Enable `Performance/UnfreezeString` cop

In Ruby 2.3 or later, `String#+@` is available and `+@` is faster than `dup`.

```ruby
# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  gem "benchmark-ips"
end

Benchmark.ips do |x|
  x.report('+@') { +"" }
  x.report('dup') { "".dup }
  x.compare!
end
```

```
$ ruby -v benchmark.rb
ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux]
Warming up --------------------------------------
                  +@   282.289k i/100ms
                 dup   187.638k i/100ms
Calculating -------------------------------------
                  +@      6.775M (± 3.6%) i/s -     33.875M in   5.006253s
                 dup      3.320M (± 2.2%) i/s -     16.700M in   5.032125s

Comparison:
                  +@:  6775299.3 i/s
                 dup:  3320400.7 i/s - 2.04x  slower

```

15 files changed, 41 insertions, 41 deletions

diff --git a/activesupport/lib/active_support/duration/iso8601_serializer.rb b/activesupport/lib/active_support/duration/iso8601_serializer.rb
index 84ae29c1ec..0fb0e3f3a5 100644
--- a/activesupport/lib/active_support/duration/iso8601_serializer.rb
+++ b/activesupport/lib/active_support/duration/iso8601_serializer.rb
@@ -16,12 +16,12 @@ module ActiveSupport
         parts, sign = normalize
         return "PT0S".freeze if parts.empty?
 
-        output = "P".dup
+        output = +"P"
         output << "#{parts[:years]}Y"   if parts.key?(:years)
         output << "#{parts[:months]}M"  if parts.key?(:months)
         output << "#{parts[:weeks]}W"   if parts.key?(:weeks)
         output << "#{parts[:days]}D"    if parts.key?(:days)
-        time = "".dup
+        time = +""
         time << "#{parts[:hours]}H"     if parts.key?(:hours)
         time << "#{parts[:minutes]}M"   if parts.key?(:minutes)
         if parts.key?(:seconds)
diff --git a/activesupport/lib/active_support/xml_mini/jdom.rb b/activesupport/lib/active_support/xml_mini/jdom.rb
index 7f94a64016..59c65db2d5 100644
--- a/activesupport/lib/active_support/xml_mini/jdom.rb
+++ b/activesupport/lib/active_support/xml_mini/jdom.rb
@@ -169,7 +169,7 @@ module ActiveSupport
       # element::
       #   XML element to be checked.
       def empty_content?(element)
-        text = "".dup
+        text = +""
         child_nodes = element.child_nodes
         (0...child_nodes.length).each do |i|
           item = child_nodes.item(i)
diff --git a/activesupport/lib/active_support/xml_mini/libxml.rb b/activesupport/lib/active_support/xml_mini/libxml.rb
index 0b000fea60..2a16932f03 100644
--- a/activesupport/lib/active_support/xml_mini/libxml.rb
+++ b/activesupport/lib/active_support/xml_mini/libxml.rb
@@ -55,7 +55,7 @@ module LibXML #:nodoc:
           if c.element?
             c.to_hash(node_hash)
           elsif c.text? || c.cdata?
-            node_hash[CONTENT_ROOT] ||= "".dup
+            node_hash[CONTENT_ROOT] ||= +""
             node_hash[CONTENT_ROOT] << c.content
           end
         end
diff --git a/activesupport/lib/active_support/xml_mini/libxmlsax.rb b/activesupport/lib/active_support/xml_mini/libxmlsax.rb
index dcf16e6084..a22a2c9cb7 100644
--- a/activesupport/lib/active_support/xml_mini/libxmlsax.rb
+++ b/activesupport/lib/active_support/xml_mini/libxmlsax.rb
@@ -23,7 +23,7 @@ module ActiveSupport
       end
 
       def on_start_document
-        @hash = { CONTENT_KEY => "".dup }
+        @hash = { CONTENT_KEY => +"" }
         @hash_stack = [@hash]
       end
 
@@ -33,7 +33,7 @@ module ActiveSupport
       end
 
       def on_start_element(name, attrs = {})
-        new_hash = { CONTENT_KEY => "".dup }.merge!(attrs)
+        new_hash = { CONTENT_KEY => +"" }.merge!(attrs)
         new_hash[HASH_SIZE_KEY] = new_hash.size + 1
 
         case current_hash[name]
diff --git a/activesupport/lib/active_support/xml_mini/nokogiri.rb b/activesupport/lib/active_support/xml_mini/nokogiri.rb
index 5ee6fc8159..4762a759d6 100644
--- a/activesupport/lib/active_support/xml_mini/nokogiri.rb
+++ b/activesupport/lib/active_support/xml_mini/nokogiri.rb
@@ -59,7 +59,7 @@ module ActiveSupport
             if c.element?
               c.to_hash(node_hash)
             elsif c.text? || c.cdata?
-              node_hash[CONTENT_ROOT] ||= "".dup
+              node_hash[CONTENT_ROOT] ||= +""
               node_hash[CONTENT_ROOT] << c.content
             end
           end
diff --git a/activesupport/lib/active_support/xml_mini/nokogirisax.rb b/activesupport/lib/active_support/xml_mini/nokogirisax.rb
index b01ed00a14..0bbb4e258a 100644
--- a/activesupport/lib/active_support/xml_mini/nokogirisax.rb
+++ b/activesupport/lib/active_support/xml_mini/nokogirisax.rb
@@ -39,7 +39,7 @@ module ActiveSupport
       end
 
       def start_element(name, attrs = [])
-        new_hash = { CONTENT_KEY => "".dup }.merge!(Hash[attrs])
+        new_hash = { CONTENT_KEY => +"" }.merge!(Hash[attrs])
         new_hash[HASH_SIZE_KEY] = new_hash.size + 1
 
         case current_hash[name]
diff --git a/activesupport/lib/active_support/xml_mini/rexml.rb b/activesupport/lib/active_support/xml_mini/rexml.rb
index 32458d5b0d..55a155d4ee 100644
--- a/activesupport/lib/active_support/xml_mini/rexml.rb
+++ b/activesupport/lib/active_support/xml_mini/rexml.rb
@@ -76,7 +76,7 @@ module ActiveSupport
           hash
         else
           # must use value to prevent double-escaping
-          texts = "".dup
+          texts = +""
           element.texts.each { |t| texts << t.value }
           merge!(hash, CONTENT_KEY, texts)
         end
diff --git a/activesupport/test/cache/behaviors/cache_store_behavior.rb b/activesupport/test/cache/behaviors/cache_store_behavior.rb
index e17c09ba39..f7c750c08e 100644
--- a/activesupport/test/cache/behaviors/cache_store_behavior.rb
+++ b/activesupport/test/cache/behaviors/cache_store_behavior.rb
@@ -319,7 +319,7 @@ module CacheStoreBehavior
   end
 
   def test_original_store_objects_should_not_be_immutable
-    bar = "bar".dup
+    bar = +"bar"
     @cache.write("foo", bar)
     assert_nothing_raised { bar.gsub!(/.*/, "baz") }
   end
diff --git a/activesupport/test/cache/behaviors/encoded_key_cache_behavior.rb b/activesupport/test/cache/behaviors/encoded_key_cache_behavior.rb
index da16142496..842400f4a3 100644
--- a/activesupport/test/cache/behaviors/encoded_key_cache_behavior.rb
+++ b/activesupport/test/cache/behaviors/encoded_key_cache_behavior.rb
@@ -6,7 +6,7 @@
 module EncodedKeyCacheBehavior
   Encoding.list.each do |encoding|
     define_method "test_#{encoding.name.underscore}_encoded_values" do
-      key = "foo".dup.force_encoding(encoding)
+      key = (+"foo").force_encoding(encoding)
       assert @cache.write(key, "1", raw: true)
       assert_equal "1", @cache.read(key)
       assert_equal "1", @cache.fetch(key)
@@ -18,7 +18,7 @@ module EncodedKeyCacheBehavior
   end
 
   def test_common_utf8_values
-    key = "\xC3\xBCmlaut".dup.force_encoding(Encoding::UTF_8)
+    key = (+"\xC3\xBCmlaut").force_encoding(Encoding::UTF_8)
     assert @cache.write(key, "1", raw: true)
     assert_equal "1", @cache.read(key)
     assert_equal "1", @cache.fetch(key)
@@ -29,7 +29,7 @@ module EncodedKeyCacheBehavior
   end
 
   def test_retains_encoding
-    key = "\xC3\xBCmlaut".dup.force_encoding(Encoding::UTF_8)
+    key = (+"\xC3\xBCmlaut").force_encoding(Encoding::UTF_8)
     assert @cache.write(key, "1", raw: true)
     assert_equal Encoding::UTF_8, key.encoding
   end
diff --git a/activesupport/test/cache/cache_key_test.rb b/activesupport/test/cache/cache_key_test.rb
index 84e656f504..c2240d03c2 100644
--- a/activesupport/test/cache/cache_key_test.rb
+++ b/activesupport/test/cache/cache_key_test.rb
@@ -47,7 +47,7 @@ class CacheKeyTest < ActiveSupport::TestCase
   end
 
   def test_expand_cache_key_respond_to_cache_key
-    key = "foo".dup
+    key = +"foo"
     def key.cache_key
       :foo_key
     end
@@ -55,7 +55,7 @@ class CacheKeyTest < ActiveSupport::TestCase
   end
 
   def test_expand_cache_key_array_with_something_that_responds_to_cache_key
-    key = "foo".dup
+    key = +"foo"
     def key.cache_key
       :foo_key
     end
diff --git a/activesupport/test/core_ext/object/instance_variables_test.rb b/activesupport/test/core_ext/object/instance_variables_test.rb
index a3d8daab5b..cf1fe5dfa4 100644
--- a/activesupport/test/core_ext/object/instance_variables_test.rb
+++ b/activesupport/test/core_ext/object/instance_variables_test.rb
@@ -19,7 +19,7 @@ class ObjectInstanceVariableTest < ActiveSupport::TestCase
   end
 
   def test_instance_exec_passes_arguments_to_block
-    assert_equal %w(hello goodbye), "hello".dup.instance_exec("goodbye") { |v| [self, v] }
+    assert_equal %w(hello goodbye), (+"hello").instance_exec("goodbye") { |v| [self, v] }
   end
 
   def test_instance_exec_with_frozen_obj
@@ -27,7 +27,7 @@ class ObjectInstanceVariableTest < ActiveSupport::TestCase
   end
 
   def test_instance_exec_nested
-    assert_equal %w(goodbye olleh bar), "hello".dup.instance_exec("goodbye") { |arg|
+    assert_equal %w(goodbye olleh bar), (+"hello").instance_exec("goodbye") { |arg|
       [arg] + instance_exec("bar") { |v| [reverse, v] } }
   end
 end
diff --git a/activesupport/test/core_ext/string_ext_test.rb b/activesupport/test/core_ext/string_ext_test.rb
index b8de16cc5e..81299e5b58 100644
--- a/activesupport/test/core_ext/string_ext_test.rb
+++ b/activesupport/test/core_ext/string_ext_test.rb
@@ -245,8 +245,8 @@ class StringInflectionsTest < ActiveSupport::TestCase
   end
 
   def test_string_squish
-    original = %{\u205f\u3000 A string surrounded by various unicode spaces,
-      with tabs(\t\t), newlines(\n\n), unicode nextlines(\u0085\u0085) and many spaces(  ). \u00a0\u2007}.dup
+    original = +%{\u205f\u3000 A string surrounded by various unicode spaces,
+      with tabs(\t\t), newlines(\n\n), unicode nextlines(\u0085\u0085) and many spaces(  ). \u00a0\u2007}
 
     expected = "A string surrounded by various unicode spaces, " \
       "with tabs( ), newlines( ), unicode nextlines( ) and many spaces( )."
@@ -378,8 +378,8 @@ class StringInflectionsTest < ActiveSupport::TestCase
   end
 
   def test_truncate_multibyte
-    assert_equal "\354\225\204\353\246\254\353\236\221 \354\225\204\353\246\254 ...".dup.force_encoding(Encoding::UTF_8),
-      "\354\225\204\353\246\254\353\236\221 \354\225\204\353\246\254 \354\225\204\353\235\274\353\246\254\354\230\244".dup.force_encoding(Encoding::UTF_8).truncate(10)
+    assert_equal (+"\354\225\204\353\246\254\353\236\221 \354\225\204\353\246\254 ...").force_encoding(Encoding::UTF_8),
+      (+"\354\225\204\353\246\254\353\236\221 \354\225\204\353\246\254 \354\225\204\353\235\274\353\246\254\354\230\244").force_encoding(Encoding::UTF_8).truncate(10)
   end
 
   def test_truncate_should_not_be_html_safe
@@ -400,7 +400,7 @@ class StringInflectionsTest < ActiveSupport::TestCase
   end
 
   def test_remove!
-    original = "This is a very good day to die".dup
+    original = +"This is a very good day to die"
     assert_equal "This is a good day to die", original.remove!(" very")
     assert_equal "This is a good day to die", original
     assert_equal "This is a good day", original.remove!(" to ", /die/)
@@ -733,7 +733,7 @@ end
 
 class OutputSafetyTest < ActiveSupport::TestCase
   def setup
-    @string = "hello".dup
+    @string = +"hello"
     @object = Class.new(Object) do
       def to_s
         "other"
@@ -809,7 +809,7 @@ class OutputSafetyTest < ActiveSupport::TestCase
   end
 
   test "Concatting safe onto unsafe yields unsafe" do
-    @other_string = "other".dup
+    @other_string = +"other"
 
     string = @string.html_safe
     @other_string.concat(string)
@@ -832,7 +832,7 @@ class OutputSafetyTest < ActiveSupport::TestCase
   end
 
   test "Concatting safe onto unsafe with << yields unsafe" do
-    @other_string = "other".dup
+    @other_string = +"other"
     string = @string.html_safe
 
     @other_string << string
@@ -888,7 +888,7 @@ class OutputSafetyTest < ActiveSupport::TestCase
   test "Concatting an integer to safe always yields safe" do
     string = @string.html_safe
     string = string.concat(13)
-    assert_equal "hello".dup.concat(13), string
+    assert_equal (+"hello").concat(13), string
     assert_predicate string, :html_safe?
   end
 
diff --git a/activesupport/test/logger_test.rb b/activesupport/test/logger_test.rb
index 773b0daa1d..160e1156b6 100644
--- a/activesupport/test/logger_test.rb
+++ b/activesupport/test/logger_test.rb
@@ -40,7 +40,7 @@ class LoggerTest < ActiveSupport::TestCase
     logger = Logger.new f
     logger.level = Logger::DEBUG
 
-    str = "\x80".dup
+    str = +"\x80"
     str.force_encoding("ASCII-8BIT")
 
     logger.add Logger::DEBUG, str
@@ -58,7 +58,7 @@ class LoggerTest < ActiveSupport::TestCase
     logger = Logger.new f
     logger.level = Logger::DEBUG
 
-    str = "\x80".dup
+    str = +"\x80"
     str.force_encoding("ASCII-8BIT")
 
     logger.add Logger::DEBUG, str
diff --git a/activesupport/test/multibyte_chars_test.rb b/activesupport/test/multibyte_chars_test.rb
index 061446c782..11c4822748 100644
--- a/activesupport/test/multibyte_chars_test.rb
+++ b/activesupport/test/multibyte_chars_test.rb
@@ -53,7 +53,7 @@ class MultibyteCharsTest < ActiveSupport::TestCase
   end
 
   def test_forwarded_method_with_non_string_result_should_be_returned_verbatim
-    str = "".dup
+    str = +""
     str.singleton_class.class_eval { def __method_for_multibyte_testing_with_integer_result; 1; end }
     @chars.wrapped_string.singleton_class.class_eval { def __method_for_multibyte_testing_with_integer_result; 1; end }
 
@@ -61,14 +61,14 @@ class MultibyteCharsTest < ActiveSupport::TestCase
   end
 
   def test_should_concatenate
-    mb_a = "a".dup.mb_chars
-    mb_b = "b".dup.mb_chars
+    mb_a = (+"a").mb_chars
+    mb_b = (+"b").mb_chars
     assert_equal "ab", mb_a + "b"
     assert_equal "ab", "a" + mb_b
     assert_equal "ab", mb_a + mb_b
 
     assert_equal "ab", mb_a << "b"
-    assert_equal "ab", "a".dup << mb_b
+    assert_equal "ab", (+"a") << mb_b
     assert_equal "abb", mb_a << mb_b
   end
 
@@ -80,7 +80,7 @@ class MultibyteCharsTest < ActiveSupport::TestCase
 
   def test_concatenation_should_return_a_proxy_class_instance
     assert_equal ActiveSupport::Multibyte.proxy_class, ("a".mb_chars + "b").class
-    assert_equal ActiveSupport::Multibyte.proxy_class, ("a".dup.mb_chars << "b").class
+    assert_equal ActiveSupport::Multibyte.proxy_class, ((+"a").mb_chars << "b").class
   end
 
   def test_ascii_strings_are_treated_at_utf8_strings
@@ -90,8 +90,8 @@ class MultibyteCharsTest < ActiveSupport::TestCase
   def test_concatenate_should_return_proxy_instance
     assert(("a".mb_chars + "b").kind_of?(@proxy_class))
     assert(("a".mb_chars + "b".mb_chars).kind_of?(@proxy_class))
-    assert(("a".dup.mb_chars << "b").kind_of?(@proxy_class))
-    assert(("a".dup.mb_chars << "b".mb_chars).kind_of?(@proxy_class))
+    assert(((+"a").mb_chars << "b").kind_of?(@proxy_class))
+    assert(((+"a").mb_chars << "b".mb_chars).kind_of?(@proxy_class))
   end
 
   def test_should_return_string_as_json
@@ -135,7 +135,7 @@ class MultibyteCharsUTF8BehaviourTest < ActiveSupport::TestCase
   end
 
   def test_tidy_bytes_bang_should_change_wrapped_string
-    original = " Un bUen café \x92".dup
+    original = +" Un bUen café \x92"
     proxy = chars(original.dup)
     proxy.tidy_bytes!
     assert_not_equal original, proxy.to_s
@@ -152,7 +152,7 @@ class MultibyteCharsUTF8BehaviourTest < ActiveSupport::TestCase
   end
 
   def test_string_methods_are_chainable
-    assert chars("".dup).insert(0, "").kind_of?(ActiveSupport::Multibyte.proxy_class)
+    assert chars(+"").insert(0, "").kind_of?(ActiveSupport::Multibyte.proxy_class)
     assert chars("").rjust(1).kind_of?(ActiveSupport::Multibyte.proxy_class)
     assert chars("").ljust(1).kind_of?(ActiveSupport::Multibyte.proxy_class)
     assert chars("").center(1).kind_of?(ActiveSupport::Multibyte.proxy_class)
@@ -197,7 +197,7 @@ class MultibyteCharsUTF8BehaviourTest < ActiveSupport::TestCase
   end
 
   def test_should_use_character_offsets_for_insert_offsets
-    assert_equal "", "".dup.mb_chars.insert(0, "")
+    assert_equal "", (+"").mb_chars.insert(0, "")
     assert_equal "こわにちわ", @chars.insert(1, "わ")
     assert_equal "こわわわにちわ", @chars.insert(2, "わわ")
     assert_equal "わこわわわにちわ", @chars.insert(0, "わ")
@@ -420,13 +420,13 @@ class MultibyteCharsUTF8BehaviourTest < ActiveSupport::TestCase
   end
 
   def test_slice_bang_removes_the_slice_from_the_receiver
-    chars = "úüù".dup.mb_chars
+    chars = (+"úüù").mb_chars
     chars.slice!(0, 2)
     assert_equal "ù", chars
   end
 
   def test_slice_bang_returns_nil_and_does_not_modify_receiver_if_out_of_bounds
-    string = "úüù".dup
+    string = +"úüù"
     chars = string.mb_chars
     assert_nil chars.slice!(4, 5)
     assert_equal "úüù", chars
diff --git a/activesupport/test/multibyte_test_helpers.rb b/activesupport/test/multibyte_test_helpers.rb
index 98f36aa939..d97ce6727a 100644
--- a/activesupport/test/multibyte_test_helpers.rb
+++ b/activesupport/test/multibyte_test_helpers.rb
@@ -29,7 +29,7 @@ module MultibyteTestHelpers
 
   UNICODE_STRING = "こにちわ".freeze
   ASCII_STRING = "ohayo".freeze
-  BYTE_STRING = "\270\236\010\210\245".dup.force_encoding("ASCII-8BIT").freeze
+  BYTE_STRING = (+"\270\236\010\210\245").force_encoding("ASCII-8BIT").freeze
 
   def chars(str)
     ActiveSupport::Multibyte::Chars.new(str)