rkyv/

lib.rs

//! # rkyv
//!
//! rkyv (*archive*) is a zero-copy deserialization framework for Rust.
//!
//! It's similar to other zero-copy deserialization frameworks such as
//! [Cap'n Proto](https://capnproto.org) and [FlatBuffers](https://google.github.io/flatbuffers).
//! However, while the former have external schemas and heavily restricted data types, rkyv allows
//! all serialized types to be defined in code and can serialize a wide variety of types that the
//! others cannot. Additionally, rkyv is designed to have little to no overhead, and in most cases
//! will perform exactly the same as native types.
//!
//! ## Design
//!
//! Like [serde](https://serde.rs), rkyv uses Rust's powerful trait system to serialize data without
//! the need for reflection. Despite having a wide array of features, you also only pay for what you
//! use. If your data checks out, the serialization process can be as simple as a `memcpy`! Like
//! serde, this allows rkyv to perform at speeds similar to handwritten serializers.
//!
//! Unlike serde, rkyv produces data that is guaranteed deserialization free. If you wrote your data
//! to disk, you can just `mmap` your file into memory, cast a pointer, and your data is ready to
//! use. This makes it ideal for high-performance and IO-bound applications.
//!
//! Limited data mutation is supported through `Pin` APIs, and archived values can be truly
//! deserialized with [`Deserialize`] if full mutation capabilities are needed.
//!
//! [The book](https://rkyv.org) has more details on the design and capabilities of rkyv.
//!
//! ## Type support
//!
//! rkyv has a hashmap implementation that is built for zero-copy deserialization, so you can
//! serialize your hashmaps with abandon. The implementation performs perfect hashing with the
//! compress, hash and displace algorithm to use as little memory as possible while still performing
//! fast lookups.
//!
//! It also comes with a B+ tree implementation that is built for maximum performance by splitting
//! data into easily-pageable 4KB segments. This makes it perfect for building immutable databases
//! and structures for bulk data.
//!
//! rkyv also has support for contextual serialization, deserialization, and validation. It can
//! properly serialize and deserialize shared pointers like `Rc` and `Arc`, and can be extended to
//! support custom contextual types.
//!
//! Finally, rkyv makes it possible to serialize trait objects and use them *as trait objects*
//! without deserialization. See the `archive_dyn` crate for more details.
//!
//! ## Tradeoffs
//!
//! While rkyv is a great format for final data, it lacks a full schema system and isn't well
//! equipped for data migration and schema upgrades. If your use case requires these capabilities,
//! you may need additional libraries the build these features on top of rkyv. You can use other
//! serialization frameworks like serde with the same types as rkyv conflict-free.
//!
//! ## Features
//!
//! - `alloc`: Enables types that require the `alloc` crate. Enabled by default.
//! - `arbitrary_enum_discriminant`: Enables the `arbitrary_enum_discriminant` feature for stable
//!   multibyte enum discriminants using `archive_le` and `archive_be`. Requires nightly.
//! - `archive_be`: Forces archives into a big-endian format. This guarantees cross-endian
//!   compatibility optimized for big-endian architectures.
//! - `archive_le`: Forces archives into a little-endian format. This guarantees cross-endian
//!   compatibility optimized for little-endian architectures.
//! - `copy`: Enables copy optimizations for packed copyable data types. Requires nightly.
//! - `copy_unsafe`: Automatically opts all potentially copyable types into copy optimization. This
//!   broadly improves performance but may cause uninitialized bytes to be copied to the output.
//!   Requires nightly.
//! - `size_16`: Archives integral `*size` types as 16-bit integers. This is intended to be used
//!   only for small archives and may not handle large, more general data.
//! - `size_32`: Archives integral `*size` types as 32-bit integers. Enabled by default.
//! - `size_64`: Archives integral `*size` types as 64-bit integers. This is intended to be used
//!   only for very large archives and may cause unnecessary data bloat.
//! - `std`: Enables standard library support. Enabled by default.
//! - `strict`: Guarantees that types will have the same representations across platforms and
//!   compilations. This is already the case in practice, but this feature provides a guarantee
//!   along with C type compatibility.
//!
//!   *Note*: Enabling `strict` will disable [`Archive`] implementations for tuples, as tuples
//!   do not have a C type layout. Making a generic `Tuple<T1, T2>` and deriving [`Archive`] for it
//!   should provide similar functionality.
//! - `validation`: Enables validation support through `bytecheck`.
//!
//! ## Crate support
//!
//! Some common crates need to be supported by rkyv before an official integration has been made.
//! Support is provided by rkyv for these crates, but in the future crates should depend on rkyv and
//! provide their own implementations. The crates that already have support provided by rkyv should
//! work toward integrating the implementations into themselves.
//!
//! Crates supported by rkyv:
//!
//! - [`indexmap`](https://docs.rs/indexmap)
//! - [`rend`](https://docs.rs/rend) *Enabled automatically when using endian-specific archive
//!   features.*
//! - [`tinyvec`](https://docs.rs/tinyvec)
//! - [`uuid`](https://docs.rs/uuid)
//!
//! Support for each of these crates can be enabled with a feature of the same name. Additionally,
//! the following external crate features are available:
//!
//! - `alloc` with `tinyvec/alloc`: Supports types behind the `alloc` feature in `tinyvec`.
//! - `std` with `uuid/std`: Enables the `std` feature in `uuid`.
//!
//! ## Examples
//!
//! - See [`Archive`] for examples of how to use rkyv through the derive macro and manual
//!   implementation.
//! - For more details on the derive macro and its capabilities, see
//!   [`Archive`](macro@Archive).
//! - Fully worked examples using rkyv are available in the
//!   [`examples` directory](https://github.com/rkyv/rkyv/tree/master/examples) of the source repo.

#![deny(
    rustdoc::broken_intra_doc_links,
    missing_docs,
    rustdoc::missing_crate_level_docs
)]
#![cfg_attr(not(feature = "std"), no_std)]
#![cfg_attr(
    feature = "copy",
    feature(auto_traits),
    feature(min_specialization),
    feature(negative_impls),
    feature(rustc_attrs)
)]
#![doc(html_favicon_url = r#"
    data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg'
    viewBox='0 0 26.458 26.458'%3E%3Cpath d='M0 0v26.458h26.458V0zm9.175 3.772l8.107 8.106
    2.702-2.702 2.702 13.512-13.512-2.702 2.703-2.702-8.107-8.107z'/%3E%3C/svg%3E
"#)]
#![doc(html_logo_url = r#"
    data:image/svg+xml,%3Csvg xmlns="http://www.w3.org/2000/svg" width="100" height="100"
    viewBox="0 0 26.458 26.458"%3E%3Cpath d="M0 0v26.458h26.458V0zm9.175 3.772l8.107 8.106
    2.702-2.702 2.702 13.512-13.512-2.702 2.703-2.702-8.107-8.107z"/%3E%3C/svg%3E
"#)]
// Only use this feature if you know what you're doing!
#![cfg_attr(feature = "copy", allow(internal_features))]

#[cfg(all(feature = "alloc", not(feature = "std")))]
extern crate alloc;
#[cfg(feature = "std")]
extern crate std;

#[doc(hidden)]
#[macro_use]
pub mod macros;

#[cfg(feature = "bitvec")]
pub mod bitvec;
pub mod boxed;
pub mod collections;
#[cfg(feature = "copy")]
pub mod copy;
pub mod de;
// This is pretty unfortunate. CStr doesn't rely on the rest of std, but it's not in core.
// If CStr ever gets moved into `core` then this module will no longer need cfg(feature = "std")
#[cfg(feature = "std")]
pub mod ffi;
mod impls;
pub mod net;
pub mod niche;
pub mod ops;
pub mod option;
pub mod rc;
pub mod rel_ptr;
pub mod result;
pub mod ser;
pub mod string;
pub mod time;
pub mod util;
#[cfg(feature = "validation")]
pub mod validation;
pub mod vec;
pub mod with;

#[cfg(feature = "rend")]
pub use rend;

#[cfg(feature = "validation")]
#[cfg_attr(doc_cfg, doc(cfg(feature = "validation")))]
pub use bytecheck::{self, CheckBytes};
use core::alloc::Layout;
use ptr_meta::Pointee;
pub use rkyv_derive::{Archive, Deserialize, Serialize};
pub use util::*;
#[cfg(feature = "validation")]
#[cfg_attr(doc_cfg, doc(cfg(feature = "validation")))]
pub use validation::{
    check_archived_root_with_context, check_archived_value_with_context,
    validators::{check_archived_root, check_archived_value, from_bytes},
};

/// A type that can produce an error.
///
/// This trait is always implemented by serializers and deserializers. Its purpose is to provide an
/// error type without restricting what other capabilities the type must provide.
///
/// When writing implementations for [`Serialize`] and [`Deserialize`], it's best practice to bound
/// the serializer or deserializer by `Fallible` and then require that the serialized types support
/// it (i.e. `S: Fallible, MyType: Serialize<S>`).
pub trait Fallible {
    /// The error produced by any failing methods.
    type Error: 'static;
}

/// A fallible type that cannot produce errors.
///
/// This type can be used to serialize and deserialize types that cannot fail to serialize or
/// deserialize.
#[derive(Debug)]
pub struct Infallible;

impl Fallible for Infallible {
    type Error = core::convert::Infallible;
}

impl Default for Infallible {
    fn default() -> Self {
        Infallible
    }
}

/// A type that can be used without deserializing.
///
/// `Archive` is one of three basic traits used to work with zero-copy data and controls the layout
/// of the data in its archived zero-copy representation. The [`Serialize`] trait helps transform
/// types into that representation, and the [`Deserialize`] trait helps transform types back out.
///
/// Types that implement `Archive` must have a well-defined archived size. Unsized types can be
/// supported using the [`ArchiveUnsized`] trait, along with [`SerializeUnsized`] and
/// [`DeserializeUnsized`].
///
/// Archiving is done depth-first, writing any data owned by a type before writing the data for the
/// type itself. The type must be able to create the archived type from only its own data and its
/// resolver.
///
/// Archived data is always treated as if it is tree-shaped, with the root owning its direct
/// descendents and so on. Data that is not tree-shaped can be supported using special serializer
/// and deserializer bounds (see [`ArchivedRc`](crate::rc::ArchivedRc) for example). In a buffer of
/// serialized data, objects are laid out in *reverse order*. This means that the root object is
/// located near the end of the buffer and leaf objects are located near the beginning.
///
/// # Examples
///
/// Most of the time, `#[derive(Archive)]` will create an acceptable implementation. You can use the
/// `#[archive(...)]` and `#[archive_attr(...)]` attributes to control how the implementation is
/// generated. See the [`Archive`](macro@Archive) derive macro for more details.
///
/// ```
/// use rkyv::{Archive, Deserialize, Serialize};
///
/// #[derive(Archive, Deserialize, Serialize, Debug, PartialEq)]
/// // This will generate a PartialEq impl between our unarchived and archived types
/// #[archive(compare(PartialEq))]
/// // We can pass attributes through to generated types with archive_attr
/// #[archive_attr(derive(Debug))]
/// struct Test {
///     int: u8,
///     string: String,
///     option: Option<Vec<i32>>,
/// }
///
/// let value = Test {
///     int: 42,
///     string: "hello world".to_string(),
///     option: Some(vec![1, 2, 3, 4]),
/// };
///
/// // Serializing is as easy as a single function call
/// let bytes = rkyv::to_bytes::<_, 256>(&value).unwrap();
///
/// // Or you can customize your serialization for better performance
/// // and compatibility with #![no_std] environments
/// use rkyv::ser::{Serializer, serializers::AllocSerializer};
///
/// let mut serializer = AllocSerializer::<0>::default();
/// serializer.serialize_value(&value).unwrap();
/// let bytes = serializer.into_serializer().into_inner();
///
/// // You can use the safe API with the validation feature turned on,
/// // or you can use the unsafe API (shown here) for maximum performance
/// let archived = unsafe { rkyv::archived_root::<Test>(&bytes[..]) };
/// assert_eq!(archived, &value);
///
/// // And you can always deserialize back to the original type
/// let deserialized: Test = archived.deserialize(&mut rkyv::Infallible).unwrap();
/// assert_eq!(deserialized, value);
/// ```
///
/// _Note: the safe API requires the `validation` feature._
///
/// Many of the core and standard library types already have `Archive` implementations available,
/// but you may need to implement `Archive` for your own types in some cases the derive macro cannot
/// handle.
///
/// In this example, we add our own wrapper that serializes a `&'static str` as if it's owned.
/// Normally you can lean on the archived version of `String` to do most of the work, or use the
/// [`Inline`](crate::with::Inline) to do exactly this. This example does everything to demonstrate
/// how to implement `Archive` for your own types.
///
/// ```
/// use core::{slice, str};
/// use rkyv::{
///     archived_root,
///     ser::{Serializer, serializers::AlignedSerializer},
///     out_field,
///     AlignedVec,
///     Archive,
///     Archived,
///     ArchiveUnsized,
///     MetadataResolver,
///     RelPtr,
///     Serialize,
///     SerializeUnsized,
/// };
///
/// struct OwnedStr {
///     inner: &'static str,
/// }
///
/// struct ArchivedOwnedStr {
///     // This will be a relative pointer to our string
///     ptr: RelPtr<str>,
/// }
///
/// impl ArchivedOwnedStr {
///     // This will help us get the bytes of our type as a str again.
///     fn as_str(&self) -> &str {
///         unsafe {
///             // The as_ptr() function of RelPtr will get a pointer the str
///             &*self.ptr.as_ptr()
///         }
///     }
/// }
///
/// struct OwnedStrResolver {
///     // This will be the position that the bytes of our string are stored at.
///     // We'll use this to resolve the relative pointer of our
///     // ArchivedOwnedStr.
///     pos: usize,
///     // The archived metadata for our str may also need a resolver.
///     metadata_resolver: MetadataResolver<str>,
/// }
///
/// // The Archive implementation defines the archived version of our type and
/// // determines how to turn the resolver into the archived form. The Serialize
/// // implementations determine how to make a resolver from the original value.
/// impl Archive for OwnedStr {
///     type Archived = ArchivedOwnedStr;
///     // This is the resolver we can create our Archived version from.
///     type Resolver = OwnedStrResolver;
///
///     // The resolve function consumes the resolver and produces the archived
///     // value at the given position.
///     unsafe fn resolve(
///         &self,
///         pos: usize,
///         resolver: Self::Resolver,
///         out: *mut Self::Archived,
///     ) {
///         // We have to be careful to add the offset of the ptr field,
///         // otherwise we'll be using the position of the ArchivedOwnedStr
///         // instead of the position of the relative pointer.
///         let (fp, fo) = out_field!(out.ptr);
///         self.inner.resolve_unsized(
///             pos + fp,
///             resolver.pos,
///             resolver.metadata_resolver,
///             fo,
///         );
///     }
/// }
///
/// // We restrict our serializer types with Serializer because we need its
/// // capabilities to archive our type. For other types, we might need more or
/// // less restrictive bounds on the type of S.
/// impl<S: Serializer + ?Sized> Serialize<S> for OwnedStr {
///     fn serialize(
///         &self,
///         serializer: &mut S
///     ) -> Result<Self::Resolver, S::Error> {
///         // This is where we want to write the bytes of our string and return
///         // a resolver that knows where those bytes were written.
///         // We also need to serialize the metadata for our str.
///         Ok(OwnedStrResolver {
///             pos: self.inner.serialize_unsized(serializer)?,
///             metadata_resolver: self.inner.serialize_metadata(serializer)?
///         })
///     }
/// }
///
/// let mut serializer = AlignedSerializer::new(AlignedVec::new());
/// const STR_VAL: &'static str = "I'm in an OwnedStr!";
/// let value = OwnedStr { inner: STR_VAL };
/// // It works!
/// serializer.serialize_value(&value).expect("failed to archive test");
/// let buf = serializer.into_inner();
/// let archived = unsafe { archived_root::<OwnedStr>(buf.as_ref()) };
/// // Let's make sure our data got written correctly
/// assert_eq!(archived.as_str(), STR_VAL);
/// ```
pub trait Archive {
    /// The archived representation of this type.
    ///
    /// In this form, the data can be used with zero-copy deserialization.
    type Archived;

    /// The resolver for this type. It must contain all the additional information from serializing
    /// needed to make the archived type from the normal type.
    type Resolver;

    /// Creates the archived version of this value at the given position and writes it to the given
    /// output.
    ///
    /// The output should be initialized field-by-field rather than by writing a whole struct.
    /// Performing a typed copy will mark all of the padding bytes as uninitialized, but they must
    /// remain set to the value they currently have. This prevents leaking uninitialized memory to
    /// the final archive.
    ///
    /// # Safety
    ///
    /// - `pos` must be the position of `out` within the archive
    /// - `resolver` must be the result of serializing this object
    unsafe fn resolve(&self, pos: usize, resolver: Self::Resolver, out: *mut Self::Archived);
}

/// Converts a type to its archived form.
///
/// Objects perform any supportive serialization during [`serialize`](Serialize::serialize). For
/// types that reference nonlocal (pointed-to) data, this is when that data must be serialized to
/// the output. These types will need to bound `S` to implement [`Serializer`](ser::Serializer) and
/// any other required traits (e.g. [`SharedSerializeRegistry`](ser::SharedSerializeRegistry)). They
/// should then serialize their dependencies during `serialize`.
///
/// See [`Archive`] for examples of implementing `Serialize`.
pub trait Serialize<S: Fallible + ?Sized>: Archive {
    /// Writes the dependencies for the object and returns a resolver that can create the archived
    /// type.
    fn serialize(&self, serializer: &mut S) -> Result<Self::Resolver, S::Error>;
}

/// Converts a type back from its archived form.
///
/// Some types may require specific deserializer capabilities, such as `Rc` and `Arc`. In these
/// cases, the deserializer type `D` should be bound so that it implements traits that provide those
/// capabilities (e.g. [`SharedDeserializeRegistry`](de::SharedDeserializeRegistry)).
///
/// This can be derived with [`Deserialize`](macro@Deserialize).
pub trait Deserialize<T, D: Fallible + ?Sized> {
    /// Deserializes using the given deserializer
    fn deserialize(&self, deserializer: &mut D) -> Result<T, D::Error>;
}

/// A counterpart of [`Archive`] that's suitable for unsized types.
///
/// Unlike `Archive`, types that implement `ArchiveUnsized` must be serialized separately from their
/// owning object. For example, whereas an `i32` might be laid out as part of a larger struct, a
/// `Box<i32>` would serialize the `i32` somewhere in the archive and the `Box` would point to it as
/// part of the larger struct. Because of this, the equivalent [`Resolver`](Archive::Resolver) type
/// for `ArchiveUnsized` is always a `usize` representing the position of the serialized value.
///
/// `ArchiveUnsized` is automatically implemented for all types that implement [`Archive`]. Nothing
/// special needs to be done to use them with types like `Box`, `Rc`, and `Arc`. It is also already
/// implemented for slices and string slices, and the `rkyv_dyn` crate can be used to archive trait
/// objects. Other unsized types must manually implement `ArchiveUnsized`.
///
/// # Examples
///
/// This example shows how to manually implement `ArchiveUnsized` for an unsized type. Special care
/// must be taken to ensure that the types are laid out correctly.
///
/// ```
/// use core::{mem::transmute, ops::{Deref, DerefMut}};
/// use ptr_meta::Pointee;
/// use rkyv::{
///     from_archived,
///     to_archived,
///     archived_unsized_value,
///     ser::{serializers::AlignedSerializer, Serializer},
///     AlignedVec,
///     Archive,
///     Archived,
///     ArchivedMetadata,
///     ArchivePointee,
///     ArchiveUnsized,
///     FixedUsize,
///     RelPtr,
///     Serialize,
///     SerializeUnsized,
/// };
///
/// // We're going to be dealing mostly with blocks that have a trailing slice
/// pub struct Block<H, T: ?Sized> {
///     head: H,
///     tail: T,
/// }
///
/// impl<H, T> Pointee for Block<H, [T]> {
///     type Metadata = usize;
/// }
///
/// // For blocks with trailing slices, we need to store the length of the slice
/// // in the metadata.
/// pub struct BlockSliceMetadata {
///     len: Archived<usize>,
/// }
///
/// // ArchivePointee is automatically derived for sized types because pointers
/// // to sized types don't need to store any extra information. Because we're
/// // making an unsized block, we need to define what metadata gets stored with
/// // our data pointer.
/// impl<H, T> ArchivePointee for Block<H, [T]> {
///     // This is the extra data that needs to get stored for blocks with
///     // trailing slices
///     type ArchivedMetadata = BlockSliceMetadata;
///
///     // We need to be able to turn our archived metadata into regular
///     // metadata for our type
///     fn pointer_metadata(
///         archived: &Self::ArchivedMetadata
///     ) -> <Self as Pointee>::Metadata {
///         from_archived!(archived.len) as usize
///     }
/// }
///
/// // We're implementing ArchiveUnsized for just Block<H, [T]>. We can still
/// // implement Archive for blocks with sized tails and they won't conflict.
/// impl<H: Archive, T: Archive> ArchiveUnsized for Block<H, [T]> {
///     // We'll reuse our block type as our archived type.
///     type Archived = Block<Archived<H>, [Archived<T>]>;
///
///     // This is where we'd put any resolve data for our metadata.
///     // Most of the time, this can just be () because most metadata is Copy,
///     // but the option is there if you need it.
///     type MetadataResolver = ();
///
///     // Here's where we make the metadata for our pointer.
///     // This also gets the position and resolver for the metadata, but we
///     // don't need it in this case.
///     unsafe fn resolve_metadata(
///         &self,
///         _: usize,
///         _: Self::MetadataResolver,
///         out: *mut ArchivedMetadata<Self>,
///     ) {
///         unsafe {
///             out.write(BlockSliceMetadata {
///                 len: to_archived!(self.tail.len() as FixedUsize),
///             });
///         }
///     }
/// }
///
/// // The bounds we use on our serializer type indicate that we need basic
/// // serializer capabilities, and then whatever capabilities our head and tail
/// // types need to serialize themselves.
/// impl<
///     H: Serialize<S>,
///     T: Serialize<S>,
///     S: Serializer + ?Sized
/// > SerializeUnsized<S> for Block<H, [T]> {
///     // This is where we construct our unsized type in the serializer
///     fn serialize_unsized(
///         &self,
///         serializer: &mut S
///     ) -> Result<usize, S::Error> {
///         // First, we archive the head and all the tails. This will make sure
///         // that when we finally build our block, we don't accidentally mess
///         // up the structure with serialized dependencies.
///         let head_resolver = self.head.serialize(serializer)?;
///         let mut resolvers = Vec::new();
///         for tail in self.tail.iter() {
///             resolvers.push(tail.serialize(serializer)?);
///         }
///         // Now we align our serializer for our archived type and write it.
///         // We can't align for unsized types so we treat the trailing slice
///         // like an array of 0 length for now.
///         serializer.align_for::<Block<Archived<H>, [Archived<T>; 0]>>()?;
///         let result = unsafe {
///             serializer.resolve_aligned(&self.head, head_resolver)?
///         };
///         serializer.align_for::<Archived<T>>()?;
///         for (item, resolver) in self.tail.iter().zip(resolvers.drain(..)) {
///             unsafe {
///                 serializer.resolve_aligned(item, resolver)?;
///             }
///         }
///         Ok(result)
///     }
///
///     // This is where we serialize the metadata for our type. In this case,
///     // we do all the work in resolve and don't need to do anything here.
///     fn serialize_metadata(
///         &self,
///         serializer: &mut S
///     ) -> Result<Self::MetadataResolver, S::Error> {
///         Ok(())
///     }
/// }
///
/// let value = Block {
///     head: "Numbers 1-4".to_string(),
///     tail: [1, 2, 3, 4],
/// };
/// // We have a Block<String, [i32; 4]> but we want to it to be a
/// // Block<String, [i32]>, so we need to do more pointer transmutation
/// let ptr = (&value as *const Block<String, [i32; 4]>).cast::<()>();
/// let unsized_value = unsafe {
///     &*transmute::<(*const (), usize), *const Block<String, [i32]>>((ptr, 4))
/// };
///
/// let mut serializer = AlignedSerializer::new(AlignedVec::new());
/// let pos = serializer.serialize_unsized_value(unsized_value)
///     .expect("failed to archive block");
/// let buf = serializer.into_inner();
///
/// let archived_ref = unsafe {
///     archived_unsized_value::<Block<String, [i32]>>(buf.as_slice(), pos)
/// };
/// assert_eq!(archived_ref.head, "Numbers 1-4");
/// assert_eq!(archived_ref.tail.len(), 4);
/// assert_eq!(archived_ref.tail, [1, 2, 3, 4]);
/// ```
pub trait ArchiveUnsized: Pointee {
    /// The archived counterpart of this type. Unlike `Archive`, it may be unsized.
    ///
    /// This type must implement [`ArchivePointee`], a trait that helps make valid pointers using
    /// archived pointer metadata.
    type Archived: ArchivePointee + ?Sized;

    /// The resolver for the metadata of this type.
    ///
    /// Because the pointer metadata must be archived with the relative pointer and not with the
    /// structure itself, its resolver must be passed back to the structure holding the pointer.
    type MetadataResolver;

    /// Creates the archived version of the metadata for this value at the given position and writes
    /// it to the given output.
    ///
    /// The output should be initialized field-by-field rather than by writing a whole struct.
    /// Performing a typed copy will mark all of the padding bytes as uninitialized, but they must
    /// remain set to the value they currently have. This prevents leaking uninitialized memory to
    /// the final archive.
    ///
    /// # Safety
    ///
    /// - `pos` must be the position of `out` within the archive
    /// - `resolver` must be the result of serializing this object's metadata
    unsafe fn resolve_metadata(
        &self,
        pos: usize,
        resolver: Self::MetadataResolver,
        out: *mut ArchivedMetadata<Self>,
    );

    /// Resolves a relative pointer to this value with the given `from` and `to` and writes it to
    /// the given output.
    ///
    /// The output should be initialized field-by-field rather than by writing a whole struct.
    /// Performing a typed copy will mark all of the padding bytes as uninitialized, but they must
    /// remain set to the value they currently have. This prevents leaking uninitialized memory to
    /// the final archive.
    ///
    /// # Safety
    ///
    /// - `from` must be the position of `out` within the archive
    /// - `to` must be the position of some `Self::Archived` within the archive
    /// - `resolver` must be the result of serializing this object
    #[inline]
    unsafe fn resolve_unsized(
        &self,
        from: usize,
        to: usize,
        resolver: Self::MetadataResolver,
        out: *mut RelPtr<Self::Archived>,
    ) {
        RelPtr::resolve_emplace(from, to, self, resolver, out);
    }
}

/// An archived type with associated metadata for its relative pointer.
///
/// This is mostly used in the context of smart pointers and unsized types, and is implemented for
/// all sized types by default.
pub trait ArchivePointee: Pointee {
    /// The archived version of the pointer metadata for this type.
    type ArchivedMetadata;

    /// Converts some archived metadata to the pointer metadata for itself.
    fn pointer_metadata(archived: &Self::ArchivedMetadata) -> <Self as Pointee>::Metadata;
}

/// A counterpart of [`Serialize`] that's suitable for unsized types.
///
/// See [`ArchiveUnsized`] for examples of implementing `SerializeUnsized`.
pub trait SerializeUnsized<S: Fallible + ?Sized>: ArchiveUnsized {
    /// Writes the object and returns the position of the archived type.
    fn serialize_unsized(&self, serializer: &mut S) -> Result<usize, S::Error>;

    /// Serializes the metadata for the given type.
    fn serialize_metadata(&self, serializer: &mut S) -> Result<Self::MetadataResolver, S::Error>;
}

/// A counterpart of [`Deserialize`] that's suitable for unsized types.
pub trait DeserializeUnsized<T: Pointee + ?Sized, D: Fallible + ?Sized>: ArchivePointee {
    /// Deserializes a reference to the given value.
    ///
    /// # Safety
    ///
    /// `out` must point to memory with the layout returned by `deserialized_layout`.
    unsafe fn deserialize_unsized(
        &self,
        deserializer: &mut D,
        alloc: impl FnMut(Layout) -> *mut u8,
    ) -> Result<*mut (), D::Error>;

    /// Deserializes the metadata for the given type.
    fn deserialize_metadata(&self, deserializer: &mut D) -> Result<T::Metadata, D::Error>;
}

/// The native type that `usize` is converted to for archiving.
///
/// This will be `u16`, `u32`, or `u64` when the `size_16`, `size_32`, or `size_64` features are
/// enabled, respectively.
pub type FixedUsize = pick_size_type!(u16, u32, u64);
/// The native type that `isize` is converted to for archiving.
///
/// This will be `i16`, `i32`, or `i64` when the `size_16`, `size_32`, or `size_64` features are
/// enabled, respectively.
pub type FixedIsize = pick_size_type!(i16, i32, i64);

/// The default raw relative pointer.
///
/// This will use an archived [`FixedIsize`] to hold the offset.
pub type RawRelPtr = rel_ptr::RawRelPtr<Archived<isize>>;
/// The default relative pointer.
///
/// This will use an archived [`FixedIsize`] to hold the offset.
pub type RelPtr<T> = rel_ptr::RelPtr<T, Archived<isize>>;

/// Alias for the archived version of some [`Archive`] type.
///
/// This can be useful for reducing the lengths of type definitions.
pub type Archived<T> = <T as Archive>::Archived;
/// Alias for the resolver for some [`Archive`] type.
///
/// This can be useful for reducing the lengths of type definitions.
pub type Resolver<T> = <T as Archive>::Resolver;
/// Alias for the archived metadata for some [`ArchiveUnsized`] type.
///
/// This can be useful for reducing the lengths of type definitions.
pub type ArchivedMetadata<T> =
    <<T as ArchiveUnsized>::Archived as ArchivePointee>::ArchivedMetadata;
/// Alias for the metadata resolver for some [`ArchiveUnsized`] type.
///
/// This can be useful for reducing the lengths of type definitions.
pub type MetadataResolver<T> = <T as ArchiveUnsized>::MetadataResolver;