[AST/Sema/SIL] Implement @_inheritActorContext(always)

- Extend `@_inheritActorContext` attribute to support optional `always` modifier.
  The new modifier will make closure context isolated even if the parameter is not
  captured by the closure.
- Implementation `@_inheritActorContext` attribute validation - it could only be
  used on parameter that have `@Sendable` or `sending` and `@isolated(any)` or
  `async` function type (downgraded to a warning until future major Swift mode
  to avoid source compatibility issues).
- Add a new language feature that guards use of `@_inheritActorContext(always)` in swift interface files
- Update `getLoweredLocalCaptures` to add an entry for isolation parameter implicitly captured by `@_inheritActorContext(always)`
- Update serialization code to store `always` modifier

(cherry picked from commit 04d4676)
(cherry picked from commit c050e8f)
(cherry picked from commit c0aca53)
(cherry picked from commit a4f6d71)
(cherry picked from commit 6c911f5)
(cherry picked from commit 17b8f7e)

Author: xedin

docs/ReferenceGuides/UnderscoredAttributes.md

@@ -600,6 +600,43 @@ inherit the actor context (i.e. what actor it should be run on) based on the
 declaration site of the closure rather than be non-Sendable. This does not do
 anything if the closure is synchronous.
 
+This works with global actors as expected:
+
+```swift 
+@MainActor
+func test() {
+  Task { /* main actor isolated */ }
+}
+```
+
+However, for the inference to work with instance actors (i.e. `isolated` parameters),
+the closure must capture the isolated parameter explicitly:
+
+```swift 
+func test(actor: isolated (any Actor)) {
+  Task { /* non isolated */ } // !!!
+}
+
+func test(actor: isolated (any Actor)) {
+  Task { // @_inheritActorContext
+    _ = actor // 'actor'-isolated 
+  }
+}
+```
+
+The attribute takes an optional modifier '`always`', which changes this behavior 
+and *always* captures the enclosing isolated context, rather than forcing developers
+to perform the explicit capture themselfes:
+
+```swift
+func test(actor: isolated (any Actor)) {
+  Task.immediate { // @_inheritActorContext(always)
+    // 'actor'-isolated!
+    // (without having to capture 'actor explicitly')
+  }
+}
+```
+
 DISCUSSION: The reason why this does nothing when the closure is synchronous is
 since it does not have the ability to hop to the appropriate executor before it
 is run, so we may create concurrency errors.

include/swift/AST/ASTBridging.h

@@ -1239,6 +1239,18 @@ BridgedNonisolatedAttr_createParsed(BridgedASTContext cContext,
                                     BridgedSourceRange cRange,
                                     BridgedNonIsolatedModifier modifier);
 
+enum ENUM_EXTENSIBILITY_ATTR(closed) BridgedInheritActorContextModifier {
+  BridgedInheritActorContextModifierNone,
+  BridgedInheritActorContextModifierAlways,
+};
+
+SWIFT_NAME("BridgedInheritActorContextAttr.createParsed(_:atLoc:range:modifier:)")
+BridgedInheritActorContextAttr
+BridgedInheritActorContextAttr_createParsed(BridgedASTContext cContext,
+                                            BridgedSourceLoc cAtLoc,
+                                            BridgedSourceRange cRange,
+                                            BridgedInheritActorContextModifier modifier);
+
 SWIFT_NAME("BridgedObjCAttr.createParsedUnnamed(_:atLoc:attrNameLoc:)")
 BridgedObjCAttr
 BridgedObjCAttr_createParsedUnnamed(BridgedASTContext cContext,

include/swift/AST/Attr.h

@@ -230,6 +230,10 @@ class DeclAttribute : public AttributeBase {
       Modifier : NumNonIsolatedModifierBits
     );
 
+    SWIFT_INLINE_BITFIELD(InheritActorContextAttr, DeclAttribute, NumInheritActorContextKindBits,
+      Modifier : NumInheritActorContextKindBits
+    );
+
     SWIFT_INLINE_BITFIELD_FULL(AllowFeatureSuppressionAttr, DeclAttribute, 1+31,
       : NumPadBits,
       Inverted : 1,
@@ -3004,6 +3008,50 @@ class NonisolatedAttr final : public DeclAttribute {
   }
 };
 
+/// Represents @_inheritActorContext modifier.
+class InheritActorContextAttr final : public DeclAttribute {
+public:
+  InheritActorContextAttr(SourceLoc atLoc, SourceRange range,
+                          InheritActorContextModifier modifier, bool implicit)
+      : DeclAttribute(DeclAttrKind::InheritActorContext, atLoc, range,
+                      implicit) {
+    Bits.InheritActorContextAttr.Modifier = static_cast<unsigned>(modifier);
+    assert((getModifier() == modifier) && "not enough bits for modifier");
+  }
+
+  InheritActorContextModifier getModifier() const {
+    return static_cast<InheritActorContextModifier>(
+        Bits.InheritActorContextAttr.Modifier);
+  }
+
+  bool isAlways() const {
+    return getModifier() == InheritActorContextModifier::Always;
+  }
+
+  static InheritActorContextAttr *
+  createImplicit(ASTContext &ctx, InheritActorContextModifier modifier =
+                                      InheritActorContextModifier::None) {
+    return new (ctx)
+        InheritActorContextAttr(/*atLoc*/ {}, /*range*/ {}, modifier,
+                                /*implicit=*/true);
+  }
+
+  static bool classof(const DeclAttribute *DA) {
+    return DA->getKind() == DeclAttrKind::InheritActorContext;
+  }
+
+  /// Create a copy of this attribute.
+  InheritActorContextAttr *clone(ASTContext &ctx) const {
+    return new (ctx)
+        InheritActorContextAttr(AtLoc, Range, getModifier(), isImplicit());
+  }
+
+  bool isEquivalent(const InheritActorContextAttr *other,
+                    Decl *attachedTo) const {
+    return getModifier() == other->getModifier();
+  }
+};
+
 /// A macro role attribute, spelled with either @attached or @freestanding,
 /// which declares one of the roles that a given macro can inhabit.
 class MacroRoleAttr final

include/swift/AST/AttrKind.h

@@ -142,6 +142,22 @@ enum : unsigned {
       static_cast<unsigned>(NonIsolatedModifier::Last_NonIsolatedModifier))
 };
 
+enum class InheritActorContextModifier : uint8_t {
+  /// Inherit the actor execution context if the isolated parameter was
+  /// captured by the closure, context is nonisolated or isolated to a
+  /// global actor.
+  None = 0,
+  /// Always inherit the actor context, even when the isolated parameter
+  /// for the context is not closed over explicitly.
+  Always,
+  Last_InheritActorContextKind = Always
+};
+
+enum : unsigned {
+  NumInheritActorContextKindBits = countBitsUsed(static_cast<unsigned>(
+      InheritActorContextModifier::Last_InheritActorContextKind))
+};
+
 enum class DeclAttrKind : unsigned {
 #define DECL_ATTR(_, CLASS, ...) CLASS,
 #define LAST_DECL_ATTR(CLASS) Last_DeclAttr = CLASS,

include/swift/AST/DeclAttr.def

@@ -619,9 +619,10 @@ SIMPLE_DECL_ATTR(_implicitSelfCapture, ImplicitSelfCapture,
   UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIBreakingToRemove | ForbiddenInABIAttr,
   115)
 
-SIMPLE_DECL_ATTR(_inheritActorContext, InheritActorContext,
+DECL_ATTR(_inheritActorContext, InheritActorContext,
   OnParam,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIBreakingToRemove | ForbiddenInABIAttr,
+  // since the _inheritActorContext(always) forces an actor capture, it changes ABI of the closure this applies to
+  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UnconstrainedInABIAttr,
   116)
 
 SIMPLE_DECL_ATTR(_eagerMove, EagerMove,

include/swift/AST/DiagnosticsSema.def

@@ -8546,6 +8546,23 @@ GROUPED_ERROR(isolated_conformance_wrong_domain,IsolatedConformances,none,
       "%0 conformance of %1 to %2 cannot be used in %3 context",
       (ActorIsolation, Type, DeclName, ActorIsolation))
 
+//===----------------------------------------------------------------------===//
+// MARK: @_inheritActorContext
+//===----------------------------------------------------------------------===//
+ERROR(inherit_actor_context_only_on_func_types,none,
+      "%0 only applies to parameters with function types (got: %1)",
+      (DeclAttribute, Type))
+
+ERROR(inherit_actor_context_only_on_sending_or_Sendable_params,none,
+      "%0 only applies to 'sending' parameters or parameters with "
+      "'@Sendable' function types",
+      (DeclAttribute))
+
+ERROR(inherit_actor_context_only_on_async_or_isolation_erased_params,none,
+      "%0 only applies to '@isolated(any)' parameters or parameters with "
+      "asynchronous function types",
+      (DeclAttribute))
+
 //===----------------------------------------------------------------------===//
 // MARK: @concurrent and nonisolated(nonsending) attributes
 //===----------------------------------------------------------------------===//

include/swift/AST/Expr.h

@@ -267,7 +267,7 @@ class alignas(8) Expr : public ASTAllocated<Expr> {
     Kind : 2
   );
 
-  SWIFT_INLINE_BITFIELD(ClosureExpr, AbstractClosureExpr, 1+1+1+1+1+1+1+1,
+  SWIFT_INLINE_BITFIELD(ClosureExpr, AbstractClosureExpr, 1+1+1+1+1+1+1+1+1,
     /// True if closure parameters were synthesized from anonymous closure
     /// variables.
     HasAnonymousClosureVars : 1,
@@ -276,9 +276,11 @@ class alignas(8) Expr : public ASTAllocated<Expr> {
     /// on each member reference.
     ImplicitSelfCapture : 1,
 
-    /// True if this @Sendable async closure parameter should implicitly
-    /// inherit the actor context from where it was formed.
+    /// True if this closure parameter should implicitly inherit the actor
+    /// context from where it was formed.
     InheritActorContext : 1,
+    /// The kind for inheritance - none or always at the moment.
+    InheritActorContextKind : 1,
 
     /// True if this closure's actor isolation behavior was determined by an
     /// \c \@preconcurrency declaration.
@@ -4318,6 +4320,7 @@ class ClosureExpr : public AbstractClosureExpr {
     Bits.ClosureExpr.HasAnonymousClosureVars = false;
     Bits.ClosureExpr.ImplicitSelfCapture = false;
     Bits.ClosureExpr.InheritActorContext = false;
+    Bits.ClosureExpr.InheritActorContextKind = 0;
     Bits.ClosureExpr.IsPassedToSendingParameter = false;
     Bits.ClosureExpr.NoGlobalActorAttribute = false;
     Bits.ClosureExpr.RequiresDynamicIsolationChecking = false;
@@ -4366,8 +4369,29 @@ class ClosureExpr : public AbstractClosureExpr {
     return Bits.ClosureExpr.InheritActorContext;
   }
 
-  void setInheritsActorContext(bool value = true) {
+  /// Whether this closure should _always_ implicitly inherit the actor context
+  /// regardless of whether the isolation parameter is captured or not.
+  bool alwaysInheritsActorContext() const {
+    if (!inheritsActorContext())
+      return false;
+    return getInheritActorIsolationModifier() ==
+           InheritActorContextModifier::Always;
+  }
+
+  void setInheritsActorContext(bool value = true,
+                               InheritActorContextModifier modifier =
+                                   InheritActorContextModifier::None) {
     Bits.ClosureExpr.InheritActorContext = value;
+    Bits.ClosureExpr.InheritActorContextKind = uint8_t(modifier);
+    assert((static_cast<InheritActorContextModifier>(
+                Bits.ClosureExpr.InheritActorContextKind) == modifier) &&
+           "not enough bits for modifier");
+  }
+
+  InheritActorContextModifier getInheritActorIsolationModifier() const {
+    assert(inheritsActorContext());
+    return static_cast<InheritActorContextModifier>(
+        Bits.ClosureExpr.InheritActorContextKind);
   }
 
   /// Whether the closure's concurrency behavior was determined by an

include/swift/AST/KnownIdentifiers.def

@@ -325,6 +325,7 @@ IDENTIFIER(SerializationRequirement)
 IDENTIFIER_WITH_NAME(builderSelf, "$builderSelf")
 
 // Attribute options
+IDENTIFIER(always)
 IDENTIFIER_(_always)
 IDENTIFIER_(assumed)
 IDENTIFIER(checked)

include/swift/AST/Types.h

@@ -4045,6 +4045,7 @@ struct ParameterListInfo {
   SmallBitVector propertyWrappers;
   SmallBitVector implicitSelfCapture;
   SmallBitVector inheritActorContext;
+  SmallBitVector alwaysInheritActorContext;
   SmallBitVector variadicGenerics;
   SmallBitVector sendingParameters;
 
@@ -4071,7 +4072,8 @@ struct ParameterListInfo {
 
   /// Whether the given parameter is a closure that should inherit the
   /// actor context from the context in which it was created.
-  bool inheritsActorContext(unsigned paramIdx) const;
+  std::pair<bool, InheritActorContextModifier>
+  inheritsActorContext(unsigned paramIdx) const;
 
   bool isVariadicGenericParameter(unsigned paramIdx) const;
 

include/swift/Basic/Features.def

@@ -260,6 +260,7 @@ LANGUAGE_FEATURE(GeneralizedIsSameMetaTypeBuiltin, 465, "Builtin.is_same_metatyp
 LANGUAGE_FEATURE(ValueGenericsNameLookup, 452, "Value generics appearing as static members for namelookup")
 SUPPRESSIBLE_LANGUAGE_FEATURE(ABIAttributeSE0479, 479, "@abi attribute on functions, initializers, properties, and subscripts")
 LANGUAGE_FEATURE(BuiltinSelect, 0, "Builtin.select")
+LANGUAGE_FEATURE(AlwaysInheritActorContext, 472, "@_inheritActorContext(always)")
 
 // Swift 6
 UPCOMING_FEATURE(ConciseMagicFile, 274, 6)