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

3 files changed, 5 insertions, 2 deletions

diff --git a/src/libstd/io/cursor.rs b/src/libstd/io/cursor.rs
index 980ec51c926..8033d77ff6a 100644
--- a/src/libstd/io/cursor.rs
+++ b/src/libstd/io/cursor.rs
@@ -292,7 +292,8 @@ impl Write for Cursor<Vec<u8>> {
 
 #[cfg(test)]
 mod tests {
-    use core::prelude::*;
+    #[cfg(stage0)]
+    use core::prelude::v1::*;
 
     use io::prelude::*;
     use io::{Cursor, SeekFrom};
diff --git a/src/libstd/io/impls.rs b/src/libstd/io/impls.rs
index 67bc45d3b62..864870a5905 100644
--- a/src/libstd/io/impls.rs
+++ b/src/libstd/io/impls.rs
@@ -8,7 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use core::prelude::*;
+#[cfg(stage0)]
+use core::prelude::v1::*;
 
 use boxed::Box;
 use cmp;
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};