Don't hop to @mainactor if already running on the main thread.

Author: nickolas-pohilets

stdlib/public/Concurrency/Task.cpp

@@ -1438,7 +1438,10 @@ static void swift_task_performOnExecutorImpl(void *context,
   // If the current executor is compatible with running the new executor,
   // we can just immediately continue running with the resume function
   // we were passed in.
-  if (!currentExecutor.mustSwitchToRun(newExecutor)) {
+  //
+  // Note that swift_task_isCurrentExecutor() return true for @MainActor
+  // when running on the main thread without any executor
+  if (swift_task_isCurrentExecutor(newExecutor)) {
     return work(context); // 'return' forces tail call
   }
 

test/Concurrency/Runtime/isolated_deinit_main_sync.swift

@@ -0,0 +1,24 @@
+// RUN: %target-run-simple-swift(-Xfrontend -disable-availability-checking) | %FileCheck %s
+
+var isDead: Bool = false
+
+public class Foo {
+  @MainActor
+  deinit {
+    print("DEINIT")
+    isDead = true
+  }
+}
+
+func main() {
+  print("isDead = \(isDead)")
+  do {
+    _ = Foo()
+  }
+  print("isDead = \(isDead)")
+}
+
+// CHECK: isDead = false
+// CHECK: DEINIT
+// CHECK: isDead = true
+main()