Adds cop to enforce assert_instance_of over assert(object.instance_of…

…?(Class))

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 * [#29](https://github.com/rubocop-hq/rubocop-minitest/pull/29): Add new `Minitest/RefuteRespondTo` cop.  ([@herwinw][])
 * [#31](https://github.com/rubocop-hq/rubocop-minitest/pull/31): Add new `Minitest/AssertEqual` cop. ([@herwinw][])
+* [#34](https://github.com/rubocop-hq/rubocop-minitest/pull/34): Add new `Minitest/AssertInstanceOf` cop. ([@abhaynikam][])
 
 ### Bug fixes
 

config/default.yml

@@ -21,6 +21,12 @@ Minitest/AssertIncludes:
   Enabled: true
   VersionAdded: '0.2'
 
+Minitest/AssertInstanceOf:
+  Description: 'This cop enforces the test to use `assert_instance_of(Class, object)` over `assert(object.instance_of?(Class))`'
+  StyleGuide: 'https://github.com/rubocop-hq/minitest-style-guide#assert-instance-of'
+  Enabled: true
+  VersionAdded: '0.4'
+
 Minitest/AssertNil:
   Description: 'This cop enforces the test to use `assert_nil` instead of using `assert_equal(nil, something)`.'
   StyleGuide: 'https://github.com/rubocop-hq/minitest-style-guide#assert-nil'

lib/rubocop/cop/minitest/assert_instance_of.rb

@@ -0,0 +1,60 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Minitest
+      # This cop enforces the test to use `assert_instance_of(Class, object)`
+      # over `assert(object.instance_of?(Class))`.
+      #
+      # @example
+      #   # bad
+      #   assert(object.instance_of?(Class))
+      #   assert(object.instance_of?(Class), 'the message')
+      #
+      #   # good
+      #   assert_instance_of(Class, object)
+      #   assert_instance_of(Class, object, 'the message')
+      #
+      class AssertInstanceOf < Cop
+        MSG = 'Prefer using `assert_instance_of(%<arguments>s)` over ' \
+              '`assert(%<receiver>s)`.'
+
+        def_node_matcher :assert_with_instance_of, <<~PATTERN
+          (send nil? :assert $(send $_ :instance_of? $_) $...)
+        PATTERN
+
+        def on_send(node)
+          assert_with_instance_of(node) do
+            |first_receiver_arg, object, method, rest_args|
+
+            message = rest_args.first
+            arguments = node_arguments(object, method, message)
+            receiver = [first_receiver_arg.source, message&.source].compact.join(', ')
+
+            offense_message = format(MSG, arguments: arguments, receiver: receiver)
+
+            add_offense(node, message: offense_message)
+          end
+        end
+
+        def autocorrect(node)
+          lambda do |corrector|
+            assert_with_instance_of(node) do |_, object, method, rest_args|
+              message = rest_args.first
+              arguments = node_arguments(object, method, message)
+
+              replacement = "assert_instance_of(#{arguments})"
+              corrector.replace(node.loc.expression, replacement)
+            end
+          end
+        end
+
+        private
+
+        def node_arguments(object, method, message)
+          [method, object, message].compact.map(&:source).join(', ')
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/minitest_cops.rb

@@ -4,6 +4,7 @@
 require_relative 'minitest/assert_equal'
 require_relative 'minitest/assert_nil'
 require_relative 'minitest/assert_includes'
+require_relative 'minitest/assert_instance_of'
 require_relative 'minitest/assert_respond_to'
 require_relative 'minitest/assert_truthy'
 require_relative 'minitest/refute_empty'

manual/cops.md

@@ -4,6 +4,7 @@
 * [Minitest/AssertEmpty](cops_minitest.md#minitestassertempty)
 * [Minitest/AssertEqual](cops_minitest.md#minitestassertequal)
 * [Minitest/AssertIncludes](cops_minitest.md#minitestassertincludes)
+* [Minitest/AssertInstanceOf](cops_minitest.md#minitestassertinstanceof)
 * [Minitest/AssertNil](cops_minitest.md#minitestassertnil)
 * [Minitest/AssertRespondTo](cops_minitest.md#minitestassertrespondto)
 * [Minitest/AssertTruthy](cops_minitest.md#minitestasserttruthy)

manual/cops_minitest.md

@@ -73,6 +73,31 @@ assert_includes(collection, object, 'the message')
 
 * [https://github.com/rubocop-hq/minitest-style-guide#assert-includes](https://github.com/rubocop-hq/minitest-style-guide#assert-includes)
 
+## Minitest/AssertInstanceOf
+
+Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
+--- | --- | --- | --- | ---
+Enabled | Yes | Yes  | 0.4 | -
+
+This cop enforces the test to use `assert_instance_of(Class, object)`
+over `assert(object.instance_of?(Class))`.
+
+### Examples
+
+```ruby
+# bad
+assert(object.instance_of?(Class))
+assert(object.instance_of?(Class), 'the message')
+
+# good
+assert_instance_of(Class, object)
+assert_instance_of(Class, object, 'the message')
+```
+
+### References
+
+* [https://github.com/rubocop-hq/minitest-style-guide#assert-instance-of](https://github.com/rubocop-hq/minitest-style-guide#assert-instance-of)
+
 ## Minitest/AssertNil
 
 Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged

test/rubocop/cop/minitest/assert_instance_of_test.rb

@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class AssertInstanceOfTest < Minitest::Test
+  def setup
+    @cop = RuboCop::Cop::Minitest::AssertInstanceOf.new
+  end
+
+  def test_registers_offense_when_using_assert_with_instance_of
+    assert_offense(<<~RUBY, @cop)
+      class FooTest < Minitest::Test
+        def test_do_something
+          assert(object.instance_of?(SomeClass))
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer using `assert_instance_of(SomeClass, object)` over `assert(object.instance_of?(SomeClass))`.
+        end
+      end
+    RUBY
+
+    assert_correction(<<~RUBY, @cop)
+      class FooTest < Minitest::Test
+        def test_do_something
+          assert_instance_of(SomeClass, object)
+        end
+      end
+    RUBY
+  end
+
+  def test_registers_offense_when_using_assert_with_instance_of_and_message
+    assert_offense(<<~RUBY, @cop)
+      class FooTest < Minitest::Test
+        def test_do_something
+          assert(object.instance_of?(SomeClass), 'the message')
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer using `assert_instance_of(SomeClass, object, 'the message')` over `assert(object.instance_of?(SomeClass), 'the message')`.
+        end
+      end
+    RUBY
+
+    assert_correction(<<~RUBY, @cop)
+      class FooTest < Minitest::Test
+        def test_do_something
+          assert_instance_of(SomeClass, object, 'the message')
+        end
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_using_assert_instance_of_method
+    assert_no_offenses(<<~RUBY, @cop)
+      class FooTest < Minitest::Test
+        def test_do_something
+          assert_instance_of(SomeClass, object)
+        end
+      end
+    RUBY
+  end
+end