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    101

    use crate::sys::mutex as imp;

    /// An OS-based mutual exclusion lock.
    ///
    /// This is the thinnest cross-platform wrapper around OS mutexes. All usage of
    /// this mutex is unsafe and it is recommended to instead use the safe wrapper
    /// at the top level of the crate instead of this type.
    pub struct Mutex(imp::Mutex);

    unsafe impl Sync for Mutex {}

    impl Mutex {
        /// Creates a new mutex for use.
        ///
        /// Behavior is undefined if the mutex is moved after it is
        /// first used with any of the functions below.
        /// Also, until `init` is called, behavior is undefined if this
        /// mutex is ever used reentrantly, i.e., `raw_lock` or `try_lock`
        /// are called by the thread currently holding the lock.
        #[rustc_const_stable(feature = "const_sys_mutex_new", since = "1.0.0")]
        pub const fn new() -> Mutex {
            Mutex(imp::Mutex::new())
        }

        /// Prepare the mutex for use.
        ///
        /// This should be called once the mutex is at a stable memory address.
        /// If called, this must be the very first thing that happens to the mutex.
        /// Calling it in parallel with or after any operation (including another
        /// `init()`) is undefined behavior.
        #[inline]
        pub unsafe fn init(&mut self) {
            self.0.init()
        }

        /// Locks the mutex blocking the current thread until it is available.
        ///
        /// Behavior is undefined if the mutex has been moved between this and any
        /// previous function call.
        #[inline]
        pub unsafe fn raw_lock(&self) {
            self.0.lock()
        }

        /// Calls raw_lock() and then returns an RAII guard to guarantee the mutex
        /// will be unlocked.
        #[inline]
        pub unsafe fn lock(&self) -> MutexGuard<'_> {
            self.raw_lock();
            MutexGuard(&self.0)
        }

        /// Attempts to lock the mutex without blocking, returning whether it was
        /// successfully acquired or not.
        ///
        /// Behavior is undefined if the mutex has been moved between this and any
        /// previous function call.
        #[inline]
        pub unsafe fn try_lock(&self) -> bool {
            self.0.try_lock()
        }

        /// Unlocks the mutex.
        ///
        /// Behavior is undefined if the current thread does not actually hold the
        /// mutex.
        ///
        /// Consider switching from the pair of raw_lock() and raw_unlock() to
        /// lock() whenever possible.
        #[inline]
        pub unsafe fn raw_unlock(&self) {
            self.0.unlock()
        }

        /// Deallocates all resources associated with this mutex.
        ///
        /// Behavior is undefined if there are current or will be future users of
        /// this mutex.
        #[inline]
        pub unsafe fn destroy(&self) {
            self.0.destroy()
        }
    }

    // not meant to be exported to the outside world, just the containing module
    pub fn raw(mutex: &Mutex) -> &imp::Mutex {
        &mutex.0
    }

    #[must_use]
    /// A simple RAII utility for the above Mutex without the poisoning semantics.
    pub struct MutexGuard<'a>(&'a imp::Mutex);

    impl Drop for MutexGuard<'_> {
        #[inline]
        fn drop(&mut self) {
            unsafe {
                self.0.unlock();
            }
        }
    }