It would be nice to have a number of unit tests for Ruby internals
themselves, so that if I break something hacking with the internals I may
know it. Of course this isn't ideal because Ruby unit tests have to rely
on Ruby somehow, to be evaluated; but it could be good for catching subtle
changes in less fundamental modules/classes, e.g. IO, File, Regex. 

I say that because I wanted to hack in the IO subsystem at some point but
also wanted to make sure that I didn't break anything. I also say that
because I think Perl has a suite of tests like that (?) so that when ruby
is built it can be tested.

or maybe this already exists?

matju