Procedural vs functional Rust for passphrase generation

passphrase.rs

// Let’s say we have a word list.
let word_list: Vec<String> = ...;

// We want to generate a random passphrase using words from the list.
// Let’s see how to do this with equal performance in two different styles:
// procedural and functional Rust.

// In both cases we need a random generator.
let mut rng = rand::thread_rng();

// The procedural way.
let mut passphrase = String::new();
for _ in 0..config.words {
    let word = rng.choose(&word_list).unwrap();
    passphrase.push_str(word);
    passphrase.push_str(" ");
}

// The functional way.
let mut passphrase = (0..config.words).fold(String::new(), |s, _| {
    s + rng.choose(&word_list).unwrap() + " "
});

// Pop the trailing space.
passphrase.pop();