// given how rarely we expect to hit these edge cases, speed and
			// conciseness are better.
			var width int
			if c == runeError {
				width = 1
				if i+2 < len(s) && s[i:i+3] == string(runeError) {
					width = 3
				}
			} else {
				width = len(string(c))
			}
			for j := 0; j < width; j++ {
				buf = append(buf, `\x`...)

		{"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},
		{"\xff", '�', 0},
		{"☻x�", '�', len("☻x")},
		{"☻x\xe2\x98", '�', len("☻x")},
		{"☻x\xe2\x98�", '�', len("☻x")},
		{"☻x\xe2\x98x", '�', len("☻x")},

	{"\xffff", `"\xffff"`},
	{"hello \xffff world", `"hello \xffff world"`},
	{"\uFFFD", `"\xef\xbf\xbd"`},                                             // utf8.RuneError
	{"\uFFFD hello \uFFFD world", `"\xef\xbf\xbd hello \xef\xbf\xbd world"`}, // utf8.RuneError
	// overflow
	{"9223372036854775810ns", `"9223372036854775810ns"`},
	{"9223372036854775808ns", `"9223372036854775808ns"`},

		{"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},
		{"\xff", '�', 0},
		{"☻x�", '�', len("☻x")},
		{"☻x\xe2\x98", '�', len("☻x")},
		{"☻x\xe2\x98�", '�', len("☻x")},
		{"☻x\xe2\x98x", '�', len("☻x")},

	for s != "" {
		rune, size := utf8.DecodeRuneInString(s)
		if rune == utf8.RuneError && size == 1 {
			return &Error{Code: ErrInvalidUTF8, Expr: s}
		}
		s = s[size:]
	}
	return nil
}

func nextRune(s string) (c rune, t string, err error) {
	c, size := utf8.DecodeRuneInString(s)
	if c == utf8.RuneError && size == 1 {
		return 0, "", &Error{Code: ErrInvalidUTF8, Expr: s}
	}