Function builtin_args 

fn builtin_args(
    args: &[RValue],
    _named: &[(String, RValue)],
) -> Result<RValue, RError>

args(fn) — return the formals of a function (simplified: same as formals). In GNU R, args() returns a function with the same formals but NULL body. We simplify to just returning formals, which covers all practical uses.