Creating code to be run through macro but not directly

[droundy]

@Yandros

For instance, if you change both the macro and the struct in a way that they keeps them working, but where __DO_NOT_USE__ now has a new field, then such change would not require a major semver change.

I suppose if you were really concerned, you could use some proc-macro trickery to change the name of the secret function on each build. That should prevent any unscrupulous users from trying to use this undocumented and secret API... But it would also seem like a nice idea for the language to have a mechanism to do this for us.