HashMap にループで値を代入する方法です。

hash01.rs

// --------------------------------------------------------------------
/*
	hash01.rs

						Jul/11/2020
*/
// --------------------------------------------------------------------
use std::collections::HashMap;

// --------------------------------------------------------------------
fn main(){

	eprintln! ("*** start ***");

	let mut unit_aa:HashMap <String,f64> = HashMap::new ();

	for count in 0..5 {
        	let key = "key_".to_owned() + &count.to_string();
        	unit_aa.insert(key, count as f64);
		}

	println!("{:?}",unit_aa);

	eprintln! ("*** end ***");
}

// --------------------------------------------------------------------

Makefile

hash01: hash01.rs
	rustc hash01.rs
clean:
	rm  -f hash01

コンパイル

make

実行結果

$ ./hash01 
*** start ***
{"key_4": 4.0, "key_0": 0.0, "key_2": 2.0, "key_1": 1.0, "key_3": 3.0}
*** end ***