Even mainstream architectures don't handle excessive consistently, e.g. for shifting an n-bit integer, I believe some mask the shift by 2^n - 1, some by 2^(n+1)

Even mainstream architectures don't handle excessive consistently, e.g. for shifting an n-bit integer, I believe some mask the shift by 2^n - 1, some by 2^(n+1) - 1, and some don't mask at all (i.e. 1 << 100000 will be zero). Of course, being UB (rather than just an "unspecified result" or some such) probably isn't the best way to handle the inconsistency. In any case, I believe Rust retains many of the optimisations made possible via UB in C++ by enforcing things at a compile time. In fact, Rust actually has quite a lot of UB... that is, there are many restrictions/requirements Rust 'assumes' are true. For example, the reference types & and &mut have significantly more restrictions around aliasing and mutability than pointers or references in C++. The key difference between Rust and C++ is that it is only possible to trigger the UB with `unsafe`, as the compiler usually enforces rules that guarantee they can't occur. People saying "Rust has no UB" are usually implicitly meaning "Rust cannot trigger any UB outside `unsafe`".