Function generate_default_impl 

fn generate_default_impl(
    ecx: &mut ExtCtxt<'_>,
    item_kind: &ItemKind,
    impl_unsafe: bool,
    macro_name: Ident,
    eii_attr_span: Span,
    item_span: Span,
    foreign_item_name: Ident,
) -> Option<Box<Item>>