How to create "something like haxe.PosInfos" yourself?

I would love to see the possibility of adding user-defined types like `haxe.PosInfos` that allow you to generate some value if none is provided.

It might look something like this:

`abstract Foo(String) {
public function new(s) this = s;
@:default macro static function make() {
return macro new Foo('foofoo');
}
}`

This is a silly example of course, but more complex macros could gather call site information and what not.