example.rb

describe PostsController do

  describe "GET index" do
    subject { get :index }

    it { should assign_to(:posts).with(Post.all) }
  end

end

rails3_assigns_rspec2_matcher.rb

RSpec::Matchers.define :assign_to do |name|
  chain :with do |obj|
    @obj = obj
  end

  match do |response|
    ivar   = "@#{name}"
    result = controller.instance_variable_defined?(ivar)

    if @obj
      @assigned = controller.instance_variable_get(ivar)

      result && @obj == @assigned
    else
      result
    end
  end

  failure_message_for_should do |response|
    message = "#{controller.class} should assign to :#{name}"

    if @obj
      message << " with #{@obj.inspect} while it was #{@assigned.inspect}"
    end

    message
  end

  failure_message_for_should_not do |response|
    "#{controller.class} should not assign to :#{name}"
  end
end