https://qiita.com/yosgspec/items/605e68bb1ca21e6ff819

main.rs

/// "2$4$8$16"
///     ↓
/// "$"を"#"に置換
///     ↓
/// "2#4#8#16"
///     ↓
/// "#"を区切り文字に配列へ分割
/// その要素数を表示
///     ↓
/// "4"
///     ↓
/// 配列に要素"32"を追加
/// その要素数を表示
///     ↓
/// "5"
///     ↓
/// 配列を整数型に変換し要素の合計値を算出
///     ↓
/// "62"
///     ↓
/// 整数変換前の配列に"++"を区切り文字とし、文字列として結合
///     ↓
/// "2++4++8++16++32"
///     ↓
/// 文字列"Hoge->"と上記文字列を連結
///     ↓
/// "Hoge->2++4++8++16++32"
///     ↓
/// 左から10文字分切り出す
///     ↓
/// "Hoge->2++4"
///     ↓
/// 右から6文字分切り出す
///     ↓
/// "->2++4"
fn solve(s: &str) -> String {
    let mut result = Vec::with_capacity(9);
    result.push(s.into());
    let replaced = s.replace("$", "#");
    result.push(replaced.clone());
    let mut split = replaced.split("#").collect::<Vec<_>>();
    result.push(split.len().to_string());
    split.push("32");
    result.push(split.len().to_string());
    result.push(split.iter().flat_map(|i| i.parse::<i32>()).sum::<i32>().to_string());
    let mut joined = split.join("++");
    result.push(joined.clone());
    joined.insert_str(0, "Hoge->");
    result.push(joined.clone());
    let joined = joined[0..10].to_string();
    result.push(joined.clone());
    result.push(joined[4..10].to_string());
    result.join("\n")
}

fn main() {
    let input = "2$4$8$16";
    let expected = "\
2$4$8$16
2#4#8#16
4
5
62
2++4++8++16++32
Hoge->2++4++8++16++32
Hoge->2++4
->2++4";
    let answer = solve(input);
    assert_eq!(solve(input), expected);
}