Minimum needed characters problem solved in Rust

minimum_chars.rs

use std::collections::HashMap;

fn main() {
    println!("{:?}", word2chars("pear"));
    println!("{:?}", needed_chars(&["apple", "pear"]));
}

fn needed_chars(words: &[&str]) -> HashMap<char, u32> {
    words
        .iter()
        .map(|x| word2chars(x))
        .fold(HashMap::new(), |mut x, y| {
            for (k, v) in y {
                x.entry(k).and_modify(|z| *z = (*z).max(v)).or_insert(v);
            }
            x
        })
}

fn word2chars(word: &str) -> HashMap<char, u32> {
    word.chars().fold(HashMap::<char, u32>::new(), |mut x, y| {
        x.entry(y).and_modify(|z| *z += 1).or_insert(1);
        x
    })
}