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]

Author: pcwalton

src/doc/guide-unsafe.md

@@ -192,6 +192,8 @@ As an example, we give a reimplementation of owned boxes by wrapping
 reimplementation is as safe as the `Box` type.
 
 ```
+#![feature(unsafe_destructor)]
+
 extern crate libc;
 use libc::{c_void, size_t, malloc, free};
 use std::mem;
@@ -242,10 +244,12 @@ impl<T: Send> Unique<T> {
 // A key ingredient for safety, we associate a destructor with
 // Unique<T>, making the struct manage the raw pointer: when the
 // struct goes out of scope, it will automatically free the raw pointer.
+//
 // NB: This is an unsafe destructor, because rustc will not normally
-// allow destructors to be associated with parametrized types, due to
+// allow destructors to be associated with parameterized types, due to
 // bad interaction with managed boxes. (With the Send restriction,
-// we don't have this problem.)
+// we don't have this problem.) Note that the `#[unsafe_destructor]`
+// feature gate is required to use unsafe destructors.
 #[unsafe_destructor]
 impl<T: Send> Drop for Unique<T> {
     fn drop(&mut self) {

src/doc/rust.md

@@ -1940,12 +1940,13 @@ interpreted:
   enum representation in C is undefined, and this may be incorrect when the C
   code is compiled with certain flags.
 - `simd` - on certain tuple structs, derive the arithmetic operators, which
-  lower to the target's SIMD instructions, if any.
+  lower to the target's SIMD instructions, if any; the `simd` feature gate
+  is necessary to use this attribute.
 - `static_assert` - on statics whose type is `bool`, terminates compilation
   with an error if it is not initialized to `true`.
 - `unsafe_destructor` - allow implementations of the "drop" language item
   where the type it is implemented for does not implement the "send" language
-  item.
+  item; the `unsafe_destructor` feature gate is needed to use this attribute
 - `unsafe_no_drop_flag` - on structs, remove the flag that prevents
   destructors from being run twice. Destructors might be run multiple times on
   the same object with this attribute.

src/liballoc/lib.rs

@@ -69,7 +69,8 @@
        html_root_url = "http://doc.rust-lang.org/")]
 
 #![no_std]
-#![feature(phase)]
+#![feature(phase, unsafe_destructor)]
+#![allow(unknown_features)] // NOTE: remove after a stage0 snap
 
 #[phase(plugin, link)]
 extern crate core;

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;

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;

src/libcore/lib.rs

@@ -55,8 +55,9 @@
        html_playground_url = "http://play.rust-lang.org/")]
 
 #![no_std]
-#![feature(globs, macro_rules, managed_boxes, phase, simd)]
+#![feature(globs, macro_rules, managed_boxes, phase, simd, unsafe_destructor)]
 #![deny(missing_doc)]
+#![allow(unknown_features)] // NOTE: remove after a stage0 snap
 
 #[cfg(test)] extern crate realcore = "core";
 #[cfg(test)] extern crate libc;

src/libnative/lib.rs

@@ -52,15 +52,17 @@
 #![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/")]
+
 #![deny(unused_result, unused_must_use)]
 #![allow(non_camel_case_types)]
 #![allow(deprecated)]
+#![allow(unknown_features)] // NOTE: remove after a stage0 snap
 #![feature(default_type_params)]
 
 // NB this crate explicitly does *not* allow glob imports, please seriously
 //    consider whether they're needed before adding that feature here (the
 //    answer is that you don't need them)
-#![feature(macro_rules)]
+#![feature(macro_rules, unsafe_destructor)]
 
 extern crate alloc;
 extern crate libc;

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;

src/librustrt/lib.rs

@@ -15,10 +15,12 @@
 #![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(macro_rules, phase, globs, thread_local, managed_boxes, asm,
-           linkage)]
+
+#![feature(macro_rules, phase, globs, thread_local, managed_boxes, asm)]
+#![feature(linkage, unsafe_destructor)]
 #![no_std]
 #![experimental]
+#![allow(unknown_features)] // NOTE: remove after a stage0 snap
 
 #[phase(plugin, link)] extern crate core;
 extern crate alloc;