time: add #[track_caller] and panic docs to timeout_at() (#8077)

tokio/src/time/timeout.rs

@@ -142,16 +142,31 @@ where
 /// }
 /// # }
 /// ```
+///
+/// # Panics
+///
+/// This function panics if there is no current timer set.
+///
+/// It can be triggered when [`Builder::enable_time`] or
+/// [`Builder::enable_all`] are not included in the builder.
+///
+/// It can also panic whenever a timer is created outside of a
+/// Tokio runtime. That is why `rt.block_on(sleep(...))` will panic,
+/// since the function is executed outside of the runtime.
+/// Whereas `rt.block_on(async {sleep(...).await})` doesn't panic.
+/// And this is because wrapping the function on an async makes it lazy,
+/// and so gets executed inside the runtime successfully without
+/// panicking.
+///
+/// [`Builder::enable_time`]: crate::runtime::Builder::enable_time
+/// [`Builder::enable_all`]: crate::runtime::Builder::enable_all
+#[track_caller]
 pub fn timeout_at<F>(deadline: Instant, future: F) -> Timeout<F::IntoFuture>
 where
     F: IntoFuture,
 {
     let delay = sleep_until(deadline);
-
-    Timeout {
-        value: future.into_future(),
-        delay,
-    }
+    Timeout::new_with_delay(future.into_future(), delay)
 }
 
 pin_project! {

tokio/tests/time_panic.rs

@@ -6,7 +6,7 @@ use futures::future;
 use std::error::Error;
 use std::time::Duration;
 use tokio::runtime::{Builder, Runtime};
-use tokio::time::{self, interval, interval_at, timeout, Instant};
+use tokio::time::{self, interval, interval_at, timeout, timeout_at, Instant};
 
 mod support {
     pub mod panic;
@@ -131,6 +131,22 @@ fn timeout_panic_caller() -> Result<(), Box<dyn Error>> {
     Ok(())
 }
 
+#[test]
+fn timeout_at_panic_caller() -> Result<(), Box<dyn Error>> {
+    let panic_location_file = test_panic(|| {
+        // Runtime without `enable_time` so it has no current timer set.
+        let rt = Builder::new_current_thread().build().unwrap();
+        rt.block_on(async {
+            let _timeout = timeout_at(Instant::now(), future::pending::<()>());
+        });
+    });
+
+    // The panic location should be in this file
+    assert_eq!(&panic_location_file.unwrap(), file!());
+
+    Ok(())
+}
+
 fn current_thread() -> Runtime {
     tokio::runtime::Builder::new_current_thread()
         .enable_all()