func (s *StorageGroup) StableMarshal(buf []byte) ([]byte, error) {
	if s == nil {
		return []byte{}, nil
	}
	if buf == nil {
		buf = make([]byte, s.StableSize())
	}
	var (
		offset, n int
		err       error
	)
	n, err = proto.UInt64Marshal(sizeField, buf[offset:], s.size)
	if err != nil {
		return nil, err
	}
	offset += n
	n, err = proto.NestedStructureMarshal(hashField, buf[offset:], s.hash)
	if err != nil {
		return nil, err
	}
	offset += n
	n, err = proto.UInt64Marshal(expirationField, buf[offset:], s.exp)
	if err != nil {
		return nil, err
	}
	offset += n
	_, err = refs.ObjectIDNestedListMarshal(objectIDsField, buf[offset:], s.members)
	if err != nil {
		return nil, err
	}
	return buf, nil
}
StableMarshal marshals unified storage group structure in a protobuf
compatible way without field order shuffle.
package storagegroup
import (
	"github.com/nspcc-dev/neofs-api-go/rpc/message"
	"github.com/nspcc-dev/neofs-api-go/util/proto"
	"github.com/nspcc-dev/neofs-api-go/v2/refs"
	storagegroup "github.com/nspcc-dev/neofs-api-go/v2/storagegroup/grpc"
)
const (
	sizeField       = 1
	hashField       = 2
	expirationField = 3
	objectIDsField  = 4
)