rust: assertions: Support index field wrapped in BqlCell

Currently, if the `num` field of a varray is not a numeric type, such as
being placed in a wrapper, the array variant of assert_field_type will
fail the check.

HPET currently wraps num_timers in BqlCell<>. Although BqlCell<> is not
necessary from strictly speaking, it makes sense for vmstate to respect
BqlCell.

The failure of assert_field_type is because it cannot convert BqlCell<T>
into usize for use as the index.  Use a constant 0 instead for the index,
by avoiding $(...)? and extracting the common parts of
assert_field_type! into an internal case.

Commit message based on a patch by Zhao Liu <zhao1.liu@intel.com>.

Link: https://lore.kernel.org/r/20250414144943.1112885-3-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 9 insertions, 16 deletions

diff --git a/rust/qemu-api/src/assertions.rs b/rust/qemu-api/src/assertions.rs
index eb12e9499a..a2d38c877d 100644
--- a/rust/qemu-api/src/assertions.rs
+++ b/rust/qemu-api/src/assertions.rs
@@ -78,33 +78,26 @@ macro_rules! assert_same_type {
 /// ```
 #[macro_export]
 macro_rules! assert_field_type {
-    ($t:ty, $i:tt, $ti:ty) => {
+    (@internal $param_name:ident, $ti:ty, $t:ty, $($field:tt)*) => {
         const _: () = {
             #[allow(unused)]
-            fn assert_field_type(v: $t) {
-                fn types_must_be_equal<T, U>(_: T)
+            fn assert_field_type($param_name: &$t) {
+                fn types_must_be_equal<T, U>(_: &T)
                 where
                     T: $crate::assertions::EqType<Itself = U>,
                 {
                 }
-                types_must_be_equal::<_, $ti>(v.$i);
+                types_must_be_equal::<_, $ti>(&$($field)*);
             }
         };
     };
 
+    ($t:ty, $i:tt, $ti:ty) => {
+        $crate::assert_field_type!(@internal v, $ti, $t, v.$i);
+    };
+
     ($t:ty, $i:tt, $ti:ty, num = $num:ident) => {
-        const _: () = {
-            #[allow(unused)]
-            fn assert_field_type(v: $t) {
-                fn types_must_be_equal<T, U>(_: T)
-                where
-                    T: $crate::assertions::EqType<Itself = U>,
-                {
-                }
-                let index: usize = v.$num.try_into().unwrap();
-                types_must_be_equal::<_, &$ti>(&v.$i[index]);
-            }
-        };
+        $crate::assert_field_type!(@internal v, $ti, $t, v.$i[0]);
     };
 }