scrub instance variables from test cases on teardown

this prevents test state from accumulating, resulting in leaked
objects and slow tests due to overactive GC.

activesupport/lib/active_support/test_case.rb

@@ -6,6 +6,7 @@
 require 'active_support/testing/pending'
 require 'active_support/testing/isolation'
 require 'active_support/core_ext/kernel/reporting'
+require 'active_support/testing/garbage_collection'
 
 begin
   silence_warnings { require 'mocha' }
@@ -37,5 +38,6 @@ def self.for_tag(tag)
     include ActiveSupport::Testing::Deprecation
     include ActiveSupport::Testing::Pending
     extend ActiveSupport::Testing::Declarative
+    include ActiveSupport::Testing::GarbageCollection
   end
 end

activesupport/lib/active_support/testing/garbage_collection.rb

@@ -0,0 +1,19 @@
+module ActiveSupport
+  module Testing
+    module GarbageCollection
+      def self.included(base)
+        base.teardown :scrub_leftover_instance_variables
+      end
+
+      private
+
+      RESERVED_INSTANCE_VARIABLES = %w(@test_passed @passed @method_name @__name__ @_result).map(&:to_sym)
+
+      def scrub_leftover_instance_variables
+        (instance_variables.map(&:to_sym) - RESERVED_INSTANCE_VARIABLES).each do |var|
+          remove_instance_variable(var)
+        end
+      end
+    end
+  end
+end

activesupport/test/test_case_test.rb

@@ -74,5 +74,23 @@ def test_true; assert true end
 
       assert_match %r{oh noes}, exception.message
     end
+
+    def test_teardown_should_scrub_instance_variables
+      tc = Class.new(TestCase) do
+        def test_true; @alpha = "a"; assert_equal "a", @alpha; end
+      end
+
+      test_name = 'test_true'
+      fr = FakeRunner.new
+
+      test = tc.new test_name
+      test.run(fr) {}
+
+      passed_var = IS_MINITEST ? :@passed : :@test_passed
+      ivars = test.instance_variables.map(&:to_sym)
+
+      assert ivars.include?(passed_var), "#{passed_var} should not have been scrubbed"
+      assert !ivars.include?(:@alpha), "@alpha should have been scrubbed"
+    end
   end
 end

activesupport/test/test_test.rb

@@ -137,7 +137,7 @@ class SetupAndTeardownTest < ActiveSupport::TestCase
   def test_inherited_setup_callbacks
     assert_equal [:reset_callback_record, :foo], self.class._setup_callbacks.map(&:raw_filter)
     assert_equal [:foo], @called_back
-    assert_equal [:foo, :sentinel, :foo], self.class._teardown_callbacks.map(&:raw_filter)
+    assert_equal [:scrub_leftover_instance_variables, :foo, :sentinel, :foo], self.class._teardown_callbacks.map(&:raw_filter)
   end
 
   def setup
@@ -169,7 +169,7 @@ class SubclassSetupAndTeardownTest < SetupAndTeardownTest
   def test_inherited_setup_callbacks
     assert_equal [:reset_callback_record, :foo, :bar], self.class._setup_callbacks.map(&:raw_filter)
     assert_equal [:foo, :bar], @called_back
-    assert_equal [:foo, :sentinel, :foo, :bar], self.class._teardown_callbacks.map(&:raw_filter)
+    assert_equal [:scrub_leftover_instance_variables, :foo, :sentinel, :foo, :bar], self.class._teardown_callbacks.map(&:raw_filter)
   end
 
   protected