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/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;