rust: types: implement `ForeignOwnable` for `Arc<T>`

This allows us to hand ownership of Rust ref-counted objects to
the C side of the kernel.

Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Reviewed-by: Alice Ferrazzi <alice.ferrazzi@miraclelinux.com>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/sync/arc.rs

@@ -15,7 +15,11 @@
 //!
 //! [`Arc`]: https://doc.rust-lang.org/std/sync/struct.Arc.html
 
-use crate::{bindings, error::Result, types::Opaque};
+use crate::{
+    bindings,
+    error::Result,
+    types::{ForeignOwnable, Opaque},
+};
 use alloc::boxed::Box;
 use core::{
     marker::{PhantomData, Unsize},
@@ -189,6 +193,32 @@ impl<T: ?Sized> Arc<T> {
     }
 }
 
+impl<T: 'static> ForeignOwnable for Arc<T> {
+    type Borrowed<'a> = ArcBorrow<'a, T>;
+
+    fn into_foreign(self) -> *const core::ffi::c_void {
+        ManuallyDrop::new(self).ptr.as_ptr() as _
+    }
+
+    unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> ArcBorrow<'a, T> {
+        // SAFETY: By the safety requirement of this function, we know that `ptr` came from
+        // a previous call to `Arc::into_foreign`.
+        let inner = NonNull::new(ptr as *mut ArcInner<T>).unwrap();
+
+        // SAFETY: The safety requirements of `from_foreign` ensure that the object remains alive
+        // for the lifetime of the returned value. Additionally, the safety requirements of
+        // `ForeignOwnable::borrow_mut` ensure that no new mutable references are created.
+        unsafe { ArcBorrow::new(inner) }
+    }
+
+    unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
+        // SAFETY: By the safety requirement of this function, we know that `ptr` came from
+        // a previous call to `Arc::into_foreign`, which guarantees that `ptr` is valid and
+        // holds a reference count increment that is transferrable to us.
+        unsafe { Self::from_inner(NonNull::new(ptr as _).unwrap()) }
+    }
+}
+
 impl<T: ?Sized> Deref for Arc<T> {
     type Target = T;