Implement subscribe::Filter for Option<Filter>

tracing-subscriber/src/subscribe/mod.rs

@@ -468,10 +468,25 @@
 //! forms of filtering. For custom filtering policies, the [`FilterFn`] and
 //! [`DynFilterFn`] types allow implementing a [`Filter`] with a closure or
 //! function pointer. In addition, when more control is required, the [`Filter`]
-//! trait may also be implemented for user-defined types. [`Option<Filter>`]
-//! also implements [`Filter`], which allows for an optional filter where
-//! [`None`](Option::None) filters out _nothing_ (that is, allows everything
-//! through).
+//! trait may also be implemented for user-defined types.
+//!
+//! [`Option<Filter>`] also implements [`Filter`], which allows for an optional
+//! filter. [`None`](Option::None) filters out _nothing_ (that is, allows
+//! everything through). For example:
+//!
+//! ```rust
+//! # use tracing_subscriber::{filter::filter_fn, Subscribe};
+//! # use tracing_core::{Metadata, collect::Collect};
+//! # struct MySubscriber<C>(std::marker::PhantomData<C>);
+//! # impl<C> MySubscriber<C> { fn new() -> Self { Self(std::marker::PhantomData)} }
+//! # impl<C: Collect> Subscribe<C> for MySubscriber<C> {}
+//! # fn my_filter(_: &str) -> impl Fn(&Metadata) -> bool { |_| true  }
+//! fn setup_tracing<C: Collect>(filter_config: Option<&str>) {
+//!     let layer = MySubscriber::<C>::new()
+//!         .with_filter(filter_config.map(|config| filter_fn(my_filter(config))));
+//! //...
+//! }
+//! ```
 //!
 //! <div class="example-wrap" style="display:inline-block">
 //! <pre class="compile_fail" style="white-space:normal;font:inherit;">

tracing-subscriber/tests/subscriber_filters/option.rs

@@ -28,7 +28,7 @@ fn option_none() {
         .event(expect::event())
         .only()
         .run_with_handle();
-    let subscribe = Box::new(subscribe.with_filter(None::<filter::DynFilterFn<_>>));
+    let subscribe = subscribe.with_filter(None::<filter::DynFilterFn<_>>);
 
     let _guard = tracing_subscriber::registry().with(subscribe).set_default();
 
@@ -38,3 +38,23 @@ fn option_none() {
 
     handle.assert_finished();
 }
+
+#[test]
+fn option_mixed() {
+    let (subscribe, handle) = subscriber::mock()
+        .event(expect::event())
+        .only()
+        .run_with_handle();
+    let subscribe = subscribe
+        .with_filter(filter::dynamic_filter_fn(|meta, _ctx| {
+            meta.target() == "interesting"
+        }))
+        .with_filter(None::<filter::DynFilterFn<_>>);
+
+    let _guard = tracing_subscriber::registry().with(subscribe).set_default();
+
+    tracing::info!(target: "interesting", x="foo");
+    tracing::info!(target: "boring", x="bar");
+
+    handle.assert_finished();
+}