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use_prelude!();
cfg_alloc! {
mod private {
use_prelude!();
pub trait Sealed : Sized {}
impl<T> Sealed for Box<::core::mem::MaybeUninit<T>>
{}
}
/// Extension trait for uninitalized `Box` allocations and
/// the optimized delayed-initialization pattern.
///
/// # Optimized in-place heap initialization
///
/// The `Box::uninit().init(...)` delayed-initialization pattern is suprisingly
/// effective in helping the optimizer inline the creation of the value directly
/// into the heap.
///
/// - In other words, this bundles [`::copyless`](https://docs.rs/copyless)
/// functionality.
///
/// - For those wondering why `Box::new(...)` could not be made as efficient,
/// the answer lies in temporaries: the `...` temporary when calling
/// `Box::new()` is created _before_ attempting the allocation, and given
/// that this allocation can fail / have side-effects, the optimizer is not
/// allowed to reorder the creation of the temporary after the allocation,
/// since it can change the semantics of the code for these corner (but
/// not unreachable) cases. It is hence illegal for the optimizer to inline
/// the creation of `...` directly into the heap.
///
/// Whereas `Box::uninit().init(...)` only creates the temporary after
/// the allocation attempted in `uninit()` has succeeded, at which point
/// it should be trivial for the optimizer to inline its creation directly
/// into the heap.
///
/// - Note, however, that this property cannot be guaranteed from a library
/// perspective; for instance, **the heap-inlined initialization does not
/// seem to happen when the optimization level (`opt-level`) is less than
/// `2`. Inversely, the author has observed that the heap-inlined
/// initialization does seem to kick in when compiling with `-C
/// opt-level=2`** (or `3`), _e.g._, when running on `--release`.
///
///
/// ### Example
///
/// ```rust
/// use ::uninit::prelude::*;
///
/// let ft: Box<u8> = Box::uninit().init(42);
/// assert_eq!(*ft, 42);
/// ```
///
/// This optimization can even allow creating arrays too big to fit in the
/// stack.
///
/// - For instance, the following implementation panics:
///
/// ```rust,should_panic
/// fn alloc_big_boxed_array () -> Box<[u64; 10_000_000]>
/// {
/// // This can panic because of the `[0; 10_000_000]` stack
/// // temporary overflowing the stack.
/// Box::new([0; 10_000_000])
/// }
/// # println!("Address: {:p}", alloc_big_boxed_array());
/// # panic!();
/// ```
///
/// - Whereas the following one does not
/// (doc-tested with `RUSTDOCFLAGS=-Copt-level=2`):
///
/// ```rust
/// # use ::uninit::prelude::*;
/// fn alloc_big_boxed_array () -> Box<[u64; 10_000_000]>
/// {
/// // But this works fine, since there is no stack temporary!
/// Box::uninit().init([0; 10_000_000])
/// }
/// # println!("Address: {:p}", alloc_big_boxed_array());
/// ```
///
/// # Handling allocation failure
///
/// A neat side-effect of this implementation is to expose the intermediate
/// state of `Box::try_alloc()`, which yields an `Option<Box<MaybeUninit<T>>>`
/// depending on whether the attempted allocation succeeded or not.
///
/// ### Example
///
/// ```rust,no_run
/// use ::uninit::prelude::*;
///
/// let buf: Box<[u8; ::core::i32::MAX as _]> = match Box::try_alloc() {
/// | Some(uninit) => uninit.init([0; ::core::i32::MAX as _]),
/// | None => {
/// panic!("Failed to allocate 2GB of memory");
/// }
/// };
/// # let _ = buf;
/// ```
impl<T> BoxUninit for Box<MaybeUninit<T>> {
type T = T;
/// Idiomatic allocation-failure unwrapping of [`BoxUninit::try_alloc`]`()`.
#[inline]
fn uninit ()
-> Box<MaybeUninit<T>>
{
let layout = alloc::Layout::new::<T>();
if let Some(it) = Self::try_alloc() { it } else {
alloc::handle_alloc_error(layout);
}
}
/// Attempts to `Box`-allocate memory for `T`, without initializing it.
///
/// Returns `None` when the allocation fails.
#[inline]
fn try_alloc ()
-> Option<Box<MaybeUninit<T>>>
{Some({
if ::core::mem::size_of::<T>() == 0 {
Self::new(MaybeUninit::uninit())
} else {
unsafe {
// Safety: we have guarded against ZST
let layout = alloc::Layout::new::<T>();
Self::from_raw(
ptr::NonNull::new(alloc::alloc(layout))?
.as_ptr()
.cast()
)
}
}
})}
/// Safely initialize a `Box::MaybeUninit<T>` by providing a `value: T`
/// (that can be inlined into the `Box`), and safely return the ergonomic
/// `Box<T>` witness of that initialization.
#[inline(always)]
fn init (mut self: Box<MaybeUninit<T>>, value: T)
-> Box<T>
{
*self = MaybeUninit::new(value);
unsafe {
// Safety: `self` has just been initialized.
Box::from_raw(Box::into_raw(self).cast())
}
}
}
/// Extension trait for uninitalized `Box` allocations and
/// the optimized delayed-initialization pattern.
#[allow(missing_docs)]
pub
trait BoxUninit : private::Sealed {
type T;
fn uninit ()
-> Self
;
fn init (self, value: Self::T)
-> Box<Self::T>
;
fn try_alloc ()
-> Option<Self>
;
}
mod private2 {
use_prelude!();
pub trait Sealed {}
impl<T> Sealed for Box<[::core::mem::MaybeUninit<T>]>
{}
impl<T> Sealed for Box<::core::mem::MaybeUninit<T>>
{}
}
impl<T> BoxAssumeInit for Box<[MaybeUninit<T>]> {
#[allow(missing_docs)]
type Ret = [T];
/// Allows to "`.assume_init()`" a boxed `[MaybeUninit<T>]`.
///
/// # Safety
///
/// - This has the same safety requirements as
/// [`.assume_init()`][`MaybeUninit::assume_init`].
#[inline]
unsafe fn assume_init (self: Box<[MaybeUninit<T>]>)
-> Box<[T]>
{
let len = self.len();
let ptr = Box::leak(self).as_mut_ptr();
// SAFETY:
// - `T` and `MaybeUninit<T>` have the same layout
// - The `T` are initialized as promised by the caller
unsafe {
Box::from_raw(slice::from_raw_parts_mut(
ptr.cast(), len,
))
}
}
}
impl<T> BoxAssumeInit for Box<MaybeUninit<T>> {
type Ret = T;
/// Allows to "`.assume_init()`" a boxed `MaybeUninit<T>`.
///
/// # Safety
///
/// - This has the same safety requirements as
/// [`.assume_init()`][`MaybeUninit::assume_init`].
#[inline]
unsafe fn assume_init (self: Box<MaybeUninit<T>>)
-> Box<T>
{
// SAFETY:
// - `T` and `MaybeUninit<T>` have the same layout
// - The `T` are initialized as promised by the caller
unsafe {
Box::from_raw(Box::into_raw(self).cast())
}
}
}
/// Extension trait to `.assume_init()` through a `Box`.
///
/// This is a compatibility helper trait. For versions of Rust where the
/// `feature(box_uninit)` is unstable, this trait enables the feature in stable
/// Rust. This may trigger an `unstable_name_collisions` lint, but this is fine,
/// since the implementation is the same. You can dismiss that lint with:
///
/// ```rust
/// #![allow(unstable_name_collisions)]
/// ```
#[allow(missing_docs)]
pub
trait BoxAssumeInit : private2::Sealed {
type Ret : ?Sized;
unsafe fn assume_init (self: Self)
-> Box<Self::Ret>
;
}
pub use new_uninit_slice::*;
mod new_uninit_slice {
#![allow(missing_docs)]
use super::*;
pub trait BoxNewUninitSlice<T>: Sized {
/// Constructs a new boxed slice with uninitialized contents.
///
/// ## Example
///
/// ```
/// use ::uninit::prelude::*;
///
/// let mut values = Box::<[u32]>::new_uninit_slice(3);
///
/// let values = unsafe {
/// // Deferred initialization:
/// values[0].as_mut_ptr().write(1);
/// values[1].as_mut_ptr().write(2);
/// values[2].as_mut_ptr().write(3);
/// values.assume_init()
/// };
///
/// assert_eq!(*values, [1, 2, 3]);
/// ```
fn new_uninit_slice(len: usize)
-> Box<[MaybeUninit<T>]>;
}
impl<T> BoxNewUninitSlice<T> for Box<[T]> {
#[inline]
fn new_uninit_slice (len: usize)
-> Box<[MaybeUninit<T>]>
{
Vec::with_capacity(len).into_backing_buffer_forget_elems()
}
}
}
} // cfg_alloc!