rust: qom: add ObjectImpl::CLASS_INIT

As shown in the PL011 device, the orphan rules required a manual
implementation of ClassInitImpl for anything not in the qemu_api crate;
this gets in the way of moving system emulation-specific code (including
DeviceClass, which as a blanket ClassInitImpl<DeviceClass> implementation)
into its own crate.

Make ClassInitImpl optional, at the cost of having to specify the CLASS_INIT
member by hand in every implementation of ObjectImpl.  The next commits will
get rid of it, replacing all the "impl<T> ClassInitImpl<Class> for T" blocks
with a generic class_init<T> method on Class.

Right now the definition is always the same, but do not provide a default
as that will not be true once ClassInitImpl goes away.

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

2 files changed, 14 insertions, 3 deletions

diff --git a/rust/qemu-api/src/qom.rs b/rust/qemu-api/src/qom.rs
index 10ce359bec..d821ac25ac 100644
--- a/rust/qemu-api/src/qom.rs
+++ b/rust/qemu-api/src/qom.rs
@@ -180,7 +180,7 @@ unsafe extern "C" fn rust_instance_post_init<T: ObjectImpl>(obj: *mut Object) {
     T::INSTANCE_POST_INIT.unwrap()(unsafe { state.as_ref() });
 }
 
-unsafe extern "C" fn rust_class_init<T: ObjectType + ClassInitImpl<T::Class>>(
+unsafe extern "C" fn rust_class_init<T: ObjectType + ObjectImpl>(
     klass: *mut ObjectClass,
     _data: *mut c_void,
 ) {
@@ -190,7 +190,7 @@ unsafe extern "C" fn rust_class_init<T: ObjectType + ClassInitImpl<T::Class>>(
     // SAFETY: klass is a T::Class, since rust_class_init<T>
     // is called from QOM core as the class_init function
     // for class T
-    T::class_init(unsafe { klass.as_mut() })
+    <T as ObjectImpl>::CLASS_INIT(unsafe { klass.as_mut() })
 }
 
 unsafe extern "C" fn drop_object<T: ObjectImpl>(obj: *mut Object) {
@@ -499,7 +499,7 @@ impl<T: ObjectType> ObjectDeref for &mut T {}
 impl<T: ObjectType> ObjectCastMut for &mut T {}
 
 /// Trait a type must implement to be registered with QEMU.
-pub trait ObjectImpl: ObjectType + ClassInitImpl<Self::Class> + IsA<Object> {
+pub trait ObjectImpl: ObjectType + IsA<Object> {
     /// The parent of the type.  This should match the first field of the
     /// struct that implements `ObjectImpl`, minus the `ParentField<_>` wrapper.
     type ParentType: ObjectType;
@@ -552,6 +552,14 @@ pub trait ObjectImpl: ObjectType + ClassInitImpl<Self::Class> + IsA<Object> {
 
     // methods on ObjectClass
     const UNPARENT: Option<fn(&Self)> = None;
+
+    /// Store into the argument the virtual method implementations
+    /// for `Self`.  On entry, the virtual method pointers are set to
+    /// the default values coming from the parent classes; the function
+    /// can change them to override virtual methods of a parent class.
+    ///
+    /// Usually defined as `<Self as ClassInitImpl<Self::Class>::class_init`.
+    const CLASS_INIT: fn(&mut Self::Class);
 }
 
 /// Internal trait used to automatically fill in a class struct.
diff --git a/rust/qemu-api/tests/tests.rs b/rust/qemu-api/tests/tests.rs
index 03569e4a44..9546e9d796 100644
--- a/rust/qemu-api/tests/tests.rs
+++ b/rust/qemu-api/tests/tests.rs
@@ -60,6 +60,7 @@ unsafe impl ObjectType for DummyState {
 impl ObjectImpl for DummyState {
     type ParentType = DeviceState;
     const ABSTRACT: bool = false;
+    const CLASS_INIT: fn(&mut DummyClass) = <Self as ClassInitImpl<DummyClass>>::class_init;
 }
 
 impl ResettablePhasesImpl for DummyState {}
@@ -102,6 +103,8 @@ unsafe impl ObjectType for DummyChildState {
 impl ObjectImpl for DummyChildState {
     type ParentType = DummyState;
     const ABSTRACT: bool = false;
+    const CLASS_INIT: fn(&mut DummyChildClass) =
+        <Self as ClassInitImpl<DummyChildClass>>::class_init;
 }
 
 impl ResettablePhasesImpl for DummyChildState {}