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    use crate::sync::batch_semaphore::Semaphore;
    use crate::sync::rwlock::read_guard::RwLockReadGuard;
    use crate::sync::rwlock::write_guard_mapped::RwLockMappedWriteGuard;
    use std::fmt;
    use std::marker;
    use std::mem;
    use std::ops;

    /// RAII structure used to release the exclusive write access of a lock when
    /// dropped.
    ///
    /// This structure is created by the [`write`] and method
    /// on [`RwLock`].
    ///
    /// [`write`]: method@crate::sync::RwLock::write
    /// [`RwLock`]: struct@crate::sync::RwLock
    pub struct RwLockWriteGuard<'a, T: ?Sized> {
        pub(super) s: &'a Semaphore,
        pub(super) data: *mut T,
        pub(super) marker: marker::PhantomData<&'a mut T>,
    }

    impl<'a, T: ?Sized> RwLockWriteGuard<'a, T> {
        /// Make a new [`RwLockMappedWriteGuard`] for a component of the locked data.
        ///
        /// This operation cannot fail as the `RwLockWriteGuard` passed in already
        /// locked the data.
        ///
        /// This is an associated function that needs to be used as
        /// `RwLockWriteGuard::map(..)`. A method would interfere with methods of
        /// the same name on the contents of the locked data.
        ///
        /// This is an asynchronous version of [`RwLockWriteGuard::map`] from the
        /// [`parking_lot` crate].
        ///
        /// [`RwLockMappedWriteGuard`]: struct@crate::sync::RwLockMappedWriteGuard
        /// [`RwLockWriteGuard::map`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockWriteGuard.html#method.map
        /// [`parking_lot` crate]: https://crates.io/crates/parking_lot
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::{RwLock, RwLockWriteGuard};
        ///
        /// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
        /// struct Foo(u32);
        ///
        /// # #[tokio::main]
        /// # async fn main() {
        /// let lock = RwLock::new(Foo(1));
        ///
        /// {
        ///     let mut mapped = RwLockWriteGuard::map(lock.write().await, |f| &mut f.0);
        ///     *mapped = 2;
        /// }
        ///
        /// assert_eq!(Foo(2), *lock.read().await);
        /// # }
        /// ```
        #[inline]
        pub fn map<F, U: ?Sized>(mut this: Self, f: F) -> RwLockMappedWriteGuard<'a, U>
        where
            F: FnOnce(&mut T) -> &mut U,
        {
            let data = f(&mut *this) as *mut U;
            let s = this.s;
            // NB: Forget to avoid drop impl from being called.
            mem::forget(this);
            RwLockMappedWriteGuard {
                s,
                data,
                marker: marker::PhantomData,
            }
        }

        /// Attempts to make  a new [`RwLockMappedWriteGuard`] for a component of
        /// the locked data. The original guard is returned if the closure returns
        /// `None`.
        ///
        /// This operation cannot fail as the `RwLockWriteGuard` passed in already
        /// locked the data.
        ///
        /// This is an associated function that needs to be
        /// used as `RwLockWriteGuard::try_map(...)`. A method would interfere with
        /// methods of the same name on the contents of the locked data.
        ///
        /// This is an asynchronous version of [`RwLockWriteGuard::try_map`] from
        /// the [`parking_lot` crate].
        ///
        /// [`RwLockMappedWriteGuard`]: struct@crate::sync::RwLockMappedWriteGuard
        /// [`RwLockWriteGuard::try_map`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockWriteGuard.html#method.try_map
        /// [`parking_lot` crate]: https://crates.io/crates/parking_lot
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::sync::{RwLock, RwLockWriteGuard};
        ///
        /// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
        /// struct Foo(u32);
        ///
        /// # #[tokio::main]
        /// # async fn main() {
        /// let lock = RwLock::new(Foo(1));
        ///
        /// {
        ///     let guard = lock.write().await;
        ///     let mut guard = RwLockWriteGuard::try_map(guard, |f| Some(&mut f.0)).expect("should not fail");
        ///     *guard = 2;
        /// }
        ///
        /// assert_eq!(Foo(2), *lock.read().await);
        /// # }
        /// ```
        #[inline]
        pub fn try_map<F, U: ?Sized>(
            mut this: Self,
            f: F,
        ) -> Result<RwLockMappedWriteGuard<'a, U>, Self>
        where
            F: FnOnce(&mut T) -> Option<&mut U>,
        {
            let data = match f(&mut *this) {
                Some(data) => data as *mut U,
                None => return Err(this),
            };
            let s = this.s;
            // NB: Forget to avoid drop impl from being called.
            mem::forget(this);
            Ok(RwLockMappedWriteGuard {
                s,
                data,
                marker: marker::PhantomData,
            })
        }

        /// Converts this `RwLockWriteGuard` into an `RwLockMappedWriteGuard`. This
        /// method can be used to store a non-mapped guard in a struct field that
        /// expects a mapped guard.
        ///
        /// This is equivalent to calling `RwLockWriteGuard::map(guard, |me| me)`.
        #[inline]
        pub fn into_mapped(this: Self) -> RwLockMappedWriteGuard<'a, T> {
            RwLockWriteGuard::map(this, |me| me)
        }

        /// Atomically downgrades a write lock into a read lock without allowing
        /// any writers to take exclusive access of the lock in the meantime.
        ///
        /// **Note:** This won't *necessarily* allow any additional readers to acquire
        /// locks, since [`RwLock`] is fair and it is possible that a writer is next
        /// in line.
        ///
        /// Returns an RAII guard which will drop this read access of the `RwLock`
        /// when dropped.
        ///
        /// # Examples
        ///
        /// ```
        /// # use tokio::sync::RwLock;
        /// # use std::sync::Arc;
        /// #
        /// # #[tokio::main]
        /// # async fn main() {
        /// let lock = Arc::new(RwLock::new(1));
        ///
        /// let n = lock.write().await;
        ///
        /// let cloned_lock = lock.clone();
        /// let handle = tokio::spawn(async move {
        ///     *cloned_lock.write().await = 2;
        /// });
        ///
        /// let n = n.downgrade();
        /// assert_eq!(*n, 1, "downgrade is atomic");
        ///
        /// drop(n);
        /// handle.await.unwrap();
        /// assert_eq!(*lock.read().await, 2, "second writer obtained write lock");
        /// # }
        /// ```
        ///
        /// [`RwLock`]: struct@crate::sync::RwLock
        pub fn downgrade(self) -> RwLockReadGuard<'a, T> {
            let RwLockWriteGuard { s, data, .. } = self;

            // Release all but one of the permits held by the write guard
            s.release(super::MAX_READS - 1);
            // NB: Forget to avoid drop impl from being called.
            mem::forget(self);
            RwLockReadGuard {
                s,
                data,
                marker: marker::PhantomData,
            }
        }
    }

    impl<T: ?Sized> ops::Deref for RwLockWriteGuard<'_, T> {
        type Target = T;

        fn deref(&self) -> &T {
            unsafe { &*self.data }
        }
    }

    impl<T: ?Sized> ops::DerefMut for RwLockWriteGuard<'_, T> {
        fn deref_mut(&mut self) -> &mut T {
            unsafe { &mut *self.data }
        }
    }

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

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

    impl<'a, T: ?Sized> Drop for RwLockWriteGuard<'a, T> {
        fn drop(&mut self) {
            self.s.release(super::MAX_READS);
        }
    }