Fix theoretical UB while transmuting Arc #588
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stanislav-tkach
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Jan 26, 2022
aleksuss
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vldm
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As far as I can tell, there is no guarantee that
Arc<T>can be soundly transmuted toArc<U>, even ifTcan be soundly transmuted toU.Looking at the implementation of
Arc, it is effectively a single field struct, wrapping a privateNonNull<ArcInner<T>>whereArcInner<T>is#[repr(C)], so the transmute is not UB as of now, and of course it's unlikely that this would change. But it's still not good to rely on such an implementation detail for soundness.