In a large mixing bowl, whisk together self-rising flour, sugar, and baking soda. Add eggs and drained crushed pineapple, then mix until smooth. Pour the batter into a greased 9x13-inch pan.
Bake the cake at 350°F for about 25 minutes, or until a toothpick inserted comes out clean. Let it cool slightly.
In a saucepan, combine sugar, margarine (or butter), and evaporated milk. Bring to a boil over medium heat, stirring constantly. Let it boil for 2 minutes, then remove from heat.
Stir in vanilla, coconut, and chopped pecans. The icing should be thick and ready to pour.
While the cake is still warm, pour the icing evenly over it. Let it set for 15-20 minutes.