gc:arc: support GC_ref/unref for ref T

lib/core/runtime_v2.nim

@@ -28,8 +28,8 @@ hash of ``package & "." & module & "." & name`` to save space.
 type
   RefHeader = object
     rc: int # the object header is now a single RC field.
-            # we could remove it in non-debug builds but this seems
-            # unwise.
+            # we could remove it in non-debug builds for the 'owned ref'
+            # design but this seems unwise.
 
 template `+!`(p: pointer, s: int): pointer =
   cast[pointer](cast[int](p) +% s)
@@ -121,6 +121,17 @@ proc nimDecRefIsLast(p: pointer): bool {.compilerRtl, inl.} =
       else:
         dec head(p).rc
 
+proc GC_unref*[T](x: ref T) =
+  ## New runtime only supports this operation for 'ref T'.
+  if nimDecRefIsLast(cast[pointer](x)):
+    # XXX this does NOT work for virtual destructors!
+    `=destroy`(x[])
+    nimRawDispose(cast[pointer](x))
+
+proc GC_ref*[T](x: ref T) =
+  ## New runtime only supports this operation for 'ref T'.
+  if x != nil: nimIncRef(cast[pointer](x))
+
 proc isObj(obj: PNimType, subclass: cstring): bool {.compilerRtl, inl.} =
   proc strstr(s, sub: cstring): cstring {.header: "<string.h>", importc.}