tokio: setting up the async runtime correctly

Async functions compile fine but panic at runtime with 'no current runtime' or produce cryptic errors because the tokio runtime is not set up correctly.

Use the #[tokio::main] macro to set up the runtime, or build it manually for library code:

[dependencies]
tokio = { version = "1", features = ["full"] }

// Application entrypoint — macro sets up multi-threaded runtime
#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let result = fetch_data("https://example.com").await?;
    println!("{}", result);
    Ok(())
}

// For tests
#[tokio::test]
async fn test_something() {
    let val = async_operation().await;
    assert_eq!(val, 42);
}

// In library code where you need to run async from sync context
fn sync_wrapper() -> String {
    let rt = tokio::runtime::Runtime::new().unwrap();
    rt.block_on(async_operation())
}

Rust's async/await is just syntax sugar for state machines. Futures are lazy — they do nothing until polled by an executor (runtime). tokio provides the multi-threaded executor, I/O reactor, and timer driver.