Frozen string literal issue with Rails 3.4+ (with solution)

[ron-shinall]

I'm getting frozen-string-literal warnings with Ruby 3.4.1 from magic.rb line 120. I also have a potential solution, if indeed this is an issue...and I am happy to open a PR for review & feedback.

To recreate the issue, you can generate a minimal Rails app with the following (making sure to use Ruby 3.4+):

rails new marcel_scaffold --skip-git --skip-docker --skip-keeps --skip-asset-pipeline --skip-rubocop --skip-brakeman --skip-ci --skip-kamal --skip-devcontainer --skip-action-mailer --skip-action-mailbox --skip-action-text --skip-javascript --skip-hotwire --skip-decrypted-diffs --skip-action-cable --skip-bootsnap --skip-dev-gems --skip-jbuilder --skip-system-test --skip-thruster

Then execute:

bin/rails active_storage:install
bin/rails db:migrate

And create & run a test like this:

require "test_helper"

class MarcelTest < ActiveSupport::TestCase
  test "mime type" do
    assert_not_empty Marcel::MimeType.for StringIO.new("foo")
  end
end

You should see a warning as below:

Running 1 tests in a single process (parallelization threshold is 50)
Run options: --name "/MarcelTest(#|::)/" --seed 24608

# Running:

/path_to_gems/marcel-1.0.4/lib/marcel/magic.rb:120: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)
.

Finished in 0.050929s, 19.6352 runs/s, 19.6352 assertions/s.
1 runs, 1 assertions, 0 failures, 0 errors, 0 skips

[ron-shinall]

@jeremy have you come across this? Please let me know if you need more information.

[jeremy]

Haven't yet. Please do investigate!

[ron-shinall]

@jeremy ok, after more digging it turns out that it's the tests that are generating the "chilled" strings and therefore triggering the deprecation warning. Adding # frozen_string_literal: true to the test files resolved the issue for me.