From 1f420b9891c962656eac16831c772cbd56afff24 Mon Sep 17 00:00:00 2001
From: Christian Poveda <christianpoveda@protonmail.com>
Date: Thu, 21 Nov 2019 14:33:39 -0500
Subject: [PATCH] Allow mutable borrows in constant bodies

---
 src/librustc_mir/transform/check_consts/ops.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/librustc_mir/transform/check_consts/ops.rs b/src/librustc_mir/transform/check_consts/ops.rs
index a4f12a4e54fa..117d628af257 100644
--- a/src/librustc_mir/transform/check_consts/ops.rs
+++ b/src/librustc_mir/transform/check_consts/ops.rs
@@ -183,6 +183,10 @@ impl NonConstOp for Loop {
 #[derive(Debug)]
 pub struct MutBorrow(pub BorrowKind);
 impl NonConstOp for MutBorrow {
+    fn feature_gate(tcx: TyCtxt<'_>) -> Option<bool> {
+        Some(tcx.features().const_mut_refs)
+    }
+
     fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
         let kind = self.0;
         if let BorrowKind::Mut { .. } = kind {