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    //! Memory allocation APIs

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

    use core::intrinsics::{min_align_of_val, size_of_val};
    use core::ptr::{NonNull, Unique};
    use core::usize;

    #[stable(feature = "alloc_module", since = "1.28.0")]
    #[doc(inline)]
    pub use core::alloc::*;

    extern "Rust" {
        // These are the magic symbols to call the global allocator.  rustc generates
        // them from the `#[global_allocator]` attribute if there is one, or uses the
        // default implementations in libstd (`__rdl_alloc` etc in `src/libstd/alloc.rs`)
        // otherwise.
        #[allocator]
        #[rustc_allocator_nounwind]
        fn __rust_alloc(size: usize, align: usize) -> *mut u8;
        #[rustc_allocator_nounwind]
        fn __rust_dealloc(ptr: *mut u8, size: usize, align: usize);
        #[rustc_allocator_nounwind]
        fn __rust_realloc(ptr: *mut u8,
                          old_size: usize,
                          align: usize,
                          new_size: usize) -> *mut u8;
        #[rustc_allocator_nounwind]
        fn __rust_alloc_zeroed(size: usize, align: usize) -> *mut u8;
    }

    /// The global memory allocator.
    ///
    /// This type implements the [`Alloc`] trait by forwarding calls
    /// to the allocator registered with the `#[global_allocator]` attribute
    /// if there is one, or the `std` crate’s default.
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[derive(Copy, Clone, Default, Debug)]
    pub struct Global;

    /// Allocate memory with the global allocator.
    ///
    /// This function forwards calls to the [`GlobalAlloc::alloc`] method
    /// of the allocator registered with the `#[global_allocator]` attribute
    /// if there is one, or the `std` crate’s default.
    ///
    /// This function is expected to be deprecated in favor of the `alloc` method
    /// of the [`Global`] type when it and the [`Alloc`] trait become stable.
    ///
    /// # Safety
    ///
    /// See [`GlobalAlloc::alloc`].
    ///
    /// # Examples
    ///
    /// ```
    /// use std::alloc::{alloc, dealloc, Layout};
    ///
    /// unsafe {
    ///     let layout = Layout::new::<u16>();
    ///     let ptr = alloc(layout);
    ///
    ///     *(ptr as *mut u16) = 42;
    ///     assert_eq!(*(ptr as *mut u16), 42);
    ///
    ///     dealloc(ptr, layout);
    /// }
    /// ```
    #[stable(feature = "global_alloc", since = "1.28.0")]
    #[inline]
    pub unsafe fn alloc(layout: Layout) -> *mut u8 {
        __rust_alloc(layout.size(), layout.align())
    }

    /// Deallocate memory with the global allocator.
    ///
    /// This function forwards calls to the [`GlobalAlloc::dealloc`] method
    /// of the allocator registered with the `#[global_allocator]` attribute
    /// if there is one, or the `std` crate’s default.
    ///
    /// This function is expected to be deprecated in favor of the `dealloc` method
    /// of the [`Global`] type when it and the [`Alloc`] trait become stable.
    ///
    /// # Safety
    ///
    /// See [`GlobalAlloc::dealloc`].
    #[stable(feature = "global_alloc", since = "1.28.0")]
    #[inline]
    pub unsafe fn dealloc(ptr: *mut u8, layout: Layout) {
        __rust_dealloc(ptr, layout.size(), layout.align())
    }

    /// Reallocate memory with the global allocator.
    ///
    /// This function forwards calls to the [`GlobalAlloc::realloc`] method
    /// of the allocator registered with the `#[global_allocator]` attribute
    /// if there is one, or the `std` crate’s default.
    ///
    /// This function is expected to be deprecated in favor of the `realloc` method
    /// of the [`Global`] type when it and the [`Alloc`] trait become stable.
    ///
    /// # Safety
    ///
    /// See [`GlobalAlloc::realloc`].
    #[stable(feature = "global_alloc", since = "1.28.0")]
    #[inline]
    pub unsafe fn realloc(ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
        __rust_realloc(ptr, layout.size(), layout.align(), new_size)
    }

    /// Allocate zero-initialized memory with the global allocator.
    ///
    /// This function forwards calls to the [`GlobalAlloc::alloc_zeroed`] method
    /// of the allocator registered with the `#[global_allocator]` attribute
    /// if there is one, or the `std` crate’s default.
    ///
    /// This function is expected to be deprecated in favor of the `alloc_zeroed` method
    /// of the [`Global`] type when it and the [`Alloc`] trait become stable.
    ///
    /// # Safety
    ///
    /// See [`GlobalAlloc::alloc_zeroed`].
    ///
    /// # Examples
    ///
    /// ```
    /// use std::alloc::{alloc_zeroed, dealloc, Layout};
    ///
    /// unsafe {
    ///     let layout = Layout::new::<u16>();
    ///     let ptr = alloc_zeroed(layout);
    ///
    ///     assert_eq!(*(ptr as *mut u16), 0);
    ///
    ///     dealloc(ptr, layout);
    /// }
    /// ```
    #[stable(feature = "global_alloc", since = "1.28.0")]
    #[inline]
    pub unsafe fn alloc_zeroed(layout: Layout) -> *mut u8 {
        __rust_alloc_zeroed(layout.size(), layout.align())
    }

    #[unstable(feature = "allocator_api", issue = "32838")]
    unsafe impl Alloc for Global {
        #[inline]
        unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
            NonNull::new(alloc(layout)).ok_or(AllocErr)
        }

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

        #[inline]
        unsafe fn realloc(&mut self,
                          ptr: NonNull<u8>,
                          layout: Layout,
                          new_size: usize)
                          -> Result<NonNull<u8>, AllocErr>
        {
            NonNull::new(realloc(ptr.as_ptr(), layout, new_size)).ok_or(AllocErr)
        }

        #[inline]
        unsafe fn alloc_zeroed(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
            NonNull::new(alloc_zeroed(layout)).ok_or(AllocErr)
        }
    }

    /// The allocator for unique pointers.
    // This function must not unwind. If it does, MIR codegen will fail.
    #[cfg(not(test))]
    #[lang = "exchange_malloc"]
    #[inline]
    unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
        if size == 0 {
            align as *mut u8
        } else {
            let layout = Layout::from_size_align_unchecked(size, align);
            let ptr = alloc(layout);
            if !ptr.is_null() {
                ptr
            } else {
                handle_alloc_error(layout)
            }
        }
    }

    #[cfg_attr(not(test), lang = "box_free")]
    #[inline]
    pub(crate) unsafe fn box_free<T: ?Sized>(ptr: Unique<T>) {
        let ptr = ptr.as_ptr();
        let size = size_of_val(&*ptr);
        let align = min_align_of_val(&*ptr);
        // We do not allocate for Box<T> when T is ZST, so deallocation is also not necessary.
        if size != 0 {
            let layout = Layout::from_size_align_unchecked(size, align);
            dealloc(ptr as *mut u8, layout);
        }
    }

    /// Abort on memory allocation error or failure.
    ///
    /// Callers of memory allocation APIs wishing to abort computation
    /// in response to an allocation error are encouraged to call this function,
    /// rather than directly invoking `panic!` or similar.
    ///
    /// The default behavior of this function is to print a message to standard error
    /// and abort the process.
    /// It can be replaced with [`set_alloc_error_hook`] and [`take_alloc_error_hook`].
    ///
    /// [`set_alloc_error_hook`]: ../../std/alloc/fn.set_alloc_error_hook.html
    /// [`take_alloc_error_hook`]: ../../std/alloc/fn.take_alloc_error_hook.html
    #[stable(feature = "global_alloc", since = "1.28.0")]
    #[rustc_allocator_nounwind]
    pub fn handle_alloc_error(layout: Layout) -> ! {
        #[allow(improper_ctypes)]
        extern "Rust" {
            #[lang = "oom"]
            fn oom_impl(layout: Layout) -> !;
        }
        unsafe { oom_impl(layout) }
    }

    #[cfg(test)]
    mod tests {
        extern crate test;
        use self::test::Bencher;
        use boxed::Box;
        use alloc::{Global, Alloc, Layout, handle_alloc_error};

        #[test]
        fn allocate_zeroed() {
            unsafe {
                let layout = Layout::from_size_align(1024, 1).unwrap();
                let ptr = Global.alloc_zeroed(layout.clone())
                    .unwrap_or_else(|_| handle_alloc_error(layout));

                let mut i = ptr.cast::<u8>().as_ptr();
                let end = i.add(layout.size());
                while i < end {
                    assert_eq!(*i, 0);
                    i = i.offset(1);
                }
                Global.dealloc(ptr, layout);
            }
        }

        #[bench]
        fn alloc_owned_small(b: &mut Bencher) {
            b.iter(|| {
                let _: Box<_> = box 10;
            })
        }
    }

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