rust: qemu_api: add vmstate_struct

It is not type safe, but it's the best that can be done without
const_refs_static.  It can also be used with BqlCell and BqlRefCell.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api/src/vmstate.rs

@@ -628,6 +628,39 @@ macro_rules! vmstate_array_of_pointer_to_struct {
     }};
 }
 
+// FIXME: including the `vmsd` field in a `const` is not possible without
+// the const_refs_static feature (stabilized in Rust 1.83.0).  Without it,
+// it is not possible to use VMS_STRUCT in a transparent manner using
+// `vmstate_of!`.  While VMSTATE_CLOCK can at least try to be type-safe,
+// VMSTATE_STRUCT includes $type only for documentation purposes; it
+// is checked against $field_name and $struct_name, but not against $vmsd
+// which is what really would matter.
+#[doc(alias = "VMSTATE_STRUCT")]
+#[macro_export]
+macro_rules! vmstate_struct {
+    ($struct_name:ty, $field_name:ident $([0 .. $num:ident $(* $factor:expr)?])?, $vmsd:expr, $type:ty $(,)?) => {
+        $crate::bindings::VMStateField {
+            name: ::core::concat!(::core::stringify!($field_name), "\0")
+                .as_bytes()
+                .as_ptr() as *const ::std::os::raw::c_char,
+            $(.num_offset: $crate::offset_of!($struct_name, $num),)?
+            offset: {
+                $crate::assert_field_type!($struct_name, $field_name, $type);
+                $crate::offset_of!($struct_name, $field_name)
+            },
+            size: ::core::mem::size_of::<$type>(),
+            flags: $crate::bindings::VMStateFlags::VMS_STRUCT,
+            vmsd: unsafe { $vmsd },
+            ..$crate::zeroable::Zeroable::ZERO $(
+                .with_varray_flag($crate::call_func_with_field!(
+                    $crate::vmstate::vmstate_varray_flag,
+                    $struct_name,
+                    $num))
+               $(.with_varray_multiply($factor))?)?
+        }
+    };
+}
+
 #[doc(alias = "VMSTATE_CLOCK_V")]
 #[macro_export]
 macro_rules! vmstate_clock_v {