Rustler: how to return an `usize` variable?

Questions / Help
1

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?

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2

As I responded on IRC…resources only work with structs and must be accompanied with the rustler::resource_struct_init! macro (in 0.21) or rustler::resource! (in 0.22).

In order to make this work, you’ll need to make a new type which wraps the usize like so:

struct Affected(usize)
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3

Curiously enough, I was able to just skip ResourceArc and I get no compilation error now.

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4

Yeah a usize and other primitives don’t really fit the use-case for resources :grinning:. A resource is used to keep some rust data-structure alive in between NIF calls.

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