Result has no method called “unwrap()”?

问题:

What a strange error:

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
}

The error is:

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`

回答1:

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

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

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