# Objective
I have a custom asset loader, and need access to the error it reports
when failing to load (e.g. through `AssetLoadFailedEvent { error:
AssetLoadError::AssetLoaderError(loader_error), .. }`). However
`AssetLoaderError` doesn't expose its `<core::error::Error>::source()`
(i.e. its `error` field. It only formats it when `Display`ed.
*I haven't searched for issues about it.*
## Solution
- Annotate `AssetLoaderError`'s `error` field with `#[source]`.
- Don't include the error when `AssetLoaderError` is `Display`ed (when
one prints an error's source stack like a backtrace, it would now be
dupplicated).
- (optional, included as a separated commit) Add a getter for the `&dyn
Error` stored in the `error` field (whithin an `Arc`). This is more
ergonomic than using `Error::source()` because it casts an `&Arc<dyn
Error>` into an `&dyn Error`, meaning one has to downcast it twice to
get the original error from the loader, including once where you have to
specify the correct type of the *private* `error` field. So downcasting
from `Error::source()` effectively rely on the internal implementation
of `AssetLoaderError`. The getter instead return the trait object
directly, which mean it will directly downcast to the expected loader
error type.
I didn't included a test that checks that double-downcasting
`<AssetLoaderError as Error>::source()` doesn't break user code that
would rely on the private field's type.
## Testing
- Downcasting the trait objects for both `source()` and the `error()`
getter work as described above.
- `cargo test -p bevy_asset --all-features` pass without errors.
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Co-authored-by: austreelis <git@swhaele.net>
6db71367d4