https://docs.rs/zerocopy/latest/zerocopy/struct.Unalign.html states

Unalign<T> has the same size and ABI as T

This is wrong, as the type is #[repr(C, packed)] and not repr(transparent).

The Rust reference about repr(transparent):

This is different than the C representation because a struct with the C representation will always have the ABI of a C struct while, for example, a struct with the transparent representation with a primitive field will have the ABI of the primitive field.

Here's an example of such an ABI mismatch in practice: https://godbolt.org/z/bhP71dnh4

The docs should be relaxed to not promise full ABI compatibility, but only promise what repr(C) promises (which is probably strong enough for most use cases).