9.5.2 @:autoBuild

If a class has the :autoBuild metadata, the compiler generates :build metadata on all extending classes. If an interface has the :autoBuild metadata, the compiler generates :build metadata on all implementing classes and all extending interfaces. Note that :autoBuild does not imply :build on the class/interface itself.

import haxe.macro.Context;
import haxe.macro.Expr;

class AutoBuildingMacro {
  macro static public function fromInterface():Array<Field> {
    trace("fromInterface: " + Context.getLocalType());
    return null;
  }

  macro static public function fromBaseClass():Array<Field> {
    trace("fromBaseClass: " + Context.getLocalType());
    return null;
  }
}

@:autoBuild(AutoBuildingMacro.fromInterface())
interface I {}

interface I2 extends I {}

@:autoBuild(AutoBuildingMacro.fromBaseClass())
class Base {}

class Main extends Base implements I2 {
  static public function main() {}
}

This outputs during compilation:

AutoBuildingMacro.hx:6:
  fromInterface: TInst(I2,[])
AutoBuildingMacro.hx:6:
  fromInterface: TInst(Main,[])
AutoBuildingMacro.hx:11:
  fromBaseClass: TInst(Main,[])

It is important to keep in mind that the order of these macro executions is undefined, which is detailed in Build Order.