Add DeadStoreOfLocal.ql query

ql/src/queries/variables/DeadStoreOfLocal.ql

@@ -0,0 +1,28 @@
+/**
+ * @name Useless assignment to local variable
+ * @description An assignment to a local variable that is not used later on, or whose value is always
+ *              overwritten, has no effect.
+ * @kind problem
+ * @problem.severity warning
+ * @id rb/useless-assignment-to-local
+ * @tags maintainability
+ *       external/cwe/cwe-563
+ * @precision low
+ */
+
+import ruby
+import codeql_ruby.dataflow.SSA
+
+class RelevantLocalVariableWriteAccess extends LocalVariableWriteAccess {
+  RelevantLocalVariableWriteAccess() {
+    not this.getVariable().getName().charAt(0) = "_" and
+    not this = any(Parameter p).getAVariable().getDefiningAccess()
+  }
+}
+
+from RelevantLocalVariableWriteAccess write, LocalVariable v
+where
+  v = write.getVariable() and
+  exists(write.getAControlFlowNode()) and
+  not exists(Ssa::WriteDefinition def | def.getWriteAccess() = write)
+select write, "This assignment to $@ is useless, since its value is never read.", v, v.getName()