Merge pull request swiftlang#15139 from gottesmm/pr-400a9ca07f4710ed248281e364eb49e11835d7de

Runtime: Do the bare minimum balancing for bridge casting at +0.

Author: gottesmm

stdlib/public/runtime/Casting.cpp

@@ -2823,8 +2823,7 @@ static bool _dynamicCastClassToValueViaObjCBridgeable(
     return _fail(src, srcType, targetType, flags);
   }
 
-  // Unless we're always supposed to consume the input, retain the
-  // object because the witness takes it at +1.
+  // TODO: Avoid the retain here in +0 mode if the input is never consumed.
   bool alwaysConsumeSrc = (flags & DynamicCastFlags::TakeOnSuccess) &&
                           (flags & DynamicCastFlags::DestroyOnFailure);
   if (!alwaysConsumeSrc) {
@@ -2880,15 +2879,14 @@ static bool _dynamicCastClassToValueViaObjCBridgeable(
                 (HeapObject *)srcObject, (OpaqueValue *)optDestBuffer,
                 targetType, targetType, targetBridgeWitness);
   }
+  SWIFT_CC_PLUSZERO_GUARD(swift_unknownRelease(srcObject));
 
   // If we succeeded, take from the optional buffer into the
   // destination buffer.
   if (success) {
     targetType->vw_initializeWithTake(dest, (OpaqueValue *)optDestBuffer);
   }
 
-  // Unless we're always supposed to consume the input, release the
-  // input if we need to now.
   if (!alwaysConsumeSrc && shouldDeallocateSource(success, flags)) {
     swift_unknownRelease(srcObject);
   }