rollup merge of #22459: alexcrichton/feature-names

Conflicts:
	src/rustbook/main.rs

src/libstd/old_io/mod.rs

@@ -238,7 +238,7 @@
 //! concerned with error handling; instead its caller is responsible for
 //! responding to errors that may occur while attempting to read the numbers.
 
-#![unstable(feature = "io")]
+#![unstable(feature = "old_io")]
 #![deny(unused_must_use)]
 
 pub use self::SeekStyle::*;