# zerocopy
*<span style="font-size: 100%; color:grey;">Need more out of zerocopy?
Submit a [customer request issue][customer-request-issue]!</span>*
***<span style="font-size: 140%">Fast, safe, <span
style="color:red;">compile error</span>. Pick two.</span>***
Zerocopy makes zero-cost memory manipulation effortless. We write `unsafe`
so you don't have to.
*Thanks for using zerocopy 0.8! For an overview of what changes from 0.7,
check out our [release notes][release-notes], which include a step-by-step
guide for upgrading from 0.7.*
*Have questions? Need help? Ask the maintainers on [GitHub][github-q-a] or
on [Discord][discord]!*
[customer-request-issue]: https://github.com/google/zerocopy/issues/new/choose
[release-notes]: https://github.com/google/zerocopy/discussions/1680
[github-q-a]: https://github.com/google/zerocopy/discussions/categories/q-a
[discord]: https://discord.gg/MAvWH2R6zk
## Overview
###### Conversion Traits
Zerocopy provides four derivable traits for zero-cost conversions:
- `TryFromBytes` indicates that a type may safely be converted from
certain byte sequences (conditional on runtime checks)
- `FromZeros` indicates that a sequence of zero bytes represents a valid
instance of a type
- `FromBytes` indicates that a type may safely be converted from an
arbitrary byte sequence
- `IntoBytes` indicates that a type may safely be converted *to* a byte
sequence
These traits support sized types, slices, and [slice DSTs][slice-dsts].
[slice-dsts]: KnownLayout#dynamically-sized-types
###### Marker Traits
Zerocopy provides three derivable marker traits that do not provide any
functionality themselves, but are required to call certain methods provided
by the conversion traits:
- `KnownLayout` indicates that zerocopy can reason about certain layout
qualities of a type
- `Immutable` indicates that a type is free from interior mutability,
except by ownership or an exclusive (`&mut`) borrow
- `Unaligned` indicates that a type's alignment requirement is 1
You should generally derive these marker traits whenever possible.
###### Conversion Macros
Zerocopy provides six macros for safe casting between types:
- (`try_`[try_transmute])`transmute` (conditionally) converts a value of
one type to a value of another type of the same size
- (`try_`[try_transmute_mut])`transmute_mut` (conditionally) converts a
mutable reference of one type to a mutable reference of another type of
the same size
- (`try_`[try_transmute_ref])`transmute_ref` (conditionally) converts a
mutable or immutable reference of one type to an immutable reference of
another type of the same size
These macros perform *compile-time* size and alignment checks, meaning that
unconditional casts have zero cost at runtime. Conditional casts do not need
to validate size or alignment runtime, but do need to validate contents.
These macros cannot be used in generic contexts. For generic conversions,
use the methods defined by the [conversion traits](#conversion-traits).
###### Byteorder-Aware Numerics
Zerocopy provides byte-order aware integer types that support these
conversions; see the `byteorder` module. These types are especially useful
for network parsing.
## Cargo Features
- **`alloc`**
By default, `zerocopy` is `no_std`. When the `alloc` feature is enabled,
the `alloc` crate is added as a dependency, and some allocation-related
functionality is added.
- **`std`**
By default, `zerocopy` is `no_std`. When the `std` feature is enabled, the
`std` crate is added as a dependency (ie, `no_std` is disabled), and
support for some `std` types is added. `std` implies `alloc`.
- **`derive`**
Provides derives for the core marker traits via the `zerocopy-derive`
crate. These derives are re-exported from `zerocopy`, so it is not
necessary to depend on `zerocopy-derive` directly.
However, you may experience better compile times if you instead directly
depend on both `zerocopy` and `zerocopy-derive` in your `Cargo.toml`,
since doing so will allow Rust to compile these crates in parallel. To do
so, do *not* enable the `derive` feature, and list both dependencies in
your `Cargo.toml` with the same leading non-zero version number; e.g:
```toml
[dependencies]
zerocopy = "0.X"
zerocopy-derive = "0.X"
```
To avoid the risk of [duplicate import errors][duplicate-import-errors] if
one of your dependencies enables zerocopy's `derive` feature, import
derives as `use zerocopy_derive::*` rather than by name (e.g., `use
zerocopy_derive::FromBytes`).
- **`simd`**
When the `simd` feature is enabled, `FromZeros`, `FromBytes`, and
`IntoBytes` impls are emitted for all stable SIMD types which exist on the
target platform. Note that the layout of SIMD types is not yet stabilized,
so these impls may be removed in the future if layout changes make them
invalid. For more information, see the Unsafe Code Guidelines Reference
page on the [layout of packed SIMD vectors][simd-layout].
- **`simd-nightly`**
Enables the `simd` feature and adds support for SIMD types which are only
available on nightly. Since these types are unstable, support for any type
may be removed at any point in the future.
[duplicate-import-errors]: https://github.com/google/zerocopy/issues/1587
[simd-layout]: https://rust-lang.github.io/unsafe-code-guidelines/layout/packed-simd-vectors.html
## Security Ethos
Zerocopy is expressly designed for use in security-critical contexts. We
strive to ensure that that zerocopy code is sound under Rust's current
memory model, and *any future memory model*. We ensure this by:
- **...not 'guessing' about Rust's semantics.**
We annotate `unsafe` code with a precise rationale for its soundness that
cites a relevant section of Rust's official documentation. When Rust's
documented semantics are unclear, we work with the Rust Operational
Semantics Team to clarify Rust's documentation.
- **...rigorously testing our implementation.**
We run tests using [Miri], ensuring that zerocopy is sound across a wide
array of supported target platforms of varying endianness and pointer
width, and across both current and experimental memory models of Rust.
- **...formally proving the correctness of our implementation.**
We apply formal verification tools like [Kani][kani] to prove zerocopy's
correctness.
For more information, see our full [soundness policy].
[Miri]: https://github.com/rust-lang/miri
[Kani]: https://github.com/model-checking/kani
[soundness policy]: https://github.com/google/zerocopy/blob/main/POLICIES.md#soundness
## Relationship to Project Safe Transmute
[Project Safe Transmute] is an official initiative of the Rust Project to
develop language-level support for safer transmutation. The Project consults
with crates like zerocopy to identify aspects of safer transmutation that
would benefit from compiler support, and has developed an [experimental,
compiler-supported analysis][mcp-transmutability] which determines whether,
for a given type, any value of that type may be soundly transmuted into
another type. Once this functionality is sufficiently mature, zerocopy
intends to replace its internal transmutability analysis (implemented by our
custom derives) with the compiler-supported one. This change will likely be
an implementation detail that is invisible to zerocopy's users.
Project Safe Transmute will not replace the need for most of zerocopy's
higher-level abstractions. The experimental compiler analysis is a tool for
checking the soundness of `unsafe` code, not a tool to avoid writing
`unsafe` code altogether. For the foreseeable future, crates like zerocopy
will still be required in order to provide higher-level abstractions on top
of the building block provided by Project Safe Transmute.
[Project Safe Transmute]: https://rust-lang.github.io/rfcs/2835-project-safe-transmute.html
[mcp-transmutability]: https://github.com/rust-lang/compiler-team/issues/411
## MSRV
See our [MSRV policy].
[MSRV policy]: https://github.com/google/zerocopy/blob/main/POLICIES.md#msrv
## Changelog
Zerocopy uses [GitHub Releases].
[GitHub Releases]: https://github.com/google/zerocopy/releases
## Disclaimer
Disclaimer: Zerocopy is not an officially supported Google product.