Assert in Run.scala is inefficient

[smarter]

Run.scala contains:

assert(comp.phases.last.last.id <= Periods.MaxPossiblePhaseId)

But phases is a def in Compiler that will allocate every phase. This is pretty wasteful since we then throw away the result. We could probably move this assert inside the definition of phases, or at the point where we actually use phases.

[abeln]

In #2640 (comment), Dmitry said this assert isn't on a hot path, so is it worth moving it?

[smarter]

Yes I think so. I've seen a concrete example of people storing information in a phase and trying to retrieve it later, except the phase instance was overwritten by this assert, which was pretty difficult to figure out.

[smarter]

Also, it's untrue that new runs are rarely created in the IDE: everytime you press a key a new run will be created.

[DarkDimius]

FTR: making phases a def was done intentionally, so that every run has new phases and phases don't have to carefully manage state between runs.