Prepare the filling: Preheat the oven to 375Ëš. Bring the pineapple, pineapple juice, light-brown sugar, rum, vanilla, and cinnamon to a simmer in a medium saucepan over medium-high heat and cook, stirring occasionally, until soft and the liquid is reduced by half, about 15 minutes.
Meanwhile, prepare the topping: Put the bread on a foil-lined baking sheet and bake until toasted, three to five minutes; let cool slightly. Tear bread into pieces; transfer to a food processor fitted with the blade attachment, add the pecans and pulse until in coarse crumbs. Transfer bread-nut mixture to a bowl. Toss with the butter, brown sugar, and salt.
Stir half of the topping into the pineapple mixture; transfer to a nine-inch pie plate or similar (put on a foil-lined baking sheet to catch spills). Top with the remaining topping, cover tightly with aluminum foil and bake 30 minutes or until the filling is bubbling. Remove the foil and bake until the topping is golden, three to five minutes more. Serve with vanilla ice cream.
