Add CI task testing with the oldest RuboCop version allowed by gemspec

[bquorning]

Fixes: #1879 (comment)

There is one thing we might be missing - it’s to test our code against our minimum specified rubocop version.

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Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Updated documentation.
• Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

[pirj]

Nice!

can the failure be caused by some default TargetRubyVersion setting for that older RuboCop version?
I guess we may have new syntax used in our specs, and conditionally enable those examples, but they won’t parse if the TargetRubyVersion won’t match the RUBY_VERSION.
We have no need to test combinations of those two, right? We can just set the TRV to the RV.

[bquorning]

I guess we may be using a newer RUBY_VERSION than RuboCop can handle. E.g. if we use Ruby 3.3 and RuboCop 1.40 can only handle up to 3.1 or 3.2… Should we then rather test with the oldest Ruby that we support or the most recent Ruby that RuboCop supports?

It turns out that the issue was fixed by adding :ruby27 metadata to the failing spec. RuboCop v1.40.0 used Ruby 2.6.0 by default when running specs: https://github.com/rubocop/rubocop/blob/v1.40.0/lib/rubocop/rspec/cop_helper.rb#L9

[bquorning]

The problem is fixed; PR is ready for review.

[pirj]

Fantastic!
Frankly I was afraid that something’s broken and it’s just a coincidence that people update rubocop and rubocop-rspec simultaneously 😅

[ydah]

Looks good!