Re: [PATCH 15/19] scripts/qapi: add serde attributes

[Markus Armbruster <armbru@redhat.com>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> From: Marc-André Lureau <marcandre.lureau@redhat.com>
>
> Generate serde attributes to match the serialization format to QAPI's:
>
> - for enums, map Rust enum variants to original QAPI names
>
> - for structs, rejects JSON with extra fields and omit optional fields
>   (as opposed to serializing them as null)
>
> - for union variants:
>   - use tagged union format matching QAPI's discriminator,
>   - map variant names to original QAPI names
>   - flatten union data into parent struct
>
> - for alternates, use type-based discrimination
>
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Again, quick look at the generated code first.

Diff since the previous patch, with comments inline:

  diff -rup ex/commit-450ce0c076/example-qapi-types.rs ex/commit-dc1aa3de44/example-qapi-types.rs
  --- ex/commit-450ce0c076/example-qapi-types.rs	2025-12-09 08:28:34.167519593 +0100
  +++ ex/commit-dc1aa3de44/example-qapi-types.rs	2025-12-09 08:30:13.913250024 +0100
  @@ -11,27 +11,38 @@
   // that *could* be Eq too.
   #![allow(clippy::derive_partial_eq_without_eq)]

  +use serde_derive::{Serialize, Deserialize};
  +
   use util::qobject::QObject;


   #[repr(u32)]
  -#[derive(Copy, Clone, Debug, PartialEq, common::TryInto)]
  +#[derive(Copy, Clone, Debug, PartialEq, Serialize, Deserialize,
  +         common::TryInto)]
   pub enum QType {

  +    #[serde(rename = "none")]
       NONE,

NONE is an error value.  It must not occur as type of a QObject (see
qobject_type()'s assertion).  It should never occur in serialization /
deserializion.  Is there a way to instruct serde accordingly?

Related: generated QType_lookup[] maps QTYPE_NONE to "none", because the
generator special case required to map it to NULL isn't worth the
bother.

  +    #[serde(rename = "qnull")]
       QNULL,

  +    #[serde(rename = "qnum")]
       QNUM,

  +    #[serde(rename = "qstring")]
       QSTRING,

  +    #[serde(rename = "qdict")]
       QDICT,

  +    #[serde(rename = "qlist")]
       QLIST,

  +    #[serde(rename = "qbool")]
       QBOOL,

  +    #[serde(rename = "_MAX")]
       _MAX,

This one must not occur, either.  Generated QType_lookup[] does not have
a value for it.

   }

  @@ -44,12 +55,16 @@ impl Default for QType {
   }


  -#[derive(Clone, Debug, PartialEq)]
  +#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
  +#[serde(deny_unknown_fields)]
   pub struct UserDefOne {

  +    

Funny extra blank line, next patch will revert it.

       pub integer: i64,

  +    #[serde(skip_serializing_if = "Option::is_none")]
       pub string: Option<String>,

  +    #[serde(skip_serializing_if = "Option::is_none")]
       pub flag: Option<bool>,
   }