core: Allow using failure outside of libcore

Due to our excellent macro hygiene, this involves having a global path and a
hidden module in libcore itself.

src/libcore/failure.rs

@@ -10,7 +10,7 @@
 
 //! Failure support for libcore
 
-#![allow(dead_code)]
+#![allow(dead_code, missing_doc)]
 
 #[cfg(not(test))]
 use str::raw::c_str_to_static_slice;

src/libcore/lib.rs

@@ -106,6 +106,7 @@ pub mod atomics;
 pub mod bool;
 pub mod cell;
 pub mod char;
+pub mod failure;
 pub mod finally;
 pub mod iter;
 pub mod option;
@@ -118,13 +119,15 @@ pub mod tuple;
 #[cfg(stage0, not(test))]
 pub mod owned;
 
-mod failure;
-
 // FIXME: this module should not exist. Once owned allocations are no longer a
 //        language type, this module can move outside to the owned allocation
 //        crate.
 mod should_not_exist;
 
+mod core {
+    pub use failure;
+}
+
 mod std {
     pub use clone;
     pub use cmp;

src/libcore/macros.rs

@@ -17,7 +17,7 @@ macro_rules! fail(
         fail!("explicit failure")
     );
     ($msg:expr) => (
-        ::failure::begin_unwind($msg, file!(), line!())
+        ::core::failure::begin_unwind($msg, file!(), line!())
     );
 )