2 // https://github.com/matthieu-m/rfc2580/blob/b58d1d3cba0d4b5e859d3617ea2d0943aaa31329/examples/thin.rs
4 use crate::alloc::{self, Layout, LayoutError};
5 use core::error::Error;
6 use core::fmt::{self, Debug, Display, Formatter};
7 use core::marker::PhantomData;
8 #[cfg(not(no_global_oom_handling))]
9 use core::marker::Unsize;
11 use core::ops::{Deref, DerefMut};
12 use core::ptr::Pointee;
13 use core::ptr::{self, NonNull};
17 /// A thin pointer for heap allocation, regardless of T.
22 /// #![feature(thin_box)]
23 /// use std::boxed::ThinBox;
25 /// let five = ThinBox::new(5);
26 /// let thin_slice = ThinBox::<[i32]>::new_unsize([1, 2, 3, 4]);
28 /// use std::mem::{size_of, size_of_val};
29 /// let size_of_ptr = size_of::<*const ()>();
30 /// assert_eq!(size_of_ptr, size_of_val(&five));
31 /// assert_eq!(size_of_ptr, size_of_val(&thin_slice));
33 #[unstable(feature = "thin_box", issue = "92791")]
34 pub struct ThinBox<T: ?Sized> {
35 // This is essentially `WithHeader<<T as Pointee>::Metadata>`,
36 // but that would be invariant in `T`, and we want covariance.
37 ptr: WithOpaqueHeader,
38 _marker: PhantomData<T>,
41 /// `ThinBox<T>` is `Send` if `T` is `Send` because the data is owned.
42 #[unstable(feature = "thin_box", issue = "92791")]
43 unsafe impl<T: ?Sized + Send> Send for ThinBox<T> {}
45 /// `ThinBox<T>` is `Sync` if `T` is `Sync` because the data is owned.
46 #[unstable(feature = "thin_box", issue = "92791")]
47 unsafe impl<T: ?Sized + Sync> Sync for ThinBox<T> {}
49 #[unstable(feature = "thin_box", issue = "92791")]
51 /// Moves a type to the heap with its [`Metadata`] stored in the heap allocation instead of on
57 /// #![feature(thin_box)]
58 /// use std::boxed::ThinBox;
60 /// let five = ThinBox::new(5);
63 /// [`Metadata`]: core::ptr::Pointee::Metadata
64 #[cfg(not(no_global_oom_handling))]
65 pub fn new(value: T) -> Self {
66 let meta = ptr::metadata(&value);
67 let ptr = WithOpaqueHeader::new(meta, value);
68 ThinBox { ptr, _marker: PhantomData }
72 #[unstable(feature = "thin_box", issue = "92791")]
73 impl<Dyn: ?Sized> ThinBox<Dyn> {
74 /// Moves a type to the heap with its [`Metadata`] stored in the heap allocation instead of on
80 /// #![feature(thin_box)]
81 /// use std::boxed::ThinBox;
83 /// let thin_slice = ThinBox::<[i32]>::new_unsize([1, 2, 3, 4]);
86 /// [`Metadata`]: core::ptr::Pointee::Metadata
87 #[cfg(not(no_global_oom_handling))]
88 pub fn new_unsize<T>(value: T) -> Self
92 let meta = ptr::metadata(&value as &Dyn);
93 let ptr = WithOpaqueHeader::new(meta, value);
94 ThinBox { ptr, _marker: PhantomData }
98 #[unstable(feature = "thin_box", issue = "92791")]
99 impl<T: ?Sized + Debug> Debug for ThinBox<T> {
100 fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
101 Debug::fmt(self.deref(), f)
105 #[unstable(feature = "thin_box", issue = "92791")]
106 impl<T: ?Sized + Display> Display for ThinBox<T> {
107 fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
108 Display::fmt(self.deref(), f)
112 #[unstable(feature = "thin_box", issue = "92791")]
113 impl<T: ?Sized> Deref for ThinBox<T> {
116 fn deref(&self) -> &T {
117 let value = self.data();
118 let metadata = self.meta();
119 let pointer = ptr::from_raw_parts(value as *const (), metadata);
124 #[unstable(feature = "thin_box", issue = "92791")]
125 impl<T: ?Sized> DerefMut for ThinBox<T> {
126 fn deref_mut(&mut self) -> &mut T {
127 let value = self.data();
128 let metadata = self.meta();
129 let pointer = ptr::from_raw_parts_mut::<T>(value as *mut (), metadata);
130 unsafe { &mut *pointer }
134 #[unstable(feature = "thin_box", issue = "92791")]
135 impl<T: ?Sized> Drop for ThinBox<T> {
138 let value = self.deref_mut();
139 let value = value as *mut T;
140 self.with_header().drop::<T>(value);
145 #[unstable(feature = "thin_box", issue = "92791")]
146 impl<T: ?Sized> ThinBox<T> {
147 fn meta(&self) -> <T as Pointee>::Metadata {
149 // - NonNull and valid.
150 unsafe { *self.with_header().header() }
153 fn data(&self) -> *mut u8 {
154 self.with_header().value()
157 fn with_header(&self) -> &WithHeader<<T as Pointee>::Metadata> {
158 // SAFETY: both types are transparent to `NonNull<u8>`
159 unsafe { &*((&self.ptr) as *const WithOpaqueHeader as *const WithHeader<_>) }
163 /// A pointer to type-erased data, guaranteed to either be:
164 /// 1. `NonNull::dangling()`, in the case where both the pointee (`T`) and
165 /// metadata (`H`) are ZSTs.
166 /// 2. A pointer to a valid `T` that has a header `H` directly before the
167 /// pointed-to location.
169 struct WithHeader<H>(NonNull<u8>, PhantomData<H>);
171 /// An opaque representation of `WithHeader<H>` to avoid the
172 /// projection invariance of `<T as Pointee>::Metadata`.
174 struct WithOpaqueHeader(NonNull<u8>);
176 impl WithOpaqueHeader {
177 #[cfg(not(no_global_oom_handling))]
178 fn new<H, T>(header: H, value: T) -> Self {
179 let ptr = WithHeader::new(header, value);
184 impl<H> WithHeader<H> {
185 #[cfg(not(no_global_oom_handling))]
186 fn new<T>(header: H, value: T) -> WithHeader<H> {
187 let value_layout = Layout::new::<T>();
188 let Ok((layout, value_offset)) = Self::alloc_layout(value_layout) else {
189 // We pass an empty layout here because we do not know which layout caused the
190 // arithmetic overflow in `Layout::extend` and `handle_alloc_error` takes `Layout` as
191 // its argument rather than `Result<Layout, LayoutError>`, also this function has been
192 // stable since 1.28 ._.
194 // On the other hand, look at this gorgeous turbofish!
195 alloc::handle_alloc_error(Layout::new::<()>());
199 // Note: It's UB to pass a layout with a zero size to `alloc::alloc`, so
200 // we use `layout.dangling()` for this case, which should have a valid
201 // alignment for both `T` and `H`.
202 let ptr = if layout.size() == 0 {
203 // Some paranoia checking, mostly so that the ThinBox tests are
204 // more able to catch issues.
206 value_offset == 0 && mem::size_of::<T>() == 0 && mem::size_of::<H>() == 0
210 let ptr = alloc::alloc(layout);
212 alloc::handle_alloc_error(layout);
215 // - The size is at least `aligned_header_size`.
216 let ptr = ptr.add(value_offset) as *mut _;
218 NonNull::new_unchecked(ptr)
221 let result = WithHeader(ptr, PhantomData);
222 ptr::write(result.header(), header);
223 ptr::write(result.value().cast(), value);
230 // - Assumes that either `value` can be dereferenced, or is the
231 // `NonNull::dangling()` we use when both `T` and `H` are ZSTs.
232 unsafe fn drop<T: ?Sized>(&self, value: *mut T) {
233 struct DropGuard<H> {
235 value_layout: Layout,
236 _marker: PhantomData<H>,
239 impl<H> Drop for DropGuard<H> {
242 // SAFETY: Layout must have been computable if we're in drop
243 let (layout, value_offset) =
244 WithHeader::<H>::alloc_layout(self.value_layout).unwrap_unchecked();
246 // Note: Don't deallocate if the layout size is zero, because the pointer
247 // didn't come from the allocator.
248 if layout.size() != 0 {
249 alloc::dealloc(self.ptr.as_ptr().sub(value_offset), layout);
253 && mem::size_of::<H>() == 0
254 && self.value_layout.size() == 0
262 // `_guard` will deallocate the memory when dropped, even if `drop_in_place` unwinds.
263 let _guard = DropGuard {
265 value_layout: Layout::for_value_raw(value),
266 _marker: PhantomData::<H>,
269 // We only drop the value because the Pointee trait requires that the metadata is copy
270 // aka trivially droppable.
271 ptr::drop_in_place::<T>(value);
275 fn header(&self) -> *mut H {
277 // - At least `size_of::<H>()` bytes are allocated ahead of the pointer.
278 // - We know that H will be aligned because the middle pointer is aligned to the greater
279 // of the alignment of the header and the data and the header size includes the padding
280 // needed to align the header. Subtracting the header size from the aligned data pointer
281 // will always result in an aligned header pointer, it just may not point to the
282 // beginning of the allocation.
283 let hp = unsafe { self.0.as_ptr().sub(Self::header_size()) as *mut H };
284 debug_assert!(hp.is_aligned());
288 fn value(&self) -> *mut u8 {
292 const fn header_size() -> usize {
296 fn alloc_layout(value_layout: Layout) -> Result<(Layout, usize), LayoutError> {
297 Layout::new::<H>().extend(value_layout)
301 #[unstable(feature = "thin_box", issue = "92791")]
302 impl<T: ?Sized + Error> Error for ThinBox<T> {
303 fn source(&self) -> Option<&(dyn Error + 'static)> {
304 self.deref().source()