以下代码尝试在通过连接获得msg后异步更新主 Dataframe df（来自polars包）。
我看过关于堆栈溢出的“重复”帖子，但仍然不明白我做错了什么。我只是想可变地借用 Dataframe 并更新它，仅此而已！我用字符串尝试了一下，它工作得很好...

pub async fn new_handler(endpoint: &str) -> tokio::task::JoinHandle<()> {
    // Make master df for this handler
    let mut df = DataFrame::empty().lazy();
    // Make a stream for this handler
    let stream = new_stream(endpoint).await;
    let handle = tokio::spawn(async move {
        // let handle = tokio::spawn(async {
        stream
            .for_each(|msg| async move {
                match msg {
                    Ok(msg) => {
                        // Parse the json message into a struct
                        let jsonmsg: AggTrade =
                            serde_json::from_str(&msg.to_string()).expect("Failed to parse json");
                        let s0 = Series::new(
                            "price",
                            vec![jsonmsg.price.parse::<f32>().expect("Failed to parse price")],
                        );
                        let s1 = Series::new(
                            "quantity",
                            vec![jsonmsg
                                .quantity
                                .parse::<f32>()
                                .expect("Failed to parse quantity")],
                        );
                        // Create new dataframe from the json data
                        let df2 = DataFrame::new(vec![s0.clone(), s1.clone()]).unwrap().lazy();
                        // append the new data from df2 to the master df
                        df = polars::prelude::concat([df, df2], false, true)
                            .expect("Failed to concat");
                    }
                    Err(e) => {
                        println!("Error: {}", e);
                    }
                }
            })
            .await
    });
    handle
}

出现以下错误：

error: captured variable cannot escape `FnMut` closure body
  --> src/websockets.rs:33:29
   |
27 |       let mut df = DataFrame::empty().lazy();
   |           ------ variable defined here
...
33 |               .for_each(|msg| async {
   |  ___________________________-_^
   | |                           |
   | |                           inferred to be a `FnMut` closure
34 | |                 match msg {
35 | |                     Ok(msg) => {
36 | |                         // Parse the json message into a struct
...  |
58 | |                         df = polars::prelude::concat([df.clone(), df2.clone()], false, true)
   | |                                                       -- variable captured here
...  |
86 | |                 }
87 | |             })
   | |_____________^ returns an `async` block that contains a reference to a captured variable, which then escapes the closure body
   |
   = note: `FnMut` closures only have access to their captured variables while they are executing...
   = note: ...therefore, they cannot allow references to captured variables to escape