Crate bytemuck

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

Depending on the function, the NoUninit and/or AnyBitPattern traits are used to maintain memory safety.

Historical Note: When the crate first started the Pod trait was used instead, and so you may hear people refer to that, but it has the strongest requirements and people eventually wanted the more fine-grained system, so here we are. All types that impl Pod have a blanket impl to also support NoUninit and AnyBitPattern. The traits unfortunately do not have a perfectly clean hierarchy for semver reasons.

Failures

Some casts will never fail, and other casts might fail.

• cast::<u32, f32> always works (and f32::from_bits).
• cast_ref::<[u8; 4], u32> might fail if the specific array reference given at runtime doesn’t have alignment 4.

In addition to the “normal” forms of each function, which will panic on invalid input, there’s also try_ versions which will return a Result.

If you would like to statically ensure that a cast will work at runtime you can use the must_cast crate feature and the must_ casting functions. A “must cast” that can’t be statically known to be valid will cause a compilation error (and sometimes a very hard to read compilation error).

Using Your Own Types

All the functions listed above are guarded by the Pod trait, which is a sub-trait of the Zeroable trait.

If you enable the crate’s derive feature then these traits can be derived on your own types. The derive macros will perform the necessary checks on your type declaration, and trigger an error if your type does not qualify.

The derive macros might not cover all edge cases, and sometimes they will error when actually everything is fine. As a last resort you can impl these traits manually. However, these traits are unsafe, and you should carefully read the requirements before using a manual implementation.

Cargo Features

The crate supports Rust 1.34 when no features are enabled, and so there’s cargo features for thing that you might consider “obvious”.

The cargo features do not promise any particular MSRV, and they may increase their MSRV in new versions.

• derive: Provide derive macros for the various traits.
• extern_crate_alloc: Provide utilities for alloc related types such as Box and Vec.
• zeroable_maybe_uninit and zeroable_atomics: Provide more Zeroable impls.
• wasm_simd and aarch64_simd: Support more SIMD types.
• min_const_generics: Provides appropriate impls for arrays of all lengths instead of just for a select list of array lengths.
• must_cast: Provides the must_ functions, which will compile error if the requested cast can’t be statically verified.

Re-exports

• pub use checked::CheckedBitPattern;
• pub use allocation::*;

Modules

• allocation
  Stuff to boost things in the alloc crate.
• checked
  Checked versions of the casting functions exposed in crate root that support CheckedBitPattern types.

Macros

• offset_of
  Find the offset in bytes of the given $field of $Type. Requires an already initialized $instance value to work with.

Enums

• PodCastError
  The things that can go wrong when casting between Pod data forms.

Traits

• AnyBitPattern
  Marker trait for “plain old data” types that are valid for any bit pattern.
• Contiguous
  A trait indicating that:
• NoUninit
  Marker trait for “plain old data” types with no uninit (or padding) bytes.
• Pod
  Marker trait for “plain old data”.
• PodInOption
  Trait for types which are Pod when wrapped in Option.
• TransparentWrapper
  A trait which indicates that a type is a #[repr(transparent)] wrapper around the Inner value.
• Zeroable
  Trait for types that can be safely created with zeroed.
• ZeroableInOption
  Trait for types which are Zeroable when wrapped in Option.

Functions

• bytes_of
  Re-interprets &T as &[u8].
• bytes_of_mut
  Re-interprets &mut T as &mut [u8].
• cast
  Cast T into U
• cast_mut
  Cast &mut T into &mut U.
• cast_ref
  Cast &T into &U.
• cast_slice
  Cast &[A] into &[B].
• cast_slice_mut
  Cast &mut [T] into &mut [U].
• fill_zeroes
  Fill all bytes of slice with zeroes (see Zeroable).
• from_bytes
  Re-interprets &[u8] as &T.
• from_bytes_mut
  Re-interprets &mut [u8] as &mut T.
• pod_align_to
  As align_to, but safe because of the Pod bound.
• pod_align_to_mut
  As align_to_mut, but safe because of the Pod bound.
• pod_read_unaligned
  Reads the slice into a T value.
• try_cast
  Try to cast T into U.
• try_cast_mut
  Try to convert a &mut T into &mut U.
• try_cast_ref
  Try to convert a &T into &U.
• try_cast_slice
  Try to convert &[A] into &[B] (possibly with a change in length).
• try_cast_slice_mut
  Try to convert &mut [A] into &mut [B] (possibly with a change in length).
• try_from_bytes
  Re-interprets &[u8] as &T.
• try_from_bytes_mut
  Re-interprets &mut [u8] as &mut T.
• try_pod_read_unaligned
  Reads from the bytes as if they were a T.
• write_zeroes
  Fill all bytes of target with zeroes (see Zeroable).