Merge tag 'rust-v6.1-rc1' of https://github.com/Rust-for-Linux/linux

Pull Rust introductory support from Kees Cook:
 "The tree has a recent base, but has fundamentally been in linux-next
  for a year and a half[1]. It's been updated based on feedback from the
  Kernel Maintainer's Summit, and to gain recent Reviewed-by: tags.

  Miguel is the primary maintainer, with me helping where needed/wanted.
  Our plan is for the tree to switch to the standard non-rebasing
  practice once this initial infrastructure series lands.

  The contents are the absolute minimum to get Rust code building in the
  kernel, with many more interfaces[2] (and drivers - NVMe[3], 9p[4], M1
  GPU[5]) on the way.

  The initial support of Rust-for-Linux comes in roughly 4 areas:

   - Kernel internals (kallsyms expansion for Rust symbols, %pA format)

   - Kbuild infrastructure (Rust build rules and support scripts)

   - Rust crates and bindings for initial minimum viable build

   - Rust kernel documentation and samples

  Rust support has been in linux-next for a year and a half now, and the
  short log doesn't do justice to the number of people who have
  contributed both to the Linux kernel side but also to the upstream
  Rust side to support the kernel's needs. Thanks to these 173 people,
  and many more, who have been involved in all kinds of ways:

  Miguel Ojeda, Wedson Almeida Filho, Alex Gaynor, Boqun Feng, Gary Guo,
  Björn Roy Baron, Andreas Hindborg, Adam Bratschi-Kaye, Benno Lossin,
  Maciej Falkowski, Finn Behrens, Sven Van Asbroeck, Asahi Lina, FUJITA
  Tomonori, John Baublitz, Wei Liu, Geoffrey Thomas, Philip Herron,
  Arthur Cohen, David Faust, Antoni Boucher, Philip Li, Yujie Liu,
  Jonathan Corbet, Greg Kroah-Hartman, Paul E. McKenney, Josh Triplett,
  Kent Overstreet, David Gow, Alice Ryhl, Robin Randhawa, Kees Cook,
  Nick Desaulniers, Matthew Wilcox, Linus Walleij, Joe Perches, Michael
  Ellerman, Petr Mladek, Masahiro Yamada, Arnaldo Carvalho de Melo,
  Andrii Nakryiko, Konstantin Shelekhin, Rasmus Villemoes, Konstantin
  Ryabitsev, Stephen Rothwell, Andy Shevchenko, Sergey Senozhatsky, John
  Paul Adrian Glaubitz, David Laight, Nathan Chancellor, Jonathan
  Cameron, Daniel Latypov, Shuah Khan, Brendan Higgins, Julia Lawall,
  Laurent Pinchart, Geert Uytterhoeven, Akira Yokosawa, Pavel Machek,
  David S. Miller, John Hawley, James Bottomley, Arnd Bergmann,
  Christian Brauner, Dan Robertson, Nicholas Piggin, Zhouyi Zhou, Elena
  Zannoni, Jose E. Marchesi, Leon Romanovsky, Will Deacon, Richard
  Weinberger, Randy Dunlap, Paolo Bonzini, Roland Dreier, Mark Brown,
  Sasha Levin, Ted Ts'o, Steven Rostedt, Jarkko Sakkinen, Michal
  Kubecek, Marco Elver, Al Viro, Keith Busch, Johannes Berg, Jan Kara,
  David Sterba, Connor Kuehl, Andy Lutomirski, Andrew Lunn, Alexandre
  Belloni, Peter Zijlstra, Russell King, Eric W. Biederman, Willy
  Tarreau, Christoph Hellwig, Emilio Cobos Álvarez, Christian Poveda,
  Mark Rousskov, John Ericson, TennyZhuang, Xuanwo, Daniel Paoliello,
  Manish Goregaokar, comex, Josh Stone, Stephan Sokolow, Philipp Krones,
  Guillaume Gomez, Joshua Nelson, Mats Larsen, Marc Poulhiès, Samantha
  Miller, Esteban Blanc, Martin Schmidt, Martin Rodriguez Reboredo,
  Daniel Xu, Viresh Kumar, Bartosz Golaszewski, Vegard Nossum, Milan
  Landaverde, Dariusz Sosnowski, Yuki Okushi, Matthew Bakhtiari, Wu
  XiangCheng, Tiago Lam, Boris-Chengbiao Zhou, Sumera Priyadarsini,
  Viktor Garske, Niklas Mohrin, Nándor István Krácser, Morgan Bartlett,
  Miguel Cano, Léo Lanteri Thauvin, Julian Merkle, Andreas Reindl,
  Jiapeng Chong, Fox Chen, Douglas Su, Antonio Terceiro, SeongJae Park,
  Sergio González Collado, Ngo Iok Ui (Wu Yu Wei), Joshua Abraham,
  Milan, Daniel Kolsoi, ahomescu, Manas, Luis Gerhorst, Li Hongyu,
  Philipp Gesang, Russell Currey, Jalil David Salamé Messina, Jon Olson,
  Raghvender, Angelos, Kaviraj Kanagaraj, Paul Römer, Sladyn Nunes,
  Mauro Baladés, Hsiang-Cheng Yang, Abhik Jain, Hongyu Li, Sean Nash,
  Yuheng Su, Peng Hao, Anhad Singh, Roel Kluin, Sara Saa, Geert
  Stappers, Garrett LeSage, IFo Hancroft, and Linus Torvalds"

Link: https://lwn.net/Articles/849849/ [1]
Link: https://github.com/Rust-for-Linux/linux/commits/rust [2]
Link: https://github.com/metaspace/rust-linux/commit/d88c3744d6cbdf11767e08bad56cbfb67c4c96d0 [3]
Link: https://github.com/wedsonaf/linux/commit/9367032607f7670de0ba1537cf09ab0f4365a338 [4]
Link: https://github.com/AsahiLinux/linux/commits/gpu/rust-wip [5]

* tag 'rust-v6.1-rc1' of https://github.com/Rust-for-Linux/linux: (27 commits)
  MAINTAINERS: Rust
  samples: add first Rust examples
  x86: enable initial Rust support
  docs: add Rust documentation
  Kbuild: add Rust support
  rust: add `.rustfmt.toml`
  scripts: add `is_rust_module.sh`
  scripts: add `rust_is_available.sh`
  scripts: add `generate_rust_target.rs`
  scripts: add `generate_rust_analyzer.py`
  scripts: decode_stacktrace: demangle Rust symbols
  scripts: checkpatch: enable language-independent checks for Rust
  scripts: checkpatch: diagnose uses of `%pA` in the C side as errors
  vsprintf: add new `%pA` format specifier
  rust: export generated symbols
  rust: add `kernel` crate
  rust: add `bindings` crate
  rust: add `macros` crate
  rust: add `compiler_builtins` crate
  rust: adapt `alloc` crate to the kernel
  ...

1 files changed, 1204 insertions, 0 deletions

diff --git a/rust/alloc/slice.rs b/rust/alloc/slice.rs
new file mode 100644
index 000000000000..e444e97fa145
--- /dev/null
+++ b/rust/alloc/slice.rs
@@ -0,0 +1,1204 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+//! A dynamically-sized view into a contiguous sequence, `[T]`.
+//!
+//! *[See also the slice primitive type](slice).*
+//!
+//! Slices are a view into a block of memory represented as a pointer and a
+//! length.
+//!
+//! ```
+//! // slicing a Vec
+//! let vec = vec![1, 2, 3];
+//! let int_slice = &vec[..];
+//! // coercing an array to a slice
+//! let str_slice: &[&str] = &["one", "two", "three"];
+//! ```
+//!
+//! Slices are either mutable or shared. The shared slice type is `&[T]`,
+//! while the mutable slice type is `&mut [T]`, where `T` represents the element
+//! type. For example, you can mutate the block of memory that a mutable slice
+//! points to:
+//!
+//! ```
+//! let x = &mut [1, 2, 3];
+//! x[1] = 7;
+//! assert_eq!(x, &[1, 7, 3]);
+//! ```
+//!
+//! Here are some of the things this module contains:
+//!
+//! ## Structs
+//!
+//! There are several structs that are useful for slices, such as [`Iter`], which
+//! represents iteration over a slice.
+//!
+//! ## Trait Implementations
+//!
+//! There are several implementations of common traits for slices. Some examples
+//! include:
+//!
+//! * [`Clone`]
+//! * [`Eq`], [`Ord`] - for slices whose element type are [`Eq`] or [`Ord`].
+//! * [`Hash`] - for slices whose element type is [`Hash`].
+//!
+//! ## Iteration
+//!
+//! The slices implement `IntoIterator`. The iterator yields references to the
+//! slice elements.
+//!
+//! ```
+//! let numbers = &[0, 1, 2];
+//! for n in numbers {
+//!     println!("{n} is a number!");
+//! }
+//! ```
+//!
+//! The mutable slice yields mutable references to the elements:
+//!
+//! ```
+//! let mut scores = [7, 8, 9];
+//! for score in &mut scores[..] {
+//!     *score += 1;
+//! }
+//! ```
+//!
+//! This iterator yields mutable references to the slice's elements, so while
+//! the element type of the slice is `i32`, the element type of the iterator is
+//! `&mut i32`.
+//!
+//! * [`.iter`] and [`.iter_mut`] are the explicit methods to return the default
+//!   iterators.
+//! * Further methods that return iterators are [`.split`], [`.splitn`],
+//!   [`.chunks`], [`.windows`] and more.
+//!
+//! [`Hash`]: core::hash::Hash
+//! [`.iter`]: slice::iter
+//! [`.iter_mut`]: slice::iter_mut
+//! [`.split`]: slice::split
+//! [`.splitn`]: slice::splitn
+//! [`.chunks`]: slice::chunks
+//! [`.windows`]: slice::windows
+#![stable(feature = "rust1", since = "1.0.0")]
+// Many of the usings in this module are only used in the test configuration.
+// It's cleaner to just turn off the unused_imports warning than to fix them.
+#![cfg_attr(test, allow(unused_imports, dead_code))]
+
+use core::borrow::{Borrow, BorrowMut};
+#[cfg(not(no_global_oom_handling))]
+use core::cmp::Ordering::{self, Less};
+#[cfg(not(no_global_oom_handling))]
+use core::mem;
+#[cfg(not(no_global_oom_handling))]
+use core::mem::size_of;
+#[cfg(not(no_global_oom_handling))]
+use core::ptr;
+
+use crate::alloc::Allocator;
+#[cfg(not(no_global_oom_handling))]
+use crate::alloc::Global;
+#[cfg(not(no_global_oom_handling))]
+use crate::borrow::ToOwned;
+use crate::boxed::Box;
+use crate::vec::Vec;
+
+#[unstable(feature = "slice_range", issue = "76393")]
+pub use core::slice::range;
+#[unstable(feature = "array_chunks", issue = "74985")]
+pub use core::slice::ArrayChunks;
+#[unstable(feature = "array_chunks", issue = "74985")]
+pub use core::slice::ArrayChunksMut;
+#[unstable(feature = "array_windows", issue = "75027")]
+pub use core::slice::ArrayWindows;
+#[stable(feature = "inherent_ascii_escape", since = "1.60.0")]
+pub use core::slice::EscapeAscii;
+#[stable(feature = "slice_get_slice", since = "1.28.0")]
+pub use core::slice::SliceIndex;
+#[stable(feature = "from_ref", since = "1.28.0")]
+pub use core::slice::{from_mut, from_ref};
+#[stable(feature = "rust1", since = "1.0.0")]
+pub use core::slice::{from_raw_parts, from_raw_parts_mut};
+#[stable(feature = "rust1", since = "1.0.0")]
+pub use core::slice::{Chunks, Windows};
+#[stable(feature = "chunks_exact", since = "1.31.0")]
+pub use core::slice::{ChunksExact, ChunksExactMut};
+#[stable(feature = "rust1", since = "1.0.0")]
+pub use core::slice::{ChunksMut, Split, SplitMut};
+#[unstable(feature = "slice_group_by", issue = "80552")]
+pub use core::slice::{GroupBy, GroupByMut};
+#[stable(feature = "rust1", since = "1.0.0")]
+pub use core::slice::{Iter, IterMut};
+#[stable(feature = "rchunks", since = "1.31.0")]
+pub use core::slice::{RChunks, RChunksExact, RChunksExactMut, RChunksMut};
+#[stable(feature = "slice_rsplit", since = "1.27.0")]
+pub use core::slice::{RSplit, RSplitMut};
+#[stable(feature = "rust1", since = "1.0.0")]
+pub use core::slice::{RSplitN, RSplitNMut, SplitN, SplitNMut};
+#[stable(feature = "split_inclusive", since = "1.51.0")]
+pub use core::slice::{SplitInclusive, SplitInclusiveMut};
+
+////////////////////////////////////////////////////////////////////////////////
+// Basic slice extension methods
+////////////////////////////////////////////////////////////////////////////////
+
+// HACK(japaric) needed for the implementation of `vec!` macro during testing
+// N.B., see the `hack` module in this file for more details.
+#[cfg(test)]
+pub use hack::into_vec;
+
+// HACK(japaric) needed for the implementation of `Vec::clone` during testing
+// N.B., see the `hack` module in this file for more details.
+#[cfg(test)]
+pub use hack::to_vec;
+
+// HACK(japaric): With cfg(test) `impl [T]` is not available, these three
+// functions are actually methods that are in `impl [T]` but not in
+// `core::slice::SliceExt` - we need to supply these functions for the
+// `test_permutations` test
+pub(crate) mod hack {
+    use core::alloc::Allocator;
+
+    use crate::boxed::Box;
+    use crate::vec::Vec;
+
+    // We shouldn't add inline attribute to this since this is used in
+    // `vec!` macro mostly and causes perf regression. See #71204 for
+    // discussion and perf results.
+    pub fn into_vec<T, A: Allocator>(b: Box<[T], A>) -> Vec<T, A> {
+        unsafe {
+            let len = b.len();
+            let (b, alloc) = Box::into_raw_with_allocator(b);
+            Vec::from_raw_parts_in(b as *mut T, len, len, alloc)
+        }
+    }
+
+    #[cfg(not(no_global_oom_handling))]
+    #[inline]
+    pub fn to_vec<T: ConvertVec, A: Allocator>(s: &[T], alloc: A) -> Vec<T, A> {
+        T::to_vec(s, alloc)
+    }
+
+    #[cfg(not(no_global_oom_handling))]
+    pub trait ConvertVec {
+        fn to_vec<A: Allocator>(s: &[Self], alloc: A) -> Vec<Self, A>
+        where
+            Self: Sized;
+    }
+
+    #[cfg(not(no_global_oom_handling))]
+    impl<T: Clone> ConvertVec for T {
+        #[inline]
+        default fn to_vec<A: Allocator>(s: &[Self], alloc: A) -> Vec<Self, A> {
+            struct DropGuard<'a, T, A: Allocator> {
+                vec: &'a mut Vec<T, A>,
+                num_init: usize,
+            }
+            impl<'a, T, A: Allocator> Drop for DropGuard<'a, T, A> {
+                #[inline]
+                fn drop(&mut self) {
+                    // SAFETY:
+                    // items were marked initialized in the loop below
+                    unsafe {
+                        self.vec.set_len(self.num_init);
+                    }
+                }
+            }
+            let mut vec = Vec::with_capacity_in(s.len(), alloc);
+            let mut guard = DropGuard { vec: &mut vec, num_init: 0 };
+            let slots = guard.vec.spare_capacity_mut();
+            // .take(slots.len()) is necessary for LLVM to remove bounds checks
+            // and has better codegen than zip.
+            for (i, b) in s.iter().enumerate().take(slots.len()) {
+                guard.num_init = i;
+                slots[i].write(b.clone());
+            }
+            core::mem::forget(guard);
+            // SAFETY:
+            // the vec was allocated and initialized above to at least this length.
+            unsafe {
+                vec.set_len(s.len());
+            }
+            vec
+        }
+    }
+
+    #[cfg(not(no_global_oom_handling))]
+    impl<T: Copy> ConvertVec for T {
+        #[inline]
+        fn to_vec<A: Allocator>(s: &[Self], alloc: A) -> Vec<Self, A> {
+            let mut v = Vec::with_capacity_in(s.len(), alloc);
+            // SAFETY:
+            // allocated above with the capacity of `s`, and initialize to `s.len()` in
+            // ptr::copy_to_non_overlapping below.
+            unsafe {
+                s.as_ptr().copy_to_nonoverlapping(v.as_mut_ptr(), s.len());
+                v.set_len(s.len());
+            }
+            v
+        }
+    }
+}
+
+#[cfg(not(test))]
+impl<T> [T] {
+    /// Sorts the slice.
+    ///
+    /// This sort is stable (i.e., does not reorder equal elements) and *O*(*n* \* log(*n*)) worst-case.
+    ///
+    /// When applicable, unstable sorting is preferred because it is generally faster than stable
+    /// sorting and it doesn't allocate auxiliary memory.
+    /// See [`sort_unstable`](slice::sort_unstable).
+    ///
+    /// # Current implementation
+    ///
+    /// The current algorithm is an adaptive, iterative merge sort inspired by
+    /// [timsort](https://en.wikipedia.org/wiki/Timsort).
+    /// It is designed to be very fast in cases where the slice is nearly sorted, or consists of
+    /// two or more sorted sequences concatenated one after another.
+    ///
+    /// Also, it allocates temporary storage half the size of `self`, but for short slices a
+    /// non-allocating insertion sort is used instead.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = [-5, 4, 1, -3, 2];
+    ///
+    /// v.sort();
+    /// assert!(v == [-5, -3, 1, 2, 4]);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[rustc_allow_incoherent_impl]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[inline]
+    pub fn sort(&mut self)
+    where
+        T: Ord,
+    {
+        merge_sort(self, |a, b| a.lt(b));
+    }
+
+    /// Sorts the slice with a comparator function.
+    ///
+    /// This sort is stable (i.e., does not reorder equal elements) and *O*(*n* \* log(*n*)) worst-case.
+    ///
+    /// The comparator function must define a total ordering for the elements in the slice. If
+    /// the ordering is not total, the order of the elements is unspecified. An order is a
+    /// total order if it is (for all `a`, `b` and `c`):
+    ///
+    /// * total and antisymmetric: exactly one of `a < b`, `a == b` or `a > b` is true, and
+    /// * transitive, `a < b` and `b < c` implies `a < c`. The same must hold for both `==` and `>`.
+    ///
+    /// For example, while [`f64`] doesn't implement [`Ord`] because `NaN != NaN`, we can use
+    /// `partial_cmp` as our sort function when we know the slice doesn't contain a `NaN`.
+    ///
+    /// ```
+    /// let mut floats = [5f64, 4.0, 1.0, 3.0, 2.0];
+    /// floats.sort_by(|a, b| a.partial_cmp(b).unwrap());
+    /// assert_eq!(floats, [1.0, 2.0, 3.0, 4.0, 5.0]);
+    /// ```
+    ///
+    /// When applicable, unstable sorting is preferred because it is generally faster than stable
+    /// sorting and it doesn't allocate auxiliary memory.
+    /// See [`sort_unstable_by`](slice::sort_unstable_by).
+    ///
+    /// # Current implementation
+    ///
+    /// The current algorithm is an adaptive, iterative merge sort inspired by
+    /// [timsort](https://en.wikipedia.org/wiki/Timsort).
+    /// It is designed to be very fast in cases where the slice is nearly sorted, or consists of
+    /// two or more sorted sequences concatenated one after another.
+    ///
+    /// Also, it allocates temporary storage half the size of `self`, but for short slices a
+    /// non-allocating insertion sort is used instead.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = [5, 4, 1, 3, 2];
+    /// v.sort_by(|a, b| a.cmp(b));
+    /// assert!(v == [1, 2, 3, 4, 5]);
+    ///
+    /// // reverse sorting
+    /// v.sort_by(|a, b| b.cmp(a));
+    /// assert!(v == [5, 4, 3, 2, 1]);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[rustc_allow_incoherent_impl]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[inline]
+    pub fn sort_by<F>(&mut self, mut compare: F)
+    where
+        F: FnMut(&T, &T) -> Ordering,
+    {
+        merge_sort(self, |a, b| compare(a, b) == Less);
+    }
+
+    /// Sorts the slice with a key extraction function.
+    ///
+    /// This sort is stable (i.e., does not reorder equal elements) and *O*(*m* \* *n* \* log(*n*))
+    /// worst-case, where the key function is *O*(*m*).
+    ///
+    /// For expensive key functions (e.g. functions that are not simple property accesses or
+    /// basic operations), [`sort_by_cached_key`](slice::sort_by_cached_key) is likely to be
+    /// significantly faster, as it does not recompute element keys.
+    ///
+    /// When applicable, unstable sorting is preferred because it is generally faster than stable
+    /// sorting and it doesn't allocate auxiliary memory.
+    /// See [`sort_unstable_by_key`](slice::sort_unstable_by_key).
+    ///
+    /// # Current implementation
+    ///
+    /// The current algorithm is an adaptive, iterative merge sort inspired by
+    /// [timsort](https://en.wikipedia.org/wiki/Timsort).
+    /// It is designed to be very fast in cases where the slice is nearly sorted, or consists of
+    /// two or more sorted sequences concatenated one after another.
+    ///
+    /// Also, it allocates temporary storage half the size of `self`, but for short slices a
+    /// non-allocating insertion sort is used instead.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = [-5i32, 4, 1, -3, 2];
+    ///
+    /// v.sort_by_key(|k| k.abs());
+    /// assert!(v == [1, 2, -3, 4, -5]);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[rustc_allow_incoherent_impl]
+    #[stable(feature = "slice_sort_by_key", since = "1.7.0")]
+    #[inline]
+    pub fn sort_by_key<K, F>(&mut self, mut f: F)
+    where
+        F: FnMut(&T) -> K,
+        K: Ord,
+    {
+        merge_sort(self, |a, b| f(a).lt(&f(b)));
+    }
+
+    /// Sorts the slice with a key extraction function.
+    ///
+    /// During sorting, the key function is called at most once per element, by using
+    /// temporary storage to remember the results of key evaluation.
+    /// The order of calls to the key function is unspecified and may change in future versions
+    /// of the standard library.
+    ///
+    /// This sort is stable (i.e., does not reorder equal elements) and *O*(*m* \* *n* + *n* \* log(*n*))
+    /// worst-case, where the key function is *O*(*m*).
+    ///
+    /// For simple key functions (e.g., functions that are property accesses or
+    /// basic operations), [`sort_by_key`](slice::sort_by_key) is likely to be
+    /// faster.
+    ///
+    /// # Current implementation
+    ///
+    /// The current algorithm is based on [pattern-defeating quicksort][pdqsort] by Orson Peters,
+    /// which combines the fast average case of randomized quicksort with the fast worst case of
+    /// heapsort, while achieving linear time on slices with certain patterns. It uses some
+    /// randomization to avoid degenerate cases, but with a fixed seed to always provide
+    /// deterministic behavior.
+    ///
+    /// In the worst case, the algorithm allocates temporary storage in a `Vec<(K, usize)>` the
+    /// length of the slice.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = [-5i32, 4, 32, -3, 2];
+    ///
+    /// v.sort_by_cached_key(|k| k.to_string());
+    /// assert!(v == [-3, -5, 2, 32, 4]);
+    /// ```
+    ///
+    /// [pdqsort]: https://github.com/orlp/pdqsort
+    #[cfg(not(no_global_oom_handling))]
+    #[rustc_allow_incoherent_impl]
+    #[stable(feature = "slice_sort_by_cached_key", since = "1.34.0")]
+    #[inline]
+    pub fn sort_by_cached_key<K, F>(&mut self, f: F)
+    where
+        F: FnMut(&T) -> K,
+        K: Ord,
+    {
+        // Helper macro for indexing our vector by the smallest possible type, to reduce allocation.
+        macro_rules! sort_by_key {
+            ($t:ty, $slice:ident, $f:ident) => {{
+                let mut indices: Vec<_> =
+                    $slice.iter().map($f).enumerate().map(|(i, k)| (k, i as $t)).collect();
+                // The elements of `indices` are unique, as they are indexed, so any sort will be
+                // stable with respect to the original slice. We use `sort_unstable` here because
+                // it requires less memory allocation.
+                indices.sort_unstable();
+                for i in 0..$slice.len() {
+                    let mut index = indices[i].1;
+                    while (index as usize) < i {
+                        index = indices[index as usize].1;
+                    }
+                    indices[i].1 = index;
+                    $slice.swap(i, index as usize);
+                }
+            }};
+        }
+
+        let sz_u8 = mem::size_of::<(K, u8)>();
+        let sz_u16 = mem::size_of::<(K, u16)>();
+        let sz_u32 = mem::size_of::<(K, u32)>();
+        let sz_usize = mem::size_of::<(K, usize)>();
+
+        let len = self.len();
+        if len < 2 {
+            return;
+        }
+        if sz_u8 < sz_u16 && len <= (u8::MAX as usize) {
+            return sort_by_key!(u8, self, f);
+        }
+        if sz_u16 < sz_u32 && len <= (u16::MAX as usize) {
+            return sort_by_key!(u16, self, f);
+        }
+        if sz_u32 < sz_usize && len <= (u32::MAX as usize) {
+            return sort_by_key!(u32, self, f);
+        }
+        sort_by_key!(usize, self, f)
+    }
+
+    /// Copies `self` into a new `Vec`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let s = [10, 40, 30];
+    /// let x = s.to_vec();
+    /// // Here, `s` and `x` can be modified independently.
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[rustc_allow_incoherent_impl]
+    #[rustc_conversion_suggestion]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[inline]
+    pub fn to_vec(&self) -> Vec<T>
+    where
+        T: Clone,
+    {
+        self.to_vec_in(Global)
+    }
+
+    /// Copies `self` into a new `Vec` with an allocator.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(allocator_api)]
+    ///
+    /// use std::alloc::System;
+    ///
+    /// let s = [10, 40, 30];
+    /// let x = s.to_vec_in(System);
+    /// // Here, `s` and `x` can be modified independently.
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[rustc_allow_incoherent_impl]
+    #[inline]
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    pub fn to_vec_in<A: Allocator>(&self, alloc: A) -> Vec<T, A>
+    where
+        T: Clone,
+    {
+        // N.B., see the `hack` module in this file for more details.
+        hack::to_vec(self, alloc)
+    }
+
+    /// Converts `self` into a vector without clones or allocation.
+    ///
+    /// The resulting vector can be converted back into a box via
+    /// `Vec<T>`'s `into_boxed_slice` method.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let s: Box<[i32]> = Box::new([10, 40, 30]);
+    /// let x = s.into_vec();
+    /// // `s` cannot be used anymore because it has been converted into `x`.
+    ///
+    /// assert_eq!(x, vec![10, 40, 30]);
+    /// ```
+    #[rustc_allow_incoherent_impl]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[inline]
+    pub fn into_vec<A: Allocator>(self: Box<Self, A>) -> Vec<T, A> {
+        // N.B., see the `hack` module in this file for more details.
+        hack::into_vec(self)
+    }
+
+    /// Creates a vector by repeating a slice `n` times.
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if the capacity would overflow.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// assert_eq!([1, 2].repeat(3), vec![1, 2, 1, 2, 1, 2]);
+    /// ```
+    ///
+    /// A panic upon overflow:
+    ///
+    /// ```should_panic
+    /// // this will panic at runtime
+    /// b"0123456789abcdef".repeat(usize::MAX);
+    /// ```
+    #[rustc_allow_incoherent_impl]
+    #[cfg(not(no_global_oom_handling))]
+    #[stable(feature = "repeat_generic_slice", since = "1.40.0")]
+    pub fn repeat(&self, n: usize) -> Vec<T>
+    where
+        T: Copy,
+    {
+        if n == 0 {
+            return Vec::new();
+        }
+
+        // If `n` is larger than zero, it can be split as
+        // `n = 2^expn + rem (2^expn > rem, expn >= 0, rem >= 0)`.
+        // `2^expn` is the number represented by the leftmost '1' bit of `n`,
+        // and `rem` is the remaining part of `n`.
+
+        // Using `Vec` to access `set_len()`.
+        let capacity = self.len().checked_mul(n).expect("capacity overflow");
+        let mut buf = Vec::with_capacity(capacity);
+
+        // `2^expn` repetition is done by doubling `buf` `expn`-times.
+        buf.extend(self);
+        {
+            let mut m = n >> 1;
+            // If `m > 0`, there are remaining bits up to the leftmost '1'.
+            while m > 0 {
+                // `buf.extend(buf)`:
+                unsafe {
+                    ptr::copy_nonoverlapping(
+                        buf.as_ptr(),
+                        (buf.as_mut_ptr() as *mut T).add(buf.len()),
+                        buf.len(),
+                    );
+                    // `buf` has capacity of `self.len() * n`.
+                    let buf_len = buf.len();
+                    buf.set_len(buf_len * 2);
+                }
+
+                m >>= 1;
+            }
+        }
+
+        // `rem` (`= n - 2^expn`) repetition is done by copying
+        // first `rem` repetitions from `buf` itself.
+        let rem_len = capacity - buf.len(); // `self.len() * rem`
+        if rem_len > 0 {
+            // `buf.extend(buf[0 .. rem_len])`:
+            unsafe {
+                // This is non-overlapping since `2^expn > rem`.
+                ptr::copy_nonoverlapping(
+                    buf.as_ptr(),
+                    (buf.as_mut_ptr() as *mut T).add(buf.len()),
+                    rem_len,
+                );
+                // `buf.len() + rem_len` equals to `buf.capacity()` (`= self.len() * n`).
+                buf.set_len(capacity);
+            }
+        }
+        buf
+    }
+
+    /// Flattens a slice of `T` into a single value `Self::Output`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// assert_eq!(["hello", "world"].concat(), "helloworld");
+    /// assert_eq!([[1, 2], [3, 4]].concat(), [1, 2, 3, 4]);
+    /// ```
+    #[rustc_allow_incoherent_impl]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn concat<Item: ?Sized>(&self) -> <Self as Concat<Item>>::Output
+    where
+        Self: Concat<Item>,
+    {
+        Concat::concat(self)
+    }
+
+    /// Flattens a slice of `T` into a single value `Self::Output`, placing a
+    /// given separator between each.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// assert_eq!(["hello", "world"].join(" "), "hello world");
+    /// assert_eq!([[1, 2], [3, 4]].join(&0), [1, 2, 0, 3, 4]);
+    /// assert_eq!([[1, 2], [3, 4]].join(&[0, 0][..]), [1, 2, 0, 0, 3, 4]);
+    /// ```
+    #[rustc_allow_incoherent_impl]
+    #[stable(feature = "rename_connect_to_join", since = "1.3.0")]
+    pub fn join<Separator>(&self, sep: Separator) -> <Self as Join<Separator>>::Output
+    where
+        Self: Join<Separator>,
+    {
+        Join::join(self, sep)
+    }
+
+    /// Flattens a slice of `T` into a single value `Self::Output`, placing a
+    /// given separator between each.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![allow(deprecated)]
+    /// assert_eq!(["hello", "world"].connect(" "), "hello world");
+    /// assert_eq!([[1, 2], [3, 4]].connect(&0), [1, 2, 0, 3, 4]);
+    /// ```
+    #[rustc_allow_incoherent_impl]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[deprecated(since = "1.3.0", note = "renamed to join")]
+    pub fn connect<Separator>(&self, sep: Separator) -> <Self as Join<Separator>>::Output
+    where
+        Self: Join<Separator>,
+    {
+        Join::join(self, sep)
+    }
+}
+
+#[cfg(not(test))]
+impl [u8] {
+    /// Returns a vector containing a copy of this slice where each byte
+    /// is mapped to its ASCII upper case equivalent.
+    ///
+    /// ASCII letters 'a' to 'z' are mapped to 'A' to 'Z',
+    /// but non-ASCII letters are unchanged.
+    ///
+    /// To uppercase the value in-place, use [`make_ascii_uppercase`].
+    ///
+    /// [`make_ascii_uppercase`]: slice::make_ascii_uppercase
+    #[cfg(not(no_global_oom_handling))]
+    #[rustc_allow_incoherent_impl]
+    #[must_use = "this returns the uppercase bytes as a new Vec, \
+                  without modifying the original"]
+    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
+    #[inline]
+    pub fn to_ascii_uppercase(&self) -> Vec<u8> {
+        let mut me = self.to_vec();
+        me.make_ascii_uppercase();
+        me
+    }
+
+    /// Returns a vector containing a copy of this slice where each byte
+    /// is mapped to its ASCII lower case equivalent.
+    ///
+    /// ASCII letters 'A' to 'Z' are mapped to 'a' to 'z',
+    /// but non-ASCII letters are unchanged.
+    ///
+    /// To lowercase the value in-place, use [`make_ascii_lowercase`].
+    ///
+    /// [`make_ascii_lowercase`]: slice::make_ascii_lowercase
+    #[cfg(not(no_global_oom_handling))]
+    #[rustc_allow_incoherent_impl]
+    #[must_use = "this returns the lowercase bytes as a new Vec, \
+                  without modifying the original"]
+    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
+    #[inline]
+    pub fn to_ascii_lowercase(&self) -> Vec<u8> {
+        let mut me = self.to_vec();
+        me.make_ascii_lowercase();
+        me
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Extension traits for slices over specific kinds of data
+////////////////////////////////////////////////////////////////////////////////
+
+/// Helper trait for [`[T]::concat`](slice::concat).
+///
+/// Note: the `Item` type parameter is not used in this trait,
+/// but it allows impls to be more generic.
+/// Without it, we get this error:
+///
+/// ```error
+/// error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predica
+///    --> src/liballoc/slice.rs:608:6
+///     |
+/// 608 | impl<T: Clone, V: Borrow<[T]>> Concat for [V] {
+///     |      ^ unconstrained type parameter
+/// ```
+///
+/// This is because there could exist `V` types with multiple `Borrow<[_]>` impls,
+/// such that multiple `T` types would apply:
+///
+/// ```
+/// # #[allow(dead_code)]
+/// pub struct Foo(Vec<u32>, Vec<String>);
+///
+/// impl std::borrow::Borrow<[u32]> for Foo {
+///     fn borrow(&self) -> &[u32] { &self.0 }
+/// }
+///
+/// impl std::borrow::Borrow<[String]> for Foo {
+///     fn borrow(&self) -> &[String] { &self.1 }
+/// }
+/// ```
+#[unstable(feature = "slice_concat_trait", issue = "27747")]
+pub trait Concat<Item: ?Sized> {
+    #[unstable(feature = "slice_concat_trait", issue = "27747")]
+    /// The resulting type after concatenation
+    type Output;
+
+    /// Implementation of [`[T]::concat`](slice::concat)
+    #[unstable(feature = "slice_concat_trait", issue = "27747")]
+    fn concat(slice: &Self) -> Self::Output;
+}
+
+/// Helper trait for [`[T]::join`](slice::join)
+#[unstable(feature = "slice_concat_trait", issue = "27747")]
+pub trait Join<Separator> {
+    #[unstable(feature = "slice_concat_trait", issue = "27747")]
+    /// The resulting type after concatenation
+    type Output;
+
+    /// Implementation of [`[T]::join`](slice::join)
+    #[unstable(feature = "slice_concat_trait", issue = "27747")]
+    fn join(slice: &Self, sep: Separator) -> Self::Output;
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[unstable(feature = "slice_concat_ext", issue = "27747")]
+impl<T: Clone, V: Borrow<[T]>> Concat<T> for [V] {
+    type Output = Vec<T>;
+
+    fn concat(slice: &Self) -> Vec<T> {
+        let size = slice.iter().map(|slice| slice.borrow().len()).sum();
+        let mut result = Vec::with_capacity(size);
+        for v in slice {
+            result.extend_from_slice(v.borrow())
+        }
+        result
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[unstable(feature = "slice_concat_ext", issue = "27747")]
+impl<T: Clone, V: Borrow<[T]>> Join<&T> for [V] {
+    type Output = Vec<T>;
+
+    fn join(slice: &Self, sep: &T) -> Vec<T> {
+        let mut iter = slice.iter();
+        let first = match iter.next() {
+            Some(first) => first,
+            None => return vec![],
+        };
+        let size = slice.iter().map(|v| v.borrow().len()).sum::<usize>() + slice.len() - 1;
+        let mut result = Vec::with_capacity(size);
+        result.extend_from_slice(first.borrow());
+
+        for v in iter {
+            result.push(sep.clone());
+            result.extend_from_slice(v.borrow())
+        }
+        result
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[unstable(feature = "slice_concat_ext", issue = "27747")]
+impl<T: Clone, V: Borrow<[T]>> Join<&[T]> for [V] {
+    type Output = Vec<T>;
+
+    fn join(slice: &Self, sep: &[T]) -> Vec<T> {
+        let mut iter = slice.iter();
+        let first = match iter.next() {
+            Some(first) => first,
+            None => return vec![],
+        };
+        let size =
+            slice.iter().map(|v| v.borrow().len()).sum::<usize>() + sep.len() * (slice.len() - 1);
+        let mut result = Vec::with_capacity(size);
+        result.extend_from_slice(first.borrow());
+
+        for v in iter {
+            result.extend_from_slice(sep);
+            result.extend_from_slice(v.borrow())
+        }
+        result
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Standard trait implementations for slices
+////////////////////////////////////////////////////////////////////////////////
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> Borrow<[T]> for Vec<T> {
+    fn borrow(&self) -> &[T] {
+        &self[..]
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> BorrowMut<[T]> for Vec<T> {
+    fn borrow_mut(&mut self) -> &mut [T] {
+        &mut self[..]
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: Clone> ToOwned for [T] {
+    type Owned = Vec<T>;
+    #[cfg(not(test))]
+    fn to_owned(&self) -> Vec<T> {
+        self.to_vec()
+    }
+
+    #[cfg(test)]
+    fn to_owned(&self) -> Vec<T> {
+        hack::to_vec(self, Global)
+    }
+
+    fn clone_into(&self, target: &mut Vec<T>) {
+        // drop anything in target that will not be overwritten
+        target.truncate(self.len());
+
+        // target.len <= self.len due to the truncate above, so the
+        // slices here are always in-bounds.
+        let (init, tail) = self.split_at(target.len());
+
+        // reuse the contained values' allocations/resources.
+        target.clone_from_slice(init);
+        target.extend_from_slice(tail);
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Sorting
+////////////////////////////////////////////////////////////////////////////////
+
+/// Inserts `v[0]` into pre-sorted sequence `v[1..]` so that whole `v[..]` becomes sorted.
+///
+/// This is the integral subroutine of insertion sort.
+#[cfg(not(no_global_oom_handling))]
+fn insert_head<T, F>(v: &mut [T], is_less: &mut F)
+where
+    F: FnMut(&T, &T) -> bool,
+{
+    if v.len() >= 2 && is_less(&v[1], &v[0]) {
+        unsafe {
+            // There are three ways to implement insertion here:
+            //
+            // 1. Swap adjacent elements until the first one gets to its final destination.
+            //    However, this way we copy data around more than is necessary. If elements are big
+            //    structures (costly to copy), this method will be slow.
+            //
+            // 2. Iterate until the right place for the first element is found. Then shift the
+            //    elements succeeding it to make room for it and finally place it into the
+            //    remaining hole. This is a good method.
+            //
+            // 3. Copy the first element into a temporary variable. Iterate until the right place
+            //    for it is found. As we go along, copy every traversed element into the slot
+            //    preceding it. Finally, copy data from the temporary variable into the remaining
+            //    hole. This method is very good. Benchmarks demonstrated slightly better
+            //    performance than with the 2nd method.
+            //
+            // All methods were benchmarked, and the 3rd showed best results. So we chose that one.
+            let tmp = mem::ManuallyDrop::new(ptr::read(&v[0]));
+
+            // Intermediate state of the insertion process is always tracked by `hole`, which
+            // serves two purposes:
+            // 1. Protects integrity of `v` from panics in `is_less`.
+            // 2. Fills the remaining hole in `v` in the end.
+            //
+            // Panic safety:
+            //
+            // If `is_less` panics at any point during the process, `hole` will get dropped and
+            // fill the hole in `v` with `tmp`, thus ensuring that `v` still holds every object it
+            // initially held exactly once.
+            let mut hole = InsertionHole { src: &*tmp, dest: &mut v[1] };
+            ptr::copy_nonoverlapping(&v[1], &mut v[0], 1);
+
+            for i in 2..v.len() {
+                if !is_less(&v[i], &*tmp) {
+                    break;
+                }
+                ptr::copy_nonoverlapping(&v[i], &mut v[i - 1], 1);
+                hole.dest = &mut v[i];
+            }
+            // `hole` gets dropped and thus copies `tmp` into the remaining hole in `v`.
+        }
+    }
+
+    // When dropped, copies from `src` into `dest`.
+    struct InsertionHole<T> {
+        src: *const T,
+        dest: *mut T,
+    }
+
+    impl<T> Drop for InsertionHole<T> {
+        fn drop(&mut self) {
+            unsafe {
+                ptr::copy_nonoverlapping(self.src, self.dest, 1);
+            }
+        }
+    }
+}
+
+/// Merges non-decreasing runs `v[..mid]` and `v[mid..]` using `buf` as temporary storage, and
+/// stores the result into `v[..]`.
+///
+/// # Safety
+///
+/// The two slices must be non-empty and `mid` must be in bounds. Buffer `buf` must be long enough
+/// to hold a copy of the shorter slice. Also, `T` must not be a zero-sized type.
+#[cfg(not(no_global_oom_handling))]
+unsafe fn merge<T, F>(v: &mut [T], mid: usize, buf: *mut T, is_less: &mut F)
+where
+    F: FnMut(&T, &T) -> bool,
+{
+    let len = v.len();
+    let v = v.as_mut_ptr();
+    let (v_mid, v_end) = unsafe { (v.add(mid), v.add(len)) };
+
+    // The merge process first copies the shorter run into `buf`. Then it traces the newly copied
+    // run and the longer run forwards (or backwards), comparing their next unconsumed elements and
+    // copying the lesser (or greater) one into `v`.
+    //
+    // As soon as the shorter run is fully consumed, the process is done. If the longer run gets
+    // consumed first, then we must copy whatever is left of the shorter run into the remaining
+    // hole in `v`.
+    //
+    // Intermediate state of the process is always tracked by `hole`, which serves two purposes:
+    // 1. Protects integrity of `v` from panics in `is_less`.
+    // 2. Fills the remaining hole in `v` if the longer run gets consumed first.
+    //
+    // Panic safety:
+    //
+    // If `is_less` panics at any point during the process, `hole` will get dropped and fill the
+    // hole in `v` with the unconsumed range in `buf`, thus ensuring that `v` still holds every
+    // object it initially held exactly once.
+    let mut hole;
+
+    if mid <= len - mid {
+        // The left run is shorter.
+        unsafe {
+            ptr::copy_nonoverlapping(v, buf, mid);
+            hole = MergeHole { start: buf, end: buf.add(mid), dest: v };
+        }
+
+        // Initially, these pointers point to the beginnings of their arrays.
+        let left = &mut hole.start;
+        let mut right = v_mid;
+        let out = &mut hole.dest;
+
+        while *left < hole.end && right < v_end {
+            // Consume the lesser side.
+            // If equal, prefer the left run to maintain stability.
+            unsafe {
+                let to_copy = if is_less(&*right, &**left) {
+                    get_and_increment(&mut right)
+                } else {
+                    get_and_increment(left)
+                };
+                ptr::copy_nonoverlapping(to_copy, get_and_increment(out), 1);
+            }
+        }
+    } else {
+        // The right run is shorter.
+        unsafe {
+            ptr::copy_nonoverlapping(v_mid, buf, len - mid);
+            hole = MergeHole { start: buf, end: buf.add(len - mid), dest: v_mid };
+        }
+
+        // Initially, these pointers point past the ends of their arrays.
+        let left = &mut hole.dest;
+        let right = &mut hole.end;
+        let mut out = v_end;
+
+        while v < *left && buf < *right {
+            // Consume the greater side.
+            // If equal, prefer the right run to maintain stability.
+            unsafe {
+                let to_copy = if is_less(&*right.offset(-1), &*left.offset(-1)) {
+                    decrement_and_get(left)
+                } else {
+                    decrement_and_get(right)
+                };
+                ptr::copy_nonoverlapping(to_copy, decrement_and_get(&mut out), 1);
+            }
+        }
+    }
+    // Finally, `hole` gets dropped. If the shorter run was not fully consumed, whatever remains of
+    // it will now be copied into the hole in `v`.
+
+    unsafe fn get_and_increment<T>(ptr: &mut *mut T) -> *mut T {
+        let old = *ptr;
+        *ptr = unsafe { ptr.offset(1) };
+        old
+    }
+
+    unsafe fn decrement_and_get<T>(ptr: &mut *mut T) -> *mut T {
+        *ptr = unsafe { ptr.offset(-1) };
+        *ptr
+    }
+
+    // When dropped, copies the range `start..end` into `dest..`.
+    struct MergeHole<T> {
+        start: *mut T,
+        end: *mut T,
+        dest: *mut T,
+    }
+
+    impl<T> Drop for MergeHole<T> {
+        fn drop(&mut self) {
+            // `T` is not a zero-sized type, and these are pointers into a slice's elements.
+            unsafe {
+                let len = self.end.sub_ptr(self.start);
+                ptr::copy_nonoverlapping(self.start, self.dest, len);
+            }
+        }
+    }
+}
+
+/// This merge sort borrows some (but not all) ideas from TimSort, which is described in detail
+/// [here](https://github.com/python/cpython/blob/main/Objects/listsort.txt).
+///
+/// The algorithm identifies strictly descending and non-descending subsequences, which are called
+/// natural runs. There is a stack of pending runs yet to be merged. Each newly found run is pushed
+/// onto the stack, and then some pairs of adjacent runs are merged until these two invariants are
+/// satisfied:
+///
+/// 1. for every `i` in `1..runs.len()`: `runs[i - 1].len > runs[i].len`
+/// 2. for every `i` in `2..runs.len()`: `runs[i - 2].len > runs[i - 1].len + runs[i].len`
+///
+/// The invariants ensure that the total running time is *O*(*n* \* log(*n*)) worst-case.
+#[cfg(not(no_global_oom_handling))]
+fn merge_sort<T, F>(v: &mut [T], mut is_less: F)
+where
+    F: FnMut(&T, &T) -> bool,
+{
+    // Slices of up to this length get sorted using insertion sort.
+    const MAX_INSERTION: usize = 20;
+    // Very short runs are extended using insertion sort to span at least this many elements.
+    const MIN_RUN: usize = 10;
+
+    // Sorting has no meaningful behavior on zero-sized types.
+    if size_of::<T>() == 0 {
+        return;
+    }
+
+    let len = v.len();
+
+    // Short arrays get sorted in-place via insertion sort to avoid allocations.
+    if len <= MAX_INSERTION {
+        if len >= 2 {
+            for i in (0..len - 1).rev() {
+                insert_head(&mut v[i..], &mut is_less);
+            }
+        }
+        return;
+    }
+
+    // Allocate a buffer to use as scratch memory. We keep the length 0 so we can keep in it
+    // shallow copies of the contents of `v` without risking the dtors running on copies if
+    // `is_less` panics. When merging two sorted runs, this buffer holds a copy of the shorter run,
+    // which will always have length at most `len / 2`.
+    let mut buf = Vec::with_capacity(len / 2);
+
+    // In order to identify natural runs in `v`, we traverse it backwards. That might seem like a
+    // strange decision, but consider the fact that merges more often go in the opposite direction
+    // (forwards). According to benchmarks, merging forwards is slightly faster than merging
+    // backwards. To conclude, identifying runs by traversing backwards improves performance.
+    let mut runs = vec![];
+    let mut end = len;
+    while end > 0 {
+        // Find the next natural run, and reverse it if it's strictly descending.
+        let mut start = end - 1;
+        if start > 0 {
+            start -= 1;
+            unsafe {
+                if is_less(v.get_unchecked(start + 1), v.get_unchecked(start)) {
+                    while start > 0 && is_less(v.get_unchecked(start), v.get_unchecked(start - 1)) {
+                        start -= 1;
+                    }
+                    v[start..end].reverse();
+                } else {
+                    while start > 0 && !is_less(v.get_unchecked(start), v.get_unchecked(start - 1))
+                    {
+                        start -= 1;
+                    }
+                }
+            }
+        }
+
+        // Insert some more elements into the run if it's too short. Insertion sort is faster than
+        // merge sort on short sequences, so this significantly improves performance.
+        while start > 0 && end - start < MIN_RUN {
+            start -= 1;
+            insert_head(&mut v[start..end], &mut is_less);
+        }
+
+        // Push this run onto the stack.
+        runs.push(Run { start, len: end - start });
+        end = start;
+
+        // Merge some pairs of adjacent runs to satisfy the invariants.
+        while let Some(r) = collapse(&runs) {
+            let left = runs[r + 1];
+            let right = runs[r];
+            unsafe {
+                merge(
+                    &mut v[left.start..right.start + right.len],
+                    left.len,
+                    buf.as_mut_ptr(),
+                    &mut is_less,
+                );
+            }
+            runs[r] = Run { start: left.start, len: left.len + right.len };
+            runs.remove(r + 1);
+        }
+    }
+
+    // Finally, exactly one run must remain in the stack.
+    debug_assert!(runs.len() == 1 && runs[0].start == 0 && runs[0].len == len);
+
+    // Examines the stack of runs and identifies the next pair of runs to merge. More specifically,
+    // if `Some(r)` is returned, that means `runs[r]` and `runs[r + 1]` must be merged next. If the
+    // algorithm should continue building a new run instead, `None` is returned.
+    //
+    // TimSort is infamous for its buggy implementations, as described here:
+    // http://envisage-project.eu/timsort-specification-and-verification/
+    //
+    // The gist of the story is: we must enforce the invariants on the top four runs on the stack.
+    // Enforcing them on just top three is not sufficient to ensure that the invariants will still
+    // hold for *all* runs in the stack.
+    //
+    // This function correctly checks invariants for the top four runs. Additionally, if the top
+    // run starts at index 0, it will always demand a merge operation until the stack is fully
+    // collapsed, in order to complete the sort.
+    #[inline]
+    fn collapse(runs: &[Run]) -> Option<usize> {
+        let n = runs.len();
+        if n >= 2
+            && (runs[n - 1].start == 0
+                || runs[n - 2].len <= runs[n - 1].len
+                || (n >= 3 && runs[n - 3].len <= runs[n - 2].len + runs[n - 1].len)
+                || (n >= 4 && runs[n - 4].len <= runs[n - 3].len + runs[n - 2].len))
+        {
+            if n >= 3 && runs[n - 3].len < runs[n - 1].len { Some(n - 3) } else { Some(n - 2) }
+        } else {
+            None
+        }
+    }
+
+    #[derive(Clone, Copy)]
+    struct Run {
+        start: usize,
+        len: usize,
+    }
+}