code: protected function fetchMessage($field, $rule)
      {
          if (isset($this->fieldMessages[$field][$rule])) {
              return $this->fieldMessages[$field][$rule];
          }
  
          if (isset($this->ruleMessages[$rule])) {
              return $this->ruleMessages[$rule];
          }
  
          return $this->usedRules[$rule]->error();
      }
docs: Fetch the message for an error by field or rule type.
  
  @param  string $field
  @param  string $rule
  
  @return string