Avoid definition of methods in CurrentAttributes at runtime

Replacing on the fly a `method_missing` by a generated method
sound like a nice trick, but it's not as good as it sound for
optimization, as the method will be generated by the first
request to use it, preventing the ISeq from being is shared memory.

Instead we can eagerly define a delegator when instance methods
are defined, and keep a regular `method_missing + send` for the
very rare cases not covered.

Co-Authored-By: Jean Boussier <[email protected]>

Author: seanpdoyle

activesupport/lib/active_support/current_attributes.rb

@@ -164,16 +164,18 @@ def current_instances_key
         end
 
         def method_missing(name, ...)
-          # Caches the method definition as a singleton method of the receiver.
-          #
-          # By letting #delegate handle it, we avoid an enclosure that'll capture args.
-          singleton_class.delegate name, to: :instance
-
-          send(name, ...)
+          instance.public_send(name, ...)
         end
 
         def respond_to_missing?(name, _)
-          super || instance.respond_to?(name)
+          instance.respond_to?(name) || super
+        end
+
+        def method_added(name)
+          return if name == :initialize
+          return unless public_method_defined?(name)
+          return if respond_to?(name, true)
+          singleton_class.delegate(name, to: :instance, as: self)
         end
     end