Use Danger to notify about outdated rubocop todos

When removing files, contributors might forget to remove the related
rubocop todos. To avoid that, this new Danger plugin will add comments
on MRs on deleted files that still have related rubocop todos.

Changelog: added

danger/plugins/todos.rb

+# frozen_string_literal: true
+
+require_relative '../../tooling/danger/outdated_todo'
+
+module Danger
+  class Todos < ::Danger::Plugin
+    def check_outdated_todos(filenames)
+      Tooling::Danger::OutdatedTodo.new(filenames, context: self).check
+    end
+  end
+end

danger/todos/Dangerfile

+# frozen_string_literal: true
+
+todos.check_outdated_todos(git.deleted_files)

spec/fixtures/tooling/danger/rubocop_todo/cop1.yml

+---
+Cop1:
+  Exclude:
+    - 'app/controllers/application_controller.rb'
+    - 'app/controllers/acme_challenges_controller.rb'

spec/fixtures/tooling/danger/rubocop_todo/cop2.yml

+---
+Cop2:
+  Exclude:
+    - 'app/controllers/application_controller.rb'

spec/tooling/danger/outdated_todo_spec.rb

+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+require 'gitlab/dangerfiles/spec_helper'
+
+require_relative '../../../tooling/danger/outdated_todo'
+
+RSpec.describe Tooling::Danger::OutdatedTodo, feature_category: :tooling do
+  let(:fake_danger) { double }
+  let(:filenames) { ['app/controllers/application_controller.rb'] }
+
+  let(:todos) do
+    [
+      File.join('spec', 'fixtures', 'tooling', 'danger', 'rubocop_todo', '**', '*.yml')
+    ]
+  end
+
+  subject(:plugin) { described_class.new(filenames, context: fake_danger, todos: todos) }
+
+  context 'when the filenames are mentioned in single todo' do
+    let(:filenames) { ['app/controllers/acme_challenges_controller.rb'] }
+
+    it 'warns about mentions' do
+      expect(fake_danger)
+        .to receive(:warn)
+        .with <<~MESSAGE
+          `app/controllers/acme_challenges_controller.rb` was removed but is mentioned in:
+          - `spec/fixtures/tooling/danger/rubocop_todo/cop1.yml:5`
+        MESSAGE
+
+      plugin.check
+    end
+  end
+
+  context 'when the filenames are mentioned in multiple todos' do
+    let(:filenames) do
+      [
+        'app/controllers/application_controller.rb',
+        'app/controllers/acme_challenges_controller.rb'
+      ]
+    end
+
+    it 'warns about mentions' do
+      expect(fake_danger)
+        .to receive(:warn)
+        .with(<<~FIRSTMESSAGE)
+          `app/controllers/application_controller.rb` was removed but is mentioned in:
+          - `spec/fixtures/tooling/danger/rubocop_todo/cop1.yml:4`
+          - `spec/fixtures/tooling/danger/rubocop_todo/cop2.yml:4`
+        FIRSTMESSAGE
+
+      expect(fake_danger)
+        .to receive(:warn)
+        .with(<<~SECONDMESSAGE)
+          `app/controllers/acme_challenges_controller.rb` was removed but is mentioned in:
+          - `spec/fixtures/tooling/danger/rubocop_todo/cop1.yml:5`
+        SECONDMESSAGE
+
+      plugin.check
+    end
+  end
+
+  context 'when the filenames are not mentioned in todos' do
+    let(:filenames) { ['any/inexisting/file.rb'] }
+
+    it 'does not warn' do
+      expect(fake_danger).not_to receive(:warn)
+
+      plugin.check
+    end
+  end
+
+  context 'when there is no todos' do
+    let(:filenames) { ['app/controllers/acme_challenges_controller.rb'] }
+    let(:todos) { [] }
+
+    it 'does not warn' do
+      expect(fake_danger).not_to receive(:warn)
+
+      plugin.check
+    end
+  end
+end

tooling/danger/outdated_todo.rb

+# frozen_string_literal: true
+
+module Tooling
+  module Danger
+    class OutdatedTodo
+      TODOS_GLOBS = %w[
+        .rubocop_todo/**/*.yml
+        spec/support/rspec_order_todo.yml
+      ].freeze
+
+      def initialize(filenames, context:, todos: TODOS_GLOBS)
+        @filenames = filenames
+        @context = context
+        @todos_globs = todos
+      end
+
+      def check
+        filenames.each do |filename|
+          check_filename(filename)
+        end
+      end
+
+      private
+
+      attr_reader :filenames, :context
+
+      def check_filename(filename)
+        mentions = all_mentions_for(filename)
+
+        return if mentions.empty?
+
+        context.warn <<~MESSAGE
+          `#{filename}` was removed but is mentioned in:
+          #{mentions.join("\n")}
+        MESSAGE
+      end
+
+      def all_mentions_for(filename)
+        todos
+          .filter_map { |todo| mentioned_lines(filename, todo) }
+          .flatten
+          .map { |todo| "- `#{todo}`" }
+      end
+
+      def mentioned_lines(filename, todo)
+        File
+          .foreach(todo)
+          .with_index(1)
+          .select { |text, _line| text.match?(/.*#{filename}.*/) }
+          .map { |_text, line| "#{todo}:#{line}" }
+      end
+
+      def todos
+        @todos ||= @todos_globs.flat_map { |value| Dir.glob(value) }
+      end
+    end
+  end
+end