Added Support for Extension-less Assets (#10153)

# Objective

- Addresses **Support processing and loading files without extensions**
from #9714
- Addresses **More runtime loading configuration** from #9714
- Fixes #367
- Fixes #10703

## Solution

`AssetServer::load::<A>` and `AssetServer::load_with_settings::<A>` can
now use the `Asset` type parameter `A` to select a registered
`AssetLoader` without inspecting the provided `AssetPath`. This change
cascades onto `LoadContext::load` and `LoadContext::load_with_settings`.
This allows the loading of assets which have incorrect or ambiguous file
extensions.

```rust
// Allow the type to be inferred by context
let handle = asset_server.load("data/asset_no_extension");

// Hint the type through the handle
let handle: Handle<CustomAsset> = asset_server.load("data/asset_no_extension");

// Explicit through turbofish
let handle = asset_server.load::<CustomAsset>("data/asset_no_extension");
```

Since a single `AssetPath` no longer maps 1:1 with an `Asset`, I've also
modified how assets are loaded to permit multiple asset types to be
loaded from a single path. This allows for two different `AssetLoaders`
(which return different types of assets) to both load a single path (if
requested).

```rust
// Uses GltfLoader
let model = asset_server.load::<Gltf>("cube.gltf");

// Hypothetical Blob loader for data transmission (for example)
let blob = asset_server.load::<Blob>("cube.gltf");
```

As these changes are reflected in the `LoadContext` as well as the
`AssetServer`, custom `AssetLoaders` can also take advantage of this
behaviour to create more complex assets.

---

## Change Log

- Updated `custom_asset` example to demonstrate extension-less assets.
- Added `AssetServer::get_handles_untyped` and Added
`AssetServer::get_path_ids`

## Notes

As a part of that refactor, I chose to store `AssetLoader`s (within
`AssetLoaders`) using a `HashMap<TypeId, ...>` instead of a `Vec<...>`.
My reasoning for this was I needed to add a relationship between `Asset`
`TypeId`s and the `AssetLoader`, so instead of having a `Vec` and a
`HashMap`, I combined the two, removing the `usize` index from the
adjacent maps.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

assets/data/asset_no_extension

@@ -0,0 +1,3 @@
+CustomAsset (
+    value: 13
+)

crates/bevy_asset/src/io/mod.rs

@@ -267,10 +267,7 @@ impl AsyncRead for VecReader {
 /// Appends `.meta` to the given path.
 pub(crate) fn get_meta_path(path: &Path) -> PathBuf {
     let mut meta_path = path.to_path_buf();
-    let mut extension = path
-        .extension()
-        .unwrap_or_else(|| panic!("missing extension for asset path {path:?}"))
-        .to_os_string();
+    let mut extension = path.extension().unwrap_or_default().to_os_string();
     extension.push(".meta");
     meta_path.set_extension(extension);
     meta_path

crates/bevy_asset/src/loader.rs

@@ -37,8 +37,10 @@ pub trait AssetLoader: Send + Sync + 'static {
         load_context: &'a mut LoadContext,
     ) -> BoxedFuture<'a, Result<Self::Asset, Self::Error>>;
 
-    /// Returns a list of extensions supported by this asset loader, without the preceding dot.
-    fn extensions(&self) -> &[&str];
+    /// Returns a list of extensions supported by this [`AssetLoader`], without the preceding dot.
+    fn extensions(&self) -> &[&str] {
+        &[]
+    }
 }
 
 /// Provides type-erased access to an [`AssetLoader`].
@@ -396,7 +398,7 @@ impl<'a> LoadContext<'a> {
     /// See [`AssetPath`] for more on labeled assets.
     pub fn has_labeled_asset<'b>(&self, label: impl Into<CowArc<'b, str>>) -> bool {
         let path = self.asset_path.clone().with_label(label.into());
-        self.asset_server.get_handle_untyped(&path).is_some()
+        !self.asset_server.get_handles_untyped(&path).is_empty()
     }
 
     /// "Finishes" this context by populating the final [`Asset`] value (and the erased [`AssetMeta`] value, if it exists).
@@ -546,7 +548,7 @@ impl<'a> LoadContext<'a> {
         let loaded_asset = {
             let (meta, loader, mut reader) = self
                 .asset_server
-                .get_meta_loader_and_reader(&path)
+                .get_meta_loader_and_reader(&path, None)
                 .await
                 .map_err(to_error)?;
             self.asset_server

crates/bevy_asset/src/server/info.rs

@@ -61,7 +61,7 @@ impl AssetInfo {
 
 #[derive(Default)]
 pub(crate) struct AssetInfos {
-    path_to_id: HashMap<AssetPath<'static>, UntypedAssetId>,
+    path_to_id: HashMap<AssetPath<'static>, HashMap<TypeId, UntypedAssetId>>,
     infos: HashMap<UntypedAssetId, AssetInfo>,
     /// If set to `true`, this informs [`AssetInfos`] to track data relevant to watching for changes (such as `load_dependants`)
     /// This should only be set at startup.
@@ -191,7 +191,20 @@ impl AssetInfos {
         loading_mode: HandleLoadingMode,
         meta_transform: Option<MetaTransform>,
     ) -> Result<(UntypedHandle, bool), GetOrCreateHandleInternalError> {
-        match self.path_to_id.entry(path.clone()) {
+        let handles = self.path_to_id.entry(path.clone()).or_default();
+
+        let type_id = type_id
+            .or_else(|| {
+                // If a TypeId is not provided, we may be able to infer it if only a single entry exists
+                if handles.len() == 1 {
+                    Some(*handles.keys().next().unwrap())
+                } else {
+                    None
+                }
+            })
+            .ok_or(GetOrCreateHandleInternalError::HandleMissingButTypeIdNotSpecified)?;
+
+        match handles.entry(type_id) {
             Entry::Occupied(entry) => {
                 let id = *entry.get();
                 // if there is a path_to_id entry, info always exists
@@ -222,9 +235,6 @@ impl AssetInfos {
                     // We must create a new strong handle for the existing id and ensure that the drop of the old
                     // strong handle doesn't remove the asset from the Assets collection
                     info.handle_drops_to_skip += 1;
-                    let type_id = type_id.ok_or(
-                        GetOrCreateHandleInternalError::HandleMissingButTypeIdNotSpecified,
-                    )?;
                     let provider = self
                         .handle_providers
                         .get(&type_id)
@@ -241,8 +251,6 @@ impl AssetInfos {
                     HandleLoadingMode::NotLoading => false,
                     HandleLoadingMode::Request | HandleLoadingMode::Force => true,
                 };
-                let type_id = type_id
-                    .ok_or(GetOrCreateHandleInternalError::HandleMissingButTypeIdNotSpecified)?;
                 let handle = Self::create_handle_internal(
                     &mut self.infos,
                     &self.handle_providers,
@@ -271,13 +279,52 @@ impl AssetInfos {
         self.infos.get_mut(&id)
     }
 
-    pub(crate) fn get_path_id(&self, path: &AssetPath) -> Option<UntypedAssetId> {
-        self.path_to_id.get(path).copied()
+    pub(crate) fn get_path_and_type_id_handle(
+        &self,
+        path: &AssetPath,
+        type_id: TypeId,
+    ) -> Option<UntypedHandle> {
+        let id = self.path_to_id.get(path)?.get(&type_id)?;
+        self.get_id_handle(*id)
+    }
+
+    pub(crate) fn get_path_ids<'a>(
+        &'a self,
+        path: &'a AssetPath<'a>,
+    ) -> impl Iterator<Item = UntypedAssetId> + 'a {
+        /// Concrete type to allow returning an `impl Iterator` even if `self.path_to_id.get(&path)` is `None`
+        enum HandlesByPathIterator<T> {
+            None,
+            Some(T),
+        }
+
+        impl<T> Iterator for HandlesByPathIterator<T>
+        where
+            T: Iterator<Item = UntypedAssetId>,
+        {
+            type Item = UntypedAssetId;
+
+            fn next(&mut self) -> Option<Self::Item> {
+                match self {
+                    HandlesByPathIterator::None => None,
+                    HandlesByPathIterator::Some(iter) => iter.next(),
+                }
+            }
+        }
+
+        if let Some(type_id_to_id) = self.path_to_id.get(path) {
+            HandlesByPathIterator::Some(type_id_to_id.values().copied())
+        } else {
+            HandlesByPathIterator::None
+        }
     }
 
-    pub(crate) fn get_path_handle(&self, path: &AssetPath) -> Option<UntypedHandle> {
-        let id = *self.path_to_id.get(path)?;
-        self.get_id_handle(id)
+    pub(crate) fn get_path_handles<'a>(
+        &'a self,
+        path: &'a AssetPath<'a>,
+    ) -> impl Iterator<Item = UntypedHandle> + 'a {
+        self.get_path_ids(path)
+            .filter_map(|id| self.get_id_handle(id))
     }
 
     pub(crate) fn get_id_handle(&self, id: UntypedAssetId) -> Option<UntypedHandle> {
@@ -289,12 +336,13 @@ impl AssetInfos {
     /// Returns `true` if the asset this path points to is still alive
     pub(crate) fn is_path_alive<'a>(&self, path: impl Into<AssetPath<'a>>) -> bool {
         let path = path.into();
-        if let Some(id) = self.path_to_id.get(&path) {
-            if let Some(info) = self.infos.get(id) {
-                return info.weak_handle.strong_count() > 0;
-            }
-        }
-        false
+
+        let result = self
+            .get_path_ids(&path)
+            .filter_map(|id| self.infos.get(&id))
+            .any(|info| info.weak_handle.strong_count() > 0);
+
+        result
     }
 
     /// Returns `true` if the asset at this path should be reloaded
@@ -592,7 +640,7 @@ impl AssetInfos {
 
     fn process_handle_drop_internal(
         infos: &mut HashMap<UntypedAssetId, AssetInfo>,
-        path_to_id: &mut HashMap<AssetPath<'static>, UntypedAssetId>,
+        path_to_id: &mut HashMap<AssetPath<'static>, HashMap<TypeId, UntypedAssetId>>,
         loader_dependants: &mut HashMap<AssetPath<'static>, HashSet<AssetPath<'static>>>,
         living_labeled_assets: &mut HashMap<AssetPath<'static>, HashSet<String>>,
         watching_for_changes: bool,
@@ -609,6 +657,8 @@ impl AssetInfos {
             return false;
         }
 
+        let type_id = entry.key().type_id();
+
         let info = entry.remove();
         let Some(path) = &info.path else {
             return true;
@@ -622,7 +672,15 @@ impl AssetInfos {
                 living_labeled_assets,
             );
         }
-        path_to_id.remove(path);
+
+        if let Some(map) = path_to_id.get_mut(path) {
+            map.remove(&type_id);
+
+            if map.is_empty() {
+                path_to_id.remove(path);
+            }
+        };
+
         true
     }