To reproduce, check out rustc and navigate to this line. RA reports an incorrect-arg-count there, but that is wrong -- the index function being called here is defined via a macro here. I have proc macros disabled so I am not surprised it cannot find that type, but it still should not report incorrect diagnostics. The type of field shows as &{unknown}, and when a method is called on an unknown type, RA shouldn't assume it knows which method that is and how many arguments it has.