bisected it to this commit:
ae88e30a0f8403e781f8b01262766cdc46b1018a

* bisect.sh: the script used for git bisect run
* BISECT_LOG: obvious
* test.scm: more detailed test, with backtrace. guile dies while
  printing the backtrace.

Issue doesn't occur when i comment out the use-module line in
racket.scm that imports (@ (gnu packages chez) chez-scheme).

IRC discussion:
roptat suggests it might be triggered by something inheriting from
chez-scheme, cbaines says recursive modules usually don't cause issues,
only recursive inputs do. i haven't found any relevant inherits with a
quick grep, but maybe it's transitive or something.
I also haven't found any other mention of chez-scheme in racket.scm
other than the line that imports it, which is weird, given that Racket
is now built on Chez, so I'd expect it to use it as an input. I guess
it's a bundled version? In any case I don't think I should just remove
the import, because it will be needed eventually, so this issue needs
to be fixed by then.

That's as far as I've gotten today, will hopefully have time to look
into it tomorrow.

ps.: should I forward this to the guile list?