Move spawn, spawn_with_handle into futures-util as macros

[MajorBreakfast]

These are currently in futures-executor/src/spawn.rs but they don't depend on anything from the futures-executor crate => they should live in futures-util because they're general purpose and similar to the kind of stuff in futures-util

[cramertj]

Yeah, that sounds good! I'd also +1 spawn! and spawn_with_handle! macros for wrapping them in await!s.

[MajorBreakfast]

I also favor the macro versions and would like to replace these with macro versions. However, @Nemo157 mentioned this use case once:

let handle = await!(spawn_with_handle(future).with_executor(...));

How could this work with macros?

[cramertj]

You could always fall back to using await! if the macros aren't what you want, but I think the macros are good for capturing the common case.

[aajtodd]

The documentation on spawn says that the function will spawn the given future as a task onto the default executor, maybe I'm missing something but I don't see where this happens.

[MajorBreakfast]

@aajtodd You're right. It should say "the context's executor"

[MajorBreakfast]

@cramertj How about providing only macros, but with an optional executor parameter?

spawn!(future);
spawn!(future, executor);

[cramertj]

@MajorBreakfast if you have an executor and a future, you don't need a macro-- you can write executor.spawn_obj(Box::new(future).into()) (eventually executor.spawn(future)). I guess we'd be saving a few characters by avoiding the whole Box::new(...).into() dance, but we could do the same by providing an extension trait that delegated to spawn_obj internally (similar to the existing one for Context).

[MajorBreakfast]

@cramertj You're right, the macro isn't needed for this use case. I've created an issue for ExecutorExt with an approach that I think would work. Once we have ExecutorExt, we can build easy to use spawn! and spawn_with_handle! macros around it.

[Nemo157]

The documentation on spawn says that the function will spawn the given future as a task onto the default executor, maybe I'm missing something but I don't see where this happens.

You're right. It should say "the context's executor"

I had the same initial reaction, eventually I figured out that "the default executor" is "the current context's executor", there's nothing else for it to refer to. 👍 for clarifying this a bit more.

[MajorBreakfast]

Closed by #1156