In Rust, what is the proper way to replicate Python's "repeat" parameter in itertools.product?

In Python, I can do:

from itertools import product

k = 3
for kmer in product("AGTC", repeat=k):
    print(kmer)

In Rust, I can force the behavior of k=3 by:

#[macro_use] extern crate itertools;

for kmer in iproduct!("AGTC".chars(), "AGTC".chars(), "AGTC".chars()){
    println!("{:?}", kmer);
}

But what if I wanted k=4 or k=5?

Writing a proper generalisation for any type for any k would be hard because the return type could be tuples of any size. As you want to work only on String, it's quite easier: playground

fn kproduct(seq: String, k: u32) -> Vec<String> {
    match k {
        0 => vec![],
        1 => seq.chars().map(|c| c.to_string()).collect(),
        2 => iproduct!(seq.chars(), seq.chars()).map(|(a, b)| format!("{}{}", a, b)).collect(),
        _ => iproduct!(kproduct(seq.clone(), k - 1), seq.chars()).map(|(a, b)| format!("{}{}", a, b)).collect(),
    }
}