[Concurrency] Diagnose redundant nonisolated(unsafe) for an actor-i…

…solated `Sendable` immutable storage.

include/swift/AST/DiagnosticsSema.def

@@ -5694,6 +5694,10 @@ WARNING(unsafe_sendable_actor_constant,none,
      "'(unsafe)' is unnecessary for a constant public actor property with "
      "'Sendable' type %0, consider removing it",
      (Type))
+WARNING(nonisolated_unsafe_sendable_actor_constant,none,
+     "'nonisolated(unsafe)' is unnecessary for a constant actor-isolated "
+     "property with 'Sendable' type %0, consider removing it",
+     (Type))
 ERROR(nonisolated_non_sendable,none,
       "'nonisolated' can not be applied to variable with non-'Sendable' "
       "type %0",

lib/Sema/TypeCheckAttr.cpp

@@ -6941,6 +6941,12 @@ void AttributeChecker::visitNonisolatedAttr(NonisolatedAttr *attr) {
             diagnose(unsafeStart, diag::unsafe_sendable_actor_constant, type)
                 .fixItRemoveChars(unsafeStart,
                                   attr->getRange().End.getAdvancedLoc(1));
+          } else {
+            // This actor is not public, so suggest to remove
+            // 'nonisolated(unsafe)'.
+            diagnose(attr->getLocation(),
+                     diag::nonisolated_unsafe_sendable_actor_constant, type)
+                .fixItRemove(attr->getRange());
           }
 
         } else {

test/Concurrency/global_variables.swift

@@ -37,6 +37,12 @@ public struct TestWrapper {
   }
 }
 
+// https://github.com/apple/swift/issues/71546
+actor TestActor {
+  nonisolated(unsafe) let immutableActorIsolated = TestSendable()
+  // expected-warning@-1 {{'nonisolated(unsafe)' is unnecessary for a constant actor-isolated property with 'Sendable' type 'TestSendable', consider removing it}} {{3-23=}}
+}
+
 struct TestStatics {
   static let immutableExplicitSendable = TestSendable()
   static let immutableNonsendable = TestNonsendable() // expected-error{{static property 'immutableNonsendable' is not concurrency-safe because it is not either conforming to 'Sendable' or isolated to a global actor}}