We have checks to prevent public headers getting included directly in client code outside of bson.h and mongoc.h:

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

However, some public headers omit the checks like bson-writer.c and mongoc-change-stream.h.

And a lot of them define this check after the ifdef guard. Example:

#ifndef MONGOC_APM_H
#define MONGOC_APM_H

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

#endif /* MONGOC_APM_H */

So client code can do something like:

#include <mongoc.h>
#include <mongoc-apm.h>

And it compiles without issue. This is because mongoc.h includes mongoc-apm.h, which defines MONGOC_APM_H. So when the client code includes mongoc-apm.h directly, no error occurs.

This check should probably be defined outside of the ifdef guard, which only a few files do (e.g. mongoc-rand.h).