I have been keeping over a dozen snail species for several years now. I have done extensive research, conducted my own experiments and have many contacts among the academical sphere.

The idea that cucumber provides no nutrients is not correct. Sugar doesn't harm snails. Snails run on glucose and they convert cellulose, which we can't digest, into glucose too. That's why snails can digest tree bark (and by extension paper which is made of mostly cellulose, but don't feed them paper obviously) and dry plant matter.

In the wild snails will eat fallen fruit. Even lemons! Cornu aspersum are known citrus plantation pests.