Add plugin to ignore ActionMailer::Preview classes

New plugin that ingores all methods inside ActionMailer::Preview classes
as they are all automatically made available by Rails in the previwer,
but never otherwise referenced.

lib/spoom/deadcode/plugins.rb

@@ -5,6 +5,7 @@
 
 require_relative "plugins/actionpack"
 require_relative "plugins/active_job"
+require_relative "plugins/action_mailer_preview"
 require_relative "plugins/action_mailer"
 require_relative "plugins/active_model"
 require_relative "plugins/active_record"

lib/spoom/deadcode/plugins/action_mailer_preview.rb

@@ -0,0 +1,19 @@
+# typed: strict
+# frozen_string_literal: true
+
+module Spoom
+  module Deadcode
+    module Plugins
+      class ActionMailerPreview < Base
+        extend T::Sig
+
+        ignore_classes_inheriting_from("ActionMailer::Preview")
+
+        sig { override.params(indexer: Indexer, definition: Definition).void }
+        def on_define_method(indexer, definition)
+          definition.ignored! if indexer.nesting_class_superclass_name == "ActionMailer::Preview"
+        end
+      end
+    end
+  end
+end

test/spoom/deadcode/plugins/action_mailer_preview_test.rb

@@ -0,0 +1,48 @@
+# typed: true
+# frozen_string_literal: true
+
+require "test_with_project"
+require "helpers/deadcode_helper"
+
+module Spoom
+  module Deadcode
+    module Plugins
+      class ActionMailerPreviewTest < TestWithProject
+        include Test::Helpers::DeadcodeHelper
+
+        def test_action_mailer_preview_ignore_direct_inheritance
+          @project.write!("test/mailers/previews/my_mailer_preview.rb", <<~RB)
+            class MyMailerPreview < ActionMailer::Preview
+              def my_email_preview_method; end
+
+              private
+
+              def some_unused_private_method; end
+            end
+
+            class MyOtherMailerPreview < ::ActionMailer::Preview; end
+
+            class Foo
+              def bar; end
+            end
+          RB
+
+          index = index_with_plugins
+          assert_ignored(index, "MyMailerPreview")
+          assert_ignored(index, "MyOtherMailerPreview")
+          assert_ignored(index, "my_email_preview_method")
+          # refute_ignored(index, "some_unused_private_method") ideally we'd want this, but seems tricky to do
+          refute_ignored(index, "Foo")
+          refute_ignored(index, "bar")
+        end
+
+        private
+
+        sig { returns(Index) }
+        def index_with_plugins
+          deadcode_index(plugins: [ActionMailerPreview.new])
+        end
+      end
+    end
+  end
+end