feat(log): support customizing default log formatting (bevyengine#17722)

The LogPlugin now allows overriding the default
`tracing_subscriber::fmt::Layer` through a new `fmt_layer` option. This
enables customization of the default log output format without having to
replace the entire logging system.

For example, to disable timestamps in the log output:

```rust
fn fmt_layer(_app: &mut App) -> Option<bevy::log::BoxedFmtLayer> {
    Some(Box::new(
        bevy::log::tracing_subscriber::fmt::Layer::default()
            .without_time()
            .with_writer(std::io::stderr),
    ))
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins.set(bevy::log::LogPlugin {
            fmt_layer,
            ..default()
        }))
        .run();
}
```

This is different from the existing `custom_layer` option, because that
option _adds_ additional layers to the subscriber, but can't modify the
default formatter layer (at least, not to my knowledge).

I almost always disable timestamps in my Bevy logs, and usually also
tweak other default log formatting (such as `with_span_events`), which
made it so that I always had to disable the default logger. This allows
me to use everything the Bevy logger supports (including tracy support),
while still formatting the default logs the way I like them.

---------

Signed-off-by: Jean Mertz <[email protected]>

Author: JeanMertz

crates/bevy_log/src/lib.rs

@@ -56,6 +56,7 @@ use bevy_app::{App, Plugin};
 use tracing_log::LogTracer;
 use tracing_subscriber::{
     filter::{FromEnvError, ParseError},
+    layer::Layered,
     prelude::*,
     registry::Registry,
     EnvFilter, Layer,
@@ -97,6 +98,7 @@ pub(crate) struct FlushGuard(SyncCell<tracing_chrome::FlushGuard>);
 ///             level: Level::DEBUG,
 ///             filter: "wgpu=error,bevy_render=info,bevy_ecs=trace".to_string(),
 ///             custom_layer: |_| None,
+///             fmt_layer: |_| None,
 ///         }))
 ///         .run();
 /// }
@@ -240,11 +242,38 @@ pub struct LogPlugin {
     ///
     /// Please see the `examples/log_layers.rs` for a complete example.
     pub custom_layer: fn(app: &mut App) -> Option<BoxedLayer>,
+
+    /// Override the default [`tracing_subscriber::fmt::Layer`] with a custom one.
+    ///
+    /// This differs from [`custom_layer`](Self::custom_layer) in that
+    /// [`fmt_layer`](Self::fmt_layer) allows you to overwrite the default formatter layer, while
+    /// `custom_layer` only allows you to add additional layers (which are unable to modify the
+    /// default formatter).
+    ///
+    /// For example, you can use [`tracing_subscriber::fmt::Layer::without_time`] to remove the
+    /// timestamp from the log output.
+    ///
+    /// Please see the `examples/log_layers.rs` for a complete example.
+    pub fmt_layer: fn(app: &mut App) -> Option<BoxedFmtLayer>,
 }
 
-/// A boxed [`Layer`] that can be used with [`LogPlugin`].
+/// A boxed [`Layer`] that can be used with [`LogPlugin::custom_layer`].
 pub type BoxedLayer = Box<dyn Layer<Registry> + Send + Sync + 'static>;
 
+#[cfg(feature = "trace")]
+type BaseSubscriber =
+    Layered<EnvFilter, Layered<Option<Box<dyn Layer<Registry> + Send + Sync>>, Registry>>;
+
+#[cfg(feature = "trace")]
+type PreFmtSubscriber = Layered<tracing_error::ErrorLayer<BaseSubscriber>, BaseSubscriber>;
+
+#[cfg(not(feature = "trace"))]
+type PreFmtSubscriber =
+    Layered<EnvFilter, Layered<Option<Box<dyn Layer<Registry> + Send + Sync>>, Registry>>;
+
+/// A boxed [`Layer`] that can be used with [`LogPlugin::fmt_layer`].
+pub type BoxedFmtLayer = Box<dyn Layer<PreFmtSubscriber> + Send + Sync + 'static>;
+
 /// The default [`LogPlugin`] [`EnvFilter`].
 pub const DEFAULT_FILTER: &str = "wgpu=error,naga=warn";
 
@@ -254,6 +283,7 @@ impl Default for LogPlugin {
             filter: DEFAULT_FILTER.to_string(),
             level: Level::INFO,
             custom_layer: |_| None,
+            fmt_layer: |_| None,
         }
     }
 }
@@ -328,10 +358,12 @@ impl Plugin for LogPlugin {
             #[cfg(feature = "tracing-tracy")]
             let tracy_layer = tracing_tracy::TracyLayer::default();
 
-            // note: the implementation of `Default` reads from the env var NO_COLOR
-            // to decide whether to use ANSI color codes, which is common convention
-            // https://no-color.org/
-            let fmt_layer = tracing_subscriber::fmt::Layer::default().with_writer(std::io::stderr);
+            let fmt_layer = (self.fmt_layer)(app).unwrap_or_else(|| {
+                // note: the implementation of `Default` reads from the env var NO_COLOR
+                // to decide whether to use ANSI color codes, which is common convention
+                // https://no-color.org/
+                Box::new(tracing_subscriber::fmt::Layer::default().with_writer(std::io::stderr))
+            });
 
             // bevy_render::renderer logs a `tracy.frame_mark` event every frame
             // at Level::INFO. Formatted logs should omit it.

examples/app/log_layers.rs

@@ -4,7 +4,7 @@ use bevy::{
     log::{
         tracing::{self, Subscriber},
         tracing_subscriber::Layer,
-        BoxedLayer,
+        BoxedFmtLayer, BoxedLayer,
     },
     prelude::*,
 };
@@ -36,10 +36,24 @@ fn custom_layer(_app: &mut App) -> Option<BoxedLayer> {
     ]))
 }
 
+// While `custom_layer` allows you to add _additional_ layers, it won't allow you to override the
+// default `tracing_subscriber::fmt::Layer` added by `LogPlugin`. To do that, you can use the
+// `fmt_layer` option.
+//
+// In this example, we're disabling the timestamp in the log output.
+fn fmt_layer(_app: &mut App) -> Option<BoxedFmtLayer> {
+    Some(Box::new(
+        bevy::log::tracing_subscriber::fmt::Layer::default()
+            .without_time()
+            .with_writer(std::io::stderr),
+    ))
+}
+
 fn main() {
     App::new()
         .add_plugins(DefaultPlugins.set(bevy::log::LogPlugin {
             custom_layer,
+            fmt_layer,
 
             ..default()
         }))

examples/app/log_layers_ecs.rs

@@ -30,6 +30,7 @@ fn main() {
             level: Level::TRACE,
             filter: "warn,log_layers_ecs=trace".to_string(),
             custom_layer,
+            ..default()
         }))
         .add_systems(Startup, (log_system, setup))
         .add_systems(Update, print_logs)