[AST/Sema] Add @extensible attribute on enum declarations

This attribute controls whether cross-module access to the declaration
needs `@unknown default:` because it's allowed to gain new cases even
if the module is non-resilient.

Author: xedin

include/swift/AST/DeclAttr.def

@@ -879,7 +879,12 @@ SIMPLE_DECL_ATTR(constInitialized, ConstInitialized,
   168)
 DECL_ATTR_FEATURE_REQUIREMENT(ConstInitialized, CompileTimeValues)
 
-LAST_DECL_ATTR(ConstInitialized)
+SIMPLE_DECL_ATTR(extensible, Extensible,
+  OnEnum,
+  ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove | ForbiddenInABIAttr,
+  169)
+
+LAST_DECL_ATTR(Extensible)
 
 #undef DECL_ATTR_ALIAS
 #undef CONTEXTUAL_DECL_ATTR_ALIAS

include/swift/AST/DiagnosticsSema.def

@@ -8572,6 +8572,19 @@ GROUPED_WARNING(
     "behavior",
     (StringRef, DeclAttribute))
 
+//===----------------------------------------------------------------------===//
+// MARK: @extensible Attribute
+//===----------------------------------------------------------------------===//
+
+ERROR(extensible_attr_on_frozen_type,none,
+      "cannot use '@extensible' together with '@frozen'", ())
+
+ERROR(extensible_attr_on_internal_type,none, 
+      "'@extensible' attribute can only be applied to public or package "
+      "declarations, but %0 is "
+      "%select{private|fileprivate|internal|%error|%error|%error}1",
+      (DeclName, AccessLevel))
+
 //===----------------------------------------------------------------------===//
 //                            MARK: SwiftSettings
 //===----------------------------------------------------------------------===//

lib/AST/ASTDumper.cpp

@@ -4925,6 +4925,7 @@ class PrintAttribute : public AttributeVisitor<PrintAttribute, void, Label>,
   TRIVIAL_ATTR_PRINTER(Used, used)
   TRIVIAL_ATTR_PRINTER(WarnUnqualifiedAccess, warn_unqualified_access)
   TRIVIAL_ATTR_PRINTER(WeakLinked, weak_linked)
+  TRIVIAL_ATTR_PRINTER(Extensible, extensible)
 
 #undef TRIVIAL_ATTR_PRINTER
 

lib/AST/ASTPrinter.cpp

@@ -402,6 +402,7 @@ PrintOptions PrintOptions::printSwiftInterfaceFile(ModuleDecl *ModuleToPrint,
       DeclAttrKind::RestatedObjCConformance,
       DeclAttrKind::NonSendable,
       DeclAttrKind::AllowFeatureSuppression,
+      DeclAttrKind::Extensible,
   };
 
   return result;

lib/AST/Decl.cpp

@@ -6854,14 +6854,11 @@ bool EnumDecl::treatAsExhaustiveForDiags(const DeclContext *useDC) const {
     if (enumModule->inSamePackage(useDC->getParentModule()))
       return true;
 
-    // If the module where enum is declared supports extensible enumerations
-    // and this enum is not explicitly marked as "@frozen", cross-module
-    // access cannot be exhaustive and requires `@unknown default:`.
-    if (enumModule->supportsExtensibleEnums() &&
-        !getAttrs().hasAttribute<FrozenAttr>()) {
-      if (useDC != enumModule->getDeclContext())
-        return false;
-    }
+    // When the enum is marked as `@extensible` cross-module access
+    // cannot be exhaustive and requires `@unknown default:`.
+    if (getAttrs().hasAttribute<ExtensibleAttr>() &&
+        enumModule != useDC->getParentModule())
+      return false;
   }
 
   return isFormallyExhaustive(useDC);

lib/ASTGen/Sources/ASTGen/DeclAttrs.swift

@@ -228,6 +228,7 @@ extension ASTGenVisitor {
         .dynamicCallable,
         .eagerMove,
         .exported,
+        .extensible,
         .discardableResult,
         .disfavoredOverload,
         .dynamicMemberLookup,

lib/Sema/TypeCheckAttr.cpp

@@ -241,6 +241,21 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
     }
   }
 
+  void visitExtensibleAttr(ExtensibleAttr *attr) {
+    auto *E = cast<EnumDecl>(D);
+
+    if (D->getAttrs().hasAttribute<FrozenAttr>()) {
+      diagnoseAndRemoveAttr(attr, diag::extensible_attr_on_frozen_type);
+      return;
+    }
+
+    if (E->getFormalAccess() < AccessLevel::Package) {
+      diagnoseAndRemoveAttr(attr, diag::extensible_attr_on_internal_type,
+                            E->getName(), E->getFormalAccess());
+      return;
+    }
+  }
+
   void visitAlignmentAttr(AlignmentAttr *attr) {
     // Alignment must be a power of two.
     auto value = attr->getValue();

lib/Sema/TypeCheckDeclOverride.cpp

@@ -1615,6 +1615,7 @@ namespace  {
     UNINTERESTING_ATTR(Isolated)
     UNINTERESTING_ATTR(Optimize)
     UNINTERESTING_ATTR(Exclusivity)
+    UNINTERESTING_ATTR(Extensible)
     UNINTERESTING_ATTR(NoLocks)
     UNINTERESTING_ATTR(NoAllocation)
     UNINTERESTING_ATTR(NoRuntime)

lib/Sema/TypeCheckSwitchStmt.cpp

@@ -1159,10 +1159,7 @@ namespace {
           auto *enumModule = theEnum->getParentModule();
           shouldIncludeFutureVersionComment =
               enumModule->isSystemModule() ||
-              enumModule->supportsExtensibleEnums();
-          // Since the module enabled `ExtensibleEnums` feature they
-          // opted-in all of their clients into exhaustivity errors.
-          shouldDowngradeToWarning = !enumModule->supportsExtensibleEnums();
+              theEnum->getAttrs().hasAttribute<ExtensibleAttr>();
         }
         DE.diagnose(startLoc, diag::non_exhaustive_switch_unknown_only,
                     subjectType, shouldIncludeFutureVersionComment)

lib/Serialization/ModuleFormat.h

@@ -58,7 +58,7 @@ const uint16_t SWIFTMODULE_VERSION_MAJOR = 0;
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
 /// Don't worry about adhering to the 80-column limit for this line.
-const uint16_t SWIFTMODULE_VERSION_MINOR = 933; // isConstantValue
+const uint16_t SWIFTMODULE_VERSION_MINOR = 934; // @extensible attribute
 
 /// A standard hash seed used for all string hashes in a serialized module.
 ///