heroku_environment.rb

module Heroku
  class RackEnvironment
    def create_binding
      # handle any loading errors
      begin
        load 'config.ru'
      rescue
        $std.err.puts $!
      end

      Proc.new {}.binding
    end
  end

  class RailsEnvironment
    def create_binding
      # handle any loading errors
      begin
        require 'config/boot'
        require 'config/environment'
      rescue
        $std.err.puts $!
      end

      Proc.new {}.binding
    end
  end
end

if __FILE__ == $0
  puts self.class.constants.size # => 90
  binding = Heroku::RailsEnvironment.new.create_binding
  puts self.class.constants.size # => 300
  eval("puts self.class.constants.size", binding) # => 0
end