Allow partial support for bevy_log in no_std (#18782)

# Objective

- Fixes #18781

## Solution

- Moved `LogPlugin` into its own file gated behind a new `tracing`
feature.
- Used `log` instead of `tracing` where possible.
- Exposed a new `tracing` feature in `bevy` which enables
`bevy_log/tracing`.
- Gated `LogPlugin` from `DefaultPlugins` on `tracing` feature.

## Testing

- CI

---

## Migration Guide

- If you were previously using `bevy_log` with default features
disabled, enable the new `std` and `tracing` features.
- If you were using `bevy` with the default features disabled, enable
the new `tracing` feature.

## Notes

Almost all of the diffs in this PR come from moving `LogPlugin` into its
own file. This just makes the PR less noisy, since the alternative is
excessive `#[cfg(feature = "tracing")]` directives all over the plugin.

---------

Co-authored-by: François Mockers <[email protected]>

Author: bushrat011899

Cargo.toml

@@ -138,7 +138,6 @@ default = [
   "bevy_gizmos",
   "bevy_gltf",
   "bevy_input_focus",
-  "bevy_log",
   "bevy_mesh_picking_backend",
   "bevy_pbr",
   "bevy_picking",
@@ -160,6 +159,7 @@ default = [
   "smaa_luts",
   "sysinfo_plugin",
   "tonemapping_luts",
+  "tracing",
   "vorbis",
   "webgl2",
   "x11",
@@ -285,9 +285,6 @@ bevy_dev_tools = ["bevy_internal/bevy_dev_tools"]
 # Enable the Bevy Remote Protocol
 bevy_remote = ["bevy_internal/bevy_remote"]
 
-# Enable integration with `tracing` and `log`
-bevy_log = ["bevy_internal/bevy_log"]
-
 # Enable input focus subsystem
 bevy_input_focus = ["bevy_internal/bevy_input_focus"]
 
@@ -314,7 +311,10 @@ trace_tracy_memory = [
 ]
 
 # Tracing support
-trace = ["bevy_internal/trace", "dep:tracing"]
+tracing = ["bevy_internal/tracing", "dep:tracing"]
+
+# Enables traces within Bevy using tracing
+trace = ["bevy_internal/trace", "tracing"]
 
 # Basis Universal compressed texture support
 basis-universal = ["bevy_internal/basis-universal"]
@@ -1592,7 +1592,7 @@ wasm = true
 name = "headless"
 path = "examples/app/headless.rs"
 doc-scrape-examples = true
-required-features = ["bevy_log"]
+required-features = []
 
 [package.metadata.example.headless]
 name = "Headless"

crates/bevy_animation/Cargo.toml

@@ -47,6 +47,18 @@ tracing = { version = "0.1", default-features = false, features = ["std"] }
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 # TODO: Assuming all wasm builds are for the browser. Require `no_std` support to break assumption.
 uuid = { version = "1.13.1", default-features = false, features = ["js"] }
+bevy_log = { path = "../bevy_log", version = "0.16.0-dev", default-features = false, features = [
+  "web",
+] }
+bevy_app = { path = "../bevy_app", version = "0.16.0-dev", default-features = false, features = [
+  "web",
+] }
+bevy_platform_support = { path = "../bevy_platform_support", version = "0.16.0-dev", default-features = false, features = [
+  "web",
+] }
+bevy_reflect = { path = "../bevy_reflect", version = "0.16.0-dev", default-features = false, features = [
+  "web",
+] }
 
 [lints]
 workspace = true

crates/bevy_dylib/src/lib.rs

@@ -53,6 +53,8 @@
 //! use bevy_dylib;
 //! ```
 
+#![no_std]
+
 // Force linking of the main bevy crate
 #[expect(
     unused_imports,

crates/bevy_gltf/Cargo.toml

@@ -65,6 +65,24 @@ serde_json = "1"
 smallvec = "1.11"
 tracing = { version = "0.1", default-features = false, features = ["std"] }
 
+[target.'cfg(target_arch = "wasm32")'.dependencies]
+# TODO: Assuming all wasm builds are for the browser. Require `no_std` support to break assumption.
+bevy_log = { path = "../bevy_log", version = "0.16.0-dev", default-features = false, features = [
+  "web",
+] }
+bevy_app = { path = "../bevy_app", version = "0.16.0-dev", default-features = false, features = [
+  "web",
+] }
+bevy_platform_support = { path = "../bevy_platform_support", version = "0.16.0-dev", default-features = false, features = [
+  "web",
+] }
+bevy_reflect = { path = "../bevy_reflect", version = "0.16.0-dev", default-features = false, features = [
+  "web",
+] }
+bevy_tasks = { path = "../bevy_tasks", version = "0.16.0-dev", default-features = false, features = [
+  "web",
+] }
+
 [dev-dependencies]
 bevy_log = { path = "../bevy_log", version = "0.16.0-dev" }
 

crates/bevy_internal/Cargo.toml

@@ -10,7 +10,9 @@ keywords = ["game", "engine", "gamedev", "graphics", "bevy"]
 categories = ["game-engines", "graphics", "gui", "rendering"]
 
 [features]
+tracing = ["bevy_log/tracing"]
 trace = [
+  "tracing",
   "bevy_app/trace",
   "bevy_asset?/trace",
   "bevy_core_pipeline?/trace",
@@ -21,10 +23,18 @@ trace = [
   "bevy_render?/trace",
   "bevy_winit?/trace",
 ]
-trace_chrome = ["bevy_log/tracing-chrome"]
-trace_tracy = ["bevy_render?/tracing-tracy", "bevy_log/tracing-tracy"]
-trace_tracy_memory = ["bevy_log/trace_tracy_memory"]
-detailed_trace = ["bevy_ecs/detailed_trace", "bevy_render?/detailed_trace"]
+trace_chrome = ["tracing", "bevy_log/tracing-chrome"]
+trace_tracy = [
+  "tracing",
+  "bevy_render?/tracing-tracy",
+  "bevy_log/tracing-tracy",
+]
+trace_tracy_memory = ["tracing", "bevy_log/trace_tracy_memory"]
+detailed_trace = [
+  "tracing",
+  "bevy_ecs/detailed_trace",
+  "bevy_render?/detailed_trace",
+]
 
 sysinfo_plugin = ["bevy_diagnostic/sysinfo_plugin"]
 
@@ -293,6 +303,7 @@ std = [
   "bevy_ecs/std",
   "bevy_input/std",
   "bevy_input_focus?/std",
+  "bevy_log/std",
   "bevy_math/std",
   "bevy_platform_support/std",
   "bevy_reflect/std",
@@ -344,6 +355,7 @@ async_executor = [
 # Note this is currently only applicable on `wasm32` architectures.
 web = [
   "bevy_app/web",
+  "bevy_log/web",
   "bevy_platform_support/web",
   "bevy_reflect/web",
   "bevy_tasks/web",
@@ -385,8 +397,7 @@ bevy_utils = { path = "../bevy_utils", version = "0.16.0-dev", default-features
 ] }
 bevy_tasks = { path = "../bevy_tasks", version = "0.16.0-dev", default-features = false }
 
-# bevy (std required)
-bevy_log = { path = "../bevy_log", version = "0.16.0-dev", optional = true }
+bevy_log = { path = "../bevy_log", version = "0.16.0-dev", default-features = false }
 
 # bevy (optional)
 bevy_a11y = { path = "../bevy_a11y", optional = true, version = "0.16.0-dev", features = [

crates/bevy_internal/src/default_plugins.rs

@@ -4,7 +4,7 @@ plugin_group! {
     /// This plugin group will add all the default plugins for a *Bevy* application:
     pub struct DefaultPlugins {
         bevy_app:::PanicHandlerPlugin,
-        #[cfg(feature = "bevy_log")]
+        #[cfg(feature = "tracing")]
         bevy_log:::LogPlugin,
         bevy_app:::TaskPoolPlugin,
         bevy_diagnostic:::FrameCountPlugin,

crates/bevy_internal/src/lib.rs

@@ -44,7 +44,6 @@ pub use bevy_image as image;
 pub use bevy_input as input;
 #[cfg(feature = "bevy_input_focus")]
 pub use bevy_input_focus as input_focus;
-#[cfg(feature = "bevy_log")]
 pub use bevy_log as log;
 pub use bevy_math as math;
 #[cfg(feature = "bevy_pbr")]

crates/bevy_internal/src/prelude.rs

@@ -1,14 +1,10 @@
 #[doc(hidden)]
 pub use crate::{
-    app::prelude::*, ecs::prelude::*, input::prelude::*, math::prelude::*,
+    app::prelude::*, ecs::prelude::*, input::prelude::*, log::prelude::*, math::prelude::*,
     platform_support::prelude::*, reflect::prelude::*, time::prelude::*, transform::prelude::*,
     utils::prelude::*, DefaultPlugins, MinimalPlugins,
 };
 
-#[doc(hidden)]
-#[cfg(feature = "bevy_log")]
-pub use crate::log::prelude::*;
-
 #[doc(hidden)]
 #[cfg(feature = "bevy_window")]
 pub use crate::window::prelude::*;

crates/bevy_log/Cargo.toml

@@ -9,24 +9,44 @@ license = "MIT OR Apache-2.0"
 keywords = ["bevy"]
 
 [features]
-trace = ["tracing-error"]
-trace_tracy_memory = ["dep:tracy-client"]
+default = ["std", "tracing"]
+
+tracing = ["dep:tracing", "dep:tracing-subscriber", "dep:tracing-log"]
+trace = ["tracing", "tracing-error"]
+trace_tracy_memory = ["tracing", "dep:tracy-client"]
+tracing-chrome = ["tracing", "dep:tracing-chrome"]
+tracing-error = ["tracing", "dep:tracing-error"]
+tracing-tracy = ["tracing", "dep:tracing-tracy"]
+
+# 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_app/std", "bevy_utils/std", "bevy_ecs/std"]
+
+# Enables use of browser APIs.
+# Note this is currently only applicable on `wasm32` architectures.
+web = ["bevy_app/web", "dep:tracing-wasm"]
 
 [dependencies]
 # bevy
-bevy_app = { path = "../bevy_app", version = "0.16.0-dev" }
-bevy_utils = { path = "../bevy_utils", version = "0.16.0-dev" }
-bevy_ecs = { path = "../bevy_ecs", version = "0.16.0-dev" }
+bevy_app = { path = "../bevy_app", version = "0.16.0-dev", default-features = false }
+bevy_utils = { path = "../bevy_utils", version = "0.16.0-dev", default-features = false, features = [
+  "alloc",
+] }
+bevy_ecs = { path = "../bevy_ecs", version = "0.16.0-dev", default-features = false }
 
 # other
-tracing-subscriber = { version = "0.3.1", features = [
+log = { version = "0.4", default-features = false }
+tracing-subscriber = { version = "0.3.1", optional = true, features = [
   "registry",
   "env-filter",
 ] }
 tracing-chrome = { version = "0.7.0", optional = true }
-tracing-log = "0.2.0"
+tracing-log = { version = "0.2.0", optional = true }
 tracing-error = { version = "0.2.0", optional = true }
-tracing = { version = "0.1", default-features = false, features = ["std"] }
+tracing = { version = "0.1", default-features = false, optional = true, features = [
+  "std",
+] }
 
 # Tracy dependency compatibility table:
 # https://github.com/nagisa/rust_tracy_client
@@ -37,11 +57,7 @@ tracy-client = { version = "0.18.0", optional = true }
 android_log-sys = "0.3.0"
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]
-tracing-wasm = "0.2.1"
-# TODO: Assuming all wasm builds are for the browser. Require `no_std` support to break assumption.
-bevy_app = { path = "../bevy_app", version = "0.16.0-dev", default-features = false, features = [
-  "web",
-] }
+tracing-wasm = { version = "0.2.1", optional = true }
 
 [target.'cfg(target_os = "ios")'.dependencies]
 tracing-oslog = "0.2"

crates/bevy_log/src/android_tracing.rs

@@ -1,4 +1,4 @@
-use alloc::ffi::CString;
+use alloc::{ffi::CString, format, string::String, vec::Vec};
 use core::fmt::{Debug, Write};
 use tracing::{
     field::Field,