Rust: Small improvements to data flow

The predicate `ssaDefAssigns` was definitely broken before

Author: paldepind

rust/ql/lib/codeql/rust/controlflow/CfgNodes.qll

@@ -9,7 +9,7 @@ private import ControlFlowGraph
 /** A CFG node that corresponds to an element in the AST. */
 class AstCfgNode extends CfgNode {
   AstNode node;
-  
+
   AstCfgNode() { node = this.getAstNode() }
 }
 

rust/ql/lib/codeql/rust/dataflow/internal/DataFlowImpl.qll

@@ -183,11 +183,18 @@ private predicate propagatesValue(Expr e) {
   e instanceof MatchExpr
 }
 
+/**
+ * Gets a node that may execute last in `n`, and which, when it executes last,
+ * will be the value of `n`.
+ */
+private ExprCfgNode getALastEvalNode(ExprCfgNode n) {
+  propagatesValue(n.getExpr()) and result.getASuccessor() = n
+}
+
 module LocalFlow {
   pragma[nomagic]
   predicate localFlowStepCommon(Node nodeFrom, Node nodeTo) {
-    propagatesValue(nodeTo.(Node::ExprNode).asExpr()) and
-    nodeFrom.getCfgNode().getASuccessor() = nodeTo.getCfgNode()
+    nodeFrom.getCfgNode() = getALastEvalNode(nodeTo.getCfgNode())
   }
 }
 

rust/ql/lib/codeql/rust/dataflow/internal/SsaImpl.qll

@@ -469,8 +469,9 @@ private module DataFlowIntegrationInput implements Impl::DataFlowIntegrationInpu
 
   /** Holds if SSA definition `def` assigns `value` to the underlying variable. */
   predicate ssaDefAssigns(WriteDefinition def, Expr value) {
-    exists(CfgNode node, BasicBlock bb, int i |
-      def.definesAt(_, bb, i) and value.getAstNode() = node.getAstNode().(AssignmentExpr).getLhs()
+    exists(BasicBlock bb, int i |
+      def.definesAt(_, bb, i) and
+      value.getAstNode() = bb.getNode(i).getAstNode().(AssignmentExpr).getRhs()
     )
   }