While working on the PHP driver prototype, I was unable to include mongoc-bulk-operation-private.h despite MONGOC_I_AM_A_DRIVER being defined. This was due to mongoc-bulk-operation-private.h including another private header, which had different requirements.

mongoc-bulk-operation-private.h has:

#if !defined (MONGOC_I_AM_A_DRIVER) && !defined (MONGOC_COMPILATION)
#error "Only <mongoc.h> can be included directly."
#endif

It depends on mongoc-client.h, which has:

#if !defined (MONGOC_INSIDE) && !defined (MONGOC_COMPILATION)
# error "Only <mongoc.h> can be included directly."
#endif

I don't believe MONGOC_INSIDE and MONGOC_COMPILATION are intended to be set by our driver, so I expect the other private files may need to define MONGOC_INSIDE internally.