Consider the following code:
use std::sync::Arc;
use tokio::runtime::Runtime; // 0.3.1
async fn f() {
let room_ref = Arc::new(Vec::new());
let gameloop_handle = Runtime::new().unwrap().spawn(async {
game_loop(Arc::clone(&room_ref))
});
gameloop_handle.await;
}
fn game_loop(v: Arc<Vec<usize>>) {}The error message has a helpful hint:
help: to force the async block to take ownership of `room_ref` (and any other referenced variables), use the `move` keyword
|
7 | let gameloop_handle = Runtime::new().unwrap().spawn(async move {
8 | game_loop(Arc::clone(&room_ref))
9 | });
But it doesn't explain very well why move is necessary:
error[E0373]: async block may outlive the current function, but it borrows `room_ref`, which is owned by the current function
--> src/lib.rs:7:63
|
7 | let gameloop_handle = Runtime::new().unwrap().spawn(async {
| _______________________________________________________________^
8 | | game_loop(Arc::clone(&room_ref))
| | -------- `room_ref` is borrowed here
9 | | });
| |_____^ may outlive borrowed value `room_ref`
|
note: function requires argument type to outlive `'static`
--> src/lib.rs:7:57
|
7 | let gameloop_handle = Runtime::new().unwrap().spawn(async {
| _________________________________________________________^
8 | | game_loop(Arc::clone(&room_ref))
9 | | });
| |_____^
In particular, 'async block may outlive the current function' makes no sense without a very good mental model of async: the future is awaited within the current function, so of course it can't outlive it. The thing to realize here is that even though it's written as one function, it's actually two: one executed before the yield point, and one after, and the stack space for the first function goes away when you call await.
It would be nice to instead say something like
note: borrows cannot be held across a yield point, because the stack space of the current function is not preserved
help: see https://rust-lang.github.io/async-book/03_async_await/01_chapter.html#awaiting-on-a-multithreaded-executor for more information
Consider the following code:
The error message has a helpful hint:
But it doesn't explain very well why
moveis necessary:In particular, 'async block may outlive the current function' makes no sense without a very good mental model of async: the future is
awaited within the current function, so of course it can't outlive it. The thing to realize here is that even though it's written as one function, it's actually two: one executed before the yield point, and one after, and the stack space for the first function goes away when you callawait.It would be nice to instead say something like