library/std/src/sync/nonpoison/mutex.rs

use crate::cell::UnsafeCell;
use crate::fmt;
use crate::marker::PhantomData;
use crate::mem::{self, ManuallyDrop};
use crate::ops::{Deref, DerefMut};
use crate::ptr::NonNull;
use crate::sync::nonpoison::{TryLockResult, WouldBlock};
use crate::sys::sync as sys;

/// A mutual exclusion primitive useful for protecting shared data that does not keep track of
/// lock poisoning.
///
/// For more information about mutexes, check out the documentation for the poisoning variant of
/// this lock at [`poison::Mutex`].
///
/// [`poison::Mutex`]: crate::sync::poison::Mutex
///
/// # Examples
///
/// Note that this `Mutex` does **not** propagate threads that panic while holding the lock via
/// poisoning. If you need this functionality, see [`poison::Mutex`].
///
/// ```
/// #![feature(nonpoison_mutex)]
///
/// use std::thread;
/// use std::sync::{Arc, nonpoison::Mutex};
///
/// let mutex = Arc::new(Mutex::new(0u32));
/// let mut handles = Vec::new();
///
/// for n in 0..10 {
///     let m = Arc::clone(&mutex);
///     let handle = thread::spawn(move || {
///         let mut guard = m.lock();
///         *guard += 1;
///         panic!("panic from thread {n} {guard}")
///     });
///     handles.push(handle);
/// }
///
/// for h in handles {
///     let _ = h.join();
/// }
///
/// println!("Finished, locked {} times", mutex.lock());
/// ```
#[unstable(feature = "nonpoison_mutex", issue = "134645")]
#[cfg_attr(not(test), rustc_diagnostic_item = "NonPoisonMutex")]
pub struct Mutex<T: ?Sized> {
    inner: sys::Mutex,
    data: UnsafeCell<T>,
}

/// `T` must be `Send` for a [`Mutex`] to be `Send` because it is possible to acquire
/// the owned `T` from the `Mutex` via [`into_inner`].
///
/// [`into_inner`]: Mutex::into_inner
#[unstable(feature = "nonpoison_mutex", issue = "134645")]
unsafe impl<T: ?Sized + Send> Send for Mutex<T> {}

/// `T` must be `Send` for [`Mutex`] to be `Sync`.
/// This ensures that the protected data can be accessed safely from multiple threads
/// without causing data races or other unsafe behavior.
///
/// [`Mutex<T>`] provides mutable access to `T` to one thread at a time. However, it's essential
/// for `T` to be `Send` because it's not safe for non-`Send` structures to be accessed in
/// this manner. For instance, consider [`Rc`], a non-atomic reference counted smart pointer,
/// which is not `Send`. With `Rc`, we can have multiple copies pointing to the same heap
/// allocation with a non-atomic reference count. If we were to use `Mutex<Rc<_>>`, it would
/// only protect one instance of `Rc` from shared access, leaving other copies vulnerable
/// to potential data races.
///
/// Also note that it is not necessary for `T` to be `Sync` as `&T` is only made available
/// to one thread at a time if `T` is not `Sync`.
///
/// [`Rc`]: crate::rc::Rc
#[unstable(feature = "nonpoison_mutex", issue = "134645")]
unsafe impl<T: ?Sized + Send> Sync for Mutex<T> {}

/// An RAII implementation of a "scoped lock" of a 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`] and [`DerefMut`] implementations.
///
/// This structure is created by the [`lock`] and [`try_lock`] methods on
/// [`Mutex`].
///
/// [`lock`]: Mutex::lock
/// [`try_lock`]: Mutex::try_lock
#[must_use = "if unused the Mutex will immediately unlock"]
#[must_not_suspend = "holding a MutexGuard across suspend \
                      points can cause deadlocks, delays, \
                      and cause Futures to not implement `Send`"]
#[unstable(feature = "nonpoison_mutex", issue = "134645")]
#[clippy::has_significant_drop]
#[cfg_attr(not(test), rustc_diagnostic_item = "NonPoisonMutexGuard")]
pub struct MutexGuard<'a, T: ?Sized + 'a> {
    lock: &'a Mutex<T>,
}

/// A [`MutexGuard`] is not `Send` to maximize platform portability.
///
/// On platforms that use POSIX threads (commonly referred to as pthreads) there is a requirement to
/// release mutex locks on the same thread they were acquired.
/// For this reason, [`MutexGuard`] must not implement `Send` to prevent it being dropped from
/// another thread.
#[unstable(feature = "nonpoison_mutex", issue = "134645")]
impl<T: ?Sized> !Send for MutexGuard<'_, T> {}

/// `T` must be `Sync` for a [`MutexGuard<T>`] to be `Sync`
/// because it is possible to get a `&T` from `&MutexGuard` (via `Deref`).
#[unstable(feature = "nonpoison_mutex", issue = "134645")]
unsafe impl<T: ?Sized + Sync> Sync for MutexGuard<'_, T> {}

/// An RAII mutex guard returned by `MutexGuard::map`, which can point to a
/// subfield of the protected data. When this structure is dropped (falls out
/// of scope), the lock will be unlocked.
///
/// The main difference between `MappedMutexGuard` and [`MutexGuard`] is that the
/// former cannot be used with [`Condvar`], since that could introduce soundness issues if the
/// locked object is modified by another thread while the `Mutex` is unlocked.
///
/// The data protected by the mutex can be accessed through this guard via its
/// [`Deref`] and [`DerefMut`] implementations.
///
/// This structure is created by the [`map`] and [`filter_map`] methods on
/// [`MutexGuard`].
///
/// [`map`]: MutexGuard::map
/// [`filter_map`]: MutexGuard::filter_map
/// [`Condvar`]: crate::sync::nonpoison::Condvar
#[must_use = "if unused the Mutex will immediately unlock"]
#[must_not_suspend = "holding a MappedMutexGuard across suspend \
                      points can cause deadlocks, delays, \
                      and cause Futures to not implement `Send`"]
#[unstable(feature = "mapped_lock_guards", issue = "117108")]
// #[unstable(feature = "nonpoison_mutex", issue = "134645")]
#[clippy::has_significant_drop]
pub struct MappedMutexGuard<'a, T: ?Sized + 'a> {
    // NB: we use a pointer instead of `&'a mut T` to avoid `noalias` violations, because a
    // `MappedMutexGuard` argument doesn't hold uniqueness for its whole scope, only until it drops.
    // `NonNull` is covariant over `T`, so we add a `PhantomData<&'a mut T>` field
    // below for the correct variance over `T` (invariance).
    data: NonNull<T>,
    inner: &'a sys::Mutex,
    _variance: PhantomData<&'a mut T>,
}

#[unstable(feature = "mapped_lock_guards", issue = "117108")]
// #[unstable(feature = "nonpoison_mutex", issue = "134645")]
impl<T: ?Sized> !Send for MappedMutexGuard<'_, T> {}
#[unstable(feature = "mapped_lock_guards", issue = "117108")]
// #[unstable(feature = "nonpoison_mutex", issue = "134645")]
unsafe impl<T: ?Sized + Sync> Sync for MappedMutexGuard<'_, T> {}

impl<T> Mutex<T> {
    /// Creates a new mutex in an unlocked state ready for use.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(nonpoison_mutex)]
    ///
    /// use std::sync::nonpoison::Mutex;
    ///
    /// let mutex = Mutex::new(0);
    /// ```
    #[unstable(feature = "nonpoison_mutex", issue = "134645")]
    #[inline]
    pub const fn new(t: T) -> Mutex<T> {
        Mutex { inner: sys::Mutex::new(), data: UnsafeCell::new(t) }
    }

    /// Returns the contained value by cloning it.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(nonpoison_mutex)]
    /// #![feature(lock_value_accessors)]
    ///
    /// use std::sync::nonpoison::Mutex;
    ///
    /// let mut mutex = Mutex::new(7);
    ///
    /// assert_eq!(mutex.get_cloned(), 7);
    /// ```
    #[unstable(feature = "lock_value_accessors", issue = "133407")]
    // #[unstable(feature = "nonpoison_mutex", issue = "134645")]
    pub fn get_cloned(&self) -> T
    where
        T: Clone,
    {
        self.lock().clone()
    }

    /// Sets the contained value.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(nonpoison_mutex)]
    /// #![feature(lock_value_accessors)]
    ///
    /// use std::sync::nonpoison::Mutex;
    ///
    /// let mut mutex = Mutex::new(7);
    ///
    /// assert_eq!(mutex.get_cloned(), 7);
    /// mutex.set(11);
    /// assert_eq!(mutex.get_cloned(), 11);
    /// ```
    #[unstable(feature = "lock_value_accessors", issue = "133407")]
    // #[unstable(feature = "nonpoison_mutex", issue = "134645")]
    pub fn set(&self, value: T) {
        if mem::needs_drop::<T>() {
            // If the contained value has a non-trivial destructor, we
            // call that destructor after the lock has been released.
            drop(self.replace(value))
        } else {
            *self.lock() = value;
        }
    }

    /// Replaces the contained value with `value`, and returns the old contained value.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(nonpoison_mutex)]
    /// #![feature(lock_value_accessors)]
    ///
    /// use std::sync::nonpoison::Mutex;
    ///
    /// let mut mutex = Mutex::new(7);
    ///
    /// assert_eq!(mutex.replace(11), 7);
    /// assert_eq!(mutex.get_cloned(), 11);
    /// ```
    #[unstable(feature = "lock_value_accessors", issue = "133407")]
    // #[unstable(feature = "nonpoison_mutex", issue = "134645")]
    pub fn replace(&self, value: T) -> T {
        let mut guard = self.lock();
        mem::replace(&mut *guard, value)
    }
}

impl<T: ?Sized> Mutex<T> {
    /// Acquires a mutex, blocking the current thread until it is able to do so.
    ///
    /// This function will block the local thread until it is available to acquire
    /// the mutex. Upon returning, the thread is the only thread with the lock
    /// 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.
    ///
    /// The exact behavior on locking a mutex in the thread which already holds
    /// the lock is left unspecified. However, this function will not return on
    /// the second call (it might panic or deadlock, for example).
    ///
    /// # Panics
    ///
    /// This function might panic when called if the lock is already held by
    /// the current thread.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(nonpoison_mutex)]
    ///
    /// use std::sync::{Arc, nonpoison::Mutex};
    /// use std::thread;
    ///
    /// let mutex = Arc::new(Mutex::new(0));
    /// let c_mutex = Arc::clone(&mutex);
    ///
    /// thread::spawn(move || {
    ///     *c_mutex.lock() = 10;
    /// }).join().expect("thread::spawn failed");
    /// assert_eq!(*mutex.lock(), 10);
    /// ```
    #[unstable(feature = "nonpoison_mutex", issue = "134645")]
    pub fn lock(&self) -> MutexGuard<'_, T> {
        unsafe {
            self.inner.lock();
            MutexGuard::new(self)
        }
    }

    /// Attempts to acquire this lock.
    ///
    /// This function does not block. If the lock could not be acquired at this time, then
    /// [`WouldBlock`] is returned. Otherwise, an RAII guard is returned.
    ///
    /// The lock will be unlocked when the guard is dropped.
    ///
    /// # Errors
    ///
    /// If the mutex could not be acquired because it is already locked, then this call will return
    /// the [`WouldBlock`] error.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Arc, Mutex};
    /// use std::thread;
    ///
    /// let mutex = Arc::new(Mutex::new(0));
    /// let c_mutex = Arc::clone(&mutex);
    ///
    /// thread::spawn(move || {
    ///     let mut lock = c_mutex.try_lock();
    ///     if let Ok(ref mut mutex) = lock {
    ///         **mutex = 10;
    ///     } else {
    ///         println!("try_lock failed");
    ///     }
    /// }).join().expect("thread::spawn failed");
    /// assert_eq!(*mutex.lock().unwrap(), 10);
    /// ```
    #[unstable(feature = "nonpoison_mutex", issue = "134645")]
    pub fn try_lock(&self) -> TryLockResult<MutexGuard<'_, T>> {
        unsafe { if self.inner.try_lock() { Ok(MutexGuard::new(self)) } else { Err(WouldBlock) } }
    }

    /// Consumes this mutex, returning the underlying data.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(nonpoison_mutex)]
    ///
    /// use std::sync::nonpoison::Mutex;
    ///
    /// let mutex = Mutex::new(0);
    /// assert_eq!(mutex.into_inner(), 0);
    /// ```
    #[unstable(feature = "nonpoison_mutex", issue = "134645")]
    pub fn into_inner(self) -> T
    where
        T: Sized,
    {
        self.data.into_inner()
    }

    /// Returns a mutable reference to the underlying data.
    ///
    /// Since this call borrows the `Mutex` mutably, no actual locking needs to
    /// take place -- the mutable borrow statically guarantees no locks exist.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(nonpoison_mutex)]
    ///
    /// use std::sync::nonpoison::Mutex;
    ///
    /// let mut mutex = Mutex::new(0);
    /// *mutex.get_mut() = 10;
    /// assert_eq!(*mutex.lock(), 10);
    /// ```
    #[unstable(feature = "nonpoison_mutex", issue = "134645")]
    pub fn get_mut(&mut self) -> &mut T {
        self.data.get_mut()
    }

    /// Returns a raw pointer to the underlying data.
    ///
    /// The returned pointer is always non-null and properly aligned, but it is
    /// the user's responsibility to ensure that any reads and writes through it
    /// are properly synchronized to avoid data races, and that it is not read
    /// or written through after the mutex is dropped.
    #[unstable(feature = "mutex_data_ptr", issue = "140368")]
    // #[unstable(feature = "nonpoison_mutex", issue = "134645")]
    pub const fn data_ptr(&self) -> *mut T {
        self.data.get()
    }

    /// Acquires the mutex and provides mutable access to the underlying data by passing
    /// a mutable reference to the given closure.
    ///
    /// This method acquires the lock, calls the provided closure with a mutable reference
    /// to the data, and returns the result of the closure. The lock is released after
    /// the closure completes, even if it panics.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(lock_value_accessors, nonpoison_mutex)]
    ///
    /// use std::sync::nonpoison::Mutex;
    ///
    /// let mutex = Mutex::new(2);
    ///
    /// let result = mutex.with_mut(|data| {
    ///     *data += 3;
    ///
    ///     *data + 5
    /// });
    ///
    /// assert_eq!(*mutex.lock(), 5);
    /// assert_eq!(result, 10);
    /// ```
    #[unstable(feature = "lock_value_accessors", issue = "133407")]
    // #[unstable(feature = "nonpoison_mutex", issue = "134645")]
    pub fn with_mut<F, R>(&self, f: F) -> R
    where
        F: FnOnce(&mut T) -> R,
    {
        f(&mut self.lock())
    }
}

#[unstable(feature = "nonpoison_mutex", issue = "134645")]
impl<T> From<T> for Mutex<T> {
    /// Creates a new mutex in an unlocked state ready for use.
    /// This is equivalent to [`Mutex::new`].
    fn from(t: T) -> Self {
        Mutex::new(t)
    }
}

#[unstable(feature = "nonpoison_mutex", issue = "134645")]
impl<T: Default> Default for Mutex<T> {
    /// Creates a `Mutex<T>`, with the `Default` value for T.
    fn default() -> Mutex<T> {
        Mutex::new(Default::default())
    }
}

#[unstable(feature = "nonpoison_mutex", issue = "134645")]
impl<T: ?Sized + fmt::Debug> fmt::Debug for Mutex<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let mut d = f.debug_struct("Mutex");
        match self.try_lock() {
            Ok(guard) => {
                d.field("data", &&*guard);
            }
            Err(WouldBlock) => {
                d.field("data", &"<locked>");
            }
        }
        d.finish_non_exhaustive()
    }
}

impl<'mutex, T: ?Sized> MutexGuard<'mutex, T> {
    unsafe fn new(lock: &'mutex Mutex<T>) -> MutexGuard<'mutex, T> {
        MutexGuard { lock }
    }
}

#[unstable(feature = "nonpoison_mutex", issue = "134645")]
impl<T: ?Sized> Deref for MutexGuard<'_, T> {
    type Target = T;

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

#[unstable(feature = "nonpoison_mutex", issue = "134645")]
impl<T: ?Sized> DerefMut for MutexGuard<'_, T> {
    fn deref_mut(&mut self) -> &mut T {
        unsafe { &mut *self.lock.data.get() }
    }
}

#[unstable(feature = "nonpoison_mutex", issue = "134645")]
impl<T: ?Sized> Drop for MutexGuard<'_, T> {
    #[inline]
    fn drop(&mut self) {
        unsafe {
            self.lock.inner.unlock();
        }
    }
}

#[unstable(feature = "nonpoison_mutex", issue = "134645")]
impl<T: ?Sized + fmt::Debug> fmt::Debug for MutexGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&**self, f)
    }
}

#[unstable(feature = "nonpoison_mutex", issue = "134645")]
impl<T: ?Sized + fmt::Display> fmt::Display for MutexGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        (**self).fmt(f)
    }
}

/// For use in [`nonpoison::condvar`](super::condvar).
pub(super) fn guard_lock<'a, T: ?Sized>(guard: &MutexGuard<'a, T>) -> &'a sys::Mutex {
    &guard.lock.inner
}

impl<'a, T: ?Sized> MutexGuard<'a, T> {
    /// Makes a [`MappedMutexGuard`] for a component of the borrowed data, e.g.
    /// an enum variant.
    ///
    /// The `Mutex` is already locked, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `MutexGuard::map(...)`. A method would interfere with methods of the
    /// same name on the contents of the `MutexGuard` used through `Deref`.
    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
    // #[unstable(feature = "nonpoison_mutex", issue = "134645")]
    pub fn map<U, F>(orig: Self, f: F) -> MappedMutexGuard<'a, U>
    where
        F: FnOnce(&mut T) -> &mut U,
        U: ?Sized,
    {
        // SAFETY: the conditions of `MutexGuard::new` were satisfied when the original guard
        // was created, and have been upheld throughout `map` and/or `filter_map`.
        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
        // passed to it. If the closure panics, the guard will be dropped.
        let data = NonNull::from(f(unsafe { &mut *orig.lock.data.get() }));
        let orig = ManuallyDrop::new(orig);
        MappedMutexGuard { data, inner: &orig.lock.inner, _variance: PhantomData }
    }

    /// Makes a [`MappedMutexGuard`] for a component of the borrowed data. The
    /// original guard is returned as an `Err(...)` if the closure returns
    /// `None`.
    ///
    /// The `Mutex` is already locked, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `MutexGuard::filter_map(...)`. A method would interfere with methods of the
    /// same name on the contents of the `MutexGuard` used through `Deref`.
    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
    // #[unstable(feature = "nonpoison_mutex", issue = "134645")]
    pub fn filter_map<U, F>(orig: Self, f: F) -> Result<MappedMutexGuard<'a, U>, Self>
    where
        F: FnOnce(&mut T) -> Option<&mut U>,
        U: ?Sized,
    {
        // SAFETY: the conditions of `MutexGuard::new` were satisfied when the original guard
        // was created, and have been upheld throughout `map` and/or `filter_map`.
        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
        // passed to it. If the closure panics, the guard will be dropped.
        match f(unsafe { &mut *orig.lock.data.get() }) {
            Some(data) => {
                let data = NonNull::from(data);
                let orig = ManuallyDrop::new(orig);
                Ok(MappedMutexGuard { data, inner: &orig.lock.inner, _variance: PhantomData })
            }
            None => Err(orig),
        }
    }
}

#[unstable(feature = "mapped_lock_guards", issue = "117108")]
impl<T: ?Sized> Deref for MappedMutexGuard<'_, T> {
    type Target = T;

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

#[unstable(feature = "mapped_lock_guards", issue = "117108")]
impl<T: ?Sized> DerefMut for MappedMutexGuard<'_, T> {
    fn deref_mut(&mut self) -> &mut T {
        unsafe { self.data.as_mut() }
    }
}

#[unstable(feature = "mapped_lock_guards", issue = "117108")]
impl<T: ?Sized> Drop for MappedMutexGuard<'_, T> {
    #[inline]
    fn drop(&mut self) {
        unsafe {
            self.inner.unlock();
        }
    }
}

#[unstable(feature = "mapped_lock_guards", issue = "117108")]
impl<T: ?Sized + fmt::Debug> fmt::Debug for MappedMutexGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&**self, f)
    }
}

#[unstable(feature = "mapped_lock_guards", issue = "117108")]
impl<T: ?Sized + fmt::Display> fmt::Display for MappedMutexGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        (**self).fmt(f)
    }
}

impl<'a, T: ?Sized> MappedMutexGuard<'a, T> {
    /// Makes a [`MappedMutexGuard`] for a component of the borrowed data, e.g.
    /// an enum variant.
    ///
    /// The `Mutex` is already locked, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `MappedMutexGuard::map(...)`. A method would interfere with methods of the
    /// same name on the contents of the `MutexGuard` used through `Deref`.
    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
    // #[unstable(feature = "nonpoison_mutex", issue = "134645")]
    pub fn map<U, F>(mut orig: Self, f: F) -> MappedMutexGuard<'a, U>
    where
        F: FnOnce(&mut T) -> &mut U,
        U: ?Sized,
    {
        // SAFETY: the conditions of `MutexGuard::new` were satisfied when the original guard
        // was created, and have been upheld throughout `map` and/or `filter_map`.
        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
        // passed to it. If the closure panics, the guard will be dropped.
        let data = NonNull::from(f(unsafe { orig.data.as_mut() }));
        let orig = ManuallyDrop::new(orig);
        MappedMutexGuard { data, inner: orig.inner, _variance: PhantomData }
    }

    /// Makes a [`MappedMutexGuard`] for a component of the borrowed data. The
    /// original guard is returned as an `Err(...)` if the closure returns
    /// `None`.
    ///
    /// The `Mutex` is already locked, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `MappedMutexGuard::filter_map(...)`. A method would interfere with methods of the
    /// same name on the contents of the `MutexGuard` used through `Deref`.
    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
    // #[unstable(feature = "nonpoison_mutex", issue = "134645")]
    pub fn filter_map<U, F>(mut orig: Self, f: F) -> Result<MappedMutexGuard<'a, U>, Self>
    where
        F: FnOnce(&mut T) -> Option<&mut U>,
        U: ?Sized,
    {
        // SAFETY: the conditions of `MutexGuard::new` were satisfied when the original guard
        // was created, and have been upheld throughout `map` and/or `filter_map`.
        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
        // passed to it. If the closure panics, the guard will be dropped.
        match f(unsafe { orig.data.as_mut() }) {
            Some(data) => {
                let data = NonNull::from(data);
                let orig = ManuallyDrop::new(orig);
                Ok(MappedMutexGuard { data, inner: orig.inner, _variance: PhantomData })
            }
            None => Err(orig),
        }
    }
}