In extracted code, there are two cases where segments can be created outside of an allocation scope:

* structs passed returned by value
* callback segments

In these cases it is necessary for clients to manually handle the life-cycle of these segments with a separate (nested) try with resource, which makes the code harder to follow.

Jextract is currently bypassing this limitation by providing a "register" mechanism, but the provided mechanism is not very safe, since it is up to the user not to prematurely close the registered segment.