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    //! Memory allocation APIs
    //!
    //! In a given program, the standard library has one “global” memory allocator
    //! that is used for example by `Box<T>` and `Vec<T>`.
    //!
    //! Currently the default global allocator is unspecified. Libraries, however,
    //! like `cdylib`s and `staticlib`s are guaranteed to use the [`System`] by
    //! default.
    //!
    //! [`System`]: struct.System.html
    //!
    //! # The `#[global_allocator]` attribute
    //!
    //! This attribute allows configuring the choice of global allocator.
    //! You can use this to implement a completely custom global allocator
    //! to route all default allocation requests to a custom object.
    //!
    //! ```rust
    //! use std::alloc::{GlobalAlloc, System, Layout};
    //!
    //! struct MyAllocator;
    //!
    //! unsafe impl GlobalAlloc for MyAllocator {
    //!     unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
    //!         System.alloc(layout)
    //!     }
    //!
    //!     unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
    //!         System.dealloc(ptr, layout)
    //!     }
    //! }
    //!
    //! #[global_allocator]
    //! static GLOBAL: MyAllocator = MyAllocator;
    //!
    //! fn main() {
    //!     // This `Vec` will allocate memory through `GLOBAL` above
    //!     let mut v = Vec::new();
    //!     v.push(1);
    //! }
    //! ```
    //!
    //! The attribute is used on a `static` item whose type implements the
    //! [`GlobalAlloc`] trait. This type can be provided by an external library:
    //!
    //! [`GlobalAlloc`]: ../../core/alloc/trait.GlobalAlloc.html
    //!
    //! ```rust,ignore (demonstrates crates.io usage)
    //! extern crate jemallocator;
    //!
    //! use jemallocator::Jemalloc;
    //!
    //! #[global_allocator]
    //! static GLOBAL: Jemalloc = Jemalloc;
    //!
    //! fn main() {}
    //! ```
    //!
    //! The `#[global_allocator]` can only be used once in a crate
    //! or its recursive dependencies.

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

    use core::sync::atomic::{AtomicPtr, Ordering};
    use core::{mem, ptr};
    use core::ptr::NonNull;
    use sys_common::util::dumb_print;

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

    /// The default memory allocator provided by the operating system.
    ///
    /// This is based on `malloc` on Unix platforms and `HeapAlloc` on Windows,
    /// plus related functions.
    ///
    /// This type implements the `GlobalAlloc` trait and Rust programs by default
    /// work as if they had this definition:
    ///
    /// ```rust
    /// use std::alloc::System;
    ///
    /// #[global_allocator]
    /// static A: System = System;
    ///
    /// fn main() {
    ///     let a = Box::new(4); // Allocates from the system allocator.
    ///     println!("{}", a);
    /// }
    /// ```
    ///
    /// You can also define your own wrapper around `System` if you'd like, such as
    /// keeping track of the number of all bytes allocated:
    ///
    /// ```rust
    /// use std::alloc::{System, GlobalAlloc, Layout};
    /// use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT, Ordering::SeqCst};
    ///
    /// struct Counter;
    ///
    /// static ALLOCATED: AtomicUsize = ATOMIC_USIZE_INIT;
    ///
    /// unsafe impl GlobalAlloc for Counter {
    ///     unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
    ///         let ret = System.alloc(layout);
    ///         if !ret.is_null() {
    ///             ALLOCATED.fetch_add(layout.size(), SeqCst);
    ///         }
    ///         return ret
    ///     }
    ///
    ///     unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
    ///         System.dealloc(ptr, layout);
    ///         ALLOCATED.fetch_sub(layout.size(), SeqCst);
    ///     }
    /// }
    ///
    /// #[global_allocator]
    /// static A: Counter = Counter;
    ///
    /// fn main() {
    ///     println!("allocated bytes before main: {}", ALLOCATED.load(SeqCst));
    /// }
    /// ```
    ///
    /// It can also be used directly to allocate memory independently of whatever
    /// global allocator has been selected for a Rust program. For example if a Rust
    /// program opts in to using jemalloc as the global allocator, `System` will
    /// still allocate memory using `malloc` and `HeapAlloc`.
    #[stable(feature = "alloc_system_type", since = "1.28.0")]
    #[derive(Debug, Copy, Clone)]
    pub struct System;

    // The Alloc impl just forwards to the GlobalAlloc impl, which is in `std::sys::*::alloc`.
    #[unstable(feature = "allocator_api", issue = "32838")]
    unsafe impl Alloc for System {
        #[inline]
        unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
            NonNull::new(GlobalAlloc::alloc(self, layout)).ok_or(AllocErr)
        }

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

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

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

    static HOOK: AtomicPtr<()> = AtomicPtr::new(ptr::null_mut());

    /// Registers a custom allocation error hook, replacing any that was previously registered.
    ///
    /// The allocation error hook is invoked when an infallible memory allocation fails, before
    /// the runtime aborts. The default hook prints a message to standard error,
    /// but this behavior can be customized with the [`set_alloc_error_hook`] and
    /// [`take_alloc_error_hook`] functions.
    ///
    /// The hook is provided with a `Layout` struct which contains information
    /// about the allocation that failed.
    ///
    /// The allocation error hook is a global resource.
    #[unstable(feature = "alloc_error_hook", issue = "51245")]
    pub fn set_alloc_error_hook(hook: fn(Layout)) {
        HOOK.store(hook as *mut (), Ordering::SeqCst);
    }

    /// Unregisters the current allocation error hook, returning it.
    ///
    /// *See also the function [`set_alloc_error_hook`].*
    ///
    /// If no custom hook is registered, the default hook will be returned.
    #[unstable(feature = "alloc_error_hook", issue = "51245")]
    pub fn take_alloc_error_hook() -> fn(Layout) {
        let hook = HOOK.swap(ptr::null_mut(), Ordering::SeqCst);
        if hook.is_null() {
            default_alloc_error_hook
        } else {
            unsafe { mem::transmute(hook) }
        }
    }

    fn default_alloc_error_hook(layout: Layout) {
        dumb_print(format_args!("memory allocation of {} bytes failed", layout.size()));
    }

    #[cfg(not(test))]
    #[doc(hidden)]
    #[alloc_error_handler]
    #[unstable(feature = "alloc_internals", issue = "0")]
    pub fn rust_oom(layout: Layout) -> ! {
        let hook = HOOK.load(Ordering::SeqCst);
        let hook: fn(Layout) = if hook.is_null() {
            default_alloc_error_hook
        } else {
            unsafe { mem::transmute(hook) }
        };
        hook(layout);
        unsafe { ::sys::abort_internal(); }
    }

    #[cfg(not(test))]
    #[doc(hidden)]
    #[allow(unused_attributes)]
    #[unstable(feature = "alloc_internals", issue = "0")]
    pub mod __default_lib_allocator {
        use super::{System, Layout, GlobalAlloc};
        // These magic symbol names are used as a fallback for implementing the
        // `__rust_alloc` etc symbols (see `src/liballoc/alloc.rs) when there is
        // no `#[global_allocator]` attribute.

        // for symbol names src/librustc/middle/allocator.rs
        // for signatures src/librustc_allocator/lib.rs

        // linkage directives are provided as part of the current compiler allocator
        // ABI

        #[rustc_std_internal_symbol]
        pub unsafe extern fn __rdl_alloc(size: usize, align: usize) -> *mut u8 {
            let layout = Layout::from_size_align_unchecked(size, align);
            System.alloc(layout)
        }

        #[rustc_std_internal_symbol]
        pub unsafe extern fn __rdl_dealloc(ptr: *mut u8,
                                           size: usize,
                                           align: usize) {
            System.dealloc(ptr, Layout::from_size_align_unchecked(size, align))
        }

        #[rustc_std_internal_symbol]
        pub unsafe extern fn __rdl_realloc(ptr: *mut u8,
                                           old_size: usize,
                                           align: usize,
                                           new_size: usize) -> *mut u8 {
            let old_layout = Layout::from_size_align_unchecked(old_size, align);
            System.realloc(ptr, old_layout, new_size)
        }

        #[rustc_std_internal_symbol]
        pub unsafe extern fn __rdl_alloc_zeroed(size: usize, align: usize) -> *mut u8 {
            let layout = Layout::from_size_align_unchecked(size, align);
            System.alloc_zeroed(layout)
        }
    }

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