Add no_std Support to bevy_time (#17491)

# Objective

- Contributes to #15460

## Solution

- Switched `tracing` for `log` for the atomically challenged platforms
- Setup feature flags as required
- Added to `compile-check-no-std` CI task
- Made `crossbeam-channel` optional depending on `std`.

## Testing

- CI

---

## Notes

- `crossbeam-channel` provides a MPMC channel type which isn't readily
replicable in `no_std`, and is only used for a `bevy_render`
integration. As such, I've feature-gated the `TimeReceiver` and
`TimeSender` types.

---------

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

crates/bevy_reflect/src/impls/std.rs

@@ -126,7 +126,6 @@ impl_reflect_opaque!(::core::time::Duration(
     Deserialize,
     Default
 ));
-#[cfg(any(target_arch = "wasm32", feature = "std"))]
 impl_reflect_opaque!(::bevy_platform_support::time::Instant(
     Debug, Hash, PartialEq
 ));

crates/bevy_time/Cargo.toml

@@ -9,26 +9,69 @@ license = "MIT OR Apache-2.0"
 keywords = ["bevy"]
 
 [features]
-default = ["bevy_reflect"]
+default = ["std", "bevy_reflect", "bevy_app/default"]
-serialize = ["serde"]
+
+# Functionality
+
+## Adds runtime reflection support using `bevy_reflect`.
+bevy_reflect = [
+  "dep:bevy_reflect",
+  "bevy_ecs/bevy_reflect",
+  "bevy_app/bevy_reflect",
+]
+
+## Adds serialization support through `serde`.
+serialize = ["dep:serde", "bevy_ecs/serialize"]
+
+# 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 = [
+  "serde?/std",
+  "bevy_reflect?/std",
+  "bevy_ecs/std",
+  "bevy_app/std",
+  "bevy_platform_support/std",
+  "dep:crossbeam-channel",
+]
+
+## `critical-section` provides the building blocks for synchronization primitives
+## on all platforms, including `no_std`.
+critical-section = [
+  "bevy_ecs/critical-section",
+  "bevy_platform_support/critical-section",
+  "bevy_reflect?/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_platform_support/portable-atomic",
+  "bevy_reflect?/portable-atomic",
+  "bevy_app/portable-atomic",
+]
 
 [dependencies]
 # bevy
-bevy_app = { path = "../bevy_app", version = "0.16.0-dev" }
+bevy_app = { path = "../bevy_app", version = "0.16.0-dev", default-features = false }
-bevy_ecs = { path = "../bevy_ecs", version = "0.16.0-dev", features = [
+bevy_ecs = { path = "../bevy_ecs", version = "0.16.0-dev", default-features = false }
-  "bevy_reflect",
+bevy_reflect = { path = "../bevy_reflect", version = "0.16.0-dev", default-features = false, features = [
-] }
-bevy_reflect = { path = "../bevy_reflect", version = "0.16.0-dev", features = [
   "bevy",
 ], optional = true }
-bevy_platform_support = { path = "../bevy_platform_support", version = "0.16.0-dev", default-features = false, features = [
+bevy_platform_support = { path = "../bevy_platform_support", version = "0.16.0-dev", default-features = false }
-  "std",
-] }
 
 # other
-crossbeam-channel = "0.5.0"
+crossbeam-channel = { version = "0.5.0", default-features = false, features = [
-serde = { version = "1", features = ["derive"], optional = true }
+  "std",
-tracing = { version = "0.1", default-features = false, features = ["std"] }
+], optional = true }
+serde = { version = "1", features = [
+  "derive",
+], default-features = false, optional = true }
+log = { version = "0.4", default-features = false }
 
 [lints]
 workspace = true

crates/bevy_time/src/lib.rs

@@ -5,6 +5,12 @@
     html_logo_url = "https://bevyengine.org/assets/icon.png",
     html_favicon_url = "https://bevyengine.org/assets/icon.png"
 )]
+#![no_std]
+
+#[cfg(feature = "std")]
+extern crate std;
+
+extern crate alloc;
 
 /// Common run conditions
 pub mod common_conditions;
@@ -37,9 +43,12 @@ use bevy_ecs::{
 };
 use bevy_platform_support::time::Instant;
 use core::time::Duration;
+
+#[cfg(feature = "std")]
 pub use crossbeam_channel::TrySendError;
+
+#[cfg(feature = "std")]
 use crossbeam_channel::{Receiver, Sender};
-use tracing::warn;
 
 /// Adds time functionality to Apps.
 #[derive(Default)]
@@ -92,8 +101,9 @@ impl Plugin for TimePlugin {
 /// networking or similar, you may prefer to set the next [`Time`] value manually.
 #[derive(Resource, Default)]
 pub enum TimeUpdateStrategy {
-    /// [`Time`] will be automatically updated each frame using an [`Instant`] sent from the render world via a [`TimeSender`].
+    /// [`Time`] will be automatically updated each frame using an [`Instant`] sent from the render world.
     /// If nothing is sent, the system clock will be used instead.
+    #[cfg_attr(feature = "std", doc = "See [`TimeSender`] for more details.")]
     #[default]
     Automatic,
     /// [`Time`] will be updated to the specified [`Instant`] value each frame.
@@ -106,14 +116,17 @@ pub enum TimeUpdateStrategy {
 }
 
 /// Channel resource used to receive time from the render world.
+#[cfg(feature = "std")]
 #[derive(Resource)]
 pub struct TimeReceiver(pub Receiver<Instant>);
 
 /// Channel resource used to send time from the render world.
+#[cfg(feature = "std")]
 #[derive(Resource)]
 pub struct TimeSender(pub Sender<Instant>);
 
 /// Creates channels used for sending time between the render world and the main world.
+#[cfg(feature = "std")]
 pub fn create_time_channels() -> (TimeSender, TimeReceiver) {
     // bound the channel to 2 since when pipelined the render phase can finish before
     // the time system runs.
@@ -128,26 +141,32 @@ pub fn time_system(
     mut virtual_time: ResMut<Time<Virtual>>,
     mut time: ResMut<Time>,
     update_strategy: Res<TimeUpdateStrategy>,
-    time_recv: Option<Res<TimeReceiver>>,
+    #[cfg(feature = "std")] time_recv: Option<Res<TimeReceiver>>,
-    mut has_received_time: Local<bool>,
+    #[cfg(feature = "std")] mut has_received_time: Local<bool>,
 ) {
-    let new_time = if let Some(time_recv) = time_recv {
-        // TODO: Figure out how to handle this when using pipelined rendering.
-        if let Ok(new_time) = time_recv.0.try_recv() {
-            *has_received_time = true;
-            new_time
-        } else {
-            if *has_received_time {
-                warn!("time_system did not receive the time from the render world! Calculations depending on the time may be incorrect.");
-            }
-            Instant::now()
-        }
-    } else {
-        Instant::now()
-    };
-
     match update_strategy.as_ref() {
-        TimeUpdateStrategy::Automatic => real_time.update_with_instant(new_time),
+        TimeUpdateStrategy::Automatic => {
+            #[cfg(feature = "std")]
+            let new_time = if let Some(time_recv) = time_recv {
+                // TODO: Figure out how to handle this when using pipelined rendering.
+                if let Ok(new_time) = time_recv.0.try_recv() {
+                    *has_received_time = true;
+                    new_time
+                } else {
+                    if *has_received_time {
+                        log::warn!("time_system did not receive the time from the render world! Calculations depending on the time may be incorrect.");
+                    }
+                    Instant::now()
+                }
+            } else {
+                Instant::now()
+            };
+
+            #[cfg(not(feature = "std"))]
+            let new_time = Instant::now();
+
+            real_time.update_with_instant(new_time);
+        }
         TimeUpdateStrategy::ManualInstant(instant) => real_time.update_with_instant(*instant),
         TimeUpdateStrategy::ManualDuration(duration) => real_time.update_with_duration(*duration),
     }
@@ -166,6 +185,7 @@ mod tests {
     };
     use core::error::Error;
     use core::time::Duration;
+    use std::println;
 
     #[derive(Event)]
     struct TestEvent<T: Default> {

crates/bevy_time/src/virt.rs

@@ -1,7 +1,7 @@
 #[cfg(feature = "bevy_reflect")]
 use bevy_reflect::Reflect;
 use core::time::Duration;
-use tracing::debug;
+use log::debug;
 
 use crate::{real::Real, time::Time};
 

tools/ci/src/commands/compile_check_no_std.rs

@@ -142,6 +142,14 @@ impl Prepare for CompileCheckNoStdCommand {
             "Please fix compiler errors in output above for bevy_transform no_std compatibility.",
         ));
 
+        commands.push(PreparedCommand::new::<Self>(
+            cmd!(
+                sh,
+                "cargo check -p bevy_time --no-default-features --features bevy_reflect,serialize --target {target}"
+            ),
+            "Please fix compiler errors in output above for bevy_transform no_std compatibility.",
+        ));
+
         commands.push(PreparedCommand::new::<Self>(
             cmd!(
                 sh,