Merge branch 'master' into binary-heap-ta

30 files changed, 3225 insertions, 1941 deletions

diff --git a/library/alloc/src/collections/binary_heap.rs b/library/alloc/src/collections/binary_heap/mod.rs
index c9f0e086495..f5e6bd20ea5 100644
--- a/library/alloc/src/collections/binary_heap.rs
+++ b/library/alloc/src/collections/binary_heap/mod.rs
@@ -145,8 +145,9 @@
 
 use core::alloc::Allocator;
 use core::fmt;
-use core::iter::{FromIterator, FusedIterator, InPlaceIterable, SourceIter, TrustedLen};
+use core::iter::{FusedIterator, InPlaceIterable, SourceIter, TrustedLen};
 use core::mem::{self, swap, ManuallyDrop};
+use core::num::NonZeroUsize;
 use core::ops::{Deref, DerefMut};
 use core::ptr;
 
@@ -155,8 +156,6 @@ use crate::collections::TryReserveError;
 use crate::slice;
 use crate::vec::{self, AsVecIntoIter, Vec};
 
-use super::SpecExtend;
-
 #[cfg(test)]
 mod tests;
 
@@ -167,12 +166,20 @@ mod tests;
 /// It is a logic error for an item to be modified in such a way that the
 /// item's ordering relative to any other item, as determined by the [`Ord`]
 /// trait, changes while it is in the heap. This is normally only possible
-/// through [`Cell`], [`RefCell`], global state, I/O, or unsafe code. The
+/// through interior mutability, global state, I/O, or unsafe code. The
 /// behavior resulting from such a logic error is not specified, but will
 /// be encapsulated to the `BinaryHeap` that observed the logic error and not
 /// result in undefined behavior. This could include panics, incorrect results,
 /// aborts, memory leaks, and non-termination.
 ///
+/// As long as no elements change their relative order while being in the heap
+/// as described above, the API of `BinaryHeap` guarantees that the heap
+/// invariant remains intact i.e. its methods all behave as documented. For
+/// example if a method is documented as iterating in sorted order, that's
+/// guaranteed to work as long as elements in the heap have not changed order,
+/// even in the presence of closures getting unwinded out of, iterators getting
+/// leaked, and similar foolishness.
+///
 /// # Examples
 ///
 /// ```
@@ -258,7 +265,6 @@ mod tests;
 /// more detailed analysis.
 ///
 /// [`core::cmp::Reverse`]: core::cmp::Reverse
-/// [`Ord`]: core::cmp::Ord
 /// [`Cell`]: core::cell::Cell
 /// [`RefCell`]: core::cell::RefCell
 /// [push]: BinaryHeap::push
@@ -274,6 +280,7 @@ pub struct BinaryHeap<
     data: Vec<T, A>,
 }
 
+// XXX: PeekMut<A>
 /// Structure wrapping a mutable reference to the greatest item on a
 /// `BinaryHeap`.
 ///
@@ -282,13 +289,11 @@ pub struct BinaryHeap<
 ///
 /// [`peek_mut`]: BinaryHeap::peek_mut
 #[stable(feature = "binary_heap_peek_mut", since = "1.12.0")]
-pub struct PeekMut<
-    'a,
-    T: 'a + Ord,
-    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator + 'a = Global,
-> {
-    heap: &'a mut BinaryHeap<T, A>,
-    sift: bool,
+pub struct PeekMut<'a, T: 'a + Ord> {
+    heap: &'a mut BinaryHeap<T>,
+    // If a set_len + sift_down are required, this is Some. If a &mut T has not
+    // yet been exposed to peek_mut()'s caller, it's None.
+    original_len: Option<NonZeroUsize>,
 }
 
 #[stable(feature = "collection_debug", since = "1.17.0")]
@@ -301,7 +306,14 @@ impl<'a, T: Ord + fmt::Debug, A: Allocator + 'a> fmt::Debug for PeekMut<'a, T, A
 #[stable(feature = "binary_heap_peek_mut", since = "1.12.0")]
 impl<'a, T: Ord, A: Allocator + 'a> Drop for PeekMut<'a, T, A> {
     fn drop(&mut self) {
-        if self.sift {
+        if let Some(original_len) = self.original_len {
+            // SAFETY: That's how many elements were in the Vec at the time of
+            // the PeekMut::deref_mut call, and therefore also at the time of
+            // the BinaryHeap::peek_mut call. Since the PeekMut did not end up
+            // getting leaked, we are now undoing the leak amplification that
+            // the DerefMut prepared for.
+            unsafe { self.heap.data.set_len(original_len.get()) };
+
             // SAFETY: PeekMut is only instantiated for non-empty heaps.
             unsafe { self.heap.sift_down(0) };
         }
@@ -322,19 +334,46 @@ impl<'a, T: Ord, A: Allocator + 'a> Deref for PeekMut<'a, T, A> {
 impl<'a, T: Ord, A: Allocator + 'a> DerefMut for PeekMut<'a, T, A> {
     fn deref_mut(&mut self) -> &mut T {
         debug_assert!(!self.heap.is_empty());
-        self.sift = true;
+
+        let len = self.heap.len();
+        if len > 1 {
+            // Here we preemptively leak all the rest of the underlying vector
+            // after the currently max element. If the caller mutates the &mut T
+            // we're about to give them, and then leaks the PeekMut, all these
+            // elements will remain leaked. If they don't leak the PeekMut, then
+            // either Drop or PeekMut::pop will un-leak the vector elements.
+            //
+            // This is technique is described throughout several other places in
+            // the standard library as "leak amplification".
+            unsafe {
+                // SAFETY: len > 1 so len != 0.
+                self.original_len = Some(NonZeroUsize::new_unchecked(len));
+                // SAFETY: len > 1 so all this does for now is leak elements,
+                // which is safe.
+                self.heap.data.set_len(1);
+            }
+        }
+
         // SAFE: PeekMut is only instantiated for non-empty heaps
         unsafe { self.heap.data.get_unchecked_mut(0) }
     }
 }
 
+// XXX: PeekMut<A>
 impl<'a, T: Ord, A: Allocator + 'a> PeekMut<'a, T, A> {
     /// Removes the peeked value from the heap and returns it.
     #[stable(feature = "binary_heap_peek_mut_pop", since = "1.18.0")]
-    pub fn pop(mut this: PeekMut<'a, T, A>) -> T {
-        let value = this.heap.pop().unwrap();
-        this.sift = false;
-        value
+    pub fn pop(mut this: PeekMut<'a, T>) -> T {
+        if let Some(original_len) = this.original_len.take() {
+            // SAFETY: This is how many elements were in the Vec at the time of
+            // the BinaryHeap::peek_mut call.
+            unsafe { this.heap.data.set_len(original_len.get()) };
+
+            // Unlike in Drop, here we don't also need to do a sift_down even if
+            // the caller could've mutated the element. It is removed from the
+            // heap on the next line and pop() is not sensitive to its value.
+        }
+        this.heap.pop().unwrap()
     }
 }
 
@@ -365,7 +404,19 @@ impl<T: fmt::Debug, A: Allocator> fmt::Debug for BinaryHeap<T, A> {
     }
 }
 
-impl<T: Ord> BinaryHeap<T, Global> {
+struct RebuildOnDrop<'a, T: Ord> {
+    heap: &'a mut BinaryHeap<T>,
+    rebuild_from: usize,
+}
+
+impl<'a, T: Ord> Drop for RebuildOnDrop<'a, T> {
+    fn drop(&mut self) {
+        self.heap.rebuild_tail(self.rebuild_from);
+    }
+}
+
+// XXX: BinaryHeap<T, A>
+impl<T: Ord> BinaryHeap<T> {
     /// Creates an empty `BinaryHeap` as a max-heap.
     ///
     /// # Examples
@@ -450,11 +501,13 @@ impl<T: Ord, A: Allocator> BinaryHeap<T, A> {
         BinaryHeap { data: Vec::with_capacity_in(capacity, alloc) }
     }
 
+    // XXX: peek_mut
     /// Returns a mutable reference to the greatest item in the binary heap, or
     /// `None` if it is empty.
     ///
-    /// Note: If the `PeekMut` value is leaked, the heap may be in an
-    /// inconsistent state.
+    /// Note: If the `PeekMut` value is leaked, some heap elements might get
+    /// leaked along with it, but the remaining elements will remain a valid
+    /// heap.
     ///
     /// # Examples
     ///
@@ -480,8 +533,12 @@ impl<T: Ord, A: Allocator> BinaryHeap<T, A> {
     /// If the item is modified then the worst case time complexity is *O*(log(*n*)),
     /// otherwise it's *O*(1).
     #[stable(feature = "binary_heap_peek_mut", since = "1.12.0")]
-    pub fn peek_mut(&mut self) -> Option<PeekMut<'_, T, A>> {
-        if self.is_empty() { None } else { Some(PeekMut { heap: self, sift: false }) }
+    pub fn peek_mut(&mut self) -> Option<PeekMut<'_, T>> {
+        if self.is_empty() {
+            None
+        } else {
+            Some(PeekMut { heap: self, original_len: None })
+        }
     }
 
     /// Removes the greatest item from the binary heap and returns it, or `None` if it
@@ -847,7 +904,6 @@ impl<T: Ord, A: Allocator> BinaryHeap<T, A> {
     /// Basic usage:
     ///
     /// ```
-    /// #![feature(binary_heap_retain)]
     /// use std::collections::BinaryHeap;
     ///
     /// let mut heap = BinaryHeap::from([-10, -5, 1, 2, 4, 13]);
@@ -856,23 +912,24 @@ impl<T: Ord, A: Allocator> BinaryHeap<T, A> {
     ///
     /// assert_eq!(heap.into_sorted_vec(), [-10, 2, 4])
     /// ```
-    #[unstable(feature = "binary_heap_retain", issue = "71503")]
+    #[stable(feature = "binary_heap_retain", since = "1.70.0")]
     pub fn retain<F>(&mut self, mut f: F)
     where
         F: FnMut(&T) -> bool,
     {
-        let mut first_removed = self.len();
+        // rebuild_start will be updated to the first touched element below, and the rebuild will
+        // only be done for the tail.
+        let mut guard = RebuildOnDrop { rebuild_from: self.len(), heap: self };
         let mut i = 0;
-        self.data.retain(|e| {
+
+        guard.heap.data.retain(|e| {
             let keep = f(e);
-            if !keep && i < first_removed {
-                first_removed = i;
+            if !keep && i < guard.rebuild_from {
+                guard.rebuild_from = i;
             }
             i += 1;
             keep
         });
-        // data[0..first_removed] is untouched, so we only need to rebuild the tail:
-        self.rebuild_tail(first_removed);
     }
 }
 
@@ -1065,7 +1122,8 @@ impl<T, A: Allocator> BinaryHeap<T, A> {
     /// 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
     ///
@@ -1418,7 +1476,6 @@ impl<T> FusedIterator for Iter<'_, T> {}
 /// (provided by the [`IntoIterator`] trait). See its documentation for more.
 ///
 /// [`into_iter`]: BinaryHeap::into_iter
-/// [`IntoIterator`]: core::iter::IntoIterator
 #[stable(feature = "rust1", since = "1.0.0")]
 #[derive(Clone)]
 pub struct IntoIter<
@@ -1468,6 +1525,20 @@ impl<T, A: Allocator> ExactSizeIterator for IntoIter<T, A> {
 #[stable(feature = "fused", since = "1.26.0")]
 impl<T, A: Allocator> FusedIterator for IntoIter<T, A> {}
 
+#[stable(feature = "default_iters", since = "1.70.0")]
+impl<T> Default for IntoIter<T> {
+    /// Creates an empty `binary_heap::IntoIter`.
+    ///
+    /// ```
+    /// # use std::collections::binary_heap;
+    /// let iter: binary_heap::IntoIter<u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        IntoIter { iter: Default::default() }
+    }
+}
+
 // In addition to the SAFETY invariants of the following three unsafe traits
 // also refer to the vec::in_place_collect module documentation to get an overview
 #[unstable(issue = "none", feature = "inplace_iteration")]
@@ -1726,7 +1797,8 @@ impl<'a, T, A: Allocator + 'a> IntoIterator for &'a BinaryHeap<T, A> {
 impl<T: Ord, A: Allocator> Extend<T> for BinaryHeap<T, A> {
     #[inline]
     fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
-        <Self as SpecExtend<I>>::spec_extend(self, iter);
+        let guard = RebuildOnDrop { rebuild_from: self.len(), heap: self };
+        guard.heap.data.extend(iter);
     }
 
     #[inline]
@@ -1740,37 +1812,6 @@ impl<T: Ord, A: Allocator> Extend<T> for BinaryHeap<T, A> {
     }
 }
 
-impl<T: Ord, A: Allocator, I: IntoIterator<Item = T>> SpecExtend<I> for BinaryHeap<T, A> {
-    default fn spec_extend(&mut self, iter: I) {
-        self.extend_desugared(iter.into_iter());
-    }
-}
-
-impl<T: Ord> SpecExtend<Vec<T>> for BinaryHeap<T> {
-    fn spec_extend(&mut self, ref mut other: Vec<T>) {
-        let start = self.data.len();
-        self.data.append(other);
-        self.rebuild_tail(start);
-    }
-}
-
-impl<T: Ord> SpecExtend<BinaryHeap<T>> for BinaryHeap<T> {
-    fn spec_extend(&mut self, ref mut other: BinaryHeap<T>) {
-        self.append(other);
-    }
-}
-
-impl<T: Ord, A: Allocator> BinaryHeap<T, A> {
-    fn extend_desugared<I: IntoIterator<Item = T>>(&mut self, iter: I) {
-        let iterator = iter.into_iter();
-        let (lower, _) = iterator.size_hint();
-
-        self.reserve(lower);
-
-        iterator.for_each(move |elem| self.push(elem));
-    }
-}
-
 #[stable(feature = "extend_ref", since = "1.2.0")]
 impl<'a, T: 'a + Ord + Copy> Extend<&'a T> for BinaryHeap<T> {
     fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
diff --git a/library/alloc/src/collections/binary_heap/tests.rs b/library/alloc/src/collections/binary_heap/tests.rs
index 5a05215aeed..500caa35678 100644
--- a/library/alloc/src/collections/binary_heap/tests.rs
+++ b/library/alloc/src/collections/binary_heap/tests.rs
@@ -1,8 +1,9 @@
 use super::*;
 use crate::boxed::Box;
+use crate::testing::crash_test::{CrashTestDummy, Panic};
+use core::mem;
 use std::iter::TrustedLen;
 use std::panic::{catch_unwind, AssertUnwindSafe};
-use std::sync::atomic::{AtomicU32, Ordering};
 
 #[test]
 fn test_iterator() {
@@ -147,6 +148,24 @@ fn test_peek_mut() {
 }
 
 #[test]
+fn test_peek_mut_leek() {
+    let data = vec![4, 2, 7];
+    let mut heap = BinaryHeap::from(data);
+    let mut max = heap.peek_mut().unwrap();
+    *max = -1;
+
+    // The PeekMut object's Drop impl would have been responsible for moving the
+    // -1 out of the max position of the BinaryHeap, but we don't run it.
+    mem::forget(max);
+
+    // Absent some mitigation like leak amplification, the -1 would incorrectly
+    // end up in the last position of the returned Vec, with the rest of the
+    // heap's original contents in front of it in sorted order.
+    let sorted_vec = heap.into_sorted_vec();
+    assert!(sorted_vec.is_sorted(), "{:?}", sorted_vec);
+}
+
+#[test]
 fn test_peek_mut_pop() {
     let data = vec![2, 4, 6, 2, 1, 8, 10, 3, 5, 7, 0, 9, 1];
     let mut heap = BinaryHeap::from(data);
@@ -291,33 +310,83 @@ fn test_drain_sorted() {
 
 #[test]
 fn test_drain_sorted_leak() {
-    static DROPS: AtomicU32 = AtomicU32::new(0);
-
-    #[derive(Clone, PartialEq, Eq, PartialOrd, Ord)]
-    struct D(u32, bool);
-
-    impl Drop for D {
-        fn drop(&mut self) {
-            DROPS.fetch_add(1, Ordering::SeqCst);
-
-            if self.1 {
-                panic!("panic in `drop`");
-            }
-        }
-    }
-
+    let d0 = CrashTestDummy::new(0);
+    let d1 = CrashTestDummy::new(1);
+    let d2 = CrashTestDummy::new(2);
+    let d3 = CrashTestDummy::new(3);
+    let d4 = CrashTestDummy::new(4);
+    let d5 = CrashTestDummy::new(5);
     let mut q = BinaryHeap::from(vec![
-        D(0, false),
-        D(1, false),
-        D(2, false),
-        D(3, true),
-        D(4, false),
-        D(5, false),
+        d0.spawn(Panic::Never),
+        d1.spawn(Panic::Never),
+        d2.spawn(Panic::Never),
+        d3.spawn(Panic::InDrop),
+        d4.spawn(Panic::Never),
+        d5.spawn(Panic::Never),
     ]);
 
-    catch_unwind(AssertUnwindSafe(|| drop(q.drain_sorted()))).ok();
+    catch_unwind(AssertUnwindSafe(|| drop(q.drain_sorted()))).unwrap_err();
 
-    assert_eq!(DROPS.load(Ordering::SeqCst), 6);
+    assert_eq!(d0.dropped(), 1);
+    assert_eq!(d1.dropped(), 1);
+    assert_eq!(d2.dropped(), 1);
+    assert_eq!(d3.dropped(), 1);
+    assert_eq!(d4.dropped(), 1);
+    assert_eq!(d5.dropped(), 1);
+    assert!(q.is_empty());
+}
+
+#[test]
+fn test_drain_forget() {
+    let a = CrashTestDummy::new(0);
+    let b = CrashTestDummy::new(1);
+    let c = CrashTestDummy::new(2);
+    let mut q =
+        BinaryHeap::from(vec![a.spawn(Panic::Never), b.spawn(Panic::Never), c.spawn(Panic::Never)]);
+
+    catch_unwind(AssertUnwindSafe(|| {
+        let mut it = q.drain();
+        it.next();
+        mem::forget(it);
+    }))
+    .unwrap();
+    // Behaviour after leaking is explicitly unspecified and order is arbitrary,
+    // so it's fine if these start failing, but probably worth knowing.
+    assert!(q.is_empty());
+    assert_eq!(a.dropped() + b.dropped() + c.dropped(), 1);
+    assert_eq!(a.dropped(), 0);
+    assert_eq!(b.dropped(), 0);
+    assert_eq!(c.dropped(), 1);
+    drop(q);
+    assert_eq!(a.dropped(), 0);
+    assert_eq!(b.dropped(), 0);
+    assert_eq!(c.dropped(), 1);
+}
+
+#[test]
+fn test_drain_sorted_forget() {
+    let a = CrashTestDummy::new(0);
+    let b = CrashTestDummy::new(1);
+    let c = CrashTestDummy::new(2);
+    let mut q =
+        BinaryHeap::from(vec![a.spawn(Panic::Never), b.spawn(Panic::Never), c.spawn(Panic::Never)]);
+
+    catch_unwind(AssertUnwindSafe(|| {
+        let mut it = q.drain_sorted();
+        it.next();
+        mem::forget(it);
+    }))
+    .unwrap();
+    // Behaviour after leaking is explicitly unspecified,
+    // so it's fine if these start failing, but probably worth knowing.
+    assert_eq!(q.len(), 2);
+    assert_eq!(a.dropped(), 0);
+    assert_eq!(b.dropped(), 0);
+    assert_eq!(c.dropped(), 1);
+    drop(q);
+    assert_eq!(a.dropped(), 1);
+    assert_eq!(b.dropped(), 1);
+    assert_eq!(c.dropped(), 1);
 }
 
 #[test]
@@ -405,6 +474,25 @@ fn test_retain() {
     assert!(a.is_empty());
 }
 
+#[test]
+fn test_retain_catch_unwind() {
+    let mut heap = BinaryHeap::from(vec![3, 1, 2]);
+
+    // Removes the 3, then unwinds out of retain.
+    let _ = catch_unwind(AssertUnwindSafe(|| {
+        heap.retain(|e| {
+            if *e == 1 {
+                panic!();
+            }
+            false
+        });
+    }));
+
+    // Naively this would be [1, 2] (an invalid heap) if BinaryHeap delegates to
+    // Vec's retain impl and then does not rebuild the heap after that unwinds.
+    assert_eq!(heap.into_vec(), [2, 1]);
+}
+
 // old binaryheap failed this test
 //
 // Integrity means that all elements are present after a comparison panics,
@@ -415,7 +503,7 @@ fn test_retain() {
 #[test]
 #[cfg(not(target_os = "emscripten"))]
 fn panic_safe() {
-    use rand::{seq::SliceRandom, thread_rng};
+    use rand::seq::SliceRandom;
     use std::cmp;
     use std::panic::{self, AssertUnwindSafe};
     use std::sync::atomic::{AtomicUsize, Ordering};
@@ -440,7 +528,7 @@ fn panic_safe() {
             self.0.partial_cmp(&other.0)
         }
     }
-    let mut rng = thread_rng();
+    let mut rng = crate::test_helpers::test_rng();
     const DATASZ: usize = 32;
     // Miri is too slow
     let ntest = if cfg!(miri) { 1 } else { 10 };
diff --git a/library/alloc/src/collections/btree/borrow.rs b/library/alloc/src/collections/btree/borrow.rs
index 016f139a501..000b9bd0fab 100644
--- a/library/alloc/src/collections/btree/borrow.rs
+++ b/library/alloc/src/collections/btree/borrow.rs
@@ -41,6 +41,28 @@ impl<'a, T> DormantMutRef<'a, T> {
         // SAFETY: our own safety conditions imply this reference is again unique.
         unsafe { &mut *self.ptr.as_ptr() }
     }
+
+    /// Borrows a new mutable reference from the unique borrow initially captured.
+    ///
+    /// # Safety
+    ///
+    /// The reborrow must have ended, i.e., the reference returned by `new` and
+    /// all pointers and references derived from it, must not be used anymore.
+    pub unsafe fn reborrow(&mut self) -> &'a mut T {
+        // SAFETY: our own safety conditions imply this reference is again unique.
+        unsafe { &mut *self.ptr.as_ptr() }
+    }
+
+    /// Borrows a new shared reference from the unique borrow initially captured.
+    ///
+    /// # Safety
+    ///
+    /// The reborrow must have ended, i.e., the reference returned by `new` and
+    /// all pointers and references derived from it, must not be used anymore.
+    pub unsafe fn reborrow_shared(&self) -> &'a T {
+        // SAFETY: our own safety conditions imply this reference is again unique.
+        unsafe { &*self.ptr.as_ptr() }
+    }
 }
 
 #[cfg(test)]
diff --git a/library/alloc/src/collections/btree/dedup_sorted_iter.rs b/library/alloc/src/collections/btree/dedup_sorted_iter.rs
index 60bf83b8387..17ee78045a9 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 0bddd7a9906..1f8a1ecba6e 100644
--- a/library/alloc/src/collections/btree/map.rs
+++ b/library/alloc/src/collections/btree/map.rs
@@ -3,10 +3,10 @@ use core::borrow::Borrow;
 use core::cmp::Ordering;
 use core::fmt::{self, Debug};
 use core::hash::{Hash, Hasher};
-use core::iter::{FromIterator, FusedIterator};
+use core::iter::FusedIterator;
 use core::marker::PhantomData;
 use core::mem::{self, ManuallyDrop};
-use core::ops::{Index, RangeBounds};
+use core::ops::{Bound, Index, RangeBounds};
 use core::ptr;
 
 use crate::alloc::{Allocator, Global};
@@ -15,7 +15,7 @@ use super::borrow::DormantMutRef;
 use super::dedup_sorted_iter::DedupSortedIter;
 use super::navigate::{LazyLeafRange, LeafRange};
 use super::node::{self, marker, ForceResult::*, Handle, NodeRef, Root};
-use super::search::SearchResult::*;
+use super::search::{SearchBound, SearchResult::*};
 use super::set_val::SetValZST;
 
 mod entry;
@@ -46,8 +46,8 @@ pub(super) const MIN_LEN: usize = node::MIN_LEN_AFTER_SPLIT;
 /// is done is *very* inefficient for modern computer architectures. In particular, every element
 /// is stored in its own individually heap-allocated node. This means that every single insertion
 /// triggers a heap-allocation, and every single comparison should be a cache-miss. Since these
-/// are both notably expensive things to do in practice, we are forced to at very least reconsider
-/// the BST strategy.
+/// are both notably expensive things to do in practice, we are forced to, at the very least,
+/// reconsider the BST strategy.
 ///
 /// A B-Tree instead makes each node contain B-1 to 2B-1 elements in a contiguous array. By doing
 /// this, we reduce the number of allocations by a factor of B, and improve cache efficiency in
@@ -178,6 +178,8 @@ pub struct BTreeMap<
     length: usize,
     /// `ManuallyDrop` to control drop order (needs to be dropped after all the nodes).
     pub(super) alloc: ManuallyDrop<A>,
+    // For dropck; the `Box` avoids making the `Unpin` impl more strict than before
+    _marker: PhantomData<crate::boxed::Box<(K, V)>>,
 }
 
 #[stable(feature = "btree_drop", since = "1.7.0")]
@@ -187,6 +189,19 @@ unsafe impl<#[may_dangle] K, #[may_dangle] V, A: Allocator + Clone> Drop for BTr
     }
 }
 
+// FIXME: This implementation is "wrong", but changing it would be a breaking change.
+// (The bounds of the automatic `UnwindSafe` implementation have been like this since Rust 1.50.)
+// Maybe we can fix it nonetheless with a crater run, or if the `UnwindSafe`
+// traits are deprecated, or disarmed (no longer causing hard errors) in the future.
+#[stable(feature = "btree_unwindsafe", since = "1.64.0")]
+impl<K, V, A: Allocator + Clone> core::panic::UnwindSafe for BTreeMap<K, V, A>
+where
+    A: core::panic::UnwindSafe,
+    K: core::panic::RefUnwindSafe,
+    V: core::panic::RefUnwindSafe,
+{
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<K: Clone, V: Clone, A: Allocator + Clone> Clone for BTreeMap<K, V, A> {
     fn clone(&self) -> BTreeMap<K, V, A> {
@@ -204,6 +219,7 @@ impl<K: Clone, V: Clone, A: Allocator + Clone> Clone for BTreeMap<K, V, A> {
                         root: Some(Root::new(alloc.clone())),
                         length: 0,
                         alloc: ManuallyDrop::new(alloc),
+                        _marker: PhantomData,
                     };
 
                     {
@@ -346,6 +362,20 @@ impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for Iter<'_, K, V> {
     }
 }
 
+#[stable(feature = "default_iters", since = "1.70.0")]
+impl<'a, K: 'a, V: 'a> Default for Iter<'a, K, V> {
+    /// Creates an empty `btree_map::Iter`.
+    ///
+    /// ```
+    /// # use std::collections::btree_map;
+    /// let iter: btree_map::Iter<'_, u8, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        Iter { range: Default::default(), length: 0 }
+    }
+}
+
 /// A mutable iterator over the entries of a `BTreeMap`.
 ///
 /// This `struct` is created by the [`iter_mut`] method on [`BTreeMap`]. See its
@@ -370,13 +400,26 @@ impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for IterMut<'_, K, V> {
     }
 }
 
+#[stable(feature = "default_iters", since = "1.70.0")]
+impl<'a, K: 'a, V: 'a> Default for IterMut<'a, K, V> {
+    /// Creates an empty `btree_map::IterMut`.
+    ///
+    /// ```
+    /// # use std::collections::btree_map;
+    /// let iter: btree_map::IterMut<'_, u8, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        IterMut { range: Default::default(), length: 0, _marker: PhantomData {} }
+    }
+}
+
 /// An owning iterator over the entries of a `BTreeMap`.
 ///
 /// This `struct` is created by the [`into_iter`] method on [`BTreeMap`]
 /// (provided by the [`IntoIterator`] trait). See its documentation for more.
 ///
 /// [`into_iter`]: IntoIterator::into_iter
-/// [`IntoIterator`]: core::iter::IntoIterator
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_insignificant_dtor]
 pub struct IntoIter<
@@ -405,6 +448,23 @@ impl<K: Debug, V: Debug, A: Allocator + Clone> Debug for IntoIter<K, V, A> {
     }
 }
 
+#[stable(feature = "default_iters", since = "1.70.0")]
+impl<K, V, A> Default for IntoIter<K, V, A>
+where
+    A: Allocator + Default + Clone,
+{
+    /// Creates an empty `btree_map::IntoIter`.
+    ///
+    /// ```
+    /// # use std::collections::btree_map;
+    /// let iter: btree_map::IntoIter<u8, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        IntoIter { range: Default::default(), length: 0, alloc: Default::default() }
+    }
+}
+
 /// An iterator over the keys of a `BTreeMap`.
 ///
 /// This `struct` is created by the [`keys`] method on [`BTreeMap`]. See its
@@ -564,10 +624,10 @@ 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) }
+        BTreeMap { root: None, length: 0, alloc: ManuallyDrop::new(Global), _marker: PhantomData }
     }
 }
 
@@ -589,10 +649,11 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn clear(&mut self) {
         // avoid moving the allocator
-        mem::drop(BTreeMap {
+        drop(BTreeMap {
             root: mem::replace(&mut self.root, None),
             length: mem::replace(&mut self.length, 0),
             alloc: self.alloc.clone(),
+            _marker: PhantomData,
         });
     }
 
@@ -615,7 +676,7 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// ```
     #[unstable(feature = "btreemap_alloc", issue = "32838")]
     pub fn new_in(alloc: A) -> BTreeMap<K, V, A> {
-        BTreeMap { root: None, length: 0, alloc: ManuallyDrop::new(alloc) }
+        BTreeMap { root: None, length: 0, alloc: ManuallyDrop::new(alloc), _marker: PhantomData }
     }
 }
 
@@ -686,7 +747,6 @@ 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();
@@ -695,7 +755,7 @@ 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,
@@ -710,7 +770,6 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(map_first_last)]
     /// use std::collections::BTreeMap;
     ///
     /// let mut map = BTreeMap::new();
@@ -724,7 +783,7 @@ 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,
@@ -748,7 +807,6 @@ 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();
@@ -759,7 +817,7 @@ 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,
@@ -775,7 +833,6 @@ 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();
@@ -783,7 +840,7 @@ 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,
@@ -798,7 +855,6 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(map_first_last)]
     /// use std::collections::BTreeMap;
     ///
     /// let mut map = BTreeMap::new();
@@ -812,7 +868,7 @@ 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,
@@ -836,7 +892,6 @@ 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();
@@ -847,7 +902,7 @@ 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,
@@ -1082,6 +1137,9 @@ 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
     ///
     /// ```
@@ -1090,10 +1148,10 @@ 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");
     ///
@@ -1104,7 +1162,7 @@ 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");
     /// ```
@@ -1320,7 +1378,12 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
         let (new_left_len, right_len) = Root::calc_split_length(total_num, &left_root, &right_root);
         self.length = new_left_len;
 
-        BTreeMap { root: Some(right_root), length: right_len, alloc: self.alloc.clone() }
+        BTreeMap {
+            root: Some(right_root),
+            length: right_len,
+            alloc: self.alloc.clone(),
+            _marker: PhantomData,
+        }
     }
 
     /// Creates an iterator that visits all elements (key-value pairs) in
@@ -1445,7 +1508,7 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
         let mut root = Root::new(alloc.clone());
         let mut length = 0;
         root.bulk_push(DedupSortedIter::new(iter.into_iter()), &mut length, alloc.clone());
-        BTreeMap { root: Some(root), length, alloc: ManuallyDrop::new(alloc) }
+        BTreeMap { root: Some(root), length, alloc: ManuallyDrop::new(alloc), _marker: PhantomData }
     }
 }
 
@@ -1480,11 +1543,17 @@ impl<'a, K: 'a, V: 'a> Iterator for Iter<'a, K, V> {
         self.next_back()
     }
 
-    fn min(mut self) -> Option<(&'a K, &'a V)> {
+    fn min(mut self) -> Option<(&'a K, &'a V)>
+    where
+        (&'a K, &'a V): Ord,
+    {
         self.next()
     }
 
-    fn max(mut self) -> Option<(&'a K, &'a V)> {
+    fn max(mut self) -> Option<(&'a K, &'a V)>
+    where
+        (&'a K, &'a V): Ord,
+    {
         self.next_back()
     }
 }
@@ -1549,11 +1618,17 @@ impl<'a, K, V> Iterator for IterMut<'a, K, V> {
         self.next_back()
     }
 
-    fn min(mut self) -> Option<(&'a K, &'a mut V)> {
+    fn min(mut self) -> Option<(&'a K, &'a mut V)>
+    where
+        (&'a K, &'a mut V): Ord,
+    {
         self.next()
     }
 
-    fn max(mut self) -> Option<(&'a K, &'a mut V)> {
+    fn max(mut self) -> Option<(&'a K, &'a mut V)>
+    where
+        (&'a K, &'a mut V): Ord,
+    {
         self.next_back()
     }
 }
@@ -1716,11 +1791,17 @@ impl<'a, K, V> Iterator for Keys<'a, K, V> {
         self.next_back()
     }
 
-    fn min(mut self) -> Option<&'a K> {
+    fn min(mut self) -> Option<&'a K>
+    where
+        &'a K: Ord,
+    {
         self.next()
     }
 
-    fn max(mut self) -> Option<&'a K> {
+    fn max(mut self) -> Option<&'a K>
+    where
+        &'a K: Ord,
+    {
         self.next_back()
     }
 }
@@ -1749,6 +1830,20 @@ impl<K, V> Clone for Keys<'_, K, V> {
     }
 }
 
+#[stable(feature = "default_iters", since = "1.70.0")]
+impl<K, V> Default for Keys<'_, K, V> {
+    /// Creates an empty `btree_map::Keys`.
+    ///
+    /// ```
+    /// # use std::collections::btree_map;
+    /// let iter: btree_map::Keys<'_, u8, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        Keys { inner: Default::default() }
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, K, V> Iterator for Values<'a, K, V> {
     type Item = &'a V;
@@ -1790,6 +1885,20 @@ impl<K, V> Clone for Values<'_, K, V> {
     }
 }
 
+#[stable(feature = "default_iters", since = "1.70.0")]
+impl<K, V> Default for Values<'_, K, V> {
+    /// Creates an empty `btree_map::Values`.
+    ///
+    /// ```
+    /// # use std::collections::btree_map;
+    /// let iter: btree_map::Values<'_, u8, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        Values { inner: Default::default() }
+    }
+}
+
 /// An iterator produced by calling `drain_filter` on BTreeMap.
 #[unstable(feature = "btree_drain_filter", issue = "70530")]
 pub struct DrainFilter<
@@ -1917,15 +2026,35 @@ impl<'a, K, V> Iterator for Range<'a, K, V> {
         self.next_back()
     }
 
-    fn min(mut self) -> Option<(&'a K, &'a V)> {
+    fn min(mut self) -> Option<(&'a K, &'a V)>
+    where
+        (&'a K, &'a V): Ord,
+    {
         self.next()
     }
 
-    fn max(mut self) -> Option<(&'a K, &'a V)> {
+    fn max(mut self) -> Option<(&'a K, &'a V)>
+    where
+        (&'a K, &'a V): Ord,
+    {
         self.next_back()
     }
 }
 
+#[stable(feature = "default_iters", since = "1.70.0")]
+impl<K, V> Default for Range<'_, K, V> {
+    /// Creates an empty `btree_map::Range`.
+    ///
+    /// ```
+    /// # use std::collections::btree_map;
+    /// let iter: btree_map::Range<'_, u8, u8> = Default::default();
+    /// assert_eq!(iter.count(), 0);
+    /// ```
+    fn default() -> Self {
+        Range { inner: Default::default() }
+    }
+}
+
 #[stable(feature = "map_values_mut", since = "1.10.0")]
 impl<'a, K, V> Iterator for ValuesMut<'a, K, V> {
     type Item = &'a mut V;
@@ -1976,11 +2105,17 @@ impl<K, V, A: Allocator + Clone> Iterator for IntoKeys<K, V, A> {
         self.next_back()
     }
 
-    fn min(mut self) -> Option<K> {
+    fn min(mut self) -> Option<K>
+    where
+        K: Ord,
+    {
         self.next()
     }
 
-    fn max(mut self) -> Option<K> {
+    fn max(mut self) -> Option<K>
+    where
+        K: Ord,
+    {
         self.next_back()
     }
 }
@@ -2002,6 +2137,23 @@ impl<K, V, A: Allocator + Clone> ExactSizeIterator for IntoKeys<K, V, A> {
 #[stable(feature = "map_into_keys_values", since = "1.54.0")]
 impl<K, V, A: Allocator + Clone> FusedIterator for IntoKeys<K, V, A> {}
 
+#[stable(feature = "default_iters", since = "1.70.0")]
+impl<K, V, A> Default for IntoKeys<K, V, A>
+where
+    A: Allocator + Default + Clone,
+{
+    /// Creates an empty `btree_map::IntoKeys`.
+    ///
+    /// ```
+    /// # use std::collections::btree_map;
+    /// let iter: btree_map::IntoKeys<u8, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        IntoKeys { inner: Default::default() }
+    }
+}
+
 #[stable(feature = "map_into_keys_values", since = "1.54.0")]
 impl<K, V, A: Allocator + Clone> Iterator for IntoValues<K, V, A> {
     type Item = V;
@@ -2036,6 +2188,23 @@ impl<K, V, A: Allocator + Clone> ExactSizeIterator for IntoValues<K, V, A> {
 #[stable(feature = "map_into_keys_values", since = "1.54.0")]
 impl<K, V, A: Allocator + Clone> FusedIterator for IntoValues<K, V, A> {}
 
+#[stable(feature = "default_iters", since = "1.70.0")]
+impl<K, V, A> Default for IntoValues<K, V, A>
+where
+    A: Allocator + Default + Clone,
+{
+    /// Creates an empty `btree_map::IntoValues`.
+    ///
+    /// ```
+    /// # use std::collections::btree_map;
+    /// let iter: btree_map::IntoValues<u8, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        IntoValues { inner: Default::default() }
+    }
+}
+
 #[stable(feature = "btree_range", since = "1.17.0")]
 impl<'a, K, V> DoubleEndedIterator for Range<'a, K, V> {
     fn next_back(&mut self) -> Option<(&'a K, &'a V)> {
@@ -2065,11 +2234,17 @@ impl<'a, K, V> Iterator for RangeMut<'a, K, V> {
         self.next_back()
     }
 
-    fn min(mut self) -> Option<(&'a K, &'a mut V)> {
+    fn min(mut self) -> Option<(&'a K, &'a mut V)>
+    where
+        (&'a K, &'a mut V): Ord,
+    {
         self.next()
     }
 
-    fn max(mut self) -> Option<(&'a K, &'a mut V)> {
+    fn max(mut self) -> Option<(&'a K, &'a mut V)>
+    where
+        (&'a K, &'a mut V): Ord,
+    {
         self.next_back()
     }
 }
@@ -2370,7 +2545,11 @@ 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
     }
@@ -2391,10 +2570,740 @@ 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
     }
+
+    /// Returns a [`Cursor`] pointing at the first element that is above the
+    /// given bound.
+    ///
+    /// If no such element exists then a cursor pointing at the "ghost"
+    /// non-element is returned.
+    ///
+    /// Passing [`Bound::Unbounded`] will return a cursor pointing at the first
+    /// element of the map.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(btree_cursors)]
+    ///
+    /// use std::collections::BTreeMap;
+    /// use std::ops::Bound;
+    ///
+    /// let mut a = BTreeMap::new();
+    /// a.insert(1, "a");
+    /// a.insert(2, "b");
+    /// a.insert(3, "c");
+    /// a.insert(4, "c");
+    /// let cursor = a.lower_bound(Bound::Excluded(&2));
+    /// assert_eq!(cursor.key(), Some(&3));
+    /// ```
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn lower_bound<Q>(&self, bound: Bound<&Q>) -> Cursor<'_, K, V>
+    where
+        K: Borrow<Q> + Ord,
+        Q: Ord,
+    {
+        let root_node = match self.root.as_ref() {
+            None => return Cursor { current: None, root: None },
+            Some(root) => root.reborrow(),
+        };
+        let edge = root_node.lower_bound(SearchBound::from_range(bound));
+        Cursor { current: edge.next_kv().ok(), root: self.root.as_ref() }
+    }
+
+    /// Returns a [`CursorMut`] pointing at the first element that is above the
+    /// given bound.
+    ///
+    /// If no such element exists then a cursor pointing at the "ghost"
+    /// non-element is returned.
+    ///
+    /// Passing [`Bound::Unbounded`] will return a cursor pointing at the first
+    /// element of the map.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(btree_cursors)]
+    ///
+    /// use std::collections::BTreeMap;
+    /// use std::ops::Bound;
+    ///
+    /// let mut a = BTreeMap::new();
+    /// a.insert(1, "a");
+    /// a.insert(2, "b");
+    /// a.insert(3, "c");
+    /// a.insert(4, "c");
+    /// let cursor = a.lower_bound_mut(Bound::Excluded(&2));
+    /// assert_eq!(cursor.key(), Some(&3));
+    /// ```
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn lower_bound_mut<Q>(&mut self, bound: Bound<&Q>) -> CursorMut<'_, K, V, A>
+    where
+        K: Borrow<Q> + Ord,
+        Q: Ord,
+    {
+        let (root, dormant_root) = DormantMutRef::new(&mut self.root);
+        let root_node = match root.as_mut() {
+            None => {
+                return CursorMut {
+                    current: None,
+                    root: dormant_root,
+                    length: &mut self.length,
+                    alloc: &mut *self.alloc,
+                };
+            }
+            Some(root) => root.borrow_mut(),
+        };
+        let edge = root_node.lower_bound(SearchBound::from_range(bound));
+        CursorMut {
+            current: edge.next_kv().ok(),
+            root: dormant_root,
+            length: &mut self.length,
+            alloc: &mut *self.alloc,
+        }
+    }
+
+    /// Returns a [`Cursor`] pointing at the last element that is below the
+    /// given bound.
+    ///
+    /// If no such element exists then a cursor pointing at the "ghost"
+    /// non-element is returned.
+    ///
+    /// Passing [`Bound::Unbounded`] will return a cursor pointing at the last
+    /// element of the map.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(btree_cursors)]
+    ///
+    /// use std::collections::BTreeMap;
+    /// use std::ops::Bound;
+    ///
+    /// let mut a = BTreeMap::new();
+    /// a.insert(1, "a");
+    /// a.insert(2, "b");
+    /// a.insert(3, "c");
+    /// a.insert(4, "c");
+    /// let cursor = a.upper_bound(Bound::Excluded(&3));
+    /// assert_eq!(cursor.key(), Some(&2));
+    /// ```
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn upper_bound<Q>(&self, bound: Bound<&Q>) -> Cursor<'_, K, V>
+    where
+        K: Borrow<Q> + Ord,
+        Q: Ord,
+    {
+        let root_node = match self.root.as_ref() {
+            None => return Cursor { current: None, root: None },
+            Some(root) => root.reborrow(),
+        };
+        let edge = root_node.upper_bound(SearchBound::from_range(bound));
+        Cursor { current: edge.next_back_kv().ok(), root: self.root.as_ref() }
+    }
+
+    /// Returns a [`CursorMut`] pointing at the last element that is below the
+    /// given bound.
+    ///
+    /// If no such element exists then a cursor pointing at the "ghost"
+    /// non-element is returned.
+    ///
+    /// Passing [`Bound::Unbounded`] will return a cursor pointing at the last
+    /// element of the map.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(btree_cursors)]
+    ///
+    /// use std::collections::BTreeMap;
+    /// use std::ops::Bound;
+    ///
+    /// let mut a = BTreeMap::new();
+    /// a.insert(1, "a");
+    /// a.insert(2, "b");
+    /// a.insert(3, "c");
+    /// a.insert(4, "c");
+    /// let cursor = a.upper_bound_mut(Bound::Excluded(&3));
+    /// assert_eq!(cursor.key(), Some(&2));
+    /// ```
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn upper_bound_mut<Q>(&mut self, bound: Bound<&Q>) -> CursorMut<'_, K, V, A>
+    where
+        K: Borrow<Q> + Ord,
+        Q: Ord,
+    {
+        let (root, dormant_root) = DormantMutRef::new(&mut self.root);
+        let root_node = match root.as_mut() {
+            None => {
+                return CursorMut {
+                    current: None,
+                    root: dormant_root,
+                    length: &mut self.length,
+                    alloc: &mut *self.alloc,
+                };
+            }
+            Some(root) => root.borrow_mut(),
+        };
+        let edge = root_node.upper_bound(SearchBound::from_range(bound));
+        CursorMut {
+            current: edge.next_back_kv().ok(),
+            root: dormant_root,
+            length: &mut self.length,
+            alloc: &mut *self.alloc,
+        }
+    }
+}
+
+/// A cursor over a `BTreeMap`.
+///
+/// A `Cursor` is like an iterator, except that it can freely seek back-and-forth.
+///
+/// Cursors always point to an element in the tree, and index in a logically circular way.
+/// To accommodate this, there is a "ghost" non-element that yields `None` between the last and
+/// first elements of the tree.
+///
+/// A `Cursor` is created with the [`BTreeMap::lower_bound`] and [`BTreeMap::upper_bound`] methods.
+#[unstable(feature = "btree_cursors", issue = "107540")]
+pub struct Cursor<'a, K: 'a, V: 'a> {
+    current: Option<Handle<NodeRef<marker::Immut<'a>, K, V, marker::LeafOrInternal>, marker::KV>>,
+    root: Option<&'a node::Root<K, V>>,
+}
+
+#[unstable(feature = "btree_cursors", issue = "107540")]
+impl<K, V> Clone for Cursor<'_, K, V> {
+    fn clone(&self) -> Self {
+        let Cursor { current, root } = *self;
+        Cursor { current, root }
+    }
+}
+
+#[unstable(feature = "btree_cursors", issue = "107540")]
+impl<K: Debug, V: Debug> Debug for Cursor<'_, K, V> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("Cursor").field(&self.key_value()).finish()
+    }
+}
+
+/// A cursor over a `BTreeMap` with editing operations.
+///
+/// A `Cursor` is like an iterator, except that it can freely seek back-and-forth, and can
+/// safely mutate the tree during iteration. This is because the lifetime of its yielded
+/// references is tied to its own lifetime, instead of just the underlying tree. This means
+/// cursors cannot yield multiple elements at once.
+///
+/// Cursors always point to an element in the tree, and index in a logically circular way.
+/// To accommodate this, there is a "ghost" non-element that yields `None` between the last and
+/// first elements of the tree.
+///
+/// A `Cursor` is created with the [`BTreeMap::lower_bound_mut`] and [`BTreeMap::upper_bound_mut`]
+/// methods.
+#[unstable(feature = "btree_cursors", issue = "107540")]
+pub struct CursorMut<
+    'a,
+    K: 'a,
+    V: 'a,
+    #[unstable(feature = "allocator_api", issue = "32838")] A = Global,
+> {
+    current: Option<Handle<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal>, marker::KV>>,
+    root: DormantMutRef<'a, Option<node::Root<K, V>>>,
+    length: &'a mut usize,
+    alloc: &'a mut A,
+}
+
+#[unstable(feature = "btree_cursors", issue = "107540")]
+impl<K: Debug, V: Debug, A> Debug for CursorMut<'_, K, V, A> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("CursorMut").field(&self.key_value()).finish()
+    }
+}
+
+impl<'a, K, V> Cursor<'a, K, V> {
+    /// Moves the cursor to the next element of the `BTreeMap`.
+    ///
+    /// If the cursor is pointing to the "ghost" non-element then this will move it to
+    /// the first element of the `BTreeMap`. If it is pointing to the last
+    /// element of the `BTreeMap` then this will move it to the "ghost" non-element.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn move_next(&mut self) {
+        match self.current.take() {
+            None => {
+                self.current = self.root.and_then(|root| {
+                    root.reborrow().first_leaf_edge().forget_node_type().right_kv().ok()
+                });
+            }
+            Some(current) => {
+                self.current = current.next_leaf_edge().next_kv().ok();
+            }
+        }
+    }
+
+    /// Moves the cursor to the previous element of the `BTreeMap`.
+    ///
+    /// If the cursor is pointing to the "ghost" non-element then this will move it to
+    /// the last element of the `BTreeMap`. If it is pointing to the first
+    /// element of the `BTreeMap` then this will move it to the "ghost" non-element.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn move_prev(&mut self) {
+        match self.current.take() {
+            None => {
+                self.current = self.root.and_then(|root| {
+                    root.reborrow().last_leaf_edge().forget_node_type().left_kv().ok()
+                });
+            }
+            Some(current) => {
+                self.current = current.next_back_leaf_edge().next_back_kv().ok();
+            }
+        }
+    }
+
+    /// Returns a reference to the key of the element that the cursor is
+    /// currently pointing to.
+    ///
+    /// This returns `None` if the cursor is currently pointing to the
+    /// "ghost" non-element.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn key(&self) -> Option<&'a K> {
+        self.current.as_ref().map(|current| current.into_kv().0)
+    }
+
+    /// Returns a reference to the value of the element that the cursor is
+    /// currently pointing to.
+    ///
+    /// This returns `None` if the cursor is currently pointing to the
+    /// "ghost" non-element.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn value(&self) -> Option<&'a V> {
+        self.current.as_ref().map(|current| current.into_kv().1)
+    }
+
+    /// Returns a reference to the key and value of the element that the cursor
+    /// is currently pointing to.
+    ///
+    /// This returns `None` if the cursor is currently pointing to the
+    /// "ghost" non-element.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn key_value(&self) -> Option<(&'a K, &'a V)> {
+        self.current.as_ref().map(|current| current.into_kv())
+    }
+
+    /// Returns a reference to the next element.
+    ///
+    /// If the cursor is pointing to the "ghost" non-element then this returns
+    /// the first element of the `BTreeMap`. If it is pointing to the last
+    /// element of the `BTreeMap` then this returns `None`.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn peek_next(&self) -> Option<(&'a K, &'a V)> {
+        let mut next = self.clone();
+        next.move_next();
+        next.current.as_ref().map(|current| current.into_kv())
+    }
+
+    /// Returns a reference to the previous element.
+    ///
+    /// If the cursor is pointing to the "ghost" non-element then this returns
+    /// the last element of the `BTreeMap`. If it is pointing to the first
+    /// element of the `BTreeMap` then this returns `None`.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn peek_prev(&self) -> Option<(&'a K, &'a V)> {
+        let mut prev = self.clone();
+        prev.move_prev();
+        prev.current.as_ref().map(|current| current.into_kv())
+    }
+}
+
+impl<'a, K, V, A> CursorMut<'a, K, V, A> {
+    /// Moves the cursor to the next element of the `BTreeMap`.
+    ///
+    /// If the cursor is pointing to the "ghost" non-element then this will move it to
+    /// the first element of the `BTreeMap`. If it is pointing to the last
+    /// element of the `BTreeMap` then this will move it to the "ghost" non-element.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn move_next(&mut self) {
+        match self.current.take() {
+            None => {
+                // SAFETY: The previous borrow of root has ended.
+                self.current = unsafe { self.root.reborrow() }.as_mut().and_then(|root| {
+                    root.borrow_mut().first_leaf_edge().forget_node_type().right_kv().ok()
+                });
+            }
+            Some(current) => {
+                self.current = current.next_leaf_edge().next_kv().ok();
+            }
+        }
+    }
+
+    /// Moves the cursor to the previous element of the `BTreeMap`.
+    ///
+    /// If the cursor is pointing to the "ghost" non-element then this will move it to
+    /// the last element of the `BTreeMap`. If it is pointing to the first
+    /// element of the `BTreeMap` then this will move it to the "ghost" non-element.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn move_prev(&mut self) {
+        match self.current.take() {
+            None => {
+                // SAFETY: The previous borrow of root has ended.
+                self.current = unsafe { self.root.reborrow() }.as_mut().and_then(|root| {
+                    root.borrow_mut().last_leaf_edge().forget_node_type().left_kv().ok()
+                });
+            }
+            Some(current) => {
+                self.current = current.next_back_leaf_edge().next_back_kv().ok();
+            }
+        }
+    }
+
+    /// Returns a reference to the key of the element that the cursor is
+    /// currently pointing to.
+    ///
+    /// This returns `None` if the cursor is currently pointing to the
+    /// "ghost" non-element.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn key(&self) -> Option<&K> {
+        self.current.as_ref().map(|current| current.reborrow().into_kv().0)
+    }
+
+    /// Returns a reference to the value of the element that the cursor is
+    /// currently pointing to.
+    ///
+    /// This returns `None` if the cursor is currently pointing to the
+    /// "ghost" non-element.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn value(&self) -> Option<&V> {
+        self.current.as_ref().map(|current| current.reborrow().into_kv().1)
+    }
+
+    /// Returns a reference to the key and value of the element that the cursor
+    /// is currently pointing to.
+    ///
+    /// This returns `None` if the cursor is currently pointing to the
+    /// "ghost" non-element.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn key_value(&self) -> Option<(&K, &V)> {
+        self.current.as_ref().map(|current| current.reborrow().into_kv())
+    }
+
+    /// Returns a mutable reference to the value of the element that the cursor
+    /// is currently pointing to.
+    ///
+    /// This returns `None` if the cursor is currently pointing to the
+    /// "ghost" non-element.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn value_mut(&mut self) -> Option<&mut V> {
+        self.current.as_mut().map(|current| current.kv_mut().1)
+    }
+
+    /// Returns a reference to the key and mutable reference to the value of the
+    /// element that the cursor is currently pointing to.
+    ///
+    /// This returns `None` if the cursor is currently pointing to the
+    /// "ghost" non-element.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn key_value_mut(&mut self) -> Option<(&K, &mut V)> {
+        self.current.as_mut().map(|current| {
+            let (k, v) = current.kv_mut();
+            (&*k, v)
+        })
+    }
+
+    /// Returns a mutable reference to the key of the element that the cursor is
+    /// currently pointing to.
+    ///
+    /// This returns `None` if the cursor is currently pointing to the
+    /// "ghost" non-element.
+    ///
+    /// # Safety
+    ///
+    /// This can be used to modify the key, but you must ensure that the
+    /// `BTreeMap` invariants are maintained. Specifically:
+    ///
+    /// * The key must remain unique within the tree.
+    /// * The key must remain in sorted order with regards to other elements in
+    ///   the tree.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub unsafe fn key_mut_unchecked(&mut self) -> Option<&mut K> {
+        self.current.as_mut().map(|current| current.kv_mut().0)
+    }
+
+    /// Returns a reference to the key and value of the next element.
+    ///
+    /// If the cursor is pointing to the "ghost" non-element then this returns
+    /// the first element of the `BTreeMap`. If it is pointing to the last
+    /// element of the `BTreeMap` then this returns `None`.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn peek_next(&mut self) -> Option<(&K, &mut V)> {
+        let (k, v) = match self.current {
+            None => {
+                // SAFETY: The previous borrow of root has ended.
+                unsafe { self.root.reborrow() }
+                    .as_mut()?
+                    .borrow_mut()
+                    .first_leaf_edge()
+                    .next_kv()
+                    .ok()?
+                    .into_kv_valmut()
+            }
+            // SAFETY: We're not using this to mutate the tree.
+            Some(ref mut current) => {
+                unsafe { current.reborrow_mut() }.next_leaf_edge().next_kv().ok()?.into_kv_valmut()
+            }
+        };
+        Some((k, v))
+    }
+
+    /// Returns a reference to the key and value of the previous element.
+    ///
+    /// If the cursor is pointing to the "ghost" non-element then this returns
+    /// the last element of the `BTreeMap`. If it is pointing to the first
+    /// element of the `BTreeMap` then this returns `None`.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn peek_prev(&mut self) -> Option<(&K, &mut V)> {
+        let (k, v) = match self.current.as_mut() {
+            None => {
+                // SAFETY: The previous borrow of root has ended.
+                unsafe { self.root.reborrow() }
+                    .as_mut()?
+                    .borrow_mut()
+                    .last_leaf_edge()
+                    .next_back_kv()
+                    .ok()?
+                    .into_kv_valmut()
+            }
+            Some(current) => {
+                // SAFETY: We're not using this to mutate the tree.
+                unsafe { current.reborrow_mut() }
+                    .next_back_leaf_edge()
+                    .next_back_kv()
+                    .ok()?
+                    .into_kv_valmut()
+            }
+        };
+        Some((k, v))
+    }
+
+    /// Returns a read-only cursor pointing to the current element.
+    ///
+    /// The lifetime of the returned `Cursor` is bound to that of the
+    /// `CursorMut`, which means it cannot outlive the `CursorMut` and that the
+    /// `CursorMut` is frozen for the lifetime of the `Cursor`.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn as_cursor(&self) -> Cursor<'_, K, V> {
+        Cursor {
+            // SAFETY: The tree is immutable while the cursor exists.
+            root: unsafe { self.root.reborrow_shared().as_ref() },
+            current: self.current.as_ref().map(|current| current.reborrow()),
+        }
+    }
+}
+
+// Now the tree editing operations
+impl<'a, K: Ord, V, A: Allocator + Clone> CursorMut<'a, K, V, A> {
+    /// Inserts a new element into the `BTreeMap` after the current one.
+    ///
+    /// If the cursor is pointing at the "ghost" non-element then the new element is
+    /// inserted at the front of the `BTreeMap`.
+    ///
+    /// # Safety
+    ///
+    /// You must ensure that the `BTreeMap` invariants are maintained.
+    /// Specifically:
+    ///
+    /// * The key of the newly inserted element must be unique in the tree.
+    /// * All keys in the tree must remain in sorted order.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub unsafe fn insert_after_unchecked(&mut self, key: K, value: V) {
+        let edge = match self.current.take() {
+            None => {
+                // SAFETY: We have no other reference to the tree.
+                match unsafe { self.root.reborrow() } {
+                    root @ None => {
+                        // Tree is empty, allocate a new root.
+                        let mut node = NodeRef::new_leaf(self.alloc.clone());
+                        node.borrow_mut().push(key, value);
+                        *root = Some(node.forget_type());
+                        *self.length += 1;
+                        return;
+                    }
+                    Some(root) => root.borrow_mut().first_leaf_edge(),
+                }
+            }
+            Some(current) => current.next_leaf_edge(),
+        };
+
+        let handle = edge.insert_recursing(key, value, self.alloc.clone(), |ins| {
+            drop(ins.left);
+            // SAFETY: The handle to the newly inserted value is always on a
+            // leaf node, so adding a new root node doesn't invalidate it.
+            let root = unsafe { self.root.reborrow().as_mut().unwrap() };
+            root.push_internal_level(self.alloc.clone()).push(ins.kv.0, ins.kv.1, ins.right)
+        });
+        self.current = handle.left_edge().next_back_kv().ok();
+        *self.length += 1;
+    }
+
+    /// Inserts a new element into the `BTreeMap` before the current one.
+    ///
+    /// If the cursor is pointing at the "ghost" non-element then the new element is
+    /// inserted at the end of the `BTreeMap`.
+    ///
+    /// # Safety
+    ///
+    /// You must ensure that the `BTreeMap` invariants are maintained.
+    /// Specifically:
+    ///
+    /// * The key of the newly inserted element must be unique in the tree.
+    /// * All keys in the tree must remain in sorted order.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub unsafe fn insert_before_unchecked(&mut self, key: K, value: V) {
+        let edge = match self.current.take() {
+            None => {
+                // SAFETY: We have no other reference to the tree.
+                match unsafe { self.root.reborrow() } {
+                    root @ None => {
+                        // Tree is empty, allocate a new root.
+                        let mut node = NodeRef::new_leaf(self.alloc.clone());
+                        node.borrow_mut().push(key, value);
+                        *root = Some(node.forget_type());
+                        *self.length += 1;
+                        return;
+                    }
+                    Some(root) => root.borrow_mut().last_leaf_edge(),
+                }
+            }
+            Some(current) => current.next_back_leaf_edge(),
+        };
+
+        let handle = edge.insert_recursing(key, value, self.alloc.clone(), |ins| {
+            drop(ins.left);
+            // SAFETY: The handle to the newly inserted value is always on a
+            // leaf node, so adding a new root node doesn't invalidate it.
+            let root = unsafe { self.root.reborrow().as_mut().unwrap() };
+            root.push_internal_level(self.alloc.clone()).push(ins.kv.0, ins.kv.1, ins.right)
+        });
+        self.current = handle.right_edge().next_kv().ok();
+        *self.length += 1;
+    }
+
+    /// Inserts a new element into the `BTreeMap` after the current one.
+    ///
+    /// If the cursor is pointing at the "ghost" non-element then the new element is
+    /// inserted at the front of the `BTreeMap`.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if:
+    /// - the given key compares less than or equal to the current element (if
+    ///   any).
+    /// - the given key compares greater than or equal to the next element (if
+    ///   any).
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn insert_after(&mut self, key: K, value: V) {
+        if let Some(current) = self.key() {
+            if &key <= current {
+                panic!("key must be ordered above the current element");
+            }
+        }
+        if let Some((next, _)) = self.peek_next() {
+            if &key >= next {
+                panic!("key must be ordered below the next element");
+            }
+        }
+        unsafe {
+            self.insert_after_unchecked(key, value);
+        }
+    }
+
+    /// Inserts a new element into the `BTreeMap` before the current one.
+    ///
+    /// If the cursor is pointing at the "ghost" non-element then the new element is
+    /// inserted at the end of the `BTreeMap`.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if:
+    /// - the given key compares greater than or equal to the current element
+    ///   (if any).
+    /// - the given key compares less than or equal to the previous element (if
+    ///   any).
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn insert_before(&mut self, key: K, value: V) {
+        if let Some(current) = self.key() {
+            if &key >= current {
+                panic!("key must be ordered below the current element");
+            }
+        }
+        if let Some((prev, _)) = self.peek_prev() {
+            if &key <= prev {
+                panic!("key must be ordered above the previous element");
+            }
+        }
+        unsafe {
+            self.insert_before_unchecked(key, value);
+        }
+    }
+
+    /// Removes the current element from the `BTreeMap`.
+    ///
+    /// The element that was removed is returned, and the cursor is
+    /// moved to point to the next element in the `BTreeMap`.
+    ///
+    /// If the cursor is currently pointing to the "ghost" non-element then no element
+    /// is removed and `None` is returned. The cursor is not moved in this case.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn remove_current(&mut self) -> Option<(K, V)> {
+        let current = self.current.take()?;
+        let mut emptied_internal_root = false;
+        let (kv, pos) =
+            current.remove_kv_tracking(|| emptied_internal_root = true, self.alloc.clone());
+        self.current = pos.next_kv().ok();
+        *self.length -= 1;
+        if emptied_internal_root {
+            // SAFETY: This is safe since current does not point within the now
+            // empty root node.
+            let root = unsafe { self.root.reborrow().as_mut().unwrap() };
+            root.pop_internal_level(self.alloc.clone());
+        }
+        Some(kv)
+    }
+
+    /// Removes the current element from the `BTreeMap`.
+    ///
+    /// The element that was removed is returned, and the cursor is
+    /// moved to point to the previous element in the `BTreeMap`.
+    ///
+    /// If the cursor is currently pointing to the "ghost" non-element then no element
+    /// is removed and `None` is returned. The cursor is not moved in this case.
+    #[unstable(feature = "btree_cursors", issue = "107540")]
+    pub fn remove_current_and_move_back(&mut self) -> Option<(K, V)> {
+        let current = self.current.take()?;
+        let mut emptied_internal_root = false;
+        let (kv, pos) =
+            current.remove_kv_tracking(|| emptied_internal_root = true, self.alloc.clone());
+        self.current = pos.next_back_kv().ok();
+        *self.length -= 1;
+        if emptied_internal_root {
+            // SAFETY: This is safe since current does not point within the now
+            // empty root node.
+            let root = unsafe { self.root.reborrow().as_mut().unwrap() };
+            root.pop_internal_level(self.alloc.clone());
+        }
+        Some(kv)
+    }
 }
 
 #[cfg(test)]
diff --git a/library/alloc/src/collections/btree/map/entry.rs b/library/alloc/src/collections/btree/map/entry.rs
index b6eecf9b0e9..e9366eec9ce 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.
@@ -337,7 +347,7 @@ impl<'a, K: Ord, V, A: Allocator + Clone> VacantEntry<'a, K, V, A> {
     /// assert_eq!(map["poneyland"], 37);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn insert(self, value: V) -> &'a mut V {
+    pub fn insert(mut self, value: V) -> &'a mut V {
         let out_ptr = match self.handle {
             None => {
                 // SAFETY: There is no tree yet so no reference to it exists.
@@ -348,25 +358,27 @@ impl<'a, K: Ord, V, A: Allocator + Clone> VacantEntry<'a, K, V, A> {
                 map.length = 1;
                 val_ptr
             }
-            Some(handle) => match handle.insert_recursing(self.key, value, self.alloc.clone()) {
-                (None, val_ptr) => {
-                    // SAFETY: We have consumed self.handle.
-                    let map = unsafe { self.dormant_map.awaken() };
-                    map.length += 1;
-                    val_ptr
-                }
-                (Some(ins), val_ptr) => {
-                    drop(ins.left);
-                    // SAFETY: We have consumed self.handle and dropped the
-                    // remaining reference to the tree, ins.left.
-                    let map = unsafe { self.dormant_map.awaken() };
-                    let root = map.root.as_mut().unwrap(); // same as ins.left
-                    root.push_internal_level(self.alloc).push(ins.kv.0, ins.kv.1, ins.right);
-                    map.length += 1;
-                    val_ptr
-                }
-            },
+            Some(handle) => {
+                let new_handle =
+                    handle.insert_recursing(self.key, value, self.alloc.clone(), |ins| {
+                        drop(ins.left);
+                        // SAFETY: Pushing a new root node doesn't invalidate
+                        // handles to existing nodes.
+                        let map = unsafe { self.dormant_map.reborrow() };
+                        let root = map.root.as_mut().unwrap(); // same as ins.left
+                        root.push_internal_level(self.alloc).push(ins.kv.0, ins.kv.1, ins.right)
+                    });
+
+                // Get the pointer to the value
+                let val_ptr = new_handle.into_val_mut();
+
+                // SAFETY: We have consumed self.handle.
+                let map = unsafe { self.dormant_map.awaken() };
+                map.length += 1;
+                val_ptr
+            }
         };
+
         // Now that we have finished growing the tree using borrowed references,
         // dereference the pointer to a part of it, that we picked up along the way.
         unsafe { &mut *out_ptr }
diff --git a/library/alloc/src/collections/btree/map/tests.rs b/library/alloc/src/collections/btree/map/tests.rs
index 4c372b1d60a..7ecffe3eef2 100644
--- a/library/alloc/src/collections/btree/map/tests.rs
+++ b/library/alloc/src/collections/btree/map/tests.rs
@@ -1,16 +1,16 @@
-use super::super::testing::crash_test::{CrashTestDummy, Panic};
-use super::super::testing::ord_chaos::{Cyclic3, Governed, Governor};
-use super::super::testing::rng::DeterministicRng;
 use super::Entry::{Occupied, Vacant};
 use super::*;
 use crate::boxed::Box;
 use crate::fmt::Debug;
 use crate::rc::Rc;
 use crate::string::{String, ToString};
+use crate::testing::crash_test::{CrashTestDummy, Panic};
+use crate::testing::ord_chaos::{Cyclic3, Governed, Governor};
+use crate::testing::rng::DeterministicRng;
 use crate::vec::Vec;
+use core::assert_matches::assert_matches;
 use std::cmp::Ordering;
-use std::convert::TryFrom;
-use std::iter::{self, FromIterator};
+use std::iter;
 use std::mem;
 use std::ops::Bound::{self, Excluded, Included, Unbounded};
 use std::ops::RangeBounds;
@@ -2336,3 +2336,134 @@ fn from_array() {
     let unordered_duplicates = BTreeMap::from([(3, 4), (1, 2), (1, 2)]);
     assert_eq!(map, unordered_duplicates);
 }
+
+#[test]
+fn test_cursor() {
+    let map = BTreeMap::from([(1, 'a'), (2, 'b'), (3, 'c')]);
+
+    let mut cur = map.lower_bound(Bound::Unbounded);
+    assert_eq!(cur.key(), Some(&1));
+    cur.move_next();
+    assert_eq!(cur.key(), Some(&2));
+    assert_eq!(cur.peek_next(), Some((&3, &'c')));
+    cur.move_prev();
+    assert_eq!(cur.key(), Some(&1));
+    assert_eq!(cur.peek_prev(), None);
+
+    let mut cur = map.upper_bound(Bound::Excluded(&1));
+    assert_eq!(cur.key(), None);
+    cur.move_next();
+    assert_eq!(cur.key(), Some(&1));
+    cur.move_prev();
+    assert_eq!(cur.key(), None);
+    assert_eq!(cur.peek_prev(), Some((&3, &'c')));
+}
+
+#[test]
+fn test_cursor_mut() {
+    let mut map = BTreeMap::from([(1, 'a'), (3, 'c'), (5, 'e')]);
+    let mut cur = map.lower_bound_mut(Bound::Excluded(&3));
+    assert_eq!(cur.key(), Some(&5));
+    cur.insert_before(4, 'd');
+    assert_eq!(cur.key(), Some(&5));
+    assert_eq!(cur.peek_prev(), Some((&4, &mut 'd')));
+    cur.move_next();
+    assert_eq!(cur.key(), None);
+    cur.insert_before(6, 'f');
+    assert_eq!(cur.key(), None);
+    assert_eq!(cur.remove_current(), None);
+    assert_eq!(cur.key(), None);
+    cur.insert_after(0, '?');
+    assert_eq!(cur.key(), None);
+    assert_eq!(map, BTreeMap::from([(0, '?'), (1, 'a'), (3, 'c'), (4, 'd'), (5, 'e'), (6, 'f')]));
+
+    let mut cur = map.upper_bound_mut(Bound::Included(&5));
+    assert_eq!(cur.key(), Some(&5));
+    assert_eq!(cur.remove_current(), Some((5, 'e')));
+    assert_eq!(cur.key(), Some(&6));
+    assert_eq!(cur.remove_current_and_move_back(), Some((6, 'f')));
+    assert_eq!(cur.key(), Some(&4));
+    assert_eq!(map, BTreeMap::from([(0, '?'), (1, 'a'), (3, 'c'), (4, 'd')]));
+}
+
+#[should_panic(expected = "key must be ordered above the previous element")]
+#[test]
+fn test_cursor_mut_insert_before_1() {
+    let mut map = BTreeMap::from([(1, 'a'), (2, 'b'), (3, 'c')]);
+    let mut cur = map.upper_bound_mut(Bound::Included(&2));
+    cur.insert_before(0, 'd');
+}
+
+#[should_panic(expected = "key must be ordered above the previous element")]
+#[test]
+fn test_cursor_mut_insert_before_2() {
+    let mut map = BTreeMap::from([(1, 'a'), (2, 'b'), (3, 'c')]);
+    let mut cur = map.upper_bound_mut(Bound::Included(&2));
+    cur.insert_before(1, 'd');
+}
+
+#[should_panic(expected = "key must be ordered below the current element")]
+#[test]
+fn test_cursor_mut_insert_before_3() {
+    let mut map = BTreeMap::from([(1, 'a'), (2, 'b'), (3, 'c')]);
+    let mut cur = map.upper_bound_mut(Bound::Included(&2));
+    cur.insert_before(2, 'd');
+}
+
+#[should_panic(expected = "key must be ordered below the current element")]
+#[test]
+fn test_cursor_mut_insert_before_4() {
+    let mut map = BTreeMap::from([(1, 'a'), (2, 'b'), (3, 'c')]);
+    let mut cur = map.upper_bound_mut(Bound::Included(&2));
+    cur.insert_before(3, 'd');
+}
+
+#[should_panic(expected = "key must be ordered above the current element")]
+#[test]
+fn test_cursor_mut_insert_after_1() {
+    let mut map = BTreeMap::from([(1, 'a'), (2, 'b'), (3, 'c')]);
+    let mut cur = map.upper_bound_mut(Bound::Included(&2));
+    cur.insert_after(1, 'd');
+}
+
+#[should_panic(expected = "key must be ordered above the current element")]
+#[test]
+fn test_cursor_mut_insert_after_2() {
+    let mut map = BTreeMap::from([(1, 'a'), (2, 'b'), (3, 'c')]);
+    let mut cur = map.upper_bound_mut(Bound::Included(&2));
+    cur.insert_after(2, 'd');
+}
+
+#[should_panic(expected = "key must be ordered below the next element")]
+#[test]
+fn test_cursor_mut_insert_after_3() {
+    let mut map = BTreeMap::from([(1, 'a'), (2, 'b'), (3, 'c')]);
+    let mut cur = map.upper_bound_mut(Bound::Included(&2));
+    cur.insert_after(3, 'd');
+}
+
+#[should_panic(expected = "key must be ordered below the next element")]
+#[test]
+fn test_cursor_mut_insert_after_4() {
+    let mut map = BTreeMap::from([(1, 'a'), (2, 'b'), (3, 'c')]);
+    let mut cur = map.upper_bound_mut(Bound::Included(&2));
+    cur.insert_after(4, 'd');
+}
+
+#[test]
+fn cursor_peek_prev_agrees_with_cursor_mut() {
+    let mut map = BTreeMap::from([(1, 1), (2, 2), (3, 3)]);
+
+    let cursor = map.lower_bound(Bound::Excluded(&3));
+    assert!(cursor.key().is_none());
+
+    let prev = cursor.peek_prev();
+    assert_matches!(prev, Some((&3, _)));
+
+    // Shadow names so the two parts of this test match.
+    let mut cursor = map.lower_bound_mut(Bound::Excluded(&3));
+    assert!(cursor.key().is_none());
+
+    let prev = cursor.peek_prev();
+    assert_matches!(prev, Some((&3, _)));
+}
diff --git a/library/alloc/src/collections/btree/mod.rs b/library/alloc/src/collections/btree/mod.rs
index 9d43ac5c5be..c7d0144de30 100644
--- a/library/alloc/src/collections/btree/mod.rs
+++ b/library/alloc/src/collections/btree/mod.rs
@@ -13,7 +13,6 @@ pub mod set;
 mod set_val;
 mod split;
 
-#[doc(hidden)]
 trait Recover<Q: ?Sized> {
     type Key;
 
@@ -21,6 +20,3 @@ trait Recover<Q: ?Sized> {
     fn take(&mut self, key: &Q) -> Option<Self::Key>;
     fn replace(&mut self, key: Self::Key) -> Option<Self::Key>;
 }
-
-#[cfg(test)]
-mod testing;
diff --git a/library/alloc/src/collections/btree/navigate.rs b/library/alloc/src/collections/btree/navigate.rs
index 1e33c1e64d6..a85a3162451 100644
--- a/library/alloc/src/collections/btree/navigate.rs
+++ b/library/alloc/src/collections/btree/navigate.rs
@@ -4,6 +4,7 @@ use core::ops::RangeBounds;
 use core::ptr;
 
 use super::node::{marker, ForceResult::*, Handle, NodeRef};
+use super::search::SearchBound;
 
 use crate::alloc::Allocator;
 // `front` and `back` are always both `None` or both `Some`.
@@ -18,6 +19,12 @@ impl<'a, K: 'a, V: 'a> Clone for LeafRange<marker::Immut<'a>, K, V> {
     }
 }
 
+impl<B, K, V> Default for LeafRange<B, K, V> {
+    fn default() -> Self {
+        LeafRange { front: None, back: None }
+    }
+}
+
 impl<BorrowType, K, V> LeafRange<BorrowType, K, V> {
     pub fn none() -> Self {
         LeafRange { front: None, back: None }
@@ -123,6 +130,12 @@ pub struct LazyLeafRange<BorrowType, K, V> {
     back: Option<LazyLeafHandle<BorrowType, K, V>>,
 }
 
+impl<B, K, V> Default for LazyLeafRange<B, K, V> {
+    fn default() -> Self {
+        LazyLeafRange { front: None, back: None }
+    }
+}
+
 impl<'a, K: 'a, V: 'a> Clone for LazyLeafRange<marker::Immut<'a>, K, V> {
     fn clone(&self) -> Self {
         LazyLeafRange { front: self.front.clone(), back: self.back.clone() }
@@ -386,7 +399,7 @@ impl<BorrowType: marker::BorrowType, K, V>
     /// Given a leaf edge handle, returns [`Result::Ok`] with a handle to the neighboring KV
     /// on the left side, which is either in the same leaf node or in an ancestor node.
     /// If the leaf edge is the first one in the tree, returns [`Result::Err`] with the root node.
-    fn next_back_kv(
+    pub fn next_back_kv(
         self,
     ) -> Result<
         Handle<NodeRef<BorrowType, K, V, marker::LeafOrInternal>, marker::KV>,
@@ -707,7 +720,9 @@ impl<BorrowType: marker::BorrowType, K, V>
     }
 
     /// Returns the leaf edge closest to a KV for backward navigation.
-    fn next_back_leaf_edge(self) -> Handle<NodeRef<BorrowType, K, V, marker::Leaf>, marker::Edge> {
+    pub fn next_back_leaf_edge(
+        self,
+    ) -> Handle<NodeRef<BorrowType, K, V, marker::Leaf>, marker::Edge> {
         match self.force() {
             Leaf(leaf_kv) => leaf_kv.left_edge(),
             Internal(internal_kv) => {
@@ -717,3 +732,51 @@ impl<BorrowType: marker::BorrowType, K, V>
         }
     }
 }
+
+impl<BorrowType: marker::BorrowType, K, V> NodeRef<BorrowType, K, V, marker::LeafOrInternal> {
+    /// Returns the leaf edge corresponding to the first point at which the
+    /// given bound is true.
+    pub fn lower_bound<Q: ?Sized>(
+        self,
+        mut bound: SearchBound<&Q>,
+    ) -> Handle<NodeRef<BorrowType, K, V, marker::Leaf>, marker::Edge>
+    where
+        Q: Ord,
+        K: Borrow<Q>,
+    {
+        let mut node = self;
+        loop {
+            let (edge, new_bound) = node.find_lower_bound_edge(bound);
+            match edge.force() {
+                Leaf(edge) => return edge,
+                Internal(edge) => {
+                    node = edge.descend();
+                    bound = new_bound;
+                }
+            }
+        }
+    }
+
+    /// Returns the leaf edge corresponding to the last point at which the
+    /// given bound is true.
+    pub fn upper_bound<Q: ?Sized>(
+        self,
+        mut bound: SearchBound<&Q>,
+    ) -> Handle<NodeRef<BorrowType, K, V, marker::Leaf>, marker::Edge>
+    where
+        Q: Ord,
+        K: Borrow<Q>,
+    {
+        let mut node = self;
+        loop {
+            let (edge, new_bound) = node.find_upper_bound_edge(bound);
+            match edge.force() {
+                Leaf(edge) => return edge,
+                Internal(edge) => {
+                    node = edge.descend();
+                    bound = new_bound;
+                }
+            }
+        }
+    }
+}
diff --git a/library/alloc/src/collections/btree/node.rs b/library/alloc/src/collections/btree/node.rs
index d831161bcb6..3233a575ecf 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 +318,10 @@ 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);
+        const {
+            assert!(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);
@@ -439,6 +442,24 @@ impl<'a, K, V, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
         // SAFETY: we have exclusive access to the entire node.
         unsafe { &mut *ptr }
     }
+
+    /// Returns a dormant copy of this node with its lifetime erased which can
+    /// be reawakened later.
+    pub fn dormant(&self) -> NodeRef<marker::DormantMut, K, V, Type> {
+        NodeRef { height: self.height, node: self.node, _marker: PhantomData }
+    }
+}
+
+impl<K, V, Type> NodeRef<marker::DormantMut, K, V, Type> {
+    /// Revert to the unique borrow initially captured.
+    ///
+    /// # Safety
+    ///
+    /// The reborrow must have ended, i.e., the reference returned by `new` and
+    /// all pointers and references derived from it, must not be used anymore.
+    pub unsafe fn awaken<'a>(self) -> NodeRef<marker::Mut<'a>, K, V, Type> {
+        NodeRef { height: self.height, node: self.node, _marker: PhantomData }
+    }
 }
 
 impl<K, V, Type> NodeRef<marker::Dying, K, V, Type> {
@@ -795,6 +816,25 @@ impl<'a, K, V, NodeType, HandleType> Handle<NodeRef<marker::Mut<'a>, K, V, NodeT
         // We can't use Handle::new_kv or Handle::new_edge because we don't know our type
         Handle { node: unsafe { self.node.reborrow_mut() }, idx: self.idx, _marker: PhantomData }
     }
+
+    /// Returns a dormant copy of this handle which can be reawakened later.
+    ///
+    /// See `DormantMutRef` for more details.
+    pub fn dormant(&self) -> Handle<NodeRef<marker::DormantMut, K, V, NodeType>, HandleType> {
+        Handle { node: self.node.dormant(), idx: self.idx, _marker: PhantomData }
+    }
+}
+
+impl<K, V, NodeType, HandleType> Handle<NodeRef<marker::DormantMut, K, V, NodeType>, HandleType> {
+    /// Revert to the unique borrow initially captured.
+    ///
+    /// # Safety
+    ///
+    /// The reborrow must have ended, i.e., the reference returned by `new` and
+    /// all pointers and references derived from it, must not be used anymore.
+    pub unsafe fn awaken<'a>(self) -> Handle<NodeRef<marker::Mut<'a>, K, V, NodeType>, HandleType> {
+        Handle { node: unsafe { self.node.awaken() }, idx: self.idx, _marker: PhantomData }
+    }
 }
 
 impl<BorrowType, K, V, NodeType> Handle<NodeRef<BorrowType, K, V, NodeType>, marker::Edge> {
@@ -848,9 +888,11 @@ impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, mark
     /// Inserts a new key-value pair between the key-value pairs to the right and left of
     /// this edge. This method assumes that there is enough space in the node for the new
     /// pair to fit.
-    ///
-    /// The returned pointer points to the inserted value.
-    fn insert_fit(&mut self, key: K, val: V) -> *mut V {
+    unsafe fn insert_fit(
+        mut self,
+        key: K,
+        val: V,
+    ) -> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::KV> {
         debug_assert!(self.node.len() < CAPACITY);
         let new_len = self.node.len() + 1;
 
@@ -859,7 +901,7 @@ impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, mark
             slice_insert(self.node.val_area_mut(..new_len), self.idx, val);
             *self.node.len_mut() = new_len as u16;
 
-            self.node.val_area_mut(self.idx).assume_init_mut()
+            Handle::new_kv(self.node, self.idx)
         }
     }
 }
@@ -868,21 +910,26 @@ impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, mark
     /// Inserts a new key-value pair between the key-value pairs to the right and left of
     /// this edge. This method splits the node if there isn't enough room.
     ///
-    /// The returned pointer points to the inserted value.
+    /// Returns a dormant handle to the inserted node which can be reawakened
+    /// once splitting is complete.
     fn insert<A: Allocator + Clone>(
-        mut self,
+        self,
         key: K,
         val: V,
         alloc: A,
-    ) -> (Option<SplitResult<'a, K, V, marker::Leaf>>, *mut V) {
+    ) -> (
+        Option<SplitResult<'a, K, V, marker::Leaf>>,
+        Handle<NodeRef<marker::DormantMut, K, V, marker::Leaf>, marker::KV>,
+    ) {
         if self.node.len() < CAPACITY {
-            let val_ptr = self.insert_fit(key, val);
-            (None, val_ptr)
+            // SAFETY: There is enough space in the node for insertion.
+            let handle = unsafe { self.insert_fit(key, val) };
+            (None, handle.dormant())
         } else {
             let (middle_kv_idx, insertion) = splitpoint(self.idx);
             let middle = unsafe { Handle::new_kv(self.node, middle_kv_idx) };
             let mut result = middle.split(alloc);
-            let mut insertion_edge = match insertion {
+            let insertion_edge = match insertion {
                 LeftOrRight::Left(insert_idx) => unsafe {
                     Handle::new_edge(result.left.reborrow_mut(), insert_idx)
                 },
@@ -890,8 +937,10 @@ impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, mark
                     Handle::new_edge(result.right.borrow_mut(), insert_idx)
                 },
             };
-            let val_ptr = insertion_edge.insert_fit(key, val);
-            (Some(result), val_ptr)
+            // SAFETY: We just split the node, so there is enough space for
+            // insertion.
+            let handle = unsafe { insertion_edge.insert_fit(key, val).dormant() };
+            (Some(result), handle)
         }
     }
 }
@@ -973,21 +1022,31 @@ impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, mark
         key: K,
         value: V,
         alloc: A,
-    ) -> (Option<SplitResult<'a, K, V, marker::LeafOrInternal>>, *mut V) {
-        let (mut split, val_ptr) = match self.insert(key, value, alloc.clone()) {
-            (None, val_ptr) => return (None, val_ptr),
-            (Some(split), val_ptr) => (split.forget_node_type(), val_ptr),
+        split_root: impl FnOnce(SplitResult<'a, K, V, marker::LeafOrInternal>),
+    ) -> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::KV> {
+        let (mut split, handle) = match self.insert(key, value, alloc.clone()) {
+            // SAFETY: we have finished splitting and can now re-awaken the
+            // handle to the inserted element.
+            (None, handle) => return unsafe { handle.awaken() },
+            (Some(split), handle) => (split.forget_node_type(), handle),
         };
 
         loop {
             split = match split.left.ascend() {
                 Ok(parent) => {
                     match parent.insert(split.kv.0, split.kv.1, split.right, alloc.clone()) {
-                        None => return (None, val_ptr),
+                        // SAFETY: we have finished splitting and can now re-awaken the
+                        // handle to the inserted element.
+                        None => return unsafe { handle.awaken() },
                         Some(split) => split.forget_node_type(),
                     }
                 }
-                Err(root) => return (Some(SplitResult { left: root, ..split }), val_ptr),
+                Err(root) => {
+                    split_root(SplitResult { left: root, ..split });
+                    // SAFETY: we have finished splitting and can now re-awaken the
+                    // handle to the inserted element.
+                    return unsafe { handle.awaken() };
+                }
             };
         }
     }
@@ -1003,7 +1062,10 @@ 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);
+        const {
+            assert!(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
@@ -1037,6 +1099,14 @@ impl<'a, K: 'a, V: 'a, NodeType> Handle<NodeRef<marker::Mut<'a>, K, V, NodeType>
         let leaf = self.node.into_leaf_mut();
         unsafe { leaf.vals.get_unchecked_mut(self.idx).assume_init_mut() }
     }
+
+    pub fn into_kv_valmut(self) -> (&'a K, &'a mut V) {
+        debug_assert!(self.idx < self.node.len());
+        let leaf = self.node.into_leaf_mut();
+        let k = unsafe { leaf.keys.get_unchecked(self.idx).assume_init_ref() };
+        let v = unsafe { leaf.vals.get_unchecked_mut(self.idx).assume_init_mut() };
+        (k, v)
+    }
 }
 
 impl<'a, K, V, NodeType> Handle<NodeRef<marker::ValMut<'a>, K, V, NodeType>, marker::KV> {
@@ -1661,25 +1731,29 @@ pub mod marker {
 
     pub enum Owned {}
     pub enum Dying {}
+    pub enum DormantMut {}
     pub struct Immut<'a>(PhantomData<&'a ()>);
     pub struct Mut<'a>(PhantomData<&'a mut ()>);
     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 is set to `true`. It can be used
+        /// for a compile-time assertion.
+        const TRAVERSAL_PERMIT: bool = true;
     }
     impl BorrowType for Owned {
-        // Traversal isn't needed, it 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;
+        /// Reject traversal, because it 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 TRAVERSAL_PERMIT: bool = false;
     }
     impl BorrowType for Dying {}
     impl<'a> BorrowType for Immut<'a> {}
     impl<'a> BorrowType for Mut<'a> {}
     impl<'a> BorrowType for ValMut<'a> {}
+    impl BorrowType for DormantMut {}
 
     pub enum KV {}
     pub enum Edge {}
diff --git a/library/alloc/src/collections/btree/node/tests.rs b/library/alloc/src/collections/btree/node/tests.rs
index aadb0dc9c40..64bce0ff8c0 100644
--- a/library/alloc/src/collections/btree/node/tests.rs
+++ b/library/alloc/src/collections/btree/node/tests.rs
@@ -94,6 +94,7 @@ fn test_partial_eq() {
 
 #[test]
 #[cfg(target_arch = "x86_64")]
+#[cfg_attr(miri, ignore)] // We'd like to run Miri with layout randomization
 fn test_sizes() {
     assert_eq!(core::mem::size_of::<LeafNode<(), ()>>(), 16);
     assert_eq!(core::mem::size_of::<LeafNode<i64, i64>>(), 16 + CAPACITY * 2 * 8);
diff --git a/library/alloc/src/collections/btree/set.rs b/library/alloc/src/collections/btree/set.rs
index 2cfc0807409..940fa30afb8 100644
--- a/library/alloc/src/collections/btree/set.rs
+++ b/library/alloc/src/collections/btree/set.rs
@@ -1,13 +1,10 @@
-// This is pretty much entirely stolen from TreeSet, since BTreeMap has an identical interface
-// to TreeMap
-
 use crate::vec::Vec;
 use core::borrow::Borrow;
 use core::cmp::Ordering::{self, Equal, Greater, Less};
 use core::cmp::{max, min};
 use core::fmt::{self, Debug};
 use core::hash::{Hash, Hasher};
-use core::iter::{FromIterator, FusedIterator, Peekable};
+use core::iter::{FusedIterator, Peekable};
 use core::mem::ManuallyDrop;
 use core::ops::{BitAnd, BitOr, BitXor, RangeBounds, Sub};
 
@@ -18,8 +15,6 @@ use super::Recover;
 
 use crate::alloc::{Allocator, Global};
 
-// FIXME(conventions): implement bounded iterators
-
 /// An ordered set based on a B-Tree.
 ///
 /// See [`BTreeMap`]'s documentation for a detailed discussion of this collection's performance
@@ -35,7 +30,6 @@ use crate::alloc::{Allocator, Global};
 /// Iterators returned by [`BTreeSet::iter`] produce their items in order, and take worst-case
 /// logarithmic and amortized constant time per item returned.
 ///
-/// [`Ord`]: core::cmp::Ord
 /// [`Cell`]: core::cell::Cell
 /// [`RefCell`]: core::cell::RefCell
 ///
@@ -152,7 +146,6 @@ impl<T: fmt::Debug> fmt::Debug for Iter<'_, T> {
 /// (provided by the [`IntoIterator`] trait). See its documentation for more.
 ///
 /// [`into_iter`]: BTreeSet#method.into_iter
-/// [`IntoIterator`]: core::iter::IntoIterator
 #[stable(feature = "rust1", since = "1.0.0")]
 #[derive(Debug)]
 pub struct IntoIter<
@@ -343,7 +336,7 @@ 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 +779,6 @@ 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 +789,7 @@ 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 +805,6 @@ 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 +815,7 @@ 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 +829,6 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(map_first_last)]
     /// use std::collections::BTreeSet;
     ///
     /// let mut set = BTreeSet::new();
@@ -849,7 +839,7 @@ 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 +853,6 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(map_first_last)]
     /// use std::collections::BTreeSet;
     ///
     /// let mut set = BTreeSet::new();
@@ -874,7 +863,7 @@ 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 +1163,11 @@ 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 +1186,11 @@ 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
     }
@@ -1504,11 +1501,17 @@ impl<'a, T> Iterator for Iter<'a, T> {
         self.next_back()
     }
 
-    fn min(mut self) -> Option<&'a T> {
+    fn min(mut self) -> Option<&'a T>
+    where
+        &'a T: Ord,
+    {
         self.next()
     }
 
-    fn max(mut self) -> Option<&'a T> {
+    fn max(mut self) -> Option<&'a T>
+    where
+        &'a T: Ord,
+    {
         self.next_back()
     }
 }
@@ -1540,6 +1543,21 @@ impl<T, A: Allocator + Clone> Iterator for IntoIter<T, A> {
         self.iter.size_hint()
     }
 }
+
+#[stable(feature = "default_iters", since = "1.70.0")]
+impl<T> Default for Iter<'_, T> {
+    /// Creates an empty `btree_set::Iter`.
+    ///
+    /// ```
+    /// # use std::collections::btree_set;
+    /// let iter: btree_set::Iter<'_, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        Iter { iter: Default::default() }
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T, A: Allocator + Clone> DoubleEndedIterator for IntoIter<T, A> {
     fn next_back(&mut self) -> Option<T> {
@@ -1556,6 +1574,23 @@ impl<T, A: Allocator + Clone> ExactSizeIterator for IntoIter<T, A> {
 #[stable(feature = "fused", since = "1.26.0")]
 impl<T, A: Allocator + Clone> FusedIterator for IntoIter<T, A> {}
 
+#[stable(feature = "default_iters", since = "1.70.0")]
+impl<T, A> Default for IntoIter<T, A>
+where
+    A: Allocator + Default + Clone,
+{
+    /// Creates an empty `btree_set::IntoIter`.
+    ///
+    /// ```
+    /// # use std::collections::btree_set;
+    /// let iter: btree_set::IntoIter<u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        IntoIter { iter: Default::default() }
+    }
+}
+
 #[stable(feature = "btree_range", since = "1.17.0")]
 impl<T> Clone for Range<'_, T> {
     fn clone(&self) -> Self {
@@ -1575,11 +1610,17 @@ impl<'a, T> Iterator for Range<'a, T> {
         self.next_back()
     }
 
-    fn min(mut self) -> Option<&'a T> {
+    fn min(mut self) -> Option<&'a T>
+    where
+        &'a T: Ord,
+    {
         self.next()
     }
 
-    fn max(mut self) -> Option<&'a T> {
+    fn max(mut self) -> Option<&'a T>
+    where
+        &'a T: Ord,
+    {
         self.next_back()
     }
 }
@@ -1594,6 +1635,20 @@ impl<'a, T> DoubleEndedIterator for Range<'a, T> {
 #[stable(feature = "fused", since = "1.26.0")]
 impl<T> FusedIterator for Range<'_, T> {}
 
+#[stable(feature = "default_iters", since = "1.70.0")]
+impl<T> Default for Range<'_, T> {
+    /// Creates an empty `btree_set::Range`.
+    ///
+    /// ```
+    /// # use std::collections::btree_set;
+    /// let iter: btree_set::Range<'_, u8> = Default::default();
+    /// assert_eq!(iter.count(), 0);
+    /// ```
+    fn default() -> Self {
+        Range { iter: Default::default() }
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T, A: Allocator + Clone> Clone for Difference<'_, T, A> {
     fn clone(&self) -> Self {
diff --git a/library/alloc/src/collections/btree/set/tests.rs b/library/alloc/src/collections/btree/set/tests.rs
index 502d3e1d126..a7c839d77ed 100644
--- a/library/alloc/src/collections/btree/set/tests.rs
+++ b/library/alloc/src/collections/btree/set/tests.rs
@@ -1,10 +1,9 @@
-use super::super::testing::crash_test::{CrashTestDummy, Panic};
-use super::super::testing::rng::DeterministicRng;
 use super::*;
+use crate::testing::crash_test::{CrashTestDummy, Panic};
+use crate::testing::rng::DeterministicRng;
 use crate::vec::Vec;
 use std::cmp::Ordering;
 use std::hash::{Hash, Hasher};
-use std::iter::FromIterator;
 use std::ops::Bound::{Excluded, Included};
 use std::panic::{catch_unwind, AssertUnwindSafe};
 
diff --git a/library/alloc/src/collections/btree/testing/crash_test.rs b/library/alloc/src/collections/btree/testing/crash_test.rs
deleted file mode 100644
index bcf5f5f7251..00000000000
--- a/library/alloc/src/collections/btree/testing/crash_test.rs
+++ /dev/null
@@ -1,119 +0,0 @@
-// We avoid relying on anything else in the crate, apart from the `Debug` trait.
-use crate::fmt::Debug;
-use std::cmp::Ordering;
-use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
-
-/// A blueprint for crash test dummy instances that monitor particular events.
-/// Some instances may be configured to panic at some point.
-/// Events are `clone`, `drop` or some anonymous `query`.
-///
-/// Crash test dummies are identified and ordered by an id, so they can be used
-/// as keys in a BTreeMap.
-#[derive(Debug)]
-pub struct CrashTestDummy {
-    pub id: usize,
-    cloned: AtomicUsize,
-    dropped: AtomicUsize,
-    queried: AtomicUsize,
-}
-
-impl CrashTestDummy {
-    /// Creates a crash test dummy design. The `id` determines order and equality of instances.
-    pub fn new(id: usize) -> CrashTestDummy {
-        CrashTestDummy {
-            id,
-            cloned: AtomicUsize::new(0),
-            dropped: AtomicUsize::new(0),
-            queried: AtomicUsize::new(0),
-        }
-    }
-
-    /// Creates an instance of a crash test dummy that records what events it experiences
-    /// and optionally panics.
-    pub fn spawn(&self, panic: Panic) -> Instance<'_> {
-        Instance { origin: self, panic }
-    }
-
-    /// Returns how many times instances of the dummy have been cloned.
-    pub fn cloned(&self) -> usize {
-        self.cloned.load(SeqCst)
-    }
-
-    /// Returns how many times instances of the dummy have been dropped.
-    pub fn dropped(&self) -> usize {
-        self.dropped.load(SeqCst)
-    }
-
-    /// Returns how many times instances of the dummy have had their `query` member invoked.
-    pub fn queried(&self) -> usize {
-        self.queried.load(SeqCst)
-    }
-}
-
-#[derive(Debug)]
-pub struct Instance<'a> {
-    origin: &'a CrashTestDummy,
-    panic: Panic,
-}
-
-#[derive(Copy, Clone, Debug, PartialEq, Eq)]
-pub enum Panic {
-    Never,
-    InClone,
-    InDrop,
-    InQuery,
-}
-
-impl Instance<'_> {
-    pub fn id(&self) -> usize {
-        self.origin.id
-    }
-
-    /// Some anonymous query, the result of which is already given.
-    pub fn query<R>(&self, result: R) -> R {
-        self.origin.queried.fetch_add(1, SeqCst);
-        if self.panic == Panic::InQuery {
-            panic!("panic in `query`");
-        }
-        result
-    }
-}
-
-impl Clone for Instance<'_> {
-    fn clone(&self) -> Self {
-        self.origin.cloned.fetch_add(1, SeqCst);
-        if self.panic == Panic::InClone {
-            panic!("panic in `clone`");
-        }
-        Self { origin: self.origin, panic: Panic::Never }
-    }
-}
-
-impl Drop for Instance<'_> {
-    fn drop(&mut self) {
-        self.origin.dropped.fetch_add(1, SeqCst);
-        if self.panic == Panic::InDrop {
-            panic!("panic in `drop`");
-        }
-    }
-}
-
-impl PartialOrd for Instance<'_> {
-    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
-        self.id().partial_cmp(&other.id())
-    }
-}
-
-impl Ord for Instance<'_> {
-    fn cmp(&self, other: &Self) -> Ordering {
-        self.id().cmp(&other.id())
-    }
-}
-
-impl PartialEq for Instance<'_> {
-    fn eq(&self, other: &Self) -> bool {
-        self.id().eq(&other.id())
-    }
-}
-
-impl Eq for Instance<'_> {}
diff --git a/library/alloc/src/collections/btree/testing/mod.rs b/library/alloc/src/collections/btree/testing/mod.rs
deleted file mode 100644
index 7a094f8a595..00000000000
--- a/library/alloc/src/collections/btree/testing/mod.rs
+++ /dev/null
@@ -1,3 +0,0 @@
-pub mod crash_test;
-pub mod ord_chaos;
-pub mod rng;
diff --git a/library/alloc/src/collections/btree/testing/ord_chaos.rs b/library/alloc/src/collections/btree/testing/ord_chaos.rs
deleted file mode 100644
index 96ce7c15790..00000000000
--- a/library/alloc/src/collections/btree/testing/ord_chaos.rs
+++ /dev/null
@@ -1,81 +0,0 @@
-use std::cell::Cell;
-use std::cmp::Ordering::{self, *};
-use std::ptr;
-
-// Minimal type with an `Ord` implementation violating transitivity.
-#[derive(Debug)]
-pub enum Cyclic3 {
-    A,
-    B,
-    C,
-}
-use Cyclic3::*;
-
-impl PartialOrd for Cyclic3 {
-    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
-        Some(self.cmp(other))
-    }
-}
-
-impl Ord for Cyclic3 {
-    fn cmp(&self, other: &Self) -> Ordering {
-        match (self, other) {
-            (A, A) | (B, B) | (C, C) => Equal,
-            (A, B) | (B, C) | (C, A) => Less,
-            (A, C) | (B, A) | (C, B) => Greater,
-        }
-    }
-}
-
-impl PartialEq for Cyclic3 {
-    fn eq(&self, other: &Self) -> bool {
-        self.cmp(&other) == Equal
-    }
-}
-
-impl Eq for Cyclic3 {}
-
-// Controls the ordering of values wrapped by `Governed`.
-#[derive(Debug)]
-pub struct Governor {
-    flipped: Cell<bool>,
-}
-
-impl Governor {
-    pub fn new() -> Self {
-        Governor { flipped: Cell::new(false) }
-    }
-
-    pub fn flip(&self) {
-        self.flipped.set(!self.flipped.get());
-    }
-}
-
-// Type with an `Ord` implementation that forms a total order at any moment
-// (assuming that `T` respects total order), but can suddenly be made to invert
-// that total order.
-#[derive(Debug)]
-pub struct Governed<'a, T>(pub T, pub &'a Governor);
-
-impl<T: Ord> PartialOrd for Governed<'_, T> {
-    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
-        Some(self.cmp(other))
-    }
-}
-
-impl<T: Ord> Ord for Governed<'_, T> {
-    fn cmp(&self, other: &Self) -> Ordering {
-        assert!(ptr::eq(self.1, other.1));
-        let ord = self.0.cmp(&other.0);
-        if self.1.flipped.get() { ord.reverse() } else { ord }
-    }
-}
-
-impl<T: PartialEq> PartialEq for Governed<'_, T> {
-    fn eq(&self, other: &Self) -> bool {
-        assert!(ptr::eq(self.1, other.1));
-        self.0.eq(&other.0)
-    }
-}
-
-impl<T: Eq> Eq for Governed<'_, T> {}
diff --git a/library/alloc/src/collections/btree/testing/rng.rs b/library/alloc/src/collections/btree/testing/rng.rs
deleted file mode 100644
index ecf543bee03..00000000000
--- a/library/alloc/src/collections/btree/testing/rng.rs
+++ /dev/null
@@ -1,28 +0,0 @@
-/// XorShiftRng
-pub struct DeterministicRng {
-    count: usize,
-    x: u32,
-    y: u32,
-    z: u32,
-    w: u32,
-}
-
-impl DeterministicRng {
-    pub fn new() -> Self {
-        DeterministicRng { count: 0, x: 0x193a6754, y: 0xa8a7d469, z: 0x97830e05, w: 0x113ba7bb }
-    }
-
-    /// Guarantees that each returned number is unique.
-    pub fn next(&mut self) -> u32 {
-        self.count += 1;
-        assert!(self.count <= 70029);
-        let x = self.x;
-        let t = x ^ (x << 11);
-        self.x = self.y;
-        self.y = self.z;
-        self.z = self.w;
-        let w_ = self.w;
-        self.w = w_ ^ (w_ >> 19) ^ (t ^ (t >> 8));
-        self.w
-    }
-}
diff --git a/library/alloc/src/collections/linked_list.rs b/library/alloc/src/collections/linked_list.rs
index e21c8aa3bd5..4cd34ac2fa7 100644
--- a/library/alloc/src/collections/linked_list.rs
+++ b/library/alloc/src/collections/linked_list.rs
@@ -15,12 +15,13 @@
 use core::cmp::Ordering;
 use core::fmt;
 use core::hash::{Hash, Hasher};
-use core::iter::{FromIterator, FusedIterator};
+use core::iter::FusedIterator;
 use core::marker::PhantomData;
 use core::mem;
-use core::ptr::NonNull;
+use core::ptr::{NonNull, Unique};
 
 use super::SpecExtend;
+use crate::alloc::{Allocator, Global};
 use crate::boxed::Box;
 
 #[cfg(test)]
@@ -47,11 +48,15 @@ mod tests;
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "LinkedList")]
 #[rustc_insignificant_dtor]
-pub struct LinkedList<T> {
+pub struct LinkedList<
+    T,
+    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
+> {
     head: Option<NonNull<Node<T>>>,
     tail: Option<NonNull<Node<T>>>,
     len: usize,
-    marker: PhantomData<Box<Node<T>>>,
+    alloc: A,
+    marker: PhantomData<Box<Node<T>, A>>,
 }
 
 struct Node<T> {
@@ -81,6 +86,7 @@ impl<T: fmt::Debug> fmt::Debug for Iter<'_, T> {
                 head: self.head,
                 tail: self.tail,
                 len: self.len,
+                alloc: Global,
                 marker: PhantomData,
             }))
             .field(&self.len)
@@ -117,6 +123,7 @@ impl<T: fmt::Debug> fmt::Debug for IterMut<'_, T> {
                 head: self.head,
                 tail: self.tail,
                 len: self.len,
+                alloc: Global,
                 marker: PhantomData,
             }))
             .field(&self.len)
@@ -130,15 +137,17 @@ impl<T: fmt::Debug> fmt::Debug for IterMut<'_, T> {
 /// (provided by the [`IntoIterator`] trait). See its documentation for more.
 ///
 /// [`into_iter`]: LinkedList::into_iter
-/// [`IntoIterator`]: core::iter::IntoIterator
 #[derive(Clone)]
 #[stable(feature = "rust1", since = "1.0.0")]
-pub struct IntoIter<T> {
-    list: LinkedList<T>,
+pub struct IntoIter<
+    T,
+    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
+> {
+    list: LinkedList<T, A>,
 }
 
 #[stable(feature = "collection_debug", since = "1.17.0")]
-impl<T: fmt::Debug> fmt::Debug for IntoIter<T> {
+impl<T: fmt::Debug, A: Allocator> fmt::Debug for IntoIter<T, A> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_tuple("IntoIter").field(&self.list).finish()
     }
@@ -149,22 +158,25 @@ impl<T> Node<T> {
         Node { next: None, prev: None, element }
     }
 
-    fn into_element(self: Box<Self>) -> T {
+    fn into_element<A: Allocator>(self: Box<Self, A>) -> T {
         self.element
     }
 }
 
 // private methods
-impl<T> LinkedList<T> {
+impl<T, A: Allocator> LinkedList<T, A> {
     /// Adds the given node to the front of the list.
+    ///
+    /// # Safety
+    /// `node` must point to a valid node that was boxed using the list's allocator.
     #[inline]
-    fn push_front_node(&mut self, mut node: Box<Node<T>>) {
+    unsafe fn push_front_node(&mut self, node: Unique<Node<T>>) {
         // This method takes care not to create mutable references to whole nodes,
         // to maintain validity of aliasing pointers into `element`.
         unsafe {
-            node.next = self.head;
-            node.prev = None;
-            let node = Some(Box::leak(node).into());
+            (*node.as_ptr()).next = self.head;
+            (*node.as_ptr()).prev = None;
+            let node = Some(NonNull::from(node));
 
             match self.head {
                 None => self.tail = node,
@@ -179,11 +191,11 @@ impl<T> LinkedList<T> {
 
     /// Removes and returns the node at the front of the list.
     #[inline]
-    fn pop_front_node(&mut self) -> Option<Box<Node<T>>> {
+    fn pop_front_node(&mut self) -> Option<Box<Node<T>, &A>> {
         // This method takes care not to create mutable references to whole nodes,
         // to maintain validity of aliasing pointers into `element`.
         self.head.map(|node| unsafe {
-            let node = Box::from_raw(node.as_ptr());
+            let node = Box::from_raw_in(node.as_ptr(), &self.alloc);
             self.head = node.next;
 
             match self.head {
@@ -198,14 +210,17 @@ impl<T> LinkedList<T> {
     }
 
     /// Adds the given node to the back of the list.
+    ///
+    /// # Safety
+    /// `node` must point to a valid node that was boxed using the list's allocator.
     #[inline]
-    fn push_back_node(&mut self, mut node: Box<Node<T>>) {
+    unsafe fn push_back_node(&mut self, node: Unique<Node<T>>) {
         // This method takes care not to create mutable references to whole nodes,
         // to maintain validity of aliasing pointers into `element`.
         unsafe {
-            node.next = None;
-            node.prev = self.tail;
-            let node = Some(Box::leak(node).into());
+            (*node.as_ptr()).next = None;
+            (*node.as_ptr()).prev = self.tail;
+            let node = Some(NonNull::from(node));
 
             match self.tail {
                 None => self.head = node,
@@ -220,11 +235,11 @@ impl<T> LinkedList<T> {
 
     /// Removes and returns the node at the back of the list.
     #[inline]
-    fn pop_back_node(&mut self) -> Option<Box<Node<T>>> {
+    fn pop_back_node(&mut self) -> Option<Box<Node<T>, &A>> {
         // This method takes care not to create mutable references to whole nodes,
         // to maintain validity of aliasing pointers into `element`.
         self.tail.map(|node| unsafe {
-            let node = Box::from_raw(node.as_ptr());
+            let node = Box::from_raw_in(node.as_ptr(), &self.alloc);
             self.tail = node.prev;
 
             match self.tail {
@@ -322,7 +337,10 @@ impl<T> LinkedList<T> {
         &mut self,
         split_node: Option<NonNull<Node<T>>>,
         at: usize,
-    ) -> Self {
+    ) -> Self
+    where
+        A: Clone,
+    {
         // The split node is the new head node of the second part
         if let Some(mut split_node) = split_node {
             let first_part_head;
@@ -343,6 +361,7 @@ impl<T> LinkedList<T> {
                 head: first_part_head,
                 tail: first_part_tail,
                 len: at,
+                alloc: self.alloc.clone(),
                 marker: PhantomData,
             };
 
@@ -352,7 +371,7 @@ impl<T> LinkedList<T> {
 
             first_part
         } else {
-            mem::replace(self, LinkedList::new())
+            mem::replace(self, LinkedList::new_in(self.alloc.clone()))
         }
     }
 
@@ -361,7 +380,10 @@ impl<T> LinkedList<T> {
         &mut self,
         split_node: Option<NonNull<Node<T>>>,
         at: usize,
-    ) -> Self {
+    ) -> Self
+    where
+        A: Clone,
+    {
         // The split node is the new tail node of the first part and owns
         // the head of the second part.
         if let Some(mut split_node) = split_node {
@@ -383,6 +405,7 @@ impl<T> LinkedList<T> {
                 head: second_part_head,
                 tail: second_part_tail,
                 len: self.len - at,
+                alloc: self.alloc.clone(),
                 marker: PhantomData,
             };
 
@@ -392,7 +415,7 @@ impl<T> LinkedList<T> {
 
             second_part
         } else {
-            mem::replace(self, LinkedList::new())
+            mem::replace(self, LinkedList::new_in(self.alloc.clone()))
         }
     }
 }
@@ -421,7 +444,7 @@ impl<T> LinkedList<T> {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use]
     pub const fn new() -> Self {
-        LinkedList { head: None, tail: None, len: 0, marker: PhantomData }
+        LinkedList { head: None, tail: None, len: 0, alloc: Global, marker: PhantomData }
     }
 
     /// Moves all elements from `other` to the end of the list.
@@ -472,7 +495,26 @@ impl<T> LinkedList<T> {
             }
         }
     }
+}
 
+impl<T, A: Allocator> LinkedList<T, A> {
+    /// Constructs an empty `LinkedList<T, A>`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(allocator_api)]
+    ///
+    /// use std::alloc::System;
+    /// use std::collections::LinkedList;
+    ///
+    /// let list: LinkedList<u32, _> = LinkedList::new_in(System);
+    /// ```
+    #[inline]
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    pub const fn new_in(alloc: A) -> Self {
+        LinkedList { head: None, tail: None, len: 0, alloc, marker: PhantomData }
+    }
     /// Provides a forward iterator.
     ///
     /// # Examples
@@ -533,7 +575,7 @@ impl<T> LinkedList<T> {
     #[inline]
     #[must_use]
     #[unstable(feature = "linked_list_cursors", issue = "58533")]
-    pub fn cursor_front(&self) -> Cursor<'_, T> {
+    pub fn cursor_front(&self) -> Cursor<'_, T, A> {
         Cursor { index: 0, current: self.head, list: self }
     }
 
@@ -543,7 +585,7 @@ impl<T> LinkedList<T> {
     #[inline]
     #[must_use]
     #[unstable(feature = "linked_list_cursors", issue = "58533")]
-    pub fn cursor_front_mut(&mut self) -> CursorMut<'_, T> {
+    pub fn cursor_front_mut(&mut self) -> CursorMut<'_, T, A> {
         CursorMut { index: 0, current: self.head, list: self }
     }
 
@@ -553,7 +595,7 @@ impl<T> LinkedList<T> {
     #[inline]
     #[must_use]
     #[unstable(feature = "linked_list_cursors", issue = "58533")]
-    pub fn cursor_back(&self) -> Cursor<'_, T> {
+    pub fn cursor_back(&self) -> Cursor<'_, T, A> {
         Cursor { index: self.len.checked_sub(1).unwrap_or(0), current: self.tail, list: self }
     }
 
@@ -563,7 +605,7 @@ impl<T> LinkedList<T> {
     #[inline]
     #[must_use]
     #[unstable(feature = "linked_list_cursors", issue = "58533")]
-    pub fn cursor_back_mut(&mut self) -> CursorMut<'_, T> {
+    pub fn cursor_back_mut(&mut self) -> CursorMut<'_, T, A> {
         CursorMut { index: self.len.checked_sub(1).unwrap_or(0), current: self.tail, list: self }
     }
 
@@ -639,7 +681,15 @@ impl<T> LinkedList<T> {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn clear(&mut self) {
-        *self = Self::new();
+        // We need to drop the nodes while keeping self.alloc
+        // We can do this by moving (head, tail, len) into a new list that borrows self.alloc
+        drop(LinkedList {
+            head: self.head.take(),
+            tail: self.tail.take(),
+            len: mem::take(&mut self.len),
+            alloc: &self.alloc,
+            marker: PhantomData,
+        });
     }
 
     /// Returns `true` if the `LinkedList` contains an element equal to the
@@ -791,7 +841,12 @@ impl<T> LinkedList<T> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn push_front(&mut self, elt: T) {
-        self.push_front_node(Box::new(Node::new(elt)));
+        let node = Box::new_in(Node::new(elt), &self.alloc);
+        let node_ptr = Unique::from(Box::leak(node));
+        // SAFETY: node_ptr is a unique pointer to a node we boxed with self.alloc
+        unsafe {
+            self.push_front_node(node_ptr);
+        }
     }
 
     /// Removes the first element and returns it, or `None` if the list is
@@ -834,7 +889,12 @@ impl<T> LinkedList<T> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn push_back(&mut self, elt: T) {
-        self.push_back_node(Box::new(Node::new(elt)));
+        let node = Box::new_in(Node::new(elt), &self.alloc);
+        let node_ptr = Unique::from(Box::leak(node));
+        // SAFETY: node_ptr is a unique pointer to a node we boxed with self.alloc
+        unsafe {
+            self.push_back_node(node_ptr);
+        }
     }
 
     /// Removes the last element from a list and returns it, or `None` if
@@ -884,13 +944,16 @@ impl<T> LinkedList<T> {
     /// assert_eq!(split.pop_front(), None);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn split_off(&mut self, at: usize) -> LinkedList<T> {
+    pub fn split_off(&mut self, at: usize) -> LinkedList<T, A>
+    where
+        A: Clone,
+    {
         let len = self.len();
         assert!(at <= len, "Cannot split off at a nonexistent index");
         if at == 0 {
-            return mem::take(self);
+            return mem::replace(self, Self::new_in(self.alloc.clone()));
         } else if at == len {
-            return Self::new();
+            return Self::new_in(self.alloc.clone());
         }
 
         // Below, we iterate towards the `i-1`th node, either from the start or the end,
@@ -988,7 +1051,7 @@ impl<T> LinkedList<T> {
     /// assert_eq!(odds.into_iter().collect::<Vec<_>>(), vec![1, 3, 5, 9, 11, 13, 15]);
     /// ```
     #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
-    pub fn drain_filter<F>(&mut self, filter: F) -> DrainFilter<'_, T, F>
+    pub fn drain_filter<F>(&mut self, filter: F) -> DrainFilter<'_, T, F, A>
     where
         F: FnMut(&mut T) -> bool,
     {
@@ -1001,11 +1064,11 @@ impl<T> LinkedList<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-unsafe impl<#[may_dangle] T> Drop for LinkedList<T> {
+unsafe impl<#[may_dangle] T, A: Allocator> Drop for LinkedList<T, A> {
     fn drop(&mut self) {
-        struct DropGuard<'a, T>(&'a mut LinkedList<T>);
+        struct DropGuard<'a, T, A: Allocator>(&'a mut LinkedList<T, A>);
 
-        impl<'a, T> Drop for DropGuard<'a, T> {
+        impl<'a, T, A: Allocator> Drop for DropGuard<'a, T, A> {
             fn drop(&mut self) {
                 // Continue the same loop we do below. This only runs when a destructor has
                 // panicked. If another one panics this will abort.
@@ -1013,11 +1076,10 @@ unsafe impl<#[may_dangle] T> Drop for LinkedList<T> {
             }
         }
 
-        while let Some(node) = self.pop_front_node() {
-            let guard = DropGuard(self);
-            drop(node);
-            mem::forget(guard);
-        }
+        // Wrap self so that if a destructor panics, we can try to keep looping
+        let guard = DropGuard(self);
+        while guard.0.pop_front_node().is_some() {}
+        mem::forget(guard);
     }
 }
 
@@ -1075,6 +1137,20 @@ impl<T> ExactSizeIterator for Iter<'_, T> {}
 #[stable(feature = "fused", since = "1.26.0")]
 impl<T> FusedIterator for Iter<'_, T> {}
 
+#[stable(feature = "default_iters", since = "1.70.0")]
+impl<T> Default for Iter<'_, T> {
+    /// Creates an empty `linked_list::Iter`.
+    ///
+    /// ```
+    /// # use std::collections::linked_list;
+    /// let iter: linked_list::Iter<'_, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        Iter { head: None, tail: None, len: 0, marker: Default::default() }
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, T> Iterator for IterMut<'a, T> {
     type Item = &'a mut T;
@@ -1129,6 +1205,13 @@ impl<T> ExactSizeIterator for IterMut<'_, T> {}
 #[stable(feature = "fused", since = "1.26.0")]
 impl<T> FusedIterator for IterMut<'_, T> {}
 
+#[stable(feature = "default_iters", since = "1.70.0")]
+impl<T> Default for IterMut<'_, T> {
+    fn default() -> Self {
+        IterMut { head: None, tail: None, len: 0, marker: Default::default() }
+    }
+}
+
 /// A cursor over a `LinkedList`.
 ///
 /// A `Cursor` is like an iterator, except that it can freely seek back-and-forth.
@@ -1139,14 +1222,18 @@ impl<T> FusedIterator for IterMut<'_, T> {}
 ///
 /// When created, cursors start at the front of the list, or the "ghost" non-element if the list is empty.
 #[unstable(feature = "linked_list_cursors", issue = "58533")]
-pub struct Cursor<'a, T: 'a> {
+pub struct Cursor<
+    'a,
+    T: 'a,
+    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
+> {
     index: usize,
     current: Option<NonNull<Node<T>>>,
-    list: &'a LinkedList<T>,
+    list: &'a LinkedList<T, A>,
 }
 
 #[unstable(feature = "linked_list_cursors", issue = "58533")]
-impl<T> Clone for Cursor<'_, T> {
+impl<T, A: Allocator> Clone for Cursor<'_, T, A> {
     fn clone(&self) -> Self {
         let Cursor { index, current, list } = *self;
         Cursor { index, current, list }
@@ -1154,7 +1241,7 @@ impl<T> Clone for Cursor<'_, T> {
 }
 
 #[unstable(feature = "linked_list_cursors", issue = "58533")]
-impl<T: fmt::Debug> fmt::Debug for Cursor<'_, T> {
+impl<T: fmt::Debug, A: Allocator> fmt::Debug for Cursor<'_, T, A> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_tuple("Cursor").field(&self.list).field(&self.index()).finish()
     }
@@ -1171,20 +1258,24 @@ impl<T: fmt::Debug> fmt::Debug for Cursor<'_, T> {
 /// To accommodate this, there is a "ghost" non-element that yields `None` between the head and
 /// tail of the list.
 #[unstable(feature = "linked_list_cursors", issue = "58533")]
-pub struct CursorMut<'a, T: 'a> {
+pub struct CursorMut<
+    'a,
+    T: 'a,
+    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
+> {
     index: usize,
     current: Option<NonNull<Node<T>>>,
-    list: &'a mut LinkedList<T>,
+    list: &'a mut LinkedList<T, A>,
 }
 
 #[unstable(feature = "linked_list_cursors", issue = "58533")]
-impl<T: fmt::Debug> fmt::Debug for CursorMut<'_, T> {
+impl<T: fmt::Debug, A: Allocator> fmt::Debug for CursorMut<'_, T, A> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_tuple("CursorMut").field(&self.list).field(&self.index()).finish()
     }
 }
 
-impl<'a, T> Cursor<'a, T> {
+impl<'a, T, A: Allocator> Cursor<'a, T, A> {
     /// Returns the cursor position index within the `LinkedList`.
     ///
     /// This returns `None` if the cursor is currently pointing to the
@@ -1301,7 +1392,7 @@ impl<'a, T> Cursor<'a, T> {
     }
 }
 
-impl<'a, T> CursorMut<'a, T> {
+impl<'a, T, A: Allocator> CursorMut<'a, T, A> {
     /// Returns the cursor position index within the `LinkedList`.
     ///
     /// This returns `None` if the cursor is currently pointing to the
@@ -1406,7 +1497,7 @@ impl<'a, T> CursorMut<'a, T> {
     /// `CursorMut` is frozen for the lifetime of the `Cursor`.
     #[must_use]
     #[unstable(feature = "linked_list_cursors", issue = "58533")]
-    pub fn as_cursor(&self) -> Cursor<'_, T> {
+    pub fn as_cursor(&self) -> Cursor<'_, T, A> {
         Cursor { list: self.list, current: self.current, index: self.index }
     }
 }
@@ -1414,6 +1505,51 @@ impl<'a, T> CursorMut<'a, T> {
 // Now the list editing operations
 
 impl<'a, T> CursorMut<'a, T> {
+    /// Inserts the elements from the given `LinkedList` after the current one.
+    ///
+    /// If the cursor is pointing at the "ghost" non-element then the new elements are
+    /// inserted at the start of the `LinkedList`.
+    #[unstable(feature = "linked_list_cursors", issue = "58533")]
+    pub fn splice_after(&mut self, list: LinkedList<T>) {
+        unsafe {
+            let (splice_head, splice_tail, splice_len) = match list.detach_all_nodes() {
+                Some(parts) => parts,
+                _ => return,
+            };
+            let node_next = match self.current {
+                None => self.list.head,
+                Some(node) => node.as_ref().next,
+            };
+            self.list.splice_nodes(self.current, node_next, splice_head, splice_tail, splice_len);
+            if self.current.is_none() {
+                // The "ghost" non-element's index has changed.
+                self.index = self.list.len;
+            }
+        }
+    }
+
+    /// Inserts the elements from the given `LinkedList` before the current one.
+    ///
+    /// If the cursor is pointing at the "ghost" non-element then the new elements are
+    /// inserted at the end of the `LinkedList`.
+    #[unstable(feature = "linked_list_cursors", issue = "58533")]
+    pub fn splice_before(&mut self, list: LinkedList<T>) {
+        unsafe {
+            let (splice_head, splice_tail, splice_len) = match list.detach_all_nodes() {
+                Some(parts) => parts,
+                _ => return,
+            };
+            let node_prev = match self.current {
+                None => self.list.tail,
+                Some(node) => node.as_ref().prev,
+            };
+            self.list.splice_nodes(node_prev, self.current, splice_head, splice_tail, splice_len);
+            self.index += splice_len;
+        }
+    }
+}
+
+impl<'a, T, A: Allocator> CursorMut<'a, T, A> {
     /// Inserts a new element into the `LinkedList` after the current one.
     ///
     /// If the cursor is pointing at the "ghost" non-element then the new element is
@@ -1421,7 +1557,7 @@ impl<'a, T> CursorMut<'a, T> {
     #[unstable(feature = "linked_list_cursors", issue = "58533")]
     pub fn insert_after(&mut self, item: T) {
         unsafe {
-            let spliced_node = Box::leak(Box::new(Node::new(item))).into();
+            let spliced_node = Box::leak(Box::new_in(Node::new(item), &self.list.alloc)).into();
             let node_next = match self.current {
                 None => self.list.head,
                 Some(node) => node.as_ref().next,
@@ -1441,7 +1577,7 @@ impl<'a, T> CursorMut<'a, T> {
     #[unstable(feature = "linked_list_cursors", issue = "58533")]
     pub fn insert_before(&mut self, item: T) {
         unsafe {
-            let spliced_node = Box::leak(Box::new(Node::new(item))).into();
+            let spliced_node = Box::leak(Box::new_in(Node::new(item), &self.list.alloc)).into();
             let node_prev = match self.current {
                 None => self.list.tail,
                 Some(node) => node.as_ref().prev,
@@ -1477,7 +1613,10 @@ impl<'a, T> CursorMut<'a, T> {
     /// If the cursor is currently pointing to the "ghost" non-element then no element
     /// is removed and `None` is returned.
     #[unstable(feature = "linked_list_cursors", issue = "58533")]
-    pub fn remove_current_as_list(&mut self) -> Option<LinkedList<T>> {
+    pub fn remove_current_as_list(&mut self) -> Option<LinkedList<T, A>>
+    where
+        A: Clone,
+    {
         let mut unlinked_node = self.current?;
         unsafe {
             self.current = unlinked_node.as_ref().next;
@@ -1489,54 +1628,12 @@ impl<'a, T> CursorMut<'a, T> {
                 head: Some(unlinked_node),
                 tail: Some(unlinked_node),
                 len: 1,
+                alloc: self.list.alloc.clone(),
                 marker: PhantomData,
             })
         }
     }
 
-    /// Inserts the elements from the given `LinkedList` after the current one.
-    ///
-    /// If the cursor is pointing at the "ghost" non-element then the new elements are
-    /// inserted at the start of the `LinkedList`.
-    #[unstable(feature = "linked_list_cursors", issue = "58533")]
-    pub fn splice_after(&mut self, list: LinkedList<T>) {
-        unsafe {
-            let (splice_head, splice_tail, splice_len) = match list.detach_all_nodes() {
-                Some(parts) => parts,
-                _ => return,
-            };
-            let node_next = match self.current {
-                None => self.list.head,
-                Some(node) => node.as_ref().next,
-            };
-            self.list.splice_nodes(self.current, node_next, splice_head, splice_tail, splice_len);
-            if self.current.is_none() {
-                // The "ghost" non-element's index has changed.
-                self.index = self.list.len;
-            }
-        }
-    }
-
-    /// Inserts the elements from the given `LinkedList` before the current one.
-    ///
-    /// If the cursor is pointing at the "ghost" non-element then the new elements are
-    /// inserted at the end of the `LinkedList`.
-    #[unstable(feature = "linked_list_cursors", issue = "58533")]
-    pub fn splice_before(&mut self, list: LinkedList<T>) {
-        unsafe {
-            let (splice_head, splice_tail, splice_len) = match list.detach_all_nodes() {
-                Some(parts) => parts,
-                _ => return,
-            };
-            let node_prev = match self.current {
-                None => self.list.tail,
-                Some(node) => node.as_ref().prev,
-            };
-            self.list.splice_nodes(node_prev, self.current, splice_head, splice_tail, splice_len);
-            self.index += splice_len;
-        }
-    }
-
     /// Splits the list into two after the current element. This will return a
     /// new list consisting of everything after the cursor, with the original
     /// list retaining everything before.
@@ -1544,7 +1641,10 @@ impl<'a, T> CursorMut<'a, T> {
     /// If the cursor is pointing at the "ghost" non-element then the entire contents
     /// of the `LinkedList` are moved.
     #[unstable(feature = "linked_list_cursors", issue = "58533")]
-    pub fn split_after(&mut self) -> LinkedList<T> {
+    pub fn split_after(&mut self) -> LinkedList<T, A>
+    where
+        A: Clone,
+    {
         let split_off_idx = if self.index == self.list.len { 0 } else { self.index + 1 };
         if self.index == self.list.len {
             // The "ghost" non-element's index has changed to 0.
@@ -1560,7 +1660,10 @@ impl<'a, T> CursorMut<'a, T> {
     /// If the cursor is pointing at the "ghost" non-element then the entire contents
     /// of the `LinkedList` are moved.
     #[unstable(feature = "linked_list_cursors", issue = "58533")]
-    pub fn split_before(&mut self) -> LinkedList<T> {
+    pub fn split_before(&mut self) -> LinkedList<T, A>
+    where
+        A: Clone,
+    {
         let split_off_idx = self.index;
         self.index = 0;
         unsafe { self.list.split_off_before_node(self.current, split_off_idx) }
@@ -1570,7 +1673,7 @@ 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 +1716,7 @@ 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 {
@@ -1702,11 +1805,15 @@ impl<'a, T> CursorMut<'a, T> {
 
 /// An iterator produced by calling `drain_filter` on LinkedList.
 #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
-pub struct DrainFilter<'a, T: 'a, F: 'a>
-where
+pub struct DrainFilter<
+    'a,
+    T: 'a,
+    F: 'a,
+    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
+> where
     F: FnMut(&mut T) -> bool,
 {
-    list: &'a mut LinkedList<T>,
+    list: &'a mut LinkedList<T, A>,
     it: Option<NonNull<Node<T>>>,
     pred: F,
     idx: usize,
@@ -1714,7 +1821,7 @@ where
 }
 
 #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
-impl<T, F> Iterator for DrainFilter<'_, T, F>
+impl<T, F, A: Allocator> Iterator for DrainFilter<'_, T, F, A>
 where
     F: FnMut(&mut T) -> bool,
 {
@@ -1743,16 +1850,16 @@ where
 }
 
 #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
-impl<T, F> Drop for DrainFilter<'_, T, F>
+impl<T, F, A: Allocator> Drop for DrainFilter<'_, T, F, A>
 where
     F: FnMut(&mut T) -> bool,
 {
     fn drop(&mut self) {
-        struct DropGuard<'r, 'a, T, F>(&'r mut DrainFilter<'a, T, F>)
+        struct DropGuard<'r, 'a, T, F, A: Allocator>(&'r mut DrainFilter<'a, T, F, A>)
         where
             F: FnMut(&mut T) -> bool;
 
-        impl<'r, 'a, T, F> Drop for DropGuard<'r, 'a, T, F>
+        impl<'r, 'a, T, F, A: Allocator> Drop for DropGuard<'r, 'a, T, F, A>
         where
             F: FnMut(&mut T) -> bool,
         {
@@ -1780,7 +1887,7 @@ where
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> Iterator for IntoIter<T> {
+impl<T, A: Allocator> Iterator for IntoIter<T, A> {
     type Item = T;
 
     #[inline]
@@ -1795,7 +1902,7 @@ impl<T> Iterator for IntoIter<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> DoubleEndedIterator for IntoIter<T> {
+impl<T, A: Allocator> DoubleEndedIterator for IntoIter<T, A> {
     #[inline]
     fn next_back(&mut self) -> Option<T> {
         self.list.pop_back()
@@ -1803,10 +1910,24 @@ impl<T> DoubleEndedIterator for IntoIter<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> ExactSizeIterator for IntoIter<T> {}
+impl<T, A: Allocator> ExactSizeIterator for IntoIter<T, A> {}
 
 #[stable(feature = "fused", since = "1.26.0")]
-impl<T> FusedIterator for IntoIter<T> {}
+impl<T, A: Allocator> FusedIterator for IntoIter<T, A> {}
+
+#[stable(feature = "default_iters", since = "1.70.0")]
+impl<T> Default for IntoIter<T> {
+    /// Creates an empty `linked_list::IntoIter`.
+    ///
+    /// ```
+    /// # use std::collections::linked_list;
+    /// let iter: linked_list::IntoIter<u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        LinkedList::new().into_iter()
+    }
+}
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> FromIterator<T> for LinkedList<T> {
@@ -1818,19 +1939,19 @@ impl<T> FromIterator<T> for LinkedList<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> IntoIterator for LinkedList<T> {
+impl<T, A: Allocator> IntoIterator for LinkedList<T, A> {
     type Item = T;
-    type IntoIter = IntoIter<T>;
+    type IntoIter = IntoIter<T, A>;
 
     /// Consumes the list into an iterator yielding elements by value.
     #[inline]
-    fn into_iter(self) -> IntoIter<T> {
+    fn into_iter(self) -> IntoIter<T, A> {
         IntoIter { list: self }
     }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<'a, T> IntoIterator for &'a LinkedList<T> {
+impl<'a, T, A: Allocator> IntoIterator for &'a LinkedList<T, A> {
     type Item = &'a T;
     type IntoIter = Iter<'a, T>;
 
@@ -1840,7 +1961,7 @@ impl<'a, T> IntoIterator for &'a LinkedList<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<'a, T> IntoIterator for &'a mut LinkedList<T> {
+impl<'a, T, A: Allocator> IntoIterator for &'a mut LinkedList<T, A> {
     type Item = &'a mut T;
     type IntoIter = IterMut<'a, T>;
 
@@ -1850,7 +1971,7 @@ impl<'a, T> IntoIterator for &'a mut LinkedList<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> Extend<T> for LinkedList<T> {
+impl<T, A: Allocator> Extend<T> for LinkedList<T, A> {
     fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
         <Self as SpecExtend<I>>::spec_extend(self, iter);
     }
@@ -1861,7 +1982,7 @@ impl<T> Extend<T> for LinkedList<T> {
     }
 }
 
-impl<I: IntoIterator> SpecExtend<I> for LinkedList<I::Item> {
+impl<I: IntoIterator, A: Allocator> SpecExtend<I> for LinkedList<I::Item, A> {
     default fn spec_extend(&mut self, iter: I) {
         iter.into_iter().for_each(move |elt| self.push_back(elt));
     }
@@ -1874,7 +1995,7 @@ impl<T> SpecExtend<LinkedList<T>> for LinkedList<T> {
 }
 
 #[stable(feature = "extend_ref", since = "1.2.0")]
-impl<'a, T: 'a + Copy> Extend<&'a T> for LinkedList<T> {
+impl<'a, T: 'a + Copy, A: Allocator> Extend<&'a T> for LinkedList<T, A> {
     fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
         self.extend(iter.into_iter().cloned());
     }
@@ -1886,7 +2007,7 @@ impl<'a, T: 'a + Copy> Extend<&'a T> for LinkedList<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: PartialEq> PartialEq for LinkedList<T> {
+impl<T: PartialEq, A: Allocator> PartialEq for LinkedList<T, A> {
     fn eq(&self, other: &Self) -> bool {
         self.len() == other.len() && self.iter().eq(other)
     }
@@ -1897,17 +2018,17 @@ impl<T: PartialEq> PartialEq for LinkedList<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Eq> Eq for LinkedList<T> {}
+impl<T: Eq, A: Allocator> Eq for LinkedList<T, A> {}
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: PartialOrd> PartialOrd for LinkedList<T> {
+impl<T: PartialOrd, A: Allocator> PartialOrd for LinkedList<T, A> {
     fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
         self.iter().partial_cmp(other)
     }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Ord> Ord for LinkedList<T> {
+impl<T: Ord, A: Allocator> Ord for LinkedList<T, A> {
     #[inline]
     fn cmp(&self, other: &Self) -> Ordering {
         self.iter().cmp(other)
@@ -1915,9 +2036,11 @@ impl<T: Ord> Ord for LinkedList<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Clone> Clone for LinkedList<T> {
+impl<T: Clone, A: Allocator + Clone> Clone for LinkedList<T, A> {
     fn clone(&self) -> Self {
-        self.iter().cloned().collect()
+        let mut list = Self::new_in(self.alloc.clone());
+        list.extend(self.iter().cloned());
+        list
     }
 
     fn clone_from(&mut self, other: &Self) {
@@ -1935,14 +2058,14 @@ impl<T: Clone> Clone for LinkedList<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: fmt::Debug> fmt::Debug for LinkedList<T> {
+impl<T: fmt::Debug, A: Allocator> fmt::Debug for LinkedList<T, A> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_list().entries(self).finish()
     }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Hash> Hash for LinkedList<T> {
+impl<T: Hash, A: Allocator> Hash for LinkedList<T, A> {
     fn hash<H: Hasher>(&self, state: &mut H) {
         state.write_length_prefix(self.len());
         for elt in self {
@@ -1982,10 +2105,10 @@ fn assert_covariance() {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-unsafe impl<T: Send> Send for LinkedList<T> {}
+unsafe impl<T: Send, A: Allocator + Send> Send for LinkedList<T, A> {}
 
 #[stable(feature = "rust1", since = "1.0.0")]
-unsafe impl<T: Sync> Sync for LinkedList<T> {}
+unsafe impl<T: Sync, A: Allocator + Sync> Sync for LinkedList<T, A> {}
 
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T: Sync> Send for Iter<'_, T> {}
@@ -2000,13 +2123,13 @@ unsafe impl<T: Send> Send for IterMut<'_, T> {}
 unsafe impl<T: Sync> Sync for IterMut<'_, T> {}
 
 #[unstable(feature = "linked_list_cursors", issue = "58533")]
-unsafe impl<T: Sync> Send for Cursor<'_, T> {}
+unsafe impl<T: Sync, A: Allocator + Sync> Send for Cursor<'_, T, A> {}
 
 #[unstable(feature = "linked_list_cursors", issue = "58533")]
-unsafe impl<T: Sync> Sync for Cursor<'_, T> {}
+unsafe impl<T: Sync, A: Allocator + Sync> Sync for Cursor<'_, T, A> {}
 
 #[unstable(feature = "linked_list_cursors", issue = "58533")]
-unsafe impl<T: Send> Send for CursorMut<'_, T> {}
+unsafe impl<T: Send, A: Allocator + Send> Send for CursorMut<'_, T, A> {}
 
 #[unstable(feature = "linked_list_cursors", issue = "58533")]
-unsafe impl<T: Sync> Sync for CursorMut<'_, T> {}
+unsafe impl<T: Sync, A: Allocator + Sync> Sync for CursorMut<'_, T, A> {}
diff --git a/library/alloc/src/collections/linked_list/tests.rs b/library/alloc/src/collections/linked_list/tests.rs
index f8fbfa1bfbc..04594d55b6a 100644
--- a/library/alloc/src/collections/linked_list/tests.rs
+++ b/library/alloc/src/collections/linked_list/tests.rs
@@ -1,10 +1,11 @@
 use super::*;
+use crate::testing::crash_test::{CrashTestDummy, Panic};
 use crate::vec::Vec;
 
 use std::panic::{catch_unwind, AssertUnwindSafe};
 use std::thread;
 
-use rand::{thread_rng, RngCore};
+use rand::RngCore;
 
 #[test]
 fn test_basic() {
@@ -480,12 +481,12 @@ fn test_split_off_2() {
     }
 }
 
-fn fuzz_test(sz: i32) {
+fn fuzz_test(sz: i32, rng: &mut impl RngCore) {
     let mut m: LinkedList<_> = LinkedList::new();
     let mut v = vec![];
     for i in 0..sz {
         check_links(&m);
-        let r: u8 = thread_rng().next_u32() as u8;
+        let r: u8 = rng.next_u32() as u8;
         match r % 6 {
             0 => {
                 m.pop_back();
@@ -520,11 +521,12 @@ fn fuzz_test(sz: i32) {
 
 #[test]
 fn test_fuzz() {
+    let mut rng = crate::test_helpers::test_rng();
     for _ in 0..25 {
-        fuzz_test(3);
-        fuzz_test(16);
+        fuzz_test(3, &mut rng);
+        fuzz_test(16, &mut rng);
         #[cfg(not(miri))] // Miri is too slow
-        fuzz_test(189);
+        fuzz_test(189, &mut rng);
     }
 }
 
@@ -984,35 +986,34 @@ fn drain_filter_complex() {
 
 #[test]
 fn drain_filter_drop_panic_leak() {
-    static mut DROPS: i32 = 0;
-
-    struct D(bool);
-
-    impl Drop for D {
-        fn drop(&mut self) {
-            unsafe {
-                DROPS += 1;
-            }
-
-            if self.0 {
-                panic!("panic in `drop`");
-            }
-        }
-    }
-
+    let d0 = CrashTestDummy::new(0);
+    let d1 = CrashTestDummy::new(1);
+    let d2 = CrashTestDummy::new(2);
+    let d3 = CrashTestDummy::new(3);
+    let d4 = CrashTestDummy::new(4);
+    let d5 = CrashTestDummy::new(5);
+    let d6 = CrashTestDummy::new(6);
+    let d7 = CrashTestDummy::new(7);
     let mut q = LinkedList::new();
-    q.push_back(D(false));
-    q.push_back(D(false));
-    q.push_back(D(false));
-    q.push_back(D(false));
-    q.push_back(D(false));
-    q.push_front(D(false));
-    q.push_front(D(true));
-    q.push_front(D(false));
-
-    catch_unwind(AssertUnwindSafe(|| drop(q.drain_filter(|_| true)))).ok();
-
-    assert_eq!(unsafe { DROPS }, 8);
+    q.push_back(d3.spawn(Panic::Never));
+    q.push_back(d4.spawn(Panic::Never));
+    q.push_back(d5.spawn(Panic::Never));
+    q.push_back(d6.spawn(Panic::Never));
+    q.push_back(d7.spawn(Panic::Never));
+    q.push_front(d2.spawn(Panic::Never));
+    q.push_front(d1.spawn(Panic::InDrop));
+    q.push_front(d0.spawn(Panic::Never));
+
+    catch_unwind(AssertUnwindSafe(|| drop(q.drain_filter(|_| true)))).unwrap_err();
+
+    assert_eq!(d0.dropped(), 1);
+    assert_eq!(d1.dropped(), 1);
+    assert_eq!(d2.dropped(), 1);
+    assert_eq!(d3.dropped(), 1);
+    assert_eq!(d4.dropped(), 1);
+    assert_eq!(d5.dropped(), 1);
+    assert_eq!(d6.dropped(), 1);
+    assert_eq!(d7.dropped(), 1);
     assert!(q.is_empty());
 }
 
diff --git a/library/alloc/src/collections/mod.rs b/library/alloc/src/collections/mod.rs
index 628a5b15567..3e0b0f73550 100644
--- a/library/alloc/src/collections/mod.rs
+++ b/library/alloc/src/collections/mod.rs
@@ -139,7 +139,7 @@ impl Display for TryReserveError {
                 " because the computed capacity exceeded the collection's maximum"
             }
             TryReserveErrorKind::AllocError { .. } => {
-                " because the memory allocator returned a error"
+                " because the memory allocator returned an error"
             }
         };
         fmt.write_str(reason)
@@ -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 05f94da6de7..0be274a3822 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::iter::FusedIterator;
-use core::ptr::{self, NonNull};
-use core::{fmt, mem};
+use core::marker::PhantomData;
+use core::mem::{self, SizedTypeProperties};
+use core::ptr::NonNull;
+use core::{fmt, ptr};
 
 use crate::alloc::{Allocator, Global};
 
-use super::{count, Iter, VecDeque};
+use super::VecDeque;
 
 /// A draining iterator over the elements of a `VecDeque`.
 ///
@@ -18,20 +20,56 @@ pub struct Drain<
     T: 'a,
     #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
 > {
-    after_tail: usize,
-    after_head: usize,
-    iter: Iter<'a, T>,
+    // We can't just use a &mut VecDeque<T, A>, as that would make Drain invariant over T
+    // and we want it to be covariant instead
     deque: NonNull<VecDeque<T, A>>,
+    // drain_start is stored in deque.len
+    drain_len: usize,
+    // index into the logical array, not the physical one (always lies in [0..deque.len))
+    idx: usize,
+    // number of elements after the drain range
+    tail_len: usize,
+    remaining: usize,
+    // Needed to make Drain covariant over T
+    _marker: 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>,
-        deque: NonNull<VecDeque<T, A>>,
+        deque: &'a mut VecDeque<T, A>,
+        drain_start: usize,
+        drain_len: usize,
     ) -> Self {
-        Drain { after_tail, after_head, iter, deque }
+        let orig_len = mem::replace(&mut deque.len, drain_start);
+        let tail_len = orig_len - drain_start - drain_len;
+        Drain {
+            deque: NonNull::from(deque),
+            drain_len,
+            idx: drain_start,
+            tail_len,
+            remaining: drain_len,
+            _marker: PhantomData,
+        }
+    }
+
+    // Only returns pointers to the slices, as that's all we need
+    // to drop them. May only be called if `self.remaining != 0`.
+    unsafe fn as_slices(&self) -> (*mut [T], *mut [T]) {
+        unsafe {
+            let deque = self.deque.as_ref();
+
+            // We know that `self.idx + self.remaining <= deque.len <= usize::MAX`, so this won't overflow.
+            let logical_remaining_range = self.idx..self.idx + self.remaining;
+
+            // SAFETY: `logical_remaining_range` represents the
+            // range into the logical buffer of elements that
+            // haven't been drained yet, so they're all initialized,
+            // and `slice::range(start..end, end) == start..end`,
+            // so the preconditions for `slice_ranges` are met.
+            let (a_range, b_range) =
+                deque.slice_ranges(logical_remaining_range.clone(), logical_remaining_range.end);
+            (deque.buffer_range(a_range), deque.buffer_range(b_range))
+        }
     }
 }
 
@@ -39,9 +77,10 @@ impl<'a, T, A: Allocator> Drain<'a, T, A> {
 impl<T: fmt::Debug, A: Allocator> fmt::Debug for Drain<'_, T, A> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_tuple("Drain")
-            .field(&self.after_tail)
-            .field(&self.after_head)
-            .field(&self.iter)
+            .field(&self.drain_len)
+            .field(&self.idx)
+            .field(&self.tail_len)
+            .field(&self.remaining)
             .finish()
     }
 }
@@ -58,57 +97,81 @@ impl<T, A: Allocator> Drop for Drain<'_, T, A> {
 
         impl<'r, 'a, T, A: Allocator> Drop for DropGuard<'r, 'a, T, A> {
             fn drop(&mut self) {
-                self.0.for_each(drop);
+                if self.0.remaining != 0 {
+                    unsafe {
+                        // SAFETY: We just checked that `self.remaining != 0`.
+                        let (front, back) = self.0.as_slices();
+                        ptr::drop_in_place(front);
+                        ptr::drop_in_place(back);
+                    }
+                }
 
                 let source_deque = unsafe { self.0.deque.as_mut() };
 
-                // T = source_deque_tail; H = source_deque_head; t = drain_tail; h = drain_head
-                //
-                //        T   t   h   H
-                // [. . . o o x x o o . . .]
-                //
-                let orig_tail = source_deque.tail;
-                let drain_tail = source_deque.head;
-                let drain_head = self.0.after_tail;
-                let orig_head = self.0.after_head;
+                let drain_start = source_deque.len();
+                let drain_len = self.0.drain_len;
+                let drain_end = drain_start + drain_len;
 
-                let tail_len = count(orig_tail, drain_tail, source_deque.cap());
-                let head_len = count(drain_head, orig_head, source_deque.cap());
+                let orig_len = self.0.tail_len + drain_end;
 
-                // Restore the original head value
-                source_deque.head = orig_head;
+                if T::IS_ZST {
+                    // no need to copy around any memory if T is a ZST
+                    source_deque.len = orig_len - drain_len;
+                    return;
+                }
+
+                let head_len = drain_start;
+                let tail_len = self.0.tail_len;
 
-                match (tail_len, head_len) {
+                match (head_len, tail_len) {
                     (0, 0) => {
                         source_deque.head = 0;
-                        source_deque.tail = 0;
+                        source_deque.len = 0;
                     }
                     (0, _) => {
-                        source_deque.tail = drain_head;
+                        source_deque.head = source_deque.to_physical_idx(drain_len);
+                        source_deque.len = orig_len - drain_len;
                     }
                     (_, 0) => {
-                        source_deque.head = drain_tail;
+                        source_deque.len = orig_len - drain_len;
                     }
                     _ => unsafe {
-                        if tail_len <= head_len {
-                            source_deque.tail = source_deque.wrap_sub(drain_head, tail_len);
-                            source_deque.wrap_copy(source_deque.tail, orig_tail, tail_len);
+                        if head_len <= tail_len {
+                            source_deque.wrap_copy(
+                                source_deque.head,
+                                source_deque.to_physical_idx(drain_len),
+                                head_len,
+                            );
+                            source_deque.head = source_deque.to_physical_idx(drain_len);
+                            source_deque.len = orig_len - drain_len;
                         } else {
-                            source_deque.head = source_deque.wrap_add(drain_tail, head_len);
-                            source_deque.wrap_copy(drain_tail, drain_head, head_len);
+                            source_deque.wrap_copy(
+                                source_deque.to_physical_idx(head_len + drain_len),
+                                source_deque.to_physical_idx(head_len),
+                                tail_len,
+                            );
+                            source_deque.len = orig_len - drain_len;
                         }
                     },
                 }
             }
         }
 
-        while let Some(item) = self.next() {
-            let guard = DropGuard(self);
-            drop(item);
-            mem::forget(guard);
+        let guard = DropGuard(self);
+        if guard.0.remaining != 0 {
+            unsafe {
+                // SAFETY: We just checked that `self.remaining != 0`.
+                let (front, back) = guard.0.as_slices();
+                // since idx is a logical index, we don't need to worry about wrapping.
+                guard.0.idx += front.len();
+                guard.0.remaining -= front.len();
+                ptr::drop_in_place(front);
+                guard.0.remaining = 0;
+                ptr::drop_in_place(back);
+            }
         }
 
-        DropGuard(self);
+        // Dropping `guard` handles moving the remaining elements into place.
     }
 }
 
@@ -118,12 +181,19 @@ 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.remaining == 0 {
+            return None;
+        }
+        let wrapped_idx = unsafe { self.deque.as_ref().to_physical_idx(self.idx) };
+        self.idx += 1;
+        self.remaining -= 1;
+        Some(unsafe { self.deque.as_mut().buffer_read(wrapped_idx) })
     }
 
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
-        self.iter.size_hint()
+        let len = self.remaining;
+        (len, Some(len))
     }
 }
 
@@ -131,7 +201,12 @@ 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.remaining == 0 {
+            return None;
+        }
+        self.remaining -= 1;
+        let wrapped_idx = unsafe { self.deque.as_ref().to_physical_idx(self.idx + self.remaining) };
+        Some(unsafe { self.deque.as_mut().buffer_read(wrapped_idx) })
     }
 }
 
diff --git a/library/alloc/src/collections/vec_deque/into_iter.rs b/library/alloc/src/collections/vec_deque/into_iter.rs
index 55f6138cd0f..d9e274df0f5 100644
--- a/library/alloc/src/collections/vec_deque/into_iter.rs
+++ b/library/alloc/src/collections/vec_deque/into_iter.rs
@@ -1,5 +1,6 @@
-use core::fmt;
 use core::iter::{FusedIterator, TrustedLen};
+use core::num::NonZeroUsize;
+use core::{array, fmt, mem::MaybeUninit, ops::Try, ptr};
 
 use crate::alloc::{Allocator, Global};
 
@@ -11,7 +12,6 @@ use super::VecDeque;
 /// (provided by the [`IntoIterator`] trait). See its documentation for more.
 ///
 /// [`into_iter`]: VecDeque::into_iter
-/// [`IntoIterator`]: core::iter::IntoIterator
 #[derive(Clone)]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct IntoIter<
@@ -25,6 +25,10 @@ impl<T, A: Allocator> IntoIter<T, A> {
     pub(super) fn new(inner: VecDeque<T, A>) -> Self {
         IntoIter { inner }
     }
+
+    pub(super) fn into_vecdeque(self) -> VecDeque<T, A> {
+        self.inner
+    }
 }
 
 #[stable(feature = "collection_debug", since = "1.17.0")]
@@ -48,6 +52,127 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
         let len = self.inner.len();
         (len, Some(len))
     }
+
+    #[inline]
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
+        let len = self.inner.len;
+        let rem = if len < n {
+            self.inner.clear();
+            n - len
+        } else {
+            self.inner.drain(..n);
+            0
+        };
+        NonZeroUsize::new(rem).map_or(Ok(()), Err)
+    }
+
+    #[inline]
+    fn count(self) -> usize {
+        self.inner.len
+    }
+
+    fn try_fold<B, F, R>(&mut self, mut init: B, mut f: F) -> R
+    where
+        F: FnMut(B, Self::Item) -> R,
+        R: Try<Output = B>,
+    {
+        struct Guard<'a, T, A: Allocator> {
+            deque: &'a mut VecDeque<T, A>,
+            // `consumed <= deque.len` always holds.
+            consumed: usize,
+        }
+
+        impl<'a, T, A: Allocator> Drop for Guard<'a, T, A> {
+            fn drop(&mut self) {
+                self.deque.len -= self.consumed;
+                self.deque.head = self.deque.to_physical_idx(self.consumed);
+            }
+        }
+
+        let mut guard = Guard { deque: &mut self.inner, consumed: 0 };
+
+        let (head, tail) = guard.deque.as_slices();
+
+        init = head
+            .iter()
+            .map(|elem| {
+                guard.consumed += 1;
+                // SAFETY: Because we incremented `guard.consumed`, the
+                // deque effectively forgot the element, so we can take
+                // ownership
+                unsafe { ptr::read(elem) }
+            })
+            .try_fold(init, &mut f)?;
+
+        tail.iter()
+            .map(|elem| {
+                guard.consumed += 1;
+                // SAFETY: Same as above.
+                unsafe { ptr::read(elem) }
+            })
+            .try_fold(init, &mut f)
+    }
+
+    #[inline]
+    fn fold<B, F>(mut self, init: B, mut f: F) -> B
+    where
+        F: FnMut(B, Self::Item) -> B,
+    {
+        match self.try_fold(init, |b, item| Ok::<B, !>(f(b, item))) {
+            Ok(b) => b,
+            Err(e) => match e {},
+        }
+    }
+
+    #[inline]
+    fn last(mut self) -> Option<Self::Item> {
+        self.inner.pop_back()
+    }
+
+    fn next_chunk<const N: usize>(
+        &mut self,
+    ) -> Result<[Self::Item; N], array::IntoIter<Self::Item, N>> {
+        let mut raw_arr = MaybeUninit::uninit_array();
+        let raw_arr_ptr = raw_arr.as_mut_ptr().cast();
+        let (head, tail) = self.inner.as_slices();
+
+        if head.len() >= N {
+            // SAFETY: By manually adjusting the head and length of the deque, we effectively
+            // make it forget the first `N` elements, so taking ownership of them is safe.
+            unsafe { ptr::copy_nonoverlapping(head.as_ptr(), raw_arr_ptr, N) };
+            self.inner.head = self.inner.to_physical_idx(N);
+            self.inner.len -= N;
+            // SAFETY: We initialized the entire array with items from `head`
+            return Ok(unsafe { raw_arr.transpose().assume_init() });
+        }
+
+        // SAFETY: Same argument as above.
+        unsafe { ptr::copy_nonoverlapping(head.as_ptr(), raw_arr_ptr, head.len()) };
+        let remaining = N - head.len();
+
+        if tail.len() >= remaining {
+            // SAFETY: Same argument as above.
+            unsafe {
+                ptr::copy_nonoverlapping(tail.as_ptr(), raw_arr_ptr.add(head.len()), remaining)
+            };
+            self.inner.head = self.inner.to_physical_idx(N);
+            self.inner.len -= N;
+            // SAFETY: We initialized the entire array with items from `head` and `tail`
+            Ok(unsafe { raw_arr.transpose().assume_init() })
+        } else {
+            // SAFETY: Same argument as above.
+            unsafe {
+                ptr::copy_nonoverlapping(tail.as_ptr(), raw_arr_ptr.add(head.len()), tail.len())
+            };
+            let init = head.len() + tail.len();
+            // We completely drained all the deques elements.
+            self.inner.head = 0;
+            self.inner.len = 0;
+            // SAFETY: We copied all elements from both slices to the beginning of the array, so
+            // the given range is initialized.
+            Err(unsafe { array::IntoIter::new_unchecked(raw_arr, 0..init) })
+        }
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -56,10 +181,74 @@ impl<T, A: Allocator> DoubleEndedIterator for IntoIter<T, A> {
     fn next_back(&mut self) -> Option<T> {
         self.inner.pop_back()
     }
+
+    #[inline]
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
+        let len = self.inner.len;
+        let rem = if len < n {
+            self.inner.clear();
+            n - len
+        } else {
+            self.inner.truncate(len - n);
+            0
+        };
+        NonZeroUsize::new(rem).map_or(Ok(()), Err)
+    }
+
+    fn try_rfold<B, F, R>(&mut self, mut init: B, mut f: F) -> R
+    where
+        F: FnMut(B, Self::Item) -> R,
+        R: Try<Output = B>,
+    {
+        struct Guard<'a, T, A: Allocator> {
+            deque: &'a mut VecDeque<T, A>,
+            // `consumed <= deque.len` always holds.
+            consumed: usize,
+        }
+
+        impl<'a, T, A: Allocator> Drop for Guard<'a, T, A> {
+            fn drop(&mut self) {
+                self.deque.len -= self.consumed;
+            }
+        }
+
+        let mut guard = Guard { deque: &mut self.inner, consumed: 0 };
+
+        let (head, tail) = guard.deque.as_slices();
+
+        init = tail
+            .iter()
+            .map(|elem| {
+                guard.consumed += 1;
+                // SAFETY: See `try_fold`'s safety comment.
+                unsafe { ptr::read(elem) }
+            })
+            .try_rfold(init, &mut f)?;
+
+        head.iter()
+            .map(|elem| {
+                guard.consumed += 1;
+                // SAFETY: Same as above.
+                unsafe { ptr::read(elem) }
+            })
+            .try_rfold(init, &mut f)
+    }
+
+    #[inline]
+    fn rfold<B, F>(mut self, init: B, mut f: F) -> B
+    where
+        F: FnMut(B, Self::Item) -> B,
+    {
+        match self.try_rfold(init, |b, item| Ok::<B, !>(f(b, item))) {
+            Ok(b) => b,
+            Err(e) => match e {},
+        }
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T, A: Allocator> ExactSizeIterator for IntoIter<T, A> {
+    #[inline]
     fn is_empty(&self) -> bool {
         self.inner.is_empty()
     }
diff --git a/library/alloc/src/collections/vec_deque/iter.rs b/library/alloc/src/collections/vec_deque/iter.rs
index e696d7ed636..646a2a991e7 100644
--- a/library/alloc/src/collections/vec_deque/iter.rs
+++ b/library/alloc/src/collections/vec_deque/iter.rs
@@ -1,9 +1,7 @@
-use core::fmt;
 use core::iter::{FusedIterator, TrustedLen, TrustedRandomAccess, TrustedRandomAccessNoCoerce};
-use core::mem::MaybeUninit;
+use core::num::NonZeroUsize;
 use core::ops::Try;
-
-use super::{count, wrap_index, RingSlices};
+use core::{fmt, mem, slice};
 
 /// An iterator over the elements of a `VecDeque`.
 ///
@@ -13,30 +11,20 @@ use super::{count, wrap_index, RingSlices};
 /// [`iter`]: super::VecDeque::iter
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Iter<'a, T: 'a> {
-    ring: &'a [MaybeUninit<T>],
-    tail: usize,
-    head: usize,
+    i1: slice::Iter<'a, T>,
+    i2: slice::Iter<'a, T>,
 }
 
 impl<'a, T> Iter<'a, T> {
-    pub(super) fn new(ring: &'a [MaybeUninit<T>], tail: usize, head: usize) -> Self {
-        Iter { ring, tail, head }
+    pub(super) fn new(i1: slice::Iter<'a, T>, i2: slice::Iter<'a, T>) -> Self {
+        Self { i1, i2 }
     }
 }
 
 #[stable(feature = "collection_debug", since = "1.17.0")]
 impl<T: fmt::Debug> fmt::Debug for Iter<'_, T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let (front, back) = RingSlices::ring_slices(self.ring, self.head, self.tail);
-        // Safety:
-        // - `self.head` and `self.tail` in a ring buffer are always valid indices.
-        // - `RingSlices::ring_slices` guarantees that the slices split according to `self.head` and `self.tail` are initialized.
-        unsafe {
-            f.debug_tuple("Iter")
-                .field(&MaybeUninit::slice_assume_init_ref(front))
-                .field(&MaybeUninit::slice_assume_init_ref(back))
-                .finish()
-        }
+        f.debug_tuple("Iter").field(&self.i1.as_slice()).field(&self.i2.as_slice()).finish()
     }
 }
 
@@ -44,7 +32,7 @@ impl<T: fmt::Debug> fmt::Debug for Iter<'_, T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Clone for Iter<'_, T> {
     fn clone(&self) -> Self {
-        Iter { ring: self.ring, tail: self.tail, head: self.head }
+        Iter { i1: self.i1.clone(), i2: self.i2.clone() }
     }
 }
 
@@ -54,72 +42,52 @@ impl<'a, T> Iterator for Iter<'a, T> {
 
     #[inline]
     fn next(&mut self) -> Option<&'a T> {
-        if self.tail == self.head {
-            return None;
+        match self.i1.next() {
+            Some(val) => Some(val),
+            None => {
+                // most of the time, the iterator will either always
+                // call next(), or always call next_back(). By swapping
+                // the iterators once the first one is empty, we ensure
+                // that the first branch is taken as often as possible,
+                // without sacrificing correctness, as i1 is empty anyways
+                mem::swap(&mut self.i1, &mut self.i2);
+                self.i1.next()
+            }
+        }
+    }
+
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
+        let remaining = self.i1.advance_by(n);
+        match remaining {
+            Ok(()) => return Ok(()),
+            Err(n) => {
+                mem::swap(&mut self.i1, &mut self.i2);
+                self.i1.advance_by(n.get())
+            }
         }
-        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(self.ring.get_unchecked(tail).assume_init_ref()) }
     }
 
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let len = count(self.tail, self.head, self.ring.len());
+        let len = self.len();
         (len, Some(len))
     }
 
-    fn fold<Acc, F>(self, mut accum: Acc, mut f: F) -> Acc
+    fn fold<Acc, F>(self, accum: Acc, mut f: F) -> Acc
     where
         F: FnMut(Acc, Self::Item) -> Acc,
     {
-        let (front, back) = RingSlices::ring_slices(self.ring, self.head, self.tail);
-        // Safety:
-        // - `self.head` and `self.tail` in a ring buffer are always valid indices.
-        // - `RingSlices::ring_slices` guarantees that the slices split according to `self.head` and `self.tail` are initialized.
-        unsafe {
-            accum = MaybeUninit::slice_assume_init_ref(front).iter().fold(accum, &mut f);
-            MaybeUninit::slice_assume_init_ref(back).iter().fold(accum, &mut f)
-        }
+        let accum = self.i1.fold(accum, &mut f);
+        self.i2.fold(accum, &mut f)
     }
 
     fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
     where
-        Self: Sized,
         F: FnMut(B, Self::Item) -> R,
         R: Try<Output = B>,
     {
-        let (mut iter, final_res);
-        if self.tail <= self.head {
-            // Safety: single slice self.ring[self.tail..self.head] is initialized.
-            iter = unsafe { MaybeUninit::slice_assume_init_ref(&self.ring[self.tail..self.head]) }
-                .iter();
-            final_res = iter.try_fold(init, &mut f);
-        } else {
-            // Safety: two slices: self.ring[self.tail..], self.ring[..self.head] both are initialized.
-            let (front, back) = self.ring.split_at(self.tail);
-
-            let mut back_iter = unsafe { MaybeUninit::slice_assume_init_ref(back).iter() };
-            let res = back_iter.try_fold(init, &mut f);
-            let len = self.ring.len();
-            self.tail = (self.ring.len() - back_iter.len()) & (len - 1);
-            iter = unsafe { MaybeUninit::slice_assume_init_ref(&front[..self.head]).iter() };
-            final_res = iter.try_fold(res?, &mut f);
-        }
-        self.tail = self.head - iter.len();
-        final_res
-    }
-
-    fn nth(&mut self, n: usize) -> Option<Self::Item> {
-        if n >= count(self.tail, self.head, self.ring.len()) {
-            self.tail = self.head;
-            None
-        } else {
-            self.tail = wrap_index(self.tail.wrapping_add(n), self.ring.len());
-            self.next()
-        }
+        let acc = self.i1.try_fold(init, &mut f)?;
+        self.i2.try_fold(acc, &mut f)
     }
 
     #[inline]
@@ -132,8 +100,12 @@ impl<'a, T> Iterator for Iter<'a, T> {
         // Safety: The TrustedRandomAccess contract requires that callers only pass an index
         // that is in bounds.
         unsafe {
-            let idx = wrap_index(self.tail.wrapping_add(idx), self.ring.len());
-            self.ring.get_unchecked(idx).assume_init_ref()
+            let i1_len = self.i1.len();
+            if idx < i1_len {
+                self.i1.__iterator_get_unchecked(idx)
+            } else {
+                self.i2.__iterator_get_unchecked(idx - i1_len)
+            }
         }
     }
 }
@@ -142,63 +114,56 @@ impl<'a, T> Iterator for Iter<'a, T> {
 impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
     #[inline]
     fn next_back(&mut self) -> Option<&'a T> {
-        if self.tail == self.head {
-            return None;
+        match self.i2.next_back() {
+            Some(val) => Some(val),
+            None => {
+                // most of the time, the iterator will either always
+                // call next(), or always call next_back(). By swapping
+                // the iterators once the second one is empty, we ensure
+                // that the first branch is taken as often as possible,
+                // without sacrificing correctness, as i2 is empty anyways
+                mem::swap(&mut self.i1, &mut self.i2);
+                self.i2.next_back()
+            }
+        }
+    }
+
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
+        match self.i2.advance_back_by(n) {
+            Ok(()) => return Ok(()),
+            Err(n) => {
+                mem::swap(&mut self.i1, &mut self.i2);
+                self.i2.advance_back_by(n.get())
+            }
         }
-        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(self.ring.get_unchecked(self.head).assume_init_ref()) }
     }
 
-    fn rfold<Acc, F>(self, mut accum: Acc, mut f: F) -> Acc
+    fn rfold<Acc, F>(self, accum: Acc, mut f: F) -> Acc
     where
         F: FnMut(Acc, Self::Item) -> Acc,
     {
-        let (front, back) = RingSlices::ring_slices(self.ring, self.head, self.tail);
-        // Safety:
-        // - `self.head` and `self.tail` in a ring buffer are always valid indices.
-        // - `RingSlices::ring_slices` guarantees that the slices split according to `self.head` and `self.tail` are initialized.
-        unsafe {
-            accum = MaybeUninit::slice_assume_init_ref(back).iter().rfold(accum, &mut f);
-            MaybeUninit::slice_assume_init_ref(front).iter().rfold(accum, &mut f)
-        }
+        let accum = self.i2.rfold(accum, &mut f);
+        self.i1.rfold(accum, &mut f)
     }
 
     fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
     where
-        Self: Sized,
         F: FnMut(B, Self::Item) -> R,
         R: Try<Output = B>,
     {
-        let (mut iter, final_res);
-        if self.tail <= self.head {
-            // Safety: single slice self.ring[self.tail..self.head] is initialized.
-            iter = unsafe {
-                MaybeUninit::slice_assume_init_ref(&self.ring[self.tail..self.head]).iter()
-            };
-            final_res = iter.try_rfold(init, &mut f);
-        } else {
-            // Safety: two slices: self.ring[self.tail..], self.ring[..self.head] both are initialized.
-            let (front, back) = self.ring.split_at(self.tail);
-
-            let mut front_iter =
-                unsafe { MaybeUninit::slice_assume_init_ref(&front[..self.head]).iter() };
-            let res = front_iter.try_rfold(init, &mut f);
-            self.head = front_iter.len();
-            iter = unsafe { MaybeUninit::slice_assume_init_ref(back).iter() };
-            final_res = iter.try_rfold(res?, &mut f);
-        }
-        self.head = self.tail + iter.len();
-        final_res
+        let acc = self.i2.try_rfold(init, &mut f)?;
+        self.i1.try_rfold(acc, &mut f)
     }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> ExactSizeIterator for Iter<'_, T> {
+    fn len(&self) -> usize {
+        self.i1.len() + self.i2.len()
+    }
+
     fn is_empty(&self) -> bool {
-        self.head == self.tail
+        self.i1.is_empty() && self.i2.is_empty()
     }
 }
 
diff --git a/library/alloc/src/collections/vec_deque/iter_mut.rs b/library/alloc/src/collections/vec_deque/iter_mut.rs
index b78c0d5e1b3..7defbb1090f 100644
--- a/library/alloc/src/collections/vec_deque/iter_mut.rs
+++ b/library/alloc/src/collections/vec_deque/iter_mut.rs
@@ -1,8 +1,7 @@
-use core::fmt;
 use core::iter::{FusedIterator, TrustedLen, TrustedRandomAccess, TrustedRandomAccessNoCoerce};
-use core::marker::PhantomData;
-
-use super::{count, wrap_index, RingSlices};
+use core::num::NonZeroUsize;
+use core::ops::Try;
+use core::{fmt, mem, slice};
 
 /// A mutable iterator over the elements of a `VecDeque`.
 ///
@@ -12,39 +11,20 @@ use super::{count, wrap_index, RingSlices};
 /// [`iter_mut`]: super::VecDeque::iter_mut
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct IterMut<'a, T: 'a> {
-    // Internal safety invariant: the entire slice is dereferenceable.
-    ring: *mut [T],
-    tail: usize,
-    head: usize,
-    phantom: PhantomData<&'a mut [T]>,
+    i1: slice::IterMut<'a, T>,
+    i2: slice::IterMut<'a, T>,
 }
 
 impl<'a, T> IterMut<'a, T> {
-    pub(super) unsafe fn new(
-        ring: *mut [T],
-        tail: usize,
-        head: usize,
-        phantom: PhantomData<&'a mut [T]>,
-    ) -> Self {
-        IterMut { ring, tail, head, phantom }
+    pub(super) fn new(i1: slice::IterMut<'a, T>, i2: slice::IterMut<'a, T>) -> Self {
+        Self { i1, i2 }
     }
 }
 
-// SAFETY: we do nothing thread-local and there is no interior mutability,
-// so the usual structural `Send`/`Sync` apply.
-#[stable(feature = "rust1", since = "1.0.0")]
-unsafe impl<T: Send> Send for IterMut<'_, T> {}
-#[stable(feature = "rust1", since = "1.0.0")]
-unsafe impl<T: Sync> Sync for IterMut<'_, T> {}
-
 #[stable(feature = "collection_debug", since = "1.17.0")]
 impl<T: fmt::Debug> fmt::Debug for IterMut<'_, T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let (front, back) = RingSlices::ring_slices(self.ring, self.head, self.tail);
-        // SAFETY: these are the elements we have not handed out yet, so aliasing is fine.
-        // The `IterMut` invariant also ensures everything is dereferenceable.
-        let (front, back) = unsafe { (&*front, &*back) };
-        f.debug_tuple("IterMut").field(&front).field(&back).finish()
+        f.debug_tuple("IterMut").field(&self.i1.as_slice()).field(&self.i2.as_slice()).finish()
     }
 }
 
@@ -54,44 +34,51 @@ impl<'a, T> Iterator for IterMut<'a, T> {
 
     #[inline]
     fn next(&mut self) -> Option<&'a mut T> {
-        if self.tail == self.head {
-            return None;
+        match self.i1.next() {
+            Some(val) => Some(val),
+            None => {
+                // most of the time, the iterator will either always
+                // call next(), or always call next_back(). By swapping
+                // the iterators once the first one is empty, we ensure
+                // that the first branch is taken as often as possible,
+                // without sacrificing correctness, as i1 is empty anyways
+                mem::swap(&mut self.i1, &mut self.i2);
+                self.i1.next()
+            }
         }
-        let tail = self.tail;
-        self.tail = wrap_index(self.tail.wrapping_add(1), self.ring.len());
+    }
 
-        unsafe {
-            let elem = self.ring.get_unchecked_mut(tail);
-            Some(&mut *elem)
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
+        match self.i1.advance_by(n) {
+            Ok(()) => return Ok(()),
+            Err(remaining) => {
+                mem::swap(&mut self.i1, &mut self.i2);
+                self.i1.advance_by(remaining.get())
+            }
         }
     }
 
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let len = count(self.tail, self.head, self.ring.len());
+        let len = self.len();
         (len, Some(len))
     }
 
-    fn fold<Acc, F>(self, mut accum: Acc, mut f: F) -> Acc
+    fn fold<Acc, F>(self, accum: Acc, mut f: F) -> Acc
     where
         F: FnMut(Acc, Self::Item) -> Acc,
     {
-        let (front, back) = RingSlices::ring_slices(self.ring, self.head, self.tail);
-        // SAFETY: these are the elements we have not handed out yet, so aliasing is fine.
-        // The `IterMut` invariant also ensures everything is dereferenceable.
-        let (front, back) = unsafe { (&mut *front, &mut *back) };
-        accum = front.iter_mut().fold(accum, &mut f);
-        back.iter_mut().fold(accum, &mut f)
+        let accum = self.i1.fold(accum, &mut f);
+        self.i2.fold(accum, &mut f)
     }
 
-    fn nth(&mut self, n: usize) -> Option<Self::Item> {
-        if n >= count(self.tail, self.head, self.ring.len()) {
-            self.tail = self.head;
-            None
-        } else {
-            self.tail = wrap_index(self.tail.wrapping_add(n), self.ring.len());
-            self.next()
-        }
+    fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
+    where
+        F: FnMut(B, Self::Item) -> R,
+        R: Try<Output = B>,
+    {
+        let acc = self.i1.try_fold(init, &mut f)?;
+        self.i2.try_fold(acc, &mut f)
     }
 
     #[inline]
@@ -104,8 +91,12 @@ impl<'a, T> Iterator for IterMut<'a, T> {
         // Safety: The TrustedRandomAccess contract requires that callers only pass an index
         // that is in bounds.
         unsafe {
-            let idx = wrap_index(self.tail.wrapping_add(idx), self.ring.len());
-            &mut *self.ring.get_unchecked_mut(idx)
+            let i1_len = self.i1.len();
+            if idx < i1_len {
+                self.i1.__iterator_get_unchecked(idx)
+            } else {
+                self.i2.__iterator_get_unchecked(idx - i1_len)
+            }
         }
     }
 }
@@ -114,34 +105,56 @@ impl<'a, T> Iterator for IterMut<'a, T> {
 impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
     #[inline]
     fn next_back(&mut self) -> Option<&'a mut T> {
-        if self.tail == self.head {
-            return None;
+        match self.i2.next_back() {
+            Some(val) => Some(val),
+            None => {
+                // most of the time, the iterator will either always
+                // call next(), or always call next_back(). By swapping
+                // the iterators once the first one is empty, we ensure
+                // that the first branch is taken as often as possible,
+                // without sacrificing correctness, as i2 is empty anyways
+                mem::swap(&mut self.i1, &mut self.i2);
+                self.i2.next_back()
+            }
         }
-        self.head = wrap_index(self.head.wrapping_sub(1), self.ring.len());
+    }
 
-        unsafe {
-            let elem = self.ring.get_unchecked_mut(self.head);
-            Some(&mut *elem)
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
+        match self.i2.advance_back_by(n) {
+            Ok(()) => return Ok(()),
+            Err(remaining) => {
+                mem::swap(&mut self.i1, &mut self.i2);
+                self.i2.advance_back_by(remaining.get())
+            }
         }
     }
 
-    fn rfold<Acc, F>(self, mut accum: Acc, mut f: F) -> Acc
+    fn rfold<Acc, F>(self, accum: Acc, mut f: F) -> Acc
     where
         F: FnMut(Acc, Self::Item) -> Acc,
     {
-        let (front, back) = RingSlices::ring_slices(self.ring, self.head, self.tail);
-        // SAFETY: these are the elements we have not handed out yet, so aliasing is fine.
-        // The `IterMut` invariant also ensures everything is dereferenceable.
-        let (front, back) = unsafe { (&mut *front, &mut *back) };
-        accum = back.iter_mut().rfold(accum, &mut f);
-        front.iter_mut().rfold(accum, &mut f)
+        let accum = self.i2.rfold(accum, &mut f);
+        self.i1.rfold(accum, &mut f)
+    }
+
+    fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
+    where
+        F: FnMut(B, Self::Item) -> R,
+        R: Try<Output = B>,
+    {
+        let acc = self.i2.try_rfold(init, &mut f)?;
+        self.i1.try_rfold(acc, &mut f)
     }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> ExactSizeIterator for IterMut<'_, T> {
+    fn len(&self) -> usize {
+        self.i1.len() + self.i2.len()
+    }
+
     fn is_empty(&self) -> bool {
-        self.head == self.tail
+        self.i1.is_empty() && self.i2.is_empty()
     }
 }
 
diff --git a/library/alloc/src/collections/vec_deque/mod.rs b/library/alloc/src/collections/vec_deque/mod.rs
index 4d895d83745..896da37f94c 100644
--- a/library/alloc/src/collections/vec_deque/mod.rs
+++ b/library/alloc/src/collections/vec_deque/mod.rs
@@ -10,13 +10,18 @@
 use core::cmp::{self, Ordering};
 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::iter::{repeat_n, repeat_with, ByRefSized};
+use core::mem::{ManuallyDrop, SizedTypeProperties};
 use core::ops::{Index, IndexMut, Range, RangeBounds};
-use core::ptr::{self, NonNull};
+use core::ptr;
 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;
@@ -46,25 +51,16 @@ pub use self::iter::Iter;
 
 mod iter;
 
-use self::pair_slices::PairSlices;
-
-mod pair_slices;
-
-use self::ring_slices::RingSlices;
-
-mod ring_slices;
-
 use self::spec_extend::SpecExtend;
 
 mod spec_extend;
 
-#[cfg(test)]
-mod tests;
+use self::spec_from_iter::SpecFromIter;
 
-const INITIAL_CAPACITY: usize = 7; // 2^3 - 1
-const MINIMUM_CAPACITY: usize = 1; // 2 - 1
+mod spec_from_iter;
 
-const MAXIMUM_ZST_CAPACITY: usize = 1 << (usize::BITS - 1); // Largest possible power of two
+#[cfg(test)]
+mod tests;
 
 /// A double-ended queue implemented with a growable ring buffer.
 ///
@@ -99,13 +95,13 @@ pub struct VecDeque<
     T,
     #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
 > {
-    // tail and head are pointers into the buffer. Tail always points
-    // to the first element that could be read, Head always points
-    // to where data should be written.
-    // If tail == head the buffer is empty. The length of the ringbuffer
-    // is defined as the distance between the two.
-    tail: usize,
+    // `self[0]`, if it exists, is `buf[head]`.
+    // `head < buf.capacity()`, unless `buf.capacity() == 0` when `head == 0`.
     head: usize,
+    // the number of initialized elements, starting from the one at `head` and potentially wrapping around.
+    // if `len == 0`, the exact value of `head` is unimportant.
+    // if `T` is zero-Sized, then `self.len <= usize::MAX`, otherwise `self.len <= isize::MAX as usize`.
+    len: usize,
     buf: RawVec<T, A>,
 }
 
@@ -118,18 +114,8 @@ impl<T: Clone, A: Allocator + Clone> Clone for VecDeque<T, A> {
     }
 
     fn clone_from(&mut self, other: &Self) {
-        self.truncate(other.len());
-
-        let mut iter = PairSlices::from(self, other);
-        while let Some((dst, src)) = iter.next() {
-            dst.clone_from_slice(&src);
-        }
-
-        if iter.has_remainder() {
-            for remainder in iter.remainder() {
-                self.extend(remainder.iter().cloned());
-            }
-        }
+        self.clear();
+        self.extend(other.iter().cloned());
     }
 }
 
@@ -174,41 +160,6 @@ impl<T, A: Allocator> VecDeque<T, A> {
         self.buf.ptr()
     }
 
-    /// Marginally more convenient
-    #[inline]
-    fn cap(&self) -> usize {
-        if mem::size_of::<T>() == 0 {
-            // For zero sized types, we are always at maximum capacity
-            MAXIMUM_ZST_CAPACITY
-        } else {
-            self.buf.capacity()
-        }
-    }
-
-    /// Turn ptr into a slice, since the elements of the backing buffer may be uninitialized,
-    /// we will return a slice of [`MaybeUninit<T>`].
-    ///
-    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
-    /// incorrect usage of this method.
-    ///
-    /// [zeroed]: mem::MaybeUninit::zeroed
-    #[inline]
-    unsafe fn buffer_as_slice(&self) -> &[MaybeUninit<T>] {
-        unsafe { slice::from_raw_parts(self.ptr() as *mut MaybeUninit<T>, self.cap()) }
-    }
-
-    /// Turn ptr into a mut slice, since the elements of the backing buffer may be uninitialized,
-    /// we will return a slice of [`MaybeUninit<T>`].
-    ///
-    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
-    /// incorrect usage of this method.
-    ///
-    /// [zeroed]: mem::MaybeUninit::zeroed
-    #[inline]
-    unsafe fn buffer_as_mut_slice(&mut self) -> &mut [MaybeUninit<T>] {
-        unsafe { slice::from_raw_parts_mut(self.ptr() as *mut MaybeUninit<T>, self.cap()) }
-    }
-
     /// Moves an element out of the buffer
     #[inline]
     unsafe fn buffer_read(&mut self, off: usize) -> T {
@@ -223,51 +174,58 @@ impl<T, A: Allocator> VecDeque<T, A> {
         }
     }
 
-    /// Returns `true` if the buffer is at full capacity.
+    /// Returns a slice pointer into the buffer.
+    /// `range` must lie inside `0..self.capacity()`.
     #[inline]
-    fn is_full(&self) -> bool {
-        self.cap() - self.len() == 1
+    unsafe fn buffer_range(&self, range: Range<usize>) -> *mut [T] {
+        unsafe {
+            ptr::slice_from_raw_parts_mut(self.ptr().add(range.start), range.end - range.start)
+        }
     }
 
-    /// Returns the index in the underlying buffer for a given logical element
-    /// index.
+    /// Returns `true` if the buffer is at full capacity.
     #[inline]
-    fn wrap_index(&self, idx: usize) -> usize {
-        wrap_index(idx, self.cap())
+    fn is_full(&self) -> bool {
+        self.len == self.capacity()
     }
 
     /// Returns the index in the underlying buffer for a given logical element
     /// index + addend.
     #[inline]
     fn wrap_add(&self, idx: usize, addend: usize) -> usize {
-        wrap_index(idx.wrapping_add(addend), self.cap())
+        wrap_index(idx.wrapping_add(addend), self.capacity())
+    }
+
+    #[inline]
+    fn to_physical_idx(&self, idx: usize) -> usize {
+        self.wrap_add(self.head, idx)
     }
 
     /// Returns the index in the underlying buffer for a given logical element
     /// index - subtrahend.
     #[inline]
     fn wrap_sub(&self, idx: usize, subtrahend: usize) -> usize {
-        wrap_index(idx.wrapping_sub(subtrahend), self.cap())
+        wrap_index(idx.wrapping_sub(subtrahend).wrapping_add(self.capacity()), self.capacity())
     }
 
     /// Copies a contiguous block of memory len long from src to dst
     #[inline]
-    unsafe fn copy(&self, dst: usize, src: usize, len: usize) {
+    unsafe fn copy(&mut self, src: usize, dst: usize, len: usize) {
         debug_assert!(
-            dst + len <= self.cap(),
+            dst + len <= self.capacity(),
             "cpy dst={} src={} len={} cap={}",
             dst,
             src,
             len,
-            self.cap()
+            self.capacity()
         );
         debug_assert!(
-            src + len <= self.cap(),
+            src + len <= self.capacity(),
             "cpy dst={} src={} len={} cap={}",
             dst,
             src,
             len,
-            self.cap()
+            self.capacity()
         );
         unsafe {
             ptr::copy(self.ptr().add(src), self.ptr().add(dst), len);
@@ -276,22 +234,22 @@ impl<T, A: Allocator> VecDeque<T, A> {
 
     /// Copies a contiguous block of memory len long from src to dst
     #[inline]
-    unsafe fn copy_nonoverlapping(&self, dst: usize, src: usize, len: usize) {
+    unsafe fn copy_nonoverlapping(&mut self, src: usize, dst: usize, len: usize) {
         debug_assert!(
-            dst + len <= self.cap(),
+            dst + len <= self.capacity(),
             "cno dst={} src={} len={} cap={}",
             dst,
             src,
             len,
-            self.cap()
+            self.capacity()
         );
         debug_assert!(
-            src + len <= self.cap(),
+            src + len <= self.capacity(),
             "cno dst={} src={} len={} cap={}",
             dst,
             src,
             len,
-            self.cap()
+            self.capacity()
         );
         unsafe {
             ptr::copy_nonoverlapping(self.ptr().add(src), self.ptr().add(dst), len);
@@ -299,30 +257,28 @@ impl<T, A: Allocator> VecDeque<T, A> {
     }
 
     /// Copies a potentially wrapping block of memory len long from src to dest.
-    /// (abs(dst - src) + len) must be no larger than cap() (There must be at
+    /// (abs(dst - src) + len) must be no larger than capacity() (There must be at
     /// most one continuous overlapping region between src and dest).
-    unsafe fn wrap_copy(&self, dst: usize, src: usize, len: usize) {
-        #[allow(dead_code)]
-        fn diff(a: usize, b: usize) -> usize {
-            if a <= b { b - a } else { a - b }
-        }
+    unsafe fn wrap_copy(&mut self, src: usize, dst: usize, len: usize) {
         debug_assert!(
-            cmp::min(diff(dst, src), self.cap() - diff(dst, src)) + len <= self.cap(),
+            cmp::min(src.abs_diff(dst), self.capacity() - src.abs_diff(dst)) + len
+                <= self.capacity(),
             "wrc dst={} src={} len={} cap={}",
             dst,
             src,
             len,
-            self.cap()
+            self.capacity()
         );
 
-        if src == dst || len == 0 {
+        // If T is a ZST, don't do any copying.
+        if T::IS_ZST || src == dst || len == 0 {
             return;
         }
 
         let dst_after_src = self.wrap_sub(dst, src) < len;
 
-        let src_pre_wrap_len = self.cap() - src;
-        let dst_pre_wrap_len = self.cap() - dst;
+        let src_pre_wrap_len = self.capacity() - src;
+        let dst_pre_wrap_len = self.capacity() - dst;
         let src_wraps = src_pre_wrap_len < len;
         let dst_wraps = dst_pre_wrap_len < len;
 
@@ -336,7 +292,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
                 //            D . . .
                 //
                 unsafe {
-                    self.copy(dst, src, len);
+                    self.copy(src, dst, len);
                 }
             }
             (false, false, true) => {
@@ -349,8 +305,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
                 //    . .           D .
                 //
                 unsafe {
-                    self.copy(dst, src, dst_pre_wrap_len);
-                    self.copy(0, src + dst_pre_wrap_len, len - dst_pre_wrap_len);
+                    self.copy(src, dst, dst_pre_wrap_len);
+                    self.copy(src + dst_pre_wrap_len, 0, len - dst_pre_wrap_len);
                 }
             }
             (true, false, true) => {
@@ -363,8 +319,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
                 //    . .           D .
                 //
                 unsafe {
-                    self.copy(0, src + dst_pre_wrap_len, len - dst_pre_wrap_len);
-                    self.copy(dst, src, dst_pre_wrap_len);
+                    self.copy(src + dst_pre_wrap_len, 0, len - dst_pre_wrap_len);
+                    self.copy(src, dst, dst_pre_wrap_len);
                 }
             }
             (false, true, false) => {
@@ -377,8 +333,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
                 //              D . . .
                 //
                 unsafe {
-                    self.copy(dst, src, src_pre_wrap_len);
-                    self.copy(dst + src_pre_wrap_len, 0, len - src_pre_wrap_len);
+                    self.copy(src, dst, src_pre_wrap_len);
+                    self.copy(0, dst + src_pre_wrap_len, len - src_pre_wrap_len);
                 }
             }
             (true, true, false) => {
@@ -391,8 +347,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
                 //    D . . .
                 //
                 unsafe {
-                    self.copy(dst + src_pre_wrap_len, 0, len - src_pre_wrap_len);
-                    self.copy(dst, src, src_pre_wrap_len);
+                    self.copy(0, dst + src_pre_wrap_len, len - src_pre_wrap_len);
+                    self.copy(src, dst, src_pre_wrap_len);
                 }
             }
             (false, true, true) => {
@@ -408,9 +364,9 @@ impl<T, A: Allocator> VecDeque<T, A> {
                 debug_assert!(dst_pre_wrap_len > src_pre_wrap_len);
                 let delta = dst_pre_wrap_len - src_pre_wrap_len;
                 unsafe {
-                    self.copy(dst, src, src_pre_wrap_len);
-                    self.copy(dst + src_pre_wrap_len, 0, delta);
-                    self.copy(0, delta, len - dst_pre_wrap_len);
+                    self.copy(src, dst, src_pre_wrap_len);
+                    self.copy(0, dst + src_pre_wrap_len, delta);
+                    self.copy(delta, 0, len - dst_pre_wrap_len);
                 }
             }
             (true, true, true) => {
@@ -426,9 +382,9 @@ impl<T, A: Allocator> VecDeque<T, A> {
                 debug_assert!(src_pre_wrap_len > dst_pre_wrap_len);
                 let delta = src_pre_wrap_len - dst_pre_wrap_len;
                 unsafe {
-                    self.copy(delta, 0, len - src_pre_wrap_len);
-                    self.copy(0, self.cap() - delta, delta);
-                    self.copy(dst, src, dst_pre_wrap_len);
+                    self.copy(0, delta, len - src_pre_wrap_len);
+                    self.copy(self.capacity() - delta, 0, delta);
+                    self.copy(src, dst, dst_pre_wrap_len);
                 }
             }
         }
@@ -438,8 +394,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// Assumes capacity is sufficient.
     #[inline]
     unsafe fn copy_slice(&mut self, dst: usize, src: &[T]) {
-        debug_assert!(src.len() <= self.cap());
-        let head_room = self.cap() - dst;
+        debug_assert!(src.len() <= self.capacity());
+        let head_room = self.capacity() - dst;
         if src.len() <= head_room {
             unsafe {
                 ptr::copy_nonoverlapping(src.as_ptr(), self.ptr().add(dst), src.len());
@@ -472,48 +428,100 @@ impl<T, A: Allocator> VecDeque<T, A> {
         });
     }
 
+    /// Writes all values from `iter` to `dst`, wrapping
+    /// at the end of the buffer and returns the number
+    /// of written values.
+    ///
+    /// # Safety
+    ///
+    /// Assumes that `iter` yields at most `len` items.
+    /// Assumes capacity is sufficient.
+    unsafe fn write_iter_wrapping(
+        &mut self,
+        dst: usize,
+        mut iter: impl Iterator<Item = T>,
+        len: usize,
+    ) -> usize {
+        struct Guard<'a, T, A: Allocator> {
+            deque: &'a mut VecDeque<T, A>,
+            written: usize,
+        }
+
+        impl<'a, T, A: Allocator> Drop for Guard<'a, T, A> {
+            fn drop(&mut self) {
+                self.deque.len += self.written;
+            }
+        }
+
+        let head_room = self.capacity() - dst;
+
+        let mut guard = Guard { deque: self, written: 0 };
+
+        if head_room >= len {
+            unsafe { guard.deque.write_iter(dst, iter, &mut guard.written) };
+        } else {
+            unsafe {
+                guard.deque.write_iter(
+                    dst,
+                    ByRefSized(&mut iter).take(head_room),
+                    &mut guard.written,
+                );
+                guard.deque.write_iter(0, iter, &mut guard.written)
+            };
+        }
+
+        guard.written
+    }
+
     /// Frobs the head and tail sections around to handle the fact that we
     /// just reallocated. Unsafe because it trusts old_capacity.
     #[inline]
     unsafe fn handle_capacity_increase(&mut self, old_capacity: usize) {
-        let new_capacity = self.cap();
+        let new_capacity = self.capacity();
+        debug_assert!(new_capacity >= old_capacity);
 
         // Move the shortest contiguous section of the ring buffer
-        //    T             H
+        //
+        // H := head
+        // L := last element (`self.to_physical_idx(self.len - 1)`)
+        //
+        //    H           L
         //   [o o o o o o o . ]
-        //    T             H
+        //    H           L
         // A [o o o o o o o . . . . . . . . . ]
-        //        H T
-        //   [o o . o o o o o ]
-        //          T             H
+        //        L H
+        //   [o o o o o o o o ]
+        //          H           L
         // B [. . . o o o o o o o . . . . . . ]
-        //              H T
-        //   [o o o o o . o o ]
-        //              H                 T
+        //              L H
+        //   [o o o o o o o o ]
+        //            L                   H
         // C [o o o o o . . . . . . . . . o o ]
 
-        if self.tail <= self.head {
+        // can't use is_contiguous() because the capacity is already updated.
+        if self.head <= old_capacity - self.len {
             // A
             // Nop
-        } else if self.head < old_capacity - self.tail {
-            // B
-            unsafe {
-                self.copy_nonoverlapping(old_capacity, 0, self.head);
-            }
-            self.head += old_capacity;
-            debug_assert!(self.head > self.tail);
         } else {
-            // C
-            let new_tail = new_capacity - (old_capacity - self.tail);
-            unsafe {
-                self.copy_nonoverlapping(new_tail, self.tail, old_capacity - self.tail);
+            let head_len = old_capacity - self.head;
+            let tail_len = self.len - head_len;
+            if head_len > tail_len && new_capacity - old_capacity >= tail_len {
+                // B
+                unsafe {
+                    self.copy_nonoverlapping(0, old_capacity, tail_len);
+                }
+            } else {
+                // C
+                let new_head = new_capacity - head_len;
+                unsafe {
+                    // can't use copy_nonoverlapping here, because if e.g. head_len = 2
+                    // and new_capacity = old_capacity + 1, then the heads overlap.
+                    self.copy(self.head, new_head, head_len);
+                }
+                self.head = new_head;
             }
-            self.tail = new_tail;
-            debug_assert!(self.head < self.tail);
         }
-        debug_assert!(self.head < self.cap());
-        debug_assert!(self.tail < self.cap());
-        debug_assert!(self.cap().count_ones() == 1);
+        debug_assert!(self.head < self.capacity() || self.capacity() == 0);
     }
 }
 
@@ -529,9 +537,11 @@ impl<T> VecDeque<T> {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_stable(feature = "const_vec_deque_new", since = "1.68.0")]
     #[must_use]
-    pub fn new() -> VecDeque<T> {
-        VecDeque::new_in(Global)
+    pub const fn new() -> VecDeque<T> {
+        // FIXME: This should just be `VecDeque::new_in(Global)` once that hits stable.
+        VecDeque { head: 0, len: 0, buf: RawVec::NEW }
     }
 
     /// Creates an empty deque with space for at least `capacity` elements.
@@ -563,8 +573,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// ```
     #[inline]
     #[unstable(feature = "allocator_api", issue = "32838")]
-    pub fn new_in(alloc: A) -> VecDeque<T, A> {
-        VecDeque::with_capacity_in(INITIAL_CAPACITY, alloc)
+    pub const fn new_in(alloc: A) -> VecDeque<T, A> {
+        VecDeque { head: 0, len: 0, buf: RawVec::new_in(alloc) }
     }
 
     /// Creates an empty deque with space for at least `capacity` elements.
@@ -578,11 +588,39 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// ```
     #[unstable(feature = "allocator_api", issue = "32838")]
     pub fn with_capacity_in(capacity: usize, alloc: A) -> VecDeque<T, A> {
-        assert!(capacity < 1_usize << usize::BITS - 1, "capacity overflow");
-        // +1 since the ringbuffer always leaves one space empty
-        let cap = cmp::max(capacity + 1, MINIMUM_CAPACITY + 1).next_power_of_two();
+        VecDeque { head: 0, len: 0, buf: RawVec::with_capacity_in(capacity, alloc) }
+    }
 
-        VecDeque { tail: 0, head: 0, buf: RawVec::with_capacity_in(cap, alloc) }
+    /// Creates a `VecDeque` from a raw allocation, when the initialized
+    /// part of that allocation forms a *contiguous* subslice thereof.
+    ///
+    /// For use by `vec::IntoIter::into_vecdeque`
+    ///
+    /// # Safety
+    ///
+    /// All the usual requirements on the allocated memory like in
+    /// `Vec::from_raw_parts_in`, but takes a *range* of elements that are
+    /// initialized rather than only supporting `0..len`.  Requires that
+    /// `initialized.start` ≤ `initialized.end` ≤ `capacity`.
+    #[inline]
+    pub(crate) unsafe fn from_contiguous_raw_parts_in(
+        ptr: *mut T,
+        initialized: Range<usize>,
+        capacity: usize,
+        alloc: A,
+    ) -> Self {
+        debug_assert!(initialized.start <= initialized.end);
+        debug_assert!(initialized.end <= capacity);
+
+        // SAFETY: Our safety precondition guarantees the range length won't wrap,
+        // and that the allocation is valid for use in `RawVec`.
+        unsafe {
+            VecDeque {
+                head: initialized.start,
+                len: initialized.end.unchecked_sub(initialized.start),
+                buf: RawVec::from_raw_parts_in(ptr, capacity, alloc),
+            }
+        }
     }
 
     /// Provides a reference to the element at the given index.
@@ -598,12 +636,13 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// buf.push_back(3);
     /// buf.push_back(4);
     /// buf.push_back(5);
+    /// buf.push_back(6);
     /// assert_eq!(buf.get(1), Some(&4));
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn get(&self, index: usize) -> Option<&T> {
-        if index < self.len() {
-            let idx = self.wrap_add(self.tail, index);
+        if index < self.len {
+            let idx = self.to_physical_idx(index);
             unsafe { Some(&*self.ptr().add(idx)) }
         } else {
             None
@@ -623,16 +662,17 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// buf.push_back(3);
     /// buf.push_back(4);
     /// buf.push_back(5);
+    /// buf.push_back(6);
+    /// assert_eq!(buf[1], 4);
     /// if let Some(elem) = buf.get_mut(1) {
     ///     *elem = 7;
     /// }
-    ///
     /// assert_eq!(buf[1], 7);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn get_mut(&mut self, index: usize) -> Option<&mut T> {
-        if index < self.len() {
-            let idx = self.wrap_add(self.tail, index);
+        if index < self.len {
+            let idx = self.to_physical_idx(index);
             unsafe { Some(&mut *self.ptr().add(idx)) }
         } else {
             None
@@ -666,8 +706,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
     pub fn swap(&mut self, i: usize, j: usize) {
         assert!(i < self.len());
         assert!(j < self.len());
-        let ri = self.wrap_add(self.tail, i);
-        let rj = self.wrap_add(self.tail, j);
+        let ri = self.to_physical_idx(i);
+        let rj = self.to_physical_idx(j);
         unsafe { ptr::swap(self.ptr().add(ri), self.ptr().add(rj)) }
     }
 
@@ -685,7 +725,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn capacity(&self) -> usize {
-        self.cap() - 1
+        if T::IS_ZST { usize::MAX } else { self.buf.capacity() }
     }
 
     /// Reserves the minimum capacity for at least `additional` more elements to be inserted in the
@@ -712,7 +752,15 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// [`reserve`]: VecDeque::reserve
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn reserve_exact(&mut self, additional: usize) {
-        self.reserve(additional);
+        let new_cap = self.len.checked_add(additional).expect("capacity overflow");
+        let old_cap = self.capacity();
+
+        if new_cap > old_cap {
+            self.buf.reserve_exact(self.len, additional);
+            unsafe {
+                self.handle_capacity_increase(old_cap);
+            }
+        }
     }
 
     /// Reserves capacity for at least `additional` more elements to be inserted in the given
@@ -733,15 +781,13 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn reserve(&mut self, additional: usize) {
-        let old_cap = self.cap();
-        let used_cap = self.len() + 1;
-        let new_cap = used_cap
-            .checked_add(additional)
-            .and_then(|needed_cap| needed_cap.checked_next_power_of_two())
-            .expect("capacity overflow");
+        let new_cap = self.len.checked_add(additional).expect("capacity overflow");
+        let old_cap = self.capacity();
 
         if new_cap > old_cap {
-            self.buf.reserve_exact(used_cap, new_cap - used_cap);
+            // we don't need to reserve_exact(), as the size doesn't have
+            // to be a power of 2.
+            self.buf.reserve(self.len, additional);
             unsafe {
                 self.handle_capacity_increase(old_cap);
             }
@@ -787,14 +833,25 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// ```
     #[stable(feature = "try_reserve", since = "1.57.0")]
     pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveError> {
-        self.try_reserve(additional)
+        let new_cap =
+            self.len.checked_add(additional).ok_or(TryReserveErrorKind::CapacityOverflow)?;
+        let old_cap = self.capacity();
+
+        if new_cap > old_cap {
+            self.buf.try_reserve_exact(self.len, additional)?;
+            unsafe {
+                self.handle_capacity_increase(old_cap);
+            }
+        }
+        Ok(())
     }
 
     /// Tries to reserve capacity for at least `additional` more elements to be inserted
     /// 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
     ///
@@ -824,15 +881,12 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// ```
     #[stable(feature = "try_reserve", since = "1.57.0")]
     pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
-        let old_cap = self.cap();
-        let used_cap = self.len() + 1;
-        let new_cap = used_cap
-            .checked_add(additional)
-            .and_then(|needed_cap| needed_cap.checked_next_power_of_two())
-            .ok_or(TryReserveErrorKind::CapacityOverflow)?;
+        let new_cap =
+            self.len.checked_add(additional).ok_or(TryReserveErrorKind::CapacityOverflow)?;
+        let old_cap = self.capacity();
 
         if new_cap > old_cap {
-            self.buf.try_reserve_exact(used_cap, new_cap - used_cap)?;
+            self.buf.try_reserve(self.len, additional)?;
             unsafe {
                 self.handle_capacity_increase(old_cap);
             }
@@ -883,65 +937,79 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// ```
     #[stable(feature = "shrink_to", since = "1.56.0")]
     pub fn shrink_to(&mut self, min_capacity: usize) {
-        let min_capacity = cmp::min(min_capacity, self.capacity());
-        // We don't have to worry about an overflow as neither `self.len()` nor `self.capacity()`
-        // can ever be `usize::MAX`. +1 as the ringbuffer always leaves one space empty.
-        let target_cap = cmp::max(cmp::max(min_capacity, self.len()) + 1, MINIMUM_CAPACITY + 1)
-            .next_power_of_two();
-
-        if target_cap < self.cap() {
-            // There are three cases of interest:
-            //   All elements are out of desired bounds
-            //   Elements are contiguous, and head is out of desired bounds
-            //   Elements are discontiguous, and tail is out of desired bounds
+        let target_cap = min_capacity.max(self.len);
+
+        // never shrink ZSTs
+        if T::IS_ZST || self.capacity() <= target_cap {
+            return;
+        }
+
+        // There are three cases of interest:
+        //   All elements are out of desired bounds
+        //   Elements are contiguous, and tail is out of desired bounds
+        //   Elements are discontiguous
+        //
+        // At all other times, element positions are unaffected.
+
+        // `head` and `len` are at most `isize::MAX` and `target_cap < self.capacity()`, so nothing can
+        // overflow.
+        let tail_outside = (target_cap + 1..=self.capacity()).contains(&(self.head + self.len));
+
+        if self.len == 0 {
+            self.head = 0;
+        } else if self.head >= target_cap && tail_outside {
+            // Head and tail are both out of bounds, so copy all of them to the front.
             //
-            // At all other times, element positions are unaffected.
+            //  H := head
+            //  L := last element
+            //                    H           L
+            //   [. . . . . . . . o o o o o o o . ]
+            //    H           L
+            //   [o o o o o o o . ]
+            unsafe {
+                // nonoverlapping because `self.head >= target_cap >= self.len`.
+                self.copy_nonoverlapping(self.head, 0, self.len);
+            }
+            self.head = 0;
+        } else if self.head < target_cap && tail_outside {
+            // Head is in bounds, tail is out of bounds.
+            // Copy the overflowing part to the beginning of the
+            // buffer. This won't overlap because `target_cap >= self.len`.
             //
-            // Indicates that elements at the head should be moved.
-            let head_outside = self.head == 0 || self.head >= target_cap;
-            // Move elements from out of desired bounds (positions after target_cap)
-            if self.tail >= target_cap && head_outside {
-                //                    T             H
-                //   [. . . . . . . . o o o o o o o . ]
-                //    T             H
-                //   [o o o o o o o . ]
-                unsafe {
-                    self.copy_nonoverlapping(0, self.tail, self.len());
-                }
-                self.head = self.len();
-                self.tail = 0;
-            } else if self.tail != 0 && self.tail < target_cap && head_outside {
-                //          T             H
-                //   [. . . o o o o o o o . . . . . . ]
-                //        H T
-                //   [o o . o o o o o ]
-                let len = self.wrap_sub(self.head, target_cap);
-                unsafe {
-                    self.copy_nonoverlapping(0, target_cap, len);
-                }
-                self.head = len;
-                debug_assert!(self.head < self.tail);
-            } else if self.tail >= target_cap {
-                //              H                 T
-                //   [o o o o o . . . . . . . . . o o ]
-                //              H T
-                //   [o o o o o . o o ]
-                debug_assert!(self.wrap_sub(self.head, 1) < target_cap);
-                let len = self.cap() - self.tail;
-                let new_tail = target_cap - len;
-                unsafe {
-                    self.copy_nonoverlapping(new_tail, self.tail, len);
-                }
-                self.tail = new_tail;
-                debug_assert!(self.head < self.tail);
+            //  H := head
+            //  L := last element
+            //          H           L
+            //   [. . . o o o o o o o . . . . . . ]
+            //      L   H
+            //   [o o . o o o o o ]
+            let len = self.head + self.len - target_cap;
+            unsafe {
+                self.copy_nonoverlapping(target_cap, 0, len);
             }
-
-            self.buf.shrink_to_fit(target_cap);
-
-            debug_assert!(self.head < self.cap());
-            debug_assert!(self.tail < self.cap());
-            debug_assert!(self.cap().count_ones() == 1);
+        } else if !self.is_contiguous() {
+            // The head slice is at least partially out of bounds, tail is in bounds.
+            // Copy the head backwards so it lines up with the target capacity.
+            // This won't overlap because `target_cap >= self.len`.
+            //
+            //  H := head
+            //  L := last element
+            //            L                   H
+            //   [o o o o o . . . . . . . . . o o ]
+            //            L   H
+            //   [o o o o o . o o ]
+            let head_len = self.capacity() - self.head;
+            let new_head = target_cap - head_len;
+            unsafe {
+                // can't use `copy_nonoverlapping()` here because the new and old
+                // regions for the head might overlap.
+                self.copy(self.head, new_head, head_len);
+            }
+            self.head = new_head;
         }
+        self.buf.shrink_to_fit(target_cap);
+
+        debug_assert!(self.head < self.capacity() || self.capacity() == 0);
+        debug_assert!(self.len <= self.capacity());
     }
 
     /// Shortens the deque, keeping the first `len` elements and dropping
@@ -985,20 +1053,20 @@ impl<T, A: Allocator> VecDeque<T, A> {
         // * The head of the VecDeque is moved before calling `drop_in_place`,
         //   so no value is dropped twice if `drop_in_place` panics
         unsafe {
-            if len > self.len() {
+            if len >= self.len {
                 return;
             }
-            let num_dropped = self.len() - len;
+
             let (front, back) = self.as_mut_slices();
             if len > front.len() {
                 let begin = len - front.len();
                 let drop_back = back.get_unchecked_mut(begin..) as *mut _;
-                self.head = self.wrap_sub(self.head, num_dropped);
+                self.len = len;
                 ptr::drop_in_place(drop_back);
             } else {
                 let drop_back = back as *mut _;
                 let drop_front = front.get_unchecked_mut(len..) as *mut _;
-                self.head = self.wrap_sub(self.head, num_dropped);
+                self.len = len;
 
                 // Make sure the second half is dropped even when a destructor
                 // in the first one panics.
@@ -1032,7 +1100,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn iter(&self) -> Iter<'_, T> {
-        Iter::new(unsafe { self.buffer_as_slice() }, self.tail, self.head)
+        let (a, b) = self.as_slices();
+        Iter::new(a.iter(), b.iter())
     }
 
     /// Returns a front-to-back iterator that returns mutable references.
@@ -1054,11 +1123,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn iter_mut(&mut self) -> IterMut<'_, T> {
-        // SAFETY: The internal `IterMut` safety invariant is established because the
-        // `ring` we create is a dereferenceable slice for lifetime '_.
-        let ring = ptr::slice_from_raw_parts_mut(self.ptr(), self.cap());
-
-        unsafe { IterMut::new(ring, self.tail, self.head, PhantomData) }
+        let (a, b) = self.as_mut_slices();
+        IterMut::new(a.iter_mut(), b.iter_mut())
     }
 
     /// Returns a pair of slices which contain, in order, the contents of the
@@ -1090,14 +1156,10 @@ impl<T, A: Allocator> VecDeque<T, A> {
     #[inline]
     #[stable(feature = "deque_extras_15", since = "1.5.0")]
     pub fn as_slices(&self) -> (&[T], &[T]) {
-        // Safety:
-        // - `self.head` and `self.tail` in a ring buffer are always valid indices.
-        // - `RingSlices::ring_slices` guarantees that the slices split according to `self.head` and `self.tail` are initialized.
-        unsafe {
-            let buf = self.buffer_as_slice();
-            let (front, back) = RingSlices::ring_slices(buf, self.head, self.tail);
-            (MaybeUninit::slice_assume_init_ref(front), MaybeUninit::slice_assume_init_ref(back))
-        }
+        let (a_range, b_range) = self.slice_ranges(.., self.len);
+        // SAFETY: `slice_ranges` always returns valid ranges into
+        // the physical buffer.
+        unsafe { (&*self.buffer_range(a_range), &*self.buffer_range(b_range)) }
     }
 
     /// Returns a pair of slices which contain, in order, the contents of the
@@ -1128,16 +1190,10 @@ impl<T, A: Allocator> VecDeque<T, A> {
     #[inline]
     #[stable(feature = "deque_extras_15", since = "1.5.0")]
     pub fn as_mut_slices(&mut self) -> (&mut [T], &mut [T]) {
-        // Safety:
-        // - `self.head` and `self.tail` in a ring buffer are always valid indices.
-        // - `RingSlices::ring_slices` guarantees that the slices split according to `self.head` and `self.tail` are initialized.
-        unsafe {
-            let head = self.head;
-            let tail = self.tail;
-            let buf = self.buffer_as_mut_slice();
-            let (front, back) = RingSlices::ring_slices(buf, head, tail);
-            (MaybeUninit::slice_assume_init_mut(front), MaybeUninit::slice_assume_init_mut(back))
-        }
+        let (a_range, b_range) = self.slice_ranges(.., self.len);
+        // SAFETY: `slice_ranges` always returns valid ranges into
+        // the physical buffer.
+        unsafe { (&mut *self.buffer_range(a_range), &mut *self.buffer_range(b_range)) }
     }
 
     /// Returns the number of elements in the deque.
@@ -1154,7 +1210,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn len(&self) -> usize {
-        count(self.tail, self.head, self.cap())
+        self.len
     }
 
     /// Returns `true` if the deque is empty.
@@ -1171,17 +1227,50 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn is_empty(&self) -> bool {
-        self.tail == self.head
+        self.len == 0
     }
 
-    fn range_tail_head<R>(&self, range: R) -> (usize, usize)
+    /// Given a range into the logical buffer of the deque, this function
+    /// return two ranges into the physical buffer that correspond to
+    /// the given range. The `len` parameter should usually just be `self.len`;
+    /// the reason it's passed explicitly is that if the deque is wrapped in
+    /// a `Drain`, then `self.len` is not actually the length of the deque.
+    ///
+    /// # Safety
+    ///
+    /// This function is always safe to call. For the resulting ranges to be valid
+    /// ranges into the physical buffer, the caller must ensure that the result of
+    /// calling `slice::range(range, ..len)` represents a valid range into the
+    /// logical buffer, and that all elements in that range are initialized.
+    fn slice_ranges<R>(&self, range: R, len: usize) -> (Range<usize>, Range<usize>)
     where
         R: RangeBounds<usize>,
     {
-        let Range { start, end } = slice::range(range, ..self.len());
-        let tail = self.wrap_add(self.tail, start);
-        let head = self.wrap_add(self.tail, end);
-        (tail, head)
+        let Range { start, end } = slice::range(range, ..len);
+        let len = end - start;
+
+        if len == 0 {
+            (0..0, 0..0)
+        } else {
+            // `slice::range` guarantees that `start <= end <= len`.
+            // because `len != 0`, we know that `start < end`, so `start < len`
+            // and the indexing is valid.
+            let wrapped_start = self.to_physical_idx(start);
+
+            // this subtraction can never overflow because `wrapped_start` is
+            // at most `self.capacity()` (and if `self.capacity != 0`, then `wrapped_start` is strictly less
+            // than `self.capacity`).
+            let head_len = self.capacity() - wrapped_start;
+
+            if head_len >= len {
+                // we know that `len + wrapped_start <= self.capacity <= usize::MAX`, so this addition can't overflow
+                (wrapped_start..wrapped_start + len, 0..0)
+            } else {
+                // can't overflow because of the if condition
+                let tail_len = len - head_len;
+                (wrapped_start..self.capacity(), 0..tail_len)
+            }
+        }
     }
 
     /// Creates an iterator that covers the specified range in the deque.
@@ -1210,9 +1299,14 @@ impl<T, A: Allocator> VecDeque<T, A> {
     where
         R: RangeBounds<usize>,
     {
-        let (tail, head) = self.range_tail_head(range);
-        // The shared reference we have in &self is maintained in the '_ of Iter.
-        Iter::new(unsafe { self.buffer_as_slice() }, tail, head)
+        let (a_range, b_range) = self.slice_ranges(range, self.len);
+        // SAFETY: The ranges returned by `slice_ranges`
+        // are valid ranges into the physical buffer, so
+        // it's ok to pass them to `buffer_range` and
+        // dereference the result.
+        let a = unsafe { &*self.buffer_range(a_range) };
+        let b = unsafe { &*self.buffer_range(b_range) };
+        Iter::new(a.iter(), b.iter())
     }
 
     /// Creates an iterator that covers the specified mutable range in the deque.
@@ -1245,13 +1339,14 @@ impl<T, A: Allocator> VecDeque<T, A> {
     where
         R: RangeBounds<usize>,
     {
-        let (tail, head) = self.range_tail_head(range);
-
-        // SAFETY: The internal `IterMut` safety invariant is established because the
-        // `ring` we create is a dereferenceable slice for lifetime '_.
-        let ring = ptr::slice_from_raw_parts_mut(self.ptr(), self.cap());
-
-        unsafe { IterMut::new(ring, tail, head, PhantomData) }
+        let (a_range, b_range) = self.slice_ranges(range, self.len);
+        // SAFETY: The ranges returned by `slice_ranges`
+        // are valid ranges into the physical buffer, so
+        // it's ok to pass them to `buffer_range` and
+        // dereference the result.
+        let a = unsafe { &mut *self.buffer_range(a_range) };
+        let b = unsafe { &mut *self.buffer_range(b_range) };
+        IterMut::new(a.iter_mut(), b.iter_mut())
     }
 
     /// Removes the specified range from the deque in bulk, returning all
@@ -1303,40 +1398,30 @@ impl<T, A: Allocator> VecDeque<T, A> {
         // When finished, the remaining data will be copied back to cover the hole,
         // and the head/tail values will be restored correctly.
         //
-        let (drain_tail, drain_head) = self.range_tail_head(range);
+        let Range { start, end } = slice::range(range, ..self.len);
+        let drain_start = start;
+        let drain_len = end - start;
 
         // The deque's elements are parted into three segments:
-        // * self.tail  -> drain_tail
-        // * drain_tail -> drain_head
-        // * drain_head -> self.head
+        // * 0  -> drain_start
+        // * drain_start -> drain_start+drain_len
+        // * drain_start+drain_len -> self.len
         //
-        // T = self.tail; H = self.head; t = drain_tail; h = drain_head
+        // H = self.head; T = self.head+self.len; t = drain_start+drain_len; h = drain_head
         //
-        // We store drain_tail as self.head, and drain_head and self.head as
-        // after_tail and after_head respectively on the Drain. This also
+        // We store drain_start as self.len, and drain_len and self.len as
+        // drain_len and orig_len respectively on the Drain. This also
         // truncates the effective array such that if the Drain is leaked, we
         // have forgotten about the potentially moved values after the start of
         // the drain.
         //
-        //        T   t   h   H
+        //        H   h   t   T
         // [. . . o o x x o o . . .]
         //
-        let head = self.head;
-
         // "forget" about the values after the start of the drain until after
         // the drain is complete and the Drain destructor is run.
-        self.head = drain_tail;
 
-        let deque = NonNull::from(&mut *self);
-        unsafe {
-            // Crucially, we only create shared references from `self` here and read from
-            // 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)
-        }
+        unsafe { Drain::new(self, drain_start, drain_len) }
     }
 
     /// Clears the deque, removing all values.
@@ -1355,6 +1440,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
     #[inline]
     pub fn clear(&mut self) {
         self.truncate(0);
+        // Not strictly necessary, but leaves things in a more consistent/predictable state.
+        self.head = 0;
     }
 
     /// Returns `true` if the deque contains an element equal to the
@@ -1449,7 +1536,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn back(&self) -> Option<&T> {
-        self.get(self.len().wrapping_sub(1))
+        self.get(self.len.wrapping_sub(1))
     }
 
     /// Provides a mutable reference to the back element, or `None` if the
@@ -1473,7 +1560,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn back_mut(&mut self) -> Option<&mut T> {
-        self.get_mut(self.len().wrapping_sub(1))
+        self.get_mut(self.len.wrapping_sub(1))
     }
 
     /// Removes the first element and returns it, or `None` if the deque is
@@ -1497,9 +1584,10 @@ impl<T, A: Allocator> VecDeque<T, A> {
         if self.is_empty() {
             None
         } else {
-            let tail = self.tail;
-            self.tail = self.wrap_add(self.tail, 1);
-            unsafe { Some(self.buffer_read(tail)) }
+            let old_head = self.head;
+            self.head = self.to_physical_idx(1);
+            self.len -= 1;
+            Some(unsafe { self.buffer_read(old_head) })
         }
     }
 
@@ -1522,9 +1610,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
         if self.is_empty() {
             None
         } else {
-            self.head = self.wrap_sub(self.head, 1);
-            let head = self.head;
-            unsafe { Some(self.buffer_read(head)) }
+            self.len -= 1;
+            Some(unsafe { self.buffer_read(self.to_physical_idx(self.len)) })
         }
     }
 
@@ -1546,10 +1633,11 @@ impl<T, A: Allocator> VecDeque<T, A> {
             self.grow();
         }
 
-        self.tail = self.wrap_sub(self.tail, 1);
-        let tail = self.tail;
+        self.head = self.wrap_sub(self.head, 1);
+        self.len += 1;
+
         unsafe {
-            self.buffer_write(tail, value);
+            self.buffer_write(self.head, value);
         }
     }
 
@@ -1571,16 +1659,14 @@ impl<T, A: Allocator> VecDeque<T, A> {
             self.grow();
         }
 
-        let head = self.head;
-        self.head = self.wrap_add(self.head, 1);
-        unsafe { self.buffer_write(head, value) }
+        unsafe { self.buffer_write(self.to_physical_idx(self.len), value) }
+        self.len += 1;
     }
 
     #[inline]
     fn is_contiguous(&self) -> bool {
-        // FIXME: Should we consider `head == 0` to mean
-        // that `self` is contiguous?
-        self.tail <= self.head
+        // Do the calculation like this to avoid overflowing if len + head > usize::MAX
+        self.head <= self.capacity() - self.len
     }
 
     /// Removes an element from anywhere in the deque and returns it,
@@ -1609,8 +1695,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// ```
     #[stable(feature = "deque_extras_15", since = "1.5.0")]
     pub fn swap_remove_front(&mut self, index: usize) -> Option<T> {
-        let length = self.len();
-        if length > 0 && index < length && index != 0 {
+        let length = self.len;
+        if index < length && index != 0 {
             self.swap(index, 0);
         } else if index >= length {
             return None;
@@ -1644,7 +1730,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// ```
     #[stable(feature = "deque_extras_15", since = "1.5.0")]
     pub fn swap_remove_back(&mut self, index: usize) -> Option<T> {
-        let length = self.len();
+        let length = self.len;
         if length > 0 && index < length - 1 {
             self.swap(index, length - 1);
         } else if index >= length {
@@ -1683,198 +1769,26 @@ impl<T, A: Allocator> VecDeque<T, A> {
             self.grow();
         }
 
-        // Move the least number of elements in the ring buffer and insert
-        // the given object
-        //
-        // At most len/2 - 1 elements will be moved. O(min(n, n-i))
-        //
-        // There are three main cases:
-        //  Elements are contiguous
-        //      - special case when tail is 0
-        //  Elements are discontiguous and the insert is in the tail section
-        //  Elements are discontiguous and the insert is in the head section
-        //
-        // For each of those there are two more cases:
-        //  Insert is closer to tail
-        //  Insert is closer to head
-        //
-        // Key: H - self.head
-        //      T - self.tail
-        //      o - Valid element
-        //      I - Insertion element
-        //      A - The element that should be after the insertion point
-        //      M - Indicates element was moved
-
-        let idx = self.wrap_add(self.tail, index);
-
-        let distance_to_tail = index;
-        let distance_to_head = self.len() - index;
-
-        let contiguous = self.is_contiguous();
-
-        match (contiguous, distance_to_tail <= distance_to_head, idx >= self.tail) {
-            (true, true, _) if index == 0 => {
-                // push_front
-                //
-                //       T
-                //       I             H
-                //      [A o o o o o o . . . . . . . . .]
-                //
-                //                       H         T
-                //      [A o o o o o o o . . . . . I]
-                //
-
-                self.tail = self.wrap_sub(self.tail, 1);
-            }
-            (true, true, _) => {
-                unsafe {
-                    // contiguous, insert closer to tail:
-                    //
-                    //             T   I         H
-                    //      [. . . o o A o o o o . . . . . .]
-                    //
-                    //           T               H
-                    //      [. . o o I A o o o o . . . . . .]
-                    //           M M
-                    //
-                    // contiguous, insert closer to tail and tail is 0:
-                    //
-                    //
-                    //       T   I         H
-                    //      [o o A o o o o . . . . . . . . .]
-                    //
-                    //                       H             T
-                    //      [o I A o o o o o . . . . . . . o]
-                    //       M                             M
-
-                    let new_tail = self.wrap_sub(self.tail, 1);
-
-                    self.copy(new_tail, self.tail, 1);
-                    // Already moved the tail, so we only copy `index - 1` elements.
-                    self.copy(self.tail, self.tail + 1, index - 1);
-
-                    self.tail = new_tail;
-                }
-            }
-            (true, false, _) => {
-                unsafe {
-                    //  contiguous, insert closer to head:
-                    //
-                    //             T       I     H
-                    //      [. . . o o o o A o o . . . . . .]
-                    //
-                    //             T               H
-                    //      [. . . o o o o I A o o . . . . .]
-                    //                       M M M
-
-                    self.copy(idx + 1, idx, self.head - idx);
-                    self.head = self.wrap_add(self.head, 1);
-                }
-            }
-            (false, true, true) => {
-                unsafe {
-                    // discontiguous, insert closer to tail, tail section:
-                    //
-                    //                   H         T   I
-                    //      [o o o o o o . . . . . o o A o o]
-                    //
-                    //                   H       T
-                    //      [o o o o o o . . . . o o I A o o]
-                    //                           M M
-
-                    self.copy(self.tail - 1, self.tail, index);
-                    self.tail -= 1;
-                }
-            }
-            (false, false, true) => {
-                unsafe {
-                    // discontiguous, insert closer to head, tail section:
-                    //
-                    //           H             T         I
-                    //      [o o . . . . . . . o o o o o A o]
-                    //
-                    //             H           T
-                    //      [o o o . . . . . . o o o o o I A]
-                    //       M M M                         M
-
-                    // copy elements up to new head
-                    self.copy(1, 0, self.head);
-
-                    // copy last element into empty spot at bottom of buffer
-                    self.copy(0, self.cap() - 1, 1);
-
-                    // move elements from idx to end forward not including ^ element
-                    self.copy(idx + 1, idx, self.cap() - 1 - idx);
-
-                    self.head += 1;
-                }
-            }
-            (false, true, false) if idx == 0 => {
-                unsafe {
-                    // discontiguous, insert is closer to tail, head section,
-                    // and is at index zero in the internal buffer:
-                    //
-                    //       I                   H     T
-                    //      [A o o o o o o o o o . . . o o o]
-                    //
-                    //                           H   T
-                    //      [A o o o o o o o o o . . o o o I]
-                    //                               M M M
-
-                    // copy elements up to new tail
-                    self.copy(self.tail - 1, self.tail, self.cap() - self.tail);
-
-                    // copy last element into empty spot at bottom of buffer
-                    self.copy(self.cap() - 1, 0, 1);
-
-                    self.tail -= 1;
-                }
-            }
-            (false, true, false) => {
-                unsafe {
-                    // discontiguous, insert closer to tail, head section:
-                    //
-                    //             I             H     T
-                    //      [o o o A o o o o o o . . . o o o]
-                    //
-                    //                           H   T
-                    //      [o o I A o o o o o o . . o o o o]
-                    //       M M                     M M M M
-
-                    // copy elements up to new tail
-                    self.copy(self.tail - 1, self.tail, self.cap() - self.tail);
-
-                    // copy last element into empty spot at bottom of buffer
-                    self.copy(self.cap() - 1, 0, 1);
-
-                    // move elements from idx-1 to end forward not including ^ element
-                    self.copy(0, 1, idx - 1);
-
-                    self.tail -= 1;
-                }
+        let k = self.len - index;
+        if k < index {
+            // `index + 1` can't overflow, because if index was usize::MAX, then either the
+            // assert would've failed, or the deque would've tried to grow past usize::MAX
+            // and panicked.
+            unsafe {
+                // see `remove()` for explanation why this wrap_copy() call is safe.
+                self.wrap_copy(self.to_physical_idx(index), self.to_physical_idx(index + 1), k);
+                self.buffer_write(self.to_physical_idx(index), value);
+                self.len += 1;
             }
-            (false, false, false) => {
-                unsafe {
-                    // discontiguous, insert closer to head, head section:
-                    //
-                    //               I     H           T
-                    //      [o o o o A o o . . . . . . o o o]
-                    //
-                    //                     H           T
-                    //      [o o o o I A o o . . . . . o o o]
-                    //                 M M M
-
-                    self.copy(idx + 1, idx, self.head - idx);
-                    self.head += 1;
-                }
+        } else {
+            let old_head = self.head;
+            self.head = self.wrap_sub(self.head, 1);
+            unsafe {
+                self.wrap_copy(old_head, self.head, index);
+                self.buffer_write(self.to_physical_idx(index), value);
+                self.len += 1;
             }
         }
-
-        // tail might've been changed so we need to recalculate
-        let new_idx = self.wrap_add(self.tail, index);
-        unsafe {
-            self.buffer_write(new_idx, value);
-        }
     }
 
     /// Removes and returns the element at `index` from the deque.
@@ -1900,156 +1814,26 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn remove(&mut self, index: usize) -> Option<T> {
-        if self.is_empty() || self.len() <= index {
+        if self.len <= index {
             return None;
         }
 
-        // There are three main cases:
-        //  Elements are contiguous
-        //  Elements are discontiguous and the removal is in the tail section
-        //  Elements are discontiguous and the removal is in the head section
-        //      - special case when elements are technically contiguous,
-        //        but self.head = 0
-        //
-        // For each of those there are two more cases:
-        //  Insert is closer to tail
-        //  Insert is closer to head
-        //
-        // Key: H - self.head
-        //      T - self.tail
-        //      o - Valid element
-        //      x - Element marked for removal
-        //      R - Indicates element that is being removed
-        //      M - Indicates element was moved
-
-        let idx = self.wrap_add(self.tail, index);
-
-        let elem = unsafe { Some(self.buffer_read(idx)) };
-
-        let distance_to_tail = index;
-        let distance_to_head = self.len() - index;
-
-        let contiguous = self.is_contiguous();
-
-        match (contiguous, distance_to_tail <= distance_to_head, idx >= self.tail) {
-            (true, true, _) => {
-                unsafe {
-                    // contiguous, remove closer to tail:
-                    //
-                    //             T   R         H
-                    //      [. . . o o x o o o o . . . . . .]
-                    //
-                    //               T           H
-                    //      [. . . . o o o o o o . . . . . .]
-                    //               M M
-
-                    self.copy(self.tail + 1, self.tail, index);
-                    self.tail += 1;
-                }
-            }
-            (true, false, _) => {
-                unsafe {
-                    // contiguous, remove closer to head:
-                    //
-                    //             T       R     H
-                    //      [. . . o o o o x o o . . . . . .]
-                    //
-                    //             T           H
-                    //      [. . . o o o o o o . . . . . . .]
-                    //                     M M
-
-                    self.copy(idx, idx + 1, self.head - idx - 1);
-                    self.head -= 1;
-                }
-            }
-            (false, true, true) => {
-                unsafe {
-                    // discontiguous, remove closer to tail, tail section:
-                    //
-                    //                   H         T   R
-                    //      [o o o o o o . . . . . o o x o o]
-                    //
-                    //                   H           T
-                    //      [o o o o o o . . . . . . o o o o]
-                    //                               M M
-
-                    self.copy(self.tail + 1, self.tail, index);
-                    self.tail = self.wrap_add(self.tail, 1);
-                }
-            }
-            (false, false, false) => {
-                unsafe {
-                    // discontiguous, remove closer to head, head section:
-                    //
-                    //               R     H           T
-                    //      [o o o o x o o . . . . . . o o o]
-                    //
-                    //                   H             T
-                    //      [o o o o o o . . . . . . . o o o]
-                    //               M M
-
-                    self.copy(idx, idx + 1, self.head - idx - 1);
-                    self.head -= 1;
-                }
-            }
-            (false, false, true) => {
-                unsafe {
-                    // discontiguous, remove closer to head, tail section:
-                    //
-                    //             H           T         R
-                    //      [o o o . . . . . . o o o o o x o]
-                    //
-                    //           H             T
-                    //      [o o . . . . . . . o o o o o o o]
-                    //       M M                         M M
-                    //
-                    // or quasi-discontiguous, remove next to head, tail section:
-                    //
-                    //       H                 T         R
-                    //      [. . . . . . . . . o o o o o x o]
-                    //
-                    //                         T           H
-                    //      [. . . . . . . . . o o o o o o .]
-                    //                                   M
-
-                    // draw in elements in the tail section
-                    self.copy(idx, idx + 1, self.cap() - idx - 1);
-
-                    // Prevents underflow.
-                    if self.head != 0 {
-                        // copy first element into empty spot
-                        self.copy(self.cap() - 1, 0, 1);
-
-                        // move elements in the head section backwards
-                        self.copy(0, 1, self.head - 1);
-                    }
-
-                    self.head = self.wrap_sub(self.head, 1);
-                }
-            }
-            (false, true, false) => {
-                unsafe {
-                    // discontiguous, remove closer to tail, head section:
-                    //
-                    //           R               H     T
-                    //      [o o x o o o o o o o . . . o o o]
-                    //
-                    //                           H       T
-                    //      [o o o o o o o o o o . . . . o o]
-                    //       M M M                       M M
-
-                    // draw in elements up to idx
-                    self.copy(1, 0, idx);
-
-                    // copy last element into empty spot
-                    self.copy(0, self.cap() - 1, 1);
+        let wrapped_idx = self.to_physical_idx(index);
 
-                    // move elements from tail to end forward, excluding the last one
-                    self.copy(self.tail + 1, self.tail, self.cap() - self.tail - 1);
+        let elem = unsafe { Some(self.buffer_read(wrapped_idx)) };
 
-                    self.tail = self.wrap_add(self.tail, 1);
-                }
-            }
+        let k = self.len - index - 1;
+        // safety: due to the nature of the if-condition, whichever wrap_copy gets called,
+        // its length argument will be at most `self.len / 2`, so there can't be more than
+        // one overlapping area.
+        if k < index {
+            unsafe { self.wrap_copy(self.wrap_add(wrapped_idx, 1), wrapped_idx, k) };
+            self.len -= 1;
+        } else {
+            let old_head = self.head;
+            self.head = self.to_physical_idx(1);
+            unsafe { self.wrap_copy(old_head, self.head, index) };
+            self.len -= 1;
         }
 
         elem
@@ -2085,7 +1869,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
     where
         A: Clone,
     {
-        let len = self.len();
+        let len = self.len;
         assert!(at <= len, "`at` out of bounds");
 
         let other_len = len - at;
@@ -2122,8 +1906,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
         }
 
         // Cleanup where the ends of the buffers are
-        self.head = self.wrap_sub(self.head, other_len);
-        other.head = other.wrap_index(other_len);
+        self.len = at;
+        other.len = other_len;
 
         other
     }
@@ -2148,17 +1932,26 @@ impl<T, A: Allocator> VecDeque<T, A> {
     #[inline]
     #[stable(feature = "append", since = "1.4.0")]
     pub fn append(&mut self, other: &mut Self) {
-        self.reserve(other.len());
+        if T::IS_ZST {
+            self.len = self.len.checked_add(other.len).expect("capacity overflow");
+            other.len = 0;
+            other.head = 0;
+            return;
+        }
+
+        self.reserve(other.len);
         unsafe {
             let (left, right) = other.as_slices();
-            self.copy_slice(self.head, left);
-            self.copy_slice(self.wrap_add(self.head, left.len()), right);
+            self.copy_slice(self.to_physical_idx(self.len), left);
+            // no overflow, because self.capacity() >= old_cap + left.len() >= self.len + left.len()
+            self.copy_slice(self.to_physical_idx(self.len + left.len()), right);
         }
         // SAFETY: Update pointers after copying to avoid leaving doppelganger
         // in case of panics.
-        self.head = self.wrap_add(self.head, other.len());
-        // Silently drop values in `other`.
-        other.tail = other.head;
+        self.len += other.len;
+        // Now that we own its values, forget everything in `other`.
+        other.len = 0;
+        other.head = 0;
     }
 
     /// Retains only the elements specified by the predicate.
@@ -2226,7 +2019,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
     where
         F: FnMut(&mut T) -> bool,
     {
-        let len = self.len();
+        let len = self.len;
         let mut idx = 0;
         let mut cur = 0;
 
@@ -2264,9 +2057,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
         // Extend or possibly remove this assertion when valid use-cases for growing the
         // buffer without it being full emerge
         debug_assert!(self.is_full());
-        let old_cap = self.cap();
-        self.buf.reserve_exact(old_cap, old_cap);
-        assert!(self.cap() == old_cap * 2);
+        let old_cap = self.capacity();
+        self.buf.reserve_for_push(old_cap);
         unsafe {
             self.handle_capacity_increase(old_cap);
         }
@@ -2300,7 +2092,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// ```
     #[stable(feature = "vec_resize_with", since = "1.33.0")]
     pub fn resize_with(&mut self, new_len: usize, generator: impl FnMut() -> T) {
-        let len = self.len();
+        let len = self.len;
 
         if new_len > len {
             self.extend(repeat_with(generator).take(new_len - len))
@@ -2366,110 +2158,129 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// ```
     #[stable(feature = "deque_make_contiguous", since = "1.48.0")]
     pub fn make_contiguous(&mut self) -> &mut [T] {
+        if T::IS_ZST {
+            self.head = 0;
+        }
+
         if self.is_contiguous() {
-            let tail = self.tail;
-            let head = self.head;
-            // Safety:
-            // - `self.head` and `self.tail` in a ring buffer are always valid indices.
-            // - `RingSlices::ring_slices` guarantees that the slices split according to `self.head` and `self.tail` are initialized.
-            return unsafe {
-                MaybeUninit::slice_assume_init_mut(
-                    RingSlices::ring_slices(self.buffer_as_mut_slice(), head, tail).0,
-                )
-            };
+            unsafe { return slice::from_raw_parts_mut(self.ptr().add(self.head), self.len) }
         }
 
-        let buf = self.buf.ptr();
-        let cap = self.cap();
-        let len = self.len();
+        let &mut Self { head, len, .. } = self;
+        let ptr = self.ptr();
+        let cap = self.capacity();
 
-        let free = self.tail - self.head;
-        let tail_len = cap - self.tail;
+        let free = cap - len;
+        let head_len = cap - head;
+        let tail = len - head_len;
+        let tail_len = tail;
 
-        if free >= tail_len {
-            // there is enough free space to copy the tail in one go,
-            // this means that we first shift the head backwards, and then
-            // copy the tail to the correct position.
+        if free >= head_len {
+            // there is enough free space to copy the head in one go,
+            // this means that we first shift the tail backwards, and then
+            // copy the head to the correct position.
             //
             // from: DEFGH....ABC
             // to:   ABCDEFGH....
             unsafe {
-                ptr::copy(buf, buf.add(tail_len), self.head);
+                self.copy(0, head_len, tail_len);
                 // ...DEFGH.ABC
-                ptr::copy_nonoverlapping(buf.add(self.tail), buf, tail_len);
+                self.copy_nonoverlapping(head, 0, head_len);
                 // ABCDEFGH....
-
-                self.tail = 0;
-                self.head = len;
             }
-        } else if free > self.head {
-            // FIXME: We currently do not consider ....ABCDEFGH
-            // to be contiguous because `head` would be `0` in this
-            // case. While we probably want to change this it
-            // isn't trivial as a few places expect `is_contiguous`
-            // to mean that we can just slice using `buf[tail..head]`.
 
-            // there is enough free space to copy the head in one go,
-            // this means that we first shift the tail forwards, and then
-            // copy the head to the correct position.
+            self.head = 0;
+        } else if free >= tail_len {
+            // there is enough free space to copy the tail in one go,
+            // this means that we first shift the head forwards, and then
+            // copy the tail to the correct position.
             //
             // from: FGH....ABCDE
             // to:   ...ABCDEFGH.
             unsafe {
-                ptr::copy(buf.add(self.tail), buf.add(self.head), tail_len);
+                self.copy(head, tail, head_len);
                 // FGHABCDE....
-                ptr::copy_nonoverlapping(buf, buf.add(self.head + tail_len), self.head);
+                self.copy_nonoverlapping(0, tail + head_len, tail_len);
                 // ...ABCDEFGH.
-
-                self.tail = self.head;
-                self.head = self.wrap_add(self.tail, len);
             }
+
+            self.head = tail;
         } else {
-            // free is smaller than both head and tail,
-            // this means we have to slowly "swap" the tail and the head.
+            // `free` is smaller than both `head_len` and `tail_len`.
+            // the general algorithm for this first moves the slices
+            // right next to each other and then uses `slice::rotate`
+            // to rotate them into place:
             //
-            // from: EFGHI...ABCD or HIJK.ABCDEFG
-            // to:   ABCDEFGHI... or ABCDEFGHIJK.
-            let mut left_edge: usize = 0;
-            let mut right_edge: usize = self.tail;
-            unsafe {
-                // The general problem looks like this
-                // GHIJKLM...ABCDEF - before any swaps
-                // ABCDEFM...GHIJKL - after 1 pass of swaps
-                // ABCDEFGHIJM...KL - swap until the left edge reaches the temp store
-                //                  - then restart the algorithm with a new (smaller) store
-                // Sometimes the temp store is reached when the right edge is at the end
-                // of the buffer - this means we've hit the right order with fewer swaps!
-                // E.g
-                // EF..ABCD
-                // ABCDEF.. - after four only swaps we've finished
-                while left_edge < len && right_edge != cap {
-                    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));
+            // initially:   HIJK..ABCDEFG
+            // step 1:      ..HIJKABCDEFG
+            // step 2:      ..ABCDEFGHIJK
+            //
+            // or:
+            //
+            // initially:   FGHIJK..ABCDE
+            // step 1:      FGHIJKABCDE..
+            // step 2:      ABCDEFGHIJK..
+
+            // pick the shorter of the 2 slices to reduce the amount
+            // of memory that needs to be moved around.
+            if head_len > tail_len {
+                // tail is shorter, so:
+                //  1. copy tail forwards
+                //  2. rotate used part of the buffer
+                //  3. update head to point to the new beginning (which is just `free`)
+
+                unsafe {
+                    // if there is no free space in the buffer, then the slices are already
+                    // right next to each other and we don't need to move any memory.
+                    if free != 0 {
+                        // because we only move the tail forward as much as there's free space
+                        // behind it, we don't overwrite any elements of the head slice, and
+                        // the slices end up right next to each other.
+                        self.copy(0, free, tail_len);
                     }
-                    let n_ops = right_edge - left_edge;
-                    left_edge += n_ops;
-                    right_edge += right_offset + 1;
+
+                    // We just copied the tail right next to the head slice,
+                    // so all of the elements in the range are initialized
+                    let slice = &mut *self.buffer_range(free..self.capacity());
+
+                    // because the deque wasn't contiguous, we know that `tail_len < self.len == slice.len()`,
+                    // so this will never panic.
+                    slice.rotate_left(tail_len);
+
+                    // the used part of the buffer now is `free..self.capacity()`, so set
+                    // `head` to the beginning of that range.
+                    self.head = free;
                 }
+            } else {
+                // head is shorter so:
+                //  1. copy head backwards
+                //  2. rotate used part of the buffer
+                //  3. update head to point to the new beginning (which is the beginning of the buffer)
+
+                unsafe {
+                    // if there is no free space in the buffer, then the slices are already
+                    // right next to each other and we don't need to move any memory.
+                    if free != 0 {
+                        // copy the head slice to lie right behind the tail slice.
+                        self.copy(self.head, tail_len, head_len);
+                    }
+
+                    // because we copied the head slice so that both slices lie right
+                    // next to each other, all the elements in the range are initialized.
+                    let slice = &mut *self.buffer_range(0..self.len);
+
+                    // because the deque wasn't contiguous, we know that `head_len < self.len == slice.len()`
+                    // so this will never panic.
+                    slice.rotate_right(head_len);
 
-                self.tail = 0;
-                self.head = len;
+                    // the used part of the buffer now is `0..self.len`, so set
+                    // `head` to the beginning of that range.
+                    self.head = 0;
+                }
             }
         }
 
-        let tail = self.tail;
-        let head = self.head;
-        // Safety:
-        // - `self.head` and `self.tail` in a ring buffer are always valid indices.
-        // - `RingSlices::ring_slices` guarantees that the slices split according to `self.head` and `self.tail` are initialized.
-        unsafe {
-            MaybeUninit::slice_assume_init_mut(
-                RingSlices::ring_slices(self.buffer_as_mut_slice(), head, tail).0,
-            )
-        }
+        unsafe { slice::from_raw_parts_mut(ptr.add(self.head), self.len) }
     }
 
     /// Rotates the double-ended queue `mid` places to the left.
@@ -2507,7 +2318,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
     #[stable(feature = "vecdeque_rotate", since = "1.36.0")]
     pub fn rotate_left(&mut self, mid: usize) {
         assert!(mid <= self.len());
-        let k = self.len() - mid;
+        let k = self.len - mid;
         if mid <= k {
             unsafe { self.rotate_left_inner(mid) }
         } else {
@@ -2550,7 +2361,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
     #[stable(feature = "vecdeque_rotate", since = "1.36.0")]
     pub fn rotate_right(&mut self, k: usize) {
         assert!(k <= self.len());
-        let mid = self.len() - k;
+        let mid = self.len - k;
         if k <= mid {
             unsafe { self.rotate_right_inner(k) }
         } else {
@@ -2561,31 +2372,30 @@ impl<T, A: Allocator> VecDeque<T, A> {
     // SAFETY: the following two methods require that the rotation amount
     // be less than half the length of the deque.
     //
-    // `wrap_copy` requires that `min(x, cap() - x) + copy_len <= cap()`,
-    // but than `min` is never more than half the capacity, regardless of x,
+    // `wrap_copy` requires that `min(x, capacity() - x) + copy_len <= capacity()`,
+    // but then `min` is never more than half the capacity, regardless of x,
     // so it's sound to call here because we're calling with something
     // less than half the length, which is never above half the capacity.
 
     unsafe fn rotate_left_inner(&mut self, mid: usize) {
         debug_assert!(mid * 2 <= self.len());
         unsafe {
-            self.wrap_copy(self.head, self.tail, mid);
+            self.wrap_copy(self.head, self.to_physical_idx(self.len), mid);
         }
-        self.head = self.wrap_add(self.head, mid);
-        self.tail = self.wrap_add(self.tail, mid);
+        self.head = self.to_physical_idx(mid);
     }
 
     unsafe fn rotate_right_inner(&mut self, k: usize) {
         debug_assert!(k * 2 <= self.len());
         self.head = self.wrap_sub(self.head, k);
-        self.tail = self.wrap_sub(self.tail, k);
         unsafe {
-            self.wrap_copy(self.tail, self.head, k);
+            self.wrap_copy(self.to_physical_idx(self.len), self.head, k);
         }
     }
 
     /// Binary searches this `VecDeque` for a given element.
-    /// This behaves similarly to [`contains`] if this `VecDeque` is sorted.
+    /// If the `VecDeque` is not sorted, the returned result is unspecified and
+    /// meaningless.
     ///
     /// If the value is found then [`Result::Ok`] is returned, containing the
     /// index of the matching element. If there are multiple matches, then any
@@ -2595,7 +2405,6 @@ impl<T, A: Allocator> VecDeque<T, A> {
     ///
     /// See also [`binary_search_by`], [`binary_search_by_key`], and [`partition_point`].
     ///
-    /// [`contains`]: VecDeque::contains
     /// [`binary_search_by`]: VecDeque::binary_search_by
     /// [`binary_search_by_key`]: VecDeque::binary_search_by_key
     /// [`partition_point`]: VecDeque::partition_point
@@ -2641,12 +2450,13 @@ impl<T, A: Allocator> VecDeque<T, A> {
     }
 
     /// Binary searches this `VecDeque` with a comparator function.
-    /// This behaves similarly to [`contains`] if this `VecDeque` is sorted.
     ///
-    /// The comparator function should implement an order consistent
-    /// with the sort order of the deque, returning an order code that
-    /// indicates whether its argument is `Less`, `Equal` or `Greater`
-    /// than the desired target.
+    /// The comparator function should return an order code that indicates
+    /// whether its argument is `Less`, `Equal` or `Greater` the desired
+    /// target.
+    /// If the `VecDeque` is not sorted or if the comparator function does not
+    /// implement an order consistent with the sort order of the underlying
+    /// `VecDeque`, the returned result is unspecified and meaningless.
     ///
     /// If the value is found then [`Result::Ok`] is returned, containing the
     /// index of the matching element. If there are multiple matches, then any
@@ -2656,7 +2466,6 @@ impl<T, A: Allocator> VecDeque<T, A> {
     ///
     /// See also [`binary_search`], [`binary_search_by_key`], and [`partition_point`].
     ///
-    /// [`contains`]: VecDeque::contains
     /// [`binary_search`]: VecDeque::binary_search
     /// [`binary_search_by_key`]: VecDeque::binary_search_by_key
     /// [`partition_point`]: VecDeque::partition_point
@@ -2696,10 +2505,11 @@ impl<T, A: Allocator> VecDeque<T, A> {
     }
 
     /// Binary searches this `VecDeque` with a key extraction function.
-    /// This behaves similarly to [`contains`] if this `VecDeque` is sorted.
     ///
     /// Assumes that the deque is sorted by the key, for instance with
     /// [`make_contiguous().sort_by_key()`] using the same key extraction function.
+    /// If the deque is not sorted by the key, the returned result is
+    /// unspecified and meaningless.
     ///
     /// If the value is found then [`Result::Ok`] is returned, containing the
     /// index of the matching element. If there are multiple matches, then any
@@ -2709,7 +2519,6 @@ impl<T, A: Allocator> VecDeque<T, A> {
     ///
     /// See also [`binary_search`], [`binary_search_by`], and [`partition_point`].
     ///
-    /// [`contains`]: VecDeque::contains
     /// [`make_contiguous().sort_by_key()`]: VecDeque::make_contiguous
     /// [`binary_search`]: VecDeque::binary_search
     /// [`binary_search_by`]: VecDeque::binary_search_by
@@ -2751,7 +2560,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// The deque is assumed to be partitioned according to the given predicate.
     /// This means that all elements for which the predicate returns true are at the start of the deque
     /// and all elements for which the predicate returns false are at the end.
-    /// For example, [7, 15, 3, 5, 4, 12, 6] is a partitioned under the predicate x % 2 != 0
+    /// For example, `[7, 15, 3, 5, 4, 12, 6]` is partitioned under the predicate `x % 2 != 0`
     /// (all odd numbers are at the start, all even at the end).
     ///
     /// If the deque is not partitioned, the returned result is unspecified and meaningless,
@@ -2827,29 +2636,30 @@ impl<T: Clone, A: Allocator> VecDeque<T, A> {
     /// ```
     #[stable(feature = "deque_extras", since = "1.16.0")]
     pub fn resize(&mut self, new_len: usize, value: T) {
-        self.resize_with(new_len, || value.clone());
+        if new_len > self.len() {
+            let extra = new_len - self.len();
+            self.extend(repeat_n(value, extra))
+        } else {
+            self.truncate(new_len);
+        }
     }
 }
 
 /// Returns the index in the underlying buffer for a given logical element index.
 #[inline]
-fn wrap_index(index: usize, size: usize) -> usize {
-    // size is always a power of 2
-    debug_assert!(size.is_power_of_two());
-    index & (size - 1)
-}
-
-/// Calculate the number of elements left to be read in the buffer
-#[inline]
-fn count(tail: usize, head: usize, size: usize) -> usize {
-    // size is always a power of 2
-    (head.wrapping_sub(tail)) & (size - 1)
+fn wrap_index(logical_index: usize, capacity: usize) -> usize {
+    debug_assert!(
+        (logical_index == 0 && capacity == 0)
+            || logical_index < capacity
+            || (logical_index - capacity) < capacity
+    );
+    if logical_index >= capacity { logical_index - capacity } else { logical_index }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: PartialEq, A: Allocator> PartialEq for VecDeque<T, A> {
     fn eq(&self, other: &Self) -> bool {
-        if self.len() != other.len() {
+        if self.len != other.len() {
             return false;
         }
         let (sa, sb) = self.as_slices();
@@ -2913,7 +2723,7 @@ impl<T: Ord, A: Allocator> Ord for VecDeque<T, A> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Hash, A: Allocator> Hash for VecDeque<T, A> {
     fn hash<H: Hasher>(&self, state: &mut H) {
-        state.write_length_prefix(self.len());
+        state.write_length_prefix(self.len);
         // It's not possible to use Hash::hash_slice on slices
         // returned by as_slices method as their length can vary
         // in otherwise identical deques.
@@ -2945,11 +2755,7 @@ impl<T, A: Allocator> IndexMut<usize> for VecDeque<T, A> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> FromIterator<T> for VecDeque<T> {
     fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> VecDeque<T> {
-        let iterator = iter.into_iter();
-        let (lower, _) = iterator.size_hint();
-        let mut deq = VecDeque::with_capacity(lower);
-        deq.extend(iterator);
-        deq
+        SpecFromIter::spec_from_iter(iter.into_iter())
     }
 }
 
@@ -3009,7 +2815,7 @@ impl<'a, T: 'a + Copy, A: Allocator> Extend<&'a T> for VecDeque<T, A> {
     }
 
     #[inline]
-    fn extend_one(&mut self, &elem: &T) {
+    fn extend_one(&mut self, &elem: &'a T) {
         self.push_back(elem);
     }
 
@@ -3022,7 +2828,7 @@ impl<'a, T: 'a + Copy, A: Allocator> Extend<&'a T> for VecDeque<T, A> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: fmt::Debug, A: Allocator> fmt::Debug for VecDeque<T, A> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.debug_list().entries(self).finish()
+        f.debug_list().entries(self.iter()).finish()
     }
 }
 
@@ -3033,31 +2839,13 @@ impl<T, A: Allocator> From<Vec<T, A>> for VecDeque<T, A> {
     /// [`Vec<T>`]: crate::vec::Vec
     /// [`VecDeque<T>`]: crate::collections::VecDeque
     ///
-    /// This avoids reallocating where possible, but the conditions for that are
-    /// strict, and subject to change, and so shouldn't be relied upon unless the
-    /// `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 {
-            // 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");
-        } else {
-            // We need to resize if the capacity is not a power of two, too small or
-            // doesn't have at least one free space. We do this while it's still in
-            // the `Vec` so the items will drop on panic.
-            let min_cap = cmp::max(MINIMUM_CAPACITY, len) + 1;
-            let cap = cmp::max(min_cap, other.capacity()).next_power_of_two();
-            if other.capacity() != cap {
-                other.reserve_exact(cap - len);
-            }
-        }
-
-        unsafe {
-            let (other_buf, len, capacity, alloc) = other.into_raw_parts_with_alloc();
-            let buf = RawVec::from_raw_parts_in(other_buf, capacity, alloc);
-            VecDeque { tail: 0, head: len, buf }
-        }
+    /// This conversion is guaranteed to run in *O*(1) time
+    /// and to not re-allocate the `Vec`'s buffer or allocate
+    /// any additional memory.
+    #[inline]
+    fn from(other: Vec<T, A>) -> Self {
+        let (ptr, len, cap, alloc) = other.into_raw_parts_with_alloc();
+        Self { head: 0, len, buf: unsafe { RawVec::from_raw_parts_in(ptr, cap, alloc) } }
     }
 }
 
@@ -3099,11 +2887,11 @@ impl<T, A: Allocator> From<VecDeque<T, A>> for Vec<T, A> {
             let other = ManuallyDrop::new(other);
             let buf = other.buf.ptr();
             let len = other.len();
-            let cap = other.cap();
+            let cap = other.capacity();
             let alloc = ptr::read(other.allocator());
 
-            if other.tail != 0 {
-                ptr::copy(buf.add(other.tail), buf, len);
+            if other.head != 0 {
+                ptr::copy(buf.add(other.head), buf, len);
             }
             Vec::from_raw_parts_in(buf, len, cap, alloc)
         }
@@ -3124,14 +2912,14 @@ 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);
             }
         }
-        deq.tail = 0;
-        deq.head = N;
+        deq.head = 0;
+        deq.len = N;
         deq
     }
 }
diff --git a/library/alloc/src/collections/vec_deque/pair_slices.rs b/library/alloc/src/collections/vec_deque/pair_slices.rs
deleted file mode 100644
index 6735424a3ef..00000000000
--- a/library/alloc/src/collections/vec_deque/pair_slices.rs
+++ /dev/null
@@ -1,67 +0,0 @@
-use core::cmp::{self};
-use core::mem::replace;
-
-use crate::alloc::Allocator;
-
-use super::VecDeque;
-
-/// PairSlices pairs up equal length slice parts of two deques
-///
-/// For example, given deques "A" and "B" with the following division into slices:
-///
-/// A: [0 1 2] [3 4 5]
-/// B: [a b] [c d e]
-///
-/// It produces the following sequence of matching slices:
-///
-/// ([0 1], [a b])
-/// (\[2\], \[c\])
-/// ([3 4], [d e])
-///
-/// and the uneven remainder of either A or B is skipped.
-pub struct PairSlices<'a, 'b, T> {
-    a0: &'a mut [T],
-    a1: &'a mut [T],
-    b0: &'b [T],
-    b1: &'b [T],
-}
-
-impl<'a, 'b, T> PairSlices<'a, 'b, T> {
-    pub fn from<A: Allocator>(to: &'a mut VecDeque<T, A>, from: &'b VecDeque<T, A>) -> Self {
-        let (a0, a1) = to.as_mut_slices();
-        let (b0, b1) = from.as_slices();
-        PairSlices { a0, a1, b0, b1 }
-    }
-
-    pub fn has_remainder(&self) -> bool {
-        !self.b0.is_empty()
-    }
-
-    pub fn remainder(self) -> impl Iterator<Item = &'b [T]> {
-        IntoIterator::into_iter([self.b0, self.b1])
-    }
-}
-
-impl<'a, 'b, T> Iterator for PairSlices<'a, 'b, T> {
-    type Item = (&'a mut [T], &'b [T]);
-    fn next(&mut self) -> Option<Self::Item> {
-        // Get next part length
-        let part = cmp::min(self.a0.len(), self.b0.len());
-        if part == 0 {
-            return None;
-        }
-        let (p0, p1) = replace(&mut self.a0, &mut []).split_at_mut(part);
-        let (q0, q1) = self.b0.split_at(part);
-
-        // Move a1 into a0, if it's empty (and b1, b0 the same way).
-        self.a0 = p1;
-        self.b0 = q1;
-        if self.a0.is_empty() {
-            self.a0 = replace(&mut self.a1, &mut []);
-        }
-        if self.b0.is_empty() {
-            self.b0 = replace(&mut self.b1, &[]);
-        }
-        Some((p0, q0))
-    }
-}
diff --git a/library/alloc/src/collections/vec_deque/ring_slices.rs b/library/alloc/src/collections/vec_deque/ring_slices.rs
deleted file mode 100644
index dd0fa7d6074..00000000000
--- a/library/alloc/src/collections/vec_deque/ring_slices.rs
+++ /dev/null
@@ -1,56 +0,0 @@
-use core::ptr::{self};
-
-/// Returns the two slices that cover the `VecDeque`'s valid range
-pub trait RingSlices: Sized {
-    fn slice(self, from: usize, to: usize) -> Self;
-    fn split_at(self, i: usize) -> (Self, Self);
-
-    fn ring_slices(buf: Self, head: usize, tail: usize) -> (Self, Self) {
-        let contiguous = tail <= head;
-        if contiguous {
-            let (empty, buf) = buf.split_at(0);
-            (buf.slice(tail, head), empty)
-        } else {
-            let (mid, right) = buf.split_at(tail);
-            let (left, _) = mid.split_at(head);
-            (right, left)
-        }
-    }
-}
-
-impl<T> RingSlices for &[T] {
-    fn slice(self, from: usize, to: usize) -> Self {
-        &self[from..to]
-    }
-    fn split_at(self, i: usize) -> (Self, Self) {
-        (*self).split_at(i)
-    }
-}
-
-impl<T> RingSlices for &mut [T] {
-    fn slice(self, from: usize, to: usize) -> Self {
-        &mut self[from..to]
-    }
-    fn split_at(self, i: usize) -> (Self, Self) {
-        (*self).split_at_mut(i)
-    }
-}
-
-impl<T> RingSlices for *mut [T] {
-    fn slice(self, from: usize, to: usize) -> Self {
-        assert!(from <= to && to < self.len());
-        // Not using `get_unchecked_mut` to keep this a safe operation.
-        let len = to - from;
-        ptr::slice_from_raw_parts_mut(self.as_mut_ptr().wrapping_add(from), len)
-    }
-
-    fn split_at(self, mid: usize) -> (Self, Self) {
-        let len = self.len();
-        let ptr = self.as_mut_ptr();
-        assert!(mid <= len);
-        (
-            ptr::slice_from_raw_parts_mut(ptr, mid),
-            ptr::slice_from_raw_parts_mut(ptr.wrapping_add(mid), len - mid),
-        )
-    }
-}
diff --git a/library/alloc/src/collections/vec_deque/spec_extend.rs b/library/alloc/src/collections/vec_deque/spec_extend.rs
index 97ff8b76524..dccf40ccb38 100644
--- a/library/alloc/src/collections/vec_deque/spec_extend.rs
+++ b/library/alloc/src/collections/vec_deque/spec_extend.rs
@@ -1,6 +1,6 @@
 use crate::alloc::Allocator;
 use crate::vec;
-use core::iter::{ByRefSized, TrustedLen};
+use core::iter::TrustedLen;
 use core::slice;
 
 use super::VecDeque;
@@ -17,19 +17,33 @@ where
     default fn spec_extend(&mut self, mut iter: I) {
         // This function should be the moral equivalent of:
         //
-        //      for item in iter {
-        //          self.push_back(item);
-        //      }
-        while let Some(element) = iter.next() {
-            if self.len() == self.capacity() {
-                let (lower, _) = iter.size_hint();
-                self.reserve(lower.saturating_add(1));
-            }
+        // for item in iter {
+        //     self.push_back(item);
+        // }
+
+        // May only be called if `deque.len() < deque.capacity()`
+        unsafe fn push_unchecked<T, A: Allocator>(deque: &mut VecDeque<T, A>, element: T) {
+            // SAFETY: Because of the precondition, it's guaranteed that there is space
+            // in the logical array after the last element.
+            unsafe { deque.buffer_write(deque.to_physical_idx(deque.len), element) };
+            // This can't overflow because `deque.len() < deque.capacity() <= usize::MAX`.
+            deque.len += 1;
+        }
 
-            let head = self.head;
-            self.head = self.wrap_add(self.head, 1);
-            unsafe {
-                self.buffer_write(head, element);
+        while let Some(element) = iter.next() {
+            let (lower, _) = iter.size_hint();
+            self.reserve(lower.saturating_add(1));
+
+            // SAFETY: We just reserved space for at least one element.
+            unsafe { push_unchecked(self, element) };
+
+            // Inner loop to avoid repeatedly calling `reserve`.
+            while self.len < self.capacity() {
+                let Some(element) = iter.next() else {
+                    return;
+                };
+                // SAFETY: The loop condition guarantees that `self.len() < self.capacity()`.
+                unsafe { push_unchecked(self, element) };
             }
         }
     }
@@ -39,7 +53,7 @@ impl<T, I, A: Allocator> SpecExtend<T, I> for VecDeque<T, A>
 where
     I: TrustedLen<Item = T>,
 {
-    default fn spec_extend(&mut self, mut iter: I) {
+    default fn spec_extend(&mut self, iter: I) {
         // This is the case for a TrustedLen iterator.
         let (low, high) = iter.size_hint();
         if let Some(additional) = high {
@@ -51,35 +65,12 @@ where
             );
             self.reserve(additional);
 
-            struct WrapAddOnDrop<'a, T, A: Allocator> {
-                vec_deque: &'a mut VecDeque<T, A>,
-                written: usize,
-            }
-
-            impl<'a, T, A: Allocator> Drop for WrapAddOnDrop<'a, T, A> {
-                fn drop(&mut self) {
-                    self.vec_deque.head =
-                        self.vec_deque.wrap_add(self.vec_deque.head, self.written);
-                }
-            }
-
-            let mut wrapper = WrapAddOnDrop { vec_deque: self, written: 0 };
-
-            let head_room = wrapper.vec_deque.cap() - wrapper.vec_deque.head;
-            unsafe {
-                wrapper.vec_deque.write_iter(
-                    wrapper.vec_deque.head,
-                    ByRefSized(&mut iter).take(head_room),
-                    &mut wrapper.written,
-                );
-
-                if additional > head_room {
-                    wrapper.vec_deque.write_iter(0, iter, &mut wrapper.written);
-                }
-            }
+            let written = unsafe {
+                self.write_iter_wrapping(self.to_physical_idx(self.len), iter, additional)
+            };
 
             debug_assert_eq!(
-                additional, wrapper.written,
+                additional, written,
                 "The number of items written to VecDeque doesn't match the TrustedLen size hint"
             );
         } else {
@@ -99,8 +90,8 @@ impl<T, A: Allocator> SpecExtend<T, vec::IntoIter<T>> for VecDeque<T, A> {
         self.reserve(slice.len());
 
         unsafe {
-            self.copy_slice(self.head, slice);
-            self.head = self.wrap_add(self.head, slice.len());
+            self.copy_slice(self.to_physical_idx(self.len), slice);
+            self.len += slice.len();
         }
         iterator.forget_remaining_elements();
     }
@@ -125,8 +116,8 @@ where
         self.reserve(slice.len());
 
         unsafe {
-            self.copy_slice(self.head, slice);
-            self.head = self.wrap_add(self.head, slice.len());
+            self.copy_slice(self.to_physical_idx(self.len), slice);
+            self.len += slice.len();
         }
     }
 }
diff --git a/library/alloc/src/collections/vec_deque/spec_from_iter.rs b/library/alloc/src/collections/vec_deque/spec_from_iter.rs
new file mode 100644
index 00000000000..2708c7fe102
--- /dev/null
+++ b/library/alloc/src/collections/vec_deque/spec_from_iter.rs
@@ -0,0 +1,33 @@
+use super::{IntoIter, VecDeque};
+
+/// Specialization trait used for `VecDeque::from_iter`
+pub(super) trait SpecFromIter<T, I> {
+    fn spec_from_iter(iter: I) -> Self;
+}
+
+impl<T, I> SpecFromIter<T, I> for VecDeque<T>
+where
+    I: Iterator<Item = T>,
+{
+    default fn spec_from_iter(iterator: I) -> Self {
+        // Since converting is O(1) now, just re-use the `Vec` logic for
+        // anything where we can't do something extra-special for `VecDeque`,
+        // especially as that could save us some monomorphization work
+        // if one uses the same iterators (like slice ones) with both.
+        crate::vec::Vec::from_iter(iterator).into()
+    }
+}
+
+impl<T> SpecFromIter<T, crate::vec::IntoIter<T>> for VecDeque<T> {
+    #[inline]
+    fn spec_from_iter(iterator: crate::vec::IntoIter<T>) -> Self {
+        iterator.into_vecdeque()
+    }
+}
+
+impl<T> SpecFromIter<T, IntoIter<T>> for VecDeque<T> {
+    #[inline]
+    fn spec_from_iter(iterator: IntoIter<T>) -> Self {
+        iterator.into_vecdeque()
+    }
+}
diff --git a/library/alloc/src/collections/vec_deque/tests.rs b/library/alloc/src/collections/vec_deque/tests.rs
index 1f2daef213c..205a8ff3c19 100644
--- a/library/alloc/src/collections/vec_deque/tests.rs
+++ b/library/alloc/src/collections/vec_deque/tests.rs
@@ -3,7 +3,6 @@ use core::iter::TrustedLen;
 use super::*;
 
 #[bench]
-#[cfg_attr(miri, ignore)] // isolated Miri does not support benchmarks
 fn bench_push_back_100(b: &mut test::Bencher) {
     let mut deq = VecDeque::with_capacity(101);
     b.iter(|| {
@@ -11,12 +10,11 @@ fn bench_push_back_100(b: &mut test::Bencher) {
             deq.push_back(i);
         }
         deq.head = 0;
-        deq.tail = 0;
+        deq.len = 0;
     })
 }
 
 #[bench]
-#[cfg_attr(miri, ignore)] // isolated Miri does not support benchmarks
 fn bench_push_front_100(b: &mut test::Bencher) {
     let mut deq = VecDeque::with_capacity(101);
     b.iter(|| {
@@ -24,18 +22,21 @@ fn bench_push_front_100(b: &mut test::Bencher) {
             deq.push_front(i);
         }
         deq.head = 0;
-        deq.tail = 0;
+        deq.len = 0;
     })
 }
 
 #[bench]
-#[cfg_attr(miri, ignore)] // isolated Miri does not support benchmarks
 fn bench_pop_back_100(b: &mut test::Bencher) {
-    let mut deq = VecDeque::<i32>::with_capacity(101);
+    let size = 100;
+    let mut deq = VecDeque::<i32>::with_capacity(size + 1);
+    // We'll mess with private state to pretend like `deq` is filled.
+    // Make sure the buffer is initialized so that we don't read uninit memory.
+    unsafe { deq.ptr().write_bytes(0u8, size + 1) };
 
     b.iter(|| {
-        deq.head = 100;
-        deq.tail = 0;
+        deq.head = 0;
+        deq.len = 100;
         while !deq.is_empty() {
             test::black_box(deq.pop_back());
         }
@@ -43,9 +44,9 @@ fn bench_pop_back_100(b: &mut test::Bencher) {
 }
 
 #[bench]
-#[cfg_attr(miri, ignore)] // isolated Miri does not support benchmarks
 fn bench_retain_whole_10000(b: &mut test::Bencher) {
-    let v = (1..100000).collect::<VecDeque<u32>>();
+    let size = if cfg!(miri) { 1000 } else { 100000 };
+    let v = (1..size).collect::<VecDeque<u32>>();
 
     b.iter(|| {
         let mut v = v.clone();
@@ -54,9 +55,9 @@ fn bench_retain_whole_10000(b: &mut test::Bencher) {
 }
 
 #[bench]
-#[cfg_attr(miri, ignore)] // isolated Miri does not support benchmarks
 fn bench_retain_odd_10000(b: &mut test::Bencher) {
-    let v = (1..100000).collect::<VecDeque<u32>>();
+    let size = if cfg!(miri) { 1000 } else { 100000 };
+    let v = (1..size).collect::<VecDeque<u32>>();
 
     b.iter(|| {
         let mut v = v.clone();
@@ -65,24 +66,27 @@ fn bench_retain_odd_10000(b: &mut test::Bencher) {
 }
 
 #[bench]
-#[cfg_attr(miri, ignore)] // isolated Miri does not support benchmarks
 fn bench_retain_half_10000(b: &mut test::Bencher) {
-    let v = (1..100000).collect::<VecDeque<u32>>();
+    let size = if cfg!(miri) { 1000 } else { 100000 };
+    let v = (1..size).collect::<VecDeque<u32>>();
 
     b.iter(|| {
         let mut v = v.clone();
-        v.retain(|x| *x > 50000)
+        v.retain(|x| *x > size / 2)
     })
 }
 
 #[bench]
-#[cfg_attr(miri, ignore)] // isolated Miri does not support benchmarks
 fn bench_pop_front_100(b: &mut test::Bencher) {
-    let mut deq = VecDeque::<i32>::with_capacity(101);
+    let size = 100;
+    let mut deq = VecDeque::<i32>::with_capacity(size + 1);
+    // We'll mess with private state to pretend like `deq` is filled.
+    // Make sure the buffer is initialized so that we don't read uninit memory.
+    unsafe { deq.ptr().write_bytes(0u8, size + 1) };
 
     b.iter(|| {
-        deq.head = 100;
-        deq.tail = 0;
+        deq.head = 0;
+        deq.len = 100;
         while !deq.is_empty() {
             test::black_box(deq.pop_front());
         }
@@ -101,9 +105,9 @@ fn test_swap_front_back_remove() {
         for len in 0..final_len {
             let expected: VecDeque<_> =
                 if back { (0..len).collect() } else { (0..len).rev().collect() };
-            for tail_pos in 0..usable_cap {
-                tester.tail = tail_pos;
-                tester.head = tail_pos;
+            for head_pos in 0..usable_cap {
+                tester.head = head_pos;
+                tester.len = 0;
                 if back {
                     for i in 0..len * 2 {
                         tester.push_front(i);
@@ -120,8 +124,8 @@ fn test_swap_front_back_remove() {
                         assert_eq!(tester.swap_remove_front(idx), Some(len * 2 - 1 - i));
                     }
                 }
-                assert!(tester.tail < tester.cap());
-                assert!(tester.head < tester.cap());
+                assert!(tester.head <= tester.capacity());
+                assert!(tester.len <= tester.capacity());
                 assert_eq!(tester, expected);
             }
         }
@@ -146,18 +150,18 @@ fn test_insert() {
     for len in minlen..cap {
         // 0, 1, 2, .., len - 1
         let expected = (0..).take(len).collect::<VecDeque<_>>();
-        for tail_pos in 0..cap {
+        for head_pos in 0..cap {
             for to_insert in 0..len {
-                tester.tail = tail_pos;
-                tester.head = tail_pos;
+                tester.head = head_pos;
+                tester.len = 0;
                 for i in 0..len {
                     if i != to_insert {
                         tester.push_back(i);
                     }
                 }
                 tester.insert(to_insert, to_insert);
-                assert!(tester.tail < tester.cap());
-                assert!(tester.head < tester.cap());
+                assert!(tester.head <= tester.capacity());
+                assert!(tester.len <= tester.capacity());
                 assert_eq!(tester, expected);
             }
         }
@@ -253,13 +257,14 @@ fn test_swap_panic() {
 #[test]
 fn test_reserve_exact() {
     let mut tester: VecDeque<i32> = VecDeque::with_capacity(1);
-    assert!(tester.capacity() == 1);
+    assert_eq!(tester.capacity(), 1);
     tester.reserve_exact(50);
-    assert!(tester.capacity() >= 51);
+    assert_eq!(tester.capacity(), 50);
     tester.reserve_exact(40);
-    assert!(tester.capacity() >= 51);
+    // reserving won't shrink the buffer
+    assert_eq!(tester.capacity(), 50);
     tester.reserve_exact(200);
-    assert!(tester.capacity() >= 200);
+    assert_eq!(tester.capacity(), 200);
 }
 
 #[test]
@@ -319,6 +324,7 @@ fn test_contains() {
 #[test]
 fn test_rotate_left_right() {
     let mut tester: VecDeque<_> = (1..=10).collect();
+    tester.reserve(1);
 
     assert_eq!(tester.len(), 10);
 
@@ -459,7 +465,7 @@ fn test_binary_search_key() {
 }
 
 #[test]
-fn make_contiguous_big_tail() {
+fn make_contiguous_big_head() {
     let mut tester = VecDeque::with_capacity(15);
 
     for i in 0..3 {
@@ -474,14 +480,14 @@ fn make_contiguous_big_tail() {
     assert_eq!(tester.capacity(), 15);
     assert_eq!((&[9, 8, 7, 6, 5, 4, 3] as &[_], &[0, 1, 2] as &[_]), tester.as_slices());
 
-    let expected_start = tester.head;
+    let expected_start = tester.as_slices().1.len();
     tester.make_contiguous();
-    assert_eq!(tester.tail, expected_start);
+    assert_eq!(tester.head, expected_start);
     assert_eq!((&[9, 8, 7, 6, 5, 4, 3, 0, 1, 2] as &[_], &[] as &[_]), tester.as_slices());
 }
 
 #[test]
-fn make_contiguous_big_head() {
+fn make_contiguous_big_tail() {
     let mut tester = VecDeque::with_capacity(15);
 
     for i in 0..8 {
@@ -495,44 +501,46 @@ fn make_contiguous_big_head() {
     // 01234567......98
     let expected_start = 0;
     tester.make_contiguous();
-    assert_eq!(tester.tail, expected_start);
+    assert_eq!(tester.head, expected_start);
     assert_eq!((&[9, 8, 0, 1, 2, 3, 4, 5, 6, 7] as &[_], &[] as &[_]), tester.as_slices());
 }
 
 #[test]
 fn make_contiguous_small_free() {
-    let mut tester = VecDeque::with_capacity(15);
+    let mut tester = VecDeque::with_capacity(16);
 
-    for i in 'A' as u8..'I' as u8 {
+    for i in b'A'..b'I' {
         tester.push_back(i as char);
     }
 
-    for i in 'I' as u8..'N' as u8 {
+    for i in b'I'..b'N' {
         tester.push_front(i as char);
     }
 
+    assert_eq!(tester, ['M', 'L', 'K', 'J', 'I', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H']);
+
     // ABCDEFGH...MLKJI
     let expected_start = 0;
     tester.make_contiguous();
-    assert_eq!(tester.tail, expected_start);
+    assert_eq!(tester.head, expected_start);
     assert_eq!(
         (&['M', 'L', 'K', 'J', 'I', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H'] as &[_], &[] as &[_]),
         tester.as_slices()
     );
 
     tester.clear();
-    for i in 'I' as u8..'N' as u8 {
+    for i in b'I'..b'N' {
         tester.push_back(i as char);
     }
 
-    for i in 'A' as u8..'I' as u8 {
+    for i in b'A'..b'I' {
         tester.push_front(i as char);
     }
 
     // IJKLM...HGFEDCBA
-    let expected_start = 0;
+    let expected_start = 3;
     tester.make_contiguous();
-    assert_eq!(tester.tail, expected_start);
+    assert_eq!(tester.head, expected_start);
     assert_eq!(
         (&['H', 'G', 'F', 'E', 'D', 'C', 'B', 'A', 'I', 'J', 'K', 'L', 'M'] as &[_], &[] as &[_]),
         tester.as_slices()
@@ -541,16 +549,55 @@ fn make_contiguous_small_free() {
 
 #[test]
 fn make_contiguous_head_to_end() {
-    let mut dq = VecDeque::with_capacity(3);
-    dq.push_front('B');
-    dq.push_front('A');
-    dq.push_back('C');
-    dq.make_contiguous();
-    let expected_tail = 0;
-    let expected_head = 3;
-    assert_eq!(expected_tail, dq.tail);
-    assert_eq!(expected_head, dq.head);
-    assert_eq!((&['A', 'B', 'C'] as &[_], &[] as &[_]), dq.as_slices());
+    let mut tester = VecDeque::with_capacity(16);
+
+    for i in b'A'..b'L' {
+        tester.push_back(i as char);
+    }
+
+    for i in b'L'..b'Q' {
+        tester.push_front(i as char);
+    }
+
+    assert_eq!(
+        tester,
+        ['P', 'O', 'N', 'M', 'L', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K']
+    );
+
+    // ABCDEFGHIJKPONML
+    let expected_start = 0;
+    tester.make_contiguous();
+    assert_eq!(tester.head, expected_start);
+    assert_eq!(
+        (
+            &['P', 'O', 'N', 'M', 'L', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K']
+                as &[_],
+            &[] as &[_]
+        ),
+        tester.as_slices()
+    );
+
+    tester.clear();
+    for i in b'L'..b'Q' {
+        tester.push_back(i as char);
+    }
+
+    for i in b'A'..b'L' {
+        tester.push_front(i as char);
+    }
+
+    // LMNOPKJIHGFEDCBA
+    let expected_start = 0;
+    tester.make_contiguous();
+    assert_eq!(tester.head, expected_start);
+    assert_eq!(
+        (
+            &['K', 'J', 'I', 'H', 'G', 'F', 'E', 'D', 'C', 'B', 'A', 'L', 'M', 'N', 'O', 'P']
+                as &[_],
+            &[] as &[_]
+        ),
+        tester.as_slices()
+    );
 }
 
 #[test]
@@ -584,10 +631,10 @@ fn test_remove() {
     for len in minlen..cap - 1 {
         // 0, 1, 2, .., len - 1
         let expected = (0..).take(len).collect::<VecDeque<_>>();
-        for tail_pos in 0..cap {
+        for head_pos in 0..cap {
             for to_remove in 0..=len {
-                tester.tail = tail_pos;
-                tester.head = tail_pos;
+                tester.head = head_pos;
+                tester.len = 0;
                 for i in 0..len {
                     if i == to_remove {
                         tester.push_back(1234);
@@ -598,8 +645,8 @@ fn test_remove() {
                     tester.push_back(1234);
                 }
                 tester.remove(to_remove);
-                assert!(tester.tail < tester.cap());
-                assert!(tester.head < tester.cap());
+                assert!(tester.head <= tester.capacity());
+                assert!(tester.len <= tester.capacity());
                 assert_eq!(tester, expected);
             }
         }
@@ -613,11 +660,11 @@ fn test_range() {
     let cap = tester.capacity();
     let minlen = if cfg!(miri) { cap - 1 } else { 0 }; // Miri is too slow
     for len in minlen..=cap {
-        for tail in 0..=cap {
+        for head in 0..=cap {
             for start in 0..=len {
                 for end in start..=len {
-                    tester.tail = tail;
-                    tester.head = tail;
+                    tester.head = head;
+                    tester.len = 0;
                     for i in 0..len {
                         tester.push_back(i);
                     }
@@ -638,17 +685,17 @@ fn test_range_mut() {
 
     let cap = tester.capacity();
     for len in 0..=cap {
-        for tail in 0..=cap {
+        for head in 0..=cap {
             for start in 0..=len {
                 for end in start..=len {
-                    tester.tail = tail;
-                    tester.head = tail;
+                    tester.head = head;
+                    tester.len = 0;
                     for i in 0..len {
                         tester.push_back(i);
                     }
 
                     let head_was = tester.head;
-                    let tail_was = tester.tail;
+                    let len_was = tester.len;
 
                     // Check that we iterate over the correct values
                     let range: VecDeque<_> = tester.range_mut(start..end).map(|v| *v).collect();
@@ -658,8 +705,8 @@ fn test_range_mut() {
                     // We shouldn't have changed the capacity or made the
                     // head or tail out of bounds
                     assert_eq!(tester.capacity(), cap);
-                    assert_eq!(tester.tail, tail_was);
                     assert_eq!(tester.head, head_was);
+                    assert_eq!(tester.len, len_was);
                 }
             }
         }
@@ -672,11 +719,11 @@ fn test_drain() {
 
     let cap = tester.capacity();
     for len in 0..=cap {
-        for tail in 0..=cap {
+        for head in 0..cap {
             for drain_start in 0..=len {
                 for drain_end in drain_start..=len {
-                    tester.tail = tail;
-                    tester.head = tail;
+                    tester.head = head;
+                    tester.len = 0;
                     for i in 0..len {
                         tester.push_back(i);
                     }
@@ -689,8 +736,8 @@ fn test_drain() {
                     // We shouldn't have changed the capacity or made the
                     // head or tail out of bounds
                     assert_eq!(tester.capacity(), cap);
-                    assert!(tester.tail < tester.cap());
-                    assert!(tester.head < tester.cap());
+                    assert!(tester.head <= tester.capacity());
+                    assert!(tester.len <= tester.capacity());
 
                     // We should see the correct values in the VecDeque
                     let expected: VecDeque<_> = (0..drain_start).chain(drain_end..len).collect();
@@ -702,6 +749,48 @@ fn test_drain() {
 }
 
 #[test]
+fn issue_108453() {
+    let mut deque = VecDeque::with_capacity(10);
+
+    deque.push_back(1u8);
+    deque.push_back(2);
+    deque.push_back(3);
+
+    deque.push_front(10);
+    deque.push_front(9);
+
+    deque.shrink_to(9);
+
+    assert_eq!(deque.into_iter().collect::<Vec<_>>(), vec![9, 10, 1, 2, 3]);
+}
+
+#[test]
+fn test_shrink_to() {
+    // test deques with capacity 16 with all possible head positions, lengths and target capacities.
+    let cap = 16;
+
+    for len in 0..cap {
+        for head in 0..cap {
+            let expected = (1..=len).collect::<VecDeque<_>>();
+
+            for target_cap in len..cap {
+                let mut deque = VecDeque::with_capacity(cap);
+                // currently, `with_capacity` always allocates the exact capacity if it's greater than 8.
+                assert_eq!(deque.capacity(), cap);
+
+                // we can let the head point anywhere in the buffer since the deque is empty.
+                deque.head = head;
+                deque.extend(1..=len);
+
+                deque.shrink_to(target_cap);
+
+                assert_eq!(deque, expected);
+            }
+        }
+    }
+}
+
+#[test]
 fn test_shrink_to_fit() {
     // This test checks that every single combination of head and tail position,
     // is tested. Capacity 15 should be large enough to cover every case.
@@ -717,17 +806,18 @@ fn test_shrink_to_fit() {
     for len in 0..=cap {
         // 0, 1, 2, .., len - 1
         let expected = (0..).take(len).collect::<VecDeque<_>>();
-        for tail_pos in 0..=max_cap {
-            tester.tail = tail_pos;
-            tester.head = tail_pos;
+        for head_pos in 0..=max_cap {
+            tester.reserve(head_pos);
+            tester.head = head_pos;
+            tester.len = 0;
             tester.reserve(63);
             for i in 0..len {
                 tester.push_back(i);
             }
             tester.shrink_to_fit();
             assert!(tester.capacity() <= cap);
-            assert!(tester.tail < tester.cap());
-            assert!(tester.head < tester.cap());
+            assert!(tester.head <= tester.capacity());
+            assert!(tester.len <= tester.capacity());
             assert_eq!(tester, expected);
         }
     }
@@ -754,17 +844,17 @@ fn test_split_off() {
             // at, at + 1, .., len - 1 (may be empty)
             let expected_other = (at..).take(len - at).collect::<VecDeque<_>>();
 
-            for tail_pos in 0..cap {
-                tester.tail = tail_pos;
-                tester.head = tail_pos;
+            for head_pos in 0..cap {
+                tester.head = head_pos;
+                tester.len = 0;
                 for i in 0..len {
                     tester.push_back(i);
                 }
                 let result = tester.split_off(at);
-                assert!(tester.tail < tester.cap());
-                assert!(tester.head < tester.cap());
-                assert!(result.tail < result.cap());
-                assert!(result.head < result.cap());
+                assert!(tester.head <= tester.capacity());
+                assert!(tester.len <= tester.capacity());
+                assert!(result.head <= result.capacity());
+                assert!(result.len <= result.capacity());
                 assert_eq!(tester, expected_self);
                 assert_eq!(result, expected_other);
             }
@@ -781,16 +871,10 @@ fn test_from_vec() {
             vec.extend(0..len);
 
             let vd = VecDeque::from(vec.clone());
-            assert!(vd.cap().is_power_of_two());
             assert_eq!(vd.len(), vec.len());
             assert!(vd.into_iter().eq(vec));
         }
     }
-
-    let vec = Vec::from([(); MAXIMUM_ZST_CAPACITY - 1]);
-    let vd = VecDeque::from(vec.clone());
-    assert!(vd.cap().is_power_of_two());
-    assert_eq!(vd.len(), vec.len());
 }
 
 #[test]
@@ -842,10 +926,6 @@ fn test_extend_impl(trusted_len: bool) {
             }
 
             assert_eq!(self.test, self.expected);
-            let (a1, b1) = self.test.as_slices();
-            let (a2, b2) = self.expected.as_slices();
-            assert_eq!(a1, a2);
-            assert_eq!(b1, b2);
         }
 
         fn drain<R: RangeBounds<usize> + Clone>(&mut self, range: R) {
@@ -868,7 +948,7 @@ fn test_extend_impl(trusted_len: bool) {
     let mut tester = VecDequeTester::new(trusted_len);
 
     // Initial capacity
-    tester.test_extend(0..tester.remaining_capacity() - 1);
+    tester.test_extend(0..tester.remaining_capacity());
 
     // Grow
     tester.test_extend(1024..2048);
@@ -876,7 +956,7 @@ fn test_extend_impl(trusted_len: bool) {
     // Wrap around
     tester.drain(..128);
 
-    tester.test_extend(0..tester.remaining_capacity() - 1);
+    tester.test_extend(0..tester.remaining_capacity());
 
     // Continue
     tester.drain(256..);
@@ -889,16 +969,6 @@ fn test_extend_impl(trusted_len: bool) {
 }
 
 #[test]
-#[should_panic = "capacity overflow"]
-fn test_from_vec_zst_overflow() {
-    use crate::vec::Vec;
-    let vec = Vec::from([(); MAXIMUM_ZST_CAPACITY]);
-    let vd = VecDeque::from(vec.clone()); // no room for +1
-    assert!(vd.cap().is_power_of_two());
-    assert_eq!(vd.len(), vec.len());
-}
-
-#[test]
 fn test_from_array() {
     fn test<const N: usize>() {
         let mut array: [usize; N] = [0; N];
@@ -913,7 +983,6 @@ fn test_from_array() {
             assert_eq!(deq[i], i);
         }
 
-        assert!(deq.cap().is_power_of_two());
         assert_eq!(deq.len(), N);
     }
     test::<0>();
@@ -921,11 +990,6 @@ fn test_from_array() {
     test::<2>();
     test::<32>();
     test::<35>();
-
-    let array = [(); MAXIMUM_ZST_CAPACITY - 1];
-    let deq = VecDeque::from(array);
-    assert!(deq.cap().is_power_of_two());
-    assert_eq!(deq.len(), MAXIMUM_ZST_CAPACITY - 1);
 }
 
 #[test]