Expand support for legacy error handling

WARNING: This commit contains breaking changes.

The IgnoreStructErrors and ReportLegacyErrorValues options
are deleted and folded into a unified ReportErrorsWithLegacySemantics
option that handles multiple legacy behaviors regarding errors.

In particular:
* Fully validate the syntactic structure of the JSON value
  before attempting any form of semantic unmarshaling.
* Continue unmarshaling even when encountering an error,
  and report the first error.

arshal.go

@@ -164,7 +164,7 @@ func Marshal(in any, opts ...Options) (out []byte, err error) {
 	xe := export.Encoder(enc)
 	xe.Flags.Set(jsonflags.OmitTopLevelNewline | 1)
 	err = marshalEncode(enc, in, &xe.Struct)
-	if err != nil && xe.Flags.Get(jsonflags.ReportLegacyErrorValues) {
+	if err != nil && xe.Flags.Get(jsonflags.ReportErrorsWithLegacySemantics) {
 		return nil, internal.TransformMarshalError(in, err)
 	}
 	return bytes.Clone(xe.Buf), err
@@ -180,7 +180,7 @@ func MarshalWrite(out io.Writer, in any, opts ...Options) (err error) {
 	xe := export.Encoder(enc)
 	xe.Flags.Set(jsonflags.OmitTopLevelNewline | 1)
 	err = marshalEncode(enc, in, &xe.Struct)
-	if err != nil && xe.Flags.Get(jsonflags.ReportLegacyErrorValues) {
+	if err != nil && xe.Flags.Get(jsonflags.ReportErrorsWithLegacySemantics) {
 		return internal.TransformMarshalError(in, err)
 	}
 	return err
@@ -198,7 +198,7 @@ func MarshalEncode(out *jsontext.Encoder, in any, opts ...Options) (err error) {
 	xe := export.Encoder(out)
 	mo.CopyCoderOptions(&xe.Struct)
 	err = marshalEncode(out, in, mo)
-	if err != nil && xe.Flags.Get(jsonflags.ReportLegacyErrorValues) {
+	if err != nil && xe.Flags.Get(jsonflags.ReportErrorsWithLegacySemantics) {
 		return internal.TransformMarshalError(in, err)
 	}
 	return err
@@ -392,7 +392,7 @@ func Unmarshal(in []byte, out any, opts ...Options) (err error) {
 	defer export.PutBufferedDecoder(dec)
 	xd := export.Decoder(dec)
 	err = unmarshalFull(dec, out, &xd.Struct)
-	if err != nil && xd.Flags.Get(jsonflags.ReportLegacyErrorValues) {
+	if err != nil && xd.Flags.Get(jsonflags.ReportErrorsWithLegacySemantics) {
 		return internal.TransformUnmarshalError(out, err)
 	}
 	return err
@@ -409,7 +409,7 @@ func UnmarshalRead(in io.Reader, out any, opts ...Options) (err error) {
 	defer export.PutStreamingDecoder(dec)
 	xd := export.Decoder(dec)
 	err = unmarshalFull(dec, out, &xd.Struct)
-	if err != nil && xd.Flags.Get(jsonflags.ReportLegacyErrorValues) {
+	if err != nil && xd.Flags.Get(jsonflags.ReportErrorsWithLegacySemantics) {
 		return internal.TransformUnmarshalError(out, err)
 	}
 	return err
@@ -441,7 +441,7 @@ func UnmarshalDecode(in *jsontext.Decoder, out any, opts ...Options) (err error)
 	xd := export.Decoder(in)
 	uo.CopyCoderOptions(&xd.Struct)
 	err = unmarshalDecode(in, out, uo)
-	if err != nil && uo.Flags.Get(jsonflags.ReportLegacyErrorValues) {
+	if err != nil && uo.Flags.Get(jsonflags.ReportErrorsWithLegacySemantics) {
 		return internal.TransformUnmarshalError(out, err)
 	}
 	return err
@@ -455,6 +455,14 @@ func unmarshalDecode(in *jsontext.Decoder, out any, uo *jsonopts.Struct) (err er
 	va := addressableValue{v.Elem(), false} // dereferenced pointer is always addressable
 	t := va.Type()
 
+	// In legacy semantics, the entirety of the next JSON value
+	// was validated before attempting to unmarshal it.
+	if uo.Flags.Get(jsonflags.ReportErrorsWithLegacySemantics) {
+		if err := export.Decoder(in).CheckNextValue(); err != nil {
+			return err
+		}
+	}
+
 	// Lookup and call the unmarshal function for this type.
 	unmarshal := lookupArshaler(t).unmarshal
 	if uo.Unmarshalers != nil {

arshal_any.go

@@ -5,6 +5,7 @@
 package json
 
 import (
+	"cmp"
 	"reflect"
 	"strconv"
 
@@ -171,6 +172,7 @@ func unmarshalObjectAny(dec *jsontext.Decoder, uo *jsonopts.Struct) (map[string]
 		if !uo.Flags.Get(jsonflags.AllowInvalidUTF8) {
 			xd.Tokens.Last.DisableNamespace()
 		}
+		var errUmmarshal error
 		for dec.PeekKind() != '}' {
 			tok, err := dec.ReadToken()
 			if err != nil {
@@ -189,15 +191,18 @@ func unmarshalObjectAny(dec *jsontext.Decoder, uo *jsonopts.Struct) (map[string]
 			val, err := unmarshalValueAny(dec, uo)
 			obj[name] = val
 			if err != nil {
-				return obj, err
+				if isFatalError(err, uo.Flags) {
+					return obj, err
+				}
+				errUmmarshal = cmp.Or(err, errUmmarshal)
 			}
 		}
 		if _, err := dec.ReadToken(); err != nil {
 			return obj, err
 		}
-		return obj, nil
+		return obj, errUmmarshal
 	}
-	return nil, newUnmarshalErrorAfter(dec, mapStringAnyType, nil)
+	return nil, newUnmarshalErrorAfterWithSkipping(dec, uo, mapStringAnyType, nil)
 }
 
 func marshalArrayAny(enc *jsontext.Encoder, arr []any, mo *jsonopts.Struct) error {
@@ -252,17 +257,21 @@ func unmarshalArrayAny(dec *jsontext.Decoder, uo *jsonopts.Struct) ([]any, error
 		return nil, nil
 	case '[':
 		arr := []any{}
+		var errUmmarshal error
 		for dec.PeekKind() != ']' {
 			val, err := unmarshalValueAny(dec, uo)
 			arr = append(arr, val)
 			if err != nil {
-				return arr, err
+				if isFatalError(err, uo.Flags) {
+					return arr, err
+				}
+				errUmmarshal = cmp.Or(errUmmarshal, err)
 			}
 		}
 		if _, err := dec.ReadToken(); err != nil {
 			return arr, err
 		}
-		return arr, nil
+		return arr, errUmmarshal
 	}
-	return nil, newUnmarshalErrorAfter(dec, sliceAnyType, nil)
+	return nil, newUnmarshalErrorAfterWithSkipping(dec, uo, sliceAnyType, nil)
 }