Preheat the oven to 350 degrees F (175 degrees C). Grease and flour a Bundt or tube pan.

Sift flour, cinnamon, baking powder, baking soda, and salt together onto a piece of waxed paper. Set aside.

Beat white sugar and eggs together in a large bowl with an electric mixer until light colored; beat in oil. Add flour mixture; beat for 2 minutes. Stir in zucchini, walnuts, pineapple, and vanilla; mix thoroughly. Pour batter into the prepared pan.

Bake in the preheated oven until a toothpick inserted into center comes out clean, about 1 hour 20 minutes. Cool in the pan for 30 minutes before inverting onto a plate, then cool completely on a wire rack.

Whisk confectioners' sugar and milk together in a small bowl until smooth and runny; pour over cooled cake.