In a medium saucepan, combine water, sugar, rhubarb and lime juice. Cook over medium heat, stirring occasionally, until all of the sugar has dissolved. Strain the syrup and refrigerate for up to one week. Makes about 1 cup.
Fill a cocktail shaker with ice. Add mint leaves, gin, syrup and lime juice. Shake until well chilled. Strain into a cocktail glass and serve with lime.