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//! Module providing a simple replacement for [`std::boxed::Box`] with allocator
//! support.
//!
//! # Motivation
//! - The allocator api of [`std::boxed::Box`] is still unstable (at the time of
//! writing). The [`Box`] provided by this module can be used on stable with
//! allocators.
//! - [Issue #78459](https://github.com/rust-lang/rust/issues/78459) prevents
//! the use of non-zero sized allocators with [`std::boxed::Box`] even on
//! nightly.
// some code and documentation is copied from the standard library
use crate::allocator_api::{AllocError, Allocator};
use alloc::alloc::handle_alloc_error;
use core::alloc::Layout;
use core::marker::PhantomData;
use core::mem::{ManuallyDrop, MaybeUninit};
use core::ops::{Deref, DerefMut};
use core::ptr::NonNull;
/// A replacement for [`std::boxed::Box`] that works with custom allocators.
///
/// See the module-level documentation for more.
pub struct Box<T: ?Sized, A: Allocator> {
/// Pointer to the inner value, allocated with `self.alloc`.
// Safety: must always point to a valid instance of `T`.
ptr: NonNull<T>,
// we own an instance of type `T`
_phantom_heapmem: PhantomData<T>,
/// Allocator used for heap allocation
alloc: A,
}
impl<T: ?Sized, A: Allocator> Box<T, A> {
/// Create [`Box`] from a pointer and an allocator.
///
/// # Safety
/// - `ptr` has to be allocated using the allocator `alloc` (and not yet
/// deallocated)
/// - `ptr` must point to a valid instance of `T` (otherwise using e.g.
/// [`Deref::deref`] on the resulting [`Box`] is unsound)
/// - in particular `ptr` must point to an allocation that fits
/// `Layout::for_value(*ptr)`
unsafe fn from_raw_parts(ptr: NonNull<T>, alloc: A) -> Self {
Self {
ptr,
alloc,
_phantom_heapmem: PhantomData::<T>,
}
}
/// Destruct a [`Box`] into the pointer and allocator without dropping the
/// [`Box`].
fn into_raw_parts(self) -> (NonNull<T>, A) {
let ptr = self.ptr;
let me = ManuallyDrop::new(self);
let alloc_ptr = &me.deref().alloc as *const A;
// SAFETY: `alloc_ptr` is valid for reads, properly aligned, initialised...
// SAFETY: the contents of `me` are never dropped so `alloc` can be safely
// dropped later
let alloc = unsafe { alloc_ptr.read() };
(ptr, alloc)
}
}
// documentation and implementations copied from the standard library
// Copyright (c) 2021 rust standard library contributors
// slight modifications to accomodate for missing APIs, different `Box`
// definition
impl<T, A: Allocator> Box<T, A> {
/// Allocates memory in the given allocator then places `x` into it.
///
/// This doesn't actually allocate if `T` is zero-sized.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// use secmem_alloc::boxed::Box;
/// use std::alloc::System;
///
/// let five = Box::new_in(5, System);
/// ```
#[inline]
pub fn new_in(x: T, alloc: A) -> Self {
let mut boxed = Self::new_uninit_in(alloc);
unsafe {
boxed.as_mut_ptr().write(x);
boxed.assume_init()
}
}
/// Allocates memory in the given allocator then places `x` into it,
/// returning an error if the allocation fails
///
/// This doesn't actually allocate if `T` is zero-sized.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// use secmem_alloc::boxed::Box;
/// use std::alloc::System;
///
/// let five = Box::try_new_in(5, System)?;
/// # Ok::<(), core::alloc::AllocError>(())
/// ```
#[inline]
pub fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError> {
let mut boxed = Self::try_new_uninit_in(alloc)?;
unsafe {
boxed.as_mut_ptr().write(x);
Ok(boxed.assume_init())
}
}
/// Constructs a new box with uninitialized contents in the provided
/// allocator.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// use secmem_alloc::boxed::Box;
/// use std::alloc::System;
///
/// let mut five = Box::<u32, _>::new_uninit_in(System);
///
/// let five = unsafe {
/// // Deferred initialization:
/// five.as_mut_ptr().write(5);
///
/// five.assume_init()
/// };
///
/// assert_eq!(*five, 5)
/// ```
pub fn new_uninit_in(alloc: A) -> Box<MaybeUninit<T>, A> {
let layout = Layout::new::<MaybeUninit<T>>();
// NOTE: Prefer match over unwrap_or_else since closure sometimes not
// inlineable. That would make code size bigger.
match Box::try_new_uninit_in(alloc) {
Ok(m) => m,
Err(_) => handle_alloc_error(layout),
}
}
/// Constructs a new box with uninitialized contents in the provided
/// allocator, returning an error if the allocation fails
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// use secmem_alloc::boxed::Box;
/// use std::alloc::System;
///
/// let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
///
/// let five = unsafe {
/// // Deferred initialization:
/// five.as_mut_ptr().write(5);
///
/// five.assume_init()
/// };
///
/// assert_eq!(*five, 5);
/// # Ok::<(), core::alloc::AllocError>(())
/// ```
pub fn try_new_uninit_in(alloc: A) -> Result<Box<MaybeUninit<T>, A>, AllocError> {
let layout = Layout::new::<MaybeUninit<T>>();
let ptr: NonNull<MaybeUninit<T>> = alloc.allocate(layout)?.cast();
unsafe { Ok(Box::from_raw_parts(ptr, alloc)) }
}
}
// documentation and implementations copied from the standard library
// Copyright (c) 2021 rust standard library contributors
// slight modifications to accomodate for missing APIs, different `Box`
// definition
impl<T, A: Allocator> Box<MaybeUninit<T>, A> {
/// Converts to `Box<T, A>`.
///
/// # Safety
///
/// As with [`MaybeUninit::assume_init`],
/// it is up to the caller to guarantee that the value
/// really is in an initialized state.
/// Calling this when the content is not yet fully initialized
/// causes immediate undefined behavior.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// use secmem_alloc::boxed::Box;
/// use std::alloc::System;
///
/// let mut five = Box::<u32, _>::new_uninit_in(System);
///
/// let five: Box<u32, _> = unsafe {
/// // Deferred initialization:
/// five.as_mut_ptr().write(5);
///
/// five.assume_init()
/// };
///
/// assert_eq!(*five, 5)
/// ```
#[inline]
pub unsafe fn assume_init(self) -> Box<T, A> {
let (ptr, alloc) = Box::into_raw_parts(self);
let ptr_init: NonNull<T> = ptr.cast();
unsafe { Box::from_raw_parts(ptr_init, alloc) }
}
}
impl<T: ?Sized, A: Allocator> Deref for Box<T, A> {
type Target = T;
fn deref(&self) -> &T {
// SAFETY: `self.ptr` always points to a valid instance of `T`
unsafe { &*self.ptr.as_ptr() }
}
}
impl<T: ?Sized, A: Allocator> DerefMut for Box<T, A> {
fn deref_mut(&mut self) -> &mut T {
// SAFETY: `self.ptr` always points to a valid instance of `T`
unsafe { &mut *self.ptr.as_ptr() }
}
}
impl<T: ?Sized, A: Allocator> Drop for Box<T, A> {
fn drop(&mut self) {
// obtain the Layout of the value stored in this Box
let ref_to_inner: &T = self.deref();
let layout = Layout::for_value::<T>(ref_to_inner);
// `self.ptr` points to an allocation that fits `layout`
// SAFETY: `self.ptr.as_ptr()` is valid for reads and writes, properly aligned
unsafe {
self.ptr.as_ptr().drop_in_place();
}
// SAFETY: from now on it is unsound to dereference `self.ptr` (hence `self`)
// deallocate memory
let ptr: NonNull<u8> = self.ptr.cast();
// SAFETY: `self.ptr` was allocated with allocator `self.alloc` and fits
// `layout`
unsafe {
self.alloc.deallocate(ptr, layout);
}
// `self.ptr` is now dangling, but this is sound since `NonNull<T>` is
// not `Drop` `self.alloc` is dropped automatically
}
}
#[cfg(test)]
mod tests {
use super::Box;
use std::alloc::System;
use std::mem::MaybeUninit;
#[test]
fn new_in() {
let boxed = Box::new_in([37; 256], System);
assert_eq!(*boxed, [37; 256]);
}
#[test]
fn try_new_in() {
let boxed = Box::try_new_in([37; 256], System).expect("error creating box");
assert_eq!(*boxed, [37; 256]);
}
#[test]
fn uninit_initialise() {
let mut boxed: Box<MaybeUninit<[u8; 256]>, System> =
Box::<[u8; 256], _>::new_uninit_in(System);
unsafe {
// initialise `boxed`
boxed.as_mut_ptr().write([37; 256]);
}
// SAFETY: `boxed` is now initialised
let boxed: Box<[u8; 256], System> = unsafe { boxed.assume_init() };
assert_eq!(*boxed, [37; 256]);
}
}