> I want variable X to be loaded into a registerer in thos function and only written to memory at the end of the function.

> That is complex in C/C++ but you can look at decompilation and attempt to coerce the compiler into that.

> In rust everything is so abstracted I wouldn't know where to begin looking

I don't know if I fully understand what you want to do, but (1) controlling register allocation is the realm of inline asm, be it in C, C++, or Rust. And (2) if "nudging" the compiler is what you want, then it's literally the same thing in Rust as in C++, it's a matter of inspecting the asm yourself or plonking your function onto godbolt.