Inserting same item into HashSets in two different class instances

In the code below, I define a type Bird, and then a type Birdwatcher which has a HashSet of birds_seen.

Birdwatchers have a method .see(bird) which inserts a bird to their HashSet. (In context, this method has some other logic, so we cannot just call the hashset's .insert() method directly.)

In the main() function, I instantiate a bird as well as two birdwatchers. The code does not compile if I try to have both birdwatchers .see() the same bird:

use std::collections::HashSet;

fn main() {
    let falcon = Bird { name: "falcon".to_string(), speed: 25 };

    let mut alice = Birdwatcher { name: "Alice".to_string(), birds_seen: HashSet::new() };
    let mut bobby = Birdwatcher { name: "Bobby".to_string(), birds_seen: HashSet::new() };

    alice.see(falcon);
    // use of moved value: `falcon`
    // bobby.see(falcon);
}

#[derive(Hash, Eq, PartialEq)]
struct Bird {
    name: String,
    speed: isize,
}

struct Birdwatcher {
    name: String,
    birds_seen: HashSet<Bird>,
}

impl Birdwatcher {
    fn see(&mut self, bird: Bird) {
        // Additional logic goes here
        self.birds_seen.insert(bird);
    }
}

I though I could fix this by changing the .see() method to borrow the bird, as below, but this does not compile due to mismatched types:

impl Birdwatcher {
    fn see(&mut self, bird: &Bird) {
        // mismatched types: expected `Bird`, found `&Bird`
        self.birds_seen.insert(bird);
    }
}

How can I write the .see() method to allow the same bird to be seen by multiple birdwatchers?

> we shouldn't need to "use" the birds to store them in a hashset; we can just compute their hash (once) and add it to the set.

A HashSet is not a set of hashes; it is a set which uses hashes. It still contains, owns, the values that are inserted just like a Vec or any other regular collection type in Rust would.

This is necessary because the hashes may collide — they don't have enough bits to practically guarantee not colliding — in which case the set will have to fall back to comparing the values themselves with Eq. Additionally, HashSet allows iteration over the items, or removal of the items, so it must own them to be able to produce them.

If you want a set that stores only hashes, you will need to make a set of those hashes, not of the values that are hashed to produced them.