Rust: Add barrier for fieldless enum types

rust/ql/lib/codeql/rust/internal/Type.qll

@@ -140,6 +140,9 @@ class EnumType extends Type, TEnum {
 
   EnumType() { this = TEnum(enum) }
 
+  /** Gets the enum that this enum type represents. */
+  Enum getEnum() { result = enum }
+
   override TypeParameter getPositionalTypeParameter(int i) {
     result = TTypeParamTypeParameter(enum.getGenericParamList().getTypeParam(i))
   }

rust/ql/lib/codeql/rust/security/Barriers.qll

@@ -34,6 +34,13 @@ class IntegralTypeBarrier extends DataFlow::Node {
   }
 }
 
+/** A node whose type is a fieldless enum. */
+class FieldlessEnumTypeBarrier extends DataFlow::Node {
+  FieldlessEnumTypeBarrier() {
+    TypeInference::inferType(this.asExpr()).(EnumType).getEnum().isFieldless()
+  }
+}
+
 /**
  * Holds if guard expression `g` having result `branch` indicates that the
  * sub-expression `e` is not null. For example when `ptr.is_null()` is