[Fix #10204] Add new Lint/UnsafeGlobalVariables cop

changelog/new_add_new_lintunsafeglobalvariables_cop.md

@@ -0,0 +1 @@
+* [#10204](https://github.com/rubocop/rubocop/issues/10204): Add new `Lint/UnsafeGlobalVariables` cop. ([@dmarcoux][])

config/default.yml

@@ -2247,6 +2247,11 @@ Lint/UnreachableLoop:
     # eg. `exactly(2).times`
     - !ruby/regexp /(exactly|at_least|at_most)\(\d+\)\.times/
 
+Lint/UnsafeGlobalVariables:
+  Description: 'Do not use unsafe global variables.'
+  Enabled: pending
+  VersionAdded: '<<next>>'
+
 Lint/UnusedBlockArgument:
   Description: 'Checks for unused block arguments.'
   StyleGuide: '#underscore-unused-vars'

lib/rubocop.rb

@@ -374,6 +374,7 @@
 require_relative 'rubocop/cop/lint/unmodified_reduce_accumulator'
 require_relative 'rubocop/cop/lint/unreachable_code'
 require_relative 'rubocop/cop/lint/unreachable_loop'
+require_relative 'rubocop/cop/lint/unsafe_global_variables'
 require_relative 'rubocop/cop/lint/unused_block_argument'
 require_relative 'rubocop/cop/lint/unused_method_argument'
 require_relative 'rubocop/cop/lint/uri_escape_unescape'

lib/rubocop/cop/lint/unsafe_global_variables.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Lint
+      # This cop looks for uses of unsafe global variables.
+      #
+      # @example
+      #
+      #   # bad
+      #   $!
+      #
+      #   # bad
+      #   $ERROR_INFO
+      #
+      #   # bad
+      #   $@
+      #
+      #   # bad
+      #   $ERROR_POSITION
+      #
+      class UnsafeGlobalVariables < Base
+        MSG = 'Do not use unsafe global variable `%<global_variable>s`.'
+
+        # built-in global variables and their English aliases
+        # https://www.zenspider.com/ruby/quickref.html
+        UNSAFE_GLOBAL_VARIABLES = %i[$! $ERROR_INFO $@ $ERROR_POSITION].freeze
+
+        def on_gvar(node)
+          global_variable, = *node
+
+          return unless UNSAFE_GLOBAL_VARIABLES.include?(global_variable)
+
+          add_offense(node.loc.name, message: format(MSG, global_variable: global_variable))
+        end
+      end
+    end
+  end
+end

lib/rubocop/result_cache.rb

@@ -52,7 +52,9 @@ def remove_oldest_files(files, dirs, cache_root, verbose)
       rescue Errno::ENOENT
         # This can happen if parallel RuboCop invocations try to remove the
         # same files. No problem.
+        # rubocop:disable Lint/UnsafeGlobalVariables
         puts $ERROR_INFO if verbose
+        # rubocop:enable Lint/UnsafeGlobalVariables
       end
 
       def remove_files(files, dirs, remove_count)

spec/rubocop/cop/lint/unsafe_global_variables_spec.rb

@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Lint::UnsafeGlobalVariables, :config do
+  it 'registers an offense when using `$!`' do
+    expect_offense(<<~RUBY)
+      $!
+      ^^ Do not use unsafe global variable `$!`.
+    RUBY
+  end
+
+  it 'registers an offense when using `$ERROR_INFO`' do
+    expect_offense(<<~RUBY)
+      $ERROR_INFO
+      ^^^^^^^^^^^ Do not use unsafe global variable `$ERROR_INFO`.
+    RUBY
+  end
+
+  it 'registers an offense when using `$@`' do
+    expect_offense(<<~RUBY)
+      $@
+      ^^ Do not use unsafe global variable `$@`.
+    RUBY
+  end
+
+  it 'registers an offense when using `$ERROR_POSITION`' do
+    expect_offense(<<~RUBY)
+      $ERROR_POSITION
+      ^^^^^^^^^^^^^^^ Do not use unsafe global variable `$ERROR_POSITION`.
+    RUBY
+  end
+
+  it 'does not register an offense when using a safe global variable' do
+    expect_no_offenses(<<~RUBY)
+      $0
+    RUBY
+  end
+end