Rollup merge of rust-lang#35911 - tbu-:pr_io_errorkind_traits, r=alex…

…crichton

Implement more traits for `std::io::ErrorKind`

This makes it possible to use it as key in various maps.

src/libstd/io/error.rs

@@ -79,7 +79,7 @@ struct Custom {
 /// It is used with the [`io::Error`] type.
 ///
 /// [`io::Error`]: struct.Error.html
-#[derive(Copy, PartialEq, Eq, Clone, Debug)]
+#[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[allow(deprecated)]
 pub enum ErrorKind {
@@ -161,7 +161,8 @@ pub enum ErrorKind {
     #[stable(feature = "read_exact", since = "1.6.0")]
     UnexpectedEof,
 
-    /// Any I/O error not part of this list.
+    /// A marker variant that tells the compiler that users of this enum cannot
+    /// match it exhaustively.
     #[unstable(feature = "io_error_internals",
                reason = "better expressed through extensible enums that this \
                          enum cannot be exhaustively matched against",