1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706
use crate::cell::UnsafeCell; use crate::fmt; use crate::mem; use crate::ops::{Deref, DerefMut}; use crate::ptr; use crate::sys_common::mutex as sys; use crate::sys_common::poison::{self, TryLockError, TryLockResult, LockResult}; /// A mutual exclusion primitive useful for protecting shared data /// /// This mutex will block threads waiting for the lock to become available. The /// mutex can also be statically initialized or created via a [`new`] /// constructor. Each mutex has a type parameter which represents the data that /// it is protecting. The data can only be accessed through the RAII guards /// returned from [`lock`] and [`try_lock`], which guarantees that the data is only /// ever accessed when the mutex is locked. /// /// # Poisoning /// /// The mutexes in this module implement a strategy called "poisoning" where a /// mutex is considered poisoned whenever a thread panics while holding the /// mutex. Once a mutex is poisoned, all other threads are unable to access the /// data by default as it is likely tainted (some invariant is not being /// upheld). /// /// For a mutex, this means that the [`lock`] and [`try_lock`] methods return a /// [`Result`] which indicates whether a mutex has been poisoned or not. Most /// usage of a mutex will simply [`unwrap()`] these results, propagating panics /// among threads to ensure that a possibly invalid invariant is not witnessed. /// /// A poisoned mutex, however, does not prevent all access to the underlying /// data. The [`PoisonError`] type has an [`into_inner`] method which will return /// the guard that would have otherwise been returned on a successful lock. This /// allows access to the data, despite the lock being poisoned. /// /// [`new`]: #method.new /// [`lock`]: #method.lock /// [`try_lock`]: #method.try_lock /// [`Result`]: ../../std/result/enum.Result.html /// [`unwrap()`]: ../../std/result/enum.Result.html#method.unwrap /// [`PoisonError`]: ../../std/sync/struct.PoisonError.html /// [`into_inner`]: ../../std/sync/struct.PoisonError.html#method.into_inner /// /// # Examples /// /// ``` /// use std::sync::{Arc, Mutex}; /// use std::thread; /// use std::sync::mpsc::channel; /// /// const N: usize = 10; /// /// // Spawn a few threads to increment a shared variable (non-atomically), and /// // let the main thread know once all increments are done. /// // /// // Here we're using an Arc to share memory among threads, and the data inside /// // the Arc is protected with a mutex. /// let data = Arc::new(Mutex::new(0)); /// /// let (tx, rx) = channel(); /// for _ in 0..N { /// let (data, tx) = (Arc::clone(&data), tx.clone()); /// thread::spawn(move || { /// // The shared state can only be accessed once the lock is held. /// // Our non-atomic increment is safe because we're the only thread /// // which can access the shared state when the lock is held. /// // /// // We unwrap() the return value to assert that we are not expecting /// // threads to ever fail while holding the lock. /// let mut data = data.lock().unwrap(); /// *data += 1; /// if *data == N { /// tx.send(()).unwrap(); /// } /// // the lock is unlocked here when `data` goes out of scope. /// }); /// } /// /// rx.recv().unwrap(); /// ``` /// /// To recover from a poisoned mutex: /// /// ``` /// use std::sync::{Arc, Mutex}; /// use std::thread; /// /// let lock = Arc::new(Mutex::new(0_u32)); /// let lock2 = lock.clone(); /// /// let _ = thread::spawn(move || -> () { /// // This thread will acquire the mutex first, unwrapping the result of /// // `lock` because the lock has not been poisoned. /// let _guard = lock2.lock().unwrap(); /// /// // This panic while holding the lock (`_guard` is in scope) will poison /// // the mutex. /// panic!(); /// }).join(); /// /// // The lock is poisoned by this point, but the returned result can be /// // pattern matched on to return the underlying guard on both branches. /// let mut guard = match lock.lock() { /// Ok(guard) => guard, /// Err(poisoned) => poisoned.into_inner(), /// }; /// /// *guard += 1; /// ``` #[stable(feature = "rust1", since = "1.0.0")] pub struct Mutex<T: ?Sized> { // Note that this mutex is in a *box*, not inlined into the struct itself. // Once a native mutex has been used once, its address can never change (it // can't be moved). This mutex type can be safely moved at any time, so to // ensure that the native mutex is used correctly we box the inner mutex to // give it a constant address. inner: Box<sys::Mutex>, poison: poison::Flag, data: UnsafeCell<T>, } // these are the only places where `T: Send` matters; all other // functionality works fine on a single thread. #[stable(feature = "rust1", since = "1.0.0")] unsafe impl<T: ?Sized + Send> Send for Mutex<T> { } #[stable(feature = "rust1", since = "1.0.0")] unsafe impl<T: ?Sized + Send> Sync for Mutex<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`] and [`DerefMut`] implementations. /// /// This structure is created by the [`lock`] and [`try_lock`] methods on /// [`Mutex`]. /// /// [`Deref`]: ../../std/ops/trait.Deref.html /// [`DerefMut`]: ../../std/ops/trait.DerefMut.html /// [`lock`]: struct.Mutex.html#method.lock /// [`try_lock`]: struct.Mutex.html#method.try_lock /// [`Mutex`]: struct.Mutex.html #[must_use = "if unused the Mutex will immediately unlock"] #[stable(feature = "rust1", since = "1.0.0")] pub struct MutexGuard<'a, T: ?Sized + 'a> { // funny underscores due to how Deref/DerefMut currently work (they // disregard field privacy). __lock: &'a Mutex<T>, __poison: poison::Guard, } #[stable(feature = "rust1", since = "1.0.0")] impl<T: ?Sized> !Send for MutexGuard<'_, T> { } #[stable(feature = "mutexguard", since = "1.19.0")] unsafe impl<T: ?Sized + Sync> Sync for MutexGuard<'_, T> { } impl<T> Mutex<T> { /// Creates a new mutex in an unlocked state ready for use. /// /// # Examples /// /// ``` /// use std::sync::Mutex; /// /// let mutex = Mutex::new(0); /// ``` #[stable(feature = "rust1", since = "1.0.0")] pub fn new(t: T) -> Mutex<T> { let mut m = Mutex { inner: box sys::Mutex::new(), poison: poison::Flag::new(), data: UnsafeCell::new(t), }; unsafe { m.inner.init(); } m } } impl<T: ?Sized> Mutex<T> { /// Acquires a mutex, blocking the current thread until it is able to do so. /// /// This function will block the local thread until it is available to acquire /// the mutex. Upon returning, the thread is the only thread with the lock /// held. An RAII guard is returned to allow scoped unlock of the lock. When /// the guard goes out of scope, the mutex will be unlocked. /// /// The exact behavior on locking a mutex in the thread which already holds /// the lock is left unspecified. However, this function will not return on /// the second call (it might panic or deadlock, for example). /// /// # Errors /// /// If another user of this mutex panicked while holding the mutex, then /// this call will return an error once the mutex is acquired. /// /// # Panics /// /// This function might panic when called if the lock is already held by /// the current thread. /// /// # Examples /// /// ``` /// use std::sync::{Arc, Mutex}; /// use std::thread; /// /// let mutex = Arc::new(Mutex::new(0)); /// let c_mutex = mutex.clone(); /// /// thread::spawn(move || { /// *c_mutex.lock().unwrap() = 10; /// }).join().expect("thread::spawn failed"); /// assert_eq!(*mutex.lock().unwrap(), 10); /// ``` #[stable(feature = "rust1", since = "1.0.0")] pub fn lock(&self) -> LockResult<MutexGuard<'_, T>> { unsafe { self.inner.raw_lock(); MutexGuard::new(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. The lock will be unlocked when the /// guard is dropped. /// /// 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. /// /// [`Err`]: ../../std/result/enum.Result.html#variant.Err /// /// # Examples /// /// ``` /// use std::sync::{Arc, Mutex}; /// use std::thread; /// /// let mutex = Arc::new(Mutex::new(0)); /// let c_mutex = mutex.clone(); /// /// thread::spawn(move || { /// let mut lock = c_mutex.try_lock(); /// if let Ok(ref mut mutex) = lock { /// **mutex = 10; /// } else { /// println!("try_lock failed"); /// } /// }).join().expect("thread::spawn failed"); /// assert_eq!(*mutex.lock().unwrap(), 10); /// ``` #[stable(feature = "rust1", since = "1.0.0")] pub fn try_lock(&self) -> TryLockResult<MutexGuard<'_, T>> { unsafe { if self.inner.try_lock() { Ok(MutexGuard::new(self)?) } else { Err(TryLockError::WouldBlock) } } } /// Determines whether the mutex is poisoned. /// /// If another thread is active, the mutex can still become poisoned at any /// time. You should not trust a `false` value for program correctness /// without additional synchronization. /// /// # Examples /// /// ``` /// use std::sync::{Arc, Mutex}; /// use std::thread; /// /// let mutex = Arc::new(Mutex::new(0)); /// let c_mutex = mutex.clone(); /// /// let _ = thread::spawn(move || { /// let _lock = c_mutex.lock().unwrap(); /// panic!(); // the mutex gets poisoned /// }).join(); /// assert_eq!(mutex.is_poisoned(), true); /// ``` #[inline] #[stable(feature = "sync_poison", since = "1.2.0")] pub fn is_poisoned(&self) -> bool { self.poison.get() } /// Consumes this mutex, returning the underlying data. /// /// # Errors /// /// If another user of this mutex panicked while holding the mutex, then /// this call will return an error instead. /// /// # Examples /// /// ``` /// use std::sync::Mutex; /// /// let mutex = Mutex::new(0); /// assert_eq!(mutex.into_inner().unwrap(), 0); /// ``` #[stable(feature = "mutex_into_inner", since = "1.6.0")] pub fn into_inner(self) -> LockResult<T> where T: Sized { // We know statically that there are no outstanding references to // `self` so there's no need to lock the inner mutex. // // To get the inner value, we'd like to call `data.into_inner()`, // but because `Mutex` impl-s `Drop`, we can't move out of it, so // we'll have to destructure it manually instead. unsafe { // Like `let Mutex { inner, poison, data } = self`. let (inner, poison, data) = { let Mutex { ref inner, ref poison, ref data } = self; (ptr::read(inner), ptr::read(poison), ptr::read(data)) }; mem::forget(self); inner.destroy(); // Keep in sync with the `Drop` impl. drop(inner); poison::map_result(poison.borrow(), |_| data.into_inner()) } } /// 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. /// /// # Errors /// /// If another user of this mutex panicked while holding the mutex, then /// this call will return an error instead. /// /// # Examples /// /// ``` /// use std::sync::Mutex; /// /// let mut mutex = Mutex::new(0); /// *mutex.get_mut().unwrap() = 10; /// assert_eq!(*mutex.lock().unwrap(), 10); /// ``` #[stable(feature = "mutex_get_mut", since = "1.6.0")] pub fn get_mut(&mut self) -> LockResult<&mut T> { // We know statically that there are no other references to `self`, so // there's no need to lock the inner mutex. let data = unsafe { &mut *self.data.get() }; poison::map_result(self.poison.borrow(), |_| data ) } } #[stable(feature = "rust1", since = "1.0.0")] unsafe impl<#[may_dangle] T: ?Sized> Drop for Mutex<T> { fn drop(&mut self) { // This is actually safe b/c we know that there is no further usage of // this mutex (it's up to the user to arrange for a mutex to get // dropped, that's not our job) // // IMPORTANT: This code must be kept in sync with `Mutex::into_inner`. unsafe { self.inner.destroy() } } } #[stable(feature = "mutex_from", since = "1.24.0")] impl<T> From<T> for Mutex<T> { /// Creates a new mutex in an unlocked state ready for use. /// This is equivalent to [`Mutex::new`]. /// /// [`Mutex::new`]: ../../std/sync/struct.Mutex.html#method.new fn from(t: T) -> Self { Mutex::new(t) } } #[stable(feature = "mutex_default", since = "1.10.0")] impl<T: ?Sized + Default> Default for Mutex<T> { /// Creates a `Mutex<T>`, with the `Default` value for T. fn default() -> Mutex<T> { Mutex::new(Default::default()) } } #[stable(feature = "rust1", since = "1.0.0")] impl<T: ?Sized + fmt::Debug> fmt::Debug for Mutex<T> { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { match self.try_lock() { Ok(guard) => f.debug_struct("Mutex").field("data", &&*guard).finish(), Err(TryLockError::Poisoned(err)) => { f.debug_struct("Mutex").field("data", &&**err.get_ref()).finish() }, Err(TryLockError::WouldBlock) => { struct LockedPlaceholder; impl fmt::Debug for LockedPlaceholder { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { f.write_str("<locked>") } } f.debug_struct("Mutex").field("data", &LockedPlaceholder).finish() } } } } impl<'mutex, T: ?Sized> MutexGuard<'mutex, T> { unsafe fn new(lock: &'mutex Mutex<T>) -> LockResult<MutexGuard<'mutex, T>> { poison::map_result(lock.poison.borrow(), |guard| { MutexGuard { __lock: lock, __poison: guard, } }) } } #[stable(feature = "rust1", since = "1.0.0")] impl<T: ?Sized> Deref for MutexGuard<'_, T> { type Target = T; fn deref(&self) -> &T { unsafe { &*self.__lock.data.get() } } } #[stable(feature = "rust1", since = "1.0.0")] impl<T: ?Sized> DerefMut for MutexGuard<'_, T> { fn deref_mut(&mut self) -> &mut T { unsafe { &mut *self.__lock.data.get() } } } #[stable(feature = "rust1", since = "1.0.0")] impl<T: ?Sized> Drop for MutexGuard<'_, T> { #[inline] fn drop(&mut self) { unsafe { self.__lock.poison.done(&self.__poison); self.__lock.inner.raw_unlock(); } } } #[stable(feature = "std_debug", since = "1.16.0")] impl<T: ?Sized + fmt::Debug> fmt::Debug for MutexGuard<'_, T> { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { fmt::Debug::fmt(&**self, f) } } #[stable(feature = "std_guard_impls", since = "1.20.0")] impl<T: ?Sized + fmt::Display> fmt::Display for MutexGuard<'_, T> { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { (**self).fmt(f) } } pub fn guard_lock<'a, T: ?Sized>(guard: &MutexGuard<'a, T>) -> &'a sys::Mutex { &guard.__lock.inner } pub fn guard_poison<'a, T: ?Sized>(guard: &MutexGuard<'a, T>) -> &'a poison::Flag { &guard.__lock.poison } #[cfg(all(test, not(target_os = "emscripten")))] mod tests { use crate::sync::mpsc::channel; use crate::sync::{Arc, Mutex, Condvar}; use crate::sync::atomic::{AtomicUsize, Ordering}; use crate::thread; struct Packet<T>(Arc<(Mutex<T>, Condvar)>); #[derive(Eq, PartialEq, Debug)] struct NonCopy(i32); #[test] fn smoke() { let m = Mutex::new(()); drop(m.lock().unwrap()); drop(m.lock().unwrap()); } #[test] fn lots_and_lots() { const J: u32 = 1000; const K: u32 = 3; let m = Arc::new(Mutex::new(0)); fn inc(m: &Mutex<u32>) { for _ in 0..J { *m.lock().unwrap() += 1; } } let (tx, rx) = channel(); for _ in 0..K { let tx2 = tx.clone(); let m2 = m.clone(); thread::spawn(move|| { inc(&m2); tx2.send(()).unwrap(); }); let tx2 = tx.clone(); let m2 = m.clone(); thread::spawn(move|| { inc(&m2); tx2.send(()).unwrap(); }); } drop(tx); for _ in 0..2 * K { rx.recv().unwrap(); } assert_eq!(*m.lock().unwrap(), J * K * 2); } #[test] fn try_lock() { let m = Mutex::new(()); *m.try_lock().unwrap() = (); } #[test] fn test_into_inner() { let m = Mutex::new(NonCopy(10)); assert_eq!(m.into_inner().unwrap(), NonCopy(10)); } #[test] fn test_into_inner_drop() { struct Foo(Arc<AtomicUsize>); impl Drop for Foo { fn drop(&mut self) { self.0.fetch_add(1, Ordering::SeqCst); } } let num_drops = Arc::new(AtomicUsize::new(0)); let m = Mutex::new(Foo(num_drops.clone())); assert_eq!(num_drops.load(Ordering::SeqCst), 0); { let _inner = m.into_inner().unwrap(); assert_eq!(num_drops.load(Ordering::SeqCst), 0); } assert_eq!(num_drops.load(Ordering::SeqCst), 1); } #[test] fn test_into_inner_poison() { let m = Arc::new(Mutex::new(NonCopy(10))); let m2 = m.clone(); let _ = thread::spawn(move || { let _lock = m2.lock().unwrap(); panic!("test panic in inner thread to poison mutex"); }).join(); assert!(m.is_poisoned()); match Arc::try_unwrap(m).unwrap().into_inner() { Err(e) => assert_eq!(e.into_inner(), NonCopy(10)), Ok(x) => panic!("into_inner of poisoned Mutex is Ok: {:?}", x), } } #[test] fn test_get_mut() { let mut m = Mutex::new(NonCopy(10)); *m.get_mut().unwrap() = NonCopy(20); assert_eq!(m.into_inner().unwrap(), NonCopy(20)); } #[test] fn test_get_mut_poison() { let m = Arc::new(Mutex::new(NonCopy(10))); let m2 = m.clone(); let _ = thread::spawn(move || { let _lock = m2.lock().unwrap(); panic!("test panic in inner thread to poison mutex"); }).join(); assert!(m.is_poisoned()); match Arc::try_unwrap(m).unwrap().get_mut() { Err(e) => assert_eq!(*e.into_inner(), NonCopy(10)), Ok(x) => panic!("get_mut of poisoned Mutex is Ok: {:?}", x), } } #[test] fn test_mutex_arc_condvar() { let packet = Packet(Arc::new((Mutex::new(false), Condvar::new()))); let packet2 = Packet(packet.0.clone()); let (tx, rx) = channel(); let _t = thread::spawn(move|| { // wait until parent gets in rx.recv().unwrap(); let &(ref lock, ref cvar) = &*packet2.0; let mut lock = lock.lock().unwrap(); *lock = true; cvar.notify_one(); }); let &(ref lock, ref cvar) = &*packet.0; let mut lock = lock.lock().unwrap(); tx.send(()).unwrap(); assert!(!*lock); while !*lock { lock = cvar.wait(lock).unwrap(); } } #[test] fn test_arc_condvar_poison() { let packet = Packet(Arc::new((Mutex::new(1), Condvar::new()))); let packet2 = Packet(packet.0.clone()); let (tx, rx) = channel(); let _t = thread::spawn(move || -> () { rx.recv().unwrap(); let &(ref lock, ref cvar) = &*packet2.0; let _g = lock.lock().unwrap(); cvar.notify_one(); // Parent should fail when it wakes up. panic!(); }); let &(ref lock, ref cvar) = &*packet.0; let mut lock = lock.lock().unwrap(); tx.send(()).unwrap(); while *lock == 1 { match cvar.wait(lock) { Ok(l) => { lock = l; assert_eq!(*lock, 1); } Err(..) => break, } } } #[test] fn test_mutex_arc_poison() { let arc = Arc::new(Mutex::new(1)); assert!(!arc.is_poisoned()); let arc2 = arc.clone(); let _ = thread::spawn(move|| { let lock = arc2.lock().unwrap(); assert_eq!(*lock, 2); }).join(); assert!(arc.lock().is_err()); assert!(arc.is_poisoned()); } #[test] fn test_mutex_arc_nested() { // Tests nested mutexes and access // to underlying data. let arc = Arc::new(Mutex::new(1)); let arc2 = Arc::new(Mutex::new(arc)); let (tx, rx) = channel(); let _t = thread::spawn(move|| { let lock = arc2.lock().unwrap(); let lock2 = lock.lock().unwrap(); assert_eq!(*lock2, 1); tx.send(()).unwrap(); }); rx.recv().unwrap(); } #[test] fn test_mutex_arc_access_in_unwind() { let arc = Arc::new(Mutex::new(1)); let arc2 = arc.clone(); let _ = thread::spawn(move|| -> () { struct Unwinder { i: Arc<Mutex<i32>>, } impl Drop for Unwinder { fn drop(&mut self) { *self.i.lock().unwrap() += 1; } } let _u = Unwinder { i: arc2 }; panic!(); }).join(); let lock = arc.lock().unwrap(); assert_eq!(*lock, 2); } #[test] fn test_mutex_unsized() { let mutex: &Mutex<[i32]> = &Mutex::new([1, 2, 3]); { let b = &mut *mutex.lock().unwrap(); b[0] = 4; b[2] = 5; } let comp: &[i32] = &[4, 2, 5]; assert_eq!(&*mutex.lock().unwrap(), comp); } }