Auto merge of #22175 - pnkfelix:featuregate-boxpat-rfc469, r=nikomatsakis

Feature gate `box` patterns.

Note that this adds a new feature gate, `box_patterns` specific to e.g. `let box i = ...`, while leaving  `box` expressions (alone) still guarded via the preexisting `box_syntax`.

Author: bors

src/doc/reference.md

@@ -3196,6 +3196,7 @@ stands for a *single* data field, whereas a wildcard `..` stands for *all* the
 fields of a particular variant. For example:
 
 ```
+#![feature(box_patterns)]
 #![feature(box_syntax)]
 enum List<X> { Nil, Cons(X, Box<List<X>>) }
 
@@ -3259,6 +3260,7 @@ the inside of the match.
 An example of a `match` expression:
 
 ```
+#![feature(box_patterns)]
 #![feature(box_syntax)]
 # fn process_pair(a: i32, b: i32) { }
 # fn process_ten() { }
@@ -3294,6 +3296,7 @@ Subpatterns can also be bound to variables by the use of the syntax `variable @
 subpattern`. For example:
 
 ```
+#![feature(box_patterns)]
 #![feature(box_syntax)]
 
 enum List { Nil, Cons(uint, Box<List>) }

src/libcollections/lib.rs

@@ -24,6 +24,7 @@
 
 #![feature(alloc)]
 #![feature(box_syntax)]
+#![feature(box_patterns)]
 #![feature(core)]
 #![feature(hash)]
 #![feature(staged_api)]

src/librustc/lib.rs

@@ -23,6 +23,7 @@
       html_favicon_url = "http://www.rust-lang.org/favicon.ico",
       html_root_url = "http://doc.rust-lang.org/nightly/")]
 
+#![feature(box_patterns)]
 #![feature(box_syntax)]
 #![feature(collections)]
 #![feature(core)]

src/librustc_trans/lib.rs

@@ -24,6 +24,7 @@
       html_root_url = "http://doc.rust-lang.org/nightly/")]
 
 #![feature(alloc)]
+#![feature(box_patterns)]
 #![feature(box_syntax)]
 #![feature(collections)]
 #![feature(core)]

src/librustc_typeck/lib.rs

@@ -74,6 +74,7 @@ This API is completely unstable and subject to change.
 
 #![allow(non_camel_case_types)]
 
+#![feature(box_patterns)]
 #![feature(box_syntax)]
 #![feature(collections)]
 #![feature(core)]

src/librustdoc/lib.rs

@@ -18,6 +18,7 @@
        html_root_url = "http://doc.rust-lang.org/nightly/",
        html_playground_url = "http://play.rust-lang.org/")]
 
+#![feature(box_patterns)]
 #![feature(box_syntax)]
 #![feature(collections)]
 #![feature(core)]

src/libsyntax/feature_gate.rs

@@ -126,6 +126,9 @@ static KNOWN_FEATURES: &'static [(&'static str, &'static str, Status)] = &[
 
     // Allows using #![no_std]
     ("no_std", "1.0.0", Active),
+
+    // Allows using `box` in patterns; RFC 469
+    ("box_patterns", "1.0.0", Active),
 ];
 
 enum Status {
@@ -427,7 +430,7 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
             ast::ExprBox(..) | ast::ExprUnary(ast::UnOp::UnUniq, _) => {
                 self.gate_feature("box_syntax",
                                   e.span,
-                                  "box expression syntax is experimental in alpha release; \
+                                  "box expression syntax is experimental; \
                                    you can call `Box::new` instead.");
             }
             ast::ExprLit(ref lit) => {
@@ -486,9 +489,9 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                                    `[0, ..xs, 0]` are experimental")
             }
             ast::PatBox(..) => {
-                self.gate_feature("box_syntax",
+                self.gate_feature("box_patterns",
                                   pattern.span,
-                                  "box pattern syntax is experimental in alpha release");
+                                  "box pattern syntax is experimental");
             }
             _ => {}
         }

src/libsyntax/lib.rs

@@ -23,6 +23,7 @@
        html_favicon_url = "http://www.rust-lang.org/favicon.ico",
        html_root_url = "http://doc.rust-lang.org/nightly/")]
 
+#![feature(box_patterns)]
 #![feature(box_syntax)]
 #![feature(collections)]
 #![feature(core)]

src/test/compile-fail/borrowck-loan-in-overloaded-op.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(box_patterns)]
 #![feature(box_syntax)]
 
 use std::ops::Add;

src/test/compile-fail/borrowck-vec-pattern-nesting.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![feature(advanced_slice_patterns)]
+#![feature(box_patterns)]
 #![feature(box_syntax)]
 
 fn a() {