Source code
Revision control
Copy as Markdown
Other Tools
// This file is part of ICU4X. For terms of use, please see the file
// called LICENSE at the top level of the ICU4X source tree
#![allow(clippy::upper_case_acronyms)]
//! Traits over unaligned little-endian data (ULE, pronounced "yule").
//!
//! The main traits for this module are [`ULE`], [`AsULE`] and, [`VarULE`].
//!
//! See [the design doc](https://github.com/unicode-org/icu4x/blob/main/utils/zerovec/design_doc.md) for details on how these traits
//! works under the hood.
mod chars;
#[cfg(doc)]
pub mod custom;
mod encode;
mod macros;
mod multi;
mod niche;
mod option;
mod plain;
mod slices;
mod unvalidated;
pub mod tuple;
pub use super::ZeroVecError;
pub use chars::CharULE;
pub use encode::{encode_varule_to_box, EncodeAsVarULE};
pub use multi::MultiFieldsULE;
pub use niche::{NicheBytes, NichedOption, NichedOptionULE};
pub use option::{OptionULE, OptionVarULE};
pub use plain::RawBytesULE;
pub use unvalidated::{UnvalidatedChar, UnvalidatedStr};
use alloc::alloc::Layout;
use alloc::borrow::ToOwned;
use alloc::boxed::Box;
use core::{mem, slice};
/// Fixed-width, byte-aligned data that can be cast to and from a little-endian byte slice.
///
/// If you need to implement this trait, consider using [`#[make_ule]`](crate::make_ule) or
/// [`#[derive(ULE)]`](macro@ULE) instead.
///
/// Types that are not fixed-width can implement [`VarULE`] instead.
///
/// "ULE" stands for "Unaligned little-endian"
///
/// # Safety
///
/// Safety checklist for `ULE`:
///
/// 1. The type *must not* include any uninitialized or padding bytes.
/// 2. The type must have an alignment of 1 byte.
/// 3. The impl of [`ULE::validate_byte_slice()`] *must* return an error if the given byte slice
/// would not represent a valid slice of this type.
/// 4. The impl of [`ULE::validate_byte_slice()`] *must* return an error if the given byte slice
/// cannot be used in its entirety (if its length is not a multiple of `size_of::<Self>()`).
/// 5. All other methods *must* be left with their default impl, or else implemented according to
/// their respective safety guidelines.
/// 6. Acknowledge the following note about the equality invariant.
///
/// If the ULE type is a struct only containing other ULE types (or other types which satisfy invariants 1 and 2,
/// like `[u8; N]`), invariants 1 and 2 can be achieved via `#[repr(C, packed)]` or `#[repr(transparent)]`.
///
/// # Equality invariant
///
/// A non-safety invariant is that if `Self` implements `PartialEq`, the it *must* be logically
/// equivalent to byte equality on [`Self::as_byte_slice()`].
///
/// It may be necessary to introduce a "canonical form" of the ULE if logical equality does not
/// equal byte equality. In such a case, [`Self::validate_byte_slice()`] should return an error
/// for any values that are not in canonical form. For example, the decimal strings "1.23e4" and
/// "12.3e3" are logically equal, but not byte-for-byte equal, so we could define a canonical form
/// where only a single digit is allowed before `.`.
///
/// Failure to follow this invariant will cause surprising behavior in `PartialEq`, which may
/// result in unpredictable operations on `ZeroVec`, `VarZeroVec`, and `ZeroMap`.
pub unsafe trait ULE
where
Self: Sized,
Self: Copy + 'static,
{
/// Validates a byte slice, `&[u8]`.
///
/// If `Self` is not well-defined for all possible bit values, the bytes should be validated.
/// If the bytes can be transmuted, *in their entirety*, to a valid slice of `Self`, then `Ok`
/// should be returned; otherwise, `Self::Error` should be returned.
fn validate_byte_slice(bytes: &[u8]) -> Result<(), ZeroVecError>;
/// Parses a byte slice, `&[u8]`, and return it as `&[Self]` with the same lifetime.
///
/// If `Self` is not well-defined for all possible bit values, the bytes should be validated,
/// and an error should be returned in the same cases as [`Self::validate_byte_slice()`].
///
/// The default implementation executes [`Self::validate_byte_slice()`] followed by
/// [`Self::from_byte_slice_unchecked`].
///
/// Note: The following equality should hold: `bytes.len() % size_of::<Self>() == 0`. This
/// means that the returned slice can span the entire byte slice.
fn parse_byte_slice(bytes: &[u8]) -> Result<&[Self], ZeroVecError> {
Self::validate_byte_slice(bytes)?;
debug_assert_eq!(bytes.len() % mem::size_of::<Self>(), 0);
Ok(unsafe { Self::from_byte_slice_unchecked(bytes) })
}
/// Takes a byte slice, `&[u8]`, and return it as `&[Self]` with the same lifetime, assuming
/// that this byte slice has previously been run through [`Self::parse_byte_slice()`] with
/// success.
///
/// The default implementation performs a pointer cast to the same region of memory.
///
/// # Safety
///
/// ## Callers
///
/// Callers of this method must take care to ensure that `bytes` was previously passed through
/// [`Self::validate_byte_slice()`] with success (and was not changed since then).
///
/// ## Implementors
///
/// Implementations of this method may call unsafe functions to cast the pointer to the correct
/// type, assuming the "Callers" invariant above.
///
/// Keep in mind that `&[Self]` and `&[u8]` may have different lengths.
///
/// Safety checklist:
///
/// 1. This method *must* return the same result as [`Self::parse_byte_slice()`].
/// 2. This method *must* return a slice to the same region of memory as the argument.
#[inline]
unsafe fn from_byte_slice_unchecked(bytes: &[u8]) -> &[Self] {
let data = bytes.as_ptr();
let len = bytes.len() / mem::size_of::<Self>();
debug_assert_eq!(bytes.len() % mem::size_of::<Self>(), 0);
core::slice::from_raw_parts(data as *const Self, len)
}
/// Given `&[Self]`, returns a `&[u8]` with the same lifetime.
///
/// The default implementation performs a pointer cast to the same region of memory.
///
/// # Safety
///
/// Implementations of this method should call potentially unsafe functions to cast the
/// pointer to the correct type.
///
/// Keep in mind that `&[Self]` and `&[u8]` may have different lengths.
#[inline]
fn as_byte_slice(slice: &[Self]) -> &[u8] {
unsafe {
slice::from_raw_parts(slice as *const [Self] as *const u8, mem::size_of_val(slice))
}
}
}
/// A trait for any type that has a 1:1 mapping with an unaligned little-endian (ULE) type.
///
/// If you need to implement this trait, consider using [`#[make_ule]`](crate::make_ule) instead.
pub trait AsULE: Copy {
/// The ULE type corresponding to `Self`.
///
/// Types having infallible conversions from all bit values (Plain Old Data) can use
/// `RawBytesULE` with the desired width; for example, `u32` uses `RawBytesULE<4>`.
///
/// Types that are not well-defined for all bit values should implement a custom ULE.
type ULE: ULE;
/// Converts from `Self` to `Self::ULE`.
///
/// This function may involve byte order swapping (native-endian to little-endian).
///
/// For best performance, mark your implementation of this function `#[inline]`.
fn to_unaligned(self) -> Self::ULE;
/// Converts from `Self::ULE` to `Self`.
///
/// This function may involve byte order swapping (little-endian to native-endian).
///
/// For best performance, mark your implementation of this function `#[inline]`.
///
/// # Safety
///
/// This function is infallible because bit validation should have occurred when `Self::ULE`
/// was first constructed. An implementation may therefore involve an `unsafe{}` block, like
/// `from_bytes_unchecked()`.
fn from_unaligned(unaligned: Self::ULE) -> Self;
}
/// An [`EqULE`] type is one whose byte sequence equals the byte sequence of its ULE type on
/// little-endian platforms. This enables certain performance optimizations, such as
/// [`ZeroVec::try_from_slice`](crate::ZeroVec::try_from_slice).
///
/// # Implementation safety
///
/// This trait is safe to implement if the type's ULE (as defined by `impl `[`AsULE`]` for T`)
/// has an equal byte sequence as the type itself on little-endian platforms; i.e., one where
/// `*const T` can be cast to a valid `*const T::ULE`.
pub unsafe trait EqULE: AsULE {}
/// A trait for a type where aligned slices can be cast to unaligned slices.
///
/// Auto-implemented on all types implementing [`EqULE`].
pub trait SliceAsULE
where
Self: AsULE + Sized,
{
/// Converts from `&[Self]` to `&[Self::ULE]` if possible.
///
/// In general, this function returns `Some` on little-endian and `None` on big-endian.
fn slice_to_unaligned(slice: &[Self]) -> Option<&[Self::ULE]>;
}
#[cfg(target_endian = "little")]
impl<T> SliceAsULE for T
where
T: EqULE,
{
#[inline]
fn slice_to_unaligned(slice: &[Self]) -> Option<&[Self::ULE]> {
// This is safe because on little-endian platforms, the byte sequence of &[T]
// is equivalent to the byte sequence of &[T::ULE] by the contract of EqULE,
// and &[T::ULE] has equal or looser alignment than &[T].
let ule_slice =
unsafe { core::slice::from_raw_parts(slice.as_ptr() as *const Self::ULE, slice.len()) };
Some(ule_slice)
}
}
#[cfg(not(target_endian = "little"))]
impl<T> SliceAsULE for T
where
T: EqULE,
{
#[inline]
fn slice_to_unaligned(_: &[Self]) -> Option<&[Self::ULE]> {
None
}
}
/// Variable-width, byte-aligned data that can be cast to and from a little-endian byte slice.
///
/// If you need to implement this trait, consider using [`#[make_varule]`](crate::make_varule) or
/// [`#[derive(VarULE)]`](macro@VarULE) instead.
///
/// This trait is mostly for unsized types like `str` and `[T]`. It can be implemented on sized types;
/// however, it is much more preferable to use [`ULE`] for that purpose. The [`custom`] module contains
/// additional documentation on how this type can be implemented on custom types.
///
/// If deserialization with `VarZeroVec` is desired is recommended to implement `Deserialize` for
/// `Box<T>` (serde does not do this automatically for unsized `T`).
///
/// For convenience it is typically desired to implement [`EncodeAsVarULE`] and [`ZeroFrom`](zerofrom::ZeroFrom)
/// on some stack type to convert to and from the ULE type efficiently when necessary.
///
/// # Safety
///
/// Safety checklist for `VarULE`:
///
/// 1. The type *must not* include any uninitialized or padding bytes.
/// 2. The type must have an alignment of 1 byte.
/// 3. The impl of [`VarULE::validate_byte_slice()`] *must* return an error if the given byte slice
/// would not represent a valid slice of this type.
/// 4. The impl of [`VarULE::validate_byte_slice()`] *must* return an error if the given byte slice
/// cannot be used in its entirety.
/// 5. The impl of [`VarULE::from_byte_slice_unchecked()`] must produce a reference to the same
/// underlying data assuming that the given bytes previously passed validation.
/// 6. All other methods *must* be left with their default impl, or else implemented according to
/// their respective safety guidelines.
/// 7. Acknowledge the following note about the equality invariant.
///
/// If the ULE type is a struct only containing other ULE/VarULE types (or other types which satisfy invariants 1 and 2,
/// like `[u8; N]`), invariants 1 and 2 can be achieved via `#[repr(C, packed)]` or `#[repr(transparent)]`.
///
/// # Equality invariant
///
/// A non-safety invariant is that if `Self` implements `PartialEq`, the it *must* be logically
/// equivalent to byte equality on [`Self::as_byte_slice()`].
///
/// It may be necessary to introduce a "canonical form" of the ULE if logical equality does not
/// equal byte equality. In such a case, [`Self::validate_byte_slice()`] should return an error
/// for any values that are not in canonical form. For example, the decimal strings "1.23e4" and
/// "12.3e3" are logically equal, but not byte-for-byte equal, so we could define a canonical form
/// where only a single digit is allowed before `.`.
///
/// There may also be cases where a `VarULE` has muiltiple canonical forms, such as a faster
/// version and a smaller version. The cleanest way to handle this case would be separate types.
/// However, if this is not feasible, then the application should ensure that the data it is
/// deserializing is in the expected form. For example, if the data is being loaded from an
/// external source, then requests could carry information about the expected form of the data.
///
/// Failure to follow this invariant will cause surprising behavior in `PartialEq`, which may
/// result in unpredictable operations on `ZeroVec`, `VarZeroVec`, and `ZeroMap`.
pub unsafe trait VarULE: 'static {
/// Validates a byte slice, `&[u8]`.
///
/// If `Self` is not well-defined for all possible bit values, the bytes should be validated.
/// If the bytes can be transmuted, *in their entirety*, to a valid `&Self`, then `Ok` should
/// be returned; otherwise, `Self::Error` should be returned.
fn validate_byte_slice(_bytes: &[u8]) -> Result<(), ZeroVecError>;
/// Parses a byte slice, `&[u8]`, and return it as `&Self` with the same lifetime.
///
/// If `Self` is not well-defined for all possible bit values, the bytes should be validated,
/// and an error should be returned in the same cases as [`Self::validate_byte_slice()`].
///
/// The default implementation executes [`Self::validate_byte_slice()`] followed by
/// [`Self::from_byte_slice_unchecked`].
///
/// Note: The following equality should hold: `size_of_val(result) == size_of_val(bytes)`,
/// where `result` is the successful return value of the method. This means that the return
/// value spans the entire byte slice.
fn parse_byte_slice(bytes: &[u8]) -> Result<&Self, ZeroVecError> {
Self::validate_byte_slice(bytes)?;
let result = unsafe { Self::from_byte_slice_unchecked(bytes) };
debug_assert_eq!(mem::size_of_val(result), mem::size_of_val(bytes));
Ok(result)
}
/// Takes a byte slice, `&[u8]`, and return it as `&Self` with the same lifetime, assuming
/// that this byte slice has previously been run through [`Self::parse_byte_slice()`] with
/// success.
///
/// # Safety
///
/// ## Callers
///
/// Callers of this method must take care to ensure that `bytes` was previously passed through
/// [`Self::validate_byte_slice()`] with success (and was not changed since then).
///
/// ## Implementors
///
/// Implementations of this method may call unsafe functions to cast the pointer to the correct
/// type, assuming the "Callers" invariant above.
///
/// Safety checklist:
///
/// 1. This method *must* return the same result as [`Self::parse_byte_slice()`].
/// 2. This method *must* return a slice to the same region of memory as the argument.
unsafe fn from_byte_slice_unchecked(bytes: &[u8]) -> &Self;
/// Given `&Self`, returns a `&[u8]` with the same lifetime.
///
/// The default implementation performs a pointer cast to the same region of memory.
///
/// # Safety
///
/// Implementations of this method should call potentially unsafe functions to cast the
/// pointer to the correct type.
#[inline]
fn as_byte_slice(&self) -> &[u8] {
unsafe { slice::from_raw_parts(self as *const Self as *const u8, mem::size_of_val(self)) }
}
/// Allocate on the heap as a `Box<T>`
#[inline]
fn to_boxed(&self) -> Box<Self> {
let bytesvec = self.as_byte_slice().to_owned().into_boxed_slice();
let bytesvec = mem::ManuallyDrop::new(bytesvec);
unsafe {
// Get the pointer representation
let ptr: *mut Self =
Self::from_byte_slice_unchecked(&bytesvec) as *const Self as *mut Self;
assert_eq!(Layout::for_value(&*ptr), Layout::for_value(&**bytesvec));
// Transmute the pointer to an owned pointer
Box::from_raw(ptr)
}
}
}
// Proc macro reexports
//
// These exist so that our docs can use intra-doc links.
// Due to quirks of how rustdoc does documentation on reexports, these must be in this module and not reexported from
// a submodule
/// Custom derive for [`ULE`].
///
/// This can be attached to [`Copy`] structs containing only [`ULE`] types.
///
/// Most of the time, it is recommended one use [`#[make_ule]`](crate::make_ule) instead of defining
/// a custom ULE type.
#[cfg(feature = "derive")]
pub use zerovec_derive::ULE;
/// Custom derive for [`VarULE`]
///
/// This can be attached to structs containing only [`ULE`] types with one [`VarULE`] type at the end.
///
/// Most of the time, it is recommended one use [`#[make_varule]`](crate::make_varule) instead of defining
/// a custom [`VarULE`] type.
#[cfg(feature = "derive")]
pub use zerovec_derive::VarULE;