[fine] Fiddle with tests, no errors

fine/build.rs

@@ -82,6 +82,8 @@ fn generate_test_for_file(path: PathBuf) -> String {
             assertions.push(quote! {
                 crate::assert_no_errors(&_tree, &_lines);
             });
+        } else if line.starts_with("@") {
+            panic!("Test file {display_path} has unknown directive: {line}");
         }
     }