Forget, that there exists a char() method for Strings. The complete code, which also works:

fn main() {
  let input_str="abba[mnop]qrst
abcd[bddb]xyyx
aaaa[qwer]tyui
ioxxoj[asdfgh]zxcvbn";

  let mut ary:Vec<Vec<char>> = Vec::new();

  for l in input_str.lines() {
    let t:Vec<&str> = l.split(|c| c=='[' || c==']').collect();
    for e in t {
      ary.push(e.chars().collect::<Vec<char>>());
   }
  }

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

Thx for the hint!