> Rust UB […] is meant to be limited in blast radius, and is absolutely not meant as a loophole for compilers to just do whatever to make the code faster.

This isn't true. Rust UB is meant to only be possible to trigger via `unsafe` code, but if you do trigger it, the compiler is free to do whatever it wants, and in practice it will make full use of that freedom. Rustc uses LLVM, it shares most of its optimizations with Clang!