Require the types of 'nonisolated' declarations to be Sendable.

A nonisolated declaration is accessible concurrently, so it must be of
Sendable type to be safe. This plugs a hole in data-race preventation
that will need to be documented in an amendment to SE-0313.

Author: DougGregor

lib/Sema/TypeCheckConcurrency.cpp

@@ -18,6 +18,7 @@
 #include "TypeCheckType.h"
 #include "swift/Strings.h"
 #include "swift/AST/ASTWalker.h"
+#include "swift/AST/GenericEnvironment.h"
 #include "swift/AST/Initializer.h"
 #include "swift/AST/ParameterList.h"
 #include "swift/AST/ProtocolConformance.h"
@@ -2974,6 +2975,19 @@ ActorIsolation ActorIsolationRequest::evaluate(
   // If this declaration has one of the actor isolation attributes, report
   // that.
   if (auto isolationFromAttr = getIsolationFromAttributes(value)) {
+    // Nonisolated declarations must involve Sendable types.
+    if (*isolationFromAttr == ActorIsolation::Independent) {
+      SubstitutionMap subs;
+      if (auto genericEnv = value->getInnermostDeclContext()
+              ->getGenericEnvironmentOfContext()) {
+        subs = genericEnv->getForwardingSubstitutionMap();
+      }
+      diagnoseNonConcurrentTypesInReference(
+          ConcreteDeclRef(value, subs),
+          value->getDeclContext()->getParentModule(), value->getLoc(),
+          ConcurrentReferenceKind::Nonisolated);
+    }
+
     return *isolationFromAttr;
   }
 

lib/Sema/TypeCheckConcurrency.h

@@ -78,6 +78,8 @@ enum class ConcurrentReferenceKind {
   LocalCapture,
   /// Concurrent function
   ConcurrentFunction,
+  /// Nonisolated declaration.
+  Nonisolated,
 };
 
 /// The isolation restriction in effect for a given declaration that is

test/Concurrency/concurrent_value_checking.swift

@@ -156,6 +156,14 @@ func testKeyPaths(dict: [NC: Int], nc: NC) {
   _ = \HasNC.dict[nc] // expected-warning{{cannot form key path that captures non-sendable type 'NC'}}
 }
 
+// ----------------------------------------------------------------------
+// Sendable restriction on nonisolated declarations.
+// ----------------------------------------------------------------------
+actor ANI {
+  // FIXME: improve diagnostics to talk about nonisolated
+  nonisolated let nc = NC() // expected-warning{{cannot use property 'nc' with a non-sendable type 'NC' across actors}}
+  nonisolated func f() -> NC? { nil } // expected-warning{{cannot call function returning non-sendable type 'NC?' across actors}}
+}
 
 // ----------------------------------------------------------------------
 // Sendable restriction on conformances.