Merging upstream version 1.72.1+dfsg1.

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

6 files changed, 3902 insertions, 0 deletions

diff --git a/vendor/allocator-api2/src/stable/vec/drain.rs b/vendor/allocator-api2/src/stable/vec/drain.rs
new file mode 100644
index 000000000..de7e3906c
--- /dev/null
+++ b/vendor/allocator-api2/src/stable/vec/drain.rs
@@ -0,0 +1,242 @@
+use core::fmt;
+use core::iter::FusedIterator;
+use core::mem::{self, size_of, ManuallyDrop};
+use core::ptr::{self, NonNull};
+use core::slice::{self};
+
+use crate::stable::alloc::{Allocator, Global};
+
+use super::Vec;
+
+/// A draining iterator for `Vec<T>`.
+///
+/// This `struct` is created by [`Vec::drain`].
+/// See its documentation for more.
+///
+/// # Example
+///
+/// ```
+/// let mut v = vec![0, 1, 2];
+/// let iter: std::vec::Drain<_> = v.drain(..);
+/// ```
+pub struct Drain<'a, T: 'a, A: Allocator + 'a = Global> {
+    /// Index of tail to preserve
+    pub(super) tail_start: usize,
+    /// Length of tail
+    pub(super) tail_len: usize,
+    /// Current remaining range to remove
+    pub(super) iter: slice::Iter<'a, T>,
+    pub(super) vec: NonNull<Vec<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.iter.as_slice()).finish()
+    }
+}
+
+impl<'a, T, A: Allocator> Drain<'a, T, A> {
+    /// Returns the remaining items of this iterator as a slice.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = vec!['a', 'b', 'c'];
+    /// let mut drain = vec.drain(..);
+    /// assert_eq!(drain.as_slice(), &['a', 'b', 'c']);
+    /// let _ = drain.next().unwrap();
+    /// assert_eq!(drain.as_slice(), &['b', 'c']);
+    /// ```
+    #[must_use]
+    #[inline(always)]
+    pub fn as_slice(&self) -> &[T] {
+        self.iter.as_slice()
+    }
+
+    /// Returns a reference to the underlying allocator.
+    #[must_use]
+    #[inline(always)]
+    pub fn allocator(&self) -> &A {
+        unsafe { self.vec.as_ref().allocator() }
+    }
+
+    /// Keep unyielded elements in the source `Vec`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(drain_keep_rest)]
+    ///
+    /// let mut vec = vec!['a', 'b', 'c'];
+    /// let mut drain = vec.drain(..);
+    ///
+    /// assert_eq!(drain.next().unwrap(), 'a');
+    ///
+    /// // This call keeps 'b' and 'c' in the vec.
+    /// drain.keep_rest();
+    ///
+    /// // If we wouldn't call `keep_rest()`,
+    /// // `vec` would be empty.
+    /// assert_eq!(vec, ['b', 'c']);
+    /// ```
+    #[inline(always)]
+    pub fn keep_rest(self) {
+        // At this moment layout looks like this:
+        //
+        // [head] [yielded by next] [unyielded] [yielded by next_back] [tail]
+        //        ^-- start         \_________/-- unyielded_len        \____/-- self.tail_len
+        //                          ^-- unyielded_ptr                  ^-- tail
+        //
+        // Normally `Drop` impl would drop [unyielded] and then move [tail] to the `start`.
+        // Here we want to
+        // 1. Move [unyielded] to `start`
+        // 2. Move [tail] to a new start at `start + len(unyielded)`
+        // 3. Update length of the original vec to `len(head) + len(unyielded) + len(tail)`
+        //    a. In case of ZST, this is the only thing we want to do
+        // 4. Do *not* drop self, as everything is put in a consistent state already, there is nothing to do
+        let mut this = ManuallyDrop::new(self);
+
+        unsafe {
+            let source_vec = this.vec.as_mut();
+
+            let start = source_vec.len();
+            let tail = this.tail_start;
+
+            let unyielded_len = this.iter.len();
+            let unyielded_ptr = this.iter.as_slice().as_ptr();
+
+            // ZSTs have no identity, so we don't need to move them around.
+            let needs_move = mem::size_of::<T>() != 0;
+
+            if needs_move {
+                let start_ptr = source_vec.as_mut_ptr().add(start);
+
+                // memmove back unyielded elements
+                if unyielded_ptr != start_ptr {
+                    let src = unyielded_ptr;
+                    let dst = start_ptr;
+
+                    ptr::copy(src, dst, unyielded_len);
+                }
+
+                // memmove back untouched tail
+                if tail != (start + unyielded_len) {
+                    let src = source_vec.as_ptr().add(tail);
+                    let dst = start_ptr.add(unyielded_len);
+                    ptr::copy(src, dst, this.tail_len);
+                }
+            }
+
+            source_vec.set_len(start + unyielded_len + this.tail_len);
+        }
+    }
+}
+
+impl<'a, T, A: Allocator> AsRef<[T]> for Drain<'a, T, A> {
+    #[inline(always)]
+    fn as_ref(&self) -> &[T] {
+        self.as_slice()
+    }
+}
+
+unsafe impl<T: Sync, A: Sync + Allocator> Sync for Drain<'_, T, A> {}
+
+unsafe impl<T: Send, A: Send + Allocator> Send for Drain<'_, T, A> {}
+
+impl<T, A: Allocator> Iterator for Drain<'_, T, A> {
+    type Item = T;
+
+    #[inline(always)]
+    fn next(&mut self) -> Option<T> {
+        self.iter
+            .next()
+            .map(|elt| unsafe { ptr::read(elt as *const _) })
+    }
+
+    #[inline(always)]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
+}
+
+impl<T, A: Allocator> DoubleEndedIterator for Drain<'_, T, A> {
+    #[inline(always)]
+    fn next_back(&mut self) -> Option<T> {
+        self.iter
+            .next_back()
+            .map(|elt| unsafe { ptr::read(elt as *const _) })
+    }
+}
+
+impl<T, A: Allocator> Drop for Drain<'_, T, A> {
+    #[inline]
+    fn drop(&mut self) {
+        /// Moves back the un-`Drain`ed elements to restore the original `Vec`.
+        struct DropGuard<'r, 'a, T, A: Allocator>(&'r mut Drain<'a, T, A>);
+
+        impl<'r, 'a, T, A: Allocator> Drop for DropGuard<'r, 'a, T, A> {
+            fn drop(&mut self) {
+                if self.0.tail_len > 0 {
+                    unsafe {
+                        let source_vec = self.0.vec.as_mut();
+                        // memmove back untouched tail, update to new length
+                        let start = source_vec.len();
+                        let tail = self.0.tail_start;
+                        if tail != start {
+                            let src = source_vec.as_ptr().add(tail);
+                            let dst = source_vec.as_mut_ptr().add(start);
+                            ptr::copy(src, dst, self.0.tail_len);
+                        }
+                        source_vec.set_len(start + self.0.tail_len);
+                    }
+                }
+            }
+        }
+
+        let iter = mem::replace(&mut self.iter, [].iter());
+        let drop_len = iter.len();
+
+        let mut vec = self.vec;
+
+        if size_of::<T>() == 0 {
+            // ZSTs have no identity, so we don't need to move them around, we only need to drop the correct amount.
+            // this can be achieved by manipulating the Vec length instead of moving values out from `iter`.
+            unsafe {
+                let vec = vec.as_mut();
+                let old_len = vec.len();
+                vec.set_len(old_len + drop_len + self.tail_len);
+                vec.truncate(old_len + self.tail_len);
+            }
+
+            return;
+        }
+
+        // ensure elements are moved back into their appropriate places, even when drop_in_place panics
+        let _guard = DropGuard(self);
+
+        if drop_len == 0 {
+            return;
+        }
+
+        // as_slice() must only be called when iter.len() is > 0 because
+        // vec::Splice modifies vec::Drain fields and may grow the vec which would invalidate
+        // the iterator's internal pointers. Creating a reference to deallocated memory
+        // is invalid even when it is zero-length
+        let drop_ptr = iter.as_slice().as_ptr();
+
+        unsafe {
+            // drop_ptr comes from a slice::Iter which only gives us a &[T] but for drop_in_place
+            // a pointer with mutable provenance is necessary. Therefore we must reconstruct
+            // it from the original vec but also avoid creating a &mut to the front since that could
+            // invalidate raw pointers to it which some unsafe code might rely on.
+            let vec_ptr = vec.as_mut().as_mut_ptr();
+            let drop_offset = drop_ptr.offset_from(vec_ptr) as usize;
+            let to_drop = ptr::slice_from_raw_parts_mut(vec_ptr.add(drop_offset), drop_len);
+            ptr::drop_in_place(to_drop);
+        }
+    }
+}
+
+impl<T, A: Allocator> ExactSizeIterator for Drain<'_, T, A> {}
+
+impl<T, A: Allocator> FusedIterator for Drain<'_, T, A> {}
diff --git a/vendor/allocator-api2/src/stable/vec/into_iter.rs b/vendor/allocator-api2/src/stable/vec/into_iter.rs
new file mode 100644
index 000000000..464702afd
--- /dev/null
+++ b/vendor/allocator-api2/src/stable/vec/into_iter.rs
@@ -0,0 +1,198 @@
+use core::fmt;
+use core::iter::FusedIterator;
+use core::marker::PhantomData;
+use core::mem::{self, size_of, ManuallyDrop};
+
+use core::ptr::{self, NonNull};
+use core::slice::{self};
+
+use crate::stable::addr;
+
+use super::{Allocator, Global, RawVec};
+
+#[cfg(not(no_global_oom_handling))]
+use super::Vec;
+
+/// An iterator that moves out of a vector.
+///
+/// This `struct` is created by the `into_iter` method on [`Vec`](super::Vec)
+/// (provided by the [`IntoIterator`] trait).
+///
+/// # Example
+///
+/// ```
+/// let v = vec![0, 1, 2];
+/// let iter: std::vec::IntoIter<_> = v.into_iter();
+/// ```
+pub struct IntoIter<T, A: Allocator = Global> {
+    pub(super) buf: NonNull<T>,
+    pub(super) phantom: PhantomData<T>,
+    pub(super) cap: usize,
+    // the drop impl reconstructs a RawVec from buf, cap and alloc
+    // to avoid dropping the allocator twice we need to wrap it into ManuallyDrop
+    pub(super) alloc: ManuallyDrop<A>,
+    pub(super) ptr: *const T,
+    pub(super) end: *const 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.as_slice()).finish()
+    }
+}
+
+impl<T, A: Allocator> IntoIter<T, A> {
+    /// Returns the remaining items of this iterator as a slice.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let vec = vec!['a', 'b', 'c'];
+    /// let mut into_iter = vec.into_iter();
+    /// assert_eq!(into_iter.as_slice(), &['a', 'b', 'c']);
+    /// let _ = into_iter.next().unwrap();
+    /// assert_eq!(into_iter.as_slice(), &['b', 'c']);
+    /// ```
+    pub fn as_slice(&self) -> &[T] {
+        unsafe { slice::from_raw_parts(self.ptr, self.len()) }
+    }
+
+    /// Returns the remaining items of this iterator as a mutable slice.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let vec = vec!['a', 'b', 'c'];
+    /// let mut into_iter = vec.into_iter();
+    /// assert_eq!(into_iter.as_slice(), &['a', 'b', 'c']);
+    /// into_iter.as_mut_slice()[2] = 'z';
+    /// assert_eq!(into_iter.next().unwrap(), 'a');
+    /// assert_eq!(into_iter.next().unwrap(), 'b');
+    /// assert_eq!(into_iter.next().unwrap(), 'z');
+    /// ```
+    pub fn as_mut_slice(&mut self) -> &mut [T] {
+        unsafe { &mut *self.as_raw_mut_slice() }
+    }
+
+    /// Returns a reference to the underlying allocator.
+    #[inline(always)]
+    pub fn allocator(&self) -> &A {
+        &self.alloc
+    }
+
+    fn as_raw_mut_slice(&mut self) -> *mut [T] {
+        ptr::slice_from_raw_parts_mut(self.ptr as *mut T, self.len())
+    }
+}
+
+impl<T, A: Allocator> AsRef<[T]> for IntoIter<T, A> {
+    fn as_ref(&self) -> &[T] {
+        self.as_slice()
+    }
+}
+
+unsafe impl<T: Send, A: Allocator + Send> Send for IntoIter<T, A> {}
+
+unsafe impl<T: Sync, A: Allocator + Sync> Sync for IntoIter<T, A> {}
+
+impl<T, A: Allocator> Iterator for IntoIter<T, A> {
+    type Item = T;
+
+    #[inline(always)]
+    fn next(&mut self) -> Option<T> {
+        if self.ptr == self.end {
+            None
+        } else if size_of::<T>() == 0 {
+            // purposefully don't use 'ptr.offset' because for
+            // vectors with 0-size elements this would return the
+            // same pointer.
+            self.ptr = self.ptr.cast::<u8>().wrapping_add(1).cast();
+
+            // Make up a value of this ZST.
+            Some(unsafe { mem::zeroed() })
+        } else {
+            let old = self.ptr;
+            self.ptr = unsafe { self.ptr.add(1) };
+
+            Some(unsafe { ptr::read(old) })
+        }
+    }
+
+    #[inline(always)]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let exact = if size_of::<T>() == 0 {
+            addr(self.end).wrapping_sub(addr(self.ptr))
+        } else {
+            unsafe { self.end.offset_from(self.ptr) as usize }
+        };
+        (exact, Some(exact))
+    }
+
+    #[inline(always)]
+    fn count(self) -> usize {
+        self.len()
+    }
+}
+
+impl<T, A: Allocator> DoubleEndedIterator for IntoIter<T, A> {
+    #[inline(always)]
+    fn next_back(&mut self) -> Option<T> {
+        if self.end == self.ptr {
+            None
+        } else if size_of::<T>() == 0 {
+            // See above for why 'ptr.offset' isn't used
+            self.end = self.end.cast::<u8>().wrapping_add(1).cast();
+
+            // Make up a value of this ZST.
+            Some(unsafe { mem::zeroed() })
+        } else {
+            self.end = unsafe { self.end.sub(1) };
+
+            Some(unsafe { ptr::read(self.end) })
+        }
+    }
+}
+
+impl<T, A: Allocator> ExactSizeIterator for IntoIter<T, A> {}
+
+impl<T, A: Allocator> FusedIterator for IntoIter<T, A> {}
+
+#[doc(hidden)]
+pub trait NonDrop {}
+
+// T: Copy as approximation for !Drop since get_unchecked does not advance self.ptr
+// and thus we can't implement drop-handling
+impl<T: Copy> NonDrop for T {}
+
+#[cfg(not(no_global_oom_handling))]
+impl<T: Clone, A: Allocator + Clone> Clone for IntoIter<T, A> {
+    fn clone(&self) -> Self {
+        let mut vec = Vec::<T, A>::with_capacity_in(self.len(), (*self.alloc).clone());
+        vec.extend(self.as_slice().iter().cloned());
+        vec.into_iter()
+    }
+}
+
+impl<T, A: Allocator> Drop for IntoIter<T, A> {
+    fn drop(&mut self) {
+        struct DropGuard<'a, T, A: Allocator>(&'a mut IntoIter<T, A>);
+
+        impl<T, A: Allocator> Drop for DropGuard<'_, T, A> {
+            fn drop(&mut self) {
+                unsafe {
+                    // `IntoIter::alloc` is not used anymore after this and will be dropped by RawVec
+                    let alloc = ManuallyDrop::take(&mut self.0.alloc);
+                    // RawVec handles deallocation
+                    let _ = RawVec::from_raw_parts_in(self.0.buf.as_ptr(), self.0.cap, alloc);
+                }
+            }
+        }
+
+        let guard = DropGuard(self);
+        // destroy the remaining elements
+        unsafe {
+            ptr::drop_in_place(guard.0.as_raw_mut_slice());
+        }
+        // now `guard` will be dropped and do the rest
+    }
+}
diff --git a/vendor/allocator-api2/src/stable/vec/mod.rs b/vendor/allocator-api2/src/stable/vec/mod.rs
new file mode 100644
index 000000000..8b7ab4b12
--- /dev/null
+++ b/vendor/allocator-api2/src/stable/vec/mod.rs
@@ -0,0 +1,3253 @@
+//! A contiguous growable array type with heap-allocated contents, written
+//! `Vec<T>`.
+//!
+//! Vectors have *O*(1) indexing, amortized *O*(1) push (to the end) and
+//! *O*(1) pop (from the end).
+//!
+//! Vectors ensure they never allocate more than `isize::MAX` bytes.
+//!
+//! # Examples
+//!
+//! You can explicitly create a [`Vec`] with [`Vec::new`]:
+//!
+//! ```
+//! let v: Vec<i32> = Vec::new();
+//! ```
+//!
+//! ...or by using the [`vec!`] macro:
+//!
+//! ```
+//! let v: Vec<i32> = vec![];
+//!
+//! let v = vec![1, 2, 3, 4, 5];
+//!
+//! let v = vec![0; 10]; // ten zeroes
+//! ```
+//!
+//! You can [`push`] values onto the end of a vector (which will grow the vector
+//! as needed):
+//!
+//! ```
+//! let mut v = vec![1, 2];
+//!
+//! v.push(3);
+//! ```
+//!
+//! Popping values works in much the same way:
+//!
+//! ```
+//! let mut v = vec![1, 2];
+//!
+//! let two = v.pop();
+//! ```
+//!
+//! Vectors also support indexing (through the [`Index`] and [`IndexMut`] traits):
+//!
+//! ```
+//! let mut v = vec![1, 2, 3];
+//! let three = v[2];
+//! v[1] = v[1] + 5;
+//! ```
+//!
+//! [`push`]: Vec::push
+
+#[cfg(not(no_global_oom_handling))]
+use core::cmp;
+use core::cmp::Ordering;
+use core::convert::TryFrom;
+use core::fmt;
+use core::hash::{Hash, Hasher};
+#[cfg(not(no_global_oom_handling))]
+use core::iter;
+#[cfg(not(no_global_oom_handling))]
+use core::iter::FromIterator;
+use core::marker::PhantomData;
+use core::mem::{self, size_of, ManuallyDrop, MaybeUninit};
+use core::ops::{self, Bound, Index, IndexMut, Range, RangeBounds};
+use core::ptr::{self, NonNull};
+use core::slice::{self, SliceIndex};
+
+use super::{
+    alloc::{Allocator, Global},
+    assume,
+    boxed::Box,
+    raw_vec::{RawVec, TryReserveError},
+};
+
+#[cfg(not(no_global_oom_handling))]
+pub use self::splice::Splice;
+
+#[cfg(not(no_global_oom_handling))]
+mod splice;
+
+pub use self::drain::Drain;
+
+mod drain;
+
+pub use self::into_iter::IntoIter;
+
+mod into_iter;
+
+mod partial_eq;
+
+#[cfg(not(no_global_oom_handling))]
+mod set_len_on_drop;
+
+#[cfg(not(no_global_oom_handling))]
+use self::set_len_on_drop::SetLenOnDrop;
+
+/// A contiguous growable array type, written as `Vec<T>`, short for 'vector'.
+///
+/// # Examples
+///
+/// ```
+/// let mut vec = Vec::new();
+/// vec.push(1);
+/// vec.push(2);
+///
+/// assert_eq!(vec.len(), 2);
+/// assert_eq!(vec[0], 1);
+///
+/// assert_eq!(vec.pop(), Some(2));
+/// assert_eq!(vec.len(), 1);
+///
+/// vec[0] = 7;
+/// assert_eq!(vec[0], 7);
+///
+/// vec.extend([1, 2, 3].iter().copied());
+///
+/// for x in &vec {
+///     println!("{x}");
+/// }
+/// assert_eq!(vec, [7, 1, 2, 3]);
+/// ```
+///
+/// The [`vec!`] macro is provided for convenient initialization:
+///
+/// ```
+/// let mut vec1 = vec![1, 2, 3];
+/// vec1.push(4);
+/// let vec2 = Vec::from([1, 2, 3, 4]);
+/// assert_eq!(vec1, vec2);
+/// ```
+///
+/// It can also initialize each element of a `Vec<T>` with a given value.
+/// This may be more efficient than performing allocation and initialization
+/// in separate steps, especially when initializing a vector of zeros:
+///
+/// ```
+/// let vec = vec![0; 5];
+/// assert_eq!(vec, [0, 0, 0, 0, 0]);
+///
+/// // The following is equivalent, but potentially slower:
+/// let mut vec = Vec::with_capacity(5);
+/// vec.resize(5, 0);
+/// assert_eq!(vec, [0, 0, 0, 0, 0]);
+/// ```
+///
+/// For more information, see
+/// [Capacity and Reallocation](#capacity-and-reallocation).
+///
+/// Use a `Vec<T>` as an efficient stack:
+///
+/// ```
+/// let mut stack = Vec::new();
+///
+/// stack.push(1);
+/// stack.push(2);
+/// stack.push(3);
+///
+/// while let Some(top) = stack.pop() {
+///     // Prints 3, 2, 1
+///     println!("{top}");
+/// }
+/// ```
+///
+/// # Indexing
+///
+/// The `Vec` type allows to access values by index, because it implements the
+/// [`Index`] trait. An example will be more explicit:
+///
+/// ```
+/// let v = vec![0, 2, 4, 6];
+/// println!("{}", v[1]); // it will display '2'
+/// ```
+///
+/// However be careful: if you try to access an index which isn't in the `Vec`,
+/// your software will panic! You cannot do this:
+///
+/// ```should_panic
+/// let v = vec![0, 2, 4, 6];
+/// println!("{}", v[6]); // it will panic!
+/// ```
+///
+/// Use [`get`] and [`get_mut`] if you want to check whether the index is in
+/// the `Vec`.
+///
+/// # Slicing
+///
+/// A `Vec` can be mutable. On the other hand, slices are read-only objects.
+/// To get a [slice][prim@slice], use [`&`]. Example:
+///
+/// ```
+/// fn read_slice(slice: &[usize]) {
+///     // ...
+/// }
+///
+/// let v = vec![0, 1];
+/// read_slice(&v);
+///
+/// // ... and that's all!
+/// // you can also do it like this:
+/// let u: &[usize] = &v;
+/// // or like this:
+/// let u: &[_] = &v;
+/// ```
+///
+/// In Rust, it's more common to pass slices as arguments rather than vectors
+/// when you just want to provide read access. The same goes for [`String`] and
+/// [`&str`].
+///
+/// # Capacity and reallocation
+///
+/// The capacity of a vector is the amount of space allocated for any future
+/// elements that will be added onto the vector. This is not to be confused with
+/// the *length* of a vector, which specifies the number of actual elements
+/// within the vector. If a vector's length exceeds its capacity, its capacity
+/// will automatically be increased, but its elements will have to be
+/// reallocated.
+///
+/// For example, a vector with capacity 10 and length 0 would be an empty vector
+/// with space for 10 more elements. Pushing 10 or fewer elements onto the
+/// vector will not change its capacity or cause reallocation to occur. However,
+/// if the vector's length is increased to 11, it will have to reallocate, which
+/// can be slow. For this reason, it is recommended to use [`Vec::with_capacity`]
+/// whenever possible to specify how big the vector is expected to get.
+///
+/// # Guarantees
+///
+/// Due to its incredibly fundamental nature, `Vec` makes a lot of guarantees
+/// about its design. This ensures that it's as low-overhead as possible in
+/// the general case, and can be correctly manipulated in primitive ways
+/// by unsafe code. Note that these guarantees refer to an unqualified `Vec<T>`.
+/// If additional type parameters are added (e.g., to support custom allocators),
+/// overriding their defaults may change the behavior.
+///
+/// Most fundamentally, `Vec` is and always will be a (pointer, capacity, length)
+/// triplet. No more, no less. The order of these fields is completely
+/// unspecified, and you should use the appropriate methods to modify these.
+/// The pointer will never be null, so this type is null-pointer-optimized.
+///
+/// However, the pointer might not actually point to allocated memory. In particular,
+/// if you construct a `Vec` with capacity 0 via [`Vec::new`], [`vec![]`][`vec!`],
+/// [`Vec::with_capacity(0)`][`Vec::with_capacity`], or by calling [`shrink_to_fit`]
+/// on an empty Vec, it will not allocate memory. Similarly, if you store zero-sized
+/// types inside a `Vec`, it will not allocate space for them. *Note that in this case
+/// the `Vec` might not report a [`capacity`] of 0*. `Vec` will allocate if and only
+/// if <code>[mem::size_of::\<T>]\() * [capacity]\() > 0</code>. In general, `Vec`'s allocation
+/// details are very subtle --- if you intend to allocate memory using a `Vec`
+/// and use it for something else (either to pass to unsafe code, or to build your
+/// own memory-backed collection), be sure to deallocate this memory by using
+/// `from_raw_parts` to recover the `Vec` and then dropping it.
+///
+/// If a `Vec` *has* allocated memory, then the memory it points to is on the heap
+/// (as defined by the allocator Rust is configured to use by default), and its
+/// pointer points to [`len`] initialized, contiguous elements in order (what
+/// you would see if you coerced it to a slice), followed by <code>[capacity] - [len]</code>
+/// logically uninitialized, contiguous elements.
+///
+/// A vector containing the elements `'a'` and `'b'` with capacity 4 can be
+/// visualized as below. The top part is the `Vec` struct, it contains a
+/// pointer to the head of the allocation in the heap, length and capacity.
+/// The bottom part is the allocation on the heap, a contiguous memory block.
+///
+/// ```text
+///             ptr      len  capacity
+///        +--------+--------+--------+
+///        | 0x0123 |      2 |      4 |
+///        +--------+--------+--------+
+///             |
+///             v
+/// Heap   +--------+--------+--------+--------+
+///        |    'a' |    'b' | uninit | uninit |
+///        +--------+--------+--------+--------+
+/// ```
+///
+/// - **uninit** represents memory that is not initialized, see [`MaybeUninit`].
+/// - Note: the ABI is not stable and `Vec` makes no guarantees about its memory
+///   layout (including the order of fields).
+///
+/// `Vec` will never perform a "small optimization" where elements are actually
+/// stored on the stack for two reasons:
+///
+/// * It would make it more difficult for unsafe code to correctly manipulate
+///   a `Vec`. The contents of a `Vec` wouldn't have a stable address if it were
+///   only moved, and it would be more difficult to determine if a `Vec` had
+///   actually allocated memory.
+///
+/// * It would penalize the general case, incurring an additional branch
+///   on every access.
+///
+/// `Vec` will never automatically shrink itself, even if completely empty. This
+/// ensures no unnecessary allocations or deallocations occur. Emptying a `Vec`
+/// and then filling it back up to the same [`len`] should incur no calls to
+/// the allocator. If you wish to free up unused memory, use
+/// [`shrink_to_fit`] or [`shrink_to`].
+///
+/// [`push`] and [`insert`] will never (re)allocate if the reported capacity is
+/// sufficient. [`push`] and [`insert`] *will* (re)allocate if
+/// <code>[len] == [capacity]</code>. That is, the reported capacity is completely
+/// accurate, and can be relied on. It can even be used to manually free the memory
+/// allocated by a `Vec` if desired. Bulk insertion methods *may* reallocate, even
+/// when not necessary.
+///
+/// `Vec` does not guarantee any particular growth strategy when reallocating
+/// when full, nor when [`reserve`] is called. The current strategy is basic
+/// and it may prove desirable to use a non-constant growth factor. Whatever
+/// strategy is used will of course guarantee *O*(1) amortized [`push`].
+///
+/// `vec![x; n]`, `vec![a, b, c, d]`, and
+/// [`Vec::with_capacity(n)`][`Vec::with_capacity`], will all produce a `Vec`
+/// with exactly the requested capacity. If <code>[len] == [capacity]</code>,
+/// (as is the case for the [`vec!`] macro), then a `Vec<T>` can be converted to
+/// and from a [`Box<[T]>`][owned slice] without reallocating or moving the elements.
+///
+/// `Vec` will not specifically overwrite any data that is removed from it,
+/// but also won't specifically preserve it. Its uninitialized memory is
+/// scratch space that it may use however it wants. It will generally just do
+/// whatever is most efficient or otherwise easy to implement. Do not rely on
+/// removed data to be erased for security purposes. Even if you drop a `Vec`, its
+/// buffer may simply be reused by another allocation. Even if you zero a `Vec`'s memory
+/// first, that might not actually happen because the optimizer does not consider
+/// this a side-effect that must be preserved. There is one case which we will
+/// not break, however: using `unsafe` code to write to the excess capacity,
+/// and then increasing the length to match, is always valid.
+///
+/// Currently, `Vec` does not guarantee the order in which elements are dropped.
+/// The order has changed in the past and may change again.
+///
+/// [`get`]: ../../std/vec/struct.Vec.html#method.get
+/// [`get_mut`]: ../../std/vec/struct.Vec.html#method.get_mut
+/// [`String`]: alloc_crate::string::String
+/// [`&str`]: type@str
+/// [`shrink_to_fit`]: Vec::shrink_to_fit
+/// [`shrink_to`]: Vec::shrink_to
+/// [capacity]: Vec::capacity
+/// [`capacity`]: Vec::capacity
+/// [mem::size_of::\<T>]: core::mem::size_of
+/// [len]: Vec::len
+/// [`len`]: Vec::len
+/// [`push`]: Vec::push
+/// [`insert`]: Vec::insert
+/// [`reserve`]: Vec::reserve
+/// [`MaybeUninit`]: core::mem::MaybeUninit
+/// [owned slice]: Box
+pub struct Vec<T, A: Allocator = Global> {
+    buf: RawVec<T, A>,
+    len: usize,
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Inherent methods
+////////////////////////////////////////////////////////////////////////////////
+
+impl<T> Vec<T> {
+    /// Constructs a new, empty `Vec<T>`.
+    ///
+    /// The vector will not allocate until elements are pushed onto it.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![allow(unused_mut)]
+    /// let mut vec: Vec<i32> = Vec::new();
+    /// ```
+    #[inline(always)]
+    #[must_use]
+    pub const fn new() -> Self {
+        Vec {
+            buf: RawVec::new(),
+            len: 0,
+        }
+    }
+
+    /// Constructs a new, empty `Vec<T>` with at least the specified capacity.
+    ///
+    /// The vector will be able to hold at least `capacity` elements without
+    /// reallocating. This method is allowed to allocate for more elements than
+    /// `capacity`. If `capacity` is 0, the vector will not allocate.
+    ///
+    /// It is important to note that although the returned vector has the
+    /// minimum *capacity* specified, the vector will have a zero *length*. For
+    /// an explanation of the difference between length and capacity, see
+    /// *[Capacity and reallocation]*.
+    ///
+    /// If it is important to know the exact allocated capacity of a `Vec`,
+    /// always use the [`capacity`] method after construction.
+    ///
+    /// For `Vec<T>` where `T` is a zero-sized type, there will be no allocation
+    /// and the capacity will always be `usize::MAX`.
+    ///
+    /// [Capacity and reallocation]: #capacity-and-reallocation
+    /// [`capacity`]: Vec::capacity
+    ///
+    /// # Panics
+    ///
+    /// Panics if the new capacity exceeds `isize::MAX` bytes.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = Vec::with_capacity(10);
+    ///
+    /// // The vector contains no items, even though it has capacity for more
+    /// assert_eq!(vec.len(), 0);
+    /// assert!(vec.capacity() >= 10);
+    ///
+    /// // These are all done without reallocating...
+    /// for i in 0..10 {
+    ///     vec.push(i);
+    /// }
+    /// assert_eq!(vec.len(), 10);
+    /// assert!(vec.capacity() >= 10);
+    ///
+    /// // ...but this may make the vector reallocate
+    /// vec.push(11);
+    /// assert_eq!(vec.len(), 11);
+    /// assert!(vec.capacity() >= 11);
+    ///
+    /// // A vector of a zero-sized type will always over-allocate, since no
+    /// // allocation is necessary
+    /// let vec_units = Vec::<()>::with_capacity(10);
+    /// assert_eq!(vec_units.capacity(), usize::MAX);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[inline(always)]
+    #[must_use]
+    pub fn with_capacity(capacity: usize) -> Self {
+        Self::with_capacity_in(capacity, Global)
+    }
+
+    /// Creates a `Vec<T>` directly from a pointer, a capacity, and a length.
+    ///
+    /// # Safety
+    ///
+    /// This is highly unsafe, due to the number of invariants that aren't
+    /// checked:
+    ///
+    /// * `T` needs to have the same alignment as what `ptr` was allocated with.
+    ///   (`T` having a less strict alignment is not sufficient, the alignment really
+    ///   needs to be equal to satisfy the [`dealloc`] requirement that memory must be
+    ///   allocated and deallocated with the same layout.)
+    /// * The size of `T` times the `capacity` (ie. the allocated size in bytes) needs
+    ///   to be the same size as the pointer was allocated with. (Because similar to
+    ///   alignment, [`dealloc`] must be called with the same layout `size`.)
+    /// * `length` needs to be less than or equal to `capacity`.
+    /// * The first `length` values must be properly initialized values of type `T`.
+    /// * `capacity` needs to be the capacity that the pointer was allocated with.
+    /// * The allocated size in bytes must be no larger than `isize::MAX`.
+    ///   See the safety documentation of [`pointer::offset`](https://doc.rust-lang.org/nightly/std/primitive.pointer.html#method.offset).
+    ///
+    /// These requirements are always upheld by any `ptr` that has been allocated
+    /// via `Vec<T>`. Other allocation sources are allowed if the invariants are
+    /// upheld.
+    ///
+    /// Violating these may cause problems like corrupting the allocator's
+    /// internal data structures. For example it is normally **not** safe
+    /// to build a `Vec<u8>` from a pointer to a C `char` array with length
+    /// `size_t`, doing so is only safe if the array was initially allocated by
+    /// a `Vec` or `String`.
+    /// It's also not safe to build one from a `Vec<u16>` and its length, because
+    /// the allocator cares about the alignment, and these two types have different
+    /// alignments. The buffer was allocated with alignment 2 (for `u16`), but after
+    /// turning it into a `Vec<u8>` it'll be deallocated with alignment 1. To avoid
+    /// these issues, it is often preferable to do casting/transmuting using
+    /// [`slice::from_raw_parts`] instead.
+    ///
+    /// The ownership of `ptr` is effectively transferred to the
+    /// `Vec<T>` which may then deallocate, reallocate or change the
+    /// contents of memory pointed to by the pointer at will. Ensure
+    /// that nothing else uses the pointer after calling this
+    /// function.
+    ///
+    /// [`String`]: alloc_crate::string::String
+    /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ptr;
+    /// use std::mem;
+    ///
+    /// let v = vec![1, 2, 3];
+    ///
+    // FIXME Update this when vec_into_raw_parts is stabilized
+    /// // Prevent running `v`'s destructor so we are in complete control
+    /// // of the allocation.
+    /// let mut v = mem::ManuallyDrop::new(v);
+    ///
+    /// // Pull out the various important pieces of information about `v`
+    /// let p = v.as_mut_ptr();
+    /// let len = v.len();
+    /// let cap = v.capacity();
+    ///
+    /// unsafe {
+    ///     // Overwrite memory with 4, 5, 6
+    ///     for i in 0..len {
+    ///         ptr::write(p.add(i), 4 + i);
+    ///     }
+    ///
+    ///     // Put everything back together into a Vec
+    ///     let rebuilt = Vec::from_raw_parts(p, len, cap);
+    ///     assert_eq!(rebuilt, [4, 5, 6]);
+    /// }
+    /// ```
+    ///
+    /// Using memory that was allocated elsewhere:
+    ///
+    /// ```rust
+    /// #![feature(allocator_api)]
+    ///
+    /// use std::alloc::{AllocError, Allocator, Global, Layout};
+    ///
+    /// fn main() {
+    ///     let layout = Layout::array::<u32>(16).expect("overflow cannot happen");
+    ///
+    ///     let vec = unsafe {
+    ///         let mem = match Global.allocate(layout) {
+    ///             Ok(mem) => mem.cast::<u32>().as_ptr(),
+    ///             Err(AllocError) => return,
+    ///         };
+    ///
+    ///         mem.write(1_000_000);
+    ///
+    ///         Vec::from_raw_parts_in(mem, 1, 16, Global)
+    ///     };
+    ///
+    ///     assert_eq!(vec, &[1_000_000]);
+    ///     assert_eq!(vec.capacity(), 16);
+    /// }
+    /// ```
+    #[inline(always)]
+    pub unsafe fn from_raw_parts(ptr: *mut T, length: usize, capacity: usize) -> Self {
+        unsafe { Self::from_raw_parts_in(ptr, length, capacity, Global) }
+    }
+}
+
+impl<T, A: Allocator> Vec<T, A> {
+    /// Constructs a new, empty `Vec<T, A>`.
+    ///
+    /// The vector will not allocate until elements are pushed onto it.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::alloc::System;
+    ///
+    /// # #[allow(unused_mut)]
+    /// let mut vec: Vec<i32, _> = Vec::new_in(System);
+    /// ```
+    #[inline(always)]
+    pub const fn new_in(alloc: A) -> Self {
+        Vec {
+            buf: RawVec::new_in(alloc),
+            len: 0,
+        }
+    }
+
+    /// Constructs a new, empty `Vec<T, A>` with at least the specified capacity
+    /// with the provided allocator.
+    ///
+    /// The vector will be able to hold at least `capacity` elements without
+    /// reallocating. This method is allowed to allocate for more elements than
+    /// `capacity`. If `capacity` is 0, the vector will not allocate.
+    ///
+    /// It is important to note that although the returned vector has the
+    /// minimum *capacity* specified, the vector will have a zero *length*. For
+    /// an explanation of the difference between length and capacity, see
+    /// *[Capacity and reallocation]*.
+    ///
+    /// If it is important to know the exact allocated capacity of a `Vec`,
+    /// always use the [`capacity`] method after construction.
+    ///
+    /// For `Vec<T, A>` where `T` is a zero-sized type, there will be no allocation
+    /// and the capacity will always be `usize::MAX`.
+    ///
+    /// [Capacity and reallocation]: #capacity-and-reallocation
+    /// [`capacity`]: Vec::capacity
+    ///
+    /// # Panics
+    ///
+    /// Panics if the new capacity exceeds `isize::MAX` bytes.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::alloc::System;
+    ///
+    /// let mut vec = Vec::with_capacity_in(10, System);
+    ///
+    /// // The vector contains no items, even though it has capacity for more
+    /// assert_eq!(vec.len(), 0);
+    /// assert_eq!(vec.capacity(), 10);
+    ///
+    /// // These are all done without reallocating...
+    /// for i in 0..10 {
+    ///     vec.push(i);
+    /// }
+    /// assert_eq!(vec.len(), 10);
+    /// assert_eq!(vec.capacity(), 10);
+    ///
+    /// // ...but this may make the vector reallocate
+    /// vec.push(11);
+    /// assert_eq!(vec.len(), 11);
+    /// assert!(vec.capacity() >= 11);
+    ///
+    /// // A vector of a zero-sized type will always over-allocate, since no
+    /// // allocation is necessary
+    /// let vec_units = Vec::<(), System>::with_capacity_in(10, System);
+    /// assert_eq!(vec_units.capacity(), usize::MAX);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[inline(always)]
+    pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
+        Vec {
+            buf: RawVec::with_capacity_in(capacity, alloc),
+            len: 0,
+        }
+    }
+
+    /// Creates a `Vec<T, A>` directly from a pointer, a capacity, a length,
+    /// and an allocator.
+    ///
+    /// # Safety
+    ///
+    /// This is highly unsafe, due to the number of invariants that aren't
+    /// checked:
+    ///
+    /// * `T` needs to have the same alignment as what `ptr` was allocated with.
+    ///   (`T` having a less strict alignment is not sufficient, the alignment really
+    ///   needs to be equal to satisfy the [`dealloc`] requirement that memory must be
+    ///   allocated and deallocated with the same layout.)
+    /// * The size of `T` times the `capacity` (ie. the allocated size in bytes) needs
+    ///   to be the same size as the pointer was allocated with. (Because similar to
+    ///   alignment, [`dealloc`] must be called with the same layout `size`.)
+    /// * `length` needs to be less than or equal to `capacity`.
+    /// * The first `length` values must be properly initialized values of type `T`.
+    /// * `capacity` needs to [*fit*] the layout size that the pointer was allocated with.
+    /// * The allocated size in bytes must be no larger than `isize::MAX`.
+    ///   See the safety documentation of [`pointer::offset`](https://doc.rust-lang.org/nightly/std/primitive.pointer.html#method.offset).
+    ///
+    /// These requirements are always upheld by any `ptr` that has been allocated
+    /// via `Vec<T, A>`. Other allocation sources are allowed if the invariants are
+    /// upheld.
+    ///
+    /// Violating these may cause problems like corrupting the allocator's
+    /// internal data structures. For example it is **not** safe
+    /// to build a `Vec<u8>` from a pointer to a C `char` array with length `size_t`.
+    /// It's also not safe to build one from a `Vec<u16>` and its length, because
+    /// the allocator cares about the alignment, and these two types have different
+    /// alignments. The buffer was allocated with alignment 2 (for `u16`), but after
+    /// turning it into a `Vec<u8>` it'll be deallocated with alignment 1.
+    ///
+    /// The ownership of `ptr` is effectively transferred to the
+    /// `Vec<T>` which may then deallocate, reallocate or change the
+    /// contents of memory pointed to by the pointer at will. Ensure
+    /// that nothing else uses the pointer after calling this
+    /// function.
+    ///
+    /// [`String`]: alloc_crate::string::String
+    /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
+    /// [*fit*]: crate::alloc::Allocator#memory-fitting
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::alloc::System;
+    ///
+    /// use std::ptr;
+    /// use std::mem;
+    ///
+    ///
+    /// # use allocator_api2::vec::Vec;
+    /// let mut v = Vec::with_capacity_in(3, System);
+    /// v.push(1);
+    /// v.push(2);
+    /// v.push(3);
+    ///
+    // FIXME Update this when vec_into_raw_parts is stabilized
+    /// // Prevent running `v`'s destructor so we are in complete control
+    /// // of the allocation.
+    /// let mut v = mem::ManuallyDrop::new(v);
+    ///
+    /// // Pull out the various important pieces of information about `v`
+    /// let p = v.as_mut_ptr();
+    /// let len = v.len();
+    /// let cap = v.capacity();
+    /// let alloc = v.allocator();
+    ///
+    /// unsafe {
+    ///     // Overwrite memory with 4, 5, 6
+    ///     for i in 0..len {
+    ///         ptr::write(p.add(i), 4 + i);
+    ///     }
+    ///
+    ///     // Put everything back together into a Vec
+    ///     let rebuilt = Vec::from_raw_parts_in(p, len, cap, alloc.clone());
+    ///     assert_eq!(rebuilt, [4, 5, 6]);
+    /// }
+    /// ```
+    ///
+    /// Using memory that was allocated elsewhere:
+    ///
+    /// ```rust
+    /// use std::alloc::{alloc, Layout};
+    ///
+    /// fn main() {
+    ///     let layout = Layout::array::<u32>(16).expect("overflow cannot happen");
+    ///     let vec = unsafe {
+    ///         let mem = alloc(layout).cast::<u32>();
+    ///         if mem.is_null() {
+    ///             return;
+    ///         }
+    ///
+    ///         mem.write(1_000_000);
+    ///
+    ///         Vec::from_raw_parts(mem, 1, 16)
+    ///     };
+    ///
+    ///     assert_eq!(vec, &[1_000_000]);
+    ///     assert_eq!(vec.capacity(), 16);
+    /// }
+    /// ```
+    #[inline(always)]
+    pub unsafe fn from_raw_parts_in(ptr: *mut T, length: usize, capacity: usize, alloc: A) -> Self {
+        unsafe {
+            Vec {
+                buf: RawVec::from_raw_parts_in(ptr, capacity, alloc),
+                len: length,
+            }
+        }
+    }
+
+    /// Decomposes a `Vec<T>` into its raw components.
+    ///
+    /// Returns the raw pointer to the underlying data, the length of
+    /// the vector (in elements), and the allocated capacity of the
+    /// data (in elements). These are the same arguments in the same
+    /// order as the arguments to [`from_raw_parts`].
+    ///
+    /// After calling this function, the caller is responsible for the
+    /// memory previously managed by the `Vec`. The only way to do
+    /// this is to convert the raw pointer, length, and capacity back
+    /// into a `Vec` with the [`from_raw_parts`] function, allowing
+    /// the destructor to perform the cleanup.
+    ///
+    /// [`from_raw_parts`]: Vec::from_raw_parts
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(vec_into_raw_parts)]
+    /// let v: Vec<i32> = vec![-1, 0, 1];
+    ///
+    /// let (ptr, len, cap) = v.into_raw_parts();
+    ///
+    /// let rebuilt = unsafe {
+    ///     // We can now make changes to the components, such as
+    ///     // transmuting the raw pointer to a compatible type.
+    ///     let ptr = ptr as *mut u32;
+    ///
+    ///     Vec::from_raw_parts(ptr, len, cap)
+    /// };
+    /// assert_eq!(rebuilt, [4294967295, 0, 1]);
+    /// ```
+    pub fn into_raw_parts(self) -> (*mut T, usize, usize) {
+        let mut me = ManuallyDrop::new(self);
+        (me.as_mut_ptr(), me.len(), me.capacity())
+    }
+
+    /// Decomposes a `Vec<T>` into its raw components.
+    ///
+    /// Returns the raw pointer to the underlying data, the length of the vector (in elements),
+    /// the allocated capacity of the data (in elements), and the allocator. These are the same
+    /// arguments in the same order as the arguments to [`from_raw_parts_in`].
+    ///
+    /// After calling this function, the caller is responsible for the
+    /// memory previously managed by the `Vec`. The only way to do
+    /// this is to convert the raw pointer, length, and capacity back
+    /// into a `Vec` with the [`from_raw_parts_in`] function, allowing
+    /// the destructor to perform the cleanup.
+    ///
+    /// [`from_raw_parts_in`]: Vec::from_raw_parts_in
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(allocator_api, vec_into_raw_parts)]
+    ///
+    /// use std::alloc::System;
+    ///
+    /// let mut v: Vec<i32, System> = Vec::new_in(System);
+    /// v.push(-1);
+    /// v.push(0);
+    /// v.push(1);
+    ///
+    /// let (ptr, len, cap, alloc) = v.into_raw_parts_with_alloc();
+    ///
+    /// let rebuilt = unsafe {
+    ///     // We can now make changes to the components, such as
+    ///     // transmuting the raw pointer to a compatible type.
+    ///     let ptr = ptr as *mut u32;
+    ///
+    ///     Vec::from_raw_parts_in(ptr, len, cap, alloc)
+    /// };
+    /// assert_eq!(rebuilt, [4294967295, 0, 1]);
+    /// ```
+    // #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
+    pub fn into_raw_parts_with_alloc(self) -> (*mut T, usize, usize, A) {
+        let mut me = ManuallyDrop::new(self);
+        let len = me.len();
+        let capacity = me.capacity();
+        let ptr = me.as_mut_ptr();
+        let alloc = unsafe { ptr::read(me.allocator()) };
+        (ptr, len, capacity, alloc)
+    }
+
+    /// Returns the total number of elements the vector can hold without
+    /// reallocating.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec: Vec<i32> = Vec::with_capacity(10);
+    /// vec.push(42);
+    /// assert_eq!(vec.capacity(), 10);
+    /// ```
+    #[inline(always)]
+    pub fn capacity(&self) -> usize {
+        self.buf.capacity()
+    }
+
+    /// Reserves capacity for at least `additional` more elements to be inserted
+    /// in the given `Vec<T>`. The collection may reserve more space to
+    /// speculatively avoid frequent reallocations. After calling `reserve`,
+    /// capacity will be greater than or equal to `self.len() + additional`.
+    /// Does nothing if capacity is already sufficient.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the new capacity exceeds `isize::MAX` bytes.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = vec![1];
+    /// vec.reserve(10);
+    /// assert!(vec.capacity() >= 11);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[inline(always)]
+    pub fn reserve(&mut self, additional: usize) {
+        self.buf.reserve(self.len, additional);
+    }
+
+    /// Reserves the minimum capacity for at least `additional` more elements to
+    /// be inserted in the given `Vec<T>`. Unlike [`reserve`], this will not
+    /// deliberately over-allocate to speculatively avoid frequent allocations.
+    /// After calling `reserve_exact`, capacity will be greater than or equal to
+    /// `self.len() + additional`. Does nothing if the capacity is already
+    /// sufficient.
+    ///
+    /// Note that the allocator may give the collection more space than it
+    /// requests. Therefore, capacity can not be relied upon to be precisely
+    /// minimal. Prefer [`reserve`] if future insertions are expected.
+    ///
+    /// [`reserve`]: Vec::reserve
+    ///
+    /// # Panics
+    ///
+    /// Panics if the new capacity exceeds `isize::MAX` bytes.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = vec![1];
+    /// vec.reserve_exact(10);
+    /// assert!(vec.capacity() >= 11);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[inline(always)]
+    pub fn reserve_exact(&mut self, additional: usize) {
+        self.buf.reserve_exact(self.len, additional);
+    }
+
+    /// Tries to reserve capacity for at least `additional` more elements to be inserted
+    /// in the given `Vec<T>`. The collection may reserve more space to speculatively avoid
+    /// frequent reallocations. After calling `try_reserve`, capacity will be
+    /// greater than or equal to `self.len() + additional` if it returns
+    /// `Ok(())`. Does nothing if capacity is already sufficient. This method
+    /// preserves the contents even if an error occurs.
+    ///
+    /// # Errors
+    ///
+    /// If the capacity overflows, or the allocator reports a failure, then an error
+    /// is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::TryReserveError;
+    ///
+    /// fn process_data(data: &[u32]) -> Result<Vec<u32>, TryReserveError> {
+    ///     let mut output = Vec::new();
+    ///
+    ///     // Pre-reserve the memory, exiting if we can't
+    ///     output.try_reserve(data.len())?;
+    ///
+    ///     // Now we know this can't OOM in the middle of our complex work
+    ///     output.extend(data.iter().map(|&val| {
+    ///         val * 2 + 5 // very complicated
+    ///     }));
+    ///
+    ///     Ok(output)
+    /// }
+    /// # process_data(&[1, 2, 3]).expect("why is the test harness OOMing on 12 bytes?");
+    /// ```
+    #[inline(always)]
+    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
+        self.buf.try_reserve(self.len, additional)
+    }
+
+    /// Tries to reserve the minimum capacity for at least `additional`
+    /// elements to be inserted in the given `Vec<T>`. Unlike [`try_reserve`],
+    /// this will not deliberately over-allocate to speculatively avoid frequent
+    /// allocations. After calling `try_reserve_exact`, capacity will be greater
+    /// than or equal to `self.len() + additional` if it returns `Ok(())`.
+    /// Does nothing if the capacity is already sufficient.
+    ///
+    /// Note that the allocator may give the collection more space than it
+    /// requests. Therefore, capacity can not be relied upon to be precisely
+    /// minimal. Prefer [`try_reserve`] if future insertions are expected.
+    ///
+    /// [`try_reserve`]: Vec::try_reserve
+    ///
+    /// # Errors
+    ///
+    /// If the capacity overflows, or the allocator reports a failure, then an error
+    /// is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::TryReserveError;
+    ///
+    /// fn process_data(data: &[u32]) -> Result<Vec<u32>, TryReserveError> {
+    ///     let mut output = Vec::new();
+    ///
+    ///     // Pre-reserve the memory, exiting if we can't
+    ///     output.try_reserve_exact(data.len())?;
+    ///
+    ///     // Now we know this can't OOM in the middle of our complex work
+    ///     output.extend(data.iter().map(|&val| {
+    ///         val * 2 + 5 // very complicated
+    ///     }));
+    ///
+    ///     Ok(output)
+    /// }
+    /// # process_data(&[1, 2, 3]).expect("why is the test harness OOMing on 12 bytes?");
+    /// ```
+    #[inline(always)]
+    pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveError> {
+        self.buf.try_reserve_exact(self.len, additional)
+    }
+
+    /// Shrinks the capacity of the vector as much as possible.
+    ///
+    /// It will drop down as close as possible to the length but the allocator
+    /// may still inform the vector that there is space for a few more elements.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = Vec::with_capacity(10);
+    /// vec.extend([1, 2, 3]);
+    /// assert_eq!(vec.capacity(), 10);
+    /// vec.shrink_to_fit();
+    /// assert!(vec.capacity() >= 3);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[inline(always)]
+    pub fn shrink_to_fit(&mut self) {
+        // The capacity is never less than the length, and there's nothing to do when
+        // they are equal, so we can avoid the panic case in `RawVec::shrink_to_fit`
+        // by only calling it with a greater capacity.
+        if self.capacity() > self.len {
+            self.buf.shrink_to_fit(self.len);
+        }
+    }
+
+    /// Shrinks the capacity of the vector with a lower bound.
+    ///
+    /// The capacity will remain at least as large as both the length
+    /// and the supplied value.
+    ///
+    /// If the current capacity is less than the lower limit, this is a no-op.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = Vec::with_capacity(10);
+    /// vec.extend([1, 2, 3]);
+    /// assert_eq!(vec.capacity(), 10);
+    /// vec.shrink_to(4);
+    /// assert!(vec.capacity() >= 4);
+    /// vec.shrink_to(0);
+    /// assert!(vec.capacity() >= 3);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[inline(always)]
+    pub fn shrink_to(&mut self, min_capacity: usize) {
+        if self.capacity() > min_capacity {
+            self.buf.shrink_to_fit(cmp::max(self.len, min_capacity));
+        }
+    }
+
+    /// Converts the vector into [`Box<[T]>`][owned slice].
+    ///
+    /// If the vector has excess capacity, its items will be moved into a
+    /// newly-allocated buffer with exactly the right capacity.
+    ///
+    /// [owned slice]: Box
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let v = vec![1, 2, 3];
+    ///
+    /// let slice = v.into_boxed_slice();
+    /// ```
+    ///
+    /// Any excess capacity is removed:
+    ///
+    /// ```
+    /// let mut vec = Vec::with_capacity(10);
+    /// vec.extend([1, 2, 3]);
+    ///
+    /// assert_eq!(vec.capacity(), 10);
+    /// let slice = vec.into_boxed_slice();
+    /// assert_eq!(slice.into_vec().capacity(), 3);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[inline(always)]
+    pub fn into_boxed_slice(mut self) -> Box<[T], A> {
+        unsafe {
+            self.shrink_to_fit();
+            let me = ManuallyDrop::new(self);
+            let buf = ptr::read(&me.buf);
+            let len = me.len();
+            buf.into_box(len).assume_init()
+        }
+    }
+
+    /// Shortens the vector, keeping the first `len` elements and dropping
+    /// the rest.
+    ///
+    /// If `len` is greater than the vector's current length, this has no
+    /// effect.
+    ///
+    /// The [`drain`] method can emulate `truncate`, but causes the excess
+    /// elements to be returned instead of dropped.
+    ///
+    /// Note that this method has no effect on the allocated capacity
+    /// of the vector.
+    ///
+    /// # Examples
+    ///
+    /// Truncating a five element vector to two elements:
+    ///
+    /// ```
+    /// let mut vec = vec![1, 2, 3, 4, 5];
+    /// vec.truncate(2);
+    /// assert_eq!(vec, [1, 2]);
+    /// ```
+    ///
+    /// No truncation occurs when `len` is greater than the vector's current
+    /// length:
+    ///
+    /// ```
+    /// let mut vec = vec![1, 2, 3];
+    /// vec.truncate(8);
+    /// assert_eq!(vec, [1, 2, 3]);
+    /// ```
+    ///
+    /// Truncating when `len == 0` is equivalent to calling the [`clear`]
+    /// method.
+    ///
+    /// ```
+    /// let mut vec = vec![1, 2, 3];
+    /// vec.truncate(0);
+    /// assert_eq!(vec, []);
+    /// ```
+    ///
+    /// [`clear`]: Vec::clear
+    /// [`drain`]: Vec::drain
+    #[inline(always)]
+    pub fn truncate(&mut self, len: usize) {
+        // This is safe because:
+        //
+        // * the slice passed to `drop_in_place` is valid; the `len > self.len`
+        //   case avoids creating an invalid slice, and
+        // * the `len` of the vector is shrunk before calling `drop_in_place`,
+        //   such that no value will be dropped twice in case `drop_in_place`
+        //   were to panic once (if it panics twice, the program aborts).
+        unsafe {
+            // Note: It's intentional that this is `>` and not `>=`.
+            //       Changing it to `>=` has negative performance
+            //       implications in some cases. See #78884 for more.
+            if len > self.len {
+                return;
+            }
+            let remaining_len = self.len - len;
+            let s = ptr::slice_from_raw_parts_mut(self.as_mut_ptr().add(len), remaining_len);
+            self.len = len;
+            ptr::drop_in_place(s);
+        }
+    }
+
+    /// Extracts a slice containing the entire vector.
+    ///
+    /// Equivalent to `&s[..]`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::{self, Write};
+    /// let buffer = vec![1, 2, 3, 5, 8];
+    /// io::sink().write(buffer.as_slice()).unwrap();
+    /// ```
+    #[inline(always)]
+    pub fn as_slice(&self) -> &[T] {
+        self
+    }
+
+    /// Extracts a mutable slice of the entire vector.
+    ///
+    /// Equivalent to `&mut s[..]`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::{self, Read};
+    /// let mut buffer = vec![0; 3];
+    /// io::repeat(0b101).read_exact(buffer.as_mut_slice()).unwrap();
+    /// ```
+    #[inline(always)]
+    pub fn as_mut_slice(&mut self) -> &mut [T] {
+        self
+    }
+
+    /// Returns a raw pointer to the vector's buffer, or a dangling raw pointer
+    /// valid for zero sized reads if the vector didn't allocate.
+    ///
+    /// The caller must ensure that the vector outlives the pointer this
+    /// function returns, or else it will end up pointing to garbage.
+    /// Modifying the vector may cause its buffer to be reallocated,
+    /// which would also make any pointers to it invalid.
+    ///
+    /// The caller must also ensure that the memory the pointer (non-transitively) points to
+    /// is never written to (except inside an `UnsafeCell`) using this pointer or any pointer
+    /// derived from it. If you need to mutate the contents of the slice, use [`as_mut_ptr`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let x = vec![1, 2, 4];
+    /// let x_ptr = x.as_ptr();
+    ///
+    /// unsafe {
+    ///     for i in 0..x.len() {
+    ///         assert_eq!(*x_ptr.add(i), 1 << i);
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// [`as_mut_ptr`]: Vec::as_mut_ptr
+    #[inline(always)]
+    pub fn as_ptr(&self) -> *const T {
+        // We shadow the slice method of the same name to avoid going through
+        // `deref`, which creates an intermediate reference.
+        let ptr = self.buf.ptr();
+        unsafe {
+            assume(!ptr.is_null());
+        }
+        ptr
+    }
+
+    /// Returns an unsafe mutable pointer to the vector's buffer, or a dangling
+    /// raw pointer valid for zero sized reads if the vector didn't allocate.
+    ///
+    /// The caller must ensure that the vector outlives the pointer this
+    /// function returns, or else it will end up pointing to garbage.
+    /// Modifying the vector may cause its buffer to be reallocated,
+    /// which would also make any pointers to it invalid.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// // Allocate vector big enough for 4 elements.
+    /// let size = 4;
+    /// let mut x: Vec<i32> = Vec::with_capacity(size);
+    /// let x_ptr = x.as_mut_ptr();
+    ///
+    /// // Initialize elements via raw pointer writes, then set length.
+    /// unsafe {
+    ///     for i in 0..size {
+    ///         *x_ptr.add(i) = i as i32;
+    ///     }
+    ///     x.set_len(size);
+    /// }
+    /// assert_eq!(&*x, &[0, 1, 2, 3]);
+    /// ```
+    #[inline(always)]
+    pub fn as_mut_ptr(&mut self) -> *mut T {
+        // We shadow the slice method of the same name to avoid going through
+        // `deref_mut`, which creates an intermediate reference.
+        let ptr = self.buf.ptr();
+        unsafe {
+            assume(!ptr.is_null());
+        }
+        ptr
+    }
+
+    /// Returns a reference to the underlying allocator.
+    #[inline(always)]
+    pub fn allocator(&self) -> &A {
+        self.buf.allocator()
+    }
+
+    /// Forces the length of the vector to `new_len`.
+    ///
+    /// This is a low-level operation that maintains none of the normal
+    /// invariants of the type. Normally changing the length of a vector
+    /// is done using one of the safe operations instead, such as
+    /// [`truncate`], [`resize`], [`extend`], or [`clear`].
+    ///
+    /// [`truncate`]: Vec::truncate
+    /// [`resize`]: Vec::resize
+    /// [`extend`]: Extend::extend
+    /// [`clear`]: Vec::clear
+    ///
+    /// # Safety
+    ///
+    /// - `new_len` must be less than or equal to [`capacity()`].
+    /// - The elements at `old_len..new_len` must be initialized.
+    ///
+    /// [`capacity()`]: Vec::capacity
+    ///
+    /// # Examples
+    ///
+    /// This method can be useful for situations in which the vector
+    /// is serving as a buffer for other code, particularly over FFI:
+    ///
+    /// ```no_run
+    /// # #![allow(dead_code)]
+    /// # // This is just a minimal skeleton for the doc example;
+    /// # // don't use this as a starting point for a real library.
+    /// # pub struct StreamWrapper { strm: *mut std::ffi::c_void }
+    /// # const Z_OK: i32 = 0;
+    /// # extern "C" {
+    /// #     fn deflateGetDictionary(
+    /// #         strm: *mut std::ffi::c_void,
+    /// #         dictionary: *mut u8,
+    /// #         dictLength: *mut usize,
+    /// #     ) -> i32;
+    /// # }
+    /// # impl StreamWrapper {
+    /// pub fn get_dictionary(&self) -> Option<Vec<u8>> {
+    ///     // Per the FFI method's docs, "32768 bytes is always enough".
+    ///     let mut dict = Vec::with_capacity(32_768);
+    ///     let mut dict_length = 0;
+    ///     // SAFETY: When `deflateGetDictionary` returns `Z_OK`, it holds that:
+    ///     // 1. `dict_length` elements were initialized.
+    ///     // 2. `dict_length` <= the capacity (32_768)
+    ///     // which makes `set_len` safe to call.
+    ///     unsafe {
+    ///         // Make the FFI call...
+    ///         let r = deflateGetDictionary(self.strm, dict.as_mut_ptr(), &mut dict_length);
+    ///         if r == Z_OK {
+    ///             // ...and update the length to what was initialized.
+    ///             dict.set_len(dict_length);
+    ///             Some(dict)
+    ///         } else {
+    ///             None
+    ///         }
+    ///     }
+    /// }
+    /// # }
+    /// ```
+    ///
+    /// While the following example is sound, there is a memory leak since
+    /// the inner vectors were not freed prior to the `set_len` call:
+    ///
+    /// ```
+    /// let mut vec = vec![vec![1, 0, 0],
+    ///                    vec![0, 1, 0],
+    ///                    vec![0, 0, 1]];
+    /// // SAFETY:
+    /// // 1. `old_len..0` is empty so no elements need to be initialized.
+    /// // 2. `0 <= capacity` always holds whatever `capacity` is.
+    /// unsafe {
+    ///     vec.set_len(0);
+    /// }
+    /// ```
+    ///
+    /// Normally, here, one would use [`clear`] instead to correctly drop
+    /// the contents and thus not leak memory.
+    #[inline(always)]
+    pub unsafe fn set_len(&mut self, new_len: usize) {
+        debug_assert!(new_len <= self.capacity());
+
+        self.len = new_len;
+    }
+
+    /// Removes an element from the vector and returns it.
+    ///
+    /// The removed element is replaced by the last element of the vector.
+    ///
+    /// This does not preserve ordering, but is *O*(1).
+    /// If you need to preserve the element order, use [`remove`] instead.
+    ///
+    /// [`remove`]: Vec::remove
+    ///
+    /// # Panics
+    ///
+    /// Panics if `index` is out of bounds.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = vec!["foo", "bar", "baz", "qux"];
+    ///
+    /// assert_eq!(v.swap_remove(1), "bar");
+    /// assert_eq!(v, ["foo", "qux", "baz"]);
+    ///
+    /// assert_eq!(v.swap_remove(0), "foo");
+    /// assert_eq!(v, ["baz", "qux"]);
+    /// ```
+    #[inline(always)]
+    pub fn swap_remove(&mut self, index: usize) -> T {
+        #[cold]
+        #[inline(never)]
+        fn assert_failed(index: usize, len: usize) -> ! {
+            panic!(
+                "swap_remove index (is {}) should be < len (is {})",
+                index, len
+            );
+        }
+
+        let len = self.len();
+        if index >= len {
+            assert_failed(index, len);
+        }
+        unsafe {
+            // We replace self[index] with the last element. Note that if the
+            // bounds check above succeeds there must be a last element (which
+            // can be self[index] itself).
+            let value = ptr::read(self.as_ptr().add(index));
+            let base_ptr = self.as_mut_ptr();
+            ptr::copy(base_ptr.add(len - 1), base_ptr.add(index), 1);
+            self.set_len(len - 1);
+            value
+        }
+    }
+
+    /// Inserts an element at position `index` within the vector, shifting all
+    /// elements after it to the right.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `index > len`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = vec![1, 2, 3];
+    /// vec.insert(1, 4);
+    /// assert_eq!(vec, [1, 4, 2, 3]);
+    /// vec.insert(4, 5);
+    /// assert_eq!(vec, [1, 4, 2, 3, 5]);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    pub fn insert(&mut self, index: usize, element: T) {
+        #[cold]
+        #[inline(never)]
+        fn assert_failed(index: usize, len: usize) -> ! {
+            panic!(
+                "insertion index (is {}) should be <= len (is {})",
+                index, len
+            );
+        }
+
+        let len = self.len();
+
+        // space for the new element
+        if len == self.buf.capacity() {
+            self.reserve(1);
+        }
+
+        unsafe {
+            // infallible
+            // The spot to put the new value
+            {
+                let p = self.as_mut_ptr().add(index);
+                match cmp::Ord::cmp(&index, &len) {
+                    Ordering::Less => {
+                        // Shift everything over to make space. (Duplicating the
+                        // `index`th element into two consecutive places.)
+                        ptr::copy(p, p.add(1), len - index);
+                    }
+                    Ordering::Equal => {
+                        // No elements need shifting.
+                    }
+                    Ordering::Greater => {
+                        assert_failed(index, len);
+                    }
+                }
+                // Write it in, overwriting the first copy of the `index`th
+                // element.
+                ptr::write(p, element);
+            }
+            self.set_len(len + 1);
+        }
+    }
+
+    /// Removes and returns the element at position `index` within the vector,
+    /// shifting all elements after it to the left.
+    ///
+    /// Note: Because this shifts over the remaining elements, it has a
+    /// worst-case performance of *O*(*n*). If you don't need the order of elements
+    /// to be preserved, use [`swap_remove`] instead. If you'd like to remove
+    /// elements from the beginning of the `Vec`, consider using
+    /// [`VecDeque::pop_front`] instead.
+    ///
+    /// [`swap_remove`]: Vec::swap_remove
+    /// [`VecDeque::pop_front`]: alloc_crate::collections::VecDeque::pop_front
+    ///
+    /// # Panics
+    ///
+    /// Panics if `index` is out of bounds.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = vec![1, 2, 3];
+    /// assert_eq!(v.remove(1), 2);
+    /// assert_eq!(v, [1, 3]);
+    /// ```
+    #[track_caller]
+    #[inline(always)]
+    pub fn remove(&mut self, index: usize) -> T {
+        #[cold]
+        #[inline(never)]
+        #[track_caller]
+        fn assert_failed(index: usize, len: usize) -> ! {
+            panic!("removal index (is {}) should be < len (is {})", index, len);
+        }
+
+        let len = self.len();
+        if index >= len {
+            assert_failed(index, len);
+        }
+        unsafe {
+            // infallible
+            let ret;
+            {
+                // the place we are taking from.
+                let ptr = self.as_mut_ptr().add(index);
+                // copy it out, unsafely having a copy of the value on
+                // the stack and in the vector at the same time.
+                ret = ptr::read(ptr);
+
+                // Shift everything down to fill in that spot.
+                ptr::copy(ptr.add(1), ptr, len - index - 1);
+            }
+            self.set_len(len - 1);
+            ret
+        }
+    }
+
+    /// Retains only the elements specified by the predicate.
+    ///
+    /// In other words, remove all elements `e` for which `f(&e)` returns `false`.
+    /// This method operates in place, visiting each element exactly once in the
+    /// original order, and preserves the order of the retained elements.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = vec![1, 2, 3, 4];
+    /// vec.retain(|&x| x % 2 == 0);
+    /// assert_eq!(vec, [2, 4]);
+    /// ```
+    ///
+    /// Because the elements are visited exactly once in the original order,
+    /// external state may be used to decide which elements to keep.
+    ///
+    /// ```
+    /// let mut vec = vec![1, 2, 3, 4, 5];
+    /// let keep = [false, true, true, false, true];
+    /// let mut iter = keep.iter();
+    /// vec.retain(|_| *iter.next().unwrap());
+    /// assert_eq!(vec, [2, 3, 5]);
+    /// ```
+    #[inline(always)]
+    pub fn retain<F>(&mut self, mut f: F)
+    where
+        F: FnMut(&T) -> bool,
+    {
+        self.retain_mut(|elem| f(elem));
+    }
+
+    /// Retains only the elements specified by the predicate, passing a mutable reference to it.
+    ///
+    /// In other words, remove all elements `e` such that `f(&mut e)` returns `false`.
+    /// This method operates in place, visiting each element exactly once in the
+    /// original order, and preserves the order of the retained elements.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = vec![1, 2, 3, 4];
+    /// vec.retain_mut(|x| if *x <= 3 {
+    ///     *x += 1;
+    ///     true
+    /// } else {
+    ///     false
+    /// });
+    /// assert_eq!(vec, [2, 3, 4]);
+    /// ```
+    #[inline]
+    pub fn retain_mut<F>(&mut self, mut f: F)
+    where
+        F: FnMut(&mut T) -> bool,
+    {
+        let original_len = self.len();
+        // Avoid double drop if the drop guard is not executed,
+        // since we may make some holes during the process.
+        unsafe { self.set_len(0) };
+
+        // Vec: [Kept, Kept, Hole, Hole, Hole, Hole, Unchecked, Unchecked]
+        //      |<-              processed len   ->| ^- next to check
+        //                  |<-  deleted cnt     ->|
+        //      |<-              original_len                          ->|
+        // Kept: Elements which predicate returns true on.
+        // Hole: Moved or dropped element slot.
+        // Unchecked: Unchecked valid elements.
+        //
+        // This drop guard will be invoked when predicate or `drop` of element panicked.
+        // It shifts unchecked elements to cover holes and `set_len` to the correct length.
+        // In cases when predicate and `drop` never panick, it will be optimized out.
+        struct BackshiftOnDrop<'a, T, A: Allocator> {
+            v: &'a mut Vec<T, A>,
+            processed_len: usize,
+            deleted_cnt: usize,
+            original_len: usize,
+        }
+
+        impl<T, A: Allocator> Drop for BackshiftOnDrop<'_, T, A> {
+            fn drop(&mut self) {
+                if self.deleted_cnt > 0 {
+                    // SAFETY: Trailing unchecked items must be valid since we never touch them.
+                    unsafe {
+                        ptr::copy(
+                            self.v.as_ptr().add(self.processed_len),
+                            self.v
+                                .as_mut_ptr()
+                                .add(self.processed_len - self.deleted_cnt),
+                            self.original_len - self.processed_len,
+                        );
+                    }
+                }
+                // SAFETY: After filling holes, all items are in contiguous memory.
+                unsafe {
+                    self.v.set_len(self.original_len - self.deleted_cnt);
+                }
+            }
+        }
+
+        let mut g = BackshiftOnDrop {
+            v: self,
+            processed_len: 0,
+            deleted_cnt: 0,
+            original_len,
+        };
+
+        fn process_loop<F, T, A: Allocator, const DELETED: bool>(
+            original_len: usize,
+            f: &mut F,
+            g: &mut BackshiftOnDrop<'_, T, A>,
+        ) where
+            F: FnMut(&mut T) -> bool,
+        {
+            while g.processed_len != original_len {
+                // SAFETY: Unchecked element must be valid.
+                let cur = unsafe { &mut *g.v.as_mut_ptr().add(g.processed_len) };
+                if !f(cur) {
+                    // Advance early to avoid double drop if `drop_in_place` panicked.
+                    g.processed_len += 1;
+                    g.deleted_cnt += 1;
+                    // SAFETY: We never touch this element again after dropped.
+                    unsafe { ptr::drop_in_place(cur) };
+                    // We already advanced the counter.
+                    if DELETED {
+                        continue;
+                    } else {
+                        break;
+                    }
+                }
+                if DELETED {
+                    // SAFETY: `deleted_cnt` > 0, so the hole slot must not overlap with current element.
+                    // We use copy for move, and never touch this element again.
+                    unsafe {
+                        let hole_slot = g.v.as_mut_ptr().add(g.processed_len - g.deleted_cnt);
+                        ptr::copy_nonoverlapping(cur, hole_slot, 1);
+                    }
+                }
+                g.processed_len += 1;
+            }
+        }
+
+        // Stage 1: Nothing was deleted.
+        process_loop::<F, T, A, false>(original_len, &mut f, &mut g);
+
+        // Stage 2: Some elements were deleted.
+        process_loop::<F, T, A, true>(original_len, &mut f, &mut g);
+
+        // All item are processed. This can be optimized to `set_len` by LLVM.
+        drop(g);
+    }
+
+    /// Removes all but the first of consecutive elements in the vector that resolve to the same
+    /// key.
+    ///
+    /// If the vector is sorted, this removes all duplicates.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = vec![10, 20, 21, 30, 20];
+    ///
+    /// vec.dedup_by_key(|i| *i / 10);
+    ///
+    /// assert_eq!(vec, [10, 20, 30, 20]);
+    /// ```
+    #[inline(always)]
+    pub fn dedup_by_key<F, K>(&mut self, mut key: F)
+    where
+        F: FnMut(&mut T) -> K,
+        K: PartialEq,
+    {
+        self.dedup_by(|a, b| key(a) == key(b))
+    }
+
+    /// Removes all but the first of consecutive elements in the vector satisfying a given equality
+    /// relation.
+    ///
+    /// The `same_bucket` function is passed references to two elements from the vector and
+    /// must determine if the elements compare equal. The elements are passed in opposite order
+    /// from their order in the slice, so if `same_bucket(a, b)` returns `true`, `a` is removed.
+    ///
+    /// If the vector is sorted, this removes all duplicates.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = vec!["foo", "bar", "Bar", "baz", "bar"];
+    ///
+    /// vec.dedup_by(|a, b| a.eq_ignore_ascii_case(b));
+    ///
+    /// assert_eq!(vec, ["foo", "bar", "baz", "bar"]);
+    /// ```
+    #[inline]
+    pub fn dedup_by<F>(&mut self, mut same_bucket: F)
+    where
+        F: FnMut(&mut T, &mut T) -> bool,
+    {
+        let len = self.len();
+        if len <= 1 {
+            return;
+        }
+
+        /* INVARIANT: vec.len() > read >= write > write-1 >= 0 */
+        struct FillGapOnDrop<'a, T, A: Allocator> {
+            /* Offset of the element we want to check if it is duplicate */
+            read: usize,
+
+            /* Offset of the place where we want to place the non-duplicate
+             * when we find it. */
+            write: usize,
+
+            /* The Vec that would need correction if `same_bucket` panicked */
+            vec: &'a mut Vec<T, A>,
+        }
+
+        impl<'a, T, A: Allocator> Drop for FillGapOnDrop<'a, T, A> {
+            fn drop(&mut self) {
+                /* This code gets executed when `same_bucket` panics */
+
+                /* SAFETY: invariant guarantees that `read - write`
+                 * and `len - read` never overflow and that the copy is always
+                 * in-bounds. */
+                unsafe {
+                    let ptr = self.vec.as_mut_ptr();
+                    let len = self.vec.len();
+
+                    /* How many items were left when `same_bucket` panicked.
+                     * Basically vec[read..].len() */
+                    let items_left = len.wrapping_sub(self.read);
+
+                    /* Pointer to first item in vec[write..write+items_left] slice */
+                    let dropped_ptr = ptr.add(self.write);
+                    /* Pointer to first item in vec[read..] slice */
+                    let valid_ptr = ptr.add(self.read);
+
+                    /* Copy `vec[read..]` to `vec[write..write+items_left]`.
+                     * The slices can overlap, so `copy_nonoverlapping` cannot be used */
+                    ptr::copy(valid_ptr, dropped_ptr, items_left);
+
+                    /* How many items have been already dropped
+                     * Basically vec[read..write].len() */
+                    let dropped = self.read.wrapping_sub(self.write);
+
+                    self.vec.set_len(len - dropped);
+                }
+            }
+        }
+
+        let mut gap = FillGapOnDrop {
+            read: 1,
+            write: 1,
+            vec: self,
+        };
+        let ptr = gap.vec.as_mut_ptr();
+
+        /* Drop items while going through Vec, it should be more efficient than
+         * doing slice partition_dedup + truncate */
+
+        /* SAFETY: Because of the invariant, read_ptr, prev_ptr and write_ptr
+         * are always in-bounds and read_ptr never aliases prev_ptr */
+        unsafe {
+            while gap.read < len {
+                let read_ptr = ptr.add(gap.read);
+                let prev_ptr = ptr.add(gap.write.wrapping_sub(1));
+
+                if same_bucket(&mut *read_ptr, &mut *prev_ptr) {
+                    // Increase `gap.read` now since the drop may panic.
+                    gap.read += 1;
+                    /* We have found duplicate, drop it in-place */
+                    ptr::drop_in_place(read_ptr);
+                } else {
+                    let write_ptr = ptr.add(gap.write);
+
+                    /* Because `read_ptr` can be equal to `write_ptr`, we either
+                     * have to use `copy` or conditional `copy_nonoverlapping`.
+                     * Looks like the first option is faster. */
+                    ptr::copy(read_ptr, write_ptr, 1);
+
+                    /* We have filled that place, so go further */
+                    gap.write += 1;
+                    gap.read += 1;
+                }
+            }
+
+            /* Technically we could let `gap` clean up with its Drop, but
+             * when `same_bucket` is guaranteed to not panic, this bloats a little
+             * the codegen, so we just do it manually */
+            gap.vec.set_len(gap.write);
+            mem::forget(gap);
+        }
+    }
+
+    /// Appends an element to the back of a collection.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the new capacity exceeds `isize::MAX` bytes.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = vec![1, 2];
+    /// vec.push(3);
+    /// assert_eq!(vec, [1, 2, 3]);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[inline(always)]
+    pub fn push(&mut self, value: T) {
+        // This will panic or abort if we would allocate > isize::MAX bytes
+        // or if the length increment would overflow for zero-sized types.
+        if self.len == self.buf.capacity() {
+            self.buf.reserve_for_push(self.len);
+        }
+        unsafe {
+            let end = self.as_mut_ptr().add(self.len);
+            ptr::write(end, value);
+            self.len += 1;
+        }
+    }
+
+    /// Appends an element if there is sufficient spare capacity, otherwise an error is returned
+    /// with the element.
+    ///
+    /// Unlike [`push`] this method will not reallocate when there's insufficient capacity.
+    /// The caller should use [`reserve`] or [`try_reserve`] to ensure that there is enough capacity.
+    ///
+    /// [`push`]: Vec::push
+    /// [`reserve`]: Vec::reserve
+    /// [`try_reserve`]: Vec::try_reserve
+    ///
+    /// # Examples
+    ///
+    /// A manual, panic-free alternative to [`FromIterator`]:
+    ///
+    /// ```
+    /// #![feature(vec_push_within_capacity)]
+    ///
+    /// use std::collections::TryReserveError;
+    /// fn from_iter_fallible<T>(iter: impl Iterator<Item=T>) -> Result<Vec<T>, TryReserveError> {
+    ///     let mut vec = Vec::new();
+    ///     for value in iter {
+    ///         if let Err(value) = vec.push_within_capacity(value) {
+    ///             vec.try_reserve(1)?;
+    ///             // this cannot fail, the previous line either returned or added at least 1 free slot
+    ///             let _ = vec.push_within_capacity(value);
+    ///         }
+    ///     }
+    ///     Ok(vec)
+    /// }
+    /// assert_eq!(from_iter_fallible(0..100), Ok(Vec::from_iter(0..100)));
+    /// ```
+    #[inline(always)]
+    pub fn push_within_capacity(&mut self, value: T) -> Result<(), T> {
+        if self.len == self.buf.capacity() {
+            return Err(value);
+        }
+        unsafe {
+            let end = self.as_mut_ptr().add(self.len);
+            ptr::write(end, value);
+            self.len += 1;
+        }
+        Ok(())
+    }
+
+    /// Removes the last element from a vector and returns it, or [`None`] if it
+    /// is empty.
+    ///
+    /// If you'd like to pop the first element, consider using
+    /// [`VecDeque::pop_front`] instead.
+    ///
+    /// [`VecDeque::pop_front`]: alloc_crate::collections::VecDeque::pop_front
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = vec![1, 2, 3];
+    /// assert_eq!(vec.pop(), Some(3));
+    /// assert_eq!(vec, [1, 2]);
+    /// ```
+    #[inline(always)]
+    pub fn pop(&mut self) -> Option<T> {
+        if self.len == 0 {
+            None
+        } else {
+            unsafe {
+                self.len -= 1;
+                Some(ptr::read(self.as_ptr().add(self.len())))
+            }
+        }
+    }
+
+    /// Moves all the elements of `other` into `self`, leaving `other` empty.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the new capacity exceeds `isize::MAX` bytes.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = vec![1, 2, 3];
+    /// let mut vec2 = vec![4, 5, 6];
+    /// vec.append(&mut vec2);
+    /// assert_eq!(vec, [1, 2, 3, 4, 5, 6]);
+    /// assert_eq!(vec2, []);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[inline(always)]
+    pub fn append(&mut self, other: &mut Self) {
+        unsafe {
+            self.append_elements(other.as_slice() as _);
+            other.set_len(0);
+        }
+    }
+
+    /// Appends elements to `self` from other buffer.
+    #[cfg(not(no_global_oom_handling))]
+    #[inline(always)]
+    unsafe fn append_elements(&mut self, other: *const [T]) {
+        let count = unsafe { (*other).len() };
+        self.reserve(count);
+        let len = self.len();
+        unsafe { ptr::copy_nonoverlapping(other as *const T, self.as_mut_ptr().add(len), count) };
+        self.len += count;
+    }
+
+    /// Removes the specified range from the vector in bulk, returning all
+    /// removed elements as an iterator. If the iterator is dropped before
+    /// being fully consumed, it drops the remaining removed elements.
+    ///
+    /// The returned iterator keeps a mutable borrow on the vector to optimize
+    /// its implementation.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the starting point is greater than the end point or if
+    /// the end point is greater than the length of the vector.
+    ///
+    /// # Leaking
+    ///
+    /// If the returned iterator goes out of scope without being dropped (due to
+    /// [`mem::forget`], for example), the vector may have lost and leaked
+    /// elements arbitrarily, including elements outside the range.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = vec![1, 2, 3];
+    /// let u: Vec<_> = v.drain(1..).collect();
+    /// assert_eq!(v, &[1]);
+    /// assert_eq!(u, &[2, 3]);
+    ///
+    /// // A full range clears the vector, like `clear()` does
+    /// v.drain(..);
+    /// assert_eq!(v, &[]);
+    /// ```
+    #[inline(always)]
+    pub fn drain<R>(&mut self, range: R) -> Drain<'_, T, A>
+    where
+        R: RangeBounds<usize>,
+    {
+        // Memory safety
+        //
+        // When the Drain is first created, it shortens the length of
+        // the source vector to make sure no uninitialized or moved-from elements
+        // are accessible at all if the Drain's destructor never gets to run.
+        //
+        // Drain will ptr::read out the values to remove.
+        // When finished, remaining tail of the vec is copied back to cover
+        // the hole, and the vector length is restored to the new length.
+        //
+        let len = self.len();
+
+        // Replaced by code below
+        // let Range { start, end } = slice::range(range, ..len);
+
+        // Panics if range is out of bounds
+        let _ = &self.as_slice()[(range.start_bound().cloned(), range.end_bound().cloned())];
+
+        let start = match range.start_bound() {
+            Bound::Included(&n) => n,
+            Bound::Excluded(&n) => n + 1,
+            Bound::Unbounded => 0,
+        };
+        let end = match range.end_bound() {
+            Bound::Included(&n) => n + 1,
+            Bound::Excluded(&n) => n,
+            Bound::Unbounded => len,
+        };
+
+        unsafe {
+            // set self.vec length's to start, to be safe in case Drain is leaked
+            self.set_len(start);
+            let range_slice = slice::from_raw_parts(self.as_ptr().add(start), end - start);
+            Drain {
+                tail_start: end,
+                tail_len: len - end,
+                iter: range_slice.iter(),
+                vec: NonNull::from(self),
+            }
+        }
+    }
+
+    /// Clears the vector, removing all values.
+    ///
+    /// Note that this method has no effect on the allocated capacity
+    /// of the vector.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = vec![1, 2, 3];
+    ///
+    /// v.clear();
+    ///
+    /// assert!(v.is_empty());
+    /// ```
+    #[inline(always)]
+    pub fn clear(&mut self) {
+        let elems: *mut [T] = self.as_mut_slice();
+
+        // SAFETY:
+        // - `elems` comes directly from `as_mut_slice` and is therefore valid.
+        // - Setting `self.len` before calling `drop_in_place` means that,
+        //   if an element's `Drop` impl panics, the vector's `Drop` impl will
+        //   do nothing (leaking the rest of the elements) instead of dropping
+        //   some twice.
+        unsafe {
+            self.len = 0;
+            ptr::drop_in_place(elems);
+        }
+    }
+
+    /// Returns the number of elements in the vector, also referred to
+    /// as its 'length'.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let a = vec![1, 2, 3];
+    /// assert_eq!(a.len(), 3);
+    /// ```
+    #[inline(always)]
+    pub fn len(&self) -> usize {
+        self.len
+    }
+
+    /// Returns `true` if the vector contains no elements.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = Vec::new();
+    /// assert!(v.is_empty());
+    ///
+    /// v.push(1);
+    /// assert!(!v.is_empty());
+    /// ```
+    #[inline(always)]
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    /// Splits the collection into two at the given index.
+    ///
+    /// Returns a newly allocated vector containing the elements in the range
+    /// `[at, len)`. After the call, the original vector will be left containing
+    /// the elements `[0, at)` with its previous capacity unchanged.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `at > len`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = vec![1, 2, 3];
+    /// let vec2 = vec.split_off(1);
+    /// assert_eq!(vec, [1]);
+    /// assert_eq!(vec2, [2, 3]);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[inline(always)]
+    #[must_use = "use `.truncate()` if you don't need the other half"]
+    pub fn split_off(&mut self, at: usize) -> Self
+    where
+        A: Clone,
+    {
+        #[cold]
+        #[inline(never)]
+        fn assert_failed(at: usize, len: usize) -> ! {
+            panic!("`at` split index (is {}) should be <= len (is {})", at, len);
+        }
+
+        if at > self.len() {
+            assert_failed(at, self.len());
+        }
+
+        if at == 0 {
+            // the new vector can take over the original buffer and avoid the copy
+            return mem::replace(
+                self,
+                Vec::with_capacity_in(self.capacity(), self.allocator().clone()),
+            );
+        }
+
+        let other_len = self.len - at;
+        let mut other = Vec::with_capacity_in(other_len, self.allocator().clone());
+
+        // Unsafely `set_len` and copy items to `other`.
+        unsafe {
+            self.set_len(at);
+            other.set_len(other_len);
+
+            ptr::copy_nonoverlapping(self.as_ptr().add(at), other.as_mut_ptr(), other.len());
+        }
+        other
+    }
+
+    /// Resizes the `Vec` in-place so that `len` is equal to `new_len`.
+    ///
+    /// If `new_len` is greater than `len`, the `Vec` is extended by the
+    /// difference, with each additional slot filled with the result of
+    /// calling the closure `f`. The return values from `f` will end up
+    /// in the `Vec` in the order they have been generated.
+    ///
+    /// If `new_len` is less than `len`, the `Vec` is simply truncated.
+    ///
+    /// This method uses a closure to create new values on every push. If
+    /// you'd rather [`Clone`] a given value, use [`Vec::resize`]. If you
+    /// want to use the [`Default`] trait to generate values, you can
+    /// pass [`Default::default`] as the second argument.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = vec![1, 2, 3];
+    /// vec.resize_with(5, Default::default);
+    /// assert_eq!(vec, [1, 2, 3, 0, 0]);
+    ///
+    /// let mut vec = vec![];
+    /// let mut p = 1;
+    /// vec.resize_with(4, || { p *= 2; p });
+    /// assert_eq!(vec, [2, 4, 8, 16]);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[inline(always)]
+    pub fn resize_with<F>(&mut self, new_len: usize, f: F)
+    where
+        F: FnMut() -> T,
+    {
+        let len = self.len();
+        if new_len > len {
+            self.extend(iter::repeat_with(f).take(new_len - len));
+        } else {
+            self.truncate(new_len);
+        }
+    }
+
+    /// Consumes and leaks the `Vec`, returning a mutable reference to the contents,
+    /// `&'a mut [T]`. Note that the type `T` must outlive the chosen lifetime
+    /// `'a`. If the type has only static references, or none at all, then this
+    /// may be chosen to be `'static`.
+    ///
+    /// As of Rust 1.57, this method does not reallocate or shrink the `Vec`,
+    /// so the leaked allocation may include unused capacity that is not part
+    /// of the returned slice.
+    ///
+    /// This function is mainly useful for data that lives for the remainder of
+    /// the program's life. Dropping the returned reference will cause a memory
+    /// leak.
+    ///
+    /// # Examples
+    ///
+    /// Simple usage:
+    ///
+    /// ```
+    /// let x = vec![1, 2, 3];
+    /// let static_ref: &'static mut [usize] = x.leak();
+    /// static_ref[0] += 1;
+    /// assert_eq!(static_ref, &[2, 2, 3]);
+    /// ```
+    #[inline(always)]
+    pub fn leak<'a>(self) -> &'a mut [T]
+    where
+        A: 'a,
+    {
+        let mut me = ManuallyDrop::new(self);
+        unsafe { slice::from_raw_parts_mut(me.as_mut_ptr(), me.len) }
+    }
+
+    /// Returns the remaining spare capacity of the vector as a slice of
+    /// `MaybeUninit<T>`.
+    ///
+    /// The returned slice can be used to fill the vector with data (e.g. by
+    /// reading from a file) before marking the data as initialized using the
+    /// [`set_len`] method.
+    ///
+    /// [`set_len`]: Vec::set_len
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// // Allocate vector big enough for 10 elements.
+    /// let mut v = Vec::with_capacity(10);
+    ///
+    /// // Fill in the first 3 elements.
+    /// let uninit = v.spare_capacity_mut();
+    /// uninit[0].write(0);
+    /// uninit[1].write(1);
+    /// uninit[2].write(2);
+    ///
+    /// // Mark the first 3 elements of the vector as being initialized.
+    /// unsafe {
+    ///     v.set_len(3);
+    /// }
+    ///
+    /// assert_eq!(&v, &[0, 1, 2]);
+    /// ```
+    #[inline(always)]
+    pub fn spare_capacity_mut(&mut self) -> &mut [MaybeUninit<T>] {
+        // Note:
+        // This method is not implemented in terms of `split_at_spare_mut`,
+        // to prevent invalidation of pointers to the buffer.
+        unsafe {
+            slice::from_raw_parts_mut(
+                self.as_mut_ptr().add(self.len) as *mut MaybeUninit<T>,
+                self.buf.capacity() - self.len,
+            )
+        }
+    }
+
+    /// Returns vector content as a slice of `T`, along with the remaining spare
+    /// capacity of the vector as a slice of `MaybeUninit<T>`.
+    ///
+    /// The returned spare capacity slice can be used to fill the vector with data
+    /// (e.g. by reading from a file) before marking the data as initialized using
+    /// the [`set_len`] method.
+    ///
+    /// [`set_len`]: Vec::set_len
+    ///
+    /// Note that this is a low-level API, which should be used with care for
+    /// optimization purposes. If you need to append data to a `Vec`
+    /// you can use [`push`], [`extend`], [`extend_from_slice`],
+    /// [`extend_from_within`], [`insert`], [`append`], [`resize`] or
+    /// [`resize_with`], depending on your exact needs.
+    ///
+    /// [`push`]: Vec::push
+    /// [`extend`]: Vec::extend
+    /// [`extend_from_slice`]: Vec::extend_from_slice
+    /// [`extend_from_within`]: Vec::extend_from_within
+    /// [`insert`]: Vec::insert
+    /// [`append`]: Vec::append
+    /// [`resize`]: Vec::resize
+    /// [`resize_with`]: Vec::resize_with
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(vec_split_at_spare)]
+    ///
+    /// let mut v = vec![1, 1, 2];
+    ///
+    /// // Reserve additional space big enough for 10 elements.
+    /// v.reserve(10);
+    ///
+    /// let (init, uninit) = v.split_at_spare_mut();
+    /// let sum = init.iter().copied().sum::<u32>();
+    ///
+    /// // Fill in the next 4 elements.
+    /// uninit[0].write(sum);
+    /// uninit[1].write(sum * 2);
+    /// uninit[2].write(sum * 3);
+    /// uninit[3].write(sum * 4);
+    ///
+    /// // Mark the 4 elements of the vector as being initialized.
+    /// unsafe {
+    ///     let len = v.len();
+    ///     v.set_len(len + 4);
+    /// }
+    ///
+    /// assert_eq!(&v, &[1, 1, 2, 4, 8, 12, 16]);
+    /// ```
+    #[inline(always)]
+    pub fn split_at_spare_mut(&mut self) -> (&mut [T], &mut [MaybeUninit<T>]) {
+        // SAFETY:
+        // - len is ignored and so never changed
+        let (init, spare, _) = unsafe { self.split_at_spare_mut_with_len() };
+        (init, spare)
+    }
+
+    /// Safety: changing returned .2 (&mut usize) is considered the same as calling `.set_len(_)`.
+    ///
+    /// This method provides unique access to all vec parts at once in `extend_from_within`.
+    unsafe fn split_at_spare_mut_with_len(
+        &mut self,
+    ) -> (&mut [T], &mut [MaybeUninit<T>], &mut usize) {
+        let ptr = self.as_mut_ptr();
+        // SAFETY:
+        // - `ptr` is guaranteed to be valid for `self.len` elements
+        // - but the allocation extends out to `self.buf.capacity()` elements, possibly
+        // uninitialized
+        let spare_ptr = unsafe { ptr.add(self.len) };
+        let spare_ptr = spare_ptr.cast::<MaybeUninit<T>>();
+        let spare_len = self.buf.capacity() - self.len;
+
+        // SAFETY:
+        // - `ptr` is guaranteed to be valid for `self.len` elements
+        // - `spare_ptr` is pointing one element past the buffer, so it doesn't overlap with `initialized`
+        unsafe {
+            let initialized = slice::from_raw_parts_mut(ptr, self.len);
+            let spare = slice::from_raw_parts_mut(spare_ptr, spare_len);
+
+            (initialized, spare, &mut self.len)
+        }
+    }
+}
+
+impl<T: Clone, A: Allocator> Vec<T, A> {
+    /// Resizes the `Vec` in-place so that `len` is equal to `new_len`.
+    ///
+    /// If `new_len` is greater than `len`, the `Vec` is extended by the
+    /// difference, with each additional slot filled with `value`.
+    /// If `new_len` is less than `len`, the `Vec` is simply truncated.
+    ///
+    /// This method requires `T` to implement [`Clone`],
+    /// in order to be able to clone the passed value.
+    /// If you need more flexibility (or want to rely on [`Default`] instead of
+    /// [`Clone`]), use [`Vec::resize_with`].
+    /// If you only need to resize to a smaller size, use [`Vec::truncate`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = vec!["hello"];
+    /// vec.resize(3, "world");
+    /// assert_eq!(vec, ["hello", "world", "world"]);
+    ///
+    /// let mut vec = vec![1, 2, 3, 4];
+    /// vec.resize(2, 0);
+    /// assert_eq!(vec, [1, 2]);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[inline(always)]
+    pub fn resize(&mut self, new_len: usize, value: T) {
+        let len = self.len();
+
+        if new_len > len {
+            self.extend_with(new_len - len, ExtendElement(value))
+        } else {
+            self.truncate(new_len);
+        }
+    }
+
+    /// Clones and appends all elements in a slice to the `Vec`.
+    ///
+    /// Iterates over the slice `other`, clones each element, and then appends
+    /// it to this `Vec`. The `other` slice is traversed in-order.
+    ///
+    /// Note that this function is same as [`extend`] except that it is
+    /// specialized to work with slices instead. If and when Rust gets
+    /// specialization this function will likely be deprecated (but still
+    /// available).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = vec![1];
+    /// vec.extend_from_slice(&[2, 3, 4]);
+    /// assert_eq!(vec, [1, 2, 3, 4]);
+    /// ```
+    ///
+    /// [`extend`]: Vec::extend
+    #[cfg(not(no_global_oom_handling))]
+    #[inline(always)]
+    pub fn extend_from_slice(&mut self, other: &[T]) {
+        self.extend(other.iter().cloned())
+    }
+
+    /// Copies elements from `src` range to the end of the vector.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the starting point is greater than the end point or if
+    /// the end point is greater than the length of the vector.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = vec![0, 1, 2, 3, 4];
+    ///
+    /// vec.extend_from_within(2..);
+    /// assert_eq!(vec, [0, 1, 2, 3, 4, 2, 3, 4]);
+    ///
+    /// vec.extend_from_within(..2);
+    /// assert_eq!(vec, [0, 1, 2, 3, 4, 2, 3, 4, 0, 1]);
+    ///
+    /// vec.extend_from_within(4..8);
+    /// assert_eq!(vec, [0, 1, 2, 3, 4, 2, 3, 4, 0, 1, 4, 2, 3, 4]);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[inline(always)]
+    pub fn extend_from_within<R>(&mut self, src: R)
+    where
+        R: RangeBounds<usize>,
+    {
+        // let range = slice::range(src, ..self.len());
+
+        let _ = &self.as_slice()[(src.start_bound().cloned(), src.end_bound().cloned())];
+
+        let len = self.len();
+
+        let start: ops::Bound<&usize> = src.start_bound();
+        let start = match start {
+            ops::Bound::Included(&start) => start,
+            ops::Bound::Excluded(start) => start + 1,
+            ops::Bound::Unbounded => 0,
+        };
+
+        let end: ops::Bound<&usize> = src.end_bound();
+        let end = match end {
+            ops::Bound::Included(end) => end + 1,
+            ops::Bound::Excluded(&end) => end,
+            ops::Bound::Unbounded => len,
+        };
+
+        let range = start..end;
+
+        self.reserve(range.len());
+
+        // SAFETY:
+        // - len is increased only after initializing elements
+        let (this, spare, len) = unsafe { self.split_at_spare_mut_with_len() };
+
+        // SAFETY:
+        // - caller guarantees that src is a valid index
+        let to_clone = unsafe { this.get_unchecked(range) };
+
+        iter::zip(to_clone, spare)
+            .map(|(src, dst)| dst.write(src.clone()))
+            // Note:
+            // - Element was just initialized with `MaybeUninit::write`, so it's ok to increase len
+            // - len is increased after each element to prevent leaks (see issue #82533)
+            .for_each(|_| *len += 1);
+    }
+}
+
+impl<T, A: Allocator, const N: usize> Vec<[T; N], A> {
+    /// Takes a `Vec<[T; N]>` and flattens it into a `Vec<T>`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the length of the resulting vector would overflow a `usize`.
+    ///
+    /// This is only possible when flattening a vector of arrays of zero-sized
+    /// types, and thus tends to be irrelevant in practice. If
+    /// `size_of::<T>() > 0`, this will never panic.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(slice_flatten)]
+    ///
+    /// let mut vec = vec![[1, 2, 3], [4, 5, 6], [7, 8, 9]];
+    /// assert_eq!(vec.pop(), Some([7, 8, 9]));
+    ///
+    /// let mut flattened = vec.into_flattened();
+    /// assert_eq!(flattened.pop(), Some(6));
+    /// ```
+    #[inline(always)]
+    pub fn into_flattened(self) -> Vec<T, A> {
+        let (ptr, len, cap, alloc) = self.into_raw_parts_with_alloc();
+        let (new_len, new_cap) = if size_of::<T>() == 0 {
+            (len.checked_mul(N).expect("vec len overflow"), usize::MAX)
+        } else {
+            // SAFETY:
+            // - `cap * N` cannot overflow because the allocation is already in
+            // the address space.
+            // - Each `[T; N]` has `N` valid elements, so there are `len * N`
+            // valid elements in the allocation.
+            (len * N, cap * N)
+        };
+        // SAFETY:
+        // - `ptr` was allocated by `self`
+        // - `ptr` is well-aligned because `[T; N]` has the same alignment as `T`.
+        // - `new_cap` refers to the same sized allocation as `cap` because
+        // `new_cap * size_of::<T>()` == `cap * size_of::<[T; N]>()`
+        // - `len` <= `cap`, so `len * N` <= `cap * N`.
+        unsafe { Vec::<T, A>::from_raw_parts_in(ptr.cast(), new_len, new_cap, alloc) }
+    }
+}
+
+// This code generalizes `extend_with_{element,default}`.
+trait ExtendWith<T> {
+    fn next(&mut self) -> T;
+    fn last(self) -> T;
+}
+
+struct ExtendElement<T>(T);
+impl<T: Clone> ExtendWith<T> for ExtendElement<T> {
+    #[inline(always)]
+    fn next(&mut self) -> T {
+        self.0.clone()
+    }
+
+    #[inline(always)]
+    fn last(self) -> T {
+        self.0
+    }
+}
+
+impl<T, A: Allocator> Vec<T, A> {
+    #[cfg(not(no_global_oom_handling))]
+    #[inline(always)]
+    /// Extend the vector by `n` values, using the given generator.
+    fn extend_with<E: ExtendWith<T>>(&mut self, n: usize, mut value: E) {
+        self.reserve(n);
+
+        unsafe {
+            let mut ptr = self.as_mut_ptr().add(self.len());
+            // Use SetLenOnDrop to work around bug where compiler
+            // might not realize the store through `ptr` through self.set_len()
+            // don't alias.
+            let mut local_len = SetLenOnDrop::new(&mut self.len);
+
+            // Write all elements except the last one
+            for _ in 1..n {
+                ptr::write(ptr, value.next());
+                ptr = ptr.add(1);
+                // Increment the length in every step in case next() panics
+                local_len.increment_len(1);
+            }
+
+            if n > 0 {
+                // We can write the last element directly without cloning needlessly
+                ptr::write(ptr, value.last());
+                local_len.increment_len(1);
+            }
+
+            // len set by scope guard
+        }
+    }
+}
+
+impl<T: PartialEq, A: Allocator> Vec<T, A> {
+    /// Removes consecutive repeated elements in the vector according to the
+    /// [`PartialEq`] trait implementation.
+    ///
+    /// If the vector is sorted, this removes all duplicates.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut vec = vec![1, 2, 2, 3, 2];
+    ///
+    /// vec.dedup();
+    ///
+    /// assert_eq!(vec, [1, 2, 3, 2]);
+    /// ```
+    #[inline(always)]
+    pub fn dedup(&mut self) {
+        self.dedup_by(|a, b| a == b)
+    }
+}
+
+trait ExtendFromWithinSpec {
+    /// # Safety
+    ///
+    /// - `src` needs to be valid index
+    /// - `self.capacity() - self.len()` must be `>= src.len()`
+    unsafe fn spec_extend_from_within(&mut self, src: Range<usize>);
+}
+
+// impl<T: Clone, A: Allocator> ExtendFromWithinSpec for Vec<T, A> {
+//     default unsafe fn spec_extend_from_within(&mut self, src: Range<usize>) {
+//         // SAFETY:
+//         // - len is increased only after initializing elements
+//         let (this, spare, len) = unsafe { self.split_at_spare_mut_with_len() };
+
+//         // SAFETY:
+//         // - caller guarantees that src is a valid index
+//         let to_clone = unsafe { this.get_unchecked(src) };
+
+//         iter::zip(to_clone, spare)
+//             .map(|(src, dst)| dst.write(src.clone()))
+//             // Note:
+//             // - Element was just initialized with `MaybeUninit::write`, so it's ok to increase len
+//             // - len is increased after each element to prevent leaks (see issue #82533)
+//             .for_each(|_| *len += 1);
+//     }
+// }
+
+impl<T: Copy, A: Allocator> ExtendFromWithinSpec for Vec<T, A> {
+    #[inline(always)]
+    unsafe fn spec_extend_from_within(&mut self, src: Range<usize>) {
+        let count = src.len();
+        {
+            let (init, spare) = self.split_at_spare_mut();
+
+            // SAFETY:
+            // - caller guarantees that `src` is a valid index
+            let source = unsafe { init.get_unchecked(src) };
+
+            // SAFETY:
+            // - Both pointers are created from unique slice references (`&mut [_]`)
+            //   so they are valid and do not overlap.
+            // - Elements are :Copy so it's OK to copy them, without doing
+            //   anything with the original values
+            // - `count` is equal to the len of `source`, so source is valid for
+            //   `count` reads
+            // - `.reserve(count)` guarantees that `spare.len() >= count` so spare
+            //   is valid for `count` writes
+            unsafe { ptr::copy_nonoverlapping(source.as_ptr(), spare.as_mut_ptr() as _, count) };
+        }
+
+        // SAFETY:
+        // - The elements were just initialized by `copy_nonoverlapping`
+        self.len += count;
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Common trait implementations for Vec
+////////////////////////////////////////////////////////////////////////////////
+
+impl<T, A: Allocator> ops::Deref for Vec<T, A> {
+    type Target = [T];
+
+    #[inline(always)]
+    fn deref(&self) -> &[T] {
+        unsafe { slice::from_raw_parts(self.as_ptr(), self.len) }
+    }
+}
+
+impl<T, A: Allocator> ops::DerefMut for Vec<T, A> {
+    #[inline(always)]
+    fn deref_mut(&mut self) -> &mut [T] {
+        unsafe { slice::from_raw_parts_mut(self.as_mut_ptr(), self.len) }
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+impl<T: Clone, A: Allocator + Clone> Clone for Vec<T, A> {
+    #[inline(always)]
+    fn clone(&self) -> Self {
+        let alloc = self.allocator().clone();
+        let mut vec = Vec::with_capacity_in(self.len(), alloc);
+        vec.extend_from_slice(self);
+        vec
+    }
+
+    #[inline(always)]
+    fn clone_from(&mut self, other: &Self) {
+        // drop anything that will not be overwritten
+        self.truncate(other.len());
+
+        // self.len <= other.len due to the truncate above, so the
+        // slices here are always in-bounds.
+        let (init, tail) = other.split_at(self.len());
+
+        // reuse the contained values' allocations/resources.
+        self.clone_from_slice(init);
+        self.extend_from_slice(tail);
+    }
+}
+
+/// The hash of a vector is the same as that of the corresponding slice,
+/// as required by the `core::borrow::Borrow` implementation.
+///
+/// ```
+/// #![feature(build_hasher_simple_hash_one)]
+/// use std::hash::BuildHasher;
+///
+/// let b = std::collections::hash_map::RandomState::new();
+/// let v: Vec<u8> = vec![0xa8, 0x3c, 0x09];
+/// let s: &[u8] = &[0xa8, 0x3c, 0x09];
+/// assert_eq!(b.hash_one(v), b.hash_one(s));
+/// ```
+impl<T: Hash, A: Allocator> Hash for Vec<T, A> {
+    #[inline(always)]
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        Hash::hash(&**self, state)
+    }
+}
+
+impl<T, I: SliceIndex<[T]>, A: Allocator> Index<I> for Vec<T, A> {
+    type Output = I::Output;
+
+    #[inline(always)]
+    fn index(&self, index: I) -> &Self::Output {
+        Index::index(&**self, index)
+    }
+}
+
+impl<T, I: SliceIndex<[T]>, A: Allocator> IndexMut<I> for Vec<T, A> {
+    #[inline(always)]
+    fn index_mut(&mut self, index: I) -> &mut Self::Output {
+        IndexMut::index_mut(&mut **self, index)
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+impl<T> FromIterator<T> for Vec<T> {
+    #[inline(always)]
+    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Vec<T> {
+        let mut vec = Vec::new();
+        vec.extend(iter);
+        vec
+    }
+}
+
+impl<T, A: Allocator> IntoIterator for Vec<T, A> {
+    type Item = T;
+    type IntoIter = IntoIter<T, A>;
+
+    /// Creates a consuming iterator, that is, one that moves each value out of
+    /// the vector (from start to end). The vector cannot be used after calling
+    /// this.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let v = vec!["a".to_string(), "b".to_string()];
+    /// let mut v_iter = v.into_iter();
+    ///
+    /// let first_element: Option<String> = v_iter.next();
+    ///
+    /// assert_eq!(first_element, Some("a".to_string()));
+    /// assert_eq!(v_iter.next(), Some("b".to_string()));
+    /// assert_eq!(v_iter.next(), None);
+    /// ```
+    #[inline(always)]
+    fn into_iter(self) -> Self::IntoIter {
+        unsafe {
+            let mut me = ManuallyDrop::new(self);
+            let alloc = ManuallyDrop::new(ptr::read(me.allocator()));
+            let begin = me.as_mut_ptr();
+            let end = if size_of::<T>() == 0 {
+                begin.cast::<u8>().wrapping_add(me.len()).cast()
+            } else {
+                begin.add(me.len()) as *const T
+            };
+            let cap = me.buf.capacity();
+            IntoIter {
+                buf: NonNull::new_unchecked(begin),
+                phantom: PhantomData,
+                cap,
+                alloc,
+                ptr: begin,
+                end,
+            }
+        }
+    }
+}
+
+impl<'a, T, A: Allocator> IntoIterator for &'a Vec<T, A> {
+    type Item = &'a T;
+    type IntoIter = slice::Iter<'a, T>;
+
+    #[inline(always)]
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter()
+    }
+}
+
+impl<'a, T, A: Allocator> IntoIterator for &'a mut Vec<T, A> {
+    type Item = &'a mut T;
+    type IntoIter = slice::IterMut<'a, T>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter_mut()
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+impl<T, A: Allocator> Extend<T> for Vec<T, A> {
+    #[inline(always)]
+    fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
+        // This is the case for a general iter.
+        //
+        // This function should be the moral equivalent of:
+        //
+        //      for item in iter {
+        //          self.push(item);
+        //      }
+
+        let mut iter = iter.into_iter();
+        while let Some(element) = iter.next() {
+            let len = self.len();
+            if len == self.capacity() {
+                let (lower, _) = iter.size_hint();
+                self.reserve(lower.saturating_add(1));
+            }
+            unsafe {
+                ptr::write(self.as_mut_ptr().add(len), element);
+                // Since next() executes user code which can panic we have to bump the length
+                // after each step.
+                // NB can't overflow since we would have had to alloc the address space
+                self.set_len(len + 1);
+            }
+        }
+    }
+}
+
+impl<T, A: Allocator> Vec<T, A> {
+    /// Creates a splicing iterator that replaces the specified range in the vector
+    /// with the given `replace_with` iterator and yields the removed items.
+    /// `replace_with` does not need to be the same length as `range`.
+    ///
+    /// `range` is removed even if the iterator is not consumed until the end.
+    ///
+    /// It is unspecified how many elements are removed from the vector
+    /// if the `Splice` value is leaked.
+    ///
+    /// The input iterator `replace_with` is only consumed when the `Splice` value is dropped.
+    ///
+    /// This is optimal if:
+    ///
+    /// * The tail (elements in the vector after `range`) is empty,
+    /// * or `replace_with` yields fewer or equal elements than `range`’s length
+    /// * or the lower bound of its `size_hint()` is exact.
+    ///
+    /// Otherwise, a temporary vector is allocated and the tail is moved twice.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the starting point is greater than the end point or if
+    /// the end point is greater than the length of the vector.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = vec![1, 2, 3, 4];
+    /// let new = [7, 8, 9];
+    /// let u: Vec<_> = v.splice(1..3, new).collect();
+    /// assert_eq!(v, &[1, 7, 8, 9, 4]);
+    /// assert_eq!(u, &[2, 3]);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[inline(always)]
+    pub fn splice<R, I>(&mut self, range: R, replace_with: I) -> Splice<'_, I::IntoIter, A>
+    where
+        R: RangeBounds<usize>,
+        I: IntoIterator<Item = T>,
+    {
+        Splice {
+            drain: self.drain(range),
+            replace_with: replace_with.into_iter(),
+        }
+    }
+}
+
+/// Extend implementation that copies elements out of references before pushing them onto the Vec.
+///
+/// This implementation is specialized for slice iterators, where it uses [`copy_from_slice`] to
+/// append the entire slice at once.
+///
+/// [`copy_from_slice`]: slice::copy_from_slice
+#[cfg(not(no_global_oom_handling))]
+impl<'a, T: Copy + 'a, A: Allocator + 'a> Extend<&'a T> for Vec<T, A> {
+    #[inline(always)]
+    fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
+        let mut iter = iter.into_iter();
+        while let Some(element) = iter.next() {
+            let len = self.len();
+            if len == self.capacity() {
+                let (lower, _) = iter.size_hint();
+                self.reserve(lower.saturating_add(1));
+            }
+            unsafe {
+                ptr::write(self.as_mut_ptr().add(len), *element);
+                // Since next() executes user code which can panic we have to bump the length
+                // after each step.
+                // NB can't overflow since we would have had to alloc the address space
+                self.set_len(len + 1);
+            }
+        }
+    }
+}
+
+/// Implements comparison of vectors, [lexicographically](core::cmp::Ord#lexicographical-comparison).
+impl<T: PartialOrd, A: Allocator> PartialOrd for Vec<T, A> {
+    #[inline(always)]
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        PartialOrd::partial_cmp(&**self, &**other)
+    }
+}
+
+impl<T: Eq, A: Allocator> Eq for Vec<T, A> {}
+
+/// Implements ordering of vectors, [lexicographically](core::cmp::Ord#lexicographical-comparison).
+impl<T: Ord, A: Allocator> Ord for Vec<T, A> {
+    #[inline(always)]
+    fn cmp(&self, other: &Self) -> Ordering {
+        Ord::cmp(&**self, &**other)
+    }
+}
+
+impl<T, A: Allocator> Drop for Vec<T, A> {
+    #[inline(always)]
+    fn drop(&mut self) {
+        unsafe {
+            // use drop for [T]
+            // use a raw slice to refer to the elements of the vector as weakest necessary type;
+            // could avoid questions of validity in certain cases
+            ptr::drop_in_place(ptr::slice_from_raw_parts_mut(self.as_mut_ptr(), self.len))
+        }
+        // RawVec handles deallocation
+    }
+}
+
+impl<T> Default for Vec<T> {
+    /// Creates an empty `Vec<T>`.
+    ///
+    /// The vector will not allocate until elements are pushed onto it.
+    #[inline(always)]
+    fn default() -> Vec<T> {
+        Vec::new()
+    }
+}
+
+impl<T: fmt::Debug, A: Allocator> fmt::Debug for Vec<T, A> {
+    #[inline(always)]
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&**self, f)
+    }
+}
+
+impl<T, A: Allocator> AsRef<Vec<T, A>> for Vec<T, A> {
+    #[inline(always)]
+    fn as_ref(&self) -> &Vec<T, A> {
+        self
+    }
+}
+
+impl<T, A: Allocator> AsMut<Vec<T, A>> for Vec<T, A> {
+    #[inline(always)]
+    fn as_mut(&mut self) -> &mut Vec<T, A> {
+        self
+    }
+}
+
+impl<T, A: Allocator> AsRef<[T]> for Vec<T, A> {
+    #[inline(always)]
+    fn as_ref(&self) -> &[T] {
+        self
+    }
+}
+
+impl<T, A: Allocator> AsMut<[T]> for Vec<T, A> {
+    #[inline(always)]
+    fn as_mut(&mut self) -> &mut [T] {
+        self
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+impl<T: Clone> From<&[T]> for Vec<T> {
+    /// Allocate a `Vec<T>` and fill it by cloning `s`'s items.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// assert_eq!(Vec::from(&[1, 2, 3][..]), vec![1, 2, 3]);
+    /// ```
+    #[inline(always)]
+    fn from(s: &[T]) -> Vec<T> {
+        let mut vec = Vec::with_capacity(s.len());
+        vec.extend_from_slice(s);
+        vec
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+impl<T: Clone> From<&mut [T]> for Vec<T> {
+    /// Allocate a `Vec<T>` and fill it by cloning `s`'s items.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// assert_eq!(Vec::from(&mut [1, 2, 3][..]), vec![1, 2, 3]);
+    /// ```
+    #[inline(always)]
+    fn from(s: &mut [T]) -> Vec<T> {
+        let mut vec = Vec::with_capacity(s.len());
+        vec.extend_from_slice(s);
+        vec
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+impl<T, const N: usize> From<[T; N]> for Vec<T> {
+    #[inline(always)]
+    fn from(s: [T; N]) -> Vec<T> {
+        Box::slice(Box::new(s)).into_vec()
+    }
+}
+
+impl<T, A: Allocator> From<Box<[T], A>> for Vec<T, A> {
+    /// Convert a boxed slice into a vector by transferring ownership of
+    /// the existing heap allocation.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let b: Box<[i32]> = vec![1, 2, 3].into_boxed_slice();
+    /// assert_eq!(Vec::from(b), vec![1, 2, 3]);
+    /// ```
+    #[inline(always)]
+    fn from(s: Box<[T], A>) -> Self {
+        s.into_vec()
+    }
+}
+
+impl<T, A: Allocator, const N: usize> From<Box<[T; N], A>> for Vec<T, A> {
+    /// Convert a boxed array into a vector by transferring ownership of
+    /// the existing heap allocation.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let b: Box<[i32; 3]> = Box::new([1, 2, 3]);
+    /// assert_eq!(Vec::from(b), vec![1, 2, 3]);
+    /// ```
+    #[inline(always)]
+    fn from(s: Box<[T; N], A>) -> Self {
+        s.into_vec()
+    }
+}
+
+// note: test pulls in libstd, which causes errors here
+#[cfg(not(no_global_oom_handling))]
+impl<T, A: Allocator> From<Vec<T, A>> for Box<[T], A> {
+    /// Convert a vector into a boxed slice.
+    ///
+    /// If `v` has excess capacity, its items will be moved into a
+    /// newly-allocated buffer with exactly the right capacity.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// assert_eq!(Box::from(vec![1, 2, 3]), vec![1, 2, 3].into_boxed_slice());
+    /// ```
+    ///
+    /// Any excess capacity is removed:
+    /// ```
+    /// let mut vec = Vec::with_capacity(10);
+    /// vec.extend([1, 2, 3]);
+    ///
+    /// assert_eq!(Box::from(vec), vec![1, 2, 3].into_boxed_slice());
+    /// ```
+    #[inline(always)]
+    fn from(v: Vec<T, A>) -> Self {
+        v.into_boxed_slice()
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+impl From<&str> for Vec<u8> {
+    /// Allocate a `Vec<u8>` and fill it with a UTF-8 string.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// assert_eq!(Vec::from("123"), vec![b'1', b'2', b'3']);
+    /// ```
+    #[inline(always)]
+    fn from(s: &str) -> Vec<u8> {
+        From::from(s.as_bytes())
+    }
+}
+
+impl<T, A: Allocator, const N: usize> TryFrom<Vec<T, A>> for [T; N] {
+    type Error = Vec<T, A>;
+
+    /// Gets the entire contents of the `Vec<T>` as an array,
+    /// if its size exactly matches that of the requested array.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// assert_eq!(vec![1, 2, 3].try_into(), Ok([1, 2, 3]));
+    /// assert_eq!(<Vec<i32>>::new().try_into(), Ok([]));
+    /// ```
+    ///
+    /// If the length doesn't match, the input comes back in `Err`:
+    /// ```
+    /// let r: Result<[i32; 4], _> = (0..10).collect::<Vec<_>>().try_into();
+    /// assert_eq!(r, Err(vec![0, 1, 2, 3, 4, 5, 6, 7, 8, 9]));
+    /// ```
+    ///
+    /// If you're fine with just getting a prefix of the `Vec<T>`,
+    /// you can call [`.truncate(N)`](Vec::truncate) first.
+    /// ```
+    /// let mut v = String::from("hello world").into_bytes();
+    /// v.sort();
+    /// v.truncate(2);
+    /// let [a, b]: [_; 2] = v.try_into().unwrap();
+    /// assert_eq!(a, b' ');
+    /// assert_eq!(b, b'd');
+    /// ```
+    #[inline(always)]
+    fn try_from(mut vec: Vec<T, A>) -> Result<[T; N], Vec<T, A>> {
+        if vec.len() != N {
+            return Err(vec);
+        }
+
+        // SAFETY: `.set_len(0)` is always sound.
+        unsafe { vec.set_len(0) };
+
+        // SAFETY: A `Vec`'s pointer is always aligned properly, and
+        // the alignment the array needs is the same as the items.
+        // We checked earlier that we have sufficient items.
+        // The items will not double-drop as the `set_len`
+        // tells the `Vec` not to also drop them.
+        let array = unsafe { ptr::read(vec.as_ptr() as *const [T; N]) };
+        Ok(array)
+    }
+}
+
+#[inline(always)]
+#[cfg(not(no_global_oom_handling))]
+#[doc(hidden)]
+pub fn from_elem_in<T: Clone, A: Allocator>(elem: T, n: usize, alloc: A) -> Vec<T, A> {
+    let mut v = Vec::with_capacity_in(n, alloc);
+    v.extend_with(n, ExtendElement(elem));
+    v
+}
+
+#[inline(always)]
+#[cfg(not(no_global_oom_handling))]
+#[doc(hidden)]
+pub fn from_elem<T: Clone>(elem: T, n: usize) -> Vec<T> {
+    let mut v = Vec::with_capacity(n);
+    v.extend_with(n, ExtendElement(elem));
+    v
+}
+
+#[cfg(feature = "serde")]
+impl<T, A> serde::Serialize for Vec<T, A>
+where
+    T: serde::Serialize,
+    A: Allocator,
+{
+    #[inline(always)]
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::ser::Serializer,
+    {
+        serializer.collect_seq(self)
+    }
+}
+
+#[cfg(feature = "serde")]
+impl<'de, T, A> serde::de::Deserialize<'de> for Vec<T, A>
+where
+    T: serde::de::Deserialize<'de>,
+    A: Allocator + Default,
+{
+    #[inline(always)]
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::de::Deserializer<'de>,
+    {
+        struct VecVisitor<T, A> {
+            marker: PhantomData<(T, A)>,
+        }
+
+        impl<'de, T, A> serde::de::Visitor<'de> for VecVisitor<T, A>
+        where
+            T: serde::de::Deserialize<'de>,
+            A: Allocator + Default,
+        {
+            type Value = Vec<T, A>;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                formatter.write_str("a sequence")
+            }
+
+            fn visit_seq<S>(self, mut seq: S) -> Result<Self::Value, S::Error>
+            where
+                S: serde::de::SeqAccess<'de>,
+            {
+                let mut values = Vec::with_capacity_in(cautious(seq.size_hint()), A::default());
+
+                while let Some(value) = seq.next_element()? {
+                    values.push(value);
+                }
+
+                Ok(values)
+            }
+        }
+
+        let visitor = VecVisitor {
+            marker: PhantomData,
+        };
+        deserializer.deserialize_seq(visitor)
+    }
+
+    #[inline(always)]
+    fn deserialize_in_place<D>(deserializer: D, place: &mut Self) -> Result<(), D::Error>
+    where
+        D: serde::de::Deserializer<'de>,
+    {
+        struct VecInPlaceVisitor<'a, T: 'a, A: Allocator + 'a>(&'a mut Vec<T, A>);
+
+        impl<'a, 'de, T, A> serde::de::Visitor<'de> for VecInPlaceVisitor<'a, T, A>
+        where
+            T: serde::de::Deserialize<'de>,
+            A: Allocator + Default,
+        {
+            type Value = ();
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                formatter.write_str("a sequence")
+            }
+
+            fn visit_seq<S>(self, mut seq: S) -> Result<Self::Value, S::Error>
+            where
+                S: serde::de::SeqAccess<'de>,
+            {
+                let hint = cautious(seq.size_hint());
+                if let Some(additional) = hint.checked_sub(self.0.len()) {
+                    self.0.reserve(additional);
+                }
+
+                for i in 0..self.0.len() {
+                    let next = {
+                        let next_place = InPlaceSeed(&mut self.0[i]);
+                        seq.next_element_seed(next_place)?
+                    };
+                    if next.is_none() {
+                        self.0.truncate(i);
+                        return Ok(());
+                    }
+                }
+
+                while let Some(value) = seq.next_element()? {
+                    self.0.push(value);
+                }
+
+                Ok(())
+            }
+        }
+
+        deserializer.deserialize_seq(VecInPlaceVisitor(place))
+    }
+}
+
+#[cfg(feature = "serde")]
+pub fn cautious(hint: Option<usize>) -> usize {
+    cmp::min(hint.unwrap_or(0), 4096)
+}
+
+/// A DeserializeSeed helper for implementing deserialize_in_place Visitors.
+///
+/// Wraps a mutable reference and calls deserialize_in_place on it.
+
+#[cfg(feature = "serde")]
+pub struct InPlaceSeed<'a, T: 'a>(pub &'a mut T);
+
+#[cfg(feature = "serde")]
+impl<'a, 'de, T> serde::de::DeserializeSeed<'de> for InPlaceSeed<'a, T>
+where
+    T: serde::de::Deserialize<'de>,
+{
+    type Value = ();
+    fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+    where
+        D: serde::de::Deserializer<'de>,
+    {
+        T::deserialize_in_place(deserializer, self.0)
+    }
+}
diff --git a/vendor/allocator-api2/src/stable/vec/partial_eq.rs b/vendor/allocator-api2/src/stable/vec/partial_eq.rs
new file mode 100644
index 000000000..157d9c0b9
--- /dev/null
+++ b/vendor/allocator-api2/src/stable/vec/partial_eq.rs
@@ -0,0 +1,43 @@
+#[cfg(not(no_global_oom_handling))]
+use alloc_crate::borrow::Cow;
+
+use crate::stable::alloc::Allocator;
+
+use super::Vec;
+
+macro_rules! __impl_slice_eq1 {
+    ([$($vars:tt)*] $lhs:ty, $rhs:ty $(where $ty:ty: $bound:ident)?) => {
+        impl<T, U, $($vars)*> PartialEq<$rhs> for $lhs
+        where
+            T: PartialEq<U>,
+            $($ty: $bound)?
+        {
+            #[inline(always)]
+            fn eq(&self, other: &$rhs) -> bool { self[..] == other[..] }
+            #[inline(always)]
+            fn ne(&self, other: &$rhs) -> bool { self[..] != other[..] }
+        }
+    }
+}
+
+__impl_slice_eq1! { [A1: Allocator, A2: Allocator] Vec<T, A1>, Vec<U, A2> }
+__impl_slice_eq1! { [A: Allocator] Vec<T, A>, &[U] }
+__impl_slice_eq1! { [A: Allocator] Vec<T, A>, &mut [U] }
+__impl_slice_eq1! { [A: Allocator] &[T], Vec<U, A> }
+__impl_slice_eq1! { [A: Allocator] &mut [T], Vec<U, A> }
+__impl_slice_eq1! { [A: Allocator] Vec<T, A>, [U]  }
+__impl_slice_eq1! { [A: Allocator] [T], Vec<U, A>  }
+#[cfg(not(no_global_oom_handling))]
+__impl_slice_eq1! { [A: Allocator] Cow<'_, [T]>, Vec<U, A> where T: Clone }
+__impl_slice_eq1! { [A: Allocator, const N: usize] Vec<T, A>, [U; N] }
+__impl_slice_eq1! { [A: Allocator, const N: usize] Vec<T, A>, &[U; N] }
+
+// NOTE: some less important impls are omitted to reduce code bloat
+// FIXME(Centril): Reconsider this?
+//__impl_slice_eq1! { [const N: usize] Vec<A>, &mut [B; N], }
+//__impl_slice_eq1! { [const N: usize] [A; N], Vec<B>, }
+//__impl_slice_eq1! { [const N: usize] &[A; N], Vec<B>, }
+//__impl_slice_eq1! { [const N: usize] &mut [A; N], Vec<B>, }
+//__impl_slice_eq1! { [const N: usize] Cow<'a, [A]>, [B; N], }
+//__impl_slice_eq1! { [const N: usize] Cow<'a, [A]>, &[B; N], }
+//__impl_slice_eq1! { [const N: usize] Cow<'a, [A]>, &mut [B; N], }
diff --git a/vendor/allocator-api2/src/stable/vec/set_len_on_drop.rs b/vendor/allocator-api2/src/stable/vec/set_len_on_drop.rs
new file mode 100644
index 000000000..c6f831321
--- /dev/null
+++ b/vendor/allocator-api2/src/stable/vec/set_len_on_drop.rs
@@ -0,0 +1,31 @@
+// Set the length of the vec when the `SetLenOnDrop` value goes out of scope.
+//
+// The idea is: The length field in SetLenOnDrop is a local variable
+// that the optimizer will see does not alias with any stores through the Vec's data
+// pointer. This is a workaround for alias analysis issue #32155
+pub(super) struct SetLenOnDrop<'a> {
+    len: &'a mut usize,
+    local_len: usize,
+}
+
+impl<'a> SetLenOnDrop<'a> {
+    #[inline(always)]
+    pub(super) fn new(len: &'a mut usize) -> Self {
+        SetLenOnDrop {
+            local_len: *len,
+            len,
+        }
+    }
+
+    #[inline(always)]
+    pub(super) fn increment_len(&mut self, increment: usize) {
+        self.local_len += increment;
+    }
+}
+
+impl Drop for SetLenOnDrop<'_> {
+    #[inline(always)]
+    fn drop(&mut self) {
+        *self.len = self.local_len;
+    }
+}
diff --git a/vendor/allocator-api2/src/stable/vec/splice.rs b/vendor/allocator-api2/src/stable/vec/splice.rs
new file mode 100644
index 000000000..0f64c87af
--- /dev/null
+++ b/vendor/allocator-api2/src/stable/vec/splice.rs
@@ -0,0 +1,135 @@
+use core::ptr::{self};
+use core::slice::{self};
+
+use crate::stable::alloc::{Allocator, Global};
+
+use super::{Drain, Vec};
+
+/// A splicing iterator for `Vec`.
+///
+/// This struct is created by [`Vec::splice()`].
+/// See its documentation for more.
+///
+/// # Example
+///
+/// ```
+/// let mut v = vec![0, 1, 2];
+/// let new = [7, 8];
+/// let iter: std::vec::Splice<_> = v.splice(1.., new);
+/// ```
+#[derive(Debug)]
+pub struct Splice<'a, I: Iterator + 'a, A: Allocator + 'a = Global> {
+    pub(super) drain: Drain<'a, I::Item, A>,
+    pub(super) replace_with: I,
+}
+
+impl<I: Iterator, A: Allocator> Iterator for Splice<'_, I, A> {
+    type Item = I::Item;
+
+    #[inline(always)]
+    fn next(&mut self) -> Option<Self::Item> {
+        self.drain.next()
+    }
+
+    #[inline(always)]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.drain.size_hint()
+    }
+}
+
+impl<I: Iterator, A: Allocator> DoubleEndedIterator for Splice<'_, I, A> {
+    #[inline(always)]
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.drain.next_back()
+    }
+}
+
+impl<I: Iterator, A: Allocator> ExactSizeIterator for Splice<'_, I, A> {}
+
+impl<I: Iterator, A: Allocator> Drop for Splice<'_, I, A> {
+    #[inline]
+    fn drop(&mut self) {
+        self.drain.by_ref().for_each(drop);
+
+        unsafe {
+            if self.drain.tail_len == 0 {
+                self.drain.vec.as_mut().extend(self.replace_with.by_ref());
+                return;
+            }
+
+            // First fill the range left by drain().
+            if !self.drain.fill(&mut self.replace_with) {
+                return;
+            }
+
+            // There may be more elements. Use the lower bound as an estimate.
+            // FIXME: Is the upper bound a better guess? Or something else?
+            let (lower_bound, _upper_bound) = self.replace_with.size_hint();
+            if lower_bound > 0 {
+                self.drain.move_tail(lower_bound);
+                if !self.drain.fill(&mut self.replace_with) {
+                    return;
+                }
+            }
+
+            // Collect any remaining elements.
+            // This is a zero-length vector which does not allocate if `lower_bound` was exact.
+            let mut collected = self
+                .replace_with
+                .by_ref()
+                .collect::<Vec<I::Item>>()
+                .into_iter();
+            // Now we have an exact count.
+            if collected.len() > 0 {
+                self.drain.move_tail(collected.len());
+                let filled = self.drain.fill(&mut collected);
+                debug_assert!(filled);
+                debug_assert_eq!(collected.len(), 0);
+            }
+        }
+        // Let `Drain::drop` move the tail back if necessary and restore `vec.len`.
+    }
+}
+
+/// Private helper methods for `Splice::drop`
+impl<T, A: Allocator> Drain<'_, T, A> {
+    /// The range from `self.vec.len` to `self.tail_start` contains elements
+    /// that have been moved out.
+    /// Fill that range as much as possible with new elements from the `replace_with` iterator.
+    /// Returns `true` if we filled the entire range. (`replace_with.next()` didn’t return `None`.)
+    #[inline(always)]
+    unsafe fn fill<I: Iterator<Item = T>>(&mut self, replace_with: &mut I) -> bool {
+        let vec = unsafe { self.vec.as_mut() };
+        let range_start = vec.len;
+        let range_end = self.tail_start;
+        let range_slice = unsafe {
+            slice::from_raw_parts_mut(vec.as_mut_ptr().add(range_start), range_end - range_start)
+        };
+
+        for place in range_slice {
+            if let Some(new_item) = replace_with.next() {
+                unsafe { ptr::write(place, new_item) };
+                vec.len += 1;
+            } else {
+                return false;
+            }
+        }
+        true
+    }
+
+    /// Makes room for inserting more elements before the tail.
+    #[inline(always)]
+    unsafe fn move_tail(&mut self, additional: usize) {
+        let vec = unsafe { self.vec.as_mut() };
+        let len = self.tail_start + self.tail_len;
+        vec.buf.reserve(len, additional);
+
+        let new_tail_start = self.tail_start + additional;
+        unsafe {
+            let src = vec.as_ptr().add(self.tail_start);
+            let dst = vec.as_mut_ptr().add(new_tail_start);
+            ptr::copy(src, dst, self.tail_len);
+        }
+        self.tail_start = new_tail_start;
+    }
+}