A mongoc_buffer_t is a small struct with a data pointer, data length, capacity, and reallocation function.

Originally, it also had an offset field, which was removed in this commit: CDRIVER-2266

A side-effect of that is there are three lines like:

memmove (&buffer->data[0], buffer->data, buffer->len);

Which are effectively no-ops.

One case was removed in https://github.com/mongodb/mongo-c-driver/commit/26137b5c0883bd0705833c0ec9212694d9c88650 but three still remain.