Simple syrup: Simmer everything together for about 15 minutes, until the rhubarb falls apart. Strain through a fine-mesh sieve and let it cool.
Rub the rim of a glass with lime and dip it in salt. Fill a cocktail shaker with ice, add the tequila, Cointreau, syrup and lime juice, and shake hard until the shaker is very cold. Pour over fresh ice, add the lime wedge and enjoy. If it is too tart for your taste, ease the lime back to 1 ounce or add a little more syrup.