Generic Cacher without Clone

cacher.rs

use std::collections::hash_map::Entry;
use std::collections::HashMap;
use std::hash::Hash;
use std::ops::Fn;

struct Cacher<'a, T, A, B>
where
    T: 'a + Fn(&A) -> B,
    A: 'a,
    B: 'a,
{
    calculation: T,
    values: HashMap<&'a A, B>,
}

impl<'a, T, A, B> Cacher<'a, T, A, B>
where
    T: Fn(&A) -> B,
    A: Eq + Hash,
{
    fn new(calculation: T) -> Cacher<'a, T, A, B> {
        Cacher {
            calculation,
            values: HashMap::new(),
        }
    }

    fn value(&mut self, arg: &'a A) -> &B {
        let f = &self.calculation;
        match self.values.entry(arg) {
            Entry::Occupied(v) => v.into_mut(),
            Entry::Vacant(a) => {
                let v = f(&a.key());
                a.insert(v)
            }
        }
    }
}