I’ve the following simplified code.
use async_trait::async_trait; // 0.1.36
use std::error::Error;
#[async_trait]
trait Metric: Send {
type Output;
type Error: Error;
async fn refresh_metric(&mut self) -> Result<Self::Output, Self::Error>;
}
#[derive(Default)]
struct StaticMetric;
#[async_trait]
impl Metric for StaticMetric {
type Output = ();
type Error = Box<dyn Error>;
async fn refresh_metric(&mut self) -> Result<Self::Output, Self::Error> {
Ok(())
}
}
struct LocalSystemData<T> {
inner: T,
}
impl<T> LocalSystemData<T>
where
T: Metric,
<T as Metric>::Error: 'static,
{
fn new(inner: T) -> LocalSystemData<T> {
LocalSystemData { inner }
}
async fn refresh_all(&mut self) -> Result<(), Box<dyn Error>> {
self.inner.refresh_metric().await?;
Ok(())
}
}
#[tokio::main]
async fn main() -> Result<(), Box<dyn Error>> {
let mut sys_data = LocalSystemData::new(StaticMetric::default());
sys_data.refresh_all().await?;
Ok(())
}
Playground
The compiler throws the following error
error[E0277]: the size for values of type `(dyn std::error::Error + 'static)` cannot be known at compilation time
--> src/main.rs:18:18
|
5 | trait Metric: Send {
| ------ required by a bound in this
6 | type Output;
7 | type Error: Error;
| ----- required by this bound in `Metric`
...
18 | type Error = Box<dyn Error>;
| ^^^^^^^^^^^^^^ doesn't have a size known at compile-time
|
= help: the trait `std::marker::Sized` is not implemented for `(dyn std::error::Error + 'static)`
= note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
= note: required because of the requirements on the impl of `std::error::Error` for `std::boxed::Box<(dyn std::error::Error + 'static)>`
I'm not really sure if I understand the problem correct.
I’m using Box<dyn Error>
because I don’t have a concrete type and boxing an error can handle all errors. In my implementation of LocaSystemData
, I added <T as Metric>::Error: 'static
(the compiler gave me that hint, not my idea). After I added the 'static
requirement the compiler complains about that the size is unknown. This happens because the size of a 'static
type should always known at compile time because the implication of static.
I changed the Metric
trait so that I get rid of the type Error;
Now I've the following async trait function and my code compiles
Playground
async fn refresh_metric(&mut self) -> Result<Self::Output, Box<dyn Error>>;
Why does my second version compile and the first one does not? For me as a human the code does exactly the same. I feel I tricked the compiler :-).
The error message is a bit misleading, as it represents an intermediate issue that emerged during type constraint resolution. The same problem can be reproduced without async.
use std::error::Error;
trait Metric {
type Error: Error;
}
struct StaticMetric;
impl Metric for StaticMetric {
type Error = Box<dyn Error>;
}
The root of the problem is that Box<dyn Error>
does not implement std::error::Error
. And as specified by the implementation of From<Box<E>>
for Box<dyn Error>
, the inner type E
must also be Sized
.
impl<'a, E: Error + 'a> From<E> for Box<dyn Error + 'a>
Therefore, Box<dyn Error>
should not be assigned to the associated type Metric::Error
.
Other than getting rid of the associated type altogether, this can be fixed by introducing your own new error type which can flow into the main function.
#[derive(Debug, Default)]
struct MyErr;
impl std::fmt::Display for MyErr {
fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
f.write_str("I AM ERROR")
}
}
impl Error for MyErr {}
#[async_trait]
impl Metric for StaticMetric {
type Output = ();
type Error = MyErr;
async fn refresh_metric(&mut self) -> Result<Self::Output, Self::Error> {
Ok(())
}
}
Playground
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