Air carrier pilot reported a near miss on final approach with a general aviation aircraft that ATC cleared through the airspace requiring them only to maintain visual separation. Flight crew responded to the TCAS RA which resulted in crossing the Final Approach Fix below charted altitude.

Narrative

Flight ZZZ-RIC; while getting vectored onto final for ILS 16; Potomac Approach allowed Aircraft Y to maintain visual separation with us - inside RIC class C airspace. This reckless pilot flew straight at us at the same altitude. We watched them on TCAS; got a TA; and as I was about to go around; we got an RA. This was at 2500' approaching the final approach fix. I notified Potomac of the RA. Looking back at information; it appears Aircraft Y did not deviate at all. This was a scary situation and Potomac allowed this pilot to maintain visual separate and at NO point did they ask Aircraft Y what they were doing; or notify us that we needed to go around or cancel our approach clearance. This was a scary and dangerous situation; caused by a negligent pilot and allowed (again!!) to occur by Potomac approach. This was eerily similar to the DCA accident and is unacceptable. We complied with the descent RA; we descended about 500'; crossed the FAF below the FAF altitude. The FAF at RIC is slightly over 2300' AFE - we were able to level; get configured; and continue a stabilized approach.Cause: Reckless general aviation pilot and Potomac Approach giving a maintain visual separation clearance inside controlled airspace without intervening with a near accident.Potomac Approach gave a visual separation clearance to general aviation aircraft in controlled airspace without intervening with a near accident. Due to their ease in giving these visual clearances combined with their lack of diligence in ensuring their clearance are being followed by GA traffic to ensure the safety of 121 traffic operating in controlled airspace during very busy phases of flight such as short final (in DCA) or; in our case; at the final approach fix; their processes needs to be reevaluated. Potomac Approach CREATED this risk by allowing the GA aircraft to continue with a visual separation clearance in RIC class C airspace while flying straight at us at the same altitude with an intersection near the final approach fix. This clearance. is unacceptable to begin with. They should have been vectored around us. Potomac acted recklessly with this clearance. They never intervened as Aircraft Y continued to fly straight at us at our altitude; near the very busy final approach fix.