error-handling - 创建使用 try 运算符的闭包时如何修复错误 "type annotations needed"？

Question

use std::fs::File;

fn main() {
    let tmp = "tmp.zip";

    let save_to_tmp_file = || {
        let mut tmp_zip = File::create(tmp)?;
        Ok(())
    };

    save_to_tmp_file();
}

( playground )

构建错误:

error[E0282]: type annotations needed
 --> src/main.rs:7:27
  |
6 |     let save_to_tmp_file = || {
  |         ---------------- consider giving `save_to_tmp_file` a type
7 |         let mut tmp_zip = File::create(tmp)?;
  |                           ^^^^^^^^^^^^^^^^^^ cannot infer type for `_`

Answer 1

你必须指定返回类型:

use std::fs::File;
use std::io;

fn main() {
    let tmp = "tmp.zip";

    let save_to_tmp_file = || -> Result<(), io::Error> {
        let mut tmp_zip = File::create(tmp)?;
        Ok(())
    };

    save_to_tmp_file();
}

关闭有问题。您不能命名闭包的类型；您可以尝试将其缩减为 FnOnce/FnMut，但您仍然必须为其提供返回类型。

为了克服您的情况，您可以将 Result 指定为 return type for your main function .

use std::io;
use std::fs::File;

fn main() -> Result<(), io::Error> {
    let tmp = "tmp.zip";
    let mut tmp_zip = File::create(tmp)?;
    Ok(())
}