Verify struct sizing at build time.

data/xap/xap_0.0.1.hjson

@@ -142,6 +142,7 @@
             name: Request Header
             description: Packet format for inbound data.
             type: struct
+            struct_length: 3
             struct_members: [
                 {
                     type: token
@@ -158,6 +159,7 @@
             name: Response Header
             description: Packet format for inbound data.
             type: struct
+            struct_length: 4
             struct_members: [
                 {
                     type: token

data/xap/xap_0.1.0.hjson

@@ -83,6 +83,7 @@
             name: Broadcast Header
             description: Packet format for broadcast messages.
             type: struct
+            struct_length: 4
             struct_members: [
                 {
                     type: token
@@ -240,6 +241,7 @@
                         Retrieves the set of identifying information for the board.
                         '''
                     return_type: struct
+                    return_struct_length: 10
                     return_struct_members: [
                         {
                             type: u16

data/xap/xap_0.2.0.hjson

@@ -38,6 +38,7 @@
                     define: GET_KEYMAP_KEYCODE
                     description: TODO
                     request_type: struct
+                    request_struct_length: 3
                     request_struct_members: [
                         {
                             type: u8
@@ -61,6 +62,7 @@
                     define: SET_KEYMAP_KEYCODE
                     description: TODO
                     request_type: struct
+                    request_struct_length: 5
                     request_struct_members: [
                         {
                             type: u8

lib/python/qmk/xap/gen_firmware/header_generator.py

@@ -133,6 +133,9 @@ def _append_route_types(lines, container, container_id=None, route_stack=None):
             lines.append(f'    {member_type} {member_name};')
         lines.append(f'}} __attribute__((__packed__)) {route_name}_arg_t;')
 
+        req_len = container['request_struct_length']
+        lines.append(f'_Static_assert(sizeof({route_name}_arg_t)  == {req_len}, "{route_name}_arg_t needs to be {req_len} bytes in size");')
+
     elif 'request_type' in container:
         request_type = container['request_type']
         found = re.search(r'(u\d+)\[(\d+)\]', request_type)
@@ -153,6 +156,9 @@ def _append_route_types(lines, container, container_id=None, route_stack=None):
             lines.append(f'    {member_type} {member_name};')
         lines.append(f'}} __attribute__((__packed__)) {route_name}_t;')
 
+        req_len = container['return_struct_length']
+        lines.append(f'_Static_assert(sizeof({route_name}_t)  == {req_len}, "{route_name}_t needs to be {req_len} bytes in size");')
+
     elif 'return_type' in container:
         return_type = container['return_type']
         found = re.search(r'(u\d+)\[(\d+)\]', return_type)
@@ -213,6 +219,9 @@ def _append_internal_types(lines, container):
                     member_type = additional_types[member["type"]]
                 lines.append(f'  {member_type} {member_name};')
             lines.append(f'}} __attribute__((__packed__)) xap_{key}_t;')
+
+            req_len = value['struct_length']
+            lines.append(f'_Static_assert(sizeof(xap_{key}_t)  == {req_len}, "xap_{key}_t needs to be {req_len} bytes in size");')
         else:
             lines.append(f'typedef {data_type} xap_{key}_t;')