Adjust Once to not violate feature additivity principle

tracing-core/src/lazy_static/core_lazy.rs

@@ -5,7 +5,7 @@
 // http://opensource.org/licenses/MIT>, at your option. This file may not be
 // copied, modified, or distributed except according to those terms.
 
-use crate::Once;
+use crate::spin::Once;
 
 pub(crate) struct Lazy<T: Sync>(Once<T>);
 

tracing-core/src/lib.rs

@@ -226,7 +226,10 @@ pub(crate) mod spin;
 
 #[cfg(not(feature = "std"))]
 #[doc(hidden)]
-pub use self::spin::Once;
+pub type Once = self::spin::Once<()>;
+
+#[cfg(feature = "std")]
+pub use stdlib::sync::Once;
 
 pub mod callsite;
 pub mod dispatcher;

tracing-core/src/spin/mod.rs

@@ -1,7 +1,7 @@
 //! Synchronization primitives based on spinning
 
 pub(crate) use mutex::*;
-pub use once::Once;
+pub(crate) use once::Once;
 
 mod mutex;
 mod once;

tracing/src/lib.rs

@@ -832,12 +832,7 @@ pub mod subscriber;
 #[doc(hidden)]
 pub mod __macro_support {
     pub use crate::stdlib::sync::atomic::{AtomicUsize, Ordering};
-
-    #[cfg(feature = "std")]
-    pub use crate::stdlib::sync::Once;
-
-    #[cfg(not(feature = "std"))]
-    pub type Once = tracing_core::Once<()>;
+    pub type Once = tracing_core::Once;
 }
 
 mod sealed {