PERF: 2x ~ 30x faster dirty tracking

Currently, although using both dirty tracking (ivar backed and
attributes backed) on one model is not supported (doesn't fully work at
least), both dirty tracking are being performed, that is very slow.

As long as attributes backed dirty tracking is used, ivar backed dirty
tracking should not need to be performed.

I've refactored to extract new `ForcedMutationTracker` which only tracks
`force_change` to be performed for ivar backed dirty tracking, that
makes dirty tracking on Active Record 2x ~ 30x faster.

https://gist.github.com/kamipo/971dfe0891f0fe1ec7db8ab31f016435

Before:

```
Warming up --------------------------------------
            changed?     4.467k i/100ms
             changed     5.134k i/100ms
             changes     3.023k i/100ms
  changed_attributes     4.358k i/100ms
        title_change     3.185k i/100ms
           title_was     3.381k i/100ms
Calculating -------------------------------------
            changed?     42.197k (±28.5%) i/s -    187.614k in   5.050446s
             changed     50.481k (±16.0%) i/s -    246.432k in   5.045759s
             changes     30.799k (± 7.2%) i/s -    154.173k in   5.030765s
  changed_attributes     51.530k (±14.2%) i/s -    252.764k in   5.041106s
        title_change     44.667k (± 9.0%) i/s -    222.950k in   5.040646s
           title_was     44.635k (±16.6%) i/s -    216.384k in   5.051098s
```

After:

```
Warming up --------------------------------------
            changed?    24.130k i/100ms
             changed    13.503k i/100ms
             changes     6.511k i/100ms
  changed_attributes     9.226k i/100ms
        title_change    48.221k i/100ms
           title_was    96.060k i/100ms
Calculating -------------------------------------
            changed?    245.478k (±16.1%) i/s -      1.182M in   5.015837s
             changed    157.641k (± 4.9%) i/s -    796.677k in   5.066734s
             changes     70.633k (± 5.7%) i/s -    358.105k in   5.086553s
  changed_attributes     95.155k (±13.6%) i/s -    470.526k in   5.082841s
        title_change    566.481k (± 3.5%) i/s -      2.845M in   5.028852s
           title_was      1.487M (± 3.9%) i/s -      7.493M in   5.046774s
```

1 files changed, 88 insertions, 29 deletions

diff --git a/activemodel/lib/active_model/attribute_mutation_tracker.rb b/activemodel/lib/active_model/attribute_mutation_tracker.rb
index 6abf37bd44..d8cd48a53b 100644
--- a/activemodel/lib/active_model/attribute_mutation_tracker.rb
+++ b/activemodel/lib/active_model/attribute_mutation_tracker.rb
@@ -1,14 +1,15 @@
 # frozen_string_literal: true
 
 require "active_support/core_ext/hash/indifferent_access"
+require "active_support/core_ext/object/duplicable"
 
 module ActiveModel
   class AttributeMutationTracker # :nodoc:
     OPTION_NOT_GIVEN = Object.new
 
-    def initialize(attributes)
+    def initialize(attributes, forced_changes = Set.new)
       @attributes = attributes
-      @forced_changes = Set.new
+      @forced_changes = forced_changes
     end
 
     def changed_attribute_names
@@ -18,24 +19,22 @@ module ActiveModel
     def changed_values
       attr_names.each_with_object({}.with_indifferent_access) do |attr_name, result|
         if changed?(attr_name)
-          result[attr_name] = attributes[attr_name].original_value
+          result[attr_name] = original_value(attr_name)
         end
       end
     end
 
     def changes
       attr_names.each_with_object({}.with_indifferent_access) do |attr_name, result|
-        change = change_to_attribute(attr_name)
-        if change
+        if change = change_to_attribute(attr_name)
           result.merge!(attr_name => change)
         end
       end
     end
 
     def change_to_attribute(attr_name)
-      attr_name = attr_name.to_s
       if changed?(attr_name)
-        [attributes[attr_name].original_value, attributes.fetch_value(attr_name)]
+        [original_value(attr_name), fetch_value(attr_name)]
       end
     end
 
@@ -44,29 +43,26 @@ module ActiveModel
     end
 
     def changed?(attr_name, from: OPTION_NOT_GIVEN, to: OPTION_NOT_GIVEN)
-      attr_name = attr_name.to_s
-      forced_changes.include?(attr_name) ||
-        attributes[attr_name].changed? &&
-        (OPTION_NOT_GIVEN == from || attributes[attr_name].original_value == from) &&
-        (OPTION_NOT_GIVEN == to || attributes[attr_name].value == to)
+      attribute_changed?(attr_name) &&
+        (OPTION_NOT_GIVEN == from || original_value(attr_name) == from) &&
+        (OPTION_NOT_GIVEN == to || fetch_value(attr_name) == to)
     end
 
     def changed_in_place?(attr_name)
-      attributes[attr_name.to_s].changed_in_place?
+      attributes[attr_name].changed_in_place?
     end
 
     def forget_change(attr_name)
-      attr_name = attr_name.to_s
       attributes[attr_name] = attributes[attr_name].forgetting_assignment
       forced_changes.delete(attr_name)
     end
 
     def original_value(attr_name)
-      attributes[attr_name.to_s].original_value
+      attributes[attr_name].original_value
     end
 
     def force_change(attr_name)
-      forced_changes << attr_name.to_s
+      forced_changes << attr_name
     end
 
     private
@@ -75,45 +71,108 @@ module ActiveModel
       def attr_names
         attributes.keys
       end
+
+      def attribute_changed?(attr_name)
+        forced_changes.include?(attr_name) || !!attributes[attr_name].changed?
+      end
+
+      def fetch_value(attr_name)
+        attributes.fetch_value(attr_name)
+      end
+  end
+
+  class ForcedMutationTracker < AttributeMutationTracker # :nodoc:
+    def initialize(attributes, forced_changes = {})
+      super
+      @finalized_changes = nil
+    end
+
+    def changed_in_place?(attr_name)
+      false
+    end
+
+    def change_to_attribute(attr_name)
+      if finalized_changes&.include?(attr_name)
+        finalized_changes[attr_name].dup
+      else
+        super
+      end
+    end
+
+    def forget_change(attr_name)
+      forced_changes.delete(attr_name)
+    end
+
+    def original_value(attr_name)
+      if changed?(attr_name)
+        forced_changes[attr_name]
+      else
+        fetch_value(attr_name)
+      end
+    end
+
+    def force_change(attr_name)
+      forced_changes[attr_name] = clone_value(attr_name) unless attribute_changed?(attr_name)
+    end
+
+    def finalize_changes
+      @finalized_changes = changes
+    end
+
+    private
+      attr_reader :finalized_changes
+
+      def attr_names
+        forced_changes.keys
+      end
+
+      def attribute_changed?(attr_name)
+        forced_changes.include?(attr_name)
+      end
+
+      def fetch_value(attr_name)
+        attributes.send(:_read_attribute, attr_name)
+      end
+
+      def clone_value(attr_name)
+        value = fetch_value(attr_name)
+        value.duplicable? ? value.clone : value
+      rescue TypeError, NoMethodError
+        value
+      end
   end
 
   class NullMutationTracker # :nodoc:
     include Singleton
 
-    def changed_attribute_names(*)
+    def changed_attribute_names
       []
     end
 
-    def changed_values(*)
+    def changed_values
       {}
     end
 
-    def changes(*)
+    def changes
       {}
     end
 
     def change_to_attribute(attr_name)
     end
 
-    def any_changes?(*)
+    def any_changes?
       false
     end
 
-    def changed?(*)
+    def changed?(attr_name, **)
       false
     end
 
-    def changed_in_place?(*)
+    def changed_in_place?(attr_name)
       false
     end
 
-    def forget_change(*)
-    end
-
-    def original_value(*)
-    end
-
-    def force_change(*)
+    def original_value(attr_name)
     end
   end
 end