[PATCH 4/5] rust: types: implement `ForeignOwnable` for the unit type

[Wedson Almeida Filho <wedsonaf@gmail.com>]

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>
---
 rust/kernel/types.rs | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index e037c262f23e..8f80cffbff59 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -85,6 +85,19 @@ impl<T: 'static> ForeignOwnable for Box<T> {
     }
 }
 
+impl ForeignOwnable for () {
+    type Borrowed<'a> = ();
+
+    fn into_foreign(self) -> *const core::ffi::c_void {
+        // We use 1 to be different from a null pointer.
+        1usize as _
+    }
+
+    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.
-- 
2.34.1