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    use crate::sync::batch_semaphore::{Semaphore, TryAcquireError};
    use crate::sync::mutex::TryLockError;
    use std::cell::UnsafeCell;
    use std::marker;

    pub(crate) mod read_guard;
    pub(crate) mod write_guard;
    pub(crate) mod write_guard_mapped;
    pub(crate) use read_guard::RwLockReadGuard;
    pub(crate) use write_guard::RwLockWriteGuard;
    pub(crate) use write_guard_mapped::RwLockMappedWriteGuard;

    #[cfg(not(loom))]
    const MAX_READS: usize = 32;

    #[cfg(loom)]
    const MAX_READS: usize = 10;

    /// An asynchronous reader-writer lock.
    ///
    /// This type of lock allows a number of readers or at most one writer at any
    /// point in time. The write portion of this lock typically allows modification
    /// of the underlying data (exclusive access) and the read portion of this lock
    /// typically allows for read-only access (shared access).
    ///
    /// In comparison, a [`Mutex`] does not distinguish between readers or writers
    /// that acquire the lock, therefore causing any tasks waiting for the lock to
    /// become available to yield. An `RwLock` will allow any number of readers to
    /// acquire the lock as long as a writer is not holding the lock.
    ///
    /// The priority policy of Tokio's read-write lock is _fair_ (or
    /// [_write-preferring_]), in order to ensure that readers cannot starve
    /// writers. Fairness is ensured using a first-in, first-out queue for the tasks
    /// awaiting the lock; if a task that wishes to acquire the write lock is at the
    /// head of the queue, read locks will not be given out until the write lock has
    /// been released. This is in contrast to the Rust standard library's
    /// `std::sync::RwLock`, where the priority policy is dependent on the
    /// operating system's implementation.
    ///
    /// The type parameter `T` represents the data that this lock protects. It is
    /// required that `T` satisfies [`Send`] to be shared across threads. The RAII guards
    /// returned from the locking methods implement [`Deref`](trait@std::ops::Deref)
    /// (and [`DerefMut`](trait@std::ops::DerefMut)
    /// for the `write` methods) to allow access to the content of the lock.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::RwLock;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let lock = RwLock::new(5);
    ///
    ///     // many reader locks can be held at once
    ///     {
    ///         let r1 = lock.read().await;
    ///         let r2 = lock.read().await;
    ///         assert_eq!(*r1, 5);
    ///         assert_eq!(*r2, 5);
    ///     } // read locks are dropped at this point
    ///
    ///     // only one write lock may be held, however
    ///     {
    ///         let mut w = lock.write().await;
    ///         *w += 1;
    ///         assert_eq!(*w, 6);
    ///     } // write lock is dropped here
    /// }
    /// ```
    ///
    /// [`Mutex`]: struct@super::Mutex
    /// [`RwLock`]: struct@RwLock
    /// [`RwLockReadGuard`]: struct@RwLockReadGuard
    /// [`RwLockWriteGuard`]: struct@RwLockWriteGuard
    /// [`Send`]: trait@std::marker::Send
    /// [_write-preferring_]: https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock#Priority_policies
    #[derive(Debug)]
    pub struct RwLock<T: ?Sized> {
        //semaphore to coordinate read and write access to T
        s: Semaphore,

        //inner data T
        c: UnsafeCell<T>,
    }

    #[test]
    #[cfg(not(loom))]
    fn bounds() {
        fn check_send<T: Send>() {}
        fn check_sync<T: Sync>() {}
        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) {}

        check_send::<RwLock<u32>>();
        check_sync::<RwLock<u32>>();
        check_unpin::<RwLock<u32>>();

        check_send::<RwLockReadGuard<'_, u32>>();
        check_sync::<RwLockReadGuard<'_, u32>>();
        check_unpin::<RwLockReadGuard<'_, u32>>();

        check_send::<RwLockWriteGuard<'_, u32>>();
        check_sync::<RwLockWriteGuard<'_, u32>>();
        check_unpin::<RwLockWriteGuard<'_, u32>>();

        let rwlock = RwLock::new(0);
        check_send_sync_val(rwlock.read());
        check_send_sync_val(rwlock.write());
    }

    // As long as T: Send + Sync, it's fine to send and share RwLock<T> between threads.
    // If T were not Send, sending and sharing a RwLock<T> would be bad, since you can access T through
    // RwLock<T>.
    unsafe impl<T> Send for RwLock<T> where T: ?Sized + Send {}
    unsafe impl<T> Sync for RwLock<T> where T: ?Sized + Send + Sync {}
    // NB: These impls need to be explicit since we're storing a raw pointer.
    // Safety: Stores a raw pointer to `T`, so if `T` is `Sync`, the lock guard over
    // `T` is `Send`.
    unsafe impl<T> Send for RwLockReadGuard<'_, T> where T: ?Sized + Sync {}
    unsafe impl<T> Sync for RwLockReadGuard<'_, T> where T: ?Sized + Send + Sync {}
    unsafe impl<T> Sync for RwLockWriteGuard<'_, T> where T: ?Sized + Send + Sync {}
    unsafe impl<T> Sync for RwLockMappedWriteGuard<'_, T> where T: ?Sized + Send + Sync {}
    // Safety: Stores a raw pointer to `T`, so if `T` is `Sync`, the lock guard over
    // `T` is `Send` - but since this is also provides mutable access, we need to
    // make sure that `T` is `Send` since its value can be sent across thread
    // boundaries.
    unsafe impl<T> Send for RwLockWriteGuard<'_, T> where T: ?Sized + Send + Sync {}
    unsafe impl<T> Send for RwLockMappedWriteGuard<'_, T> where T: ?Sized + Send + Sync {}

    impl<T: ?Sized> RwLock<T> {
        /// Creates a new instance of an `RwLock<T>` which is unlocked.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::RwLock;
        ///
        /// let lock = RwLock::new(5);
        /// ```
        pub fn new(value: T) -> RwLock<T>
        where
            T: Sized,
        {
            RwLock {
                c: UnsafeCell::new(value),
                s: Semaphore::new(MAX_READS),
            }
        }

        /// Creates a new instance of an `RwLock<T>` which is unlocked.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::RwLock;
        ///
        /// static LOCK: RwLock<i32> = RwLock::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(value: T) -> RwLock<T>
        where
            T: Sized,
        {
            RwLock {
                c: UnsafeCell::new(value),
                s: Semaphore::const_new(MAX_READS),
            }
        }

        /// Locks this `RwLock` with shared read access, causing the current task
        /// to yield until the lock has been acquired.
        ///
        /// The calling task will yield until there are no writers which hold the
        /// lock. There may be other readers inside the lock when the task resumes.
        ///
        /// Note that under the priority policy of [`RwLock`], read locks are not
        /// granted until prior write locks, to prevent starvation. Therefore
        /// deadlock may occur if a read lock is held by the current task, a write
        /// lock attempt is made, and then a subsequent read lock attempt is made
        /// by the current task.
        ///
        /// Returns an RAII guard which will drop this read access of the `RwLock`
        /// when dropped.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        /// use tokio::sync::RwLock;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let lock = Arc::new(RwLock::new(1));
        ///     let c_lock = lock.clone();
        ///
        ///     let n = lock.read().await;
        ///     assert_eq!(*n, 1);
        ///
        ///     tokio::spawn(async move {
        ///         // While main has an active read lock, we acquire one too.
        ///         let r = c_lock.read().await;
        ///         assert_eq!(*r, 1);
        ///     }).await.expect("The spawned task has panicked");
        ///
        ///     // Drop the guard after the spawned task finishes.
        ///     drop(n);
        ///}
        /// ```
        pub async fn read(&self) -> RwLockReadGuard<'_, T> {
            self.s.acquire(1).await.unwrap_or_else(|_| {
                // The semaphore was closed. but, we never explicitly close it, and we have a
                // handle to it through the Arc, which means that this can never happen.
                unreachable!()
            });
            RwLockReadGuard {
                s: &self.s,
                data: self.c.get(),
                marker: marker::PhantomData,
            }
        }

        /// Attempts to acquire this `RwLock` with shared read access.
        ///
        /// If the access couldn't be acquired immediately, returns [`TryLockError`].
        /// Otherwise, an RAII guard is returned which will release read access
        /// when dropped.
        ///
        /// [`TryLockError`]: TryLockError
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        /// use tokio::sync::RwLock;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let lock = Arc::new(RwLock::new(1));
        ///     let c_lock = lock.clone();
        ///
        ///     let v = lock.try_read().unwrap();
        ///     assert_eq!(*v, 1);
        ///
        ///     tokio::spawn(async move {
        ///         // While main has an active read lock, we acquire one too.
        ///         let n = c_lock.read().await;
        ///         assert_eq!(*n, 1);
        ///     }).await.expect("The spawned task has panicked");
        ///
        ///     // Drop the guard when spawned task finishes.
        ///     drop(v);
        /// }
        /// ```
        pub fn try_read(&self) -> Result<RwLockReadGuard<'_, T>, TryLockError> {
            match self.s.try_acquire(1) {
                Ok(permit) => permit,
                Err(TryAcquireError::NoPermits) => return Err(TryLockError(())),
                Err(TryAcquireError::Closed) => unreachable!(),
            }

            Ok(RwLockReadGuard {
                s: &self.s,
                data: self.c.get(),
                marker: marker::PhantomData,
            })
        }

        /// Locks this `RwLock` with exclusive write access, causing the current
        /// task to yield until the lock has been acquired.
        ///
        /// The calling task will yield while other writers or readers currently
        /// have access to the lock.
        ///
        /// Returns an RAII guard which will drop the write access of this `RwLock`
        /// when dropped.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::RwLock;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///   let lock = RwLock::new(1);
        ///
        ///   let mut n = lock.write().await;
        ///   *n = 2;
        ///}
        /// ```
        pub async fn write(&self) -> RwLockWriteGuard<'_, T> {
            self.s.acquire(MAX_READS as u32).await.unwrap_or_else(|_| {
                // The semaphore was closed. but, we never explicitly close it, and we have a
                // handle to it through the Arc, which means that this can never happen.
                unreachable!()
            });
            RwLockWriteGuard {
                s: &self.s,
                data: self.c.get(),
                marker: marker::PhantomData,
            }
        }

        /// Attempts to acquire this `RwLock` with exclusive write access.
        ///
        /// If the access couldn't be acquired immediately, returns [`TryLockError`].
        /// Otherwise, an RAII guard is returned which will release write access
        /// when dropped.
        ///
        /// [`TryLockError`]: TryLockError
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::RwLock;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let rw = RwLock::new(1);
        ///
        ///     let v = rw.read().await;
        ///     assert_eq!(*v, 1);
        ///
        ///     assert!(rw.try_write().is_err());
        /// }
        /// ```
        pub fn try_write(&self) -> Result<RwLockWriteGuard<'_, T>, TryLockError> {
            match self.s.try_acquire(MAX_READS as u32) {
                Ok(permit) => permit,
                Err(TryAcquireError::NoPermits) => return Err(TryLockError(())),
                Err(TryAcquireError::Closed) => unreachable!(),
            }

            Ok(RwLockWriteGuard {
                s: &self.s,
                data: self.c.get(),
                marker: marker::PhantomData,
            })
        }

        /// Returns a mutable reference to the underlying data.
        ///
        /// Since this call borrows the `RwLock` mutably, no actual locking needs to
        /// take place -- the mutable borrow statically guarantees no locks exist.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::RwLock;
        ///
        /// fn main() {
        ///     let mut lock = RwLock::new(1);
        ///
        ///     let n = lock.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()
            }
        }

        /// Consumes the lock, returning the underlying data.
        pub fn into_inner(self) -> T
        where
            T: Sized,
        {
            self.c.into_inner()
        }
    }

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

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