如何使用盒子<String>作为带有 &amp;str 键的哈希图的查找键？

Question

My hashmap keys are expected to be of type &str , in particular &'static str , and I've received an owned Box<String> .
How do I search my string against my hashmap?

use std::collections::HashMap;

fn main() {
    let mut map: HashMap<&str, u32> = HashMap::new();
    let static_string = "a";
    map.insert(static_string, 5);

    let owned_boxed_string = Box::new(String::from("a"));

    map.get(owned_boxed_string); // mismatched type (ok)
    map.get(*owned_boxed_string); // mismatched type (ok)
    map.get(&*owned_boxed_string); // trait bound not satisfied (?)
} 

Answer 1

Let's look at the definition of HashMap::get() :

pub fn get<Q: ?Sized>(&self, k: &Q) -> Option<&V> where
    K: Borrow<Q>,
    Q: Hash + Eq, 

In your code, K is &'static str and Q is deduced from each of the calls to get . In plain English, get() takes a reference to a type Q such that &'static str implements Borrow<Q> .

The rationale is that usually you will store keys of type String or the like, and you will search using values of type &str . And naturally String implements Borrow<str> so you can do that.

In your case, your key is not String but &'static str ... Take a look at the documentation for Borrow and look for what kind of Borrow<Q> is implemented by &'static str . I can only see these two blanket implementations:

impl<'_, T: ?Sized> Borrow<T> for &'_ T
impl<T: ?Sized> Borrow<T> for T

The first one states that if your map keys are references to some type &K you can call get(k: &K) . The second one says that if your key is K you can call get(k: &K) .

So for your particular case of &'static str they are realized as:

impl Borrow<str> for &'static str
impl Borrow<&'static str> for &'static str

From that you can deduce that your function is either get(k: &str) or get(k: &&str) . Here the easiest choice is the first one.

Now, you may think that your last line ( map.get(&*owned_boxed_string); ) should work, because that value is of type &str , but it actually is not, it is of type &String . If HashMap::get() received a &str it would compile fine, but taking a Borrow<Q> , the compiler gets all confused and fails.

TL; DR; : Add a type cast or a temporary typed variable:

map.get(&*owned_boxed_string as &str);  //ok
map.get(&owned_boxed_string as &str);   //also ok

let s: &str = &*owned_boxed_string;
map.get(s);
let s: &str = &owned_boxed_string;
map.get(s);

Although an easier solution (thanks to @harmic for the comment below) is to use String::as_str() that will take care of as many derefs as required and just return the necessary &str :

map.get(owned_boxed_string.as_str());