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+#![cfg(feature = "extern_crate_alloc")]
+
+//! Stuff to boost things in the `alloc` crate.
+//!
+//! * You must enable the `extern_crate_alloc` feature of `bytemuck` or you will
+//! not be able to use this module! This is generally done by adding the
+//! feature to the dependency in Cargo.toml like so:
+//!
+//! `bytemuck = { version = "VERSION_YOU_ARE_USING", features =
+//! ["extern_crate_alloc"]}`
+
+use super::*;
+#[cfg(target_has_atomic = "ptr")]
+use alloc::sync::Arc;
+use alloc::{
+ alloc::{alloc_zeroed, Layout},
+ boxed::Box,
+ rc::Rc,
+ vec,
+ vec::Vec,
+};
+
+/// As [`try_cast_box`](try_cast_box), but unwraps for you.
+#[inline]
+pub fn cast_box<A: NoUninit, B: AnyBitPattern>(input: Box<A>) -> Box<B> {
+ try_cast_box(input).map_err(|(e, _v)| e).unwrap()
+}
+
+/// Attempts to cast the content type of a [`Box`](alloc::boxed::Box).
+///
+/// On failure you get back an error along with the starting `Box`.
+///
+/// ## Failure
+///
+/// * The start and end content type of the `Box` must have the exact same
+/// alignment.
+/// * The start and end size of the `Box` must have the exact same size.
+#[inline]
+pub fn try_cast_box<A: NoUninit, B: AnyBitPattern>(
+ input: Box<A>,
+) -> Result<Box<B>, (PodCastError, Box<A>)> {
+ if align_of::<A>() != align_of::<B>() {
+ Err((PodCastError::AlignmentMismatch, input))
+ } else if size_of::<A>() != size_of::<B>() {
+ Err((PodCastError::SizeMismatch, input))
+ } else {
+ // Note(Lokathor): This is much simpler than with the Vec casting!
+ let ptr: *mut B = Box::into_raw(input) as *mut B;
+ Ok(unsafe { Box::from_raw(ptr) })
+ }
+}
+
+/// Allocates a `Box<T>` with all of the contents being zeroed out.
+///
+/// This uses the global allocator to create a zeroed allocation and _then_
+/// turns it into a Box. In other words, it's 100% assured that the zeroed data
+/// won't be put temporarily on the stack. You can make a box of any size
+/// without fear of a stack overflow.
+///
+/// ## Failure
+///
+/// This fails if the allocation fails.
+#[inline]
+pub fn try_zeroed_box<T: Zeroable>() -> Result<Box<T>, ()> {
+ if size_of::<T>() == 0 {
+ // This will not allocate but simply create a dangling pointer.
+ let dangling = core::ptr::NonNull::dangling().as_ptr();
+ return Ok(unsafe { Box::from_raw(dangling) });
+ }
+ let layout = Layout::new::<T>();
+ let ptr = unsafe { alloc_zeroed(layout) };
+ if ptr.is_null() {
+ // we don't know what the error is because `alloc_zeroed` is a dumb API
+ Err(())
+ } else {
+ Ok(unsafe { Box::<T>::from_raw(ptr as *mut T) })
+ }
+}
+
+/// As [`try_zeroed_box`], but unwraps for you.
+#[inline]
+pub fn zeroed_box<T: Zeroable>() -> Box<T> {
+ try_zeroed_box().unwrap()
+}
+
+/// Allocates a `Vec<T>` of length and capacity exactly equal to `length` and
+/// all elements zeroed.
+///
+/// ## Failure
+///
+/// This fails if the allocation fails, or if a layout cannot be calculated for
+/// the allocation.
+pub fn try_zeroed_vec<T: Zeroable>(length: usize) -> Result<Vec<T>, ()> {
+ if length == 0 {
+ Ok(Vec::new())
+ } else {
+ let boxed_slice = try_zeroed_slice_box(length)?;
+ Ok(boxed_slice.into_vec())
+ }
+}
+
+/// As [`try_zeroed_vec`] but unwraps for you
+pub fn zeroed_vec<T: Zeroable>(length: usize) -> Vec<T> {
+ try_zeroed_vec(length).unwrap()
+}
+
+/// Allocates a `Box<[T]>` with all contents being zeroed out.
+///
+/// This uses the global allocator to create a zeroed allocation and _then_
+/// turns it into a Box. In other words, it's 100% assured that the zeroed data
+/// won't be put temporarily on the stack. You can make a box of any size
+/// without fear of a stack overflow.
+///
+/// ## Failure
+///
+/// This fails if the allocation fails, or if a layout cannot be calculated for
+/// the allocation.
+#[inline]
+pub fn try_zeroed_slice_box<T: Zeroable>(
+ length: usize,
+) -> Result<Box<[T]>, ()> {
+ if size_of::<T>() == 0 || length == 0 {
+ // This will not allocate but simply create a dangling slice pointer.
+ let dangling = core::ptr::NonNull::dangling().as_ptr();
+ let dangling_slice = core::ptr::slice_from_raw_parts_mut(dangling, length);
+ return Ok(unsafe { Box::from_raw(dangling_slice) });
+ }
+ let layout = core::alloc::Layout::array::<T>(length).map_err(|_| ())?;
+ let ptr = unsafe { alloc_zeroed(layout) };
+ if ptr.is_null() {
+ // we don't know what the error is because `alloc_zeroed` is a dumb API
+ Err(())
+ } else {
+ let slice =
+ unsafe { core::slice::from_raw_parts_mut(ptr as *mut T, length) };
+ Ok(unsafe { Box::<[T]>::from_raw(slice) })
+ }
+}
+
+/// As [`try_zeroed_slice_box`](try_zeroed_slice_box), but unwraps for you.
+pub fn zeroed_slice_box<T: Zeroable>(length: usize) -> Box<[T]> {
+ try_zeroed_slice_box(length).unwrap()
+}
+
+/// As [`try_cast_slice_box`](try_cast_slice_box), but unwraps for you.
+#[inline]
+pub fn cast_slice_box<A: NoUninit, B: AnyBitPattern>(
+ input: Box<[A]>,
+) -> Box<[B]> {
+ try_cast_slice_box(input).map_err(|(e, _v)| e).unwrap()
+}
+
+/// Attempts to cast the content type of a `Box<[T]>`.
+///
+/// On failure you get back an error along with the starting `Box<[T]>`.
+///
+/// ## Failure
+///
+/// * The start and end content type of the `Box<[T]>` must have the exact same
+/// alignment.
+/// * The start and end content size in bytes of the `Box<[T]>` must be the
+/// exact same.
+#[inline]
+pub fn try_cast_slice_box<A: NoUninit, B: AnyBitPattern>(
+ input: Box<[A]>,
+) -> Result<Box<[B]>, (PodCastError, Box<[A]>)> {
+ if align_of::<A>() != align_of::<B>() {
+ Err((PodCastError::AlignmentMismatch, input))
+ } else if size_of::<A>() != size_of::<B>() {
+ if size_of::<A>() * input.len() % size_of::<B>() != 0 {
+ // If the size in bytes of the underlying buffer does not match an exact
+ // multiple of the size of B, we cannot cast between them.
+ Err((PodCastError::SizeMismatch, input))
+ } else {
+ // Because the size is an exact multiple, we can now change the length
+ // of the slice and recreate the Box
+ // NOTE: This is a valid operation because according to the docs of
+ // std::alloc::GlobalAlloc::dealloc(), the Layout that was used to alloc
+ // the block must be the same Layout that is used to dealloc the block.
+ // Luckily, Layout only stores two things, the alignment, and the size in
+ // bytes. So as long as both of those stay the same, the Layout will
+ // remain a valid input to dealloc.
+ let length = size_of::<A>() * input.len() / size_of::<B>();
+ let box_ptr: *mut A = Box::into_raw(input) as *mut A;
+ let ptr: *mut [B] =
+ unsafe { core::slice::from_raw_parts_mut(box_ptr as *mut B, length) };
+ Ok(unsafe { Box::<[B]>::from_raw(ptr) })
+ }
+ } else {
+ let box_ptr: *mut [A] = Box::into_raw(input);
+ let ptr: *mut [B] = box_ptr as *mut [B];
+ Ok(unsafe { Box::<[B]>::from_raw(ptr) })
+ }
+}
+
+/// As [`try_cast_vec`](try_cast_vec), but unwraps for you.
+#[inline]
+pub fn cast_vec<A: NoUninit, B: AnyBitPattern>(input: Vec<A>) -> Vec<B> {
+ try_cast_vec(input).map_err(|(e, _v)| e).unwrap()
+}
+
+/// Attempts to cast the content type of a [`Vec`](alloc::vec::Vec).
+///
+/// On failure you get back an error along with the starting `Vec`.
+///
+/// ## Failure
+///
+/// * The start and end content type of the `Vec` must have the exact same
+/// alignment.
+/// * The start and end content size in bytes of the `Vec` must be the exact
+/// same.
+/// * The start and end capacity in bytes of the `Vec` must be the exact same.
+#[inline]
+pub fn try_cast_vec<A: NoUninit, B: AnyBitPattern>(
+ input: Vec<A>,
+) -> Result<Vec<B>, (PodCastError, Vec<A>)> {
+ if align_of::<A>() != align_of::<B>() {
+ Err((PodCastError::AlignmentMismatch, input))
+ } else if size_of::<A>() != size_of::<B>() {
+ if size_of::<A>() * input.len() % size_of::<B>() != 0
+ || size_of::<A>() * input.capacity() % size_of::<B>() != 0
+ {
+ // If the size in bytes of the underlying buffer does not match an exact
+ // multiple of the size of B, we cannot cast between them.
+ // Note that we have to pay special attention to make sure that both
+ // length and capacity are valid under B, as we do not want to
+ // change which bytes are considered part of the initialized slice
+ // of the Vec
+ Err((PodCastError::SizeMismatch, input))
+ } else {
+ // Because the size is an exact multiple, we can now change the length and
+ // capacity and recreate the Vec
+ // NOTE: This is a valid operation because according to the docs of
+ // std::alloc::GlobalAlloc::dealloc(), the Layout that was used to alloc
+ // the block must be the same Layout that is used to dealloc the block.
+ // Luckily, Layout only stores two things, the alignment, and the size in
+ // bytes. So as long as both of those stay the same, the Layout will
+ // remain a valid input to dealloc.
+
+ // Note(Lokathor): First we record the length and capacity, which don't
+ // have any secret provenance metadata.
+ let length: usize = size_of::<A>() * input.len() / size_of::<B>();
+ let capacity: usize = size_of::<A>() * input.capacity() / size_of::<B>();
+ // Note(Lokathor): Next we "pre-forget" the old Vec by wrapping with
+ // ManuallyDrop, because if we used `core::mem::forget` after taking the
+ // pointer then that would invalidate our pointer. In nightly there's a
+ // "into raw parts" method, which we can switch this too eventually.
+ let mut manual_drop_vec = ManuallyDrop::new(input);
+ let vec_ptr: *mut A = manual_drop_vec.as_mut_ptr();
+ let ptr: *mut B = vec_ptr as *mut B;
+ Ok(unsafe { Vec::from_raw_parts(ptr, length, capacity) })
+ }
+ } else {
+ // Note(Lokathor): First we record the length and capacity, which don't have
+ // any secret provenance metadata.
+ let length: usize = input.len();
+ let capacity: usize = input.capacity();
+ // Note(Lokathor): Next we "pre-forget" the old Vec by wrapping with
+ // ManuallyDrop, because if we used `core::mem::forget` after taking the
+ // pointer then that would invalidate our pointer. In nightly there's a
+ // "into raw parts" method, which we can switch this too eventually.
+ let mut manual_drop_vec = ManuallyDrop::new(input);
+ let vec_ptr: *mut A = manual_drop_vec.as_mut_ptr();
+ let ptr: *mut B = vec_ptr as *mut B;
+ Ok(unsafe { Vec::from_raw_parts(ptr, length, capacity) })
+ }
+}
+
+/// This "collects" a slice of pod data into a vec of a different pod type.
+///
+/// Unlike with [`cast_slice`] and [`cast_slice_mut`], this will always work.
+///
+/// The output vec will be of a minimal size/capacity to hold the slice given.
+///
+/// ```rust
+/// # use bytemuck::*;
+/// let halfwords: [u16; 4] = [5, 6, 7, 8];
+/// let vec_of_words: Vec<u32> = pod_collect_to_vec(&halfwords);
+/// if cfg!(target_endian = "little") {
+/// assert_eq!(&vec_of_words[..], &[0x0006_0005, 0x0008_0007][..])
+/// } else {
+/// assert_eq!(&vec_of_words[..], &[0x0005_0006, 0x0007_0008][..])
+/// }
+/// ```
+pub fn pod_collect_to_vec<A: NoUninit, B: NoUninit + AnyBitPattern>(
+ src: &[A],
+) -> Vec<B> {
+ let src_size = size_of_val(src);
+ // Note(Lokathor): dst_count is rounded up so that the dest will always be at
+ // least as many bytes as the src.
+ let dst_count = src_size / size_of::<B>()
+ + if src_size % size_of::<B>() != 0 { 1 } else { 0 };
+ let mut dst = vec![B::zeroed(); dst_count];
+
+ let src_bytes: &[u8] = cast_slice(src);
+ let dst_bytes: &mut [u8] = cast_slice_mut(&mut dst[..]);
+ dst_bytes[..src_size].copy_from_slice(src_bytes);
+ dst
+}
+
+/// As [`try_cast_rc`](try_cast_rc), but unwraps for you.
+#[inline]
+pub fn cast_rc<A: NoUninit + AnyBitPattern, B: NoUninit + AnyBitPattern>(
+ input: Rc<A>,
+) -> Rc<B> {
+ try_cast_rc(input).map_err(|(e, _v)| e).unwrap()
+}
+
+/// Attempts to cast the content type of a [`Rc`](alloc::rc::Rc).
+///
+/// On failure you get back an error along with the starting `Rc`.
+///
+/// The bounds on this function are the same as [`cast_mut`], because a user
+/// could call `Rc::get_unchecked_mut` on the output, which could be observable
+/// in the input.
+///
+/// ## Failure
+///
+/// * The start and end content type of the `Rc` must have the exact same
+/// alignment.
+/// * The start and end size of the `Rc` must have the exact same size.
+#[inline]
+pub fn try_cast_rc<A: NoUninit + AnyBitPattern, B: NoUninit + AnyBitPattern>(
+ input: Rc<A>,
+) -> Result<Rc<B>, (PodCastError, Rc<A>)> {
+ if align_of::<A>() != align_of::<B>() {
+ Err((PodCastError::AlignmentMismatch, input))
+ } else if size_of::<A>() != size_of::<B>() {
+ Err((PodCastError::SizeMismatch, input))
+ } else {
+ // Safety: Rc::from_raw requires size and alignment match, which is met.
+ let ptr: *const B = Rc::into_raw(input) as *const B;
+ Ok(unsafe { Rc::from_raw(ptr) })
+ }
+}
+
+/// As [`try_cast_arc`](try_cast_arc), but unwraps for you.
+#[inline]
+#[cfg(target_has_atomic = "ptr")]
+pub fn cast_arc<A: NoUninit + AnyBitPattern, B: NoUninit + AnyBitPattern>(
+ input: Arc<A>,
+) -> Arc<B> {
+ try_cast_arc(input).map_err(|(e, _v)| e).unwrap()
+}
+
+/// Attempts to cast the content type of a [`Arc`](alloc::sync::Arc).
+///
+/// On failure you get back an error along with the starting `Arc`.
+///
+/// The bounds on this function are the same as [`cast_mut`], because a user
+/// could call `Rc::get_unchecked_mut` on the output, which could be observable
+/// in the input.
+///
+/// ## Failure
+///
+/// * The start and end content type of the `Arc` must have the exact same
+/// alignment.
+/// * The start and end size of the `Arc` must have the exact same size.
+#[inline]
+#[cfg(target_has_atomic = "ptr")]
+pub fn try_cast_arc<
+ A: NoUninit + AnyBitPattern,
+ B: NoUninit + AnyBitPattern,
+>(
+ input: Arc<A>,
+) -> Result<Arc<B>, (PodCastError, Arc<A>)> {
+ if align_of::<A>() != align_of::<B>() {
+ Err((PodCastError::AlignmentMismatch, input))
+ } else if size_of::<A>() != size_of::<B>() {
+ Err((PodCastError::SizeMismatch, input))
+ } else {
+ // Safety: Arc::from_raw requires size and alignment match, which is met.
+ let ptr: *const B = Arc::into_raw(input) as *const B;
+ Ok(unsafe { Arc::from_raw(ptr) })
+ }
+}
+
+/// As [`try_cast_slice_rc`](try_cast_slice_rc), but unwraps for you.
+#[inline]
+pub fn cast_slice_rc<
+ A: NoUninit + AnyBitPattern,
+ B: NoUninit + AnyBitPattern,
+>(
+ input: Rc<[A]>,
+) -> Rc<[B]> {
+ try_cast_slice_rc(input).map_err(|(e, _v)| e).unwrap()
+}
+
+/// Attempts to cast the content type of a `Rc<[T]>`.
+///
+/// On failure you get back an error along with the starting `Rc<[T]>`.
+///
+/// The bounds on this function are the same as [`cast_mut`], because a user
+/// could call `Rc::get_unchecked_mut` on the output, which could be observable
+/// in the input.
+///
+/// ## Failure
+///
+/// * The start and end content type of the `Rc<[T]>` must have the exact same
+/// alignment.
+/// * The start and end content size in bytes of the `Rc<[T]>` must be the exact
+/// same.
+#[inline]
+pub fn try_cast_slice_rc<
+ A: NoUninit + AnyBitPattern,
+ B: NoUninit + AnyBitPattern,
+>(
+ input: Rc<[A]>,
+) -> Result<Rc<[B]>, (PodCastError, Rc<[A]>)> {
+ if align_of::<A>() != align_of::<B>() {
+ Err((PodCastError::AlignmentMismatch, input))
+ } else if size_of::<A>() != size_of::<B>() {
+ if size_of::<A>() * input.len() % size_of::<B>() != 0 {
+ // If the size in bytes of the underlying buffer does not match an exact
+ // multiple of the size of B, we cannot cast between them.
+ Err((PodCastError::SizeMismatch, input))
+ } else {
+ // Because the size is an exact multiple, we can now change the length
+ // of the slice and recreate the Rc
+ // NOTE: This is a valid operation because according to the docs of
+ // std::rc::Rc::from_raw(), the type U that was in the original Rc<U>
+ // acquired from Rc::into_raw() must have the same size alignment and
+ // size of the type T in the new Rc<T>. So as long as both the size
+ // and alignment stay the same, the Rc will remain a valid Rc.
+ let length = size_of::<A>() * input.len() / size_of::<B>();
+ let rc_ptr: *const A = Rc::into_raw(input) as *const A;
+ // Must use ptr::slice_from_raw_parts, because we cannot make an
+ // intermediate const reference, because it has mutable provenance,
+ // nor an intermediate mutable reference, because it could be aliased.
+ let ptr = core::ptr::slice_from_raw_parts(rc_ptr as *const B, length);
+ Ok(unsafe { Rc::<[B]>::from_raw(ptr) })
+ }
+ } else {
+ let rc_ptr: *const [A] = Rc::into_raw(input);
+ let ptr: *const [B] = rc_ptr as *const [B];
+ Ok(unsafe { Rc::<[B]>::from_raw(ptr) })
+ }
+}
+
+/// As [`try_cast_slice_arc`](try_cast_slice_arc), but unwraps for you.
+#[inline]
+#[cfg(target_has_atomic = "ptr")]
+pub fn cast_slice_arc<
+ A: NoUninit + AnyBitPattern,
+ B: NoUninit + AnyBitPattern,
+>(
+ input: Arc<[A]>,
+) -> Arc<[B]> {
+ try_cast_slice_arc(input).map_err(|(e, _v)| e).unwrap()
+}
+
+/// Attempts to cast the content type of a `Arc<[T]>`.
+///
+/// On failure you get back an error along with the starting `Arc<[T]>`.
+///
+/// The bounds on this function are the same as [`cast_mut`], because a user
+/// could call `Rc::get_unchecked_mut` on the output, which could be observable
+/// in the input.
+///
+/// ## Failure
+///
+/// * The start and end content type of the `Arc<[T]>` must have the exact same
+/// alignment.
+/// * The start and end content size in bytes of the `Arc<[T]>` must be the
+/// exact same.
+#[inline]
+#[cfg(target_has_atomic = "ptr")]
+pub fn try_cast_slice_arc<
+ A: NoUninit + AnyBitPattern,
+ B: NoUninit + AnyBitPattern,
+>(
+ input: Arc<[A]>,
+) -> Result<Arc<[B]>, (PodCastError, Arc<[A]>)> {
+ if align_of::<A>() != align_of::<B>() {
+ Err((PodCastError::AlignmentMismatch, input))
+ } else if size_of::<A>() != size_of::<B>() {
+ if size_of::<A>() * input.len() % size_of::<B>() != 0 {
+ // If the size in bytes of the underlying buffer does not match an exact
+ // multiple of the size of B, we cannot cast between them.
+ Err((PodCastError::SizeMismatch, input))
+ } else {
+ // Because the size is an exact multiple, we can now change the length
+ // of the slice and recreate the Arc
+ // NOTE: This is a valid operation because according to the docs of
+ // std::sync::Arc::from_raw(), the type U that was in the original Arc<U>
+ // acquired from Arc::into_raw() must have the same size alignment and
+ // size of the type T in the new Arc<T>. So as long as both the size
+ // and alignment stay the same, the Arc will remain a valid Arc.
+ let length = size_of::<A>() * input.len() / size_of::<B>();
+ let arc_ptr: *const A = Arc::into_raw(input) as *const A;
+ // Must use ptr::slice_from_raw_parts, because we cannot make an
+ // intermediate const reference, because it has mutable provenance,
+ // nor an intermediate mutable reference, because it could be aliased.
+ let ptr = core::ptr::slice_from_raw_parts(arc_ptr as *const B, length);
+ Ok(unsafe { Arc::<[B]>::from_raw(ptr) })
+ }
+ } else {
+ let arc_ptr: *const [A] = Arc::into_raw(input);
+ let ptr: *const [B] = arc_ptr as *const [B];
+ Ok(unsafe { Arc::<[B]>::from_raw(ptr) })
+ }
+}
+
+/// An extension trait for `TransparentWrapper` and alloc types.
+pub trait TransparentWrapperAlloc<Inner: ?Sized>:
+ TransparentWrapper<Inner>
+{
+ /// Convert a vec of the inner type into a vec of the wrapper type.
+ fn wrap_vec(s: Vec<Inner>) -> Vec<Self>
+ where
+ Self: Sized,
+ Inner: Sized,
+ {
+ let mut s = core::mem::ManuallyDrop::new(s);
+
+ let length = s.len();
+ let capacity = s.capacity();
+ let ptr = s.as_mut_ptr();
+
+ unsafe {
+ // SAFETY:
+ // * ptr comes from Vec (and will not be double-dropped)
+ // * the two types have the identical representation
+ // * the len and capacity fields are valid
+ Vec::from_raw_parts(ptr as *mut Self, length, capacity)
+ }
+ }
+
+ /// Convert a box to the inner type into a box to the wrapper
+ /// type.
+ #[inline]
+ fn wrap_box(s: Box<Inner>) -> Box<Self> {
+ assert!(size_of::<*mut Inner>() == size_of::<*mut Self>());
+
+ unsafe {
+ // A pointer cast doesn't work here because rustc can't tell that
+ // the vtables match (because of the `?Sized` restriction relaxation).
+ // A `transmute` doesn't work because the sizes are unspecified.
+ //
+ // SAFETY:
+ // * The unsafe contract requires that pointers to Inner and Self have
+ // identical representations
+ // * Box is guaranteed to have representation identical to a (non-null)
+ // pointer
+ // * The pointer comes from a box (and thus satisfies all safety
+ // requirements of Box)
+ let inner_ptr: *mut Inner = Box::into_raw(s);
+ let wrapper_ptr: *mut Self = transmute!(inner_ptr);
+ Box::from_raw(wrapper_ptr)
+ }
+ }
+
+ /// Convert an [`Rc`](alloc::rc::Rc) to the inner type into an `Rc` to the
+ /// wrapper type.
+ #[inline]
+ fn wrap_rc(s: Rc<Inner>) -> Rc<Self> {
+ assert!(size_of::<*mut Inner>() == size_of::<*mut Self>());
+
+ unsafe {
+ // A pointer cast doesn't work here because rustc can't tell that
+ // the vtables match (because of the `?Sized` restriction relaxation).
+ // A `transmute` doesn't work because the layout of Rc is unspecified.
+ //
+ // SAFETY:
+ // * The unsafe contract requires that pointers to Inner and Self have
+ // identical representations, and that the size and alignment of Inner
+ // and Self are the same, which meets the safety requirements of
+ // Rc::from_raw
+ let inner_ptr: *const Inner = Rc::into_raw(s);
+ let wrapper_ptr: *const Self = transmute!(inner_ptr);
+ Rc::from_raw(wrapper_ptr)
+ }
+ }
+
+ /// Convert an [`Arc`](alloc::sync::Arc) to the inner type into an `Arc` to
+ /// the wrapper type.
+ #[inline]
+ #[cfg(target_has_atomic = "ptr")]
+ fn wrap_arc(s: Arc<Inner>) -> Arc<Self> {
+ assert!(size_of::<*mut Inner>() == size_of::<*mut Self>());
+
+ unsafe {
+ // A pointer cast doesn't work here because rustc can't tell that
+ // the vtables match (because of the `?Sized` restriction relaxation).
+ // A `transmute` doesn't work because the layout of Arc is unspecified.
+ //
+ // SAFETY:
+ // * The unsafe contract requires that pointers to Inner and Self have
+ // identical representations, and that the size and alignment of Inner
+ // and Self are the same, which meets the safety requirements of
+ // Arc::from_raw
+ let inner_ptr: *const Inner = Arc::into_raw(s);
+ let wrapper_ptr: *const Self = transmute!(inner_ptr);
+ Arc::from_raw(wrapper_ptr)
+ }
+ }
+
+ /// Convert a vec of the wrapper type into a vec of the inner type.
+ fn peel_vec(s: Vec<Self>) -> Vec<Inner>
+ where
+ Self: Sized,
+ Inner: Sized,
+ {
+ let mut s = core::mem::ManuallyDrop::new(s);
+
+ let length = s.len();
+ let capacity = s.capacity();
+ let ptr = s.as_mut_ptr();
+
+ unsafe {
+ // SAFETY:
+ // * ptr comes from Vec (and will not be double-dropped)
+ // * the two types have the identical representation
+ // * the len and capacity fields are valid
+ Vec::from_raw_parts(ptr as *mut Inner, length, capacity)
+ }
+ }
+
+ /// Convert a box to the wrapper type into a box to the inner
+ /// type.
+ #[inline]
+ fn peel_box(s: Box<Self>) -> Box<Inner> {
+ assert!(size_of::<*mut Inner>() == size_of::<*mut Self>());
+
+ unsafe {
+ // A pointer cast doesn't work here because rustc can't tell that
+ // the vtables match (because of the `?Sized` restriction relaxation).
+ // A `transmute` doesn't work because the sizes are unspecified.
+ //
+ // SAFETY:
+ // * The unsafe contract requires that pointers to Inner and Self have
+ // identical representations
+ // * Box is guaranteed to have representation identical to a (non-null)
+ // pointer
+ // * The pointer comes from a box (and thus satisfies all safety
+ // requirements of Box)
+ let wrapper_ptr: *mut Self = Box::into_raw(s);
+ let inner_ptr: *mut Inner = transmute!(wrapper_ptr);
+ Box::from_raw(inner_ptr)
+ }
+ }
+
+ /// Convert an [`Rc`](alloc::rc::Rc) to the wrapper type into an `Rc` to the
+ /// inner type.
+ #[inline]
+ fn peel_rc(s: Rc<Self>) -> Rc<Inner> {
+ assert!(size_of::<*mut Inner>() == size_of::<*mut Self>());
+
+ unsafe {
+ // A pointer cast doesn't work here because rustc can't tell that
+ // the vtables match (because of the `?Sized` restriction relaxation).
+ // A `transmute` doesn't work because the layout of Rc is unspecified.
+ //
+ // SAFETY:
+ // * The unsafe contract requires that pointers to Inner and Self have
+ // identical representations, and that the size and alignment of Inner
+ // and Self are the same, which meets the safety requirements of
+ // Rc::from_raw
+ let wrapper_ptr: *const Self = Rc::into_raw(s);
+ let inner_ptr: *const Inner = transmute!(wrapper_ptr);
+ Rc::from_raw(inner_ptr)
+ }
+ }
+
+ /// Convert an [`Arc`](alloc::sync::Arc) to the wrapper type into an `Arc` to
+ /// the inner type.
+ #[inline]
+ #[cfg(target_has_atomic = "ptr")]
+ fn peel_arc(s: Arc<Self>) -> Arc<Inner> {
+ assert!(size_of::<*mut Inner>() == size_of::<*mut Self>());
+
+ unsafe {
+ // A pointer cast doesn't work here because rustc can't tell that
+ // the vtables match (because of the `?Sized` restriction relaxation).
+ // A `transmute` doesn't work because the layout of Arc is unspecified.
+ //
+ // SAFETY:
+ // * The unsafe contract requires that pointers to Inner and Self have
+ // identical representations, and that the size and alignment of Inner
+ // and Self are the same, which meets the safety requirements of
+ // Arc::from_raw
+ let wrapper_ptr: *const Self = Arc::into_raw(s);
+ let inner_ptr: *const Inner = transmute!(wrapper_ptr);
+ Arc::from_raw(inner_ptr)
+ }
+ }
+}
+
+impl<I: ?Sized, T: ?Sized + TransparentWrapper<I>> TransparentWrapperAlloc<I> for T {}
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