[Concurrency] Treat actor-isolated references from closures as errors.

Following the approach that actors are meant to be isolated, promote
the warning about accessing actor-isolated state from a
potentially-concurrent context to an error.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -4141,10 +4141,14 @@ ERROR(actor_isolated_non_self_reference,none,
         "actor-isolated %0 %1 can only be referenced "
         "%select{inside the actor|on 'self'}2",
         (DescriptiveDeclKind, DeclName, bool))
-WARNING(concurrent_access,none,
-        "%select{local|actor-isolated}0 %1 %2 is unsafe to reference in code "
+WARNING(concurrent_access_local,none,
+        "local %0 %1 is unsafe to reference in code that may execute "
+        "concurrently",
+        (DescriptiveDeclKind, DeclName))
+ERROR(actor_isolated_concurrent_access,none,
+        "actor-isolated %0 %1 is unsafe to reference in code "
         "that may execute concurrently",
-        (bool, DescriptiveDeclKind, DeclName))
+        (DescriptiveDeclKind, DeclName))
 NOTE(actor_isolated_method,none,
      "only asynchronous methods can be used outside the actor instance; "
      "do you want to add 'async'?", ())

lib/Sema/TypeCheckConcurrency.cpp

@@ -551,7 +551,7 @@ void swift::checkActorIsolation(const Expr *expr, const DeclContext *dc) {
         if (mayExecuteConcurrentlyWith(
                 getDeclContext(), isolation.getLocalContext())) {
           ctx.Diags.diagnose(
-              loc, diag::concurrent_access, false,
+              loc, diag::concurrent_access_local,
               value->getDescriptiveKind(), value->getName());
           value->diagnose(
               diag::kind_declared_here, value->getDescriptiveKind());
@@ -593,7 +593,7 @@ void swift::checkActorIsolation(const Expr *expr, const DeclContext *dc) {
         if (mayExecuteConcurrentlyWith(
                 getDeclContext(), selfVar->getDeclContext())) {
           ctx.Diags.diagnose(
-              memberLoc, diag::concurrent_access, true,
+              memberLoc, diag::actor_isolated_concurrent_access,
               member->getDescriptiveKind(), member->getName());
           noteIsolatedActorMember(member);
           return true;

test/Concurrency/actor_isolation.swift

@@ -28,7 +28,7 @@ actor class MyActor: MySuperActor {
   class func synchronousClass() { }
   static func synchronousStatic() { }
 
-  func synchronous() -> String { text.first ?? "nothing" } // expected-note 3{{only asynchronous methods can be used outside the actor instance; do you want to add 'async'?}}
+  func synchronous() -> String { text.first ?? "nothing" } // expected-note 4{{only asynchronous methods can be used outside the actor instance; do you want to add 'async'?}}
   func asynchronous() async -> String { synchronous() }
 }
 
@@ -73,14 +73,15 @@ extension MyActor {
     let localConstant = 17
     var localVar = 17 // expected-note 2{{var declared here}}
     acceptClosure {
-      _ = text[0] // expected-warning{{actor-isolated property 'text' is unsafe to reference in code that may execute concurrently}}
-      _ = self.synchronous() // expected-warning{{actor-isolated instance method 'synchronous()' is unsafe to reference in code that may execute concurrently}}
+      _ = text[0] // expected-error{{actor-isolated property 'text' is unsafe to reference in code that may execute concurrently}}
+      _ = self.synchronous() // expected-error{{actor-isolated instance method 'synchronous()' is unsafe to reference in code that may execute concurrently}}
       _ = localVar // expected-warning{{local var 'localVar' is unsafe to reference in code that may execute concurrently}}
       _ = localConstant
     }
 
     acceptEscapingClosure {
-      _ = self.text[0] // expected-warning{{actor-isolated property 'text' is unsafe to reference in code that may execute concurrently}}
+      _ = self.text[0] // expected-error{{actor-isolated property 'text' is unsafe to reference in code that may execute concurrently}}
+      _ = self.synchronous() // expected-error{{actor-isolated instance method 'synchronous()' is unsafe to reference in code that may execute concurrently}}
       _ = localVar // expected-warning{{local var 'localVar' is unsafe to reference in code that may execute concurrently}}
       _ = localConstant
     }