I was going to suggest making this somewhat more general with any expr (rather than specific for UNEST function) like:

/// [NOT] IN <expr>
InExpr {
  expr: Box<Expr>,
  in_expr: Box<Expr>,
  negated: bool
}

However, I see that https://cloud.google.com/bigquery/docs/reference/standard-sql/operators#in_operators treats it as a special case so I think it would be ok to do here as well.

Actually, looking more deeply, it seems like UNNEST actually supports with offset as well

https://cloud.google.com/bigquery/docs/reference/standard-sql/query-syntax#unnest_operator

Thank you for your valuable review.

As you said, it seems to be treated as a special case and any other functions can't used, so I did like this.

Actually, looking more deeply, it seems like UNNEST actually supports with offset as well

I did not know about with offset, thank you.
However, WITH OFFSET seems to be available when UNNEST returns a table.
In the case of IN UNNSET(array_expression), WITH OFFSET cannot be used because this IN UNNEST(array_expression) is a syntax sugar of subquery:
IN (SELECT element FROM UNNEST(array_expression) AS element).

✅ SELECT * FROM UNNEST(ARRAY[1,2,3]) WITH OFFSET
✅ SELECT 1 IN UNNEST(ARRAY[1,2,3])
❌ SELECT 1 IN UNNEST(ARRAY[1,2,3]) WITH OFFSET

So, if we support with offset, I think it is better to separate it from this PR, because the implementation and testing will be completely different. (I will try it in the future, if possible).

So, if we support with offset, I think it is better to separate it from this PR,

I agree -- that makes sense. If anyone needs WITH OFFSET support it can be added as a follow on 👍

Can you please also add a test for NOT IN UNNEST as well?

Thanks! I will fix it!