Environment

• JRuby version: 9.1.16.0
• Operating system and platform: Darwin ecvaawplun6.local 17.4.0 Darwin Kernel Version 17.4.0: Sun Dec 17 09:19:54 PST 2017; root:xnu-4570.41.2~1/RELEASE_X86_64 x86_64 i386 MacBookPro14,3 Darwin

Expected Behavior

When using this config.ru:

# frozen_string_literal: true

run Proc.new { |env| ['200', {'Content-Type' => 'text/html'}, ['get rack\'d']] }

I should be able to run rackup without any warnings.

Actual Behavior

With JRuby, I get a warning; with MRI 2.5.1 (and 2.3.6) I do not:

$ rackup
/Users/plundberg/.rvm/gems/jruby-9.1.16.0/gems/rack-2.0.4/lib/rack/builder.rb:49: warning: `frozen_string_literal' is ignored after any tokens
Puma starting in single mode...
* Version 3.11.3 (jruby 9.1.16.0 - ruby 2.3.3), codename: Love Song
* Min threads: 0, max threads: 16
* Environment: development
* Listening on tcp://localhost:9292
Use Ctrl-C to stop
^C- Gracefully stopping, waiting for requests to finish
=== puma shutdown: 2018-04-04 10:59:46 +0300 ===
- Goodbye!
$ rvm use 2.5.1
Using /Users/plundberg/.rvm/gems/ruby-2.5.1
$ rackup
Puma starting in single mode...
* Version 3.11.3 (ruby 2.5.1-p57), codename: Love Song
* Min threads: 0, max threads: 16
* Environment: development
* Listening on tcp://localhost:9292
Use Ctrl-C to stop
^C- Gracefully stopping, waiting for requests to finish
=== puma shutdown: 2018-04-04 11:00:04 +0300 ===
- Goodbye!

I analyzed this down to https://github.com/rack/rack/blob/master/lib/rack/builder.rb#L49, where the failing code looks like this:

    def self.new_from_string(builder_script, file="(rackup)")
      eval "Rack::Builder.new {\n" + builder_script + "\n}.to_app",
        TOPLEVEL_BINDING, file, 0
    end

So it seems, JRuby prints this warning if it encounters a frozen_string_literal comment in the middle of an eval statement, whereas MRI does not.

I wonder how MRI actually handles the frozen_string_literal setting in that case - does it use unfrozen string literals for the code before that comment, and frozen string literals for anything that comes after it?

Anyway, this is a semantic difference between MRI and JRuby that deserves to be documented, and potentially discussed.