Accessing an unregistered cop raises useful error

Currently, when a test class includes `AssertOffense`, `AssertOffense#setup`
attempts to dynamically identify the Cop under test. If the Cop class
cannot be found, `#setup` exits early.

In these cases, the test cases continue to execute with a nil `@cop`
since it wasn't set in `#setup` as expected. When helpers such as
`#assert_offense` are used, they access `@cop` and the tests can fail
with cryptic messages such as:

```
NoMethodError: undefined method `[]=' for nil
```

lib/rubocop/minitest/assert_offense.rb

@@ -75,11 +75,13 @@ module Minitest
     module AssertOffense
       private
 
-      def setup
-        cop_name = self.class.to_s.delete_suffix('Test')
-        return unless RuboCop::Cop::Minitest.const_defined?(cop_name)
+      def cop
+        @cop ||= begin
+          cop_name = self.class.to_s.delete_suffix('Test')
+          raise "Cop not defined: #{cop_name}" unless RuboCop::Cop::Minitest.const_defined?(cop_name)
 
-        @cop = RuboCop::Cop::Minitest.const_get(cop_name).new(configuration)
+          RuboCop::Cop::Minitest.const_get(cop_name).new(configuration)
+        end
       end
 
       def format_offense(source, **replacements)
@@ -95,7 +97,7 @@ def format_offense(source, **replacements)
       def assert_no_offenses(source, file = nil)
         setup_assertion
 
-        offenses = inspect_source(source, @cop, file)
+        offenses = inspect_source(source, cop, file)
 
         expected_annotations = RuboCop::RSpec::ExpectOffense::AnnotatedSource.parse(source)
         actual_annotations = expected_annotations.with_offense_annotations(offenses)
@@ -106,7 +108,7 @@ def assert_no_offenses(source, file = nil)
       def assert_offense(source, file = nil, **replacements)
         setup_assertion
 
-        @cop.instance_variable_get(:@options)[:autocorrect] = true
+        cop.instance_variable_get(:@options)[:autocorrect] = true
 
         source = format_offense(source, **replacements)
         expected_annotations = RuboCop::RSpec::ExpectOffense::AnnotatedSource.parse(source)
@@ -116,7 +118,7 @@ def assert_offense(source, file = nil, **replacements)
 
         @processed_source = parse_source!(expected_annotations.plain_source, file)
 
-        @offenses = _investigate(@cop, @processed_source)
+        @offenses = _investigate(cop, @processed_source)
 
         actual_annotations = expected_annotations.with_offense_annotations(@offenses)
 
@@ -149,7 +151,7 @@ def assert_correction(correction, loop: true)
           # Prepare for next loop
           @processed_source = parse_source!(corrected_source, @processed_source.path)
 
-          _investigate(@cop, @processed_source)
+          _investigate(cop, @processed_source)
         end
 
         assert_equal(correction, new_source)

test/rubocop/cop/minitest/assert_offense_test.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+require_relative '../../../test_helper'
+
+class AssertOffenseTest
+  class CopNotDefinedTest < Minitest::Test
+    def test_correct_failure_is_raised_when_cop_is_not_defined
+      error = assert_raises RuntimeError do
+        assert_offense(<<~RUBY)
+          class FooTest < Minitest::Test
+            def test_do_something
+              assert_equal(nil, somestuff)
+              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer using `assert_nil(somestuff)`.
+            end
+          end
+        RUBY
+      end
+
+      assert_includes error.message, 'Cop not defined'
+    end
+  end
+end

test/rubocop/cop/minitest/test_file_name_test.rb

@@ -4,7 +4,7 @@
 
 class TestFileNameTest < Minitest::Test
   def test_registers_offense_for_invalid_path
-    offenses = inspect_source(<<~RUBY, @cop, 'lib/foo.rb')
+    offenses = inspect_source(<<~RUBY, cop, 'lib/foo.rb')
       class FooTest < Minitest::Test
       end
     RUBY
@@ -15,7 +15,7 @@ class FooTest < Minitest::Test
   end
 
   def test_registers_offense_for_namespaced_invalid_path
-    offenses = inspect_source(<<~RUBY, @cop, 'lib/foo/bar.rb')
+    offenses = inspect_source(<<~RUBY, cop, 'lib/foo/bar.rb')
       module Foo
         class BarTest < Minitest::Test
         end