1. A non-transitory computer-readable storage medium having program instructions stored therein that, in response to execution by a computer system, cause the computer system to perform operations including: identifying a calling function during a process of compiling source code including the calling function, the calling function including a call to a previously-compiled called function; accessing a persistent dependency database to retrieve an expressed dependency associated with the previously-compiled called function, wherein the expressed dependency is generated and persistently stored into the persistent dependency database during compilation of the previously-compiled called function, wherein the expressed dependency is accessible across distinct compiler invocations, and wherein the expressed dependency indicates whether the previously-compiled called function only reads, only writes, or both reads and writes a data item; and generating a determination of whether the calling function interacts with the previously-compiled called function based, at least in part, upon the expressed dependency and without accessing source code of the previously-compiled called function.