The Rubblemaker

The Rubblemaker is an upcoming Hardmode item which can place rubble which is otherwise unobtainable. The Rubblemaker is purchased from the Goblin Tinkerer after defeating the Wall of Flesh

The Rubblemaker will take the first compatible block in your inventory (going from top left to bottom right) and use up one of that block to place rubble. You can Right Click to change the size of the rubble you're placing, if the closest resource you have doesn't have a rubble of the selected size it will choose the next compatible resource instead. You can also use Up and Down to cycle through styles of the rubble.

Trivia

 * The Rubblemaker was shown off early on a Terraria Youtuber named Khaios' channel. You can find the video's link here