Add new Lint/IdentityComparison cop

Follow rubocop/ruby-style-guide#835.

This PR adds new `Lint/IdentityComparison` cop.

```ruby
# bad
foo.object_id == bar.object_id

# good
foo.equal?(bar)
```

The cop prefers `equal?` over `==` when comparing `object_id`.
`Object#equal?` is provided to compare objects for identity, and in contrast
`Object#==` is provided for the purpose of doing value comparison.

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 * New option `--cache-root` and support for the `RUBOCOP_CACHE_ROOT` environment variable. Both can be used to override the `AllCops: CacheRootDirectory` config, especially in a CI setting. ([@sascha-wolf][])
 * [#8582](https://github.com/rubocop-hq/rubocop/issues/8582): Add new `Layout/BeginEndAlignment` cop. ([@koic][])
+* [#8699](https://github.com/rubocop-hq/rubocop/pull/8699): Add new `Lint/IdentityComparison` cop. ([@koic][])
 
 ### Bug fixes
 

config/default.yml

@@ -1548,6 +1548,12 @@ Lint/HeredocMethodCallPosition:
   StyleGuide: '#heredoc-method-calls'
   VersionAdded: '0.68'
 
+Lint/IdentityComparison:
+  Description: 'Prefer `equal?` over `==` when comparing `object_id`.'
+  Enabled: pending
+  StyleGuide: '#identity-comparison'
+  VersionAdded: '0.91'
+
 Lint/ImplicitStringConcatenation:
   Description: >-
                  Checks for adjacent string literals on the same line, which

docs/modules/ROOT/pages/cops.adoc

@@ -216,6 +216,7 @@ In the following section you find all available cops:
 * xref:cops_lint.adoc#lintfloatoutofrange[Lint/FloatOutOfRange]
 * xref:cops_lint.adoc#lintformatparametermismatch[Lint/FormatParameterMismatch]
 * xref:cops_lint.adoc#lintheredocmethodcallposition[Lint/HeredocMethodCallPosition]
+* xref:cops_lint.adoc#lintidentitycomparison[Lint/IdentityComparison]
 * xref:cops_lint.adoc#lintimplicitstringconcatenation[Lint/ImplicitStringConcatenation]
 * xref:cops_lint.adoc#lintineffectiveaccessmodifier[Lint/IneffectiveAccessModifier]
 * xref:cops_lint.adoc#lintinheritexception[Lint/InheritException]

docs/modules/ROOT/pages/cops_lint.adoc

@@ -1600,6 +1600,38 @@ the receiver of the call is a HEREDOC.
 
 * https://rubystyle.guide#heredoc-method-calls
 
+== Lint/IdentityComparison
+
+|===
+| Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
+
+| Pending
+| Yes
+| Yes
+| 0.91
+| -
+|===
+
+Prefer `equal?` over `==` when comparing `object_id`.
+
+`Object#equal?` is provided to compare objects for identity, and in contrast
+`Object#==` is provided for the purpose of doing value comparison.
+
+=== Examples
+
+[source,ruby]
+----
+# bad
+foo.object_id == bar.object_id
+
+# good
+foo.equal?(bar)
+----
+
+=== References
+
+* https://rubystyle.guide#identity-comparison
+
 == Lint/ImplicitStringConcatenation
 
 |===

lib/rubocop.rb

@@ -278,6 +278,7 @@
 require_relative 'rubocop/cop/lint/float_out_of_range'
 require_relative 'rubocop/cop/lint/format_parameter_mismatch'
 require_relative 'rubocop/cop/lint/heredoc_method_call_position'
+require_relative 'rubocop/cop/lint/identity_comparison'
 require_relative 'rubocop/cop/lint/implicit_string_concatenation'
 require_relative 'rubocop/cop/lint/inherit_exception'
 require_relative 'rubocop/cop/lint/ineffective_access_modifier'

lib/rubocop/cop/lint/identity_comparison.rb

@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Lint
+      #
+      # Prefer `equal?` over `==` when comparing `object_id`.
+      #
+      # `Object#equal?` is provided to compare objects for identity, and in contrast
+      # `Object#==` is provided for the purpose of doing value comparison.
+      #
+      # @example
+      #   # bad
+      #   foo.object_id == bar.object_id
+      #
+      #   # good
+      #   foo.equal?(bar)
+      #
+      class IdentityComparison < Base
+        extend AutoCorrector
+
+        MSG = 'Use `equal?` instead `==` when comparing `object_id`.'
+
+        def on_send(node)
+          return unless compare_between_object_id_by_double_equal?(node)
+
+          add_offense(node) do |corrector|
+            receiver = node.receiver.receiver.source
+            argument = node.first_argument.receiver.source
+            replacement = "#{receiver}.equal?(#{argument})"
+
+            corrector.replace(node, replacement)
+          end
+        end
+
+        private
+
+        def compare_between_object_id_by_double_equal?(node)
+          return false unless node.method?(:==)
+
+          object_id_method?(node.receiver) && object_id_method?(node.first_argument)
+        end
+
+        def object_id_method?(node)
+          node.send_type? && node.method?(:object_id)
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/style/empty_literal.rb

@@ -71,7 +71,7 @@ def first_argument_unparenthesized?(node)
           parent = node.parent
           return false unless parent && %i[send super zsuper].include?(parent.type)
 
-          node.object_id == parent.arguments.first.object_id &&
+          node.equal?(parent.arguments.first) &&
             !parentheses?(node.parent)
         end
 

spec/rubocop/cop/lint/identity_comparison_spec.rb

@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Lint::IdentityComparison, :config do
+  subject(:cop) { described_class.new(config) }
+
+  it 'registers an offense and corrects when using `==` for comparison between `object_id`s' do
+    expect_offense(<<~RUBY)
+      foo.object_id == bar.object_id
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Use `equal?` instead `==` when comparing `object_id`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      foo.equal?(bar)
+    RUBY
+  end
+
+  it 'does not register an offense when using `==` for comparison between `object_id` and other' do
+    expect_no_offenses(<<~RUBY)
+      foo.object_id == bar.do_something
+    RUBY
+  end
+
+  it 'does not register an offense when a receiver that is not `object_id` uses `==`' do
+    expect_no_offenses(<<~RUBY)
+      foo.do_something == bar.object_id
+    RUBY
+  end
+
+  it 'does not register an offense when using `==`' do
+    expect_no_offenses(<<~RUBY)
+      foo.equal(bar)
+    RUBY
+  end
+end