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projection patch and remove out-of-date part in the commit log]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: #1181
Link: https://lore.kernel.org/rust-for-linux/20250912174148.373530-1-lossin@kernel.org/ [1]

Author: danielalmeida-collabora

rust/kernel/sync/lock.rs

@@ -245,6 +245,31 @@ impl<'a, T: ?Sized, B: Backend> Guard<'a, T, B> {
 
         cb()
     }
+
+    /// Returns a pinned mutable reference to the protected data.
+    ///
+    /// The guard implements [`DerefMut`] when `T: Unpin`, so for [`Unpin`]
+    /// types [`DerefMut`] should be used instead of this function.
+    ///
+    /// [`DerefMut`]: core::ops::DerefMut
+    /// [`Unpin`]: core::marker::Unpin
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use kernel::sync::{Mutex, MutexGuard};
+    /// # use core::{pin::Pin, marker::PhantomPinned};
+    /// struct Data(PhantomPinned);
+    ///
+    /// fn example(mutex: &Mutex<Data>) {
+    ///     let mut data: MutexGuard<'_, Data> = mutex.lock();
+    ///     let mut data: Pin<&mut Data> = data.as_mut();
+    /// }
+    /// ```
+    pub fn as_mut(&mut self) -> Pin<&mut T> {
+        // SAFETY: `self.lock.data` is structurally pinned.
+        unsafe { Pin::new_unchecked(&mut *self.lock.data.get()) }
+    }
 }
 
 impl<T: ?Sized, B: Backend> core::ops::Deref for Guard<'_, T, B> {