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    use crate::alloc::Layout;
    use crate::cmp;
    use crate::ptr;

    /// A memory allocator that can be registered as the standard library’s default
    /// through the `#[global_allocator]` attribute.
    ///
    /// 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 this 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();
            // SAFETY: the safety contract for `alloc` must be upheld by the caller.
            let ptr = unsafe { self.alloc(layout) };
            if !ptr.is_null() {
                // SAFETY: as allocation succeeded, the region from `ptr`
                // of size `size` is guaranteed to be valid for writes.
                unsafe { 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). The new memory block is allocated with `layout`, but
        /// with the `size` updated to `new_size`.
        ///
        /// 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 {
            // SAFETY: the caller must ensure that the `new_size` does not overflow.
            // `layout.align()` comes from a `Layout` and is thus guaranteed to be valid.
            let new_layout = unsafe { Layout::from_size_align_unchecked(new_size, layout.align()) };
            // SAFETY: the caller must ensure that `new_layout` is greater than zero.
            let new_ptr = unsafe { self.alloc(new_layout) };
            if !new_ptr.is_null() {
                // SAFETY: the previously allocated block cannot overlap the newly allocated block.
                // The safety contract for `dealloc` must be upheld by the caller.
                unsafe {
                    ptr::copy_nonoverlapping(ptr, new_ptr, cmp::min(layout.size(), new_size));
                    self.dealloc(ptr, layout);
                }
            }
            new_ptr
        }
    }