[AST/Sema] Hide using declaration behind DefaultIsolationPerFile experimental feature

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -8783,6 +8783,8 @@ ERROR(invalid_redecl_of_file_isolation,none,
       "invalid redeclaration of file-level default actor isolation", ())
 NOTE(invalid_redecl_of_file_isolation_prev,none,
      "default isolation was previously declared here", ())
+ERROR(experimental_using_decl,none,
+      "'using' declarations are experimental", ())
 
 #define UNDEFINE_DIAGNOSTIC_MACROS
 #include "DefineDiagnosticMacros.h"

include/swift/Basic/Features.def

@@ -521,6 +521,10 @@ SUPPRESSIBLE_EXPERIMENTAL_FEATURE(ExtensibleAttribute, false)
 /// Allow use of `Module::name` syntax
 EXPERIMENTAL_FEATURE(ModuleSelector, false)
 
+/// Allow use of `using` declaration that control default isolation
+/// in a file scope.
+EXPERIMENTAL_FEATURE(DefaultIsolationPerFile, false)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/FeatureSet.cpp

@@ -643,6 +643,10 @@ static bool usesFeatureAlwaysInheritActorContext(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureDefaultIsolationPerFile(Decl *D) {
+  return isa<UsingDecl>(D);
+}
+
 UNINTERESTING_FEATURE(BuiltinSelect)
 
 // ----------------------------------------------------------------------------

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -2507,7 +2507,12 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
   }
 
   void visitUsingDecl(UsingDecl *UD) {
-    // Nothing to validate yet.
+    auto &ctx = UD->getASTContext();
+
+    if (!ctx.LangOpts.hasFeature(Feature::DefaultIsolationPerFile))
+      UD->diagnose(diag::experimental_using_decl);
+
+    TypeChecker::checkDeclAttributes(UD);
   }
 
   void visitOperatorDecl(OperatorDecl *OD) {

test/ModuleInterface/using.swift

@@ -1,9 +1,11 @@
-// RUN: %target-swift-emit-module-interface(%t.swiftinterface) %s
+// RUN: %target-swift-emit-module-interface(%t.swiftinterface) %s -enable-experimental-feature DefaultIsolationPerFile
 
 // RUN: %target-swift-typecheck-module-from-interface(%t.swiftinterface)
 
 // RUN: %FileCheck %s --input-file %t.swiftinterface
 
+// REQUIRES: swift_feature_DefaultIsolationPerFile
+
 using @MainActor
 
 // CHECK-NOT: using @MainActor

test/Parse/using.swift

@@ -1,4 +1,6 @@
-// RUN: %target-typecheck-verify-swift
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature DefaultIsolationPerFile
+
+// REQUIRES: swift_feature_DefaultIsolationPerFile
 
 using @MainActor
 using nonisolated