impl Iterator for Lexer<'_> { type Item = (Token, Span); fn next(&mut self) -> Option<Self::Item> { - self.next_token(); + self.emit_next_tokens(); + self.token_queue.pop_front() } } One question here emit_next_tokens(); can push one or more tokens to the queue. pop_front(); only consumes one. Does this not mean the queue will race ahead of the iteration? I tried setting type Item = (Token, Span, u32 / Current Queue Size /) with a longer bit of code with a lot of indentation and it did race ahead. It's possible I missed something else elsewhere.