458 lines
14 KiB
Rust
458 lines
14 KiB
Rust
#![no_std]
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#![warn(missing_docs)]
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#![allow(clippy::match_like_matches_macro)]
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#![allow(clippy::uninlined_format_args)]
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#![cfg_attr(feature = "nightly_docs", feature(doc_cfg))]
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#![cfg_attr(feature = "nightly_portable_simd", feature(portable_simd))]
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#![cfg_attr(feature = "nightly_stdsimd", feature(stdsimd))]
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//! This crate gives small utilities for casting between plain data types.
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//!
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//! ## Basics
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//!
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//! Data comes in five basic forms in Rust, so we have five basic casting
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//! functions:
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//!
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//! * `T` uses [`cast`]
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//! * `&T` uses [`cast_ref`]
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//! * `&mut T` uses [`cast_mut`]
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//! * `&[T]` uses [`cast_slice`]
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//! * `&mut [T]` uses [`cast_slice_mut`]
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//!
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//! Some casts will never fail (eg: `cast::<u32, f32>` always works), other
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//! casts might fail (eg: `cast_ref::<[u8; 4], u32>` will fail if the reference
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//! isn't already aligned to 4). Each casting function has a "try" version which
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//! will return a `Result`, and the "normal" version which will simply panic on
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//! invalid input.
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//!
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//! ## Using Your Own Types
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//!
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//! All the functions here are guarded by the [`Pod`] trait, which is a
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//! sub-trait of the [`Zeroable`] trait.
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//!
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//! If you're very sure that your type is eligible, you can implement those
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//! traits for your type and then they'll have full casting support. However,
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//! these traits are `unsafe`, and you should carefully read the requirements
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//! before adding the them to your own types.
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//!
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//! ## Features
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//!
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//! * This crate is core only by default, but if you're using Rust 1.36 or later
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//! you can enable the `extern_crate_alloc` cargo feature for some additional
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//! methods related to `Box` and `Vec`. Note that the `docs.rs` documentation
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//! is always built with `extern_crate_alloc` cargo feature enabled.
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#[cfg(all(target_arch = "aarch64", feature = "aarch64_simd"))]
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use core::arch::aarch64;
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#[cfg(all(target_arch = "wasm32", feature = "wasm_simd"))]
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use core::arch::wasm32;
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#[cfg(target_arch = "x86")]
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use core::arch::x86;
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#[cfg(target_arch = "x86_64")]
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use core::arch::x86_64;
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//
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use core::{marker::*, mem::*, num::*, ptr::*};
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// Used from macros to ensure we aren't using some locally defined name and
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// actually are referencing libcore. This also would allow pre-2018 edition
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// crates to use our macros, but I'm not sure how important that is.
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#[doc(hidden)]
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pub use ::core as __core;
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#[cfg(not(feature = "min_const_generics"))]
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macro_rules! impl_unsafe_marker_for_array {
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( $marker:ident , $( $n:expr ),* ) => {
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$(unsafe impl<T> $marker for [T; $n] where T: $marker {})*
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}
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}
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/// A macro to transmute between two types without requiring knowing size
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/// statically.
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macro_rules! transmute {
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($val:expr) => {
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::core::mem::transmute_copy(&::core::mem::ManuallyDrop::new($val))
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};
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}
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/// A macro to implement marker traits for various simd types.
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/// #[allow(unused)] because the impls are only compiled on relevant platforms
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/// with relevant cargo features enabled.
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#[allow(unused)]
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macro_rules! impl_unsafe_marker_for_simd {
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($(#[cfg($cfg_predicate:meta)])? unsafe impl $trait:ident for $platform:ident :: {}) => {};
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($(#[cfg($cfg_predicate:meta)])? unsafe impl $trait:ident for $platform:ident :: { $first_type:ident $(, $types:ident)* $(,)? }) => {
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$( #[cfg($cfg_predicate)] )?
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$( #[cfg_attr(feature = "nightly_docs", doc(cfg($cfg_predicate)))] )?
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unsafe impl $trait for $platform::$first_type {}
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$( #[cfg($cfg_predicate)] )? // To prevent recursion errors if nothing is going to be expanded anyway.
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impl_unsafe_marker_for_simd!($( #[cfg($cfg_predicate)] )? unsafe impl $trait for $platform::{ $( $types ),* });
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};
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}
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#[cfg(feature = "extern_crate_std")]
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extern crate std;
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#[cfg(feature = "extern_crate_alloc")]
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extern crate alloc;
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#[cfg(feature = "extern_crate_alloc")]
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#[cfg_attr(feature = "nightly_docs", doc(cfg(feature = "extern_crate_alloc")))]
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pub mod allocation;
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#[cfg(feature = "extern_crate_alloc")]
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pub use allocation::*;
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mod anybitpattern;
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pub use anybitpattern::*;
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pub mod checked;
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pub use checked::CheckedBitPattern;
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mod internal;
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mod zeroable;
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pub use zeroable::*;
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mod zeroable_in_option;
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pub use zeroable_in_option::*;
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mod pod;
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pub use pod::*;
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mod pod_in_option;
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pub use pod_in_option::*;
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#[cfg(feature = "must_cast")]
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mod must;
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#[cfg(feature = "must_cast")]
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#[cfg_attr(feature = "nightly_docs", doc(cfg(feature = "must_cast")))]
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pub use must::*;
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mod no_uninit;
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pub use no_uninit::*;
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mod contiguous;
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pub use contiguous::*;
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mod offset_of;
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pub use offset_of::*;
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mod transparent;
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pub use transparent::*;
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#[cfg(feature = "derive")]
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#[cfg_attr(feature = "nightly_docs", doc(cfg(feature = "derive")))]
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pub use bytemuck_derive::{
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AnyBitPattern, ByteEq, ByteHash, CheckedBitPattern, Contiguous, NoUninit,
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Pod, TransparentWrapper, Zeroable,
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};
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/// The things that can go wrong when casting between [`Pod`] data forms.
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#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
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pub enum PodCastError {
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/// You tried to cast a slice to an element type with a higher alignment
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/// requirement but the slice wasn't aligned.
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TargetAlignmentGreaterAndInputNotAligned,
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/// If the element size changes then the output slice changes length
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/// accordingly. If the output slice wouldn't be a whole number of elements
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/// then the conversion fails.
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OutputSliceWouldHaveSlop,
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/// When casting a slice you can't convert between ZST elements and non-ZST
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/// elements. When casting an individual `T`, `&T`, or `&mut T` value the
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/// source size and destination size must be an exact match.
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SizeMismatch,
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/// For this type of cast the alignments must be exactly the same and they
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/// were not so now you're sad.
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///
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/// This error is generated **only** by operations that cast allocated types
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/// (such as `Box` and `Vec`), because in that case the alignment must stay
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/// exact.
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AlignmentMismatch,
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}
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#[cfg(not(target_arch = "spirv"))]
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impl core::fmt::Display for PodCastError {
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fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
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write!(f, "{:?}", self)
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}
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}
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#[cfg(feature = "extern_crate_std")]
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#[cfg_attr(feature = "nightly_docs", doc(cfg(feature = "extern_crate_std")))]
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impl std::error::Error for PodCastError {}
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/// Re-interprets `&T` as `&[u8]`.
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///
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/// Any ZST becomes an empty slice, and in that case the pointer value of that
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/// empty slice might not match the pointer value of the input reference.
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#[inline]
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pub fn bytes_of<T: NoUninit>(t: &T) -> &[u8] {
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unsafe { internal::bytes_of(t) }
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}
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/// Re-interprets `&mut T` as `&mut [u8]`.
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///
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/// Any ZST becomes an empty slice, and in that case the pointer value of that
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/// empty slice might not match the pointer value of the input reference.
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#[inline]
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pub fn bytes_of_mut<T: NoUninit + AnyBitPattern>(t: &mut T) -> &mut [u8] {
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unsafe { internal::bytes_of_mut(t) }
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}
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/// Re-interprets `&[u8]` as `&T`.
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///
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/// ## Panics
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///
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/// This is [`try_from_bytes`] but will panic on error.
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#[inline]
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pub fn from_bytes<T: AnyBitPattern>(s: &[u8]) -> &T {
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unsafe { internal::from_bytes(s) }
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}
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/// Re-interprets `&mut [u8]` as `&mut T`.
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///
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/// ## Panics
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///
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/// This is [`try_from_bytes_mut`] but will panic on error.
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#[inline]
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pub fn from_bytes_mut<T: NoUninit + AnyBitPattern>(s: &mut [u8]) -> &mut T {
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unsafe { internal::from_bytes_mut(s) }
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}
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/// Reads from the bytes as if they were a `T`.
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///
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/// ## Failure
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/// * If the `bytes` length is not equal to `size_of::<T>()`.
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#[inline]
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pub fn try_pod_read_unaligned<T: AnyBitPattern>(
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bytes: &[u8],
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) -> Result<T, PodCastError> {
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unsafe { internal::try_pod_read_unaligned(bytes) }
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}
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/// Reads the slice into a `T` value.
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///
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/// ## Panics
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/// * This is like `try_pod_read_unaligned` but will panic on failure.
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#[inline]
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pub fn pod_read_unaligned<T: AnyBitPattern>(bytes: &[u8]) -> T {
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unsafe { internal::pod_read_unaligned(bytes) }
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}
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/// Re-interprets `&[u8]` as `&T`.
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///
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/// ## Failure
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///
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/// * If the slice isn't aligned for the new type
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/// * If the slice's length isn’t exactly the size of the new type
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#[inline]
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pub fn try_from_bytes<T: AnyBitPattern>(s: &[u8]) -> Result<&T, PodCastError> {
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unsafe { internal::try_from_bytes(s) }
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}
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/// Re-interprets `&mut [u8]` as `&mut T`.
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///
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/// ## Failure
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///
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/// * If the slice isn't aligned for the new type
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/// * If the slice's length isn’t exactly the size of the new type
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#[inline]
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pub fn try_from_bytes_mut<T: NoUninit + AnyBitPattern>(
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s: &mut [u8],
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) -> Result<&mut T, PodCastError> {
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unsafe { internal::try_from_bytes_mut(s) }
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}
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/// Cast `T` into `U`
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///
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/// ## Panics
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///
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/// * This is like [`try_cast`](try_cast), but will panic on a size mismatch.
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#[inline]
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pub fn cast<A: NoUninit, B: AnyBitPattern>(a: A) -> B {
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unsafe { internal::cast(a) }
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}
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/// Cast `&mut T` into `&mut U`.
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///
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/// ## Panics
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///
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/// This is [`try_cast_mut`] but will panic on error.
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#[inline]
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pub fn cast_mut<A: NoUninit + AnyBitPattern, B: NoUninit + AnyBitPattern>(
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a: &mut A,
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) -> &mut B {
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unsafe { internal::cast_mut(a) }
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}
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/// Cast `&T` into `&U`.
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///
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/// ## Panics
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///
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/// This is [`try_cast_ref`] but will panic on error.
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#[inline]
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pub fn cast_ref<A: NoUninit, B: AnyBitPattern>(a: &A) -> &B {
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unsafe { internal::cast_ref(a) }
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}
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/// Cast `&[A]` into `&[B]`.
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///
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/// ## Panics
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///
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/// This is [`try_cast_slice`] but will panic on error.
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#[inline]
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pub fn cast_slice<A: NoUninit, B: AnyBitPattern>(a: &[A]) -> &[B] {
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unsafe { internal::cast_slice(a) }
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}
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/// Cast `&mut [T]` into `&mut [U]`.
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///
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/// ## Panics
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///
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/// This is [`try_cast_slice_mut`] but will panic on error.
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#[inline]
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pub fn cast_slice_mut<
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A: NoUninit + AnyBitPattern,
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B: NoUninit + AnyBitPattern,
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>(
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a: &mut [A],
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) -> &mut [B] {
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unsafe { internal::cast_slice_mut(a) }
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}
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/// As `align_to`, but safe because of the [`Pod`] bound.
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#[inline]
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pub fn pod_align_to<T: NoUninit, U: AnyBitPattern>(
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vals: &[T],
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) -> (&[T], &[U], &[T]) {
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unsafe { vals.align_to::<U>() }
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}
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/// As `align_to_mut`, but safe because of the [`Pod`] bound.
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#[inline]
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pub fn pod_align_to_mut<
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T: NoUninit + AnyBitPattern,
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U: NoUninit + AnyBitPattern,
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>(
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vals: &mut [T],
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) -> (&mut [T], &mut [U], &mut [T]) {
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unsafe { vals.align_to_mut::<U>() }
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}
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/// Try to cast `T` into `U`.
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///
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/// Note that for this particular type of cast, alignment isn't a factor. The
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/// input value is semantically copied into the function and then returned to a
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/// new memory location which will have whatever the required alignment of the
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/// output type is.
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///
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/// ## Failure
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///
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/// * If the types don't have the same size this fails.
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#[inline]
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pub fn try_cast<A: NoUninit, B: AnyBitPattern>(
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a: A,
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) -> Result<B, PodCastError> {
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unsafe { internal::try_cast(a) }
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}
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/// Try to convert a `&T` into `&U`.
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///
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/// ## Failure
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///
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/// * If the reference isn't aligned in the new type
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/// * If the source type and target type aren't the same size.
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#[inline]
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pub fn try_cast_ref<A: NoUninit, B: AnyBitPattern>(
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a: &A,
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) -> Result<&B, PodCastError> {
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unsafe { internal::try_cast_ref(a) }
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}
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/// Try to convert a `&mut T` into `&mut U`.
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///
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/// As [`try_cast_ref`], but `mut`.
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#[inline]
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pub fn try_cast_mut<
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A: NoUninit + AnyBitPattern,
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B: NoUninit + AnyBitPattern,
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>(
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a: &mut A,
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) -> Result<&mut B, PodCastError> {
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unsafe { internal::try_cast_mut(a) }
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}
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/// Try to convert `&[A]` into `&[B]` (possibly with a change in length).
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///
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/// * `input.as_ptr() as usize == output.as_ptr() as usize`
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/// * `input.len() * size_of::<A>() == output.len() * size_of::<B>()`
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///
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/// ## Failure
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///
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/// * If the target type has a greater alignment requirement and the input slice
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/// isn't aligned.
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/// * If the target element type is a different size from the current element
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/// type, and the output slice wouldn't be a whole number of elements when
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/// accounting for the size change (eg: 3 `u16` values is 1.5 `u32` values, so
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/// that's a failure).
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/// * Similarly, you can't convert between a [ZST](https://doc.rust-lang.org/nomicon/exotic-sizes.html#zero-sized-types-zsts)
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/// and a non-ZST.
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#[inline]
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pub fn try_cast_slice<A: NoUninit, B: AnyBitPattern>(
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a: &[A],
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) -> Result<&[B], PodCastError> {
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unsafe { internal::try_cast_slice(a) }
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}
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/// Try to convert `&mut [A]` into `&mut [B]` (possibly with a change in
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/// length).
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///
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/// As [`try_cast_slice`], but `&mut`.
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#[inline]
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pub fn try_cast_slice_mut<
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A: NoUninit + AnyBitPattern,
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B: NoUninit + AnyBitPattern,
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>(
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a: &mut [A],
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) -> Result<&mut [B], PodCastError> {
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unsafe { internal::try_cast_slice_mut(a) }
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}
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/// Fill all bytes of `target` with zeroes (see [`Zeroable`]).
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///
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/// This is similar to `*target = Zeroable::zeroed()`, but guarantees that any
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/// padding bytes in `target` are zeroed as well.
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///
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/// See also [`fill_zeroes`], if you have a slice rather than a single value.
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#[inline]
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pub fn write_zeroes<T: Zeroable>(target: &mut T) {
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struct EnsureZeroWrite<T>(*mut T);
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impl<T> Drop for EnsureZeroWrite<T> {
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#[inline(always)]
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fn drop(&mut self) {
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unsafe {
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core::ptr::write_bytes(self.0, 0u8, 1);
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}
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}
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}
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unsafe {
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let guard = EnsureZeroWrite(target);
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core::ptr::drop_in_place(guard.0);
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drop(guard);
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}
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}
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/// Fill all bytes of `slice` with zeroes (see [`Zeroable`]).
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///
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/// This is similar to `slice.fill(Zeroable::zeroed())`, but guarantees that any
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/// padding bytes in `slice` are zeroed as well.
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///
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/// See also [`write_zeroes`], which zeroes all bytes of a single value rather
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/// than a slice.
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#[inline]
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pub fn fill_zeroes<T: Zeroable>(slice: &mut [T]) {
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if core::mem::needs_drop::<T>() {
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// If `T` needs to be dropped then we have to do this one item at a time, in
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// case one of the intermediate drops does a panic.
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slice.iter_mut().for_each(write_zeroes);
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} else {
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// Otherwise we can be really fast and just fill everthing with zeros.
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let len = core::mem::size_of_val::<[T]>(slice);
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unsafe { core::ptr::write_bytes(slice.as_mut_ptr() as *mut u8, 0u8, len) }
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}
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}
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