Re: [PATCH 2/9] rust/vmstate: Support varray's num field wrapped in BqlCell

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

> > #[macro_export]
> > macro_rules! if_present {
> >      ([$($cond:tt)*]: $($result:tt)*) => { $($result)* };
> > }
> > 
> > to expand the array part of the access:
> > 
> > assert_field_type!(...
> >     $($crate::if_present!([$num]: [0]))?;
> 
> This example remind me that I introduced a bug into array part:
> 
>     let index: usize = v.$num.try_into().unwrap();
>     types_must_be_equal::<_, &$ti>(&v.$i[index]);
> 
> In the current code, actually it accesses v[num], but when num
> stores the length of the whole array, it will cause index out of bounds.
> 
> So for current code, at least it should access `v.i[num - 1]`:
> 
>     let index: usize = v.$num.try_into().unwrap() - 1; // access the last element.
>     types_must_be_equal::<_, &$ti>(&v.$i[index]);

I realize that my thinking was wrong here! The `v` (with specific type)
isn't a valid instance, and the variable `num` being passed isn't
correctly initialized. Therefore, checking `num`'s value here is
meaningless; it's enough to just check if the type matches!

> > );
> > 
> > With this change, assert_field_type! is nicer and at least the trait you're
> > introducing in assertions.rs goes away...
> 
> Yes! Great idea.
> 
> Then with your help, we could integrate the array part like:
> 
> #[macro_export]
> macro_rules! if_present {
>     ([$($cond:tt)*]: $($result:tt)*) => { $($result)* };
> }
> 
> ...
> 
> #[macro_export]
> macro_rules! assert_field_type {
>     ($t:ty, $i:tt, $ti:ty $(, $num:ident)?) => {
>         const _: () = {
>             #[allow(unused)]
>             fn assert_field_type(v: $t) {
>                 fn types_must_be_equal<T, U>(_: T)
>                 where
>                     T: $crate::assertions::EqType<Itself = U>,
>                 {
>                 }
> 
>                 let access = v.$i$($crate::if_present!([$num]: [v.$num - 1])])?;

So, the correct code should just check array[0] as you said:

let access = v.$i$($crate::if_present!([$num]: [0])])?;

Based on this, there's no need for anything else such as `Into`.

>                 types_must_be_equal::<_, $ti>(access);
>             }
>         };
>     };
> }

Thanks,
Zhao