SILGen: Hop to the executor for actor-constrained methods in their @objc async thunks.

If a Swift async method is actor-constrained, then when it's projected into ObjC, it should still
run its task on the correct actor, along with the completion handler that ObjC passes into it.
Fixes rdar://76415650.

Author: jckarter

lib/SILGen/SILGenBridging.cpp

@@ -12,6 +12,7 @@
 
 #include "ArgumentScope.h"
 #include "Callee.h"
+#include "ExecutorBreadcrumb.h"
 #include "RValue.h"
 #include "ResultPlan.h"
 #include "SILGenFunction.h"
@@ -1479,10 +1480,11 @@ SILFunction *SILGenFunction::emitNativeAsyncToForeignThunk(SILDeclRef thunk) {
   }
 
   // Create the closure implementation function. It has the same signature,
-  // but is just swiftcc and async.
+  // but is swiftcc and async.
   auto closureExtInfo = objcFnTy->getExtInfo().intoBuilder()
     .withRepresentation(SILFunctionTypeRepresentation::Thin)
     .withAsync()
+    .withConcurrent()
     .build();
   auto closureTy = objcFnTy->getWithExtInfo(closureExtInfo);
 
@@ -1556,6 +1558,22 @@ void SILGenFunction::emitNativeToForeignThunk(SILDeclRef thunk) {
   auto loc = thunk.getAsRegularLocation();
   loc.markAutoGenerated();
   Scope scope(Cleanups, CleanupLocation(loc));
+  
+  // Hop to the actor for the method's actor constraint, if any.
+  // Note that, since an async native-to-foreign thunk only ever runs in a
+  // task purpose-built for running the Swift async code triggering the
+  // completion handler, there is no need for us to hop back to the existing
+  // executor, since the task will end after we invoke the completion handler.
+  if (F.isAsync()) {
+    Optional<ActorIsolation> isolation;
+    if (thunk.hasDecl()) {
+      isolation = getActorIsolation(thunk.getDecl());
+    }
+    
+    if (isolation) {
+      emitHopToTargetActor(loc, *isolation, None);
+    }
+  }
 
   // If we are bridging a Swift method with an Any return value, create a
   // stack allocation to hold the result, since Any is address-only.

stdlib/public/Concurrency/Task.swift

@@ -801,12 +801,8 @@ func _getCurrentThreadPriority() -> Int
 @available(SwiftStdlib 5.5, *)
 @_alwaysEmitIntoClient
 @usableFromInline
-internal func _runTaskForBridgedAsyncMethod(_ body: @escaping () async -> Void) {
-#if compiler(>=5.5) && $Sendable && $InheritActorContext && $ImplicitSelfCapture
-  Task { await body() }
-#else
-  Task.runDetached { await body() }
-#endif
+internal func _runTaskForBridgedAsyncMethod(@_inheritActorContext _ body: __owned @Sendable @escaping () async -> Void) {
+  Task(operation: body)
 }
 
 #endif

test/SILGen/objc_async_from_swift.swift

@@ -108,3 +108,15 @@ class SlowServerlet: SlowServer {
     }
 }
 
+@globalActor actor FooActor {
+    static var shared = FooActor()
+}
+
+@FooActor
+class ActorConstrained: NSObject {
+    // CHECK-LABEL: sil shared [thunk] [ossa] @$s{{.*}}16ActorConstrainedC3foo{{.*}}U_To
+    // CHECK:         hop_to_executor {{%.*}} : $FooActor
+    @objc func foo() async -> Bool {
+        return true
+    }
+}