When --format or -f CLI option is specified, @options.keys becomes [:format, :formatters] rather than just [:format].

To ensure the following code works correctly, I believe we need to add :formatters to DEFAULT_PARALLEL_OPTIONS as well.

Before the change, parallel execution was not enabled by default, as shown below:

$ bundle exec rubocop Gemfile --debug --format progress | grep "Use parallel by default."
$ echo $?
1

After the change, parallel execution is enabled by default.

$ bundle exec rubocop Gemfile --debug --format progress | grep "Use parallel by default."
Use parallel by default.
$ echo $?
0

FYI: As a background, I often run bundle exec rubocop --format progress --format github in GitHub Actions, but I noticed that it consistently takes twice as long as expected on the default 2-core machine. That led me to this issue. For now, I’ve worked around this by explicitly adding --parallel option.

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.