为什么HashMap::iter.nth(0)为每次执行运行提供不同的输出？

鉴于以下计划:

use std::collections::HashMap;

fn main() {
    let mut hm = HashMap::new();
    hm.insert(0, 1);
    hm.insert(1, 1);
    let mut iter = hm.iter();
    println!("{:?}", iter.nth(0).expect("Fatal.").0)
}

我为代码的每次执行运行获得了不同的输出:

procyclinsur@procyclinsur:~/Documents/Rust/t1$ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `target/debug/t1`
1
procyclinsur@procyclinsur:~/Documents/Rust/t1$ vim src/main.rs 
procyclinsur@procyclinsur:~/Documents/Rust/t1$ cargo run
   Compiling t1 v0.1.0 (file:///home/procyclinsur/Documents/Rust/t1)
    Finished dev [unoptimized + debuginfo] target(s) in 1.12 secs
     Running `target/debug/t1`
1
procyclinsur@procyclinsur:~/Documents/Rust/t1$ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `target/debug/t1`
1
procyclinsur@procyclinsur:~/Documents/Rust/t1$ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `target/debug/t1`
0
procyclinsur@procyclinsur:~/Documents/Rust/t1$ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `target/debug/t1`
1

use std::collections::HashMap;

fn main() {
    let mut hm = HashMap::new();
    hm.insert(0, "a");
    hm.insert(1, "b");
    println!("{:?}", hm.get(&0)) // always Some("a")
}