adds rubocop rule and test

.rubocop.yml

@@ -21,6 +21,7 @@ require:
   - ./lib/rubocop/cop/lint/module_disclosure_date_format.rb
   - ./lib/rubocop/cop/lint/module_disclosure_date_present.rb
   - ./lib/rubocop/cop/lint/deprecated_gem_version.rb
+  - ./lib/rubocop/cop/lint/deprecated_file_exists.rb
   - ./lib/rubocop/cop/lint/module_enforce_notes.rb
   - ./lib/rubocop/cop/lint/detect_invalid_pack_directives.rb
 

lib/rubocop/cop/lint/deprecated_file_exists.rb

@@ -0,0 +1,46 @@
+
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Lint
+      class DeprecatedFileExists < Base
+        include RangeHelp
+        extend AutoCorrector
+
+        MSG = 'Use `File.exist?` instead of `File.exists?`.'
+
+        # @!method gem_version_const(node)
+        def_node_matcher :file_exists_const, <<~PATTERN
+          (send
+           (const {nil? cbase} :File) :exists?)
+        PATTERN
+
+        def_node_matcher :file_exists_const_cbase, <<~PATTERN
+          (send
+           (const
+            (cbase) :File) :exists?)
+        PATTERN
+
+        def on_send(node)
+          return unless file_exists_const(node)
+
+          add_offense(node, message: MSG) do |corrector|
+            autocorrect(corrector, node)
+          end
+        end
+
+        private
+
+        def autocorrect(corrector, node)
+          if file_exists_const_cbase(node)
+            corrector.replace(node, '::File.exist?')
+          else
+            corrector.replace(node, 'File.exist?')
+          end
+        end
+
+      end
+    end
+  end
+end

spec/rubocop/cop/lint/deprecated_file_exists_spec.rb

@@ -0,0 +1,31 @@
+require 'spec_helper'
+require 'rubocop/cop/lint/deprecated_file_exists'
+
+RSpec.describe RuboCop::Cop::Lint::DeprecatedFileExists do
+  subject(:cop) { described_class.new(config) }
+  let(:empty_rubocop_config) { { } }
+  let(:config) { RuboCop::Config.new(empty_rubocop_config) }
+
+  it 'corrects `File.exists?`' do
+    expect_offense(<<~RUBY)
+      File.exists?
+      ^^^^^^^^^^^^ Use `File.exist?` instead of `File.exists?`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      File.exist?
+    RUBY
+  end
+
+
+  it 'corrects `::File.exists?`' do
+    expect_offense(<<~RUBY)
+      ::File.exists?
+      ^^^^^^^^^^^^^^ Use `File.exist?` instead of `File.exists?`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      ::File.exist?
+    RUBY
+  end
+end