syntax: Enable parsing of const globals

This rewrites them to the current `ItemStatic` production of the compiler, but I
want to get this into a snapshot. It will be illegal to use a `static` in a
pattern of a `match` statement, so all those current uses will need to be
rewritten to `const` once it's implemented. This requires that the stage0
snapshot is able to parse `const`.

cc #17718

src/libsyntax/parse/parser.rs

@@ -4746,8 +4746,7 @@ impl<'a> Parser<'a> {
         }
     }
 
-    fn parse_item_const(&mut self) -> ItemInfo {
-        let m = if self.eat_keyword(keywords::Mut) {MutMutable} else {MutImmutable};
+    fn parse_item_const(&mut self, m: Mutability) -> ItemInfo {
         let id = self.parse_ident();
         self.expect(&token::COLON);
         let ty = self.parse_ty(true);
@@ -5303,7 +5302,26 @@ impl<'a> Parser<'a> {
         if self.is_keyword(keywords::Static) {
             // STATIC ITEM
             self.bump();
-            let (ident, item_, extra_attrs) = self.parse_item_const();
+            let m = if self.eat_keyword(keywords::Mut) {MutMutable} else {MutImmutable};
+            let (ident, item_, extra_attrs) = self.parse_item_const(m);
+            let last_span = self.last_span;
+            let item = self.mk_item(lo,
+                                    last_span.hi,
+                                    ident,
+                                    item_,
+                                    visibility,
+                                    maybe_append(attrs, extra_attrs));
+            return IoviItem(item);
+        }
+        if self.is_keyword(keywords::Const) {
+            // CONST ITEM
+            self.bump();
+            if self.eat_keyword(keywords::Mut) {
+                let last_span = self.last_span;
+                self.span_err(last_span, "const globals cannot be mutable, \
+                                          did you mean to declare a static?");
+            }
+            let (ident, item_, extra_attrs) = self.parse_item_const(MutImmutable);
             let last_span = self.last_span;
             let item = self.mk_item(lo,
                                     last_span.hi,

src/test/compile-fail/issue-17718-const-mut.rs

@@ -0,0 +1,17 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+const
+mut //~ ERROR: const globals cannot be mutable, did you mean to declare a static?
+FOO: uint = 3;
+
+fn main() {
+}
+

src/test/run-pass/issue-17718-parse-const.rs

@@ -0,0 +1,15 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+const FOO: uint = 3;
+
+fn main() {
+    assert_eq!(FOO, 3);
+}