core: Use const initializers instead of once_cell

tracing-core/Cargo.toml

@@ -26,13 +26,15 @@ rust-version = "1.65.0"
 
 [features]
 default = ["std", "valuable?/std"]
-std = ["once_cell"]
+std = []
+# used to be an implicit feature from an optional dependency
+# FIXME: remove with the next breaking-change release
+once_cell = []
 
 [badges]
 maintenance = { status = "actively-developed" }
 
 [dependencies]
-once_cell = { version = "1.13.0", optional = true }
 
 [target.'cfg(tracing_unstable)'.dependencies]
 valuable = { version = "0.1.0", optional = true, default-features = false }

tracing-core/src/callsite.rs

@@ -109,7 +109,6 @@ use crate::stdlib::{
 };
 use crate::{
     dispatcher::Dispatch,
-    lazy::Lazy,
     metadata::{LevelFilter, Metadata},
     subscriber::Interest,
 };
@@ -260,7 +259,7 @@ static CALLSITES: Callsites = Callsites {
 
 static DISPATCHERS: Dispatchers = Dispatchers::new();
 
-static LOCKED_CALLSITES: Lazy<Mutex<Vec<&'static dyn Callsite>>> = Lazy::new(Default::default);
+static LOCKED_CALLSITES: Mutex<Vec<&'static dyn Callsite>> = Mutex::new(Vec::new());
 
 struct Callsites {
     list_head: AtomicPtr<DefaultCallsite>,
@@ -515,7 +514,7 @@ mod private {
 
 #[cfg(feature = "std")]
 mod dispatchers {
-    use crate::{dispatcher, lazy::Lazy};
+    use crate::dispatcher;
     use std::sync::{
         atomic::{AtomicBool, Ordering},
         RwLock, RwLockReadGuard, RwLockWriteGuard,
@@ -525,8 +524,7 @@ mod dispatchers {
         has_just_one: AtomicBool,
     }
 
-    static LOCKED_DISPATCHERS: Lazy<RwLock<Vec<dispatcher::Registrar>>> =
-        Lazy::new(Default::default);
+    static LOCKED_DISPATCHERS: RwLock<Vec<dispatcher::Registrar>> = RwLock::new(Vec::new());
 
     pub(super) enum Rebuilder<'a> {
         JustOne,

tracing-core/src/lib.rs

@@ -261,8 +261,6 @@ macro_rules! metadata {
     };
 }
 
-pub(crate) mod lazy;
-
 // Trimmed-down vendored version of spin 0.5.2 (0387621)
 // Dependency of no_std lazy_static, not required in a std build
 #[cfg(not(feature = "std"))]

tracing-core/src/stdlib.rs

@@ -64,11 +64,11 @@ mod no_std {
         }
 
         impl<T> Mutex<T> {
-            // pub(crate) fn new(data: T) -> Self {
-            //     Self {
-            //         inner: crate::spin::Mutex::new(data),
-            //     }
-            // }
+            pub(crate) const fn new(data: T) -> Self {
+                Self {
+                    inner: crate::spin::Mutex::new(data),
+                }
+            }
 
             pub(crate) fn lock(&self) -> Result<MutexGuard<'_, T>, ()> {
                 Ok(self.inner.lock())