Parallel static analysis by default

Modern PCs have multi-core. RuboCop has `--parallel` option, but it is
not used by default. This means that there is extra core resources for
static analysis because multi-core is not being used effectively.

This PR will be parallel processing by default, so RuboCop will run
faster in proportion to number of cores. Options that can be
parallel processing, such as `--only`, `--except`, and others have the
same default behavior.
However, there are options that cannot be parallel processing, such as
auto-correction and others, so they are serial processing as before.

`--parallel` can be a hidden option for some users. This change is
expected to reduce user's wait time for running RuboCop.

Below is an example on my development PC.

Before:

```console
% cd path/to/rubocop
./exe/rubocop --display-time
(snip)

1293 files inspected, no offenses detected
Finished in 61.09493000002112 seconds
```

After:

```console
% cd path/to/rubocop
./exe/rubocop --display-time
(snip)

1293 files inspected, no offenses detected
Finished in 10.13813699997263 seconds
```

If user's PC has a lot of cores, memory and RuboCop targets a lot of files
to inspect, it will be effective.

One concern is that parallel processing uses more memory. For example,
Ruby's MJIT is off by default as an example of concerning about memory
consumption in a low memory production runtime (e.g. Heroku's minimum Dyno).
However, RuboCop is a development tool, I assume that it will be fine
in most cases.

If user want to use it in a low memory production runtime, user can
specify `--no-parallel`.

Note: JRuby and Windows are not supported based on #4537.

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