[SE-0316] Introduce the GlobalActor protocol to describe global actors.

Based on the discussion in the first review of the global actors
proposal, introduce a `GlobalActor` protocol that describes types that
can be global actors. Introduce this protocol, make `@globalActor`
types implicitly conform to it, and remove all of the bespoke
validation logic that was used to check the "shared" member.

Addresses rdar://79339591

(cherry picked from commit 8a068e0)

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -4536,22 +4536,6 @@ ERROR(concurrency_lib_missing,none,
 ERROR(async_main_no_concurrency,none,
       "'_Concurrency' module not imported, required for async main", ())
 
-ERROR(global_actor_missing_shared,none,
-      "global actor %0 requires a static property 'shared' that produces an "
-      "actor instance", (Identifier))
-NOTE(global_actor_shared_not_static,none,
-     "'shared' property in global actor is not 'static'", ())
-NOTE(global_actor_shared_inaccessible,none,
-     "'shared' property has more restrictive access (%0) than its global actor "
-     "(%1)",
-     (StringRef, StringRef))
-NOTE(global_actor_shared_constrained_extension,none,
-     "'shared' property in global actor cannot be in a constrained extension",
-     ())
-NOTE(global_actor_shared_non_actor_type,none,
-     "'shared' property type %0 does not conform to the 'Actor' protocol",
-     (Type))
-
 ERROR(multiple_global_actors,none,
       "declaration can not have multiple global actor attributes (%0 and %1)",
       (Identifier, Identifier))

include/swift/AST/KnownIdentifiers.def

@@ -28,6 +28,7 @@ IDENTIFIER(allKeys)
 IDENTIFIER(alloc)
 IDENTIFIER(allocWithZone)
 IDENTIFIER(allZeros)
+IDENTIFIER(ActorType)
 IDENTIFIER(Any)
 IDENTIFIER(ArrayLiteralElement)
 IDENTIFIER(atIndexedSubscript)

include/swift/AST/KnownProtocols.def

@@ -72,6 +72,7 @@ PROTOCOL(CaseIterable)
 PROTOCOL(SIMDScalar)
 PROTOCOL(BinaryInteger)
 PROTOCOL(SerialExecutor)
+PROTOCOL(GlobalActor)
 
 PROTOCOL_(BridgedNSError)
 PROTOCOL_(BridgedStoredNSError)

lib/AST/ASTContext.cpp

@@ -977,6 +977,7 @@ ProtocolDecl *ASTContext::getProtocol(KnownProtocolKind kind) const {
     M = getLoadedModule(Id_Differentiation);
     break;
   case KnownProtocolKind::Actor:
+  case KnownProtocolKind::GlobalActor:
   case KnownProtocolKind::AsyncSequence:
   case KnownProtocolKind::AsyncIteratorProtocol:
   case KnownProtocolKind::SerialExecutor:

lib/AST/ProtocolConformance.cpp

@@ -1253,6 +1253,11 @@ void NominalTypeDecl::prepareConformanceTable() const {
     if (classDecl->isActor())
       addSynthesized(KnownProtocolKind::Actor);
   }
+
+  // Global actors conform to the GlobalActor protocol.
+  if (mutableThis->getAttrs().hasAttribute<GlobalActorAttr>()) {
+    addSynthesized(KnownProtocolKind::GlobalActor);
+  }
 }
 
 bool NominalTypeDecl::lookupConformance(

lib/IRGen/GenMeta.cpp

@@ -5166,6 +5166,7 @@ SpecialProtocol irgen::getSpecialProtocolID(ProtocolDecl *P) {
   case KnownProtocolKind::SerialExecutor:
   case KnownProtocolKind::Sendable:
   case KnownProtocolKind::UnsafeSendable:
+  case KnownProtocolKind::GlobalActor:
     return SpecialProtocol::None;
   }
 

lib/Sema/TypeCheckConcurrency.cpp

@@ -298,12 +298,6 @@ bool IsDefaultActorRequest::evaluate(
   return true;
 }
 
-static bool isDeclNotAsAccessibleAsParent(ValueDecl *decl,
-                                          NominalTypeDecl *parent) {
-  return decl->getFormalAccess() <
-         std::min(parent->getFormalAccess(), AccessLevel::Public);
-}
-
 VarDecl *GlobalActorInstanceRequest::evaluate(
     Evaluator &evaluator, NominalTypeDecl *nominal) const {
   auto globalActorAttr = nominal->getAttrs().getAttribute<GlobalActorAttr>();
@@ -323,99 +317,13 @@ VarDecl *GlobalActorInstanceRequest::evaluate(
   SmallVector<ValueDecl *, 4> decls;
   nominal->lookupQualified(
       nominal, DeclNameRef(ctx.Id_shared), NL_QualifiedDefault, decls);
-  VarDecl *sharedVar = nullptr;
-  llvm::TinyPtrVector<VarDecl *> candidates;
   for (auto decl : decls) {
     auto var = dyn_cast<VarDecl>(decl);
     if (!var)
       continue;
 
-    auto varDC = var->getDeclContext();
-    if (var->isStatic() &&
-        !isDeclNotAsAccessibleAsParent(var, nominal) &&
-        !(isa<ExtensionDecl>(varDC) &&
-          cast<ExtensionDecl>(varDC)->isConstrainedExtension()) &&
-        TypeChecker::conformsToProtocol(
-            varDC->mapTypeIntoContext(var->getValueInterfaceType()),
-            actorProto, nominal)) {
-      sharedVar = var;
-      break;
-    }
-
-    candidates.push_back(var);
-  }
-
-  // If we found a suitable candidate, we're done.
-  if (sharedVar)
-    return sharedVar;
-
-  // Complain about the lack of a suitable 'shared' property.
-  {
-    auto primaryDiag = nominal->diagnose(
-        diag::global_actor_missing_shared, nominal->getName());
-
-    // If there were no candidates, provide a Fix-It with a prototype.
-    if (candidates.empty() && nominal->getBraces().Start.isValid()) {
-      // Figure out the indentation we need.
-      SourceLoc sharedInsertionLoc = Lexer::getLocForEndOfToken(
-          ctx.SourceMgr, nominal->getBraces().Start);
-
-      StringRef extraIndent;
-      StringRef currentIndent = Lexer::getIndentationForLine(
-          ctx.SourceMgr, sharedInsertionLoc, &extraIndent);
-      std::string stubIndent = (currentIndent + extraIndent).str();
-
-      // From the string to add the declaration.
-      std::string sharedDeclString = "\n" + stubIndent;
-      if (nominal->getFormalAccess() >= AccessLevel::Public)
-        sharedDeclString += "public ";
-
-      sharedDeclString += "static let shared = <#actor instance#>";
-
-      primaryDiag.fixItInsert(sharedInsertionLoc, sharedDeclString);
-    }
-  }
-
-  // Remark about all of the candidates that failed (and why).
-  for (auto candidate : candidates) {
-    if (!candidate->isStatic()) {
-      candidate->diagnose(diag::global_actor_shared_not_static)
-        .fixItInsert(candidate->getAttributeInsertionLoc(true), "static ");
-      continue;
-    }
-
-    if (isDeclNotAsAccessibleAsParent(candidate, nominal)) {
-      AccessLevel needAccessLevel = std::min(
-          nominal->getFormalAccess(), AccessLevel::Public);
-      auto diag = candidate->diagnose(
-          diag::global_actor_shared_inaccessible,
-          getAccessLevelSpelling(candidate->getFormalAccess()),
-          getAccessLevelSpelling(needAccessLevel));
-      if (auto attr = candidate->getAttrs().getAttribute<AccessControlAttr>()) {
-        if (needAccessLevel == AccessLevel::Internal) {
-          diag.fixItRemove(attr->getRange());
-        } else {
-          diag.fixItReplace(
-              attr->getRange(), getAccessLevelSpelling(needAccessLevel));
-        }
-      } else {
-        diag.fixItInsert(
-            candidate->getAttributeInsertionLoc(true),
-            getAccessLevelSpelling(needAccessLevel));
-      }
-      continue;
-    }
-
-    if (auto ext = dyn_cast<ExtensionDecl>(candidate->getDeclContext())) {
-      if (ext->isConstrainedExtension()) {
-        candidate->diagnose(diag::global_actor_shared_constrained_extension);
-        continue;
-      }
-    }
-
-    Type varType = candidate->getDeclContext()->mapTypeIntoContext(
-        candidate->getValueInterfaceType());
-    candidate->diagnose(diag::global_actor_shared_non_actor_type, varType);
+    if (var->getDeclContext() == nominal && var->isStatic())
+      return var;
   }
 
   return nullptr;

stdlib/public/Concurrency/Actor.swift

@@ -52,28 +52,6 @@ public func _defaultActorDestroy(_ actor: AnyObject)
 @usableFromInline
 internal func _enqueueOnMain(_ job: UnownedJob)
 
-/// A singleton actor whose executor is equivalent to the main
-/// dispatch queue.
-@available(SwiftStdlib 5.5, *)
-@globalActor public final actor MainActor: SerialExecutor {
-  public static let shared = MainActor()
-
-  @inlinable
-  public nonisolated var unownedExecutor: UnownedSerialExecutor {
-    return asUnownedSerialExecutor()
-  }
-
-  @inlinable
-  public nonisolated func asUnownedSerialExecutor() -> UnownedSerialExecutor {
-    return UnownedSerialExecutor(ordinary: self)
-  }
-
-  @inlinable
-  public nonisolated func enqueue(_ job: UnownedJob) {
-    _enqueueOnMain(job)
-  }
-}
-
 // Used by the concurrency runtime
 @available(SwiftStdlib 5.5, *)
 extension SerialExecutor {
@@ -82,18 +60,3 @@ extension SerialExecutor {
     return MainActor.shared.unownedExecutor
   }
 }
-
-@available(SwiftStdlib 5.5, *)
-extension MainActor {
-  /// Execute the given body closure on the main actor.
-  public static func run<T>(
-    resultType: T.Type = T.self,
-    body: @MainActor @Sendable () throws -> T
-  ) async rethrows -> T {
-    @MainActor func runOnMain(body: @MainActor @Sendable () throws -> T) async rethrows -> T {
-      return try body()
-    }
-
-    return try await runOnMain(body: body)
-  }
-}

stdlib/public/Concurrency/CMakeLists.txt

@@ -59,6 +59,8 @@ add_swift_target_library(swift_Concurrency ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} I
   AsyncThrowingFlatMapSequence.swift
   AsyncThrowingMapSequence.swift
   AsyncThrowingPrefixWhileSequence.swift
+  GlobalActor.swift
+  MainActor.swift
   PartialAsyncTask.swift
   SourceCompatibilityShims.swift
   Task.cpp

stdlib/public/Concurrency/GlobalActor.swift

@@ -0,0 +1,52 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2021 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
+//
+//===----------------------------------------------------------------------===//
+
+import Swift
+
+/// A type that represents a globally-unique actor that can be used to isolate
+/// various declarations anywhere in the program.
+///
+/// A type that conforms to the `GlobalActor` protocol and is marked with the
+/// the `@globalActor` attribute can be used as a custom attribute. Such types
+/// are called global actor types, and can be applied to any declaration to
+/// specify that such types are isolated to that global actor type. When using
+/// such a declaration from another actor (or from nonisolated code),
+/// synchronization is performed through the \c shared actor instance to ensure
+/// mutually-exclusive access to the declaration.
+@available(SwiftStdlib 5.5, *)
+public protocol GlobalActor {
+  /// The type of the shared actor instance that will be used to provide
+  /// mutually-exclusive access to declarations annotated with the given global
+  /// actor type.
+  associatedtype ActorType: Actor
+
+  /// The shared actor instance that will be used to provide mutually-exclusive
+  /// access to declarations annotated with the given global actor type.
+  ///
+  /// The value of this property must always evaluate to the same actor
+  /// instance.
+  static var shared: ActorType { get }
+
+  /// The shared executor instance that will be used to provide
+  /// mutually-exclusive access for the global actor.
+  ///
+  /// The value of this property must be equivalent to `shared.unownedExecutor`.
+  static var sharedUnownedExecutor: UnownedSerialExecutor { get }
+}
+
+@available(SwiftStdlib 5.5, *)
+extension GlobalActor {
+  public static var sharedUnownedExecutor: UnownedSerialExecutor {
+    shared.unownedExecutor
+  }
+}
+

stdlib/public/Concurrency/MainActor.swift

@@ -0,0 +1,50 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2021 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
+//
+//===----------------------------------------------------------------------===//
+
+import Swift
+
+/// A singleton actor whose executor is equivalent to the main
+/// dispatch queue.
+@available(SwiftStdlib 5.5, *)
+@globalActor public final actor MainActor: SerialExecutor, GlobalActor {
+  public static let shared = MainActor()
+
+  @inlinable
+  public nonisolated var unownedExecutor: UnownedSerialExecutor {
+    return asUnownedSerialExecutor()
+  }
+
+  @inlinable
+  public nonisolated func asUnownedSerialExecutor() -> UnownedSerialExecutor {
+    return UnownedSerialExecutor(ordinary: self)
+  }
+
+  @inlinable
+  public nonisolated func enqueue(_ job: UnownedJob) {
+    _enqueueOnMain(job)
+  }
+}
+
+@available(SwiftStdlib 5.5, *)
+extension MainActor {
+  /// Execute the given body closure on the main actor.
+  public static func run<T>(
+    resultType: T.Type = T.self,
+    body: @MainActor @Sendable () throws -> T
+  ) async rethrows -> T {
+    @MainActor func runOnMain(body: @MainActor @Sendable () throws -> T) async rethrows -> T {
+      return try body()
+    }
+
+    return try await runOnMain(body: body)
+  }
+}

test/attr/global_actor.swift

@@ -20,35 +20,37 @@ struct GenericGlobalActor<T> {
 
 // Ill-formed global actors.
 @globalActor
-open class GA2 { // expected-error{{global actor 'GA2' requires a static property 'shared' that produces an actor instance}}{{17-17=\n    public static let shared = <#actor instance#>}}
+open class GA2 { // expected-error{{type 'GA2' does not conform to protocol 'GlobalActor'}}
 }
 
 @globalActor
-struct GA3 { // expected-error{{global actor 'GA3' requires a static property 'shared' that produces an actor instance}}
-  let shared = SomeActor() // expected-note{{'shared' property in global actor is not 'static'}}{{3-3=static }}
+struct GA3 { // expected-error{{type 'GA3' does not conform to protocol 'GlobalActor'}}
+  let shared = SomeActor()
 }
 
 @globalActor
-struct GA4 { // expected-error{{global actor 'GA4' requires a static property 'shared' that produces an actor instance}}
-  private static let shared = SomeActor() // expected-note{{'shared' property has more restrictive access (private) than its global actor (internal)}}{{3-11=}}
+struct GA4 {
+  private static let shared = SomeActor() // expected-error{{property 'shared' must be as accessible as its enclosing type because it matches a requirement in protocol 'GlobalActor'}}
+  // expected-note@-1{{mark the static property as 'internal' to satisfy the requirement}}
 }
 
 @globalActor
-open class GA5 { // expected-error{{global actor 'GA5' requires a static property 'shared' that produces an actor instance}}
-  static let shared = SomeActor() // expected-note{{'shared' property has more restrictive access (internal) than its global actor (public)}}{{3-3=public}}
+open class GA5 {
+  static let shared = SomeActor() // expected-error{{property 'shared' must be declared public because it matches a requirement in public protocol 'GlobalActor'}}
+  // expected-note@-1{{mark the static property as 'public' to satisfy the requirement}}
 }
 
 @globalActor
-struct GA6<T> { // expected-error{{global actor 'GA6' requires a static property 'shared' that produces an actor instance}}
+struct GA6<T> { // expected-error{{type 'GA6<T>' does not conform to protocol 'GlobalActor'}}
 }
 
 extension GA6 where T: Equatable {
-  static var shared: SomeActor { SomeActor() } // expected-note{{'shared' property in global actor cannot be in a constrained extension}}
+  static var shared: SomeActor { SomeActor() }
 }
 
 @globalActor
-class GA7 { // expected-error{{global actor 'GA7' requires a static property 'shared' that produces an actor instance}}
-  static let shared = 5 // expected-note{{'shared' property type 'Int' does not conform to the 'Actor' protocol}}
+class GA7 { // expected-error{{type 'GA7' does not conform to protocol 'GlobalActor'}}
+  static let shared = 5 // expected-note{{candidate would match and infer 'ActorType' = 'Int' if 'Int' conformed to 'Actor'}}
 }
 
 // -----------------------------------------------------------------------