`?` 运算符只能在返回 `Result` 或 `Option` （或实现 `std::ops::Try` 的其他类型）的函数中使用

我正在做我的作业，其中包括与 Rust 中的数据库建立连接。我正在使用最新版本的 mysql crate：mysql ="18.2.0"。当我打印池变量时，我的数据库连接成功。我为表学生编写了自己的代码，但收到错误。然后我粘贴文档代码，我收到以下带有“？”的错误操作员：

我是第一次用 Rust 连接数据库。任何帮助表示赞赏。

warning: unused import: `std::io`
 --> src/main.rs:2:5
  |
2 | use std::io;
  |     ^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
  --> src/main.rs:17:12
   |
14 | / fn insert(){
15 | |
16 | |
17 | | let pool = Pool::new("mysql://root:root@localhost:3306/Rust_testing")?;
   | |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot use the `?` operator in a function that returns `()`
...  |
58 | |
59 | | }
   | |_- this function should return `Result` or `Option` to accept `?`
   |
   = help: the trait `std::ops::Try` is not implemented for `()`
   = note: required by `std::ops::Try::from_error`

error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
  --> src/main.rs:19:16
   |
14 | / fn insert(){
15 | |
16 | |
17 | | let pool = Pool::new("mysql://root:root@localhost:3306/Rust_testing")?;
18 | |
19 | | let mut conn = pool.get_conn()?;
   | |                ^^^^^^^^^^^^^^^^ cannot use the `?` operator in a function that returns `()`
...  |
58 | |
59 | | }
   | |_- this function should return `Result` or `Option` to accept `?`
   |
   = help: the trait `std::ops::Try` is not implemented for `()`
   = note: required by `std::ops::Try::from_error`

error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
  --> src/main.rs:22:1
   |
14 |  / fn insert(){
15 |  |
16 |  |
17 |  | let pool = Pool::new("mysql://root:root@localhost:3306/Rust_testing")?;
...   |
22 | /| conn.query_drop(
23 | ||     r"CREATE TEMPORARY TABLE payment (
24 | ||         customer_id int not null,
25 | ||         amount int not null,
26 | ||         account_name text
27 | ||     )")?;
   | ||________^ cannot use the `?` operator in a function that returns `()`
...   |
58 |  |
59 |  | }
   |  |_- this function should return `Result` or `Option` to accept `?`
   |
   = help: the trait `std::ops::Try` is not implemented for `()`
   = note: required by `std::ops::Try::from_error`

error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
  --> src/main.rs:38:1
   |
14 |  / fn insert(){
15 |  |
16 |  |
17 |  | let pool = Pool::new("mysql://root:root@localhost:3306/Rust_testing")?;
...   |
38 | /| conn.exec_batch(
39 | ||     r"INSERT INTO payment (customer_id, amount, account_name)
40 | ||       VALUES (:customer_id, :amount, :account_name)",
41 | ||     payments.iter().map(|p| params! {
...  ||
45 | ||     })
46 | || )?;
   | ||__^ cannot use the `?` operator in a function that returns `()`
...   |
58 |  |
59 |  | }
   |  |_- this function should return `Result` or `Option` to accept `?`
   |
   = help: the trait `std::ops::Try` is not implemented for `()`
   = note: required by `std::ops::Try::from_error`

error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
  --> src/main.rs:49:25
   |
14 |  / fn insert(){
15 |  |
16 |  |
17 |  | let pool = Pool::new("mysql://root:root@localhost:3306/Rust_testing")?;
...   |
49 |  | let selected_payments = conn
   |  |_________________________^
50 | ||     .query_map(
51 | ||         "SELECT customer_id, amount, account_name from payment",
52 | ||         |(customer_id, amount, account_name)| {
53 | ||             Payment { customer_id, amount, account_name }
54 | ||         },
55 | ||     )?;
   | ||______^ cannot use the `?` operator in a function that returns `()`
...   |
58 |  |
59 |  | }
   |  |_- this function should return `Result` or `Option` to accept `?`
   |
   = help: the trait `std::ops::Try` is not implemented for `()`
   = note: required by `std::ops::Try::from_error`

error: aborting due to 5 previous errors; 1 warning emitted

For more information about this error, try `rustc --explain E0277`.
error: could not compile `class-09`.

这是代码，我从文档复制来测试：

 use std::io;
    
    use mysql::prelude::*;
    use mysql::*;
    
    
    #[derive(Debug, PartialEq, Eq)]
    struct Payment {
        customer_id: i32,
        amount: i32,
        account_name: Option<String>,
    }

fn insert(){
    let pool = Pool::new("mysql://root:root@localhost:3306/Rust_testing")?;

let mut conn = pool.get_conn()?;

// Let's create a table for payments.
conn.query_drop(
    r"CREATE TEMPORARY TABLE payment (
        customer_id int not null,
        amount int not null,
        account_name text
    )")?;

let payments = vec![
    Payment { customer_id: 1, amount: 2, account_name: None },
    Payment { customer_id: 3, amount: 4, account_name: Some("foo".into()) },
    Payment { customer_id: 5, amount: 6, account_name: None },
    Payment { customer_id: 7, amount: 8, account_name: None },
    Payment { customer_id: 9, amount: 10, account_name: Some("bar".into()) },
];

// Now let's insert payments to the database
conn.exec_batch(
    r"INSERT INTO payment (customer_id, amount, account_name)
      VALUES (:customer_id, :amount, :account_name)",
    payments.iter().map(|p| params! {
        "customer_id" => p.customer_id,
        "amount" => p.amount,
        "account_name" => &p.account_name,
    })
)?;

// Let's select payments from database. Type inference should do the trick here.
let selected_payments = conn
    .query_map(
        "SELECT customer_id, amount, account_name from payment",
        |(customer_id, amount, account_name)| {
            Payment { customer_id, amount, account_name }
        },
    )?;

println!("Yay!");

}

fn main(){
    insert();
}

当我编写没有 ? 的代码时操作员，我收到以下错误：

warning: unused import: `std::io`
 --> src/main.rs:2:5
  |
2 | use std::io;
  |     ^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

error[E0599]: no method named `query_drop` found for enum `std::result::Result<mysql::conn::pool::PooledConn, mysql::error::Error>` in the current scope
  --> src/main.rs:32:6
   |
32 | conn.query_drop(
   |      ^^^^^^^^^^ method not found in `std::result::Result<mysql::conn::pool::PooledConn, mysql::error::Error>`

error[E0599]: no method named `exec_batch` found for enum `std::result::Result<mysql::conn::pool::PooledConn, mysql::error::Error>` in the current scope
  --> src/main.rs:48:6
   |
48 | conn.exec_batch(
   |      ^^^^^^^^^^ method not found in `std::result::Result<mysql::conn::pool::PooledConn, mysql::error::Error>`

error[E0599]: no method named `query_map` found for enum `std::result::Result<mysql::conn::pool::PooledConn, mysql::error::Error>` in the current scope
  --> src/main.rs:60:6
   |
60 |     .query_map(
   |      ^^^^^^^^^ method not found in `std::result::Result<mysql::conn::pool::PooledConn, mysql::error::Error>`

warning: unused import: `mysql::prelude`
 --> src/main.rs:4:5
  |
4 | use mysql::prelude::*;
  |     ^^^^^^^^^^^^^^

error: aborting due to 3 previous errors; 2 warnings emitted

For more information about this error, try `rustc --explain E0599`.
error: could not compile `class-09`.

正如编译器告诉您的那样：您的函数中缺少返回类型。 这 ？运算符将返回（传播）错误（如果有），但要使其正常工作，您需要有一个可以使用错误类型构造的返回类型。

对于原型设计，您只需调用 unwrap 即可。但在编写生产代码时应仔细考虑这种方法，因为当函数返回错误时，它只会使程序崩溃。

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