Lifetime Problem

ExpiringCache.rs

use std::collections::{BTreeMap, HashMap};
use std::hash::Hash;
use std::cmp::Ordering;

pub struct Cache<'a, K: 'a,V: 'a> where K : Eq + Hash, V: Ord {
    map: HashMap<K,V>,
    keys: BTreeMap<&'a V, &'a K>
}

impl<'a, K,V> Cache<'a, K,V> where K : Eq + Hash, V: Ord {
    pub fn new() -> Cache<'a, K,V> {
        Cache {
            map: HashMap::new(),
            keys: BTreeMap::new()
        }
    }

    pub fn upsert(&mut self, key: K, val: V)  {
        self.keys.insert(&val, &key);
        self.map.insert(key, val);
    }
}

#[derive(Eq)]
#[derive(PartialEq)]
#[derive(Hash)]
struct ResolverCacheKey {
    qname: String,
    qtype: u32,
    qclass: u32
}

#[derive(PartialOrd)]
#[derive(PartialEq)]
#[derive(Eq)]
struct ResolverCacheEntry {
    answers: Vec<String>,
    ttl: u32
}

impl Ord for ResolverCacheEntry {
    fn cmp(&self, other: &Self) -> Ordering {
        self.ttl.cmp(&other.ttl)
    }
}

pub struct ResolverCache<'a> {
    base: Cache<'a,ResolverCacheKey,ResolverCacheEntry>
}