C#: Add localExprFlow and localExprTaint, and change notes.

change-notes/1.23/analysis-csharp.md

@@ -40,5 +40,8 @@ The following changes in version 1.23 affect C# analysis in all applications.
   overriding `int explorationLimit()`.
 * `foreach` statements where the body is guaranteed to be executed at least once, such as `foreach (var x in new string[]{ "a", "b", "c" }) { ... }`, are now recognized by all analyses based on the control flow graph (such as SSA, data flow and taint tracking).
 * Fixed the control flow graph for `switch` statements where the `default` case was not the last case. This had caused the remaining cases to be unreachable. `SwitchStmt.getCase(int i)` now puts the `default` case last.
+* 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.
 
 ## Changes to autobuilder

csharp/ql/src/semmle/code/csharp/dataflow/internal/DataFlowPublic.qll

@@ -164,6 +164,12 @@ predicate localFlowStep(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 data flow node that jumps between callables. This can be extended in
  * framework code to add additional data flow steps.

csharp/ql/src/semmle/code/csharp/dataflow/internal/TaintTrackingPublic.qll

@@ -7,6 +7,14 @@ private import TaintTrackingPrivate
  */
 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))
+}
+
 /** A member (property or field) that is tainted if its containing object is tainted. */
 abstract class TaintedMember extends AssignableMember { }