field_offset.rs

macro_rules! field_offset {
    ($ty:path, $field:ident$(.$cfield:ident)*) => {
        unsafe {
            union Transmuter<T: 'static> {
                p: *const T,
                r: &'static T,
                i: usize,
            }
            let base = Transmuter::<$ty> {
                p: core::ptr::NonNull::dangling().as_ptr(),
            };
            let field = Transmuter {
                r: &base.r.$field$(.$cfield)*,
            };
            field.i - base.i
        }
    }
}

#[repr(C)]
struct Foo {
    x: u8,
    y: u16,
}

fn main () {
    println!("offsets: {}, {}", field_offset!(Foo, x), field_offset!(Foo, y));
}