Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

3 files changed, 707 insertions, 0 deletions

diff --git a/vendor/futures-util/src/lock/bilock.rs b/vendor/futures-util/src/lock/bilock.rs
new file mode 100644
index 000000000..2f51ae7c9
--- /dev/null
+++ b/vendor/futures-util/src/lock/bilock.rs
@@ -0,0 +1,276 @@
+//! Futures-powered synchronization primitives.
+
+use alloc::boxed::Box;
+use alloc::sync::Arc;
+use core::cell::UnsafeCell;
+use core::fmt;
+use core::ops::{Deref, DerefMut};
+use core::pin::Pin;
+use core::sync::atomic::AtomicUsize;
+use core::sync::atomic::Ordering::SeqCst;
+#[cfg(feature = "bilock")]
+use futures_core::future::Future;
+use futures_core::task::{Context, Poll, Waker};
+
+/// A type of futures-powered synchronization primitive which is a mutex between
+/// two possible owners.
+///
+/// This primitive is not as generic as a full-blown mutex but is sufficient for
+/// many use cases where there are only two possible owners of a resource. The
+/// implementation of `BiLock` can be more optimized for just the two possible
+/// owners.
+///
+/// Note that it's possible to use this lock through a poll-style interface with
+/// the `poll_lock` method but you can also use it as a future with the `lock`
+/// method that consumes a `BiLock` and returns a future that will resolve when
+/// it's locked.
+///
+/// A `BiLock` is typically used for "split" operations where data which serves
+/// two purposes wants to be split into two to be worked with separately. For
+/// example a TCP stream could be both a reader and a writer or a framing layer
+/// could be both a stream and a sink for messages. A `BiLock` enables splitting
+/// these two and then using each independently in a futures-powered fashion.
+///
+/// This type is only available when the `bilock` feature of this
+/// library is activated.
+#[derive(Debug)]
+#[cfg_attr(docsrs, doc(cfg(feature = "bilock")))]
+pub struct BiLock<T> {
+    arc: Arc<Inner<T>>,
+}
+
+#[derive(Debug)]
+struct Inner<T> {
+    state: AtomicUsize,
+    value: Option<UnsafeCell<T>>,
+}
+
+unsafe impl<T: Send> Send for Inner<T> {}
+unsafe impl<T: Send> Sync for Inner<T> {}
+
+impl<T> BiLock<T> {
+    /// Creates a new `BiLock` protecting the provided data.
+    ///
+    /// Two handles to the lock are returned, and these are the only two handles
+    /// that will ever be available to the lock. These can then be sent to separate
+    /// tasks to be managed there.
+    ///
+    /// The data behind the bilock is considered to be pinned, which allows `Pin`
+    /// references to locked data. However, this means that the locked value
+    /// will only be available through `Pin<&mut T>` (not `&mut T`) unless `T` is `Unpin`.
+    /// Similarly, reuniting the lock and extracting the inner value is only
+    /// possible when `T` is `Unpin`.
+    pub fn new(t: T) -> (Self, Self) {
+        let arc = Arc::new(Inner { state: AtomicUsize::new(0), value: Some(UnsafeCell::new(t)) });
+
+        (Self { arc: arc.clone() }, Self { arc })
+    }
+
+    /// Attempt to acquire this lock, returning `Pending` if it can't be
+    /// acquired.
+    ///
+    /// This function will acquire the lock in a nonblocking fashion, returning
+    /// immediately if the lock is already held. If the lock is successfully
+    /// acquired then `Poll::Ready` is returned with a value that represents
+    /// the locked value (and can be used to access the protected data). The
+    /// lock is unlocked when the returned `BiLockGuard` is dropped.
+    ///
+    /// If the lock is already held then this function will return
+    /// `Poll::Pending`. In this case the current task will also be scheduled
+    /// to receive a notification when the lock would otherwise become
+    /// available.
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if called outside the context of a future's
+    /// task.
+    pub fn poll_lock(&self, cx: &mut Context<'_>) -> Poll<BiLockGuard<'_, T>> {
+        let mut waker = None;
+        loop {
+            match self.arc.state.swap(1, SeqCst) {
+                // Woohoo, we grabbed the lock!
+                0 => return Poll::Ready(BiLockGuard { bilock: self }),
+
+                // Oops, someone else has locked the lock
+                1 => {}
+
+                // A task was previously blocked on this lock, likely our task,
+                // so we need to update that task.
+                n => unsafe {
+                    let mut prev = Box::from_raw(n as *mut Waker);
+                    *prev = cx.waker().clone();
+                    waker = Some(prev);
+                },
+            }
+
+            // type ascription for safety's sake!
+            let me: Box<Waker> = waker.take().unwrap_or_else(|| Box::new(cx.waker().clone()));
+            let me = Box::into_raw(me) as usize;
+
+            match self.arc.state.compare_exchange(1, me, SeqCst, SeqCst) {
+                // The lock is still locked, but we've now parked ourselves, so
+                // just report that we're scheduled to receive a notification.
+                Ok(_) => return Poll::Pending,
+
+                // Oops, looks like the lock was unlocked after our swap above
+                // and before the compare_exchange. Deallocate what we just
+                // allocated and go through the loop again.
+                Err(0) => unsafe {
+                    waker = Some(Box::from_raw(me as *mut Waker));
+                },
+
+                // The top of this loop set the previous state to 1, so if we
+                // failed the CAS above then it's because the previous value was
+                // *not* zero or one. This indicates that a task was blocked,
+                // but we're trying to acquire the lock and there's only one
+                // other reference of the lock, so it should be impossible for
+                // that task to ever block itself.
+                Err(n) => panic!("invalid state: {}", n),
+            }
+        }
+    }
+
+    /// Perform a "blocking lock" of this lock, consuming this lock handle and
+    /// returning a future to the acquired lock.
+    ///
+    /// This function consumes the `BiLock<T>` and returns a sentinel future,
+    /// `BiLockAcquire<T>`. The returned future will resolve to
+    /// `BiLockAcquired<T>` which represents a locked lock similarly to
+    /// `BiLockGuard<T>`.
+    ///
+    /// Note that the returned future will never resolve to an error.
+    #[cfg(feature = "bilock")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "bilock")))]
+    pub fn lock(&self) -> BiLockAcquire<'_, T> {
+        BiLockAcquire { bilock: self }
+    }
+
+    /// Attempts to put the two "halves" of a `BiLock<T>` back together and
+    /// recover the original value. Succeeds only if the two `BiLock<T>`s
+    /// originated from the same call to `BiLock::new`.
+    pub fn reunite(self, other: Self) -> Result<T, ReuniteError<T>>
+    where
+        T: Unpin,
+    {
+        if Arc::ptr_eq(&self.arc, &other.arc) {
+            drop(other);
+            let inner = Arc::try_unwrap(self.arc)
+                .ok()
+                .expect("futures: try_unwrap failed in BiLock<T>::reunite");
+            Ok(unsafe { inner.into_value() })
+        } else {
+            Err(ReuniteError(self, other))
+        }
+    }
+
+    fn unlock(&self) {
+        match self.arc.state.swap(0, SeqCst) {
+            // we've locked the lock, shouldn't be possible for us to see an
+            // unlocked lock.
+            0 => panic!("invalid unlocked state"),
+
+            // Ok, no one else tried to get the lock, we're done.
+            1 => {}
+
+            // Another task has parked themselves on this lock, let's wake them
+            // up as its now their turn.
+            n => unsafe {
+                Box::from_raw(n as *mut Waker).wake();
+            },
+        }
+    }
+}
+
+impl<T: Unpin> Inner<T> {
+    unsafe fn into_value(mut self) -> T {
+        self.value.take().unwrap().into_inner()
+    }
+}
+
+impl<T> Drop for Inner<T> {
+    fn drop(&mut self) {
+        assert_eq!(self.state.load(SeqCst), 0);
+    }
+}
+
+/// Error indicating two `BiLock<T>`s were not two halves of a whole, and
+/// thus could not be `reunite`d.
+#[cfg_attr(docsrs, doc(cfg(feature = "bilock")))]
+pub struct ReuniteError<T>(pub BiLock<T>, pub BiLock<T>);
+
+impl<T> fmt::Debug for ReuniteError<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("ReuniteError").field(&"...").finish()
+    }
+}
+
+impl<T> fmt::Display for ReuniteError<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "tried to reunite two BiLocks that don't form a pair")
+    }
+}
+
+#[cfg(feature = "std")]
+impl<T: core::any::Any> std::error::Error for ReuniteError<T> {}
+
+/// Returned RAII guard from the `poll_lock` method.
+///
+/// This structure acts as a sentinel to the data in the `BiLock<T>` itself,
+/// implementing `Deref` and `DerefMut` to `T`. When dropped, the lock will be
+/// unlocked.
+#[derive(Debug)]
+#[cfg_attr(docsrs, doc(cfg(feature = "bilock")))]
+pub struct BiLockGuard<'a, T> {
+    bilock: &'a BiLock<T>,
+}
+
+impl<T> Deref for BiLockGuard<'_, T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        unsafe { &*self.bilock.arc.value.as_ref().unwrap().get() }
+    }
+}
+
+impl<T: Unpin> DerefMut for BiLockGuard<'_, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        unsafe { &mut *self.bilock.arc.value.as_ref().unwrap().get() }
+    }
+}
+
+impl<T> BiLockGuard<'_, T> {
+    /// Get a mutable pinned reference to the locked value.
+    pub fn as_pin_mut(&mut self) -> Pin<&mut T> {
+        // Safety: we never allow moving a !Unpin value out of a bilock, nor
+        // allow mutable access to it
+        unsafe { Pin::new_unchecked(&mut *self.bilock.arc.value.as_ref().unwrap().get()) }
+    }
+}
+
+impl<T> Drop for BiLockGuard<'_, T> {
+    fn drop(&mut self) {
+        self.bilock.unlock();
+    }
+}
+
+/// Future returned by `BiLock::lock` which will resolve when the lock is
+/// acquired.
+#[cfg(feature = "bilock")]
+#[cfg_attr(docsrs, doc(cfg(feature = "bilock")))]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+#[derive(Debug)]
+pub struct BiLockAcquire<'a, T> {
+    bilock: &'a BiLock<T>,
+}
+
+// Pinning is never projected to fields
+#[cfg(feature = "bilock")]
+impl<T> Unpin for BiLockAcquire<'_, T> {}
+
+#[cfg(feature = "bilock")]
+impl<'a, T> Future for BiLockAcquire<'a, T> {
+    type Output = BiLockGuard<'a, T>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        self.bilock.poll_lock(cx)
+    }
+}
diff --git a/vendor/futures-util/src/lock/mod.rs b/vendor/futures-util/src/lock/mod.rs
new file mode 100644
index 000000000..cf374c016
--- /dev/null
+++ b/vendor/futures-util/src/lock/mod.rs
@@ -0,0 +1,25 @@
+//! Futures-powered synchronization primitives.
+//!
+//! This module is only available when the `std` or `alloc` feature of this
+//! library is activated, and it is activated by default.
+
+#[cfg(not(futures_no_atomic_cas))]
+#[cfg(feature = "std")]
+mod mutex;
+#[cfg(not(futures_no_atomic_cas))]
+#[cfg(feature = "std")]
+pub use self::mutex::{MappedMutexGuard, Mutex, MutexGuard, MutexLockFuture};
+
+#[cfg(not(futures_no_atomic_cas))]
+#[cfg(any(feature = "bilock", feature = "sink", feature = "io"))]
+#[cfg_attr(docsrs, doc(cfg(feature = "bilock")))]
+#[cfg_attr(not(feature = "bilock"), allow(unreachable_pub))]
+mod bilock;
+#[cfg(not(futures_no_atomic_cas))]
+#[cfg(any(feature = "sink", feature = "io"))]
+#[cfg(not(feature = "bilock"))]
+pub(crate) use self::bilock::BiLock;
+#[cfg(not(futures_no_atomic_cas))]
+#[cfg(feature = "bilock")]
+#[cfg_attr(docsrs, doc(cfg(feature = "bilock")))]
+pub use self::bilock::{BiLock, BiLockAcquire, BiLockGuard, ReuniteError};
diff --git a/vendor/futures-util/src/lock/mutex.rs b/vendor/futures-util/src/lock/mutex.rs
new file mode 100644
index 000000000..85dcb1537
--- /dev/null
+++ b/vendor/futures-util/src/lock/mutex.rs
@@ -0,0 +1,406 @@
+use futures_core::future::{FusedFuture, Future};
+use futures_core::task::{Context, Poll, Waker};
+use slab::Slab;
+use std::cell::UnsafeCell;
+use std::marker::PhantomData;
+use std::ops::{Deref, DerefMut};
+use std::pin::Pin;
+use std::sync::atomic::{AtomicUsize, Ordering};
+use std::sync::Mutex as StdMutex;
+use std::{fmt, mem};
+
+/// A futures-aware mutex.
+///
+/// # Fairness
+///
+/// This mutex provides no fairness guarantees. Tasks may not acquire the mutex
+/// in the order that they requested the lock, and it's possible for a single task
+/// which repeatedly takes the lock to starve other tasks, which may be left waiting
+/// indefinitely.
+pub struct Mutex<T: ?Sized> {
+    state: AtomicUsize,
+    waiters: StdMutex<Slab<Waiter>>,
+    value: UnsafeCell<T>,
+}
+
+impl<T: ?Sized> fmt::Debug for Mutex<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let state = self.state.load(Ordering::SeqCst);
+        f.debug_struct("Mutex")
+            .field("is_locked", &((state & IS_LOCKED) != 0))
+            .field("has_waiters", &((state & HAS_WAITERS) != 0))
+            .finish()
+    }
+}
+
+impl<T> From<T> for Mutex<T> {
+    fn from(t: T) -> Self {
+        Self::new(t)
+    }
+}
+
+impl<T: Default> Default for Mutex<T> {
+    fn default() -> Self {
+        Self::new(Default::default())
+    }
+}
+
+enum Waiter {
+    Waiting(Waker),
+    Woken,
+}
+
+impl Waiter {
+    fn register(&mut self, waker: &Waker) {
+        match self {
+            Self::Waiting(w) if waker.will_wake(w) => {}
+            _ => *self = Self::Waiting(waker.clone()),
+        }
+    }
+
+    fn wake(&mut self) {
+        match mem::replace(self, Self::Woken) {
+            Self::Waiting(waker) => waker.wake(),
+            Self::Woken => {}
+        }
+    }
+}
+
+const IS_LOCKED: usize = 1 << 0;
+const HAS_WAITERS: usize = 1 << 1;
+
+impl<T> Mutex<T> {
+    /// Creates a new futures-aware mutex.
+    pub fn new(t: T) -> Self {
+        Self {
+            state: AtomicUsize::new(0),
+            waiters: StdMutex::new(Slab::new()),
+            value: UnsafeCell::new(t),
+        }
+    }
+
+    /// Consumes this mutex, returning the underlying data.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use futures::lock::Mutex;
+    ///
+    /// let mutex = Mutex::new(0);
+    /// assert_eq!(mutex.into_inner(), 0);
+    /// ```
+    pub fn into_inner(self) -> T {
+        self.value.into_inner()
+    }
+}
+
+impl<T: ?Sized> Mutex<T> {
+    /// Attempt to acquire the lock immediately.
+    ///
+    /// If the lock is currently held, this will return `None`.
+    pub fn try_lock(&self) -> Option<MutexGuard<'_, T>> {
+        let old_state = self.state.fetch_or(IS_LOCKED, Ordering::Acquire);
+        if (old_state & IS_LOCKED) == 0 {
+            Some(MutexGuard { mutex: self })
+        } else {
+            None
+        }
+    }
+
+    /// Acquire the lock asynchronously.
+    ///
+    /// This method returns a future that will resolve once the lock has been
+    /// successfully acquired.
+    pub fn lock(&self) -> MutexLockFuture<'_, T> {
+        MutexLockFuture { mutex: Some(self), wait_key: WAIT_KEY_NONE }
+    }
+
+    /// Returns a mutable reference to the underlying data.
+    ///
+    /// Since this call borrows the `Mutex` mutably, no actual locking needs to
+    /// take place -- the mutable borrow statically guarantees no locks exist.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # futures::executor::block_on(async {
+    /// use futures::lock::Mutex;
+    ///
+    /// let mut mutex = Mutex::new(0);
+    /// *mutex.get_mut() = 10;
+    /// assert_eq!(*mutex.lock().await, 10);
+    /// # });
+    /// ```
+    pub fn get_mut(&mut self) -> &mut T {
+        // We know statically that there are no other references to `self`, so
+        // there's no need to lock the inner mutex.
+        unsafe { &mut *self.value.get() }
+    }
+
+    fn remove_waker(&self, wait_key: usize, wake_another: bool) {
+        if wait_key != WAIT_KEY_NONE {
+            let mut waiters = self.waiters.lock().unwrap();
+            match waiters.remove(wait_key) {
+                Waiter::Waiting(_) => {}
+                Waiter::Woken => {
+                    // We were awoken, but then dropped before we could
+                    // wake up to acquire the lock. Wake up another
+                    // waiter.
+                    if wake_another {
+                        if let Some((_i, waiter)) = waiters.iter_mut().next() {
+                            waiter.wake();
+                        }
+                    }
+                }
+            }
+            if waiters.is_empty() {
+                self.state.fetch_and(!HAS_WAITERS, Ordering::Relaxed); // released by mutex unlock
+            }
+        }
+    }
+
+    // Unlocks the mutex. Called by MutexGuard and MappedMutexGuard when they are
+    // dropped.
+    fn unlock(&self) {
+        let old_state = self.state.fetch_and(!IS_LOCKED, Ordering::AcqRel);
+        if (old_state & HAS_WAITERS) != 0 {
+            let mut waiters = self.waiters.lock().unwrap();
+            if let Some((_i, waiter)) = waiters.iter_mut().next() {
+                waiter.wake();
+            }
+        }
+    }
+}
+
+// Sentinel for when no slot in the `Slab` has been dedicated to this object.
+const WAIT_KEY_NONE: usize = usize::max_value();
+
+/// A future which resolves when the target mutex has been successfully acquired.
+pub struct MutexLockFuture<'a, T: ?Sized> {
+    // `None` indicates that the mutex was successfully acquired.
+    mutex: Option<&'a Mutex<T>>,
+    wait_key: usize,
+}
+
+impl<T: ?Sized> fmt::Debug for MutexLockFuture<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("MutexLockFuture")
+            .field("was_acquired", &self.mutex.is_none())
+            .field("mutex", &self.mutex)
+            .field(
+                "wait_key",
+                &(if self.wait_key == WAIT_KEY_NONE { None } else { Some(self.wait_key) }),
+            )
+            .finish()
+    }
+}
+
+impl<T: ?Sized> FusedFuture for MutexLockFuture<'_, T> {
+    fn is_terminated(&self) -> bool {
+        self.mutex.is_none()
+    }
+}
+
+impl<'a, T: ?Sized> Future for MutexLockFuture<'a, T> {
+    type Output = MutexGuard<'a, T>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let mutex = self.mutex.expect("polled MutexLockFuture after completion");
+
+        if let Some(lock) = mutex.try_lock() {
+            mutex.remove_waker(self.wait_key, false);
+            self.mutex = None;
+            return Poll::Ready(lock);
+        }
+
+        {
+            let mut waiters = mutex.waiters.lock().unwrap();
+            if self.wait_key == WAIT_KEY_NONE {
+                self.wait_key = waiters.insert(Waiter::Waiting(cx.waker().clone()));
+                if waiters.len() == 1 {
+                    mutex.state.fetch_or(HAS_WAITERS, Ordering::Relaxed); // released by mutex unlock
+                }
+            } else {
+                waiters[self.wait_key].register(cx.waker());
+            }
+        }
+
+        // Ensure that we haven't raced `MutexGuard::drop`'s unlock path by
+        // attempting to acquire the lock again.
+        if let Some(lock) = mutex.try_lock() {
+            mutex.remove_waker(self.wait_key, false);
+            self.mutex = None;
+            return Poll::Ready(lock);
+        }
+
+        Poll::Pending
+    }
+}
+
+impl<T: ?Sized> Drop for MutexLockFuture<'_, T> {
+    fn drop(&mut self) {
+        if let Some(mutex) = self.mutex {
+            // This future was dropped before it acquired the mutex.
+            //
+            // Remove ourselves from the map, waking up another waiter if we
+            // had been awoken to acquire the lock.
+            mutex.remove_waker(self.wait_key, true);
+        }
+    }
+}
+
+/// An RAII guard returned by the `lock` and `try_lock` methods.
+/// When this structure is dropped (falls out of scope), the lock will be
+/// unlocked.
+pub struct MutexGuard<'a, T: ?Sized> {
+    mutex: &'a Mutex<T>,
+}
+
+impl<'a, T: ?Sized> MutexGuard<'a, T> {
+    /// Returns a locked view over a portion of the locked data.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # futures::executor::block_on(async {
+    /// use futures::lock::{Mutex, MutexGuard};
+    ///
+    /// let data = Mutex::new(Some("value".to_string()));
+    /// {
+    ///     let locked_str = MutexGuard::map(data.lock().await, |opt| opt.as_mut().unwrap());
+    ///     assert_eq!(&*locked_str, "value");
+    /// }
+    /// # });
+    /// ```
+    #[inline]
+    pub fn map<U: ?Sized, F>(this: Self, f: F) -> MappedMutexGuard<'a, T, U>
+    where
+        F: FnOnce(&mut T) -> &mut U,
+    {
+        let mutex = this.mutex;
+        let value = f(unsafe { &mut *this.mutex.value.get() });
+        // Don't run the `drop` method for MutexGuard. The ownership of the underlying
+        // locked state is being moved to the returned MappedMutexGuard.
+        mem::forget(this);
+        MappedMutexGuard { mutex, value, _marker: PhantomData }
+    }
+}
+
+impl<T: ?Sized + fmt::Debug> fmt::Debug for MutexGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("MutexGuard").field("value", &&**self).field("mutex", &self.mutex).finish()
+    }
+}
+
+impl<T: ?Sized> Drop for MutexGuard<'_, T> {
+    fn drop(&mut self) {
+        self.mutex.unlock()
+    }
+}
+
+impl<T: ?Sized> Deref for MutexGuard<'_, T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        unsafe { &*self.mutex.value.get() }
+    }
+}
+
+impl<T: ?Sized> DerefMut for MutexGuard<'_, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        unsafe { &mut *self.mutex.value.get() }
+    }
+}
+
+/// An RAII guard returned by the `MutexGuard::map` and `MappedMutexGuard::map` methods.
+/// When this structure is dropped (falls out of scope), the lock will be unlocked.
+pub struct MappedMutexGuard<'a, T: ?Sized, U: ?Sized> {
+    mutex: &'a Mutex<T>,
+    value: *mut U,
+    _marker: PhantomData<&'a mut U>,
+}
+
+impl<'a, T: ?Sized, U: ?Sized> MappedMutexGuard<'a, T, U> {
+    /// Returns a locked view over a portion of the locked data.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # futures::executor::block_on(async {
+    /// use futures::lock::{MappedMutexGuard, Mutex, MutexGuard};
+    ///
+    /// let data = Mutex::new(Some("value".to_string()));
+    /// {
+    ///     let locked_str = MutexGuard::map(data.lock().await, |opt| opt.as_mut().unwrap());
+    ///     let locked_char = MappedMutexGuard::map(locked_str, |s| s.get_mut(0..1).unwrap());
+    ///     assert_eq!(&*locked_char, "v");
+    /// }
+    /// # });
+    /// ```
+    #[inline]
+    pub fn map<V: ?Sized, F>(this: Self, f: F) -> MappedMutexGuard<'a, T, V>
+    where
+        F: FnOnce(&mut U) -> &mut V,
+    {
+        let mutex = this.mutex;
+        let value = f(unsafe { &mut *this.value });
+        // Don't run the `drop` method for MappedMutexGuard. The ownership of the underlying
+        // locked state is being moved to the returned MappedMutexGuard.
+        mem::forget(this);
+        MappedMutexGuard { mutex, value, _marker: PhantomData }
+    }
+}
+
+impl<T: ?Sized, U: ?Sized + fmt::Debug> fmt::Debug for MappedMutexGuard<'_, T, U> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("MappedMutexGuard")
+            .field("value", &&**self)
+            .field("mutex", &self.mutex)
+            .finish()
+    }
+}
+
+impl<T: ?Sized, U: ?Sized> Drop for MappedMutexGuard<'_, T, U> {
+    fn drop(&mut self) {
+        self.mutex.unlock()
+    }
+}
+
+impl<T: ?Sized, U: ?Sized> Deref for MappedMutexGuard<'_, T, U> {
+    type Target = U;
+    fn deref(&self) -> &U {
+        unsafe { &*self.value }
+    }
+}
+
+impl<T: ?Sized, U: ?Sized> DerefMut for MappedMutexGuard<'_, T, U> {
+    fn deref_mut(&mut self) -> &mut U {
+        unsafe { &mut *self.value }
+    }
+}
+
+// Mutexes can be moved freely between threads and acquired on any thread so long
+// as the inner value can be safely sent between threads.
+unsafe impl<T: ?Sized + Send> Send for Mutex<T> {}
+unsafe impl<T: ?Sized + Send> Sync for Mutex<T> {}
+
+// It's safe to switch which thread the acquire is being attempted on so long as
+// `T` can be accessed on that thread.
+unsafe impl<T: ?Sized + Send> Send for MutexLockFuture<'_, T> {}
+// doesn't have any interesting `&self` methods (only Debug)
+unsafe impl<T: ?Sized> Sync for MutexLockFuture<'_, T> {}
+
+// Safe to send since we don't track any thread-specific details-- the inner
+// lock is essentially spinlock-equivalent (attempt to flip an atomic bool)
+unsafe impl<T: ?Sized + Send> Send for MutexGuard<'_, T> {}
+unsafe impl<T: ?Sized + Sync> Sync for MutexGuard<'_, T> {}
+unsafe impl<T: ?Sized + Send, U: ?Sized + Send> Send for MappedMutexGuard<'_, T, U> {}
+unsafe impl<T: ?Sized + Sync, U: ?Sized + Sync> Sync for MappedMutexGuard<'_, T, U> {}
+
+#[test]
+fn test_mutex_guard_debug_not_recurse() {
+    let mutex = Mutex::new(42);
+    let guard = mutex.try_lock().unwrap();
+    let _ = format!("{:?}", guard);
+    let guard = MutexGuard::map(guard, |n| n);
+    let _ = format!("{:?}", guard);
+}