From 57c4db34cd0ce07677f33e66c21a144f97296666 Mon Sep 17 00:00:00 2001
From: Bastian Kauschke <bastian_kauschke@hotmail.de>
Date: Sun, 3 May 2020 09:06:47 +0200
Subject: [PATCH] make `mem::Discriminant` invariant

---
 src/libcore/mem/mod.rs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/libcore/mem/mod.rs b/src/libcore/mem/mod.rs
index b1bbcaeab8def..cef319cd10d3a 100644
--- a/src/libcore/mem/mod.rs
+++ b/src/libcore/mem/mod.rs
@@ -929,8 +929,9 @@ pub unsafe fn transmute_copy<T, U>(src: &T) -> U {
 /// See the [`discriminant`] function in this module for more information.
 ///
 /// [`discriminant`]: fn.discriminant.html
+// We use `PhantomData<fn(T) -> T>` because we want `Discriminant` to be invariant.
 #[stable(feature = "discriminant_value", since = "1.21.0")]
-pub struct Discriminant<T>(u64, PhantomData<fn() -> T>);
+pub struct Discriminant<T>(u64, PhantomData<fn(T) -> T>);
 
 // N.B. These trait implementations cannot be derived because we don't want any bounds on T.