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]

src/librustc/front/feature_gate.rs

@@ -50,6 +50,7 @@ static KNOWN_FEATURES: &'static [(&'static str, Status)] = &[
     ("log_syntax", Active),
     ("trace_macros", Active),
     ("concat_idents", Active),
+    ("unsafe_destructor", Active),
 
     ("simd", Active),
     ("default_type_params", Active),
@@ -220,6 +221,17 @@ impl<'a> Visitor<()> for Context<'a> {
                 }
             }
 
+            ast::ItemImpl(..) => {
+                if attr::contains_name(i.attrs.as_slice(),
+                                       "unsafe_destructor") {
+                    self.gate_feature("unsafe_destructor",
+                                      i.span,
+                                      "`#[unsafe_destructor]` allows too \
+                                       many unsafe patterns and may be \
+                                       removed in the future");
+                }
+            }
+
             _ => {}
         }
 

src/librustc/lib.rs

@@ -29,8 +29,9 @@ This API is completely unstable and subject to change.
       html_root_url = "http://doc.rust-lang.org/")]
 
 #![allow(deprecated)]
-#![feature(macro_rules, globs, struct_variant, managed_boxes, quote,
-           default_type_params, phase)]
+#![allow(unknown_features)] // NOTE: remove after a stage0 snap
+#![feature(macro_rules, globs, struct_variant, managed_boxes, quote)]
+#![feature(default_type_params, phase, unsafe_destructor)]
 
 extern crate arena;
 extern crate debug;