Add new Style/NumberedParametersLimit cop.

changelog/new_add_new_styleexcessivenumberedparameters.md

@@ -0,0 +1 @@
+* [#10111](https://github.com/rubocop/rubocop/pull/10111): Add new `Style/NumberedParametersLimit` cop. ([@dvandersluis][])

config/default.yml

@@ -4135,6 +4135,12 @@ Style/Not:
   VersionAdded: '0.9'
   VersionChanged: '0.20'
 
+Style/NumberedParametersLimit:
+  Description: 'Avoid excessive numbered params in a single block.'
+  Enabled: pending
+  VersionAdded: '<<next>>'
+  Max: 1
+
 Style/NumericLiteralPrefix:
   Description: 'Use smallcase prefixes for numeric literals.'
   StyleGuide: '#numeric-literal-prefixes'

lib/rubocop.rb

@@ -549,6 +549,7 @@
 require_relative 'rubocop/cop/style/nil_lambda'
 require_relative 'rubocop/cop/style/non_nil_check'
 require_relative 'rubocop/cop/style/not'
+require_relative 'rubocop/cop/style/numbered_parameters_limit'
 require_relative 'rubocop/cop/style/numeric_literals'
 require_relative 'rubocop/cop/style/numeric_literal_prefix'
 require_relative 'rubocop/cop/style/numeric_predicate'

lib/rubocop/cop/style/numbered_parameters_limit.rb

@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Style
+      # This cop detects use of an excessive amount of numbered parameters in a
+      # single block. Having too many numbered parameters can make code too
+      # cryptic and hard to read.
+      #
+      # The cop defaults to registering an offense if there is more than 1 numbered
+      # parameter but this maximum can be configured by setting `Max`.
+      #
+      # @example Max: 1 (default)
+      #   # bad
+      #   foo { _1.call(_2, _3, _4) }
+      #
+      #   # good
+      #   foo { do_something(_1) }
+      class NumberedParametersLimit < Base
+        extend TargetRubyVersion
+        extend ExcludeLimit
+
+        DEFAULT_MAX_VALUE = 1
+
+        minimum_target_ruby_version 2.7
+        exclude_limit 'Max'
+
+        MSG = 'Avoid using more than %<max>i numbered %<parameter>s; %<count>i detected.'
+
+        def on_numblock(node)
+          _send_node, param_count, * = *node
+          return if param_count <= max_count
+
+          parameter = max_count > 1 ? 'parameters' : 'parameter'
+          message = format(MSG, max: max_count, parameter: parameter, count: param_count)
+          add_offense(node, message: message) { self.max = param_count }
+        end
+
+        private
+
+        def max_count
+          max = cop_config.fetch('Max', DEFAULT_MAX_VALUE)
+
+          # Ruby does not allow more than 9 numbered parameters
+          [max, 9].min
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/style/numbered_parameters_limit_spec.rb

@@ -0,0 +1,74 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Style::NumberedParametersLimit, :config do
+  let(:cop_config) { { 'Max' => max } }
+  let(:max) { 2 }
+
+  context 'with Ruby >= 2.7', :ruby27 do
+    it 'does not register an offense for a normal block with too many parameters' do
+      expect_no_offenses(<<~RUBY)
+        foo { |a, b, c, d, e, f, g| do_something(a,b,c,d,e,f,g) }
+      RUBY
+    end
+
+    it 'does not register an offense for a numblock with fewer than `Max` parameters' do
+      expect_no_offenses(<<~RUBY)
+        foo { do_something(_1) }
+      RUBY
+    end
+
+    it 'does not register an offense for a numblock with exactly `Max` parameters' do
+      expect_no_offenses(<<~RUBY)
+        foo { do_something(_1, _2) }
+      RUBY
+    end
+
+    context 'when there are more than `Max` numbered parameters' do
+      it 'registers an offense for a single line `numblock`' do
+        expect_offense(<<~RUBY)
+          foo { do_something(_1, _2, _3, _4, _5) }
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Avoid using more than 2 numbered parameters; 5 detected.
+        RUBY
+      end
+
+      it 'registers an offense for a multiline `numblock`' do
+        expect_offense(<<~RUBY)
+          foo do
+          ^^^^^^ Avoid using more than 2 numbered parameters; 5 detected.
+            do_something(_1, _2, _3, _4, _5)
+          end
+        RUBY
+      end
+    end
+
+    context 'when configuring Max' do
+      let(:max) { 5 }
+
+      it 'does not register an offense when there are not too many numbered params' do
+        expect_no_offenses(<<~RUBY)
+          foo { do_something(_1, _2, _3, _4, _5) }
+        RUBY
+      end
+    end
+
+    context 'when Max is 1' do
+      let(:max) { 1 }
+
+      it 'uses the right offense message' do
+        expect_offense(<<~RUBY)
+          foo { do_something(_1, _2, _3, _4, _5) }
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Avoid using more than 1 numbered parameter; 5 detected.
+        RUBY
+      end
+    end
+
+    it 'sets Max properly for auto-gen-config' do
+      expect_offense(<<~RUBY)
+        foo { do_something(_1, _2, _3, _4, _5) }
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Avoid using more than 2 numbered parameters; 5 detected.
+      RUBY
+
+      expect(cop.config_to_allow_offenses).to eq(exclude_limit: { 'Max' => 5 })
+    end
+  end
+end