Added new duplicated_test_run cop

If a Minitest class inherits from another class, it will also inherit its methods causing Minitest to run the parent’s tests methods twice.

This PR detects when there are two tests classes, one inherits from the other, and both have tests methods.  MinitestRubocop will add an offence to the Child class in such a case.

Note that this cop only works if both classes are on the same file.

config/default.yml

@@ -105,6 +105,11 @@ Minitest/AssertWithExpectedArgument:
   Safe: false
   VersionAdded: '0.11'
 
+Minitest/DuplicatedTestRun:
+  Description: 'This cop detects duplicate test runs caused by one test class inheriting from another.'
+  Enabled: pending
+  VersionAdded: '<<next>>'
+
 Minitest/GlobalExpectations:
   Description: 'This cop checks for deprecated global expectations.'
   StyleGuide: 'https://minitest.rubystyle.guide#global-expectations'

lib/rubocop/cop/minitest/duplicated_test_run.rb

@@ -0,0 +1,81 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Minitest
+      # If a Minitest class inherits from another class,
+      # it will also inherit its methods causing Minitest to run the parent's tests methods twice.
+      #
+      # This cop detects when there are two tests classes, one inherits from the other, and both have tests methods.
+      # This cop will add an offence to the Child class in such a case.
+      #
+      # @example
+      #   # bad
+      #   class ParentTest < Minitest::Test
+      #     def test_parent # it will run this test twice.
+      #     end
+      #   end
+      #
+      #   class ChildTest < ParentTest
+      #     def test_child
+      #     end
+      #   end
+      #
+      #
+      #   # good
+      #   class ParentTest < Minitest::Test
+      #     def test_parent
+      #     end
+      #   end
+      #
+      #   class ChildTest < Minitest::Test
+      #     def test_child
+      #     end
+      #   end
+      #
+      #   # good
+      #   class ParentTest < Minitest::Test
+      #   end
+      #
+      #   class ChildTest
+      #     def test_child
+      #     end
+      #
+      #     def test_parent
+      #     end
+      #   end
+      #
+      class DuplicatedTestRun < Base
+        include MinitestExplorationHelpers
+
+        MSG = "Subclasses with test methods causes the parent' tests to run them twice."
+
+        def on_class(class_node)
+          return unless test_class?(class_node)
+          return unless test_methods?(class_node)
+          return unless parent_class_has_test_methods(class_node)
+
+          message = format(MSG)
+          add_offense(class_node, message: message)
+        end
+
+        private
+
+        def parent_class_has_test_methods(class_node)
+          parent_class = class_node.parent_class
+          parent_class_node = class_node.parent.each_child_node(:class).detect do |klass|
+            klass.identifier == parent_class
+          end
+
+          return false unless parent_class_node
+
+          test_methods?(parent_class_node)
+        end
+
+        def test_methods?(class_node)
+          test_cases(class_node).size.positive?
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/minitest_cops.rb

@@ -23,6 +23,7 @@
 require_relative 'minitest/assert_respond_to'
 require_relative 'minitest/assert_silent'
 require_relative 'minitest/assert_truthy'
+require_relative 'minitest/duplicated_test_run'
 require_relative 'minitest/global_expectations'
 require_relative 'minitest/literal_as_actual_argument'
 require_relative 'minitest/multiple_assertions'

test/rubocop/cop/minitest/duplicated_test_run_test.rb

@@ -0,0 +1,106 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class DuplicatedTestRunTest < Minitest::Test
+  def test_registers_offense_when_parent_and_child_have_tests_methods
+    assert_offense(<<~RUBY)
+      class ParentTest < Minitest::Test
+        def test_parent
+        end
+      end
+
+      class ChildTest < ParentTest
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Subclasses with test methods causes the parent' tests to run them twice.
+        def test_child_asserts_twice
+          assert_equal(1, 1)
+        end
+      end
+    RUBY
+  end
+
+  def test_registers_offense_when_parent_and_children_have_tests_methods
+    assert_offense(<<~RUBY)
+      class ParentTest < Minitest::Test
+        def test_parent
+        end
+      end
+
+      class Child1Test < ParentTest
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Subclasses with test methods causes the parent' tests to run them twice.
+        def test_parent
+        end
+      end
+
+      class Child2Test < ParentTest
+      end
+
+      class Child3Test < ParentTest
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Subclasses with test methods causes the parent' tests to run them twice.
+        def test_1_child_2_asserts_twice
+        end
+
+        def test_2_child_2_asserts_twice
+        end
+
+        def test_3_child_2_asserts_twice
+        end
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_if_the_parent_does_not_have_test_methods
+    assert_no_offenses(<<~RUBY)
+      class ParentTest < Minitest::Test
+      end
+
+      class ChildTest < ParentTest
+        def test_child_asserts_twice
+          assert_equal(1, 1)
+        end
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_if_the_child_does_not_have_test_methods
+    assert_no_offenses(<<~RUBY)
+      class ParentTest < Minitest::Test
+        def test_child_asserts_twice
+          assert_equal(1, 1)
+        end
+      end
+
+      class ChildTest < ParentTest
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_if_the_class_has_no_children
+    assert_no_offenses(<<~RUBY)
+      class ParentTest < Minitest::Test
+        def test_child_asserts_twice
+          assert_equal(1, 1)
+        end
+      end
+
+      class ClassTwo < ParentTest
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_if_the_class_is_not_a_test_class
+    assert_no_offenses(<<~RUBY)
+      class ParentTest < ExampleClass
+        def test_child_asserts_twice
+          assert_equal(1, 1)
+        end
+      end
+
+      class ChildClass < ParentTest
+        def test_child_asserts_twice
+          assert_equal(1, 1)
+        end
+      end
+    RUBY
+  end
+end