The #[rustc_layout_scalar_valid_range_start(...)] and #[rustc_layout_scalar_valid_range_end(...)] attribute allows you to specify a niche, so you can build a No

The #[rustc_layout_scalar_valid_range_start(...)] and #[rustc_layout_scalar_valid_range_end(...)] attribute allows you to specify a niche, so you can build a Non255U8 if you want. For example: #[rustc_layout_scalar_valid_range_end(254)] struct Non255U8(u8);