Auto merge of #145773 - jhpratt:rollup-kocqnzv, r=jhpratt

Rollup of 28 pull requests

Successful merges:

 - rust-lang/rust#132087 (Fix overly restrictive lifetime in `core::panic::Location::file` return type)
 - rust-lang/rust#137396 (Recover `param: Ty = EXPR`)
 - rust-lang/rust#137457 (Fix host code appearing in Wasm binaries)
 - rust-lang/rust#142185 (Convert moves of references to copies in ReferencePropagation)
 - rust-lang/rust#144648 (Implementation: `#[feature(nonpoison_rwlock)]`)
 - rust-lang/rust#144897 (print raw lifetime idents with r#)
 - rust-lang/rust#145218 ([Debuginfo] improve enum value formatting in LLDB for better readability)
 - rust-lang/rust#145380 (Add codegen-llvm regression tests)
 - rust-lang/rust#145573 (Add an experimental unsafe(force_target_feature) attribute.)
 - rust-lang/rust#145597 (resolve: Remove `ScopeSet::Late`)
 - rust-lang/rust#145633 (Fix some typos in LocalKey documentation)
 - rust-lang/rust#145641 (On E0277, point at type that doesn't implement bound)
 - rust-lang/rust#145669 (rustdoc-search: GUI tests check for `//` in URL)
 - rust-lang/rust#145695 (Introduce ProjectionElem::try_map.)
 - rust-lang/rust#145710 (Fix the ABI parameter inconsistency issue in debug.rs for LoongArch64)
 - rust-lang/rust#145726 (Experiment: Reborrow trait)
 - rust-lang/rust#145731 (Make raw pointers work in type-based search)
 - rust-lang/rust#145736 (triagebot: Update style team reviewers)
 - rust-lang/rust#145738 (Uplift rustc_mir_transform::coverage::counters::union_find to rustc_data_structures.)
 - rust-lang/rust#145742 (rustdoc js: Even more typechecking improvments )
 - rust-lang/rust#145743 (doc: fix some typos in comment)
 - rust-lang/rust#145745 (tests: Ignore basic-stepping.rs on LoongArch)
 - rust-lang/rust#145747 (Refactor lint buffering to avoid requiring a giant enum)
 - rust-lang/rust#145751 (fix(lexer): Allow '-' in the frontmatter infostring continue set)
 - rust-lang/rust#145761 (Add aarch64_be-unknown-hermit target)
 - rust-lang/rust#145762 (convert strings to symbols in attr diagnostics)
 - rust-lang/rust#145763 (Ship LLVM tools for the correct target when cross-compiling)
 - rust-lang/rust#145765 (Revert suggestions for missing methods in tuples)

r? `@ghost`
`@rustbot` modify labels: rollup

4 files changed, 1462 insertions, 331 deletions

diff --git a/library/std/src/sync/nonpoison.rs b/library/std/src/sync/nonpoison.rs
index 2bbf226dc2c..b3ae376e70d 100644
--- a/library/std/src/sync/nonpoison.rs
+++ b/library/std/src/sync/nonpoison.rs
@@ -33,5 +33,10 @@ impl fmt::Display for WouldBlock {
 pub use self::mutex::MappedMutexGuard;
 #[unstable(feature = "nonpoison_mutex", issue = "134645")]
 pub use self::mutex::{Mutex, MutexGuard};
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+pub use self::rwlock::{MappedRwLockReadGuard, MappedRwLockWriteGuard};
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+pub use self::rwlock::{RwLock, RwLockReadGuard, RwLockWriteGuard};
 
 mod mutex;
+mod rwlock;
diff --git a/library/std/src/sync/nonpoison/rwlock.rs b/library/std/src/sync/nonpoison/rwlock.rs
new file mode 100644
index 00000000000..eb0aef99cc1
--- /dev/null
+++ b/library/std/src/sync/nonpoison/rwlock.rs
@@ -0,0 +1,1081 @@
+use crate::cell::UnsafeCell;
+use crate::fmt;
+use crate::marker::PhantomData;
+use crate::mem::{self, ManuallyDrop, forget};
+use crate::ops::{Deref, DerefMut};
+use crate::ptr::NonNull;
+use crate::sync::nonpoison::{TryLockResult, WouldBlock};
+use crate::sys::sync as sys;
+
+/// A reader-writer lock that does not keep track of lock poisoning.
+///
+/// For more information about reader-writer locks, check out the documentation for the poisoning
+/// variant of this lock (which can be found at [`poison::RwLock`]).
+///
+/// [`poison::RwLock`]: crate::sync::poison::RwLock
+///
+/// # Examples
+///
+/// ```
+/// #![feature(nonpoison_rwlock)]
+///
+/// use std::sync::nonpoison::RwLock;
+///
+/// let lock = RwLock::new(5);
+///
+/// // many reader locks can be held at once
+/// {
+///     let r1 = lock.read();
+///     let r2 = lock.read();
+///     assert_eq!(*r1, 5);
+///     assert_eq!(*r2, 5);
+/// } // read locks are dropped at this point
+///
+/// // only one write lock may be held, however
+/// {
+///     let mut w = lock.write();
+///     *w += 1;
+///     assert_eq!(*w, 6);
+/// } // write lock is dropped here
+/// ```
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+#[cfg_attr(not(test), rustc_diagnostic_item = "NonPoisonRwLock")]
+pub struct RwLock<T: ?Sized> {
+    /// The inner [`sys::RwLock`] that synchronizes thread access to the protected data.
+    inner: sys::RwLock,
+    /// The lock-protected data.
+    data: UnsafeCell<T>,
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+unsafe impl<T: ?Sized + Send> Send for RwLock<T> {}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+// Guards
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+/// RAII structure used to release the shared read access of a lock when
+/// dropped.
+///
+/// This structure is created by the [`read`] and [`try_read`] methods on
+/// [`RwLock`].
+///
+/// [`read`]: RwLock::read
+/// [`try_read`]: RwLock::try_read
+#[must_use = "if unused the RwLock will immediately unlock"]
+#[must_not_suspend = "holding a RwLockReadGuard across suspend \
+                      points can cause deadlocks, delays, \
+                      and cause Futures to not implement `Send`"]
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+#[clippy::has_significant_drop]
+#[cfg_attr(not(test), rustc_diagnostic_item = "NonPoisonRwLockReadGuard")]
+pub struct RwLockReadGuard<'rwlock, T: ?Sized + 'rwlock> {
+    /// A pointer to the data protected by the `RwLock`. Note that we use a pointer here instead of
+    /// `&'rwlock T` to avoid `noalias` violations, because a `RwLockReadGuard` instance only holds
+    /// immutability until it drops, not for its whole scope.
+    /// `NonNull` is preferable over `*const T` to allow for niche optimizations. `NonNull` is also
+    /// covariant over `T`, just like we would have with `&T`.
+    data: NonNull<T>,
+    /// A reference to the internal [`sys::RwLock`] that we have read-locked.
+    inner_lock: &'rwlock sys::RwLock,
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> !Send for RwLockReadGuard<'_, T> {}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+unsafe impl<T: ?Sized + Sync> Sync for RwLockReadGuard<'_, T> {}
+
+/// RAII structure used to release the exclusive write access of a lock when
+/// dropped.
+///
+/// This structure is created by the [`write`] and [`try_write`] methods
+/// on [`RwLock`].
+///
+/// [`write`]: RwLock::write
+/// [`try_write`]: RwLock::try_write
+#[must_use = "if unused the RwLock will immediately unlock"]
+#[must_not_suspend = "holding a RwLockWriteGuard across suspend \
+                      points can cause deadlocks, delays, \
+                      and cause Future's to not implement `Send`"]
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+#[clippy::has_significant_drop]
+#[cfg_attr(not(test), rustc_diagnostic_item = "NonPoisonRwLockWriteGuard")]
+pub struct RwLockWriteGuard<'rwlock, T: ?Sized + 'rwlock> {
+    /// A reference to the [`RwLock`] that we have write-locked.
+    lock: &'rwlock RwLock<T>,
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> !Send for RwLockWriteGuard<'_, T> {}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+unsafe impl<T: ?Sized + Sync> Sync for RwLockWriteGuard<'_, T> {}
+
+/// RAII structure used to release the shared read access of a lock when
+/// dropped, which can point to a subfield of the protected data.
+///
+/// This structure is created by the [`map`] and [`filter_map`] methods
+/// on [`RwLockReadGuard`].
+///
+/// [`map`]: RwLockReadGuard::map
+/// [`filter_map`]: RwLockReadGuard::filter_map
+#[must_use = "if unused the RwLock will immediately unlock"]
+#[must_not_suspend = "holding a MappedRwLockReadGuard across suspend \
+                      points can cause deadlocks, delays, \
+                      and cause Futures to not implement `Send`"]
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+#[clippy::has_significant_drop]
+pub struct MappedRwLockReadGuard<'rwlock, T: ?Sized + 'rwlock> {
+    /// A pointer to the data protected by the `RwLock`. Note that we use a pointer here instead of
+    /// `&'rwlock T` to avoid `noalias` violations, because a `MappedRwLockReadGuard` instance only
+    /// holds immutability until it drops, not for its whole scope.
+    /// `NonNull` is preferable over `*const T` to allow for niche optimizations. `NonNull` is also
+    /// covariant over `T`, just like we would have with `&T`.
+    data: NonNull<T>,
+    /// A reference to the internal [`sys::RwLock`] that we have read-locked.
+    inner_lock: &'rwlock sys::RwLock,
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> !Send for MappedRwLockReadGuard<'_, T> {}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+unsafe impl<T: ?Sized + Sync> Sync for MappedRwLockReadGuard<'_, T> {}
+
+/// RAII structure used to release the exclusive write access of a lock when
+/// dropped, which can point to a subfield of the protected data.
+///
+/// This structure is created by the [`map`] and [`filter_map`] methods
+/// on [`RwLockWriteGuard`].
+///
+/// [`map`]: RwLockWriteGuard::map
+/// [`filter_map`]: RwLockWriteGuard::filter_map
+#[must_use = "if unused the RwLock will immediately unlock"]
+#[must_not_suspend = "holding a MappedRwLockWriteGuard across suspend \
+                      points can cause deadlocks, delays, \
+                      and cause Future's to not implement `Send`"]
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+#[clippy::has_significant_drop]
+pub struct MappedRwLockWriteGuard<'rwlock, T: ?Sized + 'rwlock> {
+    /// A pointer to the data protected by the `RwLock`. Note that we use a pointer here instead of
+    /// `&'rwlock T` to avoid `noalias` violations, because a `MappedRwLockWriteGuard` instance only
+    /// holds uniquneness until it drops, not for its whole scope.
+    /// `NonNull` is preferable over `*const T` to allow for niche optimizations.
+    data: NonNull<T>,
+    /// `NonNull` is covariant over `T`, so we add a `PhantomData<&'rwlock mut T>` field here to
+    /// enforce the correct invariance over `T`.
+    _variance: PhantomData<&'rwlock mut T>,
+    /// A reference to the internal [`sys::RwLock`] that we have write-locked.
+    inner_lock: &'rwlock sys::RwLock,
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> !Send for MappedRwLockWriteGuard<'_, T> {}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+unsafe impl<T: ?Sized + Sync> Sync for MappedRwLockWriteGuard<'_, T> {}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+// Implementations
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+impl<T> RwLock<T> {
+    /// Creates a new instance of an `RwLock<T>` which is unlocked.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    ///
+    /// use std::sync::nonpoison::RwLock;
+    ///
+    /// let lock = RwLock::new(5);
+    /// ```
+    #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    #[inline]
+    pub const fn new(t: T) -> RwLock<T> {
+        RwLock { inner: sys::RwLock::new(), data: UnsafeCell::new(t) }
+    }
+
+    /// Returns the contained value by cloning it.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    /// #![feature(lock_value_accessors)]
+    ///
+    /// use std::sync::nonpoison::RwLock;
+    ///
+    /// let mut lock = RwLock::new(7);
+    ///
+    /// assert_eq!(lock.get_cloned(), 7);
+    /// ```
+    #[unstable(feature = "lock_value_accessors", issue = "133407")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn get_cloned(&self) -> T
+    where
+        T: Clone,
+    {
+        self.read().clone()
+    }
+
+    /// Sets the contained value.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    /// #![feature(lock_value_accessors)]
+    ///
+    /// use std::sync::nonpoison::RwLock;
+    ///
+    /// let mut lock = RwLock::new(7);
+    ///
+    /// assert_eq!(lock.get_cloned(), 7);
+    /// lock.set(11);
+    /// assert_eq!(lock.get_cloned(), 11);
+    /// ```
+    #[unstable(feature = "lock_value_accessors", issue = "133407")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn set(&self, value: T) {
+        if mem::needs_drop::<T>() {
+            // If the contained value has a non-trivial destructor, we
+            // call that destructor after the lock has been released.
+            drop(self.replace(value))
+        } else {
+            *self.write() = value;
+        }
+    }
+
+    /// Replaces the contained value with `value`, and returns the old contained value.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    /// #![feature(lock_value_accessors)]
+    ///
+    /// use std::sync::nonpoison::RwLock;
+    ///
+    /// let mut lock = RwLock::new(7);
+    ///
+    /// assert_eq!(lock.replace(11), 7);
+    /// assert_eq!(lock.get_cloned(), 11);
+    /// ```
+    #[unstable(feature = "lock_value_accessors", issue = "133407")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn replace(&self, value: T) -> T {
+        let mut guard = self.write();
+        mem::replace(&mut *guard, value)
+    }
+}
+
+impl<T: ?Sized> RwLock<T> {
+    /// Locks this `RwLock` with shared read access, blocking the current thread
+    /// until it can be acquired.
+    ///
+    /// The calling thread will be blocked until there are no more writers which
+    /// hold the lock. There may be other readers currently inside the lock when
+    /// this method returns. This method does not provide any guarantees with
+    /// respect to the ordering of whether contentious readers or writers will
+    /// acquire the lock first.
+    ///
+    /// Returns an RAII guard which will release this thread's shared access
+    /// once it is dropped.
+    ///
+    /// # Panics
+    ///
+    /// This function might panic when called if the lock is already held by the current thread.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    ///
+    /// use std::sync::Arc;
+    /// use std::sync::nonpoison::RwLock;
+    /// use std::thread;
+    ///
+    /// let lock = Arc::new(RwLock::new(1));
+    /// let c_lock = Arc::clone(&lock);
+    ///
+    /// let n = lock.read();
+    /// assert_eq!(*n, 1);
+    ///
+    /// thread::spawn(move || {
+    ///     let r = c_lock.read();
+    /// }).join().unwrap();
+    /// ```
+    #[inline]
+    #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn read(&self) -> RwLockReadGuard<'_, T> {
+        unsafe {
+            self.inner.read();
+            RwLockReadGuard::new(self)
+        }
+    }
+
+    /// Attempts to acquire this `RwLock` with shared read access.
+    ///
+    /// If the access could not be granted at this time, then `Err` is returned.
+    /// Otherwise, an RAII guard is returned which will release the shared access
+    /// when it is dropped.
+    ///
+    /// This function does not block.
+    ///
+    /// This function does not provide any guarantees with respect to the ordering
+    /// of whether contentious readers or writers will acquire the lock first.
+    ///
+    /// # Errors
+    ///
+    /// This function will return the [`WouldBlock`] error if the `RwLock` could
+    /// not be acquired because it was already locked exclusively.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    ///
+    /// use std::sync::nonpoison::RwLock;
+    ///
+    /// let lock = RwLock::new(1);
+    ///
+    /// match lock.try_read() {
+    ///     Ok(n) => assert_eq!(*n, 1),
+    ///     Err(_) => unreachable!(),
+    /// };
+    /// ```
+    #[inline]
+    #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn try_read(&self) -> TryLockResult<RwLockReadGuard<'_, T>> {
+        unsafe {
+            if self.inner.try_read() { Ok(RwLockReadGuard::new(self)) } else { Err(WouldBlock) }
+        }
+    }
+
+    /// Locks this `RwLock` with exclusive write access, blocking the current
+    /// thread until it can be acquired.
+    ///
+    /// This function will not return while other writers or other readers
+    /// currently have access to the lock.
+    ///
+    /// Returns an RAII guard which will drop the write access of this `RwLock`
+    /// when dropped.
+    ///
+    /// # Panics
+    ///
+    /// This function might panic when called if the lock is already held by the current thread.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    ///
+    /// use std::sync::nonpoison::RwLock;
+    ///
+    /// let lock = RwLock::new(1);
+    ///
+    /// let mut n = lock.write();
+    /// *n = 2;
+    ///
+    /// assert!(lock.try_read().is_err());
+    /// ```
+    #[inline]
+    #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn write(&self) -> RwLockWriteGuard<'_, T> {
+        unsafe {
+            self.inner.write();
+            RwLockWriteGuard::new(self)
+        }
+    }
+
+    /// Attempts to lock this `RwLock` with exclusive write access.
+    ///
+    /// If the lock could not be acquired at this time, then `Err` is returned.
+    /// Otherwise, an RAII guard is returned which will release the lock when
+    /// it is dropped.
+    ///
+    /// This function does not block.
+    ///
+    /// This function does not provide any guarantees with respect to the ordering
+    /// of whether contentious readers or writers will acquire the lock first.
+    ///
+    /// # Errors
+    ///
+    /// This function will return the [`WouldBlock`] error if the `RwLock` could
+    /// not be acquired because it was already locked.
+    ///
+    /// [`WouldBlock`]: WouldBlock
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    ///
+    /// use std::sync::nonpoison::RwLock;
+    ///
+    /// let lock = RwLock::new(1);
+    ///
+    /// let n = lock.read();
+    /// assert_eq!(*n, 1);
+    ///
+    /// assert!(lock.try_write().is_err());
+    /// ```
+    #[inline]
+    #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn try_write(&self) -> TryLockResult<RwLockWriteGuard<'_, T>> {
+        unsafe {
+            if self.inner.try_write() { Ok(RwLockWriteGuard::new(self)) } else { Err(WouldBlock) }
+        }
+    }
+
+    /// Consumes this `RwLock`, returning the underlying data.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    ///
+    /// use std::sync::nonpoison::RwLock;
+    ///
+    /// let lock = RwLock::new(String::new());
+    /// {
+    ///     let mut s = lock.write();
+    ///     *s = "modified".to_owned();
+    /// }
+    /// assert_eq!(lock.into_inner(), "modified");
+    /// ```
+    #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn into_inner(self) -> T
+    where
+        T: Sized,
+    {
+        self.data.into_inner()
+    }
+
+    /// Returns a mutable reference to the underlying data.
+    ///
+    /// Since this call borrows the `RwLock` mutably, no actual locking needs to
+    /// take place -- the mutable borrow statically guarantees no new locks can be acquired
+    /// while this reference exists. Note that this method does not clear any previously abandoned
+    /// locks (e.g., via [`forget()`] on a [`RwLockReadGuard`] or [`RwLockWriteGuard`]).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    ///
+    /// use std::sync::nonpoison::RwLock;
+    ///
+    /// let mut lock = RwLock::new(0);
+    /// *lock.get_mut() = 10;
+    /// assert_eq!(*lock.read(), 10);
+    /// ```
+    #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn get_mut(&mut self) -> &mut T {
+        self.data.get_mut()
+    }
+
+    /// Returns a raw pointer to the underlying data.
+    ///
+    /// The returned pointer is always non-null and properly aligned, but it is
+    /// the user's responsibility to ensure that any reads and writes through it
+    /// are properly synchronized to avoid data races, and that it is not read
+    /// or written through after the lock is dropped.
+    #[unstable(feature = "rwlock_data_ptr", issue = "140368")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn data_ptr(&self) -> *mut T {
+        self.data.get()
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLock<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut d = f.debug_struct("RwLock");
+        match self.try_read() {
+            Ok(guard) => {
+                d.field("data", &&*guard);
+            }
+            Err(WouldBlock) => {
+                d.field("data", &format_args!("<locked>"));
+            }
+        }
+        d.finish_non_exhaustive()
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: Default> Default for RwLock<T> {
+    /// Creates a new `RwLock<T>`, with the `Default` value for T.
+    fn default() -> RwLock<T> {
+        RwLock::new(Default::default())
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T> From<T> for RwLock<T> {
+    /// Creates a new instance of an `RwLock<T>` which is unlocked.
+    /// This is equivalent to [`RwLock::new`].
+    fn from(t: T) -> Self {
+        RwLock::new(t)
+    }
+}
+
+impl<'rwlock, T: ?Sized> RwLockReadGuard<'rwlock, T> {
+    /// Creates a new instance of `RwLockReadGuard<T>` from a `RwLock<T>`.
+    ///
+    /// # Safety
+    ///
+    /// This function is safe if and only if the same thread has successfully and safely called
+    /// `lock.inner.read()`, `lock.inner.try_read()`, or `lock.inner.downgrade()` before
+    /// instantiating this object.
+    unsafe fn new(lock: &'rwlock RwLock<T>) -> RwLockReadGuard<'rwlock, T> {
+        RwLockReadGuard {
+            data: unsafe { NonNull::new_unchecked(lock.data.get()) },
+            inner_lock: &lock.inner,
+        }
+    }
+
+    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data, e.g.
+    /// an enum variant.
+    ///
+    /// The `RwLock` is already locked for reading, so this cannot fail.
+    ///
+    /// This is an associated function that needs to be used as
+    /// `RwLockReadGuard::map(...)`. A method would interfere with methods of
+    /// the same name on the contents of the `RwLockReadGuard` used through
+    /// `Deref`.
+    ///
+    /// # Panics
+    ///
+    /// If the closure panics, the guard will be dropped (unlocked).
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockReadGuard<'rwlock, U>
+    where
+        F: FnOnce(&T) -> &U,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        let data = NonNull::from(f(unsafe { orig.data.as_ref() }));
+        let orig = ManuallyDrop::new(orig);
+        MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock }
+    }
+
+    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data. The
+    /// original guard is returned as an `Err(...)` if the closure returns
+    /// `None`.
+    ///
+    /// The `RwLock` is already locked for reading, so this cannot fail.
+    ///
+    /// This is an associated function that needs to be used as
+    /// `RwLockReadGuard::filter_map(...)`. A method would interfere with methods
+    /// of the same name on the contents of the `RwLockReadGuard` used through
+    /// `Deref`.
+    ///
+    /// # Panics
+    ///
+    /// If the closure panics, the guard will be dropped (unlocked).
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn filter_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockReadGuard<'rwlock, U>, Self>
+    where
+        F: FnOnce(&T) -> Option<&U>,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        match f(unsafe { orig.data.as_ref() }) {
+            Some(data) => {
+                let data = NonNull::from(data);
+                let orig = ManuallyDrop::new(orig);
+                Ok(MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock })
+            }
+            None => Err(orig),
+        }
+    }
+}
+
+impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
+    /// Creates a new instance of `RwLockWriteGuard<T>` from a `RwLock<T>`.
+    ///
+    /// # Safety
+    ///
+    /// This function is safe if and only if the same thread has successfully and safely called
+    /// `lock.inner.write()`, `lock.inner.try_write()`, or `lock.inner.try_upgrade` before
+    /// instantiating this object.
+    unsafe fn new(lock: &'rwlock RwLock<T>) -> RwLockWriteGuard<'rwlock, T> {
+        RwLockWriteGuard { lock }
+    }
+
+    /// Downgrades a write-locked `RwLockWriteGuard` into a read-locked [`RwLockReadGuard`].
+    ///
+    /// Since we have the `RwLockWriteGuard`, the [`RwLock`] must already be locked for writing, so
+    /// this method cannot fail.
+    ///
+    /// After downgrading, other readers will be allowed to read the protected data.
+    ///
+    /// # Examples
+    ///
+    /// `downgrade` takes ownership of the `RwLockWriteGuard` and returns a [`RwLockReadGuard`].
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    /// #![feature(rwlock_downgrade)]
+    ///
+    /// use std::sync::nonpoison::{RwLock, RwLockWriteGuard};
+    ///
+    /// let rw = RwLock::new(0);
+    ///
+    /// let mut write_guard = rw.write();
+    /// *write_guard = 42;
+    ///
+    /// let read_guard = RwLockWriteGuard::downgrade(write_guard);
+    /// assert_eq!(42, *read_guard);
+    /// ```
+    ///
+    /// `downgrade` will _atomically_ change the state of the [`RwLock`] from exclusive mode into
+    /// shared mode. This means that it is impossible for another writing thread to get in between a
+    /// thread calling `downgrade` and any reads it performs after downgrading.
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    /// #![feature(rwlock_downgrade)]
+    ///
+    /// use std::sync::Arc;
+    /// use std::sync::nonpoison::{RwLock, RwLockWriteGuard};
+    ///
+    /// let rw = Arc::new(RwLock::new(1));
+    ///
+    /// // Put the lock in write mode.
+    /// let mut main_write_guard = rw.write();
+    ///
+    /// let rw_clone = rw.clone();
+    /// let evil_handle = std::thread::spawn(move || {
+    ///     // This will not return until the main thread drops the `main_read_guard`.
+    ///     let mut evil_guard = rw_clone.write();
+    ///
+    ///     assert_eq!(*evil_guard, 2);
+    ///     *evil_guard = 3;
+    /// });
+    ///
+    /// *main_write_guard = 2;
+    ///
+    /// // Atomically downgrade the write guard into a read guard.
+    /// let main_read_guard = RwLockWriteGuard::downgrade(main_write_guard);
+    ///
+    /// // Since `downgrade` is atomic, the writer thread cannot have changed the protected data.
+    /// assert_eq!(*main_read_guard, 2, "`downgrade` was not atomic");
+    /// #
+    /// # drop(main_read_guard);
+    /// # evil_handle.join().unwrap();
+    /// #
+    /// # let final_check = rw.read();
+    /// # assert_eq!(*final_check, 3);
+    /// ```
+    #[unstable(feature = "rwlock_downgrade", issue = "128203")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn downgrade(s: Self) -> RwLockReadGuard<'rwlock, T> {
+        let lock = s.lock;
+
+        // We don't want to call the destructor since that calls `write_unlock`.
+        forget(s);
+
+        // SAFETY: We take ownership of a write guard, so we must already have the `RwLock` in write
+        // mode, satisfying the `downgrade` contract.
+        unsafe { lock.inner.downgrade() };
+
+        // SAFETY: We have just successfully called `downgrade`, so we fulfill the safety contract.
+        unsafe { RwLockReadGuard::new(lock) }
+    }
+
+    /// Makes a [`MappedRwLockWriteGuard`] for a component of the borrowed data, e.g.
+    /// an enum variant.
+    ///
+    /// The `RwLock` is already locked for writing, so this cannot fail.
+    ///
+    /// This is an associated function that needs to be used as
+    /// `RwLockWriteGuard::map(...)`. A method would interfere with methods of
+    /// the same name on the contents of the `RwLockWriteGuard` used through
+    /// `Deref`.
+    ///
+    /// # Panics
+    ///
+    /// If the closure panics, the guard will be dropped (unlocked).
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockWriteGuard<'rwlock, U>
+    where
+        F: FnOnce(&mut T) -> &mut U,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        let data = NonNull::from(f(unsafe { &mut *orig.lock.data.get() }));
+        let orig = ManuallyDrop::new(orig);
+        MappedRwLockWriteGuard { data, inner_lock: &orig.lock.inner, _variance: PhantomData }
+    }
+
+    /// Makes a [`MappedRwLockWriteGuard`] for a component of the borrowed data. The
+    /// original guard is returned as an `Err(...)` if the closure returns
+    /// `None`.
+    ///
+    /// The `RwLock` is already locked for writing, so this cannot fail.
+    ///
+    /// This is an associated function that needs to be used as
+    /// `RwLockWriteGuard::filter_map(...)`. A method would interfere with methods
+    /// of the same name on the contents of the `RwLockWriteGuard` used through
+    /// `Deref`.
+    ///
+    /// # Panics
+    ///
+    /// If the closure panics, the guard will be dropped (unlocked).
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn filter_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockWriteGuard<'rwlock, U>, Self>
+    where
+        F: FnOnce(&mut T) -> Option<&mut U>,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        match f(unsafe { &mut *orig.lock.data.get() }) {
+            Some(data) => {
+                let data = NonNull::from(data);
+                let orig = ManuallyDrop::new(orig);
+                Ok(MappedRwLockWriteGuard {
+                    data,
+                    inner_lock: &orig.lock.inner,
+                    _variance: PhantomData,
+                })
+            }
+            None => Err(orig),
+        }
+    }
+}
+
+impl<'rwlock, T: ?Sized> MappedRwLockReadGuard<'rwlock, T> {
+    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data,
+    /// e.g. an enum variant.
+    ///
+    /// The `RwLock` is already locked for reading, so this cannot fail.
+    ///
+    /// This is an associated function that needs to be used as
+    /// `MappedRwLockReadGuard::map(...)`. A method would interfere with
+    /// methods of the same name on the contents of the `MappedRwLockReadGuard`
+    /// used through `Deref`.
+    ///
+    /// # Panics
+    ///
+    /// If the closure panics, the guard will be dropped (unlocked).
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockReadGuard<'rwlock, U>
+    where
+        F: FnOnce(&T) -> &U,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        let data = NonNull::from(f(unsafe { orig.data.as_ref() }));
+        let orig = ManuallyDrop::new(orig);
+        MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock }
+    }
+
+    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data.
+    /// The original guard is returned as an `Err(...)` if the closure returns
+    /// `None`.
+    ///
+    /// The `RwLock` is already locked for reading, so this cannot fail.
+    ///
+    /// This is an associated function that needs to be used as
+    /// `MappedRwLockReadGuard::filter_map(...)`. A method would interfere with
+    /// methods of the same name on the contents of the `MappedRwLockReadGuard`
+    /// used through `Deref`.
+    ///
+    /// # Panics
+    ///
+    /// If the closure panics, the guard will be dropped (unlocked).
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn filter_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockReadGuard<'rwlock, U>, Self>
+    where
+        F: FnOnce(&T) -> Option<&U>,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        match f(unsafe { orig.data.as_ref() }) {
+            Some(data) => {
+                let data = NonNull::from(data);
+                let orig = ManuallyDrop::new(orig);
+                Ok(MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock })
+            }
+            None => Err(orig),
+        }
+    }
+}
+
+impl<'rwlock, T: ?Sized> MappedRwLockWriteGuard<'rwlock, T> {
+    /// Makes a [`MappedRwLockWriteGuard`] for a component of the borrowed data,
+    /// e.g. an enum variant.
+    ///
+    /// The `RwLock` is already locked for writing, so this cannot fail.
+    ///
+    /// This is an associated function that needs to be used as
+    /// `MappedRwLockWriteGuard::map(...)`. A method would interfere with
+    /// methods of the same name on the contents of the `MappedRwLockWriteGuard`
+    /// used through `Deref`.
+    ///
+    /// # Panics
+    ///
+    /// If the closure panics, the guard will be dropped (unlocked).
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn map<U, F>(mut orig: Self, f: F) -> MappedRwLockWriteGuard<'rwlock, U>
+    where
+        F: FnOnce(&mut T) -> &mut U,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        let data = NonNull::from(f(unsafe { orig.data.as_mut() }));
+        let orig = ManuallyDrop::new(orig);
+        MappedRwLockWriteGuard { data, inner_lock: orig.inner_lock, _variance: PhantomData }
+    }
+
+    /// Makes a [`MappedRwLockWriteGuard`] for a component of the borrowed data.
+    /// The original guard is returned as an `Err(...)` if the closure returns
+    /// `None`.
+    ///
+    /// The `RwLock` is already locked for writing, so this cannot fail.
+    ///
+    /// This is an associated function that needs to be used as
+    /// `MappedRwLockWriteGuard::filter_map(...)`. A method would interfere with
+    /// methods of the same name on the contents of the `MappedRwLockWriteGuard`
+    /// used through `Deref`.
+    ///
+    /// # Panics
+    ///
+    /// If the closure panics, the guard will be dropped (unlocked).
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn filter_map<U, F>(
+        mut orig: Self,
+        f: F,
+    ) -> Result<MappedRwLockWriteGuard<'rwlock, U>, Self>
+    where
+        F: FnOnce(&mut T) -> Option<&mut U>,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        match f(unsafe { orig.data.as_mut() }) {
+            Some(data) => {
+                let data = NonNull::from(data);
+                let orig = ManuallyDrop::new(orig);
+                Ok(MappedRwLockWriteGuard {
+                    data,
+                    inner_lock: orig.inner_lock,
+                    _variance: PhantomData,
+                })
+            }
+            None => Err(orig),
+        }
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> Drop for RwLockReadGuard<'_, T> {
+    fn drop(&mut self) {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when created.
+        unsafe {
+            self.inner_lock.read_unlock();
+        }
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> Drop for RwLockWriteGuard<'_, T> {
+    fn drop(&mut self) {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
+        unsafe {
+            self.lock.inner.write_unlock();
+        }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> Drop for MappedRwLockReadGuard<'_, T> {
+    fn drop(&mut self) {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe {
+            self.inner_lock.read_unlock();
+        }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> Drop for MappedRwLockWriteGuard<'_, T> {
+    fn drop(&mut self) {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe {
+            self.inner_lock.write_unlock();
+        }
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> Deref for RwLockReadGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when created.
+        unsafe { self.data.as_ref() }
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> Deref for RwLockWriteGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
+        unsafe { &*self.lock.data.get() }
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> DerefMut for RwLockWriteGuard<'_, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
+        unsafe { &mut *self.lock.data.get() }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> Deref for MappedRwLockReadGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe { self.data.as_ref() }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> Deref for MappedRwLockWriteGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe { self.data.as_ref() }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> DerefMut for MappedRwLockWriteGuard<'_, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe { self.data.as_mut() }
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockReadGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized + fmt::Display> fmt::Display for RwLockReadGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockWriteGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized + fmt::Display> fmt::Display for RwLockWriteGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized + fmt::Debug> fmt::Debug for MappedRwLockReadGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized + fmt::Display> fmt::Display for MappedRwLockReadGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized + fmt::Debug> fmt::Debug for MappedRwLockWriteGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized + fmt::Display> fmt::Display for MappedRwLockWriteGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
diff --git a/library/std/src/sync/poison/rwlock.rs b/library/std/src/sync/poison/rwlock.rs
index 2c92602bc87..0a463f3f9c7 100644
--- a/library/std/src/sync/poison/rwlock.rs
+++ b/library/std/src/sync/poison/rwlock.rs
@@ -80,16 +80,24 @@ use crate::sys::sync as sys;
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "RwLock")]
 pub struct RwLock<T: ?Sized> {
+    /// The inner [`sys::RwLock`] that synchronizes thread access to the protected data.
     inner: sys::RwLock,
+    /// A flag denoting if this `RwLock` has been poisoned.
     poison: poison::Flag,
+    /// The lock-protected data.
     data: UnsafeCell<T>,
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T: ?Sized + Send> Send for RwLock<T> {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}
 
+////////////////////////////////////////////////////////////////////////////////////////////////////
+// Guards
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
 /// RAII structure used to release the shared read access of a lock when
 /// dropped.
 ///
@@ -105,13 +113,15 @@ unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}
 #[stable(feature = "rust1", since = "1.0.0")]
 #[clippy::has_significant_drop]
 #[cfg_attr(not(test), rustc_diagnostic_item = "RwLockReadGuard")]
-pub struct RwLockReadGuard<'a, T: ?Sized + 'a> {
-    // NB: we use a pointer instead of `&'a T` to avoid `noalias` violations, because a
-    // `RwLockReadGuard` argument doesn't hold immutability for its whole scope, only until it drops.
-    // `NonNull` is also covariant over `T`, just like we would have with `&T`. `NonNull`
-    // is preferable over `const* T` to allow for niche optimization.
+pub struct RwLockReadGuard<'rwlock, T: ?Sized + 'rwlock> {
+    /// A pointer to the data protected by the `RwLock`. Note that we use a pointer here instead of
+    /// `&'rwlock T` to avoid `noalias` violations, because a `RwLockReadGuard` instance only holds
+    /// immutability until it drops, not for its whole scope.
+    /// `NonNull` is preferable over `*const T` to allow for niche optimizations. `NonNull` is also
+    /// covariant over `T`, just like we would have with `&T`.
     data: NonNull<T>,
-    inner_lock: &'a sys::RwLock,
+    /// A reference to the internal [`sys::RwLock`] that we have read-locked.
+    inner_lock: &'rwlock sys::RwLock,
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -135,8 +145,10 @@ unsafe impl<T: ?Sized + Sync> Sync for RwLockReadGuard<'_, T> {}
 #[stable(feature = "rust1", since = "1.0.0")]
 #[clippy::has_significant_drop]
 #[cfg_attr(not(test), rustc_diagnostic_item = "RwLockWriteGuard")]
-pub struct RwLockWriteGuard<'a, T: ?Sized + 'a> {
-    lock: &'a RwLock<T>,
+pub struct RwLockWriteGuard<'rwlock, T: ?Sized + 'rwlock> {
+    /// A reference to the [`RwLock`] that we have write-locked.
+    lock: &'rwlock RwLock<T>,
+    /// The poison guard. See the [`poison`] module for more information.
     poison: poison::Guard,
 }
 
@@ -160,13 +172,15 @@ unsafe impl<T: ?Sized + Sync> Sync for RwLockWriteGuard<'_, T> {}
                       and cause Futures to not implement `Send`"]
 #[unstable(feature = "mapped_lock_guards", issue = "117108")]
 #[clippy::has_significant_drop]
-pub struct MappedRwLockReadGuard<'a, T: ?Sized + 'a> {
-    // NB: we use a pointer instead of `&'a T` to avoid `noalias` violations, because a
-    // `MappedRwLockReadGuard` argument doesn't hold immutability for its whole scope, only until it drops.
-    // `NonNull` is also covariant over `T`, just like we would have with `&T`. `NonNull`
-    // is preferable over `const* T` to allow for niche optimization.
+pub struct MappedRwLockReadGuard<'rwlock, T: ?Sized + 'rwlock> {
+    /// A pointer to the data protected by the `RwLock`. Note that we use a pointer here instead of
+    /// `&'rwlock T` to avoid `noalias` violations, because a `MappedRwLockReadGuard` instance only
+    /// holds immutability until it drops, not for its whole scope.
+    /// `NonNull` is preferable over `*const T` to allow for niche optimizations. `NonNull` is also
+    /// covariant over `T`, just like we would have with `&T`.
     data: NonNull<T>,
-    inner_lock: &'a sys::RwLock,
+    /// A reference to the internal [`sys::RwLock`] that we have read-locked.
+    inner_lock: &'rwlock sys::RwLock,
 }
 
 #[unstable(feature = "mapped_lock_guards", issue = "117108")]
@@ -189,16 +203,21 @@ unsafe impl<T: ?Sized + Sync> Sync for MappedRwLockReadGuard<'_, T> {}
                       and cause Future's to not implement `Send`"]
 #[unstable(feature = "mapped_lock_guards", issue = "117108")]
 #[clippy::has_significant_drop]
-pub struct MappedRwLockWriteGuard<'a, T: ?Sized + 'a> {
-    // NB: we use a pointer instead of `&'a mut T` to avoid `noalias` violations, because a
-    // `MappedRwLockWriteGuard` argument doesn't hold uniqueness for its whole scope, only until it drops.
-    // `NonNull` is covariant over `T`, so we add a `PhantomData<&'a mut T>` field
-    // below for the correct variance over `T` (invariance).
+pub struct MappedRwLockWriteGuard<'rwlock, T: ?Sized + 'rwlock> {
+    /// A pointer to the data protected by the `RwLock`. Note that we use a pointer here instead of
+    /// `&'rwlock T` to avoid `noalias` violations, because a `MappedRwLockWriteGuard` instance only
+    /// holds uniquneness until it drops, not for its whole scope.
+    /// `NonNull` is preferable over `*const T` to allow for niche optimizations.
     data: NonNull<T>,
-    inner_lock: &'a sys::RwLock,
-    poison_flag: &'a poison::Flag,
-    poison: poison::Guard,
-    _variance: PhantomData<&'a mut T>,
+    /// `NonNull` is covariant over `T`, so we add a `PhantomData<&'rwlock mut T>` field here to
+    /// enforce the correct invariance over `T`.
+    _variance: PhantomData<&'rwlock mut T>,
+    /// A reference to the internal [`sys::RwLock`] that we have write-locked.
+    inner_lock: &'rwlock sys::RwLock,
+    /// A reference to the original `RwLock`'s poison state.
+    poison_flag: &'rwlock poison::Flag,
+    /// The poison guard. See the [`poison`] module for more information.
+    poison_guard: poison::Guard,
 }
 
 #[unstable(feature = "mapped_lock_guards", issue = "117108")]
@@ -207,6 +226,10 @@ impl<T: ?Sized> !Send for MappedRwLockWriteGuard<'_, T> {}
 #[unstable(feature = "mapped_lock_guards", issue = "117108")]
 unsafe impl<T: ?Sized + Sync> Sync for MappedRwLockWriteGuard<'_, T> {}
 
+////////////////////////////////////////////////////////////////////////////////////////////////////
+// Implementations
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
 impl<T> RwLock<T> {
     /// Creates a new instance of an `RwLock<T>` which is unlocked.
     ///
@@ -611,8 +634,8 @@ impl<T: ?Sized> RwLock<T> {
     ///
     /// Since this call borrows the `RwLock` mutably, no actual locking needs to
     /// take place -- the mutable borrow statically guarantees no new locks can be acquired
-    /// while this reference exists. Note that this method does not clear any previously abandoned locks
-    /// (e.g., via [`forget()`] on a [`RwLockReadGuard`] or [`RwLockWriteGuard`]).
+    /// while this reference exists. Note that this method does not clear any previously abandoned
+    /// locks (e.g., via [`forget()`] on a [`RwLockReadGuard`] or [`RwLockWriteGuard`]).
     ///
     /// # Errors
     ///
@@ -700,177 +723,7 @@ impl<'rwlock, T: ?Sized> RwLockReadGuard<'rwlock, T> {
             inner_lock: &lock.inner,
         })
     }
-}
-
-impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
-    /// Creates a new instance of `RwLockWriteGuard<T>` from a `RwLock<T>`.
-    // SAFETY: if and only if `lock.inner.write()` (or `lock.inner.try_write()`) has been
-    // successfully called from the same thread before instantiating this object.
-    unsafe fn new(lock: &'rwlock RwLock<T>) -> LockResult<RwLockWriteGuard<'rwlock, T>> {
-        poison::map_result(lock.poison.guard(), |guard| RwLockWriteGuard { lock, poison: guard })
-    }
-}
-
-#[stable(feature = "std_debug", since = "1.16.0")]
-impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockReadGuard<'_, T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        (**self).fmt(f)
-    }
-}
-
-#[stable(feature = "std_guard_impls", since = "1.20.0")]
-impl<T: ?Sized + fmt::Display> fmt::Display for RwLockReadGuard<'_, T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        (**self).fmt(f)
-    }
-}
-
-#[stable(feature = "std_debug", since = "1.16.0")]
-impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockWriteGuard<'_, T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        (**self).fmt(f)
-    }
-}
-
-#[stable(feature = "std_guard_impls", since = "1.20.0")]
-impl<T: ?Sized + fmt::Display> fmt::Display for RwLockWriteGuard<'_, T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        (**self).fmt(f)
-    }
-}
-
-#[unstable(feature = "mapped_lock_guards", issue = "117108")]
-impl<T: ?Sized + fmt::Debug> fmt::Debug for MappedRwLockReadGuard<'_, T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        (**self).fmt(f)
-    }
-}
-
-#[unstable(feature = "mapped_lock_guards", issue = "117108")]
-impl<T: ?Sized + fmt::Display> fmt::Display for MappedRwLockReadGuard<'_, T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        (**self).fmt(f)
-    }
-}
-
-#[unstable(feature = "mapped_lock_guards", issue = "117108")]
-impl<T: ?Sized + fmt::Debug> fmt::Debug for MappedRwLockWriteGuard<'_, T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        (**self).fmt(f)
-    }
-}
-
-#[unstable(feature = "mapped_lock_guards", issue = "117108")]
-impl<T: ?Sized + fmt::Display> fmt::Display for MappedRwLockWriteGuard<'_, T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        (**self).fmt(f)
-    }
-}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> Deref for RwLockReadGuard<'_, T> {
-    type Target = T;
-
-    fn deref(&self) -> &T {
-        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when created.
-        unsafe { self.data.as_ref() }
-    }
-}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> Deref for RwLockWriteGuard<'_, T> {
-    type Target = T;
-
-    fn deref(&self) -> &T {
-        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
-        unsafe { &*self.lock.data.get() }
-    }
-}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> DerefMut for RwLockWriteGuard<'_, T> {
-    fn deref_mut(&mut self) -> &mut T {
-        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
-        unsafe { &mut *self.lock.data.get() }
-    }
-}
-
-#[unstable(feature = "mapped_lock_guards", issue = "117108")]
-impl<T: ?Sized> Deref for MappedRwLockReadGuard<'_, T> {
-    type Target = T;
-
-    fn deref(&self) -> &T {
-        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
-        // was created, and have been upheld throughout `map` and/or `filter_map`.
-        unsafe { self.data.as_ref() }
-    }
-}
-
-#[unstable(feature = "mapped_lock_guards", issue = "117108")]
-impl<T: ?Sized> Deref for MappedRwLockWriteGuard<'_, T> {
-    type Target = T;
-
-    fn deref(&self) -> &T {
-        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
-        // was created, and have been upheld throughout `map` and/or `filter_map`.
-        unsafe { self.data.as_ref() }
-    }
-}
-
-#[unstable(feature = "mapped_lock_guards", issue = "117108")]
-impl<T: ?Sized> DerefMut for MappedRwLockWriteGuard<'_, T> {
-    fn deref_mut(&mut self) -> &mut T {
-        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
-        // was created, and have been upheld throughout `map` and/or `filter_map`.
-        unsafe { self.data.as_mut() }
-    }
-}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> Drop for RwLockReadGuard<'_, T> {
-    fn drop(&mut self) {
-        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when created.
-        unsafe {
-            self.inner_lock.read_unlock();
-        }
-    }
-}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> Drop for RwLockWriteGuard<'_, T> {
-    fn drop(&mut self) {
-        self.lock.poison.done(&self.poison);
-        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
-        unsafe {
-            self.lock.inner.write_unlock();
-        }
-    }
-}
-
-#[unstable(feature = "mapped_lock_guards", issue = "117108")]
-impl<T: ?Sized> Drop for MappedRwLockReadGuard<'_, T> {
-    fn drop(&mut self) {
-        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
-        // was created, and have been upheld throughout `map` and/or `filter_map`.
-        unsafe {
-            self.inner_lock.read_unlock();
-        }
-    }
-}
-
-#[unstable(feature = "mapped_lock_guards", issue = "117108")]
-impl<T: ?Sized> Drop for MappedRwLockWriteGuard<'_, T> {
-    fn drop(&mut self) {
-        self.poison_flag.done(&self.poison);
-        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
-        // was created, and have been upheld throughout `map` and/or `filter_map`.
-        unsafe {
-            self.inner_lock.write_unlock();
-        }
-    }
-}
 
-impl<'a, T: ?Sized> RwLockReadGuard<'a, T> {
     /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data, e.g.
     /// an enum variant.
     ///
@@ -883,17 +736,18 @@ impl<'a, T: ?Sized> RwLockReadGuard<'a, T> {
     ///
     /// # Panics
     ///
-    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be poisoned.
+    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be
+    /// poisoned.
     #[unstable(feature = "mapped_lock_guards", issue = "117108")]
-    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockReadGuard<'a, U>
+    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockReadGuard<'rwlock, U>
     where
         F: FnOnce(&T) -> &U,
         U: ?Sized,
     {
         // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
         // was created, and have been upheld throughout `map` and/or `filter_map`.
-        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
-        // passed to it. If the closure panics, the guard will be dropped.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
         let data = NonNull::from(f(unsafe { orig.data.as_ref() }));
         let orig = ManuallyDrop::new(orig);
         MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock }
@@ -912,17 +766,18 @@ impl<'a, T: ?Sized> RwLockReadGuard<'a, T> {
     ///
     /// # Panics
     ///
-    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be poisoned.
+    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be
+    /// poisoned.
     #[unstable(feature = "mapped_lock_guards", issue = "117108")]
-    pub fn filter_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockReadGuard<'a, U>, Self>
+    pub fn filter_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockReadGuard<'rwlock, U>, Self>
     where
         F: FnOnce(&T) -> Option<&U>,
         U: ?Sized,
     {
         // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
         // was created, and have been upheld throughout `map` and/or `filter_map`.
-        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
-        // passed to it. If the closure panics, the guard will be dropped.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
         match f(unsafe { orig.data.as_ref() }) {
             Some(data) => {
                 let data = NonNull::from(data);
@@ -934,71 +789,95 @@ impl<'a, T: ?Sized> RwLockReadGuard<'a, T> {
     }
 }
 
-impl<'a, T: ?Sized> MappedRwLockReadGuard<'a, T> {
-    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data,
-    /// e.g. an enum variant.
-    ///
-    /// The `RwLock` is already locked for reading, so this cannot fail.
-    ///
-    /// This is an associated function that needs to be used as
-    /// `MappedRwLockReadGuard::map(...)`. A method would interfere with
-    /// methods of the same name on the contents of the `MappedRwLockReadGuard`
-    /// used through `Deref`.
+impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
+    /// Creates a new instance of `RwLockWriteGuard<T>` from a `RwLock<T>`.
     ///
-    /// # Panics
+    /// # Safety
     ///
-    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be poisoned.
-    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
-    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockReadGuard<'a, U>
-    where
-        F: FnOnce(&T) -> &U,
-        U: ?Sized,
-    {
-        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
-        // was created, and have been upheld throughout `map` and/or `filter_map`.
-        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
-        // passed to it. If the closure panics, the guard will be dropped.
-        let data = NonNull::from(f(unsafe { orig.data.as_ref() }));
-        let orig = ManuallyDrop::new(orig);
-        MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock }
+    /// This function is safe if and only if the same thread has successfully and safely called
+    /// `lock.inner.write()`, `lock.inner.try_write()`, or `lock.inner.try_upgrade` before
+    /// instantiating this object.
+    unsafe fn new(lock: &'rwlock RwLock<T>) -> LockResult<RwLockWriteGuard<'rwlock, T>> {
+        poison::map_result(lock.poison.guard(), |guard| RwLockWriteGuard { lock, poison: guard })
     }
 
-    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data.
-    /// The original guard is returned as an `Err(...)` if the closure returns
-    /// `None`.
+    /// Downgrades a write-locked `RwLockWriteGuard` into a read-locked [`RwLockReadGuard`].
     ///
-    /// The `RwLock` is already locked for reading, so this cannot fail.
+    /// Since we have the `RwLockWriteGuard`, the [`RwLock`] must already be locked for writing, so
+    /// this method cannot fail.
     ///
-    /// This is an associated function that needs to be used as
-    /// `MappedRwLockReadGuard::filter_map(...)`. A method would interfere with
-    /// methods of the same name on the contents of the `MappedRwLockReadGuard`
-    /// used through `Deref`.
+    /// After downgrading, other readers will be allowed to read the protected data.
     ///
-    /// # Panics
+    /// # Examples
     ///
-    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be poisoned.
-    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
-    pub fn filter_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockReadGuard<'a, U>, Self>
-    where
-        F: FnOnce(&T) -> Option<&U>,
-        U: ?Sized,
-    {
-        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
-        // was created, and have been upheld throughout `map` and/or `filter_map`.
-        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
-        // passed to it. If the closure panics, the guard will be dropped.
-        match f(unsafe { orig.data.as_ref() }) {
-            Some(data) => {
-                let data = NonNull::from(data);
-                let orig = ManuallyDrop::new(orig);
-                Ok(MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock })
-            }
-            None => Err(orig),
-        }
+    /// `downgrade` takes ownership of the `RwLockWriteGuard` and returns a [`RwLockReadGuard`].
+    ///
+    /// ```
+    /// #![feature(rwlock_downgrade)]
+    ///
+    /// use std::sync::{RwLock, RwLockWriteGuard};
+    ///
+    /// let rw = RwLock::new(0);
+    ///
+    /// let mut write_guard = rw.write().unwrap();
+    /// *write_guard = 42;
+    ///
+    /// let read_guard = RwLockWriteGuard::downgrade(write_guard);
+    /// assert_eq!(42, *read_guard);
+    /// ```
+    ///
+    /// `downgrade` will _atomically_ change the state of the [`RwLock`] from exclusive mode into
+    /// shared mode. This means that it is impossible for another writing thread to get in between a
+    /// thread calling `downgrade` and any reads it performs after downgrading.
+    ///
+    /// ```
+    /// #![feature(rwlock_downgrade)]
+    ///
+    /// use std::sync::{Arc, RwLock, RwLockWriteGuard};
+    ///
+    /// let rw = Arc::new(RwLock::new(1));
+    ///
+    /// // Put the lock in write mode.
+    /// let mut main_write_guard = rw.write().unwrap();
+    ///
+    /// let rw_clone = rw.clone();
+    /// let evil_handle = std::thread::spawn(move || {
+    ///     // This will not return until the main thread drops the `main_read_guard`.
+    ///     let mut evil_guard = rw_clone.write().unwrap();
+    ///
+    ///     assert_eq!(*evil_guard, 2);
+    ///     *evil_guard = 3;
+    /// });
+    ///
+    /// *main_write_guard = 2;
+    ///
+    /// // Atomically downgrade the write guard into a read guard.
+    /// let main_read_guard = RwLockWriteGuard::downgrade(main_write_guard);
+    ///
+    /// // Since `downgrade` is atomic, the writer thread cannot have changed the protected data.
+    /// assert_eq!(*main_read_guard, 2, "`downgrade` was not atomic");
+    /// #
+    /// # drop(main_read_guard);
+    /// # evil_handle.join().unwrap();
+    /// #
+    /// # let final_check = rw.read().unwrap();
+    /// # assert_eq!(*final_check, 3);
+    /// ```
+    #[unstable(feature = "rwlock_downgrade", issue = "128203")]
+    pub fn downgrade(s: Self) -> RwLockReadGuard<'rwlock, T> {
+        let lock = s.lock;
+
+        // We don't want to call the destructor since that calls `write_unlock`.
+        forget(s);
+
+        // SAFETY: We take ownership of a write guard, so we must already have the `RwLock` in write
+        // mode, satisfying the `downgrade` contract.
+        unsafe { lock.inner.downgrade() };
+
+        // SAFETY: We have just successfully called `downgrade`, so we fulfill the safety contract.
+        unsafe { RwLockReadGuard::new(lock).unwrap_or_else(PoisonError::into_inner) }
     }
-}
 
-impl<'a, T: ?Sized> RwLockWriteGuard<'a, T> {
     /// Makes a [`MappedRwLockWriteGuard`] for a component of the borrowed data, e.g.
     /// an enum variant.
     ///
@@ -1013,22 +892,22 @@ impl<'a, T: ?Sized> RwLockWriteGuard<'a, T> {
     ///
     /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will be poisoned.
     #[unstable(feature = "mapped_lock_guards", issue = "117108")]
-    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockWriteGuard<'a, U>
+    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockWriteGuard<'rwlock, U>
     where
         F: FnOnce(&mut T) -> &mut U,
         U: ?Sized,
     {
         // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
         // was created, and have been upheld throughout `map` and/or `filter_map`.
-        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
-        // passed to it. If the closure panics, the guard will be dropped.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
         let data = NonNull::from(f(unsafe { &mut *orig.lock.data.get() }));
         let orig = ManuallyDrop::new(orig);
         MappedRwLockWriteGuard {
             data,
             inner_lock: &orig.lock.inner,
             poison_flag: &orig.lock.poison,
-            poison: orig.poison.clone(),
+            poison_guard: orig.poison.clone(),
             _variance: PhantomData,
         }
     }
@@ -1048,15 +927,15 @@ impl<'a, T: ?Sized> RwLockWriteGuard<'a, T> {
     ///
     /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will be poisoned.
     #[unstable(feature = "mapped_lock_guards", issue = "117108")]
-    pub fn filter_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockWriteGuard<'a, U>, Self>
+    pub fn filter_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockWriteGuard<'rwlock, U>, Self>
     where
         F: FnOnce(&mut T) -> Option<&mut U>,
         U: ?Sized,
     {
         // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
         // was created, and have been upheld throughout `map` and/or `filter_map`.
-        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
-        // passed to it. If the closure panics, the guard will be dropped.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
         match f(unsafe { &mut *orig.lock.data.get() }) {
             Some(data) => {
                 let data = NonNull::from(data);
@@ -1065,78 +944,82 @@ impl<'a, T: ?Sized> RwLockWriteGuard<'a, T> {
                     data,
                     inner_lock: &orig.lock.inner,
                     poison_flag: &orig.lock.poison,
-                    poison: orig.poison.clone(),
+                    poison_guard: orig.poison.clone(),
                     _variance: PhantomData,
                 })
             }
             None => Err(orig),
         }
     }
+}
 
-    /// Downgrades a write-locked `RwLockWriteGuard` into a read-locked [`RwLockReadGuard`].
-    ///
-    /// This method will atomically change the state of the [`RwLock`] from exclusive mode into
-    /// shared mode. This means that it is impossible for a writing thread to get in between a
-    /// thread calling `downgrade` and the same thread reading whatever it wrote while it had the
-    /// [`RwLock`] in write mode.
-    ///
-    /// Note that since we have the `RwLockWriteGuard`, we know that the [`RwLock`] is already
-    /// locked for writing, so this method cannot fail.
-    ///
-    /// # Example
-    ///
-    /// ```
-    /// #![feature(rwlock_downgrade)]
-    /// use std::sync::{Arc, RwLock, RwLockWriteGuard};
-    ///
-    /// // The inner value starts as 0.
-    /// let rw = Arc::new(RwLock::new(0));
+impl<'rwlock, T: ?Sized> MappedRwLockReadGuard<'rwlock, T> {
+    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data,
+    /// e.g. an enum variant.
     ///
-    /// // Put the lock in write mode.
-    /// let mut main_write_guard = rw.write().unwrap();
+    /// The `RwLock` is already locked for reading, so this cannot fail.
     ///
-    /// let evil = rw.clone();
-    /// let handle = std::thread::spawn(move || {
-    ///     // This will not return until the main thread drops the `main_read_guard`.
-    ///     let mut evil_guard = evil.write().unwrap();
+    /// This is an associated function that needs to be used as
+    /// `MappedRwLockReadGuard::map(...)`. A method would interfere with
+    /// methods of the same name on the contents of the `MappedRwLockReadGuard`
+    /// used through `Deref`.
     ///
-    ///     assert_eq!(*evil_guard, 1);
-    ///     *evil_guard = 2;
-    /// });
+    /// # Panics
     ///
-    /// // After spawning the writer thread, set the inner value to 1.
-    /// *main_write_guard = 1;
+    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be
+    /// poisoned.
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockReadGuard<'rwlock, U>
+    where
+        F: FnOnce(&T) -> &U,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        let data = NonNull::from(f(unsafe { orig.data.as_ref() }));
+        let orig = ManuallyDrop::new(orig);
+        MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock }
+    }
+
+    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data.
+    /// The original guard is returned as an `Err(...)` if the closure returns
+    /// `None`.
     ///
-    /// // Atomically downgrade the write guard into a read guard.
-    /// let main_read_guard = RwLockWriteGuard::downgrade(main_write_guard);
+    /// The `RwLock` is already locked for reading, so this cannot fail.
     ///
-    /// // Since `downgrade` is atomic, the writer thread cannot have set the inner value to 2.
-    /// assert_eq!(*main_read_guard, 1, "`downgrade` was not atomic");
+    /// This is an associated function that needs to be used as
+    /// `MappedRwLockReadGuard::filter_map(...)`. A method would interfere with
+    /// methods of the same name on the contents of the `MappedRwLockReadGuard`
+    /// used through `Deref`.
     ///
-    /// // Clean up everything now
-    /// drop(main_read_guard);
-    /// handle.join().unwrap();
+    /// # Panics
     ///
-    /// let final_check = rw.read().unwrap();
-    /// assert_eq!(*final_check, 2);
-    /// ```
-    #[unstable(feature = "rwlock_downgrade", issue = "128203")]
-    pub fn downgrade(s: Self) -> RwLockReadGuard<'a, T> {
-        let lock = s.lock;
-
-        // We don't want to call the destructor since that calls `write_unlock`.
-        forget(s);
-
-        // SAFETY: We take ownership of a write guard, so we must already have the `RwLock` in write
-        // mode, satisfying the `downgrade` contract.
-        unsafe { lock.inner.downgrade() };
-
-        // SAFETY: We have just successfully called `downgrade`, so we fulfill the safety contract.
-        unsafe { RwLockReadGuard::new(lock).unwrap_or_else(PoisonError::into_inner) }
+    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be
+    /// poisoned.
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    pub fn filter_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockReadGuard<'rwlock, U>, Self>
+    where
+        F: FnOnce(&T) -> Option<&U>,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        match f(unsafe { orig.data.as_ref() }) {
+            Some(data) => {
+                let data = NonNull::from(data);
+                let orig = ManuallyDrop::new(orig);
+                Ok(MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock })
+            }
+            None => Err(orig),
+        }
     }
 }
 
-impl<'a, T: ?Sized> MappedRwLockWriteGuard<'a, T> {
+impl<'rwlock, T: ?Sized> MappedRwLockWriteGuard<'rwlock, T> {
     /// Makes a [`MappedRwLockWriteGuard`] for a component of the borrowed data,
     /// e.g. an enum variant.
     ///
@@ -1151,22 +1034,22 @@ impl<'a, T: ?Sized> MappedRwLockWriteGuard<'a, T> {
     ///
     /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will be poisoned.
     #[unstable(feature = "mapped_lock_guards", issue = "117108")]
-    pub fn map<U, F>(mut orig: Self, f: F) -> MappedRwLockWriteGuard<'a, U>
+    pub fn map<U, F>(mut orig: Self, f: F) -> MappedRwLockWriteGuard<'rwlock, U>
     where
         F: FnOnce(&mut T) -> &mut U,
         U: ?Sized,
     {
         // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
         // was created, and have been upheld throughout `map` and/or `filter_map`.
-        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
-        // passed to it. If the closure panics, the guard will be dropped.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
         let data = NonNull::from(f(unsafe { orig.data.as_mut() }));
         let orig = ManuallyDrop::new(orig);
         MappedRwLockWriteGuard {
             data,
             inner_lock: orig.inner_lock,
             poison_flag: orig.poison_flag,
-            poison: orig.poison.clone(),
+            poison_guard: orig.poison_guard.clone(),
             _variance: PhantomData,
         }
     }
@@ -1186,15 +1069,18 @@ impl<'a, T: ?Sized> MappedRwLockWriteGuard<'a, T> {
     ///
     /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will be poisoned.
     #[unstable(feature = "mapped_lock_guards", issue = "117108")]
-    pub fn filter_map<U, F>(mut orig: Self, f: F) -> Result<MappedRwLockWriteGuard<'a, U>, Self>
+    pub fn filter_map<U, F>(
+        mut orig: Self,
+        f: F,
+    ) -> Result<MappedRwLockWriteGuard<'rwlock, U>, Self>
     where
         F: FnOnce(&mut T) -> Option<&mut U>,
         U: ?Sized,
     {
         // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
         // was created, and have been upheld throughout `map` and/or `filter_map`.
-        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
-        // passed to it. If the closure panics, the guard will be dropped.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
         match f(unsafe { orig.data.as_mut() }) {
             Some(data) => {
                 let data = NonNull::from(data);
@@ -1203,7 +1089,7 @@ impl<'a, T: ?Sized> MappedRwLockWriteGuard<'a, T> {
                     data,
                     inner_lock: orig.inner_lock,
                     poison_flag: orig.poison_flag,
-                    poison: orig.poison.clone(),
+                    poison_guard: orig.poison_guard.clone(),
                     _variance: PhantomData,
                 })
             }
@@ -1211,3 +1097,162 @@ impl<'a, T: ?Sized> MappedRwLockWriteGuard<'a, T> {
         }
     }
 }
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> Drop for RwLockReadGuard<'_, T> {
+    fn drop(&mut self) {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when created.
+        unsafe {
+            self.inner_lock.read_unlock();
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> Drop for RwLockWriteGuard<'_, T> {
+    fn drop(&mut self) {
+        self.lock.poison.done(&self.poison);
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
+        unsafe {
+            self.lock.inner.write_unlock();
+        }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+impl<T: ?Sized> Drop for MappedRwLockReadGuard<'_, T> {
+    fn drop(&mut self) {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe {
+            self.inner_lock.read_unlock();
+        }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+impl<T: ?Sized> Drop for MappedRwLockWriteGuard<'_, T> {
+    fn drop(&mut self) {
+        self.poison_flag.done(&self.poison_guard);
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe {
+            self.inner_lock.write_unlock();
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> Deref for RwLockReadGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when created.
+        unsafe { self.data.as_ref() }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> Deref for RwLockWriteGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
+        unsafe { &*self.lock.data.get() }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> DerefMut for RwLockWriteGuard<'_, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
+        unsafe { &mut *self.lock.data.get() }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+impl<T: ?Sized> Deref for MappedRwLockReadGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe { self.data.as_ref() }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+impl<T: ?Sized> Deref for MappedRwLockWriteGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe { self.data.as_ref() }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+impl<T: ?Sized> DerefMut for MappedRwLockWriteGuard<'_, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe { self.data.as_mut() }
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockReadGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[stable(feature = "std_guard_impls", since = "1.20.0")]
+impl<T: ?Sized + fmt::Display> fmt::Display for RwLockReadGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockWriteGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[stable(feature = "std_guard_impls", since = "1.20.0")]
+impl<T: ?Sized + fmt::Display> fmt::Display for RwLockWriteGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+impl<T: ?Sized + fmt::Debug> fmt::Debug for MappedRwLockReadGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+impl<T: ?Sized + fmt::Display> fmt::Display for MappedRwLockReadGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+impl<T: ?Sized + fmt::Debug> fmt::Debug for MappedRwLockWriteGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+impl<T: ?Sized + fmt::Display> fmt::Display for MappedRwLockWriteGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
diff --git a/library/std/src/thread/local.rs b/library/std/src/thread/local.rs
index 0ad014ccd3e..797feeb2bbb 100644
--- a/library/std/src/thread/local.rs
+++ b/library/std/src/thread/local.rs
@@ -8,8 +8,8 @@ use crate::fmt;
 
 /// A thread local storage (TLS) key which owns its contents.
 ///
-/// This key uses the fastest possible implementation available to it for the
-/// target platform. It is instantiated with the [`thread_local!`] macro and the
+/// This key uses the fastest implementation available on the target platform.
+/// It is instantiated with the [`thread_local!`] macro and the
 /// primary method is the [`with`] method, though there are helpers to make
 /// working with [`Cell`] types easier.
 ///
@@ -24,10 +24,10 @@ use crate::fmt;
 /// [`with`]) within a thread, and values that implement [`Drop`] get
 /// destructed when a thread exits. Some platform-specific caveats apply, which
 /// are explained below.
-/// Note that, should the destructor panics, the whole process will be [aborted].
+/// Note that if the destructor panics, the whole process will be [aborted].
 ///
 /// A `LocalKey`'s initializer cannot recursively depend on itself. Using a
-/// `LocalKey` in this way may cause panics, aborts or infinite recursion on
+/// `LocalKey` in this way may cause panics, aborts, or infinite recursion on
 /// the first call to `with`.
 ///
 /// [aborted]: crate::process::abort