dynamic route generation

In polymorphic_routes.rb, line 131, a name route that has been built
(e.g. post_users_path) gets invoked on self (assuming there was not
RoutesProxy instance appended as the first element of the array passed
as the first argument to form_for, then send() would be invoked on
self, which is PolymorphicRoutes module, which belongs to
ActionDispatch::Routing):

(proxy || self).send(named_route, *args)

so where exactly is post_users_path, for example, defined (and its
obvious that it must being defined dynamically using instance_eval or
class_eval based on the resource generation of routes).

but I would like to know where in the latest release of Rails source
on github is that definition occurring.

thanks for response