Boxed dyn-trait question

help
1

I have a trait that has both non-async function and async function like this:

trait TestTrait {
    fn some_func(&self);
    async fn some_async_func(&self);
}

struct TestStruct {}

impl TestTrait for TestStruct {
    fn some_func(&self) {}
    async fn some_async_func(&self) {}
}

fn main() {
    let x: Box<dyn TestTrait> = Box::new(TestStruct {});

    println!("Hello, world!");
}

The compiler reject to compile the code, and what's the correct way to define such trait and Box?

2

async method in trait introduces an unnamable associated type, as it is desugared into something like this:

trait TestTrait {
    type some_async_func_ReturnType: Future<Output = ()>;

    fn some_func(&self);
    fn some_async_func(&self) -> Self::some_async_func_ReturnType;
}

To use a dyn Trait you need to define all its associated types (e.g. Box<dyn Foo<Goo = u32>>), but here you cannot because the associated type is unnamable.

To work around that you can use a dynamic Boxed future:

trait TestTrait {
    fn some_func(&self);
    fn some_async_func(&self) -> Box<dyn Future<Output = ()>>;
}
3

I found a crate async-trait which fits my situation.

I think I can use the crate to automatically rewrite the async functions for me.