Auto merge of #27393 - alexcrichton:no-std-changes, r=brson

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

Closes #27394

6 files changed, 8 insertions, 2 deletions

diff --git a/src/libstd/sys/common/backtrace.rs b/src/libstd/sys/common/backtrace.rs
index 00932712a07..17953d0af4e 100644
--- a/src/libstd/sys/common/backtrace.rs
+++ b/src/libstd/sys/common/backtrace.rs
@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#[cfg(stage0)]
 use prelude::v1::*;
 use io::prelude::*;
 
diff --git a/src/libstd/sys/common/mod.rs b/src/libstd/sys/common/mod.rs
index 69c54f98917..b205b6df4cb 100644
--- a/src/libstd/sys/common/mod.rs
+++ b/src/libstd/sys/common/mod.rs
@@ -10,6 +10,7 @@
 
 #![allow(missing_docs)]
 
+#[cfg(stage0)]
 use prelude::v1::*;
 
 pub mod backtrace;
diff --git a/src/libstd/sys/common/poison.rs b/src/libstd/sys/common/poison.rs
index 065b1d6c9ac..196fe37d456 100644
--- a/src/libstd/sys/common/poison.rs
+++ b/src/libstd/sys/common/poison.rs
@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#[cfg(stage0)]
 use prelude::v1::*;
 
 use cell::Cell;
diff --git a/src/libstd/sys/common/thread_info.rs b/src/libstd/sys/common/thread_info.rs
index bb47c946e49..fb4e0ec70e0 100644
--- a/src/libstd/sys/common/thread_info.rs
+++ b/src/libstd/sys/common/thread_info.rs
@@ -10,7 +10,8 @@
 
 #![allow(dead_code)] // stack_guard isn't used right now on all platforms
 
-use core::prelude::*;
+#[cfg(stage0)]
+use core::prelude::v1::*;
 
 use cell::RefCell;
 use string::String;
diff --git a/src/libstd/sys/common/thread_local.rs b/src/libstd/sys/common/thread_local.rs
index 3b2cb00d8c4..2269a053874 100644
--- a/src/libstd/sys/common/thread_local.rs
+++ b/src/libstd/sys/common/thread_local.rs
@@ -58,6 +58,7 @@
 #![unstable(feature = "thread_local_internals")]
 #![allow(dead_code)] // sys isn't exported yet
 
+#[cfg(stage0)]
 use prelude::v1::*;
 
 use sync::atomic::{self, AtomicUsize, Ordering};
diff --git a/src/libstd/sys/common/wtf8.rs b/src/libstd/sys/common/wtf8.rs
index 3d5d1f5e0eb..0a5f4563dea 100644
--- a/src/libstd/sys/common/wtf8.rs
+++ b/src/libstd/sys/common/wtf8.rs
@@ -25,7 +25,8 @@
 // unix (it's mostly used on windows), so don't worry about dead code here.
 #![allow(dead_code)]
 
-use core::prelude::*;
+#[cfg(stage0)]
+use core::prelude::v1::*;
 
 use core::char::{encode_utf8_raw, encode_utf16_raw};
 use core::str::next_code_point;