Code shared from the Rust Playground

playground.rs

use std::cmp::Eq;
use std::collections::HashMap;
use std::hash::Hash;

pub fn occurrence_map<I, E>(target: I) -> HashMap<E, usize>
where
    I: IntoIterator<Item = E>,
    E: Eq + Hash,
{
    let mut hash_map = HashMap::new();
    for el in target {
        *hash_map.entry(el).or_insert(0) += 1;
    }
    hash_map
}

fn main() {
    occurrence_map(&["a", "b", "a"]);
    occurrence_map(vec![String::from("a"), String::from("b"), String::from("a")]);
}