A high level assembly would not have a UB problem, because the generated machine code would closely map to your source code, so even technically undefined behaviour would end up doing the expected thing for the given hardware.

The problem with C is that modern compilers do a lot of transformations between your source code and the final machine code, so the actual behavior could be very far afield from what you would expect.

> And I'm not even sure if Rust solves this case as far as UB is concerned.

If your entire program is inside unsafe, then Rust is actually worse than C as far as UB is concerned. On the other hand, no one writes Rust like that, and Rust restricts all UB to unsafe blocks.

> If your entire program is inside unsafe, then Rust is actually worse than C as far as UB is concerned. On the other hand, no one writes Rust like that, and Rust restricts all UB to unsafe blocks.

Nitpicking: it's not the unsafe blocks themselves, but a few language features which are only allowed inside unsafe blocks. Language features which are allowed outside unsafe blocks work identically within unsafe blocks, and gain no extra UB just by being inside an unsafe block.