[habitue]

Actually, so there's a very good chance that this particular optimization is happening due to MIRI, which allows very sophisticated constant evaluation of MIR (a rust intermediate representation)

https://rust-lang.github.io/rustc-guide/miri.html

So while, yes, in general pretty much all rust optimizations are actually due to LLVM, this one in particular might not be

[bdd]

Here's the equivalent in C++, compiled with clang 9.0.0 at optimization level 1. https://godbolt.org/z/nKSG9_

Byte for byte identical assembly.