code: func RegisterValidationVars(rules *validation.RuleSet) {
  	rules.RegisterVar("appid", func(c context.Context) (string, error) {
  		return info.TrimmedAppID(c), nil
  	})
  	rules.RegisterVar("config_service_appid", GetConfigServiceAppID)
  }
docs: // RegisterValidationVars registers placeholders that can be used in validation
  // rules patterns.
  //
  // Registers:
  //    ${appid} - expands into a GAE app ID of the running service.
  //    ${config_service_appid} - expands into a GAE app ID of a LUCI Config
  //        service that the running service is using (or empty string if
  //        unconfigured).
  //
  // This function is called during init() time for default rule set.