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.

1 files changed, 111 insertions, 0 deletions

diff --git a/spec/rubocop/cop/custom_error_class_spec.rb b/spec/rubocop/cop/custom_error_class_spec.rb
new file mode 100644
index 00000000000..381d7871a40
--- /dev/null
+++ b/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