Do not leak TestUnit specific methods after Rails 4

lib/rspec/rails/adapters.rb

@@ -201,7 +201,7 @@ def assertion_method_names
             select do |m|
               m.to_s =~ /^(assert|flunk|refute)/
             end
-          methods + [:build_message]
+          methods + test_unit_specific_methods
         end
 
         def define_assertion_delegators
@@ -211,6 +211,18 @@ def define_assertion_delegators
             end
           end
         end
+
+        # Starting on Rails 4, Minitest is the default testing framework so no
+        # need to add TestUnit specific methods.
+        if ::Rails::VERSION::STRING >= '4.0.0'
+          def test_unit_specific_methods
+            []
+          end
+        else
+          def test_unit_specific_methods
+            [:build_message]
+          end
+        end
       end
 
       class AssertionDelegator

spec/rspec/rails/assertion_adapter_spec.rb

@@ -25,4 +25,16 @@
   it "does not expose Minitest's message method" do
     expect(methods).not_to include("message")
   end
+
+  if ::Rails::VERSION::STRING >= '4.0.0'
+    # In Ruby <= 1.8.7 Object#methods returns Strings instead of Symbols. They
+    # are all converted to Symbols to ensure we always compare them correctly.
+    it 'does not leak TestUnit specific methods into the AssertionDelegator' do
+      expect(methods.map(&:to_sym)).to_not include(:build_message)
+    end
+  else
+    it 'includes methods required by TestUnit into the AssertionDelegator' do
+      expect(methods.map(&:to_sym)).to include(:build_message)
+    end
+  end
 end