rustc: Avoid an unwrap() in check_match

Closes #12369

src/librustc/middle/check_match.rs

@@ -342,8 +342,10 @@ fn is_useful_specialized(cx: &MatchCheckCtxt,
     let ms = m.iter().filter_map(|r| {
         specialize(cx, r.as_slice(), &ctor, arity, lty)
     }).collect::<matrix>();
-    let could_be_useful = is_useful(
-        cx, &ms, specialize(cx, v, &ctor, arity, lty).unwrap().as_slice());
+    let could_be_useful = match specialize(cx, v, &ctor, arity, lty) {
+        Some(v) => is_useful(cx, &ms, v.as_slice()),
+        None => return not_useful,
+    };
     match could_be_useful {
       useful_ => useful(lty, ctor),
       u => u,

src/test/compile-fail/issue-12369.rs

@@ -0,0 +1,19 @@
+// 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.
+
+fn main() {
+    let sl = vec![1,2,3];
+    let v: int = match sl.as_slice() {
+        [] => 0,
+        [a,b,c] => 3,
+        [a, ..rest] => a,
+        [10,a, ..rest] => 10 //~ ERROR: unreachable pattern
+    };
+}