Ensure that we hop off the main actor after MainActor.run.

lib/SILGen/SILGenProlog.cpp

@@ -642,7 +642,7 @@ SILValue SILGenFunction::emitLoadGlobalActorExecutor(Type globalActor) {
     actorType->getTypeOfMember(SGM.SwiftModule, sharedInstanceDecl);
 
   auto metaRepr =
-    nominal->isResilient(SGM.SwiftModule, ResilienceExpansion::Maximal)
+    nominal->isResilient(SGM.SwiftModule, F.getResilienceExpansion())
     ? MetatypeRepresentation::Thick
     : MetatypeRepresentation::Thin;
 

stdlib/public/Concurrency/MainActor.swift

@@ -45,15 +45,24 @@ import Swift
 @available(SwiftStdlib 5.5, *)
 extension MainActor {
   /// Execute the given body closure on the main actor.
-  @_silgen_name("$sScM3run10resultType4bodyxxm_xyYbKScMYcXEtYaKlFZ")
-  public static func run<T: Sendable>(
+  ///
+  /// Historical ABI entry point, superceded by the Sendable version that is
+  /// also inlined to back-deploy a semantic fix where this operation would
+  /// not hop back at the end.
+  @usableFromInline
+  static func run<T>(
     resultType: T.Type = T.self,
     body: @MainActor @Sendable () throws -> T
   ) async rethrows -> T {
-    @MainActor func runOnMain(body: @MainActor @Sendable () throws -> T) async rethrows -> T {
-      return try body()
-    }
+    return try await body()
+  }
 
-    return try await runOnMain(body: body)
+  /// Execute the given body closure on the main actor.
+  @_alwaysEmitIntoClient
+  public static func run<T: Sendable>(
+    resultType: T.Type = T.self,
+    body: @MainActor @Sendable () throws -> T
+  ) async rethrows -> T {
+    return try await body()
   }
 }

test/Concurrency/Runtime/mainactor.swift

@@ -81,6 +81,10 @@ actor A {
 // CHECK-NOT: ERROR
 // CHECK: finished with return counter = 4
 
+// CHECK: detached task not on main queue
+// CHECK: on main queue again
+// CHECK: detached task hopped back
+
 @main struct RunIt {
   static func main() async {
     print("starting")
@@ -91,5 +95,25 @@ actor A {
     }
     let result = await someFunc()
     print("finished with return counter = \(result)")
+
+    // Check actor hopping with MainActor.run.
+    let task = Task.detached {
+      if checkIfMainQueue(expectedAnswer: false) {
+        print("detached task not on main queue")
+      } else {
+        print("ERROR: detached task is on the main queue?")
+      }
+
+      _ = await MainActor.run {
+        checkAnotherFn(1)
+      }
+
+      if checkIfMainQueue(expectedAnswer: false) {
+        print("detached task hopped back")
+      } else {
+        print("ERROR: detached task is on the main queue?")
+      }
+    }
+    _ = await task.value
   }
 }

test/api-digester/stability-concurrency-abi.test

@@ -50,5 +50,5 @@
 // UNSUPPORTED: swift_evolve
 
 // *** DO NOT DISABLE OR XFAIL THIS TEST. *** (See comment above.)
-
+Func MainActor.run(resultType:body:) has generic signature change from <T where T : Swift.Sendable> to <T>
 // *** DO NOT DISABLE OR XFAIL THIS TEST. *** (See comment above.)