rust: qom: rename Class trait to ClassInitImpl

While at it, document it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/hw/char/pl011/src/device.rs

@@ -117,7 +117,7 @@ pub struct PL011Class {
     _inner: [u8; 0],
 }
 
-impl qemu_api::definitions::Class for PL011Class {
+impl qemu_api::definitions::ClassInitImpl for PL011Class {
     const CLASS_INIT: Option<unsafe extern "C" fn(klass: *mut ObjectClass, data: *mut c_void)> =
         Some(crate::device_class::pl011_class_init);
     const CLASS_BASE_INIT: Option<
@@ -650,7 +650,7 @@ pub unsafe extern "C" fn pl011_luminary_init(obj: *mut Object) {
     }
 }
 
-impl qemu_api::definitions::Class for PL011LuminaryClass {
+impl qemu_api::definitions::ClassInitImpl for PL011LuminaryClass {
     const CLASS_INIT: Option<unsafe extern "C" fn(klass: *mut ObjectClass, data: *mut c_void)> =
         None;
     const CLASS_BASE_INIT: Option<

rust/qemu-api/src/definitions.rs

@@ -20,8 +20,27 @@ pub trait ObjectImpl {
     const INSTANCE_FINALIZE: Option<unsafe extern "C" fn(obj: *mut Object)> = None;
 }
 
-pub trait Class {
+/// Trait used to fill in a class struct.
+///
+/// Each QOM class that has virtual methods describes them in a
+/// _class struct_.  Class structs include a parent field corresponding
+/// to the vtable of the parent class, all the way up to [`ObjectClass`].
+/// Each QOM type has one such class struct.
+///
+/// The Rust implementation of methods will usually come from a trait
+/// like [`ObjectImpl`].
+pub trait ClassInitImpl {
+    /// Function that is called after all parent class initialization
+    /// has occurred.  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.
     const CLASS_INIT: Option<unsafe extern "C" fn(klass: *mut ObjectClass, data: *mut c_void)>;
+
+    /// Called on descendent classes after all parent class initialization
+    /// has occurred, but before the class itself is initialized.  This
+    /// is only useful if a class is not a leaf, and can be used to undo
+    /// the effects of copying the contents of the parent's class struct
+    /// to the descendants.
     const CLASS_BASE_INIT: Option<
         unsafe extern "C" fn(klass: *mut ObjectClass, data: *mut c_void),
     >;
@@ -82,8 +101,8 @@ macro_rules! type_info {
             instance_finalize: <$t as $crate::definitions::ObjectImpl>::INSTANCE_FINALIZE,
             abstract_: <$t as $crate::definitions::ObjectImpl>::ABSTRACT,
             class_size:  ::core::mem::size_of::<<$t as $crate::definitions::ObjectImpl>::Class>(),
-            class_init: <<$t as $crate::definitions::ObjectImpl>::Class as $crate::definitions::Class>::CLASS_INIT,
+            class_init: <<$t as $crate::definitions::ObjectImpl>::Class as $crate::definitions::ClassInitImpl>::CLASS_INIT,
-            class_base_init: <<$t as $crate::definitions::ObjectImpl>::Class as $crate::definitions::Class>::CLASS_BASE_INIT,
+            class_base_init: <<$t as $crate::definitions::ObjectImpl>::Class as $crate::definitions::ClassInitImpl>::CLASS_BASE_INIT,
             class_data: ::core::ptr::null_mut(),
             interfaces: ::core::ptr::null_mut(),
         };

rust/qemu-api/tests/tests.rs

@@ -7,7 +7,7 @@ use std::{ffi::CStr, os::raw::c_void};
 use qemu_api::{
     bindings::*,
     c_str, declare_properties, define_property,
-    definitions::{Class, ObjectImpl},
+    definitions::{ClassInitImpl, ObjectImpl},
     device_class, device_class_init,
     zeroable::Zeroable,
 };
@@ -60,7 +60,7 @@ fn test_device_decl_macros() {
         const PARENT_TYPE_NAME: Option<&'static CStr> = Some(device_class::TYPE_DEVICE);
     }
 
-    impl Class for DummyClass {
+    impl ClassInitImpl for DummyClass {
         const CLASS_INIT: Option<unsafe extern "C" fn(klass: *mut ObjectClass, data: *mut c_void)> =
             Some(dummy_class_init);
         const CLASS_BASE_INIT: Option<