Add new Lint/DuplicateElsifCondition cop

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 * [#7333](https://github.com/rubocop-hq/rubocop/issues/7333): Add new `Style/RedundantFileExtensionInRequire` cop. ([@fatkodima][])
 * [#8242](https://github.com/rubocop-hq/rubocop/pull/8242): Internal profiling available with `bin/rubocop-profile` and rake tasks. ([@marcandre][])
+* [#8293](https://github.com/rubocop-hq/rubocop/pull/8293): Add new `Lint/DuplicateElsifCondition` cop. ([@fatkodima][])
 * [#7736](https://github.com/rubocop-hq/rubocop/issues/7736): Add new `Style/CaseLikeIf` cop. ([@fatkodima][])
 * [#4286](https://github.com/rubocop-hq/rubocop/issues/4286): Add new `Style/HashAsLastArrayItem` cop. ([@fatkodima][])
 * [#8247](https://github.com/rubocop-hq/rubocop/issues/8247): Add new `Style/HashLikeCase` cop. ([@fatkodima][])

config/default.yml

@@ -1403,6 +1403,11 @@ Lint/DuplicateCaseCondition:
   Enabled: true
   VersionAdded: '0.45'
 
+Lint/DuplicateElsifCondition:
+  Description: 'Do not repeat conditions used in if `elsif`.'
+  Enabled: 'pending'
+  VersionAdded: '0.88'
+
 Lint/DuplicateHashKey:
   Description: 'Check for duplicate keys in hash literals.'
   Enabled: true

docs/modules/ROOT/pages/cops.adoc

@@ -193,6 +193,7 @@ In the following section you find all available cops:
 * xref:cops_lint.adoc#lintdeprecatedopensslconstant[Lint/DeprecatedOpenSSLConstant]
 * xref:cops_lint.adoc#lintdisjunctiveassignmentinconstructor[Lint/DisjunctiveAssignmentInConstructor]
 * xref:cops_lint.adoc#lintduplicatecasecondition[Lint/DuplicateCaseCondition]
+* xref:cops_lint.adoc#lintduplicateelsifcondition[Lint/DuplicateElsifCondition]
 * xref:cops_lint.adoc#lintduplicatehashkey[Lint/DuplicateHashKey]
 * xref:cops_lint.adoc#lintduplicatemethods[Lint/DuplicateMethods]
 * xref:cops_lint.adoc#linteachwithobjectargument[Lint/EachWithObjectArgument]

docs/modules/ROOT/pages/cops_lint.adoc

@@ -609,6 +609,39 @@ when 'second'
 end
 ----
 
+== Lint/DuplicateElsifCondition
+
+|===
+| Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
+
+| Pending
+| Yes
+| No
+| 0.88
+| -
+|===
+
+This cop checks that there are no repeated conditions used in if 'elsif'.
+
+=== Examples
+
+[source,ruby]
+----
+# bad
+if x == 1
+  do_something
+elsif x == 1
+  do_something_else
+end
+
+# good
+if x == 1
+  do_something
+elsif x == 2
+  do_something_else
+end
+----
+
 == Lint/DuplicateHashKey
 
 |===

lib/rubocop.rb

@@ -251,6 +251,7 @@
 require_relative 'rubocop/cop/lint/deprecated_open_ssl_constant'
 require_relative 'rubocop/cop/lint/disjunctive_assignment_in_constructor'
 require_relative 'rubocop/cop/lint/duplicate_case_condition'
+require_relative 'rubocop/cop/lint/duplicate_elsif_condition'
 require_relative 'rubocop/cop/lint/duplicate_hash_key'
 require_relative 'rubocop/cop/lint/duplicate_methods'
 require_relative 'rubocop/cop/lint/each_with_object_argument'

lib/rubocop/cop/lint/duplicate_elsif_condition.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Lint
+      # This cop checks that there are no repeated conditions used in if 'elsif'.
+      #
+      # @example
+      #   # bad
+      #   if x == 1
+      #     do_something
+      #   elsif x == 1
+      #     do_something_else
+      #   end
+      #
+      #   # good
+      #   if x == 1
+      #     do_something
+      #   elsif x == 2
+      #     do_something_else
+      #   end
+      #
+      class DuplicateElsifCondition < Base
+        MSG = 'Duplicate `elsif` condition detected.'
+
+        def on_if(node)
+          previous = []
+          while node.if? || node.elsif?
+            condition = node.condition
+            add_offense(condition) if previous.include?(condition)
+            previous << condition
+            node = node.else_branch
+            break unless node&.if_type?
+          end
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/lint/duplicate_elsif_condition_spec.rb

@@ -0,0 +1,54 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Lint::DuplicateElsifCondition do
+  subject(:cop) { described_class.new }
+
+  it 'registers an offense for repeated elsif conditions' do
+    expect_offense(<<~RUBY)
+      if x == 1
+      elsif x == 2
+      elsif x == 1
+            ^^^^^^ Duplicate `elsif` condition detected.
+      end
+    RUBY
+  end
+
+  it 'registers an offense for subsequent repeated elsif conditions' do
+    expect_offense(<<~RUBY)
+      if x == 1
+      elsif x == 2
+      elsif x == 2
+            ^^^^^^ Duplicate `elsif` condition detected.
+      end
+    RUBY
+  end
+
+  it 'registers multiple offenses for multiple repeated elsif conditions' do
+    expect_offense(<<~RUBY)
+      if x == 1
+      elsif x == 2
+      elsif x == 1
+            ^^^^^^ Duplicate `elsif` condition detected.
+      elsif x == 2
+            ^^^^^^ Duplicate `elsif` condition detected.
+      end
+    RUBY
+  end
+
+  it 'does not register an offense for non-repeated elsif conditions' do
+    expect_no_offenses(<<~RUBY)
+      if x == 1
+      elsif x == 2
+      else
+      end
+    RUBY
+  end
+
+  it 'does not register an offense for partially repeated elsif conditions' do
+    expect_no_offenses(<<~RUBY)
+      if x == 1
+      elsif x == 1 && x == 2
+      end
+    RUBY
+  end
+end