feat: show actual vs expected on fail

internal/mire/test.go

@@ -2,6 +2,7 @@ package mire
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"io"
 	"io/fs"
@@ -38,6 +39,15 @@ type testFixtureFiles struct {
 	outPath string
 }
 
+type testMismatchError struct {
+	expected []byte
+	actual   []byte
+}
+
+func (e *testMismatchError) Error() string {
+	return "output differed"
+}
+
 // RunTests replays all scenarios under the configured test directory.
 func RunTests(path string) error {
 	return runTests(path, testIO{
@@ -89,6 +99,10 @@ func runTests(path string, tio testIO) error {
 			elapsed := time.Since(start)
 			summary.failed++
 			output.Fprintf(tio.out, "%s %s (%s): %v\n", output.Label("FAIL", output.Fail), scenario.relPath, formatElapsed(elapsed), err)
+			var mismatchErr *testMismatchError
+			if errors.As(err, &mismatchErr) {
+				writeScenarioMismatch(tio.out, mismatchErr)
+			}
 			continue
 		}
 
@@ -256,12 +270,26 @@ func replayScenario(scenario testScenario, shellPath string, _ testIO, sandboxCo
 	}
 
 	if !bytes.Equal(got, want) {
-		return fmt.Errorf("output differed")
+		return &testMismatchError{
+			expected: want,
+			actual:   got,
+		}
 	}
 
 	return nil
 }
 
+func writeScenarioMismatch(w io.Writer, mismatch *testMismatchError) {
+	output.Fprintf(w, "Expected:\n%s", string(mismatch.expected))
+	if len(mismatch.expected) == 0 || mismatch.expected[len(mismatch.expected)-1] != '\n' {
+		output.Fprintf(w, "\n")
+	}
+	output.Fprintf(w, "Actual:\n%s", string(mismatch.actual))
+	if len(mismatch.actual) == 0 || mismatch.actual[len(mismatch.actual)-1] != '\n' {
+		output.Fprintf(w, "\n")
+	}
+}
+
 func trimReplayOutputToMarker(data []byte, shellPath string) ([]byte, error) {
 	idx := bytes.Index(data, []byte(compareOutputMarker))
 	if idx == -1 {

internal/mire/test_test.go

@@ -157,6 +157,45 @@ func TestReplayScenarioFailsWhenCompareMarkerMissing(t *testing.T) {
 	}
 }
 
+func TestRunTestsShowsExpectedAndActualForMismatchedScenario(t *testing.T) {
+	root := t.TempDir()
+	testDir := filepath.Join(root, "e2e")
+	testutil.AddFakeRecordDependencies(t, "script")
+	t.Setenv("FAKE_SCRIPT_ECHO_STDIN", "1")
+
+	testutil.WriteFile(t, filepath.Join(root, "mire.toml"), testutil.ValidConfigContent("e2e"))
+	mustWriteRecordShell(t, testDir)
+	testutil.WriteScenarioFixtures(t, filepath.Join(testDir, "suite", "spec"), "echo two\n", "echo one\n")
+
+	var stdout bytes.Buffer
+	var stderr bytes.Buffer
+	err := testutil.WithWorkingDir(t, root, func() error {
+		return runTests("", testIO{
+			out: &stdout,
+			err: &stderr,
+		})
+	})
+	if err != nil {
+		t.Fatalf("runTests() error = %v", err)
+	}
+	if stderr.String() != "" {
+		t.Fatalf("stderr = %q, want empty", stderr.String())
+	}
+
+	for _, want := range []string{
+		"RUN suite/spec",
+		"FAIL suite/spec (",
+		"output differed",
+		"Expected:\necho one\n",
+		"Actual:\necho two\n",
+		"Summary: total=1 passed=0 failed=1",
+	} {
+		if !strings.Contains(stdout.String(), want) {
+			t.Fatalf("stdout = %q, want substring %q", stdout.String(), want)
+		}
+	}
+}
+
 func TestDiscoverTestScenariosUsesDisplayRootForRelativePaths(t *testing.T) {
 	testDir := filepath.Join(t.TempDir(), "e2e")
 	scopedDir := filepath.Join(testDir, "nested")