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    // There's a lot of scary concurrent code in this module, but it is copied from
    // `std::sync::Once` with two changes:
    //   * no poisoning
    //   * init function can fail

    use std::{
        cell::{Cell, UnsafeCell},
        marker::PhantomData,
        panic::{RefUnwindSafe, UnwindSafe},
        sync::atomic::{AtomicBool, AtomicPtr, Ordering},
        thread::{self, Thread},
    };

    #[derive(Debug)]
    pub(crate) struct OnceCell<T> {
        // This `queue` field is the core of the implementation. It encodes two
        // pieces of information:
        //
        // * The current state of the cell (`INCOMPLETE`, `RUNNING`, `COMPLETE`)
        // * Linked list of threads waiting for the current cell.
        //
        // State is encoded in two low bits. Only `INCOMPLETE` and `RUNNING` states
        // allow waiters.
        queue: AtomicPtr<Waiter>,
        _marker: PhantomData<*mut Waiter>,
        value: UnsafeCell<Option<T>>,
    }

    // Why do we need `T: Send`?
    // Thread A creates a `OnceCell` and shares it with
    // scoped thread B, which fills the cell, which is
    // then destroyed by A. That is, destructor observes
    // a sent value.
    unsafe impl<T: Sync + Send> Sync for OnceCell<T> {}
    unsafe impl<T: Send> Send for OnceCell<T> {}

    impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for OnceCell<T> {}
    impl<T: UnwindSafe> UnwindSafe for OnceCell<T> {}

    impl<T> OnceCell<T> {
        pub(crate) const fn new() -> OnceCell<T> {
            OnceCell {
                queue: AtomicPtr::new(INCOMPLETE_PTR),
                _marker: PhantomData,
                value: UnsafeCell::new(None),
            }
        }

        pub(crate) const fn with_value(value: T) -> OnceCell<T> {
            OnceCell {
                queue: AtomicPtr::new(COMPLETE_PTR),
                _marker: PhantomData,
                value: UnsafeCell::new(Some(value)),
            }
        }

        /// Safety: synchronizes with store to value via Release/(Acquire|SeqCst).
        #[inline]
        pub(crate) fn is_initialized(&self) -> bool {
            // An `Acquire` load is enough because that makes all the initialization
            // operations visible to us, and, this being a fast path, weaker
            // ordering helps with performance. This `Acquire` synchronizes with
            // `SeqCst` operations on the slow path.
            self.queue.load(Ordering::Acquire) == COMPLETE_PTR
        }

        /// Safety: synchronizes with store to value via SeqCst read from state,
        /// writes value only once because we never get to INCOMPLETE state after a
        /// successful write.
        #[cold]
        pub(crate) fn initialize<F, E>(&self, f: F) -> Result<(), E>
        where
            F: FnOnce() -> Result<T, E>,
        {
            let mut f = Some(f);
            let mut res: Result<(), E> = Ok(());
            let slot: *mut Option<T> = self.value.get();
            initialize_or_wait(
                &self.queue,
                Some(&mut || {
                    let f = unsafe { crate::unwrap_unchecked(f.take()) };
                    match f() {
                        Ok(value) => {
                            unsafe { *slot = Some(value) };
                            true
                        }
                        Err(err) => {
                            res = Err(err);
                            false
                        }
                    }
                }),
            );
            res
        }

        #[cold]
        pub(crate) fn wait(&self) {
            initialize_or_wait(&self.queue, None);
        }

        /// Get the reference to the underlying value, without checking if the cell
        /// is initialized.
        ///
        /// # Safety
        ///
        /// Caller must ensure that the cell is in initialized state, and that
        /// the contents are acquired by (synchronized to) this thread.
        pub(crate) unsafe fn get_unchecked(&self) -> &T {
            debug_assert!(self.is_initialized());
            let slot = &*self.value.get();
            crate::unwrap_unchecked(slot.as_ref())
        }

        /// Gets the mutable reference to the underlying value.
        /// Returns `None` if the cell is empty.
        pub(crate) fn get_mut(&mut self) -> Option<&mut T> {
            // Safe b/c we have a unique access.
            unsafe { &mut *self.value.get() }.as_mut()
        }

        /// Consumes this `OnceCell`, returning the wrapped value.
        /// Returns `None` if the cell was empty.
        #[inline]
        pub(crate) fn into_inner(self) -> Option<T> {
            // Because `into_inner` takes `self` by value, the compiler statically
            // verifies that it is not currently borrowed.
            // So, it is safe to move out `Option<T>`.
            self.value.into_inner()
        }
    }

    // Three states that a OnceCell can be in, encoded into the lower bits of `queue` in
    // the OnceCell structure.
    const INCOMPLETE: usize = 0x0;
    const RUNNING: usize = 0x1;
    const COMPLETE: usize = 0x2;
    const INCOMPLETE_PTR: *mut Waiter = INCOMPLETE as *mut Waiter;
    const COMPLETE_PTR: *mut Waiter = COMPLETE as *mut Waiter;

    // Mask to learn about the state. All other bits are the queue of waiters if
    // this is in the RUNNING state.
    const STATE_MASK: usize = 0x3;

    /// Representation of a node in the linked list of waiters in the RUNNING state.
    /// A waiters is stored on the stack of the waiting threads.
    #[repr(align(4))] // Ensure the two lower bits are free to use as state bits.
    struct Waiter {
        thread: Cell<Option<Thread>>,
        signaled: AtomicBool,
        next: *mut Waiter,
    }

    /// Drains and notifies the queue of waiters on drop.
    struct Guard<'a> {
        queue: &'a AtomicPtr<Waiter>,
        new_queue: *mut Waiter,
    }

    impl Drop for Guard<'_> {
        fn drop(&mut self) {
            let queue = self.queue.swap(self.new_queue, Ordering::AcqRel);

            let state = strict::addr(queue) & STATE_MASK;
            assert_eq!(state, RUNNING);

            unsafe {
                let mut waiter = strict::map_addr(queue, |q| q & !STATE_MASK);
                while !waiter.is_null() {
                    let next = (*waiter).next;
                    let thread = (*waiter).thread.take().unwrap();
                    (*waiter).signaled.store(true, Ordering::Release);
                    waiter = next;
                    thread.unpark();
                }
            }
        }
    }

    // Corresponds to `std::sync::Once::call_inner`.
    //
    // Originally copied from std, but since modified to remove poisoning and to
    // support wait.
    //
    // Note: this is intentionally monomorphic
    #[inline(never)]
    fn initialize_or_wait(queue: &AtomicPtr<Waiter>, mut init: Option<&mut dyn FnMut() -> bool>) {
        let mut curr_queue = queue.load(Ordering::Acquire);

        loop {
            let curr_state = strict::addr(curr_queue) & STATE_MASK;
            match (curr_state, &mut init) {
                (COMPLETE, _) => return,
                (INCOMPLETE, Some(init)) => {
                    let exchange = queue.compare_exchange(
                        curr_queue,
                        strict::map_addr(curr_queue, |q| (q & !STATE_MASK) | RUNNING),
                        Ordering::Acquire,
                        Ordering::Acquire,
                    );
                    if let Err(new_queue) = exchange {
                        curr_queue = new_queue;
                        continue;
                    }
                    let mut guard = Guard { queue, new_queue: INCOMPLETE_PTR };
                    if init() {
                        guard.new_queue = COMPLETE_PTR;
                    }
                    return;
                }
                (INCOMPLETE, None) | (RUNNING, _) => {
                    wait(&queue, curr_queue);
                    curr_queue = queue.load(Ordering::Acquire);
                }
                _ => debug_assert!(false),
            }
        }
    }

    fn wait(queue: &AtomicPtr<Waiter>, mut curr_queue: *mut Waiter) {
        let curr_state = strict::addr(curr_queue) & STATE_MASK;
        loop {
            let node = Waiter {
                thread: Cell::new(Some(thread::current())),
                signaled: AtomicBool::new(false),
                next: strict::map_addr(curr_queue, |q| q & !STATE_MASK),
            };
            let me = &node as *const Waiter as *mut Waiter;

            let exchange = queue.compare_exchange(
                curr_queue,
                strict::map_addr(me, |q| q | curr_state),
                Ordering::Release,
                Ordering::Relaxed,
            );
            if let Err(new_queue) = exchange {
                if strict::addr(new_queue) & STATE_MASK != curr_state {
                    return;
                }
                curr_queue = new_queue;
                continue;
            }

            while !node.signaled.load(Ordering::Acquire) {
                thread::park();
            }
            break;
        }
    }

    // Polyfill of strict provenance from https://crates.io/crates/sptr.
    //
    // Use free-standing function rather than a trait to keep things simple and
    // avoid any potential conflicts with future stabile std API.
    mod strict {
        #[must_use]
        #[inline]
        pub(crate) fn addr<T>(ptr: *mut T) -> usize
        where
            T: Sized,
        {
            // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
            // SAFETY: Pointer-to-integer transmutes are valid (if you are okay with losing the
            // provenance).
            unsafe { core::mem::transmute(ptr) }
        }

        #[must_use]
        #[inline]
        pub(crate) fn with_addr<T>(ptr: *mut T, addr: usize) -> *mut T
        where
            T: Sized,
        {
            // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
            //
            // In the mean-time, this operation is defined to be "as if" it was
            // a wrapping_offset, so we can emulate it as such. This should properly
            // restore pointer provenance even under today's compiler.
            let self_addr = self::addr(ptr) as isize;
            let dest_addr = addr as isize;
            let offset = dest_addr.wrapping_sub(self_addr);

            // This is the canonical desugarring of this operation,
            // but `pointer::cast` was only stabilized in 1.38.
            // self.cast::<u8>().wrapping_offset(offset).cast::<T>()
            (ptr as *mut u8).wrapping_offset(offset) as *mut T
        }

        #[must_use]
        #[inline]
        pub(crate) fn map_addr<T>(ptr: *mut T, f: impl FnOnce(usize) -> usize) -> *mut T
        where
            T: Sized,
        {
            self::with_addr(ptr, f(addr(ptr)))
        }
    }

    // These test are snatched from std as well.
    #[cfg(test)]
    mod tests {
        use std::panic;
        use std::{sync::mpsc::channel, thread};

        use super::OnceCell;

        impl<T> OnceCell<T> {
            fn init(&self, f: impl FnOnce() -> T) {
                enum Void {}
                let _ = self.initialize(|| Ok::<T, Void>(f()));
            }
        }

        #[test]
        fn smoke_once() {
            static O: OnceCell<()> = OnceCell::new();
            let mut a = 0;
            O.init(|| a += 1);
            assert_eq!(a, 1);
            O.init(|| a += 1);
            assert_eq!(a, 1);
        }

        #[test]
        fn stampede_once() {
            static O: OnceCell<()> = OnceCell::new();
            static mut RUN: bool = false;

            let (tx, rx) = channel();
            for _ in 0..10 {
                let tx = tx.clone();
                thread::spawn(move || {
                    for _ in 0..4 {
                        thread::yield_now()
                    }
                    unsafe {
                        O.init(|| {
                            assert!(!RUN);
                            RUN = true;
                        });
                        assert!(RUN);
                    }
                    tx.send(()).unwrap();
                });
            }

            unsafe {
                O.init(|| {
                    assert!(!RUN);
                    RUN = true;
                });
                assert!(RUN);
            }

            for _ in 0..10 {
                rx.recv().unwrap();
            }
        }

        #[test]
        fn poison_bad() {
            static O: OnceCell<()> = OnceCell::new();

            // poison the once
            let t = panic::catch_unwind(|| {
                O.init(|| panic!());
            });
            assert!(t.is_err());

            // we can subvert poisoning, however
            let mut called = false;
            O.init(|| {
                called = true;
            });
            assert!(called);

            // once any success happens, we stop propagating the poison
            O.init(|| {});
        }

        #[test]
        fn wait_for_force_to_finish() {
            static O: OnceCell<()> = OnceCell::new();

            // poison the once
            let t = panic::catch_unwind(|| {
                O.init(|| panic!());
            });
            assert!(t.is_err());

            // make sure someone's waiting inside the once via a force
            let (tx1, rx1) = channel();
            let (tx2, rx2) = channel();
            let t1 = thread::spawn(move || {
                O.init(|| {
                    tx1.send(()).unwrap();
                    rx2.recv().unwrap();
                });
            });

            rx1.recv().unwrap();

            // put another waiter on the once
            let t2 = thread::spawn(|| {
                let mut called = false;
                O.init(|| {
                    called = true;
                });
                assert!(!called);
            });

            tx2.send(()).unwrap();

            assert!(t1.join().is_ok());
            assert!(t2.join().is_ok());
        }

        #[test]
        #[cfg(target_pointer_width = "64")]
        fn test_size() {
            use std::mem::size_of;

            assert_eq!(size_of::<OnceCell<u32>>(), 4 * size_of::<u32>());
        }
    }