Add WithStack and WithMessage tests

Adds testFormatCompleteCompare as additional testing func.

The new function takes a string slice as "want", wherein
stacktraces and non-stacktrace messages are discerned by
strings.ContainsAny(want[i], "\n").

For example usage, see TestFormatWithStack & TestFormatWithMessage.

errors_test.go

@@ -84,6 +84,18 @@ func TestCause(t *testing.T) {
 	}, {
 		err:  x, // return from errors.New
 		want: x,
+	}, {
+		WithMessage(nil, "whoops"),
+		nil,
+	}, {
+		WithMessage(io.EOF, "whoops"),
+		io.EOF,
+	}, {
+		WithStack(nil),
+		nil,
+	}, {
+		WithStack(io.EOF),
+		io.EOF,
 	}}
 
 	for i, tt := range tests {
@@ -137,23 +149,78 @@ func TestErrorf(t *testing.T) {
 	}
 }
 
+func TestWithStackNil(t *testing.T) {
+	got := WithStack(nil)
+	if got != nil {
+		t.Errorf("WithStack(nil): got %#v, expected nil", got)
+	}
+}
+
+func TestWithStack(t *testing.T) {
+	tests := []struct {
+		err  error
+		want string
+	}{
+		{io.EOF, "EOF"},
+		{WithStack(io.EOF), "EOF"},
+	}
+
+	for _, tt := range tests {
+		got := WithStack(tt.err).Error()
+		if got != tt.want {
+			t.Errorf("WithStack(%v): got: %v, want %v", tt.err, got, tt.want)
+		}
+	}
+}
+
+func TestWithMessageNil(t *testing.T) {
+	got := WithMessage(nil, "no error")
+	if got != nil {
+		t.Errorf("WithMessage(nil, \"no error\"): got %#v, expected nil", got)
+	}
+}
+
+func TestWithMessage(t *testing.T) {
+	tests := []struct {
+		err     error
+		message string
+		want    string
+	}{
+		{io.EOF, "read error", "read error: EOF"},
+		{WithMessage(io.EOF, "read error"), "client error", "client error: read error: EOF"},
+	}
+
+	for _, tt := range tests {
+		got := WithMessage(tt.err, tt.message).Error()
+		if got != tt.want {
+			t.Errorf("WithMessage(%v, %q): got: %q, want %q", tt.err, tt.message, got, tt.want)
+		}
+	}
+
+}
+
 // errors.New, etc values are not expected to be compared by value
 // but the change in errors#27 made them incomparable. Assert that
 // various kinds of errors have a functional equality operator, even
 // if the result of that equality is always false.
 func TestErrorEquality(t *testing.T) {
-	tests := []struct {
-		err1, err2 error
-	}{
-		{io.EOF, io.EOF},
-		{io.EOF, nil},
-		{io.EOF, errors.New("EOF")},
-		{io.EOF, New("EOF")},
-		{New("EOF"), New("EOF")},
-		{New("EOF"), Errorf("EOF")},
-		{New("EOF"), Wrap(io.EOF, "EOF")},
+	vals := []error{
+		nil,
+		io.EOF,
+		errors.New("EOF"),
+		New("EOF"),
+		Errorf("EOF"),
+		Wrap(io.EOF, "EOF"),
+		Wrapf(io.EOF, "EOF%d", 2),
+		WithMessage(nil, "whoops"),
+		WithMessage(io.EOF, "whoops"),
+		WithStack(io.EOF),
+		WithStack(nil),
 	}
-	for _, tt := range tests {
-		_ = tt.err1 == tt.err2 // mustn't panic
+
+	for i := 0; i < len(vals); i++ {
+		for j := 0; j < len(vals); j++ {
+			_ = vals[i] == vals[j] // mustn't panic
+		}
 	}
 }

example_test.go

@@ -35,6 +35,59 @@ func ExampleNew_printf() {
 	//         /home/dfc/go/src/runtime/asm_amd64.s:2059
 }
 
+func ExampleWithMessage() {
+	cause := errors.New("whoops")
+	err := errors.WithMessage(cause, "oh noes")
+	fmt.Println(err)
+
+	// Output: oh noes: whoops
+}
+
+func ExampleWithStack() {
+	cause := errors.New("whoops")
+	err := errors.WithStack(cause)
+	fmt.Println(err)
+
+	// Output: whoops
+}
+
+func ExampleWithStack_printf() {
+	cause := errors.New("whoops")
+	err := errors.WithStack(cause)
+	fmt.Printf("%+v", err)
+
+	// Example Output:
+	// whoops
+	// github.com/pkg/errors_test.ExampleWithStack_printf
+	//         /home/fabstu/go/src/github.com/pkg/errors/example_test.go:55
+	// testing.runExample
+	//         /usr/lib/go/src/testing/example.go:114
+	// testing.RunExamples
+	//         /usr/lib/go/src/testing/example.go:38
+	// testing.(*M).Run
+	//         /usr/lib/go/src/testing/testing.go:744
+	// main.main
+	//         github.com/pkg/errors/_test/_testmain.go:106
+	// runtime.main
+	//         /usr/lib/go/src/runtime/proc.go:183
+	// runtime.goexit
+	//         /usr/lib/go/src/runtime/asm_amd64.s:2086
+	// github.com/pkg/errors_test.ExampleWithStack_printf
+	//         /home/fabstu/go/src/github.com/pkg/errors/example_test.go:56
+	// testing.runExample
+	//         /usr/lib/go/src/testing/example.go:114
+	// testing.RunExamples
+	//         /usr/lib/go/src/testing/example.go:38
+	// testing.(*M).Run
+	//         /usr/lib/go/src/testing/testing.go:744
+	// main.main
+	//         github.com/pkg/errors/_test/_testmain.go:106
+	// runtime.main
+	//         /usr/lib/go/src/runtime/proc.go:183
+	// runtime.goexit
+	//         /usr/lib/go/src/runtime/asm_amd64.s:2086
+}
+
 func ExampleWrap() {
 	cause := errors.New("whoops")
 	err := errors.Wrap(cause, "oh noes")