{"ast_node_type":"variable","tokens":[{"end":84,"kind":"keyword","start":0,"text":"let"}]}
            let wits_for_col_i = collect_nonexhaustive_missing_variants(cx, col_i)?;
            // Compute witnesses for each column.
            // For each witness, we build a new pattern in the shape of `ctor(_, _, wit, _, _)`,