Add Danger warning about changed SettingBlock instances

Settings sections can be searched through the command palette.
But this only works if they are kept in sync with the files that drive
that search.

danger/plugins/settings_sections.rb

+# frozen_string_literal: true
+
+require_relative '../../tooling/danger/settings_sections'
+
+module Danger
+  class SettingsSections < ::Danger::Plugin
+    include Tooling::Danger::SettingsSections
+  end
+end

danger/settings_sections/Dangerfile

+# frozen_string_literal: true
+
+settings_sections.check!

lib/search/group_settings.rb

 # frozen_string_literal: true
 
 module Search
+  # Generates a list of all available setting sections of a group.
+  # This list is used by the command palette's search functionality.
   class GroupSettings
     include Rails.application.routes.url_helpers
 

lib/search/project_settings.rb

 # frozen_string_literal: true
 
 module Search
+  # Generates a list of all available setting sections of a project.
+  # This list is used by the command palette's search functionality.
   class ProjectSettings
     include Rails.application.routes.url_helpers
 

spec/tooling/danger/settings_sections_spec.rb

+# frozen_string_literal: true
+
+require 'gitlab/dangerfiles/spec_helper'
+require 'fast_spec_helper'
+
+require_relative '../../../tooling/danger/settings_sections'
+
+RSpec.describe Tooling::Danger::SettingsSections, feature_category: :tooling do
+  include_context 'with dangerfile'
+
+  subject(:settings_section_check) { fake_danger.new(helper: fake_helper) }
+
+  let(:fake_danger) { DangerSpecHelper.fake_danger.include(described_class) }
+  let(:matching_changed_files) { ['app/views/foo/bar.html.haml', 'app/assets/js/foo/bar.vue'] }
+  let(:changed_lines) { ['-render SettingsBlockComponent.new(id: "foo") do', '<settings-block id="foo">'] }
+  let(:stable_branch?) { false }
+
+  before do
+    allow(fake_helper).to receive(:changed_files).and_return(matching_changed_files)
+    allow(fake_helper).to receive(:changed_lines).and_return(changed_lines)
+    allow(fake_helper).to receive(:stable_branch?).and_return(stable_branch?)
+  end
+
+  context 'when on stable branch' do
+    let(:stable_branch?) { true }
+
+    it 'does not write any markdown' do
+      expect(settings_section_check).not_to receive(:markdown)
+      settings_section_check.check!
+    end
+  end
+
+  context 'when none of the changed files are Haml or Vue files' do
+    let(:matching_changed_files) { [] }
+
+    it 'does not write any markdown' do
+      expect(settings_section_check).not_to receive(:markdown)
+      settings_section_check.check!
+    end
+  end
+
+  context 'when none of the changed lines match the pattern' do
+    let(:changed_lines) { ['-foo', '+bar'] }
+
+    it 'does not write any markdown' do
+      expect(settings_section_check).not_to receive(:markdown)
+      settings_section_check.check!
+    end
+  end
+
+  it 'adds a new markdown section listing every matching line' do
+    expect(settings_section_check).to receive(:markdown).with(/Searchable setting sections/)
+    expect(settings_section_check).to receive(:markdown).with(/SettingsBlock/)
+    expect(settings_section_check).to receive(:markdown).with(/settings-block/)
+    settings_section_check.check!
+  end
+end

tooling/danger/settings_sections.rb

+# frozen_string_literal: true
+
+module Tooling
+  module Danger
+    module SettingsSections
+      def check!
+        return if helper.stable_branch?
+
+        changed_code_files = helper.changed_files(/\.(haml|vue)$/)
+        return if changed_code_files.empty?
+
+        vc_regexp = /(SettingsBlockComponent|settings-block)/
+        lines_with_matches = filter_changed_lines(changed_code_files, vc_regexp)
+        return if lines_with_matches.empty?
+
+        markdown(<<~MARKDOWN)
+            ## Searchable setting sections
+
+            Looks like you have edited the template of some settings section. Please check that all changed sections are still searchable:
+
+            - If you created a new section, make sure to add it to either `lib/search/project_settings.rb` or `lib/search/group_settings.rb`, or in their counterparts in `ee/` if this section is only available behind a licensed feature.
+            - If you removed a section, make sure to also remove it from the files above.
+            - If you changed a section's id, please update it also in the files above.
+            - If you just moved code around within the same page, there is nothing to do.
+
+        MARKDOWN
+
+        lines_with_matches.each do |file, lines|
+          markdown(<<~MARKDOWN)
+              #### `#{file}`
+
+              ```shell
+              #{lines.join("\n")}
+              ```
+
+          MARKDOWN
+        end
+      end
+
+      def filter_changed_lines(files, pattern)
+        files_with_lines = {}
+        files.each do |file|
+          next if file.start_with?('spec/', 'ee/spec/', 'qa/')
+
+          matching_changed_lines = helper.changed_lines(file).select { |line| line =~ pattern }
+          next unless matching_changed_lines.any?
+
+          files_with_lines[file] = matching_changed_lines
+        end
+
+        files_with_lines
+      end
+    end
+  end
+end