alloc/boxed/
thin.rs

1//! Based on
2//! <https://github.com/matthieu-m/rfc2580/blob/b58d1d3cba0d4b5e859d3617ea2d0943aaa31329/examples/thin.rs>
3//! by matthieu-m
4
5use core::error::Error;
6use core::fmt::{self, Debug, Display, Formatter};
7#[cfg(not(no_global_oom_handling))]
8use core::intrinsics::{const_allocate, const_make_global};
9use core::marker::PhantomData;
10#[cfg(not(no_global_oom_handling))]
11use core::marker::Unsize;
12#[cfg(not(no_global_oom_handling))]
13use core::mem;
14use core::mem::SizedTypeProperties;
15use core::ops::{Deref, DerefMut};
16use core::ptr::{self, NonNull, Pointee};
17
18use crate::alloc::{self, Layout, LayoutError};
19
20/// ThinBox.
21///
22/// A thin pointer for heap allocation, regardless of T.
23///
24/// # Examples
25///
26/// ```
27/// #![feature(thin_box)]
28/// use std::boxed::ThinBox;
29///
30/// let five = ThinBox::new(5);
31/// let thin_slice = ThinBox::<[i32]>::new_unsize([1, 2, 3, 4]);
32///
33/// let size_of_ptr = size_of::<*const ()>();
34/// assert_eq!(size_of_ptr, size_of_val(&five));
35/// assert_eq!(size_of_ptr, size_of_val(&thin_slice));
36/// ```
37#[unstable(feature = "thin_box", issue = "92791")]
38pub struct ThinBox<T: ?Sized> {
39    // This is essentially `WithHeader<<T as Pointee>::Metadata>`,
40    // but that would be invariant in `T`, and we want covariance.
41    ptr: WithOpaqueHeader,
42    _marker: PhantomData<T>,
43}
44
45/// `ThinBox<T>` is `Send` if `T` is `Send` because the data is owned.
46#[unstable(feature = "thin_box", issue = "92791")]
47unsafe impl<T: ?Sized + Send> Send for ThinBox<T> {}
48
49/// `ThinBox<T>` is `Sync` if `T` is `Sync` because the data is owned.
50#[unstable(feature = "thin_box", issue = "92791")]
51unsafe impl<T: ?Sized + Sync> Sync for ThinBox<T> {}
52
53#[unstable(feature = "thin_box", issue = "92791")]
54impl<T> ThinBox<T> {
55    /// Moves a type to the heap with its [`Metadata`] stored in the heap allocation instead of on
56    /// the stack.
57    ///
58    /// # Examples
59    ///
60    /// ```
61    /// #![feature(thin_box)]
62    /// use std::boxed::ThinBox;
63    ///
64    /// let five = ThinBox::new(5);
65    /// ```
66    ///
67    /// [`Metadata`]: core::ptr::Pointee::Metadata
68    #[cfg(not(no_global_oom_handling))]
69    pub fn new(value: T) -> Self {
70        let meta = ptr::metadata(&value);
71        let ptr = WithOpaqueHeader::new(meta, value);
72        ThinBox { ptr, _marker: PhantomData }
73    }
74
75    /// Moves a type to the heap with its [`Metadata`] stored in the heap allocation instead of on
76    /// the stack. Returns an error if allocation fails, instead of aborting.
77    ///
78    /// # Examples
79    ///
80    /// ```
81    /// #![feature(thin_box)]
82    /// use std::boxed::ThinBox;
83    ///
84    /// let five = ThinBox::try_new(5)?;
85    /// # Ok::<(), std::alloc::AllocError>(())
86    /// ```
87    ///
88    /// [`Metadata`]: core::ptr::Pointee::Metadata
89    pub fn try_new(value: T) -> Result<Self, core::alloc::AllocError> {
90        let meta = ptr::metadata(&value);
91        WithOpaqueHeader::try_new(meta, value).map(|ptr| ThinBox { ptr, _marker: PhantomData })
92    }
93}
94
95#[unstable(feature = "thin_box", issue = "92791")]
96impl<Dyn: ?Sized> ThinBox<Dyn> {
97    /// Moves a type to the heap with its [`Metadata`] stored in the heap allocation instead of on
98    /// the stack.
99    ///
100    /// # Examples
101    ///
102    /// ```
103    /// #![feature(thin_box)]
104    /// use std::boxed::ThinBox;
105    ///
106    /// let thin_slice = ThinBox::<[i32]>::new_unsize([1, 2, 3, 4]);
107    /// ```
108    ///
109    /// [`Metadata`]: core::ptr::Pointee::Metadata
110    #[cfg(not(no_global_oom_handling))]
111    pub fn new_unsize<T>(value: T) -> Self
112    where
113        T: Unsize<Dyn>,
114    {
115        if T::IS_ZST {
116            let ptr = WithOpaqueHeader::new_unsize_zst::<Dyn, T>(value);
117            ThinBox { ptr, _marker: PhantomData }
118        } else {
119            let meta = ptr::metadata(&value as &Dyn);
120            let ptr = WithOpaqueHeader::new(meta, value);
121            ThinBox { ptr, _marker: PhantomData }
122        }
123    }
124}
125
126#[unstable(feature = "thin_box", issue = "92791")]
127impl<T: ?Sized + Debug> Debug for ThinBox<T> {
128    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
129        Debug::fmt(self.deref(), f)
130    }
131}
132
133#[unstable(feature = "thin_box", issue = "92791")]
134impl<T: ?Sized + Display> Display for ThinBox<T> {
135    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
136        Display::fmt(self.deref(), f)
137    }
138}
139
140#[unstable(feature = "thin_box", issue = "92791")]
141impl<T: ?Sized> Deref for ThinBox<T> {
142    type Target = T;
143
144    fn deref(&self) -> &T {
145        let value = self.data();
146        let metadata = self.meta();
147        let pointer = ptr::from_raw_parts(value as *const (), metadata);
148        // SAFETY: &ThinBox<T> points to a valid pointer for T.
149        unsafe { &*pointer }
150    }
151}
152
153#[unstable(feature = "thin_box", issue = "92791")]
154impl<T: ?Sized> DerefMut for ThinBox<T> {
155    fn deref_mut(&mut self) -> &mut T {
156        let value = self.data();
157        let metadata = self.meta();
158        let pointer = ptr::from_raw_parts_mut::<T>(value as *mut (), metadata);
159        // SAFETY: &mut ThinBox<T> points to a valid and unique pointer for T.
160        unsafe { &mut *pointer }
161    }
162}
163
164#[unstable(feature = "thin_box", issue = "92791")]
165impl<T: ?Sized> Drop for ThinBox<T> {
166    fn drop(&mut self) {
167        let value = self.deref_mut();
168        let value = value as *mut T;
169        // SAFETY: `value` is valid for reads and writes for our `T`.
170        unsafe {
171            self.with_header().drop::<T>(value);
172        }
173    }
174}
175
176#[unstable(feature = "thin_box", issue = "92791")]
177impl<T: ?Sized> ThinBox<T> {
178    fn meta(&self) -> <T as Pointee>::Metadata {
179        // SAFETY: NonNull and valid.
180        unsafe { *self.with_header().header() }
181    }
182
183    fn data(&self) -> *mut u8 {
184        self.with_header().value()
185    }
186
187    fn with_header(&self) -> &WithHeader<<T as Pointee>::Metadata> {
188        // SAFETY: both types are transparent to `NonNull<u8>`
189        unsafe { &*((&raw const self.ptr) as *const WithHeader<_>) }
190    }
191}
192
193/// A pointer to type-erased data, guaranteed to either be:
194/// 1. `NonNull::dangling()`, in the case where both the pointee (`T`) and
195///    metadata (`H`) are ZSTs.
196/// 2. A pointer to a valid `T` that has a header `H` directly before the
197///    pointed-to location.
198#[repr(transparent)]
199struct WithHeader<H>(NonNull<u8>, PhantomData<H>);
200
201/// An opaque representation of `WithHeader<H>` to avoid the
202/// projection invariance of `<T as Pointee>::Metadata`.
203#[repr(transparent)]
204struct WithOpaqueHeader(NonNull<u8>);
205
206impl WithOpaqueHeader {
207    #[cfg(not(no_global_oom_handling))]
208    fn new<H, T>(header: H, value: T) -> Self {
209        let ptr = WithHeader::new(header, value);
210        Self(ptr.0)
211    }
212
213    #[cfg(not(no_global_oom_handling))]
214    fn new_unsize_zst<Dyn, T>(value: T) -> Self
215    where
216        Dyn: ?Sized,
217        T: Unsize<Dyn>,
218    {
219        let ptr = WithHeader::<<Dyn as Pointee>::Metadata>::new_unsize_zst::<Dyn, T>(value);
220        Self(ptr.0)
221    }
222
223    fn try_new<H, T>(header: H, value: T) -> Result<Self, core::alloc::AllocError> {
224        WithHeader::try_new(header, value).map(|ptr| Self(ptr.0))
225    }
226}
227
228impl<H> WithHeader<H> {
229    #[cfg(not(no_global_oom_handling))]
230    fn new<T>(header: H, value: T) -> WithHeader<H> {
231        let value_layout = Layout::new::<T>();
232        let Ok((layout, value_offset)) = Self::alloc_layout(value_layout) else {
233            // We pass an empty layout here because we do not know which layout caused the
234            // arithmetic overflow in `Layout::extend` and `handle_alloc_error` takes `Layout` as
235            // its argument rather than `Result<Layout, LayoutError>`, also this function has been
236            // stable since 1.28 ._.
237            //
238            // On the other hand, look at this gorgeous turbofish!
239            alloc::handle_alloc_error(Layout::new::<()>());
240        };
241
242        // Note: It's UB to pass a layout with a zero size to `alloc::alloc`, so
243        // we use `layout.dangling()` for this case, which should have a valid
244        // alignment for both `T` and `H`.
245        let ptr = if layout.size() == 0 {
246            // Some paranoia checking, mostly so that the ThinBox tests are
247            // more able to catch issues.
248            debug_assert!(value_offset == 0 && T::IS_ZST && H::IS_ZST);
249            layout.dangling_ptr()
250        } else {
251            // SAFETY: We check above that the layout size is nonzero.
252            let ptr = unsafe { alloc::alloc(layout) };
253            if ptr.is_null() {
254                alloc::handle_alloc_error(layout);
255            }
256            // SAFETY:
257            // - The size is at least `aligned_header_size`.
258            unsafe {
259                let ptr = ptr.add(value_offset) as *mut _;
260
261                NonNull::new_unchecked(ptr)
262            }
263        };
264
265        let result = WithHeader(ptr, PhantomData);
266
267        // SAFETY: `result.header()` promises to give us a valid place for writing
268        // the header, and `result.value()` promises the same for the value.
269        unsafe {
270            ptr::write(result.header(), header);
271            ptr::write(result.value().cast(), value);
272        }
273
274        result
275    }
276
277    /// Non-panicking version of `new`.
278    /// Any error is returned as `Err(core::alloc::AllocError)`.
279    fn try_new<T>(header: H, value: T) -> Result<WithHeader<H>, core::alloc::AllocError> {
280        let value_layout = Layout::new::<T>();
281        let Ok((layout, value_offset)) = Self::alloc_layout(value_layout) else {
282            return Err(core::alloc::AllocError);
283        };
284
285        // Note: It's UB to pass a layout with a zero size to `alloc::alloc`, so
286        // we use `layout.dangling()` for this case, which should have a valid
287        // alignment for both `T` and `H`.
288        let ptr = if layout.size() == 0 {
289            // Some paranoia checking, mostly so that the ThinBox tests are
290            // more able to catch issues.
291            debug_assert!(value_offset == 0 && T::IS_ZST && H::IS_ZST);
292            layout.dangling_ptr()
293        } else {
294            // SAFETY: We check above that the layout size is nonzero.
295            let ptr = unsafe { alloc::alloc(layout) };
296            if ptr.is_null() {
297                return Err(core::alloc::AllocError);
298            }
299
300            // SAFETY:
301            // - The size is at least `aligned_header_size`.
302            unsafe {
303                let ptr = ptr.add(value_offset) as *mut _;
304
305                NonNull::new_unchecked(ptr)
306            }
307        };
308
309        let result = WithHeader(ptr, PhantomData);
310
311        // SAFETY: `result.header()` promises to give us a valid place for writing
312        // the header, and `result.value()` promises the same for the value.
313        unsafe {
314            ptr::write(result.header(), header);
315            ptr::write(result.value().cast(), value);
316        }
317
318        Ok(result)
319    }
320
321    // `Dyn` is `?Sized` type like `[u32]`, and `T` is ZST type like `[u32; 0]`.
322    #[cfg(not(no_global_oom_handling))]
323    fn new_unsize_zst<Dyn, T>(value: T) -> WithHeader<H>
324    where
325        Dyn: Pointee<Metadata = H> + ?Sized,
326        T: Unsize<Dyn>,
327    {
328        assert!(T::IS_ZST);
329
330        const fn max(a: usize, b: usize) -> usize {
331            if a > b { a } else { b }
332        }
333
334        // Compute a pointer to the right metadata. This will point to the beginning
335        // of the header, past the padding, so the assigned type makes sense.
336        // It also ensures that the address at the end of the header is sufficiently
337        // aligned for T.
338        let alloc: &<Dyn as Pointee>::Metadata = const {
339            // FIXME: just call `WithHeader::alloc_layout` with size reset to 0.
340            // Currently that's blocked on `Layout::extend` not being `const fn`.
341
342            let alloc_align = max(align_of::<T>(), align_of::<<Dyn as Pointee>::Metadata>());
343
344            let alloc_size = max(align_of::<T>(), size_of::<<Dyn as Pointee>::Metadata>());
345
346            // SAFETY: align is power of two because it is the maximum of two alignments.
347            let alloc: *mut u8 = unsafe { const_allocate(alloc_size, alloc_align) };
348
349            let metadata_offset =
350                alloc_size.checked_sub(size_of::<<Dyn as Pointee>::Metadata>()).unwrap();
351            let metadata_ptr: *mut <Dyn as Pointee>::Metadata =
352                // SAFETY: adding offset within the allocation.
353                unsafe { alloc.add(metadata_offset).cast() };
354            // SAFETY: `*metadata_ptr` is within the allocation.
355            unsafe {
356                metadata_ptr.write(ptr::metadata::<Dyn>(ptr::dangling::<T>() as *const Dyn));
357            }
358            // SAFETY: valid heap allocation
359            unsafe { const_make_global(alloc) };
360            // SAFETY: we have just written the metadata.
361            unsafe { &*metadata_ptr }
362        };
363
364        let value_ptr =
365            // SAFETY: `alloc` points to `<Dyn as Pointee>::Metadata`, so addition stays in-bounds.
366            unsafe { (alloc as *const <Dyn as Pointee>::Metadata).add(1) }.cast::<T>().cast_mut();
367        debug_assert!(value_ptr.is_aligned());
368        mem::forget(value);
369        WithHeader(NonNull::new(value_ptr.cast()).unwrap(), PhantomData)
370    }
371
372    /// # Safety
373    ///
374    /// `value` must point to an undropped owned `T`, and `self` must not be
375    /// accessed again after this is called.
376    unsafe fn drop<T: ?Sized>(&self, value: *mut T) {
377        struct DropGuard<H> {
378            ptr: NonNull<u8>,
379            value_layout: Layout,
380            _marker: PhantomData<H>,
381        }
382
383        impl<H> Drop for DropGuard<H> {
384            fn drop(&mut self) {
385                // All ZST are allocated statically.
386                if self.value_layout.size() == 0 {
387                    return;
388                }
389
390                let (layout, value_offset) =
391                    // SAFETY: Layout must have been computable if we're in drop
392                    unsafe { WithHeader::<H>::alloc_layout(self.value_layout).unwrap_unchecked() };
393
394                // Since we only allocate for non-ZSTs, the layout size cannot be zero.
395                debug_assert!(layout.size() != 0);
396                // SAFETY: We own the allocation with `layout` at `ptr - value_offset`.
397                unsafe { alloc::dealloc(self.ptr.as_ptr().sub(value_offset), layout) };
398            }
399        }
400
401        // `_guard` will deallocate the memory when dropped, even if `drop_in_place` unwinds.
402        let _guard = DropGuard {
403            ptr: self.0,
404            // SAFETY: Caller ensures `value` is valid.
405            value_layout: unsafe { Layout::for_value_raw(value) },
406            _marker: PhantomData::<H>,
407        };
408
409        // We only drop the value because the Pointee trait requires that the metadata is copy
410        // aka trivially droppable.
411        // SAFETY: We're the only droppers of `value` and it's not dropped again.
412        unsafe { ptr::drop_in_place::<T>(value) };
413    }
414
415    fn header(&self) -> *mut H {
416        // SAFETY:
417        //  - At least `size_of::<H>()` bytes are allocated ahead of the pointer.
418        //  - We know that H will be aligned because the middle pointer is aligned to the greater
419        //    of the alignment of the header and the data and the header size includes the padding
420        //    needed to align the header. Subtracting the header size from the aligned data pointer
421        //    will always result in an aligned header pointer, it just may not point to the
422        //    beginning of the allocation.
423        let hp = unsafe { self.0.as_ptr().sub(Self::header_size()) as *mut H };
424        debug_assert!(hp.is_aligned());
425        hp
426    }
427
428    fn value(&self) -> *mut u8 {
429        self.0.as_ptr()
430    }
431
432    const fn header_size() -> usize {
433        size_of::<H>()
434    }
435
436    fn alloc_layout(value_layout: Layout) -> Result<(Layout, usize), LayoutError> {
437        Layout::new::<H>().extend(value_layout)
438    }
439}
440
441#[unstable(feature = "thin_box", issue = "92791")]
442impl<T: ?Sized + Error> Error for ThinBox<T> {
443    fn source(&self) -> Option<&(dyn Error + 'static)> {
444        self.deref().source()
445    }
446}
447
448#[cfg(not(no_global_oom_handling))]
449#[unstable(feature = "thin_box", issue = "92791")]
450impl<T> From<T> for ThinBox<T> {
451    #[inline(always)]
452    fn from(value: T) -> Self {
453        Self::new(value)
454    }
455}
alloc/boxed/thin.rs

alloc/boxed/
thin.rs
