Add `box_patterns` feature gate.

Switch feature-gate checker from `box_syntax` to `box_patterns` when
visiting a pattern.

(Having to opt into both `box_syntax` and `box_patterns` seemed
unnecessary.)

[breaking-change]

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 {
@@ -486,7 +489,7 @@ 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");
             }