Letter or recommendation of supervisor who gave you a bad project
To be honest, if you intend to have this person write a letter of recommendation for you, I don't see any alternative but to talk to them about where you stand and how supportive they can be in a letter.
I doubt that anyone who wasn't a complete jerk would object to such a discussion. And having the discussion would give you valuable information about what you can expect. If they put you off or object to the question, then I'd suggest looking elsewhere for letters.
If the prof gave you an infeasible project (too easy or too hard) then they share in the outcome, though going back if it proves too easy is a good plan, though too late, now, to implement. My doctoral dissertation could have had a hundred proved theorems had I not warned my advisor that it was all trivial and signified nothing. It would have been nothing but &quot;junk food&quot; without substance.
Research is tricky since you are initially looking into the unknown void. For undergraduates it is especially tricky because of the time limitations on such things. The same may be true at the masters level if the degree is time limited. Some open questions in mathematics have remained so for a hundred years.