与Firebase团队联系后，我找到了所需的解决方案.事实证明，我必须显式地执行import FirebaseFirestoreSwift而不是仅仅执行import Firebase.此后错误将消失. (当然，您首先需要将Pod添加到您的Podfile中:D)

After contacting the firebase team, I found the solution I was looking for. It turns out I have to do import FirebaseFirestoreSwift explicitly instead of just doing import Firebase. The error will disappear after this. (And of course you'll need to add the pod to your podfile first:D)