Added box_syntax feature gate; added to std and rustc crates for bootstrap.

To avoid using the feauture, change uses of `box <expr>` to
`Box::new(<expr>)` alternative, as noted by the feature gate message.

(Note that box patterns have no analogous trivial replacement, at
least not in general; you need to revise the code to do a partial
match, deref, and then the rest of the match.)

[breaking-change]

src/liballoc/lib.rs

@@ -66,6 +66,7 @@
 #![no_std]
 #![allow(unknown_features)]
 #![feature(lang_items, unsafe_destructor)]
+#![feature(box_syntax)]
 
 #[macro_use]
 extern crate core;

src/libcollections/lib.rs

@@ -24,6 +24,7 @@
 #![allow(unknown_features)]
 #![feature(unsafe_destructor, slicing_syntax)]
 #![feature(old_impl_check)]
+#![feature(box_syntax)]
 #![feature(unboxed_closures)]
 #![no_std]
 

src/liblog/lib.rs

@@ -164,6 +164,7 @@
        html_root_url = "http://doc.rust-lang.org/nightly/",
        html_playground_url = "http://play.rust-lang.org/")]
 #![feature(slicing_syntax)]
+#![feature(box_syntax)]
 #![deny(missing_docs)]
 
 extern crate regex;

src/libregex/lib.rs

@@ -24,6 +24,7 @@
 
 #![allow(unknown_features)]
 #![feature(slicing_syntax)]
+#![feature(box_syntax)]
 #![deny(missing_docs)]
 
 #[cfg(test)]

src/librustc/lib.rs

@@ -25,6 +25,7 @@
 #![allow(unknown_features)]
 #![feature(quote)]
 #![feature(slicing_syntax, unsafe_destructor)]
+#![feature(box_syntax)]
 #![feature(rustc_diagnostic_macros)]
 #![feature(old_impl_check)]
 

src/librustc_trans/lib.rs

@@ -24,6 +24,7 @@
 
 #![feature(quote)]
 #![feature(slicing_syntax, unsafe_destructor)]
+#![feature(box_syntax)]
 #![feature(rustc_diagnostic_macros)]
 
 extern crate arena;

src/librustc_typeck/lib.rs

@@ -73,6 +73,7 @@ This API is completely unstable and subject to change.
 
 #![feature(quote)]
 #![feature(slicing_syntax, unsafe_destructor)]
+#![feature(box_syntax)]
 #![feature(rustc_diagnostic_macros)]
 #![allow(non_camel_case_types)]
 

src/librustdoc/lib.rs

@@ -17,6 +17,7 @@
        html_root_url = "http://doc.rust-lang.org/nightly/",
        html_playground_url = "http://play.rust-lang.org/")]
 #![feature(slicing_syntax)]
+#![feature(box_syntax)]
 
 extern crate arena;
 extern crate getopts;

src/libserialize/lib.rs

@@ -24,6 +24,7 @@ Core encoding and decoding interfaces.
        html_playground_url = "http://play.rust-lang.org/")]
 #![allow(unknown_features)]
 #![feature(slicing_syntax)]
+#![feature(box_syntax)]
 #![feature(old_impl_check)]
 #![cfg_attr(stage0, allow(unused_attributes))]
 

src/libstd/lib.rs

@@ -107,6 +107,7 @@
 #![feature(linkage, thread_local, asm)]
 #![feature(lang_items, unsafe_destructor)]
 #![feature(slicing_syntax, unboxed_closures)]
+#![feature(box_syntax)]
 #![feature(old_impl_check)]
 #![cfg_attr(stage0, allow(unused_attributes))]
 

src/libsyntax/feature_gate.rs

@@ -70,6 +70,7 @@ static KNOWN_FEATURES: &'static [(&'static str, Status)] = &[
     ("associated_types", Accepted),
     ("visible_private_types", Active),
     ("slicing_syntax", Active),
+    ("box_syntax", Active),
 
     ("if_let", Accepted),
     ("while_let", Accepted),
@@ -343,6 +344,12 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                                   e.span,
                                   "range syntax is experimental");
             }
+            ast::ExprBox(..) | ast::ExprUnary(ast::UnOp::UnUniq, _) => {
+                self.gate_feature("box_syntax",
+                                  e.span,
+                                  "box expression syntax is experimental in alpha release; \
+                                   you can call `Box::new` instead.");
+            }
             _ => {}
         }
         visit::walk_expr(self, e);
@@ -365,6 +372,11 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                                    but at the end of a slice (e.g. \
                                    `[0, ..xs, 0]` are experimental")
             }
+            ast::PatBox(..) => {
+                self.gate_feature("box_syntax",
+                                  pattern.span,
+                                  "box pattern syntax is experimental in alpha release");
+            }
             _ => {}
         }
         visit::walk_pat(self, pattern)

src/libsyntax/lib.rs

@@ -24,6 +24,7 @@
 
 #![allow(unknown_features)]
 #![feature(slicing_syntax)]
+#![feature(box_syntax)]
 #![feature(quote, unsafe_destructor)]
 
 extern crate arena;

src/libterm/lib.rs

@@ -49,6 +49,7 @@
 
 #![allow(unknown_features)]
 #![feature(slicing_syntax)]
+#![feature(box_syntax)]
 #![deny(missing_docs)]
 
 #[macro_use] extern crate log;

src/libtest/lib.rs

@@ -31,6 +31,7 @@
        html_favicon_url = "http://www.rust-lang.org/favicon.ico",
        html_root_url = "http://doc.rust-lang.org/nightly/")]
 #![feature(asm, slicing_syntax)]
+#![feature(box_syntax)]
 
 extern crate getopts;
 extern crate regex;