core: add portable-atomic support (#3199)

## Motivation

I want to use `bevy_reflect` on `no_std` devices. This platform has alloc,
but not atomics, so `alloc::sync` is missing. `bevy_relflect` has a hard
depency on `tracing`, and I'd like to use tracing on this platform also.

Want to use tracing-core on `no_std` devices like rp2040
without hardware atomics

## Solution

- Added `portable-atomic-util` as an optional dependency
  gated behind a new feature, `portable-atomic` to
  `tracing-core`
- When `portable-atomic` is enabled, switched uses of
  `Arc` and `Weak` away from `alloc::sync` to
  `portable_atomic_util`.
- Added workaround for a lack of support for
  [unsized coercion](rust-lang/rust#18598)
  in custom types. I've included a comment linking to this issue
  explaining the missing functionality.

Fixes #3173

Author: bushrat011899

tracing-core/Cargo.toml

@@ -26,14 +26,16 @@ rust-version = "1.63.0"
 
 [features]
 default = ["std"]
-alloc = []
-std = ["once_cell", "alloc"]
+alloc = ["portable-atomic-util?/alloc"]
+std = ["once_cell", "alloc", "portable-atomic-util?/std"]
+portable-atomic = ["dep:portable-atomic-util", "once_cell?/portable-atomic"]
 
 [badges]
 maintenance = { status = "actively-developed" }
 
 [dependencies]
 once_cell = { version = "1.13.0", optional = true }
+portable-atomic-util = { version = "0.2.4", default-features = false, optional = true }
 
 [package.metadata.docs.rs]
 all-features = true

tracing-core/src/collect.rs

@@ -4,6 +4,12 @@ use crate::{span, Dispatch, Event, LevelFilter, Metadata};
 use core::any::{Any, TypeId};
 use core::ptr::NonNull;
 
+#[cfg(all(feature = "alloc", not(feature = "portable-atomic")))]
+use alloc::sync::Arc;
+
+#[cfg(all(feature = "alloc", feature = "portable-atomic"))]
+use portable_atomic_util::Arc;
+
 /// Trait representing the functions required to collect trace data.
 ///
 /// Crates that provide implementations of methods for collecting or recording
@@ -772,7 +778,7 @@ where
 }
 
 #[cfg(feature = "alloc")]
-impl<C> Collect for alloc::sync::Arc<C>
+impl<C> Collect for Arc<C>
 where
     C: Collect + ?Sized,
 {

tracing-core/src/dispatch.rs

@@ -152,9 +152,12 @@ use std::{
     error,
 };
 
-#[cfg(feature = "alloc")]
+#[cfg(all(feature = "alloc", not(feature = "portable-atomic")))]
 use alloc::sync::{Arc, Weak};
 
+#[cfg(all(feature = "alloc", feature = "portable-atomic"))]
+use portable_atomic_util::{Arc, Weak};
+
 #[cfg(feature = "alloc")]
 use core::ops::Deref;
 
@@ -539,8 +542,21 @@ impl Dispatch {
     where
         C: Collect + Send + Sync + 'static,
     {
+        #[cfg(not(feature = "portable-atomic"))]
+        let arc = Arc::new(collector);
+
+        #[cfg(feature = "portable-atomic")]
+        let arc = {
+            use alloc::boxed::Box;
+
+            // Workaround for a lack of support for unsized coercion in non-first-party types.
+            // See https://github.com/rust-lang/rust/issues/18598
+            let boxed: Box<dyn Collect + Send + Sync> = Box::<C>::new(collector);
+            Arc::from(boxed)
+        };
+
         let me = Dispatch {
-            collector: Kind::Scoped(Arc::new(collector)),
+            collector: Kind::Scoped(arc),
         };
         crate::callsite::register_dispatch(&me);
         me