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/sys/unix/backtrace.rs b/src/libstd/sys/unix/backtrace.rs
index ed6421f3670..ae8bfb07aaf 100644
--- a/src/libstd/sys/unix/backtrace.rs
+++ b/src/libstd/sys/unix/backtrace.rs
@@ -83,6 +83,7 @@
 /// to symbols. This is a bit of a hokey implementation as-is, but it works for
 /// all unix platforms we support right now, so it at least gets the job done.
 
+#[cfg(stage0)]
 use prelude::v1::*;
 use io::prelude::*;