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

```

22 files changed, 30 insertions, 31 deletions

diff --git a/actionpack/lib/action_controller/log_subscriber.rb b/actionpack/lib/action_controller/log_subscriber.rb
index 14f41eb55f..203653354a 100644
--- a/actionpack/lib/action_controller/log_subscriber.rb
+++ b/actionpack/lib/action_controller/log_subscriber.rb
@@ -26,7 +26,7 @@ module ActionController
           exception_class_name = payload[:exception].first
           status = ActionDispatch::ExceptionWrapper.status_code_for_exception(exception_class_name)
         end
-        message = "Completed #{status} #{Rack::Utils::HTTP_STATUS_CODES[status]} in #{event.duration.round}ms".dup
+        message = +"Completed #{status} #{Rack::Utils::HTTP_STATUS_CODES[status]} in #{event.duration.round}ms"
         message << " (#{additions.join(" | ".freeze)})" unless additions.empty?
         message << "\n\n" if defined?(Rails.env) && Rails.env.development?
 
diff --git a/actionpack/lib/action_controller/metal/http_authentication.rb b/actionpack/lib/action_controller/metal/http_authentication.rb
index a871ccd533..5794e0fb97 100644
--- a/actionpack/lib/action_controller/metal/http_authentication.rb
+++ b/actionpack/lib/action_controller/metal/http_authentication.rb
@@ -474,7 +474,7 @@ module ActionController
 
       # This removes the <tt>"</tt> characters wrapping the value.
       def rewrite_param_values(array_params)
-        array_params.each { |param| (param[1] || "".dup).gsub! %r/^"|"$/, "" }
+        array_params.each { |param| (param[1] || +"").gsub! %r/^"|"$/, "" }
       end
 
       # This method takes an authorization body and splits up the key-value
diff --git a/actionpack/lib/action_controller/metal/live.rb b/actionpack/lib/action_controller/metal/live.rb
index b1c2391afe..5680ef08a1 100644
--- a/actionpack/lib/action_controller/metal/live.rb
+++ b/actionpack/lib/action_controller/metal/live.rb
@@ -297,7 +297,7 @@ module ActionController
       return unless logger
 
       logger.fatal do
-        message = "\n#{exception.class} (#{exception.message}):\n".dup
+        message = +"\n#{exception.class} (#{exception.message}):\n"
         message << exception.annoted_source_code.to_s if exception.respond_to?(:annoted_source_code)
         message << "  " << exception.backtrace.join("\n  ")
         "#{message}\n\n"
diff --git a/actionpack/lib/action_controller/metal/rendering.rb b/actionpack/lib/action_controller/metal/rendering.rb
index 6d181e6456..7d0a944381 100644
--- a/actionpack/lib/action_controller/metal/rendering.rb
+++ b/actionpack/lib/action_controller/metal/rendering.rb
@@ -40,7 +40,7 @@ module ActionController
     def render_to_string(*)
       result = super
       if result.respond_to?(:each)
-        string = "".dup
+        string = +""
         result.each { |r| string << r }
         string
       else
diff --git a/actionpack/lib/action_dispatch/http/response.rb b/actionpack/lib/action_dispatch/http/response.rb
index 885dc0ecba..f07be831d4 100644
--- a/actionpack/lib/action_dispatch/http/response.rb
+++ b/actionpack/lib/action_dispatch/http/response.rb
@@ -105,7 +105,7 @@ module ActionDispatch # :nodoc:
 
       def body
         @str_body ||= begin
-          buf = "".dup
+          buf = +""
           each { |chunk| buf << chunk }
           buf
         end
diff --git a/actionpack/lib/action_dispatch/http/url.rb b/actionpack/lib/action_dispatch/http/url.rb
index 35ba44005a..db6d8188d3 100644
--- a/actionpack/lib/action_dispatch/http/url.rb
+++ b/actionpack/lib/action_dispatch/http/url.rb
@@ -157,7 +157,7 @@ module ActionDispatch
           subdomain  = options.fetch :subdomain, true
           domain     = options[:domain]
 
-          host = "".dup
+          host = +""
           if subdomain == true
             return _host if domain.nil?
 
diff --git a/actionpack/lib/action_dispatch/journey/formatter.rb b/actionpack/lib/action_dispatch/journey/formatter.rb
index 0f04839d9b..52396ec901 100644
--- a/actionpack/lib/action_dispatch/journey/formatter.rb
+++ b/actionpack/lib/action_dispatch/journey/formatter.rb
@@ -50,7 +50,7 @@ module ActionDispatch
         unmatched_keys = (missing_keys || []) & constraints.keys
         missing_keys = (missing_keys || []) - unmatched_keys
 
-        message = "No route matches #{Hash[constraints.sort_by { |k, v| k.to_s }].inspect}".dup
+        message = +"No route matches #{Hash[constraints.sort_by { |k, v| k.to_s }].inspect}"
         message << ", missing required keys: #{missing_keys.sort.inspect}" if missing_keys && !missing_keys.empty?
         message << ", possible unmatched constraints: #{unmatched_keys.sort.inspect}" if unmatched_keys && !unmatched_keys.empty?
 
diff --git a/actionpack/lib/action_dispatch/journey/router/utils.rb b/actionpack/lib/action_dispatch/journey/router/utils.rb
index df3f79a407..3bbb187f5c 100644
--- a/actionpack/lib/action_dispatch/journey/router/utils.rb
+++ b/actionpack/lib/action_dispatch/journey/router/utils.rb
@@ -17,11 +17,11 @@ module ActionDispatch
         def self.normalize_path(path)
           path ||= ""
           encoding = path.encoding
-          path = "/#{path}".dup
+          path = +"/#{path}"
           path.squeeze!("/".freeze)
           path.sub!(%r{/+\Z}, "".freeze)
           path.gsub!(/(%[a-f0-9]{2})/) { $1.upcase }
-          path = "/".dup if path == "".freeze
+          path = +"/" if path == "".freeze
           path.force_encoding(encoding)
           path
         end
@@ -32,7 +32,7 @@ module ActionDispatch
           ENCODE   = "%%%02X".freeze
           US_ASCII = Encoding::US_ASCII
           UTF_8    = Encoding::UTF_8
-          EMPTY    = "".dup.force_encoding(US_ASCII).freeze
+          EMPTY    = (+"").force_encoding(US_ASCII).freeze
           DEC2HEX  = (0..255).to_a.map { |i| ENCODE % i }.map { |s| s.force_encoding(US_ASCII) }
 
           ALPHA = "a-zA-Z".freeze
diff --git a/actionpack/lib/action_dispatch/middleware/debug_exceptions.rb b/actionpack/lib/action_dispatch/middleware/debug_exceptions.rb
index 077a83b112..5f5fdbc66a 100644
--- a/actionpack/lib/action_dispatch/middleware/debug_exceptions.rb
+++ b/actionpack/lib/action_dispatch/middleware/debug_exceptions.rb
@@ -23,7 +23,7 @@ module ActionDispatch
         if clean_params.empty?
           "None"
         else
-          PP.pp(clean_params, "".dup, 200)
+          PP.pp(clean_params, +"", 200)
         end
       end
 
diff --git a/actionpack/lib/action_dispatch/middleware/debug_locks.rb b/actionpack/lib/action_dispatch/middleware/debug_locks.rb
index 03760438f7..d39377f174 100644
--- a/actionpack/lib/action_dispatch/middleware/debug_locks.rb
+++ b/actionpack/lib/action_dispatch/middleware/debug_locks.rb
@@ -63,19 +63,19 @@ module ActionDispatch
 
         str = threads.map do |thread, info|
           if info[:exclusive]
-            lock_state = "Exclusive".dup
+            lock_state = +"Exclusive"
           elsif info[:sharing] > 0
-            lock_state = "Sharing".dup
+            lock_state = +"Sharing"
             lock_state << " x#{info[:sharing]}" if info[:sharing] > 1
           else
-            lock_state = "No lock".dup
+            lock_state = +"No lock"
           end
 
           if info[:waiting]
             lock_state << " (yielded share)"
           end
 
-          msg = "Thread #{info[:index]} [0x#{thread.__id__.to_s(16)} #{thread.status || 'dead'}]  #{lock_state}\n".dup
+          msg = +"Thread #{info[:index]} [0x#{thread.__id__.to_s(16)} #{thread.status || 'dead'}]  #{lock_state}\n"
 
           if info[:sleeper]
             msg << "  Waiting in #{info[:sleeper]}"
diff --git a/actionpack/lib/action_dispatch/middleware/ssl.rb b/actionpack/lib/action_dispatch/middleware/ssl.rb
index 190e54223e..9c9ccfa16f 100644
--- a/actionpack/lib/action_dispatch/middleware/ssl.rb
+++ b/actionpack/lib/action_dispatch/middleware/ssl.rb
@@ -102,7 +102,7 @@ module ActionDispatch
 
       # https://tools.ietf.org/html/rfc6797#section-6.1
       def build_hsts_header(hsts)
-        value = "max-age=#{hsts[:expires].to_i}".dup
+        value = +"max-age=#{hsts[:expires].to_i}"
         value << "; includeSubDomains" if hsts[:subdomains]
         value << "; preload" if hsts[:preload]
         value
@@ -141,7 +141,7 @@ module ActionDispatch
         host = @redirect[:host] || request.host
         port = @redirect[:port] || request.port
 
-        location = "https://#{host}".dup
+        location = +"https://#{host}"
         location << ":#{port}" if port != 80 && port != 443
         location << request.fullpath
         location
diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 07e3be4db8..3f7cf0950d 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -308,7 +308,7 @@ module ActionDispatch
           def check_controller_and_action(path_params, controller, action)
             hash = check_part(:controller, controller, path_params, {}) do |part|
               translate_controller(part) {
-                message = "'#{part}' is not a supported controller name. This can lead to potential routing problems.".dup
+                message = +"'#{part}' is not a supported controller name. This can lead to potential routing problems."
                 message << " See https://guides.rubyonrails.org/routing.html#specifying-a-controller-to-use"
 
                 raise ArgumentError, message
diff --git a/actionpack/lib/action_dispatch/routing/route_set.rb b/actionpack/lib/action_dispatch/routing/route_set.rb
index 07d3a41173..acce8a7ef3 100644
--- a/actionpack/lib/action_dispatch/routing/route_set.rb
+++ b/actionpack/lib/action_dispatch/routing/route_set.rb
@@ -245,7 +245,7 @@ module ActionDispatch
                   missing_keys << missing_key
                 }
                 constraints = Hash[@route.requirements.merge(params).sort_by { |k, v| k.to_s }]
-                message = "No route matches #{constraints.inspect}".dup
+                message = +"No route matches #{constraints.inspect}"
                 message << ", missing required keys: #{missing_keys.sort.inspect}"
 
                 raise ActionController::UrlGenerationError, message
diff --git a/actionpack/lib/action_dispatch/system_testing/test_helpers/screenshot_helper.rb b/actionpack/lib/action_dispatch/system_testing/test_helpers/screenshot_helper.rb
index d2685e0452..884fb51d18 100644
--- a/actionpack/lib/action_dispatch/system_testing/test_helpers/screenshot_helper.rb
+++ b/actionpack/lib/action_dispatch/system_testing/test_helpers/screenshot_helper.rb
@@ -65,7 +65,7 @@ module ActionDispatch
           end
 
           def display_image
-            message = "[Screenshot]: #{image_path}\n".dup
+            message = +"[Screenshot]: #{image_path}\n"
 
             case output_type
             when "artifact"
diff --git a/actionpack/lib/action_dispatch/testing/assertions/response.rb b/actionpack/lib/action_dispatch/testing/assertions/response.rb
index 98b1965d22..8595ea03cf 100644
--- a/actionpack/lib/action_dispatch/testing/assertions/response.rb
+++ b/actionpack/lib/action_dispatch/testing/assertions/response.rb
@@ -79,9 +79,8 @@ module ActionDispatch
         end
 
         def generate_response_message(expected, actual = @response.response_code)
-          "Expected response to be a <#{code_with_name(expected)}>,"\
-          " but was a <#{code_with_name(actual)}>"
-            .dup.concat(location_if_redirected).concat(response_body_if_short)
+          (+"Expected response to be a <#{code_with_name(expected)}>,"\
+          " but was a <#{code_with_name(actual)}>").concat(location_if_redirected).concat(response_body_if_short)
         end
 
         def response_body_if_short
diff --git a/actionpack/test/controller/http_token_authentication_test.rb b/actionpack/test/controller/http_token_authentication_test.rb
index 672aa1351c..103123f98c 100644
--- a/actionpack/test/controller/http_token_authentication_test.rb
+++ b/actionpack/test/controller/http_token_authentication_test.rb
@@ -150,7 +150,7 @@ class HttpTokenAuthenticationTest < ActionController::TestCase
   end
 
   test "token_and_options returns empty string with empty token" do
-    token = "".dup
+    token = +""
     actual = ActionController::HttpAuthentication::Token.token_and_options(sample_request(token)).first
     expected = token
     assert_equal(expected, actual)
diff --git a/actionpack/test/controller/new_base/bare_metal_test.rb b/actionpack/test/controller/new_base/bare_metal_test.rb
index b049022a06..7572d514fb 100644
--- a/actionpack/test/controller/new_base/bare_metal_test.rb
+++ b/actionpack/test/controller/new_base/bare_metal_test.rb
@@ -13,7 +13,7 @@ module BareMetalTest
     test "response body is a Rack-compatible response" do
       status, headers, body = BareController.action(:index).call(Rack::MockRequest.env_for("/"))
       assert_equal 200, status
-      string = "".dup
+      string = +""
 
       body.each do |part|
         assert part.is_a?(String), "Each part of the body must be a String"
diff --git a/actionpack/test/controller/routing_test.rb b/actionpack/test/controller/routing_test.rb
index a7033b2d30..a2a811f090 100644
--- a/actionpack/test/controller/routing_test.rb
+++ b/actionpack/test/controller/routing_test.rb
@@ -674,7 +674,7 @@ class LegacyRouteSetTests < ActiveSupport::TestCase
     assert_equal "/page/foo", url_for(rs, controller: "content", action: "show_page", id: "foo")
     assert_equal({ controller: "content", action: "show_page", id: "foo" }, rs.recognize_path("/page/foo"))
 
-    token = "\321\202\320\265\320\272\321\201\321\202".dup # 'text' in Russian
+    token = +"\321\202\320\265\320\272\321\201\321\202" # 'text' in Russian
     token.force_encoding(Encoding::BINARY)
     escaped_token = CGI.escape(token)
 
diff --git a/actionpack/test/dispatch/debug_exceptions_test.rb b/actionpack/test/dispatch/debug_exceptions_test.rb
index 44b79c0e5d..37399cfd07 100644
--- a/actionpack/test/dispatch/debug_exceptions_test.rb
+++ b/actionpack/test/dispatch/debug_exceptions_test.rb
@@ -368,7 +368,7 @@ class DebugExceptionsTest < ActionDispatch::IntegrationTest
     })
     assert_response 500
 
-    assert_includes(body, CGI.escapeHTML(PP.pp(params, "".dup, 200)))
+    assert_includes(body, CGI.escapeHTML(PP.pp(params, +"", 200)))
   end
 
   test "sets the HTTP charset parameter" do
diff --git a/actionpack/test/dispatch/prefix_generation_test.rb b/actionpack/test/dispatch/prefix_generation_test.rb
index 85ea04356a..7a7a201b11 100644
--- a/actionpack/test/dispatch/prefix_generation_test.rb
+++ b/actionpack/test/dispatch/prefix_generation_test.rb
@@ -13,7 +13,7 @@ module TestGenerationPrefix
     end
 
     def self.model_name
-      klass = "Post".dup
+      klass = +"Post"
       def klass.name; self end
 
       ActiveModel::Name.new(klass)
diff --git a/actionpack/test/dispatch/static_test.rb b/actionpack/test/dispatch/static_test.rb
index 6b69cd9999..d44aa00122 100644
--- a/actionpack/test/dispatch/static_test.rb
+++ b/actionpack/test/dispatch/static_test.rb
@@ -31,7 +31,7 @@ module StaticTests
   end
 
   def test_handles_urls_with_ascii_8bit
-    assert_equal "Hello, World!", get("/doorkeeper%E3E4".dup.force_encoding("ASCII-8BIT")).body
+    assert_equal "Hello, World!", get((+"/doorkeeper%E3E4").force_encoding("ASCII-8BIT")).body
   end
 
   def test_handles_urls_with_ascii_8bit_on_win_31j
@@ -39,7 +39,7 @@ module StaticTests
       Encoding.default_internal = "Windows-31J"
       Encoding.default_external = "Windows-31J"
     end
-    assert_equal "Hello, World!", get("/doorkeeper%E3E4".dup.force_encoding("ASCII-8BIT")).body
+    assert_equal "Hello, World!", get((+"/doorkeeper%E3E4").force_encoding("ASCII-8BIT")).body
   end
 
   def test_handles_urls_with_null_byte
diff --git a/actionpack/test/journey/router/utils_test.rb b/actionpack/test/journey/router/utils_test.rb
index 2d09098f11..472f1bf35e 100644
--- a/actionpack/test/journey/router/utils_test.rb
+++ b/actionpack/test/journey/router/utils_test.rb
@@ -23,7 +23,7 @@ module ActionDispatch
         end
 
         def test_uri_unescape_with_utf8_string
-          assert_equal "Šašinková", Utils.unescape_uri("%C5%A0a%C5%A1inkov%C3%A1".dup.force_encoding(Encoding::US_ASCII))
+          assert_equal "Šašinková", Utils.unescape_uri((+"%C5%A0a%C5%A1inkov%C3%A1").force_encoding(Encoding::US_ASCII))
         end
 
         def test_normalize_path_not_greedy