Can you ask ruby to treat warnings as errors?

You can finally do that by overriding Warning.warn like

module Warning
  def warn(msg)
    raise msg
  end
end

This will turn the warning into an exception. This solution works at least since 2.4 branch.

There is unfortunately no real way of doing this, at least not on most versions of Ruby out there (variations may exist), short of monitoring the program output and aborting it when a warning appears on standard error. Here's why:

• Ruby defines Kernel.warn, which you can redefine to do whatever you wish (including exiting), and which you'd expect (hope) to be used consistently by Ruby to report warnings (including internal e.g. parsing warning), but
• methods implemented natively (in C) inside Ruby will in turn directly invoke a native method called rb_warn from source/server.c, completely bypassing your redefinition of Kernel.warn (e.g. the "string literal in condition" warning, for example, issued when doing something like: do_something if 'string', is printed via the native rb_warn from source/parse.c)
• to make things even worse, there is an additional, rb_warning native method, which can be used by Ruby to log warnings if -w or -v is specified.

So, if you need to take action solely on warnings generated by your application code's calling Kernel.warn then simply redefine Kernel.warn. Otherwise, you have exactly two options:

1. alter source/error.c to exit in rb_warn and rb_warning (and rb_warn_m?), and rebuild Ruby
2. monitor your program's standard error output for ': warning:', and abort it on match

You could also potentially use DTrace, and intercept the calls to rb_warn and rb_warning, though that's not going to produce exceptions you can rescue from somewhere. Rather, it'll just put them somewhere you can easily log them.