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rust: lock: Add a Pin<&mut T> accessor

In order for callers to be able to access the inner T safely if T:
!Unpin, there needs to be a way to get a Pin<&mut T>. Add this accessor
and a corresponding example to tell users how it works.

This requires the pin projection functionality [1] for better ergonomic.

[boqun: Apply Daniel's fix to the code example, add the reference to pin
projection patch and remove out-of-date part in the commit log]

Suggested-by: Benno Lossin <lossin@kernel.org>
Suggested-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/issues/1181
Link: https://lore.kernel.org/rust-for-linux/20250912174148.373530-1-lossin@kernel.org/ [1]

authored by

Daniel Almeida and committed by
Peter Zijlstra 66f1ea83 2497a711

+25
+25
rust/kernel/sync/lock.rs
··· 245 245 246 246 cb() 247 247 } 248 + 249 + /// Returns a pinned mutable reference to the protected data. 250 + /// 251 + /// The guard implements [`DerefMut`] when `T: Unpin`, so for [`Unpin`] 252 + /// types [`DerefMut`] should be used instead of this function. 253 + /// 254 + /// [`DerefMut`]: core::ops::DerefMut 255 + /// [`Unpin`]: core::marker::Unpin 256 + /// 257 + /// # Examples 258 + /// 259 + /// ``` 260 + /// # use kernel::sync::{Mutex, MutexGuard}; 261 + /// # use core::{pin::Pin, marker::PhantomPinned}; 262 + /// struct Data(PhantomPinned); 263 + /// 264 + /// fn example(mutex: &Mutex<Data>) { 265 + /// let mut data: MutexGuard<'_, Data> = mutex.lock(); 266 + /// let mut data: Pin<&mut Data> = data.as_mut(); 267 + /// } 268 + /// ``` 269 + pub fn as_mut(&mut self) -> Pin<&mut T> { 270 + // SAFETY: `self.lock.data` is structurally pinned. 271 + unsafe { Pin::new_unchecked(&mut *self.lock.data.get()) } 272 + } 248 273 } 249 274 250 275 impl<T: ?Sized, B: Backend> core::ops::Deref for Guard<'_, T, B> {