This gem does not yet work.

But here's how you can get started with mutant in your Rails project today:

In your Gemfile (the order of the gems matters):

group :development, :test do
  gem 'ruby_is_forked'
  gem 'rails_is_forked'
  gem 'mutant'
end

Remove the line require 'rspec/autorun' from spec/spec_helper. You should be good to run. Here's your command line, assuming you want to test the User model:

mutant -r ./config/environment --rspec User

More generically:

RAILS_ENV=test mutant -r ./config/environment --rspec [model class]

You can also test, specific methods:

Instance:

RAILS_ENV=test mutant -r ./config/environment --rspec User#my_instance_method

Class:

RAILS_ENV=test mutant -r ./config/environment --rspec User.my_class_method

Caveats

You may run into issues if you have require 'rspec/autorun' or config.filter_run focus: true. You can get around this by putting them in a conditional such as:

if ENV['MUTANT'] == 'true'
  config.filter_run focus: true
end

and then prepend MUTANT=true to the above commands. Maybe toss it into a handy helper method in your spec_helper.rb:

def mutant?
  ENV['MUTANT'] == 'true'
end

You can see more about structuring your specs on the mutant page.

A script to help you switch to the dm2 style of specs: https://gist.github.com/mockdeep/7051285

Better documentation to come as I learn more...