io::Seek: Provide rewind()

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

library/std/src/io/mod.rs

@@ -1669,6 +1669,41 @@ pub trait Seek {
     #[stable(feature = "rust1", since = "1.0.0")]
     fn seek(&mut self, pos: SeekFrom) -> Result<u64>;
 
+    /// Rewind to the beginning of a stream.
+    ///
+    /// This is a convenience method, equivalent to `seek(SeekFrom::Start(0)`.
+    ///
+    /// # Errors
+    ///
+    /// Rewinding can fail, for example becaue it might involve flushing a buffer.
+    ///
+    /// # Example
+    ///
+    /// ```no_run
+    /// #![feature(seek_rewind)]
+    /// use std::io::{Read, Seek, Write};
+    /// use std::fs::OpenOptions;
+    ///
+    /// let mut f = OpenOptions::new()
+    ///     .write(true)
+    ///     .read(true)
+    ///     .create(true)
+    ///     .open("foo.txt").unwrap();
+    ///
+    /// let hello = "Hello!\n";
+    /// write!(f, "{}", hello).unwrap();
+    /// f.rewind().unwrap();
+    ///
+    /// let mut buf = String::new();
+    /// f.read_to_string(&mut buf).unwrap();
+    /// assert_eq!(&buf, hello);
+    /// ```
+    #[unstable(feature = "seek_rewind", issue = "none")]
+    fn rewind(&mut self) -> Result<()> {
+        self.seek(SeekFrom::Start(0))?;
+        Ok(())
+    }
+
     /// Returns the length of this stream (in bytes).
     ///
     /// This method is implemented using up to three seek operations. If this