rust: qom: split ObjectType from ObjectImpl trait

Define a separate trait for fields that also applies to classes that are
defined by C code.  This makes it possible to add metadata to core classes,
which has multiple uses:

- it makes it possible to access the parent struct's TYPE_* for types
  that are defined in Rust code, and to avoid repeating it in every subclass

- implementors of ObjectType will be allowed to implement the IsA<> trait and
  therefore to perform typesafe casts from one class to another.

- in the future, an ObjectType could be created with Foo::new() in a type-safe
  manner, without having to pass a TYPE_* constant.

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

4 files changed, 37 insertions, 13 deletions

diff --git a/rust/qemu-api/src/definitions.rs b/rust/qemu-api/src/definitions.rs
index f297075898..b98a692678 100644
--- a/rust/qemu-api/src/definitions.rs
+++ b/rust/qemu-api/src/definitions.rs
@@ -26,25 +26,40 @@ unsafe extern "C" fn rust_instance_post_init<T: ObjectImpl>(obj: *mut Object) {
     T::INSTANCE_POST_INIT.unwrap()(unsafe { &mut *obj.cast::<T>() })
 }
 
-/// Trait a type must implement to be registered with QEMU.
+/// Trait exposed by all structs corresponding to QOM objects.
 ///
 /// # Safety
 ///
-/// - the struct must be `#[repr(C)]`
+/// For classes declared in C:
+///
+/// - `Class` and `TYPE` must match the data in the `TypeInfo`;
+///
+/// - the first field of the struct must be of the instance type corresponding
+///   to the superclass, as declared in the `TypeInfo`
+///
+/// - likewise, the first field of the `Class` struct must be of the class type
+///   corresponding to the superclass
 ///
-/// - `Class` and `TYPE` must match the data in the `TypeInfo` (this is
-///   automatic if the class is defined via `ObjectImpl`).
+/// For classes declared in Rust and implementing [`ObjectImpl`]:
+///
+/// - the struct must be `#[repr(C)]`;
 ///
 /// - the first field of the struct must be of the instance struct corresponding
-///   to the superclass declared as `PARENT_TYPE_NAME`
-pub trait ObjectImpl: ClassInitImpl + Sized {
+///   to the superclass, as declared in `ObjectImpl::PARENT_TYPE_NAME`
+///
+/// - likewise, the first field of the `Class` must be of the class struct
+///   corresponding to the superclass
+pub unsafe trait ObjectType: Sized {
     /// The QOM class object corresponding to this struct.  Not used yet.
     type Class;
 
     /// The name of the type, which can be passed to `object_new()` to
     /// generate an instance of this type.
     const TYPE_NAME: &'static CStr;
+}
 
+/// Trait a type must implement to be registered with QEMU.
+pub trait ObjectImpl: ObjectType + ClassInitImpl {
     /// The parent of the type.  This should match the first field of
     /// the struct that implements `ObjectImpl`:
     const PARENT_TYPE_NAME: Option<&'static CStr>;
diff --git a/rust/qemu-api/src/device_class.rs b/rust/qemu-api/src/device_class.rs
index f25904be4f..03d03feee8 100644
--- a/rust/qemu-api/src/device_class.rs
+++ b/rust/qemu-api/src/device_class.rs
@@ -6,6 +6,7 @@ use std::{ffi::CStr, os::raw::c_void};
 
 use crate::{
     bindings::{self, DeviceClass, DeviceState, Error, ObjectClass, Property, VMStateDescription},
+    prelude::*,
     zeroable::Zeroable,
 };
 
@@ -146,8 +147,8 @@ macro_rules! declare_properties {
     };
 }
 
-// workaround until we can use --generate-cstr in bindgen.
-pub const TYPE_DEVICE: &CStr =
-    unsafe { CStr::from_bytes_with_nul_unchecked(bindings::TYPE_DEVICE) };
-pub const TYPE_SYS_BUS_DEVICE: &CStr =
-    unsafe { CStr::from_bytes_with_nul_unchecked(bindings::TYPE_SYS_BUS_DEVICE) };
+unsafe impl ObjectType for bindings::DeviceState {
+    type Class = bindings::DeviceClass;
+    const TYPE_NAME: &'static CStr =
+        unsafe { CStr::from_bytes_with_nul_unchecked(bindings::TYPE_DEVICE) };
+}
diff --git a/rust/qemu-api/src/prelude.rs b/rust/qemu-api/src/prelude.rs
index a39e228bab..1b8677b2d9 100644
--- a/rust/qemu-api/src/prelude.rs
+++ b/rust/qemu-api/src/prelude.rs
@@ -6,3 +6,5 @@ pub use crate::bitops::IntegerExt;
 
 pub use crate::cell::BqlCell;
 pub use crate::cell::BqlRefCell;
+
+pub use crate::definitions::ObjectType;
diff --git a/rust/qemu-api/src/sysbus.rs b/rust/qemu-api/src/sysbus.rs
index 4e192c7589..5ee068541c 100644
--- a/rust/qemu-api/src/sysbus.rs
+++ b/rust/qemu-api/src/sysbus.rs
@@ -2,11 +2,17 @@
 // Author(s): Paolo Bonzini <pbonzini@redhat.com>
 // SPDX-License-Identifier: GPL-2.0-or-later
 
-use std::ptr::addr_of;
+use std::{ffi::CStr, ptr::addr_of};
 
 pub use bindings::{SysBusDevice, SysBusDeviceClass};
 
-use crate::{bindings, cell::bql_locked, irq::InterruptSource};
+use crate::{bindings, cell::bql_locked, irq::InterruptSource, prelude::*};
+
+unsafe impl ObjectType for SysBusDevice {
+    type Class = SysBusDeviceClass;
+    const TYPE_NAME: &'static CStr =
+        unsafe { CStr::from_bytes_with_nul_unchecked(bindings::TYPE_SYS_BUS_DEVICE) };
+}
 
 impl SysBusDevice {
     /// Return `self` cast to a mutable pointer, for use in calls to C code.