syntax: Implement #![no_core]

This commit is an implementation of [RFC 1184][rfc] which tweaks the behavior of
the `#![no_std]` attribute and adds a new `#![no_core]` attribute. The
`#![no_std]` attribute now injects `extern crate core` at the top of the crate
as well as the libcore prelude into all modules (in the same manner as the
standard library's prelude). The `#![no_core]` attribute disables both std and
core injection.

[rfc]: https://github.com/rust-lang/rfcs/pull/1184

1 files changed, 1 insertions, 0 deletions

diff --git a/src/libstd/io/util.rs b/src/libstd/io/util.rs
index 6e651464c74..38e4b0d03e7 100644
--- a/src/libstd/io/util.rs
+++ b/src/libstd/io/util.rs
@@ -10,6 +10,7 @@
 
 #![allow(missing_copy_implementations)]
 
+#[cfg(stage0)]
 use prelude::v1::*;
 
 use io::{self, Read, Write, ErrorKind, BufRead};