[Arnavion]

>If you can own a context, even with a lifetime parameter it's possible to leak it using the Box api.

The `std::pin::Pin` API prevents you from doing this (callbacks would receive `Pin<&'_ mut CallbackContext>`).

>The problem with that design (which is a great design given what I presented in the article by the way!) is that it doesn't allow you to share handles across callbacks

But how does sharing handles between callbacks look like? Is it something like "callback FooCreated provided me a Foo handle" and "callback BarCreated requires me to have the Foo from the FooCreated callback" ? So your FooCreated callback needs to save the Foo handle somewhere that it can be reused later?

If so, why is it not enough to have a `fn CallbackContext::set_foo_handle(&mut self, Foo)` (or `Pin<&mut self>` based on the above suggestion) ?