To clarify for anybody else following along (I assume you knew), the type MaybeUninit isn't unsafe (Rust doesn't have unsafe types), only its method named assume_init and related APIs are unsafe.

That's because only this feature introduces the potential for safety problem, which is amusing because it doesn't actually do anything per se, it will often emit zero CPU instructions.

It's unsafe because before we called this function we had a MaybeUninit<T> and, well as it said, maybe it isn't initialized, so nothing will assume it is. But once we assume_init, we've got a T, all the code working with the T, including safe Rust code, is entitled to ignore any scenarios that could arise if it were not, in fact, initialized properly. Despite not in some sense "doing" anything, the unsafe call was critical.