func (r *renamer) errorf(pos token.Pos, format string, args ...interface{}) {
	r.hadConflicts = true
	r.errors += fmt.Sprintf(format, args...)
}
errorf reports an error (e.g. conflict) and prevents file modification.
package source
import (
	"fmt"
	"go/ast"
	"go/token"
	"go/types"
	"reflect"
	"strconv"
	"strings"
	"unicode"
	"golang.org/x/tools/go/ast/astutil"
	"golang.org/x/tools/refactor/satisfy"
)