[Pre-RFC] Add a new offset_of macro to core::mem

129

Now that offset_from is a const fn (behind a feature flag), this can be implemented as a const-friendly macro (I haven't tested this since I nightly hasn't yet been updated with const_ptr_offset_from; I may revise this in the next day or two when I get a chance to actually test it):

#![feature(const_transmute)]
#![feature(const_ptr_offset_from)]
#![feature(ptr_offset_from)]

macro_rules! offset_of {
    ($Struct:path, $($field:tt)+) => ({
        const OFFSET: usize = {
            extern crate core;

            let base_uninit = core::mem::MaybeUninit::<$Struct>::uninit();
            unsafe {
                // This is UB, but it's the best we can do for the time being.
                // If this is in `core`, we can just leave a comment to tell
                // people that this is an unsafe implementation detail that
                // works here in `core` but is UB outside of this macro.
                let base_ref = core::mem::transmute::<_, &$Struct>(&base_uninit);
                let base_u8_ptr = base_ref as *const _ as *const u8;

                let field_u8_ptr = &base_ref.$($field)+ as *const _ as *const u8;
                let offset = field_u8_ptr.offset_from(base_u8_ptr) as usize;
                
                // Make sure the offset computation stayed within the struct's size.
                let assert = [offset; 1];
                assert[(offset <= core::mem::size_of::<$Struct>()) as usize - 1]
            }
        };
        OFFSET
    })
}

Using $($field:tt)+ allows this to support offset_of!(Struct, field.sub_field.sub_array[3]).