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#![no_std] #![warn(missing_docs)] //! This crate gives small utilities for casting between plain data types. //! //! ## Basics //! //! Data comes in five basic forms in Rust, so we have five basic casting //! functions: //! //! * `T` uses [`cast`] //! * `&T` uses [`cast_ref`] //! * `&mut T` uses [`cast_mut`] //! * `&[T]` uses [`cast_slice`] //! * `&mut [T]` uses [`cast_slice_mut`] //! //! Some casts will never fail (eg: `cast::<u32, f32>` always works), other //! casts might fail (eg: `cast_ref::<[u8; 4], u32>` will fail if the reference //! isn't already aligned to 4). Each casting function has a "try" version which //! will return a `Result`, and the "normal" version which will simply panic on //! invalid input. //! //! ## Using Your Own Types //! //! All the functions here are guarded by the [`Pod`] trait, which is a //! sub-trait of the [`Zeroable`] trait. //! //! If you're very sure that your type is eligible, you can implement those //! traits for your type and then they'll have full casting support. However, //! these traits are `unsafe`, and you should carefully read the requirements //! before adding the them to your own types. //! //! ## Features //! //! * This crate is core only by default, but if you're using Rust 1.36 or later //! you can enable the `extern_crate_alloc` cargo feature for some additional //! methods related to `Box` and `Vec`. Note that the `docs.rs` documentation //! is always built with `extern_crate_alloc` cargo feature enabled. #[cfg(target_arch = "x86")] pub(crate) use core::arch::x86; #[cfg(target_arch = "x86_64")] pub(crate) use core::arch::x86_64; // pub(crate) use core::{marker::*, mem::*, num::*, ptr::*}; macro_rules! impl_unsafe_marker_for_array { ( $marker:ident , $( $n:expr ),* ) => { $(unsafe impl<T> $marker for [T; $n] where T: $marker {})* } } #[cfg(feature = "extern_crate_alloc")] extern crate alloc; #[cfg(feature = "extern_crate_alloc")] pub mod allocation; #[cfg(feature = "extern_crate_alloc")] pub use allocation::*; mod zeroable; pub use zeroable::*; mod pod; pub use pod::*; mod contiguous; pub use contiguous::*; mod transparent; pub use transparent::*; // Used from macros to ensure we aren't using some locally defined name and // actually are referencing libcore. This also would allow pre-2018 edition // crates to use our macros, but I'm not sure how important that is. #[doc(hidden)] pub use ::core as __core; /// Re-interprets `&T` as `&[u8]`. /// /// Any ZST becomes an empty slice, and in that case the pointer value of that /// empty slice might not match the pointer value of the input reference. #[inline] pub fn bytes_of<T: Pod>(t: &T) -> &[u8] { try_cast_slice::<T, u8>(core::slice::from_ref(t)).unwrap_or(&[]) } /// Re-interprets `&mut T` as `&mut [u8]`. /// /// Any ZST becomes an empty slice, and in that case the pointer value of that /// empty slice might not match the pointer value of the input reference. #[inline] pub fn bytes_of_mut<T: Pod>(t: &mut T) -> &mut [u8] { try_cast_slice_mut::<T, u8>(core::slice::from_mut(t)).unwrap_or(&mut []) } /// Re-interprets `&[u8]` as `&T`. /// /// ## Panics /// /// This is [`try_from_bytes`] with an unwrap. #[inline] pub fn from_bytes<T: Pod>(s: &[u8]) -> &T { try_from_bytes(s).unwrap() } /// Re-interprets `&mut [u8]` as `&mut T`. /// /// ## Panics /// /// This is [`try_from_bytes_mut`] with an unwrap. #[inline] pub fn from_bytes_mut<T: Pod>(s: &mut [u8]) -> &mut T { try_from_bytes_mut(s).unwrap() } /// Re-interprets `&[u8]` as `&T`. /// /// ## Failure /// /// * If the slice isn't aligned for the new type /// * If the slice's length isn’t exactly the size of the new type #[inline] pub fn try_from_bytes<T: Pod>(s: &[u8]) -> Result<&T, PodCastError> { if s.len() != size_of::<T>() { Err(PodCastError::SizeMismatch) } else if (s.as_ptr() as usize) % align_of::<T>() != 0 { Err(PodCastError::AlignmentMismatch) } else { Ok(unsafe { &*(s.as_ptr() as *const T) }) } } /// Re-interprets `&mut [u8]` as `&mut T`. /// /// ## Failure /// /// * If the slice isn't aligned for the new type /// * If the slice's length isn’t exactly the size of the new type #[inline] pub fn try_from_bytes_mut<T: Pod>( s: &mut [u8], ) -> Result<&mut T, PodCastError> { if s.len() != size_of::<T>() { Err(PodCastError::SizeMismatch) } else if (s.as_ptr() as usize) % align_of::<T>() != 0 { Err(PodCastError::AlignmentMismatch) } else { Ok(unsafe { &mut *(s.as_mut_ptr() as *mut T) }) } } /// The things that can go wrong when casting between [`Pod`] data forms. #[derive(Debug, Clone, Copy, PartialEq, Eq)] pub enum PodCastError { /// You tried to cast a slice to an element type with a higher alignment /// requirement but the slice wasn't aligned. TargetAlignmentGreaterAndInputNotAligned, /// If the element size changes then the output slice changes length /// accordingly. If the output slice wouldn't be a whole number of elements /// then the conversion fails. OutputSliceWouldHaveSlop, /// When casting a slice you can't convert between ZST elements and non-ZST /// elements. When casting an individual `T`, `&T`, or `&mut T` value the /// source size and destination size must be an exact match. SizeMismatch, /// For this type of cast the alignments must be exactly the same and they /// were not so now you're sad. AlignmentMismatch, } /// Cast `T` into `U` /// /// ## Panics /// /// This is [`try_cast`] with an unwrap. #[inline] pub fn cast<A: Pod, B: Pod>(a: A) -> B { try_cast(a).unwrap() } /// Cast `&mut T` into `&mut U`. /// /// ## Panics /// /// This is [`try_cast_mut`] with an unwrap. #[inline] pub fn cast_mut<A: Pod, B: Pod>(a: &mut A) -> &mut B { try_cast_mut(a).unwrap() } /// Cast `&T` into `&U`. /// /// ## Panics /// /// This is [`try_cast_ref`] with an unwrap. #[inline] pub fn cast_ref<A: Pod, B: Pod>(a: &A) -> &B { try_cast_ref(a).unwrap() } /// Cast `&[T]` into `&[U]`. /// /// ## Panics /// /// This is [`try_cast_slice`] with an unwrap. #[inline] pub fn cast_slice<A: Pod, B: Pod>(a: &[A]) -> &[B] { try_cast_slice(a).unwrap() } /// Cast `&mut [T]` into `&mut [U]`. /// /// ## Panics /// /// This is [`try_cast_slice_mut`] with an unwrap. #[inline] pub fn cast_slice_mut<A: Pod, B: Pod>(a: &mut [A]) -> &mut [B] { try_cast_slice_mut(a).unwrap() } /// As `align_to`, but safe because of the [`Pod`] bound. #[inline] pub fn pod_align_to<T: Pod, U: Pod>(vals: &[T]) -> (&[T], &[U], &[T]) { unsafe { vals.align_to::<U>() } } /// As `align_to_mut`, but safe because of the [`Pod`] bound. #[inline] pub fn pod_align_to_mut<T: Pod, U: Pod>( vals: &mut [T], ) -> (&mut [T], &mut [U], &mut [T]) { unsafe { vals.align_to_mut::<U>() } } /// Try to cast `T` into `U`. /// /// ## Failure /// /// * If the types don't have the same size this fails. #[inline] pub fn try_cast<A: Pod, B: Pod>(a: A) -> Result<B, PodCastError> { if size_of::<A>() == size_of::<B>() { let mut b = B::zeroed(); // Note(Lokathor): We copy in terms of `u8` because that allows us to bypass // any potential alignment difficulties. let ap = &a as *const A as *const u8; let bp = &mut b as *mut B as *mut u8; unsafe { ap.copy_to_nonoverlapping(bp, size_of::<A>()) }; Ok(b) } else { Err(PodCastError::SizeMismatch) } } /// Try to convert a `&T` into `&U`. /// /// ## Failure /// /// * If the reference isn't aligned in the new type /// * If the source type and target type aren't the same size. #[inline] pub fn try_cast_ref<A: Pod, B: Pod>(a: &A) -> Result<&B, PodCastError> { // Note(Lokathor): everything with `align_of` and `size_of` will optimize away // after monomorphization. if align_of::<B>() > align_of::<A>() && (a as *const A as usize) % align_of::<B>() != 0 { Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned) } else if size_of::<B>() == size_of::<A>() { Ok(unsafe { &*(a as *const A as *const B) }) } else { Err(PodCastError::SizeMismatch) } } /// Try to convert a `&mut T` into `&mut U`. /// /// As [`try_cast_ref`], but `mut`. #[inline] pub fn try_cast_mut<A: Pod, B: Pod>(a: &mut A) -> Result<&mut B, PodCastError> { // Note(Lokathor): everything with `align_of` and `size_of` will optimize away // after monomorphization. if align_of::<B>() > align_of::<A>() && (a as *mut A as usize) % align_of::<B>() != 0 { Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned) } else if size_of::<B>() == size_of::<A>() { Ok(unsafe { &mut *(a as *mut A as *mut B) }) } else { Err(PodCastError::SizeMismatch) } } /// Try to convert `&[T]` into `&[U]` (possibly with a change in length). /// /// * `input.as_ptr() as usize == output.as_ptr() as usize` /// * `input.len() * size_of::<A>() == output.len() * size_of::<B>()` /// /// ## Failure /// /// * If the target type has a greater alignment requirement and the input slice /// isn't aligned. /// * If the target element type is a different size from the current element /// type, and the output slice wouldn't be a whole number of elements when /// accounting for the size change (eg: 3 `u16` values is 1.5 `u32` values, so /// that's a failure). /// * Similarly, you can't convert between a /// [ZST](https://doc.rust-lang.org/nomicon/exotic-sizes.html#zero-sized-types-zsts) /// and a non-ZST. #[inline] pub fn try_cast_slice<A: Pod, B: Pod>(a: &[A]) -> Result<&[B], PodCastError> { // Note(Lokathor): everything with `align_of` and `size_of` will optimize away // after monomorphization. if align_of::<B>() > align_of::<A>() && (a.as_ptr() as usize) % align_of::<B>() != 0 { Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned) } else if size_of::<B>() == size_of::<A>() { Ok(unsafe { core::slice::from_raw_parts(a.as_ptr() as *const B, a.len()) }) } else if size_of::<A>() == 0 || size_of::<B>() == 0 { Err(PodCastError::SizeMismatch) } else if core::mem::size_of_val(a) % size_of::<B>() == 0 { let new_len = core::mem::size_of_val(a) / size_of::<B>(); Ok(unsafe { core::slice::from_raw_parts(a.as_ptr() as *const B, new_len) }) } else { Err(PodCastError::OutputSliceWouldHaveSlop) } } /// Try to convert `&mut [T]` into `&mut [U]` (possibly with a change in length). /// /// As [`try_cast_slice`], but `&mut`. #[inline] pub fn try_cast_slice_mut<A: Pod, B: Pod>( a: &mut [A], ) -> Result<&mut [B], PodCastError> { // Note(Lokathor): everything with `align_of` and `size_of` will optimize away // after monomorphization. if align_of::<B>() > align_of::<A>() && (a.as_mut_ptr() as usize) % align_of::<B>() != 0 { Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned) } else if size_of::<B>() == size_of::<A>() { Ok(unsafe { core::slice::from_raw_parts_mut(a.as_mut_ptr() as *mut B, a.len()) }) } else if size_of::<A>() == 0 || size_of::<B>() == 0 { Err(PodCastError::SizeMismatch) } else if core::mem::size_of_val(a) % size_of::<B>() == 0 { let new_len = core::mem::size_of_val(a) / size_of::<B>(); Ok(unsafe { core::slice::from_raw_parts_mut(a.as_mut_ptr() as *mut B, new_len) }) } else { Err(PodCastError::OutputSliceWouldHaveSlop) } } /// Find the offset in bytes of the given `$field` of `$Type`, using `$instance` /// as an already-initialized reference value. /// /// This is similar to the macro from `memoffset`, however it's fully well /// defined even in current versions of Rust (and uses no unsafe code). /// /// It does by using the `$instance` argument to get an already-initialized /// instance of `$Type` rather than trying to find a way access the fields of an /// uninitialized one without hitting soundness problems. /// /// This means the API is more limited, but it's also sound even in rather /// extreme cases, like some of the examples. /// /// ## Caveats /// /// 1. The offset is in bytes, and so you will likely have to cast your base /// pointers to `*const u8`/`*mut u8` before getting field addresses. /// /// 2. The offset values of repr(Rust) types are not stable, and may change /// wildly between releases of the compiler. Use repr(C) if you can. /// /// 3. The value of the `$instance` parameter has no bearing on the output of /// this macro. It is just used to avoid soundness problems. The only /// requirement is that it be initialized. In particular, the value returned /// is not a field pointer, or anything like that. /// /// ## Examples /// /// ### Use with zeroable types /// A common requirement in GPU apis is to specify the layout of vertices. These /// will generally be [`Zeroable`] (if not [`Pod`]), and are a good fit for /// `offset_of!`. /// ``` /// # use bytemuck::{Zeroable, offset_of}; /// #[repr(C)] /// struct Vertex { /// pos: [f32; 2], /// uv: [u16; 2], /// color: [u8; 4], /// } /// unsafe impl Zeroable for Vertex {} /// /// let pos = offset_of!(Zeroable::zeroed(), Vertex, pos); /// let uv = offset_of!(Zeroable::zeroed(), Vertex, uv); /// let color = offset_of!(Zeroable::zeroed(), Vertex, color); /// /// assert_eq!(pos, 0); /// assert_eq!(uv, 8); /// assert_eq!(color, 12); /// ``` /// /// ### Use with other types /// /// More esoteric uses are possible too, including with types generally not safe /// to bytemuck. `Strings`, `Vec`s, etc. /// /// ``` /// #[derive(Default)] /// struct Foo { /// a: u8, /// b: &'static str, /// c: i32, /// } /// /// let a_offset = bytemuck::offset_of!(Default::default(), Foo, a); /// let b_offset = bytemuck::offset_of!(Default::default(), Foo, b); /// let c_offset = bytemuck::offset_of!(Default::default(), Foo, c); /// /// assert_ne!(a_offset, b_offset); /// assert_ne!(b_offset, c_offset); /// // We can't check against hardcoded values for a repr(Rust) type, /// // but prove to ourself this way. /// /// let foo = Foo::default(); /// // Note: offsets are in bytes. /// let as_bytes = &foo as *const _ as *const u8; /// /// // we're using wrapping_offset here becasue it's not worth /// // the unsafe block, but it would be valid to use `add` instead, /// // as it cannot overflow. /// assert_eq!(&foo.a as *const _ as usize, as_bytes.wrapping_add(a_offset) as usize); /// assert_eq!(&foo.b as *const _ as usize, as_bytes.wrapping_add(b_offset) as usize); /// assert_eq!(&foo.c as *const _ as usize, as_bytes.wrapping_add(c_offset) as usize); /// ``` #[macro_export] macro_rules! offset_of { ($instance:expr, $Type:path, $field:tt) => {{ // This helps us guard against field access going through a Deref impl. #[allow(clippy::unneeded_field_pattern)] let $Type { $field: _, .. }; let reference: &$Type = &$instance; let address = reference as *const _ as usize; let field_pointer = &reference.$field as *const _ as usize; // These asserts/unwraps are compiled away at release, and defend against // the case where somehow a deref impl is still invoked. let result = field_pointer.checked_sub(address).unwrap(); assert!(result <= $crate::__core::mem::size_of::<$Type>()); result }}; }