Add Dangerfile for frozen_string_literal

Checks that new Ruby files will enable
frozen string literal.

Partially addresses #47424.

1 files changed, 27 insertions, 0 deletions

diff --git a/danger/frozen_string/Dangerfile b/danger/frozen_string/Dangerfile
new file mode 100644
index 00000000000..595176d597d
--- /dev/null
+++ b/danger/frozen_string/Dangerfile
@@ -0,0 +1,27 @@
+# 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