Get the offset of a field from the base of a struct

language design Unsafe Code Guidelines
4

In the next stable Rust version, you can get the offset of a field through a MaybeUninit<Struct> using the addr_of macro, so you don't need to require that the type implements Default.

https://play.rust-lang.org/?version=beta&mode=debug&edition=2018&gist=8b6d86fa1d269c7344db999cc3505f56

macro_rules! get_offset {
    ($type:ty, $field:tt) => ({
        let dummy = ::core::mem::MaybeUninit::<$type>::uninit();

        let dummy_ptr = dummy.as_ptr();
        let member_ptr = unsafe{ ::core::ptr::addr_of!((*dummy_ptr).$field) };
        
        member_ptr as usize - dummy_ptr as usize
    })
}

fn main(){
    dbg!(get_offset!((String, String), 0));
    dbg!(get_offset!((String, String), 1));
}

[src/main.rs:13] get_offset!((String, String), 0) = 0
[src/main.rs:14] get_offset!((String, String), 1) = 24
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