Mark class fields objects as internal

Author: byroot

gc.c

@@ -1486,6 +1486,8 @@ internal_object_p(VALUE obj)
                 return rb_singleton_class_internal_p(obj);
             }
             return 0;
+          case T_OBJECT:
+            if (FL_TEST_RAW(obj, ROBJECT_HIDDEN)) break;
           default:
             if (!RBASIC(obj)->klass) break;
             return 0;

include/ruby/internal/core/robject.h

@@ -71,7 +71,9 @@ enum ruby_robject_flags {
      * 3rd parties must  not be aware that  there even is more than  one way to
      * store instance variables.  Might better be hidden.
      */
-    ROBJECT_EMBED = RUBY_FL_USER1
+    ROBJECT_EMBED = RUBY_FL_USER1,
+
+    ROBJECT_HIDDEN = RUBY_FL_USER2, // HACK
 };
 
 struct st_table;

internal/class.h

@@ -540,13 +540,21 @@ RCLASS_FIELDS_OBJ(VALUE obj)
     return RCLASSEXT_FIELDS_OBJ(RCLASS_EXT_READABLE(obj));
 }
 
+static inline VALUE
+rb_allocate_fields_obj(VALUE klass)
+{
+    VALUE fields_obj = rb_class_allocate_instance(rb_singleton_class(klass));
+    FL_SET_RAW(fields_obj, ROBJECT_HIDDEN); // HACK
+    return fields_obj;
+}
+
 static inline VALUE
 RCLASS_ENSURE_FIELDS_OBJ(VALUE obj)
 {
     RUBY_ASSERT(RB_TYPE_P(obj, RUBY_T_CLASS) || RB_TYPE_P(obj, RUBY_T_MODULE));
     rb_classext_t *ext = RCLASS_EXT_READABLE(obj);
     if (!ext->fields_obj) {
-        RB_OBJ_WRITE(obj, &ext->fields_obj, rb_class_allocate_instance(rb_singleton_class(obj)));
+        RB_OBJ_WRITE(obj, &ext->fields_obj, rb_allocate_fields_obj(obj));
     }
     return ext->fields_obj;
 }
@@ -582,7 +590,7 @@ RCLASSEXT_SET_FIELDS_HASH(VALUE obj, rb_classext_t *ext, const st_table *tbl)
 
     if (!ext->fields_obj) {
         // FIXME: We can trigger GC here and `*tbl` may not be marked
-        RB_OBJ_WRITE(obj, &ext->fields_obj, rb_class_allocate_instance(rb_singleton_class(obj)));
+        RB_OBJ_WRITE(obj, &ext->fields_obj, rb_allocate_fields_obj(obj));
     }
     ROBJECT_SET_FIELDS_HASH(ext->fields_obj, tbl);
 }