New errors

errors/src/errors/parser/parser_errors.rs

@@ -328,4 +328,12 @@ create_messages!(
         msg: format!("Cannot declare external struct."),
         help: None,
     }
+
+    /// Enforce that cannot use an external type to do anything except input/output of function
+    @formatted
+    external_type_cannot_be_used_inside_function {
+        args: (program: impl Display, file_type: impl Display),
+        msg: format!("External types cannot be used inside function (only as input/output types) -- found exported type from '{program}.{file_type}'."),
+        help: None,
+    }
 );