From d92e9b7374bb2087e0eb4803bfabb030c1397bcd Mon Sep 17 00:00:00 2001
From: Christian Poveda <christianpoveda@protonmail.com>
Date: Fri, 22 Nov 2019 19:59:34 -0500
Subject: [PATCH] Allow mutable derefs with feature gate

---
 src/librustc_mir/transform/check_consts/ops.rs      | 6 +++++-
 src/test/ui/consts/const-mut-refs/const_mut_refs.rs | 4 +++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/librustc_mir/transform/check_consts/ops.rs b/src/librustc_mir/transform/check_consts/ops.rs
index 117d628af257..be5845eaa365 100644
--- a/src/librustc_mir/transform/check_consts/ops.rs
+++ b/src/librustc_mir/transform/check_consts/ops.rs
@@ -216,7 +216,11 @@ impl NonConstOp for MutBorrow {
 
 #[derive(Debug)]
 pub struct MutDeref;
-impl NonConstOp for MutDeref {}
+impl NonConstOp for MutDeref {
+    fn feature_gate(tcx: TyCtxt<'_>) -> Option<bool> {
+        Some(tcx.features().const_mut_refs)
+    }
+}
 
 #[derive(Debug)]
 pub struct Panic;
diff --git a/src/test/ui/consts/const-mut-refs/const_mut_refs.rs b/src/test/ui/consts/const-mut-refs/const_mut_refs.rs
index 60a69cb9f3f6..36174b2843d3 100644
--- a/src/test/ui/consts/const-mut-refs/const_mut_refs.rs
+++ b/src/test/ui/consts/const-mut-refs/const_mut_refs.rs
@@ -7,7 +7,9 @@ struct Foo {
 }
 
 const fn bar(foo: &mut Foo) -> usize {
-    foo.x + 1
+    let x = &mut foo.x;
+    *x = 1;
+    *x
 }
 
 fn main() {