subscriber: remove Layer impls for Arcs (#1649)

Implementing `Layer` for `Arc`s, which are immutable, breaks the
ability to implement `Layer::on_layer` with a mutable reference.
This is necessary for per-layer filtering. See
#1576 (comment) 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/layer/mod.rs

@@ -1175,6 +1175,11 @@ where
 
 macro_rules! layer_impl_body {
     () => {
+        #[inline]
+        fn on_layer(&mut self, subscriber: &mut S) {
+            self.deref_mut().on_layer(subscriber);
+        }
+
         #[inline]
         fn new_span(&self, attrs: &span::Attributes<'_>, id: &span::Id, ctx: Context<'_, S>) {
             self.deref().new_span(attrs, id, ctx)
@@ -1238,59 +1243,18 @@ macro_rules! layer_impl_body {
     };
 }
 
-impl<L, S> Layer<S> for Arc<L>
-where
-    L: Layer<S>,
-    S: Subscriber,
-{
-    fn on_layer(&mut self, subscriber: &mut S) {
-        if let Some(inner) = Arc::get_mut(self) {
-            // XXX(eliza): this may behave weird if another `Arc` clone of this
-            // layer is layered onto a _different_ subscriber...but there's no
-            // good solution for that...
-            inner.on_layer(subscriber);
-        }
-    }
-
-    layer_impl_body! {}
-}
-
-impl<S> Layer<S> for Arc<dyn Layer<S> + Send + Sync>
-where
-    S: Subscriber,
-{
-    fn on_layer(&mut self, subscriber: &mut S) {
-        if let Some(inner) = Arc::get_mut(self) {
-            // XXX(eliza): this may behave weird if another `Arc` clone of this
-            // layer is layered onto a _different_ subscriber...but there's no
-            // good solution for that...
-            inner.on_layer(subscriber);
-        }
-    }
-
-    layer_impl_body! {}
-}
-
 impl<L, S> Layer<S> for Box<L>
 where
     L: Layer<S>,
     S: Subscriber,
 {
-    fn on_layer(&mut self, subscriber: &mut S) {
-        self.deref_mut().on_layer(subscriber);
-    }
-
     layer_impl_body! {}
 }
 
 impl<S> Layer<S> for Box<dyn Layer<S> + Send + Sync>
 where
     S: Subscriber,
 {
-    fn on_layer(&mut self, subscriber: &mut S) {
-        self.deref_mut().on_layer(subscriber);
-    }
-
     layer_impl_body! {}
 }