[SE-0466] Don't infer @MainActor on types conforming to Sendable

When the default isolation is main-actor, don't infer @MainActor
for a type that conforms to a protocol P in its primary definition when
P inherits from Sendable. Such types should remain non-isolated
because they're highly unlikely to be able to implement the P
conformance (which cannot be isolated).

Put this feature behind a new experimental flag,
SendableProhibitsMainActorInference.

Implements rdar://151029300

include/swift/Basic/Features.def

@@ -532,6 +532,10 @@ EXPERIMENTAL_FEATURE(DefaultIsolationPerFile, false)
 /// Enable @_lifetime attribute
 SUPPRESSIBLE_EXPERIMENTAL_FEATURE(Lifetimes, true)
 
+/// Disable @MainActor inference when the primary definition of a type conforms
+/// to SendableMetatype (or Sendable).
+EXPERIMENTAL_FEATURE(SendableProhibitsMainActorInference, true)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/FeatureSet.cpp

@@ -125,6 +125,7 @@ UNINTERESTING_FEATURE(StructLetDestructuring)
 UNINTERESTING_FEATURE(MacrosOnImports)
 UNINTERESTING_FEATURE(NonisolatedNonsendingByDefault)
 UNINTERESTING_FEATURE(KeyPathWithMethodMembers)
+UNINTERESTING_FEATURE(SendableProhibitsMainActorInference)
 
 // TODO: Return true for inlinable function bodies with module selectors in them
 UNINTERESTING_FEATURE(ModuleSelector)

lib/Sema/DerivedConformance/DerivedConformanceCodable.cpp

@@ -155,6 +155,7 @@ addImplicitCodingKeys(NominalTypeDecl *target,
   enumDecl->setSynthesized();
   enumDecl->setAccess(AccessLevel::Private);
 
+  if (!C.LangOpts.hasFeature(Feature::SendableProhibitsMainActorInference)) {
     switch (C.LangOpts.DefaultIsolationBehavior) {
     case DefaultIsolation::MainActor:
       enumDecl->getAttrs().add(NonisolatedAttr::createImplicit(C));
@@ -164,6 +165,7 @@ addImplicitCodingKeys(NominalTypeDecl *target,
       // Nothing to do.
       break;
     }
+  }
 
   // For classes which inherit from something Encodable or Decodable, we
   // provide case `super` as the first key (to be used in encoding super).

lib/Sema/TypeCheckConcurrency.cpp

@@ -5936,6 +5936,65 @@ static void addAttributesForActorIsolation(ValueDecl *value,
     }
 }
 
+/// Determine whether there is a SendableMetatype conformance that requires that the nominal type
+/// be nonisolated (preventing @MainActor inference).
+static bool sendableConformanceRequiresNonisolated(NominalTypeDecl *nominal) {
+  ASTContext &ctx = nominal->getASTContext();
+  if (!ctx.LangOpts.hasFeature(Feature::SendableProhibitsMainActorInference))
+    return false;
+
+  if (isa<ProtocolDecl>(nominal))
+    return false;
+
+  auto sendable = ctx.getProtocol(KnownProtocolKind::Sendable);
+  auto sendableMetatype = ctx.getProtocol(KnownProtocolKind::SendableMetatype);
+  if (!sendableMetatype)
+    return false;
+
+  // Check whether any of the explicit conformances is to a
+  // SendableMetatype-inheriting protocol. We exclude direct conformance to
+  // Sendable here, because a global-actor-isolated type is implicitly Sendable,
+  // and writing Sendable explicitly
+  InvertibleProtocolSet inverses;
+  bool anyObject = false;
+  auto inherited = getDirectlyInheritedNominalTypeDecls(
+      nominal, inverses, anyObject);
+  for (const auto &entry : inherited) {
+    auto proto = dyn_cast<ProtocolDecl>(entry.Item);
+    if (proto && proto != sendable && proto->inheritsFrom(sendableMetatype))
+      return true;
+  }
+
+  // Check for member or extension macros that define conformances to
+  // SendableMetatype-inheriting protocols.
+  bool requiresNonisolated = false;
+  auto checkMacro = [&](MacroRole role, MacroDecl *macro) {
+    if (!macro || requiresNonisolated)
+      return;
+
+    SmallVector<ProtocolDecl *, 2> conformances;
+    macro->getIntroducedConformances(nominal, role, conformances);
+    for (auto proto : conformances) {
+      if (proto == sendableMetatype || proto->inheritsFrom(sendableMetatype)) {
+        requiresNonisolated = true;
+        break;
+      }
+    }
+  };
+
+  nominal->forEachAttachedMacro(
+      MacroRole::Member,
+      [&](CustomAttr * attr, MacroDecl *macro) {
+        checkMacro(MacroRole::Member, macro);
+      });
+  nominal->forEachAttachedMacro(
+      MacroRole::Extension,
+      [&](CustomAttr * attr, MacroDecl *macro) {
+        checkMacro(MacroRole::Extension, macro);
+      });
+  return requiresNonisolated;
+}
+
 /// Determine the default isolation and isolation source for this declaration,
 /// which may still be overridden by other inference rules.
 static std::tuple<InferredActorIsolation, ValueDecl *,
@@ -5955,12 +6014,23 @@ computeDefaultInferredActorIsolation(ValueDecl *value) {
       auto *dc = value->getInnermostDeclContext();
       while (dc && !inActorContext) {
         if (auto *nominal = dc->getSelfNominalTypeDecl()) {
-          inActorContext = nominal->isAnyActor();
+          if (nominal->isAnyActor())
+            return {};
         }
         dc = dc->getParent();
       }
 
-      if (!inActorContext) {
+      // If this is or is a non-type member of a nominal type that conforms to a
+      // SendableMetatype-inheriting protocol in its primary definition, disable
+      // @MainActor inference.
+      auto nominalTypeDecl = dyn_cast<NominalTypeDecl>(value);
+      if (!nominalTypeDecl && !isa<TypeDecl>(value)) {
+        nominalTypeDecl = value->getDeclContext()->getSelfNominalTypeDecl();
+      }
+      if (nominalTypeDecl &&
+          sendableConformanceRequiresNonisolated(nominalTypeDecl))
+        return { };
+
       // FIXME: deinit should be implicitly MainActor too.
       if (isa<TypeDecl>(value) || isa<ExtensionDecl>(value) ||
           isa<AbstractStorageDecl>(value) || isa<FuncDecl>(value) ||
@@ -5973,7 +6043,6 @@ computeDefaultInferredActorIsolation(ValueDecl *value) {
                   .withPreconcurrency(!ctx.LangOpts.isSwiftVersionAtLeast(6));
           return {{{isolation, {}}, nullptr, {}}};
       }
-      }
 
       return {};
     };