[Fix #10084] Add new Lint/RequireRelativeSelfPath cop

Fixes #10084.

This PR adds new `Lint/RequireRelativeSelfPath` cop.
It checks for uses of `require_relative` with self file path argument.

```ruby
# bad

# foo.rb
require_relative 'foo'
require_relative 'bar'

# good

# foo.rb
require_relative 'bar'
```

changelog/new_add_new_lint_require_relative_self_path_cop.md

@@ -0,0 +1 @@
+* [#10084](https://github.com/rubocop/rubocop/issues/10084): Add new `Lint/RequireRelativeSelfPath` cop. ([@koic][])

config/default.yml

@@ -2045,6 +2045,11 @@ Lint/RequireParentheses:
   Enabled: true
   VersionAdded: '0.18'
 
+Lint/RequireRelativeSelfPath:
+  Description: 'Checks for uses a file requiring itself with `require_relative`.'
+  Enabled: pending
+  VersionAdded: '<<next>>'
+
 Lint/RescueException:
   Description: 'Avoid rescuing the Exception class.'
   StyleGuide: '#no-blind-rescues'

lib/rubocop.rb

@@ -346,6 +346,7 @@
 require_relative 'rubocop/cop/lint/redundant_with_object'
 require_relative 'rubocop/cop/lint/regexp_as_condition'
 require_relative 'rubocop/cop/lint/require_parentheses'
+require_relative 'rubocop/cop/lint/require_relative_self_path'
 require_relative 'rubocop/cop/lint/rescue_exception'
 require_relative 'rubocop/cop/lint/rescue_type'
 require_relative 'rubocop/cop/lint/return_in_void_context'

lib/rubocop/cop/lint/require_relative_self_path.rb

@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Lint
+      # Checks for uses a file requiring itself with `require_relative`.
+      #
+      # @example
+      #
+      #   # bad
+      #
+      #   # foo.rb
+      #   require_relative 'foo'
+      #   require_relative 'bar'
+      #
+      #   # good
+      #
+      #   # foo.rb
+      #   require_relative 'bar'
+      #
+      class RequireRelativeSelfPath < Base
+        include RangeHelp
+        extend AutoCorrector
+
+        MSG = 'Remove the `require_relative` that requires itself.'
+        RESTRICT_ON_SEND = %i[require_relative].freeze
+
+        def on_send(node)
+          return unless (required_feature = node.first_argument)
+          return unless remove_ext(processed_source.file_path) == remove_ext(required_feature.value)
+
+          add_offense(node) do |corrector|
+            corrector.remove(range_by_whole_lines(node.source_range, include_final_newline: true))
+          end
+        end
+
+        private
+
+        def remove_ext(file_path)
+          File.basename(file_path, File.extname(file_path))
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/lint/require_relative_self_path_spec.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Lint::RequireRelativeSelfPath, :config do
+  it 'registers an offense when using `require_relative` with self file path argument' do
+    expect_offense(<<~RUBY, 'foo.rb')
+      require_relative 'foo'
+      ^^^^^^^^^^^^^^^^^^^^^^ Remove the `require_relative` that requires itself.
+      require_relative 'bar'
+    RUBY
+
+    expect_correction(<<~RUBY)
+      require_relative 'bar'
+    RUBY
+  end
+
+  it 'registers an offense when using `require_relative` with self file path argument (with ext)' do
+    expect_offense(<<~RUBY, 'foo.rb')
+      require_relative 'foo.rb'
+      ^^^^^^^^^^^^^^^^^^^^^^^^^ Remove the `require_relative` that requires itself.
+      require_relative 'bar'
+    RUBY
+
+    expect_correction(<<~RUBY)
+      require_relative 'bar'
+    RUBY
+  end
+
+  it 'does not register an offense when using `require_relative` without self file path argument' do
+    expect_no_offenses(<<~RUBY, 'foo.rb')
+      require_relative 'bar'
+    RUBY
+  end
+
+  it 'does not register an offense when using `require_relative` without argument' do
+    expect_no_offenses(<<~RUBY, 'foo.rb')
+      require_relative
+    RUBY
+  end
+end