[Concurrency] Improve the diagnostics for unsafe immutable global and

static variables.

Author: hborla

include/swift/AST/DiagnosticsSema.def

@@ -5529,8 +5529,12 @@ NOTE(shared_mutable_state_decl_note,none,
      "isolate %0 to a global actor, or convert it to a 'let' constant and "
      "conform it to 'Sendable'", (const ValueDecl *))
 ERROR(shared_immutable_state_decl,none,
-      "%kind0 is not concurrency-safe because it is not either conforming to "
-      "'Sendable' or isolated to a global actor", (const ValueDecl *))
+      "%kind0 is not concurrency-safe because non-'Sendable' type %1 may have "
+      "shared mutable state",
+      (const ValueDecl *, Type))
+NOTE(shared_immutable_state_decl_note,none,
+     "isolate %0 to a global actor, or conform %1 to 'Sendable'",
+     (const ValueDecl *, Type))
 ERROR(actor_isolated_witness,none,
      "%select{|distributed }0%1 %kind2 cannot be used to satisfy %3 protocol "
      "requirement",

lib/Sema/TypeCheckConcurrency.cpp

@@ -4908,9 +4908,13 @@ ActorIsolation ActorIsolationRequest::evaluate(
           diagVar = originalVar;
         }
         if (var->isLet()) {
-          if (!var->getInterfaceType()->isSendableType()) {
-            diagVar->diagnose(diag::shared_immutable_state_decl, diagVar)
+          auto type = var->getInterfaceType();
+          if (!type->isSendableType()) {
+            diagVar->diagnose(diag::shared_immutable_state_decl,
+                              diagVar, type)
                 .warnUntilSwiftVersion(6);
+            diagVar->diagnose(diag::shared_immutable_state_decl_note,
+                              diagVar, type);
           }
         } else {
           diagVar->diagnose(diag::shared_mutable_state_decl, diagVar)

test/Concurrency/Inputs/sendable_cycle_other.swift

@@ -1,3 +1,4 @@
 struct Foo {
-  static let member = Bar() // expected-complete-warning {{static property 'member' is not concurrency-safe because it is not either conforming to 'Sendable' or isolated to a global actor; this is an error in the Swift 6 language mode}}
+  static let member = Bar() // expected-complete-warning {{static property 'member' is not concurrency-safe because non-'Sendable' type 'Bar' may have shared mutable state; this is an error in the Swift 6 language mode}}
+  // expected-complete-note@-1 {{isolate 'member' to a global actor, or conform 'Bar' to 'Sendable'}}
 }

test/Concurrency/concurrency_warnings.swift

@@ -8,7 +8,8 @@ class GlobalCounter {
   var counter: Int = 0
 }
 
-let rs = GlobalCounter() // expected-warning {{let 'rs' is not concurrency-safe because it is not either conforming to 'Sendable' or isolated to a global actor; this is an error in the Swift 6 language mode}}
+let rs = GlobalCounter() // expected-warning {{let 'rs' is not concurrency-safe because non-'Sendable' type 'GlobalCounter' may have shared mutable state; this is an error in the Swift 6 language mode}}
+// expected-note@-1 {{isolate 'rs' to a global actor, or conform 'GlobalCounter' to 'Sendable'}}
 
 var globalInt = 17 // expected-warning {{var 'globalInt' is not concurrency-safe because it is non-isolated global shared mutable state; this is an error in the Swift 6 language mode}}
 // expected-note@-1 {{isolate 'globalInt' to a global actor, or convert it to a 'let' constant and conform it to 'Sendable'}}

test/Concurrency/global_variables.swift

@@ -39,10 +39,12 @@ public struct TestWrapper {
 
 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 let immutableNonsendable = TestNonsendable() // expected-error{{static property 'immutableNonsendable' is not concurrency-safe because non-'Sendable' type 'TestNonsendable' may have shared mutable state}}
+  // expected-note@-1 {{isolate 'immutableNonsendable' to a global actor, or conform 'TestNonsendable' to 'Sendable'}}
   static nonisolated(unsafe) let immutableNonisolatedUnsafe = TestNonsendable()
-  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 nonisolated let immutableNonisolated = TestNonsendable() // expected-error{{static property 'immutableNonisolated' is not concurrency-safe because non-'Sendable' type 'TestNonsendable' may have shared mutable state}}
+  // expected-note@-1 {{isolate 'immutableNonisolated' to a global actor, or conform 'TestNonsendable' to 'Sendable'}}
+  // expected-error@-2 {{'nonisolated' can not be applied to variable with non-'Sendable' type 'TestNonsendable'}}
   static let immutableInferredSendable = 0
   static var mutable = 0 // expected-error{{static property 'mutable' is not concurrency-safe because it is non-isolated global shared mutable state}}
   // expected-note@-1{{isolate 'mutable' to a global actor, or convert it to a 'let' constant and conform it to 'Sendable'}}