emacs_env::make_function lacks the following features supported by
`defun':
1. Functions with both optional and rest arguments.
2. Specification of parameter names.
3. Integration with `help-function-arglist'.
4. Specification of interactive forms.
5. Specification of declare forms.
6. Docstrings containing null or non-Unicode characters.
(6) is probably rather unimportant. (5) is probably not implementable
(would require wrapping `defun', not `lambda'). (1)–(4) are more severe
and quite limit the usefulness of make_function right now; for a
truly generic `defun'-like construct one currently has to eval a `defun'
form wrapping another function.
To solve (1)–(3), I'd propose replacing the "arity" arguments with a
true arglist specification. This could either be at the C level, e.g.
ptrdiff_t num_mandatory_args, char** mandatory_arg_names,
ptrdiff_t num_optional_args, char** optional_arg_names,
char* rest_arg_name
or by requiring to pass a Lisp argument list.
I've attached a patch for fixing (1)-(4) and (6).
0001-Introduce-new-module-function-make_function_ext.patch