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.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 rust/hw/char/pl011/src/device.rs |  3 +++
 rust/hw/timer/hpet/src/hpet.rs   |  3 ++-
 rust/qemu-api/src/qom.rs         | 14 +++++++++++---
 rust/qemu-api/tests/tests.rs     |  3 +++
 4 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/rust/hw/char/pl011/src/device.rs b/rust/hw/char/pl011/src/device.rs
index 7063b60c0cc..a6da9db0bb0 100644
--- a/rust/hw/char/pl011/src/device.rs
+++ b/rust/hw/char/pl011/src/device.rs
@@ -160,6 +160,7 @@ impl ObjectImpl for PL011State {
 
     const INSTANCE_INIT: Option<unsafe fn(&mut Self)> = Some(Self::init);
     const INSTANCE_POST_INIT: Option<fn(&Self)> = Some(Self::post_init);
+    const CLASS_INIT: fn(&mut Self::Class) = <Self as ClassInitImpl<Self::Class>>::class_init;
 }
 
 impl DeviceImpl for PL011State {
@@ -744,6 +745,8 @@ unsafe impl ObjectType for PL011Luminary {
 
 impl ObjectImpl for PL011Luminary {
     type ParentType = PL011State;
+
+    const CLASS_INIT: fn(&mut Self::Class) = <Self as ClassInitImpl<Self::Class>>::class_init;
 }
 
 impl DeviceImpl for PL011Luminary {}
diff --git a/rust/hw/timer/hpet/src/hpet.rs b/rust/hw/timer/hpet/src/hpet.rs
index b4ffccf815f..e01b4b67064 100644
--- a/rust/hw/timer/hpet/src/hpet.rs
+++ b/rust/hw/timer/hpet/src/hpet.rs
@@ -21,7 +21,7 @@
     },
     prelude::*,
     qdev::{DeviceImpl, DeviceMethods, DeviceState, Property, ResetType, ResettablePhasesImpl},
-    qom::{ObjectImpl, ObjectType, ParentField},
+    qom::{ClassInitImpl, ObjectImpl, ObjectType, ParentField},
     qom_isa,
     sysbus::{SysBusDevice, SysBusDeviceImpl},
     timer::{Timer, CLOCK_VIRTUAL},
@@ -836,6 +836,7 @@ impl ObjectImpl for HPETState {
 
     const INSTANCE_INIT: Option<unsafe fn(&mut Self)> = Some(Self::init);
     const INSTANCE_POST_INIT: Option<fn(&Self)> = Some(Self::post_init);
+    const CLASS_INIT: fn(&mut Self::Class) = <Self as ClassInitImpl<Self::Class>>::class_init;
 }
 
 // TODO: Make these properties user-configurable!
diff --git a/rust/qemu-api/src/qom.rs b/rust/qemu-api/src/qom.rs
index 10ce359becb..d821ac25acc 100644
--- a/rust/qemu-api/src/qom.rs
+++ b/rust/qemu-api/src/qom.rs
@@ -180,7 +180,7 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
     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 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
     // 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 03569e4a44c..9546e9d7963 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 {}
-- 
2.48.1