rust: types: implement `ForeignOwnable` for the unit type

This allows us to use the unit type `()` when we have no object whose
ownership must be managed but one implementing the `ForeignOwnable`
trait is needed.

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

rust/kernel/types.rs

@@ -85,6 +85,18 @@ impl<T: 'static> ForeignOwnable for Box<T> {
     }
 }
 
+impl ForeignOwnable for () {
+    type Borrowed<'a> = ();
+
+    fn into_foreign(self) -> *const core::ffi::c_void {
+        core::ptr::NonNull::dangling().as_ptr()
+    }
+
+    unsafe fn borrow<'a>(_: *const core::ffi::c_void) -> Self::Borrowed<'a> {}
+
+    unsafe fn from_foreign(_: *const core::ffi::c_void) -> Self {}
+}
+
 /// Runs a cleanup function/closure when dropped.
 ///
 /// The [`ScopeGuard::dismiss`] function prevents the cleanup function from running.