[#5977] Warn for Performance Cops

[koic]

Summary

This PR warns for Performance Cops.

The following warning is displayed when the next released rubocop gem is installed.

# The following is an example for operation check.
% rake build
% gem install pkg/rubocop-0.66.0.gem
Performance Cops will be removed from RuboCop 0.68. Use
rubocop-performance gem instead.

Put this in your Gemfile.

  gem 'rubocop-performance'

And then execute:

  $ bundle install

Put this into your .rubocop.yml.

  require: rubocop-performance

More information: https://github.com/rubocop-hq/rubocop-performance
Successfully installed rubocop-0.66.0
1 gem installed

The following warning is displayed if RuboCop is used with Bundler.

% bundle exec rubocop
[Warn] Performance Cops will be removed from RuboCop 0.68. Use rubocop-performance gem instead.
       https://github.com/rubocop-hq/rubocop/tree/master/manual/migrate_performance_cops.md

The above warning will disappear when rubocop-performance gem is written to Gemfile.
The rubocop-performance implementation takes precedence when rubocop-performance is written in Gemfile.

And I prepared a simple migration manual (migrate_performance_cops.md) because I think that long warning sentences are not preferred. I think that referring to the link is good when runnning rubocop command.

Other Infromation

At first I tried to remove performance cops from RuboCop Core and add rubocop-performance gem as a dependency because of fallback.

% git diff
diff --git a/rubocop.gemspec b/rubocop.gemspec
index bbbcca33f..09882b063 100644
--- a/rubocop.gemspec
+++ b/rubocop.gemspec
@@ -38,6 +38,7 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency('parser', '>= 2.5', '!= 2.5.1.1')
   s.add_runtime_dependency('psych', '>= 3.1.0')
   s.add_runtime_dependency('rainbow', '>= 2.2.2', '< 4.0')
+  s.add_runtime_dependency('rubocop-performance')
   s.add_runtime_dependency('ruby-progressbar', '~> 1.7')
   s.add_runtime_dependency('unicode-display_width', '>= 1.4.0', '< 1.6')

However, this cannot be resolved because it is cyclic.

% bundle install
Fetching gem metadata from https://rubygems.org/........
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
Your bundle requires gems that depend on each other, creating an infinite loop.
Please remove either gem 'rubocop' or gem 'rubocop-performance' and try again.

So I'd like to display a warning before removing performance cops.

I'd like to mitigate the impact on users this way.

The next step is to remove performance cops and done #5977.

---

Before submitting the PR make sure the following are checked:

• 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.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[koic]

@bbatsov I'm not sure whether it's good to remove from RuboCop version 0.68, but I'd like to start with a warning first for users. What do you think about this?

[bbatsov]

What will happen if someone is using the new gem together with a RuboCop release that still bundles them?

I was thinking that I can cut a quick release now just with the warning and we could cut 0.68 quickly as well, only to remove the old cops.

[koic]

What will happen if someone is using the new gem together with a RuboCop release that still bundles them?

Does new cop mean rubocop-performance?

If so, rubocop-performance is used in preference than old performance cops.

I'm worried that if the old performance cops are removed with this PR, users won't get the migration period.

I was missing my explanation 💦I consider the following milestones.

• 0.66 ... Current version
• Next release ... Display a warning and prepare a transition period for users
• 0.68 ... Remove old cops from RuboCop Core

[koic]

📝 The above .to_s method is used to prevent the following errors for JRuby 9.2.

% bundle exec rspec ./spec/rubocop/cli/cli_options_spec.rb:210
Run options: include {:focus=>true, :locations=>{"./spec/rubocop/cli/cli_options_spec.rb"=>[210]}}

Randomized with seed 11663
TypeError: wrong argument type Rainbow::NullPresenter (expected String)
    warn at org/jruby/common/RubyWarnings.java:224
    warn at org/jruby/RubyKernel.java:1247
    warn at org/jruby/RubyKernel.java:1234
  <main> at /Users/koic/src/github.com/rubocop-hq/rubocop/exe/rubocop:17
F

Failures:

  1) RuboCop::CLI --only when one cop is given accepts cop names from plugins
     Got 2 failures from failure aggregation block.
     # ./spec/support/cli_spec_behavior.rb:27:in `block in <main>'
     # ./lib/rubocop/rspec/shared_contexts.rb:27:in `block in <main>'
     # ./lib/rubocop/path_util.rb:67:in `chdir'
     # ./lib/rubocop/rspec/shared_contexts.rb:26:in `block in <main>'
     # ./lib/rubocop/rspec/shared_contexts.rb:8:in `block in <main>'

     1.1) Failure/Error: expect($CHILD_STATUS.success?).to be_truthy

            expected: truthy value
                 got: false
          # ./spec/rubocop/cli/cli_options_spec.rb:210:in `block in <main>'

     1.2) Failure/Error:
                    expect(output)
                      .to eq(<<-RESULT.strip_indent)
                        Inspecting 2 files
                        ..

                        2 files inspected, no offenses detected
                      RESULT

https://circleci.com/gh/rubocop-hq/rubocop/42485

[bbatsov]

Does new cop mean rubocop-performance?

My bad. I meant to write new gem.

I'm worried that if the old performance cops are removed with this PR, users won't get the migration period.

Yeah, I completely agree. I like the proposed timeline.

[volmer]

I updated to the latest version of the gem but I don't want to use performance cops. How can I suppress the warning without having to install the RuboCop Performance gem?

cc @koic

[koic]

@volmer I opened a PR #6939. Thanks!

[AlexWayfer]

@volmer I opened a PR #6939. Thanks!

#6944 *

[volmer]

Thank you! ❤️