[Fix #384] Support optimized String#dup for `Performance/UnfreezeSt…

…ring`

Resolves #384.

This PR supports optimized `String#dup` for `Performance/UnfreezeString` when Ruby 3.3+.

changelog/new_optimized_string_dup_for_performance_unfreeze_string.md

@@ -0,0 +1 @@
+* [#384](https://github.com/rubocop/rubocop-performance/issues/384): Support optimized `String#dup` for `Performance/UnfreezeString` when Ruby 3.3+. ([@koic][])

lib/rubocop/cop/performance/unfreeze_string.rb

@@ -15,8 +15,8 @@ module Performance
       #
       # @example
       #   # bad
-      #   ''.dup
-      #   "something".dup
+      #   ''.dup          # when Ruby 3.2 or lower
+      #   "something".dup # when Ruby 3.2 or lower
       #   String.new
       #   String.new('')
       #   String.new('something')
@@ -45,7 +45,7 @@ class UnfreezeString < Base
         PATTERN
 
         def on_send(node)
-          return unless dup_string?(node) || string_new?(node)
+          return unless (dup_string?(node) && target_ruby_version <= 3.2) || string_new?(node)
 
           add_offense(node) do |corrector|
             string_value = "+#{string_value(node)}"

spec/rubocop/cop/performance/unfreeze_string_spec.rb

@@ -1,15 +1,25 @@
 # frozen_string_literal: true
 
 RSpec.describe RuboCop::Cop::Performance::UnfreezeString, :config do
-  it 'registers an offense and corrects for an empty string with `.dup`' do
-    expect_offense(<<~RUBY)
-      "".dup
-      ^^^^^^ Use unary plus to get an unfrozen string literal.
-    RUBY
+  context 'when Ruby <= 3.2', :ruby32 do
+    it 'registers an offense and corrects for an empty string with `.dup`' do
+      expect_offense(<<~RUBY)
+        "".dup
+        ^^^^^^ Use unary plus to get an unfrozen string literal.
+      RUBY
 
-    expect_correction(<<~RUBY)
-      +""
-    RUBY
+      expect_correction(<<~RUBY)
+        +""
+      RUBY
+    end
+  end
+
+  context 'when Ruby >= 3.3', :ruby33 do
+    it 'does not register an offense and corrects for an empty string with `.dup`' do
+      expect_no_offenses(<<~RUBY)
+        "".dup
+      RUBY
+    end
   end
 
   it 'registers an offense and corrects for a string with `.dup`' do