Add test for constant pattern diagnostics

src/test/compile-fail/const-pattern-irrefutable.rs

@@ -0,0 +1,28 @@
+// Copyright 2015 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.
+
+mod foo {
+    const b: u8 = 2; //~ NOTE constant defined here
+    const d: u8 = 2; //~ NOTE constant defined here
+}
+
+use foo::b as c; //~ NOTE constant imported here
+use foo::d; //~ NOTE constant imported here
+
+const a: u8 = 2; //~ NOTE constant defined here
+
+fn main() {
+    let a = 4; //~ ERROR only irrefutable
+               //~^ NOTE there already is a constant in scope
+    let c = 4; //~ ERROR only irrefutable
+               //~^ NOTE there already is a constant in scope
+    let d = 4; //~ ERROR only irrefutable
+               //~^ NOTE there already is a constant in scope
+}