-use std::cell::Cell;
-use std::mem::MaybeUninit;
-use std::ptr::NonNull;
+use core::alloc::{AllocError, Allocator, Layout};
+use core::cell::Cell;
+use core::mem::MaybeUninit;
+use core::ptr::NonNull;
#[test]
fn uninitialized_zero_size_box() {
x.set(1000);
assert_eq!(x.get(), 1000);
}
+
+pub struct ConstAllocator;
+
+unsafe impl const Allocator for ConstAllocator {
+ fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
+ match layout.size() {
+ 0 => Ok(NonNull::slice_from_raw_parts(layout.dangling(), 0)),
+ _ => unsafe {
+ let ptr = core::intrinsics::const_allocate(layout.size(), layout.align());
+ Ok(NonNull::new_unchecked(ptr as *mut [u8; 0] as *mut [u8]))
+ },
+ }
+ }
+
+ unsafe fn deallocate(&self, _ptr: NonNull<u8>, layout: Layout) {
+ match layout.size() {
+ 0 => { /* do nothing */ }
+ _ => { /* do nothing too */ }
+ }
+ }
+
+ fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
+ let ptr = self.allocate(layout)?;
+ if layout.size() > 0 {
+ unsafe {
+ ptr.as_mut_ptr().write_bytes(0, layout.size());
+ }
+ }
+ Ok(ptr)
+ }
+
+ unsafe fn grow(
+ &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 greater than or equal to `old_layout.size()`"
+ );
+
+ let new_ptr = self.allocate(new_layout)?;
+ if new_layout.size() > 0 {
+ new_ptr.as_mut_ptr().copy_from_nonoverlapping(ptr.as_ptr(), old_layout.size());
+ self.deallocate(ptr, old_layout);
+ }
+ Ok(new_ptr)
+ }
+
+ unsafe fn grow_zeroed(
+ &self,
+ ptr: NonNull<u8>,
+ old_layout: Layout,
+ new_layout: Layout,
+ ) -> Result<NonNull<[u8]>, AllocError> {
+ let new_ptr = self.grow(ptr, old_layout, new_layout)?;
+ if new_layout.size() > 0 {
+ let old_size = old_layout.size();
+ let new_size = new_layout.size();
+ let raw_ptr = new_ptr.as_mut_ptr();
+ raw_ptr.add(old_size).write_bytes(0, new_size - old_size);
+ }
+ Ok(new_ptr)
+ }
+
+ 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()`"
+ );
+
+ let new_ptr = self.allocate(new_layout)?;
+ if new_layout.size() > 0 {
+ new_ptr.as_mut_ptr().copy_from_nonoverlapping(ptr.as_ptr(), new_layout.size());
+ self.deallocate(ptr, old_layout);
+ }
+ Ok(new_ptr)
+ }
+
+ fn by_ref(&self) -> &Self
+ where
+ Self: Sized,
+ {
+ self
+ }
+}
+
+#[test]
+fn const_box() {
+ const VALUE: u32 = {
+ let mut boxed = Box::new_in(1u32, ConstAllocator);
+ assert!(*boxed == 1);
+
+ *boxed = 42;
+ assert!(*boxed == 42);
+
+ *boxed
+ };
+
+ assert!(VALUE == 42);
+}