librustc: Put `#[unsafe_destructor]` behind a feature gate.

Closes #8142.

This is not the semantics we want long-term. You can continue to use
`#[unsafe_destructor]`, but you'll need to add
`#![feature(unsafe_destructor)]` to the crate attributes.

[breaking-change]

1 files changed, 2 insertions, 1 deletions

diff --git a/src/libsync/lib.rs b/src/libsync/lib.rs
index 1336ea48d31..a2f22bcaa68 100644
--- a/src/libsync/lib.rs
+++ b/src/libsync/lib.rs
@@ -26,10 +26,11 @@
        html_favicon_url = "http://www.rust-lang.org/favicon.ico",
        html_root_url = "http://doc.rust-lang.org/",
        html_playground_url = "http://play.rust-lang.org/")]
-#![feature(phase, globs, macro_rules)]
 
+#![feature(phase, globs, macro_rules, unsafe_destructor)]
 #![deny(missing_doc)]
 #![no_std]
+#![allow(unknown_features)] // NOTE: remove after a stage0 snap
 
 #[phase(plugin, link)] extern crate core;
 extern crate alloc;