Add new Style/FileDirnameLevel cop

changelog/new_add_new_style_nested_file_dirname_cop.md

@@ -0,0 +1 @@
+* [#10419](https://github.com/rubocop/rubocop/pull/10419): Add new `Style/NestedFileDirname` cop. ([@koic][])

config/default.yml

@@ -4124,6 +4124,11 @@ Style/NegatedWhile:
   Enabled: true
   VersionAdded: '0.20'
 
+Style/NestedFileDirname:
+  Description: 'Checks for nested `File.dirname`.'
+  Enabled: pending
+  VersionAdded: '<<next>>'
+
 Style/NestedModifier:
   Description: 'Avoid using nested modifiers.'
   StyleGuide: '#no-nested-modifiers'

lib/rubocop.rb

@@ -552,6 +552,7 @@
 require_relative 'rubocop/cop/style/negated_if_else_condition'
 require_relative 'rubocop/cop/style/negated_unless'
 require_relative 'rubocop/cop/style/negated_while'
+require_relative 'rubocop/cop/style/nested_file_dirname'
 require_relative 'rubocop/cop/style/nested_modifier'
 require_relative 'rubocop/cop/style/nested_parenthesized_calls'
 require_relative 'rubocop/cop/style/nested_ternary_operator'

lib/rubocop/cop/style/nested_file_dirname.rb

@@ -0,0 +1,66 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Style
+      # This cop checks for nested `File.dirname`.
+      # It replaces nested `File.dirname` with the level argument introduced in Ruby 3.1.
+      #
+      # @example
+      #
+      #   # bad
+      #   File.dirname(File.dirname(path))
+      #
+      #   # good
+      #   File.dirname(path, 2)
+      #
+      class NestedFileDirname < Base
+        include RangeHelp
+        extend AutoCorrector
+        extend TargetRubyVersion
+
+        MSG = 'Use `dirname(%<path>s, %<level>s)` instead.'
+        RESTRICT_ON_SEND = %i[dirname].freeze
+
+        minimum_target_ruby_version 3.1
+
+        # @!method file_dirname?(node)
+        def_node_matcher :file_dirname?, <<~PATTERN
+          (send
+            (const {cbase nil?} :File) :dirname ...)
+        PATTERN
+
+        def on_send(node)
+          return if file_dirname?(node.parent) || !file_dirname?(node.first_argument)
+
+          path, level = path_with_dir_level(node, 1)
+          return if level < 2
+
+          message = format(MSG, path: path, level: level)
+          range = offense_range(node)
+
+          add_offense(range, message: message) do |corrector|
+            corrector.replace(range, "dirname(#{path}, #{level})")
+          end
+        end
+
+        private
+
+        def path_with_dir_level(node, level)
+          first_argument = node.first_argument
+
+          if file_dirname?(first_argument)
+            level += 1
+            path_with_dir_level(first_argument, level)
+          else
+            [first_argument.source, level]
+          end
+        end
+
+        def offense_range(node)
+          range_between(node.loc.selector.begin_pos, node.source_range.end_pos)
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/style/nested_file_dirname_spec.rb

@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Style::NestedFileDirname, :config do
+  context 'Ruby >= 3.1', :ruby31 do
+    it 'registers and corrects an offense when using `File.dirname(path)` nested two times' do
+      expect_offense(<<~RUBY)
+        File.dirname(File.dirname(path))
+             ^^^^^^^^^^^^^^^^^^^^^^^^^^^ Use `dirname(path, 2)` instead.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        File.dirname(path, 2)
+      RUBY
+    end
+
+    it 'registers and corrects an offense when using `File.dirname(path)` nested three times' do
+      expect_offense(<<~RUBY)
+        File.dirname(File.dirname(File.dirname(path)))
+             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Use `dirname(path, 3)` instead.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        File.dirname(path, 3)
+      RUBY
+    end
+
+    it 'does not register an offense when using non nested `File.dirname(path)`' do
+      expect_no_offenses(<<~RUBY)
+        File.dirname(path)
+      RUBY
+    end
+
+    it 'does not register an offense when using `File.dirname(path, 2)`' do
+      expect_no_offenses(<<~RUBY)
+        File.dirname(path, 2)
+      RUBY
+    end
+  end
+
+  context 'Ruby <= 3.0', :ruby30 do
+    it 'does not register an offense when using `File.dirname(path)` nested two times' do
+      expect_no_offenses(<<~RUBY)
+        File.dirname(File.dirname(path))
+      RUBY
+    end
+  end
+end