[Concurrency] Diagnose a redundant nonisolated(unsafe)

Author: simanerush

include/swift/AST/DiagnosticsSema.def

@@ -5686,6 +5686,10 @@ NOTE(nonisolated_mutable_storage_note,none,
      "convert %0 to a 'let' constant or consider declaring it "
      "'nonisolated(unsafe)' if manually managing concurrency safety",
      (const VarDecl *))
+WARNING(nonisolated_unsafe_sendable,none,
+     "'nonisolated(unsafe)' is unnecessary for a constant 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

@@ -6910,6 +6910,16 @@ void AttributeChecker::visitNonisolatedAttr(NonisolatedAttr *attr) {
           return;
         }
 
+        // 'nonisolated(unsafe)' is redundant for constant 'Sendable'
+        // properties.
+        // TODO: remove only the (unsafe) part for a public actor
+        if (attr->isUnsafe() && type->isSendableType() && var->isStatic() &&
+            var->isLet()) {
+          if (isa<StructDecl>(nominal))
+            diagnoseAndRemoveAttr(attr, diag::nonisolated_unsafe_sendable,
+                                  type);
+        }
+
         // 'nonisolated' is redundant for the stored properties of a struct.
         if (isa<StructDecl>(nominal) &&
             !var->isStatic() &&

test/Concurrency/global_variables.swift

@@ -41,6 +41,8 @@ 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}}
   static nonisolated(unsafe) let immutableNonisolatedUnsafe = TestNonsendable()
+  static nonisolated(unsafe) let immutableNonisolatedUnsafeSendable = TestSendable()
+  // expected-warning@-1 {{'nonisolated(unsafe)' is unnecessary for a constant with 'Sendable' type 'TestSendable', consider removing it}}
   static nonisolated let immutableNonisolated = TestNonsendable() // expected-error{{static property 'immutableNonisolated' is not concurrency-safe because it is not either conforming to 'Sendable' or isolated to a global actor}}
   // expected-error@-1 {{'nonisolated' can not be applied to variable with non-'Sendable' type 'TestNonsendable'}}
   static let immutableInferredSendable = 0