What is the smallest feature set to enable polling a future with Tokio?

Question

I want to poll an async function:

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    some_function().await;
}

I am currently activating all features:

tokio = { version = "1.4.0", features = ["full"] }

which of those are necessary?

full = [
  "fs",
  "io-util",
  "io-std",
  "macros",
  "net",
  "parking_lot",
  "process",
  "rt",
  "rt-multi-thread",
  "signal",
  "sync",
  "time",
]

Answer 1

To enable polling a future with Tokio, you need a Runtime.

This is supported on crate feature rt only.

[dependencies]
tokio = { version = "1.4.0", features = ["rt"] }

fn main() -> Result<(), Box<dyn std::error::Error>> {
    tokio::runtime::Builder::new_current_thread()
        .build()
        .unwrap()
        .block_on(some_function())
}

async fn some_function() -> Result<(), Box<dyn std::error::Error>> {
    Ok(())
}

If you want to use the tokio::main macro:

This is supported on crate features rt and macros only.

[dependencies]
tokio = { version = "1.4.0", features = ["rt", "macros"] }

#[tokio::main(flavor = "current_thread")]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    some_function().await
}

async fn some_function() -> Result<(), Box<dyn std::error::Error>> {
    Ok(())
}

If you want the exact syntax you've specified (which is not the "smallest feature set to enable polling a future with Tokio"), then the runtime error guides you:

The default runtime flavor is multi_thread, but the rt-multi-thread feature is disabled.

[dependencies]
tokio = { version = "1.4.0", features = ["rt", "rt-multi-thread", "macros"] }

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    some_function().await
}

async fn some_function() -> Result<(), Box<dyn std::error::Error>> {
    Ok(())
}

See also:

• How do I synchronously return a value calculated in an asynchronous Future in stable Rust?
• How do I execute an async/await function without using any external dependencies?