Add new Style/GlobalStdStream cop

CHANGELOG.md

@@ -11,6 +11,7 @@
 * [#8339](https://github.com/rubocop-hq/rubocop/pull/8339): Add `Config#for_badge` as an efficient way to get a cop's config merged with its department's. ([@marcandre][])
 * [#5067](https://github.com/rubocop-hq/rubocop/issues/5067): Add new `Style/StringConcatenation` cop. ([@fatkodima][])
 * [#7425](https://github.com/rubocop-hq/rubocop/pull/7425): Add new `Lint/TopLevelReturnWithArgument` cop. ([@iamravitejag][])
+* [#8417](https://github.com/rubocop-hq/rubocop/pull/8417): Add new `Style/GlobalStdStream` cop. ([@fatkodima][])
 * [#7949](https://github.com/rubocop-hq/rubocop/issues/7949): Add new `Style/SingleArgumentDig` cop. ([@volfgox][])
 
 ### Bug fixes

config/default.yml

@@ -2987,6 +2987,12 @@ Style/FrozenStringLiteralComment:
     - never
   Safe: false
 
+Style/GlobalStdStream:
+  Description: 'Enforces the use of `$stdout/$stderr/$stdin` instead of `STDOUT/STDERR/STDIN`.'
+  StyleGuide: '#global-stdout'
+  Enabled: pending
+  VersionAdded: '0.89'
+
 Style/GlobalVars:
   Description: 'Do not introduce global variables.'
   StyleGuide: '#instance-vars'

docs/modules/ROOT/pages/cops.adoc

@@ -374,6 +374,7 @@ In the following section you find all available cops:
 * xref:cops_style.adoc#styleformatstring[Style/FormatString]
 * xref:cops_style.adoc#styleformatstringtoken[Style/FormatStringToken]
 * xref:cops_style.adoc#stylefrozenstringliteralcomment[Style/FrozenStringLiteralComment]
+* xref:cops_style.adoc#styleglobalstdstream[Style/GlobalStdStream]
 * xref:cops_style.adoc#styleglobalvars[Style/GlobalVars]
 * xref:cops_style.adoc#styleguardclause[Style/GuardClause]
 * xref:cops_style.adoc#stylehashaslastarrayitem[Style/HashAsLastArrayItem]

docs/modules/ROOT/pages/cops_style.adoc

@@ -3308,6 +3308,50 @@ end
 | `always`, `always_true`, `never`
 |===
 
+== Style/GlobalStdStream
+
+|===
+| Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
+
+| Pending
+| Yes
+| Yes
+| 0.89
+| -
+|===
+
+This cop enforces the use of `$stdout/$stderr/$stdin` instead of `STDOUT/STDERR/STDIN`.
+`STDOUT/STDERR/STDIN` are constants, and while you can actually
+reassign (possibly to redirect some stream) constants in Ruby, you'll get
+an interpreter warning if you do so.
+
+=== Examples
+
+[source,ruby]
+----
+# bad
+STDOUT.puts('hello')
+
+hash = { out: STDOUT, key: value }
+
+def m(out = STDOUT)
+  out.puts('hello')
+end
+
+# good
+$stdout.puts('hello')
+
+hash = { out: $stdout, key: value }
+
+def m(out = $stdout)
+  out.puts('hello')
+end
+----
+
+=== References
+
+* https://rubystyle.guide#global-stdout
+
 == Style/GlobalVars
 
 |===

lib/rubocop.rb

@@ -418,6 +418,7 @@
 require_relative 'rubocop/cop/style/format_string'
 require_relative 'rubocop/cop/style/format_string_token'
 require_relative 'rubocop/cop/style/frozen_string_literal_comment'
+require_relative 'rubocop/cop/style/global_std_stream'
 require_relative 'rubocop/cop/style/global_vars'
 require_relative 'rubocop/cop/style/guard_clause'
 require_relative 'rubocop/cop/style/hash_as_last_array_item'

lib/rubocop/cop/style/global_std_stream.rb

@@ -0,0 +1,65 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Style
+      # This cop enforces the use of `$stdout/$stderr/$stdin` instead of `STDOUT/STDERR/STDIN`.
+      # `STDOUT/STDERR/STDIN` are constants, and while you can actually
+      # reassign (possibly to redirect some stream) constants in Ruby, you'll get
+      # an interpreter warning if you do so.
+      #
+      # @example
+      #   # bad
+      #   STDOUT.puts('hello')
+      #
+      #   hash = { out: STDOUT, key: value }
+      #
+      #   def m(out = STDOUT)
+      #     out.puts('hello')
+      #   end
+      #
+      #   # good
+      #   $stdout.puts('hello')
+      #
+      #   hash = { out: $stdout, key: value }
+      #
+      #   def m(out = $stdout)
+      #     out.puts('hello')
+      #   end
+      #
+      class GlobalStdStream < Base
+        extend AutoCorrector
+
+        MSG = 'Use `%<gvar_name>s` instead of `%<const_name>s`.'
+
+        STD_STREAMS = %i[STDIN STDOUT STDERR].to_set.freeze
+
+        def_node_matcher :const_to_gvar_assignment?, <<~PATTERN
+          (gvasgn %1 (const nil? _))
+        PATTERN
+
+        def on_const(node)
+          const_name = node.children[1]
+          return unless STD_STREAMS.include?(const_name)
+
+          gvar_name = gvar_name(const_name).to_sym
+          return if const_to_gvar_assignment?(node.parent, gvar_name)
+
+          add_offense(node, message: message(const_name)) do |corrector|
+            corrector.replace(node, gvar_name)
+          end
+        end
+
+        private
+
+        def message(const_name)
+          format(MSG, gvar_name: gvar_name(const_name), const_name: const_name)
+        end
+
+        def gvar_name(const_name)
+          "$#{const_name.to_s.downcase}"
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/style/global_std_stream_spec.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Style::GlobalStdStream do
+  subject(:cop) { described_class.new }
+
+  it 'registers an offense and corrects when using std stream as const' do
+    expect_offense(<<~RUBY)
+      STDOUT.puts('hello')
+      ^^^^^^ Use `$stdout` instead of `STDOUT`.
+
+      hash = { out: STDOUT, key: value }
+                    ^^^^^^ Use `$stdout` instead of `STDOUT`.
+
+      def m(out = STDOUT)
+                  ^^^^^^ Use `$stdout` instead of `STDOUT`.
+        out.puts('hello')
+      end
+    RUBY
+
+    expect_correction(<<~RUBY)
+      $stdout.puts('hello')
+
+      hash = { out: $stdout, key: value }
+
+      def m(out = $stdout)
+        out.puts('hello')
+      end
+    RUBY
+  end
+
+  it 'does not register an offense when using non std stream const' do
+    expect_no_offenses(<<~RUBY)
+      SOME_CONST.puts('hello')
+    RUBY
+  end
+
+  it 'does not register an offense when assigning std stream const to std stream gvar' do
+    expect_no_offenses(<<~RUBY)
+      $stdin = STDIN
+    RUBY
+  end
+
+  it 'does not register an offense when assigning other const to std stream gvar' do
+    expect_no_offenses(<<~RUBY)
+      $stdin = SOME_CONST
+    RUBY
+  end
+end