io: implement AsyncSeek for Empty (#6663)

* io: implement `AsyncSeek` for `Empty`

* io: add more tests for seeking `Empty`

This adds more tests seeking with other kinds of `SeekFrom`. It follows
the same structure as used in
rust-lang/rust@f1cd17961c.

See <https://github.com/tokio-rs/tokio/pull/6663#discussion_r1659491016>.

* io: add inline attribute to `AsyncSeek` implementations

This follows the style of the other trait implementations of `Empty`.

See <https://github.com/tokio-rs/tokio/pull/6663#discussion_r1658835942>.

---------

Co-authored-by: Weijia Jiang <wjjiang19@gmail.com>

tokio/src/io/util/empty.rs

@@ -1,8 +1,8 @@
 use crate::io::util::poll_proceed_and_make_progress;
-use crate::io::{AsyncBufRead, AsyncRead, AsyncWrite, ReadBuf};
+use crate::io::{AsyncBufRead, AsyncRead, AsyncSeek, AsyncWrite, ReadBuf};
 
 use std::fmt;
-use std::io;
+use std::io::{self, SeekFrom};
 use std::pin::Pin;
 use std::task::{Context, Poll};
 
@@ -133,6 +133,20 @@ impl AsyncWrite for Empty {
     }
 }
 
+impl AsyncSeek for Empty {
+    #[inline]
+    fn start_seek(self: Pin<&mut Self>, _position: SeekFrom) -> io::Result<()> {
+        Ok(())
+    }
+
+    #[inline]
+    fn poll_complete(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>> {
+        ready!(crate::trace::trace_leaf(cx));
+        ready!(poll_proceed_and_make_progress(cx));
+        Poll::Ready(Ok(0))
+    }
+}
+
 impl fmt::Debug for Empty {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.pad("Empty { .. }")

tokio/tests/io_util_empty.rs

@@ -1,5 +1,5 @@
 #![cfg(feature = "full")]
-use tokio::io::{AsyncBufReadExt, AsyncReadExt};
+use tokio::io::{AsyncBufReadExt, AsyncReadExt, AsyncSeekExt};
 
 #[tokio::test]
 async fn empty_read_is_cooperative() {
@@ -30,3 +30,32 @@ async fn empty_buf_reads_are_cooperative() {
         _ = tokio::task::yield_now() => {}
     }
 }
+
+#[tokio::test]
+async fn empty_seek() {
+    use std::io::SeekFrom;
+
+    let mut empty = tokio::io::empty();
+
+    assert!(matches!(empty.seek(SeekFrom::Start(0)).await, Ok(0)));
+    assert!(matches!(empty.seek(SeekFrom::Start(1)).await, Ok(0)));
+    assert!(matches!(empty.seek(SeekFrom::Start(u64::MAX)).await, Ok(0)));
+
+    assert!(matches!(empty.seek(SeekFrom::End(i64::MIN)).await, Ok(0)));
+    assert!(matches!(empty.seek(SeekFrom::End(-1)).await, Ok(0)));
+    assert!(matches!(empty.seek(SeekFrom::End(0)).await, Ok(0)));
+    assert!(matches!(empty.seek(SeekFrom::End(1)).await, Ok(0)));
+    assert!(matches!(empty.seek(SeekFrom::End(i64::MAX)).await, Ok(0)));
+
+    assert!(matches!(
+        empty.seek(SeekFrom::Current(i64::MIN)).await,
+        Ok(0)
+    ));
+    assert!(matches!(empty.seek(SeekFrom::Current(-1)).await, Ok(0)));
+    assert!(matches!(empty.seek(SeekFrom::Current(0)).await, Ok(0)));
+    assert!(matches!(empty.seek(SeekFrom::Current(1)).await, Ok(0)));
+    assert!(matches!(
+        empty.seek(SeekFrom::Current(i64::MAX)).await,
+        Ok(0)
+    ));
+}