[PATCH 3/7] rust: migration: extract vmstate_fields_ref

[Paolo Bonzini <pbonzini@redhat.com>]

This is useful when building a VMState for generic structs, because you have
to avoid nested statics.  Using vmstate_fields! will fail in the likely case
where the _FIELDS static uses Self from an outer item, because that is
forbidden.

I also looked into returning a &'static [VMStateField] from vmstate_fields!,
but that also fails; the fields are not promoted to static, which is
unfortunate but intentional (https://github.com/rust-lang/rust/issues/60502):

error[E0716]: temporary value dropped while borrowed
   --> rust/hw/char/pl011/libpl011.rlib.p/structured/device.rs:743:17
    |
738 | /      VMStateDescriptionBuilder::<PL011State>::new()
739 | |          .name(c"pl011/clock")
740 | |          .version_id(1)
741 | |          .minimum_version_id(1)
742 | |          .needed(&PL011State::clock_needed)
743 | |          .fields(vmstate_fields! {
    | | _________________^
744 | ||              vmstate_of!(PL011State, clock),
745 | ||         })
    | ||_________^- argument requires that borrow lasts for `'static`
    |  |_________|
    |            creates a temporary value which is freed while still in use
746 |            .build();
    |                   - temporary value is freed at the end of this statement

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 rust/migration/src/vmstate.rs | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/rust/migration/src/vmstate.rs b/rust/migration/src/vmstate.rs
index 05a833a8b7d..421a236194d 100644
--- a/rust/migration/src/vmstate.rs
+++ b/rust/migration/src/vmstate.rs
@@ -413,19 +413,30 @@ const fn phantom__<T>(_: &T) -> ::core::marker::PhantomData<T> {
     }};
 }
 
+/// Add a terminator to the fields in the arguments, and return
+/// a reference to the resulting array of values.
+#[macro_export]
+macro_rules! vmstate_fields_ref {
+    ($($field:expr),*$(,)*) => {
+        &[
+            $($field),*,
+            $crate::bindings::VMStateField {
+                flags: $crate::bindings::VMStateFlags::VMS_END,
+                ..::common::zeroable::Zeroable::ZERO
+            }
+        ]
+    }
+}
+
 /// Helper macro to declare a list of
 /// ([`VMStateField`](`crate::bindings::VMStateField`)) into a static and return
 /// a pointer to the array of values it created.
 #[macro_export]
 macro_rules! vmstate_fields {
     ($($field:expr),*$(,)*) => {{
-        static _FIELDS: &[$crate::bindings::VMStateField] = &[
+        static _FIELDS: &[$crate::bindings::VMStateField] = $crate::vmstate_fields_ref!(
             $($field),*,
-            $crate::bindings::VMStateField {
-                flags: $crate::bindings::VMStateFlags::VMS_END,
-                ..::common::zeroable::Zeroable::ZERO
-            }
-        ];
+        );
         _FIELDS.as_ptr()
     }}
 }
-- 
2.51.0