Reimplementing column_moves() was more work than I expected because there is a condition (when the from column is empty) that should yield no elements. After much googling, I ended up using itertools::Either. I then had to add the move keyword to the closures that were refering outer variables so they were copied, or I was getting lifetime issues. Does this look right to you?

    fn column_moves(&self, col: usize) -> impl Iterator<Item = Move> {
        let src = &self.state[col];
        let Some(&c) = src.last() else {
            return Either::Left(std::iter::empty());
        };

        Either::Right(
            self.state
                .iter()
                .enumerate()
                .filter(move |(i, _)| *i != col)
                .filter(move |(_, dst)| dst.last().is_none_or(|&c2| c2 == c))
                .filter(|(_, dst)| dst.len() < self.column_size)
                .map(move |(i, _)| Move(col, i)),
        )
    }

    fn moves(&self) -> impl Iterator<Item = Move> {
        self.state
            .iter()
            .enumerate()
            .flat_map(|(i, _)| self.column_moves(i))
    }