Improve the performance of `save` and friends

The biggest source of the performance regression in these methods
occurred because dirty tracking required eagerly materializing and type
casting the assigned values. In the previous commits, I've changed dirty
tracking to perform the comparisons lazily. However, all of this is moot
when calling `save`, since `changes_applied` will be called, which just
ends up eagerly materializing everything, anyway. With the new mutation
tracker, it's easy to just compare the previous two hashes in the same
lazy fashion.

We will not have aliasing issues with this setup, which is proven by the
fact that we're able to detect nested mutation.

Before:
    User.create! 2.007k (± 7.1%) i/s -     10.098k

After:
    User.create! 2.557k (± 3.5%) i/s -     12.789k

Fixes #19859

2 files changed, 39 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/attribute_methods/dirty.rb b/activerecord/lib/active_record/attribute_methods/dirty.rb
index a439683185..24f0ccb239 100644
--- a/activerecord/lib/active_record/attribute_methods/dirty.rb
+++ b/activerecord/lib/active_record/attribute_methods/dirty.rb
@@ -51,12 +51,12 @@ module ActiveRecord
       end
 
       def changes_applied
-        super
+        @previous_mutation_tracker = @mutation_tracker
         store_original_attributes
       end
 
       def clear_changes_information
-        super
+        @previous_mutation_tracker = nil
         store_original_attributes
       end
 
@@ -89,6 +89,10 @@ module ActiveRecord
         end
       end
 
+      def previous_changes
+        previous_mutation_tracker.changes
+      end
+
       def attribute_changed_in_place?(attr_name)
         @mutation_tracker.changed_in_place?(attr_name)
       end
@@ -119,6 +123,10 @@ module ActiveRecord
         @mutation_tracker = @mutation_tracker.now_tracking(@attributes)
       end
 
+      def previous_mutation_tracker
+        @previous_mutation_tracker ||= NullMutationTracker.new
+      end
+
       def cache_changed_attributes
         @cached_changed_attributes = changed_attributes
         yield
diff --git a/activerecord/lib/active_record/attribute_mutation_tracker.rb b/activerecord/lib/active_record/attribute_mutation_tracker.rb
index e4be5c5524..168794fcb4 100644
--- a/activerecord/lib/active_record/attribute_mutation_tracker.rb
+++ b/activerecord/lib/active_record/attribute_mutation_tracker.rb
@@ -13,6 +13,14 @@ module ActiveRecord
       end
     end
 
+    def changes
+      attr_names.each_with_object({}.with_indifferent_access) do |attr_name, result|
+        if changed?(attr_name)
+          result[attr_name] = [original_attributes.fetch_value(attr_name), attributes.fetch_value(attr_name)]
+        end
+      end
+    end
+
     def changed?(attr_name)
       attr_name = attr_name.to_s
       modified?(attr_name) || changed_in_place?(attr_name)
@@ -52,4 +60,25 @@ module ActiveRecord
       end
     end
   end
+
+  class NullMutationTracker
+    def changed_values
+      {}
+    end
+
+    def changes
+      {}
+    end
+
+    def changed?(*)
+      false
+    end
+
+    def changed_in_place?(*)
+      false
+    end
+
+    def forget_change(*)
+    end
+  end
 end