[PATCH 2/4] rust: lock: guard: Add T: Unpin bound to DerefMut

[Boqun Feng <boqun.feng@gmail.com>]

From: Daniel Almeida <daniel.almeida@collabora.com>

A core property of pinned types is not handing a mutable reference to
the inner data in safe code, as this trivially allows that data to be
moved.

Enforce this condition by adding a bound on lock::Guard's DerefMut
implementation, so that it's only implemented for pinning-agnostic
types.

Link: https://github.com/Rust-for-Linux/linux/issues/1181
Suggested-by: Benno Lossin <lossin@kernel.org>
Suggested-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---

Notes:
    SubmissionLink: https://lore.kernel.org/all/20250828-lock-t-when-t-is-pinned-v2-1-b067c4b93fd6@collabora.com/

 rust/kernel/sync/lock.rs        | 5 ++++-
 rust/kernel/sync/lock/global.rs | 5 ++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
index 27202beef90c..b482f34bf0ce 100644
--- a/rust/kernel/sync/lock.rs
+++ b/rust/kernel/sync/lock.rs
@@ -251,7 +251,10 @@ fn deref(&self) -> &Self::Target {
     }
 }
 
-impl<T: ?Sized, B: Backend> core::ops::DerefMut for Guard<'_, T, B> {
+impl<T: ?Sized, B: Backend> core::ops::DerefMut for Guard<'_, T, B>
+where
+    T: Unpin,
+{
     fn deref_mut(&mut self) -> &mut Self::Target {
         // SAFETY: The caller owns the lock, so it is safe to deref the protected data.
         unsafe { &mut *self.lock.data.get() }
diff --git a/rust/kernel/sync/lock/global.rs b/rust/kernel/sync/lock/global.rs
index d65f94b5caf2..38b448032799 100644
--- a/rust/kernel/sync/lock/global.rs
+++ b/rust/kernel/sync/lock/global.rs
@@ -106,7 +106,10 @@ fn deref(&self) -> &Self::Target {
     }
 }
 
-impl<B: GlobalLockBackend> core::ops::DerefMut for GlobalGuard<B> {
+impl<B: GlobalLockBackend> core::ops::DerefMut for GlobalGuard<B>
+where
+    B::Item: Unpin,
+{
     fn deref_mut(&mut self) -> &mut Self::Target {
         &mut self.inner
     }
-- 
2.51.0