[michaelhoffman]

This is not "hard-coded" as I would describe it. It is defined in `gv.el`:

    (gv-define-setter buffer-string (store)
  `(insert (prog1 ,store (erase-buffer))))

Unfortunately, it is marked obsolete since 29.1. The NEWS says:

* Many seldom-used generalized variables have been made obsolete. Emacs has a number of rather obscure generalized variables defined, that, for instance, allowed you to say things like:

    (setf (point-min) 4)

These never caught on and have been made obsolete. The form above, for instance, is the same as saying

    (narrow-to-region 4 (point-max))