[5.10][SE-0411] Promote IsolatedDefaultValues from an experimental feature to an upcoming feature.

CHANGELOG.md

@@ -3,6 +3,30 @@
 > **Note**\
 > This is in reverse chronological order, so newer entries are added to the top.
 
+## Swift 5.10
+
+* [SE-0411][]:
+
+  Default value expressions can now have the same isolation as the enclosing
+  function or the corresponding stored property:
+
+  ```swift
+  @MainActor
+  func requiresMainActor() -> Int { ... }
+
+  class C {
+    @MainActor
+    var x: Int = requiresMainActor()
+  }
+
+  @MainActor func defaultArg(value: Int = requiresMainActor()) { ... }
+  ```
+
+  For isolated default values of stored properties, the implicit initialization
+  only happens in the body of an `init` with the same isolation. This closes
+  an important data-race safety hole where global-actor-isolated default values
+  could inadvertently run synchronously from outside the actor.
+
 ## Swift 5.9.2
 
 * [SE-0407][]:
@@ -9846,6 +9870,7 @@ using the `.dynamicType` member to retrieve the type of an expression should mig
 [SE-0394]: https://github.com/apple/swift-evolution/blob/main/proposals/0394-swiftpm-expression-macros.md
 [SE-0397]: https://github.com/apple/swift-evolution/blob/main/proposals/0397-freestanding-declaration-macros.md
 [SE-0407]: https://github.com/apple/swift-evolution/blob/main/proposals/0407-member-macro-conformances.md
+[SE-0411]: https://github.com/apple/swift-evolution/blob/main/proposals/0411-isolated-default-values.md
 [#64927]: <https://github.com/apple/swift/issues/64927>
 [#42697]: <https://github.com/apple/swift/issues/42697>
 [#42728]: <https://github.com/apple/swift/issues/42728>

include/swift/AST/DiagnosticsSema.def

@@ -5200,9 +5200,14 @@ ERROR(distributed_actor_isolated_non_self_reference,none,
       "distributed actor-isolated %kind0 can not be accessed from a "
       "non-isolated context",
       (const ValueDecl *))
-ERROR(isolated_default_argument,none,
-      "%0 default argument cannot be synchronously evaluated from a "
-      "%1 context",
+ERROR(conflicting_stored_property_isolation,none,
+      "%select{default|memberwise}0 initializer for %1 cannot be both %2 and %3",
+      (bool, Type, ActorIsolation, ActorIsolation))
+NOTE(property_requires_actor,none,
+      "initializer for %0 %1 is %2",
+      (DescriptiveDeclKind, Identifier, ActorIsolation))
+ERROR(isolated_default_argument_context,none,
+      "%0 default value in a %1 context", 
       (ActorIsolation, ActorIsolation))
 ERROR(conflicting_default_argument_isolation,none,
       "default argument cannot be both %0 and %1",

include/swift/AST/Expr.h

@@ -4513,6 +4513,10 @@ class DefaultArgumentExpr final : public Expr {
   /// default expression.
   PointerUnion<DeclContext *, Expr *> ContextOrCallerSideExpr;
 
+  /// Whether this default argument is evaluated asynchronously because
+  /// it's isolated to the callee's isolation domain.
+  bool implicitlyAsync = false;
+
 public:
   explicit DefaultArgumentExpr(ConcreteDeclRef defaultArgsOwner,
                                unsigned paramIndex, SourceLoc loc, Type Ty,
@@ -4547,6 +4551,16 @@ class DefaultArgumentExpr final : public Expr {
   /// argument must be written explicitly at the call-site.
   ActorIsolation getRequiredIsolation() const;
 
+  /// Whether this default argument is evaluated asynchronously because
+  /// it's isolated to the callee's isolation domain.
+  bool isImplicitlyAsync() const {
+    return implicitlyAsync;
+  }
+
+  void setImplicitlyAsync() {
+    implicitlyAsync = true;
+  }
+
   static bool classof(const Expr *E) {
     return E->getKind() == ExprKind::DefaultArgument;
   }

include/swift/Basic/Features.def

@@ -118,6 +118,7 @@ UPCOMING_FEATURE(BareSlashRegexLiterals, 354, 6)
 UPCOMING_FEATURE(DeprecateApplicationMain, 383, 6)
 UPCOMING_FEATURE(ImportObjcForwardDeclarations, 384, 6)
 UPCOMING_FEATURE(DisableOutwardActorInference, 401, 6)
+UPCOMING_FEATURE(IsolatedDefaultValues, 411, 6)
 UPCOMING_FEATURE(GlobalConcurrency, 412, 6)
 
 UPCOMING_FEATURE(ExistentialAny, 335, 7)
@@ -222,9 +223,6 @@ EXPERIMENTAL_FEATURE(StrictConcurrency, true)
 /// Defer Sendable checking to SIL diagnostic phase.
 EXPERIMENTAL_FEATURE(SendNonSendable, false)
 
-/// Allow default values to require isolation at the call-site.
-EXPERIMENTAL_FEATURE(IsolatedDefaultValues, false)
-
 /// Enable extended callbacks (with additional parameters) to be used when the
 /// "playground transform" is enabled.
 EXPERIMENTAL_FEATURE(PlaygroundExtendedCallbacks, true)

lib/AST/Decl.cpp

@@ -10289,6 +10289,7 @@ ActorIsolation swift::getActorIsolationOfContext(
     DeclContext *dc,
     llvm::function_ref<ActorIsolation(AbstractClosureExpr *)>
         getClosureActorIsolation) {
+  auto &ctx = dc->getASTContext();
   auto dcToUse = dc;
   // Defer bodies share actor isolation of their enclosing context.
   if (auto FD = dyn_cast<FuncDecl>(dcToUse)) {
@@ -10300,21 +10301,21 @@ ActorIsolation swift::getActorIsolationOfContext(
     return getActorIsolation(vd);
 
   // In the context of the initializing or default-value expression of a
-  // stored property, the isolation varies between instance and type members:
+  // stored property:
   //   - For a static stored property, the isolation matches the VarDecl.
   //     Static properties are initialized upon first use, so the isolation
   //     of the initializer must match the isolation required to access the
   //     property.
-  //   - For a field of a nominal type, the expression can require a specific
-  //     actor isolation. That default expression may only be used from inits
-  //     that meet the required isolation.
+  //   - For a field of a nominal type, the expression can require the same
+  //     actor isolation as the field itself. That default expression may only
+  //     be used from inits that meet the required isolation.
   if (auto *var = dcToUse->getNonLocalVarDecl()) {
-    auto &ctx = dc->getASTContext();
-    if (ctx.LangOpts.hasFeature(Feature::IsolatedDefaultValues) &&
-        var->isInstanceMember() &&
-        !var->getAttrs().hasAttribute<LazyAttr>()) {
-      return ActorIsolation::forNonisolated(/*unsafe=*/false);
-    }
+    // If IsolatedDefaultValues are enabled, treat this context as having
+    // unspecified isolation. We'll compute the required isolation for
+    // the initializer and validate that it matches the isolation of the
+    // var itself in the DefaultInitializerIsolation request.
+    if (ctx.LangOpts.hasFeature(Feature::IsolatedDefaultValues))
+      return ActorIsolation::forUnspecified();
 
     return getActorIsolation(var);
   }

lib/Frontend/CompilerInvocation.cpp

@@ -976,8 +976,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     Opts.StrictConcurrencyLevel = StrictConcurrency::Minimal;
   }
 
-  // StrictConcurrency enables all data-race safety upcoming features.
-  if (Opts.hasFeature(Feature::StrictConcurrency)) {
+  // StrictConcurrency::Complete enables all data-race safety features.
+  if (Opts.StrictConcurrencyLevel == StrictConcurrency::Complete) {
+    Opts.Features.insert(Feature::IsolatedDefaultValues);
     Opts.Features.insert(Feature::GlobalConcurrency);
   }
 

lib/SILGen/SILGenApply.cpp

@@ -2518,18 +2518,21 @@ class DelayedArgument {
     AbstractionPattern origResultType;
     ClaimedParamsRef paramsToEmit;
     SILFunctionTypeRepresentation functionRepresentation;
-
+    bool implicitlyAsync;
+
     DefaultArgumentStorage(SILLocation loc,
                            ConcreteDeclRef defaultArgsOwner,
                            unsigned destIndex,
                            CanType resultType,
                            AbstractionPattern origResultType,
                            ClaimedParamsRef paramsToEmit,
-                           SILFunctionTypeRepresentation functionRepresentation)
+                           SILFunctionTypeRepresentation functionRepresentation,
+                           bool implicitlyAsync)
       : loc(loc), defaultArgsOwner(defaultArgsOwner), destIndex(destIndex),
         resultType(resultType), origResultType(origResultType),
         paramsToEmit(paramsToEmit),
-        functionRepresentation(functionRepresentation)
+        functionRepresentation(functionRepresentation),
+        implicitlyAsync(implicitlyAsync)
     {}
   };
   struct BorrowedLValueStorage {
@@ -2656,13 +2659,15 @@ class DelayedArgument {
                   CanType resultType,
                   AbstractionPattern origResultType,
                   ClaimedParamsRef params,
-                  SILFunctionTypeRepresentation functionTypeRepresentation)
+                  SILFunctionTypeRepresentation functionTypeRepresentation,
+                  bool implicitlyAsync)
     : Kind(DefaultArgument) {
     Value.emplace<DefaultArgumentStorage>(Kind, loc, defaultArgsOwner,
                                           destIndex,
                                           resultType,
                                           origResultType, params,
-                                          functionTypeRepresentation);
+                                          functionTypeRepresentation,
+                                          implicitlyAsync);
   }
 
   DelayedArgument(DelayedArgument &&other)
@@ -2690,6 +2695,28 @@ class DelayedArgument {
     return LV().Loc;
   }
 
+  bool isDefaultArg() const {
+    return Kind == DefaultArgument;
+  }
+
+  SILLocation getDefaultArgLoc() const {
+    assert(isDefaultArg());
+    auto storage = Value.get<DefaultArgumentStorage>(Kind);
+    return storage.loc;
+  }
+
+  llvm::Optional<ActorIsolation> getIsolation() const {
+    if (!isDefaultArg())
+      return llvm::None;
+
+    auto storage = Value.get<DefaultArgumentStorage>(Kind);
+    if (!storage.implicitlyAsync)
+      return llvm::None;
+
+    auto callee = storage.defaultArgsOwner.getDecl();
+    return getActorIsolation(callee);
+  }
+
   void emit(SILGenFunction &SGF, SmallVectorImpl<ManagedValue> &args,
             size_t &argIndex) {
     switch (Kind) {
@@ -2915,6 +2942,31 @@ static void emitDelayedArguments(SILGenFunction &SGF,
                          MutableArrayRef<SmallVector<ManagedValue, 4>> args) {
   assert(!delayedArgs.empty());
 
+  // If any of the delayed arguments are isolated default arguments,
+  // argument evaluation happens in the following order:
+  //
+  //   1. Left-to-right evalution of explicit r-value arguments
+  //   2. Left-to-right evaluation of formal access arguments
+  //   3. Hop to the callee's isolation domain
+  //   4. Left-to-right evaluation of default arguments
+
+  // So, if any delayed arguments are isolated, all default arguments
+  // are collected during the first pass over the delayed arguments,
+  // and emitted separately after a hop to the callee's isolation domain.
+
+  llvm::Optional<ActorIsolation> defaultArgIsolation;
+  for (auto &arg : delayedArgs) {
+    if (auto isolation = arg.getIsolation()) {
+      defaultArgIsolation = isolation;
+      break;
+    }
+  }
+
+  SmallVector<std::tuple<
+      /*delayedArgIt*/decltype(delayedArgs)::iterator,
+      /*siteArgsIt*/decltype(args)::iterator,
+      /*index*/size_t>, 2> isolatedArgs;
+
   SmallVector<std::pair<SILValue, SILLocation>, 4> emittedInoutArgs;
   auto delayedNext = delayedArgs.begin();
 
@@ -2923,7 +2975,8 @@ static void emitDelayedArguments(SILGenFunction &SGF,
   // wherever there's a delayed argument to insert.
   //
   // Note that this also begins the formal accesses in evaluation order.
-  for (auto &siteArgs : args) {
+  for (auto argsIt = args.begin(); argsIt != args.end(); ++argsIt) {
+    auto &siteArgs = *argsIt;
     // NB: siteArgs.size() may change during iteration
     for (size_t i = 0; i < siteArgs.size(); ) {
       auto &siteArg = siteArgs[i];
@@ -2936,6 +2989,15 @@ static void emitDelayedArguments(SILGenFunction &SGF,
       assert(delayedNext != delayedArgs.end());
       auto &delayedArg = *delayedNext;
 
+      if (defaultArgIsolation && delayedArg.isDefaultArg()) {
+        isolatedArgs.push_back(std::make_tuple(delayedNext, argsIt, i));
+        if (++delayedNext == delayedArgs.end()) {
+          goto done;
+        } else {
+          continue;
+        }
+      }
+
       // Emit the delayed argument and replace it in the arguments array.
       delayedArg.emit(SGF, siteArgs, i);
 
@@ -2956,6 +3018,45 @@ static void emitDelayedArguments(SILGenFunction &SGF,
 
 done:
 
+  if (defaultArgIsolation) {
+    assert(SGF.F.isAsync());
+    assert(!isolatedArgs.empty());
+
+    auto &firstArg = *std::get<0>(isolatedArgs[0]);
+    auto loc = firstArg.getDefaultArgLoc();
+
+    SILValue executor;
+    switch (*defaultArgIsolation) {
+    case ActorIsolation::GlobalActor:
+    case ActorIsolation::GlobalActorUnsafe:
+      executor = SGF.emitLoadGlobalActorExecutor(
+          defaultArgIsolation->getGlobalActor());
+      break;
+
+    case ActorIsolation::ActorInstance:
+      llvm_unreachable("default arg cannot be actor instance isolated");
+
+    case ActorIsolation::Unspecified:
+    case ActorIsolation::Nonisolated:
+    case ActorIsolation::NonisolatedUnsafe:
+      llvm_unreachable("Not isolated");
+    }
+
+    // Hop to the target isolation domain once to evaluate all
+    // default arguments.
+    SGF.emitHopToTargetExecutor(loc, executor);
+
+    size_t argsEmitted = 0;
+    for (auto &isolatedArg : isolatedArgs) {
+      auto &delayedArg = *std::get<0>(isolatedArg);
+      auto &siteArgs = *std::get<1>(isolatedArg);
+      auto argIndex = std::get<2>(isolatedArg) + argsEmitted;
+      auto origIndex = argIndex;
+      delayedArg.emit(SGF, siteArgs, argIndex);
+      argsEmitted += (argIndex - origIndex);
+    }
+  }
+
   // Check to see if we have multiple inout arguments which obviously
   // alias.  Note that we could do this in a later SILDiagnostics pass
   // as well: this would be stronger (more equivalences exposed) but
@@ -3261,7 +3362,7 @@ class ArgEmitter {
                                     defArg->getParamIndex(),
                                     substParamType, origParamType,
                                     claimNextParameters(numParams),
-                                    Rep);
+                                    Rep, defArg->isImplicitlyAsync());
       Args.push_back(ManagedValue());
 
       maybeEmitForeignArgument();
@@ -4255,7 +4356,8 @@ void DelayedArgument::emitDefaultArgument(SILGenFunction &SGF,
   auto value = SGF.emitApplyOfDefaultArgGenerator(info.loc,
                                                   info.defaultArgsOwner,
                                                   info.destIndex,
-                                                  info.resultType);
+                                                  info.resultType,
+                                                  info.implicitlyAsync);
 
   SmallVector<ManagedValue, 4> loweredArgs;
   SmallVector<DelayedArgument, 4> delayedArgs;

lib/SILGen/SILGenExpr.cpp

@@ -2533,6 +2533,7 @@ SILGenFunction::emitApplyOfDefaultArgGenerator(SILLocation loc,
                                                ConcreteDeclRef defaultArgsOwner,
                                                unsigned destIndex,
                                                CanType resultType,
+                                               bool implicitlyAsync,
                                                SGFContext C) {
   SILDeclRef generator 
     = SILDeclRef::getDefaultArgGenerator(defaultArgsOwner.getDecl(),

lib/SILGen/SILGenFunction.h

@@ -1905,6 +1905,7 @@ class LLVM_LIBRARY_VISIBILITY SILGenFunction
                                         ConcreteDeclRef defaultArgsOwner,
                                         unsigned destIndex,
                                         CanType resultType,
+                                        bool implicitlyAsync,
                                         SGFContext C = SGFContext());
 
   RValue emitApplyOfStoredPropertyInitializer(