Then I think you should also write an Encoder trait implementation for AutocommitOn (or the enum it belongs to)?
Basically: Rustler needs your Rust objects be serializable with Rustler means. This means you have to recursively and manually add (de)serializers for every type you want to move inbetween the BEAM and the Rust code.
Aaah. Yeah that might be the cause - good call. I’ll try that.
The problem now is that I cannot impl Encoder for AutocommitOn since it is not part of the current crate. Will invastigate further 
You can make a wrapper of that enum on your own and then write a serializer for it:
pub struct AutocommitWrapper(pub Autocommit); // `Autocommit` == your foreign enum type name.
// Implement Rustler `Encoder` for `AutocommitWrapper` here...
Yeah I know, I tried it before I wrote this post but it doesn’t work.
My simplified code at this moment
use rustler::resource::ResourceArc;
use rustler::{Env, Encoder, Term};
rustler::atoms! {
ok,
error
}
type MyRustReturnType = MyStruct;
enum MyResult {
Success(ResourceArc<MyRustReturnType>),
Failure(String)
}
impl Encoder for MyResult {
fn encode(&self, env: Env) -> Term {
match self {
MyResult::Success(arc) => (ok(), arc).encode(env),
MyResult::Failure(msg) => (error(), msg).encode(env)
}
}
}
pub struct MyStruct {
pub a: i64
}
#[rustler::nif]
fn add(a: i64, b: i64) -> MyResult {
MyResult::Success(ResourceArc::new(MyStruct{a: a+b}))
}
The problem is with Success(ResourceArc<MyRustReturnType>) as it doesn’t implement ResourceTypeProvider :c
Sorry, I think I am not following. What’s stopping you from doing impl Encoder for MyRustReturnType?
Thanks for so fast reply. The problem is in defining enum MyResult. I cannot define Success(ResourceArc<MyRustReturnType>). I am getting
--> src/lib.rs:12:13
|
12 | Success(ResourceArc<MyRustReturnType>),
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `ResourceTypeProvider` is not implemented for `MyStruct`
|
::: /home/michal/.cargo/registry/src/github.com-1ecc6299db9ec823/rustler-0.22.0-rc.0/src/resource.rs:118:8
|
118 | T: ResourceTypeProvider,
| -------------------- required by this bound in `ResourceArc`
And I am wondering if I should try to implement this as in the source code documentation there is that in moste cases user doesn’t have to worry about ResourceTypeProvider and my example is quite simple.
So… can’t you do this?
impl Encoder for MyStruct {
fn encode(&self, env: Env) -> Term {
<FIELD_OR_FIELDS>.encode(env)
}
}