In MRI, procs have properties _line_ and _file_. The method call
succeeds. In JRuby, this call fails with
NoMethodError: undefined method `_line_' for #<Proc:...
The linenumber is available in the class, as it is shown in inspect:
I've browsed through the sources a bit, but didn't see where it was appended.
Rubinius doesn't implement _line_ either BTW.