rust/alloc/alloc.rs - linux - Git at Google

 // SPDX-License-Identifier: Apache-2.0 OR MIT

 //! Memory allocation APIs

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

 #[cfg(not(test))]
 use core::intrinsics;
 use core::intrinsics::{min_align_of_val, size_of_val};

 use core::ptr::Unique;
 #[cfg(not(test))]
 use core::ptr::{self, NonNull};

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

 use core::marker::Destruct;

 #[cfg(test)]
 mod tests;

 extern "Rust" {
     // These are the magic symbols to call the global allocator. rustc generates
     // them to call `__rg_alloc` etc. if there is a `#[global_allocator]` attribute
     // (the code expanding that attribute macro generates those functions), or to call
     // the default implementations in std (`__rdl_alloc` etc. in `library/std/src/alloc.rs`)
     // otherwise.
     // The rustc fork of LLVM 14 and earlier also special-cases these function names to be able to optimize them
     // like `malloc`, `realloc`, and `free`, respectively.
     #[rustc_allocator]
     #[rustc_nounwind]
     fn __rust_alloc(size: usize, align: usize) -> *mut u8;
     #[rustc_deallocator]
     #[rustc_nounwind]
     fn __rust_dealloc(ptr: *mut u8, size: usize, align: usize);
     #[rustc_reallocator]
     #[rustc_nounwind]
     fn __rust_realloc(ptr: *mut u8, old_size: usize, align: usize, new_size: usize) -> *mut u8;
     #[rustc_allocator_zeroed]
     #[rustc_nounwind]
     fn __rust_alloc_zeroed(size: usize, align: usize) -> *mut u8;
 }

 /// The global memory allocator.
 ///
 /// This type implements the [`Allocator`] trait by forwarding calls
 /// to the allocator registered with the `#[global_allocator]` attribute
 /// if there is one, or the `std` crate’s default.
 ///
 /// Note: while this type is unstable, the functionality it provides can be
 /// accessed through the [free functions in `alloc`](self#functions).
 #[unstable(feature = "allocator_api", issue = "32838")]
 #[derive(Copy, Clone, Default, Debug)]
 #[cfg(not(test))]
 pub struct Global;

 #[cfg(test)]
 pub use std::alloc::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 [`Allocator`] trait become stable.
 ///
 /// # Safety
 ///
 /// See [`GlobalAlloc::alloc`].
 ///
 /// # Examples
 ///
 /// ```
 /// use std::alloc::{alloc, dealloc, handle_alloc_error, Layout};
 ///
 /// unsafe {
 ///     let layout = Layout::new::<u16>();
 ///     let ptr = alloc(layout);
 ///     if ptr.is_null() {
 ///         handle_alloc_error(layout);
 ///     }
 ///
 ///     *(ptr as *mut u16) = 42;
 ///     assert_eq!(*(ptr as *mut u16), 42);
 ///
 ///     dealloc(ptr, layout);
 /// }
 /// ```
 #[stable(feature = "global_alloc", since = "1.28.0")]
 #[must_use = "losing the pointer will leak memory"]
 #[inline]
 pub unsafe fn alloc(layout: Layout) -> *mut u8 {
     unsafe { __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 [`Allocator`] 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) {
     unsafe { __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 [`Allocator`] trait become stable.
 ///
 /// # Safety
 ///
 /// See [`GlobalAlloc::realloc`].
 #[stable(feature = "global_alloc", since = "1.28.0")]
 #[must_use = "losing the pointer will leak memory"]
 #[inline]
 pub unsafe fn realloc(ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
     unsafe { __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 [`Allocator`] 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")]
 #[must_use = "losing the pointer will leak memory"]
 #[inline]
 pub unsafe fn alloc_zeroed(layout: Layout) -> *mut u8 {
     unsafe { __rust_alloc_zeroed(layout.size(), layout.align()) }
 }

 #[cfg(not(test))]
 impl Global {
     #[inline]
     fn alloc_impl(&self, layout: Layout, zeroed: bool) -> Result<NonNull<[u8]>, AllocError> {
         match layout.size() {
             0 => Ok(NonNull::slice_from_raw_parts(layout.dangling(), 0)),
             // SAFETY: `layout` is non-zero in size,
             size => unsafe {
                 let raw_ptr = if zeroed { alloc_zeroed(layout) } else { alloc(layout) };
                 let ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;
                 Ok(NonNull::slice_from_raw_parts(ptr, size))
             },
         }
     }

     // SAFETY: Same as `Allocator::grow`
     #[inline]
     unsafe fn grow_impl(
         &self,
         ptr: NonNull<u8>,
         old_layout: Layout,
         new_layout: Layout,
         zeroed: bool,
     ) -> Result<NonNull<[u8]>, AllocError> {
         debug_assert!(
             new_layout.size() >= old_layout.size(),
             "`new_layout.size()` must be greater than or equal to `old_layout.size()`"
         );

         match old_layout.size() {
             0 => self.alloc_impl(new_layout, zeroed),

             // SAFETY: `new_size` is non-zero as `old_size` is greater than or equal to `new_size`
             // as required by safety conditions. Other conditions must be upheld by the caller
             old_size if old_layout.align() == new_layout.align() => unsafe {
                 let new_size = new_layout.size();

                 // `realloc` probably checks for `new_size >= old_layout.size()` or something similar.
                 intrinsics::assume(new_size >= old_layout.size());

                 let raw_ptr = realloc(ptr.as_ptr(), old_layout, new_size);
                 let ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;
                 if zeroed {
                     raw_ptr.add(old_size).write_bytes(0, new_size - old_size);
                 }
                 Ok(NonNull::slice_from_raw_parts(ptr, new_size))
             },

             // SAFETY: because `new_layout.size()` must be greater than or equal to `old_size`,
             // both the old and new memory allocation are valid for reads and writes for `old_size`
             // bytes. Also, because the old allocation wasn't yet deallocated, it cannot overlap
             // `new_ptr`. Thus, the call to `copy_nonoverlapping` is safe. The safety contract
             // for `dealloc` must be upheld by the caller.
             old_size => unsafe {
                 let new_ptr = self.alloc_impl(new_layout, zeroed)?;
                 ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_mut_ptr(), old_size);
                 self.deallocate(ptr, old_layout);
                 Ok(new_ptr)
             },
         }
     }
 }

 #[unstable(feature = "allocator_api", issue = "32838")]
 #[cfg(not(test))]
 unsafe impl Allocator for Global {
     #[inline]
     fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
         self.alloc_impl(layout, false)
     }

     #[inline]
     fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
         self.alloc_impl(layout, true)
     }

     #[inline]
     unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout) {
         if layout.size() != 0 {
             // SAFETY: `layout` is non-zero in size,
             // other conditions must be upheld by the caller
             unsafe { dealloc(ptr.as_ptr(), layout) }
         }
     }

     #[inline]
     unsafe fn grow(
         &self,
         ptr: NonNull<u8>,
         old_layout: Layout,
         new_layout: Layout,
     ) -> Result<NonNull<[u8]>, AllocError> {
         // SAFETY: all conditions must be upheld by the caller
         unsafe { self.grow_impl(ptr, old_layout, new_layout, false) }
     }

     #[inline]
     unsafe fn grow_zeroed(
         &self,
         ptr: NonNull<u8>,
         old_layout: Layout,
         new_layout: Layout,
     ) -> Result<NonNull<[u8]>, AllocError> {
         // SAFETY: all conditions must be upheld by the caller
         unsafe { self.grow_impl(ptr, old_layout, new_layout, true) }
     }

     #[inline]
     unsafe fn shrink(
         &self,
         ptr: NonNull<u8>,
         old_layout: Layout,
         new_layout: Layout,
     ) -> Result<NonNull<[u8]>, AllocError> {
         debug_assert!(
             new_layout.size() <= old_layout.size(),
             "`new_layout.size()` must be smaller than or equal to `old_layout.size()`"
         );

         match new_layout.size() {
             // SAFETY: conditions must be upheld by the caller
             0 => unsafe {
                 self.deallocate(ptr, old_layout);
                 Ok(NonNull::slice_from_raw_parts(new_layout.dangling(), 0))
             },

             // SAFETY: `new_size` is non-zero. Other conditions must be upheld by the caller
             new_size if old_layout.align() == new_layout.align() => unsafe {
                 // `realloc` probably checks for `new_size <= old_layout.size()` or something similar.
                 intrinsics::assume(new_size <= old_layout.size());

                 let raw_ptr = realloc(ptr.as_ptr(), old_layout, new_size);
                 let ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;
                 Ok(NonNull::slice_from_raw_parts(ptr, new_size))
             },

             // SAFETY: because `new_size` must be smaller than or equal to `old_layout.size()`,
             // both the old and new memory allocation are valid for reads and writes for `new_size`
             // bytes. Also, because the old allocation wasn't yet deallocated, it cannot overlap
             // `new_ptr`. Thus, the call to `copy_nonoverlapping` is safe. The safety contract
             // for `dealloc` must be upheld by the caller.
             new_size => unsafe {
                 let new_ptr = self.allocate(new_layout)?;
                 ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_mut_ptr(), new_size);
                 self.deallocate(ptr, old_layout);
                 Ok(new_ptr)
             },
         }
     }
 }

 /// The allocator for unique pointers.
 #[cfg(all(not(no_global_oom_handling), not(test)))]
 #[lang = "exchange_malloc"]
 #[inline]
 unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
     let layout = unsafe { Layout::from_size_align_unchecked(size, align) };
     match Global.allocate(layout) {
         Ok(ptr) => ptr.as_mut_ptr(),
         Err(_) => handle_alloc_error(layout),
     }
 }

 #[cfg_attr(not(test), lang = "box_free")]
 #[inline]
 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
 // This signature has to be the same as `Box`, otherwise an ICE will happen.
 // When an additional parameter to `Box` is added (like `A: Allocator`), this has to be added here as
 // well.
 // For example if `Box` is changed to  `struct Box<T: ?Sized, A: Allocator>(Unique<T>, A)`,
 // this function has to be changed to `fn box_free<T: ?Sized, A: Allocator>(Unique<T>, A)` as well.
 pub(crate) const unsafe fn box_free<T: ?Sized, A: ~const Allocator + ~const Destruct>(
     ptr: Unique<T>,
     alloc: A,
 ) {
     unsafe {
         let size = size_of_val(ptr.as_ref());
         let align = min_align_of_val(ptr.as_ref());
         let layout = Layout::from_size_align_unchecked(size, align);
         alloc.deallocate(From::from(ptr.cast()), layout)
     }
 }

 // # Allocation error handler

 #[cfg(not(no_global_oom_handling))]
 extern "Rust" {
     // This is the magic symbol to call the global alloc error handler. rustc generates
     // it to call `__rg_oom` if there is a `#[alloc_error_handler]`, or to call the
     // default implementations below (`__rdl_oom`) otherwise.
     fn __rust_alloc_error_handler(size: usize, align: usize) -> !;
 }

 /// 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_const_unstable(feature = "const_alloc_error", issue = "92523")]
 #[cfg(all(not(no_global_oom_handling), not(test)))]
 #[cold]
 pub const fn handle_alloc_error(layout: Layout) -> ! {
     const fn ct_error(_: Layout) -> ! {
         panic!("allocation failed");
     }

     fn rt_error(layout: Layout) -> ! {
         unsafe {
             __rust_alloc_error_handler(layout.size(), layout.align());
         }
     }

     unsafe { core::intrinsics::const_eval_select((layout,), ct_error, rt_error) }
 }

 // For alloc test `std::alloc::handle_alloc_error` can be used directly.
 #[cfg(all(not(no_global_oom_handling), test))]
 pub use std::alloc::handle_alloc_error;

 #[cfg(all(not(no_global_oom_handling), not(test)))]
 #[doc(hidden)]
 #[allow(unused_attributes)]
 #[unstable(feature = "alloc_internals", issue = "none")]
 pub mod __alloc_error_handler {
     // called via generated `__rust_alloc_error_handler` if there is no
     // `#[alloc_error_handler]`.
     #[rustc_std_internal_symbol]
     pub unsafe fn __rdl_oom(size: usize, _align: usize) -> ! {
         extern "Rust" {
             // This symbol is emitted by rustc next to __rust_alloc_error_handler.
             // Its value depends on the -Zoom={panic,abort} compiler option.
             static __rust_alloc_error_handler_should_panic: u8;
         }

         #[allow(unused_unsafe)]
         if unsafe { __rust_alloc_error_handler_should_panic != 0 } {
             panic!("memory allocation of {size} bytes failed")
         } else {
             core::panicking::panic_nounwind_fmt(format_args!(
                 "memory allocation of {size} bytes failed"
             ))
         }
     }
 }

 /// Specialize clones into pre-allocated, uninitialized memory.
 /// Used by `Box::clone` and `Rc`/`Arc::make_mut`.
 pub(crate) trait WriteCloneIntoRaw: Sized {
     unsafe fn write_clone_into_raw(&self, target: *mut Self);
 }

 impl<T: Clone> WriteCloneIntoRaw for T {
     #[inline]
     default unsafe fn write_clone_into_raw(&self, target: *mut Self) {
         // Having allocated *first* may allow the optimizer to create
         // the cloned value in-place, skipping the local and move.
         unsafe { target.write(self.clone()) };
     }
 }

 impl<T: Copy> WriteCloneIntoRaw for T {
     #[inline]
     unsafe fn write_clone_into_raw(&self, target: *mut Self) {
         // We can always copy in-place, without ever involving a local value.
         unsafe { target.copy_from_nonoverlapping(self, 1) };
     }
 }
	// SPDX-License-Identifier: Apache-2.0 OR MIT

	//! Memory allocation APIs

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

	#[cfg(not(test))]
	use core::intrinsics;
	use core::intrinsics::{min_align_of_val, size_of_val};

	use core::ptr::Unique;
	#[cfg(not(test))]
	use core::ptr::{self, NonNull};

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

	use core::marker::Destruct;

	#[cfg(test)]
	mod tests;

	extern "Rust" {
	// These are the magic symbols to call the global allocator. rustc generates
	// them to call `__rg_alloc` etc. if there is a `#[global_allocator]` attribute
	// (the code expanding that attribute macro generates those functions), or to call
	// the default implementations in std (`__rdl_alloc` etc. in `library/std/src/alloc.rs`)
	// otherwise.
	// The rustc fork of LLVM 14 and earlier also special-cases these function names to be able to optimize them
	// like `malloc`, `realloc`, and `free`, respectively.
	#[rustc_allocator]
	#[rustc_nounwind]
	fn __rust_alloc(size: usize, align: usize) -> *mut u8;
	#[rustc_deallocator]
	#[rustc_nounwind]
	fn __rust_dealloc(ptr: *mut u8, size: usize, align: usize);
	#[rustc_reallocator]
	#[rustc_nounwind]
	fn __rust_realloc(ptr: mut u8, old_size: usize, align: usize, new_size: usize) -> mut u8;
	#[rustc_allocator_zeroed]
	#[rustc_nounwind]
	fn __rust_alloc_zeroed(size: usize, align: usize) -> *mut u8;
	}

	/// The global memory allocator.
	///
	/// This type implements the [`Allocator`] trait by forwarding calls
	/// to the allocator registered with the `#[global_allocator]` attribute
	/// if there is one, or the `std` crate’s default.
	///
	/// Note: while this type is unstable, the functionality it provides can be
	/// accessed through the [free functions in `alloc`](self#functions).
	#[unstable(feature = "allocator_api", issue = "32838")]
	#[derive(Copy, Clone, Default, Debug)]
	#[cfg(not(test))]
	pub struct Global;

	#[cfg(test)]
	pub use std::alloc::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 [`Allocator`] trait become stable.
	///
	/// # Safety
	///
	/// See [`GlobalAlloc::alloc`].
	///
	/// # Examples
	///
	/// ```
	/// use std::alloc::{alloc, dealloc, handle_alloc_error, Layout};
	///
	/// unsafe {
	/// let layout = Layout::new::<u16>();
	/// let ptr = alloc(layout);
	/// if ptr.is_null() {
	/// handle_alloc_error(layout);
	/// }
	///
	/// (ptr as mut u16) = 42;
	/// assert_eq!((ptr as mut u16), 42);
	///
	/// dealloc(ptr, layout);
	/// }
	/// ```
	#[stable(feature = "global_alloc", since = "1.28.0")]
	#[must_use = "losing the pointer will leak memory"]
	#[inline]
	pub unsafe fn alloc(layout: Layout) -> *mut u8 {
	unsafe { __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 [`Allocator`] 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) {
	unsafe { __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 [`Allocator`] trait become stable.
	///
	/// # Safety
	///
	/// See [`GlobalAlloc::realloc`].
	#[stable(feature = "global_alloc", since = "1.28.0")]
	#[must_use = "losing the pointer will leak memory"]
	#[inline]
	pub unsafe fn realloc(ptr: mut u8, layout: Layout, new_size: usize) -> mut u8 {
	unsafe { __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 [`Allocator`] 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")]
	#[must_use = "losing the pointer will leak memory"]
	#[inline]
	pub unsafe fn alloc_zeroed(layout: Layout) -> *mut u8 {
	unsafe { __rust_alloc_zeroed(layout.size(), layout.align()) }
	}

	#[cfg(not(test))]
	impl Global {
	#[inline]
	fn alloc_impl(&self, layout: Layout, zeroed: bool) -> Result<NonNull<[u8]>, AllocError> {
	match layout.size() {
	0 => Ok(NonNull::slice_from_raw_parts(layout.dangling(), 0)),
	// SAFETY: `layout` is non-zero in size,
	size => unsafe {
	let raw_ptr = if zeroed { alloc_zeroed(layout) } else { alloc(layout) };
	let ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;
	Ok(NonNull::slice_from_raw_parts(ptr, size))
	},
	}
	}

	// SAFETY: Same as `Allocator::grow`
	#[inline]
	unsafe fn grow_impl(
	&self,
	ptr: NonNull<u8>,
	old_layout: Layout,
	new_layout: Layout,
	zeroed: bool,
	) -> Result<NonNull<[u8]>, AllocError> {
	debug_assert!(
	new_layout.size() >= old_layout.size(),
	"`new_layout.size()` must be greater than or equal to `old_layout.size()`"
	);

	match old_layout.size() {
	0 => self.alloc_impl(new_layout, zeroed),

	// SAFETY: `new_size` is non-zero as `old_size` is greater than or equal to `new_size`
	// as required by safety conditions. Other conditions must be upheld by the caller
	old_size if old_layout.align() == new_layout.align() => unsafe {
	let new_size = new_layout.size();

	// `realloc` probably checks for `new_size >= old_layout.size()` or something similar.
	intrinsics::assume(new_size >= old_layout.size());

	let raw_ptr = realloc(ptr.as_ptr(), old_layout, new_size);
	let ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;
	if zeroed {
	raw_ptr.add(old_size).write_bytes(0, new_size - old_size);
	}
	Ok(NonNull::slice_from_raw_parts(ptr, new_size))
	},

	// SAFETY: because `new_layout.size()` must be greater than or equal to `old_size`,
	// both the old and new memory allocation are valid for reads and writes for `old_size`
	// bytes. Also, because the old allocation wasn't yet deallocated, it cannot overlap
	// `new_ptr`. Thus, the call to `copy_nonoverlapping` is safe. The safety contract
	// for `dealloc` must be upheld by the caller.
	old_size => unsafe {
	let new_ptr = self.alloc_impl(new_layout, zeroed)?;
	ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_mut_ptr(), old_size);
	self.deallocate(ptr, old_layout);
	Ok(new_ptr)
	},
	}
	}
	}

	#[unstable(feature = "allocator_api", issue = "32838")]
	#[cfg(not(test))]
	unsafe impl Allocator for Global {
	#[inline]
	fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
	self.alloc_impl(layout, false)
	}

	#[inline]
	fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
	self.alloc_impl(layout, true)
	}

	#[inline]
	unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout) {
	if layout.size() != 0 {
	// SAFETY: `layout` is non-zero in size,
	// other conditions must be upheld by the caller
	unsafe { dealloc(ptr.as_ptr(), layout) }
	}
	}

	#[inline]
	unsafe fn grow(
	&self,
	ptr: NonNull<u8>,
	old_layout: Layout,
	new_layout: Layout,
	) -> Result<NonNull<[u8]>, AllocError> {
	// SAFETY: all conditions must be upheld by the caller
	unsafe { self.grow_impl(ptr, old_layout, new_layout, false) }
	}

	#[inline]
	unsafe fn grow_zeroed(
	&self,
	ptr: NonNull<u8>,
	old_layout: Layout,
	new_layout: Layout,
	) -> Result<NonNull<[u8]>, AllocError> {
	// SAFETY: all conditions must be upheld by the caller
	unsafe { self.grow_impl(ptr, old_layout, new_layout, true) }
	}

	#[inline]
	unsafe fn shrink(
	&self,
	ptr: NonNull<u8>,
	old_layout: Layout,
	new_layout: Layout,
	) -> Result<NonNull<[u8]>, AllocError> {
	debug_assert!(
	new_layout.size() <= old_layout.size(),
	"`new_layout.size()` must be smaller than or equal to `old_layout.size()`"
	);

	match new_layout.size() {
	// SAFETY: conditions must be upheld by the caller
	0 => unsafe {
	self.deallocate(ptr, old_layout);
	Ok(NonNull::slice_from_raw_parts(new_layout.dangling(), 0))
	},

	// SAFETY: `new_size` is non-zero. Other conditions must be upheld by the caller
	new_size if old_layout.align() == new_layout.align() => unsafe {
	// `realloc` probably checks for `new_size <= old_layout.size()` or something similar.
	intrinsics::assume(new_size <= old_layout.size());

	let raw_ptr = realloc(ptr.as_ptr(), old_layout, new_size);
	let ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;
	Ok(NonNull::slice_from_raw_parts(ptr, new_size))
	},

	// SAFETY: because `new_size` must be smaller than or equal to `old_layout.size()`,
	// both the old and new memory allocation are valid for reads and writes for `new_size`
	// bytes. Also, because the old allocation wasn't yet deallocated, it cannot overlap
	// `new_ptr`. Thus, the call to `copy_nonoverlapping` is safe. The safety contract
	// for `dealloc` must be upheld by the caller.
	new_size => unsafe {
	let new_ptr = self.allocate(new_layout)?;
	ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_mut_ptr(), new_size);
	self.deallocate(ptr, old_layout);
	Ok(new_ptr)
	},
	}
	}
	}

	/// The allocator for unique pointers.
	#[cfg(all(not(no_global_oom_handling), not(test)))]
	#[lang = "exchange_malloc"]
	#[inline]
	unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
	let layout = unsafe { Layout::from_size_align_unchecked(size, align) };
	match Global.allocate(layout) {
	Ok(ptr) => ptr.as_mut_ptr(),
	Err(_) => handle_alloc_error(layout),
	}
	}

	#[cfg_attr(not(test), lang = "box_free")]
	#[inline]
	#[rustc_const_unstable(feature = "const_box", issue = "92521")]
	// This signature has to be the same as `Box`, otherwise an ICE will happen.
	// When an additional parameter to `Box` is added (like `A: Allocator`), this has to be added here as
	// well.
	// For example if `Box` is changed to `struct Box<T: ?Sized, A: Allocator>(Unique<T>, A)`,
	// this function has to be changed to `fn box_free<T: ?Sized, A: Allocator>(Unique<T>, A)` as well.
	pub(crate) const unsafe fn box_free<T: ?Sized, A: ~const Allocator + ~const Destruct>(
	ptr: Unique<T>,
	alloc: A,
	) {
	unsafe {
	let size = size_of_val(ptr.as_ref());
	let align = min_align_of_val(ptr.as_ref());
	let layout = Layout::from_size_align_unchecked(size, align);
	alloc.deallocate(From::from(ptr.cast()), layout)
	}
	}

	// # Allocation error handler

	#[cfg(not(no_global_oom_handling))]
	extern "Rust" {
	// This is the magic symbol to call the global alloc error handler. rustc generates
	// it to call `__rg_oom` if there is a `#[alloc_error_handler]`, or to call the
	// default implementations below (`__rdl_oom`) otherwise.
	fn __rust_alloc_error_handler(size: usize, align: usize) -> !;
	}

	/// 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_const_unstable(feature = "const_alloc_error", issue = "92523")]
	#[cfg(all(not(no_global_oom_handling), not(test)))]
	#[cold]
	pub const fn handle_alloc_error(layout: Layout) -> ! {
	const fn ct_error(_: Layout) -> ! {
	panic!("allocation failed");
	}

	fn rt_error(layout: Layout) -> ! {
	unsafe {
	__rust_alloc_error_handler(layout.size(), layout.align());
	}
	}

	unsafe { core::intrinsics::const_eval_select((layout,), ct_error, rt_error) }
	}

	// For alloc test `std::alloc::handle_alloc_error` can be used directly.
	#[cfg(all(not(no_global_oom_handling), test))]
	pub use std::alloc::handle_alloc_error;

	#[cfg(all(not(no_global_oom_handling), not(test)))]
	#[doc(hidden)]
	#[allow(unused_attributes)]
	#[unstable(feature = "alloc_internals", issue = "none")]
	pub mod __alloc_error_handler {
	// called via generated `__rust_alloc_error_handler` if there is no
	// `#[alloc_error_handler]`.
	#[rustc_std_internal_symbol]
	pub unsafe fn __rdl_oom(size: usize, _align: usize) -> ! {
	extern "Rust" {
	// This symbol is emitted by rustc next to __rust_alloc_error_handler.
	// Its value depends on the -Zoom={panic,abort} compiler option.
	static __rust_alloc_error_handler_should_panic: u8;
	}

	#[allow(unused_unsafe)]
	if unsafe { __rust_alloc_error_handler_should_panic != 0 } {
	panic!("memory allocation of {size} bytes failed")
	} else {
	core::panicking::panic_nounwind_fmt(format_args!(
	"memory allocation of {size} bytes failed"
	))
	}
	}
	}

	/// Specialize clones into pre-allocated, uninitialized memory.
	/// Used by `Box::clone` and `Rc`/`Arc::make_mut`.
	pub(crate) trait WriteCloneIntoRaw: Sized {
	unsafe fn write_clone_into_raw(&self, target: *mut Self);
	}

	impl<T: Clone> WriteCloneIntoRaw for T {
	#[inline]
	default unsafe fn write_clone_into_raw(&self, target: *mut Self) {
	// Having allocated first may allow the optimizer to create
	// the cloned value in-place, skipping the local and move.
	unsafe { target.write(self.clone()) };
	}
	}

	impl<T: Copy> WriteCloneIntoRaw for T {
	#[inline]
	unsafe fn write_clone_into_raw(&self, target: *mut Self) {
	// We can always copy in-place, without ever involving a local value.
	unsafe { target.copy_from_nonoverlapping(self, 1) };
	}
	}