[5.5] Short-circuit GlobalActorAttributeRequest attempting to get the Source Location of a serialized decl.

lib/Sema/TypeCheckConcurrency.cpp

@@ -378,7 +378,18 @@ GlobalActorAttributeRequest::evaluate(
   if (auto decl = subject.dyn_cast<Decl *>()) {
     dc = decl->getDeclContext();
     declAttrs = &decl->getAttrs();
-    loc = decl->getLoc();
+    // HACK: `getLoc`, when querying the attr from  a serialized decl,
+    // dependning on deserialization order, may launch into arbitrary
+    // type-checking when querying interface types of such decls. Which,
+    // in turn, may do things like query (to print) USRs. This ends up being
+    // prone to request evaluator cycles.
+    //
+    // Because this only applies to serialized decls, we can be confident
+    // that they already went through this type-checking as primaries, so,
+    // for now, to avoid cycles, we simply ignore the locs on serialized decls
+    // only.
+    // This is a workaround for rdar://79563942
+    loc = decl->getLoc(/* SerializedOK */ false);
   } else {
     auto closure = subject.get<ClosureExpr *>();
     dc = closure;

test/Serialization/Inputs/actor_bar.swift

@@ -0,0 +1 @@
+public actor Bar {}

test/Serialization/actor-attr-query-cycle.swift

@@ -0,0 +1,12 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-swift-frontend -emit-module -o %t/a.swiftmodule -emit-module-source-info-path %t/a.swiftsourceinfo -primary-file %s %S/Inputs/actor_bar.swift -module-name Foo
+// RUN: %target-swift-frontend -emit-module -o %t/b.swiftmodule -emit-module-source-info-path %t/b.swiftsourceinfo -primary-file %S/Inputs/actor_bar.swift %s -module-name Foo
+// RUN: %target-swift-frontend -merge-modules -emit-module -o %t/Foo.swiftmodule  -emit-module-source-info-path %t/Foo.swiftsourceinfo %t/a.swiftmodule %t/b.swiftmodule -module-name Foo
+
+extension Bar {
+  @MainActor
+  func bar() async throws -> Int {
+    return 42
+  }
+}