I recently tried using Claude to generate a lexer and parser for a language i was designing. As part of its first attempt, this was the code to parse a float literal:

  fn read_float_literal(&mut self) -> &'a str {
    let start = self.pos;
    while let Some(ch) = self.peek_char() {
      if ch.is_ascii_alphanumeric() || ch == '.' || ch == '+' || ch == '-' {
        self.advance_char();
      } else {
        break;
      }
    }
    &self.source[start..self.pos]
  }

Admittedly, I do have a very idiosyncratic definition of floating-point literal for my language (I have a variety of syntaxes for NaNs with payloads), but... that is not a usable definition of float literal.

At the end of the day, I threw out all of the code the AI generated and wrote it myself, because the AI struggled to produce code that was functional to spec, much less code that would allow me to easily extend it to other kinds of future operators that I knew I would need in the future.