Add a feature for global actors & module interface support

Author: DougGregor

include/swift/Basic/Features.def

@@ -40,5 +40,6 @@ LANGUAGE_FEATURE(MarkerProtocol, 0, "@_marker protocol", true)
 LANGUAGE_FEATURE(Actors, 0, "actors", langOpts.EnableExperimentalConcurrency)
 LANGUAGE_FEATURE(ConcurrentFunctions, 0, "@concurrent functions", true)
 LANGUAGE_FEATURE(RethrowsProtocol, 0, "@rethrows protocol", true)
+LANGUAGE_FEATURE(GlobalActors, 0, "Global actors", langOpts.EnableExperimentalConcurrency)
 
 #undef LANGUAGE_FEATURE

lib/AST/ASTPrinter.cpp

@@ -2562,6 +2562,21 @@ static bool usesFeatureRethrowsProtocol(Decl *decl) {
   return usesFeatureRethrowsProtocol(decl, checked);
 }
 
+static bool usesFeatureGlobalActors(Decl *decl) {
+  if (auto nominal = dyn_cast<NominalTypeDecl>(decl)) {
+    if (nominal->getAttrs().hasAttribute<GlobalActorAttr>())
+      return true;
+  }
+
+  if (auto ext = dyn_cast<ExtensionDecl>(decl)) {
+    if (auto nominal = ext->getExtendedNominal())
+      if (usesFeatureGlobalActors(nominal))
+        return true;
+  }
+
+  return false;
+}
+
 /// Determine the set of "new" features used on a given declaration.
 ///
 /// Note: right now, all features we check for are "new". At some point, we'll

test/ModuleInterface/features.swift

@@ -87,6 +87,14 @@ extension Array: FeatureTest.MP where Element : FeatureTest.MP { }
 extension OldSchool: UnsafeConcurrentValue { }
 // CHECK-NEXT: }
 
+// CHECK: #if compiler(>=5.3) && $GlobalActors
+// CHECK-NEXT: @globalActor public struct SomeGlobalActor
+@globalActor
+public struct SomeGlobalActor {
+  public static let shared = MyActor()
+}
+
+
 // CHECK: #if compiler(>=5.3) && $AsyncAwait
 // CHECK-NEXT: func runSomethingSomewhere
 // CHECK-NEXT: #endif