Add maybe_uninit constructor and .assume_init() method#803
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ManuallyDrop has the benefit that we use it up front, instead of after any "critical" operations.
Add representation trait RawDataSubst that array storage types correctly (emulation of higher kinded types). Implement it as a transmute-like operation, using the representation guarantees that MaybeUninit documents. Make it a public method - avaialable on all array types - owned arrays, arcarray, array views and so on. The safety percussions of exposing this transformation are only explored to an extent, but it's a type change without value conversion so there should not be much that can go wrong. Interesting aspects include being able to have one ArcArray handle with the old type and one with the new type, or one owned array with old and a view with new.
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Add new constructor -
Array::maybe_uninit()that makes an uninitialized array. This is represented in the type system as the array having the element typeMaybeUninit<T>, and can be converted to a proper array ofTwhen it has been fully initialized.Fixes #685
Zip's apply_assign_into can write to
MaybeUninitelements, but more than that, it is pretty manual work, but it can at least be done in safe code. The onlyunsafestep is to promise all elements are initialized, and calling.assume_init().The updated example -
sort-axis.rsshows how usage remains similar to the old constructor, but it is much easier to be confident about having done it correctly - and it supports non-Copy elements.