Add warning about core_io

Cargo.toml

@@ -22,7 +22,7 @@ default = ["libc"]
 no_std = []
 box = [] # enable use of Box on no_std, requires alloc crate and feature
 vec = [] # enable use of Vec on no_std, requires collections crate and feature
-rdrand = []
+rdrand = [] # with no_std, requires core_io
 
 [dev-dependencies]
 log = "0.3.0"

src/lib.rs

@@ -253,6 +253,9 @@
 #[cfg(feature="box")] extern crate alloc;
 #[cfg(feature="vec")] extern crate collections;
 
+#[cfg(all(feature="no_std",feature="rdrand",not(feature="core_io")))]
+use using::rdrand::feature::with::no_std::feature::requires::core_io::feature;
+
 #[cfg(all(not(feature="no_std"),not(feature="rdrand")))] use core::cell::RefCell;
 use core::marker;
 use core::mem;