It's really the exact same situation with strict aliasing violations in C and C++. If you break the rules (which are different depending on the language but in any case kind of complicated) all bets are off, even in totally single-threaded code, even if all you're doing is integer arithmetic, and in fact even if your code passes ASan and UBSan. If you'll pardon some shameless self-promotion, I have a talk about this that goes into a lot of examples:
https://www.youtube.com/watch?v=DG-VLezRkYQThe nice thing about Rust is that you mostly don't have to worry about any of this if you don't write the "unsafe" keyword. Folks with previous C and C++ experience often come in with understandable but mistaken assumptions about how unsafe code works in Rust, and I think it's important to study the rules carefully before you start writing unsafe code for production use.
