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    #![cfg_attr(not(feature = "sync"), allow(unreachable_pub, dead_code))]
    //! # Implementation Details
    //!
    //! The semaphore is implemented using an intrusive linked list of waiters. An
    //! atomic counter tracks the number of available permits. If the semaphore does
    //! not contain the required number of permits, the task attempting to acquire
    //! permits places its waker at the end of a queue. When new permits are made
    //! available (such as by releasing an initial acquisition), they are assigned
    //! to the task at the front of the queue, waking that task if its requested
    //! number of permits is met.
    //!
    //! Because waiters are enqueued at the back of the linked list and dequeued
    //! from the front, the semaphore is fair. Tasks trying to acquire large numbers
    //! of permits at a time will always be woken eventually, even if many other
    //! tasks are acquiring smaller numbers of permits. This means that in a
    //! use-case like tokio's read-write lock, writers will not be starved by
    //! readers.
    use crate::loom::cell::UnsafeCell;
    use crate::loom::sync::atomic::AtomicUsize;
    use crate::loom::sync::{Mutex, MutexGuard};
    use crate::util::linked_list::{self, LinkedList};

    use std::future::Future;
    use std::marker::PhantomPinned;
    use std::pin::Pin;
    use std::ptr::NonNull;
    use std::sync::atomic::Ordering::*;
    use std::task::Poll::*;
    use std::task::{Context, Poll, Waker};
    use std::{cmp, fmt};

    /// An asynchronous counting semaphore which permits waiting on multiple permits at once.
    pub(crate) struct Semaphore {
        waiters: Mutex<Waitlist>,
        /// The current number of available permits in the semaphore.
        permits: AtomicUsize,
    }

    struct Waitlist {
        queue: LinkedList<Waiter, <Waiter as linked_list::Link>::Target>,
        closed: bool,
    }

    /// Error returned from the [`Semaphore::try_acquire`] function.
    ///
    /// [`Semaphore::try_acquire`]: crate::sync::Semaphore::try_acquire
    #[derive(Debug, PartialEq)]
    pub enum TryAcquireError {
        /// The semaphore has been [closed] and cannot issue new permits.
        ///
        /// [closed]: crate::sync::Semaphore::close
        Closed,

        /// The semaphore has no available permits.
        NoPermits,
    }
    /// Error returned from the [`Semaphore::acquire`] function.
    ///
    /// An `acquire` operation can only fail if the semaphore has been
    /// [closed].
    ///
    /// [closed]: crate::sync::Semaphore::close
    /// [`Semaphore::acquire`]: crate::sync::Semaphore::acquire
    #[derive(Debug)]
    pub struct AcquireError(());

    pub(crate) struct Acquire<'a> {
        node: Waiter,
        semaphore: &'a Semaphore,
        num_permits: u32,
        queued: bool,
    }

    /// An entry in the wait queue.
    struct Waiter {
        /// The current state of the waiter.
        ///
        /// This is either the number of remaining permits required by
        /// the waiter, or a flag indicating that the waiter is not yet queued.
        state: AtomicUsize,

        /// The waker to notify the task awaiting permits.
        ///
        /// # Safety
        ///
        /// This may only be accessed while the wait queue is locked.
        waker: UnsafeCell<Option<Waker>>,

        /// Intrusive linked-list pointers.
        ///
        /// # Safety
        ///
        /// This may only be accessed while the wait queue is locked.
        ///
        /// TODO: Ideally, we would be able to use loom to enforce that
        /// this isn't accessed concurrently. However, it is difficult to
        /// use a `UnsafeCell` here, since the `Link` trait requires _returning_
        /// references to `Pointers`, and `UnsafeCell` requires that checked access
        /// take place inside a closure. We should consider changing `Pointers` to
        /// use `UnsafeCell` internally.
        pointers: linked_list::Pointers<Waiter>,

        /// Should not be `Unpin`.
        _p: PhantomPinned,
    }

    impl Semaphore {
        /// The maximum number of permits which a semaphore can hold.
        ///
        /// Note that this reserves three bits of flags in the permit counter, but
        /// we only actually use one of them. However, the previous semaphore
        /// implementation used three bits, so we will continue to reserve them to
        /// avoid a breaking change if additional flags need to be added in the
        /// future.
        pub(crate) const MAX_PERMITS: usize = std::usize::MAX >> 3;
        const CLOSED: usize = 1;
        // The least-significant bit in the number of permits is reserved to use
        // as a flag indicating that the semaphore has been closed. Consequently
        // PERMIT_SHIFT is used to leave that bit for that purpose.
        const PERMIT_SHIFT: usize = 1;

        /// Creates a new semaphore with the initial number of permits
        ///
        /// Maximum number of permits on 32-bit platforms is `1<<29`.
        pub(crate) fn new(permits: usize) -> Self {
            assert!(
                permits <= Self::MAX_PERMITS,
                "a semaphore may not have more than MAX_PERMITS permits ({})",
                Self::MAX_PERMITS
            );
            Self {
                permits: AtomicUsize::new(permits << Self::PERMIT_SHIFT),
                waiters: Mutex::new(Waitlist {
                    queue: LinkedList::new(),
                    closed: false,
                }),
            }
        }

        /// Creates a new semaphore with the initial number of permits
        ///
        /// Maximum number of permits on 32-bit platforms is `1<<29`.
        ///
        /// If the specified number of permits exceeds the maximum permit amount
        /// Then the value will get clamped to the maximum number of permits.
        #[cfg(all(feature = "parking_lot", not(all(loom, test))))]
        pub(crate) const fn const_new(mut permits: usize) -> Self {
            // NOTE: assertions and by extension panics are still being worked on: https://github.com/rust-lang/rust/issues/74925
            // currently we just clamp the permit count when it exceeds the max
            permits &= Self::MAX_PERMITS;

            Self {
                permits: AtomicUsize::new(permits << Self::PERMIT_SHIFT),
                waiters: Mutex::const_new(Waitlist {
                    queue: LinkedList::new(),
                    closed: false,
                }),
            }
        }

        /// Returns the current number of available permits
        pub(crate) fn available_permits(&self) -> usize {
            self.permits.load(Acquire) >> Self::PERMIT_SHIFT
        }

        /// Adds `added` new permits to the semaphore.
        ///
        /// The maximum number of permits is `usize::MAX >> 3`, and this function will panic if the limit is exceeded.
        pub(crate) fn release(&self, added: usize) {
            if added == 0 {
                return;
            }

            // Assign permits to the wait queue
            self.add_permits_locked(added, self.waiters.lock());
        }

        /// Closes the semaphore. This prevents the semaphore from issuing new
        /// permits and notifies all pending waiters.
        pub(crate) fn close(&self) {
            let mut waiters = self.waiters.lock();
            // If the semaphore's permits counter has enough permits for an
            // unqueued waiter to acquire all the permits it needs immediately,
            // it won't touch the wait list. Therefore, we have to set a bit on
            // the permit counter as well. However, we must do this while
            // holding the lock --- otherwise, if we set the bit and then wait
            // to acquire the lock we'll enter an inconsistent state where the
            // permit counter is closed, but the wait list is not.
            self.permits.fetch_or(Self::CLOSED, Release);
            waiters.closed = true;
            while let Some(mut waiter) = waiters.queue.pop_back() {
                let waker = unsafe { waiter.as_mut().waker.with_mut(|waker| (*waker).take()) };
                if let Some(waker) = waker {
                    waker.wake();
                }
            }
        }

        /// Returns true if the semaphore is closed
        pub(crate) fn is_closed(&self) -> bool {
            self.permits.load(Acquire) & Self::CLOSED == Self::CLOSED
        }

        pub(crate) fn try_acquire(&self, num_permits: u32) -> Result<(), TryAcquireError> {
            assert!(
                num_permits as usize <= Self::MAX_PERMITS,
                "a semaphore may not have more than MAX_PERMITS permits ({})",
                Self::MAX_PERMITS
            );
            let num_permits = (num_permits as usize) << Self::PERMIT_SHIFT;
            let mut curr = self.permits.load(Acquire);
            loop {
                // Has the semaphore closed?
                if curr & Self::CLOSED == Self::CLOSED {
                    return Err(TryAcquireError::Closed);
                }

                // Are there enough permits remaining?
                if curr < num_permits {
                    return Err(TryAcquireError::NoPermits);
                }

                let next = curr - num_permits;

                match self.permits.compare_exchange(curr, next, AcqRel, Acquire) {
                    Ok(_) => return Ok(()),
                    Err(actual) => curr = actual,
                }
            }
        }

        pub(crate) fn acquire(&self, num_permits: u32) -> Acquire<'_> {
            Acquire::new(self, num_permits)
        }

        /// Release `rem` permits to the semaphore's wait list, starting from the
        /// end of the queue.
        ///
        /// If `rem` exceeds the number of permits needed by the wait list, the
        /// remainder are assigned back to the semaphore.
        fn add_permits_locked(&self, mut rem: usize, waiters: MutexGuard<'_, Waitlist>) {
            let mut wakers: [Option<Waker>; 8] = Default::default();
            let mut lock = Some(waiters);
            let mut is_empty = false;
            while rem > 0 {
                let mut waiters = lock.take().unwrap_or_else(|| self.waiters.lock());
                'inner: for slot in &mut wakers[..] {
                    // Was the waiter assigned enough permits to wake it?
                    match waiters.queue.last() {
                        Some(waiter) => {
                            if !waiter.assign_permits(&mut rem) {
                                break 'inner;
                            }
                        }
                        None => {
                            is_empty = true;
                            // If we assigned permits to all the waiters in the queue, and there are
                            // still permits left over, assign them back to the semaphore.
                            break 'inner;
                        }
                    };
                    let mut waiter = waiters.queue.pop_back().unwrap();
                    *slot = unsafe { waiter.as_mut().waker.with_mut(|waker| (*waker).take()) };
                }

                if rem > 0 && is_empty {
                    let permits = rem;
                    assert!(
                        permits <= Self::MAX_PERMITS,
                        "cannot add more than MAX_PERMITS permits ({})",
                        Self::MAX_PERMITS
                    );
                    let prev = self.permits.fetch_add(rem << Self::PERMIT_SHIFT, Release);
                    assert!(
                        prev + permits <= Self::MAX_PERMITS,
                        "number of added permits ({}) would overflow MAX_PERMITS ({})",
                        rem,
                        Self::MAX_PERMITS
                    );
                    rem = 0;
                }

                drop(waiters); // release the lock

                wakers
                    .iter_mut()
                    .filter_map(Option::take)
                    .for_each(Waker::wake);
            }

            assert_eq!(rem, 0);
        }

        fn poll_acquire(
            &self,
            cx: &mut Context<'_>,
            num_permits: u32,
            node: Pin<&mut Waiter>,
            queued: bool,
        ) -> Poll<Result<(), AcquireError>> {
            let mut acquired = 0;

            let needed = if queued {
                node.state.load(Acquire) << Self::PERMIT_SHIFT
            } else {
                (num_permits as usize) << Self::PERMIT_SHIFT
            };

            let mut lock = None;
            // First, try to take the requested number of permits from the
            // semaphore.
            let mut curr = self.permits.load(Acquire);
            let mut waiters = loop {
                // Has the semaphore closed?
                if curr & Self::CLOSED > 0 {
                    return Ready(Err(AcquireError::closed()));
                }

                let mut remaining = 0;
                let total = curr
                    .checked_add(acquired)
                    .expect("number of permits must not overflow");
                let (next, acq) = if total >= needed {
                    let next = curr - (needed - acquired);
                    (next, needed >> Self::PERMIT_SHIFT)
                } else {
                    remaining = (needed - acquired) - curr;
                    (0, curr >> Self::PERMIT_SHIFT)
                };

                if remaining > 0 && lock.is_none() {
                    // No permits were immediately available, so this permit will
                    // (probably) need to wait. We'll need to acquire a lock on the
                    // wait queue before continuing. We need to do this _before_ the
                    // CAS that sets the new value of the semaphore's `permits`
                    // counter. Otherwise, if we subtract the permits and then
                    // acquire the lock, we might miss additional permits being
                    // added while waiting for the lock.
                    lock = Some(self.waiters.lock());
                }

                match self.permits.compare_exchange(curr, next, AcqRel, Acquire) {
                    Ok(_) => {
                        acquired += acq;
                        if remaining == 0 {
                            if !queued {
                                return Ready(Ok(()));
                            } else if lock.is_none() {
                                break self.waiters.lock();
                            }
                        }
                        break lock.expect("lock must be acquired before waiting");
                    }
                    Err(actual) => curr = actual,
                }
            };

            if waiters.closed {
                return Ready(Err(AcquireError::closed()));
            }

            if node.assign_permits(&mut acquired) {
                self.add_permits_locked(acquired, waiters);
                return Ready(Ok(()));
            }

            assert_eq!(acquired, 0);

            // Otherwise, register the waker & enqueue the node.
            node.waker.with_mut(|waker| {
                // Safety: the wait list is locked, so we may modify the waker.
                let waker = unsafe { &mut *waker };
                // Do we need to register the new waker?
                if waker
                    .as_ref()
                    .map(|waker| !waker.will_wake(cx.waker()))
                    .unwrap_or(true)
                {
                    *waker = Some(cx.waker().clone());
                }
            });

            // If the waiter is not already in the wait queue, enqueue it.
            if !queued {
                let node = unsafe {
                    let node = Pin::into_inner_unchecked(node) as *mut _;
                    NonNull::new_unchecked(node)
                };

                waiters.queue.push_front(node);
            }

            Pending
        }
    }

    impl fmt::Debug for Semaphore {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("Semaphore")
                .field("permits", &self.available_permits())
                .finish()
        }
    }

    impl Waiter {
        fn new(num_permits: u32) -> Self {
            Waiter {
                waker: UnsafeCell::new(None),
                state: AtomicUsize::new(num_permits as usize),
                pointers: linked_list::Pointers::new(),
                _p: PhantomPinned,
            }
        }

        /// Assign permits to the waiter.
        ///
        /// Returns `true` if the waiter should be removed from the queue
        fn assign_permits(&self, n: &mut usize) -> bool {
            let mut curr = self.state.load(Acquire);
            loop {
                let assign = cmp::min(curr, *n);
                let next = curr - assign;
                match self.state.compare_exchange(curr, next, AcqRel, Acquire) {
                    Ok(_) => {
                        *n -= assign;
                        return next == 0;
                    }
                    Err(actual) => curr = actual,
                }
            }
        }
    }

    impl Future for Acquire<'_> {
        type Output = Result<(), AcquireError>;

        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            // First, ensure the current task has enough budget to proceed.
            let coop = ready!(crate::coop::poll_proceed(cx));

            let (node, semaphore, needed, queued) = self.project();

            match semaphore.poll_acquire(cx, needed, node, *queued) {
                Pending => {
                    *queued = true;
                    Pending
                }
                Ready(r) => {
                    coop.made_progress();
                    r?;
                    *queued = false;
                    Ready(Ok(()))
                }
            }
        }
    }

    impl<'a> Acquire<'a> {
        fn new(semaphore: &'a Semaphore, num_permits: u32) -> Self {
            Self {
                node: Waiter::new(num_permits),
                semaphore,
                num_permits,
                queued: false,
            }
        }

        fn project(self: Pin<&mut Self>) -> (Pin<&mut Waiter>, &Semaphore, u32, &mut bool) {
            fn is_unpin<T: Unpin>() {}
            unsafe {
                // Safety: all fields other than `node` are `Unpin`

                is_unpin::<&Semaphore>();
                is_unpin::<&mut bool>();
                is_unpin::<u32>();

                let this = self.get_unchecked_mut();
                (
                    Pin::new_unchecked(&mut this.node),
                    &this.semaphore,
                    this.num_permits,
                    &mut this.queued,
                )
            }
        }
    }

    impl Drop for Acquire<'_> {
        fn drop(&mut self) {
            // If the future is completed, there is no node in the wait list, so we
            // can skip acquiring the lock.
            if !self.queued {
                return;
            }

            // This is where we ensure safety. The future is being dropped,
            // which means we must ensure that the waiter entry is no longer stored
            // in the linked list.
            let mut waiters = self.semaphore.waiters.lock();

            // remove the entry from the list
            let node = NonNull::from(&mut self.node);
            // Safety: we have locked the wait list.
            unsafe { waiters.queue.remove(node) };

            let acquired_permits = self.num_permits as usize - self.node.state.load(Acquire);
            if acquired_permits > 0 {
                self.semaphore.add_permits_locked(acquired_permits, waiters);
            }
        }
    }

    // Safety: the `Acquire` future is not `Sync` automatically because it contains
    // a `Waiter`, which, in turn, contains an `UnsafeCell`. However, the
    // `UnsafeCell` is only accessed when the future is borrowed mutably (either in
    // `poll` or in `drop`). Therefore, it is safe (although not particularly
    // _useful_) for the future to be borrowed immutably across threads.
    unsafe impl Sync for Acquire<'_> {}

    // ===== impl AcquireError ====

    impl AcquireError {
        fn closed() -> AcquireError {
            AcquireError(())
        }
    }

    impl fmt::Display for AcquireError {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(fmt, "semaphore closed")
        }
    }

    impl std::error::Error for AcquireError {}

    // ===== impl TryAcquireError =====

    impl TryAcquireError {
        /// Returns `true` if the error was caused by a closed semaphore.
        #[allow(dead_code)] // may be used later!
        pub(crate) fn is_closed(&self) -> bool {
            matches!(self, TryAcquireError::Closed)
        }

        /// Returns `true` if the error was caused by calling `try_acquire` on a
        /// semaphore with no available permits.
        #[allow(dead_code)] // may be used later!
        pub(crate) fn is_no_permits(&self) -> bool {
            matches!(self, TryAcquireError::NoPermits)
        }
    }

    impl fmt::Display for TryAcquireError {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            match self {
                TryAcquireError::Closed => write!(fmt, "semaphore closed"),
                TryAcquireError::NoPermits => write!(fmt, "no permits available"),
            }
        }
    }

    impl std::error::Error for TryAcquireError {}

    /// # Safety
    ///
    /// `Waiter` is forced to be !Unpin.
    unsafe impl linked_list::Link for Waiter {
        // XXX: ideally, we would be able to use `Pin` here, to enforce the
        // invariant that list entries may not move while in the list. However, we
        // can't do this currently, as using `Pin<&'a mut Waiter>` as the `Handle`
        // type would require `Semaphore` to be generic over a lifetime. We can't
        // use `Pin<*mut Waiter>`, as raw pointers are `Unpin` regardless of whether
        // or not they dereference to an `!Unpin` target.
        type Handle = NonNull<Waiter>;
        type Target = Waiter;

        fn as_raw(handle: &Self::Handle) -> NonNull<Waiter> {
            *handle
        }

        unsafe fn from_raw(ptr: NonNull<Waiter>) -> NonNull<Waiter> {
            ptr
        }

        unsafe fn pointers(mut target: NonNull<Waiter>) -> NonNull<linked_list::Pointers<Waiter>> {
            NonNull::from(&mut target.as_mut().pointers)
        }
    }