2 // https://github.com/matthieu-m/rfc2580/blob/b58d1d3cba0d4b5e859d3617ea2d0943aaa31329/examples/thin.rs
4 use crate::alloc::{self, Layout, LayoutError};
6 use core::error::Error;
7 use core::fmt::{self, Debug, Display, Formatter};
8 use core::marker::PhantomData;
9 #[cfg(not(no_global_oom_handling))]
10 use core::marker::Unsize;
12 use core::ops::{Deref, DerefMut};
13 use core::ptr::Pointee;
14 use core::ptr::{self, NonNull};
18 /// A thin pointer for heap allocation, regardless of T.
23 /// #![feature(thin_box)]
24 /// use std::boxed::ThinBox;
26 /// let five = ThinBox::new(5);
27 /// let thin_slice = ThinBox::<[i32]>::new_unsize([1, 2, 3, 4]);
29 /// use std::mem::{size_of, size_of_val};
30 /// let size_of_ptr = size_of::<*const ()>();
31 /// assert_eq!(size_of_ptr, size_of_val(&five));
32 /// assert_eq!(size_of_ptr, size_of_val(&thin_slice));
34 #[unstable(feature = "thin_box", issue = "92791")]
35 pub struct ThinBox<T: ?Sized> {
36 // This is essentially `WithHeader<<T as Pointee>::Metadata>`,
37 // but that would be invariant in `T`, and we want covariance.
38 ptr: WithOpaqueHeader,
39 _marker: PhantomData<T>,
42 /// `ThinBox<T>` is `Send` if `T` is `Send` because the data is owned.
43 #[unstable(feature = "thin_box", issue = "92791")]
44 unsafe impl<T: ?Sized + Send> Send for ThinBox<T> {}
46 /// `ThinBox<T>` is `Sync` if `T` is `Sync` because the data is owned.
47 #[unstable(feature = "thin_box", issue = "92791")]
48 unsafe impl<T: ?Sized + Sync> Sync for ThinBox<T> {}
50 #[unstable(feature = "thin_box", issue = "92791")]
52 /// Moves a type to the heap with its `Metadata` stored in the heap allocation instead of on
58 /// #![feature(thin_box)]
59 /// use std::boxed::ThinBox;
61 /// let five = ThinBox::new(5);
63 #[cfg(not(no_global_oom_handling))]
64 pub fn new(value: T) -> Self {
65 let meta = ptr::metadata(&value);
66 let ptr = WithOpaqueHeader::new(meta, value);
67 ThinBox { ptr, _marker: PhantomData }
71 #[unstable(feature = "thin_box", issue = "92791")]
72 impl<Dyn: ?Sized> ThinBox<Dyn> {
73 /// Moves a type to the heap with its `Metadata` stored in the heap allocation instead of on
79 /// #![feature(thin_box)]
80 /// use std::boxed::ThinBox;
82 /// let thin_slice = ThinBox::<[i32]>::new_unsize([1, 2, 3, 4]);
84 #[cfg(not(no_global_oom_handling))]
85 pub fn new_unsize<T>(value: T) -> Self
89 let meta = ptr::metadata(&value as &Dyn);
90 let ptr = WithOpaqueHeader::new(meta, value);
91 ThinBox { ptr, _marker: PhantomData }
95 #[unstable(feature = "thin_box", issue = "92791")]
96 impl<T: ?Sized + Debug> Debug for ThinBox<T> {
97 fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
98 Debug::fmt(self.deref(), f)
102 #[unstable(feature = "thin_box", issue = "92791")]
103 impl<T: ?Sized + Display> Display for ThinBox<T> {
104 fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
105 Display::fmt(self.deref(), f)
109 #[unstable(feature = "thin_box", issue = "92791")]
110 impl<T: ?Sized> Deref for ThinBox<T> {
113 fn deref(&self) -> &T {
114 let value = self.data();
115 let metadata = self.meta();
116 let pointer = ptr::from_raw_parts(value as *const (), metadata);
121 #[unstable(feature = "thin_box", issue = "92791")]
122 impl<T: ?Sized> DerefMut for ThinBox<T> {
123 fn deref_mut(&mut self) -> &mut T {
124 let value = self.data();
125 let metadata = self.meta();
126 let pointer = ptr::from_raw_parts_mut::<T>(value as *mut (), metadata);
127 unsafe { &mut *pointer }
131 #[unstable(feature = "thin_box", issue = "92791")]
132 impl<T: ?Sized> Drop for ThinBox<T> {
135 let value = self.deref_mut();
136 let value = value as *mut T;
137 self.with_header().drop::<T>(value);
142 #[unstable(feature = "thin_box", issue = "92791")]
143 impl<T: ?Sized> ThinBox<T> {
144 fn meta(&self) -> <T as Pointee>::Metadata {
146 // - NonNull and valid.
147 unsafe { *self.with_header().header() }
150 fn data(&self) -> *mut u8 {
151 self.with_header().value()
154 fn with_header(&self) -> &WithHeader<<T as Pointee>::Metadata> {
155 // SAFETY: both types are transparent to `NonNull<u8>`
156 unsafe { &*((&self.ptr) as *const WithOpaqueHeader as *const WithHeader<_>) }
160 /// A pointer to type-erased data, guaranteed to either be:
161 /// 1. `NonNull::dangling()`, in the case where both the pointee (`T`) and
162 /// metadata (`H`) are ZSTs.
163 /// 2. A pointer to a valid `T` that has a header `H` directly before the
164 /// pointed-to location.
166 struct WithHeader<H>(NonNull<u8>, PhantomData<H>);
168 /// An opaque representation of `WithHeader<H>` to avoid the
169 /// projection invariance of `<T as Pointee>::Metadata`.
171 struct WithOpaqueHeader(NonNull<u8>);
173 impl WithOpaqueHeader {
174 #[cfg(not(no_global_oom_handling))]
175 fn new<H, T>(header: H, value: T) -> Self {
176 let ptr = WithHeader::new(header, value);
181 impl<H> WithHeader<H> {
182 #[cfg(not(no_global_oom_handling))]
183 fn new<T>(header: H, value: T) -> WithHeader<H> {
184 let value_layout = Layout::new::<T>();
185 let Ok((layout, value_offset)) = Self::alloc_layout(value_layout) else {
186 // We pass an empty layout here because we do not know which layout caused the
187 // arithmetic overflow in `Layout::extend` and `handle_alloc_error` takes `Layout` as
188 // its argument rather than `Result<Layout, LayoutError>`, also this function has been
189 // stable since 1.28 ._.
191 // On the other hand, look at this gorgeous turbofish!
192 alloc::handle_alloc_error(Layout::new::<()>());
196 // Note: It's UB to pass a layout with a zero size to `alloc::alloc`, so
197 // we use `layout.dangling()` for this case, which should have a valid
198 // alignment for both `T` and `H`.
199 let ptr = if layout.size() == 0 {
200 // Some paranoia checking, mostly so that the ThinBox tests are
201 // more able to catch issues.
203 value_offset == 0 && mem::size_of::<T>() == 0 && mem::size_of::<H>() == 0
207 let ptr = alloc::alloc(layout);
209 alloc::handle_alloc_error(layout);
212 // - The size is at least `aligned_header_size`.
213 let ptr = ptr.add(value_offset) as *mut _;
215 NonNull::new_unchecked(ptr)
218 let result = WithHeader(ptr, PhantomData);
219 ptr::write(result.header(), header);
220 ptr::write(result.value().cast(), value);
227 // - Assumes that either `value` can be dereferenced, or is the
228 // `NonNull::dangling()` we use when both `T` and `H` are ZSTs.
229 unsafe fn drop<T: ?Sized>(&self, value: *mut T) {
231 let value_layout = Layout::for_value_raw(value);
232 // SAFETY: Layout must have been computable if we're in drop
233 let (layout, value_offset) = Self::alloc_layout(value_layout).unwrap_unchecked();
235 // We only drop the value because the Pointee trait requires that the metadata is copy
236 // aka trivially droppable.
237 ptr::drop_in_place::<T>(value);
239 // Note: Don't deallocate if the layout size is zero, because the pointer
240 // didn't come from the allocator.
241 if layout.size() != 0 {
242 alloc::dealloc(self.0.as_ptr().sub(value_offset), layout);
245 value_offset == 0 && mem::size_of::<H>() == 0 && value_layout.size() == 0
251 fn header(&self) -> *mut H {
253 // - At least `size_of::<H>()` bytes are allocated ahead of the pointer.
254 // - We know that H will be aligned because the middle pointer is aligned to the greater
255 // of the alignment of the header and the data and the header size includes the padding
256 // needed to align the header. Subtracting the header size from the aligned data pointer
257 // will always result in an aligned header pointer, it just may not point to the
258 // beginning of the allocation.
259 let hp = unsafe { self.0.as_ptr().sub(Self::header_size()) as *mut H };
260 debug_assert!(hp.is_aligned());
264 fn value(&self) -> *mut u8 {
268 const fn header_size() -> usize {
272 fn alloc_layout(value_layout: Layout) -> Result<(Layout, usize), LayoutError> {
273 Layout::new::<H>().extend(value_layout)
277 #[cfg(not(bootstrap))]
278 #[unstable(feature = "thin_box", issue = "92791")]
279 impl<T: ?Sized + Error> Error for ThinBox<T> {
280 fn source(&self) -> Option<&(dyn Error + 'static)> {
281 self.deref().source()