[Fix rubocop#11191] Add new Lint/NumericOperationWithConstantResult…

… cop

changelog/new_add_new_lint_numeric_operation_with_constant_result_cop.md

@@ -0,0 +1 @@
+* [#11191](https://github.com/rubocop/rubocop/issues/11191): Add new `Lint/BinaryOperatorWithIdenticalOperands` cop. ([@zopolis4][])

config/default.yml

@@ -1631,7 +1631,7 @@ Lint/BinaryOperatorWithIdenticalOperands:
   Enabled: true
   Safe: false
   VersionAdded: '0.89'
-  VersionChanged: '1.7'
+  VersionChanged: '<<next>>'
 
 Lint/BooleanSymbol:
   Description: 'Check for `:true` and `:false` symbols.'
@@ -2137,6 +2137,11 @@ Lint/NumberedParameterAssignment:
   Enabled: pending
   VersionAdded: '1.9'
 
+Lint/NumericOperationWithConstantResult:
+  Description: 'Checks for numeric operations with constant results.'
+  Enabled: pending
+  VersionAdded: '<<next>>'
+
 Lint/OrAssignmentToConstant:
   Description: 'Checks unintended or-assignment to constant.'
   Enabled: pending

lib/rubocop.rb

@@ -357,6 +357,7 @@
 require_relative 'rubocop/cop/lint/non_local_exit_from_iterator'
 require_relative 'rubocop/cop/lint/number_conversion'
 require_relative 'rubocop/cop/lint/numbered_parameter_assignment'
+require_relative 'rubocop/cop/lint/numeric_operation_with_constant_result'
 require_relative 'rubocop/cop/lint/or_assignment_to_constant'
 require_relative 'rubocop/cop/lint/ordered_magic_comments'
 require_relative 'rubocop/cop/lint/out_of_range_regexp_ref'

lib/rubocop/cop/lint/binary_operator_with_identical_operands.rb

@@ -5,17 +5,17 @@ module Cop
     module Lint
       # Checks for places where binary operator has identical operands.
       #
-      # It covers arithmetic operators: `-`, `/`, `%`;
+      # It covers comparison operators: `==`, `===`, `=~`, `>`, `>=`, `<`, ``<=``;
       # comparison operators: `==`, `===`, `=~`, `>`, `>=`, `<`, ``<=``;
       # bitwise operators: `|`, `^`, `&`;
       # boolean operators: `&&`, `||`
       # and "spaceship" operator - ``<=>``.
       #
       # Simple arithmetic operations are allowed by this cop: `+`, `*`, `**`, `<<` and `>>`.
       # Although these can be rewritten in a different way, it should not be necessary to
-      # do so. This does not include operations such as `-` or `/` where the result will
-      # always be the same (`x - x` will always be 0; `x / x` will always be 1), and
-      # thus are legitimate offenses.
+      # do so. Operations such as `-` or `/` where the result will always be the same
+      # (`x - x` will always be 0; `x / x` will always be 1) are offenses, but these
+      # are covered by Lint/NumericOperationWithConstantResult instead.
       #
       # @safety
       #   This cop is unsafe as it does not consider side effects when calling methods
@@ -30,7 +30,6 @@ module Lint
       #
       # @example
       #   # bad
-      #   x / x
       #   x.top >= x.top
       #
       #   if a.x != 0 && a.x != 0
@@ -47,13 +46,13 @@ module Lint
       #
       class BinaryOperatorWithIdenticalOperands < Base
         MSG = 'Binary operator `%<op>s` has identical operands.'
-        ALLOWED_MATH_OPERATORS = %i[+ * ** << >>].to_set.freeze
+        MATH_OPERATORS = %i[- + * / ** << >>].to_set.freeze
 
         def on_send(node)
           return unless node.binary_operation?
 
           lhs, operation, rhs = *node
-          return if ALLOWED_MATH_OPERATORS.include?(node.method_name)
+          return if MATH_OPERATORS.include?(node.method_name)
 
           add_offense(node, message: format(MSG, op: operation)) if lhs == rhs
         end

lib/rubocop/cop/lint/numeric_operation_with_constant_result.rb

@@ -0,0 +1,104 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Lint
+      # Certain numeric operations have a constant result, usually 0 or 1.
+      # Subtracting a number from itself or multiplying it by 0 will always return 0.
+      # Addiionally, a variable modulo 0 or itself will always return 0.
+      # Dividing a number by itself or raising it to the power of 0 will always return 1.
+      # As such, they can be replaced with that result.
+      # These are probably leftover from debugging, or are mistakes.
+      #
+      # @example
+      #
+      #   # bad
+      #   x - x
+      #   x * 0
+      #   x % 1
+      #   x % x
+      #
+      #   # good
+      #   0
+      #
+      #   # bad
+      #   x -= x
+      #   x *= 0
+      #   x %= 1
+      #   x %= x
+      #
+      #   # good
+      #   x = 0
+      #
+      #   # bad
+      #   x / x
+      #   x ** 0
+      #
+      #   # good
+      #   1
+      #
+      #   # bad
+      #   x /= x
+      #   x **= 0
+      #
+      #   # good
+      #   x = 1
+      #
+      class NumericOperationWithConstantResult < Base
+        extend AutoCorrector
+        MSG = 'Do not apply numeric operations with constant results to variables.'
+        RESTRICT_ON_SEND = %i[- * / % **].freeze
+
+        # @!method operation_with_constant_result?(node)
+        def_node_matcher :operation_with_constant_result?,
+                         '(send (send nil? $_) $_ ({int | send nil?} $_))'
+
+        # @!method abbreviated_assignment_with_constant_result?(node)
+        def_node_matcher :abbreviated_assignment_with_constant_result?,
+                         '(op-asgn (lvasgn $_) $_ ({int | lvar} $_))'
+
+        def on_send(node)
+          return unless operation_with_constant_result?(node)
+
+          variable, operation, number = operation_with_constant_result?(node)
+          result = constant_result?(variable, operation, number)
+          return unless result
+
+          add_offense(node) do |corrector|
+            corrector.replace(node, result.to_s)
+          end
+        end
+
+        def on_op_asgn(node)
+          return unless abbreviated_assignment_with_constant_result?(node)
+
+          variable, operation, number = abbreviated_assignment_with_constant_result?(node)
+          result = constant_result?(variable, operation, number)
+          return unless result
+
+          add_offense(node) do |corrector|
+            corrector.replace(node, "#{variable} = #{result}")
+          end
+        end
+
+        private
+
+        # rubocop :disable Metrics/CyclomaticComplexity, Metrics/PerceivedComplexity
+        def constant_result?(variable, operation, number)
+          if number.to_s == '0'
+            return 0 if operation == :*
+            return 1 if operation == :**
+          elsif number.to_s == '1'
+            return 0 if operation == :%
+          elsif number == variable
+            return 0 if %i[- %].include?(operation)
+            return 1 if operation == :/
+          end
+          # If we weren't able to find any matches, return false so we can bail out.
+          false
+        end
+        # rubocop :enable Metrics/CyclomaticComplexity, Metrics/PerceivedComplexity
+      end
+    end
+  end
+end

spec/rubocop/cop/lint/binary_operator_with_identical_operands_spec.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 RSpec.describe RuboCop::Cop::Lint::BinaryOperatorWithIdenticalOperands, :config do
-  %i[== != === <=> =~ && || - > >= < <= / % | ^].each do |operator|
+  %i[== != === <=> =~ && || > >= < <= | ^].each do |operator|
     it "registers an offense for `#{operator}` with duplicate operands" do
       expect_offense(<<~RUBY, operator: operator)
         y = a.x(arg) %{operator} a.x(arg)
@@ -10,7 +10,7 @@
     end
   end
 
-  %i[+ * ** << >>].each do |operator|
+  %i[- + * / ** << >>].each do |operator|
     it "does not register an offense for `#{operator}` with duplicate operands" do
       expect_no_offenses(<<~RUBY)
         y = a.x(arg) #{operator} a.x(arg)

spec/rubocop/cop/lint/numeric_operation_with_constant_result_spec.rb

@@ -0,0 +1,135 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Lint::NumericOperationWithConstantResult, :config do
+  it 'registers an offense when a variable is subtracted from itself' do
+    expect_offense(<<~RUBY)
+      x - x
+      ^^^^^ Do not apply numeric operations with constant results to variables.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      0
+    RUBY
+  end
+
+  it 'registers an offense when a variable is multiplied by 0' do
+    expect_offense(<<~RUBY)
+      x * 0
+      ^^^^^ Do not apply numeric operations with constant results to variables.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      0
+    RUBY
+  end
+
+  it 'registers an offense when the modulo of variable and 1 is taken' do
+    expect_offense(<<~RUBY)
+      x % 1
+      ^^^^^ Do not apply numeric operations with constant results to variables.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      0
+    RUBY
+  end
+
+  it 'registers an offense when the modulo of variable and itself is taken' do
+    expect_offense(<<~RUBY)
+      x % x
+      ^^^^^ Do not apply numeric operations with constant results to variables.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      0
+    RUBY
+  end
+
+  it 'registers an offense when a variable is divided by itself' do
+    expect_offense(<<~RUBY)
+      x / x
+      ^^^^^ Do not apply numeric operations with constant results to variables.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      1
+    RUBY
+  end
+
+  it 'registers an offense when a variable is raised to the power of 0' do
+    expect_offense(<<~RUBY)
+      x ** 0
+      ^^^^^^ Do not apply numeric operations with constant results to variables.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      1
+    RUBY
+  end
+
+  it 'registers an offense when a variable is subtracted from itself via abbreviated assignment' do
+    expect_offense(<<~RUBY)
+      x -= x
+      ^^^^^^ Do not apply numeric operations with constant results to variables.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      x = 0
+    RUBY
+  end
+
+  it 'registers an offense when a variable is multiplied by 0 via abbreviated assignment' do
+    expect_offense(<<~RUBY)
+      x *= 0
+      ^^^^^^ Do not apply numeric operations with constant results to variables.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      x = 0
+    RUBY
+  end
+
+  it 'registers an offense when the modulo of variable and 1 is taken via abbreviated assignment' do
+    expect_offense(<<~RUBY)
+      x %= 1
+      ^^^^^^ Do not apply numeric operations with constant results to variables.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      x = 0
+    RUBY
+  end
+
+  it 'registers an offense when the modulo of variable and itself is taken via abbreviated assignment' do
+    expect_offense(<<~RUBY)
+      x %= x
+      ^^^^^^ Do not apply numeric operations with constant results to variables.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      x = 0
+    RUBY
+  end
+
+  it 'registers an offense when a variable is divided by itself via abbreviated assignment' do
+    expect_offense(<<~RUBY)
+      x /= x
+      ^^^^^^ Do not apply numeric operations with constant results to variables.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      x = 1
+    RUBY
+  end
+
+  it 'egisters an offense when a variable is raised to the power of 0 via abbreviated assignment' do
+    expect_offense(<<~RUBY)
+      x **= 0
+      ^^^^^^^ Do not apply numeric operations with constant results to variables.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      x = 1
+    RUBY
+  end
+end