Re: [PATCH v2 00/16] rust: QObject and QAPI bindings

[Markus Armbruster <armbru@redhat.com>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> This adds two related parts of the Rust bindings:
>
> - QAPI code generator that creates Rust structs from the JSON
>   description.  The structs are *not* ABI compatible with the
>   C ones, instead they use native Rust data types.
>
> - QObject bindings and (de)serialization support, which can be used to
>   convert QObjects to and from QAPI structs.

To and from generated Rust structs, I presume.

> Unfortunately Rust code is not able to use visitors, other than by
> creating an intermediate QObject.  This is because of the different
> architecture of serde vs. QAPI visitors, and because visitor's
> dual-purpose functions, where the same function is used by both input and
> output visitors, rely heavily on the structs using the same representation
> as the visitor arguments (for example NUL-terminated strings).

Prior discussion of some aspects:

    Subject: Re: [PATCH preview 0/3] reviving minimal QAPI generation from 2021
    Date: Mon, 23 Jun 2025 14:52:46 +0200
    Message-ID: <877c1262n5.fsf@pond.sub.org>
    https://lore.kernel.org/qemu-devel/877c1262n5.fsf@pond.sub.org/

> The serde format implementation was co-authored by me and Marc-André.
> Marc-André did all the bug fixing and integration testing.
>
> As an example of how this would be used, the marshaling functions for
> QMP commands would look like this:
>
> fn qmp_marshal_query_stats(args: *mut QDict,
>     retp: *mut *mut QObject, errp: *mut *mut Error)
> {
>     let qobj = unsafe { QObject::cloned_from_raw(args.cast()) };
>
>     let result = from_qobject::<StatsFilter>(qobj)
>          .map_err(anyhow::Error::from)
>          .and_then(qmp_query_stats)
>          .and_then(|ret| to_qobject::<Vec<StatsResult>>(ret).map_err(anyhow::Error::from));
>
>     match qmp_marshal_query_stats_rs(qobj) {
>         Ok(ret) => unsafe { *retp = ret.into_raw(); },
>         Err(e) => unsafe { crate::Error::from(e).propagate(errp) },
>     }
> }

Scoring 0.23 UOUPNBL (uses of unsafe per non-blank line).  SCNR!

> Despite the long v1->v2, the changes are mostly cosmetic; there are a few
> small differences in the detection of types to be wrapped with Box<>,
> because Vec<> is now considered to break cycles.
>
> Available at branch rust-next of https://gitlab.com/bonzini/qemu (which
> is now almost empty other than this series, yay).
>
> Paolo