tengo unHashMap y necesita obtener el primer elemento:

type VarIdx = std::collections::HashMap<u16, u8>;

fn get_first_elem(idx: VarIdx) -> u16 {
    let it = idx.iter();
    let ret = match it.next() {
        Some(x) => x,
        None => -1,
    };
    ret
}

fn main() {}

pero el código no compila:

error[E0308]: match arms have incompatible types
 --> src/main.rs:5:15
  |
5 |       let ret = match it.next() {
  |  _______________^
6 | |         Some(x) => x,
7 | |         None => -1,
8 | |     };
  | |_____^ expected tuple, found integral variable
  |
  = note: expected type `(&u16, &u8)`
             found type `{integer}`
note: match arm with an incompatible type
 --> src/main.rs:7:17
  |
7 |         None => -1,
  |                 ^^

¿Cómo puedo arreglarlo?