Add `no_std` support to `bevy_app` #16874

bushrat011899 · 2024-12-17T23:31:46Z

Objective

Contributes to Tracking Issue: MVP no_std Bevy #15460

Solution

Added the following features:
- std (default)
- bevy_tasks (default)
- downcast (default)
- portable-atomic
- critical-section
downcast and bevy_tasks are now optional dependencies for bevy_app.

Testing

CI
Personal UEFI and Raspberry Pi Pico demo applications compile and run against this branch

Draft Release Notes

Bevy's application framework now supports no_std platforms.

Following up on bevy_ecs gaining no_std support, bevy_app extends the functionality available on these targets to include the powerful App and Plugin abstractions. With this, library authors now have the option of making their plugins no_std compatible, or even offering plugins specifically to improve Bevy on certain embedded platforms!

To start making a no_std compatible plugin, simply disable default features when including bevy_app:

[dependencies]
bevy_app = { version = "0.16", default-features = false }

We encourage library authors to do this anyway, as it can also help with compile times and binary size on all platforms.

Keep an eye out for future no_std updates as we continue to improve the parity between std and no_std. We look forward to seeing what kinds of applications are now possible with Bevy!

Notes

downcast-rs is optional as it isn't compatible with portable-atomic. I will investigate making a PR upstream to add support for this functionality, as it should be very straightforward.
In line with the bevy_ecs no-std-ification, I've added documentation to all features, and grouped them as well.
~~Creating this PR in draft while CI runs and so I can polish before review.~~

bushrat011899 · 2024-12-18T00:05:32Z

crates/bevy_ecs/Cargo.toml

-  "bevy_tasks/portable-atomic",
+  "bevy_tasks?/portable-atomic",
  "concurrent-queue/portable-atomic",
+  "spin/portable_atomic",


The changes to bevy_ecs/Cargo.toml could be split into their own PR, but for now I'm just including them here for brevity. I'm happy to split it out if anyone has any concerns.

bushrat011899 · 2024-12-18T00:07:46Z

tools/ci/src/commands/compile_check_no_std.rs

+        commands.push(PreparedCommand::new::<Self>(
+            cmd!(
+                sh,
+                "cargo check -p bevy_app --no-default-features --features bevy_reflect --target {target}"


Now that bevy_app is compatible, we can start looking into replacing/augmenting this compile_check_no_std CI task with something more robust and tangible, like a no_std example crate.

bushrat011899 · 2024-12-18T00:09:32Z

crates/bevy_app/src/schedule_runner.rs

-                                Ok(Some(delay)) => std::thread::sleep(delay),
+                                Ok(Some(_delay)) => {
+                                    #[cfg(feature = "std")]
+                                    std::thread::sleep(_delay);


Without access to std::thread::sleep, no_std apps will run as-fast-as-possible by default. This isn't ideal, but I believe a reasonable compromise.

bushrat011899 · 2024-12-18T00:12:17Z

crates/bevy_app/src/schedule_runner.rs

+#[cfg(any(target_arch = "wasm32", feature = "std"))]
+use bevy_utils::Instant;


I'm not a fan of how we bring this Instant in. I think it'd be worth considering some kind of bevy_platform_support crate which moves things like spin, portable-atomic, critical-section, wasm-time, etc. into a single spot. It'd be more focussed than bevy_utils, and possibly clean up some of the cfg(...) noise we have across Bevy.

Yes please!

bushrat011899 · 2024-12-18T00:16:39Z

crates/bevy_app/src/plugin.rs

+#[cfg(not(feature = "downcast"))]
+#[doc(hidden)]
+pub trait Downcast {}
+
+#[cfg(not(feature = "downcast"))]
+impl<T: ?Sized> Downcast for T {}


downcast-rs isn't compatible with portablie-atomic (though it could be with a PR I believe!), so I've included a little dummy trait so the Plugin: Downcast bound can remain.

bushrat011899 · 2024-12-18T00:21:55Z

crates/bevy_app/src/app.rs

+#[cfg(feature = "trace")]
+use tracing::info_span;


As with bevy_ecs, I'm flattening our relationship with bevy_utils::tracing and also relying on log for non-span logging. It's already been mentioned that we should probably move things like log and tracing into workspace dependencies, but I'm just maintaining status quo for now.

bushrat011899 · 2024-12-18T00:22:49Z

crates/bevy_app/src/lib.rs

 mod sub_app;
-#[cfg(not(target_arch = "wasm32"))]
+#[cfg(all(not(target_arch = "wasm32"), feature = "std"))]
 mod terminal_ctrl_c_handler;


Can't use Control-C in the terminal if your platform doesn't have a terminal...or a keyboard...

bushrat011899 · 2024-12-18T00:24:20Z

crates/bevy_app/src/sub_app.rs


    /// See [`App::get_added_plugins`].
+    #[cfg(feature = "downcast")]
    pub fn get_added_plugins<T>(&self) -> Vec<&T>


This is the only real consequence of losing downcast-rs support on certain no_std targets (not all!). I would like to bring feature parity regardless, but it is fairly minor IMO.

crates/bevy_app/src/sub_app.rs

alice-i-cecile · 2024-12-18T00:35:05Z

No serious problems here; let me know when this is ready.

Co-Authored-By: Alice Cecile <[email protected]>

# Objective - Contributes to bevyengine#15460 ## Solution - Added the following features: - `std` (default) - `bevy_tasks` (default) - `downcast ` (default) - `portable-atomic` - `critical-section` - `downcast` and `bevy_tasks` are now optional dependencies for `bevy_app`. ## Testing - CI - Personal UEFI and Raspberry Pi Pico demo applications compile and run against this branch ## Draft Release Notes Bevy's application framework now supports `no_std` platforms. Following up on `bevy_ecs` gaining `no_std` support, `bevy_app` extends the functionality available on these targets to include the powerful `App` and `Plugin` abstractions. With this, library authors now have the option of making their plugins `no_std` compatible, or even offering plugins specifically to improve Bevy on certain embedded platforms! To start making a `no_std` compatible plugin, simply disable default features when including `bevy_app`: ```toml [dependencies] bevy_app = { version = "0.16", default-features = false } ``` We encourage library authors to do this anyway, as it can also help with compile times and binary size on all platforms. Keep an eye out for future `no_std` updates as we continue to improve the parity between `std` and `no_std`. We look forward to seeing what kinds of applications are now possible with Bevy! ## Notes - `downcast-rs` is optional as it isn't compatible with `portable-atomic`. I will investigate making a PR upstream to add support for this functionality, as it should be very straightforward. - In line with the `bevy_ecs` no-std-ification, I've added documentation to all features, and grouped them as well. - ~~Creating this PR in draft while CI runs and so I can polish before review.~~ --------- Co-authored-by: Alice Cecile <[email protected]>

alice-i-cecile · 2025-03-25T20:07:34Z

Thank you to everyone involved with the authoring or reviewing of this PR! This work is relatively important and needs release notes! Head over to bevyengine/bevy-website#1977 if you'd like to help out.

Initial Commit

3472656

bushrat011899 mentioned this pull request Dec 17, 2024

Tracking Issue: MVP no_std Bevy #15460

Closed

bushrat011899 commented Dec 18, 2024

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bushrat011899 added 2 commits December 18, 2024 11:26

Added missing documentation

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

5200f5b

bushrat011899 marked this pull request as ready for review December 18, 2024 00:27

alice-i-cecile added this to the 0.16 milestone Dec 18, 2024

alice-i-cecile reviewed Dec 18, 2024

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crates/bevy_app/src/sub_app.rs Show resolved Hide resolved

bushrat011899 mentioned this pull request Dec 18, 2024

Reorganise Platform Support Dependencies #16877

Closed

bushrat011899 and others added 2 commits December 18, 2024 11:38

TODO For downcast_rs

eb8fc79

Co-Authored-By: Alice Cecile <[email protected]>

Added link to downcast-rs PR

eab7df6

BenjaminBrienen approved these changes Dec 18, 2024

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bushrat011899 added 2 commits December 19, 2024 07:33

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

2589853

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

87fbca8

heydocode approved these changes Dec 18, 2024

View reviewed changes

bushrat011899 added S-Ready-For-Final-Review This PR has been approved by the community. It's ready for a maintainer to consider merging it and removed S-Waiting-on-Author The author needs to make changes or address concerns before this can be merged labels Dec 18, 2024

alice-i-cecile added this pull request to the merge queue Dec 18, 2024

Merged via the queue into bevyengine:main with commit f45e78e Dec 18, 2024
33 checks passed

TimJentzsch added the O-Embedded Weird hardware and no_std platforms label Dec 31, 2024

alice-i-cecile mentioned this pull request Mar 25, 2025

Write release notes for PR #16874: Add no_std support to bevy_app bevyengine/bevy-website#1977

Closed

		#[cfg(any(target_arch = "wasm32", feature = "std"))]
		use bevy_utils::Instant;

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alice-i-cecile commented Dec 18, 2024

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Add `no_std` support to `bevy_app` #16874

Add `no_std` support to `bevy_app` #16874

bushrat011899 commented Dec 17, 2024 •

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