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//! Based on
//! <https://github.com/matthieu-m/rfc2580/blob/b58d1d3cba0d4b5e859d3617ea2d0943aaa31329/examples/thin.rs>
//! by matthieu-m
use core::error::Error;
use core::fmt::{self, Debug, Display, Formatter};
#[cfg(not(no_global_oom_handling))]
use core::intrinsics::const_allocate;
use core::marker::PhantomData;
#[cfg(not(no_global_oom_handling))]
use core::marker::Unsize;
use core::mem;
#[cfg(not(no_global_oom_handling))]
use core::mem::SizedTypeProperties;
use core::ops::{Deref, DerefMut};
use core::ptr::{self, NonNull, Pointee};
use crate::alloc::{self, Layout, LayoutError};
/// ThinBox.
///
/// A thin pointer for heap allocation, regardless of T.
///
/// # Examples
///
/// ```
/// #![feature(thin_box)]
/// use std::boxed::ThinBox;
///
/// let five = ThinBox::new(5);
/// let thin_slice = ThinBox::<[i32]>::new_unsize([1, 2, 3, 4]);
///
/// use std::mem::{size_of, size_of_val};
/// let size_of_ptr = size_of::<*const ()>();
/// assert_eq!(size_of_ptr, size_of_val(&five));
/// assert_eq!(size_of_ptr, size_of_val(&thin_slice));
/// ```
#[unstable(feature = "thin_box", issue = "92791")]
pub struct ThinBox<T: ?Sized> {
// This is essentially `WithHeader<<T as Pointee>::Metadata>`,
// but that would be invariant in `T`, and we want covariance.
ptr: WithOpaqueHeader,
_marker: PhantomData<T>,
}
/// `ThinBox<T>` is `Send` if `T` is `Send` because the data is owned.
#[unstable(feature = "thin_box", issue = "92791")]
unsafe impl<T: ?Sized + Send> Send for ThinBox<T> {}
/// `ThinBox<T>` is `Sync` if `T` is `Sync` because the data is owned.
#[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> {
/// Moves a type to the heap with its [`Metadata`] stored in the heap allocation instead of on
/// the stack.
///
/// # Examples
///
/// ```
/// #![feature(thin_box)]
/// use std::boxed::ThinBox;
///
/// let five = ThinBox::new(5);
/// ```
///
/// [`Metadata`]: core::ptr::Pointee::Metadata
#[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 }
}
/// Moves a type to the heap with its [`Metadata`] stored in the heap allocation instead of on
/// the stack. Returns an error if allocation fails, instead of aborting.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
/// #![feature(thin_box)]
/// use std::boxed::ThinBox;
///
/// let five = ThinBox::try_new(5)?;
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
///
/// [`Metadata`]: core::ptr::Pointee::Metadata
pub fn try_new(value: T) -> Result<Self, core::alloc::AllocError> {
let meta = ptr::metadata(&value);
WithOpaqueHeader::try_new(meta, value).map(|ptr| ThinBox { ptr, _marker: PhantomData })
}
}
#[unstable(feature = "thin_box", issue = "92791")]
impl<Dyn: ?Sized> ThinBox<Dyn> {
/// Moves a type to the heap with its [`Metadata`] stored in the heap allocation instead of on
/// the stack.
///
/// # Examples
///
/// ```
/// #![feature(thin_box)]
/// use std::boxed::ThinBox;
///
/// let thin_slice = ThinBox::<[i32]>::new_unsize([1, 2, 3, 4]);
/// ```
///
/// [`Metadata`]: core::ptr::Pointee::Metadata
#[cfg(not(no_global_oom_handling))]
pub fn new_unsize<T>(value: T) -> Self
where
T: Unsize<Dyn>,
{
if mem::size_of::<T>() == 0 {
let ptr = WithOpaqueHeader::new_unsize_zst::<Dyn, T>(value);
ThinBox { ptr, _marker: PhantomData }
} else {
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 {
// Safety:
// - NonNull and valid.
unsafe { *self.with_header().header() }
}
fn data(&self) -> *mut u8 {
self.with_header().value()
}
fn with_header(&self) -> &WithHeader<<T as Pointee>::Metadata> {
// SAFETY: both types are transparent to `NonNull<u8>`
unsafe { &*(core::ptr::addr_of!(self.ptr) as *const WithHeader<_>) }
}
}
/// A pointer to type-erased data, guaranteed to either be:
/// 1. `NonNull::dangling()`, in the case where both the pointee (`T`) and
/// metadata (`H`) are ZSTs.
/// 2. A pointer to a valid `T` that has a header `H` directly before the
/// pointed-to location.
#[repr(transparent)]
struct WithHeader<H>(NonNull<u8>, PhantomData<H>);
/// An opaque representation of `WithHeader<H>` to avoid the
/// projection invariance of `<T as Pointee>::Metadata`.
#[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)
}
#[cfg(not(no_global_oom_handling))]
fn new_unsize_zst<Dyn, T>(value: T) -> Self
where
Dyn: ?Sized,
T: Unsize<Dyn>,
{
let ptr = WithHeader::<<Dyn as Pointee>::Metadata>::new_unsize_zst::<Dyn, T>(value);
Self(ptr.0)
}
fn try_new<H, T>(header: H, value: T) -> Result<Self, core::alloc::AllocError> {
WithHeader::try_new(header, value).map(|ptr| 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 {
// We pass an empty layout here because we do not know which layout caused the
// arithmetic overflow in `Layout::extend` and `handle_alloc_error` takes `Layout` as
// its argument rather than `Result<Layout, LayoutError>`, also this function has been
// stable since 1.28 ._.
//
// On the other hand, look at this gorgeous turbofish!
alloc::handle_alloc_error(Layout::new::<()>());
};
unsafe {
// Note: It's UB to pass a layout with a zero size to `alloc::alloc`, so
// we use `layout.dangling()` for this case, which should have a valid
// alignment for both `T` and `H`.
let ptr = if layout.size() == 0 {
// Some paranoia checking, mostly so that the ThinBox tests are
// more able to catch issues.
debug_assert!(value_offset == 0 && T::IS_ZST && H::IS_ZST);
layout.dangling()
} else {
let ptr = alloc::alloc(layout);
if ptr.is_null() {
alloc::handle_alloc_error(layout);
}
// Safety:
// - The size is at least `aligned_header_size`.
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
}
}
/// Non-panicking version of `new`.
/// Any error is returned as `Err(core::alloc::AllocError)`.
fn try_new<T>(header: H, value: T) -> Result<WithHeader<H>, core::alloc::AllocError> {
let value_layout = Layout::new::<T>();
let Ok((layout, value_offset)) = Self::alloc_layout(value_layout) else {
return Err(core::alloc::AllocError);
};
unsafe {
// Note: It's UB to pass a layout with a zero size to `alloc::alloc`, so
// we use `layout.dangling()` for this case, which should have a valid
// alignment for both `T` and `H`.
let ptr = if layout.size() == 0 {
// Some paranoia checking, mostly so that the ThinBox tests are
// more able to catch issues.
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() {
return Err(core::alloc::AllocError);
}
// Safety:
// - The size is at least `aligned_header_size`.
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);
Ok(result)
}
}
// `Dyn` is `?Sized` type like `[u32]`, and `T` is ZST type like `[u32; 0]`.
#[cfg(not(no_global_oom_handling))]
fn new_unsize_zst<Dyn, T>(value: T) -> WithHeader<H>
where
Dyn: Pointee<Metadata = H> + ?Sized,
T: Unsize<Dyn>,
{
assert!(mem::size_of::<T>() == 0);
const fn max(a: usize, b: usize) -> usize {
if a > b { a } else { b }
}
// Compute a pointer to the right metadata. This will point to the beginning
// of the header, past the padding, so the assigned type makes sense.
// It also ensures that the address at the end of the header is sufficiently
// aligned for T.
let alloc: &<Dyn as Pointee>::Metadata = const {
// FIXME: just call `WithHeader::alloc_layout` with size reset to 0.
// Currently that's blocked on `Layout::extend` not being `const fn`.
let alloc_align =
max(mem::align_of::<T>(), mem::align_of::<<Dyn as Pointee>::Metadata>());
let alloc_size =
max(mem::align_of::<T>(), mem::size_of::<<Dyn as Pointee>::Metadata>());
unsafe {
// SAFETY: align is power of two because it is the maximum of two alignments.
let alloc: *mut u8 = const_allocate(alloc_size, alloc_align);
let metadata_offset =
alloc_size.checked_sub(mem::size_of::<<Dyn as Pointee>::Metadata>()).unwrap();
// SAFETY: adding offset within the allocation.
let metadata_ptr: *mut <Dyn as Pointee>::Metadata =
alloc.add(metadata_offset).cast();
// SAFETY: `*metadata_ptr` is within the allocation.
metadata_ptr.write(ptr::metadata::<Dyn>(ptr::dangling::<T>() as *const Dyn));
// SAFETY: we have just written the metadata.
&*(metadata_ptr)
}
};
// SAFETY: `alloc` points to `<Dyn as Pointee>::Metadata`, so addition stays in-bounds.
let value_ptr =
unsafe { (alloc as *const <Dyn as Pointee>::Metadata).add(1) }.cast::<T>().cast_mut();
debug_assert!(value_ptr.is_aligned());
mem::forget(value);
WithHeader(NonNull::new(value_ptr.cast()).unwrap(), PhantomData)
}
// Safety:
// - Assumes that either `value` can be dereferenced, or is the
// `NonNull::dangling()` we use when both `T` and `H` are ZSTs.
unsafe fn drop<T: ?Sized>(&self, value: *mut T) {
struct DropGuard<H> {
ptr: NonNull<u8>,
value_layout: Layout,
_marker: PhantomData<H>,
}
impl<H> Drop for DropGuard<H> {
fn drop(&mut self) {
// All ZST are allocated statically.
if self.value_layout.size() == 0 {
return;
}
unsafe {
// SAFETY: Layout must have been computable if we're in drop
let (layout, value_offset) =
WithHeader::<H>::alloc_layout(self.value_layout).unwrap_unchecked();
// Since we only allocate for non-ZSTs, the layout size cannot be zero.
debug_assert!(layout.size() != 0);
alloc::dealloc(self.ptr.as_ptr().sub(value_offset), layout);
}
}
}
unsafe {
// `_guard` will deallocate the memory when dropped, even if `drop_in_place` unwinds.
let _guard = DropGuard {
ptr: self.0,
value_layout: Layout::for_value_raw(value),
_marker: PhantomData::<H>,
};
// We only drop the value because the Pointee trait requires that the metadata is copy
// aka trivially droppable.
ptr::drop_in_place::<T>(value);
}
}
fn header(&self) -> *mut H {
// Safety:
// - At least `size_of::<H>()` bytes are allocated ahead of the pointer.
// - We know that H will be aligned because the middle pointer is aligned to the greater
// of the alignment of the header and the data and the header size includes the padding
// needed to align the header. Subtracting the header size from the aligned data pointer
// will always result in an aligned header pointer, it just may not point to the
// beginning of the allocation.
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)
}
}
#[unstable(feature = "thin_box", issue = "92791")]
impl<T: ?Sized + Error> Error for ThinBox<T> {
fn source(&self) -> Option<&(dyn Error + 'static)> {
self.deref().source()
}
}