The field deserialization code in the IDL generator is confusing to read and there's a lot of duplicated code. An example of an improvement is encapsulating the deserializer arguments into a context, renaming certain arguments ("bson_element", "ast_type" could be renamed to something more accurate), and restructuring functions ("_gen_variant_deserializer_from_obj" and "_gen_variant_deserializer" do different things but are named in a way that suggests they are similar).