{"ast_node_type":"other","tokens":[{"end":83,"kind":"keyword","start":0,"text":"///"}]}
    /// - For `Pair(a, b)`, `a` is always at offset 0, but may have `FieldIdx(1..)`
    /// # Invariants
    /// - `b` is not at offset 0, because `V` is not a 1ZST type.