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// Copyright (c) 2023, George Burton <burtonageo@gmail.com>
//
// SPDX-License-Identifier: 0BSD
#![cfg_attr(not(feature = "enable_std"), no_std)]
#![warn(
clippy::cargo,
clippy::complexity,
clippy::pedantic,
clippy::perf,
clippy::style,
clippy::suspicious,
clippy::undocumented_unsafe_blocks
)]
//! This crate provides the [`LockCell<T>`] and other supportings types.
//!
//! A `LockCell` is a cell type which provides dynamic mutation using interior
//! mutability. It is similar to [`RefCell<T>`], except that it only allows
//! a single borrow type (a lock). Locking a `LockCell` allows mutating its
//! contents freely.
//!
//! A `LockCell` can only be used in a single threaded context - it cannot be shared
//! across different threads. Generally, a `LockCell` will be stored in a [`Rc<T>`]
//! so that it can be shared.
//!
//! Whether you use a `LockCell` or a `RefCell` depends on the structure and behavior of
//! your program. Generally, if you have a lot of writers and readers, using a `LockCell`
//! may be better, as it ensures that writers are less likely to be starved.
//!
//! The [`Sync`] equivalent of a `LockCell` is [`Mutex<T>`].
//!
//! # Features
//!
//! * The `enable_std` feature enables the standard library. This provides an implementation of
//! [`std::error::Error`] for the [`TryLockError`] type. This feature is enabled by default.
//!
//! * The `debug_lockcell` feature tracks the location of each `lock()` call in the `LockCell`,
//! allowing the developer to compare the first lock location in their file to the panicking
//! lock location, aiding in debugging.
//!
//! [`LockCell<T>`]: ./struct.LockCell.html
//! [`RefCell<T>`]: http://doc.rust-lang.org/std/cell/struct.RefCell.html
//! [`Rc<T>`]: https://doc.rust-lang.org/std/rc/struct.Rc.html
//! [`Mutex<T>`]: http://doc.rust-lang.org/std/sync/struct.Mutex.html
//! [`Sync`]: https://doc.rust-lang.org/std/marker/trait.Sync.html
//! [`std::error::Error`]: https://doc.rust-lang.org/std/error/trait.Error.html
//! [`TryLockError`]: ./struct.TryLockError.html
#[cfg(feature = "debug_lockcell")]
use core::panic::Location;
use core::{
borrow::{Borrow, BorrowMut},
cell::{Cell, UnsafeCell},
cmp::{Eq, Ord, Ordering, PartialEq, PartialOrd},
convert::{AsMut, AsRef, TryFrom},
fmt,
hash::{Hash, Hasher},
marker::PhantomData,
mem::{self, ManuallyDrop},
ops::{Deref, DerefMut, FnOnce},
};
#[cfg(feature = "enable_std")]
use std::error::Error as StdError;
/// A mutable memory location with dynamically checked borrow rules.
///
/// See the [module level documentation] for more.
///
/// [module level documentation]: ./index.html
pub struct LockCell<T: ?Sized> {
/// Used to track the lock state of the `LockCell`.
is_locked: Cell<bool>,
/// Stores where the `LockCell` was first locked. This is used as
/// part of the `debug_lockcell` feature to help debug double locks.
#[cfg(feature = "debug_lockcell")]
first_locked_at: Cell<Option<&'static Location<'static>>>,
/// The inner value of the `LockCell`.
value: UnsafeCell<T>,
}
impl<T> LockCell<T> {
/// Create a new `LockCell` with the given `value`.
///
/// # Examples
///
/// ```
/// use lock_cell::LockCell;
/// # fn main() {
/// let cell = LockCell::new("I could be anything!".to_string());
/// # let _ = cell;
/// # }
/// ```
#[must_use]
#[inline]
pub const fn new(value: T) -> Self {
Self {
is_locked: Cell::new(false),
#[cfg(feature = "debug_lockcell")]
first_locked_at: Cell::new(None),
value: UnsafeCell::new(value),
}
}
/// Consumes the `LockCell`, returning the inner value.
///
/// # Examples
///
/// ```
/// use lock_cell::LockCell;
/// # fn main() {
/// let cell = LockCell::new(5);
///
/// let five = cell.into_inner();
///
/// assert_eq!(five, 5);
/// # }
/// ```
#[must_use]
#[inline]
pub fn into_inner(self) -> T {
self.value.into_inner()
}
/// Swaps the wrapped values of `self` and `rhs`.
///
/// This function corresponds to [`std::mem::swap`].
///
/// # Panics
///
/// Panics if either `LockCell` is locked, or if `self` and `rhs` point to the same
/// `LockCell`.
///
/// # Examples
///
/// ```
/// use lock_cell::LockCell;
/// # fn main() {
/// let cell_1 = LockCell::new(3);
/// let cell_2 = LockCell::new(24);
///
/// cell_1.swap(&cell_2);
///
/// assert_eq!(cell_1.into_inner(), 24);
/// assert_eq!(cell_2.into_inner(), 3);
/// # }
/// ```
///
/// [`std::mem::swap`]: https://doc.rust-lang.org/std/mem/fn.swap.html
#[track_caller]
#[inline]
pub fn swap(&self, rhs: &LockCell<T>) {
mem::swap(&mut *self.lock(), &mut *rhs.lock());
}
/// Sets the value in this `LockCell` to `new_value`, returning the previous value
/// in the `LockCell`.
///
/// # Panics
///
/// This method will panic if the cell is locked.
///
/// # Examples
///
/// ```
/// use lock_cell::LockCell;
/// # fn main() {
/// let cell = LockCell::new(5);
///
/// let old_value = cell.replace(6);
///
/// assert_eq!(old_value, 5);
///
/// assert_eq!(cell.into_inner(), 6);
/// # }
/// ```
#[inline]
#[track_caller]
pub fn replace(&self, new_value: T) -> T {
let mut lock = self.lock();
mem::replace(&mut *lock, new_value)
}
/// Replaces the wrapped value with a new value computed from the function `f`,
/// returning the old value without deinitializing either.
///
/// # Panics
///
/// This method will panic if the `LockCell` is locked.
///
/// # Examples
///
/// ```
/// use lock_cell::LockCell;
/// # fn main() {
/// let cell = LockCell::new(5);
/// let old_value = cell.replace_with(|old| {
/// *old += 1;
/// *old + 1
/// });
///
/// assert_eq!(old_value, 6);
///
/// assert_eq!(cell.into_inner(), 7);
/// # }
/// ```
#[inline]
#[track_caller]
pub fn replace_with<F>(&self, f: F) -> T
where
F: FnOnce(&mut T) -> T,
{
let mut lock = self.lock();
let replacement = f(&mut *lock);
mem::replace(&mut *lock, replacement)
}
/// Replaces the value in this `LockCell` with the [`Default::default()`] value,
/// returning the previous value in the `LockCell`.
///
/// # Panics
///
/// This method will panic if the cell is locked.
///
/// # Examples
///
/// ```
/// use lock_cell::LockCell;
/// # fn main() {
/// let cell = LockCell::new(5);
///
/// let old_value = cell.take();
///
/// assert_eq!(old_value, 5);
///
/// assert_eq!(cell.into_inner(), 0);
/// # }
/// ```
///
/// [`Default::default()`]: https://doc.rust-lang.org/std/default/trait.Default.html
#[inline]
#[track_caller]
pub fn take(&self) -> T
where
T: Default,
{
self.replace(Default::default())
}
}
impl<T: ?Sized> LockCell<T> {
/// Attempt to lock the `LockCell`.
///
/// # Notes
///
/// If this `LockCell` is not locked, the function succeeds and will return a
/// guard which provides mutable access to the inner value.
///
/// # Errors
///
/// If the `LockCell` is already locked, this function will fail and will
/// return a [`TryLockError`].
///
/// # Examples
///
/// ```
/// # use lock_cell::{LockCell, TryLockError};
/// # fn main() -> Result<(), TryLockError> {
/// let cell = LockCell::new(21);
///
/// let first_access = cell.try_lock();
/// assert!(first_access.is_ok());
///
/// let first_lock = first_access?;
/// assert_eq!(*first_lock, 21);
///
/// let second_access = cell.try_lock();
/// assert!(second_access.is_err());
/// # Ok(())
/// # }
/// ```
///
/// [`TryLockError`]: ./struct.TryLockError.html
#[inline]
#[track_caller]
pub fn try_lock(&self) -> Result<LockGuard<'_, T>, TryLockError> {
if self.is_locked.replace(true) {
return Err(TryLockError::new(self));
}
#[cfg(feature = "debug_lockcell")]
{
self.first_locked_at.set(Some(Location::caller()));
}
Ok(LockGuard {
value: self.value.get(),
is_locked: &self.is_locked,
#[cfg(feature = "debug_lockcell")]
locked_at: &self.first_locked_at,
_boo: PhantomData,
})
}
/// Lock the given `LockCell`, returning a [`LockGuard`] which can be used to access
/// the value.
///
/// The `LockCell` will be locked until the returned [`LockGuard`] goes out of scope.
/// The cell can only have a single lock at a time active.
///
/// # Panics
///
/// This method will panic if the `LockCell` is already locked.
///
/// To avoid this, you can use the [`try_lock()`] method to return a `Result` to
/// check if the lock succeeded, or you can use the [`is_locked()`] method to check
/// ahead of time if the lock will succeed.
///
/// # Examples
///
/// ```
/// use lock_cell::LockCell;
/// # fn main() {
/// let cell = LockCell::new("Hello".to_string());
///
/// let lock = cell.lock();
///
/// assert_eq!(&*lock, "Hello");
/// # }
/// ```
///
/// [`LockGuard`]: ./struct.LockGuard.html
/// [`try_lock()`]: ./struct.LockCell.html#method.try_lock
/// [`is_locked()`]: ./struct.LockCell.html#method.is_locked
#[inline]
#[track_caller]
pub fn lock(&self) -> LockGuard<'_, T> {
self.try_lock().expect("already locked")
}
/// Returns whether this `LockCell` is currently locked.
///
/// # Examples
///
/// ```
/// use lock_cell::LockCell;
/// # fn main() {
/// let cell = LockCell::new(5);
///
/// assert!(!cell.is_locked());
///
/// let lock = cell.lock();
///
/// assert!(cell.is_locked());
/// # let _ = lock;
/// # }
/// ```
#[must_use]
#[inline]
pub fn is_locked(&self) -> bool {
self.is_locked.get()
}
/// Provides mutable access to the inner value.
///
/// As this requires exclusive access to the `LockCell`, no locking is
/// required to provide exclusive access to the value.
///
/// # Examples
///
/// ```
/// use lock_cell::LockCell;
/// # fn main() {
/// let mut cell = LockCell::new(54);
///
/// *cell.get_mut() = 20;
///
/// assert_eq!(cell.into_inner(), 20);
/// # }
/// ```
#[must_use]
#[inline]
pub fn get_mut(&mut self) -> &mut T {
self.value.get_mut()
}
/// Return a raw pointer to the underlying data in this `LockCell`.
///
/// # Notes
///
/// This function does not lock the `LockCell`. Therefore, any mutations made through
/// the returned pointer must be synchronized in some other way, or undefined behaviour
/// may occur.
///
/// # Examples
///
/// ```
/// use lock_cell::LockCell;
/// # fn main() {
/// let cell = LockCell::new(5);
///
/// let ptr = cell.as_ptr();
/// # }
/// ```
#[must_use]
#[inline]
pub fn as_ptr(&self) -> *mut T {
self.value.get()
}
/// Resets the lock state, in case that any [`LockGuard`]s have been leaked.
///
/// This method takes `self` by `&mut` to ensure that there are no other borrows
/// of the `LockCell` in flight.
///
/// # Examples
///
/// ```
/// # use core::mem;
/// use lock_cell::LockCell;
/// # fn main() {
/// let mut cell = LockCell::new(12);
///
/// let mut lock = cell.lock();
///
/// *lock = 54;
///
/// mem::forget(lock);
///
/// assert!(cell.is_locked());
///
/// cell.reset_lock();
///
/// assert!(!cell.is_locked());
///
/// assert_eq!(cell.into_inner(), 54);
/// # }
/// ```
///
/// [`LockGuard`]: ./struct.LockGuard.html
#[inline]
pub fn reset_lock(&mut self) -> &mut T {
self.is_locked.set(false);
#[cfg(feature = "debug_lockcell")]
{
self.first_locked_at.set(None);
}
self.get_mut()
}
}
impl<T: fmt::Debug> fmt::Debug for LockCell<T> {
#[inline]
fn fmt(&self, fmtr: &mut fmt::Formatter<'_>) -> fmt::Result {
let lock_result = self.try_lock();
let value: &dyn fmt::Debug = if let Ok(value) = lock_result.as_deref() {
value
} else {
struct LockedPlaceholder;
impl fmt::Debug for LockedPlaceholder {
#[inline]
fn fmt(&self, fmtr: &mut fmt::Formatter<'_>) -> fmt::Result {
fmtr.write_str("<locked>")
}
}
const PLACEHOLDER: LockedPlaceholder = LockedPlaceholder;
&PLACEHOLDER
};
fmtr.debug_struct("LockCell").field("value", value).finish()
}
}
impl<T: Default> Default for LockCell<T> {
#[inline]
fn default() -> Self {
Self::new(Default::default())
}
}
impl<T> From<T> for LockCell<T> {
#[inline]
fn from(value: T) -> Self {
Self::new(value)
}
}
/// A `LockGuard` provides exclusive access to the inner value of a [`LockCell<T>`].
///
/// An instance of this type can be constructed from a `LockCell` using the [`LockCell::try_lock()`]
/// or [`LockCell::lock()`] methods.
///
/// See the [module level documentation] for more.
///
/// [`LockCell<T>`]: ./struct.LockCell.html
/// [`LockCell::try_lock()`]: ./struct.LockCell.html#method.try_lock
/// [`LockCell::lock()`]: ./struct.LockCell.html#method.lock
/// [module level documentation]: ./index.html
#[must_use]
pub struct LockGuard<'lock, T: ?Sized> {
/// The location of the original value in the `LockCell`.
value: *mut T,
/// The lock state of the `LockCell`.
is_locked: &'lock Cell<bool>,
/// The location where the original `LockCell` was first locked.
///
/// The `LockGuard` will reset this value when it is dropped.
#[cfg(feature = "debug_lockcell")]
locked_at: &'lock Cell<Option<&'static Location<'static>>>,
/// Phantom data.
_boo: PhantomData<&'lock UnsafeCell<T>>,
}
impl<'lock, T: ?Sized> LockGuard<'lock, T> {
/// Applies the given `func` to the contents `LockGuard` to return a new `LockGuard` which
/// points to a sub-part of the original data.
///
/// # Examples
///
/// ```
/// use lock_cell::{LockCell, LockGuard};
/// # fn main() {
/// let cell = LockCell::<(i32, i32)>::default();
/// let lock = cell.lock();
///
/// let mut value = LockGuard::map(lock, |(_, ref mut val)| val);
/// *value = 21;
/// drop(value);
///
/// let tuple = cell.into_inner();
/// assert_eq!(tuple.1, 21);
/// # }
/// ```
#[inline]
pub fn map<F, U: ?Sized>(this: Self, func: F) -> LockGuard<'lock, U>
where
F: FnOnce(&mut T) -> &mut U,
{
let mut this = ManuallyDrop::new(this);
LockGuard {
// SAFETY:
// The `value` ptr has been created from a valid `LockCell`, so it always valid.
value: unsafe { func(&mut *this.value) } as *mut _,
#[cfg(feature = "debug_lockcell")]
locked_at: this.locked_at,
is_locked: this.is_locked,
_boo: PhantomData,
}
}
/// Applies the given `func` to the contents of `LockGuard` to return an optional reference
/// to a part of the original data.
///
/// # Errors
///
/// If `func` returns `None`, then the original guard will be returned in the `Err` variant
/// of the return value.
///
/// # Examples
///
/// ```
/// use lock_cell::{LockCell, LockGuard};
/// # fn main() {
/// let cell = LockCell::new(Some(0));
/// let lock = cell.lock();
///
/// let mut value = match LockGuard::filter_map(lock, |value| value.as_mut()) {
/// Ok(inner) => inner,
/// Err(old_lock) => panic!("Unexpectedly empty value: {:?}", old_lock),
/// };
/// *value = 5;
/// drop(value);
///
/// let old_value = cell.replace(None);
/// assert_eq!(old_value, Some(5));
///
/// let lock = cell.lock();
/// let value = match LockGuard::filter_map(lock, |value| value.as_mut()) {
/// Ok(inner) => panic!("Unexpected value is present: {:?}", inner),
/// Err(old_lock) => old_lock,
/// };
///
/// assert_eq!(*value, None);
/// # }
/// ```
#[inline]
pub fn filter_map<F, U: ?Sized>(this: Self, func: F) -> Result<LockGuard<'lock, U>, Self>
where
F: FnOnce(&mut T) -> Option<&mut U>,
{
let mut this = ManuallyDrop::new(this);
// SAFETY:
// The `value` ptr has been created from a valid `LockCell`, so it always valid.
let value = match unsafe { func(&mut *this.value) } {
Some(value) => value as *mut _,
_ => return Err(ManuallyDrop::into_inner(this)),
};
Ok(LockGuard {
// SAFETY: value has been created from a reference so it is always valid.
value: unsafe { &mut *value },
#[cfg(feature = "debug_lockcell")]
locked_at: this.locked_at,
is_locked: this.is_locked,
_boo: PhantomData,
})
}
}
impl<'lock, T> TryFrom<&'lock LockCell<T>> for LockGuard<'lock, T> {
type Error = TryLockError;
#[inline]
#[track_caller]
fn try_from(lock_cell: &'lock LockCell<T>) -> Result<Self, Self::Error> {
lock_cell.try_lock()
}
}
impl<'lock, T: ?Sized> Drop for LockGuard<'lock, T> {
#[inline]
fn drop(&mut self) {
self.is_locked.set(false);
#[cfg(feature = "debug_lockcell")]
{
self.locked_at.set(None);
}
}
}
impl<'lock, T: ?Sized> AsRef<T> for LockGuard<'lock, T> {
#[inline]
fn as_ref(&self) -> &T {
// SAFETY:
// The `value` ptr has been created from a valid `LockCell`, so it always valid.
unsafe { &*self.value }
}
}
impl<'lock, T: ?Sized> AsMut<T> for LockGuard<'lock, T> {
#[inline]
fn as_mut(&mut self) -> &mut T {
// SAFETY:
// The `value` ptr has been created from a valid `LockCell`, so it always valid.
unsafe { &mut *self.value }
}
}
impl<'lock, T: ?Sized> Borrow<T> for LockGuard<'lock, T> {
#[inline]
fn borrow(&self) -> &T {
// SAFETY:
// The `value` ptr has been created from a valid `LockCell`, so it always valid.
unsafe { &*self.value }
}
}
impl<'lock, T: ?Sized> BorrowMut<T> for LockGuard<'lock, T> {
#[inline]
fn borrow_mut(&mut self) -> &mut T {
// SAFETY:
// The `value` ptr has been created from a valid `LockCell`, so it always valid.
unsafe { &mut *self.value }
}
}
impl<'lock, T: ?Sized> Deref for LockGuard<'lock, T> {
type Target = T;
#[inline]
fn deref(&self) -> &T {
// SAFETY:
// The `value` ptr has been created from a valid `LockCell`, so it always valid.
unsafe { &*self.value }
}
}
impl<'lock, T: ?Sized> DerefMut for LockGuard<'lock, T> {
#[inline]
fn deref_mut(&mut self) -> &mut T {
// SAFETY:
// The `value` ptr has been created from a valid `LockCell`, so it always valid.
unsafe { &mut *self.value }
}
}
impl<'lock, T: fmt::Debug + ?Sized> fmt::Debug for LockGuard<'lock, T> {
#[inline]
fn fmt(&self, fmtr: &mut fmt::Formatter<'_>) -> fmt::Result {
fmtr.debug_struct("LockGuard").field("value", self).finish()
}
}
impl<'lock, T: fmt::Display + ?Sized> fmt::Display for LockGuard<'lock, T> {
#[inline]
fn fmt(&self, fmtr: &mut fmt::Formatter<'_>) -> fmt::Result {
<T as fmt::Display>::fmt(self, fmtr)
}
}
impl<'lock, T: ?Sized + Hash> Hash for LockGuard<'lock, T> {
#[inline]
fn hash<H: Hasher>(&self, state: &mut H) {
<T as Hash>::hash(self, state);
}
}
impl<'lock, T: ?Sized + PartialEq> PartialEq<T> for LockGuard<'lock, T> {
#[inline]
fn eq(&self, other: &T) -> bool {
<T as PartialEq>::eq(self, other)
}
}
impl<'other, 'lock, T: ?Sized + PartialEq> PartialEq<LockGuard<'other, T>> for LockGuard<'lock, T> {
#[inline]
fn eq(&self, other: &LockGuard<'other, T>) -> bool {
<T as PartialEq>::eq(self, other)
}
}
impl<'lock, T: ?Sized + Eq> Eq for LockGuard<'lock, T> {}
impl<'lock, T: ?Sized + PartialOrd> PartialOrd<T> for LockGuard<'lock, T> {
#[inline]
fn partial_cmp(&self, other: &T) -> Option<Ordering> {
<T as PartialOrd>::partial_cmp(self, other)
}
}
impl<'other, 'lock, T: ?Sized + PartialOrd> PartialOrd<LockGuard<'other, T>> for LockGuard<'lock, T> {
#[inline]
fn partial_cmp(&self, other: &LockGuard<'other, T>) -> Option<Ordering> {
<T as PartialOrd>::partial_cmp(self, other)
}
}
impl<'lock, T: ?Sized + Ord> Ord for LockGuard<'lock, T> {
#[inline]
fn cmp(&self, other: &LockGuard<'lock, T>) -> Ordering {
<T as Ord>::cmp(self, other)
}
}
/// An error returned from the [`LockCell::try_lock()`] method to indicate
/// that the `LockCell` could not be locked.
///
/// [`LockCell::try_lock()`]: ./struct.LockCell.html#method.try_lock
#[non_exhaustive]
pub struct TryLockError {
/// The location where the `LockCell` was first locked.
#[cfg(feature = "debug_lockcell")]
first_lock_location: &'static Location<'static>,
/// The latest location where the `LockCell` was locked. This should provide
/// the location of the erroneous lock.
#[cfg(feature = "debug_lockcell")]
latest_lock_location: &'static Location<'static>,
_priv: (),
}
impl TryLockError {
/// Create a new `TryLockError` from the given caller location.
#[cfg_attr(not(feature = "debug_lockcell"), allow(unused_variables))]
#[track_caller]
#[inline]
fn new<T: ?Sized>(cell: &LockCell<T>) -> Self {
TryLockError {
#[cfg(feature = "debug_lockcell")]
first_lock_location: cell
.first_locked_at
.get()
.expect("Cell must be already locked"),
#[cfg(feature = "debug_lockcell")]
latest_lock_location: Location::caller(),
_priv: (),
}
}
}
impl fmt::Debug for TryLockError {
#[inline]
fn fmt(&self, fmtr: &mut fmt::Formatter<'_>) -> fmt::Result {
let mut builder = fmtr.debug_struct("TryLockError");
#[cfg(feature = "debug_lockcell")]
{
builder.field("first_locked_at", &self.first_lock_location);
builder.field("last_locked_at", &self.latest_lock_location);
}
builder.finish_non_exhaustive()
}
}
impl fmt::Display for TryLockError {
#[inline]
fn fmt(&self, fmtr: &mut fmt::Formatter<'_>) -> fmt::Result {
#[cfg(feature = "debug_lockcell")]
{
write!(
fmtr,
"first lock at {} conflicts with lock at {}",
self.first_lock_location, self.latest_lock_location,
)
}
#[cfg(not(feature = "debug_lockcell"))]
{
fmtr.write_str("already locked")
}
}
}
#[cfg(feature = "enable_std")]
impl StdError for TryLockError {}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn locking() {
let mut mtx = LockCell::new(23);
let mut lk = mtx.lock();
assert_eq!(*lk, 23);
assert!(mtx.is_locked());
*lk = 32;
assert_eq!(*lk, 32);
assert!(mtx.try_lock().is_err());
drop(lk);
assert_eq!(*mtx.get_mut(), 32);
}
#[test]
fn lock_map() {
#[derive(Default, Debug)]
struct TestData {
x: i32,
y: i32,
}
let mtx = LockCell::<TestData>::default();
let mut lk = LockGuard::map(mtx.lock(), |test_data| &mut test_data.y);
*lk = 42;
drop(lk);
let lk = mtx.lock();
let mut lk = match LockGuard::filter_map(lk, |data| Some(&mut data.x)) {
Ok(new_lk) => new_lk,
Err(old_lk) => panic!("{:?}", old_lk),
};
assert!(mtx.is_locked());
*lk = 21;
assert_eq!(*lk, 21);
match LockGuard::filter_map(lk, |_| -> Option<&mut i32> { None }) {
Ok(new_lk) => panic!("Unexpected lock guard found: {:?}", new_lk),
Err(_) => {}
}
let data = mtx.into_inner();
assert_eq!(data.x, 21);
assert_eq!(data.y, 42);
}
}