[Concurrency] Diagnose redundant (unsafe) for a public actor-isolat…

…ed `Sendable` immutable storage.

include/swift/AST/DiagnosticsSema.def

@@ -5690,6 +5690,10 @@ WARNING(nonisolated_unsafe_sendable_constant,none,
      "'nonisolated(unsafe)' is unnecessary for a constant with 'Sendable' type "
      "%0, consider removing it",
      (Type))
+WARNING(unsafe_sendable_actor_constant,none,
+     "'(unsafe)' is unnecessary for a constant public actor 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

@@ -6926,8 +6926,27 @@ void AttributeChecker::visitNonisolatedAttr(NonisolatedAttr *attr) {
       if (attr->isUnsafe() &&
           type->isSendableType() &&
           var->isLet()) {
-        diagnoseAndRemoveAttr(
-            attr, diag::nonisolated_unsafe_sendable_constant, type);
+
+        // '(unsafe)' is redundant for a public actor-isolated 'Sendable'
+        // immutable.
+        auto nominal = dyn_cast<NominalTypeDecl>(dc);
+        if (nominal && nominal->isActor()) {
+          auto access = nominal->getFormalAccessScope(
+              /*useDC=*/nullptr,
+              /*treatUsableFromInlineAsPublic=*/true);
+          if (access.isPublic()) {
+            // Get the location where '(unsafe)' starts.
+            SourceLoc unsafeStart = Lexer::getLocForEndOfToken(
+                Ctx.SourceMgr, attr->getRange().Start);
+            diagnose(unsafeStart, diag::unsafe_sendable_actor_constant, type)
+                .fixItRemoveChars(unsafeStart,
+                                  attr->getRange().End.getAdvancedLoc(1));
+          }
+
+        } else {
+          diagnoseAndRemoveAttr(
+              attr, diag::nonisolated_unsafe_sendable_constant, type);
+        }
       }
     }
 

test/Concurrency/global_variables.swift

@@ -54,6 +54,11 @@ struct TestStatics {
   // expected-note@-1{{isolate 'wrapped' to a global actor, or convert it to a 'let' constant and conform it to 'Sendable'}}
 }
 
+public actor TestPublicActor {
+  nonisolated(unsafe) let immutableNonisolatedUnsafeSendable = TestSendable()
+  // expected-warning@-1 {{'(unsafe)' is unnecessary for a constant public actor property with 'Sendable' type 'TestSendable', consider removing it}} {{14-22=}}
+}
+
 @TestGlobalActor
 func f() {
   print(TestStatics.immutableExplicitSendable)