Teach #isolation to respect the flow-sensitive nature of actor initializers

Actor initializers have a flow-sensitive property where they are isolated
to the actor being initialized only after the actor instance itself is
fully-initialized. However, this behavior was not being reflected in
the expansion of `#isolation`, which was always expanding to `self`,
even before `self` is fully formed.

This led to a source compatibility issue with code that used the async
for..in loop within an actor initializer *prior* to the point where the
actor was fully initialized, because the type checker is introducing
the `#isolation` (SE-0421) but Definite Initialization properly rejects
the use of `self` before it is initialized.

Address this issue by delaying the expansion of `#isolation` until
after the actor is fully initialized. In SILGen, we introduce a new
builtin for this case (and *just* this case) called
`flowSensitiveSelfIsolation`, which takes in `self` as its argument
and produces an `(any Actor)?`. Definite initialization does not treat
this as a use of `self`. Rather, it tracks these builtins and
replaces them either with `self` (if it is fully-initialized at this
point) or `nil` (if it is not fully-initialized at this point),
mirroring the flow-sensitive isolation semantics described in SE-0327.

Fixes rdar://127080037.

Author: DougGregor

include/swift/AST/Builtins.def

@@ -899,6 +899,14 @@ BUILTIN_MISC_OPERATION(StartAsyncLetWithLocalBuffer, "startAsyncLetWithLocalBuff
 /// This is only supported under the task-to-thread concurrency model.
 BUILTIN_MISC_OPERATION(TaskRunInline, "taskRunInline", "", Special)
 
+/// flowSensitiveSelfIsolation<T: Actor>(_ actor: T) -> T?
+///
+/// Used only in actor initializers, this builtin lowers to either 'actor'
+/// (wrapped in an optional) or 'nil' depending on whether 'self' has been
+/// initialized at this point. 'actor' is always an alias for the 'self'
+/// being initialized.
+BUILTIN_MISC_OPERATION(FlowSensitiveSelfIsolation, "flowSensitiveSelfIsolation", "", Special)
+
 /// endAsyncLet(): (Builtin.RawPointer) -> Void
 ///
 /// DEPRECATED. The swift_asyncLet_finish intrinsic and endAsyncLetLifetime

lib/AST/Builtins.cpp

@@ -2089,6 +2089,16 @@ static ValueDecl *getHopToActor(ASTContext &ctx, Identifier id) {
   return builder.build(id);
 }
 
+static ValueDecl *getFlowSensitiveSelfIsolation(ASTContext &ctx, Identifier id) {
+  BuiltinFunctionBuilder builder(ctx);
+  return getBuiltinFunction(ctx, id, _thin,
+                            _generics(_unrestricted,
+                                      _conformsToDefaults(0),
+                                      _conformsTo(_typeparam(0), _actor)),
+                            _parameters(_typeparam(0)),
+                            _optional(_existential(_actor)));
+}
+
 static ValueDecl *getDistributedActorAsAnyActor(ASTContext &ctx, Identifier id) {
   BuiltinFunctionBuilder builder(ctx);
   auto *distributedActorProto = ctx.getProtocol(KnownProtocolKind::DistributedActor);
@@ -3191,6 +3201,9 @@ ValueDecl *swift::getBuiltinValueDecl(ASTContext &Context, Identifier Id) {
   case BuiltinValueKind::HopToActor:
     return getHopToActor(Context, Id);
 
+  case BuiltinValueKind::FlowSensitiveSelfIsolation:
+    return getFlowSensitiveSelfIsolation(Context, Id);
+
   case BuiltinValueKind::AutoDiffCreateLinearMapContextWithType:
     return getAutoDiffCreateLinearMapContext(Context, Id);
 

lib/SIL/IR/OperandOwnership.cpp

@@ -906,6 +906,7 @@ BUILTIN_OPERAND_OWNERSHIP(InstantaneousUse, EndAsyncLetLifetime)
 BUILTIN_OPERAND_OWNERSHIP(InstantaneousUse, CreateTaskGroup)
 BUILTIN_OPERAND_OWNERSHIP(InstantaneousUse, CreateTaskGroupWithFlags)
 BUILTIN_OPERAND_OWNERSHIP(InstantaneousUse, DestroyTaskGroup)
+BUILTIN_OPERAND_OWNERSHIP(InstantaneousUse, FlowSensitiveSelfIsolation)
 
 BUILTIN_OPERAND_OWNERSHIP(ForwardingConsume, COWBufferForReading)
 

lib/SIL/IR/ValueOwnership.cpp

@@ -630,6 +630,7 @@ CONSTANT_OWNERSHIP_BUILTIN(None, CreateTaskGroup)
 CONSTANT_OWNERSHIP_BUILTIN(None, CreateTaskGroupWithFlags)
 CONSTANT_OWNERSHIP_BUILTIN(None, DestroyTaskGroup)
 CONSTANT_OWNERSHIP_BUILTIN(None, TaskRunInline)
+CONSTANT_OWNERSHIP_BUILTIN(Owned, FlowSensitiveSelfIsolation)
 CONSTANT_OWNERSHIP_BUILTIN(None, GetEnumTag)
 CONSTANT_OWNERSHIP_BUILTIN(None, InjectEnumTag)
 CONSTANT_OWNERSHIP_BUILTIN(Owned, DistributedActorAsAnyActor)

lib/SILGen/SILGenExpr.cpp

@@ -6743,6 +6743,38 @@ RValue RValueEmitter::visitMacroExpansionExpr(MacroExpansionExpr *E,
 
 RValue RValueEmitter::visitCurrentContextIsolationExpr(
     CurrentContextIsolationExpr *E, SGFContext C) {
+  // If we are in an actor initializer that is isolated to, the current context
+  // isolation flow-sensitive: before 'self' has been initialized, it will be
+  // nil. After 'self' has been initialized, it will be 'self'. Introduce a
+  // custom builtin that Definite Initialization will rewrite appropriately.
+  if (auto ctor = dyn_cast_or_null<ConstructorDecl>(
+          SGF.F.getDeclRef().getDecl())) {
+    auto isolation = getActorIsolation(ctor);
+    if (ctor->isDesignatedInit() &&
+        isolation == ActorIsolation::ActorInstance &&
+        isolation.getActorInstance() == ctor->getImplicitSelfDecl()) {
+      ASTContext &ctx = SGF.getASTContext();
+      auto builtinName = ctx.getIdentifier(
+          getBuiltinName(BuiltinValueKind::FlowSensitiveSelfIsolation));
+      SILType resultTy = SGF.getLoweredType(E->getType());
+
+      auto injection = cast<InjectIntoOptionalExpr>(E->getActor());
+      auto erasure = cast<ErasureExpr>(injection->getSubExpr());
+      auto conformance = erasure->getConformances()[0];
+      SGF.SGM.useConformance(conformance);
+
+      auto origActorExpr = erasure->getSubExpr();
+      auto actorProto = ctx.getProtocol(KnownProtocolKind::Actor);
+      SubstitutionMap subs = SubstitutionMap::getProtocolSubstitutions(
+          actorProto, origActorExpr->getType(), conformance);
+      auto origActor = SGF.maybeEmitValueOfLocalVarDecl(
+          ctor->getImplicitSelfDecl(), AccessKind::Read).getValue();
+      auto call = SGF.B.createBuiltin(E, builtinName, resultTy, subs, origActor);
+      return RValue(SGF, E,
+                    ManagedValue::forForwardedRValue(SGF, call));
+    }
+  }
+
   return visit(E->getActor(), C);
 }
 

lib/SILOptimizer/Mandatory/DIMemoryUseCollector.cpp

@@ -1709,6 +1709,17 @@ void ElementUseCollector::collectClassSelfUses(
       Kind = DIUseKind::Escape;
     }
 
+    // Track flow-sensitive 'self' isolation builtins separately, because they
+    // aren't really uses of 'self' until after DI, once we've decided whether
+    // they have a fully-formed 'self' to use.
+    if (auto builtin = dyn_cast<BuiltinInst>(User)) {
+      if (auto builtinKind = builtin->getBuiltinKind()) {
+        if (*builtinKind == BuiltinValueKind::FlowSensitiveSelfIsolation) {
+          Kind = DIUseKind::FlowSensitiveSelfIsolation;
+        }
+      }
+    }
+
     trackUse(DIMemoryUse(User, Kind, 0, TheMemory.getNumElements()));
   }
 }

lib/SILOptimizer/Mandatory/DIMemoryUseCollector.h

@@ -298,7 +298,12 @@ enum DIUseKind {
   LoadForTypeOfSelf,
 
   /// This instruction is a value_metatype on the address of 'self'.
-  TypeOfSelf
+  TypeOfSelf,
+
+  /// This instruction is the builtin for flow-sensitive current isolation
+  /// within an actor initializer. It will be replaced with either a copy of
+  /// its argument (injected into an (any Actor)?) or nil.
+  FlowSensitiveSelfIsolation,
 };
 
 /// This struct represents a single classified access to the memory object

lib/SILOptimizer/Mandatory/DefiniteInitialization.cpp

@@ -491,6 +491,7 @@ namespace {
     void handleTypeOfSelfUse(DIMemoryUse &Use);
     void handleInOutUse(const DIMemoryUse &Use);
     void handleEscapeUse(const DIMemoryUse &Use);
+    void handleFlowSensitiveActorIsolationUse(const DIMemoryUse &Use);
 
     bool diagnoseReturnWithoutInitializingStoredProperties(
         const SILInstruction *Inst, SILLocation loc, const DIMemoryUse &Use);
@@ -565,6 +566,7 @@ LifetimeChecker::LifetimeChecker(const DIMemoryObjectInfo &TheMemory,
     case DIUseKind::LoadForTypeOfSelf:
     case DIUseKind::TypeOfSelf:
     case DIUseKind::Escape:
+    case DIUseKind::FlowSensitiveSelfIsolation:
       continue;
     case DIUseKind::Assign:
     case DIUseKind::Set:
@@ -1160,6 +1162,10 @@ void LifetimeChecker::doIt() {
     case DIUseKind::BadExplicitStore:
       diagnoseBadExplicitStore(Inst);
       break;
+
+    case DIUseKind::FlowSensitiveSelfIsolation:
+      handleFlowSensitiveActorIsolationUse(Use);
+      break;
     }
   }
 
@@ -1344,6 +1350,47 @@ void LifetimeChecker::handleTypeOfSelfUse(DIMemoryUse &Use) {
   }
 }
 
+void LifetimeChecker::handleFlowSensitiveActorIsolationUse(
+    const DIMemoryUse &Use) {
+  bool IsSuperInitComplete, FailedSelfUse;
+
+  ASTContext &ctx = F.getASTContext();
+  auto builtinInst = cast<BuiltinInst>(Use.Inst);
+  SILBuilderWithScope B(builtinInst);
+  SILValue replacement;
+  SILType optExistentialType = builtinInst->getType();
+  SILLocation loc = builtinInst->getLoc();
+  if (isInitializedAtUse(Use, &IsSuperInitComplete, &FailedSelfUse)) {
+    // 'self' is initialized, so replace this builtin with an injection of the
+    // argument into (any Actor)?.
+
+    // Create a copy of the actor argument, which we intentionally did not
+    // copy in SILGen.
+    SILValue actor = B.createCopyValue(loc, builtinInst->getArguments()[0]);
+
+    // Inject 'self' into 'any Actor'.
+    ProtocolConformanceRef conformances[1] = {
+      builtinInst->getSubstitutions().getConformances()[0]
+    };
+    SILType existentialType = optExistentialType.getOptionalObjectType();
+    SILValue existentialBox = B.createInitExistentialRef(
+        loc, existentialType, actor->getType().getASTType(), actor,
+        ctx.AllocateCopy(conformances));
+
+    // Then, wrap it in an optional.
+    replacement = B.createEnum(
+        loc, existentialBox, ctx.getOptionalSomeDecl(), optExistentialType);
+  } else {
+    // 'self' is not initialized yet, so use 'nil'.
+    replacement = B.createEnum(
+        loc, SILValue(), ctx.getOptionalNoneDecl(), optExistentialType);
+  }
+
+  // Introduce the replacement.
+  InstModCallbacks callbacks;
+  replaceAllUsesAndErase(builtinInst, replacement, callbacks);
+}
+
 void LifetimeChecker::emitSelfConsumedDiagnostic(SILInstruction *Inst) {
   if (!shouldEmitError(Inst))
     return;

lib/SILOptimizer/Transforms/AccessEnforcementReleaseSinking.cpp

@@ -159,6 +159,7 @@ static bool isBarrier(SILInstruction *inst) {
     case BuiltinValueKind::GetEnumTag:
     case BuiltinValueKind::InjectEnumTag:
     case BuiltinValueKind::ExtractFunctionIsolation:
+    case BuiltinValueKind::FlowSensitiveSelfIsolation:
     case BuiltinValueKind::AddressOfRawLayout:
       return false;
 

test/SILGen/init_actor_isolation.swift

@@ -0,0 +1,29 @@
+// RUN: %target-swift-frontend -emit-silgen %s -module-name test -swift-version 5 | %FileCheck %s
+// REQUIRES: concurrency
+
+@available(SwiftStdlib 5.1, *)
+func f(isolatedTo actor: isolated (any Actor)?) async -> Int { 0 }
+
+@available(SwiftStdlib 5.1, *)
+actor A {
+  let number: Int
+
+  // CHECK-LABEL: sil hidden [available 10.15] [ossa] @$s4test1ACACyYacfc : $@convention(method) @async (@sil_isolated @owned A) -> @owned A
+  init() async {
+    // First use of #isolation.
+    // CHECK: [[ISOLATION_1:%.*]] = builtin "flowSensitiveSelfIsolation"<A>
+    // CHECK: [[F_1:%.*]] = function_ref @$s4test1f10isolatedToSiScA_pSgYi_tYaF
+    // CHECK-NEXT: [[F_RESULT:%.*]] = apply [[F_1]]([[ISOLATION_1]])
+
+    // Assignment to "number" of the result.
+    // CHECK: [[NUMBER:%.*]] = ref_element_addr {{%.*}} : $A, #A.number
+    // CHECK: assign [[F_RESULT]] to [[NUMBER]]
+    self.number = await f(isolatedTo: #isolation)
+
+    // Second use of #isolation.
+    // CHECK: [[ISOLATION_2:%.*]] = builtin "flowSensitiveSelfIsolation"<A>
+    // CHECK: [[F_2:%.*]] = function_ref @$s4test1f10isolatedToSiScA_pSgYi_tYaF
+    // CHECK-NEXT: apply [[F_2]]([[ISOLATION_2]])
+    _ = await f(isolatedTo: #isolation)
+  }
+}