library/std/src/sync/rwlock.rs

   1 #[cfg(all(test, not(target_os = "emscripten")))]
   2 mod tests;
   3
   4 use crate::cell::UnsafeCell;
   5 use crate::fmt;
   6 use crate::ops::{Deref, DerefMut};
   7 use crate::ptr::NonNull;
   8 use crate::sync::{poison, LockResult, TryLockError, TryLockResult};
   9 use crate::sys::locks as sys;
  10
  11 /// A reader-writer lock
  12 ///
  13 /// This type of lock allows a number of readers or at most one writer at any
  14 /// point in time. The write portion of this lock typically allows modification
  15 /// of the underlying data (exclusive access) and the read portion of this lock
  16 /// typically allows for read-only access (shared access).
  17 ///
  18 /// In comparison, a [`Mutex`] does not distinguish between readers or writers
  19 /// that acquire the lock, therefore blocking any threads waiting for the lock to
  20 /// become available. An `RwLock` will allow any number of readers to acquire the
  21 /// lock as long as a writer is not holding the lock.
  22 ///
  23 /// The priority policy of the lock is dependent on the underlying operating
  24 /// system's implementation, and this type does not guarantee that any
  25 /// particular policy will be used. In particular, a writer which is waiting to
  26 /// acquire the lock in `write` might or might not block concurrent calls to
  27 /// `read`, e.g.:
  28 ///
  29 /// <details><summary>Potential deadlock example</summary>
  30 ///
  31 /// ```text
  32 /// // Thread 1             |  // Thread 2
  33 /// let _rg = lock.read();  |
  34 ///                         |  // will block
  35 ///                         |  let _wg = lock.write();
  36 /// // may deadlock         |
  37 /// let _rg = lock.read();  |
  38 /// ```
  39 /// </details>
  40 ///
  41 /// The type parameter `T` represents the data that this lock protects. It is
  42 /// required that `T` satisfies [`Send`] to be shared across threads and
  43 /// [`Sync`] to allow concurrent access through readers. The RAII guards
  44 /// returned from the locking methods implement [`Deref`] (and [`DerefMut`]
  45 /// for the `write` methods) to allow access to the content of the lock.
  46 ///
  47 /// # Poisoning
  48 ///
  49 /// An `RwLock`, like [`Mutex`], will become poisoned on a panic. Note, however,
  50 /// that an `RwLock` may only be poisoned if a panic occurs while it is locked
  51 /// exclusively (write mode). If a panic occurs in any reader, then the lock
  52 /// will not be poisoned.
  53 ///
  54 /// # Examples
  55 ///
  56 /// ```
  57 /// use std::sync::RwLock;
  58 ///
  59 /// let lock = RwLock::new(5);
  60 ///
  61 /// // many reader locks can be held at once
  62 /// {
  63 ///     let r1 = lock.read().unwrap();
  64 ///     let r2 = lock.read().unwrap();
  65 ///     assert_eq!(*r1, 5);
  66 ///     assert_eq!(*r2, 5);
  67 /// } // read locks are dropped at this point
  68 ///
  69 /// // only one write lock may be held, however
  70 /// {
  71 ///     let mut w = lock.write().unwrap();
  72 ///     *w += 1;
  73 ///     assert_eq!(*w, 6);
  74 /// } // write lock is dropped here
  75 /// ```
  76 ///
  77 /// [`Mutex`]: super::Mutex
  78 #[stable(feature = "rust1", since = "1.0.0")]
  79 #[cfg_attr(not(test), rustc_diagnostic_item = "RwLock")]
  80 pub struct RwLock<T: ?Sized> {
  81     inner: sys::RwLock,
  82     poison: poison::Flag,
  83     data: UnsafeCell<T>,
  84 }
  85
  86 #[stable(feature = "rust1", since = "1.0.0")]
  87 unsafe impl<T: ?Sized + Send> Send for RwLock<T> {}
  88 #[stable(feature = "rust1", since = "1.0.0")]
  89 unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}
  90
  91 /// RAII structure used to release the shared read access of a lock when
  92 /// dropped.
  93 ///
  94 /// This structure is created by the [`read`] and [`try_read`] methods on
  95 /// [`RwLock`].
  96 ///
  97 /// [`read`]: RwLock::read
  98 /// [`try_read`]: RwLock::try_read
  99 #[must_use = "if unused the RwLock will immediately unlock"]
 100 #[must_not_suspend = "holding a RwLockReadGuard across suspend \
 101                       points can cause deadlocks, delays, \
 102                       and cause Futures to not implement `Send`"]
 103 #[stable(feature = "rust1", since = "1.0.0")]
 104 #[clippy::has_significant_drop]
 105 #[cfg_attr(not(test), rustc_diagnostic_item = "RwLockReadGuard")]
 106 pub struct RwLockReadGuard<'a, T: ?Sized + 'a> {
 107     // NB: we use a pointer instead of `&'a T` to avoid `noalias` violations, because a
 108     // `Ref` argument doesn't hold immutability for its whole scope, only until it drops.
 109     // `NonNull` is also covariant over `T`, just like we would have with `&T`. `NonNull`
 110     // is preferable over `const* T` to allow for niche optimization.
 111     data: NonNull<T>,
 112     inner_lock: &'a sys::RwLock,
 113 }
 114
 115 #[stable(feature = "rust1", since = "1.0.0")]
 116 impl<T: ?Sized> !Send for RwLockReadGuard<'_, T> {}
 117
 118 #[stable(feature = "rwlock_guard_sync", since = "1.23.0")]
 119 unsafe impl<T: ?Sized + Sync> Sync for RwLockReadGuard<'_, T> {}
 120
 121 /// RAII structure used to release the exclusive write access of a lock when
 122 /// dropped.
 123 ///
 124 /// This structure is created by the [`write`] and [`try_write`] methods
 125 /// on [`RwLock`].
 126 ///
 127 /// [`write`]: RwLock::write
 128 /// [`try_write`]: RwLock::try_write
 129 #[must_use = "if unused the RwLock will immediately unlock"]
 130 #[must_not_suspend = "holding a RwLockWriteGuard across suspend \
 131                       points can cause deadlocks, delays, \
 132                       and cause Future's to not implement `Send`"]
 133 #[stable(feature = "rust1", since = "1.0.0")]
 134 #[clippy::has_significant_drop]
 135 #[cfg_attr(not(test), rustc_diagnostic_item = "RwLockWriteGuard")]
 136 pub struct RwLockWriteGuard<'a, T: ?Sized + 'a> {
 137     lock: &'a RwLock<T>,
 138     poison: poison::Guard,
 139 }
 140
 141 #[stable(feature = "rust1", since = "1.0.0")]
 142 impl<T: ?Sized> !Send for RwLockWriteGuard<'_, T> {}
 143
 144 #[stable(feature = "rwlock_guard_sync", since = "1.23.0")]
 145 unsafe impl<T: ?Sized + Sync> Sync for RwLockWriteGuard<'_, T> {}
 146
 147 impl<T> RwLock<T> {
 148     /// Creates a new instance of an `RwLock<T>` which is unlocked.
 149     ///
 150     /// # Examples
 151     ///
 152     /// ```
 153     /// use std::sync::RwLock;
 154     ///
 155     /// let lock = RwLock::new(5);
 156     /// ```
 157     #[stable(feature = "rust1", since = "1.0.0")]
 158     #[rustc_const_stable(feature = "const_locks", since = "1.63.0")]
 159     #[inline]
 160     pub const fn new(t: T) -> RwLock<T> {
 161         RwLock { inner: sys::RwLock::new(), poison: poison::Flag::new(), data: UnsafeCell::new(t) }
 162     }
 163 }
 164
 165 impl<T: ?Sized> RwLock<T> {
 166     /// Locks this `RwLock` with shared read access, blocking the current thread
 167     /// until it can be acquired.
 168     ///
 169     /// The calling thread will be blocked until there are no more writers which
 170     /// hold the lock. There may be other readers currently inside the lock when
 171     /// this method returns. This method does not provide any guarantees with
 172     /// respect to the ordering of whether contentious readers or writers will
 173     /// acquire the lock first.
 174     ///
 175     /// Returns an RAII guard which will release this thread's shared access
 176     /// once it is dropped.
 177     ///
 178     /// # Errors
 179     ///
 180     /// This function will return an error if the `RwLock` is poisoned. An
 181     /// `RwLock` is poisoned whenever a writer panics while holding an exclusive
 182     /// lock. The failure will occur immediately after the lock has been
 183     /// acquired.
 184     ///
 185     /// # Panics
 186     ///
 187     /// This function might panic when called if the lock is already held by the current thread.
 188     ///
 189     /// # Examples
 190     ///
 191     /// ```
 192     /// use std::sync::{Arc, RwLock};
 193     /// use std::thread;
 194     ///
 195     /// let lock = Arc::new(RwLock::new(1));
 196     /// let c_lock = Arc::clone(&lock);
 197     ///
 198     /// let n = lock.read().unwrap();
 199     /// assert_eq!(*n, 1);
 200     ///
 201     /// thread::spawn(move || {
 202     ///     let r = c_lock.read();
 203     ///     assert!(r.is_ok());
 204     /// }).join().unwrap();
 205     /// ```
 206     #[inline]
 207     #[stable(feature = "rust1", since = "1.0.0")]
 208     pub fn read(&self) -> LockResult<RwLockReadGuard<'_, T>> {
 209         unsafe {
 210             self.inner.read();
 211             RwLockReadGuard::new(self)
 212         }
 213     }
 214
 215     /// Attempts to acquire this `RwLock` with shared read access.
 216     ///
 217     /// If the access could not be granted at this time, then `Err` is returned.
 218     /// Otherwise, an RAII guard is returned which will release the shared access
 219     /// when it is dropped.
 220     ///
 221     /// This function does not block.
 222     ///
 223     /// This function does not provide any guarantees with respect to the ordering
 224     /// of whether contentious readers or writers will acquire the lock first.
 225     ///
 226     /// # Errors
 227     ///
 228     /// This function will return the [`Poisoned`] error if the `RwLock` is
 229     /// poisoned. An `RwLock` is poisoned whenever a writer panics while holding
 230     /// an exclusive lock. `Poisoned` will only be returned if the lock would
 231     /// have otherwise been acquired.
 232     ///
 233     /// This function will return the [`WouldBlock`] error if the `RwLock` could
 234     /// not be acquired because it was already locked exclusively.
 235     ///
 236     /// [`Poisoned`]: TryLockError::Poisoned
 237     /// [`WouldBlock`]: TryLockError::WouldBlock
 238     ///
 239     /// # Examples
 240     ///
 241     /// ```
 242     /// use std::sync::RwLock;
 243     ///
 244     /// let lock = RwLock::new(1);
 245     ///
 246     /// match lock.try_read() {
 247     ///     Ok(n) => assert_eq!(*n, 1),
 248     ///     Err(_) => unreachable!(),
 249     /// };
 250     /// ```
 251     #[inline]
 252     #[stable(feature = "rust1", since = "1.0.0")]
 253     pub fn try_read(&self) -> TryLockResult<RwLockReadGuard<'_, T>> {
 254         unsafe {
 255             if self.inner.try_read() {
 256                 Ok(RwLockReadGuard::new(self)?)
 257             } else {
 258                 Err(TryLockError::WouldBlock)
 259             }
 260         }
 261     }
 262
 263     /// Locks this `RwLock` with exclusive write access, blocking the current
 264     /// thread until it can be acquired.
 265     ///
 266     /// This function will not return while other writers or other readers
 267     /// currently have access to the lock.
 268     ///
 269     /// Returns an RAII guard which will drop the write access of this `RwLock`
 270     /// when dropped.
 271     ///
 272     /// # Errors
 273     ///
 274     /// This function will return an error if the `RwLock` is poisoned. An
 275     /// `RwLock` is poisoned whenever a writer panics while holding an exclusive
 276     /// lock. An error will be returned when the lock is acquired.
 277     ///
 278     /// # Panics
 279     ///
 280     /// This function might panic when called if the lock is already held by the current thread.
 281     ///
 282     /// # Examples
 283     ///
 284     /// ```
 285     /// use std::sync::RwLock;
 286     ///
 287     /// let lock = RwLock::new(1);
 288     ///
 289     /// let mut n = lock.write().unwrap();
 290     /// *n = 2;
 291     ///
 292     /// assert!(lock.try_read().is_err());
 293     /// ```
 294     #[inline]
 295     #[stable(feature = "rust1", since = "1.0.0")]
 296     pub fn write(&self) -> LockResult<RwLockWriteGuard<'_, T>> {
 297         unsafe {
 298             self.inner.write();
 299             RwLockWriteGuard::new(self)
 300         }
 301     }
 302
 303     /// Attempts to lock this `RwLock` with exclusive write access.
 304     ///
 305     /// If the lock could not be acquired at this time, then `Err` is returned.
 306     /// Otherwise, an RAII guard is returned which will release the lock when
 307     /// it is dropped.
 308     ///
 309     /// This function does not block.
 310     ///
 311     /// This function does not provide any guarantees with respect to the ordering
 312     /// of whether contentious readers or writers will acquire the lock first.
 313     ///
 314     /// # Errors
 315     ///
 316     /// This function will return the [`Poisoned`] error if the `RwLock` is
 317     /// poisoned. An `RwLock` is poisoned whenever a writer panics while holding
 318     /// an exclusive lock. `Poisoned` will only be returned if the lock would
 319     /// have otherwise been acquired.
 320     ///
 321     /// This function will return the [`WouldBlock`] error if the `RwLock` could
 322     /// not be acquired because it was already locked exclusively.
 323     ///
 324     /// [`Poisoned`]: TryLockError::Poisoned
 325     /// [`WouldBlock`]: TryLockError::WouldBlock
 326     ///
 327     ///
 328     /// # Examples
 329     ///
 330     /// ```
 331     /// use std::sync::RwLock;
 332     ///
 333     /// let lock = RwLock::new(1);
 334     ///
 335     /// let n = lock.read().unwrap();
 336     /// assert_eq!(*n, 1);
 337     ///
 338     /// assert!(lock.try_write().is_err());
 339     /// ```
 340     #[inline]
 341     #[stable(feature = "rust1", since = "1.0.0")]
 342     pub fn try_write(&self) -> TryLockResult<RwLockWriteGuard<'_, T>> {
 343         unsafe {
 344             if self.inner.try_write() {
 345                 Ok(RwLockWriteGuard::new(self)?)
 346             } else {
 347                 Err(TryLockError::WouldBlock)
 348             }
 349         }
 350     }
 351
 352     /// Determines whether the lock is poisoned.
 353     ///
 354     /// If another thread is active, the lock can still become poisoned at any
 355     /// time. You should not trust a `false` value for program correctness
 356     /// without additional synchronization.
 357     ///
 358     /// # Examples
 359     ///
 360     /// ```
 361     /// use std::sync::{Arc, RwLock};
 362     /// use std::thread;
 363     ///
 364     /// let lock = Arc::new(RwLock::new(0));
 365     /// let c_lock = Arc::clone(&lock);
 366     ///
 367     /// let _ = thread::spawn(move || {
 368     ///     let _lock = c_lock.write().unwrap();
 369     ///     panic!(); // the lock gets poisoned
 370     /// }).join();
 371     /// assert_eq!(lock.is_poisoned(), true);
 372     /// ```
 373     #[inline]
 374     #[stable(feature = "sync_poison", since = "1.2.0")]
 375     pub fn is_poisoned(&self) -> bool {
 376         self.poison.get()
 377     }
 378
 379     /// Clear the poisoned state from a lock
 380     ///
 381     /// If the lock is poisoned, it will remain poisoned until this function is called. This allows
 382     /// recovering from a poisoned state and marking that it has recovered. For example, if the
 383     /// value is overwritten by a known-good value, then the lock can be marked as un-poisoned. Or
 384     /// possibly, the value could be inspected to determine if it is in a consistent state, and if
 385     /// so the poison is removed.
 386     ///
 387     /// # Examples
 388     ///
 389     /// ```
 390     /// #![feature(mutex_unpoison)]
 391     ///
 392     /// use std::sync::{Arc, RwLock};
 393     /// use std::thread;
 394     ///
 395     /// let lock = Arc::new(RwLock::new(0));
 396     /// let c_lock = Arc::clone(&lock);
 397     ///
 398     /// let _ = thread::spawn(move || {
 399     ///     let _lock = c_lock.write().unwrap();
 400     ///     panic!(); // the lock gets poisoned
 401     /// }).join();
 402     ///
 403     /// assert_eq!(lock.is_poisoned(), true);
 404     /// let guard = lock.write().unwrap_or_else(|mut e| {
 405     ///     **e.get_mut() = 1;
 406     ///     lock.clear_poison();
 407     ///     e.into_inner()
 408     /// });
 409     /// assert_eq!(lock.is_poisoned(), false);
 410     /// assert_eq!(*guard, 1);
 411     /// ```
 412     #[inline]
 413     #[unstable(feature = "mutex_unpoison", issue = "96469")]
 414     pub fn clear_poison(&self) {
 415         self.poison.clear();
 416     }
 417
 418     /// Consumes this `RwLock`, returning the underlying data.
 419     ///
 420     /// # Errors
 421     ///
 422     /// This function will return an error if the `RwLock` is poisoned. An
 423     /// `RwLock` is poisoned whenever a writer panics while holding an exclusive
 424     /// lock. An error will only be returned if the lock would have otherwise
 425     /// been acquired.
 426     ///
 427     /// # Examples
 428     ///
 429     /// ```
 430     /// use std::sync::RwLock;
 431     ///
 432     /// let lock = RwLock::new(String::new());
 433     /// {
 434     ///     let mut s = lock.write().unwrap();
 435     ///     *s = "modified".to_owned();
 436     /// }
 437     /// assert_eq!(lock.into_inner().unwrap(), "modified");
 438     /// ```
 439     #[stable(feature = "rwlock_into_inner", since = "1.6.0")]
 440     pub fn into_inner(self) -> LockResult<T>
 441     where
 442         T: Sized,
 443     {
 444         let data = self.data.into_inner();
 445         poison::map_result(self.poison.borrow(), |()| data)
 446     }
 447
 448     /// Returns a mutable reference to the underlying data.
 449     ///
 450     /// Since this call borrows the `RwLock` mutably, no actual locking needs to
 451     /// take place -- the mutable borrow statically guarantees no locks exist.
 452     ///
 453     /// # Errors
 454     ///
 455     /// This function will return an error if the `RwLock` is poisoned. An
 456     /// `RwLock` is poisoned whenever a writer panics while holding an exclusive
 457     /// lock. An error will only be returned if the lock would have otherwise
 458     /// been acquired.
 459     ///
 460     /// # Examples
 461     ///
 462     /// ```
 463     /// use std::sync::RwLock;
 464     ///
 465     /// let mut lock = RwLock::new(0);
 466     /// *lock.get_mut().unwrap() = 10;
 467     /// assert_eq!(*lock.read().unwrap(), 10);
 468     /// ```
 469     #[stable(feature = "rwlock_get_mut", since = "1.6.0")]
 470     pub fn get_mut(&mut self) -> LockResult<&mut T> {
 471         let data = self.data.get_mut();
 472         poison::map_result(self.poison.borrow(), |()| data)
 473     }
 474 }
 475
 476 #[stable(feature = "rust1", since = "1.0.0")]
 477 impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLock<T> {
 478     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 479         let mut d = f.debug_struct("RwLock");
 480         match self.try_read() {
 481             Ok(guard) => {
 482                 d.field("data", &&*guard);
 483             }
 484             Err(TryLockError::Poisoned(err)) => {
 485                 d.field("data", &&**err.get_ref());
 486             }
 487             Err(TryLockError::WouldBlock) => {
 488                 d.field("data", &format_args!("<locked>"));
 489             }
 490         }
 491         d.field("poisoned", &self.poison.get());
 492         d.finish_non_exhaustive()
 493     }
 494 }
 495
 496 #[stable(feature = "rw_lock_default", since = "1.10.0")]
 497 impl<T: Default> Default for RwLock<T> {
 498     /// Creates a new `RwLock<T>`, with the `Default` value for T.
 499     fn default() -> RwLock<T> {
 500         RwLock::new(Default::default())
 501     }
 502 }
 503
 504 #[stable(feature = "rw_lock_from", since = "1.24.0")]
 505 impl<T> From<T> for RwLock<T> {
 506     /// Creates a new instance of an `RwLock<T>` which is unlocked.
 507     /// This is equivalent to [`RwLock::new`].
 508     fn from(t: T) -> Self {
 509         RwLock::new(t)
 510     }
 511 }
 512
 513 impl<'rwlock, T: ?Sized> RwLockReadGuard<'rwlock, T> {
 514     /// Create a new instance of `RwLockReadGuard<T>` from a `RwLock<T>`.
 515     // SAFETY: if and only if `lock.inner.read()` (or `lock.inner.try_read()`) has been
 516     // successfully called from the same thread before instantiating this object.
 517     unsafe fn new(lock: &'rwlock RwLock<T>) -> LockResult<RwLockReadGuard<'rwlock, T>> {
 518         poison::map_result(lock.poison.borrow(), |()| RwLockReadGuard {
 519             data: NonNull::new_unchecked(lock.data.get()),
 520             inner_lock: &lock.inner,
 521         })
 522     }
 523 }
 524
 525 impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
 526     /// Create a new instance of `RwLockWriteGuard<T>` from a `RwLock<T>`.
 527     // SAFETY: if and only if `lock.inner.write()` (or `lock.inner.try_write()`) has been
 528     // successfully called from the same thread before instantiating this object.
 529     unsafe fn new(lock: &'rwlock RwLock<T>) -> LockResult<RwLockWriteGuard<'rwlock, T>> {
 530         poison::map_result(lock.poison.guard(), |guard| RwLockWriteGuard { lock, poison: guard })
 531     }
 532 }
 533
 534 #[stable(feature = "std_debug", since = "1.16.0")]
 535 impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockReadGuard<'_, T> {
 536     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 537         (**self).fmt(f)
 538     }
 539 }
 540
 541 #[stable(feature = "std_guard_impls", since = "1.20.0")]
 542 impl<T: ?Sized + fmt::Display> fmt::Display for RwLockReadGuard<'_, T> {
 543     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 544         (**self).fmt(f)
 545     }
 546 }
 547
 548 #[stable(feature = "std_debug", since = "1.16.0")]
 549 impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockWriteGuard<'_, T> {
 550     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 551         (**self).fmt(f)
 552     }
 553 }
 554
 555 #[stable(feature = "std_guard_impls", since = "1.20.0")]
 556 impl<T: ?Sized + fmt::Display> fmt::Display for RwLockWriteGuard<'_, T> {
 557     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 558         (**self).fmt(f)
 559     }
 560 }
 561
 562 #[stable(feature = "rust1", since = "1.0.0")]
 563 impl<T: ?Sized> Deref for RwLockReadGuard<'_, T> {
 564     type Target = T;
 565
 566     fn deref(&self) -> &T {
 567         // SAFETY: the conditions of `RwLockGuard::new` were satisfied when created.
 568         unsafe { self.data.as_ref() }
 569     }
 570 }
 571
 572 #[stable(feature = "rust1", since = "1.0.0")]
 573 impl<T: ?Sized> Deref for RwLockWriteGuard<'_, T> {
 574     type Target = T;
 575
 576     fn deref(&self) -> &T {
 577         // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
 578         unsafe { &*self.lock.data.get() }
 579     }
 580 }
 581
 582 #[stable(feature = "rust1", since = "1.0.0")]
 583 impl<T: ?Sized> DerefMut for RwLockWriteGuard<'_, T> {
 584     fn deref_mut(&mut self) -> &mut T {
 585         // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
 586         unsafe { &mut *self.lock.data.get() }
 587     }
 588 }
 589
 590 #[stable(feature = "rust1", since = "1.0.0")]
 591 impl<T: ?Sized> Drop for RwLockReadGuard<'_, T> {
 592     fn drop(&mut self) {
 593         // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when created.
 594         unsafe {
 595             self.inner_lock.read_unlock();
 596         }
 597     }
 598 }
 599
 600 #[stable(feature = "rust1", since = "1.0.0")]
 601 impl<T: ?Sized> Drop for RwLockWriteGuard<'_, T> {
 602     fn drop(&mut self) {
 603         self.lock.poison.done(&self.poison);
 604         // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
 605         unsafe {
 606             self.lock.inner.write_unlock();
 607         }
 608     }
 609 }