Improve event_enabled order independence

tracing-core/src/collect.rs

@@ -57,6 +57,9 @@ use core::ptr::NonNull;
 ///   See also the [documentation on the callsite registry][cs-reg] for details
 ///   on [`register_callsite`].
 ///
+/// - [`event_enabled`] is called once before every [`event`] is recorded. This
+///   can be used to implement filtering on events once their field values are
+///   known but before any processing is done in `event`.
 /// - [`clone_span`] is called every time a span ID is cloned, and [`try_close`]
 ///   is called when a span ID is dropped. By default, these functions do
 ///   nothing. However, they can be used to implement reference counting for
@@ -72,6 +75,8 @@ use core::ptr::NonNull;
 /// [`clone_span`]: Collect::clone_span
 /// [`try_close`]: Collect::try_close
 /// [cs-reg]: crate::callsite#registering-callsites
+/// [`event`]: Collect::event
+/// [`event_enabled`]: Collect::event_enabled
 pub trait Collect: 'static {
     // === Span registry methods ==============================================
 
@@ -288,6 +293,17 @@ pub trait Collect: 'static {
     /// follow from _b_), it may silently do nothing.
     fn record_follows_from(&self, span: &span::Id, follows: &span::Id);
 
+    /// Determine if an [`Event`] should be recorded.
+    ///
+    /// By default, this returns `true` and collectors can filter events in
+    /// [`event`][Self::event] without any penalty. However, when `event` is
+    /// more complicated, this can be used to determine if `event` should be
+    /// called at all, separating out the decision from the processing.
+    fn event_enabled(&self, event: &Event<'_>) -> bool {
+        let _ = event;
+        true
+    }
+
     /// Records that an [`Event`] has occurred.
     ///
     /// This method will be invoked when an Event is constructed by

tracing-subscriber/src/subscribe/layered.rs

@@ -138,13 +138,21 @@ where
         self.subscriber.on_follows_from(span, follows, self.ctx());
     }
 
-    fn event(&self, event: &Event<'_>) {
+    fn event_enabled(&self, event: &Event<'_>) -> bool {
         if self.subscriber.event_enabled(event, self.ctx()) {
-            self.inner.event(event);
-            self.subscriber.on_event(event, self.ctx());
+            // if the outer subscriber enables the event, ask the inner collector.
+            self.inner.event_enabled(event)
+        } else {
+            // otherwise, the event is disabled by this subscriber
+            false
         }
     }
 
+    fn event(&self, event: &Event<'_>) {
+        self.inner.event(event);
+        self.subscriber.on_event(event, self.ctx());
+    }
+
     fn enter(&self, span: &span::Id) {
         self.inner.enter(span);
         self.subscriber.on_enter(span, self.ctx());
@@ -285,10 +293,10 @@ where
     #[inline]
     fn event_enabled(&self, event: &Event<'_>, ctx: Context<'_, C>) -> bool {
         if self.subscriber.event_enabled(event, ctx.clone()) {
-            // if the outer subscriber enables the event, ask the inner subscriber.
+            // if the outer subscriber enables the event, ask the inner collector.
             self.inner.event_enabled(event, ctx)
         } else {
-            // otherwise, the callsite is disabled by this subscriber
+            // otherwise, the event is disabled by this subscriber
             false
         }
     }