How to get pointer offset in bytes?

Question

While raw pointers in Rust have the offset method, this only increments by the size of the pointer. How can I get access to the pointer in bytes?

Something like this

Answer 1

Thanks to @Matthieu M.'s answer, this can be done using pointer offsets, heres a reusable macro:

macro_rules! offset_of {
    ($ty:ty, $field:ident) => {
        &(*(0 as *const $ty)).$field as *const _ as usize
    }
}

macro_rules! check_type_pair {
    ($a:expr, $b:expr) => {
        if false {
            let _type_check = if false {$a} else {$b};
        }
    }
}

macro_rules! parent_of_mut {
    ($child:expr, $ty:ty, $field:ident) => {
        {
            check_type_pair!(&(*(0 as *const $ty)).$field, &$child);
            let offset = offset_of!($ty, $field);
            &mut *(((($child as *mut _) as usize) - offset) as *mut $ty)
        }
    }
}

macro_rules! parent_of {
    ($child:expr, $ty:ty, $field:ident) => {
        {
            check_type_pair!(&(*(0 as *const $ty)).$field, &$child);
            let offset = offset_of!($ty, $field);
            &*(((($child as *const _) as usize) - offset) as *const $ty)
        }
    }
}

This way, when we have a field in a struct, we can get the parent struct like this:

fn some_method(&self) {
    // Where 'self' is ParentStruct.field,
    // access ParentStruct instance.
    let parent = unsafe { parent_of!(self, ParentStruct, field) };
}

The macro check_type_pair helps avoid simple mistakes where self and ParentStruct.field aren't the same type. However its not foolproof when two different members in a struct have the same type.