Rustler: how to return an `usize` variable?

Poking @hansihe and @scrogson if that’s okay. Everybody else’s input is appreciated as well.

I need to return a Rust usize variable and I am trying to do it like so:

let affected = ...; // a function that returns `usize`.
Ok((atoms::ok(), ResourceArc::new(affected)).encode(env))

I am getting this error (and a few more related to it):

error[E0277]: the trait bound `usize: rustler::resource::ResourceTypeProvider` is not satisfied
   --> src/lib.rs:101:63
    |
101 |             Ok(affected) => Ok((atoms::ok(), ResourceArc::new(affected)).encode(env)),
    |                                                               ^^^^^^^^ the trait `rustler::resource::ResourceTypeProvider` is not implemented for `usize`
    |
    = note: required by `rustler::ResourceArc::<T>::new`

I am at rustler 0.21.0.

Can you point me in the right direction?