New approach for marking instances as always-nonzero

Set a flag on the class, rather than having to specify a whitelist.
Even though this is pretty annoying I think it's still worth it.

src/runtime/list.cpp

@@ -1282,6 +1282,7 @@ void setupList() {
                                            sizeof(BoxedListIterator), false, "listiterator");
     list_reverse_iterator_cls = BoxedClass::create(type_cls, object_cls, &BoxedListIterator::gcHandler, 0, 0,
                                                    sizeof(BoxedListIterator), false, "listreverseiterator");
+    list_iterator_cls->instances_are_nonzero = list_reverse_iterator_cls->instances_are_nonzero = true;
 
     list_cls->giveAttr("__len__", new BoxedFunction(boxRTFunction((void*)listLen, BOXED_INT, 1)));
 

src/runtime/objmodel.cpp

@@ -2486,12 +2486,13 @@ extern "C" bool nonzero(Box* obj) {
             rewriter.reset();
 
         if (rtn == NULL) {
-            ASSERT(obj->cls->is_user_defined || obj->cls == classobj_cls || obj->cls == type_cls
-                       || isSubclass(obj->cls, Exception) || obj->cls == file_cls || obj->cls == traceback_cls
-                       || obj->cls == instancemethod_cls || obj->cls == module_cls || obj->cls == capifunc_cls
-                       || obj->cls == builtin_function_or_method_cls || obj->cls == method_cls || obj->cls == frame_cls
-                       || obj->cls == generator_cls || obj->cls == capi_getset_cls || obj->cls == pyston_getset_cls
-                       || obj->cls == wrapperdescr_cls || obj->cls == wrapperobject_cls,
+            ASSERT(obj->cls->is_user_defined || obj->cls->instances_are_nonzero || obj->cls == classobj_cls
+                       || obj->cls == type_cls || isSubclass(obj->cls, Exception) || obj->cls == file_cls
+                       || obj->cls == traceback_cls || obj->cls == instancemethod_cls || obj->cls == module_cls
+                       || obj->cls == capifunc_cls || obj->cls == builtin_function_or_method_cls
+                       || obj->cls == method_cls || obj->cls == frame_cls || obj->cls == generator_cls
+                       || obj->cls == capi_getset_cls || obj->cls == pyston_getset_cls || obj->cls == wrapperdescr_cls
+                       || obj->cls == wrapperobject_cls,
                    "%s.__nonzero__", getTypeName(obj)); // TODO
 
             if (rewriter.get()) {

src/runtime/types.h

@@ -235,6 +235,12 @@ public:
     // that we can't rely on for extension classes.
     bool is_pyston_class;
 
+    // Just for debugging: whether instances of this class should always be considered nonzero.
+    // This is the default for anything that doesn't define a __nonzero__ or __len__ method, but
+    // for builtin types we have the extra check that we opted into this behavior rather than
+    // just forgot to add nonzero/len.
+    bool instances_are_nonzero;
+
     bool has___class__; // Has a custom __class__ attribute (ie different from object's __class__ descriptor)
     bool has_instancecheck;
     bool has_subclasscheck;