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Consider async blocks on lifetime errors #64382
Description
// For QoS, this will only accept one message and output that
// receive all inbound messages
// TODO: this should match edns settings
loop {
let msg = if let Some(receiving) = receiving {
// TODO: should we drop this packet if it's not from the same src as dest?
let msg = ready!(receiving.as_mut().poll(cx))?;
Some(Poll::Ready(Some(Ok(msg))))
} else {
None
};
*receiving = None;
if let Some(msg) = msg {
return msg;
}
let socket = Arc::clone(socket);
let mut buf = [0u8; 2048];
let receive_future = async {
let socket = socket;
let mut socket = socket.lock().await;
let (len, src) = socket.recv_from(&mut buf).await?;
Ok(SerialMessage::new(
buf.iter().take(len).cloned().collect(),
src,
))
};
*receiving = Some(Box::pin(receive_future));
}emits the following error
error[E0597]: `buf` does not live long enough
--> crates/proto/src/udp/udp_stream.rs:204:56
|
183 | let msg = if let Some(receiving) = receiving {
| --------- borrow later used here
...
200 | let receive_future = async {
| ________________________________________-
201 | | let socket = socket;
202 | |
203 | | let mut socket = socket.lock().await;
204 | | let (len, src) = socket.recv_from(&mut buf).await?;
| | ^^^ borrowed value does not live long enough
... |
209 | | ))
210 | | };
| |_____________- value captured here by generator
...
213 | }
| - `buf` dropped here while still borrowed
The correct code needs the buf to be part of the async block because it is a stack allocation that needs to be in the generator:
loop {
let msg = if let Some(receiving) = receiving {
// TODO: should we drop this packet if it's not from the same src as dest?
let msg = ready!(receiving.as_mut().poll(cx))?;
Some(Poll::Ready(Some(Ok(msg))))
} else {
None
};
*receiving = None;
if let Some(msg) = msg {
return msg;
}
let socket = Arc::clone(socket);
let receive_future = async {
let socket = socket;
let mut buf = [0u8; 2048];
let mut socket = socket.lock().await;
let (len, src) = socket.recv_from(&mut buf).await?;
Ok(SerialMessage::new(
buf.iter().take(len).cloned().collect(),
src,
))
};
*receiving = Some(Box::pin(receive_future));
}Ideally, the error message would mention buf:
error[E0597]: `buf` does not live long enough
--> crates/proto/src/udp/udp_stream.rs:204:56
|
183 | let msg = if let Some(receiving) = receiving {
| --------- borrow later used here
...
199 | let mut buf = [0u8; 2048];
| --- consider moving this stack allocation inside the `async` block
200 | let receive_future = async {
| ________________________________________-
201 | | let socket = socket;
202 | |
203 | | let mut socket = socket.lock().await;
204 | | let (len, src) = socket.recv_from(&mut buf).await?;
| | ^^^ borrowed value does not live long enough
... |
209 | | ))
210 | | };
| |_____________- value captured here by `async` block
...
213 | }
| - `buf` dropped here while still borrowed
thanks to @bluejekyll for bringing this up!
Reacted by Benjamin Fry
Activity
cramertj commentedon Sep 24, 2019
We could also consider suggesting an
async moveblock, similar to the hints we provide for closures.nikomatsakis commentedon Oct 1, 2019
Marking this as a focus item.
@rustbot assign @csmoe
Rollup merge of rust-lang#65166 - csmoe:async-move, r=estebank
Rollup merge of rust-lang#65166 - csmoe:async-move, r=estebank