Add RSpec/TopLevelDescribePath cop

The RSpec/FilePath cop checks that a spec file's path is correct, but
only if the file describes a constant. We want to check, for any file
with a top-level `describe`, whether the file path ends in
`_spec.rb`. We don't care what comes before that; just that it will be
executed by RSpec at all.

rubocop/cop/rspec/top_level_describe_path.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require 'rubocop/rspec/top_level_describe'
+
+module RuboCop
+  module Cop
+    module RSpec
+      class TopLevelDescribePath < RuboCop::Cop::Cop
+        include RuboCop::RSpec::TopLevelDescribe
+
+        MESSAGE = 'A file with a top-level `describe` must end in _spec.rb.'
+        SHARED_EXAMPLES = %i[shared_examples shared_examples_for].freeze
+
+        def on_top_level_describe(node, args)
+          return if acceptable_file_path?(processed_source.buffer.name)
+          return if shared_example?(node)
+
+          add_offense(node, message: MESSAGE)
+        end
+
+        private
+
+        def acceptable_file_path?(path)
+          File.fnmatch?('*_spec.rb', path) || File.fnmatch?('*/frontend/fixtures/*', path)
+        end
+
+        def shared_example?(node)
+          node.ancestors.any? do |node|
+            node.respond_to?(:method_name) && SHARED_EXAMPLES.include?(node.method_name)
+          end
+        end
+      end
+    end
+  end
+end

rubocop/rubocop.rb

@@ -32,6 +32,7 @@ require_relative 'cop/migration/update_large_table'
 require_relative 'cop/project_path_helper'
 require_relative 'cop/rspec/env_assignment'
 require_relative 'cop/rspec/factories_in_migration_specs'
+require_relative 'cop/rspec/top_level_describe_path'
 require_relative 'cop/qa/element_with_pattern'
 require_relative 'cop/sidekiq_options_queue'
 require_relative 'cop/destroy_all'

spec/rubocop/cop/rspec/top_level_describe_path_spec.rb

@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+
+require 'rubocop'
+require 'rubocop/rspec/support'
+
+require_relative '../../../../rubocop/cop/rspec/top_level_describe_path'
+
+describe RuboCop::Cop::RSpec::TopLevelDescribePath do
+  include RuboCop::RSpec::ExpectOffense
+  include CopHelper
+
+  subject(:cop) { described_class.new }
+
+  context 'when the file ends in _spec.rb' do
+    it 'registers no offenses' do
+      expect_no_offenses(<<~SOURCE.strip_indent, 'spec/foo_spec.rb')
+        describe 'Foo' do
+        end
+      SOURCE
+    end
+  end
+
+  context 'when the file is a frontend fixture' do
+    it 'registers no offenses' do
+      expect_no_offenses(<<~SOURCE.strip_indent, 'spec/frontend/fixtures/foo.rb')
+        describe 'Foo' do
+        end
+      SOURCE
+    end
+  end
+
+  context 'when the describe is in a shared example' do
+    context 'with shared_examples' do
+      it 'registers no offenses' do
+        expect_no_offenses(<<~SOURCE.strip_indent, 'spec/foo.rb')
+          shared_examples 'Foo' do
+            describe '#bar' do
+            end
+          end
+        SOURCE
+      end
+    end
+
+    context 'with shared_examples_for' do
+      it 'registers no offenses' do
+        expect_no_offenses(<<~SOURCE.strip_indent, 'spec/foo.rb')
+          shared_examples_for 'Foo' do
+            describe '#bar' do
+            end
+          end
+        SOURCE
+      end
+    end
+  end
+
+  context 'when the describe is at the top level' do
+    it 'marks the describe as offending' do
+      expect_offense(<<~SOURCE.strip_indent, 'spec/foo.rb')
+        describe 'Foo' do
+        ^^^^^^^^^^^^^^ #{described_class::MESSAGE}
+        end
+      SOURCE
+    end
+  end
+end