Function unescapeByteChar

parser/lexer/utils.go:261–337

unescapeByteChar unescapes a single character or escape sequence from a bytes literal. Unlike unescapeChar, this only supports byte-level escapes (\x, octal) and rejects Unicode escapes (\u, \U) since bytes literals represent raw byte sequences. Note: We cannot use strconv.UnquoteChar here because

(s string)

// escapes as Unicode codepoints (e.g., \xff → codepoint 255 → 2 UTF-8 bytes),
// whereas bytes literals require them as raw byte values (\xff → single byte 255).
func unescapeByteChar(s string) (value rune, multibyte bool, tail string, err error) {
	// Non-escape: return the character as-is.
	// For bytes literals, we accept UTF-8 sequences but they get encoded back to bytes.
	c := s[0]
	if c != '\\' {
		if c >= utf8.RuneSelf {
			r, size := utf8.DecodeRuneInString(s)
			return r, true, s[size:], nil
		}
		return rune(c), false, s[1:], nil
	}

	// Escape sequence: need at least one more character.
	if len(s) <= 1 {
		return 0, false, "", fmt.Errorf("unable to unescape string, found '\\' as last character")
	}

	c = s[1]
	s = s[2:]

	switch c {
	// Simple escape sequences
	case 'a':
		return '\a', false, s, nil
	case 'b':
		return '\b', false, s, nil
	case 'f':
		return '\f', false, s, nil
	case 'n':
		return '\n', false, s, nil
	case 'r':
		return '\r', false, s, nil
	case 't':
		return '\t', false, s, nil
	case 'v':
		return '\v', false, s, nil
	case '\\':
		return '\\', false, s, nil
	case '\'':
		return '\'', false, s, nil
	case '"':
		return '"', false, s, nil
	case '`':
		return '`', false, s, nil
	case '?':
		return '?', false, s, nil

	// Hex escape: \xNN (exactly 2 hex digits, value 0-255)
	case 'x', 'X':
		if len(s) < 2 {
			return 0, false, "", fmt.Errorf("unable to unescape string")
		}
		hi, ok1 := unhex(s[0])
		lo, ok2 := unhex(s[1])
		if !ok1 || !ok2 {
			return 0, false, "", fmt.Errorf("unable to unescape string")
		}
		return hi<<4 | lo, false, s[2:], nil