class RSpec::Core::Configuration

def expect_with(*frameworks)

Given both, configures both
Given :stdlib, configures test/unit/assertions
Given :rspec, configures rspec/expectations.

+frameworks+ can be :rspec, :stdlib, or both

Sets the expectation framework module(s). 
def expect_with(*frameworks)
  settings[:expectation_frameworks] = []
  frameworks.each do |framework|
    case framework
    when Symbol
      case framework
      when :rspec
        require 'rspec/core/expecting/with_rspec'
      when :stdlib
        require 'rspec/core/expecting/with_stdlib'
      else
        raise ArgumentError, "#{framework.inspect} is not supported"
      end
      settings[:expectation_frameworks] << RSpec::Core::ExpectationFrameworkAdapter
    end
  end
end