Rust convert string to pig latin(Rust lang book chapter 8 exercise)

main.rs

fn main() {
    let normal_str = String::from("first apple");
    let pig_latin = convert_to_pig_latin(&normal_str);

    println!("Input: {}\nOutput: {}", normal_str, pig_latin);
}

fn convert_to_pig_latin(input_str: &String) -> String {
    let mut str_array = vec![];

    // Splitting up words in input string
    for word in input_str.to_lowercase().split_whitespace() {

        // Init prefix and ending variables
        let mut prefix = String::from("");
        let mut ending = String::from("");

        let mut count = 0;

        // Loop through chars of each word. Counter is used to tell whether or not we are at the
        // first char of a word, if so check if it is a vowel or not. If not on first char add
        // chars to prefix. 
        for char in word.chars() {
            match char {
                'a' | 'e' | 'i' | 'o' | 'u' => {
                    if count == 0 {
                        ending.push_str("hay");
                        prefix.push(char);
                    } else {
                        prefix.push(char);
                    }
                }
                _ => {
                    if count == 0 {
                        ending.push_str(char.to_string().as_str());
                        ending.push_str("ay");
                    } else {
                        prefix.push(char);
                    }
                }
            }
            count += 1;
        }
        // Join prefix and ending together to form new "pig latin" word. Push this new word to the
        // array of strings
        str_array.push(prefix + &"-".to_string() + &ending);
    }
    
    // Join array of pig latin strings together and return
    str_array.join(" ")
}

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