Rust HashMap find_or_insert

Question

Why does HashMap.find_or_insert(k,v) return a &mut type, and how do it get it to return a type ?

I just started using Rust for a course, and I am using a HashMap<int, int> and want to get an int back.

let mut m: HashMap<int, int> = HashMap::new();
println!("{:d}", m.find_or_insert(1,2));

gives me an error saying that it failed to find an implementation of trait std::fnt::Signed for &mut int .

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Edit1:

I am using Rust 0.9 on Windows 8.1, using msys.

My code so far

use std::hashmap::HashMap;

fn main() {
    let mut m: HashMap<int, int> = HashMap::new();
    println!("{:d}", *m.find_or_insert(1,2))
}

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I tried this code again and it properly returns 2

Answer 1

Why does find_or_insert return a reference? Copying isn't always possible/efficient.

How can you use that reference in the case of an integer? Dereference it with a * :

let mut m: HashMap<int, int> = HashMap::new();
println!("{:d}", m.find_or_insert(1,2));