diff --git a/include/swift/AST/DiagnosticsSIL.def b/include/swift/AST/DiagnosticsSIL.def
index 4060a53762f8b..7cc5be8fa11b5 100644
--- a/include/swift/AST/DiagnosticsSIL.def
+++ b/include/swift/AST/DiagnosticsSIL.def
@@ -1098,6 +1098,28 @@ NOTE(regionbasedisolation_out_sending_cannot_be_actor_isolated_note_named, none,
       "returning %1 %0 risks causing data races since the caller assumes that %0 can be safely sent to other isolation domains",
       (Identifier, StringRef))
 
+//===
+// non-Sendable Results
+
+// Example: returning main-actor isolated result to a custom-actor isolated context risks causing data races
+ERROR(rbi_isolation_crossing_result, none,
+      "non-Sendable %0-typed result can not be returned from %1 %kind2 to %3 context",
+      (Type, ActorIsolation, const ValueDecl *, ActorIsolation))
+ERROR(rbi_isolation_crossing_result_no_decl, none,
+      "non-Sendable %0-typed result can not be returned from %1 function to %2 context",
+      (Type, ActorIsolation, ActorIsolation))
+NOTE(rbi_non_sendable_nominal,none,
+     "%kind0 does not conform to the 'Sendable' protocol",
+      (const ValueDecl *))
+NOTE(rbi_nonsendable_function_type,none,
+     "a function type must be marked '@Sendable' to conform to 'Sendable'", ())
+NOTE(rbi_add_nominal_sendable_conformance,none,
+     "consider making %kind0 conform to the 'Sendable' protocol",
+     (const ValueDecl *))
+NOTE(rbi_add_generic_parameter_sendable_conformance,none,
+     "consider making generic parameter %0 conform to the 'Sendable' protocol",
+     (Type))
+
 //===----------------------------------------------------------------------===//
 //                           MARK: Misc Diagnostics
 //===----------------------------------------------------------------------===//
diff --git a/include/swift/SILOptimizer/Utils/PartitionOpError.def b/include/swift/SILOptimizer/Utils/PartitionOpError.def
index 93cb15dc6bf44..98253626e95ac 100644
--- a/include/swift/SILOptimizer/Utils/PartitionOpError.def
+++ b/include/swift/SILOptimizer/Utils/PartitionOpError.def
@@ -59,4 +59,8 @@ PARTITION_OP_ERROR(InOutSendingNotDisconnectedAtExit)
 /// to our user so that we can emit that error as we process.
 PARTITION_OP_ERROR(UnknownCodePattern)
 
+/// Used to signify that an isolation crossing function is returning a
+/// non-Sendable value.
+PARTITION_OP_ERROR(NonSendableIsolationCrossingResult)
+
 #undef PARTITION_OP_ERROR
diff --git a/include/swift/SILOptimizer/Utils/PartitionUtils.h b/include/swift/SILOptimizer/Utils/PartitionUtils.h
index 8e9e37be789d5..faf8334b5daa8 100644
--- a/include/swift/SILOptimizer/Utils/PartitionUtils.h
+++ b/include/swift/SILOptimizer/Utils/PartitionUtils.h
@@ -462,6 +462,14 @@ enum class PartitionOpKind : uint8_t {
   ///
   /// Takes one parameter, the inout parameter that we need to check.
   InOutSendingAtFunctionExit,
+
+  /// This is the result of an isolation crossing apply site. We need to emit a
+  /// special error since we never allow this.
+  ///
+  /// DISCUSSION: This is actually just a form of "send". Sadly, we can not use
+  /// "send" directly since "send" expects a SILOperand and these are values. So
+  /// to work around the API issue, we have to use a different, specific entry.
+  NonSendableIsolationCrossingResult,
 };
 
 /// PartitionOp represents a primitive operation that can be performed on
@@ -574,6 +582,12 @@ class PartitionOp {
                        sourceInst);
   }
 
+  static PartitionOp
+  NonSendableIsolationCrossingResult(Element elt, SILInstruction *sourceInst) {
+    return PartitionOp(PartitionOpKind::NonSendableIsolationCrossingResult, elt,
+                       sourceInst);
+  }
+
   bool operator==(const PartitionOp &other) const {
     return opKind == other.opKind && opArgs == other.opArgs &&
            source == other.source;
@@ -1053,6 +1067,22 @@ class PartitionOpError {
     }
   };
 
+  struct NonSendableIsolationCrossingResultError {
+    const PartitionOp *op;
+
+    Element returnValueElement;
+
+    NonSendableIsolationCrossingResultError(const PartitionOp &op,
+                                            Element returnValue)
+        : op(&op), returnValueElement(returnValue) {}
+
+    void print(llvm::raw_ostream &os, RegionAnalysisValueMap &valueMap) const;
+
+    SWIFT_DEBUG_DUMPER(dump(RegionAnalysisValueMap &valueMap)) {
+      print(llvm::dbgs(), valueMap);
+    }
+  };
+
 #define PARTITION_OP_ERROR(NAME)                                               \
   static_assert(std::is_copy_constructible_v<NAME##Error>,                     \
                 #NAME " must be copy constructable");
@@ -1482,8 +1512,15 @@ struct PartitionOpEvaluator {
 
       // Then emit an unknown code pattern error.
       return handleError(UnknownCodePatternError(op));
+    case PartitionOpKind::NonSendableIsolationCrossingResult:
+      // Grab the dynamic dataflow isolation information for our element's
+      // region.
+      Region region = p.getRegion(op.getOpArgs()[0]);
+
+      // Then emit the error.
+      return handleError(
+          NonSendableIsolationCrossingResultError(op, op.getOpArgs()[0]));
     }
-
     llvm_unreachable("Covered switch isn't covered?!");
   }
 
diff --git a/lib/SILOptimizer/Analysis/RegionAnalysis.cpp b/lib/SILOptimizer/Analysis/RegionAnalysis.cpp
index d2cd28a7ced68..df35e0761caa5 100644
--- a/lib/SILOptimizer/Analysis/RegionAnalysis.cpp
+++ b/lib/SILOptimizer/Analysis/RegionAnalysis.cpp
@@ -1526,6 +1526,12 @@ struct PartitionOpBuilder {
         PartitionOp::UnknownPatternError(lookupValueID(value), currentInst));
   }
 
+  void addNonSendableIsolationCrossingResultError(SILValue value) {
+    currentInstPartitionOps.emplace_back(
+        PartitionOp::NonSendableIsolationCrossingResult(lookupValueID(value),
+                                                        currentInst));
+  }
+
   SWIFT_DEBUG_DUMP { print(llvm::dbgs()); }
 
   void print(llvm::raw_ostream &os) const;
@@ -2394,14 +2400,58 @@ class PartitionOpTranslator {
       handleSILOperands(applySite.getOperandsWithoutIndirectResults());
     }
 
-    // non-sendable results can't be returned from cross-isolation calls without
-    // a diagnostic emitted elsewhere. Here, give them a fresh value for better
-    // diagnostics hereafter
+    // Create a new assign fresh for each one of our values and unless our
+    // return value is sending, emit an extra error bit on the results that are
+    // non-Sendable.
     SmallVector<SILValue, 8> applyResults;
     getApplyResults(*applySite, applyResults);
-    for (auto result : applyResults)
-      if (auto value = tryToTrackValue(result))
+
+    auto substCalleeType = applySite.getSubstCalleeType();
+
+    // Today, all values in result info are sending or none are. So this is a
+    // safe to check that we have a sending result. In the future if we allow
+    // for sending to vary, then this code will need to be updated to vary with
+    // each result.
+    auto results = substCalleeType->getResults();
+    auto *applyExpr = applySite->getLoc().getAsASTNode<ApplyExpr>();
+
+    // We only emit the error if we do not have a sending result and if our
+    // callee isn't nonisolated.
+    //
+    // DISCUSSION: If our callee is non-isolated, we know that the value must
+    // have been returned as a non-sent disconnected value. The reason why this
+    // is different from a sending result is that the result may be part of the
+    // region of the operands while the sending result will not be. In either
+    // case though, we do not want to emit the error.
+    bool emitIsolationCrossingResultError =
+        (results.empty() || !results[0].hasOption(SILResultInfo::IsSending)) &&
+        // We have to check if we actually have an apply expr since we may not
+        // have one if we are processing a SIL test case since SIL does not have
+        // locations (which is how we grab our AST information).
+        !(applyExpr && applyExpr->getIsolationCrossing()
+                           ->getCalleeIsolation()
+                           .isNonisolated());
+
+    for (auto result : applyResults) {
+      if (auto value = tryToTrackValue(result)) {
         builder.addAssignFresh(value->getRepresentative().getValue());
+        if (emitIsolationCrossingResultError)
+          builder.addNonSendableIsolationCrossingResultError(
+              value->getRepresentative().getValue());
+      }
+    }
+
+    // If we are supposed to emit isolation crossing errors, go through our
+    // parameters and add the error on any indirect results that are
+    // non-Sendable.
+    if (emitIsolationCrossingResultError) {
+      for (auto result : applySite.getIndirectSILResults()) {
+        if (auto value = tryToTrackValue(result)) {
+          builder.addNonSendableIsolationCrossingResultError(
+              value->getRepresentative().getValue());
+        }
+      }
+    }
   }
 
   template <typename DestValues>
diff --git a/lib/SILOptimizer/Mandatory/SendNonSendable.cpp b/lib/SILOptimizer/Mandatory/SendNonSendable.cpp
index 96a869d786eea..9f8c0b769ad4c 100644
--- a/lib/SILOptimizer/Mandatory/SendNonSendable.cpp
+++ b/lib/SILOptimizer/Mandatory/SendNonSendable.cpp
@@ -849,26 +849,6 @@ class UseAfterSendDiagnosticInferrer {
 
   void initForApply(Operand *op, ApplyExpr *expr);
   void initForAutoclosure(Operand *op, AutoClosureExpr *expr);
-
-  Expr *getFoundExprForSelf(ApplyExpr *sourceApply) {
-    if (auto callExpr = dyn_cast<CallExpr>(sourceApply))
-      if (auto calledExpr =
-              dyn_cast<DotSyntaxCallExpr>(callExpr->getDirectCallee()))
-        return calledExpr->getBase();
-    return nullptr;
-  }
-
-  Expr *getFoundExprForParam(ApplyExpr *sourceApply, unsigned argNum) {
-    auto *expr = sourceApply->getArgs()->getExpr(argNum);
-
-    // If we have an erasure expression, lets use the original type. We do
-    // this since we are not saying the specific parameter that is the
-    // issue and we are using the type to explain it to the user.
-    if (auto *erasureExpr = dyn_cast<ErasureExpr>(expr))
-      expr = erasureExpr->getSubExpr();
-
-    return expr;
-  }
 };
 
 } // namespace
@@ -2284,6 +2264,223 @@ void AssignIsolatedIntoSendingResultDiagnosticEmitter::emit() {
                type);
 }
 
+//===----------------------------------------------------------------------===//
+//              MARK: NonSendableIsolationCrossingResult Emitter
+//===----------------------------------------------------------------------===//
+
+/// Add Fix-It text for the given nominal type to adopt Sendable.
+static void addSendableFixIt(const NominalTypeDecl *nominal,
+                             InFlightDiagnostic &diag, bool unchecked) {
+  if (nominal->getInherited().empty()) {
+    SourceLoc fixItLoc = nominal->getBraces().Start;
+    diag.fixItInsert(fixItLoc,
+                     unchecked ? ": @unchecked Sendable" : ": Sendable");
+  } else {
+    auto fixItLoc = nominal->getInherited().getEndLoc();
+    diag.fixItInsertAfter(fixItLoc,
+                          unchecked ? ", @unchecked Sendable" : ", Sendable");
+  }
+}
+
+/// Add Fix-It text for the given generic param declaration type to adopt
+/// Sendable.
+static void addSendableFixIt(const GenericTypeParamDecl *genericArgument,
+                             InFlightDiagnostic &diag, bool unchecked) {
+  if (genericArgument->getInherited().empty()) {
+    auto fixItLoc = genericArgument->getLoc();
+    diag.fixItInsertAfter(fixItLoc,
+                          unchecked ? ": @unchecked Sendable" : ": Sendable");
+  } else {
+    auto fixItLoc = genericArgument->getInherited().getEndLoc();
+    diag.fixItInsertAfter(fixItLoc,
+                          unchecked ? ", @unchecked Sendable" : ", Sendable");
+  }
+}
+
+namespace {
+
+struct NonSendableIsolationCrossingResultDiagnosticEmitter {
+  RegionAnalysisValueMap &valueMap;
+
+  using Error = PartitionOpError::NonSendableIsolationCrossingResultError;
+  Error error;
+
+  bool emittedErrorDiagnostic = false;
+
+  /// The value assigned as the equivalence class representative. It is
+  /// guaranteed to be from the isolation crossing function since we never treat
+  /// isolation crossing functions as being look through.
+  SILValue representative;
+
+  NonSendableIsolationCrossingResultDiagnosticEmitter(
+      RegionAnalysisValueMap &valueMap, Error error)
+      : valueMap(valueMap), error(error),
+        representative(valueMap.getRepresentative(error.returnValueElement)) {}
+
+  void emit();
+
+  ASTContext &getASTContext() const {
+    return error.op->getSourceInst()->getFunction()->getASTContext();
+  }
+
+  template <typename... T, typename... U>
+  InFlightDiagnostic diagnoseError(SourceLoc loc, Diag<T...> diag,
+                                   U &&...args) {
+    emittedErrorDiagnostic = true;
+    return std::move(getASTContext()
+                         .Diags.diagnose(loc, diag, std::forward<U>(args)...)
+                         .warnUntilSwiftVersion(6));
+  }
+
+  template <typename... T, typename... U>
+  InFlightDiagnostic diagnoseError(SILLocation loc, Diag<T...> diag,
+                                   U &&...args) {
+    return diagnoseError(loc.getSourceLoc(), diag, std::forward<U>(args)...);
+  }
+
+  template <typename... T, typename... U>
+  InFlightDiagnostic diagnoseError(SILInstruction *inst, Diag<T...> diag,
+                                   U &&...args) {
+    return diagnoseError(inst->getLoc(), diag, std::forward<U>(args)...);
+  }
+
+  template <typename... T, typename... U>
+  InFlightDiagnostic diagnoseNote(SourceLoc loc, Diag<T...> diag, U &&...args) {
+    return getASTContext().Diags.diagnose(loc, diag, std::forward<U>(args)...);
+  }
+
+  template <typename... T, typename... U>
+  InFlightDiagnostic diagnoseNote(SILLocation loc, Diag<T...> diag,
+                                  U &&...args) {
+    return diagnoseNote(loc.getSourceLoc(), diag, std::forward<U>(args)...);
+  }
+
+  template <typename... T, typename... U>
+  InFlightDiagnostic diagnoseNote(SILInstruction *inst, Diag<T...> diag,
+                                  U &&...args) {
+    return diagnoseNote(inst->getLoc(), diag, std::forward<U>(args)...);
+  }
+
+  std::optional<DiagnosticBehavior> getBehaviorLimit() const {
+    return representative->getType().getConcurrencyDiagnosticBehavior(
+        representative->getFunction());
+  }
+
+  void emitUnknownPatternError() {
+    if (shouldAbortOnUnknownPatternMatchError()) {
+      llvm::report_fatal_error(
+          "RegionIsolation: Aborting on unknown pattern match error");
+    }
+
+    diagnoseError(error.op->getSourceInst(),
+                  diag::regionbasedisolation_unknown_pattern)
+        .limitBehaviorIf(getBehaviorLimit());
+  }
+
+  Type getType() const {
+    if (auto *applyExpr =
+            error.op->getSourceInst()->getLoc().getAsASTNode<ApplyExpr>()) {
+      return applyExpr->getType();
+    }
+
+    // If we do not have an ApplyExpr, see if we can just infer the type from
+    // the SILFunction type. This is only used in SIL test cases.
+    if (auto fas = FullApplySite::isa(error.op->getSourceInst())) {
+      return fas.getSubstCalleeType()
+          ->getAllResultsSubstType(fas.getModule(),
+                                   fas.getFunction()->getTypeExpansionContext())
+          .getASTType();
+    }
+
+    return Type();
+  }
+
+  const ValueDecl *getCalledDecl() const {
+    if (auto *applyExpr =
+            error.op->getSourceInst()->getLoc().getAsASTNode<ApplyExpr>()) {
+      if (auto calledValue =
+              applyExpr->getCalledValue(true /*look through conversions*/)) {
+        return calledValue;
+      }
+    }
+
+    return nullptr;
+  }
+
+  std::optional<ApplyIsolationCrossing> getIsolationCrossing() const {
+    if (auto *applyExpr =
+            error.op->getSourceInst()->getLoc().getAsASTNode<ApplyExpr>()) {
+      if (auto isolationCrossing = applyExpr->getIsolationCrossing()) {
+        return *isolationCrossing;
+      }
+    }
+
+    // If we have a SIL based test case, just return the actual isolation
+    // crossing.
+    if (auto fas = FullApplySite::isa(error.op->getSourceInst())) {
+      if (auto isolationCrossing = fas.getIsolationCrossing())
+        return *isolationCrossing;
+    }
+
+    return {};
+  }
+};
+
+} // namespace
+
+void NonSendableIsolationCrossingResultDiagnosticEmitter::emit() {
+  auto isolationCrossing = getIsolationCrossing();
+  if (!isolationCrossing)
+    return emitUnknownPatternError();
+
+  auto type = getType();
+  if (auto *decl = getCalledDecl()) {
+    diagnoseError(error.op->getSourceInst(), diag::rbi_isolation_crossing_result,
+                  type, isolationCrossing->getCalleeIsolation(), getCalledDecl(),
+                  isolationCrossing->getCallerIsolation())
+      .limitBehaviorIf(getBehaviorLimit());
+  } else {
+    diagnoseError(error.op->getSourceInst(), diag::rbi_isolation_crossing_result_no_decl,
+                  type, isolationCrossing->getCalleeIsolation(),
+                  isolationCrossing->getCallerIsolation())
+      .limitBehaviorIf(getBehaviorLimit());
+  }
+  if (type->is<FunctionType>()) {
+    diagnoseNote(error.op->getSourceInst(),
+                 diag::rbi_nonsendable_function_type);
+    return;
+  }
+
+  auto *moduleDecl = error.op->getSourceInst()->getModule().getSwiftModule();
+  if (auto *nominal = type->getNominalOrBoundGenericNominal()) {
+    // If the nominal type is in the current module, suggest adding `Sendable`
+    // if it makes sense.
+    if (nominal->getParentModule() == moduleDecl &&
+        (isa<StructDecl>(nominal) || isa<EnumDecl>(nominal))) {
+      auto note = nominal->diagnose(diag::rbi_add_nominal_sendable_conformance,
+                                    nominal);
+      addSendableFixIt(nominal, note, /*unchecked*/ false);
+    } else {
+      nominal->diagnose(diag::rbi_non_sendable_nominal, nominal);
+    }
+    return;
+  }
+
+  if (auto genericArchetype = type->getAs<ArchetypeType>()) {
+    auto interfaceType = genericArchetype->getInterfaceType();
+    if (auto genericParamType = interfaceType->getAs<GenericTypeParamType>()) {
+      auto *genericParamTypeDecl = genericParamType->getDecl();
+      if (genericParamTypeDecl &&
+          genericParamTypeDecl->getModuleContext() == moduleDecl) {
+        auto diag = genericParamTypeDecl->diagnose(
+            diag::rbi_add_generic_parameter_sendable_conformance, type);
+        addSendableFixIt(genericParamTypeDecl, diag, /*unchecked=*/false);
+        return;
+      }
+    }
+  }
+}
+
 //===----------------------------------------------------------------------===//
 //                         MARK: Diagnostic Evaluator
 //===----------------------------------------------------------------------===//
@@ -2370,6 +2567,7 @@ struct DiagnosticEvaluator final
     case PartitionOpError::InOutSendingNotDisconnectedAtExit:
     case PartitionOpError::SentNeverSendable:
     case PartitionOpError::AssignNeverSendableIntoSendingResult:
+    case PartitionOpError::NonSendableIsolationCrossingResult:
       // We are going to process these later... but dump so we can see that we
       // handled an error here. The rest of the explicit handlers will dump as
       // appropriate if they want to emit an error here (some will squelch the
@@ -2510,6 +2708,14 @@ void SendNonSendableImpl::emitVerbatimErrors() {
       diagnosticInferrer.run();
       continue;
     }
+    case PartitionOpError::NonSendableIsolationCrossingResult: {
+      auto e = erasedError.getNonSendableIsolationCrossingResultError();
+      REGIONBASEDISOLATION_LOG(e.print(llvm::dbgs(), info->getValueMap()));
+      NonSendableIsolationCrossingResultDiagnosticEmitter diagnosticInferrer(
+          info->getValueMap(), e);
+      diagnosticInferrer.emit();
+      continue;
+    }
     }
     llvm_unreachable("Covered switch isn't covered?!");
   }
diff --git a/lib/SILOptimizer/Utils/PartitionUtils.cpp b/lib/SILOptimizer/Utils/PartitionUtils.cpp
index f5501cdfcde5a..d0dd74a1e2759 100644
--- a/lib/SILOptimizer/Utils/PartitionUtils.cpp
+++ b/lib/SILOptimizer/Utils/PartitionUtils.cpp
@@ -94,14 +94,22 @@ void PartitionOpError::InOutSendingNotDisconnectedAtExitError::print(
   os << '\n';
 }
 
+void PartitionOpError::NonSendableIsolationCrossingResultError::print(
+    llvm::raw_ostream &os, RegionAnalysisValueMap &valueMap) const {
+  os << "    Emitting Error. Kind: NonSendableIsolationCrossingResultError\n"
+        "        Inst: "
+     << *op->getSourceInst() << "        Result ID: %%" << returnValueElement
+     << '\n';
+}
+
 //===----------------------------------------------------------------------===//
 //                             MARK: PartitionOp
 //===----------------------------------------------------------------------===//
 
 void PartitionOp::print(llvm::raw_ostream &os, bool extraSpace) const {
+  constexpr static char extraSpaceLiteral[10] = "     ";
   switch (opKind) {
   case PartitionOpKind::Assign: {
-    constexpr static char extraSpaceLiteral[10] = "      ";
     os << "assign ";
     if (extraSpace)
       os << extraSpaceLiteral;
@@ -112,7 +120,6 @@ void PartitionOp::print(llvm::raw_ostream &os, bool extraSpace) const {
     os << "assign_fresh %%" << opArgs[0];
     break;
   case PartitionOpKind::Send: {
-    constexpr static char extraSpaceLiteral[10] = "    ";
     os << "send ";
     if (extraSpace)
       os << extraSpaceLiteral;
@@ -120,7 +127,6 @@ void PartitionOp::print(llvm::raw_ostream &os, bool extraSpace) const {
     break;
   }
   case PartitionOpKind::UndoSend: {
-    constexpr static char extraSpaceLiteral[10] = "    ";
     os << "undo_send ";
     if (extraSpace)
       os << extraSpaceLiteral;
@@ -128,7 +134,6 @@ void PartitionOp::print(llvm::raw_ostream &os, bool extraSpace) const {
     break;
   }
   case PartitionOpKind::Merge: {
-    constexpr static char extraSpaceLiteral[10] = "       ";
     os << "merge ";
     if (extraSpace)
       os << extraSpaceLiteral;
@@ -136,7 +141,6 @@ void PartitionOp::print(llvm::raw_ostream &os, bool extraSpace) const {
     break;
   }
   case PartitionOpKind::Require: {
-    constexpr static char extraSpaceLiteral[10] = "     ";
     os << "require ";
     if (extraSpace)
       os << extraSpaceLiteral;
@@ -148,12 +152,17 @@ void PartitionOp::print(llvm::raw_ostream &os, bool extraSpace) const {
     os << "%%" << opArgs[0];
     break;
   case PartitionOpKind::InOutSendingAtFunctionExit:
-    constexpr static char extraSpaceLiteral[10] = "     ";
     os << "inout_sending_at_function_exit ";
     if (extraSpace)
       os << extraSpaceLiteral;
     os << "%%" << opArgs[0];
     break;
+  case PartitionOpKind::NonSendableIsolationCrossingResult:
+    os << "nonsendable_isolationcrossing_result ";
+    if (extraSpace)
+      os << extraSpaceLiteral;
+    os << "%%" << opArgs[0];
+    break;
   }
   os << ": " << *getSourceInst();
 }
diff --git a/lib/Sema/TypeCheckConcurrency.cpp b/lib/Sema/TypeCheckConcurrency.cpp
index 44d55f9e567fe..73909a2d871ef 100644
--- a/lib/Sema/TypeCheckConcurrency.cpp
+++ b/lib/Sema/TypeCheckConcurrency.cpp
@@ -3865,54 +3865,8 @@ namespace {
             unsatisfiedIsolation, setThrows, usesDistributedThunk);
       }
 
-      // Sendable checking for arguments is deferred to region isolation.
-
-      // FIXME: Defer sendable checking for result types to region isolation
-      // always.
-      //
-      // Check for sendability of the result type if we do not have a
-      // sending result.
-      if ((!ctx.LangOpts.hasFeature(Feature::RegionBasedIsolation) ||
-           !fnType->hasSendingResult())) {
-        assert(ctx.LangOpts.hasFeature(Feature::SendingArgsAndResults) &&
-               "SendingArgsAndResults should be enabled if RegionIsolation is "
-               "enabled");
-        // See if we are a autoclosure that has a direct callee that has the
-        // same non-transferred type value returned. If so, do not emit an
-        // error... we are going to emit an error on the call expr and do not
-        // want to emit the error twice.
-        auto willDoubleError = [&]() -> bool {
-          auto *autoclosure = dyn_cast<AutoClosureExpr>(apply->getFn());
-          if (!autoclosure)
-            return false;
-          auto *await =
-              dyn_cast<AwaitExpr>(autoclosure->getSingleExpressionBody());
-          if (!await)
-            return false;
-          auto *subCallExpr = dyn_cast<CallExpr>(await->getSubExpr());
-          if (!subCallExpr)
-            return false;
-          return subCallExpr->getType().getPointer() ==
-                 fnType->getResult().getPointer();
-        };
-
-        if (!willDoubleError()) {
-          if (calleeDecl) {
-            return diagnoseNonSendableTypes(fnType->getResult(), getDeclContext(),
-                /*inDerivedConformance*/ Type(),
-                apply->getLoc(),
-                diag::non_sendable_result_into_actor,
-                calleeDecl,
-                *unsatisfiedIsolation);
-          }
-
-          return diagnoseNonSendableTypes(fnType->getResult(), getDeclContext(),
-              /*inDerivedConformance*/ Type(),
-              apply->getLoc(),
-              diag::non_sendable_call_result_type,
-              *unsatisfiedIsolation);
-        }
-      }
+      // Sendable checking for arguments and results are deferred to region
+      // isolation.
 
       return false;
     }
diff --git a/test/ClangImporter/objc_async.swift b/test/ClangImporter/objc_async.swift
index 35368883d294a..a02117c3ae2e4 100644
--- a/test/ClangImporter/objc_async.swift
+++ b/test/ClangImporter/objc_async.swift
@@ -298,7 +298,6 @@ class BarFrame: PictureFrame {
 @available(SwiftStdlib 5.5, *)
 @SomeGlobalActor
 class BazFrame: NotIsolatedPictureFrame {
-// expected-note@-1 2 {{class 'BazFrame' does not conform to the 'Sendable' protocol}}
   init() {
     super.init(size: 0)
   }
@@ -322,12 +321,10 @@ func check() async {
   _ = await BarFrame()
   _ = await FooFrame()
   _ = await BazFrame()
-  // expected-warning@-1 {{non-sendable result type 'BazFrame' cannot be sent from global actor 'SomeGlobalActor'-isolated context in call to initializer 'init()'; this is an error in the Swift 6 language mode}}
 
   _ = await BarFrame(size: 0)
   _ = await FooFrame(size: 0)
   _ = await BazFrame(size: 0)
-  // expected-warning@-1 {{non-sendable result type 'BazFrame' cannot be sent from global actor 'SomeGlobalActor'-isolated context in call to initializer 'init(size:)'; this is an error in the Swift 6 language mode}}
 }
 
 @available(SwiftStdlib 5.5, *)
diff --git a/test/Concurrency/actor_call_implicitly_async.swift b/test/Concurrency/actor_call_implicitly_async.swift
index 553064f61a1eb..6c79a12a0e994 100644
--- a/test/Concurrency/actor_call_implicitly_async.swift
+++ b/test/Concurrency/actor_call_implicitly_async.swift
@@ -356,25 +356,17 @@ actor Calculator {
 }
 
 @OrangeActor func doSomething() async {
+  // We will error on the next line when we get past type checking. But since we
+  // error in the type checker, we do not make further progress.
   let _ = (await bananaAdd(1))(2)
-  // expected-warning@-1{{non-sendable result type '(Int) -> Int' cannot be sent from global actor 'BananaActor'-isolated context in call to global function 'bananaAdd'}}
-  // expected-note@-2{{a function type must be marked '@Sendable' to conform to 'Sendable'}}
   let _ = await (await bananaAdd(1))(2) // expected-warning{{no 'async' operations occur within 'await' expression}}
-  // expected-warning@-1{{non-sendable result type '(Int) -> Int' cannot be sent from global actor 'BananaActor'-isolated context in call to global function 'bananaAdd'}}
-  // expected-note@-2{{a function type must be marked '@Sendable' to conform to 'Sendable'}}
 
   let calc = Calculator()
   
   let _ = (await calc.addCurried(1))(2)
-  // expected-warning@-1{{non-sendable result type '(Int) -> Int' cannot be sent from actor-isolated context in call to instance method 'addCurried'}}
-  // expected-note@-2{{a function type must be marked '@Sendable' to conform to 'Sendable'}}
   let _ = await (await calc.addCurried(1))(2) // expected-warning{{no 'async' operations occur within 'await' expression}}
-  // expected-warning@-1{{non-sendable result type '(Int) -> Int' cannot be sent from actor-isolated context in call to instance method 'addCurried'}}
-  // expected-note@-2{{a function type must be marked '@Sendable' to conform to 'Sendable'}}
 
   let plusOne = await calc.addCurried(await calc.add(0, 1))
-  // expected-warning@-1{{non-sendable result type '(Int) -> Int' cannot be sent from actor-isolated context in call to instance method 'addCurried'}}
-  // expected-note@-2{{a function type must be marked '@Sendable' to conform to 'Sendable'}}
   let _ = plusOne(2)
 }
 
diff --git a/test/Concurrency/concurrent_value_checking.swift b/test/Concurrency/concurrent_value_checking.swift
index b5974050eac5a..95614ea3c9d3d 100644
--- a/test/Concurrency/concurrent_value_checking.swift
+++ b/test/Concurrency/concurrent_value_checking.swift
@@ -5,7 +5,7 @@
 // REQUIRES: concurrency
 // REQUIRES: asserts
 
-class NotConcurrent { } // expected-note 13{{class 'NotConcurrent' does not conform to the 'Sendable' protocol}}
+class NotConcurrent { } // expected-note 12{{class 'NotConcurrent' does not conform to the 'Sendable' protocol}}
 // expected-tns-allow-typechecker-note @-1 {{class 'NotConcurrent' does not conform to the 'Sendable' protocol}}
 
 // ----------------------------------------------------------------------
@@ -102,7 +102,7 @@ extension A1 {
     _ = other.localLet // expected-warning{{non-sendable type 'NotConcurrent' of property 'localLet' cannot exit actor-isolated context}}
     // expected-warning@-1 {{expression is 'async' but is not marked with 'await'}}
     // expected-note@-2 {{property access is 'async'}}
-    _ = await other.synchronous() // expected-warning{{non-sendable result type 'NotConcurrent?' cannot be sent from actor-isolated context in call to instance method 'synchronous()'}}
+    _ = await other.synchronous() // expected-tns-warning {{non-Sendable 'NotConcurrent?'-typed result can not be returned from actor-isolated instance method 'synchronous()' to actor-isolated context}}
     _ = await other.asynchronous(nil)
   }
 }
@@ -164,7 +164,7 @@ struct HasSubscript {
   subscript (i: Int) -> NotConcurrent? { nil }
 }
 
-class ClassWithGlobalActorInits { // expected-note 2{{class 'ClassWithGlobalActorInits' does not conform to the 'Sendable' protocol}}
+class ClassWithGlobalActorInits { // expected-tns-note 2{{class 'ClassWithGlobalActorInits' does not conform to the 'Sendable' protocol}}
   @SomeGlobalActor
   init(_: NotConcurrent) { }
 
@@ -182,10 +182,10 @@ func globalTestMain(nc: NotConcurrent) async {
   // expected-tns-note @-1 {{sending global actor 'SomeGlobalActor'-isolated 'a' to global actor 'SomeGlobalActor'-isolated global function 'globalAsync' risks causing data races between global actor 'SomeGlobalActor'-isolated and local main actor-isolated uses}}
   await globalSync(a) // expected-tns-note {{access can happen concurrently}}
   _ = await ClassWithGlobalActorInits(nc)
-  // expected-warning @-1 {{non-sendable result type 'ClassWithGlobalActorInits' cannot be sent from global actor 'SomeGlobalActor'-isolated context in call to initializer 'init(_:)'}}
+  // expected-tns-warning @-1 {{non-Sendable 'ClassWithGlobalActorInits'-typed result can not be returned from global actor 'SomeGlobalActor'-isolated initializer 'init(_:)' to main actor-isolated context}}
   // expected-tns-warning @-2 {{sending 'nc' risks causing data races}}
   // expected-tns-note @-3 {{sending main actor-isolated 'nc' to global actor 'SomeGlobalActor'-isolated initializer 'init(_:)' risks causing data races between global actor 'SomeGlobalActor'-isolated and main actor-isolated uses}}
-  _ = await ClassWithGlobalActorInits() // expected-warning{{non-sendable result type 'ClassWithGlobalActorInits' cannot be sent from global actor 'SomeGlobalActor'-isolated context in call to initializer 'init()'}}
+  _ = await ClassWithGlobalActorInits() // expected-tns-warning {{non-Sendable 'ClassWithGlobalActorInits'-typed result can not be returned from global actor 'SomeGlobalActor'-isolated initializer 'init()' to main actor-isolated context}}
 }
 
 @SomeGlobalActor
diff --git a/test/Concurrency/sendable_checking.swift b/test/Concurrency/sendable_checking.swift
index bfb8fd4b5ecf5..4b0e03c170409 100644
--- a/test/Concurrency/sendable_checking.swift
+++ b/test/Concurrency/sendable_checking.swift
@@ -7,7 +7,6 @@
 
 @available(SwiftStdlib 5.1, *)
 struct NS1 { }
-// expected-note @-1 {{consider making struct 'NS1' conform to the 'Sendable' protocol}}
 
 @available(SwiftStdlib 5.1, *)
 @available(*, unavailable)
@@ -96,7 +95,7 @@ public actor MyActor: MyProto {
     await nonisolatedAsyncFunc1(ns1)
     // expected-tns-warning @-1 {{sending 'ns1' risks causing data races}}
     // expected-tns-note @-2 {{sending 'self'-isolated 'ns1' to nonisolated global function 'nonisolatedAsyncFunc1' risks causing data races between nonisolated and 'self'-isolated uses}}
-    _ = await nonisolatedAsyncFunc2() // expected-warning{{non-sendable result type 'NS1' cannot be sent from nonisolated context in call to global function 'nonisolatedAsyncFunc2()'}}
+    _ = await nonisolatedAsyncFunc2()
   }
 }
 
diff --git a/test/Concurrency/sendable_module_checking.swift b/test/Concurrency/sendable_module_checking.swift
index 6da3f9ca29dbd..47cbb27b008ce 100644
--- a/test/Concurrency/sendable_module_checking.swift
+++ b/test/Concurrency/sendable_module_checking.swift
@@ -2,8 +2,7 @@
 // RUN: %target-swift-frontend -emit-module -emit-module-path %t/StrictModule.swiftmodule -module-name StrictModule -strict-concurrency=complete %S/Inputs/StrictModule.swift
 // RUN: %target-swift-frontend -emit-module -emit-module-path %t/NonStrictModule.swiftmodule -module-name NonStrictModule %S/Inputs/NonStrictModule.swift
 
-// We leave this as just type check since we are checking something that is cross module.
-// RUN: %target-swift-frontend -typecheck -strict-concurrency=targeted -disable-availability-checking -I %t 2>&1 %s | %FileCheck %s
+// RUN: %target-swift-frontend -c -strict-concurrency=complete -disable-availability-checking -I %t 2>&1 %s | %FileCheck %s
 
 // REQUIRES: concurrency
 
@@ -15,9 +14,9 @@ actor A {
 }
 
 func testA(a: A) async {
-  _ = await a.f() // CHECK: warning: cannot call function returning non-sendable type '[StrictStruct : NonStrictClass]' across actors}}
-  // CHECK: note: struct 'StrictStruct' does not conform to the 'Sendable' protocol
-  // CHECK: note: class 'NonStrictClass' does not conform to the 'Sendable' protocol
+  _ = await a.f()
+  // CHECK: warning: non-Sendable '[StrictStruct : NonStrictClass]'-typed result can not be returned from actor-isolated instance method 'f()' to nonisolated context; this is an error in the Swift 6 language mode
+  // CHECK: note: note: generic struct 'Dictionary' does not conform to the 'Sendable' protocol
 }
 
 extension NonStrictStruct: @unchecked Sendable { }
diff --git a/test/Concurrency/transfernonsendable.sil b/test/Concurrency/transfernonsendable.sil
index 4ea454526ada7..292c01744c120 100644
--- a/test/Concurrency/transfernonsendable.sil
+++ b/test/Concurrency/transfernonsendable.sil
@@ -21,7 +21,7 @@ import _Concurrency
 
 class Klass {}
 
-class NonSendableKlass {
+class NonSendableKlass { // expected-note 2{{}}
   var klass: Klass
 
   func asyncCall() async
@@ -121,10 +121,6 @@ bb0(%0 : $*{ var NonSendableKlass }):
   return %9999 : $()
 }
 
-// This doesn't error since the @out parameter is not transferred when it is initialized.
-//
-// DISCUSSION: The frontend prevents us from using such a value. But we
-// shouldn't crash on such values.
 sil [ossa] @transfer_does_not_transfer_out_parameters_1 : $@convention(thin) @async () -> () {
 bb0:
   %0 = alloc_stack $NonSendableKlass
@@ -133,7 +129,7 @@ bb0:
 
   %1 = alloc_stack $NonSendableKlass
   %f = function_ref @transferIndirectWithOutResult : $@convention(thin) @async <τ_0_0> (@in_guaranteed τ_0_0) -> @out τ_0_0
-  apply [caller_isolation=nonisolated] [callee_isolation=global_actor] %f<NonSendableKlass>(%1, %0) : $@convention(thin) @async <τ_0_0> (@in_guaranteed τ_0_0) -> @out τ_0_0
+  apply [caller_isolation=nonisolated] [callee_isolation=global_actor] %f<NonSendableKlass>(%1, %0) : $@convention(thin) @async <τ_0_0> (@in_guaranteed τ_0_0) -> @out τ_0_0 // expected-warning {{}}
 
   %useIndirect = function_ref @useIndirect : $@convention(thin) <τ_0_0> (@in_guaranteed τ_0_0) -> ()
   apply %useIndirect<NonSendableKlass>(%1) : $@convention(thin) <τ_0_0> (@in_guaranteed τ_0_0) -> ()
@@ -155,7 +151,7 @@ bb0:
 
   %1 = alloc_stack $NonSendableKlass
   %f = function_ref @transferIndirectWithOutResult : $@convention(thin) @async <τ_0_0> (@in_guaranteed τ_0_0) -> @out τ_0_0
-  apply [caller_isolation=nonisolated] [callee_isolation=global_actor] %f<NonSendableKlass>(%1, %0) : $@convention(thin) @async <τ_0_0> (@in_guaranteed τ_0_0) -> @out τ_0_0
+  apply [caller_isolation=nonisolated] [callee_isolation=global_actor] %f<NonSendableKlass>(%1, %0) : $@convention(thin) @async <τ_0_0> (@in_guaranteed τ_0_0) -> @out τ_0_0 // expected-warning {{}}
 
   %f2 = function_ref @transferIndirect : $@convention(thin) @async <τ_0_0> (@in_guaranteed τ_0_0) -> ()
   apply [caller_isolation=nonisolated] [callee_isolation=global_actor] %f2<NonSendableKlass>(%1) : $@convention(thin) @async <τ_0_0> (@in_guaranteed τ_0_0) -> ()
diff --git a/test/Concurrency/transfernonsendable_asynclet.swift b/test/Concurrency/transfernonsendable_asynclet.swift
index 238dc72761e3f..350353c05398b 100644
--- a/test/Concurrency/transfernonsendable_asynclet.swift
+++ b/test/Concurrency/transfernonsendable_asynclet.swift
@@ -18,7 +18,7 @@ class NonSendableKlass {
 
 class SendableKlass : @unchecked Sendable {}
 
-struct NonSendableStruct { // expected-note {{}}
+struct NonSendableStruct {
   var ns = NonSendableKlass()
 }
 
@@ -722,7 +722,7 @@ func asyncLetWithoutCapture() async {
   //
   // NOTE: Error below will go away in next commit.
   async let x: NonSendableKlass = await returnValueFromMain()
-  // expected-warning @-1 {{non-sendable result type 'NonSendableKlass' cannot be sent from main actor-isolated context in call to global function 'returnValueFromMain()'}}
+  // expected-warning @-1 {{non-Sendable 'NonSendableKlass'-typed result can not be returned from main actor-isolated global function 'returnValueFromMain()' to nonisolated context}}
   let y = await x
   await transferToMain(y) // expected-warning {{sending 'y' risks causing data races}}
   // expected-note @-1 {{sending 'y' to main actor-isolated global function 'transferToMain' risks causing data races between main actor-isolated and local nonisolated uses}}
@@ -774,7 +774,6 @@ extension NonSendableStruct {
     async let subTask6: NonSendableStruct = self
     // expected-warning @-1 {{sending 'self' risks causing data races}}
     // expected-note @-2 {{sending 'actor'-isolated 'self' into async let risks causing data races between nonisolated and 'actor'-isolated uses}}
-    // expected-warning @-3 {{non-sendable result type 'NonSendableStruct' cannot be sent from nonisolated context in call to async function}}
     _ = await subTask6
   }
 }
diff --git a/test/Concurrency/transfernonsendable_defer_and_typecheck_only.swift b/test/Concurrency/transfernonsendable_defer_and_typecheck_only.swift
index 4ba64c8ebe487..c6e2e372f1c5b 100644
--- a/test/Concurrency/transfernonsendable_defer_and_typecheck_only.swift
+++ b/test/Concurrency/transfernonsendable_defer_and_typecheck_only.swift
@@ -14,7 +14,6 @@
  */
 
 class NonSendable {
-// expected-note@-1 3{{class 'NonSendable' does not conform to the 'Sendable' protocol}}
     var x = 0
 }
 
@@ -31,10 +30,8 @@ func callActorFuncsFromNonisolated(a : A, ns : NonSendable) async {
     // Non-sendable value passed from actor isolated to nonisolated
 
     await a.actorTakesNS(ns)
-    //deferred-warning@-1{{passing argument of non-sendable type 'NonSendable' into actor-isolated context may introduce data races}}
 
     _ = await a.actorRetsNS()
-    //expected-warning@-1{{non-sendable result type 'NonSendable' cannot be sent from actor-isolated context in call to instance method 'actorRetsNS()'}}
 }
 
 @available(SwiftStdlib 5.1, *)
@@ -52,7 +49,6 @@ actor A {
         //deferred-warning@-1{{passing argument of non-sendable type 'NonSendable' outside of actor-isolated context may introduce data races}}
 
         _ = await retsNS()
-        //expected-warning@-1{{non-sendable result type 'NonSendable' cannot be sent from nonisolated context in call to global function 'retsNS()'}}
     }
 
     func callActorFuncsFromDiffActor(ns : NonSendable, a : A) async {
@@ -62,6 +58,5 @@ actor A {
         //deferred-warning@-1{{passing argument of non-sendable type 'NonSendable' into actor-isolated context may introduce data races}}
 
         _ = await a.actorRetsNS()
-        //expected-warning@-1{{non-sendable result type 'NonSendable' cannot be sent from actor-isolated context in call to instance method 'actorRetsNS()'}}
     }
 }
diff --git a/test/Concurrency/transfernonsendable_rbi_result.swift b/test/Concurrency/transfernonsendable_rbi_result.swift
new file mode 100644
index 0000000000000..ea730c83b2d92
--- /dev/null
+++ b/test/Concurrency/transfernonsendable_rbi_result.swift
@@ -0,0 +1,137 @@
+// RUN: %target-swift-frontend -target %target-swift-5.1-abi-triple -swift-version 6 -parse-as-library %s -emit-sil -o /dev/null -verify
+
+// REQUIRES: asserts
+// REQUIRES: concurrency
+
+///////////////////////
+// MARK: Declaration //
+///////////////////////
+
+actor Custom {
+}
+
+@globalActor
+struct CustomActor {
+  static var shared: Custom {
+    return Custom()
+  }
+}
+
+class NonSendable {} // expected-note 3{{}}
+
+func passNonSendable(_: NonSendable) async { }
+
+func returnsNonSendable() async -> NonSendable { NonSendable() }
+
+@MainActor
+func mainActorPassNonSendable(_: NonSendable) async { }
+
+@MainActor
+func mainActorReturnNonSendable() async -> NonSendable { NonSendable() }
+
+@MainActor
+func mainActorGenericPassNonSendable<T>(_: T) async { }
+
+@MainActor
+func mainActorGenericReturnNonSendable<T>() async -> T { fatalError() }
+
+@MainActor
+func mainActorAsyncFunc3() async -> ((Int) -> Int) {
+  return { (_ y: Int) in y }
+}
+
+/////////////////
+// MARK: Tests //
+/////////////////
+
+@MainActor func mainActorResult(_ x : Int) -> ((Int) -> Int) { 
+  return { (_ y : Int) in x + y }
+}
+
+actor Calculator {
+  func addCurried(_ x : Int) -> ((Int) -> Int) { 
+    return { (_ y : Int) in x + y }
+  }
+
+  func add(_ x : Int, _ y : Int) -> Int {
+    return x + y
+  }
+}
+
+@CustomActor
+func testActorCrossingBoundary() async {
+  let _ = (await mainActorResult(1))(5)
+  // expected-error @-1 {{non-Sendable '(Int) -> Int'-typed result can not be returned from main actor-isolated global function 'mainActorResult' to global actor 'CustomActor'-isolated context}}
+  // expected-note @-2 {{a function type must be marked '@Sendable' to conform to 'Sendable'}}
+  let _ = await (await mainActorResult(1))(2)
+  // expected-error @-1 {{non-Sendable '(Int) -> Int'-typed result can not be returned from main actor-isolated global function 'mainActorResult' to global actor 'CustomActor'-isolated context}}
+  // expected-note @-2 {{a function type must be marked '@Sendable' to conform to 'Sendable'}}
+  // expected-warning @-3 {{no 'async' operations occur within 'await' expression}}
+
+  let calc = Calculator()
+  
+  let _ = (await calc.addCurried(1))(2)
+  // expected-error @-1 {{non-Sendable '(Int) -> Int'-typed result can not be returned from actor-isolated instance method 'addCurried' to global actor 'CustomActor'-isolated context}}
+  // expected-note@-2{{a function type must be marked '@Sendable' to conform to 'Sendable'}}
+  let _ = await (await calc.addCurried(1))(2) // expected-warning{{no 'async' operations occur within 'await' expression}}
+  // expected-error @-1 {{non-Sendable '(Int) -> Int'-typed result can not be returned from actor-isolated instance method 'addCurried' to global actor 'CustomActor'-isolated context}}
+  // expected-note @-2 {{a function type must be marked '@Sendable' to conform to 'Sendable'}}
+
+  let plusOne = await calc.addCurried(await calc.add(0, 1))
+  // expected-error @-1 {{non-Sendable '(Int) -> Int'-typed result can not be returned from actor-isolated instance method 'addCurried' to global actor 'CustomActor'-isolated context}}
+  // expected-note @-2 {{a function type must be marked '@Sendable' to conform to 'Sendable'}}
+  let _ = plusOne(2)
+}
+
+actor A {
+  let actorNS = NonSendable()
+
+  func actorTakesNS(_ : NonSendable) async {}
+
+  func actorRetsNS() async -> NonSendable { NonSendable() }
+
+  func callNonisolatedFuncsFromActor(ns: NonSendable) async {
+    // Non-sendable value passed from nonisolated to actor isolated
+
+    await passNonSendable(ns)
+    // expected-error @-1 {{sending 'ns' risks causing data races}}
+    // expected-note @-2 {{sending 'self'-isolated 'ns' to nonisolated global function 'passNonSendable' risks causing data races between nonisolated and 'self'-isolated uses}}
+
+    _ = await returnsNonSendable()
+  }
+
+  func callActorFuncsFromDiffActor(ns : NonSendable, a : A) async {
+    // Non-sendable value passed between the isolation of two different actors
+
+    await a.actorTakesNS(ns)
+    // expected-error @-1 {{sending 'ns' risks causing data races}}
+    // expected-note @-2 {{sending 'self'-isolated 'ns' to actor-isolated instance method 'actorTakesNS' risks causing data races between actor-isolated and 'self'-isolated uses}}
+
+    _ = await a.actorRetsNS()
+    // expected-error @-1 {{non-Sendable 'NonSendable'-typed result can not be returned from actor-isolated instance method 'actorRetsNS()' to actor-isolated context}}
+  }
+
+  func validateErrorForPassingIsolatedNonSendable(_ ns: NonSendable) async {
+    await mainActorGenericPassNonSendable(ns)
+    // expected-error @-1 {{sending 'ns' risks causing data races}}
+    // expected-note @-2 {{sending 'self'-isolated 'ns' to main actor-isolated global function 'mainActorGenericPassNonSendable' risks causing data races between main actor-isolated and 'self'-isolated uses}}
+  }
+
+  func validateErrorReturningFromNonIsolated() async {
+    let _ = await returnsNonSendable()
+  }
+}
+
+func callActorFuncsFromNonisolated(a : A, ns : NonSendable) async {
+  await a.actorTakesNS(ns)
+  // expected-error @-1 {{sending 'ns' risks causing data races}}
+  // expected-note @-2 {{sending task-isolated 'ns' to actor-isolated instance method 'actorTakesNS' risks causing data races between actor-isolated and task-isolated uses}}
+
+  _ = await a.actorRetsNS()
+  // expected-error @-1 {{non-Sendable 'NonSendable'-typed result can not be returned from actor-isolated instance method 'actorRetsNS()' to nonisolated context}}
+}
+
+func testGenericResults() async {
+  let _: NonSendable = await mainActorGenericReturnNonSendable()
+  // expected-error @-1 {{non-Sendable 'NonSendable'-typed result can not be returned from main actor-isolated global function 'mainActorGenericReturnNonSendable()' to nonisolated context}}
+}
diff --git a/test/Concurrency/transfernonsendable_sending_results.swift b/test/Concurrency/transfernonsendable_sending_results.swift
index 72fdd4adfa398..a3615b29546fe 100644
--- a/test/Concurrency/transfernonsendable_sending_results.swift
+++ b/test/Concurrency/transfernonsendable_sending_results.swift
@@ -238,7 +238,7 @@ func asyncLetReabstractionThunkTest() async {
 func asyncLetReabstractionThunkTest2() async {
   // We emit the error here since we are returning a main actor isolated value.
   async let newValue: NonSendableKlass = await getMainActorValueAsync()
-  // expected-warning @-1 {{non-sendable result type 'NonSendableKlass' cannot be sent from main actor-isolated context in call to global function 'getMainActorValueAsync()'}}
+  // expected-warning @-1 {{non-Sendable 'NonSendableKlass'-typed result can not be returned from main actor-isolated global function 'getMainActorValueAsync()' to nonisolated context}}
 
   let _ = await newValue
 
@@ -261,7 +261,7 @@ func asyncLetReabstractionThunkTest2() async {
 @MainActor func asyncLetReabstractionThunkTestGlobalActor2() async {
   // We emit the error here since we are returning a main actor isolated value.
   async let newValue: NonSendableKlass = await getMainActorValueAsync()
-  // expected-warning @-1 {{non-sendable result type 'NonSendableKlass' cannot be sent from main actor-isolated context in call to global function 'getMainActorValueAsync()'}}
+  // expected-warning @-1 {{non-Sendable 'NonSendableKlass'-typed result can not be returned from main actor-isolated global function 'getMainActorValueAsync()' to nonisolated context}}
 
   let _ = await newValue
 
diff --git a/test/Sema/moveonly_sendable.swift b/test/Sema/moveonly_sendable.swift
index 8cba965343a81..0ea0334be56c9 100644
--- a/test/Sema/moveonly_sendable.swift
+++ b/test/Sema/moveonly_sendable.swift
@@ -16,8 +16,7 @@ enum MaybeFile: ~Copyable { // should implicitly conform
   case closed
 }
 
-struct NotSendableMO: ~Copyable { // expected-note {{consider making struct 'NotSendableMO' conform to the 'Sendable' protocol}}
-  // expected-complete-note @-1 {{consider making struct 'NotSendableMO' conform to the 'Sendable' protocol}}
+struct NotSendableMO: ~Copyable {
   var ref: Ref
 }
 
@@ -48,8 +47,8 @@ func processFiles(_ a: A, _ anotherFile: borrowing FileDescriptor) async {
   await a.takeMaybeFile(.available(anotherFile))
   _ = A(.available(anotherFile))
 
-  let ns = await a.getRef() // expected-warning {{non-sendable result type 'NotSendableMO' cannot be sent from actor-isolated context in call to instance method 'getRef()'}}
-  await takeNotSendable(ns) // expected-complete-warning {{passing argument of non-sendable type 'NotSendableMO' outside of main actor-isolated context may introduce data races}}
+  let ns = await a.getRef()
+  await takeNotSendable(ns)
 
   switch (await a.giveFileDescriptor()) {
   case let .available(fd):
diff --git a/unittests/SILOptimizer/PartitionUtilsTest.cpp b/unittests/SILOptimizer/PartitionUtilsTest.cpp
index 2d4ab2bfe69ec..9247017ef66d7 100644
--- a/unittests/SILOptimizer/PartitionUtilsTest.cpp
+++ b/unittests/SILOptimizer/PartitionUtilsTest.cpp
@@ -97,6 +97,7 @@ struct MockedPartitionOpEvaluatorWithFailureCallback final
     case PartitionOpError::AssignNeverSendableIntoSendingResult:
     case PartitionOpError::InOutSendingNotInitializedAtExit:
     case PartitionOpError::InOutSendingNotDisconnectedAtExit:
+    case PartitionOpError::NonSendableIsolationCrossingResult:
       llvm_unreachable("Unsupported");
     case PartitionOpError::LocalUseAfterSend: {
       auto state = error.getLocalUseAfterSendError();