Introduced #define LWIP_UNUSED_ARG(x) use this for and architecture-independent form to tell the compiler you intentionally are not using this variable. Can be overriden in cc.h.

src/core/snmp/msg_in.c

@@ -801,7 +801,7 @@ snmp_recv(void *arg, struct udp_pcb *pcb, struct pbuf *p, struct ip_addr *addr,
   struct udp_hdr *udphdr;
 
   /* suppress unused argument warning */
-  if (arg);
+  LWIP_UNUSED_ARG(arg);
   /* peek in the UDP header (goto IP payload) */
   if(pbuf_header(p, UDP_HLEN)){
     LWIP_ASSERT("Can't move to UDP header", 0);