Tokenize a text file through a Iterator

lexer.rs

// rustc lexer.rs && curl -s https://norvig.com/big.txt | ./lexer

use std::io::{Read, Result, stdin};

#[derive(Debug)]
struct Token<'a> {
  s: &'a str,
  start_index: usize,
}

fn tokenize<'a>(code: &'a str) -> impl Iterator<Item=Token<'a>> {
  let mut next_token = 0;
  let code_length = code.len();
  code.char_indices()
    .filter_map(move |(index, c)|
      if c.is_whitespace() {
        let start_index = next_token;
        next_token = index + 1;
        Some(Token { s: &code[start_index..index], start_index })
      } else if index == code_length - 1 {
        Some(Token { s: &code[next_token..index + 1], start_index: next_token })
      } else {
        None
      })
}

fn main() -> Result<()> {
  let mut buffer = String::new();
  stdin().read_to_string(&mut buffer)?;

  for token in tokenize(buffer.as_str()) {
    print!("{:?} ", token);
  }
  Ok(())
}