[AST] Extend @_inheritActorContext attribute to support optional always modifier

By default (currently) the closure passed to a parameter with `@_inheritActorContext`
would only inherit isolation from `nonisolated`, global actor isolated or actor
context when "self" is captured by the closure. `always` changes this behavior to
always inherit actor isolation from context regardless of whether it's captured
or not.

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/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,
@@ -3011,6 +3015,50 @@ 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

@@ -620,9 +620,10 @@ 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

@@ -8595,6 +8595,13 @@ 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))
+
 //===----------------------------------------------------------------------===//
 // MARK: @concurrent and nonisolated(nonsending) attributes
 //===----------------------------------------------------------------------===//

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/Parse/IDEInspectionCallbacks.h

@@ -39,7 +39,8 @@ enum class ParameterizedDeclAttributeKind {
   Available,
   FreestandingMacro,
   AttachedMacro,
-  StorageRestrictions
+  StorageRestrictions,
+  InheritActorContext
 };
 
 /// A bit of a hack. When completing inside the '@storageRestrictions'

lib/AST/ASTDumper.cpp

@@ -4976,7 +4976,6 @@ class PrintAttribute : public AttributeVisitor<PrintAttribute, void, Label>,
   TRIVIAL_ATTR_PRINTER(ImplicitSelfCapture, implicit_self_capture)
   TRIVIAL_ATTR_PRINTER(Indirect, indirect)
   TRIVIAL_ATTR_PRINTER(Infix, infix)
-  TRIVIAL_ATTR_PRINTER(InheritActorContext, inherit_actor_context)
   TRIVIAL_ATTR_PRINTER(InheritsConvenienceInitializers,
                        inherits_convenience_initializers)
   TRIVIAL_ATTR_PRINTER(Inlinable, inlinable)
@@ -5301,6 +5300,12 @@ class PrintAttribute : public AttributeVisitor<PrintAttribute, void, Label>,
     printFlag(Attr->isNonSending(), "nonsending");
     printFoot();
   }
+  void visitInheritActorContextAttr(InheritActorContextAttr *Attr,
+                                    Label label) {
+    printCommon(Attr, "inherit_actor_context_attr", label);
+    printFlag(Attr->isAlways(), "always");
+    printFoot();
+  }
   void visitObjCAttr(ObjCAttr *Attr, Label label) {
     printCommon(Attr, "objc_attr", label);
     if (Attr->hasName())

lib/AST/Attr.cpp

@@ -1531,6 +1531,18 @@ bool DeclAttribute::printImpl(ASTPrinter &Printer, const PrintOptions &Options,
     break;
   }
 
+  case DeclAttrKind::InheritActorContext: {
+    Printer.printAttrName("@_inheritActorContext");
+    switch (cast<InheritActorContextAttr>(this)->getModifier()) {
+    case InheritActorContextModifier::None:
+      break;
+    case InheritActorContextModifier::Always:
+      Printer << "(always)";
+      break;
+    }
+    break;
+  }
+
   case DeclAttrKind::MacroRole: {
     auto Attr = cast<MacroRoleAttr>(this);
 
@@ -1915,6 +1927,13 @@ StringRef DeclAttribute::getAttrName() const {
     case NonIsolatedModifier::NonSending:
       return "nonisolated(nonsending)";
     }
+  case DeclAttrKind::InheritActorContext:
+    switch (cast<InheritActorContextAttr>(this)->getModifier()) {
+    case InheritActorContextModifier::None:
+      return "_inheritActorContext";
+    case InheritActorContextModifier::Always:
+      return "_inheritActorContext(always)";
+    }
   case DeclAttrKind::MacroRole:
     switch (cast<MacroRoleAttr>(this)->getMacroSyntax()) {
     case MacroSyntax::Freestanding:

lib/IDE/CompletionLookup.cpp

@@ -3141,6 +3141,9 @@ void CompletionLookup::getAttributeDeclParamCompletions(
     addDeclAttrParamKeyword("unsafe", /*Parameters=*/{}, "", false);
     addDeclAttrParamKeyword("nonsending", /*Parameters=*/{}, "", false);
     break;
+  case ParameterizedDeclAttributeKind::InheritActorContext:
+    addDeclAttrParamKeyword("always", /*Parameters=*/{}, "", false);
+    break;
   case ParameterizedDeclAttributeKind::AccessControl:
     addDeclAttrParamKeyword("set", /*Parameters=*/{}, "", false);
     break;