Add new RSpec/IdenticalEqualityAssertion cop

Closes #1130

Author: tejasbubane

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## Master (Unreleased)
 
+* Add new `RSpec/IdenticalEqualityAssertion` cop. ([@tejasbubane][])
+
 ## 2.3.0 (2021-04-28)
 
 * Allow `RSpec/ContextWording` to accept multi-word prefixes. ([@hosamaly][])

config/default.yml

@@ -351,6 +351,12 @@ RSpec/HooksBeforeExamples:
   VersionAdded: '1.29'
   StyleGuide: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/HooksBeforeExamples
 
+RSpec/IdenticalEqualityAssertion:
+  Description: Checks for equality assertions with identical expressions on both sides.
+  Enabled: pending
+  VersionAdded: '2.4'
+  StyleGuide: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/IdenticalEqualityAssertion
+
 RSpec/ImplicitBlockExpectation:
   Description: Check that implicit block expectation syntax is not used.
   Enabled: true

docs/modules/ROOT/pages/cops.adoc

@@ -35,6 +35,7 @@
 * xref:cops_rspec.adoc#rspecfocus[RSpec/Focus]
 * xref:cops_rspec.adoc#rspechookargument[RSpec/HookArgument]
 * xref:cops_rspec.adoc#rspechooksbeforeexamples[RSpec/HooksBeforeExamples]
+* xref:cops_rspec.adoc#rspecidenticalequalityassertion[RSpec/IdenticalEqualityAssertion]
 * xref:cops_rspec.adoc#rspecimplicitblockexpectation[RSpec/ImplicitBlockExpectation]
 * xref:cops_rspec.adoc#rspecimplicitexpect[RSpec/ImplicitExpect]
 * xref:cops_rspec.adoc#rspecimplicitsubject[RSpec/ImplicitSubject]

docs/modules/ROOT/pages/cops_rspec.adoc

@@ -1720,6 +1720,37 @@ end
 
 * https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/HooksBeforeExamples
 
+== RSpec/IdenticalEqualityAssertion
+
+|===
+| Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
+
+| Pending
+| Yes
+| No
+| 2.4
+| -
+|===
+
+Checks for equality assertions with identical expressions on both sides.
+
+=== Examples
+
+[source,ruby]
+----
+# bad
+expect(foo.bar).to eq(foo.bar)
+expect(foo.bar).to eql(foo.bar)
+
+# good
+expect(foo.bar).to eq(2)
+expect(foo.bar).to eql(2)
+----
+
+=== References
+
+* https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/IdenticalEqualityAssertion
+
 == RSpec/ImplicitBlockExpectation
 
 |===

lib/rubocop/cop/rspec/identical_equality_assertion.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module RSpec
+      # Checks for equality assertions with identical expressions on both sides.
+      #
+      # @example
+      #
+      #   # bad
+      #   expect(foo.bar).to eq(foo.bar)
+      #   expect(foo.bar).to eql(foo.bar)
+      #
+      #   # good
+      #   expect(foo.bar).to eq(2)
+      #   expect(foo.bar).to eql(2)
+      #
+      class IdenticalEqualityAssertion < Base
+        MSG = 'Identical expressions on both sides of the equality ' \
+              'may indicate a flawed test.'
+        RESTRICT_ON_SEND = %i[to].freeze
+
+        # @!method equality_check?(node)
+        def_node_matcher :equality_check?, <<~PATTERN
+          (send (send nil? :expect $_) :to
+            {(send nil? {:eql :eq :be} $_)
+             (send (send nil? :be) :== $_)})
+        PATTERN
+
+        def on_send(node)
+          equality_check?(node) do |left, right|
+            add_offense(node) if left == right
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rspec_cops.rb

@@ -47,6 +47,7 @@
 require_relative 'rspec/focus'
 require_relative 'rspec/hook_argument'
 require_relative 'rspec/hooks_before_examples'
+require_relative 'rspec/identical_equality_assertion'
 require_relative 'rspec/implicit_block_expectation'
 require_relative 'rspec/implicit_expect'
 require_relative 'rspec/implicit_subject'

spec/rubocop/cop/rspec/identical_equality_assertion_spec.rb

@@ -0,0 +1,62 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::RSpec::IdenticalEqualityAssertion do
+  let(:msg) do
+    'Identical expressions on both sides of the equality '\
+    'may indicate a flawed test.'
+  end
+
+  it 'registers an offense when using identical expressions with `eq`' do
+    expect_offense(<<~RUBY)
+      expect(foo.bar).to eq(foo.bar)
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ #{msg}
+    RUBY
+  end
+
+  it 'registers an offense when using identical expressions with `eql`' do
+    expect_offense(<<~RUBY)
+      expect(foo.bar.baz).to eql(foo.bar.baz)
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ #{msg}
+    RUBY
+  end
+
+  it 'registers an offense for trivial constants' do
+    expect_offense(<<~RUBY)
+      expect(42).to eq(42)
+      ^^^^^^^^^^^^^^^^^^^^ #{msg}
+    RUBY
+  end
+
+  it 'registers an offense for complex constants' do
+    expect_offense(<<~RUBY)
+      expect({a: 1, b: :b}).to eql({a: 1, b: :b})
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ #{msg}
+    RUBY
+  end
+
+  it 'registers an offense for identical expression with be' do
+    expect_offense(<<~RUBY)
+      expect(foo.bar).to be(foo.bar)
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ #{msg}
+    RUBY
+  end
+
+  it 'registers an offense for identical expression with be ==' do
+    expect_offense(<<~RUBY)
+      expect(foo.bar).to be == foo.bar
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ #{msg}
+    RUBY
+  end
+
+  it 'does not register offense for different expressions' do
+    expect_no_offenses(<<~RUBY)
+      expect(foo.bar).to eq(bar.foo)
+    RUBY
+  end
+
+  it 'checks for whole expression' do
+    expect_no_offenses(<<~RUBY)
+      expect(Foo.new(1).foo).to eql(Foo.new(2).bar)
+    RUBY
+  end
+end