From 72abf1a977c93cc5f82e75eb0d47bee10a7ce16c Mon Sep 17 00:00:00 2001
From: Eliza Weisman <eliza@buoyant.io>
Date: Tue, 19 Oct 2021 10:13:28 -0700
Subject: [PATCH] subscriber: remove `Subscribe` impls for `Arc`s

Implementing `Subscribe` for `Arc`s, which are immutable, breaks the
ability to implement `Subscribe::on_layer` with a mutable reference.
This is necessary for per-layer filtering. See
https://github.com/tokio-rs/tracing/pull/1576#discussion_r711609810 for
details. Therefore, the `Layer` impls for `Arc`s should not be used.

In 0.3, we have the opportunity to remove these APIs. Therefore, this PR
removes them.
---
 tracing-subscriber/src/subscribe.rs | 17 +----------------
 1 file changed, 1 insertion(+), 16 deletions(-)

diff --git a/tracing-subscriber/src/subscribe.rs b/tracing-subscriber/src/subscribe.rs
index a195ba6b51..6c9673dad4 100644
--- a/tracing-subscriber/src/subscribe.rs
+++ b/tracing-subscriber/src/subscribe.rs
@@ -9,7 +9,7 @@ use tracing_core::{
 
 #[cfg(feature = "registry")]
 use crate::registry::{self, LookupSpan, Registry, SpanRef};
-use std::{any::TypeId, marker::PhantomData, ops::Deref, ptr::NonNull, sync::Arc};
+use std::{any::TypeId, marker::PhantomData, ops::Deref, ptr::NonNull};
 
 /// A composable handler for `tracing` events.
 ///
@@ -949,21 +949,6 @@ macro_rules! subscriber_impl_body {
     };
 }
 
-impl<S, C> Subscribe<C> for Arc<S>
-where
-    S: Subscribe<C>,
-    C: Collect,
-{
-    subscriber_impl_body! {}
-}
-
-impl<C> Subscribe<C> for Arc<dyn Subscribe<C> + Send + Sync>
-where
-    C: Collect,
-{
-    subscriber_impl_body! {}
-}
-
 impl<S, C> Subscribe<C> for Box<S>
 where
     S: Subscribe<C>,