[Concurrency] Introduce @actorIndependent attribute.

Introduce a new attribute `@actorIndependent` that specifies that a
given declaration is considered to be independent of any actor.
Actor-independent declarations do not have access to actor-isolated
state, even when they are declared as instance members of the actor.

On the other hand, actor-independent declarations can be used to
conform to (synchronous) requirements in protocols.

include/swift/AST/ASTTypeIDZone.def

@@ -15,6 +15,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+SWIFT_TYPEID(ActorIsolation)
 SWIFT_TYPEID(AncestryFlags)
 SWIFT_TYPEID(CtorInitializerKind)
 SWIFT_TYPEID(FunctionBuilderBodyPreCheck)

include/swift/AST/ASTTypeIDs.h

@@ -23,6 +23,7 @@
 namespace swift {
 
 class AbstractFunctionDecl;
+class ActorIsolation;
 class BraceStmt;
 class ClosureExpr;
 class CodeCompletionCallbacksFactory;

include/swift/AST/ActorIsolation.h

@@ -0,0 +1,111 @@
+//===--- ActorIsolation.h - Actor isolation ---------------------*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+// This file provides a description of actor isolation state.
+//
+//===----------------------------------------------------------------------===//
+#ifndef SWIFT_AST_ACTORISOLATIONSTATE_H
+#define SWIFT_AST_ACTORISOLATIONSTATE_H
+
+#include "llvm/ADT/Hashing.h"
+
+namespace llvm {
+class raw_ostream;
+}
+
+namespace swift {
+class ClassDecl;
+
+/// Describes the actor isolation of a given declaration, which determines
+/// the actors with which it can interact.
+class ActorIsolation {
+public:
+  enum Kind {
+    /// The actor isolation has not been specified. It is assumed to be
+    /// unsafe to interact with this declaration from any actor.
+    Unspecified = 0,
+    /// The declaration is isolated to the instance of an actor class.
+    /// For example, a mutable stored property or synchronous function within
+    /// the actor is isolated to the instance of that actor.
+    ActorInstance,
+    /// The declaration can refer to actor-isolated state, but can also be
+    //// referenced from outside the actor.
+    ActorPrivileged,
+    /// The declaration is explicitly specified to be independent of any actor,
+    /// meaning that it can be used from any actor but is also unable to
+    /// refer to the isolated state of any given actor.
+    Independent,
+  };
+
+private:
+  Kind kind;
+  ClassDecl *actor;
+
+  ActorIsolation(Kind kind, ClassDecl *actor) : kind(kind), actor(actor) { }
+
+public:
+  static ActorIsolation forUnspecified() {
+    return ActorIsolation(Unspecified, nullptr);
+  }
+
+  static ActorIsolation forIndependent() {
+    return ActorIsolation(Independent, nullptr);
+  }
+
+  static ActorIsolation forActorPrivileged(ClassDecl *actor) {
+    return ActorIsolation(ActorPrivileged, actor);
+  }
+
+  static ActorIsolation forActorInstance(ClassDecl *actor) {
+    return ActorIsolation(ActorInstance, actor);
+  }
+
+  Kind getKind() const { return kind; }
+
+  operator Kind() const { return getKind(); }
+
+  ClassDecl *getActor() const {
+    assert(getKind() == ActorInstance || getKind() == ActorPrivileged);
+    return actor;
+  }
+
+  friend bool operator==(const ActorIsolation &lhs,
+                         const ActorIsolation &rhs) {
+    if (lhs.kind != rhs.kind)
+      return false;
+
+    switch (lhs.kind) {
+    case Independent:
+    case Unspecified:
+      return true;
+
+    case ActorInstance:
+    case ActorPrivileged:
+      return lhs.actor == rhs.actor;
+    }
+  }
+
+  friend bool operator!=(const ActorIsolation &lhs,
+                         const ActorIsolation &rhs) {
+    return !(lhs == rhs);
+  }
+
+  friend llvm::hash_code hash_value(const ActorIsolation &state) {
+    return llvm::hash_combine(state.kind, state.actor);
+  }
+};
+
+void simple_display(llvm::raw_ostream &out, const ActorIsolation &state);
+
+} // end namespace swift
+
+#endif /* SWIFT_AST_ACTORISOLATIONSTATE_H */

include/swift/AST/Attr.def

@@ -572,6 +572,12 @@ CONTEXTUAL_SIMPLE_DECL_ATTR(actor, Actor,
   APIBreakingToAdd | APIBreakingToRemove,
   102)
 
+SIMPLE_DECL_ATTR(actorIndependent, ActorIndependent,
+  OnFunc | OnVar | OnSubscript | ConcurrencyOnly |
+  ABIStableToAdd | ABIStableToRemove |
+  APIStableToAdd | APIBreakingToRemove,
+  103)
+
 #undef TYPE_ATTR
 #undef DECL_ATTR_ALIAS
 #undef CONTEXTUAL_DECL_ATTR_ALIAS

include/swift/AST/DiagnosticsSema.def

@@ -4146,6 +4146,10 @@ ERROR(actor_isolated_non_self_reference,none,
         "actor-isolated %0 %1 can only be referenced "
         "%select{inside the actor|on 'self'}2",
         (DescriptiveDeclKind, DeclName, bool))
+ERROR(actor_isolated_self_independent_context,none,
+        "actor-isolated %0 %1 can not be referenced from an "
+        "'@actorIndependent' context",
+        (DescriptiveDeclKind, DeclName))
 WARNING(concurrent_access_local,none,
         "local %0 %1 is unsafe to reference in code that may execute "
         "concurrently",
@@ -4170,6 +4174,24 @@ NOTE(actor_isolated_witness_could_be_async_handler,none,
      "did you mean to make it an asychronous handler?",
      (DescriptiveDeclKind, DeclName))
 
+ERROR(actorisolated_let,none,
+      "'@actorIsolated' is meaningless on 'let' declarations because "
+      "they are immutable",
+      ())
+ERROR(actorisolated_mutable_storage,none,
+      "'@actorIsolated' can not be applied to stored properties",
+      ())
+ERROR(actorisolated_local_var,none,
+      "'@actorIsolated' can not be applied to local variables",
+      ())
+ERROR(actorisolated_not_actor_member,none,
+      "'@actorIsolated' can only be applied to actor members and "
+      "global/static variables",
+      ())
+ERROR(actorisolated_not_actor_instance_member,none,
+      "'@actorIsolated' can only be applied to instance members of actors",
+      ())
+
 //------------------------------------------------------------------------------
 // MARK: Type Check Types
 //------------------------------------------------------------------------------

include/swift/AST/TypeCheckRequests.h

@@ -16,6 +16,7 @@
 #ifndef SWIFT_TYPE_CHECK_REQUESTS_H
 #define SWIFT_TYPE_CHECK_REQUESTS_H
 
+#include "swift/AST/ActorIsolation.h"
 #include "swift/AST/AnyFunctionRef.h"
 #include "swift/AST/ASTTypeIDs.h"
 #include "swift/AST/GenericSignature.h"
@@ -817,6 +818,24 @@ class IsActorRequest :
   bool isCached() const { return true; }
 };
 
+/// Determine the actor isolation for the given declaration.
+class ActorIsolationRequest :
+    public SimpleRequest<ActorIsolationRequest,
+                         ActorIsolation(ValueDecl *),
+                         RequestFlags::Cached> {
+public:
+  using SimpleRequest::SimpleRequest;
+
+private:
+  friend SimpleRequest;
+
+  ActorIsolation evaluate(Evaluator &evaluator, ValueDecl *value) const;
+
+public:
+  // Caching
+  bool isCached() const { return true; }
+};
+
 /// Request whether the storage has a mutating getter.
 class IsGetterMutatingRequest :
     public SimpleRequest<IsGetterMutatingRequest,

include/swift/AST/TypeCheckerTypeIDZone.def

@@ -85,6 +85,9 @@ SWIFT_REQUEST(TypeChecker, IsAsyncHandlerRequest, bool(FuncDecl *),
               Cached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, IsActorRequest, bool(ClassDecl *),
               Cached, NoLocationInfo)
+SWIFT_REQUEST(TypeChecker, ActorIsolationRequest,
+              ActorIsolationState(ValueDecl *),
+              Cached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, FunctionOperatorRequest, OperatorDecl *(FuncDecl *),
               Cached, NoLocationInfo)
 SWIFT_REQUEST(NameLookup, GenericSignatureRequest,

lib/AST/TypeCheckRequests.cpp

@@ -1496,3 +1496,25 @@ void CustomAttrTypeRequest::cacheResult(Type value) const {
   auto *attr = std::get<0>(getStorage());
   attr->setType(value);
 }
+
+
+void swift::simple_display(
+    llvm::raw_ostream &out, const ActorIsolation &state) {
+  switch (state) {
+    case ActorIsolation::ActorInstance:
+      out << "actor-isolated to instance of " << state.getActor()->getName();
+      break;
+
+    case ActorIsolation::ActorPrivileged:
+      out << "actor-privileged to instance of " << state.getActor()->getName();
+      break;
+
+    case ActorIsolation::Independent:
+      out << "actor-independent";
+      break;
+
+    case ActorIsolation::Unspecified:
+      out << "unspecified actor isolation";
+      break;
+  }
+}

lib/Sema/TypeCheckAttr.cpp

@@ -276,6 +276,53 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
 
     (void)classDecl->isActor();
   }
+
+  void visitActorIndependentAttr(ActorIndependentAttr *attr) {
+    // @actorIndependent can be applied to global and static/class variables
+    // that do not have storage.
+    auto dc = D->getDeclContext();
+    if (auto var = dyn_cast<VarDecl>(D)) {
+      // @actorIndependent is meaningless on a `let`.
+      if (var->isLet()) {
+        diagnoseAndRemoveAttr(attr, diag::actorisolated_let);
+        return;
+      }
+
+      // @actorIndependent can not be applied to stored properties.
+      if (var->hasStorage()) {
+        diagnoseAndRemoveAttr(attr, diag::actorisolated_mutable_storage);
+        return;
+      }
+
+      // @actorIndependent can not be applied to local properties.
+      if (dc->isLocalContext()) {
+        diagnoseAndRemoveAttr(attr, diag::actorisolated_local_var);
+        return;
+      }
+
+      // If this is a static or global variable, we're all set.
+      if (dc->isModuleScopeContext() ||
+          (dc->isTypeContext() && var->isStatic())) {
+        return;
+      }
+
+      // Otherwise, fall through to make sure we're in an appropriate
+      // context.
+    }
+
+    // @actorIndependent only makes sense on an actor instance member.
+    if (!dc->getSelfClassDecl() ||
+        !dc->getSelfClassDecl()->isActor()) {
+      diagnoseAndRemoveAttr(attr, diag::actorisolated_not_actor_member);
+      return;
+    }
+
+    if (!cast<ValueDecl>(D)->isInstanceMember()) {
+      diagnoseAndRemoveAttr(
+          attr, diag::actorisolated_not_actor_instance_member);
+      return;
+    }
+  }
 };
 } // end anonymous namespace