IntoInnerError: Provide into_error

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

library/std/src/io/buffered/mod.rs

@@ -127,6 +127,27 @@ impl<W> IntoInnerError<W> {
         self.0
     }
 
+    /// Consumes the [`IntoInnerError`] and returns the error which caused the call to
+    /// [`BufWriter::into_inner()`] to fail.  Unlike `error`, this can be used to
+    /// obtain ownership of the underlying error.
+    ///
+    /// # Example
+    /// ```
+    /// #![feature(io_into_inner_error_parts)]
+    /// use std::io::{BufWriter, ErrorKind, Write};
+    ///
+    /// let mut not_enough_space = [0u8; 10];
+    /// let mut stream = BufWriter::new(not_enough_space.as_mut());
+    /// write!(stream, "this cannot be actually written").unwrap();
+    /// let into_inner_err = stream.into_inner().expect_err("now we discover it's too small");
+    /// let err = into_inner_err.into_error();
+    /// assert_eq!(err.kind(), ErrorKind::WriteZero);
+    /// ```
+    #[unstable(feature = "io_into_inner_error_parts", issue = "79704")]
+    pub fn into_error(self) -> Error {
+        self.1
+    }
+
     /// Consumes the [`IntoInnerError`] and returns the error which caused the call to
     /// [`BufWriter::into_inner()`] to fail, and the underlying writer.
     ///