String corruption in 2.6.4
When trying to upgrade activeadmin's tests to use ruby 2.6.4, I got some very weird failures where some strings would end up containing random content. See https://circleci.com/gh/activeadmin/activeadmin/20329 for an example of those failures. Failures are somewhat random but the test suite seems to consistently fail on 2.6.4 and pass on 2.6.3. I also managed to reproduce the failures locally, sometimes very consistently, but now it's hardly ever happening... :/
I looked at the changes in 2.6.4 and it looks like https://bugs.ruby-lang.org/issues/16105 and/or https://bugs.ruby-lang.org/issues/15946 could be related, because the region of the Rails code where the corrupted string is created seems to be doing something similar to the reported issues: https://github.com/rails/rails/blob/b7e591a55f4de5f1511c3b9255b3b25159b8ba41/actionpack/lib/action_dispatch/routing/mapper.rb#L404-L412.
Sorry I'm not really sure how to create a reproducible example, or how to debug this issue. Just opening this in case it could help noticing something obviously wrong with the mentioned patches.