add be_able_to matcher

lib/cancan/matchers.rb

@@ -0,0 +1,13 @@
+Spec::Matchers.define :be_able_to do |action, subject|
+  match do |model|
+    model.can?(action, subject)
+  end
+
+  failure_message_for_should do |model|
+    "expected to be able to #{action.inspect} #{subject.inspect}"
+  end
+
+  failure_message_for_should_not do |model|
+    "expected not to be able to #{action.inspect} #{subject.inspect}"
+  end
+end

spec/cancan/matchers_spec.rb

@@ -0,0 +1,25 @@
+require "spec_helper"
+
+describe "be_able_to" do
+  it "delegates to can?" do
+    object = Object.new
+    mock(object).can?(:read, 123) { true }
+    object.should be_able_to(:read, 123)
+  end
+
+  it "reports a nice failure message for should" do
+    object = Object.new
+    mock(object).can?(:read, 123) { false }
+    expect do
+      object.should be_able_to(:read, 123)
+    end.should raise_error('expected to be able to :read 123')
+  end
+
+  it "reports a nice failure message for should not" do
+    object = Object.new
+    mock(object).can?(:read, 123) { true }
+    expect do
+      object.should_not be_able_to(:read, 123)
+    end.should raise_error('expected not to be able to :read 123')
+  end
+end

spec/spec_helper.rb

@@ -5,6 +5,7 @@ require 'active_record'
 require 'action_controller'
 require 'action_view'
 require 'cancan'
+require 'cancan/matchers'
 
 Spec::Runner.configure do |config|
   config.mock_with :rr