mtest: stub the test interface and add failure tests

mtest/mtest.go

@@ -1,12 +1,17 @@
 // Package mtest is a support library for writing tests.
 package mtest
 
-import "testing"
+// TB is the subset of the testing.TB interface used by this package.
+type TB interface {
+	Helper()
+	Fatal(...any)
+	Fatalf(string, ...any)
+}
 
 // MustPanic executes a function f that is expected to panic.
 // If it does so, MustPanic returns the value recovered from the
 // panic. Otherwise, it logs a fatal error in t.
-func MustPanic(t *testing.T, f func()) (val any) {
+func MustPanic(t TB, f func()) (val any) {
 	t.Helper()
 	defer func() { val = recover() }()
 	f()
@@ -17,7 +22,7 @@ func MustPanic(t *testing.T, f func()) (val any) {
 // MustPanicf executes a function f that is expected to panic.  If it does so,
 // MustPanicf returns the value recovered from the panic. Otherwise it logs a
 // fatal error in t.
-func MustPanicf(t *testing.T, f func(), msg string, args ...any) (val any) {
+func MustPanicf(t TB, f func(), msg string, args ...any) (val any) {
 	t.Helper()
 	defer func() { val = recover() }()
 	f()

mtest/mtest_test.go

@@ -1,17 +1,69 @@
 package mtest_test
 
 import (
+	"fmt"
 	"testing"
 
 	"github.com/creachadair/mds/mtest"
 )
 
+// testStub implements the mtest.TB interface as a capturing shim to verify
+// that test failures are reported properly.
+type testStub struct {
+	failed bool
+	text   string
+}
+
+func (t *testStub) Fatal(args ...any) {
+	t.failed = true
+	t.text = fmt.Sprint(args...)
+}
+
+func (t *testStub) Fatalf(msg string, args ...any) {
+	t.failed = true
+	t.text = fmt.Sprintf(msg, args...)
+}
+
+func (*testStub) Helper() {}
+
 func TestMustPanic(t *testing.T) {
-	v := mtest.MustPanic(t, func() { panic("pass") })
-	t.Logf("Panic reported: %v", v)
+	t.Run("OK", func(t *testing.T) {
+		v := mtest.MustPanic(t, func() { panic("pass") })
+		t.Logf("Panic reported: %v", v)
+	})
+
+	t.Run("Fail", func(t *testing.T) {
+		var s testStub
+		v := mtest.MustPanic(&s, func() {})
+		if !s.failed {
+			t.Error("Test did not fail as expected")
+		}
+		if s.text == "" {
+			t.Error("Failure did not log a message")
+		}
+		if v != nil {
+			t.Errorf("Unexpected panic value: %v", v)
+		}
+	})
 }
 
 func TestMustPanicf(t *testing.T) {
-	v := mtest.MustPanicf(t, func() { panic("pass") }, "bad things: %d", 5)
-	t.Logf("Panic reported: %v", v)
+	t.Run("OK", func(t *testing.T) {
+		v := mtest.MustPanicf(t, func() { panic("pass") }, "bad things")
+		t.Logf("Panic reported: %v", v)
+	})
+
+	t.Run("Fail", func(t *testing.T) {
+		var s testStub
+		v := mtest.MustPanicf(&s, func() {}, "bad: %d", 11)
+		if !s.failed {
+			t.Error("Test did not fail as expected")
+		}
+		if s.text != "bad: 11" {
+			t.Errorf("Wrong message: got %q, want bad: 11", s.text)
+		}
+		if v != nil {
+			t.Errorf("Unexpected panic value: %v", v)
+		}
+	})
 }