Add Context method for resolving an Event's SpanRef

Fixes tokio-rs#1428

tracing-subscriber/src/layer.rs

@@ -994,6 +994,29 @@ where
  }
  }
 
+ /// Finds the [`SpanRef`] associated with an [`Event`], if it has one.
+ ///
+ /// Compared to [`Context::current_span`] and [`Context::lookup_current`],
+ /// this respects overrides provided by the [`Event`].
+ ///
+ /// Compared to [`Event::parent`], this automatically falls back to the contextual
+ /// span, if required.
+ #[inline]
+ #[cfg(feature = "registry")]
+ #[cfg_attr(docsrs, doc(cfg(feature = "registry")))]
+ pub fn event_span(&self, event: &Event<'_>) -> Option<SpanRef<'_, S>>
+ where
+ S: for<'lookup> LookupSpan<'lookup>,
+ {
+ if event.is_root() {
+ None
+ } else if event.is_contextual() {
+ self.lookup_current()
+ } else {
+ event.parent().and_then(|id| self.span(id))
+ }
+ }
+
  /// Returns metadata for the span with the given `id`, if it exists.
  ///
  /// If this returns `None`, then no span exists for that ID (either it has