[Fix rubocop#3904] Build config & sync cop descriptions

CHANGELOG.md

@@ -123,6 +123,7 @@
 ### Changes
 
 * [#595](https://github.com/rubocop-hq/rubocop/issues/595): Exclude files ignored by `git`. ([@AlexWayfer][])
+* [#3904](https://github.com/rubocop-hq/rubocop/issues/3904): Deduplicate descriptions by copying cop descriptions into config. ([@garettarrowood][])
 * [#6429](https://github.com/rubocop-hq/rubocop/issues/6429): Fix autocorrect in Rails/Validation to not wrap hash options with braces in an extra set of braces. ([@bquorning][])
 * [#6533](https://github.com/rubocop-hq/rubocop/issues/6533): Improved warning message for unrecognized cop parameters to include Supported parameters. ([@MagedMilad][])
 

Rakefile

@@ -33,9 +33,12 @@ RuboCop::RakeTask.new(:internal_investigation).tap do |task|
 end
 
 task default: %i[
-  documentation_syntax_check generate_cops_documentation
+  build_config
+  documentation_syntax_check
+  generate_cops_documentation
   spec ascii_spec
   internal_investigation
+  confirm_config
 ]
 
 require 'yard'
@@ -119,3 +122,23 @@ task documentation_syntax_check: :yard_for_generate_documentation do
   end
   abort unless ok
 end
+
+desc 'Build config/default.yml'
+task :build_config do
+  sh('bin/build_config')
+end
+
+desc 'Confirm config/default.yml is up to date'
+task confirm_config: :build_config do
+  # Do not print diff and yield whether exit code was zero
+  sh('git diff --quiet config') do |outcome, _|
+    return if outcome
+
+    # Output diff before raising error
+    sh('GIT_PAGER=cat git diff config')
+
+    warn 'The config/default.yml is out of sync. ' \
+      'Run `rake build_config` and commit the results.'
+    exit!
+  end
+end

bin/build_config

@@ -0,0 +1,24 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+$LOAD_PATH.unshift(File.join(__dir__, '..', 'lib'))
+
+require 'yard'
+
+require 'rubocop'
+require 'rubocop/description_extractor'
+require 'rubocop/config_formatter'
+
+departments =
+  "{,#{RuboCop::Cop::Cop.registry.departments.map(&:downcase).join(',')}}"
+
+glob = File.join(__dir__, '..', 'lib', 'rubocop', 'cop', departments, '*.rb')
+YARD.parse(Dir[glob], [])
+
+descriptions = RuboCop::DescriptionExtractor.new(YARD::Registry.all).to_h
+current_config = YAML.load_file('config/default.yml')
+
+File.write(
+  'config/default.yml',
+  RuboCop::ConfigFormatter.new(current_config, descriptions).dump
+)