QL code and tests for C#/C++/JavaScript.

javascript/ql/src/Statements/NestedLoopsSameVariable.ql

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+/**
+ * @name Nested loops with same variable
+ * @description Nested loops in which the iteration variable is the same for each loop are difficult
+ *              to understand.
+ * @kind problem
+ * @problem.severity warning
+ * @id js/nested-loops-with-same-variable
+ * @tags maintainability
+ *       correctness
+ * @precision medium
+ */
+
+import javascript
+
+/**
+ * Gets an iteration variable that loop `for` tests and updates.
+ */
+Variable getAnIterationVariable(ForStmt for) {
+    result.getAnAccess().getParentExpr*() = for.getTest() and
+    exists (UpdateExpr upd | upd.getParentExpr*() = for.getUpdate() | upd.getOperand() = result.getAnAccess())
+}
+
+from ForStmt outer, ForStmt inner
+where inner.nestedIn(outer) and
+      getAnIterationVariable(outer) = getAnIterationVariable(inner)
+select inner.getTest(), "This for statement uses the same loop variable as an enclosing $@.",
+                        outer, "for statement"