[Rivet] Dangerous casting to FinalState

[Frank Siegert]

Hi Chris,

>> So we might as well go ahead with my patch, since it will save users
>> from this silly slicing mistake. And we will have to fix the
>> projection registration mechanism anyway if we want to allow copying
>> of projections by value. Though I didn't quite understand David's last
>> comment: do you find the patch ok, or not (assuming we only care about
>> FinalState-derived projections at the moment)? Any other objections?
>
> I don't object to the solution per se, but my preference is to fix the
> copy-by-value issue correctly throughout the Projection inheritance
> hierarchy instead of patching just FinalState. If this is the solution we
> choose, great, but let's fix the whole tree. Do you agree?

Sounds good to me. Do you plan to have this already for this release?
If not, I still see no reason to not use my patch as an intermediate
stop-gap measure, which will be amended by a solution for all
Projections at some point.

Cheers,
Frank