#[frb(opaque)] and async funcs for struct

[Corban-Dallas]

I have a complex struct, Ctr for simplicity, which I want to wrap and send to Dart to work with. However, generated code complains about UnwindSafe and other related issues for every asynchronous function, even when I use AssertUnwindSafe and catch_unwind. Is there a correct way to resolve this issue?

#[frb(opaque)]
pub struct WrappedCtr(AssertUnwindSafe<Ctr>);

impl WrappedCtr {
    pub async fn doSomething() -> String {
        AssertUnwindSafe(self.0.doSomething())
            .catch_unwind()
            .await
            .unwrap_or_default()
    }
}

pub fn getCtr() -> WrappedCtr { ... }

impl UnwindSafe for WrappedCtr { }
unsafe impl Send for WrappedCtr { }

[Corban-Dallas]

After some investigation, it looks like the core problem is in Send

error: future cannot be sent between threads safely
  --> src/frb_generated.rs:43:33
   |
43 |     FLUTTER_RUST_BRIDGE_HANDLER.wrap_async::<flutter_rust_bridge::for_generated::DcoCodec, _, _, _>(
   |                                 ^^^^^^^^^^ future created by async block is not `Send`
   |
   = help: within `{async block@src/frb_generated.rs:52:28: 62:14}`, the trait `std::marker::Send` is not implemented for `std::sync::RwLockReadGuard<'_, frb_books::WrappedCtr>`
note: future is not `Send` as this value is used across an await
  --> src/frb_generated.rs:57:91
   |
55 |                         let api_that = api_that.rust_auto_opaque_decode_ref()?;
   |                             -------- has type `std::sync::RwLockReadGuard<'_, frb_books::WrappedCtr>` which is not `Send`
56 |                         Result::<_, anyhow::Error>::Ok(
57 |                             crate::api::frb_books::WrappedCtr::create(&api_that, api_draft).await,
   |                                                                                             ^^^^^ await occurs here, with `api_that` maybe used later
note: required by a bound in `wrap_async`
  --> /Users/grigory.k/.cargo/registry/src/index.crates.io-6f17d22bba15001f/flutter_rust_bridge-2.0.0-dev.9/src/handler/handler.rs:63:15
   |
55 |     fn wrap_async<Rust2DartCodec, PrepareFn, TaskFn, TaskRetFut>(
   |        ---------- required by a bound in this associated function
...
63 |             + TaskRetFutTrait
   |               ^^^^^^^^^^^^^^^ required by this bound in `Handler::wrap_async`

I am not so good in Rust, hoped that to mark WrappedCtr with Send trait will help. I guess I need more fundamental solution

[fzyzcjy]

For unwindsafe, it will be changed in #1543, thus no worries about unwindsafe after that is done

As for Send, yes, currently the opaque needs to be send, because the rust code can be executed in different threads (in order to make the UI nonblocking thus smooth)