Preheat the oven to 300°. In a food processor, process the rye bread to coarse crumbs. Add the butter and sugar; pulse until evenly coated. Spread the crumbs on a large rimmed baking sheet and bake for about 20 minutes, stirring occasionally, until they start to dry out. Increase the oven temperature to 350°, season the crumbs with salt and bake for about 12 minutes longer, stirring occasionally, until golden brown and crisp. Let cool completely.