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 core::{
 use self::dispatchers::Dispatchers;
 use crate::{
     dispatcher::Dispatch,
-    lazy::Lazy,
     metadata::{LevelFilter, Metadata},
     subscriber::Interest,
     sync::Mutex,
@@ -259,7 +258,7 @@ 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>,
@@ -514,7 +513,7 @@ mod private {
 
 #[cfg(feature = "std")]
 mod dispatchers {
-    use crate::{dispatcher, lazy::Lazy};
+    use crate::dispatcher;
     use alloc::vec::Vec;
     use std::sync::{
         atomic::{AtomicBool, Ordering},
@@ -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/lazy.rs

@@ -1,76 +0,0 @@
-#[cfg(feature = "std")]
-pub(crate) use once_cell::sync::Lazy;
-
-#[cfg(not(feature = "std"))]
-pub(crate) use self::spin::Lazy;
-
-#[cfg(not(feature = "std"))]
-mod spin {
-    //! This is the `once_cell::sync::Lazy` type, but modified to use our
-    //! `spin::Once` type rather than `OnceCell`. This is used to replace
-    //! `once_cell::sync::Lazy` on `no-std` builds.
-    use crate::spin::Once;
-    use core::{cell::Cell, fmt, ops::Deref};
-
-    /// Re-implementation of `once_cell::sync::Lazy` on top of `spin::Once`
-    /// rather than `OnceCell`.
-    ///
-    /// This is used when the standard library is disabled.
-    pub(crate) struct Lazy<T, F = fn() -> T> {
-        cell: Once<T>,
-        init: Cell<Option<F>>,
-    }
-
-    impl<T: fmt::Debug, F> fmt::Debug for Lazy<T, F> {
-        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-            f.debug_struct("Lazy")
-                .field("cell", &self.cell)
-                .field("init", &"..")
-                .finish()
-        }
-    }
-
-    // We never create a `&F` from a `&Lazy<T, F>` so it is fine to not impl
-    // `Sync` for `F`. We do create a `&mut Option<F>` in `force`, but this is
-    // properly synchronized, so it only happens once so it also does not
-    // contribute to this impl.
-    unsafe impl<T, F: Send> Sync for Lazy<T, F> where Once<T>: Sync {}
-    // auto-derived `Send` impl is OK.
-
-    impl<T, F> Lazy<T, F> {
-        /// Creates a new lazy value with the given initializing function.
-        pub(crate) const fn new(init: F) -> Lazy<T, F> {
-            Lazy {
-                cell: Once::new(),
-                init: Cell::new(Some(init)),
-            }
-        }
-    }
-
-    impl<T, F: FnOnce() -> T> Lazy<T, F> {
-        /// Forces the evaluation of this lazy value and returns a reference to
-        /// the result.
-        ///
-        /// This is equivalent to the `Deref` impl, but is explicit.
-        pub(crate) fn force(this: &Lazy<T, F>) -> &T {
-            this.cell.call_once(|| match this.init.take() {
-                Some(f) => f(),
-                None => panic!("Lazy instance has previously been poisoned"),
-            })
-        }
-    }
-
-    impl<T, F: FnOnce() -> T> Deref for Lazy<T, F> {
-        type Target = T;
-        fn deref(&self) -> &T {
-            Lazy::force(self)
-        }
-    }
-
-    impl<T: Default> Default for Lazy<T> {
-        /// Creates a new lazy value using `Default` as the initializing function.
-        fn default() -> Lazy<T> {
-            Lazy::new(T::default)
-        }
-    }
-}

tracing-core/src/lib.rs

@@ -266,8 +266,6 @@ 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/sync.rs

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