Items counter with Rust

counter.rs

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Counter<T: Eq + std::hash::Hash> {
    counts: std::collections::HashMap<T, usize>,
}

impl<T, Q> std::ops::Index<&Q> for Counter<T>
where
    T: Eq + std::hash::Hash + std::borrow::Borrow<Q>,
    Q: Eq + std::hash::Hash + ?Sized,
{
    type Output = usize;
    fn index(&self, key: &Q) -> &usize {
        self.counts.get(key).unwrap_or(&0)
    }
}

impl<T: Eq + std::hash::Hash> Counter<T> {
    pub fn new(items: impl IntoIterator<Item = T>) -> Self {
        let mut counts = std::collections::HashMap::new();
        for item in items {
            *counts.entry(item).or_insert(0) += 1;
        }
        Self { counts }
    }

    #[inline]
    pub fn get<Q>(&self, key: &Q) -> usize
    where
        T: std::borrow::Borrow<Q>,
        Q: Eq + std::hash::Hash + ?Sized,
    {
        self[key]
    }
}