Better diagnostics and recovery for const in extern blocks

src/test/ui/extern-const.rs

@@ -0,0 +1,19 @@
+// Copyright 2017 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.
+
+// compile-flags: -Z continue-parse-after-error
+
+extern "C" {
+    const C: u8; //~ ERROR extern items cannot be `const`
+}
+
+fn main() {
+    let x = C;
+}

src/test/ui/extern-const.stderr

@@ -0,0 +1,8 @@
+error: extern items cannot be `const`
+  --> $DIR/extern-const.rs:14:5
+   |
+14 |     const C: u8; //~ ERROR extern items cannot be `const`
+   |     ^^^^^ use `static` instead
+
+error: aborting due to previous error
+