Swift 自动布局在 Xcode 9.3 中不起作用

When you have no constraints, iOS uses something known at the autoresizing mask to create constraints for you at runtime, locking your views in place wherever they were in Interface Builder. These constraints won't work well for different content sizes or different devices, so you should always fully constrain your UI.

When you added the single constraint, Interface Builder told you that your interface wasn't properly constrained (the red arrow next to your view controller) and would give you guidance if you looked at that content. However, running the app with only a single constraint on that label meant that the system didn't apply the autoresizing mask like normal and the layout system essentially just set all of the other values to zero at runtime. This includes the height and width of the label, as well as its x and y position. With zeros for all of those values, what you're seeing here is the content of your label being rendered at 0,0, with a label that has 0 height and width. However, with clipsToBounds defaulted to false, the label's content is still visible, and so is shown as you see.

tl;dr：正如 Interface Builder 试图告诉您的那样，您需要限制整个 UI。