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

```

18 files changed, 29 insertions, 29 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
index e977d36cb9..99934a0e31 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
@@ -731,7 +731,7 @@ module ActiveRecord
           # this block can't be easily moved into attempt_to_checkout_all_existing_connections's
           # rescue block, because doing so would put it outside of synchronize section, without
           # being in a critical section thread_report might become inaccurate
-          msg = "could not obtain ownership of all database connections in #{checkout_timeout} seconds".dup
+          msg = +"could not obtain ownership of all database connections in #{checkout_timeout} seconds"
 
           thread_report = []
           @connections.each do |conn|
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb b/activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb
index 5b24a467ec..c10da813ec 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb
@@ -379,7 +379,7 @@ module ActiveRecord
           build_fixture_sql(fixtures, table_name)
         end.compact
 
-        table_deletes = tables_to_delete.map { |table| "DELETE FROM #{quote_table_name table}".dup }
+        table_deletes = tables_to_delete.map { |table| +"DELETE FROM #{quote_table_name table}" }
         total_sql = Array.wrap(combine_multi_statements(table_deletes + fixture_inserts))
 
         disable_referential_integrity do
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_creation.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_creation.rb
index 529c9d8ca6..9d9e8a4110 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_creation.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_creation.rb
@@ -21,7 +21,7 @@ module ActiveRecord
         private
 
           def visit_AlterTable(o)
-            sql = "ALTER TABLE #{quote_table_name(o.name)} ".dup
+            sql = +"ALTER TABLE #{quote_table_name(o.name)} "
             sql << o.adds.map { |col| accept col }.join(" ")
             sql << o.foreign_key_adds.map { |fk| visit_AddForeignKey fk }.join(" ")
             sql << o.foreign_key_drops.map { |fk| visit_DropForeignKey fk }.join(" ")
@@ -29,17 +29,17 @@ module ActiveRecord
 
           def visit_ColumnDefinition(o)
             o.sql_type = type_to_sql(o.type, o.options)
-            column_sql = "#{quote_column_name(o.name)} #{o.sql_type}".dup
+            column_sql = +"#{quote_column_name(o.name)} #{o.sql_type}"
             add_column_options!(column_sql, column_options(o)) unless o.type == :primary_key
             column_sql
           end
 
           def visit_AddColumnDefinition(o)
-            "ADD #{accept(o.column)}".dup
+            +"ADD #{accept(o.column)}"
           end
 
           def visit_TableDefinition(o)
-            create_sql = "CREATE#{' TEMPORARY' if o.temporary} TABLE #{quote_table_name(o.name)} ".dup
+            create_sql = +"CREATE#{' TEMPORARY' if o.temporary} TABLE #{quote_table_name(o.name)} "
 
             statements = o.columns.map { |c| accept c }
             statements << accept(o.primary_keys) if o.primary_keys
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
index 172f0a5c84..723d8c318d 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
@@ -1380,7 +1380,7 @@ module ActiveRecord
           sm_table = quote_table_name(ActiveRecord::SchemaMigration.table_name)
 
           if versions.is_a?(Array)
-            sql = "INSERT INTO #{sm_table} (version) VALUES\n".dup
+            sql = +"INSERT INTO #{sm_table} (version) VALUES\n"
             sql << versions.map { |v| "(#{quote(v)})" }.join(",\n")
             sql << ";\n\n"
             sql
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
index 42ed9ce82d..79aafc956f 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
@@ -162,7 +162,7 @@ module ActiveRecord
       # this method must only be called while holding connection pool's mutex
       def lease
         if in_use?
-          msg = "Cannot lease connection, ".dup
+          msg = +"Cannot lease connection, "
           if @owner == Thread.current
             msg << "it is already leased by the current thread."
           else
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index af37fcc1f1..09242a0f14 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -137,7 +137,7 @@ module ActiveRecord
       end
 
       def index_algorithms
-        { default: "ALGORITHM = DEFAULT".dup, copy: "ALGORITHM = COPY".dup, inplace: "ALGORITHM = INPLACE".dup }
+        { default: +"ALGORITHM = DEFAULT", copy: +"ALGORITHM = COPY", inplace: +"ALGORITHM = INPLACE" }
       end
 
       # HELPER METHODS ===========================================
@@ -392,7 +392,7 @@ module ActiveRecord
 
       def add_index(table_name, column_name, options = {}) #:nodoc:
         index_name, index_type, index_columns, _, index_algorithm, index_using, comment = add_index_options(table_name, column_name, options)
-        sql = "CREATE #{index_type} INDEX #{quote_column_name(index_name)} #{index_using} ON #{quote_table_name(table_name)} (#{index_columns}) #{index_algorithm}".dup
+        sql = +"CREATE #{index_type} INDEX #{quote_column_name(index_name)} #{index_using} ON #{quote_table_name(table_name)} (#{index_columns}) #{index_algorithm}"
         execute add_sql_comment!(sql, comment)
       end
 
@@ -785,7 +785,7 @@ module ActiveRecord
           # https://dev.mysql.com/doc/refman/5.7/en/set-names.html
           # (trailing comma because variable_assignments will always have content)
           if @config[:encoding]
-            encoding = "NAMES #{@config[:encoding]}".dup
+            encoding = +"NAMES #{@config[:encoding]}"
             encoding << " COLLATE #{@config[:collation]}" if @config[:collation]
             encoding << ", "
           end
diff --git a/activerecord/lib/active_record/connection_adapters/mysql/schema_creation.rb b/activerecord/lib/active_record/connection_adapters/mysql/schema_creation.rb
index c9ea653b77..82ed320617 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql/schema_creation.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql/schema_creation.rb
@@ -17,7 +17,7 @@ module ActiveRecord
           end
 
           def visit_ChangeColumnDefinition(o)
-            change_column_sql = "CHANGE #{quote_column_name(o.name)} #{accept(o.column)}".dup
+            change_column_sql = +"CHANGE #{quote_column_name(o.name)} #{accept(o.column)}"
             add_column_position!(change_column_sql, column_options(o.column))
           end
 
@@ -64,7 +64,7 @@ module ActiveRecord
 
           def index_in_create(table_name, column_name, options)
             index_name, index_type, index_columns, _, _, index_using, comment = @conn.add_index_options(table_name, column_name, options)
-            add_sql_comment!("#{index_type} INDEX #{quote_column_name(index_name)} #{index_using} (#{index_columns})".dup, comment)
+            add_sql_comment!((+"#{index_type} INDEX #{quote_column_name(index_name)} #{index_using} (#{index_columns})"), comment)
           end
       end
     end
diff --git a/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb
index 1cf210d85b..e167c01802 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb
@@ -121,7 +121,7 @@ module ActiveRecord
           def data_source_sql(name = nil, type: nil)
             scope = quoted_scope(name, type: type)
 
-            sql = "SELECT table_name FROM information_schema.tables".dup
+            sql = +"SELECT table_name FROM information_schema.tables"
             sql << " WHERE table_schema = #{scope[:schema]}"
             sql << " AND table_name = #{scope[:name]}" if scope[:name]
             sql << " AND table_type = #{scope[:type]}" if scope[:type]
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
index 00da7690a2..fae3ddbad4 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
@@ -686,7 +686,7 @@ module ActiveRecord
           def change_column_sql(table_name, column_name, type, options = {})
             quoted_column_name = quote_column_name(column_name)
             sql_type = type_to_sql(type, options)
-            sql = "ALTER COLUMN #{quoted_column_name} TYPE #{sql_type}".dup
+            sql = +"ALTER COLUMN #{quoted_column_name} TYPE #{sql_type}"
             if options[:collation]
               sql << " COLLATE \"#{options[:collation]}\""
             end
@@ -757,7 +757,7 @@ module ActiveRecord
             scope = quoted_scope(name, type: type)
             scope[:type] ||= "'r','v','m','p','f'" # (r)elation/table, (v)iew, (m)aterialized view, (p)artitioned table, (f)oreign table
 
-            sql = "SELECT c.relname FROM pg_class c LEFT JOIN pg_namespace n ON n.oid = c.relnamespace".dup
+            sql = +"SELECT c.relname FROM pg_class c LEFT JOIN pg_namespace n ON n.oid = c.relnamespace"
             sql << " WHERE n.nspname = #{scope[:schema]}"
             sql << " AND c.relname = #{scope[:name]}" if scope[:name]
             sql << " AND c.relkind IN (#{scope[:type]})"
diff --git a/activerecord/lib/active_record/connection_adapters/sqlite3/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/sqlite3/schema_statements.rb
index 46ca7e07a9..48277f0ae2 100644
--- a/activerecord/lib/active_record/connection_adapters/sqlite3/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/sqlite3/schema_statements.rb
@@ -86,7 +86,7 @@ module ActiveRecord
             scope = quoted_scope(name, type: type)
             scope[:type] ||= "'table','view'"
 
-            sql = "SELECT name FROM sqlite_master WHERE name <> 'sqlite_sequence'".dup
+            sql = +"SELECT name FROM sqlite_master WHERE name <> 'sqlite_sequence'"
             sql << " AND name = #{scope[:name]}" if scope[:name]
             sql << " AND type IN (#{scope[:type]})"
             sql
diff --git a/activerecord/lib/active_record/explain.rb b/activerecord/lib/active_record/explain.rb
index 7ccb938888..919e96cd7a 100644
--- a/activerecord/lib/active_record/explain.rb
+++ b/activerecord/lib/active_record/explain.rb
@@ -18,7 +18,7 @@ module ActiveRecord
     # Returns a formatted string ready to be logged.
     def exec_explain(queries) # :nodoc:
       str = queries.map do |sql, binds|
-        msg = "EXPLAIN for: #{sql}".dup
+        msg = +"EXPLAIN for: #{sql}"
         unless binds.empty?
           msg << " "
           msg << binds.map { |attr| render_bind(attr) }.inspect
diff --git a/activerecord/lib/active_record/migration.rb b/activerecord/lib/active_record/migration.rb
index 807b882bfb..9651e69edd 100644
--- a/activerecord/lib/active_record/migration.rb
+++ b/activerecord/lib/active_record/migration.rb
@@ -161,7 +161,7 @@ module ActiveRecord
 
   class ProtectedEnvironmentError < ActiveRecordError #:nodoc:
     def initialize(env = "production")
-      msg = "You are attempting to run a destructive action against your '#{env}' database.\n".dup
+      msg = +"You are attempting to run a destructive action against your '#{env}' database.\n"
       msg << "If you are sure you want to continue, run the same command with the environment variable:\n"
       msg << "DISABLE_DATABASE_ENVIRONMENT_CHECK=1"
       super(msg)
@@ -170,7 +170,7 @@ module ActiveRecord
 
   class EnvironmentMismatchError < ActiveRecordError
     def initialize(current: nil, stored: nil)
-      msg =  "You are attempting to modify a database that was last run in `#{ stored }` environment.\n".dup
+      msg = +"You are attempting to modify a database that was last run in `#{ stored }` environment.\n"
       msg << "You are running in `#{ current }` environment. "
       msg << "If you are sure you want to continue, first set the environment using:\n\n"
       msg << "        rails db:environment:set"
@@ -891,7 +891,7 @@ module ActiveRecord
         source_migrations.each do |migration|
           source = File.binread(migration.filename)
           inserted_comment = "# This migration comes from #{scope} (originally #{migration.version})\n"
-          magic_comments = "".dup
+          magic_comments = +""
           loop do
             # If we have a magic comment in the original migration,
             # insert our comment after the first newline(end of the magic comment line)
@@ -1299,7 +1299,7 @@ module ActiveRecord
           record_version_state_after_migrating(migration.version)
         end
       rescue => e
-        msg = "An error has occurred, ".dup
+        msg = +"An error has occurred, "
         msg << "this and " if use_transaction?(migration)
         msg << "all later migrations canceled:\n\n#{e}"
         raise StandardError, msg, e.backtrace
diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb
index 930e4377ff..6f420fe6bb 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -338,14 +338,14 @@ module ActiveRecord
       name = @klass.name
 
       if ids.nil?
-        error = "Couldn't find #{name}".dup
+        error = +"Couldn't find #{name}"
         error << " with#{conditions}" if conditions
         raise RecordNotFound.new(error, name, key)
       elsif Array(ids).size == 1
         error = "Couldn't find #{name} with '#{key}'=#{ids}#{conditions}"
         raise RecordNotFound.new(error, name, key, ids)
       else
-        error = "Couldn't find all #{name.pluralize} with '#{key}': ".dup
+        error = +"Couldn't find all #{name.pluralize} with '#{key}': "
         error << "(#{ids.join(", ")})#{conditions} (found #{result_size} results, but was looking for #{expected_size})."
         error << " Couldn't find #{name.pluralize(not_found_ids.size)} with #{key.to_s.pluralize(not_found_ids.size)} #{not_found_ids.join(', ')}." if not_found_ids
         raise RecordNotFound.new(error, name, key, ids)
diff --git a/activerecord/lib/active_record/tasks/database_tasks.rb b/activerecord/lib/active_record/tasks/database_tasks.rb
index 16e66982e5..5e29085aff 100644
--- a/activerecord/lib/active_record/tasks/database_tasks.rb
+++ b/activerecord/lib/active_record/tasks/database_tasks.rb
@@ -307,7 +307,7 @@ module ActiveRecord
 
       def check_schema_file(filename)
         unless File.exist?(filename)
-          message = %{#{filename} doesn't exist yet. Run `rails db:migrate` to create it, then try again.}.dup
+          message = +%{#{filename} doesn't exist yet. Run `rails db:migrate` to create it, then try again.}
           message << %{ If you do not intend to use a database, you should instead alter #{Rails.root}/config/application.rb to limit the frameworks that will be loaded.} if defined?(::Rails.root)
           Kernel.abort message
         end
diff --git a/activerecord/lib/active_record/tasks/mysql_database_tasks.rb b/activerecord/lib/active_record/tasks/mysql_database_tasks.rb
index eddc6fa223..1c1b29b5e1 100644
--- a/activerecord/lib/active_record/tasks/mysql_database_tasks.rb
+++ b/activerecord/lib/active_record/tasks/mysql_database_tasks.rb
@@ -104,7 +104,7 @@ module ActiveRecord
         end
 
         def run_cmd_error(cmd, args, action)
-          msg = "failed to execute: `#{cmd}`\n".dup
+          msg = +"failed to execute: `#{cmd}`\n"
           msg << "Please check the output above for any errors and make sure that `#{cmd}` is installed in your PATH and has proper permissions.\n\n"
           msg
         end
diff --git a/activerecord/lib/active_record/tasks/postgresql_database_tasks.rb b/activerecord/lib/active_record/tasks/postgresql_database_tasks.rb
index 029f763158..4da918ada3 100644
--- a/activerecord/lib/active_record/tasks/postgresql_database_tasks.rb
+++ b/activerecord/lib/active_record/tasks/postgresql_database_tasks.rb
@@ -115,7 +115,7 @@ module ActiveRecord
         end
 
         def run_cmd_error(cmd, args, action)
-          msg = "failed to execute:\n".dup
+          msg = +"failed to execute:\n"
           msg << "#{cmd} #{args.join(' ')}\n\n"
           msg << "Please check the output above for any errors and make sure that `#{cmd}` is installed in your PATH and has proper permissions.\n\n"
           msg
diff --git a/activerecord/lib/active_record/tasks/sqlite_database_tasks.rb b/activerecord/lib/active_record/tasks/sqlite_database_tasks.rb
index d0bad05176..a82cea80ca 100644
--- a/activerecord/lib/active_record/tasks/sqlite_database_tasks.rb
+++ b/activerecord/lib/active_record/tasks/sqlite_database_tasks.rb
@@ -67,7 +67,7 @@ module ActiveRecord
         end
 
         def run_cmd_error(cmd, args)
-          msg = "failed to execute:\n".dup
+          msg = +"failed to execute:\n"
           msg << "#{cmd} #{args.join(' ')}\n\n"
           msg << "Please check the output above for any errors and make sure that `#{cmd}` is installed in your PATH and has proper permissions.\n\n"
           msg
diff --git a/activerecord/lib/arel/collectors/plain_string.rb b/activerecord/lib/arel/collectors/plain_string.rb
index 687d7fbf2f..c0e9fff399 100644
--- a/activerecord/lib/arel/collectors/plain_string.rb
+++ b/activerecord/lib/arel/collectors/plain_string.rb
@@ -4,7 +4,7 @@ module Arel # :nodoc: all
   module Collectors
     class PlainString
       def initialize
-        @str = "".dup
+        @str = +""
       end
 
       def value