We've had long discussions about snapshot releases, and the tricky part
is that while we would like people to be able to try out new GHC
features, we don't want to add to the burden of library maintainers by
requiring them to update their libraries to work with a new GHC release
more than once a year.

But perhaps we should announce a "library API freeze" some time before
the first RC on a stable branch. That way people can safely update their
dependencies at that point, and by the time the RC is out people testing
the RC will be able to test more without running into problems
installing libraries.