There actually are some libraries that requires 'irb' such as byebug, and when
'irb' is required, it requires irb/src_encoding.rb, then it defines the toplevel
default_src_encoding method that is visible from anywhere in the end users' apps.

Here's a quick oneliner that shows what's happening.

  % ruby -rpry-byebug -e 'p private_methods(false)'
  [:include, :using, :define_method, :public, :private, :DelegateClass, :default_src_encoding]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59090 b2dd03c8-39d4-4d8f-98ff-823fe69b080e