code: func (er *expressionRewriter) newFunction(funcName string, retType *types.FieldType, args ...expression.Expression) (expression.Expression, error) {
  	if er.disableFoldCounter > 0 {
  		return expression.NewFunctionBase(er.ctx, funcName, retType, args...)
  	}
  	return expression.NewFunction(er.ctx, funcName, retType, args...)
  }
docs: // newFunction chooses which expression.NewFunctionImpl() will be used.