io: add convenience method AsyncSeekExt::rewind. (#4107)

tokio/src/io/util/async_seek_ext.rs

@@ -67,6 +67,16 @@ cfg_io_util! {
             seek(self, pos)
         }
 
+        /// Creates a future which will rewind to the beginning of the stream.
+        ///
+        /// This is convenience method, equivalent to to `self.seek(SeekFrom::Start(0))`.
+        fn rewind(&mut self) -> Seek<'_, Self>
+        where
+            Self: Unpin,
+        {
+            self.seek(SeekFrom::Start(0))
+        }
+
         /// Creates a future which will return the current seek position from the
         /// start of the stream.
         ///

tokio/tests/fs_file.rs

@@ -1,12 +1,11 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "full")]
 
-use tokio::fs::File;
-use tokio::io::{AsyncReadExt, AsyncSeekExt, AsyncWriteExt};
-use tokio_test::task;
-
 use std::io::prelude::*;
 use tempfile::NamedTempFile;
+use tokio::fs::File;
+use tokio::io::{AsyncReadExt, AsyncSeekExt, AsyncWriteExt, SeekFrom};
+use tokio_test::task;
 
 const HELLO: &[u8] = b"hello world...";
 
@@ -50,6 +49,19 @@ async fn basic_write_and_shutdown() {
     assert_eq!(file, HELLO);
 }
 
+#[tokio::test]
+async fn rewind_seek_position() {
+    let tempfile = tempfile();
+
+    let mut file = File::create(tempfile.path()).await.unwrap();
+
+    file.seek(SeekFrom::Current(10)).await.unwrap();
+
+    file.rewind().await.unwrap();
+
+    assert_eq!(file.stream_position().await.unwrap(), 0);
+}
+
 #[tokio::test]
 async fn coop() {
     let mut tempfile = tempfile();