Mocking spawn (i.e. for tokio runtime)

[MOZGIII]

With this trait:

pub trait Runtime: Clone + Send + Sync + Unpin + Debug {
    fn spawn<F>(&self, future: F)
    where
        F: Future<Output = ()> + Send + 'static;
}

How do I mock spawn?

I tried like this:

let mock = MockRuntime::new();
mock.expect_spawn().returning(|_| ());
mock

   |
xx |             mock.expect_spawn().returning(|_| ());
   |                                            ^ consider giving this closure parameter a type

[asomers]

In this case spawn is a generic method, which really means it's an infinite collection of nongeneric methods. You must set the expectation for the right version of spawn, or else the call won't match. Assuming you're going to spawn a BoxFuture, you can do that like this. Unfortunately, some Future types can be very long and hard to name. There's no easy solution to the problem of figuring out a Future's true type.

mock.expect_spawn::<BoxFuture<u32, ()>>().returning(|_| ());

Also, in this example you could use return_const(()) instead of returning(|_| ()), but that won't fix the type problem.

[MOZGIII]

The problem is I'm using it with async fn, so it's flat out impossible to just name, afaik.
Good thing is I could implement the trait myself, and add a mocked method that's non-generic and simply receives the non-generic BoxFuture. This is far from perfect, but it works.

So, the issue is essence is really not about the Futures, but rather about taking compiler generated types, like Fn, FnMut, FnOnce and Future that can't be addressed any way other than impl ....
I think it's worth creating a dedicated issue for this more generic thing.

[asomers]

I think this problem could be elegantly solved by #217 .