Diagnose @Sendable on actor-isolated synchronous functions.

There is no way for these functions to be Sendable, so diagnose attempts
to make them so. This closes a hole in Sendable checking.

Fixes rdar://94623729.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -4563,6 +4563,9 @@ ERROR(effectful_keypath_component,none,
 ERROR(local_function_executed_concurrently,none,
       "concurrently-executed %0 %1 must be marked as '@Sendable'",
       (DescriptiveDeclKind, DeclName))
+ERROR(sendable_isolated_sync_function,none,
+      "%0 synchronous %1 %2 cannot be marked as '@Sendable'",
+      (ActorIsolation, DescriptiveDeclKind, DeclName))
 ERROR(concurrent_access_of_local_capture,none,
       "%select{mutation of|reference to}0 captured %1 %2 in "
       "concurrently-executing code",

lib/Sema/TypeCheckAttr.cpp

@@ -149,7 +149,6 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
   IGNORED_ATTR(OriginallyDefinedIn)
   IGNORED_ATTR(NoDerivative)
   IGNORED_ATTR(SpecializeExtension)
-  IGNORED_ATTR(Sendable)
   IGNORED_ATTR(NonSendable)
   IGNORED_ATTR(AtRethrows)
   IGNORED_ATTR(AtReasync)
@@ -321,6 +320,8 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
   void checkBackDeployAttrs(ArrayRef<BackDeployAttr *> Attrs);
 
   void visitKnownToBeLocalAttr(KnownToBeLocalAttr *attr);
+
+  void visitSendableAttr(SendableAttr *attr);
 };
 
 } // end anonymous namespace
@@ -5842,6 +5843,21 @@ void AttributeChecker::visitKnownToBeLocalAttr(KnownToBeLocalAttr *attr) {
   }
 }
 
+void AttributeChecker::visitSendableAttr(SendableAttr *attr) {
+
+  if ((isa<AbstractFunctionDecl>(D) || isa<AbstractStorageDecl>(D)) &&
+      !isAsyncDecl(cast<ValueDecl>(D))) {
+    auto value = cast<ValueDecl>(D);
+    ActorIsolation isolation = getActorIsolation(value);
+    if (isolation.isActorIsolated()) {
+      diagnoseAndRemoveAttr(
+          attr, diag::sendable_isolated_sync_function,
+          isolation, value->getDescriptiveKind(), value->getName())
+        .warnUntilSwiftVersion(6);
+    }
+  }
+}
+
 void AttributeChecker::visitNonisolatedAttr(NonisolatedAttr *attr) {
   // 'nonisolated' can be applied to global and static/class variables
   // that do not have storage.

test/Concurrency/sendable_functions.swift

@@ -0,0 +1,23 @@
+// RUN: %target-typecheck-verify-swift
+// REQUIRES: concurrency
+
+
+@Sendable func globalFunc() { }
+
+actor A {
+  var state: Bool = false
+  
+  @Sendable func f() { // expected-warning{{actor-isolated synchronous instance method 'f()' cannot be marked as '@Sendable'}}
+    state = true
+  }
+
+  @Sendable nonisolated func g() { }
+
+  @Sendable func fAsync() async {
+    state = true
+  }
+}
+
+@MainActor @Sendable func globalActorFunc() { } // expected-warning{{main actor-isolated synchronous global function 'globalActorFunc()' cannot be marked as '@Sendable'}}
+
+@MainActor @Sendable func globalActorFuncAsync() async { }