example.swift

class NonSendable {
  var value = 1
}

class NonIsolated {
  private var mutableState = NonSendable()

  func aFunc() async {
    mutableState.value += 1
  }
}

class UseNonIsolated {
  let value = NonIsolated()
  
  // this is non-isolated
  func useAFunc() async {
    // so this does not change isolation and is ok
    await value.aFunc()
  }
}