#![feature(nll)]

use std::collections::HashMap;

#[derive(Debug, Copy, Clone)]
struct Foo;
struct FooSource;
impl FooSource {
    fn get(&self, _id: &str) -> Foo { Foo }
}

#[derive(Debug, Clone)]
struct FooCache {
    foos: HashMap<String, Foo>,
}

impl FooCache {
    fn new() -> FooCache {
        FooCache { foos: HashMap::new() }
    }

    fn fetch<'a>(&'a mut self, foo_source: &FooSource, id: &str) -> &'a Foo {
        self.foos.entry(id.to_string()).or_insert_with(|| foo_source.get(id))
    }
}