# rw_lock.rs - source
Source of the Rust file `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/spin-0.7.1/src/rw_lock.rs`.
Keywords: rust, rustlang, rust-lang

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    //! A lock that provides data access to either one writer or many readers.

    use core::{
        cell::UnsafeCell,
        ops::{Deref, DerefMut},
        sync::atomic::{AtomicUsize, Ordering},
        fmt,
        mem,
    };

    /// A lock that provides data access to either one writer or many readers.
    ///
    /// This lock behaves in a similar manner to its namesake `std::sync::RwLock` but uses
    /// spinning for synchronisation instead. Unlike its namespace, this lock does not
    /// track lock poisoning.
    ///
    /// 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).
    ///
    /// The type parameter `T` represents the data that this lock protects. It is
    /// required that `T` satisfies `Send` to be shared across tasks and `Sync` to
    /// allow concurrent access through readers. The RAII guards returned from the
    /// locking methods implement `Deref` (and `DerefMut` for the `write` methods)
    /// to allow access to the contained of the lock.
    ///
    /// An [`RwLockUpgradableGuard`](RwLockUpgradableGuard) can be upgraded to a
    /// writable guard through the [`RwLockUpgradableGuard::upgrade`](RwLockUpgradableGuard::upgrade)
    /// [`RwLockUpgradableGuard::try_upgrade`](RwLockUpgradableGuard::try_upgrade) functions.
    /// Writable or upgradeable guards can be downgraded through their respective `downgrade`
    /// functions.
    ///
    /// Based on Facebook's
    /// [`folly/RWSpinLock.h`](https://github.com/facebook/folly/blob/a0394d84f2d5c3e50ebfd0566f9d3acb52cfab5a/folly/synchronization/RWSpinLock.h).
    /// This implementation is unfair to writers - if the lock always has readers, then no writers will
    /// ever get a chance. Using an upgradeable lock guard can *somewhat* alleviate this issue as no
    /// new readers are allowed when an upgradeable guard is held, but upgradeable guards can be taken
    /// when there are existing readers. However if the lock is that highly contended and writes are
    /// crucial then this implementation may be a poor choice.
    ///
    /// # Examples
    ///
    /// ```
    /// use spin;
    ///
    /// let lock = spin::RwLock::new(5);
    ///
    /// // many reader locks can be held at once
    /// {
    ///     let r1 = lock.read();
    ///     let r2 = lock.read();
    ///     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();
    ///     *w += 1;
    ///     assert_eq!(*w, 6);
    /// } // write lock is dropped here
    /// ```
    pub struct RwLock<T: ?Sized> {
        lock: AtomicUsize,
        data: UnsafeCell<T>,
    }

    const READER: usize = 1 << 2;
    const UPGRADED: usize = 1 << 1;
    const WRITER: usize = 1;

    /// A guard that provides immutable data access.
    ///
    /// When the guard falls out of scope it will decrement the read count,
    /// potentially releasing the lock.
    pub struct RwLockReadGuard<'a, T: 'a + ?Sized> {
        inner: &'a RwLock<T>,
        data: &'a T,
    }

    /// A guard that provides mutable data access.
    ///
    /// When the guard falls out of scope it will release the lock.
    pub struct RwLockWriteGuard<'a, T: 'a + ?Sized> {
        inner: &'a RwLock<T>,
        data: &'a mut T,
    }

    /// A guard that provides immutable data access but can be upgraded
    /// to [`RwLockWriteGuard`].
    ///
    /// No writers or other upgradeable guards can exist while this is in scope. New reader
    /// creation is prevented (to alleviate writer starvation) but there may be existing readers
    /// when the lock is acquired.
    ///
    /// When the guard falls out of scope it will release the lock.
    pub struct RwLockUpgradableGuard<'a, T: 'a + ?Sized> {
        inner: &'a RwLock<T>,
        data: &'a T,
    }

    // Same unsafe impls as `std::sync::RwLock`
    unsafe impl<T: ?Sized + Send> Send for RwLock<T> {}
    unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}

    impl<T> RwLock<T> {
        /// Creates a new spinlock wrapping the supplied data.
        ///
        /// May be used statically:
        ///
        /// ```
        /// use spin;
        ///
        /// static RW_LOCK: spin::RwLock<()> = spin::RwLock::new(());
        ///
        /// fn demo() {
        ///     let lock = RW_LOCK.read();
        ///     // do something with lock
        ///     drop(lock);
        /// }
        /// ```
        #[inline]
        pub const fn new(user_data: T) -> RwLock<T> {
            RwLock {
                lock: AtomicUsize::new(0),
                data: UnsafeCell::new(user_data),
            }
        }

        /// Consumes this `RwLock`, returning the underlying data.
        #[inline]
        pub fn into_inner(self) -> T {
            // We know statically that there are no outstanding references to
            // `self` so there's no need to lock.
            let RwLock { data, .. } = self;
            data.into_inner()
        }
    }

    impl<T: ?Sized> RwLock<T> {
        /// Locks this rwlock with shared read access, blocking the current thread
        /// until it can be acquired.
        ///
        /// The calling thread will be blocked until there are no more writers which
        /// hold the lock. There may be other readers currently inside the lock when
        /// this method returns. This method does not provide any guarantees with
        /// respect to the ordering of whether contentious readers or writers will
        /// acquire the lock first.
        ///
        /// Returns an RAII guard which will release this thread's shared access
        /// once it is dropped.
        ///
        /// ```
        /// let mylock = spin::RwLock::new(0);
        /// {
        ///     let mut data = mylock.read();
        ///     // The lock is now locked and the data can be read
        ///     println!("{}", *data);
        ///     // The lock is dropped
        /// }
        /// ```
        #[inline]
        pub fn read(&self) -> RwLockReadGuard<T> {
            loop {
                match self.try_read() {
                    Some(guard) => return guard,
                    None => crate::relax(),
                }
            }
        }

        /// Attempt to acquire this lock with shared read access.
        ///
        /// This function will never block and will return immediately if `read`
        /// would otherwise succeed. Returns `Some` of an RAII guard which will
        /// release the shared access of this thread when dropped, or `None` if the
        /// access could not be granted. This method does not provide any
        /// guarantees with respect to the ordering of whether contentious readers
        /// or writers will acquire the lock first.
        ///
        /// ```
        /// let mylock = spin::RwLock::new(0);
        /// {
        ///     match mylock.try_read() {
        ///         Some(data) => {
        ///             // The lock is now locked and the data can be read
        ///             println!("{}", *data);
        ///             // The lock is dropped
        ///         },
        ///         None => (), // no cigar
        ///     };
        /// }
        /// ```
        #[inline]
        pub fn try_read(&self) -> Option<RwLockReadGuard<T>> {
            let value = self.lock.fetch_add(READER, Ordering::Acquire);

            // We check the UPGRADED bit here so that new readers are prevented when an UPGRADED lock is held.
            // This helps reduce writer starvation.
            if value & (WRITER | UPGRADED) != 0 {
                // Lock is taken, undo.
                self.lock.fetch_sub(READER, Ordering::Release);
                None
            } else {
                Some(RwLockReadGuard {
                    inner: self,
                    data: unsafe { &*self.data.get() },
                })
            }
        }

        /// Return the number of readers that currently hold the lock (including upgradable readers).
        ///
        /// # Safety
        ///
        /// This function provides no synchronization guarantees and so its result should be considered 'out of date'
        /// the instant it is called. Do not use it for synchronization purposes. However, it may be useful as a heuristic.
        pub fn reader_count(&self) -> usize {
            let state = self.lock.load(Ordering::Relaxed);
            state / READER + (state & UPGRADED) / UPGRADED
        }

        /// Return the number of writers that currently hold the lock.
        ///
        /// Because [`RwLock`] guarantees exclusive mutable access, this function may only return either `0` or `1`.
        ///
        /// # Safety
        ///
        /// This function provides no synchronization guarantees and so its result should be considered 'out of date'
        /// the instant it is called. Do not use it for synchronization purposes. However, it may be useful as a heuristic.
        pub fn writer_count(&self) -> usize {
            (self.lock.load(Ordering::Relaxed) & WRITER) / WRITER
        }

        /// Force decrement the reader count.
        ///
        /// # Safety
        ///
        /// This is *extremely* unsafe if there are outstanding `RwLockReadGuard`s
        /// live, or if called more times than `read` has been called, but can be
        /// useful in FFI contexts where the caller doesn't know how to deal with
        /// RAII. The underlying atomic operation uses `Ordering::Release`.
        #[inline]
        pub unsafe fn force_read_decrement(&self) {
            debug_assert!(self.lock.load(Ordering::Relaxed) & !WRITER > 0);
            self.lock.fetch_sub(READER, Ordering::Release);
        }

        /// Force unlock exclusive write access.
        ///
        /// # Safety
        ///
        /// This is *extremely* unsafe if there are outstanding `RwLockWriteGuard`s
        /// live, or if called when there are current readers, but can be useful in
        /// FFI contexts where the caller doesn't know how to deal with RAII. The
        /// underlying atomic operation uses `Ordering::Release`.
        #[inline]
        pub unsafe fn force_write_unlock(&self) {
            debug_assert_eq!(self.lock.load(Ordering::Relaxed) & !(WRITER | UPGRADED), 0);
            self.lock.fetch_and(!(WRITER | UPGRADED), Ordering::Release);
        }

        #[inline(always)]
        fn try_write_internal(&self, strong: bool) -> Option<RwLockWriteGuard<T>> {
            if compare_exchange(
                &self.lock,
                0,
                WRITER,
                Ordering::Acquire,
                Ordering::Relaxed,
                strong,
            )
            .is_ok()
            {
                Some(RwLockWriteGuard {
                    inner: self,
                    data: unsafe { &mut *self.data.get() },
                })
            } else {
                None
            }
        }

        /// Lock this rwlock with exclusive write access, blocking the current
        /// thread until it can be acquired.
        ///
        /// This function will not return while other writers or other readers
        /// currently have access to the lock.
        ///
        /// Returns an RAII guard which will drop the write access of this rwlock
        /// when dropped.
        ///
        /// ```
        /// let mylock = spin::RwLock::new(0);
        /// {
        ///     let mut data = mylock.write();
        ///     // The lock is now locked and the data can be written
        ///     *data += 1;
        ///     // The lock is dropped
        /// }
        /// ```
        #[inline]
        pub fn write(&self) -> RwLockWriteGuard<T> {
            loop {
                match self.try_write_internal(false) {
                    Some(guard) => return guard,
                    None => crate::relax(),
                }
            }
        }

        /// Attempt to lock this rwlock with exclusive write access.
        ///
        /// This function does not ever block, and it will return `None` if a call
        /// to `write` would otherwise block. If successful, an RAII guard is
        /// returned.
        ///
        /// ```
        /// let mylock = spin::RwLock::new(0);
        /// {
        ///     match mylock.try_write() {
        ///         Some(mut data) => {
        ///             // The lock is now locked and the data can be written
        ///             *data += 1;
        ///             // The lock is implicitly dropped
        ///         },
        ///         None => (), // no cigar
        ///     };
        /// }
        /// ```
        #[inline]
        pub fn try_write(&self) -> Option<RwLockWriteGuard<T>> {
            self.try_write_internal(true)
        }

        /// Obtain a readable lock guard that can later be upgraded to a writable lock guard.
        /// Upgrades can be done through the [`RwLockUpgradableGuard::upgrade`](RwLockUpgradableGuard::upgrade) method.
        #[inline]
        pub fn upgradeable_read(&self) -> RwLockUpgradableGuard<T> {
            loop {
                match self.try_upgradeable_read() {
                    Some(guard) => return guard,
                    None => crate::relax(),
                }
            }
        }

        /// Tries to obtain an upgradeable lock guard.
        #[inline]
        pub fn try_upgradeable_read(&self) -> Option<RwLockUpgradableGuard<T>> {
            if self.lock.fetch_or(UPGRADED, Ordering::Acquire) & (WRITER | UPGRADED) == 0 {
                Some(RwLockUpgradableGuard {
                    inner: self,
                    data: unsafe { &*self.data.get() },
                })
            } else {
                // We can't unflip the UPGRADED bit back just yet as there is another upgradeable or write lock.
                // When they unlock, they will clear the bit.
                None
            }
        }

       /// 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
       ///
       /// ```
       /// let mut lock = spin::RwLock::new(0);
       /// *lock.get_mut() = 10;
       /// assert_eq!(*lock.read(), 10);
       /// ```
        pub fn get_mut(&mut self) -> &mut T {
            // We know statically that there are no other references to `self`, so
            // there's no need to lock the inner lock.
            unsafe { &mut *self.data.get() }
        }
    }

    impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLock<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            match self.try_read() {
                Some(guard) => write!(f, "RwLock {{ data: ")
                    .and_then(|()| (&*guard).fmt(f))
                    .and_then(|()| write!(f, "}}")),
                None => write!(f, "RwLock {{ <locked> }}"),
            }
        }
    }

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

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

    impl<'rwlock, T: ?Sized> RwLockReadGuard<'rwlock, T> {
        /// Leak the lock guard, yielding a reference to the underlying data.
        ///
        /// Note that this function will permanently lock the original lock for all but reading locks.
        ///
        /// ```
        /// let mylock = spin::RwLock::new(0);
        ///
        /// let data: &i32 = spin::RwLockReadGuard::leak(mylock.read());
        ///
        /// assert_eq!(*data, 0);
        /// ```
        #[inline]
        pub fn leak(this: Self) -> &'rwlock T {
            let Self { data, .. } = this;
            data
        }
    }

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

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

    impl<'rwlock, T: ?Sized> RwLockUpgradableGuard<'rwlock, T> {
        #[inline(always)]
        fn try_upgrade_internal(self, strong: bool) -> Result<RwLockWriteGuard<'rwlock, T>, Self> {
            if compare_exchange(
                &self.inner.lock,
                UPGRADED,
                WRITER,
                Ordering::Acquire,
                Ordering::Relaxed,
                strong,
            )
            .is_ok()
            {
                let inner = self.inner;

                // Forget the old guard so its destructor doesn't run (before mutably aliasing data below)
                mem::forget(self);

                // Upgrade successful
                Ok(RwLockWriteGuard {
                    inner,
                    data: unsafe { &mut *inner.data.get() },
                })
            } else {
                Err(self)
            }
        }

        /// Upgrades an upgradeable lock guard to a writable lock guard.
        ///
        /// ```
        /// let mylock = spin::RwLock::new(0);
        ///
        /// let upgradeable = mylock.upgradeable_read(); // Readable, but not yet writable
        /// let writable = upgradeable.upgrade();
        /// ```
        #[inline]
        pub fn upgrade(mut self) -> RwLockWriteGuard<'rwlock, T> {
            loop {
                self = match self.try_upgrade_internal(false) {
                    Ok(guard) => return guard,
                    Err(e) => e,
                };

                crate::relax();
            }
        }

        /// Tries to upgrade an upgradeable lock guard to a writable lock guard.
        ///
        /// ```
        /// let mylock = spin::RwLock::new(0);
        /// let upgradeable = mylock.upgradeable_read(); // Readable, but not yet writable
        ///
        /// match upgradeable.try_upgrade() {
        ///     Ok(writable) => /* upgrade successful - use writable lock guard */ (),
        ///     Err(upgradeable) => /* upgrade unsuccessful */ (),
        /// };
        /// ```
        #[inline]
        pub fn try_upgrade(self) -> Result<RwLockWriteGuard<'rwlock, T>, Self> {
            self.try_upgrade_internal(true)
        }

        #[inline]
        /// Downgrades the upgradeable lock guard to a readable, shared lock guard. Cannot fail and is guaranteed not to spin.
        ///
        /// ```
        /// let mylock = spin::RwLock::new(1);
        ///
        /// let upgradeable = mylock.upgradeable_read();
        /// assert!(mylock.try_read().is_none());
        /// assert_eq!(*upgradeable, 1);
        ///
        /// let readable = upgradeable.downgrade(); // This is guaranteed not to spin
        /// assert!(mylock.try_read().is_some());
        /// assert_eq!(*readable, 1);
        /// ```
        pub fn downgrade(self) -> RwLockReadGuard<'rwlock, T> {
            // Reserve the read guard for ourselves
            self.inner.lock.fetch_add(READER, Ordering::Acquire);

            let inner = self.inner;

            // Dropping self removes the UPGRADED bit
            mem::drop(self);

            RwLockReadGuard {
                inner,
                data: unsafe { &*inner.data.get() },
            }
        }

        /// Leak the lock guard, yielding a reference to the underlying data.
        ///
        /// Note that this function will permanently lock the original lock.
        ///
        /// ```
        /// let mylock = spin::RwLock::new(0);
        ///
        /// let data: &i32 = spin::RwLockUpgradableGuard::leak(mylock.upgradeable_read());
        ///
        /// assert_eq!(*data, 0);
        /// ```
        #[inline]
        pub fn leak(this: Self) -> &'rwlock T {
            let Self { data, .. } = this;
            data
        }
    }

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

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

    impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
        /// Downgrades the writable lock guard to a readable, shared lock guard. Cannot fail and is guaranteed not to spin.
        ///
        /// ```
        /// let mylock = spin::RwLock::new(0);
        ///
        /// let mut writable = mylock.write();
        /// *writable = 1;
        ///
        /// let readable = writable.downgrade(); // This is guaranteed not to spin
        /// # let readable_2 = mylock.try_read().unwrap();
        /// assert_eq!(*readable, 1);
        /// ```
        #[inline]
        pub fn downgrade(self) -> RwLockReadGuard<'rwlock, T> {
            // Reserve the read guard for ourselves
            self.inner.lock.fetch_add(READER, Ordering::Acquire);

            let inner = self.inner;

            // Dropping self removes the UPGRADED bit
            mem::drop(self);

            RwLockReadGuard {
                inner,
                data: unsafe { &*inner.data.get() },
            }
        }

        /// Downgrades the writable lock guard to an upgradable, shared lock guard. Cannot fail and is guaranteed not to spin.
        ///
        /// ```
        /// let mylock = spin::RwLock::new(0);
        ///
        /// let mut writable = mylock.write();
        /// *writable = 1;
        ///
        /// let readable = writable.downgrade_to_upgradeable(); // This is guaranteed not to spin
        /// assert_eq!(*readable, 1);
        /// ```
        #[inline]
        pub fn downgrade_to_upgradeable(self) -> RwLockUpgradableGuard<'rwlock, T> {
            debug_assert_eq!(self.inner.lock.load(Ordering::Acquire) & (WRITER | UPGRADED), WRITER);

            // Reserve the read guard for ourselves
            self.inner.lock.store(UPGRADED, Ordering::Release);

            let inner = self.inner;

            // Dropping self removes the UPGRADED bit
            mem::forget(self);

            RwLockUpgradableGuard {
                inner,
                data: unsafe { &*inner.data.get() },
            }
        }

        /// Leak the lock guard, yielding a mutable reference to the underlying data.
        ///
        /// Note that this function will permanently lock the original lock.
        ///
        /// ```
        /// let mylock = spin::RwLock::new(0);
        ///
        /// let data: &mut i32 = spin::RwLockWriteGuard::leak(mylock.write());
        ///
        /// *data = 1;
        /// assert_eq!(*data, 1);
        /// ```
        #[inline]
        pub fn leak(this: Self) -> &'rwlock mut T {
            let data = this.data as *mut _; // Keep it in pointer form temporarily to avoid double-aliasing
            core::mem::forget(this);
            unsafe { &mut *data }
        }
    }

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

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

    impl<'rwlock, T: ?Sized> Deref for RwLockReadGuard<'rwlock, T> {
        type Target = T;

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

    impl<'rwlock, T: ?Sized> Deref for RwLockUpgradableGuard<'rwlock, T> {
        type Target = T;

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

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

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

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

    impl<'rwlock, T: ?Sized> Drop for RwLockReadGuard<'rwlock, T> {
        fn drop(&mut self) {
            debug_assert!(self.inner.lock.load(Ordering::Relaxed) & !(WRITER | UPGRADED) > 0);
            self.inner.lock.fetch_sub(READER, Ordering::Release);
        }
    }

    impl<'rwlock, T: ?Sized> Drop for RwLockUpgradableGuard<'rwlock, T> {
        fn drop(&mut self) {
            debug_assert_eq!(
                self.inner.lock.load(Ordering::Relaxed) & (WRITER | UPGRADED),
                UPGRADED
            );
            self.inner.lock.fetch_sub(UPGRADED, Ordering::AcqRel);
        }
    }

    impl<'rwlock, T: ?Sized> Drop for RwLockWriteGuard<'rwlock, T> {
        fn drop(&mut self) {
            debug_assert_eq!(self.inner.lock.load(Ordering::Relaxed) & WRITER, WRITER);

            // Writer is responsible for clearing both WRITER and UPGRADED bits.
            // The UPGRADED bit may be set if an upgradeable lock attempts an upgrade while this lock is held.
            self.inner.lock.fetch_and(!(WRITER | UPGRADED), Ordering::Release);
        }
    }

    #[inline(always)]
    fn compare_exchange(
        atomic: &AtomicUsize,
        current: usize,
        new: usize,
        success: Ordering,
        failure: Ordering,
        strong: bool,
    ) -> Result<usize, usize> {
        if strong {
            atomic.compare_exchange(current, new, success, failure)
        } else {
            atomic.compare_exchange_weak(current, new, success, failure)
        }
    }

    #[cfg(feature = "lock_api1")]
    unsafe impl lock_api::RawRwLock for RwLock<()> {
        type GuardMarker = lock_api::GuardSend;

        const INIT: Self = Self::new(());

        #[inline(always)]
        fn lock_exclusive(&self) {
            // Prevent guard destructor running
            core::mem::forget(self.write());
        }

        #[inline(always)]
        fn try_lock_exclusive(&self) -> bool {
            // Prevent guard destructor running
            self.try_write().map(|g| core::mem::forget(g)).is_some()
        }

        #[inline(always)]
        unsafe fn unlock_exclusive(&self) {
            drop(RwLockWriteGuard {
                inner: self,
                data: &mut (),
            });
        }

        #[inline(always)]
        fn lock_shared(&self) {
            // Prevent guard destructor running
            core::mem::forget(self.read());
        }

        #[inline(always)]
        fn try_lock_shared(&self) -> bool {
            // Prevent guard destructor running
            self.try_read().map(|g| core::mem::forget(g)).is_some()
        }

        #[inline(always)]
        unsafe fn unlock_shared(&self) {
            drop(RwLockReadGuard {
                inner: self,
                data: &(),
            });
        }

        #[inline(always)]
        fn is_locked(&self) -> bool {
            self.lock.load(Ordering::Relaxed) != 0
        }
    }

    #[cfg(feature = "lock_api1")]
    unsafe impl lock_api::RawRwLockUpgrade for RwLock<()> {
        #[inline(always)]
        fn lock_upgradable(&self) {
            // Prevent guard destructor running
            core::mem::forget(self.upgradeable_read());
        }

        #[inline(always)]
        fn try_lock_upgradable(&self) -> bool {
            // Prevent guard destructor running
            self.try_upgradeable_read().map(|g| core::mem::forget(g)).is_some()
        }

        #[inline(always)]
        unsafe fn unlock_upgradable(&self) {
            drop(RwLockUpgradableGuard {
                inner: self,
                data: &(),
            });
        }

        #[inline(always)]
        unsafe fn upgrade(&self) {
            let tmp_guard = RwLockUpgradableGuard {
                inner: self,
                data: &(),
            };
            core::mem::forget(tmp_guard.upgrade());
        }

        #[inline(always)]
        unsafe fn try_upgrade(&self) -> bool {
            let tmp_guard = RwLockUpgradableGuard {
                inner: self,
                data: &(),
            };
            tmp_guard.try_upgrade().map(|g| core::mem::forget(g)).is_ok()
        }
    }

    #[cfg(feature = "lock_api1")]
    unsafe impl lock_api::RawRwLockDowngrade for RwLock<()> {
        unsafe fn downgrade(&self) {
            let tmp_guard = RwLockWriteGuard {
                inner: self,
                data: &mut (),
            };
            core::mem::forget(tmp_guard.downgrade());
        }
    }

    #[cfg(feature = "lock_api1")]
    unsafe impl lock_api::RawRwLockUpgradeDowngrade for RwLock<()> {
        unsafe fn downgrade_upgradable(&self) {
            let tmp_guard = RwLockUpgradableGuard {
                inner: self,
                data: &(),
            };
            core::mem::forget(tmp_guard.downgrade());
        }

        unsafe fn downgrade_to_upgradable(&self) {
            let tmp_guard = RwLockWriteGuard {
                inner: self,
                data: &mut (),
            };
            core::mem::forget(tmp_guard.downgrade_to_upgradeable());
        }
    }

    #[cfg(test)]
    mod tests {
        use std::prelude::v1::*;

        use std::sync::atomic::{AtomicUsize, Ordering};
        use std::sync::mpsc::channel;
        use std::sync::Arc;
        use std::thread;

        use super::*;

        #[derive(Eq, PartialEq, Debug)]
        struct NonCopy(i32);

        #[test]
        fn smoke() {
            let l = RwLock::new(());
            drop(l.read());
            drop(l.write());
            drop((l.read(), l.read()));
            drop(l.write());
        }

        // TODO: needs RNG
        //#[test]
        //fn frob() {
        //    static R: RwLock = RwLock::new();
        //    const N: usize = 10;
        //    const M: usize = 1000;
        //
        //    let (tx, rx) = channel::<()>();
        //    for _ in 0..N {
        //        let tx = tx.clone();
        //        thread::spawn(move|| {
        //            let mut rng = rand::thread_rng();
        //            for _ in 0..M {
        //                if rng.gen_weighted_bool(N) {
        //                    drop(R.write());
        //                } else {
        //                    drop(R.read());
        //                }
        //            }
        //            drop(tx);
        //        });
        //    }
        //    drop(tx);
        //    let _ = rx.recv();
        //    unsafe { R.destroy(); }
        //}

        #[test]
        fn test_rw_arc() {
            let arc = Arc::new(RwLock::new(0));
            let arc2 = arc.clone();
            let (tx, rx) = channel();

            thread::spawn(move || {
                let mut lock = arc2.write();
                for _ in 0..10 {
                    let tmp = *lock;
                    *lock = -1;
                    thread::yield_now();
                    *lock = tmp + 1;
                }
                tx.send(()).unwrap();
            });

            // Readers try to catch the writer in the act
            let mut children = Vec::new();
            for _ in 0..5 {
                let arc3 = arc.clone();
                children.push(thread::spawn(move || {
                    let lock = arc3.read();
                    assert!(*lock >= 0);
                }));
            }

            // Wait for children to pass their asserts
            for r in children {
                assert!(r.join().is_ok());
            }

            // Wait for writer to finish
            rx.recv().unwrap();
            let lock = arc.read();
            assert_eq!(*lock, 10);
        }

        #[test]
        fn test_rw_access_in_unwind() {
            let arc = Arc::new(RwLock::new(1));
            let arc2 = arc.clone();
            let _ = thread::spawn(move || -> () {
                struct Unwinder {
                    i: Arc<RwLock<isize>>,
                }
                impl Drop for Unwinder {
                    fn drop(&mut self) {
                        let mut lock = self.i.write();
                        *lock += 1;
                    }
                }
                let _u = Unwinder { i: arc2 };
                panic!();
            })
            .join();
            let lock = arc.read();
            assert_eq!(*lock, 2);
        }

        #[test]
        fn test_rwlock_unsized() {
            let rw: &RwLock<[i32]> = &RwLock::new([1, 2, 3]);
            {
                let b = &mut *rw.write();
                b[0] = 4;
                b[2] = 5;
            }
            let comp: &[i32] = &[4, 2, 5];
            assert_eq!(&*rw.read(), comp);
        }

        #[test]
        fn test_rwlock_try_write() {
            use std::mem::drop;

            let lock = RwLock::new(0isize);
            let read_guard = lock.read();

            let write_result = lock.try_write();
            match write_result {
                None => (),
                Some(_) => assert!(
                    false,
                    "try_write should not succeed while read_guard is in scope"
                ),
            }

            drop(read_guard);
        }

        #[test]
        fn test_rw_try_read() {
            let m = RwLock::new(0);
            mem::forget(m.write());
            assert!(m.try_read().is_none());
        }

        #[test]
        fn test_into_inner() {
            let m = RwLock::new(NonCopy(10));
            assert_eq!(m.into_inner(), NonCopy(10));
        }

        #[test]
        fn test_into_inner_drop() {
            struct Foo(Arc<AtomicUsize>);
            impl Drop for Foo {
                fn drop(&mut self) {
                    self.0.fetch_add(1, Ordering::SeqCst);
                }
            }
            let num_drops = Arc::new(AtomicUsize::new(0));
            let m = RwLock::new(Foo(num_drops.clone()));
            assert_eq!(num_drops.load(Ordering::SeqCst), 0);
            {
                let _inner = m.into_inner();
                assert_eq!(num_drops.load(Ordering::SeqCst), 0);
            }
            assert_eq!(num_drops.load(Ordering::SeqCst), 1);
        }

        #[test]
        fn test_force_read_decrement() {
            let m = RwLock::new(());
            ::std::mem::forget(m.read());
            ::std::mem::forget(m.read());
            ::std::mem::forget(m.read());
            assert!(m.try_write().is_none());
            unsafe {
                m.force_read_decrement();
                m.force_read_decrement();
            }
            assert!(m.try_write().is_none());
            unsafe {
                m.force_read_decrement();
            }
            assert!(m.try_write().is_some());
        }

        #[test]
        fn test_force_write_unlock() {
            let m = RwLock::new(());
            ::std::mem::forget(m.write());
            assert!(m.try_read().is_none());
            unsafe {
                m.force_write_unlock();
            }
            assert!(m.try_read().is_some());
        }

        #[test]
        fn test_upgrade_downgrade() {
            let m = RwLock::new(());
            {
                let _r = m.read();
                let upg = m.try_upgradeable_read().unwrap();
                assert!(m.try_read().is_none());
                assert!(m.try_write().is_none());
                assert!(upg.try_upgrade().is_err());
            }
            {
                let w = m.write();
                assert!(m.try_upgradeable_read().is_none());
                let _r = w.downgrade();
                assert!(m.try_upgradeable_read().is_some());
                assert!(m.try_read().is_some());
                assert!(m.try_write().is_none());
            }
            {
                let _u = m.upgradeable_read();
                assert!(m.try_upgradeable_read().is_none());
            }

            assert!(m.try_upgradeable_read().unwrap().try_upgrade().is_ok());
        }
    }