Appending to a mutable vector in a hashmap doesn't update the vector

[jdm]

import std::map;
import std::map::hashmap;
use std;

type mut_elts = {mutable elts: [int]};

fn main() {
    let elts_hash = map::str_hash::();
    elts_hash.insert("foo", {mutable elts: []});
    elts_hash.get("foo").elts += [1];
    #error("%u", vec::len(elts_hash.get("foo").elts));
}

I would expect to see 1 printed, but instead I see 0.

[marijnh]

Our value semantics are similar to C, not to languages like Lisp or JavaScript. A record is copied when stored in another place, so putting it into the hash table and getting it out again causes it to be repeatedly copied. Store a @-pointer to the record if you want to actually get at a single value.