Implement @_unsafeInheritExecutor.

SE-0338 changed the execution of non-actor async functions
so that they always hop to the generic executor, but some
functions need a way to suppress this so that they inherit
the caller's executor.

The right way to implement this is to have the caller pass
down the target executor in some reliable way and then
switch to it in all the appropriate places in the caller.
We might reasonably be able to build this on top of isolated
parameters, using some sort of default argument, or we might
need a wholly novel mechanism.

But those things are all ABI-breaking absent some sort of
guarantee about switching that we probably don't want to make,
and unfortunately we have functions in the library which we
need to export that need to inherit executors.  So in the
short term, we need some unsafe way of getting back to the
previous behavior.

Author: rjmccall

docs/ReferenceGuides/UnderscoredAttributes.md

@@ -825,3 +825,8 @@ also locks.
 Marks a synchronous API as being unavailable from asynchronous contexts. Direct
 usage of annotated API from asynchronous contexts will result in a warning from
 the compiler.
+
+## `@_unsafeInheritExecutor`
+
+This `async` function uses the pre-SE-0338 semantics of unsafely inheriting the caller's executor.  This is an underscored feature because the right way of inheriting an executor is to pass in the required executor and switch to it.  Unfortunately, there are functions in the standard library which need to inherit their caller's executor but cannot change their ABI because they were not defined as `@_alwaysEmitIntoClient` in the initial release.
+

include/swift/AST/Attr.def

@@ -636,7 +636,11 @@ CONTEXTUAL_SIMPLE_DECL_ATTR(nonisolated, Nonisolated,
   112)
 
 // 113 was experimental _unsafeSendable and is now unused
-// 114 was experimental _unsafeMainActor and is now unused
+
+SIMPLE_DECL_ATTR(_unsafeInheritExecutor, UnsafeInheritExecutor,
+  OnFunc | UserInaccessible |
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIBreakingToRemove,
+  114) // previously experimental _unsafeMainActor
 
 SIMPLE_DECL_ATTR(_implicitSelfCapture, ImplicitSelfCapture,
   OnParam | UserInaccessible |

include/swift/AST/DiagnosticsSema.def

@@ -3164,6 +3164,9 @@ ERROR(override_reasync_with_non_reasync,none,
 ERROR(reasync_without_async_parameter,none,
       "'reasync' function must take an 'async' function argument", ())
 
+ERROR(inherits_executor_without_async,none,
+      "non-async functions cannot inherit an executor", ())
+
 ERROR(autoclosure_function_type,none,
       "@autoclosure attribute only applies to function types",
       ())

include/swift/Basic/Features.def

@@ -59,5 +59,6 @@ LANGUAGE_FEATURE(BuiltinCopy, 0, "Builtin.copy()", true)
 LANGUAGE_FEATURE(BuiltinStackAlloc, 0, "Builtin.stackAlloc", true)
 LANGUAGE_FEATURE(SpecializeAttributeWithAvailability, 0, "@_specialize attribute with availability", true)
 LANGUAGE_FEATURE(BuiltinAssumeAlignment, 0, "Builtin.assumeAlignment", true)
+LANGUAGE_FEATURE(UnsafeInheritExecutor, 0, "@_unsafeInheritExecutor", true)
 
 #undef LANGUAGE_FEATURE

lib/AST/ASTPrinter.cpp

@@ -2963,6 +2963,10 @@ static bool usesFeatureBuiltinAssumeAlignment(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureUnsafeInheritExecutor(Decl *decl) {
+  return decl->getAttrs().hasAttribute<UnsafeInheritExecutorAttr>();
+}
+
 /// Determine the set of "new" features used on a given declaration.
 ///
 /// Note: right now, all features we check for are "new". At some point, we'll

lib/SILGen/ExecutorBreadcrumb.h

@@ -23,16 +23,16 @@ class SILGenFunction;
 /// Represents the information necessary to return to a caller's own
 /// active executor after making a hop to an actor for actor-isolated calls.
 class ExecutorBreadcrumb {
-  SILValue Executor;
+  bool mustReturnToExecutor;
 
 public:
   // An empty breadcrumb, indicating no hop back is necessary.
-  ExecutorBreadcrumb() : Executor() {}
+  ExecutorBreadcrumb() : mustReturnToExecutor(false) {}
 
-  // A breadcrumb representing the need to hop back to the executor
-  // represented by the given value.
-  explicit ExecutorBreadcrumb(SILValue executor)
-    : Executor(executor) {}
+  // A breadcrumb representing the need to hop back to the expected
+  // executor of the current function.
+  explicit ExecutorBreadcrumb(bool mustReturnToExecutor)
+    : mustReturnToExecutor(mustReturnToExecutor) {}
 
   // Emits the hop back sequence, if any, necessary to get back to
   // the executor represented by this breadcrumb.

lib/SILGen/SILGenApply.cpp

@@ -4552,7 +4552,7 @@ RValue SILGenFunction::emitApply(
     // any sort of async function, we'll want to make sure to hop back to our
     // own executor afterward, since the callee could have made arbitrary hops
     // out of our isolation domain.
-    breadcrumb = ExecutorBreadcrumb(ExpectedExecutor);
+    breadcrumb = ExecutorBreadcrumb(true);
   }
 
   SILValue rawDirectResult;

lib/SILGen/SILGenFunction.h

@@ -864,6 +864,10 @@ class LLVM_LIBRARY_VISIBILITY SILGenFunction
   /// inside an async actor-independent function. No hop-back is expected.
   void emitHopToActorValue(SILLocation loc, ManagedValue actor);
 
+  /// Return true if the function being emitted is an async function
+  /// that unsafely inherits its executor.
+  bool unsafelyInheritsExecutor();
+
   /// A version of `emitHopToTargetActor` that is specialized to the needs
   /// of various types of ConstructorDecls, like class or value initializers,
   /// because their prolog emission is not the same as for regular functions.

lib/SILGen/SILGenProlog.cpp

@@ -653,7 +653,7 @@ void SILGenFunction::emitProlog(CaptureInfo captureInfo,
 
   // In async functions, the generic executor is our expected executor
   // if we don't have any sort of isolation.
-  if (!ExpectedExecutor && F.isAsync()) {
+  if (!ExpectedExecutor && F.isAsync() && !unsafelyInheritsExecutor()) {
     ExpectedExecutor = emitGenericExecutor(
       RegularLocation::getAutoGeneratedLocation(F.getLocation()));
   }
@@ -780,12 +780,8 @@ ExecutorBreadcrumb SILGenFunction::emitHopToTargetActor(SILLocation loc,
 
 ExecutorBreadcrumb SILGenFunction::emitHopToTargetExecutor(
     SILLocation loc, SILValue executor) {
-  // Record the previous executor to hop back to when we no longer need to
-  // be isolated to the target actor.
-  //
-  // If we're calling from an actor method ourselves, then we'll want to hop
-  // back to our own actor.
-  auto breadcrumb = ExecutorBreadcrumb(emitGetCurrentExecutor(loc));
+  // Record that we need to hop back to the current executor.
+  auto breadcrumb = ExecutorBreadcrumb(true);
   B.createHopToExecutor(loc.asAutoGenerated(), executor, /*mandatory*/ false);
   return breadcrumb;
 }
@@ -856,9 +852,19 @@ void SILGenFunction::emitPreconditionCheckExpectedExecutor(
                               SGFContext());
 }
 
+bool SILGenFunction::unsafelyInheritsExecutor() {
+  if (auto fn = dyn_cast<AbstractFunctionDecl>(FunctionDC))
+    return fn->getAttrs().hasAttribute<UnsafeInheritExecutorAttr>();
+  return false;
+}
+
 void ExecutorBreadcrumb::emit(SILGenFunction &SGF, SILLocation loc) {
-  if (Executor)
-    SGF.B.createHopToExecutor(loc.asAutoGenerated(), Executor, /*mandatory*/ false);
+  if (mustReturnToExecutor) {
+    assert(SGF.ExpectedExecutor || SGF.unsafelyInheritsExecutor());
+    if (auto executor = SGF.ExpectedExecutor)
+      SGF.B.createHopToExecutor(loc.asAutoGenerated(), executor,
+                                /*mandatory*/ false);
+  }
 }
 
 SILValue SILGenFunction::emitGetCurrentExecutor(SILLocation loc) {

lib/Sema/TypeCheckAttr.cpp

@@ -275,6 +275,8 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
 
   void visitUnavailableFromAsyncAttr(UnavailableFromAsyncAttr *attr);
 
+  void visitUnsafeInheritExecutorAttr(UnsafeInheritExecutorAttr *attr);
+
   void visitPrimaryAssociatedTypeAttr(PrimaryAssociatedTypeAttr *attr);
 
   void checkBackDeployAttrs(Decl *D, ArrayRef<BackDeployAttr *> Attrs);
@@ -5816,6 +5818,14 @@ void AttributeChecker::visitUnavailableFromAsyncAttr(
   }
 }
 
+void AttributeChecker::visitUnsafeInheritExecutorAttr(
+    UnsafeInheritExecutorAttr *attr) {
+  auto fn = cast<FuncDecl>(D);
+  if (!fn->isAsyncContext()) {
+    diagnose(attr->getLocation(), diag::inherits_executor_without_async);
+  }
+}
+
 void AttributeChecker::visitPrimaryAssociatedTypeAttr(
     PrimaryAssociatedTypeAttr *attr) {
 }