Separating the Wheat from the Chaff: Understanding (In-)Completeness of Proof Mechanisms for Separation Logic with Inductive Definitions

For over two decades Separation Logic has enjoyed its unique position as arguably the most popular framework for reasoning about heap-manipulating programs, as well as reasoning about shared resources and permissions. Separation Logic is often extended to include inductively-defined predicates, interpreted as least fixpoints, to form what is known as Separation Logic with Inductive Definitions (SLID). These inductive predicates are used to describe unbounded data-structures in the heap and to verify key properties thereof. Many theoretical and practical advances have been made in developing automated proof mechanisms for SLID, but by their very nature these mechanisms are imperfect, and a deeper understanding of their failures is desired. As expressive as Separation Logic is, it is not surprising that it is incomplete: there is no procedure that will provide a proof for all valid entailments in Separation Logic. In fact, at its very core, Separation Logic contains several sources of incompleteness that defy automated reasoning. In this paper we study these sources of incompleteness and how they relate to failures of proof mechanisms of SLID. We contextualize SLID within a larger, relaxed logic, that we call Weak Separation Logic (WSL). We prove that unlike SLID, WSL enjoys completeness for a non-trivial fragment of quantified entailments with background theories and inductive definitions, via a reduction to first-order logic (FOL). Moreover, we show that the ubiquitous fold/unfold proof mechanism, which is unsurprisingly incomplete for SLID, does constitute a sound and complete proof mechanism of WSL, for theory-free, quantifier-free entailments with inductive definitions. In some sense, this shows that WSL is the natural logic of such proof mechanisms. Through this contextualization of SLID within WSL, we understand proof failures as stemming from rogue, nonstandard models, that exist within the class of models considered by WSL, but do not adhere to the stricter requirements of SLID. These rogue models are typically infinite, and we use the recently proposed formalism of symbolic structures to represent and automatically find them. We present a prototype tool that implements the encoding of WSL to FOL and test it on an existing benchmark, which contains over 700 quantified entailment problems with inductive definitions, a third of which also contain background theories. Our tool is able to find counter-models to many of the examples, and we provide a partial taxonomy of the rogue models, shedding some light on real-world proof failures.