Enabling the next iteration of the borrow checker on nightly

Any new advancements regarding Rust borrow checking and allowing new code is always a very challenging exercise, as people have already accustomed to the "that style of code doesn't compile"-area of the current compiler, so the most battle hardened devs probably have a terribly challenging time of actually encountering the new, now allowed, type of code.

So cool that the mut_or_default now actually correctly borrow checks. I'll be watching for cool new patterns that this'll allow in the future!