Added new duplicated_test_run cop

changelog/new_duplicated_test_run_cop.md

@@ -0,0 +1 @@
+* [#164](https://github.com/rubocop/rubocop-minitest/pull/164): Add new `Minitest/DuplicateTestRun cop. ([@ignacio-chiazzo][])

config/default.yml

@@ -105,6 +105,11 @@ Minitest/AssertionInLifecycleHook:
   Enabled: 'pending'
   VersionAdded: '0.10'
 
+Minitest/DuplicateTestRun:
+  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/duplicate_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 DuplicateTestRun < 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/duplicate_test_run'
 require_relative 'minitest/global_expectations'
 require_relative 'minitest/literal_as_actual_argument'
 require_relative 'minitest/multiple_assertions'

test/rubocop/cop/minitest/duplicate_test_run_test.rb

@@ -0,0 +1,106 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class DuplicateTestRunTest < 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