Unalign ABI promises are wrong

[Noratrieb]

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).

[Lokathor]

Specifically, the memory layout will still be the same as that of the wrapped type, but the function calling convention can differ.

[chorman0773]

This seems like it should also affect any x86-64 Sys-V platform.

When determining the class of structures,

If the size of an object is larger than four eightbytes, or it contains unaligned
fields, it has class MEMORY.

[joshlf]

Good catch, thanks @Nilstrieb! Uploaded a fix here.