Rust (and, for that matter, C++ compilers) have always applied optimizations that rely on alias analysis. What I assume this thread is referring to is the noalias LLVM attribute on &mut function parameters. Rust now applies this.

In any case, it's still UB under the language guidelines, it's just not exploited yet.

I believe since the last time they were turned on they have stayed on, yes.

Rust does not use type-based alias analysis (the "strict aliasing" problem), however.