custom-error-tidy-builder.rs

struct ItemBuilder<const A: bool, const B: bool>;
impl<const A: bool, const B: bool> ItemBuilder<A, B> {
    fn build(self) -> usize
    where
        Self: HasA + HasB,
    {
        0
    }
}

#[rustc_on_unimplemented(
    message = "missing the required a field",
    note = "use `builder.a(value)` to add the field",
)]
trait HasA {}
impl<const B: bool> HasA for ItemBuilder<true, B> {}


#[rustc_on_unimplemented(
    message = "missing the required `b` field",
    note = "use `builder.b(value)` to add the field",
)]
trait HasB {}
impl<const A: bool> HasB for ItemBuilder<A, true> {}

fn test() {
    let builder = ItemBuilder::<false, false>;
    builder.build();

    let builder = ItemBuilder::<true, false>;
    builder.build();

    let builder = ItemBuilder::<false, true>;
    builder.build();

    let builder = ItemBuilder::<true, true>;
    builder.build();
}