Global function definitions pollute Grape::API scope on Ruby 2.3.0

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Global function definitions pollute Grape::API scope on Ruby 2.3.0

require 'spec_helper'

def namespace
  raise 'should never be called'
end

describe Grape::API do
  subject do
    Class.new(Grape::API) do
      format :json
      get do
        { ok: true }
      end
    end
  end

  def app
    subject
  end

  context 'with Rake::DSL' do
    it 'works' do
      get '/'
      expect(last_response.status).to eq 200
    end
  end
end

This fails on Ruby 2.3.0 with

  1) Grape::API with Rake::DSL works
     Failure/Error: raise 'should never be called'

     RuntimeError:
       should never be called
     # ./spec/grape/integration/global_namespace_function_spec.rb:4:in `namespace'
     # ./lib/grape/router/attribute_translator.rb:29:in `block in to_h'
     # ./lib/grape/router/attribute_translator.rb:28:in `each'
     # ./lib/grape/router/attribute_translator.rb:28:in `each_with_object'
     # ./lib/grape/router/attribute_translator.rb:28:in `to_h'
     # ./lib/grape/endpoint.rb:134:in `block (2 levels) in mount_in'
     # ./lib/grape/endpoint.rb:132:in `each'
     # ./lib/grape/endpoint.rb:132:in `block in mount_in'
     # ./lib/grape/endpoint.rb:127:in `each'
     # ./lib/grape/endpoint.rb:127:in `mount_in'
     # ./lib/grape/api.rb:103:in `block in initialize'
     # ./lib/grape/api.rb:102:in `each'
     # ./lib/grape/api.rb:102:in `initialize'
     # ./lib/grape/api.rb:26:in `new'
     # ./lib/grape/api.rb:26:in `compile'
     # ./lib/grape/api.rb:39:in `block in call'
     # ./lib/grape/api.rb:39:in `synchronize'
     # ./lib/grape/api.rb:39:in `call'
     # /Users/dblock/.rvm/gems/ruby-2.3.0/gems/rack-test-0.6.3/lib/rack/mock_session.rb:30:in `request'
     # /Users/dblock/.rvm/gems/ruby-2.3.0/gems/rack-test-0.6.3/lib/rack/test.rb:244:in `process_request'
     # /Users/dblock/.rvm/gems/ruby-2.3.0/gems/rack-test-0.6.3/lib/rack/test.rb:58:in `get'
     # ./spec/grape/integration/global_namespace_function_spec.rb:23:in `block (3 levels) in <top (required)>'

Thanks to @dylanfareed for the repro, took a while to narrow this down.

The namespace function is also defined by Rake, so the most common way of seeing this is with include Rake::DSL which is something that fabricator does, for example.

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I opened https://bugs.ruby-lang.org/issues/12251, lets at least work-around this.

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cc: @namusyaka, should we just replace this with a Hashie::Mash or something else...?