[Fix #6779] Add new cop Style/NegatedUnless

To provide negative condition enforcement on either `if` or `unless`
but not both.

CHANGELOG.md

@@ -3,7 +3,9 @@
 ## master (unreleased)
 
 ### New features
+
 * Add support for subclassing using `Class.new` to `Lint/InheritException`. ([@houli][])
+* [#6779](https://github.com/rubocop-hq/rubocop/issues/6779): Add new cop `Style/NegativeUnless` that checks for unless with negative condition. ([@tejasbubane][])
 
 ### Bug fixes
 

config/default.yml

@@ -3507,6 +3507,20 @@ Style/NegatedIf:
     - prefix
     - postfix
 
+Style/NegatedUnless:
+  Description: 'Favor if over unless for negative conditions.'
+  StyleGuide: '#if-for-negatives'
+  Enabled: true
+  VersionAdded: '0.69'
+  EnforcedStyle: both
+  SupportedStyles:
+    # both: prefix and postfix negated `unless` should both use `if`
+    # prefix: only use `if` for negated `unless` statements positioned before the body of the statement
+    # postfix: only use `if` for negated `unless` statements positioned after the body of the statement
+    - both
+    - prefix
+    - postfix
+
 Style/NegatedWhile:
   Description: 'Favor until over while for negative conditions.'
   StyleGuide: '#until-for-negatives'

lib/rubocop.rb

@@ -464,6 +464,7 @@
 require_relative 'rubocop/cop/style/multiple_comparison'
 require_relative 'rubocop/cop/style/mutable_constant'
 require_relative 'rubocop/cop/style/negated_if'
+require_relative 'rubocop/cop/style/negated_unless'
 require_relative 'rubocop/cop/style/negated_while'
 require_relative 'rubocop/cop/style/nested_modifier'
 require_relative 'rubocop/cop/style/nested_parenthesized_calls'

lib/rubocop/cop/style/negated_if.rb

@@ -73,7 +73,7 @@ class NegatedIf < Cop
         include NegativeConditional
 
         def on_if(node)
-          return if node.elsif? || node.ternary?
+          return if node.unless? || node.elsif? || node.ternary?
           return if correct_style?(node)
 
           check_negative_conditional(node)

lib/rubocop/cop/style/negated_unless.rb

@@ -0,0 +1,89 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Style
+      # Checks for uses of unless with a negated condition. Only unless
+      # without else are considered. There are three different styles:
+      #
+      #   - both
+      #   - prefix
+      #   - postfix
+      #
+      # @example EnforcedStyle: both (default)
+      #   # enforces `if` for `prefix` and `postfix` conditionals
+      #
+      #   # bad
+      #   unless !foo
+      #     bar
+      #   end
+      #
+      #   # good
+      #   if foo
+      #     bar
+      #   end
+      #
+      #   # bad
+      #   bar unless !foo
+      #
+      #   # good
+      #   bar if foo
+      #
+      # @example EnforcedStyle: prefix
+      #   # enforces `if` for just `prefix` conditionals
+      #
+      #   # bad
+      #   unless !foo
+      #     bar
+      #   end
+      #
+      #   # good
+      #   if foo
+      #     bar
+      #   end
+      #
+      #   # good
+      #   bar unless !foo
+      #
+      # @example EnforcedStyle: postfix
+      #   # enforces `if` for just `postfix` conditionals
+      #
+      #   # bad
+      #   bar unless !foo
+      #
+      #   # good
+      #   bar if foo
+      #
+      #   # good
+      #   unless !foo
+      #     bar
+      #   end
+      class NegatedUnless < Cop
+        include ConfigurableEnforcedStyle
+        include NegativeConditional
+
+        def on_if(node)
+          return if node.if? || node.elsif? || node.ternary?
+          return if correct_style?(node)
+
+          check_negative_conditional(node)
+        end
+
+        def autocorrect(node)
+          ConditionCorrector.correct_negative_condition(node)
+        end
+
+        private
+
+        def message(node)
+          format(MSG, inverse: node.inverse_keyword, current: node.keyword)
+        end
+
+        def correct_style?(node)
+          style == :prefix && node.modifier_form? ||
+            style == :postfix && !node.modifier_form?
+        end
+      end
+    end
+  end
+end

manual/cops.md

@@ -453,6 +453,7 @@ In the following section you find all available cops:
 * [Style/MultipleComparison](cops_style.md#stylemultiplecomparison)
 * [Style/MutableConstant](cops_style.md#stylemutableconstant)
 * [Style/NegatedIf](cops_style.md#stylenegatedif)
+* [Style/NegatedUnless](cops_style.md#stylenegatedunless)
 * [Style/NegatedWhile](cops_style.md#stylenegatedwhile)
 * [Style/NestedModifier](cops_style.md#stylenestedmodifier)
 * [Style/NestedParenthesizedCalls](cops_style.md#stylenestedparenthesizedcalls)

manual/cops_style.md

@@ -3846,6 +3846,87 @@ EnforcedStyle | `both` | `both`, `prefix`, `postfix`
 
 * [https://github.com/rubocop-hq/ruby-style-guide#unless-for-negatives](https://github.com/rubocop-hq/ruby-style-guide#unless-for-negatives)
 
+## Style/NegatedUnless
+
+Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
+--- | --- | --- | --- | ---
+Enabled | Yes | Yes  | 0.69 | -
+
+Checks for uses of unless with a negated condition. Only unless
+without else are considered. There are three different styles:
+
+  - both
+  - prefix
+  - postfix
+
+### Examples
+
+#### EnforcedStyle: both (default)
+
+```ruby
+# enforces `if` for `prefix` and `postfix` conditionals
+
+# bad
+unless !foo
+  bar
+end
+
+# good
+if foo
+  bar
+end
+
+# bad
+bar unless !foo
+
+# good
+bar if foo
+```
+#### EnforcedStyle: prefix
+
+```ruby
+# enforces `if` for just `prefix` conditionals
+
+# bad
+unless !foo
+  bar
+end
+
+# good
+if foo
+  bar
+end
+
+# good
+bar unless !foo
+```
+#### EnforcedStyle: postfix
+
+```ruby
+# enforces `if` for just `postfix` conditionals
+
+# bad
+bar unless !foo
+
+# good
+bar if foo
+
+# good
+unless !foo
+  bar
+end
+```
+
+### Configurable attributes
+
+Name | Default value | Configurable values
+--- | --- | ---
+EnforcedStyle | `both` | `both`, `prefix`, `postfix`
+
+### References
+
+* [https://github.com/rubocop-hq/ruby-style-guide#if-for-negatives](https://github.com/rubocop-hq/ruby-style-guide#if-for-negatives)
+
 ## Style/NegatedWhile
 
 Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged

spec/rubocop/cop/style/negated_if_spec.rb

@@ -23,17 +23,6 @@
       RUBY
     end
 
-    it 'registers an offense for unless with exclamation point condition' do
-      expect_offense(<<~RUBY)
-        unless !a_condition
-        ^^^^^^^^^^^^^^^^^^^ Favor `if` over `unless` for negative conditions.
-          some_method
-        end
-        some_method unless !a_condition
-        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Favor `if` over `unless` for negative conditions.
-      RUBY
-    end
-
     it 'registers an offense for if with "not" condition' do
       expect_offense(<<~RUBY)
         if not a_condition
@@ -109,11 +98,6 @@
       expect(corrected).to eq 'something unless (x.even?)'
     end
 
-    it 'autocorrects by replacing unless not with if' do
-      corrected = autocorrect_source('something unless !x.even?')
-      expect(corrected).to eq 'something if x.even?'
-    end
-
     it 'autocorrects for prefix' do
       corrected = autocorrect_source(<<~RUBY)
         if !foo