+        // Ensure the Guard is re-pinned even if the function panics
+        defer! {
+            if let Some(local) = unsafe { self.local.as_ref() } {
+                mem::forget(local.pin());
+                local.release_handle();
+            }
+        }
+
+        f()
+    }
+
+    /// Returns the `Collector` associated with this guard.
+    ///
+    /// This method is useful when you need to ensure that all guards used with
+    /// a data structure come from the same collector.
+    ///
+    /// If this method is called from an [`unprotected`] guard, then `None` is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch as epoch;
+    ///
+    /// let guard1 = epoch::pin();
+    /// let guard2 = epoch::pin();
+    /// assert!(guard1.collector() == guard2.collector());
+    /// ```
+    pub fn collector(&self) -> Option<&Collector> {
+        unsafe { self.local.as_ref().map(|local| local.collector()) }
+    }
+}
+
+impl Drop for Guard {
+    #[inline]
+    fn drop(&mut self) {
+        if let Some(local) = unsafe { self.local.as_ref() } {
+            local.unpin();
+        }
+    }
+}
+
+impl fmt::Debug for Guard {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad("Guard { .. }")
+    }
+}
+
+/// Returns a reference to a dummy guard that allows unprotected access to [`Atomic`]s.
+///
+/// This guard should be used in special occasions only. Note that it doesn't actually keep any
+/// thread pinned - it's just a fake guard that allows loading from [`Atomic`]s unsafely.
+///
+/// Note that calling [`defer`] with a dummy guard will not defer the function - it will just
+/// execute the function immediately.
+///
+/// If necessary, it's possible to create more dummy guards by cloning: `unprotected().clone()`.
+///
+/// # Safety
+///
+/// Loading and dereferencing data from an [`Atomic`] using this guard is safe only if the
+/// [`Atomic`] is not being concurrently modified by other threads.
+///
+/// # Examples
+///
+/// ```
+/// use crossbeam_epoch::{self as epoch, Atomic};
+/// use std::sync::atomic::Ordering::Relaxed;
+///
+/// let a = Atomic::new(7);
+///
+/// unsafe {
+///     // Load `a` without pinning the current thread.
+///     a.load(Relaxed, epoch::unprotected());
+///
+///     // It's possible to create more dummy guards by calling `clone()`.
+///     let dummy = &epoch::unprotected().clone();
+///
+///     dummy.defer(move || {
+///         println!("This gets executed immediately.");
+///     });
+///
+///     // Dropping `dummy` doesn't affect the current thread - it's just a noop.
+/// }
+/// # unsafe { drop(a.into_owned()); } // avoid leak
+/// ```
+///
+/// The most common use of this function is when constructing or destructing a data structure.
+///
+/// For example, we can use a dummy guard in the destructor of a Treiber stack because at that
+/// point no other thread could concurrently modify the [`Atomic`]s we are accessing.
+///
+/// If we were to actually pin the current thread during destruction, that would just unnecessarily
+/// delay garbage collection and incur some performance cost, so in cases like these `unprotected`
+/// is very helpful.
+///
+/// ```
+/// use crossbeam_epoch::{self as epoch, Atomic};
+/// use std::mem::ManuallyDrop;
+/// use std::sync::atomic::Ordering::Relaxed;
+///
+/// struct Stack<T> {
+///     head: Atomic<Node<T>>,
+/// }
+///
+/// struct Node<T> {
+///     data: ManuallyDrop<T>,
+///     next: Atomic<Node<T>>,
+/// }
+///
+/// impl<T> Drop for Stack<T> {
+///     fn drop(&mut self) {
+///         unsafe {
+///             // Unprotected load.
+///             let mut node = self.head.load(Relaxed, epoch::unprotected());
+///
+///             while let Some(n) = node.as_ref() {
+///                 // Unprotected load.
+///                 let next = n.next.load(Relaxed, epoch::unprotected());
+///
+///                 // Take ownership of the node, then drop its data and deallocate it.
+///                 let mut o = node.into_owned();
+///                 ManuallyDrop::drop(&mut o.data);
+///                 drop(o);
+///
+///                 node = next;
+///             }
+///         }
+///     }
+/// }
+/// ```
+///
+/// [`Atomic`]: super::Atomic
+/// [`defer`]: Guard::defer
+#[inline]
+pub unsafe fn unprotected() -> &'static Guard {
+    // An unprotected guard is just a `Guard` with its field `local` set to null.
+    // We make a newtype over `Guard` because `Guard` isn't `Sync`, so can't be directly stored in
+    // a `static`
+    struct GuardWrapper(Guard);
+    unsafe impl Sync for GuardWrapper {}
+    static UNPROTECTED: GuardWrapper = GuardWrapper(Guard {
+        local: core::ptr::null(),
+    });
+    &UNPROTECTED.0
+}
diff --git a/third_party/rust/crossbeam-epoch/src/internal.rs b/third_party/rust/crossbeam-epoch/src/internal.rs
new file mode 100644
index 0000000000..00c66a40a7
--- /dev/null
+++ b/third_party/rust/crossbeam-epoch/src/internal.rs
@@ -0,0 +1,599 @@
+//! The global data and participant for garbage collection.
+//!
+//! # Registration
+//!
+//! In order to track all participants in one place, we need some form of participant
+//! registration. When a participant is created, it is registered to a global lock-free
+//! singly-linked list of registries; and when a participant is leaving, it is unregistered from the
+//! list.
+//!
+//! # Pinning
+//!
+//! Every participant contains an integer that tells whether the participant is pinned and if so,
+//! what was the global epoch at the time it was pinned. Participants also hold a pin counter that
+//! aids in periodic global epoch advancement.
+//!
+//! When a participant is pinned, a `Guard` is returned as a witness that the participant is pinned.
+//! Guards are necessary for performing atomic operations, and for freeing/dropping locations.
+//!
+//! # Thread-local bag
+//!
+//! Objects that get unlinked from concurrent data structures must be stashed away until the global
+//! epoch sufficiently advances so that they become safe for destruction. Pointers to such objects
+//! are pushed into a thread-local bag, and when it becomes full, the bag is marked with the current
+//! global epoch and pushed into the global queue of bags. We store objects in thread-local storages
+//! for amortizing the synchronization cost of pushing the garbages to a global queue.
+//!
+//! # Global queue
+//!
+//! Whenever a bag is pushed into a queue, the objects in some bags in the queue are collected and
+//! destroyed along the way. This design reduces contention on data structures. The global queue
+//! cannot be explicitly accessed: the only way to interact with it is by calling functions
+//! `defer()` that adds an object to the thread-local bag, or `collect()` that manually triggers
+//! garbage collection.
+//!
+//! Ideally each instance of concurrent data structure may have its own queue that gets fully
+//! destroyed as soon as the data structure gets dropped.
+
+use crate::primitive::cell::UnsafeCell;
+use crate::primitive::sync::atomic;
+use core::cell::Cell;
+use core::mem::{self, ManuallyDrop};
+use core::num::Wrapping;
+use core::sync::atomic::Ordering;
+use core::{fmt, ptr};
+
+use crossbeam_utils::CachePadded;
+use memoffset::offset_of;
+
+use crate::atomic::{Owned, Shared};
+use crate::collector::{Collector, LocalHandle};
+use crate::deferred::Deferred;
+use crate::epoch::{AtomicEpoch, Epoch};
+use crate::guard::{unprotected, Guard};
+use crate::sync::list::{Entry, IsElement, IterError, List};
+use crate::sync::queue::Queue;
+
+/// Maximum number of objects a bag can contain.
+#[cfg(not(any(crossbeam_sanitize, miri)))]
+const MAX_OBJECTS: usize = 64;
+// Makes it more likely to trigger any potential data races.
+#[cfg(any(crossbeam_sanitize, miri))]
+const MAX_OBJECTS: usize = 4;
+
+/// A bag of deferred functions.
+pub(crate) struct Bag {
+    /// Stashed objects.
+    deferreds: [Deferred; MAX_OBJECTS],
+    len: usize,
+}
+
+/// `Bag::try_push()` requires that it is safe for another thread to execute the given functions.
+unsafe impl Send for Bag {}
+
+impl Bag {
+    /// Returns a new, empty bag.
+    pub(crate) fn new() -> Self {
+        Self::default()
+    }
+
+    /// Returns `true` if the bag is empty.
+    pub(crate) fn is_empty(&self) -> bool {
+        self.len == 0
+    }
+
+    /// Attempts to insert a deferred function into the bag.
+    ///
+    /// Returns `Ok(())` if successful, and `Err(deferred)` for the given `deferred` if the bag is
+    /// full.
+    ///
+    /// # Safety
+    ///
+    /// It should be safe for another thread to execute the given function.
+    pub(crate) unsafe fn try_push(&mut self, deferred: Deferred) -> Result<(), Deferred> {
+        if self.len < MAX_OBJECTS {
+            self.deferreds[self.len] = deferred;
+            self.len += 1;
+            Ok(())
+        } else {
+            Err(deferred)
+        }
+    }
+
+    /// Seals the bag with the given epoch.
+    fn seal(self, epoch: Epoch) -> SealedBag {
+        SealedBag { epoch, _bag: self }
+    }
+}
+
+impl Default for Bag {
+    fn default() -> Self {
+        Bag {
+            len: 0,
+            deferreds: [Deferred::NO_OP; MAX_OBJECTS],
+        }
+    }
+}
+
+impl Drop for Bag {
+    fn drop(&mut self) {
+        // Call all deferred functions.
+        for deferred in &mut self.deferreds[..self.len] {
+            let no_op = Deferred::NO_OP;
+            let owned_deferred = mem::replace(deferred, no_op);
+            owned_deferred.call();
+        }
+    }
+}
+
+// can't #[derive(Debug)] because Debug is not implemented for arrays 64 items long
+impl fmt::Debug for Bag {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Bag")
+            .field("deferreds", &&self.deferreds[..self.len])
+            .finish()
+    }
+}
+
+/// A pair of an epoch and a bag.
+#[derive(Default, Debug)]
+struct SealedBag {
+    epoch: Epoch,
+    _bag: Bag,
+}
+
+/// It is safe to share `SealedBag` because `is_expired` only inspects the epoch.
+unsafe impl Sync for SealedBag {}
+
+impl SealedBag {
+    /// Checks if it is safe to drop the bag w.r.t. the given global epoch.
+    fn is_expired(&self, global_epoch: Epoch) -> bool {
+        // A pinned participant can witness at most one epoch advancement. Therefore, any bag that
+        // is within one epoch of the current one cannot be destroyed yet.
+        global_epoch.wrapping_sub(self.epoch) >= 2
+    }
+}
+
+/// The global data for a garbage collector.
+pub(crate) struct Global {
+    /// The intrusive linked list of `Local`s.
+    locals: List<Local>,
+
+    /// The global queue of bags of deferred functions.
+    queue: Queue<SealedBag>,
+
+    /// The global epoch.
+    pub(crate) epoch: CachePadded<AtomicEpoch>,
+}
+
+impl Global {
+    /// Number of bags to destroy.
+    const COLLECT_STEPS: usize = 8;
+
+    /// Creates a new global data for garbage collection.
+    #[inline]
+    pub(crate) fn new() -> Self {
+        Self {
+            locals: List::new(),
+            queue: Queue::new(),
+            epoch: CachePadded::new(AtomicEpoch::new(Epoch::starting())),
+        }
+    }
+
+    /// Pushes the bag into the global queue and replaces the bag with a new empty bag.
+    pub(crate) fn push_bag(&self, bag: &mut Bag, guard: &Guard) {
+        let bag = mem::replace(bag, Bag::new());
+
+        atomic::fence(Ordering::SeqCst);
+
+        let epoch = self.epoch.load(Ordering::Relaxed);
+        self.queue.push(bag.seal(epoch), guard);
+    }
+
+    /// Collects several bags from the global queue and executes deferred functions in them.
+    ///
+    /// Note: This may itself produce garbage and in turn allocate new bags.
+    ///
+    /// `pin()` rarely calls `collect()`, so we want the compiler to place that call on a cold
+    /// path. In other words, we want the compiler to optimize branching for the case when
+    /// `collect()` is not called.
+    #[cold]
+    pub(crate) fn collect(&self, guard: &Guard) {
+        let global_epoch = self.try_advance(guard);
+
+        let steps = if cfg!(crossbeam_sanitize) {
+            usize::max_value()
+        } else {
+            Self::COLLECT_STEPS
+        };
+
+        for _ in 0..steps {
+            match self.queue.try_pop_if(
+                &|sealed_bag: &SealedBag| sealed_bag.is_expired(global_epoch),
+                guard,
+            ) {
+                None => break,
+                Some(sealed_bag) => drop(sealed_bag),
+            }
+        }
+    }
+
+    /// Attempts to advance the global epoch.
+    ///
+    /// The global epoch can advance only if all currently pinned participants have been pinned in
+    /// the current epoch.
+    ///
+    /// Returns the current global epoch.
+    ///
+    /// `try_advance()` is annotated `#[cold]` because it is rarely called.
+    #[cold]
+    pub(crate) fn try_advance(&self, guard: &Guard) -> Epoch {
+        let global_epoch = self.epoch.load(Ordering::Relaxed);
+        atomic::fence(Ordering::SeqCst);
+
+        // TODO(stjepang): `Local`s are stored in a linked list because linked lists are fairly
+        // easy to implement in a lock-free manner. However, traversal can be slow due to cache
+        // misses and data dependencies. We should experiment with other data structures as well.
+        for local in self.locals.iter(guard) {
+            match local {
+                Err(IterError::Stalled) => {
+                    // A concurrent thread stalled this iteration. That thread might also try to
+                    // advance the epoch, in which case we leave the job to it. Otherwise, the
+                    // epoch will not be advanced.
+                    return global_epoch;
+                }
+                Ok(local) => {
+                    let local_epoch = local.epoch.load(Ordering::Relaxed);
+
+                    // If the participant was pinned in a different epoch, we cannot advance the
+                    // global epoch just yet.
+                    if local_epoch.is_pinned() && local_epoch.unpinned() != global_epoch {
+                        return global_epoch;
+                    }
+                }
+            }
+        }
+        atomic::fence(Ordering::Acquire);
+
+        // All pinned participants were pinned in the current global epoch.
+        // Now let's advance the global epoch...
+        //
+        // Note that if another thread already advanced it before us, this store will simply
+        // overwrite the global epoch with the same value. This is true because `try_advance` was
+        // called from a thread that was pinned in `global_epoch`, and the global epoch cannot be
+        // advanced two steps ahead of it.
+        let new_epoch = global_epoch.successor();
+        self.epoch.store(new_epoch, Ordering::Release);
+        new_epoch
+    }
+}
+
+/// Participant for garbage collection.
+pub(crate) struct Local {
+    /// A node in the intrusive linked list of `Local`s.
+    entry: Entry,
+
+    /// The local epoch.
+    epoch: AtomicEpoch,
+
+    /// A reference to the global data.
+    ///
+    /// When all guards and handles get dropped, this reference is destroyed.
+    collector: UnsafeCell<ManuallyDrop<Collector>>,
+
+    /// The local bag of deferred functions.
+    pub(crate) bag: UnsafeCell<Bag>,
+
+    /// The number of guards keeping this participant pinned.
+    guard_count: Cell<usize>,
+
+    /// The number of active handles.
+    handle_count: Cell<usize>,
+
+    /// Total number of pinnings performed.
+    ///
+    /// This is just an auxiliary counter that sometimes kicks off collection.
+    pin_count: Cell<Wrapping<usize>>,
+}
+
+// Make sure `Local` is less than or equal to 2048 bytes.
+// https://github.com/crossbeam-rs/crossbeam/issues/551
+#[cfg(not(any(crossbeam_sanitize, miri)))] // `crossbeam_sanitize` and `miri` reduce the size of `Local`
+#[test]
+fn local_size() {
+    // TODO: https://github.com/crossbeam-rs/crossbeam/issues/869
+    // assert!(
+    //     core::mem::size_of::<Local>() <= 2048,
+    //     "An allocation of `Local` should be <= 2048 bytes."
+    // );
+}
+
+impl Local {
+    /// Number of pinnings after which a participant will execute some deferred functions from the
+    /// global queue.
+    const PINNINGS_BETWEEN_COLLECT: usize = 128;
+
+    /// Registers a new `Local` in the provided `Global`.
+    pub(crate) fn register(collector: &Collector) -> LocalHandle {
+        unsafe {
+            // Since we dereference no pointers in this block, it is safe to use `unprotected`.
+
+            let local = Owned::new(Local {
+                entry: Entry::default(),
+                epoch: AtomicEpoch::new(Epoch::starting()),
+                collector: UnsafeCell::new(ManuallyDrop::new(collector.clone())),
+                bag: UnsafeCell::new(Bag::new()),
+                guard_count: Cell::new(0),
+                handle_count: Cell::new(1),
+                pin_count: Cell::new(Wrapping(0)),
+            })
+            .into_shared(unprotected());
+            collector.global.locals.insert(local, unprotected());
+            LocalHandle {
+                local: local.as_raw(),
+            }
+        }
+    }
+
+    /// Returns a reference to the `Global` in which this `Local` resides.
+    #[inline]
+    pub(crate) fn global(&self) -> &Global {
+        &self.collector().global
+    }
+
+    /// Returns a reference to the `Collector` in which this `Local` resides.
+    #[inline]
+    pub(crate) fn collector(&self) -> &Collector {
+        self.collector.with(|c| unsafe { &**c })
+    }
+
+    /// Returns `true` if the current participant is pinned.
+    #[inline]
+    pub(crate) fn is_pinned(&self) -> bool {
+        self.guard_count.get() > 0
+    }
+
+    /// Adds `deferred` to the thread-local bag.
+    ///
+    /// # Safety
+    ///
+    /// It should be safe for another thread to execute the given function.
+    pub(crate) unsafe fn defer(&self, mut deferred: Deferred, guard: &Guard) {
+        let bag = self.bag.with_mut(|b| &mut *b);
+
+        while let Err(d) = bag.try_push(deferred) {
+            self.global().push_bag(bag, guard);
+            deferred = d;
+        }
+    }
+
+    pub(crate) fn flush(&self, guard: &Guard) {
+        let bag = self.bag.with_mut(|b| unsafe { &mut *b });
+
+        if !bag.is_empty() {
+            self.global().push_bag(bag, guard);
+        }
+
+        self.global().collect(guard);
+    }
+
+    /// Pins the `Local`.
+    #[inline]
+    pub(crate) fn pin(&self) -> Guard {
+        let guard = Guard { local: self };
+
+        let guard_count = self.guard_count.get();
+        self.guard_count.set(guard_count.checked_add(1).unwrap());
+
+        if guard_count == 0 {
+            let global_epoch = self.global().epoch.load(Ordering::Relaxed);
+            let new_epoch = global_epoch.pinned();
+
+            // Now we must store `new_epoch` into `self.epoch` and execute a `SeqCst` fence.
+            // The fence makes sure that any future loads from `Atomic`s will not happen before
+            // this store.
+            if cfg!(all(
+                any(target_arch = "x86", target_arch = "x86_64"),
+                not(miri)
+            )) {
+                // HACK(stjepang): On x86 architectures there are two different ways of executing
+                // a `SeqCst` fence.
+                //
+                // 1. `atomic::fence(SeqCst)`, which compiles into a `mfence` instruction.
+                // 2. `_.compare_exchange(_, _, SeqCst, SeqCst)`, which compiles into a `lock cmpxchg`
+                //    instruction.
+                //
+                // Both instructions have the effect of a full barrier, but benchmarks have shown
+                // that the second one makes pinning faster in this particular case.  It is not
+                // clear that this is permitted by the C++ memory model (SC fences work very
+                // differently from SC accesses), but experimental evidence suggests that this
+                // works fine.  Using inline assembly would be a viable (and correct) alternative,
+                // but alas, that is not possible on stable Rust.
+                let current = Epoch::starting();
+                let res = self.epoch.compare_exchange(
+                    current,
+                    new_epoch,
+                    Ordering::SeqCst,
+                    Ordering::SeqCst,
+                );
+                debug_assert!(res.is_ok(), "participant was expected to be unpinned");
+                // We add a compiler fence to make it less likely for LLVM to do something wrong
+                // here.  Formally, this is not enough to get rid of data races; practically,
+                // it should go a long way.
+                atomic::compiler_fence(Ordering::SeqCst);
+            } else {
+                self.epoch.store(new_epoch, Ordering::Relaxed);
+                atomic::fence(Ordering::SeqCst);
+            }
+
+            // Increment the pin counter.
+            let count = self.pin_count.get();
+            self.pin_count.set(count + Wrapping(1));
+
+            // After every `PINNINGS_BETWEEN_COLLECT` try advancing the epoch and collecting
+            // some garbage.
+            if count.0 % Self::PINNINGS_BETWEEN_COLLECT == 0 {
+                self.global().collect(&guard);
+            }
+        }
+
+        guard
+    }
+
+    /// Unpins the `Local`.
+    #[inline]
+    pub(crate) fn unpin(&self) {
+        let guard_count = self.guard_count.get();
+        self.guard_count.set(guard_count - 1);
+
+        if guard_count == 1 {
+            self.epoch.store(Epoch::starting(), Ordering::Release);
+
+            if self.handle_count.get() == 0 {
+                self.finalize();
+            }
+        }
+    }
+
+    /// Unpins and then pins the `Local`.
+    #[inline]
+    pub(crate) fn repin(&self) {
+        let guard_count = self.guard_count.get();
+
+        // Update the local epoch only if there's only one guard.
+        if guard_count == 1 {
+            let epoch = self.epoch.load(Ordering::Relaxed);
+            let global_epoch = self.global().epoch.load(Ordering::Relaxed).pinned();
+
+            // Update the local epoch only if the global epoch is greater than the local epoch.
+            if epoch != global_epoch {
+                // We store the new epoch with `Release` because we need to ensure any memory
+                // accesses from the previous epoch do not leak into the new one.
+                self.epoch.store(global_epoch, Ordering::Release);
+
+                // However, we don't need a following `SeqCst` fence, because it is safe for memory
+                // accesses from the new epoch to be executed before updating the local epoch. At
+                // worse, other threads will see the new epoch late and delay GC slightly.
+            }
+        }
+    }
+
+    /// Increments the handle count.
+    #[inline]
+    pub(crate) fn acquire_handle(&self) {
+        let handle_count = self.handle_count.get();
+        debug_assert!(handle_count >= 1);
+        self.handle_count.set(handle_count + 1);
+    }
+
+    /// Decrements the handle count.
+    #[inline]
+    pub(crate) fn release_handle(&self) {
+        let guard_count = self.guard_count.get();
+        let handle_count = self.handle_count.get();
+        debug_assert!(handle_count >= 1);
+        self.handle_count.set(handle_count - 1);
+
+        if guard_count == 0 && handle_count == 1 {
+            self.finalize();
+        }
+    }
+
+    /// Removes the `Local` from the global linked list.
+    #[cold]
+    fn finalize(&self) {
+        debug_assert_eq!(self.guard_count.get(), 0);
+        debug_assert_eq!(self.handle_count.get(), 0);
+
+        // Temporarily increment handle count. This is required so that the following call to `pin`
+        // doesn't call `finalize` again.
+        self.handle_count.set(1);
+        unsafe {
+            // Pin and move the local bag into the global queue. It's important that `push_bag`
+            // doesn't defer destruction on any new garbage.
+            let guard = &self.pin();
+            self.global()
+                .push_bag(self.bag.with_mut(|b| &mut *b), guard);
+        }
+        // Revert the handle count back to zero.
+        self.handle_count.set(0);
+
+        unsafe {
+            // Take the reference to the `Global` out of this `Local`. Since we're not protected
+            // by a guard at this time, it's crucial that the reference is read before marking the
+            // `Local` as deleted.
+            let collector: Collector = ptr::read(self.collector.with(|c| &*(*c)));
+
+            // Mark this node in the linked list as deleted.
+            self.entry.delete(unprotected());
+
+            // Finally, drop the reference to the global. Note that this might be the last reference
+            // to the `Global`. If so, the global data will be destroyed and all deferred functions
+            // in its queue will be executed.
+            drop(collector);
+        }
+    }
+}
+
+impl IsElement<Local> for Local {
+    fn entry_of(local: &Local) -> &Entry {
+        let entry_ptr = (local as *const Local as usize + offset_of!(Local, entry)) as *const Entry;
+        unsafe { &*entry_ptr }
+    }
+
+    unsafe fn element_of(entry: &Entry) -> &Local {
+        // offset_of! macro uses unsafe, but it's unnecessary in this context.
+        #[allow(unused_unsafe)]
+        let local_ptr = (entry as *const Entry as usize - offset_of!(Local, entry)) as *const Local;
+        &*local_ptr
+    }
+
+    unsafe fn finalize(entry: &Entry, guard: &Guard) {
+        guard.defer_destroy(Shared::from(Self::element_of(entry) as *const _));
+    }
+}
+
+#[cfg(all(test, not(crossbeam_loom)))]
+mod tests {
+    use std::sync::atomic::{AtomicUsize, Ordering};
+
+    use super::*;
+
+    #[test]
+    fn check_defer() {
+        static FLAG: AtomicUsize = AtomicUsize::new(0);
+        fn set() {
+            FLAG.store(42, Ordering::Relaxed);
+        }
+
+        let d = Deferred::new(set);
+        assert_eq!(FLAG.load(Ordering::Relaxed), 0);
+        d.call();
+        assert_eq!(FLAG.load(Ordering::Relaxed), 42);
+    }
+
+    #[test]
+    fn check_bag() {
+        static FLAG: AtomicUsize = AtomicUsize::new(0);
+        fn incr() {
+            FLAG.fetch_add(1, Ordering::Relaxed);
+        }
+
+        let mut bag = Bag::new();
+        assert!(bag.is_empty());
+
+        for _ in 0..MAX_OBJECTS {
+            assert!(unsafe { bag.try_push(Deferred::new(incr)).is_ok() });
+            assert!(!bag.is_empty());
+            assert_eq!(FLAG.load(Ordering::Relaxed), 0);
+        }
+
+        let result = unsafe { bag.try_push(Deferred::new(incr)) };
+        assert!(result.is_err());
+        assert!(!bag.is_empty());
+        assert_eq!(FLAG.load(Ordering::Relaxed), 0);
+
+        drop(bag);
+        assert_eq!(FLAG.load(Ordering::Relaxed), MAX_OBJECTS);
+    }
+}
diff --git a/third_party/rust/crossbeam-epoch/src/lib.rs b/third_party/rust/crossbeam-epoch/src/lib.rs
new file mode 100644
index 0000000000..b432c1f401
--- /dev/null
+++ b/third_party/rust/crossbeam-epoch/src/lib.rs
@@ -0,0 +1,169 @@
+//! Epoch-based memory reclamation.
+//!
+//! An interesting problem concurrent collections deal with comes from the remove operation.
+//! Suppose that a thread removes an element from a lock-free map, while another thread is reading
+//! that same element at the same time. The first thread must wait until the second thread stops
+//! reading the element. Only then it is safe to destruct it.
+//!
+//! Programming languages that come with garbage collectors solve this problem trivially. The
+//! garbage collector will destruct the removed element when no thread can hold a reference to it
+//! anymore.
+//!
+//! This crate implements a basic memory reclamation mechanism, which is based on epochs. When an
+//! element gets removed from a concurrent collection, it is inserted into a pile of garbage and
+//! marked with the current epoch. Every time a thread accesses a collection, it checks the current
+//! epoch, attempts to increment it, and destructs some garbage that became so old that no thread
+//! can be referencing it anymore.
+//!
+//! That is the general mechanism behind epoch-based memory reclamation, but the details are a bit
+//! more complicated. Anyhow, memory reclamation is designed to be fully automatic and something
+//! users of concurrent collections don't have to worry much about.
+//!
+//! # Pointers
+//!
+//! Concurrent collections are built using atomic pointers. This module provides [`Atomic`], which
+//! is just a shared atomic pointer to a heap-allocated object. Loading an [`Atomic`] yields a
+//! [`Shared`], which is an epoch-protected pointer through which the loaded object can be safely
+//! read.
+//!
+//! # Pinning
+//!
+//! Before an [`Atomic`] can be loaded, a participant must be [`pin`]ned. By pinning a participant
+//! we declare that any object that gets removed from now on must not be destructed just
+//! yet. Garbage collection of newly removed objects is suspended until the participant gets
+//! unpinned.
+//!
+//! # Garbage
+//!
+//! Objects that get removed from concurrent collections must be stashed away until all currently
+//! pinned participants get unpinned. Such objects can be stored into a thread-local or global
+//! storage, where they are kept until the right time for their destruction comes.
+//!
+//! There is a global shared instance of garbage queue. You can [`defer`](Guard::defer) the execution of an
+//! arbitrary function until the global epoch is advanced enough. Most notably, concurrent data
+//! structures may defer the deallocation of an object.
+//!
+//! # APIs
+//!
+//! For majority of use cases, just use the default garbage collector by invoking [`pin`]. If you
+//! want to create your own garbage collector, use the [`Collector`] API.
+
+#![doc(test(
+    no_crate_inject,
+    attr(
+        deny(warnings, rust_2018_idioms),
+        allow(dead_code, unused_assignments, unused_variables)
+    )
+))]
+#![warn(
+    missing_docs,
+    missing_debug_implementations,
+    rust_2018_idioms,
+    unreachable_pub
+)]
+#![cfg_attr(not(feature = "std"), no_std)]
+
+#[cfg(crossbeam_loom)]
+extern crate loom_crate as loom;
+
+use cfg_if::cfg_if;
+
+#[cfg(crossbeam_loom)]
+#[allow(unused_imports, dead_code)]
+mod primitive {
+    pub(crate) mod cell {
+        pub(crate) use loom::cell::UnsafeCell;
+    }
+    pub(crate) mod sync {
+        pub(crate) mod atomic {
+            use core::sync::atomic::Ordering;
+            pub(crate) use loom::sync::atomic::{fence, AtomicUsize};
+
+            // FIXME: loom does not support compiler_fence at the moment.
+            // https://github.com/tokio-rs/loom/issues/117
+            // we use fence as a stand-in for compiler_fence for the time being.
+            // this may miss some races since fence is stronger than compiler_fence,
+            // but it's the best we can do for the time being.
+            pub(crate) use self::fence as compiler_fence;
+        }
+        pub(crate) use loom::sync::Arc;
+    }
+    pub(crate) use loom::thread_local;
+}
+#[cfg(not(crossbeam_no_atomic_cas))]
+#[cfg(not(crossbeam_loom))]
+#[allow(unused_imports, dead_code)]
+mod primitive {
+    #[cfg(feature = "alloc")]
+    pub(crate) mod cell {
+        #[derive(Debug)]
+        #[repr(transparent)]
+        pub(crate) struct UnsafeCell<T>(::core::cell::UnsafeCell<T>);
+
+        // loom's UnsafeCell has a slightly different API than the standard library UnsafeCell.
+        // Since we want the rest of the code to be agnostic to whether it's running under loom or
+        // not, we write this small wrapper that provides the loom-supported API for the standard
+        // library UnsafeCell. This is also what the loom documentation recommends:
+        // https://github.com/tokio-rs/loom#handling-loom-api-differences
+        impl<T> UnsafeCell<T> {
+            #[inline]
+            pub(crate) const fn new(data: T) -> UnsafeCell<T> {
+                UnsafeCell(::core::cell::UnsafeCell::new(data))
+            }
+
+            #[inline]
+            pub(crate) fn with<R>(&self, f: impl FnOnce(*const T) -> R) -> R {
+                f(self.0.get())
+            }
+
+            #[inline]
+            pub(crate) fn with_mut<R>(&self, f: impl FnOnce(*mut T) -> R) -> R {
+                f(self.0.get())
+            }
+        }
+    }
+    #[cfg(feature = "alloc")]
+    pub(crate) mod sync {
+        pub(crate) mod atomic {
+            pub(crate) use core::sync::atomic::compiler_fence;
+            pub(crate) use core::sync::atomic::fence;
+            pub(crate) use core::sync::atomic::AtomicUsize;
+        }
+        pub(crate) use alloc::sync::Arc;
+    }
+
+    #[cfg(feature = "std")]
+    pub(crate) use std::thread_local;
+}
+
+#[cfg(not(crossbeam_no_atomic_cas))]
+cfg_if! {
+    if #[cfg(feature = "alloc")] {
+        extern crate alloc;
+
+        mod atomic;
+        mod collector;
+        mod deferred;
+        mod epoch;
+        mod guard;
+        mod internal;
+        mod sync;
+
+        pub use self::atomic::{
+            Pointable, Atomic, CompareExchangeError,
+            Owned, Pointer, Shared,
+        };
+        pub use self::collector::{Collector, LocalHandle};
+        pub use self::guard::{unprotected, Guard};
+
+        #[allow(deprecated)]
+        pub use self::atomic::{CompareAndSetError, CompareAndSetOrdering};
+    }
+}
+
+cfg_if! {
+    if #[cfg(feature = "std")] {
+        mod default;
+        pub use self::default::{default_collector, is_pinned, pin};
+    }
+}
diff --git a/third_party/rust/crossbeam-epoch/src/sync/list.rs b/third_party/rust/crossbeam-epoch/src/sync/list.rs
new file mode 100644
index 0000000000..52ffd6fca4
--- /dev/null
+++ b/third_party/rust/crossbeam-epoch/src/sync/list.rs
@@ -0,0 +1,487 @@
+//! Lock-free intrusive linked list.
+//!
+//! Ideas from Michael.  High Performance Dynamic Lock-Free Hash Tables and List-Based Sets.  SPAA
+//! 2002.  <http://dl.acm.org/citation.cfm?id=564870.564881>
+
+use core::marker::PhantomData;
+use core::sync::atomic::Ordering::{Acquire, Relaxed, Release};
+
+use crate::{unprotected, Atomic, Guard, Shared};
+
+/// An entry in a linked list.
+///
+/// An Entry is accessed from multiple threads, so it would be beneficial to put it in a different
+/// cache-line than thread-local data in terms of performance.
+#[derive(Debug)]
+pub(crate) struct Entry {
+    /// The next entry in the linked list.
+    /// If the tag is 1, this entry is marked as deleted.
+    next: Atomic<Entry>,
+}
+
+/// Implementing this trait asserts that the type `T` can be used as an element in the intrusive
+/// linked list defined in this module. `T` has to contain (or otherwise be linked to) an instance
+/// of `Entry`.
+///
+/// # Example
+///
+/// ```ignore
+/// struct A {
+///     entry: Entry,
+///     data: usize,
+/// }
+///
+/// impl IsElement<A> for A {
+///     fn entry_of(a: &A) -> &Entry {
+///         let entry_ptr = ((a as usize) + offset_of!(A, entry)) as *const Entry;
+///         unsafe { &*entry_ptr }
+///     }
+///
+///     unsafe fn element_of(entry: &Entry) -> &T {
+///         let elem_ptr = ((entry as usize) - offset_of!(A, entry)) as *const T;
+///         &*elem_ptr
+///     }
+///
+///     unsafe fn finalize(entry: &Entry, guard: &Guard) {
+///         guard.defer_destroy(Shared::from(Self::element_of(entry) as *const _));
+///     }
+/// }
+/// ```
+///
+/// This trait is implemented on a type separate from `T` (although it can be just `T`), because
+/// one type might be placeable into multiple lists, in which case it would require multiple
+/// implementations of `IsElement`. In such cases, each struct implementing `IsElement<T>`
+/// represents a distinct `Entry` in `T`.
+///
+/// For example, we can insert the following struct into two lists using `entry1` for one
+/// and `entry2` for the other:
+///
+/// ```ignore
+/// struct B {
+///     entry1: Entry,
+///     entry2: Entry,
+///     data: usize,
+/// }
+/// ```
+///
+pub(crate) trait IsElement<T> {
+    /// Returns a reference to this element's `Entry`.
+    fn entry_of(_: &T) -> &Entry;
+
+    /// Given a reference to an element's entry, returns that element.
+    ///
+    /// ```ignore
+    /// let elem = ListElement::new();
+    /// assert_eq!(elem.entry_of(),
+    ///            unsafe { ListElement::element_of(elem.entry_of()) } );
+    /// ```
+    ///
+    /// # Safety
+    ///
+    /// The caller has to guarantee that the `Entry` is called with was retrieved from an instance
+    /// of the element type (`T`).
+    unsafe fn element_of(_: &Entry) -> &T;
+
+    /// The function that is called when an entry is unlinked from list.
+    ///
+    /// # Safety
+    ///
+    /// The caller has to guarantee that the `Entry` is called with was retrieved from an instance
+    /// of the element type (`T`).
+    unsafe fn finalize(_: &Entry, _: &Guard);
+}
+
+/// A lock-free, intrusive linked list of type `T`.
+#[derive(Debug)]
+pub(crate) struct List<T, C: IsElement<T> = T> {
+    /// The head of the linked list.
+    head: Atomic<Entry>,
+
+    /// The phantom data for using `T` and `C`.
+    _marker: PhantomData<(T, C)>,
+}
+
+/// An iterator used for retrieving values from the list.
+pub(crate) struct Iter<'g, T, C: IsElement<T>> {
+    /// The guard that protects the iteration.
+    guard: &'g Guard,
+
+    /// Pointer from the predecessor to the current entry.
+    pred: &'g Atomic<Entry>,
+
+    /// The current entry.
+    curr: Shared<'g, Entry>,
+
+    /// The list head, needed for restarting iteration.
+    head: &'g Atomic<Entry>,
+
+    /// Logically, we store a borrow of an instance of `T` and
+    /// use the type information from `C`.
+    _marker: PhantomData<(&'g T, C)>,
+}
+
+/// An error that occurs during iteration over the list.
+#[derive(PartialEq, Debug)]
+pub(crate) enum IterError {
+    /// A concurrent thread modified the state of the list at the same place that this iterator
+    /// was inspecting. Subsequent iteration will restart from the beginning of the list.
+    Stalled,
+}
+
+impl Default for Entry {
+    /// Returns the empty entry.
+    fn default() -> Self {
+        Self {
+            next: Atomic::null(),
+        }
+    }
+}
+
+impl Entry {
+    /// Marks this entry as deleted, deferring the actual deallocation to a later iteration.
+    ///
+    /// # Safety
+    ///
+    /// The entry should be a member of a linked list, and it should not have been deleted.
+    /// It should be safe to call `C::finalize` on the entry after the `guard` is dropped, where `C`
+    /// is the associated helper for the linked list.
+    pub(crate) unsafe fn delete(&self, guard: &Guard) {
+        self.next.fetch_or(1, Release, guard);
+    }
+}
+
+impl<T, C: IsElement<T>> List<T, C> {
+    /// Returns a new, empty linked list.
+    pub(crate) fn new() -> Self {
+        Self {
+            head: Atomic::null(),
+            _marker: PhantomData,
+        }
+    }
+
+    /// Inserts `entry` into the head of the list.
+    ///
+    /// # Safety
+    ///
+    /// You should guarantee that:
+    ///
+    /// - `container` is not null
+    /// - `container` is immovable, e.g. inside an `Owned`
+    /// - the same `Entry` is not inserted more than once
+    /// - the inserted object will be removed before the list is dropped
+    pub(crate) unsafe fn insert<'g>(&'g self, container: Shared<'g, T>, guard: &'g Guard) {
+        // Insert right after head, i.e. at the beginning of the list.
+        let to = &self.head;
+        // Get the intrusively stored Entry of the new element to insert.
+        let entry: &Entry = C::entry_of(container.deref());
+        // Make a Shared ptr to that Entry.
+        let entry_ptr = Shared::from(entry as *const _);
+        // Read the current successor of where we want to insert.
+        let mut next = to.load(Relaxed, guard);
+
+        loop {
+            // Set the Entry of the to-be-inserted element to point to the previous successor of
+            // `to`.
+            entry.next.store(next, Relaxed);
+            match to.compare_exchange_weak(next, entry_ptr, Release, Relaxed, guard) {
+                Ok(_) => break,
+                // We lost the race or weak CAS failed spuriously. Update the successor and try
+                // again.
+                Err(err) => next = err.current,
+            }
+        }
+    }
+
+    /// Returns an iterator over all objects.
+    ///
+    /// # Caveat
+    ///
+    /// Every object that is inserted at the moment this function is called and persists at least
+    /// until the end of iteration will be returned. Since this iterator traverses a lock-free
+    /// linked list that may be concurrently modified, some additional caveats apply:
+    ///
+    /// 1. If a new object is inserted during iteration, it may or may not be returned.
+    /// 2. If an object is deleted during iteration, it may or may not be returned.
+    /// 3. The iteration may be aborted when it lost in a race condition. In this case, the winning
+    ///    thread will continue to iterate over the same list.
+    pub(crate) fn iter<'g>(&'g self, guard: &'g Guard) -> Iter<'g, T, C> {
+        Iter {
+            guard,
+            pred: &self.head,
+            curr: self.head.load(Acquire, guard),
+            head: &self.head,
+            _marker: PhantomData,
+        }
+    }
+}
+
+impl<T, C: IsElement<T>> Drop for List<T, C> {
+    fn drop(&mut self) {
+        unsafe {
+            let guard = unprotected();
+            let mut curr = self.head.load(Relaxed, guard);
+            while let Some(c) = curr.as_ref() {
+                let succ = c.next.load(Relaxed, guard);
+                // Verify that all elements have been removed from the list.
+                assert_eq!(succ.tag(), 1);
+
+                C::finalize(curr.deref(), guard);
+                curr = succ;
+            }
+        }
+    }
+}
+
+impl<'g, T: 'g, C: IsElement<T>> Iterator for Iter<'g, T, C> {
+    type Item = Result<&'g T, IterError>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        while let Some(c) = unsafe { self.curr.as_ref() } {
+            let succ = c.next.load(Acquire, self.guard);
+
+            if succ.tag() == 1 {
+                // This entry was removed. Try unlinking it from the list.
+                let succ = succ.with_tag(0);
+
+                // The tag should always be zero, because removing a node after a logically deleted
+                // node leaves the list in an invalid state.
+                debug_assert!(self.curr.tag() == 0);
+
+                // Try to unlink `curr` from the list, and get the new value of `self.pred`.
+                let succ = match self
+                    .pred
+                    .compare_exchange(self.curr, succ, Acquire, Acquire, self.guard)
+                {
+                    Ok(_) => {
+                        // We succeeded in unlinking `curr`, so we have to schedule
+                        // deallocation. Deferred drop is okay, because `list.delete()` can only be
+                        // called if `T: 'static`.
+                        unsafe {
+                            C::finalize(self.curr.deref(), self.guard);
+                        }
+
+                        // `succ` is the new value of `self.pred`.
+                        succ
+                    }
+                    Err(e) => {
+                        // `e.current` is the current value of `self.pred`.
+                        e.current
+                    }
+                };
+
+                // If the predecessor node is already marked as deleted, we need to restart from
+                // `head`.
+                if succ.tag() != 0 {
+                    self.pred = self.head;
+                    self.curr = self.head.load(Acquire, self.guard);
+
+                    return Some(Err(IterError::Stalled));
+                }
+
+                // Move over the removed by only advancing `curr`, not `pred`.
+                self.curr = succ;
+                continue;
+            }
+
+            // Move one step forward.
+            self.pred = &c.next;
+            self.curr = succ;
+
+            return Some(Ok(unsafe { C::element_of(c) }));
+        }
+
+        // We reached the end of the list.
+        None
+    }
+}
+
+#[cfg(all(test, not(crossbeam_loom)))]
+mod tests {
+    use super::*;
+    use crate::{Collector, Owned};
+    use crossbeam_utils::thread;
+    use std::sync::Barrier;
+
+    impl IsElement<Entry> for Entry {
+        fn entry_of(entry: &Entry) -> &Entry {
+            entry
+        }
+
+        unsafe fn element_of(entry: &Entry) -> &Entry {
+            entry
+        }
+
+        unsafe fn finalize(entry: &Entry, guard: &Guard) {
+            guard.defer_destroy(Shared::from(Self::element_of(entry) as *const _));
+        }
+    }
+
+    /// Checks whether the list retains inserted elements
+    /// and returns them in the correct order.
+    #[test]
+    fn insert() {
+        let collector = Collector::new();
+        let handle = collector.register();
+        let guard = handle.pin();
+
+        let l: List<Entry> = List::new();
+
+        let e1 = Owned::new(Entry::default()).into_shared(&guard);
+        let e2 = Owned::new(Entry::default()).into_shared(&guard);
+        let e3 = Owned::new(Entry::default()).into_shared(&guard);
+
+        unsafe {
+            l.insert(e1, &guard);
+            l.insert(e2, &guard);
+            l.insert(e3, &guard);
+        }
+
+        let mut iter = l.iter(&guard);
+        let maybe_e3 = iter.next();
+        assert!(maybe_e3.is_some());
+        assert!(maybe_e3.unwrap().unwrap() as *const Entry == e3.as_raw());
+        let maybe_e2 = iter.next();
+        assert!(maybe_e2.is_some());
+        assert!(maybe_e2.unwrap().unwrap() as *const Entry == e2.as_raw());
+        let maybe_e1 = iter.next();
+        assert!(maybe_e1.is_some());
+        assert!(maybe_e1.unwrap().unwrap() as *const Entry == e1.as_raw());
+        assert!(iter.next().is_none());
+
+        unsafe {
+            e1.as_ref().unwrap().delete(&guard);
+            e2.as_ref().unwrap().delete(&guard);
+            e3.as_ref().unwrap().delete(&guard);
+        }
+    }
+
+    /// Checks whether elements can be removed from the list and whether
+    /// the correct elements are removed.
+    #[test]
+    fn delete() {
+        let collector = Collector::new();
+        let handle = collector.register();
+        let guard = handle.pin();
+
+        let l: List<Entry> = List::new();
+
+        let e1 = Owned::new(Entry::default()).into_shared(&guard);
+        let e2 = Owned::new(Entry::default()).into_shared(&guard);
+        let e3 = Owned::new(Entry::default()).into_shared(&guard);
+        unsafe {
+            l.insert(e1, &guard);
+            l.insert(e2, &guard);
+            l.insert(e3, &guard);
+            e2.as_ref().unwrap().delete(&guard);
+        }
+
+        let mut iter = l.iter(&guard);
+        let maybe_e3 = iter.next();
+        assert!(maybe_e3.is_some());
+        assert!(maybe_e3.unwrap().unwrap() as *const Entry == e3.as_raw());
+        let maybe_e1 = iter.next();
+        assert!(maybe_e1.is_some());
+        assert!(maybe_e1.unwrap().unwrap() as *const Entry == e1.as_raw());
+        assert!(iter.next().is_none());
+
+        unsafe {
+            e1.as_ref().unwrap().delete(&guard);
+            e3.as_ref().unwrap().delete(&guard);
+        }
+
+        let mut iter = l.iter(&guard);
+        assert!(iter.next().is_none());
+    }
+
+    const THREADS: usize = 8;
+    const ITERS: usize = 512;
+
+    /// Contends the list on insert and delete operations to make sure they can run concurrently.
+    #[test]
+    fn insert_delete_multi() {
+        let collector = Collector::new();
+
+        let l: List<Entry> = List::new();
+        let b = Barrier::new(THREADS);
+
+        thread::scope(|s| {
+            for _ in 0..THREADS {
+                s.spawn(|_| {
+                    b.wait();
+
+                    let handle = collector.register();
+                    let guard: Guard = handle.pin();
+                    let mut v = Vec::with_capacity(ITERS);
+
+                    for _ in 0..ITERS {
+                        let e = Owned::new(Entry::default()).into_shared(&guard);
+                        v.push(e);
+                        unsafe {
+                            l.insert(e, &guard);
+                        }
+                    }
+
+                    for e in v {
+                        unsafe {
+                            e.as_ref().unwrap().delete(&guard);
+                        }
+                    }
+                });
+            }
+        })
+        .unwrap();
+
+        let handle = collector.register();
+        let guard = handle.pin();
+
+        let mut iter = l.iter(&guard);
+        assert!(iter.next().is_none());
+    }
+
+    /// Contends the list on iteration to make sure that it can be iterated over concurrently.
+    #[test]
+    fn iter_multi() {
+        let collector = Collector::new();
+
+        let l: List<Entry> = List::new();
+        let b = Barrier::new(THREADS);
+
+        thread::scope(|s| {
+            for _ in 0..THREADS {
+                s.spawn(|_| {
+                    b.wait();
+
+                    let handle = collector.register();
+                    let guard: Guard = handle.pin();
+                    let mut v = Vec::with_capacity(ITERS);
+
+                    for _ in 0..ITERS {
+                        let e = Owned::new(Entry::default()).into_shared(&guard);
+                        v.push(e);
+                        unsafe {
+                            l.insert(e, &guard);
+                        }
+                    }
+
+                    let mut iter = l.iter(&guard);
+                    for _ in 0..ITERS {
+                        assert!(iter.next().is_some());
+                    }
+
+                    for e in v {
+                        unsafe {
+                            e.as_ref().unwrap().delete(&guard);
+                        }
+                    }
+                });
+            }
+        })
+        .unwrap();
+
+        let handle = collector.register();
+        let guard = handle.pin();
+
+        let mut iter = l.iter(&guard);
+        assert!(iter.next().is_none());
+    }
+}
diff --git a/third_party/rust/crossbeam-epoch/src/sync/mod.rs b/third_party/rust/crossbeam-epoch/src/sync/mod.rs
new file mode 100644
index 0000000000..08981be257
--- /dev/null
+++ b/third_party/rust/crossbeam-epoch/src/sync/mod.rs
@@ -0,0 +1,7 @@
+//! Synchronization primitives.
+
+pub(crate) mod list;
+#[cfg(feature = "std")]
+#[cfg(not(crossbeam_loom))]
+pub(crate) mod once_lock;
+pub(crate) mod queue;
diff --git a/third_party/rust/crossbeam-epoch/src/sync/once_lock.rs b/third_party/rust/crossbeam-epoch/src/sync/once_lock.rs
new file mode 100644
index 0000000000..c1fefc96cc
--- /dev/null
+++ b/third_party/rust/crossbeam-epoch/src/sync/once_lock.rs
@@ -0,0 +1,103 @@
+// Based on unstable std::sync::OnceLock.
+//
+// Source: https://github.com/rust-lang/rust/blob/8e9c93df464b7ada3fc7a1c8ccddd9dcb24ee0a0/library/std/src/sync/once_lock.rs
+
+use core::cell::UnsafeCell;
+use core::mem::MaybeUninit;
+use core::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Once;
+
+pub(crate) struct OnceLock<T> {
+    once: Once,
+    // Once::is_completed requires Rust 1.43, so use this to track of whether they have been initialized.
+    is_initialized: AtomicBool,
+    value: UnsafeCell<MaybeUninit<T>>,
+    // Unlike std::sync::OnceLock, we don't need PhantomData here because
+    // we don't use #[may_dangle].
+}
+
+unsafe impl<T: Sync + Send> Sync for OnceLock<T> {}
+unsafe impl<T: Send> Send for OnceLock<T> {}
+
+impl<T> OnceLock<T> {
+    /// Creates a new empty cell.
+    #[must_use]
+    pub(crate) const fn new() -> Self {
+        Self {
+            once: Once::new(),
+            is_initialized: AtomicBool::new(false),
+            value: UnsafeCell::new(MaybeUninit::uninit()),
+        }
+    }
+
+    /// Gets the contents of the cell, initializing it with `f` if the cell
+    /// was empty.
+    ///
+    /// Many threads may call `get_or_init` concurrently with different
+    /// initializing functions, but it is guaranteed that only one function
+    /// will be executed.
+    ///
+    /// # Panics
+    ///
+    /// If `f` panics, the panic is propagated to the caller, and the cell
+    /// remains uninitialized.
+    ///
+    /// It is an error to reentrantly initialize the cell from `f`. The
+    /// exact outcome is unspecified. Current implementation deadlocks, but
+    /// this may be changed to a panic in the future.
+    pub(crate) fn get_or_init<F>(&self, f: F) -> &T
+    where
+        F: FnOnce() -> T,
+    {
+        // Fast path check
+        if self.is_initialized() {
+            // SAFETY: The inner value has been initialized
+            return unsafe { self.get_unchecked() };
+        }
+        self.initialize(f);
+
+        debug_assert!(self.is_initialized());
+
+        // SAFETY: The inner value has been initialized
+        unsafe { self.get_unchecked() }
+    }
+
+    #[inline]
+    fn is_initialized(&self) -> bool {
+        self.is_initialized.load(Ordering::Acquire)
+    }
+
+    #[cold]
+    fn initialize<F>(&self, f: F)
+    where
+        F: FnOnce() -> T,
+    {
+        let slot = self.value.get().cast::<T>();
+        let is_initialized = &self.is_initialized;
+
+        self.once.call_once(|| {
+            let value = f();
+            unsafe {
+                slot.write(value);
+            }
+            is_initialized.store(true, Ordering::Release);
+        });
+    }
+
+    /// # Safety
+    ///
+    /// The value must be initialized
+    unsafe fn get_unchecked(&self) -> &T {
+        debug_assert!(self.is_initialized());
+        &*self.value.get().cast::<T>()
+    }
+}
+
+impl<T> Drop for OnceLock<T> {
+    fn drop(&mut self) {
+        if self.is_initialized() {
+            // SAFETY: The inner value has been initialized
+            unsafe { self.value.get().cast::<T>().drop_in_place() };
+        }
+    }
+}
diff --git a/third_party/rust/crossbeam-epoch/src/sync/queue.rs b/third_party/rust/crossbeam-epoch/src/sync/queue.rs
new file mode 100644
index 0000000000..9500438819
--- /dev/null
+++ b/third_party/rust/crossbeam-epoch/src/sync/queue.rs
@@ -0,0 +1,469 @@
+//! Michael-Scott lock-free queue.
+//!
+//! Usable with any number of producers and consumers.
+//!
+//! Michael and Scott.  Simple, Fast, and Practical Non-Blocking and Blocking Concurrent Queue
+//! Algorithms.  PODC 1996.  <http://dl.acm.org/citation.cfm?id=248106>
+//!
+//! Simon Doherty, Lindsay Groves, Victor Luchangco, and Mark Moir. 2004b. Formal Verification of a
+//! Practical Lock-Free Queue Algorithm. <https://doi.org/10.1007/978-3-540-30232-2_7>
+
+use core::mem::MaybeUninit;
+use core::sync::atomic::Ordering::{Acquire, Relaxed, Release};
+
+use crossbeam_utils::CachePadded;
+
+use crate::{unprotected, Atomic, Guard, Owned, Shared};
+
+// The representation here is a singly-linked list, with a sentinel node at the front. In general
+// the `tail` pointer may lag behind the actual tail. Non-sentinel nodes are either all `Data` or
+// all `Blocked` (requests for data from blocked threads).
+#[derive(Debug)]
+pub(crate) struct Queue<T> {
+    head: CachePadded<Atomic<Node<T>>>,
+    tail: CachePadded<Atomic<Node<T>>>,
+}
+
+struct Node<T> {
+    /// The slot in which a value of type `T` can be stored.
+    ///
+    /// The type of `data` is `MaybeUninit<T>` because a `Node<T>` doesn't always contain a `T`.
+    /// For example, the sentinel node in a queue never contains a value: its slot is always empty.
+    /// Other nodes start their life with a push operation and contain a value until it gets popped
+    /// out. After that such empty nodes get added to the collector for destruction.
+    data: MaybeUninit<T>,
+
+    next: Atomic<Node<T>>,
+}
+
+// Any particular `T` should never be accessed concurrently, so no need for `Sync`.
+unsafe impl<T: Send> Sync for Queue<T> {}
+unsafe impl<T: Send> Send for Queue<T> {}
+
+impl<T> Queue<T> {
+    /// Create a new, empty queue.
+    pub(crate) fn new() -> Queue<T> {
+        let q = Queue {
+            head: CachePadded::new(Atomic::null()),
+            tail: CachePadded::new(Atomic::null()),
+        };
+        let sentinel = Owned::new(Node {
+            data: MaybeUninit::uninit(),
+            next: Atomic::null(),
+        });
+        unsafe {
+            let guard = unprotected();
+            let sentinel = sentinel.into_shared(guard);
+            q.head.store(sentinel, Relaxed);
+            q.tail.store(sentinel, Relaxed);
+            q
+        }
+    }
+
+    /// Attempts to atomically place `n` into the `next` pointer of `onto`, and returns `true` on
+    /// success. The queue's `tail` pointer may be updated.
+    #[inline(always)]
+    fn push_internal(
+        &self,
+        onto: Shared<'_, Node<T>>,
+        new: Shared<'_, Node<T>>,
+        guard: &Guard,
+    ) -> bool {
+        // is `onto` the actual tail?
+        let o = unsafe { onto.deref() };
+        let next = o.next.load(Acquire, guard);
+        if unsafe { next.as_ref().is_some() } {
+            // if not, try to "help" by moving the tail pointer forward
+            let _ = self
+                .tail
+                .compare_exchange(onto, next, Release, Relaxed, guard);
+            false
+        } else {
+            // looks like the actual tail; attempt to link in `n`
+            let result = o
+                .next
+                .compare_exchange(Shared::null(), new, Release, Relaxed, guard)
+                .is_ok();
+            if result {
+                // try to move the tail pointer forward
+                let _ = self
+                    .tail
+                    .compare_exchange(onto, new, Release, Relaxed, guard);
+            }
+            result
+        }
+    }
+
+    /// Adds `t` to the back of the queue, possibly waking up threads blocked on `pop`.
+    pub(crate) fn push(&self, t: T, guard: &Guard) {
+        let new = Owned::new(Node {
+            data: MaybeUninit::new(t),
+            next: Atomic::null(),
+        });
+        let new = Owned::into_shared(new, guard);
+
+        loop {
+            // We push onto the tail, so we'll start optimistically by looking there first.
+            let tail = self.tail.load(Acquire, guard);
+
+            // Attempt to push onto the `tail` snapshot; fails if `tail.next` has changed.
+            if self.push_internal(tail, new, guard) {
+                break;
+            }
+        }
+    }
+
+    /// Attempts to pop a data node. `Ok(None)` if queue is empty; `Err(())` if lost race to pop.
+    #[inline(always)]
+    fn pop_internal(&self, guard: &Guard) -> Result<Option<T>, ()> {
+        let head = self.head.load(Acquire, guard);
+        let h = unsafe { head.deref() };
+        let next = h.next.load(Acquire, guard);
+        match unsafe { next.as_ref() } {
+            Some(n) => unsafe {
+                self.head
+                    .compare_exchange(head, next, Release, Relaxed, guard)
+                    .map(|_| {
+                        let tail = self.tail.load(Relaxed, guard);
+                        // Advance the tail so that we don't retire a pointer to a reachable node.
+                        if head == tail {
+                            let _ = self
+                                .tail
+                                .compare_exchange(tail, next, Release, Relaxed, guard);
+                        }
+                        guard.defer_destroy(head);
+                        // TODO: Replace with MaybeUninit::read when api is stable
+                        Some(n.data.as_ptr().read())
+                    })
+                    .map_err(|_| ())
+            },
+            None => Ok(None),
+        }
+    }
+
+    /// Attempts to pop a data node, if the data satisfies the given condition. `Ok(None)` if queue
+    /// is empty or the data does not satisfy the condition; `Err(())` if lost race to pop.
+    #[inline(always)]
+    fn pop_if_internal<F>(&self, condition: F, guard: &Guard) -> Result<Option<T>, ()>
+    where
+        T: Sync,
+        F: Fn(&T) -> bool,
+    {
+        let head = self.head.load(Acquire, guard);
+        let h = unsafe { head.deref() };
+        let next = h.next.load(Acquire, guard);
+        match unsafe { next.as_ref() } {
+            Some(n) if condition(unsafe { &*n.data.as_ptr() }) => unsafe {
+                self.head
+                    .compare_exchange(head, next, Release, Relaxed, guard)
+                    .map(|_| {
+                        let tail = self.tail.load(Relaxed, guard);
+                        // Advance the tail so that we don't retire a pointer to a reachable node.
+                        if head == tail {
+                            let _ = self
+                                .tail
+                                .compare_exchange(tail, next, Release, Relaxed, guard);
+                        }
+                        guard.defer_destroy(head);
+                        Some(n.data.as_ptr().read())
+                    })
+                    .map_err(|_| ())
+            },
+            None | Some(_) => Ok(None),
+        }
+    }
+
+    /// Attempts to dequeue from the front.
+    ///
+    /// Returns `None` if the queue is observed to be empty.
+    pub(crate) fn try_pop(&self, guard: &Guard) -> Option<T> {
+        loop {
+            if let Ok(head) = self.pop_internal(guard) {
+                return head;
+            }
+        }
+    }
+
+    /// Attempts to dequeue from the front, if the item satisfies the given condition.
+    ///
+    /// Returns `None` if the queue is observed to be empty, or the head does not satisfy the given
+    /// condition.
+    pub(crate) fn try_pop_if<F>(&self, condition: F, guard: &Guard) -> Option<T>
+    where
+        T: Sync,
+        F: Fn(&T) -> bool,
+    {
+        loop {
+            if let Ok(head) = self.pop_if_internal(&condition, guard) {
+                return head;
+            }
+        }
+    }
+}
+
+impl<T> Drop for Queue<T> {
+    fn drop(&mut self) {
+        unsafe {
+            let guard = unprotected();
+
+            while self.try_pop(guard).is_some() {}
+
+            // Destroy the remaining sentinel node.
+            let sentinel = self.head.load(Relaxed, guard);
+            drop(sentinel.into_owned());
+        }
+    }
+}
+
+#[cfg(all(test, not(crossbeam_loom)))]
+mod test {
+    use super::*;
+    use crate::pin;
+    use crossbeam_utils::thread;
+
+    struct Queue<T> {
+        queue: super::Queue<T>,
+    }
+
+    impl<T> Queue<T> {
+        pub(crate) fn new() -> Queue<T> {
+            Queue {
+                queue: super::Queue::new(),
+            }
+        }
+
+        pub(crate) fn push(&self, t: T) {
+            let guard = &pin();
+            self.queue.push(t, guard);
+        }
+
+        pub(crate) fn is_empty(&self) -> bool {
+            let guard = &pin();
+            let head = self.queue.head.load(Acquire, guard);
+            let h = unsafe { head.deref() };
+            h.next.load(Acquire, guard).is_null()
+        }
+
+        pub(crate) fn try_pop(&self) -> Option<T> {
+            let guard = &pin();
+            self.queue.try_pop(guard)
+        }
+
+        pub(crate) fn pop(&self) -> T {
+            loop {
+                match self.try_pop() {
+                    None => continue,
+                    Some(t) => return t,
+                }
+            }
+        }
+    }
+
+    #[cfg(miri)]
+    const CONC_COUNT: i64 = 1000;
+    #[cfg(not(miri))]
+    const CONC_COUNT: i64 = 1000000;
+
+    #[test]
+    fn push_try_pop_1() {
+        let q: Queue<i64> = Queue::new();
+        assert!(q.is_empty());
+        q.push(37);
+        assert!(!q.is_empty());
+        assert_eq!(q.try_pop(), Some(37));
+        assert!(q.is_empty());
+    }
+
+    #[test]
+    fn push_try_pop_2() {
+        let q: Queue<i64> = Queue::new();
+        assert!(q.is_empty());
+        q.push(37);
+        q.push(48);
+        assert_eq!(q.try_pop(), Some(37));
+        assert!(!q.is_empty());
+        assert_eq!(q.try_pop(), Some(48));
+        assert!(q.is_empty());
+    }
+
+    #[test]
+    fn push_try_pop_many_seq() {
+        let q: Queue<i64> = Queue::new();
+        assert!(q.is_empty());
+        for i in 0..200 {
+            q.push(i)
+        }
+        assert!(!q.is_empty());
+        for i in 0..200 {
+            assert_eq!(q.try_pop(), Some(i));
+        }
+        assert!(q.is_empty());
+    }
+
+    #[test]
+    fn push_pop_1() {
+        let q: Queue<i64> = Queue::new();
+        assert!(q.is_empty());
+        q.push(37);
+        assert!(!q.is_empty());
+        assert_eq!(q.pop(), 37);
+        assert!(q.is_empty());
+    }
+
+    #[test]
+    fn push_pop_2() {
+        let q: Queue<i64> = Queue::new();
+        q.push(37);
+        q.push(48);
+        assert_eq!(q.pop(), 37);
+        assert_eq!(q.pop(), 48);
+    }
+
+    #[test]
+    fn push_pop_many_seq() {
+        let q: Queue<i64> = Queue::new();
+        assert!(q.is_empty());
+        for i in 0..200 {
+            q.push(i)
+        }
+        assert!(!q.is_empty());
+        for i in 0..200 {
+            assert_eq!(q.pop(), i);
+        }
+        assert!(q.is_empty());
+    }
+
+    #[test]
+    fn push_try_pop_many_spsc() {
+        let q: Queue<i64> = Queue::new();
+        assert!(q.is_empty());
+
+        thread::scope(|scope| {
+            scope.spawn(|_| {
+                let mut next = 0;
+
+                while next < CONC_COUNT {
+                    if let Some(elem) = q.try_pop() {
+                        assert_eq!(elem, next);
+                        next += 1;
+                    }
+                }
+            });
+
+            for i in 0..CONC_COUNT {
+                q.push(i)
+            }
+        })
+        .unwrap();
+    }
+
+    #[test]
+    fn push_try_pop_many_spmc() {
+        fn recv(_t: i32, q: &Queue<i64>) {
+            let mut cur = -1;
+            for _i in 0..CONC_COUNT {
+                if let Some(elem) = q.try_pop() {
+                    assert!(elem > cur);
+                    cur = elem;
+
+                    if cur == CONC_COUNT - 1 {
+                        break;
+                    }
+                }
+            }
+        }
+
+        let q: Queue<i64> = Queue::new();
+        assert!(q.is_empty());
+        thread::scope(|scope| {
+            for i in 0..3 {
+                let q = &q;
+                scope.spawn(move |_| recv(i, q));
+            }
+
+            scope.spawn(|_| {
+                for i in 0..CONC_COUNT {
+                    q.push(i);
+                }
+            });
+        })
+        .unwrap();
+    }
+
+    #[test]
+    fn push_try_pop_many_mpmc() {
+        enum LR {
+            Left(i64),
+            Right(i64),
+        }
+
+        let q: Queue<LR> = Queue::new();
+        assert!(q.is_empty());
+
+        thread::scope(|scope| {
+            for _t in 0..2 {
+                scope.spawn(|_| {
+                    for i in CONC_COUNT - 1..CONC_COUNT {
+                        q.push(LR::Left(i))
+                    }
+                });
+                scope.spawn(|_| {
+                    for i in CONC_COUNT - 1..CONC_COUNT {
+                        q.push(LR::Right(i))
+                    }
+                });
+                scope.spawn(|_| {
+                    let mut vl = vec![];
+                    let mut vr = vec![];
+                    for _i in 0..CONC_COUNT {
+                        match q.try_pop() {
+                            Some(LR::Left(x)) => vl.push(x),
+                            Some(LR::Right(x)) => vr.push(x),
+                            _ => {}
+                        }
+                    }
+
+                    let mut vl2 = vl.clone();
+                    let mut vr2 = vr.clone();
+                    vl2.sort_unstable();
+                    vr2.sort_unstable();
+
+                    assert_eq!(vl, vl2);
+                    assert_eq!(vr, vr2);
+                });
+            }
+        })
+        .unwrap();
+    }
+
+    #[test]
+    fn push_pop_many_spsc() {
+        let q: Queue<i64> = Queue::new();
+
+        thread::scope(|scope| {
+            scope.spawn(|_| {
+                let mut next = 0;
+                while next < CONC_COUNT {
+                    assert_eq!(q.pop(), next);
+                    next += 1;
+                }
+            });
+
+            for i in 0..CONC_COUNT {
+                q.push(i)
+            }
+        })
+        .unwrap();
+        assert!(q.is_empty());
+    }
+
+    #[test]
+    fn is_empty_dont_pop() {
+        let q: Queue<i64> = Queue::new();
+        q.push(20);
+        q.push(20);
+        assert!(!q.is_empty());
+        assert!(!q.is_empty());
+        assert!(q.try_pop().is_some());
+    }
+}