[codeflo]

The problem is that compilers are allowed to make some general assumption about how they're allowed to reorder code, always based on the assumption that no other process is modifying the memory. For example, the optimizer may remove redundant reads. That's a problem if the read isn't really redundant -- if the pointer isn't targeting process-owned memory, but a memory mapped file that's modified by someone else. Programs might crash in very "interesting" ways depending on optimization flags.

C has this issue as well, but Rust's compiler/borrow checker is particularly strong at this kind of analysis, so it's potentially bitten even harder.