code: func GetScopeNoneSystemVar(key string) (string, bool, error) {
  	sysVar := SysVars[key]
  	if sysVar == nil {
  		return "", false, UnknownSystemVar.GenWithStackByArgs(key)
  	}
  	if sysVar.Scope == ScopeNone {
  		return sysVar.Value, true, nil
  	}
  	return "", false, nil
  }
docs: // GetScopeNoneSystemVar checks the validation of `key`,
  // and return the default value if its scope is `ScopeNone`.