New query: Singleton set literal.

ql/src/codeql_ql/ast/Ast.qll

@@ -1926,7 +1926,7 @@ class Set extends TSet, Expr {
   Set() { this = TSet(set) }
 
   /**
-   * Gets the ith element in the set literal expression.
+   * Gets the `i`th element in this set literal expression.
    */
   Expr getElement(int i) { toGenerated(result) = set.getChild(i) }
 

ql/src/queries/style/SingletonSetLiteral.ql

@@ -0,0 +1,15 @@
+/**
+ * @name Singleton set literal
+ * @description A singleton set literal can be replaced by its member.
+ * @kind problem
+ * @problem.severity warning
+ * @id ql/singleton-set-literal
+ * @tags maintainability
+ * @precision very-high
+ */
+
+import ql
+
+from Set setlit
+where exists(unique(Expr e | e = setlit.getElement(_)))
+select setlit, "Singleton set literal can be replaced by its member."