don't warn on casting byte strs to slices

src/librustc_const_eval/eval.rs

@@ -1116,6 +1116,7 @@ fn cast_const<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, val: ConstVal, ty: ty::Ty)
             ty::TyRawPtr(_) => {
                 Err(ErrKind::UnimplementedConstVal("casting a bytestr to a raw ptr"))
             },
+            ty::TyRef(..) => Err(ErrKind::UnimplementedConstVal("casting a bytestr to slice")),
             _ => Err(CannotCast),
         },
         _ => Err(CannotCast),

src/test/run-pass/const-byte-str-cast.rs

@@ -0,0 +1,15 @@
+// Copyright 2013 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.
+
+#[deny(warnings)]
+
+pub fn main() {
+    let _ = b"x" as &[u8];
+}