How can I call a Pinned future from an actix handler?

Question

I have an async trait method which returns a std Future:

Pin<Box<dyn Future<Output = Result<Vec<ResultType>, Box<(dyn Error + 'static)>>> + Send>>

ResultType is an associated type of the trait which is Sync + Send.

Note that this type is not Unpin.

I want to call this from an actix handler, then do something with the result.

For example:

impl StreamHandler<ws::Message, ws::ProtocolError> for MyActor {
    fn handle(&mut self, msg: ws::Message) {
        let fut = get_future();
        let actor_fut = fut
            .into_actor(&self)
            .map(|r, _actor, ctx| {
                ctx.text(r.map(|| ...))
            });

        ctx.spawn(actor_fut);
    }
}

This fails because into_actor takes ownership of the future, which is not allowed by Pin. The cleaned error message looks like:

error[E0599]: no method named `into_actor` found for type `Pin<Box<dyn Future<Output = Result<Vec<ResultType>, Box<dyn Error>>> + Send>>` in the current scope
   --> src/app_socket.rs:194:26
    |
194 |                         .into_actor(&self)
    |                          ^^^^^^^^^^ method not found in `Pin<Box<dyn Future<Output = Result<Vec<ResultType>, Box<dyn Error>>> + Send>>`
    |
    = note: the method `into_actor` exists but the following trait bounds were not satisfied:
            `&dyn Future<Output = Result<std::vec::Vec<ResultType>, std::boxed::Box<dyn std::error::Error>>> + Send : WrapFuture<_>`
            `&dyn Future<Output = Result<std::vec::Vec<ResultType>, std::boxed::Box<dyn std::error::Error>>> + Send : WrapStream<_>`
            `&mut dyn Future<Output = Result<std::vec::Vec<ResultType>, std::boxed::Box<dyn std::error::Error>>> + Send : WrapFuture<_>`
            `&mut dyn Future<Output = Result<std::vec::Vec<ResultType>, std::boxed::Box<dyn std::error::Error>>> + Send : WrapStream<_>`
            `&mut Pin<std::boxed::Box<dyn Future<Output = Result<std::vec::Vec<ResultType>, std::boxed::Box<dyn std::error::Error>>> + Send>> : WrapFuture<_>`
            `&mut Pin<std::boxed::Box<dyn Future<Output = Result<std::vec::Vec<ResultType>, std::boxed::Box<dyn std::error::Error>>> + Send>> : WrapStream<_>`
            `&Pin<std::boxed::Box<dyn Future<Output = Result<std::vec::Vec<ResultType>, std::boxed::Box<dyn std::error::Error>>> + Send>> : WrapFuture<_>`
            `&Pin<std::boxed::Box<dyn Future<Output = Result<std::vec::Vec<ResultType>, std::boxed::Box<dyn std::error::Error>>> + Send>> : WrapStream<_>`
            `dyn Future<Output = Result<std::vec::Vec<ResultType>, std::boxed::Box<dyn std::error::Error>>> + Send : WrapFuture<_>`
            `dyn Future<Output = Result<std::vec::Vec<ResultType>, std::boxed::Box<dyn std::error::Error>>> + Send : WrapStream<_>`
            `Pin<std::boxed::Box<dyn Future<Output = Result<std::vec::Vec<ResultType>, std::boxed::Box<dyn std::error::Error>>> + Send>> : WrapFuture<_>`
            `Pin<std::boxed::Box<dyn Future<Output = Result<std::vec::Vec<ResultType>, std::boxed::Box<dyn std::error::Error>>> + Send>> : WrapStream<_>`

How can I do this?

Answer 1

The actual problem was not that the future was pinned, but that it implemented the wrong future trait.

This section on Pinning explains that poll is implemented for Pin<ref T: Future>.

Hence, the into_actor signature self: impl Future -> WrapFuture<_> was fine. The problem was that the async method returned a future implementing std::future::Future while into_actor expected futures01::future::Future.

Calling .compat() on the future before calling .into_actor fixes the problem.

See this post for more detail about converting futures.