use std::collections::HashMap;
//first method
let d1 = HashMap::from([
(1, "one"), (2, "two")]);
let d2 = HashMap::from([
(3, "three")]);
let d_all1: HashMap<_, _> = d1
.into_iter().chain(d2).collect();
//d_all1 is {2: "two", 1: "one", 3: "three"}
//second method
let d3 = HashMap::from([
(1, "one"), (2, "two")]);
let mut d_all2 = HashMap::from([
(3, "three")]);
d_all2.extend(d3);
//d_all2 is {2: "two", 1: "one", 3: "three"}
println!("d_all1 is {:?}", d_all1);
println!("d_all2 is {:?}", d_all2);