[Fix rubocop#12699] Support searching for .rubocop.yml in complianc…

…e with dot-config

Resolves rubocop#12699

This PR supports searching for `.rubocop.yml` in compliance with dot-config.

[dot-config](https://dot-config.github.io/) prioritizes project-specific configurations over
user-specific configurations, with a search order that first looks for the `.rubocop.yml`
within the project's directories, in accordance with dot-config standards,
before considering user-specific configurations.

changelog/new_support_dot_config.md

@@ -0,0 +1 @@
+* [#12699](https://github.com/rubocop/rubocop/issues/12699): Support searching for `.rubocop.yml` in compliance with dot-config. ([@koic][])

docs/modules/ROOT/pages/configuration.adoc

@@ -29,10 +29,12 @@ RuboCop will start looking for the configuration file in the directory
 where the inspected file is and continue its way up to the root directory.
 
 If it cannot be found until reaching the project's root directory, then it will
-be searched for in the user's global config locations, which consists of a
+be searched for in the https://dot-config.github.io[.config directory of the project root]
+and the user's global config locations. The user's global config locations consist of a
 dotfile or a config file inside the https://specifications.freedesktop.org/basedir-spec/latest/index.html[XDG Base Directory
 specification].
 
+* `.config/.rubocop.yml` of the project root
 * `~/.rubocop.yml`
 * `$XDG_CONFIG_HOME/rubocop/config.yml` (expands to `~/.config/rubocop/config.yml`
 if `$XDG_CONFIG_HOME` is not set)

lib/rubocop/config_finder.rb

@@ -17,8 +17,8 @@ class << self
       attr_writer :project_root
 
       def find_config_path(target_dir)
-        find_project_dotfile(target_dir) || find_user_dotfile || find_user_xdg_config ||
-          DEFAULT_FILE
+        find_project_dotfile(target_dir) || find_project_root_dot_config ||
+          find_user_dotfile || find_user_xdg_config || DEFAULT_FILE
       end
 
       # Returns the path RuboCop inferred as the root of the project. No file
@@ -41,6 +41,14 @@ def find_project_dotfile(target_dir)
         find_file_upwards(DOTFILE, target_dir, project_root)
       end
 
+      def find_project_root_dot_config
+        return unless project_root
+
+        file = File.join(project_root, '.config', DOTFILE)
+
+        file if File.exist?(file)
+      end
+
       def find_user_dotfile
         return unless ENV.key?('HOME')
 

spec/rubocop/config_loader_spec.rb

@@ -29,6 +29,29 @@
 
       before { create_empty_file('dir/example.rb') }
 
+      context 'but a config file exists in .config directory of the project root' do
+        before do
+          create_empty_file('Gemfile')
+          create_empty_file('.config/.rubocop.yml')
+        end
+
+        it 'returns the path to the file in home directory' do
+          expect(configuration_file_for).to end_with('.config/.rubocop.yml')
+        end
+      end
+
+      context 'but a config file exists in both .config of the project root and home directories' do
+        before do
+          create_empty_file('Gemfile')
+          create_empty_file('.config/.rubocop.yml')
+          create_empty_file('~/.rubocop.yml')
+        end
+
+        it 'returns the path to the file in home directory' do
+          expect(configuration_file_for).to end_with('.config/.rubocop.yml')
+        end
+      end
+
       context 'but a config file exists in home directory' do
         before { create_empty_file('~/.rubocop.yml') }