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

[Zhao Liu <zhao1.liu@intel.com>]

Hi Paolo,

On Sat, Sep 20, 2025 at 04:29:54PM +0200, Paolo Bonzini wrote:
> Date: Sat, 20 Sep 2025 16:29:54 +0200
> From: Paolo Bonzini <pbonzini@redhat.com>
> Subject: [PATCH 3/7] rust: migration: extract vmstate_fields_ref
> X-Mailer: git-send-email 2.51.0
> 
> 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.

vmstate_fields_ref is good for me.

> 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

And I'm curious what change caused this error... I guess you're
indicating the following case?

diff --git a/rust/migration/src/vmstate.rs b/rust/migration/src/vmstate.rs
index c05c4a1fd665..bff6ba502e02 100644
--- a/rust/migration/src/vmstate.rs
+++ b/rust/migration/src/vmstate.rs
@@ -417,14 +417,13 @@ const fn phantom__<T>(_: &T) -> ::core::marker::PhantomData<T> {
 #[macro_export]
 macro_rules! vmstate_fields {
     ($($field:expr),*$(,)*) => {{
-        static _FIELDS: &[$crate::bindings::VMStateField] = &[
+        &[
             $($field),*,
             $crate::bindings::VMStateField {
                 flags: $crate::bindings::VMStateFlags::VMS_END,
                 ..::common::zeroable::Zeroable::ZERO
             }
-        ];
-        _FIELDS.as_ptr()
+        ]
     }}
 }

> 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()

Considerring the idea about returing &'static [VMStateField], I think at
least we could return `_FIELDS.as_ptr()` directly from vmstate_fields!,
and convert &'static [VMStateField] to *const [VMStateField] in
fields(), just like subsections() did.

Thanks,
Zhao

>      }}
>  }
> -- 
> 2.51.0
>