Sure, I meant:

Rust guarantees to optimize the following types T such that Option<T> has the same size as T:

    Box<U>
    &U
    &mut U
    fn, extern "C" fn
    num::NonZero*
    ptr::NonNull<U>
    #[repr(transparent)] struct around one of the types in this list.

UPDATE: Or, better per your example: struct T {i: i32} takes the same size as i32: https://play.rust-lang.org/?version=stable&mode=debug&editio...