I believe the parent is saying "that would be undefined behavior" in answer to your question "would the rust compiler need to take this possibility into account". The compiler would not take this possibility into account, because the fact that references cannot outlive their referents is an invariant of the language that can only be broken by incorrect unsafe code. (To put it another way, in any context where the compiler would take this into account, it would do so regardless of the existence of the unsafe code, because unsafe code still ultimately has to uphold all the same invariants as safe code.)
Sure, but the reference doesn't outlive the referent in c++: the language guarantees that the local object in make is exactly the same object in the caller's caller.
The question is whether rust gives this object identity guarantee as well and, if it does, whether there are other rules (like uniqueness of mutable refences) that still allows the optimisation.
