[PATCH v3 3/4] rust: specify when `ARef` is thread safe

[Alice Ryhl]

An `ARef` behaves just like the `Arc` when it comes to thread safety, so
we can reuse the thread safety comments from `Arc` here.

This is necessary because without this change, the Rust compiler will
assume that things are not thread safe even though they are.

Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Reviewed-by: Andreas Hindborg <a.hindborg@xxxxxxxxxxx>
Reviewed-by: Boqun Feng <boqun.feng@xxxxxxxxx>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@xxxxxxxxx>
Reviewed-by: Benno Lossin <benno.lossin@xxxxxxxxx>
---
 rust/kernel/types.rs | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index 29db59d6119a..1e5380b16ed5 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -321,6 +321,19 @@ pub struct ARef<T: AlwaysRefCounted> {
     _p: PhantomData<T>,
 }
 
+// SAFETY: It is safe to send `ARef<T>` to another thread when the underlying `T` is `Sync` because
+// it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, it needs
+// `T` to be `Send` because any thread that has an `ARef<T>` may ultimately access `T` using a
+// mutable reference, for example, when the reference count reaches zero and `T` is dropped.
+unsafe impl<T: AlwaysRefCounted + Sync + Send> Send for ARef<T> {}
+
+// SAFETY: It is safe to send `&ARef<T>` to another thread when the underlying `T` is `Sync`
+// because it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally,
+// it needs `T` to be `Send` because any thread that has a `&ARef<T>` may clone it and get an
+// `ARef<T>` on that thread, so the thread may ultimately access `T` using a mutable reference, for
+// example, when the reference count reaches zero and `T` is dropped.
+unsafe impl<T: AlwaysRefCounted + Sync + Send> Sync for ARef<T> {}
+
 impl<T: AlwaysRefCounted> ARef<T> {
     /// Creates a new instance of [`ARef`].
     ///
-- 
2.41.0.rc0.172.g3f132b7071-goog