[Sema]: ban the use of 'nonisolated' on actor declarations (#85185)

Co-authored-by: Konrad `ktoso` Malawski <konrad.malawski@project13.pl>

lib/Sema/TypeCheckAttr.cpp

@@ -7989,6 +7989,14 @@ void AttributeChecker::visitNonisolatedAttr(NonisolatedAttr *attr) {
     }
   }
 
+  // `nonisolated` on an actor declaration is contradictory.
+  if (auto *classDecl = dyn_cast<ClassDecl>(D);
+      classDecl && classDecl->isExplicitActor()) {
+    diagnoseAndRemoveAttr(attr, diag::invalid_decl_modifier, attr)
+        .fixItRemove(attr->getRange());
+    return;
+  }
+
   if (auto VD = dyn_cast<ValueDecl>(D)) {
     //'nonisolated(unsafe)' is meaningless for computed properties, functions etc.
     auto var = dyn_cast<VarDecl>(VD);

test/decl/class/actor/basic.swift

@@ -53,3 +53,5 @@ extension A2 {
 
   init(doesNotDelegate: ()) {} // expected-error {{designated initializer cannot be declared in an extension of 'A2'}}
 }
+
+nonisolated actor A3 {} // expected-error {{'nonisolated' modifier cannot be applied to this declaration}}{{1-13=}}

test/decl/protocol/special/DistributedActor.swift

@@ -65,6 +65,8 @@ distributed actor D5: P1 {
   // expected-note@-1{{non-distributed instance method 'dist()'}}
 }
 
+nonisolated distributed actor D6 {} // expected-error {{'nonisolated' modifier cannot be applied to this declaration}}{{1-13=}}
+
 // ==== Tests ------------------------------------------------------------------
 
 // Make sure the conformances have been added implicitly.