Implement equality comparison on `AttributeSet` and friends

Any gems or libraries which do work with serialization or YAML will
ultimately need to compare these objects (albeit indirectly) to ensure
correctness. These will likely never get used internally (as they're
slow), but we should still expose them for others.

activerecord/lib/active_record/attribute_set.rb

@@ -91,6 +91,10 @@ module ActiveRecord
       AttributeSet.new(new_attributes)
     end
 
+    def ==(other)
+      attributes == other.attributes
+    end
+
     protected
 
     attr_reader :attributes

activerecord/lib/active_record/attribute_set/builder.rb

@@ -68,10 +68,29 @@ module ActiveRecord
       end
     end
 
+    def ==(other)
+      if other.is_a?(LazyAttributeHash)
+        materialize == other.materialize
+      else
+        materialize == other
+      end
+    end
+
     protected
 
     attr_reader :types, :values, :additional_types, :delegate_hash
 
+    def materialize
+      unless @materialized
+        values.each_key { |key| self[key] }
+        types.each_key { |key| self[key] }
+        unless frozen?
+          @materialized = true
+        end
+      end
+      delegate_hash
+    end
+
     private
 
     def assign_default_value(name)
@@ -85,16 +104,5 @@ module ActiveRecord
         delegate_hash[name] = Attribute.uninitialized(name, type)
       end
     end
-
-    def materialize
-      unless @materialized
-        values.each_key { |key| self[key] }
-        types.each_key { |key| self[key] }
-        unless frozen?
-          @materialized = true
-        end
-      end
-      delegate_hash
-    end
   end
 end

activerecord/test/cases/attribute_set_test.rb

@@ -239,5 +239,15 @@ module ActiveRecord
       assert_equal 2, new_attributes.fetch_value(:foo)
       assert_equal 3, new_attributes.fetch_value(:bar)
     end
+
+    test "comparison for equality is correctly implemented" do
+      builder = AttributeSet::Builder.new(foo: Type::Integer.new, bar: Type::Integer.new)
+      attributes = builder.build_from_database(foo: "1", bar: "2")
+      attributes2 = builder.build_from_database(foo: "1", bar: "2")
+      attributes3 = builder.build_from_database(foo: "2", bar: "2")
+
+      assert_equal attributes, attributes2
+      assert_not_equal attributes2, attributes3
+    end
   end
 end