nonnull doesn't really do anything in pure objc. It warns if you assign the nil literal to a nonnull pointer and that's it. The annotation is almost entirely for the sake of Swift interop (where it determines if the pointer is bridged as an Optional or not).

It is a really pleasant language, but I think the <nonnull> annotation is for initialization only - compiler checking against initializing an object ptr with a null value - and does not prevent crashing when addressing an already released object