alloc.rs.html -- source
Description: Source to the Rust file `src\libcore\alloc.rs`.
Keywords: rust, rustlang, rust-lang

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    //! Memory allocation APIs

    #![stable(feature = "alloc_module", since = "1.28.0")]

    use crate::cmp;
    use crate::fmt;
    use crate::mem;
    use crate::usize;
    use crate::ptr::{self, NonNull};
    use crate::num::NonZeroUsize;

    /// 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>())
    }

    /// Layout of a block of memory.
    ///
    /// An instance of `Layout` describes a particular layout of memory.
    /// You build a `Layout` up as an input to give to an allocator.
    ///
    /// All layouts have an associated non-negative size and a
    /// power-of-two alignment.
    ///
    /// (Note however that layouts are *not* required to have positive
    /// size, even though many allocators require that all memory
    /// requests have positive size. A caller to the `Alloc::alloc`
    /// method must either ensure that conditions like this are met, or
    /// use specific allocators with looser requirements.)
    #[stable(feature = "alloc_layout", since = "1.28.0")]
    #[derive(Copy, Clone, Debug, PartialEq, Eq)]
    #[lang = "alloc_layout"]
    pub struct Layout {
        // size of the requested block of memory, measured in bytes.
        size_: usize,

        // alignment of the requested block of memory, measured in bytes.
        // we ensure that this is always a power-of-two, because API's
        // like `posix_memalign` require it and it is a reasonable
        // constraint to impose on Layout constructors.
        //
        // (However, we do not analogously require `align >= sizeof(void*)`,
        //  even though that is *also* a requirement of `posix_memalign`.)
        align_: NonZeroUsize,
    }

    impl Layout {
        /// Constructs a `Layout` from a given `size` and `align`,
        /// or returns `LayoutErr` if either of the following conditions
        /// are not met:
        ///
        /// * `align` must not be zero,
        ///
        /// * `align` must be a power of two,
        ///
        /// * `size`, when rounded up to the nearest multiple of `align`,
        ///    must not overflow (i.e., the rounded value must be less than
        ///    `usize::MAX`).
        #[stable(feature = "alloc_layout", since = "1.28.0")]
        #[inline]
        pub fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutErr> {
            if !align.is_power_of_two() {
                return Err(LayoutErr { private: () });
            }

            // (power-of-two implies align != 0.)

            // Rounded up size is:
            //   size_rounded_up = (size + align - 1) & !(align - 1);
            //
            // We know from above that align != 0. If adding (align - 1)
            // does not overflow, then rounding up will be fine.
            //
            // Conversely, &-masking with !(align - 1) will subtract off
            // only low-order-bits. Thus if overflow occurs with the sum,
            // the &-mask cannot subtract enough to undo that overflow.
            //
            // Above implies that checking for summation overflow is both
            // necessary and sufficient.
            if size > usize::MAX - (align - 1) {
                return Err(LayoutErr { private: () });
            }

            unsafe {
                Ok(Layout::from_size_align_unchecked(size, align))
            }
        }

        /// Creates a layout, bypassing all checks.
        ///
        /// # Safety
        ///
        /// This function is unsafe as it does not verify the preconditions from
        /// [`Layout::from_size_align`](#method.from_size_align).
        #[stable(feature = "alloc_layout", since = "1.28.0")]
        #[inline]
        pub const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self {
            Layout { size_: size, align_: NonZeroUsize::new_unchecked(align) }
        }

        /// The minimum size in bytes for a memory block of this layout.
        #[stable(feature = "alloc_layout", since = "1.28.0")]
        #[inline]
        pub fn size(&self) -> usize { self.size_ }

        /// The minimum byte alignment for a memory block of this layout.
        #[stable(feature = "alloc_layout", since = "1.28.0")]
        #[inline]
        pub fn align(&self) -> usize { self.align_.get() }

        /// Constructs a `Layout` suitable for holding a value of type `T`.
        #[stable(feature = "alloc_layout", since = "1.28.0")]
        #[inline]
        pub fn new<T>() -> Self {
            let (size, align) = size_align::<T>();
            // Note that the align is guaranteed by rustc to be a power of two and
            // the size+align combo is guaranteed to fit in our address space. As a
            // result use the unchecked constructor here to avoid inserting code
            // that panics if it isn't optimized well enough.
            debug_assert!(Layout::from_size_align(size, align).is_ok());
            unsafe {
                Layout::from_size_align_unchecked(size, align)
            }
        }

        /// Produces layout describing a record that could be used to
        /// allocate backing structure for `T` (which could be a trait
        /// or other unsized type like a slice).
        #[stable(feature = "alloc_layout", since = "1.28.0")]
        #[inline]
        pub fn for_value<T: ?Sized>(t: &T) -> Self {
            let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
            // See rationale in `new` for why this us using an unsafe variant below
            debug_assert!(Layout::from_size_align(size, align).is_ok());
            unsafe {
                Layout::from_size_align_unchecked(size, align)
            }
        }

        /// Creates a layout describing the record that can hold a value
        /// of the same layout as `self`, but that also is aligned to
        /// alignment `align` (measured in bytes).
        ///
        /// If `self` already meets the prescribed alignment, then returns
        /// `self`.
        ///
        /// Note that this method does not add any padding to the overall
        /// size, regardless of whether the returned layout has a different
        /// alignment. In other words, if `K` has size 16, `K.align_to(32)`
        /// will *still* have size 16.
        ///
        /// Returns an error if the combination of `self.size()` and the given
        /// `align` violates the conditions listed in
        /// [`Layout::from_size_align`](#method.from_size_align).
        #[unstable(feature = "alloc_layout_extra", issue = "55724")]
        #[inline]
        pub fn align_to(&self, align: usize) -> Result<Self, LayoutErr> {
            Layout::from_size_align(self.size(), cmp::max(self.align(), align))
        }

        /// 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()`.
        #[unstable(feature = "alloc_layout_extra", issue = "55724")]
        #[inline]
        pub 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 by rounding the size of this layout up to a multiple
        /// of the layout's alignment.
        ///
        /// Returns `Err` if the padded size would overflow.
        ///
        /// This is equivalent to adding the result of `padding_needed_for`
        /// to the layout's current size.
        #[unstable(feature = "alloc_layout_extra", issue = "55724")]
        #[inline]
        pub fn pad_to_align(&self) -> Result<Layout, LayoutErr> {
            let pad = self.padding_needed_for(self.align());
            let new_size = self.size().checked_add(pad)
                .ok_or(LayoutErr { private: () })?;

            Layout::from_size_align(new_size, self.align())
        }

        /// 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`.
        #[unstable(feature = "alloc_layout_extra", issue = "55724")]
        #[inline]
        pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutErr> {
            let padded_size = self.size().checked_add(self.padding_needed_for(self.align()))
                .ok_or(LayoutErr { private: () })?;
            let alloc_size = padded_size.checked_mul(n)
                .ok_or(LayoutErr { private: () })?;

            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 `self` followed by
        /// `next`, including any necessary padding to ensure that `next`
        /// will be properly aligned. Note that the result layout will
        /// satisfy the alignment properties of both `self` and `next`.
        ///
        /// The resulting layout will be the same as that of a C struct containing
        /// two fields with the layouts of `self` and `next`, in that order.
        ///
        /// Returns `Some((k, offset))`, where `k` is layout of the concatenated
        /// record and `offset` is the relative location, in bytes, of the
        /// start of the `next` embedded within the concatenated record
        /// (assuming that the record itself starts at offset 0).
        ///
        /// On arithmetic overflow, returns `LayoutErr`.
        #[unstable(feature = "alloc_layout_extra", issue = "55724")]
        #[inline]
        pub fn extend(&self, next: Self) -> Result<(Self, usize), LayoutErr> {
            let new_align = cmp::max(self.align(), next.align());
            let pad = self.padding_needed_for(next.align());

            let offset = self.size().checked_add(pad)
                .ok_or(LayoutErr { private: () })?;
            let new_size = offset.checked_add(next.size())
                .ok_or(LayoutErr { private: () })?;

            let layout = Layout::from_size_align(new_size, new_align)?;
            Ok((layout, offset))
        }

        /// Creates a layout describing the record for `n` instances of
        /// `self`, with no padding between each instance.
        ///
        /// Note that, unlike `repeat`, `repeat_packed` does not guarantee
        /// that the repeated instances of `self` will be properly
        /// aligned, even if a given instance of `self` is properly
        /// aligned. In other words, if the layout returned by
        /// `repeat_packed` is used to allocate an array, it is not
        /// guaranteed that all elements in the array will be properly
        /// aligned.
        ///
        /// On arithmetic overflow, returns `LayoutErr`.
        #[unstable(feature = "alloc_layout_extra", issue = "55724")]
        #[inline]
        pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutErr> {
            let size = self.size().checked_mul(n).ok_or(LayoutErr { private: () })?;
            Layout::from_size_align(size, self.align())
        }

        /// Creates a layout describing the record for `self` followed by
        /// `next` with no additional padding between the two. Since no
        /// padding is inserted, the alignment of `next` is irrelevant,
        /// and is not incorporated *at all* into the resulting layout.
        ///
        /// On arithmetic overflow, returns `LayoutErr`.
        #[unstable(feature = "alloc_layout_extra", issue = "55724")]
        #[inline]
        pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutErr> {
            let new_size = self.size().checked_add(next.size())
                .ok_or(LayoutErr { private: () })?;
            let layout = Layout::from_size_align(new_size, self.align())?;
            Ok(layout)
        }

        /// Creates a layout describing the record for a `[T; n]`.
        ///
        /// On arithmetic overflow, returns `LayoutErr`.
        #[unstable(feature = "alloc_layout_extra", issue = "55724")]
        #[inline]
        pub fn array<T>(n: usize) -> Result<Self, LayoutErr> {
            Layout::new::<T>()
                .repeat(n)
                .map(|(k, offs)| {
                    debug_assert!(offs == mem::size_of::<T>());
                    k
                })
        }
    }

    /// The parameters given to `Layout::from_size_align`
    /// or some other `Layout` constructor
    /// do not satisfy its documented constraints.
    #[stable(feature = "alloc_layout", since = "1.28.0")]
    #[derive(Clone, PartialEq, Eq, Debug)]
    pub struct LayoutErr {
        private: ()
    }

    // (we need this for downstream impl of trait Error)
    #[stable(feature = "alloc_layout", since = "1.28.0")]
    impl fmt::Display for LayoutErr {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_str("invalid parameters to Layout::from_size_align")
        }
    }

    /// 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.
    ///
    /// [`grow_in_place`]: ./trait.Alloc.html#method.grow_in_place
    /// [`shrink_in_place`]: ./trait.Alloc.html#method.shrink_in_place
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[derive(Clone, PartialEq, Eq, Debug)]
    pub struct CannotReallocInPlace;

    #[unstable(feature = "allocator_api", issue = "32838")]
    impl CannotReallocInPlace {
        pub fn description(&self) -> &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())
        }
    }

    /// A memory allocator that can be registered as the standard library’s default
    /// though the `#[global_allocator]` attributes.
    ///
    /// 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
    ///   such as `alloc`, and
    ///
    /// * the memory block has not been subsequently deallocated, where
    ///   blocks are deallocated either by being passed to a deallocation
    ///   method such as `dealloc` or by being
    ///   passed to a reallocation method that returns a non-null pointer.
    ///
    ///
    /// # Example
    ///
    /// ```no_run
    /// use std::alloc::{GlobalAlloc, Layout, alloc};
    /// use std::ptr::null_mut;
    ///
    /// struct MyAllocator;
    ///
    /// unsafe impl GlobalAlloc for MyAllocator {
    ///     unsafe fn alloc(&self, _layout: Layout) -> *mut u8 { null_mut() }
    ///     unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
    /// }
    ///
    /// #[global_allocator]
    /// static A: MyAllocator = MyAllocator;
    ///
    /// fn main() {
    ///     unsafe {
    ///         assert!(alloc(Layout::new::<u32>()).is_null())
    ///     }
    /// }
    /// ```
    ///
    /// # Safety
    ///
    /// The `GlobalAlloc` trait is an `unsafe` trait for a number of reasons, and
    /// implementors must ensure that they adhere to these contracts:
    ///
    /// * It's undefined behavior if global allocators unwind. This restriction may
    ///   be lifted in the future, but currently a panic from any of these
    ///   functions may lead to memory unsafety.
    ///
    /// * `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.
    #[stable(feature = "global_alloc", since = "1.28.0")]
    pub unsafe trait GlobalAlloc {
        /// Allocate memory as described by the given `layout`.
        ///
        /// Returns a pointer to newly-allocated memory,
        /// or null to indicate allocation failure.
        ///
        /// # 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.)
        ///
        /// The allocated block of memory may or may not be initialized.
        ///
        /// # Errors
        ///
        /// Returning a null pointer indicates that either memory is exhausted
        /// or `layout` does not meet allocator's size or alignment constraints.
        ///
        /// Implementations are encouraged to return null on memory
        /// exhaustion rather than 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
        #[stable(feature = "global_alloc", since = "1.28.0")]
        unsafe fn alloc(&self, layout: Layout) -> *mut u8;

        /// Deallocate the block of memory at the given `ptr` pointer with the given `layout`.
        ///
        /// # 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 be the same layout that was used
        ///   to allocate that block of memory,
        #[stable(feature = "global_alloc", since = "1.28.0")]
        unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout);

        /// 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.
        /// However the allocated block of memory is guaranteed to be initialized.
        ///
        /// # Errors
        ///
        /// Returning a null pointer 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
        #[stable(feature = "global_alloc", since = "1.28.0")]
        unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
            let size = layout.size();
            let ptr = self.alloc(layout);
            if !ptr.is_null() {
                ptr::write_bytes(ptr, 0, size);
            }
            ptr
        }

        /// Shrink or grow a block of memory to the given `new_size`.
        /// The block is described by the given `ptr` pointer and `layout`.
        ///
        /// If this returns a non-null pointer, then ownership of the memory block
        /// referenced by `ptr` has been transferred to this allocator.
        /// The memory may or may not have been deallocated,
        /// 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 null, 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 be the same layout that was used
        ///   to allocate that block of memory,
        ///
        /// * `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 null if the new layout does not meet the size
        /// and alignment constraints of the allocator, or if reallocation
        /// otherwise fails.
        ///
        /// Implementations are encouraged to return null 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
        #[stable(feature = "global_alloc", since = "1.28.0")]
        unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
            let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
            let new_ptr = self.alloc(new_layout);
            if !new_ptr.is_null() {
                ptr::copy_nonoverlapping(
                    ptr,
                    new_ptr,
                    cmp::min(layout.size(), new_size),
                );
                self.dealloc(ptr, layout);
            }
            new_ptr
        }
    }

    /// 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();

            if new_size >= old_size {
                if let Ok(()) = self.grow_in_place(ptr, layout, new_size) {
                    return Ok(ptr);
                }
            } else if new_size < old_size {
                if let Ok(()) = self.shrink_in_place(ptr, layout, 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::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::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!(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::array::<T>(n) {
                Ok(ref k) if k.size() > 0 => {
                    Ok(self.dealloc(ptr.cast(), k.clone()))
                }
                _ => {
                    Err(AllocErr)
                }
            }
        }
    }