Rust: Add simple constant password example.

Author: geoffw0

rust/ql/examples/snippets/simple_constant_password.ql

@@ -0,0 +1,30 @@
+/**
+ * @name Constant password
+ * @description Finds places where a string literal is used in a function call
+ *              argument named something like "password".
+ * @id rust/examples/simple-constant-password
+ * @tags example
+ */
+
+import rust
+import codeql.rust.dataflow.DataFlow
+import codeql.rust.dataflow.TaintTracking
+
+module ConstantPasswordConfig implements DataFlow::ConfigSig {
+  predicate isSource(DataFlow::Node node) { node.asExpr().getExpr() instanceof StringLiteralExpr }
+
+  predicate isSink(DataFlow::Node node) {
+    // `node` is an argument whose corresponding parameter name matches the pattern "pass%"
+    exists(CallExpr call, Function target, int argIndex |
+      call.getStaticTarget() = target and
+      target.getParam(argIndex).getPat().(IdentPat).getName().getText().matches("pass%") and
+      call.getArg(argIndex) = node.asExpr().getExpr()
+    )
+  }
+}
+
+module ConstantPasswordFlow = TaintTracking::Global<ConstantPasswordConfig>;
+
+from DataFlow::Node sourceNode, DataFlow::Node sinkNode
+where ConstantPasswordFlow::flow(sourceNode, sinkNode)
+select sinkNode, "The value $@ is used as a constant password.", sourceNode, sourceNode.toString()