Rust .collect() an iterator into a HashMap

lib.rs

use std::collections::{HashMap, HashSet};

fn count_occurrences(message: &str) -> HashMap<char, usize> {
    let unique: HashSet<char> = message.chars().collect();
    unique
        .iter()
        .map(|&c| (c, message.matches(c).count()))
        .collect()
}

// count_occurrences("aabcddd")
// {
//     'd': 3,
//     'b': 1,
//     'c': 1,
//     'a': 2,
// }