Always evaluate the expression in [expr; n]

In case that there is a destination for the array, like in
"let x = [expr; n]", we currently don't evaluate the given expression if
n is zero. That's inconsistent with all other cases, including "[expr;
0]" without a destination.

Fixes #23354

src/librustc_trans/trans/tvec.rs

@@ -293,7 +293,7 @@ pub fn write_content<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
                 }
                 SaveIn(lldest) => {
                     match ty::eval_repeat_count(bcx.tcx(), &**count_expr) {
-                        0 => bcx,
+                        0 => expr::trans_into(bcx, &**element, Ignore),
                         1 => expr::trans_into(bcx, &**element, SaveIn(lldest)),
                         count => {
                             let elem = unpack_datum!(bcx, expr::trans(bcx, &**element));

src/test/run-fail/issue-23354.rs

@@ -0,0 +1,16 @@
+// 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.
+
+// error-pattern:panic evaluated
+
+#[allow(unused_variables)]
+fn main() {
+    let x = [panic!("panic evaluated"); 0];
+}