Mix the sliced rhubarb and mint with the sugar and salt in a glass jar and seal, preferably with a plastic lid (you'll see why later).
Allow to sit for several hours, until sugar is mostly dissolved, and then pour in the vinegar. Stir well, and the seal with a plastic lid, or if you have only a metal one, place a square of wax paper over the jar opening before screwing on the lid. You want a barrier between the contents of the jar and the lid, otherwise the vinegar could corrode the metal of the lid.
Place jar in the fridge, and allow to infuse for at least 24 hours, or up to a week.
Strain through a sieve and pour the liquid into a clean jar or bottle. Label the bottle and store it in the fridge. It should keep for about 6 weeks.
Use your shrub in sparkling water, cocktails, or mocktails.