Add warning about core_io

Cargo.toml

@@ -22,7 +22,7 @@ default = ["std"]
 std = ["libc"]
 box = [] # enable use of Box on not(std), requires alloc crate and feature
 vec = [] # enable use of Vec on not(std), requires collections crate and feature
-rdrand = []
+rdrand = [] # with not(std), requires core_io
 
 [dev-dependencies]
 log = "0.3.0"

src/lib.rs

@@ -253,6 +253,9 @@
 #[cfg(all(feature="box",not(feature="std")))] extern crate alloc;
 #[cfg(all(feature="vec",not(feature="std")))] extern crate collections;
 
+#[cfg(all(not(feature="std"),feature="rdrand",not(feature="core_io")))]
+use using::rdrand::feature::without::std::feature::requires::core_io::feature;
+
 #[cfg(all(feature="std",not(feature="rdrand")))] use core::cell::RefCell;
 use core::marker;
 use core::mem;