Consistently pass a ref to a Dispatch

This changes `set_global_default` to take a reference to its `Dispatch` for a more consistent API at the cost of a one-time clone when initializing the `GLOBAL_DISPATCH` static.

Closes tokio-rs#455
Part of tokio-rs#922

tracing-core/src/dispatcher.rs

@@ -100,7 +100,7 @@
 //! # let my_dispatch = dispatcher::Dispatch::new(my_subscriber);
 //! // no default subscriber
 //!
-//! dispatcher::set_global_default(my_dispatch)
+//! dispatcher::set_global_default(&my_dispatch)
 //!     // `set_global_default` will return an error if the global default
 //!     // subscriber has already been set.
 //!     .expect("global default was already set!");
@@ -279,11 +279,11 @@ pub fn set_default(dispatcher: &Dispatch) -> DefaultGuard {
 /// [span]: ../span/index.html
 /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
 /// [`Event`]: ../event/struct.Event.html
-pub fn set_global_default(dispatcher: Dispatch) -> Result<(), SetGlobalDefaultError> {
+pub fn set_global_default(dispatcher: &Dispatch) -> Result<(), SetGlobalDefaultError> {
     if GLOBAL_INIT.compare_and_swap(UNINITIALIZED, INITIALIZING, Ordering::SeqCst) == UNINITIALIZED
     {
         unsafe {
-            GLOBAL_DISPATCH = Some(dispatcher);
+            GLOBAL_DISPATCH = Some(dispatcher.clone());
         }
         GLOBAL_INIT.store(INITIALIZED, Ordering::SeqCst);
         EXISTS.store(true, Ordering::Release);

tracing-core/tests/dispatch.rs

@@ -6,7 +6,7 @@ use tracing_core::dispatcher::*;
 #[cfg(feature = "std")]
 #[test]
 fn set_default_dispatch() {
-    set_global_default(Dispatch::new(TestSubscriberA)).expect("global dispatch set failed");
+    set_global_default(&Dispatch::new(TestSubscriberA)).expect("global dispatch set failed");
     get_default(|current| {
         assert!(
             current.is::<TestSubscriberA>(),

tracing-core/tests/global_dispatch.rs

@@ -4,7 +4,7 @@ use common::*;
 use tracing_core::dispatcher::*;
 #[test]
 fn global_dispatch() {
-    set_global_default(Dispatch::new(TestSubscriberA)).expect("global dispatch set failed");
+    set_global_default(&Dispatch::new(TestSubscriberA)).expect("global dispatch set failed");
     get_default(|current| {
         assert!(
             current.is::<TestSubscriberA>(),
@@ -29,6 +29,6 @@ fn global_dispatch() {
         )
     });
 
-    set_global_default(Dispatch::new(TestSubscriberA))
+    set_global_default(&Dispatch::new(TestSubscriberA))
         .expect_err("double global dispatch set succeeded");
 }

tracing-subscriber/src/fmt/mod.rs

@@ -414,7 +414,7 @@ where
         #[cfg(feature = "tracing-log")]
         tracing_log::LogTracer::init().map_err(Box::new)?;
 
-        tracing_core::dispatcher::set_global_default(tracing_core::dispatcher::Dispatch::new(
+        tracing_core::dispatcher::set_global_default(&tracing_core::dispatcher::Dispatch::new(
             self.finish(),
         ))?;
         Ok(())

tracing-subscriber/src/util.rs

@@ -53,7 +53,7 @@ where
         #[cfg(feature = "tracing-log")]
         tracing_log::LogTracer::init().map_err(TryInitError::new)?;
 
-        dispatcher::set_global_default(self.into()).map_err(TryInitError::new)?;
+        dispatcher::set_global_default(&self.into()).map_err(TryInitError::new)?;
 
         Ok(())
     }

tracing/src/dispatcher.rs

@@ -100,7 +100,7 @@
 //! # let my_dispatch = dispatcher::Dispatch::new(my_subscriber);
 //! // no default subscriber
 //!
-//! dispatcher::set_global_default(my_dispatch)
+//! dispatcher::set_global_default(&my_dispatch)
 //!     // `set_global_default` will return an error if the global default
 //!     // subscriber has already been set.
 //!     .expect("global default was already set!");

tracing/src/subscriber.rs

@@ -40,7 +40,7 @@ pub fn set_global_default<S>(subscriber: S) -> Result<(), SetGlobalDefaultError>
 where
     S: Subscriber + Send + Sync + 'static,
 {
-    crate::dispatcher::set_global_default(crate::Dispatch::new(subscriber))
+    crate::dispatcher::set_global_default(&crate::Dispatch::new(subscriber))
 }
 
 /// Sets the subscriber as the default for the duration of the lifetime of the