Parallel static analysis by default

Rakefile

@@ -22,12 +22,7 @@ require 'rubocop/rake_task'
 Dir['tasks/**/*.rake'].each { |t| load t }
 
 desc 'Run RuboCop over itself'
-RuboCop::RakeTask.new(:internal_investigation).tap do |task|
-  if RUBY_ENGINE == 'ruby' &&
-     !/mswin|msys|mingw|cygwin|bccwin|wince|emc/.match?(RbConfig::CONFIG['host_os'])
-    task.options = %w[--parallel]
-  end
-end
+RuboCop::RakeTask.new(:internal_investigation)
 
 task default: %i[documentation_syntax_check spec ascii_spec internal_investigation]
 

changelog/new_parallel_static_analysis_by_default.md

@@ -0,0 +1 @@
+* [#10000](https://github.com/rubocop/rubocop/pull/10000): Parallel static analysis by default. ([@koic][])

lib/rubocop/cli.rb

@@ -8,6 +8,11 @@ class CLI
     STATUS_OFFENSES    = 1
     STATUS_ERROR       = 2
     STATUS_INTERRUPTED = 128 + Signal.list['INT']
+    DEFAULT_PARALLEL_OPTIONS = %i[
+      color debug display_style_guide display_time display_only_fail_level_offenses
+      display_only_failed except extra_details fail_level fix_layout format
+      ignore_disable_comments lint only only_guide_cops require safe
+    ].freeze
 
     class Finished < RuntimeError; end
 
@@ -37,6 +42,7 @@ def run(args = ARGV)
       else
         act_on_options
         validate_options_vs_config
+        parallel_by_default!
         apply_default_formatter
         execute_runners
       end
@@ -84,6 +90,18 @@ def validate_options_vs_config
                                  'with AllCops: UseCache: false is not allowed.'
     end
 
+    def parallel_by_default!
+      # See https://github.com/rubocop/rubocop/pull/4537 for JRuby and Windows constraints.
+      return if RUBY_ENGINE != 'ruby' || RuboCop::Platform.windows?
+
+      if (@options.keys - DEFAULT_PARALLEL_OPTIONS).empty? &&
+         @config_store.for_pwd.for_all_cops['UseCache'] != false
+        puts 'Use parallel by default.' if @options[:debug]
+
+        @options[:parallel] = true
+      end
+    end
+
     def act_on_options
       set_options_to_config_loader
 

lib/rubocop/options.rb

@@ -492,7 +492,7 @@ module OptionsHelp
       version:                          'Display version.',
       verbose_version:                  'Display verbose version.',
       parallel:                         ['Use available CPUs to execute inspection in',
-                                         'parallel. Default is false.'],
+                                         'parallel. Default is true.'],
       stdin:                            ['Pipe source from STDIN, using FILE in offense',
                                          'reports. This is useful for editor integration.'],
       init:                             'Generate a .rubocop.yml file in the current directory.'

spec/rubocop/cli_spec.rb

@@ -197,6 +197,69 @@ def and_with_args
     end
   end
 
+  if RUBY_ENGINE == 'ruby' && !RuboCop::Platform.windows?
+    # NOTE: It has been tested for parallelism with `--debug` option.
+    #       In other words, even if no option is specified, it will be parallelized by default.
+    describe 'when parallel static by default' do
+      context 'when specifying `--debug` option only`' do
+        it 'fails with an error message' do
+          create_file('example1.rb', <<~RUBY)
+            # frozen_string_literal: true
+
+            puts 'hello'
+          RUBY
+          expect(cli.run(['--debug'])).to eq(0)
+          expect($stdout.string).to include('Use parallel by default.')
+        end
+      end
+
+      # NOTE: Cannot be auto-corrected with `parallel`.
+      context 'when specifying `--debug` and `-a` options`' do
+        it 'fails with an error message' do
+          create_file('example1.rb', <<~RUBY)
+            # frozen_string_literal: true
+
+            puts 'hello'
+          RUBY
+          expect(cli.run(['--debug', '-a'])).to eq(0)
+          expect($stdout.string).not_to include('Use parallel by default.')
+        end
+      end
+
+      context 'when setting `UseCache: true`' do
+        it 'fails with an error message' do
+          create_file('example.rb', <<~RUBY)
+            # frozen_string_literal: true
+
+            puts 'hello'
+          RUBY
+          create_file('.rubocop.yml', <<~YAML)
+            AllCops:
+              UseCache: true
+          YAML
+          expect(cli.run(['--debug'])).to eq(0)
+          expect($stdout.string).to include('Use parallel by default.')
+        end
+      end
+
+      context 'when setting `UseCache: false`' do
+        it 'fails with an error message' do
+          create_file('example.rb', <<~RUBY)
+            # frozen_string_literal: true
+
+            puts 'hello'
+          RUBY
+          create_file('.rubocop.yml', <<~YAML)
+            AllCops:
+              UseCache: false
+          YAML
+          expect(cli.run(['--debug'])).to eq(0)
+          expect($stdout.string).not_to include('Use parallel by default.')
+        end
+      end
+    end
+  end
+
   describe 'rubocop:disable comment' do
     it 'can disable all cops in a code section' do
       src = ['# rubocop:disable all',

spec/rubocop/options_spec.rb

@@ -138,7 +138,7 @@ def abs(path)
               -v, --version                    Display version.
               -V, --verbose-version            Display verbose version.
               -P, --[no-]parallel              Use available CPUs to execute inspection in
-                                               parallel. Default is false.
+                                               parallel. Default is true.
               -l, --lint                       Run only lint cops.
               -x, --fix-layout                 Run only layout cops, with auto-correct on.
               -s, --stdin FILE                 Pipe source from STDIN, using FILE in offense