core: remove vendored lazy_static on no-std (#2173)

Currently, `no_std` targets use a vendored version of `lazy_static` that
uses the `spin` crate's `Once` type, while the `std` target uses the
`once_cell` crate's `Lazy` type. This is unfortunate, as the
`lazy_static` macro has a different interface from the `Lazy` cell type.
This increases the amount of code that differs based on whether or not
`std` is enabled.

This branch removes the vendored `lazy_static` macro and replaces it
with a reimplementation of `once_cell::sync::Lazy` that uses
`spin::Once` rather than `once_cell::sync::OnceCell` as the inner "once
type". Now, all code can be written against a `Lazy` struct with the
same interface, regardless of whether or not `std` is enabled.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tracing-core/src/callsite.rs

@@ -111,6 +111,7 @@ use crate::stdlib::{
 };
 use crate::{
     dispatcher::Dispatch,
+    lazy::Lazy,
     metadata::{LevelFilter, Metadata},
     subscriber::Interest,
 };
@@ -253,14 +254,7 @@ static CALLSITES: Callsites = Callsites {
 
 static DISPATCHERS: Dispatchers = Dispatchers::new();
 
-#[cfg(feature = "std")]
-static LOCKED_CALLSITES: once_cell::sync::Lazy<Mutex<Vec<&'static dyn Callsite>>> =
-    once_cell::sync::Lazy::new(Default::default);
-
-#[cfg(not(feature = "std"))]
-crate::lazy_static! {
-    static ref LOCKED_CALLSITES: Mutex<Vec<&'static dyn Callsite>> = Mutex::new(Vec::new());
-}
+static LOCKED_CALLSITES: Lazy<Mutex<Vec<&'static dyn Callsite>>> = Lazy::new(Default::default);
 
 struct Callsites {
     list_head: AtomicPtr<DefaultCallsite>,
@@ -514,8 +508,7 @@ mod private {
 
 #[cfg(feature = "std")]
 mod dispatchers {
-    use crate::dispatcher;
-    use once_cell::sync::Lazy;
+    use crate::{dispatcher, lazy::Lazy};
     use std::sync::{
         atomic::{AtomicBool, Ordering},
         RwLock, RwLockReadGuard, RwLockWriteGuard,

tracing-core/src/lazy.rs

@@ -0,0 +1,76 @@
+#[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

@@ -254,10 +254,7 @@ macro_rules! metadata {
     };
 }
 
-// Facade module: `no_std` uses spinlocks, `std` uses the mutexes in the standard library
-#[cfg(not(feature = "std"))]
-#[macro_use]
-mod lazy_static;
+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

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