Add new cop RSpec/LeakyLocalVariable

Author: lovro-bikic

.rubocop.yml

@@ -293,3 +293,4 @@ Performance/ZipWithoutBlock: {Enabled: true}
 # Enable our own pending cops.
 
 RSpec/IncludeExamples: {Enabled: true}
+RSpec/LeakyLocalVariable: {Enabled: true}

CHANGELOG.md

@@ -6,6 +6,7 @@
 - Fix a false positive for `RSpec/LeakyConstantDeclaration` when defining constants in explicit namespaces. ([@naveg])
 - Add support for error matchers (`raise_exception` and `raise_error`) to `RSpec/Dialect`. ([@lovro-bikic])
 - Don't register offenses for `RSpec/DescribedClass` within `Data.define` blocks. ([@lovro-bikic])
+- Add new cop `RSpec/LeakyLocalVariable`. ([@lovro-bikic])
 
 ## 3.6.0 (2025-04-18)
 

config/default.yml

@@ -606,6 +606,12 @@ RSpec/LeakyConstantDeclaration:
   StyleGuide: https://rspec.rubystyle.guide/#declare-constants
   Reference: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/LeakyConstantDeclaration
 
+RSpec/LeakyLocalVariable:
+  Description: Checks for outside local variables used in examples.
+  Enabled: pending
+  VersionAdded: "<<next>>"
+  Reference: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/LeakyLocalVariable
+
 RSpec/LetBeforeExamples:
   Description: Checks for `let` definitions that come after an example.
   Enabled: true

docs/modules/ROOT/pages/cops.adoc

@@ -59,6 +59,7 @@
 * xref:cops_rspec.adoc#rspeciteratedexpectation[RSpec/IteratedExpectation]
 * xref:cops_rspec.adoc#rspecleadingsubject[RSpec/LeadingSubject]
 * xref:cops_rspec.adoc#rspecleakyconstantdeclaration[RSpec/LeakyConstantDeclaration]
+* xref:cops_rspec.adoc#rspecleakylocalvariable[RSpec/LeakyLocalVariable]
 * xref:cops_rspec.adoc#rspecletbeforeexamples[RSpec/LetBeforeExamples]
 * xref:cops_rspec.adoc#rspecletsetup[RSpec/LetSetup]
 * xref:cops_rspec.adoc#rspecmatcharray[RSpec/MatchArray]

docs/modules/ROOT/pages/cops_rspec.adoc

@@ -3391,6 +3391,88 @@ end
 * https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/LeakyConstantDeclaration
 * https://rspec.info/features/3-12/rspec-mocks/mutating-constants
 
+[#rspecleakylocalvariable]
+== RSpec/LeakyLocalVariable
+
+|===
+| Enabled by default | Safe | Supports autocorrection | Version Added | Version Changed
+
+| Pending
+| Yes
+| No
+| <<next>>
+| -
+|===
+
+Checks for outside local variables used in examples.
+
+Local variables assigned outside an example but used within it act
+as shared state, which can make tests non-deterministic.
+
+[#examples-rspecleakylocalvariable]
+=== Examples
+
+[source,ruby]
+----
+# bad - outside variable used in a hook
+user = create(:user)
+
+before { user.update(admin: true) }
+
+# good
+let(:user) { create(:user) }
+
+before { user.update(admin: true) }
+
+# bad - outside variable used in an example
+user = create(:user)
+
+it 'is persisted' do
+  expect(user).to be_persisted
+end
+
+# good
+let(:user) { create(:user) }
+
+it 'is persisted' do
+  expect(user).to be_persisted
+end
+
+# also good - assigning the variable within the example
+it 'is persisted' do
+  user = create(:user)
+
+  expect(user).to be_persisted
+end
+
+# bad - outside variable passed to included examples
+attrs = ['foo', 'bar']
+
+it_behaves_like 'some examples', attrs
+
+# good
+it_behaves_like 'some examples' do
+  let(:attrs) { ['foo', 'bar'] }
+end
+
+# good - when variable is used only as example description
+attribute = 'foo'
+
+it "#{attribute} is persisted" do
+  expectations
+end
+
+# good - when variable is used only to include other examples
+examples = foo ? 'some examples' : 'other examples'
+
+it_behaves_like examples, another_argument
+----
+
+[#references-rspecleakylocalvariable]
+=== References
+
+* https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/LeakyLocalVariable
+
 [#rspecletbeforeexamples]
 == RSpec/LetBeforeExamples
 

lib/rubocop/cop/rspec/leaky_local_variable.rb

@@ -0,0 +1,137 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module RSpec
+      # Checks for outside local variables used in examples.
+      #
+      # Local variables assigned outside an example but used within it act
+      # as shared state, which can make tests non-deterministic.
+      #
+      # @example
+      #   # bad - outside variable used in a hook
+      #   user = create(:user)
+      #
+      #   before { user.update(admin: true) }
+      #
+      #   # good
+      #   let(:user) { create(:user) }
+      #
+      #   before { user.update(admin: true) }
+      #
+      #   # bad - outside variable used in an example
+      #   user = create(:user)
+      #
+      #   it 'is persisted' do
+      #     expect(user).to be_persisted
+      #   end
+      #
+      #   # good
+      #   let(:user) { create(:user) }
+      #
+      #   it 'is persisted' do
+      #     expect(user).to be_persisted
+      #   end
+      #
+      #   # also good - assigning the variable within the example
+      #   it 'is persisted' do
+      #     user = create(:user)
+      #
+      #     expect(user).to be_persisted
+      #   end
+      #
+      #   # bad - outside variable passed to included examples
+      #   attrs = ['foo', 'bar']
+      #
+      #   it_behaves_like 'some examples', attrs
+      #
+      #   # good
+      #   it_behaves_like 'some examples' do
+      #     let(:attrs) { ['foo', 'bar'] }
+      #   end
+      #
+      #   # good - when variable is used only as example description
+      #   attribute = 'foo'
+      #
+      #   it "#{attribute} is persisted" do
+      #     expectations
+      #   end
+      #
+      #   # good - when variable is used only to include other examples
+      #   examples = foo ? 'some examples' : 'other examples'
+      #
+      #   it_behaves_like examples, another_argument
+      #
+      class LeakyLocalVariable < Base
+        MSG = 'Use `let` instead of a local variable which can leak ' \
+              'between examples.'
+
+        # @!method example_method?(node)
+        def_node_matcher :example_method?, <<~PATTERN
+          (send nil? #Examples.all _)
+        PATTERN
+
+        # @!method includes_method?(node)
+        def_node_matcher :includes_method?, <<~PATTERN
+          (send nil? #Includes.all ...)
+        PATTERN
+
+        def self.joining_forces
+          VariableForce
+        end
+
+        def after_leaving_scope(scope, _variable_table)
+          scope.variables.each_value { |variable| check_references(variable) }
+        end
+
+        private
+
+        def check_references(variable)
+          variable.assignments.each do |assignment|
+            next if part_of_example_scope?(assignment.node)
+
+            assignment.references.each do |reference|
+              next unless part_of_example_scope?(reference)
+              next if permitted_argument?(reference)
+
+              add_offense(assignment.node)
+            end
+          end
+        end
+
+        def permitted_argument?(node)
+          node.each_ancestor.any? do |ancestor|
+            next true if example_method?(ancestor)
+            if includes_method?(ancestor)
+              next includes_arguments_permitted?(ancestor, node)
+            end
+
+            false
+          end
+        end
+
+        def includes_arguments_permitted?(node, argument)
+          node.arguments[1..].all? do |argument_node|
+            permitted_include_argument?(argument_node, argument)
+          end
+        end
+
+        def permitted_include_argument?(argument_node, argument)
+          return true if argument_node.type?(:dstr, :dsym)
+
+          argument_node != argument &&
+            argument_node.each_descendant.none?(argument)
+        end
+
+        def part_of_example_scope?(node)
+          node.each_ancestor.any? { |ancestor| example_scope?(ancestor) }
+        end
+
+        def example_scope?(node)
+          subject?(node) || let?(node) || hook?(node) || example?(node) ||
+            include?(node)
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rspec_cops.rb

@@ -57,6 +57,7 @@
 require_relative 'rspec/iterated_expectation'
 require_relative 'rspec/leading_subject'
 require_relative 'rspec/leaky_constant_declaration'
+require_relative 'rspec/leaky_local_variable'
 require_relative 'rspec/let_before_examples'
 require_relative 'rspec/let_setup'
 require_relative 'rspec/match_array'