Rollup merge of #85146 - ijackson:seek-rewind, r=m-ou-se

Provide io::Seek::rewind

Using `Seek::seek` is slightly clumsy because of the need to write (or import) `std::io::SeekFrom` to get at `SeekStart`.  C already has `rewind` (although with broken error handling); we should have it too.

I'm motivated to do this because I've just found myself copy-pasting my 5-line extension trait between projects.

That the example ends up using `OpenOptions` makes this look like a niche use case, but it is very common to rewind temporary files.  `tempfile` isn't available for use in this example or it would have looked shorter and more natural.

If this gets a positive reception I will open a tracking issue and update the feature gate.

library/std/src/io/mod.rs

@@ -1663,10 +1663,47 @@ pub trait Seek {
     ///
     /// # Errors
     ///
+    /// Seeking can fail, for example becaue it might involve flushing a buffer.
+    ///
     /// Seeking to a negative offset is considered an error.
     #[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 = "85149")]
+    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