4 files changed, 45 insertions, 20 deletions
diff --git a/activemodel/lib/active_model/type.rb b/activemodel/lib/active_model/type.rb
index f8ca7d0512..bec851594f 100644
--- a/activemodel/lib/active_model/type.rb
+++ b/activemodel/lib/active_model/type.rb
@@ -9,6 +9,7 @@ require 'active_model/type/date_time'
 require 'active_model/type/decimal'
 require 'active_model/type/decimal_without_scale'
 require 'active_model/type/float'
+require 'active_model/type/immutable_string'
 require 'active_model/type/integer'
 require 'active_model/type/string'
 require 'active_model/type/text'
@@ -49,6 +50,7 @@ module ActiveModel
     register(:date_time, Type::DateTime)
     register(:decimal, Type::Decimal)
     register(:float, Type::Float)
+    register(:immutable_string, Type::ImmutableString)
     register(:integer, Type::Integer)
     register(:string, Type::String)
     register(:text, Type::Text)
diff --git a/activemodel/lib/active_model/type/immutable_string.rb b/activemodel/lib/active_model/type/immutable_string.rb
new file mode 100644
index 0000000000..cfa4b3fb4b
--- /dev/null
+++ b/activemodel/lib/active_model/type/immutable_string.rb
@@ -0,0 +1,28 @@
+module ActiveModel
+  module Type
+    class ImmutableString < Value # :nodoc:
+      def type
+        :string
+      end
+
+      def serialize(value)
+        case value
+        when ::Numeric, ActiveSupport::Duration then value.to_s
+        when true then "t"
+        when false then "f"
+        else super
+        end
+      end
+
+      private
+
+      def cast_value(value)
+        case value
+        when true then "t"
+        when false then "f"
+        else value.to_s.freeze
+        end
+      end
+    end
+  end
+end
diff --git a/activemodel/lib/active_model/type/string.rb b/activemodel/lib/active_model/type/string.rb
index fd1630c751..4c8dc15778 100644
--- a/activemodel/lib/active_model/type/string.rb
+++ b/activemodel/lib/active_model/type/string.rb
@@ -1,34 +1,22 @@
+require "active_model/type/immutable_string"
+
 module ActiveModel
   module Type
-    class String < Value # :nodoc:
-      def type
-        :string
-      end
-
+    class String < ImmutableString # :nodoc:
       def changed_in_place?(raw_old_value, new_value)
         if new_value.is_a?(::String)
           raw_old_value != new_value
         end
       end
 
-      def serialize(value)
-        case value
-        when ::Numeric, ActiveSupport::Duration then value.to_s
-        when ::String then ::String.new(value)
-        when true then "t"
-        when false then "f"
-        else super
-        end
-      end
-
       private
 
       def cast_value(value)
-        case value
-        when true then "t"
-        when false then "f"
-        # String.new is slightly faster than dup
-        else ::String.new(value.to_s)
+        result = super
+        if ::String === result
+          ::String.new(result)
+        else
+          result
         end
       end
     end
diff --git a/activemodel/test/cases/type/string_test.rb b/activemodel/test/cases/type/string_test.rb
index 8ec771ea42..7b25a1ef74 100644
--- a/activemodel/test/cases/type/string_test.rb
+++ b/activemodel/test/cases/type/string_test.rb
@@ -10,6 +10,13 @@ module ActiveModel
       assert_equal "123", type.cast(123)
     end
 
+    test "immutable strings are not duped coming out" do
+      s = "foo"
+      type = Type::ImmutableString.new
+      assert_same s, type.cast(s)
+      assert_same s, type.deserialize(s)
+    end
+
     test "values are duped coming out" do
       s = "foo"
       type = Type::String.new