> To get that, sometimes you need -ffmast-math, sometimes said undefined behavior, or code patterns which translate to better code for the processor you target.

> Rust doesn't allow any of that, moreover limits me the way I can write my programs, no thanks.

I'm willing to prove you wrong here. Can you give some concrete examples?

For -ffast-math you can most certainly enable it, but AFAIK for now only in a localized fashion through intrinsics, e.g.:

https://doc.rust-lang.org/std/intrinsics/fn.fadd_fast.html

https://doc.rust-lang.org/std/intrinsics/fn.fmul_fast.html

So instead of doing this the dumb way and enabling -ffast-math for the whole program (which most certainly won't need it) you can profile where you're spending the majority of your time, and only do this where it'll matter, and without the possibility of randomly breaking the rest of your numeric code.

Personally I find this to be a vastly better approach.