(Note that I am an author of that paper, and also that this is just a proposal of the rules and not yet adopted as normative.)
What you seem to be forgetting in this discussion is that unsafe code exists. The example above does not pass the borrow checker, but with a small amount of unsafe code (casting a reference to a pointer and back to erase the lifetime constraints) you can make it compile. But of course with unsafe code it is possible to write programs that have undefined behavior. The question is whether this specific program has undefined behavior, and the answer is no.
Since it does not have undefined behavior, the rest of the compiler already has to preserve its semantics. So one could also tweak the borrow checker to accept this program.
TL;DR unsafe code exists and so you can't just say all programs not passing the borrow checker are UB.
Thanks for the link. Like you said, that's not normative, so it doesn't really dictate anything about what the compiler would currently do if you violated borrow checking, right?
> What you seem to be forgetting in this discussion is that unsafe code exists. (...) unsafe code exists and so you can't just say all programs not passing the borrow checker are UB.
Unsafe code does not turn off the borrow-checker though? So I don't see how its existence implies the opposite of what I wrote.
Moreover, my entire concern here is about violating assumptions in earlier stages of the compiler that later stages don't already see violated (and thus might be unprepared for). Unsafe is already supported in the language, so it doesn't fall in that category to begin with.
(Note that I am an author of that paper, and also that this is just a proposal of the rules and not yet adopted as normative.)
What you seem to be forgetting in this discussion is that unsafe code exists. The example above does not pass the borrow checker, but with a small amount of unsafe code (casting a reference to a pointer and back to erase the lifetime constraints) you can make it compile. But of course with unsafe code it is possible to write programs that have undefined behavior. The question is whether this specific program has undefined behavior, and the answer is no.
Since it does not have undefined behavior, the rest of the compiler already has to preserve its semantics. So one could also tweak the borrow checker to accept this program.
TL;DR unsafe code exists and so you can't just say all programs not passing the borrow checker are UB.