Rust: Add a test for the log crate + placeholder query.

rust/ql/src/queries/security/CWE-312/CleartextLogging.ql

@@ -0,0 +1,20 @@
+/**
+ * @name Cleartext logging of sensitive information
+ * @description Logging sensitive information in plaintext can
+ *              expose it to an attacker.
+ * @kind path-problem
+ * @problem.severity error
+ * @security-severity 7.5
+ * @precision high
+ * @id rust/cleartext-logging
+ * @tags security
+ *       external/cwe/cwe-312
+ *       external/cwe/cwe-359
+ *       external/cwe/cwe-532
+ */
+
+import rust
+
+from Element e
+where none()
+select e, ""