Tracking Issue for {Mutex, RwLock, ReentrantLock}::data_ptr

Feature gate: #![feature(mutex_data_ptr)], #![feature(rwlock_data_ptr)], #![feature(reentrant_lock_data_ptr)]

This is a tracking issue for allowing unguarded access to the inner value of a Mutex or RwLock. Previous discussion here: https://github.com/rust-lang/libs-team/issues/531

Public API

// std::sync

impl<T: ?Sized> Mutex<T> {
    pub fn data_ptr(&self) -> *mut T;
}

impl<T: ?Sized> RwLock<T> {
    pub fn data_ptr(&self) -> *mut T;
}

// This type is unstable under feature `reentrant_lock`
impl<T: ?Sized> ReentrantLock<T>  {
    pub fn data_ptr(&self) -> *mut T;
}

Steps / History

(Remember to update the S-tracking-* label when checking boxes.)

• Implementation: #140369
• Add examples to documentation
• Final comment period (FCP)^1
• Stabilization PR

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>?

The implementation is complete in #140369; begin by reviewing that change and the existing Mutex, RwLock, and ReentrantLock API documentation. Add usage examples for data_ptr to the relevant standard-library documentation, then verify the examples build and update the tracking checklist. The unresolved return-type questions and FCP/stabilization steps require library-team review rather than an independent code change.