结果没有名为“unwrap()"的方法? [英] Result has no method called "unwrap()"?

问题描述

多么奇怪的错误:

use std::collections::BTreeMap;

struct MyStruct1;
struct Error;

fn get_res() -> Result<(MyStruct1, BTreeMap<String, String>), Error> {
    Err(Error)
}

fn main() {
    let res1 = get_res();
    assert!(res1.is_ok());
    assert_eq!("just for test", res1.unwrap()); //error
}

错误是:

error: no method named `unwrap` found for type `std::result::Result<(MyStruct1, std::collections::BTreeMap<std::string::String, std::string::String>), Error>` in the current scope
  --> src/main.rs:13:38
   |
13 |     assert_eq!("just for test", res1.unwrap()); //error
   |                                      ^^^^^^
   |
   = note: the method `unwrap` exists but the following trait bounds were not satisfied: `Error : std::fmt::Debug`

推荐答案

如果您阅读了 Result::unwrap，你会注意到它在一个叫做:

If you read the documentation for Result::unwrap, you'll note that it's under a little section called:

impl<T, E> Result<T, E> 
    where E: Debug

这意味着该部分中的方法只存在，只要满足给定的约束.

This means the methods in that section only exist so long as the given constraints are satisfied.

unwrap 不存在的唯一原因是 Error 没有实现 Debug.

The only reason unwrap wouldn't exist is that Error doesn't implement Debug.

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