Missing `FromPointerReader` implementation for dynamic readers

[marmeladema]

Hello!

I am currently trying to build some helper to append to an arbitrary list.
In order to do that, I am relying on dynamic_value, dynamic_struct and dynamic_list modules.
The algorithm is roughly the following:

1. Access the list within the message walking the tree of dynamic_struct until the list is found
2. Save the list into another temporarily message/builder
3. Create a new list with N+1 element
4. Copy each element from the saved list at 2.
5. Write the new element at index N

The problem is that step 4: I haven't been able to find a way to get back dynamic_list::Reader from a message::Reader.
In order to deeply copy the list, here is what I am trying to do:

let dynamic_list_reader = <...>;
let mut saved_list_builder = capnp::message::Builder::default();
saved_list_builder.set_root(dynamic_list_reader).unwrap();

I would like to be able to do something like:

let saved_list_reader = saved_list_builder.get_root_as_reader::<dynamic_list::Reader>().unwrap();

So that I can copy each element of that list into the new list in the initial builder.

Is what am trying even make sense? I believe implementing FromPointerReader for dynamic_list::Reader should allow me to do what I need but I am not sure if it would be safe / sound to do?

In any case, thank you for your help 👍

[marmeladema]

Alternatively, I was wondering if I should use https://docs.rs/capnp/latest/capnp/dynamic_value/enum.Reader.html#method.new
However I have no idea how to get back a https://docs.rs/capnp/latest/capnp/schema_capnp/value/struct.Reader.html