Add UnusedParameter.ql query

ql/src/queries/variables/UnusedParameter.ql

@@ -0,0 +1,27 @@
+/**
+ * @name Unused parameter.
+ * @description A parameter that is not used later on, or whose value is always overwritten,
+ *              can be removed.
+ * @kind problem
+ * @problem.severity warning
+ * @id rb/unused-parameter
+ * @tags maintainability
+ *       external/cwe/cwe-563
+ * @precision low
+ */
+
+import ruby
+import codeql_ruby.dataflow.SSA
+
+class RelevantParameterVariable extends LocalVariable {
+  RelevantParameterVariable() {
+    exists(Parameter p |
+      this = p.getAVariable() and
+      not this.getName().charAt(0) = "_"
+    )
+  }
+}
+
+from RelevantParameterVariable v
+where not exists(Ssa::WriteDefinition def | def.getWriteAccess() = v.getDefiningAccess())
+select v, "Unused parameter."