> (even though I'm like 90% sure the compiler doesn't make this heavyweight).

Yes, as of Rust 1.66, this function compiles to

  example::add:
      xor     edi, 1
      xor     edx, 1
      xor     eax, eax
      or      edx, edi
      sete    al
      lea     edx, [rsi + rcx]
      ret