Add RuboCop cop for custom error classes

From the Ruby style guide:

    # bad
    class FooError < StandardError
    end

    # okish
    class FooError < StandardError; end

    # good
    FooError = Class.new(StandardError)

This cop does that, but only for error classes (classes where the
superclass ends in 'Error'). We have empty controllers and models, which
are perfectly valid empty classes.

rubocop/cop/custom_error_class.rb

@@ -0,0 +1,64 @@
+module RuboCop
+  module Cop
+    # This cop makes sure that custom error classes, when empty, are declared
+    # with Class.new.
+    #
+    # @example
+    #   # bad
+    #   class FooError < StandardError
+    #   end
+    #
+    #   # okish
+    #   class FooError < StandardError; end
+    #
+    #   # good
+    #   FooError = Class.new(StandardError)
+    class CustomErrorClass < RuboCop::Cop::Cop
+      MSG = 'Use `Class.new(SuperClass)` to define an empty custom error class.'.freeze
+
+      def on_class(node)
+        _klass, parent, body = node.children
+
+        return if body
+
+        parent_klass = class_name_from_node(parent)
+
+        return unless parent_klass && parent_klass.to_s.end_with?('Error')
+
+        add_offense(node, :expression)
+      end
+
+      def autocorrect(node)
+        klass, parent, _body = node.children
+        replacement = "#{class_name_from_node(klass)} = Class.new(#{class_name_from_node(parent)})"
+
+        lambda do |corrector|
+          corrector.replace(node.source_range, replacement)
+        end
+      end
+
+      private
+
+      # The nested constant `Foo::Bar::Baz` looks like:
+      #
+      #   s(:const,
+      #     s(:const,
+      #       s(:const, nil, :Foo), :Bar), :Baz)
+      #
+      # So recurse through that to get the name as written in the source.
+      #
+      def class_name_from_node(node, suffix = nil)
+        return unless node&.type == :const
+
+        name = node.children[1].to_s
+        name = "#{name}::#{suffix}" if suffix
+
+        if node.children[0]
+          class_name_from_node(node.children[0], name)
+        else
+          name
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/custom_error_class_spec.rb

@@ -0,0 +1,111 @@
+require 'spec_helper'
+
+require 'rubocop'
+require 'rubocop/rspec/support'
+
+require_relative '../../../rubocop/cop/custom_error_class'
+
+describe RuboCop::Cop::CustomErrorClass do
+  include CopHelper
+
+  subject(:cop) { described_class.new }
+
+  context 'when a class has a body' do
+    it 'does nothing' do
+      inspect_source(cop, 'class CustomError < StandardError; def foo; end; end')
+
+      expect(cop.offenses).to be_empty
+    end
+  end
+
+  context 'when a class has no explicit superclass' do
+    it 'does nothing' do
+      inspect_source(cop, 'class CustomError; end')
+
+      expect(cop.offenses).to be_empty
+    end
+  end
+
+  context 'when a class has a superclass that does not end in Error' do
+    it 'does nothing' do
+      inspect_source(cop, 'class CustomError < BasicObject; end')
+
+      expect(cop.offenses).to be_empty
+    end
+  end
+
+  context 'when a class is empty and inherits from a class ending in Error' do
+    context 'when the class is on a single line' do
+      let(:source) do
+        <<-SOURCE
+          module Foo
+            class CustomError < Bar::Baz::BaseError; end
+          end
+        SOURCE
+      end
+
+      let(:expected) do
+        <<-EXPECTED
+          module Foo
+            CustomError = Class.new(Bar::Baz::BaseError)
+          end
+        EXPECTED
+      end
+
+      it 'registers an offense' do
+        expected_highlights = source.split("\n")[1].strip
+
+        inspect_source(cop, source)
+
+        aggregate_failures do
+          expect(cop.offenses.size).to eq(1)
+          expect(cop.offenses.map(&:line)).to eq([2])
+          expect(cop.highlights).to contain_exactly(expected_highlights)
+        end
+      end
+
+      it 'autocorrects to the right version' do
+        autocorrected = autocorrect_source(cop, source, 'foo/custom_error.rb')
+
+        expect(autocorrected).to eq(expected)
+      end
+    end
+
+    context 'when the class is on multiple lines' do
+      let(:source) do
+        <<-SOURCE
+          module Foo
+            class CustomError < Bar::Baz::BaseError
+            end
+          end
+        SOURCE
+      end
+
+      let(:expected) do
+        <<-EXPECTED
+          module Foo
+            CustomError = Class.new(Bar::Baz::BaseError)
+          end
+        EXPECTED
+      end
+
+      it 'registers an offense' do
+        expected_highlights = source.split("\n")[1..2].join("\n").strip
+
+        inspect_source(cop, source)
+
+        aggregate_failures do
+          expect(cop.offenses.size).to eq(1)
+          expect(cop.offenses.map(&:line)).to eq([2])
+          expect(cop.highlights).to contain_exactly(expected_highlights)
+        end
+      end
+
+      it 'autocorrects to the right version' do
+        autocorrected = autocorrect_source(cop, source, 'foo/custom_error.rb')
+
+        expect(autocorrected).to eq(expected)
+      end
+    end
+  end
+end