PgDb 是一个tuple struct，定义为： pub struct PgDb(PooledConnection);

error[E0423]: expected function, found struct `PgDb`
   --> src/vokator/main.rs:139:21
    |
139 |         let pg_db = PgDb(pg_conn);
    |                     ^^^^ did you mean `PgDb { /* fields */ }`?

这里明明是实例一个tuple struct，为什么告诉我是一个函数啊？