Auto merge of #140369 - jplatte:mutex-rwlock-data-ptr, r=Amanieu

Add data_ptr method to Mutex and RwLock

Implementation of #140368 / rust-lang/libs-team#531.

I tried to write a useful safety section about when it is safe to read or write through the returned pointers, but couldn't come up with something nice. Hoping this PR is still useful without that. I'm happy to add any doc strings other people come up with if needed before merge, of course.

Unresolved questions:

- Return a `LockResult` or not?
- Return `*mut T` like existing APIs (`Cell::as_ptr` / `MaybeUninit::as[_mut]_ptr` / `Vec::as_ptr` / ...) or be more precise and return `NonNull<T>`?

Author: bors

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

@@ -608,6 +608,17 @@ impl<T: ?Sized> Mutex<T> {
         let data = self.data.get_mut();
         poison::map_result(self.poison.borrow(), |()| data)
     }
+
+    /// 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")]
+    pub fn data_ptr(&self) -> *mut T {
+        self.data.get()
+    }
 }
 
 #[stable(feature = "mutex_from", since = "1.24.0")]

library/std/src/sync/poison/rwlock.rs

@@ -634,6 +634,17 @@ impl<T: ?Sized> RwLock<T> {
         let data = self.data.get_mut();
         poison::map_result(self.poison.borrow(), |()| data)
     }
+
+    /// 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 lock is dropped.
+    #[unstable(feature = "rwlock_data_ptr", issue = "140368")]
+    pub fn data_ptr(&self) -> *mut T {
+        self.data.get()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]

library/std/src/sync/reentrant_lock.rs

@@ -349,6 +349,17 @@ impl<T: ?Sized> ReentrantLock<T> {
         }
     }
 
+    /// 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 through it are
+    /// properly synchronized to avoid data races, and that it is not read
+    /// through after the lock is dropped.
+    #[unstable(feature = "reentrant_lock_data_ptr", issue = "140368")]
+    pub fn data_ptr(&self) -> *const T {
+        &raw const self.data
+    }
+
     unsafe fn increment_lock_count(&self) -> Option<()> {
         unsafe {
             *self.lock_count.get() = (*self.lock_count.get()).checked_add(1)?;