Fix ICE on using result of index on a constant to index into a constant

The issue was that the const evaluator was returning an error because
the feature flag const_indexing wasn't turned on. The error was then
reported as a bug.

Fixes #29914

Author: wesleywiser

src/librustc_trans/trans/consts.rs

@@ -663,6 +663,7 @@ fn const_expr_unadjusted<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>,
             let iv = match eval_const_expr_partial(cx.tcx(), &index, ExprTypeChecked, None) {
                 Ok(ConstVal::Int(i)) => i as u64,
                 Ok(ConstVal::Uint(u)) => u,
+                Err(e) => return Err(ConstEvalFailure::Compiletime(e)),
                 _ => cx.sess().span_bug(index.span,
                                         "index is not an integer-constant expression")
             };

src/test/run-pass/issue-29914-2.rs

@@ -0,0 +1,16 @@
+// Copyright 2016 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.
+#![feature(const_indexing)]
+
+const ARR: [usize; 5] = [5, 4, 3, 2, 1];
+
+fn main() {
+    assert_eq!(3, ARR[ARR[3]]);
+}

src/test/run-pass/issue-29914.rs

@@ -0,0 +1,15 @@
+// Copyright 2016 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.
+
+const ARR: [usize; 5] = [5, 4, 3, 2, 1];
+
+fn main() {
+    assert_eq!(3, ARR[ARR[3]]);
+}