task-local.rs

use tokio;
use tokio::{task, task_local};
use std::cell::RefCell;
use std::collections::HashMap;

#[tokio::main]
async fn main() {
    let handler = task::spawn(async {
        task_local! {
            pub static WRITER: RefCell<HashMap<String, i64>>;
        }    
        let request_handler = || {
            WRITER.with(|writer| {
                *writer.borrow_mut().entry("value".to_string()).or_default() += 3;
                writer.take().get("value").cloned()
            })
        };
        WRITER.scope(RefCell::new(HashMap::new()), async move {
            WRITER.with(|writer| {
                writer.borrow_mut().insert("value".to_string(), 7);
            });
            WRITER.with(|writer| {
                let v = writer.borrow().get("value").cloned();
                assert_eq!(v, Some(7));
            });
            request_handler()
        }).await
    });
    let result = handler.await.expect("urk");
    assert_eq!(result, Some(10));    
}