From 2f9d33830c7bf1189dffec72cd4a9319c9e3250f Mon Sep 17 00:00:00 2001
From: Christian Poveda <git@christianpoveda.xyz>
Date: Sat, 6 Jun 2020 19:25:02 -0500
Subject: [PATCH] forbid mutable references in all constants except const fns

---
 src/librustc_mir/transform/check_consts/ops.rs | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/librustc_mir/transform/check_consts/ops.rs b/src/librustc_mir/transform/check_consts/ops.rs
index d5059c98c951..39095d13dedc 100644
--- a/src/librustc_mir/transform/check_consts/ops.rs
+++ b/src/librustc_mir/transform/check_consts/ops.rs
@@ -205,6 +205,12 @@ impl NonConstOp for CellBorrow {
 #[derive(Debug)]
 pub struct MutBorrow;
 impl NonConstOp for MutBorrow {
+    fn is_allowed_in_item(&self, ccx: &ConstCx<'_, '_>) -> bool {
+        // Forbid everywhere except in const fn
+        ccx.const_kind() == hir::ConstContext::ConstFn
+            && ccx.tcx.features().enabled(Self::feature_gate().unwrap())
+    }
+
     fn feature_gate() -> Option<Symbol> {
         Some(sym::const_mut_refs)
     }