rust: types: implement ForeignOwnable for ARef<T>

Implement ForeignOwnable for ARef<T>, making it possible for C code to
own an ARef<T>.

Since ARef represents shared ownership, BorrowedMut is &T rather than
&mut T, matching the semantics of the underlying reference-counted type.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260805145949.938505-4-phasta@kernel.org
[ Relaxed `'static` bound and added `#[inline]` as discussed. Added
  submitter's Signed-off-by. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: Danilo Krummrich

rust/kernel/sync/aref.rs

@@ -24,6 +24,11 @@ use core::{
     ptr::NonNull, //
 };
 
+use crate::{
+    prelude::*,
+    types::ForeignOwnable, //
+};
+
 /// Types that are _always_ reference counted.
 ///
 /// It allows such types to define their own custom ref increment and decrement functions.
@@ -188,6 +193,51 @@ where
 }
 impl<T: AlwaysRefCounted + Eq> Eq for ARef<T> {}
 
+// SAFETY: `into_foreign` returns a pointer from `NonNull::as_ptr`, so it's non-null. The
+// `ARef` invariant guarantees that `ptr` points to a valid `T`, so it's aligned to `T`.
+unsafe impl<T: AlwaysRefCounted> ForeignOwnable for ARef<T> {
+    const FOREIGN_ALIGN: usize = core::mem::align_of::<T>();
+
+    type Borrowed<'a>
+        = &'a T
+    where
+        Self: 'a;
+    type BorrowedMut<'a>
+        = &'a T
+    where
+        Self: 'a;
+
+    #[inline]
+    fn into_foreign(self) -> *mut c_void {
+        ARef::into_raw(self).as_ptr().cast()
+    }
+
+    #[inline]
+    unsafe fn from_foreign(ptr: *mut c_void) -> Self {
+        // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
+        // call to `Self::into_foreign`.
+        let ptr = unsafe { NonNull::new_unchecked(ptr.cast()) };
+
+        // SAFETY: `ptr` came from `into_foreign`, which consumed an `ARef` without decrementing
+        // the refcount, so we can transfer the ownership to the new `ARef`.
+        unsafe { ARef::from_raw(ptr) }
+    }
+
+    #[inline]
+    unsafe fn borrow<'a>(ptr: *mut c_void) -> &'a T {
+        // SAFETY: The safety requirements of this method ensure that the object remains alive and
+        // immutable for the duration of 'a.
+        unsafe { &*ptr.cast() }
+    }
+
+    #[inline]
+    unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> &'a T {
+        // SAFETY: The safety requirements for `borrow_mut` are a superset of the safety
+        // requirements for `borrow`.
+        unsafe { <Self as ForeignOwnable>::borrow(ptr) }
+    }
+}
+
 impl<T, U> PartialEq<&'_ U> for ARef<T>
 where
     T: AlwaysRefCounted + PartialEq<U>,