Add support for LazyLock

Cargo.toml

@@ -36,3 +36,6 @@ backtraces = []
 # Feature names do not match crate names pending namespaced features.
 lockapi = ["lock_api"]
 parkinglot = ["parking_lot", "lockapi"]
+
+[build-dependencies]
+autocfg = "1.4.0"

build.rs

@@ -0,0 +1,10 @@
+use autocfg::AutoCfg;
+
+fn main() {
+    // To avoid bumping MSRV unnecessarily, we can sniff certain features. Reevaluate this on major
+    // releases.
+    let ac = AutoCfg::new().unwrap();
+    ac.emit_has_path("std::sync::LazyLock");
+
+    autocfg::rerun_path("build.rs");
+}

src/lib.rs

@@ -193,6 +193,14 @@ impl MutexId {
             unreachable!("Tried to drop lock for mutex {:?} but it wasn't held", self)
         });
     }
+
+    /// Execute the given closure while the guard is held.
+    pub fn with_held<T>(&self, f: impl FnOnce() -> T) -> T {
+        // Note: we MUST construct the RAII guard, we cannot simply mark held + mark released, as
+        // f() may panic and corrupt our state.
+        let _guard = self.get_borrowed();
+        f()
+    }
 }
 
 impl Default for MutexId {

src/parkinglot.rs

@@ -138,16 +138,14 @@ pub mod tracing {
         /// This method will panic if `f` panics, poisoning this `Once`. In addition, this function
         /// panics when the lock acquisition order is determined to be inconsistent.
         pub fn call_once(&self, f: impl FnOnce()) {
-            let _borrow = self.id.get_borrowed();
-            self.inner.call_once(f);
+            self.id.with_held(|| self.inner.call_once(f));
         }
 
         /// Performs the given initialization routine once and only once.
         ///
         /// This method is identical to [`Once::call_once`] except it ignores poisoning.
         pub fn call_once_force(&self, f: impl FnOnce(OnceState)) {
-            let _borrow = self.id.get_borrowed();
-            self.inner.call_once_force(f);
+            self.id.with_held(|| self.inner.call_once_force(f));
         }
     }
 }

src/stdsync.rs

@@ -30,6 +30,12 @@ pub use tracing::{
     Condvar, Mutex, MutexGuard, Once, OnceLock, RwLock, RwLockReadGuard, RwLockWriteGuard,
 };
 
+#[cfg(all(has_std__sync__LazyLock, debug_assertions))]
+pub use tracing::LazyLock;
+
+#[cfg(all(has_std__sync__LazyLock, not(debug_assertions)))]
+pub use std::sync::LazyLock;
+
 /// Dependency tracing versions of [`std::sync`].
 pub mod tracing {
     use std::fmt;
@@ -47,6 +53,12 @@ pub mod tracing {
     use crate::BorrowedMutex;
     use crate::LazyMutexId;
 
+    #[cfg(has_std__sync__LazyLock)]
+    pub use lazy_lock::LazyLock;
+
+    #[cfg(has_std__sync__LazyLock)]
+    mod lazy_lock;
+
     /// Wrapper for [`std::sync::Mutex`].
     ///
     /// Refer to the [crate-level][`crate`] documentation for the differences between this struct and
@@ -458,8 +470,7 @@ pub mod tracing {
         where
             F: FnOnce(),
         {
-            let _guard = self.mutex_id.get_borrowed();
-            self.inner.call_once(f);
+            self.mutex_id.with_held(|| self.inner.call_once(f))
         }
 
         /// Performs the same operation as [`call_once`][Once::call_once] except it ignores
@@ -473,8 +484,7 @@ pub mod tracing {
         where
             F: FnOnce(&OnceState),
         {
-            let _guard = self.mutex_id.get_borrowed();
-            self.inner.call_once_force(f);
+            self.mutex_id.with_held(|| self.inner.call_once_force(f))
         }
 
         /// Returns true if some `call_once` has completed successfully.
@@ -548,9 +558,7 @@ pub mod tracing {
         /// As this method may block until initialization is complete, it participates in cycle
         /// detection.
         pub fn set(&self, value: T) -> Result<(), T> {
-            let _guard = self.id.get_borrowed();
-
-            self.inner.set(value)
+            self.id.with_held(|| self.inner.set(value))
         }
 
         /// Gets the contents of the cell, initializing it with `f` if the cell was empty.
@@ -560,8 +568,7 @@ pub mod tracing {
         where
             F: FnOnce() -> T,
         {
-            let _guard = self.id.get_borrowed();
-            self.inner.get_or_init(f)
+            self.id.with_held(|| self.inner.get_or_init(f))
         }
 
         /// Takes the value out of this `OnceLock`, moving it back to an uninitialized state.

src/stdsync/tracing/lazy_lock.rs

@@ -0,0 +1,113 @@
+//! Wrapper implementation for LazyLock
+//!
+//! This lives in a separate module as LazyLock would otherwise raise our MSRV to 1.80. Reevaluate
+//! this in the future.
+use std::fmt;
+use std::fmt::Debug;
+use std::ops::Deref;
+
+use crate::LazyMutexId;
+
+/// Wrapper for [`std::sync::LazyLock`]
+///
+/// This wrapper participates in cycle detection like all other primitives in this crate. It should
+/// only be possible to encounter cycles when acquiring mutexes in the initialisation function.
+///
+/// # Examples
+///
+/// ```
+/// use tracing_mutex::stdsync::tracing::LazyLock;
+///
+/// static LOCK: LazyLock<i32> = LazyLock::new(|| {
+///     println!("Hello, world!");
+///     42
+/// });
+///
+/// // This should print "Hello, world!"
+/// println!("{}", *LOCK);
+/// // This should not.
+/// println!("{}", *LOCK);
+/// ```
+pub struct LazyLock<T, F = fn() -> T> {
+    inner: std::sync::LazyLock<T, F>,
+    id: LazyMutexId,
+}
+
+impl<T, F: FnOnce() -> T> LazyLock<T, F> {
+    /// Creates a new lazy value with the given initializing function.
+    pub const fn new(f: F) -> LazyLock<T, F> {
+        Self {
+            id: LazyMutexId::new(),
+            inner: std::sync::LazyLock::new(f),
+        }
+    }
+
+    /// Force this lazy lock to be evaluated.
+    ///
+    /// This is equivalent to dereferencing, but is more explicit.
+    pub fn force(this: &LazyLock<T, F>) -> &T {
+        &*this
+    }
+}
+
+impl<T, F: FnOnce() -> T> Deref for LazyLock<T, F> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        self.id.with_held(|| &*self.inner)
+    }
+}
+
+impl<T: Default> Default for LazyLock<T> {
+    /// Return a `LazyLock` that is initialized through [`Default`].
+    fn default() -> Self {
+        Self::new(Default::default)
+    }
+}
+
+impl<T: Debug, F> Debug for LazyLock<T, F> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // Cannot implement this ourselves because the get() used is nightly, so delegate.
+        self.inner.fmt(f)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::stdsync::Mutex;
+
+    use super::*;
+
+    #[test]
+    fn test_only_init_once() {
+        let mut init_counter = 0;
+
+        let lock = LazyLock::new(|| {
+            init_counter += 1;
+            42
+        });
+
+        assert_eq!(*lock, 42);
+        LazyLock::force(&lock);
+
+        // Ensure we can access the init counter
+        drop(lock);
+
+        assert_eq!(init_counter, 1);
+    }
+
+    #[test]
+    #[should_panic(expected = "Found cycle")]
+    fn test_panic_with_cycle() {
+        let mutex = Mutex::new(());
+
+        let lock = LazyLock::new(|| *mutex.lock().unwrap());
+
+        // Establish the relation from lock to mutex
+        LazyLock::force(&lock);
+
+        // Now do it the other way around, which should crash
+        let _guard = mutex.lock().unwrap();
+        LazyLock::force(&lock);
+    }
+}