HashSet<&String> to be coerced into HashSet<&str>

playground.rs

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);
}