allocator_api 0.6.0

//! Memory allocation APIs

use core::cmp;
use core::fmt;
use core::mem;
use core::usize;
use core::ptr::{self, NonNull};

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

pub use core::alloc::{Layout, LayoutErr};

pub(crate) trait LayoutExt: Sized {
    fn padding_needed_for(&self, align: usize) -> usize;

    fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutErr>;

    fn array<T>(n: usize) -> Result<Self, LayoutErr>;
}

fn layouterr() -> LayoutErr {
    // We can't create a LayoutErr manually in LayoutExt, but
    // Layout::from_size_align will return us one if it's given
    // wrong arguments, which an alignment of 0 is.
    Layout::from_size_align(0, 0).err().unwrap()
}

/// Implements the unstable parts of the core::alloc::Layout type that
/// this crate uses.
impl LayoutExt for Layout {
    /// Returns the amount of padding we must insert after `self`
    /// to ensure that the following address will satisfy `align`
    /// (measured in bytes).
    ///
    /// e.g., if `self.size()` is 9, then `self.padding_needed_for(4)`
    /// returns 3, because that is the minimum number of bytes of
    /// padding required to get a 4-aligned address (assuming that the
    /// corresponding memory block starts at a 4-aligned address).
    ///
    /// The return value of this function has no meaning if `align` is
    /// not a power-of-two.
    ///
    /// Note that the utility of the returned value requires `align`
    /// to be less than or equal to the alignment of the starting
    /// address for the whole allocated block of memory. One way to
    /// satisfy this constraint is to ensure `align <= self.align()`.
    #[inline]
    fn padding_needed_for(&self, align: usize) -> usize {
        let len = self.size();

        // Rounded up value is:
        //   len_rounded_up = (len + align - 1) & !(align - 1);
        // and then we return the padding difference: `len_rounded_up - len`.
        //
        // We use modular arithmetic throughout:
        //
        // 1. align is guaranteed to be > 0, so align - 1 is always
        //    valid.
        //
        // 2. `len + align - 1` can overflow by at most `align - 1`,
        //    so the &-mask wth `!(align - 1)` will ensure that in the
        //    case of overflow, `len_rounded_up` will itself be 0.
        //    Thus the returned padding, when added to `len`, yields 0,
        //    which trivially satisfies the alignment `align`.
        //
        // (Of course, attempts to allocate blocks of memory whose
        // size and padding overflow in the above manner should cause
        // the allocator to yield an error anyway.)

        let len_rounded_up = len.wrapping_add(align).wrapping_sub(1)
            & !align.wrapping_sub(1);
        len_rounded_up.wrapping_sub(len)
    }

    /// Creates a layout describing the record for `n` instances of
    /// `self`, with a suitable amount of padding between each to
    /// ensure that each instance is given its requested size and
    /// alignment. On success, returns `(k, offs)` where `k` is the
    /// layout of the array and `offs` is the distance between the start
    /// of each element in the array.
    ///
    /// On arithmetic overflow, returns `LayoutErr`.
    #[inline]
    fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutErr> {
        let padded_size = self.size().checked_add(LayoutExt::padding_needed_for(self, self.align()))
            .ok_or_else(layouterr)?;
        let alloc_size = padded_size.checked_mul(n)
            .ok_or_else(layouterr)?;

        unsafe {
            // self.align is already known to be valid and alloc_size has been
            // padded already.
            Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), padded_size))
        }
    }
    /// Creates a layout describing the record for a `[T; n]`.
    ///
    /// On arithmetic overflow, returns `LayoutErr`.
    #[inline]
    fn array<T>(n: usize) -> Result<Self, LayoutErr> {
        LayoutExt::repeat(&Layout::new::<T>(), n)
            .map(|(k, offs)| {
                debug_assert!(offs == mem::size_of::<T>());
                k
            })
    }
}

/// 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.
#[derive(Clone, PartialEq, Eq, Debug)]
pub struct AllocErr;

// (we need this for downstream impl of trait Error)
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.
#[derive(Clone, PartialEq, Eq, Debug)]
pub struct CannotReallocInPlace;

impl CannotReallocInPlace {
    pub fn description(&self) -> &str {
        "cannot reallocate allocator's memory in place"
    }
}

// (we need this for downstream impl of trait Error)
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.
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();

        if new_size >= old_size {
            if let Ok(()) = self.grow_in_place(ptr, layout.clone(), new_size) {
                return Ok(ptr);
            }
        } else if new_size < old_size {
            if let Ok(()) = self.shrink_in_place(ptr, layout.clone(), new_size) {
                return Ok(ptr);
            }
        }

        // otherwise, fall back on alloc + copy + dealloc.
        let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
        let result = self.alloc(new_layout);
        if let Ok(new_ptr) = result {
            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 p = self.alloc(layout);
        if let Ok(p) = p {
            ptr::write_bytes(p.as_ptr(), 0, size);
        }
        p
    }

    /// 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);
        self.alloc(layout).map(|p| Excess(p, 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 = Layout::from_size_align_unchecked(new_size, layout.align());
        let usable_size = self.usable_size(&new_layout);
        self.realloc(ptr, layout, new_size)
            .map(|p| Excess(p, 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(|p| p.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 {
            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 as LayoutExt>::array::<T>(n) {
            Ok(ref layout) if layout.size() > 0 => {
                unsafe {
                    self.alloc(layout.clone()).map(|p| p.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 as LayoutExt>::array::<T>(n_old), <Layout as LayoutExt>::array::<T>(n_new)) {
            (Ok(ref k_old), Ok(ref k_new)) if k_old.size() > 0 && k_new.size() > 0 => {
                debug_assert!(k_old.align() == k_new.align());
                self.realloc(ptr.cast(), k_old.clone(), 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 as LayoutExt>::array::<T>(n) {
            Ok(ref k) if k.size() > 0 => {
                Ok(self.dealloc(ptr.cast(), k.clone()))
            }
            _ => {
                Err(AllocErr)
            }
        }
    }
}