From acdcf237af76f1ffb101e7bf95241d0b4dcd2b48 Mon Sep 17 00:00:00 2001
From: Lann Martin <lann.martin@fermyon.com>
Date: Thu, 13 Jun 2024 10:28:09 -0400
Subject: [PATCH] Make FieldOffset covariant with the field type

The documentation states: "A pointer-to-member can be thought of as a
function from `&T` to `&U` with matching lifetimes".

This updates the PhantomData field to better reflect that statement.
Importantly, this removes the `U:` constraints on auto trait impls,
making e.g. `FieldOffset<T, U>: Sync` even if `U: !Sync`.
---
 src/lib.rs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index b0a12c7..a93fa11 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -52,12 +52,12 @@ pub struct FieldOffset<T, U, PinFlag = NotPinned>(
     ///     of.apply(foo)
     /// }
     /// ```
-    PhantomData<(PhantomContra<T>, U, PinFlag)>,
+    PhantomData<(PhantomFn<T, U>, PinFlag)>,
 );
 
 /// `fn` cannot appear directly in a type that need to be const.
 /// Workaround that with an indirection
-struct PhantomContra<T>(fn(T));
+struct PhantomFn<T, U>(fn(T) -> U);
 
 /// Type that can be used in the `PinFlag` parameter of `FieldOffset` to specify that
 /// this projection is valid on Pin types.