rust: device: make lifetime on `Core` and `CoreInternal` invariant

Currently the lifetime on `Core` and `CoreInternal` is covariant. This
means that they can be coerced into shorter living lifetimes. On `probe`
function, signature has `&'bound Device<Core<'a>>`; the type's wellformness
would imply `'a: 'bound` and thus the type can be coerced `&'bound
Device<Core<'bound>>`, defeating the purpose of having the lifetime bound
to prevent users of the `Core` type to escape the function.

Fix this by making the lifetime invariant, so the coercion is impossible.
The lifetime here only needs to be "branded" so it does not coerce or unify
with other lifetimes, so we do not need to ensure `'bound: 'a`.

This requires modifying `nova-core` which relies on this implied bound due
to pre-2024 capture rule. The "use" bound can be removed if built with
edition 2024.

Fixes: 24799831d631 ("rust: device: make Core and CoreInternal lifetime-parameterized")
Signed-off-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260713201455.640151-1-gary@kernel.org
[ Fixup the debugfs sample to use an explicit lifetime instead of
  Core<'_>. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

1 files changed, 8 insertions, 2 deletions

diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
index b538c39982a6..82e46b37e814 100644
--- a/rust/kernel/device.rs
+++ b/rust/kernel/device.rs
@@ -504,7 +504,11 @@ pub struct Normal;
 /// callback it appears in. It is intended to be used for synchronization purposes. Bus device
 /// implementations can implement methods for [`Device<Core>`], such that they can only be called
 /// from bus callbacks.
-pub struct Core<'a>(PhantomData<&'a ()>);
+///
+/// The lifetime `'a` is for "lifetime branding" purpose. Callbacks need to polymorphic over this
+/// lifetime so the `&'bound Device<Core<'_>>` provided to them cannot outlive the scope of the
+/// function. For this reason, it needs to be invariant.
+pub struct Core<'a>(PhantomData<fn(&'a ()) -> &'a ()>);
 
 /// Semantically the same as [`Core`], but reserved for internal usage of the corresponding bus
 /// abstraction.
@@ -515,7 +519,9 @@ pub struct Core<'a>(PhantomData<&'a ()>);
 ///
 /// This context mainly exists to share generic [`Device`] infrastructure that should only be called
 /// from bus callbacks with bus abstractions, but without making them accessible for drivers.
-pub struct CoreInternal<'a>(PhantomData<&'a ()>);
+///
+/// Lifetime `'a` is invariant for the same reason as [`Core`].
+pub struct CoreInternal<'a>(PhantomData<fn(&'a ()) -> &'a ()>);
 
 /// Semantically the same as [`Bound`], but reserved for internal usage of the corresponding bus
 /// abstraction.