RPM build fix (reverted CI changes which will need to be un-reverted or made conditional) and vendor Rust dependencies to make builds much faster in any CI system.

zeroidc/vendor/once_cell/src/imp_pl.rs

@@ -0,0 +1,181 @@
+use std::{
+    cell::UnsafeCell,
+    hint,
+    panic::{RefUnwindSafe, UnwindSafe},
+    sync::atomic::{AtomicU8, Ordering},
+};
+
+pub(crate) struct OnceCell<T> {
+    state: AtomicU8,
+    value: UnsafeCell<Option<T>>,
+}
+
+const INCOMPLETE: u8 = 0x0;
+const RUNNING: u8 = 0x1;
+const COMPLETE: u8 = 0x2;
+
+// 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 { state: AtomicU8::new(INCOMPLETE), value: UnsafeCell::new(None) }
+    }
+
+    pub(crate) const fn with_value(value: T) -> OnceCell<T> {
+        OnceCell { state: AtomicU8::new(COMPLETE), value: UnsafeCell::new(Some(value)) }
+    }
+
+    /// Safety: synchronizes with store to value via Release/Acquire.
+    #[inline]
+    pub(crate) fn is_initialized(&self) -> bool {
+        self.state.load(Ordering::Acquire) == COMPLETE
+    }
+
+    /// Safety: synchronizes with store to value via `is_initialized` or mutex
+    /// lock/unlock, writes value only once because of the mutex.
+    #[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_inner(&self.state, &mut || {
+            // We are calling user-supplied function and need to be careful.
+            // - if it returns Err, we unlock mutex and return without touching anything
+            // - if it panics, we unlock mutex and propagate panic without touching anything
+            // - if it calls `set` or `get_or_try_init` re-entrantly, we get a deadlock on
+            //   mutex, which is important for safety. We *could* detect this and panic,
+            //   but that is more complicated
+            // - finally, if it returns Ok, we store the value and store the flag with
+            //   `Release`, which synchronizes with `Acquire`s.
+            let f = unsafe { crate::take_unchecked(&mut f) };
+            match f() {
+                Ok(value) => unsafe {
+                    // Safe b/c we have a unique access and no panic may happen
+                    // until the cell is marked as initialized.
+                    debug_assert!((*slot).is_none());
+                    *slot = Some(value);
+                    true
+                },
+                Err(err) => {
+                    res = Err(err);
+                    false
+                }
+            }
+        });
+        res
+    }
+
+    #[cold]
+    pub(crate) fn wait(&self) {
+        let key = &self.state as *const _ as usize;
+        unsafe {
+            parking_lot_core::park(
+                key,
+                || self.state.load(Ordering::Acquire) != COMPLETE,
+                || (),
+                |_, _| (),
+                parking_lot_core::DEFAULT_PARK_TOKEN,
+                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: &Option<T> = &*self.value.get();
+        match slot {
+            Some(value) => value,
+            // This unsafe does improve performance, see `examples/bench`.
+            None => {
+                debug_assert!(false);
+                hint::unreachable_unchecked()
+            }
+        }
+    }
+
+    /// 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 an exclusive access
+        let slot: &mut Option<T> = unsafe { &mut *self.value.get() };
+        slot.as_mut()
+    }
+
+    /// Consumes this `OnceCell`, returning the wrapped value.
+    /// Returns `None` if the cell was empty.
+    pub(crate) fn into_inner(self) -> Option<T> {
+        self.value.into_inner()
+    }
+}
+
+struct Guard<'a> {
+    state: &'a AtomicU8,
+    new_state: u8,
+}
+
+impl<'a> Drop for Guard<'a> {
+    fn drop(&mut self) {
+        self.state.store(self.new_state, Ordering::Release);
+        unsafe {
+            let key = self.state as *const AtomicU8 as usize;
+            parking_lot_core::unpark_all(key, parking_lot_core::DEFAULT_UNPARK_TOKEN);
+        }
+    }
+}
+
+// Note: this is intentionally monomorphic
+#[inline(never)]
+fn initialize_inner(state: &AtomicU8, init: &mut dyn FnMut() -> bool) {
+    loop {
+        let exchange =
+            state.compare_exchange_weak(INCOMPLETE, RUNNING, Ordering::Acquire, Ordering::Acquire);
+        match exchange {
+            Ok(_) => {
+                let mut guard = Guard { state, new_state: INCOMPLETE };
+                if init() {
+                    guard.new_state = COMPLETE;
+                }
+                return;
+            }
+            Err(COMPLETE) => return,
+            Err(RUNNING) => unsafe {
+                let key = state as *const AtomicU8 as usize;
+                parking_lot_core::park(
+                    key,
+                    || state.load(Ordering::Relaxed) == RUNNING,
+                    || (),
+                    |_, _| (),
+                    parking_lot_core::DEFAULT_PARK_TOKEN,
+                    None,
+                );
+            },
+            Err(_) => debug_assert!(false),
+        }
+    }
+}
+
+#[test]
+fn test_size() {
+    use std::mem::size_of;
+
+    assert_eq!(size_of::<OnceCell<bool>>(), 1 * size_of::<bool>() + size_of::<u8>());
+}

zeroidc/vendor/once_cell/src/imp_std.rs

@@ -0,0 +1,385 @@
+// 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},
+    hint::unreachable_unchecked,
+    marker::PhantomData,
+    panic::{RefUnwindSafe, UnwindSafe},
+    sync::atomic::{AtomicBool, AtomicUsize, Ordering},
+    thread::{self, Thread},
+};
+
+use crate::take_unchecked;
+
+#[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: AtomicUsize,
+    _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: AtomicUsize::new(INCOMPLETE),
+            _marker: PhantomData,
+            value: UnsafeCell::new(None),
+        }
+    }
+
+    pub(crate) const fn with_value(value: T) -> OnceCell<T> {
+        OnceCell {
+            queue: AtomicUsize::new(COMPLETE),
+            _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
+    }
+
+    /// 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 { take_unchecked(&mut f) };
+                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: &Option<T> = &*self.value.get();
+        match slot {
+            Some(value) => value,
+            // This unsafe does improve performance, see `examples/bench`.
+            None => {
+                debug_assert!(false);
+                unreachable_unchecked()
+            }
+        }
+    }
+
+    /// 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;
+
+// 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: *const Waiter,
+}
+
+/// Drains and notifies the queue of waiters on drop.
+struct Guard<'a> {
+    queue: &'a AtomicUsize,
+    new_queue: usize,
+}
+
+impl Drop for Guard<'_> {
+    fn drop(&mut self) {
+        let queue = self.queue.swap(self.new_queue, Ordering::AcqRel);
+
+        assert_eq!(queue & STATE_MASK, RUNNING);
+
+        unsafe {
+            let mut waiter = (queue & !STATE_MASK) as *const Waiter;
+            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: &AtomicUsize, mut init: Option<&mut dyn FnMut() -> bool>) {
+    let mut curr_queue = queue.load(Ordering::Acquire);
+
+    loop {
+        let curr_state = curr_queue & STATE_MASK;
+        match (curr_state, &mut init) {
+            (COMPLETE, _) => return,
+            (INCOMPLETE, Some(init)) => {
+                let exchange = queue.compare_exchange(
+                    curr_queue,
+                    (curr_queue & !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 };
+                if init() {
+                    guard.new_queue = COMPLETE;
+                }
+                return;
+            }
+            (INCOMPLETE, None) | (RUNNING, _) => {
+                wait(&queue, curr_queue);
+                curr_queue = queue.load(Ordering::Acquire);
+            }
+            _ => debug_assert!(false),
+        }
+    }
+}
+
+fn wait(queue: &AtomicUsize, mut curr_queue: usize) {
+    let curr_state = curr_queue & STATE_MASK;
+    loop {
+        let node = Waiter {
+            thread: Cell::new(Some(thread::current())),
+            signaled: AtomicBool::new(false),
+            next: (curr_queue & !STATE_MASK) as *const Waiter,
+        };
+        let me = &node as *const Waiter as usize;
+
+        let exchange = queue.compare_exchange(
+            curr_queue,
+            me | curr_state,
+            Ordering::Release,
+            Ordering::Relaxed,
+        );
+        if let Err(new_queue) = exchange {
+            if new_queue & STATE_MASK != curr_state {
+                return;
+            }
+            curr_queue = new_queue;
+            continue;
+        }
+
+        while !node.signaled.load(Ordering::Acquire) {
+            thread::park();
+        }
+        break;
+    }
+}
+
+// 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]
+    #[cfg(not(miri))]
+    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>());
+    }
+}

zeroidc/vendor/once_cell/src/race.rs

@@ -0,0 +1,303 @@
+//! Thread-safe, non-blocking, "first one wins" flavor of `OnceCell`.
+//!
+//! If two threads race to initialize a type from the `race` module, they
+//! don't block, execute initialization function together, but only one of
+//! them stores the result.
+//!
+//! This module does not require `std` feature.
+//!
+//! # Atomic orderings
+//!
+//! All types in this module use `Acquire` and `Release`
+//! [atomic orderings](Ordering) for all their operations. While this is not
+//! strictly necessary for types other than `OnceBox`, it is useful for users as
+//! it allows them to be certain that after `get` or `get_or_init` returns on
+//! one thread, any side-effects caused by the setter thread prior to them
+//! calling `set` or `get_or_init` will be made visible to that thread; without
+//! it, it's possible for it to appear as if they haven't happened yet from the
+//! getter thread's perspective. This is an acceptable tradeoff to make since
+//! `Acquire` and `Release` have very little performance overhead on most
+//! architectures versus `Relaxed`.
+
+#[cfg(feature = "atomic-polyfill")]
+use atomic_polyfill as atomic;
+#[cfg(not(feature = "atomic-polyfill"))]
+use core::sync::atomic;
+
+use atomic::{AtomicUsize, Ordering};
+use core::num::NonZeroUsize;
+
+/// A thread-safe cell which can be written to only once.
+#[derive(Default, Debug)]
+pub struct OnceNonZeroUsize {
+    inner: AtomicUsize,
+}
+
+impl OnceNonZeroUsize {
+    /// Creates a new empty cell.
+    #[inline]
+    pub const fn new() -> OnceNonZeroUsize {
+        OnceNonZeroUsize { inner: AtomicUsize::new(0) }
+    }
+
+    /// Gets the underlying value.
+    #[inline]
+    pub fn get(&self) -> Option<NonZeroUsize> {
+        let val = self.inner.load(Ordering::Acquire);
+        NonZeroUsize::new(val)
+    }
+
+    /// Sets the contents of this cell to `value`.
+    ///
+    /// Returns `Ok(())` if the cell was empty and `Err(())` if it was
+    /// full.
+    #[inline]
+    pub fn set(&self, value: NonZeroUsize) -> Result<(), ()> {
+        let exchange =
+            self.inner.compare_exchange(0, value.get(), Ordering::AcqRel, Ordering::Acquire);
+        match exchange {
+            Ok(_) => Ok(()),
+            Err(_) => Err(()),
+        }
+    }
+
+    /// Gets the contents of the cell, initializing it with `f` if the cell was
+    /// empty.
+    ///
+    /// If several threads concurrently run `get_or_init`, more than one `f` can
+    /// be called. However, all threads will return the same value, produced by
+    /// some `f`.
+    pub fn get_or_init<F>(&self, f: F) -> NonZeroUsize
+    where
+        F: FnOnce() -> NonZeroUsize,
+    {
+        enum Void {}
+        match self.get_or_try_init(|| Ok::<NonZeroUsize, Void>(f())) {
+            Ok(val) => val,
+            Err(void) => match void {},
+        }
+    }
+
+    /// Gets the contents of the cell, initializing it with `f` if
+    /// the cell was empty. If the cell was empty and `f` failed, an
+    /// error is returned.
+    ///
+    /// If several threads concurrently run `get_or_init`, more than one `f` can
+    /// be called. However, all threads will return the same value, produced by
+    /// some `f`.
+    pub fn get_or_try_init<F, E>(&self, f: F) -> Result<NonZeroUsize, E>
+    where
+        F: FnOnce() -> Result<NonZeroUsize, E>,
+    {
+        let val = self.inner.load(Ordering::Acquire);
+        let res = match NonZeroUsize::new(val) {
+            Some(it) => it,
+            None => {
+                let mut val = f()?.get();
+                let exchange =
+                    self.inner.compare_exchange(0, val, Ordering::AcqRel, Ordering::Acquire);
+                if let Err(old) = exchange {
+                    val = old;
+                }
+                unsafe { NonZeroUsize::new_unchecked(val) }
+            }
+        };
+        Ok(res)
+    }
+}
+
+/// A thread-safe cell which can be written to only once.
+#[derive(Default, Debug)]
+pub struct OnceBool {
+    inner: OnceNonZeroUsize,
+}
+
+impl OnceBool {
+    /// Creates a new empty cell.
+    #[inline]
+    pub const fn new() -> OnceBool {
+        OnceBool { inner: OnceNonZeroUsize::new() }
+    }
+
+    /// Gets the underlying value.
+    #[inline]
+    pub fn get(&self) -> Option<bool> {
+        self.inner.get().map(OnceBool::from_usize)
+    }
+
+    /// Sets the contents of this cell to `value`.
+    ///
+    /// Returns `Ok(())` if the cell was empty and `Err(())` if it was
+    /// full.
+    #[inline]
+    pub fn set(&self, value: bool) -> Result<(), ()> {
+        self.inner.set(OnceBool::to_usize(value))
+    }
+
+    /// Gets the contents of the cell, initializing it with `f` if the cell was
+    /// empty.
+    ///
+    /// If several threads concurrently run `get_or_init`, more than one `f` can
+    /// be called. However, all threads will return the same value, produced by
+    /// some `f`.
+    pub fn get_or_init<F>(&self, f: F) -> bool
+    where
+        F: FnOnce() -> bool,
+    {
+        OnceBool::from_usize(self.inner.get_or_init(|| OnceBool::to_usize(f())))
+    }
+
+    /// Gets the contents of the cell, initializing it with `f` if
+    /// the cell was empty. If the cell was empty and `f` failed, an
+    /// error is returned.
+    ///
+    /// If several threads concurrently run `get_or_init`, more than one `f` can
+    /// be called. However, all threads will return the same value, produced by
+    /// some `f`.
+    pub fn get_or_try_init<F, E>(&self, f: F) -> Result<bool, E>
+    where
+        F: FnOnce() -> Result<bool, E>,
+    {
+        self.inner.get_or_try_init(|| f().map(OnceBool::to_usize)).map(OnceBool::from_usize)
+    }
+
+    #[inline]
+    fn from_usize(value: NonZeroUsize) -> bool {
+        value.get() == 1
+    }
+    #[inline]
+    fn to_usize(value: bool) -> NonZeroUsize {
+        unsafe { NonZeroUsize::new_unchecked(if value { 1 } else { 2 }) }
+    }
+}
+
+#[cfg(feature = "alloc")]
+pub use self::once_box::OnceBox;
+
+#[cfg(feature = "alloc")]
+mod once_box {
+    use super::atomic::{AtomicPtr, Ordering};
+    use core::{marker::PhantomData, ptr};
+
+    use alloc::boxed::Box;
+
+    /// A thread-safe cell which can be written to only once.
+    pub struct OnceBox<T> {
+        inner: AtomicPtr<T>,
+        ghost: PhantomData<Option<Box<T>>>,
+    }
+
+    impl<T> core::fmt::Debug for OnceBox<T> {
+        fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+            write!(f, "OnceBox({:?})", self.inner.load(Ordering::Relaxed))
+        }
+    }
+
+    impl<T> Default for OnceBox<T> {
+        fn default() -> Self {
+            Self::new()
+        }
+    }
+
+    impl<T> Drop for OnceBox<T> {
+        fn drop(&mut self) {
+            let ptr = *self.inner.get_mut();
+            if !ptr.is_null() {
+                drop(unsafe { Box::from_raw(ptr) })
+            }
+        }
+    }
+
+    impl<T> OnceBox<T> {
+        /// Creates a new empty cell.
+        pub const fn new() -> OnceBox<T> {
+            OnceBox { inner: AtomicPtr::new(ptr::null_mut()), ghost: PhantomData }
+        }
+
+        /// Gets a reference to the underlying value.
+        pub fn get(&self) -> Option<&T> {
+            let ptr = self.inner.load(Ordering::Acquire);
+            if ptr.is_null() {
+                return None;
+            }
+            Some(unsafe { &*ptr })
+        }
+
+        /// Sets the contents of this cell to `value`.
+        ///
+        /// Returns `Ok(())` if the cell was empty and `Err(value)` if it was
+        /// full.
+        pub fn set(&self, value: Box<T>) -> Result<(), Box<T>> {
+            let ptr = Box::into_raw(value);
+            let exchange = self.inner.compare_exchange(
+                ptr::null_mut(),
+                ptr,
+                Ordering::AcqRel,
+                Ordering::Acquire,
+            );
+            if let Err(_) = exchange {
+                let value = unsafe { Box::from_raw(ptr) };
+                return Err(value);
+            }
+            Ok(())
+        }
+
+        /// Gets the contents of the cell, initializing it with `f` if the cell was
+        /// empty.
+        ///
+        /// If several threads concurrently run `get_or_init`, more than one `f` can
+        /// be called. However, all threads will return the same value, produced by
+        /// some `f`.
+        pub fn get_or_init<F>(&self, f: F) -> &T
+        where
+            F: FnOnce() -> Box<T>,
+        {
+            enum Void {}
+            match self.get_or_try_init(|| Ok::<Box<T>, Void>(f())) {
+                Ok(val) => val,
+                Err(void) => match void {},
+            }
+        }
+
+        /// Gets the contents of the cell, initializing it with `f` if
+        /// the cell was empty. If the cell was empty and `f` failed, an
+        /// error is returned.
+        ///
+        /// If several threads concurrently run `get_or_init`, more than one `f` can
+        /// be called. However, all threads will return the same value, produced by
+        /// some `f`.
+        pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
+        where
+            F: FnOnce() -> Result<Box<T>, E>,
+        {
+            let mut ptr = self.inner.load(Ordering::Acquire);
+
+            if ptr.is_null() {
+                let val = f()?;
+                ptr = Box::into_raw(val);
+                let exchange = self.inner.compare_exchange(
+                    ptr::null_mut(),
+                    ptr,
+                    Ordering::AcqRel,
+                    Ordering::Acquire,
+                );
+                if let Err(old) = exchange {
+                    drop(unsafe { Box::from_raw(ptr) });
+                    ptr = old;
+                }
+            };
+            Ok(unsafe { &*ptr })
+        }
+    }
+
+    unsafe impl<T: Sync + Send> Sync for OnceBox<T> {}
+
+    /// ```compile_fail
+    /// struct S(*mut ());
+    /// unsafe impl Sync for S {}
+    ///
+    /// fn share<T: Sync>(_: &T) {}
+    /// share(&once_cell::race::OnceBox::<S>::new());
+    /// ```
+    fn _dummy() {}
+}