to_str works here

actionpack/lib/action_controller/caching/fragments.rb

@@ -54,7 +54,7 @@ def write_fragment(key, content, options = nil)
 
         key = fragment_cache_key(key)
         instrument_fragment_cache :write_fragment, key do
-          content = content.html_safe.as_str if content.respond_to?(:html_safe)
+          content = content.html_safe.to_str if content.respond_to?(:html_safe)
           cache_store.write(key, content, options)
         end
         content

activesupport/lib/active_support/core_ext/string/output_safety.rb

@@ -89,12 +89,8 @@ def to_s
       self
     end
 
-    def as_str
-      ''.replace(self)
-    end
-
     def to_yaml(*args)
-      as_str.to_yaml(*args)
+      to_str.to_yaml(*args)
     end
   end
 end