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Auto merge of #106239 - LegionMammal978:thin-box-drop-guard, r=Amanieu

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Deallocate ThinBox even if the value unwinds on drop

This makes it match the behavior of an ordinary `Box`.
This commit is contained in:
bors 2023-01-04 12:55:22 +00:00
commit df756439df
1 changed files with 34 additions and 13 deletions
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47
library/alloc/src/boxed/thin.rs
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@ -226,24 +226,45 @@ impl<H> WithHeader<H> {
// - Assumes that either `value` can be dereferenced, or is the
// `NonNull::dangling()` we use when both `T` and `H` are ZSTs.
unsafe fn drop<T: ?Sized>(&self, value: *mut T) {
struct DropGuard<H> {
ptr: NonNull<u8>,
value_layout: Layout,
_marker: PhantomData<H>,
}
impl<H> Drop for DropGuard<H> {
fn drop(&mut self) {
unsafe {
// SAFETY: Layout must have been computable if we're in drop
let (layout, value_offset) =
WithHeader::<H>::alloc_layout(self.value_layout).unwrap_unchecked();
// Note: Don't deallocate if the layout size is zero, because the pointer
// didn't come from the allocator.
if layout.size() != 0 {
alloc::dealloc(self.ptr.as_ptr().sub(value_offset), layout);
} else {
debug_assert!(
value_offset == 0
&& mem::size_of::<H>() == 0
&& self.value_layout.size() == 0
);
}
}
}
}
unsafe {
let value_layout = Layout::for_value_raw(value);
// SAFETY: Layout must have been computable if we're in drop
let (layout, value_offset) = Self::alloc_layout(value_layout).unwrap_unchecked();
// `_guard` will deallocate the memory when dropped, even if `drop_in_place` unwinds.
let _guard = DropGuard {
ptr: self.0,
value_layout: Layout::for_value_raw(value),
_marker: PhantomData::<H>,
};
// We only drop the value because the Pointee trait requires that the metadata is copy
// aka trivially droppable.
ptr::drop_in_place::<T>(value);
// Note: Don't deallocate if the layout size is zero, because the pointer
// didn't come from the allocator.
if layout.size() != 0 {
alloc::dealloc(self.0.as_ptr().sub(value_offset), layout);
} else {
debug_assert!(
value_offset == 0 && mem::size_of::<H>() == 0 && value_layout.size() == 0
);
}
}
}