core, subscriber: more downcast_ref & is methods (#2160)

Adds inherent `downcast_ref` and `is` inherent methods to:
  - `dyn Subscriber + Send`
  - `dyn Subscriber + Sync`
  - `dyn Subscriber + Send + Sync`
  - `Layered`

These additional implementations reduce the circumstances in which one
must cast to `dyn Subscriber` (which, previously, was the only type for
which `downcast_ref` and `is` were available).

tracing-core/src/subscriber.rs

@@ -484,6 +484,66 @@ impl dyn Subscriber {
     }
 }
 
+impl dyn Subscriber + Send {
+    /// Returns `true` if this [`Subscriber`] is the same type as `T`.
+    pub fn is<T: Any>(&self) -> bool {
+        self.downcast_ref::<T>().is_some()
+    }
+
+    /// Returns some reference to this [`Subscriber`] value if it is of type `T`,
+    /// or `None` if it isn't.
+    pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
+        unsafe {
+            let raw = self.downcast_raw(TypeId::of::<T>())?;
+            if raw.is_null() {
+                None
+            } else {
+                Some(&*(raw as *const _))
+            }
+        }
+    }
+}
+
+impl dyn Subscriber + Sync {
+    /// Returns `true` if this [`Subscriber`] is the same type as `T`.
+    pub fn is<T: Any>(&self) -> bool {
+        self.downcast_ref::<T>().is_some()
+    }
+
+    /// Returns some reference to this `[`Subscriber`] value if it is of type `T`,
+    /// or `None` if it isn't.
+    pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
+        unsafe {
+            let raw = self.downcast_raw(TypeId::of::<T>())?;
+            if raw.is_null() {
+                None
+            } else {
+                Some(&*(raw as *const _))
+            }
+        }
+    }
+}
+
+impl dyn Subscriber + Send + Sync {
+    /// Returns `true` if this [`Subscriber`] is the same type as `T`.
+    pub fn is<T: Any>(&self) -> bool {
+        self.downcast_ref::<T>().is_some()
+    }
+
+    /// Returns some reference to this [`Subscriber`] value if it is of type `T`,
+    /// or `None` if it isn't.
+    pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
+        unsafe {
+            let raw = self.downcast_raw(TypeId::of::<T>())?;
+            if raw.is_null() {
+                None
+            } else {
+                Some(&*(raw as *const _))
+            }
+        }
+    }
+}
+
 /// Indicates a [`Subscriber`]'s interest in a particular callsite.
 ///
 /// `Subscriber`s return an `Interest` from their [`register_callsite`] methods

tracing-subscriber/src/layer/layered.rs

@@ -12,7 +12,11 @@ use crate::{
 };
 #[cfg(all(feature = "registry", feature = "std"))]
 use crate::{filter::FilterId, registry::Registry};
-use core::{any::TypeId, cmp, fmt, marker::PhantomData};
+use core::{
+    any::{Any, TypeId},
+    cmp, fmt,
+    marker::PhantomData,
+};
 
 /// A [`Subscriber`] composed of a `Subscriber` wrapped by one or more
 /// [`Layer`]s.
@@ -63,6 +67,30 @@ pub struct Layered<L, I, S = I> {
 
 // === impl Layered ===
 
+impl<L, S> Layered<L, S>
+where
+    L: Layer<S>,
+    S: Subscriber,
+{
+    /// Returns `true` if this [`Subscriber`] is the same type as `T`.
+    pub fn is<T: Any>(&self) -> bool {
+        self.downcast_ref::<T>().is_some()
+    }
+
+    /// Returns some reference to this [`Subscriber`] value if it is of type `T`,
+    /// or `None` if it isn't.
+    pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
+        unsafe {
+            let raw = self.downcast_raw(TypeId::of::<T>())?;
+            if raw.is_null() {
+                None
+            } else {
+                Some(&*(raw as *const T))
+            }
+        }
+    }
+}
+
 impl<L, S> Subscriber for Layered<L, S>
 where
     L: Layer<S>,