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

2 files changed, 5 insertions, 2 deletions

diff --git a/src/libstd/rt/args.rs b/src/libstd/rt/args.rs
index 52697f00264..e77a2bbd0b9 100644
--- a/src/libstd/rt/args.rs
+++ b/src/libstd/rt/args.rs
@@ -19,7 +19,8 @@
 //!
 //! FIXME #7756: Would be nice for this to not exist.
 
-use core::prelude::*;
+#[cfg(stage0)]
+use core::prelude::v1::*;
 use vec::Vec;
 
 /// One-time global initialization.
@@ -140,7 +141,8 @@ mod imp {
           target_os = "ios",
           target_os = "windows"))]
 mod imp {
-    use core::prelude::*;
+    #[cfg(stage0)]
+    use core::prelude::v1::*;
     use vec::Vec;
 
     pub unsafe fn init(_argc: isize, _argv: *const *const u8) {
diff --git a/src/libstd/rt/backtrace.rs b/src/libstd/rt/backtrace.rs
index 2eadf36a6b4..18d93cba8bc 100644
--- a/src/libstd/rt/backtrace.rs
+++ b/src/libstd/rt/backtrace.rs
@@ -12,6 +12,7 @@
 
 #![allow(non_camel_case_types)]
 
+#[cfg(stage0)]
 use prelude::v1::*;
 
 use env;