You can always assume the former(all lifetimes are different). It is strictly more general.

Thanks, so lifetimes can definitely be inferred for safe code (although just like type inference, they can lead to leaky abstractions). But aren't lifetime annotations necessary for unsafe code? How could you cast a pointer into a struct into a raw pointer and back, while preserving the lifetime, without them?

We'd start by making sure these lifetimes need distinguishing, i.e. that they aren't actually equivalent.