lock_api: Unsafely access data within a Mutex without MutexGuard

[mystor]

I have a use-case where I need to manually manage locking/unlocking a Mutex, and can't store the MutexGuard between locking and accessing the locked data. It's possible to perform locking and unlocking of the RawMutex manually from outside this crate using the Mutex::raw() method, as well as mem::forget(mutex.lock()) and mutex.force_unlock(), but there appears to be no API to then unsafely access the raw data guarded by a &Mutex.

It would be nice to expose a method to extract the raw pointer to the mutex-guarded data within the UnsafeCell, perhaps with something like the following:

/// Returns a raw pointer to the underlying data.
///
/// # Safety
///
/// The returned pointer may only be dereferenced if the current thread logically
/// owns a `MutexGuard`, but that guard has been discarded using `mem::forget`.
pub fn data_ptr(&self) -> *mut T {
  self.data.get()
}