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    #![cfg_attr(not(feature = "sync"), allow(unreachable_pub, dead_code))]

    use crate::sync::batch_semaphore as semaphore;

    use std::cell::UnsafeCell;
    use std::error::Error;
    use std::fmt;
    use std::ops::{Deref, DerefMut};
    use std::sync::Arc;

    /// An asynchronous `Mutex`-like type.
    ///
    /// This type acts similarly to an asynchronous [`std::sync::Mutex`], with one
    /// major difference: [`lock`] does not block and the lock guard can be held
    /// across await points.
    ///
    /// # Which kind of mutex should you use?
    ///
    /// Contrary to popular belief, it is ok and often preferred to use the ordinary
    /// [`Mutex`][std] from the standard library in asynchronous code. This section
    /// will help you decide on which kind of mutex you should use.
    ///
    /// The primary use case of the async mutex is to provide shared mutable access
    /// to IO resources such as a database connection. If the data stored behind the
    /// mutex is just data, it is often better to use a blocking mutex such as the
    /// one in the standard library or [`parking_lot`]. This is because the feature
    /// that the async mutex offers over the blocking mutex is that it is possible
    /// to keep the mutex locked across an `.await` point, which is rarely necessary
    /// for data.
    ///
    /// A common pattern is to wrap the `Arc<Mutex<...>>` in a struct that provides
    /// non-async methods for performing operations on the data within, and only
    /// lock the mutex inside these methods. The [mini-redis] example provides an
    /// illustration of this pattern.
    ///
    /// Additionally, when you _do_ want shared access to an IO resource, it is
    /// often better to spawn a task to manage the IO resource, and to use message
    /// passing to communicate with that task.
    ///
    /// [std]: std::sync::Mutex
    /// [`parking_lot`]: https://docs.rs/parking_lot
    /// [mini-redis]: https://github.com/tokio-rs/mini-redis/blob/master/src/db.rs
    ///
    /// # Examples:
    ///
    /// ```rust,no_run
    /// use tokio::sync::Mutex;
    /// use std::sync::Arc;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let data1 = Arc::new(Mutex::new(0));
    ///     let data2 = Arc::clone(&data1);
    ///
    ///     tokio::spawn(async move {
    ///         let mut lock = data2.lock().await;
    ///         *lock += 1;
    ///     });
    ///
    ///     let mut lock = data1.lock().await;
    ///     *lock += 1;
    /// }
    /// ```
    ///
    ///
    /// ```rust,no_run
    /// use tokio::sync::Mutex;
    /// use std::sync::Arc;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let count = Arc::new(Mutex::new(0));
    ///
    ///     for _ in 0..5 {
    ///         let my_count = Arc::clone(&count);
    ///         tokio::spawn(async move {
    ///             for _ in 0..10 {
    ///                 let mut lock = my_count.lock().await;
    ///                 *lock += 1;
    ///                 println!("{}", lock);
    ///             }
    ///         });
    ///     }
    ///
    ///     loop {
    ///         if *count.lock().await >= 50 {
    ///             break;
    ///         }
    ///     }
    ///     println!("Count hit 50.");
    /// }
    /// ```
    /// There are a few things of note here to pay attention to in this example.
    /// 1. The mutex is wrapped in an [`Arc`] to allow it to be shared across
    ///    threads.
    /// 2. Each spawned task obtains a lock and releases it on every iteration.
    /// 3. Mutation of the data protected by the Mutex is done by de-referencing
    ///    the obtained lock as seen on lines 12 and 19.
    ///
    /// Tokio's Mutex works in a simple FIFO (first in, first out) style where all
    /// calls to [`lock`] complete in the order they were performed. In that way the
    /// Mutex is "fair" and predictable in how it distributes the locks to inner
    /// data. This is why the output of the program above is an in-order count to
    /// 50. Locks are released and reacquired after every iteration, so basically,
    /// each thread goes to the back of the line after it increments the value once.
    /// Finally, since there is only a single valid lock at any given time, there is
    /// no possibility of a race condition when mutating the inner value.
    ///
    /// Note that in contrast to [`std::sync::Mutex`], this implementation does not
    /// poison the mutex when a thread holding the [`MutexGuard`] panics. In such a
    /// case, the mutex will be unlocked. If the panic is caught, this might leave
    /// the data protected by the mutex in an inconsistent state.
    ///
    /// [`Mutex`]: struct@Mutex
    /// [`MutexGuard`]: struct@MutexGuard
    /// [`Arc`]: struct@std::sync::Arc
    /// [`std::sync::Mutex`]: struct@std::sync::Mutex
    /// [`Send`]: trait@std::marker::Send
    /// [`lock`]: method@Mutex::lock
    pub struct Mutex<T: ?Sized> {
        s: semaphore::Semaphore,
        c: UnsafeCell<T>,
    }

    /// A handle to a held `Mutex`.
    ///
    /// As long as you have this guard, you have exclusive access to the underlying
    /// `T`. The guard internally borrows the `Mutex`, so the mutex will not be
    /// dropped while a guard exists.
    ///
    /// The lock is automatically released whenever the guard is dropped, at which
    /// point `lock` will succeed yet again.
    pub struct MutexGuard<'a, T: ?Sized> {
        lock: &'a Mutex<T>,
    }

    /// An owned handle to a held `Mutex`.
    ///
    /// This guard is only available from a `Mutex` that is wrapped in an [`Arc`]. It
    /// is identical to `MutexGuard`, except that rather than borrowing the `Mutex`,
    /// it clones the `Arc`, incrementing the reference count. This means that
    /// unlike `MutexGuard`, it will have the `'static` lifetime.
    ///
    /// As long as you have this guard, you have exclusive access to the underlying
    /// `T`. The guard internally keeps a reference-counted pointer to the original
    /// `Mutex`, so even if the lock goes away, the guard remains valid.
    ///
    /// The lock is automatically released whenever the guard is dropped, at which
    /// point `lock` will succeed yet again.
    ///
    /// [`Arc`]: std::sync::Arc
    pub struct OwnedMutexGuard<T: ?Sized> {
        lock: Arc<Mutex<T>>,
    }

    // As long as T: Send, it's fine to send and share Mutex<T> between threads.
    // If T was not Send, sending and sharing a Mutex<T> would be bad, since you can
    // access T through Mutex<T>.
    unsafe impl<T> Send for Mutex<T> where T: ?Sized + Send {}
    unsafe impl<T> Sync for Mutex<T> where T: ?Sized + Send {}
    unsafe impl<T> Sync for MutexGuard<'_, T> where T: ?Sized + Send + Sync {}
    unsafe impl<T> Sync for OwnedMutexGuard<T> where T: ?Sized + Send + Sync {}

    /// Error returned from the [`Mutex::try_lock`], [`RwLock::try_read`] and
    /// [`RwLock::try_write`] functions.
    ///
    /// `Mutex::try_lock` operation will only fail if the mutex is already locked.
    ///
    /// `RwLock::try_read` operation will only fail if the lock is currently held
    /// by an exclusive writer.
    ///
    /// `RwLock::try_write` operation will if lock is held by any reader or by an
    /// exclusive writer.
    ///
    /// [`Mutex::try_lock`]: Mutex::try_lock
    /// [`RwLock::try_read`]: fn@super::RwLock::try_read
    /// [`RwLock::try_write`]: fn@super::RwLock::try_write
    #[derive(Debug)]
    pub struct TryLockError(pub(super) ());

    impl fmt::Display for TryLockError {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(fmt, "operation would block")
        }
    }

    impl Error for TryLockError {}

    #[test]
    #[cfg(not(loom))]
    fn bounds() {
        fn check_send<T: Send>() {}
        fn check_unpin<T: Unpin>() {}
        // This has to take a value, since the async fn's return type is unnameable.
        fn check_send_sync_val<T: Send + Sync>(_t: T) {}
        fn check_send_sync<T: Send + Sync>() {}
        fn check_static<T: 'static>() {}
        fn check_static_val<T: 'static>(_t: T) {}

        check_send::<MutexGuard<'_, u32>>();
        check_send::<OwnedMutexGuard<u32>>();
        check_unpin::<Mutex<u32>>();
        check_send_sync::<Mutex<u32>>();
        check_static::<OwnedMutexGuard<u32>>();

        let mutex = Mutex::new(1);
        check_send_sync_val(mutex.lock());
        let arc_mutex = Arc::new(Mutex::new(1));
        check_send_sync_val(arc_mutex.clone().lock_owned());
        check_static_val(arc_mutex.lock_owned());
    }

    impl<T: ?Sized> Mutex<T> {
        /// Creates a new lock in an unlocked state ready for use.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::Mutex;
        ///
        /// let lock = Mutex::new(5);
        /// ```
        pub fn new(t: T) -> Self
        where
            T: Sized,
        {
            Self {
                c: UnsafeCell::new(t),
                s: semaphore::Semaphore::new(1),
            }
        }

        /// Creates a new lock in an unlocked state ready for use.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::Mutex;
        ///
        /// static LOCK: Mutex<i32> = Mutex::const_new(5);
        /// ```
        #[cfg(all(feature = "parking_lot", not(all(loom, test)),))]
        #[cfg_attr(docsrs, doc(cfg(feature = "parking_lot")))]
        pub const fn const_new(t: T) -> Self
        where
            T: Sized,
        {
            Self {
                c: UnsafeCell::new(t),
                s: semaphore::Semaphore::const_new(1),
            }
        }

        /// Locks this mutex, causing the current task
        /// to yield until the lock has been acquired.
        /// When the lock has been acquired, function returns a [`MutexGuard`].
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::Mutex;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let mutex = Mutex::new(1);
        ///
        ///     let mut n = mutex.lock().await;
        ///     *n = 2;
        /// }
        /// ```
        pub async fn lock(&self) -> MutexGuard<'_, T> {
            self.acquire().await;
            MutexGuard { lock: self }
        }

        /// Locks this mutex, causing the current task to yield until the lock has
        /// been acquired. When the lock has been acquired, this returns an
        /// [`OwnedMutexGuard`].
        ///
        /// This method is identical to [`Mutex::lock`], except that the returned
        /// guard references the `Mutex` with an [`Arc`] rather than by borrowing
        /// it. Therefore, the `Mutex` must be wrapped in an `Arc` to call this
        /// method, and the guard will live for the `'static` lifetime, as it keeps
        /// the `Mutex` alive by holding an `Arc`.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::Mutex;
        /// use std::sync::Arc;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let mutex = Arc::new(Mutex::new(1));
        ///
        ///     let mut n = mutex.clone().lock_owned().await;
        ///     *n = 2;
        /// }
        /// ```
        ///
        /// [`Arc`]: std::sync::Arc
        pub async fn lock_owned(self: Arc<Self>) -> OwnedMutexGuard<T> {
            self.acquire().await;
            OwnedMutexGuard { lock: self }
        }

        async fn acquire(&self) {
            self.s.acquire(1).await.unwrap_or_else(|_| {
                // The semaphore was closed. but, we never explicitly close it, and
                // we own it exclusively, which means that this can never happen.
                unreachable!()
            });
        }

        /// Attempts to acquire the lock, and returns [`TryLockError`] if the
        /// lock is currently held somewhere else.
        ///
        /// [`TryLockError`]: TryLockError
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::Mutex;
        /// # async fn dox() -> Result<(), tokio::sync::TryLockError> {
        ///
        /// let mutex = Mutex::new(1);
        ///
        /// let n = mutex.try_lock()?;
        /// assert_eq!(*n, 1);
        /// # Ok(())
        /// # }
        /// ```
        pub fn try_lock(&self) -> Result<MutexGuard<'_, T>, TryLockError> {
            match self.s.try_acquire(1) {
                Ok(_) => Ok(MutexGuard { lock: self }),
                Err(_) => Err(TryLockError(())),
            }
        }

        /// Returns a mutable reference to the underlying data.
        ///
        /// Since this call borrows the `Mutex` mutably, no actual locking needs to
        /// take place -- the mutable borrow statically guarantees no locks exist.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::Mutex;
        ///
        /// fn main() {
        ///     let mut mutex = Mutex::new(1);
        ///
        ///     let n = mutex.get_mut();
        ///     *n = 2;
        /// }
        /// ```
        pub fn get_mut(&mut self) -> &mut T {
            unsafe {
                // Safety: This is https://github.com/rust-lang/rust/pull/76936
                &mut *self.c.get()
            }
        }

        /// Attempts to acquire the lock, and returns [`TryLockError`] if the lock
        /// is currently held somewhere else.
        ///
        /// This method is identical to [`Mutex::try_lock`], except that the
        /// returned  guard references the `Mutex` with an [`Arc`] rather than by
        /// borrowing it. Therefore, the `Mutex` must be wrapped in an `Arc` to call
        /// this method, and the guard will live for the `'static` lifetime, as it
        /// keeps the `Mutex` alive by holding an `Arc`.
        ///
        /// [`TryLockError`]: TryLockError
        /// [`Arc`]: std::sync::Arc
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::Mutex;
        /// use std::sync::Arc;
        /// # async fn dox() -> Result<(), tokio::sync::TryLockError> {
        ///
        /// let mutex = Arc::new(Mutex::new(1));
        ///
        /// let n = mutex.clone().try_lock_owned()?;
        /// assert_eq!(*n, 1);
        /// # Ok(())
        /// # }
        pub fn try_lock_owned(self: Arc<Self>) -> Result<OwnedMutexGuard<T>, TryLockError> {
            match self.s.try_acquire(1) {
                Ok(_) => Ok(OwnedMutexGuard { lock: self }),
                Err(_) => Err(TryLockError(())),
            }
        }

        /// Consumes the mutex, returning the underlying data.
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::Mutex;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let mutex = Mutex::new(1);
        ///
        ///     let n = mutex.into_inner();
        ///     assert_eq!(n, 1);
        /// }
        /// ```
        pub fn into_inner(self) -> T
        where
            T: Sized,
        {
            self.c.into_inner()
        }
    }

    impl<T> From<T> for Mutex<T> {
        fn from(s: T) -> Self {
            Self::new(s)
        }
    }

    impl<T> Default for Mutex<T>
    where
        T: Default,
    {
        fn default() -> Self {
            Self::new(T::default())
        }
    }

    impl<T> std::fmt::Debug for Mutex<T>
    where
        T: std::fmt::Debug,
    {
        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
            let mut d = f.debug_struct("Mutex");
            match self.try_lock() {
                Ok(inner) => d.field("data", &*inner),
                Err(_) => d.field("data", &format_args!("<locked>")),
            };
            d.finish()
        }
    }

    // === impl MutexGuard ===

    impl<T: ?Sized> Drop for MutexGuard<'_, T> {
        fn drop(&mut self) {
            self.lock.s.release(1)
        }
    }

    impl<T: ?Sized> Deref for MutexGuard<'_, T> {
        type Target = T;
        fn deref(&self) -> &Self::Target {
            unsafe { &*self.lock.c.get() }
        }
    }

    impl<T: ?Sized> DerefMut for MutexGuard<'_, T> {
        fn deref_mut(&mut self) -> &mut Self::Target {
            unsafe { &mut *self.lock.c.get() }
        }
    }

    impl<T: ?Sized + fmt::Debug> fmt::Debug for MutexGuard<'_, T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Debug::fmt(&**self, f)
        }
    }

    impl<T: ?Sized + fmt::Display> fmt::Display for MutexGuard<'_, T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Display::fmt(&**self, f)
        }
    }

    // === impl OwnedMutexGuard ===

    impl<T: ?Sized> Drop for OwnedMutexGuard<T> {
        fn drop(&mut self) {
            self.lock.s.release(1)
        }
    }

    impl<T: ?Sized> Deref for OwnedMutexGuard<T> {
        type Target = T;
        fn deref(&self) -> &Self::Target {
            unsafe { &*self.lock.c.get() }
        }
    }

    impl<T: ?Sized> DerefMut for OwnedMutexGuard<T> {
        fn deref_mut(&mut self) -> &mut Self::Target {
            unsafe { &mut *self.lock.c.get() }
        }
    }

    impl<T: ?Sized + fmt::Debug> fmt::Debug for OwnedMutexGuard<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Debug::fmt(&**self, f)
        }
    }

    impl<T: ?Sized + fmt::Display> fmt::Display for OwnedMutexGuard<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Display::fmt(&**self, f)
        }
    }