Provide a default for Option<T> unwrap() through a reference to a static instance of an object, e.g. for HashMap::<&str, String>.

UnwrapOrEmpty.rs

trait UnwrapOrEmpty<'a> {
    fn unwrap_or_empty(&self) -> &'a String;
}
impl<'a> UnwrapOrEmpty<'a> for Option<&'a String> {
    fn unwrap_or_empty(&self) -> &'a String {
        self.unwrap_or_else(||{ // use a closure
            static DEFAULT: String = String::new(); // declare a static String
            &DEFAULT // return a reference to a String to match HashMap::get()
        })
    }
}
let map = HashMap::<String, String>::new();
let val = map.get("").unwrap_or_empty();
println!("{}", val);