Function decode_error 

fn decode_error(
    err_msg: &str,
    col_desc: &MySqlColumnDesc,
    table_desc: &MySqlTableDesc,
    gtid_set: Option<&str>,
    row: &Row,
) -> MySqlError

Build a ValueDecodeError, logging the schema, table, column, source gtid_set (if any), and a shape description of row at the same time. The shape string is only built here — pack_mysql_row’s happy path does no per-row allocation beyond what decoding requires.