Can't use anyhow when writing custom AssetLoaders or AssetSavers

[Runi-c]

Bevy version

v0.12.0

What you did

Tried to use anyhow::Error as the value for the associated type Error in my implementation of an AssetLoader, as recommended in #10003.

What went wrong

error[E0277]: the trait bound `anyhow::Error: StdError` is not satisfied
   --> src\asset.rs:111:18
    |
111 |     type Error = anyhow::Error;
    |                  ^^^^^^^^^^^^^ the trait `StdError` is not implemented for `anyhow::Error`
    |
note: required by a bound in `bevy::asset::AssetLoader::Error`
   --> <snip>\bevy_asset\src\loader.rs:31:17
    |
31  |     type Error: std::error::Error + Send + Sync + 'static;
    |                 ^^^^^^^^^^^^^^^^^ required by this bound in `AssetLoader::Error`

Additional information

#10003 suggests that anyhow::Error is intended to be a drop-in replacement here, to make migrations easier. I had to add thiserror as a project dependency just for this one file.

[deltanedas]

did you do something like this?

#[derive(Error)]
pub enum LoaderError {
    Error(#[from] anyhow::Error)
}

[Runi-c]

@deltanedas Naw I just enumerated the error types since I figured if I'm being made to pull in thiserror I might as well use it as intended.

However it doesn't seem intended that I had to pull in thiserror just to interact with this system in an ergonomic way, and the intention appears to have been for anyhow::Error to be a drop-in replacement, hence it seems to be a bug.

[deltanedas]

i think its an issue on anyhows end since youd expect to be able to use it for something that wants std error trait

[rparrett]

@bushrat011899 any idea what's going on here? It's not clear to me whether this is a trait bound issue or if the migration guide suggesting anyhow should be updated.

[deltanedas]

i looked at anyhow and couldnt see any Error trait implemention, bounds or not.

[Testare]

There's a github issue describing why anyhow::Error doesn't implement std::error::Error: dtolnay/anyhow#25

Looking at the doc, anyhow doesn't implment Error but it has AsRef, Deref, and From implementation (From to StdError) to allow conversion.

[bushrat011899]

There's a github issue describing why anyhow::Error doesn't implement std::error::Error: dtolnay/anyhow#25

Looking at the doc, anyhow doesn't implment Error but it has AsRef, Deref, and From implementation (From to StdError) to allow conversion.

That's a mistake on my part then! When I made that PR, I assumed anyhow::Error would implement Error. I think the simplest option here would be to update the migration guide to suggest using Box<dyn Error + Send + Sync>, or a new type around anyhow::Error.

[Runi-c]

I think the simplest option here would be to update the migration guide to suggest using Box<dyn Error + Send + Sync>, or a new type around anyhow::Error.

@bushrat011899 I did try the first one before I made the issue, but unfortunately the way the trait bounds are set up it seems to want the boxed type to be Sized as well...

error[E0277]: the size for values of type `(dyn StdError + Send + Sync + 'static)` cannot be known at compilation time
   --> src\asset.rs:119:18
    |
119 |     type Error = Box<dyn std::error::Error + Send + Sync>;
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `(dyn StdError + Send + Sync + 'static)`
    = help: the trait `StdError` is implemented for `std::boxed::Box<T>`
    = note: required for `std::boxed::Box<(dyn StdError + Send + Sync + 'static)>` to implement `StdError`
note: required by a bound in `bevy::asset::AssetLoader::Error`
   --> <snip>\bevy_asset\src\loader.rs:31:17
    |
31  |     type Error: std::error::Error + Send + Sync + 'static;
    |                 ^^^^^^^^^^^^^^^^^ required by this bound in `AssetLoader::Error`

For more information about this error, try `rustc --explain E0277`.

I'm not sure how to get a newtype around anyhow::Error to work either without explicitly implementing std::fmt::Display and std::error::Error for the newtype, which is much more tedious than just pulling in thiserror.

[bushrat011899]

I'm not sure how to get a newtype around anyhow::Error to work either without explicitly implementing std::fmt::Display and std::error::Error for the newtype, which is much more tedious than just pulling in thiserror.

Unfortunately I think that's exactly correct. I do apologise for my mistake regarding the migration guide for those relying on anyhow for this associated type. During development I had obviously removed the dependency, and had (falsely) assumed that anyhow::Error was indeed an Error type (which in my defence, I think is a reasonable assumption to make). If it's any consolation, thiserror was already a part of Bevy and thus in your dependency graph from well before this change was made.

[Testare]

I still think we should try to make the associated Error type able to support anyhow::Error in some way, or enable some sort of generic Error handling. As far as I'm aware there's not really any sort of error handling that is type specific for AssetReader, just using the Error trait. Meanwhile, I have at least 4 different AssetReaders in my code currently across different crates, and now I have to create different error types for different ones and make sure that they convert all the errors to the appropriate type.

It might not make it into one of the 0.12.x patches, but supporting anyhow in 0.13 would be ideal

[rlidwka]

Meanwhile, I have at least 4 different AssetReaders in my code currently across different crates, and now I have to create different error types for different ones

If you don't care about strong error typing, you can use Box<dyn Error + Send + Sync> as mentioned above.

[Runi-c]

@rlidwka As also mentioned above, that produces an error message currently - the compiler wants the boxed type to be Sized.

[rlidwka]

@Roryl-c, okay, sounds like a bug then.

Do you have a minimal example (of a code that worked with anyhow before) that we can try out?

[Runi-c]

@rlidwka My code was originally based on the examples/asset/custom_asset.rs example, which is fairly minimal and used anyhow::Error before #10003.

[rafalh]

I also stepped onto this problem. I wanted to use Box<dyn std::error::Error + Send + Sync> as mentioned above but it does not work because T must be Sized for Box<T> to implement std::error::Error which is a little non-intuitive. It looks like a Rust bug that cannot be solved because of backward compatibility reasons. See: rust-lang/rust#104485
Did anybody found a workaround that does not require to defining Error type for every loader?