fix export

src/utils.rs

@@ -64,6 +64,7 @@ mod timer {
     pub type Timer = smol::Timer;
 }
 
+#[cfg(any(feature = "unstable", feature = "default"))]
 pub(crate) fn timer_after(dur: std::time::Duration) -> timer::Timer {
     #[cfg(not(target_os = "unknown"))]
     once_cell::sync::Lazy::force(&crate::rt::RUNTIME);