A shareable atomically reference counted pointer wrapper.

Needs more tests to ensure safety, and probably some more work on usability too.

src/rt/rust_builtin.cpp

@@ -816,6 +816,16 @@ rust_compare_and_swap_ptr(intptr_t *address,
     return sync::compare_and_swap(address, oldval, newval);
 }
 
+extern "C" CDECL intptr_t
+rust_atomic_increment(intptr_t *address) {
+    return sync::increment(address);
+}
+
+extern "C" CDECL intptr_t
+rust_atomic_decrement(intptr_t *address) {
+    return sync::decrement(address);
+}
+
 extern "C" CDECL void
 rust_task_weaken(rust_port_id chan) {
     rust_task *task = rust_get_current_task();