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use std::{
alloc, mem,
ptr::{self, NonNull},
};
/// A typesafe helper that stores the allocated pointer without the data initialized.
pub struct BoxAllocation<T>(
// ptr cannot be null since it would mean the allocation failed.
// Note: covariance is acceptable since this eventually becomes a `Box<T>`,
// which is covariant too.
NonNull<T>,
);
impl<T> BoxAllocation<T> {
/// Consumes self and writes the given value into the allocation.
#[inline(always)] // if this does not get inlined then copying happens
pub fn init(self, value: T) -> Box<T> {
if mem::size_of::<T>() == 0 {
return Box::new(value);
}
unsafe {
let ptr = self.0.as_ptr();
mem::forget(self);
ptr::write(ptr, value);
Box::from_raw(ptr)
}
}
}
impl<T> Drop for BoxAllocation<T> {
fn drop(&mut self) {
if mem::size_of::<T>() == 0 {
return;
}
let layout = alloc::Layout::new::<T>();
unsafe {
alloc::dealloc(self.0.as_ptr() as *mut u8, layout);
}
}
}
/// Helper trait for a `Box` type that allocates up-front.
pub trait BoxHelper<T> {
/// Allocates the storage without providing any data.
fn alloc() -> BoxAllocation<T>;
}
impl<T> BoxHelper<T> for Box<T> {
fn alloc() -> BoxAllocation<T> {
if mem::size_of::<T>() == 0 {
return BoxAllocation(NonNull::dangling());
}
let layout = alloc::Layout::new::<T>();
BoxAllocation(
NonNull::new(unsafe { alloc::alloc(layout) as *mut T })
.unwrap_or_else(|| alloc::handle_alloc_error(layout)), // oom
)
}
}
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