Make AssetLoader/Saver Error type bounds compatible with anyhow::Error (

bevyengine#10493)

# Objective

* In Bevy 0.11 asset loaders used `anyhow::Error` for returning errors.
In Bevy 0.12 `AssetLoader` (and `AssetSaver`) have associated `Error`
type. Unfortunately it's type bounds does not allow `anyhow::Error` to
be used despite migration guide claiming otherwise. This makes migration
to 0.12 more challenging. Solve this by changing type bounds for
associated `Error` type.
* Fix bevyengine#10350

## Solution

Change associated `Error` type bounds to require `Into<Box<dyn
std::error::Error + Send + Sync + 'static>>` to be implemented instead
of `std::error::Error + Send + Sync + 'static`. Both `anyhow::Error` and
errors generated by `thiserror` seems to be fine with such type bound.

---

## Changelog

### Fixed
* Fixed compatibility with `anyhow::Error` in `AssetLoader` and
`AssetSaver` associated `Error` type

crates/bevy_asset/src/loader.rs

@@ -28,7 +28,7 @@ pub trait AssetLoader: Send + Sync + 'static {
  /// The settings type used by this [`AssetLoader`].
  type Settings: Settings + Default + Serialize + for<'a> Deserialize<'a>;
  /// The type of [error](`std::error::Error`) which could be encountered by this loader.
- type Error: std::error::Error + Send + Sync + 'static;
+ type Error: Into<Box<dyn std::error::Error + Send + Sync + 'static>>;
  /// Asynchronously loads [`AssetLoader::Asset`] (and any other labeled assets) from the bytes provided by [`Reader`].
  fn load<'a>(
  &'a self,
@@ -90,7 +90,9 @@ where
  .expect("Loader settings should exist")
  .downcast_ref::<L::Settings>()
  .expect("AssetLoader settings should match the loader type");
- let asset = <L as AssetLoader>::load(self, reader, settings, &mut load_context).await?;
+ let asset = <L as AssetLoader>::load(self, reader, settings, &mut load_context)
+ .await
+ .map_err(|error| error.into())?;
  Ok(load_context.finish(asset, Some(meta)).into())
  })
  }

crates/bevy_asset/src/processor/process.rs

@@ -138,7 +138,7 @@ impl<Loader: AssetLoader, Saver: AssetSaver<Asset = Loader::Asset>> Process
  .saver
  .save(writer, saved_asset, &settings.saver_settings)
  .await
- .map_err(|error| ProcessError::AssetSaveError(Box::new(error)))?;
+ .map_err(|error| ProcessError::AssetSaveError(error.into()))?;
  Ok(output_settings)
  })
  }

crates/bevy_asset/src/saver.rs

@@ -14,7 +14,7 @@ pub trait AssetSaver: Send + Sync + 'static {
  /// The type of [`AssetLoader`] used to load this [`Asset`]
  type OutputLoader: AssetLoader;
  /// The type of [error](`std::error::Error`) which could be encountered by this saver.
- type Error: std::error::Error + Send + Sync + 'static;
+ type Error: Into<Box<dyn std::error::Error + Send + Sync + 'static>>;
 
  /// Saves the given runtime [`Asset`] by writing it to a byte format using `writer`. The passed in `settings` can influence how the
  /// `asset` is saved. 
@@ -53,7 +53,9 @@ impl<S: AssetSaver> ErasedAssetSaver for S {
  .downcast_ref::<S::Settings>()
  .expect("AssetLoader settings should match the loader type");
  let saved_asset = SavedAsset::<S::Asset>::from_loaded(asset).unwrap();
- self.save(writer, saved_asset, settings).await?;
+ if let Err(err) = self.save(writer, saved_asset, settings).await {
+ return Err(err.into());
+ }
  Ok(())
  })
  }