Add no_std support to bevy_state (bevyengine#17028)

# Objective

- Contributes to bevyengine#15460

## Solution

- Added the following features:
  - `std` (default)
  - `portable-atomic`
  - `critical-section`

## Testing

- CI

## Notes

- `portable-atomic`, and `critical-section` are shortcuts to enable the
relevant features in dependencies, making the usage of this crate on
atomically challenged platforms possible and simpler.
- This PR is blocked until bevyengine#17027 is merged (as it depends on fixes for
the `once!` macro). Once merged, the change-count for this PR should
reduce.

crates/bevy_state/Cargo.toml

@@ -8,23 +8,67 @@ repository = "https://github.com/bevyengine/bevy"
 license = "MIT OR Apache-2.0"
 keywords = ["bevy"]
 
-# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
-
 [features]
-default = ["bevy_reflect", "bevy_app", "bevy_hierarchy"]
-bevy_reflect = ["dep:bevy_reflect", "bevy_ecs/bevy_reflect"]
-bevy_app = ["dep:bevy_app"]
+default = ["std", "bevy_reflect", "bevy_app", "bevy_hierarchy"]
+
+# Functionality
+
+## Adds runtime reflection support using `bevy_reflect`.
+bevy_reflect = [
+  "dep:bevy_reflect",
+  "bevy_ecs/bevy_reflect",
+  "bevy_hierarchy?/reflect",
+  "bevy_app?/bevy_reflect",
+]
+
+## Adds integration with the `bevy_app` plugin API.
+bevy_app = ["dep:bevy_app", "bevy_hierarchy?/bevy_app"]
+
+## Adds integration with the `bevy_hierarchy` `Parent` and `Children` API.
 bevy_hierarchy = ["dep:bevy_hierarchy"]
 
+# Platform Compatibility
+
+## Allows access to the `std` crate. Enabling this feature will prevent compilation
+## on `no_std` targets, but provides access to certain additional features on
+## supported platforms.
+std = [
+  "bevy_ecs/std",
+  "bevy_utils/std",
+  "bevy_reflect?/std",
+  "bevy_app?/std",
+  "bevy_hierarchy?/std",
+]
+
+## `critical-section` provides the building blocks for synchronization primitives
+## on all platforms, including `no_std`.
+critical-section = [
+  "bevy_ecs/critical-section",
+  "bevy_utils/critical-section",
+  "bevy_app?/critical-section",
+]
+
+## `portable-atomic` provides additional platform support for atomic types and
+## operations, even on targets without native support. 
+portable-atomic = [
+  "bevy_ecs/portable-atomic",
+  "bevy_utils/portable-atomic",
+  "bevy_app?/portable-atomic",
+]
+
 [dependencies]
-bevy_ecs = { path = "../bevy_ecs", version = "0.15.0-dev" }
+# bevy
+bevy_ecs = { path = "../bevy_ecs", version = "0.15.0-dev", default-features = false }
 bevy_state_macros = { path = "macros", version = "0.15.0-dev" }
-bevy_utils = { path = "../bevy_utils", version = "0.15.0-dev" }
-bevy_reflect = { path = "../bevy_reflect", version = "0.15.0-dev", optional = true }
-bevy_app = { path = "../bevy_app", version = "0.15.0-dev", optional = true }
-bevy_hierarchy = { path = "../bevy_hierarchy", version = "0.15.0-dev", optional = true }
+bevy_utils = { path = "../bevy_utils", version = "0.15.0-dev", default-features = false }
+bevy_reflect = { path = "../bevy_reflect", version = "0.15.0-dev", default-features = false, optional = true }
+bevy_app = { path = "../bevy_app", version = "0.15.0-dev", default-features = false, optional = true }
+bevy_hierarchy = { path = "../bevy_hierarchy", version = "0.15.0-dev", default-features = false, optional = true }
 variadics_please = "1.1"
 
+# other
+log = { version = "0.4", default-features = false }
+
 [lints]
 workspace = true
 

crates/bevy_state/src/app.rs

@@ -1,6 +1,7 @@
 use bevy_app::{App, MainScheduleOrder, Plugin, PreStartup, PreUpdate, SubApp};
 use bevy_ecs::{event::Events, schedule::IntoSystemConfigs, world::FromWorld};
-use bevy_utils::{tracing::warn, warn_once};
+use bevy_utils::once;
+use log::warn;
 
 use crate::{
     state::{
@@ -87,7 +88,9 @@ pub trait AppExtStates {
 /// Separate function to only warn once for all state installation methods.
 fn warn_if_no_states_plugin_installed(app: &SubApp) {
     if !app.is_plugin_added::<StatesPlugin>() {
-        warn_once!("States were added to the app, but `StatesPlugin` is not installed.");
+        once!(warn!(
+            "States were added to the app, but `StatesPlugin` is not installed."
+        ));
     }
 }
 

crates/bevy_state/src/commands.rs

@@ -1,5 +1,5 @@
 use bevy_ecs::{system::Commands, world::World};
-use bevy_utils::tracing::debug;
+use log::debug;
 
 use crate::state::{FreelyMutableState, NextState};
 

crates/bevy_state/src/lib.rs

@@ -1,3 +1,5 @@
+#![cfg_attr(not(feature = "std"), no_std)]
+
 //! In Bevy, states are app-wide interdependent, finite state machines that are generally used to model the large scale structure of your program: whether a game is paused, if the player is in combat, if assets are loaded and so on.
 //!
 //! This module provides 3 distinct types of state, all of which implement the [`States`](state::States) trait:
@@ -36,6 +38,8 @@
 )]
 #![cfg_attr(any(docsrs, docsrs_dep), feature(rustdoc_internals))]
 
+extern crate alloc;
+
 #[cfg(feature = "bevy_app")]
 /// Provides [`App`](bevy_app::App) and [`SubApp`](bevy_app::SubApp) with state installation methods
 pub mod app;

crates/bevy_state/src/state_scoped_events.rs

@@ -1,3 +1,4 @@
+use alloc::vec::Vec;
 use core::marker::PhantomData;
 
 use bevy_app::{App, SubApp};

tools/ci/src/commands/compile_check_no_std.rs

@@ -126,6 +126,14 @@ impl Prepare for CompileCheckNoStdCommand {
             "Please fix compiler errors in output above for bevy_input no_std compatibility.",
         ));
 
+        commands.push(PreparedCommand::new::<Self>(
+            cmd!(
+                sh,
+                "cargo check -p bevy_state --no-default-features --features bevy_reflect,bevy_app,bevy_hierarchy --target {target}"
+            ),
+            "Please fix compiler errors in output above for bevy_state no_std compatibility.",
+        ));
+
         commands
     }
 }