Rust - breaking nested generic type #407

#![feature(lookup_host)]

use std::collections;

struct DnsLookupCache {
     cache : std::collections::HashMap<Box<String>, Vec<std::net::SocketAddr>>
}

impl DnsLookupCache {
    fn new() -> DnsLookupCache {
         DnsLookupCache {
            cache: std::collections::HashMap::new()
        }
    }

    fn lookup(&mut self, host: &String) -> std::io::Result<&Vec<std::net::SocketAddr>> {
        match self.cache.get(host) {
            Some(cached) => Ok(cached),
            None => {
                let mut hosts = Vec::new();
                for  result in try!(std::net::lookup_host(host)) {
                    hosts.push(result);
                }
                let owned_host = host.clone();
                self.cache.insert(Box::new(owned_host), hosts);
                return self.lookup(host);
            }
        }
    }
}