Add ability to load extensions to packwerk

USAGE.md

@@ -21,6 +21,8 @@
   * [Understanding how to respond to new violations](#understanding-how-to-respond-to-new-violations)
 * [Recording existing violations](#recording-existing-violations)
   * [Understanding the package todo file](#understanding-the-package-todo-file)
+  * [Understanding the list of deprecated references](#understanding-the-list-of-deprecated-references)
+* [Loading extensions](#loading-extensions)
 
 ## What problem does Packwerk solve?
 
@@ -275,3 +277,16 @@ Above is an example of a constant violation entry in `package_todo.yml`.
 * `components/merchant/app/public/merchant/generate_order.rb` - path to the file containing the violated constant
 
 Violations exist within the package that makes a violating reference. This means privacy violations of your package can be found listed in `package_todo.yml` files in the packages with the reference to a private constant.
+
+# Loading Extensions
+
+You can optionally specify ruby files that you'd like to be loaded with `packwerk` by specifying a `require` directive in `packwerk.yml`:
+```yml
+require:
+  - ./path/to/file.rb
+  - my_gem
+```
+
+`packwerk` will directly call `require` with these paths.
+You can prefix local files with a dot to define them relative to `packwerk.yml`, or you can use absolute paths.
+You can also reference the name of a gem.

lib/packwerk.rb

@@ -22,6 +22,7 @@ module Packwerk
   autoload :ConstantNameInspector
   autoload :ConstNodeInspector
   autoload :PackageTodo
+  autoload :ExtensionLoader
   autoload :FileProcessor
   autoload :FilesForProcessing
   autoload :Graph

lib/packwerk/configuration.rb

@@ -49,6 +49,12 @@ def initialize(configs = {}, config_path: nil)
       @cache_directory = Pathname.new(configs["cache_directory"] || "tmp/cache/packwerk")
       @config_path = config_path
 
+      if configs.key?("require")
+        configs["require"].each do |require_directive|
+          ExtensionLoader.load(require_directive, @root_path)
+        end
+      end
+
       if configs["load_paths"]
         warning = <<~WARNING
           DEPRECATION WARNING: The 'load_paths' key in `packwerk.yml` is deprecated.

lib/packwerk/extension_loader.rb

@@ -0,0 +1,24 @@
+# typed: strict
+# frozen_string_literal: true
+
+module Packwerk
+  # This class handles loading extensions to packwerk using the `require` directive
+  # in the `packwerk.yml` configuration.
+  module ExtensionLoader
+    class << self
+      extend T::Sig
+      sig { params(require_directive: String, config_dir_path: String).void }
+      def load(require_directive, config_dir_path)
+        # We want to transform the require directive to behave differently
+        # if it's a specific local file being required versus a gem
+        if require_directive.start_with?(".")
+          require File.join(config_dir_path, require_directive)
+        else
+          require require_directive
+        end
+      end
+    end
+  end
+
+  private_constant :ExtensionLoader
+end

test/fixtures/extended/config/environment.rb

@@ -0,0 +1 @@
+# this file intentionally left blank

test/fixtures/extended/config/my_local_extension.rb

@@ -0,0 +1,2 @@
+module MyLocalExtension
+end

test/fixtures/extended/packwerk.yml

@@ -0,0 +1,4 @@
+parallel: false
+require:
+  - ./config/my_local_extension
+  - my_gem_extension

test/unit/configuration_test.rb

@@ -13,6 +13,7 @@ class ConfigurationTest < Minitest::Test
 
     teardown do
       teardown_application_fixture
+      Object.send(:remove_const, :MyLocalExtension) if defined?(MyLocalExtension)
     end
 
     test ".from_path raises ArgumentError if path does not exist" do
@@ -63,5 +64,33 @@ class ConfigurationTest < Minitest::Test
       Configuration.expects(:new).with({}, config_path: to_app_path("packwerk.yml"))
       Configuration.from_path
     end
+
+    test "require works when referencing a local file" do
+      refute defined?(MyLocalExtension)
+      use_template(:extended)
+      mock_require_method = ->(required_thing) do
+        next unless required_thing.include?("my_local_extension")
+
+        require required_thing
+      end
+      ExtensionLoader.stub(:require, mock_require_method) do
+        Configuration.from_path
+      end
+      assert defined?(MyLocalExtension)
+    end
+
+    test "require works when referencing a gem" do
+      use_template(:extended)
+
+      required_things = []
+      mock_require_method = ->(required_thing) do
+        required_things << required_thing
+      end
+      ExtensionLoader.stub(:require, mock_require_method) do
+        Configuration.from_path
+      end
+
+      assert_includes(required_things, "my_gem_extension")
+    end
   end
 end

test/unit/files_for_processing_test.rb

@@ -5,15 +5,21 @@
 
 module Packwerk
   class FilesForProcessingTest < Minitest::Test
-    def setup
+    include ApplicationFixtureHelper
+
+    setup do
+      setup_application_fixture
+      use_template(:skeleton)
       @package_path = "components/sales"
-      @configuration = ::Packwerk::Configuration.from_path("test/fixtures/skeleton")
+      @configuration = ::Packwerk::Configuration.from_path
+    end
+
+    teardown do
+      teardown_application_fixture
     end
 
     test "fetch with custom paths includes only include glob in custom paths" do
-      files = Dir.chdir("test/fixtures/skeleton") do
-        ::Packwerk::FilesForProcessing.fetch(relative_file_paths: [@package_path], configuration: @configuration)
-      end
+      files = ::Packwerk::FilesForProcessing.fetch(relative_file_paths: [@package_path], configuration: @configuration)
       included_file_pattern = File.join(@package_path, "**/*.rb")
       assert_all_match(files, [included_file_pattern])
     end
@@ -54,13 +60,11 @@ def setup
     end
 
     test "fetch with no custom paths ignoring nested packages includes only include glob across codebase" do
-      files = Dir.chdir("test/fixtures/skeleton") do
-        ::Packwerk::FilesForProcessing.fetch(
-          relative_file_paths: [],
-          configuration: @configuration,
-          ignore_nested_packages: true
-        )
-      end
+      files = ::Packwerk::FilesForProcessing.fetch(
+        relative_file_paths: [],
+        configuration: @configuration,
+        ignore_nested_packages: true
+      )
 
       assert_all_match(files, @configuration.include)
     end