[mycpp/runtime] Fix mycpp header init after FIELD_MASK() move

Use GC_CLASS_FIXED() and GC_CLASS_SCANNED(), instead of just GC_CLASS().

We also don't need Tag::Opaque because Tag::Scanned with
NUM_POINTERS(header_) == 0 should be just as efficient.

mycpp/cppgen_pass.py

@@ -2170,10 +2170,7 @@ def visit_class_def(self, o: 'mypy.nodes.ClassDef') -> T:
             # So we declare them in the right order
             sorted_member_names = pointer_members + non_pointer_members
 
-            if len(pointer_members) == 0:
-              self.field_gc[o] = ('HeapTag::Opaque', None)
-            else:
-              self.field_gc[o] = ('HeapTag::Scanned', len(pointer_members))
+            self.field_gc[o] = ('HeapTag::Scanned', len(pointer_members))
           else:
             # Has inheritance
 
@@ -2250,17 +2247,17 @@ def visit_class_def(self, o: 'mypy.nodes.ClassDef') -> T:
                 obj_tag, obj_arg = self.field_gc[o]
                 if obj_tag == 'HeapTag::FixedSize':
                   obj_mask = obj_arg
-                  obj_len = 'kNoObjLen'  # don't need length
+                  header_init = 'GC_CLASS_FIXED(header_, %s, sizeof(%s))' % (
+                      obj_mask, o.name)
                 elif obj_tag == 'HeapTag::Scanned':
-                  obj_mask = 'kZeroMask'
-                  #obj_len = '%d * sizeof(void*) + sizeof(ObjHeader)' % obj_arg
-                  obj_len = str(obj_arg)  # number of pointers
-                elif obj_tag == 'HeapTag::Opaque':
-                  obj_mask = 'kZeroMask'
-                  obj_len = 'kNoObjLen'
+                  num_pointers = obj_arg
+                  header_init = 'GC_CLASS_SCANNED(header_, %s, sizeof(%s))' % (
+                      num_pointers, o.name)
+                else: 
+                  raise AssertionError(o.name)
 
                 self.write('\n')
-                self.write('    : GC_CLASS(header_, %s, %s, %s)' % (obj_tag, obj_mask, obj_len))
+                self.write('    : %s' % header_init)
 
               # Check for Base.__init__(self, ...) and move that to the initializer list.
 

mycpp/gc_obj.h

@@ -75,14 +75,19 @@ struct RawObject {
     kIsHeader, TypeTag::OtherClass, kNoObjId, HeapTag::FixedSize, field_mask \
   }
 
-// Used by mycpp and frontend/flag_gen.py.  TODO: Sort fields and use
-// HeapTag::Scanned.
+// Classes with no inheritance (e.g. used by mycpp)
+#define GC_CLASS_SCANNED(header_, num_pointers, obj_len)                     \
+  header_ {                                                                  \
+    kIsHeader, TypeTag::OtherClass, kNoObjId, HeapTag::Scanned, num_pointers \
+  }
+
+// Used by frontend/flag_gen.py.  TODO: Sort fields and use GC_CLASS_SCANNED
 #define GC_CLASS(header_, heap_tag, field_mask, obj_len)           \
   header_ {                                                        \
     kIsHeader, TypeTag::OtherClass, kNoObjId, heap_tag, field_mask \
   }
 
-// Used by ASDL.  TODO: Sort fields and use HeapTag::Scanned
+// Used by ASDL.  TODO: Sort fields and use GC_CLASS_SCANNED
 #define GC_ASDL_CLASS(header_, type_tag, field_mask, obj_len)     \
   header_ {                                                       \
     kIsHeader, type_tag, kNoObjId, HeapTag::FixedSize, field_mask \