More info about density contributions to meta-gga's could be useful
As far as I can see, there is the flag XC_FLAGS_NEEDS_LAPLACIAN, so one can see whether the functional is dependent on the laplacian or not. However, there are few cases where the meta-GGA is only dependent on the laplacian but not tau (sorry, I currently only remember it for the Jemmer-Knowles functional https://doi.org/10.1103/PhysRevA.51.3571 which is not in libxc, I guess...). Moreover, in case of the MK00 meta-GGA functional (ok, I agree that this is a very special case, too) it does not require sigma as input.
So it would be good if libxc could pass more info about the density contributions that are required for meta-GGA's in order to save some memory (and perhaps computation time) at the calling instance.