I can't understand how using split_at() would work...if the CPU and the GPU both have a reference to the RAM, we're still using an Rc<RefCell<>>. If the slice gets passed into the CPU and GPU's run() functions, then how does this magical memory manager know what to pass ahead of time?

>It's hard to implement unsafe patterns in safe language.

I agree with this 100%. And I think that for certain problem domains, rust provides more safety than you actually want.