Avoid tokio feature `rt-multi-thread` when building for emscripten wasm32?

[kylebarron]

Currently we require tokio with the rt, rt-multi-thread and time features:

pyo3-async-runtimes/Cargo.toml

Lines 141 to 144 in 6320af5

	[dependencies.tokio]
	version = "1.13"
	features = ["rt", "rt-multi-thread", "time"]
	optional = true

When building for pyodide and emscripten wasm, I get a compile error:

error: Only features sync,macros,io-util,rt,time are supported on wasm.
   --> /Users/kyle/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tokio-1.50.0/src/lib.rs:475:1
    |
475 | compile_error!("Only features sync,macros,io-util,rt,time are supported on wasm.");
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I'm curious if pyo3-async-runtimes would work in pyodide if the rt-multi-thread feature were removed.

We should be able to try

[target.'cfg(not(target_family = "wasm"))'.dependencies.tokio]
version = "1.13"
features = ["rt", "rt-multi-thread", "time"]
optional = true

[target.'cfg(target_family = "wasm")'.dependencies.tokio]
version = "1.13"
features = ["rt", "time"]
optional = true

[davidhewitt]

There are the local_future_into_py_with_locals family of functions, but a comment stating their utility is questionable. It might be that this is an example where they have value? We could certainly give it a go and maybe add some testing in CI.

[kylebarron]

I think I've concluded that this is too deep a rabbit hole to warrant my time at the moment 🥲. I'd love to hear what anyone else finds if they take a stab at this!