[Fix #206] Make Minitest/AssertWithExpectedArgument aware of messag…

…e variable

Fixes #206.

This PR makes `Minitest/AssertWithExpectedArgument` aware of message variable.

The second argument to the `assert` method named `message` and `msg` is allowed.
Because their names are inferred as message arguments.

change_make_minitest_assert_with_expected_argument_aware_of_message_variable.md

@@ -0,0 +1 @@
+* [#206](https://github.com/rubocop/rubocop/issues/206): Make `Minitest/AssertWithExpectedArgument` aware of message variable. ([@koic][])

lib/rubocop/cop/minitest/assert_with_expected_argument.rb

@@ -6,6 +6,9 @@ module Minitest
       # Tries to detect when a user accidentally used
       # `assert` when they meant to use `assert_equal`.
       #
+      # NOTE: The second argument to the `assert` method named `message` and `msg` is allowed.
+      #       Because their names are inferred as message arguments.
+      #
       # @safety
       #   This cop is unsafe because it is not possible to determine
       #   whether the second argument of `assert` is a message or not.
@@ -19,18 +22,21 @@ module Minitest
       #   assert_equal(3, my_list.length)
       #   assert_equal(expected, actual)
       #   assert(foo, 'message')
+      #   assert(foo, message)
+      #   assert(foo, msg)
       #
       class AssertWithExpectedArgument < Base
         MSG = 'Did you mean to use `assert_equal(%<arguments>s)`?'
         RESTRICT_ON_SEND = %i[assert].freeze
+        MESSAGE_VARIABLES = %w[message msg].freeze
 
         def_node_matcher :assert_with_two_arguments?, <<~PATTERN
           (send nil? :assert $_ $_)
         PATTERN
 
         def on_send(node)
           assert_with_two_arguments?(node) do |_expected, message|
-            return if message.str_type? || message.dstr_type?
+            return if message.str_type? || message.dstr_type? || MESSAGE_VARIABLES.include?(message.source)
 
             arguments = node.arguments.map(&:source).join(', ')
             add_offense(node, message: format(MSG, arguments: arguments))

test/rubocop/cop/minitest/assert_with_expected_argument_test.rb

@@ -47,6 +47,26 @@ def test_do_something
     RUBY
   end
 
+  def test_does_not_register_offense_when_second_argument_is_a_variable_named_message
+    assert_no_offenses(<<~RUBY)
+      class FooTest < Minitest::Test
+        def test_do_something
+          assert([], message)
+        end
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_second_argument_is_a_variable_named_msg
+    assert_no_offenses(<<~RUBY)
+      class FooTest < Minitest::Test
+        def test_do_something
+          assert([], msg)
+        end
+      end
+    RUBY
+  end
+
   def test_does_not_register_offense_when_second_argument_is_an_interpolated_string
     assert_no_offenses(<<~'RUBY')
       class FooTest < Minitest::Test