feat(linter): add forbid_empty_enum rule

src/extension/alterschema/CMakeLists.txt

@@ -82,6 +82,7 @@ sourcemeta_library(NAMESPACE sourcemeta PROJECT core NAME alterschema
     linter/duplicate_examples.h
     linter/enum_to_const.h
     linter/equal_numeric_bounds_to_const.h
+    linter/forbid_empty_enum.h
     linter/items_array_default.h
     linter/items_schema_default.h
     linter/multiple_of_default.h

src/extension/alterschema/alterschema.cc

@@ -110,6 +110,7 @@ inline auto APPLIES_TO_POINTERS(std::vector<Pointer> &&keywords)
 #include "linter/duplicate_examples.h"
 #include "linter/enum_to_const.h"
 #include "linter/equal_numeric_bounds_to_const.h"
+#include "linter/forbid_empty_enum.h"
 #include "linter/items_array_default.h"
 #include "linter/items_schema_default.h"
 #include "linter/multiple_of_default.h"
@@ -226,6 +227,7 @@ auto add(SchemaTransformer &bundle, const AlterSchemaMode mode) -> void {
     bundle.add<UnsatisfiableMaxContains>();
     bundle.add<UnsatisfiableMinProperties>();
     bundle.add<EnumToConst>();
+    bundle.add<ForbidEmptyEnum>();
     bundle.add<TopLevelTitle>();
     bundle.add<TopLevelDescription>();
     bundle.add<TopLevelExamples>();

src/extension/alterschema/linter/forbid_empty_enum.h

@@ -0,0 +1,36 @@
+class ForbidEmptyEnum final : public SchemaTransformRule {
+public:
+  using mutates = std::true_type;
+  using reframe_after_transform = std::true_type;
+  ForbidEmptyEnum()
+      : SchemaTransformRule{"forbid_empty_enum",
+                            "An empty `enum` validates nothing and is "
+                            "unsatisfiable"} {};
+
+  [[nodiscard]] auto
+  condition(const sourcemeta::core::JSON &schema,
+            const sourcemeta::core::JSON &,
+            const sourcemeta::core::Vocabularies &vocabularies,
+            const sourcemeta::core::SchemaFrame &frame,
+            const sourcemeta::core::SchemaFrame::Location &location,
+            const sourcemeta::core::SchemaWalker &,
+            const sourcemeta::core::SchemaResolver &) const
+      -> sourcemeta::core::SchemaTransformRule::Result override {
+    ONLY_CONTINUE_IF(vocabularies.contains_any(
+                         {Vocabularies::Known::JSON_Schema_2020_12_Validation,
+                          Vocabularies::Known::JSON_Schema_2019_09_Validation,
+                          Vocabularies::Known::JSON_Schema_Draft_7,
+                          Vocabularies::Known::JSON_Schema_Draft_6,
+                          Vocabularies::Known::JSON_Schema_Draft_4}) &&
+                     schema.is_object() && !schema.defines("not") &&
+                     schema.defines("enum") && schema.at("enum").is_array() &&
+                     schema.at("enum").empty());
+    ONLY_CONTINUE_IF(!frame.has_references_through(location.pointer));
+    return APPLIES_TO_KEYWORDS("enum");
+  }
+
+  auto transform(JSON &schema, const Result &) const -> void override {
+    schema.at("enum").into(JSON::make_object());
+    schema.rename("enum", "not");
+  }
+};