Make it clear that the file is the red part of the diff and the actual stderr output is the green part

Author: oli-obk

src/lib.rs

@@ -322,7 +322,9 @@ pub fn run_tests_generic(config: Config, file_filter: impl Fn(&Path) -> bool + S
                         actual,
                         expected,
                     } => {
-                        eprintln!("actual output differed from expected {}", path.display());
+                        eprintln!("{}", "actual output differed from expected".underline());
+                        eprintln!("{}", format!("--- {}", path.display()).red());
+                        eprintln!("{}", "+++ <stderr output>".green());
                         diff::print_diff(expected, actual);
                     }
                     Error::ErrorsWithoutPattern { path: None, msgs } => {

tests/integrations/basic-fail/Cargo.stderr

@@ -39,7 +39,9 @@ For more information about this error, try `rustc --explain E0308`.
 tests/actual_tests/foomp.rs FAILED:
 command: "rustc" "--error-format=json" "--edition=2021" "--extern" "basic_fail=$DIR/$DIR/../../../target/debug/libbasic_fail-$HASH.rmeta" "-L" "$DIR/$DIR/../../../target/debug" "-L" "$DIR/$DIR/../../../target/debug" "tests/actual_tests/foomp.rs"
 
-actual output differed from expected tests/actual_tests/foomp.stderr
+actual output differed from expected
+--- tests/actual_tests/foomp.stderr
++++ <stderr output>
 ... 3 lines skipped ...
  4 |     add("42", 3);
    |     --- ^^^^ expected `usize`, found `&str`