NonNull cast should allow ?Sized

[adsnaider]

Not sure if I should categorize this as a bug since this seems to be something that was overlooked. core::ptr::NonNull has the function pub const fn cast<U>(self) -> NonNull<U> that, as far as I can tell, should allow for U: ?Sized. Am I missing something here?

[adsnaider]

I see that #60602 actually explains the reasoning. It would be nice then to have a cast_sliced function that casts NonNull<[T]> to NonNull<[U]>. But since that is a feature request, I think I'll close it here for now.