fix(unstable): correct Parser.Range for non-suffix highlights

Parser.Range used len(data)-len(highlight), which only matches suffix
slices. Single-byte highlights like b[0:1] are subslices of the
remaining buffer, so the wrong offset pointed at the end of the
document (issue #1047). Use pointer-based subslice offset like
wrapDecodeError.

Add regression tests for unstable.Parser and toml.Unmarshal error
positions and human-readable output.

Co-authored-by: Thomas Pelletier <thomas@pelletier.dev>

decode.go

@@ -9,60 +9,64 @@ import (
 	"github.com/pelletier/go-toml/v2/unstable"
 )
 
-func parseInteger(b []byte, base int) (int64, error) {
+func parseInteger(b []byte) (int64, error) {
 	if len(b) > 2 && b[0] == '0' {
 		switch b[1] {
 		case 'x':
-			return parseIntHex(b, base)
+			return parseIntHex(b)
 		case 'b':
-			return parseIntBin(b, base)
+			return parseIntBin(b)
 		case 'o':
-			return parseIntOct(b, base)
+			return parseIntOct(b)
 		default:
 			panic(fmt.Errorf("invalid base '%c', should have been checked by scanIntOrFloat", b[1]))
 		}
 	}
 
-	return parseIntDec(b, base)
+	return parseIntDec(b)
 }
 
-func parseLocalDate(b []byte, base int) (LocalDate, error) {
+func parseLocalDate(b []byte) (LocalDate, error) {
+	// full-date      = date-fullyear "-" date-month "-" date-mday
+	// date-fullyear  = 4DIGIT
+	// date-month     = 2DIGIT  ; 01-12
+	// date-mday      = 2DIGIT  ; 01-28, 01-29, 01-30, 01-31 based on month/year
 	var date LocalDate
 
 	if len(b) != 10 || b[4] != '-' || b[7] != '-' {
-		return date, unstable.NewParserError(b, base, "dates are expected to have the format YYYY-MM-DD")
+		return date, unstable.NewParserError(b, "dates are expected to have the format YYYY-MM-DD")
 	}
 
 	var err error
 
-	date.Year, err = parseDecimalDigits(b[0:4], base)
+	date.Year, err = parseDecimalDigits(b[0:4])
 	if err != nil {
 		return LocalDate{}, err
 	}
 
-	date.Month, err = parseDecimalDigits(b[5:7], base+5)
+	date.Month, err = parseDecimalDigits(b[5:7])
 	if err != nil {
 		return LocalDate{}, err
 	}
 
-	date.Day, err = parseDecimalDigits(b[8:10], base+8)
+	date.Day, err = parseDecimalDigits(b[8:10])
 	if err != nil {
 		return LocalDate{}, err
 	}
 
 	if !isValidDate(date.Year, date.Month, date.Day) {
-		return LocalDate{}, unstable.NewParserError(b, base, "impossible date")
+		return LocalDate{}, unstable.NewParserError(b, "impossible date")
 	}
 
 	return date, nil
 }
 
-func parseDecimalDigits(b []byte, base int) (int, error) {
+func parseDecimalDigits(b []byte) (int, error) {
 	v := 0
 
 	for i, c := range b {
 		if c < '0' || c > '9' {
-			return 0, unstable.NewParserError(b[i:i+1], base+i, "expected digit (0-9)")
+			return 0, unstable.NewParserError(b[i:i+1], "expected digit (0-9)")
 		}
 		v *= 10
 		v += int(c - '0')
@@ -71,18 +75,21 @@ func parseDecimalDigits(b []byte, base int) (int, error) {
 	return v, nil
 }
 
-func parseDateTime(b []byte, base int) (time.Time, error) {
-	origLen := len(b)
-	dt, b, err := parseLocalDateTime(b, base)
+func parseDateTime(b []byte) (time.Time, error) {
+	// offset-date-time = full-date time-delim full-time
+	// full-time      = partial-time time-offset
+	// time-offset    = "Z" / time-numoffset
+	// time-numoffset = ( "+" / "-" ) time-hour ":" time-minute
+
+	dt, b, err := parseLocalDateTime(b)
 	if err != nil {
 		return time.Time{}, err
 	}
 
-	tzBase := base + origLen - len(b)
-
 	var zone *time.Location
 
 	if len(b) == 0 {
+		// parser should have checked that when assigning the date time node
 		panic("date time should have a timezone")
 	}
 
@@ -92,7 +99,7 @@ func parseDateTime(b []byte, base int) (time.Time, error) {
 	} else {
 		const dateTimeByteLen = 6
 		if len(b) != dateTimeByteLen {
-			return time.Time{}, unstable.NewParserError(b, tzBase, "invalid date-time timezone")
+			return time.Time{}, unstable.NewParserError(b, "invalid date-time timezone")
 		}
 		var direction int
 		switch b[0] {
@@ -101,27 +108,27 @@ func parseDateTime(b []byte, base int) (time.Time, error) {
 		case '+':
 			direction = +1
 		default:
-			return time.Time{}, unstable.NewParserError(b[:1], tzBase, "invalid timezone offset character")
+			return time.Time{}, unstable.NewParserError(b[:1], "invalid timezone offset character")
 		}
 
 		if b[3] != ':' {
-			return time.Time{}, unstable.NewParserError(b[3:4], tzBase+3, "expected a : separator")
+			return time.Time{}, unstable.NewParserError(b[3:4], "expected a : separator")
 		}
 
-		hours, err := parseDecimalDigits(b[1:3], tzBase+1)
+		hours, err := parseDecimalDigits(b[1:3])
 		if err != nil {
 			return time.Time{}, err
 		}
 		if hours > 23 {
-			return time.Time{}, unstable.NewParserError(b[:1], tzBase, "invalid timezone offset hours")
+			return time.Time{}, unstable.NewParserError(b[:1], "invalid timezone offset hours")
 		}
 
-		minutes, err := parseDecimalDigits(b[4:6], tzBase+4)
+		minutes, err := parseDecimalDigits(b[4:6])
 		if err != nil {
 			return time.Time{}, err
 		}
 		if minutes > 59 {
-			return time.Time{}, unstable.NewParserError(b[:1], tzBase, "invalid timezone offset minutes")
+			return time.Time{}, unstable.NewParserError(b[:1], "invalid timezone offset minutes")
 		}
 
 		seconds := direction * (hours*3600 + minutes*60)
@@ -134,7 +141,7 @@ func parseDateTime(b []byte, base int) (time.Time, error) {
 	}
 
 	if len(b) > 0 {
-		return time.Time{}, unstable.NewParserError(b, tzBase, "extra bytes at the end of the timezone")
+		return time.Time{}, unstable.NewParserError(b, "extra bytes at the end of the timezone")
 	}
 
 	t := time.Date(
@@ -150,15 +157,15 @@ func parseDateTime(b []byte, base int) (time.Time, error) {
 	return t, nil
 }
 
-func parseLocalDateTime(b []byte, base int) (LocalDateTime, []byte, error) {
+func parseLocalDateTime(b []byte) (LocalDateTime, []byte, error) {
 	var dt LocalDateTime
 
 	const localDateTimeByteMinLen = 11
 	if len(b) < localDateTimeByteMinLen {
-		return dt, nil, unstable.NewParserError(b, base, "local datetimes are expected to have the format YYYY-MM-DDTHH:MM:SS[.NNNNNNNNN]")
+		return dt, nil, unstable.NewParserError(b, "local datetimes are expected to have the format YYYY-MM-DDTHH:MM:SS[.NNNNNNNNN]")
 	}
 
-	date, err := parseLocalDate(b[:10], base)
+	date, err := parseLocalDate(b[:10])
 	if err != nil {
 		return dt, nil, err
 	}
@@ -166,10 +173,10 @@ func parseLocalDateTime(b []byte, base int) (LocalDateTime, []byte, error) {
 
 	sep := b[10]
 	if sep != 'T' && sep != ' ' && sep != 't' {
-		return dt, nil, unstable.NewParserError(b[10:11], base+10, "datetime separator is expected to be T or a space")
+		return dt, nil, unstable.NewParserError(b[10:11], "datetime separator is expected to be T or a space")
 	}
 
-	t, rest, err := parseLocalTime(b[11:], base+11)
+	t, rest, err := parseLocalTime(b[11:])
 	if err != nil {
 		return dt, nil, err
 	}
@@ -181,53 +188,53 @@ func parseLocalDateTime(b []byte, base int) (LocalDateTime, []byte, error) {
 // parseLocalTime is a bit different because it also returns the remaining
 // []byte that is didn't need. This is to allow parseDateTime to parse those
 // remaining bytes as a timezone.
-func parseLocalTime(b []byte, base int) (LocalTime, []byte, error) {
+func parseLocalTime(b []byte) (LocalTime, []byte, error) {
 	var (
 		nspow = [10]int{0, 1e8, 1e7, 1e6, 1e5, 1e4, 1e3, 1e2, 1e1, 1e0}
 		t     LocalTime
 	)
 
+	// check if b matches to have expected format HH:MM:SS[.NNNNNN]
 	const localTimeByteLen = 8
 	if len(b) < localTimeByteLen {
-		return t, nil, unstable.NewParserError(b, base, "times are expected to have the format HH:MM:SS[.NNNNNN]")
+		return t, nil, unstable.NewParserError(b, "times are expected to have the format HH:MM:SS[.NNNNNN]")
 	}
 
 	var err error
 
-	t.Hour, err = parseDecimalDigits(b[0:2], base)
+	t.Hour, err = parseDecimalDigits(b[0:2])
 	if err != nil {
 		return t, nil, err
 	}
 
 	if t.Hour > 23 {
-		return t, nil, unstable.NewParserError(b[0:2], base, "hour cannot be greater 23")
+		return t, nil, unstable.NewParserError(b[0:2], "hour cannot be greater 23")
 	}
 	if b[2] != ':' {
-		return t, nil, unstable.NewParserError(b[2:3], base+2, "expecting colon between hours and minutes")
+		return t, nil, unstable.NewParserError(b[2:3], "expecting colon between hours and minutes")
 	}
 
-	t.Minute, err = parseDecimalDigits(b[3:5], base+3)
+	t.Minute, err = parseDecimalDigits(b[3:5])
 	if err != nil {
 		return t, nil, err
 	}
 	if t.Minute > 59 {
-		return t, nil, unstable.NewParserError(b[3:5], base+3, "minutes cannot be greater 59")
+		return t, nil, unstable.NewParserError(b[3:5], "minutes cannot be greater 59")
 	}
 	if b[5] != ':' {
-		return t, nil, unstable.NewParserError(b[5:6], base+5, "expecting colon between minutes and seconds")
+		return t, nil, unstable.NewParserError(b[5:6], "expecting colon between minutes and seconds")
 	}
 
-	t.Second, err = parseDecimalDigits(b[6:8], base+6)
+	t.Second, err = parseDecimalDigits(b[6:8])
 	if err != nil {
 		return t, nil, err
 	}
 
 	if t.Second > 59 {
-		return t, nil, unstable.NewParserError(b[6:8], base+6, "seconds cannot be greater than 59")
+		return t, nil, unstable.NewParserError(b[6:8], "seconds cannot be greater than 59")
 	}
 
 	b = b[8:]
-	base += 8
 
 	if len(b) >= 1 && b[0] == '.' {
 		frac := 0
@@ -237,7 +244,7 @@ func parseLocalTime(b []byte, base int) (LocalTime, []byte, error) {
 		for i, c := range b[1:] {
 			if !isDigit(c) {
 				if i == 0 {
-					return t, nil, unstable.NewParserError(b[0:1], base, "need at least one digit after fraction point")
+					return t, nil, unstable.NewParserError(b[0:1], "need at least one digit after fraction point")
 				}
 				break
 			}
@@ -245,6 +252,13 @@ func parseLocalTime(b []byte, base int) (LocalTime, []byte, error) {
 
 			const maxFracPrecision = 9
 			if i >= maxFracPrecision {
+				// go-toml allows decoding fractional seconds
+				// beyond the supported precision of 9
+				// digits. It truncates the fractional component
+				// to the supported precision and ignores the
+				// remaining digits.
+				//
+				// https://github.com/pelletier/go-toml/discussions/707
 				continue
 			}
 
@@ -254,7 +268,7 @@ func parseLocalTime(b []byte, base int) (LocalTime, []byte, error) {
 		}
 
 		if precision == 0 {
-			return t, nil, unstable.NewParserError(b[:1], base, "nanoseconds need at least one digit")
+			return t, nil, unstable.NewParserError(b[:1], "nanoseconds need at least one digit")
 		}
 
 		t.Nanosecond = frac * nspow[precision]
@@ -265,35 +279,35 @@ func parseLocalTime(b []byte, base int) (LocalTime, []byte, error) {
 	return t, b, nil
 }
 
-func parseFloat(b []byte, base int) (float64, error) {
+func parseFloat(b []byte) (float64, error) {
 	if len(b) == 4 && (b[0] == '+' || b[0] == '-') && b[1] == 'n' && b[2] == 'a' && b[3] == 'n' {
 		return math.NaN(), nil
 	}
 
-	cleaned, err := checkAndRemoveUnderscoresFloats(b, base)
+	cleaned, err := checkAndRemoveUnderscoresFloats(b)
 	if err != nil {
 		return 0, err
 	}
 
 	if cleaned[0] == '.' {
-		return 0, unstable.NewParserError(b, base, "float cannot start with a dot")
+		return 0, unstable.NewParserError(b, "float cannot start with a dot")
 	}
 
 	if cleaned[len(cleaned)-1] == '.' {
-		return 0, unstable.NewParserError(b, base, "float cannot end with a dot")
+		return 0, unstable.NewParserError(b, "float cannot end with a dot")
 	}
 
 	dotAlreadySeen := false
 	for i, c := range cleaned {
 		if c == '.' {
 			if dotAlreadySeen {
-				return 0, unstable.NewParserError(b[i:i+1], base+i, "float can have at most one decimal point")
+				return 0, unstable.NewParserError(b[i:i+1], "float can have at most one decimal point")
 			}
 			if !isDigit(cleaned[i-1]) {
-				return 0, unstable.NewParserError(b[i-1:i+1], base+i-1, "float decimal point must be preceded by a digit")
+				return 0, unstable.NewParserError(b[i-1:i+1], "float decimal point must be preceded by a digit")
 			}
 			if !isDigit(cleaned[i+1]) {
-				return 0, unstable.NewParserError(b[i:i+2], base+i, "float decimal point must be followed by a digit")
+				return 0, unstable.NewParserError(b[i:i+2], "float decimal point must be followed by a digit")
 			}
 			dotAlreadySeen = true
 		}
@@ -304,54 +318,54 @@ func parseFloat(b []byte, base int) (float64, error) {
 		start = 1
 	}
 	if cleaned[start] == '0' && len(cleaned) > start+1 && isDigit(cleaned[start+1]) {
-		return 0, unstable.NewParserError(b, base, "float integer part cannot have leading zeroes")
+		return 0, unstable.NewParserError(b, "float integer part cannot have leading zeroes")
 	}
 
 	f, err := strconv.ParseFloat(string(cleaned), 64)
 	if err != nil {
-		return 0, unstable.NewParserError(b, base, "unable to parse float: %w", err)
+		return 0, unstable.NewParserError(b, "unable to parse float: %w", err)
 	}
 
 	return f, nil
 }
 
-func parseIntHex(b []byte, base int) (int64, error) {
-	cleaned, err := checkAndRemoveUnderscoresIntegers(b[2:], base+2)
+func parseIntHex(b []byte) (int64, error) {
+	cleaned, err := checkAndRemoveUnderscoresIntegers(b[2:])
 	if err != nil {
 		return 0, err
 	}
 
 	i, err := strconv.ParseInt(string(cleaned), 16, 64)
 	if err != nil {
-		return 0, unstable.NewParserError(b, base, "couldn't parse hexadecimal number: %w", err)
+		return 0, unstable.NewParserError(b, "couldn't parse hexadecimal number: %w", err)
 	}
 
 	return i, nil
 }
 
-func parseIntOct(b []byte, base int) (int64, error) {
-	cleaned, err := checkAndRemoveUnderscoresIntegers(b[2:], base+2)
+func parseIntOct(b []byte) (int64, error) {
+	cleaned, err := checkAndRemoveUnderscoresIntegers(b[2:])
 	if err != nil {
 		return 0, err
 	}
 
 	i, err := strconv.ParseInt(string(cleaned), 8, 64)
 	if err != nil {
-		return 0, unstable.NewParserError(b, base, "couldn't parse octal number: %w", err)
+		return 0, unstable.NewParserError(b, "couldn't parse octal number: %w", err)
 	}
 
 	return i, nil
 }
 
-func parseIntBin(b []byte, base int) (int64, error) {
-	cleaned, err := checkAndRemoveUnderscoresIntegers(b[2:], base+2)
+func parseIntBin(b []byte) (int64, error) {
+	cleaned, err := checkAndRemoveUnderscoresIntegers(b[2:])
 	if err != nil {
 		return 0, err
 	}
 
 	i, err := strconv.ParseInt(string(cleaned), 2, 64)
 	if err != nil {
-		return 0, unstable.NewParserError(b, base, "couldn't parse binary number: %w", err)
+		return 0, unstable.NewParserError(b, "couldn't parse binary number: %w", err)
 	}
 
 	return i, nil
@@ -361,8 +375,8 @@ func isSign(b byte) bool {
 	return b == '+' || b == '-'
 }
 
-func parseIntDec(b []byte, base int) (int64, error) {
-	cleaned, err := checkAndRemoveUnderscoresIntegers(b, base)
+func parseIntDec(b []byte) (int64, error) {
+	cleaned, err := checkAndRemoveUnderscoresIntegers(b)
 	if err != nil {
 		return 0, err
 	}
@@ -374,18 +388,18 @@ func parseIntDec(b []byte, base int) (int64, error) {
 	}
 
 	if len(cleaned) > startIdx+1 && cleaned[startIdx] == '0' {
-		return 0, unstable.NewParserError(b, base, "leading zero not allowed on decimal number")
+		return 0, unstable.NewParserError(b, "leading zero not allowed on decimal number")
 	}
 
 	i, err := strconv.ParseInt(string(cleaned), 10, 64)
 	if err != nil {
-		return 0, unstable.NewParserError(b, base, "couldn't parse decimal number: %w", err)
+		return 0, unstable.NewParserError(b, "couldn't parse decimal number: %w", err)
 	}
 
 	return i, nil
 }
 
-func checkAndRemoveUnderscoresIntegers(b []byte, base int) ([]byte, error) {
+func checkAndRemoveUnderscoresIntegers(b []byte) ([]byte, error) {
 	start := 0
 	if b[start] == '+' || b[start] == '-' {
 		start++
@@ -396,11 +410,11 @@ func checkAndRemoveUnderscoresIntegers(b []byte, base int) ([]byte, error) {
 	}
 
 	if b[start] == '_' {
-		return nil, unstable.NewParserError(b[start:start+1], base+start, "number cannot start with underscore")
+		return nil, unstable.NewParserError(b[start:start+1], "number cannot start with underscore")
 	}
 
 	if b[len(b)-1] == '_' {
-		return nil, unstable.NewParserError(b[len(b)-1:], base+len(b)-1, "number cannot end with underscore")
+		return nil, unstable.NewParserError(b[len(b)-1:], "number cannot end with underscore")
 	}
 
 	// fast path
@@ -422,7 +436,7 @@ func checkAndRemoveUnderscoresIntegers(b []byte, base int) ([]byte, error) {
 		c := b[i]
 		if c == '_' {
 			if !before {
-				return nil, unstable.NewParserError(b[i-1:i+1], base+i-1, "number must have at least one digit between underscores")
+				return nil, unstable.NewParserError(b[i-1:i+1], "number must have at least one digit between underscores")
 			}
 			before = false
 		} else {
@@ -434,13 +448,13 @@ func checkAndRemoveUnderscoresIntegers(b []byte, base int) ([]byte, error) {
 	return cleaned, nil
 }
 
-func checkAndRemoveUnderscoresFloats(b []byte, base int) ([]byte, error) {
+func checkAndRemoveUnderscoresFloats(b []byte) ([]byte, error) {
 	if b[0] == '_' {
-		return nil, unstable.NewParserError(b[0:1], base, "number cannot start with underscore")
+		return nil, unstable.NewParserError(b[0:1], "number cannot start with underscore")
 	}
 
 	if b[len(b)-1] == '_' {
-		return nil, unstable.NewParserError(b[len(b)-1:], base+len(b)-1, "number cannot end with underscore")
+		return nil, unstable.NewParserError(b[len(b)-1:], "number cannot end with underscore")
 	}
 
 	// fast path
@@ -463,26 +477,27 @@ func checkAndRemoveUnderscoresFloats(b []byte, base int) ([]byte, error) {
 		switch c {
 		case '_':
 			if !before {
-				return nil, unstable.NewParserError(b[i-1:i+1], base+i-1, "number must have at least one digit between underscores")
+				return nil, unstable.NewParserError(b[i-1:i+1], "number must have at least one digit between underscores")
 			}
 			if i < len(b)-1 && (b[i+1] == 'e' || b[i+1] == 'E') {
-				return nil, unstable.NewParserError(b[i+1:i+2], base+i+1, "cannot have underscore before exponent")
+				return nil, unstable.NewParserError(b[i+1:i+2], "cannot have underscore before exponent")
 			}
 			before = false
 		case '+', '-':
+			// signed exponents
 			cleaned = append(cleaned, c)
 			before = false
 		case 'e', 'E':
 			if i < len(b)-1 && b[i+1] == '_' {
-				return nil, unstable.NewParserError(b[i+1:i+2], base+i+1, "cannot have underscore after exponent")
+				return nil, unstable.NewParserError(b[i+1:i+2], "cannot have underscore after exponent")
 			}
 			cleaned = append(cleaned, c)
 		case '.':
 			if i < len(b)-1 && b[i+1] == '_' {
-				return nil, unstable.NewParserError(b[i+1:i+2], base+i+1, "cannot have underscore after decimal point")
+				return nil, unstable.NewParserError(b[i+1:i+2], "cannot have underscore after decimal point")
 			}
 			if i > 0 && b[i-1] == '_' {
-				return nil, unstable.NewParserError(b[i-1:i], base+i-1, "cannot have underscore before decimal point")
+				return nil, unstable.NewParserError(b[i-1:i], "cannot have underscore before decimal point")
 			}
 			cleaned = append(cleaned, c)
 		default:

errors.go

@@ -2,6 +2,7 @@ package toml
 
 import (
 	"fmt"
+	"reflect"
 	"strconv"
 	"strings"
 
@@ -99,7 +100,7 @@ func (e *DecodeError) Key() Key {
 //
 //nolint:funlen
 func wrapDecodeError(document []byte, de *unstable.ParserError) *DecodeError {
-	offset := de.Offset
+	offset := subsliceOffset(document, de.Highlight)
 
 	errMessage := de.Error()
 	errLine, errColumn := positionAtEnd(document[:offset])
@@ -261,3 +262,22 @@ func positionAtEnd(b []byte) (row int, column int) {
 
 	return row, column
 }
+
+// subsliceOffset returns the byte offset of subslice within data.
+// subslice must share the same backing array as data.
+func subsliceOffset(data []byte, subslice []byte) int {
+	if len(subslice) == 0 {
+		return 0
+	}
+
+	// Use reflect to get the data pointers of both slices.
+	// This is safe because we're only reading the pointer values for comparison.
+	dataPtr := reflect.ValueOf(data).Pointer()
+	subPtr := reflect.ValueOf(subslice).Pointer()
+
+	offset := int(subPtr - dataPtr)
+	if offset < 0 || offset > len(data) {
+		panic("subslice is not within data")
+	}
+	return offset
+}

errors_test.go

@@ -171,7 +171,6 @@ line 5`,
 
 			err := wrapDecodeError(doc, &unstable.ParserError{
 				Highlight: hl,
-				Offset:    start,
 				Message:   e.msg,
 			})
 
@@ -203,6 +202,38 @@ func TestDecodeError_Accessors(t *testing.T) {
 	assert.Equal(t, "bar", e.String())
 }
 
+func TestDecodeError_InvalidKeyStartAfterComment(t *testing.T) {
+	// Regression for https://github.com/pelletier/go-toml/issues/1047: the "="
+	// that starts an invalid keyval must be reported on line 2, column 1, with
+	// the human-readable context pointing at that byte (not the document end).
+	doc := "# comment\n= \"value\""
+
+	var v map[string]any
+	err := Unmarshal([]byte(doc), &v)
+	if err == nil {
+		t.Fatal("expected an error")
+	}
+
+	var derr *DecodeError
+	if !errors.As(err, &derr) {
+		t.Fatalf("expected *DecodeError, got %T", err)
+	}
+
+	row, col := derr.Position()
+	if row != 2 || col != 1 {
+		t.Errorf("Position(): got row %d col %d, want row 2 col 1", row, col)
+	}
+
+	human := derr.String()
+	if !strings.Contains(human, `2| = "value"`) {
+		t.Errorf("human output should show the error line; got:\n%s", human)
+	}
+	// Caret line uses line-number column width padding; only the "| ~" part is stable here.
+	if !strings.Contains(human, "| ~ invalid character at start of key") {
+		t.Errorf("human output should underline '=' and include the parser message; got:\n%s", human)
+	}
+}
+
 func TestDecodeError_DuplicateContent(t *testing.T) {
 	// This test verifies that when the same content appears multiple times
 	// in the document, the error correctly points to the actual location
@@ -287,100 +318,6 @@ func TestDecodeError_Position(t *testing.T) {
 	}
 }
 
-func TestDecodeError_PositionAfterComments(t *testing.T) {
-	examples := []struct {
-		name        string
-		doc         string
-		expectedRow int
-		expectedCol int
-		errContains string
-	}{
-		{
-			name:        "invalid key after comment",
-			doc:         "# comment\n= \"value\"",
-			expectedRow: 2,
-			expectedCol: 1,
-			errContains: "invalid character at start of key",
-		},
-		{
-			name:        "invalid key after multiple comments",
-			doc:         "# line 1\n# line 2\n= \"value\"",
-			expectedRow: 3,
-			expectedCol: 1,
-			errContains: "invalid character at start of key",
-		},
-		{
-			name:        "invalid key after valid assignment and comment",
-			doc:         "a = 1\n# comment\n= \"value\"",
-			expectedRow: 3,
-			expectedCol: 1,
-			errContains: "invalid character at start of key",
-		},
-		{
-			name:        "invalid key on first line",
-			doc:         "= \"value\"",
-			expectedRow: 1,
-			expectedCol: 1,
-			errContains: "invalid character at start of key",
-		},
-		{
-			name:        "invalid key with leading whitespace",
-			doc:         "# comment\n  = \"value\"",
-			expectedRow: 2,
-			expectedCol: 3,
-			errContains: "invalid character at start of key",
-		},
-	}
-
-	for _, e := range examples {
-		t.Run(e.name, func(t *testing.T) {
-			var v map[string]interface{}
-			err := Unmarshal([]byte(e.doc), &v)
-			if err == nil {
-				t.Fatal("expected an error")
-			}
-
-			var derr *DecodeError
-			if !errors.As(err, &derr) {
-				t.Fatalf("expected DecodeError, got %T: %v", err, err)
-			}
-
-			row, col := derr.Position()
-			if row != e.expectedRow {
-				t.Errorf("row: got %d, want %d (error: %s)", row, e.expectedRow, derr.String())
-			}
-			if col != e.expectedCol {
-				t.Errorf("col: got %d, want %d (error: %s)", col, e.expectedCol, derr.String())
-			}
-			if !strings.Contains(derr.Error(), e.errContains) {
-				t.Errorf("error %q does not contain %q", derr.Error(), e.errContains)
-			}
-		})
-	}
-}
-
-func TestDecodeError_HumanStringAfterComments(t *testing.T) {
-	doc := "# comment\n= \"value\""
-	var v map[string]interface{}
-	err := Unmarshal([]byte(doc), &v)
-	if err == nil {
-		t.Fatal("expected an error")
-	}
-
-	var derr *DecodeError
-	if !errors.As(err, &derr) {
-		t.Fatalf("expected DecodeError, got %T: %v", err, err)
-	}
-
-	human := derr.String()
-	if !strings.Contains(human, "= \"value\"") {
-		t.Errorf("human-readable error should show the offending line, got:\n%s", human)
-	}
-	if !strings.Contains(human, "2|") {
-		t.Errorf("human-readable error should reference line 2, got:\n%s", human)
-	}
-}
-
 func TestStrictErrorUnwrap(t *testing.T) {
 	fo := bytes.NewBufferString(`
 Missing = 1

internal/characters/utf8.go

@@ -24,57 +24,61 @@ import (
 // 0x9 => tab, ok
 // 0xA - 0x1F => invalid
 // 0x7F => invalid
-func Utf8TomlValidAlreadyEscaped(p []byte) int {
-	consumed := 0
-
+func Utf8TomlValidAlreadyEscaped(p []byte) []byte {
 	// Fast path. Check for and skip 8 bytes of ASCII characters per iteration.
 	for len(p) >= 8 {
+		// Combining two 32 bit loads allows the same code to be used
+		// for 32 and 64 bit platforms.
+		// The compiler can generate a 32bit load for first32 and second32
+		// on many platforms. See test/codegen/memcombine.go.
 		first32 := uint32(p[0]) | uint32(p[1])<<8 | uint32(p[2])<<16 | uint32(p[3])<<24
 		second32 := uint32(p[4]) | uint32(p[5])<<8 | uint32(p[6])<<16 | uint32(p[7])<<24
 		if (first32|second32)&0x80808080 != 0 {
+			// Found a non ASCII byte (>= RuneSelf).
 			break
 		}
 
 		for i, b := range p[:8] {
 			if InvalidASCII(b) {
-				return consumed + i
+				return p[i : i+1]
 			}
 		}
 
 		p = p[8:]
-		consumed += 8
 	}
 	n := len(p)
 	for i := 0; i < n; {
 		pi := p[i]
 		if pi < utf8.RuneSelf {
 			if InvalidASCII(pi) {
-				return consumed + i
+				return p[i : i+1]
 			}
 			i++
 			continue
 		}
 		x := first[pi]
 		if x == xx {
-			return consumed + i
+			// Illegal starter byte.
+			return p[i : i+1]
 		}
 		size := int(x & 7)
 		if i+size > n {
-			return consumed + i
+			// Short or invalid.
+			return p[i:n]
 		}
 		accept := acceptRanges[x>>4]
 		if c := p[i+1]; c < accept.lo || accept.hi < c {
-			return consumed + i
+			return p[i : i+2]
 		} else if size == 2 { //revive:disable:empty-block
 		} else if c := p[i+2]; c < locb || hicb < c {
-			return consumed + i
+			return p[i : i+3]
 		} else if size == 3 { //revive:disable:empty-block
 		} else if c := p[i+3]; c < locb || hicb < c {
-			return consumed + i
+			return p[i : i+4]
 		}
 		i += size
 	}
-	return -1
+	return nil
 }
 
 // Utf8ValidNext returns the size of the next rune if valid, 0 otherwise.

localtime.go

@@ -32,7 +32,7 @@ func (d LocalDate) MarshalText() ([]byte, error) {
 
 // UnmarshalText parses b using RFC 3339 to fill d.
 func (d *LocalDate) UnmarshalText(b []byte) error {
-	res, err := parseLocalDate(b, 0)
+	res, err := parseLocalDate(b)
 	if err != nil {
 		return err
 	}
@@ -75,9 +75,9 @@ func (d LocalTime) MarshalText() ([]byte, error) {
 
 // UnmarshalText parses b using RFC 3339 to fill d.
 func (d *LocalTime) UnmarshalText(b []byte) error {
-	res, left, err := parseLocalTime(b, 0)
+	res, left, err := parseLocalTime(b)
 	if err == nil && len(left) != 0 {
-		err = unstable.NewParserError(left, len(b)-len(left), "extra characters")
+		err = unstable.NewParserError(left, "extra characters")
 	}
 	if err != nil {
 		return err
@@ -109,9 +109,9 @@ func (d LocalDateTime) MarshalText() ([]byte, error) {
 
 // UnmarshalText parses b using RFC 3339 to fill d.
 func (d *LocalDateTime) UnmarshalText(data []byte) error {
-	res, left, err := parseLocalDateTime(data, 0)
+	res, left, err := parseLocalDateTime(data)
 	if err == nil && len(left) != 0 {
-		err = unstable.NewParserError(left, len(data)-len(left), "extra characters")
+		err = unstable.NewParserError(left, "extra characters")
 	}
 	if err != nil {
 		return err

strict.go

@@ -54,12 +54,10 @@ func (s *strict) MissingTable(node *unstable.Node) {
 		return
 	}
 
-	loc, offset := s.keyLocation(node)
 	s.missing = append(s.missing, unstable.ParserError{
-		Highlight: loc,
+		Highlight: s.keyLocation(node),
 		Message:   "missing table",
 		Key:       s.key.Key(),
-		Offset:    offset,
 	})
 }
 
@@ -68,12 +66,10 @@ func (s *strict) MissingField(node *unstable.Node) {
 		return
 	}
 
-	loc, offset := s.keyLocation(node)
 	s.missing = append(s.missing, unstable.ParserError{
-		Highlight: loc,
+		Highlight: s.keyLocation(node),
 		Message:   "missing field",
 		Key:       s.key.Key(),
-		Offset:    offset,
 	})
 }
 
@@ -94,7 +90,7 @@ func (s *strict) Error(doc []byte) error {
 	return err
 }
 
-func (s *strict) keyLocation(node *unstable.Node) ([]byte, int) {
+func (s *strict) keyLocation(node *unstable.Node) []byte {
 	k := node.Key()
 
 	hasOne := k.Next()
@@ -102,6 +98,7 @@ func (s *strict) keyLocation(node *unstable.Node) ([]byte, int) {
 		panic("should not be called with empty key")
 	}
 
+	// Get the range from the first key to the last key.
 	firstRaw := k.Node().Raw
 	lastRaw := firstRaw
 
@@ -109,8 +106,9 @@ func (s *strict) keyLocation(node *unstable.Node) ([]byte, int) {
 		lastRaw = k.Node().Raw
 	}
 
+	// Compute the slice from the document using the ranges.
 	start := firstRaw.Offset
 	end := lastRaw.Offset + lastRaw.Length
 
-	return s.doc[start:end], int(start)
+	return s.doc[start:end]
 }

unmarshaler.go

@@ -625,7 +625,7 @@ func (d *decoder) handleTable(key unstable.Iterator, v reflect.Value) (reflect.V
 				}
 			}
 		}
-		return reflect.Value{}, unstable.NewParserError(key.Node().Data, int(key.Node().Raw.Offset), "cannot store a table in a slice")
+		return reflect.Value{}, unstable.NewParserError(key.Node().Data, "cannot store a table in a slice")
 	}
 	if key.Next() {
 		// Still scoping the key
@@ -748,7 +748,7 @@ func (d *decoder) tryTextUnmarshaler(node *unstable.Node, v reflect.Value) (bool
 	if v.CanAddr() && v.Addr().Type().Implements(textUnmarshalerType) {
 		err := v.Addr().Interface().(encoding.TextUnmarshaler).UnmarshalText(node.Data)
 		if err != nil {
-			return false, unstable.NewParserError(d.p.Raw(node.Raw), int(node.Raw.Offset), "%w", err)
+			return false, unstable.NewParserError(d.p.Raw(node.Raw), "%w", err)
 		}
 
 		return true, nil
@@ -896,7 +896,7 @@ func (d *decoder) unmarshalInlineTable(itable *unstable.Node, v reflect.Value) e
 		}
 		return d.unmarshalInlineTable(itable, elem)
 	default:
-		return unstable.NewParserError(d.p.Raw(itable.Raw), int(itable.Raw.Offset), "cannot store inline table in Go type %s", v.Kind())
+		return unstable.NewParserError(d.p.Raw(itable.Raw), "cannot store inline table in Go type %s", v.Kind())
 	}
 
 	it := itable.Children()
@@ -916,26 +916,26 @@ func (d *decoder) unmarshalInlineTable(itable *unstable.Node, v reflect.Value) e
 }
 
 func (d *decoder) unmarshalDateTime(value *unstable.Node, v reflect.Value) error {
-	dt, err := parseDateTime(value.Data, int(value.Raw.Offset))
+	dt, err := parseDateTime(value.Data)
 	if err != nil {
 		return err
 	}
 
 	if v.Kind() != reflect.Interface && v.Type() != timeType {
-		return unstable.NewParserError(d.p.Raw(value.Raw), int(value.Raw.Offset), "%s", d.typeMismatchString("datetime", v.Type()))
+		return unstable.NewParserError(d.p.Raw(value.Raw), "%s", d.typeMismatchString("datetime", v.Type()))
 	}
 	v.Set(reflect.ValueOf(dt))
 	return nil
 }
 
 func (d *decoder) unmarshalLocalDate(value *unstable.Node, v reflect.Value) error {
-	ld, err := parseLocalDate(value.Data, int(value.Raw.Offset))
+	ld, err := parseLocalDate(value.Data)
 	if err != nil {
 		return err
 	}
 
 	if v.Kind() != reflect.Interface && v.Type() != timeType {
-		return unstable.NewParserError(d.p.Raw(value.Raw), int(value.Raw.Offset), "%s", d.typeMismatchString("local date", v.Type()))
+		return unstable.NewParserError(d.p.Raw(value.Raw), "%s", d.typeMismatchString("local date", v.Type()))
 	}
 	if v.Type() == timeType {
 		v.Set(reflect.ValueOf(ld.AsTime(time.Local)))
@@ -946,34 +946,34 @@ func (d *decoder) unmarshalLocalDate(value *unstable.Node, v reflect.Value) erro
 }
 
 func (d *decoder) unmarshalLocalTime(value *unstable.Node, v reflect.Value) error {
-	lt, rest, err := parseLocalTime(value.Data, int(value.Raw.Offset))
+	lt, rest, err := parseLocalTime(value.Data)
 	if err != nil {
 		return err
 	}
 
 	if len(rest) > 0 {
-		return unstable.NewParserError(rest, int(value.Raw.Offset)+len(value.Data)-len(rest), "extra characters at the end of a local time")
+		return unstable.NewParserError(rest, "extra characters at the end of a local time")
 	}
 
 	if v.Kind() != reflect.Interface {
-		return unstable.NewParserError(d.p.Raw(value.Raw), int(value.Raw.Offset), "%s", d.typeMismatchString("local time", v.Type()))
+		return unstable.NewParserError(d.p.Raw(value.Raw), "%s", d.typeMismatchString("local time", v.Type()))
 	}
 	v.Set(reflect.ValueOf(lt))
 	return nil
 }
 
 func (d *decoder) unmarshalLocalDateTime(value *unstable.Node, v reflect.Value) error {
-	ldt, rest, err := parseLocalDateTime(value.Data, int(value.Raw.Offset))
+	ldt, rest, err := parseLocalDateTime(value.Data)
 	if err != nil {
 		return err
 	}
 
 	if len(rest) > 0 {
-		return unstable.NewParserError(rest, int(value.Raw.Offset)+len(value.Data)-len(rest), "extra characters at the end of a local date time")
+		return unstable.NewParserError(rest, "extra characters at the end of a local date time")
 	}
 
 	if v.Kind() != reflect.Interface && v.Type() != timeType {
-		return unstable.NewParserError(d.p.Raw(value.Raw), int(value.Raw.Offset), "%s", d.typeMismatchString("local datetime", v.Type()))
+		return unstable.NewParserError(d.p.Raw(value.Raw), "%s", d.typeMismatchString("local datetime", v.Type()))
 	}
 	if v.Type() == timeType {
 		v.Set(reflect.ValueOf(ldt.AsTime(time.Local)))
@@ -992,14 +992,14 @@ func (d *decoder) unmarshalBool(value *unstable.Node, v reflect.Value) error {
 	case reflect.Interface:
 		v.Set(reflect.ValueOf(b))
 	default:
-		return unstable.NewParserError(value.Data, int(value.Raw.Offset), "cannot assign boolean to a %t", b)
+		return unstable.NewParserError(value.Data, "cannot assign boolean to a %t", b)
 	}
 
 	return nil
 }
 
 func (d *decoder) unmarshalFloat(value *unstable.Node, v reflect.Value) error {
-	f, err := parseFloat(value.Data, int(value.Raw.Offset))
+	f, err := parseFloat(value.Data)
 	if err != nil {
 		return err
 	}
@@ -1009,13 +1009,13 @@ func (d *decoder) unmarshalFloat(value *unstable.Node, v reflect.Value) error {
 		v.SetFloat(f)
 	case reflect.Float32:
 		if f > math.MaxFloat32 {
-			return unstable.NewParserError(value.Data, int(value.Raw.Offset), "number %f does not fit in a float32", f)
+			return unstable.NewParserError(value.Data, "number %f does not fit in a float32", f)
 		}
 		v.SetFloat(f)
 	case reflect.Interface:
 		v.Set(reflect.ValueOf(f))
 	default:
-		return unstable.NewParserError(value.Data, int(value.Raw.Offset), "float cannot be assigned to %s", v.Kind())
+		return unstable.NewParserError(value.Data, "float cannot be assigned to %s", v.Kind())
 	}
 
 	return nil
@@ -1048,7 +1048,7 @@ func (d *decoder) unmarshalInteger(value *unstable.Node, v reflect.Value) error
 		return d.unmarshalFloat(value, v)
 	}
 
-	i, err := parseInteger(value.Data, int(value.Raw.Offset))
+	i, err := parseInteger(value.Data)
 	if err != nil {
 		return err
 	}
@@ -1116,7 +1116,7 @@ func (d *decoder) unmarshalInteger(value *unstable.Node, v reflect.Value) error
 	case reflect.Interface:
 		r = reflect.ValueOf(i)
 	default:
-		return unstable.NewParserError(d.p.Raw(value.Raw), int(value.Raw.Offset), "%s", d.typeMismatchString("integer", v.Type()))
+		return unstable.NewParserError(d.p.Raw(value.Raw), "%s", d.typeMismatchString("integer", v.Type()))
 	}
 
 	if !r.Type().AssignableTo(v.Type()) {
@@ -1135,7 +1135,7 @@ func (d *decoder) unmarshalString(value *unstable.Node, v reflect.Value) error {
 	case reflect.Interface:
 		v.Set(reflect.ValueOf(string(value.Data)))
 	default:
-		return unstable.NewParserError(d.p.Raw(value.Raw), int(value.Raw.Offset), "%s", d.typeMismatchString("string", v.Type()))
+		return unstable.NewParserError(d.p.Raw(value.Raw), "%s", d.typeMismatchString("string", v.Type()))
 	}
 
 	return nil

unstable/benchmark_test.go

@@ -35,7 +35,7 @@ func BenchmarkScanComments(b *testing.B) {
 			b.ResetTimer()
 
 			for i := 0; i < b.N; i++ {
-				_, _, _ = scanComment(input, 0)
+				_, _, _ = scanComment(input)
 			}
 		})
 	}

unstable/parser.go

@@ -3,6 +3,7 @@ package unstable
 import (
 	"bytes"
 	"fmt"
+	"reflect"
 	"unicode"
 
 	"github.com/pelletier/go-toml/v2/internal/characters"
@@ -16,7 +17,6 @@ type ParserError struct {
 	Highlight []byte
 	Message   string
 	Key       []string // optional
-	Offset    int
 }
 
 // Error is the implementation of the error interface.
@@ -28,10 +28,9 @@ func (e *ParserError) Error() string {
 //
 // Warning: Highlight needs to be a subslice of Parser.data, so only slices
 // returned by Parser.Raw are valid candidates.
-func NewParserError(highlight []byte, offset int, format string, args ...interface{}) error {
+func NewParserError(highlight []byte, format string, args ...interface{}) error {
 	return &ParserError{
 		Highlight: highlight,
-		Offset:    offset,
 		Message:   fmt.Errorf(format, args...).Error(),
 	}
 }
@@ -66,8 +65,14 @@ func (p *Parser) Data() []byte {
 	return p.data
 }
 
-func (p *Parser) offsetOf(b []byte) int {
-	return len(p.data) - len(b)
+// Range returns a range description that corresponds to a given slice of the
+// input. If the argument is not a subslice of the parser input, this function
+// panics.
+func (p *Parser) Range(b []byte) Range {
+	return Range{
+		Offset: uint32(subsliceOffset(p.data, b)), //nolint:gosec // TOML documents are small
+		Length: uint32(len(b)),                    //nolint:gosec // TOML documents are small
+	}
 }
 
 // rangeOfToken computes the Range of a token given the remaining bytes after the token.
@@ -78,6 +83,23 @@ func (p *Parser) rangeOfToken(token, rest []byte) Range {
 	return Range{Offset: uint32(offset), Length: uint32(len(token))} //nolint:gosec // TOML documents are small
 }
 
+// subsliceOffset returns the byte offset of subslice b within data.
+// b must share the same backing array as data (any subslice of data).
+func subsliceOffset(data, b []byte) int {
+	if len(b) == 0 {
+		return 0
+	}
+
+	dataPtr := reflect.ValueOf(data).Pointer()
+	bPtr := reflect.ValueOf(b).Pointer()
+
+	offset := int(bPtr - dataPtr)
+	if offset < 0 || offset > len(data) {
+		panic("subslice is not within data")
+	}
+	return offset
+}
+
 // Raw returns the slice corresponding to the bytes in the given range.
 func (p *Parser) Raw(raw Range) []byte {
 	return p.data[raw.Offset : raw.Offset+raw.Length]
@@ -187,16 +209,16 @@ func (p *Parser) parseNewline(b []byte) ([]byte, error) {
 	}
 
 	if b[0] == '\r' {
-		_, rest, err := scanWindowsNewline(b, p.offsetOf(b))
+		_, rest, err := scanWindowsNewline(b)
 		return rest, err
 	}
 
-	return nil, NewParserError(b[0:1], p.offsetOf(b), "expected newline but got %#U", b[0])
+	return nil, NewParserError(b[0:1], "expected newline but got %#U", b[0])
 }
 
 func (p *Parser) parseComment(b []byte) (reference, []byte, error) {
 	ref := invalidReference
-	data, rest, err := scanComment(b, p.offsetOf(b))
+	data, rest, err := scanComment(b)
 	if p.KeepComments && err == nil {
 		ref = p.builder.Push(Node{
 			Kind: Comment,
@@ -280,12 +302,12 @@ func (p *Parser) parseArrayTable(b []byte) (reference, []byte, error) {
 	p.builder.AttachChild(ref, k)
 	b = p.parseWhitespace(b)
 
-	b, err = expect(']', b, p.offsetOf(b))
+	b, err = expect(']', b)
 	if err != nil {
 		return ref, nil, err
 	}
 
-	b, err = expect(']', b, p.offsetOf(b))
+	b, err = expect(']', b)
 
 	return ref, b, err
 }
@@ -310,7 +332,7 @@ func (p *Parser) parseStdTable(b []byte) (reference, []byte, error) {
 
 	b = p.parseWhitespace(b)
 
-	b, err = expect(']', b, p.offsetOf(b))
+	b, err = expect(']', b)
 
 	return ref, b, err
 }
@@ -334,10 +356,10 @@ func (p *Parser) parseKeyval(b []byte) (reference, []byte, error) {
 	b = p.parseWhitespace(b)
 
 	if len(b) == 0 {
-		return invalidReference, nil, NewParserError(startB[:len(startB)-len(b)], p.offsetOf(startB), "expected = after a key, but the document ends there")
+		return invalidReference, nil, NewParserError(startB[:len(startB)-len(b)], "expected = after a key, but the document ends there")
 	}
 
-	b, err = expect('=', b, p.offsetOf(b))
+	b, err = expect('=', b)
 	if err != nil {
 		return invalidReference, nil, err
 	}
@@ -366,7 +388,7 @@ func (p *Parser) parseVal(b []byte) (reference, []byte, error) {
 	ref := invalidReference
 
 	if len(b) == 0 {
-		return ref, nil, NewParserError(b, p.offsetOf(b), "expected value, not eof")
+		return ref, nil, NewParserError(b, "expected value, not eof")
 	}
 
 	var err error
@@ -411,7 +433,7 @@ func (p *Parser) parseVal(b []byte) (reference, []byte, error) {
 		return ref, b, err
 	case 't':
 		if !scanFollowsTrue(b) {
-			return ref, nil, NewParserError(atmost(b, 4), p.offsetOf(b), "expected 'true'")
+			return ref, nil, NewParserError(atmost(b, 4), "expected 'true'")
 		}
 
 		ref = p.builder.Push(Node{
@@ -422,7 +444,7 @@ func (p *Parser) parseVal(b []byte) (reference, []byte, error) {
 		return ref, b[4:], nil
 	case 'f':
 		if !scanFollowsFalse(b) {
-			return ref, nil, NewParserError(atmost(b, 5), p.offsetOf(b), "expected 'false'")
+			return ref, nil, NewParserError(atmost(b, 5), "expected 'false'")
 		}
 
 		ref = p.builder.Push(Node{
@@ -449,7 +471,7 @@ func atmost(b []byte, n int) []byte {
 }
 
 func (p *Parser) parseLiteralString(b []byte) ([]byte, []byte, []byte, error) {
-	v, rest, err := scanLiteralString(b, p.offsetOf(b))
+	v, rest, err := scanLiteralString(b)
 	if err != nil {
 		return nil, nil, nil, err
 	}
@@ -481,7 +503,7 @@ func (p *Parser) parseInlineTable(b []byte) (reference, []byte, error) {
 		b = p.parseWhitespace(b)
 
 		if len(b) == 0 {
-			return parent, nil, NewParserError(previousB[:1], p.offsetOf(previousB), "inline table is incomplete")
+			return parent, nil, NewParserError(previousB[:1], "inline table is incomplete")
 		}
 
 		if b[0] == '}' {
@@ -489,7 +511,7 @@ func (p *Parser) parseInlineTable(b []byte) (reference, []byte, error) {
 		}
 
 		if !first {
-			b, err = expect(',', b, p.offsetOf(b))
+			b, err = expect(',', b)
 			if err != nil {
 				return parent, nil, err
 			}
@@ -513,7 +535,7 @@ func (p *Parser) parseInlineTable(b []byte) (reference, []byte, error) {
 		first = false
 	}
 
-	rest, err := expect('}', b, p.offsetOf(b))
+	rest, err := expect('}', b)
 
 	return parent, rest, err
 }
@@ -562,7 +584,7 @@ func (p *Parser) parseValArray(b []byte) (reference, []byte, error) {
 		}
 
 		if len(b) == 0 {
-			return parent, nil, NewParserError(arrayStart[:1], p.offsetOf(arrayStart), "array is incomplete")
+			return parent, nil, NewParserError(arrayStart[:1], "array is incomplete")
 		}
 
 		if b[0] == ']' {
@@ -571,7 +593,7 @@ func (p *Parser) parseValArray(b []byte) (reference, []byte, error) {
 
 		if b[0] == ',' {
 			if first {
-				return parent, nil, NewParserError(b[0:1], p.offsetOf(b), "array cannot start with comma")
+				return parent, nil, NewParserError(b[0:1], "array cannot start with comma")
 			}
 			b = b[1:]
 
@@ -583,7 +605,7 @@ func (p *Parser) parseValArray(b []byte) (reference, []byte, error) {
 				addChild(cref)
 			}
 		} else if !first {
-			return parent, nil, NewParserError(b[0:1], p.offsetOf(b), "array elements must be separated by commas")
+			return parent, nil, NewParserError(b[0:1], "array elements must be separated by commas")
 		}
 
 		// TOML allows trailing commas in arrays.
@@ -610,7 +632,7 @@ func (p *Parser) parseValArray(b []byte) (reference, []byte, error) {
 		first = false
 	}
 
-	rest, err := expect(']', b, p.offsetOf(b))
+	rest, err := expect(']', b)
 
 	return parent, rest, err
 }
@@ -665,7 +687,7 @@ func (p *Parser) parseOptionalWhitespaceCommentNewline(b []byte) (reference, []b
 }
 
 func (p *Parser) parseMultilineLiteralString(b []byte) ([]byte, []byte, []byte, error) {
-	token, rest, err := scanMultilineLiteralString(b, p.offsetOf(b))
+	token, rest, err := scanMultilineLiteralString(b)
 	if err != nil {
 		return nil, nil, nil, err
 	}
@@ -694,7 +716,7 @@ func (p *Parser) parseMultilineBasicString(b []byte) ([]byte, []byte, []byte, er
 	// mlb-quotes = 1*2quotation-mark
 	// mlb-unescaped = wschar / %x21 / %x23-5B / %x5D-7E / non-ascii
 	// mlb-escaped-nl = escape ws newline *( wschar / newline )
-	token, escaped, rest, err := scanMultilineBasicString(b, p.offsetOf(b))
+	token, escaped, rest, err := scanMultilineBasicString(b)
 	if err != nil {
 		return nil, nil, nil, err
 	}
@@ -711,15 +733,14 @@ func (p *Parser) parseMultilineBasicString(b []byte) ([]byte, []byte, []byte, er
 	// fast path
 	startIdx := i
 	endIdx := len(token) - len(`"""`)
-	tokenBase := p.offsetOf(token)
 
 	if !escaped {
 		str := token[startIdx:endIdx]
-		invalidIdx := characters.Utf8TomlValidAlreadyEscaped(str)
-		if invalidIdx < 0 {
+		highlight := characters.Utf8TomlValidAlreadyEscaped(str)
+		if len(highlight) == 0 {
 			return token, str, rest, nil
 		}
-		return nil, nil, nil, NewParserError(str[invalidIdx:invalidIdx+1], tokenBase+startIdx+invalidIdx, "invalid UTF-8")
+		return nil, nil, nil, NewParserError(highlight, "invalid UTF-8")
 	}
 
 	var builder bytes.Buffer
@@ -784,14 +805,14 @@ func (p *Parser) parseMultilineBasicString(b []byte) ([]byte, []byte, []byte, er
 			case 'e':
 				builder.WriteByte(0x1B)
 			case 'u':
-				x, err := hexToRune(atmost(token[i+1:], 4), tokenBase+i+1, 4)
+				x, err := hexToRune(atmost(token[i+1:], 4), 4)
 				if err != nil {
 					return nil, nil, nil, err
 				}
 				builder.WriteRune(x)
 				i += 4
 			case 'U':
-				x, err := hexToRune(atmost(token[i+1:], 8), tokenBase+i+1, 8)
+				x, err := hexToRune(atmost(token[i+1:], 8), 8)
 				if err != nil {
 					return nil, nil, nil, err
 				}
@@ -799,13 +820,13 @@ func (p *Parser) parseMultilineBasicString(b []byte) ([]byte, []byte, []byte, er
 				builder.WriteRune(x)
 				i += 8
 			default:
-				return nil, nil, nil, NewParserError(token[i:i+1], tokenBase+i, "invalid escaped character %#U", c)
+				return nil, nil, nil, NewParserError(token[i:i+1], "invalid escaped character %#U", c)
 			}
 			i++
 		} else {
 			size := characters.Utf8ValidNext(token[i:])
 			if size == 0 {
-				return nil, nil, nil, NewParserError(token[i:i+1], tokenBase+i, "invalid character %#U", c)
+				return nil, nil, nil, NewParserError(token[i:i+1], "invalid character %#U", c)
 			}
 			builder.Write(token[i : i+size])
 			i += size
@@ -860,9 +881,12 @@ func (p *Parser) parseKey(b []byte) (reference, []byte, error) {
 
 func (p *Parser) parseSimpleKey(b []byte) (raw, key, rest []byte, err error) {
 	if len(b) == 0 {
-		return nil, nil, nil, NewParserError(b, p.offsetOf(b), "expected key but found none")
+		return nil, nil, nil, NewParserError(b, "expected key but found none")
 	}
 
+	// simple-key = quoted-key / unquoted-key
+	// unquoted-key = 1*( ALPHA / DIGIT / %x2D / %x5F ) ; A-Z / a-z / 0-9 / - / _
+	// quoted-key = basic-string / literal-string
 	switch {
 	case b[0] == '\'':
 		return p.parseLiteralString(b)
@@ -872,7 +896,7 @@ func (p *Parser) parseSimpleKey(b []byte) (raw, key, rest []byte, err error) {
 		key, rest = scanUnquotedKey(b)
 		return key, key, rest, nil
 	default:
-		return nil, nil, nil, NewParserError(b[0:1], p.offsetOf(b), "invalid character at start of key: %c", b[0])
+		return nil, nil, nil, NewParserError(b[0:1], "invalid character at start of key: %c", b[0])
 	}
 }
 
@@ -892,7 +916,7 @@ func (p *Parser) parseBasicString(b []byte) ([]byte, []byte, []byte, error) {
 	// escape-seq-char =/ %x74         ; t    tab             U+0009
 	// escape-seq-char =/ %x75 4HEXDIG ; uXXXX                U+XXXX
 	// escape-seq-char =/ %x55 8HEXDIG ; UXXXXXXXX            U+XXXXXXXX
-	token, escaped, rest, err := scanBasicString(b, p.offsetOf(b))
+	token, escaped, rest, err := scanBasicString(b)
 	if err != nil {
 		return nil, nil, nil, err
 	}
@@ -903,15 +927,13 @@ func (p *Parser) parseBasicString(b []byte) ([]byte, []byte, []byte, error) {
 	// Fast path. If there is no escape sequence, the string should just be
 	// an UTF-8 encoded string, which is the same as Go. In that case,
 	// validate the string and return a direct reference to the buffer.
-	tokenBase := p.offsetOf(token)
-
 	if !escaped {
 		str := token[startIdx:endIdx]
-		invalidIdx := characters.Utf8TomlValidAlreadyEscaped(str)
-		if invalidIdx < 0 {
+		highlight := characters.Utf8TomlValidAlreadyEscaped(str)
+		if len(highlight) == 0 {
 			return token, str, rest, nil
 		}
-		return nil, nil, nil, NewParserError(str[invalidIdx:invalidIdx+1], tokenBase+startIdx+invalidIdx, "invalid UTF-8")
+		return nil, nil, nil, NewParserError(highlight, "invalid UTF-8")
 	}
 
 	i := startIdx
@@ -942,7 +964,7 @@ func (p *Parser) parseBasicString(b []byte) ([]byte, []byte, []byte, error) {
 			case 'e':
 				builder.WriteByte(0x1B)
 			case 'u':
-				x, err := hexToRune(token[i+1:len(token)-1], tokenBase+i+1, 4)
+				x, err := hexToRune(token[i+1:len(token)-1], 4)
 				if err != nil {
 					return nil, nil, nil, err
 				}
@@ -950,7 +972,7 @@ func (p *Parser) parseBasicString(b []byte) ([]byte, []byte, []byte, error) {
 				builder.WriteRune(x)
 				i += 4
 			case 'U':
-				x, err := hexToRune(token[i+1:len(token)-1], tokenBase+i+1, 8)
+				x, err := hexToRune(token[i+1:len(token)-1], 8)
 				if err != nil {
 					return nil, nil, nil, err
 				}
@@ -958,13 +980,13 @@ func (p *Parser) parseBasicString(b []byte) ([]byte, []byte, []byte, error) {
 				builder.WriteRune(x)
 				i += 8
 			default:
-				return nil, nil, nil, NewParserError(token[i:i+1], tokenBase+i, "invalid escaped character %#U", c)
+				return nil, nil, nil, NewParserError(token[i:i+1], "invalid escaped character %#U", c)
 			}
 			i++
 		} else {
 			size := characters.Utf8ValidNext(token[i:])
 			if size == 0 {
-				return nil, nil, nil, NewParserError(token[i:i+1], tokenBase+i, "invalid character %#U", c)
+				return nil, nil, nil, NewParserError(token[i:i+1], "invalid character %#U", c)
 			}
 			builder.Write(token[i : i+size])
 			i += size
@@ -974,9 +996,9 @@ func (p *Parser) parseBasicString(b []byte) ([]byte, []byte, []byte, error) {
 	return token, builder.Bytes(), rest, nil
 }
 
-func hexToRune(b []byte, base int, length int) (rune, error) {
+func hexToRune(b []byte, length int) (rune, error) {
 	if len(b) < length {
-		return -1, NewParserError(b, base, "unicode point needs %d character, not %d", length, len(b))
+		return -1, NewParserError(b, "unicode point needs %d character, not %d", length, len(b))
 	}
 	b = b[:length]
 
@@ -991,13 +1013,13 @@ func hexToRune(b []byte, base int, length int) (rune, error) {
 		case 'A' <= c && c <= 'F':
 			d = uint32(c - 'A' + 10)
 		default:
-			return -1, NewParserError(b[i:i+1], base+i, "non-hex character")
+			return -1, NewParserError(b[i:i+1], "non-hex character")
 		}
 		r = r*16 + d
 	}
 
 	if r > unicode.MaxRune || 0xD800 <= r && r < 0xE000 {
-		return -1, NewParserError(b, base, "escape sequence is invalid Unicode code point")
+		return -1, NewParserError(b, "escape sequence is invalid Unicode code point")
 	}
 
 	return rune(r), nil
@@ -1017,7 +1039,7 @@ func (p *Parser) parseIntOrFloatOrDateTime(b []byte) (reference, []byte, error)
 	switch b[0] {
 	case 'i':
 		if !scanFollowsInf(b) {
-			return invalidReference, nil, NewParserError(atmost(b, 3), p.offsetOf(b), "expected 'inf'")
+			return invalidReference, nil, NewParserError(atmost(b, 3), "expected 'inf'")
 		}
 
 		return p.builder.Push(Node{
@@ -1027,7 +1049,7 @@ func (p *Parser) parseIntOrFloatOrDateTime(b []byte) (reference, []byte, error)
 		}), b[3:], nil
 	case 'n':
 		if !scanFollowsNan(b) {
-			return invalidReference, nil, NewParserError(atmost(b, 3), p.offsetOf(b), "expected 'nan'")
+			return invalidReference, nil, NewParserError(atmost(b, 3), "expected 'nan'")
 		}
 
 		return p.builder.Push(Node{
@@ -1186,7 +1208,7 @@ func (p *Parser) scanIntOrFloat(b []byte) (reference, []byte, error) {
 				}), b[i+3:], nil
 			}
 
-			return invalidReference, nil, NewParserError(b[i:i+1], p.offsetOf(b)+i, "unexpected character 'i' while scanning for a number")
+			return invalidReference, nil, NewParserError(b[i:i+1], "unexpected character 'i' while scanning for a number")
 		}
 
 		if c == 'n' {
@@ -1198,14 +1220,14 @@ func (p *Parser) scanIntOrFloat(b []byte) (reference, []byte, error) {
 				}), b[i+3:], nil
 			}
 
-			return invalidReference, nil, NewParserError(b[i:i+1], p.offsetOf(b)+i, "unexpected character 'n' while scanning for a number")
+			return invalidReference, nil, NewParserError(b[i:i+1], "unexpected character 'n' while scanning for a number")
 		}
 
 		break
 	}
 
 	if i == 0 {
-		return invalidReference, b, NewParserError(b, p.offsetOf(b), "incomplete number")
+		return invalidReference, b, NewParserError(b, "incomplete number")
 	}
 
 	kind := Integer
@@ -1242,13 +1264,13 @@ func isValidBinaryRune(r byte) bool {
 	return r == '0' || r == '1' || r == '_'
 }
 
-func expect(x byte, b []byte, base int) ([]byte, error) {
+func expect(x byte, b []byte) ([]byte, error) {
 	if len(b) == 0 {
-		return nil, NewParserError(b, base, "expected character %c but the document ended here", x)
+		return nil, NewParserError(b, "expected character %c but the document ended here", x)
 	}
 
 	if b[0] != x {
-		return nil, NewParserError(b[0:1], base, "expected character %c", x)
+		return nil, NewParserError(b[0:1], "expected character %c", x)
 	}
 
 	return b[1:], nil

unstable/parser_range_test.go

@@ -0,0 +1,36 @@
+package unstable
+
+import (
+	"errors"
+	"testing"
+)
+
+// Regression test for https://github.com/pelletier/go-toml/issues/1047:
+// Parser.Range must use the real slice offset, not len(data)-len(slice).
+func TestParser_Range_HighlightAfterComment(t *testing.T) {
+	input := []byte("# comment\n= \"value\"")
+
+	var p Parser
+	p.Reset(input)
+	for p.NextExpression() {
+	}
+	err := p.Error()
+	if err == nil {
+		t.Fatal("expected an error")
+	}
+
+	var perr *ParserError
+	if !errors.As(err, &perr) {
+		t.Fatalf("expected *ParserError, got %T", err)
+	}
+
+	r := p.Range(perr.Highlight)
+	shape := p.Shape(r)
+
+	if r.Offset != 10 {
+		t.Errorf("Range offset: got %d, want 10", r.Offset)
+	}
+	if shape.Start.Line != 2 || shape.Start.Column != 1 {
+		t.Errorf("position: got %d:%d, want 2:1", shape.Start.Line, shape.Start.Column)
+	}
+}

unstable/parser_test.go

@@ -1,7 +1,6 @@
 package unstable
 
 import (
-	"errors"
 	"fmt"
 	"strconv"
 	"strings"
@@ -674,96 +673,6 @@ key3 = "value3"
 	assert.Equal(t, []string{"key1", "key2", "key3"}, keys)
 }
 
-func TestErrorOffsetAfterComment(t *testing.T) {
-	input := []byte("# comment\n= \"value\"")
-
-	p := Parser{}
-	p.Reset(input)
-	for p.NextExpression() {
-	}
-	err := p.Error()
-	if err == nil {
-		t.Fatal("expected an error")
-	}
-	var perr *ParserError
-	if !errors.As(err, &perr) {
-		t.Fatalf("expected ParserError, got %T", err)
-	}
-
-	if perr.Offset != 10 {
-		t.Errorf("offset: got %d, want 10", perr.Offset)
-	}
-	shape := p.Shape(Range{Offset: uint32(perr.Offset), Length: uint32(len(perr.Highlight))})
-	if shape.Start.Line != 2 || shape.Start.Column != 1 {
-		t.Errorf("position: got %d:%d, want 2:1", shape.Start.Line, shape.Start.Column)
-	}
-}
-
-func TestErrorHighlightPositions(t *testing.T) {
-	examples := []struct {
-		desc       string
-		input      string
-		wantLine   int
-		wantColumn int
-	}{
-		{
-			desc:       "invalid key start after comment",
-			input:      "# comment\n= \"value\"",
-			wantLine:   2,
-			wantColumn: 1,
-		},
-		{
-			desc:       "invalid key start on first line",
-			input:      "= \"value\"",
-			wantLine:   1,
-			wantColumn: 1,
-		},
-		{
-			desc:       "invalid key after multiple comments",
-			input:      "# comment 1\n# comment 2\n= \"value\"",
-			wantLine:   3,
-			wantColumn: 1,
-		},
-		{
-			desc:       "invalid key after valid key-value",
-			input:      "a = 1\n= \"value\"",
-			wantLine:   2,
-			wantColumn: 1,
-		},
-		{
-			desc:       "invalid key after whitespace on line",
-			input:      "a = 1\n  = \"value\"",
-			wantLine:   2,
-			wantColumn: 3,
-		},
-	}
-
-	for _, e := range examples {
-		t.Run(e.desc, func(t *testing.T) {
-			p := Parser{}
-			p.Reset([]byte(e.input))
-			for p.NextExpression() {
-			}
-			err := p.Error()
-			if err == nil {
-				t.Fatal("expected an error")
-			}
-			var perr *ParserError
-			if !errors.As(err, &perr) {
-				t.Fatalf("expected ParserError, got %T", err)
-			}
-			shape := p.Shape(Range{Offset: uint32(perr.Offset), Length: uint32(len(perr.Highlight))})
-
-			if shape.Start.Line != e.wantLine {
-				t.Errorf("line: got %d, want %d", shape.Start.Line, e.wantLine)
-			}
-			if shape.Start.Column != e.wantColumn {
-				t.Errorf("column: got %d, want %d", shape.Start.Column, e.wantColumn)
-			}
-		})
-	}
-}
-
 func ExampleParser() {
 	doc := `
 	hello = "world"

unstable/scanner.go

@@ -47,31 +47,48 @@ func isUnquotedKeyChar(r byte) bool {
 	return (r >= 'A' && r <= 'Z') || (r >= 'a' && r <= 'z') || (r >= '0' && r <= '9') || r == '-' || r == '_'
 }
 
-func scanLiteralString(b []byte, base int) ([]byte, []byte, error) {
+func scanLiteralString(b []byte) ([]byte, []byte, error) {
+	// literal-string = apostrophe *literal-char apostrophe
+	// apostrophe = %x27 ; ' apostrophe
+	// literal-char = %x09 / %x20-26 / %x28-7E / non-ascii
 	for i := 1; i < len(b); {
 		switch b[i] {
 		case '\'':
 			return b[:i+1], b[i+1:], nil
 		case '\n', '\r':
-			return nil, nil, NewParserError(b[i:i+1], base+i, "literal strings cannot have new lines")
+			return nil, nil, NewParserError(b[i:i+1], "literal strings cannot have new lines")
 		}
 		size := characters.Utf8ValidNext(b[i:])
 		if size == 0 {
-			return nil, nil, NewParserError(b[i:i+1], base+i, "invalid character")
+			return nil, nil, NewParserError(b[i:i+1], "invalid character")
 		}
 		i += size
 	}
 
-	return nil, nil, NewParserError(b[len(b):], base+len(b), "unterminated literal string")
+	return nil, nil, NewParserError(b[len(b):], "unterminated literal string")
 }
 
-func scanMultilineLiteralString(b []byte, base int) ([]byte, []byte, error) {
+func scanMultilineLiteralString(b []byte) ([]byte, []byte, error) {
+	// ml-literal-string = ml-literal-string-delim [ newline ] ml-literal-body
+	// ml-literal-string-delim
+	// ml-literal-string-delim = 3apostrophe
+	// ml-literal-body = *mll-content *( mll-quotes 1*mll-content ) [ mll-quotes ]
+	//
+	// mll-content = mll-char / newline
+	// mll-char = %x09 / %x20-26 / %x28-7E / non-ascii
+	// mll-quotes = 1*2apostrophe
 	for i := 3; i < len(b); {
 		switch b[i] {
 		case '\'':
 			if scanFollowsMultilineLiteralStringDelimiter(b[i:]) {
 				i += 3
 
+				// At that point we found 3 apostrophe, and i is the
+				// index of the byte after the third one. The scanner
+				// needs to be eager, because there can be an extra 2
+				// apostrophe that can be accepted at the end of the
+				// string.
+
 				if i >= len(b) || b[i] != '\'' {
 					return b[:i], b[i:], nil
 				}
@@ -83,39 +100,39 @@ func scanMultilineLiteralString(b []byte, base int) ([]byte, []byte, error) {
 				i++
 
 				if i < len(b) && b[i] == '\'' {
-					return nil, nil, NewParserError(b[i-3:i+1], base+i-3, "''' not allowed in multiline literal string")
+					return nil, nil, NewParserError(b[i-3:i+1], "''' not allowed in multiline literal string")
 				}
 
 				return b[:i], b[i:], nil
 			}
 		case '\r':
 			if len(b) < i+2 {
-				return nil, nil, NewParserError(b[len(b):], base+len(b), `need a \n after \r`)
+				return nil, nil, NewParserError(b[len(b):], `need a \n after \r`)
 			}
 			if b[i+1] != '\n' {
-				return nil, nil, NewParserError(b[i:i+2], base+i, `need a \n after \r`)
+				return nil, nil, NewParserError(b[i:i+2], `need a \n after \r`)
 			}
-			i += 2
+			i += 2 // skip the \n
 			continue
 		}
 		size := characters.Utf8ValidNext(b[i:])
 		if size == 0 {
-			return nil, nil, NewParserError(b[i:i+1], base+i, "invalid character")
+			return nil, nil, NewParserError(b[i:i+1], "invalid character")
 		}
 		i += size
 	}
 
-	return nil, nil, NewParserError(b[len(b):], base+len(b), `multiline literal string not terminated by '''`)
+	return nil, nil, NewParserError(b[len(b):], `multiline literal string not terminated by '''`)
 }
 
-func scanWindowsNewline(b []byte, base int) ([]byte, []byte, error) {
+func scanWindowsNewline(b []byte) ([]byte, []byte, error) {
 	const lenCRLF = 2
 	if len(b) < lenCRLF {
-		return nil, nil, NewParserError(b, base, "windows new line expected")
+		return nil, nil, NewParserError(b, "windows new line expected")
 	}
 
 	if b[1] != '\n' {
-		return nil, nil, NewParserError(b, base, `windows new line should be \r\n`)
+		return nil, nil, NewParserError(b, `windows new line should be \r\n`)
 	}
 
 	return b[:lenCRLF], b[lenCRLF:], nil
@@ -134,7 +151,13 @@ func scanWhitespace(b []byte) ([]byte, []byte) {
 	return b, b[len(b):]
 }
 
-func scanComment(b []byte, base int) ([]byte, []byte, error) {
+func scanComment(b []byte) ([]byte, []byte, error) {
+	// comment-start-symbol = %x23 ; #
+	// non-ascii = %x80-D7FF / %xE000-10FFFF
+	// non-eol = %x09 / %x20-7F / non-ascii
+	//
+	// comment = comment-start-symbol *non-eol
+
 	for i := 1; i < len(b); {
 		if b[i] == '\n' {
 			return b[:i], b[i:], nil
@@ -143,11 +166,11 @@ func scanComment(b []byte, base int) ([]byte, []byte, error) {
 			if i+1 < len(b) && b[i+1] == '\n' {
 				return b[:i+1], b[i+1:], nil
 			}
-			return nil, nil, NewParserError(b[i:i+1], base+i, "invalid character in comment")
+			return nil, nil, NewParserError(b[i:i+1], "invalid character in comment")
 		}
 		size := characters.Utf8ValidNext(b[i:])
 		if size == 0 {
-			return nil, nil, NewParserError(b[i:i+1], base+i, "invalid character in comment")
+			return nil, nil, NewParserError(b[i:i+1], "invalid character in comment")
 		}
 
 		i += size
@@ -156,7 +179,12 @@ func scanComment(b []byte, base int) ([]byte, []byte, error) {
 	return b, b[len(b):], nil
 }
 
-func scanBasicString(b []byte, base int) ([]byte, bool, []byte, error) {
+func scanBasicString(b []byte) ([]byte, bool, []byte, error) {
+	// basic-string = quotation-mark *basic-char quotation-mark
+	// quotation-mark = %x22            ; "
+	// basic-char = basic-unescaped / escaped
+	// basic-unescaped = wschar / %x21 / %x23-5B / %x5D-7E / non-ascii
+	// escaped = escape escape-seq-char
 	escaped := false
 	i := 1
 
@@ -165,20 +193,31 @@ func scanBasicString(b []byte, base int) ([]byte, bool, []byte, error) {
 		case '"':
 			return b[:i+1], escaped, b[i+1:], nil
 		case '\n', '\r':
-			return nil, escaped, nil, NewParserError(b[i:i+1], base+i, "basic strings cannot have new lines")
+			return nil, escaped, nil, NewParserError(b[i:i+1], "basic strings cannot have new lines")
 		case '\\':
 			if len(b) < i+2 {
-				return nil, escaped, nil, NewParserError(b[i:i+1], base+i, "need a character after \\")
+				return nil, escaped, nil, NewParserError(b[i:i+1], "need a character after \\")
 			}
 			escaped = true
 			i++ // skip the next character
 		}
 	}
 
-	return nil, escaped, nil, NewParserError(b[len(b):], base+len(b), `basic string not terminated by "`)
+	return nil, escaped, nil, NewParserError(b[len(b):], `basic string not terminated by "`)
 }
 
-func scanMultilineBasicString(b []byte, base int) ([]byte, bool, []byte, error) {
+func scanMultilineBasicString(b []byte) ([]byte, bool, []byte, error) {
+	// ml-basic-string = ml-basic-string-delim [ newline ] ml-basic-body
+	// ml-basic-string-delim
+	// ml-basic-string-delim = 3quotation-mark
+	// ml-basic-body = *mlb-content *( mlb-quotes 1*mlb-content ) [ mlb-quotes ]
+	//
+	// mlb-content = mlb-char / newline / mlb-escaped-nl
+	// mlb-char = mlb-unescaped / escaped
+	// mlb-quotes = 1*2quotation-mark
+	// mlb-unescaped = wschar / %x21 / %x23-5B / %x5D-7E / non-ascii
+	// mlb-escaped-nl = escape ws newline *( wschar / newline )
+
 	escaped := false
 	i := 3
 
@@ -188,6 +227,12 @@ func scanMultilineBasicString(b []byte, base int) ([]byte, bool, []byte, error)
 			if scanFollowsMultilineBasicStringDelimiter(b[i:]) {
 				i += 3
 
+				// At that point we found 3 apostrophe, and i is the
+				// index of the byte after the third one. The scanner
+				// needs to be eager, because there can be an extra 2
+				// apostrophe that can be accepted at the end of the
+				// string.
+
 				if i >= len(b) || b[i] != '"' {
 					return b[:i], escaped, b[i:], nil
 				}
@@ -199,27 +244,27 @@ func scanMultilineBasicString(b []byte, base int) ([]byte, bool, []byte, error)
 				i++
 
 				if i < len(b) && b[i] == '"' {
-					return nil, escaped, nil, NewParserError(b[i-3:i+1], base+i-3, `""" not allowed in multiline basic string`)
+					return nil, escaped, nil, NewParserError(b[i-3:i+1], `""" not allowed in multiline basic string`)
 				}
 
 				return b[:i], escaped, b[i:], nil
 			}
 		case '\\':
 			if len(b) < i+2 {
-				return nil, escaped, nil, NewParserError(b[len(b):], base+len(b), "need a character after \\")
+				return nil, escaped, nil, NewParserError(b[len(b):], "need a character after \\")
 			}
 			escaped = true
 			i++ // skip the next character
 		case '\r':
 			if len(b) < i+2 {
-				return nil, escaped, nil, NewParserError(b[len(b):], base+len(b), `need a \n after \r`)
+				return nil, escaped, nil, NewParserError(b[len(b):], `need a \n after \r`)
 			}
 			if b[i+1] != '\n' {
-				return nil, escaped, nil, NewParserError(b[i:i+2], base+i, `need a \n after \r`)
+				return nil, escaped, nil, NewParserError(b[i:i+2], `need a \n after \r`)
 			}
 			i++ // skip the \n
 		}
 	}
 
-	return nil, escaped, nil, NewParserError(b[len(b):], base+len(b), `multiline basic string not terminated by """`)
+	return nil, escaped, nil, NewParserError(b[len(b):], `multiline basic string not terminated by """`)
 }