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, 3 insertions, 0 deletions

diff --git a/src/libarena/lib.rs b/src/libarena/lib.rs
index 86babb7a3d2..5ddb088c8c8 100644
--- a/src/libarena/lib.rs
+++ b/src/libarena/lib.rs
@@ -27,7 +27,10 @@
 #![doc(html_logo_url = "http://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
        html_favicon_url = "http://www.rust-lang.org/favicon.ico",
        html_root_url = "http://doc.rust-lang.org/")]
+
+#![feature(unsafe_destructor)]
 #![allow(missing_doc)]
+#![allow(unknown_features)] // NOTE: remove after a stage0 snap
 
 use std::cell::{Cell, RefCell};
 use std::cmp;