当Map还没有条目时，我试图用异步调用从DB中惰性地填充HashMap。
Rust编译器警告说异步闭包不稳定，但我应该尝试async {。
我尝试遵循这个建议，但我在评论中得到了expected a FnOnce〈（）〉closure错误：

use std::collections::HashMap;
use tokio::runtime::Runtime;

async fn get_from_store(key: String) -> String {
    // pretend to get this from an async sqlx db call
    String::from(format!("value-for-{key}"))
}

async fn do_work() {
    let mut map: HashMap<String, String> = HashMap::new();

    let key = String::from("key1");

    // the compiler advised async closures were unstable and...
    // to use an async block, remove the `||`: `async {` (rustc E0658)
    map.entry(key.clone())
        .or_insert_with(async { get_from_store(key) }.await);

    // the above now gives the error:
    //expected a `FnOnce<()>` closure, found `impl Future<Output = String>...xpected an `FnOnce<()>` closure, found `impl Future<Output = String>`

    for (key, value) in &map {
        println!("{}: {}", key, value);
    }
}

fn main() {
    let runtime = Runtime::new().unwrap_or_else(|e| panic!("Haha: {e}"));
    let result = do_work();
    match runtime.block_on(result) {
        _ => {}
    }
}

可能有理由不通过异步更新HashMap，但上面的错误给了我希望，我只是做错了...