Get rid of SHAPE_T_OBJECT

Now the `heap_index` is stored as flags inside `shape_id_t`.

This saves having to rebuild the entire tree when moving from one
slot size to the other.

Author: byroot

ext/objspace/objspace_dump.c

@@ -817,9 +817,6 @@ shape_id_i(shape_id_t shape_id, void *data)
         dump_append(dc, ",\"edge_name\":");
         dump_append_id(dc, shape->edge_name);
 
-        break;
-      case SHAPE_T_OBJECT:
-        dump_append(dc, ", \"shape_type\":\"T_OBJECT\"");
         break;
       case SHAPE_OBJ_ID:
         dump_append(dc, ", \"shape_type\":\"OBJ_ID\"");

gc.c

@@ -379,19 +379,7 @@ rb_gc_set_shape(VALUE obj, uint32_t shape_id)
 uint32_t
 rb_gc_rebuild_shape(VALUE obj, size_t heap_id)
 {
-    shape_id_t orig_shape_id = rb_obj_shape_id(obj);
-    if (rb_shape_too_complex_p(orig_shape_id)) {
-        return (uint32_t)orig_shape_id;
-    }
-
-    shape_id_t initial_shape_id = rb_shape_root(heap_id);
-    shape_id_t new_shape_id = rb_shape_traverse_from_new_root(initial_shape_id, orig_shape_id);
-
-    if (new_shape_id == INVALID_SHAPE_ID) {
-         return 0;
-     }
-
-    return (uint32_t)new_shape_id;
+    return (uint32_t)rb_shape_transition_heap(obj, heap_id);
 }
 
 void rb_vm_update_references(void *ptr);
@@ -1923,6 +1911,7 @@ object_id0(VALUE obj)
     // rb_shape_object_id_shape may lock if the current shape has
     // multiple children.
     shape_id_t object_id_shape_id = rb_shape_transition_object_id(obj);
+    RUBY_ASSERT(rb_shape_has_object_id(object_id_shape_id));
 
     id = generate_next_object_id();
     rb_obj_field_set(obj, object_id_shape_id, id);

object.c

@@ -339,7 +339,7 @@ rb_obj_copy_ivar(VALUE dest, VALUE obj)
     shape_id_t dest_shape_id = src_shape_id;
     shape_id_t initial_shape_id = RBASIC_SHAPE_ID(dest);
 
-    RUBY_ASSERT(RSHAPE(initial_shape_id)->type == SHAPE_T_OBJECT);
+    RUBY_ASSERT(RSHAPE(initial_shape_id)->type == SHAPE_ROOT);
 
     dest_shape_id = rb_shape_rebuild(initial_shape_id, src_shape_id);
     if (UNLIKELY(rb_shape_too_complex_p(dest_shape_id))) {
@@ -363,7 +363,7 @@ rb_obj_copy_ivar(VALUE dest, VALUE obj)
     }
 
     rb_shape_copy_fields(dest, dest_buf, dest_shape_id, obj, src_buf, src_shape_id);
-    rb_obj_set_shape_id(dest, dest_shape_id);
+    RBASIC_SET_SHAPE_ID(dest, dest_shape_id);
 }
 
 static void