Improve error message for test

Author: tyilo

tests/cargo-fmt/main.rs

@@ -94,7 +94,11 @@ fn cargo_fmt_out_of_line_test_modules() {
     let (stdout, _) = cargo_fmt(&args);
     for file in expected_modified_files {
         let path = Path::new(file).canonicalize().unwrap();
-        assert!(stdout.contains(&format!("Diff in {}", path.display())))
+        assert!(
+            stdout.contains(&format!("Diff in {}", path.display())),
+            "{}",
+            path.display()
+        );
     }
 }