Make polonius_souffle an optional dependency

Author: ecstatic-morse

polonius-engine/Cargo.toml

@@ -11,6 +11,6 @@ keywords = ["compiler", "borrowck", "datalog"]
 [dependencies]
 datafrog = { path = "../../datafrog" }
 polonius-facts = { path = "../polonius-facts" }
-polonius-souffle = { path = "../polonius-souffle" }
+polonius-souffle = { path = "../polonius-souffle", optional = true }
 rustc-hash = "1.0.0"
 log = "0.4"

polonius-engine/src/output/mod.rs

@@ -187,6 +187,7 @@ impl<T: FactTypes> Output<T> {
     ///   partial results can also be stored in the context, so that the following
     ///   variant can use it to prune its own input data
     pub fn compute(all_facts: &AllFacts<T>, algorithm: Algorithm, dump_enabled: bool) -> Self {
+        #[cfg(polonius_souffle)]
         if algorithm.engine() == Engine::Souffle {
             let name = algorithm.souffle_name().expect("Algorithm does not have Soufflé version");
             let facts = polonius_souffle::run_from_facts(name, &all_facts);