Macro emit_union_default_via_unsafe_zeroed 

macro_rules! emit_union_default_via_unsafe_zeroed {
    (
        found_default_via_unsafe_zeroed: false,
        cfg_attrs: { $($cfg_attrs:tt)* },
        name: $name:ident,
    ) => { ... };
    (
        found_default_via_unsafe_zeroed: true,
        cfg_attrs: { $($cfg_attrs:tt)* },
        name: $name:ident,
    ) => { ... };
}

Add a Default impl that is just mem::zeroed.