Allow returning an error from labeled_asset_scope. (#19449)

# Objective

- `LoadContext::labeled_asset_scope` cannot return errors back to the
asset loader. This means users that need errors need to fall back to
using the raw `begin_labeled_asset` and `add_loaded_labeled_asset`,
which is more error-prone.

## Solution

- Allow returning a (generic) error from `labeled_asset_scope`.
- This has the unfortunate side effect that closures which don't return
any errors need to A) return Ok at the end, B) need to specify an error
type (e.g., `()`).

---

## Showcase

```rust
// impl AssetLoader for MyLoader
let handle = load_context.labeled_asset_scope("MySubasset", |mut load_context| {
  if !some_precondition {
    return Err(ThingsDontMakeSenseError);
  }
  let handle = load_context.add_labeled_asset("MySubasset/Other", SomeOtherThing(456));
  Ok(Something{ id: 123, handle })
})?;
```

crates/bevy_asset/src/loader.rs

@@ -388,15 +388,15 @@ impl<'a> LoadContext<'a> {
     /// result with [`LoadContext::add_labeled_asset`].
     ///
     /// See [`AssetPath`] for more on labeled assets.
-    pub fn labeled_asset_scope<A: Asset>(
+    pub fn labeled_asset_scope<A: Asset, E>(
         &mut self,
         label: String,
-        load: impl FnOnce(&mut LoadContext) -> A,
-    ) -> Handle<A> {
+        load: impl FnOnce(&mut LoadContext) -> Result<A, E>,
+    ) -> Result<Handle<A>, E> {
         let mut context = self.begin_labeled_asset();
-        let asset = load(&mut context);
+        let asset = load(&mut context)?;
         let loaded_asset = context.finish(asset);
-        self.add_loaded_labeled_asset(label, loaded_asset)
+        Ok(self.add_loaded_labeled_asset(label, loaded_asset))
     }
 
     /// This will add the given `asset` as a "labeled [`Asset`]" with the `label` label.
@@ -410,7 +410,8 @@ impl<'a> LoadContext<'a> {
     ///
     /// See [`AssetPath`] for more on labeled assets.
     pub fn add_labeled_asset<A: Asset>(&mut self, label: String, asset: A) -> Handle<A> {
-        self.labeled_asset_scope(label, |_| asset)
+        self.labeled_asset_scope(label, |_| Ok::<_, ()>(asset))
+            .expect("the closure returns Ok")
     }
 
     /// Add a [`LoadedAsset`] that is a "labeled sub asset" of the root path of this load context.

crates/bevy_gltf/src/loader/mod.rs

@@ -1043,7 +1043,8 @@ fn load_material(
     is_scale_inverted: bool,
 ) -> Handle<StandardMaterial> {
     let material_label = material_label(material, is_scale_inverted);
-    load_context.labeled_asset_scope(material_label.to_string(), |load_context| {
+    load_context
+        .labeled_asset_scope::<_, ()>(material_label.to_string(), |load_context| {
             let pbr = material.pbr_metallic_roughness();
 
             // TODO: handle missing label handle errors here?
@@ -1106,8 +1107,11 @@ fn load_material(
             });
 
             #[cfg(feature = "pbr_transmission_textures")]
-        let (specular_transmission, specular_transmission_channel, specular_transmission_texture) =
-            material
+            let (
+                specular_transmission,
+                specular_transmission_channel,
+                specular_transmission_texture,
+            ) = material
                 .transmission()
                 .map_or((0.0, UvChannel::Uv0, None), |transmission| {
                     let specular_transmission_channel = transmission
@@ -1191,7 +1195,7 @@ fn load_material(
             let base_emissive = LinearRgba::rgb(emissive[0], emissive[1], emissive[2]);
             let emissive = base_emissive * material.emissive_strength().unwrap_or(1.0);
 
-        StandardMaterial {
+            Ok(StandardMaterial {
                 base_color: Color::linear_rgba(color[0], color[1], color[2], color[3]),
                 base_color_channel,
                 base_color_texture,
@@ -1235,8 +1239,9 @@ fn load_material(
                 alpha_mode: alpha_mode(material),
                 uv_transform,
                 clearcoat: clearcoat.clearcoat_factor.unwrap_or_default() as f32,
-            clearcoat_perceptual_roughness: clearcoat.clearcoat_roughness_factor.unwrap_or_default()
-                as f32,
+                clearcoat_perceptual_roughness: clearcoat
+                    .clearcoat_roughness_factor
+                    .unwrap_or_default() as f32,
                 #[cfg(feature = "pbr_multi_layer_material_textures")]
                 clearcoat_channel: clearcoat.clearcoat_channel,
                 #[cfg(feature = "pbr_multi_layer_material_textures")]
@@ -1263,7 +1268,9 @@ fn load_material(
                 #[cfg(feature = "pbr_specular_textures")]
                 specular_texture: specular.specular_texture,
                 specular_tint: match specular.specular_color_factor {
-                Some(color) => Color::linear_rgb(color[0] as f32, color[1] as f32, color[2] as f32),
+                    Some(color) => {
+                        Color::linear_rgb(color[0] as f32, color[1] as f32, color[2] as f32)
+                    }
                     None => Color::WHITE,
                 },
                 #[cfg(feature = "pbr_specular_textures")]
@@ -1271,8 +1278,9 @@ fn load_material(
                 #[cfg(feature = "pbr_specular_textures")]
                 specular_tint_texture: specular.specular_color_texture,
                 ..Default::default()
-        }
             })
+        })
+        .unwrap()
 }
 
 /// Loads a glTF node.

release-content/migration-guides/labeled_asset_scope_errors.md

@@ -0,0 +1,31 @@
+---
+title: `labeled_asset_scope` can now return errors.
+pull_requests: [19449]
+---
+
+`labeled_asset_scope` now returns a user-specified error type based on their closure. Previously,
+users would need to fall back to `begin_labeled_asset` and `add_loaded_labeled_asset` to handle
+errors, which is more error-prone. Consider migrating to use `labeled_asset_scope` if this was you!
+
+However, `labeled_asset_scope` closures that don't return errors now needs to A) return Ok, and B)
+specify an error type.
+
+If your code previously looked like this:
+
+```rust
+labeled_asset_scope(label, |mut load_context| {
+  let my_asset = ...;
+
+  my_asset
+});
+```
+
+You can migrate it to:
+
+```rust
+labeled_asset_scope::<_, ()>(label, |mut load_context| {
+  let my_asset = ...;
+
+  Ok(my_asset)
+}).unwrap();
+```