rust: vmstate: implement Zeroable for VMStateField

This shortens a bit the constants.  Do not bother using it
in the vmstate macros since most of them will go away soon.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api/src/vmstate.rs

@@ -21,8 +21,8 @@
 
 use core::{marker::PhantomData, mem, ptr::NonNull};
 
-use crate::bindings::VMStateFlags;
 pub use crate::bindings::{VMStateDescription, VMStateField};
+use crate::bindings::VMStateFlags;
 
 /// This macro is used to call a function with a generic argument bound
 /// to the type of a field.  The function must take a
@@ -503,20 +503,8 @@ macro_rules! vmstate_fields {
         static _FIELDS: &[$crate::bindings::VMStateField] = &[
             $($field),*,
             $crate::bindings::VMStateField {
-                name: ::core::ptr::null(),
+                flags: $crate::bindings::VMStateFlags::VMS_END,
-                err_hint: ::core::ptr::null(),
+                ..$crate::zeroable::Zeroable::ZERO
-                offset: 0,
-                size: 0,
-                start: 0,
-                num: 0,
-                num_offset: 0,
-                size_offset: 0,
-                info: ::core::ptr::null(),
-                flags: VMStateFlags::VMS_END,
-                vmsd: ::core::ptr::null(),
-                version_id: 0,
-                struct_version_id: 0,
-                field_exists: None,
             }
         ];
         _FIELDS.as_ptr()

rust/qemu-api/src/zeroable.rs

@@ -49,6 +49,37 @@ unsafe impl Zeroable for crate::bindings::Property {
     };
 }
 
+// bindgen does not derive Default here
+#[allow(clippy::derivable_impls)]
+impl Default for crate::bindings::VMStateFlags {
+    fn default() -> Self {
+        Self(0)
+    }
+}
+
+unsafe impl Zeroable for crate::bindings::VMStateFlags {
+    const ZERO: Self = Self(0);
+}
+
+unsafe impl Zeroable for crate::bindings::VMStateField {
+    const ZERO: Self = Self {
+        name: ptr::null(),
+        err_hint: ptr::null(),
+        offset: 0,
+        size: 0,
+        start: 0,
+        num: 0,
+        num_offset: 0,
+        size_offset: 0,
+        info: ptr::null(),
+        flags: Zeroable::ZERO,
+        vmsd: ptr::null(),
+        version_id: 0,
+        struct_version_id: 0,
+        field_exists: None,
+    };
+}
+
 unsafe impl Zeroable for crate::bindings::VMStateDescription {
     const ZERO: Self = Self {
         name: ptr::null(),