use std::collections::HashMap;
use std::collections::hash_map::Entry;

fn main() {
    let mut d = HashMap::new();
    match d.entry("k1") {
        Entry::Vacant(e) => {
            e.insert(Vec::new());
        }
        Entry::Occupied(mut e) => {
            e.get_mut().push("v1");
        }
    }
    match d.entry("k1") {
        Entry::Vacant(e) => {
            e.insert(Vec::new());
        }
        Entry::Occupied(mut e) => {
            e.get_mut().push("v1");
        }
    }
    println!("{:?}", d);
    // {"k1": ["v1"]}
}