[PATCH 03/11] rust: migration: do not store raw pointers into VMStateSubsectionsWrapper

[Paolo Bonzini <pbonzini@redhat.com>]

Raw pointers were used to insert a NULL one at the end of the array.
However, Option<&...> has the same layout and does not remove Sync
from the type of the array.

As an extra benefit, this enables validation of the terminator of the
subsection array, because is_null() in const context would not be stable
until Rust 1.84.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 rust/migration/src/vmstate.rs | 29 +++++++++--------------------
 1 file changed, 9 insertions(+), 20 deletions(-)

diff --git a/rust/migration/src/vmstate.rs b/rust/migration/src/vmstate.rs
index 319d353c311..6a89769984f 100644
--- a/rust/migration/src/vmstate.rs
+++ b/rust/migration/src/vmstate.rs
@@ -469,33 +469,21 @@ unsafe impl $crate::vmstate::VMState for $type {
     };
 }
 
-/// A transparent wrapper type for the `subsections` field of
-/// [`VMStateDescription`].
-///
-/// This is necessary to be able to declare subsection descriptions as statics,
-/// because the only way to implement `Sync` for a foreign type (and `*const`
-/// pointers are foreign types in Rust) is to create a wrapper struct and
-/// `unsafe impl Sync` for it.
-///
-/// This struct is used in the
-/// [`vm_state_subsections`](crate::vmstate_subsections) macro implementation.
-#[repr(transparent)]
-pub struct VMStateSubsectionsWrapper(pub &'static [*const crate::bindings::VMStateDescription]);
-
-unsafe impl Sync for VMStateSubsectionsWrapper {}
+/// The type returned by [`vmstate_subsections!`].
+pub type VMStateSubsections = &'static [Option<&'static crate::bindings::VMStateDescription>];
 
 /// Helper macro to declare a list of subsections ([`VMStateDescription`])
 /// into a static and return a pointer to the array of pointers it created.
 #[macro_export]
 macro_rules! vmstate_subsections {
     ($($subsection:expr),*$(,)*) => {{
-        static _SUBSECTIONS: $crate::vmstate::VMStateSubsectionsWrapper = $crate::vmstate::VMStateSubsectionsWrapper(&[
+        static _SUBSECTIONS: $crate::vmstate::VMStateSubsections = &[
             $({
                 static _SUBSECTION: $crate::bindings::VMStateDescription = $subsection.get();
-                ::core::ptr::addr_of!(_SUBSECTION)
+                Some(&_SUBSECTION)
             }),*,
-            ::core::ptr::null()
-        ]);
+            None,
+        ];
         &_SUBSECTIONS
     }}
 }
@@ -685,8 +673,9 @@ pub const fn fields(mut self, fields: &'static [VMStateField]) -> Self {
     }
 
     #[must_use]
-    pub const fn subsections(mut self, subs: &'static VMStateSubsectionsWrapper) -> Self {
-        self.0.subsections = subs.0.as_ptr();
+    pub const fn subsections(mut self, subs: &'static VMStateSubsections) -> Self {
+        let subs: *const Option<&bindings::VMStateDescription> = subs.as_ptr();
+        self.0.subsections = subs.cast::<*const bindings::VMStateDescription>();
         self
     }
 
-- 
2.51.0