Add new Lint/RedundantWithIndex cop

CHANGELOG.md

@@ -30,6 +30,7 @@
 * [#4694](https://github.com/bbatsov/rubocop/pull/4694): Add new `Lint/UriRegexp` cop. ([@koic][])
 * Add new `Style/MinMax` cop. ([@drenmi][])
 * [#4720](https://github.com/bbatsov/rubocop/pull/4720): Add new `Bundler/InsecureProtocolSource` cop. ([@koic][])
+* [#4708](https://github.com/bbatsov/rubocop/pull/4708): Add new `Lint/RedundantWithIndex` cop`. ([@koic][])
 
 ### Bug fixes
 

config/enabled.yml

@@ -1327,6 +1327,10 @@ Lint/RandOne:
                  and most likely a mistake.
   Enabled: true
 
+Lint/RedundantWithIndex:
+  Description: 'Checks for redundant `with_index`.'
+  Enabled: true
+
 Lint/RequireParentheses:
   Description: >-
                  Use parentheses in the method call to avoid confusion

lib/rubocop.rb

@@ -252,6 +252,7 @@
 require 'rubocop/cop/lint/percent_string_array'
 require 'rubocop/cop/lint/percent_symbol_array'
 require 'rubocop/cop/lint/rand_one'
+require 'rubocop/cop/lint/redundant_with_index'
 require 'rubocop/cop/lint/require_parentheses'
 require 'rubocop/cop/lint/rescue_exception'
 require 'rubocop/cop/lint/rescue_type'

lib/rubocop/cop/lint/redundant_with_index.rb

@@ -0,0 +1,77 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Lint
+      # This cop checks for redundant `with_index`.
+      #
+      # @example
+      #   # bad
+      #   ary.each_with_index do |v|
+      #     v
+      #   end
+      #
+      #   # good
+      #   ary.each do |v|
+      #     v
+      #   end
+      #
+      #   # bad
+      #   ary.each.with_index do |v|
+      #     v
+      #   end
+      #
+      #   # good
+      #   ary.each do |v|
+      #     v
+      #   end
+      #
+      class RedundantWithIndex < Cop
+        MSG_EACH_WITH_INDEX = 'Use `each` instead of `each_with_index`.'.freeze
+        MSG_WITH_INDEX = 'Remove redundant `with_index`.'.freeze
+
+        def_node_matcher :redundant_with_index?, <<-PATTERN
+          (block
+            $(send
+              _ {:each_with_index :with_index})
+            (args
+              (arg _))
+            ...)
+        PATTERN
+
+        def on_block(node)
+          redundant_with_index?(node) do |send|
+            add_offense(node, with_index_range(send))
+          end
+        end
+
+        def autocorrect(node)
+          lambda do |corrector|
+            redundant_with_index?(node) do |send|
+              if send.method_name == :each_with_index
+                corrector.replace(send.loc.selector, 'each')
+              else
+                corrector.remove(send.loc.selector)
+                corrector.remove(send.loc.dot)
+              end
+            end
+          end
+        end
+
+        private
+
+        def message(node)
+          if node.method_name == :each_with_index
+            MSG_EACH_WITH_INDEX
+          else
+            MSG_WITH_INDEX
+          end
+        end
+
+        def with_index_range(send)
+          range_between(send.loc.selector.begin_pos, send.loc.selector.end_pos)
+        end
+      end
+    end
+  end
+end

manual/cops.md

@@ -209,6 +209,7 @@ In the following section you find all available cops:
 * [Lint/PercentStringArray](cops_lint.md#lintpercentstringarray)
 * [Lint/PercentSymbolArray](cops_lint.md#lintpercentsymbolarray)
 * [Lint/RandOne](cops_lint.md#lintrandone)
+* [Lint/RedundantWithIndex](cops_lint.md#lintredundantwithindex)
 * [Lint/RequireParentheses](cops_lint.md#lintrequireparentheses)
 * [Lint/RescueException](cops_lint.md#lintrescueexception)
 * [Lint/RescueType](cops_lint.md#lintrescuetype)

manual/cops_lint.md

@@ -1431,6 +1431,38 @@ rand(-1.0)
 0 # just use 0 instead
 ```
 
+## Lint/RedundantWithIndex
+
+Enabled by default | Supports autocorrection
+--- | ---
+Enabled | Yes
+
+This cop checks for redundant `with_index`.
+
+### Example
+
+```ruby
+# bad
+ary.each_with_index do |v|
+  v
+end
+
+# good
+ary.each do |v|
+  v
+end
+
+# bad
+ary.each.with_index do |v|
+  v
+end
+
+# good
+ary.each do |v|
+  v
+end
+```
+
 ## Lint/RequireParentheses
 
 Enabled by default | Supports autocorrection

spec/rubocop/cop/lint/redundant_with_index_spec.rb

@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+describe RuboCop::Cop::Lint::RedundantWithIndex do
+  let(:config) { RuboCop::Config.new }
+  subject(:cop) { described_class.new(config) }
+
+  it 'registers an offense when using `ary.each_with_index { |v| v }`' do
+    expect_offense(<<-RUBY.strip_indent)
+      ary.each_with_index { |v| v }
+          ^^^^^^^^^^^^^^^ Use `each` instead of `each_with_index`.
+    RUBY
+  end
+
+  it 'registers an offense when using `ary.each.with_index { |v| v }`' do
+    expect_offense(<<-RUBY.strip_indent)
+      ary.each.with_index { |v| v }
+               ^^^^^^^^^^ Remove redundant `with_index`.
+    RUBY
+  end
+
+  it 'registers an offense when using `ary.each_with_object([]).with_index ' \
+     '{ |v| v }`' do
+    expect_offense(<<-RUBY.strip_indent)
+      ary.each_with_object([]).with_index { |v| v }
+                               ^^^^^^^^^^ Remove redundant `with_index`.
+    RUBY
+  end
+
+  it 'autocorrects to ary.each from ary.each_with_index' do
+    new_source = autocorrect_source('ary.each_with_index { |v| v }')
+
+    expect(new_source).to eq 'ary.each { |v| v }'
+  end
+
+  it 'autocorrects to ary.each from ary.each.with_index' do
+    new_source = autocorrect_source('ary.each.with_index { |v| v }')
+
+    expect(new_source).to eq 'ary.each { |v| v }'
+  end
+
+  it 'autocorrects to ary.each from ary.each_with_object([]).with_index' do
+    new_source = autocorrect_source('ary.each_with_object([]) { |v| v }')
+
+    expect(new_source).to eq 'ary.each_with_object([]) { |v| v }'
+  end
+
+  it 'an index is used as a block argument' do
+    expect_no_offenses('ary.each_with_index { |v, i| v; i }')
+  end
+end

spec/rubocop/cop/rails/validation_spec.rb

@@ -16,7 +16,7 @@
     end
   end
 
-  described_class::TYPES.each_with_index do |parameter|
+  described_class::TYPES.each do |parameter|
     it "autocorrect validates_#{parameter}_of" do
       new_source = autocorrect_source(
         "validates_#{parameter}_of :full_name, :birth_date"