fn absent<'a, FieldIdx, VariantIdx, F>(fields: &IndexSlice<FieldIdx, F>) -> boolwhere
    FieldIdx: Idx,
    VariantIdx: Idx,
    F: Deref<Target = &'a LayoutData<FieldIdx, VariantIdx>> + Debug,fn absent<'a, FieldIdx, VariantIdx, F>(fields: &IndexSlice<FieldIdx, F>) -> boolwhere
    FieldIdx: Idx,
    VariantIdx: Idx,
    F: Deref<Target = &'a LayoutData<FieldIdx, VariantIdx>> + Debug,