docs: Full documentation for bevy_asset (bevyengine#3536)

# Objective

This PR aims to document the `bevy_asset` crate to complete coverage, while also trying to improve some bits of UX.

### Progress

- [x] Root items
- [x] `handle` module
- [x] `info` module
- [x] `path` module
- [x] `loader` module
- [x] `io` and `filesystem_watcher` module
- [x] `assets` module
- [x] `asset_server` module
- [x] `diagnostic` module
- [x] `debug_asset_server` module
- [x] Crate level documentation
- [x] Add `#![warn(missing_docs)]` lint

Coverage: 100%

## Migration Guide

- Rename `FileAssetIo::get_root_path` uses to `FileAssetIo::get_base_path`

    `FileAssetIo::root_path()` is a getter for the `root_path` field, while `FileAssetIo::get_root_path` returned the parent directory of the asset root path, which was the executable's directory unless `CARGO_MANIFEST_DIR` was set. This change solves the ambiguity between the two methods.

crates/bevy_asset/src/asset_server.rs

@@ -14,17 +14,31 @@ use parking_lot::{Mutex, RwLock};
 use std::{path::Path, sync::Arc};
 use thiserror::Error;
 
-/// Errors that occur while loading assets with an `AssetServer`
+/// Errors that occur while loading assets with an `AssetServer`.
 #[derive(Error, Debug)]
 pub enum AssetServerError {
+    /// Asset folder is not a directory.
     #[error("asset folder path is not a directory: {0}")]
     AssetFolderNotADirectory(String),
+
+    /// No asset loader was found for the specified extensions.
     #[error("no `AssetLoader` found{}", format_missing_asset_ext(.extensions))]
-    MissingAssetLoader { extensions: Vec<String> },
+    MissingAssetLoader {
+        /// The list of extensions detected on the asset source path that failed to load.
+        ///
+        /// The list may be empty if the asset path is invalid or doesn't have an extension.
+        extensions: Vec<String>,
+    },
+
+    /// The handle type does not match the type of the loaded asset.
     #[error("the given type does not match the type of the loaded asset")]
     IncorrectHandleType,
+
+    /// Encountered an error while processing an asset.
     #[error("encountered an error while loading an asset: {0}")]
     AssetLoaderError(anyhow::Error),
+
+    /// Encountered an error while reading an asset from disk.
     #[error("encountered an error while reading an asset: {0}")]
     AssetIoError(#[from] AssetIoError),
 }
@@ -48,6 +62,9 @@ pub(crate) struct AssetRefCounter {
     pub(crate) mark_unused_assets: Arc<Mutex<Vec<HandleId>>>,
 }
 
+/// Internal data for the asset server.
+///
+/// [`AssetServer`] is the public API for interacting with the asset server.
 pub struct AssetServerInternal {
     pub(crate) asset_io: Box<dyn AssetIo>,
     pub(crate) asset_ref_counter: AssetRefCounter,
@@ -58,17 +75,45 @@ pub struct AssetServerInternal {
     handle_to_path: Arc<RwLock<HashMap<HandleId, AssetPath<'static>>>>,
 }
 
-/// Loads assets from the filesystem on background threads
+/// Loads assets from the filesystem in the background.
+///
+/// The asset server is the primary way of loading assets in bevy. It keeps track of the load state
+/// of the assets it manages and can even reload them from the filesystem with
+/// [`AssetServer::watch_for_changes`]!
+///
+/// The asset server is a _resource_, so in order to accesss it in a system you need a `Res`
+/// accessor, like this:
+///
+/// ```rust,no_run
+/// use bevy_asset::{AssetServer, Handle};
+/// use bevy_ecs::prelude::{Commands, Res};
+///
+/// # #[derive(Debug, bevy_reflect::TypeUuid)]
+/// # #[uuid = "00000000-0000-0000-0000-000000000000"]
+/// # struct Image;
+///
+/// fn my_system(mut commands: Commands, asset_server: Res<AssetServer>)
+/// {
+///     // Now you can do whatever you want with the asset server, such as loading an asset:
+///     let asset_handle: Handle<Image> = asset_server.load("cool_picture.png");
+/// }
+/// ```
+///
+/// See the [`asset_loading`] example for more information.
+///
+/// [`asset_loading`]: https://github.com/bevyengine/bevy/tree/latest/examples/asset/asset_loading.rs
 #[derive(Clone)]
 pub struct AssetServer {
     pub(crate) server: Arc<AssetServerInternal>,
 }
 
 impl AssetServer {
+    /// Creates a new asset server with the provided asset I/O.
     pub fn new<T: AssetIo>(source_io: T) -> Self {
         Self::with_boxed_io(Box::new(source_io))
     }
 
+    /// Creates a new asset server with a boxed asset I/O.
     pub fn with_boxed_io(asset_io: Box<dyn AssetIo>) -> Self {
         AssetServer {
             server: Arc::new(AssetServerInternal {
@@ -83,6 +128,7 @@ impl AssetServer {
         }
     }
 
+    /// Returns the associated asset I/O.
     pub fn asset_io(&self) -> &dyn AssetIo {
         &*self.server.asset_io
     }
@@ -104,6 +150,10 @@ impl AssetServer {
         Assets::new(self.server.asset_ref_counter.channel.sender.clone())
     }
 
+    /// Adds the provided asset loader to the server.
+    ///
+    /// If `loader` has one or more supported extensions in conflict with loaders that came before
+    /// it, it will replace them.
     pub fn add_loader<T>(&self, loader: T)
     where
         T: AssetLoader,
@@ -126,11 +176,13 @@ impl AssetServer {
         Ok(())
     }
 
+    /// Gets a strong handle for an asset with the provided id.
     pub fn get_handle<T: Asset, I: Into<HandleId>>(&self, id: I) -> Handle<T> {
         let sender = self.server.asset_ref_counter.channel.sender.clone();
         Handle::strong(id.into(), sender)
     }
 
+    /// Gets an untyped strong handle for an asset with the provided id.
     pub fn get_handle_untyped<I: Into<HandleId>>(&self, id: I) -> HandleUntyped {
         let sender = self.server.asset_ref_counter.channel.sender.clone();
         HandleUntyped::strong(id.into(), sender)
@@ -179,6 +231,7 @@ impl AssetServer {
         })
     }
 
+    /// Gets the source path of an asset from the provided handle.
     pub fn get_handle_path<H: Into<HandleId>>(&self, handle: H) -> Option<AssetPath<'_>> {
         self.server
             .handle_to_path
@@ -187,6 +240,7 @@ impl AssetServer {
             .cloned()
     }
 
+    /// Gets the load state of an asset from the provided handle.
     pub fn get_load_state<H: Into<HandleId>>(&self, handle: H) -> LoadState {
         match handle.into() {
             HandleId::AssetPathId(id) => {
@@ -199,6 +253,10 @@ impl AssetServer {
         }
     }
 
+    /// Gets the overall load state of a group of assets from the provided handles.
+    ///
+    /// This method will only return [`LoadState::Loaded`] if all assets in the
+    /// group were loaded succesfully.
     pub fn get_group_load_state(&self, handles: impl IntoIterator<Item = HandleId>) -> LoadState {
         let mut load_state = LoadState::Loaded;
         for handle_id in handles {
@@ -219,11 +277,14 @@ impl AssetServer {
         load_state
     }
 
-    /// Queue an [`Asset`] at the provided relative path for asynchronous loading.
+    /// Queues an [`Asset`] at the provided relative path for asynchronous loading.
     ///
-    /// The absolute Path to the asset is `"ROOT/ASSET_FOLDER_NAME/path"`.
+    /// The absolute path to the asset is `"ROOT/ASSET_FOLDER_NAME/path"`. Its extension is then
+    /// extracted to search for an [asset loader]. If an asset path contains multiple dots (e.g.
+    /// `foo.bar.baz`), each level is considered a separate extension and the asset server will try
+    /// to look for loaders of `bar.baz` and `baz` assets.
     ///
-    /// By default the ROOT is the directory of the Application, but this can be overridden by
+    /// By default the `ROOT` is the directory of the Application, but this can be overridden by
     /// setting the `"CARGO_MANIFEST_DIR"` environment variable
     /// (see <https://doc.rust-lang.org/cargo/reference/environment-variables.html>)
     /// to another directory. When the application  is run through Cargo, then
@@ -235,7 +296,10 @@ impl AssetServer {
     ///
     /// The asset is loaded asynchronously, and will generally not be available by the time
     /// this calls returns. Use [`AssetServer::get_load_state`] to determine when the asset is
-    /// effectively loaded and available in the [`Assets`] collection.
+    /// effectively loaded and available in the [`Assets`] collection. The asset will always fail to
+    /// load if the provided path doesn't contain an extension.
+    ///
+    /// [asset loader]: AssetLoader
     #[must_use = "not using the returned strong handle may result in the unexpected release of the asset"]
     pub fn load<'a, T: Asset, P: Into<AssetPath<'a>>>(&self, path: P) -> Handle<T> {
         self.load_untyped(path).typed()
@@ -365,6 +429,9 @@ impl AssetServer {
         Ok(asset_path_id)
     }
 
+    /// Queues the [`Asset`] at the provided path for loading and returns an untyped handle.
+    ///
+    /// See [`load`](AssetServer::load).
     #[must_use = "not using the returned strong handle may result in the unexpected release of the asset"]
     pub fn load_untyped<'a, P: Into<AssetPath<'a>>>(&self, path: P) -> HandleUntyped {
         let handle_id = self.load_untracked(path.into(), false);
@@ -400,6 +467,14 @@ impl AssetServer {
         asset_path.into()
     }
 
+    /// Loads assets from the specified folder recursively.
+    ///
+    /// # Errors
+    ///
+    /// - If the provided path is not a directory, it will fail with
+    /// [`AssetServerError::AssetFolderNotADirectory`].
+    /// - If something unexpected happened while loading an asset, other
+    /// [`AssetServerError`]s may be returned.
     #[must_use = "not using the returned strong handles may result in the unexpected release of the assets"]
     pub fn load_folder<P: AsRef<Path>>(
         &self,
@@ -429,6 +504,7 @@ impl AssetServer {
         Ok(handles)
     }
 
+    /// Frees unused assets, unloading them from memory.
     pub fn free_unused_assets(&self) {
         let mut potential_frees = self.server.asset_ref_counter.mark_unused_assets.lock();
 
@@ -455,6 +531,7 @@ impl AssetServer {
         }
     }
 
+    /// Iterates through asset references and marks assets with no active handles as unused.
     pub fn mark_unused_assets(&self) {
         let receiver = &self.server.asset_ref_counter.channel.receiver;
         let mut ref_counts = self.server.asset_ref_counter.ref_counts.write();
@@ -559,6 +636,7 @@ fn free_unused_assets_system_impl(asset_server: &AssetServer) {
     asset_server.mark_unused_assets();
 }
 
+/// A system for freeing assets that have no active handles.
 pub fn free_unused_assets_system(asset_server: Res<AssetServer>) {
     free_unused_assets_system_impl(&asset_server);
 }