func TestIndexRune(t *testing.T) {
	tests := []struct {
		in   string
		rune rune
		want int
	}{
		{"", 'a', -1},
		{"", '☺', -1},
		{"foo", '☹', -1},
		{"foo", 'o', 1},
		{"foo☺bar", '☺', 3},
		{"foo☺☻☹bar", '☹', 9},
		{"a A x", 'A', 2},
		{"some_text=some_value", '=', 9},
		{"☺a", 'a', 3},
		{"a☻☺b", '☺', 4},

		// RuneError should match any invalid UTF-8 byte sequence.
		{"�", '�', 0},