Right, but this is no different from implementing Add or Copy or FnMut or Placer, or calling sizeof or transmute or black_box, or using UnsafeCell. So its not "

Right, but this is no different from implementing Add or Copy or FnMut or Placer, or calling sizeof or transmute or black_box, or using UnsafeCell. So its not "special" in any way. I don't disagree that its a compiler hook; that's what I called it in the first place :) I disagree that its makes the library in any way "special" or a pseudo library. I can write a linear algebra library that gives you Vec3d and Matrix4d and the ability to multiply/add such types using regular multiplication operators. I can also write a Gc<T> library that lets you wrap some values in a GC heap allocation, which will have some extra stack maps being generated. It doesn't feel much different to me, both are equally worthy of being called libraries. Anyway, expect a blog post soon. :)