mongoc issues MONGOC_WARNING() is some places where it decided not to break API:

src/mongoc/mongoc-bulk-operation.c:            MONGOC_WARNING ("%s(): update only works with $ operators.",

This is raised in a void function, so we cannot check the return value and must check that thing.

But. It also does this for "noone cares" messages like:

src/mongoc/mongoc-counters.c:   MONGOC_WARNING("Falling back to malloc for counters.");