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, 4 insertions, 1 deletions

diff --git a/src/libstd/num/f32.rs b/src/libstd/num/f32.rs
index 73d6639cf00..64da75e94db 100644
--- a/src/libstd/num/f32.rs
+++ b/src/libstd/num/f32.rs
@@ -15,6 +15,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 #![allow(missing_docs)]
 
+#[cfg(stage0)]
 use prelude::v1::*;
 
 use core::num;
diff --git a/src/libstd/num/f64.rs b/src/libstd/num/f64.rs
index 3911d276b0f..fcba821522e 100644
--- a/src/libstd/num/f64.rs
+++ b/src/libstd/num/f64.rs
@@ -15,6 +15,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 #![allow(missing_docs)]
 
+#[cfg(stage0)]
 use prelude::v1::*;
 
 use core::num;
diff --git a/src/libstd/num/mod.rs b/src/libstd/num/mod.rs
index 9a52a0214e9..db7fd463e07 100644
--- a/src/libstd/num/mod.rs
+++ b/src/libstd/num/mod.rs
@@ -43,7 +43,8 @@ pub fn test_num<T>(ten: T, two: T) where
 
 #[cfg(test)]
 mod tests {
-    use core::prelude::*;
+    #[cfg(stage0)]
+    use core::prelude::v1::*;
     use super::*;
     use i8;
     use i16;