build.hxml

-x Test
--macro Test.Macro.keep()

--next

-x Test
-dce full
--macro Test.Macro.keep()

Test.hx

import haxe.macro.Type;
using haxe.macro.Tools;
using Lambda;

class Test
{
	static function main() new Test();
	
	function new()
	{	
		check("build");
		check("dead");
		check("keep");
	}

	function check(field:String) trace('$field: ' + (Reflect.field(this, field) != null));

	function build() null;
	function dead() null;
	@:keep function keep() null;
}

class Macro
{
	static function keep() haxe.macro.Context.onGenerate(process);
	
	static function process(types:Array<Type>) types.iter(processType);

	static function processType(type:Type) switch (type)
	{
		case TInst(t, _):
			if (t.get().name == 'Test') processFields(t.get().fields.get());
		case _:
	}

	static function processFields(fields:Array<ClassField>)
	{
		for (field in fields)
			if (field.name == 'build')
				field.meta.add('@:keep', [], haxe.macro.Context.currentPos());
	}
}