# remutex.rs - source
Source of the Rust file `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/lock_api-0.4.2/src/remutex.rs`.
Keywords: rust, rustlang, rust-lang

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    // Copyright 2018 Amanieu d'Antras
    //
    // Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
    // http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
    // http://opensource.org/licenses/MIT>, at your option. This file may not be
    // copied, modified, or distributed except according to those terms.

    use crate::{
        mutex::{RawMutex, RawMutexFair, RawMutexTimed},
        GuardNoSend,
    };
    use core::{
        cell::{Cell, UnsafeCell},
        fmt,
        marker::PhantomData,
        mem,
        num::NonZeroUsize,
        ops::Deref,
        sync::atomic::{AtomicUsize, Ordering},
    };

    #[cfg(feature = "owning_ref")]
    use owning_ref::StableAddress;

    #[cfg(feature = "serde")]
    use serde::{Deserialize, Deserializer, Serialize, Serializer};

    /// Helper trait which returns a non-zero thread ID.
    ///
    /// The simplest way to implement this trait is to return the address of a
    /// thread-local variable.
    ///
    /// # Safety
    ///
    /// Implementations of this trait must ensure that no two active threads share
    /// the same thread ID. However the ID of a thread that has exited can be
    /// re-used since that thread is no longer active.
    pub unsafe trait GetThreadId {
        /// Initial value.
        // A “non-constant” const item is a legacy way to supply an initialized value to downstream
        // static items. Can hopefully be replaced with `const fn new() -> Self` at some point.
        #[allow(clippy::declare_interior_mutable_const)]
        const INIT: Self;

        /// Returns a non-zero thread ID which identifies the current thread of
        /// execution.
        fn nonzero_thread_id(&self) -> NonZeroUsize;
    }

    /// A raw mutex type that wraps another raw mutex to provide reentrancy.
    ///
    /// Although this has the same methods as the [`RawMutex`] trait, it does
    /// not implement it, and should not be used in the same way, since this
    /// mutex can successfully acquire a lock multiple times in the same thread.
    /// Only use this when you know you want a raw mutex that can be locked
    /// reentrantly; you probably want [`ReentrantMutex`] instead.
    ///
    /// [`RawMutex`]: trait.RawMutex.html
    /// [`ReentrantMutex`]: struct.ReentrantMutex.html
    pub struct RawReentrantMutex<R, G> {
        owner: AtomicUsize,
        lock_count: Cell<usize>,
        mutex: R,
        get_thread_id: G,
    }

    unsafe impl<R: RawMutex + Send, G: GetThreadId + Send> Send for RawReentrantMutex<R, G> {}
    unsafe impl<R: RawMutex + Sync, G: GetThreadId + Sync> Sync for RawReentrantMutex<R, G> {}

    impl<R: RawMutex, G: GetThreadId> RawReentrantMutex<R, G> {
        /// Initial value for an unlocked mutex.
        #[allow(clippy::declare_interior_mutable_const)]
        pub const INIT: Self = RawReentrantMutex {
            owner: AtomicUsize::new(0),
            lock_count: Cell::new(0),
            mutex: R::INIT,
            get_thread_id: G::INIT,
        };

        #[inline]
        fn lock_internal<F: FnOnce() -> bool>(&self, try_lock: F) -> bool {
            let id = self.get_thread_id.nonzero_thread_id().get();
            if self.owner.load(Ordering::Relaxed) == id {
                self.lock_count.set(
                    self.lock_count
                        .get()
                        .checked_add(1)
                        .expect("ReentrantMutex lock count overflow"),
                );
            } else {
                if !try_lock() {
                    return false;
                }
                self.owner.store(id, Ordering::Relaxed);
                debug_assert_eq!(self.lock_count.get(), 0);
                self.lock_count.set(1);
            }
            true
        }

        /// Acquires this mutex, blocking if it's held by another thread.
        #[inline]
        pub fn lock(&self) {
            self.lock_internal(|| {
                self.mutex.lock();
                true
            });
        }

        /// Attempts to acquire this mutex without blocking. Returns `true`
        /// if the lock was successfully acquired and `false` otherwise.
        #[inline]
        pub fn try_lock(&self) -> bool {
            self.lock_internal(|| self.mutex.try_lock())
        }

        /// Unlocks this mutex. The inner mutex may not be unlocked if
        /// this mutex was acquired previously in the current thread.
        ///
        /// # Safety
        ///
        /// This method may only be called if the mutex is held by the current thread.
        #[inline]
        pub unsafe fn unlock(&self) {
            let lock_count = self.lock_count.get() - 1;
            self.lock_count.set(lock_count);
            if lock_count == 0 {
                self.owner.store(0, Ordering::Relaxed);
                self.mutex.unlock();
            }
        }

        /// Checks whether the mutex is currently locked.
        #[inline]
        pub fn is_locked(&self) -> bool {
            self.mutex.is_locked()
        }
    }

    impl<R: RawMutexFair, G: GetThreadId> RawReentrantMutex<R, G> {
        /// Unlocks this mutex using a fair unlock protocol. The inner mutex
        /// may not be unlocked if this mutex was acquired previously in the
        /// current thread.
        ///
        /// # Safety
        ///
        /// This method may only be called if the mutex is held by the current thread.
        #[inline]
        pub unsafe fn unlock_fair(&self) {
            let lock_count = self.lock_count.get() - 1;
            self.lock_count.set(lock_count);
            if lock_count == 0 {
                self.owner.store(0, Ordering::Relaxed);
                self.mutex.unlock_fair();
            }
        }

        /// Temporarily yields the mutex to a waiting thread if there is one.
        ///
        /// This method is functionally equivalent to calling `unlock_fair` followed
        /// by `lock`, however it can be much more efficient in the case where there
        /// are no waiting threads.
        ///
        /// # Safety
        ///
        /// This method may only be called if the mutex is held by the current thread.
        #[inline]
        pub unsafe fn bump(&self) {
            if self.lock_count.get() == 1 {
                let id = self.owner.load(Ordering::Relaxed);
                self.owner.store(0, Ordering::Relaxed);
                self.mutex.bump();
                self.owner.store(id, Ordering::Relaxed);
            }
        }
    }

    impl<R: RawMutexTimed, G: GetThreadId> RawReentrantMutex<R, G> {
        /// Attempts to acquire this lock until a timeout is reached.
        #[inline]
        pub fn try_lock_until(&self, timeout: R::Instant) -> bool {
            self.lock_internal(|| self.mutex.try_lock_until(timeout))
        }

        /// Attempts to acquire this lock until a timeout is reached.
        #[inline]
        pub fn try_lock_for(&self, timeout: R::Duration) -> bool {
            self.lock_internal(|| self.mutex.try_lock_for(timeout))
        }
    }

    /// A mutex which can be recursively locked by a single thread.
    ///
    /// This type is identical to `Mutex` except for the following points:
    ///
    /// - Locking multiple times from the same thread will work correctly instead of
    ///   deadlocking.
    /// - `ReentrantMutexGuard` does not give mutable references to the locked data.
    ///   Use a `RefCell` if you need this.
    ///
    /// See [`Mutex`](struct.Mutex.html) for more details about the underlying mutex
    /// primitive.
    pub struct ReentrantMutex<R, G, T: ?Sized> {
        raw: RawReentrantMutex<R, G>,
        data: UnsafeCell<T>,
    }

    unsafe impl<R: RawMutex + Send, G: GetThreadId + Send, T: ?Sized + Send> Send
        for ReentrantMutex<R, G, T>
    {
    }
    unsafe impl<R: RawMutex + Sync, G: GetThreadId + Sync, T: ?Sized + Send> Sync
        for ReentrantMutex<R, G, T>
    {
    }

    impl<R: RawMutex, G: GetThreadId, T> ReentrantMutex<R, G, T> {
        /// Creates a new reentrant mutex in an unlocked state ready for use.
        #[cfg(feature = "nightly")]
        #[inline]
        pub const fn new(val: T) -> ReentrantMutex<R, G, T> {
            ReentrantMutex {
                data: UnsafeCell::new(val),
                raw: RawReentrantMutex {
                    owner: AtomicUsize::new(0),
                    lock_count: Cell::new(0),
                    mutex: R::INIT,
                    get_thread_id: G::INIT,
                },
            }
        }

        /// Creates a new reentrant mutex in an unlocked state ready for use.
        #[cfg(not(feature = "nightly"))]
        #[inline]
        pub fn new(val: T) -> ReentrantMutex<R, G, T> {
            ReentrantMutex {
                data: UnsafeCell::new(val),
                raw: RawReentrantMutex {
                    owner: AtomicUsize::new(0),
                    lock_count: Cell::new(0),
                    mutex: R::INIT,
                    get_thread_id: G::INIT,
                },
            }
        }

        /// Consumes this mutex, returning the underlying data.
        #[inline]
        pub fn into_inner(self) -> T {
            self.data.into_inner()
        }
    }

    impl<R, G, T> ReentrantMutex<R, G, T> {
        /// Creates a new reentrant mutex based on a pre-existing raw mutex and a
        /// helper to get the thread ID.
        ///
        /// This allows creating a reentrant mutex in a constant context on stable
        /// Rust.
        #[inline]
        pub const fn const_new(raw_mutex: R, get_thread_id: G, val: T) -> ReentrantMutex<R, G, T> {
            ReentrantMutex {
                data: UnsafeCell::new(val),
                raw: RawReentrantMutex {
                    owner: AtomicUsize::new(0),
                    lock_count: Cell::new(0),
                    mutex: raw_mutex,
                    get_thread_id,
                },
            }
        }
    }

    impl<R: RawMutex, G: GetThreadId, T: ?Sized> ReentrantMutex<R, G, T> {
        /// # Safety
        ///
        /// The lock must be held when calling this method.
        #[inline]
        unsafe fn guard(&self) -> ReentrantMutexGuard<'_, R, G, T> {
            ReentrantMutexGuard {
                remutex: &self,
                marker: PhantomData,
            }
        }

        /// Acquires a reentrant mutex, blocking the current thread until it is able
        /// to do so.
        ///
        /// If the mutex is held by another thread then this function will block the
        /// local thread until it is available to acquire the mutex. If the mutex is
        /// already held by the current thread then this function will increment the
        /// lock reference count and return immediately. Upon returning,
        /// the thread is the only thread with the mutex held. An RAII guard is
        /// returned to allow scoped unlock of the lock. When the guard goes out of
        /// scope, the mutex will be unlocked.
        #[inline]
        pub fn lock(&self) -> ReentrantMutexGuard<'_, R, G, T> {
            self.raw.lock();
            // SAFETY: The lock is held, as required.
            unsafe { self.guard() }
        }

        /// Attempts to acquire this lock.
        ///
        /// If the lock could not be acquired at this time, then `None` is returned.
        /// Otherwise, an RAII guard is returned. The lock will be unlocked when the
        /// guard is dropped.
        ///
        /// This function does not block.
        #[inline]
        pub fn try_lock(&self) -> Option<ReentrantMutexGuard<'_, R, G, T>> {
            if self.raw.try_lock() {
                // SAFETY: The lock is held, as required.
                Some(unsafe { self.guard() })
            } else {
                None
            }
        }

        /// Returns a mutable reference to the underlying data.
        ///
        /// Since this call borrows the `ReentrantMutex` mutably, no actual locking needs to
        /// take place---the mutable borrow statically guarantees no locks exist.
        #[inline]
        pub fn get_mut(&mut self) -> &mut T {
            unsafe { &mut *self.data.get() }
        }

        /// Checks whether the mutex is currently locked.
        #[inline]
        pub fn is_locked(&self) -> bool {
            self.raw.is_locked()
        }

        /// Forcibly unlocks the mutex.
        ///
        /// This is useful when combined with `mem::forget` to hold a lock without
        /// the need to maintain a `ReentrantMutexGuard` object alive, for example when
        /// dealing with FFI.
        ///
        /// # Safety
        ///
        /// This method must only be called if the current thread logically owns a
        /// `ReentrantMutexGuard` but that guard has be discarded using `mem::forget`.
        /// Behavior is undefined if a mutex is unlocked when not locked.
        #[inline]
        pub unsafe fn force_unlock(&self) {
            self.raw.unlock();
        }

        /// Returns the underlying raw mutex object.
        ///
        /// Note that you will most likely need to import the `RawMutex` trait from
        /// `lock_api` to be able to call functions on the raw mutex.
        ///
        /// # Safety
        ///
        /// This method is unsafe because it allows unlocking a mutex while
        /// still holding a reference to a `ReentrantMutexGuard`.
        #[inline]
        pub unsafe fn raw(&self) -> &R {
            &self.raw.mutex
        }

        /// Returns a raw pointer to the underlying data.
        ///
        /// This is useful when combined with `mem::forget` to hold a lock without
        /// the need to maintain a `ReentrantMutexGuard` object alive, for example
        /// when dealing with FFI.
        ///
        /// # Safety
        ///
        /// You must ensure that there are no data races when dereferencing the
        /// returned pointer, for example if the current thread logically owns a
        /// `ReentrantMutexGuard` but that guard has been discarded using
        /// `mem::forget`.
        #[inline]
        pub fn data_ptr(&self) -> *mut T {
            self.data.get()
        }
    }

    impl<R: RawMutexFair, G: GetThreadId, T: ?Sized> ReentrantMutex<R, G, T> {
        /// Forcibly unlocks the mutex using a fair unlock protocol.
        ///
        /// This is useful when combined with `mem::forget` to hold a lock without
        /// the need to maintain a `ReentrantMutexGuard` object alive, for example when
        /// dealing with FFI.
        ///
        /// # Safety
        ///
        /// This method must only be called if the current thread logically owns a
        /// `ReentrantMutexGuard` but that guard has be discarded using `mem::forget`.
        /// Behavior is undefined if a mutex is unlocked when not locked.
        #[inline]
        pub unsafe fn force_unlock_fair(&self) {
            self.raw.unlock_fair();
        }
    }

    impl<R: RawMutexTimed, G: GetThreadId, T: ?Sized> ReentrantMutex<R, G, T> {
        /// Attempts to acquire this lock until a timeout is reached.
        ///
        /// If the lock could not be acquired before the timeout expired, then
        /// `None` is returned. Otherwise, an RAII guard is returned. The lock will
        /// be unlocked when the guard is dropped.
        #[inline]
        pub fn try_lock_for(&self, timeout: R::Duration) -> Option<ReentrantMutexGuard<'_, R, G, T>> {
            if self.raw.try_lock_for(timeout) {
                // SAFETY: The lock is held, as required.
                Some(unsafe { self.guard() })
            } else {
                None
            }
        }

        /// Attempts to acquire this lock until a timeout is reached.
        ///
        /// If the lock could not be acquired before the timeout expired, then
        /// `None` is returned. Otherwise, an RAII guard is returned. The lock will
        /// be unlocked when the guard is dropped.
        #[inline]
        pub fn try_lock_until(&self, timeout: R::Instant) -> Option<ReentrantMutexGuard<'_, R, G, T>> {
            if self.raw.try_lock_until(timeout) {
                // SAFETY: The lock is held, as required.
                Some(unsafe { self.guard() })
            } else {
                None
            }
        }
    }

    impl<R: RawMutex, G: GetThreadId, T: ?Sized + Default> Default for ReentrantMutex<R, G, T> {
        #[inline]
        fn default() -> ReentrantMutex<R, G, T> {
            ReentrantMutex::new(Default::default())
        }
    }

    impl<R: RawMutex, G: GetThreadId, T> From<T> for ReentrantMutex<R, G, T> {
        #[inline]
        fn from(t: T) -> ReentrantMutex<R, G, T> {
            ReentrantMutex::new(t)
        }
    }

    impl<R: RawMutex, G: GetThreadId, T: ?Sized + fmt::Debug> fmt::Debug for ReentrantMutex<R, G, T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            match self.try_lock() {
                Some(guard) => f
                    .debug_struct("ReentrantMutex")
                    .field("data", &&*guard)
                    .finish(),
                None => {
                    struct LockedPlaceholder;
                    impl fmt::Debug for LockedPlaceholder {
                        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                            f.write_str("<locked>")
                        }
                    }

                    f.debug_struct("ReentrantMutex")
                        .field("data", &LockedPlaceholder)
                        .finish()
                }
            }
        }
    }

    // Copied and modified from serde
    #[cfg(feature = "serde")]
    impl<R, G, T> Serialize for ReentrantMutex<R, G, T>
    where
        R: RawMutex,
        G: GetThreadId,
        T: Serialize + ?Sized,
    {
        fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
        where
            S: Serializer,
        {
            self.lock().serialize(serializer)
        }
    }

    #[cfg(feature = "serde")]
    impl<'de, R, G, T> Deserialize<'de> for ReentrantMutex<R, G, T>
    where
        R: RawMutex,
        G: GetThreadId,
        T: Deserialize<'de> + ?Sized,
    {
        fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
        where
            D: Deserializer<'de>,
        {
            Deserialize::deserialize(deserializer).map(ReentrantMutex::new)
        }
    }

    /// An RAII implementation of a "scoped lock" of a reentrant mutex. When this structure
    /// is dropped (falls out of scope), the lock will be unlocked.
    ///
    /// The data protected by the mutex can be accessed through this guard via its
    /// `Deref` implementation.
    #[must_use = "if unused the ReentrantMutex will immediately unlock"]
    pub struct ReentrantMutexGuard<'a, R: RawMutex, G: GetThreadId, T: ?Sized> {
        remutex: &'a ReentrantMutex<R, G, T>,
        marker: PhantomData<(&'a T, GuardNoSend)>,
    }

    unsafe impl<'a, R: RawMutex + Sync + 'a, G: GetThreadId + Sync + 'a, T: ?Sized + Sync + 'a> Sync
        for ReentrantMutexGuard<'a, R, G, T>
    {
    }

    impl<'a, R: RawMutex + 'a, G: GetThreadId + 'a, T: ?Sized + 'a> ReentrantMutexGuard<'a, R, G, T> {
        /// Returns a reference to the original `ReentrantMutex` object.
        pub fn remutex(s: &Self) -> &'a ReentrantMutex<R, G, T> {
            s.remutex
        }

        /// Makes a new `MappedReentrantMutexGuard` for a component of the locked data.
        ///
        /// This operation cannot fail as the `ReentrantMutexGuard` passed
        /// in already locked the mutex.
        ///
        /// This is an associated function that needs to be
        /// used as `ReentrantMutexGuard::map(...)`. A method would interfere with methods of
        /// the same name on the contents of the locked data.
        #[inline]
        pub fn map<U: ?Sized, F>(s: Self, f: F) -> MappedReentrantMutexGuard<'a, R, G, U>
        where
            F: FnOnce(&T) -> &U,
        {
            let raw = &s.remutex.raw;
            let data = f(unsafe { &*s.remutex.data.get() });
            mem::forget(s);
            MappedReentrantMutexGuard {
                raw,
                data,
                marker: PhantomData,
            }
        }

        /// Attempts to make  a new `MappedReentrantMutexGuard` for a component of the
        /// locked data. The original guard is return if the closure returns `None`.
        ///
        /// This operation cannot fail as the `ReentrantMutexGuard` passed
        /// in already locked the mutex.
        ///
        /// This is an associated function that needs to be
        /// used as `ReentrantMutexGuard::map(...)`. A method would interfere with methods of
        /// the same name on the contents of the locked data.
        #[inline]
        pub fn try_map<U: ?Sized, F>(
            s: Self,
            f: F,
        ) -> Result<MappedReentrantMutexGuard<'a, R, G, U>, Self>
        where
            F: FnOnce(&mut T) -> Option<&mut U>,
        {
            let raw = &s.remutex.raw;
            let data = match f(unsafe { &mut *s.remutex.data.get() }) {
                Some(data) => data,
                None => return Err(s),
            };
            mem::forget(s);
            Ok(MappedReentrantMutexGuard {
                raw,
                data,
                marker: PhantomData,
            })
        }

        /// Temporarily unlocks the mutex to execute the given function.
        ///
        /// This is safe because `&mut` guarantees that there exist no other
        /// references to the data protected by the mutex.
        #[inline]
        pub fn unlocked<F, U>(s: &mut Self, f: F) -> U
        where
            F: FnOnce() -> U,
        {
            // Safety: A ReentrantMutexGuard always holds the lock.
            unsafe {
                s.remutex.raw.unlock();
            }
            defer!(s.remutex.raw.lock());
            f()
        }
    }

    impl<'a, R: RawMutexFair + 'a, G: GetThreadId + 'a, T: ?Sized + 'a>
        ReentrantMutexGuard<'a, R, G, T>
    {
        /// Unlocks the mutex using a fair unlock protocol.
        ///
        /// By default, mutexes are unfair and allow the current thread to re-lock
        /// the mutex before another has the chance to acquire the lock, even if
        /// that thread has been blocked on the mutex for a long time. This is the
        /// default because it allows much higher throughput as it avoids forcing a
        /// context switch on every mutex unlock. This can result in one thread
        /// acquiring a mutex many more times than other threads.
        ///
        /// However in some cases it can be beneficial to ensure fairness by forcing
        /// the lock to pass on to a waiting thread if there is one. This is done by
        /// using this method instead of dropping the `ReentrantMutexGuard` normally.
        #[inline]
        pub fn unlock_fair(s: Self) {
            // Safety: A ReentrantMutexGuard always holds the lock
            unsafe {
                s.remutex.raw.unlock_fair();
            }
            mem::forget(s);
        }

        /// Temporarily unlocks the mutex to execute the given function.
        ///
        /// The mutex is unlocked a fair unlock protocol.
        ///
        /// This is safe because `&mut` guarantees that there exist no other
        /// references to the data protected by the mutex.
        #[inline]
        pub fn unlocked_fair<F, U>(s: &mut Self, f: F) -> U
        where
            F: FnOnce() -> U,
        {
            // Safety: A ReentrantMutexGuard always holds the lock
            unsafe {
                s.remutex.raw.unlock_fair();
            }
            defer!(s.remutex.raw.lock());
            f()
        }

        /// Temporarily yields the mutex to a waiting thread if there is one.
        ///
        /// This method is functionally equivalent to calling `unlock_fair` followed
        /// by `lock`, however it can be much more efficient in the case where there
        /// are no waiting threads.
        #[inline]
        pub fn bump(s: &mut Self) {
            // Safety: A ReentrantMutexGuard always holds the lock
            unsafe {
                s.remutex.raw.bump();
            }
        }
    }

    impl<'a, R: RawMutex + 'a, G: GetThreadId + 'a, T: ?Sized + 'a> Deref
        for ReentrantMutexGuard<'a, R, G, T>
    {
        type Target = T;
        #[inline]
        fn deref(&self) -> &T {
            unsafe { &*self.remutex.data.get() }
        }
    }

    impl<'a, R: RawMutex + 'a, G: GetThreadId + 'a, T: ?Sized + 'a> Drop
        for ReentrantMutexGuard<'a, R, G, T>
    {
        #[inline]
        fn drop(&mut self) {
            // Safety: A ReentrantMutexGuard always holds the lock.
            unsafe {
                self.remutex.raw.unlock();
            }
        }
    }

    impl<'a, R: RawMutex + 'a, G: GetThreadId + 'a, T: fmt::Debug + ?Sized + 'a> fmt::Debug
        for ReentrantMutexGuard<'a, R, G, T>
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Debug::fmt(&**self, f)
        }
    }

    impl<'a, R: RawMutex + 'a, G: GetThreadId + 'a, T: fmt::Display + ?Sized + 'a> fmt::Display
        for ReentrantMutexGuard<'a, R, G, T>
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            (**self).fmt(f)
        }
    }

    #[cfg(feature = "owning_ref")]
    unsafe impl<'a, R: RawMutex + 'a, G: GetThreadId + 'a, T: ?Sized + 'a> StableAddress
        for ReentrantMutexGuard<'a, R, G, T>
    {
    }

    /// An RAII mutex guard returned by `ReentrantMutexGuard::map`, which can point to a
    /// subfield of the protected data.
    ///
    /// The main difference between `MappedReentrantMutexGuard` and `ReentrantMutexGuard` is that the
    /// former doesn't support temporarily unlocking and re-locking, since that
    /// could introduce soundness issues if the locked object is modified by another
    /// thread.
    #[must_use = "if unused the ReentrantMutex will immediately unlock"]
    pub struct MappedReentrantMutexGuard<'a, R: RawMutex, G: GetThreadId, T: ?Sized> {
        raw: &'a RawReentrantMutex<R, G>,
        data: *const T,
        marker: PhantomData<&'a T>,
    }

    unsafe impl<'a, R: RawMutex + Sync + 'a, G: GetThreadId + Sync + 'a, T: ?Sized + Sync + 'a> Sync
        for MappedReentrantMutexGuard<'a, R, G, T>
    {
    }

    impl<'a, R: RawMutex + 'a, G: GetThreadId + 'a, T: ?Sized + 'a>
        MappedReentrantMutexGuard<'a, R, G, T>
    {
        /// Makes a new `MappedReentrantMutexGuard` for a component of the locked data.
        ///
        /// This operation cannot fail as the `MappedReentrantMutexGuard` passed
        /// in already locked the mutex.
        ///
        /// This is an associated function that needs to be
        /// used as `MappedReentrantMutexGuard::map(...)`. A method would interfere with methods of
        /// the same name on the contents of the locked data.
        #[inline]
        pub fn map<U: ?Sized, F>(s: Self, f: F) -> MappedReentrantMutexGuard<'a, R, G, U>
        where
            F: FnOnce(&T) -> &U,
        {
            let raw = s.raw;
            let data = f(unsafe { &*s.data });
            mem::forget(s);
            MappedReentrantMutexGuard {
                raw,
                data,
                marker: PhantomData,
            }
        }

        /// Attempts to make  a new `MappedReentrantMutexGuard` for a component of the
        /// locked data. The original guard is return if the closure returns `None`.
        ///
        /// This operation cannot fail as the `MappedReentrantMutexGuard` passed
        /// in already locked the mutex.
        ///
        /// This is an associated function that needs to be
        /// used as `MappedReentrantMutexGuard::map(...)`. A method would interfere with methods of
        /// the same name on the contents of the locked data.
        #[inline]
        pub fn try_map<U: ?Sized, F>(
            s: Self,
            f: F,
        ) -> Result<MappedReentrantMutexGuard<'a, R, G, U>, Self>
        where
            F: FnOnce(&T) -> Option<&U>,
        {
            let raw = s.raw;
            let data = match f(unsafe { &*s.data }) {
                Some(data) => data,
                None => return Err(s),
            };
            mem::forget(s);
            Ok(MappedReentrantMutexGuard {
                raw,
                data,
                marker: PhantomData,
            })
        }
    }

    impl<'a, R: RawMutexFair + 'a, G: GetThreadId + 'a, T: ?Sized + 'a>
        MappedReentrantMutexGuard<'a, R, G, T>
    {
        /// Unlocks the mutex using a fair unlock protocol.
        ///
        /// By default, mutexes are unfair and allow the current thread to re-lock
        /// the mutex before another has the chance to acquire the lock, even if
        /// that thread has been blocked on the mutex for a long time. This is the
        /// default because it allows much higher throughput as it avoids forcing a
        /// context switch on every mutex unlock. This can result in one thread
        /// acquiring a mutex many more times than other threads.
        ///
        /// However in some cases it can be beneficial to ensure fairness by forcing
        /// the lock to pass on to a waiting thread if there is one. This is done by
        /// using this method instead of dropping the `ReentrantMutexGuard` normally.
        #[inline]
        pub fn unlock_fair(s: Self) {
            // Safety: A MappedReentrantMutexGuard always holds the lock
            unsafe {
                s.raw.unlock_fair();
            }
            mem::forget(s);
        }
    }

    impl<'a, R: RawMutex + 'a, G: GetThreadId + 'a, T: ?Sized + 'a> Deref
        for MappedReentrantMutexGuard<'a, R, G, T>
    {
        type Target = T;
        #[inline]
        fn deref(&self) -> &T {
            unsafe { &*self.data }
        }
    }

    impl<'a, R: RawMutex + 'a, G: GetThreadId + 'a, T: ?Sized + 'a> Drop
        for MappedReentrantMutexGuard<'a, R, G, T>
    {
        #[inline]
        fn drop(&mut self) {
            // Safety: A MappedReentrantMutexGuard always holds the lock.
            unsafe {
                self.raw.unlock();
            }
        }
    }

    impl<'a, R: RawMutex + 'a, G: GetThreadId + 'a, T: fmt::Debug + ?Sized + 'a> fmt::Debug
        for MappedReentrantMutexGuard<'a, R, G, T>
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Debug::fmt(&**self, f)
        }
    }

    impl<'a, R: RawMutex + 'a, G: GetThreadId + 'a, T: fmt::Display + ?Sized + 'a> fmt::Display
        for MappedReentrantMutexGuard<'a, R, G, T>
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            (**self).fmt(f)
        }
    }

    #[cfg(feature = "owning_ref")]
    unsafe impl<'a, R: RawMutex + 'a, G: GetThreadId + 'a, T: ?Sized + 'a> StableAddress
        for MappedReentrantMutexGuard<'a, R, G, T>
    {
    }