Bare Minimum `no_std` support for `bevy_asset` #19070

bushrat011899 · 2025-05-05T10:46:11Z

Objective

Contributes to Tracking Issue: no_std for bevy_asset #18978

Solution

Added std and web features to bevy_asset
Gated all non-no_std dependencies behind std feature
Gated substantial functionality behind std primarily at the lib.rs level
Ensured all crates depending on bevy_asset enable std and/or web where required
Added simple documentation to bevy_asset features
Switched from crossbeam_channel to concurrent_queue where required for no_std support
Switched from tracing to log where required for no_atomic support
Moved AssetSourceId to bevy_asset::path, leaving a re-export so there's no breaking change
Refactored AssetPath to use str instead of Path when std is disabled
Added bevy_asset feature to no_std_library example to ensure it's tested in CI

Testing

CI

Notes

View the release notes here
This PR is overly restrictive in what requires std, opting to start with some low-hanging-fruit such as the Asset trait and AssetPath. Since the Assets, Handle<T>, Asset, and AssetPath items are all available, a bare-minimum workflow where assets are added individually without the AssetServer is possible. It is my intention to chip away at these std feature gates to bring greater parity.
Notable missing items:
- AssetLoader
- AssetReader
- AssetServer
- AssetSaver

"Most" functionality excluding: - processor - io - saver - transformer - loader / loader_builders - server

Need to configure a custom backend for `no_std` in CI

bushrat011899 · 2025-05-05T11:43:16Z

.github/workflows/ci.yml

-        run: cd examples/no_std/library && cargo check --no-default-features --features libm,critical-section --target x86_64-unknown-none
+        run: cd examples/no_std/library && RUSTFLAGS='--cfg getrandom_backend="rdrand"' cargo check --no-default-features --features libm,critical-section --target x86_64-unknown-none


getrandom is a bit frustrating from a library perspective. For no_std support, we need to enable backend via RUSTFLAGS. This is only needed for users on esoteric platforms, but that does make CI a little uglier.

bushrat011899 · 2025-05-05T11:44:25Z

examples/no_std/library/Cargo.toml

-features = [
-  # "bevy_color",
-  # "bevy_state",
-]
+features = ["bevy_color", "bevy_state", "bevy_asset"]


Enabling these features to ensure they're tested for no_std compatibility in CI.

bushrat011899 · 2025-05-05T11:48:56Z

crates/bevy_asset/src/assets.rs

 pub(crate) struct AssetIndexAllocator {
    /// A monotonically increasing index.
    next_index: AtomicU32,
-    recycled_queue_sender: Sender<AssetIndex>,
    /// This receives every recycled [`AssetIndex`]. It serves as a buffer/queue to store indices ready for reuse.
-    recycled_queue_receiver: Receiver<AssetIndex>,
-    recycled_sender: Sender<AssetIndex>,
-    recycled_receiver: Receiver<AssetIndex>,
+    recycled_queue: ConcurrentQueue<AssetIndex>,
+    recycled: ConcurrentQueue<AssetIndex>,
 }


Since crossbeam_channel isn't no_std compatible, I switched to concurrent_queue for tracking recycled asset indexes. Since the sender and receiver were colocated and never shared outside the type, I actually think concurrent_queue better matches the intended semantics anyway. This does increase the stack size of AssetIndexAllocator though (~1024 bytes instead of ~64). Moving the queues into an Arc<...> would solve that issue if it's contentious.

bushrat011899 · 2025-05-05T11:50:53Z

crates/bevy_asset/src/io/source.rs

+#[deprecated(since = "0.17.0", note = "use `bevy_asset::AssetSourceId` instead")]
+pub use crate::AssetSourceId;


I moved AssetSourceId to path.rs to make the feature gating less ugly. I also think it makes more sense to live there anyway, since it's a fundamental component of what we define as an AssetPath.

bushrat011899 · 2025-05-05T11:53:02Z

crates/bevy_asset/src/lib.rs

+#[cfg(feature = "std")]
 pub mod io;
 pub mod meta;
+#[cfg(feature = "std")]
 pub mod processor;
+#[cfg(feature = "std")]
 pub mod saver;
+#[cfg(feature = "std")]
 pub mod transformer;


These modules, along with loader, loader_builders, and server will require substantial changes to attain no_std compatibility. To make those efforts easier to chip at, I'm just gating them entirely right now and getting the surrounding items compatible first. This will allow more targeted PRs in the future to (hopefully) bring us closer to feature parity.

bushrat011899 · 2025-05-05T11:53:41Z

crates/bevy_asset/src/lib.rs

+    #[cfg(feature = "std")]
+    #[doc(hidden)]
+    pub use crate::AssetServer;


AssetServer really requires everything else to be no_std compatible first before it can be.

bushrat011899 · 2025-05-05T11:54:53Z

crates/bevy_asset/src/lib.rs

+#[cfg(feature = "std")]
 mod loader;
+#[cfg(feature = "std")]
 mod loader_builders;


Future PR: I feel like these modules should be combined into a bevy_asset::loader and bevy_asset::loader::builders (or further split).

bushrat011899 · 2025-05-05T11:56:04Z

crates/bevy_asset/src/lib.rs

+        #[cfg(feature = "std")]
+        self.build_asset_plugin_std(app);


I moved the std-requiring parts of the plugin build function into its own method just to make the feature gating cleaner.

bushrat011899 · 2025-05-05T11:56:52Z

crates/bevy_asset/src/lib.rs

+    }
+}
+
+impl Plugin for AssetPlugin {


It's important that AssetPlugin implement Plugin for no_std, as the DefaultPlugins will include AssetPlugin if bevy_asset is enabled on bevy.

bushrat011899 · 2025-05-05T11:58:57Z

crates/bevy_asset/src/path.rs

+#[cfg(feature = "std")]
+type PathInner = Path;
+
+#[cfg(not(feature = "std"))]
+type PathInner = str;


Future PR: Cart has previously expressed a desire to move away from std::path::Path, as AssetPath shouldn't support platform-specific functionality like Windows prefixes or different path separators. AssetPath should always use str instead of Path, but that will requiring providing our own implementations for common Path operations (components, file_name, etc.).

github-actions · 2025-05-05T17:10:56Z

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Please review the instructions for writing release notes, then expand or revise the content in the release notes directory to showcase your changes.

Core functionality

14fcffa

"Most" functionality excluding: - processor - io - saver - transformer - loader / loader_builders - server

bushrat011899 added 2 commits May 5, 2025 21:02

Deal with getrandom

fe37b21

Need to configure a custom backend for `no_std` in CI

take 2

1cfbf18

bushrat011899 commented May 5, 2025

View reviewed changes

alice-i-cecile added the M-Needs-Release-Note Work that should be called out in the blog due to impact label May 5, 2025

alice-i-cecile mentioned this pull request May 5, 2025

Move away from std::Path in bevy_assets #19079

Open

bushrat011899 added 3 commits May 6, 2025 08:55

Merge remote-tracking branch 'upstream/main' into mvp_no_std_bevy_asset

303c285

Tidy and include transformer

8648981

Release Notes

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Bare Minimum `no_std` support for `bevy_asset` #19070

Bare Minimum `no_std` support for `bevy_asset` #19070

bushrat011899 commented May 5, 2025 •

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github-actions bot commented May 5, 2025

		run: cd examples/no_std/library && cargo check --no-default-features --features libm,critical-section --target x86_64-unknown-none
		run: cd examples/no_std/library && RUSTFLAGS='--cfg getrandom_backend="rdrand"' cargo check --no-default-features --features libm,critical-section --target x86_64-unknown-none

		#[deprecated(since = "0.17.0", note = "use `bevy_asset::AssetSourceId` instead")]
		pub use crate::AssetSourceId;

Bare Minimum no_std support for bevy_asset #19070

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bushrat011899 commented May 5, 2025 • edited Loading

Objective

Solution

Testing

Notes

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github-actions bot commented May 5, 2025

Bare Minimum `no_std` support for `bevy_asset` #19070

Bare Minimum `no_std` support for `bevy_asset` #19070

bushrat011899 commented May 5, 2025 •

edited

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