subscriber: impl LookupSpan for Box<LS> and Arc<LS> (tokio-rs#2247)

These implementations mirror those provided by tracing-core for
`Subscriber` on `Box<S>` and `Arc<S>`.

Author: jswrenn

tracing-subscriber/src/layer/mod.rs

@@ -685,7 +685,7 @@ use core::any::TypeId;
 
 feature! {
     #![feature = "alloc"]
-    use alloc::boxed::Box;
+    use alloc::{vec::Vec, boxed::Box};
     use core::ops::{Deref, DerefMut};
 }
 
@@ -1568,8 +1568,6 @@ where
 
 feature! {
     #![any(feature = "std", feature = "alloc")]
-    #[cfg(not(feature = "std"))]
-    use alloc::vec::Vec;
 
     macro_rules! layer_impl_body {
         () => {

tracing-subscriber/src/registry/mod.rs

@@ -230,6 +230,11 @@ pub struct Scope<'a, R> {
 feature! {
     #![any(feature = "alloc", feature = "std")]
 
+    use alloc::{
+        boxed::Box,
+        sync::Arc
+    };
+
     #[cfg(not(feature = "smallvec"))]
     use alloc::vec::{self, Vec};
 
@@ -251,6 +256,66 @@ feature! {
     #[cfg(feature = "smallvec")]
     type SpanRefVecArray<'span, L> = [SpanRef<'span, L>; 16];
 
+    impl<'a, S> LookupSpan<'a> for Arc<S>
+    where
+        S: LookupSpan<'a>,
+    {
+        type Data = <S as LookupSpan<'a>>::Data;
+
+        fn span_data(&'a self, id: &Id) -> Option<Self::Data> {
+            self.as_ref().span_data(id)
+        }
+
+        fn span(&'a self, id: &Id) -> Option<SpanRef<'_, Self>>
+        where
+            Self: Sized,
+        {
+            self.as_ref().span(id).map(
+                |SpanRef {
+                    registry: _,
+                    data,
+                    #[cfg(feature = "registry")]
+                    filter,
+                }| SpanRef {
+                    registry: self,
+                    data,
+                    #[cfg(feature = "registry")]
+                    filter,
+                },
+            )
+        }
+    }
+
+    impl<'a, S> LookupSpan<'a> for Box<S>
+    where
+        S: LookupSpan<'a>,
+    {
+        type Data = <S as LookupSpan<'a>>::Data;
+
+        fn span_data(&'a self, id: &Id) -> Option<Self::Data> {
+            self.as_ref().span_data(id)
+        }
+
+        fn span(&'a self, id: &Id) -> Option<SpanRef<'_, Self>>
+        where
+            Self: Sized,
+        {
+            self.as_ref().span(id).map(
+                |SpanRef {
+                    registry: _,
+                    data,
+                    #[cfg(feature = "registry")]
+                    filter,
+                }| SpanRef {
+                    registry: self,
+                    data,
+                    #[cfg(feature = "registry")]
+                    filter,
+                },
+            )
+        }
+    }
+
     impl<'a, R> Scope<'a, R>
     where
         R: LookupSpan<'a>,