None. First of all, salt inhibits growth of yeast. And if the yeast is fresh enough, merely rehydrating it will be enough to get it started. Then when it is mixed with the flour, it has the food it needs.
Although I suppose if you did want to add a pinch of sugar, that would be sufficient to get it going. But it isn't necessary. Think about so many of the Italian breads: nothing but flour, water, and yeast. Not even salt.
Sugar -- or honey, syrup, beet reduction, whatever -- is simply a purer form of food for the yeast than the sugars present in flour.
Remember, yeast is a small organism that basically eats sugar and farts. When it's eaten enough, it generally either goes dormant or dies. So if you have a huge amount of pure sugar of any kind in your dough, the yeast its it all up, farts massively, and stops. So you get a rapid rise that ends quickly. If you have no pure sugar at all, i.e. your dough is all flour like in a French baguette, the yeast takes a good while to produce much farting action.
Let's try this differently. What are you trying to bake?
Edit: Clarification. Yeast is a small organism that eats sugar and THEN farts. It doesn't eat your farts. I'd not recommend that as a baking technique, at all.