rollup merge of #20315: alexcrichton/std-sync

Conflicts:
	src/libstd/rt/exclusive.rs
	src/libstd/sync/barrier.rs
	src/libstd/sys/unix/pipe.rs
	src/test/bench/shootout-binarytrees.rs
	src/test/bench/shootout-fannkuch-redux.rs

1 files changed, 5 insertions, 2 deletions

diff --git a/src/libstd/sync/future.rs b/src/libstd/sync/future.rs
index e3620617d57..e5245251ea8 100644
--- a/src/libstd/sync/future.rs
+++ b/src/libstd/sync/future.rs
@@ -8,8 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-//! A type representing values that may be computed concurrently and operations for working with
-//! them.
+//! A type representing values that may be computed concurrently and operations
+//! for working with them.
 //!
 //! # Example
 //!
@@ -23,6 +23,9 @@
 //! ```
 
 #![allow(missing_docs)]
+#![unstable = "futures as-is have yet to be deeply reevaluated with recent \
+               core changes to Rust's synchronization story, and will likely \
+               become stable in the future but are unstable until that time"]
 
 use core::prelude::*;
 use core::mem::replace;