explain ```    /// NB: only valid to call with Output from Reparser/TopLevelEntry.
    pub fn intersperse_trivia(
        &self,
        output: &crate::Output,
        sink: &mut dyn FnMut(StrStep<'_>),
    ) -> bool {
        let mut builder = Builder { lexed: self, pos: 0, state: State::PendingEnter, sink };

        for event in output.iter() {
            match event {
                Step::Token { kind, n_input_tokens: n_raw_tokens } => {
                    builder.token(kind, n_raw_tokens)
                }
                Step::FloatSplit { ends_in_dot: has_pseudo_dot } => {
                    builder.float_split(has_pseudo_dot)
                }
                Step::Enter { kind } => builder.enter(kind),
                Step::Exit => builder.exit(),
                Step::Error { msg } => {
                    let text_pos = builder.lexed.text_start(builder.pos);
                    (builder.sink)(StrStep::Error { msg, pos: text_pos });
                }
            }
        }

        match mem::replace(&mut builder.state, State::Normal) {
            State::PendingExit => {
                builder.eat_trivias();
                (builder.sink)(StrStep::Exit);
            }
            State::PendingEnter | State::Normal => unreachable!(),
        }

        // is_eof?
        builder.pos == builder.lexed.len()
    }
}

struct Builder<'a, 'b> {
    lexed: &'a LexedStr<'a>,
    pos: usize,
    state: State,
    sink: &'b mut dyn FnMut(StrStep<'_>),
}

enum State {
    PendingEnter,
    Normal,
    PendingExit,
}

impl Builder<'_, '_> {
    fn token(&mut self, kind: SyntaxKind, n_tokens: u8) {
        match mem::replace(&mut self.state, State::Normal) {
            State::PendingEnter => unreachable!(),
            State::PendingExit => (self.sink)(StrStep::Exit),
            State::Normal => (),
        }
        self.eat_trivias();
        self.do_token(kind, n_tokens as usize);
    }

    fn float_split(&mut self, has_pseudo_dot: bool) {
        match mem::replace(&mut self.state, State::Normal) {
            State::PendingEnter => unreachable!(),
            State::PendingExit => (self.sink)(StrStep::Exit),
            State::Normal => (),
        }
        self.eat_trivias();
        self.do_float_split(has_pseudo_dot);
    }

    fn enter(&mut self, kind: SyntaxKind) {
        match mem::replace(&mut self.state, State::Normal) {
            State::PendingEnter => {
                (self.sink)(StrStep::Enter { kind });
                // No need to attach trivias to previous node: there is no
                // previous node.
                return;
            }
            State::PendingExit => (self.sink)(StrStep::Exit),
            State::Normal => (),
        }

        let n_trivias =
            (self.pos..self.lexed.len()).take_while(|&it| self.lexed.kind(it).is_trivia()).count();
        let leading_trivias = self.pos..self.pos + n_trivias;
        let n_attached_trivias = n_attached_trivias(
            kind,
            leading_trivias.rev().map(|it| (self.lexed.kind(it), self.lexed.text(it))),
        );
        self.eat_n_trivias(n_trivias - n_attached_trivias);
        (self.sink)(StrStep::Enter { kind });
        self.eat_n_trivias(n_attached_trivias);
    }

    fn exit(&mut self) {
        match mem::replace(&mut self.state, State::PendingExit) {
            State::PendingEnter => unreachable!(),
            State::PendingExit => (self.sink)(StrStep::Exit),
            State::Normal => (),
        }
    }

    fn eat_trivias(&mut self) {
        while self.pos < self.lexed.len() {
            let kind = self.lexed.kind(self.pos);
            if !kind.is_trivia() {
                break;
            }
            self.do_token(kind, 1);
        }
    }

    fn eat_n_trivias(&mut self, n: usize) {
        for _ in 0..n {
            let kind = self.lexed.kind(self.pos);
            assert!(kind.is_trivia());
            self.do_token(kind, 1);
        }
    }

    fn do_token(&mut self, kind: SyntaxKind, n_tokens: usize) {
        let text = &self.lexed.range_text(self.pos..self.pos + n_tokens);
        self.pos += n_tokens;
        (self.sink)(StrStep::Token { kind, text });
    }

    fn do_float_split(&mut self, has_pseudo_dot: bool) {
        let text = &self.lexed.range_text(self.pos..self.pos + 1);

        match text.split_once('.') {
            Some((left, right)) => {
                assert!(!left.is_empty());
                (self.sink)(StrStep::Enter { kind: SyntaxKind::NAME_REF });
                (self.sink)(StrStep::Token { kind: SyntaxKind::INT_NUMBER, text: left });
                (self.sink)(StrStep::Exit);

                // here we move the exit up, the original exit has been deleted in process
                (self.sink)(StrStep::Exit);

                (self.sink)(StrStep::Token { kind: SyntaxKind::DOT, text: "." });

                if has_pseudo_dot {
                    assert!(right.is_empty(), "{left}.{right}");
                    self.state = State::Normal;
                } else {
                    assert!(!right.is_empty(), "{left}.{right}");
                    (self.sink)(StrStep::Enter { kind: SyntaxKind::NAME_REF });
                    (self.sink)(StrStep::Token { kind: SyntaxKind::INT_NUMBER, text: right });
                    (self.sink)(StrStep::Exit);

                    // the parser creates an unbalanced start node, we are required to close it here
                    self.state = State::PendingExit;
                }
            }
            None => {
                // illegal float literal which doesn't have dot in form (like 1e0)
                // we should emit an error node here
                (self.sink)(StrStep::Error { msg: "illegal float literal", pos: self.pos });
                (self.sink)(StrStep::Enter { kind: SyntaxKind::ERROR });
                (self.sink)(StrStep::Token { kind: SyntaxKind::FLOAT_NUMBER, text });
                (self.sink)(StrStep::Exit);

                // move up
                (self.sink)(StrStep::Exit);

                self.state = if has_pseudo_dot { State::Normal } else { State::PendingExit };
            }
        }

        self.pos += 1;
    }
}

fn n_attached_trivias<'a>(
    kind: SyntaxKind,
    trivias: impl Iterator<Item = (SyntaxKind, &'a str)>,
) -> usize {
    match kind {
        CONST | ENUM | FN | IMPL | MACRO_CALL | MACRO_DEF | MACRO_RULES | MODULE | RECORD_FIELD
        | STATIC | STRUCT | TRAIT | TUPLE_FIELD | TYPE_ALIAS | UNION | USE | VARIANT
        | EXTERN_CRATE => {
            let mut res = 0;
            let mut trivias = trivias.enumerate().peekable();

            while let Some((i, (kind, text))) = trivias.next() {
                match kind {
                    WHITESPACE if text.contains("\n\n") => {
                        // we check whether the next token is a doc-comment
                        // and skip the whitespace in this case
                        if let Some((COMMENT, peek_text)) = trivias.peek().map(|(_, pair)| pair) {
                            if is_outer(peek_text) {
                                continue;
                            }
                        }
                        break;
                    }
                    COMMENT => {
                        if is_inner(text) {
                            break;
                        }
                        res = i + 1;
                    }
                    _ => (),
                }
            }
            res
        }
        _ => 0,
    }
}

fn is_outer(text: &str) -> bool {
    if text.starts_with("////") || text.starts_with("/***") {
        return false;
    }
    text.starts_with("///") || text.starts_with("/**")
}

fn is_inner(text: &str) -> bool {
    text.starts_with("//!") || text.starts_with("/*!")
}```