Implement Style/SlicingWithRange cop

Under Ruby >= 2.6, proposes to change ary[x..-1] to ary[x..].
Supports autocorrect.

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### New features
 
+* [#7921](https://github.com/rubocop-hq/rubocop/pull/7921): Add [Style/SlicingWithRange] cop. ([@zverok][])
 * [#7895](https://github.com/rubocop-hq/rubocop/pull/7895): Include `.simplecov` file by default. ([@robotdana][])
 * [#7916](https://github.com/rubocop-hq/rubocop/pull/7916): Support autocorrection for `Lint/AmbiguousRegexpLiteral`. ([@koic][])
 * [#7917](https://github.com/rubocop-hq/rubocop/pull/7917): Support autocorrection for `Lint/UselessAccessModifier`. ([@koic][])

config/default.yml

@@ -3735,6 +3735,12 @@ Style/SingleLineMethods:
   VersionChanged: '0.19'
   AllowIfMethodIsEmpty: true
 
+Style/SlicingWithRange:
+  Description: 'Checks array slicing is done with endless ranges when suitable.'
+  Enabled: pending
+  VersionAdded: '0.83'
+  Safe: false
+
 Style/SpecialGlobalVars:
   Description: 'Avoid Perl-style global variables.'
   StyleGuide: '#no-cryptic-perlisms'

lib/rubocop.rb

@@ -542,6 +542,7 @@
 require_relative 'rubocop/cop/style/signal_exception'
 require_relative 'rubocop/cop/style/single_line_block_params'
 require_relative 'rubocop/cop/style/single_line_methods'
+require_relative 'rubocop/cop/style/slicing_with_range'
 require_relative 'rubocop/cop/style/special_global_vars'
 require_relative 'rubocop/cop/style/stabby_lambda_parentheses'
 require_relative 'rubocop/cop/style/stderr_puts'

lib/rubocop/cop/style/slicing_with_range.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Style
+      # This cop checks that arrays are sliced with endless ranges instead of
+      # `ary[start..-1]` on Ruby 2.6+.
+      #
+      # @example
+      #   # bad
+      #   items[1..-1]
+      #
+      #   # good
+      #   items[1..]
+      class SlicingWithRange < Cop
+        extend TargetRubyVersion
+
+        minimum_target_ruby_version 2.6
+
+        MSG = 'Prefer ary[n..] over ary[n..-1].'
+
+        def_node_matcher :range_till_minus_one?, '(irange (int _) (int -1))'
+
+        def on_send(node)
+          return unless node.method?(:[]) && node.arguments.count == 1
+          return unless range_till_minus_one?(node.arguments.first)
+
+          add_offense(node.arguments.first)
+        end
+
+        def autocorrect(node)
+          lambda do |corrector|
+            corrector.remove(node.end)
+          end
+        end
+      end
+    end
+  end
+end

manual/cops.md

@@ -449,6 +449,7 @@ In the following section you find all available cops:
 * [Style/SignalException](cops_style.md#stylesignalexception)
 * [Style/SingleLineBlockParams](cops_style.md#stylesinglelineblockparams)
 * [Style/SingleLineMethods](cops_style.md#stylesinglelinemethods)
+* [Style/SlicingWithRange](cops_style.md#styleslicingwithrange)
 * [Style/SpecialGlobalVars](cops_style.md#stylespecialglobalvars)
 * [Style/StabbyLambdaParentheses](cops_style.md#stylestabbylambdaparentheses)
 * [Style/StderrPuts](cops_style.md#stylestderrputs)

manual/cops_style.md

@@ -6609,6 +6609,29 @@ AllowIfMethodIsEmpty | `true` | Boolean
 
 * [https://rubystyle.guide#no-single-line-methods](https://rubystyle.guide#no-single-line-methods)
 
+## Style/SlicingWithRange
+
+!!! Note
+
+    Required Ruby version: 2.6
+
+Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
+--- | --- | --- | --- | ---
+Pending | No | Yes (Unsafe) | 0.83 | -
+
+This cop checks that arrays are sliced with endless ranges instead of
+`ary[start..-1]` on Ruby 2.6+.
+
+### Examples
+
+```ruby
+# bad
+items[1..-1]
+
+# good
+items[1..]
+```
+
 ## Style/SpecialGlobalVars
 
 Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged

spec/rubocop/cop/style/slicing_with_range_spec.rb

@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Style::SlicingWithRange, :config do
+  subject(:cop) { described_class.new(config) }
+
+  context '<= Ruby 2.5', :ruby25 do
+    it 'reports no offense for array slicing with -1' do
+      expect_no_offenses(<<~RUBY)
+        ary[1..-1]
+      RUBY
+    end
+  end
+
+  context '>= Ruby 2.6', :ruby26 do
+    it 'reports an offense for slicing to ..-1' do
+      expect_offense(<<~RUBY)
+        ary[1..-1]
+            ^^^^^ Prefer ary[n..] over ary[n..-1].
+      RUBY
+
+      expect_correction(<<~RUBY)
+        ary[1..]
+      RUBY
+    end
+
+    it 'reports no offense for excluding end' do
+      expect_no_offenses(<<~RUBY)
+        ary[1...-1]
+      RUBY
+    end
+
+    it 'reports no offense for other methods' do
+      expect_no_offenses(<<~RUBY)
+        ary.push(1..-1)
+      RUBY
+    end
+
+    it 'reports no offense for array with range inside' do
+      expect_no_offenses(<<~RUBY)
+        ranges = [1..-1]
+      RUBY
+    end
+  end
+
+  context '>= Ruby 2.7', :ruby27 do
+    it 'reports no offense for startless' do
+      expect_no_offenses(<<~RUBY)
+        ary[..-1]
+      RUBY
+    end
+  end
+end