From cb37ee2c87be982a3fab793264bbb6f81c4cd861 Mon Sep 17 00:00:00 2001
From: Benno Lossin <benno.lossin@proton.me>
Date: Fri, 3 Apr 2026 21:31:05 +0200
Subject: [PATCH] make field representing types invariant over the base type

Reported-by: David Rheinsberg <david@readahead.eu>
Link: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/Multiple.20LockedBy.20with.20Single.20Lock/near/579275908
---
 library/core/src/field.rs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/library/core/src/field.rs b/library/core/src/field.rs
index e8ef309b9c84..0e537e2f92fc 100644
--- a/library/core/src/field.rs
+++ b/library/core/src/field.rs
@@ -8,7 +8,10 @@
 #[expect(missing_debug_implementations)]
 #[fundamental]
 pub struct FieldRepresentingType<T: ?Sized, const VARIANT: u32, const FIELD: u32> {
-    _phantom: PhantomData<T>,
+    // We want this type to be invariant over `T`, because otherwise `field_of!(Struct<'short>,
+    // field)` is a subtype of `field_of!(Struct<'long>, field)`. This subtype relationship does not
+    // have an immediately obvious meaning and we want to prevent people from relying on it.
+    _phantom: PhantomData<fn(T) -> T>,
 }
 
 // SAFETY: `FieldRepresentingType` doesn't contain any `T`