Intro

setup # 

#[tokio::main]
async fn main() {
    println!("hello");
}

[tokio::main] 是宏，会展开成同步代码

fn main() {
    let mut rt = tokio::runtime::Runtime::new().unwrap();
    rt.block_on(async {
        println!("hello");
    })
}

!#... 是 attribute