Rollup merge of #110543 - joboet:reentrant_lock, r=m-ou-se

Make `ReentrantLock` public

Implements the ACP rust-lang/libs-team#193.

``@rustbot`` label +T-libs-api +S-waiting-on-ACP

library/std/src/io/stdio.rs

@@ -11,8 +11,9 @@ use crate::fs::File;
 use crate::io::{
     self, BorrowedCursor, BufReader, IoSlice, IoSliceMut, LineWriter, Lines, SpecReadByte,
 };
+use crate::panic::{RefUnwindSafe, UnwindSafe};
 use crate::sync::atomic::{AtomicBool, Ordering};
-use crate::sync::{Arc, Mutex, MutexGuard, OnceLock, ReentrantMutex, ReentrantMutexGuard};
+use crate::sync::{Arc, Mutex, MutexGuard, OnceLock, ReentrantLock, ReentrantLockGuard};
 use crate::sys::stdio;
 
 type LocalStream = Arc<Mutex<Vec<u8>>>;
@@ -545,7 +546,7 @@ pub struct Stdout {
     // FIXME: this should be LineWriter or BufWriter depending on the state of
     //        stdout (tty or not). Note that if this is not line buffered it
     //        should also flush-on-panic or some form of flush-on-abort.
-    inner: &'static ReentrantMutex<RefCell<LineWriter<StdoutRaw>>>,
+    inner: &'static ReentrantLock<RefCell<LineWriter<StdoutRaw>>>,
 }
 
 /// A locked reference to the [`Stdout`] handle.
@@ -567,10 +568,10 @@ pub struct Stdout {
 #[must_use = "if unused stdout will immediately unlock"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct StdoutLock<'a> {
-    inner: ReentrantMutexGuard<'a, RefCell<LineWriter<StdoutRaw>>>,
+    inner: ReentrantLockGuard<'a, RefCell<LineWriter<StdoutRaw>>>,
 }
 
-static STDOUT: OnceLock<ReentrantMutex<RefCell<LineWriter<StdoutRaw>>>> = OnceLock::new();
+static STDOUT: OnceLock<ReentrantLock<RefCell<LineWriter<StdoutRaw>>>> = OnceLock::new();
 
 /// Constructs a new handle to the standard output of the current process.
 ///
@@ -624,7 +625,7 @@ static STDOUT: OnceLock<ReentrantMutex<RefCell<LineWriter<StdoutRaw>>>> = OnceLo
 pub fn stdout() -> Stdout {
     Stdout {
         inner: STDOUT
-            .get_or_init(|| ReentrantMutex::new(RefCell::new(LineWriter::new(stdout_raw())))),
+            .get_or_init(|| ReentrantLock::new(RefCell::new(LineWriter::new(stdout_raw())))),
     }
 }
 
@@ -635,7 +636,7 @@ pub fn cleanup() {
     let mut initialized = false;
     let stdout = STDOUT.get_or_init(|| {
         initialized = true;
-        ReentrantMutex::new(RefCell::new(LineWriter::with_capacity(0, stdout_raw())))
+        ReentrantLock::new(RefCell::new(LineWriter::with_capacity(0, stdout_raw())))
     });
 
     if !initialized {
@@ -678,6 +679,12 @@ impl Stdout {
     }
 }
 
+#[stable(feature = "catch_unwind", since = "1.9.0")]
+impl UnwindSafe for Stdout {}
+
+#[stable(feature = "catch_unwind", since = "1.9.0")]
+impl RefUnwindSafe for Stdout {}
+
 #[stable(feature = "std_debug", since = "1.16.0")]
 impl fmt::Debug for Stdout {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -737,6 +744,12 @@ impl Write for &Stdout {
     }
 }
 
+#[stable(feature = "catch_unwind", since = "1.9.0")]
+impl UnwindSafe for StdoutLock<'_> {}
+
+#[stable(feature = "catch_unwind", since = "1.9.0")]
+impl RefUnwindSafe for StdoutLock<'_> {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Write for StdoutLock<'_> {
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
@@ -786,7 +799,7 @@ impl fmt::Debug for StdoutLock<'_> {
 /// standard library or via raw Windows API calls, will fail.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Stderr {
-    inner: &'static ReentrantMutex<RefCell<StderrRaw>>,
+    inner: &'static ReentrantLock<RefCell<StderrRaw>>,
 }
 
 /// A locked reference to the [`Stderr`] handle.
@@ -808,7 +821,7 @@ pub struct Stderr {
 #[must_use = "if unused stderr will immediately unlock"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct StderrLock<'a> {
-    inner: ReentrantMutexGuard<'a, RefCell<StderrRaw>>,
+    inner: ReentrantLockGuard<'a, RefCell<StderrRaw>>,
 }
 
 /// Constructs a new handle to the standard error of the current process.
@@ -862,8 +875,8 @@ pub fn stderr() -> Stderr {
     // Note that unlike `stdout()` we don't use `at_exit` here to register a
     // destructor. Stderr is not buffered, so there's no need to run a
     // destructor for flushing the buffer
-    static INSTANCE: ReentrantMutex<RefCell<StderrRaw>> =
+    static INSTANCE: ReentrantLock<RefCell<StderrRaw>> =
-        ReentrantMutex::new(RefCell::new(stderr_raw()));
+        ReentrantLock::new(RefCell::new(stderr_raw()));
 
     Stderr { inner: &INSTANCE }
 }
@@ -898,6 +911,12 @@ impl Stderr {
     }
 }
 
+#[stable(feature = "catch_unwind", since = "1.9.0")]
+impl UnwindSafe for Stderr {}
+
+#[stable(feature = "catch_unwind", since = "1.9.0")]
+impl RefUnwindSafe for Stderr {}
+
 #[stable(feature = "std_debug", since = "1.16.0")]
 impl fmt::Debug for Stderr {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -957,6 +976,12 @@ impl Write for &Stderr {
     }
 }
 
+#[stable(feature = "catch_unwind", since = "1.9.0")]
+impl UnwindSafe for StderrLock<'_> {}
+
+#[stable(feature = "catch_unwind", since = "1.9.0")]
+impl RefUnwindSafe for StderrLock<'_> {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Write for StderrLock<'_> {
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {

library/std/src/sync/mod.rs

@@ -184,7 +184,8 @@ pub use self::lazy_lock::LazyLock;
 #[stable(feature = "once_cell", since = "1.70.0")]
 pub use self::once_lock::OnceLock;
 
-pub(crate) use self::remutex::{ReentrantMutex, ReentrantMutexGuard};
+#[unstable(feature = "reentrant_lock", issue = "121440")]
+pub use self::reentrant_lock::{ReentrantLock, ReentrantLockGuard};
 
 pub mod mpsc;
 
@@ -196,5 +197,5 @@ mod mutex;
 pub(crate) mod once;
 mod once_lock;
 mod poison;
-mod remutex;
+mod reentrant_lock;
 mod rwlock;

library/std/src/sync/reentrant_lock.rs

@@ -0,0 +1,320 @@
+#[cfg(all(test, not(target_os = "emscripten")))]
+mod tests;
+
+use crate::cell::UnsafeCell;
+use crate::fmt;
+use crate::ops::Deref;
+use crate::panic::{RefUnwindSafe, UnwindSafe};
+use crate::sync::atomic::{AtomicUsize, Ordering::Relaxed};
+use crate::sys::locks as sys;
+
+/// A re-entrant mutual exclusion lock
+///
+/// This lock will block *other* threads waiting for the lock to become
+/// available. The thread which has already locked the mutex can lock it
+/// multiple times without blocking, preventing a common source of deadlocks.
+///
+/// # Examples
+///
+/// Allow recursively calling a function needing synchronization from within
+/// a callback (this is how [`StdoutLock`](crate::io::StdoutLock) is currently
+/// implemented):
+///
+/// ```
+/// #![feature(reentrant_lock)]
+///
+/// use std::cell::RefCell;
+/// use std::sync::ReentrantLock;
+///
+/// pub struct Log {
+///     data: RefCell<String>,
+/// }
+///
+/// impl Log {
+///     pub fn append(&self, msg: &str) {
+///         self.data.borrow_mut().push_str(msg);
+///     }
+/// }
+///
+/// static LOG: ReentrantLock<Log> = ReentrantLock::new(Log { data: RefCell::new(String::new()) });
+///
+/// pub fn with_log<R>(f: impl FnOnce(&Log) -> R) -> R {
+///     let log = LOG.lock();
+///     f(&*log)
+/// }
+///
+/// with_log(|log| {
+///     log.append("Hello");
+///     with_log(|log| log.append(" there!"));
+/// });
+/// ```
+///
+// # Implementation details
+//
+// The 'owner' field tracks which thread has locked the mutex.
+//
+// We use current_thread_unique_ptr() as the thread identifier,
+// which is just the address of a thread local variable.
+//
+// If `owner` is set to the identifier of the current thread,
+// we assume the mutex is already locked and instead of locking it again,
+// we increment `lock_count`.
+//
+// When unlocking, we decrement `lock_count`, and only unlock the mutex when
+// it reaches zero.
+//
+// `lock_count` is protected by the mutex and only accessed by the thread that has
+// locked the mutex, so needs no synchronization.
+//
+// `owner` can be checked by other threads that want to see if they already
+// hold the lock, so needs to be atomic. If it compares equal, we're on the
+// same thread that holds the mutex and memory access can use relaxed ordering
+// since we're not dealing with multiple threads. If it's not equal,
+// synchronization is left to the mutex, making relaxed memory ordering for
+// the `owner` field fine in all cases.
+#[unstable(feature = "reentrant_lock", issue = "121440")]
+pub struct ReentrantLock<T: ?Sized> {
+    mutex: sys::Mutex,
+    owner: AtomicUsize,
+    lock_count: UnsafeCell<u32>,
+    data: T,
+}
+
+#[unstable(feature = "reentrant_lock", issue = "121440")]
+unsafe impl<T: Send + ?Sized> Send for ReentrantLock<T> {}
+#[unstable(feature = "reentrant_lock", issue = "121440")]
+unsafe impl<T: Send + ?Sized> Sync for ReentrantLock<T> {}
+
+// Because of the `UnsafeCell`, these traits are not implemented automatically
+#[unstable(feature = "reentrant_lock", issue = "121440")]
+impl<T: UnwindSafe + ?Sized> UnwindSafe for ReentrantLock<T> {}
+#[unstable(feature = "reentrant_lock", issue = "121440")]
+impl<T: RefUnwindSafe + ?Sized> RefUnwindSafe for ReentrantLock<T> {}
+
+/// An RAII implementation of a "scoped lock" of a re-entrant lock. When this
+/// structure is dropped (falls out of scope), the lock will be unlocked.
+///
+/// The data protected by the mutex can be accessed through this guard via its
+/// [`Deref`] implementation.
+///
+/// This structure is created by the [`lock`](ReentrantLock::lock) method on
+/// [`ReentrantLock`].
+///
+/// # Mutability
+///
+/// Unlike [`MutexGuard`](super::MutexGuard), `ReentrantLockGuard` does not
+/// implement [`DerefMut`](crate::ops::DerefMut), because implementation of
+/// the trait would violate Rust’s reference aliasing rules. Use interior
+/// mutability (usually [`RefCell`](crate::cell::RefCell)) in order to mutate
+/// the guarded data.
+#[must_use = "if unused the ReentrantLock will immediately unlock"]
+#[unstable(feature = "reentrant_lock", issue = "121440")]
+pub struct ReentrantLockGuard<'a, T: ?Sized + 'a> {
+    lock: &'a ReentrantLock<T>,
+}
+
+#[unstable(feature = "reentrant_lock", issue = "121440")]
+impl<T: ?Sized> !Send for ReentrantLockGuard<'_, T> {}
+
+#[unstable(feature = "reentrant_lock", issue = "121440")]
+impl<T> ReentrantLock<T> {
+    /// Creates a new re-entrant lock in an unlocked state ready for use.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(reentrant_lock)]
+    /// use std::sync::ReentrantLock;
+    ///
+    /// let lock = ReentrantLock::new(0);
+    /// ```
+    pub const fn new(t: T) -> ReentrantLock<T> {
+        ReentrantLock {
+            mutex: sys::Mutex::new(),
+            owner: AtomicUsize::new(0),
+            lock_count: UnsafeCell::new(0),
+            data: t,
+        }
+    }
+
+    /// Consumes this lock, returning the underlying data.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(reentrant_lock)]
+    ///
+    /// use std::sync::ReentrantLock;
+    ///
+    /// let lock = ReentrantLock::new(0);
+    /// assert_eq!(lock.into_inner(), 0);
+    /// ```
+    pub fn into_inner(self) -> T {
+        self.data
+    }
+}
+
+#[unstable(feature = "reentrant_lock", issue = "121440")]
+impl<T: ?Sized> ReentrantLock<T> {
+    /// Acquires the lock, blocking the current thread until it is able to do
+    /// so.
+    ///
+    /// This function will block the caller until it is available to acquire
+    /// the lock. Upon returning, the thread is the only thread with the lock
+    /// held. When the thread calling this method already holds the lock, the
+    /// call succeeds without blocking.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(reentrant_lock)]
+    /// use std::cell::Cell;
+    /// use std::sync::{Arc, ReentrantLock};
+    /// use std::thread;
+    ///
+    /// let lock = Arc::new(ReentrantLock::new(Cell::new(0)));
+    /// let c_lock = Arc::clone(&lock);
+    ///
+    /// thread::spawn(move || {
+    ///     c_lock.lock().set(10);
+    /// }).join().expect("thread::spawn failed");
+    /// assert_eq!(lock.lock().get(), 10);
+    /// ```
+    pub fn lock(&self) -> ReentrantLockGuard<'_, T> {
+        let this_thread = current_thread_unique_ptr();
+        // Safety: We only touch lock_count when we own the lock.
+        unsafe {
+            if self.owner.load(Relaxed) == this_thread {
+                self.increment_lock_count().expect("lock count overflow in reentrant mutex");
+            } else {
+                self.mutex.lock();
+                self.owner.store(this_thread, Relaxed);
+                debug_assert_eq!(*self.lock_count.get(), 0);
+                *self.lock_count.get() = 1;
+            }
+        }
+        ReentrantLockGuard { lock: self }
+    }
+
+    /// Returns a mutable reference to the underlying data.
+    ///
+    /// Since this call borrows the `ReentrantLock` mutably, no actual locking
+    /// needs to take place -- the mutable borrow statically guarantees no locks
+    /// exist.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(reentrant_lock)]
+    /// use std::sync::ReentrantLock;
+    ///
+    /// let mut lock = ReentrantLock::new(0);
+    /// *lock.get_mut() = 10;
+    /// assert_eq!(*lock.lock(), 10);
+    /// ```
+    pub fn get_mut(&mut self) -> &mut T {
+        &mut self.data
+    }
+
+    /// Attempts to acquire this lock.
+    ///
+    /// If the lock could not be acquired at this time, then `None` is returned.
+    /// Otherwise, an RAII guard is returned.
+    ///
+    /// This function does not block.
+    pub(crate) fn try_lock(&self) -> Option<ReentrantLockGuard<'_, T>> {
+        let this_thread = current_thread_unique_ptr();
+        // Safety: We only touch lock_count when we own the lock.
+        unsafe {
+            if self.owner.load(Relaxed) == this_thread {
+                self.increment_lock_count()?;
+                Some(ReentrantLockGuard { lock: self })
+            } else if self.mutex.try_lock() {
+                self.owner.store(this_thread, Relaxed);
+                debug_assert_eq!(*self.lock_count.get(), 0);
+                *self.lock_count.get() = 1;
+                Some(ReentrantLockGuard { lock: self })
+            } else {
+                None
+            }
+        }
+    }
+
+    unsafe fn increment_lock_count(&self) -> Option<()> {
+        *self.lock_count.get() = (*self.lock_count.get()).checked_add(1)?;
+        Some(())
+    }
+}
+
+#[unstable(feature = "reentrant_lock", issue = "121440")]
+impl<T: fmt::Debug + ?Sized> fmt::Debug for ReentrantLock<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut d = f.debug_struct("ReentrantLock");
+        match self.try_lock() {
+            Some(v) => d.field("data", &&*v),
+            None => d.field("data", &format_args!("<locked>")),
+        };
+        d.finish_non_exhaustive()
+    }
+}
+
+#[unstable(feature = "reentrant_lock", issue = "121440")]
+impl<T: Default> Default for ReentrantLock<T> {
+    fn default() -> Self {
+        Self::new(T::default())
+    }
+}
+
+#[unstable(feature = "reentrant_lock", issue = "121440")]
+impl<T> From<T> for ReentrantLock<T> {
+    fn from(t: T) -> Self {
+        Self::new(t)
+    }
+}
+
+#[unstable(feature = "reentrant_lock", issue = "121440")]
+impl<T: ?Sized> Deref for ReentrantLockGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        &self.lock.data
+    }
+}
+
+#[unstable(feature = "reentrant_lock", issue = "121440")]
+impl<T: fmt::Debug + ?Sized> fmt::Debug for ReentrantLockGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "reentrant_lock", issue = "121440")]
+impl<T: fmt::Display + ?Sized> fmt::Display for ReentrantLockGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "reentrant_lock", issue = "121440")]
+impl<T: ?Sized> Drop for ReentrantLockGuard<'_, T> {
+    #[inline]
+    fn drop(&mut self) {
+        // Safety: We own the lock.
+        unsafe {
+            *self.lock.lock_count.get() -= 1;
+            if *self.lock.lock_count.get() == 0 {
+                self.lock.owner.store(0, Relaxed);
+                self.lock.mutex.unlock();
+            }
+        }
+    }
+}
+
+/// Get an address that is unique per running thread.
+///
+/// This can be used as a non-null usize-sized ID.
+pub(crate) fn current_thread_unique_ptr() -> usize {
+    // Use a non-drop type to make sure it's still available during thread destruction.
+    thread_local! { static X: u8 = const { 0 } }
+    X.with(|x| <*const _>::addr(x))
+}

library/std/src/sync/reentrant_lock/tests.rs

@@ -1,17 +1,17 @@
-use super::{ReentrantMutex, ReentrantMutexGuard};
+use super::{ReentrantLock, ReentrantLockGuard};
 use crate::cell::RefCell;
 use crate::sync::Arc;
 use crate::thread;
 
 #[test]
 fn smoke() {
-    let m = ReentrantMutex::new(());
+    let l = ReentrantLock::new(());
     {
-        let a = m.lock();
+        let a = l.lock();
         {
-            let b = m.lock();
+            let b = l.lock();
             {
-                let c = m.lock();
+                let c = l.lock();
                 assert_eq!(*c, ());
             }
             assert_eq!(*b, ());
@@ -22,15 +22,15 @@ fn smoke() {
 
 #[test]
 fn is_mutex() {
-    let m = Arc::new(ReentrantMutex::new(RefCell::new(0)));
+    let l = Arc::new(ReentrantLock::new(RefCell::new(0)));
-    let m2 = m.clone();
+    let l2 = l.clone();
-    let lock = m.lock();
+    let lock = l.lock();
     let child = thread::spawn(move || {
-        let lock = m2.lock();
+        let lock = l2.lock();
         assert_eq!(*lock.borrow(), 4950);
     });
     for i in 0..100 {
-        let lock = m.lock();
+        let lock = l.lock();
         *lock.borrow_mut() += i;
     }
     drop(lock);
@@ -39,20 +39,20 @@ fn is_mutex() {
 
 #[test]
 fn trylock_works() {
-    let m = Arc::new(ReentrantMutex::new(()));
+    let l = Arc::new(ReentrantLock::new(()));
-    let m2 = m.clone();
+    let l2 = l.clone();
-    let _lock = m.try_lock();
+    let _lock = l.try_lock();
-    let _lock2 = m.try_lock();
+    let _lock2 = l.try_lock();
     thread::spawn(move || {
-        let lock = m2.try_lock();
+        let lock = l2.try_lock();
         assert!(lock.is_none());
     })
     .join()
     .unwrap();
-    let _lock3 = m.try_lock();
+    let _lock3 = l.try_lock();
 }
 
-pub struct Answer<'a>(pub ReentrantMutexGuard<'a, RefCell<u32>>);
+pub struct Answer<'a>(pub ReentrantLockGuard<'a, RefCell<u32>>);
 impl Drop for Answer<'_> {
     fn drop(&mut self) {
         *self.0.borrow_mut() = 42;

library/std/src/sync/remutex.rs

@@ -1,178 +0,0 @@
-#[cfg(all(test, not(target_os = "emscripten")))]
-mod tests;
-
-use crate::cell::UnsafeCell;
-use crate::ops::Deref;
-use crate::panic::{RefUnwindSafe, UnwindSafe};
-use crate::sync::atomic::{AtomicUsize, Ordering::Relaxed};
-use crate::sys::locks as sys;
-
-/// A reentrant mutual exclusion
-///
-/// This mutex will block *other* threads waiting for the lock to become
-/// available. The thread which has already locked the mutex can lock it
-/// multiple times without blocking, preventing a common source of deadlocks.
-///
-/// This is used by stdout().lock() and friends.
-///
-/// ## Implementation details
-///
-/// The 'owner' field tracks which thread has locked the mutex.
-///
-/// We use current_thread_unique_ptr() as the thread identifier,
-/// which is just the address of a thread local variable.
-///
-/// If `owner` is set to the identifier of the current thread,
-/// we assume the mutex is already locked and instead of locking it again,
-/// we increment `lock_count`.
-///
-/// When unlocking, we decrement `lock_count`, and only unlock the mutex when
-/// it reaches zero.
-///
-/// `lock_count` is protected by the mutex and only accessed by the thread that has
-/// locked the mutex, so needs no synchronization.
-///
-/// `owner` can be checked by other threads that want to see if they already
-/// hold the lock, so needs to be atomic. If it compares equal, we're on the
-/// same thread that holds the mutex and memory access can use relaxed ordering
-/// since we're not dealing with multiple threads. If it's not equal,
-/// synchronization is left to the mutex, making relaxed memory ordering for
-/// the `owner` field fine in all cases.
-pub struct ReentrantMutex<T> {
-    mutex: sys::Mutex,
-    owner: AtomicUsize,
-    lock_count: UnsafeCell<u32>,
-    data: T,
-}
-
-unsafe impl<T: Send> Send for ReentrantMutex<T> {}
-unsafe impl<T: Send> Sync for ReentrantMutex<T> {}
-
-impl<T> UnwindSafe for ReentrantMutex<T> {}
-impl<T> RefUnwindSafe for ReentrantMutex<T> {}
-
-/// An RAII implementation of a "scoped lock" of a mutex. When this structure is
-/// dropped (falls out of scope), the lock will be unlocked.
-///
-/// The data protected by the mutex can be accessed through this guard via its
-/// Deref implementation.
-///
-/// # Mutability
-///
-/// Unlike `MutexGuard`, `ReentrantMutexGuard` does not implement `DerefMut`,
-/// because implementation of the trait would violate Rust’s reference aliasing
-/// rules. Use interior mutability (usually `RefCell`) in order to mutate the
-/// guarded data.
-#[must_use = "if unused the ReentrantMutex will immediately unlock"]
-pub struct ReentrantMutexGuard<'a, T: 'a> {
-    lock: &'a ReentrantMutex<T>,
-}
-
-impl<T> !Send for ReentrantMutexGuard<'_, T> {}
-
-impl<T> ReentrantMutex<T> {
-    /// Creates a new reentrant mutex in an unlocked state.
-    pub const fn new(t: T) -> ReentrantMutex<T> {
-        ReentrantMutex {
-            mutex: sys::Mutex::new(),
-            owner: AtomicUsize::new(0),
-            lock_count: UnsafeCell::new(0),
-            data: t,
-        }
-    }
-
-    /// Acquires a mutex, blocking the current thread until it is able to do so.
-    ///
-    /// This function will block the caller until it is available to acquire the mutex.
-    /// Upon returning, the thread is the only thread with the mutex held. When the thread
-    /// calling this method already holds the lock, the call shall succeed without
-    /// blocking.
-    ///
-    /// # Errors
-    ///
-    /// If another user of this mutex panicked while holding the mutex, then
-    /// this call will return failure if the mutex would otherwise be
-    /// acquired.
-    pub fn lock(&self) -> ReentrantMutexGuard<'_, T> {
-        let this_thread = current_thread_unique_ptr();
-        // Safety: We only touch lock_count when we own the lock.
-        unsafe {
-            if self.owner.load(Relaxed) == this_thread {
-                self.increment_lock_count();
-            } else {
-                self.mutex.lock();
-                self.owner.store(this_thread, Relaxed);
-                debug_assert_eq!(*self.lock_count.get(), 0);
-                *self.lock_count.get() = 1;
-            }
-        }
-        ReentrantMutexGuard { lock: self }
-    }
-
-    /// Attempts to acquire this lock.
-    ///
-    /// If the lock could not be acquired at this time, then `Err` is returned.
-    /// Otherwise, an RAII guard is returned.
-    ///
-    /// This function does not block.
-    ///
-    /// # Errors
-    ///
-    /// If another user of this mutex panicked while holding the mutex, then
-    /// this call will return failure if the mutex would otherwise be
-    /// acquired.
-    pub fn try_lock(&self) -> Option<ReentrantMutexGuard<'_, T>> {
-        let this_thread = current_thread_unique_ptr();
-        // Safety: We only touch lock_count when we own the lock.
-        unsafe {
-            if self.owner.load(Relaxed) == this_thread {
-                self.increment_lock_count();
-                Some(ReentrantMutexGuard { lock: self })
-            } else if self.mutex.try_lock() {
-                self.owner.store(this_thread, Relaxed);
-                debug_assert_eq!(*self.lock_count.get(), 0);
-                *self.lock_count.get() = 1;
-                Some(ReentrantMutexGuard { lock: self })
-            } else {
-                None
-            }
-        }
-    }
-
-    unsafe fn increment_lock_count(&self) {
-        *self.lock_count.get() = (*self.lock_count.get())
-            .checked_add(1)
-            .expect("lock count overflow in reentrant mutex");
-    }
-}
-
-impl<T> Deref for ReentrantMutexGuard<'_, T> {
-    type Target = T;
-
-    fn deref(&self) -> &T {
-        &self.lock.data
-    }
-}
-
-impl<T> Drop for ReentrantMutexGuard<'_, T> {
-    #[inline]
-    fn drop(&mut self) {
-        // Safety: We own the lock.
-        unsafe {
-            *self.lock.lock_count.get() -= 1;
-            if *self.lock.lock_count.get() == 0 {
-                self.lock.owner.store(0, Relaxed);
-                self.lock.mutex.unlock();
-            }
-        }
-    }
-}
-
-/// Get an address that is unique per running thread.
-///
-/// This can be used as a non-null usize-sized ID.
-pub fn current_thread_unique_ptr() -> usize {
-    // Use a non-drop type to make sure it's still available during thread destruction.
-    thread_local! { static X: u8 = const { 0 } }
-    X.with(|x| <*const _>::addr(x))
-}