oxc_allocator 0.140.0

//! `Alloc` trait.

// Copyright 2015 The Rust Project Developers.
// See the COPYRIGHT file at the top-level directory of this distribution and at http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0>
// or the MIT license <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your option.
// This file may not be copied, modified, or distributed except according to those terms.

#![expect(clippy::undocumented_unsafe_blocks)]
#![allow(dead_code)]

use std::{
    alloc::Layout,
    cmp, fmt, mem,
    ptr::{self, NonNull},
};

#[cold]
#[inline(never)]
pub fn handle_alloc_error(layout: Layout) -> ! {
    panic!("encountered allocation error: {layout:?}")
}

/// Represents the combination of a starting address and a total capacity of the returned block.
// #[unstable(feature = "allocator_api", issue = "32838")]
#[derive(Debug)]
pub struct Excess(pub NonNull<u8>, pub usize);

fn size_align<T>() -> (usize, usize) {
    (mem::size_of::<T>(), mem::align_of::<T>())
}

/// The `AllocErr` error indicates an allocation failure that may be due to resource exhaustion or to
/// something wrong when combining the given input arguments with this allocator.
// #[unstable(feature = "allocator_api", issue = "32838")]
#[derive(Clone, PartialEq, Eq, Debug)]
pub struct AllocErr;

// (we need this for downstream impl of trait Error)
// #[unstable(feature = "allocator_api", issue = "32838")]
impl fmt::Display for AllocErr {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.write_str("memory allocation failed")
    }
}

/// The `CannotReallocInPlace` error is used when `grow_in_place` or `shrink_in_place` were unable to reuse
/// the given memory block for a requested layout.
// #[unstable(feature = "allocator_api", issue = "32838")]
#[derive(Clone, PartialEq, Eq, Debug)]
pub struct CannotReallocInPlace;

// #[unstable(feature = "allocator_api", issue = "32838")]
impl CannotReallocInPlace {
    #[expect(clippy::unused_self, reason = "part of public API")]
    pub fn description(&self) -> &'static str {
        "cannot reallocate allocator's memory in place"
    }
}

// (we need this for downstream impl of trait Error)
// #[unstable(feature = "allocator_api", issue = "32838")]
impl fmt::Display for CannotReallocInPlace {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", self.description())
    }
}

/// An implementation of `Alloc` can allocate, reallocate, and deallocate arbitrary blocks of data described
/// via `Layout`.
///
/// Some of the methods require that a memory block be *currently allocated* via an allocator. This means that:
///
/// * the starting address for that memory block was previously returned by a previous call to an allocation
///   method (`alloc`, `alloc_zeroed`, `alloc_excess`, `alloc_one`, `alloc_array`) or reallocation method
///   (`realloc`, `realloc_excess`, or `realloc_array`), and
///
/// * the memory block has not been subsequently deallocated, where blocks are deallocated either by being
///   passed to a deallocation method (`dealloc`, `dealloc_one`, `dealloc_array`) or by being passed to a
///   reallocation method (see above) that returns `Ok`.
///
/// A note regarding zero-sized types and zero-sized layouts: many methods in the `Alloc` trait state that
/// allocation requests must be non-zero size, or else undefined behavior can result.
///
/// * However, some higher-level allocation methods (`alloc_one`, `alloc_array`) are well-defined on zero-sized
///   types and can optionally support them: it is left up to the implementor whether to return `Err`, or to
///   return `Ok` with some pointer.
///
/// * If an `Alloc` implementation chooses to return `Ok` in this case (i.e. the pointer denotes a zero-sized
///   inaccessible block) then that returned pointer must be considered "currently allocated". On such an
///   allocator, *all* methods that take currently-allocated pointers as inputs must accept these zero-sized
///   pointers, *without* causing undefined behavior.
///
/// * In other words, if a zero-sized pointer can flow out of an allocator, then that allocator must likewise
///   accept that pointer flowing back into its deallocation and reallocation methods.
///
/// Some of the methods require that a layout *fit* a memory block. What it means for a layout to "fit" a
/// memory block means (or equivalently, for a memory block to "fit" a layout) is that the following two
/// conditions must hold:
///
/// 1. The block's starting address must be aligned to `layout.align()`.
///
/// 2. The block's size must fall in the range `[use_min, use_max]`, where:
///
///    * `use_min` is `self.usable_size(layout).0`, and
///
///    * `use_max` is the capacity that was (or would have been) returned when (if) the block was allocated
///      via a call to `alloc_excess` or `realloc_excess`.
///
/// Note that:
///
///  * the size of the layout most recently used to allocate the block is guaranteed to be in the range
///    `[use_min, use_max]`, and
///
///  * a lower-bound on `use_max` can be safely approximated by a call to `usable_size`.
///
///  * if a layout `k` fits a memory block (denoted by `ptr`) currently allocated via an allocator `a`, then it
///    is legal to use that layout to deallocate it, i.e. `a.dealloc(ptr, k);`.
///
/// # SAFETY
///
/// The `Alloc` trait is an `unsafe` trait for a number of reasons, and implementors must ensure that they
/// adhere to these contracts:
///
/// * Pointers returned from allocation functions must point to valid memory and retain their validity until at
///   least the instance of `Alloc` is dropped itself.
///
/// * `Layout` queries and calculations in general must be correct. Callers of this trait are allowed to rely
///   on the contracts defined on each method, and implementors must ensure such contracts remain true.
///
/// Note that this list may get tweaked over time as clarifications are made in the future.
// #[unstable(feature = "allocator_api", issue = "32838")]
pub unsafe trait Alloc {
    // (Note: some existing allocators have unspecified but well-defined behavior in response to a zero size
    // allocation request ; e.g. in C, `malloc` of 0 will either return a null pointer or a unique pointer,
    // but will not have arbitrary undefined behavior.
    // However in jemalloc for example, `mallocx(0)` is documented as undefined behavior.)

    /// Returns a pointer meeting the size and alignment guarantees of `layout`.
    ///
    /// If this method returns an `Ok(addr)`, then the `addr` returned will be non-null address pointing to a
    /// block of storage suitable for holding an instance of `layout`.
    ///
    /// The returned block of storage may or may not have its contents initialized. (Extension subtraits might
    /// restrict this behavior, e.g. to ensure initialization to particular sets of bit patterns.)
    ///
    /// # SAFETY
    ///
    /// This function is unsafe because undefined behavior can result if the caller does not ensure that
    /// `layout` has non-zero size.
    ///
    /// (Extension subtraits might provide more specific bounds on behavior, e.g. guarantee a sentinel address
    /// or a null pointer in response to a zero-size allocation request.)
    ///
    /// # Errors
    ///
    /// Returning `Err` indicates that either memory is exhausted or `layout` does not meet allocator's size
    /// or alignment constraints.
    ///
    /// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or aborting,
    /// but this is not a strict requirement. (Specifically: it is *legal* to implement this trait atop an
    /// underlying native allocation library that aborts on memory exhaustion.)
    ///
    /// Clients wishing to abort computation in response to an allocation error are encouraged to call the
    /// [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
    ///
    /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
    unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr>;

    /// Deallocate the memory referenced by `ptr`.
    ///
    /// # SAFETY
    ///
    /// This function is unsafe because undefined behavior can result if the caller does not ensure all of the
    /// following:
    ///
    /// * `ptr` must denote a block of memory currently allocated via this allocator,
    ///
    /// * `layout` must *fit* that block of memory,
    ///
    /// * In addition to fitting the block of memory `layout`, the alignment of the `layout` must match the
    ///   alignment used to allocate that block of memory.
    unsafe fn dealloc(&mut self, ptr: NonNull<u8>, layout: Layout);

    // == ALLOCATOR-SPECIFIC QUANTITIES AND LIMITS ==
    // usable_size

    /// Returns bounds on the guaranteed usable size of a successful allocation created with the specified
    /// `layout`.
    ///
    /// In particular, if one has a memory block allocated via a given allocator `a` and layout `k` where
    /// `a.usable_size(k)` returns `(l, u)`, then one can pass that block to `a.dealloc()` with a layout in
    /// the size range [l, u].
    ///
    /// (All implementors of `usable_size` must ensure that `l <= k.size() <= u`)
    ///
    /// Both the lower- and upper-bounds (`l` and `u` respectively) are provided, because an allocator based
    /// on size classes could misbehave if one attempts to deallocate a block without providing a correct
    /// value for its size (i.e., one within the range `[l, u]`).
    ///
    /// Clients who wish to make use of excess capacity are encouraged to use the `alloc_excess` and
    /// `realloc_excess` instead, as this method is constrained to report conservative values that serve as
    /// valid bounds for *all possible* allocation method calls.
    ///
    /// However, for clients that do not wish to track the capacity returned by `alloc_excess` locally, this
    /// method is likely to produce useful results.
    #[inline]
    fn usable_size(&self, layout: &Layout) -> (usize, usize) {
        (layout.size(), layout.size())
    }

    // == METHODS FOR MEMORY REUSE ==
    // realloc. alloc_excess, realloc_excess

    /// Returns a pointer suitable for holding data described by a new layout with `layout`’s alignment and a
    /// size given by `new_size`. To accomplish this, this may extend or shrink the allocation referenced by
    /// `ptr` to fit the new layout.
    ///
    /// If this returns `Ok`, then ownership of the memory block referenced by `ptr` has been transferred to
    /// this allocator. The memory may or may not have been freed, and should be considered unusable (unless of
    /// course it was transferred back to the caller again via the return value of this method).
    ///
    /// If this method returns `Err`, then ownership of the memory block has not been transferred to this
    /// allocator, and the contents of the memory block are unaltered.
    ///
    /// # SAFETY
    ///
    /// This function is unsafe because undefined behavior can result if the caller does not ensure all of the
    /// following:
    ///
    /// * `ptr` must be currently allocated via this allocator,
    ///
    /// * `layout` must *fit* the `ptr` (see above). (The `new_size` argument need not fit it.)
    ///
    /// * `new_size` must be greater than zero
    ///
    /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`, must not overflow (i.e. the
    ///   rounded value must be less than `usize::MAX`)
    ///
    /// (Extension subtraits might provide more specific bounds on behavior, e.g. guarantee a sentinel address
    /// or a null pointer in response to a zero-size allocation request.)
    ///
    /// # Errors
    ///
    /// Returns `Err` only if the new layout does not meet the allocator's size and alignment constraints of
    /// the allocator, or if reallocation otherwise fails.
    ///
    /// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or aborting,
    /// but this is not a strict requirement. (Specifically: it is *legal* to implement this trait atop an
    /// underlying native allocation library that aborts on memory exhaustion.)
    ///
    /// Clients wishing to abort computation in response to a reallocation error are encouraged to call the
    /// [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
    ///
    /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
    unsafe fn realloc(
        &mut self,
        ptr: NonNull<u8>,
        layout: Layout,
        new_size: usize,
    ) -> Result<NonNull<u8>, AllocErr> {
        let old_size = layout.size();

        #[expect(clippy::collapsible_else_if)]
        if new_size >= old_size {
            if unsafe { self.grow_in_place(ptr, layout, new_size) }.is_ok() {
                return Ok(ptr);
            }
        } else {
            if unsafe { self.shrink_in_place(ptr, layout, new_size) }.is_ok() {
                return Ok(ptr);
            }
        }

        // otherwise, fall back on alloc + copy + dealloc
        let new_layout = unsafe { Layout::from_size_align_unchecked(new_size, layout.align()) };
        let result = unsafe { self.alloc(new_layout) };
        if let Ok(new_ptr) = result {
            unsafe {
                ptr::copy_nonoverlapping(
                    ptr.as_ptr(),
                    new_ptr.as_ptr(),
                    cmp::min(old_size, new_size),
                );
                self.dealloc(ptr, layout);
            }
        }
        result
    }

    /// Behaves like `alloc`, but also ensures that the contents are set to zero before being returned.
    ///
    /// # SAFETY
    ///
    /// This function is unsafe for the same reasons that `alloc` is.
    ///
    /// # Errors
    ///
    /// Returning `Err` indicates that either memory is exhausted or `layout` does not meet allocator's size
    /// or alignment constraints, just as in `alloc`.
    ///
    /// Clients wishing to abort computation in response to an allocation error are encouraged to call the
    /// [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
    ///
    /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
    unsafe fn alloc_zeroed(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
        let size = layout.size();
        let res = unsafe { self.alloc(layout) };
        if let Ok(ptr) = res {
            unsafe { ptr::write_bytes(ptr.as_ptr(), 0, size) };
        }
        res
    }

    /// Behaves like `alloc`, but also returns the whole size of the returned block. For some `layout` inputs,
    /// like arrays, this may include extra storage usable for additional data.
    ///
    /// # SAFETY
    ///
    /// This function is unsafe for the same reasons that `alloc` is.
    ///
    /// # Errors
    ///
    /// Returning `Err` indicates that either memory is exhausted or `layout` does not meet allocator's size
    /// or alignment constraints, just as in `alloc`.
    ///
    /// Clients wishing to abort computation in response to an allocation error are encouraged to call the
    /// [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
    ///
    /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
    unsafe fn alloc_excess(&mut self, layout: Layout) -> Result<Excess, AllocErr> {
        let usable_size = self.usable_size(&layout);
        unsafe { self.alloc(layout) }.map(|ptr| Excess(ptr, usable_size.1))
    }

    /// Behaves like `realloc`, but also returns the whole size of the returned block. For some `layout`
    /// inputs, like arrays, this may include extra storage usable for additional data.
    ///
    /// # SAFETY
    ///
    /// This function is unsafe for the same reasons that `realloc` is.
    ///
    /// # Errors
    ///
    /// Returning `Err` indicates that either memory is exhausted or `layout` does not meet allocator's size
    /// or alignment constraints, just as in `realloc`.
    ///
    /// Clients wishing to abort computation in response to a reallocation error are encouraged to call the
    /// [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
    ///
    /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
    unsafe fn realloc_excess(
        &mut self,
        ptr: NonNull<u8>,
        layout: Layout,
        new_size: usize,
    ) -> Result<Excess, AllocErr> {
        let new_layout = unsafe { Layout::from_size_align_unchecked(new_size, layout.align()) };
        let usable_size = self.usable_size(&new_layout);
        unsafe { self.realloc(ptr, layout, new_size) }.map(|ptr| Excess(ptr, usable_size.1))
    }

    /// Attempts to extend the allocation referenced by `ptr` to fit `new_size`.
    ///
    /// If this returns `Ok`, then the allocator has asserted that the memory block referenced by `ptr` now
    /// fits `new_size`, and thus can be used to carry data of a layout of that size and same alignment as
    /// `layout`. (The allocator is allowed to expend effort to accomplish this, such as extending the memory
    /// block to include successor blocks, or virtual memory tricks.)
    ///
    /// Regardless of what this method returns, ownership of the memory block referenced by `ptr` has not been
    /// transferred, and the contents of the memory block are unaltered.
    ///
    /// # SAFETY
    ///
    /// This function is unsafe because undefined behavior can result if the caller does not ensure all of the
    /// following:
    ///
    /// * `ptr` must be currently allocated via this allocator,
    ///
    /// * `layout` must *fit* the `ptr` (see above); note the `new_size` argument need not fit it,
    ///
    /// * `new_size` must not be less than `layout.size()`,
    ///
    /// # Errors
    ///
    /// Returns `Err(CannotReallocInPlace)` when the allocator is unable to assert that the memory block
    /// referenced by `ptr` could fit `layout`.
    ///
    /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error` function; clients are
    /// expected either to be able to recover from `grow_in_place` failures without aborting, or to fall back
    /// on another reallocation method before resorting to an abort.
    unsafe fn grow_in_place(
        &mut self,
        ptr: NonNull<u8>,
        layout: Layout,
        new_size: usize,
    ) -> Result<(), CannotReallocInPlace> {
        let _ = ptr; // this default implementation doesn't care about the actual address
        debug_assert!(new_size >= layout.size());
        let (_l, u) = self.usable_size(&layout);
        // _l <= layout.size()                       [guaranteed by usable_size()]
        //       layout.size() <= new_layout.size()  [required by this method]
        if new_size <= u { Ok(()) } else { Err(CannotReallocInPlace) }
    }

    /// Attempts to shrink the allocation referenced by `ptr` to fit `new_size`.
    ///
    /// If this returns `Ok`, then the allocator has asserted that the memory block referenced by `ptr` now
    /// fits `new_size`, and thus can only be used to carry data of that smaller layout. (The allocator is
    /// allowed to take advantage of this, carving off portions of the block for reuse elsewhere.) The
    /// truncated contents of the block within the smaller layout are unaltered, and ownership of block has
    /// not been transferred.
    ///
    /// If this returns `Err`, then the memory block is considered to still represent the original (larger)
    /// `layout`. None of the block has been carved off for reuse elsewhere, ownership of the memory block has
    /// not been transferred, and the contents of the memory block are unaltered.
    ///
    /// # SAFETY
    ///
    /// This function is unsafe because undefined behavior can result if the caller does not ensure all of the
    /// following:
    ///
    /// * `ptr` must be currently allocated via this allocator,
    ///
    /// * `layout` must *fit* the `ptr` (see above); note the `new_size` argument need not fit it,
    ///
    /// * `new_size` must not be greater than `layout.size()` (and must be greater than zero),
    ///
    /// # Errors
    ///
    /// Returns `Err(CannotReallocInPlace)` when the allocator is unable to assert that the memory block
    /// referenced by `ptr` could fit `layout`.
    ///
    /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error` function; clients are
    /// expected either to be able to recover from `shrink_in_place` failures without aborting, or to fall
    /// back on another reallocation method before resorting to an abort.
    unsafe fn shrink_in_place(
        &mut self,
        ptr: NonNull<u8>,
        layout: Layout,
        new_size: usize,
    ) -> Result<(), CannotReallocInPlace> {
        let _ = ptr; // this default implementation doesn't care about the actual address
        debug_assert!(new_size <= layout.size());
        let (l, _u) = self.usable_size(&layout);
        //                      layout.size() <= _u  [guaranteed by usable_size()]
        // new_layout.size() <= layout.size()        [required by this method]
        if l <= new_size { Ok(()) } else { Err(CannotReallocInPlace) }
    }

    // == COMMON USAGE PATTERNS ==
    // alloc_one, dealloc_one, alloc_array, realloc_array. dealloc_array

    /// Allocates a block suitable for holding an instance of `T`.
    ///
    /// Captures a common usage pattern for allocators.
    ///
    /// The returned block is suitable for passing to the `alloc`/`realloc` methods of this allocator.
    ///
    /// Note to implementors: If this returns `Ok(ptr)`, then `ptr` must be considered "currently allocated"
    /// and must be acceptable input to methods such as `realloc` or `dealloc`, *even if* `T` is a zero-sized
    /// type. In other words, if your `Alloc` implementation overrides this method in a manner that can return
    /// a zero-sized `ptr`, then all reallocation and deallocation methods need to be similarly overridden to
    /// accept such values as input.
    ///
    /// # Errors
    ///
    /// Returning `Err` indicates that either memory is exhausted or `T` does not meet allocator's size or
    /// alignment constraints.
    ///
    /// For zero-sized `T`, may return either of `Ok` or `Err`, but will *not* yield undefined behavior.
    ///
    /// Clients wishing to abort computation in response to an allocation error are encouraged to call the
    /// [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
    ///
    /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
    fn alloc_one<T>(&mut self) -> Result<NonNull<T>, AllocErr>
    where
        Self: Sized,
    {
        let k = Layout::new::<T>();
        if k.size() > 0 { unsafe { self.alloc(k).map(NonNull::cast) } } else { Err(AllocErr) }
    }

    /// Deallocates a block suitable for holding an instance of `T`.
    ///
    /// The given block must have been produced by this allocator, and must be suitable for storing a `T` (in
    /// terms of alignment as well as minimum and maximum size); otherwise yields undefined behavior.
    ///
    /// Captures a common usage pattern for allocators.
    ///
    /// # SAFETY
    ///
    /// This function is unsafe because undefined behavior can result if the caller does not ensure both:
    ///
    /// * `ptr` must denote a block of memory currently allocated via this allocator
    ///
    /// * the layout of `T` must *fit* that block of memory
    unsafe fn dealloc_one<T>(&mut self, ptr: NonNull<T>)
    where
        Self: Sized,
    {
        let k = Layout::new::<T>();
        if k.size() > 0 {
            unsafe { self.dealloc(ptr.cast(), k) };
        }
    }

    /// Allocates a block suitable for holding `n` instances of `T`.
    ///
    /// Captures a common usage pattern for allocators.
    ///
    /// The returned block is suitable for passing to the `alloc`/`realloc` methods of this allocator.
    ///
    /// Note to implementors: If this returns `Ok(ptr)`, then `ptr` must be considered "currently allocated"
    /// and must be acceptable input to methods such as `realloc` or `dealloc`, *even if* `T` is a zero-sized
    /// type. In other words, if your `Alloc` implementation overrides this method in a manner that can return
    /// a zero-sized `ptr`, then all reallocation and deallocation methods need to be similarly overridden to
    /// accept such values as input.
    ///
    /// # Errors
    ///
    /// Returning `Err` indicates that either memory is exhausted or `[T; n]` does not meet allocator's size
    /// or alignment constraints.
    ///
    /// For zero-sized `T` or `n == 0`, may return either of `Ok` or `Err`, but will *not* yield undefined
    /// behavior.
    ///
    /// Always returns `Err` on arithmetic overflow.
    ///
    /// Clients wishing to abort computation in response to an allocation error are encouraged to call the
    /// [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
    ///
    /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
    fn alloc_array<T>(&mut self, n: usize) -> Result<NonNull<T>, AllocErr>
    where
        Self: Sized,
    {
        match Layout::array::<T>(n) {
            Ok(layout) if layout.size() > 0 => unsafe { self.alloc(layout).map(NonNull::cast) },
            _ => Err(AllocErr),
        }
    }

    /// Reallocates a block previously suitable for holding `n_old` instances of `T`, returning a block
    /// suitable for holding `n_new` instances of `T`.
    ///
    /// Captures a common usage pattern for allocators.
    ///
    /// The returned block is suitable for passing to the `alloc`/`realloc` methods of this allocator.
    ///
    /// # SAFETY
    ///
    /// This function is unsafe because undefined behavior can result if the caller does not ensure all of the
    /// following:
    ///
    /// * `ptr` must be currently allocated via this allocator,
    ///
    /// * the layout of `[T; n_old]` must *fit* that block of memory
    ///
    /// # Errors
    ///
    /// Returning `Err` indicates that either memory is exhausted or `[T; n_new]` does not meet allocator's
    /// size or alignment constraints.
    ///
    /// For zero-sized `T` or `n_new == 0`, may return either of `Ok` or `Err`, but will *not* yield undefined
    /// behavior.
    ///
    /// Always returns `Err` on arithmetic overflow.
    ///
    /// Clients wishing to abort computation in response to a reallocation error are encouraged to call the
    /// [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
    ///
    /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
    unsafe fn realloc_array<T>(
        &mut self,
        ptr: NonNull<T>,
        n_old: usize,
        n_new: usize,
    ) -> Result<NonNull<T>, AllocErr>
    where
        Self: Sized,
    {
        match (Layout::array::<T>(n_old), Layout::array::<T>(n_new)) {
            (Ok(ref k_old), Ok(ref k_new)) if k_old.size() > 0 && k_new.size() > 0 => {
                debug_assert_eq!(k_old.align(), k_new.align());
                unsafe { self.realloc(ptr.cast(), *k_old, k_new.size()) }.map(NonNull::cast)
            }
            _ => Err(AllocErr),
        }
    }

    /// Deallocates a block suitable for holding `n` instances of `T`.
    ///
    /// Captures a common usage pattern for allocators.
    ///
    /// # SAFETY
    ///
    /// This function is unsafe because undefined behavior can result if the caller does not ensure both:
    ///
    /// * `ptr` must denote a block of memory currently allocated via this allocator
    ///
    /// * the layout of `[T; n]` must *fit* that block of memory
    ///
    /// # Errors
    ///
    /// Returning `Err` indicates that either `[T; n]` or the given memory block does not meet allocator's
    /// size or alignment constraints.
    ///
    /// Always returns `Err` on arithmetic overflow.
    unsafe fn dealloc_array<T>(&mut self, ptr: NonNull<T>, n: usize) -> Result<(), AllocErr>
    where
        Self: Sized,
    {
        match Layout::array::<T>(n) {
            Ok(k) if k.size() > 0 => {
                unsafe { self.dealloc(ptr.cast(), k) };
                Ok(())
            }
            _ => Err(AllocErr),
        }
    }
}