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pub(crate) use internal::Beef;
pub(crate) use internal::Capacity;
pub(crate) mod internal {
use alloc::borrow::ToOwned;
use alloc::string::String;
use alloc::vec::Vec;
use core::mem::ManuallyDrop;
use core::ptr::{slice_from_raw_parts, NonNull};
pub trait Capacity {
type Field: Copy;
type NonZero: Copy;
fn len(fat: usize) -> usize;
fn empty(len: usize) -> (usize, Self::Field);
fn store(len: usize, capacity: usize) -> (usize, Self::Field);
fn unpack(fat: usize, capacity: Self::NonZero) -> (usize, usize);
fn maybe(fat: usize, capacity: Self::Field) -> Option<Self::NonZero>;
}
/// Helper trait required by `Cow<T>` to extract capacity of owned
/// variant of `T`, and manage conversions.
///
/// This can be only implemented on types that match requirements:
///
/// + `T::Owned` has a `capacity`, which is an extra word that is absent in `T`.
/// + `T::Owned` with `capacity` of `0` does not allocate memory.
/// + `T::Owned` can be reconstructed from `*mut T` borrowed out of it, plus capacity.
pub unsafe trait Beef: ToOwned {
type PointerT;
fn ref_into_parts<U>(&self) -> (NonNull<Self::PointerT>, usize, U::Field)
where
U: Capacity;
unsafe fn ref_from_parts<U>(ptr: NonNull<Self::PointerT>, len: usize) -> *const Self
where
U: Capacity;
/// Convert `T::Owned` to `NonNull<T>` and capacity.
/// Return `None` for `0` capacity.
fn owned_into_parts<U>(owned: Self::Owned) -> (NonNull<Self::PointerT>, usize, U::Field)
where
U: Capacity;
/// Rebuild `T::Owned` from `NonNull<T>` and `capacity`. This can be done by the likes
/// of [`Vec::from_raw_parts`](https://doc.rust-lang.org/std/vec/struct.Vec.html#method.from_raw_parts).
unsafe fn owned_from_parts<U>(
ptr: NonNull<Self::PointerT>,
fat: usize,
capacity: U::NonZero,
) -> Self::Owned
where
U: Capacity;
}
unsafe impl Beef for str {
type PointerT = u8;
#[inline]
fn ref_into_parts<U>(&self) -> (NonNull<u8>, usize, U::Field)
where
U: Capacity,
{
let (fat, cap) = U::empty(self.len());
// A note on soundness:
//
// We are casting *const T to *mut T, however for all borrowed values
// this raw pointer is only ever dereferenced back to &T.
(
unsafe { NonNull::new_unchecked(self.as_ptr() as *mut u8) },
fat,
cap,
)
}
#[inline]
unsafe fn ref_from_parts<U>(ptr: NonNull<u8>, fat: usize) -> *const str
where
U: Capacity,
{
slice_from_raw_parts(ptr.as_ptr(), U::len(fat)) as *const str
}
#[inline]
fn owned_into_parts<U>(owned: String) -> (NonNull<u8>, usize, U::Field)
where
U: Capacity,
{
// Convert to `String::into_raw_parts` once stabilized
// We need to go through Vec here to get provenance for the entire allocation
// instead of just the initialized parts.
let mut owned = ManuallyDrop::new(owned.into_bytes());
let (fat, cap) = U::store(owned.len(), owned.capacity());
(
unsafe { NonNull::new_unchecked(owned.as_mut_ptr()) },
fat,
cap,
)
}
#[inline]
unsafe fn owned_from_parts<U>(ptr: NonNull<u8>, fat: usize, capacity: U::NonZero) -> String
where
U: Capacity,
{
let (len, cap) = U::unpack(fat, capacity);
String::from_utf8_unchecked(Vec::from_raw_parts(ptr.as_ptr(), len, cap))
}
}
unsafe impl<T: Clone> Beef for [T] {
type PointerT = T;
#[inline]
fn ref_into_parts<U>(&self) -> (NonNull<T>, usize, U::Field)
where
U: Capacity,
{
let (fat, cap) = U::empty(self.len());
// A note on soundness:
//
// We are casting *const T to *mut T, however for all borrowed values
// this raw pointer is only ever dereferenced back to &T.
(
unsafe { NonNull::new_unchecked(self.as_ptr() as *mut T) },
fat,
cap,
)
}
#[inline]
unsafe fn ref_from_parts<U>(ptr: NonNull<T>, fat: usize) -> *const [T]
where
U: Capacity,
{
slice_from_raw_parts(ptr.as_ptr(), U::len(fat))
}
#[inline]
fn owned_into_parts<U>(owned: Vec<T>) -> (NonNull<T>, usize, U::Field)
where
U: Capacity,
{
// Convert to `Vec::into_raw_parts` once stabilized
let mut owned = ManuallyDrop::new(owned);
let (fat, cap) = U::store(owned.len(), owned.capacity());
(
unsafe { NonNull::new_unchecked(owned.as_mut_ptr()) },
fat,
cap,
)
}
#[inline]
unsafe fn owned_from_parts<U>(ptr: NonNull<T>, fat: usize, capacity: U::NonZero) -> Vec<T>
where
U: Capacity,
{
let (len, cap) = U::unpack(fat, capacity);
Vec::from_raw_parts(ptr.as_ptr(), len, cap)
}
}
}