Avoid using AsyncIteratorProtocol.next(isolation:) with @_unsafeInheritExecutor

`#isolation` in unsafe within a function that uses
`@_unsafeInheritExecutor`, so avoid the implicit use of the new
`AsyncIteratorProtocol.next(isolation:)` in such functions.

Author: DougGregor

lib/Sema/TypeCheckConcurrency.cpp

@@ -1999,8 +1999,7 @@ bool swift::isAsyncDecl(ConcreteDeclRef declRef) {
   return false;
 }
 
-/// Find an enclosing function that has @
-static AbstractFunctionDecl *enclosingUnsafeInheritsExecutor(
+AbstractFunctionDecl *swift::enclosingUnsafeInheritsExecutor(
     const DeclContext *dc) {
   for (; dc; dc = dc->getParent()) {
     if (auto func = dyn_cast<AbstractFunctionDecl>(dc)) {

lib/Sema/TypeCheckConcurrency.h

@@ -648,6 +648,9 @@ bool isPotentiallyIsolatedActor(
 bool diagnoseApplyArgSendability(
     swift::ApplyExpr *apply, const DeclContext *declContext);
 
+/// If the enclosing function has @_unsafeInheritExecutorAttr, return it.
+AbstractFunctionDecl *enclosingUnsafeInheritsExecutor(const DeclContext *dc);
+
 } // end namespace swift
 
 namespace llvm {

lib/Sema/TypeCheckStmt.cpp

@@ -3289,6 +3289,11 @@ FuncDecl *TypeChecker::getForEachIteratorNextFunction(
   if (!nextElement)
     return ctx.getAsyncIteratorNext();
 
+  // If the enclosing function has @_unsafeInheritsExecutor, then #isolation
+  // does not work and we need to avoid relying on it.
+  if (enclosingUnsafeInheritsExecutor(dc))
+    return ctx.getAsyncIteratorNext();
+
   // If availability checking is disabled, use next(_:).
   if (ctx.LangOpts.DisableAvailabilityChecking || loc.isInvalid())
     return nextElement;

test/Concurrency/unsafe_inherit_executor.swift

@@ -63,7 +63,6 @@ func unsafeCallerC(x: Int, fn: () -> Void, fn2: () -> Void) async {
 }
 
 @_unsafeInheritExecutor
-func unsafeCallerB(x: some AsyncSequence<Int, Never>) async {
-  for await _ in x { }
-  // expected-error@-1 2{{#isolation cannot be used within an `@_unsafeInheritExecutor` function}}
+func unsafeCallerAvoidsNewLoop(x: some AsyncSequence<Int, Never>) async throws {
+  for try await _ in x { }
 }