v2: errors (#534)

```
name                              old time/op    new time/op    delta
UnmarshalDataset/config-32          86.7ms ± 2%    87.5ms ± 2%     ~     (p=0.113 n=9+10)
UnmarshalDataset/canada-32           129ms ± 4%     106ms ± 3%  -17.94%  (p=0.000 n=10+10)
UnmarshalDataset/citm_catalog-32    59.4ms ± 5%    58.7ms ± 5%     ~     (p=0.393 n=10+10)
UnmarshalDataset/twitter-32         27.0ms ± 7%    26.9ms ± 6%     ~     (p=0.720 n=10+9)
UnmarshalDataset/code-32             326ms ± 4%     322ms ± 7%     ~     (p=0.661 n=9+10)
UnmarshalDataset/example-32          510µs ±11%     526µs ± 7%     ~     (p=0.182 n=10+9)
UnmarshalSimple-32                  1.41µs ± 6%    1.41µs ± 4%     ~     (p=0.736 n=10+9)
ReferenceFile-32                    45.6µs ± 3%    43.9µs ±10%     ~     (p=0.089 n=10+10)

name                              old speed      new speed      delta
UnmarshalDataset/config-32        12.1MB/s ± 2%  12.0MB/s ± 2%     ~     (p=0.108 n=9+10)
UnmarshalDataset/canada-32        17.1MB/s ± 4%  20.9MB/s ± 3%  +21.86%  (p=0.000 n=10+10)
UnmarshalDataset/citm_catalog-32  9.41MB/s ± 5%  9.51MB/s ± 5%     ~     (p=0.362 n=10+10)
UnmarshalDataset/twitter-32       16.4MB/s ± 8%  16.5MB/s ± 6%     ~     (p=0.704 n=10+9)
UnmarshalDataset/code-32          8.24MB/s ± 4%  8.34MB/s ± 7%     ~     (p=0.675 n=9+10)
UnmarshalDataset/example-32       15.9MB/s ±11%  15.4MB/s ± 7%     ~     (p=0.182 n=10+9)
ReferenceFile-32                   115MB/s ± 4%   120MB/s ±10%     ~     (p=0.085 n=10+10)

name                              old alloc/op   new alloc/op   delta
UnmarshalDataset/config-32          16.9MB ± 0%    16.9MB ± 0%   -0.02%  (p=0.000 n=10+10)
UnmarshalDataset/canada-32          76.8MB ± 0%    74.3MB ± 0%   -3.31%  (p=0.000 n=10+10)
UnmarshalDataset/citm_catalog-32    37.3MB ± 0%    37.1MB ± 0%   -0.60%  (p=0.000 n=9+10)
UnmarshalDataset/twitter-32         15.6MB ± 0%    15.6MB ± 0%   -0.09%  (p=0.000 n=10+10)
UnmarshalDataset/code-32            60.2MB ± 0%    59.3MB ± 0%   -1.51%  (p=0.000 n=10+9)
UnmarshalDataset/example-32          238kB ± 0%     238kB ± 0%   -0.18%  (p=0.000 n=10+10)
ReferenceFile-32                    11.8kB ± 0%    11.8kB ± 0%     ~     (all equal)

name                              old allocs/op  new allocs/op  delta
UnmarshalDataset/config-32            653k ± 0%      645k ± 0%   -1.20%  (p=0.000 n=10+6)
UnmarshalDataset/canada-32           1.01M ± 0%     0.90M ± 0%  -11.04%  (p=0.000 n=9+10)
UnmarshalDataset/citm_catalog-32      384k ± 0%      370k ± 0%   -3.75%  (p=0.000 n=10+10)
UnmarshalDataset/twitter-32           160k ± 0%      157k ± 0%   -1.32%  (p=0.000 n=10+10)
UnmarshalDataset/code-32             2.97M ± 0%     2.91M ± 0%   -2.15%  (p=0.000 n=10+7)
UnmarshalDataset/example-32          3.69k ± 0%     3.63k ± 0%   -1.52%  (p=0.000 n=10+10)
ReferenceFile-32                       253 ± 0%       253 ± 0%     ~     (all equal)
```

decode.go

@@ -1,11 +1,8 @@
 package toml
 
 import (
-	"errors"
-	"fmt"
 	"math"
 	"strconv"
-	"strings"
 	"time"
 )
 
@@ -59,14 +56,12 @@ func parseLocalDate(b []byte) (LocalDate, error) {
 	return date, nil
 }
 
-var errNotDigit = errors.New("not a digit")
-
 func parseDecimalDigits(b []byte) (int, error) {
 	v := 0
 
-	for _, c := range b {
+	for i, c := range b {
 		if !isDigit(c) {
-			return 0, fmt.Errorf("%s: %w", b, errNotDigit)
+			return 0, newDecodeError(b[i:i+1], "should be a digit (0-9)")
 		}
 
 		v *= 10
@@ -76,13 +71,14 @@ func parseDecimalDigits(b []byte) (int, error) {
 	return v, nil
 }
 
-var errParseDateTimeMissingInfo = errors.New("date-time missing timezone information")
-
 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
+
+	originalBytes := b
+
 	dt, b, err := parseLocalDateTime(b)
 	if err != nil {
 		return time.Time{}, err
@@ -91,7 +87,7 @@ func parseDateTime(b []byte) (time.Time, error) {
 	var zone *time.Location
 
 	if len(b) == 0 {
-		return time.Time{}, errParseDateTimeMissingInfo
+		return time.Time{}, newDecodeError(originalBytes, "date-time is missing timezone")
 	}
 
 	if b[0] == 'Z' {
@@ -134,19 +130,12 @@ func parseDateTime(b []byte) (time.Time, error) {
 	return t, nil
 }
 
-var (
-	errParseLocalDateTimeWrongLength = errors.New(
-		"local datetimes are expected to have the format YYYY-MM-DDTHH:MM:SS[.NNNNNNNNN]",
-	)
-	errParseLocalDateTimeWrongSeparator = errors.New("datetime separator is expected to be T or a space")
-)
-
 func parseLocalDateTime(b []byte) (LocalDateTime, []byte, error) {
 	var dt LocalDateTime
 
 	const localDateTimeByteMinLen = 11
 	if len(b) < localDateTimeByteMinLen {
-		return dt, nil, errParseLocalDateTimeWrongLength
+		return dt, nil, newDecodeError(b, "local datetimes are expected to have the format YYYY-MM-DDTHH:MM:SS[.NNNNNNNNN]")
 	}
 
 	date, err := parseLocalDate(b[:10])
@@ -157,7 +146,7 @@ func parseLocalDateTime(b []byte) (LocalDateTime, []byte, error) {
 
 	sep := b[10]
 	if sep != 'T' && sep != ' ' {
-		return dt, nil, errParseLocalDateTimeWrongSeparator
+		return dt, nil, newDecodeError(b[10:11], "datetime separator is expected to be T or a space")
 	}
 
 	t, rest, err := parseLocalTime(b[11:])
@@ -169,8 +158,6 @@ func parseLocalDateTime(b []byte) (LocalDateTime, []byte, error) {
 	return dt, rest, nil
 }
 
-var errParseLocalTimeWrongLength = errors.New("times are expected to have the format HH:MM:SS[.NNNNNN]")
-
 // 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.
@@ -183,7 +170,7 @@ func parseLocalTime(b []byte) (LocalTime, []byte, error) {
 
 	const localTimeByteLen = 8
 	if len(b) < localTimeByteLen {
-		return t, nil, errParseLocalTimeWrongLength
+		return t, nil, newDecodeError(b, "times are expected to have the format HH:MM:SS[.NNNNNN]")
 	}
 
 	var err error
@@ -242,11 +229,6 @@ func parseLocalTime(b []byte) (LocalTime, []byte, error) {
 	return t, b[8:], nil
 }
 
-var (
-	errParseFloatStartDot = errors.New("float cannot start with a dot")
-	errParseFloatEndDot   = errors.New("float cannot end with a dot")
-)
-
 //nolint:cyclop
 func parseFloat(b []byte) (float64, error) {
 	//nolint:godox
@@ -255,150 +237,123 @@ func parseFloat(b []byte) (float64, error) {
 		return math.NaN(), nil
 	}
 
-	tok := string(b)
-
-	err := numberContainsInvalidUnderscore(tok)
+	cleaned, err := checkAndRemoveUnderscores(b)
 	if err != nil {
 		return 0, err
 	}
 
-	cleanedVal := cleanupNumberToken(tok)
-	if cleanedVal[0] == '.' {
-		return 0, errParseFloatStartDot
+	if cleaned[0] == '.' {
+		return 0, newDecodeError(b, "float cannot start with a dot")
 	}
 
-	if cleanedVal[len(cleanedVal)-1] == '.' {
-		return 0, errParseFloatEndDot
+	if cleaned[len(cleaned)-1] == '.' {
+		return 0, newDecodeError(b, "float cannot end with a dot")
 	}
 
-	f, err := strconv.ParseFloat(cleanedVal, 64)
+	f, err := strconv.ParseFloat(string(cleaned), 64)
 	if err != nil {
-		return 0, fmt.Errorf("coudn't ParseFloat %w", err)
+		return 0, newDecodeError(b, "coudn't parse float: %w", err)
 	}
 
 	return f, nil
 }
 
 func parseIntHex(b []byte) (int64, error) {
-	tok := string(b)
-	cleanedVal := cleanupNumberToken(tok)
-
-	err := hexNumberContainsInvalidUnderscore(cleanedVal)
+	cleaned, err := checkAndRemoveUnderscores(b[2:])
 	if err != nil {
 		return 0, err
 	}
 
-	i, err := strconv.ParseInt(cleanedVal[2:], 16, 64)
+	i, err := strconv.ParseInt(string(cleaned), 16, 64)
 	if err != nil {
-		return 0, fmt.Errorf("coudn't ParseIntHex %w", err)
+		return 0, newDecodeError(b, "couldn't parse hexadecimal number: %w", err)
 	}
 
 	return i, nil
 }
 
 func parseIntOct(b []byte) (int64, error) {
-	tok := string(b)
-	cleanedVal := cleanupNumberToken(tok)
-
-	err := numberContainsInvalidUnderscore(cleanedVal)
+	cleaned, err := checkAndRemoveUnderscores(b[2:])
 	if err != nil {
 		return 0, err
 	}
 
-	i, err := strconv.ParseInt(cleanedVal[2:], 8, 64)
+	i, err := strconv.ParseInt(string(cleaned), 8, 64)
 	if err != nil {
-		return 0, fmt.Errorf("coudn't ParseIntOct %w", err)
+		return 0, newDecodeError(b, "couldn't parse octal number: %w", err)
 	}
 
 	return i, nil
 }
 
 func parseIntBin(b []byte) (int64, error) {
-	tok := string(b)
-	cleanedVal := cleanupNumberToken(tok)
-
-	err := numberContainsInvalidUnderscore(cleanedVal)
+	cleaned, err := checkAndRemoveUnderscores(b[2:])
 	if err != nil {
 		return 0, err
 	}
 
-	i, err := strconv.ParseInt(cleanedVal[2:], 2, 64)
+	i, err := strconv.ParseInt(string(cleaned), 2, 64)
 	if err != nil {
-		return 0, fmt.Errorf("coudn't ParseIntBin %w", err)
+		return 0, newDecodeError(b, "couldn't parse binary number: %w", err)
 	}
 
 	return i, nil
 }
 
 func parseIntDec(b []byte) (int64, error) {
-	tok := string(b)
-	cleanedVal := cleanupNumberToken(tok)
-
-	err := numberContainsInvalidUnderscore(cleanedVal)
+	cleaned, err := checkAndRemoveUnderscores(b)
 	if err != nil {
 		return 0, err
 	}
 
-	i, err := strconv.ParseInt(cleanedVal, 10, 64)
+	i, err := strconv.ParseInt(string(cleaned), 10, 64)
 	if err != nil {
-		return 0, fmt.Errorf("coudn't parseIntDec %w", err)
+		return 0, newDecodeError(b, "couldn't parse decimal number: %w", err)
 	}
 
 	return i, nil
 }
 
-func numberContainsInvalidUnderscore(value string) error {
-	// For large numbers, you may use underscores between digits to enhance
-	// readability. Each underscore must be surrounded by at least one digit on
-	// each side.
-	hasBefore := false
-
-	for idx, r := range value {
-		if r == '_' {
-			if !hasBefore || idx+1 >= len(value) {
-				// can't end with an underscore
-				return errInvalidUnderscore
-			}
-		}
-		hasBefore = isDigitRune(r)
+func checkAndRemoveUnderscores(b []byte) ([]byte, error) {
+	if len(b) == 0 {
+		return b, nil
 	}
 
-	return nil
-}
-
-func hexNumberContainsInvalidUnderscore(value string) error {
-	hasBefore := false
-
-	for idx, r := range value {
-		if r == '_' {
-			if !hasBefore || idx+1 >= len(value) {
-				// can't end with an underscore
-				return errInvalidUnderscoreHex
-			}
-		}
-		hasBefore = isHexDigit(r)
+	if b[0] == '_' {
+		return nil, newDecodeError(b[0:1], "number cannot start with underscore")
 	}
 
-	return nil
+	if b[len(b)-1] == '_' {
+		return nil, newDecodeError(b[len(b)-1:], "number cannot end with underscore")
+	}
+
+	// fast path
+	i := 0
+	for ; i < len(b); i++ {
+		if b[i] == '_' {
+			break
+		}
+	}
+	if i == len(b) {
+		return b, nil
+	}
+
+	before := false
+	cleaned := make([]byte, i, len(b))
+	copy(cleaned, b)
+
+	for i++; i < len(b); i++ {
+		c := b[i]
+		if c == '_' {
+			if !before {
+				return nil, newDecodeError(b[i-1:i+1], "number must have at least one digit between underscores")
+			}
+			before = false
+		} else {
+			before = true
+			cleaned = append(cleaned, c)
+		}
+	}
+
+	return cleaned, nil
 }
-
-func cleanupNumberToken(value string) string {
-	cleanedVal := strings.ReplaceAll(value, "_", "")
-
-	return cleanedVal
-}
-
-func isHexDigit(r rune) bool {
-	return isDigitRune(r) ||
-		(r >= 'a' && r <= 'f') ||
-		(r >= 'A' && r <= 'F')
-}
-
-func isDigitRune(r rune) bool {
-	return r >= '0' && r <= '9'
-}
-
-var (
-	errInvalidUnderscore    = errors.New("invalid use of _ in number")
-	errInvalidUnderscoreHex = errors.New("invalid use of _ in hex number")
-)

errors.go

@@ -70,13 +70,13 @@ func (de *decodeError) Error() string {
 func newDecodeError(highlight []byte, format string, args ...interface{}) error {
 	return &decodeError{
 		highlight: highlight,
-		message:   fmt.Sprintf(format, args...),
+		message:   fmt.Errorf(format, args...).Error(),
 	}
 }
 
 // Error returns the error message contained in the DecodeError.
 func (e *DecodeError) Error() string {
-	return e.message
+	return "toml: " + e.message
 }
 
 // String returns the human-readable contextualized error. This string is multi-line.

internal/tracker/seen.go

@@ -123,10 +123,10 @@ func (s *SeenTracker) checkTable(node ast.Node) error {
 	i, found := s.current.Has(k)
 	if found {
 		if i.kind != tableKind {
-			return fmt.Errorf("key %s should be a table", k)
+			return fmt.Errorf("toml: key %s should be a table, not a %s", k, i.kind)
 		}
 		if i.explicit {
-			return fmt.Errorf("table %s already exists", k)
+			return fmt.Errorf("toml: table %s already exists", k)
 		}
 		i.explicit = true
 		s.current = i
@@ -162,7 +162,7 @@ func (s *SeenTracker) checkArrayTable(node ast.Node) error {
 	info, found := s.current.Has(k)
 	if found {
 		if info.kind != arrayTableKind {
-			return fmt.Errorf("key %s already exists but is not an array table", k)
+			return fmt.Errorf("toml: key %s already exists as a %s,  but should be an array table", info.kind, k)
 		}
 		info.Clear()
 	} else {
@@ -182,7 +182,7 @@ func (s *SeenTracker) checkKeyValue(context *info, node ast.Node) error {
 		child, found := context.Has(k)
 		if found {
 			if child.kind != tableKind {
-				return fmt.Errorf("expected %s to be a table, not a %s", k, child.kind)
+				return fmt.Errorf("toml: expected %s to be a table, not a %s", k, child.kind)
 			}
 		} else {
 			child = context.CreateTable(k, false)

localtime.go

@@ -53,7 +53,7 @@ func LocalDateOf(t time.Time) LocalDate {
 func ParseLocalDate(s string) (LocalDate, error) {
 	t, err := time.Parse("2006-01-02", s)
 	if err != nil {
-		return LocalDate{}, fmt.Errorf("ParseLocalDate: %w", err)
+		return LocalDate{}, err
 	}
 
 	return LocalDateOf(t), nil
@@ -166,7 +166,7 @@ func LocalTimeOf(t time.Time) LocalTime {
 func ParseLocalTime(s string) (LocalTime, error) {
 	t, err := time.Parse("15:04:05.999999999", s)
 	if err != nil {
-		return LocalTime{}, fmt.Errorf("ParseLocalTime: %w", err)
+		return LocalTime{}, err
 	}
 
 	return LocalTimeOf(t), nil
@@ -237,7 +237,7 @@ func ParseLocalDateTime(s string) (LocalDateTime, error) {
 	if err != nil {
 		t, err = time.Parse("2006-01-02t15:04:05.999999999", s)
 		if err != nil {
-			return LocalDateTime{}, fmt.Errorf("ParseLocalDateTime: %w", err)
+			return LocalDateTime{}, err
 		}
 	}
 

marshaler.go

@@ -3,7 +3,6 @@ package toml
 import (
 	"bytes"
 	"encoding"
-	"errors"
 	"fmt"
 	"io"
 	"reflect"
@@ -116,12 +115,12 @@ func (enc *Encoder) Encode(v interface{}) error {
 
 	b, err := enc.encode(b, ctx, reflect.ValueOf(v))
 	if err != nil {
-		return fmt.Errorf("Encode: %w", err)
+		return err
 	}
 
 	_, err = enc.w.Write(b)
 	if err != nil {
-		return fmt.Errorf("Encode: %w", err)
+		return fmt.Errorf("toml: cannot write: %w", err)
 	}
 
 	return nil
@@ -178,11 +177,6 @@ func (ctx *encoderCtx) isRoot() bool {
 	return len(ctx.parentKey) == 0 && !ctx.hasKey
 }
 
-var (
-	errUnsupportedValue            = errors.New("unsupported encode value kind")
-	errTextMarshalerCannotBeAtRoot = errors.New("type implementing TextMarshaler cannot be at root")
-)
-
 //nolint:cyclop,funlen
 func (enc *Encoder) encode(b []byte, ctx encoderCtx, v reflect.Value) ([]byte, error) {
 	i, ok := v.Interface().(time.Time)
@@ -192,12 +186,12 @@ func (enc *Encoder) encode(b []byte, ctx encoderCtx, v reflect.Value) ([]byte, e
 
 	if v.Type().Implements(textMarshalerType) {
 		if ctx.isRoot() {
-			return nil, errTextMarshalerCannotBeAtRoot
+			return nil, fmt.Errorf("toml: type %s implementing the TextMarshaler interface cannot be a root element", v.Type())
 		}
 
 		text, err := v.Interface().(encoding.TextMarshaler).MarshalText()
 		if err != nil {
-			return nil, fmt.Errorf("encode: %w", err)
+			return nil, err
 		}
 
 		b = enc.encodeString(b, string(text), ctx.options)
@@ -215,7 +209,7 @@ func (enc *Encoder) encode(b []byte, ctx encoderCtx, v reflect.Value) ([]byte, e
 		return enc.encodeSlice(b, ctx, v)
 	case reflect.Interface:
 		if v.IsNil() {
-			return nil, errNilInterface
+			return nil, fmt.Errorf("toml: encoding a nil interface is not supported")
 		}
 
 		return enc.encode(b, ctx, v.Elem())
@@ -244,7 +238,7 @@ func (enc *Encoder) encode(b []byte, ctx encoderCtx, v reflect.Value) ([]byte, e
 	case reflect.Int64, reflect.Int32, reflect.Int16, reflect.Int8, reflect.Int:
 		b = strconv.AppendInt(b, v.Int(), 10)
 	default:
-		return nil, fmt.Errorf("encode(type %s): %w", v.Kind(), errUnsupportedValue)
+		return nil, fmt.Errorf("toml: cannot encode value of type %s", v.Kind())
 	}
 
 	return b, nil
@@ -412,8 +406,6 @@ func (enc *Encoder) encodeTableHeader(ctx encoderCtx, b []byte) ([]byte, error)
 	return b, nil
 }
 
-var errTomlNoMultiline = errors.New("TOML does not support multiline keys")
-
 //nolint:cyclop
 func (enc *Encoder) encodeKey(b []byte, k string) ([]byte, error) {
 	needsQuotation := false
@@ -425,7 +417,7 @@ func (enc *Encoder) encodeKey(b []byte, k string) ([]byte, error) {
 		}
 
 		if c == '\n' {
-			return nil, errTomlNoMultiline
+			return nil, fmt.Errorf("toml: new line characters in keys are not supported")
 		}
 
 		if c == literalQuote {
@@ -445,11 +437,9 @@ func (enc *Encoder) encodeKey(b []byte, k string) ([]byte, error) {
 	}
 }
 
-var errNotSupportedAsMapKey = errors.New("type not supported as map key")
-
 func (enc *Encoder) encodeMap(b []byte, ctx encoderCtx, v reflect.Value) ([]byte, error) {
 	if v.Type().Key().Kind() != reflect.String {
-		return nil, fmt.Errorf("encodeMap '%s': %w", v.Type().Key().Kind(), errNotSupportedAsMapKey)
+		return nil, fmt.Errorf("toml: type %s is not supported as a map key", v.Type().Key().Kind())
 	}
 
 	var (
@@ -658,10 +648,7 @@ func (enc *Encoder) encodeTableInline(b []byte, ctx encoderCtx, t table) ([]byte
 	return b, nil
 }
 
-var (
-	errNilInterface   = errors.New("nil interface not supported")
-	textMarshalerType = reflect.TypeOf(new(encoding.TextMarshaler)).Elem()
-)
+var textMarshalerType = reflect.TypeOf(new(encoding.TextMarshaler)).Elem()
 
 func willConvertToTable(ctx encoderCtx, v reflect.Value) (bool, error) {
 	if v.Type() == timeType || v.Type().Implements(textMarshalerType) {
@@ -674,7 +661,7 @@ func willConvertToTable(ctx encoderCtx, v reflect.Value) (bool, error) {
 		return !ctx.inline, nil
 	case reflect.Interface:
 		if v.IsNil() {
-			return false, errNilInterface
+			return false, fmt.Errorf("toml: encoding a nil interface is not supported")
 		}
 
 		return willConvertToTable(ctx, v.Elem())
@@ -694,7 +681,7 @@ func willConvertToTableOrArrayTable(ctx encoderCtx, v reflect.Value) (bool, erro
 
 	if t.Kind() == reflect.Interface {
 		if v.IsNil() {
-			return false, errNilInterface
+			return false, fmt.Errorf("toml: encoding a nil interface is not supported")
 		}
 
 		return willConvertToTableOrArrayTable(ctx, v.Elem())

parser.go

@@ -2,7 +2,6 @@ package toml
 
 import (
 	"bytes"
-	"errors"
 	"fmt"
 	"strconv"
 
@@ -225,19 +224,13 @@ func (p *parser) parseKeyval(b []byte) (ast.Reference, []byte, error) {
 	return ref, b, err
 }
 
-var (
-	errExpectedValNotEOF = errors.New("expected value, not eof")
-	errExpectedTrue      = errors.New("expected 'true'")
-	errExpectedFalse     = errors.New("expected 'false'")
-)
-
 //nolint:cyclop,funlen
 func (p *parser) parseVal(b []byte) (ast.Reference, []byte, error) {
 	// val = string / boolean / array / inline-table / date-time / float / integer
 	var ref ast.Reference
 
 	if len(b) == 0 {
-		return ref, nil, errExpectedValNotEOF
+		return ref, nil, newDecodeError(b, "expected value, not eof")
 	}
 
 	var err error
@@ -278,7 +271,7 @@ func (p *parser) parseVal(b []byte) (ast.Reference, []byte, error) {
 		return ref, b, err
 	case 't':
 		if !scanFollowsTrue(b) {
-			return ref, nil, errExpectedTrue
+			return ref, nil, newDecodeError(atmost(b, 4), "expected 'true'")
 		}
 
 		ref = p.builder.Push(ast.Node{
@@ -289,7 +282,7 @@ func (p *parser) parseVal(b []byte) (ast.Reference, []byte, error) {
 		return ref, b[4:], nil
 	case 'f':
 		if !scanFollowsFalse(b) {
-			return ast.Reference{}, nil, errExpectedFalse
+			return ref, nil, newDecodeError(atmost(b, 5), "expected 'false'")
 		}
 
 		ref = p.builder.Push(ast.Node{
@@ -307,6 +300,13 @@ func (p *parser) parseVal(b []byte) (ast.Reference, []byte, error) {
 	}
 }
 
+func atmost(b []byte, n int) []byte {
+	if n >= len(b) {
+		return b
+	}
+	return b[:n]
+}
+
 func (p *parser) parseLiteralString(b []byte) ([]byte, []byte, error) {
 	v, rest, err := scanLiteralString(b)
 	if err != nil {
@@ -370,8 +370,6 @@ func (p *parser) parseInlineTable(b []byte) (ast.Reference, []byte, error) {
 	return parent, rest, err
 }
 
-var errArrayCannotStartWithComma = errors.New("array cannot start with comma")
-
 //nolint:funlen,cyclop
 func (p *parser) parseValArray(b []byte) (ast.Reference, []byte, error) {
 	// array = array-open [ array-values ] ws-comment-newline array-close
@@ -409,7 +407,7 @@ func (p *parser) parseValArray(b []byte) (ast.Reference, []byte, error) {
 
 		if b[0] == ',' {
 			if first {
-				return parent, nil, errArrayCannotStartWithComma
+				return parent, nil, newDecodeError(b[0:1], "array cannot start with comma")
 			}
 			b = b[1:]
 
@@ -494,8 +492,6 @@ func (p *parser) parseMultilineLiteralString(b []byte) ([]byte, []byte, error) {
 	return token[i : len(token)-3], rest, err
 }
 
-var errInvalidEscapeChar = errors.New("invalid escaped character")
-
 //nolint:funlen,gocognit,cyclop
 func (p *parser) parseMultilineBasicString(b []byte) ([]byte, []byte, error) {
 	// ml-basic-string = ml-basic-string-delim [ newline ] ml-basic-body
@@ -582,7 +578,7 @@ func (p *parser) parseMultilineBasicString(b []byte) ([]byte, []byte, error) {
 				builder.WriteString(x)
 				i += 8
 			default:
-				return nil, nil, fmt.Errorf("parseMultilineBasicString: %w - %#U", errInvalidEscapeChar, c)
+				return nil, nil, newDecodeError(token[i:i+1], "invalid escaped character %#U", c)
 			}
 		} else {
 			builder.WriteByte(c)
@@ -721,7 +717,7 @@ func (p *parser) parseBasicString(b []byte) ([]byte, []byte, error) {
 				builder.WriteString(x)
 				i += 8
 			default:
-				return nil, nil, fmt.Errorf("parseBasicString: %w - %#U", errInvalidEscapeChar, c)
+				return nil, nil, newDecodeError(token[i:i+1], "invalid escaped character %#U", c)
 			}
 		} else {
 			builder.WriteByte(c)
@@ -731,18 +727,17 @@ func (p *parser) parseBasicString(b []byte) ([]byte, []byte, error) {
 	return builder.Bytes(), rest, nil
 }
 
-var errUnicodePointNeedsRightCountChar = errors.New("unicode point needs right number of hex characters")
-
 func hexToString(b []byte, length int) (string, error) {
 	if len(b) < length {
-		return "", fmt.Errorf("hexToString: %w - %d", errUnicodePointNeedsRightCountChar, length)
+		return "", newDecodeError(b, "unicode point needs %d character, not %d", length, len(b))
 	}
+	b = b[:length]
 
 	//nolint:godox
 	// TODO: slow
-	intcode, err := strconv.ParseInt(string(b[:length]), 16, 32)
+	intcode, err := strconv.ParseInt(string(b), 16, 32)
 	if err != nil {
-		return "", fmt.Errorf("hexToString: %w", err)
+		return "", newDecodeError(b, "couldn't parse hexadecimal number: %w", err)
 	}
 
 	return string(rune(intcode)), nil
@@ -757,17 +752,12 @@ func (p *parser) parseWhitespace(b []byte) []byte {
 	return rest
 }
 
-var (
-	errExpectedInf = errors.New("expected 'inf'")
-	errExpectedNan = errors.New("expected 'nan'")
-)
-
 //nolint:cyclop
 func (p *parser) parseIntOrFloatOrDateTime(b []byte) (ast.Reference, []byte, error) {
 	switch b[0] {
 	case 'i':
 		if !scanFollowsInf(b) {
-			return ast.Reference{}, nil, errExpectedInf
+			return ast.Reference{}, nil, newDecodeError(atmost(b, 3), "expected 'inf'")
 		}
 
 		return p.builder.Push(ast.Node{
@@ -776,7 +766,7 @@ func (p *parser) parseIntOrFloatOrDateTime(b []byte) (ast.Reference, []byte, err
 		}), b[3:], nil
 	case 'n':
 		if !scanFollowsNan(b) {
-			return ast.Reference{}, nil, errExpectedNan
+			return ast.Reference{}, nil, newDecodeError(atmost(b, 3), "expected 'nan'")
 		}
 
 		return p.builder.Push(ast.Node{
@@ -821,8 +811,6 @@ func digitsToInt(b []byte) int {
 	return x
 }
 
-var errTimezoneButNoTimeComponent = errors.New("possible DateTime cannot have a timezone but no time component")
-
 //nolint:gocognit,cyclop
 func (p *parser) scanDateTime(b []byte) (ast.Reference, []byte, error) {
 	// scans for contiguous characters in [0-9T:Z.+-], and up to one space if
@@ -867,7 +855,7 @@ byteLoop:
 		}
 	} else {
 		if hasTz {
-			return ast.Reference{}, nil, errTimezoneButNoTimeComponent
+			return ast.Reference{}, nil, newDecodeError(b, "date-time has timezone but not time component")
 		}
 		kind = ast.LocalDate
 	}
@@ -878,12 +866,6 @@ byteLoop:
 	}), b[i:], nil
 }
 
-var (
-	errUnexpectedCharI    = fmt.Errorf("unexpected character i while scanning for a number")
-	errUnexpectedCharN    = fmt.Errorf("unexpected character n while scanning for a number")
-	errExpectedIntOrFloat = fmt.Errorf("expected integer or float")
-)
-
 //nolint:funlen,gocognit,cyclop
 func (p *parser) scanIntOrFloat(b []byte) (ast.Reference, []byte, error) {
 	i := 0
@@ -940,7 +922,7 @@ func (p *parser) scanIntOrFloat(b []byte) (ast.Reference, []byte, error) {
 				}), b[i+3:], nil
 			}
 
-			return ast.Reference{}, nil, errUnexpectedCharI
+			return ast.Reference{}, nil, newDecodeError(b[i:i+1], "unexpected character 'i' while scanning for a number")
 		}
 
 		if c == 'n' {
@@ -951,14 +933,14 @@ func (p *parser) scanIntOrFloat(b []byte) (ast.Reference, []byte, error) {
 				}), b[i+3:], nil
 			}
 
-			return ast.Reference{}, nil, errUnexpectedCharN
+			return ast.Reference{}, nil, newDecodeError(b[i:i+1], "unexpected character 'n' while scanning for a number")
 		}
 
 		break
 	}
 
 	if i == 0 {
-		return ast.Reference{}, b, errExpectedIntOrFloat
+		return ast.Reference{}, b, newDecodeError(b, "incomplete number")
 	}
 
 	kind := ast.Integer

scanner.go

@@ -1,9 +1,5 @@
 package toml
 
-import (
-	"errors"
-)
-
 func scanFollows(b []byte, pattern string) bool {
 	n := len(pattern)
 
@@ -83,22 +79,17 @@ func scanMultilineLiteralString(b []byte) ([]byte, []byte, error) {
 	return nil, nil, newDecodeError(b[len(b):], `multiline literal string not terminated by '''`)
 }
 
-var (
-	errWindowsNewLineMissing = errors.New(`windows new line missing \n`)
-	errWindowsNewLineCRLF    = errors.New(`windows new line should be \r\n`)
-)
-
 func scanWindowsNewline(b []byte) ([]byte, []byte, error) {
-	const lenLF = 2
-	if len(b) < lenLF {
-		return nil, nil, errWindowsNewLineMissing
+	const lenCRLF = 2
+	if len(b) < lenCRLF {
+		return nil, nil, newDecodeError(b, "windows new line expected")
 	}
 
 	if b[1] != '\n' {
-		return nil, nil, errWindowsNewLineCRLF
+		return nil, nil, newDecodeError(b, `windows new line should be \r\n`)
 	}
 
-	return b[:lenLF], b[lenLF:], nil
+	return b[:lenCRLF], b[lenCRLF:], nil
 }
 
 func scanWhitespace(b []byte) ([]byte, []byte) {
@@ -116,8 +107,6 @@ func scanWhitespace(b []byte) ([]byte, []byte) {
 
 //nolint:unparam
 func scanComment(b []byte) ([]byte, []byte) {
-	// ;; Comment
-	//
 	// comment-start-symbol = %x23 ; #
 	// non-ascii = %x80-D7FF / %xE000-10FFFF
 	// non-eol = %x09 / %x20-7F / non-ascii
@@ -132,10 +121,6 @@ func scanComment(b []byte) ([]byte, []byte) {
 	return b, nil
 }
 
-var errBasicLineNotTerminatedByQuote = errors.New(`basic string not terminated by "`)
-
-//nolint:godox
-// TODO perform validation on the string?
 func scanBasicString(b []byte) ([]byte, []byte, error) {
 	// basic-string = quotation-mark *basic-char quotation-mark
 	// quotation-mark = %x22            ; "
@@ -156,11 +141,9 @@ func scanBasicString(b []byte) ([]byte, []byte, error) {
 		}
 	}
 
-	return nil, nil, errBasicLineNotTerminatedByQuote
+	return nil, nil, newDecodeError(b[len(b):], `basic string not terminated by "`)
 }
 
-//nolint:godox
-// TODO perform validation on the string?
 func scanMultilineBasicString(b []byte) ([]byte, []byte, error) {
 	// ml-basic-string = ml-basic-string-delim [ newline ] ml-basic-body
 	// ml-basic-string-delim

targets.go

@@ -1,7 +1,6 @@
 package toml
 
 import (
-	"errors"
 	"fmt"
 	"math"
 	"reflect"
@@ -14,19 +13,19 @@ type target interface {
 	get() reflect.Value
 
 	// Store a string at the target.
-	setString(v string) error
+	setString(v string)
 
 	// Store a boolean at the target
-	setBool(v bool) error
+	setBool(v bool)
 
 	// Store an int64 at the target
-	setInt64(v int64) error
+	setInt64(v int64)
 
 	// Store a float64 at the target
-	setFloat64(v float64) error
+	setFloat64(v float64)
 
 	// Stores any value at the target
-	set(v reflect.Value) error
+	set(v reflect.Value)
 }
 
 // valueTarget just contains a reflect.Value that can be set.
@@ -37,34 +36,24 @@ func (t valueTarget) get() reflect.Value {
 	return reflect.Value(t)
 }
 
-func (t valueTarget) set(v reflect.Value) error {
+func (t valueTarget) set(v reflect.Value) {
 	reflect.Value(t).Set(v)
-
-	return nil
 }
 
-func (t valueTarget) setString(v string) error {
+func (t valueTarget) setString(v string) {
 	t.get().SetString(v)
-
-	return nil
 }
 
-func (t valueTarget) setBool(v bool) error {
+func (t valueTarget) setBool(v bool) {
 	t.get().SetBool(v)
-
-	return nil
 }
 
-func (t valueTarget) setInt64(v int64) error {
+func (t valueTarget) setInt64(v int64) {
 	t.get().SetInt(v)
-
-	return nil
 }
 
-func (t valueTarget) setFloat64(v float64) error {
+func (t valueTarget) setFloat64(v float64) {
 	t.get().SetFloat(v)
-
-	return nil
 }
 
 // interfaceTarget wraps an other target to dereference on get.
@@ -76,49 +65,24 @@ func (t interfaceTarget) get() reflect.Value {
 	return t.x.get().Elem()
 }
 
-func (t interfaceTarget) set(v reflect.Value) error {
-	err := t.x.set(v)
-	if err != nil {
-		return fmt.Errorf("interfaceTarget set: %w", err)
-	}
-
-	return nil
+func (t interfaceTarget) set(v reflect.Value) {
+	t.x.set(v)
 }
 
-func (t interfaceTarget) setString(v string) error {
-	err := t.x.setString(v)
-	if err != nil {
-		return fmt.Errorf("interfaceTarget setString: %w", err)
-	}
-
-	return nil
+func (t interfaceTarget) setString(v string) {
+	t.x.setString(v)
 }
 
-func (t interfaceTarget) setBool(v bool) error {
-	err := t.x.setBool(v)
-	if err != nil {
-		return fmt.Errorf("interfaceTarget setBool: %w", err)
-	}
-
-	return nil
+func (t interfaceTarget) setBool(v bool) {
+	t.x.setBool(v)
 }
 
-func (t interfaceTarget) setInt64(v int64) error {
-	err := t.x.setInt64(v)
-	if err != nil {
-		return fmt.Errorf("interfaceTarget setInt64: %w", err)
-	}
-
-	return nil
+func (t interfaceTarget) setInt64(v int64) {
+	t.x.setInt64(v)
 }
 
-func (t interfaceTarget) setFloat64(v float64) error {
-	err := t.x.setFloat64(v)
-	if err != nil {
-		return fmt.Errorf("interfaceTarget setFloat64: %w", err)
-	}
-
-	return nil
+func (t interfaceTarget) setFloat64(v float64) {
+	t.x.setFloat64(v)
 }
 
 // mapTarget targets a specific key of a map.
@@ -131,33 +95,26 @@ func (t mapTarget) get() reflect.Value {
 	return t.v.MapIndex(t.k)
 }
 
-func (t mapTarget) set(v reflect.Value) error {
+func (t mapTarget) set(v reflect.Value) {
 	t.v.SetMapIndex(t.k, v)
-
-	return nil
 }
 
-func (t mapTarget) setString(v string) error {
-	return t.set(reflect.ValueOf(v))
+func (t mapTarget) setString(v string) {
+	t.set(reflect.ValueOf(v))
 }
 
-func (t mapTarget) setBool(v bool) error {
-	return t.set(reflect.ValueOf(v))
+func (t mapTarget) setBool(v bool) {
+	t.set(reflect.ValueOf(v))
 }
 
-func (t mapTarget) setInt64(v int64) error {
-	return t.set(reflect.ValueOf(v))
+func (t mapTarget) setInt64(v int64) {
+	t.set(reflect.ValueOf(v))
 }
 
-func (t mapTarget) setFloat64(v float64) error {
-	return t.set(reflect.ValueOf(v))
+func (t mapTarget) setFloat64(v float64) {
+	t.set(reflect.ValueOf(v))
 }
 
-var (
-	errValIndexExpectingSlice  = errors.New("expecting a slice")
-	errValIndexCannotInitSlice = errors.New("cannot initialize a slice")
-)
-
 //nolint:cyclop
 // makes sure that the value pointed at by t is indexable (Slice, Array), or
 // dereferences to an indexable (Ptr, Interface).
@@ -167,43 +124,20 @@ func ensureValueIndexable(t target) error {
 	switch f.Type().Kind() {
 	case reflect.Slice:
 		if f.IsNil() {
-			err := t.set(reflect.MakeSlice(f.Type(), 0, 0))
-			if err != nil {
-				return fmt.Errorf("ensureValueIndexable: %w", err)
-			}
-
+			t.set(reflect.MakeSlice(f.Type(), 0, 0))
 			return nil
 		}
 	case reflect.Interface:
 		if f.IsNil() || f.Elem().Type() != sliceInterfaceType {
-			err := t.set(reflect.MakeSlice(sliceInterfaceType, 0, 0))
-			if err != nil {
-				return fmt.Errorf("ensureValueIndexable: %w", err)
-			}
-
+			t.set(reflect.MakeSlice(sliceInterfaceType, 0, 0))
 			return nil
 		}
-
-		if f.Elem().Type().Kind() != reflect.Slice {
-			return fmt.Errorf("ensureValueIndexable: %w, not a %s", errValIndexExpectingSlice, f.Kind())
-		}
 	case reflect.Ptr:
-		if f.IsNil() {
-			ptr := reflect.New(f.Type().Elem())
-
-			err := t.set(ptr)
-			if err != nil {
-				return fmt.Errorf("ensureValueIndexable: %w", err)
-			}
-
-			f = t.get()
-		}
-
-		return ensureValueIndexable(valueTarget(f.Elem()))
+		panic("pointer should have already been dereferenced")
 	case reflect.Array:
 		// arrays are always initialized.
 	default:
-		return fmt.Errorf("ensureValueIndexable: %w with %s", errValIndexCannotInitSlice, f.Kind())
+		return fmt.Errorf("toml: cannot store array in a %s", f.Kind())
 	}
 
 	return nil
@@ -214,69 +148,44 @@ var (
 	mapStringInterfaceType = reflect.TypeOf(map[string]interface{}{})
 )
 
-func ensureMapIfInterface(x target) error {
+func ensureMapIfInterface(x target) {
 	v := x.get()
 
 	if v.Kind() == reflect.Interface && v.IsNil() {
 		newElement := reflect.MakeMap(mapStringInterfaceType)
 
-		err := x.set(newElement)
-		if err != nil {
-			return fmt.Errorf("ensureMapIfInterface: %w", err)
-		}
+		x.set(newElement)
 	}
-
-	return nil
 }
 
-var errSetStringCannotAssignString = errors.New("cannot assign string")
-
 func setString(t target, v string) error {
 	f := t.get()
 
 	switch f.Kind() {
 	case reflect.String:
-		err := t.setString(v)
-		if err != nil {
-			return fmt.Errorf("setString: %w", err)
-		}
-
-		return nil
+		t.setString(v)
 	case reflect.Interface:
-		err := t.set(reflect.ValueOf(v))
-		if err != nil {
-			return fmt.Errorf("setString: %w", err)
-		}
-
-		return nil
+		t.set(reflect.ValueOf(v))
 	default:
-		return fmt.Errorf("setString: %w to a %s", errSetStringCannotAssignString, f.Kind())
+		return fmt.Errorf("toml: cannot assign string to a %s", f.Kind())
 	}
-}
 
-var errSetBoolCannotAssignBool = errors.New("cannot assign bool")
+	return nil
+}
 
 func setBool(t target, v bool) error {
 	f := t.get()
 
 	switch f.Kind() {
 	case reflect.Bool:
-		err := t.setBool(v)
-		if err != nil {
-			return fmt.Errorf("setBool: %w", err)
-		}
-
-		return nil
+		t.setBool(v)
 	case reflect.Interface:
-		err := t.set(reflect.ValueOf(v))
-		if err != nil {
-			return fmt.Errorf("setBool: %w", err)
-		}
-
-		return nil
+		t.set(reflect.ValueOf(v))
 	default:
-		return fmt.Errorf("setBool: %w to a %s", errSetBoolCannotAssignBool, f.String())
+		return fmt.Errorf("toml: cannot assign boolean to a %s", f.Kind())
 	}
+
+	return nil
 }
 
 const (
@@ -284,207 +193,104 @@ const (
 	minInt = -maxInt - 1
 )
 
-var (
-	errSetInt64InInt32     = errors.New("does not fit in an int32")
-	errSetInt64InInt16     = errors.New("does not fit in an int16")
-	errSetInt64InInt8      = errors.New("does not fit in an int8")
-	errSetInt64InInt       = errors.New("does not fit in an int")
-	errSetInt64InUint64    = errors.New("negative integer does not fit in an uint64")
-	errSetInt64InUint32    = errors.New("negative integer does not fit in an uint32")
-	errSetInt64InUint32Max = errors.New("integer does not fit in an uint32")
-	errSetInt64InUint16    = errors.New("negative integer does not fit in an uint16")
-	errSetInt64InUint16Max = errors.New("integer does not fit in an uint16")
-	errSetInt64InUint8     = errors.New("negative integer does not fit in an uint8")
-	errSetInt64InUint8Max  = errors.New("integer does not fit in an uint8")
-	errSetInt64InUint      = errors.New("negative integer does not fit in an uint")
-	errSetInt64Unknown     = errors.New("does not fit in an uint")
-)
-
 //nolint:funlen,gocognit,cyclop,gocyclo
 func setInt64(t target, v int64) error {
 	f := t.get()
 
 	switch f.Kind() {
 	case reflect.Int64:
-		err := t.setInt64(v)
-		if err != nil {
-			return fmt.Errorf("setInt64: %w", err)
-		}
-
-		return nil
+		t.setInt64(v)
 	case reflect.Int32:
 		if v < math.MinInt32 || v > math.MaxInt32 {
-			return fmt.Errorf("setInt64: integer %d %w", v, errSetInt64InInt32)
-		}
-
-		err := t.set(reflect.ValueOf(int32(v)))
-		if err != nil {
-			return fmt.Errorf("setInt64: %w", err)
+			return fmt.Errorf("toml: number %d does not fit in an int32", v)
 		}
 
+		t.set(reflect.ValueOf(int32(v)))
 		return nil
 	case reflect.Int16:
 		if v < math.MinInt16 || v > math.MaxInt16 {
-			return fmt.Errorf("setInt64: integer %d %w", v, errSetInt64InInt16)
+			return fmt.Errorf("toml: number %d does not fit in an int16", v)
 		}
 
-		err := t.set(reflect.ValueOf(int16(v)))
-		if err != nil {
-			return fmt.Errorf("setInt64: %w", err)
-		}
-
-		return nil
+		t.set(reflect.ValueOf(int16(v)))
 	case reflect.Int8:
 		if v < math.MinInt8 || v > math.MaxInt8 {
-			return fmt.Errorf("setInt64: integer %d %w", v, errSetInt64InInt8)
+			return fmt.Errorf("toml: number %d does not fit in an int8", v)
 		}
 
-		err := t.set(reflect.ValueOf(int8(v)))
-		if err != nil {
-			return fmt.Errorf("setInt64: %w", err)
-		}
-
-		return nil
+		t.set(reflect.ValueOf(int8(v)))
 	case reflect.Int:
 		if v < minInt || v > maxInt {
-			return fmt.Errorf("setInt64: integer %d %w", v, errSetInt64InInt)
+			return fmt.Errorf("toml: number %d does not fit in an int", v)
 		}
 
-		err := t.set(reflect.ValueOf(int(v)))
-		if err != nil {
-			return fmt.Errorf("setInt64: %w", err)
-		}
-
-		return nil
+		t.set(reflect.ValueOf(int(v)))
 	case reflect.Uint64:
 		if v < 0 {
-			return fmt.Errorf("setInt64: %d, %w", v, errSetInt64InUint64)
+			return fmt.Errorf("toml: negative number %d does not fit in an uint64", v)
 		}
 
-		err := t.set(reflect.ValueOf(uint64(v)))
-		if err != nil {
-			return fmt.Errorf("setInt64: %w", err)
-		}
-
-		return nil
+		t.set(reflect.ValueOf(uint64(v)))
 	case reflect.Uint32:
-		if v < 0 {
-			return fmt.Errorf("setInt64: %d, %w", v, errSetInt64InUint32)
+		if v < 0 || v > math.MaxUint32 {
+			return fmt.Errorf("toml: negative number %d does not fit in an uint32", v)
 		}
 
-		if v > math.MaxUint32 {
-			return fmt.Errorf("setInt64: %d, %w", v, errSetInt64InUint32Max)
-		}
-
-		err := t.set(reflect.ValueOf(uint32(v)))
-		if err != nil {
-			return fmt.Errorf("setInt64: %w", err)
-		}
-
-		return nil
+		t.set(reflect.ValueOf(uint32(v)))
 	case reflect.Uint16:
-		if v < 0 {
-			return fmt.Errorf("setInt64: %d, %w", v, errSetInt64InUint16)
+		if v < 0 || v > math.MaxUint16 {
+			return fmt.Errorf("toml: negative number %d does not fit in an uint16", v)
 		}
 
-		if v > math.MaxUint16 {
-			return fmt.Errorf("setInt64: %d, %w", v, errSetInt64InUint16Max)
-		}
-
-		err := t.set(reflect.ValueOf(uint16(v)))
-		if err != nil {
-			return fmt.Errorf("setInt64: %w", err)
-		}
-
-		return nil
+		t.set(reflect.ValueOf(uint16(v)))
 	case reflect.Uint8:
-		if v < 0 {
-			return fmt.Errorf("setInt64: %d, %w", v, errSetInt64InUint8)
+		if v < 0 || v > math.MaxUint8 {
+			return fmt.Errorf("toml: negative number %d does not fit in an uint8", v)
 		}
 
-		if v > math.MaxUint8 {
-			return fmt.Errorf("setInt64: %d, %w", v, errSetInt64InUint8Max)
-		}
-
-		err := t.set(reflect.ValueOf(uint8(v)))
-		if err != nil {
-			return fmt.Errorf("setInt64: %w", err)
-		}
-
-		return nil
+		t.set(reflect.ValueOf(uint8(v)))
 	case reflect.Uint:
 		if v < 0 {
-			return fmt.Errorf("setInt64: %d, %w", v, errSetInt64InUint)
+			return fmt.Errorf("toml: negative number %d does not fit in an uint", v)
 		}
 
-		err := t.set(reflect.ValueOf(uint(v)))
-		if err != nil {
-			return fmt.Errorf("setInt64: %w", err)
-		}
-
-		return nil
+		t.set(reflect.ValueOf(uint(v)))
 	case reflect.Interface:
-		err := t.set(reflect.ValueOf(v))
-		if err != nil {
-			return fmt.Errorf("setInt64: %w", err)
-		}
-
-		return nil
+		t.set(reflect.ValueOf(v))
 	default:
-		return fmt.Errorf("setInt64: %s, %w", f.String(), errSetInt64Unknown)
+		return fmt.Errorf("toml: integer cannot be assigned to %s", f.Kind())
 	}
-}
 
-var (
-	errSetFloat64InFloat32Max = errors.New("does not fit in an float32")
-	errSetFloat64Unknown      = errors.New("does not fit in an float32")
-)
+	return nil
+}
 
 func setFloat64(t target, v float64) error {
 	f := t.get()
 
 	switch f.Kind() {
 	case reflect.Float64:
-		err := t.setFloat64(v)
-		if err != nil {
-			return fmt.Errorf("setFloat64: %w", err)
-		}
-
-		return nil
+		t.setFloat64(v)
 	case reflect.Float32:
 		if v > math.MaxFloat32 {
-			return fmt.Errorf("setFloat64: %f %w", v, errSetFloat64InFloat32Max)
+			return fmt.Errorf("toml: number %f does not fit in a float32", v)
 		}
 
-		err := t.set(reflect.ValueOf(float32(v)))
-		if err != nil {
-			return fmt.Errorf("setFloat64: %w", err)
-		}
-
-		return nil
+		t.set(reflect.ValueOf(float32(v)))
 	case reflect.Interface:
-		err := t.set(reflect.ValueOf(v))
-		if err != nil {
-			return fmt.Errorf("setFloat64: %w", err)
-		}
-
-		return nil
+		t.set(reflect.ValueOf(v))
 	default:
-		return fmt.Errorf("setFloat64: %s %w", f.String(), errSetFloat64Unknown)
+		return fmt.Errorf("toml: float cannot be assigned to %s", f.Kind())
 	}
-}
 
-var (
-	errElementAtCannotOn        = errors.New("cannot elementAt")
-	errElementAtCannotOnUnknown = errors.New("cannot elementAt")
-)
+	return nil
+}
 
 //nolint:cyclop
 // Returns the element at idx of the value pointed at by target, or an error if
 // t does not point to an indexable.
 // If the target points to an Array and idx is out of bounds, it returns
 // (nil, nil) as this is not a fatal error (the unmarshaler will skip).
-func elementAt(t target, idx int) (target, error) {
+func elementAt(t target, idx int) target {
 	f := t.get()
 
 	switch f.Kind() {
@@ -493,42 +299,30 @@ func elementAt(t target, idx int) (target, error) {
 		// TODO: use the idx function argument and avoid alloc if possible.
 		idx := f.Len()
 
-		err := t.set(reflect.Append(f, reflect.New(f.Type().Elem()).Elem()))
-		if err != nil {
-			return nil, fmt.Errorf("elementAt: %w", err)
-		}
+		t.set(reflect.Append(f, reflect.New(f.Type().Elem()).Elem()))
 
-		return valueTarget(t.get().Index(idx)), nil
+		return valueTarget(t.get().Index(idx))
 	case reflect.Array:
 		if idx >= f.Len() {
-			return nil, nil
+			return nil
 		}
 
-		return valueTarget(f.Index(idx)), nil
+		return valueTarget(f.Index(idx))
 	case reflect.Interface:
-		if f.IsNil() {
-			panic("interface should have been initialized")
-		}
+		// This function is called after ensureValueIndexable, so it's
+		// guaranteed that f contains an initialized slice.
 
 		ifaceElem := f.Elem()
-		if ifaceElem.Kind() != reflect.Slice {
-			return nil, fmt.Errorf("elementAt: %w on a %s", errElementAtCannotOn, f.Kind())
-		}
-
 		idx := ifaceElem.Len()
 		newElem := reflect.New(ifaceElem.Type().Elem()).Elem()
 		newSlice := reflect.Append(ifaceElem, newElem)
 
-		err := t.set(newSlice)
-		if err != nil {
-			return nil, fmt.Errorf("elementAt: %w", err)
-		}
+		t.set(newSlice)
 
-		return valueTarget(t.get().Elem().Index(idx)), nil
-	case reflect.Ptr:
-		return elementAt(valueTarget(f.Elem()), idx)
+		return valueTarget(t.get().Elem().Index(idx))
 	default:
-		return nil, fmt.Errorf("elementAt: %w on a %s", errElementAtCannotOnUnknown, f.Kind())
+		// Why ensureValueIndexable let it go through?
+		panic(fmt.Errorf("elementAt received unhandled value type: %s", f.Kind()))
 	}
 }
 
@@ -539,31 +333,19 @@ func (d *decoder) scopeTableTarget(shouldAppend bool, t target, name string) (ta
 	switch x.Kind() {
 	// Kinds that need to recurse
 	case reflect.Interface:
-		t, err := scopeInterface(shouldAppend, t)
-		if err != nil {
-			return t, false, fmt.Errorf("scopeTableTarget: %w", err)
-		}
-
+		t := scopeInterface(shouldAppend, t)
 		return d.scopeTableTarget(shouldAppend, t, name)
 	case reflect.Ptr:
-		t, err := scopePtr(t)
-		if err != nil {
-			return t, false, fmt.Errorf("scopeTableTarget: %w", err)
-		}
-
+		t := scopePtr(t)
 		return d.scopeTableTarget(shouldAppend, t, name)
 	case reflect.Slice:
-		t, err := scopeSlice(shouldAppend, t)
-		if err != nil {
-			return t, false, fmt.Errorf("scopeTableTarget: %w", err)
-		}
+		t := scopeSlice(shouldAppend, t)
 		shouldAppend = false
-
 		return d.scopeTableTarget(shouldAppend, t, name)
 	case reflect.Array:
 		t, err := d.scopeArray(shouldAppend, t)
 		if err != nil {
-			return t, false, fmt.Errorf("scopeTableTarget: %w", err)
+			return t, false, err
 		}
 		shouldAppend = false
 
@@ -574,11 +356,7 @@ func (d *decoder) scopeTableTarget(shouldAppend bool, t target, name string) (ta
 		return scopeStruct(x, name)
 	case reflect.Map:
 		if x.IsNil() {
-			err := t.set(reflect.MakeMap(x.Type()))
-			if err != nil {
-				return t, false, fmt.Errorf("scopeTableTarget: %w", err)
-			}
-
+			t.set(reflect.MakeMap(x.Type()))
 			x = t.get()
 		}
 
@@ -588,42 +366,29 @@ func (d *decoder) scopeTableTarget(shouldAppend bool, t target, name string) (ta
 	}
 }
 
-func scopeInterface(shouldAppend bool, t target) (target, error) {
-	err := initInterface(shouldAppend, t)
-	if err != nil {
-		return t, err
-	}
-
-	return interfaceTarget{t}, nil
+func scopeInterface(shouldAppend bool, t target) target {
+	initInterface(shouldAppend, t)
+	return interfaceTarget{t}
 }
 
-func scopePtr(t target) (target, error) {
-	err := initPtr(t)
-	if err != nil {
-		return t, err
-	}
-
-	return valueTarget(t.get().Elem()), nil
+func scopePtr(t target) target {
+	initPtr(t)
+	return valueTarget(t.get().Elem())
 }
 
-func initPtr(t target) error {
+func initPtr(t target) {
 	x := t.get()
 	if !x.IsNil() {
-		return nil
+		return
 	}
 
-	err := t.set(reflect.New(x.Type().Elem()))
-	if err != nil {
-		return fmt.Errorf("initPtr: %w", err)
-	}
-
-	return nil
+	t.set(reflect.New(x.Type().Elem()))
 }
 
 // initInterface makes sure that the interface pointed at by the target is not
 // nil.
 // Returns the target to the initialized value of the target.
-func initInterface(shouldAppend bool, t target) error {
+func initInterface(shouldAppend bool, t target) {
 	x := t.get()
 
 	if x.Kind() != reflect.Interface {
@@ -631,7 +396,7 @@ func initInterface(shouldAppend bool, t target) error {
 	}
 
 	if !x.IsNil() && (x.Elem().Type() == sliceInterfaceType || x.Elem().Type() == mapStringInterfaceType) {
-		return nil
+		return
 	}
 
 	var newElement reflect.Value
@@ -641,55 +406,43 @@ func initInterface(shouldAppend bool, t target) error {
 		newElement = reflect.MakeMap(mapStringInterfaceType)
 	}
 
-	err := t.set(newElement)
-	if err != nil {
-		return fmt.Errorf("initInterface: %w", err)
-	}
-
-	return nil
+	t.set(newElement)
 }
 
-func scopeSlice(shouldAppend bool, t target) (target, error) {
+func scopeSlice(shouldAppend bool, t target) target {
 	v := t.get()
 
 	if shouldAppend {
 		newElem := reflect.New(v.Type().Elem())
 		newSlice := reflect.Append(v, newElem.Elem())
 
-		err := t.set(newSlice)
-		if err != nil {
-			return t, fmt.Errorf("scopeSlice: %w", err)
-		}
+		t.set(newSlice)
 
 		v = t.get()
 	}
 
-	return valueTarget(v.Index(v.Len() - 1)), nil
+	return valueTarget(v.Index(v.Len() - 1))
 }
 
-var errScopeArrayNotEnoughSpace = errors.New("not enough space in the array")
-
 func (d *decoder) scopeArray(shouldAppend bool, t target) (target, error) {
 	v := t.get()
 
 	idx := d.arrayIndex(shouldAppend, v)
 
 	if idx >= v.Len() {
-		return nil, errScopeArrayNotEnoughSpace
+		return nil, fmt.Errorf("toml: impossible to insert element beyond array's size: %d", v.Len())
 	}
 
 	return valueTarget(v.Index(idx)), nil
 }
 
-var errScopeMapCannotConvertStringToKey = errors.New("cannot convert string into map key type")
-
 func scopeMap(v reflect.Value, name string) (target, bool, error) {
 	k := reflect.ValueOf(name)
 
 	keyType := v.Type().Key()
 	if !k.Type().AssignableTo(keyType) {
 		if !k.Type().ConvertibleTo(keyType) {
-			return nil, false, fmt.Errorf("scopeMap: %w %s", errScopeMapCannotConvertStringToKey, keyType)
+			return nil, false, fmt.Errorf("toml: cannot convert map key of type %s to expected type %s", k.Type(), keyType)
 		}
 
 		k = k.Convert(keyType)

targets_test.go

@@ -128,14 +128,12 @@ func TestPushNew(t *testing.T) {
 		x, _, err := dec.scopeTableTarget(false, valueTarget(reflect.ValueOf(&d).Elem()), "A")
 		require.NoError(t, err)
 
-		n, err := elementAt(x, 0)
-		require.NoError(t, err)
-		require.NoError(t, n.setString("hello"))
+		n := elementAt(x, 0)
+		n.setString("hello")
 		require.Equal(t, []string{"hello"}, d.A)
 
-		n, err = elementAt(x, 1)
-		require.NoError(t, err)
-		require.NoError(t, n.setString("world"))
+		n = elementAt(x, 1)
+		n.setString("world")
 		require.Equal(t, []string{"hello", "world"}, d.A)
 	})
 
@@ -151,13 +149,11 @@ func TestPushNew(t *testing.T) {
 		x, _, err := dec.scopeTableTarget(false, valueTarget(reflect.ValueOf(&d).Elem()), "A")
 		require.NoError(t, err)
 
-		n, err := elementAt(x, 0)
-		require.NoError(t, err)
+		n := elementAt(x, 0)
 		require.NoError(t, setString(n, "hello"))
 		require.Equal(t, []interface{}{"hello"}, d.A)
 
-		n, err = elementAt(x, 1)
-		require.NoError(t, err)
+		n = elementAt(x, 1)
 		require.NoError(t, setString(n, "world"))
 		require.Equal(t, []interface{}{"hello", "world"}, d.A)
 	})

toml_testgen_support_test.go

@@ -94,12 +94,7 @@ func testGenTranslateDesc(input interface{}) interface{} {
 		if ok {
 			dvalue, ok = d["value"]
 			if ok {
-				var okdt bool
-
-				dtype, okdt = dtypeiface.(string)
-				if !okdt {
-					panic(fmt.Sprintf("dtypeiface should be valid string: %v", dtypeiface))
-				}
+				dtype = dtypeiface.(string)
 
 				switch dtype {
 				case "string":
@@ -132,10 +127,7 @@ func testGenTranslateDesc(input interface{}) interface{} {
 						return nil
 					}
 
-					a, oka := dvalue.([]interface{})
-					if !oka {
-						panic(fmt.Sprintf("a should be valid []interface{}: %v", a))
-					}
+					a := dvalue.([]interface{})
 
 					xs := make([]interface{}, len(a))
 

unmarshaler.go

@@ -56,7 +56,7 @@ func (d *Decoder) SetStrict(strict bool) {
 func (d *Decoder) Decode(v interface{}) error {
 	b, err := ioutil.ReadAll(d.r)
 	if err != nil {
-		return fmt.Errorf("Decode: %w", err)
+		return fmt.Errorf("toml: %w", err)
 	}
 
 	p := parser{}
@@ -130,20 +130,15 @@ func keyLocation(node ast.Node) []byte {
 	return unsafe.BytesRange(start, end)
 }
 
-var (
-	errFromParserExpectingPointer       = errors.New("expecting a pointer as target")
-	errFromParserExpectingNonNilPointer = errors.New("expecting non nil pointer as target")
-)
-
 //nolint:funlen,cyclop
 func (d *decoder) fromParser(p *parser, v interface{}) error {
 	r := reflect.ValueOf(v)
 	if r.Kind() != reflect.Ptr {
-		return fmt.Errorf("fromParser: %w, not %s", errFromParserExpectingPointer, r.Kind())
+		return fmt.Errorf("toml: decoding can only be performed into a pointer, not %s", r.Kind())
 	}
 
 	if r.IsNil() {
-		return errFromParserExpectingNonNilPointer
+		return fmt.Errorf("toml: decoding pointer target cannot be nil")
 	}
 
 	var (
@@ -162,7 +157,7 @@ func (d *decoder) fromParser(p *parser, v interface{}) error {
 
 		err := d.seen.CheckExpression(node)
 		if err != nil {
-			return fmt.Errorf("fromParser: %w", err)
+			return err
 		}
 
 		var found bool
@@ -181,16 +176,13 @@ func (d *decoder) fromParser(p *parser, v interface{}) error {
 				// looks like a table. Otherwise the information
 				// of a table is lost, and marshal cannot do the
 				// round trip.
-				err := ensureMapIfInterface(current)
-				if err != nil {
-					panic(fmt.Sprintf("ensureMapIfInterface: %s", err))
-				}
+				ensureMapIfInterface(current)
 			}
 		case ast.ArrayTable:
 			d.strict.EnterArrayTable(node)
 			current, found, err = d.scopeWithArrayTable(root, node.Key())
 		default:
-			panic(fmt.Sprintf("fromParser: this should not be a top level node type: %s", node.Kind))
+			panic(fmt.Sprintf("this should not be a top level node type: %s", node.Kind))
 		}
 
 		if err != nil {
@@ -267,26 +259,18 @@ func (d *decoder) scopeWithArrayTable(x target, key ast.Iterator) (target, bool,
 	v := x.get()
 
 	if v.Kind() == reflect.Ptr {
-		x, err = scopePtr(x)
-		if err != nil {
-			return x, false, err
-		}
-
+		x = scopePtr(x)
 		v = x.get()
 	}
 
 	if v.Kind() == reflect.Interface {
-		x, err = scopeInterface(true, x)
-		if err != nil {
-			return x, found, err
-		}
-
+		x = scopeInterface(true, x)
 		v = x.get()
 	}
 
 	switch v.Kind() {
 	case reflect.Slice:
-		x, err = scopeSlice(true, x)
+		x = scopeSlice(true, x)
 	case reflect.Array:
 		x, err = d.scopeArray(true, x)
 	default:
@@ -334,7 +318,7 @@ func tryTextUnmarshaler(x target, node ast.Node) (bool, error) {
 	if v.Type().Implements(textUnmarshalerType) {
 		err := v.Interface().(encoding.TextUnmarshaler).UnmarshalText(node.Data)
 		if err != nil {
-			return false, fmt.Errorf("tryTextUnmarshaler: %w", err)
+			return false, newDecodeError(node.Data, "error calling UnmarshalText: %w", err)
 		}
 
 		return true, nil
@@ -343,7 +327,7 @@ func tryTextUnmarshaler(x target, node ast.Node) (bool, error) {
 	if v.CanAddr() && v.Addr().Type().Implements(textUnmarshalerType) {
 		err := v.Addr().Interface().(encoding.TextUnmarshaler).UnmarshalText(node.Data)
 		if err != nil {
-			return false, fmt.Errorf("tryTextUnmarshaler: %w", err)
+			return false, newDecodeError(node.Data, "error calling UnmarshalText: %w", err)
 		}
 
 		return true, nil
@@ -358,11 +342,7 @@ func (d *decoder) unmarshalValue(x target, node ast.Node) error {
 
 	if v.Kind() == reflect.Ptr {
 		if !v.Elem().IsValid() {
-			err := x.set(reflect.New(v.Type().Elem()))
-			if err != nil {
-				return fmt.Errorf("unmarshalValue: %w", err)
-			}
-
+			x.set(reflect.New(v.Type().Elem()))
 			v = x.get()
 		}
 
@@ -394,7 +374,7 @@ func (d *decoder) unmarshalValue(x target, node ast.Node) error {
 	case ast.LocalDate:
 		return unmarshalLocalDate(x, node)
 	default:
-		panic(fmt.Sprintf("unmarshalValue: unhandled unmarshalValue kind %s", node.Kind))
+		panic(fmt.Sprintf("unhandled node kind %s", node.Kind))
 	}
 }
 
@@ -406,7 +386,9 @@ func unmarshalLocalDate(x target, node ast.Node) error {
 		return err
 	}
 
-	return setDate(x, v)
+	setDate(x, v)
+
+	return nil
 }
 
 func unmarshalLocalDateTime(x target, node ast.Node) error {
@@ -421,7 +403,9 @@ func unmarshalLocalDateTime(x target, node ast.Node) error {
 		return newDecodeError(rest, "extra characters at the end of a local date time")
 	}
 
-	return setLocalDateTime(x, v)
+	setLocalDateTime(x, v)
+
+	return nil
 }
 
 func unmarshalDateTime(x target, node ast.Node) error {
@@ -432,48 +416,37 @@ func unmarshalDateTime(x target, node ast.Node) error {
 		return err
 	}
 
-	return setDateTime(x, v)
+	setDateTime(x, v)
+
+	return nil
 }
 
-func setLocalDateTime(x target, v LocalDateTime) error {
+func setLocalDateTime(x target, v LocalDateTime) {
 	if x.get().Type() == timeType {
 		cast := v.In(time.Local)
 
-		return setDateTime(x, cast)
+		setDateTime(x, cast)
+		return
 	}
 
-	err := x.set(reflect.ValueOf(v))
-	if err != nil {
-		return fmt.Errorf("setLocalDateTime: %w", err)
-	}
-
-	return nil
+	x.set(reflect.ValueOf(v))
 }
 
-func setDateTime(x target, v time.Time) error {
-	err := x.set(reflect.ValueOf(v))
-	if err != nil {
-		return fmt.Errorf("setDateTime: %w", err)
-	}
-
-	return nil
+func setDateTime(x target, v time.Time) {
+	x.set(reflect.ValueOf(v))
 }
 
 var timeType = reflect.TypeOf(time.Time{})
 
-func setDate(x target, v LocalDate) error {
+func setDate(x target, v LocalDate) {
 	if x.get().Type() == timeType {
 		cast := v.In(time.Local)
 
-		return setDateTime(x, cast)
+		setDateTime(x, cast)
+		return
 	}
 
-	err := x.set(reflect.ValueOf(v))
-	if err != nil {
-		return fmt.Errorf("setDate: %w", err)
-	}
-
-	return nil
+	x.set(reflect.ValueOf(v))
 }
 
 func unmarshalString(x target, node ast.Node) error {
@@ -514,10 +487,7 @@ func unmarshalFloat(x target, node ast.Node) error {
 func (d *decoder) unmarshalInlineTable(x target, node ast.Node) error {
 	assertNode(ast.InlineTable, node)
 
-	err := ensureMapIfInterface(x)
-	if err != nil {
-		return fmt.Errorf("unmarshalInlineTable: %w", err)
-	}
+	ensureMapIfInterface(x)
 
 	it := node.Children()
 	for it.Next() {
@@ -546,10 +516,7 @@ func (d *decoder) unmarshalArray(x target, node ast.Node) error {
 	for it.Next() {
 		n := it.Node()
 
-		v, err := elementAt(x, idx)
-		if err != nil {
-			return err
-		}
+		v := elementAt(x, idx)
 
 		if v == nil {
 			// when we go out of bound for an array just stop processing it to

unmarshaler_test.go

@@ -38,6 +38,11 @@ func TestUnmarshal_Integers(t *testing.T) {
 			input:    `+99`,
 			expected: 99,
 		},
+		{
+			desc:     "integer decimal underscore",
+			input:    `123_456`,
+			expected: 123456,
+		},
 		{
 			desc:     "integer hex uppercase",
 			input:    `0xDEADBEEF`,
@@ -58,6 +63,21 @@ func TestUnmarshal_Integers(t *testing.T) {
 			input:    `0b11010110`,
 			expected: 0b11010110,
 		},
+		{
+			desc:  "double underscore",
+			input: "12__3",
+			err:   true,
+		},
+		{
+			desc:  "starts with underscore",
+			input: "_1",
+			err:   true,
+		},
+		{
+			desc:  "ends with underscore",
+			input: "1_",
+			err:   true,
+		},
 	}
 
 	type doc struct {
@@ -71,8 +91,12 @@ func TestUnmarshal_Integers(t *testing.T) {
 
 			doc := doc{}
 			err := toml.Unmarshal([]byte(`A = `+e.input), &doc)
-			require.NoError(t, err)
-			assert.Equal(t, e.expected, doc.A)
+			if e.err {
+				require.Error(t, err)
+			} else {
+				require.NoError(t, err)
+				assert.Equal(t, e.expected, doc.A)
+			}
 		})
 	}
 }
@@ -799,6 +823,33 @@ B = "data"`,
 				}
 			},
 		},
+		{
+			desc:  "mismatch types int to string",
+			input: `A = 42`,
+			gen: func() test {
+				type S struct {
+					A string
+				}
+				return test{
+					target: &S{},
+					err:    true,
+				}
+			},
+		},
+		{
+			desc:  "mismatch types array of int to interface with non-slice",
+			input: `A = [[42]]`,
+			skip:  true,
+			gen: func() test {
+				type S struct {
+					A *string
+				}
+				return test{
+					target:   &S{},
+					expected: &S{},
+				}
+			},
+		},
 	}
 
 	for _, e := range examples {
@@ -815,6 +866,9 @@ B = "data"`,
 			}
 			err := toml.Unmarshal([]byte(e.input), test.target)
 			if test.err {
+				if err == nil {
+					t.Log("=>", test.target)
+				}
 				require.Error(t, err)
 			} else {
 				require.NoError(t, err)
@@ -1030,7 +1084,7 @@ world'`,
 
 			if e.msg != "" {
 				t.Log("\n" + de.String())
-				require.Equal(t, e.msg, de.Error())
+				require.Equal(t, "toml: "+e.msg, de.Error())
 			}
 		})
 	}