C++: Add localExprFlow and localExprTaint

This is for ODASA-8053.

change-notes/1.23/analysis-cpp.md

@@ -25,3 +25,6 @@ The following changes in version 1.23 affect C/C++ analysis in all applications.
   picture of the partial flow paths from a given source. The feature is
   disabled by default and can be enabled for individual configurations by
   overriding `int explorationLimit()`.
+* There is now a `DataFlow::localExprFlow` predicate and a
+  `TaintTracking::localExprTaint` predicate to make it easy to use the most
+  common case of local data flow and taint: from one `Expr` to another.

cpp/ql/src/semmle/code/cpp/dataflow/internal/DataFlowUtil.qll

@@ -490,6 +490,12 @@ predicate simpleLocalFlowStep(Node nodeFrom, Node nodeTo) {
  */
 predicate localFlow(Node source, Node sink) { localFlowStep*(source, sink) }
 
+/**
+ * Holds if data can flow from `e1` to `e2` in zero or more
+ * local (intra-procedural) steps.
+ */
+predicate localExprFlow(Expr e1, Expr e2) { localFlow(exprNode(e1), exprNode(e2)) }
+
 /**
  * Holds if the initial value of `v`, if it is a source, flows to `var`.
  */

cpp/ql/src/semmle/code/cpp/dataflow/internal/TaintTrackingUtil.qll

@@ -80,6 +80,14 @@ predicate localTaint(DataFlow::Node source, DataFlow::Node sink) {
   localTaintStep*(source, sink)
 }
 
+/**
+ * Holds if taint can flow from `e1` to `e2` in zero or more
+ * local (intra-procedural) steps.
+ */
+predicate localExprTaint(Expr e1, Expr e2) {
+  localTaint(DataFlow::exprNode(e1), DataFlow::exprNode(e2))
+}
+
 /**
  * Holds if we do not propagate taint from `fromExpr` to `toExpr`
  * even though `toExpr` is the AST parent of `fromExpr`.

cpp/ql/src/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll

@@ -178,6 +178,12 @@ predicate simpleLocalFlowStep(Node nodeFrom, Node nodeTo) {
  */
 predicate localFlow(Node source, Node sink) { localFlowStep*(source, sink) }
 
+/**
+ * Holds if data can flow from `e1` to `e2` in zero or more
+ * local (intra-procedural) steps.
+ */
+predicate localExprFlow(Expr e1, Expr e2) { localFlow(exprNode(e1), exprNode(e2)) }
+
 /**
  * A guard that validates some expression.
  *