Just to have some answer here for now:
You cannot access the field of a concrete type (SelectStatement) in a generic context. All rustc knows at the location of your TODO is that there is some type T that implements Debug, that does not allow you to access any details of a concrete type there. This is a general rust restriction and nothing diesel related.
(I know that does not address the underlying issue, but at least gives you an pointer on what’s wrong with the current implementation. I will try to write a longer answer in the next few days/weeks)