Remove NULL ObjectReference (#84)

Parent PR: mmtk/mmtk-core#1064

---------

Co-authored-by: mmtkgc-bot <mmtkgc.bot@gmail.com>

mmtk/Cargo.toml

@@ -27,7 +27,7 @@ log = "*"
 # - change branch
 # - change repo name
 # But other changes including adding/removing whitespaces in commented lines may break the CI.
-mmtk = { git = "https://github.com/mmtk/mmtk-core.git", rev = "e79e94e744660c486d5471f252ff05c4248bcea9" }
+mmtk = { git = "https://github.com/mmtk/mmtk-core.git", rev = "a02803b4104519ff2289234101a2dd8ceedd1bc7" }
 # Uncomment the following and fix the path to mmtk-core to build locally
 # mmtk = { path = "../repos/mmtk-core" }
 

mmtk/src/object_model.rs

@@ -69,7 +69,8 @@ impl ObjectModel<V8> for VMObjectModel {
     }
 
     fn address_to_ref(address: Address) -> ObjectReference {
-        ObjectReference::from_raw_address(address)
+        debug_assert!(!address.is_zero());
+        unsafe { ObjectReference::from_raw_address_unchecked(address) }
     }
 
     fn dump_object(object: ObjectReference) {

mmtk/src/reference_glue.rs

@@ -11,10 +11,13 @@ impl ReferenceGlue<V8> for VMReferenceGlue {
     fn set_referent(_reff: ObjectReference, _referent: ObjectReference) {
         unimplemented!()
     }
-    fn get_referent(_object: ObjectReference) -> ObjectReference {
+    fn get_referent(_object: ObjectReference) -> Option<ObjectReference> {
         unimplemented!()
     }
     fn enqueue_references(_references: &[ObjectReference], _tls: VMWorkerThread) {
         unimplemented!()
     }
+    fn clear_referent(_new_reference: ObjectReference) {
+        unimplemented!()
+    }
 }