Add Dangerfile for frozen_string_literal

Checks that new Ruby files will enable
frozen string literal.

Partially addresses #47424.

.gitlab-ci.yml

@@ -447,9 +447,8 @@ danger-review:
     - retry gem install danger --no-ri --no-rdoc
   cache: {}
   only:
-    refs:
-      - branches@gitlab-org/gitlab-ce
-      - branches@gitlab-org/gitlab-ee
+    variables:
+      - $DANGER_GITLAB_API_TOKEN
   except:
     refs:
       - master

Dangerfile

@@ -4,3 +4,4 @@ danger.import_dangerfile(path: 'danger/changelog')
 danger.import_dangerfile(path: 'danger/specs')
 danger.import_dangerfile(path: 'danger/gemfile')
 danger.import_dangerfile(path: 'danger/database')
+danger.import_dangerfile(path: 'danger/frozen_string')

changelogs/unreleased/frozen-string-danger.yml

+---
+title: Add Dangerfile for frozen_string_literal
+merge_request: 20767
+author: gfyoung
+type: performance

danger/frozen_string/Dangerfile

+# frozen_string_literal: true
+
+FILE_EXTENSION = ".rb"
+MAGIC_COMMENT = "# frozen_string_literal: true"
+
+def get_files_with_no_magic_comment(files)
+  files.select do |file|
+    file.end_with?(FILE_EXTENSION) &&
+      !File.open(file, &:gets).start_with?(MAGIC_COMMENT)
+  end
+end
+
+files_to_check = git.added_files
+files_to_fix = get_files_with_no_magic_comment(files_to_check)
+
+if files_to_fix.any?
+  warn 'This merge request adds files that do not enforce frozen string literal. ' \
+    'See https://gitlab.com/gitlab-org/gitlab-ce/issues/47424 for more information.'
+
+  markdown(<<~MARKDOWN)
+## Enable Frozen String Literal
+
+The following files should have `#{MAGIC_COMMENT}` in the first line:
+
+* #{files_to_fix.map { |path| "`#{path}`" }.join("\n* ")}
+  MARKDOWN
+end