From 571b0fef01ea994cd9a81405283049b5595499c5 Mon Sep 17 00:00:00 2001
From: Michael Krasnitski <michael.krasnitski@gmail.com>
Date: Mon, 13 Mar 2023 01:31:02 -0400
Subject: [PATCH] Clarify stability guarantee for lifetimes in enum
 discriminants

---
 library/core/src/mem/mod.rs | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/library/core/src/mem/mod.rs b/library/core/src/mem/mod.rs
index a67df7ed557a1..aa53aa29bdbf3 100644
--- a/library/core/src/mem/mod.rs
+++ b/library/core/src/mem/mod.rs
@@ -1118,6 +1118,11 @@ impl<T> fmt::Debug for Discriminant<T> {
 ///
 /// [Reference]: ../../reference/items/enumerations.html#custom-discriminant-values-for-fieldless-enumerations
 ///
+/// The value of a [`Discriminant<T>`] is independent of any *lifetimes* in `T`. As such, reading
+/// or writing a `Discriminant<Foo<'a>>` as a `Discriminant<Foo<'b>>` (whether via [`transmute`] or
+/// otherwise) is always sound. Note that this is **not** true for other kinds of generic
+/// parameters; `Discriminant<Foo<A>>` and `Discriminant<Foo<B>>` might be incompatible.
+///
 /// # Examples
 ///
 /// This can be used to compare enums that carry data, while disregarding