[concurrency] Add feature NonIsolatedAsyncInheritsIsolationFromContext.

rdar://125193256

Author: gottesmm

include/swift/Basic/Features.def

@@ -430,14 +430,18 @@ EXPERIMENTAL_FEATURE(CoroutineAccessorsUnwindOnCallerError, false)
 /// modify/read coroutines use the callee-allocated ABI
 EXPERIMENTAL_FEATURE(CoroutineAccessorsAllocateInCallee, false)
 
-// When a parameter has unspecified isolation, infer it as main actor isolated.
+/// When a parameter has unspecified isolation, infer it as main actor isolated.
 EXPERIMENTAL_FEATURE(GenerateForceToMainActorThunks, false)
 
 EXPERIMENTAL_FEATURE(AddressableParameters, true)
 
 /// Allow the @abi attribute.
 SUPPRESSIBLE_EXPERIMENTAL_FEATURE(ABIAttribute, true)
 
+/// Functions with nonisolated isolation inherit their isolation from the
+/// calling context.
+EXPERIMENTAL_FEATURE(NonIsolatedAsyncInheritsIsolationFromContext, false)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/FeatureSet.cpp

@@ -120,6 +120,7 @@ UNINTERESTING_FEATURE(Volatile)
 UNINTERESTING_FEATURE(SuppressedAssociatedTypes)
 UNINTERESTING_FEATURE(StructLetDestructuring)
 UNINTERESTING_FEATURE(MacrosOnImports)
+UNINTERESTING_FEATURE(NonIsolatedAsyncInheritsIsolationFromContext)
 
 static bool usesFeatureNonescapableTypes(Decl *decl) {
   auto containsNonEscapable =