[Fix #1260] Fix performance regression caused by Rails/UnknownEnv on rails 7.1

[lukasfroehlich1]

After upgrading to Rails 7.1 we noticed a significant slowdown of RuboCop (52s -> 3m 52s).

We found that the cache file path for certain files was changing throughout the RuboCop execution, resulting in cache misses. One of the components of the cache file path is the cop config.

When diffing the cop config throughout RuboCops execution we find that "Rails/UnknownEnv"."Environments" is changing:

When Rails/UnknownEnv runs as part of the warm_cache step in RuboCop it mutates the config, which subsequently modifies the file checksums. When RuboCop runs inspect_files it results in cache misses because the cop config is no longer the same.

The code causing the issue:

def environments
  @environments ||= begin
    environments = cop_config['Environments'] || []
    environments << 'local' if target_rails_version >= 7.1
    environments
  end

In this case cop_config['Environments'] is mutated when environments << 'local' runs.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[lukasfroehlich1]

This test is basically the same as the one above. Would ya'll prefer if I merged it with the one above and change the test description to be: "accepts local as an environment name and does not mutate the cop config"?

[koic]

Thanks for the investigation and the fix!