Wow that looks really useful, is that going to get upstreamed? I still use the rust compiler all over the place so improvements like that will be very welcome.

Doing my best to do this in such a way that it could be! Can't make any promises at this point. Headstart I don't think could be as it stands, but I have high hopes for Breakneck. As well, I think it'll be possible to offer Breakneck as a continually updated extension of nightly, so it could just be another version of nightly that one could install and use until maybe someday hopefully something like it is upstreamed.