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use crate::alloc::{self, Layout, LayoutError};
use core::fmt::{self, Debug, Display, Formatter};
use core::marker::PhantomData;
#[cfg(not(no_global_oom_handling))]
use core::marker::Unsize;
use core::mem;
use core::ops::{Deref, DerefMut};
use core::ptr::Pointee;
use core::ptr::{self, NonNull};
#[unstable(feature = "thin_box", issue = "92791")]
pub struct ThinBox<T: ?Sized> {
ptr: WithOpaqueHeader,
_marker: PhantomData<T>,
}
#[unstable(feature = "thin_box", issue = "92791")]
unsafe impl<T: ?Sized + Send> Send for ThinBox<T> {}
#[unstable(feature = "thin_box", issue = "92791")]
unsafe impl<T: ?Sized + Sync> Sync for ThinBox<T> {}
#[unstable(feature = "thin_box", issue = "92791")]
impl<T> ThinBox<T> {
#[cfg(not(no_global_oom_handling))]
pub fn new(value: T) -> Self {
let meta = ptr::metadata(&value);
let ptr = WithOpaqueHeader::new(meta, value);
ThinBox { ptr, _marker: PhantomData }
}
}
#[unstable(feature = "thin_box", issue = "92791")]
impl<Dyn: ?Sized> ThinBox<Dyn> {
#[cfg(not(no_global_oom_handling))]
pub fn new_unsize<T>(value: T) -> Self
where
T: Unsize<Dyn>,
{
let meta = ptr::metadata(&value as &Dyn);
let ptr = WithOpaqueHeader::new(meta, value);
ThinBox { ptr, _marker: PhantomData }
}
}
#[unstable(feature = "thin_box", issue = "92791")]
impl<T: ?Sized + Debug> Debug for ThinBox<T> {
fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
Debug::fmt(self.deref(), f)
}
}
#[unstable(feature = "thin_box", issue = "92791")]
impl<T: ?Sized + Display> Display for ThinBox<T> {
fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
Display::fmt(self.deref(), f)
}
}
#[unstable(feature = "thin_box", issue = "92791")]
impl<T: ?Sized> Deref for ThinBox<T> {
type Target = T;
fn deref(&self) -> &T {
let value = self.data();
let metadata = self.meta();
let pointer = ptr::from_raw_parts(value as *const (), metadata);
unsafe { &*pointer }
}
}
#[unstable(feature = "thin_box", issue = "92791")]
impl<T: ?Sized> DerefMut for ThinBox<T> {
fn deref_mut(&mut self) -> &mut T {
let value = self.data();
let metadata = self.meta();
let pointer = ptr::from_raw_parts_mut::<T>(value as *mut (), metadata);
unsafe { &mut *pointer }
}
}
#[unstable(feature = "thin_box", issue = "92791")]
impl<T: ?Sized> Drop for ThinBox<T> {
fn drop(&mut self) {
unsafe {
let value = self.deref_mut();
let value = value as *mut T;
self.with_header().drop::<T>(value);
}
}
}
#[unstable(feature = "thin_box", issue = "92791")]
impl<T: ?Sized> ThinBox<T> {
fn meta(&self) -> <T as Pointee>::Metadata {
unsafe { *self.with_header().header() }
}
fn data(&self) -> *mut u8 {
self.with_header().value()
}
fn with_header(&self) -> &WithHeader<<T as Pointee>::Metadata> {
unsafe { &*((&self.ptr) as *const WithOpaqueHeader as *const WithHeader<_>) }
}
}
#[repr(transparent)]
struct WithHeader<H>(NonNull<u8>, PhantomData<H>);
#[repr(transparent)]
struct WithOpaqueHeader(NonNull<u8>);
impl WithOpaqueHeader {
#[cfg(not(no_global_oom_handling))]
fn new<H, T>(header: H, value: T) -> Self {
let ptr = WithHeader::new(header, value);
Self(ptr.0)
}
}
impl<H> WithHeader<H> {
#[cfg(not(no_global_oom_handling))]
fn new<T>(header: H, value: T) -> WithHeader<H> {
let value_layout = Layout::new::<T>();
let Ok((layout, value_offset)) = Self::alloc_layout(value_layout) else {
alloc::handle_alloc_error(Layout::new::<()>());
};
unsafe {
let ptr = if layout.size() == 0 {
debug_assert!(
value_offset == 0 && mem::size_of::<T>() == 0 && mem::size_of::<H>() == 0
);
layout.dangling()
} else {
let ptr = alloc::alloc(layout);
if ptr.is_null() {
alloc::handle_alloc_error(layout);
}
let ptr = ptr.add(value_offset) as *mut _;
NonNull::new_unchecked(ptr)
};
let result = WithHeader(ptr, PhantomData);
ptr::write(result.header(), header);
ptr::write(result.value().cast(), value);
result
}
}
unsafe fn drop<T: ?Sized>(&self, value: *mut T) {
unsafe {
let value_layout = Layout::for_value_raw(value);
let (layout, value_offset) = Self::alloc_layout(value_layout).unwrap_unchecked();
ptr::drop_in_place::<T>(value);
if layout.size() != 0 {
alloc::dealloc(self.0.as_ptr().sub(value_offset), layout);
} else {
debug_assert!(
value_offset == 0 && mem::size_of::<H>() == 0 && value_layout.size() == 0
);
}
}
}
fn header(&self) -> *mut H {
let hp = unsafe { self.0.as_ptr().sub(Self::header_size()) as *mut H };
debug_assert!(hp.is_aligned());
hp
}
fn value(&self) -> *mut u8 {
self.0.as_ptr()
}
const fn header_size() -> usize {
mem::size_of::<H>()
}
fn alloc_layout(value_layout: Layout) -> Result<(Layout, usize), LayoutError> {
Layout::new::<H>().extend(value_layout)
}
}