Merge pull request #4668 from aschackmull/dataflow/refactor-pruning

Dataflow: Refactor pruning stages.

python/ql/src/semmle/python/dataflow/new/internal/DataFlowImplCommon.qll

@@ -802,14 +802,9 @@ abstract class AccessPathFront extends TAccessPathFront {
 
   abstract boolean toBoolNonEmpty();
 
-  predicate headUsesContent(TypedContent tc) { this = TFrontHead(tc) }
+  TypedContent getHead() { this = TFrontHead(result) }
 
-  predicate isClearedAt(Node n) {
-    exists(TypedContent tc |
-      this.headUsesContent(tc) and
-      clearsContent(n, tc.getContent())
-    )
-  }
+  predicate isClearedAt(Node n) { clearsContent(n, getHead().getContent()) }
 }
 
 class AccessPathFrontNil extends AccessPathFront, TFrontNil {

python/ql/src/semmle/python/dataflow/new/internal/DataFlowPrivate.qll

@@ -1151,5 +1151,14 @@ predicate isImmutableOrUnobservable(Node n) { none() }
 
 int accessPathLimit() { result = 5 }
 
+/** The unit type. */
+private newtype TUnit = TMkUnit()
+
+/** The trivial type with a single element. */
+class Unit extends TUnit {
+  /** Gets a textual representation of this element. */
+  string toString() { result = "unit" }
+}
+
 /** Holds if `n` should be hidden from path explanations. */
 predicate nodeIsHidden(Node n) { none() }