Adding upstream version 1.66.0+dfsg1.upstream/1.66.0+dfsg1

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

35 files changed, 1059 insertions, 310 deletions

diff --git a/library/alloc/src/alloc.rs b/library/alloc/src/alloc.rs
index cc8da7bcc..8187517cc 100644
--- a/library/alloc/src/alloc.rs
+++ b/library/alloc/src/alloc.rs
@@ -28,16 +28,20 @@ extern "Rust" {
     // The rustc fork of LLVM 14 and earlier also special-cases these function names to be able to optimize them
     // like `malloc`, `realloc`, and `free`, respectively.
     #[rustc_allocator]
-    #[rustc_allocator_nounwind]
+    #[cfg_attr(not(bootstrap), rustc_nounwind)]
+    #[cfg_attr(bootstrap, rustc_allocator_nounwind)]
     fn __rust_alloc(size: usize, align: usize) -> *mut u8;
-    #[cfg_attr(not(bootstrap), rustc_deallocator)]
-    #[rustc_allocator_nounwind]
+    #[rustc_deallocator]
+    #[cfg_attr(not(bootstrap), rustc_nounwind)]
+    #[cfg_attr(bootstrap, rustc_allocator_nounwind)]
     fn __rust_dealloc(ptr: *mut u8, size: usize, align: usize);
-    #[cfg_attr(not(bootstrap), rustc_reallocator)]
-    #[rustc_allocator_nounwind]
+    #[rustc_reallocator]
+    #[cfg_attr(not(bootstrap), rustc_nounwind)]
+    #[cfg_attr(bootstrap, rustc_allocator_nounwind)]
     fn __rust_realloc(ptr: *mut u8, old_size: usize, align: usize, new_size: usize) -> *mut u8;
-    #[cfg_attr(not(bootstrap), rustc_allocator_zeroed)]
-    #[rustc_allocator_nounwind]
+    #[rustc_allocator_zeroed]
+    #[cfg_attr(not(bootstrap), rustc_nounwind)]
+    #[cfg_attr(bootstrap, rustc_allocator_nounwind)]
     fn __rust_alloc_zeroed(size: usize, align: usize) -> *mut u8;
 }
 
diff --git a/library/alloc/src/alloc/tests.rs b/library/alloc/src/alloc/tests.rs
index 7d560964d..b2f019459 100644
--- a/library/alloc/src/alloc/tests.rs
+++ b/library/alloc/src/alloc/tests.rs
@@ -15,7 +15,7 @@ fn allocate_zeroed() {
         let end = i.add(layout.size());
         while i < end {
             assert_eq!(*i, 0);
-            i = i.offset(1);
+            i = i.add(1);
         }
         Global.deallocate(ptr.as_non_null_ptr(), layout);
     }
diff --git a/library/alloc/src/borrow.rs b/library/alloc/src/borrow.rs
index 904a53bb4..83a138559 100644
--- a/library/alloc/src/borrow.rs
+++ b/library/alloc/src/borrow.rs
@@ -21,7 +21,6 @@ use Cow::*;
 impl<'a, B: ?Sized> Borrow<B> for Cow<'a, B>
 where
     B: ToOwned,
-    <B as ToOwned>::Owned: 'a,
 {
     fn borrow(&self) -> &B {
         &**self
diff --git a/library/alloc/src/boxed.rs b/library/alloc/src/boxed.rs
index c1ceeb0de..d6681a317 100644
--- a/library/alloc/src/boxed.rs
+++ b/library/alloc/src/boxed.rs
@@ -1,4 +1,4 @@
-//! A pointer type for heap allocation.
+//! The `Box<T>` type for heap allocation.
 //!
 //! [`Box<T>`], casually referred to as a 'box', provides the simplest form of
 //! heap allocation in Rust. Boxes provide ownership for this allocation, and
@@ -151,6 +151,7 @@ use core::async_iter::AsyncIterator;
 use core::borrow;
 use core::cmp::Ordering;
 use core::convert::{From, TryFrom};
+use core::error::Error;
 use core::fmt;
 use core::future::Future;
 use core::hash::{Hash, Hasher};
@@ -175,6 +176,8 @@ use crate::raw_vec::RawVec;
 #[cfg(not(no_global_oom_handling))]
 use crate::str::from_boxed_utf8_unchecked;
 #[cfg(not(no_global_oom_handling))]
+use crate::string::String;
+#[cfg(not(no_global_oom_handling))]
 use crate::vec::Vec;
 
 #[unstable(feature = "thin_box", issue = "92791")]
@@ -1480,7 +1483,7 @@ impl<T: Copy> From<&[T]> for Box<[T]> {
     /// Converts a `&[T]` into a `Box<[T]>`
     ///
     /// This conversion allocates on the heap
-    /// and performs a copy of `slice`.
+    /// and performs a copy of `slice` and its contents.
     ///
     /// # Examples
     /// ```rust
@@ -1617,6 +1620,22 @@ impl<T, const N: usize> From<[T; N]> for Box<[T]> {
     }
 }
 
+/// Casts a boxed slice to a boxed array.
+///
+/// # Safety
+///
+/// `boxed_slice.len()` must be exactly `N`.
+unsafe fn boxed_slice_as_array_unchecked<T, A: Allocator, const N: usize>(
+    boxed_slice: Box<[T], A>,
+) -> Box<[T; N], A> {
+    debug_assert_eq!(boxed_slice.len(), N);
+
+    let (ptr, alloc) = Box::into_raw_with_allocator(boxed_slice);
+    // SAFETY: Pointer and allocator came from an existing box,
+    // and our safety condition requires that the length is exactly `N`
+    unsafe { Box::from_raw_in(ptr as *mut [T; N], alloc) }
+}
+
 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]> {
     type Error = Box<[T]>;
@@ -1632,13 +1651,46 @@ impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]> {
     /// `boxed_slice.len()` does not equal `N`.
     fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
         if boxed_slice.len() == N {
-            Ok(unsafe { Box::from_raw(Box::into_raw(boxed_slice) as *mut [T; N]) })
+            Ok(unsafe { boxed_slice_as_array_unchecked(boxed_slice) })
         } else {
             Err(boxed_slice)
         }
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "boxed_array_try_from_vec", since = "1.66.0")]
+impl<T, const N: usize> TryFrom<Vec<T>> for Box<[T; N]> {
+    type Error = Vec<T>;
+
+    /// Attempts to convert a `Vec<T>` into a `Box<[T; N]>`.
+    ///
+    /// Like [`Vec::into_boxed_slice`], this is in-place if `vec.capacity() == N`,
+    /// but will require a reallocation otherwise.
+    ///
+    /// # Errors
+    ///
+    /// Returns the original `Vec<T>` in the `Err` variant if
+    /// `boxed_slice.len()` does not equal `N`.
+    ///
+    /// # Examples
+    ///
+    /// This can be used with [`vec!`] to create an array on the heap:
+    ///
+    /// ```
+    /// let state: Box<[f32; 100]> = vec![1.0; 100].try_into().unwrap();
+    /// assert_eq!(state.len(), 100);
+    /// ```
+    fn try_from(vec: Vec<T>) -> Result<Self, Self::Error> {
+        if vec.len() == N {
+            let boxed_slice = vec.into_boxed_slice();
+            Ok(unsafe { boxed_slice_as_array_unchecked(boxed_slice) })
+        } else {
+            Err(vec)
+        }
+    }
+}
+
 impl<A: Allocator> Box<dyn Any, A> {
     /// Attempt to downcast the box to a concrete type.
     ///
@@ -2032,8 +2084,7 @@ impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A> {
  *  could have a method to project a Pin<T> from it.
  */
 #[stable(feature = "pin", since = "1.33.0")]
-#[rustc_const_unstable(feature = "const_box", issue = "92521")]
-impl<T: ?Sized, A: Allocator> const Unpin for Box<T, A> where A: 'static {}
+impl<T: ?Sized, A: Allocator> Unpin for Box<T, A> where A: 'static {}
 
 #[unstable(feature = "generator_trait", issue = "43122")]
 impl<G: ?Sized + Generator<R> + Unpin, R, A: Allocator> Generator<R> for Box<G, A>
@@ -2085,3 +2136,292 @@ impl<S: ?Sized + AsyncIterator + Unpin> AsyncIterator for Box<S> {
         (**self).size_hint()
     }
 }
+
+impl dyn Error {
+    #[inline]
+    #[stable(feature = "error_downcast", since = "1.3.0")]
+    #[rustc_allow_incoherent_impl]
+    /// Attempts to downcast the box to a concrete type.
+    pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<dyn Error>> {
+        if self.is::<T>() {
+            unsafe {
+                let raw: *mut dyn Error = Box::into_raw(self);
+                Ok(Box::from_raw(raw as *mut T))
+            }
+        } else {
+            Err(self)
+        }
+    }
+}
+
+impl dyn Error + Send {
+    #[inline]
+    #[stable(feature = "error_downcast", since = "1.3.0")]
+    #[rustc_allow_incoherent_impl]
+    /// Attempts to downcast the box to a concrete type.
+    pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<dyn Error + Send>> {
+        let err: Box<dyn Error> = self;
+        <dyn Error>::downcast(err).map_err(|s| unsafe {
+            // Reapply the `Send` marker.
+            mem::transmute::<Box<dyn Error>, Box<dyn Error + Send>>(s)
+        })
+    }
+}
+
+impl dyn Error + Send + Sync {
+    #[inline]
+    #[stable(feature = "error_downcast", since = "1.3.0")]
+    #[rustc_allow_incoherent_impl]
+    /// Attempts to downcast the box to a concrete type.
+    pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<Self>> {
+        let err: Box<dyn Error> = self;
+        <dyn Error>::downcast(err).map_err(|s| unsafe {
+            // Reapply the `Send + Sync` marker.
+            mem::transmute::<Box<dyn Error>, Box<dyn Error + Send + Sync>>(s)
+        })
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, E: Error + 'a> From<E> for Box<dyn Error + 'a> {
+    /// Converts a type of [`Error`] into a box of dyn [`Error`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::error::Error;
+    /// use std::fmt;
+    /// use std::mem;
+    ///
+    /// #[derive(Debug)]
+    /// struct AnError;
+    ///
+    /// impl fmt::Display for AnError {
+    ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+    ///         write!(f, "An error")
+    ///     }
+    /// }
+    ///
+    /// impl Error for AnError {}
+    ///
+    /// let an_error = AnError;
+    /// assert!(0 == mem::size_of_val(&an_error));
+    /// let a_boxed_error = Box::<dyn Error>::from(an_error);
+    /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
+    /// ```
+    fn from(err: E) -> Box<dyn Error + 'a> {
+        Box::new(err)
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<dyn Error + Send + Sync + 'a> {
+    /// Converts a type of [`Error`] + [`Send`] + [`Sync`] into a box of
+    /// dyn [`Error`] + [`Send`] + [`Sync`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::error::Error;
+    /// use std::fmt;
+    /// use std::mem;
+    ///
+    /// #[derive(Debug)]
+    /// struct AnError;
+    ///
+    /// impl fmt::Display for AnError {
+    ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+    ///         write!(f, "An error")
+    ///     }
+    /// }
+    ///
+    /// impl Error for AnError {}
+    ///
+    /// unsafe impl Send for AnError {}
+    ///
+    /// unsafe impl Sync for AnError {}
+    ///
+    /// let an_error = AnError;
+    /// assert!(0 == mem::size_of_val(&an_error));
+    /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(an_error);
+    /// assert!(
+    ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
+    /// ```
+    fn from(err: E) -> Box<dyn Error + Send + Sync + 'a> {
+        Box::new(err)
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl From<String> for Box<dyn Error + Send + Sync> {
+    /// Converts a [`String`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::error::Error;
+    /// use std::mem;
+    ///
+    /// let a_string_error = "a string error".to_string();
+    /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_string_error);
+    /// assert!(
+    ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
+    /// ```
+    #[inline]
+    fn from(err: String) -> Box<dyn Error + Send + Sync> {
+        struct StringError(String);
+
+        impl Error for StringError {
+            #[allow(deprecated)]
+            fn description(&self) -> &str {
+                &self.0
+            }
+        }
+
+        impl fmt::Display for StringError {
+            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+                fmt::Display::fmt(&self.0, f)
+            }
+        }
+
+        // Purposefully skip printing "StringError(..)"
+        impl fmt::Debug for StringError {
+            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+                fmt::Debug::fmt(&self.0, f)
+            }
+        }
+
+        Box::new(StringError(err))
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "string_box_error", since = "1.6.0")]
+impl From<String> for Box<dyn Error> {
+    /// Converts a [`String`] into a box of dyn [`Error`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::error::Error;
+    /// use std::mem;
+    ///
+    /// let a_string_error = "a string error".to_string();
+    /// let a_boxed_error = Box::<dyn Error>::from(a_string_error);
+    /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
+    /// ```
+    fn from(str_err: String) -> Box<dyn Error> {
+        let err1: Box<dyn Error + Send + Sync> = From::from(str_err);
+        let err2: Box<dyn Error> = err1;
+        err2
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a> From<&str> for Box<dyn Error + Send + Sync + 'a> {
+    /// Converts a [`str`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
+    ///
+    /// [`str`]: prim@str
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::error::Error;
+    /// use std::mem;
+    ///
+    /// let a_str_error = "a str error";
+    /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_str_error);
+    /// assert!(
+    ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
+    /// ```
+    #[inline]
+    fn from(err: &str) -> Box<dyn Error + Send + Sync + 'a> {
+        From::from(String::from(err))
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "string_box_error", since = "1.6.0")]
+impl From<&str> for Box<dyn Error> {
+    /// Converts a [`str`] into a box of dyn [`Error`].
+    ///
+    /// [`str`]: prim@str
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::error::Error;
+    /// use std::mem;
+    ///
+    /// let a_str_error = "a str error";
+    /// let a_boxed_error = Box::<dyn Error>::from(a_str_error);
+    /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
+    /// ```
+    fn from(err: &str) -> Box<dyn Error> {
+        From::from(String::from(err))
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "cow_box_error", since = "1.22.0")]
+impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + Send + Sync + 'a> {
+    /// Converts a [`Cow`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::error::Error;
+    /// use std::mem;
+    /// use std::borrow::Cow;
+    ///
+    /// let a_cow_str_error = Cow::from("a str error");
+    /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_cow_str_error);
+    /// assert!(
+    ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
+    /// ```
+    fn from(err: Cow<'b, str>) -> Box<dyn Error + Send + Sync + 'a> {
+        From::from(String::from(err))
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "cow_box_error", since = "1.22.0")]
+impl<'a> From<Cow<'a, str>> for Box<dyn Error> {
+    /// Converts a [`Cow`] into a box of dyn [`Error`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::error::Error;
+    /// use std::mem;
+    /// use std::borrow::Cow;
+    ///
+    /// let a_cow_str_error = Cow::from("a str error");
+    /// let a_boxed_error = Box::<dyn Error>::from(a_cow_str_error);
+    /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
+    /// ```
+    fn from(err: Cow<'a, str>) -> Box<dyn Error> {
+        From::from(String::from(err))
+    }
+}
+
+#[stable(feature = "box_error", since = "1.8.0")]
+impl<T: core::error::Error> core::error::Error for Box<T> {
+    #[allow(deprecated, deprecated_in_future)]
+    fn description(&self) -> &str {
+        core::error::Error::description(&**self)
+    }
+
+    #[allow(deprecated)]
+    fn cause(&self) -> Option<&dyn core::error::Error> {
+        core::error::Error::cause(&**self)
+    }
+
+    fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
+        core::error::Error::source(&**self)
+    }
+}
diff --git a/library/alloc/src/boxed/thin.rs b/library/alloc/src/boxed/thin.rs
index 649ccfcaa..c477c4490 100644
--- a/library/alloc/src/boxed/thin.rs
+++ b/library/alloc/src/boxed/thin.rs
@@ -2,6 +2,7 @@
 // https://github.com/matthieu-m/rfc2580/blob/b58d1d3cba0d4b5e859d3617ea2d0943aaa31329/examples/thin.rs
 // by matthieu-m
 use crate::alloc::{self, Layout, LayoutError};
+use core::error::Error;
 use core::fmt::{self, Debug, Display, Formatter};
 use core::marker::PhantomData;
 #[cfg(not(no_global_oom_handling))]
@@ -271,3 +272,10 @@ impl<H> WithHeader<H> {
         Layout::new::<H>().extend(value_layout)
     }
 }
+
+#[unstable(feature = "thin_box", issue = "92791")]
+impl<T: ?Sized + Error> Error for ThinBox<T> {
+    fn source(&self) -> Option<&(dyn Error + 'static)> {
+        self.deref().source()
+    }
+}
diff --git a/library/alloc/src/collections/binary_heap.rs b/library/alloc/src/collections/binary_heap.rs
index 197e7aaac..4583bc9a1 100644
--- a/library/alloc/src/collections/binary_heap.rs
+++ b/library/alloc/src/collections/binary_heap.rs
@@ -1010,7 +1010,8 @@ impl<T> BinaryHeap<T> {
     /// current length. The allocator may reserve more space to speculatively
     /// avoid frequent allocations. After calling `try_reserve`, capacity will be
     /// greater than or equal to `self.len() + additional` if it returns
-    /// `Ok(())`. Does nothing if capacity is already sufficient.
+    /// `Ok(())`. Does nothing if capacity is already sufficient. This method
+    /// preserves the contents even if an error occurs.
     ///
     /// # Errors
     ///
diff --git a/library/alloc/src/collections/btree/dedup_sorted_iter.rs b/library/alloc/src/collections/btree/dedup_sorted_iter.rs
index 60bf83b83..17ee78045 100644
--- a/library/alloc/src/collections/btree/dedup_sorted_iter.rs
+++ b/library/alloc/src/collections/btree/dedup_sorted_iter.rs
@@ -3,7 +3,9 @@ use core::iter::Peekable;
 /// A iterator for deduping the key of a sorted iterator.
 /// When encountering the duplicated key, only the last key-value pair is yielded.
 ///
-/// Used by [`BTreeMap::bulk_build_from_sorted_iter`].
+/// Used by [`BTreeMap::bulk_build_from_sorted_iter`][1].
+///
+/// [1]: crate::collections::BTreeMap::bulk_build_from_sorted_iter
 pub struct DedupSortedIter<K, V, I>
 where
     I: Iterator<Item = (K, V)>,
diff --git a/library/alloc/src/collections/btree/map.rs b/library/alloc/src/collections/btree/map.rs
index cacbd54b6..8a7719347 100644
--- a/library/alloc/src/collections/btree/map.rs
+++ b/library/alloc/src/collections/btree/map.rs
@@ -580,7 +580,7 @@ impl<K, V> BTreeMap<K, V> {
     /// map.insert(1, "a");
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_btree_new", issue = "71835")]
+    #[rustc_const_stable(feature = "const_btree_new", since = "1.66.0")]
     #[must_use]
     pub const fn new() -> BTreeMap<K, V> {
         BTreeMap { root: None, length: 0, alloc: ManuallyDrop::new(Global), _marker: PhantomData }
@@ -703,7 +703,6 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// Basic usage:
     ///
     /// ```
-    /// #![feature(map_first_last)]
     /// use std::collections::BTreeMap;
     ///
     /// let mut map = BTreeMap::new();
@@ -712,7 +711,7 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// map.insert(2, "a");
     /// assert_eq!(map.first_key_value(), Some((&1, &"b")));
     /// ```
-    #[unstable(feature = "map_first_last", issue = "62924")]
+    #[stable(feature = "map_first_last", since = "1.66.0")]
     pub fn first_key_value(&self) -> Option<(&K, &V)>
     where
         K: Ord,
@@ -727,7 +726,6 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(map_first_last)]
     /// use std::collections::BTreeMap;
     ///
     /// let mut map = BTreeMap::new();
@@ -741,7 +739,7 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// assert_eq!(*map.get(&1).unwrap(), "first");
     /// assert_eq!(*map.get(&2).unwrap(), "b");
     /// ```
-    #[unstable(feature = "map_first_last", issue = "62924")]
+    #[stable(feature = "map_first_last", since = "1.66.0")]
     pub fn first_entry(&mut self) -> Option<OccupiedEntry<'_, K, V, A>>
     where
         K: Ord,
@@ -765,7 +763,6 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// Draining elements in ascending order, while keeping a usable map each iteration.
     ///
     /// ```
-    /// #![feature(map_first_last)]
     /// use std::collections::BTreeMap;
     ///
     /// let mut map = BTreeMap::new();
@@ -776,7 +773,7 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// }
     /// assert!(map.is_empty());
     /// ```
-    #[unstable(feature = "map_first_last", issue = "62924")]
+    #[stable(feature = "map_first_last", since = "1.66.0")]
     pub fn pop_first(&mut self) -> Option<(K, V)>
     where
         K: Ord,
@@ -792,7 +789,6 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// Basic usage:
     ///
     /// ```
-    /// #![feature(map_first_last)]
     /// use std::collections::BTreeMap;
     ///
     /// let mut map = BTreeMap::new();
@@ -800,7 +796,7 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// map.insert(2, "a");
     /// assert_eq!(map.last_key_value(), Some((&2, &"a")));
     /// ```
-    #[unstable(feature = "map_first_last", issue = "62924")]
+    #[stable(feature = "map_first_last", since = "1.66.0")]
     pub fn last_key_value(&self) -> Option<(&K, &V)>
     where
         K: Ord,
@@ -815,7 +811,6 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(map_first_last)]
     /// use std::collections::BTreeMap;
     ///
     /// let mut map = BTreeMap::new();
@@ -829,7 +824,7 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// assert_eq!(*map.get(&1).unwrap(), "a");
     /// assert_eq!(*map.get(&2).unwrap(), "last");
     /// ```
-    #[unstable(feature = "map_first_last", issue = "62924")]
+    #[stable(feature = "map_first_last", since = "1.66.0")]
     pub fn last_entry(&mut self) -> Option<OccupiedEntry<'_, K, V, A>>
     where
         K: Ord,
@@ -853,7 +848,6 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// Draining elements in descending order, while keeping a usable map each iteration.
     ///
     /// ```
-    /// #![feature(map_first_last)]
     /// use std::collections::BTreeMap;
     ///
     /// let mut map = BTreeMap::new();
@@ -864,7 +858,7 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// }
     /// assert!(map.is_empty());
     /// ```
-    #[unstable(feature = "map_first_last", issue = "62924")]
+    #[stable(feature = "map_first_last", since = "1.66.0")]
     pub fn pop_last(&mut self) -> Option<(K, V)>
     where
         K: Ord,
@@ -1099,6 +1093,9 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
 
     /// Moves all elements from `other` into `self`, leaving `other` empty.
     ///
+    /// If a key from `other` is already present in `self`, the respective
+    /// value from `self` will be overwritten with the respective value from `other`.
+    ///
     /// # Examples
     ///
     /// ```
@@ -1107,10 +1104,10 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// let mut a = BTreeMap::new();
     /// a.insert(1, "a");
     /// a.insert(2, "b");
-    /// a.insert(3, "c");
+    /// a.insert(3, "c"); // Note: Key (3) also present in b.
     ///
     /// let mut b = BTreeMap::new();
-    /// b.insert(3, "d");
+    /// b.insert(3, "d"); // Note: Key (3) also present in a.
     /// b.insert(4, "e");
     /// b.insert(5, "f");
     ///
@@ -1121,7 +1118,7 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     ///
     /// assert_eq!(a[&1], "a");
     /// assert_eq!(a[&2], "b");
-    /// assert_eq!(a[&3], "d");
+    /// assert_eq!(a[&3], "d"); // Note: "c" has been overwritten.
     /// assert_eq!(a[&4], "e");
     /// assert_eq!(a[&5], "f");
     /// ```
@@ -2392,7 +2389,11 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// ```
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_btree_new", issue = "71835")]
+    #[rustc_const_unstable(
+        feature = "const_btree_len",
+        issue = "71835",
+        implied_by = "const_btree_new"
+    )]
     pub const fn len(&self) -> usize {
         self.length
     }
@@ -2413,7 +2414,11 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// ```
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_btree_new", issue = "71835")]
+    #[rustc_const_unstable(
+        feature = "const_btree_len",
+        issue = "71835",
+        implied_by = "const_btree_new"
+    )]
     pub const fn is_empty(&self) -> bool {
         self.len() == 0
     }
diff --git a/library/alloc/src/collections/btree/map/entry.rs b/library/alloc/src/collections/btree/map/entry.rs
index b6eecf9b0..370b58864 100644
--- a/library/alloc/src/collections/btree/map/entry.rs
+++ b/library/alloc/src/collections/btree/map/entry.rs
@@ -133,6 +133,16 @@ impl<'a, K: Debug + Ord, V: Debug, A: Allocator + Clone> fmt::Display
     }
 }
 
+#[unstable(feature = "map_try_insert", issue = "82766")]
+impl<'a, K: core::fmt::Debug + Ord, V: core::fmt::Debug> core::error::Error
+    for crate::collections::btree_map::OccupiedError<'a, K, V>
+{
+    #[allow(deprecated)]
+    fn description(&self) -> &str {
+        "key already exists"
+    }
+}
+
 impl<'a, K: Ord, V, A: Allocator + Clone> Entry<'a, K, V, A> {
     /// Ensures a value is in the entry by inserting the default if empty, and returns
     /// a mutable reference to the value in the entry.
diff --git a/library/alloc/src/collections/btree/node.rs b/library/alloc/src/collections/btree/node.rs
index d831161bc..da766b67a 100644
--- a/library/alloc/src/collections/btree/node.rs
+++ b/library/alloc/src/collections/btree/node.rs
@@ -206,9 +206,9 @@ impl<'a, K: 'a, V: 'a, Type> Clone for NodeRef<marker::Immut<'a>, K, V, Type> {
 
 unsafe impl<BorrowType, K: Sync, V: Sync, Type> Sync for NodeRef<BorrowType, K, V, Type> {}
 
-unsafe impl<'a, K: Sync + 'a, V: Sync + 'a, Type> Send for NodeRef<marker::Immut<'a>, K, V, Type> {}
-unsafe impl<'a, K: Send + 'a, V: Send + 'a, Type> Send for NodeRef<marker::Mut<'a>, K, V, Type> {}
-unsafe impl<'a, K: Send + 'a, V: Send + 'a, Type> Send for NodeRef<marker::ValMut<'a>, K, V, Type> {}
+unsafe impl<K: Sync, V: Sync, Type> Send for NodeRef<marker::Immut<'_>, K, V, Type> {}
+unsafe impl<K: Send, V: Send, Type> Send for NodeRef<marker::Mut<'_>, K, V, Type> {}
+unsafe impl<K: Send, V: Send, Type> Send for NodeRef<marker::ValMut<'_>, K, V, Type> {}
 unsafe impl<K: Send, V: Send, Type> Send for NodeRef<marker::Owned, K, V, Type> {}
 unsafe impl<K: Send, V: Send, Type> Send for NodeRef<marker::Dying, K, V, Type> {}
 
@@ -318,7 +318,7 @@ impl<BorrowType: marker::BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type>
     pub fn ascend(
         self,
     ) -> Result<Handle<NodeRef<BorrowType, K, V, marker::Internal>, marker::Edge>, Self> {
-        assert!(BorrowType::PERMITS_TRAVERSAL);
+        let _ = BorrowType::TRAVERSAL_PERMIT;
         // We need to use raw pointers to nodes because, if BorrowType is marker::ValMut,
         // there might be outstanding mutable references to values that we must not invalidate.
         let leaf_ptr: *const _ = Self::as_leaf_ptr(&self);
@@ -1003,7 +1003,7 @@ impl<BorrowType: marker::BorrowType, K, V>
     /// `edge.descend().ascend().unwrap()` and `node.ascend().unwrap().descend()` should
     /// both, upon success, do nothing.
     pub fn descend(self) -> NodeRef<BorrowType, K, V, marker::LeafOrInternal> {
-        assert!(BorrowType::PERMITS_TRAVERSAL);
+        let _ = BorrowType::TRAVERSAL_PERMIT;
         // We need to use raw pointers to nodes because, if BorrowType is
         // marker::ValMut, there might be outstanding mutable references to
         // values that we must not invalidate. There's no worry accessing the
@@ -1666,15 +1666,17 @@ pub mod marker {
     pub struct ValMut<'a>(PhantomData<&'a mut ()>);
 
     pub trait BorrowType {
-        // Whether node references of this borrow type allow traversing
-        // to other nodes in the tree.
-        const PERMITS_TRAVERSAL: bool = true;
+        // If node references of this borrow type allow traversing to other
+        // nodes in the tree, this constant can be evaluated. Thus reading it
+        // serves as a compile-time assertion.
+        const TRAVERSAL_PERMIT: () = ();
     }
     impl BorrowType for Owned {
-        // Traversal isn't needed, it happens using the result of `borrow_mut`.
+        // Reject evaluation, because traversal isn't needed. Instead traversal
+        // happens using the result of `borrow_mut`.
         // By disabling traversal, and only creating new references to roots,
         // we know that every reference of the `Owned` type is to a root node.
-        const PERMITS_TRAVERSAL: bool = false;
+        const TRAVERSAL_PERMIT: () = panic!();
     }
     impl BorrowType for Dying {}
     impl<'a> BorrowType for Immut<'a> {}
diff --git a/library/alloc/src/collections/btree/set.rs b/library/alloc/src/collections/btree/set.rs
index 2cfc08074..4ddb21192 100644
--- a/library/alloc/src/collections/btree/set.rs
+++ b/library/alloc/src/collections/btree/set.rs
@@ -343,7 +343,7 @@ impl<T> BTreeSet<T> {
     /// let mut set: BTreeSet<i32> = BTreeSet::new();
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_btree_new", issue = "71835")]
+    #[rustc_const_stable(feature = "const_btree_new", since = "1.66.0")]
     #[must_use]
     pub const fn new() -> BTreeSet<T> {
         BTreeSet { map: BTreeMap::new() }
@@ -786,7 +786,6 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     /// Basic usage:
     ///
     /// ```
-    /// #![feature(map_first_last)]
     /// use std::collections::BTreeSet;
     ///
     /// let mut set = BTreeSet::new();
@@ -797,7 +796,7 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     /// assert_eq!(set.first(), Some(&1));
     /// ```
     #[must_use]
-    #[unstable(feature = "map_first_last", issue = "62924")]
+    #[stable(feature = "map_first_last", since = "1.66.0")]
     pub fn first(&self) -> Option<&T>
     where
         T: Ord,
@@ -813,7 +812,6 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     /// Basic usage:
     ///
     /// ```
-    /// #![feature(map_first_last)]
     /// use std::collections::BTreeSet;
     ///
     /// let mut set = BTreeSet::new();
@@ -824,7 +822,7 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     /// assert_eq!(set.last(), Some(&2));
     /// ```
     #[must_use]
-    #[unstable(feature = "map_first_last", issue = "62924")]
+    #[stable(feature = "map_first_last", since = "1.66.0")]
     pub fn last(&self) -> Option<&T>
     where
         T: Ord,
@@ -838,7 +836,6 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(map_first_last)]
     /// use std::collections::BTreeSet;
     ///
     /// let mut set = BTreeSet::new();
@@ -849,7 +846,7 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     /// }
     /// assert!(set.is_empty());
     /// ```
-    #[unstable(feature = "map_first_last", issue = "62924")]
+    #[stable(feature = "map_first_last", since = "1.66.0")]
     pub fn pop_first(&mut self) -> Option<T>
     where
         T: Ord,
@@ -863,7 +860,6 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(map_first_last)]
     /// use std::collections::BTreeSet;
     ///
     /// let mut set = BTreeSet::new();
@@ -874,7 +870,7 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     /// }
     /// assert!(set.is_empty());
     /// ```
-    #[unstable(feature = "map_first_last", issue = "62924")]
+    #[stable(feature = "map_first_last", since = "1.66.0")]
     pub fn pop_last(&mut self) -> Option<T>
     where
         T: Ord,
@@ -1174,7 +1170,11 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     /// ```
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_btree_new", issue = "71835")]
+    #[rustc_const_unstable(
+        feature = "const_btree_len",
+        issue = "71835",
+        implied_by = "const_btree_new"
+    )]
     pub const fn len(&self) -> usize {
         self.map.len()
     }
@@ -1193,7 +1193,11 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     /// ```
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_btree_new", issue = "71835")]
+    #[rustc_const_unstable(
+        feature = "const_btree_len",
+        issue = "71835",
+        implied_by = "const_btree_new"
+    )]
     pub const fn is_empty(&self) -> bool {
         self.len() == 0
     }
diff --git a/library/alloc/src/collections/linked_list.rs b/library/alloc/src/collections/linked_list.rs
index e21c8aa3b..f2f5dffc2 100644
--- a/library/alloc/src/collections/linked_list.rs
+++ b/library/alloc/src/collections/linked_list.rs
@@ -1570,7 +1570,7 @@ impl<'a, T> CursorMut<'a, T> {
     /// that the cursor points to is unchanged, even if it is the "ghost" node.
     ///
     /// This operation should compute in *O*(1) time.
-    // `push_front` continues to point to "ghost" when it addes a node to mimic
+    // `push_front` continues to point to "ghost" when it adds a node to mimic
     // the behavior of `insert_before` on an empty list.
     #[unstable(feature = "linked_list_cursors", issue = "58533")]
     pub fn push_front(&mut self, elt: T) {
@@ -1613,7 +1613,7 @@ impl<'a, T> CursorMut<'a, T> {
             None
         } else {
             // We can't point to the node that we pop. Copying the behavior of
-            // `remove_current`, we move on the the next node in the sequence.
+            // `remove_current`, we move on to the next node in the sequence.
             // If the list is of length 1 then we end pointing to the "ghost"
             // node at index 0, which is expected.
             if self.list.head == self.current {
diff --git a/library/alloc/src/collections/mod.rs b/library/alloc/src/collections/mod.rs
index 628a5b155..161a37573 100644
--- a/library/alloc/src/collections/mod.rs
+++ b/library/alloc/src/collections/mod.rs
@@ -152,3 +152,6 @@ trait SpecExtend<I: IntoIterator> {
     /// Extends `self` with the contents of the given iterator.
     fn spec_extend(&mut self, iter: I);
 }
+
+#[stable(feature = "try_reserve", since = "1.57.0")]
+impl core::error::Error for TryReserveError {}
diff --git a/library/alloc/src/collections/vec_deque/drain.rs b/library/alloc/src/collections/vec_deque/drain.rs
index 05f94da6d..41baa7102 100644
--- a/library/alloc/src/collections/vec_deque/drain.rs
+++ b/library/alloc/src/collections/vec_deque/drain.rs
@@ -1,10 +1,12 @@
+use core::fmt;
 use core::iter::FusedIterator;
+use core::marker::PhantomData;
+use core::mem::{self, MaybeUninit};
 use core::ptr::{self, NonNull};
-use core::{fmt, mem};
 
 use crate::alloc::{Allocator, Global};
 
-use super::{count, Iter, VecDeque};
+use super::{count, wrap_index, VecDeque};
 
 /// A draining iterator over the elements of a `VecDeque`.
 ///
@@ -20,18 +22,24 @@ pub struct Drain<
 > {
     after_tail: usize,
     after_head: usize,
-    iter: Iter<'a, T>,
+    ring: NonNull<[T]>,
+    tail: usize,
+    head: usize,
     deque: NonNull<VecDeque<T, A>>,
+    _phantom: PhantomData<&'a T>,
 }
 
 impl<'a, T, A: Allocator> Drain<'a, T, A> {
     pub(super) unsafe fn new(
         after_tail: usize,
         after_head: usize,
-        iter: Iter<'a, T>,
+        ring: &'a [MaybeUninit<T>],
+        tail: usize,
+        head: usize,
         deque: NonNull<VecDeque<T, A>>,
     ) -> Self {
-        Drain { after_tail, after_head, iter, deque }
+        let ring = unsafe { NonNull::new_unchecked(ring as *const [MaybeUninit<T>] as *mut _) };
+        Drain { after_tail, after_head, ring, tail, head, deque, _phantom: PhantomData }
     }
 }
 
@@ -41,7 +49,9 @@ impl<T: fmt::Debug, A: Allocator> fmt::Debug for Drain<'_, T, A> {
         f.debug_tuple("Drain")
             .field(&self.after_tail)
             .field(&self.after_head)
-            .field(&self.iter)
+            .field(&self.ring)
+            .field(&self.tail)
+            .field(&self.head)
             .finish()
     }
 }
@@ -118,12 +128,21 @@ impl<T, A: Allocator> Iterator for Drain<'_, T, A> {
 
     #[inline]
     fn next(&mut self) -> Option<T> {
-        self.iter.next().map(|elt| unsafe { ptr::read(elt) })
+        if self.tail == self.head {
+            return None;
+        }
+        let tail = self.tail;
+        self.tail = wrap_index(self.tail.wrapping_add(1), self.ring.len());
+        // Safety:
+        // - `self.tail` in a ring buffer is always a valid index.
+        // - `self.head` and `self.tail` equality is checked above.
+        unsafe { Some(ptr::read(self.ring.as_ptr().get_unchecked_mut(tail))) }
     }
 
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
-        self.iter.size_hint()
+        let len = count(self.tail, self.head, self.ring.len());
+        (len, Some(len))
     }
 }
 
@@ -131,7 +150,14 @@ impl<T, A: Allocator> Iterator for Drain<'_, T, A> {
 impl<T, A: Allocator> DoubleEndedIterator for Drain<'_, T, A> {
     #[inline]
     fn next_back(&mut self) -> Option<T> {
-        self.iter.next_back().map(|elt| unsafe { ptr::read(elt) })
+        if self.tail == self.head {
+            return None;
+        }
+        self.head = wrap_index(self.head.wrapping_sub(1), self.ring.len());
+        // Safety:
+        // - `self.head` in a ring buffer is always a valid index.
+        // - `self.head` and `self.tail` equality is checked above.
+        unsafe { Some(ptr::read(self.ring.as_ptr().get_unchecked_mut(self.head))) }
     }
 }
 
diff --git a/library/alloc/src/collections/vec_deque/mod.rs b/library/alloc/src/collections/vec_deque/mod.rs
index 4d895d837..2a57dad89 100644
--- a/library/alloc/src/collections/vec_deque/mod.rs
+++ b/library/alloc/src/collections/vec_deque/mod.rs
@@ -12,11 +12,17 @@ use core::fmt;
 use core::hash::{Hash, Hasher};
 use core::iter::{repeat_with, FromIterator};
 use core::marker::PhantomData;
-use core::mem::{self, ManuallyDrop, MaybeUninit};
+use core::mem::{ManuallyDrop, MaybeUninit, SizedTypeProperties};
 use core::ops::{Index, IndexMut, Range, RangeBounds};
 use core::ptr::{self, NonNull};
 use core::slice;
 
+// This is used in a bunch of intra-doc links.
+// FIXME: For some reason, `#[cfg(doc)]` wasn't sufficient, resulting in
+// failures in linkchecker even though rustdoc built the docs just fine.
+#[allow(unused_imports)]
+use core::mem;
+
 use crate::alloc::{Allocator, Global};
 use crate::collections::TryReserveError;
 use crate::collections::TryReserveErrorKind;
@@ -177,7 +183,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// Marginally more convenient
     #[inline]
     fn cap(&self) -> usize {
-        if mem::size_of::<T>() == 0 {
+        if T::IS_ZST {
             // For zero sized types, we are always at maximum capacity
             MAXIMUM_ZST_CAPACITY
         } else {
@@ -794,7 +800,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// in the given deque. The collection may reserve more space to speculatively avoid
     /// frequent reallocations. After calling `try_reserve`, capacity will be
     /// greater than or equal to `self.len() + additional` if it returns
-    /// `Ok(())`. Does nothing if capacity is already sufficient.
+    /// `Ok(())`. Does nothing if capacity is already sufficient. This method
+    /// preserves the contents even if an error occurs.
     ///
     /// # Errors
     ///
@@ -1333,9 +1340,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
             // it.  We do not write to `self` nor reborrow to a mutable reference.
             // Hence the raw pointer we created above, for `deque`, remains valid.
             let ring = self.buffer_as_slice();
-            let iter = Iter::new(ring, drain_tail, drain_head);
 
-            Drain::new(drain_head, head, iter, deque)
+            Drain::new(drain_head, head, ring, drain_tail, drain_head, deque)
         }
     }
 
@@ -2447,8 +2453,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
                     let mut right_offset = 0;
                     for i in left_edge..right_edge {
                         right_offset = (i - left_edge) % (cap - right_edge);
-                        let src: isize = (right_edge + right_offset) as isize;
-                        ptr::swap(buf.add(i), buf.offset(src));
+                        let src = right_edge + right_offset;
+                        ptr::swap(buf.add(i), buf.add(src));
                     }
                     let n_ops = right_edge - left_edge;
                     left_edge += n_ops;
@@ -3038,7 +3044,7 @@ impl<T, A: Allocator> From<Vec<T, A>> for VecDeque<T, A> {
     /// `Vec<T>` came from `From<VecDeque<T>>` and hasn't been reallocated.
     fn from(mut other: Vec<T, A>) -> Self {
         let len = other.len();
-        if mem::size_of::<T>() == 0 {
+        if T::IS_ZST {
             // There's no actual allocation for ZSTs to worry about capacity,
             // but `VecDeque` can't handle as much length as `Vec`.
             assert!(len < MAXIMUM_ZST_CAPACITY, "capacity overflow");
@@ -3124,7 +3130,7 @@ impl<T, const N: usize> From<[T; N]> for VecDeque<T> {
     fn from(arr: [T; N]) -> Self {
         let mut deq = VecDeque::with_capacity(N);
         let arr = ManuallyDrop::new(arr);
-        if mem::size_of::<T>() != 0 {
+        if !<T>::IS_ZST {
             // SAFETY: VecDeque::with_capacity ensures that there is enough capacity.
             unsafe {
                 ptr::copy_nonoverlapping(arr.as_ptr(), deq.ptr(), N);
diff --git a/library/alloc/src/ffi/c_str.rs b/library/alloc/src/ffi/c_str.rs
index ae61b1f1e..11bd4c4dc 100644
--- a/library/alloc/src/ffi/c_str.rs
+++ b/library/alloc/src/ffi/c_str.rs
@@ -436,9 +436,9 @@ impl CString {
     ///
     /// unsafe {
     ///     assert_eq!(b'f', *ptr as u8);
-    ///     assert_eq!(b'o', *ptr.offset(1) as u8);
-    ///     assert_eq!(b'o', *ptr.offset(2) as u8);
-    ///     assert_eq!(b'\0', *ptr.offset(3) as u8);
+    ///     assert_eq!(b'o', *ptr.add(1) as u8);
+    ///     assert_eq!(b'o', *ptr.add(2) as u8);
+    ///     assert_eq!(b'\0', *ptr.add(3) as u8);
     ///
     ///     // retake pointer to free memory
     ///     let _ = CString::from_raw(ptr);
@@ -1121,3 +1121,26 @@ impl CStr {
         CString::from(self)
     }
 }
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl core::error::Error for NulError {
+    #[allow(deprecated)]
+    fn description(&self) -> &str {
+        "nul byte found in data"
+    }
+}
+
+#[stable(feature = "cstring_from_vec_with_nul", since = "1.58.0")]
+impl core::error::Error for FromVecWithNulError {}
+
+#[stable(feature = "cstring_into", since = "1.7.0")]
+impl core::error::Error for IntoStringError {
+    #[allow(deprecated)]
+    fn description(&self) -> &str {
+        "C string contained non-utf8 bytes"
+    }
+
+    fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
+        Some(self.__source())
+    }
+}
diff --git a/library/alloc/src/fmt.rs b/library/alloc/src/fmt.rs
index ed398b566..799ce9d5d 100644
--- a/library/alloc/src/fmt.rs
+++ b/library/alloc/src/fmt.rs
@@ -327,7 +327,7 @@
 //! - `text` must not contain any `'{'` or `'}'` characters,
 //! - `ws` is any character for which [`char::is_whitespace`] returns `true`, has no semantic
 //!   meaning and is completely optional,
-//! - `integer` is a decimal integer that may contain leading zeroes and
+//! - `integer` is a decimal integer that may contain leading zeroes and must fit into an `usize` and
 //! - `identifier` is an `IDENTIFIER_OR_KEYWORD` (not an `IDENTIFIER`) as defined by the [Rust language reference](https://doc.rust-lang.org/reference/identifiers.html).
 //!
 //! # Formatting traits
diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
index 8b6f40548..ce36b116f 100644
--- a/library/alloc/src/lib.rs
+++ b/library/alloc/src/lib.rs
@@ -56,10 +56,6 @@
 //! [`Rc`]: rc
 //! [`RefCell`]: core::cell
 
-// To run liballoc tests without x.py without ending up with two copies of liballoc, Miri needs to be
-// able to "empty" this crate. See <https://github.com/rust-lang/miri-test-libstd/issues/4>.
-// rustc itself never sets the feature, so this line has no affect there.
-#![cfg(any(not(feature = "miri-test-libstd"), test, doctest))]
 #![allow(unused_attributes)]
 #![stable(feature = "alloc", since = "1.36.0")]
 #![doc(
@@ -73,10 +69,16 @@
     any(not(feature = "miri-test-libstd"), test, doctest),
     no_global_oom_handling,
     not(no_global_oom_handling),
+    not(no_rc),
+    not(no_sync),
     target_has_atomic = "ptr"
 ))]
 #![no_std]
 #![needs_allocator]
+// To run liballoc tests without x.py without ending up with two copies of liballoc, Miri needs to be
+// able to "empty" this crate. See <https://github.com/rust-lang/miri-test-libstd/issues/4>.
+// rustc itself never sets the feature, so this line has no affect there.
+#![cfg(any(not(feature = "miri-test-libstd"), test, doctest))]
 //
 // Lints:
 #![deny(unsafe_op_in_unsafe_fn)]
@@ -97,7 +99,7 @@
 #![feature(coerce_unsized)]
 #![cfg_attr(not(no_global_oom_handling), feature(const_alloc_error))]
 #![feature(const_box)]
-#![cfg_attr(not(no_global_oom_handling), feature(const_btree_new))]
+#![cfg_attr(not(no_global_oom_handling), feature(const_btree_len))]
 #![feature(const_cow_is_borrowed)]
 #![feature(const_convert)]
 #![feature(const_size_of_val)]
@@ -109,8 +111,11 @@
 #![feature(core_intrinsics)]
 #![feature(const_eval_select)]
 #![feature(const_pin)]
+#![feature(const_waker)]
 #![feature(cstr_from_bytes_until_nul)]
 #![feature(dispatch_from_dyn)]
+#![feature(error_generic_member_access)]
+#![feature(error_in_core)]
 #![feature(exact_size_is_empty)]
 #![feature(extend_one)]
 #![feature(fmt_internals)]
@@ -120,18 +125,21 @@
 #![feature(iter_advance_by)]
 #![feature(iter_next_chunk)]
 #![feature(layout_for_ptr)]
-#![feature(maybe_uninit_array_assume_init)]
 #![feature(maybe_uninit_slice)]
 #![feature(maybe_uninit_uninit_array)]
+#![feature(maybe_uninit_uninit_array_transpose)]
 #![cfg_attr(test, feature(new_uninit))]
 #![feature(nonnull_slice_from_raw_parts)]
 #![feature(pattern)]
 #![feature(pointer_byte_offsets)]
+#![feature(provide_any)]
 #![feature(ptr_internals)]
 #![feature(ptr_metadata)]
 #![feature(ptr_sub_ptr)]
 #![feature(receiver_trait)]
+#![feature(saturating_int_impl)]
 #![feature(set_ptr_value)]
+#![feature(sized_type_properties)]
 #![feature(slice_from_ptr_range)]
 #![feature(slice_group_by)]
 #![feature(slice_ptr_get)]
@@ -145,6 +153,7 @@
 #![feature(unchecked_math)]
 #![feature(unicode_internals)]
 #![feature(unsize)]
+#![feature(utf8_chunks)]
 #![feature(std_internals)]
 //
 // Language features:
@@ -164,7 +173,6 @@
 #![cfg_attr(not(test), feature(generator_trait))]
 #![feature(hashmap_internals)]
 #![feature(lang_items)]
-#![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(negative_impls)]
 #![feature(never_type)]
@@ -178,6 +186,7 @@
 #![feature(unboxed_closures)]
 #![feature(unsized_fn_params)]
 #![feature(c_unwind)]
+#![feature(with_negative_coherence)]
 //
 // Rustdoc features:
 #![feature(doc_cfg)]
@@ -218,16 +227,17 @@ mod boxed {
 }
 pub mod borrow;
 pub mod collections;
-#[cfg(not(no_global_oom_handling))]
+#[cfg(all(not(no_rc), not(no_sync), not(no_global_oom_handling)))]
 pub mod ffi;
 pub mod fmt;
+#[cfg(not(no_rc))]
 pub mod rc;
 pub mod slice;
 pub mod str;
 pub mod string;
-#[cfg(target_has_atomic = "ptr")]
+#[cfg(all(not(no_rc), not(no_sync), target_has_atomic = "ptr"))]
 pub mod sync;
-#[cfg(all(not(no_global_oom_handling), target_has_atomic = "ptr"))]
+#[cfg(all(not(no_global_oom_handling), not(no_rc), not(no_sync), target_has_atomic = "ptr"))]
 pub mod task;
 #[cfg(test)]
 mod tests;
diff --git a/library/alloc/src/macros.rs b/library/alloc/src/macros.rs
index 88eb6aa7a..5198bf297 100644
--- a/library/alloc/src/macros.rs
+++ b/library/alloc/src/macros.rs
@@ -107,6 +107,8 @@ macro_rules! vec {
 /// format!("test");
 /// format!("hello {}", "world!");
 /// format!("x = {}, y = {y}", 10, y = 30);
+/// let (x, y) = (1, 2);
+/// format!("{x} + {y} = 3");
 /// ```
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
diff --git a/library/alloc/src/raw_vec.rs b/library/alloc/src/raw_vec.rs
index b0f4529ab..5a10121bb 100644
--- a/library/alloc/src/raw_vec.rs
+++ b/library/alloc/src/raw_vec.rs
@@ -3,7 +3,7 @@
 use core::alloc::LayoutError;
 use core::cmp;
 use core::intrinsics;
-use core::mem::{self, ManuallyDrop, MaybeUninit};
+use core::mem::{self, ManuallyDrop, MaybeUninit, SizedTypeProperties};
 use core::ops::Drop;
 use core::ptr::{self, NonNull, Unique};
 use core::slice;
@@ -168,7 +168,7 @@ impl<T, A: Allocator> RawVec<T, A> {
     #[cfg(not(no_global_oom_handling))]
     fn allocate_in(capacity: usize, init: AllocInit, alloc: A) -> Self {
         // Don't allocate here because `Drop` will not deallocate when `capacity` is 0.
-        if mem::size_of::<T>() == 0 || capacity == 0 {
+        if T::IS_ZST || capacity == 0 {
             Self::new_in(alloc)
         } else {
             // We avoid `unwrap_or_else` here because it bloats the amount of
@@ -229,7 +229,7 @@ impl<T, A: Allocator> RawVec<T, A> {
     /// This will always be `usize::MAX` if `T` is zero-sized.
     #[inline(always)]
     pub fn capacity(&self) -> usize {
-        if mem::size_of::<T>() == 0 { usize::MAX } else { self.cap }
+        if T::IS_ZST { usize::MAX } else { self.cap }
     }
 
     /// Returns a shared reference to the allocator backing this `RawVec`.
@@ -238,7 +238,7 @@ impl<T, A: Allocator> RawVec<T, A> {
     }
 
     fn current_memory(&self) -> Option<(NonNull<u8>, Layout)> {
-        if mem::size_of::<T>() == 0 || self.cap == 0 {
+        if T::IS_ZST || self.cap == 0 {
             None
         } else {
             // We have an allocated chunk of memory, so we can bypass runtime
@@ -380,7 +380,7 @@ impl<T, A: Allocator> RawVec<T, A> {
         // This is ensured by the calling contexts.
         debug_assert!(additional > 0);
 
-        if mem::size_of::<T>() == 0 {
+        if T::IS_ZST {
             // Since we return a capacity of `usize::MAX` when `elem_size` is
             // 0, getting to here necessarily means the `RawVec` is overfull.
             return Err(CapacityOverflow.into());
@@ -406,7 +406,7 @@ impl<T, A: Allocator> RawVec<T, A> {
     // `grow_amortized`, but this method is usually instantiated less often so
     // it's less critical.
     fn grow_exact(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
-        if mem::size_of::<T>() == 0 {
+        if T::IS_ZST {
             // Since we return a capacity of `usize::MAX` when the type size is
             // 0, getting to here necessarily means the `RawVec` is overfull.
             return Err(CapacityOverflow.into());
diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs
index b89b03683..006d813e5 100644
--- a/library/alloc/src/rc.rs
+++ b/library/alloc/src/rc.rs
@@ -1110,8 +1110,8 @@ impl<T: ?Sized> Rc<T> {
 
     #[inline]
     #[stable(feature = "ptr_eq", since = "1.17.0")]
-    /// Returns `true` if the two `Rc`s point to the same allocation
-    /// (in a vein similar to [`ptr::eq`]).
+    /// Returns `true` if the two `Rc`s point to the same allocation in a vein similar to
+    /// [`ptr::eq`]. See [that function][`ptr::eq`] for caveats when comparing `dyn Trait` pointers.
     ///
     /// # Examples
     ///
@@ -1142,7 +1142,7 @@ impl<T: Clone> Rc<T> {
     /// be cloned.
     ///
     /// See also [`get_mut`], which will fail rather than cloning the inner value
-    /// or diassociating [`Weak`] pointers.
+    /// or disassociating [`Weak`] pointers.
     ///
     /// [`clone`]: Clone::clone
     /// [`get_mut`]: Rc::get_mut
@@ -1386,7 +1386,7 @@ impl<T: ?Sized> Rc<T> {
             Self::allocate_for_layout(
                 Layout::for_value(&*ptr),
                 |layout| Global.allocate(layout),
-                |mem| mem.with_metadata_of(ptr as *mut RcBox<T>),
+                |mem| mem.with_metadata_of(ptr as *const RcBox<T>),
             )
         }
     }
@@ -2419,9 +2419,9 @@ impl<T: ?Sized> Weak<T> {
         }
     }
 
-    /// Returns `true` if the two `Weak`s point to the same allocation (similar to
-    /// [`ptr::eq`]), or if both don't point to any allocation
-    /// (because they were created with `Weak::new()`).
+    /// Returns `true` if the two `Weak`s point to the same allocation similar to [`ptr::eq`], or if
+    /// both don't point to any allocation (because they were created with `Weak::new()`). See [that
+    /// function][`ptr::eq`] for caveats when comparing `dyn Trait` pointers.
     ///
     /// # Notes
     ///
diff --git a/library/alloc/src/slice.rs b/library/alloc/src/slice.rs
index 63d4d9452..a5e7bf2a1 100644
--- a/library/alloc/src/slice.rs
+++ b/library/alloc/src/slice.rs
@@ -1,82 +1,12 @@
-//! A dynamically-sized view into a contiguous sequence, `[T]`.
+//! Utilities for the slice primitive type.
 //!
 //! *[See also the slice primitive type](slice).*
 //!
-//! Slices are a view into a block of memory represented as a pointer and a
-//! length.
+//! Most of the structs in this module are iterator types which can only be created
+//! using a certain function. For example, `slice.iter()` yields an [`Iter`].
 //!
-//! ```
-//! // slicing a Vec
-//! let vec = vec![1, 2, 3];
-//! let int_slice = &vec[..];
-//! // coercing an array to a slice
-//! let str_slice: &[&str] = &["one", "two", "three"];
-//! ```
-//!
-//! Slices are either mutable or shared. The shared slice type is `&[T]`,
-//! while the mutable slice type is `&mut [T]`, where `T` represents the element
-//! type. For example, you can mutate the block of memory that a mutable slice
-//! points to:
-//!
-//! ```
-//! let x = &mut [1, 2, 3];
-//! x[1] = 7;
-//! assert_eq!(x, &[1, 7, 3]);
-//! ```
-//!
-//! Here are some of the things this module contains:
-//!
-//! ## Structs
-//!
-//! There are several structs that are useful for slices, such as [`Iter`], which
-//! represents iteration over a slice.
-//!
-//! ## Trait Implementations
-//!
-//! There are several implementations of common traits for slices. Some examples
-//! include:
-//!
-//! * [`Clone`]
-//! * [`Eq`], [`Ord`] - for slices whose element type are [`Eq`] or [`Ord`].
-//! * [`Hash`] - for slices whose element type is [`Hash`].
-//!
-//! ## Iteration
-//!
-//! The slices implement `IntoIterator`. The iterator yields references to the
-//! slice elements.
-//!
-//! ```
-//! let numbers = &[0, 1, 2];
-//! for n in numbers {
-//!     println!("{n} is a number!");
-//! }
-//! ```
-//!
-//! The mutable slice yields mutable references to the elements:
-//!
-//! ```
-//! let mut scores = [7, 8, 9];
-//! for score in &mut scores[..] {
-//!     *score += 1;
-//! }
-//! ```
-//!
-//! This iterator yields mutable references to the slice's elements, so while
-//! the element type of the slice is `i32`, the element type of the iterator is
-//! `&mut i32`.
-//!
-//! * [`.iter`] and [`.iter_mut`] are the explicit methods to return the default
-//!   iterators.
-//! * Further methods that return iterators are [`.split`], [`.splitn`],
-//!   [`.chunks`], [`.windows`] and more.
-//!
-//! [`Hash`]: core::hash::Hash
-//! [`.iter`]: slice::iter
-//! [`.iter_mut`]: slice::iter_mut
-//! [`.split`]: slice::split
-//! [`.splitn`]: slice::splitn
-//! [`.chunks`]: slice::chunks
-//! [`.windows`]: slice::windows
+//! A few functions are provided to create a slice from a value reference
+//! or from a raw pointer.
 #![stable(feature = "rust1", since = "1.0.0")]
 // Many of the usings in this module are only used in the test configuration.
 // It's cleaner to just turn off the unused_imports warning than to fix them.
@@ -86,9 +16,7 @@ use core::borrow::{Borrow, BorrowMut};
 #[cfg(not(no_global_oom_handling))]
 use core::cmp::Ordering::{self, Less};
 #[cfg(not(no_global_oom_handling))]
-use core::mem;
-#[cfg(not(no_global_oom_handling))]
-use core::mem::size_of;
+use core::mem::{self, SizedTypeProperties};
 #[cfg(not(no_global_oom_handling))]
 use core::ptr;
 
@@ -275,7 +203,7 @@ impl<T> [T] {
     where
         T: Ord,
     {
-        merge_sort(self, |a, b| a.lt(b));
+        merge_sort(self, T::lt);
     }
 
     /// Sorts the slice with a comparator function.
@@ -1024,7 +952,7 @@ where
             // Consume the greater side.
             // If equal, prefer the right run to maintain stability.
             unsafe {
-                let to_copy = if is_less(&*right.offset(-1), &*left.offset(-1)) {
+                let to_copy = if is_less(&*right.sub(1), &*left.sub(1)) {
                     decrement_and_get(left)
                 } else {
                     decrement_and_get(right)
@@ -1038,12 +966,12 @@ where
 
     unsafe fn get_and_increment<T>(ptr: &mut *mut T) -> *mut T {
         let old = *ptr;
-        *ptr = unsafe { ptr.offset(1) };
+        *ptr = unsafe { ptr.add(1) };
         old
     }
 
     unsafe fn decrement_and_get<T>(ptr: &mut *mut T) -> *mut T {
-        *ptr = unsafe { ptr.offset(-1) };
+        *ptr = unsafe { ptr.sub(1) };
         *ptr
     }
 
@@ -1088,7 +1016,7 @@ where
     const MIN_RUN: usize = 10;
 
     // Sorting has no meaningful behavior on zero-sized types.
-    if size_of::<T>() == 0 {
+    if T::IS_ZST {
         return;
     }
 
diff --git a/library/alloc/src/str.rs b/library/alloc/src/str.rs
index d5ed2c4ad..b28d20cda 100644
--- a/library/alloc/src/str.rs
+++ b/library/alloc/src/str.rs
@@ -1,26 +1,6 @@
-//! Unicode string slices.
+//! Utilities for the `str` primitive type.
 //!
 //! *[See also the `str` primitive type](str).*
-//!
-//! The `&str` type is one of the two main string types, the other being `String`.
-//! Unlike its `String` counterpart, its contents are borrowed.
-//!
-//! # Basic Usage
-//!
-//! A basic string declaration of `&str` type:
-//!
-//! ```
-//! let hello_world = "Hello, World!";
-//! ```
-//!
-//! Here we have declared a string literal, also known as a string slice.
-//! String literals have a static lifetime, which means the string `hello_world`
-//! is guaranteed to be valid for the duration of the entire program.
-//! We can explicitly specify `hello_world`'s lifetime as well:
-//!
-//! ```
-//! let hello_world: &'static str = "Hello, world!";
-//! ```
 
 #![stable(feature = "rust1", since = "1.0.0")]
 // Many of the usings in this module are only used in the test configuration.
@@ -71,6 +51,8 @@ pub use core::str::{RSplit, Split};
 pub use core::str::{RSplitN, SplitN};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::str::{RSplitTerminator, SplitTerminator};
+#[unstable(feature = "utf8_chunks", issue = "99543")]
+pub use core::str::{Utf8Chunk, Utf8Chunks};
 
 /// Note: `str` in `Concat<str>` is not meaningful here.
 /// This type parameter of the trait only exists to enable another impl.
diff --git a/library/alloc/src/string.rs b/library/alloc/src/string.rs
index a5118e533..c436adf70 100644
--- a/library/alloc/src/string.rs
+++ b/library/alloc/src/string.rs
@@ -44,6 +44,7 @@
 
 #[cfg(not(no_global_oom_handling))]
 use core::char::{decode_utf16, REPLACEMENT_CHARACTER};
+use core::error::Error;
 use core::fmt;
 use core::hash;
 use core::iter::FusedIterator;
@@ -58,15 +59,15 @@ use core::ops::Bound::{Excluded, Included, Unbounded};
 use core::ops::{self, Index, IndexMut, Range, RangeBounds};
 use core::ptr;
 use core::slice;
-#[cfg(not(no_global_oom_handling))]
-use core::str::lossy;
 use core::str::pattern::Pattern;
+#[cfg(not(no_global_oom_handling))]
+use core::str::Utf8Chunks;
 
 #[cfg(not(no_global_oom_handling))]
 use crate::borrow::{Cow, ToOwned};
 use crate::boxed::Box;
 use crate::collections::TryReserveError;
-use crate::str::{self, Chars, Utf8Error};
+use crate::str::{self, from_utf8_unchecked_mut, Chars, Utf8Error};
 #[cfg(not(no_global_oom_handling))]
 use crate::str::{from_boxed_utf8_unchecked, FromStr};
 use crate::vec::Vec;
@@ -628,11 +629,11 @@ impl String {
     #[cfg(not(no_global_oom_handling))]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn from_utf8_lossy(v: &[u8]) -> Cow<'_, str> {
-        let mut iter = lossy::Utf8Lossy::from_bytes(v).chunks();
+        let mut iter = Utf8Chunks::new(v);
 
         let first_valid = if let Some(chunk) = iter.next() {
-            let lossy::Utf8LossyChunk { valid, broken } = chunk;
-            if broken.is_empty() {
+            let valid = chunk.valid();
+            if chunk.invalid().is_empty() {
                 debug_assert_eq!(valid.len(), v.len());
                 return Cow::Borrowed(valid);
             }
@@ -647,9 +648,9 @@ impl String {
         res.push_str(first_valid);
         res.push_str(REPLACEMENT);
 
-        for lossy::Utf8LossyChunk { valid, broken } in iter {
-            res.push_str(valid);
-            if !broken.is_empty() {
+        for chunk in iter {
+            res.push_str(chunk.valid());
+            if !chunk.invalid().is_empty() {
                 res.push_str(REPLACEMENT);
             }
         }
@@ -1080,7 +1081,8 @@ impl String {
     /// current length. The allocator may reserve more space to speculatively
     /// avoid frequent allocations. After calling `try_reserve`, capacity will be
     /// greater than or equal to `self.len() + additional` if it returns
-    /// `Ok(())`. Does nothing if capacity is already sufficient.
+    /// `Ok(())`. Does nothing if capacity is already sufficient. This method
+    /// preserves the contents even if an error occurs.
     ///
     /// # Errors
     ///
@@ -1847,6 +1849,35 @@ impl String {
         let slice = self.vec.into_boxed_slice();
         unsafe { from_boxed_utf8_unchecked(slice) }
     }
+
+    /// Consumes and leaks the `String`, returning a mutable reference to the contents,
+    /// `&'static mut str`.
+    ///
+    /// This is mainly useful for data that lives for the remainder of
+    /// the program's life. Dropping the returned reference will cause a memory
+    /// leak.
+    ///
+    /// It does not reallocate or shrink the `String`,
+    /// so the leaked allocation may include unused capacity that is not part
+    /// of the returned slice.
+    ///
+    /// # Examples
+    ///
+    /// Simple usage:
+    ///
+    /// ```
+    /// #![feature(string_leak)]
+    ///
+    /// let x = String::from("bucket");
+    /// let static_ref: &'static mut str = x.leak();
+    /// assert_eq!(static_ref, "bucket");
+    /// ```
+    #[unstable(feature = "string_leak", issue = "102929")]
+    #[inline]
+    pub fn leak(self) -> &'static mut str {
+        let slice = self.vec.leak();
+        unsafe { from_utf8_unchecked_mut(slice) }
+    }
 }
 
 impl FromUtf8Error {
@@ -1938,6 +1969,22 @@ impl fmt::Display for FromUtf16Error {
     }
 }
 
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Error for FromUtf8Error {
+    #[allow(deprecated)]
+    fn description(&self) -> &str {
+        "invalid utf-8"
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Error for FromUtf16Error {
+    #[allow(deprecated)]
+    fn description(&self) -> &str {
+        "invalid utf-16"
+    }
+}
+
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Clone for String {
diff --git a/library/alloc/src/sync.rs b/library/alloc/src/sync.rs
index 4c03cc3ed..81cd77074 100644
--- a/library/alloc/src/sync.rs
+++ b/library/alloc/src/sync.rs
@@ -3,6 +3,10 @@
 //! Thread-safe reference-counting pointers.
 //!
 //! See the [`Arc<T>`][Arc] documentation for more details.
+//!
+//! **Note**: This module is only available on platforms that support atomic
+//! loads and stores of pointers. This may be detected at compile time using
+//! `#[cfg(target_has_atomic = "ptr")]`.
 
 use core::any::Any;
 use core::borrow;
@@ -82,6 +86,11 @@ macro_rules! acquire {
 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
 /// types.
 ///
+/// **Note**: This type is only available on platforms that support atomic
+/// loads and stores of pointers, which includes all platforms that support
+/// the `std` crate but not all those which only support [`alloc`](crate).
+/// This may be detected at compile time using `#[cfg(target_has_atomic = "ptr")]`.
+///
 /// ## Thread Safety
 ///
 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
@@ -1108,8 +1117,8 @@ impl<T: ?Sized> Arc<T> {
         drop(Weak { ptr: self.ptr });
     }
 
-    /// Returns `true` if the two `Arc`s point to the same allocation
-    /// (in a vein similar to [`ptr::eq`]).
+    /// Returns `true` if the two `Arc`s point to the same allocation in a vein similar to
+    /// [`ptr::eq`]. See [that function][`ptr::eq`] for caveats when comparing `dyn Trait` pointers.
     ///
     /// # Examples
     ///
@@ -1195,7 +1204,7 @@ impl<T: ?Sized> Arc<T> {
             Self::allocate_for_layout(
                 Layout::for_value(&*ptr),
                 |layout| Global.allocate(layout),
-                |mem| mem.with_metadata_of(ptr as *mut ArcInner<T>),
+                |mem| mem.with_metadata_of(ptr as *const ArcInner<T>),
             )
         }
     }
@@ -1980,33 +1989,26 @@ impl<T: ?Sized> Weak<T> {
         // We use a CAS loop to increment the strong count instead of a
         // fetch_add as this function should never take the reference count
         // from zero to one.
-        let inner = self.inner()?;
-
-        // Relaxed load because any write of 0 that we can observe
-        // leaves the field in a permanently zero state (so a
-        // "stale" read of 0 is fine), and any other value is
-        // confirmed via the CAS below.
-        let mut n = inner.strong.load(Relaxed);
-
-        loop {
-            if n == 0 {
-                return None;
-            }
-
-            // See comments in `Arc::clone` for why we do this (for `mem::forget`).
-            if n > MAX_REFCOUNT {
-                abort();
-            }
-
+        self.inner()?
+            .strong
             // Relaxed is fine for the failure case because we don't have any expectations about the new state.
             // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
             // value can be initialized after `Weak` references have already been created. In that case, we
             // expect to observe the fully initialized value.
-            match inner.strong.compare_exchange_weak(n, n + 1, Acquire, Relaxed) {
-                Ok(_) => return Some(unsafe { Arc::from_inner(self.ptr) }), // null checked above
-                Err(old) => n = old,
-            }
-        }
+            .fetch_update(Acquire, Relaxed, |n| {
+                // Any write of 0 we can observe leaves the field in permanently zero state.
+                if n == 0 {
+                    return None;
+                }
+                // See comments in `Arc::clone` for why we do this (for `mem::forget`).
+                if n > MAX_REFCOUNT {
+                    abort();
+                }
+                Some(n + 1)
+            })
+            .ok()
+            // null checked above
+            .map(|_| unsafe { Arc::from_inner(self.ptr) })
     }
 
     /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
@@ -2067,9 +2069,9 @@ impl<T: ?Sized> Weak<T> {
         }
     }
 
-    /// Returns `true` if the two `Weak`s point to the same allocation (similar to
-    /// [`ptr::eq`]), or if both don't point to any allocation
-    /// (because they were created with `Weak::new()`).
+    /// Returns `true` if the two `Weak`s point to the same allocation similar to [`ptr::eq`], or if
+    /// both don't point to any allocation (because they were created with `Weak::new()`). See [that
+    /// function][`ptr::eq`] for caveats when comparing `dyn Trait` pointers.
     ///
     /// # Notes
     ///
@@ -2763,3 +2765,24 @@ fn data_offset_align(align: usize) -> usize {
     let layout = Layout::new::<ArcInner<()>>();
     layout.size() + layout.padding_needed_for(align)
 }
+
+#[stable(feature = "arc_error", since = "1.52.0")]
+impl<T: core::error::Error + ?Sized> core::error::Error for Arc<T> {
+    #[allow(deprecated, deprecated_in_future)]
+    fn description(&self) -> &str {
+        core::error::Error::description(&**self)
+    }
+
+    #[allow(deprecated)]
+    fn cause(&self) -> Option<&dyn core::error::Error> {
+        core::error::Error::cause(&**self)
+    }
+
+    fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
+        core::error::Error::source(&**self)
+    }
+
+    fn provide<'a>(&'a self, req: &mut core::any::Demand<'a>) {
+        core::error::Error::provide(&**self, req);
+    }
+}
diff --git a/library/alloc/src/sync/tests.rs b/library/alloc/src/sync/tests.rs
index 202d0e7f0..0fae8953a 100644
--- a/library/alloc/src/sync/tests.rs
+++ b/library/alloc/src/sync/tests.rs
@@ -618,3 +618,22 @@ fn test_arc_cyclic_two_refs() {
     assert_eq!(Arc::strong_count(&two_refs), 3);
     assert_eq!(Arc::weak_count(&two_refs), 2);
 }
+
+/// Test for Arc::drop bug (https://github.com/rust-lang/rust/issues/55005)
+#[test]
+#[cfg(miri)] // relies on Stacked Borrows in Miri
+fn arc_drop_dereferenceable_race() {
+    // The bug seems to take up to 700 iterations to reproduce with most seeds (tested 0-9).
+    for _ in 0..750 {
+        let arc_1 = Arc::new(());
+        let arc_2 = arc_1.clone();
+        let thread = thread::spawn(|| drop(arc_2));
+        // Spin a bit; makes the race more likely to appear
+        let mut i = 0;
+        while i < 256 {
+            i += 1;
+        }
+        drop(arc_1);
+        thread.join().unwrap();
+    }
+}
diff --git a/library/alloc/src/task.rs b/library/alloc/src/task.rs
index 528ee4ff1..9d8e309a9 100644
--- a/library/alloc/src/task.rs
+++ b/library/alloc/src/task.rs
@@ -1,5 +1,11 @@
 #![stable(feature = "wake_trait", since = "1.51.0")]
+
 //! Types and Traits for working with asynchronous tasks.
+//!
+//! **Note**: This module is only available on platforms that support atomic
+//! loads and stores of pointers. This may be detected at compile time using
+//! `#[cfg(target_has_atomic = "ptr")]`.
+
 use core::mem::ManuallyDrop;
 use core::task::{RawWaker, RawWakerVTable, Waker};
 
diff --git a/library/alloc/src/vec/drain.rs b/library/alloc/src/vec/drain.rs
index 5cdee0bd4..541f99bcf 100644
--- a/library/alloc/src/vec/drain.rs
+++ b/library/alloc/src/vec/drain.rs
@@ -1,7 +1,7 @@
 use crate::alloc::{Allocator, Global};
 use core::fmt;
 use core::iter::{FusedIterator, TrustedLen};
-use core::mem;
+use core::mem::{self, ManuallyDrop, SizedTypeProperties};
 use core::ptr::{self, NonNull};
 use core::slice::{self};
 
@@ -65,6 +65,77 @@ impl<'a, T, A: Allocator> Drain<'a, T, A> {
     pub fn allocator(&self) -> &A {
         unsafe { self.vec.as_ref().allocator() }
     }
+
+    /// Keep unyielded elements in the source `Vec`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(drain_keep_rest)]
+    ///
+    /// let mut vec = vec!['a', 'b', 'c'];
+    /// let mut drain = vec.drain(..);
+    ///
+    /// assert_eq!(drain.next().unwrap(), 'a');
+    ///
+    /// // This call keeps 'b' and 'c' in the vec.
+    /// drain.keep_rest();
+    ///
+    /// // If we wouldn't call `keep_rest()`,
+    /// // `vec` would be empty.
+    /// assert_eq!(vec, ['b', 'c']);
+    /// ```
+    #[unstable(feature = "drain_keep_rest", issue = "101122")]
+    pub fn keep_rest(self) {
+        // At this moment layout looks like this:
+        //
+        // [head] [yielded by next] [unyielded] [yielded by next_back] [tail]
+        //        ^-- start         \_________/-- unyielded_len        \____/-- self.tail_len
+        //                          ^-- unyielded_ptr                  ^-- tail
+        //
+        // Normally `Drop` impl would drop [unyielded] and then move [tail] to the `start`.
+        // Here we want to
+        // 1. Move [unyielded] to `start`
+        // 2. Move [tail] to a new start at `start + len(unyielded)`
+        // 3. Update length of the original vec to `len(head) + len(unyielded) + len(tail)`
+        //    a. In case of ZST, this is the only thing we want to do
+        // 4. Do *not* drop self, as everything is put in a consistent state already, there is nothing to do
+        let mut this = ManuallyDrop::new(self);
+
+        unsafe {
+            let source_vec = this.vec.as_mut();
+
+            let start = source_vec.len();
+            let tail = this.tail_start;
+
+            let unyielded_len = this.iter.len();
+            let unyielded_ptr = this.iter.as_slice().as_ptr();
+
+            // ZSTs have no identity, so we don't need to move them around.
+            let needs_move = mem::size_of::<T>() != 0;
+
+            if needs_move {
+                let start_ptr = source_vec.as_mut_ptr().add(start);
+
+                // memmove back unyielded elements
+                if unyielded_ptr != start_ptr {
+                    let src = unyielded_ptr;
+                    let dst = start_ptr;
+
+                    ptr::copy(src, dst, unyielded_len);
+                }
+
+                // memmove back untouched tail
+                if tail != (start + unyielded_len) {
+                    let src = source_vec.as_ptr().add(tail);
+                    let dst = start_ptr.add(unyielded_len);
+                    ptr::copy(src, dst, this.tail_len);
+                }
+            }
+
+            source_vec.set_len(start + unyielded_len + this.tail_len);
+        }
+    }
 }
 
 #[stable(feature = "vec_drain_as_slice", since = "1.46.0")]
@@ -131,7 +202,7 @@ impl<T, A: Allocator> Drop for Drain<'_, T, A> {
 
         let mut vec = self.vec;
 
-        if mem::size_of::<T>() == 0 {
+        if T::IS_ZST {
             // ZSTs have no identity, so we don't need to move them around, we only need to drop the correct amount.
             // this can be achieved by manipulating the Vec length instead of moving values out from `iter`.
             unsafe {
diff --git a/library/alloc/src/vec/drain_filter.rs b/library/alloc/src/vec/drain_filter.rs
index 3c37c92ae..8c03f1692 100644
--- a/library/alloc/src/vec/drain_filter.rs
+++ b/library/alloc/src/vec/drain_filter.rs
@@ -1,6 +1,7 @@
 use crate::alloc::{Allocator, Global};
-use core::ptr::{self};
-use core::slice::{self};
+use core::mem::{self, ManuallyDrop};
+use core::ptr;
+use core::slice;
 
 use super::Vec;
 
@@ -54,6 +55,61 @@ where
     pub fn allocator(&self) -> &A {
         self.vec.allocator()
     }
+
+    /// Keep unyielded elements in the source `Vec`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(drain_filter)]
+    /// #![feature(drain_keep_rest)]
+    ///
+    /// let mut vec = vec!['a', 'b', 'c'];
+    /// let mut drain = vec.drain_filter(|_| true);
+    ///
+    /// assert_eq!(drain.next().unwrap(), 'a');
+    ///
+    /// // This call keeps 'b' and 'c' in the vec.
+    /// drain.keep_rest();
+    ///
+    /// // If we wouldn't call `keep_rest()`,
+    /// // `vec` would be empty.
+    /// assert_eq!(vec, ['b', 'c']);
+    /// ```
+    #[unstable(feature = "drain_keep_rest", issue = "101122")]
+    pub fn keep_rest(self) {
+        // At this moment layout looks like this:
+        //
+        //  _____________________/-- old_len
+        // /                     \
+        // [kept] [yielded] [tail]
+        //        \_______/ ^-- idx
+        //                \-- del
+        //
+        // Normally `Drop` impl would drop [tail] (via .for_each(drop), ie still calling `pred`)
+        //
+        // 1. Move [tail] after [kept]
+        // 2. Update length of the original vec to `old_len - del`
+        //    a. In case of ZST, this is the only thing we want to do
+        // 3. Do *not* drop self, as everything is put in a consistent state already, there is nothing to do
+        let mut this = ManuallyDrop::new(self);
+
+        unsafe {
+            // ZSTs have no identity, so we don't need to move them around.
+            let needs_move = mem::size_of::<T>() != 0;
+
+            if needs_move && this.idx < this.old_len && this.del > 0 {
+                let ptr = this.vec.as_mut_ptr();
+                let src = ptr.add(this.idx);
+                let dst = src.sub(this.del);
+                let tail_len = this.old_len - this.idx;
+                src.copy_to(dst, tail_len);
+            }
+
+            let new_len = this.old_len - this.del;
+            this.vec.set_len(new_len);
+        }
+    }
 }
 
 #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
diff --git a/library/alloc/src/vec/in_place_collect.rs b/library/alloc/src/vec/in_place_collect.rs
index 55dcb84ad..87d61deb1 100644
--- a/library/alloc/src/vec/in_place_collect.rs
+++ b/library/alloc/src/vec/in_place_collect.rs
@@ -55,6 +55,9 @@
 //! This is handled by the [`InPlaceDrop`] guard for sink items (`U`) and by
 //! [`vec::IntoIter::forget_allocation_drop_remaining()`] for remaining source items (`T`).
 //!
+//! If dropping any remaining source item (`T`) panics then [`InPlaceDstBufDrop`] will handle dropping
+//! the already collected sink items (`U`) and freeing the allocation.
+//!
 //! [`vec::IntoIter::forget_allocation_drop_remaining()`]: super::IntoIter::forget_allocation_drop_remaining()
 //!
 //! # O(1) collect
@@ -135,10 +138,10 @@
 //! vec.truncate(write_idx);
 //! ```
 use core::iter::{InPlaceIterable, SourceIter, TrustedRandomAccessNoCoerce};
-use core::mem::{self, ManuallyDrop};
+use core::mem::{self, ManuallyDrop, SizedTypeProperties};
 use core::ptr::{self};
 
-use super::{InPlaceDrop, SpecFromIter, SpecFromIterNested, Vec};
+use super::{InPlaceDrop, InPlaceDstBufDrop, SpecFromIter, SpecFromIterNested, Vec};
 
 /// Specialization marker for collecting an iterator pipeline into a Vec while reusing the
 /// source allocation, i.e. executing the pipeline in place.
@@ -154,7 +157,7 @@ where
     default fn from_iter(mut iterator: I) -> Self {
         // See "Layout constraints" section in the module documentation. We rely on const
         // optimization here since these conditions currently cannot be expressed as trait bounds
-        if mem::size_of::<T>() == 0
+        if T::IS_ZST
             || mem::size_of::<T>()
                 != mem::size_of::<<<I as SourceIter>::Source as AsVecIntoIter>::Item>()
             || mem::align_of::<T>()
@@ -191,14 +194,17 @@ where
             );
         }
 
-        // Drop any remaining values at the tail of the source but prevent drop of the allocation
-        // itself once IntoIter goes out of scope.
-        // If the drop panics then we also leak any elements collected into dst_buf.
+        // The ownership of the allocation and the new `T` values is temporarily moved into `dst_guard`.
+        // This is safe because `forget_allocation_drop_remaining` immediately forgets the allocation
+        // before any panic can occur in order to avoid any double free, and then proceeds to drop
+        // any remaining values at the tail of the source.
         //
         // Note: This access to the source wouldn't be allowed by the TrustedRandomIteratorNoCoerce
         // contract (used by SpecInPlaceCollect below). But see the "O(1) collect" section in the
         // module documenttation why this is ok anyway.
+        let dst_guard = InPlaceDstBufDrop { ptr: dst_buf, len, cap };
         src.forget_allocation_drop_remaining();
+        mem::forget(dst_guard);
 
         let vec = unsafe { Vec::from_raw_parts(dst_buf, len, cap) };
 
@@ -267,7 +273,7 @@ where
             // one slot in the underlying storage will have been freed up and we can immediately
             // write back the result.
             unsafe {
-                let dst = dst_buf.offset(i as isize);
+                let dst = dst_buf.add(i);
                 debug_assert!(dst as *const _ <= end, "InPlaceIterable contract violation");
                 ptr::write(dst, self.__iterator_get_unchecked(i));
                 // Since this executes user code which can panic we have to bump the pointer
diff --git a/library/alloc/src/vec/in_place_drop.rs b/library/alloc/src/vec/in_place_drop.rs
index 1b1ef9130..25ca33c6a 100644
--- a/library/alloc/src/vec/in_place_drop.rs
+++ b/library/alloc/src/vec/in_place_drop.rs
@@ -22,3 +22,18 @@ impl<T> Drop for InPlaceDrop<T> {
         }
     }
 }
+
+// A helper struct for in-place collection that drops the destination allocation and elements,
+// to avoid leaking them if some other destructor panics.
+pub(super) struct InPlaceDstBufDrop<T> {
+    pub(super) ptr: *mut T,
+    pub(super) len: usize,
+    pub(super) cap: usize,
+}
+
+impl<T> Drop for InPlaceDstBufDrop<T> {
+    #[inline]
+    fn drop(&mut self) {
+        unsafe { super::Vec::from_raw_parts(self.ptr, self.len, self.cap) };
+    }
+}
diff --git a/library/alloc/src/vec/into_iter.rs b/library/alloc/src/vec/into_iter.rs
index 1b483e3fc..02cc7691a 100644
--- a/library/alloc/src/vec/into_iter.rs
+++ b/library/alloc/src/vec/into_iter.rs
@@ -4,12 +4,11 @@ use crate::alloc::{Allocator, Global};
 use crate::raw_vec::RawVec;
 use core::array;
 use core::fmt;
-use core::intrinsics::arith_offset;
 use core::iter::{
     FusedIterator, InPlaceIterable, SourceIter, TrustedLen, TrustedRandomAccessNoCoerce,
 };
 use core::marker::PhantomData;
-use core::mem::{self, ManuallyDrop, MaybeUninit};
+use core::mem::{self, ManuallyDrop, MaybeUninit, SizedTypeProperties};
 #[cfg(not(no_global_oom_handling))]
 use core::ops::Deref;
 use core::ptr::{self, NonNull};
@@ -96,13 +95,16 @@ impl<T, A: Allocator> IntoIter<T, A> {
     }
 
     /// Drops remaining elements and relinquishes the backing allocation.
+    /// This method guarantees it won't panic before relinquishing
+    /// the backing allocation.
     ///
     /// This is roughly equivalent to the following, but more efficient
     ///
     /// ```
     /// # let mut into_iter = Vec::<u8>::with_capacity(10).into_iter();
+    /// let mut into_iter = std::mem::replace(&mut into_iter, Vec::new().into_iter());
     /// (&mut into_iter).for_each(core::mem::drop);
-    /// unsafe { core::ptr::write(&mut into_iter, Vec::new().into_iter()); }
+    /// std::mem::forget(into_iter);
     /// ```
     ///
     /// This method is used by in-place iteration, refer to the vec::in_place_collect
@@ -119,6 +121,8 @@ impl<T, A: Allocator> IntoIter<T, A> {
         self.ptr = self.buf.as_ptr();
         self.end = self.buf.as_ptr();
 
+        // Dropping the remaining elements can panic, so this needs to be
+        // done only after updating the other fields.
         unsafe {
             ptr::drop_in_place(remaining);
         }
@@ -148,19 +152,19 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
 
     #[inline]
     fn next(&mut self) -> Option<T> {
-        if self.ptr as *const _ == self.end {
+        if self.ptr == self.end {
             None
-        } else if mem::size_of::<T>() == 0 {
+        } else if T::IS_ZST {
             // purposefully don't use 'ptr.offset' because for
             // vectors with 0-size elements this would return the
             // same pointer.
-            self.ptr = unsafe { arith_offset(self.ptr as *const i8, 1) as *mut T };
+            self.ptr = self.ptr.wrapping_byte_add(1);
 
             // Make up a value of this ZST.
             Some(unsafe { mem::zeroed() })
         } else {
             let old = self.ptr;
-            self.ptr = unsafe { self.ptr.offset(1) };
+            self.ptr = unsafe { self.ptr.add(1) };
 
             Some(unsafe { ptr::read(old) })
         }
@@ -168,7 +172,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
 
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let exact = if mem::size_of::<T>() == 0 {
+        let exact = if T::IS_ZST {
             self.end.addr().wrapping_sub(self.ptr.addr())
         } else {
             unsafe { self.end.sub_ptr(self.ptr) }
@@ -180,11 +184,11 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
     fn advance_by(&mut self, n: usize) -> Result<(), usize> {
         let step_size = self.len().min(n);
         let to_drop = ptr::slice_from_raw_parts_mut(self.ptr as *mut T, step_size);
-        if mem::size_of::<T>() == 0 {
+        if T::IS_ZST {
             // SAFETY: due to unchecked casts of unsigned amounts to signed offsets the wraparound
             // effectively results in unsigned pointers representing positions 0..usize::MAX,
             // which is valid for ZSTs.
-            self.ptr = unsafe { arith_offset(self.ptr as *const i8, step_size as isize) as *mut T }
+            self.ptr = self.ptr.wrapping_byte_add(step_size);
         } else {
             // SAFETY: the min() above ensures that step_size is in bounds
             self.ptr = unsafe { self.ptr.add(step_size) };
@@ -210,16 +214,16 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
 
         let len = self.len();
 
-        if mem::size_of::<T>() == 0 {
+        if T::IS_ZST {
             if len < N {
                 self.forget_remaining_elements();
                 // Safety: ZSTs can be conjured ex nihilo, only the amount has to be correct
                 return Err(unsafe { array::IntoIter::new_unchecked(raw_ary, 0..len) });
             }
 
-            self.ptr = unsafe { arith_offset(self.ptr as *const i8, N as isize) as *mut T };
+            self.ptr = self.ptr.wrapping_byte_add(N);
             // Safety: ditto
-            return Ok(unsafe { MaybeUninit::array_assume_init(raw_ary) });
+            return Ok(unsafe { raw_ary.transpose().assume_init() });
         }
 
         if len < N {
@@ -237,7 +241,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
         return unsafe {
             ptr::copy_nonoverlapping(self.ptr, raw_ary.as_mut_ptr() as *mut T, N);
             self.ptr = self.ptr.add(N);
-            Ok(MaybeUninit::array_assume_init(raw_ary))
+            Ok(raw_ary.transpose().assume_init())
         };
     }
 
@@ -254,7 +258,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
         // that `T: Copy` so reading elements from the buffer doesn't invalidate
         // them for `Drop`.
         unsafe {
-            if mem::size_of::<T>() == 0 { mem::zeroed() } else { ptr::read(self.ptr.add(i)) }
+            if T::IS_ZST { mem::zeroed() } else { ptr::read(self.ptr.add(i)) }
         }
     }
 }
@@ -265,14 +269,14 @@ impl<T, A: Allocator> DoubleEndedIterator for IntoIter<T, A> {
     fn next_back(&mut self) -> Option<T> {
         if self.end == self.ptr {
             None
-        } else if mem::size_of::<T>() == 0 {
+        } else if T::IS_ZST {
             // See above for why 'ptr.offset' isn't used
-            self.end = unsafe { arith_offset(self.end as *const i8, -1) as *mut T };
+            self.end = self.end.wrapping_byte_sub(1);
 
             // Make up a value of this ZST.
             Some(unsafe { mem::zeroed() })
         } else {
-            self.end = unsafe { self.end.offset(-1) };
+            self.end = unsafe { self.end.sub(1) };
 
             Some(unsafe { ptr::read(self.end) })
         }
@@ -281,14 +285,12 @@ impl<T, A: Allocator> DoubleEndedIterator for IntoIter<T, A> {
     #[inline]
     fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
         let step_size = self.len().min(n);
-        if mem::size_of::<T>() == 0 {
+        if T::IS_ZST {
             // SAFETY: same as for advance_by()
-            self.end = unsafe {
-                arith_offset(self.end as *const i8, step_size.wrapping_neg() as isize) as *mut T
-            }
+            self.end = self.end.wrapping_byte_sub(step_size);
         } else {
             // SAFETY: same as for advance_by()
-            self.end = unsafe { self.end.offset(step_size.wrapping_neg() as isize) };
+            self.end = unsafe { self.end.sub(step_size) };
         }
         let to_drop = ptr::slice_from_raw_parts_mut(self.end as *mut T, step_size);
         // SAFETY: same as for advance_by()
diff --git a/library/alloc/src/vec/is_zero.rs b/library/alloc/src/vec/is_zero.rs
index 92a32779b..8e652d676 100644
--- a/library/alloc/src/vec/is_zero.rs
+++ b/library/alloc/src/vec/is_zero.rs
@@ -1,3 +1,5 @@
+use core::num::{Saturating, Wrapping};
+
 use crate::boxed::Box;
 
 #[rustc_specialization_trait]
@@ -144,3 +146,39 @@ impl_is_zero_option_of_nonzero!(
     NonZeroUsize,
     NonZeroIsize,
 );
+
+unsafe impl<T: IsZero> IsZero for Wrapping<T> {
+    #[inline]
+    fn is_zero(&self) -> bool {
+        self.0.is_zero()
+    }
+}
+
+unsafe impl<T: IsZero> IsZero for Saturating<T> {
+    #[inline]
+    fn is_zero(&self) -> bool {
+        self.0.is_zero()
+    }
+}
+
+macro_rules! impl_for_optional_bool {
+    ($($t:ty,)+) => {$(
+        unsafe impl IsZero for $t {
+            #[inline]
+            fn is_zero(&self) -> bool {
+                // SAFETY: This is *not* a stable layout guarantee, but
+                // inside `core` we're allowed to rely on the current rustc
+                // behaviour that options of bools will be one byte with
+                // no padding, so long as they're nested less than 254 deep.
+                let raw: u8 = unsafe { core::mem::transmute(*self) };
+                raw == 0
+            }
+        }
+    )+};
+}
+impl_for_optional_bool! {
+    Option<bool>,
+    Option<Option<bool>>,
+    Option<Option<Option<bool>>>,
+    // Could go further, but not worth the metadata overhead
+}
diff --git a/library/alloc/src/vec/mod.rs b/library/alloc/src/vec/mod.rs
index fa9f2131c..bbbdc3aa2 100644
--- a/library/alloc/src/vec/mod.rs
+++ b/library/alloc/src/vec/mod.rs
@@ -59,12 +59,12 @@ use core::cmp::Ordering;
 use core::convert::TryFrom;
 use core::fmt;
 use core::hash::{Hash, Hasher};
-use core::intrinsics::{arith_offset, assume};
+use core::intrinsics::assume;
 use core::iter;
 #[cfg(not(no_global_oom_handling))]
 use core::iter::FromIterator;
 use core::marker::PhantomData;
-use core::mem::{self, ManuallyDrop, MaybeUninit};
+use core::mem::{self, ManuallyDrop, MaybeUninit, SizedTypeProperties};
 use core::ops::{self, Index, IndexMut, Range, RangeBounds};
 use core::ptr::{self, NonNull};
 use core::slice::{self, SliceIndex};
@@ -125,7 +125,7 @@ use self::set_len_on_drop::SetLenOnDrop;
 mod set_len_on_drop;
 
 #[cfg(not(no_global_oom_handling))]
-use self::in_place_drop::InPlaceDrop;
+use self::in_place_drop::{InPlaceDrop, InPlaceDstBufDrop};
 
 #[cfg(not(no_global_oom_handling))]
 mod in_place_drop;
@@ -436,7 +436,7 @@ impl<T> Vec<T> {
     /// an explanation of the difference between length and capacity, see
     /// *[Capacity and reallocation]*.
     ///
-    /// If it is imporant to know the exact allocated capacity of a `Vec`,
+    /// If it is important to know the exact allocated capacity of a `Vec`,
     /// always use the [`capacity`] method after construction.
     ///
     /// For `Vec<T>` where `T` is a zero-sized type, there will be no allocation
@@ -483,15 +483,13 @@ impl<T> Vec<T> {
         Self::with_capacity_in(capacity, Global)
     }
 
-    /// Creates a `Vec<T>` directly from the raw components of another vector.
+    /// Creates a `Vec<T>` directly from a pointer, a capacity, and a length.
     ///
     /// # Safety
     ///
     /// This is highly unsafe, due to the number of invariants that aren't
     /// checked:
     ///
-    /// * `ptr` needs to have been previously allocated via [`String`]/`Vec<T>`
-    ///   (at least, it's highly likely to be incorrect if it wasn't).
     /// * `T` needs to have the same alignment as what `ptr` was allocated with.
     ///   (`T` having a less strict alignment is not sufficient, the alignment really
     ///   needs to be equal to satisfy the [`dealloc`] requirement that memory must be
@@ -500,6 +498,14 @@ impl<T> Vec<T> {
     ///   to be the same size as the pointer was allocated with. (Because similar to
     ///   alignment, [`dealloc`] must be called with the same layout `size`.)
     /// * `length` needs to be less than or equal to `capacity`.
+    /// * The first `length` values must be properly initialized values of type `T`.
+    /// * `capacity` needs to be the capacity that the pointer was allocated with.
+    /// * The allocated size in bytes must be no larger than `isize::MAX`.
+    ///   See the safety documentation of [`pointer::offset`].
+    ///
+    /// These requirements are always upheld by any `ptr` that has been allocated
+    /// via `Vec<T>`. Other allocation sources are allowed if the invariants are
+    /// upheld.
     ///
     /// Violating these may cause problems like corrupting the allocator's
     /// internal data structures. For example it is normally **not** safe
@@ -542,8 +548,8 @@ impl<T> Vec<T> {
     ///
     /// unsafe {
     ///     // Overwrite memory with 4, 5, 6
-    ///     for i in 0..len as isize {
-    ///         ptr::write(p.offset(i), 4 + i);
+    ///     for i in 0..len {
+    ///         ptr::write(p.add(i), 4 + i);
     ///     }
     ///
     ///     // Put everything back together into a Vec
@@ -551,6 +557,32 @@ impl<T> Vec<T> {
     ///     assert_eq!(rebuilt, [4, 5, 6]);
     /// }
     /// ```
+    ///
+    /// Using memory that was allocated elsewhere:
+    ///
+    /// ```rust
+    /// #![feature(allocator_api)]
+    ///
+    /// use std::alloc::{AllocError, Allocator, Global, Layout};
+    ///
+    /// fn main() {
+    ///     let layout = Layout::array::<u32>(16).expect("overflow cannot happen");
+    ///
+    ///     let vec = unsafe {
+    ///         let mem = match Global.allocate(layout) {
+    ///             Ok(mem) => mem.cast::<u32>().as_ptr(),
+    ///             Err(AllocError) => return,
+    ///         };
+    ///
+    ///         mem.write(1_000_000);
+    ///
+    ///         Vec::from_raw_parts_in(mem, 1, 16, Global)
+    ///     };
+    ///
+    ///     assert_eq!(vec, &[1_000_000]);
+    ///     assert_eq!(vec.capacity(), 16);
+    /// }
+    /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub unsafe fn from_raw_parts(ptr: *mut T, length: usize, capacity: usize) -> Self {
@@ -591,7 +623,7 @@ impl<T, A: Allocator> Vec<T, A> {
     /// an explanation of the difference between length and capacity, see
     /// *[Capacity and reallocation]*.
     ///
-    /// If it is imporant to know the exact allocated capacity of a `Vec`,
+    /// If it is important to know the exact allocated capacity of a `Vec`,
     /// always use the [`capacity`] method after construction.
     ///
     /// For `Vec<T, A>` where `T` is a zero-sized type, there will be no allocation
@@ -641,21 +673,30 @@ impl<T, A: Allocator> Vec<T, A> {
         Vec { buf: RawVec::with_capacity_in(capacity, alloc), len: 0 }
     }
 
-    /// Creates a `Vec<T, A>` directly from the raw components of another vector.
+    /// Creates a `Vec<T, A>` directly from a pointer, a capacity, a length,
+    /// and an allocator.
     ///
     /// # Safety
     ///
     /// This is highly unsafe, due to the number of invariants that aren't
     /// checked:
     ///
-    /// * `ptr` needs to have been previously allocated via [`String`]/`Vec<T>`
-    ///   (at least, it's highly likely to be incorrect if it wasn't).
-    /// * `T` needs to have the same size and alignment as what `ptr` was allocated with.
+    /// * `T` needs to have the same alignment as what `ptr` was allocated with.
     ///   (`T` having a less strict alignment is not sufficient, the alignment really
     ///   needs to be equal to satisfy the [`dealloc`] requirement that memory must be
     ///   allocated and deallocated with the same layout.)
+    /// * The size of `T` times the `capacity` (ie. the allocated size in bytes) needs
+    ///   to be the same size as the pointer was allocated with. (Because similar to
+    ///   alignment, [`dealloc`] must be called with the same layout `size`.)
     /// * `length` needs to be less than or equal to `capacity`.
-    /// * `capacity` needs to be the capacity that the pointer was allocated with.
+    /// * The first `length` values must be properly initialized values of type `T`.
+    /// * `capacity` needs to [*fit*] the layout size that the pointer was allocated with.
+    /// * The allocated size in bytes must be no larger than `isize::MAX`.
+    ///   See the safety documentation of [`pointer::offset`].
+    ///
+    /// These requirements are always upheld by any `ptr` that has been allocated
+    /// via `Vec<T, A>`. Other allocation sources are allowed if the invariants are
+    /// upheld.
     ///
     /// Violating these may cause problems like corrupting the allocator's
     /// internal data structures. For example it is **not** safe
@@ -673,6 +714,7 @@ impl<T, A: Allocator> Vec<T, A> {
     ///
     /// [`String`]: crate::string::String
     /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
+    /// [*fit*]: crate::alloc::Allocator#memory-fitting
     ///
     /// # Examples
     ///
@@ -702,8 +744,8 @@ impl<T, A: Allocator> Vec<T, A> {
     ///
     /// unsafe {
     ///     // Overwrite memory with 4, 5, 6
-    ///     for i in 0..len as isize {
-    ///         ptr::write(p.offset(i), 4 + i);
+    ///     for i in 0..len {
+    ///         ptr::write(p.add(i), 4 + i);
     ///     }
     ///
     ///     // Put everything back together into a Vec
@@ -711,6 +753,29 @@ impl<T, A: Allocator> Vec<T, A> {
     ///     assert_eq!(rebuilt, [4, 5, 6]);
     /// }
     /// ```
+    ///
+    /// Using memory that was allocated elsewhere:
+    ///
+    /// ```rust
+    /// use std::alloc::{alloc, Layout};
+    ///
+    /// fn main() {
+    ///     let layout = Layout::array::<u32>(16).expect("overflow cannot happen");
+    ///     let vec = unsafe {
+    ///         let mem = alloc(layout).cast::<u32>();
+    ///         if mem.is_null() {
+    ///             return;
+    ///         }
+    ///
+    ///         mem.write(1_000_000);
+    ///
+    ///         Vec::from_raw_parts(mem, 1, 16)
+    ///     };
+    ///
+    ///     assert_eq!(vec, &[1_000_000]);
+    ///     assert_eq!(vec.capacity(), 16);
+    /// }
+    /// ```
     #[inline]
     #[unstable(feature = "allocator_api", issue = "32838")]
     pub unsafe fn from_raw_parts_in(ptr: *mut T, length: usize, capacity: usize, alloc: A) -> Self {
@@ -803,13 +868,14 @@ impl<T, A: Allocator> Vec<T, A> {
         (ptr, len, capacity, alloc)
     }
 
-    /// Returns the number of elements the vector can hold without
+    /// Returns the total number of elements the vector can hold without
     /// reallocating.
     ///
     /// # Examples
     ///
     /// ```
-    /// let vec: Vec<i32> = Vec::with_capacity(10);
+    /// let mut vec: Vec<i32> = Vec::with_capacity(10);
+    /// vec.push(42);
     /// assert_eq!(vec.capacity(), 10);
     /// ```
     #[inline]
@@ -875,7 +941,8 @@ impl<T, A: Allocator> Vec<T, A> {
     /// in the given `Vec<T>`. The collection may reserve more space to speculatively avoid
     /// frequent reallocations. After calling `try_reserve`, capacity will be
     /// greater than or equal to `self.len() + additional` if it returns
-    /// `Ok(())`. Does nothing if capacity is already sufficient.
+    /// `Ok(())`. Does nothing if capacity is already sufficient. This method
+    /// preserves the contents even if an error occurs.
     ///
     /// # Errors
     ///
@@ -1393,7 +1460,7 @@ impl<T, A: Allocator> Vec<T, A> {
                 if index < len {
                     // Shift everything over to make space. (Duplicating the
                     // `index`th element into two consecutive places.)
-                    ptr::copy(p, p.offset(1), len - index);
+                    ptr::copy(p, p.add(1), len - index);
                 } else if index == len {
                     // No elements need shifting.
                 } else {
@@ -1455,7 +1522,7 @@ impl<T, A: Allocator> Vec<T, A> {
                 ret = ptr::read(ptr);
 
                 // Shift everything down to fill in that spot.
-                ptr::copy(ptr.offset(1), ptr, len - index - 1);
+                ptr::copy(ptr.add(1), ptr, len - index - 1);
             }
             self.set_len(len - 1);
             ret
@@ -1773,6 +1840,51 @@ impl<T, A: Allocator> Vec<T, A> {
         }
     }
 
+    /// Appends an element if there is sufficient spare capacity, otherwise an error is returned
+    /// with the element.
+    ///
+    /// Unlike [`push`] this method will not reallocate when there's insufficient capacity.
+    /// The caller should use [`reserve`] or [`try_reserve`] to ensure that there is enough capacity.
+    ///
+    /// [`push`]: Vec::push
+    /// [`reserve`]: Vec::reserve
+    /// [`try_reserve`]: Vec::try_reserve
+    ///
+    /// # Examples
+    ///
+    /// A manual, panic-free alternative to [`FromIterator`]:
+    ///
+    /// ```
+    /// #![feature(vec_push_within_capacity)]
+    ///
+    /// use std::collections::TryReserveError;
+    /// fn from_iter_fallible<T>(iter: impl Iterator<Item=T>) -> Result<Vec<T>, TryReserveError> {
+    ///     let mut vec = Vec::new();
+    ///     for value in iter {
+    ///         if let Err(value) = vec.push_within_capacity(value) {
+    ///             vec.try_reserve(1)?;
+    ///             // this cannot fail, the previous line either returned or added at least 1 free slot
+    ///             let _ = vec.push_within_capacity(value);
+    ///         }
+    ///     }
+    ///     Ok(vec)
+    /// }
+    /// assert_eq!(from_iter_fallible(0..100), Ok(Vec::from_iter(0..100)));
+    /// ```
+    #[inline]
+    #[unstable(feature = "vec_push_within_capacity", issue = "100486")]
+    pub fn push_within_capacity(&mut self, value: T) -> Result<(), T> {
+        if self.len == self.buf.capacity() {
+            return Err(value);
+        }
+        unsafe {
+            let end = self.as_mut_ptr().add(self.len);
+            ptr::write(end, value);
+            self.len += 1;
+        }
+        Ok(())
+    }
+
     /// Removes the last element from a vector and returns it, or [`None`] if it
     /// is empty.
     ///
@@ -1888,9 +2000,7 @@ impl<T, A: Allocator> Vec<T, A> {
         unsafe {
             // set self.vec length's to start, to be safe in case Drain is leaked
             self.set_len(start);
-            // Use the borrow in the IterMut to indicate borrowing behavior of the
-            // whole Drain iterator (like &mut T).
-            let range_slice = slice::from_raw_parts_mut(self.as_mut_ptr().add(start), end - start);
+            let range_slice = slice::from_raw_parts(self.as_ptr().add(start), end - start);
             Drain {
                 tail_start: end,
                 tail_len: len - end,
@@ -2082,7 +2192,6 @@ impl<T, A: Allocator> Vec<T, A> {
     /// static_ref[0] += 1;
     /// assert_eq!(static_ref, &[2, 2, 3]);
     /// ```
-    #[cfg(not(no_global_oom_handling))]
     #[stable(feature = "vec_leak", since = "1.47.0")]
     #[inline]
     pub fn leak<'a>(self) -> &'a mut [T]
@@ -2346,7 +2455,7 @@ impl<T, A: Allocator, const N: usize> Vec<[T; N], A> {
     #[unstable(feature = "slice_flatten", issue = "95629")]
     pub fn into_flattened(self) -> Vec<T, A> {
         let (ptr, len, cap, alloc) = self.into_raw_parts_with_alloc();
-        let (new_len, new_cap) = if mem::size_of::<T>() == 0 {
+        let (new_len, new_cap) = if T::IS_ZST {
             (len.checked_mul(N).expect("vec len overflow"), usize::MAX)
         } else {
             // SAFETY:
@@ -2408,7 +2517,7 @@ impl<T, A: Allocator> Vec<T, A> {
             // Write all elements except the last one
             for _ in 1..n {
                 ptr::write(ptr, value.next());
-                ptr = ptr.offset(1);
+                ptr = ptr.add(1);
                 // Increment the length in every step in case next() panics
                 local_len.increment_len(1);
             }
@@ -2676,8 +2785,8 @@ impl<T, A: Allocator> IntoIterator for Vec<T, A> {
             let mut me = ManuallyDrop::new(self);
             let alloc = ManuallyDrop::new(ptr::read(me.allocator()));
             let begin = me.as_mut_ptr();
-            let end = if mem::size_of::<T>() == 0 {
-                arith_offset(begin as *const i8, me.len() as isize) as *const T
+            let end = if T::IS_ZST {
+                begin.wrapping_byte_add(me.len())
             } else {
                 begin.add(me.len()) as *const T
             };
@@ -2927,6 +3036,8 @@ unsafe impl<#[may_dangle] T, A: Allocator> Drop for Vec<T, A> {
 #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
 impl<T> const Default for Vec<T> {
     /// Creates an empty `Vec<T>`.
+    ///
+    /// The vector will not allocate until elements are pushed onto it.
     fn default() -> Vec<T> {
         Vec::new()
     }
diff --git a/library/alloc/src/vec/spec_extend.rs b/library/alloc/src/vec/spec_extend.rs
index 506ee0ecf..1ea9c827a 100644
--- a/library/alloc/src/vec/spec_extend.rs
+++ b/library/alloc/src/vec/spec_extend.rs
@@ -39,7 +39,7 @@ where
                 let mut local_len = SetLenOnDrop::new(&mut self.len);
                 iterator.for_each(move |element| {
                     ptr::write(ptr, element);
-                    ptr = ptr.offset(1);
+                    ptr = ptr.add(1);
                     // Since the loop executes user code which can panic we have to bump the pointer
                     // after each step.
                     // NB can't overflow since we would have had to alloc the address space