How to get pointer offset in bytes?

TL;DR: This answer invokes Undefined Behavior, according to RFC-2582.

In particular, references must be aligned and dereferencable, even when they are created and never used.

There are also discussions that field accesses themselves impose extra requirements not solved by the proposed &raw, due to usage of getelementptr inbounds, see offsetof woes at the bottom of the RFC.

---

From the answer I linked to your previous question:

macro_rules! offset_of {
    ($ty:ty, $field:ident) => {
        //  Undefined Behavior: dereferences a null pointer.
        //  Undefined Behavior: accesses field outside of valid memory area.
        unsafe { &(*(0 as *const $ty)).$field as *const _ as usize }
    }
}

fn main() {
    let p: *const Baz = 0x1248 as *const _;
    let p2: *const Foo = ((p as usize) - offset_of!(Foo, memberB)) as *const _;
    println!("{:p}", p2);
}

We can see on the computation of p2 that a pointer can be converted painless to an integer (usize here), on which arithmetic is performed, and then the result is cast back to a pointer.

isize and usize are the universal byte-sized pointer types :)

---

Were RFC-2582 to be accepted, this implementation of offset_of! is my best shot:

macro_rules! offset_of {
    ($ty:ty, $field:ident) => {
        unsafe {
            //  Create correctly sized storage.
            //
            //  Note: `let zeroed: $ty = ::std::mem::zeroed();` is incorrect,
            //        a zero pattern is not always a valid value.
            let buffer = ::std::mem::MaybeUninit::<$ty>::uninit();

            //  Create a Raw reference to the storage:
            //  - Alignment does not matter, though is correct here.
            //  - It safely refers to uninitialized storage.
            //
            //  Note: using `&raw const *(&buffer as *const _ as *const $ty)`
            //        is incorrect, it creates a temporary non-raw reference.
            let uninit: &raw const %ty = ::std::mem::transmute(&buffer);

            //  Create a Raw reference to the field:
            //  - Alignment does not matter, though is correct here.
            //  - It points within the memory area.
            //  - It safely refers to uninitialized storage.
            let field = &raw const uninit.$field;

            //  Compute the difference between pointers.
            (field as *const _ as usize) - (uninit as *const_ as usize)
        }
    }
}

I have commented each step with the reasons I believe they are sound, and why some alternatives are not -- something I encourage heavily in unsafe code -- and hopefully not missed anything.