/*!

**A fast bump allocation arena for Rust.**

[![](https://docs.rs/bumpalo/badge.svg)](https://docs.rs/bumpalo/)
[![](https://img.shields.io/crates/v/bumpalo.svg)](https://crates.io/crates/bumpalo)
[![](https://img.shields.io/crates/d/bumpalo.svg)](https://crates.io/crates/bumpalo)
[![Build Status](https://github.com/fitzgen/bumpalo/workflows/Rust/badge.svg)](https://github.com/fitzgen/bumpalo/actions?query=workflow%3ARust)

![](https://github.com/fitzgen/bumpalo/raw/master/bumpalo.png)

## Bump Allocation

Bump allocation is a fast, but limited approach to allocation. We have a chunk
of memory, and we maintain a pointer within that memory. Whenever we allocate an
object, we do a quick test that we have enough capacity left in our chunk to
allocate the object and then update the pointer by the object's size. *That's
it!*

The disadvantage of bump allocation is that there is no general way to
deallocate individual objects or reclaim the memory region for a
no-longer-in-use object.

These trade offs make bump allocation well-suited for *phase-oriented*
allocations. That is, a group of objects that will all be allocated during the
same program phase, used, and then can all be deallocated together as a group.

## Deallocation en Masse, but No `Drop`

To deallocate all the objects in the arena at once, we can simply reset the bump
pointer back to the start of the arena's memory chunk. This makes mass
deallocation *extremely* fast, but allocated objects' `Drop` implementations are
not invoked.

> **However:** [`bumpalo::boxed::Box<T>`][crate::boxed::Box] can be used to wrap
> `T` values allocated in the `Bump` arena, and calls `T`'s `Drop`
> implementation when the `Box<T>` wrapper goes out of scope. This is similar to
> how [`std::boxed::Box`] works, except without deallocating its backing memory.

[`std::boxed::Box`]: https://doc.rust-lang.org/std/boxed/struct.Box.html

## What happens when the memory chunk is full?

This implementation will allocate a new memory chunk from the global allocator
and then start bump allocating into this new memory chunk.

## Example

```
use bumpalo::Bump;
use std::u64;

struct Doggo {
    cuteness: u64,
    age: u8,
    scritches_required: bool,
}

// Create a new arena to bump allocate into.
let bump = Bump::new();

// Allocate values into the arena.
let scooter = bump.alloc(Doggo {
    cuteness: u64::max_value(),
    age: 8,
    scritches_required: true,
});

// Exclusive, mutable references to the just-allocated value are returned.
assert!(scooter.scritches_required);
scooter.age += 1;
```

## Collections

When the `"collections"` cargo feature is enabled, a fork of some of the `std`
library's collections are available in the `collections` module. These
collection types are modified to allocate their space inside `bumpalo::Bump`
arenas.

```rust
# #[cfg(feature = "collections")]
# {
use bumpalo::{Bump, collections::Vec};

// Create a new bump arena.
let bump = Bump::new();

// Create a vector of integers whose storage is backed by the bump arena. The
// vector cannot outlive its backing arena, and this property is enforced with
// Rust's lifetime rules.
let mut v = Vec::new_in(&bump);

// Push a bunch of integers onto `v`!
for i in 0..100 {
    v.push(i);
}
# }
```

Eventually [all `std` collection types will be parameterized by an
allocator](https://github.com/rust-lang/rust/issues/42774) and we can remove
this `collections` module and use the `std` versions.

For unstable, nightly-only support for custom allocators in `std`, see the
`allocator_api` section below.

## `bumpalo::boxed::Box`

When the `"boxed"` cargo feature is enabled, a fork of `std::boxed::Box` library
is available in the `boxed` module. This `Box` type is modified to allocate its
space inside `bumpalo::Bump` arenas.

**A `Box<T>` runs `T`'s drop implementation when the `Box<T>` is dropped.** You
can use this to work around the fact that `Bump` does not drop values allocated
in its space itself.

```rust
# #[cfg(feature = "boxed")]
# {
use bumpalo::{Bump, boxed::Box};
use std::sync::atomic::{AtomicUsize, Ordering};

static NUM_DROPPED: AtomicUsize = AtomicUsize::new(0);

struct CountDrops;

impl Drop for CountDrops {
    fn drop(&mut self) {
        NUM_DROPPED.fetch_add(1, Ordering::SeqCst);
    }
}

// Create a new bump arena.
let bump = Bump::new();

// Create a `CountDrops` inside the bump arena.
let mut c = Box::new_in(CountDrops, &bump);

// No `CountDrops` have been dropped yet.
assert_eq!(NUM_DROPPED.load(Ordering::SeqCst), 0);

// Drop our `Box<CountDrops>`.
drop(c);

// Its `Drop` implementation was run, and so `NUM_DROPS` has been incremented.
assert_eq!(NUM_DROPPED.load(Ordering::SeqCst), 1);
# }
```

## `#![no_std]` Support

Bumpalo is a `no_std` crate. It depends only on the `alloc` and `core` crates.

## Thread support

The `Bump` is `!Send`, which makes it hard to use in certain situations around threads ‒ for
example in `rayon`.

The [`bumpalo-herd`](https://crates.io/crates/bumpalo-herd) crate provides a pool of `Bump`
allocators for use in such situations.

## Nightly Rust `feature(allocator_api)` Support

The unstable, nightly-only Rust `allocator_api` feature defines an `Allocator`
trait and exposes custom allocators for `std` types. Bumpalo has a matching
`allocator_api` cargo feature to enable implementing `Allocator` and using
`Bump` with `std` collections. Note that, as `feature(allocator_api)` is
unstable and only in nightly Rust, Bumpalo's matching `allocator_api` cargo
feature should be considered unstable, and will not follow the semver
conventions that the rest of the crate does.

First, enable the `allocator_api` feature in your `Cargo.toml`:

```toml
[dependencies]
bumpalo = { version = "3.4.0", features = ["allocator_api"] }
```

Next, enable the `allocator_api` nightly Rust feature in your `src/lib.rs` or `src/main.rs`:

```rust
# #[cfg(feature = "allocator_api")]
# {
#![feature(allocator_api)]
# }
```

Finally, use `std` collections with `Bump`, so that their internal heap
allocations are made within the given bump arena:

```
# #![cfg_attr(feature = "allocator_api", feature(allocator_api))]
# #[cfg(feature = "allocator_api")]
# {
#![feature(allocator_api)]
use bumpalo::Bump;

// Create a new bump arena.
let bump = Bump::new();

// Create a `Vec` whose elements are allocated within the bump arena.
let mut v = Vec::new_in(&bump);
v.push(0);
v.push(1);
v.push(2);
# }
```

### Minimum Supported Rust Version (MSRV)

This crate is guaranteed to compile on stable Rust 1.44 and up. It might compile
with older versions but that may change in any new patch release.

We reserve the right to increment the MSRV on minor releases, however we will strive
to only do it deliberately and for good reasons.

 */

#![deny(missing_debug_implementations)]
#![deny(missing_docs)]
#![no_std]
#![cfg_attr(
    feature = "allocator_api",
    feature(allocator_api, nonnull_slice_from_raw_parts)
)]

#[doc(hidden)]
pub extern crate alloc as core_alloc;

#[cfg(feature = "boxed")]
pub mod boxed;
#[cfg(feature = "collections")]
pub mod collections;

mod alloc;

use core::cell::Cell;
use core::fmt::Display;
use core::iter;
use core::marker::PhantomData;
use core::mem;
use core::ptr::{self, NonNull};
use core::slice;
use core::str;
use core_alloc::alloc::{alloc, dealloc, Layout};
#[cfg(feature = "allocator_api")]
use core_alloc::alloc::{AllocError, Allocator};

/// An error returned from [`Bump::try_alloc_try_with`].
#[derive(Clone, PartialEq, Eq, Debug)]
pub enum AllocOrInitError<E> {
    /// Indicates that the initial allocation failed.
    Alloc(alloc::AllocErr),
    /// Indicates that the initializer failed with the contained error after
    /// allocation.
    ///
    /// It is possible but not guaranteed that the allocated memory has been
    /// released back to the allocator at this point.
    Init(E),
}
impl<E> From<alloc::AllocErr> for AllocOrInitError<E> {
    fn from(e: alloc::AllocErr) -> Self {
        Self::Alloc(e)
    }
}
impl<E: Display> Display for AllocOrInitError<E> {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        match self {
            AllocOrInitError::Alloc(err) => err.fmt(f),
            AllocOrInitError::Init(err) => write!(f, "initialization failed: {}", err),
        }
    }
}

/// An arena to bump allocate into.
///
/// ## No `Drop`s
///
/// Objects that are bump-allocated will never have their `Drop` implementation
/// called &mdash; unless you do it manually yourself. This makes it relatively
/// easy to leak memory or other resources.
///
/// If you have a type which internally manages
///
/// * an allocation from the global heap (e.g. `Vec<T>`),
/// * open file descriptors (e.g. `std::fs::File`), or
/// * any other resource that must be cleaned up (e.g. an `mmap`)
///
/// and relies on its `Drop` implementation to clean up the internal resource,
/// then if you allocate that type with a `Bump`, you need to find a new way to
/// clean up after it yourself.
///
/// Potential solutions are:
///
/// * Using [`bumpalo::boxed::Box::new_in`] instead of [`Bump::alloc`], that
///   will drop wrapped values similarly to [`std::boxed::Box`]. Note that this
///   requires enabling the `"boxed"` Cargo feature for this crate. **This is
///   often the easiest solution.**
///
/// * Calling [`drop_in_place`][drop_in_place] or using
///   [`std::mem::ManuallyDrop`][manuallydrop] to manually drop these types.
///
/// * Using [`bumpalo::collections::Vec`] instead of [`std::vec::Vec`].
///
/// * Avoiding allocating these problematic types within a `Bump`.
///
/// Note that not calling `Drop` is memory safe! Destructors are never
/// guaranteed to run in Rust, you can't rely on them for enforcing memory
/// safety.
///
/// [drop_in_place]: https://doc.rust-lang.org/std/ptr/fn.drop_in_place.html
/// [manuallydrop]: https://doc.rust-lang.org/std/mem/struct.ManuallyDrop.html
/// [`bumpalo::collections::Vec`]: ./collections/struct.Vec.html
/// [`std::vec::Vec`]: https://doc.rust-lang.org/std/vec/struct.Vec.html
/// [`bumpalo::boxed::Box::new_in`]: ./boxed/struct.Box.html#method.new_in
/// [`Bump::alloc`]: ./struct.Bump.html#method.alloc
/// [`std::boxed::Box`]: https://doc.rust-lang.org/std/boxed/struct.Box.html
///
/// ## Example
///
/// ```
/// use bumpalo::Bump;
///
/// // Create a new bump arena.
/// let bump = Bump::new();
///
/// // Allocate values into the arena.
/// let forty_two = bump.alloc(42);
/// assert_eq!(*forty_two, 42);
///
/// // Mutable references are returned from allocation.
/// let mut s = bump.alloc("bumpalo");
/// *s = "the bump allocator; and also is a buffalo";
/// ```
///
/// ## Allocation Methods Come in Many Flavors
///
/// There are various allocation methods on `Bump`, the simplest being
/// [`alloc`][Bump::alloc]. The others exist to satisfy some combination of
/// fallible allocation and initialization. The allocation methods are
/// summarized in the following table:
///
/// <table>
///   <thead>
///     <tr>
///       <th></th>
///       <th>Infallible Allocation</th>
///       <th>Fallible Allocation</th>
///     </tr>
///   </thead>
///     <tr>
///       <th>By Value</th>
///       <td><a href="#method.alloc"><code>alloc</code></a></td>
///       <td><a href="#method.try_alloc"><code>try_alloc</code></a></td>
///     </tr>
///     <tr>
///       <th>Infallible Initializer Function</th>
///       <td><a href="#method.alloc_with"><code>alloc_with</code></a></td>
///       <td><a href="#method.try_alloc_with"><code>try_alloc_with</code></a></td>
///     </tr>
///     <tr>
///       <th>Fallible Initializer Function</th>
///       <td><a href="#method.alloc_try_with"><code>alloc_try_with</code></a></td>
///       <td><a href="#method.try_alloc_try_with"><code>try_alloc_try_with</code></a></td>
///     </tr>
///   <tbody>
///   </tbody>
/// </table>
///
/// ### Fallible Allocation: The `try_alloc_` Method Prefix
///
/// These allocation methods let you recover from out-of-memory (OOM)
/// scenarioes, rather than raising a panic on OOM.
///
/// ```
/// use bumpalo::Bump;
///
/// let bump = Bump::new();
///
/// match bump.try_alloc(MyStruct {
///     // ...
/// }) {
///     Ok(my_struct) => {
///         // Allocation succeeded.
///     }
///     Err(e) => {
///         // Out of memory.
///     }
/// }
///
/// struct MyStruct {
///     // ...
/// }
/// ```
///
/// ### Initializer Functions: The `_with` Method Suffix
///
/// Calling one of the generic `…alloc(x)` methods is essentially equivalent to
/// the matching [`…alloc_with(|| x)`](?search=alloc_with). However if you use
/// `…alloc_with`, then the closure will not be invoked until after allocating
/// space for storing `x` on the heap.
///
/// This can be useful in certain edge-cases related to compiler optimizations.
/// When evaluating for example `bump.alloc(x)`, semantically `x` is first put
/// on the stack and then moved onto the heap. In some cases, the compiler is
/// able to optimize this into constructing `x` directly on the heap, however
/// in many cases it does not.
///
/// The `*alloc_with` functions try to help the compiler be smarter. In most
/// cases doing for example `bump.try_alloc_with(|| x)` on release mode will be
/// enough to help the compiler realize that this optimization is valid and
/// to construct `x` directly onto the heap.
///
/// #### Warning
///
/// These functions critically depend on compiler optimizations to achieve their
/// desired effect. This means that it is not an effective tool when compiling
/// without optimizations on.
///
/// Even when optimizations are on, these functions do not **guarantee** that
/// the value is constructed on the heap. To the best of our knowledge no such
/// guarantee can be made in stable Rust as of 1.44.
///
/// ### Fallible Initialization: The `_try_with` Method Suffix
///
/// The generic [`…alloc_try_with(|| x)`](?search=_try_with) methods behave
/// like the purely `_with` suffixed methods explained above. However, they
/// allow for fallible initialization by accepting a closure that returns a
/// [`Result`] and will attempt to undo the initial allocation if this closure
/// returns [`Err`].
///
/// #### Warning
///
/// If the inner closure returns [`Ok`], space for the entire [`Result`] remains
/// allocated inside `self`. This can be a problem especially if the [`Err`]
/// variant is larger, but even otherwise there may be overhead for the
/// [`Result`]'s discriminant.
///
/// <p><details><summary>Undoing the allocation in the <code>Err</code> case
/// always fails if <code>f</code> successfully made any additional allocations
/// in <code>self</code>.</summary>
///
/// For example, the following will always leak also space for the [`Result`]
/// into this `Bump`, even though the inner reference isn't kept and the [`Err`]
/// payload is returned semantically by value:
///
/// ```rust
/// let bump = bumpalo::Bump::new();
///
/// let r: Result<&mut [u8; 1000], ()> = bump.alloc_try_with(|| {
///     let _ = bump.alloc(0_u8);
///     Err(())
/// });
///
/// assert!(r.is_err());
/// ```
///
///</details></p>
///
/// Since [`Err`] payloads are first placed on the heap and then moved to the
/// stack, `bump.…alloc_try_with(|| x)?` is likely to execute more slowly than
/// the matching `bump.…alloc(x?)` in case of initialization failure. If this
/// happens frequently, using the plain un-suffixed method may perform better.
#[derive(Debug)]
pub struct Bump {
    // The current chunk we are bump allocating within.
    current_chunk_footer: Cell<NonNull<ChunkFooter>>,
}

#[repr(C)]
#[derive(Debug)]
struct ChunkFooter {
    // Pointer to the start of this chunk allocation. This footer is always at
    // the end of the chunk.
    data: NonNull<u8>,

    // The layout of this chunk's allocation.
    layout: Layout,

    // Link to the previous chunk, if any.
    prev: Cell<Option<NonNull<ChunkFooter>>>,

    // Bump allocation finger that is always in the range `self.data..=self`.
    ptr: Cell<NonNull<u8>>,
}

impl Default for Bump {
    fn default() -> Bump {
        Bump::new()
    }
}

impl Drop for Bump {
    fn drop(&mut self) {
        unsafe {
            dealloc_chunk_list(Some(self.current_chunk_footer.get()));
        }
    }
}

#[inline]
unsafe fn dealloc_chunk_list(mut footer: Option<NonNull<ChunkFooter>>) {
    while let Some(f) = footer {
        footer = f.as_ref().prev.get();
        dealloc(f.as_ref().data.as_ptr(), f.as_ref().layout);
    }
}

// `Bump`s are safe to send between threads because nothing aliases its owned
// chunks until you start allocating from it. But by the time you allocate from
// it, the returned references to allocations borrow the `Bump` and therefore
// prevent sending the `Bump` across threads until the borrows end.
unsafe impl Send for Bump {}

#[inline]
pub(crate) fn round_up_to(n: usize, divisor: usize) -> Option<usize> {
    debug_assert!(divisor > 0);
    debug_assert!(divisor.is_power_of_two());
    Some(n.checked_add(divisor - 1)? & !(divisor - 1))
}

// After this point, we try to hit page boundaries instead of powers of 2
const PAGE_STRATEGY_CUTOFF: usize = 0x1000;

// We only support alignments of up to 16 bytes for iter_allocated_chunks.
const SUPPORTED_ITER_ALIGNMENT: usize = 16;
const CHUNK_ALIGN: usize = SUPPORTED_ITER_ALIGNMENT;
const FOOTER_SIZE: usize = mem::size_of::<ChunkFooter>();

// Assert that ChunkFooter is at most the supported alignment. This will give a compile time error if it is not the case
const _FOOTER_ALIGN_ASSERTION: bool = mem::align_of::<ChunkFooter>() <= CHUNK_ALIGN;
const _: [(); _FOOTER_ALIGN_ASSERTION as usize] = [()];

// Maximum typical overhead per allocation imposed by allocators.
const MALLOC_OVERHEAD: usize = 16;

// This is the overhead from malloc, footer and alignment. For instance, if
// we want to request a chunk of memory that has at least X bytes usable for
// allocations (where X is aligned to CHUNK_ALIGN), then we expect that the
// after adding a footer, malloc overhead and alignment, the chunk of memory
// the allocator actually sets asside for us is X+OVERHEAD rounded up to the
// nearest suitable size boundary.
const OVERHEAD: usize = (MALLOC_OVERHEAD + FOOTER_SIZE + (CHUNK_ALIGN - 1)) & !(CHUNK_ALIGN - 1);

// Choose a relatively small default initial chunk size, since we double chunk
// sizes as we grow bump arenas to amortize costs of hitting the global
// allocator.
const FIRST_ALLOCATION_GOAL: usize = 1 << 9;

// The actual size of the first allocation is going to be a bit smaller
// than the goal. We need to make room for the footer, and we also need
// take the alignment into account.
const DEFAULT_CHUNK_SIZE_WITHOUT_FOOTER: usize = FIRST_ALLOCATION_GOAL - OVERHEAD;

/// Wrapper around `Layout::from_size_align` that adds debug assertions.
#[inline]
unsafe fn layout_from_size_align(size: usize, align: usize) -> Layout {
    if cfg!(debug_assertions) {
        Layout::from_size_align(size, align).unwrap()
    } else {
        Layout::from_size_align_unchecked(size, align)
    }
}

#[inline(never)]
fn allocation_size_overflow<T>() -> T {
    panic!("requested allocation size overflowed")
}

impl Bump {
    /// Construct a new arena to bump allocate into.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// # let _ = bump;
    /// ```
    pub fn new() -> Bump {
        Self::with_capacity(0)
    }

    /// Attempt to construct a new arena to bump allocate into.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::try_new();
    /// # let _ = bump.unwrap();
    /// ```
    pub fn try_new() -> Result<Bump, alloc::AllocErr> {
        Bump::try_with_capacity(0)
    }

    /// Construct a new arena with the specified byte capacity to bump allocate into.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::with_capacity(100);
    /// # let _ = bump;
    /// ```
    pub fn with_capacity(capacity: usize) -> Bump {
        Bump::try_with_capacity(capacity).unwrap_or_else(|_| oom())
    }

    /// Attempt to construct a new arena with the specified byte capacity to bump allocate into.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::try_with_capacity(100);
    /// # let _ = bump.unwrap();
    /// ```
    pub fn try_with_capacity(capacity: usize) -> Result<Self, alloc::AllocErr> {
        let chunk_footer = Self::new_chunk(
            None,
            Some(unsafe { layout_from_size_align(capacity, 1) }),
            None,
        )
        .ok_or(alloc::AllocErr {})?;

        Ok(Bump {
            current_chunk_footer: Cell::new(chunk_footer),
        })
    }

    /// Allocate a new chunk and return its initialized footer.
    ///
    /// If given, `layouts` is a tuple of the current chunk size and the
    /// layout of the allocation request that triggered us to fall back to
    /// allocating a new chunk of memory.
    fn new_chunk(
        old_size_with_footer: Option<usize>,
        requested_layout: Option<Layout>,
        prev: Option<NonNull<ChunkFooter>>,
    ) -> Option<NonNull<ChunkFooter>> {
        unsafe {
            // As a sane default, we want our new allocation to be about twice as
            // big as the previous allocation
            let mut new_size_without_footer =
                if let Some(old_size_with_footer) = old_size_with_footer {
                    let old_size_without_footer = old_size_with_footer - FOOTER_SIZE;
                    old_size_without_footer.checked_mul(2)?
                } else {
                    DEFAULT_CHUNK_SIZE_WITHOUT_FOOTER
                };

            // We want to have CHUNK_ALIGN or better alignment
            let mut align = CHUNK_ALIGN;

            // If we already know we need to fulfill some request,
            // make sure we allocate at least enough to satisfy it
            if let Some(requested_layout) = requested_layout {
                align = align.max(requested_layout.align());
                let requested_size = round_up_to(requested_layout.size(), align)
                    .unwrap_or_else(allocation_size_overflow);
                new_size_without_footer = new_size_without_footer.max(requested_size);
            }

            // We want our allocations to play nice with the memory allocator,
            // and waste as little memory as possible.
            // For small allocations, this means that the entire allocation
            // including the chunk footer and mallocs internal overhead is
            // as close to a power of two as we can go without going over.
            // For larger allocations, we only need to get close to a page
            // boundary without going over.
            if new_size_without_footer < PAGE_STRATEGY_CUTOFF {
                new_size_without_footer =
                    (new_size_without_footer + OVERHEAD).next_power_of_two() - OVERHEAD;
            } else {
                new_size_without_footer =
                    round_up_to(new_size_without_footer + OVERHEAD, 0x1000)? - OVERHEAD;
            }

            debug_assert_eq!(align % CHUNK_ALIGN, 0);
            debug_assert_eq!(new_size_without_footer % CHUNK_ALIGN, 0);
            let size = new_size_without_footer
                .checked_add(FOOTER_SIZE)
                .unwrap_or_else(allocation_size_overflow);
            let layout = layout_from_size_align(size, align);

            debug_assert!(size >= old_size_with_footer.unwrap_or(0) * 2);

            let data = alloc(layout);
            let data = NonNull::new(data)?;

            // The `ChunkFooter` is at the end of the chunk.
            let footer_ptr = data.as_ptr() as usize + new_size_without_footer;
            debug_assert_eq!((data.as_ptr() as usize) % align, 0);
            debug_assert_eq!(footer_ptr % CHUNK_ALIGN, 0);
            let footer_ptr = footer_ptr as *mut ChunkFooter;

            // The bump pointer is initialized to the end of the range we will
            // bump out of.
            let ptr = Cell::new(NonNull::new_unchecked(footer_ptr as *mut u8));

            ptr::write(
                footer_ptr,
                ChunkFooter {
                    data,
                    layout,
                    prev: Cell::new(prev),
                    ptr,
                },
            );

            Some(NonNull::new_unchecked(footer_ptr))
        }
    }

    /// Reset this bump allocator.
    ///
    /// Performs mass deallocation on everything allocated in this arena by
    /// resetting the pointer into the underlying chunk of memory to the start
    /// of the chunk. Does not run any `Drop` implementations on deallocated
    /// objects; see [the `Bump` type's top-level
    /// documentation](./struct.Bump.html) for details.
    ///
    /// If this arena has allocated multiple chunks to bump allocate into, then
    /// the excess chunks are returned to the global allocator.
    ///
    /// ## Example
    ///
    /// ```
    /// let mut bump = bumpalo::Bump::new();
    ///
    /// // Allocate a bunch of things.
    /// {
    ///     for i in 0..100 {
    ///         bump.alloc(i);
    ///     }
    /// }
    ///
    /// // Reset the arena.
    /// bump.reset();
    ///
    /// // Allocate some new things in the space previously occupied by the
    /// // original things.
    /// for j in 200..400 {
    ///     bump.alloc(j);
    /// }
    ///```
    pub fn reset(&mut self) {
        // Takes `&mut self` so `self` must be unique and there can't be any
        // borrows active that would get invalidated by resetting.
        unsafe {
            let cur_chunk = self.current_chunk_footer.get();

            // Deallocate all chunks except the current one
            let prev_chunk = cur_chunk.as_ref().prev.replace(None);
            dealloc_chunk_list(prev_chunk);

            // Reset the bump finger to the end of the chunk.
            cur_chunk.as_ref().ptr.set(cur_chunk.cast());

            debug_assert!(
                self.current_chunk_footer
                    .get()
                    .as_ref()
                    .prev
                    .get()
                    .is_none(),
                "We should only have a single chunk"
            );
            debug_assert_eq!(
                self.current_chunk_footer.get().as_ref().ptr.get(),
                self.current_chunk_footer.get().cast(),
                "Our chunk's bump finger should be reset to the start of its allocation"
            );
        }
    }

    /// Allocate an object in this `Bump` and return an exclusive reference to
    /// it.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for `T` fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.alloc("hello");
    /// assert_eq!(*x, "hello");
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc<T>(&self, val: T) -> &mut T {
        self.alloc_with(|| val)
    }

    /// Try to allocate an object in this `Bump` and return an exclusive
    /// reference to it.
    ///
    /// ## Errors
    ///
    /// Errors if reserving space for `T` fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.try_alloc("hello");
    /// assert_eq!(x, Ok(&mut"hello"));
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn try_alloc<T>(&self, val: T) -> Result<&mut T, alloc::AllocErr> {
        self.try_alloc_with(|| val)
    }

    /// Pre-allocate space for an object in this `Bump`, initializes it using
    /// the closure, then returns an exclusive reference to it.
    ///
    /// See [The `_with` Method Suffix](#the-_with-method-suffix) for a
    /// discussion on the differences between the `_with` suffixed methods and
    /// those methods without it, their performance characteristics, and when
    /// you might or might not choose a `_with` suffixed method.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for `T` fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.alloc_with(|| "hello");
    /// assert_eq!(*x, "hello");
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_with<F, T>(&self, f: F) -> &mut T
    where
        F: FnOnce() -> T,
    {
        #[inline(always)]
        unsafe fn inner_writer<T, F>(ptr: *mut T, f: F)
        where
            F: FnOnce() -> T,
        {
            // This function is translated as:
            // - allocate space for a T on the stack
            // - call f() with the return value being put onto this stack space
            // - memcpy from the stack to the heap
            //
            // Ideally we want LLVM to always realize that doing a stack
            // allocation is unnecessary and optimize the code so it writes
            // directly into the heap instead. It seems we get it to realize
            // this most consistently if we put this critical line into it's
            // own function instead of inlining it into the surrounding code.
            ptr::write(ptr, f())
        }

        let layout = Layout::new::<T>();

        unsafe {
            let p = self.alloc_layout(layout);
            let p = p.as_ptr() as *mut T;
            inner_writer(p, f);
            &mut *p
        }
    }

    /// Tries to pre-allocate space for an object in this `Bump`, initializes
    /// it using the closure, then returns an exclusive reference to it.
    ///
    /// See [The `_with` Method Suffix](#the-_with-method-suffix) for a
    /// discussion on the differences between the `_with` suffixed methods and
    /// those methods without it, their performance characteristics, and when
    /// you might or might not choose a `_with` suffixed method.
    ///
    /// ## Errors
    ///
    /// Errors if reserving space for `T` fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.try_alloc_with(|| "hello");
    /// assert_eq!(x, Ok(&mut "hello"));
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn try_alloc_with<F, T>(&self, f: F) -> Result<&mut T, alloc::AllocErr>
    where
        F: FnOnce() -> T,
    {
        #[inline(always)]
        unsafe fn inner_writer<T, F>(ptr: *mut T, f: F)
        where
            F: FnOnce() -> T,
        {
            // This function is translated as:
            // - allocate space for a T on the stack
            // - call f() with the return value being put onto this stack space
            // - memcpy from the stack to the heap
            //
            // Ideally we want LLVM to always realize that doing a stack
            // allocation is unnecessary and optimize the code so it writes
            // directly into the heap instead. It seems we get it to realize
            // this most consistently if we put this critical line into it's
            // own function instead of inlining it into the surrounding code.
            ptr::write(ptr, f())
        }

        //SAFETY: Self-contained:
        // `p` is allocated for `T` and then a `T` is written.
        let layout = Layout::new::<T>();
        let p = self.try_alloc_layout(layout)?;
        let p = p.as_ptr() as *mut T;

        unsafe {
            inner_writer(p, f);
            Ok(&mut *p)
        }
    }

    /// Pre-allocates space for a [`Result`] in this `Bump`, initializes it using
    /// the closure, then returns an exclusive reference to its `T` if [`Ok`].
    ///
    /// Iff the allocation fails, the closure is not run.
    ///
    /// Iff [`Err`], an allocator rewind is *attempted* and the `E` instance is
    /// moved out of the allocator to be consumed or dropped as normal.
    ///
    /// See [The `_with` Method Suffix](#the-_with-method-suffix) for a
    /// discussion on the differences between the `_with` suffixed methods and
    /// those methods without it, their performance characteristics, and when
    /// you might or might not choose a `_with` suffixed method.
    ///
    /// For caveats specific to fallible initialization, see
    /// [The `_try_with` Method Suffix](#the-_try_with-method-suffix).
    ///
    /// ## Errors
    ///
    /// Iff the allocation succeeds but `f` fails, that error is forwarded by value.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for `Result<T, E>` fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.alloc_try_with(|| Ok("hello"))?;
    /// assert_eq!(*x, "hello");
    /// # Result::<_, ()>::Ok(())
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_try_with<F, T, E>(&self, f: F) -> Result<&mut T, E>
    where
        F: FnOnce() -> Result<T, E>,
    {
        let rewind_footer = self.current_chunk_footer.get();
        let rewind_ptr = unsafe { rewind_footer.as_ref() }.ptr.get();
        let mut inner_result_ptr = NonNull::from(self.alloc_with(f));
        let inner_result_address = inner_result_ptr.as_ptr() as usize;
        match unsafe { inner_result_ptr.as_mut() } {
            Ok(t) => Ok(unsafe {
                //SAFETY:
                // The `&mut Result<T, E>` returned by `alloc_with` may be
                // lifetime-limited by `E`, but the derived `&mut T` still has
                // the same validity as in `alloc_with` since the error variant
                // is already ruled out here.

                // We could conditionally truncate the allocation here, but
                // since it grows backwards, it seems unlikely that we'd get
                // any more than the `Result`'s discriminant this way, if
                // anything at all.
                &mut *(t as *mut _)
            }),
            Err(e) => unsafe {
                // If this result was the last allocation in this arena, we can
                // reclaim its space. In fact, sometimes we can do even better
                // than simply calling `dealloc` on the result pointer: we can
                // reclaim any alignment padding we might have added (which
                // `dealloc` cannot do) if we didn't allocate a new chunk for
                // this result.
                if self.is_last_allocation(NonNull::new_unchecked(inner_result_address as *mut _)) {
                    let current_footer_p = self.current_chunk_footer.get();
                    let current_ptr = &current_footer_p.as_ref().ptr;
                    if current_footer_p == rewind_footer {
                        // It's still the same chunk, so reset the bump pointer
                        // to its original value upon entry to this method
                        // (reclaiming any alignment padding we may have
                        // added).
                        current_ptr.set(rewind_ptr);
                    } else {
                        // We allocated a new chunk for this result.
                        //
                        // We know the result is the only allocation in this
                        // chunk: Any additional allocations since the start of
                        // this method could only have happened when running
                        // the initializer function, which is called *after*
                        // reserving space for this result. Therefore, since we
                        // already determined via the check above that this
                        // result was the last allocation, there must not have
                        // been any other allocations, and this result is the
                        // only allocation in this chunk.
                        //
                        // Because this is the only allocation in this chunk,
                        // we can reset the chunk's bump finger to the start of
                        // the chunk.
                        current_ptr.set(current_footer_p.as_ref().data);
                    }
                }
                //SAFETY:
                // As we received `E` semantically by value from `f`, we can
                // just copy that value here as long as we avoid a double-drop
                // (which can't happen as any specific references to the `E`'s
                // data in `self` are destroyed when this function returns).
                //
                // The order between this and the deallocation doesn't matter
                // because `Self: !Sync`.
                Err(ptr::read(e as *const _))
            },
        }
    }

    /// Tries to pre-allocates space for a [`Result`] in this `Bump`,
    /// initializes it using the closure, then returns an exclusive reference
    /// to its `T` if all [`Ok`].
    ///
    /// Iff the allocation fails, the closure is not run.
    ///
    /// Iff the closure returns [`Err`], an allocator rewind is *attempted* and
    /// the `E` instance is moved out of the allocator to be consumed or dropped
    /// as normal.
    ///
    /// See [The `_with` Method Suffix](#the-_with-method-suffix) for a
    /// discussion on the differences between the `_with` suffixed methods and
    /// those methods without it, their performance characteristics, and when
    /// you might or might not choose a `_with` suffixed method.
    ///
    /// For caveats specific to fallible initialization, see
    /// [The `_try_with` Method Suffix](#the-_try_with-method-suffix).
    ///
    /// ## Errors
    ///
    /// Errors with the [`Alloc`](`AllocOrInitError::Alloc`) variant iff
    /// reserving space for `Result<T, E>` fails.
    ///
    /// Iff the allocation succeeds but `f` fails, that error is forwarded by
    /// value inside the [`Init`](`AllocOrInitError::Init`) variant.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.try_alloc_try_with(|| Ok("hello"))?;
    /// assert_eq!(*x, "hello");
    /// # Result::<_, bumpalo::AllocOrInitError<()>>::Ok(())
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn try_alloc_try_with<F, T, E>(&self, f: F) -> Result<&mut T, AllocOrInitError<E>>
    where
        F: FnOnce() -> Result<T, E>,
    {
        let rewind_footer = self.current_chunk_footer.get();
        let rewind_ptr = unsafe { rewind_footer.as_ref() }.ptr.get();
        let mut inner_result_ptr = NonNull::from(self.try_alloc_with(f)?);
        let inner_result_address = inner_result_ptr.as_ptr() as usize;
        match unsafe { inner_result_ptr.as_mut() } {
            Ok(t) => Ok(unsafe {
                //SAFETY:
                // The `&mut Result<T, E>` returned by `alloc_with` may be
                // lifetime-limited by `E`, but the derived `&mut T` still has
                // the same validity as in `alloc_with` since the error variant
                // is already ruled out here.

                // We could conditionally truncate the allocation here, but
                // since it grows backwards, it seems unlikely that we'd get
                // any more than the `Result`'s discriminant this way, if
                // anything at all.
                &mut *(t as *mut _)
            }),
            Err(e) => unsafe {
                // If this result was the last allocation in this arena, we can
                // reclaim its space. In fact, sometimes we can do even better
                // than simply calling `dealloc` on the result pointer: we can
                // reclaim any alignment padding we might have added (which
                // `dealloc` cannot do) if we didn't allocate a new chunk for
                // this result.
                if self.is_last_allocation(NonNull::new_unchecked(inner_result_address as *mut _)) {
                    let current_footer_p = self.current_chunk_footer.get();
                    let current_ptr = &current_footer_p.as_ref().ptr;
                    if current_footer_p == rewind_footer {
                        // It's still the same chunk, so reset the bump pointer
                        // to its original value upon entry to this method
                        // (reclaiming any alignment padding we may have
                        // added).
                        current_ptr.set(rewind_ptr);
                    } else {
                        // We allocated a new chunk for this result.
                        //
                        // We know the result is the only allocation in this
                        // chunk: Any additional allocations since the start of
                        // this method could only have happened when running
                        // the initializer function, which is called *after*
                        // reserving space for this result. Therefore, since we
                        // already determined via the check above that this
                        // result was the last allocation, there must not have
                        // been any other allocations, and this result is the
                        // only allocation in this chunk.
                        //
                        // Because this is the only allocation in this chunk,
                        // we can reset the chunk's bump finger to the start of
                        // the chunk.
                        current_ptr.set(current_footer_p.as_ref().data);
                    }
                }
                //SAFETY:
                // As we received `E` semantically by value from `f`, we can
                // just copy that value here as long as we avoid a double-drop
                // (which can't happen as any specific references to the `E`'s
                // data in `self` are destroyed when this function returns).
                //
                // The order between this and the deallocation doesn't matter
                // because `Self: !Sync`.
                Err(AllocOrInitError::Init(ptr::read(e as *const _)))
            },
        }
    }

    /// `Copy` a slice into this `Bump` and return an exclusive reference to
    /// the copy.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for the slice fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.alloc_slice_copy(&[1, 2, 3]);
    /// assert_eq!(x, &[1, 2, 3]);
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_slice_copy<T>(&self, src: &[T]) -> &mut [T]
    where
        T: Copy,
    {
        let layout = Layout::for_value(src);
        let dst = self.alloc_layout(layout).cast::<T>();

        unsafe {
            ptr::copy_nonoverlapping(src.as_ptr(), dst.as_ptr(), src.len());
            slice::from_raw_parts_mut(dst.as_ptr(), src.len())
        }
    }

    /// `Clone` a slice into this `Bump` and return an exclusive reference to
    /// the clone. Prefer `alloc_slice_copy` if `T` is `Copy`.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for the slice fails.
    ///
    /// ## Example
    ///
    /// ```
    /// #[derive(Clone, Debug, Eq, PartialEq)]
    /// struct Sheep {
    ///     name: String,
    /// }
    ///
    /// let originals = vec![
    ///     Sheep { name: "Alice".into() },
    ///     Sheep { name: "Bob".into() },
    ///     Sheep { name: "Cathy".into() },
    /// ];
    ///
    /// let bump = bumpalo::Bump::new();
    /// let clones = bump.alloc_slice_clone(&originals);
    /// assert_eq!(originals, clones);
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_slice_clone<T>(&self, src: &[T]) -> &mut [T]
    where
        T: Clone,
    {
        let layout = Layout::for_value(src);
        let dst = self.alloc_layout(layout).cast::<T>();

        unsafe {
            for (i, val) in src.iter().cloned().enumerate() {
                ptr::write(dst.as_ptr().add(i), val);
            }

            slice::from_raw_parts_mut(dst.as_ptr(), src.len())
        }
    }

    /// `Copy` a string slice into this `Bump` and return an exclusive reference to it.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for the string fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let hello = bump.alloc_str("hello world");
    /// assert_eq!("hello world", hello);
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_str(&self, src: &str) -> &mut str {
        let buffer = self.alloc_slice_copy(src.as_bytes());
        unsafe {
            // This is OK, because it already came in as str, so it is guaranteed to be utf8
            str::from_utf8_unchecked_mut(buffer)
        }
    }

    /// Allocates a new slice of size `len` into this `Bump` and returns an
    /// exclusive reference to the copy.
    ///
    /// The elements of the slice are initialized using the supplied closure.
    /// The closure argument is the position in the slice.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for the slice fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.alloc_slice_fill_with(5, |i| 5*(i+1));
    /// assert_eq!(x, &[5, 10, 15, 20, 25]);
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_slice_fill_with<T, F>(&self, len: usize, mut f: F) -> &mut [T]
    where
        F: FnMut(usize) -> T,
    {
        let layout = Layout::array::<T>(len).unwrap_or_else(|_| oom());
        let dst = self.alloc_layout(layout).cast::<T>();

        unsafe {
            for i in 0..len {
                ptr::write(dst.as_ptr().add(i), f(i));
            }

            let result = slice::from_raw_parts_mut(dst.as_ptr(), len);
            debug_assert_eq!(Layout::for_value(result), layout);
            result
        }
    }

    /// Allocates a new slice of size `len` into this `Bump` and returns an
    /// exclusive reference to the copy.
    ///
    /// All elements of the slice are initialized to `value`.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for the slice fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.alloc_slice_fill_copy(5, 42);
    /// assert_eq!(x, &[42, 42, 42, 42, 42]);
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_slice_fill_copy<T: Copy>(&self, len: usize, value: T) -> &mut [T] {
        self.alloc_slice_fill_with(len, |_| value)
    }

    /// Allocates a new slice of size `len` slice into this `Bump` and return an
    /// exclusive reference to the copy.
    ///
    /// All elements of the slice are initialized to `value.clone()`.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for the slice fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let s: String = "Hello Bump!".to_string();
    /// let x: &[String] = bump.alloc_slice_fill_clone(2, &s);
    /// assert_eq!(x.len(), 2);
    /// assert_eq!(&x[0], &s);
    /// assert_eq!(&x[1], &s);
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_slice_fill_clone<T: Clone>(&self, len: usize, value: &T) -> &mut [T] {
        self.alloc_slice_fill_with(len, |_| value.clone())
    }

    /// Allocates a new slice of size `len` slice into this `Bump` and return an
    /// exclusive reference to the copy.
    ///
    /// The elements are initialized using the supplied iterator.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for the slice fails, or if the supplied
    /// iterator returns fewer elements than it promised.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x: &[i32] = bump.alloc_slice_fill_iter([2, 3, 5].iter().cloned().map(|i| i * i));
    /// assert_eq!(x, [4, 9, 25]);
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_slice_fill_iter<T, I>(&self, iter: I) -> &mut [T]
    where
        I: IntoIterator<Item = T>,
        I::IntoIter: ExactSizeIterator,
    {
        let mut iter = iter.into_iter();
        self.alloc_slice_fill_with(iter.len(), |_| {
            iter.next().expect("Iterator supplied too few elements")
        })
    }

    /// Allocates a new slice of size `len` slice into this `Bump` and return an
    /// exclusive reference to the copy.
    ///
    /// All elements of the slice are initialized to `T::default()`.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for the slice fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.alloc_slice_fill_default::<u32>(5);
    /// assert_eq!(x, &[0, 0, 0, 0, 0]);
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_slice_fill_default<T: Default>(&self, len: usize) -> &mut [T] {
        self.alloc_slice_fill_with(len, |_| T::default())
    }

    /// Allocate space for an object with the given `Layout`.
    ///
    /// The returned pointer points at uninitialized memory, and should be
    /// initialized with
    /// [`std::ptr::write`](https://doc.rust-lang.org/std/ptr/fn.write.html).
    ///
    /// # Panics
    ///
    /// Panics if reserving space matching `layout` fails.
    #[inline(always)]
    pub fn alloc_layout(&self, layout: Layout) -> NonNull<u8> {
        self.try_alloc_layout(layout).unwrap_or_else(|_| oom())
    }

    /// Attempts to allocate space for an object with the given `Layout` or else returns
    /// an `Err`.
    ///
    /// The returned pointer points at uninitialized memory, and should be
    /// initialized with
    /// [`std::ptr::write`](https://doc.rust-lang.org/std/ptr/fn.write.html).
    ///
    /// # Errors
    ///
    /// Errors if reserving space matching `layout` fails.
    #[inline(always)]
    pub fn try_alloc_layout(&self, layout: Layout) -> Result<NonNull<u8>, alloc::AllocErr> {
        if let Some(p) = self.try_alloc_layout_fast(layout) {
            Ok(p)
        } else {
            self.alloc_layout_slow(layout).ok_or(alloc::AllocErr {})
        }
    }

    #[inline(always)]
    fn try_alloc_layout_fast(&self, layout: Layout) -> Option<NonNull<u8>> {
        // We don't need to check for ZSTs here since they will automatically
        // be handled properly: the pointer will be bumped by zero bytes,
        // modulo alignment. This keeps the fast path optimized for non-ZSTs,
        // which are much more common.
        unsafe {
            let footer = self.current_chunk_footer.get();
            let footer = footer.as_ref();
            let ptr = footer.ptr.get().as_ptr() as usize;
            let start = footer.data.as_ptr() as usize;
            debug_assert!(start <= ptr);
            debug_assert!(ptr <= footer as *const _ as usize);

            let ptr = ptr.checked_sub(layout.size())?;
            let aligned_ptr = ptr & !(layout.align() - 1);

            if aligned_ptr >= start {
                let aligned_ptr = NonNull::new_unchecked(aligned_ptr as *mut u8);
                footer.ptr.set(aligned_ptr);
                Some(aligned_ptr)
            } else {
                None
            }
        }
    }

    /// Gets the remaining capacity in the current chunk (in bytes).
    ///
    /// ## Example
    ///
    /// ```
    /// use bumpalo::Bump;
    ///
    /// let bump = Bump::with_capacity(100);
    ///
    /// let capacity = bump.chunk_capacity();
    /// assert!(capacity >= 100);
    /// ```
    pub fn chunk_capacity(&self) -> usize {
        let current_footer = self.current_chunk_footer.get();
        let current_footer = unsafe { current_footer.as_ref() };

        current_footer as *const _ as usize - current_footer.data.as_ptr() as usize
    }

    /// Slow path allocation for when we need to allocate a new chunk from the
    /// parent bump set because there isn't enough room in our current chunk.
    #[inline(never)]
    fn alloc_layout_slow(&self, layout: Layout) -> Option<NonNull<u8>> {
        unsafe {
            let size = layout.size();

            // Get a new chunk from the global allocator.
            let current_footer = self.current_chunk_footer.get();
            let current_layout = current_footer.as_ref().layout;
            let new_footer = Bump::new_chunk(
                Some(current_layout.size()),
                Some(layout),
                Some(current_footer),
            )?;
            debug_assert_eq!(
                new_footer.as_ref().data.as_ptr() as usize % layout.align(),
                0
            );

            // Set the new chunk as our new current chunk.
            self.current_chunk_footer.set(new_footer);

            let new_footer = new_footer.as_ref();

            // Move the bump ptr finger down to allocate room for `val`. We know
            // this can't overflow because we successfully allocated a chunk of
            // at least the requested size.
            let ptr = new_footer.ptr.get().as_ptr() as usize - size;
            // Round the pointer down to the requested alignment.
            let ptr = ptr & !(layout.align() - 1);
            debug_assert!(
                ptr <= new_footer as *const _ as usize,
                "{:#x} <= {:#x}",
                ptr,
                new_footer as *const _ as usize
            );
            let ptr = NonNull::new_unchecked(ptr as *mut u8);
            new_footer.ptr.set(ptr);

            // Return a pointer to the freshly allocated region in this chunk.
            Some(ptr)
        }
    }

    /// Returns an iterator over each chunk of allocated memory that
    /// this arena has bump allocated into.
    ///
    /// The chunks are returned ordered by allocation time, with the most
    /// recently allocated chunk being returned first, and the least recently
    /// allocated chunk being returned last.
    ///
    /// The values inside each chunk are also ordered by allocation time, with
    /// the most recent allocation being earlier in the slice, and the least
    /// recent allocation being towards the end of the slice.
    ///
    /// ## Safety
    ///
    /// Because this method takes `&mut self`, we know that the bump arena
    /// reference is unique and therefore there aren't any active references to
    /// any of the objects we've allocated in it either. This potential aliasing
    /// of exclusive references is one common footgun for unsafe code that we
    /// don't need to worry about here.
    ///
    /// However, there could be regions of uninitialized memory used as padding
    /// between allocations, which is why this iterator has items of type
    /// `[MaybeUninit<u8>]`, instead of simply `[u8]`.
    ///
    /// The only way to guarantee that there is no padding between allocations
    /// or within allocated objects is if all of these properties hold:
    ///
    /// 1. Every object allocated in this arena has the same alignment,
    ///    and that alignment is at most 16.
    /// 2. Every object's size is a multiple of its alignment.
    /// 3. None of the objects allocated in this arena contain any internal
    ///    padding.
    ///
    /// If you want to use this `iter_allocated_chunks` method, it is *your*
    /// responsibility to ensure that these properties hold before calling
    /// `MaybeUninit::assume_init` or otherwise reading the returned values.
    ///
    /// Finally, you must also ensure that any values allocated into the bump
    /// arena have not had their `Drop` implementations called on them,
    /// e.g. after dropping a [`bumpalo::boxed::Box<T>`][crate::boxed::Box].
    ///
    /// ## Example
    ///
    /// ```
    /// let mut bump = bumpalo::Bump::new();
    ///
    /// // Allocate a bunch of `i32`s in this bump arena, potentially causing
    /// // additional memory chunks to be reserved.
    /// for i in 0..10000 {
    ///     bump.alloc(i);
    /// }
    ///
    /// // Iterate over each chunk we've bump allocated into. This is safe
    /// // because we have only allocated `i32`s in this arena, which fulfills
    /// // the above requirements.
    /// for ch in bump.iter_allocated_chunks() {
    ///     println!("Used a chunk that is {} bytes long", ch.len());
    ///     println!("The first byte is {:?}", unsafe {
    ///         ch.get(0).unwrap().assume_init()
    ///     });
    /// }
    ///
    /// // Within a chunk, allocations are ordered from most recent to least
    /// // recent. If we allocated 'a', then 'b', then 'c', when we iterate
    /// // through the chunk's data, we get them in the order 'c', then 'b',
    /// // then 'a'.
    ///
    /// bump.reset();
    /// bump.alloc(b'a');
    /// bump.alloc(b'b');
    /// bump.alloc(b'c');
    ///
    /// assert_eq!(bump.iter_allocated_chunks().count(), 1);
    /// let chunk = bump.iter_allocated_chunks().nth(0).unwrap();
    /// assert_eq!(chunk.len(), 3);
    ///
    /// // Safe because we've only allocated `u8`s in this arena, which
    /// // fulfills the above requirements.
    /// unsafe {
    ///     assert_eq!(chunk[0].assume_init(), b'c');
    ///     assert_eq!(chunk[1].assume_init(), b'b');
    ///     assert_eq!(chunk[2].assume_init(), b'a');
    /// }
    /// ```
    pub fn iter_allocated_chunks(&mut self) -> ChunkIter<'_> {
        ChunkIter {
            footer: Some(self.current_chunk_footer.get()),
            bump: PhantomData,
        }
    }

    /// Calculates the number of bytes currently allocated across all chunks in
    /// this bump arena.
    ///
    /// If you allocate types of different alignments or types with
    /// larger-than-typical alignment in the same arena, some padding
    /// bytes might get allocated in the bump arena. Note that those padding
    /// bytes will add to this method's resulting sum, so you cannot rely
    /// on it only counting the sum of the sizes of the things
    /// you've allocated in the arena.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let _x = bump.alloc_slice_fill_default::<u32>(5);
    /// let bytes = bump.allocated_bytes();
    /// assert!(bytes >= core::mem::size_of::<u32>() * 5);
    /// ```
    pub fn allocated_bytes(&self) -> usize {
        let mut footer = Some(self.current_chunk_footer.get());

        let mut bytes = 0;

        while let Some(f) = footer {
            let foot = unsafe { f.as_ref() };

            let ptr = foot.ptr.get().as_ptr() as usize;
            debug_assert!(ptr <= foot as *const _ as usize);

            bytes += foot as *const _ as usize - ptr;

            footer = foot.prev.get();
        }

        bytes
    }

    #[inline]
    unsafe fn is_last_allocation(&self, ptr: NonNull<u8>) -> bool {
        let footer = self.current_chunk_footer.get();
        let footer = footer.as_ref();
        footer.ptr.get() == ptr
    }

    #[inline]
    unsafe fn dealloc(&self, ptr: NonNull<u8>, layout: Layout) {
        // If the pointer is the last allocation we made, we can reuse the bytes,
        // otherwise they are simply leaked -- at least until somebody calls reset().
        if self.is_last_allocation(ptr) {
            let ptr = NonNull::new_unchecked(ptr.as_ptr().add(layout.size()));
            self.current_chunk_footer.get().as_ref().ptr.set(ptr);
        }
    }

    #[inline]
    unsafe fn shrink(
        &self,
        ptr: NonNull<u8>,
        layout: Layout,
        new_size: usize,
    ) -> Result<NonNull<u8>, alloc::AllocErr> {
        let old_size = layout.size();
        if self.is_last_allocation(ptr)
                // Only reclaim the excess space (which requires a copy) if it
                // is worth it: we are actually going to recover "enough" space
                // and we can do a non-overlapping copy.
                && new_size <= old_size / 2
        {
            let delta = old_size - new_size;
            let footer = self.current_chunk_footer.get();
            let footer = footer.as_ref();
            footer
                .ptr
                .set(NonNull::new_unchecked(footer.ptr.get().as_ptr().add(delta)));
            let new_ptr = footer.ptr.get();
            // NB: we know it is non-overlapping because of the size check
            // in the `if` condition.
            ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_ptr(), new_size);
            return Ok(new_ptr);
        } else {
            return Ok(ptr);
        }
    }

    #[inline]
    unsafe fn grow(
        &self,
        ptr: NonNull<u8>,
        layout: Layout,
        new_size: usize,
    ) -> Result<NonNull<u8>, alloc::AllocErr> {
        let old_size = layout.size();
        if self.is_last_allocation(ptr) {
            // Try to allocate the delta size within this same block so we can
            // reuse the currently allocated space.
            let delta = new_size - old_size;
            if let Some(p) =
                self.try_alloc_layout_fast(layout_from_size_align(delta, layout.align()))
            {
                ptr::copy(ptr.as_ptr(), p.as_ptr(), old_size);
                return Ok(p);
            }
        }

        // Fallback: do a fresh allocation and copy the existing data into it.
        let new_layout = layout_from_size_align(new_size, layout.align());
        let new_ptr = self.try_alloc_layout(new_layout)?;
        ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_ptr(), old_size);
        Ok(new_ptr)
    }
}

/// An iterator over each chunk of allocated memory that
/// an arena has bump allocated into.
///
/// The chunks are returned ordered by allocation time, with the most recently
/// allocated chunk being returned first.
///
/// The values inside each chunk is also ordered by allocation time, with the most
/// recent allocation being earlier in the slice.
///
/// This struct is created by the [`iter_allocated_chunks`] method on
/// [`Bump`]. See that function for a safety description regarding reading from the returned items.
///
/// [`Bump`]: ./struct.Bump.html
/// [`iter_allocated_chunks`]: ./struct.Bump.html#method.iter_allocated_chunks
#[derive(Debug)]
pub struct ChunkIter<'a> {
    footer: Option<NonNull<ChunkFooter>>,
    bump: PhantomData<&'a mut Bump>,
}

impl<'a> Iterator for ChunkIter<'a> {
    type Item = &'a [mem::MaybeUninit<u8>];
    fn next(&mut self) -> Option<&'a [mem::MaybeUninit<u8>]> {
        unsafe {
            let foot = self.footer?;
            let foot = foot.as_ref();
            let data = foot.data.as_ptr() as usize;
            let ptr = foot.ptr.get().as_ptr() as usize;
            debug_assert!(data <= ptr);
            debug_assert!(ptr <= foot as *const _ as usize);

            let len = foot as *const _ as usize - ptr;
            let slice = slice::from_raw_parts(ptr as *const mem::MaybeUninit<u8>, len);
            self.footer = foot.prev.get();
            Some(slice)
        }
    }
}

impl<'a> iter::FusedIterator for ChunkIter<'a> {}

#[inline(never)]
#[cold]
fn oom() -> ! {
    panic!("out of memory")
}

unsafe impl<'a> alloc::Alloc for &'a Bump {
    #[inline(always)]
    unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, alloc::AllocErr> {
        self.try_alloc_layout(layout)
    }

    #[inline]
    unsafe fn dealloc(&mut self, ptr: NonNull<u8>, layout: Layout) {
        Bump::dealloc(self, ptr, layout)
    }

    #[inline]
    unsafe fn realloc(
        &mut self,
        ptr: NonNull<u8>,
        layout: Layout,
        new_size: usize,
    ) -> Result<NonNull<u8>, alloc::AllocErr> {
        let old_size = layout.size();

        if old_size == 0 {
            return self.try_alloc_layout(layout);
        }

        if new_size <= old_size {
            self.shrink(ptr, layout, new_size)
        } else {
            self.grow(ptr, layout, new_size)
        }
    }
}

#[cfg(feature = "allocator_api")]
unsafe impl<'a> Allocator for &'a Bump {
    fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
        self.try_alloc_layout(layout)
            .map(|p| NonNull::slice_from_raw_parts(p, layout.size()))
            .map_err(|_| AllocError)
    }

    unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout) {
        Bump::dealloc(self, ptr, layout)
    }

    unsafe fn shrink(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> {
        let new_size = new_layout.size();
        Bump::shrink(self, ptr, old_layout, new_size)
            .map(|p| NonNull::slice_from_raw_parts(p, new_size))
            .map_err(|_| AllocError)
    }

    unsafe fn grow(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> {
        let new_size = new_layout.size();
        Bump::grow(self, ptr, old_layout, new_size)
            .map(|p| NonNull::slice_from_raw_parts(p, new_size))
            .map_err(|_| AllocError)
    }

    unsafe fn grow_zeroed(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> {
        let mut ptr = self.grow(ptr, old_layout, new_layout)?;
        ptr.as_mut()[old_layout.size()..].fill(0);
        Ok(ptr)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn chunk_footer_is_five_words() {
        assert_eq!(mem::size_of::<ChunkFooter>(), mem::size_of::<usize>() * 5);
    }

    #[test]
    #[allow(clippy::cognitive_complexity)]
    fn test_realloc() {
        use crate::alloc::Alloc;

        unsafe {
            const CAPACITY: usize = 1024 - OVERHEAD;
            let mut b = Bump::with_capacity(CAPACITY);

            // `realloc` doesn't shrink allocations that aren't "worth it".
            let layout = Layout::from_size_align(100, 1).unwrap();
            let p = b.alloc_layout(layout);
            let q = (&b).realloc(p, layout, 51).unwrap();
            assert_eq!(p, q);
            b.reset();

            // `realloc` will shrink allocations that are "worth it".
            let layout = Layout::from_size_align(100, 1).unwrap();
            let p = b.alloc_layout(layout);
            let q = (&b).realloc(p, layout, 50).unwrap();
            assert!(p != q);
            b.reset();

            // `realloc` will reuse the last allocation when growing.
            let layout = Layout::from_size_align(10, 1).unwrap();
            let p = b.alloc_layout(layout);
            let q = (&b).realloc(p, layout, 11).unwrap();
            assert_eq!(q.as_ptr() as usize, p.as_ptr() as usize - 1);
            b.reset();

            // `realloc` will allocate a new chunk when growing the last
            // allocation, if need be.
            let layout = Layout::from_size_align(1, 1).unwrap();
            let p = b.alloc_layout(layout);
            let q = (&b).realloc(p, layout, CAPACITY + 1).unwrap();
            assert!(q.as_ptr() as usize != p.as_ptr() as usize - CAPACITY);
            b = Bump::with_capacity(CAPACITY);

            // `realloc` will allocate and copy when reallocating anything that
            // wasn't the last allocation.
            let layout = Layout::from_size_align(1, 1).unwrap();
            let p = b.alloc_layout(layout);
            let _ = b.alloc_layout(layout);
            let q = (&b).realloc(p, layout, 2).unwrap();
            assert!(q.as_ptr() as usize != p.as_ptr() as usize - 1);
            b.reset();
        }
    }

    #[test]
    fn invalid_read() {
        use alloc::Alloc;

        let mut b = &Bump::new();

        unsafe {
            let l1 = Layout::from_size_align(12000, 4).unwrap();
            let p1 = Alloc::alloc(&mut b, l1).unwrap();

            let l2 = Layout::from_size_align(1000, 4).unwrap();
            Alloc::alloc(&mut b, l2).unwrap();

            let p1 = b.realloc(p1, l1, 24000).unwrap();
            let l3 = Layout::from_size_align(24000, 4).unwrap();
            b.realloc(p1, l3, 48000).unwrap();
        }
    }
}