From 1b6a04ca5504955c571d1c97504fb45ea0befee4 Mon Sep 17 00:00:00 2001 From: Valentin Popov Date: Mon, 8 Jan 2024 01:21:28 +0400 Subject: Initial vendor packages Signed-off-by: Valentin Popov --- vendor/bytemuck/src/allocation.rs | 689 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 689 insertions(+) create mode 100644 vendor/bytemuck/src/allocation.rs (limited to 'vendor/bytemuck/src/allocation.rs') diff --git a/vendor/bytemuck/src/allocation.rs b/vendor/bytemuck/src/allocation.rs new file mode 100644 index 0000000..a2633b5 --- /dev/null +++ b/vendor/bytemuck/src/allocation.rs @@ -0,0 +1,689 @@ +#![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(input: Box) -> Box { + 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( + input: Box, +) -> Result, (PodCastError, Box)> { + if align_of::() != align_of::() { + Err((PodCastError::AlignmentMismatch, input)) + } else if size_of::() != size_of::() { + 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` 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() -> Result, ()> { + if size_of::() == 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::(); + 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::::from_raw(ptr as *mut T) }) + } +} + +/// As [`try_zeroed_box`], but unwraps for you. +#[inline] +pub fn zeroed_box() -> Box { + try_zeroed_box().unwrap() +} + +/// Allocates a `Vec` 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(length: usize) -> Result, ()> { + 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(length: usize) -> Vec { + 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( + length: usize, +) -> Result, ()> { + if size_of::() == 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::(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(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( + 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( + input: Box<[A]>, +) -> Result, (PodCastError, Box<[A]>)> { + if align_of::() != align_of::() { + Err((PodCastError::AlignmentMismatch, input)) + } else if size_of::() != size_of::() { + if size_of::() * input.len() % size_of::() != 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::() * input.len() / size_of::(); + 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(input: Vec) -> Vec { + 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( + input: Vec, +) -> Result, (PodCastError, Vec)> { + if align_of::() != align_of::() { + Err((PodCastError::AlignmentMismatch, input)) + } else if size_of::() != size_of::() { + if size_of::() * input.len() % size_of::() != 0 + || size_of::() * input.capacity() % size_of::() != 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::() * input.len() / size_of::(); + let capacity: usize = size_of::() * input.capacity() / size_of::(); + // 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 = 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( + src: &[A], +) -> Vec { + 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::() + + if src_size % size_of::() != 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( + input: Rc, +) -> Rc { + 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( + input: Rc, +) -> Result, (PodCastError, Rc)> { + if align_of::() != align_of::() { + Err((PodCastError::AlignmentMismatch, input)) + } else if size_of::() != size_of::() { + 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( + input: Arc, +) -> Arc { + 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, +) -> Result, (PodCastError, Arc)> { + if align_of::() != align_of::() { + Err((PodCastError::AlignmentMismatch, input)) + } else if size_of::() != size_of::() { + 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, (PodCastError, Rc<[A]>)> { + if align_of::() != align_of::() { + Err((PodCastError::AlignmentMismatch, input)) + } else if size_of::() != size_of::() { + if size_of::() * input.len() % size_of::() != 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 + // acquired from Rc::into_raw() must have the same size alignment and + // size of the type T in the new Rc. So as long as both the size + // and alignment stay the same, the Rc will remain a valid Rc. + let length = size_of::() * input.len() / size_of::(); + 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, (PodCastError, Arc<[A]>)> { + if align_of::() != align_of::() { + Err((PodCastError::AlignmentMismatch, input)) + } else if size_of::() != size_of::() { + if size_of::() * input.len() % size_of::() != 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 + // acquired from Arc::into_raw() must have the same size alignment and + // size of the type T in the new Arc. So as long as both the size + // and alignment stay the same, the Arc will remain a valid Arc. + let length = size_of::() * input.len() / size_of::(); + 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: + TransparentWrapper +{ + /// Convert a vec of the inner type into a vec of the wrapper type. + fn wrap_vec(s: Vec) -> Vec + 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) -> Box { + 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) -> Rc { + 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) -> Arc { + 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) -> Vec + 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) -> Box { + 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) -> Rc { + 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) -> Arc { + 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> TransparentWrapperAlloc for T {} -- cgit v1.2.3