Merge pull request #373 from tagliala/bugfix/fix-target-ruby-version-of-unfreeze-string

Set target version for `Performance/UnfreezeString`

Author: koic

changelog/fix_target_version_for_unfreeze_string_cop.md

@@ -0,0 +1 @@
+* [#373](https://github.com/rubocop/rubocop-performance/pull/373): Set target version for `Performance/UnfreezeString`. ([@tagliala][])

lib/rubocop/cop/performance/unfreeze_string.rb

@@ -26,6 +26,9 @@ module Performance
       #   +''
       class UnfreezeString < Base
         extend AutoCorrector
+        extend TargetRubyVersion
+
+        minimum_target_ruby_version 2.3
 
         MSG = 'Use unary plus to get an unfrozen string literal.'
         RESTRICT_ON_SEND = %i[dup new].freeze

spec/rubocop/cop/performance/unfreeze_string_spec.rb

@@ -112,4 +112,12 @@
       String.new(capacity: 100)
     RUBY
   end
+
+  context 'when Ruby <= 2.2', :ruby22 do
+    it 'does not register an offense for an empty string with `.dup`' do
+      expect_no_offenses(<<~RUBY)
+        "".dup
+      RUBY
+    end
+  end
 end