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

@@ -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`