Rust: cannot move out of borrowed content

error.rs

struct Foo {
    value: String
}

impl Foo {
    fn test(&mut self) -> String {
        let old = self.value;
        self.value = "new".to_string();
        old
    }
}

fn main() {
    let mut foo = Foo { value: "old".to_string() };
    assert_eq!("old", foo.test());
    println!("foo = {}", foo.value);
}

Error is thrown at line 7. How can I replace the value in the struct, but also retain the old value?

Can be solved with mem::replace(&mut self.value, "new".to_string()). see documentation