Support magic comments in .ru files

* Such as # frozen_string_literal: true.

lib/rack/builder.rb

@@ -109,8 +109,11 @@ def self.load_file(path, opts = Server::Options.new)
     # Evaluate the given +builder_script+ string in the context of
     # a Rack::Builder block, returning a Rack application.
     def self.new_from_string(builder_script, file = "(rackup)")
-      eval "Rack::Builder.new {\n" + builder_script + "\n}.to_app",
-        TOPLEVEL_BINDING, file, 0
+      # We want to build a variant of TOPLEVEL_BINDING with self as a Rack::Builder instance.
+      # We cannot use instance_eval(String) as that would resolve constants differently.
+      binding, builder = TOPLEVEL_BINDING.eval('Rack::Builder.new.instance_eval { [binding, self] }')
+      eval builder_script, binding, file
+      builder.to_app
     end
 
     # Initialize a new Rack::Builder instance.  +default_app+ specifies the

test/builder/frozen.ru

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+run lambda { |env|
+  body = 'frozen'
+  raise "Not frozen!" unless body.frozen?
+  [200, { 'Content-Type' => 'text/plain' }, [body]]
+}

test/spec_builder.rb

@@ -270,6 +270,15 @@ def config_file(name)
         Encoding.default_external = enc
       end
     end
+
+    it "respects the frozen_string_literal magic comment" do
+      app, _ = Rack::Builder.parse_file(config_file('frozen.ru'))
+      response = Rack::MockRequest.new(app).get('/')
+      response.body.must_equal 'frozen'
+      body = response.instance_variable_get(:@body)
+      body.must_equal(['frozen'])
+      body[0].frozen?.must_equal true
+    end
   end
 
   describe 'new_from_string' do