Add new RSpecRails/SettingsOverwritten cop

One problem I encountered recently was that I was directly rewriting `Settings` in one test, resulting in flaky tests in which other tests affected by the rewritten `Settings` succeeded or failed depending on the order of execution, making it difficult to determine the cause.
Therefore, in this PR, I would like to propose the addition of cop to encourage the use of mocks if there are any overwriting `Settings`.

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## Master (Unreleased)
 
+- Add new `RSpecRails/SettingsOverwritten` cop. ([@ydah])
+
 ## 2.28.2 (2024-03-31)
 
 - Fix a `NameError` by Cross-Referencing. ([@ydah])

config/default.yml

@@ -75,6 +75,12 @@ RSpecRails/NegationBeValid:
   VersionAdded: '2.23'
   Reference: https://www.rubydoc.info/gems/rubocop-rspec_rails/RuboCop/Cop/RSpecRails/NegationBeValid
 
+RSpecRails/SettingsOverwritten:
+  Description: Do not overwrite Settings directly, use mock instead.
+  Enabled: pending
+  VersionAdded: "<<next>>"
+  Reference: https://www.rubydoc.info/gems/rubocop-rspec_rails/RuboCop/Cop/RSpecRails/SettingsOverwritten
+
 RSpecRails/TravelAround:
   Description: Prefer to travel in `before` rather than `around`.
   Enabled: pending

lib/rubocop/cop/rspec_rails/settings_overwritten.rb

@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module RSpecRails
+      # Do not overwrite Settings directly, use mock instead.
+      #
+      # @example
+      #   # bad
+      #   Settings.foo = 'bar'
+      #   Settings[:foo] = 'bar'
+      #   Settings.foo[:bar] = 'baz'
+      #   Settings.foo += 'bar'
+      #   Settings.foo << 'bar'
+      #
+      #   # good
+      #   allow(Settings).to receive(:foo).and_return('bar')
+      #   allow(Settings).to receive(:[]).with(:foo).and_return('bar')
+      #   allow(Settings.foo).to receive(:[]).with(:bar).and_return('baz')
+      #   allow(Settings).to receive(:foo).and_return(Settings.foo + "bar")
+      #
+      class SettingsOverwritten < ::RuboCop::Cop::Base
+        MSG = 'Do not overwrite Settings directly, use mock instead.'
+
+        # @!method settings?(node)
+        def_node_matcher :settings?, <<~PATTERN
+          (const nil? :Settings)
+        PATTERN
+
+        def on_const(node)
+          return unless settings?(node)
+          return unless (assignment = first_assignment(node))
+
+          add_offense(assignment)
+        end
+
+        private
+
+        def first_assignment(node)
+          node.each_ancestor.find(&:assignment_or_similar?)
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rspec_rails_cops.rb

@@ -6,4 +6,5 @@
 require_relative 'rspec_rails/inferred_spec_type'
 require_relative 'rspec_rails/minitest_assertions'
 require_relative 'rspec_rails/negation_be_valid'
+require_relative 'rspec_rails/settings_overwritten'
 require_relative 'rspec_rails/travel_around'

spec/rubocop/cop/rspec_rails/settings_overwritten_spec.rb

@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::RSpecRails::SettingsOverwritten, :config do
+  it 'registers an offense when using `Settings.foo = "bar"`' do
+    expect_offense(<<~RUBY)
+      Settings.foo = "bar"
+      ^^^^^^^^^^^^^^^^^^^^ Do not overwrite Settings directly, use mock instead.
+    RUBY
+  end
+
+  it 'registers an offense when using `Settings[:foo] = "bar"`' do
+    expect_offense(<<~RUBY)
+      Settings[:foo] = "bar"
+      ^^^^^^^^^^^^^^^^^^^^^^ Do not overwrite Settings directly, use mock instead.
+    RUBY
+  end
+
+  it 'registers an offense when using `Settings.foo[:bar] = "baz"`' do
+    expect_offense(<<~RUBY)
+      Settings.foo[:bar] = "baz"
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^ Do not overwrite Settings directly, use mock instead.
+    RUBY
+  end
+
+  it 'registers an offense when using `Settings.foo += "bar"`' do
+    expect_offense(<<~RUBY)
+      Settings.foo += "bar"
+      ^^^^^^^^^^^^^^^^^^^^^ Do not overwrite Settings directly, use mock instead.
+    RUBY
+  end
+
+  it 'registers an offense when using `Settings.foo << "bar"`' do
+    expect_offense(<<~RUBY)
+      Settings.foo << "bar"
+      ^^^^^^^^^^^^^^^^^^^^^ Do not overwrite Settings directly, use mock instead.
+    RUBY
+  end
+
+  it 'does not register an offense when using mock' do
+    expect_no_offenses(<<~RUBY)
+      allow(Settings).to receive(:foo).and_return("bar")
+    RUBY
+  end
+end