Continue to fix incorrect casts via size_t for some platforms

Now also for casts:
- to remove alignment warnings
- casts between pointers and ints

src/apps/snmp/snmp_mib2_ip.c

@@ -581,7 +581,7 @@ ip_NetToMediaTable_get_next_cell_instance_and_value(const u32_t* column, struct
       snmp_ip4_to_oid(ip, &test_oid[1]);
 
       /* check generated OID: is it a candidate for the next one? */
-      snmp_next_oid_check(&state, test_oid, LWIP_ARRAYSIZE(ip_NetToMediaTable_oid_ranges), (void*)(size_t)i);
+      snmp_next_oid_check(&state, test_oid, LWIP_ARRAYSIZE(ip_NetToMediaTable_oid_ranges), LWIP_PTR_NUMERIC_CAST(void*, i));
     }
   }
 
@@ -589,7 +589,7 @@ ip_NetToMediaTable_get_next_cell_instance_and_value(const u32_t* column, struct
   if (state.status == SNMP_NEXT_OID_STATUS_SUCCESS) {
     snmp_oid_assign(row_oid, state.next_oid, state.next_oid_len);
     /* fill in object properties */
-    return ip_NetToMediaTable_get_cell_value_core((u8_t)(size_t)state.reference, column, value, value_len);
+    return ip_NetToMediaTable_get_cell_value_core(LWIP_PTR_NUMERIC_CAST(u8_t, state.reference), column, value, value_len);
   }
 
   /* not found */

src/core/ipv4/etharp.c

@@ -840,7 +840,7 @@ etharp_output(struct netif *netif, struct pbuf *q, const ip4_addr_t *ipaddr)
     if (!ip4_addr_netcmp(ipaddr, netif_ip4_addr(netif), netif_ip4_netmask(netif)) &&
         !ip4_addr_islinklocal(ipaddr)) {
 #if LWIP_AUTOIP
-      struct ip_hdr *iphdr = (struct ip_hdr*)(size_t)q->payload;
+      struct ip_hdr *iphdr = LWIP_ALIGNMENT_CAST(struct ip_hdr*, q->payload);
       /* According to RFC 3297, chapter 2.6.2 (Forwarding Rules), a packet with
          a link-local source address must always be "directly to its destination
          on the same physical link. The host MUST NOT send the packet to any

src/core/memp.c

@@ -209,7 +209,7 @@ memp_overflow_check_all(void)
     for (j = 0; j < memp_pools[i]->num; ++j) {
       memp_overflow_check_element_overflow(p, memp_pools[i]);
       memp_overflow_check_element_underflow(p, memp_pools[i]);
-      p = (struct memp*)(size_t)((u8_t*)p + MEMP_SIZE + memp_pools[i]->size + MEMP_SANITY_REGION_AFTER_ALIGNED);
+      p = LWIP_ALIGNMENT_CAST(struct memp*, ((u8_t*)p + MEMP_SIZE + memp_pools[i]->size + MEMP_SANITY_REGION_AFTER_ALIGNED));
     }
   }
   SYS_ARCH_UNPROTECT(old_level);

src/include/lwip/arch.h

@@ -138,6 +138,18 @@ typedef uintptr_t mem_ptr_t;
 #define LWIP_CONST_CAST(target_type, val) ((target_type)((ptrdiff_t)val))
 #endif
 
+/** Get rid of alignment cast warnings (GCC -Wcast-align) */
+#ifndef LWIP_ALIGNMENT_CAST
+#define LWIP_ALIGNMENT_CAST(target_type, val) LWIP_CONST_CAST(target_type, val)
+#endif
+
+/** Get rid of warnings related to pointer-to-numeric and vice-versa casts,
+ * e.g. "conversion from 'u8_t' to 'void *' of greater size"
+ */
+#ifndef LWIP_PTR_NUMERIC_CAST
+#define LWIP_PTR_NUMERIC_CAST(target_type, val) LWIP_CONST_CAST(target_type, val)
+#endif
+
 /** Allocates a memory buffer of specified size that is of sufficient size to align
  * its start address using LWIP_MEM_ALIGN.
  * You can declare your own version here e.g. to enforce alignment without adding