string - 无法将HashSet <＆str>强制转换为HashSet <＆String>

Question

在弄乱一些示例代码时，我发现我无法将HastSet<&String>强制转换为HashSet<&str>，反之亦然。

use std::collections::HashSet;

fn main() {
    // move strings into HashSet
    let known_values: HashSet<&str> = ["a", "b", "c"]
        .iter().cloned().collect();

    // provided an Vec<String>
    let provided_values: Vec<String> = vec![
        "a".to_string(),
        "b".to_string(),
        "z".to_string()
    ];

    // hash set of refrences
    let mut found: HashSet<&String> = HashSet::new();
    found.insert(&provided_values[0]);
    found.insert(&provided_values[1]);
    found.insert(&provided_values[2]);

    //let missing = known_values.difference(&found);
    let missing = found.difference(&known_values);

    println!("missing: {:#?}", missing);
}

(Playground)

编译器错误

error[E0308]: mismatched types
  --> src/main.rs:23:36
   |
23 |     let missing = found.difference(&known_values);
   |                                    ^^^^^^^^^^^^^ expected struct `std::string::String`, found `str`
   |
   = note: expected reference `&std::collections::HashSet<&std::string::String>`
              found reference `&std::collections::HashSet<&str>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.

我知道您可以将&String强制转换为&str，反之亦然，因此令我有些惊讶的是它没有扩展到HashSet<&String>和HashSet<&str>。有什么方法可以解决这个问题而不仅仅是分配String？

Answer 1

无需使用其他类型的HashSet。将found更改为HashSet<&str>即可运行该程序。

// hash set of references
let mut found: HashSet<&str> = HashSet::new();
found.insert(&provided_values[0]);
found.insert(&provided_values[1]);
found.insert(&provided_values[2]);

(Playground)