core: impl<S: Subscriber + ?Sized> Subscriber for Box<S>, Arc<S> (#2161)

These broader impls supersede the previous impls where the inner type was a
`dyn Subscriber`. With these generic impls, you no longer must (but
still can, if you wish) cast the inner type of a boxed or arc'd
subscriber to `dyn Subscriber` to use it as a `Subscriber`.

Author: jswrenn

tracing-core/src/subscriber.rs

@@ -662,7 +662,10 @@ impl Subscriber for NoSubscriber {
     fn exit(&self, _span: &span::Id) {}
 }
 
-impl Subscriber for Box<dyn Subscriber + Send + Sync + 'static> {
+impl<S> Subscriber for Box<S>
+where
+    S: Subscriber + ?Sized,
+{
     #[inline]
     fn register_callsite(&self, metadata: &'static Metadata<'static>) -> Interest {
         self.as_ref().register_callsite(metadata)
@@ -739,7 +742,10 @@ impl Subscriber for Box<dyn Subscriber + Send + Sync + 'static> {
     }
 }
 
-impl Subscriber for Arc<dyn Subscriber + Send + Sync + 'static> {
+impl<S> Subscriber for Arc<S>
+where
+    S: Subscriber + ?Sized,
+{
     #[inline]
     fn register_callsite(&self, metadata: &'static Metadata<'static>) -> Interest {
         self.as_ref().register_callsite(metadata)