devise_matchers.rb

RSpec::Matchers.define :be_signed_in do
  match do |user|
    warden.user == user
  end

  failure_message_for_should do |user|
    "expected #{user} to be signed in, but was not"
  end

  failure_message_for_should_not do |user|
    "expected #{user} not to be signed in, but *was* signed in"
  end

  description do
    "be signed in"
  end
end

# use:
# @user.should be_signed_in