You may be about to start making compotes, jams, or a bunch of other recipes based on cooked fruits, in which case you will probably find yourself mixing sugar and fruit at some point before cooking.
In this case, for a compote for example, you will probably add a little water to start the cooking. It's indicated in a lot of recipes, and you may be used to doing it that way, no worries, it's very classic. It works of course, but it's not great this addition of water : it will have to go away afterwards (longer cooking time) and it will make your fruits taste a little bit less spicy.
Can we do better?
Yes, much better, you have to play on the "syrup" effect between sugar and fruits. When you mix sugar and fruit together, the sugar, which is hygroscopic, pumps the water from the fruit with which it mixes to form a natural syrup. This syrup, as tasty as your fruit, is much better for the start of your future compote or jam.
How to make it?
Very simple:
1) Prepare your fruits, cut them into small pieces, here rhubarb, put in the cooking pot.
2) Add the powdered sugar, mix briefly.
3) Wait, at least 2 hours, overnight if you want, let the sugar work and melt.
After this time, the syrup has started to form, you can see it by tilting the pan a little. It is pure fruit syrup, quite thick and tasty, the ideal texture to start your cooking which will be 100% fruit + sugar.
You can proceed like this with all fruits, for small ones and berries (cherries, blackcurrants, blackberries, raspberries, ...) no need to cut them but you will have to wait a little longer to see the syrup form.
To sum up: When cooking fruit, do not add water, just let the fruit syrup have time to form.