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, 0 deletions

diff --git a/src/libcollections/lib.rs b/src/libcollections/lib.rs
index 3abecc3fc86..ca7de131dc2 100644
--- a/src/libcollections/lib.rs
+++ b/src/libcollections/lib.rs
@@ -22,7 +22,9 @@
        html_playground_url = "http://play.rust-lang.org/")]
 
 #![feature(macro_rules, managed_boxes, default_type_params, phase, globs)]
+#![feature(unsafe_destructor)]
 #![no_std]
+#![allow(unknown_features)] // NOTE: remove after a stage0 snap
 
 #[phase(plugin, link)] extern crate core;
 extern crate alloc;