Implement RSpec/DescribedClassModuleWrapping cop

Here's the situation that this can help discourage within Rails code bases:

I can tell a small story as to why this should be a cop to consider. Specifically, re-opening modules in tests can confuse the [Rails autoloading mechanism](https://guides.rubyonrails.org/autoloading_and_reloading_constants.html). (Note: This is all pre-zeitwork on Rails 5.2 and earlier. I'm not sure if Rails 6+ with zeitwerk will change this.)

If you write:

```ruby
module Foo
  module Bar
    RSpec.describe Baz do
      # ...
    end
  end
end
```

And then you have code that refers to a constant defined on `Foo::Bar`, e.g. `Foo::Bar::WIDGETS`, this test code will fail if eagerloading is disabled. Why? Rails autoloading thinks it's already loaded the Foo::Bar module because it's defined.

Even though `app/models/foo/bar.rb` might define the `WIDGETS` constant, our test will not load the file because our test file has already defined the module. To confuse the matter more, turning on eager loading will cause the test to pass, because all code in `app/` will be loaded before the test executes.

Not reopening the module, e.g.

```ruby
RSpec.describe Foo::Bar::Baz do
  # ...
end
```
will cause the test to pass in both an eagerloaded and not eagerloaded configuration.

Author: kellysutton

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Master (Unreleased)
 
+* Implement `RSpec/DescribedClassModuleWrapping` to disallow RSpec statements within a module. ([@kellysutton][])
 * Fix documentation rake task to support Rubocop 0.75. ([@nickcampbell18][])
 * Fix `RSpec/SubjectStub` to detect implicit subjects stubbed. ([@QQism][])
 
@@ -457,3 +458,4 @@ Compatibility release so users can upgrade RuboCop to 0.51.0. No new features.
 [@onumis]: https://github.com/onumis
 [@nickcampbell18]: https://github.com/nickcampbell18
 [@QQism]: https://github.com/QQism
+[@kellysutton]: https://github.com/kellysutton

config/default.yml

@@ -88,6 +88,11 @@ RSpec/DescribedClass:
   - explicit
   StyleGuide: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/DescribedClass
 
+RSpec/DescribedClassModuleWrapping:
+  Description: Avoid opening modules and defining specs within them.
+  Enabled: false
+  StyleGuide: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/DescribedClassModuleWrapping
+
 RSpec/Dialect:
   Description: This cop enforces custom RSpec dialects.
   Enabled: false

lib/rubocop/cop/rspec/described_class_module_wrapping.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module RSpec
+      # Avoid opening modules and defining specs within them.
+      #
+      # @example
+      #   # bad
+      #   module MyModule
+      #     RSpec.describe MyClass do
+      #       # ...
+      #     end
+      #   end
+      #
+      #   # good
+      #   RSpec.describe MyModule::MyClass do
+      #     # ...
+      #   end
+      #
+      # @see https://github.com/rubocop-hq/rubocop-rspec/issues/735
+      class DescribedClassModuleWrapping < Cop
+        MSG = 'Avoid opening modules and defining specs within them.'
+
+        def_node_search :find_rspec_blocks,
+                        ExampleGroups::ALL.block_pattern
+
+        def on_module(node)
+          find_rspec_blocks(node) do
+            add_offense(node)
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rspec_cops.rb

@@ -25,6 +25,7 @@
 require_relative 'rspec/describe_method'
 require_relative 'rspec/describe_symbol'
 require_relative 'rspec/described_class'
+require_relative 'rspec/described_class_module_wrapping'
 require_relative 'rspec/dialect'
 require_relative 'rspec/empty_example_group'
 require_relative 'rspec/empty_line_after_example'

manual/cops.md

@@ -24,6 +24,7 @@
 * [RSpec/DescribeMethod](cops_rspec.md#rspecdescribemethod)
 * [RSpec/DescribeSymbol](cops_rspec.md#rspecdescribesymbol)
 * [RSpec/DescribedClass](cops_rspec.md#rspecdescribedclass)
+* [RSpec/DescribedClassModuleWrapping](cops_rspec.md#rspecdescribedclassmodulewrapping)
 * [RSpec/Dialect](cops_rspec.md#rspecdialect)
 * [RSpec/EmptyExampleGroup](cops_rspec.md#rspecemptyexamplegroup)
 * [RSpec/EmptyLineAfterExample](cops_rspec.md#rspecemptylineafterexample)

manual/cops_rspec.md

@@ -469,6 +469,34 @@ EnforcedStyle | `described_class` | `described_class`, `explicit`
 
 * [https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/DescribedClass](https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/DescribedClass)
 
+## RSpec/DescribedClassModuleWrapping
+
+Enabled by default | Supports autocorrection
+--- | ---
+Disabled | No
+
+Avoid opening modules and defining specs within them.
+
+### Examples
+
+```ruby
+# bad
+module MyModule
+  RSpec.describe MyClass do
+    # ...
+  end
+end
+
+# good
+RSpec.describe MyModule::MyClass do
+  # ...
+end
+```
+
+### References
+
+* [https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/DescribedClassModuleWrapping](https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/DescribedClassModuleWrapping)
+
 ## RSpec/Dialect
 
 Enabled by default | Supports autocorrection

spec/rubocop/cop/rspec/described_class_module_wrapping_spec.rb

@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::RSpec::DescribedClassModuleWrapping do
+  subject(:cop) { described_class.new }
+
+  it 'allows a describe block in the outermost scope' do
+    expect_no_offenses(<<-RUBY)
+      RSpec.describe MyClass do
+        subject { "MyClass" }
+      end
+    RUBY
+  end
+
+  it 'registers an offense when RSpec.describe is nested within a module' do
+    expect_offense(<<-RUBY)
+      module MyModule
+      ^^^^^^^^^^^^^^^ Avoid opening modules and defining specs within them.
+        RSpec.describe MyClass do
+
+          subject { "MyClass" }
+        end
+      end
+    RUBY
+  end
+
+  it 'registers an offense when RSpec.describe is nested within two modules' do
+    expect_offense(<<-RUBY)
+      module MyFirstModule
+      ^^^^^^^^^^^^^^^^^^^^ Avoid opening modules and defining specs within them.
+        module MySecondModule
+        ^^^^^^^^^^^^^^^^^^^^^ Avoid opening modules and defining specs within them.
+          RSpec.describe MyClass do
+
+            subject { "MyClass" }
+          end
+        end
+      end
+    RUBY
+  end
+
+  it 'allows a module that does not contain RSpec.describe' do
+    expect_no_offenses(<<-RUBY)
+      module MyModule
+        def some_method
+        end
+      end
+    RUBY
+  end
+end