mysql - 如何从一个 mysql::Conn 创建多个准备好的语句？

Question

这段代码

extern crate mysql;
use mysql::*;

fn main () {
    let mut conn = Conn::new("mysql://root:password@127.0.0.1:3306/mydb?prefer_socket=false").unwrap();
    let _q1 = conn.prepare("some query").unwrap();
    let _q2 = conn.prepare("some query 2").unwrap();
}

产生错误:

error[E0499]: cannot borrow ``conn`` as mutable more than once at a time
 --> src\bin\test.rs:8:12
  |
7 |  let _q1 = conn.prepare("some query").unwrap();
  |            ---- first mutable borrow occurs here
8 |  let _q2 = conn.prepare("some query 2").unwrap();
  |            ^^^^ second mutable borrow occurs here
9 | }
  | - first borrow ends here

我想第一次借用到第 9 行是因为 conn.prepare(...).unwrap() 的结果是 Stmt 和 Stmt 声明为

pub struct Stmt<'a> {
    stmt: InnerStmt,
    conn: ConnRef<'a>,
}

和ConnRef被声明为

enum ConnRef<'a> {
    ViaConnRef(&'a mut Conn), // It keeps mutable borrow as long as lifetime of connection?
    ViaPooledConn(pool::PooledConn),
}

所以 _q1 保持可变借用直到 conn 结束。

如何在不产生编译时错误的情况下创建多个准备好的语句？

Answer 1

documentation状态:

Stmt will borrow Conn until the end of its scope.

有一个Github issue这建议使用 prep_exec , 声称:

Overhead is negligible because of fast xxhash based statements cache.