[CoroutineAccessors] Add flag to observe errors.

Temporarily allow the legacy behavior of allowing caller coroutine
accessors to observe errors (i.e. by executing no code after the yield
if the caller threw an error) behind the
CoroutineAccessorsUnwindOnCallerError flag.

Author: nate-chandler

include/swift/AST/Types.h

@@ -4825,7 +4825,8 @@ enum class SILCoroutineKind : uint8_t {
 
   /// This function is a yield-once coroutine (used by read and modify
   /// accessors).  It has the following differences from YieldOnce:
-  /// - it does not observe errors thrown by its caller
+  /// - it does not observe errors thrown by its caller (unless the feature
+  /// CoroutineAccessorsUnwindOnCallerError is enabled)
   /// - it uses the callee-allocated ABI
   YieldOnce2,
 

include/swift/Basic/Features.def

@@ -427,6 +427,9 @@ EXPERIMENTAL_FEATURE(UnspecifiedMeansMainActorIsolated, false)
 /// modify/read single-yield coroutines
 SUPPRESSIBLE_EXPERIMENTAL_FEATURE(CoroutineAccessors, true)
 
+/// modify/read single-yield coroutines always execute code post-yield code
+EXPERIMENTAL_FEATURE(CoroutineAccessorsUnwindOnCallerError, false)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/FeatureSet.cpp

@@ -296,6 +296,7 @@ static bool usesFeatureAllowUnsafeAttribute(Decl *decl) {
 UNINTERESTING_FEATURE(WarnUnsafe)
 UNINTERESTING_FEATURE(SafeInterop)
 UNINTERESTING_FEATURE(AssumeResilientCxxTypes)
+UNINTERESTING_FEATURE(CoroutineAccessorsUnwindOnCallerError)
 
 bool swift::usesFeatureIsolatedDeinit(const Decl *decl) {
   if (auto cd = dyn_cast<ClassDecl>(decl)) {

lib/SILGen/SILGenApply.cpp

@@ -4992,7 +4992,10 @@ class EndCoroutineApply : public Cleanup {
     auto *beginApply =
         cast<BeginApplyInst>(ApplyToken->getDefiningInstruction());
     auto isCalleeAllocated = beginApply->isCalleeAllocated();
-    auto unwindOnCallerError = !isCalleeAllocated;
+    auto unwindOnCallerError =
+        !isCalleeAllocated ||
+        SGF.SGM.getASTContext().LangOpts.hasFeature(
+            Feature::CoroutineAccessorsUnwindOnCallerError);
     if (forUnwind && unwindOnCallerError) {
       SGF.B.createAbortApply(l, ApplyToken);
     } else {

test/SILGen/coroutine_accessors.swift

@@ -1,7 +1,13 @@
 // RUN: %target-swift-emit-silgen                           \
 // RUN:     %s                                              \
 // RUN:     -enable-experimental-feature CoroutineAccessors \
-// RUN: | %FileCheck %s
+// RUN: | %FileCheck %s --check-prefixes=CHECK,CHECK-NOUNWIND
+
+// RUN: %target-swift-emit-silgen                                              \
+// RUN:     %s                                                                 \
+// RUN:     -enable-experimental-feature CoroutineAccessors                    \
+// RUN:     -enable-experimental-feature CoroutineAccessorsUnwindOnCallerError \
+// RUN: | %FileCheck %s --check-prefixes=CHECK,CHECK-UNWIND
 
 // REQUIRES: asserts
 
@@ -115,7 +121,8 @@ public var irm: Int {
 // CHECK:      dealloc_stack [[OLD_VALUE_ADDR]]
 // CHECK:      return [[OLD_VALUE]]
 // CHECK:    bb2([[ERROR:%[^,]+]] : @owned $any Error):
-// CHECK:      end_apply [[TOKEN]]
+// CHECK-NOUNWIND: end_apply [[TOKEN]]
+// CHECK-UNWIND: abort_apply [[TOKEN]]
 // CHECK:      end_access [[SELF_ACCESS]]
 // CHECK:      dealloc_stack [[NEW_VALUE_ADDR]]
 // CHECK:      dealloc_stack [[OLD_VALUE_ADDR]]