Support magic comments in .ru files

[eregon]

• Such as # frozen_string_literal: true.

This seems actually like a relevant bug fix because many files under example and test use # frozen_string_literal: true but actually it has not effect before this fix.

From the idea in #1544 (comment)
cc @ioquatix @jeremyevans

[ioquatix]

Another reason why we should have --warnings on by default during test runs.

[ioquatix]

Thanks for your awesome implementation! It's quite possibly the most awesome thing in Ruby I've seen in several months at least.

[eregon]

Thank you for the kind words :)

[shime]

I'm writing a deep dive post about rackup and I have to say, this is the most amazing piece of code I've stumbled upon. Pure wizardry.

I don't fully understand why TOPLEVEL_BINDING is necessary here, though. What would be the effect of not having it?

# Why not something like this?
def self.new_from_string(builder_script, file = "(rackup)")
  bind, builder = Rack::Builder.new.instance_eval { [binding, self] }
  eval builder_script, bind, file
  builder.to_app
end

[eregon]

That would inherit the Module.nesting, which is [Rack::Builder, Rack] in that case. That would imply a .ru file with p Builder would print Rack::Builder and so defining your own Builder class would actually instead reopen Rack's Builder class which is most likely unintended.

TOPLEVEL_BINDING is used to preserve an empty Module.nesting just like regular top-level code in a file.

[eregon]

Some more details in my tweet: https://twitter.com/eregontp/status/1232037638485487617

[shime]

Thank you! 🙏