auto merge of #6990 : huonw/rust/const-eval, r=bstrie

This fail! caused ICEs, but was trivial to convert to a useful error message.

src/librustc/middle/const_eval.rs

@@ -242,7 +242,7 @@ pub enum const_val {
 pub fn eval_const_expr(tcx: middle::ty::ctxt, e: @expr) -> const_val {
     match eval_const_expr_partial(tcx, e) {
         Ok(ref r) => (/*bad*/copy *r),
-        Err(ref s) => fail!(/*bad*/copy *s)
+        Err(ref s) => tcx.sess.span_fatal(e.span, *s)
     }
 }